Electrospun nanofibers have numerous applications in various fields. Developing an electrospinning technique for large-scale nanofiber production has become more and more important, as the conventional needle electrospinning has limited productivity and is only suitable for research purpose. Upward needleless electrospinning has been shown the ability to mass produce nanofibers and it is also the most successful design for practical applications. It is expected that the solvent can be recycled effectively so that the fibers are produced with minimal impact on the environment. An efficient needleless electrospinning system to produce nanofibers from thermoplastic polymers is yet to be developed.

### **8. References**

Electrospun nanofibers have numerous applications in various fields. Developing an electrospinning technique for large-scale nanofiber production has become more and more important, as the conventional needle electrospinning has limited productivity and is only suitable for research purpose. Upward needleless electrospinning has been shown the ability to mass produce nanofibers and it is also the most successful design for practical applications. It is expected that the solvent can be recycled effectively so that the fibers are produced with minimal impact on the environment. An efficient needleless electrospinning

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Cengiz F. & Jirsak O. (2009). The effect of salt on the roller electrospinning of polyurethane nanofibers. *Fibers and Polymers,* Vol. 10, No. 2, pp. 177-84, ISSN 1229-9197 Cengiz F., Krucinska I., Gliscinska E., Chrzanowski M. & Goktepe F. (2009). Comparative

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Fang J., Niu H., Lin T. & Wang X. G. (2008). Applications of electrospun nanofibers. *Chinese*

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Kim G., Choa Y. S. & Kim W. D. (2006). Stability analysis for multi-jets electrospinning

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system to produce nanofibers from thermoplastic polymers is yet to be developed.

**7. Concluding remarks**

**8. References**

*application 1975504.*

ISSN 1687-4110

2419-22, ISSN 0167-577X

(May 2010), pp. 936-43, ISSN 1548-2634

Vol. 17, No. 4, pp. 1123-27, ISSN 1361-6528

**3**

*USA*

**Electrospinning of Metal Doped Alumina**

The world is turning into a vast technological jungle and the amount of pollution has increased proportionally to the number of industries and world population. The majority of the pollution is created by anthropogenic activities that include burning of fossil fuels, exhaust of automobiles and other combustion processes. Beginning with the 1940s, some urban cities started experiencing air pollution because of the increasing number of automobiles (Haagen-Smit, 1970). By the 1960s, cars had been manufactured in large quantities and had become popular in providing personal mobility to an increasing range of people. But in automobiles, the process of combustion of gasoline in the engine to carbon dioxide (CO2) and water was not a very efficient process (Twigg, 2007). The byproducts of combustion include residual uncombusted hydrocarbons (HCs), partial combustion product carbon monoxide (CO), nitric oxides (NOx) formed from atmospheric nitrogen during combustion, and particulate matter (PM), especially carbonaceous particulate formed in diesel engines (Shelef & McCabe, 2000). In the engine exhausts the CO level ranged from 1 to 2 vol. % while unburnt HCs ranged between 500- 1000 vppm. During the process of combustion, very high temperatures are produced by the burning of gasoline droplets that result in the thermal fixation of N2 in the air to form NOx with concentrations in the range of 100-300 vppm (Patterson& Henein,1972; Armor, 1994; Heck & Farrauto, 1995; Silver et al., 1992). NOx (where 1<x<2), consisting mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO2 ) are one among many toxic pollutants in air. NO is the most abundant nitrogen derivative present in the atmosphere, and represents 95% of the total NOx emissions (Degobert & Marshall, 1995). The total nitrogen oxides emitted in the world from automotive exhausts was 6.4 million in 2005 (EPA,2005). NOx undergoes photochemical reactions leading to the generation of smog. It has been proven to cause serious health impacts such as respiratory troubles, circulation problems, and premature death (Patterson& Henein,1972; Meulenbelt, 2007). Another major pollutant is carbon monoxide. CO poisoning is the most common type of fatal poisoning in many countries. CO adversely effects human health leading to cardiovascular and neurobehavioral problems, developmental disorders and in some cases even death (Raub et al., 2000). CO primarily occurs in urban traffic where gasoline engines often idle resulting in high concentration levels of up to 100 mg/m3

To prevent environmental pollution, it is essential to eliminate these toxic air pollutants by converting them into less harmful substances. Hence their abatement using catalysts is a very important topic of research and application. Noble metals are most commonly used as

**1. Introduction**

(Degobert & Marshall, 1995).

**Nanofibers for Catalyst Applications**

Sneha Swaminathan and George Chase

*The University of Akron,*

### **Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications**

Sneha Swaminathan and George Chase *The University of Akron, USA*

#### **1. Introduction**

36 Nanofibers – Production, Properties and Functional Applications

Wu D., Huang X., Lai X., Sun D. & Lin L. (2010). High Throughput Tip-Less Electrospinning

Yang E., Shi J. & Xue Y. (2010). Influence of electric field interference on double nozzles

Yarin A. L. & Zussman E. (2004). Upward needleless electrospinning of multiple nanofibers.

*Polymer,* Vol. 45, No. 9, (April 2004), pp. 2977-80, ISSN 1548-2634

10, No. 7, (July 2010), pp. 4221-26, ISSN 1533-4899

3688-92, ISSN 1097-4628

via a Circular Cylindrical Electrode. *Journal of Nanoscience and Nanotechnology,* Vol.

electrospinning. *Journal of Applied Polymer Science,* Vol. 116, No. 6, (June 2010), pp.

The world is turning into a vast technological jungle and the amount of pollution has increased proportionally to the number of industries and world population. The majority of the pollution is created by anthropogenic activities that include burning of fossil fuels, exhaust of automobiles and other combustion processes. Beginning with the 1940s, some urban cities started experiencing air pollution because of the increasing number of automobiles (Haagen-Smit, 1970). By the 1960s, cars had been manufactured in large quantities and had become popular in providing personal mobility to an increasing range of people. But in automobiles, the process of combustion of gasoline in the engine to carbon dioxide (CO2) and water was not a very efficient process (Twigg, 2007). The byproducts of combustion include residual uncombusted hydrocarbons (HCs), partial combustion product carbon monoxide (CO), nitric oxides (NOx) formed from atmospheric nitrogen during combustion, and particulate matter (PM), especially carbonaceous particulate formed in diesel engines (Shelef & McCabe, 2000). In the engine exhausts the CO level ranged from 1 to 2 vol. % while unburnt HCs ranged between 500- 1000 vppm. During the process of combustion, very high temperatures are produced by the burning of gasoline droplets that result in the thermal fixation of N2 in the air to form NOx with concentrations in the range of 100-300 vppm (Patterson& Henein,1972; Armor, 1994; Heck & Farrauto, 1995; Silver et al., 1992). NOx (where 1<x<2), consisting mainly of nitrogen monoxide (NO) and nitrogen dioxide (NO2 ) are one among many toxic pollutants in air. NO is the most abundant nitrogen derivative present in the atmosphere, and represents 95% of the total NOx emissions (Degobert & Marshall, 1995). The total nitrogen oxides emitted in the world from automotive exhausts was 6.4 million in 2005 (EPA,2005). NOx undergoes photochemical reactions leading to the generation of smog. It has been proven to cause serious health impacts such as respiratory troubles, circulation problems, and premature death (Patterson& Henein,1972; Meulenbelt, 2007). Another major pollutant is carbon monoxide. CO poisoning is the most common type of fatal poisoning in many countries. CO adversely effects human health leading to cardiovascular and neurobehavioral problems, developmental disorders and in some cases even death (Raub et al., 2000). CO primarily occurs in urban traffic where gasoline engines often idle resulting in high concentration levels of up to 100 mg/m3 (Degobert & Marshall, 1995).

To prevent environmental pollution, it is essential to eliminate these toxic air pollutants by converting them into less harmful substances. Hence their abatement using catalysts is a very important topic of research and application. Noble metals are most commonly used as

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 39

minimize catalyst loss. A small amount of the catalyst incorporated ceramic nanofibers are mixed with microfibers to fabricate a filter disk by a vacuum molding technique. This filter disk, termed as 'catalytic filter', is a combination of catalytic elements and filter. The catalyzed ceramic nanofiber augmented microfiber filter media can be used for two applications: reduction of NOx and oxidation of CO and for enhanced particulate removal. This chapter describes the process of fabrication of the catalytic filters and its applications in

The combination of two operations, catalysis and filtration, in one operation improves performance and reduces processing, investment and maintenance costs (Heidenreich et al., 2008). The individual nanoparticles are dispersed throughout the entire fiber and firmly anchored to the walls of the pores of the alumina support; hence their tendency to sinter and coalesce is minimized. In addition, there is free diffusional access of reactants and exit of products from the nanocatalyst, thereby facilitating the catalytic turnover of bulky organic molecules such as unburnt hydrocarbons (Hermans et al., 2001; Raja et al., 2001; Thomas et al., 2001). Nanomaterials are more effective than the monolithic support structures for two reasons. First, their extremely small size yields a tremendous surface area-to-volume ratio. Second, when materials are fabricated on the nanoscale, they achieve properties not found within their macroscopic counterparts. Both of these reasons account for the versatility and effectiveness of nanocatalysts. They also help in reducing the cost of processes which are

Fibrous materials in filter media have advantages which include high filtration efficiency and low air resistance (Tsai et al., 2002). One method to develop highly effective and efficient filters is by the addition of nanofibers. When the channels and structural elements of a filter are matched to the size of the particles or droplets to be captured, the capture efficiency improves considerably (Graham et al., 2002). Nanofibrous media have low basis weight, high permeability and small pore size that make them appropriate for a wide range of filtration applications. Nanofibers offer unique properties of high specific surface area, high porosity and good interconnectivity of pores, potential to incorporate active chemistry

Fig. 1. Electrospinning setup.

reducing NOx and CO emissions.

indispensable without them (Schlogl & Hamid, 2004).

catalysts for reducing these air pollutants and to meet the stringent federal regulations (Patterson & Henein, 1972) The global market for energy and environmental catalysts was worth \$12.2 billion in 2006, \$13.0 billion in 2009 and an estimated \$18.5 billion in 2012 at a compound annual growth rate (CAGR) of 7.4 % from 2007 to 2012 (BCC Research, 2007).

Noble metals can be used as nanocatalysts to eliminate these toxic substances from the various exhausts. Nanosize materials possess high surface energy which result in specific catalytic activity and selectivity (Zhou et al., 2004). The catalytic activity of supported metal particle catalysts is strongly dependent on the size and shape of the particles. Hence, on a mass basis nanocatalysts are very active due to their high surface areas and high specific catalytic activity (Gai et al., 2002). Nanocatalysts also optimize the use of precious metals thus minimizing the specific cost per function.

Due to high surface area to volume ratio, noble metal nanoscale metals differ in their properties from bulk materials (Narayanan & El-Sayed, 2005). These materials have potential applications in catalysis and as sensors (Takashi et al.,2005). Supported nanoparticles have been used for a variety of heterogeneous catalysis reactions such as acylation, alkane isomerisation, benzene hydrogenation, Suzuki-Miayura reaction and Heck's reaction. Nanoparticles have properties in between bulk materials and molecules and are potentially useful for new generations of chemical, optical, magnetic, and electronic devices. Well distributed nano sized noble metal particles show better potential as catalysts than bulk materials [Gniewek et al., 2008; Luo et al., 2005; . Tribolet & Kiwi-Minsker, 2005).

Metal oxide nanofibers are potential candidates as catalyst supports for heterogeneous catalytic reactions (Patel et al., 2007; Park et al., 1998; Park et al.,2008). Nanofibers can be produced by electrospinning. Electrospinning is a process in which fibers of nanoscale or submicron scale are generated, as the electrified jet (composed of a highly viscous polymer solution) is continuously stretched due to electrostatic repulsions between the surface charges. The jet continues to stretch until it either encounters a grounded surface or enough of the solvent has evaporated that the jet is no longer fluid enough to stretch (Li & Xia, 2004). The fiber diameter can be controlled by varying the process parameters such as polymer concentration, viscosity, electric field, solvent composition, flow rate, diameter and angle of spinneret (Doshi & Reneker, 1995). As catalyst supports, nanofibers can provide a means of constructing catalytic systems that have large surface area, are lightweight, are thermally and chemically stable, and result in optimization of precious metals (Takashi et al., 2005). The formation of nanoscale diameter fibers from electrospinning have been explored for high-performance filters, wound dressing materials, and biomaterial scaffolds for tissue engineering (Wei & Ma, 2004; Filatov, 2007). The structures thus produced have a high surface area to volume ratio, high length to diameter ratio, high porosity and interconnected pores, good surface functionalities, and superior mechanical performance (e.g. stiffness and tensile strength) (Doshi & Reneker, 1995; Sukigara et al., 2003; Huang et al., 2003). Figure 1 shows the schematic diagram of an electrospinning setup.

Traditionally, the catalysts are coated onto commercial monolithic supports by the wetness impregnation technique. In this method, the catalyst precursor salt is dispersed on the support surface by the impregnation of the solution of the precursor. The precursor particles are formed in the drying process, calcined to form metal-oxides and subsequently reduced to form fine supported metal particles (Zhou et al., 2004).

In the present work, an alternative approach has been developed wherein the noble metal nanocatalysts are incorporated into a ceramic nanofiber. The catalysts on ceramic nanofibers increase the overall exposed catalyst area and simultaneously immobilize the catalyst to

Fig. 1. Electrospinning setup.

catalysts for reducing these air pollutants and to meet the stringent federal regulations (Patterson & Henein, 1972) The global market for energy and environmental catalysts was worth \$12.2 billion in 2006, \$13.0 billion in 2009 and an estimated \$18.5 billion in 2012 at a compound annual growth rate (CAGR) of 7.4 % from 2007 to 2012 (BCC Research, 2007). Noble metals can be used as nanocatalysts to eliminate these toxic substances from the various exhausts. Nanosize materials possess high surface energy which result in specific catalytic activity and selectivity (Zhou et al., 2004). The catalytic activity of supported metal particle catalysts is strongly dependent on the size and shape of the particles. Hence, on a mass basis nanocatalysts are very active due to their high surface areas and high specific catalytic activity (Gai et al., 2002). Nanocatalysts also optimize the use of precious metals

Due to high surface area to volume ratio, noble metal nanoscale metals differ in their properties from bulk materials (Narayanan & El-Sayed, 2005). These materials have potential applications in catalysis and as sensors (Takashi et al.,2005). Supported nanoparticles have been used for a variety of heterogeneous catalysis reactions such as acylation, alkane isomerisation, benzene hydrogenation, Suzuki-Miayura reaction and Heck's reaction. Nanoparticles have properties in between bulk materials and molecules and are potentially useful for new generations of chemical, optical, magnetic, and electronic devices. Well distributed nano sized noble metal particles show better potential as catalysts than bulk materials [Gniewek et al., 2008; Luo et al., 2005; . Tribolet & Kiwi-Minsker, 2005). Metal oxide nanofibers are potential candidates as catalyst supports for heterogeneous catalytic reactions (Patel et al., 2007; Park et al., 1998; Park et al.,2008). Nanofibers can be produced by electrospinning. Electrospinning is a process in which fibers of nanoscale or submicron scale are generated, as the electrified jet (composed of a highly viscous polymer solution) is continuously stretched due to electrostatic repulsions between the surface charges. The jet continues to stretch until it either encounters a grounded surface or enough of the solvent has evaporated that the jet is no longer fluid enough to stretch (Li & Xia, 2004). The fiber diameter can be controlled by varying the process parameters such as polymer concentration, viscosity, electric field, solvent composition, flow rate, diameter and angle of spinneret (Doshi & Reneker, 1995). As catalyst supports, nanofibers can provide a means of constructing catalytic systems that have large surface area, are lightweight, are thermally and chemically stable, and result in optimization of precious metals (Takashi et al., 2005). The formation of nanoscale diameter fibers from electrospinning have been explored for high-performance filters, wound dressing materials, and biomaterial scaffolds for tissue engineering (Wei & Ma, 2004; Filatov, 2007). The structures thus produced have a high surface area to volume ratio, high length to diameter ratio, high porosity and interconnected pores, good surface functionalities, and superior mechanical performance (e.g. stiffness and tensile strength) (Doshi & Reneker, 1995; Sukigara et al., 2003; Huang et

al., 2003). Figure 1 shows the schematic diagram of an electrospinning setup.

to form fine supported metal particles (Zhou et al., 2004).

Traditionally, the catalysts are coated onto commercial monolithic supports by the wetness impregnation technique. In this method, the catalyst precursor salt is dispersed on the support surface by the impregnation of the solution of the precursor. The precursor particles are formed in the drying process, calcined to form metal-oxides and subsequently reduced

In the present work, an alternative approach has been developed wherein the noble metal nanocatalysts are incorporated into a ceramic nanofiber. The catalysts on ceramic nanofibers increase the overall exposed catalyst area and simultaneously immobilize the catalyst to

thus minimizing the specific cost per function.

minimize catalyst loss. A small amount of the catalyst incorporated ceramic nanofibers are mixed with microfibers to fabricate a filter disk by a vacuum molding technique. This filter disk, termed as 'catalytic filter', is a combination of catalytic elements and filter. The catalyzed ceramic nanofiber augmented microfiber filter media can be used for two applications: reduction of NOx and oxidation of CO and for enhanced particulate removal. This chapter describes the process of fabrication of the catalytic filters and its applications in reducing NOx and CO emissions.

The combination of two operations, catalysis and filtration, in one operation improves performance and reduces processing, investment and maintenance costs (Heidenreich et al., 2008). The individual nanoparticles are dispersed throughout the entire fiber and firmly anchored to the walls of the pores of the alumina support; hence their tendency to sinter and coalesce is minimized. In addition, there is free diffusional access of reactants and exit of products from the nanocatalyst, thereby facilitating the catalytic turnover of bulky organic molecules such as unburnt hydrocarbons (Hermans et al., 2001; Raja et al., 2001; Thomas et al., 2001). Nanomaterials are more effective than the monolithic support structures for two reasons. First, their extremely small size yields a tremendous surface area-to-volume ratio. Second, when materials are fabricated on the nanoscale, they achieve properties not found within their macroscopic counterparts. Both of these reasons account for the versatility and effectiveness of nanocatalysts. They also help in reducing the cost of processes which are indispensable without them (Schlogl & Hamid, 2004).

Fibrous materials in filter media have advantages which include high filtration efficiency and low air resistance (Tsai et al., 2002). One method to develop highly effective and efficient filters is by the addition of nanofibers. When the channels and structural elements of a filter are matched to the size of the particles or droplets to be captured, the capture efficiency improves considerably (Graham et al., 2002). Nanofibrous media have low basis weight, high permeability and small pore size that make them appropriate for a wide range of filtration applications. Nanofibers offer unique properties of high specific surface area, high porosity and good interconnectivity of pores, potential to incorporate active chemistry

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 41

Fig. 2. Process flow diagram for the manufacture of catalytic filter.

The purposes of a carrier polymer are to assist with the electrospinning process and to form uniform colloidal dispersions of catalyst salt particles (Esumi et al., 1994). In this case, polyvinylpyrrolidone (PVP) was chosen as the carrier polymer because PVP can form a complex with the metal ions or metal particles, which can prevent the agglomeration of the cluster, and synthesis monodisperse nanoparticles with uniform size (Qian et al., 2001). PVP also plays an important role in protecting and stabilizing the colloidal dispersions of noble

or functionality on nanoscale and high permeability for gases (Barhate & Ramakrishna, 2007; Ahn et al., 2006). The nanofibrous media have advantages over mesoporous media by avoiding the mass transfer limitation of substrates due to reduced thickness and intrafiber porosity. In particular, they are highly successful for the development of high-performance air filters. The electrospinning process in particular can be used to tailor the porosity, fiber diameter, and pore size to achieve the desired structural configuration (Barhate & Ramakrishna, 2007).

From filtration theory, submicron fibers have better filter efficiency than larger fibers at the same pressure drop in the interception and inertial impaction regimes. The interception and inertial impaction efficiencies will increase faster, more than compensating for the pressure drop increase (Graham et al., 2002). A second factor for submicron fibers is the effect of slip flow on the fiber surface. One assumption in filtration theory is that of continuous flow around the fiber with no-slip condition at the fiber surface. But, when the scale of the fiber becomes small enough that the molecular movements of the air molecules are significant in relation to the size of the fibers and the flow field, this assumption becomes invalid and slipflow model is added to extend the filtration theory.

#### **2. Experimental: fabrication of catalytic filters**

The manufacture of the catalytic filter media involves the steps shown in Fig.2. The active ingredient in the catalytic filter is the ceramic nanofibers i.e. metal incorporated alumina nanofibers which is responsible for the conversion of NOx and CO into safer products.

Over the years, ceramic nanofibers have evolved with a multitude of applications which can be categorized primarily in four ways (Ramaseshan et al., 2007):

Hence, in this research the immense potential of ceramic nanofibers is exploited for catalysis and filtration applications.

The main steps in the fabrication of a catalytic filter are:

The synthesis of ceramic nanofibers involves three main steps:

or functionality on nanoscale and high permeability for gases (Barhate & Ramakrishna, 2007; Ahn et al., 2006). The nanofibrous media have advantages over mesoporous media by avoiding the mass transfer limitation of substrates due to reduced thickness and intrafiber porosity. In particular, they are highly successful for the development of high-performance air filters. The electrospinning process in particular can be used to tailor the porosity, fiber diameter, and pore size to achieve the desired structural configuration (Barhate &

From filtration theory, submicron fibers have better filter efficiency than larger fibers at the same pressure drop in the interception and inertial impaction regimes. The interception and inertial impaction efficiencies will increase faster, more than compensating for the pressure drop increase (Graham et al., 2002). A second factor for submicron fibers is the effect of slip flow on the fiber surface. One assumption in filtration theory is that of continuous flow around the fiber with no-slip condition at the fiber surface. But, when the scale of the fiber becomes small enough that the molecular movements of the air molecules are significant in relation to the size of the fibers and the flow field, this assumption becomes invalid and slip-

The manufacture of the catalytic filter media involves the steps shown in Fig.2. The active ingredient in the catalytic filter is the ceramic nanofibers i.e. metal incorporated alumina nanofibers which is responsible for the conversion of NOx and CO into safer products. Over the years, ceramic nanofibers have evolved with a multitude of applications which can

1. Chemistry based applications: catalysis, storage batteries, membranes, fuel cells,

2. Physical change based applications: optical communications, data and magnetic

4. Biomedical applications: biosensors, bone and tissue engineering, biomechanical

Hence, in this research the immense potential of ceramic nanofibers is exploited for catalysis

4. Reduction to convert the catalyst metal oxides in the alumina nanofibers to their

1. Preparation of sol with suitable inorganic precursor and carrier polymer with suitable

2. Electrospinning the sol by adjusting and optimizing conditions such as viscosity,

3. Calcination of the composite mat to decompose the polymer template and to convert

electric field, concentration, flow rate etc. to obtain composite fibers, and

3. Materials based applications: filtration, paints, packaging, aerospace, and

2. Electrospinning to form metal oxide incorporated alumina nanofibers,

Ramakrishna, 2007).

sensors,

storage,

devices

and filtration applications.

catalytic metal state.

flow model is added to extend the filtration theory.

**2. Experimental: fabrication of catalytic filters**

The main steps in the fabrication of a catalytic filter are:

The synthesis of ceramic nanofibers involves three main steps:

concentration, mixing and temperature conditions,

the metal precursors to their respective oxides.

1. Preparation of the electrospinning solution,

3. Vacuum molding to form fibrous filter, and

be categorized primarily in four ways (Ramaseshan et al., 2007):

Fig. 2. Process flow diagram for the manufacture of catalytic filter.

The purposes of a carrier polymer are to assist with the electrospinning process and to form uniform colloidal dispersions of catalyst salt particles (Esumi et al., 1994). In this case, polyvinylpyrrolidone (PVP) was chosen as the carrier polymer because PVP can form a complex with the metal ions or metal particles, which can prevent the agglomeration of the cluster, and synthesis monodisperse nanoparticles with uniform size (Qian et al., 2001). PVP also plays an important role in protecting and stabilizing the colloidal dispersions of noble

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 43

where, Li is the length of the fiber segments with diameter xi and Lt =∑Li. The distributions

Fig. 4. SEM images of (a) PdCl2/Aluminum Acetate/PVP nanofibers and (b) PdO/Al2O3

���� √�� ��� �� ��� ���)��� �����

���)�

� � (3)

� = 54.59 nm and standard

� ��� ���

are asymmetric and may be tted with a log-normal distribution.

Fig. 3. TGA of PdCl2/Aluminum Acetate/PVP nanofibers.

a b

The frequency distributions are determined from Equation (3)

�� <sup>=</sup> � ��

and plotted in Figure 5. The average fiber diameter was x���

deviation was �� = 1.28 for the log-normal distribution.

�

nanofibers.

metals. Since soluble polymers can cover the whole surface of the catalyst, the effect of the supporting polymer may be promoted in the case of the polymer- protected colloidal metals compared with that by inorganic supports (Hirai et al., 1985). The precursor for alumina is basic aluminum acetate. The oxidation of basic aluminum acetate produces alumina. Aluminum acetate is calcined in air at 600 oC to form alumina. At this temperature, the carrier polymer degrades primarily to CO2 and H2O. The stepwise conversion is as follows: From 170 to 220 °C, condensation of the OH groups on the basic acetate occurs by the reaction (Ayral & Droguet, 1989).

2 Al (OH) (OAc)2 Al2O(OAc)4 + H2O

From, 220 to 475 °C the two carboxylates groups decompose to form amorphous alumina. Other phases of the alumina can be obtained by calcining at higher temperatures (Ayral & Droguet, 1989). This is described by the reaction

Amorphous A12O3 γAl2O3 δA12O3 θAl2O3 α Al2O3. 800 oC 1200 oC

#### **2.1 Electrospinning of palladium doped alumina nanofibers**

To optimize the electrospinning process, combinations of several parameters have to be considered. 9 wt. % solution of polyvinylpyrrolidone (PVP, Aldrich, MW 1,300,000) in ethanol (Pharmco-AAPER alcohol) was prepared. This molecular weight of PVP was used because low concentrations of PVP could be used for electrospinning when diluted with ceramic precursor solution to obtain nanofibers with small diameters. Since the carrier polymer is generally sacrificed in the end to obtain ceramic nanofibers, lower concentrations of the polymer are preferred to increase the yield of ceramic nanofibers. Aluminum acetate precursor solution (AA solution) was prepared by mixing aluminum acetate (Alfa Aesar, Basic hydrate), water and formic acid in the weight ratio of 1:2.5:1, respectively. The PVP solution and AA solution were mixed in the weight ratio of 1:2, respectively. Palladium chloride (Sigma-Aldrich, 60 % Pd) was mixed in the resulting mixture in the ratio of 5 wt% PdCl2 with respect to aluminum acetate and stirred overnight at 40 oC using a magnetic stirrer. The solution was further diluted with ethanol. The solution was electrospun with a flow rate of 2 μl/min and an electric field of 1 kV/cm. The sample was placed in a vacuum oven at room temperature for 5 hours to remove trapped solvents. The electrospun nanofibers were calcined to 600 oC to degrade the polymer and convert the metal precursors to metal oxides. From Thermogravimetric Analysis (TGA) there was significant mass loss as the temperature was ramped up to 500 oC. Above 500oC, the sample mass became constant, indicating that the organic materials (PVP, ethanol, acetate group) had degraded and the volatiles had evaporated (Figure 3). Hence, the fibers calcined to 600 oC were relatively pure metal oxide and free of PVP and residual solvent.

To obtain the fiber diameter distribution, the diameters of 100 fibers were measured from 10 Scanning Electron Microscope (SEM) images similar to those in Figure 4. The distribution was calculated using Equations (1) and (2) (Dosunmu et al., 2006; Varabhas et al., 2008):

$$\ln(\overleftarrow{\mathbf{x}\_{g}}) = \frac{1}{L\_{t}} \sum L\_{l} \ln(\mathbf{x}\_{l}) \tag{1}$$

$$\ln\left(\sigma\_{g}\right) = \sqrt{\frac{\sum (L\_{l}(\ln(\mathbf{x}\_{l}) - \ln(\mathbf{x}\_{g}^{-}))^{x})}{L\_{\text{t}}}} \tag{2}$$

metals. Since soluble polymers can cover the whole surface of the catalyst, the effect of the supporting polymer may be promoted in the case of the polymer- protected colloidal metals compared with that by inorganic supports (Hirai et al., 1985). The precursor for alumina is basic aluminum acetate. The oxidation of basic aluminum acetate produces alumina. Aluminum acetate is calcined in air at 600 oC to form alumina. At this temperature, the carrier polymer degrades primarily to CO2 and H2O. The stepwise conversion is as follows: From 170 to 220 °C, condensation of the OH groups on the basic acetate occurs by the

2 Al (OH) (OAc)2 Al2O(OAc)4 + H2O From, 220 to 475 °C the two carboxylates groups decompose to form amorphous alumina. Other phases of the alumina can be obtained by calcining at higher temperatures (Ayral &

To optimize the electrospinning process, combinations of several parameters have to be considered. 9 wt. % solution of polyvinylpyrrolidone (PVP, Aldrich, MW 1,300,000) in ethanol (Pharmco-AAPER alcohol) was prepared. This molecular weight of PVP was used because low concentrations of PVP could be used for electrospinning when diluted with ceramic precursor solution to obtain nanofibers with small diameters. Since the carrier polymer is generally sacrificed in the end to obtain ceramic nanofibers, lower concentrations of the polymer are preferred to increase the yield of ceramic nanofibers. Aluminum acetate precursor solution (AA solution) was prepared by mixing aluminum acetate (Alfa Aesar, Basic hydrate), water and formic acid in the weight ratio of 1:2.5:1, respectively. The PVP solution and AA solution were mixed in the weight ratio of 1:2, respectively. Palladium chloride (Sigma-Aldrich, 60 % Pd) was mixed in the resulting mixture in the ratio of 5 wt% PdCl2 with respect to aluminum acetate and stirred overnight at 40 oC using a magnetic stirrer. The solution was further diluted with ethanol. The solution was electrospun with a flow rate of 2 μl/min and an electric field of 1 kV/cm. The sample was placed in a vacuum oven at room temperature for 5 hours to remove trapped solvents. The electrospun nanofibers were calcined to 600 oC to degrade the polymer and convert the metal precursors to metal oxides. From Thermogravimetric Analysis (TGA) there was significant mass loss as the temperature was ramped up to 500 oC. Above 500oC, the sample mass became constant, indicating that the organic materials (PVP, ethanol, acetate group) had degraded and the volatiles had evaporated (Figure 3). Hence, the fibers calcined to 600 oC were relatively pure

800 oC 1200 oC

To obtain the fiber diameter distribution, the diameters of 100 fibers were measured from 10 Scanning Electron Microscope (SEM) images similar to those in Figure 4. The distribution was calculated using Equations (1) and (2) (Dosunmu et al., 2006; Varabhas et al., 2008):

∑ �����(��) (1)

(2)

���)�)

��

���(���

��) = � ��

������ = �∑(��(���(��)���������

Amorphous A12O3 γAl2O3 δA12O3 θAl2O3 α Al2O3.

**2.1 Electrospinning of palladium doped alumina nanofibers**

reaction (Ayral & Droguet, 1989).

Droguet, 1989). This is described by the reaction

metal oxide and free of PVP and residual solvent.

where, Li is the length of the fiber segments with diameter xi and Lt =∑Li. The distributions are asymmetric and may be tted with a log-normal distribution.

Fig. 3. TGA of PdCl2/Aluminum Acetate/PVP nanofibers.

Fig. 4. SEM images of (a) PdCl2/Aluminum Acetate/PVP nanofibers and (b) PdO/Al2O3 nanofibers.

The frequency distributions are determined from Equation (3)

$$f\_l = \frac{1}{d\_l \ln \sigma\_\theta} \frac{1}{\sqrt{2\pi}} \exp\left(-\frac{(\ln(\chi\_l) - \ln(\overline{\chi\_\theta}))^2}{2\left(\ln \sigma\_\theta\right)^2}\right) \tag{3}$$

and plotted in Figure 5. The average fiber diameter was x��� � = 54.59 nm and standard deviation was �� = 1.28 for the log-normal distribution.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 45

The 9 wt. % PVP solution and AA solution was mixed in the ratio of 1:3 by weight, respectively. Platinum chloride (PtCl2, 99.9 %, Sigma Aldrich) was dissolved in a mixture of water and ethanol. This solution contained 3.8 wt. % of platinum chloride with respect to aluminum acetate. The solution was mixed overnight at 50 oC. Then the solution was electrospun at a flow rate of 2 µl/min and an electric field of 1 kV/cm. The sample was placed in a vacuum oven at room temperature for 5 hours to remove trapped solvents. The electrospun nanofibers were calcined to 600 oC for 4 hours to obtain PtO/Al2O3. Similar to Figure 3 for PdO/Al2O3, the TGA analysis of PtO/Al2O3 showed significant mass loss occurred as the temperature was ramped up to 450 oC. Above 450 oC the sample mass became constant, indicating that the organic materials (PVP, ethanol, acetate group) had degraded and the volatile components had evaporated. Hence, the fibers calcined to 600 oC

Figure 7a shows scanning electron micrographs of PtCl2/Aluminum Acetate/ PVP composite nanofibers. From Figure 7b, the average fiber diameters of PtO/Al2O3 were in the range of 35-65 nm. After these composite nanofibers were calcined to 600 oC, they were converted to PtO/Al2O3. Figure 8 shows a sample TEM image of PtO/Al2O3 of PtO nanoparticles on the alumina nanofiber. From the TEM images the average particle size of 100 measurements of the PtO nanoparticles on the Al2O3 nanofiber was calculated to be

The length weighted log-normal frequency ber diameter distributions for PtO/Al2O3 are

Fig. 7. Scanning electron micrographs of (a) PtCl2/Aluminum Acetate/ PVP composite

�� = 53.05 nm and standard deviation of

**2.2 Electrospinning of platinum incorporated alumina nanofibers**

were relatively pure PtO/Al2O3 and free of PVP and residual solvents.

**a b**

shown in Figure 11 with average fiber diameter of ���

�� = 1.53 for the log-normal distribution.

nanofibers (b) PtO/Al2O3 nanofibers.

about 2.87±0.82 nm.

Fig. 5. Length weighted log-normal frequency distribution of PdO/Al2O3 fiber diameters.

From Transmission Electron Microscope (TEM) images (Figure 6) the average particle size of 50 measurements of the PdO nanoparticles on the Al2O3 nanofiber was 2.43±0.70 nm.

Fig. 6. Transmission electron micrographs of PdO/Al2O3 nanofibers.

**Frequency Distribution vs**

**Fiber Diameter**

Fig. 5. Length weighted log-normal frequency distribution of PdO/Al2O3 fiber diameters. From Transmission Electron Microscope (TEM) images (Figure 6) the average particle size of

0 20 40 60 80 100

**Diameter (nm)**

50 measurements of the PdO nanoparticles on the Al2O3 nanofiber was 2.43±0.70 nm.

0 0.01 0.02 0.03 0.04

**f**

Fig. 6. Transmission electron micrographs of PdO/Al2O3 nanofibers.

#### **2.2 Electrospinning of platinum incorporated alumina nanofibers**

The 9 wt. % PVP solution and AA solution was mixed in the ratio of 1:3 by weight, respectively. Platinum chloride (PtCl2, 99.9 %, Sigma Aldrich) was dissolved in a mixture of water and ethanol. This solution contained 3.8 wt. % of platinum chloride with respect to aluminum acetate. The solution was mixed overnight at 50 oC. Then the solution was electrospun at a flow rate of 2 µl/min and an electric field of 1 kV/cm. The sample was placed in a vacuum oven at room temperature for 5 hours to remove trapped solvents. The electrospun nanofibers were calcined to 600 oC for 4 hours to obtain PtO/Al2O3. Similar to Figure 3 for PdO/Al2O3, the TGA analysis of PtO/Al2O3 showed significant mass loss occurred as the temperature was ramped up to 450 oC. Above 450 oC the sample mass became constant, indicating that the organic materials (PVP, ethanol, acetate group) had degraded and the volatile components had evaporated. Hence, the fibers calcined to 600 oC were relatively pure PtO/Al2O3 and free of PVP and residual solvents.

Figure 7a shows scanning electron micrographs of PtCl2/Aluminum Acetate/ PVP composite nanofibers. From Figure 7b, the average fiber diameters of PtO/Al2O3 were in the range of 35-65 nm. After these composite nanofibers were calcined to 600 oC, they were converted to PtO/Al2O3. Figure 8 shows a sample TEM image of PtO/Al2O3 of PtO nanoparticles on the alumina nanofiber. From the TEM images the average particle size of 100 measurements of the PtO nanoparticles on the Al2O3 nanofiber was calculated to be about 2.87±0.82 nm.

The length weighted log-normal frequency ber diameter distributions for PtO/Al2O3 are shown in Figure 11 with average fiber diameter of ��� �� = 53.05 nm and standard deviation of �� = 1.53 for the log-normal distribution.

Fig. 7. Scanning electron micrographs of (a) PtCl2/Aluminum Acetate/ PVP composite nanofibers (b) PtO/Al2O3 nanofibers.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 47

9 wt % PVP solution and AA precursor were added in the ratio of 1:4 by weight, respectively. Rhodium chloride (RhCl3, 99.9 %, Sigma Aldrich) was dissolved in ethanol in the ratio of 3.7 wt % of rhodium chloride respect to the aluminum acetate. This solution was mixed overnight at the 50 oC. Using the rotating drum collector electrospinning setup, nanofiber mat was produced with a flow rate flow rate of 2 µl/min and an electric field of 1 kV/cm. The nanofibers were calcined at 600oC for 4 hrs to form Rh2O3/Al2O3 nanofibers. The TGA analysis of Rh2O3/Al2O3 precursor fibers, similar to the TGA analysis of PdO/Al2O3 in Figure 3, showed significant mass loss as the temperature ramped up to 450 oC. Above 450 oC, the sample mass became constant, indicating degradation of the PVP and

Figure 10(a) shows SEM images of RhCl3/Aluminum Acetate/ PVP composite nanofibers. After these composite nanofibers were calcined to 600 oC, they were converted to Rh2O3/Al2O3 (Figure 10b). Figure 11 shows an example TEM image of Al2O3 fibers with the Rh2O3 nanoparticles on the nanofiber. The average particle size of the Rh2O3 nanoparticles

The length weighted log-normal frequency ber diameter distribution for Rh2O3/Al2O3 is

**a b**

Fig. 10. (a) Scanning electron micrographs of of RhCl3/Aluminum Acetate/ PVP composite

�� = 61.57 nm and the standard deviation

**2.3 Electrospinning of rhodium incorporated alumina nanofibers**

evaporation of volatiles.

shown in Figure 12. The average fiber diameter is ���

is �� = 1.33 for the log-normal distribution.

nanofibers (b) Rh2O3/Al2O3 nanofibers.

was 1.16±0.35 nm.

Fig. 8. Transmission electron micrographs of PtO/Al2O3 nanofibers.

Fig. 9. Length weighted log-normal frequency distribution of PtO/Al2O3 ber diameters.

Fig. 8. Transmission electron micrographs of PtO/Al2O3 nanofibers.

0

0.005

0.01

0.015

**f**

0.02

0.025

Fig. 9. Length weighted log-normal frequency distribution of PtO/Al2O3 ber diameters.

0 20 40 60 80 100 120

**Frequency Distribution vs**

**Fiber Diameter**

**Diameter (nm)**

#### **2.3 Electrospinning of rhodium incorporated alumina nanofibers**

9 wt % PVP solution and AA precursor were added in the ratio of 1:4 by weight, respectively. Rhodium chloride (RhCl3, 99.9 %, Sigma Aldrich) was dissolved in ethanol in the ratio of 3.7 wt % of rhodium chloride respect to the aluminum acetate. This solution was mixed overnight at the 50 oC. Using the rotating drum collector electrospinning setup, nanofiber mat was produced with a flow rate flow rate of 2 µl/min and an electric field of 1 kV/cm. The nanofibers were calcined at 600oC for 4 hrs to form Rh2O3/Al2O3 nanofibers.

The TGA analysis of Rh2O3/Al2O3 precursor fibers, similar to the TGA analysis of PdO/Al2O3 in Figure 3, showed significant mass loss as the temperature ramped up to 450 oC. Above 450 oC, the sample mass became constant, indicating degradation of the PVP and evaporation of volatiles.

Figure 10(a) shows SEM images of RhCl3/Aluminum Acetate/ PVP composite nanofibers. After these composite nanofibers were calcined to 600 oC, they were converted to Rh2O3/Al2O3 (Figure 10b). Figure 11 shows an example TEM image of Al2O3 fibers with the Rh2O3 nanoparticles on the nanofiber. The average particle size of the Rh2O3 nanoparticles was 1.16±0.35 nm.

The length weighted log-normal frequency ber diameter distribution for Rh2O3/Al2O3 is shown in Figure 12. The average fiber diameter is ��� �� = 61.57 nm and the standard deviation is �� = 1.33 for the log-normal distribution.

Fig. 10. (a) Scanning electron micrographs of of RhCl3/Aluminum Acetate/ PVP composite nanofibers (b) Rh2O3/Al2O3 nanofibers.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 49

Filter media were prepared by vacuum molding an aqueous slurries of fibers and binder. The vacuum molding apparatus consists of a mixing tank, filter mold, collection tank and vacuum pump. The slurry was prepared using a mixture of microfibers, ceramic nanofibers, corn starch and alumina binder in water. Diluted acid was added to maintain the slurry at a pH ~ 6.0 and starch was added to enhance the binding capability of the binder and fibers. The slurry was mixed overnight using a stirrer to obtain a uniform dispersion of materials. It was poured into the mixing tank and agitated by bubbling into the bottom of the tank. The Plexiglas mold was located at the bottom of the mixing tank. The mold had an internal diameter of 2.3 cm, equal to the diameter of the filter. The bottom of the mold had a fine steel wire mesh to capture the fibers. A vacuum was applied to the collection tank. A hose from the collection tank to the underside of the mold transferred the vacuum pressure to the mold to pull the slurry from the mixing tank through the steel mesh. The fibers were retained on the steel mesh forming a wet fibrous filter cake. The wet cake was heated to dry

The slurry in the mixing tank consisted of 4 L of water, 0.5 g of alumina microfibers (SAFFIL HA, Thermal Ceramics), 0.05 g catalytic fibers, 0.02 g corn starch, 1 mL alumina binder (alumina rigidizer, Zircar Ceramics) and about 20 drops of dilute H2SO4 acid. The slurry was stirred for 8 hours before it was vacuum molded at 5 psi to form a filter wet filter cake. The wet cake was dried at 120 oC for 2 hours to remove moisture and calcined to 600 oC for 4 hours to set the binder. The filter disk was heated at 400 oC with hydrogen gas (20 % H2, 80

Figure 13 is a sample SEM image of the alumina microfibers. The average fiber diameter range determined by log-normal frequency distribution (Figure 14) was calculated to be ���

3.20 µm and the standard deviation was σg = 1.29 for the log-normal distribution. Alumina microfibers were used to structurally and functionally support the catalytic fibers because they can withstand high temperatures and they have a similar coefficient of expansion in

�� =

% N2) for 4 hours to convert the precious metal oxides to their native metal form.

**2.4 Formation of nanofiber based filter media**

the cake and to thermally set the binder.

comparison to the catalytic nanofibers.

Fig. 13. Scanning electron micrographs of Al2O3 microfibers.

Fig. 11. Transmission electron micrograph of Rh2O3/Al2O3 nanofibers.

Fig. 12. Length weighted log-normal ber diameter frequency distribution of of Rh2O3/Al2O3 nanofibers.

#### **2.4 Formation of nanofiber based filter media**

48 Nanofibers – Production, Properties and Functional Applications

Fig. 11. Transmission electron micrograph of Rh2O3/Al2O3 nanofibers.

Fig. 12. Length weighted log-normal ber diameter frequency distribution of of

0 20 40 60 80 100 120

**Frequency Distribution vs Fiber**

**Diameter**

**Diameter (nm)**

Rh2O3/Al2O3 nanofibers.

0

0.005

0.01

0.015

**f**

0.02

0.025

Filter media were prepared by vacuum molding an aqueous slurries of fibers and binder. The vacuum molding apparatus consists of a mixing tank, filter mold, collection tank and vacuum pump. The slurry was prepared using a mixture of microfibers, ceramic nanofibers, corn starch and alumina binder in water. Diluted acid was added to maintain the slurry at a pH ~ 6.0 and starch was added to enhance the binding capability of the binder and fibers. The slurry was mixed overnight using a stirrer to obtain a uniform dispersion of materials. It was poured into the mixing tank and agitated by bubbling into the bottom of the tank. The Plexiglas mold was located at the bottom of the mixing tank. The mold had an internal diameter of 2.3 cm, equal to the diameter of the filter. The bottom of the mold had a fine steel wire mesh to capture the fibers. A vacuum was applied to the collection tank. A hose from the collection tank to the underside of the mold transferred the vacuum pressure to the mold to pull the slurry from the mixing tank through the steel mesh. The fibers were retained on the steel mesh forming a wet fibrous filter cake. The wet cake was heated to dry the cake and to thermally set the binder.

The slurry in the mixing tank consisted of 4 L of water, 0.5 g of alumina microfibers (SAFFIL HA, Thermal Ceramics), 0.05 g catalytic fibers, 0.02 g corn starch, 1 mL alumina binder (alumina rigidizer, Zircar Ceramics) and about 20 drops of dilute H2SO4 acid. The slurry was stirred for 8 hours before it was vacuum molded at 5 psi to form a filter wet filter cake. The wet cake was dried at 120 oC for 2 hours to remove moisture and calcined to 600 oC for 4 hours to set the binder. The filter disk was heated at 400 oC with hydrogen gas (20 % H2, 80 % N2) for 4 hours to convert the precious metal oxides to their native metal form.

Figure 13 is a sample SEM image of the alumina microfibers. The average fiber diameter range determined by log-normal frequency distribution (Figure 14) was calculated to be ��� �� = 3.20 µm and the standard deviation was σg = 1.29 for the log-normal distribution. Alumina microfibers were used to structurally and functionally support the catalytic fibers because they can withstand high temperatures and they have a similar coefficient of expansion in comparison to the catalytic nanofibers.

Fig. 13. Scanning electron micrographs of Al2O3 microfibers.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 51

Porosity is a 3-dimensional measurement of the void volume of a filter material. Porosity is defined as the ratio of the volume of all of the pores of a material to the volume of the filter and is one of the parameters that describes the structure of the filter material. A special made pycnometer was used to measure the bulk porosity of the filter media by a gas expansion method. The pycnometer consists of two chambers, one (with a removable gastight lid) and the second chamber of a fixed, known internal volume - referred to as the reference volume or added volume. The device also comprises a valve to admit a gas under pressure to the second chamber, a pressure measuring device and a valved vent from the

The porosities of three of the disk shaped catalytic filters are listed in Table 1. Each listed porosity is the average of three filters of the same type. The accuracy of the special made

Permeability is defined as the volume of a fluid that will move across a known area of filter media at a constant pressure drop. It is used to describe how easily a fluid flows through a material and is measured by a Frazier Air Permeability tester (Frazier Precision Instrument Co.) modified to hold the disk shaped filters. The Frazier test is based on Darcy's law which relates the pressure drop to flow through a packed bed with the permeability coefficient as

Table 2 lists the permeabilities of the catalytic filters for measurements done in triplicate for

(4)

 ఓ ቀ ೀିಽ ቁ ൌ ொ

Type of catalytic filter Porosity±0.01

Pd/Al2O3 0.97

Pt/Al2O3 0.97

Rh/Al2O3 0.96

The catalytic filters were characterized for porosity, permeability and hardness.

**2.5 Characterization of filter media**

**2.5.1 Porosity**

second chamber.

**2.5.2 Permeability**

given by Equation (4):

where k= Permeability in m2

A=Area, in m2

each type of catalytic filter.

Po= Initial pressure, in Pa PL= Final pressure, in Pa L=Thickness of media, in m Q=Volumetric flow rate, in m3/s µ=Kinematic viscosity, in Ns/m2

pycnometer is at best ±0.01.

Table 1. Porosity of the catalytic filters.

Figure 15 (a) shows picture of a catalytic filter having diameter of 2.3 cm and height of 1.5 cm. The filter can be molded to the desired shape and dimensions using molds and meshes with the desired diameter. Figure 15 (b) are scanning electron micrographs of sections of catalytic filters. The nanofibers are entrapped by the large microfibers. This structure results in higher capture efficiencies and quality factor for a very small increase in pressure drop in comparison to a filter with only microfibers (Srinivasan, 2005).

Fig. 14. Length weighted log-normal frequency of ber diameter of Al2O3 microfibers.

Fig. 15. (a) Disk shaped catalytic filter approximately 2.3 cm in diameter and 1.2 cm thick. (b) SEM image of nanofibers mixed among microfibers in the catalytic filter.

#### **2.5 Characterization of filter media**

The catalytic filters were characterized for porosity, permeability and hardness.

#### **2.5.1 Porosity**

50 Nanofibers – Production, Properties and Functional Applications

Figure 15 (a) shows picture of a catalytic filter having diameter of 2.3 cm and height of 1.5 cm. The filter can be molded to the desired shape and dimensions using molds and meshes with the desired diameter. Figure 15 (b) are scanning electron micrographs of sections of catalytic filters. The nanofibers are entrapped by the large microfibers. This structure results in higher capture efficiencies and quality factor for a very small increase in pressure drop in

Fig. 14. Length weighted log-normal frequency of ber diameter of Al2O3 microfibers.

a b

02468

**Frequency Distribution vs**

**Fiber Diameter**

**Diameter (μm)**

Fig. 15. (a) Disk shaped catalytic filter approximately 2.3 cm in diameter and 1.2 cm thick. (b)

SEM image of nanofibers mixed among microfibers in the catalytic filter.

comparison to a filter with only microfibers (Srinivasan, 2005).

0

0.1

0.2

0.3

**f**

0.4

0.5

0.6

Porosity is a 3-dimensional measurement of the void volume of a filter material. Porosity is defined as the ratio of the volume of all of the pores of a material to the volume of the filter and is one of the parameters that describes the structure of the filter material. A special made pycnometer was used to measure the bulk porosity of the filter media by a gas expansion method. The pycnometer consists of two chambers, one (with a removable gastight lid) and the second chamber of a fixed, known internal volume - referred to as the reference volume or added volume. The device also comprises a valve to admit a gas under pressure to the second chamber, a pressure measuring device and a valved vent from the second chamber.

The porosities of three of the disk shaped catalytic filters are listed in Table 1. Each listed porosity is the average of three filters of the same type. The accuracy of the special made pycnometer is at best ±0.01.

Table 1. Porosity of the catalytic filters.

#### **2.5.2 Permeability**

Permeability is defined as the volume of a fluid that will move across a known area of filter media at a constant pressure drop. It is used to describe how easily a fluid flows through a material and is measured by a Frazier Air Permeability tester (Frazier Precision Instrument Co.) modified to hold the disk shaped filters. The Frazier test is based on Darcy's law which relates the pressure drop to flow through a packed bed with the permeability coefficient as given by Equation (4):

$$\frac{k}{\mu} \left( \frac{p\_O - p\_L}{L} \right) = \frac{Q}{A} \tag{4}$$

where k= Permeability in m2

Q=Volumetric flow rate, in m3/s

µ=Kinematic viscosity, in Ns/m2

A=Area, in m2

Table 2 lists the permeabilities of the catalytic filters for measurements done in triplicate for each type of catalytic filter.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 53

The pressure drop across the reactor was measured using a pressure transducer. The concentration of the inlet and the outlet gases were measured using a gas chromatograph (GC). The GC was connected to a computer where the peaks of the gases were displayed and their respective concentrations were calculated. Figure 17 shows the flow diagram of

a b

Fig. 16. (a) Reactor (b) Internal view of the reactor and screws.

are less able to dissociate NO than Rh (Thomas & Thomas, 1997).

The ceramic filter medium was positioned in the filter holder for measuring its activity. The gases were continuously flowed from the bottom of the reactor and exhausted from the top. The inlet concentration and the exhaust concentration were monitored by the GC. The temperature of the reactor was gradually increased to observe the light-off temperature of the NO and CO gases. The concentration profile of the inlet and outlet gases was obtained with respect to different temperatures. The maximum temperature the setup was taken to

For the Pd/Al2O3 (Figure 18) catalytic filter which contained 0.05g of the PdO/Al2O3 nanofibers the decomposition temperature of NO was 350 oC. In the case of Pt/Al2O3 (Figure 19) the filter contained 0.05g of PtO/Al2O3 nanofibers and the decomposition temperature of NO was 325 oC. The Rh/Al2O3 (Figure 20) filter decomposed NO at a much lower temperature of 250 oC. This is because NO readily dissociates on Rh {111} and N2 is adsorbed from the Rh surfaces from 200 to 300 oC (Lambert & Bridge, 1984; Hendrickx & Nieuwenhuys, 1986). On the other hand, Pt and Pd are good for oxidation reactions and but

the reactor setup.

was 450 oC.

Table 2. Permeability of the catalytic filters.

#### **2.5.3 Hardness**

A durometer (Rex Gauge Co. Model 1600 Type O) was used to measure the hardness of the filter. Type O durometers are used for very soft elastomers, textile windings, soft granular materials (Rex Durometers, 2006) and is suitable for measuring the hardness of fibrous filters. In this durometer, the load deflection curve is a straight line where 100 durometer O points equals 822 grams and zero durometer O points equals 56 g. The durometer was pressed down and held in a vertical position till the entire foot was in contact with the filter. The dial hand gave the durometer reading. The reading after 15 seconds was also noted while maintaining firm contact of the durometer foot against the filter to make sure that the value was consistent and there was no creep or cold-flow of the filter. The hardness values of the catalytic filters are given in Table 3, as the average of triplicate measurements.

Table 3. Hardness of the catalytic filters

#### **2.6 Reaction test setup**

The reaction test setup was constructed to test the activity of the catalytic filter when challenged with NO and CO gases. He, CO and NO were forced through the catalytic filter cell with controlled flow rates using mass flow controllers. To increase the velocity and to maintain the stoichiometry of NO and CO, He was used as the carrier gas. In the construction of the filter cell, two stainless steel cylindrical blocks were machined with a central hole of diameter 2.3 cm and three holes in the corners for screwing the two parts together. These two parts were bolted together. A gasket made of ceramic fibers was used for sealing the reactor. Bottom and top parts of the reactor were connected with 1/8" stainless steel tubing to the rest of the setup (Figure 16). To control the reactor temperature, the reactor was wrapped with heating tape connected to a temperature controller and insulated.

A durometer (Rex Gauge Co. Model 1600 Type O) was used to measure the hardness of the filter. Type O durometers are used for very soft elastomers, textile windings, soft granular materials (Rex Durometers, 2006) and is suitable for measuring the hardness of fibrous filters. In this durometer, the load deflection curve is a straight line where 100 durometer O points equals 822 grams and zero durometer O points equals 56 g. The durometer was pressed down and held in a vertical position till the entire foot was in contact with the filter. The dial hand gave the durometer reading. The reading after 15 seconds was also noted while maintaining firm contact of the durometer foot against the filter to make sure that the value was consistent and there was no creep or cold-flow of the filter. The hardness values of the catalytic filters are given in Table 3, as the average of

Type of catalytic filter Durometer O Points Load(g)

Pd/Al2O3 10 ±2 110

Pt/Al2O3 14±1 140

Rh/Al2O3 13±1 120

The reaction test setup was constructed to test the activity of the catalytic filter when challenged with NO and CO gases. He, CO and NO were forced through the catalytic filter cell with controlled flow rates using mass flow controllers. To increase the velocity and to maintain the stoichiometry of NO and CO, He was used as the carrier gas. In the construction of the filter cell, two stainless steel cylindrical blocks were machined with a central hole of diameter 2.3 cm and three holes in the corners for screwing the two parts together. These two parts were bolted together. A gasket made of ceramic fibers was used for sealing the reactor. Bottom and top parts of the reactor were connected with 1/8" stainless steel tubing to the rest of the setup (Figure 16). To control the reactor temperature, the reactor was wrapped with heating tape connected to a temperature controller and

Type of catalytic filter Permeability (m2)

Pd/Al2O3 1.25±0.05\*10-11

Pt/Al2O3 1.15±0.08\*10-11

Rh/Al2O3 1.4±0.1\*10-11

Table 2. Permeability of the catalytic filters.

**2.5.3 Hardness**

triplicate measurements.

**2.6 Reaction test setup**

insulated.

Table 3. Hardness of the catalytic filters

The pressure drop across the reactor was measured using a pressure transducer. The concentration of the inlet and the outlet gases were measured using a gas chromatograph (GC). The GC was connected to a computer where the peaks of the gases were displayed and their respective concentrations were calculated. Figure 17 shows the flow diagram of the reactor setup.

Fig. 16. (a) Reactor (b) Internal view of the reactor and screws.

The ceramic filter medium was positioned in the filter holder for measuring its activity. The gases were continuously flowed from the bottom of the reactor and exhausted from the top. The inlet concentration and the exhaust concentration were monitored by the GC. The temperature of the reactor was gradually increased to observe the light-off temperature of the NO and CO gases. The concentration profile of the inlet and outlet gases was obtained with respect to different temperatures. The maximum temperature the setup was taken to was 450 oC.

For the Pd/Al2O3 (Figure 18) catalytic filter which contained 0.05g of the PdO/Al2O3 nanofibers the decomposition temperature of NO was 350 oC. In the case of Pt/Al2O3 (Figure 19) the filter contained 0.05g of PtO/Al2O3 nanofibers and the decomposition temperature of NO was 325 oC. The Rh/Al2O3 (Figure 20) filter decomposed NO at a much lower temperature of 250 oC. This is because NO readily dissociates on Rh {111} and N2 is adsorbed from the Rh surfaces from 200 to 300 oC (Lambert & Bridge, 1984; Hendrickx & Nieuwenhuys, 1986). On the other hand, Pt and Pd are good for oxidation reactions and but are less able to dissociate NO than Rh (Thomas & Thomas, 1997).

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 55

Fig. 19. Concentration vs. temperature for the reaction over 0.05g of Pt/Al2O3.

200 400

Temperature (oC)

Concentration (ppm)

Concentration (ppm)

Fig. 20. Concentration vs. temperature for the reaction over 0.05g of Rh/Al2O3.

200 400

Temperature (oC)

MFC – Mass Flow Controller; MFM - Mass Flow Meter; ROT – Rotameter; P.G. – Pressure Gage; GC – Gas Chromatograph

Fig. 17. Flow diagram of the reactor setup.

Temperature (oC)

Fig. 18. Concentration vs. temperature for the reaction over 0.05g of Pd/Al2O3.

MFC – Mass Flow Controller; MFM - Mass Flow Meter; ROT – Rotameter;

Fig. 18. Concentration vs. temperature for the reaction over 0.05g of Pd/Al2O3.

200 400

Temperature (oC)

P.G. – Pressure Gage; GC – Gas Chromatograph Fig. 17. Flow diagram of the reactor setup.

Concentration (ppm)

Fig. 19. Concentration vs. temperature for the reaction over 0.05g of Pt/Al2O3.

Fig. 20. Concentration vs. temperature for the reaction over 0.05g of Rh/Al2O3.

Electrospinning of Metal Doped Alumina Nanofibers for Catalyst Applications 57

Gai, PL; Roper, R; White, MG. (2002) Recent advances in nanocatalysis research. *Current*

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#### **3. Conclusion**

In this research, the noble metals (palladium, platinum and rhodium) have been incorporated into alumina nanofiber support by sol gel processing and electrospinning techniques. The diameter of the nanofibers and the particle sizes of the noble metals are summarized in Table 4.

Table 4. Diameter of the nanofibers and catalyst particles.

These nanofibers were mixed with alumina microfibers to form a filter using vacuum molding. The catalytic filter was reduced under flowing hydrogen to reduce the noble metal oxide into their native metal form. This filter was tested in a stream of NO and CO. The NO gas was completely decomposed in the temperature range 225 ~ 400 oC depending on the type and amount of catalyst and reaction conditions. Nitrogen, carbon dioxide, and nitrous oxide gases were detected as reaction products. The electrospinning solutions for fabricating the catalyst supporting nanofibers were optimized by varying the ratio of aluminum acetate precursor solution and carrier polymer solutions to obtain maximum yield of ceramic nanofibers.

#### **4. References**

In this research, the noble metals (palladium, platinum and rhodium) have been incorporated into alumina nanofiber support by sol gel processing and electrospinning techniques. The diameter of the nanofibers and the particle sizes of the noble metals are

These nanofibers were mixed with alumina microfibers to form a filter using vacuum molding. The catalytic filter was reduced under flowing hydrogen to reduce the noble metal oxide into their native metal form. This filter was tested in a stream of NO and CO. The NO gas was completely decomposed in the temperature range 225 ~ 400 oC depending on the type and amount of catalyst and reaction conditions. Nitrogen, carbon dioxide, and nitrous oxide gases were detected as reaction products. The electrospinning solutions for fabricating the catalyst supporting nanofibers were optimized by varying the ratio of aluminum acetate precursor

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Material Fiber Diameter (nm) Catalyst Particle Diameter (nm)

PdO/Al2O3 40-70 2.43±0.70 PtO/Al2O3 35-65 2.87±0.82 Rh2O3/Al2O3 50-80 1.16±0.35

Table 4. Diameter of the nanofibers and catalyst particles.

**3. Conclusion**

**4. References**

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Connecticut.

summarized in Table 4.

**4**

*Japan*

**Chitin Nanofibers with a**

**Uniform Width of 10 to 20 nm and**

**Their Transparent Nanocomposite Films**

Shinsuke Ifuku, Antonio Norio Nakagaito and Hiroyuki Saimoto *Graduate School of Engineering, Tottori University, Koyama-cho Minami, Tottori,*

Nanofibers have been paid much attention in research community due to the recent developments in nanotechnology. Generally, nanotechnology deals with structures sized 1-100 nm in at least one dimension. Nanotechnology may create many new materials and devices with a vast range of applications such as in medicine, electronics, biomaterials and energy production. As described in some reports, the fabrication of nanofibers is the subject of much attention because of their unique characteristics such as a very large surface area to mass ratio and other properties, which allow many promising properties for advanced applications. Nanofibers have been produced by a variety of methods. Most nanofibers are artificially prepared by electrospinning process, which produces fibers from polymer solution using interactions between fluid dynamics, electrically charged surfaces and electrically charged liquids. However, the processability, biocompatibility, and biodegradability of the obtained composites are limited compared to those of natural

There is a growing interest in producing nanofibers from natural polymers, since they introduce additional functionalities such as biodegrability, biocompability, renewability and availability. A variety of nanofibers are produced in nature, such as collagen fibrils, fibroin

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in silk, deoxyribonucleic acid (DNA) with double helix, and so on.

**1. Introduction**

polymers.

grinding treatment.

	- <http://www.rexgauge.com/content/tech\_reference\_guide>.

## **Chitin Nanofibers with a Uniform Width of 10 to 20 nm and Their Transparent Nanocomposite Films**

Shinsuke Ifuku, Antonio Norio Nakagaito and Hiroyuki Saimoto *Graduate School of Engineering, Tottori University, Koyama-cho Minami, Tottori, Japan*

#### **1. Introduction**

58 Nanofibers – Production, Properties and Functional Applications

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Available from < http://bccresearch.com/report/catalysts-environment-energy-

Nanofibers have been paid much attention in research community due to the recent developments in nanotechnology. Generally, nanotechnology deals with structures sized 1-100 nm in at least one dimension. Nanotechnology may create many new materials and devices with a vast range of applications such as in medicine, electronics, biomaterials and energy production. As described in some reports, the fabrication of nanofibers is the subject of much attention because of their unique characteristics such as a very large surface area to mass ratio and other properties, which allow many promising properties for advanced applications. Nanofibers have been produced by a variety of methods. Most nanofibers are artificially prepared by electrospinning process, which produces fibers from polymer solution using interactions between fluid dynamics, electrically charged surfaces and electrically charged liquids. However, the processability, biocompatibility, and biodegradability of the obtained composites are limited compared to those of natural polymers.

There is a growing interest in producing nanofibers from natural polymers, since they introduce additional functionalities such as biodegrability, biocompability, renewability and availability. A variety of nanofibers are produced in nature, such as collagen fibrils, fibroin in silk, deoxyribonucleic acid (DNA) with double helix, and so on.

Among the variety of nanofibers, cellulose microfibrils are the most abundant natural nanofiber in nature. Recently, Abe et al. succeeded in isolating cellulose nanofibers with a uniform width of approximately 15 nm from wood (Abe et al., 2007). Wood cell walls consist of stiff cellulose microfibrils, embedded in soft matrix substances such as hemicelluloses and lignin which behave just as adhesive between microfibrils. Due to the extended crystalline structure, the microfibrils have efficient physical properties; their Young's modulus is 138 GPa (Nishino et al., 1995), their tensile strength is at least 2 GPa, based on experimental results for kraft pulp, and their thermal-expansion coefficient in the axial direction is as small as 0.1 ppm/K (Nishino et al., 2004) . These structural features of wood cell walls make the wood stiff and tough, and are helpful in supporting the huge body of a tree. Since cellulose nanofibers are embedded in the matrix phase, they can be isolated by first removing the matrix and subsequently extracted as nanofibers by a very simple grinding treatment.

Chitin Nanofibers with a Uniform Width of

**2. Preparation of chitin nanofibers 2.1 Chitin nanofibers from crab shell**

10 to 20 nm and Their Transparent Nanocomposite Films 61

In general, the crab shell has a strictly hierarchical organization which reveals various structural levels, as shown in Figure 2 (Chen et al., 2008; Giraud-guille, 1984; Raabe et al.,

crystals in the form of thinner nanofibers. These nanofibers are wrapped in protein layers. The next level in the scale consists in the clustering of some of these nanofibers into chitin/protein fibers of about 50–300 nm in diameter. The next step is the formation of a planar woven and branched network of such chitin–protein fibers with a variety of thicknesses. These strands are embedded in a variety of proteins and minerals. The minerals mainly consist of crystalline calcium carbonate. The thickness of strands vary widely among crustaceans. Furthermore, these woven and networked planes form a twisted plywood pattern. This structure is formed by the helicoidal stacking sequences of the fibrous chitinprotein layers. The thickness of the twisted plywood layer corresponds to a certain stacking

α-chitin

2005, 2006). Chitin molecules are aligned in an antiparallel manner that gives rise to

density of planes, which are gradually rotated about their normal axis.

Fig. 2. Schematic presentation of the exoskeleton structure of crab shell.

Shimahara & Takiguchi, 1998 ).

To extract chitin nanofibers from crab shell, the samples were purified by a series of chemical treatments according to the flowchart shown in Figure 3. Dried crab shell flakes of *Paralithodes camtschaticus* (Red king crab), which is commercially available as a fertilizer at low cost were used for this study as the starting material. Crab shell powder was purified according to the general method. First, crab shell flakes were treated with 2 N HCl for 2 days at room temperature to remove the mineral salts. The suspension was filtered and washed thoroughly with distilled water. The sample was refluxed in 2 N NaOH for 2 days to remove the various proteins (Gopalan & Dufresne, 2003; BeMiller & Whistler, 1962,

After cellulose, chitin is the second most abundant biomacromolecule in nature, existing mainly in exoskeletons of crabs, shrimps, and insects, and is synthesized in the amount 1010 to 1011 tons every year (Gopalan & Dufresne, 2003). Owing to its linear (1→4)β-*N*-acetyl glycosaminoglycan structure with two reactive hydroxyl groups and an acetamide group per anhydro-*N*-acetylglucosamine unit, chitin has broad potential in the design of advanced polymeric biomaterials (Figure 1). However, most of chitin is discarded as industrial food residue without effective utilization. Thus, it is important to develop efficient use of chitin as a natural and eco-friendly material.

Fig. 1. Chemical structure of cellulose and chitin.

Native chitin in crab shells has highly crystalline structure, arranged as α-chitin in an antiparallel fashion forming microfibrils strongly connected by hydrogen bonds. The chitin microfibrils consist of nanofibers about 2–5 nm in diameter and about 300 nm in length embedded in several protein matrices (Raabe et al., 2006; Chen et al., 2008). Since chitin nanofibers are considered to have great potential, various methods have been employed for their preparation, such as acid hydrolysis (Gopalan & Dufresne, 2003; Revol & Marchessault, 1993), ultrasonication (Zhao et al., 2007), and electrospinning (Min et al., 2004). However, the nanofibers obtained by these processes were different from the native chitin nanofibers in terms of width, aspect ratio, crystallinity, chemical structure, and/or width distribution. Recently, Isogai et al, have developed an efficient method to prepare completely individualized cellulose nanofibers by 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) mediated oxidation of native cellulose, followed by ultrasonic treatment. Although TEMPO mediated oxidation was applicable to crab shell α*-*chitin, the average nanocrystal length was considerably low (Fan et al., 2008a). In addition, the nanocrystals are actually chitin derivatives. They reported a procedure for preparing chitin nanofibers 3–4 nm in width from squid pen β-chitin by ultrasonication treatment under acidic conditions, however, the crystallinity of the nanofibers from the squid pen is relatively low (Fan et al., 2008b). Moreover, the biomass quantity of the pen is considerably lower than those of crab shells.

Similar to wood cell walls, the exoskeletons of crab and prawn have a strictly hierarchical structure consisting of crystalline α-chitin nanofibers and various types of proteins and minerals (mainly calcium carbonate) (Chen et al, 2008; Raabe et al, 2006; Giraud-guille, 1984). Thus, just like cellulose nanofibers, chitin nanofibers are embedded in matrix components. As Abe et al. described a preparation method can be used to isolate cellulose nanofibers from any natural plant containing lignin and hemicellulose, we considered that this isolation procedure would be applicable to any natural nanofiber source consisting of bionanofibers and other embedding matrixes. Accordingly, we here studied the extraction of natural α-chitin nanofibers with a uniform width of 10–20 nm from crab shells.

#### **2. Preparation of chitin nanofibers**

#### **2.1 Chitin nanofibers from crab shell**

60 Nanofibers – Production, Properties and Functional Applications

After cellulose, chitin is the second most abundant biomacromolecule in nature, existing mainly in exoskeletons of crabs, shrimps, and insects, and is synthesized in the amount 1010

glycosaminoglycan structure with two reactive hydroxyl groups and an acetamide group per anhydro-*N*-acetylglucosamine unit, chitin has broad potential in the design of advanced polymeric biomaterials (Figure 1). However, most of chitin is discarded as industrial food residue without effective utilization. Thus, it is important to develop efficient use of chitin as

β

α

α

*-*chitin, the average

to 1011 tons every year (Gopalan & Dufresne, 2003). Owing to its linear (1→4)-

Native chitin in crab shells has highly crystalline structure, arranged as

Although TEMPO mediated oxidation was applicable to crab shell

α

β

antiparallel fashion forming microfibrils strongly connected by hydrogen bonds. The chitin microfibrils consist of nanofibers about 2–5 nm in diameter and about 300 nm in length embedded in several protein matrices (Raabe et al., 2006; Chen et al., 2008). Since chitin nanofibers are considered to have great potential, various methods have been employed for their preparation, such as acid hydrolysis (Gopalan & Dufresne, 2003; Revol & Marchessault, 1993), ultrasonication (Zhao et al., 2007), and electrospinning (Min et al., 2004). However, the nanofibers obtained by these processes were different from the native chitin nanofibers in terms of width, aspect ratio, crystallinity, chemical structure, and/or width distribution. Recently, Isogai et al, have developed an efficient method to prepare completely individualized cellulose nanofibers by 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) mediated oxidation of native cellulose, followed by ultrasonic treatment.

nanocrystal length was considerably low (Fan et al., 2008a). In addition, the nanocrystals are actually chitin derivatives. They reported a procedure for preparing chitin nanofibers 3–4

however, the crystallinity of the nanofibers from the squid pen is relatively low (Fan et al., 2008b). Moreover, the biomass quantity of the pen is considerably lower than those of crab

Similar to wood cell walls, the exoskeletons of crab and prawn have a strictly hierarchical

minerals (mainly calcium carbonate) (Chen et al, 2008; Raabe et al, 2006; Giraud-guille, 1984). Thus, just like cellulose nanofibers, chitin nanofibers are embedded in matrix components. As Abe et al. described a preparation method can be used to isolate cellulose nanofibers from any natural plant containing lignin and hemicellulose, we considered that this isolation procedure would be applicable to any natural nanofiber source consisting of bionanofibers and other embedding matrixes. Accordingly, we here studied the extraction

a natural and eco-friendly material.

nm in width from squid pen

structure consisting of crystalline

shells.

of natural

α

Fig. 1. Chemical structure of cellulose and chitin.

In general, the crab shell has a strictly hierarchical organization which reveals various structural levels, as shown in Figure 2 (Chen et al., 2008; Giraud-guille, 1984; Raabe et al., 2005, 2006). Chitin molecules are aligned in an antiparallel manner that gives rise to α-chitin crystals in the form of thinner nanofibers. These nanofibers are wrapped in protein layers. The next level in the scale consists in the clustering of some of these nanofibers into chitin/protein fibers of about 50–300 nm in diameter. The next step is the formation of a planar woven and branched network of such chitin–protein fibers with a variety of thicknesses. These strands are embedded in a variety of proteins and minerals. The minerals mainly consist of crystalline calcium carbonate. The thickness of strands vary widely among crustaceans. Furthermore, these woven and networked planes form a twisted plywood pattern. This structure is formed by the helicoidal stacking sequences of the fibrous chitinprotein layers. The thickness of the twisted plywood layer corresponds to a certain stacking density of planes, which are gradually rotated about their normal axis.

Fig. 2. Schematic presentation of the exoskeleton structure of crab shell.

To extract chitin nanofibers from crab shell, the samples were purified by a series of chemical treatments according to the flowchart shown in Figure 3. Dried crab shell flakes of *Paralithodes camtschaticus* (Red king crab), which is commercially available as a fertilizer at low cost were used for this study as the starting material. Crab shell powder was purified according to the general method. First, crab shell flakes were treated with 2 N HCl for 2 days at room temperature to remove the mineral salts. The suspension was filtered and washed thoroughly with distilled water. The sample was refluxed in 2 N NaOH for 2 days to remove the various proteins (Gopalan & Dufresne, 2003; BeMiller & Whistler, 1962, Shimahara & Takiguchi, 1998 ).

Chitin Nanofibers with a Uniform Width of

bar is 300 nm.

10 to 20 nm and Their Transparent Nanocomposite Films 63

Fig. 4. FE-SEM image of crab shell surface after the removal of matrix. The length of scale

acidic condition is important to maintain the stable dispersion state by electrostatic

well dispersed under an acidic condition by the cationization of the amino groups on the fiber surface, which would facilitate fibrillation into 10-20 nm chitin nanofibers. Thus, the pH value of the purified chitin suspension was adjusted to approximately 3 by the addition of AcOH, and then the chitin was subjected to a grinder treatment. It should be emphasized that the chitin slurry thus obtained formed a gel after a single pass through the grinder, as the extremely large surface area leads to high dispersion property in water, suggesting that fibrillation was successfully accomplished, (Abe et al., 2007). Figure 5b and 5c show FE-SEM micrographs of the dried chitin gel. The isolated chitins were highly uniform nanofibers with a width of 10–20 nm, suggesting that the fibrillation process was facilitated in acidic solution as expected. Since broken fibers are not observed even over a wide observation area, the aspect ratios of the nanofibers are very high. These results indicate that chitin nanofibers were successfully isolated from crab shells without altering their natural shape. Thus, the never-dried process that has previously been used to extract the cellulose nanofibers from wood cell walls was here proven applicable for the preparation of 10-20 nm

α

β

β

Fan et al. reported a preparation method of chitin nanofibers from squid pen

chitin nanofibers from crab shells by mechanical grinding at pH 3–4 condition.

The degree of *N*-acetylation of the obtained chitin nanofibers were calculated from the C and N contents from the elemental analysis data by using an elemental analyzer, and was found to be 95%. Thus, the degree of substitution of amino group was just 5%, indicating that deacetylation hardly occurred after the removal of proteins with 2N NaOH, and chitin nanofibers were obtained as natural fibers. Thus, even though the ratio of amino group was only 5%, cationization of amino groups facilitated the fibrillation of chitin fibers into nanofibers and maintained the stable dispersion state in the water by electrostatic repulsions

water at pH 3–4 (Fan et al., 2008a). Cationization of C2 amino groups in

repulsions. Therefore, we considered that the purified

**2.2 Characterization of chitin nanofibers**

between the chitin nanofibers with cationic surface charges.

Fig. 3. Preparation procedure of chitin nanofibers from crab and prawn shell.

After filtration and rinsing with distilled water, the pigments and lipids in the sample were extracted by ethanol followed by filtration and rinsing with water (Shimahara & Takiguchi, 1998). It is known that almost all proteins and minerals (mainly calcium carbonate) can be removed by treatment with NaOH and HCl solutions each <0.1%. On the other hand, since complete removal of hemicelluloses from wood fiber is difficult, considerable amount of hemicelluloses remain in cellulose by alkali treatment. The yield of chitin from the crab shells was estimated to be 12.1 wt %. Since the process of drying chitin nanofibers generates strong hydrogen bonding between the nanofiber bundles, making difficult to obtain thin and uniform nanofibers, the material was kept wet after the removal of the matrix. Figure 4 shows field emission scanning electron microscope (FE-SEM) images of the crab shell surface after the removal of the matrix. The image represents the endocuticle, which makes up approximately 90 vol.% of the crab exoskeleton. We could observe that the chitin was made up of regularly structured chitin fiber networks, and the networks were found to be comprised of bundles of chitin nanofibers.

The purified wet chitin from dry crab shells was dispersed in distilled water at 1 wt %, and the slurry was passed through a grinder (MKCA6-3; Masuko Sangyo Co., Ltd.) under a neutral pH condition, with a careful adjustment of the clearance between the grinding stones. In principle, there is no direct contact between the grinding stones due to the presence of chitin suspension. The suspension of chitin fibrils was subjected to oven-drying after replacement of water by ethanol, and the obtained sheets were coated with an approximately 2 nm layer of platinum by an ion sputter coater and observed with FE-SEM. The widths of the fibers derived from crab shell after grinder treatment were widely distributed over a range 10 to 100 nm (Figure 5a). The chitin fiber network structure seems to have been broken completely by the grinder treatment after removal of the matrix substances. However the thicker fibrils of approximately 100 nm, which were bundles of nanofibers of 10–20 nm in width, still remained after grinder treatment even though the protein layers were removed under a never-dried condition.

Fig. 3. Preparation procedure of chitin nanofibers from crab and prawn shell.

comprised of bundles of chitin nanofibers.

protein layers were removed under a never-dried condition.

After filtration and rinsing with distilled water, the pigments and lipids in the sample were extracted by ethanol followed by filtration and rinsing with water (Shimahara & Takiguchi, 1998). It is known that almost all proteins and minerals (mainly calcium carbonate) can be removed by treatment with NaOH and HCl solutions each <0.1%. On the other hand, since complete removal of hemicelluloses from wood fiber is difficult, considerable amount of hemicelluloses remain in cellulose by alkali treatment. The yield of chitin from the crab shells was estimated to be 12.1 wt %. Since the process of drying chitin nanofibers generates strong hydrogen bonding between the nanofiber bundles, making difficult to obtain thin and uniform nanofibers, the material was kept wet after the removal of the matrix. Figure 4 shows field emission scanning electron microscope (FE-SEM) images of the crab shell surface after the removal of the matrix. The image represents the endocuticle, which makes up approximately 90 vol.% of the crab exoskeleton. We could observe that the chitin was made up of regularly structured chitin fiber networks, and the networks were found to be

The purified wet chitin from dry crab shells was dispersed in distilled water at 1 wt %, and the slurry was passed through a grinder (MKCA6-3; Masuko Sangyo Co., Ltd.) under a neutral pH condition, with a careful adjustment of the clearance between the grinding stones. In principle, there is no direct contact between the grinding stones due to the presence of chitin suspension. The suspension of chitin fibrils was subjected to oven-drying after replacement of water by ethanol, and the obtained sheets were coated with an approximately 2 nm layer of platinum by an ion sputter coater and observed with FE-SEM. The widths of the fibers derived from crab shell after grinder treatment were widely distributed over a range 10 to 100 nm (Figure 5a). The chitin fiber network structure seems to have been broken completely by the grinder treatment after removal of the matrix substances. However the thicker fibrils of approximately 100 nm, which were bundles of nanofibers of 10–20 nm in width, still remained after grinder treatment even though the

Fig. 4. FE-SEM image of crab shell surface after the removal of matrix. The length of scale bar is 300 nm.

Fan et al. reported a preparation method of chitin nanofibers from squid pen β-chitin in water at pH 3–4 (Fan et al., 2008a). Cationization of C2 amino groups in β-chitin under an acidic condition is important to maintain the stable dispersion state by electrostatic repulsions. Therefore, we considered that the purified α-chitin from crab shell would also be well dispersed under an acidic condition by the cationization of the amino groups on the fiber surface, which would facilitate fibrillation into 10-20 nm chitin nanofibers. Thus, the pH value of the purified chitin suspension was adjusted to approximately 3 by the addition of AcOH, and then the chitin was subjected to a grinder treatment. It should be emphasized that the chitin slurry thus obtained formed a gel after a single pass through the grinder, as the extremely large surface area leads to high dispersion property in water, suggesting that fibrillation was successfully accomplished, (Abe et al., 2007). Figure 5b and 5c show FE-SEM micrographs of the dried chitin gel. The isolated chitins were highly uniform nanofibers with a width of 10–20 nm, suggesting that the fibrillation process was facilitated in acidic solution as expected. Since broken fibers are not observed even over a wide observation area, the aspect ratios of the nanofibers are very high. These results indicate that chitin nanofibers were successfully isolated from crab shells without altering their natural shape. Thus, the never-dried process that has previously been used to extract the cellulose nanofibers from wood cell walls was here proven applicable for the preparation of 10-20 nm chitin nanofibers from crab shells by mechanical grinding at pH 3–4 condition.

#### **2.2 Characterization of chitin nanofibers**

The degree of *N*-acetylation of the obtained chitin nanofibers were calculated from the C and N contents from the elemental analysis data by using an elemental analyzer, and was found to be 95%. Thus, the degree of substitution of amino group was just 5%, indicating that deacetylation hardly occurred after the removal of proteins with 2N NaOH, and chitin nanofibers were obtained as natural fibers. Thus, even though the ratio of amino group was only 5%, cationization of amino groups facilitated the fibrillation of chitin fibers into nanofibers and maintained the stable dispersion state in the water by electrostatic repulsions between the chitin nanofibers with cationic surface charges.

Chitin Nanofibers with a Uniform Width of

α

the removal of the matrix and the grinder treatments.

original molecular structure and

nanofibers, and (c) crab shell flakes.

We succeeded in preparing

any acid.

**2.3 Chitin nanofibers from prawn shell**

α

α

crystal patterns of

10 to 20 nm and Their Transparent Nanocomposite Films 65

X-ray diffraction profiles obtained with Ni-filtered Cu Kα of (a) commercially available pure

Blackwell, 1978). Thus, the chitin nanofibers were extracted from the crab shell, and the

Fig. 7. X-ray diffraction profiles of a) commercially available chitin powder, (b) chitin

approximately 10–20 nm (Ifuku et al., 2009). The grinder treatment under acidic conditions is the key to preparing chitin nanofibers. Cationization of amino groups in the chitin by the addition of an acid is important to maintain a stable dispersion state by electrostatic repulsions, which facilitate nano-fibrillation into chitin nanofibers. However, in some cases, the acidic condition may cause significant problems for application of chitin nanofibers such as in biomedical materials, nanocomposites, electronics devices, and so on, because in general, these materials are sensitive to acid. Indeed, it is difficult to remove acidic chemicals from chitin nanofiber suspension, since chitin nanofibers are homogeneously dispersed in water to give high viscosity. Therfore preparation of chitin nanofibers under a neutral pH condition is required to expand the application of nanofibers. Here, we show the extraction of chitin nanofibers from prawn shells under neutral conditions without using

α

α

Fig. 5. SEM images of chitin nanofibers from crab shell after one pass through the grinder (a): under neutral pH condition, (b and c): under acidic condition at pH 3. The length of the scale bar is (a) 400 nm, (b) 400 nm, and (c) 200 nm, respectively.

Infrared spectra (FT-IR) recorded using potassium bromide pellets of (a) commercially available pure chitin, (b) newly prepared chitin nanofibers from crab shell, and (c) dried crab shell flakes without purification are shown in Figure 6. The spectrum of the dried crab shell flakes is very different from the other two spectra because of the matrix components included in the shell. On the other hand, the spectrum of newly prepared chitin nanofibers is in excellent agreement with the spectrum of commercial pure α-chitin, indicating that the matrix, protein and minerals were well removed by a series of processing steps. In particular, the absorption band at 1420 cm-1 derived from protein has completely disappeared, suggesting that these treatments were sufficient to eliminate all the proteins. Moreover, the OH stretching band at 3482 cm-1, NH stretching band at 3270 cm-1, amide I bands at 1661 and 1622 cm-1, and amide II band at 1559 cm-1 of the chitin nanofibers are observed. These characteristic absorption peaks in the carbonyl region corresponds to anhydrous α-chitin (Gopalan & Dufresne, 2003).

Fig. 6. FT–IR spectra of (a) commercially available chitin powder, (b) chitin nanofibers, and (c) crab shell flakes.

Fig. 5. SEM images of chitin nanofibers from crab shell after one pass through the grinder (a): under neutral pH condition, (b and c): under acidic condition at pH 3. The length of the

Infrared spectra (FT-IR) recorded using potassium bromide pellets of (a) commercially available pure chitin, (b) newly prepared chitin nanofibers from crab shell, and (c) dried crab shell flakes without purification are shown in Figure 6. The spectrum of the dried crab shell flakes is very different from the other two spectra because of the matrix components included in the shell. On the other hand, the spectrum of newly prepared chitin nanofibers

matrix, protein and minerals were well removed by a series of processing steps. In particular, the absorption band at 1420 cm-1 derived from protein has completely disappeared, suggesting that these treatments were sufficient to eliminate all the proteins. Moreover, the OH stretching band at 3482 cm-1, NH stretching band at 3270 cm-1, amide I bands at 1661 and 1622 cm-1, and amide II band at 1559 cm-1 of the chitin nanofibers are observed. These characteristic absorption peaks in the carbonyl region corresponds to

Fig. 6. FT–IR spectra of (a) commercially available chitin powder, (b) chitin nanofibers, and

α

scale bar is (a) 400 nm, (b) 400 nm, and (c) 200 nm, respectively.

is in excellent agreement with the spectrum of commercial pure

anhydrous

α

(c) crab shell flakes.

X-ray diffraction profiles obtained with Ni-filtered Cu Kα of (a) commercially available pure α-chitin, (b) newly prepared chitin nanofibers from crab shell, and (c) dried crab shell flakes derived from red king crab are shown in Figure 7. The diffraction peak at 29.6, which is typical of calcium carbonate, completely disappeared from the profile of chitin nanofibers, indicating that the mineral component was entirely removed from the newly prepared chitin nanofibers. The four diffraction peaks of chitin nanofibers observed at 9.5, 19.5, 20.9, and 23.4, which corresponds to 020, 110, 120, and 130 planes, respectively, are typical crystal patterns of α-chitin, and are closely coincident with commercial α-chitin (Minke & Blackwell, 1978). Thus, the chitin nanofibers were extracted from the crab shell, and the original molecular structure and α-chitin crystalline structure were maintained even after the removal of the matrix and the grinder treatments.

Fig. 7. X-ray diffraction profiles of a) commercially available chitin powder, (b) chitin nanofibers, and (c) crab shell flakes.

#### **2.3 Chitin nanofibers from prawn shell**

We succeeded in preparing α-chitin nanofibers from crab shells with a uniform width of approximately 10–20 nm (Ifuku et al., 2009). The grinder treatment under acidic conditions is the key to preparing chitin nanofibers. Cationization of amino groups in the chitin by the addition of an acid is important to maintain a stable dispersion state by electrostatic repulsions, which facilitate nano-fibrillation into chitin nanofibers. However, in some cases, the acidic condition may cause significant problems for application of chitin nanofibers such as in biomedical materials, nanocomposites, electronics devices, and so on, because in general, these materials are sensitive to acid. Indeed, it is difficult to remove acidic chemicals from chitin nanofiber suspension, since chitin nanofibers are homogeneously dispersed in water to give high viscosity. Therfore preparation of chitin nanofibers under a neutral pH condition is required to expand the application of nanofibers. Here, we show the extraction of chitin nanofibers from prawn shells under neutral conditions without using any acid.

Chitin Nanofibers with a Uniform Width of

crab shell.

through the grinder.

We have succeeded in isolating

10 to 20 nm and Their Transparent Nanocomposite Films 67

The plausible explanation of successful fibrillation is following. The exoskeleton of crab and prawn is made up of mainly two parts, the exocuticle and the endocuticle. Although the exocuticle has a very fine twisted plywood-type structure, the endocuticle has a much coarser structure with a thicker fiber diameter (Fig. 4). In general, approximately 90% of a crab shell is made of endocuticle (Chen et al., 2008). In contrast, the exoskeleton of *Natantia*, which has a semitransparent soft shell, including the black tiger prawn, is made up primarily of a fine exocuticle, as shown in Figure 8 (Yano, 1972, 1975 & 1977). As a result, due to the differences in the cuticle structure, fibrillation of prawn shell is easier than that of

The preparation method for chitin nanofibers from prawn shells under a neutral pH condition was applicable to other prawn shells. Figure 10 shows SEM images of the chitin nanofibers derived from (a) *Marsupenaeus japonicus* (Japanese tiger prawn) and (b) *Pandalus eous* (Alaskan pink shrimp), both of which are important food sources. These chitin nanofibers were prepared by the same preparation procedure, that is, removal of matrix components and subsequent grinding treatment under a neutral pH condition. Both chitins are also observed as uniform nanofibers with a width of 10–20 nm, which is similar to the nanofibers from black tiger prawn. This result suggests that chitin nanofibers can be obtained from other prawn species having a very fine exocuticle structure by nanofibrillation under neutral pH conditions. Since many materials are sensitive to acid

chemicals, this study will expand the application of chitin nanofibers.

Fig. 9. FE-SEM images of chitin nanofibers from black tiger prawn shell after one pass

uniform width of 10–20 nm and a high aspect ratio (Ifuku et al., 2009). In general, the drying process of chitin and cellulose fibers generates strong hydrogen bonding between these fibers after removal of the matrix, which makes it difficult to fibrillate them into nanofibers. Therefore, chitin and cellulose must be kept wet after removal of the matrix for nanofiber preparation (Abe et al, 2007; Fan et al, 2009; Ifuku et al, 2009; Iwamoto et al., 2008; Saito et al., 2006). However, this requirement causes a strong disadvantage in the commercial

**2.4 Facile preparation of chitin nanofiber from dry chitin**

α

Fresh shells of *Penaeus monodon* (black tiger prawn) was purified to prepare the chitin nanofibers. It is a widely cultured prawn species in the world and some of its shell is thrown away as industrial waste without effective utilization. Proteins and minerals were removed according to the conventional method using aqueous NaOH and HCl, respectively to extract the chitin component from the prawn shell (Shimahara & Takiguchi, 1998). The pigment component in the sample was then removed using ethanol. The yield of dry chitin from the wet prawn shells was approximately 16.7%. The degree of deacetylation (DDA) of the samples determined by the C and N content in the elemental analysis data was 7%. The SEM micrograph of the black tiger prawn shell surface after removal of the matrix components (without grinding treatment) is shown in Figure 8. This image is from the exocuticle, which is the main part of the prawn shell. Although the appearance of the prawn shell is still intact after removal of the matrix, we could observe uniform chitin nanofibers with an elaborate design.

Fig. 8. FE-SEM image of the surface of the black tiger prawn after removing matrix components.

The purified chitin suspension with a concentration of 1 wt% was crushed by a domestic blender, and passed through a grinder for nano-fibrillation without using any acid. The chitin slurry thus obtained was viscous after a single grinding treatment similar to the chitin nanofibers from crab shell. The obtained chitin slurry was observed using FE-SEM (Fig. 9). In the case of crab shell, the widths of the fibers were widely distributed over a range from 10 to 100 nm by grinder treatment under a neutral condition as described above (Fig. 5a, Ifuku et al., 2009). On the other hand, in the case of prawn shell, we could see a uniform shape of chitin nanofibers using the same extraction treatment. The chitin nanofibers were highly uniform over an extensive area (Fig. 9a), and the width of the nanofibers was approximately 10–20 nm, including a 2 nm thick platinum coating layer (Fig. 9b). The fiber thickness of 10–20nm was similar to nanofibers from crab shell fibrillated under an acidic condition (Ifuku et al., 2009). Thus, chitin nanofibers were successfully prepared from prawn shell with a uniform width under a neutral pH condition.

Fresh shells of *Penaeus monodon* (black tiger prawn) was purified to prepare the chitin nanofibers. It is a widely cultured prawn species in the world and some of its shell is thrown away as industrial waste without effective utilization. Proteins and minerals were removed according to the conventional method using aqueous NaOH and HCl, respectively to extract the chitin component from the prawn shell (Shimahara & Takiguchi, 1998). The pigment component in the sample was then removed using ethanol. The yield of dry chitin from the wet prawn shells was approximately 16.7%. The degree of deacetylation (DDA) of the samples determined by the C and N content in the elemental analysis data was 7%. The SEM micrograph of the black tiger prawn shell surface after removal of the matrix components (without grinding treatment) is shown in Figure 8. This image is from the exocuticle, which is the main part of the prawn shell. Although the appearance of the prawn shell is still intact after removal of the matrix, we could observe uniform chitin nanofibers with an elaborate

Fig. 8. FE-SEM image of the surface of the black tiger prawn after removing matrix

prawn shell with a uniform width under a neutral pH condition.

The purified chitin suspension with a concentration of 1 wt% was crushed by a domestic blender, and passed through a grinder for nano-fibrillation without using any acid. The chitin slurry thus obtained was viscous after a single grinding treatment similar to the chitin nanofibers from crab shell. The obtained chitin slurry was observed using FE-SEM (Fig. 9). In the case of crab shell, the widths of the fibers were widely distributed over a range from 10 to 100 nm by grinder treatment under a neutral condition as described above (Fig. 5a, Ifuku et al., 2009). On the other hand, in the case of prawn shell, we could see a uniform shape of chitin nanofibers using the same extraction treatment. The chitin nanofibers were highly uniform over an extensive area (Fig. 9a), and the width of the nanofibers was approximately 10–20 nm, including a 2 nm thick platinum coating layer (Fig. 9b). The fiber thickness of 10–20nm was similar to nanofibers from crab shell fibrillated under an acidic condition (Ifuku et al., 2009). Thus, chitin nanofibers were successfully prepared from

design.

components.

The plausible explanation of successful fibrillation is following. The exoskeleton of crab and prawn is made up of mainly two parts, the exocuticle and the endocuticle. Although the exocuticle has a very fine twisted plywood-type structure, the endocuticle has a much coarser structure with a thicker fiber diameter (Fig. 4). In general, approximately 90% of a crab shell is made of endocuticle (Chen et al., 2008). In contrast, the exoskeleton of *Natantia*, which has a semitransparent soft shell, including the black tiger prawn, is made up primarily of a fine exocuticle, as shown in Figure 8 (Yano, 1972, 1975 & 1977). As a result, due to the differences in the cuticle structure, fibrillation of prawn shell is easier than that of crab shell.

The preparation method for chitin nanofibers from prawn shells under a neutral pH condition was applicable to other prawn shells. Figure 10 shows SEM images of the chitin nanofibers derived from (a) *Marsupenaeus japonicus* (Japanese tiger prawn) and (b) *Pandalus eous* (Alaskan pink shrimp), both of which are important food sources. These chitin nanofibers were prepared by the same preparation procedure, that is, removal of matrix components and subsequent grinding treatment under a neutral pH condition. Both chitins are also observed as uniform nanofibers with a width of 10–20 nm, which is similar to the nanofibers from black tiger prawn. This result suggests that chitin nanofibers can be obtained from other prawn species having a very fine exocuticle structure by nanofibrillation under neutral pH conditions. Since many materials are sensitive to acid chemicals, this study will expand the application of chitin nanofibers.

Fig. 9. FE-SEM images of chitin nanofibers from black tiger prawn shell after one pass through the grinder.

#### **2.4 Facile preparation of chitin nanofiber from dry chitin**

We have succeeded in isolating α-chitin nanofibers from crab and shrimp shells with a uniform width of 10–20 nm and a high aspect ratio (Ifuku et al., 2009). In general, the drying process of chitin and cellulose fibers generates strong hydrogen bonding between these fibers after removal of the matrix, which makes it difficult to fibrillate them into nanofibers. Therefore, chitin and cellulose must be kept wet after removal of the matrix for nanofiber preparation (Abe et al, 2007; Fan et al, 2009; Ifuku et al, 2009; Iwamoto et al., 2008; Saito et al., 2006). However, this requirement causes a strong disadvantage in the commercial

Chitin Nanofibers with a Uniform Width of

respectively.

fibrillated into nanofibers (Fig. 12b and c).

10 to 20 nm and Their Transparent Nanocomposite Films 69

Fig. 11. FE-SEM images of cellulose fibers from wood pulp (a and b) before and (c) after one pass through grinder. The length of the scale bar is (a) 30,000 nm, and (b, c) 1000 nm,

removing matrices, followed by drying. It can be observed that the chitin was made up of structurally well organized bundles of nanofibers, as mentioned above. The slurry of purified dry chitin was passed through a grinder with a concentration of 1 wt % with and without acetic acid to fibrillate the bundles of chitin nanofibers. In the case of treatment under an acidic condition, the obtained chitin slurry had high viscosity, similar to the chitin nanofibers prepared by the previous method, suggesting that dried chitin was successfully fibrillated and homogeneously dispersed in acidic water with a very high surface ratio (Ifuku et al., 2009). Figure 12 shows FE-SEM images of chitin fibers after one pass through the grinder treated (a) without and (b, c) with acetic acid. In the case of grinder treatment under neutral pH condition, dried chitin could not be fibrillated at all, and aggregates of chitin nanofibers were observed (Fig. 12a). This lack of fibrillation also occurred because the drying process of purified chitin causes strong hydrogen bonding between hydroxyl, acetamide, and amino groups with strong dipole moments on the chitin fiber surface, which make it hard to fibrillate bundles of chitin nanofibers, as in the case of dry pulp fibers. On the other hand, chitin slurry treated at pH 3 was completely

Fig. 10. FE-SEM images of chitin nanofibers (a) from Japanese tiger prawn shell, and (b) from Alaskan pink shrimp shell.

production of nanofibers. If chitin nanofibers could be obtained from dry chitin, we could prepare nanofibers easily and immediately whenever required. Therefore, preparation of nanofibers from a dried pure chitin is an important goal for expanding the use of chitin nanofiber. As described above, for the preparation of nanofibers, grinder treatment under an acidic condition is the key to fibrillating the chitin effectively (Fan et al., 2008b). A small number of amino groups in the chitin are cationized by the addition of an acid, which promotes the fibrillation of chitin into nanofibers by electrostatic repulsion. Indeed, electrostatic repulsion force is applied for the fibrillation of cellulose to obtain nanofibers through TEMPO-catalyzed oxidation too (Saito et al., 2006). If the electrostatic repulsion force of amino cations can break the strong hydrogen bonds between the chitin bundles, chitin nanofibers could be obtained from pure dry chitin aggregate. Accordingly, we here show the fibrillation of dried chitin into nanofibers by a grinding method under acidic conditions (Ifuku 2010b).

First, we tried to prepare cellulose nanofibers from dry pulp fibers, as a preliminary experiment. Figure 11 shows FE-SEM micrographs of cellulose fiber (a, b) before and (c) after single pass through the grinder with a concentration of 1 wt % for fibrillation of pulp fibers into cellulose nanofibers. In Figs. 11a and 11b, it can be seen that pulp fibers consist of a bundle of cellulose nanofibers. Figure 11c shows that the pulp fibers were not fibrillated into nanofibers at all after the grinder treatment. This is obviously because the drying process causes strong hydrogen bonding between the cellulose nanofibers after the removal of matrix substances hemicelluloses and lignin, thus making it difficult to obtain cellulose nanofibers. Thus, in general, the sample should never dry out for bionanofiber preparation after the removal of matrix (Abe et al., 2007; Iwamoto et al., 2008).

Next, we tried to fibrillate dried chitin into nanofibers. Dried chitin was prepared from crab shell flakes with an α-chitin crystal structure. The chitin was purified by the removal of matrix components of proteins and calcium carbonate according to the conventional method (Shimahara & Takiguchi, 1998), followed by drying in an oven to promote hydrogen bonding interaction between chitin nanofibers. Figure 4 is FE-SEM image of a crab shell after

Fig. 10. FE-SEM images of chitin nanofibers (a) from Japanese tiger prawn shell, and (b)

production of nanofibers. If chitin nanofibers could be obtained from dry chitin, we could prepare nanofibers easily and immediately whenever required. Therefore, preparation of nanofibers from a dried pure chitin is an important goal for expanding the use of chitin nanofiber. As described above, for the preparation of nanofibers, grinder treatment under an acidic condition is the key to fibrillating the chitin effectively (Fan et al., 2008b). A small number of amino groups in the chitin are cationized by the addition of an acid, which promotes the fibrillation of chitin into nanofibers by electrostatic repulsion. Indeed, electrostatic repulsion force is applied for the fibrillation of cellulose to obtain nanofibers through TEMPO-catalyzed oxidation too (Saito et al., 2006). If the electrostatic repulsion force of amino cations can break the strong hydrogen bonds between the chitin bundles, chitin nanofibers could be obtained from pure dry chitin aggregate. Accordingly, we here show the fibrillation of dried chitin into nanofibers by a grinding method under acidic

First, we tried to prepare cellulose nanofibers from dry pulp fibers, as a preliminary experiment. Figure 11 shows FE-SEM micrographs of cellulose fiber (a, b) before and (c) after single pass through the grinder with a concentration of 1 wt % for fibrillation of pulp fibers into cellulose nanofibers. In Figs. 11a and 11b, it can be seen that pulp fibers consist of a bundle of cellulose nanofibers. Figure 11c shows that the pulp fibers were not fibrillated into nanofibers at all after the grinder treatment. This is obviously because the drying process causes strong hydrogen bonding between the cellulose nanofibers after the removal of matrix substances hemicelluloses and lignin, thus making it difficult to obtain cellulose nanofibers. Thus, in general, the sample should never dry out for bionanofiber preparation

Next, we tried to fibrillate dried chitin into nanofibers. Dried chitin was prepared from crab

matrix components of proteins and calcium carbonate according to the conventional method (Shimahara & Takiguchi, 1998), followed by drying in an oven to promote hydrogen bonding interaction between chitin nanofibers. Figure 4 is FE-SEM image of a crab shell after

after the removal of matrix (Abe et al., 2007; Iwamoto et al., 2008).

from Alaskan pink shrimp shell.

conditions (Ifuku 2010b).

shell flakes with an

α

Fig. 11. FE-SEM images of cellulose fibers from wood pulp (a and b) before and (c) after one pass through grinder. The length of the scale bar is (a) 30,000 nm, and (b, c) 1000 nm, respectively.