the present invention pertains to a process for the preparation of an anionic clay by reacting a slurry comprising aluminium trihydrate or its thermally treated form with a magnesium source and magnesium acetate . the reaction results in the direct formation of an anionic clay with acetate anions as the interlayer charge - balancing species . the reaction takes place at ambient temperature or higher . at temperatures higher than 100 ° c ., the reaction is preferably carried out under autogeneous conditions . in this method acetate anions are provided in the reaction medium as a soluble magnesium acetate salt and incorporated into the interlayer as the necessary charge balancing anion . anionic clays prepared by this method exhibit the well known properties and characteristics ( e . g . chemical analysis . powder x - ray diffraction pattern , frir , thermal decomposition characteristics , surface area , pore volume , and pore size distribution ) usually associated with acetate - containing anionic clays prepared by the customary and previously disclosed methods . upon being heated , anionic clays generally decompose to mg — al solid solutions , and at higher temperatures spinels . when used as a catalyst , an adsorbent ( for instance a so x adsorbent for catalytic cracking reactions ), or a catalyst support , the anionic clay according to the invention is usually heated during preparation and is thus in the mg — al solid solution form . during use in an fcc unit , the catalyst or adsorbent is converted from an anionic clay into mg — al solid solutions . therefore , the present invention is also directed to a process wherein an anionic clay prepared by reacting a slurry comprising aluminum trihydrate and a magnesium oxide source , is heat - treated at a temperature between 300 and 1200 ° c . to form a mg — al solid solution . the anionic clay according to the invention has a layered structure corresponding to the general formula [ mg m 2 + al n 3 + ( oh ) 2m + 2n . ]( oac − ) m . b h 2 o wherein oac = acetate , m and n have a value such that m / n = 1 to 10 , preferably 1 to 6 , and b has a value in the range of from 0 to 10 , generally a value of 2 to 6 and often a value of about 4 . it is preferred that m / n should have a value of 2 to 4 , more particularly a value close to 3 . since the process disclosed in this patent does not require washing of the product or filtering , there is no filtrate waste or gaseous emissions ( e . g . from acid decomposition ), making the process particularly environmental - friendly and more suited to the environmental constraints which are increasingly imposed on commercial operations . the product can be spray dried directly to form microspheres or can be extruded to form shaped bodies . the present invention includes the use of crystalline aluminum trihydrate ( ath ), for example gibbsites provided by reynolds aluminium company rh - 20 ® or jm huber micral ® grades . also boc ( bauxite ore concentrate ), bayente and nordstrandite are suitable aluminum trihydrates . boc is the cheapest alumina source . the alumina trihydrate is preferred to have a small particle size . in another embodiment of the invention thermally treated forms of gibbsite are used . combinations of aluminum trihydrate and thermally treated forms of aluminium trihydrate can also be used . the calcined aluminium trihydrate is readily obtained by thermally treating aluminum trlhydrate ( gibbsate ) at a temperature ranging from 100 to 800 ° c . for 15 minutes to 24 hours . in any event , the calcining temperature and time for obtaining calcined aluminum trihydrate should be sufficient to cause a measurable increase of the surface area in view of the surface area of the gibbsite as produced by the bayer process which is generally between 30 and 50 m 2 / g . it should be noted that within the concept of this invention flash calcined alumina is also considered to be a thermally treated form of aluminum trihydrate , although generally it is considered a very specific alumina . flash calcined alumina is obtained by treating aluminum trihydrate at temperatures between 800 - 1000 ° c . for very short periods of time in special industrial equipment , as is described in u . s . pat . nos . 4 , 051 , 072 and 3 , 222 , 129 . combinations of various thermally treated forms of aluminum trihydrate can also be used . preferably the aluminum source is added to the reactor in the form of a slurry . in particular it is emphasized that there is no need to use a peptizable alumina source ( gibbsite is not peptizabie ) and as a result no need to add either mineral or organic acid to vary the ph of the mixture . in the process according to this invention other aluminum sources beside aluminum trihydrate or its thermally treated forms may be added to the slurry such as oxides and hydroxides of aluminum , ( e . g . sols , flash calcined alumina , gels , pseudo - boehmite , boehmite ) aluminum salts such as aluminum nitrate , aluminum chloride , aluminum chlorohydrate and sodium aluminate . the other aluminum sources may be soluble or insoluble in water and may be added to the aluminum trihydrate and / or its thermally treated form or it may be added to the slurry separately as a solid , a solution or as a suspension . mg - bearing sources which may be used include mgo , mg ( oh ) 2 , magnesium acetate , magnesium formats , magnesium hydroxy acetate , hydromagnesite ( mg 5 ( co 3 ) 4 ( oh ) 2 ), magnesium carbonate , magnesium bicarbonate , magnesium nitrate , magnesium chloride , dolomite and sepiolite . both solid mg sources and soluble mg salts are suitable . also combinations of mg sources may be used . the magnesium source may be added to the reactor as a solid , a solution , or , preferably , as a slurry . the magnesium source may also be combined with the aluminum source before it is added to the reactor . the magnseium acetate may be added to the slurry containing aluminum trihydrate or its thermally treated form and / or magnesium source or vice versa . acetic acid may also be added to the slurry to increase the acetate concentration and / or control ph . also other metal acetate salts may be added e . g . zinc acetate provided it is acceptable that divalent cations other than mg may enter the anionic clay structure . because of its simplicity , this process is particularly suitable to be carried out in a continuous mode . thereto an aluminum source and a magnesium source are fed to a reactor and reacted in aqueous suspension to obtain an anionic clay - containing compostion . in the case of a batch process an aluminum source and a magnesium source are added to a reactor and reacted in aqueous suspension to obtain an anionic clay - containing composition . within the context of this invention a reactor is considered to be any confined zone in which the reaction between the aluminum source and magnesium source takes place . the reactor may be equipped with stirrers , baffles etc . to ensure homogeneous mixing of the reactants . the reaction can take place with or without stirring , at ambient or at elevated temperature and at atmospheric or elevated pressure . usually , a temperature between 0 and 100 ° c . is used at or above atmospheric pressure . it is preferred to carry out the process at temperatures above 50 ° c . rather than at room temperature , because this results in anionic clays with sharper peaks in the x - ray diffraction pattern than anionic clay - containing compositions obtained at room temperature . the reactor may be heated by any heating source such as a furnace , microwave , infrared sources , heating jackets ( either electrical or with a heating fluid ), lamps , etc . the aqueous suspension in the reactor may be obtained by either adding slurries of the starting materials , either combined or separate , to the reactor or adding magnesium source to a slurry of alumina trihydrate and / or its thermally treated form or vice versa and adding the resulting slurry to the reactor . it is possible to treat , for instance the aluminum trihydrate slurry at elevated temperature and then add either the mg source pro se , or add the mg source in a slurry or solution either to the reactor or the aluminum source slurry . given particular facilities which might be available , the continuous process can be conducted hydrothermally . this is particularly advantageous , because it this is faster and a higher conversion is obtained . there is no need to wash or filter the product , as unwanted ions ( e . g . sodium , ammonium , chloride , sulphate ) which are frequently encountered when using other preparation methods , are absent in the product . in a further embodiment of the invention , the process is conducted in a multistep process , e . g . a slurry of ath and mg source is treated thermally in a first reactor at a mild temperature , followed by a hydrothermal treatment in a second reactor . if desired a preformed anionic clay may be added to the reactor . the preformed clay may be recycled anionic clay from the reaction mixture or anionic clay made separately by the process according to the invention or any other process . if desired , organic or inorganic acids and bases , for example for control of the ph , may be fed to the reactor or added to either the magnesium source or the aluminum source before they are fed to the reactor . an example of a preferred ph modifier is an ammonium base , because upon drying no deleterious cations remain in the anionic clay . if desired , the anionic clay prepared by the process according to the invention may be subjeed to ion exchange . upon ion exchange the interlayer charge - balancing acetate ion is replaced with other anions . the other anions are the ones commonly present in anionic clays and include pillaring anions such as v 10 o 28 − 6 , mo 7 o 24 6 − pw 12 o 40 3 − b ( oh ) 4 − ; b 4 o 5 ( oh ) 4 2 − , hbo 4 2 − , hgao 3 2 − cro 4 2 − . examples of suitable pillaring anions are given in u . s . pat . no . 4 , 774 , 212 which is included by reference for this purpose . the ion exchange can be conducted before or after drying the anionic clay and aluminum source - containing composition formed in the slurry . the process of the invention provides wide flexibility in preparing products with a wide range of mg : al ratios . the mg : al ratio can vary from 1 to 10 , preferably from 1 to 6 , more preferred from 2 to 4 , and especially preferred to close to 3 . for some applications it is desirable to have additives , both metals and non - metals , such as rare earth metals , si , p , b , group vi , group viii , alkaline earth ( for instance ca and ba ) and / or transition metals ( for example mn , fe , ti , zr , cu , ni , zn , mo , sn ), present . the metals and non - metals can easily be deposited on the anionic clay or the solid solution according to the invention or they can be added either to the alumina source or magnesium source which are added to the reactor or added the reactor separately . suitable sources of metals or non - metals are oxides , halides or any other salt such as chlorides , nitrates etc . in the case of a multi - step process the metals and nonmetals may be added in any of the steps . is can be especially advantageous for controlling the distribution of the metals and nonmetals in the anionic clay . the present invention is illustrated by the following examples which are not to be considered limitative by any means . all examples according to the invention were made in air at 1 atm . it is also possible to prepare the anionic clays under nitrogen or carbon dioxide - free atmosphere , so that the anionic clay comprises less carbonate as a balancing anion . a commercially available sample of a mg — al acetate anionic clay was obtained from laroche chemical company . its pxrd pattern is shown for illustration in fig1 . the following example illustrates the preparation of an acetate anionic clay by coprecipitation . 5 . 14 g of magnesium nitrate and 3 . 75 g of aluminum nitrate were dissolved in 50 ml of de - ionized water . 11 . 48 g of sodium acetate were dissolved in 90 ml of de - ionized water . the nitrate solution was added drop - wise to the acetate solution , with the ph maintained at 10 by the addition of a solution made up as follows : 100 ml of de - ionised water , 12 g of ( 3n ) naoh , and 6 . 2 g of sodium acetate ( 0 . 75 n ), all under a nitrogen atmosphere at room temperature . the pxrd pattern is shown in fig2 . 4 . 28 g of magnesium acetate and 3 . 75 g of aluminum nitrate were dissolved in 50 ml of de - ionised water . 3 . 75 g of sodium acetute were dissolved in 90 ml of de - ionised water . the nitrate solution was then added dropwise to the acetate solution , with the ph maintained at 10 by the addition of a solution made up as follows : 100 ml of do - ionised water , 12 g of 3n naoh , and 6 . 2 g of sodium acetate ( 0 . 75 n ) in air at room temperature . the pxrd pattern is shown in fig3 . 4 . 28 g of magnesium acetate and 3 . 75 g of aluminum nitrate were dissolved in 50 ml of de - ionized water at 65 ° c . 3 . 75 g of sodium acetate were dissolved in 90 ml of de - ionized water , at 65 ° c . the nitrate solution was then added dropwise to the acetate solution , with the ph maintained at 10 by the addition of a solution made up as follows : 100 ml of de - ionized water , 12 g of 3n naoh and 6 . 2 g of sodium acetate ( 0 . 75 n ) in air at 65 ° c . the pxrd pattern is shown in fig4 . 2 . 14 g of magnesium acetate were dissolved in 50 ml of de - ionized water and 1 . 0 g mgo and 2 . 35 g cp 1 . 5 ® ( ex alcoa ) were added . the mixture was kept at 65 ° c . overnight . the slurry was dried in an oven at 65 ° c . the pxrd pattern of the product before drying is shown in fig5 a . the pxrd pattern of the product after drying is shown in fig5 b . 2 . 14 g of magnesium acetate were dissolved in 50 ml of do - ionized water and 2 . 0 g mgo and 2 . 35 g cp 1 . 5 ® ( ex alcoa ) were added . the mixture was kept at 65 ° c . overnight . the slurry was dried in an oven at 65 ° c . the pxrd pattern of the product is shown in fig6 and corresponds to the formation of an anionic clay with acetate as the charge balancing interlayer anion . 1 . 27 g of magnesium acetate were dissolved in 50 ml of de - ionized water , and 3 . 92 g mgo and 2 . 5 g alcoa cp ® 1 . 5 were added . the mixture was kept at 65 ° c . overnight . the slurry was dried in an oven at 65 ° c . pxrd confirmed that an anionic clay with an ( 003 ) reflection at 8 . 67 å was formed in conformity the formation of an anionic clay with acetate as the charge - balancing interlayer anion . ( see fig7 .) the product of example 5 was subjected to after calcination at 735 ° c . for 1 hr . the pxrd pattern showed that a mg — al solid solution was formed . ( see fig8 ).