the invention which is the subject matter herein is a method for the hydrogenation of a carbohydrate to its polyols comprising contacting at hydrogenation conditions an aqueous solution of the carbohydrate with hydrogen and a catalyst consisting essentially of a zerovalent group viii metal selected from the group consisting of osmium , ruthenium , palladium , and platinum dispersed on alpha - alumina , and recovering the formed polyols . this invention results from the discovery that alpha - alumina possesses remarkable hydrothermal stability under conditions necessary for the hydrogenation of aqueous solutions of carbohydrates , especially in comparison with the more commonly used gamma - alumina . thus , whereas substantial amounts of silica and gamma - alumina , which are two commonly employed support materials , dissolve in the aqueous medium during hydrogenation of carbohydrates , virtually no leaching of alpha - alumina occurs under comparable hydrogenation conditions . therefore , one advantage of this invention is that the product contains a substantially lower level of dissolved metal from the inert support described herein than that resulting from inert supports commonly employed previously in the hydrogenation of carbohydrates . another advantage of this invention is that the zerovalent metals commonly employed as a hydrogenation catalyst retain their activity on the alpha - alumina support of this invention . yet another advantage of the invention as claimed is that of the group viii metals palladium , osmium , and ruthenium are more resistant to leaching under hydrogenation conditions than other metals of this class , such as nickel . because ruthenium is both resistant to leaching and particularly catalytically active it is especially advantageous in the practice of this invention . as mentioned previously , the invention herein is concerned with a method of hydrogenating a carbohydrate to its polyols . carbohydrates are polyhydroxyaldehydes , polyhydroxyketones , or compounds that can be hydrolyzed to them . a carbohydrate that cannot be hydrolyzed to simpler compounds is called a monosaccharide . one that can be hydrolyzed to two monosaccharide molecules is called a disaccharide , and one that can be hydrolyzed to many monosaccharide molecules is called a polysaccharide . a monosaccharide may be classified according to the number of carbon atoms it contains ; a hexose is a 6 - carbon monosaccharide , a pentose is a 5 - carbon monosaccharide , and a tetrose is a 4 - carbon monosaccharide . monosaccharides are preferred among the carbohydrates which may be used in this invention , and among these the hexoses , pentoses and tetroses are the most important members , with the hexoses particularly preferred . the polyol reduction products of this invention have the formula hoch 2 ( choh ) n ch 2 oh , where n is 2 , 3 , or 4 depending upon the kind of monosaccharide used or the kind of units in the di - or polysaccharide . where n is 4 , the polyol is a hexitol ; where n is 3 , the polyol is a pentitol ; and where n is 2 , the polyol is tetritol . it is to be understood that where the carbohydrate is a disaccharide or polysaccharide , substantial hydrolysis accompanies hydrogenation to ultimately afford the polyols of this invention . the examples of carbohydrates below are cited merely for illustration , and are not intended as exhaustive of the suitable reactants which may be used in this invention . accordingly , monosaccharides that can be employed include glucose , mannose , galactose , talose , fructose , allose , altrose , idose , gulose , xylose , lyxose , ribose , arabinose , threose and erythrose . glucose and mannose are particularly preferred monosaccharides which afford sorbitol and mannitol , respectively , as their polyol reduction product . fructose is another preferred monosaccharide which affords a mixture of sorbitol and mannitol as the product . examples of disaccharides include maltose , cellobiose , sucrose and lactose . among the more abundant polysaccharides which may be employed in this invention are starch , cellulose and their degradation products . the catalysts of this invention consist essentially of a zerovalent group viii metal dispersed on alpha - alumina . among the metals which may be used are included ruthenium , osmium , palladium and platinum , with ruthenium being preferred because of its high resistance to leaching and particularly high catalytic activity under process conditions . the group viii metal is generally dispersed on alpha - alumina as the inert support by impregnating the latter with a suitable salt of the metal , calcining the salt where necessary , and by reducing it to the zerovalent metal in a hydrogen atmosphere . calcining is performed where volatiles are to be removed from the support , or where the metal salt needs to be converted , e . g ., to its oxide , to be readily reducible . in suitable cases calcination and reduction may be combined in the same step . it is to be understood that by alpha - alumina is meant alumina whose crystallinity as measured by x - ray diffraction corresponds to that characterized in astm file number 10 - 173 . because the surface area of alpha - alumina is relatively low , metal loadings are correspondingly low . in this invention the catalyst typically contains from about 1 to about 5 % metal . the aqueous solution of the carbohydrate is contacted with hydrogen and the catalyst of this invention at hydrogenation conditions . hydrogenation conditions include a pressure of at least about 200 psig , with pressures in excess of about 5000 psig generally not advantageous . in the usual case , a hydrogen pressure from about 700 to about 5000 psig is used , with a pressure from about 1000 to about 3000 psig preferred . the hydrogenation temperature will be greater than about 80 ° c ., with the upper temperature limit dictated by the onset of the decomposition of either the product or reactant . for example , in the case of glucose as the reactant and sorbitol as the product , the upper temperature limit is about 160 ° c . in practical terms , a hydrogenation temperature from about 100 ° to about 150 ° c . is preferred with one from about 110 ° to about 130 ° c . being especially advantageous . the amount of catalyst used will depend , inter alia , on the amount of metal on the support , hydrogenation pressure , and temperature . in the case of ruthenium , for example , sufficient catalyst is employed to give from about 0 . 1 to about 1 wt . % ruthenium based on the carbohydrate as monosaccharide . the method of this invention may be practiced in either a batch or a fixed mode . in the batch mode , an aqueous solution of the carbohydrate containing from about 25 to about 60 percent carbohydrates is loaded into a reactor containing , for example , the ruthenium on a alpha - alumina catalyst of this invention in an amount sufficient to give from about 0 . 1 to about 1 wt . % ruthenium based on the carbohydrate . the mixture is then heated to the desired temperature , which is from about 80 ° to about 160 ° c ., and usually from about 100 ° to about 150 ° c . after the desired reaction temperature is attained , hydrogen is admitted to a pressure from about 700 to about 5000 psig . the entire reaction mixture is then agitated to provide adequate contact among the hydrogen , catalyst , and carbohydrate . the hydrogenation is continued until there is no further hydrogen uptake , which generally is a time from about 0 . 5 to about 5 hours . the invention described is advantageously practiced in a continuous fashion using the catalyst in a fixed bed , fluidized bed , expanded bed , and so forth . in a typical operation , feedstock containing from about 25 to about 60 % of the carbohydrate ( s ) to be reduced is passed , through the bed of catalyst , which is platinum , palladium , osmium , or ruthenium on alpha - alumina , in a hydrogen atmosphere . hydrogen pressure is from about 700 to about 5000 psig , and bed temperature is generally from about 100 ° to about 150 ° c . the effluent is an aqueous solution of the formed polyol ( s ), which may be recovered by , for example , removal of water by evaporation . the examples which follow merely illustrate this invention and are not intended to limit it in any way . the following experiment was done to demonstrate the hydrothermal stability of various materials often used as an inert support for catalytically active zerovalent metals . a mixture of 50 ml of a 50 percent aqueous solution of sorbitol and 2 . 5 g of support material was held in a rotating glass - lined autoclave for 24 hours in the presence of hydrogen at 135 atmospheres and at 130 ° c . at the end of this period , solid was removed by filtration and the filtrate was analyzed for metals . the following table summarizes the results . ______________________________________leaching of inert supportssupport material dissolved support material______________________________________alpha - alumina less than 1 ppm algamma - alumina . sup . a 60 ppm algamma - alumina . sup . b 129 ppm alkieselguhr . sup . c 83 ppm si______________________________________ . sup . a 0 . 5 abd , surface area 200 m . sup . 2 / g . sup . b 0 . 3 abd , surface area 160 m . sup . 2 / g . sup . c solution of glucose was used instead of sorbitol . the results clearly show the superior hydrothermal stability of alpha - alumina relative to other commonly employed supports . an aqueous solution ( 200 ml ) of 5 . 2 % rucl 3 . 3h 2 o was mixed with 152 g alpha - alumina of surface area 3 m 2 / g , after which water was evaporated . the impregnated solid was calcined for 3 hr . under flowing nitrogen at 400 ° c ., then reduced for 4 hr . in a hydrogen stream at 400 ° c . the resulting catalyst contained about 3 % by weight ruthenium with an average crystallite size of 158a . continuous reductions were performed in a 7 / 8 &# 34 ; i . d . vertical tube reactor with a spiral preheater and with a bed of 100 cc ( 135 g ) catalyst as prepared in example 2 . the feedstock was a 50 % aqueous solution of glucose at ph about 5 . 5 . hydrogen was introduced at a 10 : 1 molar ratio relative to glucose . effluent was analyzed by gas - liquid phase chromatography for sorbitol , mannitol , fructose , and glucose . the following table , a composite of several runs , shows the effect of process variables on conversion and product distribution . ______________________________________continuous reduction of glucose with 3 % ru - alpha alumina con - selec - ver - sorbi - tivity . sup . f fruc - p . sup . at . sup . b mode . sup . c lhsv . sup . d sion . sup . e tol mannitol tose______________________________________1500 120 upflow 1 . 0 60 96 3 11500 120 upflow 0 . 5 95 96 2 01500 120 upflow 0 . 25 98 93 3 0 700 120 upflow 1 . 0 43 95 2 3 700 120 upflow 0 . 5 61 95 3 1 700 120 upflow 0 . 25 95 94 5 0 700 120 downflow 1 . 0 56 96 1 0 700 120 downflow 0 . 5 80 96 1 0 700 120 downflow 0 . 25 98 95 2 01500 120 upflow 0 . 5 95 97 01500 130 upflow 0 . 5 96 95 41500 140 upflow 0 . 5 97 92 6 700 120 upflow 1 . 0 43 95 1 31500 120 upflow 1 . 0 62 96 3 12000 120 upflow 1 . 0 72 97 1 0 700 120 downflow 1 . 0 57 95 1 01500 120 downflow 1 . 0 78 95 1 02000 120 downflow 1 . 0 85 96 1 0______________________________________ . sup . a hydrogen pressure , psig . . sup . b temperature , ° c . . sup . c feedstock passed upflow or downflow . . sup . d liquid hourly space velocity . . sup . e percent glucose reacted . . sup . f selectivity is the percentage of any one product relative to total product formation . the data show that sorbitol can be formed with a selectivity in excess of 95 % at glucose conversions about 95 %.