I
IRA
T. CLARK
Forest Products Laboratory,
U. S.
Department of Agriculture, Madison, Wis.
Hydrogenolysis of Sorbitol Glycerol can be obtained in 4070 yield from sorbitol. better than when sugar is used directly U. S. DEPARTMENT of Agriculture is devoting considerable research toward finding a process for hydrogenating sugars to glycerol. Hexose sugars can be readily hydrogenated to the corresponding sugar alcohols, but higher temperatures required for cleavage of the sugar molecules has resulted in complex mixtures that vary with catalysts and conditions used. The principal product of sugar hydrogenolysis is 1,2-propanediol with smaller amounts of glycerol, ethanediol, and several sugar alcohols and their anhydrides. Hydrogenolysis of sorbitol, however, under conditions that would result in sugar degradation to a greater variety of less valuable products, can give glycerol in 40y0 yields. T o utilize sugars obtained from wood saccharification, the kinetics of sorbitol hydrogenolysis was investigated a t temperatures of 215', 230°, and 245' C. and hydrogen pressures of 2000 to 5600 T H E
p s i . Aqueous solutions containing 40% of 99% D-sorbitol were used with calcium hydroxide promoter and a 100to 200-mesh catalyst of 50% nickel on kieselguhr in suspension. Nickel amounted to 9% and calcium hydroxide to 1% of the sorbitol. An internally stirred reactor, charged batch-wise with a liter of sorbitol solution-catalyst slurry was raised to operating pressure with hydrogen, brought rapidly to temperature, and leveled off. A continuous flow of excess hydrogen a t reaction pressure was passed upward through the stirred reaction mixture and vented through a condenser and pressure-regulating valve. Samples taken throughout the course of a run were analyzed by quantitative chromatography. Individual hexitols or pentitols were not resolved in the analysis and are, therefore, expressed as sorbitol and xylitol, respectively. The experiments indicated that in
This is
hydrogenolysis of sorbitol, a series of simultaneous and consecutive reactions occur, in which the carbon chain is cleaved chiefly to two 3-carbon glycerol fragments, but also in a 5,l arrangement to xylitol and methanol, and in a 4,2 arrangement to erythritol and ethanediol. Hydrogenolysis of carbon-carbon and carbon-hydroxyl bonds in these fragments also occurs. The 1,2- propanediol was formed principally by glycerol hydrogenolysis. Depending on readtion conditions and time, degradation could continue until glycols, primary alcohols, or hydrocarbons and water remain. Some of these reactions, were verified in hydrogenolysis of xylitol and glycerol separately. With xylitol, principally glycerol, 1,2propanediol, and ethanediol resulted. With glycerol alone, two parts of 1,2propanediol to one part of ethanediol resulted-cleavage of a hydroxyl on an end carbon in glycerol is only about
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215. C. 2000 P.S.1
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GLYCEROL
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A Figure 1. Hydrogenolysis of sorbitol gives chiefly these five products, but relative quantities and total yields are affected b y reaction conditions
Figure 2. For temperatures of 200' to 230' C., sorbitol hydrogenolysis rates for a hydrogen pressure vary with temperature according to the Arrhenius equation
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4 Figure 3. Rate of sorbitol hydrogenolysis depends on both temperature and hydrogen pressure
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The exceptions were a t higher pressures for each temperature when hydrogen apparently occupied most of the active sites on the catalyst surface and slowed reaction rates. Interaction of temperatures and hydro-
gen pressures significantly affected reaction rates and glycerol yields. Most rapid reaction rates and best yields were obtained within a characteristic pressure range for each temperature. At pressures below this range, glycerol decomposition increased. At pressures above it, the reaction departed from first order as indicated by the changing slope of the rate line to slower rates. When logarithms of first-order specific reaction-rate constants for sorbitol were plotted against reciprocals of the absolute temperature, as in Figure 2, a family of curves was obtained which showed that, for temperatures from 200" to 230' C., hydrogenolysis rates varied as functions of the absolute temperature in agreement with the Arrhenius equation
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Rates approximately doubled for a 10" C. temperature increase except above 230" C., where the rate of increase was less, perhaps because of catalyst inactivation or more rapid desorption of reactants from the catalyst surface at these temperatures. Activation energy, determined for sorbitol from experimental
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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half as difficult as cleavage of a carboncarbon bond. Figure 1 shows the distribution of products amounting to 85 to 90% of the sorbitol used. Some of the xylitol, erythritol, and glycerol, formed early in the reaction, was converted to compounds of lower molecular weight as reaction time increased. In other runs, these five products were chiefly obtained, but yields and time a t which they were obtained were affected by reaction conditions. Complete analysis of the kinetics of the hydrogenolysis reaction would be difficult; however, some conclusions were drawn by treating the data as for homogeneous reactions without considering diffusion rates and catalyst surface area and porosity. In most instances, hydrogenolysis of sorbitol and of glycerol is a first-order reaction which agrees with the equation ( 7 ) .
data, varied from 44,000 calories for a hydrogen pressure of 5600 p.s.i. to 33,000 calories for 2900 p.s.i. The runs a t a pressure of 2000 p.s.i., the line of which differed from others in Figure 2 for some unexplained reason, gave a value of 18,000 calories. Effects of the interaction of temperature and hydrogen pressure on rates of sorbitol hvdrogenolvsis are shown in Figure 3. " The hydrogenolysis rate of glycerol a t a hydrogen pressure of 2000 p.s.i. increased more rapidly than that of sorbitol with temperature increase :
Figure 4. Best glycerol yields were obtained in 2l/2 hours, at215',in90minutes and in 35 a t 230', minutes a t 245' C., at hydrogen pressures characteristic of each temperature
0.0112 0.0213 0.0311
0.0010 0,0030 0,0059
This explains the decrease in glycerol yields that accompanies increase of reaction temperature. Instead of determining reaction-rate constants for glycerol at other temperatures studied, maximum glycerol yields of sorbitol runs were used to compare effects of reaction conditions upon glycerol. Figure 4 illustrates the effect of hydrogen pressure a t 230" and 245' C. upon maximum glycerol yields obtained a t 90 and at 35 minutes, respectively. At 215' C. and a hydrogen pressure of 2000 p.s.i. maximum glycerol yield was reached in 2l/2 hours. Pressures above this slowed the reaction rate. The dotted line in Figure 4 shows glycerol yields that failed to reach a maximum in 4l/2 hours a t the higher pressures. A calcium hydroxide promoter, equivalent to 2 or 30/, of the sorbitol resulted in more rapid reactions but lower glycerol yields than when 1% of calcium hydroxide was used. Catalyst nickel, equal to 6, 9, and 12y0 of sorbitol, resulted in only slight differences in reaction rates at 215' C. a t any hydrogen pressure. At 230' C., reaction rates for all catalyst levels were from 2 to 4 times more rapid than those a t 215' C. and increased with pressure and catalyst nickel up to a hydrogen pressure of 3800 p.s.i. but were lower at 4700 than a t 3800 p s i . At the 9 and 12% levels of nickel, glycerol yields were the same and averaged 2y0 more than when 6y0levels were used. Lower catalyst levels or inactive catalysts resulted in lower glycerol yields and a greater diversity of cleavage products. literature Cited
(1) Glasstone, S.. "Textbook of Physical Chemistry," Van Nostrand, New York, 1940.
RECEIVED for review September 11, 1957 ACCEPTEDMarch 3, 1958 Work partially supported by the Ordnance Corps, Department of the Army.
