Matthews, C. N., Moser, R. E., Nature (London),215, 1230 (1967). Oro. J.. Kimball. A. P.. Arch. Biochem. Bioohvs.. 94. 217 119611. Pette;;,'A. E. G.,'Ware,'G . C., Chem. lnd. ( i o n d o n ) , 1232 (1955). Reynolds, G. A , . Humphett, W. J., Swamer, F. W., Hauser, C . R . , J. Org. Chem., 16, 165 (1951). Sanchez, R . A . , Ferris, J. P., Orgel, L. E., J . Mol. Biol., 3 0 , 223 (1967). Southgate, 6.A., Gas World, 99, 14 (May 6, 1933). Steiger, R . E., Org. Syn. 24, 9 (1944). Tatsuo, S.,Hisahara, T., Hakko Kogaku Zasshi, 48, 277 (1970).
Volker, T., Angew. Chem., 72, 379 (1960).
Received for review January 10, 1973 Accepted October 4,1973
Presented at the Division of Water, Air, and Waste Chemistry, 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972.
Hydrogenation of Xylose to Xylitol J. Wisniak,* M. Hershkowitz,
R. Leibowitz, and S. Stein
Deparfment of Chemical Engineering, University of the Negev, Beer Sheva, lsrael
Xylose has been hydrogenated with Raney nickel under a wide range of pressure, temperature, catalyst concentration, and agitation rates. The reaction follows a pseudo-first-order course with a surface-reaction-controlling step between atomically adsorbed hydrogen and adsorbed xylose at 100" and unadsorbed xylose at higher temperatures.
This work was undertaken to provide kinetic information and process variable influences on the manufacture of xylitol from xylose. Monosaccharides and disaccharides have long been used and developed as natural sweetening agents, with glucose, sucrose, and starch hydrolyzates being the most commonly utilized in the diet. For various reasons, an increasing number of people have found it necessary to substitute artificial sweeteners for these sugars. About 70 years ago, saccharin, the first artificial sweetening agent, was introduced to the market. Its sweetening capacity was about 300 times that of cane sugar, but it did have a slight bitter aftertaste that made it objectionable to some users. In 1950 sodium and calcium salts of cyclamate were introduced with about 30-40 times the sweetening capacity of sugar. The use of cyclamates rose very rapidly, reaching nearly 5% of the world consumption of sweetening agents, including sugar, by 1969, when it was ordered off the market by the FDA after indications that it may be a cancer-promoting agent. Presently, the largest selling artificial sweetener in the market, saccharin, is also under close scrutiny and the possibility of its being banned from general use is considered. The fact remains that artificial sweeteners will continue to be needed by people who have to restrict their sugar intake. A possible candidate for this job can be xylitol, a penta alcohol obtained by the hydrogenation of xylose
H-(i=o
CHzOH
H-C-OH
I
HO-C-H
I H-C-OH I
CHZOH
+ H2
-
I I HO-C-H I H-C-OH I CH,OH H-C-OH
Xylitol is very soluble in water-64.2% a t 25"; its crystals resemble those of sugar; it is stable upon storage and does not caramelize at elevated temperatures. Its sweet-
ening capacity is 20-2570 greater than that of sugar and has no insulin requirements; hence it is suitable for use by diabetics (Hennecke, 1970). In addition, it has a favorable effect on the fat balance of the body and does not elevate the sugar level in blood (Lang, 1971). It has the added attraction that its manufacture involves the use of a material, xylose, that can be readily prepared from agricultural wastes. Xylan occurs in large percentages in many industrial wastes such as corncobs, cornstalks, peanut shells, and cottonseed hulls. Hydrolysis of xylan to xylose is usually carried out by boiling the raw material with a dilute acid preferably under pressure. The liquor is neutralized with lime, decolorized and purified with active carbon or ionexchange resins, and crystallized under vacuum (Suminoe and Okamura, 1971). Very little literature is available on the kinetics of hydrogenation of xylose to xylitol and the effect of operating variables. Leikin (1963) hydrogenated xylose with 4-8070 Raney nickel and found that the pH dropped sharply during the first 30 min, except when the amount of catalyst was 8. Similar but less pronounced results were obtained when using buffers. Tomkuljak (1949) used 3-30% Raney nickel with and without methanol at atmospheric pressure and temperatures between 30 and 70" and claimed that the pH drop was caused by xylonic acid produced by a nickel-induced Cannizzaro reaction. The high-pressure range is well covered by patents (Specht, 1959; Kohno and Yamatsu, 1971; Steiner, 1971). Their general characteristics are that the hydrogenation is carried on with Raney nickel under pressures up to 50 atm, temperatures of 100-140", and basic pH. It is generally claimed that addition of a basic buffer eliminates side-product formation and facilitates crystallization. Information regarding the hydrogenation of glucose to sorbitol is more readily available and may be used as a comparison (Froment and de Groof, 1966; Brahme, et al., 1964). In general, the reaction is said to be first order in the concentration of glucose and catalyst and half order with regard to hydrogen pressure. It does not depend on Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1. 1974
75
the initial concentration of glucose and the apparent energy of activation is about 5.2 kcal/mol. Several publications have dealt with the mechanism of aldehyde hydrogenation. Anderson and MacNaughton (1942) studied the hydrogenation of butyraldehyde, acetic aldehyde, and several other aldehydes and ketones with a mixture of 80% hydrogen and 20% deuterium. They found that the mechanism is more dependent upon the temperature, ketonic addition being predominant at low temperatures and more enolic addition taking place at higher temperatures. Similar results were obtained when using nickel, platinum, and copper chromite catalysts. Their results were substantiated by Badin and Pacsu (1944), who found that the mechanism involves carbon atom 2 with catalysis similar to a general acid catalysis. The first step was considered to be the addition of a proton with the simultaneous elimination of a proton a t another part of the molecule. Babcock, et al. (1957), hydrogenated a-methylstyrene with palladium, platinum, rhodium, ruthenium, and nickel catalysts. Their findings indicated that with palladium above 3 atm the apparent rate-controlling step was the surface reaction between dissociated hydrogen and a-methylstyrene both adsorbed on different types of active sites. Below 3 atm the reactants competed for similar active sites. Results with platinum were similar to those of palladium below 3 atm. Hydrogenation of sugars under more drastic conditions may lead to cleavage of the carbon chain. With hexoses, cleavage of the chain leads mainly to the formation of glycerol and ethylene glycols (Boelhouwer, et al., 1960; Van Ling and Vlugter, 1969). The composition of the reaction mixture can be readily determined by polarimetry since xylitol is optically inactive, by colorimetry (Scott, et al., 1967), or by gas-liquid chromatography (Sweeley, et al., 1963; Weiss and Tambawala, 1972). The last procedure requires conversion of xylose and xylitol to more volatile components by silylation with a 2: 1 mixture of hexamethyldisilazane and trimethylchlorosilane in pyridine (TMS) or N-(tetramethylsily1)imidazole in pyridine (Tri Si1 Z). An alternative method is to titrate the aldehyde group with hydroxylamine hydrochloride in methanol (Patchornik, 1956). Thin-layer chromatography of sugars and their alcohols can be effected on plates covered with a mixture of 60% Kieselguhr and 4 Kieselgel G using as eluent a solution of 2-propanol, ethyl acetate, and water. The spots are afterward developed with lead tetraacetate in glacial acetic acid (Wasserman, 1963). Equipment and Materials Hydrogenation runs were made in a dead-end 3/4-gal batch hydrogenator, Model AF 150, which was manufactured by Autoclave Engineers for a working pressure of 5000 psi at 650°F. Details of the experimental setup appear in Figure 1. The reactor fitted inside a 3-kW heating mantle (1) connected to an automatic temperature-controlling instrument through a chromel-alumel thermocouple. Agitation was provided by a Dispersimax turbine (2) with a diameter 40% that of the autoclave and driven by a constant-speed Y3-h~explosion-proof electric motor (4). Pulley arrangements (3) were used to change the speed of the agitator. Cooling was provided by an internal coil (5) and by proper use of the water valve it was possible to control the temperature to within f 2 " . Funnel (11) allowed introduction of the catalyst and solution, helped by the vacuum line (6). Samples were taken with a lk-in. high-pressure tube (10) submerged in front of the turbine and provided with an external jacket cooled with ice (19) to avoid water losses by flashing. Hydrogen was bubbled 76
Ind. Eng. Chern., Prod. Res. Develop., Vol. 13,No. 1, 1974
12
U Figure 1. Hydrogenation apparatus
under the turbine through line (16). Two chromel-alumel thermocouples inserted in thermocouple well (9) allowed for temperature control and recording. Pressure gauges (12) and (7) indicated pressure in the reactor and the vacuum line. The gas pressure in the system could be vented to the outside through line (13) connected with a tee to the rupture disk line (15). Two batches of 99%+ xylose were used; the first was obtained from Sigma and the second from Okamura Mills, Osaka, Japan. The electrolytic hydrogen was reported by the vendor to be 99.9% pure. Part of the Raney nickel catalyst (W2) was manufactured according to the standard procedure (Vogel, 1966) and part purchased from Doduco Chemie, Elsenz, Germany (Actimet C). Operating Techniques A total of 1000 ml of xylose solution and the desired amount of catalyst were added to the reactor through funnel (11) and valve f, and then deaerated by applying vacuum and bubbling hydrogen through valves b, c, d, for a few minutes. Heating was effected under a slight hydrogen pressure to avoid oxidation. When the reactor temperature reached the desired level, the hydrogen pressure inside the apparatus was adjusted to the desired value with regulator c. This was considered the start of the hydrogenation run and the first sample was taken and the time, temperature, and pressure were recorded. Six to seven samples were usually taken during each run a t time intervals of 10-20 min. They were obtained by cracking valve e, the initial volume was discarded, and the next 5-10 ml was saved for analysis. The samples were filtered to remove the catalyst and analyzed for xylose content, and when necessary by glc and tlc. The simplest polarimetric method was tried at the beginning and afterward abandoned because of the large sample required and because at low xylose contents a resolution of better than 0.01" was required to have an acceptable result. An apparatus of this characteristic was not available at the time of this work. A calibration curve for the xylose employed indicated that the specific rotation was 18.8"for concentrations up to 1.5 M . It was decided then to analyze for aldehyde content by titration with hydroxylamine hydrochloride, as follows (Patchornik, 1973). To 0.1 ml of sample dissolved in absolute methanol were added a few drops of Thymol Blue and enough H&04 (0.025 N ) in absolute methanol to change the color to violet. Addition of 5 ml of a saturated solution of hydroxylamine hydrochloride in methanol changed the color
R U N 34
- I
200 P S l G 9 8 0 FPM 2 % NI
'5
4 1
fJB 1 1
0 1
I
01
,
3
as
08
1.1I
I
ID
12
SOLUTION VOLUME I M L
Figure 4. Influence of impeller position
200 P.SIG. OB0 R P M 2 2 % Nt
R P M
Figure 3. Influence of agitation rate :
1
21
to yellow which reversed again to violet on heating to almost boiling. The warm solution was then titrated with CH30Na in methanol (0.02 N) to a yellow end point. Sharper changes in color were obtained by minimizing the amount of water present by use of absolute methanol in every step needed. The saturated solution of hydroxylamine hydrochloride was prepared by adding enough crystals to methanol and titrating with CH30Na to a yellow . . color. The reactor product was filtered and purified by successive treatments with ion-exchange resins IR-120 and IRA93. The latter removed the color and raised the pH to about 9. The clear solution was afterward concentrated at 40" under vacuum up to a specific gravity of 1.38-1.40. In this range crystallization proceeded very rapidly when cooling to room temperature and adding a few milliliters of methanol.
Figure 5. Temperature influence of reaction rate constant
Or .5o
O ~
i y l o s e %No
.
P- S l G
300 150
12 2 1
h
9 8 0 R P M
450 450
15
+
200 r
22 22
525
0
I
I I 1
Results and Discussion
030
About 100 runs were made in the following range of variables: xylose concentration, 1-3.5 -44;catalyst concentration, 1-1870 Ni based on xylose; agitation, 300-1200 rpm; pressure, 100-800 psig; temperature, 80-140". The course of the reaction was followed in each case by plotting the logarithm of xylose concentration us. the time of the reaction. In every run the plot yielded a straight line so that the overall rate could be represented .by a pseudo-first-order reaction r = d C / d t = kC (1) Figure 2 shows a typical experiment. The first order reaction constant could thus be used to characterize a particular set of variables. In every case it was found that the initial p H of 6.5 dropped within 20-40 min to 4.5 and remained constant
-
20
40
60
80
100
120
T I M E , MINUTES
Figure 6. Behavior a t low temperature
afterward. Titration of the acidity indicated that it was equivalent to about 12 mequiv of HCl/l. The points indicated in every figure are usually the average of two runs under similar conditions. Homogeneous Reaction. Runs made in the absence of a catalyst under varied conditions showed no xylose conversion so that it can be concluded that within experimental conditions the homogeneous reaction was negligible and there was no catalytic effect by the metal walls of the system. Mass-Transfer Effects. In order to eliminate geometrical effects it is important to work under conditions where Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
77
0
2
4
(GRAM
10
8
6
NICKEL
wz
/ GM
12
14
16
18
MOLE X Y L O S E ) * I O O
Figure 7. Effect of catalyst concentration
I
/
25
980 R.P.M.
2.2
20
%
0 IOOOC
x
Ni
llO°C 125OC
-'.E
b:
E
15
a
10
5
0
zoo HYDROGEN
400
600
800
PRESSURE, P.S.I.A.
Figure 8. Pressure dependence
the chemical resistance is predominant. Hydrogenation runs made under variable agitation regimes showed that the rate was independent of the impeller speed, between 800 and 1200 rpm (Figure 3). This range is larger than the one found during hydrogenation of fats on a similar reactor (Wisniak and Stefanovic, 1967) indicating that hydrogen saturates the xylose solutions more easily. A possible explanation for this effect could be the lower relative viscosity of the system. Runs made under variable liquid heights showed that the optimum reaction rate was attained with 1 1. of solution; this corresponded to the turbine being located at about two-thirds of the liquid height. These results follow the general trend of gas-liquid contacting in dead-end reactors (Wisniak, et al., 1971). The influence of the impeller position appears in Figure 4. These results, together with those obtained at different catalyst loadings, indicate that mass-transfer resistance effects were probably insignificant. Temperature. The values of k at 200 psig were plotted on a log-log graph against the reciprocal of temperature between 100 and 140" (Figure 5 ) . It can be seen that in this range the Arrhenius equation is satisfied with an acti78
Ind. Eng. Chern., Prod. Res. Develop., Vol. 13, No. 1, 1974
vation energy of about 6.5 kcal/mol. Higher temperatures could not be explored because of xylose decomposition. Runs made at 80" showed an interesting behavior (Figure 6); the reaction behaved initially according to the Arrhenius extrapolated k and then the rate dropped to a negligible value. One possible explanation could be that the xylose was reacting mainly with the hydrogen originally present in the catalyst surface which was not replaced rapidly enough with hydrogen from the gas area. This behavior has been observed previously with substrates other than xylose (Csuros, et al., 1961). On investigating the hydrogenation of different chemical species, Csuros found that a marked decrease in adsorbed hydrogen occurred during the course of the reaction and that the degree of decrease was proportional to the strength of the sorption of the substrate. A more plausible explanation was to assume that xylitol was adsorbed more strongly than xylose and because of the low temperature level it did not abandon the catalyst surface. To test this hypothesis a solution containing 2 mol of xylose and 1 mol of xylitol was hydrogenated under similar conditions and it was found that the rate was negligible from the very beginning. Notice should be taken of the fact that the apparent activation energy reported includes the effect of the catalyst concentration and hydrogen pressure and that it will vary significantly with these factors (Figure 8). Its low value suggested initially some kind of mouth poisoning of the catalyst pore by a foreign substance or by hydrogen. A plot of the logarithm of k against the temperature showed that the data could be fairly well estimated by a ninth power variation in temperature. This is a much stronger dependence than the one expected by mouth poisoning. Xylose Concentration. Runs made under various initial concentrations of xylose showed that k was independent of this variable. Some of these results appear in Figure 6. Catalyst Concentration. Figure 7 shows that within the experimental range the values of k are proportional to the amount of catalyst present. This fact substantiates the basic hypothesis that mass-transfer resistances were not predominant. The W2 catalyst prepared showed lower activity than that of purchased Actimet C. It should be remembered that eight different types of Raney nickel may be manufactured with different relative activities according to the original alloy and activation process. Pressure. The effect of hydrogen pressure on k is shown in Figure 8 for three different temperature levels. Striking
differences in behavior occur as the temperature and pressure are increased. For example, a t loo" the rate will go over a maximum at about 300 psia and then decrease until a pressure above 600 psia is reached, where another jump in value takes place. These results can be partially explained by an analysis of the mechanism of the reaction, as shown below. Mechanism of the Reaction. The following conclusions were drawn from the experimental values of k that appear in Figures 3, 4, 7, and 8. (1) Mass-transfer resistance in the gas phase is negligible as the phase is essentially hydrogen. ( 2 ) Mass-transfer resistances in the liquid phase are also negligible as shown by the independence of the reaction rate on stirring rate and a linear variation with catalyst loading. The process is then controlled by the chemical steps and can be further analyzed by the reaction mechanisms of Hougen and Watson (1947) if it is assumed that the hydrogen pressure is a direct measure of hydrogen concentration at the surface of the catalyst. (3) The absorption of hydrogen is not rate controlling because the rate would vary proportional to the hydrogen concentration and inversely proportional to xylose concentration. (4) The adsorption of xylose is not rate controlling as this would require a decrease in the rate with increase in hydrogen pressure. (5) The desorption of xylitol cannot be rate controlling. If it were, the rate should grow to a constant value when the hydrogen pressure is increased. (6) The rate controlling step is not the impact of one unadsorbed species on the other adsorbed. This would require a first-order rate in hydrogen, which is not the case. The 17 major mechanisms of Hougen and Watson could thus be reduced to two: (a) reaction between atomic chemisorbed hydrogen and adsorbed xylose on similar or different active sites, surface reaction controlling
r=
(a
PHC bC
+ fa + + dC$
and (b) reaction between atomically chemisorbed hydrogen and unadsorbed xylose, surface reaction controlling
(3) It can be seen that both mechanisms require the hydrogen to be adsorbed in atomic form. This hypothesis is substantiated by what is known about the chemisorption of hydrogen on nickel, particularly Raney nickel (Csuros, et al., 1961). During its preparation, nickel is in contact with large amounts of nascent hydrogen and desorption of the same is markedly exothermic, due apparently to the heat of recombination of hydrogen atoms. Neither of the two mechanisms proposed is capable of explaining the results for the full temperature range, but it can be expected that mechanism (a) is valid a t 100" because eq 2 shows that the rate goes through a maximum and then decreases when PH is increased. At higher temperatures the hydrogen is more strongly adsorbed than the xylose and will not allow its adsorption. As shown by eq 3, the rate will become constant at high enough pressures. This is consistent with the results shown in Figure 6. The sudden increase in rate at 100" and about 800 psig indicates a possible change from mechanism ( a ) to mechanism (b) a t sufficiently high pressures. As indicated in the introduction, similar behavior has been observed during
the hydrogenation of compounds like a-methylstyrene (Babcock, et al., 1957). At hydrogen pressures over 3 atm and temperatures below 60" the hydrogenation mechanism changes from (a) to (b). As indicated before, drastic conditions may lead to cleavage of the carbon chain. This possibility, which was explored as a first explanation to the behavior at 125", was discarded after tlc and glc analyses failed to show the presence in significant amounts of products other than xylitol. Summary During the hydrogenation of xylose to xylitol with Raney nickel under conditions of pressure, temperature, catalyst concentration, and agitation that ensured negligible mass-transfer resistances, it was found that (1) the reaction followed a pseudo-first-order course; with 2% nickel, conversion of xylose to xylitol was complete at 125°C and 500 psig; ( 2 ) at a catalyst loading of 2% and 200 psig, the activation energy was 6.5 kcal/mol; (3) the ratecontrolling step involved the surface reaction between chemisorbed atomic hydrogen and adsorbed or unadsorbed xylose depending on the temperature level. Nomenclature a, b, d, f = constants C = xylose concentration
C, = concentration of adsorbed xylitol k = pseudo-first-order reaction constant, min-l PH = hydrogen pressure r = rateofreaction t = time
Literature Cited Anderson, L. C., MacNaughton, N . W., J . Amer. Chem. SOC., 64, 1456 (1942). Babcock, B.D . , Mejdell, G. T., Hougen, 0. A., AlChE J . , 3,366 (1957). Badin, E. J., Pacsu, E., J. Amer. Chem. SOC., 66, 1963 (1944). Brahme, P. H., Pai, M. U., Narshiman, G., Brit. Chem. Eng., 9, 684 (1964). Boelhouwer, C., Korf, D., Waterman, H. I., J. Appl. Chem., 10, 292 (1960). Csuros, Z.,Petro, J., Holly, S., Acta Chim. (Budapest), 29,351 (1961). Froment, G. F., de Groof, W., Recl. Conf. Colloq. Pharm. lnd., 5, 36 (1966). ,Hennecke, H., Deut. Lebensrn.-Rundsch., 66,329 (1970). Hougen. 0. A., Watson, K. M., "Chemical Process Princlpies," pp 947949, Wiley, New York, N. Y., 1947. Kohno, S., Yamatsu, I., U. S. Patent 3,558,725 (1971). Lang, K., Klin. Wochenschr., 49, 233 (1971). Leikin, E. R . . Ref. Zh., Khim., Abstract No. 24P21 (1963) Patchornik, A., Doctoral Thesis, The Hebrew University of Jerusalem, Jerusalem, 1956. Patchornik, A., personal communication, 1973. Scott, R . W., Moore, W. E., Effland, M. J., Millett, M. A,, Ana/. Biochem.. 21, 68 (1967). Specht, H., German Patent 1,066,568 (1959). Steiner, K., U. S. Patent 3,586,537 (1971). Suminoe, K., Okamura, K., U . S. Patent 3,565,687 (1971). Sweeley, C. C.,Bentley, R., Makita, M., Wells, W. W . , J . Amer. Chem. SOC., 85,2497 (1963). Tomkuljak, D., Chem. Zvesti, 3, 209 (1949). Van Ling, G.. Vlugter, J. C., J . Appl. Chem., 19,43 (1969). Vogel, A., "A Textbook of Practical Organic Chemistry," Longmans, Green and Co., London, 1966. Wasserman, L.. Hanus. H., Naturwissenschaften, 50, 351 (1963). Weiss, A. H., Tambawala, J., J. Chrornalogr. Sci., 10, 120 (1972). Wisniak, J., Stefanovic, S.,J . Arner. OilChern. Soc., 44, 545 (1967). Wisniak. J., Stefanovic, S.. Rubin, E., Hoffman, S., Talmon, Y., J . Amer. OilChern. Soc., 48,379 (1971).
Receiued for recieu: May 14, 1973 Accepted September 23,1973
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