Continuous processes for the highly selective conversion of sugars to propylene glycol or mixtures of propylene glycol and ethylene glycol
Continuous processes for making propylene glycol from ketose-yielding carbohydrates are disclosed which enhance the selectivity to propylene glycol.
1. A continuous process for converting carbohydrate-containing feed, which feed contains at least one of aldopentose and ketose-yielding carbohydrate, said ketose comprising at least one of hexaketose and pentaketose, to propylene glycol or a mixture of propylene glycol and ethylene glycol comprising:
a. continuously or intermittently passing said carbohydrate feed into a reaction zone having an aqueous-hydrogenation medium containing retro-aldol catalyst, hydrogen and hydrogenation catalyst;
b. maintaining the aqueous-hydrogenation medium in the reaction zone at hydrogenation conditions to provide a product solution comprising propylene glycol and alkitol comprising at least one of pentitol and hexitol, and optionally, ethylene glycol, said hydrogenation conditions comprising a temperature in the range of between about 225° C. to 300° C., a ratio of retro-aldol catalyst to hydrogenation catalyst, and hydrogen partial pressure that, in combination, are sufficient to convert at least about 95 percent of the carbohydrate; and
c. continuously or intermittently withdrawing product solution from said reaction zone,
wherein said carbohydrate feed is at least partially hydrated and is under a pressure sufficient to maintain partial hydration; wherein said carbohydrate feed is below a temperature of about 170° C.; and wherein said carbohydrate feed is heated to above 225° C. immediately prior to or in the reaction zone and the rate of heating of the carbohydrate feed from below 170° C. to above 225° C. is sufficient to provide a product solution having a mass ratio of total propylene glycol and ethylene glycol to said alkitol greater than about 10:1.
2. The process of claim 1 wherein the feed comprises ketose-yielding and aldose-yielding carbohydrate and a mixture of propylene glycol and ethylene glycol is contained in the product solution.
3. The process of claim 1 wherein the feed comprises sucrose.
4. The process of claim 1 wherein the product solution contains a mass ratio of 1,2-butanediol to total propylene glycol and ethylene glycol of less than about 1:10.
5. The process of claim 1 wherein the aqueous solution is maintained at a temperature of greater than about 170° C. and less than 230° C. for less than about 15 seconds prior to being passed into the aqueous-hydrogenation medium.
6. The process of claim 1 wherein the heating of the carbohydrate feed from below 170° C. to above 230° C. is at least in part by direct heat exchange by admixing the carbohydrate feed with a warmer fluid.
7. The process of claim 6 wherein the warmer fluid comprises the aqueous-hydrogenation medium.
8. The process of claim 6 wherein the admixing of the carbohydrate feed and warmer fluid involves high shear mixing.
9. The process of claim 6 wherein the admixing of the carbohydrate feed and warmer fluid involves rapid diffusional mixing.
10. The process of claim 1 wherein the heating of the carbohydrate feed from below 170° C. to above 230° C. is at least in part by indirect heat exchange.
11. The process of claim 1 wherein the heating of the carbohydrate feed from below 170° C. to above 230° C. occurs whereupon the carbohydrate is contacted with an aqueous, retro-aldol solution containing retro-aldol catalyst in the substantial absence of hydrogenation catalyst.
12. The process of claim 1 wherein the carbohydrate-containing feed contains between about 120 and 800 grams of carbohydrate per liter of aqueous-hydrogenation medium.
13. The process of claim 1 wherein the carbohydrate-containing feed contains aldose and the rate of heating of the carbohydrate feed from below 170° C. to above 225° C. is sufficiently slow to provide a product solution having provide a mass ratio of propylene glycol to ethylene glycol of greater that about 1:2.
14. The process of claim 13 wherein the heating of the carbohydrate feed from below 170° C. to above 225° C. is conducted in the presence of an isomerization catalyst or enzyme.
15. The process of claim 13 wherein the heating of the carbohydrate feed from below 170° C. to above 225° C. is conducted in the presence of retro-aldol catalyst.
16. The process of claim 1 wherein the carbohydrate-containing feed contains at least about 50 mass percent ketose-yielding carbohydrate based upon total carbohydrate in the carbohydrate feed and the rate of heating of the carbohydrate feed from 170° C. to above 225° C. is sufficiently rapid to provide a product solution having a mass ratio of propylene glycol to ethylene glycol of greater that about 3:1.