Continuous processes for the highly selective conversion of aldohexose-yielding carbohydrate to ethylene glycol

1. A highly selectivity, continuous process for converting aldohexose in an aldohexose-yielding carbohydrate feed to ethylene glycol by retro-aldol conversion of the aldohexose to glycolaldehyde and hydrogenation of glycolaldehyde to ethylene glycol, comprising:

a. continuously or intermittently passing said carbohydrate-containing feed into a reaction zone having an aqueous, hydrogenation medium containing homogeneous retro-aldol catalyst, hydrogen and hydrogenation catalyst;

b. providing at least one pH control agent to the hydrogenation zone to maintain the hydrogenation medium at a pH between about 3 and 8;

c. maintaining the aqueous, hydrogenation medium in the reaction zone at hydrogenation conditions to provide a product solution comprising ethylene glycol, said hydrogenation conditions being at retro-aldol conditions of a temperature between about 230° C. and 300° C. and providing a ratio of retro-aldol catalyst to hydrogenation catalyst, and a hydrogen partial pressure that, in combination are sufficient to provide a conversion efficiency of said aldohexose to ethylene glycol of at least 60 percent, 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-containing feed is rapidly heated to above 230° C.; and

d. continuously or intermittently withdrawing said product solution from the reaction zone, which withdrawn product contains retro-aldol catalyst.

2. The process of claim 1 wherein the aldohexose-yielding carbohydrate being passed to the aqueous medium is in an aqueous solution.

3. The process of claim 2 wherein the aqueous solution contains retro-aldol catalyst.

4. 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.

5. The process of claim 1 wherein the hydrogenation medium is at a pH between about 3.5 and 8.

6. The process of claim 1 wherein the pH control agent comprises buffer.

7. The process of claim 1 wherein the pH control agent comprises base.

8. The process of claim 1 wherein at least a portion of the retro-aldol catalyst contained in the withdrawn product solution is recycled to the reaction zone.

9. The process of claim 1 wherein the pH control agent is added to the recycle.

10. The process of claim 9 wherein the pH control agent comprises buffer.

11. The process of claim 9 wherein the pH control agent comprises base.

12. The process of claim 1 wherein the hydrogenation medium is at a pH between about 3.5 and 8.

13. The process of claim 1 wherein the aldohexose-yielding carbohydrate being passed to the aqueous medium is in an aqueous solution containing retro-aldol catalyst and having an essential absence of hydrogenation catalyst.

14. The process of claim 1 wherein the wherein the aldohexose-yielding carbohydrate being passed to the aqueous medium is in an aqueous solution and the aqueous solution contains ethylene glycol.

15. The process of claim 14 wherein the aqueous solution contains retro-aldol catalyst and has an essential absence of hydrogenation catalyst.

16. The process of claim 1 wherein the aqueous hydrogenation medium contains sodium tungstate.