Continuous processes for the selective conversion of aldohexose-yielding carbohydrate to ethylene glycol using low concentrations of retro-aldol catalyst

1. A continuous, catalytic process for producing ethylene glycol from an aldose-yielding carbohydrate-containing feed, comprising:

(a) continuously or intermittently supplying the feed to a reaction zone containing a liquid medium having therein heterogeneous, nickel-containing hydrogenation catalyst, wherein the feed is supplied at a rate of at least about 50 grams per hour of carbohydrate per liter of liquid medium, and wherein said liquid medium is at catalytic conversion conditions including the presence of dissolved hydrogen, a temperature of at least about 235° C., a pH greater than 3 and a residence time sufficient to react at least 99 mass percent of the aldose-yielding carbohydrate, wherein:

(i) the heterogeneous hydrogenation catalyst has a maximum particle dimension of less than about 100 microns, and

(ii) the hydrogenation catalyst is dispersed in the liquid medium in an amount of less than about 100 grams per liter thereby providing a spatial relationship among catalytically active hydrogenation sites in the liquid medium;

(b) continuously or intermittently supplying to the reaction zone homogeneous, tungsten-containing retro-aldol catalyst the concentration of solubilized tungsten compounds, calculated as tungsten atoms, in the liquid medium in the reactor is from about 200 to 1500 milligrams per liter, wherein the relative amounts of hydrogenation catalyst and retro-aldol catalyst are sufficient to provide, under the catalytic conversion conditions, a cumulative conversion efficiency of the aldose-containing carbohydrate to ethylene glycol of at least 75 percent for a duration of 100 hours; and

(c) continuously or intermittently withdrawing from the reaction zone a raw product stream containing ethylene glycol.

2. The process of claim 1 wherein the aldose-yielding carbohydrate comprises glucose.

3. The process of claim 1 wherein the liquid medium comprises water.

4. The process of claim 3 wherein the liquid medium comprises water and a cosolvent having a greater solubility of hydrogen than that of water.

5. The process of claim 1 wherein the hydrogenation catalyst is a supported nickel-containing catalyst on an inert support having less than 10 mass percent nickel (calculated as elemental nickel).

6. The process of claim 5 wherein the support has a surface area of less than 10 square meters per gram.

7. The process of claim 6 wherein the support is alpha-alumina.

8. The process of claim 1 wherein the hydrogenation catalyst is pretreated to attenuate the hydrogenation activity.

9. The process of claim 8 wherein the pretreatment comprises depositing tungsten-containing compound on the hydrogenation catalyst.

10. The process of claim 1 wherein the solubilized tungsten compounds in the liquid medium are in a concentration of from about 300 to 1200 milligrams per liter calculated as elemental tungsten.

11. The process of claim 1 wherein continuously or intermittently the concentration of the solubilized tungsten compound in the liquid medium is cycled between a higher and a lower concentration.

12. The process of claim 11 wherein during the cycle to a lower concentration of solubilized tungsten, the pH is increased by at least 0.5 pH units.

13. The process of claim 1 wherein the hydrogenation catalyst is a supported catalyst and is present in the liquid medium in an amount to provide from 0.1 to 3 grams of nickel (calculated as elemental nickel) per liter.

14. The process of claim 1 wherein the catalytic conversion conditions comprise a pressure of from 2500 to 20,000 kPa absolute.

15. The process of claim 1 wherein the process is conducted in a loop reactor.

16. The process of claim 1 wherein the process is conducted in a stirred bed reactor.

17. The process of claim 1 wherein a portion of the hydrogenation catalysts is withdrawn from the reactor continuously or intermittently and replaced with rejuvenated or fresh hydrogenation catalyst.

18. The process of claim 1 wherein the aldose-yielding carbohydrate-containing feed is supplied to the reactor in an amount of from 100 to 1000 grams of carbohydrate per liter of liquid medium per hour.

19. The process of claim 18 wherein the aldose-yielding carbohydrate-containing feed is supplied to the reactor at multiple points to attenuate the risk of hydrogen starvation.

20. The process of claim 1 wherein at least a portion of the homogeneous, tungsten-containing retro-aldol catalyst is added to the aldose-yielding carbohydrate-containing feed being passed to the reactor.

21. The process of claim 20 wherein the homogeneous, tungsten-containing retro-aldol catalyst has catalytic activity for isomerizing aldose.

22. The process of claim 21 wherein the homogeneous, tungsten-containing retro-aldol catalyst is partially neutralized.

23. The process of claim 1 wherein the catalytic conversion conditions comprise a pH of from 3.8 to 8.

24. The process of claim 1 wherein less than about 35 percent of the nickel on the hydrogenation catalyst is in the zero-valence state.

25. The process of claim 1 wherein the support has a maximum dimension of less than about 50 microns.

26. The process of claim 1 wherein the carbohydrate-containing feed is introduced into the reaction zone such that it is spatially dispersed.

27. The process of claim 26 wherein the mass ratio of carbohydrate to liquid in the feed is from about 0.1:1 to 0.4:1.

28. The process of claim 26 wherein feed is introduced at two or more locations in the reaction zone.

29. The process of claim 26 wherein the feed comprises carbohydrate and a portion of the liquid medium from the reaction zone that has an absence of hydrogenation catalyst.

30. The process of claim 29 wherein the portion of the liquid medium from the reaction zone for the feed is used as a motive fluid for introducing microbubbles of hydrogen into the reaction zone.