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

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.