Oxidative gas phase carbonylation of methanol to dimethyl carbonate over chloride-free Cu-impregnated zeolite Y catalysts at elevated pressure

Incipient wetness impregnation of zeolite Y with copper(II) nitrate solution and inert activation at 650 degrees C led to active catalysts for the oxidative carbonylation of methanol to dimethyl carbonate in the gas phase. Activities were measured under elevated pressure (0.4-1.6 MPa) with feed compositions of CO/MeOH/O-2 = 40/20/6-1.5 vol.% (balanced by N-2) over zeolite Y loaded with 10-17 wt.% copper. It could be shown that inert activation at 650 degrees C enhanced the activity, and that Cu loading of 14-17 wt.% gave the best performance. By combined XRD, TEM, TPR and DRIFT characterization it was found that the inert activation initiated dispersion of crystalline CuO, auto-reduction of Cu2+ to Cu+ and redistribution of copper ions with enrichment inside the supercages of the zeolite. The O-2 content of the feed was found to control the selectivity to dimethyl carbonate. Dimethyl carbonate selectivities of 70-75% were achieved within the temperature range of 140-170 degrees C at an O-2 content of 1.5 vol.%. This allowed space-time yields of dimethyl carbonate up to 632 g l(cat)(-1) h(-1) at methanol conversions of 5-12%. Formation of the main side product, dimethoxy methane, was surprisingly affected by CO, which is not in line with suggested reaction pathways. A mechanism is proposed including formation of surface carbonate structures as common intermediate. (c) 2007 Published by Elsevier B.V.