Polynuclear metal complexes as model mixed oxide catalysts: Selective chemisorption of ammonia and nitric oxide

Abstract

Model mixed oxide catalysts were prepared by supporting polynuclear metal (M 3+ and Cu 2+) complexes of the form M[(μ-OH)Cu(μ-OCH 2CH 2NR 2)] 3+ 6 (M = Al, Cr, and Fe) on Cab-O-Sil (28 wt% loading). The supported complexes exhibited both Brønsted (bridging OH groups) and Lewis (Cu 2+ ions) acidities. While the coordinatively saturated central M 3+ ion was not an available Lewis site, changing M 3+ influenced catalyst acidity through an apparent ligand effect. Reported are acidity characterizations by ammonia adsorption, first with Cu 2+ ions open to estimate overall acidity, and second with Cu 2+ ions blocked to evaluate the Brønsted acidity. Selective sorption of nitric oxide was an effective Lewis site poison at temperatures to 80 °C. Ammonia sorption to unpoisoned supported complexes revealed increasing overall acidity for the series Al to Cr to Fe (with NH 3 adsorption heats of 4.6, 8.1, and 8.2 kcal/mol, respectively). Ammonia sorption to NO-poisoned supported complexes indicated increasing Brønsted acidity with NH 3 adsorption heats of 2.6, 8.5, and 8.2 kcal/mol, respectively.