(144c) From Surface Science to Catalytic Reactions in Oxygen-Assisted Coupling of Alcohols on Ag-Modified Nanoporous Gold
Achieving high selectivity for high volume chemical synthesis is important for lowering energy consumption through reduction in waste. We report the selective synthesis of esters through catalytic O2-assisted self- and cross-coupling of alcohols using activated, support-free nanoporous gold (npAu). Fundamental studies with well defined single crystal surfaces of gold reveal the details of the complex mechanism of these reactions, which are in each case initiated by adsorbed atomic oxygen as the active site. Both well-controlled studies on npAu ingots in ultrahigh vacuum (UHV) conditions and experiments under ambient pressure catalytic conditions on both the ingots and microspherical hollow shell catalysts reveal guiding principles for controlling selectivity. Under UHV conditions the the reactions occur facilely. Under steady-state catalytic operation high, stable activity was observed for coupling in flowing gaseous reactant mixtures at atmospheric pressure and 425 K with negligible combustion. Optimum selectivity for cross-coupling to form methyl-esters from methanol is obtained in methanol-rich mixtures due to a combination of two factors: (1) the relative coverage of the respective alkoxy reactive intermediates involved, and (2) the relative facility of their C–H elimination. The relative coverage of the alkoxys is governed by van der Waal’s interactions between the alkyl groups and the surface - a suprisingly large effect due to such weak interactions. The concepts linking the fundamental mechanistic studies on the gold single crystals, the theory and the steady-state catalytic process will be discussed..