Designing the activity/selectivity of surface acidic, basic and redox active sites in the supported K 2O–V 2O 5/Al 2O 3 catalytic system

Abstract

The supported K 2O–V 2O 5/Al 2O 3 catalytic system was designed to create surfaces that were 100% acidic, 100% basic, 100% redox, mixed redox-acidic and mixed redox-basic. The resulting nature of the surface sites was controlled by the impregnation of the specific additives (K-basic or V-redox/acidic), their order of impregnation and their surface coverage. The exact locations of the surface methoxy intermediates (Al OCH 3, K OCH 3 or V OCH 3) on the mixed oxide catalyst surfaces during methanol oxidation were determined with in situ Raman spectroscopy. The surface chemistry of the various surface sites and their surface reaction intermediates were chemically probed by CH 3OH oxidation steady-state and temperature programmed surface reaction (TPSR) spectroscopy studies. The specific reactivity order and the product selectivity of the various surface sites were found to be: V OCH 3 (HCHO) ∼ Al OCH 3 (CH 3OCH 3) ≫ K OCH 3 (primarily CO 2 and minor amounts of HCHO). Formation of dimethoxy methane, (CH 3O) 2CH 2, required the presence of dual surface redox-acidic sites surface redox sites to yield H 2CO and surface acidic sites to insert the surface methoxy into H 2CO to form dimethoxy methane, (CH 3O) 2CH 2. The addition of basic surface potassium oxide to Al 2O 3 possessing surface acid sites completely suppressed reactions from the surface acidic sites and formed a surface with only basic characteristics. The addition of redox surface vanadia to the supported K 2O/Al 2O 3 catalyst was able to completely suppress reactions from surface basic sites and formed a surface with only redox characteristics. These studies demonstrate that it is possible to determine the specific surface site requirements for each reaction pathway for methanol oxidation to products, and that this informative approach should also be applicable to other reactant molecules.