DYNAMIC DESCRIPTION OF THE OXIDATION OF N-BUTANE ON VARIOUS FACES OF (VO)2P2O7 IN TERMS OF THE CRYSTALLOCHEMICAL MODEL OF ACTIVE-SITES

(VO)2P2O7 is known to be a very good industrial catalyst for the selec tive, direct oxidation of n-butane to maleic anhydride, provided that the catalyst is prepared by ways which lead to the predominant exposur e of the {100} crystal face. In a former paper (J. Catal., 122 (1990) 126) we analyzed the geometry and energetics of the {100} face in term s of the crystallochemical model of active sites (CMAS), along with al l possible butane-surface interactions and elementary steps. We indica ted the structure and performance of the active cluster site for the d iscussed reaction and its energetically most favourable pathway. Analo gous analysis is done in this work for other faces which (according to the electron microscopy studies) are exposed by (VO)2P2O7 Crystals: { 001), {010}, {021} and {012}, including the crystallographically non-e quivalent sections. It is shown that n-butane reacting on these faces encounters various geometric and energetic ''troubles'', much more imp ortant than those on {100}. They consist either in an non-convenient a rrangement of active, undersaturated oxygens around the possible adsor ption sites (hindering a bifurcated activation or bridging of the term inal methyl carbons), or in very high energy barriers for some element ary steps (especially for evolution of water and insertion or surface transfer of oxygen). These hindrances are responsible for the lower ac tivity and selectivity of faces other than {100}. Consequently the cat alytic anisotropy shown by (VO)2P2O7 in oxidation of butane is explain ed by CMAS. Based on the surface energy calculations and Curie-Wulff p lot, it is also shown that in the state close to equilibrium, the area of the {100} face comprises only about 30 % of the total external gra in area. Therefore any method of preparation which contributes to the development of the {100} face enhances strongly the activity and selec tivity of the catalyst.