ISOMERIZATION OF 1-BUTENE OVER SILICA-SUPPORTED MO(VI), W(VI), AND CR(VI)

Isomerization of 1-butene on Mo(VI)ISiO2, W(VI)/SiO2, and Cr(VI)/SiO2 was investigated by steady-state flow reactor studies and Fourier tran sform infrared spectroscopy. (eta(5)-C5H5)(2)M-2(CO)(6) or M(eta(3)-C3 H5)(4) (M = MO, W) were used as precursors in preparing Mo(VI)/SiO2 an d W(VI)/SiO2 catalysts. Cr(NO3)(3) was used to prepare Cr(VI)/SiO2. Is omerization proceeds by a Bronsted catalyzed pathway involving alkoxid e intermediates and an oxidative dehydrogenation pathway involving all ylic intermediates. Flow reactor experiments revealed that two pathway s for isomerization could be observed with Mo(VI)/SiO2: through the al koxide intermediate at T < 523 K and through the allylic intermediate at T > 573 K. W(VI)/SiO2 catalysts were more active for isomerization than Mo(VI)/SiO2 catalysts, but the demarcation of the two pathways wa s not as clear. Only the oxidative dehydrogenation pathway for isomeri zation was observed for the Cr(VI)/SiO2 catalysts at T > 473 K. In IR studies, alkoxide intermediates were observed following adsorption of 1-butene on Mo(VI)/SiO2 and W(VI)/SiO2 catalysts at 300 K; complete de sorption of the surface alkoxide occurred by 523 K. The surface specie s observed for adsorption of 1-butene on Cr(VI)/SiO2 at 300 K were typ ical of allylic C-H abstraction; these surface species transform into deep oxidation products on heating, resulting in a surface that is poi soned. Pyridine adsorption studies reveal traces of Bronsted acidity o n Mo(VI)/SiO2, W(VI)/SiO2, and Cr(VI)/SiO2. Bronsted acidity would acc ount for the alkoxide mechanism of 1-butene isomerization. The reason that these Bronsted acid sites are most active for 1-butene isomerizat ion on W(VI)/SiO2 and inactive on Cr(VI)/SiO2 is discussed. (C) 1998 A cademic Press.