Highly selective 1-heptene isomerization over vanadium grafted mesoporous molecular sieve catalysts

Abstract

Mesoporous molecular sieve supported vanadium catalysts (V-MCM-41, V-SBA-15 and V-TUD-1) with different pore structures and pore sizes were prepared via a post-synthesis grafting method using atomic layer deposition. Nitrogen physisorption and X-ray diffraction demonstrated that ordered mesoporous structures for all samples were attained after grafting procedures. UV–vis and UV–Raman spectroscopies suggested that highly dispersed vanadium units were formed on the pore wall surface. Ammonia temperature-programmed desorption and IR spectroscopy of adsorbed pyridine suggested the existence of mild acidity, containing both Lewis and Brønsted sites; the ratios of Lewis/Brønsted acid depended on the structure of the catalyst support. The isomerization of 1-heptene was employed as a reaction probe to characterize the acid properties of these catalysts with moderate acid strengths. No methyl migration isomerization was observed since a stronger acidity was required. Conversion as high as 92% at 600 K and isomer selectivity over 90% (mainly double bond shift products) over the V-SBA-15 catalyst were achieved. The catalytic performances, including activities, selectivities, 2-heptene to 3-heptene ratios, and cis-heptene/ trans-heptene ratios, over different mesoporous supported catalysts were compared and discussed in detail. The catalyst deactivation was also explored.