Effect of varying reflux durations on the physico-chemical and catalytic performance of vanadium phosphate catalysts synthesized via vanadyl hydrogen phosphate sesquihydrate

Abstract

A series of vanadyl pyrophosphate, (VO) 2P 2O 7, catalysts prepared via vanadyl hydrogen phosphate sesquihydrate precursors (VOHPO 4·1.5H 2O) was calcined in a reaction flow of 0.75% n-butane in air mixture at 733 K for 18 h. The precursors have been synthesized by refluxing vanadyl phosphate dihydrate (VOPO 4·2H 2O) with 1-butanol for different lengths of time, i.e. 8, 15 and 24 h, and the produced catalysts were denoted as VPO s-R8, VPO s-R15 and VPO s-R24, respectively. X-ray diffraction (XRD) patterns of the three catalysts showed similar diffraction pattern, comprised of a well-crystallized (VO) 2P 2O 7 phase. Brunauer–Emmett–Teller (BET) surface area measurements showed that VPO s-R24 has the highest specific surface area, i.e. 31 m 2 g −1 followed by 27 m 2 g −1 and 19 m 2 g −1 for VPO s-R15 and VPO s-R8, respectively. Inductively coupled plasma (ICP) analyses indicated that the P/V atomic ratios of these catalysts were in the optimum range in producing (VO) 2P 2O 7 phase. A small increment in the average oxidation number of the vanadium was observed as the precursor reflux duration increased. Scanning electron microscope showed the secondary structures of the catalysts with plate-like crystals in different sizes, which were agglomerated into rosette-shape clusters. The total amount of oxygen desorbed from the catalysts increased as the precursor reflux duration increased. Temperature-programmed reduction (TPR) in H 2 profiles of all the catalysts gave three reduction peaks. VPO s-R8 gave the highest total amount of oxygen removed from V 5+/V 4+ phase followed by VPO s-R15 and VPO s-R24. Catalytic tests revealed that the catalyst with lower precursor reflux duration exhibited higher selectivity but lower activity and vice versa.