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

摘要

A series of vanadyl pyrophosphate, (VCO2P2O7, catalysts prepared via vanadyl hydrogen phosphate sesquihydrate precursors (VOHPO41.5H2O) 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 dihy-drate (VOPO4-2H2O) with 1-butanol for different lengths of time, i.e. 8,15 and 24h, and the produced catalysts were denoted as VPOs-R8, VPOs-R15 and VPOs-R24, respectively. X-ray diffraction (XRD) patterns of the three catalysts showed similar diffraction pattern, comprised of a well-crystallized (VO)2P2O7 phase. Brunauer-Emmett-Teller (BET) surface area measurements showed that VPOs-R24 has the highest specific surface area, i.e. 31 m~2g~(-1) followed by 27 m~2 g~(-1) and 19m~2 g~(-1) forVPOs-R15 and VPOs-R8, respectively. Inductively coupled plasma (1CP) analyses indicated that the P/V atomic ratios of these catalysts were in the optimum range in producing (VCO2P2O7 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 H2 profiles of all the catalysts gave three reduction peaks. VPOs-R8 gave the highest total amount of oxygen removed from V~(5+)/V~(4+) phase followed by VPOs-R15 and VPOs-R24. Catalytic tests revealed that the catalyst with lower precursor reflux duration exhibited higher selectivity but lower activity and vice versa.