Heptamolybdate-intercalated CoMgAl hydrotalcites as precursors for HDS-selective hydrotreating catalysts

Abstract

Two series of unsupported CoMgMoAl hydrotreating catalysts were prepared starting from CoMgAl-terephthalate layered double hydroxides (LDHs) with nominal aluminum molar fractions (Al/(Al+Ni+Mg) ratios) 0.3 (Al30 series) and 0.5 (Al50 series), and nominal cobalt atom fractions (Co/(Co+Mg)) in the 0.2–1.0 range in each series. The materials were prepared by ion exchange with ammonium heptamolybdate, followed by calcination at 723K. Mixed oxides containing ca. 14–32wt.% molybdenum and Ni/Mo atomic ratios in the 0.5–1.6 range were obtained. Despite the observed loss in long-range ordering and, in some cases, magnesium leaching during the ion-exchange, there was evidence that molybdenum-intercalated LHDs were indeed produced.The catalysts were sulfided in situ and subsequently tested in simultaneous thiophene HDS and cyclohexene hydrogenation (OHYD reaction) in a tubular flow micro-reactor at 573 and 623K and 20bar. In each series, both the HDS and the OHYD activities of the catalysts increased with increasing Mg content, possibly due to improved Co/Mo ratio. Although there was a trend of decreasing selectivity for the HDS reaction with increasing HDS activity, the HDS selectivity of the most active catalysts was higher than that of a commercial alumina-supported CoMo catalyst.