Nickel doped hydrotalcites as catalyst precursors for the partial oxidation of light paraffins

Abstract

A series of Ni, Mg and Al containing hydrotalcite-type precursors was synthesised (Ni X Mg 72− X Al 28, X=10–72) by coprecipitation at low supersaturation at pH 8±0.2 at 60–70°C. Their calcination at 900°C in air resulted in the formation of solids consisting of spinel and divalent metal oxide phases. The catalyst activation was studied by temperature programmed reduction (TPR). Ni 2+ in the divalent metal oxide phase could be reduced in all samples, with the reduction temperatures increasing with decreasing Ni content, but only in the sample without Mg the Ni 2+ in the spinel phase were reducible. The Ni dispersion determined from the TPR results was higher for the catalysts with a lower Ni content and decreased for higher Ni contents. Catalytic tests were carried out at 700°C and at 800°C for 240 min using a C 3H 8/O 2/He-mixture at a gas hourly space velocity (GHSV) of 900 dm 3 g −1 h −1. High values for the conversion of C 3H 8 (≥73%) as well as for the selectivities to CO (≥88%) and H 2 (≥69%) were observed for all the catalysts at both reaction temperatures. At lower temperatures and with higher Ni content, these values were lower, whereas the conversion of O 2 was found to be almost complete in all experiments. The carbon content of the spent catalysts was determined by temperature programmed combustion (TPC) using thermogravimetry (TG). The tendency towards carbon formation was found to be higher at lower temperatures and for higher Ni contents. On the basis of activity, selectivity and resistance against coke formation, the Ni containing catalyst with the lowest Ni content was found to be the best catalyst.