Synthesis gas conversion to isobutane-rich hydrocarbons over a hybrid catalyst containing Beta zeolite – role of doped palladium and influence of the SiO2/Al2O3 ratio

Abstract

Hybrid catalysts composed of a copper-based methanol synthesis catalyst (M-cat) and palladium-doped Beta zeolite (Pd-Beta) showed excellent catalytic performance including activity, selectivity and stability for the conversion of synthesis gas to C₃ and C₄ paraffins, especially to isobutane. When Pd-free Beta (H-Beta) was used for the hybrid catalyst instead of Pd-Beta, the product contained olefins, a large amount of hexamethylbenzene and coke. Its catalytic activity decreased quickly because of the deposited coke. A marked induction period of zeolite was detected during reaction which happened within 10 h for the zeolite with high alumina content (SiO₂/Al₂O₃ = 38), whereas that for the zeolite with low alumina content (SiO₂/Al₂O₃ = 150) was around 100 h. The hybrid catalyst containing Pd-Beta (SiO₂/Al₂O₃ = 38) showed excellent stability for the conversion of syngas to hydrocarbons with deposition of a small amount of coke. Pd-Beta produced butane with high selectivity, whose iso/normal ratio was as high as 40/1 while keeping the selectivity of methane and ethane at 1–2% or less. Similar results involving the high stability of Pd-Beta in hydrogen gas and the quick deactivation of H-Beta were observed for dimethyl ether (DME) reaction. The most plausible reaction pathway was postulated to be: (1) methanol synthesis on a methanol synthesis catalyst (M-cat) (reversible); (2) methanol conversion to DME on zeolite (reversible); (3) hydro-conversion of DME to hydrocarbons (irreversible) by the acidic functions of Beta zeolite. Furthermore, details of the reaction pathway from DME to isobutane involving the incorporation of hydrogen species, which was supplied from the gas phase through doped Pd with a spillover effect, as well as that of metal-free Beta were postulated.