Direct conversion of syngas to light olefins (C2–C3) over a tandem catalyst CrZn–SAPO-34: Tailoring activity and stability by varying the Cr/Zn ratio and calcination temperature

A series of CrZn oxide catalysts with different Cr/Zn ratios were synthesized by co-precipitation and calcined at different temperatures. The catalysts were tested in a physical mixture with SAPO-34 in the direct conversion of syngas to olefins. X-ray diffraction, x-ray photoelectron and absorption spectroscopy, and electron microscopy were used to evaluate the impact of the Cr/Zn ratio and calcination temperature on the physicochemical properties of the resultant materials. CrZn–SAPO-34 catalysts were able to selectively convert syngas to C2–C3 hydrocarbons with a high olefin/paraffin ratio and low methane yield. However, the catalytic performance of these systems exhibited a trade-off between activity and stability in the olefin yield over time. By tuning the Cr/Zn ratio, it is possible to improve the olefin production stability, but with some compromise in the overall activity. The nonstoichiometric and inversed ZnCr2O4 spinel phase was considered to be the active phase for CO hydrogenation, while the decay in olefin production over time is correlated with abundance of ZnO on the resultant catalyst. Spectroscopic analysis demonstrated that the inversed spinel evolves toward its normal form under reaction conditions, which is accompanied by the formation of segregated ZnO; this phase may also increase the olefin hydrogenation activity. The challenge for the rational design of an optimal CrZn–SAPO-34 system is the stabilization of inversed ZnCr2O4, while minimizing the presence of the ZnO phase in the catalyst formulation.