Optimizing hollow ZSM-5 spheres (hZSM5) morphology and its intrinsic acidity for hydrogenation of CO2 to DME with copper–aluminum

Abstract

Hollow zeolites are normally made by synthesizing parent rigid zeolites form soluble silica-alumina suspension followed by removal of the aluminum-deficient centers, during which the uniformity of hollow zeolites is compromised. Herein, two uniform spherical hollow ZSM-5 (hZSM5) with distinct morphologies, monodispersed (mhZSM5) and networked (nhZSM5), were hydrothermally synthesized using premade Stöber SiO2 spheres along with structure directing agents with and without aluminum. Subsequently, both mhZSM5 and nhZSM5 supports were impregnated with mono- and multimetallic Cu-based catalysts to investigate CO2 hydrogenation with in-depth catalyst characterizations (HRTEM-EDX, Pyridine-FTIR, H2-TPR, XPS, XANES, and DRIFTS). It was evidenced that uniform and decent morphology of mhZSM5 spheres arises from the existence of framework-intrinsic Al3+. Besides, it was revealed that mhZSM5-AlCu containing both framework-intrinsic Al3+ and catalyst-extrinsic Al3+ promoter leads to an optimal DME production (33.5 mgDME·gcat.−1·h−1), which is comparable or even higher to the ones reported in several published works. Not only was mhZSM5-AlCu activity even better, but also it showed a prolonged stability compared to comZSM5-AlCu and nsZSM5-AlCu samples that were derived using commercial ZSM-5 supports with better physiochemical properties. Based on our in-situ DRIFTS, a step-wise mechanistic pathway of DME formation over mhZSM5-AlCu was proposed, assuming that the hollow ZSM-5 shell framework dehydrates methanol that is formed in-situ via CO2 hydrogenation.