ZnO modified ZSM-5 and Y zeolites fabricated by atomic layer deposition for propane conversion

Abstract

ZnO modified ZSM-5 and Y zeolites are synthesized by performing atomic layer deposition (ALD) of ZnO to HZSM-5 and HY using diethyl zinc and water as the precursors. The surface area and pore volume of ZSM-5 and Y zeolites are progressively reduced with the increasing number of ZnO ALD cycles. XRD and SEM characterization methods show that highly dispersed ZnO species are deposited on the internal and external surfaces of both zeolites. The ZnO species deposited on ZSM-5 are in an amorphous form while nano-crystallites of ZnO are present on Y zeolites after performing ≥2 cycles of ZnO ALD. XPS and TPR characterization methods reveal that isolated Zn(OH)⁺ species are predominantly formed on both zeolites after the first cycle of ZnO ALD and the ZnO clusters gradually grow larger with the increasing number of ALD cycles. The type and strength of acid sites on the parent and the ALD ZnO modified zeolites are studied by FTIR spectra of adsorbed pyridine. Incorporation of ZnO into Y zeolite by ALD completely eliminates the Brønsted acid sites and increases the number of strong Lewis acid sites. Similar effects are obtained on ALD ZnO modified ZSM-5 except that the Brønsted acid sites are only partially removed. Catalytic properties of the ALD ZnO modified zeolites are evaluated in propane conversion. Introduction of ZnO species significantly improves the activities of both zeolites. Propylene is the major reaction product on ALD ZnO modified Y zeolite while high selectivities to aromatics are achieved on ALD ZnO modified ZSM-5. These results suggest that ZnO species merely promote the dehydrogenation reaction while the subsequent oligomerization and cyclization reactions require Brønsted acid sites. For both zeolites the catalyst fabricated by only 1 or 2 cycles of ZnO ALD performs better than those fabricated by multiple cycles of ALD, indicating that isolated Zn(OH)⁺ species are more effective for the conversion of propane to propylene and aromatics.