The growth mode of ZnO on HZSM-5 substrates by atomic layer deposition and its catalytic property in the synthesis of aromatics from methanol

Abstract

A series of ZnO/HZSM-5 catalysts were prepared by atomic layer deposition (ALD) with different cycle numbers and tested for methanol to aromatics (MTA) reactions. The growth rate of Zn content monotonically decreases with increasing number of deposition cycles due to the “half-self-limiting” ALD-type growth mode for ZnO on HZSM-5 zeolite, in which the density of the regenerated reactive –OH sites in the HZSM-5 substrates was less than that of the consumed –OH sites in one cycle. The deposited ZnO existed as small nanocrystallites upon low ALD cycles. However, as the number of ALD cycles exceeds a certain value, the resulting ZnO grown on the HZSM-5 substrate exists in the form of a continuous coating with corrugated surfaces, which could effectively prevent damage to the HZSM-5 framework caused by exposure of the zeolite skeleton to steam during the reaction and regeneration processes. The ZnO/HZSM-5 catalyst treated with 40 ALD cycles was proved to be the optimum catalyst for MTA reactions, producing a nearly twofold increase in BTX aromatics (carbon yield of 60.3%) than the parent HZSM-5 (28.3% carbon yield of BTX aromatics). The catalyst prepared by ALD not only is more effective for the aromatization of methanol but also shows a more stable performance in the MTA process than that prepared by conventional methods (i.e. IWI and IE) with a comparable amount of Zn content. This better performance could be ascribed to the synergetic effect that occurred between the multinuclear oxygenated zinc sites and (ZnOH)⁺ sites formed during the ALD process, which would boost the transformation of inert low alkanes to aromatics.