Effects of Proximity-Dependent Metal Migration on Bifunctional Composites Catalyzed Syngas to Olefins
Yi Ding, Feng Jiao, Xiulian Pan, Yi Ji, Mingrun Li, Rui Si, Yang Pan, Guangjin Hou, Xinhe Bao
Abstract
The proximity of the oxide-zeolite bifunctional catalysts plays a crucial role in syngas conversion to light olefins. However, its underlying mechanism is not well understood and the optimal proximity is yet to be identified. Herein, we take ZnCrOx-SAPO-34 and MnOx-SAPO-34 as examples and show that the reaction benefits from the shortened proximity with the granules decreasing to the micrometer size due to reduced mass transport limitation. CO conversion reaches 60.0%, light olefin selectivity 75.5%, and a space time yield of light olefins 0.24 g·gcat–1·h–1 over ZnCrOx-SAPO-34. However, at nanoscale proximity, an interaction may develop between different active sites due to the migration of metal species in addition to intermediate exchange, which could modify their properties significantly. For instance, zinc species migrate to SAPO-34 and form Zn-OH preferably over Brønsted acid sites under reaction conditions, which leads to a deteriorating light olefin selectivity due to enhanced hydrogenation. This can be alleviated over zeotypes containing less acid sites. By contrast, MnOx does not migrate under reaction conditions, and the light olefin selectivity exhibits a feature of “the closer, the better” over MnOx-SAPO-34. These findings are essential for further development of analogous bifunctional catalysts.