Recognizing the Important Role of Surface Barriers in MOR Zeolite Catalyzed DME Carbonylation Reaction

Transport resistance in microporous zeolites has an important impact on their applications in catalysis. Relative to the well-known intracrystalline transport resistance, the significance of surface barriers on the catalytic performance of zeolites has not been well recognized. Herein, we report that the DME carbonylation reaction can be governed by surface barriers on zeolites, affecting both the catalyst activity and stability. The two MOR zeolites used for the investigation were synthesized by different organic structure-directing agents (OSDAs). They possess similar Si/Al ratios, diffusion lengths, Al distributions, and acidities but quite different diffusion properties. The MOR-C sample with severe transport limitations exhibits inferior apparent activity (∼50% lower) and poor stability in comparison compared with the MOR-T sample. Chemical etching of the outer layer of as-made MOR-C crystals has been proven to be an effective strategy to reduce surface barriers, enhance mass transport properties, and improve the activity and stability of the MOR catalyst. The carbonylation activity of etched MOR-C is indeed comparable to that of MOR-T. This work highlights the importance of controlling the synthetic strategy and surface barriers on zeolite crystals for the design/development of highly efficient catalysts.