Effect of External Surface Diffusion Barriers on Pt/Beta Catalyzed Isomerization of n-Pentane.

Zeolite crystals contain transport barriers on their external surface that hinder molecular uptake and processes that depend on it, such as catalysis and separations. However, the potentially dominant role of surface barriers in such processes is only emerging. In this work, we have developed a generalizable strategy to quantify the effect of surface barriers on zeolite catalysis. Isomerization of n -pentane, catalyzed by Pt/Beta, is taken as a model reaction system. Firstly, the surface modification by chemical liquid deposition of SiO 2 was carried out to control the surface barriers on zeolite Beta crystals. The deposition of SiO 2 leads to a very slight change in the physical properties of Beta crystals, but an obvious reduction in Bronsted acid sites. Diffusion measurements by the zero-length column (ZLC) method show that the apparent diffusivity of n -pentane can be more than doubled after SiO 2 deposition, indicating that the surface barriers have been weakened. Catalytic performance was tested in a fixed-bed reactor, showing that the apparent catalytic activity improved by 51-131% after SiO 2 deposition. These results provide direct proof that reducing surface barriers can be an effective route to improve zeolite catalyst performance deteriorated by transport limitations.