Understanding Zeolite Desilication by NMR Spectroscopy

Abstract

Today, zeolites play a considerable role in many industrial fields, especially in heterogeneous catalysis. Well-defined microporous structure combined with acidity provides exceptional size and shape selectivity, making zeolites indispensable in petrochemistry. However, the micropores can cause diffusion limitations and, in turn, a drop in reaction rate and selectivity. Hence, the development of modification methodologies on zeolite textural properties is one of the attention-grabbing research topics nowadays. For example, to overcome transport limitations in zeolites, the particle size can be reduced, or a system of larger auxiliary pores can be introduced [1]. One of the most promising methods for introducing secondary pores on a large scale is desilication since it is low-cost, versatile, and easy [2]. Despite its simplicity, the desilication mechanism is still a matter of discussion. In detail, it is not well-understood: 1. The influence of different species on mesopore formation kinetics 2. How aluminum is assembled back into the zeolite 3. Which types of aluminum species form throughout the treatment. The present study tries to answer these questions by relating ex-situ and in-situ NMR. The proposed ex-situ 29Si MAS NMR approach allows monitoring the development of mesoporosity and silicon extraction by analyzing Q3 and Q4 changes. The combination of ex-situ with in-situ 29Si MAS NMR study showed that the limiting step of Si extraction is the transformation of Q3 into Q2. 27Al MAS NMR combined with MQMAS showed the formation of new aluminum species after desilication. It was shown that some of the Al framework T-sites might dissolve during alkaline treatment. In-situ 27Al NMR indicates redistribution of dissolved aluminum upon desilication.