Dealumination of zeolite beta by acid leaching: a new insight with two-dimensional multi-quantum and cross polarization 27Al MAS NMR

Abstract

The influence of acid leaching on a commercial zeolite beta with concentrated solutions of HCl and HNO₃ has been investigated. The parent and modified zeolites are characterized with ICP, BET, XRD, SEM, FTIR, and ²⁷Al solid-state NMR, with conventional MAS, CP/MAS and two-dimensional multiple-quantum MAS (MQMAS) techniques. The first advantage to submitting zeolite beta to an acid leaching procedure is to remove or passivate all kinds of extra-framework impurities from aluminium clusters to silica–alumina debris. These impurities have been shown to be to some extent acidic. However, the most important effect of acid leaching with concentrated acid solutions is to remove selectively framework aluminium from positions close to defect sites and on the external surface area. Thus, the acid treatments markedly reduce the relative activity of the high external surface area so that the intrinsic properties of the beta microporous structure come into effect. The alkylation of biphenyl with propene transforms afterwards into a shape-selective process.