Influence of Alkaline Earth Cation Exchanged X-Zeolites Towards Ortho-Selectivity in Alkylation of Aromatics: Hard-Soft-Acid-Base Concept

The present work describes a novel approach through which the acid hardness/softness of NaX zeolite is varied by ion-exchanging with different alkaline divalent metal ions, resulting in different selectivity for ortho-xylene in toluene methylation. It is well known that alkali cation exchanged X-zeolites are already established as solid base catalysts for side chain alkylation of toluene to produce ethyl benzene/styrene. In this work, alkaline earth cation exchanged X-zeolites are interestingly found to be acidic catalysts for ring alkylation of toluene showing different selectivity pattern for xylenes compared to other zeolite catalysts. The alkaline earth metal ion-exchanged X zeolites (MgX, CaX, SrX and BaX) were synthesized by ion exchange method and characterized by XRD, TPD-NH3, SEM and py-FTIR. The catalytic activity and variation in ortho-xylene selectivity was correlated with chemical composition, acidity, basic strength and acid hardness/softness of zeolite catalyst. The toluene conversion was correlated with Bronsted acidity, whereas selectivity for ortho-xylene was explained on the basis of Hard-Soft-Acid-Base (HSAB) concept. Among the different alkaline earth metal ion-exchanged catalysts, SrX gave good catalytic activity with high ortho-xylene selectivity. Further, influence of Si/Al ratio, different structured Sr exchanged zeolites and amount of Sr content on toluene methylation was studied. The influence of acid hardness of the alkaline earth metal ion-exchanged X zeolite on selective synthesis of ortho product also holds true for ethyl benzene ethylation reaction. The current experimental results further confirmed the theory of HSAB concept to explain the observed selectivity of products in heterogeneous catalysis.