Mesoporous ZSM-5 zeolite catalysts prepared by desilication with organic hydroxides and comparison with NaOH leaching
Hierarchical zeolites combining micro- and mesoporosity were prepared by desilication of ZSM-5 (Si/Al = 42) in aqueous solutions of tetraalkylammonium hydroxides (TPAOH, TBAOH). Similarities between the treatment mediated by organic hydroxides and the conventional NaOH leaching comprised (i) the requirement to work with a starting zeolite in the optimal Si/Al window of 25–50 and (ii) the minor influence of the counter-cation (H+, NH4+, Na+) on the degree of mesoporosity developed. However, both desilication media presented distinctive kinetic and mechanistic characteristics. The use of organic hydroxides directly produced the protonic form of the mesoporous zeolite upon calcination, simplifying the final ion exchange with NH4NO3 characteristic of the NaOH treatment. Silicon dissolution in TPAOH (or TBAOH) was much slower than in NaOH, making the demetallation process highly controllable. The treatment in the organic base was less selective for silicon extraction, i.e. a higher amount of aluminum was leached to the solution compared to the NaOH treatment. The differences in porosity development and chemical composition of the final solid can be assigned to the effect of the bulky tetraalkylammonium cation. The optimal hierarchical ZSM-5 zeolite (treated in 1 M TPAOH at 338 K and 8 h) had a mesopore surface area of 160 m2 g−1 and the intrinsic zeolite properties were largely preserved. This sample displayed improved catalytic performance in the liquid-phase benzene alkylation with ethylene compared to the parent and NaOH-treated zeolites.
Graphical abstractHierarchical porous zeolites are prepared by desilication of ZSM-5 in aqueous solutions of tetraalkylammonium hydroxides (TPAOH and TBAOH). The treatment in organic hydroxides is compared with conventional NaOH leaching. The TPAOH-treated zeolite displays improved performance in the liquid-phase benzene alkylation with ethylene with respect to the parent and NaOH-treated zeolites.Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 364, Issues 1–2, 31 July 2009, Pages 191–198