Hierarchical H-ZSM-5 zeolite and sulfonic SBA-15: The properties of acidic H and behavior in acetylation and alkylation reactions

Abstract

A commercial H-ZSM-5 zeolite was modified by alkaline treatment using NaOH solutions in different concentrations, followed by consecutive ion exchanges with NH4NO3 solution in order to recover the acidic form of the material. According to the intensity of the treatment, mesoporosity with pore size ranged between 9 and 13 nm was generated in the zeolite. Also, the strength and number of acid sites were practically maintained, observing only a noticeable decrease for the more intense treatment. After testing in the glycerol acetylation reaction, some improvements in the activity were observed compared to the original zeolite, but no appreciable increment in the selectivity to diacetin (DA) and triacetin (TA) was reached. However, when a mesoporous SBA-15 functionalized with propylsulfonic acid groups was used as catalyst, the selectivities to DA and TA were noticeably enhanced. Moreover, the zeolites were tested in the isobutane/butene alkylation reaction in gas phase, and also in this case the modified materials did not favor the production of trimethylpentanes (TMPs) compared to the original zeolite. Characterization results allowed to conclude that the mesopores formed in the zeolite after the modification facilitated the accessibility of reactants and products to some acid sites, but these have not enough concentration, being most of them located in the micropores of the zeolite. H-D exchange in sulfonic SBA-15 prior to butene pulses demonstrated that its acidic H has not capacity for butenes adsorption. This explains the lack of activity of this catalyst in the isobutane/butene alkylation reaction.