Activity and selectivity differences of external Brønsted acid sites of single-unit-cell thick and conventional MFI and MWW zeolites

A comparison of activity and selectivity of external Brønsted acid sites in catalytic conversion of benzyl alcohol in mesitylene over unit-cell thick zeolite materials (MCM36 or pillared MWW, pillared MFI, and self-pillared pentasil (SPP)) showed that the external surface of MWW and MFI zeolites influences drastically the activity and selectivity of the parallel alkylation and etherification reactions. Pillared MWW, containing independent (not-interconnected) micropores and mesopores, catalyzed both of the parallel reactions only in the mesopores as evidenced by complete loss of the activity upon 2,6-di-tert-butylpyridine (DTBP) titration. Pillared MFI and SPP, consisting of highly interconnected micropores and mesopores, catalyzed both of the parallel reactions in the mesopores. Pillared MFI and SPP catalyzed only the etherification reaction in the micropores as illustrated by the complete suppression of alkylation and retention of residual etherification activity after DTBP titration. Moreover, it was found that the external surface of MWW zeolites favors the alkylation reaction, while the etherification reaction takes place with similar reaction rates on MFI and MWW external surfaces. The evidence shown here for the assessment of external acid sites in catalyzing parallel reactions extends the scope of observed catalytic performances in these materials beyond those reflecting transport effects and accessibility of acid sites and highlights the importance of external surface structure.