Transformations of alkyl aromatics over delaminated MCM-22 zeolites and their composites with mesoporous MCM-41 silicate

Abstract

Small (S)- and large (L)-crystalline zeolite MCM-22 precursors with the same Si/Al F ratio of 17, were synthesized and either converted to zeolites MCM-22(S/L) or delaminated. From the delaminated zeolite MCM-22 precursors delaminated zeolites MCM-22 (dl-MCM-22(S/L)) were prepared. Delaminated zeolite precursors were admixed to the synthesis mixture of mesoporous silica MCM-41 to obtain the respective dl-MCM-22/MCM-41(S/L) composite materials. The preparations were characterized by XRD, N 2 adsorption measurements, scanning electron microscopy. FT-IR spectroscopic studies of pyridine (Py) and 2,4-dimethylquinoline (2,4-DMQ) adsorption were used for determination of total (Brønsted and Lewis acidity) and external Brønsted acidity, respectively. It was found that the crystallite size of the MCM-22 precursors strongly influenced the structural, acidic and catalytic properties of the delaminated and composite materials. The effectiveness of the delamination procedure for the zeolite MCM-22(S) precursor having smaller and thinner particles is higher. At same synthesis gel compositions, the amount of mesoporous fraction in the composite materials was different. In dl-MCM-22/MCM-41(S) composite the formation of the mesoporous component was suppressed. Moreover, delamination resulted, for both delaminated and composite materials, in a similar decrease in the Brønsted acidity. The reduction in toluene disproportionation activity confirmed the above structural and acidity changes. 2,4-DMQ adsorption results proved that both the dl-MCM-22 samples and the composites possess higher amount of external Brønsted acid sites than zeolite MCM-22. The enhanced external acidity and accessibility of the active sites of these preparations appeared favorable when more basic and bulkier reactant, such as 1,2,4-trimethylbenzene was transformed.