ИСТИНА

The development of efficient zeolite catalysts requires thorough investigation of their acid properties. Infrared spectroscopy of adsorbed probe molecules is one of most utilized methods for studying acid sites of solid catalysts. Amidst convenient probe molecules pyridine is the most used one, however bulky pyridines are more compatible for characterization of acid sites on the outer surface. In this work, we consider the selection of a probe molecule for studying catalytically active sites of zeolites with MWW structure in the alkylation of benzene with propylene. Zeolite agglomerates are spherical beads formed by layered flat zeolite crystals, which consist of primary nanoplates of MWW structure having two independent pore systems. The first system passes in the plane of the plate and is limited to 10-membered rings (checkered area on the picture), the second has the form of an hourglass, which are perpendicular to the plane of the crystal, and has access to the surface of the crystal through 12-membered rings (grey area). According to published data [1], benzene alkylation with propylene proceeds at the Brønsted acid centers located in “cups” with 12-membered rings (striped area). Recrystallization of MWW zeolite was carried out in a solution of alkali and CTAB, followed by pH adjustment with hydrochloric acid [2]. As a result of recrystallization, a partial destruction of the crystals occurred, followed by the formation of the mesoporous material MCM-41 on the surface of the modified zeolite. Infrared spectra were recorded on a Nicolet Protégé 460 FTIR spectrometer equipped with vacuum line applicable for heating the samples and adsorption of probe molecules. Pyridine, 2,6-ditertbutylpyridine, 2,3,6-trimethylpyridine, 2,6-dipropylpyridine, 2,4,6-trimethylpyridine were used as probe molecules. Porous characteristics of MWW zeolites were studied by low-temperature nitrogen adsorption. Alkylation of benzene with propylene was carried out in an automated catalytic unit with a flow-type reactor made of stainless steel. Analysis of the reaction products was carried out by gas chromatography. Post-processing in an alkaline medium leads to the destruction of the spherical agglomerates, which should cause an increase in surface area. Nevertheless, according to low-temperature nitrogen adsorption data, recrystallization does not affect the specific surface area (calculated from BET), since mesopores are formed due to partial destruction of micropores. At the same time, the availability of acid sites for bulky probe molecules did not change, as evidenced by the preservation of the peak area of the vibrations of the Al-O(H)-Si group after the adsorption of 2,4,6-trimethylpyridine. Post-synthetic modification led to an increase in the catalytic activity of MWW in the reaction of benzene alkylation with propylene. This effect can be caused by an increase in the diffusion rate in crystalline zeolite agglomerates. The results show, the accessibility of acid sites on the outer surface changes depending on the size of substituted pyridines, which indicates that the most voluminous pyridine homologues do not fit in the “cups” of MWW zeolite. An important part of the work was the selection of a probe molecule for which acid sites in the "cups" are accessible, but acid sites that are not active in the alkylation of benzene with propylene are inaccessible. Based on the data obtained, a technique for determining the concentration of catalytically active sites in the alkylation of benzene with propylene by infrared spectroscopy of adsorbed probe molecules has been developed. The authors greatfully acknowledge Russian Science Foundation (project № 19-73-10160) for the financial support.