Characterization and Preparation of Chained Si Species in Zeolite Supercages

Abstract

Chained Si species were synthesized in Y-type zeolite supercages by the reaction with phenylsilane (PhSiH3) at 423 K. The preparation process was studied with the infrared spectra, and the prepared Si species were characterized with X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, and ultraviolet absorption spectra. The initial sticking reaction of SiHx species to zeolite framework was studied with a temperature-programmed desorption experiment on the deuterium-exchanged zeolite using a quadrupole mass spectrometer. Benzene molecules composed selectively with d1 species (C6H5D) were desorbed with hydrogen molecules at 360−380 K; concurrently, hydroxyl groups in supercages disappeared. These results imply that the initial grafting reaction of Si species to a HY−zeolite framework proceeds through the thermal reaction of PhSiH3 with hydroxyl groups in supercages at 298 K to yield [O]−SiH3 and benzene. The formed [O]−SiHx species were characterized with infrared spectra as a function of reaction temperature. The wavenumbers shift in Si−H species was explained by the thermal conversion of [O]−SiH3 (2180 cm-1) to [O]2−SiH2 (2208 cm-1) and [O]3−SiH (2270 cm-1). The reaction of PhSiH3 molecules with hydroxyl groups in supercages at 423 K suggests the propagation of Si species finally to yield [O]2−SixHy. The successive propagation reaction with PhSiH3 yielded Si species with Si 2p XPS signals at 101.4 and 102.3 eV, which could be assigned to polysilane families. The quantitative XPS analysis implied that polysilane families with about 30 Si atoms were produced in a zeolite super cage. The zeolite pressed between metal barrels showed intense PL spectra peaked at 340 (3.6 eV) and 460 nm (2.7 eV). The peak intensities diminished considerably with treatment with oxygen gas at 573 K for 48 h, which caused the selective enhancement of X-ray diffraction intensity at around 2θ = 6°, characteristic of a zeolite (111) face. These results induce that surface contaminants such as organic compounds can be removed by the oxygen treatment as well as the zeolite crystallinity is improved by the decrease of oxygen vacancies. Chained Si species in zeolite supercages showed intense PL spectra at around 4 eV. The Si species can be extracted in hexane at 298 K, and the extracted species also showed redshifted intense PL spectra peaked at 4.07 eV. The broad UV spectra due to the polysilane backbone structure was detected at 220−280 nm. It is concluded that polysilane families are formed in zeolite supercages and absorb excitation photon energy and relax to show PL characteristic to the Si backbone structure.