Preparation of a Water-Resistant Siliceous MCM-41 Sample, through Improvement of Crystallinity, and Its Prominent Adsorption Features

Abstract

We have prepared MCM-41 samples having good crystallinity and possessing a small amount of surface hydroxyl groups (0.48 OH nm-2) through extensive aging of a mixed solution of silica and surfactant in the temperature range from 308 to 413 K. The MCM-41 sample thus prepared exhibits excellent resistance to water, and its structure survives even after treating it in boiling water for more than 1 week. The surface and acidic properties of this sample were examined by X-ray diffraction, 29Si magic-angle spinning NMR, adsorption isotherms of dinitrogen and water molecules, and Fourier transform infrared measurements to determine the factors governing an adsorption feature and affecting the acidic property of the MCM-41 samples. The synthesized sample gives a sharp band centered at 3745 cm-1 tailing toward the lower wavenumber side. This band was deconvoluted into three components (3749, 3736, and 3715 cm-1), which were assigned to the stretching vibrations of the respective OH groups: a free OH (silanol), a geminal OH, and a terminal OH group of the hydrogen-bonded species. The data obtained by utilizing CO as a probe molecule provide useful information on the acidic nature of the surface OH groups. A linear relationship between ΔνOH (shift in wavenumber of OH stretching vibration) and νCO (stretching vibration of adsorbed CO) has been found to hold in the present system. It is concluded that the newly formed OH groups, which give strong IR bands at 3736 and 3715 cm-1, after water vapor and boiling water treatments, are responsible for the sites that are generally recognized as the strongly acidic sites existing in MCM-41, compared with the acidity of free silanol groups. The freshly prepared sample shows characteristic behavior in the differential heat of adsorption of water, qdiff, due to lateral interaction of the adsorbed water molecules. This is a different behavior from water adsorbed on the hydrated and boiled samples, indicating the homogeneous nature of the surface of the as-prepared sample for adsorbing water molecules. All properties of such materials should be discussed by taking account of the types of surface OH groups; the surface of MCM-41 having a smaller number of acid sites exhibits a prominent feature for water adsorption. From our results, it was concluded that the surface crystallinity of the sample plays a pivotal role in protecting the sample against attack by water and also in the adsorption behavior of adsorbates, such as water and CO molecules.