Template induction of supramolecular structure : synthesis and characterisation of the mesoporous molecular sieve, MCM-41

Abstract

The pure silicate mesoporous material, MCM-41 having hexagonally packed cylindrical channels with a centre to centre distance of ca. 45 A may be synthesised from a preparation containing only sodium silicate solution, a surfactant template molecule, water, and some acid. The preparation was optimised initially both for heated and unheated syntheses. The effects of aging in the gel, heating time and stirring were investigated. MCM-41 materials which were stable to calcination were prepared in an ambient temperature synthesis, with stability proportional to aging time in the gel. Heated preparations proved to have highest long range order after 3 days at lOOOC in unstirred systems. The effect of pH during synthesis was then investigated. Preparations titrated against 1 M acid to maintain a constant pH during the whole of the synthesis developed much higher long-range order, as determined by the number and intensity of the observed X-ray diffraction peaks. A small counterion effect, dependant upon the type of acid used was noted. The most highly ordered materials were prepared from preparations titrated with sulphuric acid to maintain a pH of 10 during the synthesis. Heated preparations were more ordered than those carried out at ambient temperatures, although the addition of acid also promoted order in unheated syntheses. From these highly ordered materials X-ray diffraction patterns containing up to seven peaks were obtained using synchrotron radiation. These peaks could all be indexed to a hexagonal lattice. The intensity envelope for these peaks was modelled by the expected envelope for an array of cylinders. The data could not be fitted by a model containing only one cylinder, but required two concentric cylinders of different scattering length density. This indicates the presence of three regions in the MCM-41 framework. Firstly, a denser, continuous wall structure about 6 A wide filling the regions between pores. Secondly, each pore is lined with a less dense silica region about 12 A thick. The empty holes down the centre of each channel in this material have a radius of about 7 A. The average bulk MCM-41 density calculated from the X-ray results and gas adsorption measurements was found to be low, around 0.83(5) g cm-3, with the denser part of the wall being 0.99 g cm-3 and the less dense lining of the pores 0.87 g cm-3. This low density model with porous walls is supported by results from neutron diffraction, inelastic and quasielastic neutron scattering measurements on hydrogen adsorbed in the pores of MCM-41, and by small angle neutron scattering using contrast variation on MCM-41 materials at all stages of preparation. Other possible interpretations of the data are presented and discussed. The behaviour of methane adsorbed in the channels of MCM-41 was also observed by quasielastic neutron scattering. Considerable alteration in the phase behaviour from that of bulk methane was observed. The melting point was depressed from 91 K in bulk methane to between 45 and 60 K for the confined methane in this system, and a liquid phase was still present in the pores at 180 K, around 70 K above the normal boiling point of the bulk material. Show more