Highly-controlled synthesis of organic layers on mesoporous silica: their structure and application to toxic ion adsorptions

Abstract

Since the discovery of ordered mesoporous silica, control of the structure of organic layers at the mesoscopic level has become a topic of growing interest for the modification of silica surfaces. In addition, the microstructure of functional groups on the surface also produces new problems specific to mesoporous silica. Our interest in this field consists in revealing potential problems in the synthesis of modified mesoporous silica for achieving a uniform structure, developing new synthetic routes to highly-functionalized solids and elucidating the mechanism where they work as an adsorbent. All these objectives will be described both on the mesoscopic and microscopic scales. The comparison of in-advance silane synthesis with post on-surface modification is related mainly to the uniformity in the molecular structure of the functional groups. On the other hand, the differences emerging from the comparison between grafting and direct co-condensation are often attributed to the uniformity in the dispersion of the organic groups. The density of the functional groups, which is often regarded as an important specification of the material, is discussed in detail, especially in terms of the mesoscopic uniformity, the method of dispersion (explored by changing the numbers of the functional groups per organic chain or per unit surface area and by changing the stage of synthesis where the modification is carried out) and the distribution on the solid. A refined direct co-condensation method, functionalization associated with templating, has several advantages in the exposure of all functional groups as well as in selective modification of the internal pore surface. The performance of ion adsorption is discussed in relation to the structural differences caused by the choice of synthetic method, small differences in the adsorption sites and the mesopore framework structures.