Siliceous MCM-41 samples were modified by silylation using trimethylchlorosilane (TMCS). The surface coverage of functional groups was studied systematically in this work. The role of surface silanol groups during modification was evaluated using techniques of FTIR and 29Si CP/MAS NMR. Adsorption of water and benzene on samples of various hydrophobicities was measured and compared. It was found that the maximum degree of surface attachments of trimethylsilyl (TMS) groups was about 85%, corresponding to the density of TMS groups of 1.9 per nm2. The degree of silylation is found to linearly increase with increasing pre-outgassing temperature prior to silylation. A few types of silanol groups exist on MCM-41 surfaces, among which both free and geminal ones are responsible for active silylation. Results of water adsorption show that aluminosilicate MCM-41 materials are more or less hydrophilic, giving a type IV isotherm, similar to that of nitrogen adsorption, whereas siliceous MCM-41 are hydrophobic, exhibiting a type V adsorption isotherm. The fully silylated Si−MCM-41 samples are more hydrophobic, giving a type III adsorption isotherm. Benzene adsorption on all MCM-41 samples shows type IV isotherms regardless of the surface chemistry. Capillary condensation occurs at a higher relative pressure for the silylated MCM-41 than that for the unsilylated sample, though the pore diameter was found reduced markedly by silylation. This is thought attributed to the diffusion constriction posed by the attached TMS groups. The results show that the surface chemistry plays an important role in water adsorption, whereas benzene adsorption is predominantly determined by the pore geometry of MCM-41.

No abstract

The recently discovered mesoporous molecular sieve MCM-41 was tested as an adsorbent for VOC removal. Its adsorption/desorption properties were evaluated and compared with other hydrophobic zeolites (silicalite-1 and zeolite Y) and a commercial activated carbon, BPL. The adsorption isotherms of some typical VOCs (benzene, carbon tetrachloride, and n-hexane) on MCM-41 are of type IV according to the IUPAC classification, drastically different from the other microporous adsorbents, indicating that VOCs, in the gas phase, have to be at high partial pressures in order to make the most of the new mesoporous material as an adsorbent for VOC removal. However, a proper modification of the pore openings of MCM-41 can change the isotherm types from type IV to type I without remarkable loss of the accessible pore volumes and, therefore, significantly enhance the adsorption performance at low partial pressures. Adsorption isotherms of water on these adsorbents are all of type V, demonstrating that they possess a similar hydrophobicity. Desorption of VOCs from MCM-41 could be achieved at lower temperatures (50−60 °C), while this had to be conducted at higher temperatures (100−120 °C) for microporous adsorbents, zeolites, and activated carbons.

In this paper, we report a novel route to the synthesis of high-quality, large-pore periodic mesoporous organosilicas (PMOs) using triblock copolymer P123 as a template. The novelty lies in that highly ordered PMOs can only be synthesized at a limited range of low acid concentrations, which differs from traditional approach to the synthesis of similar materials including large-pore periodic mesoporous silicas (PMSs). The role that acid plays in the synthesis of high-quality PMOs and PMSs was critically examined, compared, and interpreted.

The amine moiety is an important functionality for many applications such as enzyme immobilization on porous solid supports. In this study, mesoporous SBA-15 was functionalized by co-condensation of tetraethoxysilane (TEOS) with organosilane (aminopropyl)triethoxysilane (APTES) in a wide range of molar ratios of APTES:TEOS in the presence of triblock copolymer P123 under acidic synthetic conditions. The functionalized materials were characterized by physical adsorption, CHN elemental analysis, and various spectroscopic techniques. The data of FTIR, elemental analysis, XPS, and solid-state NMR demonstrated the incorporation of amine functional groups on the surface and inside the pore walls of the APTES-functionalized SBA-15 samples. The results of SAXS, N2 adsorption, and TEM showed the effect of APTES present in the initial synthesis mixtures on the formation of SBA-15 mesostructure such as structural ordering, pore size, and surface area. Reasons behind the observed strong adverse effect of APTES on SBA-15 mesostructure were investigated.

MCM-41 samples of various pore dimensions are synthesized. Plotting of nitrogen adsorption data at 77 K versus the statistical film thickness (comparison plot) reveals three distinct stages, with a characteristic of two points of inflection. The steep intermediate stage caused by capillary condensation occurred in the highly uniform mesopores. From the slopes of the sections before and after the condensation, the surface area of the mesopores is calculated. The linear portion of the last section is extrapolated to the adsorption axis of the comparison plot, and this intercept is used to obtain the volume of the mesopores. From the surface area and pore volume, average mesopore diameter is calculated, and the value thus obtained is in good agreement with the pore dimension obtained from powder X-ray diffraction measurements. The principle of the calculation as well as problems associated are discussed in detail.

The pore structure stability of MCM-41 materials upon hydration/dehydration was studied by XRD, 29Si MAS NMR, and gravimetric adsorption techniques. Results demonstrated that collapses of the pore structure of MCM-41 occurred upon rehydration at room temperature due to the hydrolysis of the bare Si−O−Si(Al) bonds in the presence of water vapor. Full structure collapses of MCM-41 were found to occur when a MCM-41 sample was left in air for three months. It is also suggested that care must be taken when XRD is used to evaluate the structure property of MCM-41 materials to avoid the possible adverse effects of water vapor.

No abstract