Microporous structure and enhanced hydrophobicity in methylated SiO2 for molecular separation

Abstract

Methylated microporous silica with high thermal stability and tuneable hydrophobicity was obtained by acid-catalysed sol-gel hydrolysis and condensation of mixtures of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES). The gels exhibited a trend towards smaller ultramicropores with increasing methyl content, while in addition some supermicropores were formed with sizes of around 2 nm. For low MTES concentration, dilution prior to gelation and ageing resulted in materials with clearly smaller ultramicropores, whereas only a minor effect of dilution on structure was found at high MTES concentration. The small ultramicropore size in 'diluted' materials can be associated with a higher extent of condensation of mainly TEOS monomers. Stable structures formed from MTES in an early stage of synthesis may explain the particular micropore structure of MTES-rich gels. With increasing methyl content and with dilution of the sol, the affinity of the surface to water was strongly decreased. The applicability of microporous silica in wet atmospheres may thus be improved by methylation, and their pore structure modified by adaptation of the recipe, which would be highly relevant for industrial gas and liquid separation by inorganic membranes.

Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening

“…The separation factor of the BTESE/MTES membrane probably suffers from the relatively large contribution of pores [ 1 nm, despite the fact that its average pore size is the lowest of the three membrane types. Similar observations of bimodal pore size distributions caused by MTES were made in adsorption studies on methylated silica gels [24]. For the purely bridged precursor membranes even the comparatively small difference between BTESM and BTESE membranes leads to a dramatic difference in selectivity for MeOH/H 2 O mixtures.…”

Organic groups influencing microporosity in organosilicas

“…However, the significantly lower He densities of MTMS-and PTMS-as compared to BTESE-and BTMSH-derived networks indicate that the first ones contained larger pore fractions that are inaccessible to He (pores inaccessible to He suppress the measured He density). This has also been reported for methylated silica elsewhere [3] Information on the pore entrance sizes can be obtained by comparing the uptake of methanol, 1-propanol and cyclohexane in Figure 3. These molecules are at least partially hydrophobic and their uptake is therefore not expected to depend on pore chemistry; hydrophobic molecules can enter hydrophilic and hydrophobic channels with the same ease due to the lack of polar interaction.…”

“…However, terminal methyl (-CH3) and phenyl (-C6H5) groups have also been reported to increase the effective pore opening size in membrane applications as compared to inorganic silica [22] and 3 methyl groups can increase the total pore volume [3]. Organic groups in terminal position increase the network hydrophobicity by sticking out at the pore surface [3,4,22,23].…”

“…Organic groups in terminal position increase the network hydrophobicity by sticking out at the pore surface [3,4,22,23].…”

“…replacement of Si-O-Si linkages with Si-R or Si-R-Si linkages, influences various functional properties such as mechanical flexibility and fracture resistance [1,2], hydrophobicity [3][4][5] and hydrothermal stability [5][6][7][8]. For applications that make use of the microporosity of organosilica materials, such as separation membranes, varying the nature and location of the organic groups allows tuning of micropore sizes and surface chemistries.…”