Mesoporous Silica Membranes Silylated by Fluorinated and Non-Fluorinated Alkylsilanes for the Separation of Methyl Tert-Butyl Ether from Water

Abstract

It is of great significance to separate hazardous methyl tert-butyl ether (MTBE) from water in terms of environmental protection and human health. In the present work, α-Al 2 O 3 -suppotred silica membranes were prepared by the sol-gel and dip-coating technique. Two fluorinated alkylsilanes (1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) and trifluoropropyltriethoxysilane (TFPTES)) and two non-fluorinated alkylsilanes (octyltriethoxysilane (OTES) and propyltriethoxysilane (PTES)) were adopted to silylate the silica membrane by the post-grafting method which is used for the separation of MTBE from water by pervaporation. The results show that silylation enhances the hydrophobicity of silica membranes. The silylated silica membranes are selective towards MTBE, and the MTBE/water separation factor varies with grafting agents in the order: PFOTES > TFPTES > OTES > PTES. Membranes silylated with fluorinated carbon chains seem to be more selective towards MTBE than those with non-fluorinated carbon chains. The total flux is proportional to the pore volume of silica membranes, which depends on grafting agents in the order: PTES > PFOTES > OTES > TFPTES. Considering both total flux and selectivity, the PFOTES-SiO 2 membrane is most effective in separation, with a MTBE/water separation factor of 24.6 and a total flux of 0.35 kg m −2 h −1 under a MTBE concentration of 3.0% and a feed temperature of 30 • C.There are various reports related to the application of ceramic membranes, i.e., TiO 2 , Al 2 O 3 , ZrO 2 , zeolite and SiO 2 membranes in VOCs removal from water [15][16][17][18]23,24]. As a kind of common material, silica membrane has been used in gas separation [25,26], membrane reactor [27], and desalination [28] due to its intrinsic merits, for instance, ease of preparation, tunable pore size, relatively low cost and so on. Pristine silica membrane is generally believed to be hydrothermally unstable owing to the presence of hydrophilic surface hydroxyl groups, which tend to physically adsorb water. The attack of physically adsorbed water leads to further hydrolysis and condensation of silica species, thus resulting in structure deterioration [29]. Fortunately, the hydrothermal stability of silica membrane has been significantly enhanced by replacing hydroxyl groups with hydrophobic groups [26], so that hydrophobic silica membranes can be applied in an aqueous environment such as desalination [28]. To the best of our knowledge, only a few mesoporous silica membranes have been reported previously on the separation of VOCs from aqueous solution by pervaporation [30][31][32]. Pervaporation is considered as a separation process for liquid mixture not only by dense membranes, but also by porous membranes, especially microporous and mesoporous membranes [23,24,31,32]. Compared to conventional separation methods, pervaporation has numerous advantages; for instance, it can be utilized for the separation of azeotropes and close-boiling point mixtures [33]. Pervaporation is advantageous for the separation of VOCs from water...

Progress on Silica Pervaporation Membranes in Solvent Dehydration and Solvent Recovery Processes

“…The importance of pervaporation in industries based on solvents is quite evident. However, the study by Wang et al [ 100 ] brings forth the applications centered on purification and removal of toxic chemicals from the environment. Methyl tert-butyl ether is a toxic compound requiring efficient removal to mitigate the risks posed to the ecosystem as well as counter any exposure to the general populace.…”

Effect of the Formation of Ultrathin Selective Layers on the Structure and Performance of Thin-Film Composite Chitosan/PAN Membranes for Pervaporation Dehydration

“…Membrane processes are an alternative to traditional separation methods. Pervaporation is one of the most efficient membrane methods for the separation of low molecular weight liquid mixtures, which is widely used for the dehydration of alcohols and other organic substances [ 4 , 5 , 6 , 7 , 8 , 9 ] and characterized by high efficiency with low energy consumption, a compact equipment, environmental friendliness and the ability of the automation [ 10 ]. Pervaporation is also a promising and perspective technology for the separation of azeotropic mixtures without application of additional reagents.…”

A Methyl-Modified Silica Layer Supported on Porous Ceramic Membranes for the Enhanced Separation of Methyl Tert-Butyl Ether from Aqueous Solution

“…This work obtains a higher total flux and a considerably larger MTBE/water separation factor compared to our previous work. This observation may be attributed to the possibility that the MSL possesses a higher loading of methyl groups, either on the outside surface or on the inner pore surface, compared to the membrane modified by a simple post-grafting approach in the previous work [ 35 ]. In the post-grafting method, a condensation reaction occurs between the surface hydroxyl groups on the membranes and the alkoxy groups in the modifier.…”

“…In our previous work, silica membranes were modified by different NFAS (octyltriethoxysilane (OTES) or propyltriethoxysilane (PTES)) via a post-grafting strategy. The OTES- and PTES-modified silica membranes show an MTBE/water separation factor of 15.3 and 12.0, respectively, considerably lower than that of the membranes modified by perfluorooctyltriethoxysilane (PFOTES) (24.6) [ 35 ]. This observation might be related to the fact that the NFAS molecules are less compatible with MTBE than the FAS molecules due to the relatively larger surface free energy of NFAS [ 36 ].…”