Structure–property tuning in hydrothermally stable

Authors: J E ten Elshof A P Dral

Publish Date: 2015/10/27

Volume: 79, Issue: 2, Pages: 279-294

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Abstract

Supported microporous organosilica membranes made from bridged silsesquioxane precursors by an acidcatalyzed sol–gel process have demonstrated a remarkable hydrothermal stability in pervaporation and gas separation processes making them the first generation of ceramic molecular sieving membranes with sufficient performance under industrially relevant conditions The commercial availability of various αωbistrialkoxysilylalkane and 14bistrialkoxysilylbenzene precursors facilitates the tailoring of membrane properties like pore size and surface chemistry via the choice of precursors and process variables Here we describe the engineering of sols for making supported microporous thin films discuss the thermal and hydrothermal stability of microporous organosilicas and give a short overview of the developments and applications of these membranes in liquid and gas separation processes since their first report in 2008Microporous ceramic membranes have been receiving considerable attention since the late 1980s because of their ability to separate gases and liquids on the molecular scale 1 2 3 Very high gas separation selectivities have been reported for acidcatalyzed sol–gelderived silica membranes which are typically based on the use of tetraethyl orthosilicate TEOS as precursor 4 According to the official IUPAC definition “microporous” refers to pore diameters 2 nm where the physical interaction between the transported molecule and the pore wall is significant and interactions between transported molecules are much less relevant than when the pore size would be larger The molecular transport mechanism in the microporous size range is sometimes referred to as “singlefile diffusion” because the pores are too narrow to allow parallel passage of two moleculesDue to the high thermal stability of ceramic materials microporous ceramic membranes offer an interesting route to hightemperature gas and liquid separation processes such as methane reforming steam reforming and dehydration of organic solvents and bioethanol Unfortunately amorphous microporous silica is rather unstable at high temperatures in the presence of water ie under hydrothermal conditions 5 Even operating temperatures as low as 100 °C lead to performance loss and membrane failure within days or weeks of operation The main reason for the poor stability of silica is the hydrolytic instability of the ≡Si–O–Si≡ bonds that can easily break upon reaction with water ≡Si–O–Si≡ + H2O → 2≡Si–OH leading to dissolution of membrane material pore widening and ultimately loss of membrane selectivityWater content in permeate for a hybrid organosilica membrane operating continuously at 150 °C in pervaporation of 5 wt water–95 wt nbutanol Selectivity is compared with methylated and inorganic silica membranes Reproduced from Ref 7 with permission from The Royal Society of ChemistryThe present article presents a concise overview of the main developments and state of the art in hybrid organosilica membranes for molecular separations since their first report in 2008 7 with some emphasis on the work done in our group at the University of Twente in collaboration with the University of Amsterdam and the Energy Center of the Netherlands ECN The discussion is limited to “hybrid” organosilica membranes ie 3D bonded networks containing ≡Si–O–Si≡ and ≡Si–R–Si≡ groups with homogeneous dispersion of both types of bonds on the atomic scale For a review of the sol–gel processing of amorphous microporous silica and organically modified silica membranes the reader is referred to other sources 9 10 11Bridged silsesquioxanes that have already been reported for hybrid organosilica membranes contain aliphatic or aromatic bridging groups 12 13 14 The precursors referred to in this article are αωbistriethoxysilylR compounds where R is an alkylene –C n H2n – n = 1 2 3 6 8 or 10 ethenylene –C2H2– ethynylene –C≡C– or an arylene like pphenylene –pC6H4– or dipphenylene –pC6H4–pC6H4– Cocondensation with other organically modified silicon alkoxides is also possible typically by using triethoxy organosilanes such as structure 2 in Fig 2 where R’ can be a methyl ethyl propyl or other terminating organic group In particular the methylated precursor methyltriethoxysilane MTES R’ = –CH3 that was also used to make the first methylated silica membrane 15 is being referred to several times in this paper Functionalization of the hybrid matrix by cocondensation of 12bistriethoxysilylethane BTESE with an aminefunctional silane precursor has also been reported 16 Very recently the use of more complex precursors has been reported such as structure 3 in Fig 2 17 and a triazinefunctional precursor 18 In addition to membrane modification by organic bridging and pending end groups selective doping with transition metal cations to alter the chemical environment within the hybrid organosilica matrix has also been investigated and is discussed belowIn the next section the most important design rules for obtaining hybrid organosilica sols suitable for the formation of supported thin films without large defects are explained Especially the relationship between the engineering of the sol and the final pore size and pore structure and the evolution of nano and microstructure in drying sol–gel thin films are discussed In Sect 3 the thermal and hydrothermal stability of hybrid organosilicas is discussed based on their molecular design In Sect 4 a short overview is given of the most important studies in which hybrid organosilica membranes have been employed for liquid and gas separation processes

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