Water selective zeolite membranes performances

Table 15.2 Performances of water selective zeolite membranes...

MTBE synthesis from /-butanol and methanol in a membrane reactor has been reported by Salomon et al. [2.453]. Hydrophilic zeolite membranes (mordenite or NaA) were employed to selectively remove water from the reaction atmosphere during the gas-phase synthesis of MTBE. This reaction was carried out over a bed of Amberlyst 15 catalyst packed in the inside of a zeolite tubular membrane. Prior to reaction, the zeolite membranes were characterized by measuring their performance in the separation of the equilibrium mixture containing water, methanol, /-butanol, MTBE, and isobutene. The results obtained with zeolite membrane reactors were compared with those of a fixed-bed reactor (FBR) under the same operating conditions. MTBE yields obtained with the PBMR at 334 K reached 67.6 %, under conditions, where the equilibrium value without product removal (FBR) would be 60.9%. 

Besides producing mixed-hydrocarbons (ultra-clean diesel), F-T process can also selectively produce mixed-alcohols (oxygenated fuel). The addition of mixed-alcohols into gasoline can effectively reduce HC and CO emissions. However, before directly used as fuels or blended with conventional fuels, the water content in the as-produced F-T mixed-alcohols must be reduced below 0.5wt.%. This dehydration step is essential but difficult since most of the contained alcohols form azeotropies with water. In our group, we studied the dehydration performance of microwave synthesized NaA zeolite membrane toward F-T produced mixed-alcohols [24, 25]. The membrane also showed excellent pervaporation performance toward dehydration of simulated F-T produced mixed-alcohols. The permeate consisted of only water and little methanol (< 10%) in aU the range of feed composition. This result confirmed that NaA zeolite membrane based pervaporation (or vapor permeation) process could be an effective technology for dehydration of F-T produced mixed-alcohols. 

Research has shown that a solute with a diameter closer to that of zeolite pore dimension showed higher adsorption (close fit mechanism). An important consideration for applying zeolites in drinking water treatment practice is that their size exclusion and close fit adsorption mechanism makes them effective for the removal of specific solutes. [4]. Because the Si Al ratio allows for tuning of the surface properties and the resultant electrostatic double layer such membranes could also be tuned for specific ion-selective applications, but further work is needed to fully understand the connection between zeolite chemistry and membrane performance. Osewe (2014) investigated the dissipation half-life of malathion as affected by the largest window opening for FAU, MOR, and ZSM-5s zeolites [16]. 

Several polymers other than PDMS and PVA have been examined for solid-polymer mixed-matrix pervaporation. An activated earbon-block polyether-amide mixed-matrix membrane was investigated for the separation of volatile organie eompounds (VOCs). An enhancement in both trichloroethane and water flux was noted, as well as increase in trichloroethane-water selectivity compared to the neat polymer. Zeolite-polyimide mixed-matrix membranes were explored for xylraie isomo- separation. Solid-polymer adhesion was poor, resulting in a sieve-in-a-eage morphology therefore, no separation properties that exceeded neat polymer membranes were identified. Methanol-toluene separation was successfully performed with NaX—Viton mixed-matrix membranes, though other solid-polymer combinations were attempted and failed to form an enhanced membrane. 

The incorporation of nanofillers, for instance, zeolites, represents a promising strategy for improving membrane performance in PV. Dobrak et al. [123] prepared PDMS composite membranes and investigated the effect of two types of fillers, namely, commercial zeolite silicalite (CBV 3002) and laboratory-made colloidal silicalite-1, on membrane performance in the removal of ethanol from ethanol/water mixtures through PV. Filler incorporation increased membrane stability by cross-linking. Furthermore, the PDMS membrane filled with conunercial zeolites showed a significant increase of selectivity. Incorporation of CBV 3002 fillers into a PDMS composite membrane was also found to enhance the performance in PV tests of toluene removal from water [124]. 

Another potential application for zeolite/polymer mixed-matrix membranes is the separation of various liquid chemical mixtures via pervaporation. Pervapora-tion is a promising membrane-based technique for the separation of liquid chemical mixtures, especially in azeotropic or close-boihng solutions. Polydime thy 1-siloxane (PDMS), which is a hydrophobic polymer, has been widely used as the continuous polymer matrix for preparing hydrophobic mixed-matrix membranes. To achieve good compatibility and adhesion between the zeolite particles and the PDMS polymer, ZSM-5 was incorporated into the PDMS polymer matrix, the resulting ZS M -5/ P DM S mixed-matrix membranes showed simultaneous enhancement in selectivity and flux for the separation of isopropyl alcohol from water. It was demonstrated that the separation performance of these membranes was affected by the concentration of the isopropyl alcohol in the feed [96]. 

The benefits of the use of micromembranes for the selective removal of one or more products during reaction have been demonstrated for equdibrium-limited reactions [289]. For example, the performance of hydrophilic ZSM-5 and NaA membranes over multichannel microreactors prepared from electro-discharge micromachining of commercial porous stainless steel plates was studied by Yeung et al. in the Knoevenagel condensation [290,291] and andine oxidation to azoxybenzene [292]. For such kind of reactions, the zeolite micromembrane role consists of the selective removal of water, which indeed yields higher conversions, better product purity, and a reduction in catalyst deactivation in comparison to the traditional packed bed reactor. 

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