Preparation and stability study of TMCS modified MFI zeolite for ethanol/water separation

Trimethylchlorosilane (TMCS) was used as the modification source to modify the surface of MFI zeolite membranes. The effect of TMCS concentration and modification time on the performance of MFI zeolite membranes in the separation of ethanol/water mixtures was systematically investigated. Biofuel ethanol is a kind of renewable fuel resource with the characteristics of low carbon and sulfur-free. Pervaporation is considered as a most promising energy-efficient process for ethanol extraction from the fermentation broth containing dilute alcohol. Pure-silica MFI zeolite membrane, also called silicalite-1 membrane, possesses uniform pore size of 0.55 nm and desirable hydrophobicity with the potential to achieve large flux and high separation factor for ethanol over water. However, the significant performance degradation of MFI zeolite membranes in ethanol/water mixture separation is an obstacle to industrial their application. Based on the literature investigation and our previous work, we found that the stability of membrane during pervaporation was affected by the silicon defects in MFI zeolite membranes formed under alkaline synthetic conditions. Herein, we present the Si-OH eliminated MFI membranes by a simple Trimethylchlorosilane (TMCS) modification, which can effectively prevent the chemical reaction between Si-OH groups and components, endowing the long-term pervaporation stability for the ethanol/water mixture. The effects of TMCS content and modification time on the separation performances of membranes were studied, and the obtained membranes were characterized by SEM, XRD,29Si NMR, FT-IR and contact angle measurement. The results show that the surface Si-OH groups can be effectively reduced after the modification, and obtained membranes showed improved hydrophobicity and separation stability. With the increase of silane content and modification time, the obtained MFI zeolite membranes showed decreased initial pervaporation performance but better stability. The zeolite membrane prepared with TMCS content of 0.4%(mass) and the reaction time of 2 h had the best separation performance and also showed a long-term stability for the separation of 5%(mass) ethanol/water mixture at 60℃. During the 90 h pervaporation process, the permeation flux slightly decreased from 1.83 kg•m-2•h-1 to 1.61 kg•m-2•h-1, and the separation factor decreased from 27 to 20. For the unmodified MFI zeolite membrane, however, the permeation flux dramatically decreased from 3.46 kg•m-2•h-1 to 0.6 kg•m-2•h-1, and the separation factor decreased from 30 to 3. This study showed that, the surface organic modification can be an effective method to improve the hydrophobicity and separation stability of MFI zeolite membrane. © 2021, Editorial Board of CIESC Journal. All right reserved.