Abstract:

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.

Key words: zeolite, membranes, pervaporation, TMCS, surface modified, stability

摘要：

采用三甲基氯硅烷（TMCS）作为修饰源对MFI分子筛膜进行表面改性，系统考察了TMCS浓度以及修饰时间对于MFI分子筛膜在分离乙醇/水混合物时的性能影响。SEM、XRD、²⁹Si NMR、FT-IR、接触角实验及分离实验结果表明，TMCS可以与硅羟基反应，嫁接分子筛膜表面，在消除膜表面硅缺陷的同时提高膜的疏水性及膜分离性能的稳定性。随着TMCS浓度以及反应时间的增加，修饰后MFI分子筛膜的通量及分离因子略有下降，但稳定性增强。在TMCS的浓度为0.4%（质量），修饰时间为2 h时，所得到的膜具有最佳渗透汽化分离性能，并可在60℃下分离5%（质量）乙醇/水混合物时保持良好的稳定性。在连续90 h渗透汽化分离过程中，其渗透通量稳定在1.61 kg·m^-2·h^-1 左右，分离因子保持在20以上。

关键词: 沸石, 膜, 渗透蒸发, TMCS, 表面改性, 稳定性