Convenient preparation of inexpensive sandwich-type poly(vinylidene fluoride)/molecular sieve/ethyl cellulose mixed matrix membranes and their effective pre-exploration for the selective separation of CO2 in large-scale industrial utilization

Abstract

Due to its high methane content, biogas is considered a potential replacement for natural gas as a clean and renewable energy source for future large-scale applications. In addition to CH4 (50–80%), biogas also includes CO2 (20–40%), N2 (0–5%), and other gases. Efficient capture and separation of CO2 not only enhances the calorific value of biogas but also aids in reducing greenhouse gas emissions. Utilizing membrane separation technology is an effective method for separating and capturing CO2 and N2. This technology is renowned for its controllability and efficiency. Here, a variety of ternary mixed matrix membranes consisting of poly(vinylidene fluoride)/molecular sieve/ethyl cellulose were successfully prepared using a simple layer-by-layer coating process. Among them, the porous polyvinylidene difluoride (PVDF) membrane serves as the bottom support layer, the molecular sieve (Titanate molecular sieve (TS-1) and Zeolite Socony Mobil-5 (ZSM-5)) acts as the middle selective layer, and ethyl cellulose (EC) forms the top fixed layer. These membrane materials were used in an attempt to efficiently separate CO2 from a CO2/CH4 gas mixture with a 1:1 ratio and a CO2/N2 gas mixture with a 1:1 ratio. It is encouraging to note that the PVDF/ZSM-5/EC mixed matrix membrane (with 10% ZSM-5), which is the most cost-effective among the ternary mixed matrix membranes, exhibited the best gas separation performance. For a mixture of CO2 and CH4 gases, the CO2 permeability was 891.23 Barrer, and the CO2/CH4 selectivity was 12.92. Its gas permselectivity exceeds the Robeson 2008 line. For the CO2 and N2 gas mixture, the CO2 permeability is 316.39 Barrer, and the CO2/N2 selectivity is 21.85. It exceeds the Robeson 1991 line for gas permselectivity. Although the PVDF/ZSM-5/EC mixed matrix membrane may not be highly effective in separating CO2 from a mixture of CO2 and N2 (in comparison to CO2/CH4 mixture gases), given the gas composition in biogas and natural gas, it is reasonable to conclude that our PVDF/ZSM-5/EC mixed matrix membrane is well-suited for the selective separation of CO2. This experiment is an important exploration for us to understand the transition of advanced membranes to practical applications. It lays a solid foundation for the production of PVDF/ZSM-5/EC ternary mixed matrix membranes at the lowest cost and for large-scale application in the field of biogas and natural gas purification, as well as greenhouse gas treatment from coal-fired power plants (CO2 selective separation).