Adsorptive Separation of Methane from Carbon Dioxide by Zeolite@ZIF Composite

Methane, as a greenhouse gas, is detrimental to the environment. With the increasing number of municipal solid waste (MSW) landfills, human-related methane emission levels have kept rising in the last five decades. Meanwhile, methane is the primary component of natural gas, which can be utilized as an alternative fuel source. One of the major challenges in the MSW controlling process is to purify methane from landfill gas (LFG), a mixture of CH4 (similar to 50%), CO2 (similar to 50%), and other gas molecules. In this study, a composite material containing a zeolite and a zeolitic imidazolate framework (zeolite@ZIF-95) was synthesized via a one-pot solvothermal approach. Commercial zeolite 4A as nuclei promoted the heterogeneous crystal growth during the synthesis process. The resulting composite material was characterized by XRD and IR to confirm its crystal structure and chemical composition. A layered microstructure of the synthesized zeolite@ZIF-95 composite was observed by SEM. Moreover, EDX studies confirmed that zeolite 4A was evenly dispersed inside and adsorbed outside the ZIF-95, forming a heterojunction composite. Gas sorption studies of CO2 and CH4 were carried out on zeolite 4A, ZIF-95, and zeolite@ZIF-95 composite at 273 and 298 K. The gas adsorption selectivities toward CO2 and CH4 at different temperatures were determined on the basis of single-component gas adsorption isotherms with the aid of the ideal adsorbed solution theory. Not only the selectivity of a binary mixture of CO2/CH4 (50/50) on zeolite@ZIF-95 has been improved to 24.6 (volume ratio per gram of adsorbate) at 273 K and 1 atm, comparable to other porous metal-organic framework adsorbents, but also the cost of making zeolite@ZIF-95 composite is reduced by 30%. Overall, zeolite@ZIF-95 exhibits great potential as a low-cost selective adsorbate in LFG separation. Furthermore, the versatile and large pores of ZIF-95 in the composite can be used as catalyst carriers, which will provide numerous applications for heterogeneous catalytic conversion of hydrocarbons.