Separation of Methane and Carbon Dioxide Using Zeolite@ZIF-95 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 level has kept rising in the last five decades. Meanwhile, methane is the primary component in natural gas, which can be utilized as an alternative fuel source. One of the major challenges in MSW controlling process is to purify methane from landfill gas (LFG), a mixture of CH₄ (~55%), CO₂ (~45%), and other gas molecules.In this study, a composite material contains zeolite and 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 resulted composite material was characterized by power X-ray diffraction (XRD) and infrared spectroscopy (IR) to confirm its crystal structure and chemical composition. A layered microstructure of the synthesized zeolite@95 composite was observed using scanning electron microscope (SEM). Moreover, zeolite@ZIF-95 exhibited the highest BET surface area compared to pristine zeolite and ZIF-95 crystal, confirmed by the nitrogen gas sorption isotherm analysis at 77K. Gas sorption of CO₂ and CH₄were carried out on zeolite 4A, ZIF-95, and zeolite@ZIF-95 composite at 273 K and 298 K, respectively. The gas adsorption selectivity toward CO₂ and CH₄ at different temperatures were performed based on single-component gas adsorption isotherms with the aid of the Ideal Adsorbed Solution Theory (IAST). The selectivity of CO₂/CH₄ on zeolite@ZIF-95 has been improved to 20.1 (volume ratio per gram of adsorbate), comparable to other porous metal-organic frameworks (MOFs). But the cost of making zeolite@ZIF-95 composite is less expensive than synthesizing MOFs. Overall, zeolite@ZIF-95 exhibits great potential as a low-cost selective adsorbate in landfill gas separation. Furthermore, the versatile and large pores of ZIF-95 in the composite can be used as catalyst carriers, which provides numerous applications for heterogeneous catalytic conversion of hydrocarbons.