1 INTRODUCTION
Due to the continuous and increasing use of traditional fossil fuels, the world is facing great challenges in terms of the energy crisis and environmental problems.1,2 Natural gas (mainly methane), which has a high combustion calorific value, is regarded as a promising alternative to traditional fossil fuels.3,4To date, the consumption of natural gas has occupied an important proportion of primary energy consumption (24.2%) and is still growing.5 To meet the ever-growing for demand natural gas, it is of great significance to seek more gas sources in addition to conventional gas reservoirs. As a vital component of unconventional natural gas, coal-mine methane (CMM) has been shown to contain a large amount of methane. There are ~29 to 41 billion cubic meters of CMM liberated from underground coal mines every year, which can complement conventional natural gas supplies.6However, removing the unacceptable concentrations of impurities in CMM is an important prerequisite before its commercial use, especially nitrogen.7-10
Currently, cryogenic distillation based on the boiling point difference (112 K for CH4 and 77 K for N2) is utilized as the main technology for CMM enrichment, but the high energy consumption and operation cost hinder its industrial application.11,12 To overcome these issues, adsorption-based technology is regarded as a promising strategy benefiting from its low investment cost, simple operation, flexibility, and energy conservation. However, the key to this technology is the availability of high-performance adsorbents.13,14Unfortunately, the adsorption/separation of CH4/N2 is particularly difficult due to their similar kinetic diameters (3.8 Å for CH4 and 3.6 Å for N2) and comparable polarizability (CH4: 26.0 × 10−25cm3 and N2: 17.6 × 10−25 cm3).8,10Traditional adsorbents including activated carbons and zeolites have been investigated for CH4/N2 separation, but their industrialization remains a distant option, which is limited by their low selectivity and/or poor capacity.8Considering the urgency of CH4/N2separation, new types of adsorbents, which are industrially feasible, need to be developed.
As a new type of crystalline porous material, metal–organic frameworks (MOFs) have exhibited potential application in the field of gas adsorption and separation.15-17 Due to their designability and structural and chemical adjustability, MOFs provide the opportunity to design of new materials with better gas separation performance.18-26 In regard to CH4/N2 separation, MOFs have been proven to possess high-efficiency separation performance.12,27 For example, ATC-Cu reported by Ma and co-workers exhibited a new CH4 capture benchmark of 64.9 cm3/g due to its high-density open Cu sites.12 More recently, Ni(ina)2 was observed to possess the highest ever reported CH4/N2 selectivity of 15.8 under ambient conditions.27 It is worth noting that although many MOFs show high IAST selectivity and CH4 uptake, most of them cannot meet the demands of practical industrial application and hindered due to their high toxicity, scarcity, and use of expensive metal salts and/or organic ligands, as well as poor thermal and chemical stability. Al-MOFs, which are constructed from AlO6polyhedra and an organic carboxylate linker are considered to be one of the most prospective materials for CH4/N2 separation in practical applications.28 Due to their high structural stability and large-scale synthesis, Al-MOFs are easy to commercialize.29 Al-BDC (Basolite A100) and Al-FUM (Basolite A520) have been commercialized by BASF SE. Consequently, it is necessary to discover new Al-MOFs with prominent CH4/N2 separation properties from the viewpoint of their industrial application.
Current studies on CH4/N2 separation have mostly focused on MOF materials with ultra-microporous structures (<7 Å) and non-polar/inert pore environments, which are mainly considered from the perspective of thermodynamic separation.1,2,12,27,30 In fact, the separation performance of adsorbents is affected by both thermodynamic and kinetic factors. Previous studies have proven that the adsorption kinetic behavior will play an important role when the pore size of the adsorbent is comparable to the kinetic diameter of the target gas,31-35 and sometimes exhibit a size sieve effect.36-39 Bearing this analysis in mind, it can be predicted that Al-MOFs will display both priority CH4dynamic and thermodynamic adsorption behavior and exhibit excellent CH4/N2 separation performance under dynamic conditions.
Herein, we studied an ultra-microporous MOF (MIL-120Al) with non-polar pore walls composed of para-benzene rings with a comparable pore size to the kinetic diameter of methane, which exhibits the thermodynamic-kinetic synergistic separation of CH4/N2 mixtures. Single-component adsorption isotherms and time-dependent kinetic adsorption studies on CH4 and N2 were carried out. Our results show the remarkable diffusivity and adsorption difference between CH4 and N2. The high CH4/N2 separation performance was confirmed using breakthrough experiments and pressure swing adsorption (PSA) process simulations. More importantly, this MOF can be easily regenerated and synthesized on a large-scale, and exhibits ultra-high chemical and thermal stability.