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.