1 | Introduction
Carbon dioxide capture is an effective strategy to mitigate greenhouse
effect and other related challenges.
CO2/N2 separation technology is pivotal
for post-combustion CO2 capture since the flue gas from
power plants is mainly composed of N2(70%~75%) and CO2(10%~15%) 1. Besides,
CO2 is also a harmful component for natural gas,
especially under moisture condition 2. Therefore,
separation of CO2 from CH4 is
challenging but of great significance to methane upgrading3. Compared with traditional amine-scrubbing,
adsorptive separation employing suitable porous materials is
sought-after as a result of the ease of operation and low energy
expenditure.
Metal-organic frameworks (MOFs) are promising adsorbents for their
diversified chemical compositions and porous skeletons4-6. The advancement of MOFs has motivated their
utility in gas separation and purification 7-13.
Selective adsorption on MOFs underpins isolating gases in high purity.
The core is to significantly accentuate the adsorption discrepancy of a
pair of gases (e.g., CO2/N2,
CO2/CH4) on MOFs, which is critical for
selectivity in dynamic adsorption-based separation and membrane
separation 14-18.
Cavity-occupying is an efficient
solution to narrow the pore size, and thereby to reinforce the molecular
recognition and selective adsorption of MOFs while retaining structural
integrity. This operation would be more effective in MOFs with
flexibility or with pore size significantly larger than molecule
dimensions. Ban et al. demonstrated that the confinement of ionic liquid
[bmim][Tf2N] into the sodalite nanocage of ZIF-8
can tailor the cage size of ZIF-8, impeding the passage of the larger
molecule and appreciably improve CO2/N2selectivity from 19 to 100 19. Liao et al. decorated
hydrazine into the pore of a MOF. The composite also excels in selective
adsorption of CO2 at low pressure 20.
Lin et al. loaded molecular-level polyethyleneimine into the nanopore of
MIL-101, engendering lower CO2 capacity but higher
CO2/N2 selectivity in the constrained
inner pore space 21.
MOF-74 possessing abundant open metal sites take up significant
saturable CO2 capacity. However, the large pore of
MOF-74 displays moderate CO2 selectivity over
N2 and CH4 22. It is
worthy to explore the possibility of improving CO2selectivity by cavity-occupying.
Nitrogen-containing agents show good CO2 affinity in
terms of Lewis acid-base interactions but weaker binding with
CH4 and N2 23.
Therefore, introduction of the suitable nitrogen-containing agent as a
cavity-occupant could be helpful for reducing the effective pore size of
MOF-74, and simultaneously providing compensation sites for
CO2 trapping. In this work, we seek to demonstrate a
platform based on MOF-74 for outstanding CO2 selective
adsorption. Pyrazine with dual
“para-nitrogen” atoms was introduced into the pore of MOF-74 via a
post vapor modification method (Scheme 1). One nitrogen atom of pyrazine
is bonded to the open metal ions in MOF-74 with a minimal steric
hindrance, and the other nitrogen atom of pyrazine provides potential
affinity to CO2, resulting in a stable
pyrazine-interior-embodied MOF-74 composite for boosting
CO2/N2 and
CO2/CH4 adsorption selectivity through
varying the pyrazine loading content. Given the pore environment of
MOF-74 after pyrazine modification, the selective adsorption of
pyrazine-interior-embodied MOF-74 for C4olefine/paraffine was also investigated.