Nitrogen (N2) rejection
from methane (CH4) is the most challenging step in natural
gas processing because
of the close similarity of their physical-chemical properties. For
decades, efforts to find a functioning material that can selectively
discriminate N2 had little outcome. Here, we report a molecular
trapdoor zeolite K-ZSM-25 that has the largest unit cell among all
zeolites, with the ability to capture N2 in favor of CH4 with a selectivity as high as 34. This zeolite was found
to show a temperature-regulated gas adsorption wherein gas molecules’
accessibility to the internal pores of the crystal is determined by
the effect of the gas–cation interaction on the thermal oscillation
of the “door-keeping” cation. N2 and CH4 molecules were differentiated by different admission-trigger
temperatures. A mild working temperature range of 240–300 K
was determined wherein N2 gas molecules were able to access
the internal pores of K-ZSM-25 while CH4 was rejected.
As confirmed by experimental, molecular dynamic, and ab initio density functional theory studies, the outstanding N2/CH4 selectivity is achieved within a specific temperature
range where the thermal oscillation of door-blocking K+ provides enough space only for the relatively smaller molecule (N2) to diffuse into and through the zeolite supercages. Such
temperature-regulated adsorption of the K-ZSM-25 trapdoor zeolite
opens up a new approach for rejecting N2 from CH4 in the gas industry without deploying energy-intensive cryogenic
distillation around 100 K.
Lithium (Li)-doped polycyclic aromatic hydrocarbons showed a high potential for N2 removal from natural gas. Li doping significantly increases the gas adsorption energies resulting in considerable N2 adsorption selectivity.
Gas solubility can go beyond classical
bulk-liquid Henry’s
law saturation under the nanoconfinement of a liquid phase. This concept
establishes the foundation of the current study for developing a novel
catalytic system for transformation of carbon dioxide to cyclic carbonates
at mild conditions with major emphasis on application for CO2 capture and utilization. A series of mesoporous silica-based supports
of various pore sizes and shapes grafted with a quaternary ammonium
salt is synthesized and characterized. CO2 sorption in
styrene oxide, either in bulk or nanoconfined state, as well as catalytic
reactivity for CO2 transformation into styrene carbonate,
are experimentally evaluated. The family of mesoporous catalysts with
aligned cylindrical pores (MCM-41 and SBA-15) with pore sizes ranging
from 3.5 to 9 nm exhibit enhanced sorption of CO2 in nanoconfined
styrene oxide with maximum sorption capacity taking place in MCM-41
with the smallest pore size. The catalysts with interconnected cylindrical
pores (KIT-6) with pore sizes ranging from 4.5 to 8.7 nm showed CO2 solubilities almost equal to the bulk solubility of styrene
oxide. Monte Carlo simulations revealed that the oversolubility in
styrene oxide confined complex is directly related to the density
of adsorbed solvent in the nanopore, which is less than its bulk density.
Catalytic reactivities correlate with CO2 sorption enhancement,
showing higher turnover frequencies for catalysts having higher CO2 sorption capacity. The turnover frequency is increased by
a factor of 7.5 for grafted MCM-41 with the smallest pore size with
nanoconfined styrene oxide in comparison to the homogeneous reaction
implemented in bulk.
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