A Friedel−Crafts reaction was used to obtain covalent aromatic networks with high surface area and microporosity suited for CO2 and CH4 adsorption, even at low pressures. Starting from tetraphenylmethane and formaldehyde dimethyl acetal in different concentrations, the reaction yields porous polymers which were characterized with a wealth of experimental and computational methods. Thermogravimetry, infrared spectroscopy, and solid-state NMR were used to study the material structure. The pore distributions were measured by applying nonlocal density functional theory analysis to the adsorption isotherms of N2 at 77 K and Ar at 87 K (the latter being more suited for pore widths less than 10 Å). Carbon dioxide and methane were adsorbed at 273 and 298 K to evaluate the performance of these systems in gas capture, separation, and storage. A theoretical model of the porous network was defined to describe the ordered fraction of the material, with particular attention to ultramicropores. Ar, CO2, and CH4 adsorption in this model material was simulated by Monte Carlo techniques with a purposely optimized force field
The adsorption isotherms of methane in four micro- and mesoporous materials, based on the diamond structure with (poly)phenyl chains inserted in all the C-C bonds, have been simulated with Grand Canonical Monte Carlo technique. The pressure range was extended above 250 bar and the isotherms were computed at 298, 313, and 353 K, to explore the potentiality of these materials for automotive applications, increasing the capacity of high-pressure tanks or storing a comparable amount of gas at much lower pressure. The force field employed in the simulations was optimized to fit the correct behavior of the free gas in all the pressure range and to reproduce the methane-phenyl interactions computed at high quantum mechanical level (post Hartree-Fock). All the examined materials showed a high affinity for methane, ensuring a larger storage of gas than simple compression in all the conditions: two samples exceeded the target proposed by U.S. Department of Energy for methane storage in low-pressure fuel tanks (180 cm(3) (STP)/cm(3) at 35 bar and room temperature).
This article describes the structure and the electronic properties of a series of layered perovskites of a general formula (A+)2(SnX4)−2 where X = I, Br and A+ is an organic cation, either formamidinium, 1-methylimidazolium, or phenylethylammonium. For each system, two conformations are considered, with eclipsed or staggered stacking of the adjacent inorganic layers. Geometry optimizations are performed at the density functional theory level with generalized gradient approximation (GGA) functional and semiempirical correction for dispersion energies; band profiles and bandgaps are computed including both spin orbit coupling (SOC) and correlation (GW) effects through an additive scheme. The theoretical procedures are validated by reproducing the experimental data of a well known 3D tin iodide perovskite. The results, combined with the calculations previously reported on PbI4 analogues, allow us to discuss the effect of cation, metal, and halide substitution in these systems and in particular to explore the possibility of changing the electronic bandgap as required by different applications. The balance of SOC and GW effects depends on the chemical nature of the studied perovskites and strongly influences the value of the simulated bandgap.
We present a vibrational study of PAF-302, belonging to the class of porous aromatic frameworks (PAFs), recently synthesized and applied in several applications involving gas adsorption. The precursor, tetrakis(4-bromophenyl) methane (TBPM), and the polymer were studied with FTIR and Raman spectroscopies to investigate the structure of PAF-302, whereas the system after methane adsorption was studied by FTIR, also varying the CH 4 loading, to get some hints on the strength of the interactions with adsorbed methane. Theoretical calculations of the harmonic frequencies of TBPM, methane, and methane/ aromatic model systems were performed at high theory level (MP2 with extended basis set) to support the assignment of vibrational bands and to estimate the interactions causing the observed frequency shifts upon methane adsorption. The analysis shows that the polymerization process is essentially complete and that the adsorbed CH 4 molecules interact with two phenyl rings, though stronger interactions can be envisaged. The computed interaction energies are compatible with the isosteric heats of adsorption previously measured for methane in PAF-302. A Grand Canonical Monte Carlo (GCMC) approach was used to simulate CH 4 adsorption isotherms at different temperatures (87−115 K) and in the 0−0.020 bar pressure range, thus allowing us to estimate the loading of methane in the FTIR adsorption study.
The adsorption isotherms of CO2 in several porous aromatic frameworks (PAFs) have been simulated with Grand Canonical Monte Carlo technique, to support the synthesis of new materials for efficient carbon dioxide capture and storage. The simulations covered the 0-60 bar pressure range and were repeated at 273, 298, and 323 K. The force field employed in the simulations was optimized to fit the correct behavior of the free gas and to reproduce the CO2-phenyl interactions computed at high quantum mechanical level. PAFs are based on the diamond structure, with polyaromatic chains inserted in C-C bonds. We examined four PAF-30n (n being the number of phenyl rings in the aromatic linkers), finding that PAF-302 is overall the best performing, although PAF-301 provides higher adsorbed densities at very low pressure. The CO2 adsorption then was simulated in a number of modified PAF-302, with different functional groups (aminomethane, toluene, pyridine, and imidazole) attached to the phenyl chains; different degrees of substitution (25%, 50%, and 100% derivatized rings) were considered. The effects of functionalization and the dependence on the substitution degree are carefully discussed, to determine the most promising materials at low, intermediate, and high pressures.
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