Monte Carlo Modeling of Carbon Dioxide Adsorption in Porous Aromatic Frameworks Alberto Fraccarollo, Lorenzo Canti, Leonardo Marchese, and Maurizio Cossi* Dipartimento di Scienze e Innovazione Tecnologica (DISIT), Centro Interdisciplinare Nano-SiSTeMI, Universita ̀ del Piemonte Orientale, via T. Michel 11, I-15121, Alessandria, Italy * S Supporting Information ABSTRACT: The adsorption isotherms of CO 2 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 CO 2 -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 CO 2 adsorption then was simulated in a number of modified PAF-302, with different functional groups (amino- methane, 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. 1. INTRODUCTION The ever-growing energy demand, largely satisfied by the combustion of fossil fuels, caused an escalated global CO 2 emission, which became the major cause of global warming. Besides the attempts to reduce anthropogenic emissions, it has been proposed to control atmospheric CO 2 concentration removing the gas from the air through carbon capture and storage (CCS) techniques, whose study is strongly supported by governments and scientific institutes. 1,2 In CCS technology, the adsorption in microporous materials has been considered recently for its economic and environ- mental advantages. Nanoporous dipeptide-based materials, 3 carbon-based materials, 4,5 zeolites, 6 metal-organic frameworks (MOFs), 7-9 covalent-organic frameworks (COFs), 10 zeolitic imidazolate frameworks (ZIFs), 11 and porous aromatic frame- works (PAFs) 12 are being considered as candidates to capture CO 2 . The performance of a very large number of porous solids, mainly zeolites and MOFs, have also been compared computationally with a specific protocol, to predict the energy demand and efficiency in actual plants. 13 Covalent-organic frameworks were also modeled with a variety of approaches, 14 including the so-called connectivity-based atom contribution method 15 and force fields derived from quantum-mechanical calculations (at the DFT and MP2 levels). 16 Among the newest materials, PAFs are raising a great interest as adsorbent materials, due to their very low density, large surface area, high porosity, exceptional structural properties, and high thermal and hydrothermal stability. 12,17-20 Here, we consider different members of the PAF family, collectively indicated as PAF-30n (where n =1-4 is the number of phenyl rings inserted in the C-C bonds of the diamond structure; see below). 21 Among this series, only PAF-302, sometimes referred to as PAF-1 in the literature, has been synthesized and characterized so far, although several variants and functionaliza- tions of the original PAF-302 have been studied. 22-27 Note that, although Trewin and Cooper suggested the PAF-302 might be amorphous based on the powder X-ray diffraction pattern, 28 all of the model studies about its adsorption properties were based on a crystalline structure. In their original work, Ben and co-workers proposed a method to synthesize the first high surface area PAF with dia topology (i.e., PAF-302, therein called PAF-1), 12 via a nickel(0)-catalyzed Yamamoto-type Ullmann cross-coupling. The resulting crystalline material showed a record specific surface area (SSA, 5640 m 2 /g with BET method) and an exceptional physicochemical stability; furthermore, it provided a very high uptake of carbon dioxide (1.3 g/g at 40 bar, 298 K). Recently, the record for SSA was gained by another porous aromatic polymer network, PPN-4, 29 with BET SSA as high as 6463 m 2 /g. Continuining their research, Ben and co-workers Received: January 28, 2014 Revised: March 18, 2014 Published: March 19, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 4147 dx.doi.org/10.1021/la500111a | Langmuir 2014, 30, 4147-4156