Understanding Water Adsorption and the Impact on CO2 Capture in

Subscriber access provided by Kaohsiung Medical University

C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Understanding Water Adsorption and the Impact on CO Capture in Chemically Stable Covalent Organic Frameworks YUXIN GE, Hao Zhou, Yujin Ji, Lifeng Ding, Yuanyuan Cheng, Ruiyao Wang, Siyuan Yang, Yufeng Liu, Xiaoyu Wu, and Youyong Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09033 • Publication Date (Web): 08 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Understanding Water Adsorption and the Impact on CO2 Capture in Chemically Stable Covalent Organic Frameworks Yuxin Ge,a Hao Zhou,a Yujin Ji,b Lifeng Ding,*a Yuanyuan Cheng,c Ruiyao Wang,a Siyuan Yang,a Yufeng Liu,a Xiaoyu Wu,a and Youyong Li,*b a Department of Chemistry, Xi’an JiaoTong-Liverpool University, 111 Ren’ai Road, Suzhou Dushu Lake Higher Education Town, Jiangsu Province, 215123, China b Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for CarbonBased Functional Materials & Devices, Soochow University, Suzhou, Jiangsu, 215213, China c School of Environmental Science & Engineering, Suzhou University of Science & Technology, Suzhou, 215009, China Yuxin Ge and Hao Zhou contributed equally to this work Corresponding authors: Lifeng Ding ([email protected]); Youyong Li ([email protected]) ABSTRACT: In this work, a diverse set of chemically stable COFs have been studied to understand their water adsorption behaviour and how water adsorption will affect CO2 capture performance in the COFs using Grand Canonical Monte Carlo (GCMC) simulations. Our results revealed three different types of water adsorption behaviour in the COFs: (1) initial small water cluster formation was followed by early monolayer coverage in COFs with smooth hydrophilic surface, such as TpPa-1, TpPa-NO2 and DAAQ-TFP; (2) hydrophobic functional groups presented in the hydrophilic surface of the COFs, such as COF-42, COF-43 and TpBD, could disrupt the water monolayer formation, which would delay the pore filling of water in the COFs; (3) instantaneous pore filling of water molecules could happen without any nucleation stage for COFs with relatively small pores, such as NPN-COFs. In general, hydrophilic surface and small pore sizes are two good features for COFs to be used in water adsorption applications at low relative humidity (RH). Geometric steric effect may disrupt the pore filling of water in COFs. On the other hand, NPN-COFs provide good CO2 capture performance and water tolerance to a low RH (p/p0 = 0.1) due to their small pores with 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

hydrophobic surface. It is also exciting to find out that CO2 capture in COF-300 can tolerate a high RH of 0.8 due to its super-hydrophobic surface and the unique inter-planar space for CO2 uptake. INTRODUCTION With the recent advancement in the development of novel adsorbents with large surface area and well defined pore surface/structure, such as Metal Organic Frameworks (MOFs),1 Zeolitic Imidazolate Frameworks (ZIFs)2 and Covalent Organic Frameworks (COFs),3 it is promising to design systems based on novel adsorbents to capture water2 or CO24. Water adsorption in porous materials is useful for designing water harvesting system and water adsorption driven heat exchangers/pumps.5 On the other hand, widely present moisture can be detrimental for the utilization of porous materials to capture CO2, which is of particular urgency to address climate change.6 The use of conventional porous materials, such as active carbon or zeolite water for harvesting applications at low relative humidity (RH) is difficult. They either capture little water or require high energy input to release the captured water.7 Accurate understanding of water adsorption in porous materials is essential to the design of such materials that either directly captures water vapor or resists water intrusion. MOFs/COFs have shown promising adsorption/desorption balance to be used in water harvesting system. Canivet et al.7 found out that the water adsorption in MIL-101 and MIL-100 at a low RH outperformed the traditional porous solids, zeolites and porous carbon, and the energy required for regeneration was much lower than those of zeolites. Furukawa et al.8 demonstrated that zirconium-based MOFs, such as MOF-801 and MOF-841 could exhibit high water uptake at low RH with facial hydration/dehydration cycles. On the other hand, efforts were paid to apply water stable porous materials to capture CO2 under moist conditions to address greenhouse gas emissions with minimal cost. Zhang et al.9 synthesized Zn-phbc-12a(bpe) and Zn-pbdc-121a(bpy) which showed good tolerance in the presence of moisture with CO2 uptakes up to 98 and 78 cm3 g-1, respectively. Liao et al.10 illustrated that MOFs (MAF-X25, MAF-X25ox, MAF-X27, MAF-X27ox) functionalized by monodentate hydroxide could lead to high CO2 uptakes (13.4 wt%) at a high RH of 82%. Among the novel adsorbents, COFs have gained increasing interests in recent years.11,12 This is owing to their highly ordered frameworks with high porosity as well as excellent thermal and chemical stability. Their structures are covalently constructed from light elements (e.g. H, B, C, Si, N and O).13 The hydrolytic stability of both MOFs and COFs has been a concern when they are applied in a moist rich environment.2,14 MOFs that are assembled by carboxylate-metal bonds are known to be susceptible to water attack.7,15 COFs containing boronate ester bonds are usually 2 ACS Paragon Plus Environment

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


hydrolytically instable with moderate humidity.14,16 Banerjee et al. studied a diverse set of ketoenamine based hydrolytically stable COFs for their water adsorption performance.17 Their results indicated that TpPa-serie (Tp representing triformylphloroglucinol with Pa representing phenylenediamine) COFs featured high water uptake under low RHs with excellent recyclability and low regeneration cost, which was comparable with recently reported porous materials, such as MOF-801 or MOF-804.8,18,19 To aid the design and utilization of novel COFs, molecular level simulations can provide a systematic understanding of water adsorption as well as its impact to their CO2 capture performance. In this study, we have chosen a diverse set of non-boronate based COFs, (Figure 1) which features excellent chemical and thermal stability. These COFs also feature diverse functional groups decorating their surfaces, for instance, -OEt group (COF-42 and COF-43), -NO2 groups (TpPaNO2), -F groups (TpPa-F4), and -CH3 groups (TpPa-2), as well as varied surface area, pore volume, and pore diameter, ranging from 861 m2/cm3 to 3453 m2/cm3, 0.34 cm3/g to 2.07 cm3/g and 3.51 Å to 31.71 Å, respectively. We expect these distinct differences in the COFs should provide a comprehensive insight into their water/CO2 adsorption behaviour through our computational study. Moreover, we intend to gain a molecular level understanding of water adsorption and its effect on CO2 capture performance in COFs. Such understanding will rationalize the role of surface hydrophilic/hydrophobic functionalities, pore size/geometry of COFs to either optimize water capture related applications or minimize the effect of humidity on the CO2 capture performance.

3 ACS Paragon Plus Environment


Page 4 of 27

Figure 1. Space filling models of the selected COFs in this work. Atom colors: C, grey; H, white; O, red; N, blue; F, green; Si, yellow.

COMPUTATIONAL METHODOLOGIES COFs Structures. 15 COFs whose linkage groups are mainly keto-enamine (DAAQ-TFP20, TpPa1, TpPa-2, TpBD, TpPa-F4 and TpPa-NO217), imine (COF-LZU121, COF-30022 and COF-32023), triazine (CTF-124), azodioxy (NPN-1, NPN-2 and NPN-325) and hydrazine (COF-42 and COF-4326) based were studied in this work (Figure 1). Keto-enamine based COFs have been reported to have excellent chemical, thermal and hydrolytic stability.13,27 Other COFs appear to have good chemical and thermal stability. Their hydrolytic stability remains unexplored. The research to utilize these non-boronate based COFs for water and CO2 capture is still scarce in the literature. The COFs selected in this work have diverse surface area, pore volume, and pore diameter. The characteristics, such as geometric surface area, porosity and pore size of the COFs are summarized in Table S1 and S2. GCMC simulation details. The adsorption of pure water vapor and CO2/N2 mixture (15:85) with or without hydration in the COFs were simulated using grand canonical Monte Carlo (GCMC) simulations with RASPA package.28 During the GCMC simulations, the structures of the COFs 4 ACS Paragon Plus Environment

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were treated as rigid. The Monte Carlo moves in the GCMC simulations which included translational, rotational, addition/deletion, reinsertion and identity change moves were tried with equal probability in one MC cycle. A total of 1×106 MC cycles was set for the initialization run followed by a production run of another 1×106 MC cycles. The isosteric heats of adsorption of guest molecules in the COFs was obtained at a low coverage. The geometric surface area of the COFs was calculated through Duren’s method,29 and the pore dimension and pore size distribution of the COFs was computed using Talu et al.’s method,30 both of which are implemented in RASPA. Force Fields. In this work, a combination of Leonard Jones (LJ) potential and coulomb interaction was used to address the interactions between water, CO2, N2 and the COFs. The LJ parameters for CO2 and N2 adopted were taken from the force field developed by Harris et al31 and TraPPE32, respectively. The LJ potential parameters for the framework atoms of COFs adopted were from the DREIDING force field.33 The partial charges of the framework atoms of the COFs were taken from Yang et al.’s work.34 The partial charges of the COFs that were not available in Yang et al.’s work were calculated using ChelpG method.34 Lorentz-Berthelot mixing rules were used to mix LJ interaction pairs. To address the water uptake behavior in the COFs, a suitable water model is required. There are more than 46 distinct water models that have been reported for reproducing specific properties of water.35 A set of six commonly used water models, including SPC, SPC/E, TIP4P, TIP4P_Ew, TIP5P, and TIP5P_Ew, was chosen and the validity of the water models were evaluated in our work.36–38 All the force field parameters were given in the ESI† (Table S4, S5 & S6)

5 ACS Paragon Plus Environment


Page 6 of 27

Figure 2. Experimental and simulated water adsorption isotherms in TpPa-1 (a) and TpPa-2 (b) at 298 K. Experimental and simulated CO2 (c) and N2 (d) adsorption isotherms in TpPa-1, TpPa-2 and TpPa-F4 at 298 K. Validation of the force fields. As given in Table S3, all 6 water models produce similar isosteric heats of adsorption of water in TpPa-1 and TpPa-2, which are above the latent condensation energy of water (40.66 kJ mol-1) apart from TIP5P and TIP5P_EW which give slightly lower adsorption heats in TpPa-1. This is in line with the reported hydrophilic nature of TpPa-1 and TpPa-2 in Banerjee et al.’s work.17 As shown in Figure 2 (a) and (b), the simulated water adsorption isotherms are compared with the experimental results for TpPa-1 and TpPa-2 reported by Banerjee et al..17 For TpPa-1, it is observed that TIP4P, TIP5P and TIP5P_Ew water models largely underestimated the water uptake. The inflection point (where the steep pore filling starts to happen) of the water adsorption isotherm of SPC model are much higher than those of the experimental results. SPC/E and TIP4P_Ew water models are in good agreement with the experimental data at low pressure region (0 ~ 0.6 kpa), but overestimated the water uptake at higher pressure level (0.6 ~ 3 kpa), and the inflection points (around 0.7 kpa) of the water adsorption isotherms match well with the 6 ACS Paragon Plus Environment

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


experimental results. It should be noteworthy that ideal crystals of the COFs were used in the simulation. COFs have been well known for their crystal imperfection.14,39 A large discrepancy was also found between the calculated surface areas from ideal COFs crystals and the experimental surface areas, (Table S1) which supports the imperfection of TpPa-1 and TpPa-2 or likelihood that solvent could be trapped in the COFs. As a result, the imperfect crystallinity of the COFs may contribute to the adsorption discrepancy between experimental and simulation results.40 Similar discrepancy (simulated data is about twice as the experimental data) was observed for CO2 and N2 adsorption in TpPa-1,TpPa-2 and TpPa-F4. Therefore, SPC/E model was chosen for our studies for its low computation cost and good accuracy to produce correct inflection points. RESULTS AND DISCUSSIONS GCMC simulations of water uptake in hydrophilic and hydrophobic COFs. Figure 3 presents the simulated water adsorption isotherms for 15 COFs based on SPC/E model at 298 K. It should be noted that there has not been a consensus on defining the hydrophobicity/hydrophilicity of porous materials.2 Snurr et al. suggested the hydrophobicity/hydrophilicity of a porous material could be determined by the pressure at which the steep pore filling begins (where the inflection is).38 Following this criteria, we arbitrarily classified these COFs into two categories based on the inflection points of the water adsorption isotherms: hydrophilic COFs (inflection point p/p0

Understanding Water Adsorption and the Impact on CO2 Capture in

Recommend Documents