Wenchuan Wang

Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area

Porous materials have been of intense scientific and technological interest because of their vital importance in many applications such as catalysis, gas separation, and gas storage. Great efforts in the past decade have led to the production of highly porous materials with large surface areas. In particular, the development of metal–organic frameworks (MOFs) has been especially rapid. Indeed, the highest surface area reported to date is claimed for a recently reported MOF material UMCM-2, which has a N2 uptake capacity of 1500 cm g at saturation, from which a Langmuir surface area of 6060 m g (Brunauer–Emmett–Teller (BET) surface area of 5200 m g) can be derived. Unfortunately, the high-surface-area porous MOFs usually suffer from low thermal and hydrothermal stabilities, which severely limit their applications, particularly in industry. These low stability issues could be resolved by replacing coordination bonds with stronger covalent bonds, as observed in covalent organic frameworks (COFs) or porous organic polymers. However, the COFs and porous organic polymers reported to date have lower surface areas compared to MOFs; the highest reported surface area for a COF is 4210 m g (BET) in COF103. Thus, further efforts are required to explore various strategies to achieve higher surface areas in COFs. Herein, we present a strategy that has enabled us to achieve, with the aid of computational design, a structure that possesses by far the highest surface area reported to date, as well as exceptional thermal and hydrothermal stabilities. We report the synthesis and properties of a porous aromatic framework PAF-1, which has a Langmuir surface area of 7100 m g. Besides its exceptional surface area, PAF-1 outperforms highly porous MOFs in thermal and hydrothermal stabilities, and demonstrates high uptake capacities for hydrogen (10.7 wt % at 77 K, 48 bar) and carbon dioxide (1300 mgg 1 at 298 K, 40 bar). Moreover, the super hydrophobicity and high surface area of PAF-1 result in unprecedented uptake capacities of benzene and toluene vapors at room temperature. It is well known that one of the most stable compounds in nature is diamond, in which each carbon atom is tetrahedrally connected to four neighboring atoms by covalent bonds (Figure 1a). Conceptually, replacement of the C C covalent bonds of diamond with rigid phenyl rings should not only retain a diamond-like structural stability but also allow sufficient exposure of the faces and edges of phenyl rings with the expectation of increasing the internal surface areas. By employing a multiscale theoretical method, which

Metal–Organic Frameworks with Incorporated Carbon Nanotubes: Improving Carbon Dioxide and Methane Storage Capacities by Lithium Doping

Reduction of the anthropogenic emission of CO2 is currently a top priority because CO2 emission is closely associated with climate change. Carbon capture and storage (CCS) and the development of renewable and clean energy sources are two approaches for the reduction of CO2 emission. One of the most promising alternative fuels is CH4, which is the major component of natural gas. The efficient storage of CH4 is still one of the main challenges for its widespread application. Accordingly, the development of more efficient approaches for CO2 capture and CH4 storage is critically important. Recently, metal–organic frameworks (MOFs, e.g., MOF210 and NU-100) have shown great potential for gas storage because of their high specific surface area (SSA) and functionalized pore walls. However, most MOF materials still show relatively low CO2 and CH4 uptakes. To enhance CO2 and CH4 adsorption, it is imperative to develop new materials, such as covalent organic frameworks (COFs), or to modify MOFs by using postsynthetic approaches. Herein, we focus on the latter strategy. One of the modification approaches is incorporation of carbon nanotubes (CNTs) into MOFs in order to achieve enhanced composite performance, because of the unusual mechanical and hydrophobicity properties of CNTs. Another approach is doping MOFs or COFs with electropositive metals. Recent studies indicate that the surface carboxylate functional groups of a substrate could act as nucleation sites to form MOFs by heterogeneous nucleation and crystal growth. Both experimental and theoretical investigations indicate that the H2 adsorption capacities of MOFs can be enhanced significantly by doping alkali-metal ions, in particular Li ions, to the frameworks, owing to the strong affinity of Li ions towards H2 molecules. [3d, 7] Similarly, Lan et al. also showed theoretically that doping of COFs with Li ions can significantly enhance the CH4 uptake of COFs. [8] Most recently, the multiscale simulations performed by Lan et al. indicate that Li is the best surface modifier of COFs for CO2 capture among a series of metals (Li, Na, K, Be, Mg Ca, Sc and Ti). Furthermore, their simulations show that the excess CO2 uptakes of the lithium-doped COFs can be enhanced by four to eight times compared to the undoped COFs at 298 K and 1 bar. Motivated by these experimental and theoretical results, we synthesized hybrid MOF materials by using the two modification techniques outlined above, that is, 1) incorporation of CNTs into [Cu3(C9H3O6)2(H2O)3]·x H2O ([Cu3(btc)2], HKUST-1; btc = 1,3,5-benzenetricarboxylate), which is an important MOF material owing to its open metal sides and high thermal stabilities, as well as its sorption properties, 10] and 2) doping [Cu3(btc)2] with Li + ions. We used lithium naphthalenide (LiC10H8 ) to introduce Li ions into the [Cu3(btc)2] frameworks. These frameworks have Cu 2+

Lithium‐Doped 3D Covalent Organic Frameworks: High‐Capacity Hydrogen Storage Materials

Quick on the uptake: A multiscale theoretical method predicts that the gravimetric adsorption capacities of H(2) in Li-doped covalent organic frameworks based on the building blocks shown (Li violet, H white, B pink, C green, O red, Si yellow) can reach nearly 7 % at T=298 K and p=100 bar, suggesting that these Li-doped materials are promising adsorbents for hydrogen storage.

Doping of Alkali, Alkaline-Earth, and Transition Metals in Covalent-Organic Frameworks for Enhancing CO2 Capture by First-Principles Calculations and Molecular Simulations

We use the multiscale simulation approach, which combines the first-principles calculations and grand canonical Monte Carlo simulations, to comprehensively study the doping of a series of alkali (Li, Na, and K), alkaline-earth (Be, Mg, and Ca), and transition (Sc and Ti) metals in nanoporous covalent organic frameworks (COFs), and the effects of the doped metals on CO2 capture. The results indicate that, among all the metals studied, Li, Sc, and Ti can bind with COFs stably, while Be, Mg, and Ca cannot, because the binding of Be, Mg, and Ca with COFs is very weak. Furthermore, Li, Sc, and Ti can improve the uptakes of CO2 in COFs significantly. However, the binding energy of a CO2 molecule with Sc and Ti exceeds the lower limit of chemisorptions and, thus, suffers from the difficulty of desorption. By the comparative studies above, it is found that Li is the best surface modifier of COFs for CO2 capture among all the metals studied. Therefore, we further investigate the uptakes of CO2 in the Li-doped COFs. Our simulation results show that at 298 K and 1 bar, the excess CO2 uptakes of the Li-doped COF-102 and COF-105 reach 409 and 344 mg/g, which are about eight and four times those in the nondoped ones, respectively. As the pressure increases to 40 bar, the CO2 uptakes of the Li-doped COF-102 and COF-105 reach 1349 and 2266 mg/g at 298 K, respectively, which are among the reported highest scores to date. In summary, doping of metals in porous COFs provides an efficient approach for enhancing CO2 capture.

Multiscale simulation and modelling of adsorptive processes for energy gas storage and carbon dioxide capture in porous coordination frameworks

Computational modelling is a powerful tool for the study of gas–solid interactions, and can be used both to complement experiment and design new materials. For the modelling of gas adsorption by nanoporous media, a multiscale approach can be used, in which the molecular force fields required for Grand Canonical Monte Carlo (GCMC) simulations are derived from first-principles calculations. This can result in significantly enhanced accuracy, in comparison with conventional empirical force field-based GCMC methods. In this article, we review the application of this multiscale approach to the simulation of the adsorption of hydrogen, methane and carbon dioxide in Porous Coordination Frameworks (PCFs) for the purpose of gas storage for energy transportation and Carbon Capture and Storage (CCS) technology. We also define a scheme for the design of new materials with improved adsorption performance for the storage of these gases through the combination of multiscale simulation and experimental work, and discuss some of the issues regarding gas adsorption measurement accuracy in the context of the validation of simulation results using experimental data.

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Porous covalent polymers are attracting increasing interest in the fields of gas adsorption, gas separation, and catalysis due to their fertile synthetic polymer chemistry, large internal surface areas, and ultrahigh hydrothermal stabilities. While precisely manipulating the porosities of porous organic materials for targeted applications remains challenging, we show how a large degree of diversity can be achieved in covalent organic polymers by incorporating multiple functionalities into a single framework, as is done for crystalline porous materials. Here, we synthesized 17 novel porous covalent organic polymers (COPs) with finely tuned porosities, a wide range of Brunauer-Emmett-Teller (BET) specific surface areas of 430-3624 m(2) g(-1), and a broad range of pore volumes of 0.24-3.50 cm(3) g(-1), all achieved by tailoring the length and geometry of building blocks. Furthermore, we are the first to successfully incorporate more than three distinct functional groups into one phase for porous organic materials, which has been previously demonstrated in crystalline metal-organic frameworks (MOFs). COPs decorated with multiple functional groups in one phase can lead to enhanced properties that are not simply linear combinations of the pure component properties. For instance, in the dibromobenzene-lined frameworks, the bi- and multifunctionalized COPs exhibit selectivities for carbon dioxide over nitrogen twice as large as any of the singly functionalized COPs. These multifunctionalized frameworks also exhibit a lower parasitic energy cost for carbon capture at typical flue gas conditions than any of the singly functionalized frameworks. Despite the significant improvement, these frameworks do not yet outperform the current state-of-art technology for carbon capture. Nonetheless, the tuning strategy presented here opens up avenues for the design of novel catalysts, the synthesis of functional sensors from these materials, and the improvement in the performance of existing covalent organic polymers by multifunctionalization.

Absorption of CO2 in the Ionic Liquid 1-n-Hexyl-3-methylimidazolium Tris(pentafluoroethyl)trifluorophosphate ([hmim][FEP]): A Molecular View by Computer Simulations

Using a computational screening methodology, we predicted (AIChE J. 2008, 54, 2717) that the anion tris(pentafluoroethyl)trifluorophosphate ([FEP]) should increase the solubility of CO2 in ionic liquids (ILs) relative to a wide range of conventional anions. This prediction was confirmed experimentally. In this work, we develop a united-atom force field for the [FEP] anion and use the continuous fractional component Monte Carlo (CFC MC) method to predict CO2 absorption isotherms in 1-n-hexyl-3-methylimidazolium ([hmim]) [FEP] at 298.2 and 323.2 K and pressures up to 20.0 bar. The simulated isotherms overestimate the solubility of CO2 by about 20% but capture the experimental trends quite well. Additional Monte Carlo (MC) and molecular dynamics (MD) simulations are performed to study the mechanisms of CO2 absorption in [hmim][FEP] and [hmim][PF6]. The site-site radial distribution functions (RDFs) show that CO2 is highly organized around the [PF6] anion due to its symmetry and smaller size, while less ordered distributions were found around [FEP] and [hmim]. However, more CO2 can be found in the first coordination shell of [FEP] compared with [PF6]. The structures of ILs, illustrated by P-P radial distribution functions, change very little upon the addition of as much as 50 mol % CO2. An energetic analysis shows that the van der Waals interactions between CO2 and ILs are generally larger than electrostatic interactions.

Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations

Replacement of CH4 in hydrate form with CO2 is a candidate for recovering CH4 gas from its hydrates and storing CO2. In this work, microsecond molecular dynamics simulations were performed to study the replacement mechanism of CH4 hydrate by CO2 molecules. The replacement process is found to be controlled cooperatively by the chemical potentials of guest molecules, “memory effect”, and mass transfer. The replacement pathway includes the melting of CH4 hydrate near the hydrate surface and the subsequent formation of an amorphous CO2 hydrate layer. A large number of hydrate residual rings left after the melting of CH4 hydrate facilitate the nucleation of CO2 hydrate and enhance the dynamic process, indicating the existence of so-called “memory effect”. In the dynamic aspect, the replacement process takes place near the surface of CH4 hydrate rather easily. However, as the replacement process proceeds, the formation of the amorphous layer of the CO2 hydrate provides a significant barrier to the mass transfer of the guest CH4 and CO2 molecules, which prevents the CH4 hydrate from further dissociation and slows down the replacement rate.

Microsecond Molecular Dynamics Simulations of the Kinetic Pathways of Gas Hydrate Formation from Solid Surfaces

In this paper, we report microsecond molecular dynamics simulations of the kinetic pathway of CO(2) hydrate formation triggered by hydroxylated silica surfaces. Our simulation results show that the nucleation of the CO(2) hydrate is a three-stage process. First, an icelike layer is formed closest to the substrates on the nanosecond scale. Then, on the submicrosecond timescale, a thin layer with intermediate structure is induced to compensate for the structure mismatch between the icelike layer and the final stable CO(2) hydrate. Finally, on the microsecond timescale, the nucleation of the first CO(2) hydrate motif layer is generated from the intermediate structure that acts as nucleation seeds. We also address the effects of the distance between two surfaces.

High uptakes of methane in Li-doped 3D covalent organic frameworks.

By using a multiscale theoretical method, which combines the first-principles calculation and grand canonical Monte Carlo (GCMC) simulation, we studied storage capacities of methane in 3D covalent organic frameworks (COFs) and their Li-doped compounds at T = 243 and 298 K. Our results predicted that, at T = 298 K and 35 bar, the excess gravimetric capacities of COF-102 and COF-103 reach 17.72 and 16.61 wt % (corresponding to 302 and 285 cm(3) (STP)/g)), which are in good agreement with experimental data, while the excess volumetric capacities of COF-102 and COF-103 reach 127 and 108 v (STP)/v, respectively. The high methane storage capacity of the COFs can be attributed to their ultrahigh surface areas and low densities. To further enhance the methane capacity, we investigated the impact of Li-doping on the methane storage performance of the COFs. Our first-principles calculations show that the Li cation doped in the COFs can enhance the binding of methane to the substrate significantly because of the London dispersion and the induced dipole interaction, due to the strong affinity of Li cation to methane molecules. At T = 298 K and relatively low pressures (p < 50 bar), the Li-doping method nearly doubles the methane uptakes of the COFs, compared to the nondoped materials. In particular, at T = 298 K and p = 35 bar, the methane volumetric uptakes of Li-doped COF-102 and COF-103 reach 303 and 290 v (STP)/v, respectively, which is more than 2 times those in the nondoped (127 and 108 v (STP)/v). To the best of our knowledge, the Li-doped 3D COFs show the largest methane storage uptakes at room temperature to date.