Adsorption of CO2 and CH4 molecules on the Pd-decorated C3N based sensors: A first-principles study

Adsorption of greenhouse gases (methane and carbon dioxide) on the pure and Pd-adsorbed stanene nanosheets: A theoretical study

Recent advancements in experimental approaches have made it possible to synthesize silver (Ag5) atomic quantum clusters (AQCs), which have shown a great potential in photocatalysis. This study employs the generalized gradient approximation (GGA) density functional theory (DFT) to explore the adsorption of CO2, CH4, and H2O molecules on the Ag5 AQC. Our investigations focus on the structural and electronic properties of the molecules in Ag5 AQC systems. This involves adsorption energy simulations, charge transfer, charge density difference, and the density of states for the modelled systems. Our simulations suggest that CH4 and H2O molecules exhibit higher adsorption energies on the Ag5 AQC compared to CO2 molecules. Remarkably, the presence of CH4 molecule leads to a significant deformation in the Ag5 AQC structure. The structure reforms from a bipyramidal to trapezoidal shape. This study also reveals that the Ag5 AQC donates electrons to CO2 and CH4 molecules, resulting in an oxidation state. In contrast, gaining charges from H2O molecules results in a reduced state. We believe the proposed predictions provide valuable insights for future experimental investigations of the interaction behaviour between carbon dioxide, methane, water molecules, and Ag5 sub-nanometre clusters.

The adsorption of CO, H2S and CH4 on the intrinsic SnS2 monolayer and SnS2 monolayer with S-vacancy were studied by the first principle method. The most stable adsorption geometry was determined, and the electronic structure and differential charge were calculated. The results show that the band gap value of SnS2 monolayer with S-vacancy decreases and the surface activity increases. Compared with the intrinsic SnS2 monolayer, from the adsorption energy, electron transfer and adsorption distance of CO, H2S and CH4 on SnS2 monolayer with S-vacancy, it can be seen that the S vacancy system is more sensitive to these gases. The results show that the SnS2 monolayer with S-vacancy is more sensitive to three gases, which provides a new design idea for SnS2 based gas sensor. As seen from the graphical abstract, the gas molecules (CO, H2S, CH4) on SnS2 monolayer with S vacancy has stronger adsorption energy than the intrinsic SnS2 surface, and the adsorption properties of H2S gas molecule on SnS2 monolayer with S-vacancy surface has the strongest adsorption energy −0.77 eV. It indicate that SnS2 monolayer with S-vacancy can be used as a promising gas detection material. Statement of article significance: First-principles studies were performed to investigate the electronic structures and adsorptive property of SnS2 monolayer and SnS2 monolayer with S-Vacancy with adsorbents CH4, CO and H2S molecules. The adsorption order of the three gases on the intrinsic surface is H2S > CH4 > CO. The adsorption order of the three gases on SnS2 monolayer with S-vacancy is H2S > CO > CH4. In addition, the results indicate that the adsorption energy of H2S on SnS2 monolayer with S-vacancy is strong. The work provides a new design idea for SnS2 based gas sensor.

Open AccessCCS ChemistryRESEARCH ARTICLE3 Oct 2022Fluorous Metal–Organic Frameworks with Unique Cage-in-Cage Structures Featuring Fluorophilic Pore Surfaces for Efficient C2H2/CO2 Separation Xing-Ping Fu†, Yu-Ling Wang†, Xue-Feng Zhang, Zhenjie Zhang, Chun-Ting He and Qing-Yan Liu Xing-Ping Fu† College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 Department of Ecological and Resources Engineering, Fujian Key Laboratory of Eco-Industrial Green Technology, Wuyi University, Wuyishan, Fujian 354300 , Yu-Ling Wang† College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 , Xue-Feng Zhang College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 , Zhenjie Zhang College of Chemistry and State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071 , Chun-Ting He College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 and Qing-Yan Liu *Corresponding author: E-mail Address: [email protected] College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Molecules for Ministry of Education, Jiangxi Normal University, Nanchang, Jiangxi 330022 https://doi.org/10.31635/ccschem.021.202101575 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The similarities in molecular size and physical properties of acetylene (C2H2) and carbon dioxide (CO2) produce a formidable challenge for their separation. Herein, we report two isoreticular fluorinated metal–organic frameworks (MOFs), labeled as JXNU-11(Fe2M) (M = Ni and Co), featuring unique octahedral cages encapsulated by cuboctahedral cages. JXNU-11(Fe2Ni) shows a record high C2H2-capture amount of 4.8 mmol g−1 and a long C2H2/CO2 breakthrough interval time of 55 min g−1 in an actual breakthrough experiment based on the equimolar C2H2/CO2 mixture under ambient conditions, indicating a high-performance material for C2H2 capture and C2H2/CO2 separation. Computational simulations revealed that the nanosized double-shell cages decorated with abundant fluorine and carboxylate oxygen atoms afford the optimized pore spaces for preferentially trapping C2H2, which account for the remarkable C2H2-capture capacity and the highly efficient C2H2/CO2 separation for JXNU-11(Fe2M). This work not only develops a design strategy for building the cage-in-cage structures in MOFs, but also provides universal guidance on designing porous MOFs with highly polarized and fluorophilic pore cages to capture C2H2. Download figure Download PowerPoint Introduction Acetylene (C2H2), the simplest alkyne, is an essential chemical for many common chemical products, such as acetaldehyde, acetic acid, plastic, and synthetic rubber,1 and electronic materials in the electronics industry.2 Additionally, C2H2 is an important flammable gas widely utilized for welding and metal cutting. The production of C2H2 from the oxidative coupling of natural gas is a commonly used preparation method for C2H2 gas in the petrochemical industry.3 Such a C2H2 preparation process leads to a small amount of carbon dioxide (CO2) in the C2H2 product. The presence of CO2 in C2H2 starting material has a seriously negative effect on subsequent preparation processes for the down-stream chemical products. Thus separation of C2H2/CO2 mixtures is vital in the petrochemical industry. However, the similar molecular sizes, as well as the same kinetic diameter (3.3 Å) of the linear-shaped C2H2 and CO2 molecules4–9 ( Supporting Information Table S1), cause the C2H2/CO2 separation to be a formidable task. Organic solvent extraction and cryogenic distillation are the current technologies for purifying C2H2 gas but are energy-intensive and environmentally unfriendly. Therefore, it is important to develop a highly energy-efficient and environment-friendly technology for achieving C2H2/CO2 separation. Metal–organic frameworks (MOFs) with porous and crystalline structures are fascinating porous solid materials.5–14 MOFs show highly promising potential in gaseous mixture separation due to their designable frameworks' structures and tunable pore environments.15–19 Recently, many MOFs have been utilized for the separation of C2H2/CO2 mixtures,20–29 and a few of them exhibit high-performance C2H2/CO2 separation.30–34 In the column breakthrough separation of binary gases, the breakthrough time and the captured amount for the late eluted adsorbate in the column are the important parameters for evaluating the separation efficiency of an adsorbent. The long breakthrough time for the late eluted gas generally results in a large captured amount for the late eluted gas, leading to a high separation efficiency of the adsorbent. However, in practice breakthrough experiments based on the equimolar C2H2/CO2 mixture, C2H2 breakthrough time for MOFs is generally shorter than 50 min g−1 under ambient conditions.35–40 The top-performing MOFs are SNNU-45 and ZJU-74a with the C2H2 breakthrough times of 113 and 81 min g−1,41,42 respectively. In contrast, the C2H2-captured amounts for MOFs in the packed columns are commonly less than 3 mmol g−1. SNNU-45 and ZJU-74a have the highest C2H2-capture amounts of 3.5 and 3.64 mmol g−1, respectively. It has been well demonstrated that the fluorine atoms of the inorganic anions, including MF62− (M = Si4+ and Ti4+) and NbOF52,− in the hybrid MOFs are the preferential bind sites for the acidic hydrogen atoms of the alkyne.43–47 The multiple C–H···F interactions between the C2H2 molecule and inorganic MF62− or NbOF52− groups of MOFs have resulted in an efficient C2H2/CO2 separation.47–49 Thus the fluorinated MOFs are a highly appealing platform for C2H2/CO2 separation. Unfortunately, due to the synthetic inaccessibility of the suitable fluorinated organic ligands, the fluorinated MOFs have thus been largely unexplored.50–54 Herein, we present two isostructural fluorous MOFs [termed as JXNU-11(Fe2M), M represents the divalent metal ions of Ni2+ and Co2+] with a fluorinated organic linker, featuring unique cage-in-cage structures. Remarkably, JXNU-11(Fe2Ni) shows a record-high C2H2-capture amount and a long C2H2 breakthrough time in the practice breakthrough experiment, resulting in high-performance C2H2/CO2 separation. The double-shell cages possessing highly electronegative F and O atoms in JXNU-11(Fe2M) are suitable pore cages to match the size of C2H2. The C2H2 molecules are trapped in the highly fluorophilic and polarized double-shell cages through strong host–guest interactions, which are responsible for the highly efficient C2H2/CO2 separation by JXNU-11(Fe2M). Experimental Section Synthesis of JXNU-11(Fe2Ni) A mixture of FeCl3·6H2O (8.10 mg, 0.03 mmol), Ni(NO3)3·6H2O (4.36 mg, 0.015 mmol), 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate acid (4.71 mg, 0.015 mmol), 1,3,5-tris(4-carboxyphenyl)benzene (6.58 mg, 0.015 mmol), N,N-dimethylformamide (DMF) (2 mL), and CH3COOH (0.19 mL) was capped in a 20 mL vial and heated at 100 °C for 2 days. After cooling to room temperature, brown triangle-shaped crystals were obtained. Elemental analysis for {[Fe2Ni(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]·6DMF·2.5H2O}n (C68H77F4O24.5N6Fe2Ni: 1616.74). Calcd/found: H, 4.80/4.71; C, 50.51/50.41; N, 5.19/5.02. IR data (KBr, cm−1): 3427 (m), 1662 (s), 1615 (s), 1394 (s), 1253 (w), 1185 (w), 1147 (w), 1098 (w), 1035 (s), 858 (s), 807 (w), 781 (s), 683 (w), 663 (w), 589 (m), 498 (m), 443 (s). Synthesis of JXNU-11(Fe2Co) A mixture of FeCl3·6H2O (8.10 mg, 0.03 mmol), Co(NO3)3·6H2O (4.37 mg, 0.015 mmol), 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate acid (4.71 mg, 0.015 mmol), 1,3,5-tris(4-carboxyphenyl)benzene (6.58 mg, 0.015 mmol), DMF (2 mL), and CH3COOH (0.17 mL) was capped in a 20 mL vial and heated at 100 °C for 2 days. After cooling to room temperature, brown triangle-shaped crystals were obtained. Elemental analysis for {[Fe2Co(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]·4DMF·H2O}n (C62H60F4O21N4Fe2Co: 1443.77). Calcd/found: H, 4.18/4.21; C, 51.57/51.46; N, 3.88/3.92. IR data (KBr, cm−1): 3420 (m), 1655 (s), 1612 (s), 1394 (s), 1254 (w), 1185 (w), 1146 (w), 1098 (w), 1034 (s), 859 (s), 808 (w), 781 (s), 706 (w), 663 (w), 589 (m), 500 (m), 443 (s). Methods X-ray single-crystal diffraction experiments were carried out with a Rigaku Oxford SuperNova diffractometer, and powder X-ray diffraction patterns were recorded on a Rigaku DMax 2500 powder diffractometer. Gas sorption–desorption isotherms were measured on a Micromeritics ASAP 2020 HD88 adsorption analyzer. Breakthrough experiments for separation of C2H2/CO2 (v/v, 50/50) were carried out in a fixed bed with a gas chromatograph detection system. The detailed experimental methods are provided in Supporting Information. Results and Discussion Compounds JXNU-11(Fe2M) constructed from the oxygen-centered heterometallic trimeric [Fe2M(μ3-O)(COO)6] (M = Ni2+ and Co2+) clusters (Figure 1a) were prepared. Compound JXNU-11(Fe2M) formulated as [Fe2M(μ3-O)(TFBPDC)(BTB)4/3(H2O)3]n is based on the linear 3,3′,5,5′-tetrakis(fluoro)biphenyl-4,4′-dicarboxylate (TFBPDC2−) and the triangular 1,3,5-tris(4-carboxyphenyl)benzene (BTB3−) ligands (Figure 1b) and was characterized by single-crystal X-ray diffraction ( Supporting Information Table S2). The phase purity of the bulk samples was confirmed by powder X-ray diffraction ( Supporting Information Figure S2). The metal concentrations in both compounds were determined by inductively coupled plasma atomic emission spectroscopy ( Supporting Information Table S3). The two compounds are isostructural and crystallize in a trigonal R3m space group. The trimeric [Fe2M(μ3-O)] core is coordinated by two TFBPDC2−, four BTB3−, and three water ligands ( Supporting Information Figure S1). Each [Fe2M(μ3-O)(COO)6] cluster is linked by four BTB3− ligands and two TFBPDC2− ligands to generate a three-dimensional (3D) framework. Exploration of the structure indicates the 3D framework contains two kinds of cages: a small octahedral cage formed from six [Fe2M(μ3-O)(COO)6] clusters, three TFBPDC2−, and four BTB3− ligands with a diameter of ∼12 Å (Figure 1c), and a large cage composed of 12 [Fe2M(μ3-O)(COO)6] clusters, six TFBPDC2−, and ten BTB3− ligands with a diameter of ∼26 Å, which can be described as a cuboctahedron (Figure 1d). Thus the 3D framework has close packing of the octahedral and cuboctahedral cages (Figure 1e). Such large voids in the cages in a single 3D framework lead to the generation of a twofold interpenetrated network (Figure 1f). It is remarkable that each octahedral cage is encapsulated by a large cuboctahedral cage from the interpenetrating network (Figure 1g). Such a unique cage-in-cage structure in a MOF is distinctive. The double-shell nested cages are reminiscent of the interesting Russian-doll-like cage. In addition, because of the presence of the highly polar C–F and C–O bonds and dense F atoms on the pore surfaces for these cages, double-shell nested cages exhibit highly polarized and fluorophilic character. As shown in Figure 1g, part of the voids in the large cuboctahedral cage are occupied by the small octahedral cage. The remaining pores of the total 3D framework are filled by the disordered guest solvent molecules, which occupy 64.1% and 65.6% of the volumes of unit cell for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively. JXNU-11(Fe2M) materials exhibit excellent chemical stability after exposure to air for a long time or immersion in water with pH values ranging from 3 to 11 for 24 h ( Supporting Information Figure S3). The solvent-free JXNU-11(Fe2M) was obtained through solvent-exchange and heat under vacuum ( Supporting Information Figure S4). Figure 1 | Structures of JXNU-11(Fe2M). (a) [Fe2M(μ3-O)(COO)6] unit and (b) TFBPDC2− and BTB3− ligands. (c) 3D framework of JXNU-11(Fe2M). (d) Octahedral and (e) cuboctahedral cages. (f) Twofold interpenetrating frameworks of JXNU-11(Fe2M). (g) Cage-in-cage structure in JXNU-11(Fe2M). The fluorine atoms in (b–d) are represented as green balls. Download figure Download PowerPoint The permanent porosity of JXNU-11(Fe2M) was confirmed by N2 adsorption isotherms at 77 K. The N2 sorption isotherms for both compounds have typical type I sorption behavior with the saturation sorption amounts of 561 and 556 cm3 g−1 (Figure 2a). The pore volumes for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) from the experimental N2 data are 0.87 and 0.86 cm3 g−1, respectively, which are close to the corresponding crystal structure calculated values of 0.87 and 0.89 cm3 g−1. Additionally, the pore sizes are mainly around 6.8 and 11 Å (Figure 2a, inset), in good agreement with the obtained values from the crystal structures. The Brunauer–Emmett–Teller surface areas for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) were calculated to be 2208 m2 g−1 (Langmuir surface area of 2418 m2 g−1) and 2122 m2 g−1 (Langmuir surface area of 2392 m2 g−1) ( Supporting Information Figure S5), respectively. Figure 2 | Adsorption data of JXNU-11(Fe2M). (a) N2 adsorption–desorption isotherms at 77 K and pore size distribution for JXNU-11(Fe2M). C2H2 and CO2 adsorption–desorption isotherms of JXNU-11(Fe2M) at 273 K (b) and 298 K (c). Download figure Download PowerPoint The C2H2 and CO2 sorption isotherms of JXNU-11(Fe2M) (Figures 2b and 2c) were collected to evaluate the gas separation potentials for JXNU-11(Fe2M). JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) exhibit high C2H2 adsorption capacities of 191 and 180 cm3 g−1 at 273 K and 1 bar, respectively, which are higher than the best-performance MOFs, including FJU-6-TATB (160 cm3 g−1),55 ATU-Cu (134 cm3 g−1),33 ZJU-74a (107 cm3 g−1),42 JXNU-10(Y) (94.9 cm3 g−1),27 CuI@UIO-66-(COOH)2 (71 cm3 g−1),56 and MOF-OH (68.7 cm3 g−1).57 At 298 K, the adsorption amounts of C2H2 are 118 and 107 cm3 g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively, which are larger than those of the prominent MOFs FeNi-M′MOF (96 cm3 g−1),58 JNU-1 (63 cm3 g−1),59 NKMOF-1-Ni (61 cm3 g−1),60 UTSA-300a (69 cm3 g−1),47 and CPL-NH2 (41 cm3 g−1).39 In contrast, JXNU-11(Fe2M) showed low CO2 uptakes (Figures 2b and 2c). The high uptake of C2H2 is mainly caused by the polarized and fluorophilic microporous environments made of the cage-in-cage structures that are decorated with plenty of highly electronegative fluorine and carboxylate oxygen atoms that have high affinity for the acidic H atoms of C2H2 molecules but low affinity for CO2 molecules with two terminal electronegative O atoms. Such distinct gas sorption behaviors were confirmed by grand canonical Monte Carlo (GCMC) simulations. As depicted in Figures 3a and 3b, the distribution of C2H2 molecules in JXNU-11(Fe2M) is mainly within the octahedral cage and the space between the two cages, whereas much fewer CO2 molecules were distributed within the double-shell nest. Such a phenomenon suggests the double-shell nest is the desirable space for accommodating C2H2 molecules, in agreement with the experimental finding. The linear adsorption isotherms for CO2 further suggest the low affinity to CO2 for the host frameworks. The different adsorption behaviors between C2H2 and CO2 were also evidenced by the isosteric heats of adsorption (Qst) ( Supporting Information Figure S6). The obtained Qst of C2H2 were in the range of 31.6–29.7 kJ mol−1 for JXNU-11(Fe2M), which are notably higher than those of CO2 (16.0–19.6 kJ mol−1), further reflecting the strong affinity toward C2H2. The present Qst(C2H2) values are comparable to those of FJU-6-TATB (29 kJ mol−1),55 UTSA-74 (31 kJ mol−1),37 and JXNU-5a (32.9 kJ mol−1),35 but lower than those of CuI@UIO-66-(COOH)2 (74.5 kJ mol−1),56 JNU-1 (47.6 kJ mol−1),59 and NKMOF-1-Ni (53.9 kJ mol−1).60 The evident difference in adsorption enthalpy endows JXNU-11(Fe2M) with the thermodynamic separation possibility of a C2H2/CO2 mixture. Furthermore, JXNU-11(Fe2M) retained C2H2 storage ability after adsorption/desorption cycling experiments ( Supporting Information Figure S7), confirming their excellent recyclability for C2H2 adsorption. Figure 3 | Grand canonical Monte Carlo (GCMC) adsorption simulation. Computational C2H2 (a) and CO2 (b) distribution in the double-shell nest in JXNU-11(Fe2M) at 298 K and 1 atm. Download figure Download PowerPoint To evaluate the separation performance of JXNU-11(Fe2M), ideal adsorbed solution theory was used to calculate the separation selectivity of the C2H2/CO2 (v∶v = 50∶50) mixture. At 1 bar and 298 K, the adsorption selectivities for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) are 2.7 and 2.5, respectively ( Supporting Information Figure S8), which are comparable to the leading MOFs of UPC-200(Fe)-F-H2O (2.25)32 and FJU-6-TATB (3.1).55 The C2H2/CO2 uptake ratios for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) at 298 K and 1 bar are 2.1 and 1.9, respectively, which are larger than those of noted MOFs FJU-90a (1.75),36 SIFSIX-Cu-TPA (1.7),49 and ATU-Cu (1.2).33The high C2H2 adsorption capacity and moderate selectivity of JXNU-11(Fe2M) at ambient temperature reveal the greatly promising potential for C2H2/CO2 separation. To further investigate the C2H2/CO2 separation performance of JXNU-11(Fe2M), the breakthrough experiments were performed in practical separation processes for a C2H2/CO2 mixture (v∶v = 50∶50) at 298 K and 1 atm. The breakthrough experimental results show that CO2 was eluted first from the packed column, whereas C2H2 was retained in the bed for more than 120 and 108 min g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co) (Figure 4a), respectively. The C2H2 breakthrough times for JXNU-11(Fe2M) rank among the leading MOFs ( Supporting Information Table S4). Moreover, the significant roll-up phenomenon of CO2 was observed in the breakthrough experiments, indicating a large proportion of previously adsorbed CO2 molecules can be replaced by the later fed C2H2 molecules. Such a phenomenon means that the C2H2 molecule competes better than CO2 for the binding sites of JXNU-11(Fe2M), highlighting the excellent separation efficiency of JXNU-11(Fe2M) for a C2H2/CO2 mixture. The breakthrough experiments were terminated when the concentrations of effluent gases stabilized. Figure 4 | C2H2 and CO2 separation performances. (a) Breakthrough curves of JXNU-11(Fe2M) at the flow of 2 mL min−1, 298 K, and 1 atm. Breakthrough curves of JXNU-11(Fe2Ni) at different temperatures (b) and different total flow rates at 298 K (c). Download figure Download PowerPoint The captured amounts for C2H2 during the 0∼tbreak time under the dynamic conditions are 4.8 and 4.3 mmol g−1 for JXNU-11(Fe2Ni) and JXNU-11(Fe2Co), respectively. The C2H2 captured amounts are remarkable and outperform all other MOFs, including the top-performing ZJU-74a (3.64 mmol g−1),42 SNNU-45 (3.5 mmol g−1),41 BSF-3 (2.9 mmol g−1),38 JCM-1 (2.2 mmol g−1),61 and FJU-90a (1.87 mmol g−1)36 (Figure 5 and Supporting Information Table S4). In addition, a long interval time of 55 min g−1 between C2H2 and CO2 breakthrough for JXNU-10(Fe2Ni) was obtained. Such an interval time stands out amongst MOFs and is only shorter than those of SNNU-45 (79 min g−1)41 and SIFSIX-Cu-TPA (69 min g−1).49 Furthermore, their practical separation potentials under different temperatures were evaluated. The C2H2 breakthrough times increased significantly with a decrease of temperature (Figure 4b and Supporting information Figure S9). The breakthrough times of C2H2 reached 146 (283 K) and 165 min g−1 (273 K) with a gas flow of 2 mL min−1. Figure 5 | Experimental breakthrough performance. Comparison of MOFs with top-high C2H2/CO2 breakthrough performance at 298 K and 1 atm. Download figure Download PowerPoint The separation performance of JXNU-11(Fe2M) with different gas flow rates at 298 K were also studied. With the gas flow of 2 mL min−1, the longest C2H2 breakthrough time and the largest amount of the adsorbed C2H2 were achieved for JXNU-11(Fe2M) (Figure 4c and Supporting Information Figure S10). JXNU-11(Fe2M) exhibited no appreciable changes in breakthrough times after three cycles of dynamic breakthrough experiments ( Supporting Information Figure S11), indicative of an excellent recycling separation capability. With the advantage of the modest adsorption heat of C2H2, JXNU-11(Fe2M) materials can be easily regenerated through purging the column with He gas at ambient temperature, as evidenced by the overlapped breakthrough curves ( Supporting Information Figure S11). Such results further indicate JXNU-11(Fe2M) are highly desirable microporous materials for C2H2/CO2 separation. Finally, the desorption experiments were carried out on JXNU-11(Fe2Ni). After reaching the breakthrough equilibrium, the adsorption column was purged with He gas (flow rate: 4 mL min−1) under ambient pressure and 303 K. As depicted in Figure 6a, the adsorbed CO2 molecules were released from the adsorption bed quickly. In contrast, the desorption of C2H2 gas was much slower. The productivity of C2H2 with over 95% purity was estimated from the desorption curve to be 1.98 mmol for 1 g of JXNU-11(Fe2Ni) (Figure 6b). Figure 6 | Breakthrough experiments. (a) and (b) Breakthrough curves and desorption curves of JXNU-11(Fe2Ni) based on the equimolar C2H2/CO2 mixture. Download figure Download PowerPoint To fully understand the underlying mechanism of the preferential adsorption of C2H2 over CO2 in JXNU-11(Fe2M), we performed comprehensive GCMC simulations to investigate the gas adsorption sites for JXNU-11(Fe2M). The calculated preferential binding sites of C2H2 are shown in Figures 7a and 7b. Four primary binding sites (sites I–IV) for C2H2 were found. As expected, the primary binding sites for C2H2 molecules are F and carboxylate O sites. For site I, site II, and site III, strong C−H···F hydrogen bonding notably occur between C2H2 and F atoms (H···F = 2.49–3.01 Å), which confirms the strong binding affinity of JXNU-11(Fe2M) toward C2H2. Compared with these sites, site IV exhibits strong C−H⋯O hydrogen bonds with a pair of carboxylate O atoms with the distances of 2.61 and 2.60 Å. These H⋯F and H⋯O distances are shorter than the sums of the corresponding van der Waals radii of hydrogen and fluorine (2.67 Å) or hydrogen and oxygen (2.72 Å) atoms, suggesting the substantial interactions. Moreover, the C≡C group of C2H2 interacts with the adjacent open metal sites through π⋯M interactions, and weakly interacts with the neighboring aromatic ring units of the framework through π⋯π interactions. Thus these multiple interactions between C2H2 molecules and the framework synergistically result in the remarkable affinity for C2H2. Therefore, the nanosized double-shell nests with many highly electronegative F and O atoms provide the polarized and fluorophilic pore spaces to accommodate C2H2. In contrast, the negatively charged F and O sites are not desirable sites for CO2. The presence of the repulsive interactions between the F/O atoms and the O atoms of CO2 leads to a weaker interaction between CO2 and the framework, which is consistent with the experimental results. The computational results gave an average binding energy of 31.8 kJ mol−1 for C2H2, which is much higher than that of 20.9 kJ mol−1 for CO2, further corroborating the stronger affinity toward C2H2 in the present framework. These calculated results clearly reveal that the rich F and O atoms as well as the open metal sites in the cage-in-cage structures are synergistically responsible for the preferential adsorption of C2H2 over CO2, thus affording the top-level C2H2/CO2 separation performance of JXNU-11(Fe2M). Figure 7 | Mechanism study. adsorption sites of C2H2 in JXNU-11(Fe2M). (a) I and (b) and Ni F and O in and H in C2H2. The labeled is measured in Å. Download figure Download PowerPoint two MOFs M = Ni and featuring nanosized structures and highly polarized and fluorophilic pore Such a cage-in-cage structure formed is composed of an and an large The double-shell nested cages in the MOFs provide the fascinating and the optimized pore spaces for trapping C2H2. The abundant F and O atoms of the organic ligands in JXNU-11(Fe2M) JXNU-11(Fe2M) with multiple binding sites for C2H2. The F and O atoms possessing high in JXNU-11(Fe2M) the pore surfaces of the cages result in the highly dense on the pore which are the desirable sites for trapping C2H2 with charged H atoms. As a JXNU-11(Fe2M) shows a strong binding affinity to C2H2 over CO2. The nanosized double-shell cages in JXNU-11(Fe2M) afford the optimized pore spaces for trapping C2H2, leading to an efficient C2H2/CO2 separation. The practical breakthrough performance of JXNU-11(Fe2Ni) the breakthrough time for C2H2 gas and the C2H2-capture the highest observed at ambient confirming the C2H2/CO2 separation performance of JXNU-11(Fe2M) Thus we a strategy for the design of cage-in-cage structures in MOFs and develop an efficient for the of the highly polarized and fluorophilic pore cages in the fluorous Supporting Information Supporting Information is and detailed experimental crystal data for JXNU-11(Fe2M), adsorption column breakthrough experiments, and GCMC simulation. of is no of to Information This work is by the of and and the for and of of Jiangxi of and of Acetylene and by the of K. Chemistry and of Metal–Organic and of Frameworks to MOF and on Adsorption in a as the in the of the Chemistry of and Metal–Organic Frameworks and Zhang of Frameworks Featuring Separation Metal–Organic Frameworks and for of Surfaces by in Synthesis of in a

Clarification of the molecular mechanism underlying the interaction of coal with CH4, CO2, and H2O molecules is the basis for an in-depth understanding of the states of fluid in coal and fluid-induced coal swelling/contraction. In terms of instrumental analysis, molecular simulation technology based on molecular mechanics/dynamics and quantum chemistry is a powerful tool for revealing the relationship between the structure and properties of a substance and understanding the interaction mechanisms of physical-chemical systems. In this study, the giant canonical ensemble Monte Carlo (GCMC) and molecular dynamics (MD) methods were applied to investigate the adsorption behavior of a Yanzhou coal model (C222H185N3O17S5). We explored the adsorption amounts of CH4, CO2, and H2O onto Yanzhou coal, the adsorption conformation, and the impact of oxygen-containing functional groups. Furthermore, we revealed the different adsorption mechanisms of the three substances using isosteric heat of adsorption and energy change data. (1) The adsorption isotherms of the mono-component CH4, CO2, and H2O were consistent with the Langmuir model, and their adsorption amounts showed an order of CH4 CO2>CH4. In addition, at higher temperatures, the isosteric heat of adsorption decreased; pressure had no significant effect on the heat of adsorption. (3) CH4 molecules displayed an aggregated distribution in the pores, whereas CO2 molecules were cross arranged in pairs. Regarding H2O molecules, under the influence of hydrogen bonds, the O atom pointed to surrounding H2O molecules or the H atoms of coal molecules in a regular pattern. The intermolecular distances of the three substances were 0.421, 0.553, and 0.290 nm, respectively. The radial distribution function (RDF) analysis showed that H2O molecules were arranged in the most compact fashion, forming a tight molecular layer. (4) H2O molecules showed a significantly stratified distribution around oxygen-containing functional groups on the coal surface, and the bonding strength showed a descending order of hydroxyl> carboxyl>carbonyl. In contrast, CO2 and CH4 showed only slightly stratified distributions. (5) After the adsorption of CH4, CO2, and H2O, the total energy, the energy of valence electrons, and the non-bonding interaction of the system in the Yanzhou coal model all decreased. The results regarding the decrease in the total energy of the system indicated an order of H2O>CO2>CH4 in terms of the adsorption priority of the Yanzhou coal model. The results regarding the decrease in the energy of valence electrons showed that under certain geological conditions, a pressure-induced “coal strain” could lead to a structural rearrangement during the interaction of coal with fluid to form a more stable conformation, which might be the molecular mechanism of coal swelling resulting from the interaction between fluid and coal. An analysis of the contribution of Van der Waals forces, electrostatic interactions and hydrogen bonds to the decrease in non-bonding interactions revealed the mechanism underlying the interactions between coal molecules and the three substances. The interaction between coal molecules and CH4 consisted of typical physical adsorption, whereas that between coal molecules and CO2 consisted mainly of physical adsorption combined with weak chemical adsorption. The interaction between coal molecules and H2O is physical and chemical.

Insight into the adsorption mechanisms of CH4, CO2, and H2O molecules on illite (001) surfaces: A first-principles study

Molecular dynamics simulation of hydrocarbon molecule adsorption on kaolinite (0 0 1) surface

Molecular simulation of CH4 and CO2 adsorption behavior in coal physicochemical structure model and its control mechanism

We researched the coadsorption of CO and CH4 molecule on the most stable structure of metal atom (Ni, Ag, Au, Rh, Zn, Pt) doped SnO2 (110) surface with the first principle methods. The formation energy results show that the Au/SnO2 (110) surface is the most stable structure. The adsorption energy, bond length, bond angle, density of states, electron population and charge density difference of gas molecule adsorbed on Au/SnO2 (110) surface are researched, which shows that Au/SnO2 (110) surface have excellent adsorption performance to CO and CH4 molecule. The stable adsorption of double CO on Au/SnO2 (110) surface shows that it has practical value. The research of CO and CH4 coadsorption on Au/SnO2 (110) surface shows that the Au/SnO2 (110) surface has stronger adsorption properties to CO than CH4 molecule. Our research indicates that Au/SnO2 is a potential CO sensor material.

The interaction of small gas molecules as H2, N2, CO, H2S, NH3, SO2 and CH4 on Li-functionalized graphitic carbon nitride was investigated by using the density functional theory, to explore their gas adsorption properties. The calculated Eads values of all gas molecules on [Li g-C3N4]+ show that these gas molecules present favourable interaction with the lithium atom coordinated on the sheet. NH3 and SO2 molecules present strong interactions, with Eads values of −18.60 and −9.50 kcal mol−1. The natural bond orbital analysis indicates that donor orbitals belong to the lone pairs of oxygen, nitrogen, sulphur and carbon atoms from SO2, N2, NH3, H2S, CO molecules, and acceptor orbitals (LP*) from the lithium atom. Computational studies suggest that H2, N2, CO, H2S, NH3, SO2 and CH4 molecules on [Li g-C3N4]+ present physisorption.

To investigate the adsorption mechanism of CO2 and CH4 with oxygen-containing functional groups in coal, the structural models of coal with different functional groups of carboxyl, hydroxyl, carbonyl, and ether bonds were constructed using Materials Studio software, and the adsorption isotherms, heat of adsorption, adsorption energy, energy distribution, and adsorption configurations of CO2 and CH4 adsorbed by different functional group structures were analyzed using the giant regular GCMC method. The results showed that the presence of carboxyl, hydroxyl, and carbonyl functional groups will promote the adsorption of CO2 by coal, and the intermolecular interaction energies of the model with the addition of carboxyl, hydroxyl, and carbonyl groups, and CO2 increased by 70.37%, 33.67%, and 14.26%, respectively. The order of the adsorption strength of the five structures is: AC-COOH > AC-OH > AC-O > AC > AC-OCH3, which indicates that the influence of oxygen-containing functional groups on the adsorption performance of CO2 by coal mainly depends on polarity. The presence of carboxyl, hydroxyl, and carbonyl functional groups weakens the adsorption of CH4 on coal, and the interaction energy between the model and gas molecules with the addition of carboxyl, hydroxyl, carbonyl, and ether bonds reduced by 41.83%, 36.14%, 26.22%, and 80.21%, respectively. The order of the adsorption strength of the five structures is: AC > AC-O > AC-OH > AC-COOH > AC-OCH3, which indicates that the effect of oxygen-containing functional groups on the adsorption of CH4 on coal mainly depends on hydrophobicity; the introduction of ether-bonded functional groups does not favor the adsorption of CO2 and CH4 molecules on coal. It indicates that the ether bond has a certain repulsive effect on CO2 and CH4 molecules. In different functionalized coal structures with the same carbon atom skeleton, there are different adsorption capacities for CH4 as well as CO2, but in the same functionalized structure, the adsorption magnitude relationship is CO2 > CH4, indicating that the coal body structure interacts with CO2 more than CH4.

Inspired by the recent use of two-dimensional nanomaterials as gas sensors, we used density functional theory calculations to study the adsorption of gas molecules (CH4, CO and H2O) on sandwich vanadium dioxide tablets. The results showed that of all these gases, only the CH4 gas molecule was the electron acceptor with significant charge transfer on the VO2 sheet. The adsorption energies of CH4, CO and H2O are −229.5 meV, −239.1 meV and −388.3 meV, respectively. We have also compared the adsorption energy of three different gas molecules on the VO2 surface, our calculation results show that when the three kinds of gases are adsorbed on the VO2 surface, the order of the surface adsorption energy is H2O > CO > CH4. It is also found that after adsorption of CH4, CO and H2O molecules, the electronic properties of VO2 sheet changed obviously. However, due to the strong adsorption of H2O molecule on VO2 sheet, it is difficult to desorption, which hinders its application in gas molecular sensors. The optical properties of VO2 sheet are further calculated. The absorption of CH4, CO and H2O molecules is introduced to red-shift the dielectric function of the thin film, which indicates that the optical properties of the thin film have changed significantly. According to the change of optical properties of VO2 sheet before and after molecular adsorption, VO2 can be used as a highly selective optical gas sensor for CH4, CO and H2O detection. These results provide a new approach for the potential application of VO2 based optical gas sensors.

The adsorption of CO2 and CH4 on graphitic carbon nitride (g-C3N4) functionalized with four boron atoms (g-C43N4B4) were investigated by using the density functional theory for adsorption and separation of the CO2/CH gas molecules. The calculated Eads values showed that the CO2 molecule, in comparison with the CH4 molecule, has favorable interaction with the boron cluster hosted in the sheet. The capacity of adsorption of the g-C3N4B4 complex with other small gas molecules (C2H4, H2, NH3, and SO2) was also studied observing chemisorption for the H2 molecule. The computational studies revealed physisorption and selectivity of CO2 molecule over CH4 on the sheet. This material could serve as promising adsorbent of CO2 and for natural gas purification.

Ab-initio investigation of adsorption of CO and CO2 molecules on graphene: Role of intrinsic defects on gas sensing

The interactions between boron carbide (BC) nanocluster of B16C16 and H2O, NO2, CO, and CH4 small molecules were investigated by using density functional theory (DFT) computations to exploit the structural and electronic properties of the adsorbate/cluster complexes. The calculated adsorption energies of the most stable states are -16.6, -0.17, -1.28, -0.18 eV for NO2, CO, H2O, and CH4 molecules, respectively. Meanwhile, the interactions between CO and CH4 molecules and the cluster induce dramatic changes to the cluster electronic properties so that the molecular orbital (HOMO/LUMO) gap of cluster decreased its original value. It was shown that the phenomenon leads to an increment in the electrical conductivity of the cluster at a definite temperature. Furthermore, it is revealed that the adsorptions of NO2 and H2O molecules have no significant effects on the electronic properties of the cluster. Thus, this work suggests that the investigated B16C16 nano-cage could work as a selective gas sensor device towards CO, CH4, NO2 and H2O molecules.