Adsorption Site Selective Occupation Strategy within a Metal-Organic Framework for Highly Efficient Sieving Acetylene from Carbon Dioxide.

The separation of acetylene and carbon dioxide is an essential but challenging process owing to the similar molecular sizes and physical properties of the two gas molecules. Notably, these molecules usually exhibit different orientations in the pore channel. We report an adsorption site selective occupation strategy by taking advantage of differences in orientation to sieve the C2H2 from CO2 in a judiciously designed amine‐functionalized metal–organic framework, termed CPL‐1‐NH2. In this material, the incorporation of amino groups not only occupies the adsorption sites of CO2 molecules and shields the interaction of uncoordinated oxygen atom and CO2 molecules resulting in a negligible adsorption amount and a decrease in enthalpy of adsorption but also strengthened the binding affinity toward C2H2 molecules. This material thus shows an extremely high amount of C2H2 at low pressure and a remarkably high C2H2/CO2 IAST selectivity (119) at 1 bar and 298 K.

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

An All-Purpose Porous Cleaner for Acid Gas Removal and Dehydration of Natural Gas

Porous Metal-Organic Frameworks: Promising Materials for Methane Storage

Metal-organicframeworks (MOFs) are promising gas adsorbents. Knowledgeof the behavior of gas molecules adsorbed inside MOFs is crucial foradvancing MOFs as gas capture materials. However, their behavior isnot always well understood. In this work, carbon dioxide (CO2) adsorption in the microporous α-Zn3(HCOO)6 MOF was investigated. The behavior of the CO2 moleculesinside the MOF was comprehensively studied by a combination of single-crystalX-ray diffraction (SCXRD) and multinuclear solid-state magnetic resonancespectroscopy. The locations of CO2 molecules adsorbed insidethe channels of the framework were accurately determined using SCXRD,and the framework hydrogens from the formate linkers were found toact as adsorption sites. 67Zn solid-state NMR (SSNMR) resultssuggest that CO2 adsorption does not significantly affectthe metal center environment. Variable-temperature 13CSSNMR experiments were performed to quantitatively examine guest dynamics.The results indicate that CO2 molecules adsorbed insidethe MOF channel undergo two types of anisotropic motions: a localizedrotation (or wobbling) upon the adsorption site and a twofold hoppingbetween adjacent sites located along the MOF channel. Interestingly, 13C SSNMR spectroscopy targeting adsorbed CO2 revealsnegative thermal expansion (NTE) of the framework as the temperaturerose past ca. 293 K. A comparative study shows that carbon monoxide(CO) adsorption does not induce framework shrinkage at high temperatures,suggesting that the NTE effect is guest-specific.

CO2 is the main greenhouse gas emitted from the combustion of fossil fuels and is considered a threat in the context of global warming. Carbon capture and storage (CCS) schemes embody a group of technologies for the capture of CO2 from power plants, followed by compression, transport, and permanent storage. Key advances in recent years include the further development of new types of porous materials with high affinity and selectivity toward CO2 for optimizing the energy penalty of capture. In this regard, microporous metal-organic frameworks (MOFs) represent an opportunity to create next-generation materials that are optimized for real-world applications in CO2 capture. MOFs have great potential in CCS because they can store greater amounts of CO2 than other classes of porous materials, and their chemically-adjustable organic and inorganic moieties can be carefully pre-designed to be suitable for molecular recognition of CO2. Taking into account the nature of physisorption and inherent polarity of CO2 molecules, addressing materials with both a large surface area and polar pores for strong CO2 binding affinity is an effective method. Decorating the pores of MOFs with some specific functional groups by directly using functionalized organic linkers or postsynthetic modification, that have high binding affinity to CO2 molecules, is among the most promising strategies has been pursued to achieve high-performance CO2 uptake. This review highlights the literature reported on MOFs with amide-decorated pores for CO2 capture, showing the effects of amide groups on uptake capacity, selectivity and adsorption enthalpies of CO2.

Mesoporous zirconia was prepared using the sol–gel process and the EISA method. It presents a specific surface area of 90 m2·g–1 and an interparticular porosity associated with a pore diameter of around 4–5 nm, and it crystallizes in the tetragonal symmetry. The determination of the CO2 adsorption properties (both the isotherm and adsorption enthalpies) coupled with isotherm modeling using a multi-Langmuir model and density functional theory (DFT) calculations on different representative clusters evidenced that the surface of zirconia is heterogeneous from an energetic point of view. It emerges from both the experimental and the theoretical results that (i) the adsorption sites associated with the lowest enthalpies of adsorption (between −24 and −34 kJ·mol–1) represent nearly 65–70% of the total number of the adsorption sites present on the zirconia surface. They correspond to the interactions (physisorption) between carbon dioxide and oxygen atoms or hydroxyl groups of the surface. (ii) The adsorption sites associated with higher enthalpies of adsorption (around −65 kJ·mol–1) correspond to the interactions between carbon dioxide and Zr atoms; they represent around 5% of the total amount of adsorption sites. (iii) The adsorption sites associated with high enthalpies of adsorption (below −70 kJ·mol–1) represent only a small fraction of the adsorption sites (around 10%) and correspond probably to the interaction of CO2 with structural surface defects or charged sites.

The metal-organic framework [Y(tbpp)]·nDMF (1) was synthesized from yttrium(III) nitrate and the tritopic linker tris(4'-carboxy[1,1'-biphenyl]-4-yl)phosphine (H3tbpp). The distance between the coordinating atoms of the carboxylate groups of the extended tridentate phosphine linker is more than 1.8 nm, resulting in an average pore dimension of 0.9 nm in the noninterpenetrated metal-organic framework. The material exhibits high thermal stability and permanent porosity after removal of guest molecules from the one-dimensional pore system. The desolvated compound adsorbs nitrogen, argon, hydrogen, and carbon dioxide. Favorable adsorption of CO2 over N2 is predicted using ideal adsorbed solution theory (IAST). The isosteric enthalpies of adsorption of H2 and CO2 of -7 and -22 kJ mol-1, respectively, are representative for metal-organic frameworks with no accessible strong host-guest binding sites, despite the bifunctional nature of the organic ligand. The absence of strong specific adsorption sites was confirmed by in situ powder synchrotron X-ray diffraction of the reversible isobaric CO2 sorption process. Analysis of the diffraction data indicates that the CO2 molecules in the pores are disordered and nonlocalized. Despite this, it was possible to quantify the evolution of the occupation of the pores. CO2 is adsorbed at an approximately constant below 320 K from 10% loading to full capacity at 195 K.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

As a highly promising candidate for hydrogen storage, crucial to vehicles powered by fuel cells, metal–organic frameworks (MOFs) have attracted the attention of chemists in recent decades. H2 uptak...

Open AccessCCS ChemistryRESEARCH ARTICLE1 May 2021Regulating Electronic Status of Platinum Nanoparticles by Metal–Organic Frameworks for Selective Catalysis Yu Shen†, Ting Pan†, Peng Wu, Jiawei Huang, Hongfeng Li, Islam E. Khalil, Sheng Li, Bing Zheng, Jiansheng Wu, Qiang Wang, Weina Zhang, Wei David Wei and Fengwei Huo Yu Shen† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Ting Pan† Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Peng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Jiawei Huang Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 , Hongfeng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Islam E. Khalil Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Sheng Li Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Bing Zheng Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Jiansheng Wu Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Qiang Wang Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816 , Weina Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 , Wei David Wei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Center for Catalysis, University of Florida, Gainesville, FL 32611 and Fengwei Huo *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 211816 https://doi.org/10.31635/ccschem.020.202000278 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Selective hydrogenation of alkynes to alkenes remains challenging in the field of catalysis due to the ease of over-hydrogenated of alkynes to alkanes. Favorably, the incorporation of metal nanoparticles (MNPs) into metal–organic frameworks (MOFs) provides an opportunity to adjust the surface electronic properties of MNPs for selective hydrogenation of alkynes. Herein, we used different metal-O clusters of MOFs to regulate the electronic status of platinum nanoparticles (Pt NPs) toward overhydrogenation, semihydrogenation, and unhydrogenation of phenylacetylene. Specifically, Pt/Fe-O cluster-based MOFs are found to reduce the electronic density on Pt NPs and inhibit the overhydrogenation of styrene, leading to an 80% increase in selectivity toward a semihydrogenation product (styrene). Meanwhile, Cu-O cluster-based MOFs generate high oxidation states of Pt NPs and release Cu2+ ions, which worked together to deactivate Pt NPs in the hydrogenation reaction entirely. Thus, our studies illustrate the critical role of metal-O clusters in governing chemical environments within MOFs for the precise control of selective hydrogenation of alkynes, thereby, offering appealing opportunities for designing MNPs/MOFs catalysts to prompt a variety of reactions. Download figure Download PowerPoint Introduction Selective hydrogenation of alkynes to alkenes is a key transformation reaction in industrial manufacturing of fine chemicals, pharmaceuticals, polymers, and others.1–3 Achieving high selectivity of partial hydrogenation products in an economical, mild, and environmentally benign way is still a challenge because it is easy to overhydrogenate alkynes into alkanes.4,5 Heterogeneous metal catalysts are brought into the spotlight due to their high activity and stability.6 A traditional Lindlar catalyst, composed of palladium nanoparticles (Pd NPs) modified by lead ions and quinolone additives, has been used widely in the semihydrogenation industry for decades.7 However, toxic lead ions in Lindlar catalysts hamper their applications in the green chemical industry. Recently, metal oxides, organic molecule- or metal ion-modified metal nanoparticles (MNPs), multimetallic alloys, and single-atom catalysts have been developed for improving the selectivity of semihydrogenation.8–11 However, all those materials require complicated procedures to tune the electronic status of MNPs to achieve selective hydrogenation. Thus, developing new strategies to regulate the electronic status of MNPs remains a critical issue in the field of selective hydrogenation of alkynes. As an emerging porous material, metal–organic frameworks (MOFs) offer intriguing properties, including diverse organic–inorganic compositions, facile functionalization, and uniform yet tunable cavities, showing promising application prospects in gas separation, sensor platforms, heterogeneous catalysis, and so on.12–14 Besides, MOFs have been used as hosts for MNPs, and those hybrid catalysts combine both the molecular sieving effect of MOFs matrix and the high catalytic activity of MNPs.15–21 Recently, scientists found that MOFs could be used to modulate the electronic status of MNPs.22–24 For instance, Zhao et al.25 demonstrated that controlling the electron transfer in Pt/MOFs improved the catalytic selectivity for hydrogenation of α,β-unsaturated aldehydes. Also, Xiao et al.26 observed that tuning the electron transfer between platinum nanoparticles (Pt NPs) and porphyrinic MOFs allowed an increase in the surface electron densities of Pt NPs and enhanced the alcohol oxidation. Herein, we demonstrate the use of metal-O clusters within MOFs to regulate precisely the interfacial electronic status of Pt NPs for promoting the selective hydrogenation of phenylacetylene (Scheme 1). Specifically, Cr-O, Fe-O, and Cu-O cluster-based MOFs, which had similar ligands and coordination structures, were exploited to create different chemical environments for Pt NPs. We found that Pt/Fe-O cluster-based MOFs catalysts exhibited >99% conversion of phenylacetylene and ∼80% selectivity to styrene. Meanwhile, Pt/Cr-O cluster-based MOFs catalysts showed no influence on the selectivity, and thus, resulted in overhydrogenation, with the formation of ethylbenzene. Surprisingly, Cu-O cluster-based hybrid catalysts were found to lose catalytic activity totally in the hydrogenation reaction. Further, studies using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) confirmed the essential role of metal-O clusters within MOFs in modulating the electronic status of Pt NPs in promoting the activity and selectivity of alkyne hydrogenation. Scheme 1 | Precise control of selective hydrogenation of phenylacetylene using distinct metal-O cluster-based MOFs as modulators to regulate the electronic status of Pt NPs. MOFs, metal–organic frameworks; Pt NPs, platinum nanoparticles. Download figure Download PowerPoint Experimental Methods Preparation of Pt/MOFs catalysts About 30 mL as-synthesized Pt NPs solution (0.6 mM) and 50 mg MOFs powder were stirred under 500 rpm at room temperature for 6 h. Subsequently, Pt/MOFs were collected by centrifugation at 8000 rpm for 5 min and washed twice with ethanol or methanol. Finally, the obtained Pt/MOFs powder was dried at room temperature in a vacuum oven for 12 h. Catalytic hydrogenation of phenylacetylene In a typical procedure, 10 mg of the Pt NPs catalyst was dispersed in 5 mL of methanol solution, and then 100 μL phenylacetylene was added to the above solution. Subsequently, the solution was purged with a H2 balloon. During the catalytic process, the reaction solution was stirred magnetically at room temperature for the desired reaction time. After that, the catalysts were separated by centrifugation, and the solution was analyzed by gas chromatography (GC). Results and Discussion Catalysts preparation To actualize the concept while avoiding the interference of pore size within MOFs and MNPs morphology, the hybrid Pt/MOFs catalysts were rationally designed and synthesized, where presynthesized MNPs were deposited on the MOFs surface. Pt NPs with an average size of 2.8 nm were synthesized by established methods ( Supporting Information Figure S1).27 Several stable MOFs were selected as supports to explore the effect of distinct metal-O clusters on regulating the electronic status of Pt NPs, namely, Cr-based MIL-100(Cr) and MIL-101(Cr); Fe-based MIL-100(Fe), MIL-101(Fe), and MIL-88(Fe); and Cu-based MOF, HKUST-1, and MOF nanosheets, Cu-TCPP.28–30 Pt NPs were dispersed uniformly on the MOFs surface, as revealed by transmission electron microscopy (TEM) images and energy-dispersive X-ray spectroscopy (EDS) elemental mapping data (Figures 1a–1d). As shown in the large-scale TEM images from Supporting Information Figures S2–S8, no free Pt NPs were observed in the catalysts. Additionally, powder X-ray diffraction (PXRD) patterns of Pt/MOFs composites were identical to those of the corresponding simulated MOFs, indicating that MOFs maintained their original crystal structures after the deposition of Pt NPs ( Supporting Information Figures S9–S15). An inductively coupled plasma mass spectrometry (ICP-MS) measurements confirmed the similarity of Pt concentration in the composites ( Supporting Information Table S1). Figure 1 | TEM, high-angle annular dark-field scanning TEM (HAADF-STEM), and the corresponding EDS elemental mapping images of Pt NPs anchored on different MOFs supports. (a) Pt/MIL-101(Cr), (b) Pt/MIL-101(Fe), (c) Pt/MIL-88(Fe), (d) Pt/HKUST-1. TEM, transmission electron microscopy; EDS, energy-dispersive X-ray spectroscopy, Pt NPs, platinum nanoparticles; MOFs, metal–organic frameworks. Download figure Download PowerPoint Catalytic performance Various Pt/MOFs catalysts were utilized in exploring the chemoselectivity of phenylacetylene hydrogenations (Figure 2a). In a typical catalytic reaction, Pt/MIL-100(Cr) and Pt/MIL-101(Cr) with Cr-O clusters generated mostly overhydrogenation products (ethylbenzene), which were similar to the Pt NPs catalysts (Figure 2b and Supporting Information Table S2). Surprisingly, Pt/MIL-100(Fe), Pt/MIL-101(Fe), and Pt/MIL-88(Fe) with Fe-O clusters showed a high conversion (>99%) and selectivity (∼80%) toward the semihydrogenation product (styrene; Figure 2b), while suppressing the overhydrogenation of styrene to ethylbenzene (Figure 2c). No significant decay in the selectivity was noticed even when the reaction time was prolonged to 24 h ( Supporting Information Figure S16). Pt/HKUST-1 and Pt/Cu-TCPP composite of Cu-O clusters in MOFs showed no activity of phenylacetylene and styrene hydrogenations (Figures 2b and 2c). For comparison, MOFs and the corresponding metal oxide supports were also tested for the phenylacetylene and styrene hydrogenation. As shown in Supporting Information Table S2, all MOFs support exhibited no hydrogenation activity, and Pt/metal oxide catalysts showed no hydrogenation selectivity. The evaluation of Pt/MOFs catalytic performance was based on the similar Pt NPs loading (2%) and full conversion of phenylacetylene ( Supporting Information Figure S17 and Table S1). In short, Pt/MOFs catalysts showed three distinct hydrogenation results: overhydrogenation, semihydrogenation, and unhydrogenation, indicating that metal-O clusters within MOFs functioned as modulators of Pt NPs and altered the catalytic performance of phenylacetylene hydrogenation reaction in Pt NPs. Figure 2 | Performance of various Pt/MOFs catalysts for the hydrogenation reaction yielding phenylacetylene and styrene. (a) Schematic of hydrogenation of phenylacetylene. (b) The yield of phenylacetylene hydrogenation on various catalysts. (c) The yield of styrene to ethylbenzene on various catalysts. MOFs, metal–organic frameworks. Download figure Download PowerPoint Mechanistic studies We sought to gain an understanding of how metal-O clusters within MOFs affected Pt NPs' activity and selectivity in the hydrogenation of phenylacetylene by exploring the mechanism of the electronic effects and the coordination environments of Pt NPs on MOFs. The electronic properties were experimentally confirmed by XPS and XAS spectra. The Pt 4f spectra of Pt/MIL-101(Cr) and Pt/MIL-101(Fe) showed two main peaks at 71.3 ± 0.1 and 74.6 ± 0.1 eV, corresponding to Pt 4f7/2 and Pt 4f5/2, respectively (Figure 3a).31 Interestingly, an 0.3 eV shift of Pt 4f toward the high-energy side was observed on Pt/HKUST-1, revealing the difference in the electronic status of the Pt NPs supported on HKUST-1, compared with that on MIL-101(Cr) and MIL-101(Fe). Furthermore, the Pt 4f7/2 was fitted with two components, including the predominant metallic Pt0 located in the binding energy of 71.3 eV in the spectra (denoted as red peaks) and Pt2+ at 72.3 eV (denoted as blue peaks).32 Pt/MIL-101(Cr) showed 28% amount of Pt2+ species, which was similar to that of the bare Pt NPs (27%), indicating a weak electronic interaction between Pt NPs and MIL-101(Cr).25 We speculated that this weak electronic interaction was mainly due to the less overlap of the d orbitals between Pt NPs and Cr-O clusters in MIL-101(Cr).23 Notably, the Pt2+ species ratio in Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts increased to 32% and 36%, respectively, revealing that Pt NPs on these two MOFs became electron deficient, compared with the bare Pt NPs. In comparison with the Cr 2p, Fe 2p, and Cu 2p XPS spectra of MIL-101(Cr), MIL-101(Fe), HKUST-1, and Pt/MOFs composites (Figures 3b–3d), the Cr 2p states remained unchanged after the deposition of Pt NPs, whereas noticeable increases in Fe2+ species (from 61.3% to 67.7%) and Cu+ species (from 22% to 41%) were observed. These phenomena suggested that electrons from Pt NPs were transferred to Fe-O and Cu-O clusters-based MOFs across the heterogeneous interface. As shown in the normalized X-ray absorption near-edge structure (XANES) at the Pt L-edge ( Supporting Information Figure S18), Pt/HKUST-1 exhibited a higher white line intensity than Pt/MIL-101(Fe), MIL-101(Cr), and bore Pt NPs, further confirming the electron deficiency of Pt NPs in Pt/HKUST-1.31 The local coordination environment of Pt in Pt/MOFs catalysts was further characterized by extended X-ray absorption fine structure (EXAFS; Supporting Information Figure S19). The stronger peaks at 2.6 and 1.6 Å could be assigned to Pt–Pt and Pt–O coordination, respectively (Figure 3e).33 The intensity of Pt–O peaks exhibited a gradual increase upon the deposition of Pt NPs on MIL-101(Fe), and HKUST-1 supports. The quantitative EXAFS curve fitting analysis revealed that the coordination number of Pt–O bond in Pt/MIL-101(Cr) was 0.4, while in Pt/MIL-101(Fe) and Pt/HKUST-1, it increased to 0.8 and 1.2, respectively ( Supporting Information Table S3). The formation of abundant Pt–O bonds and charge-transfer interactions demonstrated that MOFs matrix shared the same function as inorganic and organic materials to regulate the surface electronic status of Pt NPs. DFT calculations further proved that the exposed metal-O clusters on MOFs could withdraw electrons from Pt atoms in the order of MIL-101(Cr) < MIL-101(Fe) < HKUST-1 and consequently increased the Bader charge of Pt atoms (Figure 3f and Supporting Information Figure S20 and Table S4). Figure 3 | Electronic status of Pt on various Pt/MOFs catalysts for phenylacetylene hydrogenation reaction. (a) XPS profiles of Pt 4f for Pt NPs supported on a series of MOFs. The metallic Pt0 located at the binding energy of 71.3 and 74.6 eV, and oxidation state Pt2+ located at 72.3 and 75.5 eV. The ratio of Pt2+/Pt0 in various catalysts was marked. (b–d) XPS profiles of Cr 2p, Fe 2p, and Cu 2p in pure MOFs and Pt/MOFs catalysts. The ratio of reduction state of Fe2+ and Cu+ within Fe-O and Cu-O cluster-based MOFs was marked. (e) EXAFS spectra of Pt foil, PtO2, and Pt/MOFs catalysts. (f) DFT calculations of Bader charge of Pt atoms on different MOFs. (g) DFT calculations of the binding energy of H atoms on the Pt surface. (h) Photographs of 5 mg Pt/MOFs catalysts mixed with 45 mg of WO3 before and after treatment with H2 gas at 25 °C for 5 min. MOFs, metal–organic frameworks; XPS, X-ray photoelectron spectroscopy; EXAFS, extended X-ray absorption fine structure; DFT, density functional theory; WO3, tungsten oxide. Download figure Download PowerPoint The surface electronic status of Pt NPs directly correlated to the interaction with active H atoms and unsaturated molecules, thus, delivering different catalytic activities and selectivities of alkyne hydrogenation.34,35 The binding energy of active H atoms on Pt(111) (2 × 2) surface with different charges was explored by DFT calculations (Figure 3g and Supporting Information Figure S21 and Table S5). The small binding energy (−0.42 eV) of H atoms on the neutrally charged Pt surface indicated that active H atoms could readily migrate to the adsorbed phenylacetylene and/or styrene, resulting in overhydrogenation on Pt NPs and Pt/MIL-101(Cr) catalysts. When the Pt(111) surface charge increased to a positive value through the interfacial electron transfer from Pt to MOFs, the higher Pt–H binding energy (−1.2 eV) prevented the migration of active H atoms toward the adsorbed phenylacetylene and/or styrene, suppressing the overhydrogenation on Pt/MIL-101(Fe) and Pt/HKUST-1 catalysts. These DFT results were evaluated further by the color change in tungsten oxide (WO3), since active H atoms could react readily with the yellow WO3 to form dark blue HxWO3.36 As depicted in Figure 3h, the Pt/HKUST-1 with WO3 powder mixture exhibited no change in color after the H2 treatment, whereas mixing Pt/MIL-101(Cr) and Pt/MIL-101(Fe) with WO3 powders showed a different extent of color changes, which revealed the distinct binding energies of active H atoms on the Pt surface. These differences in the color change of WO3 were consistent with DFT simulations and the performance of phenylacetylene hydrogenation on distinct Pt/MOFs catalysts. Furthermore, the partially oxidized Pt could adsorb phenylacetylene tightly on its surface, while weakening its interaction with styrene molecules,34 as verified by the lower catalytic activity (5%) of Pt/MOFs(Fe) catalysts for styrene (Figure 2c). The electronic environment of Pt NPs created by Fe-O clusters allowed for the semihydrogenation of phenylacetylene while styrene intermediates preferred desorption from the Pt surface. The results mentioned above demonstrated that the chemical environment of metal-O clusters within MOFs could regulate the catalytic performance of Pt NPs in the hydrogenation of phenylacetylene. It is known that the metal ions also affect the catalytic activity and selectivity of MNPs in the hydrogenation reaction.37 Therefore, we wondered whether the trace of metal ions released during the catalytic process would affect the catalytic performance. As shown in Supporting Information Figure S22, even when Fe3+ ions were increased to 6 ppm, no apparent decay of Pt NPs in the hydrogenation activity was observed. The high conversion (99%) of phenylacetylene to ethylbenzene indicated that the Fe3+ ions could hardly modify Pt NPs to realize alkyne semihydrogenation. In contrast, 6 ppm Cu2+ ions could hinder the hydrogenation activity completely, suggesting that the activity of Pt NPs was sensitive to Cu2+ ions. Considering the potential influence of metal ions on the activity of Pt NPs, we built a core–shell structure [email protected](Cr) as catalysts to better evaluate the influences of metal ions released from MIL-101(Fe) and HKUST-1 on phenylacetylene hydrogenation reaction ( Supporting Information Figure S23). [email protected](Cr) catalysts reached 99% conversion of phenylacetylene to ethylbenzene, and the reduced reaction rate was mainly a result of the diffusion of the reactant through the MOFs channel from the surface to the active sites ( Supporting Information Figure S24). When HKUST-1 powders were introduced to the reaction, the conversion of phenylacetylene only reached 35% within 1 h, after which no further increase occurred ( Supporting Information Figure S24, red dots). This result indicated that the synergistic effect of Cu-O metal clusters and released Cu2+ ions could poison Pt NPs. However, when the MIL-101(Fe) was added to the reaction, the conversion of phenylacetylene reached 100%, and the selectivity was similar to [email protected](Cr) ( Supporting Information Figure S24, blue dots). These results confirmed that the metal-O clusters within MOFs could regulate the catalytic chemoselectivity of Pt NPs in the hydrogenation of alkynes. Furthermore, the catalytic stability of Pt/MIL-101(Fe) catalyst for phenylacetylene hydrogenation was evaluated by PXRD, TEM, and XPS characterizations. There was a slight decrease in Pt NPs content of Pt/MIL-101(Fe) after three reaction cycles (from 2.2% to 2.0%; Supporting Information Figure S25) and negligible influence on catalytic performance ( Supporting Information Figure S26), indicating the catalytic stability of Pt/MIL-101(Fe) catalyst. The crystallinity of MIL-101(Fe) changed slightly ( Supporting Information Figure S27) as the sizes, and electronic status of Pt NPs were retained ( Supporting Information Figures S28 and S29), which confirmed the stability of the Pt/MIL-101(Fe) catalyst. Conclusion We successfully demonstrated that employing different metal-O clusters within MOFs enabled the regulation of interfacial electronic structures of Pt NPs for significant improvement of selective hydrogenation of phenylacetylene. We found that Pt/Fe-O cluster-based MOFs catalysts highly favored the semihydrogenation of phenylacetylene to form styrene while Pt/Cr-O cluster-based MOFs facilitated overhydrogenation to an alkane. More importantly, Pt NPs were deactivated completely when Cu-O metal cluster-based MOFs were used as supports. Our studies affirmed that the electronic status of MNPs modified by metal-O clusters and trace poisonous metal ions released from MOFs is crucial in regulating the hydrogenation of unsaturated molecules, thus, opening up new paths for designing a series of suitable MNP/MOFs catalysts to boost other selective hydrogenation reactions of alkynes. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no conflict of interest. Funding Information This study was supported by the National Key R&D Program of China (no. 2017YFA0207201), the National Natural Science Foundation (nos. 21727808, 21574065, 21604038, 21971114, 21604040, and 51702155), the National Science Foundation for Distinguished Young Scholars (no. 21625401), and the Jiangsu Provincial Funds for Natural Science Foundation (nos. BK20160975, BK20160981, and BK20170975). Acknowledgments The authors are grateful to the Synchrotron Radiation Research Center (NSRRC) in Taiwan for their help on X-ray absorption spectroscopy measurements.

Metal-organic frameworks (MOFs) are porous crystalline materials with attractive properties for gas separation and storage. Their remarkable tunability makes it possible to create millions of MOF variations but creates the need for fast material screening to identify promising structures. Computational high-throughput screening (HTS) is a possible solution, but its usefulness is tied to accurate predictions of MOF adsorption properties. Accurate adsorption simulations often require an accurate description of electrostatic interactions, which depend on the electronic charges of the MOF atoms. HTS-compatible methods to assign charges to MOF atoms need to accurately reproduce electrostatic potentials (ESPs) and be computationally affordable, but current methods present an unsatisfactory trade-off between computational cost and accuracy. We illustrate a method to assign charges to MOF atoms based on ab initio calculations on MOF molecular building blocks. A library of building blocks with built-in charges is thus created and used by an automated MOF construction code to create hundreds of MOFs with charges "inherited" from the constituent building blocks. The molecular building block-based (MBBB) charges are similar to REPEAT charges-which are charges that reproduce ESPs obtained from ab initio calculations on crystallographic unit cells of nanoporous crystals-and thus similar predictions of adsorption loadings, heats of adsorption, and Henry's constants are obtained with either method. The presented results indicate that the MBBB method to assign charges to MOF atoms is suitable for use in computational high-throughput screening of MOFs for applications that involve adsorption of molecules such as carbon dioxide.

The efficient capture of CO2 is a critical problem for porous adsorbents. The inadequacy of conventional adsorbents has low adsorption capacity towards CO2 removal. Metal organic frame work has been considered as very effective for CO2 adsorption as it shows higher rate of CO2 adsorption at room temperature. In conventional amine processes, a comparatively high energy penalty is required, whereas a novel class of metal-organic framework by the combination of amine solvent have improve the potential of adsorption process and also the efficiency of separation. Amine-functionalized MOFs become more fascinated due to strong interaction between carbon dioxide and amine-functionalized MOF. A renewable green γCD-MOF was synthesized by using vapor diffusion method. Post-synthetic modification of γCD-MOF was done with piperazine and analyzed to expose its crystalline structure, morphology, and porous structure. The main aim of this paper is to enhance the CO2 adsorption by functionalization of inexpensive, green, nanoporous γCD-MOF and also to highlight the effects of amine-based functionalization towards potential application. Gravimetric CO2 adsorption isotherms for γCD-MOF, pip-γCD-MOF are reported up to 60 °C and found to follow a pseudo-second-order reaction. The pip-γCD-MOF confirms comparatively increased rapid adsorption rate of CO2 than that of γCD-MOF and desorption of CO2, and need less energy for regeneration. These results are the complete evidence of piperazine as an efficient amine group for increasing the CO2 adsorption uptake capacity.

Mesoporous Silica Encapsulated Metal-Organic Frameworks for Heterogeneous Catalysis

Comprehensive SummaryCarbon dioxide (CO2) capture is one of the most important aspects of reducing global warming. In terms of CO2 capture, metal‐organic frameworks (MOFs) have several advantages. However, it isn't easy to shape MOFs while maintaining their performance. Herein, we describe the development of a pellet‐shaped ultramicroporous MOF, Ni(3‐ain)2 (3‐ain = 3‐aminoinoisonicotinic acid), that is capable of selectively adsorbing CO2. Polyvinyl butyral (PVB) is used as a binder during the production of Ni(3‐ain)2 MOF pellets. The adequately shaped material can maintain its crystallinity and exhibit a high CO2 adsorption capacity (3.73 mmol·g–1) at ambient conditions, which is significantly greater than those obtained for N2 (0.63 mmol·g–1) and CO (0.90 mmol·g–1). Consequently, this material displays high IAST selectivities for CO2/N2 (26.3, 15/85, V/V) and CO2/CO (19.2, 1/99, V/V). According to the theoretical calculations, Ni(3‐ain)2 preferentially adsorbs CO2 molecules over N2 molecules and CO molecules. The results of experiments on dynamic breakthrough have demonstrated that Ni(3‐ain)2 pellets are capable of effectively separating CO2/N2 or CO2/CO mixtures under conditions of dynamic flow. Furthermore, the structured MOF materials can be synthesized in one step at kilogram scale. This work provides an avenue for the shaping of MOFs for potential industrial applications in the future.