Metal-Free Heptazine-Based Porous Polymeric Network as Highly Efficient Catalyst for CO2 Capture and Conversion.

The capture and catalytic conversion of CO2 into value-added chemicals is a promising and sustainable approach to tackle the global warming and energy crisis. The nitrogen-rich porous organic polymers are excellent materials for CO2 capture and separation. Herein, we present a nitrogen-rich heptazine-based microporous polymer for the cycloaddition reaction of CO2 with epoxides in the absence of metals and solvents. HMP-TAPA, being rich in the nitrogen site, showed a high CO2 uptake of 106.7 mg/g with an IAST selectivity of 30.79 toward CO2 over N2. Furthermore, HMP-TAPA showed high chemical and water stability without loss of any structural integrity. Besides CO2 sorption, the catalytic activity of HMP-TAPA was checked for the cycloaddition of CO2 and terminal epoxides, resulting in cyclic carbonate with high conversion (98%). They showed remarkable recyclability up to 5 cycles without loss of activity. Overall, this study represents a rare demonstration of the rational design of POPs (HMP-TAPA) for multiple applications.

Co-Catalyst-Free Chemical Fixation of CO2 into Cyclic Carbonates by using Metal-Organic Frameworks as Efficient Heterogeneous Catalysts.

The concentration of carbon dioxide (CO2 ) in the atmosphere is increasing at an alarming rate resulting in undesirable environmental issues. To mitigate this growing concentration of CO2 , selective carbon capture and storage/sequestration (CCS) are being investigated intensively. However, CCS technology is considered as an expensive and energy-intensive process. In this context, selective carbon capture and utilization (CCU) as a C1 feedstock to synthesize value-added chemicals and fuels is a promising step towards lowering the concentration of the atmospheric CO2 and for the production of high-value chemicals. Towards this direction, several strategies have been developed to convert CO2 , a Greenhouse gas (GHG) into useful chemicals by forming C-N, C-O, C-C, and C-H bonds. Among the various CO2 functionalization processes known, the cycloaddition of CO2 to epoxides has gained considerable interest owing to its 100% atom-economic nature producing cyclic carbonates or polycarbonates in high yield and selectivity. Among the various classes of catalysts studied for cycloaddition of CO2 to cyclic carbonates, porous metal-organic frameworks (MOFs) have gained a special interest due to their modular nature facilitating the introduction of a high density of Lewis acidic (LA) and CO2 -philic Lewis basic (LB) functionalities. However, most of the MOF-based catalysts reported for cycloaddition of CO2 to respective cyclic carbonates in high yields require additional co-catalyst, say tetra-n-butylammonium bromide (TBAB). On the contrary, the co-catalyst-free conversion of CO2 using rationally designed MOFs composed of both LA and LB sites is relatively less studied. In this review, we provide a comprehensive account of the research progress in the design of MOF based catalysts for environment-friendly, co-catalyst-free fixation of CO2 into cyclic carbonates.

Molecular Basis of Water Sorption Behavior of Rivaroxaban-Malonic Acid Cocrystal.

The present study aims to investigate the molecular basis of water sorption behavior of rivaroxaban-malonic acid cocrystal (RIV-MAL). It was hypothesized, that the amount of water sorbed by a crystalline solid is governed by the surface molecular environment of different crystal facets and their relative abundance to crystal surface. Water sorption behavior was measured using a dynamic vapor sorption analyzer. The surface molecular environment of different crystal facets and their relative contribution were determined using single crystal structure evaluation and face indexation analysis, respectively. The surface area-normalized water sorption for rivaroxaban (RIV), malonic acid (MAL), and RIV-MAL at 90% RH/25 °C was 0.28, 92.6, and 11.1% w/w, respectively. The crystal surface of MAL had a larger contribution (58.7%) of hydrophilic (Hphi) functional groups and showed the "highest" water sorption (92.6% w/w). On the contrary, RIV had a larger surface contribution (65.2%) of hydrophobic (Hpho) functional groups, and the smaller contribution (34.8%) of Hphi+Hpho groups exhibited the "lowest" water sorption (0.28% w/w). The "intermediate" water sorption (11.1% w/w) by RIV-MAL, as compared to RIV, was ascribed to the increased surface contribution of Hphi+Hpho groups (from 34.8 to 42.1%) and reduced hydrophobic surface contribution (from 65.2 to 57.9%). However, the significantly higher water gained (∼39-fold) by the cocrystal as compared to RIV, despite the nominal change in the surface contributions, was further attributed to the relatively stronger hydrogen bonding interactions between the surface-exposed carboxyl groups and water molecules. The study highlights that the amount of water sorbed by the cocrystal is governed by the surface molecular environment and additionally by the strength of hydrogen bonding. This investigation has implications on designing materials with a desired moisture-sorption property.

Environmentally Friendly, Co-catalyst-Free Chemical Fixation of CO2 at Mild Conditions Using Dual-Walled Nitrogen-Rich Three-Dimensional Porous Metal-Organic Frameworks.

Highly porous, polyhedral metal-organic frameworks (MOFs) of Co(II)/Ni(II), {[M6(TATAB)4(DABCO)3(H2O)3]·12DMF·9H2O} n (where M = Co(II) (1)/Ni(II) (2), H3TATAB = 4,4',4″- s-triazine-1,3,5-triyl-tri- p-aminobenzoic acid, and DABCO = 1,4-diazabicyclo[2.2.2]octane) have been synthesized solvothermally. Both MOFs 1 and 2 show a 2-fold interpenetrated 3D framework structure composed of dual-walled cages of dimension ∼ 30 Å functionalized with a high density of Lewis acidic Co(II)/Ni(II) metal sites and basic -NH- groups. Interestingly, MOF 1 shows selective adsorption of CO2 with high heat of adsorption ( Qst) value of 39.7 kJ/mol that is further supported by theoretical studies with computed binding energy (BE) of 41.17 kJ/mol. The presence of the high density of both Lewis acidic and basic sites make MOFs 1/2 ideal candidate materials to carry out co-catalyst-free cycloaddition of CO2 to epoxides. Consequently, MOFs 1/2 act as excellent recyclable catalysts for cycloaddition of CO2 to epoxides for high-yield synthesis of cyclic carbonates under co-catalyst-free mild conditions of 1 bar of CO2. Further, MOF 1 was recycled for five successive cycles without substantial loss in catalytic activity. Herein, rational design of rare examples of 3D polyhedral MOFs composed of Lewis acidic and basic sites exhibiting efficient co-catalyst-free conversion of CO2 has been demonstrated.

Palladium(II)/N-Heterocyclic Carbene Catalyzed One-Pot Sequential α-Arylation/Alkylation: Access to 3,3-Disubstituted Oxindoles.

Rationally designed fluorene-based mono- and bimetallic Pd-PEPPSI complexes were synthesized and demonstrated to be effective for the one-pot sequential α-arylation/alkylation of oxindoles. This streamlined approach offers efficient access to functionalized 3,3-disubstituted oxindoles in excellent yields (up to 89%) under mild reaction conditions.

Correlating Single Crystal Structure, Nanomechanical, and Bulk Compaction Behavior of Febuxostat Polymorphs.

Febuxostat exhibits unprecedented solid forms with a total of 40 polymorphs and pseudopolymorphs reported. Polymorphs differ in molecular arrangement and conformation, intermolecular interactions, and various physicochemical properties, including mechanical properties. Febuxostat Form Q (FXT Q) and Form H1 (FXT H1) were investigated for crystal structure, nanomechanical parameters, and bulk deformation behavior. FXT Q showed greater compressibility, densification, and plastic deformation as compared to FXT H1 at a given compaction pressure. Lower mechanical hardness of FXT Q (0.214 GPa) as compared to FXT H1 (0.310 GPa) was found to be consistent with greater compressibility and lower mean yield pressure (38 MPa) of FXT Q. Superior compaction behavior of FXT Q was attributed to the presence of active slip systems in crystals which offered greater plastic deformation. By virtue of greater compressibility and densification, FXT Q showed higher tabletability over FXT H1. Significant correlation was found with anticipation that the preferred orientation of molecular planes into a crystal lattice translated nanomechanical parameters to a bulk compaction process. Moreover, prediction of compactibility of materials based on true density or molecular packing should be carefully evaluated, as slip-planes may cause deviation in the structure-property relationship. This study supported how molecular level crystal structure confers a bridge between particle level nanomechanical parameters and bulk level deformation behavior.

Effect of differential surface anisotropy on performance of two plate shaped crystals of aspirin form I.

Differential surface anisotropy of different crystals of the same API can have a significant impact on their pharmaceutical performance. The present work investigated the impact of differential surface anisotropy of two plate-shaped crystals of aspirin (form I) on their hygroscopicity, stability and compaction behavior. These crystals differed in their predominant facets (100) and (001) and were coded as AE-100 & E-001. (100) facets exposed polar carbonyl groups which provided hydrophilicity to the facets. In contrast, (001) facets possessed hydrophobicity as they exposed non-polar aryl and methyl groups. Both the samples showed different degradation behavior, at various stability conditions (i.e. 40°C/75%RH, 30°C/90%RH and 30°C/60%RH) and different time intervals. Polar groups of aspirin have been reported to be prone to hydrolysis due to which AE-100 was less stable than E-001. Dynamic vapor sorption (DVS) analysis at different simulated stability conditions also supported this observation, wherein AE-100 showed higher moisture sorption than E-001. Both the samples having similar particle size, shape, surface area and hardness value, showed differences in their compactibility. However, milling narrowed down the predominance of facets and both the milled samples showed similar stability and compaction behavior. This study was also supported by surface free energy determination, molecular modeling and face indexation of unmilled and milled samples.

Construction of 3-Fold-Interpenetrated Three-Dimensional Metal-Organic Frameworks of Nickel(II) for Highly Efficient Capture and Conversion of Carbon Dioxide.

A series of three new isostructural metal-organic frameworks (MOFs) of nickel(II), [{Ni(muco)(bpa)(2H2O)}·2H2O] (1), [{Ni(muco)(bpe)(2H2O)}·2.5H2O] (2), and [{Ni(muco)(azopy)(2H2O)}·2H2O] (3) [where muco = trans,trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpe = 1,2-bis(4-pyridyl)ethylene, and azopy = 4,4'-bis(azobipyridine)], have been synthesized and characterized by single-crystal X-ray diffraction analysis and other physicochemical methods. Compounds 1-3 exhibit an interesting 3-fold-interpenetrated three-dimensional pillar-layered framework structure constituted of 4-coordinating (4-c) NiII nodes with {66}-neb net topology. Remarkably, in spite of 3-fold interpenetration, the structures possess one-dimensional channels with dimensions of ∼8.05 × 5.25 Å2. Gas (N2, Ar, H2, and CO2) adsorption studies of compounds 2 and 3 revealed selective adsorption properties for CO2 over other gases. In all three structures, the 4-c NiII node has two coordinated H2O molecules that can be reversibly removed by high-temperature treatment to generate a dehydrated framework composed of highly unsaturated, Lewis acidic NiII ions. Further, the activated compounds of 1-3 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 with styrene oxide, resulting in cyclic carbonate with high conversion and selectivity. Interestingly, the cycloaddition reactions of CO2 with bulky epoxides show a decrease in the activity with an increase in the alkyl chain length of the substrate due to confinement of the pore size of the MOF. The high catalytic efficiency and size-dependent selectivity for smaller epoxides show the potential utility of 1 as a promising heterogeneous catalyst for the cycloaddition of CO2. Furthermore, the catalyst can be easily separated and reused for several cycles without significant reduction in the catalytic activity as well as structural rigidity. Compounds 1-3 represent rare examples of interpenetrated MOFs exhibiting selective storage and conversion of CO2 at mild conditions.