Surface-exposed Pd nanoparticles supported over nanoporous carbon hollow tubes as an efficient heterogeneous catalyst for the C C bond formation and hydrogenation reactions

Pyrene-Based Nanoporous Covalent Organic Framework for Carboxylation of C-H Bonds with CO2 and Value-Added 2-Oxazolidinones Synthesis under Ambient Conditions

The selective carbon capture and utilization (CCU) as a one-carbon (C1) feedstock offers dual advantages for mitigating the rising atmospheric CO2 content and producing fine chemicals/fuels. In this context, herein, we report the application of a porous bipyridine-functionalized, pyrene-based covalent organic framework (Pybpy-COF) for the stable anchoring of catalytic Ag(0) nanoparticles (NPs) and its catalytic investigation for fixation of CO2 to commodity chemicals at ambient conditions. Notably, Ag@Pybpy-COF showed excellent catalytic activity for the carboxylation of various terminal alkynes to corresponding alkynyl carboxylic acids/phenylpropiolic acids via C-H bond activation under atmospheric pressure conditions. Besides, carboxylative cyclization of various propargylic amines with CO2 to generate 2-oxazolidinones, an important class of antibiotics, has also been achieved under mild conditions. This significant catalytic activity of Ag@Pybpy-COF with wide functional group tolerance is rendered by the presence of highly exposed, alkynophilic Ag(0) catalytic sites decorated on the pore walls of high surface area (787 m2 g-1) Pybpy-COF. Further, density functional theory calculations unveiled the detailed mechanistic path of the Ag@Pybpy-COF-catalyzed transformation of CO2 to alkynyl carboxylic acids and 2-oxazolidinones. Moreover, the catalyst showed high recyclability for several cycles of reuse without significant loss in its catalytic activity and structural rigidity. This work demonstrates the promising application of Pybpy-COF for stable anchoring of Ag NPs for successful transformation of CO2 to valuable commodity chemicals at ambient conditions.

Pd nanoparticles decorated on a porous Co(BDC-NH2) MOF as an effective heterogeneous catalyst for dye reduction

Herein, a new catalytic nanocomposite [Co(BDC-NH2)-Pd NPs] composed of a Co(BDC-NH2) MOF has been developed. The catalyst was prepared by modifying the synthesized porous Co(BDC-NH2) MOF with decorated Pd nanoparticles. This nanocatalyst was used as a heterogeneous catalyst in the reductive degradation of organic dyes Rhodamine B and methyl orange with NaBH4. The kinetic and thermodynamic parameters of the reactions were evaluated. The results showed that the low catalyst content could successfully catalyze the dye reduction reaction quickly (1 min). The metal-organic frameworks unique porous morphology of the Co(BDC-NH2) MOF appears to increase dye adsorption and achieve effective dye reduction. Additionally, recyclability studies of the catalyst confirmed that it could be recovered and reused for 10 consecutive reaction cycles with negligible Pd leaching and reduction in catalytic activity.

TiO2-Modified Montmorillonite-Supported Porous Carbon-Immobilized Pd Species Nanocomposite as an Efficient Catalyst for Sonogashira Reactions

In this study, a combination of the porous carbon (PCN), montmorillonite (MMT), and TiO₂ was synthesized into a composite immobilized Pd metal catalyst (TiO₂-MMT/PCN@Pd) with effective synergism improvements in catalytic performance. The successful TiO₂-pillaring modification for MMT, derivation of carbon from the biopolymer of chitosan, and immobilization of Pd species for the prepared TiO₂-MMT/PCN@Pd⁰ nanocomposites were confirmed using a combined characterization with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), N₂ adsorption-desorption isotherms, high-resolution transition electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was shown that the combination of PCN, MMT, and TiO₂ as a composite support for the stabilization of the Pd catalysts could synergistically improve the adsorption and catalytic properties. The resultant TiO₂-MMT₈₀/PCN₂₀@Pd⁰ showed a high surface area of 108.9 m²/g. Furthermore, it exhibited moderate to excellent activity (59-99% yield) and high stability (recyclable 19 times) in the liquid-solid catalytic reactions, such as the Sonogashira reactions of aryl halides (I, Br) with terminal alkynes in organic solutions. The positron annihilation lifetime spectroscopy (PALS) characterization sensitively detected the development of sub-nanoscale microdefects in the catalyst after long-term recycling service. This study provided direct evidence for the formation of some larger-sized microdefects during sequential recycling, which would act as leaching channels for loaded molecules, including active Pd species.

A novel strategy for stabilization of sub-nanometric Pd colloids on kryptofix functionalized MCM-41: nanoengineered material for Stille coupling transformation

The stabilization of sub-nanometric metal particles (< 1 nm) with suitable distribution remained challenging in the catalytic arena. Herein, an intelligent strategy was described to anchoring and stabilizing sub-nanometric Pd colloids with an average size of 0.88 nm onto Kryptofix 23 functionalized MCM-41. Then, the catalytic activity of Pd@Kryf/MCM-41 was developed in Stille coupling reaction with a turnover frequency (TOF) value of 247 h-1. The findings demonstrate that porous MCM-41 structure and high-affinity Kryptofix 23 ligand toward adsorption of Pd colloids has a vital role in stabilizing the sub-nanometric particles and subsequent catalytic activity. Overall, these results suggest that Pd@Kryf/MCM-41 is a greener, more suitable option for large-scale applications and provides new insights into the stabilization of sub-nanometric metal particles.

Mesoporous Porphyrin-Silica Nanocomposite as Solid Acid Catalyst for High Yield Synthesis of HMF in Water

Solid acid catalysts occupy a special class in heterogeneous catalysis for their efficiency in eco-friendly conversion of biomass into demanding chemicals. We synthesized porphyrin containing porous organic polymers (PorPOPs) using colloidal silica as a support. Post-modification with chlorosulfonic acid enabled sulfonic acid functionalization, and the resulting material (PorPOPS) showed excellent activity and durability for the conversion of fructose to 5-hydroxymethyl furfural (HMF) in green solvent water. PorPOPS composite was characterized by N2 sorption, FTIR, TGA, CHNS, FESEM, TEM and XPS techniques, justifying the successful synthesis of organic networks and the grafting of sulfonic acid sites (5 wt%). Furthermore, a high surface area (260 m2/g) and the presence of distinct mesopores of ~15 nm were distinctly different from the porphyrin containing sulfonated porous organic polymer (FePOP-1S). Surprisingly the hybrid PorPOPS showed an excellent yield of HMF (85%) and high selectivity (>90%) in water as compared to microporous pristine-FePOP-1S (yield of HMF = 35%). This research demonstrates the requirement of organic modification on silica surfaces to tailor the activity and selectivity of the catalysts. We foresee that this research may inspire further applications of biomass conversion in water in future environmental research.

Fabrication of Pd NPs-supported porous carbon by integrating the reducing reactivity and carbon-rich network of lignin

The renewable resource as a major feedstock to prepare porous carbon has showed many advantages compared to fossil-based materials. This study proposes a new strategy to synthesize palladium nanoparticles (Pd NPs)-supported porous carbon, utilizing both the chemical reactivity and the carbon-rich 3D network of lignin. The Pd NPs-supported porous carbons were prepared in one-pot synthesis, with Pd(NH3)2Cl2 as precursor, lignin as reducing and stabilizing agents of Pd NPs, nano SiO2 as hard-template, followed by carbonization and removal of the template. The results reveal a positive effect of Pd precursor dosage on the development and excellent texture of the Pd NPs-supported porous carbon. Accordingly, the synthesized porous carbon was proved to have large micropore volume and good micro-mesopore porous structure, revealing it a promising hydrogen adsorbent.

Liquid phase catalytic hydrogenation reduction of Cr(VI) using highly stable and active Pd/CNT catalysts coated by N-doped carbon

Liquid catalytic hydrogenation is a green and cost-effective technique for the reductive removal of pollutants in water. Supported noble metals are the most frequently used catalysts in liquid phase catalytic hydrogenation, whereas marked catalyst deactivation is commonly identified. In this study, we coated supported Pd catalyst on carbon nanotube (denoted as Pd/CNT) by different overcoatings (including SiO₂, carbon and N-doped carbon) to prevent catalyst deactivation. The activities of the coated catalysts for liquid phase catalytic hydrogenation reduction of hexavalent chromium (Cr(VI)) differed with the overcoating properties. Negligible Cr(VI) conversion was observed on SiO₂ coated Pd/CNT, while feasible Cr(VI) reduction was identified on carbon coated (denoted as Pd/CNT@C) and N-doped carbon coated catalysts (denoted as Pd/CNT@CN). Pd/CNT@CN exhibited a much higher catalytic activity than Pd/CNT@C, which was ascribed to the stronger Cr(VI) adsorption on CN overcoating. The catalytic activity of Pd/CNT@CN was positively correlated with the conductivity and hydrophilicity of CN overcoating, which could be optimized by varying carbonization temperature. Furthermore, Pd/CNT@CN retained its initial activity after ten consecutive catalyst cycles without any deactivation, whereas Pd/CNT only retained 8.2% of its initial activity, reflecting much higher catalytic stability of Pd/CNT@CN than Pd/CNT. The findings in the present study highlight that liquid catalytic reduction using Pd/CNT@CN as the catalyst is a highly stable and effective method to remove Cr(VI) in water.

Microporous Nanotubes and Nanospheres with Iron-Catechol Sites: Efficient Lewis Acid Catalyst and Support for Ag Nanoparticles in CO2 Fixation Reaction

Fe^III -containing hyper-crosslinked microporous nanotubes (FeNTs) and nanospheres (FeNSs) are synthesized through the reaction of catechol and dimethoxymethane in the presence of FeCl₃ or CF₃ SO₃ H. Both FeNTs and FeNSs demonstrate excellent catalytic activity in Lewis acid catalysis (hydrolysis and regioselective methanolysis of styrene oxide) and tandem catalysis involving a sequential oxidation-cyclization process, which selectively converts benzyl alcohol to 2-phenyl benzimidazole. Apart from Lewis acidity, the FeNTs and FeNSs also showed CO₂ uptake capacities of 2.6 and 2.2 mmol g^-1 , respectively, at a pressure of 1 atm and temperature of 273 K. Furthermore, Ag nanoparticles are immobilized successfully on the surfaces of FeNTs and FeNSs by the liquid-phase impregnation method to prepare Ag@FeNT and Ag@FeNS nanocomposites, which show high catalytic activity for the selective fixation of CO₂ to phenylacetylene to yield phenylpropiolic acid at 60 °C and 1 atm CO₂ pressure. Hence, Fe^III -catechol-containing hyper-crosslinked nanotubes and nanospheres have huge potential not only as Lewis acid catalysts, but also as excellent supports for immobilizing Ag nanoparticles in the design of a robust catalyst for the carboxylation of terminal alkynes, which has wide scope in catalysis and environmental research.

Influence of Metal Deposition and Activation Method on the Structure and Performance of Carbon Nanotube Supported Palladium Catalysts

The effects of the metal deposition and activation methods on metal particle size and distribution were investigated for carbon nanotube supported Pd catalysts. The Pd precursor was loaded by incipient wetness impregnation, ion adsorption, and deposition precipitation and was activated by thermal treatment under a nitrogen atmosphere or in the liquid phase by reduction by formaldehyde or sodium borohydride. Regardless of the metal precursor loading method, activation under a N₂ atmosphere at 500 °C led to homogeneously distributed 4 nm Pd particles. Liquid‐phase reduction by sodium borohydride provided a bimodal distribution with particle sizes of approximately 1 and >10 nm. A somewhat weaker reducing agent, formaldehyde, yielded particles approximately 1 nm in size. The activities of the catalysts for the hydrogenation of cinnamaldehyde correlated with the particle sizes.