Stone

A azine-linked covalent organic framework, COF-JLU2, was designed and synthesized by condensation of hydrazine hydrate and 1,3,5-triformylphloroglucinol under solvothermal conditions for the first time. The new covalent organic framework material combines permanent micropores, high crystallinity, good thermal and chemical stability, and abundant heteroatom activated sites in the skeleton. COF-JLU2 possesses a moderate BET surface area of over 410 m² g⁻¹ with a pore volume of 0.56 cm³ g⁻¹. Specifically, COF-JLU2 displays remarkable carbon dioxide uptake (up to 217 mg g⁻¹) and methane uptake (38 mg g⁻¹) at 273 K and 1 bar, as well as high CO₂/N₂ (77) selectivity. Furthermore, we further highlight that it exhibits a higher hydrogen storage capacity (16 mg g⁻¹) than those of reported COFs at 77 K and 1 bar.

An azine-linked covalent organic framework was constructed by condensation of hydrazine hydrate and 1,3,5-triformylphloroglucinol under solvothermal conditions. While the framework possesses moderate surface areas, it exhibits excellent capture and uptake capacities for CO₂ (217 mg g⁻¹), H₂ (16 mg g⁻¹), and CH₄ (38 mg g⁻¹) at 1 bar pressure, with high adsorption selectivity for CO₂ over N₂.

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) is one of the most attractive reactions to establish biomass-based sustainable chemical processes. Supported Au and Pt catalysts have mainly been reported for this reaction, but excess amounts of base additives are generally required, which makes the process less green. Here, we demonstrate that Pt nanoparticles loaded on functionalized carbon nanotubes (CNTs) can catalyze the aerobic oxidation of HMF to FDCA in water without any base additives. Kinetic studies suggest a tandem reaction mechanism via 2,5-diformylfuran and 5-formylfurancarboxylic acid intermediates. It has been clarified that the oxygen-containing functional groups, in particular carbonyl/quinone and/or phenol groups, on CNT surfaces play crucial roles in FDCA formation. These functional groups could enhance the adsorption of HMF as well as the reaction intermediates from water and might facilitate hydrogen transfer.

Biomass oxidation: Functionalized Pt nanoparticles supported on carbon nanotubes (Pt/CNT) catalyze the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) in water in the absence of any base additives. The alcohol group in HMF is preferentially oxidized to form 2,5-diformylfuran (DFF) as a primary product. The carbonyl/quinone and/or phenol groups on the CNTs play crucial roles in the formation of FDCA via DFF and 5-formyl-2-furancarboxylic acid (FFCA).

As one of the alternative metals to platinum, which supports a wide range of applications in chemistry and catalysis in industry, palladium increasingly receives attention because of its similar physicochemical properties. However, Pd is generally less expensive than Pt and has a richer natural reserve. Herein, some recently developed techniques for the preparation and characterization of Pd-based bimetallic catalysts are reviewed. The impact on catalytic reactions of interest, including hydrogenation, dehydrogenation, hydrogenolysis, reforming, the oxygen reduction reaction, and hydrodesulfurization are also discussed. It is shown that the catalytic performance of Pd-based bimetallic catalysts is strongly dependent on the geometric and electronic states of Pd, which can be significantly affected by blended foreign element(s). Rationalization of the structure–activity relationship can provide useful guidelines to the fine tuning of these important catalytic reactions.

Two metals are better than one: Metallic Pd represents one of the most frequently employed components in catalysis. With the incorporation of a secondary element(s), Pd-based bimetallic nanoparticles can show excellent catalytic performance in a wide range of chemical reactions. This minireview summarizes the recent developments in supported Pd bimetallic catalysts used for chemical synthesis, environment protection, and renewable energy provision.

A protocol was developed to distinguish between well-defined molecular and nanoparticle-based catalysts for the Pd-catalyzed semihydrogenation reaction of alkynes to Z-alkenes. The protocol applies quantitative partial poisoning and dynamic light scattering methods, which allow the institution of additional validation experiments. For the quantitative partial poisoning method, tetramethylthiourea (TMTU) was developed as an alternative for the standard poison ligand CS₂, and was found to be superior in its applicability. The protocol and the TMTU poison ligand were validated using the well-described [Pd^II(phenanthroline)]-catalyzed copolymerization of styrene and CO, confirming that this system is clearly operating as a well-defined molecular catalyst. The protocol was subsequently applied to three catalyst systems used for the semihydrogenation of alkynes. The first was proposed to be a molecular [Pd⁰(IMes)] catalyst that uses molecular hydrogen, but the data gathered for this system, following the new protocol, clearly showed that nanoparticles (NPs) are catalytically active. The second catalyst system studied was an N-heterocyclic carbene (NHC) Pd system for transfer semihydrogenation using formic acid as the hydrogen source, which was proposed to operate through an in situ generated molecular [Pd⁰(IMes)] catalyst in earlier studies. The investigations showed that only a small fraction of the Pd added becomes active in the catalytic reaction and that NPs are formed. However, despite these findings, a clear distinction between catalytic activity of NPs versus a molecular catalyst could not be made. The third investigated system is based on a [Pd^II(IMes)(η³-allyl)Cl] precatalyst with additive ligands. The combined data gathered for this system are multi-interpretable, but suggest that a partially deactivated molecular catalyst dominates in this reaction.

CSI: Pd -Catalyst Species Investigation: The identity of the catalyst for several Pd-catalyzed semihydrogenation reactions has been studied with an easy to use protocol to distinguish between molecular catalysts and nanoparticles.

A pluronic P123 surfactant was employed in the exfoliation and stabilization of Pd⁰-supported graphene oxide (Pd_NPs–GO; NPs=nanoparticles) nanosheets in aqueous solution during the oxidation of benzylic and aliphatic alcohols under aerobic conditions at 80 °C. Surfactants prevented the aggregation of Pd_NPs–GO sheets at 80 °C under aqueous conditions and Pd_NPs–GO/P123 adducts acted as a highly dispersive homogeneous-like catalyst. The catalytic performance of the nanocomposite material was remarkably improved, being highly stable and reusable in the aerobic oxidation of alcohols for at least nine runs.

Surfactants for stabilization: The addition of surfactants as efficient exfoliating agents of graphene oxide (GO) has a remarkable influence on the catalytic activity of Pd-supported GO nanohybrid materials for the aerobic aqueous oxidation of alcohols. The surfactant improves the solubility of reactants in water and prevents aggregation of the nanocomposite.

Selective hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural were performed with carbon nanotube-supported bimetallic NiFe (NiFe/CNT) catalysts. The combination of Ni and Fe in an appropriate atomic ratio of Ni/Fe (2.0) significantly increased the selectivity to 2,5-furandimethanol or 2,5-dimethylfuran depending on the reaction temperature. The selectivities to 2,5-furandimethanol and 2,5-dimethylfuran were as high as 96.1 % at 383 K and 91.3 % at 473 K, respectively. The characterization results confirmed that bimetallic particles with sizes less than 7 nm were formed on the catalyst. Several key molecules related to 5-hydroxymethylfurfural transformation were used to investigate the product distribution and reaction pathway. The results indicated that the formation of NiFe alloy species is beneficial to the selective cleavage of the CO bond. Recycling experiments showed that the catalyst can be easily separated with a magnet and reused several times without significant loss of activity.

Benefits of selective memory: The alloying effect of nonprecious NiFe on carbon nanotubes is beneficial to the selective hydrogenation and hydrogenolysis of 5-hydroxymethylfurfural, which gives high selectivities to 2,5-furandimethanol or 2,5-dimethylfuran depending on the reaction temperature. The catalyst can be easily separated with a magnet and reused several times without significant loss of activity.

The oxygen reduction reaction (ORR) is of key importance in the field of electrochemical energy production. Remarkable catalytic properties of “metal-free” graphene nanoribbons towards the ORR were suggested in previous reports. Herein, we show that the electrocatalytic properties of an undoped supposedly “metal-free” graphene nanoribbon towards the ORR is actually caused by metallic impurities within the graphene nanoribbons.

Going around in circles: Opening of carbon nanotubes (CNTs) by using KMnO₄ and H₂SO₄ leads to graphene nanoribbons that are full of metallic impurities from the synthesis of the CNTs and MnO₂ from the opening of the CNTs. These impurities (and not the ribbons) dominate in the observed “electrocatalysis” of the oxygen reduction reaction.

Preparation of porous materials from one-dimensional polymers is challenging because the packing of polymer chains results in a dense, non-porous arrangement. Herein, we demonstrate the remarkable adaptation of an amorphous, linear, non-porous, flexible organic polymer into a three-dimensional, highly porous, crystalline solid, as the organic component of a metal–organic framework (MOF). A polymer with aromatic dicarboxylic acids in the backbone functioned as a polymer ligand upon annealing with Zn^II, generating a polymer–metal–organic framework (polyMOF). These materials break the dogma that MOFs must be prepared from small, rigid ligands. Similarly, polyMOFs contradict conventional polymer chemistry by demonstrating that linear and amorphous polymers can be readily coaxed into a highly crystalline, porous, three-dimensional structure by coordination chemistry.

A bottom-up strategy is used to generate porosity from non-porous, one-dimensional, amorphous polymeric materials by their transformation into crystalline polyMOF materials. These materials harness the advantages of the porosity and crystallinity of MOFs along with certain attributes of the polymers, such as facile film formation and greater chemical stability.

Porous organic polymers have prospects as functional substrates for catalysis, with quite different molecular properties from inorganic substrates. Here we disclose for the first time that porous palladium(II)-polyimines are excellent catalysts for cooperatively catalyzed and enantioselective cascade reactions. In synergy with a chiral amine co-catalyst, polysubstituted cyclopentenes and spirocyclic oxindoles, including the all-carbon quaternary stereocenter, were synthesized in high yields. High diastereo- and enantioselectivities were achieved for these dynamic kinetic asymmetric transformations (DYKAT) of enals with propargylic nucleophiles.

Pt catalysts prepared over different metallic oxide supports were investigated in the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in alkaline aqueous solutions with air, to examine the combined effect of the support and base addition. The base (nature and amount) played a significant role in the degradation or oxidation of HMF. Increasing amounts of the weak NaHCO₃ base improved significantly the overall catalytic activity of Pt/TiO₂ and Pt/ZrO₂ by accelerating the oxidation steps, especially for the aldehyde group. This was highlighted by a proposed kinetic model that gave very good concentration–time fittings. Moreover, the promotion of the catalyst with bismuth yielded a PtBi/TiO₂ catalytic system with improved activity and stability. Y₂O₃ and La₂O₃ZrO₂-supported catalysts exhibited lower activity than Pt/ZrO₂, which suggests no cooperative effect of the weakly basic properties introduced and the homogeneous base. Quantitative oxidation of HMF (0.1 M) and high yields of FDCA (>99 %) were obtained in less than 5 h by using an HMF/Pt molar ratio of 100 and Na₂CO₃ as a weak base over PtBi/TiO₂ (Bi/Pt=0.22).

Back to bases: The addition of increasing amounts of NaHCO₃ soluble base improved significantly the catalytic activity of Pt/TiO₂ in HMF oxidation to FDCA by accelerating the oxidation rate of the aldehyde group. Moreover, the promotion of the catalyst with bismuth yielded a PtBi/TiO₂ catalytic system (with Na₂CO₃ as the weak base) with improved activity and stability with respect to the monometallic catalyst.

A new approach is developed for hydrogenolytic ring opening of biobased 5-hydroxymethylfurfural (HMF), dehydration product of hexoses, towards 1,6-hexanediol (HDO) under atmospheric pressure. The highest yield of HDO, 43 %, is achieved over reusable Pd/zirconium phosphate (ZrP) catalyst at 413 K in the presence of formic acid as hydrogen source. In comparison with various Brønsted and/or Lewis acidic supports, the specific Brønsted acidity on ZrP support effectively accelerated the cleavage of CO bond in a furan ring.

Hydrogenolytic ring opening of 5-hydroxymethylfurfural to 1,6-hexanediol (HDO) by catalytic transfer hydrogenation is examined over a heterogeneous palladium on zirconium phosphate (Pd/ZrP) catalyst under mild reaction conditions. Formic acid is used as hydrogen source. The ZrP catalyst is superior to other Brønsted-acidic supports in the cleavage of the CO bond, which accelerates deoxygenation of the furan ring towards HDO. The catalyst is also reusable.