Porous Organic Polymer-Driven Evolution of High-Performance Cobalt Phosphide Hybrid Nanosheets as Vanillin Hydrodeoxygenation Catalyst

Synthesis of palladium-decorated defective tungsten oxide heterostructures with enhanced photothermal catalytic activity for hydrodeoxygenation of vanillin

The selective hydrodeoxygenation (HDO) of sustainable lignocellulosic biomass plays a pivotal role in the conversion of biomass into high-value fuels and chemicals. Nevertheless, HDO for biomass upgrading always demands high temperatures and high hydrogen (H2) pressure. Photothermal catalysis has been recognized as an effective approach for boosting chemical reactions under mild conditions while maintaining superior selectivity. Herein, we report the design of palladium-decorated defective tungsten oxide (Pd/WO3-x) catalysts with enhanced photothermal catalytic performances for the efficient HDO of vanillin. Pd/WO3-x nanoflowers have been synthesized through a solvothermal/in-situ reduction two-step strategy, and they exhibit notable photoabsorption in a wide range (200-1100 nm), high photothermal conversion and efficient charge separation efficiency. Under simulated sunlight irradiation (0.3 W cm-2), Pd/WO3-x exhibits a maximum vanillin conversion up to 86.8 % with a 2-methoxy-4-methylphenol (MMP) selectivity of 100 %, which is obviously higher than that (vanillin conversion = 33.1 %, MMP selectivity = 100 %) in the oil bath at the same temperature. Such higher conversion efficiency and selectivity under sunlight should result from the synergistic integration of hot electrons and photothermal heating, both of which are derived from localized surface plasmon resonance (LSPR) in WO3-x. Importantly, Pd/WO3-x catalyst demonstrates good stability and high selectivity to MMP even after 5 cycles. This work may offer a novel viewpoint on the advancement of photothermal catalysts and the realization of photothermal catalytic biomass conversion under mild conditions.

Selective hydrogenation of vanillin over a graphene-encapsulated nitrogen-doped bimetallic magnetic Ni/Fe@NDC nano-catalyst

One of the main obstacles to the development of sustainable biomass feedstocks today is the research of selective hydrogenation of biomass platform compounds for the synthesis of high-value chemicals. This work reports on the synthesis of a Ni/Fe bimetallic catalyst with nitrogen-doped carbon serving as the carrier, hydrogen serving as the primary donor, and isopropanol serving as the reaction medium and serving as a secondary donor. Vanillin was catalytically hydrogenated to produce 4-methylguaiacol, a complete hydrogenation product, under a reaction temperature of 200 °C for four hours. A single product with a good yield (95.26% conversion and selectivity up to 99%) was achieved by the moderate conditions, offering a potential route for the catalytic hydrogenation of biomass platform compounds.

In situ construction of 3D NiMo bimetallic catalysts anchored on dendritic mesoporous silica for the upgrading of biomass derivatives

Reasonable construction of bi-function catalysts with well dispersed hydrogenation active sites and acidic sites are crucial for the hydrodeoxygenation (HDO) of biomass-derived compounds but still a huge challenge. Herein, a 3D Mo functionalized Ni-based bimetallic embedded catalyst with fine metal nanoparticles size (<6 nm) was prepared for the first time using dendritic mesoporous silica as a sacrificial template by one-pot hydrothermal synthesis and adopted in the HDO process of vanillin (VAN) upgrade to 2-methoxy-4-methylphenol (MMP). The characterization results illustrated that Mo species regulated the acidity of the catalyst and promoted the formation of Ni-Mo alloy sites. Density functional theory (DFT) calculations further unveiled that Ni-Mo alloy sites promoted the activation and dissociation of CO bond in VAN, enhanced the ability of protonation hydrogenolysis. Benefitting from the synergistic effect of the highly uniformly dispersed hydrogenation metal sites and acidic sites, nearly 100% yield of MMP could obtained over the designed catalyst under mild conditions (130 °C, 1.5 MPa H₂, 3 h, 10 wt% catalyst dosage). Additionally, the NiMo_0.1@MSN catalyst displayed robust activity for no less than 8 recycles and excellent universality for the HDO of a variety of lignin derivatives and biomass platform molecules, which provide a feasible strategy for the construction of 3D confined catalysts for the high-efficiency HDO of biomass derivatives.

The catalytic hydrodeoxygenation of bio-oil for upgradation from lignocellulosic biomass

The increasing depletion of oil resources and the environmental problems caused by using much fossil energy in the rapid development of society. The bio-oil becomes a promising alternative energy source to fossil. However, bio-oil cannot be directly utilized, owing to its high proportion of oxygenated compounds with low calorific value and poor thermal stability. Catalytic hydrodeoxygenation (HDO) is one of the most effective methods for refining oxygenated compounds from bio-oil. HDO catalysts play a crucial role in the HDO reaction. This review emphasizes the description of the main processing of HDO and various catalytic systems for bio-oil, including noble/non-noble metal catalysts, porous organic polymer catalysts, and polar solvents. A discussion based on recent studies and evaluations of different catalytic materials and mechanisms is considered. Finally, the challenges and future opportunities for the development of catalytic hydrodeoxygenation for bio-oil upgradation are looked forward.

Improving the hydrodeoxygenation activity of vanillin and its homologous compounds by employing MoO3-incorporated Co-BTC MOF-derived MoCoOx@C

Lignin-derived aryl ethers and vanillin are essential platform chemicals that fulfil the demands for renewable aromatic compounds. Herein, an efficient heterogeneous catalyst is reported for reforming vanillin via a selective hydrodeoxygenation route to 2-methoxy-4-methyl phenol (MMP), a precursor to medicinal, food, and petrochemical industries. A series of MoCoO_x@C catalysts were synthesized by decorating the Co-BTC MOF with different contents of MoO₃ rods, followed by carbonization. Among these catalysts, MoCoO_x@C-2 afforded ∼99% vanillin conversion and ∼99% MMP selectivity at 150 °C in 1.5 h in an aqueous medium. In contrast, CoO_x@C afforded ∼75% vanillin conversion and ∼85% MMP selectivity. Detailed catalyst characterization revealed that CoO_x and Co₂Mo₃O₈ were the active species contributing to the higher activity of MoCoO_x@C-2. The excellent H₂-adsorption characteristics and acidity of MoCoO_x@C-2 were beneficial to the hydrodeoxygenation of vanillin and other homologous compounds. The DFT adsorption energy calculations suggested the favourable interactions of vanillin and vanillyl alcohol with the Co₂Mo₃O₈ sites in MoCoO_x@C-2. The catalyst could be efficiently recycled 5 times, with a negligible loss in activity after the 5th cycle. These findings provide a systematic explication of the active sites of the mixed metal oxide-based MoCoO_x@C-2 catalyst for the selective hydrodeoxygenation of vanillin to MMP, which is important for the academic and industrial catalysis community.

Ring-Opening Oligomerization Mechanism of a Vanillin-Furfurylamine-Based Benzoxazine and a Mono-Azomethine Derivative

Exploring the ring-opening polymerization (ROP) mechanism of benzoxazines is a fundamental issue in benzoxazine chemistry. Though some research papers on the topic have been reported, the ROP mechanism of mono-benzoxazines is still elusive. The key point for mechanistic studies is to determine and characterize the structure and formation pathways of the products generated in ROP. In this paper, the ROP of a vanillin-furfurylamine-based benzoxazine and a mono-azomethine derivative is studied with differential scanning calorimetry, fourier transform infrared spectroscopy, nuclear magnetic resonance, and electrospray ionization mass spectrometry, respectively. The results show that the products consist of a range of cationic species, zwitterions, fragments, and series of cyclic and linear oligomers of varying molecular sizes. It is proposed that both mono-benzoxazines undergo thermally activated cationic ring-opening oligomerization via zwitterion intermediates. Upon thermal induction, multi-bond-cleavage takes place to form various zwitterionic intermediates, which react with a monomer, a fragment, or a second zwitterion by several pathways to generate cyclic and linear oligomers.

Unlocking Molecular Secrets in a Monomer-Assembly-Promoted Zn-Metalated Catalytic Porous Organic Polymer for Light-Responsive CO2 Insertion

Anthropogenic carbon dioxide (CO₂) emission is soaring day by day due to fossil fuel combustion to fulfill the daily energy requirements of our society. The CO₂ concentration should be stabilized to evade the deadly consequences of it, as climate change is one of the major consequences of greenhouse gas emission. Chemical fixation of CO₂ to other value-added chemicals requires high energy due to its stability at the highest oxidation state, creating a tremendous challenge to the scientific community to fix CO₂ and prevent global warming caused by it. In this work, we have introduced a novel monomer-assembly-directed strategy to design va isible-light-responsive conjugated Zn-metalated porous organic polymer (Zn@MA-POP) with a dynamic covalent acyl hydrazone linkage, via a one-pot condensation between the self-assembled monomer 1,3,5-benzenetricarbohydrazide (TPH) and a Zn complex (Zn@COM). We have successfully explored as-synthesized Zn@MA-POP as a potential photocatalyst in visible-light-driven CO₂ photofixation with styrene epoxide (SE) to styrene carbonate (SC). Nearly 90% desired product (SC) selectivity has been achieved with our Zn@MA-POP, which is significantly better than that for the conventional Zn@TiO₂ (∼29%) and Zn@gC₃N₄ (∼26%) photocatalytic systems. The excellent light-harvesting nature with longer lifetime minimizes the radiative recombination rate of photoexcited electrons as a result of extended π-conjugation in Zn@MA-POP and increased CO₂ uptake, eventually boosting the photocatalytic activity. Local structural results from a first-shell EXAFS analysis reveals the existence of a Zn(N₂O₄) core structure in Zn@MA-POP, which plays a pivotal role in activating the epoxide ring as well as capturing the CO₂ molecules. An in-depth study of the POP-CO₂ interaction via a density functional theory (DFT) analysis reveals two feasible interactions, Zn@MA-POP-CO₂-A and Zn@MA-POP-CO₂-B, of which the latter has a lower relative energy of 0.90 kcal/mol in comparison to the former. A density of states (DOS) calculation demonstrates the lowering of the LUMO energy (EL) of Zn@MA-POP by 0.35 and 0.42 eV, respectively, for the two feasible interactions, in comparison to Zn@COM. Moreover, the potential energy profile also unveils the spontaneous and exergonic photoconversion pathways for the SE to SC conversion. Our contribution is expected to spur further interest in the precise design of visible-light-active conjugated porous organic polymers for CO₂ photofixation to value-added chemicals.

Presenting porous-organic-polymers as next-generation invigorating materials for nanoreactors

Porous organic polymers (POPs) represent an emerging class of porous organic materials which mainly comprise organic building blocks that are interconnected via strong covalent bonds, thereby offering highly cross-linked frameworks with rigid structures and specific void spaces for accommodating guest molecules. In the past few years, POPs have garnered colossal research interest as nanoreactors for heterogeneous catalysis (thermal, photochemical, electrochemical, etc.) because of their intriguing characteristic features, such as high thermal and chemical stabilities, adjustable chemical functionalities, large surface areas, and tunable pore size distributions. This feature article provides an overview of existing research relating to diverse POP synthetic approaches (COFs, CTFs, and some amorphous POPs), the possible modification of the functionality of POPs, and their exciting application as next-generation nanoreactors. These POPs are extremely interesting, as they offer the potential for either metal-free or metalated polymer catalysts allowing photocatalytic CO₂ reduction to solar-fuel, biofuel upgrades, the conversion of waste cooking oil to bio-oil, and clean H₂ production from water, addressing many scientific and technological challenges and providing new opportunities for various specific topics in catalysis. Finally, we emphasize that the integration of various synthetic approaches and the application of POPs as nanoreactors will provide opportunities in the near future for the precision synthesis of functional materials with significant impact in both basic and applied research areas.

Phosphorus containing porous organic polymers: synthetic techniques and applications in organic synthesis and catalysis

The phosphorus-containing porous organic polymer is a trending material for the synthesis of heterogeneous catalysts. Decades of investigations have established phosphines as versatile ligands in homogeneous catalysis. Recently, phosphine-based heterogeneous catalysts were synthesized to exploit the same electronic properties while leveraging extra stability and reusability. In the last few decades, the catalysts were applied in diverse organic transformations, including hydroformylation, hydrogenation, C-C, C-N and C-X coupling, hydrosilylation, oxidative-carbonylation reactions, and so on. However, even though these polymers possess a multifunctional character, they face multiple synthetic issues in controlling the pore size, increasing the surface area, and creating a single type of active site. This review summarizes the developments in this field over the last few decades, highlighting the current limitation and future scope.

An efficient environmentally friendly CuFe2O4/SiO2 catalyst for vanillyl mandelic acid oxidation in water under atmospheric pressure and a mechanism study

Aimed at the green production of vanillin, a highly efficient environmentally friendly oxidation system was introduced to oxidize VMA with a porous CuFe₂O₄/SiO₂ component nano-catalyst in aqueous solution under atmospheric pressure.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Realizing Catalytic Acetophenone Hydrodeoxygenation with Palladium-Equipped Porous Organic Polymers

Porous organic polymers (POPs) constructed through covalent bonds have raised tremendous research interest because of their suitability to develop robust catalysts and their successful production with improved efficiency. In this work, we have designed and explored the properties and catalytic activity of a template-free-constructed, hydroxy (-OH) group-enriched porous organic polymer (Ph-POP) bearing functional Pd nanoparticles (Pd-NPs) by one-pot condensation of phloroglucinol (1,3,5-trihydroxybenzene) and terephthalaldehyde followed by solid-phase reduction with H₂. The encapsulated Pd-NPs rested within well-defined POP nanocages and remained undisturbed from aggregation and leaching. This polymer hybrid nanocage Pd@Ph-POP is found to enable efficient liquid-phase hydrodeoxygenation (HDO) of acetophenone (AP) with high selectivity (99%) of ethylbenzene (EB) and better activity than its Pd@Al₂O₃ counterpart. Our investigation demonstrates a facile, scalable, catalyst-template-free methodology for developing novel porous organic polymer catalysts and next-generation efficient greener chemical processes from platform molecules to produce value-added chemicals. With the aid of comprehensive in situ ATR-IR spectroscopy experiments, it is suggested that EB can be more easily desorbed in a solution, reflecting from the much weaker but better-resolved signal at 1494 cm^-1 in Pd@Ph-POP compared to that in Pd@Al₂O₃, which is the key determining factor in favoring an efficient catalytic mechanism. Density functional theory (DFT) calculations were performed to illustrate the detailed reaction network and explain the high catalytic activity observed for the fabricated Pd@Ph-POP catalyst in the HDO conversion of AP to EB. All of the hydrogenation routes, including direct hydrogenation by surface hydrogen, hydrogen transfer, and the keto-enol pathway, are evaluated, providing insights into the experimental observations. The presence of phenolic hydroxyl groups in the Ph-POP frame structure facilitates the hydrogen-shuttling mechanism for dehydration from the intermediate phenylethanol, which was identified as a crucial step for the formation of the final product ethylbenzene. Besides, weaker binding of the desired product ethylbenzene and lower coverage of surface hydrogen atoms on Pd@Ph-POP both contributed to inhibiting the overhydrogenation reaction and explained well the high yield of EB produced during the HDO conversion of AP on Pd@Ph-POP in this study.

Synthesis of Nanosheets Containing Uniformly Dispersed PdII Ions at an Aqueous/Aqueous Interface: Development of a Highly Active Nanosheet Catalyst for Mizoroki-Heck Reaction

A method was developed to synthesize a nanosheet at the interface of an aqueous layer of Pd^II ions and an aqueous layer of hydrophilic polymer bearing a metal coordination unit (HPMC). The nanosheet was synthesized through generation of an interface by the addition of an aqueous solution of Pd^II ions with a low specific gravity (1.03 g/cm³) to a dispersed aqueous solution of HPMC with a high specific gravity (1.50 g/cm³), resulting in rapid cross-linking at the interface. An electron probe microanalysis mapping image showed that the Pd^II ions were uniformly dispersed in the nanosheet. The nanosheet showed a high catalyst activity for the Mizoroki-Heck cross-coupling reaction with a turnover number (TON) and turnover frequency (TOF) greater than 3,333,333 and 138,889 h^-1, respectively. These are the greatest TON and TOF values reported for heterogeneous polymeric catalysts for the Mizoroki-Heck reaction.

Leveraging Cu/CuFe2O4-Catalyzed Biomass-Derived Furfural Hydrodeoxygenation: A Nanoscale Metal-Organic-Framework Template Is the Prime Key

Enormous efforts have been initiated in the production of biobased fuels and value-added chemicals via biorefinery owing to the scarcity of fossil resources and huge environmental synchronization. Herein, non-noble metal-based metal/mixed metal oxide supported on carbon employing a metal-organic framework as a sacrificial template is demonstrated for the first time in the selective hydrodeoxygenation (HDO) of biomass-derived furfural (FFR) to 2-methyl furan (MF). The aforementioned catalyst (referred to as Cu/CuFe₂O₄@C-A) exhibited extraordinary catalytic proficiency (100% selectivity toward MF) compared with the conventional Cu/CuFe₂O₄@C-B catalyst which was prepared by the wet impregnation method. High-resolution transmission electron microscopy and synchrotron X-ray diffraction studies evidenced the existence of both metal (Cu) and mixed metal oxide (CuFe₂O₄) phases, in which the metal could help in hydrogenation to alcohol and metal oxide could assist in the hydroxyl group removal step during HDO reaction. The stabilization of encapsulated metal/metal oxide nanoparticles in the carbon matrix, modulation of the electronic structure, and regulation of geometric effects in the Cu/CuFe₂O₄@C-A are thought to play an important role in its excellent catalytic performance, confirmed by X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigations. Furthermore, the structure and activity interconnection was confirmed by in situ attenuated total reflection-IR studies, which manifested the strong interfacial interaction between FFR and the Cu/CuFe₂O₄@C-A catalyst. This finding was further supported by NH₃ temperature-programmed desorption analysis, which suggested that the presence of more Lewis/weak acidic sites in this catalyst was beneficial for the hydrogenolysis step in HDO reaction. Additionally, H₂ temperature-programmed reduction studies revealed that the adsorption of H₂ was stronger on the Cu/CuFe₂O₄@C-A than that over the conventional Cu/CuFe₂O₄@C-B catalyst; thus, the former catalyst promoted activation of H₂. A detailed kinetic analysis which demonstrated the lower activation energy barrier along with dual active sites attributed for the activation of the two separate reactions in the HDO process on the Cu/CuFe₂O₄@C-A catalyst. This work has great implication in developing a highly stable catalyst for the selective upgradation of biomass without deactivation of metal sites in extended catalytic cycles and opens the door of opportunity for developing a sustainably viable catalyst in biomass refinery industries.

Visible-light-driven photo-Fenton degradation of organic pollutants by a novel porphyrin-based porous organic polymer at neutral pH

Developing novel heterogeneous photo-Fenton catalysts with high efficiency and stability, driven by visible-light rather ultraviolet light at neutral pH has been a major challenge for degradation of organic pollutants. In this work, we successfully synthesized a metalloporphyrin-based porous organic polymer (FePPOP-1) by the Sonogashira cross-coupling reaction. UV-vis absorption spectra showed FePPOP-1 exhibits a significant coverage of the natural solar irradiance spectrum. As a result, the prepared FePPOP-1 has a significantly enhanced photocatalytic activity for the visible-light-driven degradation of methylene blue. By using only 4 mg of FePPOP-1 as a catalyst, it was found that 50 mL of organic wastewater containing 70 ppm MB could be totally degraded in 80 min even at neutral pH. The effects of the initial MB, H₂O₂ concentrations, pH value and common ions on MB degradation were studied in detail. Both the catalytic mechanism of FePPOP-1 and the degradation route of MB were also proposed.

Fabrication of Ir-CoOx@mesoporous SiO2 Nanoreactors for Selective Hydrogenation of Substituted Nitroaromatics

Nanosized Ir catalysts suffer from serious side reactions and poor stability during hydrogenation of substituted nitroaromatics to produce aromatic amines. In this work, core-shell nanostructures with sub-4 nm Ir-CoO_x hybrid cores and mesoporous SiO₂ shells were designed and prepared to overcome these problems. The Ir-CoO_x hybrid cores were converted from IrCo alloy nanoparticles (NPs) inside SiO₂ through in situ calcination and reduction pretreatments. The SiO₂ mesoporous shells in Ir-CoO_x@SiO₂ nanoreactors prevented the agglomeration/sintering of IrCo NPs, while allowing the free reactants and products (big molecules). The synergy between Ir and CoO_x species improved H₂ adsorption, thus affecting the reaction rate as well as the selectivity to aromatic amines. As a result, the obtained Ir-CoO_x@SiO₂ nanocatalyst showed tremendous improvement in catalytic activity, selectivity, and stability.

Engineering of Surface Environment of Pd Nanoparticle Catalysts on Carbon Support with Pyrene-Thiol Ligands for Semihydrogenation of Alkynes

A new type of pyrene-thiol derivative-modified Pd nanoparticle (NP) catalyst on a carbon black support for the efficient semihydrogenation of alkynes to alkenes is reported herein. Colloidal Pd NPs surrounded by pyrene-thiol modifiers were prepared using the two-phase Brust method followed by impregnation of carbon black materials. Based on the structural characterization of the prepared catalyst (PyC₁₂S-Pd/VC) by NMR, UV-vis, FT-IR, TEM, HAADF-STEM, Pd K-edge XAFS, XRD, N₂ adsorption, and XPS, we show that highly dispersed Pd NPs are immobilized on the catalysts via π-π interaction between pyrene groups bound to the Pd NPs and carbon black supports. PyC₁₂S-Pd/VC efficiently catalyzes the alkyne semihydrogenation reaction while maintaining high alkene selectivity; an alkene selectivity of 94% is attained at 98% conversion after 5 h of reaction, and the selectivity was retained around 80% in 10 h of reaction. This performance is superior to that of a catalyst without pyrene groups and that of a commercial Lindlar catalyst. The steric hindrance of pyrene groups restricts access of the substrates to Pd NP surfaces, suppressing the unfavorable overhydrogenation of alkenes to alkanes, which is revealed by the solvent and substrate dependency on the catalytic performance and a DFT calculation study. Furthermore, the high selectivity and stability of PyC₁₂S-Pd/VC are caused by the strong interaction between pyrene groups and carbon supports, which prevents the separation of pyrene modifiers and the leaching or sintering of Pd NPs during the catalytic reaction. It is demonstrated that the combination of Pd NPs, pyrene-thiol modifiers, and carbon supports offers high activity, alkene selectivity, and stability in the semihydrogenation reaction.