A "Green" route to adipic acid: direct oxidation of cyclohexenes with 30 percent hydrogen peroxide

Sustainable Routes for the Synthesis of Renewable Adipic Acid from Biomass Derivatives

Adipic acid (AA) is a key industrial dicarboxylic acid intermediate used in nylon manufacturing. Unfortunately, the traditional process technology is accompanied by serious environmental pollution. Given the growing demand for adipic acid and the desire to reduce its negative impact on the environment, considerable efforts have been devoted to developing more green and friendly routes. This Review is focused on the latest advances in the sustainable preparation of AA from biomass-based platform molecules, including 5-hydroxymethylfufural, glucose, γ-valerolactone, and phenolic compounds, through biocatalysis, chemocatalysis, and the combination of both. Additionally, the development of state-of-the-art catalysts for different catalytic systems systematically is discussed and summarized, as well as their reaction mechanisms. Finally, the prospects for all preparation routes are critically evaluated and key technical challenges in the development of green and sustainable processes for the manufacture of AA are highlighted. It is hoped that the green adipic acid synthesis pathways presented can provide insights and guidance for further research into other industrial processes for the production of nylon precursors in the future.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles

La_1−xSr_xCoO₃ (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O₂ as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co²⁺ was also increased. However, Co²⁺ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol⁻¹ for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O₂ pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Nanospatial Charge Modulation of Monodispersed Polymeric Microsphere Photocatalysts for Exceptional Hydrogen Peroxide Production

Photocatalysis offers a sustainable strategy for hydrogen peroxide (H₂ O₂ ) production, which is an essential oxidant and emerging energy carrier in modern chemical industry. The development of polymer-based photocatalysts to produce H₂ O₂ has great potential but is limited by lower efficiency due to the limitation of light utilization and the low charge separation efficiency. Herein, a series of monodispersed mesoporous resorcinol-formaldehyde resin spheres (MRFS) are reported with a rational designed spatial charge distribution, exhibiting wide light absorption with a solar-to-chemical conversion (SCC) efficiency of 1.1%. Surface photovoltage microscopy (SPVM) measurements unraveled the charge separation in nanospace with uneven distribution of donor (D) and acceptor (A) sites. A density functional theory (DFT) calculation elucidated the origin of photogenerated electrons and holes. Moreover, MRFS demonstrates photocatalytic water oxidation ability. The findings in this work open a new avenue for the development of porous polymeric photocatalysts toward highly efficient solar energy conversion.

Molybdenum/Tungsten-Based Heteropoly Salts in Oxidations

Oxidation represents one of the most important and practical chemical transformations for both organic synthesis, material science and pharmaceutical area. Among the existing strategies, molybdenum/tungsten-based heteropoly salts involved oxidations with low-cost and environmentally benign terminal oxidant and thus have attracted considerable attention in recent years. In this review, we have summarized the recent development of heteropoly salts utilized in oxidations, mainly the peroxomolybdates and peroxotungstates. We wish to highlight the progress made in the past 20 years of this field. Three categories are classified according to the aggregation state of metal oxides. Special attention is paid to the catalytically active peroxometalate species generated during the oxidation process. It is helpful to shed light on the common features that enable highly efficient and selective oxidations. We aim to inspire fellow chemists to explore more functional metalates for catalytic oxidations, especially asymmetric versions. Meanwhile, we attempt to understand the design principles for the discovery of more efficient, selective and economical catalytic systems.

Au Modified F-TiO2 for Efficient Photocatalytic Synthesis of Hydrogen Peroxide

In this work, Au-modified F-TiO₂ is developed as a simple and efficient photocatalyst for H₂O₂ production under ultraviolet light. The Au/F-TiO₂ photocatalyst avoids the necessity of adding fluoride into the reaction medium for enhancing H₂O₂ synthesis, as in a pure TiO₂ reaction system. The F⁻ modification inhibits the H₂O₂ decomposition through the formation of the ≡Ti–F complex. Au is an active cocatalyst for photocatalytic H₂O₂ production. We compared the activity of TiO₂ with F⁻ modification and without F⁻ modification in the presence of Au, and found that the H₂O₂ production rate over Au/F-TiO₂ reaches four times that of Au/TiO₂. In situ electron spin resonance studies have shown that H₂O₂ is produced by stepwise single-electron oxygen reduction on the Au/F-TiO₂ photocatalyst.

Fe-S Catalyst Generated In Situ from Fe(III)- and S3•--Promoted Aerobic Oxidation of Terminal Alkenes

An iron-sulfur complex formed by the simple mixture of FeCl₃ with S₃^•- generated in situ from K₂S is developed and applied to selective aerobic oxidation of terminal alkenes. The reaction was carried out under an atmosphere of O₂ (balloon) and could proceed on a gram scale, expanding the application of S₃^•- in organic synthesis. This study also encourages us to explore the application of an Fe-S catalyst in organic reactions.

Engineering adipic acid metabolism in Pseudomonas putida

Bio-upcycling of plastics is an upcoming alternative approach for the valorization of diverse polymer waste streams that are too contaminated for traditional recycling technologies. Adipic acid and other medium-chain-length dicarboxylates are key components of many plastics including polyamides, polyesters, and polyurethanes. This study endows Pseudomonas putida KT2440 with efficient metabolism of these dicarboxylates. The dcaAKIJP genes from Acinetobacter baylyi, encoding initial uptake and activation steps for dicarboxylates, were heterologously expressed. Genomic integration of these dca genes proved to be a key factor in efficient and reliable expression. In spite of this, adaptive laboratory evolution was needed to connect these initial steps to the native metabolism of P. putida, thereby enabling growth on adipate as sole carbon source. Genome sequencing of evolved strains revealed a central role of a paa gene cluster, which encodes parts of the phenylacetate metabolic degradation pathway with parallels to adipate metabolism. Fast growth required the additional disruption of the regulator-encoding psrA, which upregulates redundant β-oxidation genes. This knowledge enabled the rational reverse engineering of a strain that can not only use adipate, but also other medium-chain-length dicarboxylates like suberate and sebacate. The reverse engineered strain grows on adipate with a rate of 0.35 ± 0.01 h^-1, reaching a final biomass yield of 0.27 ± 0.00 g_CDW g_adipate^-1. In a nitrogen-limited medium this strain produced polyhydroxyalkanoates from adipate up to 25% of its CDW. This proves its applicability for the upcycling of mixtures of polymers made from fossile resources into biodegradable counterparts.

MOF Embedded and Cu Doped CeO2 Nanostructures as Efficient Catalyst for Adipic Acid Production: Green Catalysis

Greatly efficient chemical processes are customarily based upon a catalyst activating the process pathway to achieve higher yields of a product with desired specifications. Catalysts capable of achieving good performance without compromising green credentials are a pre-requisite for the development of a sustainable process. In this study, CeO2 nanoparticles were tested for their catalytic activity with two different configurations, one as a hybrid of CeO2 nanoparticles with Zeolitic Immidazole Framework (ZIF-67) and second being doped Cu cations into CeO2 nanoparticles. Physicochemical and catalytic activity was investigated and compared for both systems. Each hybrid was synthesized by embedding the CeO2 nanoparticles into the microporous structure of ZIF-67, and Cu doped CeO2 nanoparticles were prepared by a facile hydrothermal route. As a catalytic test, it was employed for the oxidation of cyclohexene to adipic acid (AA) as an alternative to expensive noble metal-based catalysts. Heterogeneous ZIF-67/CeO2 found catalytical activity towards the oxidation of cyclohexene with nearly complete conversion of cyclohexene into AA under moderate and co-catalyst free reaction conditions, whereas Cu doped CeO2 nanoparticles have shown no catalytic activity towards cyclohexene conversion, depicting the advantages of the porous ZIF-67 structure and its synergistic effect with CeO2 nanoparticles. The large surface area catalyst could be a viable option for the green synthesis of many other chemicals.

Efficient Catalysts for the Green Synthesis of Adipic Acid from Biomass

Green synthesis of adipic acid from renewable biomass is a very attractive goal of sustainable chemistry. Herein, we report efficient catalysts for a two-step transformation of cellulose-derived glucose into adipic acid via glucaric acid. Carbon nanotube-supported platinum nanoparticles are found to work efficiently for the oxidation of glucose to glucaric acid. An activated carbon-supported bifunctional catalyst composed of rhenium oxide and palladium is discovered to be powerful for the removal of four hydroxyl groups in glucaric acid, affording adipic acid with a 99 % yield. Rhenium oxide functions for the deoxygenation but is less efficient for four hydroxyl group removal. The co-presence of palladium not only catalyzes the hydrogenation of olefin intermediates but also synergistically facilitates the deoxygenation. This work presents a green route for adipic acid synthesis and offers a bifunctional-catalysis strategy for efficient deoxygenation.

State-of-the-art palladium-catalyzed alkoxycarbonylations

State-of-the-art Pd-catalyzed alkoxycarbonylation: catalyst development and applications.

A decatungstate-based ionic liquid exhibiting a very low dielectric constant suitable for acting as a solvent and a catalyst for the oxidation of organic substrates

The use of a POM-based ionic liquid both as an efficient solvent and as a catalyst for the oxidation of organic substrates.

Oxidative cleavage of cycloalkenes using hydrogen peroxide and a tungsten-based catalyst: towards a complete mechanistic investigation

The identification of intermediates and by-products issuing from the oxidative cleavage of cycloolefins allows proposing of a reaction mechanism.

Biodegradable all polyester-based multiblock copolymer elastomers with controlled properties

A cascade polymerization method is developed here for the synthesis of environmentally-friendly biodegradable all polyester-based thermoplastic elastomers with tunable properties.

Selective Hydroperoxygenation of Olefins Realized by a Coinage Multimetallic 1-Nanometer Catalyst

The science of particles on a sub-nanometer (ca. 1 nm) scale has attracted worldwide attention. However, it has remained unexplored because of the technical difficulty in the precise synthesis of sub-nanoparticles (SNPs). We recently developed the "atom-hybridization method (AHM)" for the precise synthesis of SNPs by using a suitably designed macromolecule as a template. We have now investigated the chemical reactivity of alloy SNPs obtained by the AHM. Focusing on the coinage metal elements, we systematically evaluated the oxidation reaction of an olefin catalyzed by these SNPs. The SNPs showed high catalytic performance even under milder conditions than those used with conventional catalysts. Additionally, the hybridization of multiple elements enhanced the turnover frequency and the selectivity for the formation of the hydroperoxide derivative. We discuss the unique quantum-sized catalysts providing generally unstable hydroperoxides from the viewpoint of the miniaturization and hybridization of materials.

Adipic Acid Route: Oxidation of Cyclohexene vs. Cyclohexane

A cleaner alternative to the current inefficient oxidation of cyclohexane to adipic acid is presented. Direct oxidation of neat cyclohexene by aq. hydrogen peroxide to adipic acid is selectively achieved in good yield (46%), in the presence of the recyclable C-homoscorpionate iron(II) complex [FeCl2{κ3-HC(pz)3}] (pz = pyrazol-1-yl) and microwave irradiation, by a nitrous oxide-free protocol.

Porphyrin–Nanodiamond Hybrid Materials—Active, Stable and Reusable Cyclohexene Oxidation Catalysts

The quest for active, yet “green” non-toxic catalysts is a continuous challenge. In this work, covalently linked hybrid porphyrin–nanodiamonds were prepared via ipso nitro substitution reaction and characterized by X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy, infrared spectroscopy (IR) and thermogravimetry-differential scanning calorimetry (TG-DSC). The amine-functionalized nanodiamonds (ND@NH2) and 2-nitro-5,10,15,20-tetra(4-trifluoromethylphenyl)porphyrin covalently linked to nanodiamonds (ND@βNH-TPPpCF3) were tested using Allium cepa as a plant model, and showed neither phytotoxicity nor cytotoxicity. The hybrid nanodiamond–copper(II)–porphyrin material ND@βNH-TPPpCF3-Cu(II) was also evaluated as a reusable catalyst in cyclohexene allylic oxidation, and displayed a remarkable turnover number (TON) value of ≈265,000, using O2 as green oxidant, in the total absence of sacrificial additives, which is the highest activity ever reported for said allylic oxidation. Additionally, ND@βNH-TPPpCF3-Cu(II) could be easily separated from the reaction mixture by centrifugation, and reused in three consecutive catalytic cycles without major loss of activity.

A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands

We report a new mononuclear molybdenum(iv) complex, MoOLBISLSQ, in which LSQ (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, LBIS, and MoO2(acac)2. The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOLBISLSQ revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of LBIS and two O atoms of LSQ. The effective magnetic moment (μ eff) of MoOLBISLSQ decreased from 2.36 to 0.2 μB in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOLBISLSQ was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.

Oxidation of Long-Chain α-Olefins Using Environmentally-Friendly Oxidants

Studies on the oxidation of α-olefins via the two-stage method are presented. The new method consisted of oxidizing C30+ α-olefins with hydrogen peroxide (2 equiv.) and subsequent oxidation with oxygen. Products with high acid numbers (29–82 mgKOH/g) and saponification numbers (64–140 mgKOH/g) were obtained and compared with products obtained using only hydrogen peroxide or oxygen. It was demonstrated that H₂O₂ can be partially replaced by oxygen in the oxidative cleavage reaction of α-olefins. N-hydroxyphthalimide in combination with Co(acac)₂ demonstrated high activity in the oxidation stage using oxygen.

One-pot biocatalytic route from cycloalkanes to α,ω-dicarboxylic acids by designed Escherichia coli consortia

Aliphatic α,ω‐dicarboxylic acids (DCAs) are a class of useful chemicals that are currently produced by energy-intensive, multistage chemical oxidations that are hazardous to the environment. Therefore, the development of environmentally friendly, safe, neutral routes to DCAs is important. We report an in vivo artificially designed biocatalytic cascade process for biotransformation of cycloalkanes to DCAs. To reduce protein expression burden and redox constraints caused by multi-enzyme expression in a single microbe, the biocatalytic pathway is divided into three basic Escherichia coli cell modules. The modules possess either redox-neutral or redox-regeneration systems and are combined to form E. coli consortia for use in biotransformations. The designed consortia of E. coli containing the modules efficiently convert cycloalkanes or cycloalkanols to DCAs without addition of exogenous coenzymes. Thus, this developed biocatalytic process provides a promising alternative to the current industrial process for manufacturing DCAs.

Aliphatic α,ω-dicarboxylic acids (DCAs) are widely used chemicals that are synthesised by multistage chemical oxidations. Here, the authors report an artificially designed biocatalytic cascade for the oxidation of cycloalkanes or cycloalkanols to DCAs in the form of microbial consortia, composed of three Escherichia coli cell modules.

Polyoxotungstate ([PW11O39]7-) immobilized on mesoporous polymer for selective liquid-phase oxidation of alcohols using H2O2

Selective oxidation of alcohols is an attractive organic transformation and has received tremendous attention from the scientific community over the years. Herein, a mesoporous polymer (MP) was synthesized by a template-free solvothermal approach. The surface of the MP was functionalized with quaternary ammonium groups and polyoxotungstate anion (PW11O39 7-) was subsequently supported on the MP as a counter anion to the ammonium cation by a simple ion-exchange procedure. The structure of PW11 and PW4 complexes was confirmed by 31P NMR and FTIR analysis. The surface properties of all the catalysts synthesized were explored by various characterization techniques such as nitrogen sorption, TGA, contact angle measurement, and ICP-OES analysis. The synthesized PW11/MP catalysts were employed for selective oxidation of alcohols. Among the various PW11 supported catalysts, PW11/MP (80 : 20) demonstrated excellent catalytic activity for the oxidation of alcohols using aqueous H2O2. The PW11/MP (80 : 20) catalyst showed good catalytic activity for oxidation of a wide range of alcohols including substituted, heterocyclic and secondary alcohols. The superior catalytic activity of PW11/MP (80 : 20) is attributed to an optimum balance in the hydrophilicity/hydrophobicity in the mesoporous environment, better catalyst wettability, and enrichment of reactants in the catalytic active sites.

Recent Progress, Challenges, and Prospects in Two-Dimensional Photo-Catalyst Materials and Environmental Remediation

The successful photo-catalyst library gives significant information on feature that affects photo-catalytic performance and proposes new materials. Competency is considerably significant to form multi-functional photo-catalysts with flexible characteristics. Since recently, two-dimensional materials (2DMs) gained much attention from researchers, due to their unique thickness-dependent uses, mainly for photo-catalytic, outstanding chemical and physical properties. Photo-catalytic water splitting and hydrogen (H2) evolution by plentiful compounds as electron (e-) donors is estimated to participate in constructing clean method for solar H2-formation. Heterogeneous photo-catalysis received much research attention caused by their applications to tackle numerous energy and environmental issues. This broad review explains progress regarding 2DMs, significance in structure, and catalytic results. We will discuss in detail current progresses of approaches for adjusting 2DMs-based photo-catalysts to assess their photo-activity including doping, hetero-structure scheme, and functional formation assembly. Suggested plans, e.g., doping and sensitization of semiconducting 2DMs, increasing electrical conductance, improving catalytic active sites, strengthening interface coupling in semiconductors (SCs) 2DMs, forming nano-structures, building multi-junction nano-composites, increasing photo-stability of SCs, and using combined results of adapted approaches, are summed up. Hence, to further improve 2DMs photo-catalyst properties, hetero-structure design-based 2DMs' photo-catalyst basic mechanism is also reviewed.

Hydrogen peroxide synthesis from water and oxygen using a three-component nanohybrid photocatalyst consisting of Au particle-loaded rutile TiO2 and RuO2 with a heteroepitaxial junction

Thin heteroepitaxial (HEPI) layers of RuO₂ were selectively formed on the TiO₂ surface of Au nanoparticle-loaded rutile TiO₂ particles (RuO₂#TiO₂-Au) with an orientation of RuO₂(110)//TiO₂(110) by a hydrothermal method, and the three-component nanohybrid exhibits a high photocatalytic activity far exceeding that of Au/TiO₂ for hydrogen peroxide generation from water and oxygen due to the HEPI junction-induced unique morphology of RuO₂.