Selectivity Of Adipic Acid Research Articles

Introduced gas regulating fast and highly exothermic reactions with in situ gas generation in microreactors

AbstractThe oxidation of cyclohexanol/cyclohexanone with nitric acid as a fast and highly exothermic process was hard to control at low molar ratios of oxidant to organic substrate even in microreactors. Process of runaway gas generation was observed due to the local hot spots, which decreased the selectivity of adipic acid. Additional gas was introduced into microreactors before the reaction, changing the previous wave annular flow to the uniform slug flow. Through the microscopic high‐speed photography and CFD simulation, it was found that the internal convection made the reactant molecules in the aqueous slug distribute evenly instead of gathering near the interface before, thus eliminating the local hot spots and stabilizing the hydrodynamic characteristics. The reaction performance was evaluated under different conditions, and a higher adipic acid selectivity of 93.6% was achieved at a molar ratio of 6.7 with the combination of microreactors and conventional reactors.

Kinetic study on the reaction routes in the oxidation of K/A oil by nitric acid with microreactors

A continuous-flow microreactor system was applied to evaluate the detailed kinetics of the reaction routes involved in the oxidation of K/A oil by nitric acid, which is essential to obtain an in-depth comprehension of complex reaction mechanisms, having important guiding significance for the two-stage temperature control strategy. 6-hydroxyimino-6-nitrohexanoic acid (AMNA) was identified as a major intermediate and its presence was confirmed through UPLC-MS and 1H NMR characterizations. A multi-step reaction kinetic model was developed, revealing good prediction with the experimental results. Finally, the model was applied to investigate the effects of operating parameters on the reaction routes distribution and reaction rate. The simulation results showed that high temperature and nitric acid concentration contributed to high selectivity of adipic acid through promoting the AMNA route, and greatly reduced the required reaction time, indicating that the accurate control over the conditions in this rapid and strongly exothermic process was of paramount importance.

Highly dispersed metal incorporated hexagonal mesoporous silicates for catalytic cyclohexanone oxidation to adipic acid

Abstract Adipic acid is a dicarboxylic acid of great industrial importance, mainly used in the production of nylon-6,6 and polyurethane. The use of nitric acid as an oxidant in the industrial production of adipic acid poses significant carbon footprint to the environment. Clean adipic acid synthesis methods using a heterogeneous catalyst with H2O2 as oxidant and water as solvent have potential advantages of low catalyst cost, easy synthesis and recovery, cleanness and environmental protection. In this work, hexagonal mesoporous silicate materials were synthesized by a sol–gel method and evaluated for cyclohexanol/cyclohexanone oxidation to adipic acid. The physical and chemical properties of Fe-HMS were characterized by XRD, HR-TEM, BET and UV–Vis. The experimental results showed that Fe-HMS materials show pore sizes ranging from 2–3 nm. W- and Mo-based polyoxometalates were also evaluated and compared to the Fe-based HMS catalysts. To improve the adipic acid yield, the influence of the transition metal as well as the effect of metal loading, reaction temperature and catalyst amount on the catalytic performances of Fe-HMS have been investigated in details. When Si/Fe atomic ratio = 100, Fe-HMS catalyst shows the highest activity, with a cyclohexanone conversion of 92.3% and adipic acid selectivity of 29.4%. The reaction pathway of cyclohexanone oxidation was further proposed based on experimental data.

Synergistic hydrogen atom transfer with the active role of solvent: Preferred one-step aerobic oxidation of cyclohexane to adipic acid by N-hydroxyphthalimide

In this work, we developed an one-step aerobic oxidation of cyclohexane to prepare adipic acid, catalyzed by N-hydroxyphthalimide (NHPI) under promoter- and metal-free conditions. A significant beneficial solvent effect for synergistic reaction is observed with varying polarity and hydrogen-bonding strength: detailed study reveals that the solvent environments manipulate catalytic activity and adipic acid selectivity. Cyclic voltammetry measurements and UV–visible spectra of the NHPI catalyst are examined in various solvent environments to understand the active role of solvent in influencing the catalytic-site structure (>NOH) of the molecule. Analysis of the UV–visible spectra reveals that these differences can be rationalized by considering hydrogen-bonding with solvent molecules, which modifies the catalytic-site structure. This observation is in agreement with cyclic voltammetry results: the different reversibility of the catalytic-site (>NOH/>NO) wave shows that the catalytic activity of NHPI is related to the formation of hydrogen bonds with the active participation of solvents. Computational studies presented herein have furnished mechanistic insights into the effect of solvent environments. Specifically, we present the structures, dissociation energies, and reaction barriers from DFT studies of the reactants and reaction intermediates involved in the two types of H-abstraction on >NO catalytic-sites for the rate-determining step. The results of modeling the solvent effects using the PCM continuum solvent method predict that the resulting reaction barrier of the rate-controlling H-abstraction for cyclohexane and cyclohexanone is modified significantly: the transition state barrier of H-abstraction for cyclohexane decreases from 22.36 (in benzene) to 20.78 kcal⋅mol−1 (in acetonitrile); the α-H-abstraction barrier for cyclohexanone decreases from 21.45 to 20.53 kcal⋅mol−1. The active participation of solvent molecule results in a strong interaction between pre-reaction complex (PINO∙∙∙H∙∙∙C < ) with the migrating hydrogen and polar solvent molecules, which in turn favors the H-abstraction by a hydrogen-transfer to the >NO catalytic-sites at the transition state. The lower calculated barriers of H-abstraction for cyclohexanone oxidation approximate more closely the experimental results of the higher adipic acid selectivity. Our work provides a dimension of sustainable chemistry for the metal-free preparation of adipic acid: a conversion of 27% with 79% adipic acid selectivity is achieved over use of NHPI catalysts in CH3CN solvent.

Single-Atom Mn Active Site in a Triol-Stabilized β-Anderson Manganohexamolybdate for Enhanced Catalytic Activity towards Adipic Acid Production

Adipic acid is an important raw chemical for the commercial production of polyamides and polyesters. The traditional industrial adipic acid production utilizes nitric acid to oxidize KA oil (mixtures of cyclohexanone and cyclohexanol), leading to the emission of N2O and thus causing ozone depletion, global warming, and acid rain. Herein, we reported an organically functionalized β-isomer of Anderson polyoxometalates (POMs) nanocluster with single-atom Mn, β-{[H3NC(CH2O)3]2MnMo6O18}− (1), as a highly active catalyst to selectively catalyze the oxidation of cyclohexanone, cyclohexanol, or KA oil with atom economy use of 30% H2O2 for the eco-friendly synthesis of adipic acid. The catalyst has been characterized by single crystal and powder XRD, XPS, ESI-MS, FT-IR, and NMR. A cyclohexanone (cyclohexanol) conversion of &gt;99.9% with an adipic acid selectivity of ~97.1% (~85.3%) could be achieved over catalyst 1 with high turnover frequency of 2427.5 h−1 (2132.5 h−1). It has been demonstrated that the existence of Mn3+ atom active site in catalyst 1 and the special butterfly-shaped topology of POMs both play vital roles in the enhancement of catalytic activity.

Bio-adipic acid production by catalysed hydrogenation of muconic acid in mild operating conditions

In this work trans,trans-muconic acid was hydrogenated to adipic acid, a strategic intermediate for the industry of polyamides and high performance polymers. This study: (1) develops a new and robust analytical method for the evaluation of muconic acid conversion and adipic acid selectivity, (2) defines a standard procedure for the confirmation of the kinetic regime using experimental data and some practical criteria to exclude mass transfer limitations, (3) tests Pt/C 5% commercial catalyst for the study of muconic acid hydrogenation at different operating conditions, (4) hypothesizes a reaction pathway in aqueous media, and (5) demonstrates the possibility to recycle the catalyst without loss in activity and selectivity. The developed analytical method evaluates muconic acid conversion by UV–vis analyses, while the selectivity towards adipic acid and mono-unsatured intermediates was determined by GC/TCD analysis after derivatization of the reaction mixture. Under optimized conditions the hydrogenation reaction (70°C, 4bar of hydrogen and 10 substrate/catalyst ratio (wt/wt)) reached full conversion with full selectivity toward adipic acid in 2h. Finally, recycling tests revealed the possibility to reuse catalyst up to 10 times without loss in conversion and selectivity opening the way to a scaling up of the process.

Bifunctional H2WO4/TS-1 catalysts for direct conversion of cyclohexane to adipic acid: Active sites and reaction steps

Adipic acid production may account for 10% of the annual increase in atmospheric nitrous oxide level. Here we developed a hollow H2WO4/TS-1 bifunctional catalyst with excellent catalytic activity for the direct transformation of cyclohexane to adipic acid with 30% hydrogen peroxide by a non-HNO3 route. XRD diffraction, N2 physisorption, TEM, NH3-TPD and Raman, XPS, Infrared, and UV–visible spectroscopies were used to characterize catalysts to increase understanding of the structure of active species on the hollow TS-1 support. The enhanced catalytic activity in comparison of reference catalysts (H2WO4, H2WO4/Silicate-1 and TS-1) was explained in terms of their bifunctional catalytic sites and perfect hollow morphology with large intraparticle voids. Experimental results on the product distribution indicated that the tetrahedral Ti and surface W sites played important roles in the synergistic effect: tetrahedral Ti worked in oxidation of cyclohexane to intermediates (cyclohexanone and cyclohexanol) and surface W worked in further oxidation of late intermediates to adipic acid. Theoretical calculation results on catalytic activity of the TiOOH and WOOH active sites in the H2WO4/TS-1 for different reaction steps validate the experimental results. The catalytic activity was also correlated to the strong interaction between W sites and HTS-1 surface induced increase in acidity of the materials. Increasing calcination temperature led to structural evolution of supported active species and subsequent activity change. A cyclohexane conversion of 31% with 78% adipic acid selectivity was achieved over this catalyst calcinated at 500 ∘C. Based on the catalytic and characterization results, possible reaction pathways were thus proposed to explain high adipic acid yield over hollow H2WO4/TS-1 catalysts.

Selective oxidation of cyclohexene to adipic acid over silver supported tungsten oxide nanostructured catalysts

We have developed a new synthesis strategy to prepare ∼5 nm metallic silver nanoparticles (AgNPs) supported on tungsten oxide (WO3) nanorods with diameters between 40 and 60 nm in the presence of a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The catalyst was characterized by XRD, XPS, ICP-AES, FT-IR, Raman spectroscopy, EXAFS, SEM and TEM. The catalyst is very effective in liquid phase oxidation of cyclohexene to adipic acid with hydrogen peroxide as an oxidant. The synergy between the surface AgNPs and WO3 nanorods plays the most vital role towards this very high catalytic activity. The reusability of the catalyst which is a prerequisite for practical applications was analysed and it was found that the catalyst exhibits no significant changes in its catalytic activity even after five cycles of reuse. A cyclohexene conversion of >99.9% with an adipic acid selectivity of ∼94% was achieved over ∼5 nm AgNPs supported on the WO3 nanorod catalyst with a very high turnover frequency of ∼12 h−1.

A comparative oxidation of cyclohexane catalyzed by N-hydroxyphthalimide and ZSM-5 supported Co(II), Mn(II), Ni(II), Zn(II), Fe(III) with molecular oxygen in the absence of solvents and reductants

A catalytic system consisting of ZSM-5 supported metal ions (M/ZSM-5) and N-hydroxyphthalimide (NHPI) for the selective oxidation of cyclohexane with dioxygen was reported, and the catalytic activity of various M/ZSM-5 was investigated. Cyclohexane was successfully oxidized in good conversion, the highest conversion of cyclohexane could be obtained using Co(II)/ZSM-5 and NHPI, however, the highest selectivity of adipic acid was from Mn(II)/ZSM-5 and NHPI. The stability and recycling of the catalyst were also reviewed. © 2008 Elsevier B.V. All rights reserved.

Rational oxidation of cyclohexane to cyclohexanol, cyclohexanone and adipic acid with air over metalloporphyrin and cobalt salt

A systematic study on the effect of a catalyst on the aerobic oxidation of cyclohexane was carried out using cobalt isooctanoate, metalloporphyrin p- ClTPPCo and [p- ClTPPFe ]2 O as catalysts. The results showed that the metalloporphyrin-catalyzed system performed better than the traditional cobalt salt ( Co isooctanoate) in terms of both the reaction conversion and the selectivity of cyclohexanol and cyclohexanone (KA oil). More importantly, we discovered that bisironporphyrin complex, which was traditionally considered to be either non-active or not-so-active for hydrocarbon oxidation, excelled at 155°C, achieving high selectivity for KA oil (80%) and good reaction conversion (13.8%). This result is in sharp contrast to earlier results by others. Meanwhile, the production of adipic acid was also increased and good selectivity of adipic acid among the by-products was also obtained. Our study suggested that [p- ClTPPFe ]2 O catalyzed aerobic oxidation could be used to produce adipic acid in addition to KA oil. Some mechanistic rationales were proposed to explain the superior performance of the [p- ClTPPFe ]2 O catalyst based on its unique structural and chemical properties.

An Industrial Process for Adipic Acid Production by the Liquid-Phase Oxidation of Cyclohexanone with Molecular Oxygen

Abstract Adipic acid is produced industrially by the oxidation of cyclohexanone and cyclohexanol with nitric acid. Dinitrogen oxide (nitrous oxide, N2O), a non-CO2 greenhouse gas, is produced as a by-product in this process. We have investigated an industrial process for the production of adipic acid without the generation of N2O. When acetic acid was used as a solvent and Mn(OAc)2/Co(OAc)2 were used as catalysts under atmospheric pressure of pure O2 at 70 °C, the selectivity of adipic acid was 77% and the combined selectivity of carboxylic acids, which consisted of adipic acid, glutaric acid, and succinic acid, was 93% for the 100% conversion of cyclohexanone. We presume that adipic acid is generated via 6-oxohexanoic acid. Glutaric acid was probably generated via 5-oxopentanoic acid in the reaction path that branched at 6-oxohexanoic acid. When air containing less than 10 vol % of oxygen was used at 1.22 MPa in order to avoid explosion hazards, the selectivity of adipic acid was 78% for a 99.3% conversion of cyclohexanone. We conclude that the present reaction can be used as an industrial process for adipic acid production.

Role of cobalt molecular sieves in the liquid-phase oxidation of cyclohexane to adipic acid

The catalytic activity of CoAPO-5 in the oxidation of cyclohexane to adipic acid has been re-examined in order to specify the actual role of the solid. It was shown that the catalytic activity in acetic acid was ascribable to the leaching of cobalt. The activity of CoAPO-5 was comparable to that of cobalt(II) acetate at very low concentration. Surprisingly, a maximum of activity was found at 0.17 mmol dm−3, which corresponds to the concentration of cobalt leached from the solid in the reaction. Such a concentration is far below the concentration usually used for homogeneous catalysis. Variations in adipic acid selectivity, which followed the variations in activity, were interpreted by changes in mechanism.

Product Distributions in the Liquid-phase Oxidation of Cyclohaxane over CoAPO-5.

CoAPO-5 has been found to be a potential catalyst for the one-step liquid-phase oxidation of cyclohexane to produce cyclohexanone and adipic acid (without any promoter being added and with no induction period). Effects of various reaction conditions, including catalyst loading (10–5–2.5 × 10–3 kg), concentration of cyclobexane (1.44–4.33 × 103 mol/m3), oxygen pressure (5–15 × 105 Pa), temperature of reaction (115–135°C) and amount of acetic anhydride (0–60 × 10–6 m3) on the conversion and selectivity of liquid-phase oxidation of cyclohexane using CoAPO-5 as catalyst have been investigated. The results reveal that the selectivity of adipic acid is higher at reaction conditions of higher cyclohexane concentration, higher reaction temperature (135°C) and higher oxygen pressure (15 × 105 Pa) before 4 hrs. Accumulated water in the reaction mixture will affect conversion and selectivity of reaction if its amount is large enough. Addition of acetic anhydride increases the rate of cyclohexane oxidation and the selectivity of cyclohexylacetate (from the esterification of cyclohexanol and acetic acid) and succinic acid but lowers the selectivity of adipic acid.