Elsevier

Journal of Catalysis

Volume 378, October 2019, Pages 256-269
Journal of Catalysis

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

https://doi.org/10.1016/j.jcat.2019.08.042Get rights and content

Highlights

  • Solvents manipulate catalytic activity for the one-step preparation of adipic acid.

  • 27% cyclohexane conversion and 79% adipic acid selectivity are achieved.

  • Active participation of solvents in the different types of H-abstraction process.

  • Using molecular oxygen as oxidant without any metal element.

  • Solvent modifies the catalytic-site structure and reaction barrier.

Abstract

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/>NOradical dot) 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 >NOradical dot 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 >NOradical dot 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.

Introduction

Selective aerobic oxidation of sp3 hybridized carbon of non-activated alkanes is one of the “dream reactions” for transformation of petroleum-based feedstocks to various useful chemicals from the academic and industrial perspective [1], [2], [3]. Probably, apart from theirs high bond dissociation energy of C(sp3)single bondH bonds (90–100 kcal⋅mol−1) for hydrogen-atom transfer in alkane activation [4], the O2 molecule with a nominal double bond (1.208 Å) in its triplet ground state is also kinetically hindered to produce reactive species (O2radical dot+, O2radical dot−, O22–, HOradical dot, HOOradical dot and H2O2) toward the strong C(sp3)single bondH bonds. To overcome this impediment, given that a high activation energy (e.g., high reaction temperature >175 °C) first has to be supplied for O2 and C(sp3)single bondH bond activation, this hydrogen abstraction step by O2 is even more exothermic. Thus, it is difficult to dissipate the reaction heat and stop the process at the stage of carboxylic acids to avoid overoxidation to CO2. For instance, direct aerobic oxidation of cyclohexane (with rather poor performance) produces adipic acid (AA), which is in turn used to prepare Nylon-66 polymers and pharmaceutical intermediates. At present, the industrial conversion of cyclohexane to AA undergoes a two-step process involving the Co-catalyzed autoxidation of cyclohexane at 150–180 °C to a KA-oil (a mixture of cyclohexanone and cyclohexanol) and the nitric acid oxidation of the KA-oil to AA [5], [6]. The main drawbacks of this process are that the aerobic oxidation in the first step must be operated with less than 4% cyclohexane conversion to keep a high KA-oil selectivity (70–80%), and that the nitric acid oxidation in the second step produces a large amount of undesired global-warming substances like N2O (Scheme 1) [7], [8]. An extensive body of research on bioderived feedstocks has been directed at alternative processes of AA synthesis: these processes, however, require multiple chemical conversions and/or employ transition-metal catalysts [9], [10]. The direct Csingle bondH bond functionalization of cyclohexane to AA with O2 has therefore emerged as a promising approach to molecular construction with high atom- and step-economy in industrial chemistry.

Recently a great number of interesting aerobic cyclohexane oxidation processes to AA have been achieved by using transition-metal catalyst, because the aforementioned hurdles can be overcome by use of a redox-active metal to activate both O2 and the Csingle bondH bond. For the first time, by applying a high concentration of Co3+ acetate combined with acetaldehyde as the promoter, Tanaka et al. succeeded in achieving conversion of cyclohexane to AA under 3 MPa oxygen pressure [11]. The patent described the cyclohexane oxidation with dioxygen using a novel system composed of a solvent-soluble cobalt salt and a chromium compound, with which a high AA selectivity of about 40% was attained [12]. Simonato et al. reported one-step process for obtaining AA from cyclohexane using a stable lipophilic carboxylic-acid with low manganese and cobalt salts loadings [13]. Lü and co-workers claimed an aerobic catalytic system for the direct production of AA from cyclohexane over the Anderson-type [(C18H37)2N(CH3)2]6Mo7O24 catalyst [14]. Molecular sieve catalysts with active-site metal isolation such as FeAlPO-31 and MnTS-1 have been designed for the direct oxidation of cyclohexane to AA [15], [16]. In recent years, NHPI, a powerful carbon-radical-chain promoter, has been used as a valuable catalyst for the efficient aerobic oxidation of cyclohexane to AA in the presence of transition metal ions as co-catalysts under mild reaction conditions [17]. However, most of these catalytic systems are based on metal catalysts or co-catalysts, especially transition-metal ions such as Mn, Fe, Mo and Co, which may possibly leave toxic traces of heavy metals in the product grades intended to satisfy food- and drug-grade specifications. In the context of metal-free catalysis, N-doped carbon materials were recently discovered active in the cyclohexane oxidation with <60% AA selectivity [18], [19], [20], [21]. The development of metal-free catalytic systems for the efficient oxidation of cyclohexane to AA therefore appears appealing from the perspective of sustainable and green chemistry, which still remains a scientific challenge.

The direct aerobic oxidation of cyclohexane to AA proceeds according to a widely known free radical chain mechanism, where the elementary reaction results in homolysis not only of the desired Csingle bondH bonds but also of undesired Csingle bondC bonds (a lower bond energy). More importantly, the free radical mechanism suffers from a major limitation imposed by its intrinsic nature: the Csingle bondH bond of reaction products is very often more reactive than the starting material and formed intermediate [22]. In the past several years, a unique phthalimido-N-oxyl (PINOradical dot) radical from NHPI has been introduced to the aerobic oxidation process which enhances the initiation and propagation steps in the autoxidation cycle [23], [24], [25], [26], [27]. It is demonstrated that there is an established equilibrium ROOradical dot + NHPI = ROOH + PINOradical dot, where the reverse reaction is very fast. Thus, in order to shift the equilibrium right to form more PINOradical dot, a third component such as transition metals can be added to reduce the ROOH concentration, since the PINOradical dot radical is more reactive than the ROOradical dot counterpart in abstraction of the Csingle bondH bond hydrogen [28]. Interesting, some nonmetallic organic substrates and reaction intermediate such as alcohol, aldehyde, ketone, acid and AIBN, have recently been used as mediators combined with NHPI for ROOH activation by abstracting a hydrogen [29], [30], [31], [32]. In contrast, regarding the metal-free method, the fact that organic solvents can play critical active roles in NHPI and ROOH activation during the elementary reaction step though their effects of electronic structure on the oxidation is, however, often ignored. At present, organic solvent is usually considered to be a reagent for the dissolution of NHPI [17]. Probably because catalytic system is the presence of co-catalysts or promoters, the synergistic nature of the solvent-NHPI interface for hydrogen atom transfer (HAT) appears to be of little importance for aerobic cyclohexane oxidation to AA. For example, NHPI combined with Mn(acac)2 and Co(acac)3 in acetic acid is able to show specific catalysis for the formation of AA, where a selectivity up to 70% is obtained [33]. Therefore, any further understanding of the synergistic catalytic performance of solvent would be impossible without detailed unravelling of the solvent effects alone for the NHPI-based catalytic system (without the presence of any co-catalysts or promoters) on the reactivity and AA selectivity in the aerobic cyclohexane oxidation.

Solvent-induced C(sp3)single bondH bond activation of alkane to produce active radicals can be an alternative to improve the catalytic oxidation process, avoiding the usage of organic promoters or transition-metal co-catalysts. Reported here is a novel observation of the synergistic catalytic function of solvent as an independent organic promoters or transition-metal co-catalysts for NHPI-catalytic oxidation of cyclohexane to AA with dioxygen. In the absence of any co-catalyst, these experiments are carried out for the NHPI-catalytic aerobic oxidation of cyclohexane, also including linear paraffin in different solvents. The obtained data provide a rigorous assessment of HAT features in terms of the relative cyclohexane reactivity and AA selectivity in different solvents. The synergistic effect of CH3CN and NHPI ensures its efficient catalytic ability: 27% cyclohexane conversion with 79% selectivity for AA can be achieved at 120 °C under 1.0 MPa of dioxygen in 6 h. Theoretical calculations by DFT (density functional theory) with the PCM continuum model are made to gain important information on the structures, dissociation energies, and reaction barriers of the reactants and intermediates involving in the two types of H-abstraction on >NOradical dot catalytic-sites of PINOradical dot for solvent activation, which further entitles us to propose possible rate-determining elementary reaction steps based on the catalytic action of NHPI alone. The synergistic catalytic function of solvent disclosed in this study will not only shake the understanding of the solvent as an active participant (not a reagent for only the dissolution of NHPI) in earlier NHPI-metal catalysis process, but also provides a new dimension of sustainable chemistry for the metal-free preparation of AA.

Section snippets

Characterization techniques

Cyclic voltammetry is performed using a three-electrode configuration and an autolab electrochemical workstation (Eco Chemie, Holland). The working electrode (WE) is a glassy carbon electrode (GCE, 3-mm diameter disk), the counter electrode is a graphite rod and the reference electrode is a KCl saturated calomel electrode (SCE). All the measurements here are carried out at room temperature (25 °C) under argon atmosphere, and all potentials are reported vs the SCE. The UV–visible spectra of

Active role of solvent on NHPI-catalyzed aerobic oxidation of cyclohexane

In studying solvent effects, the solvent molecule has perhaps too often been regarded as a continuous medium rather than as an active participant in the catalytic reaction. In what follows, the oxidation of cyclohexane to AA is chosen as a model reaction to investigate the active role of solvent on NHPI-catalyzed aerobic oxidation of hydrocarbons, and the central feature of the approach is envisioned to arise from the two types of H-abstraction (cyclohexane and cyclohexanone) on >NOradical dot

Conclusion

Generally, the role of solvent is limited to the simple requirement for diluting and/or dissolving reactants in the liquid-phase catalytic reactions. Here, insights are presented into the influence of solvents (acetone, acetic acid, acetonitrile, benzonitrile, propionitrile, pyridine and benzene) on manipulation of the catalytic activity and AA selectivity for one-step aerobic oxidation of cyclohexane catalyzed by NHPI catalysts under promoter- and metal-free conditions. We found that a polar

Acknowledgments

This work was supported by the Natural Science Foundation of China (Grant No. 21576078, 21878074 and 21978078) and the Natural Science Foundation of Hunan Province (Grant No. 2016JJ2081) and Innovation Platform Open Fund of Hunan College (16K052) and Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization.

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