Selective Decomposition of Cyclohexyl Hydroperoxide using Homogeneous and Heterogeneous Cr^VI Catalysts: Optimizing the Reaction by Evaluating the Reaction Mechanism

Abstract

IntroductionNylon 6i so ne of the most widely used polymers and is produced on al arge industrial scale ( % 4million t/a). [1][2][3][4] This importantp olymer has aw ide range of applications, for example in the textile industry in the manufacture of yarn or as thin plastic film for packaging purposes. [2] On an industrial scale, the polymer is produced throughr ing-opening polymerization of e-caprolactam at high temperatures. [1,2,4] The required starting materialf or the synthesis of e-caprolactam is cyclohexanone that reacts with hydroxylamine in ac ondensation reaction to the correspondingc yclohexanone oxime and through an acid-catalyzed Beckmann rearrangement providing the desired ecaprolactam (Scheme 1). [4] The well-established industrial process to obtain cyclohexanone is the autoxidation of cyclohexane, either non-catalyzed or catalyzed by different transition metal compounds such as soluble cobalt catalysts. [5][6][7][8][9][10][11][12][13][14][15] The liquid-phaseo xidationo fc yclohexane is conducted at temperatures up to 130 8Cincombination with highpressures (125-165 8C, 8-15 bar). These conditions are appliedt os horten the retention time to avoid oxidative side reactions. For the same reason, the cyclohexane conversion is limited to 10-12 %p er cycle. [3] One common intermediate in the oxidation procedure is cyclohexyl hydroperoxide which further decomposest oam ixture of the In this study,k nown homogeneous and heterogeneous chromium(VI) catalysts ystems were investigated with respectt o the favored formation of cyclohexanone during the decomposition of cyclohexyl hydroperoxide (CHHP). The focus was on mechanistic studies using differents pectroscopic methods as well as DFT calculations to further optimize the reactionc onditions. As in previous decomposition studies, am echanism via the formation of am etal alkylperoxido intermediate is probable. In situ spectroscopics tudies revealed that in case of both the soluble and insoluble catalyst, the selectived ecomposition happens via an on-radical, non-redox mechanism at the Cr VI stage through the formation of ac yclohexylperoxychromium(-VI) complex.T he proposed mechanism is supported by thorough DFT calculations.

Operandomonitoring of mechanisms and deactivation of molecular catalysts

“…Coupled with NMR spectroscopy and DFT calculations, a promising mechanism was proposed. 59 X-ray absorption spectroscopy (XAS), measuring the energydependent fine structure of the X-ray absorption coefficient near the absorption edge of a particular element, is a powerful tool for probing the average local electronic and geometric structures of catalysts in the working state. 60,61 It is widely used in heterogeneous operando studies, however, only few cases have been presented on molecular catalysis in situ analysis.…”

“…UV-vis spectroscopy produces comparatively sensitive spectra, which can show the d-d* and charge transfer transitions of transition metal complexes and the formation of organic molecules via their n-p* or p-p* transitions. 58,59 Due to the limited structural information UV-vis can provide mainly qualitative data. However, for those compounds having UV-vis chromophores, it is still a useful tool for supplementing information gained from other spectroscopic techniques.…”

The Influence of Reaction Conditions on the Oxidation of Cyclohexane via the In-Situ Production of H2O2

“…The current industrial means of producing KA oil via the high temperature (> 150 °C) aerobic oxidation of cyclohexane typically utilises homogeneous cobalt salts and limits conversion to below 5% in order to maintain high selectivity towards desired products. Initially cyclohexyl hydroperoxide (CHHP) is the major product that decomposes to form cyclohexanone and cyclohexanol, with carboxylic acids also produced [1,2]. At higher conversion rates significant amounts of ring-opened by-products, such as 6-hydroxyhexanoic acid, in addition to a range of C 1 -C 5 (di)acids are produced [3].…”

Tackling Challenges in Industrially Relevant Homogeneous Catalysis: The Catalysis Research Laboratory (CaRLa), an Industrial–Academic Partnership

“…CHHP is not available commercially, and it is difficult to obtain larger quantities from the dedicated lab synthesis. , It can, however, be easily isolated out of the crude oxidation mixture. Using this as starting material, the pure hydroperoxide was accessible in gram amounts …”

“…The mechanism of this catalysis was evaluated and confirmed by DFT calculations, NMR, UV–vis, and IR spectroscopy as well as kinetic measurements. The reaction order and the free Gibbs energy of activation determined by the kinetic measurement were in alignment with the lowest energy pathway of the catalysis obtained by DFT calculations …”