Browsing by Author "Marthala, Venkata Ramana Reddy"

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Mechanistic studies of the vapor-phase Beckmann rearrangement on solid catalysts by in situ solid-state NMR spectroscopy
(2009) Marthala, Venkata Ramana Reddy; Hunger, Michael (Prof. Dr.)
Epsilon-Caprolactam, the main product of the Beckmann rearrangement of cyclohexanone oxime, is an intermediate in the manufacture of nylon-6. Currently, most of the industrial plants produce epsilon-caprolactam by the Beckmann rearrangement of cyclohexanone oxime using sulphuric acid or oleum as a homogeneous catalyst. This process has two main disadvantages: (i) The production of uneconomical by-product, such as ammonium sulphate, and (ii) the environmentally unfriendly corrosive reaction medium. In recent years, the demand for environmentally benign processes has been increasing due to the stringent environmental regulations. Therefore, the new route of the vapor-phase Beckmann rearrangement of cyclohexanone oxime on solid acid catalysts has gained increasing interest as an environmentally, economically and energetically favorable process. For the first time in 2003, the catalytic vapor-phase Beckmann rearrangement of cyclohexanone oxime on high-silica MFI-type zeolites has been industrialized by Sumitomo Chemical Company Ltd. in Japan. Despite the commercialization of this process, several issues are not clear and remain under debate. The main issues are: (i) The nature and location of active sites of the catalysts, (ii) the chemistry of the vapor-phase Beckmann rearrangement reaction (i.e. adsorption and desorption behavior of reactants, intermediates, products, and by-products of the reaction), (iii) the reasons for the catalyst deactivation, and (iv) the influence of additives during the vapor-phase Beckmann rearrangement. The present work mainly focussed on the above-mentioned issues. In this study, for better understanding of the reaction, mechanistic aspects of the vapor-phase Beckmann rearrangement on solid catalysts have been studied by in situ solid-state NMR spectroscopy. To study the reaction, several solid catalysts, such as MFI-type zeolites (silicalite-1, H-[B]ZSM-5 and H-ZSM-5) and mesoporous materials (SBA-15, [Al]SBA-15, H-[Si]MCM-41, and H-[Al]MCM-41) were prepared according to the procedures described in the literature. The catalysts were characterized by various techniques, such as powder X-ray diffraction, ICP-OES, N2 adsorption measurements (BET surface), SEM, and solid-state 1H, 2H, and 29Si MAS NMR spectroscopy. It has been reported that boron-containing ZSM-5 zeolite (H-[B]ZSM-5) is highly selective in the vapor-phase Beckmann rearrangement of cyclohexanone oxime. However, the nature of the active sites in this zeolite and the influence of reactant molecules on framework boron atoms were not reported in the literature. In this study, the nature of active sites and the coordination change of framework boron atoms upon adsorption of reactant molecules in H-[B]ZSM-5 (nSi / nB = 38) zeolite were studied by simultaneous 1H and 11B MAS NMR spectroscopy. For this purpose, several probe molecules characterized by different proton affinities (PA) were adsorbed on the zeolite. Probe molecules, such as pyridine, ammonia, and acetone, were loaded on H [B]ZSM-5 zeolites by an in situ flow technique. The 1H MAS NMR spectrum of dehydrated zeolite H [B]ZSM-5 consists of signals at 1.9, 2.5, and 3.2 ppm. Upon adsorption of pyridine (PA = 930 kJ mol-1) and ammonia (PA = 854 kJ mol-1) on H-[B]ZSM-5 zeolite, the signal at 2.5 ppm was mainly affected. Therefore, the signal at 2.5 ppm was assigned to the Brønsted acidic SiOH[B] groups in the vicinity of framework boron species. On the other hand, by 11B MAS NMR spectroscopy, the coordination change of B[3] species into B[4] species was investigated. It was found that the proton affinity of PA ≥ 854 kJ mol-1 is required for probe molecules to induce the coordination change of the framework B[3] species into B[4] species in H-[B]ZSM-5 zeolite. This coordination change of framework boron was accompanied by the protonation of probe molecules with PA ≥ 854 kJ mol-1. As a result of protonation, a decrease of the 11B quadrupole coupling constant from CQCC = 2.7 ± 0.1 MHz (B[3]) to CQCC ≤ 0.85 MHz (B[4]) was noticed. This PA value (≥ 854 kJ mol 1) is slightly higher (ca. 30 kJ mol-1) than the proton affinity of PA = 821 kJ mol-1 required for the protonation of probe molecules adsorbed on aluminum-containing zeolites reported in the literature. This is supported by the lower acid strength of SiOH[B] groups in H-[B]ZSM-5 zeolites than the bridging OH groups (SiOH[Al]) in aluminum-containing zeolites. In the present study, for the first time, high-speed 2H MAS NMR spectroscopy at the sample spinning rates of 25 kHz was applied to study the surface sites of deuterated catalysts and their interaction with non-deuterated reactant molecules. Deuterated catalysts, such as H/D-ZSM-5, H/D-[Al]SBA-15, and H/D-SBA-15, were successfully prepared by the H/D exchange of OH groups using D2O vapor. By 2H MAS NMR spectroscopy, suitable spectral resolution of the signals of SiOH groups (ca. 1.8 ppm) and bridging hydroxyl groups (ca. 5.7 ppm) in the 2H MAS NMR spectrum of H/D-ZSM-5 was achieved. Two reactant molecules characterized by different molecular diameters, such as cyclohexanone oxime (ca. 0.65 nm) and cyclododecanone oxime (ca. 0.9 nm), were adsorbed on H/D exchanged catalysts at room temperature. Upon adsorption of non-deuterated reactant molecules (i.e. cyclohexanone and cyclododecanone oximes), a dominating signal at ca. 12 ppm was observed in the 2H MAS NMR spectra of H/D exchanged catalysts (H/D-ZSM-5, H/D-[Al]SBA-15, and H/D-SBA-15). These results indicate that all kinds of surface sites of H/D-ZSM-5 zeolite, such as silanol groups, silanol nests, or Brønsted acid sites, which can be located on the external surface, near the pore mouth, or in the interior pores, are accessible for cyclohexanone oxime. In situ solid-state 1H and 15N MAS NMR spectroscopy were utilized to study the chemistry of the vapor-phase Beckmann rearrangement of cyclohexanone oxime on MFI-type zeolites and mesoporous MCM-41 and SBA-15-type materials. The results obtained in this work help to understand the adsorption, desorption, and reaction behavior of reactants, intermediates, products, and by-products formed during the reaction. Initially, in situ 1H MAS NMR spectroscopy was applied to study the step-wise conversion of cyclohexanone oxime-D11 on silicalite-1 (nSi / nAl = 1700) and H-ZSM-5 (nSi / nAl = 14) catalysts. Deuterated cyclohexanone oxime (i.e. cyclohexanone oxime-D11) was successfully prepared with the H/D exchange degree of ca. 83%. The vapor-phase Beckmann rearrangement of cyclohexanone oxime-D11 was performed on non-deuterated silicalite-1 and H-ZSM-5 catalysts at different reaction temperatures. The 1H MAS NMR spectroscopy provided the evidence on the formation of reaction intermediates, such as H-bonded cyclohexanone oxime and N-protonated cyclohexanone oxime on SiOH groups and bridging hydroxyl groups, respectively. From the 1H MAS NMR studies of the vapor-phase Beckmann rearrangement of cyclohexanone oxime-D11 on weakly acidic silicalite-1, the formation of epsilon-caprolactam, hydroxylamine, and 5-cyano-1-penetene was observed as product and by-product molecules of the reaction. In contrast, on strongly acidic H-ZSM-5 catalyst, the O-protonated epsilon-caprolactam was identified due to the strong adsorption of this species on bridging hydroxyl groups. Furthermore, the conversion of O-protonated epsilon-caprolactam on bridging hydroxyl groups of H-ZSM-5 led to the formation of N-protonated epsilon-caprolactam, or non- or N-protonated epsilon-aminocapric acid species. The observable chemical shift range of 15N MAS NMR spectroscopy is larger than the observable chemical shifts range of 1H MAS NMR spectroscopy. Therefore, in situ solid-state 15N NMR spectroscopy is the most suitable technique to study the reaction mechanism. Furthermore, the interpretation of 15N MAS NMR signals is less complicated than of 1H MAS NMR signals. In this work, the chemistry of the vapor-phase Beckmann rearrangement on solid catalysts was thoroughly studied by in situ 15N MAS NMR spectroscopy. The 15N MAS NMR results suggest that both SiOH and Brønsted acid sites are active in this reaction. 15N-cyclohexanone oxime interacts with SiOH groups via hydrogen bonding, which is indicated by the 15N MAS NMR signal in the range of -30 to -46 ppm, while it interacts strongly with Brønsted acid sites and form N-protonated cyclohexanone oxime as represented by the 15N MAS NMR signal in the range of -145 to -160 ppm. By-products, such as hydroxylamine and amines, were also observed on siliceous catalysts. The corresponding 15N MAS NMR signals of these species are in the range of -269 to -280, and -375 to -387 ppm, respectively. In addition, O-protonation of the main product epsilon-caprolactam was found on both siliceous and Brønsted acidic catalysts as evidenced by the 15N MAS NMR signal at -237 ppm. On siliceous catalysts, however, the O protonation of epsilon-caprolactam depends on the number of acidic Q3 (Si(OSi)3OH) silanol groups, which is ascertained by 29Si MAS NMR spectroscopy. Furthermore, the conversion of O-protonated epsilon-caprolactam into N-protonated epsilon-caprolactam, and non- or N protoanted epsilon-aminocapric acid was exclusively observed on Brønsted acidic catalysts. Generally, methanol as an additive in the reaction system improves the selectivity towards the desired product epsilon-caprolactam during the vapor-phase Beckmann rearrangement of cyclohexanone oxime. In this study, the influence of the methanol on the adsorbed species formed and the conversion of the additive methanol during the vapor-phase Beckmann rearrangement reaction were investigated by in situ 15N and 13C MAS NMR spectroscopy. On silicalite-1 and H-ZSM-5 zeolites, hydrocarbons (13C MAS NMR signals in the range of 25 to 30 ppm) were formed and deposited at increasing reaction temperatures. Furthermore, the formation of isopropyl amine (13C MAS NMR signal at 42 ppm) was observed on silicalite-1 as a result of the reaction of isobutane (13C MAS NMR signal at 25 ppm) and hydroxyl amine, which is a by-product of the Beckmann rearrangement. On strongly acidic H-ZSM-5 zeolite, methanol (13C MAS NMR signal at ca. 50 ppm) dehydrates into dimethylether (DME) and water. This water promotes the conversion of O-protonated epsilon-caprolactam into non- or N-protonated epsilon-aminocapric acid. On the mesoporous SBA-15 materials, no influence and conversion of methanol was detected by 15N and 13C MAS NMR spectroscopy.