Bismuth supported SBA-15 catalyst for vapour phase Beckmann rearrangement reaction of cyclohexanone oxime to ɛ-caprolactam

Bismuth supported SBA-15 catalyst for vapour phase Beckmann rearrangement reaction of cyclohexanone oxime to ɛ-caprolactam

Accepted Manuscript Title: Cost Effective Bismuth Supported SBA-15 Catalyst for Vapor Phase Beckmann Rearrangement Reaction of Cyclohexanone Oxime to ...

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Accepted Manuscript Title: Cost Effective Bismuth Supported SBA-15 Catalyst for Vapor Phase Beckmann Rearrangement Reaction of Cyclohexanone Oxime to ε-Caprolactam Author: Rawesh Kumar Nagasuresh Enjamuri J. K Pandey Debasis Sen S. Mazumder Asim Bhaumik Biswajit Chowdhury PII: DOI: Reference:

S0926-860X(15)00142-8 http://dx.doi.org/doi:10.1016/j.apcata.2015.02.044 APCATA 15286

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

15-1-2015 26-2-2015 28-2-2015

Please cite this article as: R. Kumar, N. Enjamuri, J.K. Pandey, D. Sen, S. Mazumder, A. Bhaumik, B. Chowdhury, Cost Effective Bismuth Supported SBA15 Catalyst for Vapor Phase Beckmann Rearrangement Reaction of Cyclohexanone Oxime to rmvarepsilon-Caprolactam, Applied Catalysis A, General (2015), http://dx.doi.org/10.1016/j.apcata.2015.02.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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*Graphical Abstract (for review)

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*Highlights (for review)

Highlights Bismuth

Supported

SBA-15

Catalyst

for

Gas

Phase

Beckmann

Rearrangement Reaction of Cyclohexanone Oxime to ε-caprolactam Rawesh Kumar1, Nagasuresh Enjamuri1, J.K Pandey2, Debasis Sen3, S. Mazumder3, Asim

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Bhaumik4 and Biswajit Chowdhury1*

1. Department of Applied Chemistry, Indian School of Mines, Dhanbad, India

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2. Central Institute of Mining and Fuel research, Brawa Road, Dhanbad, India

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3. Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

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4. Solid State Physics Division, Bhabha Atomic Research Center (BARC), Mumbai, India

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*Corresponding author: [email protected]; Tel: (+91)-326-2235663; Fax: (+91)-326-2296563

 Low cost bismuth metal precursor is used for Beckmann rearrangement reaction

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 Non-polar solvent is found beneficial than polar solvent

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 Surface silanol in the vicinity of Lewis acid centre is suggested for protonation during reaction

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Cost Effective Bismuth Supported SBA-15 Catalyst for Vapor Phase Beckmann Rearrangement Reaction of Cyclohexanone Oxime to ε-Caprolactam Rawesh Kumar1, Nagasuresh Enjamuri1, J. K Pandey2, Debasis Sen3, S. Mazumder3, Asim Bhaumik4

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and Biswajit Chowdhury1* 1. Department of Applied Chemistry, Indian School of Mines, Dhanbad, 826004, India

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2. Respiratory Protection Lab, Central Institute of Mining and Fuel research, Brawa Road, Dhanbad, 826015,India

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3. Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

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4. Solid State Physics Division, Bhabha Atomic Research Center (BARC), Mumbai, India *Corresponding author: [email protected]; Tel: (+91)-326-2235663; Fax: (+91)-326-

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2296563

ABSTRACT

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Development of environmental benign as well as cost effective catalyst is a challenge for chemical

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industry. In this work we report Bi promoted SBA-15 catalyst for vapour phase Beckmann

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rearrangement reaction of cyclohexanone oxime. The three different Bi loaded SBA-15 catalysts are prepared and characterised by BET surface area and porosity measurement, small angle X-ray scattering, wide angle X-ray Diffraction, field emission scanning electron microscope, high resolution transmission electron microscope, ultraviolet–visible spectroscope, Fourier transform infrared spectroscopy and temperature programmed desorption techniques. Further, catalytic activity is optimized by changing reaction temperature, pre-treatment temperature and pre-treatment time so that 100% cyclohexanone oxime conversion and 100% ε-caprolactam selectivity has been achieved. Role of solvent, silylation and time on stream has also been examined. The TPD profiles of all three bismuth loaded samples are mainly characterised by weak acidic sites or surface silanol groups. The FTIR 1

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profile shows hydroxyl rich surface in the vicinity of bismuth at Bi-SBA-15 (Bi/Si=1/100). Based on catalyst characterisation and activity data correlation, it can be suggested that protonation at oxime oxygen can be triggered by surface silanol in the vicinity of a Lewis acid centre bismuth. The high

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Keywords: SBA-15, Bismuth, Cyclohexanone oxime, ε-Caprolactam

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space-time-yield of caprolactum was 114.3 mol gcat-1 h-1 was obtained.

1. Introduction

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ε-Caprolactam is an important intermediate for the manufacture of synthetic fibers and engineering

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plastics[1]. The classical process for production of ε-caprolactam is ecologically and economically questionable, due to the involvement of concentrated sulfuric acid as a homogeneous catalyst during

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the reaction. Furthermore, the neutralization of the reaction mixture with ammonia or ammonium hydroxide generates large quantities of ammonium sulfate as a waste. A solution to the problem could

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be change from homogeneous to heterogeneous catalysis, wherein catalysts need not to be separated.

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Recently, Rawesh et. al. have discussed the factors affecting the cyclohexanone oxime conversion and

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ε-caprolactam selectivity over different zeolite systems [2]. Apart from the zeolite, mixed oxide [3-12] and mesoporous molecular sieves [13-24] are used frequently towards vapour phase Beckmann reaction taking cyclohexanone oxime as substrate. The use of mixed oxide is limited because of its rapid deactivation during Beckmann rearrangement reaction. Mesoporous molecular sieves are promising candidate due to high temperature stability and easy diffusion of reactants and products. Mesoporous molecular sieves containing metal like Al, B, W, Nb are being reported previously as suitable catalyst in vapor phase Beckmann rearrangement of cyclohexanone oxime reaction for production of ε-caprolactam with high yield [13-24].

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Due to the high market price of several catalysts, alternative catalysts for the production of εcaprolactam are always demanded so that cost effective as well as environmentally friendly process can be demonstrated. In recent years, low cost non-toxic bismuth compounds have been employed as eco-

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friendly mild Lewis acid catalysts system in synthetic green chemistry [25]. Bismuth belongs to group XV and has an electron configuration of [Xe] 4f145d106s26p3. Due to the weak shielding of the 4f

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electrons (Lanthanide contraction), bismuth (III) compounds exhibit Lewis acidity [26]. Herein,

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towards a cost effective catalytic system, we first time report the synthesis of Bi incorporated hexagonal mesoporous system Bi-SBA-15 (Bi/Si=1/100 to 5/100) as a suitable catalyst for synthesis of

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ε-caprolactam. The catalyst are synthesized through sol-gel method and characterized by BET surface area and porosity measurement, small angle X-ray scattering (SAXS), wide angle X-ray Diffraction

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(WAXRD), field emission scanning electron microscope (FESEM), high resolution transmission electron microscope (HRTEM), ultraviolet–visible spectroscope (UV–VIS), Fourier transform infrared

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spectroscopy (FTIR) and temperature programmed desorption (TPD) techniques. The developed

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catalysts are employed for vapour phase Beckmann rearrangement reaction of cyclohexanone oxime to

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ε-caprolactam and further the catalytic activity is optimized with bismuth loading, reaction temperature, pretreatment time and pretreatment temperature. Role of solvent and time on stream has also been examined. Effect of silylation has also been studied for verifying role of surface silanol groups in the reaction. An interesting correlation between catalytic activity and catalyst characterization results has been found.

2. Experimental:

2.1. Catalyst Preparation High quality of hexagonally ordered mesoporous Bi-SBA-15 was synthesized by using triblock copolymer as a template under acidic condition [27, 28]. In this synthetic procedure, Pluronic P123 3

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(Aldrich) (4 g) was dissolved in 150 mL distilled water and 7–8 g 35% HCl (Merck) by continuous stirring. After 4 h of stirring a clear solution was obtained and then 4 g n-butanol (Merck) was added to this solution with continuous stirring for 1 h. Then tetraethyl orthosilicate (8.4 g) (TEOS, Acros) and

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desired amount bismuth nitrate (Merck) dissolved in acetic acid were being added to this solution. The resulting gel composition of the mixture is P123 : H2O : HCl : n-Butanol : TEOS : Bi(NO3)2 6 H2O::

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0.017 : 200 : 5.4 : 1.325 : 1 : 0.01-0.05 (molar ratio). The mixture was stirred for 24 h at room

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temperature. Then the mixture was taken in a closed polypropylene bottle and aged at 373K for 24 h under static hydrothermal conditions. After hydrothermal treatment the material was filtered in hot

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conditions and then dried at 373K for 12 h in air. For template removal, the as-synthesized silica powder was slurred in an ethanol-HCl mixture (1:1) and subsequently calcined at 823K for 4 h.

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Silylated Bi-SBA-15 (Bi/Si=1/100) catalyst is prepared as mentioned by Chowdhury et. al. in previous work [29]. The catalyst was trimethylsilylated at 423 K by passing methoxytrimethylsilane vapor

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(maintained at 298 K) in an Ar stream over the catalyst for 30 min, followed by flushing with Ar at 473

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K for 5h.

2.2 Catalyst Characterization

2.2.1 BET Surface Area and Porosity Measurement: The BET S. A. and pore size analysis of the samples were measured at liquid nitrogen temperature with a Quantachrome Autosorb-1C-TPD at 77K. Pretreatment of the samples were done at 473K for 3 h under high vacuum. The surface area was determined by Brunauer-Emmett-Teller (BET) equation. Pore size distributions were calculated using NLDFT (Non-linear Density Functional Theory) model of cylindrical pore approximation.

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2.2.2 Small Angle X-Ray Scattering (SAXS): Small-angle x-ray scattering (SAXS) measurements were performed using a laboratory based SAXS instrument with Cu K X-ray source. Variation of

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scattering intensity with wave vector transfer [q=4πsin (Ɵ)/] was measured for the powder samples.

2.2.3 Wide Angle X-Ray Diffraction (WAXRD): Wide angle X-ray diffraction (WAXRD) analysis

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was carried out by using Rigaku Ultima 4 diffractometer operated at 40 kV voltage and 40 mA current

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and calibrated with a standard silicon sample, using Ni-filtered Cu Kα (λ = 0.15406 nm) radiation.

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2.2.4 Field Emission Scanning Electron Microscope (FESEM): Field emission scanning electron microscope characterisation (FESEM) was carried out by using Supra 55, Carl (Zeiss, Germany)

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microscope. Sample was supported on lacey carbon and then coated with platinum by plasma prior to

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measurement.

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2.2.5 High Resolution Transmission Electron Microscope (HRTEM) & Energy dispersive X-ray

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(EDX): The HRTEM and EDX investigation was done on JEOL JEM 2100 microscope operated at 200 KV acceleration voltage using lacey carbon coated Cu grid of 300 mess size. EDX was used from Oxford Instruments (model x-sight).

2.2.6. Ultraviolet–Visible Spectroscope (UV-VIS): DRUV Visible measurement was carried out by using Varian Cary 500 (Shimadzu) spectrophotometer. The spectra were recorded in the range of 200– 800 nm wavelength.

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2.2.7 Fourier Transform Infrared Spectroscopy (FTIR): The FT-IR measurements were carried out by using Perkin Elmer GX spectrophotometer. The spectra were recorded in the range 400-4000cm-1 using KBr pellet.

ammonia of the sample

was

done

using

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thermal

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2.2.8 Temperature Programmed Desorption (TPD): Temperature programmed desorption (TPD) of conductivity detector

(TCD)

in

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Micromeritics Chemisorb 2720 instrument. For the experiment the samples was first degassed in flow

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of Helium at a flow rate of 30 cc for 2 hour at 473K. Then the sample was saturated with 10 % NH3 in He at room temperature for 30 minutes. Afterwards the excess NH3 was flushed out in a flow of He

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(flow rate 30 cc/min) for 45 minutes. Then the temperature programmed desorption of ammonia (carrier gas He) was obtained by heating from ambient temperature to 773K at a temperature ramp of

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10oC/ min.

2.3 Vapor Phase Beckmann Rearrangement of Cyclohexanone Oxime

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The catalytic reaction was carried out in a fixed bed catalytic reactor (quartz made, 1.5 cm inner diameter). The catalyst, Bi-SBA-15 (85 mesh size) of 0.3g amount was packed into the reactor and then

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pretreatment was carried out by flowing air (moisture free, 30 ml/min flow maintained by Aalborg mass flow controller) at 673K for 4 h. The catalyst bed temperature was monitored with a thermocouple touching the catalyst bed. After the pretreatment the reactor was cooled to the desired reaction temperature and then N2 gas (purity 99.999%, 30 mL min-1) was passed through the catalyst bed for 15 min. To prepare the feed mixture, cyclohexanone oxime (Acros) was dissolved in a solvent (anhydrous benzene or ethanol or methanol) in mole ratio of 0.0088: 0.1408 (cyclohexanone oxime: solvent). To maintain this mole ratio, weight ratio of cyclohexanone oxime:

benzene: ethanol:

methanol were 1: 11: 6.48: 4.5 was prepared. The solution was injected into the reactor by a syringe pump (B/BRAUN) along with N2 (purity 99.999%) as a carrier gas (30 mL min-1). The flows of the 6

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gases were maintained by Aalborg mass flow controller. The reactor outlet was connected to a cooling trap, which was immersed into a salt–ice freezing mixture. The reactor effluent was collected at different time intervals and analyzed by a gas chromatograph (CIC, India) equipped with a flame

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ionization detector and SE-30 column. The cyclohexanone oxime conversion, ε-caprolactam

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3 Characterisation Results

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selectivity, space time yield (STY) and mass balance are calculated by following formula:

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3.1 Surface Area and Porosity Measurement: Nitrogen sorption isotherms of three different BiSBA-15 samples is plotted in Fig. 1 and the textural data (surface area, pore volume and pore diameter) are depicted Table S1. All Bi-SBA-15 samples indicate typical type IV adsorption/desorption isotherms with H1 hysteresis loop, which is characteristic of the mesoporous materials with 2Dhexagonal structure, as defined by IUPAC system. The surface area of Bi loaded SBA-15 samples is found higher than SBA-15 samples. It indicates that bismuth is successfully incorporated into silica network and expanded the silica framework. The shape of the N2 adsorption isotherm of the all BiSBA-15 samples is almost similar except for some reduction in the height of the hysteresis loop with increase in bismuth loading. This observation suggests that bismuth oxide species occupied some of the

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pores of SBA-15, showing lower surface area. The decreases of surface area and pore volume for BiSBA-15 samples with increase in loading are similar as reported in literature [27].

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3.2. Small Angle X-Ray Scattering (SAXS) & Wide Angle X-Ray Diffraction (WAXRD): Small angle X-ray scattering (SAXS) of the Bi–SBA-15 catalysts with different Bi loadings are shown in Fig.

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2. The Bi-SBA-15 catalyst exhibits an intense diffraction peak corresponding to the (10) plane at 0.64

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nm-1 which is a typical characteristic of hexagonal mesoporous silica material [30,31]. The intensity of the reflection corresponding to the (10), (11) and (20) plane shows long range order for SBA-15 type of

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material.

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Wide-angle XRD patterns of Bi–SBA-15 catalysts with different Bi loadings are depicted in Fig S1. All samples show one broad peak in the range 20° to 23° range which is attributed to amorphous silica

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[32]. No phases of pure Bi2O3 are found in the wide angle XRD pattern [33]. This indicates that even at

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highest loading bismuth oxide is well dispersed over silica surface and is not detected by wide angle

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XRD pattern. The higher dispersion of bismuth on silica surface renders the active sites available towards reactant molecules during the progress of reaction.

3.3 Field Emission Scanning Electron Microscope (FESEM) & High Resolution Transmission Electron Microscope (HR-TEM):

The FESEM image of SBA-15 and bismuth loaded SBA-15 catalysts are shown in Fig. 3. From FESEM image it is evident that loading of bismuth over SBA-15 changes the morphology of the catalyst. It is observed that morphology of undoped SBA-15 is vermicular stalks. With increasing bismuth loading till Bi-SBA-15 (Bi/Si=5/100), large scale annulations and disappearance of vermicular 8

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morphology are observed in Fig. 3 (a-d). The HRTEM image and EDX profile of Bi-SBA-15 (Bi/Si=1/100) are shown in Fig. 3 (e-g). Highly ordered hexagonal array of uniform channels can be easily observed in HRTEM image of Bi-SBA-15 (Bi/Si=1/100) sample. The presence of bismuth is

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confirmed by EDX profile.

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3.4 Ultraviolet–Visible Spectroscopy (UV-VIS):

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All bismuth-containing SBA-15 samples show an intense UV band centered around 215 nm (Fig. 4). This bond implies the presence of a ligand-to-metal charge transfer involving isolated metal sites,

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which may be in tetrahedral coordination in the silica network [34]. As loading increases, intensity of band at 215nm increases sharply. The absence of absorption band at 400 nm clearly indicates non-

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existence of Bi2O3 extra-frame work species at highest loading [35] which is similar to XRD results obtained. The UV-Vis-NIR spectra of the silylated Bi-SBA-15 (Bi/Si=1/100) catalyst show small bands

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groups (Fig S2).

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in the region of 6000–5500 cm-1 which can be assigned to the overtones and combination bands of CH3

3.5 Fourier Transform Infrared Spectroscopy (FTIR) The FTIR spectra of SBA-15 and bismuth loaded SBA-15 is shown in Fig. 5. The IR band at 1085 cm-1 and 1195 cm-1 are attributed to the asymmetric stretching vibration of framework Si-O-Si bridges for Bi-SBA-15 samples with three different loading of bismuth [27]. The strong band at 817 cm-1 is attributed to the Si-O-Si symmetric stretching [36]. The decrease of peak intensity at 817 cm-1 for Bi incorporated SBA-15 in comparison to SBA-15 catalyst might be due to the formation of Si-O-Bi moiety. The band at 960 cm-1 has been interpreted for stretching vibration of Si__OH in surface silanol groups. In metal loaded sample this peak is assigned as stretching vibration of surface silanol (Si__OH) 9

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groups which is perturbed due to nearby metal species [37]. At highest loading, 960 cm-1 peak intensity decreases which may be due to increase of extra-framework bismuth oxide species. The weak band at 1633 cm-1 and the broad band at 3500 cm-1 can be attributed to a combination of the stretching

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vibration of silanol groups or silanol ‘‘nests” with cross hydrogen-bonding interactions and the H-O-H

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stretching mode of physiosorbed water [38].

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3.6 NH3- Temperature Programmed Desorption (TPD):

Ammonia temperature programmed desorption profile amount of ammonia adsorbed of all three

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bismuth loaded SBA-15 samples is shown in Fig. 6 and Table S2 respectively. The TPD profiles of all three bismuth loaded samples are mainly characterised by weak acidic sites centred about 368K. All

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three bismuth loaded sample show low temperature acid profile (<473K) or weak acid sites whereas the

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4. Catalytic Activity and Discussion

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highest bismuth loaded SBA-15 sample has smaller density of weak acid sites.

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The catalytic activity for Beckmann rearrangement of cyclohexanone oxime is tested over Bi-SBA-15 catalyst in fixed bed reactor at ambient condition. The catalytic activity is optimized by changing bismuth loading, pretreatment time and pretreatment temperature as described below. Role of solvent, silylation and time on stream has also been examined.

4.1 Effect of Different Loading

The catalytic activities of the bismuth loaded and unloaded SBA-15 materials towards the Beckmann rearrangement reaction are shown in Table 1. Undoped SBA-15 showed 64% cyclohexanone oxime conversion and 79% ε-caprolactam selectivity. It indicates inherent acidity in the SBA-15 network 10

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which is capable of performing essential 1, 2 alkyl shifts and results in ε-caprolactam formation. The similar findings were observed in different studies based on MCM and SBA-15 materials previously [39, 14, 21]. After 4h pretreatment at 673K, Bi-SBA-15 (Bi/Si=1/100) catalyst shows superior

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at 623K reaction temperature with space time yield (STY) 97.8 mol g-1cat h-1.

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performance toward cyclohexanone oxime conversion (89.4%) and ε-caprolactam selectivity (95.7%)

4.2 Effect of Reaction Temperature

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Effect of reaction temperature on the catalytic activity is studied over Bi-SBA-15 (Bi/Si=1/100). At

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573K temperature, no reaction takes place whereas at higher temperature (673K) 100% cyclohexanone oxime conversion is achieved but ε-caprolactam selectivity were found very poor. Many unwanted

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products are appeared which may be due to decomposition products of ε-caprolactam at higher temperature [23]. It is observed that best activity with respect to cyclohexanone oxime conversion and

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ε-caprolactam selectivity is obtained at 623K temperature.

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4.3 Effect of Pretreatment Time and Pretreatment Temperature The effect of pretreatment time and pretreatment temperature on the cyclohexanone oxime conversion and ε-caprolactam selectivity over Bi-SBA-15 (Bi/Si=1/100) are shown in Table 2 and Table 3 respectively. At longer pretreatment times and higher pretreatment temperatures, sub-layer active species present in the mesopores which diffused to the surface generating more active sites [27]. So, a longer pre-treatment time (5 h) and higher pre-treatment temperature (773K) are favorable for obtaining optimum catalytic performance in this present case.

4.4 Time on Stream (TOS) study

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Time of stream of cyclohexanone oxime conversion and ε-caprolactam selectivity is studied over the Bi–SBA-15 catalyst and presented in Table S3. On increasing time on stream, cyclohexanone oxime conversion as well as ε-caprolactam selectivity decreases and at the same time cyclohexanone and

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aniline selectivity are increased. On prolong time on stream study, catalytic active sites are possibly masked by oligomers which results into decreasing cyclohexanone oxime conversion. During

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rearrangement H2O molecule is formed which can compete at oxime intermediate and gives

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cyclohexanone as hydrated product.

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4.5 Effect of Silylation

To find the role of surface hydroxyl groups, the Bi-SBA-15 (Bi/Si=1/100) catalysts are being silylated

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and the activity results are provided in Table S4. It is observed that after silylation the catalyst activity

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is decreased compared to un-silylated catalyst. It indicates that silylation possibly reduces the

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concentration of surface silanol groups rendering poor catalytic activity.

4.6 Effect of Solvent:

Role of polar and non-polar solvent is studied over Bi-SBA-15 (Bi/Si=1/100) catalyst after 4h pretreatment at 673K and product is collected at 623K reaction temperature. The catalytic activity results are shown in Table S5. Non-polar solvents are generally found inferior in most literature report of Beckmann rearrangement of cyclohexanone oxime conversion except the paper presented by B-Q. Xu et. al. and cheng et. al. [7,40]. In the present case, using non-polar benzene as a solvent, high cyclohexanone oxime conversion (89.4 %) and ε-caprolactam selectivity (95.7%) are found. In case of methanol only 1.5% cyclohexanone oxime conversion is found while by using ethanol only 10% 12

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cyclohexanone oxime conversion is noticed. As reported in literature, bismuth containing siliceous surface has good interaction with hydrocarbon (Benzene, toluene and cyclohexane) in hydrocarbon oxidation reaction [35, 41-43]. So, hydrocarbon (benzene) containing cyclohexanone oxime may be

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interacted well with catalytic active site which leads high cyclohexanone oxime conversion and εcaprolactam selectivity. By using polar solvent, catalytic activity is steeply dropped down even those

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observed over un-doped SBA-15. Cheng et. al. proposed that the reactants blocked the active sites

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responsible for Beckmann rearrangement reaction due to the competitive adsorption of polar solvents at

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catalyst acidic sites [40].

5. Proposed Reaction Mechanism:

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It is reported that Lewis acidity of bismuth (III) compounds are mainly due to poor shielding of the f-

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orbital electrons (Lanthanide contraction) [26] rendering the acceptance lone pair electron readily.

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Previously, Kyle J. Eash et. al. [44] claimed that bismuth can co-ordinate the oxygen of acetal which increases the susceptibility of the acetal carbon for nucleophilic attack by water during the deprotection

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reaction of acetals and ketals. During transformation of epoxides to aldehydes and ketones by bismuth salts, Andrew M. Anderson et. al. suggested that the rearrangement product is determined by the identity of the Lewis acid, complexation of Lewis acid site at oxygen of epoxide and migratory aptitude of the epoxide substituent [26]. H. Firouzabadi et. al. claimed the coordination of bismuth to the cyclohexanone oxime’s nitrogen increases the susceptibility of the oxime carbon to nucleophilic attack by water leading the formation of cyclohexanone [45]. In our case, water is not used in reaction medium and pretreatment of catalyst and reaction is carried out very high temperature, so nucleophilic attack by water molecule can be neglected.

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As reported earlier, during Beckmann rearrangement reaction cyclohexanone oxime is attacked by acidic sites of catalysts more preferably to oxygen center of oxime [46, 20]. NH3-TPD results showed that bismuth loaded samples are mainly characterised by weak acidic sites or surface silanol. The FTIR

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profile also showed hydroxyl rich surface in the vicinity of bismuth at Bi-SBA-15 (Bi/Si=1/100). As reported in previous literature bismuth salts are found to retain Lewis acidic character [26]. It is also

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reported that the Bronsted acid strength of the surface hydroxyl groups are enhanced by Lewis acidic

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sites [47]. So, it can be concluded that protonation of cyclohexanone oxime oxygen centre can be triggered by surface silanol in the vicinity of a Lewis acid centre or bismuth cation (Fig. 7). Further, the

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shifts of O-protonated oxime to N-protonated oxime create possible 1, 2 alkyl shift for ε-caprolactam formation. The decrease of cyclohexanone oxime conversion with increasing bismuth loading may be

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claimed to increase of extra-framework bismuth oxide as verified by decreasing pore volume and pore diameter with increasing bismuth loading. After silylation, surface hydroxyl in vicinity of bismuth is

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6. Conclusion

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possibly blocked and become the region of fast drop of catalytic activity.

We first time report cost effective, heterogeneous and environment friendly Bi-SBA-15 for vapour phase Beckmann rearrangement reaction of cyclohexanone oxime to ε-caprolactam. Activity over BiSBA-15 is optimized by changing bismuth loading, reaction temperature, pretreatment time and pretreatment temperature such that 100% cyclohexanone oxime conversion and 100% ε-caprolactam selectivity is achieved over Bi-SBA-15 (Bi/Si=1/100). At higher Bi loading the activity of catalyst was less. Non-polar solvent is found superior performance than polar. Based on the catalytic characterization and activity data, it is suggested that protonation of cyclohexanone oxime’s oxygen centre can be triggered by surface silanol in the vicinity of a Lewis acid centre bismuth. 14

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Acknowledgments R.K. would like to acknowledge Indian School of Mines, Government of India, for a junior research fellowship. B.C. would like to acknowledge UGC, Government of India, for funding (39/802/2010

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(SR)) and DST, Government of India, for funding (SR/S1/PC-10/2012). NE acknowledges DST for

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providing research fellowship.

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References:

[1] G. Dahlhoff, U. Barsnick, W.F. Hölderich, Appl. Catal. A: Gen., 210 (2001) 83–95

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[2] R. Kumar, B. Chowdhury, Ind. Eng. Chem. Res., 53 (2014) 16587–16599

[3] M. Ghiaci, A. Abbaspur, R. J. Kalbasi, Appl. Catal. A : Gen., 287 (2005) 83–88

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[4] B.-Q. Xu , S.-B. Cheng, S. Jiang, Q.-M. Zhu, Appl. Catal. A : Gen., 188 (1999) 361–368

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[5] L. Forni, G. Fornasari, C. Tosi, F. Trifirò, A. Vaccari, F. Dumeignil, J. Grimblot, Appl. Catal. A: Gen., 248 (2003) 47–57

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Figure Captions Fig. 1 Nitrogen adsorption–desorption isotherms (a) Bi-SBA-15 (Bi/Si=1/100) (b) Bi-SBA-15 (Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

ip t

Fig. 2 Small angle X-ray scattering of the Bi–SBA-15 catalysts (a) Bi-SBA-15 (Bi/Si=1/100) (b) BiSBA-15 (Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

cr

Fig. 3 FESEM image of (a) SBA-15 (b) Bi-SBA-15 (Bi/Si=1/100) (c) Bi-SBA-15 (Bi/Si=2.5/100) (d) Bi-SBA-15(Bi/Si=5/100), HRTEM image of Bi-SBA-15 (Bi/Si=1/100) (e) & (f) and EDS of Bi-SBA15 (Bi/Si=1/100) (g)

us

Fig. 4 UV-VIS band of (a) SBA-15 (b) Bi-SBA-15 (Bi/Si=1/100) (c) Bi-SBA-15(Bi/Si=2.5/100) (d) Bi-SBA-15 (Bi/Si=5/100)

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Fig. 5 FTIR spectra of (a) SBA-15 (b) Bi-SBA-15 (Bi/Si=1/100) (c) Bi-SBA-15 (Bi/Si=2.5/100) (d) Bi-SBA-15 (Bi/Si=5/100) Fig. 6 NH3 temperature programmed desorption profile of catalysts (a) Bi-SBA-15 (Bi/Si=1/100) (b) Bi-SBA-15 (Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

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Fig. 7 Proposed reaction mechanism for Beckmann rearrangement of cyclohexanone oxime to εcaprolactam conversion over Bi-SBA-15.

te

d

Table Captions

Table 1 Catalytic activity of Bi-SBA-15 at different loading towards Beckmann rearrangement reaction

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Table 2 Catalytic activity of Bi-SBA-15 (Bi/Si=1/100) at different pretreatment time towards Beckmann

rearrangement reaction

Table 3 Catalytic activity of Bi-SBA-15 (Bi/Si=1/100) at different pretreatment temperature towards

Beckmann rearrangement reaction

18

Page 20 of 25

Table

Table Table 1 Catalytic activity of Bi-SBA-15 catalyst at different loading towards Beckmann rearrangement reaction Pretreatment

Pretreatment

temperature (K)

time (min)

Conversion

STY

Selectivity (%) ε-Caprolactam Cyclohexanone

(%)

673

4h

64.3

79.2

20.8

1%Bi-SBA-15 (Bi/Si=1/100) 2.5%Bi-SBA-15 (Bi/Si=2.5/100) 5% Bi-SBA-15 (Bi/Si=5/100)

673

4h

89.4

95.8

4.2

673

4h

86.8

91.2

8.6

673

4h

85.9

89.9

9.7

Aniline mol h-1 gcat –1 0 58.2

(%) 94.3

0

97.8

98.35

0.2

90.5

96.7

0.4

88.2

96.2

us

cr

SBA-15

Mass balance

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Catalyst

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Reaction condition: Reactant (Solution of Cyclohexanone Oxime in benzene, Cyclohexanone oxime: Benzene weight ratio is (1:11), flow=5ml/h, Reaction temperature=623K, carrier gas (N2) flow = 30 ml/min Table 2 Catalytic activity of Bi-SBA-15 (Bi/Si=1/100) at different pretreatment time towards Beckmann

Catalyst

Pretreatment

Pretreatment

Conversion

temperature (K) time (min)

673

4h

673

5h

80.0 89.4 98.9

STY

Selectivity (%)

Mass balance

9.0

Aniline mol h-1 gcat –1 (%) 0 98.12 83.3

95.7

4.3

0

97.8

98.35

99.7

0.2

0.1

112.8

99.89

ε-Caprolactam Cyclohexanone 91.0

d

3h

te

673

(%)

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1%Bi-SBA-15 (Bi/Si=1/100) 1%Bi-SBA-15 (Bi/Si=1/100) 1% Bi-SBA-15 (Bi/Si=1/100)

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rearrangement reaction

Reaction condition: Reactant (Solution of Cyclohexanone Oxime in benzene, Cyclohexanone Oxime: Benzene weight ratio is (1:11), flow=5ml/h, Reaction temperature=623K, carrier gas (N2) flow = 30 ml/min Table 3 Catalytic activity of Bi-SBA-15 (Bi/Si=1/100) at different pretreatment temperature towards

Beckmann rearrangement reaction Catalyst

1%Bi-SBA-15 (Bi/Si=1/100) 1%Bi-SBA-15 (Bi/Si=1/100) 1% Bi-SBA-15 (Bi/Si=1/100)

Pretreatment

Pretreatment

temperature (K)

time (min)

Conversion

STY

Selectivity (%)

(%)

ε-Caprolactam Cyclohexanone

Mass balance

573

5h

98.9

100

0

Aniline mol h-1 gcat –1 0 113.1

673

5h

99.1

100

0

0

113.3

99.88

773

5h

100

100

0

0

114.3

99.89

(%) 99.89

Reaction condition: Reactant (Solution of Cyclohexanone Oxime in benzene, Cyclohexanone Oxime: Benzene weight ratio is (1:11), flow=5ml/h, Reaction temperature= 623K , carrier gas (N2) flow = 30 ml/min

Page 21 of 25

Figure

Figures

(c)

(b)

an

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cr

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(a)

Fig. 1 Nitrogen adsorption–desorption isotherms (a) Bi-SBA-15(Bi/Si=1/100) (b) Bi-SBA-15

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te

d

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(Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

Fig. 2 Small angle X-ray scattering of the Bi–SBA-15 catalysts (a) Bi-SBA-15(Bi/Si=1/100) (b) BiSBA-15 (Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

Page 22 of 25

ip t cr us an

Ac ce p

te

d

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Fig.3 FESEM image of (a) SBA-15 (b) Bi-SBA-15(Bi/Si=1/100) (c) Bi-SBA-15 (Bi/Si=2.5/100) (d) Bi-SBA-15 (Bi/Si=5/100), HRTEM image of Bi-SBA-15 (Bi/Si=1/100) (e) & (f) and EDS of BiSBA-15 (Bi/Si=1/100) (g)

Fig. 4 UV-VIS band of (a) SBA-15 (b) Bi-SBA-15 (Bi/Si=1/100) (c) Bi-SBA-15 (Bi/Si=2.5/100) (d) Bi-SBA-15 (Bi/Si=1/100)

Page 23 of 25

ip t cr us an M

Ac ce p

te

d

Fig. 5 FTIR spectra of (a) SBA-15 (b) Bi-SBA-15 (Bi/Si=1/100) (c) Bi-SBA-15 (Bi/Si=2.5/100) (d) Bi-SBA-15 (Bi/Si=5/100)

Fig. 6 NH3 temperature programmed desorption profile of catalysts (a) Bi-SBA-15 (Bi/Si=1/100) (b) Bi-SBA-15 (Bi/Si=2.5/100) (c) Bi-SBA-15 (Bi/Si=5/100)

Page 24 of 25

-H2O

Si

O

O

C

N O

O

N

H2O

N

us

O

Si

Bi O

O

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O

cr

OH

H2O

HO

H

N

N

N

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Fig. 7 Proposed reaction mechanism for Beckmann rearrangement of cyclohexanone oxime to ε-

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te

d

M

caprolactam conversion over Bi-SBA-15.

Page 25 of 25