Comprehensive Study for Vapor Phase Beckmann Rearrangement

Review pubs.acs.org/IECR

Comprehensive Study for Vapor Phase Beckmann Rearrangement Reaction over Zeolite Systems Rawesh Kumar and Biswajit Chowdhury* Department of Applied Chemistry, Indian School of Mines, Dhanbad-826004, Jharkhand, India ABSTRACT: Replacement of homogeneous catalysis systems by heterogeneous catalysis systems is widely accepted throughout academia and industry due to ecological concerns, difficulty of separations, and nonrecyclability of homogeneous catalysts. Among several heterogeneous catalysts, the zeolite system has been focused extensively on vapor phase Beckmann reactions for a few decades. In this article a review of the factors affecting the cyclohexanone oxime conversion and ε-caprolactam selectivity over different zeolite systems is presented. It can be concluded that several catalyst preparation parameters such as Si/Al ratio, different metal/nonmetal loading, micropore size, and postsynthesis treatment as well as catalyst preparation parameters like temperature and solvent have crucial roles in deciding cyclohexanone oxime conversion and product selectivity over microporous zeolite catalysts. In the last section, an attempt was made to summarize the reaction mechanism for this reaction over zeolite catalyst systems. This thorough study might be useful for efficient design of a microporous zeolite system toward Beckmann rearrangement reactions.

1. INTRODUCTION The Beckmann rearrangement of cyclohexanone oxime to εcaprolactam is an important process step in the production of polyamide 6, better known under the trade name “nylon-6”. The standard catalyst for industrial plants for Beckmann rearrangement is fuming sulfuric acid. In most of the cases, ammonia is used to neutralize the product mixture that yields an enormous amount of ammonia sulfate as high as 1.9 tons per ton of product,1 causing some ecological concerns in addition to economical drawbacks. Toward a greener approach, the work of Ikushima et al.2 and the first-principles molecular dynamics study of Mauro Boero et al.3 toward liquid phase Beckmann rearrangement using supercritical water are admirable but do not measure up to industrial standards. So, a permanent solution to the problem even for global scale plants could be a change from homogeneous to heterogeneous catalysis wherein vapor phase reaction is preferred due to easy removal and reusability of the catalyst. Among several heterogeneous catalysts, the zeolite system has been focused extensively on the vapor phase Beckmann reaction in the past. Here, we present a comprehensive review about the factors effecting the cyclohexanoneoxime conversion and ε-caprolactam selectivity in different zeolite systems named ETS-10, βzeolite, USY zeolites, mordenite, high silica MFI, layered silicates, ZSM-5, SAPO-11 and AlPO4-11, ferrierite, and A-type zeolite. These factors can be catogorized in two sections: the first section includes factors due to catalyst preparation parameters and the other one includes factors due to reaction parameters. This study might be useful for efficient design of microporous zeolite systems for Beckmann rearrangement reactions.

roles in deciding cyclohexanone oxime conversion and product selectivity over microporous zeolite catalyst. 2.1. Effect of Si/Al Atomic Ratio. The catalytic performance of USY zeolite for the vapor phase Beckmann rearrangement of cyclohexanone oxime was greatly improved by adjusting the SiO2/Al2O3 ratio. With a decreasing SiO2/ Al2O3ratio, the acid amount of H-USY zeolite increased which resulted in increasing cyclohexanone oxime conversion.4 The selectivity for ε-caprolactam formation increased initially with decreasing SiO2/Al2O3 ratio, and the maximum selectivity for εcaprolactam was attained at a SiO2/Al2O3 ratio of 62. On further decrease in the SiO2/Al2O3 ratio, the selectivity for εcaprolactam decreased slightly, mainly due to the increased formation of dimers and polymers.The Beckmann rearrangement was commonly assumed to take place by way of formation of O-protonated cyclohexanone oxime to be followed by a migration from carbon to nitrogen of a substituent accompanied by loss of H2O. Hydrophobic HUSY samples could not retain sufficient H2O and, thus, the ring opening of the iminium intermediate lead to nitrile formation. So, the relatively high selectivity for nitriles on H-USY samples with a high SiO2/Al2O3 ratio might be related to their hydrophobicity.4 Sato et al. reported very interesting results for this catalyst system recently.5 They studied the vapor phase Beckmann rearrangement over some MFI zeolite having Si/Al atomic ratios 8−11 000. An increasing trend of cyclohexanone oxime conversion and ε-caprolactam selectivity was observed with increasing atomic ratio of Si/Al. Even with a very high silica MFI zeolite (Si/Al atomic ratio =147 000), which was expected to possess almost no acidity, 100% cyclohexanone oxime

2. CATALYST PREPARATION PARAMETERS Several catalyst preparation parameters such as Si/Al atomic ratio, micropore size, calcination temperature, different metal/ nonmetal loading, and postsynthesis treatment have crucial

Received: Revised: Accepted: Published:

© 2014 American Chemical Society

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Industrial & Engineering Chemistry Research

Review

Table 1. Si/Al Atomic Ratio, Its Effect over Different Zeolites, and Catalytic Performance Toward Vapor Phase Beckmann Rearrangement Reaction catalyst

Si/Al

H-USY H-USY H-USY

high Si/Al ratio Si/Al until 62 Si/Al oxime

MFI, silicalite-1, A-type zeolite, ferrite

micropore size < oxime

ε-caprolactam was readsorbed on neighboring acid sites and undergoes further reaction reaction proceeds selectively on the active sites on the external surface of zeolite

conversion and 79.6% ε-caprolactam selectivity were achieved.6 So, it was concluded that the active sites of the catalyst are extremely weak acid sites that might not be detected by ammonia temperature programmed desorption (TPD) measurements. In a kinetics study Komastu et al.7 found that desorption of ε-caprolactam was the slowest reaction process in Beckmann rearrangement over zeolite. Over strong acid sites, εcaprolactam would be multiply readsorbed and desorbed during diffusion out of the zeolite and this necessarily reduced the turnover quite substantially and enhanced the undesired product formation leading to decrease in cyclohexanone oxime conversion and selectivity.8 Table 1 shows Si/Al atomic ratio, its effect over different zeolite, and catalytic performance toward vapor phase Beckmann rearrangement reaction 2.2. Effect of Pore Size. Mordenite and ZSM-5 had pore size larger than the reactant which resulted in cyclohexanone oxime penetrating into the large pores of the catalyst and reacting inside the pores. The produced ε-caprolactam was readsorbed on the neighboring acid sites and underwent a further reaction to produce undesired products mainly the oligomer of ε-caprolactam9 that lowers the ε-caprolactam selectivity.10 The micropore sizes of the MFI, ferrierite, and Atype zeolite were smaller than that of cyclohexanone oxime (about 0.73 nm); the ε-caprolactam selectivity over these catalysts was higher than mordenite and ZSM-5.10,11 These results indicate that when cyclohexanone oxime reacted on the acid sites located at the outer surface or very near the pore mouth of the catalyst, the products were easily diffused. As a result, the produced ε-caprolactam did not change to coke or its precursor. When the reaction was carried out over acid catalysts with almost the same acidity but with different pore size, the εcaprolactam selectivity significantly decreased with the micropore size.10 Table 2 represents micropore size, its effect over different zeolites, and catalytic performance toward vapor phase Beckmann rearrangement reaction 2.3. Effect of Metal/Nonmetal Host in Silica Matrixes. The aluminum in β-zeolite produces Brønsted acidity on both internal and external surface as well as Lewis acidity on the internal surface.12 H3BO3 was successfully impregnated over Hβ-zeolite (called B2O3/Hβ-zeolite) as confirmed by a new Xray diffraction (XRD) peak of B2O3 at 28.1° and decreasing surface area in BET study.13 9.09% B2O3 loaded β-zeolite showed 246% growth of weak acid sites and 123% growth of

catalytic performance due to many unwanted products, ε-caprolactam selectivity was decreased ε-caprolactam selectivity was increased

strong acid sites in the NH3-TPD profile. For the same loading, a Py-adsorbed infrared (IR) profile showed 161% increase in Brønsted acid sites and 151% increase in Lewis acid sites. The catalyst showed increase in selectivity of ε-caprolactam on 8 h time on stream (TOS) and slower deactivation of B2O3/Hβzeolite catalyst than Hβ-zeolite. B2O3/Hβ-zeolite had higher a density of terminal silanol as confirmed by Fourier transform infrared (FTIR). These terminal silanols were responsible for the higher deactivation rate of the B2O3/Hβ-zeolite14 and higher coke deposition. The deactivation factor of B2O3/Hβzeolite was found to be 15 times higher compared to a B-ZSM5 zeolite. Based on B-MFI synthesis studies, the upper limit of boron incorporation depends on the relative amount of organic cations in the template used and which can be accommodated in the porous system.15 The framework BO4 atoms, in the as synthesized samples, were attributed to the presence of large cations such as NH4+ atoms (from the template molecules), which stabilizes the tetrahedral coordination of boron.16,17 Forni et al.18 found an increasing trend of cyclohexanone oxime conversion (50−76%) and ε-caprolactam selectivity (71−95%) with Boron loading to a H3BO3/SiO2 molar ratio range of 3.7− 5.4 × 10−3, respectively. Boron in ZSM-5 is weakly bonded to the lattice, causing a removal of framework boron upon calcinations under air.19 Boron removal creates defects in the lattice and subsequent growth of silanol nests at the defect sites. Borons removed from the framework were accommodated in the extraframework space as boron oxide species which enhanced the cyclohexanone oxime conversion up to 98% against 70% of the material without extraframework boron. The activity of the BZSM-5 zeolite was increased with the calcination temperature. The highest cyclohexanone oxime conversion of about 90% and ε-caprolactam selectivity about 93% was achieved during 2 h TOS with the catalyst calcined at 600 °C temperature.20 A mild acidic treatment with 0.1 M HCl, and a mild basic treatment with a 0.33 wt % solution of aqueous ammonia (25 wt %) and aqueous ammonia nitrate (20 wt %) resulted in an increased amount of the silanol group and the silanol nest showing improved catalytic performance. It was found that adsorbed water on B-ZSM-5 had a negative effect which led to a decrease of its catalytic activity due to the hydration of boron oxide species on the surface. However, when water-adsorbed B-ZSM5 was recalcined, the high activity of the catalyst was restored. 16588

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Figure 1. Effect of calcination and water adsorption over B- ZSM-5 and catalytic performance toward the cyclohexanone oxime conversion and εcaprolactam selectivity.

Figure 2. Metal/nonmetal incorporation in ilerite, its effect over catalyst, and catalytic performace toward vapor phase Beckmann rearrangement reaction.

selectivity. Calcination temperature of Al-β-zeolite at 750 °C in lieu of 550 °C caused removal of framework aluminum as well as annealing of hydroxyl groups which was confirmed by the disappearance of the framework acidic bridged hydroxyl band and the loss of intensity of the terminal silanol bands as observed in FTIR spectra, respectively. Additionally loss of crystalinity to ∼19% was observed in XRD after this severe calcination. This modification resulted in a drop of cyclohexanone oxime conversion after 8 h TOS and fall of εcaprolactam selectivity to approximately 5%. According to Jia et al.,22 beside the loss of silanol groups, removal of aluminum from the framework resulted in a decrease in catalystic activity. This might be due to the extra framework aluminum which blocked the active sites. The role of titanium species in

Figure 1 represents the effect of calcination and water adsorption over B-ZSM-5 and catalytic performance toward the cyclohexanone oxime conversion and ε-caprolactam selectivity. Anilkumar et al. synthesized Nb-β-Zeolite (Si/Nb = 32) having weak acidity and predominantly Lewis acid sites as confirmed by TPD and pyridine adsorbed FTIR spectra, respectively. During 2 h of reaction 62% cyclohexanone oxime conversion and 89.4% ε-caprolactam selectivity were achieved. However, on longer TOS (8 h), both cyclohexanone oxime conversion and ε-caprolactam selectivity decreased down to 56% and 86%, respectively.21 Interesting results were found with Al-β-zeolite with the rise of calcination temperature toward cyclohexanone oxime conversion and ε-caprolactam 16589



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Figure 3. Postsynthesis treatment, tuning of surface profile, and catalytic performance toward vapor phase Beckmann rearrangement over ETS-10, H2O2, treated ETS-10, and β-zeolite [Adapted with permission from refs 28 (Copyright 2006 American Chemical Society) and 33 (Copyright 2008 Elsevier)].

silicalite-1 attracted the attention of the research community to Beckmann rearrangement. The cyclohexanone oxime conversion and ε-caprolactam selectivity on silicalite-1 was found to be 95.2% and 81.6%, respectively. On incorporation of Ti in the silicalite, cyclohexanone oxime conversion was increased to 100% and ε-caprolactam selectivity up to 91% due to the creation of more active centers by Ti. The cyclohexanone oxime conversion and ε-caprolactam selectivity kept increasing with increasing Ti content.23 In ilerite, sodium cations reside between silicate layers which can easily be exchanged with protons. The proton exchanged ilerite has reactive silanol groups oriented in a regular manner on its interlayer surface. Although the layered silicate itself is thermally unstable and has low surface area, the layered silicate pillared with inorganic metal oxide has high thermal stability and high surface area like zeolite. The pore structure can be tailored by the nature of the host materials and the pillaring precursors, the pillared layered silicates have potential application in catalysis. The low activity of Nb−H-ilerite and Ta/H-ilerite (impregnated) toward vapor phase Beckmann reaction was claimed to be because of its nonporosity. Si-ilerite and H-ilerite had no acidity and Nb/H-ilerite had some acidity.24−27 So, Si-ilerite and H-ilerite showed very low catalytic activity and Nb/H-ilerite had little activity. Nb-ilerite has weak and medium acid sites as well as high surface area, and Ta-ilerite has a large number of moderate acid sites. The acidities in Ta- and Nb-ilerite were attributed to hydroxyl groups (M−O−Si−OH) generated by a metal pillaring process into the ilerite layer as well as a hydrogen-bonded hydroxyl group in the interlayer. Nb-ilerite showed 100% cyclohexanone oxime conversion and ε-caprolactam selectivity >75%. The tantalum pillared-ilerite catalyst had 97.1% cyclohexanone oxime conversion and 89.1% ε-caprolactam selectivity at 350 °C. It should be also mentioned that with incrasing pillering time (96 h) of Ta-ilerite, the surface area of the catalyst increased and resulted in high ε-caprolactam selectivity (79%) against 72% of less pillered catalyst. Further Pt and Pd loading over M-lierite resulted into higher catalytic performance as well as strong resistance to deactivation. Pt and Pd loading on Tailerite caused an increasing number of relatively weak acid sites

and relatively medium acid sites, respectively. As an observation, it was found that both systems resulted in 7−8% higher ε-caprolactam selectivity but Pt/Ta-ilerite exhibited stronger resistance to deactivation (6 h) than Pd/Ta-ilerite. Also, Pt/Nb-ilertie and Pd/Nb-ilerite catalysts had 3.5% higher ε-caprolactam yield than Nb-ilerite with 8 h TOS and had a relatively strong resistance to deactivation. Figure 2 represents metal/nonmetal incorporation in ilerite, its effect over catalyst, and catalytic performace toward vapor phase Beckmann rearrangement reaction. 2.4. Effect of Postsynthesis Treatment. H2O2 treatment of ETS-10 could generate a hierarchical pore structure by partial leaching of Si and Ti atoms which subsequently form defects as well.28 The formation of mesoporosity was confirmed by increasing uptake of N2 at higher pressure with a hyperstesis loop and bright area on tranmission electron microscopy (TEM) images (Figure 3). The increase in intensity of silanol IR band after H2O2 treatment indicates that defects resulted in increasing silanol formation. It was also observed that defined mesopore channels with diameters of about 10 nm are created in ETS-10 when the treatment with H2O2 was performed under microwave irradiation. As a consequence, the external surface area was significantly increased.29 Pavel et al. again modified the ETS-10 and H2O2-treated-ETS-10 (called meso-H-ETS-10) with NH4NO3 for protanation. The reaction rate of the latter was increased almost 3-fold from 6.65 × 10−2gCL/(gETS h) to 18.8 × 10−2gCL/(gETS h) than earlier with constant selectivity toward ε-caprolactam. The group again prepared Na/K-ETS-10 and H2O2 -treated-Na/K-ETS-10 by a proton exchange method.30 Both of them showed low selectivity for εcaprolactam having reaction rates not exceeding 4.5 × 10−2gCL/(gETS h) which could be attributed to a decreased proton-donating ability and correspondingly lower acidity. Aquous ammonia solution treatment of β-zeolite could enlarge the zeolite aperture and produce roughness on the outer surface as shown in the scanning electron microscopy (SEM) image (Figure 3) due to removal of the framework Si.31,32 Silica removal causes the decrease in ratio of SiO2/Al2O3 as well as increased BET surface area.33 The Py-TPD showed significant increase in both weak acid sites and decrease in strong acid sites 16590



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Figure 4. Effect of chlorotrimethylsilane feed and methanol feed over high silica MFI influencing cyclohexanone oxime conversion and εcaprolactam selectivity.

through a long turnover stream.8 When methanol was cofed with cyclohexanone oxime, dimethyl ether was not formed due to the less acidic high silica MFI.6 In B-MFI, water molecules interacted with Brønsted acid sites and stabilized the tetrahedral coordination of boran, whereas upon water removal, the SiO(H)−B bond was weakened and boron rearranged in a trigonal coordination.39−41 After steaming with deionized water, boron atoms were extracted from the zeolite lattice forming BO3 extraframework species; a progressive treatment with dilute acid solution removes extraframework BO3 .42 The decreasing boron concentration was confirmed by the shift of the NH3-TPD profile to the lower value which was attributed to increasing Si−(OH)−Si bindings with respect to the B−(OH)−Si bindings.18 The deboronation method strongly improves the catalytic performance and time on stream due to the increasing amount of H-bonded silanol. Forni et al.18 found activity close to 65% after 7 h of highly deboranated catalyst sample while boranated samples have activity values around 30%. Proton exchanged (by ammonium salt at a pH = 10.5) or low temperature calcined and both proton exchanged and low temperature treated silicalite-143 creates SiOH type defects as well as marginal decrease in H-bonded silanol as confirmed in Si NMR and ammonia TPD. This resulted in 96% cyclohexanone oxime conversion and 87% ε-caprolactam selectivity with no deactivation until 120 h. Ammonium salt exchange at low pH (6.5) or high temperature or both of them treated silicalite-1 creates Si(OH)2 defects and larger space for coke deposition43 which resulted in fast deactivation. Cyclohexanone oxime conversion decreased to 80% at 27 h and then subsequently droped quickly to 30% during the next 5 h of reaction. Loadings of Na+ and Br− up to 2 mol % have the same catalytic activity as an unloaded sample, i.e. 99% cyclohexanone oxime conversion after 36.5 h on stream, but on further loading from 5 to 20 mol %, a faster deactivation to 98.2% cyclohexanone oxime within 30.5 h on stream was noticed. This was due to formation of the larger crystal sizes (from 170 to 270 nm) or longer diffusion pathway which leads difficult diffusion of larger product ε-caprolactam, coke formation, and deactivation of catalyst.44 Further base treatment causes disappearance of terminal silanol but increase of vicinal and

whereas Py-adsorbed IR profile showed increase in Lewis acid sites without effecting the Brønsted acid site. This resulted in a steep increase in cyclohexanone oxime conversion in the range of 57%−96.39% in 8 h TOS with small fluctuation in εcaprolactam selectivity as well as slow deactivation of the catalyst. Figure 3 represents postsynthesis treatment, tuning of surface profile, and catalytic performance toward vapor phase Beckmann rearrangement over different zeolites. Treatment with aquous HCl with either B2O3/Hβ-zeolite34 or Al-β-zeolite always resulted into loss in catalytic acitivity. In B2O3/Hβzeolite, loss in activity was majorily due to loss in crystalinity14 whereas in Al-β-zeolite, rapidly falling cyclohexanone oxime conversion droped from 97% to 66% within 8 h TOS, due to the loss of Brønsted acidic framework silanol. When silanol groups’ protons in MFI zeolite were completely exchanged by sodium or potassium ions, the rearrangement did not occur at 623 K. This suggests that the strong acid sites play important roles in the rearrangement. However, the excess amounts of strong acid sites were responsible for the coke formation.35 Sato et al.35 modified terminal silanols on the surface of a high silica MFI zeolite (Si/Al atomic ratio > 30 000) by treating it with chlorotrimethylsilane vapor. They reported that the selectivity to ε-caprolactam was changed drastically from 85 to 95% by such a treatment. Similarly, high silica MFI zeolite (Si/Al atomic ratio = 147 000) treatment with methanol vapor converted the terminal silanol into methylsilylether as confirmed by replacement of the terminal silanol peak (3740 cm−1) by two new clear methoxy bands (2970−2850 cm−1) whereas the silanol nest band (3500 cm−1) remained preserved.37 So, the terminal silanol which was responsible for byproduct formation is now masked. Therefore, as far as methanol or chlorotrimethylsilane vapor exists in the reaction system, the modification demonstrated the high selectivity to ε-caprolactam formation. These observations suggest that the silanol nests are the most favorable sites for the Beckmann rearrangement, while terminal silanols are the least favorable.38 Figure 4 shows the effect of chlorotrimethylsilane feed and methanol feed over high silica MFI influencing cyclohexanone oxime conversion and ε-caprolactam selectivity. Ammonia treatment improved the catalytic performance. It helped to keep the ε-caprolactam selectivity on a high level 16591



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showed greater resistance to deactivation than unlinked silicalite oxime conversion up to 99% and ε-caproiactam selectivity above than 95% (against 90% of untreated catalyst) difficult diffusion of product resulted into deactivation as oxime conversion drops from 99 to 98% within 30 h less oxime conversion (80% for27 h) and then oxime conversion droped to 30% within 5 h

terminal silanols disappeared, vicinal silanol and silanol nests appeared large crystal sizes were created providing longer diffusion pathways

tuning of surface profile catalytic activity

96% oxime conversion and 87% ε-caprolactam selectivity with no deactivation until 120 h

further base treatment Silicalite-1

loading of Na+ and Br− from 5 to 20 mol %

ammonium salt exchanged at pH 6.5/ high temperature calcined/both Si(OH)2 typed defectes were created having larger space for coke formation ammonium salt exchanged at pH 10.5/low temperature calcined/both SiOH type defects were created and Hbonded silanol disappeared treatment

3. INFLUENCE OF REACTION PARAMETERS Apart from catalyst preparation parameters, various reaction parameters like solvent, reaction temperature, and weight hourly space velocity (WHSV) should be considered carefully for obtaining the best cyclohexanone oxime conversion and εcaprolactam selectivity. 3.1. Effect of Solvent. In most of the catalytic systems, use of alcohol as a solvent is preferred due to easy desorption of product and conversion of byproduct responsible terminal silanol into alkoxy groups. But in micro-H-ETS-10 a decreasing trend of ε-caprolactam selectivity was observed, whereas in meso-H-ETS-10 a decreasing trend in both productivity and εcaprolactam selectivity was found. Pavel et al. hardly attributed this unexpected behavior to the low acidity of the catalytically active sites in ETS-10.30 Herrero et al.52 showed the poor catalytic performance of β-zeolite, i.e., 62% cyclohexanone oxime conversion and 63% ε-caprolactam selectivity, at 335 °C using benzene as the solvent. Among polar solvent. Dai et al.53

(99% cyclohexanone oxime conversion until 36.5 h TOS)

Table 3. Postsynthesis Treatment, Tuning of Surface Profile, and Catalytic Performance over Silicalite-1

cross-linking by silanol linker

silanol nests on the catalyst surface which in turn showed 99% conversion of cyclohexanone oxime and 95% ε-caprolactam selectivity against 90% ε-caprolactam selectivity of untreated catalys.44 Using silanol linker (1,7-dichlorooctamethyltetrasiloxane) in silicalite-1 and TS-1 for cross-linking, the additional mesoporosity along with microporosity of zeolite was generated.45,46 The additional mesoporosity did not only affect the catalytic activity, as reported earlier for cross-linked silicalite-1 nanocrystals but resulted in a drastic reduction in catalytic deactivation. The unlinked TS-1 showed a rapid deactivation within 4 h TOS, but cross-linking with 0.65 or 1.3 mmol of linker results in an increased productivity of 28% and 32%, respectively. Table 3 shows postsynthesis treatment, tuning of surface profile, and catalytic performance over silicalite-1. Alumina- and sodium-exchanged47 ZSM-5 did not show any activity toward Beckmann rearrangement reaction. H-ZSM-5 had a much higher acidity than silicalite-1, but its selectivity to ε-caprolactam was found to decrease with increasing acidity.7 Additionally, the surface area of the fresh catalyst was 416 m2/g and decreased to 334 m2/g after 55 min of TOS. This showed another factor other than acidity palying role in activity and that stronger acidity of the catalyst was responsible for more coke formation and deactivation of the catalyst in the long run. The coke on the catalyst surface was removed on heating for 1 h at 773 K in a nitrogen atmosphere. Further, when the reaction was carried out over the regenerated catalyst, the catalytic activity was almost 90% of the original one. It is well accepted that the acid centers of aluminophosphate molecular sieves originated from terminal hydroxyl bond (Al− OH, P−OH, or Si−OH) and lattice defect induced Lewis acid sites48−50 as confirmed by IR spectra of chemisorbed pyridine.51 Silica content of the sample synthesized in HF medium was higher, but it did not generate more Brønsted acidity as compared to high silica content. Low silica content SAPO-11 caused lower activity of catalyst and selectivity to εcaprolactam. This indicates that the main catalytic center responsible sites are generated by the substitution of silicon for phosphorus. Silylation of SAPO-11 resulted in low cyclohexanone oxime conversion due to passivating the external surface of the catalyst. This indicates that reaction took place mostly on the external surface of the catalyst. However, increasing hydrophobicity with silylation caused a decrease in hydrolysis product cyclohexanone.51

additional mesoporosity arrived

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Table 4. Solvent−Zeolite Interaction and Catalytic Performance over Different Zeolite Systems

was lactim−lactam conversion. Upon involving a molecule of water or methanol, the reaction barrier for the lactim−lactam transformation decreaseed from 90 to 3 kJ/mol. The addition of water to the reaction with a maximum amount of 0.1−2.5 mol H2O per mol cyclohexanone oxime has been reported over B-MFI zeolite.58,59 The small amount of water formed as well as the polar solvents can be considered to supersede polymers and the coke precursors from the catalyst surface. On addition of larger amounts of water up to 6 mol per mole cyclohexanone oxime over B-MFI-zeolite, Holderich et al. found an increase in cyclohexanone oxime conversion from 97% to 99% and ε-caprolactam selectivity from 92% to 93%.60 The effect of the water addition on the catalyst service time is also significant. Without any additional water the cyclohexanone oxime conversion of the cyclohexanone oxime was droped from 97% to 94% within 8 h TOS. By adding 6 mol water per mole cyclohexanone oxime, the conversion was droped only to 0.7% within 8 h TOS. The optimal service time was achieved with ethanol as the solvent. The drop in cyclohexanone oxime conversion after 8 h TOS was only about 3% at those process conditions.61 Polar solvents such as methanol or ethanol showed higher yields and lower deactivation rates than nonpolar solvents, such as toluene or benzene. The deactivation was noticed due to the pore blocking effect by nitrogen-containing species inside the pores of MFI borosilicate.62 Over silicalite-1, it was also found that ethanol, which has medium polarity, was effective in the formation of εcaprolactam due to the effect of OH groups. IR measurements confirmed the desorption of ε-caprolactam that was induced by ethanol molecule coverage of the catalyst surface. Alcohols with higher carbon numbers showed lower yield owing to their hydrophobic nature.5 It should be noted that basic solvents such as pyridine gave comparable yields, although those will

reported that using a higher alcohol such as 1-hexanol as a solvent, H-β-zeolite demonstrated 100% conversion of the cyclohexanone oxime and ε-caprolactam selectivity up to 96%, whereas Al containing β-zeolite resulted in complete cyclohexanone oxime conversion as well as ε-caprolactam selectivity up to 96% at 350 °C and atmospheric pressure. Dai et al. also reported that alcohols with lower carbon numbers such as ethanol showed lower ε-caprolactam yields with Al-β-zeolite, i.e., 99.5% cyclohexanone oxime conversion, but only 87% εcaprolactam selectivity with respect to longer chain alcohols. USY zeolite showed a rise in ε-caprolactam selectivity and a rapid decay of activity when benzene was used as a diluent.54−57 Alcohol treatment with Y-zeolite caused esterification reaction with the surface hydroxyl groups of Y-zeolite and/or an interaction of alcohol molecules with skeletal Brønsted acid sites via strong hydrogen bonds.4 The adsorption of methanol on H-USY of relatively low SiO2/Al2O3ratio (62) was stronger than that of 1-hexanol; in contrast, 1-hexanol adsorbed on HUSY of a high SiO2/Al2O3 ratio (390) more strongly than methanol. It is conceivable that only the relatively strong Brønsted acid sites (>623 K) in low silica H-USY (62) might be covered preferentially with 1-hexanol. Thus, the activity and selectivity for the formation of ε-caprolactam was enhanced effectively by the usage of 1-hexanol. In high silica H-USY (390), a very small number of acid sites would be covered with 1-hexanol due to their stronger interaction. This resulted in the lower activity and selectivity for ε-caprolactam compared to those of the other catalysts.4 Mordenite converted some ethanol (solvent) into diethyl ether and ethylene that would cause some deviation in ε-caprolactam selectivity. Bucko et al.38 in its DFT calculation found that polar solvent improved the εcaprolactam selectivity and cyclohexanone oxime conversion. They found that solvent participate actively in the final step of Beckmann rearrangement (catalyzed by weak acid sites) which 16593



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zeolite was noticed at a reaction temperature of 300 °C and 0.1 bar upon the adsorption of water. On the other hand, if the material was first calcined, the high activity of the catalyst was restored. At the higher temperature, 350 °C, the catalytic sites regained their high activity and ε-caprolactam selectivity. Over SAPO-11 at higher temperature, 390 °C, cyclohexanone oxime conversion was increased up to 100% but ε-caprolactam selectivity was decreased to 86%. At higher temperature, decomposition of ε-caprolactam resulted in lower ε-caprolactam selectivity but increasing 5-cyano-pent-1-ene selectivity. At lower reaction temperatures, the catalyst deactivates quickly. Cyclohexanone oxime conversion was greater at high temperatures; however, its selectivity toward ε-caprolactam decreased continually at high temperatures. At all temperatures, cyclohexanone oxime conversion decreased with time due to the deactivation of the unidimensional pore structure of SAPO11.51 3.3. Effect of WHSV and Carrier Flow Rate. Nb-ilerite, Pt/Nb-ilerite, and Pd/Nb-ilerite had a lower catalytic performance at 0.8 h−1 WHSV compared to that at 0.4 h−1. At low space velocity, ε-caprolactam yield over Pt/Nb-ilerite and Pd/ Nb-ilerite was 70% and 63% whereas over Nb-ilerite it was 60%.26 With increasing WHSV, a progressive decrease in cyclohexanone oxime conversion, increase in ε-caprolactam selectivity, and decrease in cyclohexanone and 5-cyano-pent-1ene selectivity were observed over SAPO-11.51 With increasing carrier N2 flow rate, the resisdence time of reactants over the catalyst was reduced which caused a decrease in cyclohexanone oxime conversion and ε-caprolactam selectivity.

neutralize the acid sites if present. It was explained by the extremely low acidity of silicalite-1 catalyst.5 The polar solvent can remove much of the oligomer. So, the deactivation constant from the polar solvent was found to be smaller than that from the nonpolar solvent. When methanol was used as the solvent for the rearrangement over high-silica HZSM-5 (Si/Al atomic ratio = 500), the selectivity of ε-caprolactam was increased up to 96% and the catalyst life (defined by the half-life period) was 2200 h.63 On H-ZSM-5,11 both cyclohexanone oxime conversion and ε-caprolactam selectivity had shown the highest values (82% and 88%, respectively) with ethanol among various solvents. The deactivation constant obtained from 2-propanol over high-silica HZSM-5 catalyst was still 1/100 times lower than that over the silica−alumina.9 Takahashi et al.47 showed that deactivation was due to results from the adsorption of volatile material on the acid site of highly siliceous H-ZSM-5. The addition of up to 6 mol of water per mole of cyclohexanone oxime to the reaction mixture improved the cyclohexanone oxime conversion rate and catalyst service time. Table 4 shows solvent−zeolite interaction and catalytic performance over different zeolite systems. 3.2. Effect of Reaction Temperature. Heitmann et al.14 observed very high cyclohexanone oxime conversion around 99% and ε-caprolactam selectivity varied between 56% and 76% with increasing TOS over Al-β-zeolite catalyst at 350 °C temperature and atmospheric pressure. On the contrary, by decreasing reaction temperature and pressure, the initial cyclohexanone oxime conversion reached 100% and enhancement of ε-caprolactam selectivity was up to 91% with a significant loss of activity after 8 h TOS. At 300 °C temperature and 0.1 atm pressure, reaction over β-zeolite showed a decrease in cyclohexanone oxime conversion from 100% to 21% and increase in ε-caprolactam selectivity from 92% to 97% during 8 h TOS; whereas at 350 °C temperature and 1 atm pressure, cyclohexanone oxime conversion was slightly higher with a value around 33% and ε-caprolactam selectivity was less than 90% after 8 h TOS. Over B-MFI-zeolite, temperatures higher than 250 °C should be promising due to the boiling point of cyclohexanone oxime. As reported in the literature at 300 °C, 92% cyclohexanone oxime conversion was observed. With increasing temperature, the coke precursor was desorbed easily and left the catalytic surface fresh to accommodate the new reactant molecule which resulted in increased cyclohexanone oxime conversion up to 99%. At temperatures above 350 °C, no drop in cyclohexanone oxime conversion even after 8 h TOS was observed. Temperatures higher than 380 °C resulted in a drastic reduction in ε-caprolactam selectivity due to the decomposition of ε-caprolactam and other side reactions.60 At any temperature, TS-1 was more active than silicalite whereas silicalite-1 was more rapidly deactivated than TS-1. In both cases, cyclohexanone oxime conversion and ε-caprolactam selectivity increaseed with increasing temperature.46 Over M-ilerite, cyclohexanone (by the dehydroxylamination of cyclohexanone oxime) was found as the major byproduct at 300 °C. The highest catalytic performance of M-ilerite was observed at 350 °C. The main byproduct at 400 °C was 5-cyanopent-1-ene due to the ring-opening of cyclohexanone oxime by dehydration which again affected the catalytic activity.24 A reduction of the reaction pressure resulted in a significant increase in ε-caprolactam selectivity over H-ZSM-5. At 300 °C and 0.1 bar (reduced from 1 bar), ε-caprolactam selectivity up to 95% was obtained.20 The loss of activity of the B-ZSM-5

4. REACTION MECHANISM The IR spectrum of pure cyclohexanone oxime and product εcaprolactam is shown in Figure 5. Bucko et al. in their periodic

Figure 5. IR spectra of (A) cyclohexanoneoxime and (B) εcaprolactam (Reprinted with permission from ref 66. Copyright 2003 Elsevier).

ab initio DFT calculations38 over mordenite showed that “effective” activation energy at Brønsted acid sites was 142 kJ/ mol vs 184 kJ/mol for weak acid sites represented by silanol nests. The reaction at Brønsted acid sites was about 5 orders of magnitude faster than that at a silanol nest. The activity of weak acid sites in the Beckmann rearrangement decreases in the order silanol nest > H-bonded terminal silanol groups > isolated silanol group which is in good agreement with experiment.64,65 The activation energies for the rate-controlling step of the Beckmann rearrangement increase as follows Brønsted site < silanol nest < H-bonded terminal silanol groups 16594



Review

Table 5. 1H NMR Spectra Positions That Appeared during Reaction of Cyclohexanone Oxime Rearrangement over Silicalite-1 at Different Reaction Temperatures sample silicalite-1 silicalite-1 + cyclohexanone oxime silicalite-1 + cyclohexanone oxime silicalite-1 + cyclohexanone oxime

temp Room temp 200 °C

silanol

residual ring proton of cyclohexanone oxime

hydroxyl proton of H-bonded cyclohexanone oxime

H-bonded caprolactam

protonated caprolactam

byproduct

1.8 ppm 2.5 ppm

9.3 ppm

225 °C

2.5 ppm

9.3 ppm (weaker)

12.2 ppm

14.5 ppm (weak intensity)

6.5 ppm

250 °C

2.5 ppm

disappeared

12.2 ppm

14.5 ppm (weak intensity)

6.5 ppm

Table 6. 15NCP/MAS NMR Spectra Positions that Appeared during 15N-Cyclohexanone Oxime Rearrangement over Silicalite-1 at Different Reaction Temperatures sample silicalite-1 + 15 NCyclohexanone oxime

silicalite+ 15Ncyclohexanone oxime + 13C-methanol

temp

oxime

room temp

−55 ppm

150 200 250 250

°C °C °C °C

−55 ppm disappeared disappeared −55 ppm

silanol interaction with oxime

nitrilium ion intermediate

−46 ppm disappeared disappeared

−237 ppm disappeared

ε-caprolactam

−260 ppm −260 ppm −260 ppm (reduced intensity)

O-protanated caprolactam

amines

hydroxyl amine

−237 to −243 ppm disappeared



al.70 also found that cyclohexanone oxime was adsorbed on the silanol group of surfaces. Using 15NCP/MAS NMR, Reddy Marthala et al.68 also found a peak at −46 ppm which was due to interaction of 15Ncyclohexanone oxime with the SiOH groups of silicalite-1 via hydrogen bonding during adsorption (Table 6). They carried out reaction at 200 °C and obtained valuable information about the reaction intermediate. At 200 °C, a peak at −237 ppm due to formation of nitrilium ion was observed. With increasing temperature from 200 to 225 °C, the intermediate ion peak disappeared and the product caprolactam peak at −260 ppm intensified. On further increase of temperature to 250 °C, new peaks in the range of −237 to −243 ppm due to O-protonated caprolactam, at −269 ppm due to by product hydroxylamine, and at −387 ppm due to byproduct amine were observed.69 The weak O-protonated ε-caprolactam signal at 14.5 ppm was also observed in 1H NMR that strengths the above argument.67 However, O-protonated caprolactam was more prominent in H-ZSM-5 and mesopores system like SBA-15. Byproducts like amine and hydroxyl amine were formed due to hydrolysis of protonated caprolactam. Interestingly, the mixing of 15Ncyclohexanone oxime with silicalite-1 and 13C-methanol showed disappearance of 15N-CP/MAS NMR peak of protonated εcaprolactam and byproducts. The 13C-methanol brought easy desorption of ε-caprolactam by decreasing the energy barrier for product desorption. However, due to total coverage of 13Cmethanol over silanol groups, cyclohexanone oxime conversion also droped down. On the basis of the results shown in Tables 5 and 6, the reaction mechanism can be sketched as

< isolated silanol group. It has been reported that the vicinal silanol and silanol nests are favorable for the formation of εcaprolactam, while the terminal silanol are responsible for the formation of the byproducts.60 Using FTIR, Flego et al.66 found that external silanol interact more with cyclohexanone oxime than internal silanols as confirmed by the disappearance of the external silanol IR signal at 3742 cm−1 and slow disappearance of the internal IR signal at 3730 and 3710 cm−1. The shortening of the OH bond in cyclohexanone oxime was confirmed by a shift of the O−H IR band of cyclohexanone oxime to higher energy, and abstraction of H from SiOH was noticed by a decrease in O−H IR band intensity of silanol. These observations indicate that catalyst−reagent interaction is initiated through the oxygen atom of cyclohexanone oxime and OH of silicates. Finally, it can be concluded that one OH group interacts with the “O” atom of CEOX, and the other one, with both the “N” atom and the vicinal “C” atom of the ring which facilitates the interaction of the reactant for its transformation. Among the zeolite system, silicalite-1 is the choice of most researchers working in this area to understand the reaction mechanism of Beckmann rearrangement especially cyclohexanone oxime to ε-caprolactam. FTIR, 1H MAS NMR, 2H MAS NMR, 15N-CP/MAS NMR, 13C−CP MAS NMR, and DFT are important tools to study the reaction mechanism properly.66−70 Using H1 NMR, Reddy Marthala et al.67 found that a peak at 1.8 ppm due to silanol over silicalite-1 disappeared when silicalite-1 interacted with cyclohexanone oxime (Table 5). At the same time a new peak at 9.3 ppm due to H-bonded hydroxyl of cyclohexanone oxime appeared. As reaction temperature increased, the peak at 9.3 ppm started to disappear and a new H-bonded ε-caprolactam peak at 12.2 ppm started to intensify. In DFT calculation, Lezcano-González et 16595



Review

Table 7. 1H NMR Spectra Positions That Appeared during Reaction of Cyclohexanone Oxime Rearrangement over H-ZSM-5 at Different Reaction Temperatures sample H-ZSM-5 H-ZSM-5 + cyclohexanone oxime H-ZSM-5 + cyclohexanone oxime H-ZSM-5 + cyclohexanone oxime

temp room temp 120 °C

silanol

large no. of acidic bridging OH group

1.8 ppm

4.2 ppm (strong)

residual ring proton in cyclohexanone oxime

hydroxyl proton of bridging H-bonded cyclohexanone oxime

2.5 ppm

protonated cyclohexanone oxime

protonated caprolactam

byproduct

10.4 ppm

11.2−13.0 ppm

225 °C

10.4 ppm (weak)

11.2−13.0 ppm (strong decrease)

14.2 ppm

6.5 ppm

250 °C

10.4 ppm (weak)

11.2−13.0 ppm (strong decrease)

14.2 ppm

6.5 ppm

Table 8. 15NCP/MAS NMR Spectra Positions That Appeared during 15N-Cyclohexanone Oxime Rearrangement over H-ZSM-5 at Different Reaction Temperatures sample H-ZSM-5 + 15 NCyclohex-anone oxime

temp

oxime

room temp

−55 ppm

150 200 225 250

°C °C °C °C

−55 ppm less intense disappeared Disappeared

SiOHAl interaction with oxime

ε-caprolactam

N-protanated caprolactam

O-protanated caprolactam

amine

5-cyano-1pentene

−160 ppm less intense disappeared Disappeared

−260 ppm Less intense

−347 ppm intensified Intensify

−237 ppm less intense Less intense

−364 ppm −364 ppm

−199 ppm

equation. When the reaction temperature exceeded the optimum, the coke precursor changed to coke with very low vapor pressure. This time deactivation rate is decided from the balance of the oligomerization rate and its removal rate. Possibly cokes were mainly aliphatic-type carbonaceous compounds which deposited preferentially on acid sites, and their deposition could be reduced by uniform dispersion of modifier as found by Zhang et al. over boron modified AlMCM-41.75 After reactaion, HZSM-5 was regenerated by oxygen atmosphere. Using methanol as the solvent, the cyclohexanone oxime conversion decreased with the regeneration number, whereas the caprolactam selectivity was almost constant.76 The catalyst deactivation at the fresh zeolite was largest among the regenerated zeolites. Although the crystalline structure did not change with regeneration, the acid strength of the zeolite decreased with the regeneration number. These results suggested that the acid sites in the extralattice space of the zeolite were responsible for the catalyst deactivation. The 1H NMR spectral position upon conversion of 15Ncyclohexanone oxime over H-ZSM-5 catalyst at different reaction temperature is shown in Table 7. Using 1HNMR, Reddy Marthala et al.67 found a peak at 1.8 ppm due to silanol and another intense peak at 4.2 ppm due to strong acidic bridging OH group over H-ZSM-5 were appeared. However, both peaks disappeared when H-ZSM-5 interacts with cyclohexanone oxime. At the same time two new peaks one at 10.4 ppm due to H-bonded hydroxyl of cyclohexanone oxime by bridging hydrogen of catalyst and another at 11.2 to 13 ppm due to protonated cyclohexanone oxime appeared. As reaction temperature was increased, both peaks started to disappear and two new peaks one at 14.2 ppm due to protonated caprolactam and another at 6.5 ppm due to byproduct intensified. In DFT calculation, Lezcano-González et al.70 also found that cyclohexanone oxime was adsorbed on Brønsted acid site surfaces.

Apart from the silanol concentration, the crystalinity of silicate-1 should be also considered before a conclusion is made about the reaction mechanism. Zeolite crystalinity has diffusion limitations which cause coke deposition and fast deactivation of the catalyst. To reduce the diffusion path length, Li et al.71 synthesized silicalite-1 nanocrystals (30−40 nm) using resorcinaol-formaldehyde-based aerogel, Deng et al.72 synthesized nanosized (50 nm) silicate-1 by optimizing aging time (24 h), aging temperature (100 °C), water content (65 mol), and Llysine content (0.128 mol), Kim et al.73 synthesized surfactant directed high silica MFI nanosheet (particle size 60 nm, width 2 nm), and Wei et al.74 synthesized TS-1 submicrocrystals by using nano S-1 and nano-TS-1 seeds in different TS-1 solutions. Nanosized silicate-1 structures had reduced diffusion limitation as well as high surface area. Interestingly, when the crystal size decreased to below 100 nm and the sheet width below 2 nm, silanol concentration increases, but it does not show an effect on catalytic activity because diffusion relaxation is more influential than silanol concentration. H-ZSM-5 is the second one after silicalite-1 in the zeolite family which is preferred by most researchers to study the reaction mechanism of Beckmann reaction especially cyclohexanone oxime to caprolactam conversion. When the Beckmann rearrangement was carried out over an acid catalyst like ZSM-5, the rate constant decreased exponentially due to coke formation as k(t) = k0 exp(−bt), where“k0” is the initial rate constant (m3/kg·s), “t” is the TOS, and “b”is the deactivation constant.47 The deactivation constant was decreased with the reaction temperature up to an optimum as stated by the 16596



Review

Using 15N CP/MAS NMR, Reddy Marthala et al.68 also found peak at −160 ppm due to protonated 15N-cyclohexanone oxime interacted with acidic bridging OH groups (SiOHAl) in the vicinity of framework aluminum atoms. The 15NCP/MAS NMR spectrum of conversion of 15N-cyclohexanone oxime over H-ZSM-5 catalyst at different reaction condition and expected reaction mechanism is shown in Table 8. Using 15N CP/MAS NMR, Reddy Marthala et al.69 carried out reaction at 200 °C and they got valuable information about O-protonated caprolactam having peak at −237 ppm and N-protonated caprolactam having peak at −347 ppm. Also when temperature was increased to 225 °C, a small peak at −260 ppm due to caprolactam was observed. The O-protonated caprolactam peak was deintensified, and that for N-protonated caprolactam was intensified. This indicates a 1,3-H shift of O-protonated caprolactam to N-protonated caprolactam which on hydrolysis converted into amines. On further increase of temperature up to 250 °C, the caprolactam peak was deintensified and the 5cyano-1-pentene peak was intensified. This indicates that at higher temperature, dehydration of cyclohexanone oxime leads the formation of 5-cyano-1-pentene than caprolactam. A 13Cmethanol additive in the reaction mixture can form water during methanol conversion into diethyl ether. Water formed during methanol conversion promotes the O-protonated caprolactam toward hydrolysis, and so, aminocarporic acid is formed. The O-protonated caprolactam signal at 14.2 ppm was also observed in 1H NMR that strengths the above argument.67 On the basis of the results shown in Tables 7 and 8, the reaction mechanism can be sketched as

caprolactam selectivity. Finally, a reaction mechanism is proposed by review of work over neutral catalysts like silicalite-1 and acidic catalysts like H-ZSM-5. This may be useful for efficient design of microporous zeolite systems for Beckmann rearrangement reactions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-3262235663. Fax: +91-326-2296563. Notes

The authors declare no competing financial interest.

■

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

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REFERENCES

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In 15NCP/MAS NMR spectra, 15N-cyclohexanone oximewith H-[B]-ZSM-5 showed some different peaks than H-ZSM-5. At 150 °C, a peak at −153 ppm due to silanol groups in the vicinity of framework boron atoms (SiOH[B]) was observed which were characterized by a lower acid strength in comparison with the SiOHAl groups in H-ZSM-5. This surface profile influenced the product distribution at increasing temperature. At increasing temperature, a peak at −199 ppm due to 5-cyano-1-pentene and another peak at −275 ppm due to cyclohexanone was found as usual 5-cyano-1-pentene is dehydration product and cyclohexanone is hydrolysis product of cyclohexanone oxime. Both may originate from a reversible cleavage of the unstable −Si−OH−B− bridges in H-[B]-ZSM5 catalysts.

5. CONCLUSION It can be concluded that several parameters such as micropore size, Si/Al atomic ratio, different metal/nonmetal loading, calcination temperature, and postsynthesis treatment have crucial roles in deciding cyclohexanone oxime conversion and product selectivity over zeolite catalyst. Apart from catalyst preparation parameters, reaction parameters like reaction temperature and solvent selection should also be considered for getting the best cyclohexanone oxime conversion and ε16597



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