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Catalytic Activity of Clay-Based Titanium Silicalite-1 Composite in Cyclohexanone Ammoximation Alex C. K. Yip and Xijun Hu* Department of Chemical and Biomolecular Engineering, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong
Following the recent report on the synthesis of a clay-based titanium silicalite-1 (TS-1) composite, this work provided important information on the catalytic activity of the catalyst developed for cyclohexanone ammoximation. Experimental works showed that the optimum molar ratio of cyclohexanone:H2O2:NH3 was 1:1.6:2.5. It was found that the cyclohexanone ammoximation over the clay-based catalytic composite gave the highest oxime yield when H2O2 and NH3 was continuously added and was evenly distributed in 5 doses, respectively, to the reaction mixture. The external and internal diffusions can be eliminated by using a water: t-butanol mixture as a solvent, with sufficiently fast agitation speed and having TS-1 crystal with size smaller than 500 nm. On the basis of the adsorption studies and the spectroscopic evidence, the mechanism of the cyclohexanone ammoximation over the catalyst developed are suggested to follow Langmuir-Hinshelwood and Eley-Rideal kinetics. The data acquired in this study is particularly essential for industrial reactor design. 1. Introduction Cyclohexanone oxime is one of the most important starting materials in chemical industry because its downstream product, ε-caprolactam, can be used to produce Nylon-6 from ringopening polymerization.1-3 Being the second most widely used polyamide in the world,4 Nylon-6 and its manufacturing allow cyclohexanone oxime to become a highly demanding intermediate and thus of excellent market value. However, the traditional approach of making cyclohexanone oxime is affected by low product yield and carries with it environmental concerns because of the massive generation of ammonium sulfate as a byproduct.5 A breakthrough in this process was made by the discovery of titanium silicalite-1 (TS-1) by Taramasso et al. in early 1980s.6 TS-1 is a titanium-containing zeolite with shape-selectivity which heterogeneously catalyzes a “green” reaction converting cyclohexanone to its oxime in the presence of hydrogen peroxide (H2O2) and ammonia (NH3). Despite the fact that TS-1 overcomes the environmental problems and gives excellent oxime yields at moderate temperatures,7-9 the commercial value of cyclohexanone ammoximation over TS-1 is still under question because of the expensive cost, the separation, and recovery of the TS-1 catalyst in pilot-scaled practice.10-14 To solve this problem head-on, we recently developed a clay-based TS-1 catalyic composite to fill the void of the industrial TS-1 process. In our separate works, we demonstrated that the immobilization of TS-1 on a bentonite clay support can increase the stability of the active component, thereby preventing submicrometer-sized TS-1 from catalyst deactivation due to agglomeration. This advantage was proven from the difference between the catalytic activity of the unsupported TS-1 and that of the clay-based TS-1 composite after reuse. After seven reaction cycles, the unsupported TS-1 showed a drastic deterioration in activity (39% oxime selectivity) while the claybased TS-1 composite remained catalytically active (90% selectivity). The fact that no leaching of Ti active sites was detected in both the unsupported TS-1 and the clay-based TS-1 composite after reuse suggests the catalyst stability difference is attributed to the structural unlikeness. More importantly, the * To whom correspondence should be addressed. E-mail: kexhu@ ust.hk. Tel.: (852) 2358-7134. Fax: (852) 2358-0054.
use of clay support greatly enhanced the settling ability of the catalyst and thus the clay-based TS-1 composite becomes much more durable than the conventional TS-1.15,16 This paper will focus on the catalytic activity of the claybased TS-1 composite in cyclohexanone ammoximation under different reaction conditions. By learning the effect of various reaction parameters, important fundamental details regarding the design of the cyclohexanone ammoximation process catalyzed by the clay-based TS-1 composite can be gained. The reaction mechanisms are also proposed and discussed. 2. Experimental Section 2.1. Preparation of Clay-Based TS-1 Catalytic Composite. The clay-based TS-1 catalytic composite was prepared following the procedure described in our recent works.15,16 A 1.26 g portion of tetrabutyl orthotitanate (TBOT, 97%) was first added to 13.2 g of tetraethyl orthosilicate (TEOS, 98%) in a beaker under mild stirring for 15 min at room temperature. The template solution necessary to direct the formation of TS-1 structure was prepared separately by mixing 22.07 g of tetrapropyl ammonium hydroxide (TPAOH, 1.0 M solution in water) with half the quantity of additional water required. The diluted templating solution was then added to the TBOT-TEOS mixture by a metric pump at the rate of 0.1 mL/min under moderate stirring. After the addition was finished, the entire mixture was further stirred for 1 h to ensure complete hydrolysis. Then, 2 g of bentonite and the remaining stoichiometrically required water were added to the resulting gel. The final composition of the synthesis gel was 0.36 TPAOH:1.00 TEOS:0.06 TBOT:18.81 H2O:4.00 EtOH:0.24 BuOH. The synthesis mixture was left stirring overnight. Finally, the clay-gel mixture was transferred to a Teflon-lined autoclave and heated to 175 °C (5 °C/min ramping) for 48 h under autogenous pressure. After crystallization, the products were separated by centrifuge and washed with double distilled water. The sample was then dried at 110 °C and calcined at 550 °C for 6 h with a ramping rate at 5 °C/min. The TS-1 crystals supported on the clay were 300-400 nm (the entire clay-based composite was ca. 10 µm) according to the
10.1021/ie900731s CCC: $40.75 2009 American Chemical Society Published on Web 08/10/2009
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scanning electron microscopic image16 and were used as a standard catalyst in every activity test in this work, unless specified. In addition, the composites with different size of TS-1 were prepared using a mixture of TPAOH and tetraethyl ammonium hydroxide (TEAOH) in different compositions as the template solution (TPAOH:TEAOH ∼ 1:0.1, 1:0.3, and 1:1 mol/mol) following the same procedure described above. 2.2. Characterization. The X-ray powder diffraction (XRD) pattern of the sample was obtained from a Philips PW 1830 powder diffractometer with Cu KR radiation at 40 kV and 20 mA. The 2θ angle ranged from 6 to 38°, and the scan rate used was 0.025°/min. A Fourier transform infrared (FT-IR) spectrometer, PerkinElmer Spectrum GX, was used to give the infrared spectra of the catalyst. The measurement was made using the diffuse-reflectance mode with the KBr wafer technique in the range between 500 and 1050 cm-1. To investigate the effect of reagents on the active site, the samples were freezedried after individual contact with hydrogen peroxide, ammonia, and cyclohexanone before FT-IR analysis. Diffuse reflectance UV-visible (DR-UV-vis) spectra were recorded under ambient conditions on a CARY-1E spectrophotometer with BaSO4 as the standard reference material. The BET surface area of the sample was determined by N2 adsorption at 77 K with a Coulter SA3100 surface area and pore size analyzer. The energy dispersive X-ray spectroscopy (EDS) equipped in JEOL 2010F TEM was used for chemical analysis at localized position. A scanning electron microscope (JEOL 6300F) was used to examine the crystal size in the composite. 2.3. Cyclohexanone Ammoximation. The general procedure of cyclohexanone ammoximation was carried out as follows. In a standard run, a defined amount of cyclohexanone (99%) and the water:t-butanol solvent (1:1 g/g) were first added to a glass reactor (STEM Omni-Reacto Station) equipped with a water-circulated condenser and a built-in magentic stirrer. After this, the clay-based TS-1 composite (0.33 g/g cyclohexanone) was added to the reactor. The reaction mixture was then slowly heated to 80 °C by a temperature-controlled heater. The magnetic stirrer was kept on during the course of reaction in order to provide sufficient mixing and to suspend the catalyst in the reactor. After the temperature reached 80 °C, a required amount of hydrogen peroxide (H2O2, 30%) was introduced to the reaction mixture in dropwise fashion by a metric pump over the reaction period. On the other hand, ammonia solution (NH3 · H2O, 28%) was added equally to the reaction mixture at 0, 30, 60, 90, and 120 min so that the overall reagents composition was cyclohexanone:H2O2:NH3 · H2O ) 1:1.6:2.5. The standard reaction period was 2.5 h timed as soon as H2O2 was first added to the mixture. Subsequent to the completion of reaction, the catalyst was separated by centrifuge. Cyclohexanone oxime and the remaining unreacted cyclohexanone in the upper liquid were then extracted with toluene. The concentration of cyclohexanone and cyclohexanone oxime in the extracted solution were analyzed using a gas chromatograph (Hewlett-Packard 5890 Series II; AT-wax column, 30 m × 0.25 mm ID; flame ionization detector (FID)) without pretreatment. The conversion of cyclohexanone and the selectivity to oxime were calculated as follows: conversion ) 100 - (moles of unreacted cyclohexanone/moles fed) × 100; cyclohexanone oxime selectivity ) (moles of cyclohexanone oxime/moles of cyclohexanone reacted) × 100.
Figure 1. XRD pattern of (a) the clay-based TS-1 composite and (b) the unsupported TS-1.
3. Results and Discussion 3.1. Catalyst Characterization. The clay-based TS-1 catalytic composite was synthesized by hydrothermal treatment. To confirm the key characteristics responsible for selective oxidation reactions, the composite was characterized using XRD and FT-IR to verify the presence of the MFI zeolite structure and the Ti incorporation in the TS-1 framework, correspondingly. The XRD patterns of the clay-based TS-1 composite and the unsupported pure TS-1 are presented in Figure 1a and b, respectively. The composite synthesized (Figure 1a) shows a typical TS-1 diffraction pattern with peaks that are identical to ZSM-5, in which the major characteristic peaks at 7.9, 8.9, 14.8, 23.1, and 23.9° are attributed to the basic silica MFI structure, with single sharp peaks at 2θ ) 24.4 and 29.3°. The latter is indicative of orthorhombic symmetry attributed to the incorporation of Ti in the MFI framework. The XRD pattern thus confirmed that the TS-1 crystallized on the clay support are very crystalline and have the MFI structure. It is worth mentioning that the XRD pattern obtained for the clay-based TS-1 composite is dominated by TS-1 because of its highly crystalline nature; therefore, the XRD pattern of the bentonite clay cannot be revealed after the composite is synthesized. The IR spectrum (Figure 2) of both the clay-based TS-1 composite and the unsupported TS-1 shows a characteristic band at 960 cm-1. This infrared feature is shared by other Ti-containing zeolitic structures such as TS-2,17,18 Ti-beta,19 and Ti-ZSM-4820 and is attributed to a stretching vibration of the silicon-oxygen bond in Si-O-Ti bridges.21-23 This characteristic is an indicative of titanium presence in tetrahedral framework positions arising from isomorphous substitution of silicon in the MFI structure.21,24 The IR bands at 800 and 550 cm-1 are attributed to the Si-O-Si stretching and specifically to the presence of MFItype framework, correspondingly. Together with the open framework structures with pores and channels of regular size and shape, the catalytic titanium site in the clay-based composite can catalyze the conversion of ketones to its corresponding oxime with H2O2 and NH3 (ammoximation) at a high oxime selectivity. In accord with the 960 cm-1 IR band, the DRUV-vis spectrum of the sample synthesized (Figure 3) shows a single intense absorption band at 220 nm, further confirming that the Ti atoms occupy tetrahedral positions in the TS-1 framework.25-27 Considering that DR-UV-vis technique is very sensitive to the Ti environment and the fact that no absorption bands are detected around 260 and 330 nm, the clay-based TS-1
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Table 1. Cyclohexanone Ammoximation with and without Catalytic Composite catalysta clay-based TS-1 composite clay-based S-1 compositef no catalyst
Ti/(Si + Ti) BET surface (mol %)b area (m2/g) convc (%) selecd (%) yielde (%) 5.1
307
100
97
97
296
65
24
16
59
1
99.95 purity. b With double distilled water only.
of the liquid to liquid diffusion limitation may be attributed to the nature of the solvent as illustrated by the ammoximation that uses water as the solvent (hollow triangles). In the absence of organic polar solvent, there is no linear correlation between the oxime yield and the catalyst loading. It is suggested that the low oxime yield is resulted from the significant retardation of reaction by the liquid to liquid mass transfer due to the low solubility of cyclohexanone in the aqueous phase. This result evidently indicates that the use of polar solvent, in appropriate amount, can enchance the rate of ammoximation. Moreover, the increasing trend of oxime yield with increasing external catalyst surface available, i.e. increase of the catalyst loading, supports the argument that the effect of the liquid to solid transfer process (b and d) is larger than that of the liquid to liquid transfer (a) on ammoximation. But considering that the clay-based TS-1 catalyst loading of 1080 mg gives very high oxime yield (97%), the fact that the reaction takes place at this condition is primarily assumed not to be limited by the liquid to solid transfer phenomena but instead it is strongly dependent on the rate of the chemical reaction on the active site. Table 4 further illustrates the above interpretation regarding the solvent effect with the reaction data of ammoximation using methanol (MeOH) and water:t-butanol mixture in different ratios of the solvent. These solvents were chosen because they form a single phase with the H2O2 and NH3 solutions so that the mass transfer problem associated with the different phases can be prevented. Although both methanol and water are protic molecules the result shows that both of these solvents give low oxime yield in ammoximation compared with the solvent consists of water and t-butanol. The poor oxime yield resulted from using water as the solvent is much expected because TS-1 is hydrophobic in nature thus it screens out the bulk of the water from the pore system. Moreover, as the volume of the aqueous phase is much more than that of the organic phase, the emulsified cyclohexanone bubbles are surrounded by the aqueous phase when sufficient stirring is applied. Therefore, direct diffusion of cyclohexanone from the organic phase to the TS-1 surface is not expected. This condition, in the absence of other solvent, upsets the mass transfer of reactants to the pore system leading to low activity. However, the fact that using methanol as the solvent for ammoximation gives a comparable result as using water is not anticipated. This is because the donor properties of methanol is higher than that of water therefore it is expected to exhibit higher catalytic activity according to the five-membered cyclic structure mechanism in which the methanol molecule stabilizes the Ti-peroxo complex by having coordinations with the Ti centers.32,33 One possible reason to explain the low activity associated with the methanol solvent is that the advantage of high electrophilicity of methanol is overcome by increasing the steric constraints resulted from the size of the solvent molecule which lower the diffusion rate out of the pore. The results found that the oxime yield can be increased by increasing the t-butanol content in the water:tbutanol solvent mixture. It is suggested that a solvent with water:
Figure 6. Effect of the stirring speed of the reactor on ammoximation.
t-butanol ratio of 1:1 (g/g), i.e. solvent D, is a good medium for both reagents (H2O2, NH3, and cyclohexanone) and the reaction product (cyclohexanone oxime). This is likely because solvent D has the best solvent power for both cyclohexanone and the oxime hence the liquid-liquid diffusion problem can be minimized. Considering alcohols themselves, apart from being a solvent, may also undergo oxidation with H2O2 in the prence of TS-1 catalyst to produce aldehydes or ketones,34 it is suggested that the water:t-butanol mixture in this weight ratio has good stability to hydrogen peroxide at high temperature compared with pure methanol. This is particularly true in the presence of cyclohexanone as the rate of alcohol oxidation is remarkably decreased to a point that it can be considered as negligible. Apart from the solubility of the cyclohexanone in the aqueous phase, the interface area between the organic and aqueous phases is another main factor affecting the rate of the external mass transfer processes (a, b, and d). It is expected that the interface area between the two liquid phases can be enhanced by good mixing of the reaction mixture. Figure 6 illustrates the effect of stirring speed (the agitation rate) on the oxime yield after 2.5 h reaction period. The results first demonstrate that mechanical agitation is extremely important in the reaction system as no oxime yield is recorded when the ammoximation is carried out without mixing (0 rpm). Under such static conditions (also refer to Figure 4-1), it is concluded that rate limitation by external transfer processes occur. However, a significant improvement on oxime yield is observed once mixing is provided to the system. The oxime yield is found to be increased with increasing stirring speed from 100 to 450 rpm, suggesting that the external liquid to liquid and liquid to solid diffusion become less significant at stirring speed of this range although they are still rate limiting. When the stirring speed is faster than 450 rpm, however, the ammoximation is not rate limited by the external diffusion processes as the oxime yield does not seem to be influenced by the stirring speed. The results obtained thus give evidence that in the presence of water:t-butanol and under high speed stirring (450 rpm or above), the ammoximation of cyclohexanone over clay-based TS-1 composite is not rate limited by external diffusion. To investigate the dependence of ammoximation rate on intraparticle diffusion (e and c), clay-based composites with various size of TS-1 are synthesized using tetraethyl ammonium hydroxide (TEAOH) as part of the template solution. Assuming the same structure and composition, Table 5 suggests that there is a clear correlation between the rate of ammoximation and
Ind. Eng. Chem. Res., Vol. 48, No. 18, 2009 Table 5. Effect of the TS-1 Crystal Size on the Clay Support TS-1 crystal size (nm)
oxime yield (%)
300-400a 350-500b 500-1000c >1000d
97 96 83 58
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Table 6. Catalytic Activity of the Clay-Based TS-1 Composite in Cyclohexanone Ammoximation and Cyclododecanone Ammoximation
a Sample prepared following the description in experimental section. Prepared using TPAOH:TEAOH (1:0.1 mol/mol) mixture as the template. c TPAOH:TEAOH (1:0.3 mol/mol) mixture as the template. d TPAOH:TEAOH (1:1 mol/mol) mixture as the template. b
a Estimated from C-C single bond length of 1.53 Å. b Conv (%) ) conversion of ketone. c Selec (%) ) selectivity to corresponding oxime. d Oxime yield (%) ) conv × selec × 100%.
Figure 7. Adsorption of cyclohexanone, cyclododecanone, and their oximes on the clay-based TS-1 composite at 80 °C.
the TS-1 crystal size. The catalytic composite with large TS-1 crystal (>1000 nm) exhibits low oxime yield (58%), thus suggesting that the cyclohexanone ammoximation is strongly influenced by the intraporous diffusion of the reactants and products, especially by the diffusion of cyclohexanone and its corresponding oxime formed in the TS-1 channel. It is found that the oxime yield increases with decreasing TS-1 crystal size down to 350-500 nm. The result also indicates that further reduction in TS-1 crystal size (300-400 nm) does not change the oxime yield. This gives evidence that the ammoximation over the clay-based catalytic composite with TS-1 of 300-400 nm is not governed by internal diffusion. Hence, the catalytic composite with TS-1 of this size range is suggested for kinetic experiment because the limitation of internal diffusion can be ignored such that only the catalytic reaction needs to be considered. By excluding the limitations of the internal and external diffusion, the mechanism of cyclohexanone ammoximation over the clay-based TS-1 composite can be now derived directly from the interaction between the reactants on the Ti-O-Si active site (f). The argument regarding the cyclohexanone ammoximation should take place inside the TS-1 channel is first justified from the adsorption test of cyclohexanone at reaction temperature of 80 °C (Figure 7). The bulk concentration of cyclohexanone in the water:t-butanol solvent reflects the adsorption properties of the clay-based TS-1 composite to cyclohexanone. It is found that nearly 40% of cyclohexanone is adsorbed from the bulk phase to the catalytic composite at 15 min and that no apparent desorption is observed during the tested period. Considering that the theoretical pore sizes of TS-1 with an MFI structure are 0.51 × 0.55 nm for the sinusoidal channel and 0.53 × 0.56 nm for the straight channel,35 the cyclohexanone molecule with kinetic diameter less than 3 Å is able to access to the active site inside the TS-1 pores. This is consistent with
the large adsorbed amount of cyclohexanone on the clay-based composite. The adsorption of the bulk cyclic ketone molecules to the TS-1 channel is further revealed by adsorbing cyclododecanone molecule under the same adsorption condition. As the size of cyclododecanone is too large to enter the pore of TS-1, the decrease of cyclododecanone concentration in the bulk phase is mostly caused by the adsorption on the outer surface of the clay-based TS-1 composite. The difference between the adsorbed amount of cyclohexanone and cyclododecanone confirms that the contribution of TS-1 channels on the adsorption of bulk organic molecules must be greater than that of the outer surface. The results also demonstrate that the adsorption of oximes on the clay-based TS-1 composite is weak which is favorable to product delivery after the reaction. The comparison between the catalytic activity of the claybased TS-1 composite in cyclohexanone ammoximation and cyclododecanone ammoximation is of particular interest as it gives information on the contribution of the outer and inner surfaces on the reaction. Table 6 shows that the clay-based TS-1 composite catalyzes 74% conversion of cyclododecanone and 68% selectivity to the corresponding oxime after 2.5 h. Although the pore sizes of TS-1 are about the same as those of crosssection of C5-C8 cyclic ketones, cyclododecanone molecules with kinetic diameters larger than 4.3 Å are likely excluded from the MFI structure, while only trace amounts of cyclododecanone molecules are able to be adsorbed in the pore strucutre of TS-1. In view of this steric restriction, the yield of cyclododecanone oxime confirms that the outer surface of the clay-based TS-1 composite must also have contribution on the ammoximation. This interpretation suggests that the catalytic oxidation of NH3 and H2O2 to hydroxylamine perhaps takes place inside the TS-1 pores, while the oxime is formed from noncatalyzed condensation of hydroxylamine with the ketone on the catalyst surface. Comparing the selectivity of cyclohexanone and that of cyclododecanone, it can be seen that the TS-1 pores and channels have significant effect on improving the selectivity to oxime by providing the inner surface for reaction to take place. Considering cyclohexanone molecules are able to diffuse into the pores effectively, the effect of the outer surface is considerably small compared to the inner surface and thus very high selectivity to oxime is imposed by the steric effect inside the channels. Moreoever, the trace amount of cyclododecanone adsorbed in the pore have difficulty in diffusion inside the structure due to the bulkiness of the molecules, resulting in retardation of reaction which eventually leads to low oxime yield. This is however not the case for small cyclic ketone molecules such as cyclohexanone. According to the results obtained thus far, it is concluded that both the inner and outer
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Figure 8. FT-IR spectra of (a) the clay-based TS-1 composite and that upon interaction with (b) H2O2, (c) NH3, (d) cyclohexanone, (e) H2O2 + NH3, and (f) the clay-based TS-1 composite regenerated at calcination condition after interaction with H2O2 + NH3.
surfaces of the clay-based TS-1 catalyst have contribution on cyclohexanone ammoximation in which the inner surface has more influence on oxime yield due to shape-selectivity. To identify the mechanism on the active site of the claybased TS-1 composite, the changes of the 960 cm-1 IR band upon the interaction with H2O2, NH3, and cyclohexanone are investigated individually (Figure 8). The interaction was carried out in t-butanol for 2.5 h, and the dosage of the reagents was identical to their concentration in the optimum condition of cyclohexanone ammoximation. It should be mentioned that the following interpretation on reaction mechanism is based on two major hypotheses: (1) the active site in the clay-based TS-1 composite for cyclohexanone ammoximation refers only to perfect Ti (IV) center, that is a Ti atom linked to Si atoms through four oxygen bonds in tetrahedral coordination [Ti(OSi)4]. Therefore, the Ti-OH and HO-Si pairs which may be hydrolyzed from the Ti-O-Si bridges are considered to be catalytically inactive for the reaction; (2) the active site can simultaneously adsorb more than one molecule. Figure 8a is the IR spectra of the clay-based TS-1 composite showing the strong vibration occurring at 960 cm-1 attributed to the presence of framework titanium, i.e. the Ti-O-Si bridge. The difference in the IR spectrum is recorded immediately upon interaction with H2O2 (Figure 8b). It can be seen that the 960 cm-1 charateristic band disappeared and blue-shifted to ca. 1002 cm-1 with weak IR intensity (circled). This result confirms that the adsorbed H2O2 molecules must have interaction with the active site and changes the coordination of the Ti ions in the structure. Furthermore, the typical vibration of Si-O-Si bridge at 800 cm-1 is shifted to the 785 cm-1 position with complex feature, suggesting the clay-based TS-1 composite, both the TS-1 and bentonite, may experience changes in their silica surface due to interaction with H2O2. One possible form of Si that gives rise to this IR band maybe the SiOOH group. If the interaction with H2O2 hydrolyzes the Ti-O-Si bridges to SiOOH group, the presence of its counterpart TiOOH group is very much expected. The absence of 837 cm-1 IR band which is indicative
of the TiOOH group hence concludes that the physical structure of the Ti-O-Si active site is independent of the interaction with H2O2. Considering that the Ti ion in the clay-based TS-1 composite is coordinatively unsaturated, it is possessed of Lewis acidity and is capable of adsorbing H2O2 as well as NH3 molecules. Figure 8c exhibits the IR spectrum of the composite upon NH3 dosage. Compared with the untreated clay-based TS-1 composite (Figure 8a), the vibration occurring at 960 cm-1 is virtually absent in the NH3 adsorbed sample while no additional band is observed in the spectral position nearby. Due to the hydrophobic nature of TS-1, water molecules are not expected to have influence on the Ti-O-Si band either in position or in intensity. Thus, the significant perturbation of 960 cm-1 band is only ascribable to the NH3 molecules in the NH4OH solution. The result thus confirms that the NH3 molecules interact directly with the Ti (IV) active sites and that the affinity is much higher toward Ti with respect to water. This IR manifestation agrees well with the fact that a strong base like NH3 can easily form Lewis acid-base adducts in both perfect Ti(OSi)4 and defective TiOH(OSi)3 sites. It is worth mentioning that the effect arose from the latter case is ignored because the hydrolyzed site is not considered as the active site for ammoximation according to the assumption stated previously. Coming now to the interaction with cyclohexanone, Figure 8d shows that there is no apparent spectral change compared with Figure 8a. As shown in the adsorption test earlier, the claybased TS-1 composite has high adsorption capacity of cyclohexanone (40% of the quantity in the bulk phase in 15 min) because the cyclohexanone molecules are small enough to diffuse into the pore and channel of the TS-1. Therefore, it is evident that the adsorption of cyclohexanone is significantly contributed by the inner surface. On this basis, it is hypothesized that the Ti active site inside the TS-1 supported on the clay is accessible to cyclohexanone molecules similar to H2O2 and NH3. Although the intensity of the 960 cm-1 band is slightly reduced upon interaction with cyclohexanone, the fact that there is no change in the spectral position suggests that the cyclohexanone molecules do not change the coordination of the Ti ions in the structure. Therefore, it is concluded that the cyclohexanone molecules are adsorbed on the inner surface inside the channel of the clay-based TS-1 instead of being adsorbed directly on the Ti active site. Figure 8e is the IR spectrum of the clay-based TS-1 composite obtained after interacting with NH3 and H2O2, simultaneously. After adsorbing NH3 and H2O2, the 960 cm-1 IR band disappears with an additional band appears at ca. 1002 cm-1. This phenomenon is consistent with those observed upon interaction with NH3 and H2O2 individually. The result points out that when NH3 and H2O2 coexist in the catalytic system, both reagents have interaction with the Ti active site and that an intermediate is likely formed. This fact agrees well with the literature showing that TS-1 is an efficient catalyst for the selective oxidation of NH3 to hydroxylamine (NH2OH) in the absence of ketones.29 Interestingly, Figure 8f shows that such interaction with NH3 and H2O2 is almost reversible as can be seen from the NH3 and H2O2 concurrently adsorbed sample which is regenerated at calcination condition (550 °C for 6 h). The spectral position of the characteristic IR band at 960 cm-1 is recovered upon regeneration. The same also occurs for the 800 cm-1 band. On the basis of the above interpretation, the three reactants in cyclohexanone ammoximation have different extent of interaction with the Ti active site. It is evident that H2O2 and NH3 interact directly with the Ti active site while cyclohexanone
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only demonstrates adsorption on the inner and outer surface of the catalyst. It is also certain that the mechanism of cyclohexanone ammoximation over the clay-based TS-1 catalytic composite must consist of several steps including adsorption, surface reaction, and desorption at different rates. After the reactants have been adsorbed onto the active site or other surface elsewhere, they are capable of reacting with each other in a number of ways to form the final product. According to the spectroscopic evidence and the experimental results obtained, the cycloehxanone ammoximation over the clay-based TS-1 composite most possibly accord with two mechanisms namely the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The mechanism based on LH kinetics is as follows. The H2O2 and NH3 molecules are adsorbed onto the Ti active site separately, followed by in situ surface reaction to form hydroxylamine (eq 1, 2, and 4). Concurrently, the cyclohexanone molecules is also adsorbed on the catalytically inactive site which is in close proximity to the Ti site (eq 3). The reaction then proceeds via interactions between the hydroxylamine precursor and cyclohexanone on the surface to produce cyclohexanone oxime (eq 5). Finally, the oxime product is desorbed and diffuses to the bulk phase. [A] + H2O2 T [A · H2O2]
(1)
[A] + NH3 T [A · NH3]
(2)
[B] + cyclo T [B · cyclo]
(3)
[A · H2O2] + [A · NH3] f [A · NH2OH] + [A] + H2O (4) [A · NH2OH] + [B · cyclo] f prod + [A] + [B] + H2O (5) where [A] is the Ti active site in the clay-based catalytic composite, [B] is the adsorption site on the inner and outer surface of the catalytic composite, cyclo is cyclohexanone, and prod represents cyclohexanone oxime, i.e. the desired product. In addition to the LH kinetics based on the surface reaction between two adsorbed species, a modified version which takes into account the possibility that the mechanism could also consists of reaction between an adsorbed molecule and a molecule in the bulk phase is suggested. Such a mechanism is referred to as following Eley-Rideal kinetics. Since the adsorption of H2O2 on the active site is much stronger than that of NH3 and cyclohexanone, ER kinetics of cyclohexanone ammoximation over the clay-based TS-1 composite therefore considers H2O2 to be the only adsorbed specie on the Ti active site [A] while, subsequent to its adsorption, NH3 molecules from the bulk phase come into contact and react with the adsorbed H2O2 to form hydroxylamine. Similarly, the adsorbed hydroxylamine then reacts with the cyclohexanone diffused from the bulk phase to yield oxime (eqs 1 and 6-8). Another approach is also proposed taking into consideration the cyclohexanone can be efficiently adsorbed by the large inner surface available in the clay-based TS-1 composite (eq 3). In such case, the hydroxylamine formed from the reaction between the adsorbed H2O2 and free NH3 is desorbed to the bulk phase. It then reacts with the cyclohexanone which is adsorbed on the catalyst surface, particularly on the inner surface, to form oxime. At last, the oxime product is released to the bulk phase (eqs 9-11). [A] + H2O2 T [A · H2O2]
(1)
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[A · H2O2] + NH3 f [A · NH2OH] + H2O
(6)
[A · NH2OH] + cyclo f [A · prod] + H2O
(7)
[A · prod] f [A] + prod
(8)
[B] + cyclo T [B · cyclo]
(3a)
[A · H2O2] + NH3 f NH2OH + [A] + H2O
(9)
[B · cyclo] + NH2OH f [B · prod] + H2O
(10)
[B · prod] f [B] + prod
(11)
It is worth to mentioning that both the LH and ER mechanisms of the cyclohexanone ammoximation over the clay-based TS-1 composite suggest that the oxidation of NH3 to hydroxylamine (NH2OH) in the presence of H2O2 is a catalytic process in which the Ti active site is responsible. This is followed by the noncatalyzed condensation of NH2OH with cyclohexanone to yield oxime on the catalyst surface. Using kinetic analysis, it is possible to justify the feasibility of the two proposed reaction models. The related investigation is currently in progress. 4. Conclusions In this paper, it was shown that a clay-based TS-1 catalytic composite is successfully synthesized by hydrothermal treatment. Characterization techniques include FT-IR and DRUV-vis spectroscopies demonstrate that the composite formed is of high purity and, more importantly, the titanium atoms are confirmed to be incorporated in a MFI zeolite framework resulting in specific characteristics needed for catalytic oxidation, namely cyclohexanone ammoximation. It was found that the reaction catalyzed by the clay-based TS-1 composite gives the highest oxime yield when H2O2 and NH3 are in excess. The optimum molar ratio of cyclohexanone: H2O2:NH3 is found to be 1:1.6:2.5 owing to the fact that the cyclohexanone ammoximation works best at 80 °C environment. This study also concludes that continuously adding H2O2 and evenly distributing the required amount of NH3 in five doses to the reaction mixture is the best method of reagent addition. Due to the triphasic nature of the system, the cyclohexanone ammoximation over the clay-based TS-1 composite is affected by external and internal diffusions. In the absence of t-butanol, mass transfer of cyclohexanone from the organic phase to the aqueous phase is difficult and, therefore, is the rate limiting step. This diffusion limitation can be eliminated by using a water:tbutanol mixture as a solvent to enhance the solubility of cyclohexanone in the aqueous phase. Using a water:t-butanol solvent with the optimum ratio, the apparent kinetic is assumed to be limited by the chemical reaction on the active site. In a static condition without stirring, the cyclohexanone ammoximation is limited by the liquid to solid mass transfer process. The limitation is excluded when the stirring speed is faster than 450 rpm. Moreover, the fact that the composites with TS-1 crystal size smaller than 500 nm show equivalent catalytic activity further rules out the internal diffusion problem. Therefore, this study provides a conclusive proof that, under the conditions suggested, the cyclohexanone ammoximation catalyzed by the clay-based TS-1 catalytic composite is not governed by the diffusional factors. This information is particularly important when measuring the data of intrinsic kinetics which are essential for industrial reactor design. The adsorption studies coupled with FT-IR analysis concluded that only H2O2 and NH3 have direct interaction with the Ti active
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site while cyclohexanone is adsorbed on the inner and outer surface of the catalytic composite in which adsorption inside the TS-1 channel is dominated. Accordingly, it is proposed that the Langmuir-Hinshelwood and Eley-Rideal models are the two most feasible mechanisms for the cyclohexanone ammoximation over the composite synthesized. Acknowledgment We acknowledge the financial support from the National Natural Science Foundation of China (project No. 20518001) and the Research Grants Council of Hong Kong under the grant Nos. N_HKUST620/05 and 605108. Literature Cited (1) Bell, W. K.; Haag, W. O. Synthesis of Caprolactam. US Patent 4,927,924, 1990. (2) Thomas, J. M.; Raja, R. Design of a “Green” One-Step Catalytic Production of Epsilon-Caprolactam (Precursor of Nylon-6). Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 13732. (3) Mokaya, R.; Poliakoff, M. Chemistry - A Cleaner Way to Nylon. Nature 2005, 437, 1243. (4) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes; Wiley: New York, 1992. (5) Roffia, P.; Padovan, M.; Moretti, E.; De Alberti, G. Catalytic process for preparing cyclohexanone-oxime. Europ. Pat. 1987, 208, 311. (6) Taramasso, M.; Pergo, G.; Notari, B. Preparation of Porous Crystalline Synthetic Material Comprised of Silicon and Titanium Oxides. US Patent 4,410,501, 1983. (7) Thangaraj, A.; Sivasanker, S.; Ratnasamy, P. Catalytic Properties of Crystalline Titanium Silicalites. 3. Ammoximation of Cyclohexanone. J. Catal. 1991, 131, 394. (8) LeBars, J.; Dakka, J.; Sheldon, R. A. Ammoximation of Cyclohexanone and Hydroxyaromatic Ketones Over Titanium Molecular Sieves. Appl. Catal., A 1996, 136, 69. (9) Mantegazza, M. A.; Petrini, G.; Spano, G.; Bagatin, R.; Rivetti, F. Selective Oxidations with Hydrogen Peroxide and Titanium Silicalite Catalyst. J. Mol. Catal. A: Chem. 1999, 146, 223. (10) Tuel, A. Crystallization of Titanium Silicalite (TS-1) from Gels Containing Hexanediamine and Tetrapropylammonium Bromide. Zeolites 1996, 16, 108. (11) Zhao, Q.; Bao, X. H.; Han, X. W.; Liu, X. M.; Tan, D. L.; Lin, L. W.; Guo, X. W.; Li, G.; Wang, X. S. Studies on the Crystallization Process of Titanium Silicalite-1 (TS-1) Synthesized Using Tetrapropylammonium Bromide as a Template. Mater. Chem. Phys. 2000, 66, 41. (12) Van der Pol, A. J. H. P.; Verduyn, A. J.; van Hooff, J. H. C. Why Are Some Titanium Silicalite-1 Samples Active and Others Not. Appl. Catal., A 1992, 92, 113. (13) Van der Pol, A. J. H. P.; van Hooff, J. H. C. Parameters Affecting the Synthesis of Titanium Silicalite-1. Appl. Catal., A 1992, 92, 93. (14) Zhang, G. Y.; Sterte, J.; Schoeman, B. Preparation of Colloidal Suspensions of Discrete TS-1 Crystals. Chem. Mater. 1997, 9, 210. (15) Yip, A. C. K.; Lam, F. L. Y.; Hu, X. A Heterostructured Titanium Silicalite-1 Catalytic Composite for Cyclohexanone Ammoximation. Microporous Mesoporous Mater. 2009, 120, 368. (16) Yip, A. C. K.; Lam, F. L. Y.; Hu, X.; Li, P.; Yuan, W. K. Study on the Synthesis of Clay-based Titanium Silicalite-1 Catalytic Composite. Ind. Eng. Chem. Res. 2009, 48, 5266.
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ReceiVed for reView May 6, 2009 ReVised manuscript receiVed July 26, 2009 Accepted July 27, 2009 IE900731S
