Continuous heterogeneous cyclohexanone ammoximation reaction using a monolithic TS-1/cordierite catalyst

Abstract

In this study, a hydrothermal synthesis was developed to prepare a thin titanium silicalite-1 catalyst layer in the internal channels of the honeycomb cordierite support, which was used in a continuous heterogeneous reaction. The as-prepared thin TS-1 catalyst layer was characterized by SEM, XRD, UV-vis and ATR-FTIR methods. Multiple hydrothermal syntheses were used for controlling the crystalline growth to optimize catalytic activity and lifetime. Moreover, a monolithic TS-1/cordierite reactor with external looping was utilized to investigate the characteristics of the continuous heterogeneous ammoximation reaction without the organic solvent tert-butanol (TBA). The reaction results showed that the conversion of cyclohexanone and selectivity of oxime approached more than 0.96 and 0.98, respectively. When compared with the reaction using TBA as the reaction solvent, the activity of the monolithic TS-1/cordierite showed no differences except for a longer lifetime.

---

1. Introduction

The ammoximation reaction of cyclohexanone has been demonstrated on an industrial scale, and its product, cyclohexanone oxime, is an important intermediate for producing caprolactam (CPL) and Nylon 6 after Beckmann rearrangement and polymerization, respectively. The application of titanium silicalite-1 (TS-1) catalysts has solved the challenges involved in the ammoximation of cyclohexanone reaction such as multiple reaction steps, harmful oximation agents, and large amounts of environmental polluted co-products.^1,2 Nevertheless, the use of the organic solvent tert-butanol (TBA) to intensify mass transfer has environmental drawbacks and increases industrial cost. In addition, the by-products generated in TBA can cause deactivation of the TS-1 catalyst because the micropores in the catalyst can be blocked by bulky organic solvent.^3,4 Recently, investigations into catalyst deactivation and liquid–solid separation in a slurry bed reactor for the cyclohexanone ammoximation reaction have attracted considerable attention. Therefore, a solvent-free ammoximation reaction would be preferred. However, in the case of solvent removal, the reactant mixture would be two phased and aqueous catalysis would be weakened because of the limitation of mass transfer in the liquid–liquid two phase mixture. Meanwhile, to maintain the conversion and selectivity of the reaction is still very challenging.

TS-1 with a MFI topology framework has drawn extensive attention for its excellent catalytic oxidation performance^5–10 in H₂O₂ systems under mild reaction conditions such as aromatic hydroxylation, alkene epoxidation, and ketone ammoximation. During the past decades, the investigation of TS-1 synthesis,^11,12 structural orientation preparation^13,14 and the catalytic mechanism¹⁵ have achieved significant progress. However, the utilization of a direct cyclohexanone ammoximation reaction in industrial plants has faced a rather disappointing dilemma because the expensive TS-1 catalyst powders deactivate in a short lifetime and run off with the liquid products from a slurry bed reactor.

Herein, a solvent-free route using monolithic TS-1 as catalyst was chosen for the cyclohexanone ammoximation reaction. The monolithic TS-1 catalyst, constructed by a support with many straight flow channels in parallel and a “flat” catalyst surface, was prepared by direct multi-step hydrothermal syntheses. The activity of the monolithic TS-1 catalyst applied continuously in a custom-made structured reactor was evaluated under different reaction conditions. The catalytic performance of the monolithic reactor was characterized by a low pressure drop, high-mass transfer rates, and easy scale up,^2,16,17 in the continuously heterogeneous cyclohexanone ammoximation reaction and demonstrate a similar conversion, selectivity, and superior lifetime over a conventional fixed bed reactor in large flow flux. We speculate it is because the channel size and catalyst layer thickness of the monolithic catalytic reactor are small enough to shorten the distance of mixing the bulk reactants and diffusing in the catalyst for multiphase catalytic reactions.

2. Experimental

2.1. Materials

The honeycomb cordierite supports (Jiangyin Huayin Porcelain & Machine Electricity Technology Co. Ltd., China) had parallel triangular channels with a length of about 90 mm and width of 2, 1.5 and 1 mm corresponding to 150, 250, 350 channels per square inch (cpsi), respectively. Tetrabutyl orthotitanate (TBOT, 98.5 wt%, AR), tetraethyl orthosilicate (TEOS, 98 wt%, AR), isopropanol (IPA, 98 wt%, AR) and tetrapropylammonium hydroxide (TPAOH, 25 wt%, AR) were purchased from Shanghai Aladdin Reagent Co., Ltd., China, and used as received. The reagents used in the cyclohexanone ammoximation reaction were purchased from Tianjin Guang Fu Fine Chemical Research Institute, China, including cyclohexanone (99 wt%, AR), hydrogen peroxide (H₂O₂, 30 wt%, AR), ammonia water (NH₃·H₂O, 25 wt%, AR), cyclohexanone oxime (99 wt%, AR), TBA (98 wt%, AR), toluene (99.5 wt%, AR), ethanol (99.7 wt%, AR) and deionized water.

2.2. Preparation of monolithic TS-1/cordierite catalysts

The synthesis of the TS-1 precursor was similar to the process described in the literature.¹¹ A typical synthetic procedure for the TS-1 precursor sol is as follows: 26.8 g TEOS was dissolved into 30.5 g TPAOH solution by a metric syringe pump for 30 min and hydrolyzed at 277 K with vigorous stirring for 1 hour. Then, a mixture of 1.7 g TBOT and 7.5 mL anhydrous IPA was added slowly to the abovementioned solution with the metric syringe pump over 1 hour. The resulting solution was further hydrolyzed at 277 K for 1 hour. Alcohol was removed at 358 K in 5 hours, and the volume loss of the hydrolyzed solution was compensated by adding deionized water. Finally, a transparent sol with a molar composition of 1SiO₂ : 0.04TiO₂ : 0.3TPAOH : 25H₂O was obtained for coating on the internal surface of the monolithic supports.

The monolithic cordierite supports were pretreated with deionized water in an ultrasonic bath to remove any contamination on the surface, and then calcined in air at 823 K for 6 hours. The cleaned and dried support was submerged in an autoclave filled with the as prepared TS-1 precursor sol and sealed. The next step was a hydrothermal synthesis by fastening the autoclave with a Teflon liner on a rotating bracket in an oven at 448 K and 5 rpm for 3 days. After cooling to room temperature, the monolithic TS-1/cordierite catalyst was washed with deionized water, dried overnight at 383 K, and calcined in air at a heating rate of 1 K min⁻¹ from room temperature to 823 K and maintained at 823 K for 12 hours. The hydrothermal synthesis was repeated following the same procedure in an autoclave filled with fresh TS-1 precursor and freshly prepared monolithic TS-1/cordierite catalyst. The mass of the TS-1 layer coated on monolithic cordierite was controlled by multiple hydrothermal syntheses. The solid powders settled on the bottom of the autoclave were collected for comparison with the TS-1 layers on the monolithic supports.

2.3. Characterization of the prepared catalysts

The crystallite morphologic micrograph and energy-dispersive X-ray spectroscopic (EDS) analysis of the prepared catalysts were obtained with a Hitachi S-4800 field emission scanning electron microscope (SEM) equipped with a Thermo Scientific energy dispersion X-ray fluorescence analyzer. XRD patterns were recorded with a Bruker D8 Focus using Cu Kα radiation in the 2θ angle range of 5–60° at 40 kV and 40 mA, and FT-IR spectra were recorded by a Nicolet 6700 in the attenuated total reflectance (ATR) mode and transmission mode. In addition, UV-visible measurements were performed on a Shimadzu UV-2550 using the diffused reflectance mode with BaSO₄ as a reference.

2.4. Activity and lifetime of the as-prepared monolithic TS-1/cordierite catalysts

The catalytic ammoximation reaction of cyclohexanone was carried out continuously in a custom-made glass reactor at atmospheric pressure. As shown in Fig. 1, the monolithic TS-1 catalyst was wound outside with Teflon tape and tightly pushed into the reactor. For a typical reaction using TBA as a solvent, the reactor was completely filled with TBA before heating to a temperature ranging from 323 to 353 K, and the liquid in the reactor was externally circulated through the monolithic TS-1/cordierite at controllable flow rate. Then, a mixture of ammonium hydroxide and TBA solution, cyclohexanone and H₂O₂ was separately fed into the reactor with three pumps. All the reactants were maintained at constant feeding rates for a molar ratio of cyclohexanone : H₂O₂ : NH₃·H₂O : TBA = 1 : 1.2 : 1.5 : 10.

Fig. 1 Setup for the continuous cyclohexanone ammoximation reaction using the monolithic TS-1/cordierite catalyst where, 1: cyclohexanone tank; 2: hydrogen peroxide tank; 3: ammonia water (or mixture of ammonia water and t-butanol) tank; 4: constant flow pump; 5: reserve tank; 6: peristaltic pump; 7: reactor; 8: monolithic TS-1/cordierite catalyst; 9: thermometer; 10: water bath; 11: mechanical stirrer.

However, for the reaction in the absence of TBA, the same volume of deionized water was filled into the reactor instead of TBA, and the feeding process remained unchanged as in the presence of the solvent with the exception of no feeding of TBA solvent. Samples were sucked periodically, dissolved in ethanol and analyzed by SP3420 GC (Beifen-Ruili Analytical Instrument Co. Ltd). Toluene was an internal standard; flame ionization detector and a 30 m of SE-54 capillary column were used. The start-up would last for approximately 40 hours before approaching a continuous operation.

The conversion of cyclohexanone, yield and selectivity of oxime were defined as follows:

Instantaneous conversion of cyclohexanone:

Yield of oxime:

Selectivity of oxime:

where w⁰_CYC, w_CYC and w_oxime denote the initial, the instantaneous mass fraction of cyclohexanone, and the mass fraction of cyclohexanone oxime in the reaction solution, respectively.

The activity of the monolithic TS-1/cordierite catalysts was defined as:

a = X_CYC/X⁰_CYC (4)

where X⁰_CYC denote initial steady conversions of cyclohexanone.

The space velocity was calculated based on the mass of coated TS-1 catalyst using the following definition:

WHSV = F_all/m_TS-1 (5)

where WHSV denotes the weight hourly space velocity; F_all is the feeding flow rate of all the components, g h⁻¹; m_TS-1 is the mass of coated TS-1 layer on monolithic support (g).

3. Results and discussion

3.1. Characterization of the monolithic TS-1/cordierite catalyst

First, the as-prepared monolithic TS-1/cordierite catalyst was investigated in detail. As shown schematically in Fig. 2, the catalyst contains a cordierite support and a layer of TS-1 on top. The cordierite support has a coarse inner surface of channels, as shown in Fig. 2a. After hydrothermal synthesis, a thin layer of TS-1 was deposited on the inner surface (Fig. 2b and c). From the cross-sectional view, shown in Fig. 2b, the thickness of the TS-1 crystal layer is about 3 μm after one-step crystallization. To increase the amount of TS-1 catalyst, multiple crystallizations, which could generate a thicker TS-1 layer, were performed. As shown in Fig. 2d, the thickness reaches 5 μm after two-step crystallizations. The TS-1 crystal size was about 100 nm, as revealed in Fig. 2f. As shown in Fig. 2c and e, the coarse surface was gradually smoothed through multiple crystallizations.

Fig. 2 SEM images of the monolithic TS-1/cordierite catalyst (a) top view of cordierite support; (b) cross-sectional view and (c) top view of the TS-1 layers and cordierite support after a one-step hydrothermal synthesis; (d) cross-sectional view, (e) small and (f) large magnified top view of the TS-1 layer and cordierite support after a two-step hydrothermal synthesis.

The speed of the growth of the TS-1 layer was also investigated. The thickness and weight of the TS-1 layer can be controlled by the number of crystallization steps (displayed in Fig. S1†). In the hydrothermal process, the coarse surface of the support can offer nucleation sites to promote the crystallization of TS-1. The TS-1 nanoparticles formed in the former step can act as seeds and provide nucleation sites for subsequent crystallization. Therefore, they can effectively reduce the growth time for subsequent steps.

Furthermore, XRD was applied to prove the successful synthesis of the monolithic TS-1/cordierite catalyst (Fig. 3). The XRD pattern of the TS-1 powders (Fig. 3c) was compared with the TS-1/cordierite catalysts (Fig. 3b) and the cordierite support (Fig. 3a). The XRD pattern of the TS-1/cordierite catalyst showed characteristic peaks of an MFI structure at 2θ = 7.9°, 8.9°, 23.1°, 23.9° and 24.4° (indicated by a star), which was identical with the standard TS-1 pattern.^11,18 The Ti atom peak at 29.3° (indicated by a dot) was a convincing evidence of the incorporation of in the zeolite framework. Other peaks shown in Fig. 3a were attributed to the cordierite support (indicated by a triangle) when compared with Fig. 3c. Clearly, none of the distinct peaks in Fig. 3b were different from those in Fig. 3a and c, which implied that the as-prepared catalyst had a TS-1 layer grown on the cordierite support.

Fig. 3 XRD patterns of (a) the cordierite support, (b) fresh TS-1 layer, and (c) TS-1 powders.

The FT-IR and UV-vis spectra were also employed to further confirm the incorporation of Ti atoms into the framework of the TS-1 crystals. As shown in Fig. 4, the TS-1 layer (b) and powders (c) display the characteristic transmittance bands of an MFI zeolite at 1220 cm⁻¹, 1100 cm⁻¹, and 800 cm⁻¹. The band at around 960 cm⁻¹ belongs to Ti–O–Si, which has been generally accepted as a direct proof of the incorporation of Ti atoms into the TS-1 framework.^19,20 The FT-IR results further support that the obtained layer on the monolith is composed of the TS-1 powders, and the components in the cordierite do not affect TS-1 formation in the hydrothermal synthesis. In addition, the corresponding UV-vis spectra of the same samples are shown in Fig. 5. The broad absorbance bands around 210 nm in the spectra of the TS-1 catalyst (Fig. 5b and c) belongs to the tetrahedral coordinated framework of titanium.¹⁹ A typical signal of anatase TiO₂ at 330 nm was also detected in fresh catalysts and the as-prepared TS-1 powders.

Fig. 4 FT-IR spectra of (a) the cordierite support, (b) fresh TS-1 layer, and (c) TS-1 powders.

Fig. 5 UV-vis spectra of (a) the cordierite support, (b) fresh TS-1 layer, and (c) TS-1 powders.

3.2. Characteristics of the cyclohexanone ammoximation reaction using the monolithic TS-1/cordierite catalysts

To compare the characteristics of the cyclohexanone ammoximation reaction using the monolithic TS-1/cordierite catalysts with and without TBA, a series of experiments were conducted with various influence factors such as reaction temperature, WHSV and channel dimension.

3.2.1 Effect of reaction temperature on the cyclohexanone ammoximation reaction. The cyclohexanone ammoximation reaction was carried out in a continuously operated monolithic reactor, as mentioned above. Cyclohexanone, H₂O₂, NH₃·H₂O at a molar ratio of 1 : 1.2 : 1.5 were fed to the reaction. In the reaction temperature range of 323 to 353 K, the conversion of cyclohexanone increased with an increasing temperature. When the temperature was higher than 353 K, the selectivity of oxime decreased from 0.99 to 0.96 (Fig. 6). Similarly, the influence of temperature was investigated in the presence of the co-solvent TBA. Cyclohexanone conversion and oxime selectivity approached 0.96 and 0.99, respectively, at an optimal reaction temperature of 343 K.

Fig. 6 Effect of reaction temperature on the cyclohexanone ammoximation reaction using the monolithic TS-1/cordierite catalysts. The feed molar ratio of the continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O : TBA = 1 : 1.2 : 1.5 : 10 (with TBA), cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5 (without TBA); WHSV, 6.0 h⁻¹; flow rate of external circulation, 250 mL min⁻¹; channel density of the monolith cordierite, 250 cpsi.

3.2.2 Effect of weight hourly space velocity (WHSV). WHSV, which was calculated by the weight of coated TS-1 layer using eqn (4), was also investigated to investigate its effect on the ammoximation reaction. Within a wide WHSV window from 1.4 to 10 h⁻¹, the cyclohexanone conversion and oxime selectivity without TBA at steady states were above 0.96 and greater than 0.99, respectively (Fig. 7), which were not different from that in the presence of TBA. Therefore, the weight of the loaded catalyst layer was sufficient to achieve a higher conversion of cyclohexanone and selectivity of oxime at a WHSV below 10 h⁻¹.

Fig. 7 Effect of WHSV on the cyclohexanone ammoximation reaction using monolithic TS-1/cordierite catalysts. Feed molar ratio of the continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O : TBA = 1 : 1.2 : 1.5 : 10 (with TBA), cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5 (without TBA); flow rate of external circulation, 250 mL min⁻¹; channel density of monolith cordierite, 250 cpsi; reaction temperature, 343 K.

3.2.3 Influence of the support channel size on the cyclohexanone ammoximation reaction. The monolithic TS-1/cordierite catalysts comprise supports with an array of parallel reaction channels with different size, and the bulk liquids were externally looped through the channels in a single phase or two-phase flow corresponding to the presence or absence of TBA. In contrast to the random flow patterns of multiphase in conventionally packed beds, the flows in the monolithic catalyst bed are highly ordered.¹⁶ The impact of channel size on the ammoximation reaction was tested using three kinds of monoliths with triangular channels of 2, 1.5, and 1 mm width. As shown in Fig. 8, at the same WHSV, 1 mm and 1.5 mm channels gave similar conversions and selectivities, which are higher than that found using a 2 mm channel because of the changes in mass transfer distance and liquid moving speed at the same recycle flow rate.

Fig. 8 Effect of the monolithic support channel sizes on the cyclohexanone ammoximation reaction. Feed molar ratio of the continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5; WHSV, 6.0 h⁻¹; flow rate of external circulation, 250 mL min⁻¹; reaction temperature, 343 K.

The monolithic reactor with multi-channels of a millimeter or sub-millimeter scale is different from conventional fixed bed reactors.^17,21–23 Since the hydraulic diameter of each channel is similar to a capillary, liquid–solid surface phenomena may also play a significant role in intensifying mass transfer. The activity results shown in Fig. 8 illustrate that the decrease in channel sizes enhanced the mass transfer and reaction. When the reaction was operated at the same external looping rate, liquid surface tension dominated the heterogeneous ammoximation reaction when compared with inertia force, and the formation of Taylor's flow would be beneficial for intensifying mass transfer in the millimeter-scaled channels.^24,25

3.3. Activity and lifetime of the monolithic TS-1/cordierite catalysts

Catalyst deactivation is a critical problem for the ammoximation reaction of cyclohexanone. Therefore, it is necessary to investigate the feasibility and activity stability of the monolithic TS-1/cordierite catalyst. The reaction was run for over 50 h of time on stream (TOS), and the experimental data are presented in Fig. 9. As shown in Fig. 9a–d, the conversions were nearly 0.98 at the primary steady stage and were maintained until deactivation, while the selectivities increased at the primary stage, were maintained at 0.99 for a long period, and finally decreased to 0.90. In comparison with a slurry bed reactor^4,26 and membrane reactor,^27,28 the monolithic reactor displayed much longer operating stability and feasibility in the continuous ammoximation reaction. Fig. 9b recorded the reaction data without TBA, which were similar to that with TBA, as shown in Fig. 9c. Due to the advantages of monolithic support, multi-channels in the monolithic TS-1/cordierite catalysts provided efficient mixing between the cyclohexanone in the organic phase and H₂O₂, NH₃·H₂O in the water phase without TBA, and finally intensified the multiphase reaction.

Fig. 9 Performances of different WHSVs for the continuous cyclohexanone ammoximation reaction using monolithic TS-1/cordierite catalysts. The cyclohexanone conversion, X and the oxime selectivity, S: (a) X, S, WHSV = 1.4 h⁻¹, without TBA; (b) X, S, WHSV = 6 h⁻¹, without TBA; (c) X, S, WHSV = 6 h⁻¹, with TBA; (d) X, S, WHSV = 10 h⁻¹, without TBA; (e) X, S, WHSV = 20 h⁻¹, without TBA in the ammoximation reaction; feed molar ratio of continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O : TBA = 1 : 1.2 : 1.5 : 10 (with TBA), cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5 (without TBA); flow rate of external circulation, 250 mL min⁻¹; channel density of monolith cordierite, 250 cpsi; reaction temperature, 343 K.

In Fig. 9e at a high WHSV of 20 h⁻¹, the cyclohexanone conversion and oxime selectivity at steady state was maintained at about 0.80 because the reaction is not sufficient. The activity of the monolithic TS-1/cordierite catalyst declined sharply within a short duration irrespective of the adoption of TBA in the continuous ammoximation reaction (Fig. 10). The activity was defined using eqn (3), which was high under WHSV below 10 h⁻¹ and low under WHSV up to 20 h⁻¹. Except for the high catalytic activity, the applicability of the monolithic TS-1/cordierite catalysts also depended on their lifetime and stability. The deactivation was considered to start when the activity decreased to 0.92. Fig. 10b and c illustrate that the lifetimes of the monolithic TS-1/cordierite catalysts without TBA exhibited a slightly longer lifetime than that with the solvent. In addition, TBA was beneficial to mass transfer but accelerate the deactivation of the TS-1 catalyst.⁴

Fig. 10 Variation of activity under different WHSVs for the continuous cyclohexanone ammoximation reaction using monolithic TS-1/cordierite catalysts. The activity, a: (a) , WHSV = 1.4 h⁻¹, without TBA; (b) , WHSV = 6 h⁻¹, without TBA; (c) , WHSV = 6 h⁻¹, with TBA; (d) , WHSV = 10 h⁻¹, without TBA; (e) , WHSV = 20 h⁻¹, without TBA in the ammoximation reaction; feed molar ratio of the continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O : TBA = 1 : 1.2 : 1.5 : 10 (with TBA), cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5 (without TBA); flow rate of external circulation, 250 mL min⁻¹; channel density of monolith cordierite, 250 cpsi; reaction temperature, 343 K.

From the curves in Fig. 10, the deactivation rates of monolithic TS-1/cordierite catalyst obey the trend of . The lifetime of the monolithic TS-1/cordierite catalyst strongly depends on WHSV because WHSV directly affects the formation of the precursor of coke and causes the deactivation of the TS-1 catalysts (Fig. 11). The results shown in Fig. 11 were obtained at the optimal reaction temperature for different WHSVs and maintaining the other conditions in Fig. 10. The rapid deactivation of the monolithic TS-1/cordierite catalyst occurred under high WHSV, and was directly related to the residual amounts of unreacted H₂O₂ and NH₃·H₂O, causing side reactions such as forming the precursor of coking and leaching titanium from the TS-1 framework.

Fig. 11 Variation of lifetime under different WHSVs for the continuous cyclohexanone ammoximation reaction without TBA using monolithic TS-1/cordierite catalysts. Feed molar ratio of the continuous ammoximation reaction was cyclohexanone : H₂O₂ : NH₃·H₂O = 1 : 1.2 : 1.5; flow rate of external circulation, 250 mL min⁻¹; channel density of monolith cordierite, 250 cpsi; reaction temperature, 343 K.

To illustrate the cause of deactivation, the deactivated catalyst used for TOSs of 130 hours was characterized by XRD, UV-vis and FT-IR. The XRD patterns verified that the used catalyst still possessed the typical MFI structure but the crystallinity was decreased (Fig. S2†), even though they were calcined in air at 823 K. In addition, the UV-vis spectra of the deactivated catalysts showed that the intensity of the absorbance band round 330 nm had increased (Fig. S3†). It was demonstrated that the anatase TiO₂ was easier to form due to the collapse of skeleton under alkaline reaction conditions.²⁹ The deactivation was attributed to the irreversible change in the structure of the catalyst.

EDS analyses conducted on the surface of the monolithic TS-1 layer samples revealed the presence of Si, Ti and O atoms (Fig. S4†). According to the EDS data (see Table 1), the Si/Ti molar ratio of the deactivated TS-1 layer obtained from the continuous ammoximation with TBA increases than that without TBA. The Si/Ti molar ratio of the deactivated TS-1 layer obtained from the continuous ammoximation without TBA was about 28.57, and had slight changes compared with the fresh TS-1 layer. It also proved that the degree of skeleton collapsing was affected by the alkalinity of ammoximation reaction solution. The introduction of the solvent TBA would increase the alkalinity of ammoximation reaction solution.

Table 1 EDS analysis of the various TS-1 layer samples^a

Samples	Actual Element (atom%)			Si/Ti
	O	Si	Ti
a Samples: 1, fresh TS-1 layer; 2, deactivated TS-1 layer obtained from the continuous ammoximation reaction with TBA (TOS 124 h); 3, deactivated TS-1 layer obtained from the continuous ammoximation reaction without TBA (TOS 130 h).
1	80.96 ± 1.46	18.36 ± 0.42	0.68 ± 0.12	27.00
2	75.89 ± 1.37	23.42 ± 0.32	0.70 ± 0.09	33.46
3	74.27 ± 1.43	24.86 ± 0.25	0.87 ± 0.09	28.57

---

The FT-IR spectra of the fresh TS-1 layer (Fig. 4b) and the deactivated TS-1 layer (Fig. 12) displayed all the characteristic bands of a MFI zeolite. Although the intensities of the bands around 960 and 800 cm⁻¹ decline after deactivation, the ratio of I₉₆₀/I₈₀₀ remains unchanged, which was different from that in the continuous ammoximation reaction with solvent TBA.^4,30 The bands in the range of 2900–3000 cm⁻¹, assigned to the stretching of saturated C–H bonds, disappear completely when the deactivated catalyst was calcined in air at 823 K, as shown in Fig. 12b. The high-molecular-weight byproducts block the pore mouths of TS-1 catalyst and cause fast deactivation.

Fig. 12 FT-IR spectra of (a) deactivated TS-1 layer obtained from the continuous ammoximation reaction without TBA and (b) the same deactivated TS-1 layer calcined in air at 823 K for 6 h.

4. Conclusions

The monolithic TS-1/cordierite catalysts prepared by a hydrothermal synthesis exhibited high catalytic activity in the continuous cyclohexanone ammoximation reaction. The heterogeneous cyclohexanone ammoximation reaction without the organic solvent, TBA, was intensified in the monolithic TS-1/cordierite catalyst due to the multi-channels of millimeter or sub-millimeter scale effectively decreasing the external mass transfer. Therefore, the cyclohexanone conversion and oxime selectivity within a wide WHSV range from 1.4 to 10 h⁻¹ were retained at above 0.96 and 0.98, respectively, at an optimal reaction temperature of 343 K. These processes can significantly reduce subsequent separating costs. Moreover, the lifetime of the monolithic TS-1/cordierite catalysts in the heterogeneous continuous ammoximation reaction under solvent free conditions exhibited a longer lifetime than that with TBA.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Project no. 20876109 and 21276180) and the program for Changjiang Scholars and Innovative Research Team in University (IRT0936).

References

1. G. Bellussi and M. S. Rigutto, in Studies in Surface Science and Catalysis, ed. E. M. F. P. A. J. H. van Bekkum and J. C. Jansen, Elsevier, 2001, vol. 137, pp. 911–955.
2. H. Ichihashi and H. Sato, Appl. Catal., A, 2001, 221, 359–366.
3. A. Cesana, M. A. Mantegazza and M. Pastori, J. Mol. Catal. A: Chem., 1997, 117, 367–373.
4. X. Zhang, Y. Wang and F. Xin, Appl. Catal., A, 2006, 307, 222–230.
5. J. E. Gallot, D. Trong On, M. P. Kapoor and S. Kaliaguine, Ind. Eng. Chem. Res., 1997, 36, 3458–3467.
6. F. Bonino, A. Damin, S. Bordiga, C. Lamberti and A. Zecchina, Langmuir, 2003, 19, 2155–2161.
7. F. Bonino, A. Damin, G. Ricchiardi, M. Ricci, G. Spanò, R. D'Aloisio, A. Zecchina, C. Lamberti, C. Prestipino and S. Bordiga, J. Phys. Chem. B, 2004, 108, 3573–3583.
8. A. Renken, in Basic Principles in Applied Catalysis, ed. M. Baerns, Springer, Berlin, Heidelberg, 2004, vol. 75, pp. 521–542.
9. W. O. Parker and R. Millini, J. Am. Chem. Soc., 2006, 128, 1450–1451.
10. Z. Zhong, X. Liu, R. Chen, W. Xing and N. Xu, Ind. Eng. Chem. Res., 2009, 48, 4933–4938.
11. M. Taramasso and G. Perego, Snamprogetti, US4410501A, 1983.
12. Y. G. Li, Y. M. Lee and J. F. Porter, J. Mater. Sci., 2002, 37, 1959–1965.
13. Z. Lai, G. Bonilla, I. Diaz, J. G. Nery, K. Sujaoti, M. A. Amat, E. Kokkoli, O. Terasaki, R. W. Thompson, M. Tsapatsis and D. G. Vlachos, Science, 2003, 300, 456–460.
14. Z. Zhuang, X. Jiang, D. Y. Peng, W. J. Lv and F. Xin, Mater. Res. Innovations, 2013, 17, 490–494.
15. L. Dal Pozzo, G. Fornasari and T. Monti, Catal. Commun., 2002, 3, 369–375.
16. R. K. Edvinsson and A. Cybulski, Catal. Today, 1995, 24, 173–179.
17. M. T. Kreutzer, P. Du, J. J. Heiszwolf, F. Kapteijn and J. A. Moulijn, Chem. Eng. Sci., 2001, 56, 6015–6023.
18. X. Wang, X. Zhang, H. Liu, K. L. Yeung and J. Wang, Chem. Eng. J., 2010, 156, 562–570.
19. M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, in Studies in Surface Science and Catalysis, ed. A. Z. Claudio Morterra and C. Giacomo, Elsevier, 1989, vol. 48, pp. 133–144.
20. G. Ricchiardi, A. Damin, S. Bordiga, C. Lamberti, G. Spanò, F. Rivetti and A. Zecchina, J. Am. Chem. Soc., 2001, 123, 11409–11419.
21. Z. R. Ismagilov, R. A. Shkrabina, S. A. Yashnik, N. V. Shikina, I. P. Andrievskaya, S. R. Khairulin, V. A. Ushakov, J. A. Moulijn and I. V. Babich, Catal. Today, 2001, 69, 351–356.
22. M. T. Kreutzer, P. Du, J. J. Heiszwolf, F. Kapteijn and J. A. Moulijn, Chem. Eng. Sci., 2001, 56, 6015–6023.
23. A. Stankiewicz, Chem. Eng. Sci., 2001, 56, 359–364.
24. R. K. Edvinsson and S. Irandoust, AIChE J., 1996, 42, 1815–1823.
25. N. Shao, A. Gavriilidis and P. Angeli, Chem. Eng. J., 2010, 160, 873–881.
26. N. Liu, H. Guo, X. Wang, L. Chen and Y. Chen, Chin. J. Catal., 2003, 24, 441–446.
27. Z. Li, R. Chen, W. Xing, W. Jin and N. Xu, Ind. Eng. Chem. Res., 2010, 49, 6309–6316.
28. Z. Zhong, W. Xing, X. Liu, W. Jin and N. Xu, J. Membr. Sci., 2007, 301, 67–75.
29. A. Zheng, C. Xia, Y. Xiang, M. Xin, B. Zhu, M. Lin, G. Xu and X. Shu, Catal. Commun., 2014, 45, 34–38.
30. C. Wu, Y. Wang, Z. Mi, L. Xue, W. Wu, E. Min, S. Han, F. He and S. Fu, React. Kinet. Catal. Lett., 2002, 77, 73–81.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra01789g

---