Preparation of Nanosized Silicalite-1 and Its Application in Vapor

Mar 5, 2012 - It was found that the crystal size of silicalite-1 has an effect on ... Na Li , Ying-Ying Zhang , Lang Chen , Chak-Tong Au , Shuang-Feng...
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Preparation of Nanosized Silicalite-1 and Its Application in VaporPhase Beckmann Rearrangement of Cyclohexanone Oxime Yi-Qiang Deng,† Shuang-Feng Yin,*,† and Chak-Tong Au†,‡ †

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, Hunan, China ‡ Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong ABSTRACT: Series of silicalite-1 catalysts with particle size varying from 50 to 450 nm were synthesized by hydrothermal method using tetrapropylammonium hydroxide as a template and L-lysine as an additive. The crystal size of silicalite-1 could be easily controlled by regulating the amount of L-lysine. The series of silicalite-1 and their NaOH-modified derivatives were examined as catalysts for vapor-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam in the 330−390 °C range. It was found that the crystal size of silicalite-1 has an effect on catalytic activity, ε-caprolactam selectivity, and catalyst stability. With a decline in crystal size, there was improvement of catalytic activity and catalyst stability. It is deduced that silicalite-1, of a smaller crystal size, is less inclined to pore blocking by coke and hence has better resistance to deactivation.

1. INTRODUCTION ε-Caprolactam is an important industrial chemical used for the production of Nylon 6 fibers and resins. It was reported that the world production capacity of ε-caprolactam in 2010 was 4.98 million tons.1 The chemical is mainly manufactured via the liquid-phase Beckmann rearrangement of cyclohexanone oxime catalyzed by concentrated sulfuric acid; as a byproduct, lowvalue ammonium sulfate is produced in large quantity. Needless to say, the process causes serious environment as well as corrosion concerns.2 To avoid the use of sulfuric acid, the Beckmann rearrangement of cyclohexanone oxime was tested in the vapor phase over solid acids.3−5 Among the solid acid catalysts, high-silica ZSM-5 zeolite (silicalite-1,6−8 Scheme 1)

received much attention. Nanosized zeolites were prepared by several routes. For example, nanocrystals of zeolites (MFI-, FAU-, BEA-, and SOD-type) were directly synthesized from a clear gel solution of zeolite precursors.22−26 An emulsion system with surfactants was employed to prepare MFI27 and MOR28 nanocrystals. The “confined space synthesis” method using porous carbon as a template also led to successful synthesis of various nanosized zeolites.29−31 However, these methods are highly complicated. Therefore, the development of a facile approach for the generation of silica nanoparticles of uniform size is strongly desired. In the present study, we designed and generated nanosized silicalite-1 with MFI topology that is regular six-prism in shape by a hydrothermal method using tetrapropylammonium hydroxide as a template and L-lysine as an additive. The nanosized silicalite-1 zeolites were tested as catalysts for the vapor-phase Beckmann rearrangement of cyclohexanone oxime. To the best of our knowledge, the synthesis of this kind of silicalite-1 and the study of its catalytic activity in vapor-phase Beckmann rearrangement of cyclohexanone oxime has never been reported before. We studied the effects of synthesis conditions such as sol composition, aging condition, and presence of L-lysine. Furthermore, the relationship between structure and catalytic efficiency of the silicalite zeolites was investigated.

Scheme 1. Gas Phase Beckmann Rearrangement of Cyclohexanone Oxime to ε-Caprolactam over High-Silica MFI Catalysts

and boron-substituted zeolite ZSM-5 (H-[B]ZSM-5)9−11 were found to be highly active and selective. Nonetheless, the lifetime of these catalysts was unsatisfactory. For example, even if the zeolite catalysts were treated with NaOH solution (pH = 11), there was still fast deactivation of catalysts.8,12−14 It is well-known that coking would result in fast deterioration in catalyst performance.14,15 The coke fouls the surfaces and blocks the micropores, resulting in poor diffusion efficiency.7 It is expected that if the zeolite was nanosized rather than microsized, there would be considerable improvement in catalytic activity and deactivation resistance. Recently, uniformly nanosized zeolites with dimensions less than 100 nm were found to show better catalytic and adsorption properties than the micrometer-sized ones.16−21 The nanosized zeolites exhibited efficient mass transport of guest molecules and © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Tetraethyl orthosilicate (TEOS, ≥98 wt %) as a silica source was purchased from Tianjin Damao Chemical Reagent Factory. Industrial grade tetrapropylammonium hydroxide (TPAOH, C(OH−) = 0.83 mol/L, C(Na+) = 30 ppm, C(Br−) = 0.32 mg/100 mL) was bought Received: Revised: Accepted: Published: 9492

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2.4. Catalytic Evaluation. The catalytic reaction was carried out under atmospheric pressure in a quartz tube reactor (8 mm i.d.). The catalyst (0.5 g, 20−40 mesh size) was loaded in the reactor and subjected to activation in a nitrogen stream at 400 °C for 0.5 h. Then, the catalyst was regulated to a selected temperature for reaction. First a solution of cyclohexanone oxime in ethanol (25 wt %) was injected using a syringe pump under a nitrogen flow (20 mL/min). The weight hourly space velocity (WHSV) of cyclohexanone oxime was varied from 2 to 8 h−1. The product collected at an ice/water trap was analyzed by gas chromatography (Agilent 7820A) using an Agilent ABFFAP capillary column (30 m × 0.25 mm × 0.25 μm) and FID. The recorded cyclohexanone oxime conversion, ε-caprolactam yield, and product distribution were the averaged values of data collected at different time intervals within a period of 2 or 4 h.

from Changlian Catalyst Company. Reagent grade basic amino acid L-lysine was from Sinopharm Chemical Co. Ltd. and cyclohexanone oxime (≥98 wt %) from Aladdin Reagent Inc. The gases 5% CO2/He, 5% NH3/He, He (99.999% purity), and N2 (99.999% purity) were purchased from Changsha Gas Co. All of the chemicals and reagents were used as received without further purification or treatment. 2.2. Synthesis of Different Silicalite-1. The molar composition of the gel mixture was set to 1TEOS:0.3TPAOH:xL-lysine:yH2O. The L-lysine concentration x and H2O concentration y were varied from 0 to 0.128 and 16.5 to 65.0, respectively, for the study of the influence of L-lysine and water on crystallite size. The mixing of precursors was performed at room temperature (20 °C). The as-resulted sol was aged at 80 °C under stirring for a designated period of time (denoted hereinafter as the “aging process”). Subsequently, the sol was transferred to a Teflon-lined stainless steel autoclave and subject to hydrothermal treatment at 170 °C for 72 h. After synthesis, the as-obtained solid was separated by centrifugation, washed, and dried in the air at 110 °C overnight. The removal of the template was carried out in a muffle furnace at 550 °C for 6 h at a heating rate of 2 °C/min. For comparison purposes, conventional silicalite-1 was synthesized according to the method reported by Forni et al.8 as well as by Heitmann et al.10 The treatment of silicaite-1 with NaOH aqueous solution was done according to the procedures reported by Tao et al.12 as well as by Suzuki and Okuhara.32 2.3. Catalyst Characterization. A powder X-ray diffraction (XRD) experiment was conducted on a BRUKER D8 ADVANCE diffractometer using monochromatized Cu Kα radiation. The particle size and morphology of the samples (airdried and coated with a thin gold film) were investigated using scanning electron microscopy (SEM; FEI Sirion 2000). The particle size was determined based on the SEM images by averaging the diameters of more than 100 particles according to the method reported by Kurtis et al.33 FT-IR spectra were acquired to identify the structure vibration and surface hydroxyl groups of samples. The BET (Brunauer−Emmett−Teller) surface area, pore volume, and pore size of catalysts were measured using a TriStar 3000 instrument; before each measurement, the sample was heated to 300 °C and kept at this temperature for 5 h. The BET specific surface area (SBET) was calculated from the adsorption data in the relative pressure ranging from 0.04 to 0.20. The external surface area (SEXT) was estimated using the t-plot method. Thermogravimetric (TGA) data were recorded in the air (flow rate 50 mL/min) on a PerkinElmer Diamond TG/DTA Instruments within the 20− 900 °C range, with a heating rate of 10 °C/min and initial sample weight of 10 mg. The coke amount was determined using thermogravimetry (TG). The weight loss from 300 to 900 °C in each TG profile was defined as the content of coke on a used catalyst.34 NH3-TPD experiments of catalysts were conducted on a Micromeritics 2920 II apparatus using a thermal conductivity detector (TCD). Before measurement, the sample (0.2 g for all catalysts) was treated at 360 °C for 1 h and cooled to 100 °C in a helium flow. Then, the gas flow was switched to 5% NH3/He for 30 min for NH3 adsorption. After adsorption, the gas flow was switched to pure He (60 mL/min). NH3-TPD was performed (heating rate 15 °C/min) once a stable baseline was attained. The desorbed amount was estimated by calibration of the peak area against the TCD signal of a known amount of NH3.

3. RESULTS AND DISCUSSION 3.1. Preparation of Silicalite-1. 3.1.1. Effects of Sol Aging Time and Temperature. The sol composition was 1TEOS:0.3TPAOH:35H2O. At an aging temperature of 80 °C, the effect of aging time was examined. Figure 1 shows the

Figure 1. SEM images of products prepared with an aging time of (a) 0, (b), 24, (c) 48 h, and (d) 72 h.

SEM images of the products synthesized with an aging time that fell within the 0−72 h range. It is apparent that with an increase of aging time, there is a decrease of crystal size, and the average crystal sizes of the products at aging times of 0, 24, 48, and 72 h are 450, 200, 165, and 120 nm, respectively. All of the harvested particles exhibit “six-prism shape”. Further extension of the aging time after 24 h has little effect on particle size. Then, with the aging time fixed at 24 h, the effect of aging temperature was examined. The average crystal sizes of the products prepared at aging temperature of 20 and 80 °C were found to be 320 and 200 nm, respectively (Figure 2). A further rise of aging temperature from 80 to 100 °C would cause little change in average crystal size. This result could be accounted for by the fact that both nucleation and crystal growth are fast at 80 and 100 °C, as suggested before by Mochizuki et al.17 3.1.2. Effects of Water Content. The effect of the amount of water in sol gel on the morphology of silicalite-1 was investigated. With the aging temperature and time fixed at 80 °C and 24 h, silicalite-1 was synthesized with the molar composition of water varying from 16.5 to 65. The SEM images of the products show that with an increase of water content, 9493

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3.1.3. Effects of L-Lysine Amino Acid. By controlling the amount of water in the synthesis gel and optimizing the aging process in terms of time and temperature, we obtained silicalite1 with crystal sizes ranging from 120 to 450 nm. Keeping in mind that Yokoi et al.,37,38 Atchison et al.,39 and Kurtis et al.33 synthesized monodispersed silica nanoparticles in an aqueous environment using a “seed regrowth” method in the presence of amino acid, we added L-lysine amino acid into the synthesis gel for the fabrication of nanosized silicalite-1. At a gel molar composition of 1TEOS:0.3TPAOH:35H2O, silicalite-1 was synthesized with L-lysine molar composition of 0.008, 0.016, 0.032, and 0.128, and the average particle size of silicalite-1 as estimated by SEM was 100, 56, 55, and 50 nm, respectively (Figure 4). The addition of L-lysine with a molar composition

Figure 2. SEM images of products prepared with an aging temperature of (a) 20 °C, (b) 80 °C, and (c) 100 °C.

there is a gradual increase of particle size (Figure 3); the particle sizes at water/TEOS ratios of 16.5 and 65 are ca. 120

Figure 4. SEM images of products prepared with L-lysine/Si molar ratios of (a) 0.008, (b) 0.016, (c) 0.032, and (d) 0.128.

of 0.016 effectively led to size reduction. It was pointed out that an excess amount of L-lysine would promote the aggregation of primary particles because the decrease in pH value of the synthetic solution would enhance aggregation of the primary particles, and the result was the formation of “larger-sized” silicalite-1.17 The decrease in particle size may be explained by the fact that the L-lysine molecules cover the nanoparticles’ surface (through interaction of protonated amino groups with silicates) during the growth process, leading to the control of silica particles’ size. Meanwhile, as pointed out by Yokoi et al., uniform-sized silica spheres can be synthesized by using neutral amino acid in combination with ammonia as a base catalyst under the same conditions, but the assembly of thus obtained spheres lacked regularity.37 Therefore, the formation of uniform-sized particles could be attributed to the well-packed structure that originated with the assistance of hydrogenbonding interaction between L-lysine molecules. We demonstrated that, when using L-lysine as an additive, silicalite-1 zeolites of different particle sizes (50−450 nm) could be synthesized by controlling the sol composition and aging conditions. For the silicalite-1 with a particle size of 150 nm, the crystals were well dispersed and exhibited a six-prism shape. The silicalite-1 structure is different from that of microsized crystals reported by Qi and Zhao (spherical);40 it is also different from that reported by Lee et al. (coffin-shaped)41 and Davis et al. (plate-like features).42 Furthermore, the silicalite-1 particles in our system are more uniform in size compared to those reported by Lee et al.,41 Ban et al.43 and Lin and Yates.44

Figure 3. SEM images of products prepared with a H2O/Si molar ratio of (a) 16.5, (b) 25, (c) 35, (d) 45, (e) 55, and (f) 65.

and 300 nm, respectively. It is noted that the “six-prism shape” particles were obtained only when water compositions were above 25. The pH values of synthesis gel at water/TEOS ratios of 16.5 and 65 were >13.0 and 12.5, respectively. Persson et al.35 and Watanabe et al.36 reported that an enhancement of alkalinity favored the nucleation and formation of “smallersized” zeolite. In addition, a lesser amount of water in synthesis gel (i.e., higher concentration of silicate species) promoted nucleation.17 In our case, the reduction in water amount led to a size decline of silicalite-1, in line with the observations reported in the literature. 9494

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968 and 550 cm−1 are ascribed to the stretching vibration of the Si−OH group and MFI-structured zeolite. Meanwhile, one can see that the 550 cm−1 absorption peaks of conv-s and treat-s are similar in intensity, both having an intensity much lower than that of nano-s. The results show that the addition of L-lysine in the gel did not result in a decrease of silicalite-1 crystallinity, consistent with the results of XRD characterization. As listed in Table 1, the surface area and pore volume of conv-s are 373 m2/g and 0.19 mL/g. The nano-s possesses a

3.2. XRD, FT-IR, BET, and NH3-TPD Characterization. In order to investigate the physicochemical properties of the nanosized silicalite-1 (56 nm; synthesis conditions: gel molar composition, 1TEOS:0.3TPAOH:0.016L-lysine:35H2O, aging temperature 80 °C, and aging time 24 h), XRD, FT-IR, BET, and NH3-TPD techniques were employed. As references, conventional silicalite-1 (210 nm) and conventional silicalite-1 treated with NaOH aqueous solution were also characterized. Hereafter, the nanosized silicalite-1 (56 nm), conventional silicalite-1 (210 nm), and conventional silicalite-1 treated with NaOH aqueous solution are denoted as nano-s, conv-s, and treat-s, respectively. The XRD patterns of nano-s, conv-s, and treat-s are shown in Figure 5. The XRD patterns of all samples showed five distinct

Table 1. Physical Characteristics of Catalysts catalyst nano-s conv-s treat-s used nano-s (40 h)

particle sizea (nm)

SBETb (m2/g)

SEXTc (m2/g)

Vtotald (mL/g)

56 210 210 56

437 373 402 175

199 168 190 85

0.23 0.19 0.22 0.10

a

Particle size: estimated based on SEM results. bSBET: BET surface area. cSEXT: external surface area. dVtotal: total pore volume.

surface area (437 m2/g) and pore volume (0.23 mL/g) slightly higher than those of treat-s. After reaction at 370 °C for 40 h, the nano-s catalyst declined significantly in surface area (to 175 m2/g) and in pore volume (to 0.10 mL/g). It is apparent that catalyst deactivation is mainly due to pore blocking. The results of NH3-TPD analyses showed that the silicalite-1 catalysts are fairly similar in acid amount. Across the nano-s, conv-s, and treat-s synthesized in this study, ammonia adsorption was about 0.042, 0.039, and 0.040 mmol/g, respectively, indicating that all of the silicalite-1 samples are extremely low in terms of acidity. 3.3. Catalytic Performance. 3.3.1. Vapor-Phase Beckmann Rearrangement of Cyclohexanone Oxime. Generally, silicalite-1 treated with NaOH aqueous solution exhibited better catalytic performance than conventional silicalite-1.8,15,42 Herein, nano-s, conv-s, and treat-s were examined for the vapor-phase Beckmann rearrangement of cyclohexanone oxime. First, the influence of reaction temperature (330−390 °C) on catalytic performance was investigated (Figure 7). Cyclohexanone oxime conversion increases with reaction temperature, reaching 100% at 370 °C across all of the catalysts. It is

Figure 5. XRD patterns of (a) nano-s, (b) conv-s, and (c) treat-s.

peaks at 7.98°, 8.82°, 23.18°, 24.02°, and 24.46° ascribable to (101), (020), (501), (151), and (303) reflections, respectively,45,46 which are characteristic of silicalite-1 zeolite. All three samples show peaks of similar intensities, indicating that the addition of L-lysine in the gel did not result in any reduction in silicalite-1 crystallinity. The IR spectra of nano-s, conv-s, and treat-s are shown in Figure 6. All of the samples show vibration bands at 448, 797, 1087, and 1215 cm−1, assignable to Si−O−Si bending, Si−O− Si symmetric stretching (outer SiO4 tetrahedron), Si−O−Si asymmetric stretching (inner SiO4 tetrahedron), and Si−O−Si asymmetric stretching (outer SiO4 tetrahedron) of a condensed silica network, respectively.40,46 The absorption peaks at around

Figure 7. Results (obtained at fourth hour) of vapor-phase Beckmann rearrangement of cyclohexanone oxime over nano-s, conv-s, and treat-s with a reaction temperature ranging from 330 to 390 °C. Reaction conditions: 0.5 g catalyst, and WHSV = 8 h−1. (Open symbols denote cyclohexanon oxime conversion, whereas solid symbols denote caprolactam selectivity; conv-s, circle; treat-s, square; nano-s, triangle.)

Figure 6. FT-IR spectra of (a) nano-s, (b) conv-s, and (c) treat-s. 9495

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worth pointing out that Forni et al.8 and Kath et al.47 observed similar results over high silica zeolite catalysts. The results can be ascribed to the high reaction temperature. With the high catalyst activation and tar desorption, Beckmann rearrangement over MFI-type zeolites is not limited by cyclohexanone oxime diffusion under the adopted reaction conditions. On the other hand, ε-caprolactam selectivity shows a different behavior. Increase of reaction temperature led to slight decrease in ε-caprolactam selectivity. It is deduced that the decrease in selectivity at higher temperatures is probably due to side reactions and decomposition of ε-caprolactam. The treating of conventional silicalite-1 with NaOH aqueous solution would result in an increase of ε-caprolactam selectivity, in good agreement with the results of Forni et al.8 One can see that ε-caprolactam selectivity over nano-s is higher than that over treat-s and conv-s. Kath et al.47 also reported that higher selectivity can be attained over the catalyst with a smaller crystal size. Hence, we investigated the relationship between catalytic performance and particle size using the silicalite-1 samples of different sizes. As listed in Table 2, ε-caprolactam selectivity

on the external surface as well as at the inner parts close to the mouth of the micropores. This supposition is backed up by molecular modeling demonstrating that ε-caprolactam is not capable of entering the MFI-pore system.14,48 The influence of WHSV on the catalytic performance of nano-s, conv-s, and treat-s at 370 °C are presented in Figure 8.

Table 2. Effect of Particle Size on the Catalytic Performance in the Conversion of Cyclohexanone Oxime over Silicalite1a particles size (nm)

cyclohexanone oxime conversion (%)

caprolactam selectivity (%)

caprolactam yield (%)

210 120 56

99.9 99.4 99.6

88.2 93.5 96.4

88.1 92.9 96.0

Figure 8. Results (obtained at fourth hour) of vapor Beckmann rearrangement of cyclohexanone oxime over nano-s, conv-s, and treat-s with WHSV varied from 2 to 8 h−1. Reaction conditions: 0.5 g catalyst, 370 °C. (Open symbols denote cyclohexanon oxime conversion, whereas solid symbols denote caprolactam selectivity; conv-s, circle; treat-s, square; nano-s, triangle.)

The trends of change in ε-caprolactam selectivity on increase of WHSV are similar to those caused by an increase in the reaction temperature. The stability of the three catalysts degrades with increasing WHSV, and there is a slight change in selectivity to ε-caprolactam. In the case of nano-s, cyclohexanone oxime conversion is nearly 100% at high WHSV, while the selectivity to ε-caprolactam goes through a maximum (96.4%) at WHSV = 6 h−1 and then slightly decreases (96.3%) at a WHSV of 8 h−1. On the other hand, under the reaction conditions of 0.5 g catalyst, 370 °C, and WHSV = 8 h−1, the activity of conv-s decreases to 97.3% after 4 h, but nano-s and treat-s show relatively higher stability: cyclohexanon oxime conversion is about 99.5%. 3.3.2. Catalyst Stability. Although the three catalysts are similar in initial conversion, they are markedly different in stability. It is noted in Figure 9 that conv-s has an initial activity of 100%; it decreases to 95.8% after 22 h and rapidly decreases to 86.5% after 58 h. There is certain improvement in the case of treat-s in terms of catalytic lifetime and caprolactam selectivity. In contrast, the nano-s catalyst shows a relatively longer lifetime; cyclohexanone oxime conversion is 99% after 40 h and is 97.2% after 60 h. Apparently, compared to microsized silicalite-1, nanosized silicalite-1 is more stable in catalytic performance. The long lifetime of nano-s can be explained by the facile removal of coke-generating polymeric species that leads to deactivation of the catalytic sites. This explanation is supported by a significant difference in coke contents between nano-s and the other catalysts after the same reaction period as determined by TGA. TGA results of the used catalysts (after 40 h TOS) are shown in Figure 10. The coked conv-s and treat-s catalysts show a weight loss of 6.1% and 4.7%, respectively. In comparison, the coked nano-s shows only 1.5% weight loss. The results indicate that the average rate of coke formation on nano-s was about

Reaction conditions: 0.5 g catalyst, 370 °C, and WHSV = 6 h−1. Values obtained at fourth hour. a

increases with a decrease in particle size; the result suggests that catalytic performance over the MFI-zeolites is strongly influenced by the size of crystals. The worst selectivity (around 88%) was observed over conv-s. The treating of conventional silicalite-1 with aqueous NaOH solution brought about an increase in total (from 373 to 402 m2/g) as well as external (from 168 to 190 m2/g) surface area. The increase in surface areas was due to the formation of supermicropores, and the size of micropores was unaffected.32 In our system, the most probable pore diameters of conv-s, treat-s, and nano-s as derived from BJH analysis of the N2 adsorption branch were 5.9, 6.2, 6.5 Å, respectively. Despite the about 30% Si loss of MFI zeolite, treat-s showed an ε-caprolactam selectivity of 94.3%; a similar phenomenon was also reported by Suzuki and Okuhara.32 We have demonstrated that ε-caprolactam selectivity could be 94.3% at a cyclohexanone oxime conversion of 100% over nano-s. Furthermore, compared to the cases of conv-s and treat-s, the drop of conversion after 24 h time on stream (TOS) is much less than that in the case of nano-s (see below). The results showed that with an increase of external surface area (i.e., decrease in crystal size), there is a significant rise in cyclohexanone oxime conversion. This can be considered as a strong hint that the external surface area plays an important role in the reaction. The higher conversion of cyclohexanone oxime over crystals of smaller size can be ascribed to the higher external surface area, and hence a higher specific number of catalytically active sites accessible to the reactants. We hence deduce that the Beckmann rearrangement reaction does not take place inside the micropores of the MFI-structure but rather 9496

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Figure 9. Results of vapor Beckmann rearrangement of cyclohexanone oxime over nano-s, conv-s, and treat-s. (a) Conversion vs time on stream and (b) conversion vs time on stream. (Reaction conditions: 0.5 g catalyst, WHSV = 6 h−1, 370 °C; conv-s, circle; treat-s, square; nano-s, triangle.)

and L-lysine as an additive. The addition of an appropriate amount of L-lysine in the synthesis gel suppresses particle growth, and the size of particles can be easily regulated to a size of about 50 nm. In comparison with micrometer-sized silicalite1, the nanosized silicalite-1 (about 50 nm) showed higher catalytic activity as well as better caprolactam selectivity and catalyst stability in the vapor-phase Beckmann rearrangement of cyclohexanone oxime. The reduction of silicalite-1 crystal size suppressed coke formation on the surface and/or at pore entrances, consequently raising the deactivation resistance of the catalyst.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86-731-88821310. Fax: 86-731-88821310. E-mail: sf_ [email protected]. Notes

Figure 10. TGA profiles of coked catalysts. (a) nano-s, (b) nano-s (after 60 h of reaction), (c) treat-s, (d) conv-s. (Reaction conditions: 0.5 g catalyst, 370 °C, WHSV = 6 h−1, TOS = 40 h.)

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by Hunan Provincial Natural Science Foundation of China (10JJ1003), NSFC (20873038, J0830415, E50725825), the program for New Century Excellent Talents in Universities (NCET-10-0371), and the Fundamental Research Funds for the Central Universities. C.-T.A. thanks the Hunan University for an adjunct professorship.

0.04% per hour, much lower than that (0.15%) of conv-s. From the results of TG analysis, one can see that the extent of coke deposition on nano-s after 60 h is markedly larger than that after 40 h. It is hence deduced that coke formation is gradual at the beginning and then quickens with time on stream. The deposited carbon species cover the active sites and block silicalite-1 channels, making the active sites less accessible. On the basis of the results, we deduce that the main cause of catalyst deactivation is coking. In order to verify the stability of the nano-s samples, we performed catalyst regeneration cycles at 550 °C in the air. Such operation is required to eliminate tars adsorbed on catalyst surface, since tar formation occurs mainly on the external surface of silicalite-1.8 The dimension of silicalite-1 channels prevents the formation of heavy products through a shape selective action; thus catalyst deactivation was mainly due to pore blocking and cavity inaccessibility. This operating condition is favorable for catalyst regeneration and removal of the organic residues. The catalytic results over nano-s are rather high, and we do not observe any relevant difference between catalytic performances.

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REFERENCES

(1) Hou, X. X. Market Analysis and Development Solutions for Caprolactam. Fine Chem. Intermediates 2011, 41, 19. (2) Lobo, R. Chemical diversity of zeolite catalytic sites. AIChE J. 2008, 54, 1402. (3) Bordoloia, A.; Halligudi, S. B. Catalytic properties of WOx/SBA15 for vapor-phase Beckmann rearrangement of cyclohexanone oxime. Appl. Catal. A: Gen. 2010, 379, 141. (4) Sato, H. Acidity control and catalysis of pentasil zeolites. Catal. Rev. Sci. Eng. 1997, 39, 395. (5) Dahlhoff, G.; Niederer, J. P. M.; Hö elderich, W. F. εCaprolactam: new by-product free synthesis routes. Catal. Rev. Sci. Eng. 2001, 43, 381. (6) Ichihashi, H.; Kitamura, M. Some aspects of the vapor phase Beckmann rearrangement for the production of ε-Caprolactam over high silica MFI zeolites. Catal. Today 2002, 73, 23. (7) Takahashi, T.; Nasution, M. N. A.; Kai, T. Effects of acid strength and micro pore size on ε-caprolactam selectivity and catalyst deactivation in vapor phase Beckmann rearrangement over acid solid catalysts. Appl. Catal. A: Gen. 2001, 210, 339.

4. CONCLUSIONS A series of silicalite-1 catalysts that are different in particle size (50−450 nm) were synthesized according to a hydrothermal method using tetrapropylammonium hydroxide as a template 9497

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dx.doi.org/10.1021/ie3001277 | Ind. Eng. Chem. Res. 2012, 51, 9492−9499

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the nature and location of active sites for the Beckmann rearrangement reaction in microporous materials. J. Catal. 2007, 249, 116.

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dx.doi.org/10.1021/ie3001277 | Ind. Eng. Chem. Res. 2012, 51, 9492−9499