Synthesis of High-Performanced Titanium Silicalite-1 Zeolite at Very

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Synthesis of High-Performanced Titanium Silicalite‑1 Zeolite at Very Low Usage of Tetrapropyl Ammonium Hydroxide Deng-Gao Huang,† Xian Zhang,† Tian-Wei Liu,† Chen Huang,† Bao-Hui Chen,† Cai-Wu Luo,† Eli Ruckenstein,*,‡ and Zi-Sheng Chao*,† †

College of Chemistry and Chemical Engineering, State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, China ‡ Department of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260, United States ABSTRACT: A novel method for the synthesis of high-performanced titanium silicalite-1 (TS-1) zeolite at a very low usage of tetrapropylammonium hydroxide (TPAOH) was developed in this work. The method involved a temperature-programmed hydrothermal crystallization (TPHC) of a batch with a TPAOH/SiO2 molar ratio of as low as 0.05, being 5 times smaller than that employed in the conventional synthesis, in which the TPAOH was entirely provided by a TS-1 precursor sol. The characterizations of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), diffuse reflectance ultraviolet− visible spectroscopy (DR UV−vis), and scanning electron microscopy (SEM) indicated that the TS-1 synthesized via the novel method possessed a uniform crystal size of ∼0.3 μm, a very high crystallinity and a considerably large content of framework Ti (2.33 mol %). The TS-1 had been employed as the catalyst for the ammoximation of cyclohexanone, of which the influencing factors were systematically investigated. It was shown that the TS-1 synthesized via the novel method possessed a very high performance, which is even better than for those synthesized via the conventional method both in this work and in the literature work.

1. INTRODUCTION Titanium silicalite-1 (TS-1) is receiving more and more attentions, due to its unique catalytic performance, being effective in the clean oxidative reactions of a variety of organic compounds at low temperature and with diluted hydrogen peroxide as an oxidant.1−5 Some of the reactions catalyzed by TS-1, such as, the selective oxidation of phenol to catechol and hydroquinone6,7 and the ammoximation of cyclohexanone to cyclohexanone oxime,5,8 have achieved an industrial-scaled production. Although these processes basing on TS-1 are much cleaner and efficient, compared with the conventional ones that produce inevitably a large amount of hazardous byproducts, their commercial values are still under question mainly due to the expensive cost of TS-1. In fact, the exploration to reduce the cost of TS-1 has been the research hot-button since the first invention of this zeolite. The generation of TS-1 is a result of hydrothermal crystallization from a batch containing proper Si and Ti sources, usually tetraethyl orthosilicalite (TEOS) and tetrabutyl orthosilicate (TBOT), respectively, and template, with alkalifree tetrapropyl ammonium hydroxide (TPAOH) being currently the most effective one. Among these raw materials, the template was estimated to contribute usually as large as >96.7% of the cost of TS-1.9 Therefore, in the synthesis of TS1, while cheaper Si and Ti sources have been explored,10,11 much efforts are being given to the search for inexpensive templates for the substitutes of TPAOH12−15 or to the reduction in the usage or TPAOH.15,16 Reported cheaper templates included mainly tetrabutylammonium hydroxide (TBAOH) + tetraethylammonium hydroxide (TEAOH), hexanediamine (HDA) + tetrapropylammonium bromide © 2013 American Chemical Society

(TPABr), methylamine (MA) + TPABr, HDA + n-butylamine (n-BA) and TPABr + NH3.13,14,17−21 However, it was found that the crystal sizes of TS-1 synthesized based on these templates were much larger, usually in the micrometer range (2−7 μm),19,20,22 than those based on TPAOH (0.1−0.4 μm).1,23,24 This is unfavorable, since the catalytic activity of TS1 decreased rapidly with the increase of its crystal size due to the pore diffusion resistance,25 especially for the crystal sizes being larger than ∼0.5 μm.26 In the original patent,1 TS-1 was synthesized from the batch with a molar ratio of TPAOH/SiO2 as high as 0.4−1.0. Some reports showed that, with decreasing the molar ratio of TPAOH/SiO2, the crystal size of TS-1 decreased, attaining the minimum for TPAOH/SiO2 = 0.4, and then increased.11,27 It appears that using a too small amount of TPAOH is unfavorable with regard to the crystal size of TS-1 and accordingly its catalytic performance, although the cost of TS-1 may be reduced. The decrease in the usage of TPAOH may lead to other disadvantages, such as the formation of nonuniform particles of TS-1, the lowering of crystallinity of TS-1, and the generation of extra framework Ti, which were all unfavorable to the catalytic performance of TS-1.15 A few reports showed that TS-1 with an appreciably small crystal size could be synthesized from a batch with a relatively low molar ratio of TPA+/SiO2 by adding seed into the synthesis sol. For example, Chen et al.16 synthesized the TS-1 with crystal size of 0.3−0.5 μm from a sol with a total molar ratio of TPAOH/SiO2 Received: Revised: Accepted: Published: 3762

August 8, 2012 February 13, 2013 February 14, 2013 February 14, 2013 dx.doi.org/10.1021/ie302130x | Ind. Eng. Chem. Res. 2013, 52, 3762−3772

Industrial & Engineering Chemistry Research

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Table 1. Summary for the Synthesis of Specimens A−G molar composition and pH of synthesis sol specimen

SiO2

TiO2

TPAOH

n-BA

H2O

pH

adding TS-1 precursor sol

stirring before crystallizationa

procedure for crystallizationb

A B C D Ec Fc G

1 1 1 1 1 1 1

0.033 0.033 0.033 0.033 0.033 0.033 0.033

0 0.05 0.05 0.05 0.05 0.05 0.25

0.6 0 0.6 0.6 0.6 0.6 0

35 35 35 35 35 35 25

N.D.d ∼10.3 ∼11.6 ∼11.6 ∼11.8 ∼11.8 ∼13.0

none none none none yes yes none

none none none yes none yes none

CTHC CTHC CTHC TPHC CTHC TPHC CTHC

temperature and time (K, d) (443, (443, (443, (393, (443, (393, (443,

4) 4) 4) 2)+(443, 1) 4) 2)+(443, 1) 4)

a

Stirring of synthesis sol was performed at 373 K for 1 d under refluxing. bCrystallization was performed in the sealed autoclave. CTHC: constanttemperature hydrothermal crystallization; TPHC: temperature-programmed hydrothermal crystallization. cTPAOH in the synthesis sol was entirely provided by TS-1 precursor sol. dN.D.: not determined.

constant-temperature hydrothermal crystallization (CTHC) at 363 K for 24 h. After cooling to room temperature, the final sol, namely TS-1 precursor sol, was employed, without any treatment, in the subsequent synthesis. 2.4. Synthesis of TS-1. Synthesis of Specimen A. A synthesis sol with a molar composition SiO2:0.033 TiO2:0.6 nBA:35 H2O was first prepared as described in section 2.2, except that TPAOH was replaced with n-BA. Then, the synthesis sol was charged and sealed in a Teflon-lined autoclave and subjected to a CTHC at 443 K for 4 d. The solid was finally recovered by centrifugal separation, washing with distilled water, drying overnight at 393 K, and calcination at 823 K for 12 h, giving the specimen A. Synthesis of Specimen B. A synthesis sol with a molar composition SiO2:0.033 TiO2:0.05 TPAOH:35 H2O was first prepared as described in section 2.2, and the pH of the sol was determined to be ∼10.3. Then, the same procedures for the CTHC procedure of synthesis sol and recovery of solid, as in the synthesis of specimen A, were carried out, giving the specimen B. Synthesis of Specimen C. A synthesis sol with a molar composition SiO2:0.033 TiO2:0.6 n-BA:35 H2O was first prepared as described in section 2.2, except that TPAOH was replaced with n-BA. Then, a calculated amount of TPAOH and water was added, resulting in a final synthesis sol with a molar composition SiO2:0.033 TiO2:0.05 TPAOH:0.6 n-BA:35 H2O, the pH of which was determined to be ∼11.6. After that, the final synthesis sol was subjected to the CTHC procedure, and the solid was recovered, according to the same procedures as for the synthesis of specimen A, thus giving the specimen C. Synthesis of Specimen D. The final synthesis sol used for the synthesis of specimen C was first stirred at 373 K for 1 d under refluxing. Then, the synthesis sol was transferred into a Teflon-lined autoclave, the autoclave was sealed, and the sol was subjected to a temperature-programmed hydrothermal crystallization (TPHC) at 393 K for 2 d and at 443 K for 1 d. After that, the solid was recovered, according the same procedure as in the synthesis of specimen A, giving the specimen D. Synthesis of Specimen E. A synthesis sol with a molar composition SiO2:0.033 TiO2:0.75 n-BA:38.75 H2O was first prepared as described in section 2.2, except that TPAOH was replaced with n-BA. Then, a calculated amount of TS-1 precursor sol prepared in section 2.3 was added, generating a final synthesis sol with a molar composition SiO2:0.033 TiO2:0.05 TPAOH:0.6 n-BA:35 H2O, the pH of which was determined to be ∼11.8, and entire TPAOH was provided by

= 0.12, in which a seed gel precrystallized from a clear solution with a molar composition of SiO2:0.35 TPAOH:20 H2O had been added. Zuo et al. found that the TS-1 with a crystal size of 0.6 μm × 0.4 μm × 0.25 μm could be synthesized from a sol with total molar ratio TPAOH/SiO2 = 0.16, using a mother liquid of nanosized TS-1, precrystallized from a sol with TPAOH/SiO2 = 0.25−0.35, as the seed.28 However, no information regarding the content of framework Ti of TS-1 was provided in the above seeded synthesis of TS-1. As is wellknown, a high content of framework Ti is largely desired for TS-1 to present a high activity in its catalysis application.11 In this report, we report a novel synthesis of TS-1 involving a combined employment of a temperature-programmed hydrothermal crystallization (TPHC) procedure and TS-1 precursor sol to provide the entire template (TPAOH). It has been shown that the TS-1 with small crystal size (∼0.3 μm) and high content of framework Ti (2.33 mol %) can be effectively generated from a batch with a molar ratio of TPAOH/SiO2 of as low as 0.05, which is 5 times smaller than what has been commonly used in the literature.1,3,24,29,30 The synthesized TS1 exhibits a very good catalytic performance in the ammoximation of cyclohexanone, being much better than the TS-1 synthesized with large usage of TPAOH in the literature work.

2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals involved were tetraethyl orthosilicalite (TEOS, 99.8%), tetrabutyl orthosilicate (TBOT, 99.6%), tetrapropyl ammonium hydroxide (TPAOH, 1.5 M solution in water), n-butylamine (n-BA, 99.5%), isopropanol (IPA, 99.7%), tert-butanol (99%), cyclohexanone (98.5%), H2O2 (30%), and NH3·H2O (28%). 2.2. Preparation of Synthesis Sol. Calculated amounts of TEOS, TPAOH, and H2 O were first mixed at room temperature and under strong stirring, allowing the hydrolysis of TEOS for 1 h. Then, the sol obtained was kept at ice-bath temperature and to this was added a certain amount of a solution of TBOT in IPA under strong stirring. After the mixture was stirred for another 1 h, the temperature was raised to 333 K for 1 h, and then to 358 K to evaporate the alkanol (ethanol and IPA), resulting in a synthesis sol with required batch composition. 2.3. Synthesis of TS-1 Precursor. A synthesis sol with a molar composition SiO2:0.033 TiO2:0.25 TPAOH:20 H2O, the pH of which was determined to be ∼13, was first prepared as described in section 2.2. Then, the synthesis sol was transferred and sealed into a Teflon-lined autoclave and subjected to a 3763

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the TS-1 precursor sol. After that, the final synthesis sol was subjected to the CTHC procedure, and the solid was recovered, according to the same procedures as in the synthesis of specimen A, giving the specimen E. Synthesis of Specimen F. The final synthesis sol used for the synthesis of specimen E was subjected to the TPHC procedure, and the solid was recovered, according to the same procedures as in the synthesis of specimen D, giving the specimen F. Synthesis of Specimen G:31 Calculated amounts of TEOS, TPAOH, and H2O were first mixed at room temperature and under stirring, allowing the hydrolysis of TEOS for 1 h. Then, to the resultant sol, a solution of the required quantity of TBOT in IPA was added dropwise under vigorous stirring. After the resultant mixture was stirred for ∼1 h, the temperature was raised to 358 K, allowing the evaporation of alkanol for ∼3 h. The final synthesis sol, having a molar composition SiO2:0.033 TiO2:0.25 TPAOH:25 H2O and pH of ∼13, was subjected to a CTHC procedure at 443 K for 4 d. Finally, the solid was recovered by centrifugal separation, washing with distilled water, drying overnight at 393 K, and then calcination at 823 K for 12 h, giving the specimen G. For comparison, the batch compositions and procedures for the synthesis of specimens A−G are summarized in Table 1. 2.5. Characterization of TS-1. X-ray diffraction (XRD) spectroscopy was performed on a Bruker D8 Advance X-ray diffractometer, using a Cu Kα radiation resource (λ = 1.54187 Å). The instrument was operated under the conditions of scanning voltage 40 kV, scanning current 40 mA, scanning speed = 0.5 s, and scanning step 0.01°. The XRD spectra were recorded in the range of 2θ = 5−50°. The crystallinities of specimens were calculated on the basis of the ratio of the sum of areas under the peaks at 2θ = 23.1°, 23.8°, and 24.2°, relative to the specimen with the largest sum of peak areas, which was designated as a reference and attributed a crystallinity of 100%.14 The parameters of the crystal cells were obtained via the XRD Rietveld refinement analysis, taking into account the contribution of both Kα1 and Kα2 (2:1 intensity ratio) radiation to the pseudoreflection profile. Diffuse reflectance ultraviolet−visible spectroscopy (DR UV−vis) was carried out using a Perkin-Elmer Lambda 35 spectrometer, equipped with a deuterium lamp and a tungsten lamp. The reference material was BaSO4. Fourier transform infrared spectroscopy (FT-IR) was recorded on a Varian 3100 spectrometer, equipped with a DTGS detector and a CeI beam splitter. The datas were recorded from 400 to 2000 cm−1 at a scanning number of 30 and a resolution of 4 cm−1. Scanning electron microscopy (SEM) was conducted with a JSM-6700 F scanning electron microscope operated at an accelerating voltage of 5 kV. Energy dispersive X-ray spectroscopy (EDX) was conducted over the EDS accessory of SEM instrument. N2-physisorption was performed at liquid nitrogen temperature using a Quantachrome Autosorb-1 instrument. Before the adsorption measurement, the specimen was degassed for 16 h at a temperature of 573 K under a residual pressure lower than 4 × 10−4 Pa in the degas port of the adsorption apparatus. The specific surface area was calculated by using the multipoint BET equation. The pore size and its distribution were obtained from the N2-adsorption branch by applying the SF methods. The pore volume was calculated by using the t-plot micropore analysis method.

2.6. Ammoximation of Cyclohexanone. The reaction was performed at 333−353 K and atmospheric pressure in a three-neck flask. The TS-1 synthesized above (specimens A− G) was employed as the catalyst, a 30% H2O2 aqueous solution as the oxidant and an equimolar mixture of water and tertbutanol as the solvent. The reaction batch had a molar composition of solvent: NH 3 /H 2 O 2 /cyclohexanone = 4.0:1.0:1.0:1.0−4.0:2.0:1.6:1.0, and the usage of catalyst was 7.0−13.0 g per mol cyclohexanone. After the reaction was carried out for 2−6 h, the products mixture was analyzed by a Varian 3800 gas chromatograph equipped with a flame ionization detector (FID) and a SE-54 capillary column (0.25 mm × 50 m). The specimen F was employed in the test of catalyst recycle in the ammoximation of cyclohexanone. At first, the ammoximation of cyclohexanone was run for 6 h at 353 K, using the equimolar mixture of water and tert-butanol as the solvent and NH3/H2O2/cyclohexanone (molar ratio 1.2/1.2/ 1.0) as the reactants. Then, the catalyst was separated, washed by the solvent, dried overnight at 373 K, and reused in the ammoximation of cyclohexanone under the same conditions as above.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of TS-1. Figure 1 shows the XRD patterns of specimens synthesized via various

Figure 1. XRD patterns of specimens synthesized via various methods. (a−g) Specimens A−G. Batch compositions for specimen A: SiO2:0.033 TiO2:0.6 n-BA:35 H2O; for specimen B: SiO2:0.033 TiO2:0.05 TPAOH:35 H2O; for specimens C and D: SiO2:0.033 TiO2:0.05 TPAOH:0.6 n-BA:35 H2O; for specimens E and F: SiO2:0.033 TiO2:0.05 TPAOH:0.6 n-BA:35 H2O (TS-1 precursor sol as the source of TPAOH); and for specimen G: SiO2:0.033 TiO2:0.25 TPAOH:25 H2O. Crystallization conditions for specimens A−C, E, and G: 443 K for 4 d; and for specimens E and F: 393 K for 2 d and 443 K for 1d.

methods. One can see that there is no diffraction peak but only a broad band at 15−25°, being indicative of an amorphous phase, present for the specimen A. All of specimens B−G display a set of diffraction peaks respectively at 2θ = 7.9°, 8.8°, 23.1°, 23.9°, and 24.4°, indicating the presence of the TS-1 phase.4,19,32 The broad band of the amorphous phase can also be obviously observed for the specimen B but not for the specimens C−G. This suggests that little amorphous phase may be present, and if there is any, it is highly dispersed in the specimens C−G. The crystallinities of these specimens are calculated, using the specimen with the highest crystallinity as the standard, and the results are illustrated in Figure 3. One can 3764



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see that the crystallinities for the specimens exhibit an order F > G > E > C > D > B > A. Since the Ti−O bond is longer than the Si−O bond, the incorporation of Ti(IV) into the framework of TS-1, replacing Si(IV), would result in the expansion of the TS-1 cell.12,17,33 The linear relationship between the content of framework Ti, xf, and unit cell volume of TS-1, v, can be usually expressed as follows:34

leading to a significantly higher crystallinity and content of framework Ti for the specimen G than for the specimen C. In the case where the synthesis involves both the template TPAOH with low usage and the additive n-BA, the crystallinity of TS-1 and its content of framework Ti can be promoted by introducing TPAOH into the batch in the same manner as that of the TS-1 precursor sol, relative to the introduction of TPAOH in a normal manner (specimens E vs C and F vs D, respectively). The TS-1 precursor sol has been prepared from a batch with a molar ratio of TPAOH/SiO2 = 0.25 at the pH of ∼13 and temperature of 363 K. After the TS-1 precursor sol was directly dried at 393 K, the solid obtained was identified by XRD (the diffraction pattern is not given here). It was found that only the amorphous phase was present, indicating that the TS-1 precursor sol contains no TS-1 crystals with a large enough crystal size to be detectable by XRD. This confirms that nuclei of TS-1, instead of TS-1 microcrystals that were usually used as the seed for zeolite synthesis in the literature, were generated in the TS-1 precursor sol. Since the much higher alkalinity, larger usage of TPAOH, and lower temperature were employed in the preparation of TS-1 precursor sol, the significantly large amount of nuclei can be expected to be involved in the synthesis of the specimens F and E, compared to those in the synthesis of the specimens D and C. Therefore, the TS-1 precursor sol provides additional nuclei of TS-1, besides the nuclei of TS-1 in situ-generated during the synthesis of TS-1.42,43 The crystallinity and content of framework Ti can be affected by employing the TPHC procedure, relative to the CTHC procedure (specimens F vs E and D vs C, respectively). In the synthesis of TS-1 via the TPHC procedure, the synthesis sol has been subjected to additional steps of stirring at 373 K for 1 d and hydrothermal crystallization at 393 K for 2 d before the final hydrothermal crystallization at 443 K for 1 d, whereas the synthesis sol was directly subjected to the hydrothermal crystallization at 443 K for 4 d in the CTHC procedure. The stirring accelerates the mass transfer and rearrangement of gel particles in the synthesis sol, favoring the nucleation of TS-1. The hydrothermal crystallization at low temperature (393 K), before that at high temperature (443 K), enables many more nuclei to be formed;23 meanwhile, the dissolution of nuclei, which occurs usually at high temperature, is reduced.40,41 A larger number of nuclei is therefore involved in the synthesis via the TPHC procedure than via the CTHC one, providing more chances for the insertion of Ti into the framework of TS-1; as a result, the specimens F and D possess respectively a larger content of framework Ti than the specimens E and C. It is known that the growth rate of zeolite is regulated by both the nuclei amount and crystallization temperature, and the larger amount of nuclei and/or higher crystallization temperature facilitates the rapid zeolite growth. In the synthesis of the specimens F and E, most of the nuclei have been introduced into the synthesis sol prior to the hydrothermal crystallization step, and the growth of TS-1 can occur at the very beginning of hydrothermal crystallization. A significantly larger amount of nuclei in the synthesis of the specimen F promotes so largely a rapid growth of TS-1 in a shorter time that the negative effect of low temperature, slowing the growth of TS-1, can be sufficiently compensated, compared to the synthesis of specimen E. This allows the specimen F to exhibit a higher crystallinity than the specimen E. In the synthesis of the specimens D and C, the nuclei are in situ-generated during the hydrothermal crystallization. Although a larger amount of

v = 5345.6 + 19.7xf xf = n Ti /(n Ti + nSi)

where nTi and nSi denote respectively the molar numbers of Ti and Si in the framework of TS-1. Table 2 lists the unit cell Table 2. Unit Cell Parameters and Contents of Framework Ti for the Specimens Synthesized via Various Methods specimen

a (Ǻ )

B C D E F G

20.1117 20.1030 20.1085 20.1128 20.1281 20.1284

a

unit cell parameters b (Ǻ ) c (Ǻ ) 19.9282 19.9121 19.9199 19.9573 19.9469 19.9443

13.3581 13.3982 13.4053 13.4112 13.4286 13.4282

volume (Ǻ 3)

xfa(mol %)

5353.78 5363.19 5369.63 5383.22 5391.50 5390.71

0.42 0.89 1.22 1.91 2.33 2.29

xf refers to the content of framework Ti.

parameters and volumes obtained by the XRD determination, and contents of framework Ti, calculated on the basis of the above expressions, for the specimens B−G. One can see that the contents of framework Ti for the specimens display an order F > G > E > D > C > B, with the largest value being 2.33% over the specimen F. The above results reveal that n-BA cannot be used as an effective template to generate TS-1 (specimen A), being similar to other short-chain alkyl amines like methylamine that behave as poor templates in comparison with TPA+.35,36 When TPAOH with a low usage (molar ratio TPAOH/SiO2 = 0.05) is solely employed as the template, TS-1 can be generated; however, an amorphous phase is also concomitantly formed (specimen B). The crystallinity and content of framework Ti of TS-1 can be increased significantly by employing both the template TPAOH and additive n-BA (specimen C) or by employing a high usage of TPAOH (molar ratio TPAOH/SiO2 = 0.25) (specimen G). This is because the n-BA, as a base elevates the alkalinity of synthesis sol (pHs of ∼10.3 and ∼11.6 for specimens B and C, respectively), facilitating a rapid zeolite nucleation and growth; in addition, the n-BA can interact with a variety of metallic ions including Ti4+ to form medium-strong amino complexes in aqueous solution, increasing the general mobility of the (Si−M)n+ oligomers, which is desired for the efficient crystallization of zeolite from the gel phase, through a dissolution−condensation process.35 Thus, the introduction of additive n-BA promotes the mineralization of Ti and Si species and nucleation of TS1,35,37−39 favoring the formation of TS-1 and providing a greater chance for the incorporation of Ti into the framework of TS-1 to elevate the content of framework Ti species. Since TPAOH behaves as both the alkaline and the template for the formation of TS-1, the high usage of TPAOH provides more template and also much larger alkalinity than the additive n-BA (pHs ∼13 and ∼11.6 for specimens G and C, respectively), 3765



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to estimate the level of Ti incorporated into the framework,49 and this ratio is believed to be proportional to the content of the framework Ti.46,50 However, such an application of I960/I800 ratio has to be based upon the elimination of the disturbance of the 970 cm−1 band (caused by the SiOH vibration in the amorphous silica phase) to the 960 cm−1 band,19,46 especially for a specimen with a low crystallinity. On the assumption that the concentration of the SiOH group is in proportion to the content of amorphous silica phase in the specimen, the I960/I800 ratio can be calibrated via the following equation:19 I RIR = 960 + 970 − R 0(1 − C %) I800

nuclei is also involved in the synthesis of specimen D, the time consumed in the growth of TS-1 may be not enough, especially at the lower temperature, relative to the synthesis of specimen C (2 d @ 393 K + 1 d @ 443 K for the specimen D vs 4 d @ 443 K for the specimen C). This results in a lower crystallinity of the specimen D than that of the specimen C. It appears that the amount of nuclei, introduced by the high usage of TPAOH, the TS-1 precursor sol, and the TPHC procedure, increases sequently, since the crystallinity and content of framework Ti in the specimens have been identified to follow an order G > E > D. Among all the specimens A−G, the specimen F possesses the highest crystallinity of TS-1 and largest content of framework Ti. This may be due to the fact that the coemployment of the TS-1 precursor sol and TPHC procedure provides a synergetic effect on the promotion of nucleation of TS-1. From the above discussion, it can be deduced that the amount of nuclei involved in the synthesis of the specimens follows an order F > G > E > D > C > B > A. It is particularly worth noting that the specimen G has been synthesized via the conventional method that is reported usually in the literature, which employs a much larger usage of TPAOH than for the synthesis of the specimen F. Therefore, the method developed in this work for the synthesis of specimen F, which involves the coemployment of the TPHC procedure and lower usage of TPAOH added in the manner of the TS-1 precursor sol, is much cheaper and also more effective in promoting the crystallization of TS-1 and content of framework Ti than is the conventional synthesis method, which involves the CTHC procedure and high usage of TPAOH. Figure 2 shows the FT-IR spectra of specimens synthesized via various methods. One can see that all the specimens C−G

where RIR and I960+970 /I800 represent the I960/I800 ratio after and before the calibration for the specimens B−G; R0 refers to the absorbance ratio of the band at ∼970 cm−1 to that at ∼800 cm−1 for the amorphous specimen A; C% denotes the crystallinity calculated from XRD for the specimens B−G. After the above calibration, the RIR value manifests therefore the true contribution of framework Ti to the band at 960 cm−1 and can reasonably be employed to evaluate the content of the framework Ti in a specimen. The RIR values for the specimens B−G are illustrated in Figure 3, which shows an order F > G >

Figure 3. Relative crystallinity and RIR of specimens synthesized via various methods. (RIR is the absorbance ratio, I960/I800, for the bands at ∼960 and 800 cm−1 after eliminating the disturbance of the band at ∼970 cm−1 to that at 960 cm−1. For the synthesis conditions of specimens A−G see the caption of Figure 1.).

E > D > C > B, being the same as that for the contents of framework Ti derived from the XRD determination (see Table 2). One can also see that the variation of RIR value is parallel to that of crystallinity, indicating that the insertion of Ti atoms into the framework of TS-1 is accompanied by the formation of zeolite structure. Figure 4 shows the SEM micrographs of specimens synthesized via various methods. The specimen A exhibits an amorphous morphology in the SEM micrograph (not given). The specimen B comprises a large amount of amorphous particles and a few TS-1 crystals (Figure 4a), the specimen D contains mainly TS-1 and small amounts of amorphous particles, while the specimens C and E−G consist entirely of TS-1 crystals (Figure 4b,d−f). One can see that the order for the crystallinities of specimens identified by SEM is consistent with that by XRD. The TS-1 crystals in the specimens B, C, and E have a bricklike morphology with an average dimension of ∼10 μm × 4 μm × 2 μm, 8 μm × 4.8 μm × 1.6 μm, and 8 μm × 2.5 μm × 0.5 μm, respectively, and the crystal size distributions of these specimens are all broad (Figure 4a,b,d). The morphology of TS-1 crystal in the specimen D is close to a

Figure 2. FT-IR spectra of specimens synthesized via various methods. (a−g) Specimens A−G, respectively. For the synthesis conditions of these specimens, see the caption of Figure 1.

display obviously an absorption band at ∼960 cm−1, which is usually assigned to the stretching vibration of Si−O in (Si− O)3SiOTi unit44 or that of Si−Oδ−···Tiδ+,45−47 being indicative of the incorporation of Ti into the TS-1 framework.32,45 For the specimens A and B, the bands at ∼960 cm−1 are much weaker and broaden toward a high wavenumber, with its maximum absorption appearing at ∼970 cm−1, compared to the specimens C−G. The weak band at ∼970 cm−1 is commonly ascribed to a stretching vibration mode of Si−O in a Si−OH and thus caused by the amorphous silica in the specimen A and B.19 Besides the bands at 960 cm−1, a band at ∼800 cm−1, being assigned to the symmetry stretching vibration of SiO4 units,48 is present for all the specimens A−G. The absorbance ratio for the bands at ∼960 and 800 cm−1, I960/I800, is usually employed 3766



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crystallinity of the specimen D than that of the specimen C, of which the reason has been provided in discussion of XRD result. The above results indicate that the method developed in this work for the synthesis of specimen F, which involves the coemployment of the TPHC procedure and low usage of TPAOH added in the manner of TS-1 precursor sol, has a very high efficiency in both reducing the crystal size of TS-1 and narrowing the crystal size distribution of TS-1. This method can be even more effective than the conventional synthesis method, which involves the CTHC procedure and high usage of TPAOH. Figure 5 shows the DR UV−vis spectra of specimens synthesized via various methods. One can see that all the

Figure 5. UV−vis spectra of specimens synthesized via various methods. (a−g) Specimens A−G. For the synthesis conditions of these specimens see the caption of Figure 1. Figure 4. SEM micrographs of specimens synthesized via various methods. (a−f) Specimens B−G, respectively. For the synthesis conditions of these specimens see the caption of Figure 1.

specimens display a broad absorption in the range of 200−380 nm, in which mainly three bands at ∼230, 270, and 320 nm can be identified. The band at ∼230 nm is usually ascribed to a charge transfer between tetrahedral-coordinated Ti4+ and O2− of the TS-1 zeolite framework and thus is assigned to framework Ti.33,51,52 Over the specimen A, the band at ∼230 nm is not distinct, and it becomes more and more obvious over the specimens B−F. The intensity of the band at ∼230 nm, which is indicative of the content of framework Ti, exhibits approximately an order F > G > E > D > C > B > A, being consistent with that determined by XRD. The band at ∼320 nm is usually assigned to anatase,33,34 and that at ∼270 nm is caused by the non-tetra-coordinated Ti, e.g., the partially condensed hexa-coordinated Ti species,53 the site-isolated Ti species in penta- or octahedral coordination,54 or even the Ti species partially coordinated with water molecules.34,55 Both the anatase and non-tetra-coordinated Ti belong to extraframework Ti species.54,56 The band at ∼270 nm is strong over the specimens A and B, and it becomes weak over the specimens C, D, E, and G but much weaker over the specimen F. The band at ∼320 nm is present mainly over the specimens C−F, with its intensity being larger over the specimen F than over the specimens C−E, and this band is almost absent over the specimens A, B, and G. The above variations of non-tetra-coordinated Ti and anatase Ti among the specimens occur most probably because the nontetra-coordinated Ti has a relatively low stability (there being a lack of lattice energy) relative to that of the framework Ti and anatase Ti. The higher alkalinity for the synthesis of specimens C−G (pH 11.6−13) than for the synthesis of specimen B (pH ∼10.3) may lead to a partial dissolution of non-tetracoordinated Ti and thus to its relatively low content in the

tablet with an average dimension of ∼1.3 μm × 1 μm × 1 μm and a narrow distribution of crystal size (Figure 4c). Both the specimens F and G are polycrystalline and possess the morphology of spherical aggregates of much smaller crystals. The polycrystalline is determined to have an average size ∼300 nm for the specimen F and ∼350 nm for the specimen G, with a narrow size distribution. Unlike the specimens B−D, all of which possess a smooth surface of TS-1, the specimens F and G both have a rough surface of TS-1, caused by the aggregation of smaller TS-1 crystals, especially for the specimen F. The rough surface of TS-1 can be very useful in catalysis applications, since it would provide more active centers than the smooth surface. The variations in the morphology of TS-1 crystals and their average crystal sizes are mainly due to the differences in the amount of nuclei contained in the synthesis batch. It is known that, the larger the amount of nuclei in the synthesis batch, the smaller is the average crystal size of the zeolite synthesized. As has been obtained in the discussion of XRD results, the amounts of nuclei involved in the synthesis of the specimens have an order F > G > E > D > C > B; thus, the order of average crystal size of TS-1 in the specimens should be in the opposite direction. In fact, the above SEM characterization has shown that the average crystal size of TS-1 in the specimens exhibits an order B > C > E > D > G > F, being consistent with that expected theoretically for most of the specimens. The exception occurs, however, for the specimens D and E, of which the order of average crystal sizes observed from the SEM micrographs is the reverse of that expected theoretically. This may be due that the specimen D is incompletely crystallized at a lower growth rate, relative to the specimen E, just similar as the lower 3767



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Table 3. Textural Properties of Specimens B−Ga

former specimens. It is also possible that there is a transformation from the non-tetra-coordinated Ti to the anatase Ti for the specimens C−F. In that transformation, nBA plays an important role, since anatase Ti is almost absent in the specimen B and G, both of which are synthesized in the absence of n-BA. The coordination of n-BA to Ti4+ and van der Waals interactions among the butyl groups of n-BA molecules may bring the Ti species near to each other,35 promoting the condensation of Ti species and thus the formation of anatase Ti. Noting that the anatase Ti is almost absent in the amorphous specimen A synthesized by employing n-BA and the content of anatase Ti is the largest in the specimen F, which has the smallest crystal size among all the specimens, the large surface of the small crystal of TS-1 thus promotes the transformation from the non-tetra-coordinated Ti into the anatase Ti. It must be mentioned that, although anatase is identified by DR UV−vis, it is not detected by XRD. This indicates that the anatase has a high dispersion in the TS-1 zeolite. The content of overall Ti (framework Ti + extra-framework Ti), namely xf+e, for various specimens has been determined by EDX, and the result is shown in Figure 6. On the basis of xf+e

specimens

SBET (m2/g)

Vmicro (cm3/g)

DSF (Å)

B C D E F G

168.5 378.1 376.6 400.2 443.1 429.4

0.0544 0.1572 0.1540 0.1588 0.1736 0.1681

5.15 5.11 5.21 5.29 5.36 5.32

a SBET, BET specific surface area; Vmicro., micropore volume; DSF, micropore size determined by SF method.

structured TS-1 zeolite. The slightly smaller pore size for the specimens B−D (5.11−5.21 Å) than for the specimens E−G (5.29−5.36 Å) may be due to a larger amount of extraframework Ti being present in the former specimens, which blocks partially the pores of TS-1. In the above experiments, we have identified that the TS-1 with excellent textural property can be successively synthesized at a usage of TPAOH as low as TPAOH/SiO2 = 0.05, by employing both the TPHC procedure and TS-1 precursor sol. Such a novel synthesis of TS-1 results in a greater number of nuclei being generated, even compared to the conventional synthesis that usually employs TPAOH/SiO2 > 0.25. One may wonder why TS-1 can be synthesized with much less TPAOH than that employed in the conventional method. We have preliminarily proposed a few answers to the above question, as follows. Tuel pointed out that, when TPA+ cation was the only organic molecule entrapped in the TS-1 channels and only served as a templating molecule to direct the MFI structure, the minimum amount of TPA+ required to crystallize TS-1 corresponds to the molar ratio of TPA+/SiO2 = 0.042 in the gel.57 However, in the conventional synthesis of TS-124,29,30 and most of the literature reports on the synthesis of TS-125−27 much higher molar ratios of TPA+/SiO2 were usually employed. It hints that the efficiencies of TPA+ utilized in those syntheses might have been appreciably low. In fact, Min et al. had studied the thermal stability of TPAOH on the synthesis of TS-1.58 It was found that for a crystallization time of 4 h, the decomposition ratio of TPAOH was 3.1% at a crystallization temperature of 373 K, which increased to 10.4% and 52.1% at the crystallization temperatures of 393 and 443 K, respectively. The prolonging of crystallization time also led to the obvious decomposition TPAOH, as manifested by the synthesis of TS-1 at 443 K for a different crystallization time. The decomposition ratio of TPAOH was only 9.6% for a crystallization time of 1 h, which increased largely to 89% with prolonging the crystallization time to 24 h. It can be reasonably deduced that, in our work, the synthesis of specimen F possesses a significantly higher utilizing efficiency of TPAOH to generate the nuclei than the synthesis of specimens A−E and G, since the temperature is lower for the preparation of TS-1 precursor sol (363 K) and for the crystallization of TS-1 at a shorter crystallization time (2 d @ 393 K + 1 d @ 443 K) in the synthesis of the former specimen. In addition, as is well-known, a higher temperature and/or alkalinity promotes usually the dissolution of nuclei.23 In the synthesis of specimen F, the TS-1 precursor sol has been prepared from a batch with a pH ∼13 at 363 K, and after the addition of precursor sol, the final synthesis sol had a pH of only ∼11.8, which was thus employed for the crystallization at relatively low temperature and shorter crystallization. In contrast, the specimen G was synthesized

Figure 6. Contents of framework, extra-framework, and overall Ti (xf, xe, and xf+e, respectively) for the specimens synthesized via various methods. (For the synthesis conditions of specimens B−G see the caption of Figure 1.)

and content of framework Ti, i.e., xf, which has been provided by XRD (see Table 2), the content of extra-framework, namely xe, can be quantitatively evaluated by the following equation: xe = xf + e − xf and the result is also illustrated in Figure 6. One can see that both xf+e and xf change obviously from the specimen B to G and display an order F > G > E > D > C > B. However, the xe changes slightly from the specimens B to G, with its value for the specimens C−D being larger than for the specimens E−G. It indicates that the employment of either a TS-1 precursor sol or a high usage of TPAOH in the synthesis of TS-1 helps to retard the formation of extra-framework Ti. This occurs due to the larger amount of nuclei in the synthesis batch providing more chances for the insertion of Ti into the framework of TS1 and, in turn, reducing the amount of extra-framework Ti. The textural properties of specimens B−G were determined by N2-physisorption at liquid nitrogen temperature, and the results are listed in Table 3. One can see that both the surface area and micropore volume over the specimen B−G exhibit the same order as that of the crystallinity, i.e., specimens F > G > E > C > D > B. The micropore sizes of these specimens are all in the range of 5.11−5.36 Å, which is normal for the MFI3768



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It reveals that the catalytic performance can be closely related to the average crystal size of TS-1 in the specimen. The content of extra-framework Ti changes unremarkably from the specimens B−G, indicating that the catalytic performance has not been remarkably affected by the content of extra-framework Ti in the specimen. The crystallinity and content of framework Ti over the specimens exhibit an order F > G > E > C > D > B and F > G > E > D > C > B, respectively, both of which are approximately same as that for the catalytic performance. However, an exception to the above regularity is present, i.e., the specimen D possesses a smaller crystallinity than the specimens C and E and also a lower content of framework Ti than the specimen E, whereas its catalytic performance is higher than for the specimens E and C. This occurs most probably due to the decrease in the average crystal size of TS-1 elevating the catalytic performance for the specimen D, relative to that of the specimens C and E. It indicates that both the crystallinity and content of framework Ti also affect, to some extent, the catalytic performance of the specimen. From the above discussions, one can conclude that, while increasing the crystallinity and content of framework Ti and decreasing the average crystal size and content of extra-framework Ti, the catalytic performance of TS-1 is generally improved, and among various factors, the average crystal size of TS-1 exhibits a much more remarkable effect on its catalytic performance. The effects of reaction conditions, including reaction temperature, NH3/H2O2/cyclohexanone molar ratio, reaction time, and amount of catalyst, have been examined over the specimen F as catalyst, and the results are respectively shown in Tables 5 and 6 and Figures 7 and 8. It is shown by Table 5 that

via a conventional method, which involved a much higher alkalinity (pH ∼13) and crystallization temperature as well as longer crystallization time (4 d @ 443 K). Thus, more nuclei survive in the heated basic solution in the synthesis of specimen F, being able to be utilized in the crystallization of TS-1. Another reason for the generation of a greater number of nuclei in the synthesis of specimen F may be due to the initiation of new nuclei by the TS-1 precursor sol. Epitaxy growth on the structures of the crystallites had been clearly observed by a few researchers.59 Zhang et al. found that the addition of powdered nanosize TS-1 as the crystallization seed could render a higher crystallinity of the resultant TS-1.60 It is deduced that the preformed nuclei introduced by the TS-1 precursor sol in the synthesis of specimen F would have provided a much larger surface area for further nucleation. This results in a considerably lower usage of TPAOH that has been required in the synthesis of specimen F. To attest to the above deductions for the mechanism of TS-1 synthesis via the novel method, further investigation is therefore highly desired, which is still ongoing in our research group. 3.2. Catalytic Performance of TS-1 for the Ammoximation of Cyclohexanone. The specimens B−G have been employed as the catalyst for the ammoximation of cyclohexanone, and their catalytic performances are listed in Table 4. Table 4. Catalytic Performances for the Ammoximation of Cyclohexanone over Various Specimensa specimen

conversion (%)

selectivity (%)

yield (%)

B C D E F G

4.2 8.2 22.6 8.8 98.5 98.0

80.5 91.2 95.5 91.3 99.4 99.2

3.4 7.5 21.6 8.0 97.9 97.2

Table 5. Effect of Reaction Temperature on the Ammoximation of Cyclohexanone over the Specimen Fa

a

Temperature = 353 K; reaction time = 6 h; catalyst/cyclohexanone = 10 g/mol; molar ratio NH3/H2O2/cyclohexanone = 1.2:1.2:1.0.

temp (K)

ketone conv. (%)

oxime select. (%)

oxime yield (%)

333 343 353

73.3 93.6 98.5

81.7 96.5 99.4

60.0 90.3 97.9

a

Catalyst/cyclohexanone = 10 g/mol; reaction time = 6 h; molar ratio NH3/H2O2/cyclohexanone = 1.2:1.2:1.0.

One can see that both the conversions of cyclohexanone and yields to cyclohexanone−oxime over the specimens exhibit an order F > G ≫ D > E > C > B. The specimen F, among all the specimens, has displayed the highest catalytic performance, as manifested by a yield as high as 97.9% of cyclohexanone−oxime at a 98.5% conversion of cyclohexone and a 99.4% selectivity to cyclohexanone−oxime. It is known that the catalytic performance of TS-1 can be affected by many factors, such as the crystal size, the crystallinity, and the contents of framework and extraframework Ti. The decrease in the crystal size usually increases the catalytic activity of TS-1,25,26 since the reduction in pore diffusion resistance.10,25 Higher catalytic performance can be obtained by increasing the crystallinity and/or content of framework Ti species,31 due to the large concentration of catalytic active centers. The presence of extra-framework Ti in TS-1 is often found to reduce the catalytic performance, because of its promotion to the fast decomposition of H2O256 and its possible blocking access of reacting molecules to the internal active framework Ti4+ sites,56,61 However, it is also claimed by some researchers that the extra-framework Ti species has little influence on the catalytic performance of TS1.62 In section 3.1, we have shown that the average crystal size of TS-1 for the specimens exhibits an order B > C > E > D > G > F, being just the reverse of that for the catalytic performance.

Table 6. Effect of Ketone/NH3/H2O2 Molar Ratio on the Ammoximation of Cyclohexanone over the Specimen Fa NH3/H2O2/cyclohexanone (mol/mol/mol)

ketone conv. (%)

oxime select. (%)

oxime yield (%)

1.0/1.0/1.0 1.2/1.2/1.0 1.6/1.3/1.0 2.0/1.6/1.0

64.5 98.5 99.4 99.5

99.1 99.4 99.8 99.8

63.9 97.9 99.2 99.3

a

Catalyst/cyclohexanone = 10 g/mol; reaction time = 6 h; temperature = 353 K.

the conversion of cyclohexanone and the selectivity and yield of cyclohexanone−oxime at a lower reaction temperature (333 K) are all relatively small, with values of 73.3%, 81.7%, and 60.0%, respectively, and they increase rapidly with increasing reaction temperature. A 97.9% yield of cyclohexanone−oxime at 98.5% conversion and 99.4% selectivity can be obtained at the reaction temperature of 353 K. Since the boiling point of the solvent tert-butanol (355.3 K) was already being approached, the reaction temperature was not increased further in our study. From Table 6 for a low molar ratio of NH3/H2O2/ 3769



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the reaction time. A 97.9% yield of cyclohexanone−oxime at 98.5% conversion and 99.5% selectivity is obtained for the longest reaction time of 6 h employed in this work. Figure 8 illustrates that, for using a small amount of catalyst (7 g/mol), only a low yield of cyclohexanone−oxime (70.3%) at a 73.8% conversion of cyclohexanone is obtained, although the selectivity to cyclohexanone−oxime is appreciably high (95.2%). The increase in the usage of catalyst to 8 g/mol promotes largely the yield of cyclohexanone−oxime (95.4%), due to the remarkable increase in the conversion of cyclohexanone (97.5%) and a small increase in the selectivity to cyclohexanone−oxime (97.8%). The promotion becomes smaller for the amount of catalyst increasing to 9 g/mol and much weaker for increasing further the amount of catalyst to >10 g/mol. The suitable usage of catalyst is apparently 10 g/ mol, in view of the high cost of TS-1, which provides a 97.9% yield of cyclohexanone−oxime at 98.5% conversion and 99.4% selectivity. The above results reveal that the proper conditions for ammoximation of cyclohexanone are reaction temperature = 353 K, reaction time = 6 h, NH3/H2O2/cyclohexanone = 1.2:1.2:1.0 (mol) and catalyst usage = 10 g/mol. Figure 9 illustrates the results of catalyst (specimen F) recycle in the ammoximation of cyclohexanone. One can see

Figure 7. Effect of reaction time on the ammoximation of cyclohexanone using the specimen F as catalyst. (Catalyst/cyclohexanone = 10 g/mol; temperature = 353 K; molar ratio NH3/H2O2/ cyclohexanone = 1.2:1.2:1.0.

Figure 8. Effect of catalyst (specimen F) usage on the ammoximation of cyclohexanone. (Catalyst/cyclohexanone = 10 g/mol; temperature = 353 K; reaction time = 6 h; molar ratio NH3/H2O2/cyclohexanone = 1:1.2:1.2.

Figure 9. Recycle of catalyst (specimen F) in the ammoximation of cyclohexanone. (Catalyst/cyclohexanone = 10 g/mol; temperature = 353 K; reaction time = 6 h; molar ratio NH3/H2O2/cyclohexanone = 1:1.2:1.2.

cyclohexanone = 1.0/1.0/1.0, one can see that, although the selectivity to cyclohexanone−oxime is appreciably high (99.1%), the conversion of cyclohexanone is relatively low (64.5%), and this results in a relatively low yield of cyclohexanone−oxime (63.9%). The increase in the molar ratio of NH3/H2O2/cyclohexanone to 1.2/1.2/1.0 promotes largely all the conversion (98.5%), selectivity (99.4%), and yield (97.9%). The promoting effect becomes relatively smaller with the molar ratio NH3/H2O2/cyclohexanone increasing to much higher values (1.6/1.3/1.0−2.0/1.6/1.0), by which the yields of cyclohexanone−oxime larger than 99% can be obtained. However, such a small increase in the yield of cyclohexanone−oxime at the cost of the use of larger amounts of NH3 and H2O2 may not be economic in the practical application. Figure 7 shows that the reaction efficiency is relatively small (cyclohexanone conversion 68.3%, cyclohexanone−oxime selectivity 85.5%, and yield 58.4%) for a reaction time of 2 h, and it can be promoted rapidly (cyclohexanone conversion 96.5%, cyclohexanone−oxime selectivity 98.0%, and yield 94.5%) by increasing the reaction time to 3 h. The promotion becomes smaller for increasing the reaction time to 4 h and much weaker for prolonging further

that the catalyst can be recycled with a total number up to 6, keeping its high conversion of cyclohexanone (≥98.5%), selectivity to cyclohexanone−oxime (≥99.2%) and yield of cyclohexanone−oxime (≥97.8%). It is indicates that the catalyst possesses an very good performance for the ammoximation of cyclohexanone to cyclohexanone−oxime. We also make a comparison of the catalytic performances between the specimens F and G and those reported in the literature,27,63,64 which is shown in Table 7. One can see that the catalytic performance of the specimen F has been the highest one reported up till now. Considering the extremely low usage of TPAOH in the synthesis of specimen F and its very high catalytic performance, the new method developed in this work for the synthesis of TS-1, i.e., the coemployment of TS-1 precursor sol and temperature-programmed hydrothermal procedure, would be very promising in its commercial application. 3770



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Table 7. Comparison of Catalytic Performance for Ammoximation of Cyclohexanone over TS-1 Zeolite Catalysts Synthesized Respectively in Our Work and in the Literature conditions for crystallization

conditions for oximation

catalyst

TPA+/SiO2 (mol/mol)

temp and time (K, d)

ACSa (μm)

CCatalb (g/mol)

Coxidolc (wt %)

CHd/H2O2/NH3 (mol/mol/mol)

temp (K)

time (h)

χe (%)

TS-126 TS-159 TS-160 G in this work F in this work

0.36 0.45 0.36 0.25 0.05

(448, 2) (443, 10) (443, 1−5) (443, 4) (393, 2) + (443, 1)

0.3−0.4 N.A.f 0.4−0.8 0.35 0.30

32 20 10 10 10

30 5 5 30 30

1:1.6:2.5 1:1:1.5 1:1:1.5 1:1.2:1.2 1:1.2:1.2

353 338 333 353 353

2.5 5 6 6 6

97 96.9 92.5 97.2 97.9

ACS: average crystal size of TS-1. bCCatal: usage of TS-1 zeolite catalyst. cCoxidol: concentration of H2O2. dCH: cyclohexanone. eχ: yield of oxime. N.A.: not available.

a f

(3) Mukherjee, P.; Bhaumik, A.; Kumar, R. Eco-friendly, Selective Hydroxylation of C-7 Aromatic Compounds Catalyzed by TS-1/H2O2 System under Solvent-free Solid-Liquid-Liquid-Type Triphase Conditions. Ind. Eng. Chem. Res. 2007, 46, 8657. (4) Wroblewska, A.; Ławro, E.; Milchert, E. Technological Parameter Optimization for Epoxidation of Methallyl Alcohol by Hydrogen Peroxide over TS-1 Catalyst. Ind. Eng. Chem. Res. 2006, 45, 7365. (5) Sooknoi, T.; Chitranuwatkul, V. Ammoximation of Cyclohexanone in Acetic Acid Using Titanium Silicalite-1 Catalyst: Activity and Reaction Pathway. J. Mol. Catal. A: Chem. 2005, 236, 220. (6) Bengoa, J. F.; Gallegos, N. G.; Marchetti, S. G. Influence of TS-1 Structural Properties and Operation Conditionson Benzene Catalytic Oxidation with H2O2. Microporous Mesoporous Mater. 1998, 24, 163. (7) Thangaraj, A.; Kumar, R.; Ratnasamy, P. Direct Catalytic Hydroxylation of Benzene with Hydrogen Peroxide over Titaniumsilicate Zeolites. Appl. Catal. 1990, 57, L1. (8) Tatsumi, T.; Jappar, N. Ammoximation of Cyclic Ketones on TS1 and Amorphous SiO2-TiO2. J. Catal. 1996, 161, 570. (9) Iwasaki, T.; Isaka, M.; Nakamura, H.; Yasuda, H.; Watano, M. S. Synthesis of Titanosilicate TS-1 Crystals via Mechanochemical Route Using Low Cost Materials. Microporous Mesoporous Mater. 2012, 150, 1. (10) Van der Pol, A. J. H. P.; Van Hoof, J. H. C. Parameters Affecting the Synthesis of Titanium Silicalite 1. Appl. Catal. A: Gen. 1992, 92, 93. (11) Gao, H.; Lu, W.; Chen, Q. Characterization of titanium silicalite1 prepared from aqueous TiCl3. Microporous Mesoporous Mater. 2000, 34, 307. (12) Tuel, A.; Ben Taârit, Y.; Naccache, C. Characterization of TS-1 Synthesized Using Mixtures of Tetrabutyl and Tetraethyl Ammonium Hydroxides. Zeolites 1993, 13, 454. (13) Grieneisen, J. L.; Kessler, H.; Fache, E.; Govic, A. M. L. Synthesis of TS-1 in Fluoride Medium: A New Way to a Cheap and Efficient Catalyst for Phenol Hydroxylation. Microporous Mesoporous Mater. 2000, 37, 379. (14) Xia, Q. H.; Gao, Z. Crystallization Kinetics of Pure TS-1 Zeolite Using Quaternary Ammonium Halides as Templates. Mater. Chem. Phys. 1997, 47, 225. (15) Khomane, R. B.; Kulkarni, B. D.; Paraskar, A.; Sainkar, S. R. Synthesis, Characterization and Catalytic Performance of Titanium Silicalite-1 Prepared in Micellar Media. Mater. Chem. Phys. 2002, 76, 99. (16) Chen, L.; Wang, Y. M.; He, M. Y. Hydrothermal Synthesis of Hierarchical Titanium Silicalite-1 Using Single Template. Mater. Res. Bull. 2011, 46, 698. (17) Müeller, U.; Steck, W. Ammonium-Based Alkaline-Free Synthesis of MFI-Type Boron- and Titanium Zeolites. Stud. Surf. Sci. Catal. 1994, 84, 203. (18) Chen, P.; Chen, X. B.; Chen, X. S.; Kita, H. Preparation and Catalytic Activity of Titanium Silicalite-1 Zeolite Membrane with TPABr as Template. J. Membr. Sci. 2009, 330, 369. (19) 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 Tetrapropyl ammonium Bromide as a Template. Mater. Chem. Phys. 2000, 66, 41.

4. CONCLUSION A novel method for the synthesis of high quality TS-1 has been developed in the present work. This method is characteristic of the low usage of TPAOH, being 5 times smaller than that was commonly used in the literature report. It involves the temperature-programmed hydrothermal crystallization (TPHC) of TS-1 from a batch containing both the template TPAOH and additive n-BA, in which the TPAOH is introduced in a manner of TS-1 precursor sol. The additive n-BA plays a role to increase the alkalinity, promoting the nucleation of TS1. The introduction of TS-1 precursor sol into the batch provides additional amount of TS-1 nuclei, besides the nuclei in situ generated during the synthesis of TS-1. The TPHC procedure favors the nucleation of TS-1, due to its relatively low temperature, compared to the constant-temperature hydrothermal crystallization (CTHC) procedure. By employing the novel synthesis method, the TS-1 with not only a very high crystallinity and content of framework Ti (2.33 mol %) but also a considerably small crystal size (∼300 nm) and low content of extra-framework Ti can be generated. The TS-1 synthesized by the novel method possesses a very high performance for the ammoximation of cyclohexanone, which is even better than for those synthesized via the conventional method both in this work and in the literature work. Under the conditions of catalyst usage = 10 g/mol, temperature = 353 K, reaction time = 6 h and NH3/H2O2/cyclohexanone =1.2:1.2:1.0, a 97.9% yield of cyclohexanone−oxime at a 98.5% conversion of cyclohexone and 99.4% selectivity to cyclohexanone−oxime has been achieved.

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AUTHOR INFORMATION

Corresponding Author

*(Z.-S.C.) Tel: +86-731-88713257. E-mail: zschao@yahoo. com. (E.R.) Tel: +1-716-6451179. E-mail: feaeliru@buffalo.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from the Program for Lotus Scholar in Hunan Province, P.R. China.

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REFERENCES

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Synthesis of High-Performanced Titanium Silicalite-1 Zeolite at Very

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