Xia Changjiu; Lin Min; Peng Xinxin; Zhu Bin; Xu Guangtong; Shu Xingtian

Transcription

1 Scientific Research China Petroleum Processing and Petrochemical Technology 2016, Vol. 18, No. 4, pp 1-10 December 30, 2016 Deactivation Behavior of Hollow Titanium Silicalite Zeolite in Aqueous Ammonia Solution under Simulated Industrial Cyclohexanone Ammoximation Conditions Xia Changjiu; Lin Min; Peng Xinxin; Zhu Bin; Xu Guangtong; Shu Xingtian (State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing ) Abstract: For simulating the real deactivation of hollow titanium silicalite (HTS) zeolite in commercial ammoximation process, HTS was treated by 10% NH 3 H 2 O solution at 120 in stirred autoclave. It is found that a part of HTS zeolite crystals dissolved in the hot NH 3 H 2 O solution, and the specific surface area and pore volume continuously decreased with the increase in NH 3 hydrothermal treatment time. Meanwhile, the transformation of framework Ti species into extraframework Ti species was detected by the spectroscopic methods. However, the extraframework Ti species were still in a highly dispersed state after the hydrothermal and thermal treatments as shown by TEM images, while the formation of new acid sites was not detected. Upon combining the results of characterization with catalytic performance of HTS, the main deactivation reason for this material had been determined, which might be attributed to the reduction of specific surface area and active centers after basic treatment and calcination of HTS samples. And then the possible mechanism of simulated deactivation of HTS zeolite was proposed, which could describe the elemental reaction steps much more visually and directly. Key words: deactivation, hollow titanium silicate, ammoximation, extraframework Ti, TEM 1 Introduction Hollow titanium silicalite zeolite (HTS), which has the MFI topology and sufficient mesoporous structure, was originally produced by some researchers, through the postsynthesis of common TS-1 zeolite in the presence of templates under hydrothermal conditions [1-5]. HTS has been widely applied in many environmentally friendly catalytic oxidation processes in industry, i.e. phenol hydroxylation, cyclohexanone ammoxidation, propene epoxidation, and alkane oxidation, with low concentration hydrogen peroxide solution serving as the oxidant. Compared with traditional homogeneous oxidation processes, only a few numbers of by-products and wastes were produced in line with this heterogeneous catalysis route, thus reducing the energy and raw materials consumption [6-12]. Although HTS zeolite shows even higher stability than TS-1 zeolite in ammoximation process, it will still lose its catalytic activity gradually when it is used in the alkaline media for a long time (more than half a year in industry). The deactivation phenomena on TS-1 zeolite in ammoxidation reaction were investigated by G. Petrini, et al., and three main possible reasons were identified [13-15], namely: (i) the pore occupation by coke or other by-products; (ii) the dissolution of framework Si atoms; and (iii) the transfer of framework Ti atoms to the extraframework position. The effect of pore filling by organic substances could be reduced by calcination in air, while the other two factors were irreversible. The deactivated TS-1 zeolite should be removed from the reaction system, which would increase the cost of ε-caprolactam production. In our previous work, the irreversibly deactivated HTS zeolite samples were confirmed by multiple characterization and catalytic test methods [16-17]. The formation of acidic highly dispersed amorphous TiO 2 -SiO 2 oxides, which could accelerate the decomposition of oxidant, was considered as the main reason for its deactivation [18]. However, it was very difficult to trace the deactivation be- Received date: ; Accepted date: Corresponding Author: Prof. Lin Min, Telephone: ; linmin.ripp@sinopec.com. 1

2 havior of HTS zeolite catalyst in the continuous commercial ammoxidation process. Furthermore, the deactivation rate of HTS zeolite was very slow under commercial operating conditions, making it unrealistic to investigate the elementary steps of deactivation in the laboratory. In order to simulate the real deactivation steps in this reaction, the as-synthesized HTS zeolite samples were operated under the NH 3 H 2 O hydrothermal condition and during calcination at high temperature. The aim of this work is to confirm the influencing factors and serve as a guide on how to restrict and regenerate the deactivated HTS zeolite. Further characterization was used to check out the physicochemical properties of HTS zeolite samples after hydrothermal and thermal treatments. Finally, the plausible mechanism of simulated deactivation was illustrated to help us understand the elementary steps of effects of NH 3 H 2 O hydrothermal process and calcination on the decrease in catalytic performance and related feature changes. The probing phenol hydroxylation reaction was introduced to evaluate and verify the catalytic activity of simulated deactivated HTS zeolite in accordance with their physicochemical properties. 2 Experimental 2.1 Hydrothermal treatment of HTS zeolite in NH 3 H 2 O solution HTS zeolite was produced following an existing published method, which described the preparation of high activity and stability zeolite catalysts suitable for many industrial oxidation reactions [2]. The initial gel was prepared by dissolving both of tetraethyl orthosilicate (TEOS, Aldrich) and tetrabutyl orthotitanate (TBOT, Aldrich) in an aqueous structure directing agent solution. After that, the mixture was stirred at 90 for 5 h, while a proper amount of water was added, making alcohols completely evaporated. The resulting gel was charged into an 100-mL autoclave, and heated at 170 for 3 days. The as-made white powder product was collected by filtration, and calcined at 550 for 6 h; and then the so-called TS-1 zeolite sample was obtained. Furthermore, the HTS zeolite could be formed by post-synthesis of TS-1 zeolite. The calcined HTS zeolite sample was put into a highpressure stirred autoclave, in which a proper amount of aqueous ammonia solution was added, and the mole ratio of ammonia and HTS zeolite (counted as pure silica, SiO 2 ) was about The NH 3 hydrothermal treatment temperature was 120, and the rotational speed was 400 r/min, while operating under its autogenous pressure. After few hours, the resulting simulated deactivated HTS zeolite sample was obtained. It was labeled as HTS-NH 3 -nh, with n denoting the hydrothermal reaction time. The calcined simulated deactivated HTS zeolite sample was heated at 550 for 6 h, which was labeled as HTS- NH 3 -B-nh, with n denoting the hydrothermal reaction time. 2.2 Characterization The XRD analysis was performed on a PANalytical powder diffractometer equipped with a CuKα radiation with λ= nm, under the conditions covering: a beam voltage of 40 kv, a dwell time of 500 s, and a 2θ range of between The unit cell parameters of both fresh and deactivated HTS zeolites were determined by the whole-pattern refinement Rietveld method. The N 2 physisorption isotherms were measured on a Micromeritics AS-6B apparatus, using the conventional BET and BJH methods to quantify the surface areas and pore volumes of zeolite samples. Prior to analyses, the samples were heated to a constant weight under vacuum (10-1 Pa) at 300 C for 6 h. The transmission electron microscope images were taken on a JEM-2100 microscope. The STEM images and energy dispersive X-ray spectroscopy element mapping of the Ti-containing zeolite samples were detected by a highangle annular dark field (HAADF) detector and an EDAX spectrometer on Tecnai F20 G2 S-Twin microscope. X-ray photoelectron spectroscopy equipped with a PHI model 590 spectrometer and an Al Kα radiation source was used to analyze the titanium content on the zeolite crystal surface. The UV-vis spectra were collected by a Cary 300 Agilent UV-vis spectrophotometer, with the wavelength ranging from 200 nm to 800 nm. The pyridine adsorbed IR characterization was operated in the BIO-RAD FTS infrared spectrum, with the wavenumber region ranging from cm -1 to cm -1. Ammonia temperature programmed desorption (NH 3-2

3 TPD) was performed on an AutoChem II 2920 equipment. 2.3 Catalytic activity test Liquid phase phenol hydroxylation was selected as a probe reaction to detect the catalytic activity of Ti-containing zeolite. The reaction was carried out in an 100 ml threenecked flask reactors, with stirring and heating at the same time g of catalyst, mol of phenol and 10 ml of acetone were put into every flask reactor, and were mixed homogeneously and heated under stirring by a magnetic stirrer. When the temperature was increased to 80, 0.44 mol of 30% H 2 O 2 solution was added into each reactor. After the reaction was carried out for 2 h, the liquid-phase mixtures were detected by an Agilent gas-chromatograph equipped with a HP-5 column (30 m 0.25 mm 0.25 μm) and a FID detector. rough, denoting that zeolite crystals were corroded by NH 3 H 2 O molecules at high temperature and under high pressure, and some amounts of zeolite crystals were dis- Figure 1 XRD spectra of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time a HTS-NH 3-40h; b HTS-NH 3-2h; c HTS-NH 3 -B-2h; d HTS-NH 3 -B-40h 3 Results and Discussion 3.1 Effects of NH 3 hydrothermal treatment and calcination on structural and morphological properties of zeolite samples The X-ray powder diffraction (XRD) patterns of both fresh and calcined simulated deactivated HTS zeolite samples are presented in Figure 1. It can be seen that all these zeolite samples have similar characteristic diffraction peaks, which could be used to reflect the MFI topological structure of HTS zeolite. It was obvious that the framework of HTS zeolite samples was very stable during the NH 3 hydrothermal treatment and calcination, without any structural collapse even after alkaline treatment for 40 h [19-20]. Differential scanning calorimetry (DSC) analysis revealed that some ammonia species were chemically adsorbed in the simulated deactivated HTS zeolite, which could be verified by the endothermic peak at about 260 in Figure 2. It is indicated that there might be a strong interaction between NH 3 molecules and the framework of zeolite sample, which could be related to its physical and chemical properties. Although the topological structure of simulated deactivated zeolite was still in accord with the MFI type, the morphology of both fresh and calcined samples was changed, as shown in Figure 3. The SEM images revealed that the crystal surface of deactivated zeolite samples became Figure 2 DSC spectra of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time Figure 3 SEM and TEM images of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time 3

4 solved in the alkaline solution. To identify the dissolution of HTS zeolite, the liquid samples were inspected by ICP-AES to analyze their composition. As shown in Figure 4, the silicon-containing substance was dissolved in the aqueous NH 3 H 2 O solution during the hydrothermal treatment. However, it was interesting to find that the contents of silicon species dissolved at different reaction time were almost at the similar level, indicating that there was a solubility equilibrium between the zeolite sample and NH 3 H 2 O solution at a specific temperature. results clearly indicated that the framework Si species could be dissolved in the NH 3 H 2 O solution, causing the damaged morphology and the worsening microporous structure of HTS zeolite. Table 1 XRF and BET results of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time Sample n(ti)/ n(ti+si) S BET, m 2 /g S Z, m 2 /g V pore, ml/g V micro, ml/g HTS HTS-NH HTS-NH HTS-NH HTS-NH Figure 4 ICP-AES analysis on dissolution of Si from HTS zeolite in NH 3 H 2 O solution Along with the dissolution of Si from HTS zeolite, the mole ratio of Ti/Si in the deactivated HTS zeolite samples detected by the XRF method was almost equal, and the data of some deactivated zeolite samples were smaller than those of the fresh one, as shown in Table 1, which was in good agreement with the results of ICP-AES analyses. It can be seen from the BET results that both of the specific surface area and the pore volume became smaller with an increase in ammonia hydrothermal treatment time. Compared with the fresh HTS zeolite, when the treatment time was 40 h, the value of S BET and V Pore was only 63.39% and 61.13% of that of fresh one, respectively. It was concluded that some microporous structure was dissolved, while the remaining zeolite particles were of a highly crystalline MFI topology. During the dissolving of silicon species from zeolite framework, the mesopore size was increasing with the extension of treatment time, and some macrospores were formed, as shown in Figure 5. It was found that the mesopore size of HTS-NH 3-2h sample was 12 nm, while that of fresh one was about 20 nm. And this macrospore size could be related to the open cavities as shown in the SEM images. These characterization Figure 5 Pore distribution of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time HTS-NH 3-2h; HTS-NH 3-14h; HTS-NH 3-31h; HTS-NH 3-40h; Fresh HTS zeolite 3.2 Effect of NH 3 H 2 O treatment and calcination on chemical state of Ti species Another aspect which could influence the catalytic oxidation performance of titanium-containing MFI zeolite was the chemical state and distribution of Ti species. Figure 6 shows the UV-vis spectra of both uncalcined and calcined simulated deactivated HTS zeolite samples. In both cases, there were two adsorption peaks at around 220 nm and 330 nm, which could reveal the existence of framework Ti species and bulk TiO 2 phases, respectively. Furthermore, a new adsorption peak at around 350 nm was found in the uncalcined simulated deactivated zeolite sample, while there was an adsorption peak at around 275 nm in the calcined zeolite sample, which could be ascribed to the amorphous TiO 2 -SiO 2 oxide as referred to in the literature [21-25]. 4

5 It had been demonstrated that the NH 3 molecules were chemically adsorbed in the framework of uncalcined simulated deactivated HTS zeolite, denoting that NH 3 molecules might be coordinated with the extra-framework Ti species as well in the form of Ti(OSi) x (OH) y (NH 2 ) 6-x-y. It is worth noting that the above-mentioned Ti-containing species were calcined at 550 for 6 h, when the gaseous H 2 O and NH 3 were evaporated to yield the amorphous Ti(SiO) x (OH) 6-x species. In a word, the tetrahedral framework Ti species were transformed into extraframework Ti species under the effect of NH 3 H 2 O hydrothermal treatment and calcination cm -1 to 400 cm -1, which means that a large amount of Ti species are still in the framework position. However, there are many differences in the adsorption peak positions and relative strength among different samples, i.e., the detailed results of adsorption sites at around 960 cm -1 and the relative peak strength ratio of 960 cm -1 vs 800 cm -1 (I 960 /I 800 ) in different samples are listed in Figure 8. It is widely accepted that the I 960 /I 800 ratio could reflect the content of Ti species incorporated in zeolite framework and the catalytic performance in oxidation reactions [29-30]. Figure 7 FT-IR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time Figure 6 UV-Vis spectra of simulated deactivated HTS zeolite treated under hydrothermal condition for 40 h: (a) uncalcined samples; (b) calcined samples As it is known in the TS-1 zeolite framework, the replacement of Si 4+ with Ti 4+ introduced a band at 960 cm -1 in the FT-IR spectrum, which could be usually attributed to the stretching vibration of Si-O - groups in the polarized Si-O δ- Ti δ+ bond, although this issue is still under debate [26-28]. Figure 7 shows the FT-IR spectra of the simulated deactivated HTS zeolite samples processed with different NH 3 hydrothermal treatment times. All of the four samples have the similar IR bands in the area between Figure 8 I960/I800 and peak site of simulated deactivated HTS zeolite with different hydrothermal treatment time As we can see, the ratio of I 960 /I 800 decreased with an increasing NH 3 hydrothermal treatment time. It means that a part of framework Ti species would shift to the extraframework position in this environment. Furthermore, it is interesting that the adsorption peak site at around 960 cm -1, which may be used to reveal the interactions and relative distance change between Ti and Si atoms, has shifted to higher wavenumber position after alkaline treatment. That may occur because the OH - groups in the NH 3 H 2 O solution can accelerate the hydrolytic cleavage speed of 5

6 Ti-O-Si bonds, while a large amount of Si-OH, Ti-NH 2 and Ti-OH groups are obtained. This fact means that the hydrolysis of Ti-O-Si bonds, catalyzed by the inorganic base, could change the coordination states of both Ti and Si atoms, and the content of tetrahedral framework Ti species would become less than that in the fresh HTS zeolite. To verify the conclusions that were drawn from the results of FT-IR and UV-vis spectroscopic analyses, the 29 Si MAS NMR method was used to analyze the chemical environment of Si atoms in the simulated deactivated HTS zeolite samples, as shown in Figure 9. The signals from different Si atoms in the deactivated zeolites were found at -112, -115 and -116 that corresponded to Si(OSi) 3 OH (marked as Q 3 ), Si(OSi) 4 (marked as Q 4 ) and Si(OSi) 3 (OTi) species, respectively [31-33]. The quantification of intensity ratio of Q 3 /Q 4 showed a substantial increase of surface defect content in the treated samples, which was in agreement with the results of hydroxyl IR characterization of different treated materials (as shown in Figure 10). There were relatively uniformed two peaks at around cm -1 and cm -1 for fresh HTS zeolite, while a set of wide peaks at cm -1 to cm -1 appeared in the spectra of simulated deactivated zeolite samples, denoting that many hydroxyl groups with different strength existed after NH 3 hydrothermal treatment [34]. Many literature reports have mentioned that the Q 3 /Q 4 ratio of TS-1 zeolite samples could be correlated to the Ti content incorporated into the framework, in which the framework Ti species could be coordinated with the drawbacks in the topological structure, and would reduce the content of Si-OH groups. The Q 3 /Q 4 ratio of simulated deactivated HTS zeolite samples is presented in Figure 11. It can be seen that the Q 3 /Q 4 ratio of both uncalcined and calcined simulated deactivated HTS zeolites becomes larger with the increase of treatment time under the basic condition. It is inferred that the breaking of Ti-O-Si and Si-O-Si bonds could produce a large amount of Si- OH groups, and a part of the framework Ti species were transformed into the extraframework Ti species, which could correspond to the results of UV-vis and FT-IR analyses. Upon comparing the Q 3 /Q 4 ratio of uncalcined and calcined samples treated at the same hydrothermal treated time, it is interesting to find that these ratio values of the calcined samples were slightly less than those of Figure 9 29 Si MAS NMR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time NH 3 -B-52h; NH 3 -B-23h; NH 3 -B-4h; NH 3-52h; NH 3-23h; NH 3-4h Figure 10 Hydroxyl-IR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time Figure 11 Q 3 /Q 4 ratio of simulated deactivated HTS zeolite treated at different NH 3 hydrothermal treatment time (4 h, 23 h and 52 h) With calcination; Without calcination uncalcined ones. We could infer that a certain number of Si-O-Si or Ti-O-Si bonds were formed during the calcination process via the dehydration condensation reaction between Si-OH and Ti-OH groups at high temperature. 6

7 The acidity properties of fresh and simulated deactivated HTS zeolites are shown in Table 2. Although a swarm of framework Ti species had been transformed during the hydrothermal and thermal treatment, there was almost no change in the number of both Lewis and Brönsted acid sites among the different samples. This fact means that the possible Ti-containing species, which were in the form of Ti(OSi) x (OH) 6-x as predicted by the results of UVvis analyses, had no acidity when the Ti species were in a highly dispersed state without the formation of Ti-O- Ti bonds. Even after the elapse of a long calcination time (over 12 h at 550 in air), there was no appearance of Brönsted acid sites and the increase in number of Lewis acid sites in the simulated deactivated HTS zeolite. It is obvious that the above-mentioned Ti-containing amorphous oxide is very stable, and could not be sintered to produce some acidic TiO 2 -SiO 2 amorphous oxides, as revealed by the real industrial deactivated HTS zeolite under that treatment condition. Table 2 Acidity property of simulated deactivated HTS zeolite samples treated at different hydrothermal treatment time 250 Calcination Samples Lewis acid, Brönsted acid, time, h μmol/g μmol/g Fresh HTS HTS-NH 3 -B-2 h HTS-NH 3 -B-40 h HTS-NH 3 -B-40 h HTS-NH 3 -B-40 h Catalytic performance for phenol hydroxylation reaction The results of catalytic performance of simulated deactivated HTS zeolites for the phenol hydroxylation reaction using 30% hydrogen peroxide as the oxidant are summarized in Figure 12. It can be seen that the catalytic activity of treated HTS zeolites in terms of phenol conversion decreased with an increasing NH 3 hydrothermal treatment time. Upon combining the results of characterization tests and catalytic activity tests, it is inferred that the reasons for deactivation of the simulated HTS zeolite samples may be related with three restraints, namely: (1) The first one is the reduction of the specific surface area and pore volume, in particular the reduction of microporous volume; (2) The second one is the transformation of scores of framework tetrahedral Ti species into Ti(OSi) x (OH) 6-x species, indicating to the disappearance of active sites needed for the catalytic oxidation reactions; and (3) the newly formed Ti-containing species may accelerate the decomposition of hydrogen peroxide molecules during the phenol hydroxylation reaction. Figure 12 Catalytic performance of simulated deactivated HTS zeolite samples treated at different hydrothermal treatment time It has been widely accepted that the reduction of specific surface area and active sites could influence the catalytic properties of Ti-containing zeolites. In order to determine the last factor, the catalytic performance of simulated deactivated HTS zeolites treated at different hydrothermal and thermal treatment times was studied, as shown in Figure 13. It is observed that there is almost no difference among the deactivated samples treated at different calcination time ranging from 2 12 h. This fact means that the calcination process has no obvious influence on the catalytic properties of zeolite samples, which could be Figure 13 Catalytic performance of simulated deactivated HTS zeolite samples treated at different hydrothermal and thermal treatment time HTS-NH 3-8h; HTS-NH 3-23h; HTS-NH 3-52h 7

8 well correlated with the acidity features of corresponding samples, as shown in Table 2. It was concluded that the neutral amorphous TiO 2 -SiO 2 oxide could not catalyze the reaction for decomposition of hydrogen peroxide. Therefore, the formation of new Ti-containing species could be assigned to the reason for the deactivation of the simulated HTS zeolite. 3.4 The mechanism for deactivation of simulated HTS under aqueous NH 3.H 2 O treatment condition The mechanism for deactivation of plausible simulated HTS zeolite is presented in Figure 14. When ammonia gas was dissolved in the aqueous solution, the OH - ions could be formed through the ionization reaction between ammonia and water. At the initial step, OH - ions as a nucleophilic reagent could attack the electrophilic Ti atoms in the framework of HTS zeolite, and an intermediate with negative charge was produced (marked as TS1). In this intermediate, the electron was transferred to the Ti atoms, and a new Ti-OH bond was formed. Then the electron was shifted to the Si-O bonds, causing the breaking of Ti-O bonds and the appearing of O - ions that were connected with Si atoms. The Si-O - species could electrophilically attract the H atoms in water molecules, resulting in the formation of Si-OH bonds and the regeneration of OH - ions. When the Ti-OH bonds were formed, and some small ligands, such as OH - and NH - 2 ions, might coordinate with tetrahedral Ti atoms, which could be transformed into octahedral Ti atoms, as evidenced by the results of UV-vis and FT-IR analyses. In a word, the Ti- O-Si bonds were broken upon being catalyzed by alkaline OH - groups through this circle as shown in Figure 14. The regenerated OH - ions could catalyze the hydrolysis of Ti-O-Si and Si-O-Si bonds continually. The Si(OH) 4 species could dissolve in NH 3 H 2 O solution, causing the increase of Ti/ Si ratio in the deactivated HTS zeolite Figure 14 Proposed mechanism of deactivation of simulated HTS zeolite obtained under NH 3 hydrothermal and thermal treatment condition 8

9 samples and the damage of HTS zeolite crystals. When the deactivated zeolite was calcined, a large amount of Si- OH and Ti-OH groups could be hydrated to form Ti-O-Si bonds, leading to the increase of framework Ti contents in the thermally treated deactivated HTS zeolite samples. However, the Ti atoms were still in a highly dispersed distribution state without the formation of Ti-O-Ti bonds and new acid sites. This mechanism could be in good agreement with the results of the 29 Si MAS NMR and pyridine adsorbed FT-IR analyses. 4 Conclusions The aqueous NH 3 solution at high temperature gave rise to the dissolution of silicon species from microporous HTS zeolite framework, causing the damage of a part of zeolite crystals. However, the remaining deactivated HTS zeolite crystals still retained the MFI topology, which was confirmed by the XRD, BET and XRF analyses. The reduction of specific surface area and pore volume, in particular the micropore volume, was regarded as the partial reason for the deactivation of simulated HTS zeolite samples. The transformation of tetrahedral framework Ti species into octahedral extraframework Ti species was detected by the spectroscopic methods. It was observed that a number of Ti(OSi) x (OH) y (NH 2 ) 6-x-y species were produced during alkaline hydrothermal processing. And they could be transformed into Ti(OSi) x (OH) 6-x species during high temperature calcination in air. Both of these two kinds of Ti species showed no catalytic oxidation activity. Thus, it was inferred that the decrease of framework Ti content is the second possible reason leading to its deactivation. The acidity property of deactivation of simulated HTS zeolite samples was characterized by the pyridine adsorbed IR spectroscopy. It was found that there was almost no change after alkaline hydrothermal treatment. This means the newly formed amorphous TiO 2 -SiO 2 oxides could not be assigned to the deactivation reason. In the end, according to the results of both characterization tests and catalytic activity tests, a corresponding mechanism for deactivation of HTS zeolite was proposed. Acknowledgments: Thanks a lot for the kind help from Prof. Xuhong Mu, Prof. Yibin Luo, Dr. Aiguo Zheng, Dr. Yanjuan Xiang, and all staffs conducting the characterization of materials at RIPP. This work was financially supported by the National Basic Research Program of China (973 Program, 2006CB202508), and by the China Petrochemical Corporation (SINOPEC Group ). References [1] Taramasso M, Perego G, Notari B. Preparation of porous crystalline synthetic material comprised of silicon and titanium oxides: The United States, US [P] [2] Lin M, Shu X T, Wang X Q, et al. Titanium-silicalite molecular sieve and the method for its preparation: The United States, US [P] [3] Wang Y, Lin M, Tuel A. Hollow TS-1 crystals formed via a dissolution recrystallization process[j]. Microporous and Mesoporous Materials, 2007, 102(1): [4] Shi C, Zhu B, Lin M, et al. Cyclohexane mild oxidation catalyzed by new titanosilicate with hollow structure[j]. Catalysis Today, 2011, 175(1): [5] Xia C, Zhu B, Lin M, et al. A green cyclohexanone oxidation route catalyzed by hollow titanium silicate zeolite for preparing ε-caprolactone, 6-hydroxyhexanoic acid and adipic acid[j]. China Petroleum Processing & Petrochemical Technology, 2012, 14(4): [6] Lin M, Xia C, Zhu B, et al. Green and efficient epoxidation of propylene with hydrogen peroxide (HPPO process) catalyzed by hollow TS-1 zeolite: A 1.0 kt/a pilot-scale study[j]. Chemical Engineering Journal, 2016, 295: [7] Xia C, Long L, Zhu B, et al. Enhancing the selectivity of para-dihydroxybenzene in hollow titanium silicalite zeolite catalyzed phenol hydroxylation by introducing acid-base sites[j]. Catalysis Communications, 2016, 80: [8] Robinson D J, Davies L, McGuire N, et al. Oxidation of thioethers and sulfoxides with hydrogen peroxide using TS-1 as catalyst[j]. Physical Chemistry Chemical Physics, 2000, 2(7): [9] Xia C, Ju L, Zhao Y, et al. Heterogeneous oxidation of cyclohexanone catalyzed by TS-1: Combined experimental and DFT studies[j]. Chinese Journal of Catalysis, 2015, 36(6): [10] Perego C, Carati A, Ingallina P, et al. Production of titanium containing molecular sieves and their application in catalysis[j]. Applied Catalysis A: General, 2001, 221(1): [11] Ratnasamy P, Srinivas D, Knözinger H. Active sites and reactive intermediates in titanium silicate molecular sieves[j]. Advances in Catalysis, 2004, 48:

10 [12] Dal Pozzo L, Fornasari G, Monti T. TS-1, catalytic mechanism in cyclohexanone oxime production[j]. Catalysis Communications, 2002, 3(8): [13] Kholdeeva O A, Ivanchikova I D, Guidotti M, et al. Highly selective oxidation of alkylphenols to benzoquinones with hydrogen peroxide over silica-supported titanium catalysts: titanium cluster site versus titanium single site[j]. Advanced Synthesis & Catalysis, 2009, 351(11/12): [14] Cesana A, Mantegazza M A, Pastori M. A study of the organic by-products in the cyclohexanone ammoximation[j]. Journal of Molecular Catalysis A: Chemical, 1997, 117(1): [15] Gontier S, Tuel A. Liquid-phase oxidation of aniline over various transition-metal-substituted molecular-sieves[j]. Journal of Catalysis, 1995, 157(1): [16] Xia C, Lin M, Zheng A, et al. Irreversible deactivation of hollow TS-1 zeolite caused by the formation of acidic amorphous TiO 2 -SiO 2 nanoparticles in a commercial cyclohexanone ammoximation process[j]. Journal of Catalysis, 2016, 338: [17] Xia C, Lin M, Peng X, et al. Regeneration of deactivated hollow titanium silicalite zeolite from commercial ammoximation process by encapsulating amorphous TiO 2 SiO 2 nanoparticles inside zeolite crystal[j]. Chemistry Select, 2016, 1(14): [18] Zheng A, Xia C, Xiang Y, et al. Titanium species in deactivated HTS-1 zeolite from industrial cyclohexanone ammoxidation process[j]. Catalysis Communications, 2014, 45: [19] Marra G L, Artioli G, Fitch A N, et al. Orthorhombic to monoclinic phase transition in high-ti-loaded TS-1: an attempt to locate Ti in the MFI framework by low temperature XRD[J]. Microporous and Mesoporous Materials, 2000, 40(1): [20] Thangaraj A, Eapen M J, Sivasanker S, et al. Studies on the synthesis of titanium silicalite, TS-1[J]. Zeolites, 1992, 12(8): [21] Geobaldo F, Bordiga S, Zecchina A, et al. DRS UV-Vis and EPR spectroscopy of hydroperoxo and superoxo complexes in titanium silicalite[j]. Catalysis Letters, 1992, 16(1/2): [22] Serrano D P, Sanz R, Pizarro P, et al. Improvement of the hierarchical TS-1 properties by silanization of protozeolitic units in presence of alcohols[j]. Microporous and Mesoporous Materials, 2013, 166: [23] Su J, Xiong G, Zhou J, et al. Amorphous Ti species in titanium silicalite-1: Structural features, chemical properties, and inactivation with sulfosalt[j]. Journal of Catalysis, 2012, 288: 1-7 [24] Armaroli T, Milella F, Notari B, et al. A spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity[j]. Topics in Catalysis, 2001, 15(1): [25] Muscas M, Solinas V, Gontier S, et al. Microcalorimetry studies of the acidic properties of titanium-silicalites-1[j]. Studies in Surface Science and Catalysis, 1995, 94: [26] Camblor M A, Corma A, Perez-Pariente J. Infrared spectroscopic investigation of titanium in zeolites. A new assignment of the 960 cm -1 band[j]. Journal of the Chemical Society, Chemical Communications, 1993 (6): [27] Li C, Xiong G, Liu J, et al. Identifying framework titanium in TS-1 zeolite by UV resonance Raman spectroscopy[j]. The Journal of Physical Chemistry B, 2001, 105(15): [28] Smirnov K S, van de Graaf B. On the origin of the band at 960 cm -1 in the vibrational spectra of Ti-substituted zeolites[j]. Microporous materials, 1996, 7(2): [29] Huang D G, Zhang X, Chen B H, et al. Ethanol-assistant synthesis of TS-1 containing no extra-framework Ti species[j]. Catalysis Today, 2010, 158(3): [30] Camblor M A, Corma A, Perez-Pariente J. Synthesis of titanoaluminosilicates isomorphous to zeolite beta, active as oxidation catalysts[j]. Zeolites, 1993, 13(2): [31] Van der Waal J C, Rigutto M S, Van Bekkum H. Zeolite titanium beta as a selective catalyst in the epoxidation of bulky alkenes[j]. Applied Catalysis A: General, 1998, 167(2): [32] Fan W, Duan R G, Yokoi T, et al. Synthesis, crystallization mechanism, and catalytic properties of titanium-rich TS-1 free of extraframework titanium species[j]. Journal of the American Chemical Society, 2008, 130(31): [33] Tuel A, Taarit Y B. Variable temperature 29 Si MAS NMR studies of titanosilicalite TS-1[J]. Journal of the Chemical Society, Chemical Communications, 1992 (21): [34] Bonino F, Damin A, Bordiga S, et al. Interaction of CD3 CN and pyridine with the Ti (IV) centers of TS-1 catalysts: a spectroscopic and computational study[j]. Langmuir, 2003, 19(6):