(State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

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 100083)

For simulating the real deactivation of hollow titanium silicalite (HTS) zeolite in commercial ammoximation process, HTS was treated by 10% NH3·H2O solution at 120 ℃ in stirred autoclave. It is found that a part of HTS zeolite crystals dissolved in the hot NH3·H2O solution, and the specifc surface area and pore volume continuously decreased with the increase in NH3hydrothermal 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 specifc 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.

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 identifed[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 flling 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 TiO2-SiO2oxides, which could accelerate the decomposition of oxidant, was considered as the main reason for its deactivation[18]. However, it was very diffcult to trace the deactivation be-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 NH3·H2O hydrothermal condition and during calcination at high temperature. The aim of this work is to confrm the infuencing 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 NH3·H2O 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 NH3·H2O 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, SiO2) was about 2.94. The NH3hydrothermal 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-NH3-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 HTSNH3-B-nh, withndenoting the hydrothermal reaction time.

2.2 Characterization

The XRD analysis was performed on a PANalytical powder diffractometer equipped with a CuKα radiation withλ=0.154 178 nm, under the conditions covering: a beam voltage of 40 kV, a dwell time of 500 s, and a 2θ range of between 5°—80°. The unit cell parameters of both fresh and deactivated HTS zeolites were determined by the whole-pattern refnement Rietveld method.

The N2physisorption 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 (10-1Pa) 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 feld (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 3 000 infrared spectrum, with the wavenumber region ranging from 1 400 cm-1to 1 700 cm-1. Ammonia temperature programmed desorption (NH3-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. 0.625 g of catalyst, 0.133 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% H2O2solution 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.

3 Results and Discussion

3.1 Effects of NH3hydrothermal 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 NH3hydrothermal 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 verifed by the endothermic peak at about 260 ℃ in Figure 2. It is indicated that there might be a strong interaction between NH3molecules and the framework of zeolite sample, which could be related to its physical and chemical properties.

Figure 1 XRD spectra of simulated deactivated HTS zeolite treated at different NH3hydrothermal treatment time

Figure 2 DSC spectra of simulated deactivated HTS zeolite treated at different NH3hydrothermal treatment time

Figure 3 SEM and TEM images of simulated deactivated HTS zeolite treated at different NH3hydrothermal treatment time

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 rough, denoting that zeolite crystals were corroded by NH3·H2O molecules at high temperature and under high pressure, and some amounts of zeolite crystals were dis-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 NH3·H2O solution during the hydrothermal treatment. However, it was interesting to fnd 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 NH3·H2O solution at a specifc temperature.

Figure 4 ICP-AES analysis on dissolution of Si from HTS zeolite in NH3·H2O 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 specifc 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 ofSBETandVPorewas 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-NH3-2h 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 results clearly indicated that the framework Si species could be dissolved in the NH3·H2O 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 NH3hydrothermal treatment time

Figure 5 Pore distribution of simulated deactivated HTS zeolite treated at different NH3hydrothermal treatment time

3.2 Effect of NH3·H2O treatment and calcination on chemical state of Ti species

Another aspect which could infuence 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 TiO2phases, 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 TiO2-SiO2oxide as referred to in the literature[21-25].

It had been demonstrated that the NH3molecules were chemically adsorbed in the framework of uncalcined simulated deactivated HTS zeolite, denoting that NH3molecules might be coordinated with the extra-framework Ti species as well in the form of Ti(OSi)x(OH)y(NH2)6-x-y. It is worth noting that the above-mentioned Ti-containing species were calcined at 550 ℃ for 6 h, when the gaseous H2O and NH3were evaporated to yield the amorphous Ti(SiO)x(OH)6-xspecies. In a word, the tetrahedral framework Ti species were transformed into extraframework Ti species under the effect of NH3·H2O hydrothermal treatment and calcination. 1400 cm-1to 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-1and the relative peak strength ratio of 960 cm-1vs 800 cm-1(I960/I800) in different samples are listed in Figure 8. It is widely accepted that theI960/I800ratio could refect the content of Ti species incorporated in zeolite framework and the catalytic performance in oxidation reactions[29-30].

Figure 6 UV-Vis spectra of simulated deactivated HTS zeolite treated under hydrothermal condition for 40 h: (a) uncalcined samples; (b) calcined samples

Figure 7 FT-IR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time

As it is known in the TS-1 zeolite framework, the replacement of Si4+with Ti4+introduced a band at 960 cm-1in the FT-IR spectrum, which could be usually attributed to the stretching vibration of Si-O-groups in the polarized Si-Oδ-···Tiδ+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 NH3hydrothermal 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 ofI960/I800decreased with an increasing NH3hydrothermal 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 NH3·H2O solution can accelerate the hydrolytic cleavage speed ofTi-O-Si bonds, while a large amount of Si-OH, Ti-NH2and 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, the29Si 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)3OH (marked as Q3), Si(OSi)4(marked as Q4) and Si(OSi)3(OTi) species, respectively[31-33]. The quantifcation of intensity ratio of Q3/Q4showed 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 3 750 cm-1and 3 500 cm-1for fresh HTS zeolite, while a set of wide peaks at 3 800 cm-1to 3 200 cm-1appeared in the spectra of simulated deactivated zeolite samples, denoting that many hydroxyl groups with different strength existed after NH3hydrothermal treatment[34].

Many literature reports have mentioned that the Q3/Q4ratio 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 Q3/Q4ratio of simulated deactivated HTS zeolite samples is presented in Figure 11. It can be seen that the Q3/Q4ratio 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 Q3/Q4ratio of uncalcined and calcined samples treated at the same hydrothermal treated time, it is interesting to fnd that these ratio values of the calcined samples were slightly less than those of 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.

Figure 929Si MAS NMR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time

Figure 10 Hydroxyl-IR spectra of simulated deactivated HTS zeolite treated at different hydrothermal treatment time

Figure 11 Q3/Q4ratio of simulated deactivated HTS zeolite treated at different NH3hydrothermal treatment time (4 h, 23 h and 52 h)

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 Br?nsted 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-xas predicted by the results of UV-vis analyses, had no acidity when the Ti species were in a highly dispersed state without the formation of Ti-OTi bonds. Even after the elapse of a long calcination time (over 12 h at 550℃ in air), there was no appearance of Br?nsted 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 TiO2-SiO2amorphous oxides, as revealed by the real industrial deactivated HTS zeolite under that treatment condition.

Table 2 Acidity property of simulated deactivated HTS zeolitesamples treated at different hydrothermal treatment time

3.3 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 NH3hydrothermal 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 frst one is the reduction of the specifc 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-xspecies, 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 specifc surface area and active sites could infuence 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 infuence on the catalytic properties of zeolite samples, which could bewell correlated with the acidity features of corresponding samples, as shown in Table 2. It was concluded that the neutral amorphous TiO2-SiO2oxide 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.

Figure 13 Catalytic performance of simulated deactivated HTS zeolite samples treated at different hydrothermal and thermal treatment time

3.4 The mechanism for deactivation of simulated HTS under aqueous NH3.H2O 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 NH2-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.

Figure 14 Proposed mechanism of deactivation of simulated HTS zeolite obtained under NH3hydrothermal and thermal treatment condition

The regenerated OH-ions could catalyze the hydrolysis of Ti-O-Si and Si-O-Si bonds continually. The Si(OH)4species could dissolve in NH3·H2O solution, causing the increase of Ti/ Si ratio in the deactivated HTS zeolitesamples 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 the29Si MAS NMR and pyridine adsorbed FT-IR analyses.

4 Conclusions

The aqueous NH3solution 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 confrmed 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(NH2)6-x-yspecies were produced during alkaline hydrothermal processing. And they could be transformed into Ti(OSi)x(OH)6-xspecies 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 TiO2-SiO2oxides 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 20673054).

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Received date: 2016-08-18; Accepted date: 2016-09-24.

Prof. Lin Min, Telephone:+86-10-82368801; E-mail: linmin.ripp@sinopec.com.