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Fabrication and Characterization of Titanate Nanotube Supported ZSM-5 Zeolite Composite Catalyst for Ethanol Dehydration to Ethylene
Fabrication and Characterization of Titanate Nanotube Supported ZSM-5 Zeolite Composite Catalyst for Ethanol Dehydration to Ethylene
Bulletin of the Korean Chemical Society. 2014. Feb, 35(2): 525-530
Copyright © 2014, Korea Chemical Society
  • Received : October 29, 2013
  • Accepted : November 25, 2013
  • Published : February 20, 2014
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Liangpeng Wu
Xinjun Li
Zhenhong Yuan
Yong Chen

Abstract
Titanium dioxide nanotube supported ZSM-5 zeolite composite catalyst was fabricated by decorating ZSM-5 zeolite on the hydrothermally synthesized titanium dioxide via hydrothermal process and subsequent annealing. The catalyst was characterized by X-ray powder diffraction (XRD), Transmission electron microscopy (TEM) and Nitrogen adsorption-desorption (BET). The surface acidity of the catalyst was measured by means of Fourier transform infrared (FT-IR) spectrum of pyridine adsorption. And the catalytic activity for ethanol dehydration to ethylene was evaluated in a continuous flow fixed-bed reactor. Attributed to the increase of the effective surface acid sites caused by titanium dioxide nanotube as electron acceptor, titanium dioxide nanotube supported ZSM-5 zeolite composite catalyst exhibits strongly enhanced activity for ethanol dehydration to ethylene.
Keywords
Introduction
Ethylene is one of the most common building blocks in the petrochemical industry. It can undergo many types of reactions that generate a plethora of important petrochemical products such as ethylene oxide and polyethylene. 1 The main technical process to produce ethylene at present is the thermal cracking of liquefied petroleum gas or naphtha at high temperature of 600-1000 °C. 2 The nonrenewable fossil fuels will inevitably face the problem of its reserves becoming less and less, and its final depletion is inevitable. 3 Therefore, much attention has been focused on production of petrochemicals from a non-petroleum, environmentally friendly feedstock and development of new, efficient ethylene production processes. 4 Catalytic dehydration of bioethanol to ethylene is becoming a potential and alternative route to meet the demand of ethylene with a complementary supply. 5 More importantly, ethanol can be obtained from renewable sources by fermentation of natural carbohydrates like vegetable biomass, lignocellulosic waste and sugerindustry residues. 6
The catalyst plays a crucial role in the success of the reaction in terms of the rate and the yields of ethanol dehydration to ethylene. 7 Various solid acid catalysts, including zeolite, 8 9 heteropolyacid 10 11 and transition metal oxides, 12 13 have been extensively applied to the reaction of ethanol dehydration to ethylene. One of the most widely used catalysts in industrial applications is γ-Al 2 O 3 . 14 15 The conventional γ-Al 2 O 3 catalyst is high in reaction temperature and low in ethylene selectivity. 16 However, zeolite possesses microporous structure with unique uniform pores and channel sizes, allows lower reaction temperatures and produces higher ethylene yields. 17 In particular, ethylene could be directly produced over zeolite from bioethanol aqueous solution. Over the past few decades, a tremendous effort has been made over the past decade to improve catalytic activities for ethanol dehydration to ethylene by metal ionmodified zeolite catalysts. 18 - 21
Titanium dioxide (TiO 2 ) has been found to be an effective promoter to enhance the catalytic performance on ethanol dehydration to ethylene. For example, it has been reported that the γ-Al 2 O 3 doped with 10% TiO 2 has high ethanol conversion and ethylene selectivity comparatively to γ- Al 2 O 3 . 22 4A zeolite coupled with TiO 2 exhibits better catalytic performance than 4A zeolite does. 23 And TiO 2 particlesupported zeolite composite catalyst exhibits much higher catalytic efficiency. 24 It has been reported that TiO 2 nanotube, possesses a unique combination of morphological and physicochemical properties, such as large specific surface area, mesoporous structure, high aspect ratio, and efficient electron conductivity, has exhibited immense potential in a wide variety of applications ranging from environmental purification, energy storage and gas sensing. 25 However, there is no report about the potential application of TiO 2 nanotube as a promoter of composite catalysts for ethanol dehydration to ethylene. In this paper, a novel composite structure catalyst, TiO 2 nanotube supported zeolite ( Figure 1 ), was fabricated by decorating ZSM-5 zeolite on the hydrothermally synthesized TiO 2 nanotube via a facile hydrothermal process. And the catalytic performance for ethanol dehydration to ethylene was also investigated
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Schematic representation of the structure of TiO2 nanotube supported ZSM-5 zeolite composite catalyst.
Experimental
Preparation of TiO2 Nanotube . Commercial titania powder (anatase, Panzhihua, China) was used without any purification treatment as a starting material to prepare titanate nanotube by hydrothermal method. In a typical procedure, 8 g TiO 2 powder was added to 100 mL 10 M NaOH solution in a 250 mL Teflon-lined flask under continuous magnetic stirring. The Teflon-lined flask was maintained at 110 °C for 48 h and then cooled to room temperature naturally. After the reaction, the resulting products were filtered, repeatedly washed with distilled water until the conductivity of the solution was less than 70 μs/cm. Then the precipitate was bathed with 0.1 M HCl solution for 5 h and subsequently washed by deionized water until the solution conductivity was less than 5 μs/cm. The washed samples were dried in a vacuum oven at 80 °C for 8 h and calcined at 350 °C for 1 h to obtain TiO 2 nanotube (TiO 2 NT).
Preparation of ZSM-5 Zeolite. ZSM-5 Zeolite was synthesized according to the following hydrothermal synthesis procedure. 26 In this synthesis procedure, tetraethylorthosilicate (silicon dioxide > 28.5%) and aluminum sulfate were used as Si source and Al source, respectively. Cetyltrimethyl ammonium bromide (purity > 99.9%) and tetrapropylammonium hydroxide solution (TPAOH, 20%) was used as organic template. The starting mixture was formed using the following procedures: Firstly, Al 2 (SO 4 ) 3 ·18H 2 O was dissolved in deionized water, and then 0.1 M NaOH was added into the aforementioned solution until the precipitate disappeared to get a solution. The Solution was added into a mixed solution of tetraethylorthosilicate, deionized water and template agent under constant stirring condition for 1 h to obtain a gel. The homogenous solution with the molar composition of 1Al 2 O 3 :50SiO 2 :3CTAB:7TPAOH:1500H 2 O was transferred to Teflon stainless-steel autoclaves and hydrothermally treated at 160 °C under autogenous pressure for 72 h. Finally, the resultant product was separated by filtration, washed with deionized water, dried at 80 °C for 12 h in the oven and calcined at 500 °C for 3 h in a muffle.
Preparation of TiO2NT Supported ZSM-5 Zeolite . Similar to the above hydrothermal synthesis procedure, TiO 2 NT supported ZSM-5 zeolite composite catalyst was fabricated expect that TiO 2 NT was added into the gel.
Characterizations. Transmission electron microscope (TEM) images were taken on a JEM-1010 microscope with an acceleration voltage of 100 kV. The phase structure of the catalysts was analyzed by X-ray powder diffraction (XRD, X’Pert-PRO, PANalytical, Holland) with Cu Kα radiation source (k = 0.154056 nm) operating at an accelerating voltage of 40 kV and a current of 40 mA. The patterns were recorded in the 2θ range from 5 to 80° at a scan rate of 1.5°/ min. Specific surface area, pore volume and pore diameter of the samples were measured by nitrogen adsorptiondesorption technique using an ASAP 2020 instrument (Micromeritics Instrument Corporation, USA). The types of catalyst acid sites were identified by FT-IR analysis of pyridine-sorbed samples. Firstly, the samples were pressed into self-supported wafers, set in a Pyrex glass cell pretreated at high vacuum at 250 °C for 4 h. The samples adsorbed pyridine at ambient temperature. And then, at the same temperature, the sample was submitted at vacuum for 15 min in order to minimize the amount of physisorbed pyridine. Finally, desorption experiments were conducted at elevated temperatures (350 °C) under vacuum for 10 min in order to compare the strength of Bronsted and Lewis acid sites. The spectra of pyridine bonded by Bronsted and Lewis acid sites were recorded and analyzed.
Dehydration Performance Evaluation. The catalytic performance of the catalysts was evaluated in a home-made, continuous flow fixed bed reactor (length 350 mm, inner diameter 9 mm) under atmospheric pressure. The catalyst (2 g) was placed the middle of the reactor, and nitrogen gas was fed into the reactor for 30 min. The feedstock was introduced by a pulse micro-liquid pump with a weight hourly space velocity (WHSV) of 2.2 h ╶1 into the evaporator and the reactor. The evaporator temperature was kept at 170 °C, the reactor temperature was increased from 210 °C to 420 °C. The gas phase products were analyzed by a GC (Agilent 6890) linked a TCD detector at an oven temperature of 90 °C with Ar as carrier gas. The liquid phase products were analyzed by another GC (Agilent 6820) linked a FID detector in a temperature-programmed course from 40 °C to 100 °C with N 2 as carrier gas. The conversion of ethanol (C ethanol ) is defined as the molar ratio of conversion ethanol to the total injection ethanol. The ethylene selectivity (S ethylene ) and the diethyl ether selectivity (S DEE ) are defined as the molar ratio of ethylene and diethyl ether to the total conversion ethanol, respectively.
Results and Discussion
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XRD patterns of TiO2NT supported ZSM-5 zeolite (a), ZSM-5 zeolite (b) and TiO2NT (c).
XRD patterns of TiO 2 NT, ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite are presented in Figure 2 . The diffraction peaks at about 2θ = 25.4°, 25.4°, 37.9°, 48.1°, 54.0°, 55.1° can be indexed to the (101), (004), (200), (105), (211), (204) reflection peaks of the anatase TiO 2 NT (JCPDS 74-1940). The representative peaks at 7.9°, 8.9°, 23.5°, 24° and 24.5° corresponding to typical MFI-type and the characteristic peaks of ZSM-5 zeolite (JCPDS 44-0003), which is similar to that reported by Lei et al . 27 The characteristic peaks of TiO 2 NT and ZSM-5 zeolite are appeared in the sample of TiO 2 NT supported ZSM-5 zeolite composite catalyst, but the characteristic peaks intensity of anatase TiO 2 and ZSM-5 zeolite were a little decrease.
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TEM images of TiO2NT (a), ZSM-5 zeolite (b) and TiO2NT supported ZSM-5 zeolite (c).
The TEM images of TiO 2 NT, ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite composite catalysts are shown in Figure 3 . As presented in Figure 3(a) , the anatase TiO 2 NT is approximately uniform with the diameter size of around 15-20 nm and the length ranges from hundreds of nanometers to several micrometers. ZSM-5 zeolite with approximately uniform particle size is shown in Figure 3(b) . TiO 2 NT supported ZSM-5 zeolite composite catalyst can be synthesized by a facile hydrothermal process. Due to the different electron penetrability for ZSM-5 zeolite and TiO 2 NT, it can be seen that TiO 2 NT is decorated with ZSM-5 zeolite from Figure 3(c) .
The nitrogen adsorption-desorption isotherms of ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite composite catalyst are shown in Figure 4 . The isotherms can be classified as type IV type of isotherms (IUPAC classification) and show characteristics of mesoporous materials. Textural properties including specific surface area (A BET ) and the pore volume (V P ) are calculated by the BET method. The specific surface area and the pore volume of zeolite are 261 m 2 /g and 0.16 cm 3 /g, and that of TiO 2 NT supported zeolite composite catalyst are 218 m 2 /g and 0.13 cm 3 /g, respectively. The corresponding pore size distribution curves displayed in Figure 5 were determined by the BJH method. Narrow and sharp peak of ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite is observed in the diameter range 2.5-4 nm showing uniform pore size. However, a small quantity of pores rang-ing from 6 to 12 nm is also observed on TiO 2 NT supported ZSM-5 zeolite composite catalyst.
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N2 adsorption-desorption isotherms of ZSM-5 zeolite and TiO2NT supported ZSM-5 zeolite.
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Pore size distribution of ZSM-5 zeolite and TiO2NT supported ZSM-5 zeolite.
The types of catalyst acid sites were identified by FT-IR analysis of pyridine-sorbed samples. The ring vibration of pyridine detected in the frequency range of 1400-1600 cm ╶1 commonly used to characterize the concentrations of Bronsted and Lewis acid sites. 28 The band at 1490 cm ╶1 is ascribed to the pyridine adsorbed on both Bronsted and Lewis acid sites. The bands at 1548 cm ╶1 and 1452 cm ╶1 are assigned to the pyridine molecule adsorbed on the Bronsted and the Lewis acid sites, respectively. 29 The FT-IR spectra of pyridine adsorbed on ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite composite catalyst are shown in Figure 6 . The FT-IR spectra of pyridine adsorbed reveal that the acidic sites of ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite composite catalyst are both Bronsted and Lewis acid sites. The presence of a significant concentration of acid sites in these catalysts can be attributed to their generation during the prolonged calcination period at 500 °C. It has been found that thermal decomposition of zeolitic alkylammonium cations may lead to the formation of acid sites in the zeolite structure. 30 It can be also seen that the concentration of Bronsted and Lewis acid sites on TiO 2 NT supported ZSM-5 zeolite composite catalyst is more than that on ZSM-5 Zeolite.
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FT-IR spectra pyridine adsorption on ZSM-5 zeolite and TiO2NT supported ZSM-5 zeolite at 350 °C.
Figure 7 represents ethylene formation rate, ethanol conversion, ethylene selectivity and the diethyl ether selectivity over TiO 2 NT, ZSM-5 zeolite and TiO 2 NT supported ZSM-5 zeolite composite catalyst obtained at different reaction temperature. The reaction temperature over TiO 2 NT supported ZSM-5 zeolite composite catalyst starts at 210 °C, while that over TiO 2 or ZSM-5 zeolite does at 240 °C. The conversion of ethanol and the selectivity of ethylene increase with rising reaction temperature, but the selectivity of diethyl ether behave in the opposite way. The catalytic performance for ethanol dehydration to ethylene is markedly improved on TiO 2 NT supported ZSM-5 zeolite composite catalyst, and small amount of byproducts such as diethyl ether is restrained under the same reaction condition. For TiO 2 NT supported ZSM-5 zeolite composite catalyst, the conversion of ethanol is higher than 70% as the reaction temperature is higher than 330 °C, the selectivity of ethylene is higher than 97%, whereas the catalytic performance of TiO 2 NT and ZSM-5 zeolite is much lower than that of TiO 2 NT supported ZSM-5 zeolite composite catalyst. It can be seen from Figure 7(a) and Figure 7(b) , the formation rate of ethylene, the conversion of ethanol and the ethylene selectivity on TiO 2 NT supported ZSM-5 zeolite composite catalyst keep stability above 330 °C. It is likely that the catalytic ability of TiO 2 NT supported ZSM-5 zeolite composite catalyst was nearly saturated in the reaction and the chemical reaction was not a reaction rate-controlling step.
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Ethylene formation rate (VC2H4) (a), Ethanol conversion (Cethanol) and ethylene selectivity (Sethylene) (b), and diethyl ether selectivity (SDEE) (c) vs. T over TiO2NT, ZSM-5 zeolite and TiO2NT supported ZSM-5 zeolite composite catalyst.
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Schematic diagram illustrating the increase of effective surface acid sites caused by TiO2NT as electron acceptor for TiO2NT supported ZSM-5 zeolite composite catalyst.
It is generally accepted that the dehydration reaction ethanol is an acid-catalyzed reaction. 31 32 Ethanol adsorbed on the acid center of the catalyst surface would form an oxonium ion C 2 H 5 OH 2 + , which either reacts with another C 2 H 5 OH molecule to form the foremost product of diethyl ether or dehydrates to form the subsequent ethylene. 33 Diethyl ether is generated via inter-molecular dehydration of ethanol at low reaction temperatures, and ethylene is produced via intra-molecular dehydration of ethanol at high reaction temperatures. 34
Generally, zeolite is known to possess of the suitable Bronsted and Lewis acid centers which have an important influence on the catalytic performance for ethanol dehydration and the distribution of products. In recent years, a great of researchers have been devoting to developing zeolite with different pore structures and modifying zeolite to adjust the acidity of zeolite. Bi et al. reported the improved catalytic performance of the nanoscale HZSM-5 zeolite should be attributed to the increase of acid sites on the external surface compared with that of the microscale HZSM-5. 35 Takahara et al. reported that the increase in the number of acid sites of HZSM-5 zeolite during the dehydration of ethanol could cause the dehydration activity to increase. 36 The phenomena have been observed that titanium dioxide coupled with Al 2 O 3 or zeolite can effectively improve the catalytic activity of the composite catalysts for the dehydration of ethanol to ethylene. 22 - 24 37 In this study, the catalytic performance for ethanol dehydration to ethylene is markedly improved on TiO 2 NT supported zeolite composite catalyst.
Titanium dioxide (TiO 2 ) has been found to be an effective promoter to enhance the catalytic performance on ethanol dehydration to ethylene. TiO 2 NT has attracted particular attention owing to its interesting one-dimensional hollow structure, large surface area, high pore volume and potential applications in catalysis. In comparison with TiO 2 nanoparticles, TiO 2 NT offers a larger available surface area and also provides channels for enhanced electron transfer. 38 And TiO 2 NT is usually used as an electron acceptor material. For example, it is used in Dye Sensitized Cell (DSSC) to receive the electron from the excitated dye sensitizer. 39 For TiO 2 NT supported ZSM-5 zeolite composite catalyst, At elevated temperatures, one would expect more thermal electrons from ZSM-5 zeolite to TiO 2 NT and generate more vacancies at the catalyst’ surface ( Figure 8 ), which would cause the increased effective acid sites on the surface of ZSM-5 zeolite. The amount of Bronsted and Lewis acid sites on TiO 2 NT supported ZSM-5 zeolite composite catalyst has been identified to be more than that on zeolite by FT-IR spectra of pyridine adsorbed ( Figure 6 ). Besides, the nanotube structure of composite catalyst is propitious to the reactant diffusion, prolongs the contact time of the reactant and TiO 2 NT supported ZSM-5 zeolite composite catalyst, and also facilitates unimolecular dehydration of ethanol for ethylene production. So, the increase of the effective acid sites and the pore structure of composite catalyst contribute to the enhanced activity for ethanol dehydration to ethylene.
Conclusion
TiO 2 NT supported ZSM-5 zeolite composite catalyst was fabricated by decorating ZSM-5 zeolite on the hydrothermally synthesized TiO 2 NT via hydrothermal process and subsequent annealing. Attributed to the increase of the effective surface acid sites caused by TiO 2 NT as electron acceptor and catalyst topology, TiO 2 NT supported ZSM-5 zeolite composite catalyst exhibits strongly enhanced activity for ethanol dehydration to ethylene.
Acknowledgements
The authors are grateful for the financial support from the National 973 project of China (No. 2013CB228105), Knowledge Innovation Program of Chinese Academy of Sciences (No. KSCX2-YW-G-004) and National Natural Science Foundation of China (No. 51172233). And the publication cost of this paper was supported by the Korean Chemical Society.
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