International Journal of Chemical Engineering
Volume 2019 (2019), Article ID 4936292, 11 pages
https://doi.org/10.1155/2019/4936292

Research Article

Ethanol Dehydration over WO₃/TiO₂ Catalysts Using Titania Derived from Sol-Gel and Solvothermal Methods

Anchale Tresatayawed,¹ Peangpit Glinrun,² and Bunjerd Jongsomjit¹

¹Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
²Department of Petrochemicals and Environmental Management, Faculty of Engineering, Pathumwan Institute of Technology, Phathumwan, Bangkok 10330, Thailand

Correspondence should be addressed to Bunjerd Jongsomjit; bunjerd.j@chula.ac.th

Received 23 November 2018; Accepted 12 February 2019; Published 3 March 2019

Guest Editor: Masaru Watanabe

Copyright © 2019 Anchale Tresatayawed et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The present study aims to investigate the catalytic ethanol dehydration to higher value products including ethylene, diethyl ether (DEE), and acetaldehyde. The catalysts used for this reaction were WO₃/TiO₂ catalysts having W loading of 13.5 wt.%. For a comparative study, the TiO₂ supports employed were varied by two different preparation methods including the sol-gel and solvothermal-derived TiO₂ supports, denoted as TiO₂-SG and TiO₂-SV, respectively. It is obvious that the different preparation methods essentially altered the physicochemical properties of TiO₂ supports. It was found that the TiO₂-SV exhibited higher surface area and pore volume and larger amounts of acid sites than those of TiO₂-SG. As a consequence, different characteristics of support apparently affected the catalytic properties of WO₃/TiO₂ catalysts. As expected, both catalysts WO₃/TiO₂-SG and WO₃/TiO₂-SV exhibited increased ethanol conversion with increasing temperatures from 200 to 400°C. It appeared that the highest ethanol conversion (ca. 88%) at 400°C was achieved by the WO₃/TiO₂-SV catalysts due to its high acidity. It is worth noting that the presence of WO₃ onto TiO₂-SV yielded a remarkable increase in DEE selectivity (ca. 68%) at 250°C. In summary, WO₃/TiO₂-SV catalyst is promising to convert ethanol into ethylene and DEE, having the highest ethylene yield of ca. 77% at 400°C and highest DEE yield of ca. 26% at 250°C. These can be attributed to proper pore structure, acidity, and distribution of WO₃.

1. Introduction

Recently, catalytic ethanol dehydration to produce ethylene and diethyl ether (DEE) has been paid attention due to its cleaner technology and efficient utilization of ethanol, which is a renewable raw material obtained from fermentation of biomass. For instance, the production of ethylene from ethanol is considered as an alternative way to produce ethylene, which is currently produced by the catalytic thermal cracking of petroleum feed stocks such as naphtha and dehydrogenation of ethane from natural gas. In fact, dehydration of ethanol to ethylene is a cleaner technology due to lower operating temperature, uncomplicated process, and less impurity. It is well known that ethylene is one of the most important raw materials for petrochemical industry, which is used as a starting material for production of polyethylene, ethylene oxide, vinyl acetate, ethyl benzene, etc. Considering the production of commercialized DEE at present, although it is produced from dehydration of ethanol, the process is not benign since it uses mineral liquid acids such as H₂SO₄ to catalyze the reaction. Thus, further separation and purification are required. In this case, the solid acid catalysts are preferred since they are reusable and easy to separate from the product. Although the use of DEE is much lesser than ethylene, it is very important chemical. In particular, DEE is mainly employed as a solvent for fragrance and pharmaceutical industries. In transport fuel function, DEE is applied as an ignition-improving additive in engines according to its high volatility and cetane and octane number. The blending of DEE in diesel improves the performance-emission characteristics with thermal efficiency and reduced emission of NOx, CO, and HC [1]. Hence, the production of ethylene and DEE from ethanol using suitable solid catalysts is very captivating.

Theoretically, the catalytic ethanol dehydration to ethylene and DEE requires acid sites. This reaction essentially undergoes via thermodynamic and kinetic controls. The formation of ethylene is dominated by high reaction temperature since it is an endothermic reaction, whereas DEE mainly occurs at lower reaction temperature due to its exothermic reaction. However, during dehydration of ethanol, a side reaction such as dehydrogenation can occur resulting in the formation of acetaldehyde as a byproduct. From previous works, many solid acid catalysts have been investigated in ethanol dehydration reaction including the transition metal oxides [2–4], zeolites [5], silica-alumina [6, 7], and heteropolyacids [8]. Many researchers found that the transition metal oxides such as TiO₂, ZrO₂, SiO₂, and Nb₂O₅ play an important role in heterogeneous catalysis acting as an active phase, promoter, or support of solid catalysts. Those solid catalysts have been developed on structure characteristics and acid properties to build up the product selectivity, catalytic activity, and stability. Among the transition metal oxides, TiO₂ has been widely used as a support in heterogeneous catalysts due to its suitable surface areas, thermal stability and mechanical resistance [9, 10]. Furthermore, the modification by doping of the active noble and transition metals such as Cs [11], Au/Ag/Cu [12], Al [13], Ru [14], Pt, Pd [15], Mo [16], and W [17, 18] into catalyst supports apparently affected the catalyst selectivity and activity.

In addition, the presence of tungsten (W) metal was found to be very interesting since it contributes Brønsted acid site and increases the catalyst stability and activity [19–21]. It is reported that WO₃/TiO₂ catalyst is widely used in various reactions and processes including glycerol hydrogenation, reforming, oxidation of dibenzothiophene [22], selective catalytic reduction [23], dehydration [24], and photoelectrocatalytic degradation [25]. Phung et al. [26] also reported that WO₃/TiO₂ is promising for the catalytic dehydration of ethanol to ethylene and especially DEE at low temperature. They reported that the addition of tungsten on transition metal oxide provided the Brønsted acids sites that are active to the ethanol dehydration reaction to produce ethylene and DEE and also prevent the formation of byproducts such as acetaldehyde and higher hydrocarbons. In addition, with various tungsten loadings on TiO₂, ZrO₂ and SiO₂ support, the WO₃/TiO₂ catalyst was found to be the most active in this reaction giving the highest yield of DEE. However, besides the active metals, one needs to consider on the properties of a wide variety of supports themselves. The variations of support characteristics mostly arise from different preparation methods. There are many reports focusing on using different methods to prepare various metal oxide supports including the sol-gel [27, 28] and solvothermal methods [29, 30]. In most cases, they found that different preparation methods can alter the properties of support and consequently, different catalytic properties were observed. Therefore, the effect of different preparation methods on the properties of support is crucial for better understanding.

Similarly to Phung et al. [26] study, the WO₃/TiO₂ catalyst was employed in ethanol dehydration reaction. They examined the difference in some support metal oxides such as ZrO₂ and SiO₂. However, the main goal of this present study is to develop a better understanding on different preparation methods including the sol-gel and solvothermal methods to synthesize the TiO₂ supports for WO₃ catalysts used in catalytic ethanol dehydration to ethylene and DEE. The different characteristics of TiO₂ supports and WO₃/TiO₂ catalysts were determined using various characterization techniques. The change in catalytic properties was also investigated via the catalytic ethanol dehydration in a fixed-bed microreactor at the temperature range of 200 to 400°C. Ethanol conversion and product selectivity of different WO₃/TiO₂ catalysts were reported and discussed further.

2. Experimental

2.1. Materials

Chemicals used for preparation of the catalysts were titanium ethoxide (Ti ∼ 20%) purchased from Aldrich, ethanol (99.99%) from Merck Company Ltd., titanium (IV) n-butoxide (97%), 1,4–butanediol, and tungsten (VI) chloride (99.9+% metals) from Aldrich. For the reaction study, ultrahigh purity nitrogen gas (99.99%) from Linde (Thailand) Public Company Ltd. was employed.

2.2. Preparation of TiO₂ Supports and WO₃/TiO₂ Catalysts

In this study, TiO₂ supports were synthesized using two different methods including the sol-gel and solvothermal methods as reported by Panpranot et al. [29] for a comparative study. For the sol-gel method, titanium ethoxide was used as the precursor. First, the precursor was dissolved in the excess ethanol before added to deionized water with the molar ratio of 165. The mixture solution was stirred under 20 rpm/min at room temperature at least for 2 h. The white precipitates of hydrous oxides formed instantly and separated by centrifugation. The product was redispersed in the ethanol at least 5 times following with centrifugation. The sample was dried and calcined at 450°C for 2 h at the heating rate of 10°C/min. Finally, the white powder of TiO₂ prepared by the sol-gel method was obtained and denoted as TiO₂-SG.

For the solvothermal method, 25 g of titanium (IV) n-butoxide (TNB) was used as the precursor. TNB was suspended in 100 ml of 1,4-butanediol in a test tube and placed in the autoclave. The autoclave was completely purged with nitrogen at a pressure of 30 bars before increasing the temperature to 320°C at a heating rate of 2.5°C/min and further held at that temperature for 6 h. Autogenous pressure during the reaction gradually increased as the temperature increased. After the reaction, the autoclave was cooled down to room temperature. The white powder was collected and then washed with ethanol followed by centrifugation at least 5 times. The powder was dried overnight at 110°C, and finally the white powder of TiO₂ prepared by the solvothermal method was obtained and denoted as TiO₂-SV.

In order to prepare the WO₃/TiO₂ catalysts having W loading of 13.5 wt.%, the simple incipient wetness impregnation was used. The TiO₂-SG and TiO₂-SV supports obtained as mentioned above were used. It was accomplished using the tungsten (VI) chloride as a precursor, followed by drying the catalyst sample overnight at 110°C and calcined at 400°C with a heating rate of 10°C/min for 3 h. Consequently, the obtained WO₃/TiO₂ catalysts are denoted as WO₃/TiO₂-SG and WO₃/TiO₂-SV upon the TiO₂ supports employed.

2.3. Catalysts Characterization

The support and catalysts were characterized by several techniques, which are as follows.

2.3.1. Powder X-Ray Diffraction (XRD)

The SIEMENS D-5000 X-ray diffractometer using CuKα radiation (λ = 1.54439 Å) was used to determine the crystalline phase structure of supports and catalysts. The crystalline domain sizes were calculated from the Scherrer equation. The supports and catalysts were scanned at a rate of 2.4 min⁻¹ in the range 2θ from 20 to 80 degrees with the resolution of 0.02°.

2.3.2. N₂ Physisorption

The adsorptiometer Micromeritics ASAP 2010 automated system instrument was used to determine surface area (BET method), pore volume/diameter, and pore size distribution (BJH method) by nitrogen gas adsorption-desorption at liquid nitrogen temperature at −196°C.

2.3.3. Temperature-Programmed Desorption of Ammonia (NH₃-TPD) and Carbon Dioxide (CO₂-TPD)

The Micromeritics Chemisorb 2750 Pulse chemisorption system instrument was employed to identify the acidity and basicity on supports and catalysts. The 0.03 g quartz wool and 0.05 g catalysts were packed in a quartz tube and pretreated at 500°C under He flow for 1 hr. Next, the catalyst surface was saturated with NH₃ or CO₂ in He at 40°C for 30 min. Then, the excess adsorbed gas (the physisorbed NH₃ or CO₂) was purged with He until the baseline was constant. Afterwards, the catalysts was heated from 40°C to 500°C at a heating rate of 10°C/min to desorb NH₃ or CO₂. The amount of NH₃ or CO₂ in effluent was measured via the thermal conductivity detector (TCD) signal as a function of temperature.

2.3.4. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX)

The SEM model JEOL mode JSM-6400 and EDX with stand Link Isis series 300 program were operated for analysis the morphology, element composition, and distributions over supports and catalysts.

2.3.5. X-Ray Fluorescence Analysis (XRF)

The Olympus model Vanta M Series was performed to determine the amount of tungsten loading on catalysts. XRF spectrometer has an X-ray tube with Rh anode. The spectra were collected during 120 s, with the tube operating with a current of 100 µA and a voltage of 40 kV.

2.3.6. Catalytic Ethanol Dehydration Reaction

Essentially, the ethanol dehydration reaction system reported by Krutpijit and Jongsomjit was used [31]. It was performed in a fixed-bed continuous flow microreactor having an inside diameter of 0.7 cm and length of 33 cm length. First, 0.01 g of quartz wool and 0.05 g of catalyst (or support) were packed in the middle of reactor. The catalyst was preheated by flowing N₂ with a flow rate of 60 ml/min at 200°C for 1 h under atmospheric pressure to remove the moisture and impurity on surface of catalyst prior reaction. Afterwards, the reaction was started by feeding vaporized ethanol and N₂ stream as carrier gas. The ethanol flow rate was maintained at 1.45 ml/h (weight hourly space velocity (WHSV) = 22.9 (gethanol·gcat⁻¹·h⁻¹)) controlled by a syringe pump injection. The reaction was operated at temperature varied from 200 to 400°C. After reaching steady-state condition, the reaction product compositions at reactor effluent were analyzed by a Shimadzu gas chromatography (GC8A) with flame ionization detector (FID) using DB-5 capillary column. Nitrogen with the pressure at 260 kPa was used as carrier gas in GC using the temperature of injector and detector at 150°C. The ethanol conversion was defined as follows:

The selectivity to i product was defined as follows:

The yield to i product was defined as follows:

where and are defined as the molar flow rate (mmol/min) of ethanol in feed and product, respectively. , , , and were defined as the molar flow rate of ethylene, DEE, acetaldehyde, and total products (mmol/min), respectively.

3. Results and Discussion

3.1. Characteristics

The XRD patterns of both TiO₂ supports and WO₃/TiO₂ catalysts are illustrated in Figure 1. Considering the XRD patterns of the supports including TiO₂-SG and TiO₂-SV, they exhibited the similar XRD patterns having the strong diffraction peaks located at 2θ degree of 25° (major), 38°, and 48°, which are assigned to the tetragonal anatase phase of crystalline TiO₂ [32, 33]. When the support was impregnated with 13.5 wt.% of tungsten, the XRD patterns were also similar with those of titania supports. The intensities were lower indicating the smaller crystallite size of WO₃/TiO₂ catalysts than the TiO₂ supports. Besides, the low intensity peaks were observed at 24° and 34° for both WO₃/TiO₂-SG and WO₃/TiO₂-SV catalysts, which were assigned to the formation of the WO₃ crystals with tetragonal phase [34, 35]. When compared the intensity of XRD peaks between WO₃/TiO₂-SG and WO₃/TiO₂-SV, it appeared that WO₃/TiO₂-SG exhibited lower intensity than WO₃/TiO₂-SV suggesting that the crystalline size of the former was smaller than the latter. Based on the Scherrer equation, the average crystalline size of WO₃/TiO₂-SG was smaller than WO₃/TiO₂-SV as seen in Table 1, where the TiO₂ crystalline size was in the range of 10.7–14.3 nm.

Figure 1: X-ray powder diffraction patterns of supports and catalysts.

Table 1: Physical properties of TiO₂ supports and catalysts.

Sample
SBET (m²/g)
Pore volume (cm³/g)
Pore diameter (nm)
Crystallite TiO₂ sizea (nm)
Crystallite WO₃ sizea (nm)
W contentb (wt.%)

TiO₂-SG
73
0.13
4.8
12.4
—
—

TiO₂-SV
85
0.42
16.5
15.3
—
—

WO₃/TiO₂-SG
61
0.11
5.1
10.7
6.1
16.3

WO₃/TiO₂-SV
78
0.30
13.0
14.3
9.0
18.9

aMeasured by XRD using the Scherrer equation, bmeasured by XRF.

The BET surface area (SBET), pore volume, and pore size diameter of TiO₂ supports and WO₃/TiO₂ catalysts analyzed by N₂ physisorption are shown in Table 1. The results revealed that the TiO₂-SG exhibited smaller surface area (73 m²/g) and pore volume (0.13 cm³/g) than those of TiO₂-SV BET surface area (85 m²/g) and pore volume (0.42 cm³/g). The large surface area essentially enhances catalytic activity in ethanol dehydration by increasing possibility of ethanol to attach on the acid site [28]. In addition, the surface area, pore volume, and pore size diameter decreased with the presence of tungsten due to some pore blockage [36].

Figure 2 showed the N₂ adsorption-desorption isotherms at −196°C for the TiO₂ supports and WO₃/TiO₂ catalysts. The results displayed the Type IV adsorption isotherms with the H1 hysteresis loop indicating the mesoporous structure according to the IUPACS. After loading tungsten to obtain WO₃/TiO₂-SG and WO₃/TiO₂-SV, it was found that the Type IV isotherm was still observed. The hysteresis loop moved toward lower pressure for WO₃/TiO₂-SG and WO₃/TiO₂-SV suggesting that the addition of tungsten onto TiO₂ support catalyst resulted in decreased pore volume. The pore size distribution (PSD) for the TiO₂ support catalysts and catalysts are shown in Figure 3. All catalysts were in the average pore diameter range of 2–50 nm classified as mesoporous particles. The TiO₂-SG showed the narrower pore size distribution than that of TiO₂-SV. The average pore sizes for all samples calculated by Barrett–Joyner–Halenda (BJH) are shown in Table 1, which were corresponding to the results from N₂ adsorption-desorption isotherm as seen from Figure 2.

Figure 2: Nitrogen adsorption-desorption isotherms for supports and catalysts.

Figure 3: The pore size distribution for supports and catalysts.

The morphology of supports and catalysts prepared by different methods showed the different morphologies. The TiO₂-SG formed irregular shape particles, while TiO₂-SV formed small agglomerated spherical and porous particles. When impregnated with tungsten on TiO₂-SG and TiO₂-SV supports, both samples exhibited more porous particles. This suggested that the presence of tungsten into TiO₂ resulted in an increase of porosity. The EDX mapping of WO₃/TiO₂-SG and WO₃/TiO₂-SV catalysts are illustrated in Figure 4. It shows the elemental distribution of Ti, O, and W dispersing on the external surface of catalysts.

(a)

(b)

Figure 4: Elemental distribution by EDX mapping for (a) WO₃/TiO₂-SG and (b) WO₃/TiO₂-SV catalyst.

The tungsten was well dispersed at the outer surface of both TiO₂-SG and TiO₂-SV. The weight ratios of W/Ti are also listed in Table 2. The amount of tungsten present at outer surface of TiO₂-SG was larger than TiO₂-SV. Nevertheless, according to the XRF analysis the amount of tungsten in the bulk of TiO₂-SV catalyst was larger than TiO₂-SG indicating that the distribution of tungsten for TiO₂-SV catalyst was mostly located inside the pore of catalyst.

Table 2: Elemental compositions (wt.%) on external surface of catalysts obtained from EDX.

Sample
O
Ti
W
Cl
W/Ti

TiO₂-SG
44.67
55.33
n.a.
n.a.
n.a

TiO₂-SV
44.37
55.63
n.a.
n.a.
n.a

WO₃/TiO₂-SG
30.71
35.17
33.84
0.28
0.96

WO₃/TiO₂-SV
39.00
48.25
12.36
0.39
0.26

It is well known that acidity is the key factor relating to the activity of dehydration catalysts. The acidity of supports and catalysts was evaluated by NH₃-temperature-programmed desorption as shown in Figure 5. As observed, the NH₃-TPD profiles for all samples exhibited the broad desorption peaks in range of 150–500°C.

(a)

(b)

Figure 5: NH₃-TPD profiles of TiO₂ supports and WO₃/TiO₂ catalysts.

It is known that the NH₃-TPD desorption temperature of acidic sites are classified as 3 categories. The desorption of NH₃ between 150 and 300°C is assigned as weak acidic sites, whereas the desorption between 300 and 450°C is moderate acid sites and the desorption above 450°C is strong acid sites [37]. As seen in Table 3, it indicated that the TiO₂-SG exhibited lower amount of acid site than the TiO₂-SV. However, with tungsten loading on both supports, it showed significant increase in weak and total acid sites for both WO₃/TiO₂-SG and WO₃/TiO₂-SV catalysts, which is necessary for enhancing the production of ethylene and DEE [38, 39]. In addition, it can be observed that the WO₃/TiO₂-SV catalyst exhibited the highest amount of total acid sites at 3645 µmol/g cat.

Table 3: The amount of surface acidity of supports and catalysts measured by NH₃-TPD.

Sample
NH₃ desorption (µmol/g cat)
Total acidity (µmol/g cat)

Weak
Medium
Strong

TiO₂-SG
895
662
717
2274

TiO₂-SV
1152
1232
841
3224

WO₃/TiO₂-SG
1030
1054
681
2765

WO₃/TiO₂-SV
1558
1263
823
3645

Considering the CO₂-TPD profiles as shown in Figure 6, the desorption peak displayed the narrow desorption in the temperature range of 50–200°C. Both TiO₂ supports exhibited low temperature CO₂ desorption around 87°C assigned to weak basic sites [40, 41]. The addition of tungsten into support catalysts resulted in a decrease of peak intensity around 80°C. It also revealed that WO₃/TiO₂-SG had the higher amount of basicity site than WO₃/TiO₂-SV as seen in Table 4.

(a)

(b)

Figure 6: CO₂-TPD profile of TiO₂ supports and WO₃/TiO₂ catalysts.

Table 4: The amount of surface basicity of supports and catalysts measured by CO₂-TPD.

Sample
CO₂ desorption (µmol/g cat)
Total basicity (µmol/g cat)

Weak
Medium/strong

TiO₂-SG
27
4
31

TiO₂-SV
18
2
20

WO₃/TiO₂-SG
6
1
7

WO₃/TiO₂-SV
4
0
4

The amounts of carbon deposition after reaction obtained by EDX measurement are shown in Table 5. The WO₃/TiO₂-SG and WO₃/TiO₂-SV exhibited higher amounts of carbon deposition than those of the TiO₂ supports due to their higher acidity. It is well known that high acidity yields high amount of carbon deposition.

Table 5: The amount of carbon elemental compositions (wt.%) on external surface supports and catalysts after reaction obtained from EDX (the spent catalyst used in the reaction condition at T = 200–400°C for 8.5 hr).

Sample
TiO₂-SG
TiO₂-SV
WO₃/TiO₂-SG
WO₃/TiO₂-SV

C (%)
1.0
1.2
1.8
4.5

3.2. Ethanol Dehydration Reaction

To investigate the catalytic activity and product distribution for all catalysts, the ethanol dehydration reaction in gas phase at atmospheric pressure and temperature ranging from 200°C to 400°C was performed. As seen in Table 6, the ethanol conversion for all samples increased with increasing reaction temperature indicating no deactivation of supports and catalysts. The highest ethanol conversion was achieved at 400°C for all samples. The ethanol conversion was found in order of WO₃/TiO₂-SV (87.6%) > TiO₂-SV (56.3%) > WO₃/TiO₂-SG (45.4%) > TiO₂-SG (33.9%), which are related to total amount acid sites of catalysts. It should be noted that the conversion of TiO₂-SV was still higher than that of WO₃/TiO₂-SG, which is quite interesting.

Table 6: Ethanol conversion, product selectivity, and product yield as a function of reaction temperature (the reaction condition: T = 200–400°C, WHSV = 22.9 gethanol·gcat⁻¹·h⁻¹, and catalyst weight = 0.05 (g)).

Catalyst
Temp (°C)
Ethanol conversion (%)
Product selectivity (%)
Product yield (%)

Ethylene
DEE
Acetaldehyde
Ethylene
DEE
Acetaldehyde

TIO₂-SG
200
4.5
0.0
0.0
100.0
0.0
0.0
4.5

250
8.6
1.8
0.0
98.2
0.2
0.0
8.5

300
13.2
4.0
2.6
93.4
0.5
0.3
12.3

350
32.3
11.4
1.6
87.0
3.7
0.5
28.1

400
33.9
17.7
0.9
81.4
6.0
0.3
27.6

TIO₂-SV
200
21.1
0.0
0.0
100.0
0.0
0.0
21.1

250
23.2
0.0
0.0
100.0
0.0
0.0
23.2

300
35.7
2.5
2.1
95.3
0.9
0.8
34.1

350
54.3
9.7
2.4
87.8
5.3
1.3
47.7

400
56.3
25.5
3.4
71.1
14.4
1.9
40.0

WO₃/TiO₂-SG
200
12.4
4.7
37.4
57.9
0.6
4.6
7.2

250
16.4
17.1
40.9
42.0
2.8
6.7
6.9

300
25.0
39.5
10.4
50.1
9.9
2.6
12.5

350
33.4
54.5
3.3
42.3
18.2
1.1
14.1

400
45.4
64.6
1.3
34.1
29.3
0.6
15.5

WO₃/TiO₂-SV
200
33.2
5.2
42.6
52.2
1.7
14.2
17.3

250
37.7
22.4
67.7
10.0
8.4
25.5
3.8

300
51.8
52.6
42.4
5.0
27.3
22.0
2.6

350
71.7
76.4
17.8
5.8
54.8
12.8
4.1

400
87.6
88.2
3.5
8.3
77.2
3.1
7.3

The selectivity to ethylene, DEE, and acetaldehyde are presented in Table 6. It can be observed that the TiO₂ rendered acetaldehyde as a major product. However, with the introduction of tungsten into TiO₂ supports, ethylene and DEE are turned out to be the major products at different temperature. In general, ethanol dehydration reaction can produce ethylene and DEE in 2 competitive pathways:

The reaction (4) is endothermic and favors the moderate to high temperature between 320°C and 500°C, while reaction (5) is exothermic and favors the low to moderate temperature between 150°C and 300°C [42]. The formation of the ethylene occurs by acid catalyst protonating to hydroxyl group of ethanol molecule (proton transfers from acid to O atom to form alkyloxonium ion), and then the water molecules is generated. Subsequently, an ethoxide surface group forms and deprotonates its methyl group to produce the ethylene. The DEE formation proceeds via either dissociative pathway or associative pathway [43]. The dissociative pathway is happened by one ethanol adsorption on catalyst and water elimination providing an adsorbed ethyl group. After that, the ethyl group reacts with the second ethanol and finally the DEE is formed. The associative pathway takes place from coadsorption of two ethanol reacted and formed into DEE. It is recognized that the dehydration of alcohol essentially takes place on Brønsted acid sites [39, 44], while Lewis acid sites rarely contribute for this reaction [45, 46]. The mechanism of ethanol dehydration to ethylene and DEE over the WO₃/TiO₂ catalysts is illustrated in Schemes 1 and 2, respectively. Besides, acetaldehyde is produced under the side reaction or dehydrogenation reaction in reaction (6), which is favored on the basic sites:

Scheme 1: Ethanol dehydration to ethylene over WO₃/TiO₂ catalyst.

(a)

(b)

Scheme 2: Ethanol dehydration to DEE with (a) dissociative and (b) associative pathways over WO₃/TiO₂ catalyst.

According to the experiment, it was observed that TiO₂-SG and TiO₂-SV support catalysts still showed high acetaldehyde selectivity at ca. 81°C and ca. 71°C, respectively. This is perhaps due to the high reaction temperature or the heat treatment effect to an oxidation of Lewis oxygen in pure TiO₂ structure with the basic site is more dominant than acid as seen from the pure SBA-15 catalyst used in ethanol dehydration reported by Autthanit and Jongsomjit in 2018 [47]. The mechanism of ethanol dehydrogenation to acetaldehyde over TiO₂ support is illustrated in Scheme 3. Acetaldehyde occurs by ethanol molecule firstly adsorbed on the catalyst surface. An ethoxy group is further generated and converted into acetaldehyde. When impregnated the tungsten on the TiO₂ supports, it revealed increased ethylene selectivity at high temperature and increased DEE selectivity at low temperature. The ethylene and DEE selectivities over WO₃/TiO₂-SV catalyst were higher than those of WO₃/TiO₂-SG catalyst. It was found that the highest ethylene selectivity of ca. 88% under the reaction temperature at 400°C and the highest DEE selectivity of ca. 68% under the reaction temperature at 250°C for WO₃/TiO₂-SV. The factor to increase the ethylene and DEE selectivity is the high amount of weak acid or Brønsted acid, which is the active site for ethanol dehydration after adding the tungsten on TiO₂ supports. As shown in Table 3, the weak acid sites for WO₃/TiO₂ catalyst were significantly higher than TiO₂ support catalyst, whereas the strong acid sites for TiO₂ were higher than the WO₃/TiO₂ catalyst. It is familiar that amount of weak acid site is probably correlated to Brønsted acid site, while the Lewis acid site may correlate to the strong acid site [47].

Scheme 3: Ethanol dehydrogenation to acetaldehyde over TiO2 support catalyst.

Besides two products, the acetaldehyde was formed as byproduct over WO₃/TiO₂-SG and WO₃/TiO₂-SV catalysts. The acetaldehyde selectivity over WO₃/TiO₂-SG was higher than that of WO₃/TiO₂-SV relating to higher amounts of basic site present in WO₃/TiO₂-SG catalyst. The mass balance (carbon balance) in the reaction test typically closed to 90%. Such a deviation may occur due to mainly from coke formation.

The comparison of product yield for each temperature over supports and catalysts is reported in Table 6. It showed that presence of tungsten essentially improved the ethylene and DEE yield. The highest ethylene yield was found to be ca. 77% at 400°C over WO₃/TiO₂-SV. Moreover, it was observed that the highest DEE yield of ca. 26% at 250°C was obtained with WO₃/TiO₂-SV. However, DEE yield was rather small due to the low conversion at low temperature. It should be mentioned that the highest acetaldehyde yield of ca. 48% at 350°C obtained from TiO₂-SV was observed, which is quite interesting.

4. Conclusion

The WO₃/TiO₂-SV catalyst is promising for dehydration of ethanol to ethylene and DEE having the highest ethylene of 77% at 400°C and the highest yield of 26% at 250°C. It showed that the more efficient method to synthesize TiO₂ support was the solvothermal method due to high acidity and surface area. It is worth noting that the TiO₂-SV itself also rendered the highest yield of acetaldehyde at 48% at 350°C. This support can be potentially used as support for a catalyst in dehydrogenation of ethanol to acetaldehyde.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors thank the Grant for International Research Integration: Chula Research Scholar, Ratchadaphiseksomphot Endowment Fund, Endowment Fund, Grant for Research: Government Budget, Chulalongkorn University (2018), and the National Research Council of Thailand (NRCT) for their financial support of this project.

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