Influence of acidity on the performance of silica supported tungsten oxide catalysts assessed by in situ and Operando DRIFTS

Journal Pre-proof Influence of Acidity on the Performance of Silica Supported Tungsten Oxide Catalysts Assessed by In Situ and Operando DRIFTS Thotsatham Takkawatakarn, Kongkiat Suriye, Bunjerd Jongsomjit, Joongjai Panpranot, Piyasan Praserthdam

PII:

S0920-5861(19)30495-X

DOI:

https://doi.org/10.1016/j.cattod.2019.08.062

Reference:

CATTOD 12451

To appear in:

Catalysis Today

Received Date:

16 May 2019

Revised Date:

19 August 2019

Accepted Date:

30 August 2019

Please cite this article as: Takkawatakarn T, Suriye K, Jongsomjit B, Panpranot J, Praserthdam P, Influence of Acidity on the Performance of Silica Supported Tungsten Oxide Catalysts Assessed by In Situ and Operando DRIFTS, Catalysis Today (2019), doi: https://doi.org/10.1016/j.cattod.2019.08.062

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Influence of Acidity on the Performance of Silica Supported Tungsten Oxide Catalysts Assessed by

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In Situ and Operando DRIFTS

Thotsatham Takkawatakarna, Kongkiat Suriyeb, Bunjerd Jongsomjita, Joongjai Panpranota, Piyasan Praserthdama,* of Excellence on Catalysis and Catalytic Reaction Engineering, Department of

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Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330,

Chemicals Co., Ltd., 1 Siam Cement Road, Bangsue, Bangkok 10800, Thailand

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Thailand

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*Corresponding author: [email protected]

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Graphical Abstract

Highlights: -

Monitoring the effect of Lewis acid site type II on tungsten oxide catalyst in olefin metathesis Understanding the adsorption ability of 1-butene on Lewis acid site type II Characterization of the Lewis acid site by in-situ and Operando DRIFT Using ammonia coupled with temperature program desorption to specific block on Lewis acid site type II

ABSTRACT

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The types of acidity on WOx/SiO2 catalysts prepared with different tungsten loadings (5 and 9 wt.% W) and pretreatment atmospheres (N2 and H2) were clarified by using the in situ diffuse reflectance infrared Fourier transform spectroscopy with temperature-programmed

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desorption (in situ DRIFTS-TPD) technique. Ammonia (NH3) acted as probe molecules that could block Lewis acid site type II (1620 cm-1) on the studied catalysts. The reactant feed

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containing 2% trans-2-butene or mixed feed of 2% trans-2-butene and 4% ethylene were used to investigate the reaction pathway on the WOx/SiO2 catalysts. Combining the Operando

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DRIFTS with gas chromatography-flame ionization detector (Operando DRIFTS-GC-FID) technique and in situ DRIFTS-TPD led to more deeply understanding of the correlation of acid

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sites and the primary cross metathesis reaction as the main reaction with isomerization of trans2-butene as the side reaction. The adsorption energy of pulse chemisorption of 1-butene on

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catalyst by in situ DSC, confirmed the presence of the Lewis acid site type II related to the

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opportunity of 1-butene adsorption. The results confirmed that the Lewis acid site at 1620 cm1

was crucial for the secondary metathesis reaction of 1-butene.

Keywords: Magnesium oxide, Isomerization, Secondary metathesis, 2-butene, 1-butene

1. INTRODUCTION Supported transition metal catalysts including Mo [1-4], Re [5-7] and W [8-13] have been investigated by a number of researchers in propylene production by cross metathesis of ethylene and 2-butene. It is accepted that WO3/SiO2 catalysts is one of the most successful catalysts used in metathesis reaction for propylene production because of their potential applications in industry due to the low sensitivity to trace amounts of impurities in feed stream [14, 15]. It is well known that the formation of the metal carbene active species is formed by

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the reaction between an olefin and well dispersed tungsten oxide species [13, 16, 17]. The mechanism of metathesis reaction starts from the formation of tungsten-carbene intermediates which requires acidity available from Lewis site-alkene complex located on the transition metal

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ions [17]. Chaemchuen et al. [11] suggested that Lewis acidity may additionally contribute to the formation of by-products such as 1-butene and cis-2-butene from isomerization side

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reactions.

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As mentioned above, acidity plays an important role on the performance of WOx/SiO2 catalysts. Early reports described the effects of both Lewis and Brønsted acid sites on the

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catalyst performances. There have also been continuous efforts to develop acidity characterization technique in order to be able to thoroughly classify the type of acidity such as

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using IR that can detect the acidity on catalyst surface by adsorption of some probe molecules. Ammonia as the probe molecule is another technique that can be used to classify the type of

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acid site. Based on the IR spectrum of adsorbed ammonia, we can indicate and classify the acidity into four groups including both types of Lewis acid sites and Brønsted acid sites [18, 19]. With ammonia IR technique, better understanding of the correlation of types of acidity and structure of WOx/SiO2 can be achieved. Operando diffuse reflectance infrared Fourier transform spectroscopy (Operando DRIFTS) is an analytical methodology that is used to characterize and measure the catalytic

activity under ongoing reaction [20-22]. This technique has been used to observe the surface reaction on the catalyst and identify the species of hydrocarbon that adsorbed and reacted on the WOx/SiO2 catalysts. Combining Operando DRIFTS and in situ DRIFTS contributed to demonstrate the correlation of acidity to reaction more thoroughly. The role of acidity is still unclear for the metathesis reaction on supported WO3 catalysts [23-25]. In the present study, the role of acidity and the type of acidity that affected metathesis and isomerization activity on the WOx/SiO2 catalysts and the WOx/SiO2 that have

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been reduced with H2 were studied by using the in situ DRIFTS with temperature-programmed desorption of ammonia as the probe molecule and the Operando DRIFTS-GC-FID.

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2. EXPERIMENTAL

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2.1 Catalyst Preparation

WOx/SiO2 catalysts with 5 and 9 wt.% of tungsten loadings were prepared by the

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incipient wetness impregnation of amorphous silica gel (Davisil Grade 646, supplied by Aldrich) with an aqueous solution containing the desired amount of ammonium metatungstate

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hydrate ((NH4)6H2W12O40·xH2O, 99.9%, supplied by Aldrich) as the tungsten precursor. The impregnated catalyst was dried for 2 h in ambient air and subsequently in an oven at 110oC for

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24 h, following by calcination in air at temperature greater than 570oC. For the catalyst pretreatment step, WOx/SiO2 catalysts were pretreated at 500oC for 1 h in nitrogen under

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atmospheric pressure. In case for the reduction of WOx/SiO2 catalysts, they were reduced with hydrogen at 500oC for 1 h. There samples are denoted as 5WOx/SiO2 and 9WOx/SiO2 for nonreduced catalysts and RD-5WOx/SiO2 and RD-9WOx/SiO2 for the reduced WOx/SiO2 catalysts. 2.2 Catalyst Characterization

Operando DRIFT coupled with GC-FID experiments; the Operando DRIFT-GCFID (diffuse reflectance infrared Fourier transform spectroscopy-gas chromatography with flame ionization detector) was used to monitor the surface reaction and reaction activity. The Praying MantisTM High Temperature Reaction Chambers, diffuse reflection accessory from Harrick Scientific Product Inc., with two Zinc Selenide (ZnSe) windows and one SiO2 observation window on the chamber were used in conjunction with FT-IR from Bruker (vertex70 spectrometer with a mercury cadmium telluride (MCT) detector kept at -196oC by liquid N2

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during measurement) to measure the reaction surface under controlled pressure and high temperature. The procedure of catalytic reaction testing was carried out under the controlled condition in DRIFTS cell. Firstly, the catalyst was pretreated at 500oC for 1 h under nitrogen

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atmosphere, and then cooled down to 50oC. Subsequently, probe molecule [ammonia (NH3)] was then introduced through to the cell until equilibrium followed by heating up to 500oC to

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desorb the ammonia for 30 min under nitrogen atmosphere. Next, the temperature was cooled

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to 450oC for starting the reaction testing. The reactant gas was fed to the DRIFTS cell to test the reaction. During the reaction testing, the IR spectrum was recorded every 2 min to observe the surface reaction. All IR spectra of the adsorbed species were obtained by subtracting the

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background spectrum using the OPUS software package (OPUS 7.5, Bruker Optik GmbH

GC-FID.

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2014). At the same time, the composition of the product and feed streams were analyzed by

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In situ DRIFTS experiment was performed on the sample holder cup with the probe ammonia molecule can flow through the sample in an in situ DRIFTS cell supplied by Bruker and Harrick Scientific Product Inc. as same as operando DRIFTS equipment, but it used the dome with two KBr windows and one SiO2 observation window on the chamber instead. Prior to measurement, the catalyst was preheated at 500oC for 1 h in N2 (10 ml/min-1) with a heating rate of 10oC/min under atmospheric pressure. After that, the sample was cooled down to 50oC

and IR spectrum was record as the background spectrum. Subsequently, the ammonia probe molecule (NH3) was introduced through the sample until equilibrium adsorption. After purging the physisorbed ammonia by N2 flow for 30 min, the FTIR spectra of adsorbed species on catalysts were collected simultaneously. The temperature-programmed mode was started with the ramp rate of 10oC/min and IR spectrum was recorded from 50oC to 500oC at intervals of 50oC. In situ difference scanning calorimetry (in situ DSC) was used to characterize the

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adsorption energy of olefin gas that is adsorbed on the catalysts. The 200 mg of WOx/SiO2 catalyst was packed in gas circulation cell and put into the furnace chamber in sample cell holder of C-80 Calvet Calorimetry, SETARAM Instrumentation. The olefin gas as the probe

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molecule of 25 cc (cubic centimeter) was fed into the reference cell and sample cell by nitrogen carrier (20 standard cubic centimeter per minutes; sccm) at 50oC at the same time. The total

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heat evolved was measured by 3D Calvet sensor with high sensitivity at 30 µW/mW and

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resolution of 0.10 µW. The total energy was converted by CALISTO software to analyze the energy of adsorption for the catalyst.

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Temperature–Programmed Reduction (TPR) of each sample under H2 was performed on a Micromeritics Instrument (Chemisorb 2750) apparatus. Prior to measurement,

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about 0.1 g of catalyst was preheated at 500oC for 1 h in Ar (25 ml/min-1) with a heating rate of 10oC/min and cooled down to 50oC. Then, 10% H2/Ar flow (15 ml/min-1) was introduced

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through the cell, while the catalyst was heated with a heating rate of 10oC/min from 50oC to a final temperature of 800oC and hold at this temperature for 30 min. The H2 consumption was measured by on-line thermal conductivity detector (TCD) equipped with water trap. Raman spectra of the samples were collected using a HORIBA spectrometer (MicroHR Raman) equipped with a 532 nm Nb/YAG laser of 100 mW power with the sample at room

temperature. The signal was detected by open electrode CCD detector after passage dispersion by a grating (1800 groove/mm) with edge filter to remove the backscattered light. The spectra were collected after pretreatment at 500oC in N2 for 1 h. Powder X-ray diffraction (XRD); diffractograms of all catalysts were obtained with a Bruker (D8 Advance) instrument to determine the bulk phase of the catalysts. The CuK radiation with Ni filter was used as the X-ray source to measure in the 2Ɵrange of 5-80 degrees

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with a resolution of 0.004o. X-ray photoelectron spectroscopy (XPS) was performed on an AMICUS/ESCA 3400 spectrometer to determine the composition and chemical state. The valence-band and core level

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spectra of WOx compounds were measured in an ion-pumped chamber (5x10-7 Pa).

3.1 Fundamental Characterization

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3. RESULTS AND DISCUSSION

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The XRD analysis of the 5WOx/SiO2 and 9WOx/SiO2 is shown in Figure 1A. The diffraction characteristic peaks of WO3 crystal species were systematically detected at 23.12o,

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23.60o, 24.38o, and 51.22o 2Ɵ[10, 26, 27]. The intensity of WO3 crystal peak increased with increasing tungsten loading.

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H2-TPR was used to determine the reducibility of tungsten oxide species on SiO2, 5WOx/SiO2, and 9WOx/SiO2. As can be seen in Figure 1B, the 5WOx/SiO2, and 9WOx/SiO2

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exhibited three peaks with maxima at 538oC, 765oC, and 800oC. This is a classical result for tungsten oxide on silica support [28, 29]. The H2-TPR results are in good agreement with the XRD results that increased tungsten loading resulted in increasing tungsten crystalline species and as a consequence, the H2 consumption on 9WOx/SiO2 was higher than that on 5WOx/SiO2.

The structure of tungsten oxide species located on the supported catalysts was studied by Raman spectroscopy (Figure 1C). The characteristics of Si-O-Si stretching vibration were found at 498, 602, and 1060 cm-1[30]. The Raman bands at 263-275 and 707-720 cm-1 were attributed to the deformation mode of W-O-W and bending mode of W-O, respectively [26]. The narrow peak centered at 807 cm-1 was assigned to the symmetric stretching mode of W-O. Additionally, the broad band at 970 cm-1 was assigned to the O=W=O band of the isolate tetrahedral tungsten oxide species [31], which was reported as the active species for metathesis

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[32]. Increasing of tungsten loading affected to a sharp increase of intensities at 807 and 710 cm-1 bands (crystalline species), while the 970 cm-1 band (isolated tetrahedral species) was mainly unchanged. This is correlated well with the XRD results that the WO3 crystal was more

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pronounced at higher tungsten loading (9WOx/SiO2).

To analyze the oxidation state of surface tungsten on WOx/SiO2 catalyst, XPS study

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was carried out (Figure 2). The XPS spectra showing the binding energies (BE) (W4f7/2) at

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36.5 eV can be assigned to the presence of W6+ phase and at 35.4 eV can be assigned to the W5+ phase [33-36]. The deconvoluted peaks were considered with the peak intensity ratio of

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W 4f5/2 and W 4f7/2 (I(f7/2):I(f5/2)) of 4:3, the full width at half-maximum (FWHM) of 1.5 eV, the peak intensity ratio of W 4f5/2 and W 4f7/2 of 2.1 eV, and the binding energy difference

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between W5+ 4f7/2 and W6+ 4f7/2 peak about 0.9-1.1 eV. In the present work, the studied WOx/SiO2 catalysts with non-reduced and reduced forms (with H2) were systematically studied

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to observe the oxidation state of surface tungsten. The surface compositions of WOx/SiO2 catalysts were determined by XPS and the results are shown in Table 1. For the non-reduced samples, the W6+ phase of tungsten was found on both 5WOx/SiO2 and 9WOx/SiO2. However, both 5WOx/SiO2 and 9WOx/SiO2 exhibited lower amount of W5+ phase. For the H2 reduced WOx/SiO2 catalysts, the XPS spectra showed that some part of W6+ phase was transformed to

W5+ and W4+ phase [36]. Interestingly, the amount of W5+ phase was higher on RD-5WOx/SiO2

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than that on RD-9WOx/SiO2 as shown in Table 1.

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Figure 1. (A) XRD patterns of the studied catalysts, (B) H2-TPR profiles of studied catalysts,

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(C) Raman spectra of the studied catalysts.

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3.2 NH3 Adsorption and NH3-TPD The DRIFTS spectra of NH3 adsorption on the surface catalyst are shown in Figure 3.

The quantity of acid sites can be divided into two groups. Firstly, three strong positive adsorption bands of N-H stretching vibrations were found at 3396, 3268, and 3167 cm-1 [37]. In addition, the adsorption bands at 3396 and 3268 cm-1 have been assigned to coordinated

ammonium ions (NH4+ groups), while the one at 3167 cm-1 has been attributed to coordinated ammonia molecules (Figure 3A). Secondly, four relatively weaker positive adsorption bands of NH4+ bending vibrations were found at 1680, 1620, 1470, and 1280 cm-1 (Figure 3B) [19, 38-40]. The former assigned the 1470 cm-1 and 1680 cm-1 to bending vibration of NH4+ species resulting from NH3 species that were adsorbed on Si-OH and W-OH groups at the position of Brønsted acid site, while the 1280 cm-1 and 1620 cm-1 were attributed to N-H bands bending vibration of molecularly adsorbed of NH3 on Lewis acid sites [18]. The negative bands at 3688

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and 3609 cm-1 represent the O-H stretching bands of silanol and structural hydroxyl groups [41].

Considering the DRIFTS spectra of the WOx/SiO2 catalyst comparing with the SiO2

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support, Brønsted acid sites at 1470 cm-1 and Lewis acid site at 1280 cm-1 (Type I) and 1620 cm-1 (Type II) were generated due to the introduction of tungsten oxide. Figure 3B shows that

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an increase of tungsten metal loading from 5 to 9 wt.% resulted in a remarkable increase of the

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Lewis acid sites, which agreed with the literatures [27, 42, 43] and the Brønsted acid site at 1680 cm-1 apparently decreased. The acid concentration of Lewis and Brønsted acid sites can

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be determined by the band area at the position mentioned above, which can be seen in Table 2. Interestingly, with reduction by H2 of the WOx/SiO2 catalysts, it appeared to affect both

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Lewis acid site type I and II, but this was in the opposite way. While Lewis acid site type I increased, Lewis acid site type II dramatically decreased. However, the Brønsted acid site

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remained almost unchanged. According to the chemical surface of tungsten from the XPS results, the amount of the W5+ increased, while the W6+ decreased with reduction by H2. By correlating the results of FT‐IR‐NH3 with the XPS results, it should be pointed out that the generating of W5+ onto the WOx/SiO2 catalyst resulted in an increasing of the Lewis acid site type I.

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Figure 2. (A, B) Oxidation state of tungsten oxide on yWOx/SiO2 from XPS spectra and (C,

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D) for the yWOx/SiO2 that was reduced by hydrogen.

Figure 3. In situ DRIFTS spectra of 5WOx/SiO2 and 9WOx/SiO2 in 3050-3750 and 1050-1750 cm-1 as shown in (A) and (B), respectively Temperature-programmed desorption results indicated that the intensity of both Lewis and Brønsted acid decreased with increasing the temperature (Figure 4). The intensities of Lewis acid site Type I (1280 cm-1) and Brønsted acid site (1680 and 1450 cm-1) decreased at 100-250oC and almost disappeared at 400oC, while the Lewis acid site Type II (1615 cm-1) was still detected on 5WOx/SiO2 and 9WOx/SiO2 catalysts at 500oC. According to the desorption

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results at 500oC, it is advantage to adapt this condition for studying the effect of blocking some

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acid sites to observe the reaction pathway and catalytic activity on the WOx/SiO2 catalyst.

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Figure 4. DRIFTS spectra of NH3 blocking at the position of Lewis acid site (1620 cm-1) on 5WOx/SiO2 and 9WOx/SiO2 after de-chemisorption at 500oC.

3.3 Catalytic performances of WOx/SiO2 The distribution of products obtained from different feeds (only 2% trans-2-butene balanced in N2 and mixed feed between 4% ethylene and 2% trans-2-butene balanced in N2) over the WOx/SiO2 catalysts is shown in Table 3. The main reaction and side reactions

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involved in the propylene production from different feeds are shown below in equations (1) to (5). Isomerization reaction

cis-2-butene

Trans -2-butene

1-butene

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Trans-2-butene + ethylene

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Primary cross metathesis reaction

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Trans-2-butene

(1) (2)

2 (propylene)

(3)

ethylene + C6=

(4)

propylene + C5=

(5)

Secondary metathesis reaction

1-Butene + 1-butene

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Self metathesis

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Cross metathesis 1-Butene + 2-butene

The catalytic performance of 2-butene was measured over the non-reduced catalysts to

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observe and investigate the reaction pathways from isomerization to second metathesis by using 2% trans-2-butene feed. The reaction conditions were operated at 450oC, 2.1 h-1 of WHSV at atmospheric pressure (0.1 MPa). The reaction test was stopped at 6 h of time on stream to consider the reaction pathways. As can be seen in Table 3, the main reaction that occurred over the WOx/SiO2 catalyst was isomerization of trans-2-butene to 1-butene, and then following with secondary metathesis of 1-butene. However, the secondary cross-metathesis of

1-butene and 2-butene to propylene was not predominated. As mentioned above, desorption of ammonia at 500oC also suggested that Lewis acid site type I and type II showed different behaviors. Thus, this condition is useful for studying the relation between acidity and activity. Prior to the reaction, NH3 as a probe molecule was introduced through the catalyst at 50oC for 30 min after the catalyst pretreatment step, and then heating up to 500oC under N2 atmosphere to desorb part of NH3. After that the temperature was cooled down to 450oC and the feed stream was fed subsequently. According to the FT-IR-NH3, Lewis acid site type II was blocked by

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NH3 and still presented at the surface of the WOx/SiO2 catalyst in contrast with Lewis acid site type I. Interestingly, blocking the Lewis acid site type II with ammonia affected both conversion and selectivity. Conversion of 2-butene over 5WOx/SiO2 and 9WOx/SiO2 catalysts

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essentially increased. Moreover, the secondary cross-metathesis of 1-butene and 2-butene was more pronounced and an increase of propylene was clearly observed. However, ethylene that

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was produced from 1-butene secondary self-metathesis slightly decreased, suggesting that its

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active site was not functioned. From the results, it is likely to postulate that 1-butene is preferentially adsorbed on the Lewis acid site type II than 2-butene. Thus, blocking the Lewis acid site type II can obstruct 1-butene adsorption resulting in the less pronounced secondary

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self-metathesis of 1-butene.

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The catalytic results of using 4% ethylene mixed with 2% trans-2-butene balanced in N2 feed under the same conditions of the 5WOx/SiO2 and 9WOx/SiO2 catalysts are shown in

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Table 3. When ethylene participated in the reaction, the propylene selectivity increased due to the fact that ethylene and trans-2-butene can directly react through cross metathesis reaction. In addition, the isomerization reaction of trans-2-butene to 1-butene and cis-2-butene was remarkably pronounced resulting in a decrease of the secondary metathesis reaction that can be observed from the less quantity of C5+ in product distribution. Another reason was that ethylene can be adsorbed on tungsten active sites, hence the opportunity of product such as 1-

butene from isomerization to re-adsorb on the tungsten active site decreased. Thus, the secondary metathesis on WOx/SiO2 catalyst in mixed feed was less predominated than using only trans-2-butene feed. Again, the NH3-blocking on Lewis acid site type II was also studied on the surface of WOx/SiO2 catalyst for the system of mixed feed between ethylene and trans-2-butene. It was found that similar results as seen from the use of only trans-2-butene feed were obtained (see Table 3). However, there is an interesting point from the Operando DRIFTS spectra of

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WOx/SiO2 that have been blocked by ammonia (Figures 6 and 7). The bands at 870-950 cm-1 due to the deformation vibration of CH2 on the ethylene indicated that the ethylene adsorption on the catalyst surface significantly decreased compared to the WOx/SiO2 catalyst [44]. The

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reactant adsorption on the WOx/SiO2 in system of in situ IR was used to confirm the active bands around 870-950 cm-1 as shown in Figure 5. On the other hand, the increase of intensities

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at the wavenumber ca. 3030-3235 and 1500-1480 cm-1 can be assigned to –CH2NH3+

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asymmetric NH3 stretching vibration and symmetric NH3+ deformation vibration, respectively [45-47]. From the DRIFT spectra results, it was postulated that ethylene was not adsorbed on

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the Lewis acid site II, but formed the interaction with NH3+ that adsorbed and blocked at the same position of Lewis acid site II. Considering the reaction pathways, it was possible to

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increase the yield product of C5+ because ethylene was not competitively adsorbed with trans2-butene. Moreover, ethylene interacted with ammonia on Lewis acid site II that affected to

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the decrease in the density of ethylene on the other tungsten active site. Finally, 1-butene from isomerization should have the opportunity to re-adsorb on the tungsten active site and generated C5+ product from the secondary metathesis. Figure 6 shows the Operando DRIFT spectra results of the 5WOx/SiO2 and 9WOx/SiO2, the bands of –CH2NH3+ occurring from the ammonia interacted with ethylene can be observed thorough the reaction, indicating that ammonia that was blocked at the Lewis acid site II did not desorb, but it was still adsorbed and

covered to block the function of Lewis acid site II along the reaction. In addition, the major quantities of ethylene adsorbed on Lewis acid site type II affect to increase the opportunity of trans-2-butene being adsorbed on another active site to form the metal carbene and to undergo the cross metathesis or isomerization reaction. Considering the color band around 2800-3350 cm-1 in Figures 6 and 7, it revealed the high quantities of hydrocarbon that were adsorbed on the catalyst surface with the different induction period [48]. In case of ammonia blocked on Lewis acid site type II, it can be observed the faster induction period because when major

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quantities of ethylene reacted with ammonia, it affected to increase the possibility that trans-2butene can be adsorbed on another active site and formed metal carbene faster than ethylene

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[49, 50].

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The conversion and product selectivity results of the H2-reduced catalysts are shown in Table 3. It was found that the conversion and the product selectivity were changed compared

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to the reaction that occurred on the non-reduced catalyst system. Reduction of the WOx/SiO2 catalyst with H2 in the pretreatment step at 500oC for 30 min resulted in decreased chemical

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state of tungsten on the surface of catalysts. From the XPS results, after reduction with H2, the W5+ increased around 24.5 and 39.3% on the surface of 5WOx/SiO2 and 9WOx/SiO2,

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respectively. Moreover, the results of NH3-IR showed the increase of Lewis acid site type I around 42 and 26% on 5WOx/SiO2 and 9WOx/SiO2, respectively. From the XPS and NH3-IR

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results, the correlation between chemical state of tungsten (W5+ and W6+) and Lewis acid site (type I and II) can be summarized. It is possible that W6+ could form the tungsten oxide species that generated the Lewis acid site type II, whereas W5+ was increased after H2-reduction. This possibly formed the tungsten oxide species that generated the Lewis acid site type I.

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Figure 5. DRIFTS spectrum of NH3 blocking on 9WOx/SiO2 after de-chemisorption at 500oC

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followed by adsorption of ethylene and trans-2-butene at 50oC.

Absorbance (a.u.)

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Absorbance (a.u.)

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Absorbance (a.u.)

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Figure 6. (A, C, E) Phase- and (B, D, F) time-domain FTIR spectra during reaction at 450oC; (A, B) 9W/SiO2, (C, D) 9W/SiO2 (NH3-blocking) and (E, F) RD-9W/SiO2 (NH3-blocking).

Absorbance (a.u.)

f Absorbance (a.u.)

Pr

e-

pr

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Absorbance (a.u.)

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The Operando DRIFT spectra results showed that after the sites were blocked with ammonia, ethylene can be still adsorbed on the surface of catalyst. It indicated that with decreased Lewis acid site type II, it resulted in an increase of the opportunity of ethylene adsorbed on the Lewis acid site type I that can be enhanced after H2-reduction. Figure 7 shows the intensities of ethylene at the bands of 870-950 cm-1, which were increased when the catalyst was reduced with H2. The results were related to the quantities of Lewis acid site type I that increased around 42 and 26% on 5WOx/SiO2 and 9WOx/SiO2, respectively. Considering the

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conversion and product selectivity (see Table 3), the selectivity of C5+ increased when Lewis acid site type II was blocked. In addition, when Lewis acid site type II decreased, it led to increase the quantities of ethylene remained in the system. The presence of the large amount

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of ethylene in the system inhibited the chance of trans-2-butene being adsorbed on another active site. As the result, it increases the period to form the metal carbene that can be observed

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from color band around 2800-3350 cm-1 in Figure 7.

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Additionally, it is considered in case of 5WOx/SiO2 and 9WOx/SiO2 pretreated in nitrogen. After ammonia adsorption, when increased the temperature-programmed desorption

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to 570oC, complete ammonia desorption on the WOx/SiO2 catalysts was observed. It can confirm that the catalyst after removal of desorbed ammonia from tungsten active sites can

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maintain the similar catalytic activity as seen from fresh WOx/SiO2. Thus, the activity of 5WOx/SiO2 and 9WOx/SiO2 having completely desorbed ammonia was not significantly

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changed as shown in Table 3. It is worth noting that after complete desorption of ammonia from WOx/SiO2, the performance of active site including the acid sites on position of silanol group on silica support did not significantly change. Pulse chemisorption of 1-butene on catalyst at 50oC that was packed in system of in situ DSC, can determine the adsorption energy [51] of the 1-butene that was adsorbed on the different pretreatment along with system of non-blocking and blocking with ammonia. Table

4 shows the comparison between 9WOx/SiO2 in case of non-blocking and blocking with ammonia indicating the different adsorption energy at 0.294 and 0.240 J·(g catalyst)-1, respectively. From further results, the correlation of adsorption ability of 1-butene on Lewis acid site type 2 is corresponding with those from in situ DSC. It can be observed that when Lewis acid site type 2 was blocked with ammonia, it led to a decrease in the quantity of 1butene adsorption on WOx/SiO2 catalysts. In addition, the result of 9WOx/SiO2-RD in H2 pretreatment also revealed the same correlation that quantity of Lewis acid site type 2

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conformed to the adsorption energy. The WOx/SiO2-RD exhibited the lower adsorption energy of 1-butene at 0.187 J·(g catalyst)-1 than that of WOx/SiO2. In part of ammonia blocking on WOx/SiO2-RD, it had lower adsorption energy of 1-butene at 0.147 J·(g catalyst)-1 than the

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system of non-blocking with ammonia. These results assist to confirm that when Lewis acid site type 2 was blocked with ammonia, it decreased the quantity of 1-butene adsorption on

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WOx/SiO2. On the other hand, Lewis acid site type II prefers 1-butene to adsorb on them.

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According to characterization results as mentioned above, a correlation between Lewis acid site and the reaction pathways on the yWOx/SiO2 and RD-yWOx/SiO2 was proposed.

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Considering the ammonia adsorption and desorption at 500oC, this led to specifically block the Lewis acid site type II that prefers the 1-butene adsorption. Then, ethylene that was fed

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simultaneously with trans-2-butene was adsorbed on NH3+ to generate the species of – CH2NH3+ on the position of Lewis acid site type II. In the same time, cross metathesis reaction

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of trans-2-butene and ethylene produced the propylene and isomerization of trans-2-butene produced the cis-2-butene and 1-butene occurred in the system. So, Lewis acid site type II that have been blocked with ammonia led to decrease the opportunity of 1-butene adsorption on them and eventually decreased the possibility of secondary self-metathesis of 1-butene on WOx/SiO2 catalysts as described and shown in Scheme 1. In case of RD-yWOx/SiO2 catalysts, when Lewis acid site type I increased, while Lewis acid site type II decreased, it resulted in an

increase of the opportunity of ethylene that remained in the system to react with trans-2-butene on another active site and underwent the cross-metathesis reaction to produce the propylene

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product.

Scheme 1. Proposed mechanism of correlation between Lewis acid site type II and the reaction

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4. CONCUSIONS

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pathways on the yWOx/SiO2 and RD-yWOx/SiO2

The in situ DRIFTS-TPD of NH3 was adapted to study the function of acid site that

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affects the performance of WO3/SiO2 catalyst in metathesis reaction. De-chemisorption of ammonia at 500oC was appropriately chosen as the probe molecule that can specific blocking the Lewis acid site type II (1620 cm-1). The Operando DRIFTS-GC-FID, in situ DSC and characterization results, led to achieve the correlation of Lewis acid site type II and adsorption ability of 1-butene on the WOx/SiO2 catalyst. Furthermore, combining the two methods of in situ DRIFTS-TPD and Operando DRIFTS-GC-FID was a key step to deeply understand the

correlation of acid sites to secondary metathesis reaction of 1-butene on WOx/SiO2 and RDWOx/SiO2 with ammonia blocking and non-blocking system. Considering the oxidation state and type of Lewis acidity, it can postulate that the appearance of Lewis acid site type II on WOx/SiO2 catalyst was the W6+ with fully oxide. Finally, this knowledge can be applied to design the tungsten oxide catalysts to prevent the generation of the Lewis acid site type II that affects the high secondary metathesis reaction that is the cause for generating the coke deposition on catalyst.

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ASSOCIATED CONTENT -

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ABBREVIATIONS

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Piyasan Praserthdam; 0000-0001-8021-2115

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ORCID

DRIFTS diffuse reflectance infrared Fourier transform spectroscopy; XRD X-ray powder

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diffraction; XPS X-ray photoelectron spectroscopy; TPD temperature program desorption;

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GC-FID gas chromatograph-flame ionization detector; DSC differential scanning calorimetry

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ACKNOWLEMENTS

We would to thank the SCG Chemical Co., Ltd., Thailand and The Thailand Research Fund (TRF) and Research and Researcher for Industry (RRi) for financial support.

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39.

Table 1. Surface characterization of WOx/SiO2 and RD-WOx/SiO2 catalysts by X-ray photoelectron spectroscopy (XPS)

9WOx/SiO2 9WOx/SiO2 (NH3-Blocked) RD-9WOx/SiO2 RD-9WOx/SiO2 (NH3Blocked)

1-Butene

9WOx/SiO2 9WOx/SiO2 (NH3-Blocked) RD-9WOx/SiO2 RD-9WOx/SiO2 (NH3Blocked)

Trans-2Butene

Exothermic (J·g catalyst1)

Endothermic (J·g catalyst1)

Adsorption Energy (J·g catalyst-1)

0.608

0.314

0.294

0.620

0.380

0.240

0.942

0.755

0.187

1.195

1.048

0.147

0.269

0.243

0.026

0.300

0.245

0.055

0.317

0.261

0.056

0.235

0.095

0.330

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na

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b

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Reactant

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Catalysts

Table 2. The amount of Brønsted and Lewis acid sites over the WOx/SiO2 catalyst determined from the in-situ DRFITS of NH3 adsorption at 50oC after pretreatment with different gas (N2 and H2).

Bronsted acid (a.u.)

Lewis acid (a.u.)

Bronsted acid (a.u.)

(1680 cm-1)

(1620 cm-1); Type II

(1470 cm-1)

SiO2

0.16

0.30

0.02

5WOx/SiO2

0.07

0.41

0.70

9WOx/SiO2

0.02

0.72

1.54

RD-5WOx/SiO2

0.06 (-5 %)a

0.21 (-96 %)a

0.68 (-3 %)a

0.35 (+42 %)a

RD-9WOx/SiO2

0.02 (-15 %)b

0.47 (-53 %)b

1.47 (-5 %)b

0.53 (+26 %)b

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0.04

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Catalysts

The percentage of acid site compared with 5WOx/SiO2

b

The percentage of acid site compared with 9WOx/SiO2

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a

Lewis acid (a.u.) (1280 cm-1); Type I

0.20 0.39

Table 3 Trans-2-butene conversion and product selectivity of the WOx/SiO2 and RDWOx/SiO2 under atmospheric pressure.

9WOx/SiO2 5WOx/SiO2 (NH3Blocked) 9WOx/SiO2 (NH3Blocked)

2%Tran s-2Butene balance in N2

14.6

27.2

35.1

23.8

0.9

1.3

25.4

30.3

26.7

38.8

0.7

1.9

27.7

0.6

2.1

32.0

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17. 5 13. 4 15. 3 11. 3

12.9

28.6

38.8

20.2

31.9

33.2

26.8

29.9

23.3

8.6 3 5.7 3 5.4 0 3.2 0

21.9

50.8

13.5

6.4

14.6

22.3

32.1

7.3

16.9

38.9

29.6

3.1

27.0

29.5

24.0

9.4 2

32.3

3.1

21.1

49.4

14.2

5.9 0

29.8

2.1

29.9

34.8

19.5

46.6

3.0

39.4

38.3

12.1

30.4

3.0

29.1

39.1

18.0

31.1

6.1

36.5

41.8

10.2

30.4

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RD-5WOx/SiO2 (NH3-Blocked) RD-9WOx/SiO2 (NH3-Blocked)

3.0

C6= , C6+ =

3.2

37.7

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26.4

-

na

RD-9WOx/SiO2

2Penten e

1.6

9WOx/SiO2

RD-5WOx/SiO2

Cis-2Buten e

1.1

26.8

2% Trans2Butene and 4% Ethylen e balance in N2

1Buten e

36.3

5WOx/SiO2

5WOx/SiO2 (NH3Blocked) 9WOx/SiO2 (NH3Blocked) 5WOx/SiO2 (NH3Completely Desorption) 9WOx/SiO2 (NH3Completely Desorption)

Propyle ne

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5WOx/SiO2

Ethyle ne

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Feed

Selectivity (%)

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Catalysts

Trans-2Butene conversi on (%)

9.7 3 6.0 3 9.0 0 5.1 0

Table 4 Adsorption energy of 1-butene and trans-2-butene of the 9WOx/SiO2 and 9RDWOx/SiO2 with blocking and non-blocking by ammonia.

9WOx/SiO2 9WOx/SiO2 (NH3-Blocked) RD-9WOx/SiO2 RD-9WOx/SiO2 (NH3Blocked)

1-Butene

9WOx/SiO2 9WOx/SiO2 (NH3-Blocked)

Endothermic (J·g catalyst1)

Adsorption Energy (J·g catalyst-1)

0.608

0.314

0.294

0.620

0.380

0.240

0.942

0.755

0.187

1.195

1.048

0.147

0.269

0.243

0.026

0.300

0.245

0.055

0.317

0.261

0.056

0.235

0.095

0.330

Jo

ur

na

lP

re

RD-9WOx/SiO2 RD-9WOx/SiO2 (NH3Blocked)

Trans-2Butene

Exothermic (J·g catalyst1)

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Reactant

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Catalysts