Al2O3 Catalysts for Propane

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On the Nature of Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation Gang Liu, Zhi-Jian Zhao, Tengfang Wu, Liang Zeng, and Jinlong Gong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00893 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on June 29, 2016

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On the Nature of Active Sites of VOx/Al2O3 Catalysts for Propane Dehydrogenation Gang Liu1, Zhi-Jian Zhao1, Tengfang Wu, Liang Zeng, and Jinlong Gong*

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

[1] These authors contributed equally to this work. [*] Corresponding author: [email protected]

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ABSTRACT: Supported VOx catalysts are promising for propane dehydrogenation (PDH) due to relatively superior activity and stable performance upon regeneration. However, the nature of active sites and reaction mechanism during PDH over VOx based catalysts remains elusive. This paper describes the understanding of active species through attaining various fraction of V5+/V4+/V3+ ions by adjusting surface vanadium density on an alumina support. The results presented a close relationship between Ea/TOF and the fraction of V3+ ion, indicating that the V3+ was more active for PDH. In situ DRIFTS showed the same strong adsorbed species during both propane dehydrogenation and propylene hydrogenation. The results indicated that such intermediate may correspond with V species containing C=C double bond i.e. V-C3H5, and a reaction mechanism was proposed accordingly. KEYWORDS: propane dehydrogenation (PDH), vanadium oxide, oxidation states, DRIFTS, reaction mechanism

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1. Introduction

Propylene serves as a key basic feedstock for the manufacture of various products such as polypropylene, propylene oxide, and acrylonitrile. Due to the growing demand for propylene and the vast amount of propane in shale gas deposits, propane dehydrogenation (PDH) becomes increasingly important as a propylene on-purpose production technology.1 Supported CrOx and Pt catalysts are two major commercial catalytic systems for PDH, but their activities decay gradually after successive regenerations.2-3 Moreover, the high cost of Pt as well as serious pollution issues caused by Cr restrict the further development of the PDH industry. Alternatively, supported vanadium oxide catalysts have shown promise for PDH.3-4 Open literature counts numerous studies of vanadium-based catalysts, although most of them focused on the oxidative dehydrogenation of ethane, propane or butane.5-9 However, low olefin yield is the major issue during oxidative dehydrogenation process because alkene can be easily over-oxidized to COx. Meanwhile, the operating safety as well as catalyst lifetime are also concerned. High olefin selectivity can be achieved via economical direct dehydrogenation process, and vanadium oxides can be used as the catalyst. Particularly, vanadium oxides supported on

Al2O3

exhibited

superior

performance

for

non-oxidative

butane

dehydrogenation.10-15 Additionally, Sokolov et al. compared the catalytic performance of PDH over the VOx, CrOx and PtSn catalysts, and concluded that VOx-based catalysts had superior propylene selectivity and stability after several regeneration cycles.3 3


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The mechanism of oxidative dehydrogenation of propane catalyzed by VOx has been extensively studied, which was proposed to be related with V5+/V4+ or V5+/V3+ redox cycles.7,

16

However, limited amount of studies deal with propane direct

dehydrogenation mechanism over the promising family of vanadium-based catalysts. Prior to the dehydrogenation reaction, the pentavalent vanadium is mainly present in the catalysts, while the catalyst would be reduced by hydrocarbons or hydrogen to some extent during the reaction, resulting in the formation of V4+ and V3+.17-19 However, the nature of active vanadium ion during PDH is still elusive. Harlin et al obtained higher initial activity via pretreating vanadium oxide with H2, CO or CH4 during butane dehydrogenation, and V4+ and V3+ ions were reported to be more active.14 Similar situation has been reported for CrOx-based catalysts that only Cr3+ and Cr2+ are active sites for dehydrogenation reaction among a number of identified surface species, including Cr6+, Cr5+, Cr3+ and Cr2+.20-21 To elucidate the origin of vanadium ion, it is crucial to develop tailor-made catalysts, which contain different fractions of V5+/V4+/V3+ ions. Many studies have suggested that three types of VOx species, i.e. monovanadate, polyvanadate, and V2O5 crystallite existed on the surface of Al2O3, generally depending on the ratio of support surface area and metal loading.22-23 Due to the various molecular structure, the vanadium species exhibited different reducibility. It has been proved that V-O-V and V=O bonds were more easily reduced than V-O-support.24 Thus, it is possible that different valence state distribution would be obtained on the VOx/Al2O3 catalysts upon reduction with the same condition. 4


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Detecting the intermediates formed during reaction is crucial to understand reaction mechanism, and in situ infrared spectroscopy (IR) serves as a powerful tool. Various intermediates have been identified during propane dehydrogenation. On Zn/H-MFI catalyst, zinc propyl species generated by dissociative adsorption of propane were detected by IR spectroscopic techniques.25 Meanwhile, the formation of acetates/carboxylates

was

evidenced

during

propane

dehydrogenation

over

CrOx-Al2O3 catalysts.26 The authors proposed that the acetates/carboxylates originated from isopropoxide species, which were formed via the abstraction of hydrogen from propane. Ethyl, grafted on metal ions (Ga3+, Zn2+), zinc hydrides and gallium hydrides were also detected by IR after the adsorption of ethane on Ga2O3 and ZnZSM-5, respectively.27-28 To the best of our knowledge, the IR data of propane dehydrogenation over vanadium-based catalysts is unavailable so far. This paper describes the exploration of the nature of active valence state and mechanistic aspects of PDH over the VOx/Al2O3 catalysts. We obtained different valence state distribution via tuning the surface VOx density on Al2O3, and then PDH activity was tested on the prepared catalysts. The physical-chemical properties of the catalysts were studied by X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), Raman spectroscopy, Transmission Electron Microscopy (TEM), X-ray photoelectron spectra (XPS) and electron spin-resonance spectroscopy (EPR) etc. Catalytic tests were then carried out in order to evaluate the effects of valence state on the PDH performances. Aiming at understanding the mechanism of PDH over VOx/Al2O3 catalysts, propane dehydrogenation and propylene hydrogenation were 5


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investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS).

2. Experiment 2.1 Catalyst preparation All VOx/γ-Al2O3 samples were prepared via incipient wetness impregnation with γ-Al2O3 (Adamas, 99.99%, SBET = 180 m2/g) as the support. The precursor used was NH4VO3 (99.0%, Tianjin Guangfu Technology Development Co. LTD.) dissolving in an aqueous solution of oxalic acid (99.0%, Aladdin Industrial Corporation). The γ-Al2O3 supports were impregnated in the NH4VO3 solution at room temperature for 12 h. After impregnation, the catalysts were dried at 90 °C overnight, and then calcined at 500 °C for 2 h. The prepared catalysts were named xV/Al, where x is the weight ratio of V/γ-Al2O3.

2.2 Characterization XRD measurements were performed with 2θ values between 10 and 90° by using a Rigaku C/mx-2500 diffractometer employing the graphite filtered Cu Kα radiation (λ = 1.5406 Å). Raman spectra was obtained under ambient conditions using a Renishaw inVia reflex Raman spectrometer with 325 nm and 532 nm Ar-ion laser beam. Before measurements, the samples were dried at 80 °C for 2 h. The EPR spectra was taken with a Bruker A300 spectrometer at 130 K. Selected EPR spectra was simulated with the SIMPOW program, which generated a powder spectrum

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calculated to second order. TEM was conducted to characterize the morphology of spent catalysts using a Tecnai G2 F20 transmission electron microscope at 200 KV. TGA (STA449F3 NETZSCH Corp.) was used to investigate the carbon deposition of spent catalysts. The samples were preheated at 80 °C for 0.5 h in N2 (50 mL/min), then were heated to 800 °C at a rate of 10 °C/min in air (100 mL/min). UV-Vis spectra in the range of 200-800 nm was taken on a Shimadzu UV-2550 spectrophotometer, using BaSO4 as a reference. All the samples were dried at 80 °C for 2 h before measurements. H2-TPR experiments were carried out by a Micromeritics AutoChem 2920 apparatus. Prior to the TPR experiments, 100 mg sample was pretreated at 300 °C for 1 h under Ar stream (20 mL/min). After cooling to 50 °C, the H2-TPR was performed in 10 vol% H2/Ar (30 mL/min) with a heating rate of 10 °C/min to 800 °C and the profile was registered with a thermal conductivity detector. XPS measurements were carried out on a PHI 1600 ESCA instrument (PE Company) equipped with an Al Kα X-ray radiation source (hν = 1486.6 eV). Before measurements, all the samples were reduced under a flow of H2 at 600 °C for 1 h. The binding energies were calibrated using the C 1s peak at 284.5 eV as the reference. In situ DRIFTS experiments were carried out on a Thermo Scientific Nicolet IS50 spectrometer, equipped with a Harrick Scientiﬁc DRIFT cell and a mercury−cadmium−telluride (MCT) detector cooled by liquid N2. Two major experiments were conducted with propane and propylene as feed gas respectively. All the samples were pretreated at 550 °C under 10% H2/Ar flow (50 mL/min) for 0.5 h 7


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and cooled to the desired temperature to get a background spectrum, and this spectrum was then subtracted from the sample spectra for each measurement. For propane adsorption, the experiment included following steps: (1) directing 4% C3H8/He to the sample cell at 30 °C and holding for 30 min; (2) increasing the temperature to 300 °C and staying for 60 min; (3) shutting down the C3H8 along with decreasing the temperature to 30 °C and keeping for 30 min. Regarding propylene adsorption, the procedure included following steps: (1) conducting adsorption of propylene at a flow of 4% C3H6/He for 30 min, followed by evacuation for 30 min at 30 °C; (2) subjecting the sample to 10% H2/Ar or He flow (50 mL/min) and heating to 200 °C, and holding for 30 min; (3) evacuating the resulting sample at 30 °C. The IR spectra of spent catalyst was collected after pretreating at 300 °C under He for 30 min, using KBr as background spectrum. For the spectra of chemisorbed formic acid and acetone, 20 mg catalysts were loaded into in-situ cell equipped with ZnSe windows after compression, then were preheated at 550 °C under H2/Ar. After cooling to 50 °C, 6 µL formic acid or acetone was injected and hold for 30 min. The spectrum was collected under vacuum at 50 °C, 200 °C and 300 °C respectively.

2.3 Catalytic tests Catalytic tests were carried out at the atmospheric pressure and 600 °C in a quartz fixed-bed reactor with 8 mm inner diameter and 24 cm length. 0.25 g catalyst with 20-40 mesh size distribution was loaded in the quartz tubular reactor. The catalyst was first heated to 600 °C under H2/N2, and held for 0.5 h, afterward H2/N2 was replaced by PDH reaction mixture of C3H8 (28 vol %) and H2 (28 vol %) in N2 at 8


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a total flow of 25 mL/min. The weight hourly space velocity (WHSV) of propane was 3 h-1. The product gas was analyzed by an online GC equipped with a flame ionization detector (Chromosorb 102 column) and a thermal conductivity detector (Al2O3 Plot column). The conversion of propane and selectivity to propylene were determined from Eq. (1) and Eq. (2) respectively: Con (%) = ([FC3H8]in － [FC3H8]out) / [FC3H8]in.

(1)

Sel (%) = 100 × ni × [Fi]out / (∑ni × [Fi]out).

(2)

Where i represents the hydrocarbon products in the effluent gas, ni is the number of carbon atoms of the component i, and Fi is the corresponding flow rate. The apparent activation energy (Ea) was obtained using Arrhenius plots, measuring at a series of temperatures under 10% propane conversion by adjusting the mass of catalysts.

3. Results and discussion

3.1 Structure Analysis of VOx/Al2O3 Catalysts A series of catalyst materials, including 1V/Al, 6V/Al, 12V/Al and 20V/Al were prepared, and the XRD patterns of these catalysts are shown in Figure 1. For lower metal loading (1%~12%), only diffraction lines of γ-Al2O3 (JCPDS 10-0425) were detected, suggesting that the vanadium oxide species were well dispersed on the surface of support. However, for the 20V/Al catalyst, additional reflection peaks at 2θ = 15.3°, 20.2°, 21.7°, 26.1°, 31.0°, 34.2°, 47.3°, 51.3°, 55.5°, 62.1° and 72.5° were detected, which can be assigned to the characteristic peaks of the V2O5 crystal, 9


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indicating the formation of crystalline V2O5 over the catalyst. In addition, UV-Vis spectroscopy was employed to evaluate the polymerization degree of VOx species on VOx/Al2O3 catalysts. The spectra as well as the calculated edge energy are shown in Figure S1. As expected, with increasing metal loading, the edge energy decreased from 3.9 eV to 2.5 eV, indicating the increase in polymerization degree and the growth in domain size of vanadium oxides.29-31 Previous studies have suggested that monovanadates, polyvanadate and V2O5 formed at different vanadium densities: isolated VOx was the predominant species at surface density below 1.2 V/nm2; polymerized VOx coexisted with monovanadates at 1.2 V/nm2~4.4 V/nm2; V2O5 formed at a surface VOx density above 4.4 V/nm2, and V2O5 nanoparticles participated to a large extent after VOx density above vanadia monolayer coverage (8~9 V/nm2).24, 32 In the present work, the vanadia densities of catalysts are 0.7, 3.9, 7.9, 13.1 V/nm2 for 1V/Al, 6V/Al, 12V/Al and 20V/Al respectively, implying that different VOx species are present on the synthesized catalysts. In order to verify the surface configuration of VOx species on the Al2O3 surface, both UV and visible Raman spectroscopies were employed. This technique has been successfully used to distinguish the VOx species in supported vanadium systems. Specifically, UV Raman is more sensitive for monitoring the isolated and polymerized VOx species, while visible Raman is more appropriate for the detection of crystalline V2O5.23-24 Figure 2a and b present the visible (532 nm) and UV (325 nm) Raman spectra of VOx/Al2O3 samples respectively, where several sets of Raman bands were observed: broad bands in the range of 500~700 cm-1 and at 910 cm-1, intense ones at 10


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994 cm-1, and sharp bands at around 1021 cm-1. According to previous studies, the band around 994 cm-1 was assigned to crystalline V2O5, while the other three were attributed to V-O-V (500-700 cm-1), V-O-Al (910 cm-1) and V=O (1021 cm-1) modes, respectively.24, 33 Specifically, only the Raman features at 910 cm-1 (V-O-Al) and 1021 cm-1 (V=O) in UV Raman spectra were observed on the 1V/Al catalyst, whereas no features were identified by the visible Raman because of the weak signal. Since that no V-O-V and crystalline V2O5 modes were detected, we could deduce that the isolated VO4 species were present on the 1V/Al. On the other hand, the 6V/Al catalyst and 12V/Al catalyst exhibited similar UV Raman features: in addition to the Raman bands at 910 cm-1 and 1021 cm-1, a broad band in the range of 500~700 cm-1 (V-O-V) was detected. Moreover, in the case of visible-excited Raman spectra, additional band at 994 cm-1 (crystalline V2O5) was observed on the 12V/Al sample. This implies that polyvanadate species exist at the surfaces of both 6V/Al and 12V/Al catalysts, meanwhile a small amount of V2O5 crystalline also forms on the latter one. Regarding the 20V/Al catalyst, a sharp band at 994 cm-1 (crystalline V2O5) appeared in both UV and visible Raman spectra, and V2O5 has also been detected by XRD, suggesting that V2O5 crystals are predominant on this catalyst. Therefore, a possible scheme can be drawn based on the above analysis: the 1V/Al catalyst possesses mainly isolated VOx species, while monovanadates and polyvanadate coexist on the surface of 6V/Al catalyst. Moreover, the vanadium density of 12V/Al sample is approximate to monolayer coverage, as both polymerized VOx and small V2O5 crystal are presented. For the 20V/Al catalyst, a large amount of 11


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V2O5 nanoparticles forms above the monolayer coverage. The surface mode of the studied catalysts is depicted in Scheme 1.

3.2 Distribution of valence state Each sample was analyzed by H2-TPR, and the reduction profiles are illustrated in Figure 3. Only one peak at around 500 °C was observed for all the samples, which is associated to the reduction of VOx species.34-35 The consumed H atoms per V atom was then calculated (Table 1), corresponding to 1.0, 1.4, 1.6, 1.9 for the 1V/Al, 6V/Al, 12V/Al, and 20V/Al catalysts, respectively. The results demonstrate that the catalyst with higher surface VOx density has higher reducibility. Such trend has also been reported in previous studies, the reason for which was considered to be the difference in the reducibility of V=O, V-O-Al, V-O-V bounds and V2O5 nanoparticle.24,

36

Additionally, similar phenomena has also been observed in the chromium oxide system.37-38 XPS measurements have been carried out to acquire the quantitative information of the valence distribution of vanadium on the catalyst surface. The binding energies at around 517.6 eV, 516.6 eV, and 515.7 eV were assigned to V5+, V4+, V3+ ion, respectively, and the fraction of each oxidation valence was obtained by peak deconvolution.13-14, 39 Note that the difference on the obtained Vx+ fraction between XPS and H2-TPR may be caused by deconvolution error and hydrogen correction factor, respectively. As shown in Figure 4 and Table 2, the V oxidation state was in a mixture of 5+, 4+ and 3+ after reduction, which is consistent with earlier reports.13-14 Specifically, except for the 1V/Al sample, less pentavalent vanadium was observed 12


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over the reduced catalyst with higher metal loading, suggesting that more VOx species was reduced, in agreement with the TPR results. Note that the result for1V/Al catalyst was not presented here due to the relatively low signal intensity. As discussed above, mainly isolated VOx species was present on the surface of 1V/Al catalyst, and the monomeric vanadium oxide should possess only one oxidation state upon complete reduction. The average state calculated from H2-TPR profile was 4+ and abundant V4+ ion was also observed by EPR (Figure S2).23, 40 Thus it is reasonable to speculate that almost 100% V ions exist in V4+ form over the 1V/Al sample. In addition, the V valence state does not change under reaction conditions, which was confirmed by additional XPS analysis of the 6V/Al sample after one-hour reaction (Figure S6). In summary, different fractions of V5+/V4+/V3+ ion were achieved by reducing different vanadium species, which were obtained by adjusting the vanadium density over the catalyst surface.

3.3 Catalytic performance The VOx/Al2O3 catalysts with different VOx density were tested for propane dehydrogenation at 600 °C. The activity and selectivity are illustrated in Figure 5. All the catalysts showed high selectivity towards propylene (over 87%), which could remain constant during the whole reaction, indicating that VOx has superior C-H activation ability.3-4, 29 As can be clearly seen, the initial propane conversion increased as a function of V loading up to 12% and then declined on the 20V/Al catalyst. Moreover, a dramatic decrease in activity was found, which is mainly owing to coke formation throughout the reaction.4,

29

In Figure 6a, carbonaceous material was 13


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observed covering the 12V/Al catalyst after 4 h-PDH reaction, and the amount of coke deposition (i.e., 0.054 g/gcat) was measure by TGA (Figure 6b). In order to elucidate the origin of active sites, we calculated the initial TOFperV at 600 °C based on all the vanadium atoms. Since all the V atoms were exposed to reactants and served as the active site for isolated and polymerized VOx species, the calculated TOFperV value will be the same as the TOFpersite, which is defined on a per active site basis. However, for the catalysts with crystalline V2O5, the TOFperV would be smaller than TOFpersite because only surface V atoms of V2O5 served as active sites. For better understanding, additional TOFs on the 3V/Al and 9V/Al catalysts were also measured and all the results are listed in Table 3. Apparently, the initial TOFperV grew along with V loading up to the 9V/Al sample, and then declined on the 12V/Al and 20V/Al catalysts. Since crystalline V2O5 start to form when V loading was above 12%, the actual TOFspersite on the 12V/Al and 20V/Al systems would be higher than the calculated TOFperV, while those on the catalysts with lower vanadium loading (< 9%) would be close to the calculated one. Nevertheless, instead of a constant TOFperV value, the variation of the TOFperV at low V loadings (< 9%) indicates that multiple active sites exist and the ratio between these sites change along with metal loading, consistent with XPS results discussed in section 3.2. Additionally, in the case of 1V/Al, 6V/Al, 12V/Al, and 20V/Al catalysts, the apparent activation energy declines continuously from 244 kJ/mol to 165 kJ/mol (Table 3), which further suggests the change of active sites over the sample with different vanadium loading.41 Note that the mass transfer limitation has been excluded 14


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as shown in Supporting Information Section S7. Moreover, the increasing TOFperV indicates that the fraction of active site with higher activity increases along with the rise of VOx density. As noted, the catalyst with higher VOx surface density suffers from deeper reduction, and thus possesses much more V3+ ions. The fraction of V3+ ions on the surface of 1V/Al, 6V/Al, 12V/Al, and 20V/Al catalysts are 0, 19%, 28%, 60%, respectively (see the Section 3.2). Therefore, we propose that the VOx species (V5+, V4+ and V3+ ions) on the catalysts exhibit different activity for propane dehydrogenation, and the V3+ ions are more active than V5+ and V4+ ions. In addition, the oxidation state of vanadium determines the catalytic activity, while the degree of polymerization, tuned by vanadium loading, also affects the distribution of vanadium oxidation state, which has an indirect influence on the catalyst activity. Along with incremental VOx density, the fraction of exposed V atoms increases first and then declines (due to the formation of 3-D V2O5), while the ratio of more active V3+ increases continuously. Given these two factors, the PDH activity over the VOx/Al2O3 catalyst exhibits a volcanic trend and reaches its maximum over the 12V/Al catalyst with an optimal balance between catalyst dispersion and activity of active site,42 as showed in Figure 5.

3.4 DRIFTS studies The DRIFTS experiments were performed over the 12V/Al catalyst to explore the reaction mechanism during PDH. Figure 7 shows the IR spectra obtained after adsorption of propane at 30 °C (spectra 1), and after reaction at 300 °C for 60 min (spectra 2) and then evacuation at 30 °C (spectra 3) on the 12V/Al catalyst. In the 15


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initial spectra, the bands at 2967, 1460 and 1370 cm-1, assigned to propane, were detected. Surprisingly, upon heating to 300 °C a new strong band at 1555 cm-1 emerged, suggesting that propane was involved in reaction over the catalyst. After evacuation at 30 °C, the bands at 1555 cm-1 and 1460 cm-1 kept unchanged, while the band at 2967 cm-1 disappeared because propane could barely be chemisorbed on the catalyst surface (Figure S3). These results indicate that the obvious bands at 1555 cm-1 and 1460 cm-1 must be corresponded to some strongly adsorbed species generated by propane at elevated temperature. The IR spectra gained after adsorption of propylene at 30 °C and subsequent evacuation (Figure 8, spectra 1) gives the bands at 2978, 2876, 1625, 1460 and 1381 cm-1, which are the characteristic adsorption bands of propylene. After subsequent adsorption of hydrogen at 30 °C, no significant changes were found (not shown here). However, after increasing the temperature to 200 °C, the bands at 2978 and 2876 cm-1 became negligible, while a new broad band at 1555 cm-1 appeared. And these bands remained unchanged after evacuation at 30 °C (Figure 9, spectra 2 and 3). Note that, in contrast to hydrogen, heating the sample under helium atmosphere to the same temperature only led to the disappearance of all bands, suggesting the desorption of propylene under 200 °C (Figure 9, spectra 4). In summary, strong bands at 1555 cm-1 and 1460 cm-1 were observed during PDH. Meanwhile, bands at 1555 cm-1 and 1460 cm-1 (overlapping with 1555 cm-1) were also detected during the propylene hydrogenation, which is the microscopic reverse of dehydrogenation. The results of IR spectra clearly verify that the bands at 1555 cm-1 and 1460 cm-1 are related to the 16


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intermediates during propane dehydrogenation. The band at 1460 cm-1 is very likely due to the C-H vibration, while the new peak at 1555 cm-1 is possibly initiated by the vibration of C=C double bond considering the IR band of familiar functional groups.43-44, In addition, DFT calculation showed that η-vinyl over the Pt(111), which preserves double bond nature, exhibited C=C vibration at 1547 cm-1, much close to our experimental result, further supporting our proposal.45 Although the first step of propane dehydrogenation over metal oxides catalyst is commonly considered to be the dissociative adsorption of propane, forming metal alkyl and bridging hydroxyl,28, 46-49 saturated alkyl (such as propyl) would not lead to the IR vibration as high as 1555 cm-1.50 Therefore, we propose that vanadium propyl would form at the first step of PDH, which instantly transforms to propenyl-vanadium due to instability, containing one C=C double bond and five H atoms. DFT calculations on a V doped α-Al2O3 indicate two bands from propenyl-vanadium, which are very close to experimental observations: the C=C stretching at 1566 cm-1 and C-H bending mode at 1427 cm-1 (See SI). At elevated temperatures, propylene and coke deposition were initiated from the propenyl-vanadium, and vanadium propyl might also lead to their generation directly. This hypothesis was proven by the MS signal of H2 (Figure 9), which is generated via propane dehydrogenation to propenyl, in the temperature range without detection of propylene both by MS and DRIFTS (Figure 8, spectra 2) during C3H8 TPSR. Regarding propylene hydrogenation, it was considered as the microscopic reverse of propane dehydrogenation, and shared the reverse mechanism.51-52 Heating the catalyst 17


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with strongly adsorbed propylene to 200 °C under hydrogen atmosphere produced the same intermediate as propane dehydrogenation, but only propylene desorption was observed under helium atmosphere. In the hydrogen atmosphere, propylene is first partially hydrogenated to vanadium propyl species, which then transforms to propenyl-vanadium. However, propylene cannot be hydrogenated to vanadium propyl without the co-existence of hydrogen, and it can only desorb at elevated temperatures. Because heavy coke deposition occurs during propane dehydrogenation (also demonstrated in Figure 7) and carbonaceous species typically contain C=C double bond, we check whether the peak at 1555 cm-1 is due to the carbon species. Previous study have verified that coke species mainly formed from the alkenes rather than the alkanes with relatively high temperature.26 However, no propylene was detected by IR at 300 °C during propane dehydrogenation. Besides, the IR spectra of spent catalyst containing coke deposition showed the strong band at 1585 cm-1 (Figure S4), which is the feature of unsaturated or aromatic hydrocarbons, consistent with earlier studies.26, 53

Moreover, if 1555 cm-1 peak belongs to carbon deposition, it is barely possible that

it only appears by propylene with hydrogen instead of propylene with helium. Hence, the bands at 1555 cm-1 and 1460 cm-1 are not attributed to coke deposition, which is mainly produced after propylene formation. Previous studies on chromium/alumina during propane dehydrogenation reported a band at 1555 cm-1, similiar to our results and the authors assigned this band to the formates νas (COO).26 However, in our vanadium-based system, the intermediate is unlikely to be the species with C=O bond. Both the IR spectra of adsorbed acetone 18


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and formic acid (Figure S5) showed the band at 1589 cm-1 over 50 °C, 200 °C or 300 °C, which was related to the C=O vibration. Moreover, the sample was completely reduced before experiments and the oxygen was typically coordinated with Al or V atom, and thus no excessive O can form C=O bond. All of these results illustrate that the bands at 1555 cm-1 and 1460 cm-1 may not be assigned to the species with C=O bond. Consequently, we speculate that the intermediate formed during both propane dehydrogenation and propylene hydrogenation is vanadium propenal(V-C3H5) and the proposed reaction mechanism for propane dehydrogenation over the VOx/Al2O3 catalysts is depicted in Scheme. 2.

4. Conclusions

In conclusion, the present study constitutes an in-depth attempt to explore the nature of active sites and the mechanism of propane dehydrogenation over promising VOx/Al2O3 catalysts. Different fractions of V5+/V4+/V3+ ion on the catalyst surface were obtained by altering the vanadium density on the catalyst. Specifically, the catalyst with higher VOx density is more facile to reduction, leading to higher proportion of V3+ ions on the surface. TOFpersite over the catalyst (V loading at most 9%) increases constantly. Therefore, we propose that multiple active sites exist over the VOx/Al2O3 catalyst, and V3+ is more active for propane dehydrogenation than the other V sites with different oxidation states. In situ DRFITS was employed for the investigation of the reaction mechanism. Same intermediates were detected during both propane dehydrogenation and 19


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propylene hydrogenation, indicating that they share reverse reaction mechanism. In combination with previous studies, a new intermediate i.e. propenyl-vanadium (V-C3H5) was proposed, instead of carbonyl compound or carbonaceous species. Propane activation over the catalyst surface involves the abstraction of a hydrogen atom,

then

the

formed

vanadium

propyl

was

rapidly

transformed

to

propenyl-vanadium because of instability, as observed by IR spectroscopic technique. At

elevated

temperatures,

propylene

formed

from

propenyl-vanadium

or

propyl-vanadium, then desorbed from the catalyst surface, and coke deposition was generated at the same time, leading to catalyst deactivation. The conclusions reported in this paper would help us understand the reaction mechanism in depth and further improve catalytic performance. Maximizing the exposure of vanadium atom and the fraction of V3+ ion could enhance the propane conversion.

Supporting Information

Figure S1-S6 and details of DFT calculations as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

This work was supported by the National Science Foundation of China (21222604, 21525626, and 21506149), the Program for New Century Excellent Talents in University (NCET-10-0611), the Scientific Research Foundation for the 20


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Returned Overseas Chinese Scholars (MoE), and the Program of Introducing Talents of Discipline to Universities (B06006). We thank Mr. Di Li for fruitful discussion.

References

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Tables Table 1. Consumed H atoms per V atom over the VOx/Al2O3 catalysts based on H2-TPR profiles Sample

H:V ratio

1V/Al

1.0

6V/Al

1.4

12V/Al

1.6

20V/Al

1.9

25


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Table 2. Results of peak deconvolution of the V photoelectron signal Sample

V5+/%

V4+/%

V3+/%

1V/Al

n.d[a]

n.d[a]

n.d[a]

6V/Al

42

39

19

12V/Al

31

41

28

20V/Al

17

23

60

[a] not determined here because of relatively low signal.

26


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Table 3. TOF[a], Ea and V3+ fraction over the V/Al catalysts. Sample

TOF[a] (10-3×s-1)

Ea (kJ/mol)

V3+/(V5++V4++V3+)

1V/Al

4.1

245

0

3V/Al

4.3

n.d[b]

n.d[b]

6V/Al

4.8

191

19%

9V/Al

4.9

n.d[b]

n.d[b]

12V/Al

4.5

179

28%

20V/Al

3.7

165

60%

[a] TOF was calculated based on all vanadium atoms [b] not determined

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Figures

Figure 1. XRD patterns of the VOx/Al2O3 catalysts

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Figure 2. a) Visble (λexcitation=532 nm) and b) UV (λexcitation=325 nm) Raman spectra of the VOx/Al2O3 catalysts.

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Figure 3. H2-TPR profiles of the VOx/Al2O3 catalysts.

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Figure 4. Deconvolution of the fit of the V 2p2/3 signal for the a) 6V/Al, b) 12V/Al and c) 20V/Al catalyst.

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Figure 5. a) Propane Conversion and b) propylene selectivity on the VOx/Al2O3 catalysts at different reaction time as the function of vanadium loading

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Figure 6. a) TEM image and b) TGA profile of spent 12V/Al catalyst

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Figure 7. DRIFTS spectra observed for reaction of propane over the 12V/Al catalyst 1) after propane adsorption at 30 ºC, 2) after consequent heating at 300 ºC for 60 min, 3) and evacuation at 30 ºC.

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Figure 8. DRIFTS spectra of observed 1) after adsorption of propylene at 30 ºC, 2) after heating at 200 ºC under H2/Ar for 30 min, 3) consequent evacuation at 30 ºC, 4) heating the sample with adsorbed propylene at 200 ºC under helium over the 12V/Al catalyst.

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Figure 9. Profiles of C3H6 (red line, m/e=41) and H2 (black line, m/e=2) during C3H8 temperature programmed reaction over the 12V/Al catalyst.

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Schemes

Scheme 1. Proposed surface model of VOx/Al2O3 catalysts (the green parts represent isolated and polymerized VOx species, while the yellow parts are for V2O5 nanoparticles).

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Scheme 2. Possible reaction path of propane dehydrogenation over the VOx/Al2O3 catalysts.

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Table of Contents

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Al2O3 Catalysts for Propane

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