Relationship between Surface Chemistry and Catalytic Performance of

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Relationship between Surface Chemistry and Catalytic Performance of Mesoporous #-Al2O3 Supported VOX Catalyst in Catalytic Dehydrogenation of Propane Peng Bai, Zhipeng Ma, Tingting Li, Yupeng Tian, Zhanquan Zhang, Ziyi Zhong, Wei Xing, Pingping Wu, Xin-mei Liu, and Zifeng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07779 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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ACS Applied Materials & Interfaces

Relationship between Surface Chemistry and Catalytic Performance of Mesoporous γ-Al2O3 Supported VOX Catalyst in Catalytic Dehydrogenation of Propane Peng Bai,*,† Zhipeng Ma,† Tingting Li, † Yupeng Tian, † Zhanquan Zhang, § Ziyi Zhong, ǁ Wei Xing, ‡ Pingping Wu, † Xinmei Liu, † and Zifeng Yan*,† †

State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, College of Chemical Engineering, China University of Petroleum, Qingdao 266580, China ‡

School of Science, China University of Petroleum, Qingdao 266580, China §

ǁ

Petrochina Petrochemical Research Institute, Beijing 102206, China

School of Chemical & Biomedical Engineering, Nanyang Technological University (NTU), 62 Nanyang Drive, 637459 Singapore

Keywords: mesoporous γ-Al2O3, vanadium-based catalyst, propane dehydrogenation, surface acidity, polymerized VOX species * Corresponding authors. Tel: +86-532-86981856; 86981296; Fax: +86 532 86981295.

ACS Paragon Plus Environment

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E-mail address:[email protected] (P. Bai), [email protected] (Z. Yan)


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ABSTRACT

Mesoporous γ-Al2O3 was synthesized via a cation-anion double hydrolysis approach (CADH). The synthesized mesoporous alumina displayed a relatively high surface area, a large pore volume and a narrow pore size distribution. By applying the mesoporous alumina as a support, supported vanadium catalysts were prepared and evaluated in the dehydrogenation of propane, exhibiting a superior catalytic performance over that supported on a commercial alumina. Materials were characterized with a variety of techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy,

51

V magnetic angle spinning

nuclear magnetic resonance, Raman spectroscopy, Fourier transformed infrared spectroscopy of pyridine adsorption and thermogravimetric-differential thermal analysis. The correlated structure-performance relationship of catalysts reveals that a higher crystallization temperature endows mesoporous alumina materials with more surface acid sites, favoring the formation of polymerized VOX species, which are more active than isolated ones in the propane dehydrogenation, resulting in a better catalytic performance. The established relationship between surface chemistry and catalytic performance of supported VOX catalysts suggests that a superior vanadium catalyst for propane dehydrogenation could be achieved by rationally enriching the concentration of polymeric VOX species on the catalyst, which can be realized by tuning the surface acidity of alumina support.


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1. INTRODUCTION Propene, as an important chemical feedstock, is widely used for the production of polypropylene and various propene derivatives, such as acrylonitrile, propylene oxide, cumene and acrylic acid. In industry, propene is produced on a large scale by steam cracking of naphtha, fluid catalytic cracking process (FCC), methanol to olefins (MTO) and dehydrogenation of propane (PDH).1-4 Among them, PDH is a highly profitable process by converting the low-value propane to the value-added propene. This technology is receiving increasing attention due to the fast increase in the market demand for propene and its derivatives. There are two types of processes for catalytic propane dehydrogenation, non-oxidative and oxidative propane dehydrogenation.5-8 The former has achieved commercialization, while the latter has not yet due to a number of issues remaining to be resolved, such as the low propene selectivity and safety problems. The most common commercial catalysts used in PDH are platinum-based and chromium-based catalysts. Despite the successful application in industry, both catalysts exhibit disadvantages in terms of economic and environmental issues. Platinumbased catalysts employing noble metal are costly, while chromium-based catalysts are subject to poisoning issues imposed to the environment.9-14 These challenges necessitate the development of an alternative catalyst that should be more economically advantageous and environmentally friendly.12, 15 Despite the fact that vanadium-based catalysts have been intensively investigated in the oxidative propane dehydrogenation,16-19 there are fewer reports on the application of this type of catalysts in the non-oxidative propane dehydrogenation.20-21 Ovsitser et al. reported that a silica supported VOX catalyst exhibited high stability in the PDH with a slight deactivation by 10% during 20 h of reaction. The selectivity to propene was up to 80% at a propane conversion of


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45%.20 Sokolov et al. compared the catalytic performance of MCM-41 supported VOX, CrOX and Pt-Sn/Al2O3 catalysts in the PDH. It was found that VOX/MCM-41 catalyst exhibited a high stability during several dehydrogenation-regeneration cycles, while CrOX/MCM-41 and PtSn/Al2O3 catalysts showed a gradual deactivation.21 The structure of VOX remained stable during such dehydrogenation-regeneration cycles, which may account for the high stability of VOX/MCM-41 catalyst. In addition, vanadium-based catalysts possess a lower cost than that of Pt-based catalysts, and are less hazardous to environment than Cr-based catalysts. For the vanadium-based catalysts, it has been found that a number of factors affect the surface chemistry of vanadium species, such as the preparation method, vanadium loading, promoter, and especially support characteristics.17, 22-25 The support type was found to strongly influence the distribution of different surface vanadium species, such as isolated and island species,26-28 which accounts for the change in catalytic performance of vanadium catalysts. Several supports, such as Al2O3, MgO, SiO2 and zeolites, have been attempted in propane dehydrogenation. Among them, Al2O3 has been widely used in the dehydrogenation processes due to its appropriate chemical properties, high structural stability and low cost.29-31 Being a catalyst support, the textural properties of alumina is vital to the catalytic performance of catalysts. As an emerging alumina material, mesoporous alumina has recently received intensive attention due to the superior pore structure.32-36 Vanadium oxide supported on mesoporous alumina was reported to exhibit high activity and high selectivity in the oxidative dehydrogenation of ethane, which was found to be ascribed to the high surface area and high density of surface hydroxyls on mesoporous alumina.37 We have previously developed a cation-anion double hydrolysis approach (CADH) to the synthesis of mesoporous alumina with high surface area and a pure γAl2O3 phase.38-39 In this work, mesoporous γ-Al2O3 materials were prepared via the CADH


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approach and used as the support for the preparation of supported vanadium catalysts. Catalysts were characterized by a variety of techniques and evaluated in the PDH reaction. It was found that the surface chemistry of catalysts changed significantly with the variation of the crystallization temperature used for the synthesis of mesoporous alumina, correspondingly affecting the catalytic performance of catalysts in the PDH.

2. EXPERIMENTAL SECTION 2.1. Material Preparation Mesoporous γ-Al2O3 was synthesized based on the procedure established previously with some modifications.38-39 In a typical synthesis, P123 was dissolved in deionized water to form a clear solution, and Al(NO3)3·9H2O was then added into the P123 solution under stirring. The mixture was stirring at 313 K and 343 K for 2 h respectively, followed with the addition of NaAlO2 solution which led to the formation of white precipitates immediately. After stirring at 343 K for 2 h, the resulting gel was then transferred into a stainless steel autoclave lined with a polytetrafluorethylene liner for crystallization at different temperatures for 24 h. Finally, the product was retrieved by suction filtration, washed by deionized water and dried at 373 K for 12 h. The dried solid was calcined at 773 K for 2 h with a temperature ramp rate of 1 K/min. The mesoporous γ-Al2O3 thus obtained was denoted as meso-Al2O3-XK, where X represents the crystallization temperature. The catalysts with 10 wt.% vanadium loading were prepared using the incipient wetness impregnation method. In a typical preparation, the alumina support was impregnated with an ammonium metavanadate solution, dried at 373 K for 12 h and subsequently calcined at 823 K for 10 h. After that, 0.1 wt.% of potassium was further impregnated using a potassium nitrate


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solution following the aforementioned impregnation procedure. Thus prepared catalysts were pelleted, grounded and sieved to 20-60 mesh for use. The resulting catalysts were denoted as VOX/meso-Al2O3-XK. For comparison purpose, a commercial γ-Al2O3 denoted as CS-Al2O3 was also used as support for the catalyst denoted as VOX/CS-Al2O3.

2.2. Material Characterization N2 sorption was carried out on a TriStar 3000 analyzer (Micromeritics, USA) at 77 K. Prior to the analysis, samples were degassed in vacuum at 573 K for 4 h. The specific surface area was calculated in the relative pressure of 0.05-0.25 using the Brunauer-Emmett-Teller (BET) equation. The pore volume of samples was calculated at a relative pressure P/P0 of 0.993. The pore size distribution was derived from the desorption branch of isotherms using the BarrettJoyner-Halenda (BJH) method. Powder X-ray diffraction (XRD) patterns of samples were measured on an X’Pert PRO MPD diffractiometer (Philips, Netherland) using a Cu Ka radiation (λ=0.1542 nm) at 35 kV and 40 mA. 51

V magic-angle spinning nuclear magnetic resonance (51V MAS NMR) spectra were

measured on a Bruker Arane 400M spectrometer (Bruker, Germany) operated at a resonance frequency of 105.2 MHz with a 4 mm BB spin (MAS 5K). The chemical shifts were calibrated with a reference of NaVO3 (-574.3 ppm). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI5000 VersaProbe instrument (Ulvac-Phi, Japan) employing a monochromatic 18.2 W Al Kα excitation source (pass energy of 46.95 eV, angle of 45°). The reported binding energies have been referenced to C1s at 284.8 eV. The quantitative analysis of vanadium species of different valences was obtained by calculating the ratio of the deconvoluted peak areas with a shirley background.


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Raman spectra were recorded with a DXR Raman spectrophotometer (Thermo Fisher Scientific, USA) in the powder form. The spectra were acquired at room temperature with a 532 nm excitation source. Ultraviolet-visible (UV-vis) spectra were measured on a Hitachi U-4100 spectrophotometer (Hitachi, Japan) with a diffuse reflectance spectroscopy (DRS) technique in the range of 2001000 nm. BaSO4 (AR) was used as white standards to dilute the samples to minimize the effect of the highly different extinction coefficients.40 The Fourier transform infrared spectra of pyridine adsorption (Py-FTIR) were recorded on a NEXUS FTIR (Thermo Fisher Scientific, USA) using pyridine as a probe molecule to examine the nature of surface acid sites. Prior to the measurement, the samples were degassed in vacuum at 573 K for 2 h to ensure the removal of moisture. The pyridine adsorption was conducted in a drying oven in vacuum for 24 h. The FT-IR spectra were finally recorded from 4000 to 400 cm-1 using the average record of 64 times scanning. The thermogravimetric and differential thermal analysis (TG-DTA) was measured on a HCT-1 thermal differential analyzer (Beijing Hengjiu, China) to evaluate the coke deposition of used catalysts under air flow. The analysis was conducted in the range of 278-873 K at a heating rate of 10 K/min with a ceramic crucible as reference.

2.3. Catalytic Evaluation in the Dehydrogenation of Propane The catalytic dehydrogenation of propane was conducted in a continuous-flow fixed-bed micro-reactor (Tianjin Tianda Beiyang Chemical Equipment Co.,Ltd.) at ambient pressure with 1.0 g of catalyst loaded into a stainless steel tube (Length of 500 mm, diameter of 10 mm, thickness of 3 mm). After loading catalyst, the reactor was heated to 863 K in a N2 flow of 30 mL/min, and then the gas flow was switched to a H2 flow of 30 mL/min for 2 h. After that, the


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H2 flow was closed and the reactor temperature was increased to 883 K.41-44 A mixture of N2/propane with a volume ratio of 1:4 was introduced to the reactor with a total gas flowrate of 30 mL/min. In order to investigate the long-term stability of the catalysts, the catalyst was regenerated at 833 K for 3 h in an air flow of 30 mL/min after 10 h of dehydrogenation reaction. The products were analyzed on-line by an Agilent 7820A GC with a HP-PONA capillary column and a flame ionization (FID) detector. The conversion of propane, selectivity of propene and the yield of propene were calculated by a carbon atom balance method using the following formulas, where N0 and N represent relative numbers of carbon atoms in feed and product streams, respectively. XC3H8, YC3H6 and SC3H6 represent the conversion of propane, yield of propene and selectivity of propene respectively.12, 43 Given the very small amounts of COx gases and coke produced during reaction, these byproducts were not considered for the calculation. XC3 H8 = YC3 H6 = SC3 H6 =

N0 C3H8 -NC3H8 N0 C3H8

NC3H6 N0 C3H6 N0 C3H8 NC3 H6 -N0 C3H6 N0 C3 H8 -NC3H8

(1)

(2)

(3)

3. RESULTS AND DISCUSSION 3.1.

Catalytic Performance

Fig.1 shows the catalytic performance of vanadium catalysts supported on the commercial and mesoporous alumina supports. As shown in Fig. 1, all catalysts display a similar transient response to running time in terms of propane conversion, propene yield and propene selectivity. As for propane conversion and propene yield, a relatively rapid increase to summit is observed


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within 1.5 h, followed by a gradual decrease with running time to approach their relatively steady-state values. The increase of propane conversion and propene yield at the beginning of the dehydrogenation stage may be caused by: (i) the reduced VOX species of a higher activity, (ii) the formation of the carbon species with dehydrogenation activity. The decrease of catalytic performance is due to the coverage of active VOX species by coke.41 Interestingly, the selectivity is firstly increased to its maximum value, then dropped, and followed by a slight increase to the steady state selectivity which is lower than the summit value. Among the six catalysts supported on mesoporous alumina, VOX/meso-Al2O3-373K shows the best performance in terms of propane conversion, propene yield and selectivity, while VOX/meso-Al2O3-298K has the poorest performance. More specifically, catalysts show improved catalytic performance with increase of the crystallization temperature of mesoporous alumina in the range of 298-373 K, while the catalytic performance of the catalyst decreases with the crystallization temperature further increased to 393 K. As a result, sample VOX/meso-Al2O3-373K achieved the best performance with a maximum propane conversion of 70% and a propene yield of 60% at 1 h of reaction. The conversion and yield have large fluctuations in the ranges of 30-70% and 16-60%, respectively, which may be due to the deactivation of active sites by the deposited coke.21 Compared with catalyst prepared using the commercial alumina as the support (CS-Al2O3), catalysts prepared with mesoporous alumina show better stability except VOX/meso-Al2O3393K. The activity of VOX/CS-Al2O3 drops sharply after 1h of reaction. After 3h of reaction, the catalytic activity of VOX/CS-Al2O3 becomes obviously lower than those supported on mesoporous alumina prepared in the crystallization temperature range of 298-373 K. Especially, sample VOX/meso-Al2O3-373K exhibited obviously higher activity and better stability than VOX/CS-Al2O3. The difference in the maximum propane conversion as well as propene yield is


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about 12% and 15% respectively between VOX/meso-Al2O3-373K and VOX/CS-Al2O3, which is economically significant in terms of large scale industrial processes for the propane dehydrogenation, say 1 million ton propane processed per year. The analysis of by-products produced in the process of propane dehydrogenation was conducted, and the yield and selectivity of main side-products are shown in Fig. 2. As can be seen, the main by-products include methane, ethane and ethylene, and the by-products with 4 carbon atoms or more are not observed. The yield and selectivity of all by-products are relatively lower compared with propene. Specifically, the yields of methane, ethane and ethylene show a similar variation trend to that of propene, while the selectivities of these lighter hydrocarbons exhibit a reverse trend with that of propene. In general, the catalysts supported on the mesoAl2O3 crystallized at higher temperatures (from 313 K to 373 K) tend to produce less by-products in the process of propane dehydrogenation. Moreover the catalyst supported on CS-Al2O3 produce the highest yield of by-products during the initial 2 h of dehydrogenation reaction among all catalysts.

3.2.

Textural and Crystalline Properties

Fig. 3 shows N2 adsorption/desorption isotherms and pore size distribution of catalysts and their supports. According to the IUPAC classification, all catalysts and supports present a typeIV isotherm associated with an H-2 hysteresis loop, indicating that all materials have typical inkbottle mesopores. In addition, four mesoporous alumina samples prepared at a crystallization temperature of below 373 K display a similar sorption isotherm, while with the crystallization temperature increasing to 373 K and higher, the samples possess a higher adsorption capacity in the high relative pressure range, and their pore sizes also become larger. The same trend is also observed in the adsorption/desorption isotherms of the resulting catalysts using mesoporous


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alumina samples as supports. As shown in Table 1, different catalysts exhibit slight difference in the surface area (from 228 to 199 m2/g), although larger differences among supports are observed (from 320 to 242 m2/g). For the pore volume and pore size, mesoporous alumina supports exhibit an increasing trend with increase of the crystallization temperature. Similar results for the corresponding catalysts are also observed. However, the textural difference in catalysts could not reasonably explain the significant variation of their catalytic performance. For instance, catalysts supported on mesoporous alumina samples prepared in the temperature range of 298 to 353 K exhibit similar textural properties, but with quite different catalytic behavior. Besides, sample VOX/CS-Al2O3 displays higher activity in the initial 1.5 h of time on stream than most mesoporous alumina supported samples except for VOX/meso-Al2O3-373K, although it has a relatively lower surface area and pore volume. XRD patterns of mesoporous alumina and corresponding catalysts are shown in Figure 4. As can be seen, there is no significant difference in the crystalline structure between mesoporous alumina supports and catalysts. All samples are of similar γ-Al2O3 crystalline phase. After loading vanadium onto the supports, the absence of peaks related to VOX indicates the vanadium oxides are highly dispersed. In a word, the variation of catalytic behavior of different catalyst should not be attributed to the difference in the textural and crystalline properties of samples. Other factors, like the surface chemistry, may play a crucial role in determining the catalytic performance of catalysts. Therefore, additional characterization techniques are needed to explore the relationship between the surface chemistry and catalytic performance of vanadium catalysts supported on different alumina samples.

3.3.

UV-vis Spectroscopy Analysis


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To reveal the distribution of VOX species on the alumina supports, catalysts were characterized by the UV-vis spectroscopy and the spectra are shown in Fig. 5. As is seen, for all the samples, the ligand to metal charge transfer (LMCT) bands at 224, 275 and 310 nm are identified, indicating the existence of highly dispersed isolated VOX species and V-O-V bridged low-polymerized vanadium species with tetrahedral coordination.26, 45-46 The band at ca. 350 nm is ascribed to high-polymerized tetrahedral-coordinated and distorted octahedral-coordinated vanadium species. The high intensity of 224, 275 and 310 nm bands and the relative weak absorbance at 350 nm infer that the surface vanadium species are mainly composed of lowpolymerized and isolated vanadium species. Moreover, there are no obvious bands in the range of 400 to 600 nm, demonstrating the absence of V2O5 crystals, which is in agreement with the above XRD results. According to the previous studies, the O→V charge transfer bands within VO-V bridge structure of catalysts could reflect the coordination condition of the V5+ core. The band-gap energy (Eg) as a function of wavelength ( ) are calculated using the following formula (the wavelength used for the calculation of band-gap energy is determined by the intersection point of the vertical and horizontal parts of the spectra).47-49 = 1240

(4)

As is listed in Table 2, the band-gap energy of O→V charge transfer bands in the samples decreases with the increase of the crystallization temperature in the range of 298 K to 373 K, indicating an increase of concentration of the vanadium species on the surface. However, the band-gap energy of O→V charge transfer bands increases when the crystallization temperature was further raised to 393 K, evidencing the concentration of the vanadium species decreases at such high crystallization temperature. It has reported that the ratio of polymerized to isolated vanadium species proportionally increases with increasing the concentration of vanadium


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species.28 Hence, it implies that the ratio of polymerized to isolated vanadium species could be increased with the increase of crystallization temperature in the range of 298 K to 373 K, but decreased when the temperature is further increased to 393 K.16 According to previous reports,5051

the low polymerized vanadium species is most active for the propane dehydrogenation, while

isolated vanadium species exhibits a lower dehydrogenation activity. These results may partially explain the improved catalytic performance of catalysts supported on mesoporous alumina samples with increasing the crystallization temperature in the range of 298 K to 373 K.A possible explanation is that the crystallization temperatures modify the surface properties of mesoporous alumina, which then effects the interaction between VOX and alumina surface, resulting in the difference in the distribution of the vanadium species. Besides, as is seen from Table 2, the CS-Al2O3 supported catalyst also possesses a high vanadium density, only lower than that of VOX/meso-Al2O3-373K.

3.4.

Raman Spectroscopy Analysis

Raman spectra of catalysts are shown in Fig. 6. A significant variation of Raman spectra is observed with the increase of crystallization temperature (from 298 K to 373 K), suggesting that the crystallization temperature has a significant effect on the distribution of vanadium species. A series of characteristic bands appear at 145, 225, 354, 488, 823, 942 and 1052 cm-1 in the spectra, which should be assigned to the response of vanadium species, because alumina shows no significant Raman activity in the range of 100-1200 cm-1.23 As is reported previously,45 the band at ca. 1052 cm-1 was ascribed to the V=O stretching mode of surface VOx species, demonstrating the presence of isolated tetrahedral-coordinated vanadium species. The bands at ca. 823 and ca. 488 cm-1 are assigned to the V-O-V and/or V-O-Al antisymmetric and symmetric stretching vibrations, while that at ca. 323 cm-1 is assigned to the V-O-V δ-stretching vibration, suggesting


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the presence of the polymerized vanadium species. The band at ca. 942 cm-1 is ascribed to the terminal V=O symmetric stretching vibration, indicating the polymerized structure of distorted octahedra sharing corners and/or edges.23 The absence of signal at ca. 998 cm-1 suggests that crystalline V2O5 particles are not formed, which is in line with the above UV-Vis and XRD results. For catalysts prepared with mesoporous alumina samples, it is seen that bands at ca. 823 and ca. 942 cm-1 display a shift to the higher wavenumber when the crystallization temperature is increased to 373 K (the band at ca. 942 cm-1 even shifts to 986 cm-1), which is due to the enhancement of interaction between surface vanadium species. This means that a high crystallization temperature favors a higher vandium coverage in the surface of the catalysts (lower than monolayer coverage) with an increasing quantity of polymerized VOX species.52 Raman spectra of the CS-Al2O3 supported catalyst exhibit no band at ca. 1052 cm-1, suggesting the absence of isolated vandium species in this sample. Besides, the features of VOX/CS-Al2O3 at ca. 823 and ca. 942 cm-1 show a similar shift like sample VOX/meso-Al2O3-373K, which also demonstrates a strong interation of the vanadium species on the surface of CS-Al2O3 support with a high vanadium coverage. 51

3.5. 51

V MAS NMR Analysis

V MAS NMR spectra of catalysts are shown in Fig. 7. Two types of spectra can be

essentially interpreted from the NMR spectra of catalysts supported on CS-Al2O3 and mesoporous alumina samples. Samples crystallized at relative lower temperatures (313K, 333K and 353K) and further higher temperature (393 K) exhibit dominant peaks at ca. -579 ppm and ca. -534 ppm, which are attributed to tetrahedral-coordinated V5+ vanadium species. For VOX/meso-Al2O3-373K and VOX/CS-Al2O3, the peak at ca. -353 ppm appears, which is assigned


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to distorted octahedral-coordinated or octahedral-coordinated VO6 vanadium species.53 It can be seen that the proportion of octahedral-coordinated vanadium species increases obviously when the crystallization temperature increases to 373 K, indicating the ratio between octahedralcoordinated and tetra-coordinated vanadium species is increased obviously with the increase of crystallization temperature in the range of 298 K to 373 K, but is decreased when the crystallization temperature is further higher.22,

37, 54

Koranne et al. also observed this

phenomenon when investigating the effect of the vanadium loading,55 and inferred that this occurrence would accompany with the increase of polymerized vanadium species, as well as the decrease of isolated vanadium species in quantity.

3.6.

XPS Analysis

XPS spectra of three catalysts (VOX/meso-Al2O3-313K/333K/373K) are presented in Fig. 8, and the calculated average oxidation states and the surface atomic ratios are displayed in Table 3. Both qualitative and quantitative analyses are conducted to confirm the valence and relative concentration of surface vanadium species.26, 56 As is seen, since trivalent vanadium species are not formed after calcination in air atmosphere, the peak of V2p3/2 is deconvoluted into two peaks assigned to tetravalent and pentavalent vanadium species. As shown in Table 3, the surface atomic ratio of V (IV) to V (V) increases with increasing the crystallization temperature. The binding energies of V (IV) and V (V) are slightly higher than those previuosly reported,57-59 especially for VOX/meso-Al2O3-373K, which may be due to the strong interaction between vanadium species with alumina supports, resulting in the electron transfer from vanadium atoms to the support.60 The surface atomic ratio between V and O exhibits a decreasing trend with the increase of crystallization temperature, and the calculated ratio of V to Al also decreases slightly along with the increase of the crystallization temperature, which indicates there are more surface


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aluminum and oxygen atoms not covered by VOX,61 implying a high polymerization degree of vanadium species. Therefore, the surface composition results from XPS indicate the existence of more polymerized and less isolated surface vanadium species for catalysts prepared at high crystallization temperatures, which is consistent with the findings of UV-vis and Raman analyses. Besides, XPS analyses revealed a close relationship between the crystallization temperature and valence state of surface vanadium species. The ratio of V (IV) to V (V) species are verified to increase with the increase of the crystallization temperature. It is reported that the V (V) species is less active in the dehydrogenation reaction, while V(IV) and V (III) (easily produced by the reduction of V(IV)) species are confirmed to be more active in propane dehydrogenation.62-63 Hence, the superior performance of the catalysts with alumina crystallized at higher temperatures may be partially attributed to the higher content of V (IV) species.

3.7.

Surface Acidity

The surface acidity is a key factor influencing the propane dehydrogenation reaction, being responsible for the activation and adsorption of propane molecules during the reaction.64 Fig. 9 shows the Py-FTIR spectra of mesoporous alumina and the corresponding supported vanadium catalysts. As can be seen, the mesoporous alumina materials possess solely Lewis acid sites (LAS) showing a typical band at 1450 cm-1, while all catalysts possess dominant LAS and relatively weak BAS at 1540 cm-1 except VOX/meso-Al2O3-393K. It is previously reported that LAS originate from hydroxyl groups (-OHs) on alumina support, and are produced by the dehydration of neighboring -OHs on the Al3+ ions. And BAS are derived from the introduction of VOX into alumina, where vanadium oxide incorporation occurs along with the formation of polymerized VOX species.43, 65-67


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The acidity properties of mesoporous alumina and catalysts calculated from Fig. 9 are displayed in Table 4.68 As is seen, the crystallization temperature has a strong effect on the surface acidity of catalysts. The concentration of LAS in mesoporous alumina increases with the rise of crystallization temperature (373 K limited), which may promote the formation of polymerized vanadium species in the catalysts, consistent with the previous work.69 As for the catalysts, the concentration of BAS also exhibits an increasing trend with the rise of the crystallization temperature in the range of 298-373 K except VOX/meso-Al2O3-313K. Given that the amount of Brönsted acid sites parallels with the extent of polymerization of vanadium species,65 this phenomenon confirms that more polymerized vanadium species may form onto the surface of catalyst with increasing the crystallization temperature of mesoporous alumina, which also agrees well with the above UV-vis and Raman results. The amount of LAS becomes more intense with the rise of crystallization temperature in from 298 K to 373 K. However, when further increasing the crystallization temperature to 393 K, the amount of LAS decreases. In the process of propane dehydrogenation, Lewis acid sites contribute to the activation and adsorption of propane,70 which facilitates the transfer of propane molecules to the active metal sites for dehydrogenation. Therefore, catalysts supported on mesoporous alumina samples crystallized at higher temperatures (from 298 K to 373 K) have more LAS and BAS (higher total acidity), which promotes the activation, adsorption and conversion of propane, resulting in an improved catalytic activity of catalysts. But, when the crystallization temperature is further increased to 393 K, sample VOX/meso-Al2O3-393K possesses obviously decreased amount of Lewis acid sites, probably leading to its low dehydrogenation activity.

3.8.

Coke Deposition Behavior


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TG-DTA profiles and related data of used catalysts after reaction for 10.5 h are shown in Fig. 10 and Table 5, respectively. It has been recognized that VOX species are more active for the coke deposition compared with surface acid sites and a higher degree of VOX polymerization contributes to more carbon deposition in the process of propane dehydrogenation.69 In this sense, the coke deposition behavior of catalysts not only reflects the catalytic performance of catalysts, but also reveals the polymerization degree of surface VOX species. As can be seen from Fig. 10, all TG profiles show a slight declining trend before 530 K, which is ascribed to the desorption of physically adsorbed water.71 Following that, a dramatically weight loss emerges and the DTA curves start to rise from the horizontal line, indicating the combustion of the deposited coke.72 Finally, all TG-DTA curves level off to steady-state values. As can be seen from Table 5 and the fitting curve of weight loss data in Fig. 11, for all samples except VOX/meso-Al2O3-333K, generally, the starting and summit temperatures of exothermic signals in DTA curves exhibit an increasing trend when the crystallization temperature increases from 298 K to 373 K, accompanied by an increase in the amount of coke deposition. However, when the crystallization temperature is further increased to 393 K, temperatures of exothermal peaks and the amount of coke deposition decrease. By combining the above results from UV-vis, Raman and XPS analyses, it can be readily concluded that the amount of coke deposition has an approximately proportional relationship with the polymerization degree of VOX species, which is consistent with the previous report.69 A higher crystallization temperature induced a higher polymerization degree of VOX species, leading to the increased coke deposition of spent catalysts. As for the difference in the summit temperature of exothermic signals, it should be attributed to the difference in the graphitization degree of coke, which may be caused by different types of surface vanadium carbides in different


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catalysts.73 It is interesting to note that among all tested catalysts, samples VOX/meso-Al2O3353K and VOX/meso-Al2O3-373K, although having large amounts of coke deposition (14.46 and 14.73 wt.% respectively), exhibit a high stability in terms of high propane conversion and high propene selectivity after 7 h of reaction as shown in Fig. 1. This phenomenon may be ascribed to the different location of coke deposition on different catalysts. If the coke covers the surface of active VOX sites, the catalyst may deactivate fast, resulting in a low coke amount. In contrast, if the coke can be transferred to the alumina support, the catalyst will exhibit a long-lasting behavior together with a large amount of coke deposition.

3.9.

Regeneration Behavior

The dehydrogenation-regeneration cycles of catalyst VOX/meso-Al2O3-373K was conducted to investigate the long-term stability of the catalyst. As shown in Fig. 12, the catalytic activity of the catalyst decreased with time during the first 10 h of reaction, which may be due to the deactivation caused by the carbon deposition. Then, after regeneration in air, the catalytic performance was restored with a slight decrease in the propane conversion and propene yield by about 6%, while with an increase in the propene selectivity by about 4%. This irreversible loss of the catalytic activity after the first dehydrogenation-regeneration cycle may be due to the structural changes of the VOX species in the fresh catalysts, such as the agglomeration of the VOX species.21, 74 As compared with the first dehydrogenation (DH) cycle, the stability of the catalyst was even strengthened in the second DH cycle with an increase in propane conversion by 5% and an increase in propene selectivity by 4% at the end of the 10 h of reaction compared with that in the first dehydrogenation cycle. After the 2nd DH cycle, the catalytic performance of the catalyst in the 3rd and 4th DH cycles almost show no difference (conversion of propane varies from 62% to 60%), which means the structural changes of the VOX species in the first cycle lead


20

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to the structure stabilization of the fresh catalyst. The catalyst maintains a high level of activity even in the 4th dehydrogenation cycle, clearly demonstrating the excellent long-term stability of mesoporous alumina supported vanadium catalysts.

4. CONCLUSIONS This work shows that mesoporous alumina can serve as a good catalyst support for vanadium oxide catalysts. A series of mesoporous γ-alumina samples were synthesized by a CADH method and employed as the catalyst support for propane dehydrogenation. It was found that the catalytic performance was strongly dependent on the crystallization temperature at which the mesoporous alumina support is prepared. Compared with CS-Al2O3 supported catalyst, the VOX catalyst supported on the mesoporous alumina prepared at 373 K exhibits higher activity and better stability. The gaps in the maximum propane conversion as well propene selectivity between these two samples are ca. 12 % and ca. 15 % respectively, economically significant in terms of large scale industrial processes. The correlation between the surface chemistry and catalytic performance of catalysts reveals that high crystallization temperatures (373 K limited) promotes the formation of more polymerized vanadium species and a higher proportion of V(IV) atoms ,which are the main factors resulting in the improved catalytic performance. The higher temperature produces more acid sites on mesoporous Al2O3 support, favoring the formation of polymerized VOX species. The established structure-property relationship herein could afford guidance for the rational design of alumina support to achieve a superior supported VOX catalyst for the propane dehydrogenation.


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FIGURES

Figure 1. Catalytic performance of catalysts supported on commercial alumina and mesoporous alumina supports prepared at different crystallization temperatures.


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Figure 2. The yield and selectivity of main side-products (methane, ethane and ethylene) of catalysts supported on commercial alumina and mesoporous alumina supports prepared at different crystallization temperatures.


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Figure 3. N2 adsorption-desorption isotherms and pore size distribution of catalysts and supports.

Figure 4. XRD patterns of mesoporous alumina samples before (A) and after (B) V loading. Asterisks indicate the peaks of γ-Al2O3 phase.


24

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Figure 5. UV-vis spectra of VOX/CS-Al2O3 and catalysts supported on mesoporous alumina samples.

Figure 6. Raman spectra of VOX/CS-Al2O3 and catalysts supported on mesoporous alumina samples.


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Figure 7.

51

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V MAS NMR spectra of VOX/CS-Al2O3 and catalysts supported on mesoporous

alumina samples.

Figure 8. V2p3/2 XPS spectra of mesoporous alumina supported catalysts.


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Figure 9. Py-FTIR spectra of mesoporous alumina and catalysts supported on mesoporous alumina samples.

Figure 10. TG-DTA profiles of used catalysts after reaction for 10.5 h.

Figure 11. The fitting curve for weight loss tendency of used catalysts.


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Figure 12. Catalytic performance of VOX/meso-Al2O3-373K in 4 dehydrogenation-regeneration cycles.

TABLES Table 1. Textural properties of catalysts and supports. Samples

SBET(m2/g)

DBJH(nm)

Vt(cm3/g)

CS-Al2O3

240

3.9

0.35

meso-Al2O3-298K

320

5.2

0.46

meso-Al2O3-313K

316

5.2

0.46

meso-Al2O3-333K

290

5.2

0.44

meso-Al2O3-353K

317

5.5

0.47

meso-Al2O3-373K

268

6.8

0.53

meso-Al2O3-393K

242

8.8

0.57

VOX/CS-Al2O3

220

3.9

0.20

VOX/meso-Al2O3-298K

228

5.2

0.34

VOX/meso-Al2O3-313K

223

5.2

0.34

VOX/meso-Al2O3-333K

222

5.2

0.35

VOX/meso-Al2O3-353K

238

5.2

0.36


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VOX/meso-Al2O3-373K

233

5.7

0.40

VOX/meso-Al2O3-393K

199

8.6

0.44

Table 2. Band-gap energy of O→V charge transfer band of catalysts.

Samples

λg/nm

Band-gap energy/eV

VOX/CS-Al2O3

532

2.33

VOX/meso-Al2O3-298K

515

2.41

VOX/meso-Al2O3-313K

520

2.38

VOX/meso-Al2O3-333K

525

2.36

VOX/meso-Al2O3-353K

529

2.34

VOX/meso-Al2O3-373K

536

2.31

VOX/meso-Al2O3-393K

528

2.35

Table 3. Binding energies of V(IV) and V(V) from V2p3/2, and corresponding surface atomic ratios. Samples

V(IV)/ eV

V(V)/eV

V(IV)/V(V)

Cal. V/Al

Cal. V/O

VOX/meso-Al2O3-313K

516.8

517.6

1.019

0.072

0.032

VOX/meso-Al2O3-333K

516.6

517.5

1.112

0.066

0.031

VOX/meso-Al2O3-373K

517.0

518.0

1.202

0.060

0.027

Table 4. Acidity properties of mesoporous alumina and catalysts. Samples

NLAS (mmol/g)

NBAS (µmol/g)

B/L ratio

Ntotal (µmol/g)

Ntotal/SBET (µmol/m2)

meso-Al2O3-298K

1.57

0.00

0.00

1.57

0.0049

meso-Al2O3-313K

2.18

0.00

0.00

2.18

0.0069


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Page 30 of 42

meso-Al2O3-333K

2.50

0.00

0.00

2.50

0.0086

meso-Al2O3-353K

3.11

0.00

0.00

3.11

0.0098

meso-Al2O3-373K

3.20

0.00

0.00

3.20

0.0119

meso-Al2O3-393K

3.09

0.00

0.00

3.09

0.0127

VOX/meso-Al2O3-298K

4.11

1.26

0.23

5.38

0.0236

VOX/meso-Al2O3-313K

4.60

0.84

0.14

5.44

0.0244

VOX/meso-Al2O3-333K

4.99

2.23

0.34

7.22

0.0325

VOX/meso-Al2O3-353K

5.74

2.27

0.30

8.00

0.0336

VOX/meso-Al2O3-373K

6.15

4.82

0.59

10.97

0.0471

VOX/meso-Al2O3-393K

2.83

4.80

1.28

7.63

0.0383

Table 5. TG-DTA data of used catalysts after reaction for 10.5 h. Sample

Starting exothermal temperature /K

Peak temperature /K

Amount of Coke / wt.%

VOX/meso-Al2O3-298K

537

704

11.77

VOX/meso-Al2O3-313K

538

707

12.84

VOX/meso-Al2O3-333K

536

698

12.19

VOX/meso-Al2O3-353K

547

718

14.46

VOX/meso-Al2O3-373K

575

743

14.73

VOX/meso-Al2O3-393K

579

720

10.90

VOX/CS-Al2O3

532

715

13.21

ACKNOWLEDGMENTS This work was financially supported by the Joint Funds of the National Natural Science Foundation of China and China National Petroleum Corporation (U1362202), Natural Science Foundation of China (21206195), the Fundamental Research Funds for the Central Universities


30

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14CX02050A,

14CX02123A),

Shandong

Provincial

Natural

Science

Foundation

(ZR2012BM014), the project sponsored by Scientific Research Foundation for Returned Overseas Chinese Scholars, and the Postgraduate Innovation Project of China university of petroleum (East China) (YCXJ2016038). Z. Zhong ([email protected]) is mainly working in the Institute of Chemical and Engineering Sciences under Agency for Science, Technology and Research (A*star), and also holds an adjunct associate professor position in Nanyang Technological University in Singapore.

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Relationship between Surface Chemistry and Catalytic Performance of

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