English

• Considering a wide application of vanadyl catalysts in the oxidative dehydrogenation (ODH) and non-oxidative dehydrogenation (i.e., direct dehydrogenation) of light alkanes ^[1-5], it has been regarded as a promising alternative to the commercialized Pt- or Cr-based catalysts. At present, the ODH process over vanadium-based catalysts still suffered from low activity or difficult control of over-oxidation reaction ^[6-8], thus leading to more attention being paid to the direct dehydrogenation process.

  Metal elements were widely used for modifying vanadium-based catalysts to enhance the stability and selectivity. Harlin et al. ^[9] found that Mg could effectively enhance the iso-butene selectivity and suppress the coke formation during the dehydrogenation reaction over VO_x/Al₂O₃ catalysts. Liu et al.^[10] also showed that Mg over VO_x/SBA-15 catalysts could enhance the selectivity and yield of olefins by improving the reducibility of VO_x species and decreasing the surface acidity. Reddy et al.^[11] increased the dispersion of VO_x species by adding Ce and Zr to V₂O₅/SiO₂ catalysts for alkane dehydrogenation reaction. They found that the deactivation process was effectively prevented by Ce_xZr_1−xO₂ species on the surface. Raju et al.^[12] also tested the dehydrogenation performance of V/CeO₂-ZrO₂ and concluded that the CeO₂-ZrO₂ support played a vital role in maintaining stable catalytic activity. Zhou et al.^[13] and Sasikala et al.^[14] reported that the dehydrogenation performance of the VO_x/HMS (hexagonal mesoporous silica) catalyst was significantly promoted by rare earth elements (La and Y) due to the formation of new active phase YVO₄ and LaVO₄. Yang et al.^[15] demonstrated that the incorporation of CrO_x into Al₂O₃ led to more reactive VO_x over the surface, thus obtaining higher dehydrogenation rate and selectivity. Ajayi et al.^[16] also stated that the dehydrogenation performance over Cr modified V/MCM-41 was largely promoted due to the enhanced reducibility and surface acidity which were correlated to uniformly dispersed Cr-V-O species. Ascoop et al.^[17] observed that the WO_x species on the surface could prevent the aggregation of VO_x species.

  Compared to the metal elements mentioned above, non-metallic elements might also be effective for vanadyl catalysts. Previously we proved that boron could largely enhance the dispersion of surface vanadyl species by the formation of B−O−V bonds ^[18]. However, the boron precursor was relatively expensive. Similar to boron, phosphorus was also widely used in the modification of silicate-based catalysts (such as SAPO type zeolites) due to the advantage of being less expensive for the phosphorus precursors. Besides, phosphorus oxides could form tetrahedral units through P−O bonds similar to tetrahedral silicon oxide tetrahedron and interact with SiO₂ perfectly by sharing oxygen atoms, which made it suitable for the modification of silica-based catalysts. In this work, the effect of phosphorus modification on the MCM-41 supported vanadyl catalysts was studied. The influence of phosphorus introduction on the structural properties and surface vanadyl species were systematically examined and the dehydrogenation performances were tested.

  1.   Experimental

  1.1   Synthesis of V-P-MCM-41

  The phosphorus modified V-MCM-41 catalysts were prepared as follows: Firstly, 3.6 g vanadium acetylacetonate (99%, Energy Chemical), triethyl phosphate (A.R., Sinopharm) and 60.0 g TEOS (A.R., Merck) were dissolved in 300.0 g methanol (A.R., Sinopharm) under mild stirring. At the same time, another solution was prepared via the following steps: 12.5 g cetyl trimethyl ammonium bromide (A.R., Sinopharm) was dissolved in 200.0 g deionized water under stirring followed by 75.0 g aqueous ammonia solution (25 %, A.R., Sinopharm). The methanol mixture obtained in the first step was added dropwise to the water solution and stirred vigorously for 30 min. Then the mixture was sealed in autoclaves at 120 ºC for 48 h. The obtained solids were filtered, washed with deionized water and dried overnight. After calcination (550 ºC, 6 h), the powder was pressed, crushed and sieved to small particles in 30−40 mesh. The obtained catalysts were marked as P-x (x was the Si/P ratio in the synthesis process). A reference catalyst was also prepared without addition of phosphorus precursors and was denoted as P-0.

  1.2   Characterization

  The X-ray diffraction measurements of the V-P-MCM-41 samples were performed on a Bruker D8 advance XRD system using Cu Kα radiation in the 2θ range of 1.5°−35°.

  The N₂ physisorption isotherms were measured and analyzed on an ASAP 2020 (Micromeritics) apparatus. The samples were first vacuum-degassed at 350 °C for 5 h before N₂ adsorption. The specific surface area and pore distribution were determined by BET and BJH method, respectively.

  The vanadium content of the catalysts was measured by ICP-AES (Perkin Elmer Optima 7300 V). The catalysts were fully dissolved in the mixture of concentrated HF and HNO₃ solution for testing.

  TPR experiments were carried out in the range of 100−800 °C on an AutoChem 2920 instrument equipped with a MS detector. Typically, 100 mg particles (30−40 mesh) were used for each run. After treating in dry air (500 °C, 1 h) and cooling down, the samples were reduced in a flow of 10%H₂/Ar. The heating ramp was kept at 10 °C/min.

  NH₃-TPD profiles were also obtained on the same AutoChem 2920 equipment. After dehydration in He at 600 °C for 2 h, the sample (0.1 g) was cooled down to room temperature before NH₃ adsorption. Then NH₃ desorption was conducted in the temperature range of 40−600 °C under He flow (30 mL/min). The heating ramp was kept as 10 °C/min throughout desorption.

  XPS spectra were obtained from a Thermo Fisher K-Alpha instrument. The charge calibration was based on the C 1s peak at 284.6 eV.

  O₂ pulsed adsorption was used to determine the number of vanadium sites on the surface by assuming V_surf=O_ads^[18]. The specific experimental steps and conditions were described in detail in the previous studies ^{[18, 19]}.

  The catalysts were characterized by Raman spectroscopy under dehydrated conditions on a Raman spectrometer system (Horiba Lab RAM HR800) at room temperature. The samples were excited with a 325-nm laser and the spectral resolution was 1 cm⁻¹.

  The meso-structure of the catalysts was directly observed by TEM (FEI Tecnai F20) operated at 200 kV.

  1.3   Propane dehydrogenation reaction

  The propane dehydrogenation reactions were carried out in a fixed-bed reactor under atmospheric pressure and packed with 1.0 g catalyst particles in the isothermal zone by quartz sand at both ends. The catalysts were first dehydrated by pure N₂ at 600 °C (kept for 2 h). The effluent gas was monitored by using an on-line gas chromatography. The reaction temperature was 600 °C and the total gas flow was 50 mL/min (20% propane in N₂). The propane conversion and product selectivity were calculated based on carbon atom balance method (coke already ignored).

  2.   Results and discussion

  2.1   Structure and morphology

  The XRD patterns of the obtained catalysts were given in Figure 1. All samples clearly exhibited three well-resolved diffraction peaks assigned to [100], [110] and [200] of MCM-41, indicating that the meso-structure was not dramatically changed after the introduction of vanadium and phosphorus^[20]. No bulk V₂O₅ clusters were detected as there was no distinct diffraction peak of V₂O₅ both in the small and large angle XRD profiles^[21]. The peak intensity decreased gradually with the phosphorus content, indicating the decrease of the crystallinity. The diffraction peak position at around 2° slowly shifted towards larger angle in samples containing high phosphorus amount, reflecting that the crystalline interplanar spacing increased after the introduction of phosphorus (shown in Table 1). Besides, no diffraction peaks of VOPO₄, (VO)₂P₂O₇ or other vanadyl-phosphorus species was detected, meaning the absence of bulk vanadyl-phosphorus species. This also suggested that the vanadyl species were not aggregated or polymerized after the introduction of phosphorus and the possible vanadyl-phosphorus species were also highly dispersed on the surface.

  Figure 1

  

  Figure 1.  XRD patterns of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  Table 1

  

  Table 1.  Physio-chemical properties of V-P-MCM-41 catalysts

  下载: 导出CSV

  Sample	SBET / （m2∙g−1）	Pore volume /（mL∙g−1）	d100 a /Å	α b /Å	r c /Å
  P-10	596.6	1.07	40.3	46.5	25.6
  P-30	601.5	0.85	40.2	46.4	27.1
  P-50	628.3	0.96	39. 9	46.1	27.0
  P-0	668.6	0.73	39.0	45.1	27.6
  a: [100] Crystalline interplanar spacing, calculated by Prague equation b: Cell parameter, α=2d100/30.5 c: Pore diameter

  The specific surface areas and pore distribution were measured by N₂ physisorption, as show in Figure 2. All samples showed a clear hysteresis loop at around p/p₀=0.3, suggesting the existence of ordered mesoporous structure. Besides, all samples showed a narrow pore width distribution concentrated in the range of 22−35 Å, which was typical for MCM-41 materials^[22]. More details about the specific surface and pore structure parameters were listed in Table 1. The specific surface area of the V-P-MCM-41 samples showed an obvious reductive trend alongside with the phosphorus introduction amount while the pore volume slightly increased. Besides, the interplanar spacing and cell parameter of the V-P-MCM-41 also slightly increased with the phosphorus content, suggesting that the introduction of phosphorus enlarged the crystal cell to some extents^[23].

  The TEM images of V-P-MCM-41 catalysts were given in Figure S1. Clearly could be seen were the long ordered mesopores in all the catalysts. Compared with the V-MCM-41 catalyst (Figure S1 f), there was no significant difference in the TEM images of the V-P-MCM-41 samples. This directly proved that meso-structure was not greatly influenced by the introduction of phosphorus.

  Figure 2

  

  Figure 2.  N₂ adsorption-desorption isotherms of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  2.2   Surface vanadyl species

  The reducibility of the V-P-MCM-41 catalysts was examined by H₂-TPR and the results were given in Figure 3. There was a wide main reduction peak in the range of 350−550 °C for all the samples, which should be ascribed to the direct reduction of V⁵⁺ to V³⁺ of highly dispersed vanadyl species with tetrahedral coordination. Besides, a small shoulder peak (around 600 °C) also could be observed. As the reduction temperature of VOPO₄ or other vanadyl-phosphorous species was at around 700 °C or higher and there was almost no H₂ consumption at above 700 °C, the shoulder peak should be ascribed to polymerized vanadyl species with octahedral coordination^[24-26]. This indicated that highly-dispersed vanadyl species were dominant on the surface while a small amount of aggregated vanadyl species were still present ^{[27, 28]}. Hence, the reduction peaks were deconvoluted to distinguish different vanadyl species. According to the results in Table 2, the proportion of polymerized vanadyl species decreased along with increasing phosphorous content. This suggested that the introduction of phosphorus could decrease the number of octahedrally coordinated vanadyl species, thus overall improving the reducibility of the vanadyl species.

  Figure 3

  

  Figure 3.  TPR curves of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  Table 2

  

  Table 2.  TPR, TPD and O₂ chemisorption results of V-P-MCM-41 catalysts

  下载: 导出CSV

  Sample	Va/%	Proportion of surface vanadyl species b			Surface V sites c/ （10−4 mol∙g−1）		Surface V density d/ nm−2
  		highly dispersed	polymerized
  P-10	3.49	80.5	19.5		3.19		3.21
  P-30	3.45	78.8	21.9		3.35		3.35
  P-50	3.43	75.0	25.0		3.29		3.15
  P-0	3.38	74.3	25.7		3.11		2.80
  a: measured by ICP-AESb: determined by the deconvolution results of TPR c: determined by O2 chemisorptiond: based on the surface V sites determined by O2 chemisorption and BET surface area

  The acid properties of the V-P-MCM-41 catalysts were investigated by NH₃-TPD and the results were given in Figure 4. Only one broad peak at below 200 °C was observed for the P-0 sample, suggesting the adsorption of NH₃ molecules on the surface vanadyl species ^{[29, 30]}. After the introduction of phosphorus, a peak at lower temperature (around 90 °C) appeared and its integration area increased gradually alongside the phosphorus content. The weaker acid sites should be ascribed to the vanadyl species connected with phosphorus species ^{[31, 32]}. For all the samples, the acid sites with medium/high strength were rare as the ammonia desorption profile was almost flat in the temperature range of 250−600 °C. This suggested that the introduction of phosphorus only change the weak acid sites by forming highly dispersed vanadyl-phosphorus species (Table 2).

  Figure 4

  

  Figure 4.  NH₃-TPD profiles of V-P-MCM-41 samples

  下载: 全尺寸图片 幻灯片

  To precisely measure the number of surface vanadium sites, O₂ chemisorption was used by pulsing a fixed amount of O₂ through the reduced catalysts. The O₂ signal curves were illustrated in Figure 5 and the calculated surface vanadium sites were listed in Table 2. It could be seen that the total O₂ consumption first increased along with the phosphorus content and then it slightly decreased. Accordingly, the surface vanadyl species also showed a volcano shape with the phosphorus content. The maximum amount of surface vanadium sites (3.35×10⁻⁴ mol/g) was obtained over the P-30 sample. This indicated that a suitable amount of phosphorus was beneficial for the enhancement of surface vanadium sites. Combined with the results of O₂ chemisorption and ICP-AES, the decrease of surface vanadium sites might be attributed to the occupation of surface area by the excessive phosphorous species ^{[33, 34]}. Besides, the surface vanadyl species density also exhibited a volcano shape along with the phosphorous content. This further proved that the suitable amount of phosphorous introduction could effectively enhance the vanadyl dispersion.

  Figure 5

  

  Figure 5.  O₂ chemisorption profiles of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  The chemical state of surface elements over the V-P-MCM-41 catalysts was studied by XPS and the results were summarized in Table 3. It was clear that the binding energies of O 1s and Si 2p are mainly derived from SiO₂^[35-37]. According to previous studies^{[24, 38, 39]}, the peaks of V 2p_3/2 could be deconvoluted into two peaks at 518.4 and 517.0 which should be ascribed to polymerized vanadyl species (V⁵⁺) and highly dispersed vanadyl species (V⁵⁺). As shown in Figure 6 and Table 3, a small amount of polymerized vanadyl species could still be identified, which was consistent with the TPR results. After the introduction of phosphorus, the proportion of polymerized vanadyl species first decreased along with the phosphorus content and then slightly increased. This further illustrated that the dispersion of vanadyl species was enhanced and the reducibility of the vanadyl species was improved with suitable amount of phosphorous introduction^[40].

  Table 3

  

  Table 3.  XPS results of V-P-MCM-41

  下载: 导出CSV

  Sample	E/eV			Vanadyl species distribution / %
  	O 1s	Si 2p		517.0 eV	518.4 eV
  P-10	532.6	103.3		90.4	9.6
  P-30	532.6	103.5		92.1	7.9
  P-50	532.6	103.4		88.4	11.6
  P-0	532.7	103.5		83.8	16.2

  The P 2p spectra were shown in Figure S2. Due to the low phosphorus content, it could be seen that the P-50 and P-30 catalysts all exhibited a broad peak, whose specific attribution was difficult to be distinguished. However, the peak of P-10 located at around 134.6 eV was much clearer. As all samples were synthesized in the same way, they should share similar phosphorus species. According to the P 2p spectra, most probably, the peak should be ascribed to the P−O bond in the P−Si−O composite oxide (similar to those phosphorus species with tetrahedral coordination inside the SAPO type zeolites) instead of VOPO₄ (133.8 eV) or P₂O₅ (around 135.0 eV)^{[41, 42]}, although there might still exist a small amount of V−O−P bonds in the catalysts. According to Table 4, the molar percentage of vanadium content slightly decreased due to the introduction of phosphorus while the weight percentage of vanadium increased. The surface vanadium content determined by XPS exhibited the same trend with the results of ICP-AES, suggesting that introducing a suitable amount of phosphorus contributed both to the overall vanadium loading and the surface vanadium sites.

  The form of VO_x species was further studied by Raman spectroscopy (Figure 7). The peak at 1024 cm⁻¹ attributed to the terminal V=O bond of highly dispersed tetrahedral VO_x was found on all the samples^{[43, 44]}. Besides, a shoulder peak at around 990 cm⁻¹ which should be ascribed to aggregated VO_x species that has two- or three-dimensional networks similar to bulk V₂O₅ clusters^[45-47] was also found on the P-0 sample. After the phosphorus introduction, the shoulder peak gradually disappeared along with the increasing phosphorus content. This indicated that an appropriate amount of phosphorus could enhance the vanadium dispersion. Compared to the normal V₂O₅ Raman peak which was two- or three-time higher than the peak of the V=O bond, the small peak at 485 cm⁻¹ indicated that the highly dispersed vanadyl species were dominant and there was only a small amount of polymerized vanadyl species^{[7, 21, 48]}. This indicated that the highly dispersed vanadyl species were dominant on the surface of the catalysts, which was consistent with the O₂ chemisorption and XPS results. Besides, no sharp peaks assigned to vanadyl-phosphorus species (925 and 1035 cm⁻¹) were detected in the spectra, indicating that there were no crystalline vanadyl-phosphorus species formed on the catalysts.

  Figure 6

  

  Figure 6.  Curve fitting results of V 2p_3/2 XPS spectra

  下载: 全尺寸图片 幻灯片

  Table 4

  

  Table 4.  Surface composition of V-P-MCM-41 catalysts determined by XPS

  下载: 导出CSV

  Sample	Surface composition /mol %				Surface V content
  	O	Si	P	V	w/%
  P-10	63.42	32.81	2.48	1.28	3.12
  P-30	65.12	32.45	1.14	1.29	3.17
  P-50	64.64	33.21	0.87	1.28	3.16
  P-0	65.22	33.47	−	1.31	3.03

  Figure 7

  

  Figure 7.  Raman spectra of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  According to the characterization results above, it could be concluded that the meso-structure of MCM-41 were in general preserved despite that the cell parameters were slightly increased. The introduced phosphorus was successfully incorporated into the silicate, and it had prohibited the formation of polymerized vanadyl species. Therefore, both the dispersion and reducibility of the vanadyl species were improved by the phosphorus introduction.

  2.3   Effect of phosphorus to the catalytic performance

  The as-prepared V-P-MCM-41 catalysts were tested for propane dehydrogenation and the results were given in Figures 8 and 9. All samples showed a volcano shape during the dehydrogenation process, which should be ascribed to the introduction process of vanadyl catalysts (V⁵⁺ was reduced to V⁴⁺ or V³⁺ species). The propane conversion over the P-0 sample decreased rapidly from 33% to 24% during the dehydrogenation process. However, the propane conversion over the V-P-MCM-41 samples increased and the deactivation rate was largely reduced. The propane conversion over these catalysts was closely related to the surface vanadium sites (determined by O₂ chemisorption), which was consistent with the conclusion that the surface vanadyl sites were the active center for the dehydrogenation process. The selectivity of propylene and methane were shown in Figure 9. The propylene selectivity all exhibited an increasing trend during the dehydrogenation process. It was clear that the propylene selectivity over the samples with phosphorous modification was much higher than the P-0 sample. Besides, the propylene selectivity was closely related to the phosphorous content. The initial propylene selectivity over the P-50 sample was 83.0% and it finally increased to 85.2%. However, it increased to 84.8% and 88.5% over the P-10 sample, respectively. The enhancement of propylene selectivity by phosphorous introduction was obvious. Correspondingly, the selectivity of methane over the phosphorous modified samples also declined remarkably. The methane selectivity over the P-0 sample decreased from 9.5% to 6.9% during the dehydrogenation process. Over the P-10 sample, the initial and final methane selectivity was 7.1% and 5.1%. This indicated that the side reaction of propane cracking was suppressed by the phosphorous introduction. Combined with the data of propane conversion and product selectivity, it could be concluded that both the dehydrogenation performance and stability were improved after the phosphorus introduction.

  Figure 8

  

  Figure 8.  Conversion of propane over V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  Figure 9

  

  Figure 9.  Selectivity of propylene and CH₄ over V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  The catalytic performance of supported vanadium catalysts was closely related to the vanadyl species on the surface^{[49, 50]}. After the phosphorous introduction, the vanadium loading, surface vanadyl sites amount and surface vanadyl sites density all slightly increased and reached the maximum over the P-30 sample. However, a further increase of phosphorus content was detrimental for the highly dispersed vanadyl species although the vanadium loading amount still slightly increased. The propane conversion also increased due to the increase of surface vanadium sites. The best propane dehydrogenation performance was also obtained over the P-30 sample. This indicated that there existed a suitable range for the phosphorus introduction. A suitable amount of phosphorus introduction could prohibit the formation of polymerized vanadyl species by forming V−O−P or Si−P−O bond while excessive amount of phosphorus on the surface did not help further increase the dispersion of vanadyl species due to the decrease of specific surface area. The selectivity of propylene and methane were closely related to the structure and chemical environments of vanadyl species. The isolated or highly-dispersed vanadyl species was more favorable to form propylene while the aggregated vanadyl species were less selective. After phosphorus introduction, the reducibility of the vanadyl species was improved and more vanadyl species could easily complete the catalytic cycle of dehydrogenation, resulting in the enhanced stability and selectivity. The similarity of products selectivities over P-30 and P-10 indicated that the surface vanadyl species shared similar chemical environments. The decrease of the vanadyl sites on P-10 should be strongly correlated to the mild aggregation of vanadyl species caused by the decreased specific surface areas.

  3.   Conclusions

  Phosphorus-modified V-MCM-41 catalysts were synthesized and tested for propane dehydrogenation reaction. Although the specific surface area slightly decreased after the introduction of phosphorus, the ordered mesoporous structure remained stable. Both reducibility and dispersion of the surface vanadyl species were improved. The vanadyl species were highly dispersed on the surface, and the content of polymerized vanadyl species decreased due to the presence of phosphorus species. The surface vanadium sites reached a maximum of 3.35×10⁻⁴ mol/g over the P-30 sample. The propane dehydrogenation reaction results showed that both of the catalytic performance and stability of the catalyst were obviously increased after the phosphorus introduction.

  Acknowledgments

  The authors thank the Beijing Key Laboratory of Biogas Upgrading Utilization for the support of characterization.

  
  
• 1. [1]

     CAVANI F, BALLARINI N, CERICOLA A. Oxidative dehydrogenation of ethane and propane: How far from commercial implementation?[J]. Catal Today,2007,127(1/4):113−131. doi: 10.1016/j.cattod.2007.05.009

  2. [2]

     GARTNER C A, VAN VEEN A, LERCHER J A. Oxidative dehydrogenation of ethane: Common principles and mechanistic Aspects[J]. ChemCatChem,2013,5(11):3196−3217. doi: 10.1002/cctc.201200966

  3. [3]

     CARRERO C A, SCHLOEGL R, WACHS I E, SCHOMAECKER R. Critical literature review of the kinetics for the oxidative dehydrogenation of propane over well-defined supported vanadium oxide catalysts[J]. ACS Catal,2014,4(10):3357−3380. doi: 10.1021/cs5003417

  4. [4]

     JAMES O O, MANDA S, ALELE N, CHOWDHURY B, MAITY S. Lower alkanes dehydrogenation: Strategies and reaction routes to corresponding alkenes[J]. Fuel Process Technol,2016,149:239−255. doi: 10.1016/j.fuproc.2016.04.016

  5. [5]

     LONG L L, XIA K, LANG W Z, SHEN L L, YANG Q, YAN X, GUO Y J. The comparison and optimization of zirconia, alumina, and zirconia-alumina supported PtSnIn trimetallic catalysts for propane dehydrogenation reaction[J]. J Ind Eng Chem,2017,51:271−280. doi: 10.1016/j.jiec.2017.03.012

  6. [6]

     SOKOLOV S, BYCHKOV V Y, STOYANOVA M, RODEMERCK U, BENTRUP U, LINKE D, TYULENIN Y P, KORCHAK V N, KONDRATENKO E V. Effect of VO_x species and support on coke formation and catalyst stability in nonoxidative propane dehydrogenation[J]. ChemCatChem,2015,7(11):1691−1700.

  7. [7]

     WACHS I E. Catalysis science of supported vanadium oxide catalysts[J]. Dalton Trans,2013,42:11762−11769. doi: 10.1039/c3dt50692d

  8. [8]

     DONG A H, WANG K, ZHU S Z, YANG G B, WANG X T. Facile preparation of PtSn-La/Al₂O₃ catalyst with large pore size and its improved catalytic performance for isobutane dehydrogenation[J]. Fuel Process Technol,2017,158:218−225. doi: 10.1016/j.fuproc.2017.01.004

  9. [9]

     HARLIN M E, NIEMI V M, KRAUSE A O. Alumina-supported vanadium oxide in the dehydrogenation of butanes[J]. J Catal,2000,195(1):67−78. doi: 10.1006/jcat.2000.2969

  10. [10]

      LIU Y M, CA Y, YI N, FENG W L, DAI W L, YAN S R, HE H Y, FAN K N. Vanadium oxide supported on mesoporous SBA-15 as highly selective catalysts in the oxidative dehydrogenation of propane[J]. J Catal,2004,224(2):417−428. doi: 10.1016/j.jcat.2004.03.010

  11. [11]

      REDDY B M, LAKSHMANAN P, LORIDANT S, YAMADA Y, KOBAYASHI T, LÓPEZ-CARTES C, ROJAS T C, FERNÁNDEZ A. Structural characterization and oxidative dehydrogenation activity of V₂O₅/Ce_xZr_1-xO₂/SiO₂ catalysts[J]. J Phys Chem B,2006,110(18):9140−9147. doi: 10.1021/jp061018k

  12. [12]

      RAJU G, REDDY B M, PARK S E. CO₂ promoted oxidative dehydrogenation of n-butane over VO_x/MO₂-ZrO₂ (M=Ce or Ti) catalysts[J]. J CO₂ Util,2014,5:41−46. doi: 10.1016/j.jcou.2013.12.003

  13. [13]

      ZHOU R, CAO Y, YAN S R, FAN K N. Rare earth (Y, La, Ce)-promoted V-HMS mesoporous catalysts for oxidative dehydrogenation of propane[J]. Appl Catal A: Gen,2002,236(1/2):103−111.

  14. [14]

      SASIKALA R, SUDARSAN V, KULSHRESHTHA S K. Studies on the interaction of vanadia with modified silica supports and catalytic activity for oxidative dehydrogenation of propane: Effect of support modification by Al³⁺, Zr⁴⁺ or Y³⁺[J]. Eur J Inorg Chem,2006,2006(20):4151−4156.

  15. [15]

      YANG S, IGLESIA E, BELL A T. Oxidative Dehydrogenation of Propane over V₂O₅/MoO₃/Al₂O₃ and V₂O₅/Cr₂O₃/Al₂O₃:   Structural characterization and catalytic function[J]. J Phys Chem B,2005,109(18):8987−9000. doi: 10.1021/jp040708q

  16. [16]

      AJAYI B P, JERMY B R, OGUNRONBI K E, ABUSSAUD B A, AL-KHATTAF S. n-Butane dehydrogenation over mono and bimetallic MCM-41 catalysts under oxygen free atmosphere[J]. Catal Today,2013,204:189−196. doi: 10.1016/j.cattod.2012.07.013

  17. [17]

      ASCOOP I, GALVITA V V, ALEXOPOULOS K, REYNIERS M F, VAN DER VOORT P, BLIZNUK V, MARIN G B. The role of CO₂ in the dehydrogenation of propane over WO_x-VO_x/SiO₂[J]. J Catal,2016,335:1−10. doi: 10.1016/j.jcat.2015.12.015

  18. [18]

      OYAMA S T, WENT G T, LEWIS K B, BELL A T, SOMORJAI G A. Oxygen-chemisorption and Laser Raman-spectroscopy of unsupported and silica-supported vanadium-oxide catalysts[J]. J Phys Chem,1989,93(18):6786−6790.

  19. [19]

      WANG X, ZHOU G, CHEN Z, LI Q, ZHOU H, XU C. Enhancing the vanadium dispersion on V-MCM-41 by boron modification for efficient iso-butane dehydrogenation[J]. Appl Catal A: Gen,2018,555:171−177. doi: 10.1016/j.apcata.2018.02.021

  20. [20]

      SHYLESH S, SINGH A. Vanadium-containing ordered mesoporous silicates: Does the silica source really affect the catalytic activity, structural stability, and nature of vanadium sites in V-MCM-41[J]? J Catal, 2005, 233(2): 359−371.

  21. [21]

      WANG C B, DEO G, WACHS I E. Characterization of vanadia sites in V-silicalite, vanadia-silica cogel, and silica-supported Vanadia catalysts: X-ray powder diffraction, Raman Spectroscopy, Solid-State ⁵¹V NMR, Temperature-Programmed Reduction, and Methanol Oxidation Studies[J]. J Catal, 1998,1998,178(2):640−648.

  22. [22]

      SOLSONA B, BLASCO T, LÓPEZ NIETO J M, PEñA M L, REY F, VIDAL-MOYA A. Vanadium oxide supported on mesoporous MCM-41 as selective catalysts in the oxidative dehydrogenation of alkanes[J]. J Catal,2001,203(2):443−452.

  23. [23]

      WANG X, ZHOU G, CHEN Z, JIANG W, ZHOU H. In-situ synthesis and characterization of V-MCM-41 for oxidative dehydrogenation of n-butane[J]. Microporous Mesoporous Mater,2016,223:261−267.

  24. [24]

      LIU Q L, YANG Z, LUO M S, ZHAO Z, WANG J Y, XIE Z A, GUO L. Vanadium-containing dendritic mesoporous silica nanoparticles: Multifunctional catalysts for the oxidative and non-oxidative dehydrogenation of propane to propylene[J]. Microporous Mesoporous Mater,2019,282:133−145.

  25. [25]

      LI X K, JI W J, ZHAO J, ZHANG Z, AU C T. A comparison study on the partial oxidation of n-butane and propane over VPO catalysts supported on SBA-15, MCM-41, and fumed SiO₂[J]. Appl Catal A: Gen, 2006, 306: 8−16.

  26. [26]

      ARIAS-PÉREZ S, GARCÍA-ALAMILLA R, CÁRDENAS-GALINDO M G, HANDY B E, ROBLES-ANDRADE S, SANDOVAL-ROBLES G. Evaluation of vanadium-phosphorus oxide (VPO) catalysts for the oxidative dehydrogenation of propane[J]. Ind Eng Chem Res,2009,48(3):1215−1219.

  27. [27]

      BULÁNEK R, KALUŽOVÁA, SETNIČKA M, ZUKAL A, ČIČMANEC P, MAYEROVÁ J. Study of vanadium based mesoporous silicas for oxidative dehydrogenation of propane and n-butane[J]. Catal Today,2012,179(1):149−158.

  28. [28]

      SETNIČKA M, BULÁNEK R, ČAPEK L, ČIČMANEC P. n-Butane oxidative dehydrogenation over VO_x-HMS catalyst[J]. J Mol Catal A: Chem,2011,344(1/2):1−10.

  29. [29]

      SANTRA C, SHAH S, MONDAL A, PANDEY J K, PANDA A B, MAITY S, CHOWDHURY B. Synthesis, characterization of VPO catalyst dispersed on mesoporous silica surface and catalytic activity for cyclohexane oxidation reaction[J]. Microporous Mesoporous Mater,2016,223:121−128.

  30. [30]

      PUTLURU S, RIISAGER A, FEHRMANN R. The effect of acidic and redox properties of V₂O₅/CeO₂-ZrO₂ catalysts in selective catalytic reduction of NO by NH₃[J]. Catal Lett,2009,133:370−375.

  31. [31]

      HU P, LANG W Z, YAN X, CHU L F, GUO Y J. Influence of gelation and calcination temperature on the structure-performance of porous VO_x-SiO₂ solids in non-oxidative propane dehydrogenation[J]. J Catal,2018,358:108−117.

  32. [32]

      ROSTOM S, DE LASA H I. Propane oxidative dehydrogenation using consecutive feed injections and fluidizable VO_x/gamma Al₂O₃ and VO_x/ZrO₂-gamma Al₂O₃ catalysts[J]. Ind Eng Chem Res,2017,56:13110−13125.

  33. [33]

      WACHS I E, WECKHUYSEN B M. Vanadia catalysts for selective oxidation of hydrocarbons and their derivatives structure and reactivity of surface vanadium oxide species on oxide supports[J]. Appl Catal A: Gen,1997,157(1/2):67−90.

  34. [34]

      CARRERO C A, KETURAKIS C J, ORREGO A, SCHOMACKER R, WACHS I E. Anomalous reactivity of supported V₂O₅ nanoparticles for propane oxidative dehydrogenation: Influence of the vanadium oxide precursor[J]. Dalton Trans,2013,42:12644−12653.

  35. [35]

      MURGIA V, TORRES E, GOTTIFREDI J, SHAM E. Sol-gel synthesis of V₂O₅-SiO₂ catalyst in the oxidative dehydrogenation of n-butane[J]. Appl Catal A: Gen,2006,312:134−143.

  36. [36]

      ARENA F, FRUSTERI F, MARTRA G, COLUCCIA S, PARMALIANA A. Surface structures, reduction pattern and oxygen chemisorption of V₂O₅/SiO₂catalysts[J]. J Chem Soc Faraday Trans,1997,93:3849−3854.

  37. [37]

      FUKUDOME K, IKENAGA N, MIYAKE T, SUZUKI T. Oxidative dehydrogenation of propane using lattice oxygen of vanadium oxides on silica[J]. Catal Sci Technol,2011,1:987−998.

  38. [38]

      BAI P, MA Z, LI T, TIAN Y, ZHANG Z, ZHONG Z, XING W, WU P, LIU X, YAN Z. Relationship between surface chemistry and catalytic performance of mesoporous γ-Al₂O₃ supported VO_x catalyst in catalytic dehydrogenation of propane[J]. ACS Appl Mater Interfaces,2016,8(39):25979−25990.

  39. [39]

      TIAN Y P, BAI P, LIU S M, LIU X M, YAN Z F. VO_x-K₂O/γ-Al₂O₃ catalyst for nonoxidative dehydrogenation of isobutane[J]. Fuel Process Technol,2016,151:31−39.

  40. [40]

      CHEN C, SUN M L, HU Z P, LIU Y P, ZHANG S M, YUAN Z Y. Nature of active phase of VO_x catalysts supported on SiBeta for direct dehydrogenation of propane propylene[J]. Chin J Catal,2020,41(2):276−285.

  41. [41]

      CASALETTO M P, LISI L, MATTOGNO G, PATRONO P, RUOPPOLO G. An XPS study of titania-supported vanadyl phosphate catalysts for the oxidative dehydrogenation of ethane[J]. Appl Catal A: Gen,2004,267(1/2):157−164.

  42. [42]

      HASHA D, SIERRA DE SALDARRIAGA L, SALDARRIAGA C, HATHAWAY P E, COX D F, DAVIS M E. Studies of silicoaluminophosphates with the sodalite structure[J]. J Am Chem Soc,1988,110:2127−2135.

  43. [43]

      EBERHARDT M A, PROCTOR A, HOUALLA M, HERCULES D M. Investigation of V oxidation states in reduced V/Al₂O₃ catalysts by XPS[J]. J Catal,1996,160(1):27−34.

  44. [44]

      GAO X, JEHNG J M, WACHS I E. In situ UV-vis-NIR diffuse reflectance and Raman spectroscopic studies of propane oxidation over ZrO₂-supported vanadium oxide catalysts[J]. J Catal,2002,209(1):43−50.

  45. [45]

      RESINI C, MONTANARI T, BUSCA G, JEHNG J M, WACHS I E. Comparison of alcohol and alkane oxidative dehydrogenation reactions over supported vanadium oxide catalysts: in situ infrared, Raman and UV-vis spectroscopic studies of surface alkoxide intermediates and of their surface chemistry[J]. Catal Today,2005,99(1/2):105−114.

  46. [46]

      DZWIGAJ S. Recent advances in the incorporation and identification of vanadium species in microporous materials[J]. Curr Opin Solid State Mater Sci,2003,7(6):461−470.

  47. [47]

      PIUMETTI M, BONELLI B, MASSIANI P, DZWIGAJ S, ROSSETTI I, CASALE S, GABEROVA L, ARMANDI M, GARRONE E. Effect of vanadium dispersion and support properties on the catalytic activity of V-SBA-15 and V-MCF mesoporous materials prepared by direct synthesis[J]. Catal Today,2011,176(1):458−464.

  48. [48]

      CHRISTODOULAKIS A. Molecular structure and reactivity of vanadia-based catalysts for propane oxidative dehydrogenation studied by in situ Raman spectroscopy and catalytic activity measurements[J]. J Catal,2004,222(2):293−306.

  49. [49]

      RODEMERCK U, SOKOLOV S, STOYANOVA M, BENTRUP U, LINKE D, KONDRATENKO E V. Influence of support and kind of VO_x species on isobutene selectivity and coke deposition in non-oxidative dehydrogenation of isobutane[J]. J Catal,2016,338:174−183.

  50. [50]

      OVSITSER O, SCHOMAECKER R, KONDRATENKO E V, WOLFRAM T, TRUNSCHKE A. Highly selective and stable propane dehydrogenation to propene over dispersed VO_x-species under oxygen-free and oxygen-lean conditions[J]. Catal Today,2012,192(1):16−19.

• 

  Figure FIG. 1268. 

  下载: 全尺寸图片 幻灯片
  

  Figure 1  XRD patterns of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 2  N₂ adsorption-desorption isotherms of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 3  TPR curves of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 4  NH₃-TPD profiles of V-P-MCM-41 samples

  下载: 全尺寸图片 幻灯片
  

  Figure 5  O₂ chemisorption profiles of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Curve fitting results of V 2p_3/2 XPS spectra

  下载: 全尺寸图片 幻灯片
  

  Figure 7  Raman spectra of V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Conversion of propane over V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Selectivity of propylene and CH₄ over V-P-MCM-41 catalysts

  下载: 全尺寸图片 幻灯片

  Table 1.  Physio-chemical properties of V-P-MCM-41 catalysts

  Sample	SBET / （m2∙g−1）	Pore volume /（mL∙g−1）	d100 a /Å	α b /Å	r c /Å
  P-10	596.6	1.07	40.3	46.5	25.6
  P-30	601.5	0.85	40.2	46.4	27.1
  P-50	628.3	0.96	39. 9	46.1	27.0
  P-0	668.6	0.73	39.0	45.1	27.6
  a: [100] Crystalline interplanar spacing, calculated by Prague equation b: Cell parameter, α=2d100/30.5 c: Pore diameter

  下载: 导出CSV

  Table 2.  TPR, TPD and O₂ chemisorption results of V-P-MCM-41 catalysts

  Sample	Va/%	Proportion of surface vanadyl species b			Surface V sites c/ （10−4 mol∙g−1）		Surface V density d/ nm−2
  		highly dispersed	polymerized
  P-10	3.49	80.5	19.5		3.19		3.21
  P-30	3.45	78.8	21.9		3.35		3.35
  P-50	3.43	75.0	25.0		3.29		3.15
  P-0	3.38	74.3	25.7		3.11		2.80
  a: measured by ICP-AESb: determined by the deconvolution results of TPR c: determined by O2 chemisorptiond: based on the surface V sites determined by O2 chemisorption and BET surface area

  下载: 导出CSV

  Table 3.  XPS results of V-P-MCM-41

  Sample	E/eV			Vanadyl species distribution / %
  	O 1s	Si 2p		517.0 eV	518.4 eV
  P-10	532.6	103.3		90.4	9.6
  P-30	532.6	103.5		92.1	7.9
  P-50	532.6	103.4		88.4	11.6
  P-0	532.7	103.5		83.8	16.2

  下载: 导出CSV

  Table 4.  Surface composition of V-P-MCM-41 catalysts determined by XPS

  Sample	Surface composition /mol %				Surface V content
  	O	Si	P	V	w/%
  P-10	63.42	32.81	2.48	1.28	3.12
  P-30	65.12	32.45	1.14	1.29	3.17
  P-50	64.64	33.21	0.87	1.28	3.16
  P-0	65.22	33.47	−	1.31	3.03

  下载: 导出CSV
•