C:\Users\raoh\Desktop\Paper_6.xps TJER 2012, Vol. 9, No. 1, 46-54 1. Introduction Propene is an important link between relatively cheap and abundant natural gas and many petrochem- icals and intermediates such as propylene oxide, acry- lonitrile and isopropyl alcohol. This importance has spurred a seires of recent research efforts towards _________________________________________ *Corresponding author’s e-mail: baba@squ.edu.om developing catalysts for oxidative dehydrogenation of propane as an alternative route to creating propene (Mamedov, Cortes 1995). The alternative route has the potential to lower reaction temperatures and pres- sure due to the absence of a thermodynamic equilibri- um, which is a major limitation of the non-oxidative route. However, thus far the catalysts proposed for Oxydehydrogenation of Propane over Vanadium Oxide Supported on Kieselguhr or MCM-41 BY Jibril*a, A Attaa, SA Al-Dressb, MC Al-Kinanyb and HA Al-Megrenb aPetroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33, Postal Code 123, Al-Khoud, Muscat, Oman bPetroleum & Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh - 11442, Saudi Arabia Received 7 July 2009; accepted 15 May 2011 Abstract: Supported vanadium oxide (5 wt%) on either Kieselguhr or mesoporous MCM-41 was pre- pared using impregnation method and tested as a catalyst in propane oxidative dehydrogenation (POD). The catalyst samples were characterized using X-ray elemental analysis, Brunauer-Emmett-Teller (BET) physisorption, and Z-ray Photoelectron Spectroscopy (XPS). After impregnation, the catalyst surface area decreased compared with that of the support. More drastic decrease was observed in the case of MCM-41 (77%) than the Kieselguhr supported sample (48%). There are also different degrees of vana- dium oxide-support interaction as reflected by the XPS result. Si-O binding energy of 531.5 eV was observed on MCM-41-supported sample compared with 529.5 eV for the Kieselguhr-supported sample. The catalyst tests were conducted at atmospheric pressure, with a propane to oxygen ratio of 0.7 - 3.6 and a reaction temperature of 400 - 700 °C. Oxidative dehydrogenation and combustion products were observed. Minor cracking reaction products (methane, ethane, and ethene) were also produced above 550 °C. The highest propene yield of 14% was obtained from the Kieselguhr-supported sample at 700 °C and with a C3H8/O2 ratio of 1.5. Keywords: Oxidative dehydrogenation, Propane, Propene, Vanadium oxide, Kieselguhr, MCM-41 "Kieselguhr"(MCM-41) -0 (SI 47 BY Jibril, A Atta, SA Al-Dress, MC Al-Kinany and HA Al-Megren oxydehydrogenation of propane to propene have exhibited low yields. As of publication, there is no economic motivation for developing the process on a commercial scale due to low yields (Cavani et al. 2007). Because of this low yield, the current research effort is toward developing active catalysts to conduct the reaction at temperatures lower than 600 oC with low selectivity to undesired products. To this end, a large number of supported oxides and mixed-oxides have been reported as summarized in recent reviews (Mamedov, Cortes 1995; Blasco, Lopez-Nieto 1997; Centi et al. 2001). Most of the catalysts reported, such as V-Mg-O (Mamedov, Cortes C 1995; Centi et al. 2001), Mg-V-Sb-O (Michaels et al. 1996a; Michaels et al. 1996b) and V-Nb-O (Smits et al. 1993) are based on vanadium. Others, such as those based on molybdenum, have also been shown to be active for the reaction (Stern et al. 1997). For the vanadium- based system, high selectivity to propene has been associated with the presence of monomeric or oligo- metric tetrahedral V5+ species. The bridging oxygen between vanadium and the support activates propane (Christodoulakis et al. 2004; Routray et al. 2004). In the case of V-Mg-O, three different phases - Mg3V2O8, Mg2V2O7 and MgV2O6 - have been identified. Two of them - orthovanadate (Chaar et al. 1987) and pyrovanadate (Siew et al. 1990; Ruiz-Guerrero et al. 1992) - are associated with the activation of alkane. In both phases, the catalysts reactivities depend on the difference in the nature and surface density of V-O bonds (Mamedov, Cortes 1995; Blasco, Lopez-Nieto 1997). Furthermore, the nature of the support influ- ences the surface density and reducibility of the V-O bond (Mamedov, Cortes 1995; Tsilomelekis et al. 2007; Xie et al. 2000). Silica was identified as exhibiting higher selectivity to propene than more acidic supports such as alumina and titania (Kondratenko et al. 2006; Singh et al. 2005; Grabowski 2004). High vanadia dispersion was real- ized on high surface mesoporous silica such as M41S (Pena et al. 2001; Buyevskaya et al. 2001), HMS (Knotek et al. 2007) and MCM-41 (Karakoulia et al. 2008; Jibril, Ahmed 2006; Shanahan, Holder 1999). Our previous study demonstrated the effect of intra and extra-framework positions of the Co, Ni or Mo in MCM-41 on the selectivity to propene (Jibril, Ahmed 2006). The aim of this work was to further explore the effects of silica-based supports - (i.e., Kieselguhr and MCM-41 - on the vanadium species and their cat- alytic performances in the oxidative hydrogenation of propane. Kieselguhr is diatomaceous earth (primarily silica dioxide based) that has been demonstrated to be effective as a support in oxidative dehydrogenation of propane (Jibril et al. 2006) and other reactions (Wells 1991). In addition to the catalytic test, the properties of a sample of the catalysts were studied using Brunauer-Emmett-Teller (BET), surface area, (X-Ray Fluorescene (XRF), and X-Ray Photoelectron Spectroscopy (XPS). The catalysts were found to be active for the reaction. 2. Experimental 2.1 Preparation of Catalysts The materials used in the experiment were: cetyltrimethyl ammonium chloride (C16TMACl) (Sigma) tetraethoxysilane (TEOS) (Sigma, 98%), sodium aluminate (Fluka, 98%), ammonium vanadate [(NH4)VO3] (Aldrich, 99%) and commercial Kieselguhr (BDH). High purity propane (99.95%), oxygen (99.995%), and helium (99.999%) were obtained from a local supplier and used as received. The mesoporous MCM-41 support was prepared according to a procedure reported earlier (Berndt et al. 2000): 2 g of C16TMACl was dissolved in 500 ml of distilled and deionized water containing 50 ml of 0.125 M of NaOH at room temperature. The mixture was stirred for 2 hours. To the above solution, 8 grams of sodium aluminate in 50 ml water and 5 grams of TEOS were added. The mixture was continuously stirred for 48 hours at ambient temperature. A precipi- tate was formed, filtered, washed with distilled and deionized water and dried for 10 hours at 100 ºC in a vacuum oven. To remove the organic surfactant tem- plates, the synthesized sample was refluxed in 200 ml of ethanol for 12 h. The extracted material was fil- tered, washed with ethanol and dried for 10 hours at 100 °C in a vacuum oven. MCM-41 and Kieselguhr supported vanadium oxide catalysts were prepared by wet impregnation of MCM- 41 and commercial Kieselguhr with an aqueous solu- tion containing ammonium vanadate with a concentra- tion calculated to give a total loading of 5 w% of vana- dium as an oxide. The prepared catalysts were dried at 100 °C in a vacuum oven for 16 hours before calcina- tion in the air at 700 °C for 3 hours. In the subsequent discussion, the catalysts V-oxide/MCM-41 and V- oxide/Kieselguhr are represented as V/M and V/K respectively. 2.2 Catalyst Characterization The nitrogen adsorption-desorption isotherm of MCM-41, commercial Kieselguhr, or supported vana- dium oxide catalysts samples were measured with the Micromeritics ASAP-2010 system using nitrogen sorption at 77 K. Prior to the experiments, approxi- mately 0.2 g of sample was degassed at 523 K for 10 hours. The surface areas and porosities were obtained. The surface area was calculated using a multipoint 48 Oxydehydrogenation of Propane over Vanadium Oxide Supported on Kieselguhr or MCM-41 BET model. The pore sizes were obtained using Berrett-Joyner-Halenda (BJH) and BET models. The total pore volumes were estimated at a relative pres- sure of 0.99 atm, assuming full surface saturation with nitrogen. XPS studies were recorded with a Jeol JPS 9010MC photoelectron spectrometer using MgK (1253.6eV) radiation from an X-ray source operated at 10KV and 20 mA. The measurements were performed at room temperature and the working pressure was lower than 3.5x10-7 Pa. X-ray elemental analysis studies were taken at 30 KV at room temperature using a Jeol ele- ment analyzer JAX-3201. 2.3 Catalyst Evaluation The catalyst was tested in a laboratory bench scale pilot plant fitted with mass flow meters and a fixed bed down pass flow stainless steel reactor (inside diameter = 8 mm, and length = 300 mm) placed in a cylindrical furnace equipped with a coaxial thermocouple. The catalyst zone (about 30 mm) in the middle of the reac- tor was filled with 1.0 g of catalyst diluted with an equal amount of quartz particles (100-600 mesh) in order to minimize temperature gradients. In the center of the catalyst bed, a thermocouple was installed so that it was in contact with the catalyst particles were measure the reaction temperature. The heating zones at the inlet and the outlet of the reactor was measured in a thermocouple located inside the furnace and were controlled by a temperature controller (Cole Parmer, Digi-Sense). Quartz wool was placed in the empty space in the reactor to minimize gas-phase reactions. A feed flow of 83 ml/minute with -propane to oxygen ratios ranging from 0.7 - 3.6 - at atmospheric pressure was used with helium employed as a diluent. The cat- alyst zone of 30 mm mades the contact time 4.4 sec- onds. The activities of the catalysts and product distri- bution were studied at 400, 500, 600 and 700 °C. Blank reactor runs were conducted as described above in the absence of catalysts. No significant conversions of C3H8 or O2 were observed. Carbon mass balance obtained was within ± 8%. The reactants and reaction products were passed through a heated-line to an online RGA Varian 3800 series gas chromatography equipped with a flame ion- ization detector (FID) to analyze hydrocarbon prod- ucts (CH4, C2H6, C2H4, C3H8 and C3H6) and thermal conductivity detectors (TCD) to analyze O2, CO and CO2. The analyses were based on external standard calibration. Conversions, selectivity, and carbon balance are defined below: Conversion = [Propane (in) - Propane (out)] x 100/Propane (in) Selectivity = [moles of product / moles of products] x 100 Carbon balance = [moles of C atoms (out) / moles of C atoms (in)] x 100 3. Results and Discussion 3.1 Catalyst Characterization BET surface area, pore volume, and average pore size based on the BET and BJH methods for the cata- lysts and bare MCM-41 and Kieselguhr are shown in Table 1. The impregnation of the active component and calcinations causes reductions in the BET surface areas and pore volumes of the supports. For the less porous Kieselguhr; the decrease in the area (48%) is much lower than that of mesoporous MCM-41 (77%). There are corresponding decreases in pore volumes and increases in pore diameters as was found in an ear- lier report (Liu et al. 2008). This suggests a blockage of narrow pores by vanadium oxides dispersed on the surface of the supports. X-ray elemental analysis studies are recorded using X-ray source operating at 30 kV. The analysis of sup- Table 1. BET surface areas, pore volumes and average pore diameters of the supports and catalysts (a) BET surface area (b) Pore volume (c) Average pore size determined based on the BET method (d) Average pore size determined based on the BJH method 49 BY Jibril, A Atta, SA Al-Dress, MC Al-Kinany and HA Al-Megren ported vanadium oxide on Kieselguhr showed two major peaks in SiK attributed to Kieselguhr (SiO2) and VK attributed to metal oxide as shown in Fig. 1a. Also the analysis of supported vanadium oxide on MCM-41 showed three major peaks SiK and AlK attributed to MCM-41 support and VK attributed to 0.00 1.00 2.00 3.00 4.00 5.00 6.00 Figure 1a. X-Ray elemental analysis of V-oxide/Kieselguhr support 0.00 1.00 2.00 3.00 4.00 5.00 6.00 Figure 1b. X-Ray elemental analysis of V-oxide/MCM-41 support 50 Oxydehydrogenation of Propane over Vanadium Oxide Supported on Kieselguhr or MCM-41 metal oxide as shown in Fig. 1b. The quantitative analysis as summarized in Table 2 shows that the com- position of vanadium oxide on each support is about 5 wt% after the calcinations. The results in Table 3 shows the results of XPS for the catalysts and supports. Figures 2a and 2b show two of the XPS spectra, as samples of wide and narrow scan spectra (O1s, Si2p3/2 and V 2p3/2) of V2O5/Kieselguhr. It can be speculated that the V2p3/2 binding energy of 519.1 +1eV for the V-line is due to the presence of vanadium species in an oxidation state V5+ in the vanadium samples. Its variation in the cata- lysts indicates the effect of V-O-support interactions. The XPS of the Si2p3/2 energy region shows the pres- ence of a well defined line at 102.1eV characteristics of SiO2 state of Kieselguhr and MCM-41 supports. The Al2p photoelectron spectra exhibit Al 2p1/2 at 74.5 eV, which shares characteristics of Al2O3. A strong O 1s spectral line recorded at 532 eV is attrib- uted to oxygen from the supports and supported vana- dium oxides. The O 1s line for MCM-41 support could be divided into two bands (in the curve fitting) at 531.5 eV (Si-O) and 532.2 eV (Al-O). In the case of V- oxide/Kieselguhr the two bonds are identified at 529.5 eV (Si-O) and 531.5 eV (V-O), while in the case of V- oxide/MCM-41 the three bonds are identified at 531.5 eV (V-O), 531.2 eV (Si-O) and 532.7 eV (Al-O). The variations in oxygen binding energies reflect the effects of the supports on the catalysts surface proper- ties. 3.2 Tests of Catalysts The performances of the catalysts in propane oxida- tive dehydrogenation were studied. Figure 3 summa- rizes the effects of temperature and feed compositions on propane conversions and products distributions. Propene is the desired product. In the oxidative dehy- drogenation reaction, we expect carbon poisoning effects to be minimized. This is because the oxygen in the feed stream reacts with the carbon or its precursor to produce CO/CO2. The catalysts exhibit low conver- sions at 400 °C. Both propene and CO2 appear to be primary products on V/K while only propene is a pri- mary product on V/M. The occurrence of the two reac- tions suggests the presence of sites of different reducibilities on the catalysts (Klisinska 2006). On V/K, the conversion increases exponentially to 60% at 700 °C, with a corresponding linear decrease in propene selectivity. CO was produced at 500 °C and higher temperatures with selectivities of 5 - 15%. It is perhaps a secondary product of propene further reac- tions. When the conversion increases to 5%, selectivi- ty to CO2 increases about six fold that of CO. The high selectivity to CO2 is in line with the presence of highly reducible sites. These attack the electron-rich region of propyl or propane species to produce CO2. The exponential increase in propane conversion is due to cracking of propane as evidenced by the high selectivities to methane and ethane. However, the cracking seems to be catalyst surface mediated due to Table 2. X-Ray elemental analysis Table 3 catalysts (a) Curve fitting of the 0 1s binding energy 51 BY Jibril, A Atta, SA Al-Dress, MC Al-Kinany and HA Al-Megren lower levels of conversion and selectivity to CO2 observed on V/M (Ji et al. 2008). On the V/M, propene is the only primary product. Its further reac- tion led to CO instead of the CO2 observed in the case of V/K at 500 °C. At 600 °C, selectivity to CO2 of 66% was obtained. Thus, there was CO to CO2 switch between the temperatures of 500 and 600 °C. This sug- gests the dynamic change in reducibilities of the cata- lyst’s surface. It may also be associated with an increase in oxygen mobility at higher temperature, Binding Energy (eV) Figure 2b. Narrow scan XPS spectrum of 0 1s, Si 2p/32 and V 2p3/2 of 5% V2O5 / Kiesel O1s Si 2p V 2p 0 900 800 700 600 500 400 300 200 100 Binding Energy (eV) 18000 14000 10000 6000 2000 Figure 2a. Wide scan XPS spectrum of supported 5% V205 on Kieselguhr 540 535 530 525 105 100 95 90 520 515 510 505 Oxygen Silicon Vanadium 1000 800 600 400 200 52 2 rather than CO, depending on the nature of the catalysts (Owen et al. 2008). Similar to V/K, a substantial amount of cracking products with selectivity of about 35% was observed. In order to gain further insight into the effects of catalysts on the cracking reaction, the propane/oxygen ratio was varied at 700 °C (Fig. 4). As the ratio increases (0.7 - 3.6), the conversion and selectivities to COx decrease with corresponding increase in selectiv- ities to propene. The relative amount of oxygen in the feed determines the selectivity to propene. For the oxygen-rich feed; V/K exhibits selectivity to propene of 3%. This improves to 23% when the amount of propane increases five times at a constant oxygen flow. The ratio of CO/CO2 also changes (2.8-0.7) from low to high propane flow. The distribution is different on V/M catalysts. The dominant product is CO2 at all feed compositions. The differences in COX distributions by the catalysts are in line with differences in oxygen binding energy in V/K and V/M http://(srdata.nist.gov/xps/ (NIST X-ray photoelectron spectroscopy database). 4. Conclusions Vanadium-oxide supported on Kieselguhr is more active than that supported on MCM-41 in propane Figure 4. Effect of propane / oxygen ratio on propane conversion and products selectivities for (a) V2O5 / Kieselguhr and (b) V2O5 / MCM-41 C3H8/02 Ratio Reaction Temperature, oC Figure 3. Effect of temperature on propane conversion and products selectivities for (a) V2O3/Kieselguhr and V2O3/MCM-41 53 BY Jibril, A Atta, SA Al-Dress, MC Al-Kinany and HA Al-Megren oxidative dehydrogenation at the conditions explored. The latter's surface area was more drastically decreased when it was impregnated with the vanadium oxide. Both propene and CO2 appeared to be primary products on vanadium-oxide/Kiesulguhr while only propene was the primary product on vanadium- oxide/MCM-41, suggestive of sites of different reducibilities on the catalysts. The superior perform- ance was maintained when the catalysts were further tested in both oxygen-rich and oxygen-deficient feed- stock. The higher activity and different products distri- butions of Kieselguhr supported samples is associated with its lower Si-O binding energy as evidenced by XPS. The different extent of vanadium oxide interac- tion with supports affects both its surface properties and catalytic activities. 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