Evaluation of Vanadium− Phosphorus Oxide (VPO) Catalysts for the

Ind. Eng. Chem. Res. 2009, 48, 1215–1219

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Evaluation of Vanadium-Phosphorus Oxide (VPO) Catalysts for the Oxidative Dehydrogenation of Propane Salvador Arias-Pe´rez,† Ricardo Garcı´a-Alamilla,† Marı´a G. Ca´rdenas-Galindo,‡ Brent E. Handy,‡ Sergio Robles-Andrade,† and Guillermo Sandoval-Robles*,† DiVisio´n de Estudios de Posgrado e InVestigacio´n del ITCM, JuVentino Rosas y Jess Urueta S/N, Col. Los Mangos, C.P. 89440, Cd. Madero, Tamaulipas, Me´xico, Facultad de Ciencias Quı´micas de la UASLP, AV. Dr. Manuel NaVa No. 6, Zona UniVersitaria, San Luis Potosı´, SLP, Me´xico

The catalytic activity of vanadium-phosphorus oxide (VPO) catalysts for the oxidative dehydrogenation of propane is analyzed. Two series of catalysts were synthesized in an organic medium with P/V molar ratios of 0.9 and 1.1. These materials were characterized by X-ray diffraction, N2 physisorption, NH3 adsorption microcalorimetry, decomposition of 2-propanol, and redox titration. The crystalline phase (VO)2P2O7, with a tetravalent vanadium oxidation state, was observed in all the catalysts. As a result of thermal treatment, their crystal size increased, which suggests more crystallinity, and a slight increment in the surface area (31-39 m2/g) also was observed. The microcalorimetry studies of the vanadium phosphates showed heats of adsorption for NH3 between 260 and 60 kJ/mol that can be associated with strong and medium strength acid sites, which are responsible for promoting the selectivity toward oxygenated products and COx; furthermore, it was found that larger crystallographic disorder favors the selectivity to propylene up to an 8.82% over a VPO that has been calcined at 673 K and has a P/V molar ratio of 1.1. 1. Introduction The interest in the oxidative dehydrogenation (OXDH) of light paraffines (C2-C4) for their conversion to the correspondent olefins has increased rcently, because of the increasing demand of these feedstocks for the synthesis of chemical products. Therefore, the OXDH of alkanes plays a growing role in the petrochemical industry.1-3 The partial oxidation of n-butane is commonly done in the industry over vanadium phosphates to produce maleic anhydride; however, it has also been studied over vanadium supported on several metal oxides (VOx/Al2O3, VOx/SiO2, and V/MgO have special importance in this regard).4,5 The good performance of these materials in oxidation reactions has stimulated interest in their study for the OXDH of light paraffines.6-8 To achieve a good balance between conversion and selectivity in the OXDH of hydrocarbons, the active sites in vanadiumbased catalysts must be well-dispersed, without V2O5 crystallites, and disposed toward stabilizing tetravalent vanadium.9-11 In the partial oxidation of propane to acrylic acid, propylene is the first intermediate that is formed12 over catalysts of the type Mo-V-Sb-O,13 via the abstraction of two hydrogens from the alkane by an acid-base pair: the V5+ cation (Lewis acid center) and the oxygen-associated O2- anion (basic center). Acidity is essential; however, if the acidity is too strong, it will favor the successive formation of oxygenated products.12,13 Reducing the overall acidity would have a negative effect on catalytic activity, but the controlled elimination of only the strongest acid centers would have the benefit of increasing the selectivity toward the products of interest. Redox properties are also important, and the proximity of labile oxygen to active redox centers is a critical * To whom correspondence should be addressed. Tel./Fax: +52 833 2 15 85 44. E-mail: [email protected]. † Divisio´n de Estudios de Posgrado e Investigacio´n del ITCM, Juventino Rosas y Jess Urueta S/N. ‡ Facultad de Ciencias Quı´micas de la UASLP, Av. Dr. Manuel Nava No. 6, Zona Universitaria.

issue.9 In the case of unsupported vanadium-phosphorus oxide (VPO) catalysts, the P/V ratio is important for controlling the redox and acid-base properties,14 and in TiO2 supports, the conversion decreases with the phosphorus content for the OXDH of propane.15 Considering that the redox properties and the acid-base characteristics of the VPO, in addition to the (VO)2P2O7 phase, are related to the activity and selectivity in oxidation reactions; these materials have been proposed for the reaction of OXDH of hydrocarbons C2-C4.2,12 In this work, several VPO catalysts were synthesized; in addition, the structural, textural, and acid properties were characterized using X-ray diffraction, redox titration, nitrogen (N2) physisorption, 2-propanol decomposition, and microcalorimetry, and the catalytic properties were evaluated for the propane OXDH reaction. 2. Experimental Section 2.1. Catalyst Preparation. Vanadyl phosphate catalysts were prepared in organic media to give P/V molar ratios of 0.9 and 1.1.5,14,16 Pure V2O5 (Productos Quı´micos Monterrey S.A.) was reduced in a butyl-benzyl alcohol mixture (2:1 v/v) that was refluxed for 4 h at 383 K, yielding a dark-green vanadium(IV) solution that was subsequently reacted with H3PO4 (85%, Productos Quı´micos Monterrey) at 383 K and refluxed 2 h, forming a paste that was washed with butyl alcohol and filtered under reduced pressure, dried at 398 K and calcined in inert atmosphere (N2) at 673, 773, and 873 K. Samples are designated according to the P/V ratio and calcination temperature (for example, 0.9VPO873 denotes a VPO sample with a P/V molar ratio of 0.9 that has been calcined at 873 K). 2.2. Catalyst Characterization. Crystalline structure was determined with X-ray diffraction (Phillips, X-Pert MPD), using a graphite monochromator and Cu KR radiation (λ ) 0.154056 Å; 40 kV, 30 mA).

10.1021/ie800649z CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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Figure 1. Diffractograms of vanadyl phosphate catalysts with a P/V molar ratio of 0.9: precursor (0.9VPP) and precursors calcined at 673 K (0.9VPO673), 773 K (0.9VPO773), and 873 K (0.9VPO873).

Figure 2. Diffractograms of vanadyl phosphate catalysts with a P/V molar ratio of 1.1: precursor (1.1VPP), precursors calcined at 673 K (1.1VPO673), 773 K (1.1VPO773), and 873 K (1.1VPO873).

The average oxidation state of vanadium was determined from the titration of permanganate ion.14,16,17 The nonspecific surface area, including the presence of micropores, was determined with a volumetric adsorption apparatus using nitrogen (N2) adsorption at 77 K. Prior to N2 adsorption, samples were degassed to 0.1 Pa at 473 K for 1 h. Ammonia adsorption microcalorimetry was conducted in a Tian-Calvet calorimeter at 473 K,18,19 using 0.3 g sample sizes that were degassed under vacuum (10-3 Pa) at 673 K. Tests for the decomposition of 2-propanol were performed in a fixed-bed reactor at atmospheric pressure with 50 mg of catalyst at 393 K. The feed stream of 2-propanol/N2 consisted of passing a pure nitrogen (Praxair) stream of 4.39 × 10-3 g/min through saturator with alcohol at 283 K. Reaction product analysis was performed online using a gas chromatography (Varian Star 3400 CX) that was equipped with a Poropak Q column and a flame ionization detector (FID). Catalytic activity for the OXDH of propane was evaluated with a quartz U-tube reactor (4 mm ID) loaded with 50 mg of fine powdered catalyst. Reaction conditions were 743 K and atmospheric pressure with a feed of propane/air (10 vol % C3) maintained at a gas hourly space velocity (GHSV) of 0.196 min-1. Propane, propene, and COx compositions were evaluated online using a Chrompak MicroGC that was equipped with CPSil 5 and Hayesep A columns and a thermal conductivity detector (TCD).

Table 1. Physicochemical Properties of VPO Catalysts

3. Results and Discussion Diffraction patterns for the precursor state and for VPO catalysts are shown in Figures 1 and 2. In the precursor form, the two catalyst types (0.9VPP and 1.1VPP) show the same hydrated phase (VOHPO4 · H2O).16,20 For the purposes of forming active catalysts with tetravalent vanadium, the precursors were calcined at 673, 773, and 873 K under inert atmosphere. The average vanadium oxidation states of calcined samples are reported in Table 1. With 0.9VPO873 and 1.1VPO673, the oxidation states are 4.4 and 3.6, respectively, indicating that pentavalent vanadium (highly oxidized) is prevalent in the former sample, whereas the latter sample must contain an appreciable amount of trivalent vanadium (severely reduced). As shown by XRD, the only crystalline phase detected

catalyst 0.9VPO673 0.9VPO773 0.9VPO873 1.1VPO673 1.1VPO773 1.1VPO873

average average oxidation surface area, S crystal size state of (m2/g) (Å) vanadium, NV 174 186 293 152 186 228

4.1 4.2 4.4 3.6 4.2 4.1

33 32 31 39 36 33

2-Propanol Dehydrationa (%) XC3

SC3)

Sether

28 21 13 23 19 12

100 93 90 100 87 100

0 6 10 0 13 0

a XC3 ) 2-propanol conversion, SC3) ) propylene selectivity, Sether ) di-isopropyl ether selectivity.

in all materials is the pyrophosphate (VO)2P2O7,21,22 where vanadium is tetravalent, which is the phase that is considered to be the most active one for n-butane partial oxidation reactions.1,5,14 Nevertheless, the crystallinity of our samples is strongly dependent on the calcination temperature. In Figures 1 and 2, for example, it is observed that diffraction peaks for the (200), (013), (023), and (044) planes narrow and intensify with increasing calcination temperature. It has been reported recently that slightly disordered crystalline vanadyl phosphates are more active in the conversion of n-butane to maleic anhydride than highly ordered catalysts.16,17 Crystal growth with increasing calcination temperature is evident in both sample series (see Table 1). Differential heat of adsorption profiles for NH3 at 473 K are shown in Figures 3 and 4. Total acid site densities for Qads > 100 kJ/mol are 75-100 µmol/g in all samples, although the effect of higher calcination temperature produces slightly higher densities of all acid strengths for the P/V ) 1.1 series. In both series, the initial heats of adsorption increase with calcination temperature, showing 20 µmol/g of new 200-150 kJ/mol sites for P/V ) 0.9 and 15 µmol/g of new 200-300 kJ/mol sites for P/V ) 1.1 when calcination is performed at 773 K or above. The effect of pretreatment temperature in these catalysts is related to structural changes (progressive dehydration, relative amount of crystal defects in the pyrophosphate structure), and the calorimetric results generally suggest the creation or enhancement of existing Lewis acid sites at surface unsaturated V ions. This has implications for both catalytic activity and selectivity.

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Figure 3. Differential heat of NH3 adsorption for VPO with P/V ) 0.9, calcined at 673 and 773 K.

Figure 5. Propane OXDH conversion for VPO catalysts with P/V ) 0.9.

Figure 4. Differential heat of NH3 adsorption for VPO with P/V ) 1.1, calcined at 673 K, 773 K, and 873 K.

Figure 6. Propane OXDH conversion for VPO catalysts with P/V ) 1.1.

Acidity also was evident in the 2-propanol conversion data, with three catalysts showing almost complete or complete conversion to propylene (see Table 1). A minor amount of secondary product (di-isopropyl ether) was evident with 0.9VPO773, 0.9VPO873, and 1.1VPO773. Olefin formation is expected from acid sites, and the formation of ether is also a result of an acid-mediated process of coupling between propylene and 2-propanol.23,24 On the other hand, the absence of acetone formation indicates the lack of dehydrogenation activity. In both series, alcohol conversion has a tendency to decline as the calcination temperature increases (see Table 1). The specific surface areas reported here are all greater than 30 m2/g and typical of VPO catalysts prepared in organic media. The slight reductions in surface area within each series parallel this trend, which is indicative of minor sintering behavior, although the calorimetric data would suggest the opposite, that there should be higher alcohol conversions with the appearance of newer acid sites at higher calcination temperatures. The results for the oxidative dehydrogenation of propane are presented in Figures 5-8. Product selectivities for oxidation products (COx) and dehydrogenation (propylene) are reported. All catalysts produced an unquantified amount of acrolein that was condensed and trapped out as acrylic

Figure 7. Propylene and COx selectivity for VPO with P/V ) 0.9.

acid. Under the conditions applied, in all catalysts, the conversions increased slowly with time online, steadily approaching complete conversion over a 5-h period, although the P/V ) 1.1 series seems to stabilize more rapidly. Also

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Figure 8. Propylene and COx selectivity for VPO with P/V ) 1.1.

noteworthy is the fact that the catalysts that were calcined at the highest temperature (873 K) stabilize first. Product selectivity profiles for all catalysts do not change significantly with time. The COx selectivities vary between 24%-27% for the P/V ) 0.9 series and 21%-25% for the P/V ) 1.1 series. The lowest values were recorded for the catalysts that were calcined at 873 K: 22% for 0.9VPO873 and 20% for 1.1VPO873. The propylene selectivities remained below 10% for all catalysts (see Figures 7 and 8). The highest value was 8.8% (at 92% conversion), with 1.1VPO673. The structure of the active phase in VPO catalysts is complicated and sensitive to many factors, which, in addition to the P/V molar ratio and calcination temperature, can include reactant atmosphere and time-on-stream. The propylene selectivities reported here are low and consistent with the partial oxidation behavior that is typical of VPOs. Higher olefin selectivities have been achieved using more basic support materials that facilitate the rapid removal of formed olefins before they can be attacked by surface labile oxygen.7,25,26 In the case of the VPOs studied here, their demonstrated acidity is perhaps high enough to retain olefins near redox centers and allow them to be oxidized.2,27-29 4. Conclusions Vanadyl phosphates prepared in organic media at P/V molar ratios of 0.9 and 1.1 were calcined at 673, 773, and 873 K to produce crystalline vanadyl pyrophosphates with stable surface areas above 30 m2/g. Calcination at 873 K increased the degree of crystallinity without a major loss in surface area (sintering), and acidity was maintained. Catalytic activity was considerable, yet favored COx formation over propylene, perhaps as a consequence of strong acidity. The highest propylene selectivity (8.8%) was associated with a VPO with a P/V molar ratio of 1.1 that was calcined at 673 K. Acknowledgment Financial support from CONACYT No. 181667 (postgraduate fellowship) and UASLP Project No. CO5-FAI-04-4.6 are gratefully acknowledged. Literature Cited (1) Lemonidou, A. A. Oxidative dehydrogenation of C4 hydrocarbons over VMgO catalystsskinetic investigations. Appl. Catal., A 2001, 216, 277–284.

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ReceiVed for reView April 21, 2008 ReVised manuscript receiVed November 27, 2008 Accepted December 2, 2008 IE800649Z