Selective Oxidative Dehydrogenation of Propane on Promoted

Chapter 29

Selective Oxidative Dehydrogenation of Propane on Promoted Niobium Pentoxide 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 23, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch029

R. H. H. Smits, K. Seshan, and J. R. H. Ross

Faculty of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, Netherlands

It has been shown that the activity of niobia for the oxidative dehydrogenation of propane can be increased by adding vanadium or chromium, while maintaining a high selectivity towards propylene. The vanadium-containing materials have been characterised further by XRD, TPR, laser Raman spectroscopy and XPS in combination with Ar -sputtering. The influence of various catalyst pretreatments on the catalytic performance has also been investigated. +

There is currently an increasing interest in the catalytic activation of lower alkanes. Following the initial work of Chaar et al. (1) on η-butane, a number of papers have appeared on the oxidative dehydrogenation of propane (2-10) over V-Mg-O catalysts. Propane is usually investigated because oxidative dehydrogenation may be an attractive alternative route for the production of propylene, compared to conventional dehydrogenation and cracking. Oxidative dehydrogenation of isobutane is another interesting application as the isobutylene produced could be used in the synthesis of MTBE. Research on this subject has focused on discussions of the nature of the active phase: magnesium orthovanadate or pyrovanadate (1, 5, 6). We attempted to improve the V-Mg-O catalyst system by doping it with various elements (8, 11), but these attempts were not successful. We have therefore focused our attention on niobium pentoxide. In previous papers (9,10) we have shown that high selectivities towards propylene are possible using pure niobia, but that the conversions were low. In this paper we present the results of experiments which show that the activity of niobia can be improved by adding other suitable elements, while maintaining high selectivity. These results bring the proces of oxidative dehydrogenation of propane closer to practical application. 1

Current address: University of Limerick, Plassey Technological Park, Limerick, Ireland

0097-6156/93/0523-0380$06.00/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Selective Oxidative Dehydrogenation of Propane

29. SMITS ET AL.


Experimental Niobia-based catalysts containing approximately 5 mol% of a high-valency element oxide were made by three different methods. A number of elements of which the oxides are soluble in both oxalic acid and ammonia (Ρ, V, Cr, Ge and Mo) were incorporated by evaporation of the water from an oxalic acid solution of niobium and the oxide using a rotary evaporator. Elements of which the oxides are soluble in oxalic acid but not in ammonia (Ti, Zr and Sn), were incorporated using coprecipitation: a mixed oxalic acid solution of the two elements was slowly added to an ammonium oxalate/ammonia buffer (pH 10) at 60 °C, the pH being kept around 10 by addition of ammonia. Elements of which the oxides are not soluble in oxalic acid or ammonia (Sb, Pb and Bi) were incorporated by simultaneous slow addition of a nitric acid solution of the element and an oxalic acid solution of niobium to an ammonium oxalate/ammonia buffer (pH 10) at 60 °C. The resulting precipitates were filtered and washed on a glass filter. In all cases, the powders were then dried overnight at 80 °C, followed by calcination for 5h at 630 °C. This temperature was chosen because it appeared to be optimal for unpromoted niobia (9, 10). The chemicals used in the different methods were ( N H ) H P 0 , TiO(AcAc) , N H V 0 , Cr(N0 ) .9H 0, G e 0 , ZrOCl .8H 0, ( N H ) M o 0 . 4 H 0 , SnCl .2H 0, Sb(OAc) , Pb(N0 ) and BiO(N0 ). The niobium oxalate was provided by Niobium Products Company, Inc., USA (batch AD/651). Because of the interesting results obtained with the addition of vanadium, catalysts containing 0.25, 1 and 10 wt% V 0 were prepared by evaporation as described above. In addition, two other preparation methods were also used. The first of these involved dry-mixing of the hydrated niobia (Niobium Products Company batch AD/628, 10g), N H V 0 and oxalic acid dihydrate (40g), and heating the resultant mixture at 120 °C overnight. This causes the oxalic acid to dissolve in its water of crystalisation, and then to react with the hydrated niobia and the vanadium to form niobium oxalate and vanadium(IV) oxalate. A large part of the oxalates formed dissolves; the oxalic acid evaporates slowly and the oxalates then decompose, the result being a very fine powder (VINb melt). The other method involved wet impregnation of the vanadium by evaporation of an aqueous N H V 0 solution in the presence of a niobia support (14.3 m /g) prepared by calcination of hydrated niobia for 5h at 600 °C (VINb imp.). The catalysts prepared in these ways were characterised by X-ray powder diffraction (for phase composition), Temperature Programmed Reduction (TPR, for reducibility), Ar or N adsorption (for total BET area), and X-ray fluorescence (for chemical composition). TPR experiments were performed using a TCD detector, a gas flow of 20 ml/min of 5% H in argon, a heating rate of 5 °C/min, and 100 mg of sample (0.3-0.6 mm grains). The materials were tested for catalytic activity using a flow reactor system as described previously (10), using a feed consisting of 10 vol% 0 , 30 vol% C H and 60 vol% He, a gas flow of 140 ml/min, and 600 mg of catalyst (0.3-0.6 mm grains). The tests were carried out by heating theJsample in a series of seven steps of 25 °C in the required temperature range, and carrying out a sequence of measurements during each temperature step; each temperature was maintained for two hours. Laser Raman 4

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Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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spectra were taken using a Specs Triplemate spectrometer equipped with an argon laser (wavelength 514 nm), and XPS in combination with Ar -sputtering was done on a Kratos X S A M 800 (sputtering rate 1-2 Â/min, vanadium concentrations were calculated from the corrected areas of the peaks of V2p and Nb3d). +


Results The results of XRF, BET surface area measurements and catalytic testing are summarized in Table I, while the results of catalytic testing are also shown in Figure 1. The rates displayed in Table I are the rates of reaction of propane per unit surface area as calculated from the interpolated propane conversion. Propane conversion was 3% or less, while oxygen conversion was less than 30%. Table I. Results of XRF, BET Surface Area Measurements and Catalytic Testing for Niobia Catalysts Containing Various Additives Sample Amount Surface area Rate at 550 °C Temperature Selectivity (mol% ) (m /g) (/xmnl/m *) (at 20% Q conversion) Nb O 0 72 9.9 581 2.7 P5Nb nd 35.7 71 2.8 551 Ti5Nb 4.4 544 66 28 3.0 V5Nb 4.6 11.3 353 75 8.6 Cr5Nb 4.3 43.2 372 78 1.4 Ge5Nb 4.7 41 542 2.2 48 Zr5Nb 5.9 21 542 70 5.8 Mo5Nb nd 15.7 498 88 2.0 Sn5Nb nd 16 539 18 2.9 Sb5Nb nd 20 0.8 PbSNb 4.9 17 3.2 BiSNb 8.7 18 2.2 551 14 Element relative to Nb; Prepared by calcination of hydrated niobia for 5h at 650 °C; At 350 °C; At 450 °C. a

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The BET surface areas fall in the range 10-45 m /g, with Cr5Nb and Ge5Nb having the highest areas, and V5Nb having the lowest surface area. Most surface areas are higher than that of pure niobia also when calcined at 600 °C (14.3 m /g). The X R D results showed that mixed oxides were formed with V5Nb ( V N b 0 , (12)\ Sb5Nb (SbNb0 ) and Pb5Nb (PbNb 0 ); only in the case of Ge5Nb (and possibly Pb5Nb) the pure dopant oxide was found. T T - N b 0 was found to be the major phase in all cases. Any other phase present in the catalysts was either amorphous or present in a quantity too small to be detected. In order to compare the activities of the catalysts, the temperature at which 20% oxygen conversion is reached is displayed in Table I, while the oxygen conversion as a function of temperature is shown in Figure la. The increase in activity caused by the addition of vanadium is remarkable; good results were also obtained for the chromium-containing material. In both cases, the temperature at which reaction started was 200 °C below that required for the unpromoted N b 0 . The addition of molybdenum also increased the activity, but molybdenum was lost 2

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Figure 1 Oxygen conversion and propylene selectivity as a function of temperature for niobia with various additives.



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from the catalyst, and deposited at the end of the reactor (thus resulting in an unstable catalyst). Most of the other additives had a negligible effect on the activity compared with pure niobia, while Sb and Pb caused a decrease in oxygen conversion (they did not reach 20% oxygen conversion in the temperature range investigated). The curves for the conversion of propane have the same shape as the oxygen conversion curves, while the amount of propane converted depends on the efficiency of the use of oxygen, i.e. on the selectivity towards total oxidation. The propane conversion may reach 16% at 100% oxygen conversion. The same conclusions can be drawn from the rates of reaction of propane per unit surface area shown in Table I, with the possible exception of Zr5Nb, which is slightly more active than the others, and Pb5Nb, which shows a normal reactivity towards propane. The effect of the additives on the selectivity towards propylene was more pronounced. Figure lb shows that both V5Nb and Cr5Nb, in addition to having a high activity, have high selectivities towards propylene. A V-Mg-O catalyst prepared according to the method described by Chaar et al. (2) and tested under our circumstances reached a selectivity towards propylene of less than 60% at a temperature which was 50 °C higher. Other products produced by these catalysts were CO (5-20% sel), C 0 (10-15% sel) and small amounts (less than 1%) of C 2 H and C H . In contrast, the addition of Bi or Sn was detrimental to the selectivity towards propylene, while addition of Pb or Ge also caused a decrease in selectivity over part of the temperature range. The other additives did not affect the selectivity towards propylene, although they tended to produce more total oxidation products (CO and C 0 ) than cracking products ( C H and CH ). 2

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Table II. Results of XRF, BET Surface Area Measurements and Catalytic Testing for Vanadia-Niobia Catalysts Sample Amount Surface Rate at 350 °C Temperature Selectivity code (mol%) area (m /g) (μπιοί/m .s) (at 50% 0 conversion) V0.25Nb 0.26 11.0 8.5 565 77 VINb 0.93 33.2 2.0 423 77 V5Nb 4.58 11.3 8.6 390 70 VIONb 8.64 9.4 10.3 408 75 VINb 1. fresh 0.99 16.1 4.6 423 76 melt 2. used 1.5 448 79 3. oxidized 4.0 420 76 4. reduced 1.3 452 80 5. no further treatment 1.4 448 79 VINb imD. 1.47 8.5 5.9 455 75 At 500 °C. 2

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The results of the characterisation of the series of catalysts with different vanadium loadings are presented in Table II. The X R D results showed that at higher loading of vanadium, at least one new phase is formed. In VIONb, the major phase was B(V, N b ) 0 (23), while T T - N b 0 was only a minor phase. The same conclusion can be drawn from the laser Raman spectrum of VIONb shown in Figure 2. The broad band at 600 cm' , which is typical for N b 0 (10), has 2

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become much smaller, and a new band at 950 cm" appears; this may be attributed to V = 0 bonds. In laser Raman spectra of samples with a lower vanadium concentration, only the broad band at 600 cm" was visible. It appears that the catalyst containing 0.26 mol% V 0 is much less active than the other samples. The other catalysts are more or less equally active when oxygen conversion is considered. On a per unit surface area basis, VINb is less active than V5Nb and VIONb, but this sample also has a much higher surface area. Results not given here showed that the minimum value of selectivity, which usually occurs when the oxygen conversion reaches 100%, became lower for the more active catalysts. Table II also gives results for the effect of various pretreatments on a VINb melt catalyst. After a catalyst of this type had gone through the usual temperature cycle used to test the catalyst (100% oxygen conversion is reached during the last part of this temperature cycle), its activity during a second cycle was lower than that of the fresh catalyst. However, when the catalyst which had been used once was given an oxygen treatment (lh in flowing oxygen at 600 °C), the original behaviour of the fresh catalyst was restored. When the catalyst was then reduced by hydrogen (lh in 5% H / A r at 600 °C), the activity decreased. When this catalyst was then tested again without any further treatment, the behaviour of the used catalyst was restored. From the TPR results for this catalyst shown in Figure 3 it follows that the reduction treatment mentioned above is sufficient to reduce vanadium to V . The consumption of hydrogen during the TPR was equivalent to the reduction of all the vanadium in two equal steps from V to V . A TPR of the same catalyst after use showed the same pattern, except that the first reduction peak at 506 °C was smaller. The niobium reduction peak at 890 °C corresponds to 8% of the Nb being reduced to N b . This reduction peak also occured during TPR of undoped TT- and T-Nb 0 , but not in the case of H - N b 0 (14). The results of XPS combined with Ar -sputtering on a VINb imp.-sample are shown in Figure 4. In this figure, sputtering profiles are shown for a fresh catalyst and for one which had been used for 300h at 510 °C (at this temperature 100% oxygen conversion was reached, no deactivation was observed). Since X R F showed that no vanadium was lost from the catalyst, it can be concluded that in the fresh catalyst, vanadium is present in a homogeneous layer which is at least 40-80 Â thick. Upon use, sub-surface vanadium has diffused into the bulk of the catalyst, while the surface concentration of vanadium remains the same. The vanadium at the surface was found to be mostly V in both the fresh and the used catalyst. Because of the low concentrations of vanadium at the surface and interference from the Ols peak, the presence of small amounts of reduced vanadium cannot be excluded. 1


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Discussion Since little is known about crystal structures of the low-temperature mixed oxides of niobia, it is difficult to explain from a theoretical point of view the good results obtained for doping with vanadium or chromium. It appears that the vanadium and chromium are present in the catalyst in a unique environment, which makes


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400 600 800 1000 1200 Wavenumber (cm-1) Figure 2 Laser Raman spectrum of VIONb.


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Figure 3 TPR of VINb melt.

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Figure 4 Ar -sputtering profiles as determined by XPS for a VINb imp.catalyst. Sputtering rate 1-2 Â/min.



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them selective for the oxidative dehydrogenation of propane. It is known that vanadium is a selective catalyst for this reaction only when it is present in a certain environment; in other environments (e.g. in the bulk oxide or when present on a support either as a monolayer or in multiple layers) it causes total oxidation (5). The use of chromium in selective oxidation catalysts usually results in total oxidation, although examples of selective chromium-containing oxidation catalysts exist (75). The results obtained with the variation of the vanadium content suggest that when the amount of vanadium is increased until 1 mol%, a compound is initially formed (possibly a solid solution of V in niobia) in which the vanadium is present in sites where it is not particularly active. Upon addition of more vanadium (1 mol% or more), another compound is formed which contains vanadium in an environment in which it is active and selective for the oxidative dehydrogenation of propane. This compound probably has a fixed composition, and is preferably segregated at the surface of the catalyst. Addition of more vanadium causes this layer to grow thicker, without a change in surface structure or in surface vanadium concentration and thus in activity (Figure 5). It is possible that this compound is one of the phases found in X R D of VIONb, i.e. V N b 0 (8 mol% V, half of which is V ) or fi(V, N b ) 0 (10 mol% V) (72, 13). These phases have structures similar to those of V N b 0 and H-Nb 0 , with vanadium replacing niobium in the tetrahedral sites at the junction of the octahedral blocks (16). The results for the VINb-catalyst suggest that this catalyst contains the same amount of active phase exposed at the surface per gram (T for 50% oxygen conversion for 600 mg catalyst is similar, see Table II), but that because the surface area is larger, more inactive "solid solution" is exposed at the surface, resulting in a lower activity per unit surface area. The effects of the various pretreatments to a VINb-catalyst, in combination with TPR results, give evidence that V becomes reduced to V upon use, causing a lower activity and a higher selectivity towards propylene. The presence of V = 0 bonds in the VIONb catalyst (Figure 2) does not harm the selectivity towards propylene. This is in contrast to the suggestion made by Chaar et al. (1, 2), who explain the selectivity of magnesium orthovanadate for the oxidative dehydrogenation of propane and butane from the fact that in this compound, V = 0 bonds are absent. The XPS results of the fresh catalyst show that vanadium is not present as a monolayer, since a decrease in vanadium concentration just below the surface would be expected in this case. The fact that in the used catalyst, vanadium at the surface is present in the same concentration and oxidation state as in the fresh material suggest that the compound mentioned 2

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Figure 5 Model for the formation of a layer of the active compound on a vanadia-niobia catalyst.


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above has formed in a thin layer at the surface of the catalyst during the cooling down in the reaction mixture after the temperature cycle used to test it. The XPS results also show that upon use, a redistribution of the vanadium occurs, possibly because of the destruction of the compound due to reduction. Acknowledgements We wish to thank E.M. Kool, P.H.H. in't Panhuis and A. Toebes for carrying out a substantial part of the experimental work. The hydrated niobia and the niobium oxalate used in the preparation of the catalysts were kindly provided by the Niobium Products Company, Inc., USA. One of the authors (R.H.H. Smits) thanks the Dutch Foundation for Chemical Research (SON) for financial assistance. References 1 Chaar, M.A.; Patel, D.; Kung, M.C.; Kung, H.H. J. Catal. 1987, 105, 483. 2 Chaar, M.A.; Patel, D.; Kung, H.H. J. Catal. 1988, 109, 463. 3 Nguyen, K.T.; Kung, H.H. J. Catal. 1990, 122, 415. 4 Patel, D.; Andersen, P.J.; Kung, H.H. J. Catal. 1990, 125, 132. 5 Siew Hew Sam, D.; Soenen, V.; Volta, J.C. J. Catal. 1990, 123, 417. 6 Guerrero-Ruiz, Α.; Rodriguez-Ramos, I.; Fierro, J.G.L.; Soenen, V.; Herrmann, J.M.; Volta, J.C. Stud. Surf. Sc. Catal. 1992, 72, 203. 7 Corma, Α.; López-Nieto, J.M.; Paredes, N.; Pérez, M . ; Shen, Y.; Cao H.; Suib, S.L. Stud. Surf. Sci. Catal. 1992, 72, 213. 8 Seshan, K.; Swaan, H.M.; Smits, R.H.H.; Van Ommen, J.G.; Ross, J.R.H. Stud. Surf. Sc. Catal. 1990, 55, 505. 9 Smits, R.H.H.; Seshan, K.; Ross, J.R.H. J. Chem. Soc., Chem. Comm. 1991, 8, 558. 10 Smits, R.H.H.; Seshan, K.; Ross, J.R.H. Stud. Surf. Sci. Catal. 1992, 72, 221. 11 Smits, R.H.H.; Seshan, K.; Van Ommen, J.G.; Ross, J.R.H. unpublished results. 12 Waring, J.L.; Roth, R.S. J. Res. Nat. Bur. Stand. 1965, 69A (2), 119. 13 Goldschmidt, H.J. Metallurgia 1960, 62, 211. 14 Smits, R.H.H.; Seshan, K.; Ross, J.R.H. to be published. 15 Loukah, M.; Coudurier G.; Védrine, J.C. Stud. Surf. Sci. Catal. 1992, 72, 191. 16 Wadsley, A.D.; Andersson, S. In Perspectives in Structural Chemistry; Dunitz, J.D.; Ibers, J.A.; John Wiley & Sons, 1970, Vol. 3; 19-26. RECEIVED November 6, 1992


Selective Oxidative Dehydrogenation of Propane on Promoted

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