The Oxidative Dehydrogenation of Propane over Molybdenum

Chapter 12

The Oxidative Dehydrogenation of Propane over Molybdenum-Containing Catalysts

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 28, 2017 | http://pubs.acs.org Publication Date: August 13, 1996 | doi: 10.1021/bk-1996-0638.ch012

The Effect of Support Pretreatment on Catalytic Performance 1

Azra Yasmeen, Frederic C. Meunier, and Julian R. H. Ross

Centre for Environmental Research, Department of Chemical and Environmental Science, University of Limerick, Limerick, Ireland Previous work from this laboratory has shown that of a series of molybdena catalysts supported on various supports, molybdena supported on titania was the most promising catalyst for the oxidative dehydrogenation of propane and that the yields obtained with this material compared well with those of some of the best catalysts for this reaction reported in the literature. This paper reports work which showed that an even more effective catalyst was molybdena supported on an alumina which had been calcined at higher temperatures to reduce its total surface area. The effect of different calcination temperatures of the alumina support was examined in some detail and it was found that calcination at 1200°C gave the best results. It has been shown that a coverage at least equivalent to a monolayer is necessary for the optimum performance of the catalyst. In a previous paper [1], we compared the behaviour of a series of catalysts consisting of molybdena supported on a variety of oxides for the oxidative dehydrogenation of propane to propene: C H 3

8

+ 1/2 0

2

+ H 0. 2

We found that of the materials made, the titania-supported catalyst was the most selective for comparable conversions of propane. We also found that the coverage of the support had to be more than a monolayer in order to obtain the optimum selectivity to propene. Addition of vanadium and niobium oxides to the titania-supported molybdenum oxide gave an increase in the activity of the resultant catalyst compared with that of the unpromoted material without any loss of selectivity. The catalysts examined in the previous work referred to above were prepared using oxide supports which had a variety of different surface areas rangingfrom12 to 211 m g which had been calcined at either 450 or 650 °C. Hence, as the molybdena loading in all cases was maintained constant at approximately 5 wt%, the differences between the catalysts could have been due to different surface coverages by the molybdena rather than being an inherent property of the molybdena-support combination. The present paper therefore compares the results obtained with a series of molybdenacontaining catalysts prepared using supports which had been sintered so that the surface areas were approximately constant. The data obtained show that of these materials, molybdena supported on alumina which had been calcined at 1200°C gave the highest yields of propene, the resultant yields being higher than those reported previously for molybdena on titania. Further work has therefore been carried out to examine the use of alumina as a support for molybdena and this paper thus describes the effect of molybdena loading and of the effect of calcination temperature of the support, as well as the effect of the addition of vanadia and niobia to the 2

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1

Corresponding author 0097-6156/96/0638-0170$15.00/0 © 1996 American Chemical Society Warren and Oyama; Heterogeneous Hydrocarbon Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

12. YASMEEN ET AL.

Oxidative Propane over Mo-Containing Catalysts

most selective M0/AI2O3 catalyst. It is shown that a 5 wt% M0/AI2O3 gives the optimum yield of propylene, slightly above 10%, under the conditions used for the measurements. This result compares very favourably with results obtained under the same conditions for a NiMoC>4 catalyst prepared according to the method described by Mazzochia et al. [2]. Experimental


Table I lists some of the catalysts used in this work. The supports used were titania (anatase, BDH), niobia (prepared by calcining hydrated niobia (HCST GmbH)), y-alumina (Ketjen), silica (BDH), magnesia (BDH) and zirconia (monoclinic, Gimex). Table I. Details of Mo-containing catalysts prepared with different supports

Catalyst

Temperature of calcination of support/°C

BET area of catalyst

BET area of support

/mV

1

/mV

1

Molybdenum content /wt%Mo

31.3 5.04 41 M0/T1O9 uncalcined 5.22 9.2 630 12.9 Mo/Nb Os 29.3 5.15 Mo/Al Ch 1200 47 4.42 22.4 1150 44.5 Mo/SiO? 88.0 5.4 58 Mo/MgO uncalcined 5.4 42.0 Mo/ZrO? 650 46.4 All the catalysts, except for Mo/TiC>2, were calcined at 650°C.; Mo/TiC>2 was calcined at 450°C. 9

9

The alumina, silica and zirconia were calcined at the temperatures shown so that their surface areas were of the same order as those of the titania and magnesia. The niobia, for which the area was much lower than the other samples, was calcined at 630°C to ensure that the T modification was obtained. (When hydrated niobia is heated, itfirstforms the TT phase at above about 537°C and then transforms to the T phase at about 600°C which is stable up to about 1000°C at which temperature it forms the H-phase [3J; the T-phase is characterised by a doublet in the XRD pattern at d= 3.13A.) The molybdena-containing materials were prepared by the incipient wetness technique using an aqueous solution of ammonium heptamolybdate (Rhone Poulenc, Normapur AR) maintained at 70°C, followed by drying overnight at 70°C and calcination at 650°C for two hours; an exception was the Mo/TiC>2 sample which was calcined at 450°C to avoid further sintering of the support. In all the cases shown in Table I, the Mo content was approximately 5 wt% (corresponding to about 7.5wt% M 0 O 3 ) ; in several cases, it was necessary to use a series of impregnation steps to attain this loading, chosen to be just above monolayer coverage [4]. Table EL Details of a series of 5 wt% M0-AI2O3 catalysts prepared using a series of alumina supports calcined at different temperatures.

Catalyst

Temperature of calcination of support

BET area of support

l°C

/mV

900 Mo/Al Ch Mo/Al Ch 1000 1200 Mo/Al Ch Mo/AbCh uncalcined All the catalysts were calcined at 650°C. 9

9

7

1

173 125.27 47 211

BET area of catalyst 1

/mV

110.2 94.8 29.3 189.0

Table II gives details of a further series of M0/AI2O3 catalysts which was prepared using alumina calcined at various temperatures; the Mo contents were adjusted so that the surface coverages were approximately the same as those of the samples shown in Table I, just above the monolayer

Warren and Oyama; Heterogeneous Hydrocarbon Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

171

172

HETEROGENEOUS HYDROCARBON OXIDATION

•—

Mo/Nb205


- ° — Mo/MgO Mo/Ti02 - o — Mo/A1203 ± — Mo/Zr02 Mo/A1203hsa 200

300

400

500

600

700

Temperature f°C Figure 1. Plots of propane conversion versus reaction temperature for a series of molybdena catalysts (see Table I) supported on various oxides of low surface area.



12. YASMEEN ET AL.


coverage. Finally, an alumina-supported sample containing M0O3 and V2O5 on alumina was prepared by impregnation using a solution of ammonium and ammonium heptamolybdate and another containing M 0 O 3 , V 2 O 5 and Nb2C>5 was preparedfroman oxalic acid solution of the same two salts together with niobium oxalate. The total surface areas of the supports and the Mo-containing catalysts were obtained using a Micromeretics Gemini system (BET method, N2 adsorption at 78 K). The catalytic behaviours were obtained using a quartz atmospheric pressure flow reactor (4 mm internal diameter) in which particles of the catalyst samples (600 mg, 0.3 - 0.6 mm) were held by quartz wool plugs. The temperature of the reactor was measured by an external thermocouple placed just after the catalyst. The standard reaction mixture used for the tests presented here consisted of a flow containing 29.4% propane (Air Products, 99.9%) and 9.6% oxygen (BOC, 99.9%) with the balance being helium (BOC, 99.9%). Analysis of the main products (propene and carbon oxides) was carried out using an on-line Varian 3300 gas chromatograph (TCD detector, HayesSep Q column). For each sample, the reaction temperature was raised in steps of 50°C over the range 250 to 650°C, each step being maintained for 1 h. Results and Discussion Figure 1 shows the propane conversion as a function of reaction temperature measured for the series of Mo03 catalysts prepared on different supports which had been calcined at different temperatures so that their areas were of the order of 40-50 m g (Table I). The results shown for the MgO and TK>2 supports are the same as those which were presented previously [1]. The silica-supported material had no measurable activity under the conditions used for these experiments. The order of activities of the catalysts based on the various supports was: Zr02>Nb205>Al203>MgO>Ti02»Si02. This differs slightly from the order found previously for supports of widely differing areas [1]: Zr02>Al203>Nb205>Ti02>Si02. The most significant difference between these results and those reported previously was a substantial decrease in the activity (i.e. an increase in the temperature required for a given conversion by some 50°C) of the alumina-supported material compared with that of the material prepared on a higher surface area alumina (area ca. 230 m^/g) for which the data are also shown (denoted as hsa) for comparison purposes and the total inactivity of the silica-supported material. Figure 2 shows a series of plots of the selectivity towards propene versus the conversion of propane, these plots corresponding to the data of Figure 1; the balance of the products was made up predominantly of CO2 in all cases although there was also a small quantity of oxygenated products formed. At any conversion of propane, the selectivity towards propene was highest for the aluminasupported material and the results for the other supports were in the order: Ti02>MgO>Zr02>Nb205. Compared with the results reported previously for the high-surface area alumina calcined at lower temperatures (also shown), the selectivities obtained with the low surface area alumina are significantly higher, particularly at lower conversions: the highest yield of propene (conversion x selectivity) obtained for the highest propane conversion (corresponding to approximately 100% conversion of the oxygen) with the low surface area support was ca. 10.5% . The value obtained with the low surface area support compared favorably with a value of (13.15%) obtained with a N1M0O4 material prepared according to the method given in ref. [2]. There was also a slight improvement in the yield for the M0/Z1O2 sample over what had previously been reported [1] for the same support calcined at a lower temperature but this yield was not as high as that obtained with the alumina-based material. In consequence, it was decided to examine in more detail the effect of the temperature of calcination of the support and of the M 0 O 3 loading on the catalytic properties of M0/AI2O3. Figure 3 shows the propane conversion as a function of reaction temperature for a series of samples prepared with AI2O3 supports calcined at different temperatures (Table II); also shown for comparison purposes are data obtained for the support calcined at 1200°C but without any M 0 O 3 . ^ most active sample was that which was calcined at the lowest temperature and the least active was the support alone. Corresponding selectivity vs. conversion plots are given in Figure 4 which shows that the highest selectivity for any conversion was obtained with the support calcined at the highest temperature, a gradual improvement being obtained with increasing calcination temperature. The selectivity of the support alone was much lower than those of the molybdena materials, this being a 2

_1


173

174


- A1203 1200°C


- Mo/A1203 900°C - MO/A1203 1000°C - M0/A12O3 1200°C •••

200

400

600

M0/A12O3 hsa

800

Temperature l°C Figure 3. Plots of propane conversion versus reaction temperature for molybdena catalysts (Table II) supported on alumina calcined at various temperatures.

Figure 4. Propene selectivity versus conversion over molybdena supported on alumina calcined at various temperatures for the experiments shown in Fig. 3.



12. YASMEEN ET AL.


Figure 5. Plots of propane conversion versus reaction temperature for catalysts with different loadings of molybdena supported on alumina calcined at 1200°C.

2%Mo/A1203 5%Mo/A1203 10%Mo/Al2O3 A1203 Mo03 15%Mo/A1203

Propane conversion %

Figure 6. Propene selectivity versus propane conversion for catalysts with different loadings of molybdena supported on alumina calcined at 1200°C.


175

176


result of the total oxidation of the propane to give C O 2 ; the total oxidation probably occured on acidic sites on the alumina which were covered by molybdena species with the supported molybdena catalyst. We now present equivalent data for the effect of M 0 O 3 loading on the catalytic behaviour, these having been obtained with the support calcined at 1200°C. Fig. 5 shows the propane conversion as a function of reaction temperature for various different M 0 O 3 loadings and also for pure M 0 O 3 & for the A I 2 O 3 alone. The activity of the M 0 O 3 was very low and the activities of the 2 wt% M 0 O 3 sample and the alumina support were somewhat higher but very similar; the activities of the other materials were significantly higher and increased with increasing M 0 O 3 contents. Figure 6 shows the corresponding selectivity vs. conversion plots. The selectivities of the catalyst with 2 wt% M 0 O 3 higher than those of the support alone but were considerably lower than those of the materials with higher M 0 O 3 contents. For the latter, the activity and selectivity data were significantly different only at higher conversions; above conversions of ca. 6%, the selectivities of the material with 5 wt% M 0 O 3 were the highest. This was the sample discussed above which gave a yield of 10.5% of propene. It is interesting to note that the samples with greater than 5 wt% of M 0 O 3 also gave up to ca. 5.2 % selectivity to oxygenates, this contrasting with the material with 2 wt% which gave ca. 2% oxygenates or with the support alone which gave none. We can conclude from these results that it is necessary to cover the support by molybdena species so that no uncovered alumina sites are available for total oxidation. Similar conclusions have been reached by other authors for different reactions over supported molybdena catalysts; see for example reference [5]. M


W E R E

The yields obtained with the alumina supports calcined at high temperatures were significantly better than those obtained with the same supports calcined at lower temperatures (Figures 3 and 4 and Table II) which contained more than a monolayer of molybdena, the optimum yield for the 19.5wt% M 0 O 3 / A I 2 O 3 sample being 8% (with a corresponding yield to oxygenates of 1.9%). It would therefore appear that tie improved behavior of the catalysts for which the support had been calcined at higher temperatures is not due alone to a better coverage of the support by the active phase, as discussed above, but also to a more ideal interaction between the active phase and the support, possibly due to a change in the degree of dehydration of the support or in its crystal structure with increase in calcination temperature. It is also possible that the change in pore structure of the material calcined following calcination at high temperature is responsible for improved selectivities. X-ray diffraction did not show any difference in the structures of the molybdena which waslargely amorphous. Further work is being carried out to investigate the nature of the oxide-support interaction and the effect of pore structure on the performance of the catalysts and to try to increase further their selectivities. It was shown previously [1] that the activity of a MoC^/TiC^ catalyst could be improved significantly without any significant change in the selectivity by adding either vanadia or a mixture of vanadia and M>205 to the formulation. Equivalent M 0 O 3 + V 2 O 5 and M 0 O 3 + V 2 O 5 + Nb2C>5 materials supported on low-area alumina were also prepared and tested when it was found that the propene selectivities and yields were lowered significantly by the presence of these promoters. Under the conditions of the experiments of Figures 1 and 2, the maximum conversions obtained with these materials were 15 and 14 % respectively and the corresponding selectivities to propene were 52 and 50 %. Further details of these results will be presented elsewhere [6]. We must conclude here that the interaction between the molybdena and the alumina discussed above is influenced in a different way by the incorporation of these two elements compared to the case of titania. Acknowledgements Part of this work was funded by the European Community (Human Capital and Mobility Programme, Grant NoCHRX CT92 0065.) which also provided a fellowship for F.C.M. We thank Drs. R.H.H. Smits and K. Seshan for useful discussions.

Literature Cited 1. Meunier, F.C., Yasmeen, A . and Ross, J.R.H. paper presented at the 13th Meeting of the North American Catalysis Society, Snowbird, Utah, June 1995; submitted for publication in Catal. Today.


12. YASMEEN ET AL. Oxidative Propane over Mo-Containing Catalysts 177


2. Mazzochia, C., Aboumrad, C., Diagne, C., Tempesti, E., Herrman J.M. and Thomas, G., Catal. Lett., 10(1991) 181-192. 3. Smits, R.H.H., Ph.D. thesis, University of Twente, The Netherlands (1994). 4. Grzybowska, B., Kess, P., Grzybowski, R., Wcislo,K., Barbaux, Y., and Gengember, L., Stud. Surf. Sci.Catal.,82 (1994) 151-158. 5. Fransen, T., Ph.D. thesis, University of Twente, The Netherlands (1977). 6. Yasmeen, A., Meunier, F.C., and Ross, J.R.H., to be published.


The Oxidative Dehydrogenation of Propane over Molybdenum

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