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Propane ammoxidation over SnOVOSb mixed oxide:

**preparation method and calcination effects (*)**

S. ALBONETTI(1), G. BLANCHARD(2), P. BURATTIN(2) S. MASETTI(1) and F. TRIFIRO` (1)

(1_{) Department of Industrial Chemistry and Materials, University of Bologna}

V.le Risorgimento 4, 40136 Bologna, Italy

(2_{) Rhone Poulenc Chimie - Rue De La Haie Coq 52, 93308 Aubervilliers, France} (ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)

Summary. — The aim of this work is to analyse the reactivity of a tin, vanadium,

antimony mixed oxide in the ammoxidation of propane to acrylonitrile with respect to the preparation and calcination method.

PACS 82.30 – Specific chemical reactions; reaction mechanisms. PACS 01.30.Cc – Conference proceedings.

1. – Introduction

The acrylonitrile (ACN) is produced nowadays by the ammoxidation of propene on a catalysts made of promoted Fe-Bi-Mo-O (SOHIO) or promoted Fe-Sb-O (Nitto); nevertheless, in recent years some companies have decided to invest in the ammox-idation of propane research.

One of the more interesting catalytic system for the direct ammoxidation of the paraffin is SbOVOO [1-4]. Centi et al. [2] and Nilsson et al. [3] have shown that an excess of antimony brings about an activity decrease, but also a large increase in the selectivities and in the yields in acrylonitrile and propene. The best catalyst for the synthesis of acrylonitrile from propane has a large excess of antimony (SbOV=5.0). This excess seems to quicken the transformation of the intermediate propene to acrylonitrile; on the contrary, an excess of vanadium brings about an activity increase, but also a low selectivity in acrylonitrile due to the production of propene and carbon oxides.

The SnOSbOO system has been widely studied in recent years [5] as a catalyst active in the allylic oxidation and ammoxidation: the best preparation method for these

(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.

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S.ALBONETTI,G.BLANCHARD,P.BURATTIN,S.MASETTIandF.TRIFIRO`

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compounds involves coprecipitation from a solution of Sn(IV) and Sb(V) chlorides [6]; calcination at temperatures higher than 700–900 7C (depending on the bulk antimonyO tin ratio) provokes segregation on the surface of a-Sb2O4 particles [7]; the catalytic activity strongly depends on the Sn/Sb ratio and on the calcination temperature [7, 8]. In order to make a new catalyst for the ammoxidation of acrylonitrile, we decided to mix these two systems: SbOV, in order to activate the paraffin, and SnOSb, that was already utilized to ammoxidate the relative olefin. In this paper, the effects of the method of preparation and calcination of a Sn-V-Sb mixed oxide on its catalytic performance are analysed. In particular we changed the medium in which the starting material were dissolved, utilizing ethanol, iso-butanol and water, in order to understand the interaction between the solvent and the metallic ions and to enhance the performance of the catalyst. Moreover, we made a mixture of oxide and hydroxide in n-hexane to compare the coprecipitation with a simple mixture. The catalyst with the best performance was then calcined at different temperatures and in different atmospheres in order to optimize the thermal treatment [9].

2. – Experimental

2.1. Catalyst preparation. – Every sample was prepared with a relative molar ratio Sn : V : Sb of 1 : 0.2 : 1 (this ratio was optimized in a previous work of this research group). The coprecipitation of vanadium, antimony and tin oxohydrates was achieved as follows: initially a solution of anhydrous SnCl4 in an organic medium (absolute ethyl alcohol for sample 1 and iso-butanol for sample 3) or in an acidic aqueous medium (around HCl 3M for sample 2) was prepared; then VO(acac)2and SbCl5were dissolved, in this sequence, in the solution, in order to obtain the desired Sn/V/Sb ratio. This solution was added dropwise to an aqueous solution of CH3COONH4, having an initial pH of around 7.0 (typically 2M solutions were utilized). During the precipitation of the

oxohydrates the pH, that decreased due to the release of HCl, was mantained constant by the addition of ammonia solution. The resulting precipitate was filtered, washed and dried overnight at 120 7C; then it was calcined at 350 7C for 1 hour and at 700 7C for 3 hours.

Sample 4 was prepared by a solid-state reaction between V2O5, Sb2O3 and a tin oxohydrate, prepared by precipitating an ethanolic solution of SnCl4 in a pH-controlled aqueous solution (similar to the preparation of sample 1). The precipitate of Sn(OH)4was filtered, washed and dried at 120 7C overnight

(

drying at 100–140 7C leads to the partial dehydratation of the ortho-stannic acid Sn(OH)4to the meta-stannic acid SnO(OH)4

)

, and then mixed with V2O5 and Sb2O3 by suspending the powders in vigorously stirred n-hexane. After evaporation of the solvent under reduced pressure, the mixture was dried at 120 7C overnight and finally calcined at 350 7C for 1 hour and at 700 7C for 3 hours.

Sample 1, which is the catalyst with the best performance, was re-prepared and, after the drying treatment, was then calcined up to 500 7C (sample 5) and 800 7C (sample 6) in air and to 700 7C in nitrogen (sample 7) with similar procedures.

2.2. Characterization. – The catalysts were tested in a conventional laboratory apparatus with a tubular fixed-bed reactor working at atmospheric pressure. The

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Fig. 1. – FTIR of samples 1, 2, 3 (A) and 4 (B).

analysis of propane, propene, acrylonitrile, acetonitrile and uncondensable gases were made by gas-chromatographic techniques; ammonia and hydrogen cyanide were detected by absorption and titration. The composition of the feed was: 25% propane, 20% oxygen, 10% ammonia and the remainder was helium. The catalyst (2 ml) was loaded as grains (30–40 mesh) and the contact time was around 2 s. A thermocouple, placed in the middle of the catalyst bed, was used to verify that the axial temperature profile was within 3 K.

Surface areas were determined using the B.E.T. method with nitrogen absorption at 77 K on a Carlo Erba instrument.

Fourier-transformed infrared (FT-IR) spectra in transmission were recorded using a Perkin-Elmer 7200 Fourier transform spectrometer and the KBr disk technique.

X-ray diffraction patterns (powder technique) were obtained using Ni-filtered Cu Ka radiation (l 41.542 Å) with a Philips computer controlled instrument (PW1.050/81).

3. – Results and discussion

3.1. Effects of the preparation method. – It is possible to see that the four different preparation methods lead to samples with similar surface areas (table I).

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Fig. 2. – XRD of samples 1 (A), 2 (B), 3 (C) and 4 (D).

(fig. 1A). It is possible to see the stretching absorbance of the V1O bond at around 988 cm21_{: this shifting to lower frequencies with respect to the absorbance of the} crystalline V2O5may be attributed to two effects, firstly to the interactions between the vanadyl species and the rutile type SnO2 matrix and secondly to the electronic effect due to the presence of neighboring reduced vanadium sites. The absorbance at 629 cm21 _{can be attributed to the stretching of the Sn`O bond: the shifting of this} characteristic band of the tin oxide from 645 cm21 _{is due to the interaction with} antimony. The spectra of sample 4, prepared by a mixture of oxide and hydroxide in TABLEI. – Sample prepared and surface area.

Sample Medium Temperature

(7C) Atmosphere Asur ( m2 Og ) 1 2 3 4 5 6 7 EtOH H2O i-BuOH n-hexane EtOH EtOH EtOH 700 700 700 700 500 800 700 air air air air air air nitrogen 46 54 62 32 114 32 26

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Fig. 3. – Conversion vs. temperature for samples 1, 2, 3 and 4.

hexane, (fig. 1B) is very different from the other ones: it shows the absorbance bands of

a-Sb2O4 at 745, 648, 605 and 529 cm21 and a weak one at 1020 cm21, typical of crystalline V2O5.

The diffractograms of this four samples are reported in fig. 2. The samples prepared by coprecipitation methodology (respectively, fig. 2A, B and C) are very similar and clearly show the diffractions line of the rutile-type SnO2 phase. No other phases are present, however, it can be observed that, due to the width of the diffraction lines of SnO2, the possible presence of the rutile-type VSbO4phase would be evidenced with difficulty. When the preparation is carried out by solid-state reaction (fig. 2D), the XRD patterns drastically change: the main phase present is a-Sb2O4.

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Fig. 5. – FTIR of sample 1, (A), 6 (B) and 7 (C).

Figures 3 and 4 show, respectively, conversion vs. temperature and acrylonitrile selectivity vs. conversion for these four samples. It is possible to see that, even if they reach a similar conversion, they have a different activity, due to the different working temperature range. On the contrary the acrylonitrile selectivity is very different: sample 1 (EtOH) is the best one, since it reaches a value of 60%, then there follow, in the order, sample 3 (i-BuOH), sample 2 (water) and sample 4 (n-hexane). Hence, while the catalytic data of the samples prepared by the coprecipitation technique are different, the structural characterization has not revealed meaningful differences. The use of ethanol as solvent for the dissolution of the ions brings about the formation of tin-, antimony-, and vanadyl-alkoxide, which slows down the precipitation of the different compounds, thus making the velocity uniform. In this way a better interaction among the hydroxides is favoured and a better dispersion of the active sites brings about the multifunctionality necessary to convert a saturated hydrocarbon into a functionalized molecule with a good selectivity. These kinds of equilibria do not exist with water; on the contrary iso-butanol is a sterically hindered alcohol and the alcoxides are more difficult to obtain.

The method of preparation by solid-state reaction leads to a mixture of oxides and not to a mixed oxide: in fact the characterization has revealed a segregation of the system into the single oxides. Consequently the mixture of the active sites is not good and the acrylonitrile selectivity slows down considerably.

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Fig. 6. – XRD of samples 1 (A), 6 (B) and 7 (C).

3.1. Effects of the thermal treatment. – The surface area of the sample calcined in air (table I) slows down with increasing calcination temperature, due to the elimination of microporosity; moreover the sample calcined in air has a surface area higher with respect to the one calcined at the same temperature in nitrogen.

The FTIR spectra of the sample calcined in air are quite similar (fig. 5A), it is possible to see the typical absorbance bands of the tin oxide (around 629 cm21_{) and of} vanadyl species (around 988 cm21_{) described in the previous section. The spectrum of} sample 7 (fig. 5B), calcined in nitrogen is different: the V1O band is less intense and the typical bands of antimony oxides, even if not very intense, are observed, due to a initial segregation of the phases.

The XRD of sample 5 calcined at 500 7C is similar to that of sample 1 calcined at 700 7C (fig. 6A), it is possible to see only the reflections of the rutile type SnO2, even if less crystalline. The sample calcined at 800 7C (fig. 6B) shows an increasing of the crystallinity and the segregation of a-Sb2O4 and b-Sb2O4. The sample calcined in nitrogen at 700 7C (fig. 6C) shows a segregation of the phases too (only a-Sb2O4): the formation of antimony oxide is shifted at lower temperature with respect to calcination in air.

In figs. 7 and 8 are reported, respectively, conversion vs. temperature and the acrylonitrile selectivity vs. conversion for samples 1, 5, 6 and 7. Sample 7, calcined in nitrogen, is less active with respect to one calcined in air at the same temperature and the selectivity is very low, owing to combustion; the absence of vanadyl species,

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Fig. 7. – Conversion vs. temperature for samples 1, 5, 6 and 7.

Fig. 8. – ACN selectivity vs. conversion for the samples 1, 5, 6 and 7.

Table II. – Bulk and surface relative atomic ratio of the sample calcined in air (*).

Sample Calcination temperature (7C) Atomic ratio Sn : V : Sb bulk surface 1 5 6 700 500 800 5.0 : 1.0 : 5.0 5.0 : 1.0 : 5.0 5.0 : 1.0 : 5.0 5.0 : 1.2 : 5.9 5.0 : 1.0 : 4.8 5.0 : 1.2 : 3.7 (*) The analysis of the bulk atomic ratio is made by atomic absorption.

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evidenced by FTIR analysis, seems to influence the catalytic performance in a dramatic way. The activity of sample 5, calcined in air at 500 7C, is quite high, but the selectivity is very low: the quite high surface area of this sample, calcined at low temperature, negatively influences the catalytic performance. Sample 6, calcined in air at 800 7C, is less selective in acrylonitrile with respect to sample 1, calcined at 700 7C; this decrease is caused by the system structural evolution with segregation of the antimony oxide. The best was found to be sample 1, calcined in air at 700 7C, characterized by the crystalline SnO2 phase, which incorporates vanadium and antimony, and by an amorphous SbOx; another important difference, which can explain the high selectivity

of this sample is a surface enrichment of antimony (table II) with respect of the other two samples calcined in air (antimony is known as a booster of selectivity in this reaction).

R E F E R E N C E S

[1] CENTIG., PESHEVAD. and TRIFIRO` F., Appl. Catal., 33 (1987) 343.

[2] CENTIG., TRIFIRO` F. and GRASSELLIR. K., La Chimica & L’Industria, 72 (1990) 617. [3] NILSSONR., LINDBLADT., ANDERSSONA., SONGC. and HANSENS., in New Developments in

Selective Oxidation II, edited by V. CORTESCORBERANand S. VICBELLON, Vol. 82 (Elsevier, Amsterdam) 1994, p. 293.

[4] CENTI G., GRASSELLI R. K., PATANE` E. and TRIFIRO` F., in New Developments in Selective Oxidation, edited by G. CENTIand F. TRIFIRO`, Vol. 55 (Elsevier, Amsterdam) 1990, p. 515. [5] CENTIG. and TRIFIRO` F., Cat. Rev.-Sci. Eng., 28 (1986) 165.

[6] BERRYF. J., Adv. Catal., 30 (1981) 97.

[7] HERNIMANH. J., PYKED. R. and REIDR., J. Catal., 58 (1979) 68. [8] TRIFIRO` F. and PASQUONI., La Chimica & L’Industria, 52 (1970) 228.

[9] ALBONETTI S., BLANCHARD G., BURATTIN P., CAVANI F. and TRIFIRO` F., France Patent 94-07982 1994, assigned to Rhone Poulenc Chimie.