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Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile
Applied Catalysis A: General 251 (2003) 49–59
Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile Part II. Effect of catalyst composition on catalytic performance N. Ballarini a , F. Cavani a,∗,1 , M. Cimini a , F. Trifirò a , R. Catani b , U. Cornaro b , D. Ghisletti c a
Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy b Snamprogetti SpA, via Maritano 26, 20097 S. Donato MI, Italy c EniTecnologie SpA, via Maritano 26, 20097 S. Donato MI, Italy Received 17 September 2002; received in revised form 7 April 2003; accepted 8 April 2003
Abstract Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr + V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V4+ ; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V3+ ; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Ammoxidation of propane; Ammoxidation of propylene; Acrylonitrile; Chromium, antimony, vanadium oxides
1. Introduction The ammoxidation of propane to acrylonitrile represents an alternative to the current industrial pro∗ Corresponding author. Tel.: +39-051-2093680; fax: +39-051-2093680. E-mail address: [email protected] (F. Cavani). 1 INSTM, Research Unit of Bologna.
duction of acrylonitrile via ammoxidation of propylene [1–7]. Semi-commercial and pilot plants for propane ammoxidation have been built by BP, Mitsubishi and Asahi. Each company has developed its own technology, where differences concern the catalytic system employed and the operating conditions. Catalysts active and selective for the ammoxidation of propane are either made of mixed antimonates having the rutile-type structure, like in the case of the
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00315-6
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BP system which is based on V(Al)SbO4 [8–12], or of mixed molybdates [13,14], like in the Mitsubishi (Mo/V/Nb/Te/O) or Asahi (Mo/V/Nb/Sb/O) systems. Moreover, while in former patents by Standard Oil, operation at propane-rich conditions seemed to be the preferred choice, more recent patents issued by Mitsubishi, Asahi and BP prefer propane-leaner conditions [15,16]. Also in this case recycle of the unconverted paraffin can be done, through the PSA technology which makes possible the separation of light paraffins and olefins from the other components of the stream, including molecular nitrogen. It is a peculiarity of antimonate-based catalysts that the active and selective systems are not only based on the rutile compound [17], but necessitate the presence of dispersed antimony oxide to obtain good selectivity to acrylonitrile. In fact, propylene is the reaction intermediate, and the catalyst needs allylic Oor NH-insertion properties in order to transform this intermediate to acrylonitrile, avoiding its desorption into the gas phase and its conversion to by-products. On the other hand, control of catalyst activity, of selectivity to acrylonitrile and of activity in ammonia combustion (an undesired side-reaction) is achieved by the addition of co-elements which are able to develop mixed rutile compounds together with the main components. In a related work [18, Part I of this paper] we have reported about the characterization of rutile-type Cr/V/Sb mixed oxides, and evidenced that the nature of the V species which develop is a function of the atomic ratio between components. In the present work, we report about the reactivity of Cr/V/Sb/O catalysts in propane and propylene ammoxidation, and analyze how the ratio between components affects the catalytic performance.
2. Experimental Catalysts were prepared with the coprecipitation technique, developed for the preparation of rutile SnO2 -based systems claimed by Rhodia [19]. The preparation involves the dissolution of Cr(NO3 )3 · 6H2 O, VO(acac)2 and SbCl5 in absolute ethanol; the solution is then dropped into an aqueous solution maintained at pH 7. A precipitate is obtained, which is separated from the supernatant liquid by centrifu-
gation and filtration. The solid is then dried at 120 ◦ C, and calcined in air at 700 ◦ C for 3 h, with a heating rate of 5 ◦ C/min. Catalytic tests were carried out in a laboratory stainless steel fixed-bed reactor operating at atmospheric pressure. Two milliliters of catalyst were loaded, shaped in particles having size ranging between 0.42 and 0.55 mm. The catalyst particles were mixed with an equal amount of inert particles (␣-alumina). The following reaction conditions were used: (i) propane ammoxidation-feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s; (ii) propylene ammoxidation-feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s. Products were analyzed by means of gas chromatography. A Hay-sep T column (TCD) was used for the separation of CO2 , NH3 , C3 H8 , H2 O, HCN, acrolein, acetonitrile and acrylonitrile, a MS-5A column (TCD) for the separation of O2 , N2 and CO, and a Porapak QS column (FID) for the separation of propane and propylene. 3. Results and discussion 3.1. The ammoxidation of propane: the effect of catalyst composition on catalytic performance Catalysts containing Cr, V and Sb were prepared and characterized using the Cr/Sb/O-based catalysts, studied in previous works, as the reference compounds [20,21]. Three series of catalysts were prepared, having Cr/V/Sb atomic ratios 1/x/δ (with Cr chosen as the reference element), where δ was equal to (i) 0.8–1.0 (this series of samples is here referred to as Cr/V/Sb 1/x/1), (ii) 1.8–2.1 (series of composition Cr/V/Sb 1/x/2), and (iii) 3–5 (series of composition Cr/V/Sb 1/x/3–5). In the three series of catalysts the value of x was varied between 0 and 1.0. Table 1 reports the main features of catalysts prepared. The reactivity in propane ammoxidation of samples belonging to series Cr/V/Sb 1/x/1 is summarized in Fig. 1; the conversion of propane, of ammonia and of oxygen, the selectivity to acrylonitrile and the selectivity to nitrogen from ammonia combustion are plotted for each catalyst as functions of temperature. The selectivity to the products at T ≈ 380 ◦ C is also
N. Ballarini et al. / Applied Catalysis A: General 251 (2003) 49–59 Table 1 Main characteristics of Cr/V/Sb/O catalysts with composition Cr/V/Sb 1/x/1, 1/x/2 and 1/x/3–5 Cr/V/Sb 1/x/δ (at. ratio)a
Surface area (m2 /g)
Secondary crystalline phasesb
1.0/0/0.8 1.0/0.3/1.1 1.0/0.5/0.9 1.0/0.8/1.1 1.0/0.9/0.9 1/0/1.6 1/0.2/1.7 1/0.5/1.8 1/0.8/2.0 1/0.9/1.8 0/1.0/2.1 1/0/2.8 1/0.2/5d 1/1/5d
37 59 50 32 36 67 67 49 40 40 20 54 51 72
– – – – 3.0 wt.% V2 O5 c – – – – – 26 wt.% ␣-Sb2 O4 c 14.6 wt.% Sb6 O13 c Sb6 O13 e Sb6 O13 e
a
As determined by SEM–EDX. Principal crystalline phase: rutile. c As determined by Rietveld analysis of the XRD pattern. d The effective composition was not determined analytically; atomic ratios correspond to those employed for samples preparation. e As evaluated from a qualitative analysis of the XRD patterns. b
reported in the figure. Results for samples belonging to series Cr/V/Sb 1/x/3–5 are reported in Fig. 2. Results concerning the reactivity of series Cr/V/Sb 1/x/2 have been reported in a previous work [21], but are discussed again in the present work to have a better insight of relationships between catalytic performance and chemical–physical features. The addition of V in samples belonging to series Cr/V/Sb 1/x/1 considerably increased the catalyst activity, notwithstanding the decrease of the surface area for x > 0.3. This indicates that the presence of V in Cr/V/Sb mixed oxides generates sites which are more active than those present in Cr/Sb/O systems. The conversions of ammonia and propane were limited by oxygen. Catalyst Cr/V/Sb 1/0.9/0.9 converted oxygen completely at 340 ◦ C, with a propane conversion close to 23%. This catalyst is one of the most active, non-noble-metal-based catalysts ever reported in the literature for paraffin (amm)oxidation carried out under hydrocarbon-rich conditions. The conversion of propane decreased when the temperature was increased above 350 ◦ C, as a consequence of a change in the distribution of products and of an higher oxygen consumption.
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The selectivity to acrylonitrile was low (<10%) in samples belonging to series Cr/V/Sb 1/x/1 when x was higher than 0.3, while the selectivity to propylene was higher than 10%. The selectivity to carbon monoxide increased and that to carbon dioxide decreased when the value of x was increased, while the sum of the two products remained constant. The sample having the highest V content (Cr/V/Sb 1/0.9/0.9) produced of propylene, carbon monoxide and carbon dioxide with similar selectivity. The selectivity to the other by-products (acetonitrile and cyanhydric acid) was not affected by the amount of V in the catalyst. The behavior of Cr/V/Sb 1/x/1 samples has analogies but also differences if compared to samples belonging to series Cr/V/Sb 1/x/2 [21]. Also in the latter case, in fact, an increase of V content led to an increase of activity and to a decrease of selectivity to acrylonitrile, but the latter was in all samples higher than 20% (for total conversion of oxygen), and the selectivity to propylene was lower than 8%. Samples having excess Sb, i.e. with composition Cr/V/Sb 1/x/3–5 (Fig. 2), behaved differently from samples Cr/V/Sb 1/x/1, and the catalytic performance was similar to that of catalysts belonging to series Cr/V/Sb 1/x/2 (Fig. 2) [21]. Increasing amounts of V led to an increase of activity, but the selectivity to acrylonitrile was higher than 25% for all samples, that to carbon oxides was 50–60%, and that to propylene was lower than 10%. The distribution of products was not much affected by the content of V. The degree of surface Sb enrichment, as determined by XPS, was considerably affected by the (Cr + V)/Sb ratio [18]. When the x value in samples having composition Cr/V/Sb 1/x/1 was increased, the Sb enrichment decreased [18]. This can be the reason for the low availability of NH2− -insertion sites, and for the decrease of selectivity to acrylonitrile (Fig. 1). The change in the intra-crystalline distribution of elements, instead, did not occur in the case of samples belonging to series Cr/V/Sb 1/x/2; a large excess of Sb in the outermost atomic layers of the rutile crystals was found in all samples [18]. Therefore, these catalysts always had a number of sites sufficient for the transformation of a large fraction of the unsaturated intermediate to acrylonitrile. In the case of samples belonging to series Cr/V/Sb 1/x/3–5, the presence of antimony oxide [18] provided additional sites for the
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Fig. 1. Summary of the catalytic performance of samples Cr/V/Sb 1/x/1. x = 0 (䉱), x = 0.3 (䊏), x = 0.5 (䊉), x = 0.8 (×), x = 0.9 (䉬). Bottom right figure: data collected at T = 374–386 ◦ C (except for sample Cr/V/Sb 1/0/0.8, at T = 430 ◦ C). Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s. AN, acrylonitrile; C3 = propylene; ACN, acetonitrile; CO2 , carbon dioxide; CO, carbon monoxide; HCN, cyanhydric acid.
transformation of the intermediate to acrylonitrile, rather than to carbon oxides. The catalytic performance of samples Cr/Sb 1/0.8, Cr/Sb 1/1.6, Cr/V/Sb 1.0/0.9/0.9, Cr/V/Sb 1.0/0.9/1.8 and V/Sb 1/2.1 is compared in Fig. 3. Cr/V/Sb/O catalysts were more active than V/Sb/O and Cr/Sb/O. The very high activity of three-component catalysts (especially of sample Cr/V/Sb 1/0.9/0.9) can be attributed
in part to the higher surface area of Cr/V/Sb/O samples as compared to V/Sb/O. In addition, a synergetic effect arising from the contemporaneous presence of Cr and V in the same structure can be hypothesized. A similar activity-enhancement effect, with a corresponding decrease in selectivity to acrylonitrile, has also been reported for Fe1−x Vx SbO4 catalysts [22]. The improvement occurred with respect to both FeSbO4
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Fig. 2. Summary of the catalytic performance of samples Cr/V/Sb 1/x/3–5. x = 0 (䉱), x = 0.2 (䊏), x = 1 (䊉). Bottom right figure: data collected at T = 484–495 ◦ C. Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s. AN, acrylonitrile; C3 = propylene; ACN, acetonitrile; CO2 , carbon dioxide; CO, carbon monoxide; HCN, cyanhydric acid.
and VSbO4 , with a maximum activity achieved for x ≈ 0.3, and was attributed to the generation of anionic–cationic vacancy couples as a consequence of replacement of V4+ by Fe3+ in the rutile structure. The behavior of V/Sb/O, Cr/Sb/O and Cr/V/Sb/O samples was also different in regard to the selectivity to acrylonitrile (Fig. 3, bottom). Sample Cr/V/Sb 1/0.9/0.9 yielded only traces of acrylonitrile, while sample Cr/V/Sb 1/0.9/1.8 reached a selectivity close
to that of sample V/Sb 1/2.1 only at high temperature, while being non-selective at low temperature, because of the relevant formation of propylene, carbon dioxide and cyanhydric acid. Sample Cr/V/Sb 1.0/0.9/0.9 was more active than sample Cr/V/Sb 1.0/0.9/1.8, notwithstanding the comparable values of surface area. The very high activity of this sample can be attributed to the preferential formation of the V4+ species. In fact, the characterization
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Fig. 3. Propane conversion (top) and selectivity to acrylonitrile (bottom) as functions of temperature for catalysts: V/Sb 1/2.1 (䉬), Cr/Sb 1/0.8 (䊏), Cr/Sb 1/1.6 (䊐), Cr/V/Sb 1/0.9/0.9 (䊉), Cr/V/Sb 1/0.9/1.8 (䊊). Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s.
evidenced that the relative amount of V3+ and V4+ in calcined Cr/V/Sb/O catalysts is a function of the (Cr + V)/Sb atomic ratio [18]; the amount of V4+ ions was the highest in samples having a (Cr + V)/Sb ratio above ≈1, which also are the most active catalysts among Cr/V/Sb mixed oxides here reported. The syn-
ergy effect on activity which occurs when Cr and V are in the same rutile structure might thus be related to an increase of the V4+ /V3+ ratio with respect to V/Sb/O samples, especially when the amount of Sb5+ is not enough to develop a mixed antimonate of V and Cr.
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3.2. The ammoxidation of propane: the reaction network Tests were made to determine the reaction network. The residence time was varied by changing the amount of catalyst loaded in the reactor (and loading an amount of ␣-alumina corresponding to that necessary to fill the reactor completely), while keeping the flow rate constant. Accurate control of the temperature (which was affected by the flow rate) made possible to obtain results under truly isothermal conditions (temperature 435 ◦ C). Moreover, the small contribution to conversion due to the reactor walls and to the inert material was subtracted from the catalytic results. Reaction conditions chosen were those so as to achieve very low conversions, in order to detect the formation of reaction intermediates. The results are summarized in Fig. 4, where the propane conversion and the selectivity to the products are plotted as functions of the residence time for catalyst Cr/V/Sb 1/1/5; this yielded the best selectivity to acrylonitrile, and a low selectivity to propylene and to carbon oxides. Propylene was the only primary product, while secondary products were cyanhydric acid and carbon monoxide. The formation of acetonitrile, carbon dioxide and acrylonitrile likely involved both direct and consecutive contributions. The selectivity to propylene decreased when the residence time was
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increased, with a corresponding increase in the selectivity to carbon dioxide and to acrylonitrile. Therefore, from the kinetic point of view propylene is an intermediate product; it is re-adsorbed on active sites and is converted either to acrylonitrile or to carbon dioxide. The data do not exclude the possibility of a direct pathway for the formation of acrylonitrile; this implies that a fraction of the adsorbed intermediate obtained by propane activation and (oxi)dehydrogenation is directly transformed to acrylonitrile. Acrylonitrile also underwent a consecutive transformation to carbon monoxide and to cyanhydric acid. These data confirm that the low selectivity to propylene which is obtained under usual reaction conditions is due (i) to the likely direct transformation of the adsorbed intermediate to acrylonitrile (direct acrylonitrile formation), and (ii) to the rapid re-adsorption of propylene from the gas phase and transformation to the successive products (consecutive acrylonitrile formation). This occurs with samples Cr/V/Sb 1/x/2 even in the absence of excess antimony oxide, differently from that observed in the case of the V/Sb/O system [23], because the intra-crystalline Sb enrichment in Cr/V/Sb/O rutile crystals provides the sites to perform allylic ammoxidation efficiently. Nevertheless, excess antimony oxide is useful to further increase the selectivity to acrylonitrile (as it occurs in samples Cr/V/Sb 1/x/3–5), by favoring the transformation
Fig. 4. Propane conversion (䉬) and selectivity to carbon dioxide (䉱), propylene (䊏), cyanhydric acid (䊊), acrylonitrile (䊐), acetonitrile ( ), and carbon monoxide (×), as functions of residence time. Catalyst Cr/V/Sb 1/1/5. Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; temperature 435 ◦ C.
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of propylene to acrylonitrile, rather than to carbon oxides. 3.3. The ammoxidation of propylene: the effect of catalyst composition on catalytic performance The performance of sample Cr/V/Sb 1/0.3/1.1 in propylene ammoxidation is shown in Fig. 5; the con-
version of reactants and the selectivity to the products are reported as functions of the reaction temperature. For comparison, the results obtained in propane ammoxidation with the same catalyst are also reported in Fig. 5. The reaction conditions in the two cases were different, since for propylene ammoxidation the conditions were chosen so as to reproduce industrial operation. Therefore, analogous conversions for the limiting
Fig. 5. Ammoxidation of propylene (top) and ammoxidation of propane (bottom). Conversions and selectivity as functions of temperature. Catalyst Cr/V/Sb 1/0.3/1.1. Conversion of the hydrocarbon (䉬), conversion of ammonia (䊏), conversion of oxygen (䉫), selectivity to propylene (bottom figure) (䊉), acrylonitrile (䊐), acetonitrile ( ), cyanhydric acid (䊊), carbon monoxide (×), carbon dioxide (䉱), acrolein (top figure) (䊉). Reaction conditions: propane ammoxidation-feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s; propylene ammoxidation-feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s.
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Fig. 6. Propylene conversion (filled symbols) and selectivity to acrylonitrile (open symbols) as functions of reaction temperature for catalysts Cr/V/Sb 1/x/1 (top) and Cr/V/Sb 1/x/2 (bottom). Top: Cr/V/Sb 1/0/0.8 (䉬), 1/0.3/1.1 (䉱), 1/0.9/0.9 (䊏). Bottom: Cr/V/Sb 1/0/1.6 (䉬), 1/0.2/1.7 (䉱), 1/0.5/1.8 (䊏), 1/0.9/1.8 (䊉). Reaction conditions: feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s.
reactant were achieved from the two hydrocarbons in similar ranges of temperature, despite the lower reactivity of the paraffin as compared to the olefin. Fig. 6 compares the propylene conversion and the selectivity to acrylonitrile as functions of the temperature for catalysts belonging to the two series Cr/V/Sb 1/x/1 and 1/x/2. Also from propylene, the activity increased and the selectivity to acrylonitrile decreased when the V
content in catalysts was increased. Sample Cr/V/Sb 1/0.3/1.1 reached a low selectivity to acrylonitrile from propane (around 15%, lower than that obtained with the Cr/Sb 1/0.8 sample, Fig. 1), but it did exhibit a good selectivity to acrylonitrile from propylene (around 65%). This indicates that the surface Sb enrichment was enough to provide a quick transformation of the activated olefin to acrylonitrile. When this Sb enrichment was smaller, as in the case of sample
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Cr/V/Sb 1/0.9/0.9 [18], the selectivity to acrylonitrile was very low both from propane and from propylene. On the contrary, sample Cr/V/Sb 1/0.9/1.8 gave a good selectivity to acrylonitrile from both hydrocarbons, as it was also for all samples belonging to series Cr/V/Sb 1/x/2; this was due to the presence of the surface Sb enrichment. A non-negligible selectivity to acrolein from propylene was obtained with samples Cr/V/Sb 1/x/1, and with sample Cr/V/Sb 1/0.9/1.8 (which was the most active catalyst in series Cr/V/Sb 1/x/2). On the contrary, acrolein was never obtained from propane. This difference may be interpreted by assuming that the relative amount of NH2− - and O2− -insertion sites is a function of the reaction conditions. Under conditions of oxygen starvation (as in the case of propane ammoxidation), the number of sites responsible for allylic ammoxidation was much greater than the number of sites responsible for allylic oxidation. In the case of propylene ammoxidation, instead, the larger availability of gas-phase oxygen and the lower partial pressure of ammonia led to the formation of acrolein.
4. Conclusions Rutile-type Cr/V/Sb/O catalysts are active in propane ammoxidation. A synergy effect is postulated to explain the high activity of Cr/V/Sb mixed oxides as compared to the Cr/Sb/O and V/Sb/O systems; this effect is likely related to the preferred formation of V4+ , rather than V3+ , in the presence of Cr, especially for (Cr + V)/Sb ratios higher than 1. The selectivity to acrylonitrile is a function of this parameter, too: selectivity close to 35% to acrylonitrile is obtained with samples having the lowest (Cr + V)/Sb ratio. The selectivity to propylene is very low in all cases, except for very low propane conversions. This is attributed to the intrinsic multifunctionality of Cr/V/Sb/O compounds, in which surface enrichment of Sb, in samples having (Cr + V)/Sb ratios between 0.5 and 1, confers to the catalyst the ability to transform the unsaturated intermediate to acrylonitrile. In samples having a (Cr + V)/Sb ratio lower than 0.5, excess antimony oxide also contributes to a further increase of the selectivity to acrylonitrile at the expense of carbon oxides.
Fig. 7. Simplified scheme for the ammoxidation of propane to acrylonitrile. Minor by-products have been omitted.
Table 2 Effect of catalyst composition on the selectivity to the main products in propane ammoxidation. The selectivity ranges are those obtained for total oxygen conversion. Path numbers are those indicated in Fig. 7 (Cr + V)/Sb (at. ratio)
>1 0.5–1 <0.5
Path preferred
7 > [1 + 2] > [1 + 4 + 5 + 6] 7 > [1 + (2 + 3 +) 4 + 5 + 6] > [1 + 2] 7 > [1 + 2 + 3 + 4 + 5 + 6], [1 + 4 + 5 + 6] > [1 + 2]
Selectivity (%) C3 H6
COx
C3 H3 N
10–30 <10 <5
60–80 55–70 50–60
<20 20–30 25–35
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Fig. 7 and Table 2 summarize the main results of the present work, with indication of the effect of the (Cr + V)/Sb atomic ratio on the catalytic performance in propane ammoxidation, in relation to the reaction scheme. Acknowledgements Snamprogetti SpA is acknowledged for financial support. References [1] R.K. Grasselli, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 5, Wiley-VCH, Weinheim, 1997, p. 2302. [2] Y. Moro-oka, W. Ueda, in: Catalysis, vol. 11, Royal Society of Chemistry, Cambridge, UK, 1994, p. 223. [3] R.K. Grasselli, Catal. Today 49 (1999) 141. [4] G. Centi, F. Cavani, F. Trifirò, Selective Oxidation by Heterogeneous Catalysis, Kluwer Academic Publishers/ Plenum Press, New York, 2001. [5] F. Cavani, F. Trifirò, in: R.K. Grasselli, S.T. Oyama, A.M. Gaffney, J.E. Lyons (Eds.), 3rd World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, p. 19. [6] V.D. Sokolovskii, A.A. Davydov, O.Yu. Ovsitser, Catal. Rev.-Sci. Eng. 37 (3) (1995) 425. [7] P. Arpentinier, F. Cavani, F. Trifirò, The Technology of Catalytic Oxidations, Ed. Technip, Paris, 2001.
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Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile Part II. Effect of catalyst composition on catalytic performance N. Ballarini a , F. Cavani a,∗,1 , M. Cimini a , F. Trifirò a , R. Catani b , U. Cornaro b , D. Ghisletti c a
Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy b Snamprogetti SpA, via Maritano 26, 20097 S. Donato MI, Italy c EniTecnologie SpA, via Maritano 26, 20097 S. Donato MI, Italy Received 17 September 2002; received in revised form 7 April 2003; accepted 8 April 2003
Abstract Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr + V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V4+ ; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr + V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V3+ ; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Ammoxidation of propane; Ammoxidation of propylene; Acrylonitrile; Chromium, antimony, vanadium oxides
1. Introduction The ammoxidation of propane to acrylonitrile represents an alternative to the current industrial pro∗ Corresponding author. Tel.: +39-051-2093680; fax: +39-051-2093680. E-mail address: [email protected] (F. Cavani). 1 INSTM, Research Unit of Bologna.
duction of acrylonitrile via ammoxidation of propylene [1–7]. Semi-commercial and pilot plants for propane ammoxidation have been built by BP, Mitsubishi and Asahi. Each company has developed its own technology, where differences concern the catalytic system employed and the operating conditions. Catalysts active and selective for the ammoxidation of propane are either made of mixed antimonates having the rutile-type structure, like in the case of the
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00315-6
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BP system which is based on V(Al)SbO4 [8–12], or of mixed molybdates [13,14], like in the Mitsubishi (Mo/V/Nb/Te/O) or Asahi (Mo/V/Nb/Sb/O) systems. Moreover, while in former patents by Standard Oil, operation at propane-rich conditions seemed to be the preferred choice, more recent patents issued by Mitsubishi, Asahi and BP prefer propane-leaner conditions [15,16]. Also in this case recycle of the unconverted paraffin can be done, through the PSA technology which makes possible the separation of light paraffins and olefins from the other components of the stream, including molecular nitrogen. It is a peculiarity of antimonate-based catalysts that the active and selective systems are not only based on the rutile compound [17], but necessitate the presence of dispersed antimony oxide to obtain good selectivity to acrylonitrile. In fact, propylene is the reaction intermediate, and the catalyst needs allylic Oor NH-insertion properties in order to transform this intermediate to acrylonitrile, avoiding its desorption into the gas phase and its conversion to by-products. On the other hand, control of catalyst activity, of selectivity to acrylonitrile and of activity in ammonia combustion (an undesired side-reaction) is achieved by the addition of co-elements which are able to develop mixed rutile compounds together with the main components. In a related work [18, Part I of this paper] we have reported about the characterization of rutile-type Cr/V/Sb mixed oxides, and evidenced that the nature of the V species which develop is a function of the atomic ratio between components. In the present work, we report about the reactivity of Cr/V/Sb/O catalysts in propane and propylene ammoxidation, and analyze how the ratio between components affects the catalytic performance.
2. Experimental Catalysts were prepared with the coprecipitation technique, developed for the preparation of rutile SnO2 -based systems claimed by Rhodia [19]. The preparation involves the dissolution of Cr(NO3 )3 · 6H2 O, VO(acac)2 and SbCl5 in absolute ethanol; the solution is then dropped into an aqueous solution maintained at pH 7. A precipitate is obtained, which is separated from the supernatant liquid by centrifu-
gation and filtration. The solid is then dried at 120 ◦ C, and calcined in air at 700 ◦ C for 3 h, with a heating rate of 5 ◦ C/min. Catalytic tests were carried out in a laboratory stainless steel fixed-bed reactor operating at atmospheric pressure. Two milliliters of catalyst were loaded, shaped in particles having size ranging between 0.42 and 0.55 mm. The catalyst particles were mixed with an equal amount of inert particles (␣-alumina). The following reaction conditions were used: (i) propane ammoxidation-feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s; (ii) propylene ammoxidation-feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s. Products were analyzed by means of gas chromatography. A Hay-sep T column (TCD) was used for the separation of CO2 , NH3 , C3 H8 , H2 O, HCN, acrolein, acetonitrile and acrylonitrile, a MS-5A column (TCD) for the separation of O2 , N2 and CO, and a Porapak QS column (FID) for the separation of propane and propylene. 3. Results and discussion 3.1. The ammoxidation of propane: the effect of catalyst composition on catalytic performance Catalysts containing Cr, V and Sb were prepared and characterized using the Cr/Sb/O-based catalysts, studied in previous works, as the reference compounds [20,21]. Three series of catalysts were prepared, having Cr/V/Sb atomic ratios 1/x/δ (with Cr chosen as the reference element), where δ was equal to (i) 0.8–1.0 (this series of samples is here referred to as Cr/V/Sb 1/x/1), (ii) 1.8–2.1 (series of composition Cr/V/Sb 1/x/2), and (iii) 3–5 (series of composition Cr/V/Sb 1/x/3–5). In the three series of catalysts the value of x was varied between 0 and 1.0. Table 1 reports the main features of catalysts prepared. The reactivity in propane ammoxidation of samples belonging to series Cr/V/Sb 1/x/1 is summarized in Fig. 1; the conversion of propane, of ammonia and of oxygen, the selectivity to acrylonitrile and the selectivity to nitrogen from ammonia combustion are plotted for each catalyst as functions of temperature. The selectivity to the products at T ≈ 380 ◦ C is also
N. Ballarini et al. / Applied Catalysis A: General 251 (2003) 49–59 Table 1 Main characteristics of Cr/V/Sb/O catalysts with composition Cr/V/Sb 1/x/1, 1/x/2 and 1/x/3–5 Cr/V/Sb 1/x/δ (at. ratio)a
Surface area (m2 /g)
Secondary crystalline phasesb
1.0/0/0.8 1.0/0.3/1.1 1.0/0.5/0.9 1.0/0.8/1.1 1.0/0.9/0.9 1/0/1.6 1/0.2/1.7 1/0.5/1.8 1/0.8/2.0 1/0.9/1.8 0/1.0/2.1 1/0/2.8 1/0.2/5d 1/1/5d
37 59 50 32 36 67 67 49 40 40 20 54 51 72
– – – – 3.0 wt.% V2 O5 c – – – – – 26 wt.% ␣-Sb2 O4 c 14.6 wt.% Sb6 O13 c Sb6 O13 e Sb6 O13 e
a
As determined by SEM–EDX. Principal crystalline phase: rutile. c As determined by Rietveld analysis of the XRD pattern. d The effective composition was not determined analytically; atomic ratios correspond to those employed for samples preparation. e As evaluated from a qualitative analysis of the XRD patterns. b
reported in the figure. Results for samples belonging to series Cr/V/Sb 1/x/3–5 are reported in Fig. 2. Results concerning the reactivity of series Cr/V/Sb 1/x/2 have been reported in a previous work [21], but are discussed again in the present work to have a better insight of relationships between catalytic performance and chemical–physical features. The addition of V in samples belonging to series Cr/V/Sb 1/x/1 considerably increased the catalyst activity, notwithstanding the decrease of the surface area for x > 0.3. This indicates that the presence of V in Cr/V/Sb mixed oxides generates sites which are more active than those present in Cr/Sb/O systems. The conversions of ammonia and propane were limited by oxygen. Catalyst Cr/V/Sb 1/0.9/0.9 converted oxygen completely at 340 ◦ C, with a propane conversion close to 23%. This catalyst is one of the most active, non-noble-metal-based catalysts ever reported in the literature for paraffin (amm)oxidation carried out under hydrocarbon-rich conditions. The conversion of propane decreased when the temperature was increased above 350 ◦ C, as a consequence of a change in the distribution of products and of an higher oxygen consumption.
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The selectivity to acrylonitrile was low (<10%) in samples belonging to series Cr/V/Sb 1/x/1 when x was higher than 0.3, while the selectivity to propylene was higher than 10%. The selectivity to carbon monoxide increased and that to carbon dioxide decreased when the value of x was increased, while the sum of the two products remained constant. The sample having the highest V content (Cr/V/Sb 1/0.9/0.9) produced of propylene, carbon monoxide and carbon dioxide with similar selectivity. The selectivity to the other by-products (acetonitrile and cyanhydric acid) was not affected by the amount of V in the catalyst. The behavior of Cr/V/Sb 1/x/1 samples has analogies but also differences if compared to samples belonging to series Cr/V/Sb 1/x/2 [21]. Also in the latter case, in fact, an increase of V content led to an increase of activity and to a decrease of selectivity to acrylonitrile, but the latter was in all samples higher than 20% (for total conversion of oxygen), and the selectivity to propylene was lower than 8%. Samples having excess Sb, i.e. with composition Cr/V/Sb 1/x/3–5 (Fig. 2), behaved differently from samples Cr/V/Sb 1/x/1, and the catalytic performance was similar to that of catalysts belonging to series Cr/V/Sb 1/x/2 (Fig. 2) [21]. Increasing amounts of V led to an increase of activity, but the selectivity to acrylonitrile was higher than 25% for all samples, that to carbon oxides was 50–60%, and that to propylene was lower than 10%. The distribution of products was not much affected by the content of V. The degree of surface Sb enrichment, as determined by XPS, was considerably affected by the (Cr + V)/Sb ratio [18]. When the x value in samples having composition Cr/V/Sb 1/x/1 was increased, the Sb enrichment decreased [18]. This can be the reason for the low availability of NH2− -insertion sites, and for the decrease of selectivity to acrylonitrile (Fig. 1). The change in the intra-crystalline distribution of elements, instead, did not occur in the case of samples belonging to series Cr/V/Sb 1/x/2; a large excess of Sb in the outermost atomic layers of the rutile crystals was found in all samples [18]. Therefore, these catalysts always had a number of sites sufficient for the transformation of a large fraction of the unsaturated intermediate to acrylonitrile. In the case of samples belonging to series Cr/V/Sb 1/x/3–5, the presence of antimony oxide [18] provided additional sites for the
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Fig. 1. Summary of the catalytic performance of samples Cr/V/Sb 1/x/1. x = 0 (䉱), x = 0.3 (䊏), x = 0.5 (䊉), x = 0.8 (×), x = 0.9 (䉬). Bottom right figure: data collected at T = 374–386 ◦ C (except for sample Cr/V/Sb 1/0/0.8, at T = 430 ◦ C). Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s. AN, acrylonitrile; C3 = propylene; ACN, acetonitrile; CO2 , carbon dioxide; CO, carbon monoxide; HCN, cyanhydric acid.
transformation of the intermediate to acrylonitrile, rather than to carbon oxides. The catalytic performance of samples Cr/Sb 1/0.8, Cr/Sb 1/1.6, Cr/V/Sb 1.0/0.9/0.9, Cr/V/Sb 1.0/0.9/1.8 and V/Sb 1/2.1 is compared in Fig. 3. Cr/V/Sb/O catalysts were more active than V/Sb/O and Cr/Sb/O. The very high activity of three-component catalysts (especially of sample Cr/V/Sb 1/0.9/0.9) can be attributed
in part to the higher surface area of Cr/V/Sb/O samples as compared to V/Sb/O. In addition, a synergetic effect arising from the contemporaneous presence of Cr and V in the same structure can be hypothesized. A similar activity-enhancement effect, with a corresponding decrease in selectivity to acrylonitrile, has also been reported for Fe1−x Vx SbO4 catalysts [22]. The improvement occurred with respect to both FeSbO4
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Fig. 2. Summary of the catalytic performance of samples Cr/V/Sb 1/x/3–5. x = 0 (䉱), x = 0.2 (䊏), x = 1 (䊉). Bottom right figure: data collected at T = 484–495 ◦ C. Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s. AN, acrylonitrile; C3 = propylene; ACN, acetonitrile; CO2 , carbon dioxide; CO, carbon monoxide; HCN, cyanhydric acid.
and VSbO4 , with a maximum activity achieved for x ≈ 0.3, and was attributed to the generation of anionic–cationic vacancy couples as a consequence of replacement of V4+ by Fe3+ in the rutile structure. The behavior of V/Sb/O, Cr/Sb/O and Cr/V/Sb/O samples was also different in regard to the selectivity to acrylonitrile (Fig. 3, bottom). Sample Cr/V/Sb 1/0.9/0.9 yielded only traces of acrylonitrile, while sample Cr/V/Sb 1/0.9/1.8 reached a selectivity close
to that of sample V/Sb 1/2.1 only at high temperature, while being non-selective at low temperature, because of the relevant formation of propylene, carbon dioxide and cyanhydric acid. Sample Cr/V/Sb 1.0/0.9/0.9 was more active than sample Cr/V/Sb 1.0/0.9/1.8, notwithstanding the comparable values of surface area. The very high activity of this sample can be attributed to the preferential formation of the V4+ species. In fact, the characterization
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Fig. 3. Propane conversion (top) and selectivity to acrylonitrile (bottom) as functions of temperature for catalysts: V/Sb 1/2.1 (䉬), Cr/Sb 1/0.8 (䊏), Cr/Sb 1/1.6 (䊐), Cr/V/Sb 1/0.9/0.9 (䊉), Cr/V/Sb 1/0.9/1.8 (䊊). Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s.
evidenced that the relative amount of V3+ and V4+ in calcined Cr/V/Sb/O catalysts is a function of the (Cr + V)/Sb atomic ratio [18]; the amount of V4+ ions was the highest in samples having a (Cr + V)/Sb ratio above ≈1, which also are the most active catalysts among Cr/V/Sb mixed oxides here reported. The syn-
ergy effect on activity which occurs when Cr and V are in the same rutile structure might thus be related to an increase of the V4+ /V3+ ratio with respect to V/Sb/O samples, especially when the amount of Sb5+ is not enough to develop a mixed antimonate of V and Cr.
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3.2. The ammoxidation of propane: the reaction network Tests were made to determine the reaction network. The residence time was varied by changing the amount of catalyst loaded in the reactor (and loading an amount of ␣-alumina corresponding to that necessary to fill the reactor completely), while keeping the flow rate constant. Accurate control of the temperature (which was affected by the flow rate) made possible to obtain results under truly isothermal conditions (temperature 435 ◦ C). Moreover, the small contribution to conversion due to the reactor walls and to the inert material was subtracted from the catalytic results. Reaction conditions chosen were those so as to achieve very low conversions, in order to detect the formation of reaction intermediates. The results are summarized in Fig. 4, where the propane conversion and the selectivity to the products are plotted as functions of the residence time for catalyst Cr/V/Sb 1/1/5; this yielded the best selectivity to acrylonitrile, and a low selectivity to propylene and to carbon oxides. Propylene was the only primary product, while secondary products were cyanhydric acid and carbon monoxide. The formation of acetonitrile, carbon dioxide and acrylonitrile likely involved both direct and consecutive contributions. The selectivity to propylene decreased when the residence time was
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increased, with a corresponding increase in the selectivity to carbon dioxide and to acrylonitrile. Therefore, from the kinetic point of view propylene is an intermediate product; it is re-adsorbed on active sites and is converted either to acrylonitrile or to carbon dioxide. The data do not exclude the possibility of a direct pathway for the formation of acrylonitrile; this implies that a fraction of the adsorbed intermediate obtained by propane activation and (oxi)dehydrogenation is directly transformed to acrylonitrile. Acrylonitrile also underwent a consecutive transformation to carbon monoxide and to cyanhydric acid. These data confirm that the low selectivity to propylene which is obtained under usual reaction conditions is due (i) to the likely direct transformation of the adsorbed intermediate to acrylonitrile (direct acrylonitrile formation), and (ii) to the rapid re-adsorption of propylene from the gas phase and transformation to the successive products (consecutive acrylonitrile formation). This occurs with samples Cr/V/Sb 1/x/2 even in the absence of excess antimony oxide, differently from that observed in the case of the V/Sb/O system [23], because the intra-crystalline Sb enrichment in Cr/V/Sb/O rutile crystals provides the sites to perform allylic ammoxidation efficiently. Nevertheless, excess antimony oxide is useful to further increase the selectivity to acrylonitrile (as it occurs in samples Cr/V/Sb 1/x/3–5), by favoring the transformation
Fig. 4. Propane conversion (䉬) and selectivity to carbon dioxide (䉱), propylene (䊏), cyanhydric acid (䊊), acrylonitrile (䊐), acetonitrile ( ), and carbon monoxide (×), as functions of residence time. Catalyst Cr/V/Sb 1/1/5. Reaction conditions: feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; temperature 435 ◦ C.
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of propylene to acrylonitrile, rather than to carbon oxides. 3.3. The ammoxidation of propylene: the effect of catalyst composition on catalytic performance The performance of sample Cr/V/Sb 1/0.3/1.1 in propylene ammoxidation is shown in Fig. 5; the con-
version of reactants and the selectivity to the products are reported as functions of the reaction temperature. For comparison, the results obtained in propane ammoxidation with the same catalyst are also reported in Fig. 5. The reaction conditions in the two cases were different, since for propylene ammoxidation the conditions were chosen so as to reproduce industrial operation. Therefore, analogous conversions for the limiting
Fig. 5. Ammoxidation of propylene (top) and ammoxidation of propane (bottom). Conversions and selectivity as functions of temperature. Catalyst Cr/V/Sb 1/0.3/1.1. Conversion of the hydrocarbon (䉬), conversion of ammonia (䊏), conversion of oxygen (䉫), selectivity to propylene (bottom figure) (䊉), acrylonitrile (䊐), acetonitrile ( ), cyanhydric acid (䊊), carbon monoxide (×), carbon dioxide (䉱), acrolein (top figure) (䊉). Reaction conditions: propane ammoxidation-feed composition 25 mol% propane, 10% ammonia, 20% oxygen, remainder helium; residence time 2.0 s; propylene ammoxidation-feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s.
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Fig. 6. Propylene conversion (filled symbols) and selectivity to acrylonitrile (open symbols) as functions of reaction temperature for catalysts Cr/V/Sb 1/x/1 (top) and Cr/V/Sb 1/x/2 (bottom). Top: Cr/V/Sb 1/0/0.8 (䉬), 1/0.3/1.1 (䉱), 1/0.9/0.9 (䊏). Bottom: Cr/V/Sb 1/0/1.6 (䉬), 1/0.2/1.7 (䉱), 1/0.5/1.8 (䊏), 1/0.9/1.8 (䊉). Reaction conditions: feed composition 10% propylene, 10% ammonia, 25% oxygen, remainder helium; residence time 1.0 s.
reactant were achieved from the two hydrocarbons in similar ranges of temperature, despite the lower reactivity of the paraffin as compared to the olefin. Fig. 6 compares the propylene conversion and the selectivity to acrylonitrile as functions of the temperature for catalysts belonging to the two series Cr/V/Sb 1/x/1 and 1/x/2. Also from propylene, the activity increased and the selectivity to acrylonitrile decreased when the V
content in catalysts was increased. Sample Cr/V/Sb 1/0.3/1.1 reached a low selectivity to acrylonitrile from propane (around 15%, lower than that obtained with the Cr/Sb 1/0.8 sample, Fig. 1), but it did exhibit a good selectivity to acrylonitrile from propylene (around 65%). This indicates that the surface Sb enrichment was enough to provide a quick transformation of the activated olefin to acrylonitrile. When this Sb enrichment was smaller, as in the case of sample
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Cr/V/Sb 1/0.9/0.9 [18], the selectivity to acrylonitrile was very low both from propane and from propylene. On the contrary, sample Cr/V/Sb 1/0.9/1.8 gave a good selectivity to acrylonitrile from both hydrocarbons, as it was also for all samples belonging to series Cr/V/Sb 1/x/2; this was due to the presence of the surface Sb enrichment. A non-negligible selectivity to acrolein from propylene was obtained with samples Cr/V/Sb 1/x/1, and with sample Cr/V/Sb 1/0.9/1.8 (which was the most active catalyst in series Cr/V/Sb 1/x/2). On the contrary, acrolein was never obtained from propane. This difference may be interpreted by assuming that the relative amount of NH2− - and O2− -insertion sites is a function of the reaction conditions. Under conditions of oxygen starvation (as in the case of propane ammoxidation), the number of sites responsible for allylic ammoxidation was much greater than the number of sites responsible for allylic oxidation. In the case of propylene ammoxidation, instead, the larger availability of gas-phase oxygen and the lower partial pressure of ammonia led to the formation of acrolein.
4. Conclusions Rutile-type Cr/V/Sb/O catalysts are active in propane ammoxidation. A synergy effect is postulated to explain the high activity of Cr/V/Sb mixed oxides as compared to the Cr/Sb/O and V/Sb/O systems; this effect is likely related to the preferred formation of V4+ , rather than V3+ , in the presence of Cr, especially for (Cr + V)/Sb ratios higher than 1. The selectivity to acrylonitrile is a function of this parameter, too: selectivity close to 35% to acrylonitrile is obtained with samples having the lowest (Cr + V)/Sb ratio. The selectivity to propylene is very low in all cases, except for very low propane conversions. This is attributed to the intrinsic multifunctionality of Cr/V/Sb/O compounds, in which surface enrichment of Sb, in samples having (Cr + V)/Sb ratios between 0.5 and 1, confers to the catalyst the ability to transform the unsaturated intermediate to acrylonitrile. In samples having a (Cr + V)/Sb ratio lower than 0.5, excess antimony oxide also contributes to a further increase of the selectivity to acrylonitrile at the expense of carbon oxides.
Fig. 7. Simplified scheme for the ammoxidation of propane to acrylonitrile. Minor by-products have been omitted.
Table 2 Effect of catalyst composition on the selectivity to the main products in propane ammoxidation. The selectivity ranges are those obtained for total oxygen conversion. Path numbers are those indicated in Fig. 7 (Cr + V)/Sb (at. ratio)
>1 0.5–1 <0.5
Path preferred
7 > [1 + 2] > [1 + 4 + 5 + 6] 7 > [1 + (2 + 3 +) 4 + 5 + 6] > [1 + 2] 7 > [1 + 2 + 3 + 4 + 5 + 6], [1 + 4 + 5 + 6] > [1 + 2]
Selectivity (%) C3 H6
COx
C3 H3 N
10–30 <10 <5
60–80 55–70 50–60
<20 20–30 25–35
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Fig. 7 and Table 2 summarize the main results of the present work, with indication of the effect of the (Cr + V)/Sb atomic ratio on the catalytic performance in propane ammoxidation, in relation to the reaction scheme. Acknowledgements Snamprogetti SpA is acknowledged for financial support. References [1] R.K. Grasselli, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 5, Wiley-VCH, Weinheim, 1997, p. 2302. [2] Y. Moro-oka, W. Ueda, in: Catalysis, vol. 11, Royal Society of Chemistry, Cambridge, UK, 1994, p. 223. [3] R.K. Grasselli, Catal. Today 49 (1999) 141. [4] G. Centi, F. Cavani, F. Trifirò, Selective Oxidation by Heterogeneous Catalysis, Kluwer Academic Publishers/ Plenum Press, New York, 2001. [5] F. Cavani, F. Trifirò, in: R.K. Grasselli, S.T. Oyama, A.M. Gaffney, J.E. Lyons (Eds.), 3rd World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, p. 19. [6] V.D. Sokolovskii, A.A. Davydov, O.Yu. Ovsitser, Catal. Rev.-Sci. Eng. 37 (3) (1995) 425. [7] P. Arpentinier, F. Cavani, F. Trifirò, The Technology of Catalytic Oxidations, Ed. Technip, Paris, 2001.
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