Dependence of the catalytic behavior of V—Sb-oxides in propane ammoxidation to acrylonitrile from the method of preparation

~ ELSEVIER

AA PT PA LE IY DSS C L I A: GENERAL

Applied Catalysis A: General 165 (1997) 273-290

Dependence of the catalytic behavior of V-Sb-oxides in propane ammoxidation to acrylonitrile from the method of preparation Gabriele Centi*, Paolo Mazzoli, Siglinda Perathoner Dept. of lndustrial Chemistry, University of Messina, 1-98166 Messina, Italy Received 3 April 1997; received in revised form 16 June 1997; accepted 16 June 1997

Abstract

The catalytic behavior in propane ammoxidation to acrylonitrile of V-Sb-oxides prepared by four different methods and with a Sb : V ratio in the 0.8-3.0 range was studied and analyzed in relation to the structural/composition features of these samples determined by XRD, IR and SEM-EDX techniques. In particular, the following aspects are discussed: (i) composition, (ii) local homogeneity and morphology of the samples, (iii) presence, amount and dimensions of crystalline phases, (iv) presence and nature of the amorphous phases, and (v) crystallographic and non-stoichiometric features of the ~VSbO4 phase. It is suggested that the selective behavior in propane ammoxidation is related principally to the nonstoichiometry of the ~ V S b O 4 phase and especially the creation of a Sb-rich ~VSbO4 phase, the formation of which depends on the microstructure of the catalyst, chemistry of preparation and Sb : V ratio. ~-" 1997 Elsevier Science B.V.

Keywords: Vanadium-antimony oxide; Acrylonitrile; Propane ammoxidation; Preparation

1. I n t r o d u c t i o n In recent years there has been a considerable growth in the market for acrylonitrile, a chemical widely used as a monomer and co-monomer and as intermediate for adiponitrile. Acrylonitrile is commercially produced mainly by ammoxidation of propene, but recent developments have indicated the possibility of selectire one-stage synthesis of acrylonitrile from propane using multicomponent oxides based on antimonate catalysts, especially oxides containing vanadium as a key components [ 1-6]. This reaction represents one of the few examples with a good outlook for com*Corresponding author. Tel: + 39 90 39 3134; fax: + 39 90

391518: e-mail: [email protected], 0926-860X/97/$17.00 ~) 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 6 - 8 6 0 X ( 9 7 ) 0 0 2 0 8 - 1

mercial application of a process for the synthesis of a large volume chemical using an alkane as feedstock [7]. V-Sb-oxides are characterized by the presence of the following crystalline phases: a rutile-like vanadium-antimonate ( ~ V S b O 4 ) phase and ot-Sb204 or/35 b 2 0 4 , when the Sb : V ratio is higher than about 1.0 [ 8 - 1 2 ] . /3-Sb204 is usually present when calcination temperatures are higher than about 800°C [ 11,12] due to the o~---~-5b204 transition catalyzed by vanadium, probably partially incorporated into the/~-Sb20 4 lattice [ 11,13,14]. In addition to these crystalline phases, amorphous V 5+- and SbS+-oxide supported on the former phases can also be detected by IR, SEM and X P S analyses [12,15,16]. The calcined V-Sb-oxide catalysts thus may be described as monophasic V S b O 4

274

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

or biphasic VSbO4+Sb204 systems, but on which an overlayer or patches of well dispersed V 5+- and Sb 5+oxides are also present. With increasing Sb : V ratio, there is a change in the relative amounts of the overlying, XRD amorphous phases, in addition to the change from a monophasic to biphasic crystalline system, The phase composition of V-Sb-oxide catalysts depends on several factors, including the method of preparation adopted for the synthesis of V-Sb-oxides which do not influences only the phase composition, but also features of the catalyst such as the microstructure and non-stoichiometry of the ~VSbO4 phase [12,16]. In particular, in a previous work dealing on the structure, activity and selectivity relationships in V-Sb-oxides prepared by precipitation-deposition method the importance of the formation of a Sb-rich ~VSbO4 phase was shown [17], suggesting the importance of the method of preparation in the formation of this phase. However, specific studies of the relationship between these catalyst features or in general the method of preparation and the catalytic behavior in propane ammoxidation to acrylonitrile are not available in literature. The scope of this work is to investigate this question and how the microstructure and non-stoichiometry of the ,~VSbO 4 phase are determined from the chemistry of preparation, 2. Experimental

2.1. Preparation of the catalysts Samples were prepared in the 0.8-3.0 Sb : Vatomic range using four preparation procedures. The following nomenclature will be used hereinafter to indicate these catalysts: SbVx-Y(Z), where x indicates the S b : V atomic ratio, Y the method of preparation described below and Z a suffix indicating whether the sample is that after calcination (C) or discharged (D) from the reactor after the catalytic tests in propane ammoxidation (up to a temperature of 500°C using the standard feed composition reported below). For example, the code SbV1-GS(C) indicates a calcined sample prepared with method GS and a Sb : V ratio of 1.0.

2.1.1. Method GS This method can be briefly described as a gel-solid method. To a slurry of V205 in 100 ml distilled water

maintained in an ice-bath, 30 ml of an aqueous solution of 30% H202 is added in three steps. After about 2 h under continuous stirring, initially a sol and then a gel of vanadium monoperoxide forms, with a parallel increase in the viscosity of the solution. At this stage, the viscous solution is heated up to about 100°C after adding some water to reduce viscosity; the solution is maintained under stirring and reflux. Successively, finely ground Sb203 is a added to the viscous solution in an amount sufficient to provide the required Sb : V ratio. A dark-green solution forms, which is maintained under stirring and reflux for 3 h, after which the water is removed on a hot plate and the solid is finely ground and dried at 140°C for 12 h. After further grinding and mixing, the solid is calcined in a flow of air up to 600°C (3 h) using a constant rate of increase in temperature (50°C h-l).

2.1.2. Method DAA This method can be briefly described as a deposition on antimonic acid. The first step is the preparation of the antimonic acid by adding dropwise SbC15 to an aqueous solution of 30% H20 2 maintained in an icebath. The white precipitate of antimonic acid, after ageing for 6 h, is filtered, washed three times with distilled water and then dried at 140°C overnight. The second step is the preparation of the solution of V 4+ by reduction of V205 with oxalic acid in an aqueous solution maintained under boiling for about 3 h. To the V 4+ solution is then added the antimonic acid in such an amount as to have the right S b : V ratio. The solution is maintained under stirring and reflux for 3 h. Water is then evaporated and the solid obtained, after grinding, is dried at 140°C overnight. After further grinding and mixing, the solid is calcined as above.

2.1.3. Method SSR This method can be briefly described as a solid state reaction. Commercial V205 and Sb20 3 from Merck are separately finely ground and then mixed together gently, but for enough time as to obtain an intimate homogeneous mixture. The solid is then calcined as above, but with intermediate steps of mixing of the solid each 2 h in order to enhance the contact between the two solids.

275

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

2.1.4. Method PD

following crystalline phases: VSbO4 ( S b ' V = I . 0 ) ,

This method can be briefly described as a precipitation-deposition method and is the same used to prepare catalysts cited in a previous work [17]. VC13 is dissolved in a 0.1 N aqueous HC1 solution to give a dark green transparent solution (solution A). SbCI5 is dissolved in a 3-5 N HC1 aqueous solution to give a yellow solution (solution B). The dropwise addition of solution A to solution B leads to the formation of white precipitate (mainly Sb-hydroxide) and a dark blue solution due to VO 2+ ions. The latter derive from the oxidation of V 3+ to VO 3+ due to the two electron redox reaction of Sb 5+ with V 3+ and the rapid reaction of VO 3+ with V 3+ to form VO 2+. VO 2+ is then precipitated over the Sb-hydroxide by the dropwise addition of a concentrated aqueous solution of ammonia up to basic pH. The precipitate is filtered, washed three times with distilled water and then dried at 140~C overnight. The resulting solid is then calcined as above,

V S b O 4 + 0 . 1 V205 (Sb : V=0.8), V S b O 4 - - 0 . 5 S b 2 0 4 (Sb • V=2.0) and V S b O 4 + S b 2 0 4 (Sb : V = 3 . 0 ) . The

2.2. Characterization of the samples X-ray diffraction (XRD) analysis was carried out using the powder method and a Philips PW 1840 diffractometer with C u K , radiation. TiO2 (anatase) added in calibrated amounts (30% by wt.) was used as an internal standard for quantitative determinations and as a reference for the evaluation of the unit cell parameters estimated on the basis of least-square refinement on the 8 more intense reflections of the rutile tetragonal cell: (hkl)=(110) at 20=27.4 °, (101) at 35.2", (200) at 39.1 °, (111) at 40.5 °, (211 ) at 53.7 ~', (220) at 56.3 ° , (310) at 63.9 ° and (301) at 68.3 ° . The quantitative determination of the relative amounts of the crystalline species was made by normalizing the integral area of the (110) reflection (20=27.35 °) of the rutile phase or (112) reflection of o~-Sb204 ( 2 0 = 2 9 . 0 °) with respect to the integral area of the (101)reflection of TiO2 (20=25.3 °) added in a calibrated amount (30% by wt.). This normalized amount was then divided by the normalized amount of the same phase in SbV I-PD(D) for ~VSbO4 or SbV3-DAA(D) for c~S b 2 0 4 to obtain the relative amount of that crystalline phase present in the various samples. This amount can be compared with the theorical amount estimated on the basis of the hypothesis of formation of only the

dimensions of the crystallites were determined using the Bragg equation and the width at half-height of the reflection at 20=27.35 ° (110 line of the rutile phase) or at 20=29.0 ~ (112 line of o~-8b204). Infrared (IR) analysis was carried out with a Perkin-Elmer FT-IR 1750 instrument using the KBr disc technique and calibrated amounts of the samples (5% by wt.). Spectra were recorded in air using a 2 cm " 1 resolution. Scanning electron micrographs with mapping of the V and Sb distribution using electron dispersive X-ray analysis (SEM-EDX) were recorded with a JEOL 32 instrument. The local composition of the sample was determined from the integral areas of EDX peaks of V and Sb, after calibrating the relative intensities using a low resolution image which gives values of the intensities averaged over the entire sample, which thus can be compared with the S b : V ratio determined by chemical analysis. The surface area was determined with the BET method (N2 adsorption at 77 K) using a Carlo Erba Sorpty 1750 instrument. The samples were pretreated by evacuation at 200°C. The surface areas of the samples before and after the catalytic tests were found to be nearly equivalent, taking into consideration the experimental error. The values of the surface area of the samples after the catalytic tests are reported in Table 1.

2.3. Catalytic tests Catalytic tests were carried out in a conventional plug-flow-type reactor with on-line gas-chromatoTable 1 Surface area (m2g II of the sample alter the catalylic tests of propane ammoxidation

Sb:V

Surface area (SA). meg 1

0.8 1.0 2.0 3.0

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DAA 14 12 15 io

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SSR

3 2 2 1

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

276

graphic (GC) analysis (two GCs equipped with a flame ionization and thermoconducibility detector, respectively). A m m o n i a conversion and HCN formation were instead detected by absorption in appropriate solutions and titration. The correctness of the analytical determinations was checked for each test by verification that the carbon balance (based on the propane converted) was within the cumulative mean error of the determinations ( + 8 % ) . Tests were made using 3 g of sample with particle dimensions in the 0.1-0.2 m m range and diluted in a 1 : 5 ratio using an inert support in order to ensure a homogeneous radial and axial temperature profile. The axial temperature profile was monitored by a thermocouple sliding inside a tube inserted into the catalytic bed. A continuous fixed-bed stainless steel reactor was used for the catalytic tests, after having checked the absence of significant mass and heat transfer limitations in the standard reaction conditions. Tests were made using the following feedstock: 7.1% propane, 12.8% 02 and 11.0% NH3. The total flow rate was 6 1 h -1 corresponding to a gas-space hourly velocity (GHSV) of about 3 0 0 0 h - L estimated at STP conditions and catalyst volume without the inert component. Yields and selectivities in products were determined on the basis of the moles of propane feed or converted, respectively. The following six-step procedure for the catalytic tests was used. 1. The calcined catalyst is heated up to 400°C in a helium flow. 2. Propane, oxygen and ammonia (in the concentrations indicated above) are added to the feed and then the reactor is heated up to 500°C in about 1 h.

3. Results

3.1. Catalytic behavior in propane ammoxidation Reported in Fig. 1 is the effect of reaction temperature on the catalytic behavior of S b V 1 - D A A and S b V 3 - D A A samples (Fig. l ( a ) a n d (b), respectively). In both cases the selectivity to acrylonitrile passes through a m a x i m u m for a reaction temperature of about 480°C. Similar results were found for the other samples with different Sb : V ratios and/or methods of preparation and therefore for conciseness, in the following discussion this temperature of reaction (480°C) will be used as the reference temperature

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G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

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to compare the catalytic behavior as a function of the Sb ' V ratio and method of preparation. The effect of the S b : V ratio on the catalytic behavior of S b V x - D A A samples at 480°C is summarized in Fig. 2. As the Sb " V ratio increases from 0.8 to 3.0 the selectivity to acrylonitrile increases from about 18% to above 65% with a parallel decrease in the selectivity to propene from about 43 to 8%. The selectivity to carbon oxides also decreases from about 37 to 18%, whereas the conversion of propane do not change considerably in the 15-20% range. A considerable decrease is instead noted in the side reaction of unselective conversion of ammonia to N2 and in the total conversion of ammonia. The trend in the NH3--+N2 reaction versus the S b : V atomic ratio follows inversely that in the selectivity to acrylonitrile. The same effect was noted in all catalysts prepared by different methods. It may be noted also that neither the catalyst activity and the selectivity depend on the surface area of these samples after the catalytic tests (Table 1), indicating that the activity is not linearly proportional to the surface area due to the presence of a multiphase system as discussed below. The role of the preparation method on the selectiv-

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oxides (c) at 4 8 0 C as a function of the S b : V atomic ratio in catalysts prepared with the methods DAA, GS, PD and SSR.

278

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

therein) were made using a high propane concentration and 02 and NH3 as the limiting reactants to be converted nearly completely. In the conditions selected for the catalytic tests reported in Fig. 3, the order of selectivity as a function of the preparation method does not depend significantly from the change of reactants conversion as verified in some preliminary tests, being mainly dependent on the intrinsic reactivity of the catalyst itself. It is thus preferable to compare the catalysts making the catalytic tests at iso reaction temperature and space-velocity as in Fig. 3 which evidence the intrinsic reactivity of the catalysts that change the above parameters in order to compare catalytic performances at equal conversion, but different reaction conditions. The results of Fig. 3 can be summarized as follows: 1. The preparation methods DAA and PD are relatively analogous in terms of selectivity to acrylonitrile (Fig. 3(a)), although depending on the Sb : V ratio, the samples prepared by the PD (Sb : V=2) or DAA (Sb : V=3) methods show a slightly better selectivity to acrylonitrile. In both cases a selectivity to acrylonitrile up to over 60% can be obtained for the higher Sb : V ratios. SbVxPD samples, however, show a lower formation of c a r b o n o x i d e s than S b V x - D A A s a m p l e s (Fig. 3(c)), mainly due to the higher formation of propene. Therefore, although the highest selectivity to acrylonitrile was observed for the SbV3-DAA sample, the preparation method PD can be considered slightly better for the lower formation of carbon oxides, 2. The samples prepared with the method GS show a selectivity to acrylonitrile significantly lower than that shown by the samples prepared with the methods DAA and PD (Fig. 3(a)), especially for the higher Sb : V ratios. All three samples have a similar selectivity for a S b : V = I ratio, but the maximum in selectivity to acrylonitrile for SbVxGS samples is shown by that having a Sb : V = 2 ratio instead of Sb : V=3 as for other preparation methods. The lower selectivity of samples prepared by the GS method is mainly due to the formation of carbon oxides (Fig. 3(c)), whereas the formation of propene is comparable to that of the other preparation methods (Fig. 3(b)). 3. The SbVx-SSR method of preparation gives the poorest catalytic results (lowest selectivity to acryl-

onitrile and higher carbon oxides formation), although the selectivity to propene is not significantly different from that of the other samples. 3.2. Phase composition 3.2.1. Effect of Sb : V ratio Reported in Fig. 4(A) are the IR spectra of SbVxPD(C) samples as a function of the Sb : V ratio and in Fig. 4(B) the relative XRD patterns. In the sample with S b : V = 0 . 8 , the IR spectrum (Fig. 4(A), (a)) shows the presence of crystallites of V205 (bands at 1012 and 845 cm l; Uv=o and 6vov, respectively), although the XRD pattern of the same sample (Fig. 4(B), (a)) indicates the presence of a rutile ~ W S b O 4 phase only. The Uv=oIR band falls at a slight lower frequency than in crystalline V205 (1022cm-1), probably due to low dimensions of the crystallites which, in fact, are not detected by XRD. A shoulder at about 996 cm - l is also present on the lower frequencies side of the 1012 cm - l band, indicating that part of the VS+-oxide is also present spread as a thin layer over the ~ W b S O 4 crystallites by analogy with the frequency observed for VS+-oxide spread over TiO2 [17]. The intense bands at 673 and 549 cm i are instead to ~ V b S O 4 phase (ul and u2, respectively) [18,19]. As the Sb : V ratio increases to the stoichiometric value for VSbO4 (Sb : V---~I.0), analogous IR spectra and XRD patterns are observed, although the amount of VS+-oxide crystallites decreases with respect to the sample with Sb : V=0.8. The further increase in the Sb : V ratio to 2 and 3 leads to a further decrease in the relative intensity of the band of VS+-oxide crystallites, but not its complete disappearance even for the sample with Sb : V=3.0. New bands also appear in the IR spectra of samples with Sb : V_>2: at 732 and 443 cm i (o~_Sb204) [20] (the other bands overlap with those of ,-~VSbOa perturbing their relative intensity) and a shoulder at about 830 cm -1 and possibly near 900 cm -1 Weak reflections for Sb6013 , the relative intensities of which increase with increasing Sb : V ratio, could be seen in the XRD pattern (Fig. 4(B)). This suggests that the latter two bands may be due to this phase. On the contrary, XRD reflections for o~-5b204 are absent or very weak even in the sample with Sb : V=3.0, although IR analysis shows clearly its presence for Sb : V>I.0 (band at 732

279

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

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The role of the preparation method in determining the phase composition is summarized in Fig. 5 which reports the IR spectra and XRD patterns (Fig. 5(A) and 5(B), respectively)for calcined samples with a S b : V ratio of 1.0 and 3.0 (SbVI-Y(C) and SbV3Y(C), respectively, where the symbol Y indicates the different modalities of preparation). The IR spectra of the samples with S b : V = I . 0 (Fig. 5, SbV1-Y(C)) clearly show the influence of the method of preparation on the phase composition. The IR spectrum of SbV1-DAA(C) is relatively similar to that of SbV 1-PD(C) (see above), but the relative intensity of the band for V~Os crystallites increases - and their u,._,, IR band shifts to higher frequencies (1020 cm l). Weak reflections of V205 crystallites could be detected in the XRD pattern (Fig. 5(B), (a)). Furthermore, an additional band at about 837 cln attributed to SbS+-oxide could be detected, and accordingly the XRD pattern shows the presence of weak reflections of S b 6 O l 3 . SbVI-DAA(C) with respect to SbV 1-PD(C) is thus characterized by larger crystallites of V205, a reduced fraction of well spread V 5+-oxide, and higher amount of poorly crystallized

20 (Degrees) Fig. 4. IR spectra (A) and XRD patterns (B) of SbVx-PD(C) samples as a function of the Sb : V ratio: (a) Sb : V ratio: (a) Sb : v--0.8, (b) Sb : V 1.0. (c) Sb : V=2.0 and (d) Sb : V=3.0.

and 443 cm i). This is evidence that also this phase is present in the form of crystallites with dimensions smaller than that required for detection by the XRD method, reasonably due to the effect of ~ V S b O 4 p h a s e in inhibiting the growth of larger Sb204 crystallites detectable by XRD analysis,

The IR spectra of SbV1-GS(C) and SbVI-SSR(C), in comparison with those of SbVI-DAA(C) and SbVI-PDCC). show a decrease in the intensity of the VS+-oxide IR band, but a marked increase in that o f S b 2 0 4 (intense b a n d c e n t r e d at 7 4 5 c m ~) (see a l s o below for the discussion of the type of Sb,04+ phase). The XRD pattern of SbVI-GS(C) (Fig. 5(B) (d)) confirms the relevant presence of crystalline ~S b ,4O.

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G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

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G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

however, the IR spectrum shows the clear presence of both ~- and ~-Sb204, as indicated by the doublet centred at about 740 c m - l (bands at 732 and 748 cm -1, due to USb-O in c~-and ~-Sb204, respectively) [21]. Probably only o~-Sb204 is present in SbV3-PD(C), but the presence of the intense shoulders due to 5b6013 does not allow a more precise indication. Only traces of both c~- and /3-5b204 could be detected by XRD analysis in both samples, In SbV3-GS(C)and SbV3-SSR(C), the bands of the Sb204 phase are very intense and dominate the IR spectrum (Fig. 5(A), spectra (d) and (c), respectively), The XRD pattern indicates the presence together with ~VSbO4 of ~-Sb204 in SbV3-SSR(C) and fl-Sb204 in SbV3-GS(C), although the IR spectra of these two samples are similar (only a not resolved band centred at 744 cm l), indicating the probable presence of both c~- and 9-Sb204 in two samples, although with different degrees of crystallinity of the two Sb-oxides. 3.2.3. Changes after the catalytic tests

Considerable changes in phase composition occur after the catalytic tests, although the effect depends on the modality of preparation of the samples. The results are summarized in Fig. 6 which reports the IR spectra (Fig. 6(A))and XRD patterns (Fig. 6(B))of the s a m e samples of Fig. 5, but after the catalytic tests in propane ammoxidation following the procedure reported in the experimental part. SbVI-DAA(D) and SbV1-PD(D), with respect to the corresponding samples after calcination, show a considerable reduction of the IR band of vS+-oxide which nearly completely disappears, although a weak, but sharp, band remains at 968 cm -1 in SbV1DAA(D) probably due to the formation of the side phase VOSb204 [16], which, however, is in so low amounts to be not considered determining for the catalytic behavior. The broad band in the 800900 cm ~ region indicating the presence of Sb 5+oxide also disappears, but a broad band centred at 882 cm I remains clearly visible suggesting the continued presence of well dispersed Sb 5+ sites supported on the ~VSbO4 matrix. The XRD analysis of both samples (Fig. 6(B)), on the contrary, reveals the presence of ~VSb(-)4 only. In the samples with a S b : V = 3 . 0 ratio (SbV3DAA(D) and SbV3-PD(D)), ~VSbO4, ol-Sb204 and Sb6Oi3 could be detected by XRD analysis, but in

281

SbV3-DAA(D) the reflections of ~-Sb204 and Sb6Ol3 are relatively more intense than in SbV3-PD(D). The IR spectra (Fig. 6(A)), confirm this indication and the observation already made for Sb : V--1 samples, i.e., the considerable reduction of the V 5+- and SbS+-oxide phases after the catalytic tests, but still the presence of broad bands near 1000 and 882 cm i indicating the presence of well dispersed V 5-+ and Sb 5+ on the surface of the other crystallites. It should also be noted that the difference in the nature of Sb204 species discussed above was also evident in the samples after the catalytic tests, i.e., the presence of c~- and i~-Sb204 in SbV3-PD(D) and probably only c~-Sb204 in SbV3DAA(D) The samples prepared by the SSR and GS methods, on the contrary, remain nearly unchanged with respect to the same samples before the catalytic tests, independently of the Sb : V ratio. This is indicative of a lower degree of reactivity of these catalysts towards solid state transformations driven by the catalytic reaction of propane ammoxidation. 3.3. Amount and mean dimensions ~f~ the crvstallites

The relative amounts of the crystalline phases

(~VSbO 4 and Sb204) present in the samples discharged after the catalytic tests and their dimensions were determined by XRD analysis. The data refer to the amount of the phase in the sample with respect to that present in SbV1-PD(D) for ~VSbO4 (Table 2) or SbV3-DAA(D) for o~-Sb204 (Table 3) used as reference to analyze how the relative amounts of these phases change as a function of the preparation method. Two values are reported in the Tables: the absolute amount with respect to the reference sample (AA) and the relative amount (RA) which considers that the maximum theoretical amount of the phases depends on the S b : V ratio. The theoretical amount of ~VSbO4or Sb20 4 can be estimated on the basis of the hypothesis of formation of only the following crystalline phases: VSbO 4 (Sb : V = 1.0), VSbO4+0.1 V205 (Sb : V=0.8), VSbO4+0.5 Sb204 (Sb : V--2.0) and VSbO4+Sb204 (Sb : V=3.0). The data in Tables 2 and 3 indicate the following points: (a) The sample prepared by the GS method are characterized by a significantly lower amount of crystalline ~VSbO4 and higher amount of or-

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

~.82

A [ SBV1-Y(D)

745 ~'1~653550 4461 (

~

I I

["

;I

i"

~

!

; II

/

gl

SbV3-Y(D)

.

i:

i

F

~1

ii,1 a- :! 1 i;.I ~,

I

,~

/,[

.,IL

800 600 wavenumber,cm"1

B

I

" '~'-~ki

',rfe

400

SbV1-Y(D) ~

~

,',

,...f

1000

I

d/"/r-'il

~,

<%.

1200

731 f/~ 754

~-,>~

1200

1000

/

800 600 wavenumber,cm 1

400

~ VSbO, m ~.-Sb~O4

2

~-~ ~ .~,,wf'°-~~'~ . ,'-~.~ ~'~~~/~. .-!"-~,.. , ~,..-.A~......... ._,.,L"~ .

~o

~o

SbV3-Y(D)i

I~P"V~v ~

20

i

~'o '

20 (Degrees)

~o '

~o

~ma-Sb204 VSbO, •• t~Sb204 SbsOI3

"

30

,10

50

60

20 (Degrees) Fig. 6. IR spectra (A) and XRD patterns (b) of samples with Sb : V = l . 0 (SbVI-Y(D)) and Sb : V=3.0 [SbV3-Y(D)] prepared with differer methods after the catalytic tests in propane ammoxidation: (a) Y=DAA, (b) Y--PD, (c) Y=SSR and (d) Y=GS.

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

(b) An increase in Sb : V ratio leads to a decrease in t h e a b s o l u t e a m o u n t (AA) o f ~ V S b O 4 , b u t to a n

Table 2 Absolute amount (AA) and relative amount (RA) of ~VSbO4 as a function of the Sb : V ratio and method of preparation in samples after the catalytic tests of propane ammoxidation. See text for the procedure used to estimate AA and RA values. Mean error 4-0.(/5. The reference sample for the estimation is highlighted in bold Sb:V

increase in the relative amount (RA) which takes into a c c o u n t the c h a n g e s in the m a x i m u m theoretical amount of this phases with Sb : V ratio. This indicates that an increase in antimony-oxide content promotes the formation of crystalline

Amount of crystalline ~VSbO 4 PD

DAA

GS

SSR AA

~VSbO4.

AA

RA

AA

RA

AA

RA

0.8 1.0

1.03 l.OO

1.10 t.00

1.03 1.03

1.10 1.03

0.34 0.40

( I . 3 7 0.66 0.40 0.76

0.71 0.76

2.(/

1.03

1.70

0.89

1.48

0.28

0.45

0.48

0.80

3.(1

0.89

2.06

0.72

1.68

0.31

0.52

0.38

0.88

(C) Although the d i f f e r e n c e s b e t w e e n the D A A and PD preparation methods are less accentuated

RA

with respect to the comparison with the GS and SSR methods, some differences are present. In general, the PD method leads to an increase in the relative amount of crystalline ~ V S b O 4 with respect to the D A A method. (d) An increase in S b : V ratio leads to an increase in the formation of crystalline S b 2 0 4,

Table 3 Absolute amount (AA) and relative amount (RA) of c~-Sb204 (/3Sb204 for the GS preparation) as a function of the Sb : V ratio and method of preparation in samples after the catalytic tests of propane ammoxidation. See text for the procedure used to estimate AA and RA values. Mean error ±0.08. Note: for SbVI-GS, it is not possible to indicate a value for RA, because the theoretical amount

but significant differences exist between samples.

AA

DAA

RA

AA

GS

RA

AA

AA

o.00 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0.00 0.0(1 0 . 0 0 0 . 0 0 0 . 0 0 0.22 - 0.00 0.00

2.(/ 3.0

0.00 0.16

0.32 1.00

0.46 1.00

0.72 2.60

1.03 2.60

this

phase

is

The mean dimensions of the crystallites of ~ V S b O 4 and Sb204 are summarized in Table 4.The data indi-

RA

0.8 1.0

0.00 0.16

of

o f ~ - S b 2 0 4.

SSR

RA

amount

samples. However, whereas the PD, DAA and SSR methods lead to the formation of crystalline ~,t-Sb204, the GS method leads to the formation

Amountof crystalline Sb204 PD

The

the

the highest for GS samples and the lowest for P D

is zero

Sb:V

283

cate the following aspects: (a) The S b : V ratio does not affect in a systematic way the dimensions of the crystallites o f ~ V S b O a , but Sb : V ratios higher than 1.0 lead

0 . 6 1 0.87 1 . 0 6 1.06

to an increase in the mean dimensions of the S b 2 0 4 crystallites. (b) The samples prepared with the GS and SSR methods are characterized by larger crystals of ~ V S b O 4 , whereas those prepared with method D A A have the smaller dimensions.

S b 2 0 4 , in agreement with IR analysis. Although

the SSR method leads to the higher amounts of c r y s t a l l i n e ~ V S b O 4 with respect to the GS method, the amount is lower than that obtained using the PA and D A A methods.

Table 4 Mean dimensions (±20 ~,) of the crystallites of ~VSbO4 and a-Sb204 (.3-Sb204 for the GS preparation) as a function of the Sb : V ratio and method of preparation in samples after the catalytic tests of propane ammoxidation Sb : V

Mean dimensions of crystallites, ~, PD

0.8 1.0 2.0 3.(/

DAA

GS

SSR

~VSbO4

Sb204

~VSbO4

Sb204

~VSbO4

Sb204

~VSbO4

Sb204

214 285 267 231

---183

210 204 195 267

--214 382

279 306 326 365

-350 422 486

379 423 451 353

--360 423

G. Cent±et al./Applied Catalysis A: General 165 (1997) 273-290

284

Table 5 Unit cell parameters of the/-utile~VbSO4 phase (tetragonalcell) as a functionof Sb : V ratio and method of preparation in samples after the catalytic tests of propane ammoxidation Sb : V

Unit cell parameters of rutile ~ V S b O

4

PD

0.8 1.0 2.0 3.0

DAA

a=b, A

cla

a=b, A

cla

4.6010±0.0009 4.6170±0.0015 4.63584-0.0009 4.6395±0.0013

0,6641 0,6613 0,6574 0,65677

4.6083+0.0012 4.6191+0.0010 4.6334+0.0019 4.64064,0.0015

0,6627 0,6608 0,6564 0,6565

GS

0.8 1.0 2.0 3.0

SSR

a=b, A

cla

a-b, A

cla

4.60914-0.0019 4.61764-0.0020 4.62094"0.0021 4.62114-0.0023

0,6622 0,6614 0,6600 0,6596

4.60954-0.0011 4.62084"0.0010 4.61934"0.0012 4.63024"0.0012

0,6617 0,6575 0,6583 0,6546

(c) The samples prepared with the GS and SSR methods are characterized by significantly larger dimensions of the crystals of 0~-Sb204, whereas those prepared with the method PD have the smaller dimensions.

66.0

/

~< ~- 65.5

///

~. // /~"

/" /

3.4. Unit cell characteristics o f the rutile ~ V S b 0 4 crystalline phase The unit cell parameters of the tetragonal ,~VSbO4 rutile phase as a function of the preparation method and S b : V ratio are reported in Table 5, whereas reported in Fig. 7 is the dependence of the ~VSbO4 unit cell volume on the same parameters. It is possible to make the following observations: (a) As the Sb : V ratio increases a decrease in the c/a ratio and an increase in the unit cell volume (Fig. 7) is observed. (b) The modality of preparation affects the d e p e n d e n c e of the c/a ratio and unit cell volume on the S b : V ratio. Especially the latter parameters depend little on the Sb : V ratio (above 1.0) in the SSR and GS preparations differently from the DAA and PD preparations (Fig. 8).

~ ~

/~-

65.0

64.5

~//////~e1.0

1.5

-.-oH GS - . - po ~¢~

-~

2.0

SSR

...... 2.5

3.0

Sb:Vratio Fig. 7. Unite cell volume of ~WSbO4crystallitesas a functionof the Sb : V atomic ratio and method of preparation.

3.5. Morphology and local Sb : V ratio Then morphology of the catalysts after the catalytic tests was studied by scanning electron microscopy (SEM) with an X-ray energy dispersive analysis (EDX) probe to determine the local Sb : V ratio in various parts of the samples. Due to the presence of marked surface roughness, usually from 5 to 10 determinations for morphologically equivalent areas

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

•

7- ~ '1 a

. k~

~[,~ ~ ~ ~ ' ~ w T ~ ~. t

b

i I

t

c

Fig. 8. Scanning electron microscopy (SEM)images of SbV3-Y(D) samples: (a) SSR, (b) DAA and (c) PD methods. Enlargement:

xs000, were made, and the mean error on the determination of the Sb : V ratio may be estimated as about 15%. The catalysts after the catalytic tests in propane ammoxidation prepared with the different methods and having Sb : V = I . 0 or 3.0 were characterized using this technique, The morphological and local composition characteristics of SbVx-GS(D)and SbVx-SSR(D)samples are similar. In both cases, three types of morphologies for the crystallites could be detected: (i) long hexagonal-octagonal type crystallites, sometimes inter-

285

grown, (ii) plate-like crystallites, sometimes stacked or assuming a desert rose-like structure, and (iii) powdery-like aggregates without a specific morphology (Fig. 8(a)). The first two types of crystallites are dusty, although smooth zones not covered by the powder could be clearly detected. The calcination to 850°C leads to an increased fraction of plate-like crystallites which thus can be assigned to ~-Sb204, on the basis of the increased intensity of the relative reflections observed in the XRD pattern. The platelike crystallites also prevail in SbVx-GS(D) samples. The EDX-SEM analysis evidences in both cases several local inhomogeneities in the Sb " V ratio with the presence of vanadium-rich zones (local zones with Sb : V ratio in the 0.5-0.7 range in the samples with an overall Sb : V ratio _<1.0) or Sb-rich (local zones with Sb : V ratios in the 10-20 range in the samples with an overall Sb " V ratio _>2.0). The latter zones are associated with the first two types of crystallite morphologies and confirm their assignment to S b 2 0 4 crystals, although even in the smooth zones apparently not covered by dust particles the local Sb" V ratios do not exceed a value of about 20, due to the possible partial incorporation of V inside Sb204 crystals or the presence of a thin layer of vanadium oxide on the surface. In agreement, Teller et al. [11] observed analogous crystal morphologies for V-doped S b 2 0 4 oxide. The morphological and local composition characteristics of SbVx-PD(D) and SbVx-DAA(D) samples are instead different from the above two series of catalysts. For these samples, in fact, no local inhomogeneities could be detected, i.e., the S b ' V local ratio estimated at an enlargement of x180,000 was coincident inside the experimental error with that determined at low magnitude (x500). This observation was true for the samples with Sb:V---I.0 and 3.0, even though in the latter case the presence of Sb204 crystallites was expected. SEM images of SbVx-DAA(D)samples indicate the presence, together with zones without a definite morphology of crystallites with a morphology analogous to those of ~- or ,3-Sb204, but completely covered by very small dusty or needle-like crystallites (Fig. 8(b)). Sometimes also a desert rose-like morphology appears, deriving probably from the stacking and intergrowth of small plate-like crystals which are also covered by the very small dusty or needle-like crystal-

286

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

lites. Reasonably the homogeneous local Sb : V ratio detected throughout the entire sample, even when the nominal Sb : V ratio is 3.0, is due to the presence of this microcrystalline phase covering the rose- or plate-

the basis of crystallographic studies [18,22]. V 3+ forms by the disproportional reaction

like macrocrystallites, In the SbVx-PD(D) samples, instead, no morphologically different zones were detected and the samples with both Sb : V = I . 0 and 3.0 are characterized by rough and spongy, but relatively compact, bodies clearly composed of several microcrystalline aggregates (Fig. 8(c)). No inhomogeneity in the local Sb : V composition could be detected also in these samples,

but there are also the competitive reactions of V 3÷ oxidation by gaseous oxygen and by V 5+ ions

4. Discussion

4.1. Relationship between chemistry of preparation and characteristics of V-Sb-oxides The structural and composition characteristics of V-Sb-oxides depend considerably on the method of preparation (Figs. 5 and 6). Preparation by the gel-solid(GS) method leads to the lowest amount of crystalline ~VSbO4 (Table 2 and Fig. 5) and reasonably also of amorphous÷crystalline phase, as shown by the large amount of unreacted antimony oxide in the sample with Sb : V=I.0. Furthermore, the samples prepared with this method are modified very little by the catalytic tests (compare Figs. 5 and 6) differently from PD and DAA preparations. This indicates the inertness of vanadium oxide in this case. The synthesis of ~VSbO4 requires the 'formal' reaction between V 3+ and Sb 5+ (formed by two redox reaction of V 5+ and Sb 3+, see below) to form the V(3+)-antimonate. 121Sb-M6ssbauser spectra [8] reveal that antimony in ~VSbO 4 is present only in the 5+ valence state, differently from Sb204 oxides where it is present as both Sb 3+ and Sb 5+ [21]. Analogous ~2~Sb-M6ssbauser data also were obtained for the catalysts of the present study, indicating that independently of the method of preparation antimony is present in the ~VSbO4 phase only in the Sb 5÷ valence state. Vanadium in the non-stoichiometric ,~VSbO4 phase is instead present both as V 3+ and V 4+, as shown by chemical analyses data [16] in agreement also with the composition suggested on

Sb3+

+

V 5+

~

V3+

+

SbS+

2V 3+ ÷ ½02 + 2H ÷ ---* 2V 4+ ÷ 2H20 V 3 + + V5+---+ 2V 4+

(2)

(3) (4)

which limit the effective availability of V 3+ ions to form the ~VSbO4 phase. This explains why the reaction between vanadium oxide and antimony oxide to form the ~VSbO4 phase is not complete even for Sb : V ratios much higher than 1.0 (Fig. 5) and also why the reducing conditions of the catalytic tests favour the further formation of ~VSbO4 from the unreacted oxides. The absence of this effect in the catalysts prepared by the GS method suggests that in these samples, there is a resistance to the occurrence of the solid state reaction favoured by the reducing conditions during the catalytic tests. The GS method is based on the gelification of the VS+-peroxide solution to form a viscous liquid to which Sb203 oxide particles are added. Sb203 is insoluble in the conditions of reaction, but the oxide particles are maintained in suspension by the viscosity effect. Drying of this viscous slurry leads to a vitreouslike solid with well dispersed Sb203 oxide particles. During the consecutive calcination, V 5+ reacts at the grain boundaries of Sb203 particles forming Sb 5+ ions and the reaction then proceeds by solid state diffusion. It is reasonable, however, that the V 5+ ions in the vitreous-like solid are characterized by a very low mobility and in addition there also is a considerable resistance to the diffusion of gaseous oxygen. Reaction (3) thus does not effectively compete with the reaction of formation of ,-~VSbO4, but at the same time the rate of formation of Sb 5+ ions by reaction (2) is probably too low with respect to both the competitive Sb203---*Sb204 transformation and reaction (4). Therefore, the tbrmation of ~VSbO4 is inhibited and probably the crystallites grow over a central nucleus of unreacted vitreous-like V204, as indicated by the very low intense IR bands of VS+-oxide in this sample, whereas at the same time large particles of Sb204 form. On the other hand, this vitreous-like V2Ox phase should be poorly crystalline and thus not detectable by

287

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

XRD analysis. This chemistry of preparation of SbVxGS samples thus well explains the features of these samples noted in their characterization: (i) inhomogeneities in composition, (ii) low amount of ,~VSbO4, but at the same time low amount of Vh+-oxide, (iii) high amount of Sb204 with the largest crystal dimensions and (iv) low solid state reactivity during the catalytic reaction, In the preparation by solid state reaction of Sb203 and V205 oxide particles (method SSR) analogous effects can be expected, with a reaction controlled by solid state diffusion. However, in the absence of the vitreous-like matrix the rates of the competitive gasto-solid and solid-to-solid reactions are altered. Therefore, the amount of crystalline ~VSbO4 is lower than

Method

Precursor after drying ca~c650°~c Catalyst catal tests

Sb203 ~:~VSbO,

'i i,

SSR v~o~ z . z vitreous-like Sb20 V~, ge~.~mk.~

GS

~

~

core V20, with VSbO, on the surface

~ ,.~ 1204

DAA

desertcovered rose-likebySb204 core Sb60~3withV'* crystals VSbO,I on the surface microcrystals I ~ ."~~' I ~ i:~:~-'i~ i i

in the DAA and PD methods, but significantly higher than in the GS method and parallely the amount of unreacted antimony oxide also falls in between that of these two cases. This preparation method, however, leads to very low surface areas (Table 1) and inhomogeneities in composition, both factors probably responsible for the worse catalytic performances of these sample. It should be noted, however, that the catalysts in this work were calcined at 650°C only and different results may be observed for different calcination temperatures. In fact, Nilsson et al. [23] found better catalytic performances for catalysts prepared as for SSR method, but calcined at higher temperatures, although the catalytic behavior was still significantly worse than that of samples prepared by PD and DAA methods both in terms of selectivity to acrylonitrile and especially activity in propane conversion. Shown in Fig. 9 is a schematic representation of the influence of the method of preparation on the nature of precursor compound before calcination and on the final catalyst after calcination at 650°C and catalytic tests. The samples prepared by methods PD and DAA are both characterized by homogeneous local composition, but have different morphological characteristics. SEM images of SbVx-DDA(D) samples indicate the presence of Sb204-1ike macrocrystals completely covered by microcrystals. It is reasonable to hypothesize that these characteristic morphologies derive from the Sb6013-+Sb204 transformation, because (i) this method involves the deposition of V4+-oxalate cornplexes on particles of Sb6013 and (ii) analogous morphologies have been assigned to 8b204 particles in the literature [9-11]. During calcination, the Sb6Ot3

v,*- and Sbh*/Sb3÷

mixedhydroxide PD

VSbO, microcrystals ....

~

amorphous Sb~O, Fig. 9. Schematic drawing illustrating the dependence of the morphologicalcharacteristics of SbV-Ysamples on the method of preparation. transforms tO Sb204 and V 4+ is oxidized to V 5+ which further reacts according to reaction (2), first step for the further reaction with antimony oxide to give ~-~VSbO4. It is thus reasonable that the scheletral matrices responsible for the morphology are the Sb204 particles, but at the surface of which microcrystallites of ~ V S b O 4 grow, covering the entire surface. In agreement with experimental evidence, it is expected that (i) smaller dimensions of the crystallites of ~VSbO4, (ii) larger crystallite size of Sb204 and (iii) higher proportion of/~- to ~'-Sb204 (see Fig. 5(B) and 6) will be obtained with this method than with the PD method (see below). The samples prepared by the PD method are characterized by a compact, structureless morphology constituted by aggregates of several microcrystals. Although relatively similar to the DAA method, this preparation involves the precipitation of an Sb 5+hydroxide which is then put in contact with a solution containing V 3+ ion. V 3+ reacts with Sb 5 ~ according to reaction (2) and V 5+ ions generated by this reaction further react with V 3+ ions according to reaction (4). The net effect is the deposition of V 4+ ions on an

288

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

Sb3+/Sb 5+ mixed hydroxide. There is thus an intimate contact between V- and Sb-oxides, which prevents the growth of large crystallites of the latter even in the presence of a large excess of antimony oxide. In fact, Sb204 particles start to be detectable only for a ratio of Sb : V = 3.0, differently from the other methods and the crystallites have the lowest mean dimensions found for the examined preparations (Table 3). This prevents the formation of detectable morphological structures characteristic in the presence of Sb204 crystallites, but at the same time also favours the formation of large surface areas (Table I). The analysis of the chemistry of preparation of V Sb-oxides provides information on the differences observed in the structural/composition features of these catalysts. It should be noted that the microstructure of the catalysts and especially the growth of ,-~VSbO4 microcrystals is considerably affected by the chemistry of preparation. Whereas in the cases of the DAA and PD methods, the ~VSbO4 microcrystals develop over macro- or micro-crystals of Sb204, respectively, the same is not true for the SSR and GS methods. This reflects not only on the presence of local dishomogeneities in the latter preparations, but also on the non-stoichiometry characteristics of the ~VSbO 4 phase, as discussed below,

4.2. Influence of the method of preparation on the catalytic behavior Various properties of the V-Sb-oxide catalysts change as a function of the method of preparation: (i) amount and dimensions of the crystallites of ~VSbO4 and Sb20 4 (Tables 2-4), (ii) dimensions of the unit cell of ~VSbO4 (Table 5, Fig. 7), which are related to its non-stoichiometric characteristics, (iii) presence and amount of amorphous phases (Sb 5+and VS+-oxides, amorphous ~VSbO4 and Sb20 4 oxides) and (iv) ratio between c~- and ~3-5b204 in the samples (Fig. 5 and Fig. 6). It is thus difficult to single out which of these characteristics has a primary role in influencing the catalytic behavior (Figs. 1-4), although reasonably several factors concurr in determining the global behavior. We shall limit discussion, however, to the main factor determining the catalytic behavior, A general feature of the samples prepared by all these methods, however, is that the selectivity to

acrylonitrile increases, because minor effects are present on the selectivity to carbon oxides (Fig. 3(c)). Increasing Sb : V ratio also decreases the side NH3--,N2 reaction (Fig. 2); the same feature was observed in the catalysts prepared by different methods, but for conciseness it was omitted, also because already discussed in previous works [12,15,16]. The effect is mainly due to an increased isolation of vanadium sites and has as a direct consequence an increased availability of surface ammonia species for conversion of the propene intermediate to acrylonitrile. However, this aspect is only one of those determining the selectivity to acrylonitrile. In fact, data in Fig. 3c for example indicate that the formation of carbon oxides depends only slightly on the S b : V ratio, but considerably on the method of preparation. It is thus possible to associate, as a dominant effect, different steps in the reaction network to different parameters in the preparation: (i) Sb : V affects primarily the rate of transformation of intermediate propene to acrylonitrile and (ii) the chemistry of preparation is responsible for the competitive rates of propane transformation by selective and unselective routes. With increasing Sb : V ratio, a series of changes occur in catalyst composition and structure: (i) the amount of crystalline and amorphous Sb-oxide increases, present mainly as Sb204, but also as Sb5+-oxide although the latter undergoes considerable reduction during the catalytic tests, (ii) the amount of surface spread amorphous V5+-oxide decreases, and also undergoes considerable reduction during the catalytic tests, and (iii) the cell characteristics of the ~VSbO4 phase change with an expansion of the cell volume and a decrease in the ratio of the c/a parameters of the tetragonal rutile unit cell. Supported vS+-oxide, for example on TiO2, is very active in the NH3--'*N2 oxidation reaction. In a homogeneous series of catalysts a relationship between relative amount of supported V 5+ (estimated by IR spectra deconvolution) and conversion of NH3 to N2 has been observed in the catalyst after calcination, but not when the same catalyst was analyzed after the catalytic tests in propane ammoxidation [24]. Also in the catalysts of this study, no definite relationship between catalytic behavior in propane ammoxidation and presence of supported V5+-oxide (band near 1000 cm i in tile IR spectra) in the catalysts after the catalytic tests (Fig. 6

289

G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

(A)) could be observed, although the presence of this supported phase decreases with increasing Sb : V ratio (Fig. 4 (A)). As indicated by IR studies on the reactivity of ammonia with V-Sb-oxide [25], the supported VS'--oxide reduces very fast during the catalytic reaction, but slowly reoxidizes and thus plays a role mainly during the transient catalytic behavior [26]. The presence of crystalline Sb204 which could block sites for ammonia oxidation either physically or by a chemical effect also does not appear to be a main reason for the increase in the selectivity to acrylonitrile with increasing Sb : V ratio, because no specific relationship could be observed in the catalysts examined between amount or dimensions

........................

Fig. 10. The rutile lattice with indication of the different Me-O distances,d~ and d~. The arrows indicate the direction of change in bond length when non-stoichiometric Sb-rich ~VSbO4 forms,

of Sb204 crystallites (Table 3 and Table 4) and selectivity to acrylonitrile (Fig. 3(a)). A good parallelism between selectivity to acrylonitrile (Fig. 3(a)) and unit cell volume of the rutile cell (Fig. 7)could be instead observed. The catalysts more selective to acrylonitrile are also those with the higher cell volume of the rutile cell. Furthermore, increasing the S b : V ratio, decreases the c/a ratio in all the samples (Table 5), although a less definite relationship between c/a ratio and selectivity could be observed, However, it should be noted that the more selective catalysts are those with the larger value of the parameter a of the rutile unit cell. Increasing the V4+/V 3+ ratio inside the rutile unit cell a decrease in the c/a ratio and slight contraction of the unit cell volume is observed [18], due to an increase in the V4+-V 4+ metal-metal bond interaction [27,28]. In our case, as the Sb : V ratio increases, a significant expansion of the cell volume is instead observed (Fig. 7) with a lowering of the c/a ratio (Table 5) which, however, do not parallel the change in the cell volume, largely affected, instead, by the change in the parameter a of the unit cell. The samples with the larger cell volume are those with the greater value of the parameter a. An increase in the parameter a together with a decrease in the c/a value is observed in passing from RuO~ to TiO2 [28], due to an increase in da (apical distance, see Fig. 10) and decrease in de (equatorial distance) caused by an increased electrostatic repulsion of anions caused by an excess of localized charge on the oxygens. A similar effect is reasonably present also in our case. In agreement, Teller et al. [11]

observed that the metal-oxygen octahedron is symmetric for 'normal' ~VSbO4, but somewhat squashed in ~VSbO4 epitaxially grown over ~:]-Sb204 macrocrystals and having a Sb/V ratio greater than that of 'normal' ~VSbO4. The shortest O. • -O distance passes from 2.58 A in 'normal' ~VSbO4 to 2.67 ,~ in 'epitaxially grown' ~VSbO4. The distortion on the octahedron thus does not derive from the increased metal-metal interaction as expected when V 4~ ions substituted V 3+ ions in the rutile cell, but rather from the repulsive effect of oxygen anion due to the probable formation of a Sb-rich ~VSbOa phase [11 ]. Berry et al. [29,30] suggested the presence of cation ordering in rutile FeSbO4 when the Sb : Fe ratio is greater than 1 giving rise to the formation of a trirutile superstructure formed of alternating sheets of FeSbO4 and Sb-oxide only. An analogous situation is probably present in Sb-rich ~VSbO4 phase. Landa-Canovas et al. [18] exclude the presence of cation ordering in nonstoichiometric V-Sb-oxide, but studying samples with S b : V < 1.0, whereas the possible formation of a nonstoichiometric Sb-rich ~VSbO4 phase occur for Sb : V > 1.0. A good relationship between change in unit cell parameters of the ~VSbO4phase (Fig. 8, Table 5)and dependence of the selectivity to acrylonitrile on the Sb : V ratio (Fig. 3(a)) can be noted both as a function of the Sb : V ratio and of the method of preparation. In particular, the more selective catalysts are those in which the modification of the unit cell characteristics deriving from the formation of a Sb-rich ~VSbO4 phase (trirutile-like)is more pronounced.

/ ' ~ ,

o

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G. Centi et al./Applied Catalysis A: General 165 (1997) 273-290

The model shown in Fig. 10 for the possible modification in the octahedron distances in the rutile cell due to the formation of a Sb-rich ~ V S b O 4 p h a s e

[2] V.D. Sokolovskii, A.A. Davydov, O.Yu. Ovsitser, Catal. Rev.

suggests tentatively an increase in the BrCnsted acid character of the equatorial oxygens and an increase in the lability or redox character of the apical oxygens with respect to the 'normal' ~ V S b O 4 phase. We noted by IR spectroscopy [25] that the rate of the side

[4] R. Nilson, T. Lindblad, A. Andersson, J. Catal. 148 (1994) 501. [5] J. Nilsson, A.R. Landa-Canovas,S. Hansen, A. Andersson, J. Catal. 160 (1996) 244. [6] G. Centi, R.K. Grasselli, E Trifirb, Catal. Today 13 (1992)

reaction of NH3---~N2 oxidation decreases with increasing BrCnsted acidity of the catalysts. On the other hand, the increased redox reactivity of the apical oxygens would also favour an increase in the rate of the propene to acrylonitrile transformation. Both the change in Br0nsted surface acidity of equatorial oxygens and in redox reactivity of apical oxygens concur in increasing the rate of transformation of intermediate propene to acrylonitrile and thus the selectivity to this product. Furthermore, based on the proposed mechanism of propane conversion on V-Sb-oxides [25] it is expected that the cited surface modifications favour a selective transformation of propane to propene intermediate than attack of methyl groups with formation of propionate-like species. The latter route leads mainly to carbon oxides, whereas the first is the selective one. The expected change in the surface properties of the rutile phase is thus consistent with the observed change in the catalytic behavior in going from a V-rich to an Sb-rich ~ V S b O 4 phase, both in terms of rate of transformation of intermediate propene to acrylonitrile and ratio between selective versus unselective propane activation. Formation of the Sb-rich ~ V S b O 4 phase depends on the chemistry of preparation of these catalysts and their microstructure. In particular, non-stoichiometric Sb-rich ~ V S b O 4 is observed w h e n ~ V S b O 4 microcrystals grow o v e r S b 2 0 4 micro- or macro-crystals, explaining the considerable dependence of the catalytic behavior on the method of preparation and Sb : V ratio. Further understanding of this mechanism of development of ~VSbO4 microcrystals and of their structural and non-stoichiometric features may allow better control of the catalyst microstructure and behavior in propane ammoxidation to acrylonitrile.

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