Formation and properties of the new Al₈V₁₀W₁₆O₈₅ and Fe_8−x Al _x V₁₀W₁₆O₈₅ phases with the M–Nb₂O₅ structure

Abstract

A new, previously unknown phase Al₈V₁₀W₁₆O₈₅ has been obtained from reaction taking place in the solid state. It forms continuous solid solution with Fe₈V₁₀W₁₆O₈₅ of the Fe_8−x Al _x V₁₀W₁₆O₈₅ general formula. All these phases are isostructural with M–Nb₂O₅ and (W_0.35V_0.65)₂O₅ and belong to a block structure phases with ReO₃ type blocks of 4 × 4×∞ dimensions. Al₈V₁₀W₁₆O₈₅ is tetragonal and has the lattice constants a = b = 1.9487(1) nm and c = 0.36706(4) nm. It melts incongruently at 1,183 K depositing Al₂(WO₄)₃ and WO₃. The increase of the Al³⁺ ions content in the crystal lattice of Fe₈V₁₀W₁₆O₈₅ causes the melting point increasing, and decreasing of a = b unit cell parameters with c being almost constant. IR spectra of Al₈V₁₀W₁₆O₈₅ and Fe_8−x Al _x V₁₀W₁₆O₈₅ phases have been recorded.

Introduction

The oxidative dehydrogenation (ODH) of light hydrocarbons and selective reduction of NO with NH₃ have been extensively studied over alumina or titania supported vanadium oxide catalysts [1–4]. A major challenge in the light hydrocarbons ODH catalytic technology is improvement in the alkene yields, because significant carbon oxide byproducts are also formed. One method to improve the yield of the desired product is to use metal oxide additives. The vanadium containing catalysts can be tuned with second or third component like Mo, W, Sb, Ni, and Co [1–4]. The effect of the incorporation of second or third component to V-containing catalysts results in a replace of the polyvanadate structure with less reactive V–O–X structure, leading to lower reducibility and oxidative dehydrogenation rates. In spite of extensive scientific studies on catalytic activity of multicomponent oxide systems, the knowledge on phase equilibria being established in an appropriate ternary or quaternary systems is unsatisfactory. In the case of Al₂O₃–V₂O₅–WO₃ only binary systems have been the subject of investigations. In the Al₂O₃–WO₃ system a phase with general formula Al₂(WO₄)₃ forms, crystallizing in an orthorhombic system and melting congruently at 1,527 K [5, 6]. Whereas in the Al₂O₃–V₂O₅ system triclinic AlVO₄ forms, being isostructural with FeVO₄ [7]. AlVO₄ melts incongruently at 1,018 K with deposition of α-Al₂O₃ as a solid product [7]. Literature data pertaining V₂O₅–WO₃ imply that only solid solution of the V_2−x W _x O₅ type with a x < 0.07 [8] or x ≤ 0.15 [9] solubility limit is formed. On the other hand, the Fe₂O₃–V₂O₅–WO₃ system has been the subject of extensive studies [10–16]. The components of this system react with each other to form Fe₈V₁₀W₁₆O₈₅ phase [10–12]. Fe₈V₁₀W₁₆O₈₅ crystallizes in a tetragonal system [13], but its structure is unknown. This compound melts at 1,103 K with depositing two solid products, i.e. Fe₂WO₆ and WO₃ [10]. Besides, a solid solution of V₂O₅ in Fe₂WO₆ has been found to occur in the three-component system [11]. It is also known that in the quaternary system Fe₂O₃–V₂O₅–WO₃–MoO₃ Fe₈V₁₀W_16−x Mo _x O₈₅ solid solution is formed (for x ≤ 4), isostructural with Fe₈V₁₀W₁₆O₈₅ [14]. The equal charge and close values of Fe³⁺ and Al³⁺ ionic radii (R _Al = 0.0535 nm and R _Fe = 0.0645 nm in octahedral coordination), let one expect formation of solid solutions as a result of substitution of aluminum for iron in the quaternary Fe₂O₃–Al₂O₃–V₂O₅–WO₃ system. This study shows the experimental results of the substitution of aluminum for iron in the case of Fe₈V₁₀W₁₆O₈₅ compound.

Experimental

The following materials were used for the research: V₂O₅, p.a. (POCh, Poland), WO₃, 99.9% (Fluka AG, USA), Al₂O₃, pure (POCh, Poland), α-Fe₂O₃ p.a. (POCh, Poland), MoO₃, pure (POCh, Poland), ZnO 99.9% (Sigma–Aldrich, Germany), and CoCO₃, pure (POCh, Poland).

For the experiments five samples were selected with contents corresponding to Fe_8−x Al _x V₁₀W₁₆O₈₅ formula with x = 0, 2, 4, 6, and 8. They represented the whole component concentration range of the system: Fe₈V₁₀W₁₆O₈₅–Al₈V₁₀W₁₆O₈₅. The content of V₂O₅ and WO₃ in the all mixtures was always constant and amounted to 20.00 and 64.00 mol%, respectively. The oxides weighed in suitable proportions were homogenized and calcinated at 873, 923, 973, 1023, and 1073 K in 24 h stages. After each sintering stage the samples were powdered using agate mortar and examined with the aid of XRD. The samples obtained after last heating stage were additionally examined by the IR and DTA/TG methods. Results of investigations by XRD, DTA, and IR methods allow a determination of phase composition of samples, establishing of their melting temperatures as well as melting behavior [17–20].

Two additional samples were prepared for IR investigations. ZnV₂O₆ was synthesized from oxides by sintering in 24 h stages at 823, 873, and 923 K, whereas CoMoO₄ from CoCO₃ and MoO₃ by calcination at 873, 923, and 973 K.

The DTA/TG examinations were made using an apparatus of Paulik–Paulik–Erdey type (MOM, Hungary). Samples of 500 mg were investigated in air up to the 1,273 K in quartz crucibles and at the heating rate of 10 K min⁻¹.

X-ray diffraction phase analysis of the samples was performed using a DRON-3 diffractometer (Bourevestnik, Sankt Petersburg, Russia) applying the CoKα/Fe radiation (step 0.02° 2θ, time 1 s).

The IR spectra were recorded on a SPECORD M-80 spectrometer (Carl-Zeiss Jena, Germany) in the wave-number region of 1,500–200 cm⁻¹. The samples were mixed with KBr in a wieght ratio of 1:300 and then pressed to pellets.

Results and discussion

The XRD experimental results have shown that diffraction patterns of all materials after the last calcination stage at 1,073 K, were similar to one another and to the diffractogram of Fe₈V₁₀W₁₆O₈₅ both with respect to the number and to the mutual intensity relations of the recorded diffraction lines. The angular positions of these lines were shifted with increasing the Al₂O₃ content in initial oxide mixtures toward higher angles 2θ, in comparison with the diffractogram of Fe₈V₁₀W₁₆O₈₅, i.e., they corresponded to smaller interplanar distances d. The same tendency was observed in the case of formation of solid solution of the Fe₈V₁₀W_16−x Mo _x O₈₅ type (for x ≤ 4) when the Mo⁶⁺ ions were substituted for W⁶⁺ ones [14]. The obtained results indicated that continuous substitutional solid solution of a general formula Fe_8−x Al _x V₁₀W₁₆O₈₅ is formed by an incorporation of the Al³⁺ ions into the crystal lattice of Fe₈V₁₀W₁₆O₈₅ instead of Fe³⁺. The formulae of obtained solid solution were evaluated from the content of initial oxides. Diffraction lines of the Al₈V₁₀W₁₆O₈₅ and Fe_8−x Al _x V₁₀W₁₆O₈₅ type solid solution samples (x = 2, 4, 6) recorded within 2θ (CoK_α-aver) 4–65° region were selected for indexing (program Refinement). The result of indexing of powder diffraction pattern of Al₈V₁₀W₁₆O₈₅ is presented in Table 1. The parameters and volumes of the unit cells of Al₈V₁₀W₁₆O₈₅ as well as Fe₈V₁₀W₁₂Mo₄O₈₅ [14] and Fe_8−x Al _x V₁₀W₁₆O₈₅ solid solutions are presented in Table 2.

Table 1 The result of indexing of X-ray powder diffraction pattern of the Al₈V₁₀W₁₆O₈₅

Table 2 Unit cell parameters and volumes of M–Nb₂O₅ [21], (W_0.35V_0.65)₂O₅ [22], Fe₈V₁₀W₁₆O₈₅ [13], Fe₈V₁₀W₁₂Mo₄O₈₅ [14], Al₈V₁₀W₁₆O₈₅, and Fe_8−x Al _x V₁₀W₁₆O₈₅ solid solution (x = 2, 4, 6)

Looking for isostructural compounds with Fe₈V₁₀W₁₆O₈₅, Al₈V₁₀W₁₆O₈₅, Fe_8−x Al _x V₁₀W₁₆O₈₅, and Fe₈V₁₀W_16−x Mo _x O₈₅ solid solution phases it have been taken under consideration that their general formulas can be given as M₃₄O₈₅ or 17 × M₂O₅ (M stands for metal ion) and that these compounds have very characteristic values of unit cell parameters. The literature scan has shown that all these phases are isostructural with polymorphic form of niobium(V) oxide designated as M–Nb₂O₅ [21] and (W_0.35V_0.65)₂O₅, (M₂O₅) [22]. The values of their unit cell parameters (Table 2), the same type of general formula, M₂O₅, as well as very similar powder diffraction patterns support this assumption. Figure 1 presents the crystal structure of M–Nb₂O₅ [21]. M–Nb₂O₅ and (W_0.35V_0.65)₂O₅ belong to a block structure phases with ReO₃ type blocks of 4 × 4×∞ dimensions. These phases are built up of corner and edge shared highly distorted MO₆ octahedra. An analysis of the data compiled in Table 2 indicates that with increasing the incorporation extent of the smaller Al³⁺ ions into the Fe₈V₁₀W₁₆O₈₅ structure, the unit cell parameters a = b are decreasing whereas parameter c being almost constant.

Fig. 1

The crystal structure of M–Nb₂O₅ [21]

The single phase materials obtained after the last heating stage were subjected to the DTA/TG investigation. In the DTA curve of Fe₈V₁₀W₁₆O₈₅ two endothermic effects were recorded up to 1,273 K with their onsets at 1,113 and 1,213 K. The second effect was much smaller than the first one and it was registered as a poorly pronounced remnant effect. It was in accord with literature data where the first endothermic effect was attributed to incongruent melting of Fe₈V₁₀W₁₆O₈₅ [10].

On the other hand in the DTA curves of the Fe_8−x Al _x V₁₀W₁₆O₈₅ solid solution materials (x > 0) and Al₈V₁₀W₁₆O₈₅ phase one endothermic effect was recorded up to 1,273 K. Figure 2 shows the DTA curve of Al₈V₁₀W₁₆O₈₅. The onset temperature of the endothermic effects was increasing gradually with the increase of aluminum content. Values of temperatures were 1163, 1168, 1173, and 1183 K for x = 2, 4, 6, and 8, respectively. No weight changes were recorded on the TG curves (not presented) up to the onsets of the observed endothermic effects on the DTA curves.

Fig. 2

DTA curve of Al₈V₁₀W₁₆O₈₅

In order to explain the nature of the endothermic effect on the DTA curve of Al₈V₁₀W₁₆O₈₅ and to establish its melting behavior the sample of this phase was additionally heated for 3 h at 1,213 K, i.e., at temperature close to the extremum temperature of the endothermic effect registered in the DTA curve. After heating at 1,213 K sample was cooled rapidly to room temperature. The X-ray phase analysis of the partially melted at this temperature sample (Fig. 3, graph b) showed that it comprised a mixture of WO₃, Al₂(WO₄)₃, and V₂O₅. Diffraction lines characteristic for V₂O₅ were shifted toward higher 2θ angles indicating formation of V_2−x M _x O₅ (M stands for metal ion) solid solution. At 1,213 K V₂O₅ do not exist already as solid phase, so it crystallizes from the liquid. Thus, it was concluded that Al₈V₁₀W₁₆O₈₅ melts incongruently and the solid products of its melting are WO₃ and Al₂(WO₄)₃:

$$ {\text{Al}}_{ 8} {\text{V}}_{ 10} {\text{W}}_{ 1 6} {\text{O}}_{{ 8 5({\text{s}})}} \to {\text{ WO}}_{{ 3({\text{s}})}} \, + {\text{ Al}}_{ 2} \left( {{\text{WO}}_{ 4} } \right)_{{ 3({\text{s}})}} + {\text{ liquid}} $$

Fe₈V₁₀W₁₆O₈₅, Al₈V₁₀W₁₆O₈₅ as well as the solid solution phases Fe₈V₁₀W_16−x Mo _x O₈₅ and Fe_8−x Al _x V₁₀W₁₆O₈₅ were also subjected to an investigation with the use of infra-red spectroscopy (IR). The results of literature survey [21, 22] have revealed that Fe₈V₁₀W₁₆O₈₅ type phases are isostructural with M–Nb₂O₅ and (W_0.35V_0.65)₂O₅, built up from highly distorted MO₆ octahedra. Since the VO₆ octahedra are relatively less common polyhedra, for comparison, IR investigations have been conducted also in the case of ZnV₂O₆. The crystal structure of this phase is built up from distorted VO₆ octahedra [23]. On the other hand, IR spectrum of CoMoO₄ have been recorded because this phase has crystal structure related to these ones of M–Nb₂O₅ and (W_0.35V_0.65)₂O₅ [24].

Fig. 3

Powder diffraction patterns of: (a) Al₈V₁₀W₁₆O₈₅, (b) sample of Al₈V₁₀W₁₆O₈₅ additionally heated for 3 h at 1,213 K and cooled rapidly to room temperature comprised a mixture of WO₃ (filled square), Al₂(WO₄)₃ (filled circle) and V_2−x M _x O₅ (filled diamond)

Figure 4 shows the IR spectra of CoMoO₄ (curve a), Al₈V₁₀W₁₆O₈₅ (curve b), Fe₄Al₄V₁₀W₁₆O₈₅ (curve c), Fe₈V₁₀W₁₆O₈₅ (curve d) [12], Fe₈V₁₀W₁₂Mo₄O₈₅ (curve e) [14], and ZnV₂O₆ (curve f).

Fig. 4

IR spectra of: (a) CoMoO₄, (b) Al₈V₁₀W₁₆O₈₅, (c) Fe₄Al₄V₁₀W₁₆O₈₅, (d) Fe₈V₁₀W₁₆O₈₅ [12], (e) Fe₈V₁₀W₁₂Mo₄O₈₅ [14] and (f) ZnV₂O₆

The IR spectra of all Fe₈V₁₀W₁₆O₈₅ type phases are very similar, what supports assumption that these phases are isostructural.

The IR spectrum of the Fe₈V₁₀W₁₆O₈₅ (curve d) reveals the presence of absorption bands with their maxima at 922, 654, 486, 368, and 320 cm⁻¹ [12–14]. With increasing the incorporation extent of the lighter and smaller Al³⁺ ions into the structure of Fe₈V₁₀W₁₆O₈₅ a relatively small shift of the respective absorption bands towardhigher wave-numbers was observed. Their maxima characteristic for Al₈V₁₀W₁₆O₈₅ are observed at 944, 660, 514, 412, and 334 cm⁻¹ (curve b). A broad absorption bands lying at the range of wave-numbers 1,050–800 cm⁻¹ are characteristic for all presented spectra (curves a–f) of phases built up only from distorted octahedra. It is noteworthy that broad absorption band in the IR spectrum of WO₃ covering the wave-number region of 1,000–700 cm⁻¹ is caused by stretching vibrations of W–O bonds in highly distorted WO₆ octahedra [13, 14]. Thus, the absorption band covering the wave-number region of 1,050–800 cm⁻¹ in the IR spectrum of the Fe₈V₁₀W₁₆O₈₅ type phases can be attributed to stretching vibrations of the very short M–O bonds in the MO₆ (M = W, V, Al, Fe) octahedra [19]. Another very broad absorption band covering the wave-number region of 800–550 cm⁻¹ can correspond to stretching vibrations of longer M–O bonds in MO₆ octahedra. A similar absorption band with a 640 cm⁻¹ maximum was noticed in an IR spectra of α- and γ-Fe₂WO₆, comprising WO₆ and FeO₆ octahedra in their structures [13, 14, 25]. Next absorption band recorded in the range of 550–400 cm⁻¹ can be the most likely ascribed to stretching vibrations of Fe–O and Al–O bonds in MO₆ octahedra. A similar absorption bands were noticed in the IR spectra of γ-Fe₂WO₆, FeVO₄, and AlVO₄ in the structure of which the FeO₆ or AlO₆ octahedra occur [7, 13, 14, 25]. On the other hand, bands lying within the wave-number range of 400–250 cm⁻¹ may correspond to bending vibrations of M–O bonds in MO₆ octahedra or be of a mixed character [14, 19]. An evident broadening of the absorption bands in the IR spectrum of Fe₈V₁₀W₁₂Mo₄O₈₅ (curve e) in comparison to the IR spectrum of Fe₈V₁₀W₁₆O₈₅ is undoubtedly due to an appearance of numerous additional Mo–O bonds in the structure of Fe₈V₁₀W₁₂Mo₄O₈₅.

Thus, analysis of the IR spectra of the Fe₈V₁₀W₁₆O₈₅ type phases seems to support assumption based on structural considerations that these phases are built up from MO₆ octahedra.

Conclusions

In this study, the results concerning the quaternary system Fe₂O₃–Al₂O₃–V₂O₅–WO₃ were presented for the first time. The XRD, DTA, and IR measuring techniques were used to show that:

• 1. In the ternary system Al₂O₃–V₂O₅–WO₃ new compound Al₈V₁₀W₁₆O₈₅ forms, not described earlier in literature.

• 2. The Al₈V₁₀W₁₆O₈₅ melts incongruently at 1,183 K with deposition of Al₂(WO₄)₃ and WO₃ as a solid products of melting.

• 3. In the quaternary system Fe₂O₃–Al₂O₃–V₂O₅–WO₃ continuous solid solution Fe_8−x Al _x V₁₀W₁₆O₈₅ forms, not described earlier in the literature.

• 4. Al₈V₁₀W₁₆O₈₅ and Fe₈V₁₀W₁₆O₈₅ as well as solid solutions Fe_8−x Al _x V₁₀W₁₆O₈₅ and Fe₈V₁₀W_16−x Mo _x O₈₅ are isostructural with M–Nb₂O₅ and (W_0.35V_0.65)₂O₅ and belong to block type structure phases.

• 5. With the increase of Al³⁺ content in the crystal lattice of Fe₈V₁₀W₁₆O₈₅ a decrease of the a = b parameters of the unite cell occurs, with c parameter being almost constant.

• 6. The melting temperature of the Fe_8−x Al _x V₁₀W₁₆O₈₅ solid solution increase with increasing the content of Al₂O₃ and changes from being equal to 1,113 K for x = 0 up to 1,183 K for x = 8.

• 7. Analysis of the IR spectra of the Fe₈V₁₀W_16−x Mo _x O₈₅ type phases supports assumption that these phases are built up from VO₆, WO₆, FeO₆, and AlO₆ octahedra.