Characterization by temperature programmed techniques of spent and acid treated vanadium catalysts

Caracterización por técnicas de la temperatura programada de catalizadores de vanadio desactivados y tratados con ácido

Iran David Charry, Lina María González*, Consuelo Montes de Correa

Environmental Catalysis Research Group. Universidad de Antioquia. Sede de Investigación Universitaria- SIU. A.A. 1226, Calle 62 N° 52-59. Medellín, Colombia

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Abstract

Spent samples of V₂O₅/SiO₂-γAl₂O₃ catalyst used for H₂SO₄ production were acid treated to recover vanadium. Fresh, spent, and acid treated samples were characterized by temperature-programmed techniques: TPR-H₂, TPO-O₂, TPD-NH₃ and, TGA-DTA in order to determine the effect of deactivating species on vanadium recovery. DRX and UV-Vis techniques were used to confirm several signals from the temperature-programmed profiles. The results suggest that vanadium sulfovanadates and oxides in treated samples strongly interact with the support making it difficult to get more than 86% vanadium recovery.

Keywords: V₂O₅ catalyst, catalyst deactivation, vanadium recovery, sulfovanadate species, temperature-programmed techniques.

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Resumen

Muestras de V₂O₅/SiO₂-γAl₂O₃ desactivado durante la producción de ácido sulfúrico se trataron con ácido con el fin de recuperar el vanadio. Los materiales se caracterizaron mediante técnicas de temperatura programada: TPR-H₂, TPO-O₂, TPD-NH₃, y TGA-DTA con el fin de determinar el efecto de las especies responsables de la desactivación en la recuperación del metal. Análisis por DRX y UV-Vis se utilizaron para confirmar algunas señales obtenidas en los perfiles de temperatura programada. Los resultados sugieren que la presencia de sulfovanadatos y óxidos de vanadio en las muestras tratadas hacen difícil recuperar más del 86% del vanadio presente en las muestras de catalizador.

Palabras clave: Catalizador V₂O₅, desactivación de catalizadores, recuperación de vanadio, sulfovanadatos, técnicas de temperatura programada.

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Introduction

The catalyst V₂O₅/SiO₂-γAl₂O₃ has been widely used for the oxidation of SO₂ to SO₃ to commercially produce sulfuric acid [1, 2]. In spite of its high catalytic activity, it is slowly deactivated under reaction conditions (temperatures between 450 and 600 °C) [3, 4]. Characterization of spent V₂O₅/ SiO₂-γAl₂O₃ samples evidenced the formation of some species of vanadium V⁴⁺, such as: K₄(VO)₃(SO₄), KV(SO₄)₂, Na₂VO(SO₄) and Na₄(VO)₂O(SO₄)₄ [5], which are very stable [1,2]. The disposal of deactivated vanadium catalysts has turned on an environmental problem in recent years due to its toxicity [6]. Recycling of some catalyst components, such as vanadium, silica, and alumina has been recommended instead of catalyst disposal. Additionally, the recovery of vanadium from spent catalysts is considered an additional source of vanadium, which has a large commercial demand [2]. Although several methods have been proposed for vanadium leaching from spent catalysts leading to around 90 % wt. vanadium recovery [2-9], most of them involve the use large quantities of leaching agents, are too expensive, or use hazardous organic solvents, leading to the formation of vanadium chlorides. Previously, we reported a clean and economical method for vanadium leaching from spent V₂O₅/SiO₂-γAl₂O₃ commercial catalyst using a 10 % vol. H₂SO₄ at room temperature obtaining 86 % wt. vanadium recovery [6]. The goal of this work is to characterize commercial samples of fresh (V₂O₅-F) and spent (V₂O₅-S) vanadium catalysts by temperature programmed techniques. Temperature-programmed reduction and oxidation (TPR-H₂ and TPO-O₂) shed light of a possible reduction of the active phase and the formation of oligomeric vanadium species. Ammonia temperature-programmed desorption (TPD-NH₃) and thermogravimetric analyses (TGA-DTA) were used to identify sulfovanadates formed with the alkali promoted metals. Likewise, experiments of oxidation-reduction cycles (TPO/ TPR) were carried out as a possible reactivation method. In addition, complementary techniques such as X-Ray Diffraction (XRD), and UV-VIS Diffuse Reflectance were used. The formation of sulfovanadates and the interaction of vanadia with the support are important for the efficiency of vanadium recovery.

Experimental

Catalyst materials

The ring shaped fresh and used vanadium catalyst samples were supplied by Industrias Básicas de Caldas S.A from Monsanto Enviro-Chem Systems, Inc., USA (ref. LP110). Table 1 shows the chemical analysis of fresh and spent catalyst samples.

Samples of non-supported vanadium oxide and supported over silica, γ-Al₂O₃ and silica/γ-Al₂O₃ mixture were prepared for identifying specific signals of the temperature-programmed profiles. V₂O₅ was prepared using NH₄VO₃ (11 mmol) dissolved in 11.5 mL of oxalic acid (0.91 M). The resulting solid was dried at 100 °C and calcined at 550 °C for 8 h in a muffle furnace. VO₂ was prepared by reduction of V₂O₅ in 4.47 % H₂/Ar at 700 °C for 30 min. V₂O₅ supported on SiO₂, γ-Al₂O₃ or γ-Al₂O₃/SiO₂ (weight ratio 1:40) samples were prepared by the precipitation of ammonium metavanadate (9.5 mmol) in a suspension of the support (1 g) in 5 mL of oxalic acid (0.91 M). The excess water was eliminated by evaporation in a rotavap and the solid finally dried at 100 °C for 16 h, and calcined at 450 °C for 8 h in a muffle furnace.

Catalyst characterization

Temperature-programmed experiments were carried out in a Micromeritics Autochem II equipped with a thermal conductivity detector (TCD). Catalyst samples were loaded in a U-type quartz reactor of 0.95 cm diameter. Before TPR or TPO experiments 50 mg samples were pretreated during 60 min at 100 °C under flowing argon (50 mL/min) [8] and then cooled to 40 °C. The TPR and TPO analysis were carried out at 900 °C at a heating rate of 5 °C/min in flowing 4.47 % H₂/ Ar or 5.36 % O₂/Ar, respectively (50 mL/min). In order to evaluate the re-oxidation capacity of vanadium reduced species TPO analysis of fresh catalyst samples previously reduced at 600 °C (highest oxidation temperature of SO₂ to SO₃ [1,2]) was carried out during 120 min in flowing 4.47 % H₂/Ar. During TPD-NH₃ experiments the samples were manipulated under nitrogen atmosphere. Before the analysis, the sample (around 50 mg) was heated at 500 °C in flowing helium for 30 min. After cooling, the sample was treated in flowing 3 % NH₃/He (50 mL/min) for 90 min and then 30 min in flowing Argon (30 mL/min). Then, the sample was heated (10 °C/ min) to 1100 °C. An additional analysis was performed to a surface scraped sample of spent catalyst to evaluate the possible deactivation by the formation of superficial sulfates. The thermogravimetric and differential thermal measurements (TGA-DTA) were carried out on a Universal V2.5 TGA equipment. Samples were heated (10 °C/min) in air at temperatures between 25 and 800 °C. X-ray diffraction (XRD) patterns were collected (5° < 2θ < 60°) in a Rigaku Miniplex equipped with a Cu lamp. UV- VIS diffuse reflectance spectra were collected in the range 200 - 800 nm in a Lamda 4B Perkin Elmer spectrophotometer equipped with a diffuse reflectance attachment using BaSO₄ as reference.

Leaching of vanadium with sulphuric acid

Finely ground samples (1 g) of fresh (V₂O₅-F) and spent (V₂O₅-S) commercial catalyst were treated with a solution of 10 vol %. H₂SO₄ (20 mL) at room temperature [9,10]. Then a 30 wt. % solution of H₂O₂, was added up to a pH between 6 and 7. The final solution was stirred for 2 hours. Treated materials were coded as V₂O₅-F-T and V₂O₅-S-T, respectively.

Results and discussion

H₂-TPR

Different types of oxidized species on vanadium catalysts [11,12] were identified. TPR profiles of fresh, spent and acid treated catalyst samples and their hydrogen consumption are shown in figure 1.

The fresh catalyst sample, figure 1a, exhibits a peak at 454 °C associated with the reduction of polyvanadate species bonded to catalyst alkali promoters: sodium and potassium [13,14]. The shift to 481 °C, figure 1b, observed in a spent catalyst sample can be attributed to the formation of sulphovanadates on the catalytic surface, such as KV(SO₄)₂ K₄(V^IVO)₃(SO₄), K₂V^IVO(SO₄)₃, Na₂V^IVO(SO₄)₂, Na₃(VO)₂(SO₄)₄ and K₃(VO)₂(SO₄)₅*, where (*) refers to a mixture of V⁵⁺ y V⁴⁺ compounds [3,15-18]. Formation of those compounds is favored by the presence of pyrosulfates S₂O₇^2-, which not only act as active intermediate species in the oxidation of SO₂ to SO₃ [2,15,16], but also in the sulphovanadate precipitation over the catalytic surface leading to catalyst deactivation [16,17].

An enhanced effect of the support (γ-Al₂O₃- SiO₂) in the reduction of V₂O₅ is evidenced by a consumption peak around 590 °C (Figs 1a and 1b) associated to the formation of V₆O₁₃ [18-20]. Unsupported V₂O₅ shows a signal at 640 °C. The shift to lower temperature observed in the supported sample is attributed to support interaction [18]. Notwithstanding, γ-Al₂O₃ is able to stabilize the reduced forms of vanadium, mainly V⁴⁺ compounds V-O-Al, during the deactivation of the catalyst as the signal remains in the spent catalyst, figure 1b, [21-23]. The peak around 670 °C is associated to the reduction of monomeric, polymeric, and crystalline VOx species which coexist on the surface as: V₃O₇, V₄O₉, V₆O₁₃ and V₂O₅ [7, 13, 21, 24-26]. Thus, this peak appears to be a superposition of different reduction steps of these species [18]. When catalyst samples were treated with H₂SO₄ (Figures 1c and 1d) the strong interaction between vanadium species and the support are evidenced since the signals around 570 and 612 °C remained.

TPO

TPO analysis was used to evaluate the oxidation of spent catalyst samples as a possible regeneration method. TPO of V₂O₅-F samples after TPR are shown in figure 2. After deconvolution of the signal, the fresh sample exhibits three oxidation peaks at 345 °C, 448 °C and 519 °C, figure 2a. The peaks at 448 °C and 519 °C are associated to the oxidation of VO₂ and V₆O₁₃ coexisting with V₂O₅, as shown in reaction 1 [6,19,24,25]. Similar to TPR experiments, the lower oxidation temperature compared to unsupported VO₄ is associated with the interaction between the support and VOx species [18].

The peaks around 345 °C exhibited by the fresh and spent catalyst samples are associated with the reoxidation of VOx most probably crystalline species [24]. The higher oxygen consumption of the spent catalyst sample compared with the fresh one could be attributed to the sinterization and formation of sulphovanadates of sodium and potassium during H₂SO₄ production [3,15-18,27]. Moreover, the oxidation of the fresh catalyst sample reduced at 600 °C, figure 2c, suggests that reduced vanadium compounds (V⁴⁺) are stable compared to the pentavalent initial state making it difficult catalyst re-activation. Figures 2d and 2e show that remaining vanadium species seem to be stable to oxidation since negligible oxygen consumption was observed.

UV-VIS results, figure 3, confirm the oxidation states of vanadium and the strong interaction with the support [22]. A broad band close to 250 nm corresponds to the SiO₂ support, and a shoulder around 350 nm is assigned to the ligand to metal charge transfer (LMCT) band of V⁵⁺ [20, 22, 28] confirming the presence of V₂O₅-K-SiO₂. The d-d electron transition band of V⁴⁺/ V³⁺ is found in the range 600 - 800 nm [20,22], and the absorption increased due to the presence of sulphovanadates in the spent sample, figure 3b.

TPD-NH₃

Figure 4 shows ammonia TPD profiles of V₂O₅-F, V₂O₅-S, and V₂O₅-S surface scraped samples. These samples exhibit a well resolved desorption peak in the range 670-730 °C assigned to a mixture of strong Lewis and Brönsted acid sites. The Lewis sites are unsaturated VO²⁺ vanadyls, and the Brönsted sites are associated with V-OH and the specific acidic properties of the support [22,29]. However, the increase of ammonia desorption of the spent sample compared to the fresh one, figure 4a and 4b, confirms the deposition of sulfate ions through the formation of sulfovanadates during the reaction. Furthermore, TPD-NH₃ of scrapped surface of a spent catalyst sample, figure 4c, evidences the formation of sulfovanadates.

Vanadium recovery

TPO analysis after TPR confirms the irreversibility of vanadium catalyst deactivation. Therefore, the recovery of vanadium becomes a good alternative for avoiding the release of toxic waste. After the treatment with H₂SO₄ (10 % vol.) only 86 wt. % of vanadium was extracted from the support in the spent catalyst and 92 wt. % from the fresh catalyst [6]. The presence of vanadium species was confirmed by TGA-DTA, figure 5. The small weight lost around 450 °C is associated to vanadium degradation [30], while the lost at 660 °C is assigned to bond breaking of reduced vanadium (V⁴⁺) and alkali promoters [6,13].

XRD evidences the presence of reduced V⁴⁺ species which are hard to remove during the leaching process. Peaks in the X ray diffraction pattern (not shown), at 11°, 25°, 28°, 31°, 33° and 36° are attributed to the presence of VO₂, V₆O₁₃, V₂O₅-K-SiO₂, and sulphovanadates KV(SO₄)₂ [6, 26]. Notwithstanding, the peaks associated with the presence of VO₂ and V₂O₅-K-SiO₂ also remain in the fresh catalyst after the treatment with sulphuric acid, confirming the strong interaction between vanadium species and the support.

Conclusions

The results of this study let us to conclude that alkali sulfovanates in the spent V₂O₅/SiO₂-γAl₂O₃ catalyst are hard to remove with 10 vol. % H₂SO₄. TPR-H₂ and TPO-O₂ of fresh samples treated with acid under the same conditions confirm strong interactions between the support and vanadium oxide which can also affect vanadium leaching with 10 vol. % H₂SO₄.

Acknowledgments

The authors acknowledge financial support from Universidad de Antioquia through the project "Sustainability 2009 - 2010".

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(Recibido el 03 de febrero de 2010. Aceptado el 15 de octubre de 2010)

^*Autor de correspondencia: teléfono: + 57 + 4 + 219 65 09, fax: + 57 + 4 + 219 65 65, correo electrónico:lgonzale@udea.edu.co lgonzale@udea.edu.co (L. M. González)