EFFECT OF THE ADDITION OF PHOSPHOTUNGSTIC ACID ON THE THERMAL STABILITY OF ZIRCONIUM OXIDE

EFECTO DE LA ADICIÓN DE ÁCIDO FOSFOTÚNGSTICO EN LA ESTABILIDAD TÉRMICA DEL ÓXIDO DE CIRCONIO

JUAN MANUEL HERNÁNDEZ-ENRÍQUEZ
Ph.D., Instituto Tecnológico de Ciudad Madero, Ciudad Madero Tamaulipas, México, jmanuelher@hotmail.com

RICARDO GARCÍA-ALAMILLA
Ph.D., Instituto Tecnológico de Ciudad Madero, Ciudad Madero Tamaulipas, México, rgalamilla@yahoo.com.mx

GUILLERMO SANDOVAL-ROBLES ]]> Ph.D., Instituto Tecnológico de Ciudad Madero, Ciudad Madero Tamaulipas, México, guillermosandov@gmail.com

JOSÉ AARÓN MELO-BANDA
Ph.D., Instituto Tecnológico de Ciudad Madero, Ciudad Madero Tamaulipas, México, melobanda@yahoo.com.mx

LUZ ARCELIA GARCÍA-SERRANO
Ph.D., Instituto Politécnico Nacional (CIIEMAD), México D.F., México, draluzg@gmail.com

Received for review September 6^th, 2012, accepted November 1^th, 2013, final version November, 25^th, 2013

ABSTRACT: Zirconium hydroxide [Zr(OH)₄] was synthesized by the sol-gel method using zirconium n-butoxide and 1-butanol as synthesis precursors. A part of the synthesized material was impregnated with phosphotungstic acid (H₃PW₁₂O₄₀) in order to evaluate the effect of this dopant agent on the thermal, textural and structural properties of zirconium oxide (ZrO₂). Pure and modified hydroxides were calcined in an air flow at 400, 500 and 600°C for 3 h and afterwards were characterized by thermal analysis, infrared spectroscopy, X-ray diffraction and nitrogen physisorption. Results showed that the interaction between the heteropolyanion and zirconium oxide surface produces a stabilizing effect of the crystalline structure and delays the drastic sintering of the material caused by the thermal treatment, obtaining high specific surface areas and small crystallite size for the modified zirconium oxides.

Key words: Zirconium oxide; Phosphotungstic acid; Thermal stability; Specific surface area; Crystallite size

Palabras clave: Óxido de circonio; Ácido fosfotúngstico; Estabilidad térmica; Área específica; Tamaño de cristal

1. INTRODUCTION

Metal oxides are one of the most common and outstanding categories of solid catalysts. Most heterogeneous catalysts are composed of a support and an active phase [1]. Many industrial applications require supports or catalysts that combine high specific surface areas, a suitable porous structure that allows good reactant diffusion throughout the inner particles of the catalyst, as well as high thermal stability [2].

One metal oxide with applications in diverse science and engineering fields is zirconium oxide (ZrO₂), one of the most important heterogeneous catalysts [3]. The great diversity of applications for ZrO₂ is related to its acid-base and oxido-reductive properties [4], which allow it to behave as a catalyst in alkilation, isomerization, esterification, alcohol dehydration, combustion, hydrogenation reactions, among others [5-9]. One of ZrO₂'s disadvantages is the sudden decrease of specific surface area during calcination due to a sintering process which occurs slowly when increasing the thermal treatment temperature, and results in the decrease in pore volume and increase of average diameter [10]. During thermal treatment, ZrO₂ is also subject to complete transformation from a tetragonal to a monoclinic structure when the calcination temperature reaches 600°C [11]. For zirconium oxide based materials it is important stabilize high specific surface areas and tetragonal structure, since these parameters are considered as promoters of the catalytic activity [12].

Since the early developments in the decade of 1970, chemical methods using the sol-gel route under different experimental synthesis conditions allowed suitable control of textural and structural properties in zirconium oxide [13]. During the last few years the application of new preparation methods as well as the combination of extreme critical drying techniques has resulted in high thermal stability, tetragonal structure and high specific surface area for this kind of materials [14-16]. In the same way it has been possible to stabilize the tetragonal structure of ZrO₂ with high specific surface areas by means of adding dopant agents like sulfate (SO₄^2-), phosphate (PO₄^3-) and borate (BO₃^3-) ions [17-19], as well as by the addition of metal cations especially: Mg²⁺, Ca²⁺, Sc³⁺, Y³⁺, La³⁺ and Ce⁴⁺ [2]. The amount of dopant agent required to stabilize the textural and structural parameters previously mentioned depends on the nature of the dopant agent and mainly on the preparation method.

In previous works we have reported successful preparation of ZrO₂ with high thermal stability, a defined tetragonal structure and high specific surface areas in the calcination temperature interval between 400 and 600°C, using the sol-gel method and 15% weight of PO₄^3- and BO₃^3- ions as structure-stabilizing agents. The synthesized materials were active in alcohol dehydration and light paraffin isomerization due to their excellent acid properties [20, 21].

The aim of this work is to study the thermal stability that can be induced in zirconium oxide by impregnation of precursor hydroxide with a heteropolyacid, phosphotungstic acid (H₃PW₁₂O₄₀), as well as its effect on specific surface area and crystalline structure.

2.1. Material synthesis
The synthesis of zirconium hydroxide [Zr(OH)₄] was made by the sol-gel method as previously indicated [20]. The impregnation of Zr(OH)₄ with the acid agent was performed by the incipient wetness method using a H₃PW₁₂O₄₀/ethanol solution, adding the required volume of this solution to obtain a catalytic support with 15% by weight of the dopant agent. Pure and modified hydroxides were dried at 100°C for 24 h and afterwards they were calcined under dynamic air atmosphere during 3 h at 400, 500 and 600°C. Taking calcination temperature and acid agent as reference, Table 1 lists the nomenclature used throughout this work.

2.2. Characterization techniques
The characterization of synthesized materials was carried out by thermogravimetry, infrared spectroscopy, X-ray diffraction and nitrogen physisorption.

Thermal analyses were performed in the thermogravimetric instrument TA Instruments STD 2960 Simultaneous DSC-TGA. The samples were analyzed under dry air flow (10 mL/min) at a heating rate of 10ºC/min in the temperature interval from ambient up to 900ºC.

Infrared spectroscopy was carried out in a Fourier Transform Spectrometer (Perkin-Elmer Spectrum One) with transparent wafers containing the sample to be analyzed and KBr as a binder. Spectra were recorded at a resolution of 4 cm-1 and by co-adding 16 scans.

The crystalline structure of zirconium oxides was determined in a Bruker Advance D800 diffractometer which used Cu Kα radiation (λ = 1.5406 Ǻ) and graphite monochromator in the secondary beam; the intensities of the diffraction lines were obtained in the range of 0-70° in the 2θ scale, with 0.02° steps and 2.4 s per point.

Nitrogen physisorption was used to determine the specific surface areas of the materials at the temperature of liquid nitrogen (-196°C) in a Quantachrome Autosorb-1 instrument. Prior to the measurements, samples were outgassed at 350°C for 2 h. The specific surface area was calculated using the BET equation and the BJH method was used to calculate the average pore diameter and the pore volume of the samples.

3. RESULTS AND DISSCUSION

3.1. Thermal analysis
By means of thermogravimetry, the physical and chemical processes developed in the synthesized zirconium hydroxides (Zr(OH)₄ and Zr(OH)₄-H₃PW₁₂O₄₀) versus thermal treatment temperature were studied.

In the thermogravimetric profile of Figure 1 that corresponds to Zr(OH)₄, multiple-stage decomposition of material can be seen, with three important weight losses. The first one is recorded in the interval of temperature from ambient to 200°C, and is related with the evaporation of water and alcohol that remain from synthesis, occluded up to that moment in the material's porous structure, in the curve of thermodifferential analysis such material releases are shown as endothermal signals near 50°C and 200°C [10]. The second weight loss is detected in the temperature range from 200°C to 370°C and is attributable to the combustion process of residual organic matter, related with the exothermal signals located at 258°C and 285°C in the thermodifferential profile. The last stage of weight loss recorded above 370°C could be related to de-hydroxylation of the material and the possible change of zirconium oxide from the amorphous to crystalline phase [10,22]; the thermodifferential analysis showed an exothermal change at 404°C during this weight loss. The transformation to zirconium hydroxide led to a total weight loss of 24%, which stabilized after 500°C.

The thermograms of Figure 2 corresponding to sample Zr(OH)₄-H₃PW₁₂O₄₀, show a behavior similar to the thermograms of Zr(OH)₄, with a slight shift in temperatures at which the physical and chemical processes previously described occur, which indicates the presence of a different specie in the Zr(OH)₄. The broad endothermal change centered at 493°C is attributable to a slow transition from the amorphous state of the material to a crystalline state, probably caused by the interaction between the heteropolyanion ([PW₁₂O₄₀]^3-) and zirconium atoms.

Figure 3 shows the thermogravimetric and thermodifferential analysis for phosphotungstic acid (H₃PW₁₂O₄₀). The curve of the thermogravimetric analysis shows that the most important transitions of weight loss for this material are located in the range from ambient temperature to 300°C. The thermodifferential analysis profile shows endothermal signals near 68°C and 190°C that are result of the elimination of physisorbed and structural water contained in the heteropolyacid. The exothermal peak located at 612°C is characteristic of this material and is related with the decomposition of its Keggin structure in oxides of tungsten and tungsten-phosphorus [23], this change takes place without weight loss.

3.2. Infrared spectroscopy
Table 2 summarizes the most important data of infrared spectroscopy analyses carried out to heteropolyacid and to synthesized zirconium oxides. The infrared spectroscopy study of phosphotungstic acid with a Keggin structure (H₃PW₁₂O₄₀) consists of five characteristic bands in the zone 1100-400 cm^-1: 1081, 984, 891, 795, and 593 cm^-1. The position of these bands can be assigned, in turn, to the stretching vibrations associated to P-O, W=O, W-O-W bonds and to the bending vibration movement of in and out of the plane caused by P-O bonds, respectively [25]. These signals show a slight shift when heteropolyacid is introduced in the matrix of zirconium oxide, suggesting a strong interaction between the heteropolyanion coming from H₃PW₁₂O₄₀ and the oxide surface. Although the bands slightly change in position, they do not disappear from this spectrum, only those that coincide with the absorption bandwidths shown by pure zirconium oxide in the region 750-500 cm^-1 overlap. These signals associated to Zr-O stretching also slightly modify their position; this is possibly caused by material re-crystallization [26]. In the synthesized oxides also there are signals located at 3500 and 1620 cm^-1, related with some hydroxyl groups that exist after the calcination process [27]. The hydroxyl groups contained in Zr(OH)₄ at the moment of impregnation are responsible for the fixation and stabilization of the heteropolyanion in zirconium oxide.

3.3. X-ray diffraction
By means of X-ray diffraction the crystallization process of materials thermally treated in the temperature interval between 400°C and 600°C was studied. Figure 6 shows the development of crystalline structures as a function of thermal treatment temperature. In the diffraction patterns of pure zirconium oxide it can be noted that the material Z400 is mainly composed of tetragonal crystalline structure according to pattern card number JCPDS #80-2155. Also low intensity signals are seen at 2θ = 17.55, 24.52, 28.24, 31.53, 38.61, 41.22, 45.59, 54.10, 55.54 and 65.75° that characterize the existence of zirconium oxide with a monoclinic structure if compared to card number JCPDS #7-3430. These signals intensify as the calcination temperature increases, showing an almost completely monoclinic structure in the material Z600, and the full transformation of this structure at temperatures higher than 600°C can be predicted. With these results it can be concluded that the concentration of the crystalline phases in pure zirconium oxide depends mainly on the thermal treatment temperature.

The X-ray diffraction patterns of zirconium oxide modified with phosphotungstic acid are different to those of pure zirconium oxide. In these materials there could be seen only reflections at 30.25, 35.10, 50.41, and 60.02° in the 2θ scale that are characteristic of the formation of zirconium oxide with a tetragonal structure. The material ZH400 shows an X-ray diffraction pattern with incipient signals of very wide bases that are characteristic of materials with poor crystallinity. The increase in calcination temperature generates an intensification of signals shown in diffractograms of materials ZH500 and ZH600, which is related to an increase in crystallite size. The absence of diffraction lines associated to phosphotungstic acid in materials ZH400, ZH500 and ZH600 could be related to the good dispersion ability of the acid agent in zirconium oxide as well as with its low concentration.

In Figure 6 the crystallite sizes obtained with ZrO₂ and ZrO₂-H₃PW₁₂O₄₀ calcined at different temperatures are shown. It was found that before the complete transformation from the tetragonal to the monoclinic structure take place, the crystallite size of pure zirconium oxide was between 4 and 16 nm. The Figure shows a sudden increase in the crystallite size for this material, which depends on the thermal treatment temperature. The crystallite size difference between pure oxide and the modified material with phosphotungstic acid is remarkable. The interaction between heteropolyanion and zirconium atoms limits solid state diffusion and delays crystal growth [28], obtaining values between 2 and 7 nm, making the influence of calcination temperature on this parameter less significant.

3.4. Nitrogen physisorption
Figure 7 shows specific surface areas of synthesized materials and their evolution according to thermal treatment temperature. Zirconium hydroxide [Zr(OH)₄] showed a specific surface area of 595 m²/g, which decreases to 144 m²/g when the calcination temperature of the material reaches 400°C. It is worth mentioning that major physical and chemical changes of the samples take place during thermal treatment, in the range between 100 and 400°C, which lead to a sudden decrease in the specific surface area.

The material's sintering process that occurs after 400°C causes a decrease of 88% in the specific surface area. Impregnation of Zr(OH)₄ with phosphotungstic acid showed a beneficial effect on specific surface areas obtained in the materials after the calcination process. The presence of a dopant agent reduces the loss of the material's specific surface area in the studied temperature range. Therefore, material modified with phosphotungstic acid and calcined at 600°C (ZH600) surpasses by up to six times the specific surface area value obtained with pure zirconium oxide calcined at the same temperature (Z600).

4. CONCLUSIONS

With the addition of phosphotungstic acid in the phase of hydroxide precursor, the crystallization behavior of zirconium oxide was modified. The presence of heteropolyanion in zirconium oxide inhibited monoclinic structure formation, limited diffusion in the solid state, favored stability of the tetragonal structure in the interval of temperatures studied, delayed crystal growth and also encouraged development of nanocrystalline materials with high specific surface area values. The heteropolyanion coming from phosphotungstic acid caused a thermal stability effect similar to the one obtained with SO₄^2-, PO₄^2- and BO₃^3- ions and some metal cations like Y³⁺, La³⁺ and Ce⁴⁺.

ACKNOWLEDGEMENTS

The authors are grateful to División de Estudios de Posgrado e Investigación at the Instituto Tecnológico de Ciudad Madero for the support given during the stage of post-doctoral studies, from which this research work was taken.

REFERENCES

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