Synthesis and characterization of novel ZrO2-SiO2 mixed oxides

Synthesis and characterization of novel ZrO₂-SiO₂ mixed oxides

Milena Araújo Ferreira e Santos; Ivon Pinheiro Lôbo; Rosenira Serpa da Cruz^*

Departamento de Ciências Exatas e Tecnológicas, Universidade Estadual de Santa Cruz - UESC, Rod. Jorge Amado, Km 16, CEP 45662-900, Ilhéus, BA, Brasil

ABSTRACT

In this study the mixed oxides ZrO₂-SiO₂, were synthesized by the sol-gel method with a molar ratio of 95:5 (Si/metal) and characterized by X-ray diffraction, absorption-desorption of N₂, Fourier transform infrared spectroscopy, thermal gravimetric analysis, temperature-programmed desorption of ammonia, and acidity test by titration. The synthesized materials, which are amorphous to X-rays, are composed of a mixture of micro- and mesopores. They show a higher acid strength than the separate oxides, indicating that the ZrO₂ is highly dispersed in the silica matrix.

Keywords: mixed oxides, heterogeneous catalyst, methylation, sulfation

1. Introduction

The continuous evolution of science and the general call for the protection of the environment have created an increased demand for new technologies and new materials. This becomes evident by the progress in the improvement of high performance metallic and polymeric materials, semiconductors, ceramics, nanotechnology, and catalysts^1,2. In particular, those materials which exhibit catalytic activity have attracted attention due to their broad and growing applicability in many industrial processes³, from the petrochemical industry to the food sector^4-7.

Numerous studies can be found in the literature^8-13 regarding the development of new materials with catalytic properties. Among the various solids with industrial applications, the mixed oxides are capable of combining the chemical and mechanical properties of their constituting oxides⁵. Once formed, the mixed oxides display acidic sites due to the excess negative or positive charge or due to defects in the lattice^14,15. Owing to these properties, the acidic mixed oxides play an important role in applications such as the cyclization of citronellal to isopulegol^16,17, isomerization of α-pinene and dehydration of 4-methyl-2-pentanol¹⁸, Fischer-Tropsch process¹⁹, ammonia oxidation²⁰, hydrogenation of alkenes²¹, synthesis of alcohols²², and biodiesel production²³.

Among the most frequently employed methods for the synthesis of mixed oxides are the coprecipitation²⁴, hydrothermal routes²⁵, sonochemical processes²⁶, the oxalate route²⁷, and the sol-gel process²⁸. From all these processes, the sol-gel process (SGP) has attracted attention due to its versatility in the development of new materials, owing to the simplicity and flexibility of the synthesis, which permits the preparation of highly pure and homogeneous products with controlled porosity²⁹. Moreover, this process is environmentally friendly compared to the traditional routes for the preparation of ceramics and glasses³⁰.

An important application for mixed oxides is the catalysis of the esterification reaction of the free fatty acids contained in oils and/or fats, as a preceding step in biodiesel production. At commercial scale, biodiesel is mainly produced with homogeneous alkaline catalysts, although this technology poses some disadvantages, for example, its requirement of high-quality feedstock with low content of free fatty acids and water, the difficulty of recovery and reutilization of the catalyst, and the generation of effluents, all of which contribute to increased production costs. Therefore, research activities have turned towards the development of heterogeneous catalysts that can be separated easily from the reaction medium, allow for more flexibility concerning feedstocks with higher contents of free fatty acids and water, and can be reused several times, besides avoiding the generation of larger amounts of effluents.

In this context, silica-supported zirconium oxide has emerged as catalyst because of its thermal and chemical stability, its higher resistance to deactivation compared to the isolated oxides, and its higher acidity^27,31-33.

Based on the above considerations, the objective of this work was the synthesis and characterization of the mixed oxide ZrO₂-SiO₂ as well as the modification of its acid force and its hydrophilic character, followed by an preliminary assessment of the catalyst in the esterification of oleic acid with methanol, aiming at a future application in the production of biodiesel from high free fatty acid feedstocks.

2. Experimental

2.1. Synthesis of the solids

The ZrO₂-SiO₂ catalysts were prepared by the sol-gel method³⁴, with a 95:5 molar ratio of Si/Metal. Initially, a pre-hydrolysis was performed by agitating a solution containing tetraethoxysilane (TEOS)/H₂O/ethanol/HNO₃ at 80 ºC, for 3 hours. Thereafter, appropriate amounts of a solution of the metal oxide precursor (ZrOCl₂.8H₂O) with ethanol were added, maintaining the agitation for two more hours, at room temperature. Then, H₂O and concentrated HNO₃ were added, and the resulting solution was kept at room temperature for another two hours. The gels were obtained by evaporation of the solvent and dried for 6 hours at 80 ºC, macerated, washed for removal of the chlorides, dried at 100 ºC for two hours, washed with ethanol in the soxhlet extractor, and calcined at 500 ºC for three hours. This catalyst will be referred to as ZrO₂-SiO₂.

In order to obtain methylated material, the synthesis was carried out as described above, except that a proportion of 1:1 of methyltriethoxysilane MeSi(OEt)₃/TEOS was added in the pre hydrolytic step. This catalyst will be referred to as ZrO₂-SiO₂-Me.

In order to obtain sulfated material in situ, the synthesis proceeded as described for catalyst ZrO₂-SiO₂, but with the addition of an amount of 4.1 mL sulfuric acid at the hydrolytic and condensation steps, instead of the nitric acid. This catalyst will be referred to as ZrO₂-SiO₂-SO₄^2--is.

2.2. Characterizations

The X-ray diffractional patterns with CuKa radiation was obtained using a Siemens D5000 diffractometer. The range examined was 2θ = 5 to 80º, and the speed was 4º. s^-1. The adsorption-desorption isotherms of nitrogen were obtained at -196 ºC with a Micromeritics ASAP 2010. The sample was previously treated for 17 hours at 250 ºC under nitrogen flow. The pore diameters, as well as the pore size distribution, were calculated by using the BJH (Barrett, Joyner and Halenda) for microporous solids, based on the nitrogen adsorption isotherm. The specific area was calculated with the BET (Brunauer, Emmett and Teller) equation in the region with low pressure (p/po = 0.200), and the micropore area and micropore volume were determined using the software t-plot.

Infrared spectra have been registered in the range 4000‑400 cm⁻¹ using Shimadzu spectrometer, with a resolution of 4 cm⁻¹. The samples were prepared as KBr pastilles, with a proportion of 1% of the solids. The TG-DTA analyses were conducted with a Shimadzu DTG - 60/60H. The samples of the solids were heated under nitrogen flow, from room temperatures up to 700-900 ºC, at a rate of 40 ºC min^-1, using aluminium oxide cells.

The profiles of the thermal programmed desorption of ammonia (TPD-NH₃) were obtained with a Micromeritics Chemsorb 2720. The samples were pre-treated for one hour at 300 ºC, under helium flow of 25 mL.min^-1 followed by cooling to room temperature in order to remove any physisorbed species on the surface of the sample. After this pre-treatment, the samples were submitted to a chemisorption step using ammonia in helium (9.9%, mol/mol) flow of 25 mL.min^-1 at room temperature, for 30 minutes. Thereafter, the system was purged with helium at room temperature for 30 minutes. In order to remove the physisorbed ammonia molecules, the material was treated for 30 minutes at 150 ºC under helium flow (25 mL.min^-1) and then cooled to room temperature. This step was followed by the thermal programmed desorption analysis, in which the sample was heated from room temperature to 1000 ºC, at a rate of 10 ºC min^-1 and under helium flow (25 mL.min^-1). The amount of desorbed ammonia was monitored with a thermal conductivity detector. The acidity of the catalyst was determined from the quantification of Brønsted acid sites by titration³⁵. Finally, 100 mg of the material was brought into contact with 20 mL of 0.1 mol L^-1 NaOH for three hours with light agitation. Then, samples of the basic solution were separated and titrated with 0.1 mol L^-1 HCl in order to verify the amount of NaOH which reacted with the material. Acidity was determined in mmol H⁺ per gram of material.

The synthesized materials were assessed in an esterification reaction using oleic acid (model molecule) with methanol, in a Parr reactor with a working volume of 300 mL, using the following reaction conditions: the molar ratio of acid/alcohol was 6:1, the amount of catalyst corresponded to 3% of the mass of the oleic acid, and the temperature was 120 ºC, for 3 hours. Preceding the reaction, the catalyst had been dried at 110 ºC, for 3 hours.

3. Results and Discussion

3.1. Synthesis of the solids

The solids were synthesized by means of the sol-gel process (SGP), a comparatively simple method for the synthesis of mixed oxides, which promotes high homogeneity of the metal on the surface of the matrix, better control of particle size, and higher purity³⁶.

The TEOS was chosen as precursor for presenting low hydrolysis rates, which promotes high homogeneity for mixed oxides³⁷. The solvent was ethanol, which prevents the separation of the liquid-liquid phases during the initial stage of hydrolysis, and consequently, the concentrations of water and silicate are maintained, which influence the reaction speed in the gelation step. Furthermore, ethanol facilitates the drying process due to its high vapor pressure. The drying process by slow evaporation of the ethanol as well as the thermal treatment with low heating rates led to the formation of a xerogel^38,39.

The synthesis route was catalyzed by a nitric acid (HNO₃) with a (HNO₃:TEOS-1,2x10^-4) molar ratio, in order to produce a linear polymeric system with few ramifications. The acid hydrolysis started with a pre-hydrolysis of the lesser hydrolysable precursor to ascertain the adequate control of the number of hydrolyzed sites of the TEOS. This procedure favors the condensation in linear polymer chains and high homogeneity of the oxides, because the silicon alkoxides have lower reactivity than those of the metal transition precursors³⁹.

The evaporation of ethanol and water in environmental conditions during the hydrolization/condensation step for the synthesized materials promoted the increase of the mean diameter of the pores.

Modifying the surface of the catalyst by the substitution of 50% of the TEOS by MeSi(OEt)₃ during the SGP (part 2.1- represented by ZrO₂-SiO₂- Me) produced a material with lesser hydrophilic character due to the exchange of the acidic hydrogens of the silanol groups (Si-OH) by methyl groups (-CH₃), as shown in Figure 1. Therefore, this material is expected to be more resistant to deactivation by the water produced during the esterification reaction.

The sulfation of the ZrO₂-SiO₂ material was conducted with to increase the acid force for some relevant reactions.

The in situ sulfation of the material in the SGP process was studied, where the sulfuric acid was added in the sol-stage. Therefore, the formation of sulfate groups is expected which strongly bond to the lattice. This can be explained by the interaction of the aqueous complexes of Zr⁴⁺ with the sulfate ions, forming species of the type [Zr(OH)₂(SO₄^2-)_x (H₂O)_y]_n^n(2-2x), which can interact strongly with the silica in the solution and which exhibit a negative charge under the employed synthesis conditions. It can therefore be concluded that the method of sulfation in situ provided a better distribution of the sulfated zirconium between the surface and the lattice of the material^40,41.

3.2. Characterization

The characterization was done in order to verify if the different synthesis conditions promoted the formation of mixed oxides with high dispersion of the zirconium oxide in the silica matrix and narrow pore distribution.

The analysis of the X-ray diffractional patterns for all of the materials in Figure 2 shows only wide peaks varying between 15º and 30º, which are attributed to SiO₂ in its amorphous state. This indicates that the synthesis methods used promoted a high dispersion of the ZrO₂ in the amorphous silica matrix^31,32,42,43.

As discussed in section 3.1, and suggested by the results of the X-ray diffraction, the pre-hydrolysis of the TEOS promoted the production of highly homogeneous materials.

The evaluation of the textural characterization of the solids was based on the adsorption-desorption isotherms of N₂ at -196.15 ºC and revealed different textures of the solids. Figures 3 and 4 show the isotherms of adsorption-desorption of N₂ and the distribution curves of the pore volumes for synthesized materials.

The materials showed type IV isotherms with a combination of hysteresis types (H2 and H3), which indicate the presence of micropores as well as mesopores. The total pore volume, the micropore area and the micropore volume, calculated with t-plot, confirmed these observations (Table 1).

The distribution curves of pore diameter shown in Figure 4 as well as the data in Table 1 show that all the synthesized materials have an even monomodal pore-size distribution with mean pore diameters ranging from 3.3 to 3.7 nm, which classifies them as mesoporous solids according to IUPAC^28,44.

Partial substitution of hydroxyl groups by methyl-CH₃ groups in the ZrO₂-SiO₂ material induces an increase in pore diameter (Table 1). This may be due the decrease of the interaction of the hydrogen bonds between the water and the OH-groups on the inner walls of the pores, promoting a reduction of the internal tension during the drying process⁴⁵. The insertion of sulfate groups to the ZrO₂SiO₂-SO₄^2-is materials resulted in a decrease of the specific area and increase of the mean pore diameter (Table 1). The presence of sulfate ions in the micropores partially blocks the pores, makes them inaccessible to N₂ molecules, and consequently influences the size of the area. The increase in pore diameter can be linked to the presence of sulfate ions on the micropore walls, causing tension and rupture, followed by increased micropore volume and diameter⁴².

The spectra in the infrared region shown in Figure 5 indicate the presence of a band at 1630 cm^-1 that corresponds to scissor bending vibration of the H-O-H bond of the physisorbed molecular water^27,42,46,47.

The samples showed bands centered at 1078 cm^-1 with a shoulder at ~ 1220 cm^-1 due to the asymmetric Si-O-Si stretching vibrations of transverse-optical (TO₃) mode and the longitudinal-optic (LO₃) part stretching vibrations, respectively⁴⁷. Bands observed around 800 cm^-1 and 460 cm^-1 are due to TO symmetric stretching and bending vibrations of Si-O-Si bonds, respectively^47,48.

Bands with frequency mode from 910 cm^-1 to 975 cm^-1 can be assigned to Si-O- groups in SiO₂ gel or to stretching vibration modes of the Si-O-Zr groups in ZrO₂-SiO₂ binary oxides^31,40,46. Typically, bands placed at higher frequencies are assigned to Si-O-Zr groups⁴⁷. These features confirm the results of the DRX, suggesting the production of X-ray amorphous or poor crystalline materials and incorporation of ZrO₂ into silicate matrix.

The analysis of the thermogravimetric curves of the materials in Figure 6 shows the almost constant decrease of mass in the range of 100 to 200 ºC, which is due to the elimination of water, impurities, alkoxides, and unreacted solvents. Above 700 ºC, the loss of mass is associated with the dehydroxilation of the surface²⁷.

The usually characteristic band of Si-C at 1276 cm^-1 for the ZrO₂-SiO₂-Me catalyst in the FTIR analysis (Figure 5) could not be verified due to the small amount of MeSi(OEt)₃ used for synthesis. The thermal analysis (Figure 6b) revealed the existence of a wide peak reaching from 596 to 793 ºC, which is considered to result from alcoxides that have not reacted, and from the breaking of the Si-C-bonds, an evidence for the insertion of the methyl group. Based on this information, the calcination temperature of these materials was determined, because at temperatures above 500 ºC the methylated groups decompose, compromising the reaction³⁷. Besides, it is noteworthy that the loss of water attributed to the methylated catalyst (ZrO₂-SiO₂-Me) is smaller when compared with other materials, and corresponds with the lesser hydrophilic character expected for this sample.

The thermal programmed ammonia desorption (TPD-NH₃) shown in Figure 7 for the ZrO₂-SiO₂ and ZrO₂-SiO₂-SO₄^2--is catalysts reflects the strength and distribution of the acid sites at the desorption temperature of ammonia.

The materials without sulfation (Figure 7a) showed three defined peaks at 132, 328, and 506 ºC that are related to weak, intermediate and strong acid sites, respectively. The occurrence of these sites can be attributed to the defects on the surface due to the insertion of zirconia in the silica matrix and the formation of bonds of the Si-O-Zr-O-Zr as well as the, Zr-O e Zr=O bands⁴⁹.

The presence of the desorption peaks at 310 ºC with a shoulder at 182 ºC and 618 ºC (Figure 7b) suggest, respectively, the existence of weak/intermediate and strong/superacid sites in the sulfated material.

The increase of acidity in the sulfated material was observed based on the analysis of TPD-NH₃ (Figure 7) and confirmed by the acidity test by titration of NaOH (Table 2), and revealed the presence of acid/superacid sites for the sulfated materials.

The acidity of the materials produced by the titration with NaOH, which only identifies Brønsted acid sites, is consistent with the results of the TDP-NH₃ (Figure 7), where the sulfation favored the formation of Lewis acid sites and led to a decrease of Brønsted acid sites as shown in Table 2. As the Lewis acid sites are stronger than the Brønsted acid sites, the in situ sulfated mixed oxide shows a higher concentration of strong acid sites than the standard material (ZrO₂-SiO₂). It is suggested that the increase in Lewis acid sites after sulfation is associated with the formation of Zr-SO₄⁴⁹. After the methylation of the mixed oxide ZrO₂-SiO₂, a distinct decrease of Brønstedt acidity was observed, which is probably due to the substitution of silanol groups by methyl groups at the surface.

The solids ZrO₂-SiO₂, ZrO₂-SiO₂-Me e ZrO₂-SiO₂-SO₄^2--is were tested for their catalytic activity in an esterification reaction of oleic acid with methanol, which resulted in ester conversions of 40.8, 49.8 and 49.4%, respectively. As expected, the modified materials ZrO₂-SiO₂-Me and ZrO₂-SiO₂-SO₄^2--is showed the best catalytic activity. The catalyst ZrO₂-SiO₂-Me displays a hydrophobic surface which minimizes blocking of active sites of the material, This is due to the anchoring of water molecules from the esterification reaction^46,50. For the catalyst ZrO₂-SiO₂-SO₄^2- - is, the conversion percentage in the esterification reaction is due to the acidity caused by the incorporation of sulfate into its structure.

The materials were promising in relation to their catalytic activity in the esterification reaction. In a next step, these solids will be evaluated with regard to their stability to leaching of the active species to the reaction medium and to the possibility of their reuse in the reaction. Following this, the reaction conditions will be optimized to improve ester yields.

4. Conclusions

X-ray amorphous, meso- to microporous mixed oxides were produced by the sol-gel method, with highly dispersed zirconium on the silica matrix, as verified by the techniques DRX e FTIR. In the mixed oxide ZrO₂SiO₂ - Me, the surface polarity was modified through the substitution of silanol groups by methyl groups, as evidenced by the TGA/DTA technique. The mixed oxide ZrO₂-SiO₂-SO₄^2- - is showed a higher distribution of acidic sites after sulfatation in situ, as confirmed by the TPD-NH₃ technique.

The preliminary tests of the catalytic activity showed that the modifications of the mixed oxide ZrO₂SiO₂ contributed considerably to the increase of the conversion of the esterification reaction of oleic acid with methanol. The synthesized materials can therefore be considered promising catalysts for esterification reactions in the context of biodiesel production.

Acknowledgments

We gratefully acknowledge funding provided by the Fundação de Apoio à Pesquisa do Estado da Bahia (FAPESB) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). MAFS would like to acknowledge Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for a Master's scholarship.

Received: September 2, 2013;

Revised: February 6, 2014

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Publication Dates

• Publication in this collection
  15 Apr 2014
• Date of issue
  June 2014

History

• Received
  02 Sept 2013
• Accepted
  06 Feb 2014