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Materials Science
Procedia Materials Science 1 (2012) 491 - 498
www.elsevier.com/locate/procedia
11th International Congress on Metallurgy & Materials SAM/CONAMET 2011.
Effect of the addition of boric acid to zirconia synthesized employing pore-forming agents
L. Osiglio, M. Blanco*
Centro de Investigación y Desarrollo en Ciencias Aplicadas "Dr. J. J. Ronco" (CINDECA), Departamento de Química, Facultad de Ciencias Exactas, UNLP-CCTLa Plata, CONICET, 47 N° 257, 1900 La Plata, Argentina
ELSEVIER
Abstract
The preparation and characterization of borated zirconia calcined at 320 °C was studied. The zirconia was prepared using zirconyl chloride as precursor and ammonium hydroxide as precipitating agent. Pore-forming agents during zirconia synthesis were added, specifically glucose or polyethylene glycol (PEG) of different molecular weight. After drying, an aqueous solution of boric acid was added to the zirconia. Zirconias without the addition of boric acid were also prepared. The specific surface area of the borated zirconias is higher than those of the samples without boron. The addition of PEG led to a mean pore size larger than 3 nm, while with glucose the value was somewhat lower, indicating that PEGs are more effective pore-forming agents. By X-ray diffraction, it was observed that the materials have amorphous characteristics. The FT-IR spectra showed that the washing of the samples seems to be effective in the removal of the pore-forming agents, and the bands assigned to boron species are present in the borated zirconias. On the other hand, it was estimated that all the borated zirconias presented very strong acid sites. So, solids with adequate characteristics to be used as catalysts in acid reactions were synthesized.
© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility o f SAM/ CONAMET 20 o 1, Rosario , Argentina.
Keywords: Zirconia; pore-forming agent; boric acid addition
* Corresponding author. Tel.: 54-221-421-1353; fax: 54-221-421-1353. E-mail address: mnblanco@quimica.unlp.edu.ar.
2211-8128 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/CONAMET 2011, Rosario, Argentina. doi:10.1016/j.mspro.2012.06.066
1. Introduction
The zirconium oxide (zirconia) preparation using pore-forming agents was carried out in order to obtain a catalyst support, due to the acid as well as basic properties. These properties can be modified by adding cationic or anionic substances. The addition of boric acid to the zirconium oxide has not been much studied, though it leads to materials with interesting acid characteristics, nevertheless with lower acidity than sulphated zirconia [Xu et al., 2000; Yadav and Nair, 1999]. An important number of variables must be taken into account to obtain zirconia, which covers from the preparation method till the thermal treatment, including the precursors, their concentration, pH of the synthesis, among others. On the other hand, different compounds were employed as pore-forming agents, mainly to obtain mesoporous materials, which have attracted much attention in various fields related to the preparation of catalysts, because they can improve the textural properties of the materials normally giving high specific surface areas and pore sizes. For example, the use of non-surfactant organic compounds, such as hydroxyacids and urea, has been reported for the preparation of oxides using the sol-gel method [Wei et al., 1998; Zheng et al., 2001; Pizzio, 2005].
In this work, zirconia was prepared employing the micelar method from zirconyl chloride as precursor, ammonium hydroxide as precipitating agent, final pH = 10, aging time of the hydrogel of 72 h, calcination of the samples at 320 °C. With the purpose of improving the characteristics of the materials obtained in a previous work [Osiglio et al., 2010], pore-forming agents were employed, which were added at the beginning of the hydrogel preparation. The effect of the addition of glucose or polyethylene glycol (PEG) of different molecular weight in the characteristics of the obtained materials was studied. After washing the hydrated zirconias with distilled water till chloride removal and drying at room temperature, boron was added to the zirconias employing an aqueous solution of boric acid by means of wet impregnation. The hypothesis of the work is that the addition of the pore-forming agents during the synthesis allows obtaining materials with better textural and acid characteristics, suitable for their use in catalysis.
2. Experimental
2.1. Sample preparation
Zirconia was prepared by means of the micelar method employing an aqueous solution of ZrOCl2.8H2O (Fluka) (0.56 mol Zr/dm3) as precursor, to which ammonium hydroxide 30% (Merck) was added drop-wise as precipitating agent up to a final pH of 10, under constant stirring. The obtained products were aged for 72 h and were filtered under vacuum. The precipitates were washed with distilled water to remove chlorides, up to negative reaction with AgNO3 in the washings. Pore-forming agents were employed, which were added during the hydrogel preparation. The effect of the addition of 10% (w/w) of glucose or polyethylene glycol (PEG) of molecular weight 400, 2000 and 6000 Da was studied. The supports, dried at room temperature, were impregnated with aqueous solutions of H3BO3 (Anedra p.a.) of 0.3 mol B/dm3 concentration, to obtain boron concentration in the solid of 15 % (w/w), expressed as B2O3 respect to support. Then, the suspension was evaporated to dryness at 50 °C. Afterwards, the solids were calcined in N2 atmosphere for 5 h at 320 °C. The samples corresponding to addition of glucose, PEG400, PEG2000 and PEG6000 will be named S8GB, S8P4B, S8P20B and S8P60B, respectively. In order to make a comparison, the materials without boric acid addition calcined at the same temperature above-mentioned (S8G, S8P4, S8P20, S8P60 samples, respectively) were studied.
2.2. Sample characterization
The specific surface area, the pore volume and the pore size of the solids were estimated from the adsorption-desorption isotherms of nitrogen at -196 °C, using Micromeritics equipment, model ASAP 2020, and previous degasification at 100 °C for 1 h. The X-ray diffraction (XRD) patterns were registered between 5 and 60 °20 with Philips PW-1714 equipment, using Cu Ka radiation, Ni filter, 20 mA and 40 kV in the high tension source, and a scanning rate of 1° per min. To obtain the micrographs of the solids, scanning electron microscopy (SEM) was employed, using Philips equipment, model 505, at a working potential of 15 kV, supporting the samples on graphite and metallizing with a sputtered gold film. The images were obtained with ADDAII acquisitor, with Soft Imaging System, using magnification between 300 and 5000x. The infrared (FT-IR) spectra were acquired employing Bruker IFS 66 equipment and pellets of the sample in KBr. The measurements were performed in the 400-4000 cm-1 range. The materials were potentiometrically titrated in order to estimate their acidity. To this end, 0.05 g of solid was suspended in 45 cm3 of acetonitrile. The suspension was stirred for 3 h, and then titrated with a 0.025 N solution of n-butylamine in acetonitrile. The potential variation was measured with Hanna 211 pHmeter, and double-junction electrode.
3. Results and discussion
The specific surface area determined by the Brunauer-Emmett-Teller method (SBET) of the zirconias obtained adding PEG during the synthesis decreased between 25 and 50% after calcination at 320 °C. The contribution of the microporous area estimated by the t-plot method (Smicro) also decreased, being negligible its contribution to the total area in the calcined samples (Table 1). On the other hand, the total pore volume (Vp) and the mean pore size (MPS), estimated by the Barret-Joyner-Halenda (BJH) method from the desorption branch of the isotherm, changed in a lower extent for the supports obtained employing different PEG. When boric acid was added, the samples presented higher values of Sbet and Smicro. This fact may be explained considering that the boron dispersed on the surface of the hydrated zirconium oxide acts separating the particles and inhibiting their later growth, as was reported by Zhao et al., 1997, for the Mo, W, Cu, Fe and Ni oxides supported on zirconia, thus resulting in a lower diminution of the area when the samples are calcined (7-8.5%). In turn, Vp and MPS were only slightly higher than the bare supports. The addition of the three PEG led to values of MPS higher than 3 nm, while when glucose was added the value was the lowest. This indicates that the PEGs are more effective pore-forming agents, leading to mesoporous samples, moreover if a comparison is made with the value of 2.1 nm that has been reported in a previous work for zirconia and borated zirconia without the addition of pore-forming agents [Osiglio et al., 2010].
Table 1. Textural properties of the samples calcined at 320 °C obtained using various pore-forming agents.
Sample Sbet (m2/g) Smicro (m2/g) Vp (cm3/g) MPS (nm)
S8P4 131.5 - 0.10 3.0
S8P20 166.9 14.3 0.11 3.0
S8P60 116.5 2.3 0.09 3.1
S8G 2 - - -
S8P4B 214.9 61.1 0.14 3.4
S8P20B 207.1 62.7 0.13 3.2
S8P60B 214.0 46.1 0.14 3.3
S8GB 224.0 127.6 0.11 2.4
By X-ray diffraction, the materials presented amorphous characteristics, with a broad band whose maximum is placed at a value of 20 = 30°. Crystalline phases were not detected, being the diagrams of the samples with boron similar to those of the respective supports. Though the boric acid calcined at 320 °C presented sharp peaks at 20 of 14.8° and 27.9°, corresponding to boric oxide, these peaks were not observed in the diagrams of the borated zirconias. This fact may be attributed to a good dispersion on the support as a non-crystalline phase or to the presence of crystallites small enough to give diffraction lines. Neither the crystalline modifications of zirconia (monoclinic, tetragonal, cubic) nor the metastable tetragonal phase of zirconium oxide were observed, in spite that it has been reported that its stabilization could be produced caused by the presence of impurities [Yamaguchi, 1994].
Figure 1. XRD diagrams of the samples obtained using different pore-forming agents.
The FT-IR spectra of zirconias (Figure 2a) showed a broad band between 400 and 1100 cm-1, a band at 1623 cm-1 with a shoulder at 1383 cm-1, and a wide band with maximum around 3400 cm-1. The first band can be assigned to the vibration of the (O-Zr-O) bond [Vives et al., 1999], the second band and the shoulder have been assigned to the bending vibrations of the (H-O-H) y (O-H-O) bonds, respectively, and the last one to asymmetric stretching of hydroxo and aquo (-OH) [Patel el al., 2003].
The washing of the prepared solids seems to be effective, since the characteristic peaks of PEG or glucose are not observed. PEGs have peaks between 530 and 1467 cm-1, with the most intense peak at 1115 cm-1, while glucose has numerous peaks between 619 and 1458 cm-1, with the most intense one at 1024 cm-1. Nevertheless, the possibility that a minor quantity of pore-forming agent be present is not discarded. On the other hand, the spectra of the samples with boron (Figure 2b) showed a broad band that is extended from 400 up to 1750 cm-1, in which various maxima are observed. This characteristic is the result of the overlapping of
the wide band of zirconia and those of boron species. In such zone, there is a similarity with the spectra of boric acid and boric oxide, which present the most intense peaks at 881 and 1190 cm-1, though the overlapped bands are slightly shifted with respect to those of such compounds.
Figure 2. FT-IR spectra of the samples obtained employing different pore-forming agents.
The SEM micrographs of all samples at low magnification indicate the presence of big and small particles, while lower-sized particles were mainly obtained when boron is added. Regarding the zirconia prepared using PEG 400 as pore-forming agent, it was observed in the micrographs with higher magnification that the porous structure of the original zirconia tends to disappear, obtaining a more compact structure (Figure 3).
For the support synthesized employing PEG2000, both the particles with and without boron addition maintained a spongy structure. In turn, the sample obtained adding PEG6000 presented somehow a more compact structure, even more closed when it was impregnated with boron. Finally, the sample obtained using glucose presented compact particles, both for the case of the support as well as for the sample impregnated with boron.
The titration with n-butylamine leads to comparatively estimate the strength of the acid sites present in the solids. The initial electrode potential (Ei) indicates the maximum strength of the sites, which can be classified according the following scale: Ei > 100 mV (very strong sites), 0 < Ei < 100 mV (strong sites), -100 < Ei < 0 mV (weak sites) and Ei < -100 mV (very weak sites) [Osiglio et al., 2010].
Figure 3. SEM Micrographs of the samples obtained using different pore-forming agents. Magnification: 1500x. Bar: 20 (jm.
In Figure 4a, it can be observed that, without boron addition, the S8G sample presented weak acid sites and only the S8P60 sample has very strong acid sites. It may be considered that, in this sample, the PEG chains in the sol form a reticular structure [Sun et al., 2006], interaction of the (-CH2CH2O-) groups of the PEG with hydroxyls of the hydrated zirconia occurs, obtaining a framework of the oxide with reticular morphology. In turn, with PEG2000, it can be proposed that the PEG chains in the sol form micelles, and the zirconia particles
surround them, thus giving aggregate-type morphology. On the other hand, using PEG400, the short helices of the polymer can lead to materials with something more closed morphology, which in turn results in a lower acidity. It can be remarked that preliminary experiments gave an indication that glucose presents a strong interaction with the hydrated zirconium oxide, and this may be the cause of the compact structure that leads to the lower acidity of the S8G sample.
In turn, all the borated zirconias presented very strong acid sites, though the sample obtained using glucose has a lower acidity (Figure 4b). This fact may be the result of the presence of the boron species evidenced in the FT-IR spectra, which must be well dispersed on the surface of the hydrated oxide according to what was observed by XRD, and that almost directly contribute to the acidity of the samples, because Bronsted acid sites can be generated by hydration of boron species, or the Lewis acidity of the zirconium ions can be increased by inductive effect [Ravindra et al., 2004].
0,5 1,0 1,5 2,0 0,5 1,0 1,5 2,0
meq amine/g
Figure 4. Potentiometric titration curves of the samples obtained employing different pore-forming agents.
4. Conclusions
The use of PEG as a pore-forming agent during the zirconia preparation gave efficient results, improving the textural as well as the acid properties of the samples, moreover in those samples in which boric acid was added as a zirconia modifier.
The lower effectiveness of the addition of glucose can be related to the presence of products resulting from the interaction of glucose with the hydrated oxide, which however must be in a low amount because they were not clearly evidenced in the FT-IR spectra.
The obtained results show that all the synthesized materials are solids with suitable textural and acid properties to be utilized as catalyst support or as catalysts in acid reactions.
Acknowledgements
The authors thank G. Valle, M. Theiller and E. Soto for their experimental contribution, and UNLP and CONICET for the financial support.
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