Synthesis, structure and some properties of zirconium phosphate/oxide support compositions

Journal of Thermal Analysis and Calorimetry, Jun 2012

Deposited zirconium phosphate samples on the base of silica and titania have been prepared using the sol–gel and mechanochemical methods. Porous structure, phase composition, and electrokinetic parameters have been studied by means of nitrogen adsorption–desorption, XRD, DTA-TG, FTIR, electrophoresis, and potentiometric titration. The compositions possess varied parameters of porous structure, structure of deposited phase, and electrokinetic properties depending on support nature and synthesis conditions.

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Synthesis, structure and some properties of zirconium phosphate/oxide support compositions

V. Sydorchuk 0 1 2 W. Janusz 0 1 2 S. Khalameida 0 1 2 E. Skwarek 0 1 2 J. Skubiszewska-Zieba 0 1 2 R. Leboda 0 1 2 V. Zazhigalov 0 1 2 0 J. Skubiszewska-Zieba R. Leboda Faculty of Chemistry, Maria Curie-Sklodowska University , Maria Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland 1 W. Janusz (&) E. Skwarek Department of Radiochemistry and Colloid Chemistry, Maria Curie-Skodowska University , Maria Curie-Sklodowska Sq. 3, 20-031 Lublin, Poland 2 V. Sydorchuk S. Khalameida V. Zazhigalov Institute for Sorption and Problems of Endoecology , NAS of Ukraine, 13 Naumov Str, Kiev 03164, Ukraine Deposited zirconium phosphate samples on the base of silica and titania have been prepared using the sol-gel and mechanochemical methods. Porous structure, phase composition, and electrokinetic parameters have been studied by means of nitrogen adsorption-desorption, XRD, DTA-TG, FTIR, electrophoresis, and potentiometric titration. The compositions possess varied parameters of porous structure, structure of deposited phase, and electrokinetic properties depending on support nature and synthesis conditions. - Acid salts of quadrivalent metals are the most frequently studied groups of inorganic exchangers [1, 2]. Amorphous and crystalline zirconium phosphates Zr(HPO4)2 (ZrP) also belong to this class and possess both its advantages and shortcomings [35]. Due to the high Bronsted acidity they are promising as acidic catalysts, adsorbents of cations, solid electrolytes, supports of the active phase [68] i.e., multifunctional materials. Besides, ZrP is characterized by large thermal and chemical stability, resistance to ionizing radiation. As for its ion-exchange properties, ZrP is insoluble in water, of large capacity, fast kinetics, and superior to organic exchangers. However, ZrP, first of all crystalline substances, exists as submicron particles and cannot be directly used in fixed-bed columns or any other flow-through systems because of the excessive pressure drops. Furthermore, crystalline ZrP has a low specific surface area. In contrast, amorphous ZrP is characterized by larger specific surface area and sufficiently large content of micropores which impairs its kinetic characteristics and reduces the efficiency of its application in sorption and catalysis. To overcome these drawbacks several ways have been suggested. The deposition onto granular supports with a large specific surface area and porosity is one of the most effective methods to improve ZrP properties. Among the used carriers, organic polymers can be as primarily identified [911]. At the same time, it is known that inorganic porous materials possess greater mechanical strength and do not swell in liquid media. Therefore, they are preferred as supports. Different silicas are most often used as matrices for ZrP [2, 1221]. There are some examples of the application of other inorganic materials as carriers for ZrP: active carbon [22], ceramic [23], alumina [24]. Liquid grafting to the silica surface, exfoliation of crystalline forms of ZrP by different silanes, and precipitation of ZrP on outer and inner surfaces of supports are used as a rule of deposited ZrP, synthesis. Only one work is devoted to ball milling procedure, namely for deposition of ZrP onto alumina carrier [24]. The content of deposited or incorporated ZrP in compositions is varied between 10 and 50% w/w and depends on the distribution process. On the other hand, their physico-chemical properties are also determined by the fraction of ZrP in the composition. For example, according to [14], deposited ZrP showed a maximum of ion-exchange capacity with the ZrP content about 20% w/w whereas the conductivity has the maximum at 25% w/w. The last maximum is located with 2030% w/w of inorganic oxides according the data in [24]. As a rule, the specific surface area of the silica based compositions is increased with increasing ZrP percentage [14] or passes through a maximum at 21% w/w of ZrP [18]. On the other hand, the specific surface area of c-ZrP in the composition with silica increases from a few m2 g-1 to 191 m2 g-1, as a result of exfoliation [17]. This c-ZrP/SiO2 composite exhibited a higher conversion rate of ethylbenzene to styrene (about five times) compared with bulk un-exfoliated c-ZrP. Literature review points out to the enhancement of physico-chemical characteristics of supported ZrP and the prospects of its use in catalytic, adsorption, and other processes. However, the studies are not systematic. Therefore, from our point of view, it is necessary to develop other approaches for its preparation and extend the number of potential supports. In this study, we propose some new procedures for synthesis of deposited ZrP: application of dry and wet mechanochemical deposition; deposition of ZrP in the form of dried xerogel and wet hydrogel; solgel co-precipitation of silica and ZrP by orthophosphoric acid; use of fumed oxides (silica and titania) as supports. The investigation of some physico-chemical properties of the prepared deposited samples (specific surface area and porosity, surface structure, and electrokinetics) is another task of the presented study. Fumed silica (aerosils A50 and A380) and titania (Oriana, Ukraine) with the specific surface area 45, 346, and 64 m2 g-1, respectively, were utilized as supports for preparation of deposited samples. Aerosils are amorphous and titania consists of phases of anatase and rutile. Synthesis procedures The procedures of ZrP deposition (20% w/w) were as follows: (1) Solgel co-precipitation, 4.427 g of ZrOCl2 8H2O were dissolved in 16.5 mL of water (solution 1). 40.5 mL of TEOS were mixed with 18 mL ethanol while stirring for 10 min (solution 2). Solution 1 was added dropwise to solution 2 while stirring for 10 min (mixed solution 3). 19 mL of 1 M aqueous solution of orthophosphoric acid dropwise were added to mixed solution 3 for 10 min. Then agitation stopped. Hydrogel was formed for 10 min. It was aged at 20 C for 20 h. Then the aged gel was washed with water until the absence of Cl- ions in the rinsing water and dried in air at 20 C for 72 h. The part of washed wet gel was subjected to hydrothermal treatment (HTT) at 200 C for 3 h. (2) Appropriate amounts of reagents, namely ZrP dried xerogel or hydrogel with 88% w/w humidity and fumed titania or aerosils A-50 (A-380), were subjected to mechanochemical treatment MChT (milling) at 300 rpm for 0.5 h using Pulverisette-7 (Fritsch Gmbh). The milling was carried out in air (dry milling) or in water (wet milling). Water was not added to the mill when ZrP hydrogel was used. The bulk ZrP, that used while milling, was prepared through precipitation of H3PO4 with aqueous solutions of ZrOCl2 8H2O as in p. 1. The formed colloids after wet milling were dried at 20 C for 72 h. Study of physico-chemical characteristics The starting reagents and products of their mechanochemical transformations were studied by means of X-ray powder diffraction (XRD) using a diffractometer Philips PW 1830 with CuKa-radiation. The DTA and TG curves were recorded using the apparatus Derivatograph-C (F.Paulik, J.Paulik, L.Erdey) in the temperature range 20700 C at the heating rate 10 C min-1. The FTIR spectra in the range 4000400 cm-1 were registered using the spectrometer Spectrum-One (Perkin-Elmer). The ratio of sample and KBr powders was 1:20. KBr was dried at 600 C for 2 h before measurements. The parameters of porous structure (specific surface area S, sorption pore volume Vs, micropores volume Vmi, mesopores volume Vme) were determined from the isotherms of low-temperature nitrogen adsorption obtained by means of the analyzer ASAP 2405 N (Micromeritics Instrument Corp). Outgassing temperature and duration were 150 C and 2 h, respectively. Meantime, the value of Vs was determined with the relative nitrogen pressure close to 1, the values of Vmi and Vme were calculated using the tmethod and BJH-method, respectively. The total pore volume VR was determined by impregnation of the samples, dried at 150 C, by liquid water. The following relations are valid between these parameters: Vs Vmi Vme where Vma the volume of macropores which are not filled in the process of adsorption from the vapor phase, but by impregnation from the liquid phase. The pore diameter dPSD was calculated from the curves of pore size distribution (PSD) using the desorption branch of isotherms accordingly to the BJH-method. Electrokinetic measurements Electrophoresis studies were carried out using a Zetasizer 3000 (Malvern Instruments). The suspensions of oxide (100 ppm of solid) were prepared using an ultrasonic disperser (Sonicator Misonix Inc.) before the electrokinetic measurements. The zeta potential f of the system has been determined as a function of pH for 0.001 mol dm-3 solution of NaCl. The pH values, measured by a precision digital pH meter, were adjusted by the addition of 0.1 mol dm-3 HCl or NaOH solutions. The zeta potential was calculated from electrophoretic mobility using the Smoluchowski equation. Determination of surface charge density The surface charge of titanium dioxide was calculated by comparison of the potentiometric titrations curve of the oxide suspension and the background electrolyte. These titrations of ZrP suspensions in 0.001 mol dm-3 solution of NaCl were carried out in the thermostatic Teflon vessel, under nitrogen atmosphere free of CO2, at 25 C. Measurements were performed using a PHM 240 Radiometer Research pH meter with K401 as a glass electrode and G202B as a calomel reference electrode. Potentiometric titration was carried out with the use of automatic burette Dosimat (Metrohm). The point zero of charge was determined from of surface charge density as a function of pH, pHpzc was equal to pH, when r0 = 0. Results and discussion Porous structure of compositions The isotherms of nitrogen adsorptiondesorption for the deposited samples prepared using different methods are presented in Figs. 1, 2, and 3. The parameters of porous structure which were calculated from the adsorption desorption isotherms are presented in Table 1. One can see that the initial bulk ZrP is micro-mesoporous (sample 1). Its dry mechanochemical deposition in the form of xerogel on aerosil A-380 leads to the increase of specific surface area of the resulted composition (sample 2). However, this sample can be considered as non-porous because initial support (A-380) itself is superfine and non-porous and remains in that state in the process of dry milling [25]. The use of a more coarse dispersed support, namely fumed TiO2, results in formation of rigid porous compositions as a result of dry and wet milling (samples 3, 4). In both cases milled samples contain mesopores with two maxima in the same position which are observed on the PSD curves. It can be seen that the PSD curve, showing the characteristic of the sample prepared by wet milling, has no sharp peaks. At the same time, the fraction of mesopores (column 6) for this sample is small (about 16%) but the content of macropores is prevalent (column 8). Because of this its specific surface area is three times lower compared with that of the sample subjected to dry milling. It should be noted that isotherms of nitrogen adsorptiondesorption obtained for the compositions, based on titania, contain a weakly pronounced capillar-condensation hysteresis loop (Fig. 1). Deposition of ZrP in the form of wet hydrogel leads to formation of meso-macroporous compositions with different contents of indicated pores and high value of total pore volume (samples 57). Meantime, the sample based on more dispersed A380 possesses maximal value of Vme while that prepared from lesser dispersed A50 has a maximal value of Vma. The isotherms obtained for the compositions prepared from ZrP hydrogel relate to type IV and contain a capillar-condensation hysteresis loop of type A (Figs. 1,2). It is noteworthy that the PSD curve for the composition based on A380 has one maximum but those for the samples based on lesser dispersed A50 and TiO2 possess two maxima (Figs. 1, 2). The latter is better than for the sample on the basis of TiO2 (Fig. 1 curve c). Fig. 1 Isotherms of nitrogen adsorptiondesorption and curves of PSD for the bulk ZrP (a) and the milled samples based on TiO2: ZrP xerogel air (b), ZrP xerogel water (c), ZrP hydrogel (d) Fig. 3 Isotherms of nitrogen adsorptiondesorption and curves of PSD for the bulk ZrP (a) and the samples prepared via the solgel method: 10% w/w ZrP at 20 C (b), 20% w/w ZrP at 20 C (c), and 200 C (d) It can also be seen that embedding of ZrP moieties in the siliceous framework in the stage of gelation results in decrease of both specific surface area S and volume of sorption pores Vs, and micropores Vmi (samples 8,9 in comparison with sample 11). In fact, the mesopores volume and size (Vme and dPSD, respectively) as well as the total pore volume VR are not changed. However, the value of VR exceeds that of Vs. The latter indicates formation of macropores in the composite samples. Soft HTT in the hydrogel stage unexpectedly leads to the sharp growth of S, Vs, Vme, and VR (sample 10). At the same time, micropores almost disappear. In other words, more open and larger porous, namely mesoporous, structure, which is more accessible for sorbate molecules, is formed. This can be seen from the change of the shape of adsorptiondesorption isotherms (Fig. 3) and the curves of PSD that are Fig. 2 Isotherms of nitrogen adsorptiondesorption and curves of PSD for the bulk ZrP (a) and the milled samples based on aerosils: ZrP xerogel air on A380 (b), ZrP hydrogel on A380 (c), ZrP hydrogel on A50 (d) constructed based on the isotherms (Fig. 3). Indeed, the curves of PSD for untreated samples 8 and 9 have a broad maximum around 2.7 nm, while a distinct peak at 3.7 nm is on the PSD curve for modified sample 10. It should be added that the above-described patterns of change of the porous structure largely coincide with those previously obtained for the heteropolycompounds/oxide supports compositions [26, 27]. Structure of deposited samples In accordance with the X-ray fluorescent analysis, bulk and deposited ZrP has the atom ratio Zr/P = 2:1, which corresponds to the compound Zr(HPO4)2. The XRD data show that all supported samples synthesized based on silica are X-ray amorphous. At the same time, diffractograms of the samples based on fumed titania contain reflexes attributed to anatase and rutile, i.e., they relate only to support. In fact, intensity of the reflections of support and their ratios somewhat differ for the prepared compositions. Therefore, all deposited samples contain amorphous Zr(HPO4)2. It should be added that initial bulk ZrP is also X-ray amorphous. The DT- and TG analyses indicate thermal stability of the amorphous component up to 800 C. The DTA and TG curves obtained for the deposited samples show only the final effect of adsorbed water removal in the temperature range 20200 C and have no effects related to phase transformations. The FTIR spectra depicted in Figs. 4, 5, and 6 confirm the presence of ZrP in the structure of compositions. Thus, the spectrum of bulk ZrP contains a. and b. centered around 425, 510, 595, 710, and 1038 cm-1 (Fig. 4, curve a) which are attributed to deformation and stretching vibrations of PO bonds in PO43- [4, 28, 29]. The embedding of ZrP into the siliceous framework during the solgel synthesis results in shift of a.b. 510 to 528 cm-1 and a.b. 1038 cm-1 toward 980 cm-1 (Fig. 4, curve b). The latter may be a consequence of the interaction between ZrP and the surface groups of support in narrow pores the content of which is the largest in this sample. The changes of position of indicated a.b. are smaller and lesser for samples prepared at 200 C (curve c): thus, a.b. 1038 cm-1 shifts to 1000 cm-1. The latter may be because of the fact that the hydrothermally modified sample possesses larger pores as mentioned above. In contrast, interaction of deposited ZrP with nonporous fumed oxides obviously is minimal. At least, this interaction does not appear in spectra of the samples synthesized via milling (Figs. 5, 6): the shift of a. and b. which are characteristic of ZrP is not observed. Moreover, these bands are overlapped by the intense absorption which is attributed to supports. This can be explained by larger porous structure of the matrices, which are formed from fumed oxides compared with the samples prepared Table 1 Influence of synthesis conditions on porous structure resulted compositions Zr(HPO4)2 bulk, precipitation from ZrOCl2 and H3PO4 20% Zr(HPO4)2/A-380, MChT of Zr(HPO4)2 xerogel on air, 300 rpm 0.5 h 20% Zr(HPO4)2/fumed TiO2, MChT of Zr(HPO4)2 xerogel on air, 300 rpm 0.5 h 20% Zr(HPO4)2/fumed TiO2, MChT of Zr(HPO4)2 xerogel in H2O, 300 rpm 0.5 h 20% Zr(HPO4)2/A-380, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 20% Zr(HPO4)2/A-50, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 20% Zr(HPO4)2/fumed TiO2, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 10% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS 20% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS 20% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS and HTT of hydrogel 200 C 3 h SiO2 from TEOS, solgel 5 9 0 c 1 Fig. 4 FTIR spectra for bulk ZrP (a) and the deposited samples prepared via the solgel method at 20 C (b), 200 C (c) via the solgel route. This can be clearly seen in Table 1 (column 9). Electrokinetic properties Very important parameters characterizing the electric double layer are isoelectric point pHiep and point of zero charge pHpzc. Assuming that the potential of diffuse layer equals the zeta potential, the concentration of surface groups negatively charged in pHiep point equals of surface groups which are positively charged. pHiep of bulk and some supported ZrP as a function of pH in the 0.001 M aqueous NaCl solution is depicted in Figs. 7, 8. The values of pHiep determined for all samples are collected in 2500 2000 1500 Wavenumber/cm1 Fig. 5 FTIR spectra for the milled deposited samples based on aerosils: ZrP xerogel air on A380 (a), ZrP hydrogel on A380 (b), and A50 (c) Table 2. As can be seen a zeta potential reduces with the pH increase in all cases. It has been also found that nature of support and procedure of ZrP deposition have great influence on pHiep value. The differences between the samples are expressed as in the course of zeta potentialpH depencence and in the position of the isoelectric point. The value of pHiep for bulk ZrP equal to 3.65 is consistent with literature [30]. For pure supports prepared under the same conditions, as deposited samples, pHiep has the following positions: 2.02, 2.30, 2.61, and 2.53 for milled A380, A50, titania and silica synthesized using the solgel technique, respectively. The presented values also correspond to literature data [30, 31]. It is known that Fig. 6 FTIR spectra for the bulk ZrP (a) and the milled samples based on TiO2: ZrP xerogel air (b), ZrP xerogel water (c), ZrP hydrogel (d) Fig. 7 The f potentialpH dependence for the samples ZrP/A380 Fig. 8 The f potentialpH dependence for the samples ZrP/TiO2 mixed (two-component) systems may exhibit isoelectric point values that are intermediate to those of the corresponding pure components [32]. So, in column 4 of Table 2 the pHiaedp values there are presented for the supported samples calculated by the additive rule. From Table 2 one can see that for most deposited samples (except one) pHiep, determined experimentally, is smaller than that for bulk ZrP but larger compared with the pHiaedp values. Only for the compositions prepared through milling of ZrP xerogel with titania (samples 3,4) pHiep and pHiaedp coincide. These samples have a minimum value of pHiep close to pHiep of pure support. The magnitude of difference between pHiep and pHiaedp obviously depends on the extent of coverage of support surface with ZrP. Based on this assumption, one can conclude that the surface of titania is covered with ZrP to the smallest extent. At the same time, the use of nonporous support possessing a maximal specific surface area (namely A380) leads to the increase of pHiep of the formed composition and improved coverage of the support surface. The deposition of ZrP in the form of wet hydrogel brings the same results (samples 57). The supported samples prepared using solgel procedure display an average effect (samples 810). Surface charge density at the deposited sample/ electrolyte solution interface Figures 9, 10 show the relationships of surface charge density versus pH for the electrolyte concentration 0.001 mol dm-3 for some samples. As one can possibly notice, the surface charge density r decreases with the pH increase in all cases. However, the surface charge density is rather sharply and uniformly reduced for bulk ZrP. At the same time, this parameter decreases very slowly in the range from pHpzc to pH = 7 for the silica based samples (Fig. 9). Fast jump of r is observed over this pH value. It is characteristic of SiO2/solution system [33]. The course of surface charge densitypH dependences obtained for the TiO2 based compositions is closer to that for bulk ZrP (Fig. 10). On the other hand, the point of zero charge pHpzc is approximately equal to 3.45 for bulk ZrP and all studied deposited samples independent of the nature of support and method of their preparation. The values of pHpzc and pHiep of the ZrP sample are very close(within 0.2 pH unit) that may testify that its surface is free of alkali or acid contaminants [34]. The deposited samples of ZrP on fumed TiO2 show different pHiep for ZrP hydrogel deposited on fumed TiO2 is 0.2 pH unit below pHpzc, whereas for xerogels in air and xerogels in H2O is below pHpzc 0.65 and 0.8 pH unit, respectively. This indicate that deposition of ZrP xerogel in air or in H2O on the surface of the samples there appeared groups of acidic character. However, small increase in the 0 5 10 15 2 ) 20 /cm25 (C30 ity 35 sen 40 ed 45 g rah 50 ce 55 c fra 60 uS 65 70 75 80 85 Zr(HPO4)2 bulk, precipitation from ZrOCl2 and H3PO4 20 Zr(HPO4)2/A-380, MChT of Zr(HPO4)2 xerogel on air, 300 rpm 0.5 h 20% Zr(HPO4)2fumed TiO2, MChT of Zr(HPO4)2 xerogel on air, 300 rpm 0.5 h 20% Zr(HPO4)2/fumed TiO2, MChT of Zr(HPO4)2 xerogel in H2O, 300 rpm 0.5 h 20% Zr(HPO4)2/A-380, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 20% Zr(HPO4)2/A-50, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 20% Zr(HPO4)2/fumed TiO2, MChT of Zr(HPO4)2 hydrogel, 300 rpm 0.5 h 10% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS 20% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS 20% Zr(HPO4)2/SiO2, solgel from ZrOCl2 ? TEOS and HTT of hydrogel 200 C 3 h Fig. 9 The surface charge densitypH dependence for the samples ZrP/A380 Fig. 10 The surface charge densitypH dependence for the samples ZrP/TiO2 surface charge in the sample of ZrP/fumed TiO2 xerogel/ electrolyte interface indicated that surface concentration of these groups is lower than that for sample prepared in H2O. As mentioned above, the surface charge density as a function pH of ZrP deposited on A-380 shows similar characteristic to that of the silica/electrolyte system. The comparison of pHpzc and pHiep for these systems is very difficult because the slope of surface charge as a function of ZrP/A-380 samples is very low (as in the case of silica). However, the shift of pHiep shows that for these samples on the surface, the groups with acidic properties also appear. Therefore, it can be assumed that in the deposited samples ZrP itself influences on the surface charge more than a support. The synthesis of the zirconium phosphate/oxide support compositions was carried out using two procedures: joint milling of ZrP with the support and co-precipitation of ZrP and silica during the solgel process. Deposited mesomacroporous or micro-mesoporous samples are formed using the mechanochemical and solgel methods, respectively. Amorphous zirconium phosphate located in pores interacts with varying force with the surface of supports depending on porous structure. The degree of support surface coverage is determined by nature of support and methods of zirconium phosphate deposition. The zeta potential of supported samples based on titania has a value close to the zeta potential of pure support while the samples prepared via deposition of zirconium hydrogel on aerosils possess the zeta potential equal to that for bulk zirconium phosphate. At the same time, the surface charge density of all supported samples is exceptionally determined by that of zirconium phosphate. Acknowledgements This study was supported by the European Community under a Maria Curie International Research Staff Exchange Scheme (IRSES), Project No 230790. Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.


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V. Sydorchuk, W. Janusz, S. Khalameida. Synthesis, structure and some properties of zirconium phosphate/oxide support compositions, Journal of Thermal Analysis and Calorimetry, 2012, 1009-1016, DOI: 10.1007/s10973-011-1978-7