J. MATER. CHEM., 1991, 1(4), 681-684 68 1 Sol-Gel Synthesis of Zr(HP0,),*H20 Hafida Benhamzab, Philippe Barboux,*a Ahmed Bouhaouss,b Francois-Andre Josiena and Jacques Livage" a Chimie de la Matiere Condensee, Universite P. et M. Curie, Paris, France Laboratoire de Chimie Physique, Universite Mohammed V, Rabat, Maroc Inorganic ion exchangers such as a-Zr(HPO,),*H,O are usually synthesized from aqueous solution. Amorphous precipitates are then obtained, which crystallize very slowly upon ageing at ca. 90°C in their mother liquor. Crystallization could be much faster if alkoxide precursors are used. The sol-gel synthesis of a-Zr(HPO,),-H20 from both inorganic and metallo-organic precursors has been followed by solid-state 31PMAS NMR and X-ray diffraction. Condensation mechanisms leading to the formation of a solid phase are discussed. Compared to ZrOCI, in aqueous solution, zirconium mpropoxide allows shorter reaction times and lower crystallization temperatures.This results in a material with smaller grains which should exhibit enhanced exchange properties. Keywords: Alkoxide; Sol-gel processing; Ion exchange; Phosphate; Nuclear magnetic resonance spectroscopy Acid phosphates of tetravalent metals have been extensively Experimental studied since the discovery of their ion-exchange proper tie^.'-^ The aim was to make inorganic exchange membranes whose Acid zirconium phosphates were prepared by adding H3P04 chemical resistance toward strongly acidic and basic solutions (85%) to an equal volume of the precursor solution (1 mol was expected to be higher than that of the equivalent organic dmP3).In both cases a white gelatinous precipitate is obtained materials. as soon as the solutions are mixed together. Precipitates are These phosphates correspond to the general formula kept in their mother liquor for different times at different M(HP04)2* H20 where M is a tetravalent cation (Zr, Ti, Hf, temperatures as indicated below in order to follow the crys- Ge, Sn, Pb). The a-crystalline form exhibits a layered structure tallization process. The resulting precipitate is then filtered, of planar macroanions [M(PO,),]~"-. The negatively charged washed with distilled water and dried in air at ca.60 "C. oxygen atoms are compensated by an equivalent number of 31PMAS NMR spectra were recorded at room temperature protons forming weak PO-H bonds3 These acid protons with a MSL400 Bruker spectrometer at a frequency of can be easily replaced by other cations without any structural 161.98 MHz.Samples were spun at a rate of ca. 5 kHz. change of the layered host lattice. IR absorption experiments were performed with a Perkin- Zirconium phosphates are usually synthesized by reacting Elmer 610 continuous wave spectrometer. Solid samples were concentrated phosphoric acid (H3P04, 4 mol dm-3) with an pressed into KBr pellets. TG of powders dried at 60 "C was performed in air at a aqueous solution of a zirconium salt such as ZrOC1,. A heating rate of 10 "Cmin-'.gelatinous amorphous precipitate is readily obtained.Slow crystallization then occurs when the precipitate is heated under reflux in its mother liquor. After 2 weeks of reflux, crystallites of ca. 1 pm in diameter are obtained.' Results One of the main drawbacks of aqueous solutions is that X-Ray Diffraction water is both a solvent of the inorganic salt and a chemical reagent for the hydrolysis of metal cations. It is therefore The X-ray diffraction pattern of powders obtained from rather difficult to control the hydrolysis and condensation of propoxide precursors after 1 h at 60 "C exhibits only broad aqueous molecular precursors. It would therefore be interest- peaks corresponding to the a-Zr(HPO,), * H20 phase. Crys- ing to look for more versatile precursors. tallization occurs within 1 day at 60 "C [Fig.l(a)J. It can also In the framework of the so-called sol-gel process, metal be obtained at room temperature after a few weeks when the alkoxides are known to offer many advantages despite their precipitate is left in its mother liquor. Crystallization occurs higher cost.4 (i) Metal alkoxides can be dissolved in their progressively. It is therefore not possible to define an exact parent alcohol so that hydrolysis may be carefully controlled crystallization time since it depends on the criterion of crystal- by adding small amounts of water diluted in the alcohol. (ii) line quality. The molecular structure and the functionality of these precur- Precipitates obtained from ZrOC1, in water remain sors can be designed by adding nucleophilic modifiers.It amorphous even after 1 week at 60 "C. Their X-ray diffraction therefore becomes possible to control the hydrolysis and patterns then exhibit broad peaks similar to those observed condensation reactions in order to tailor the morphology and with the alkoxide precursor after 1 h [Fig. I@)]. Crystalliz-structure of the resulting material. ation has never been observed at room temperature even after Although the classical synthesis of cr-Zr(HP04)2 *H20 from several months. aqueous solutions is very simple, alkoxide precursors also offer an opportunity to study the chemical substitution of alkoxy groups by inorganic nucleophilic reagents such as "P MAS NMR Spectroscopy PO2-. This paper reports on the reaction of phosphoric acid (85Y0 More accurate information on the local environment of in water and anhydrous) with the following zirconium precur- phosphate groups has been obtained by solid-state 31Pmagic-sors: ZrOC1, in aqueous solution; zirconium propoxide, angle-spinning NMR.The spectra are reported in Fig. 2. Zr(OPr"),, in propan-1-01. Chemical shifts are referenced to H3P04 (85%). Surprisingly, J. MATER. CHEM., 1991, VOL. 1 A B & > 10 20 30 40 10 20 30 40 61" 01" Fig. 1 X-Ray diffraction pattern of Zr(HPO,),-H,O precipitated from: A, Zr(OPr"),; B, ZrOC1, and heated for different times, (a) 1 h, (6) 1 day, (c) 8 days, at 60 "C in its mother liquor _h - I I I I I 1 1 I I 20 10 0 -10 -20 -30 -40 -50 -60 8 (PPW B -22.3 b. ---.-< 1 I I I I I I I I 20 10 0 -10 -20 -30 -40 -50 -60 6 (PPW Fig.2 31P MAS NMR spectra (shifts relative to H3PO4) for Zr(HP04)2. H20 precipitated from: A, Zr(OPr"),; B, ZrOC1, and heated for different times (a) 1 h, (6) 1 day, (c) 8 days, at 60 "C in its mother liquor. Peaks marked with * are rotation bands. Minor peaks are found around 0.0, -7.0, -14.0 and -27 ppm as discussed in the text even amorphous samples give sharp peaks, showing that phosphate groups have a rather well defined co-ordination. The NMR spectra of amorphous powders precipitated from aqueous solutions exhibit a main peak at -22.3 ppm [Fig. 2(b), t= 1 h]. Some smaller peaks are also observed on each side (6=0, -7, -14 and -27 ppm). The positions of all these peaks remain unchanged to within kO.2 ppm, which is the experimental error, when precipitates are heated at 60 "C in their mother solution.Small peaks decrease slightly in intensity while the main one does not change. The behaviour of powders precipitated from alkoxide pre- cursors is somewhat different. A main peak is again observed at -22.3 pprn with a shoulder at -19.3 ppm [Fig. 2(a), t= 1 h]. As in the case of aqueous precursors, smaller peaks can be seen on both low- and high-field sides (around 6=0.0, -7.0, -14.0 and, as traces, -27.0 ppm). These small peaks rapidly disappear upon ageing. The peak at -19.3 ppm progressively increases in intensity while the one at -22.3 ppm decreases. A single sharp peak at 19.1 ppm is observed after 1 week at 60 "C.This peak is characteristic of the crystalline C~-Z~(HPO,)~H20 phase.It obviously corresponds to a phos- phate group HPOi- triply bound to three different zirconium atoms. The small variation of the chemical shift from -22.3 ppm to 19.3 ppm after 8 h and -19.1 ppm after 1 week could be due to some modification in the second co-ordination shell of the phosphorus atom or some variation in the P-0-Zr angle. Smaller peaks which are visible in the 31P NMR spectra of amorphous samples could be assigned to phosphate groups bonded to one (6% -7ppm), two (6z -14 ppm) or even four (6%-27 ppm) zirconium atoms. Such behaviour would agree with a previous study performed on phosphatoantimonate compounds showing that the chemi- cal shift is displaced toward high fields when the connectivity of phosphate groups increase^.^ However, chemical shifts close to 0 and +1 ppm have been observed for other compounds such as KTiOPO,, in which phosphate groups are bonded to four titanium atoms.Connectivity should therefore not be the only parameter to be taken into account. The large distortion of the [PO,] tetrahedron in the KTiOPO, structure may also be responsible for the observed chemical shifts- Actually, the 31Pchemical shifts mainly depend on the amount of 0 and R bonding. They are therefore very sensitive to 0-P-0 angles.6 Infrared Absorption Infrared spectra of precipitates obtained from alkoxide precur- sors do not exhibit any vibration band corresponding to organic groups (Fig.3). This indicates that all alkoxy ligands have been completely removed by hydrolysis or phosphatis- ation and points out the high reactivity of alkoxide precursors. J. MATER. CHEM., 1991, VOL. 1 I I 4000 3000 2000 1600 1200 800 400 wavenurnber/crn -' Fig. 3 IR absorption spectra of zirconium phosphates precipitated from zirconium alkoxide after (a) 1 h (amorphous) and (b) 8 days (crystallized) Vibrations corresponding to phosphate groups are located between 1200 and 900 cm-'. They become sharper when crystallization improves. Two broad peaks at 2400 and 1250 cm- are also seen with amorphous samples. They can be assigned to P-0-H stretching and bending vibrations, re~pectively.~The peak at 2400 cm -almost disappears in the well crystallized samples although all other vibrations tend to sharpen.This could be due to the fact that P-OH groups form stronger hydrogen bonds resulting in a broaden- ing of the vibration band. Such a modification of the infrared absorption spectrum may be related to the slight change Observed in the main peak Of the 31pNMR spectrum that shifts from -22.3 to -19.1 ppm. Such a shift could also arise from some modification of the hydrogen-bond network. The infrared 'Pectra Of Powders Precipitated from aqueous ZrOC12 do not change 'POn ageing*They remain similar to that of Fig. 3(a)even after 1 week at 60 "C. Thermal Analysis TG curves are shown in Fig. 4. All samples precipitated from ZrOC1, aqueous solutions exhibit a single progressive weight loss up to 600 "C.According to literature such behaviour is typical of amorphous phosphates. Different behaviour is -5 = -10OI 0 200 400 600 800 1000 T/"C Fig. 4 Thermogravimetric analysis under ambient oxygen of zir- conium phosphate powders precipitated from: A, Zr(OPr"),; B, ZrOC1, after thermal treatment for different times (a)0, (b) 1 day, (c) 8 days, (d) 4h, at 60 "C observed with powders precipitated from alkoxides. Two weight losses are clearly visible even after a reflux of 4 h only. This is typical of the thermal decomposition of crystalline a-Zr(HP04),*H20. The first step at ca. 150 "C corresponds to the departure of intercalated water molecules, whereas the second one at ca. 550 "C corresponds to the condensation of phosphate groups into pyrophosphates as follows: Zr(HP04), *H20 -+ Zr(HP04), +H20 + ZrP207+H20 The total weight loss gives the amount of water in the precipitate.It appears to be larger for amorphous samples. This must be related to the larger surface area of these samples which are known to exhibit better exchange capacities.2 Electron Microscopy Electron micrographs of crystalline a-Zr(HPO& H20 pow-ders (Fig. 5) illustrate the effect of the thermal treatment required to obtain crystalline phases. Well crystallized samples are obtained from zirconium propoxide precursors after heat- ing the precipitate for 8 days at 60 "C. A longer thermal treatment is required to obtain the same result from aqueous solutions of ZrOC1, (t=1 week, T= 95 "C).Fig. 5 shows that crystallites are significantly smaller in the first case. Faster exchange reactions and larger capacities are therefore expected with phosphates precipitated from alkoxide precursors. Discussion Molecular Structure of the Precursors Zirconium alkoxides usually exhibit oligomeric structures via bridging alkoxide groups. Solvation by alcohol molecules is also observed in order to expand the co-ordination state of zirconium. The molecular structure of the precursor then depends on parameters such as the steric hindrance of the Fig. 5 Transmission electron micrographs of zirconium phosphate particles precipitated from: (a) Zr(OPr), and heated at 60 "C for 8 days in propanol; (b) ZrOC1, and heated at 95 "C for 8 days in water alkoxy ligand, the nature of the solvent and, of course, the concentration.According to the literature, dimers or trimers should be formed mainly when zirconium n-propoxide is dissolved in propan- l-ol. Zirconium atoms are hexaco-ordi- anted in these oligomers.8 ZrIV ions are known to form tetrameric species in aqueous solutions at low pH, [Zr4(OH)8(H20)16]8+. Zirconium atoms are co-ordinated by eight ligands, four bridging OH groups and four solvating water molecule^.^ Aqueous molecular precursors are thus more condensed and zirconium exhibits a higher co-ordination. They should therefore be less reactive than metal alkoxides. Hydrolysis and Complexation Zirconium precursors are very prone to nucleophilic reactions, and therefore may react with the nucleophilic species present in the solution, namely water molecules and phosphate groups.Hydrolysis and phosphatisation mainly depend on the positive charge of the zirconium atom, the nucleophilic power of the entering species and the ability of a protonated group to be released. (HPO,)' -groups are obviously better nucleophiles than water molecules so that in both cases, zirconium precur- sors should react with phosphoric acid rather than with water. The possibility of phosphatisation occurring prior to hydroly- sis was investigated in the case of titanium phosphates by dissolving anhydrous H3P04 (solid) in propan- l-ol. No esteri- fication was observed even after 1 week as evidenced by liquid 31PNMR. This anhydrous phosphoric acid solution reacts readily with titanium propoxide giving a precipitate.The 31P NMR spectrum of this precipitate is just the same as that of the precipitate obtained by adding an aqueous solution of H3P04 (85%).lo Chemical analysis on this precipitate dried under vacuum indicated the presence of only 0.2 of a carbon atom per titanium atom and a ratio P:Ti of 2.0, showing that alkoxy groups are removed by phosphate groups without hydrolysis. The complexing power of H3P04 towards alkoxides is greater than for aqueous precursors. This mainly arises from the ability of alkoxy groups to be protonated and released in the solution as follows: -Zr-OPr +H-OPOa- +[-Zr-0PO3l2- +ROH which is easier than -Zr-OH+H-OPO~-+[-Zr-OP03]2- +H,O The complexing phosphate group still exhibits other P-OH acidities.It can react with other zirconium precursors leading to the formation of Zr-0-P-0-Zr bridges until J. MATER. CHEM., 1991, VOL. I three P-OH have reacted. A solid network is then built in which phosphate groups are bonded to three different zir- conium atoms. Such a reaction is not so easy with aqueous solutions in which zirconium precursors are already hydrolysed giving rise to tetrameric hydroxylated species. Protonation of bridging OH groups by phosphoric acid is more difficult. Therefore, heating is required to favour this reaction in order to build a zirconium phosphate network. Alkoxide precursors react faster with phosphoric acid, and precipitates of smaller grain size are obtained at lower tem- peratures.Moreover, zirconium alkoxides would react with a wide range of nucleophilic species such as carboxylic acids or fi-diketones." It should therefore be possible to design these molecular precursors in order to tailor their functionality and chemical reactivity. Such molecular engineering is already currently performed in the sol-gel synthesis of glasses and ceramics uia chemical modification of metal alkoxides. Conclusions Alkoxide precursors appear to give better results than aqueous solutions of ZrOC1,. Crystallization of m-Zr(HP04), H,O is obtained faster and at lower temperatures. IR absorption and 31 P NMR experiments suggest that the phosphate groups are similar in all amorphous precipitates.This seems reasonable as phosphoric acid is used as a precursor in both cases. The main differences should then arise from the reactivity of zirconium precursors towards water (hydrolysis) and phos- phoric acid (complexation). References 1 A. Clearfield, in Inorganic Exchange Materials, CRC Press, Boca Raton, 1981, and references therein. 2 A. Clearfield, A. Oskarsson and C. Oskarsson, Zon Exch. Membr., 1972, 1, 9. 3 A. Ahrland and J. Albertson, Acta Chem. Scand., 1969, 23, 1446. 4 J. Livage, M. Henry and C. Sanchez, N. J. Chem., 1990, 14, 513. 5 F. Taulelle, C. Sanchez, J. Livage, A. Lachgard and Y. Piffard, J. Phys. Chem. Solids, 1988, 49, 299. 6 J. H. Letcher and J. R. Van Wazer, J. Chem. Phys., 1966,44,815. 7 S. E. Horsley, D. V. Nowell and D. T. Stewart, Spectrochim. Acta, Part A, 1974,30, 535. 8 D. C. Bradley, Coord. Chem. Rev., 1967, 2, 299. 9 G. M. Muha and P. A. Vaughan, J. Chem. Phys., 1960, 33, 194. 10 C. Schmutz, E. Basset, P. Barboux and J. Livage, unpublished results. 11 C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst. Solids, 1988, 100, 65. Paper 1/01503F; Received 28th March, 1991