J O U R NA L O F M AT E R I A L S S C I E N C E L E T T E R S 1 7 ( 1 9 9 8 ) 1 8 3 9 ± 1 8 4 2

Effect of reaction parameters on the hydrolysis of zirconium propoxide.

A study by infrared spectroscopy

F. RUBIO, J. RUBIO, J. L. OTEOInstituto de Ceramica y Vidrio. (C.S.I.C.), Arganda del Rey, Madrid, SpainE-mail: frubio@icv.csic.es

It has been well established that certain parametersaffect the morphology of zirconium oxides obtainedby hydrolysis and polymerization reactions ofzirconium alkoxides [1]. Yoldas has carried out animportant study of such parameters and concludedthat the type of alkyl group in the alkoxide,water=alkoxide ratio, molecular separation of speciesand the reaction temperature affect the particle sizeand the morphology of such zirconium oxides, andtherefore these parameters affect the sinteringbehavior and crystalline transformation [2].

The hydrolysis of zirconium alkoxides leads topreferential formation of oxo and aquo groups ratherthan true hydroxides [3]. It has been shown that theoxide content of ZrO2 is close to 80% by weight,regardless of the amount of water used during thehydrolysis. This result is different than that of otheralkoxides such as silicon, aluminum and titanium[1].

Zirconium alkoxides react rapidly with water inaccordance with these simpli®ed reactions [4]:

Zr(OR)4 � H2O! ZrO(OR)2 � 2R(OH) (1)

2ZrO(OR)2 ! ZrO2 � Zr(OR)4 (2)

where R is an alkyl group. Therefore, 1 mole Zr(OR)reacts with 1 mole H2O to give a half mole of ZrO2.For a complete reaction, 2 moles of H2O arenecessary [5]. Yoldas has shown the importance ofa controlled hydrolysis of zirconium alkoxides forobtaining products with desired properties [2].

Zirconium alkoxides are used for obtaining ZrO2,and they are widely used in conjunction with othermetal alkoxides (silicon, aluminum, titanium, etc.)for obtaining glass ®bers, high-density refractorybodies, thin ®lms, composites, catalysts and adsor-bents [4]. In these cases, the hydrolysis of differentmetal alkoxides are of different reaction rates, andthis result may allow precipitation of that hydroxideat a faster reaction rate. Therefore, the hydrolysismust not only be controlled, but it also must beknown. The infrared spectroscopy is a powerfultechnique that allows one to follow the hydrolysis ofmetal alkoxides [6, 7]. In this work, we used Fouriertransformation infrared (FTIR) spectroscopy to studythe hydrolysis of zirconium n-propoxide (Zr±OPr),the effects that produce different H2O=alkoxides andthe alcohol=alkoxide ratios on such reactions.

The hydrolysis of Zr±OPr was carried out in a200 ml beaker coupled with a refrigerator, a mech-

anical stirrer, an addition burette and a hole forsampling used for FTIR measurements. The totalvolume used in all reactions was 100 ml. Zr±OPr(Fluka 70.0%), n-propanol (n-PrOH) (Fluka, 99.9%)and water (distilled and deionized) were used asstarting materials. In this work, we studied thein¯uence of both water:alkoxide molar ratio as afunction of alcohol:alkoxide molar ratios andalcohol:alkoxide molar ratio as a function ofwater:alkoxide molar ratios. Then, for a ratio of n-PrOH:Zr±OPr of 4:1, the ratios of H2O:Zr±OPrstudied were 1:1, 2:1, 3:1 and 4:1, and for a ratio ofH2O:Zr±OPr of 4:1, the ratios of n-PrOH:Zr±OPrstudied were 4:1, 8:1, 16:1 and 32:1. In all cases, theexperimental procedure was identical: Zr±OPr andhalf of the corresponding volume of n-PrOH weremixed in the reaction beaker, and the remaining n-PrOH was mixed with the corresponding H2O in adifferent beaker; both solutions were stirred for 2 hat the reaction temperature (25� 0:2 8C). Bothbeakers were sealed to avoid alcohol evaporationas well as hydrolysis of Zr±OPr due to roomhumidity. After this time, the H2O=n-PrOH solutionwas transferred to the addition burette maintainedconstantly at the reaction temperature, and for 2 hthis solution was added dropwise to the reactingsolution while a vigorous stirring was maintained.We used the same parent alcohol (n-PrOH) to avoidester interchange reactions [3]. During the additionprocess, small volumes of reactants were extractedby a liquid microsyringe (50 ìl) and placed betweentwo infrared transparent crystals; the correspondingspectra were recorded. Ten scans were used in eachmeasurement, and the background was subtracted inall cases. An FTIR spectrophotometer (Perkin-Elmer,Model 1760x) was used with a resolution of 2 cmÿ1.The spectra were stored in a computer and thendeconvoluted in the corresponding IR bands assum-ing a Gaussian pro®le for each band; these bandswere characterized by the position (wave number),half width and intensity.

FTIR spectra of Zr±OPr, n-PrOH and a mixture ofZr±OPr=n-PrOH are shown in Fig. 1a. Because Zr±OPr is a crystalline solid, it is necessary to dissolveit in a small amount of n-PrOH to obtain a liquid[3]. The amount of this alcohol may be observed inthe purity of the Zr±OPr used (70.0%). As can beobserved in Fig. 1a, this result is in accordance withthe presence of IR bands of n-PrOH in the spectrumof Zr±OPr. Three spectral regions may be observed

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in the Zr±OPr spectrum. Between 4000±1400 cmÿ1

O±H and C±H stretching and bending vibrationsoccur. Both IR vibrations do not give any informa-tion about the hydrolysis of the zirconium alkoxide.Between 1300±800 cmÿ1, the Zr±O±C stretching inZr±OPr and the C±O stretching in n-PrOH occur.Between 800±400 cmÿ1, the corresponding bendingvibrations of Zr±O±C and C±O bonds appear [8].Because in this low wave number region the out-of-plane OH bending of n-PrOH appears, the 1300±800 cmÿ1 region is the most important region inwhich to study the hydrolysis of Zr±OPr.

In the 1200±800 cmÿ1 spectral region, the IRbands corresponding to Zr±OPr appear and arelocated at 1153, 1132, 1108, 1082 and 1006 cmÿ1

(clearly shown in Fig. 1b in the spectral range 1200±1000 cmÿ1); these bands are associated with the Zr±O±C stretching. On the other hand, the n-PrOHgives 8 IR bands lying at 1099, 1069, 1056, 1017,970, 907, 888 and 860 cmÿ1. At 1127 cmÿ1 therealso appears a slight shoulder. To obtain completeinformation about the hydrolysis process, we ®rstdeconvoluted the spectra of Zr±OPr, n-PrOH and amixture of Zr±OPr=n-PrOH. In this step, theunambiguously assigned IR bands of Zr±OPr andn-PrOH were obtained; such bands were character-ized by the three parameters of a Gaussian band;intensity, half-width and wave number. The errorbetween the deconvoluted and experimental spectrawas lower than 3% in all cases. The study of thehydrolysis of Zr±OPr was carried out by analyzingthe evolution of these three parameters, which areused to obtain the integrated intensities of thecorresponding IR bands. Because the Zr±OPr bandslocated at 1132, 1108, 1082 and 1006 cmÿ1 are veryclose to those of n-PrOH, the band that can bestudied is the well-de®ned, very strong band lying at1153 cmÿ1 (Fig. 1b).

Spectra corresponding to the hydrolysis of Zr±OPrat different reaction times are shown in Fig. 2. It can

be clearly observed that the Zr±OPr bands men-tioned above disappear, and the long time spectrumonly shows the bands corresponding to n-PrOH.Fig. 3 shows the evolution of the 1153 cmÿ1 band asa function of the hydrolysis time for differentH2O:Zr±OPr molar ratios. As the water:alkoxideratio decreases, the rate of hydrolysis also decreases,as may be expected. It can be observed in Fig. 3 thatfor H2O:Zr±OPr ratios of 4:1 and 3:1, the hydrolysisof Zr±OPr is completed in 2 and 24 h, respectively.For H2O:Zr±OPr ratios of 2:1 and 1:1, it is possibleto extrapolate times from the experimental curve andthe obtained hydrolysis that are 238 and 23.106 days,respectively.

In accordance with Reactions 1 and 2, 2 moles ofH2O are needed for a complete hydrolysis of 1 mole

Figure 1 (a) Infrared spectra of raw materials in the spectral range of 4000±400 cmÿ1 where (1) Zr±OPr; (2) n-PrOH; and (3) starting sol, (b)

infrared spectra of raw materials in the spectral range of 1200±1000 cmÿ1.

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Figure 2 FTIR spectra corresponding to the hydrolysis of Zr±OPr

(molar ratio of H2O:ZrÿOPr � 1:1) at different reaction times where

(a) 2 min; (b) 10 min; (c) 30 min; (d) 60 min; (e) 90 min; and

(f) 120 min.

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of Zr±OPr. Therefore, the only H2O:Zr-OPr ratioused in this study that does not give such completehydrolysis is the 1:1 ratio. The obtained hydrolysistimes are in accordance with the ratios used. Bradleyand Carter [5] have shown that the hydrolysis of Zr±OPr gives three kinds of polymers: unsolvatedtrimeric species, solvated dimeric species andmonomeric solvated species. For H2O:alkoxide ratioslower than one, trimeric and dimeric species arepresent in the early stages of hydrolysis. ForH2O:alkoxide ratios higher than one, solvationbecomes more effective and polymers based ondimeric and monomeric species are present. There-after, polymers based in monomeric species becomeincreasingly signi®cant as the H2O:alkoxide ratio isincreased. Bradley and Carter stopped their hydro-lysis study for a water:alkoxide ratio of 1.814 due tothe presence of residual water in the system, whichalso suggests the presence of Zr±OH groups. A widerange of water:alkoxide ratios are normally used[1, 2], however, and it is necessary to study suchhydrolysis ratios. Fig. 3 shows that it is possible tostudy the hydrolysis of Zr±OPr even when Zr±OHgroups are formed.

It is also observed in Fig. 3 that the higherhydrolysis rate occurs during the addition of theH2O=alcohol solutions, i.e., the ®rst 2 h, and thisreaction rate increases as the H2O:alkoxide ratioincreases. This result shows that, as is known, thehydrolysis of the zirconium n-propoxide is instanta-neous.

Yoldas [2] has shown the in¯uence of theH2O:alkoxide and alcohol:alkoxide ratios in theproperties of zirconium oxide materials obtainedfrom Zr±alkoxides. We have shown the differenthydrolysis rates found for different H2O:alkoxideratios, and now we are going to study the effect ofthe alcohol:alkoxide ratio or, in accordance withYoldas, the molecular separation [9]. When for agiven H2O:alkoxide ratio, the alcohol:alkoxide ratiodecreases, the concentration of the solution in-

creases, and therefore the molecular separation alsodecreases giving as a result a higher possibility ofencounters between reacting molecules. If thesemolecules have hydroxyl groups in their terminalbonds, a condensation reaction can take place whereH2O is eliminated and a metal±oxygen±metal bondis formed [10]. On the other hand, when the terminalbonds are alkyl groups, the encounters betweenmolecules cannot lead to chemical reactions. Inaccordance with Reactions 1 and 2 in the case ofzirconium alkoxides, it is not necessary that terminalgroups be hydroxyls. To study the in¯uence of thealcohol:alkoxide ratio in the hydrolysis rate of Zr±OPr, we only studied the H2O:Zr±OPr molar ratio of4:1, and, as we have noted for this ratio, thehydrolysis of Zr±OPr is completed in 2 h. The n-PrOH:Zr±OPr ratios studied were 4:1, 8:1, 16:1 and32:1.

Fig. 4 shows the FTIR spectra of the hydrolysis ofZr±OPr when the alcohol:alkoxide ratio is 32 as afunction of the reaction time. These spectra are veryclose to those shown in Fig. 2, obtained for a loweralcohol:alkoxide ratio. The spectral deconvolutiongave the results shown in Fig. 5 where the evolutionof the Zr±O±C band located at 1153 cmÿ1 could beobserved. The evolution of this band has the samebehavior for the different n-PrOH:Zr±OPr ratiosstudied. This result shows that when an excess ofwater is used for the hydrolysis of Zr±OPr, thehydrolysis rate does not depend on the dilution ofthe solution. Yoldas [2] has also shown that for agiven water:zirconium alkoxide ratio, the ZrO2

powders obtained for different alcohol:alkoxideratios have similar densi®cation curves due to ahigh degree of resistance to monoclinic-tetragonaltransformation. The results obtained in this work arein accordance with those of Yoldas.

In summary, this work has shown the possibility ofstudying the hydrolysis of zirconium n-propoxide by

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Figure 3 Time evolution of the integrated area for the 1153 cmÿ1 band

during hydrolysis of Zr±OPr for different molar ratios of H2O:Zr±OPr.

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Figure 4 FTIR spectra corresponding to the hydrolysis of Zr±OPr

(molar ratio of n-PrOH:ZrÿOPr � 32:1) at different reaction times

where (a) 2 min; (b) 10 min; (c) 30 min; (d) 60 min; (e) 120 min; and

(f) 202 min.

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means of FTIR spectroscopy. By a conventionaldeconvolution procedure, the evolution of the Zr±O±C band is followed, and their integrated intensityis used for determining the hydrolysis degree of thezirconium alkoxide. The hydrolysis rate dependsstrongly on the water:alkoxide molar ratio, beinghigher as such ratio is increased. On the other hand,

if a water-rich environment is used for the hydrolysisof the zirconium n-propoxide, the hydrolysis doesnot depend on the dilution of the alkoxide solution.

AcknowledgmentsThe authors wish to thank the Comision Interminis-terial de Ciencia y Tecnologia of Spain for ®nancialsupport under Project MAT96-0564.

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(1951) 280.

4. K . S . M A Z D I YA S N I , C . T. LY N C H and J. S . S M I T H ,

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5. D. C . B R A D L E Y and D. G . C A RT E R, Con. J. Chem. 39

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6. F. RU B I O, J. RU B I O and J. L . OT E O, Spectroscopy Lett.

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7. M . C . M ATO S , L . M . I L H A R C O and R . M . A L M E I DA ,

J. Non-Cryst. Solids. 147±148 (1992) 232.

8. R . T. C O N L E Y, `̀ Infrared Spectroscopy'' Ed. Alhambra.

9. B. E . YO L DA S, J. Non-Cryst. Solids 51 (1982) 105.

10. Idem., ibid. 82 (1986) 11.

Received 9 Julyand accepted 22 July 1998

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0 2 4 6 8 10 12

4:1

8:1

16:1

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Figure 5 Time evolution of the integrated area for the 1153 cmÿ1 band

during hydrolysis of Zr±OPr for different molar ratios of n-PrOH:Zr±

OPr.

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