_________~~~~~~~~~~ Preparation of anatase, brookite and rutile at low temperature by non-hydrolytic sol-gel methods Pascal Arnal, Robert J.P. Corriu, Dominique Leclercq, P. Hubert Mutin and AndrC Vioux* Laboratoire des pricurseurs organomitalliques de matiriaux, case 007, Universitt? Montpellier II, 34095 Montpellier Cidex 5, France Titania samples prepared by different non-hydrolytic sol-gel methods, mainly based on the etherolysis and alcoholysis of titanium tetrachloride, have been found to differ in both structure and texture. Thus, the reaction of diethyl ether with TiCl, at 110 'C affords anatase, which begins to convert into rutile only around 1000"C. The reaction of TiC1, with ethanol leads to rutile as early as 110 "C, whereas the reaction of tert-butyl alcohol at 110 "C leads to the singular formation of brookite.The development of low-temperature routes to transition-metal oxide processable materials underlies the recent works in the field of sol-gel synthesis.' In these methods based on the hydrolysis of molecular precursors, such as metal alkoxides, the major problem is control of the reaction rates which are generally too fast. An attractive solution is to use organic additives which act as chelating ligands (carboxylic acids, p-diketones, etc.) and modify the reactivity of the precursors.2 Here we propose various non-hydrolytic sol-gel routes tested in the case of titanium, chosen as a representative transition metal. As reported previously, a novel sol-gel route is provided by the thermal condensation of metal halides with metal alkox- ides:3 MX, +M(OR), -t2MO,,, +nRX (1) Alternatively, it is possible to generate the alkoxyl groups in situ by the action of alcohol4 or dialkyl ether5 on metal halides: =M-X +ROH-M-OR +HX or EM-X +ROR-tE M-OR +RX (2) Another possible variation is the direct reaction of anhydride with alkoxide precursors,6 which leads to acylation, then condensation, via the formation of an ester: M(OR),+n/2 (R'C=O),O+MO,,,+nR'COOR (3) In these non-hydrolytic reactions, the metal alkoxide, dialkyl ether, alcohol and anhydride act as oxygen donors, instead of water.The main features of these methods are: (i) low synthesis temperatures (cu. 100-150°C); (ii) a simple reaction of TiC1, [or Ti (OR),] with readily available compounds; (iii) an easily removed by-product (hydrogen halide, alkyl halide or acetate); (iv) no cosolvent required (otherwise needed to dissolve water).This paper falls into two parts. First, four typical non-hydrolytic routes to titania were compared [systems TiC1, plus Ti(OPr'),, TiCl, plus diisopropyl ether, TiC1, plus isopropyl alcohol, Ti(OPr'), plus acetic anhydride]; the structures and textures of the different samples were studied by means of X- ray diffraction (XRD) and BET measurements. Secondly, the influence of the nature of the oxygen donor on the structure and crystallisation behaviour of the TiO, precur-sors was investigated in the etherolysis and alcoholysis methods. Background Hydrothermal and sol-gel methods have attracted much inter- est in the preparation of titania powders and colloids because of the many uses of this material, as a pigment, filler and, more recently, as a membrane, anti-reflection coating, catalyst and pho toca taly st.Titania sol-gel synthesis has been developed from inorganic precursors and from metal-organic Ti(OR), precursors.' Thus, TiO, gels can be obtained by adding a weak base [e.g.Na,C03, (NH,),C03] to a solution of sodium titanate in concentrated hydrochloric acid, whereas sols can be obtained by the addition of TiC1, or TiO(NO,), to acidic aqueous solution.'*7 Moreover, stable clear sols, which tend to turn into monolithic gels, can be obtained from Ti(OR), precursors by using high water/ alkoxide ratios and inorganic acids as peptization Ti0,-based gels or colloids can also be obtained after a chemical modification of titanium alkoxides by chelating agents, such as acetic acid and acetylacetone." Titanium dioxide is known to exist in three crystalline modifications, namely rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic).Anatase and rutile are the common polymorphs of synthetic titania. The titanium atoms are octahedral in both structures. In fact, crystallisation is highly influenced by the hydrolysis conditions." During the condensation process, the formation of the kinked chains of edge-sharing octahedra corresponding to anatase appears more probable than the formation of the straight chains typical of rutile.Therefore, anatase is obtained in processes under kinetic control, whereas processes involving Ostwald ripening lead to the equilibrium phase, i.e. rutile.'* On the other hand, the brookite structure, in which each octahedron shares one edge, has not been X-ray characterised to date, to our knowledge, in sol-gel products. Usually, amorphous TiO, crystallises into anatase below 4OO0C, which is further converted to rutile from 600 to 1100"C.I3The rates of transformation are markedly influenced by particle size or the presence of imp~rities.'~ The anatase- rutile transformation may be detrimental in many applications. It involves a collapse from the relatively open anatase structure with a cell volume of 0.068 nm3 to rutile with a cell volume of 0.0624nm3, a volume change of ca.8%. So the structural transformation may be damaging for supported membranes owing to the extra stresses introduced by this volume change during sintering.15 Moreover, the anatase-rutile transformation is considered to represent the major factor in catalyst deacti- vation in selective oxidation catalysis.16 In the same way, rutile has been found to be unable to sensitise the photomineralis- ation of phenol derivatives, unlike anatase." This behaviour J. Muter. Chem., 1996, 6(12), 1925-1932 1925 appears to be associated with the concomitant decrease in specific surface area Experimental General Titanium chloride and titanium tetraisopropoxide [Ti(OPr'),] were purchased from Aldrich Titanium tetra-tert-butyloxide [Ti(OBut),] was prepared from Ti(OPf), and tert-butylacet- ate according to the literature l8 Ethers and alcohols were distilled from the appropriate drying agents prior to use All manipulations were carried out under argon, in oven-dried glassware, to preclude any hydrolysis side reaction Reactions of TiCl, with titanium isopropoxide and ethers The oxygen donor was added to titanium chloride (typically 2-5 ml) at 0 "C in a tube under argon (Table 1) An exothermic reaction occurred and in most cases a precipitate was formed, which melted on heating The tube was frozen in liquid nitrogen then sealed under vacuum The sealed tube was held at 110 "C for 7 days, then it was opened in a glove bag under argon The solid and the liquid phases were separated by filtering The solid was washed with successive portions of dichloro- methane, then dried under vacuum Reactions of titanium isopropoxide with acetic anhydride Acetic anhydride (6 4 ml, 67 mmol) was added to titanium isopropoxide (10 ml, 33 5 mmol) in a Schlenk tube under argon, an exothermic reaction occurred, which led to a yellow solution After the addition of a small amount of TiC1, (004 ml, O36mmol), the mixture was transferred to a tube, frozen in liquid nitrogen, then sealed under vacuum The tube was kept at 140°C for 7 days A white solid was obtained which was isolated by the above procedure Reactions of TiC14 with alcohols Titanium chloride was added dropwise at room temperature with stirring to an excess of alcohol (TiC1,-ROH, 1 6) under argon Viscous yellow solutions were obtained, except in the case of tert-butyl alcohol where a yellow precipitate formed initially, which then dissolved on slow heating The solutions were stirred at room temperature for 2 h to allow complete HC1 evolution Thereafter they were allowed to stand for 7 days at 110 "C, under autogenous pressure in sealed tubes C haracterisa tion methods The syneresis liquids were analysed by 'H NMR spectroscopy and by gas chromatography (GC) The elemental analyses of solids were performed by the Service Central &Analyses of CNRS (Vernaison, France) C content was determined by IR spectroscopy after high-temperature combustion, C1 content was determined by potentiometric titration, Ti content was determined by induction coupling plasma (ICP) from an aqueous solution Thermal analyses were performed at a heating rate of 10Krni11-~ in a 20 80 0,-N, mixture on a Netzsch STA 409 thermobalance In some cases the thermobal- ance was coupled with a Balzers QMG 421 mass spectrometer, thus allowing the continuous analysis of the gases evolved Speclfic surface areas were determined by the Brunauer-Emmett-Teller (BET) method, using nitrogen adsorption- desorption isotherms recorded on Micromentics ASAP 2400 analyser (estimated error d 5%) X-Ray powder diffraction (XRD) patterns were recorded with Cu-Ka radiation using a SEIFERT MZ IV diffractometer (4 scans, with 1000 digitised points and 500 ms acquisition time, 8 angle ranging from 5 to 40") The mass percentage of rutile in the mixtures was calculated using %R= 1/[ 1+0 8 (IA/IR)],where I, and IRare the intensities of the (101) reflection of anatase and the (1 10) reflection of rutile, respectively l9 Results Comparison of different non-hydrolytic methods Four preparation modes, involving parent byproducts (isopro- pyl chloride or isopropyl acetate), were compared (1) the reaction of TiCl, with Ti(OPr'), (equimolar), (2) the reaction of TiCl, with diisopropyl ether, Pf20[2 (A) and 3 (B) equiv 3, (3)the reaction of Ti(OPr'), with acetic anhydride, (CH3C0)20 (2 equiv), (4) the reaction of TiCl, with an excess of isopro- pyl alcohol Experimental details are given in Table 1 In methods 1 and 2, the reactions were carried out at 110 "C under autogenous pressure in sealed tubes, without any cosolvent In fact, when mixed at room temperature TiCl, and Ti(OPf), give chloroal- koxide precipitates which melt readily below 110 "C 2o 21 TiCl, is known to form Lewis adducts with ethers, however, with diisopropyl ether the adduct was not isolated and the precipi- tate obtained at room temperature was ascribed to the forma- tion of the substitution compound TiCI,(OPr') 22 No cosolvent was needed in method 3 either, since the reaction of Ti(OPr'), with 2 equiv of acetic anhydride is known to yield liquid Ti(OAc),(OPr'), 23 The strong evolution of HC1 (exothermic reaction) which occurred when isopropyl alcohol was added to TiCl, clearly indicated the formation of alkoxide groups In fact, whereas the reaction of alcohol with silicon tetrachloride usually leads to tetraalkoxides Si(OR), , compounds of formula TiC12(0R), .ROH (which are soluble in alcohol and are poten- tial candidates for non-hydrolytic condensation) are generally obtained from TiCl, and alcohol 24 Polycondensation took place within a few hours at 110 "C, except in the reaction of Ti(OPr'), with acetic anhydride where a temperature of 140°C was needed Note that in the latter case no solid formed in the absence of some TiCl, catalyst The total heating time was always 7 days The solids were obtained as white agglomerates (grains gathered in a mass), except in the case of alcoholysis in which a precipitate (separate grains) was obtained In all cases the GC analysis of the expelled liquid (syneresis liquid) established the expected formation of isopropyl chloride Table 1 Comparison of different non-hydrolytic sol-gel routes to TiOz preparation of the samples (7 days reaction time) ~~~~~ method reagents 1 TiCl,, Ti(OPr'),, no solvent 2A TiCl,, 2Pf20, no solvent 2B TiCl,, 3Pr'20, no solvent 3 Ti(OPr'),, 2(CH,CO),O, 1% TiCI,, no solvent 4 TiCl,, excess Pr'OH, c= 1 8 mol I-' "Time for which the formation of a solid phase was observed 1926 J Muter Chem, 1996, 6(12), 1925-1932 gel time"/h, temperaturePC ca 5, 110 ca 2, 110 ca 3, 110 ca 19, 140 ca 6, 110 appearance white agglomerate white agglomerate white agglomerate white agglomerate white precipitate TiOZ yieldb byproducts (%I Pr'C1+ hydrocarbons 90 Pr'Cl +hydrocarbons 95 Pr'Cl +hydrocarbons 91 AcOPr' 75 Pr'Cl +hydrocarbons 86 From TG, after calcination in air at 1000 "C Table 2 Comparison of different non-hydrolytic sol-gel routes to TiO,: characterisation of the different titanias dryingmethod' temperature/"C 1 110 180 2A 110 180 2B 110 3 140 4 110 ~~ ~ S/m2 g-' XRD (after XRD (after composition from elemental analysis Am/m (Yo)(from formula) Am/m (YO) (after calcination (TG) at 500 "C for 5 h) calcination at 500"C for 5 h) calcination at 950 "C) TiC1O.llCO.85~ Tic1,., 1(0pf)0.2801 .6 160 anatase anatase 100% 80% condensation Tic10.04c0.67 TiC10.04(0Pf )0.2Z01 .87 -13 -14 94% condensation TiC10.28C0.87 120 anatase anatase 86% Tic10.28(OPf )O.2g0l72% condensation .43 +rutile 14% Tic1,.14c0.41 14(0Pf)0.1401.86 -12 -16 93% condensation TiC10.09Co.10 63' anatase rutile 100% Tic10.09 (OPf )0.03O1.94 -8 -8 97% condensation Tic10.08c2. 18 47 anatase anatase 25% Tic10.08 (0Pr')0.44(0Ac)0.~401.56b -36 -39 +rutile 75% 76% condensation TiC10.07C0.03 40 anatase anatase 63 YO Tic10.07~0pf~0.0101.95 -3 -7 +rutile 37% 98% condensation Systems 1 TiCl4/Ti(0Pf),; 2A TiCl,, 2Pr1,0; 2B TiCl,, 3Pt20; 3 Ti(OPr'),, 2(CH,CO),O; 4 TiCl,, excess PfOH. Assuming an equal number of OPf and OAc groups. 174 m2 g-' after calcination in air at 200 "C for 3 h.or acetate. In methods 1,2 and 4, the presence of hydrocarbons was detected as well as the alkyl chloride; they may be ascribed to the polymerisation of the alkene arising from the partial dehydrochlorination of the alkyl chloride (both elimination of HC1 and polymerisation of alkenes are known to be catalysed by titanium chloroalkoxides).25 Table 2 sets out the compositions (based on elemental analyses) of the samples after vacuum drying for 3 h at the given temperature. The number of residual OR groups (and OAc and C1 groups) per Ti atom are calculated from carbon contents (and chloro contents), and the number of 0x0 bridges are deduced by difference (to meet valence requirement, assuming no presence of hydroxy groups). Therefore the calcu- lated formulae TiCl,(OR),O, give an estimation of the degree of condensation [condensation (%)=z/2 x 1001.The mass losses, Am/m, based on thermogravimetry (TG) from 200 to 1000°C are in rather good agreement (except for sample 4) with those calculated from the formulae (Table 2); this confirms the postulated small amount of hydroxy groups. X-Ray diffraction (XRD) indicated the incipient crystallis- ation of anatase in all the as-prepared samples. In Fig. 1, the XRD patterns of sample 1 after annealing at different tempera- tures show only the reflections of anatase. However, the progress of the anatase-rutile transformation at 950 "C (with- out holding time) was found to be quite uneven depending on the preparative method (Table 2; e.g.75% rutile in method 3 us. 100% anatase in method 1). No explicit exothermic peak associated with crystallisation or phase transformation (and not associated with mass loss) was recognized from the differential thermal analysis (DTA) curves. An additional etherolysis experiment, involving an excess of ether (3 equiv.; Table 1, method 2B), was performed for a comparison with method 4 in which an excess of alcohol was used. Anatase was produced again; however, the anatase-rutile transformation was found to be promoted (accompanying a higher degree of condensation) compared to the stoichiometric etherolysis (Table 2, methods 2A and 2B). The specific surface areas of the samples were measured by the BET method after 5 h heat treatment at 500 "C (Table 2).The low values found in methods 3 and 4 (i.e. acetate and isopropyl alcohol methods) are related to the progress of crystallisation. On the other hand, the specific surfaces found for methods 1 and 2A are 160 and 120 m2 g-', respectively (174 m2 g-' in method 2B after calcination at 200 "C for 3 h). The nitrogen adsorption-desorption isotherms obtained for methods 1 and 2A are of type IV according to the BDDT classification26 (mesoporous solids; Fig. 2). .-1801 5 10 15 20 25 30 35 40 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 e/degrees relative pressure,P/Po Fig. 1 X-Ray diffraction patterns of sample 1, arising from the reaction of TiCl, with Ti(OPr'),, after heating at (a) 180, (b) 500, (c) 950°C Fig.2 Nitrogen adsorption-desorption isotherms of sample 2A,arising from the reaction of TiCl, with Pr',O, after calcination at 500 "C for 5 h J. Mater. Chem., 1996, 6(12), 1925-1932 1927 Influence of the nature of the organic oxygen donor (ether or alcohol ) Etherolysis. Titanium chloride was reacted with diisopropyl ether, di-n-propyl ether, tetrahydrofuran (THF), diethyl ether and dimethoxyethane (DME) Table 3 shows that gel time (the time for which a solid phase was observed) depends on the dialkyl ether used and it increases in the order Pr',O <Et,O <THF <PP,O <DME The analysis of the syn- eresis liquid confirms the formation of alkyl chloride as bypro- duct (with the partial isomerisation of n-propyl chloride into isopropyl chloride in the case of di-n-propyl ether) In the case of DME no liquid could be isolated The mass losses, Am/m, measured by thermogravimetry (Table 4) are related to the amount of residual groups (organics and chloride) present In the dried samples (after vacuum drying at 180 "C), this was found to be rather low (16-22%), except for with DME In this case the high mass loss (Am/m= 47%) must be related to the absence of 1,2-dichloroethane as the syneresis liquid Apparently DME reacts mainly through the methoxy end groups, giving an inorganic-organic hybrid compound (volatile methyl chloride was not isolated) -TiCl+ CH3-0CH,CH,O-CH3 + ~Ti-OCH2CH20-Ti~ +2 CH3C1 (4) This was supported by the continuous analyses of the gases formed during the heat treatment, by coupling the thermobal- ance with a mass spectrometer, which allowed the detection of (Fig 3) (a) between 150 and 400°C 172-dichloroethane(m/z 62) and acetylene (m/z 25), as well as HCl (m/z 36) and H20 (m/z 18), (b) between 400 and 650°C CO, (m/z 44) and H20 (m/z 18) associated with the combustion In fact, the gases produced between 150 and 400°C may be accounted for by the following reactions ~T1-OCH2CH20-Ti--' +2 =TlCl+ 2 Ti-0-Ti= +C1CH2CH2C1 (5) ClCH,CH,Cl+HC=CH +2 HC1 (6) -Ti- OCH2CH20-Ti= +2=Ti- OH +HC-CH (7) 1 TG -1004 -.-. -. -. . 0 200 400 600 800 lo00 ,II, ml544 II /I I-........0 200 400 600 800 loo0 TI% Fig. 3 Thermogravimetric and differential thermal analysis in air of the sample arising from the reaction of TiCl, with DME, and continuous mass spectrometry of the gases evolved during the heat treatment Note that the precursor derived from DME was quite amorphous after vacuum drying at 180°C for 3 h, whereas incipient crystallisation of anatase was discernible in other precursors (Fig 4) However, XRD indicated that all the samples gave anatase after calcination at 500°C One can see from Table4 that the surface areas, which varied from 45 to 120m2 g-' are related to the crystallinity of the samples [Fig 5(u)] Upon heating to 950°C (without holding time), the transformation to rutile was complete only for the sample Table 3 Preparation of the samples by etherolysis of TiCl, (7 days reaction time) Ti0, yield' ether gel timea/h appearance by products (%) Pt,O ca 2 white agglomerate Pf20 ca 70 white agglomerate PtC1+ hydrocarbons Pr"C1 +Pr'C1-t hydrocarbons 95 81 THF ca 60 white agglomerate CI(CH,),Cl 89 Et,O ca 17 white agglomerate EtCl 74 DME ca 85 black powder - 80 a At 110 "C 'From TG, after calcination in air at 1000"C Table 4 Characterisation of the different titanias prepared by etherolysis of TiCl, composition ether (condensation degree)" pr'20 14(OPf)0 14O1 86 (93%)..Pr"20 Tic1, 34(OPf)0 0701 80 (90%) THF TIC10 13(ORC1)~ 1501 86' (93%) Et20 41(OEt)0 16O1 67 (83%) DME 09(ORo)0 55OO 91' (46 Yo) XRD (after vacuum XRD (after XRD (after s/mzg (after Am/m (%) (from formula) Am/m (YO) (TG) drying at 180°C) calcination at 500°C for 5 h) calcination at 950 "C) calcination at 500°C for 5 h) -12 -16 -15 -16 -16 -18 amorph + anatase amorph + anatase amorph + anatase amorph + anatase amorph + anatase amorph + anatase anatase 86% anatase 89% anatase 57% +rutile 14% +rutile 11YO +rutile 43% 120 58 73 -17 -40 -22 -47 amorph + anatase amorph amorph + anatase amorph + anatase 42% rutile 100% +rutile 58% 45 52 anatase a After vacuum drying at 180 "C 'R =(CH2)4 R =CH,CH, 1928 J Muter Chem , 1996, 6(12), 1925-1932 Pr',O I DME I...-.----...--.--...--....-...-.-.I S 10 IS 20 25 30 35 40 Bldegrees Fig.4 X-Ray diffraction patterns of the samples arising from the reactions of TiC1, with DME and with Pr',O (sample 2A),after vacuum drying at 180°C for 3 h derived from DME, whereas the transformation had just begun for the sample derived from Pf,O [Fig.5(b)]. Alcoholysis. Titania precursors were prepared from primary alcohols (ethanol, propan-1-01, butan-1-01, ethylene glycol), secondary alcohols (isopropyl alcohol, butan-2-01) and tertiary alcohols (tert-butyl alcohol, tert-amyl alcohol). Experimental details for gelation are given in Table 5. Gelation occurred within the range 0.5 h (tertiary alcohol) to 18 h (butan-1-01). In all the cases the expected alkyl chloride was identified by GC in the syneresis liquid. However, note the partial isomeris- ation of Bu'Cl into ButCl in the case of butan-2-01. The mass losses were measured by TG (Table 6).Values as low as 44% were indicative of high condensation degrees.The unique large mass loss (Am/m =32.6%) observed with ethylene glycol as the oxygen donor may be accounted for as in the above-mentioned case of DME. Table 6 indicates the components of the samples after vacuum drying at room temperature or at 11O"C, and after calcination in air at 500 and 950 "C, based on X-ray diffraction analyses (Fig. 6). Dramatic differences are observed, depending on the nature of the alcohol, as early as room temperature. Thus, anatase was visible in the samples derived from propan- 1-01, propan-2-01, butan-1-01 (and this phase was retained to 950 "C), whereas rutile was present in the samples arising from ethanol and butan-2-01.The reaction of tert-butyl alcohol with TiC1, leads to the formation of a precipitate, which is yellow and which does not melt below 110°C. However, it dissolves in an excess of alcohol. A translucent gel was obtained within 1h at 110 "C, afterwards the gel expelled all the solvent on heating, and turned into an agglomerated powder. The X-ray diffraction pattern exhibits the typical reflections of brookite and rutile (Fig. 7). The conversion of brookite to rutile was almost complete after 5 h heat treatment at 500 "C. Traces of brookite were also detected in the as-prepared xerogel derived from tert-amyl alcohol, but they disappeared after vacuum drying at 110 "C. The unique formation of brookite from tertiary alcohols led us to conduct further investigations.The role of the alcohol Prn20 THF Eta0 DME I t I I I I 1 1 II 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 eldegrees Fig. 5 X-Ray diffraction patterns of the different titanias prepared by etherolysis of TiC1, after calcination (a) at 500°C for 5 h; (b)at 950°C without holding seems to be decisive in the formation of brookite. Indeed, brookite was not formed in the absence of ButOH: neither by reacting TiC1, and Ti(OBut),, nor by reacting TiC1, and Ti(OPr'), in the presence of 4equiv. of ButCl (only anatase Table 5 Preparation of the samples by alcoholysis of TiCl, (7 days reaction time) gel time/h TiO, yield" alcohol (110 "C) appearance byproducts (%) EtOH ca. 6 white agglomerate EtCl 91 Pr"0H ca.7 white agglomerate Pr"C1 85 Bu"OH ca. 18 white agglomerate Bu"C1 83 Pr'OH Bu'OH HOCH,CH,OH Bu'OH AmtOH ca. 6 ca. 3 ca. 50 ca. 0.5 ca. 0.5 white agglomerate white agglomerate white gel white gel white gel Pr'C1 +hydrocarbons 86 Bu'Cl +Bu'Cl +hydrocarbons 88 60 CH,CH,C( CH,),Cl +hydrocarbons 87 BuTl +hydrocarbons 95 a From TG; after calcination in air at 1000 "C. J. Muter. Chem., 1996, 6(12), 1925-1932 1929 Table 6 Charactensationof the different titanias prepared by alcoholysis of TiCl, and from the reaction of TiC1, with Ti(OBu'), oxygen donor EtOH HO(CH2)20H PPOH Bu"0H Pr'OH Bu'OH Bu'OH Am'OH Ti(OBu'), composition (from elemental anal after vacuum drying at 110"C) O4(OEt)003O1 96 (97% condensation) 15(0R0)0 78O1 14 (57%) TIC10 07(OPr')o 0201 95 07(0Bu")0 04O1 94 07(0Pr')0 Olol 95 04(0Bus)0 OOSO1 97 04(0Bu')0 OOSO1 98 TIC10 os(OAm')o 0101 97 27(OBU')0 19O176 (88 5%) Amlm (YO) AmJm (%I XRD (vacuum XRD (vacuum XRD (calcined XRD (calcined S/m2 g-' (calcined (formula) (TG) dried RT) dried 1lOOC) 500"C, 5 h) 9503C) 500"C, 5 h) -3 -4 amorph +rutile amorph +rutile rutile rutile 10 -32 -33 amorph +anatase amorph +anatase amorph +anatase anatase 2% +rutile 98% 35 -4 -7 amorph +anatase amorph +anatase anatase anatase 45% 30 frutile 55% -5 -8 amorph +anatase amorph +anatase amorph +anatase anatase 64% 43 +rutile 36% -3 -7 amorph +anatase amorph +anatase anatase anatase 63% 40 +rutile 37% -2 -5 amorph +anatase +rutile amorph +anatase +rutile anatase +rutile anatase 9% +rutile 91% 16 -2 -4 amorph +rutile +brookite amorph +rutile +brookite amorph +rutile +brookite rutile 23 -2 -4 amorph +rutile +brookite amorph +rutile amorph +rutile rutile 28 -20 -19 - amorph +anatase amorph +anatase anatase 100% 64 was obtained in both cases at 110 "C).Nevertheless, upon heat treatment in ButOH at 110°C for 7 days of an amorphous sample prepared from Ti(OPr'), and TiC1, (7 h heating at 110 "C in a sealed tube), only anatase was obtained. Moreover, the composition of the precipitate formed initially in tert-butyl alcohol before dissolution did not correspond to the expected formula TiCl,(OR), -ROH.The elemental analy- sis found (C, 4.68; C1, 18.66; Ti, 36.65%) better corresponds to the hydroxide; TiC1, 68(0B~t)o 127(0H)319, or to oxotitanate, TiCl,.,8(OBut)o 12701The mass losses, Arn/rn, calculated59. from these formulae are 41 and 25%, respectively. The exper- imental value (38%) is rather close to the former; furthermore, the latter would correspond to a condensation degree as high as 80%, which is inconsistent with the dissolution of the precipitate. However, the hydroxide precipitate did not crystal- lise to brookite on heating at 180 "C, after Bu'OH was removed by filtering and drying. Discussion Elemental analyses and mass losses measured by TG permit the evaluation of the amount of residual groups and, therefore, the comparison of the condensation degrees achieved in the different non-hydrolytic methods.Note that the reaction of acetic anhydride with Ti(OPr'), hardly occurs at 140 "C, and only then in the presence of TiC1, as a Lewis-acid catalyst. The catalytic efficiency of the Lewis acid FeC1, has been reported in the case of the polycondensation of R,Si(OAc), with PhSi(OR')3,27 but this type of catalysis has never been used in the sol-gel area to our knowledge, although it may be of some interest.28 However, this condensation reaction is a mediocre method in the case of titanium, compared to the reaction of Ti(OPr'), with TiCl,, or the etherolysis and alcoholysis of TiC1,. In the latter cases the non-hydrolytic condensation degree is improved by using an excess of the oxygen donor (in Table 2, compare samples 2A and 2B) or an additional heat treatment (in Table 2, compare samples 1 and 2A after drying at 110 and 180 "C).Thus, by reacting an excess of diisopropyl ether with TiC1, at 110 "C, an amount of about one residual group (OPr' plus C1) per every eight Ti atoms was found on average in the solid (Table 2). It is interesting to compare this result to that reported for the conventional aqueous route: alkoxy groups (Pr'O) remained at more than about one group for every six Ti atoms in samples produced by the hydrolysis of Ti(OPr'), (after heat treatment at 110°C for 7 h), even though a large amount of HCl solution was used." It is known that residual alkoxy groups determine the crystallisation behaviour of TiO, precursors.As a matter of fact, steric hindrance by residual unhydrolysed isopropoxy groups, preventing crystallisation to anatase, was reported." However, it appears that huge amounts of alkyls left over, such as OCH,CH,O bridges (Tables 4 and 6), bring forward the anatase-rutile transformation (whereas the presence of OCH,CH,O bridges delayed anatase formation at lower tem- perature, as shown in Fig. 4). This behaviour might be related to the exothermic oxidation of organics (at ca. 350"C), which brings about a local increase in temperature sufficient to promote the crystallisation of rutile when the organic content is high. Moreover, one has to keep in mind that the crystallis- ation of the anatase phase and the transformation from anatase to rutile are known to also take place on ageing of gel precursors." Our experimental conditions, especially in the case of alcoholysis, are reminiscent of solvothermal conditions (a prolonged heat treatment in an autoclave in the presence of a protic ~olvent).~' The effect of the alcoholic medium on crystal- lisation is most plausible.The ability of alcohols to cleave Ti-0-Ti linkages (thus slightly reversing the condensation reaction) may be invoked. Indeed, it has been reported recently that ethanol rinses, at room temperature, are able to convert a nanocrystalline anatase precipitate to an amorphous phase.30 Moreover, note the recently reported synthesis of materials with particular structures and morphologies in alcoholic media, more particularly via glycothermal treatment.31 However, no template effect (in the sense of structure director) was observed with ethylene glycol in our case.Moreover, note that no effect was observed when ButCl was used as the solvent in the reaction of TiC1, with Ti(OPr'),, although one would have expected that it would be able to reverse condensation reactions. The formation of brookite from tertiary alcohols, as well as the early crystallisation of rutile in the precursors arising from the alcoholysis of TiC1, with ethanol and butan-2-01, are difficult to rationalise. In addition to the effect of the alcoholic medium, the crystallisation behaviour is most probably influ- enced by the initial ultrastr~cture~~ of the amorphous precur- sor.Thus, the unique formation of hydroxy groups in the reaction of tert-butyl alcohol with TiCl, might be related to the crystallisation of brookite. This formation may be accounted for by the occurrence of reaction (8) (liberation of ButCl) instead of reaction (9) (liberation of HCl), owing to the increased cationic character on the tertiary carbon group 1930 J. Muter. Chew., 1996, 6( 12), 1925-1932 Am'OH I 14- A. L. A. BuSOH A A,- Pr'OH A A A M c BunOH Am'OH I A.A A. BunOH I I, & I PPOH II L.. .* L. ..Ah-Fig. 6 X-Ray diffraction patterns of the different titanias prepared by alcoholysis of TiCl, after calcination (a)at 500 "C for 5 h, (b)at 950 "C without holding which favours the nucleophilic attack of chloride ETiCl+ Bu'OH+ rTiOH +ButCl (8) =TIC1 +ButOH+ -TiOBu'+ HCl (9) Moreover, note that the probable presence of remaining OH groups in the dried precursor makes it difficult to evaluate the condensation degree from the elemental analyses (as is con- firmed by the discrepancy between the calculated and exper- imental values of mass loss in Table 6) R ~...(....,..,.,....I....I..... 5 10 15 20 25 30 35 40 eldeg rees Fig.7 X-Ray diffraction patterns of the sample arising from the reaction of TiCl, with Bu'OH, after drying and heating at (a) 20, (b) 110, (c) 500, (d) 950 "C Conclusion This study highlights the capability of non-hydrolytic sol-gel methods in preparing Ti02 precursors with various ultra- structures which in turn induce various crystallisation behav- lours These methods enable us to produce a particular crystalline structure depending on the nature of the oxygen donor (mainly ether or alcohol), as do conventional sol-gel methods by controlling the hydrolysis conditions 30 Thus it is possible to delay the anatase-rutile transformation up to ca 950°C, or to obtain the rutile phase directly as well Note that brookite, which was obtained at low temperature by action of tert-butyl alcohol on titanium chloride, is very uncommon in synthetic products 33 Further work is needed to rationalise the action of organic 0x0 compounds A solution-chemistry study of a non-hydro- lytic sol-gel route to titania will be published in the future for the reactions of titanium tetrachlonde with Ti(OPr'), and Pr',O 34 References 1 J Livage, M Henry and C Sanchez, Prog Solid State Chem ,1988, 18,259 2 C Sanchez, J Llvage, M Henry and F Babonneau, J Non-Cryst Solids, 1988,100,65 3 R J P Corm, D Leclercq, P Lefevre, P H Mutin and A Vloux, J Mater Chem ,1992,2,673 4 R J P Corm, D Leclercq, P Lefevre, P H Mutin and A Vioux, J Non-Cryst Solids, 1992,146,301 5 P Arnal, R J P Corriu, D Leclercq, H Mutin and A Vioux, Better Ceramics through Chemistry VI, Muter Res SOC Symp Proc ,1994,271,339 6 R C Mehrotra and R Bohra, Metal Carboxylates, Academic Press, London, 1983 7 J L Woodhead, J Phys Colloq ,1986,47, C1-3 8 B E Yoldas, J Muter SCI , 1986,21, 1087 9 A Larbot, J P Fabre, C Guizard and L Cot, J Am Ceram SOC, 1989,72,257 10 (a) S Doeuff, M Henry, C Sanchez and J Livage, J Non-Cryst Solids, 1987, 89, 206, (b)A Leaustic, F Babonneau and J Livage, Chem Muter ,1989,1,248 11 K Terabe, K Kato, H Miyazaki, S Yamaguchi, A Imai and Y Iguchi, J Muter Sci ,1994,29,1617 12 J P Jolivet, in De la solution a 1 oxyde, InterEditions, Paris, 1994, P 98 13 E F Heald and C W Weiss, Am Mineral, 1972,57, 10 J Muter Chew , 1996,6( 12), 1925-1932 1931 14 M Ocaiia, J V Garcia-Ramos and C J Serna, J Am Ceram SOC, 26 S Lowel and J E Shields, Powder surface area and porosity, 15 1992,75,201 V T Zaspahs, W Van Praag, K Keizer and J R H Ross, J Muter Sci ,1992,27,1023 27 Chapman and Hall, London, 1991 K A Andnanov, N N Sokolov and E N Khrustaleva, Zh Obshch Khim , 1956,26,1102 16 G Ohven, G Ramis, G Busca and V S Escribano, J Muter 28 M Jansen and E Guenther, Chem Muter, 1995,7,2110 17 18 19 20 21 22 23 24 Chem , 1993,3,1239 A Mills, R H Davies and D Worsley, Chem SOC Rev, 1993,417 R C Mehrotra, J Am Chem SOC, 1954,76,2266 R A Spurr and H Myers, Anal Chem ,1957,29,760 D C Bradley, D C Hancock and W Wardlaw, J Chem SOC, 1952,2773 C Dijkgraaf and J P G Rousseau, Spectrochim Acta Part A, 1968,24,1213 P M Hamilton,R McBeth, W BekebredeandH H Sisler,J Am Chem SOC ,1953,75,2881 K C Pande and R C Mehrotra, 2 Anorg Allg Chem, 1957, 290,95 J S Jennings, W Wardlaw and W J R Way, J Chem SOC, 1936,637 29 30 31 32 33 34 A M Chippindale, A R Cowley and R I Walton, J Muter Chem , 1996,6,611 D C Hague and M J Mayo, J Am Ceram Soc, 1994,77,1957 M Inoue, H Otsu, H Kominami and T Inui, Ind Eng Chem Res ,1996,35,295, and references therein The ultrastructure level is considered to be the range of somewhe5e between molecular and submicron dimensions, I e 10-1000 A J D Mackenzie and D R Ulrich, Ultrastructure Processing of Advanced Ceramics, Wiley, New York 1988 I Keesmann, Z Anorg Allg Chem , 1966,346, 30, S Komarneni, R Roy and E Breval, J Am Ceram Soc , 1985,68, C41 P Arnal, R J P Corriu, D Leclercq, P H Mutin and A Vioux, Chem Muter, in the press 25 G A RasuwaJew, L M Bobinowa and V S Etlis, Tetrahedron, 1959,6,154 Paper 6/04130B, Received 12th June, 1996 1932 J Matev Chem, 1996,6(12), 1925-1932