Decomposition Processes and Characterization of theSurface Basicity of Cl- and CO3

2- Hydrotalcites

Didier Tichit,* Mohammed Naciri Bennani,† Francois Figueras,‡ andJose Rafael Ruiz§

Laboratoire des Materiaux Catalytiques et Catalyse en Chimie Organique, U.M.R 5618,C.N.R.S/E.N.S.C.M 8, rue Ecole Normale, 34296 Montpellier Cedex 5, France

Received May 26, 1997. In Final Form: November 26, 1997

Thedecompositionunder thermal treatmentsofanhydrotalcite sampleprepared fromMgandAl chlorides,thus containing Cl- and CO3

2- as compensating anions, and of the same sample totally exchanged byCO3

2-, was followedby several characterization techniques (in situXRD, 27AlNMR,TG, chemical analysis).A similar structural evolution is found, whatever the compensating anion, with a progressive collapse ofthe interlayer space up to 573 K and an expansion of the lattice a parameter related to an extraction fromthe lattice ofAl3+,whose ionic radius is slightly lower than that ofMg2+. Athigher calcination temperatures,a Mg(Al)O phase is formed and a small amount of tetrahedrally coordinated Al and a line at d ) 0.254nm in the XRD pattern are detected. On the other hand, more Al is extracted from the chlorinated thanfrom the carbonated samples, either noncalcined or calcined at increasing temperatures, when treatedwith an alkaline solution (NaOH). The aluminum is removed at the reverse of chlorine. Therefore Al ispreferably removed fromtheweakerbasic sitesnotblockedbyCl-. This obviously shows that the restorationof a calcined LDH could not be totally reversible. Nevertheless the lamellar structure is restored whendispersed in water, with a different composition than that of the starting material. The adsorption ofpyrrole studied by IR spectroscopy reveals an interaction of this probe mainly in its undissociated formwith the OH groups. Rather different species are formed depending on whether the sample contains Cl-or not. From these species, an increase of the basicity is qualitatively inferred following the removal ofCl-.

Introduction

The layered double hydroxides (LDHs) of the hydro-talcite-like family can be represented by the generalformula [M(II)1-xM(III)x(OH)2]x+[Am-

x/m]‚nH2O, whereM(III) partially substitutes M(II) metallic cations inbrucite-like octahedral layers. This induces an excess ofpositive charge balanced by exchangeable anions in theinterlayer space where water is also present to fill theavailable space. Their interest in catalysis arises fromthe easily controlled composition and the diversity of thecations available in the structure (M(II))Mg,Ni, Zn, Cu;M(III))Al,Ga,Cr,Fe).1-4 Indeed there is theopportunityto obtainby thermaldecompositionmaterialswith severalcations randomly dispersed which are found to be basiccatalysts5-9 and also used in various catalytic reactionssuchasmethanation ofCO,10,11 synthesis ofmethanol and

higheralcohols,4Fischer-Tropsch reactions,12 alkylationsof phenol,13 and selective hydrogenation of nitriles.14The thermal treatment induces, for the highly hy-

droxylated LDHs, dramatic structural and textural modi-fications extensively studied for both hydrotalcite (Mg2+/Al3+/CO3

2-)6,15,16 and takovite (Ni2+/Al3+/CO32-)17,18 minerals

in the temperature range from 473 to 973 K, where thelayered structure is transformed intoamixedoxidephase.In addition, small amounts of a M(II)doped alumina andof an aluminate spinel type phase are also found, whichlargely contribute to the specific area, the thermalstability, and the reducibility of these type of catalysts.19,20The decomposition is even more complex with LDHscontainingCu,Co, andFe, due to the occurrence of severalcrystallographic phases with different coordination andoxidation states of these cations.21 Therefore the activa-

* Correspondingauthor.Fax: +330467144349.E-mail: tichit@cit.enscm.fr.

† Present address: Laboratoire de Chimie Physique, U. M. I.Faculte des Sciences de Meknes, Meknes, Maroc.

‡ Present address: Institut de Recherches sur la Catalyse duCNRS, 2 Av. A. Einstein, 69626 Villeurbanne Cedex, France.

§ On leave from University of Cordoba (Spain).(1) Reichle, W. T. Chemtech. 1986, 1, 58.(2) Jones, W.; Chibwe, M. In Pillared Layered StructuressCurrent

Trends and Applications; Mitchell, I. V., Ed.; Elsevier Applied Science:London and New York, 1990; p 67.

(3) De Roy, B.; Forano, C.; El Malki, K.; Besse, J. P. In Synthesis ofMicroporous Materials; Occelli, M. L., Robson, H., Eds.; Van NostrandReinhold: New York, 1992; Vol. 2, p 108.

(4) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 2, 11.(5) Suzuki, E.; Ono, Y. Bull. Chem. Soc. Jpn. 1988, 61, 1008.(6) Reichle, W. T. J. Catal. 1985, 94, 547.(7) Corma, A.; Fornes, V.; Martin-Aranda, R. M.; Rey, F. J. Catal.

1992, 134, 58.(8) Corma, A.; Martin-Aranda, R. J. Catal. 1991, 130, 130.(9) Tichit, D.; Lhouty, M. H.; Guida, A.; Chiche, B. H.; Figueras, F.;

Auroux, A.; Bartalini, D.; Garrone, E. J. Catal. 1995, 151, 50.

(10) Kruissink, E. C.; van Reijen, L. L.; Ross, J. R. H. J. Chem. Soc.,Faraday Trans. 1 1981, 77, 649.

(11) Alzamora, L. E.; Ross, J. R. H.; Kruissink, E. C.; van Reijen, L.L. J. Chem. Soc., Faraday Trans. 1 1981, 77, 665.

(12) Bruce, L.; Takos, J.; Turney, T. W. In Novel Materials inHeterogeneous Catalysis; Baker, R. T., Murrell, L. l., Eds.; ACSSymposium Series 437; American Chemical Society: Washington, DC,1990; p 129.

(13) Velu, S.; Swamy, C. S. Appl. Catal., A General 1994, 119, 241.(14) Medina, F.; Tichit, D.; Coq, B.; Vaccari, A.; Thy Dung, N. J.

Catal. 1997, 167, 142.(15) Belloto,M.;Rebours,B.;Clause,O.; Lynch, J.;Bazin,D.;Elkaım,

E. J. Phys. Chem. 1996, 100, 8535.(16) Rebours, B.; d’Espinose de la Caillerie, J. B.; Clause, O. J. Am.

Chem. Soc. 1994, 116, 1707.(17) Trifiro, F.; Vaccari, A.; Clause, O. Catal. Today 1994, 21, 185.(18) Tichit, D.; Medina, F.; Coq, B.; Dutartre, R. Appl. Catal., A:

General, 1997, 159, 241.(19) Clause, O.; Goncalves Coelho, M.; Gazzano, M.; Matteuzzi, D.;

Trifiro, F.; Vaccari, A. Appl. Clay Sci. 1993, 8, 169.(20) Fornasari, G.; Gazzano, M.; Matteuzzi, D.; Trifiro, F.; Vaccari,

A. Appl. Clay Sci. 1995, 19, 69.(21) Porta, P.; Morpurgo, S.; Pettiti, I. J. Solid State Chem. 1996,

122, 324.

2086 Langmuir 1998, 14, 2086-2091

S0743-7463(97)00543-X CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 03/21/1998

tion and the regeneration of the catalysts issuing fromLDHs are complex processes whichmodify the structure,the composition, and also the acido-basicity of thesematerials. In a previous paper we pointed out theinfluence of Cl- and CO3

2- on the basicity of the mixedoxides used as catalysts in the Claisen-Schmidt conden-sation reaction of benzaldehyde and acetone.9 Catalystsof high basic strength are obtained when Cl- ions aretotally exchanged with CO3

2- in hydrotalcite and whenthe calcination temperature is in the range 673-773 K.The aim of this paper is to analyze the influence of

several treatments involved in the activation and theregeneration processes of the hydrotalcites used ascatalysts, that is, exchanges with CO3

2-, calcinations,rehydrations, and leaching with alkaline solutions ableto eliminate minor additional phases. The structuralmodifications were followed by chemical and in situ XRDanalysis. At the same time the variation of basicityinduced by the removal of Cl- was studied. The use ofseveral complementary methods is generally required tocharacterize the basicity due to the lack of an ideal probemolecule.22 Nevertheless pyrrole has been successfullyused as a probe molecule to investigate the basicity ofsolids close to hydrotalcite, like MgO23 and alumina,23,24Al2O3/MgO mixed oxides,25 or other basic solids such asalkaline exchange zeolites.26,27 Dissociated or undisso-ciated species can be formed when pyrrole interacts withbasic sites, depending on their strength. Pyrrolate anionsare thus observed when pyrrole dissociates on the strongO2- sites while pyrrole molecules are hydrogen bonded toless basic O2- or OH- sites.24 Pyrrole acts in these casesas an acid probe through its hydrogen donor property.But pyrrole is also a hydrogen acceptor through the πelectrons. This makes the spectra and the assignment ofthe peaks quite complex. However, despite these draw-backs, pyrrole has been widely used to characterize basicsolids due to its weak acidity and to the possible correla-tion, existing for the nondissociated adsorbed pyrrole,between the ν(NH) stretching frequency and the basicstrength of the solids.26,27 This makes this methodavailable forbasicitiesmeasurements. Wedecided in turnto use this probemolecule to examine the influence of theanionic exchangeon thebasicity, at least at thequalitativelevel.

Experimental Section

Materials Synthesis. Hydrotalcite (HT) was prepared bycoprecipitation of an aqueous solution containing MgCl2‚6H2Oand AlCl3‚6H2O with a total cation concentration of 1.5 M andan aqueous solution containingNaOH (2.5M) andNa2CO3 (0.05M). The pH was maintained between 8 and 10 ( 0.2, and theMg/Al ratiowasadjusted to3. Theadditionwascompletedwithin2 h under vigorous stirring. The gel was then aged overnightat 330K in themother liquor, filtered,washedwithhot deionized

water until the solution was free of chloride ions (AgNO3 test),and dried at 353 K.HTE was obtained fromHT by removing the Cl- by exchange

with CO32- anions. With this aim, 2 g of hydrotalcite was

suspended in 200 mL of a Na2CO3 solution (1.5 × 10-2 M) andstirred at 343 K for 2 h. To achieve a higher degree of exchange,after removing the solution, the solid was repeatedly suspendedin a fresh one. Afterward it was filtered off, washed, and driedat 353 K.Treatments were performed on hydrotalcite samples either

uncalcined or calcined for 5hat 573, 723, and873K. The samplepowder was suspended under magnetic stirring in 1 M or 4 MNaOH solutions, during 2 h at 300 K. After decantation,filtration, and drying at 313 K, the products were stored in air.For identification the sampleswere later noted, for instanceHT-573-1M, where 573 is the temperature of calcination and 1M isthe molarity of the NaOH solution.Characterization. Chemical analyses were performed at

the Service Central d’Analyse du CNRS (Solaize, France).The calcinations were carried out in a thin bed configuration

using a horizontal furnace flushed with an air flow of about 100mL min-1. The heating rate was 0.8 K h-1, and the finaltemperature was maintained for 5 h.The X-ray powder diffraction (XRD) patterns were recorded

on a CGR Theta 60 instrument using monochromatized Cu KR1radiation at λ ) 1.5405 Å (40 kV and 50 mA). The crystal sizeswere determined by the Scherrer equation using the averagevalue of the [003] line breadth. Structural evolutions duringthermal treatment under air were followed in situ with a high-temperature XRD attachment (Barret-Gerard system) adaptedon a CGR Theta 60 goniometer. Calcinations were performedunder flowing air (50 mL min-1) from 273 to 773 K (ramp 5 Kmin-1) by steps of 1 h every 100 K. Studies of the rehydrationwere also performed in situ on the calcined samples afterreturning to room temperature. With this aim the XRD cell isswept by flowing N2 (80 mL min-1) saturated with the vaporpressure of H2O at 273 K. Patterns were registered at definedintervals of time.Specific surface areas were determined by N2 at 77 K (BET

method) using Micromeritics ASAP 2000 equipment. Thesamples were first outgassed (2 × 10-4 Torr) overnight at 573K.Thermogravimetric analysis (TG)was carriedout inaSetaram

microbalance, operated under a flow of dry nitrogen (110 mLmin-1) at a heating rate of 5 K min-1 from 273 to 1073 K on 20mg of sample.

27MASNMRspectrawere recordedat78.17MHzwithaBrukerAM-300 spectrometer at room temperature. The length of therf pulses was 2µs, and the spinning frequency was in the range4-5 kHz. Three hundred scans were accumulated.Pyrrole adsorption was followed by means of IR spectroscopy

on self-supported wafers of hydrotalcites mounted in a Pyrexcell and evacuated overnight at 773 K after an increase intemperature of 0.5 K min-1. Freshly distilled pyrrole was thenadsorbed at room temperature for 10min followed by evacuationat room temperature for 1 h. Spectra were recorded using aNicolet 320 FT-IR spectrometer in the range between 4000 and400 cm-1.

Results

The chemical compositions and the crystallographicparameters of the initial (HT) and the exchanged sample(HTE) are given in Table 1. The molar fraction in Al isslightly lower than that in the solution, and more than10% of the anionic charge is due to Cl anions provided bythe synthesis salts. The XRD patterns are those of ahydrotalcite phase with intense diffraction peaks due toplanes [00l] (Figure 1a). The exchange of Cl- for CO3

2-

(22) Lavalley, J. C. Trends Phys. Chem. 1991, 2, 305.(23) Scokart, P. O.; Rouxhet, P. G. J. Chem. Soc., Faraday Trans.1

1980, 76, 1476.(24) Binet, C.; Jadi, A.; Lamotte, J.; Lavalley, J. C. J. Chem. Soc.,

Faraday Trans. 1996, 92, 123.(25) Lercher, J. A.; Colombier, C.; Noller, H. Z. Phys. Chem. N. F.

1982, 131, 111.(26) Huang, M.; Kaliaguine, S. J. Chem. Soc., Faraday Trans. 1992,

88, 751.(27) Barthomeuf, D. J. Phys. Chem. 1984, 88, 42.

Table 1. Some Characteristics of the Hydrotalcite Samples

samplec

(nm)a

(nm)D003(nm)

SBET(m2 g-1) chemical formula

HT 2.349 0.305 17 105 [Mg0.73Al0.27(OH)2][CO32-

0.143Cl-0.043]‚0.41H2OHTE 2.317 0.305 23 80 [Mg0.74Al0.26(OH)2][CO3

2-0.166]‚0.42H2O

Cl- and CO32- Hydrotalcites Langmuir, Vol. 14, No. 8, 1998 2087

performed in the alkaline Na2CO3 solution decreases, asexpected, the interlayer distance governedby the size andthe orientation parallel to the layers of the CO3

2- anions.The crystallinity and the average crystal sizes increase,showing that dissolution of the smaller particles is takingplace. The higher excess of anionic charge could beaccounted for by the presence of bicarbonate species.The XRD patterns registered in situ from room tem-

perature to 773 K show a similar trend for the decom-positionof these structuresas shown for example inFigure1 for HT. Up to 373 K, the departure of a small amountofphysisorbedwaterdoesnotmodify the layeredstructure.At 473 K (Figure 1b) dehydration induces a decrease ofabout 1 nm of the basal spacing. At the same time the[006] reflection disappears and the [01l] reflections areshifted and become broader, showing a disorder of thestructure and the appearance of stacking defaults. Thelayer structure ismaintainedup to 573K (Figure 1c)withan additional decrease of d003 to 0.65-0.67 nm. A greatchange is noticed at 673 K (Figure 1d) when dehydrationand decomposition of the anions are taking place. Broadlines at d ) 0.209 nm and d ) 0.148 nm correspondingto the [400] and [440] reflections of a mixed oxide phasewith theMgO type structure are detected. Its structuralparameters are lower than those in the pure MgO rock-salt structure, showing that Al3+ ions of smaller ionicradius thanthatofMg2+are inserted into thestructure.16,28Inaddition, abroadpeak,notbelonging to themixedoxide,appears atd) 0.254 nm. At 773K the sharpness and theintensities of the lines of the mixed oxide and of the peakat d ) 0.254 nm increase, in line with the progressiverearrangement of the structure. In addition to theprogressive collapse of the layered structure, the latticea parameter increases in the same temperature range(Figure 2). In the hydrotalcite lattice it is always lowerthan that in brucite (a) 0.314 nm), due to the isomorphicsubstitution ofMg2+ byAl3+. The lattice a parameter andtheAl/Mg+Al ratiowill thenobey theVegardcorrelation.4During calcination, both the increase of the lattice aparameter and the broadening of the [01l] reflections ofthe LDH suggest an extraction of Al from the layer. Thevariation of the lattice a parameter between room tem-perature and550K fitswellwith that reported previouslyin the same temperature range.28 From the Vegard

correlation, assuming that a ) 0.314 nm for brucite anda ) 0.306 nm for hydrotalcite with Al/Mg + Al ) 0.25,4it is estimated, from the increase of the a parameter, that10 wt % of Al is extracted at 573 K either from thechlorinated (HT) or from the exchanged sample (HTE).

27Al MAS NMR spectra of HT (Figure 3), show for theuncalcinedsampleasinglepeakat10.40ppmofoctahedralAl and, after calcination at 723 K, an additional peak at74 ppm, in agreement with the value reported fortetrahedral Al.29,30

A rehydration was performed at room temperature onHTE calcined in situ at 723 K. The initial XRD patternis that of the mixed Mg(Al)O oxide (Figure 4). Thestructure is unchangedafter 1hof rehydration. The [003]and [006] lines of the hydrotalcite phase are clearlydetected after 3.5 h (Figure 4b). Then the amount of thelayered structure progressively increaseswith time to thedetriment of the mixed oxide phase. After 12 h ofrehydration (Figure 4c), the reconstructed hydrotalcite isthe major phase and a residue of mixed oxide is stilldetected. The layered structure gives more narrow linesabove 24 h (Figure 4d). The width of the lines stillincreases compared to those for the fresh HTE sample,corresponding to a decrease of the average particle sizefrom 23 to 10 nm. The broadening between 2Θ ) 30 and60° in the reconstructed sample suggests the existence ofa small amount of an amorphous phase and that thestructure is less ordered than initially. Therefore therehydration process is not totally reversible. It is well-

(28) Gazzano, M.; Kagunya, W.; Matteuzzi, D.; Vaccari, A. J. Phys.Chem. B 1997, 101, 4514.

(29) McKenzie, A. L.; Fishel, C. T.; Davis, R. J. J. Catal. 1992, 138,547.

(30) Reichle,W. T.; Kang, S. Y.; Everhardt, D. S. J. Catal. 1986, 101,352.

Figure 1. XRD profile of HT recorded in situ in the XRDattachment at (a) room temperature, (b) 473 K, (c) 573 K, (d)673 K, (e) 773 K. (*) additional phase at d ) 0.254 nm.

Figure 2. Lattice parameter a versus temperature for (b) HTand (9) HTE.

Figure 3. 27Al MAS NMR spectra of (a) HT, (b) HT calcinedat 723 K, and (c) HT-573-1N.

2088 Langmuir, Vol. 14, No. 8, 1998 Tichit et al.

known that the reconstruction of the lamellar structurefor LDHs calcined at increasing temperatures needsexposure to air for increasing timeand evena rehydrationin water for those heated at 773 K.31Therefore the thermal decomposition of hydrotalcite

leads to the formation of a mixed oxide phase with theMgO rock-salt structure. An additional line at d ) 0.254nm in the XRD pattern and tetrahedrally coordinated Alare also evidenced. They have been previously assignedtoamagnesiumaluminate typephase.16,17 Furtherstudiesusingneutrondiffraction andRietveld refinements onNi/Mg/Al samples28haveshownthat therock-salt oxide latticeand the tetrahedrally coordinated Al may be accountedfor by a nonstoichiometric spinel phase, which is asupercell with the a lattice parameter twice that of theoxide. The rehydration under water vapor during 24 hleads to a recovering of a less ordered layered structurewith a smaller particle size than that of the initial sample.Leachings with NaOH. The samples either uncal-

cined or calcined up to 873 K (Table 2) were submitted toleachings with NaOH. After redispersion in water forwashing, theirXRDpatternsare characteristic of a layeredstructurewith someadditional lines in increasingamount

with the Al extracted (Figure 5). The tetrahedrallycoordinated aluminum formed under calcination is nolonger detected in the solid after leaching and successivewashing. Indeed 27AlNMRspectra of thewashed samples(Figure 3c) showat 10.25-10.40 ppm the peak of Al in theoctahedral environment, thus belonging to the LDHstructure. Two relevant points concern the chemicalcompositions of the samples after leachings (Table 2). Inall cases (i) aluminum is removedand (ii) Cl- is exchangedby CO3

2-. The total anionic charge is always larger thanthat expected to ensure the chargebalance. This suggeststhat there are no OH- ions as compensating anions andthatCO3

2- ions, providedbyair, arepreferentially trappedduring this treatment, due to thehighaffinity of theLDHsfor these species.3,4 The comparison of the differentsamples (Table 2) leads to several additional commentsconcerning particularly the amount of aluminum ex-tracted. It increases (i) on the samples calcined at 723 Kand (ii) with the concentration of the NaOH solution;indeed it is four times higher in HT-573-4M than in HT-573-1M. Otherwise, the strong influence of the chlorinecompensating anions on the dealumination process mustbe pointed out. Indeed, it is obvious that, following thesame alkaline treatments, higher amounts of aluminumare removed from the chlorinated samples. In line withthisbehavior, it is also interesting tonote that theamountsof Al and Cl removed from the HT sample, calcined atdifferent temperatures, after leaching with NaOH (1 N),are in an opposite way (Figure 6). Therefore, the dealu-mination process is boosted when Cl remains in thestructure.IRSpectroscopy of AdsorbedPyrrole. The typical

spectrum in the hydroxyl region of HT outgassed at 623K gives a broad band between 3500 and 3650 cm-1 anda shoulder at 3705 cm-1 (Figure 7), respectively assigned,by comparison to those of MgO,23 to superficial bridgedhydroxyls and to free hydroxyl groups. When outgassingis performed at 773 K, dehydroxylation goes on and theintensity of the broad bandat 3500-3650 cm-1 decreases,while that of the free hydroxyl groups is shifted towardhigher frequencies, giving a sharp band at 3715 cm-1.Pyrrole, adsorbed at its vapor pressure at room temper-ature, interacts with HT and HTE outgassed at 773 K.The most intense band is observed in both cases at 3400cm-1. ConcerningHT, several bands appear in thehigherfrequency domain: a very weak band at 3990 cm-1, a

(31) Rey, F.; Fornes, V.; Rojo, J. M. J. Chem. Soc., Faraday Trans.1992, 88, 2233.

Figure4. XRDprofiles ofHTrecorded in thehigh-temperatureXRD attachment at room temperature after (a) 1 h, (b) 3.5 h,(c) 12 h, and (d) 24 h of treatment under water.

Table 2. Cl Content and Amount of Al Extracted byLeaching of the Hydrotalcite Samples

sampleCl content

(anionic charge %)Al extracted

(wt %)

HT 13 -HT-1N 0 7HT-573-1N 1.20 9.60HT-723-1N 4.90 14.40HT-873-1N 1.25 7.75HT-573-4N 2.25 41HTE 0HTE-1N 0 4.90HTE-573-4N 0 9.75HTE-723-1N 0 12.75

Figure 5. XRD profiles after washing of (a) HT-723-1N and(b) HT-573-4N: (*) additional phase.

Cl- and CO32- Hydrotalcites Langmuir, Vol. 14, No. 8, 1998 2089

shoulder at 3725 cm-1, and a broad band between 3750and 3500 cm-1 with a maximum peaking at 3545 cm-1

(Figure 8). Between 3200 and 2500 cm-1 the main bandsare located at 3142, 3110, 3060, 2977, 2947, and 2874cm-1. They are in the spectral range of the ν(CH)vibrations.24 Following the anionic exchangewithCO3

2-,the spectra is slightly modified (Figure 8). On HTE theintensity of the band between 3750 and 3500 cm-1

increases and its maximum shifts to 3610 cm-1. At the

same time a weak band appears around 3200 cm-1 andthe intensities of those at 3060, 2977, and 2874 cm-1

increase. Spectra with similar general shapes are foundwhen pyrrole is adsorbed on different basic solids, suchas for example partially dehydroxylated alumina,23,24MgO23, or alkaline-exchanged zeolites.26,27 The band at3400 cm-1 is also found in liquid pyrrole and is thereforeassigned to the ν(NH) stretching vibrations of pyrroleinteracting with itself. Therefore, the interaction ofpyrrole with the surface is strong, since the presence ofthis band at 3400 cm-1 is indicative of a high surfacecoverage. Otherwise the combination band at 3990 cm-1

and the broad ν(OH) band in the 3750-3500-cm-1 rangeare found when pyrrole in its undissociated form ishydrogen bonded with OH species.24 The increase in theintensity of the band at 3610 cm-1 and of the differentbands in the 3200-2500-cm-1 rangewhen going fromHTto HTE could result from a higher amount of pyrroleadsorbedon the surface. Asasimilar trend isnot observedfor the band at 3400 cm-1, this evolution more certainlyaccounts for slightlydifferent interactionsbetweenpyrroleand surface basic species. Indeed bands at 3980, 3530,and 3370 cm-1 are found when a linear hydrogen-bondedpyrrole molecule interacts with bidentate surface OHgroups on partially dehydroxylated alumina.24 On theother hand, a band at 3610 cm-1 is reported for pyrroleadsorbed on MgO23 and assigned to a cyclic hydrogenbridge of a pyrrolemolecule bonded to amonodentate OHat the surface. In the 3100-2900-cm-1 range bandsprobably accounting for pyrrolate anions24 also increaseafter removal of Cl-. A band located around 3260 cm-1

on alumina24 and in the 3300-3200-cm-1 range fordifferentalkaline-exchangedXzeolites27hasbeenassignedto the NH stretching vibration of pyrrole adsorbed onstrong basic O2- sites. This ν(NH) frequency decreaseswhen the basic strength of the sites increases. The weakshoulder around 3200 cm-1 on HTE accounts, at best, fora low amount of such strong basic sites. Normally ν(CH)bands in the 1200-1600-cm-1 frequency range are ableto confirm the existence of pyrrolate anions. Particularlyν(ring) stretching vibrations of pyrrole interacting eitherwith monodentate or bidentate OH give distinct lines inthe same domain. Unfortunately there is a poor trans-mittance in this frequency range. It results fromthegreatamount of remaining carbonate species. Itmust be notedthat strong basic sites are probably blocked by thesespecies. Similar poisoning by CO3

2-, well-known in basicsolids, is also reported for MgO and Al2O3.23

Discussion

Theavailability of theMg/Alhydrotalcitesasprecursorsfor basic mixed oxides of the MgO type is wellrecognized.3-9 From the structural point of view, thememory effect often described suggests that dehydrationand decarbonation of hydrotalcite into a mixed oxide is areversible process.31,32 It takes place spontaneously inair, when the sample has been calcined at a temperaturebelow 500 K. This property has been used to exchangevaporizable anions as CO3

2- by different salts of organicand inorganic species.33-35 With this aim, a calcinationat a moderate temperature is first performed; then themixed oxide is restored by dispersion in an aqueous

(32) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34,883.

(33) Perez-Bernal, M. E.; Ruano-Casero, R.; Pinnavaia, T. J. Catal.Lett. 1991, 11, 55.

(34) Drezdon, M. A. Inorg. Chem. 1988, 27, 4628.(35) Chibwe, K.; Jones, W. J. Chem. Soc., Chem. Commun. 1989,

926.

Figure 6. Amount of Cl exchanged and of Al extracted fromHT calcined at different temperatures after leachings withNaOH.

Figure 7. Infrared spectra in the OH frequency range of HToutgassed overnight at (a) 623 K, (b) 700 K, and (c) 773 K.

Figure 8. Infrared spectra of pyrrole adsorbed on (a) HT and(b)HTE outgassed at 773K and following desorption of pyrroleduring 1 h at room temperature.

2090 Langmuir, Vol. 14, No. 8, 1998 Tichit et al.

solution containing thedesiredanion. Nevertheless,moreextensive studies involving EXAFS, in situ X-ray diffrac-tion, 27Al MAS NMR,16 and neutron diffraction28 undoubt-edly show that the reconstruction of the mixed MgO typeoxide into the lamellar structuremustbehardly reversiblebecause dramatic structural modifications are takingplace, the main one being, above 500 K, the formation ofa nonideally mixed rock-salt phase. It accounts for theexistence of tetrahedrally coordinated aluminum cationsin a supercellwith ana lattice parameter twice that of thestoichiometric oxide in a defective spinel-like structure.28Otherwise, it has been shown that the decompositionprocess and the thermal stability are influenced by theadmixture ofMg-Alhydroxycarbonates oraluminaphaseto the hydrotalcite.36 In line with these results, we alsoshow that several treatments currently involved in theactivation of a catalyst and specifically anionic exchanges,calcinations, and regenerations induce irreversible modi-fications of the structural, textural, and basic propertiesof hydrotalcite. The most important structural modifica-tion found either after dispersion of the hydrotalcite intoalkalineNa2CO3andNaOHsolutionsorafter calcinations,results from the migration of Al, which could evensegregate into alumina in the most severe conditions. Inthis field, themainresult of thiswork is toput intoevidencethat the nature of the anion, and particularly chlorine,greatly influences these processes and the basicity. Themigration of Al is initiated in the lamellar structure, asshown by the increase of the lattice a parameter forcalcination temperatures below 573 K. This is in ac-cordance with the decomposition process proposed byBellotto et al.,15 at least up to 473 K, showing that, uponheating, the layered structure is maintained but withmigration of Al cations out of the octahedral brucite typelayers to tetrahedral sites in the interlayer. We alsoevidenced by 27Al NMR the formation in the sametemperature range of tetrahedrally coordinated Al. Theexpansionof thea latticeparametershowsthat itsamount,after calcination at 573 K, is close to 10%, as foundpreviously.28 The variations of basicity are linked to ourprevious published results,9 showing that the catalyticactivity for the condensation reaction of benzaldehydeandacetone dramatically increases whenCl- is exchanged byCO3

2-. On the other hand, calorimetric and IR analysisshows that the amount of basic sites greatly increaseswhen Cl- is exchanged by CO3

2- and particularly sites ofhigh strength appear. The influence of the anions on thebasic properties has also been examined by Constantinoand Pinnavaia.32 LDH in the chloride form works as abasic catalyst for 2-methyl-3-butyn-2-ol conversion whenactivated below 523 K, but highly acid sites are formedin the mixed oxide activated up to 723 K. A mechanismwas proposed to account for these results, where Cl ionsreplace some OH ions of the hydrotalcite layer when it iscalcined above 573 K. The remaining OH groups, boundtometal centers and adjacent to the Cl ions, are Bronstedacids due to the electron withdrawing effect of the latter.Concerning the acid-base properties, this work empha-sized thecomplex interactionofpyrroleactingasaH-donor

or as a π electron-donor molecule with the hydrotalcitewhich contains a priori different types of basic sites (OH-,O2-,CO3

2-) andalsoLewisacid sites (Al3+). It is confirmedononehand thatpyrrole interactswith the surfacemainlyin its undissociated form, giving species hydrogen bondedwith eithermonodentate or bidentateOHgroups. On theother hand dissociation of pyrrole into pyrrolate anionson strong basic sites is not excluded. Last, following theexchange of CO3

2- for Cl-, ν(NH) vibrations of pyrroleadsorbedonO2- sites, generally recognizedasvery strong,are inferred from theweak ν(NH) bandaround3200 cm-1.Therefore a tendency appears toward an increase of thebasicitywhengoing fromHTtoHTE, that is, uponremovalof Cl-. Moreover it must be noted that the stronger basicsitesareprobablyblockedbyresidualCO3

2-anions. Theseresults confirmthat thenatureof the compensatinganionsis a main parameter governing the basicity of thesematerials.32 Themixed oxides issuing fromhydrotalcites,with basicities in the range of alumina and MgO, have adifferent behavior than those resulting fromamechanicalmixture of MgO and Al2O3 described previously25 wherepyrrole acted as an electron donor molecule with thesurface hydroxyls. However a comparisonwithMgOandAl2O3, whose basicities are very controversial, has notbeen considered in this work. Scokart et al.23 foundstronger basic sites on alumina than on MgO, while J. A.Lercher et al.25 found the reverse. Concerning alumina,Binet et al.24 foundan intermediatebasicitybetween thoseof alkali-metal zeolites and ceria. The comparison withsuch basic solids shows in any case that the hydrotalciteslead to an average basic strength, suggesting that themost basic sample is very close to MgO. Del Arco et al.37reported that mixed oxides obtained by calcination ofhydrotalcites at 1073 K have a surface basicity related tothat of isolated hydroxyl groups. They dissociativelyadsorbed formic acid. Despite the different activationtemperature, this seems to be in line with the results ofthis work.In conclusion, Cl--containing hydrotalcites are ir-

reversibly modified by calcination and then would behardly reactivated when they are used as catalysts orrestored in their initial lamellar form for anionic exchangereactions. Al is weakly bonded and therefore preferablyremoved fromthe structurewhennearby sites are blockedwithCl-. TheexchangeofCl- forvaporizableanionsoffersabetterpossibility of regenerationof thebasicityby simpleheat treatment. It is worth noting that dramatic effectsresult fromthepresenceof anamountofCl- correspondingtoabout15%of the total anionic charge of thehydrotalcite.This means that chlorine must be avoided when goodreversibility andhighbasicity levels are required. Pyrroleis poorly adapted for the characterization of the basic sitesof thesematerials particularlywhen slight variations areconcerned due to its complex interactionwith the surface.

Acknowledgment. Jose Rafael Ruiz thanks ELF-Atochem for the financial support as apostdoctoral fellow.

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(36) Valcheva-Traykona,M.L.;Davidova,N.P.;Weiss,A.H.J.Mater.Soc. 1993, 28, 2157.

(37) Del Arco, M.; Martin, C.; Martin, I.; Rives, V.; Trujillano, R.Spectrochim. Acta 1993, 11, 1575.

Cl- and CO32- Hydrotalcites Langmuir, Vol. 14, No. 8, 1998 2091