Langmuir 1991, 7, 2677-2681
2677
Surface Basicity of Mixed Oxides: Magnesium and Zinc Aluminates Pier Francesco Rossi,t Guido Busca,*lt Vincenzo Lorenzelli,t Mohamed Waqif,t Odette Saw,* and Jean-Claude Lavalley* Istituto di Chimica, Facolth di Ingegneria, Universitb di Genova, Razzale Kennedy, 1-16129 Cenova, Italy, and Laboratoire de Catalyse et Spectrochimie, URA CNRS 414, ISMRa, F-14050 Caen Ceder, France Received November 26,1990. In Final Form: June 24, 1991 The surface basicity of magnesium and zinc aluminates has been investigated by adsorption microcalorimetry, using hexafluoro-2-propanol as an acidic probe molecule. The data have been compared to those of the component oxides AlzO3, MgO, and ZnO as well as with two aluminate samples having an excess of bivalent ions with respect to the spinel stoichiometry. The surface properties of these materials have also been investigated by Fourier transform infrared spectroscopy of surface hydroxy groups and of adsorbed pyridine. Evidence is provided for a strong parallelism of the characteristics of the two stoichiometric spinels MgA1204 and ZnA1204. The strength and distribution of surface basic sites on these materials closely reproduce those of alumina. For stoichiometric materials the basicity scale is MgO > ZnO > MgA1204 = ZnAl204 A1203. Nonstoichiometric aluminates having an excess of bivalent ion show an increased basicity, still lower than that of the corresponding bivalent cations oxides.
Introduction Magnesium aluminates find application in the fields of heterogeneous catalysts as active phases (e.g. in SO, catalytic abatement' and in propylene oxide polymerization2) or as mechanically strong supports (e.g. for ammonia synthesis catalysts3) in the adsorption of sulfur oxides4 as well as in humidity sensor te~hnology.~ Zinc aluminates are also of interest in surface chemistry and c a t a l y s i ~ . These ~ * ~ compounds have in common the spinel structure and can consequently be considered structurally similar to transitional aluminas that have defective spineltype structures (7,6, tl, and 0 phases). However, the materials used for the above applications have frequently an excess of bivalent ions with respect to the spinel stoichiometry. Surface characterization studies generally define transitional aluminas as acidic materials while MgO and ZnO are considered typically as basic oxides.819 So, magnesium and zinc aluminates can be viewed as constituted by a typical Lewis acidic matrix (that of transitional aluminas) modified by ions arising from basic oxides. Recently, we proposed a method to characterize through adsorption microcalorimetry the surface basicity of some metal oxides.I0 In this paper we use the same method, coupled with Fourier transform infrared (FT-IR) investigations of adsorbed probe-molecules, to characterize the surface chemistry of zinc and magnesium aluminates, in ~~
t
~
UniversitB. de Genova.
Laboratoire de Catalyse et Spectrochimie.
(1)Bhattacharyye,A. A.; Woltermann,G. M., Yoo, J. S.; Karch, J. A.; Cormier, W. E. Ind. Eng. Chem. Res. 1988,27, 1356. (2)Kohjiya, S.;Sato, T.; Nakeyama, T.; Yamashita, S. Makromol. Chem. Rapid Commun. 1981,2, 233. (3)Reforming Catalystsfor the Production ofdmmonia; The British Sulphur Co. Ltd.: London, 1988. (4) Waqif, M.; Saur, 0.; Lavalley, J. C.; Wang, Y.;Morrow, B. A. Appl. Catal. 1991, 71, 319. (6) Shimizu, Y.;Arai, H.; Seiyama, T. Sensors Actuators 1985, 7, 11. (6)Tanabe, K.; Shimazu, Ke.; Hattori, H. J. Catal. 1979,57, 35. (7)Chaumette, P.;Courthy, Ph.; Barbier, J.; Fortin, T.; Lavalley, J. C.; Chauvin, C.; Kiennemann, A.; Idriss, H.; Sneeden, R. P. A.; Denise, B. h o c . 9th ICC, 1988; p 585. (8) Tanabe, K.; Misono, M.; Ono,Y.; Hattori, H. New Solid Acids and Bases; Elsevier: Amsterdam, 1989. (9)Auroux, A.; Gervasini, A. J. Phys. Chem. 1990,94, 6371. (10)Rossi, P.F.; Busca, G.; Lorenzelli, V.; Lion, M.; Lavalley, J. C. J. Catal. 1988, 109, 378.
0743-7463/91/2407-2677$02.50/0
order to check the surface acidlbase character of mixed oxides arising from an acidic and a basic component. The investigation is extended to the two spinel-type compounds and to the three corresponding binary oxides (MgO, ZnO, and &03), as well as to two materials having an excess of bivalent cations with respect to the spinel stoichiometry, used in the industry as catalyst supports.
Experimental Section Alumina (RhSne-Poulenc),magnesiumoxide (CarloErba)and stoichiometric magensium aluminate (Dow) are commercial products. Others have been supplied by Institut Franqais du PBtrol (IFP). They have been prepared by coprecipitation from water solutions of the mixed nitrates under controlled pH and temperature conditions, followed by washing, drying, and calcination. Analytic grade commercial materials were used. The preparation of the zinc aluminates has been the object of a publication.1l The elemental compositionof these materials has been checked by atomic absorption spectroscopy to correspond to the formulas given in Table I. Before any adsorption experiment the solids were activated by heating under air at 450 O C for 2 h and then evacuated 1 h at 450 O C . For IR studies, they were used in the form of selfsupported pellets. The IR spectra have been recorded by Nicolet MX1 FT-IR instruments, using heatable, evacuable quartz IR cells (NaC1windows). Microcalorimetric experiments were carried out at room temperature with a Tian-Calvet heat-flow microcalorimeter, equipped with a Setaram NV724 amplifier nanovoltmeter and a Servotrace-Seframrecorder. The sample weight was between 0.3and 0.8 g. Gravimetric experimentswere carried out on a McBain microbalance at room temperature. Surface areas have been measured by the BET method while phase composition has been determined by X-ray diffraction (XRD) analysis, using a Phillips 1130 instrument (Co Ka radiation).
Results and Discussion Before studying the surface basicity, we report the data relative to the structural characterization, based on XRD data, and to the qualitative surface characterization by IR (11) Tehevenot, F.; Szymanski, R.; Chaumette, P. Calys Clay Miner.
1989, 37, 395.
0 1991 American Chemical Society
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2678 Langmuir, Vol. 7, No. 11,1991
spectra of surface hydroxy groups and of adsorbed pyridine, used as a probe molecule for acid sites identification. Structural Characterization. The structural characterization of these samples has been performed using XRD, by comparison with the JCPDS tables of the pure compounds. The alumina preparation is a very poorly crystallized y form (JCPDS table 10-425); ZnO and MgO are the pure stable phases (zincite,JCPDS table 36-1451, and periclase, JCPDS table 4-0829). The XRD patterns of the two magnesium aluminate samples show only the peaks of the mineral spinel, MgAl204 (JCPDS table 211152),MgO being not detected at all. This is not surprising, because it is known that MgA1204 can dissolve small amounts of MgO leading to nonstoichiometric solid solution spinels.12 The XRD patterns of zinc aluminates show together the diffraction lines of the spinel ZnAl204 (gahnite, JCPDS table 5-669) and of ZnO. The strong difference in line width and height of the aluminate peaks (broad and strong) and of the oxide (sharp and weak) suggests that together with a predominant microcrystalline spinel phase, few big crystals of zinc oxide are present. Surprisingly, the relative intensity of the ZnO diffraction peaks is stronger in the nominally stoichiometric sample than in the nonstoichiometric one, while it does not exceed a few percent in both cases. It is reasonable to conclude that, although strictly speaking the Zn alumiante samples are not monophasic, the surface is almost entirely due to a more or less stoichiometric spinel phase. Surface Characterization. (a) Surface Hydroxy Groups. The FT-IR spectra of all samples after activation show sharp bands in the 3800-3600 cm-l range due to the stretching vibrations of surface free hydroxy groups. Some of them also present bands in the 1500-1300 cm-l range due to residual carbonate species. The spectra of the pure oxides in the UOH region agree with those reported in the literat~re.'~On alumina, bands are observed at 3790 (shoulder), 3773,3720, and 3675 cm-l, on MgO essentially a single band is observed a t 3750 cm-l, while on ZnO, bands are observed at 3670,3640, and 3618 cm-l. In Figure 1, the spectra in the UOH region of the metal aluminate samples, after activation, are reported. On both stoichiometric metal aluminates a strong band is observed (centered at 3735 cm-l on MgA1204 and at 3695 cm-1 on ZnAl204) with small shonlders (3750 and 3700 cm-l in the case of MgA1204,3735 cm-1 in the case of ZnAlzO4). The spectra agree with those previously reported for MgA120414 and with that of another metal aluminate having the normal spinel structure C0A1204.'~ In all three cases a main band is observed, split in two components not well resolved. According to our previous discussionl~we can propose assignments of the bands observed in the region 3750-3725 cm-l to OH bonded to an octahedral AP+ ion, while the other components (3750 cm-l on MgA1204 and 3695 cm-l on ZnAlzO4) can be due to OH bonded to the bivalent metal. The spectrum of the nonstoichiometric zinc aluminate is virtually identical to that of the stoichiometric compound, while in the case of nonstoichiometric magnesium aluminate the shoulder at 3700 cm-' (12) Yet-Ming Chiang; Kingery, W. D. J. Am. Ceram. SOC.1989, 72, 271. Yoo, J. S.; Karch, J. A.; Radlowski, C. E.; Bhattacharyya, A. A. In Catalytic Science and Technology; Yoshida, S., et al., Eds.;VCH: Weinheim, 1990; Vol. 1, p 183. (13) Boehm,H. P.;Knozinger,H. In Catalysis Science and Technology; Springer-Verlag: Berlin, 1983; Vol. 4, p 39. (14) Morterra, C.; Ghiotti, G.;Boccuzzi, F.; Coluccia, S. J.Catal. 1978, 51,299. (15) Busca, G.; Lorenzelli, V.; Guidetti, R.; Sanchez Escribano, V. J. Catal. 1991, 131, 167.
L
0
3850
3750
3550
3650
NlVlnUmblP
3450
cm-i.
Figure 1. FT-IRspectra (OH stretchingregion)of presseddisks of (a) MgA1~04,(b) Mgl.lN204.1, (c) ZnA404, and (d)Zn1..4204.~ after activation in vacuum at 723 K. I
C
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Figure 2. FT-IRspectra of pyridine adsorbed on (a) MgAlnO4, (b) Mg,~A120~.~, F d (c) ZnAlzO4, after contact at room temperature and following evacuation at 423 K.
is much more evident than on the compound MgA1204. This shoulder can be due to bridging OH groups. (b) Adsorption of Pyridine. In Figure 2 we report the spectra of residual adsorbed species after pyridine adsorption at room temperature and following evacuation a t 423 K on MgA1204, on Z d l 2 0 4 , and on the sample Mg1.lA12O4.1. This procedure has been adopted in order to remove weakly bound species. Two bands belonging to the 8a vibrational mode of two different adsorbed pyridine species are observed on both stoichiometric spinels: they occur at 1623, 1608 cm-1 on MgAlzO4 and a t 1622,1611 cm-' on ZnAlzO4. Similar spectra are observed on the two nonstoichiometricspinels although the spectrum on Zn1.6A1204.5 presents extremely weak bands, due to a very small adsorbed amount. The spectra of pyridine adsorbed on the two stoichiometric compounds are again similar each other and resemble that observed on the isostructural compound C0A1204.15 It has previously been shown that a band at 1620-1625 cm-l is due to the 8a vibrational mode of pyridine bonded to Ala+ in an overall tetrahedral coordination, as observed over several defective spinel-
Langmuir, Vol. 7, No. 11, 1991 2679
Surface Basicity of Mixed Oxides type transitional aluminas.16J6 Consequently,the presence of this band could be related to the disorder in cation distribution in these spinels. According to structural data, in fact, these compounds present some degree of inversion in their spinel structure.17J8 The lower frequency band, detected at 1611 cm-1 on ZnAl2O4 and at 1608 cm-l on MgAl2O4, as well as on the isostructural C0&04,'5 is not observed on pure aluminas and can be assigned to pyridine coordinated on the bivalent cation.16 Pyridilie adsorbed over octahedral A13+is no more detectable in these conditions, because it desorbes under evacuation due to the weakness of the adsorptive bond. The study of surface hydroxy groups and adsorbed pyridine indicatesthat the surfaces of different normal spinels have similar structures and composition. The surfaces of these compounds expose both the bivalent ions and the trivalent one ( A P ) , and there is evidence at the surface of apartial inversion of the spinel structure. Consequently, their surface composition refleots tfie bulk composition. Spectroscopic features assignable to the surface of the pure oxides are observed neither on the stoichiometric compounds nor on the nonstoichiometric preparhtions. This is particularly evident in the case of the compound Zn~A1204.6,in spite of its significant excess of bivalent ions. On this surface, neither typical ZnO OH groups nor pyridine adsorbed on ZnO (vibrational mode 8a 1605 cm-l le) can be found. We can conclude that the surface properties of the metal oxides MgO and ZnO do not contribute to the surface of nonstoichiometric magnesium and zinc aluminates, probably because the excess of the bivalent ion is essentially involved in the formation of B nonstoichiometric spinel phase (solid solutions of the oxides in the spinel phases), as it is well-known for magnesium aluminates.lJ2 Adsorption of Hexafluoro-2-propanol. As discussed in our previous paper,1° the reliability of the present microcalorimetric method for basicity characterization implies that adsorption of the probe molecule (hexafluoro2-propanol, hereinafter HFIP) occurs dissociatively selectively on basic sites. This has been previously established for a number of binary oxides.10*20*21 In Figure 3 and 4 the spectra of the adsorbed species arising from HFIP adsorption taken near saturation coverage on our materials are shown. The spectra are similar to each other and indicate that the adsorption is totally dissociative in all cases. In fact, no bands associated to the in-plane COH deformation vibration can be seen in the region 14501300 cm-l, where one only sharp band, at 1380-1365 cm-l in all cases, is detected, due to the in-plane CH deformation mode. The COH deformation band for the unassociated (non-H-bonded)alcohol is observed sharp at 1308cm-' in ccl4 solution while for the associated pure material it can be found broad at 1430 cm-l. In all cases a new band due to surface OH groups is produced by dissociation of the alcohol: it falls at 3650-3600 cm-l except on MgO where created OH groups contribute in part to the residual OH groups absorbing at 3750 cm-l and in part are H bonded ~ and adsorb in the 3600-3400-cm-lregion, broad. u c bands are also observed in all cases between 2910 and 2875 cm-l. (16)Morterra, C.; Chiorino, C.; Ghiotti,G.; Garrone, E. J. Chem. Soc., Faraday T ~ o M1. 1979, 75,271. (17)Aweletti, C.: Pew. F.;Porta. P. J . Chem SOC.,Faraday Trans I 1977,73r1972. (18)Coaley, R. F.;Reed,J. S . J. Am. Ceram. SOC.1972,55, 395. (19)Lavallev. J. C.: Caillod. J. J. Chim. Phvs. 1980. 77.373 and 379. (20)BenaiG, M.;Saur, 0.;Lavalley, J. C. k t e r . &em. 1982,7,699. (21)R w i , P. F.;Bueca, G.; Lorenzelli, V.; Saur, 0.; Lavalley, J. C. Langmurr 1987,3,52.
0 ' 1540
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Figure 3. FT-IRspectra of the adsorbed specie6 arising from hexafluoro-2-propanol adsorption on (a) A1203, (b)MgAl44, (c) Mgl.lA1204.1, and (4MgO.
n
u
c n L
n
1500
1400
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$
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cm -1 .
Figure 4. FT-IRspectra of the adsorbed species arising from hexafluoro-2-propanoladsorptionon (a)ZnAlz04, (b)Z n d l ~ O 4 . ~ , and (c) ZnO.
These data indicate that adsorption of HFIP is almost completely dissociative on all surfaces up to saturation coverages. The gravimetric data of hexafluoro-2-propanol adsorption on the samples are summarized in Table I. As previously shown, the slightly smaller amounts measured volumetrically (Figure 5) than gravimetrically can be ascribed to the different procedures used in the two experimental setups. The total adsorbed amount ranges between 2.8 and 3.6 pmol/m2 on all surfaces except ZnO where the value is much higher. In all cases the adsorption is predominantly irreversible,accordingto the strong heats evolved (Figures 6-8). The volumetric HIFP adsorption isotherms, reported in Figure 5 , show in all cases a region of very strong interaction, corresponding to a very low equilibrium pressure, undetectable in our conditions; a plateau region, corresponding to weak adsorption, follows in the cases of both stoichiometricand nonstoichiometricaluminates that show a Langmuir-type isotherm (type I in the BDDT classificationn), while only in the case of ZnO the isotherm appears to be intermediate between type I and type I1 in the BDDT classification.22 A higher site heterogeneity is consequently present on ZnO surface. This difference is ~~
(22)Gregg, S.J.; Sing, K. S. W. Adsorption,Surface Area and Porosity; Academic Press: New York, 1982.
2680 Langmuir, Vol. 7,No. 11, 1991 Table I.
n(ad)b total irrev 0
Rossi et
41.
Characteristics and Gravimetric AdsorDtion Data tor Metal Oxide Samdes
3.6 2.9
4.7 4.6
3.6 3.0
2.9 2.3
3.5 2.9
3.4 3.0
3.4 3.1
m2/g. n(ad) = adsorbed hexafluoro-2-propanol molecules (pmol/m2).
7
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Figure 5. Volumetric isotherms of hexafluoro-2-propanol adsorption on metal oxides: (*) AlZOa;).( MgO; (0)ZnO; ( 0 ) MgA1204; ( 0 )Mg1.1Al~O4.1; (A)ZnA404; (A)Znl.sAlzO4.~.
0
I
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2
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Figure 7. Differential heats of hexafluoro-2-propanoladsorption on aluminum and zinc compounds (symbols as in Figure 5).
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Figure 6. Calorimetric isotherms of hexafluoro-2-propanol adsorption on metal oxides (symbols as in Figure 5). probably in relation with the anomalously high total adsorbed amount of HFIP measured gravimetrically on this material. We can see that the amount adsorbed at 0 Torr on ZnO is similar to that adsorbed on the other materials (near 3 pmol/m2). However, onlyon ZnO is this firststrong and irreversible adsorption step followed by another step with detectable equilibrium vapor pressures. The calorimetric adsorption isotherms (Figure 6) indicate that at the same equilibrium pressure the adsorption heats per unit area are similar in the cases of the stoichiometric compounds ZnAlzOr, MgA1204, and A1203. As for Zn compounds, extremely high adsorption heats are observed for ZnO while the nonstoichiometric aluminate shows higher integral adsorption heats t h m the stoichiometric one. So, the total heat evolved a t saturation increases with increasing Zn content (ZnO > Zn1.6A1204.5 > ZnAl204 > A1203). This is confirmed by the analysis of the differential adsorption heats (Figure 7). In the case of ZnO, whose measurements are less accurate because of the much lower surface area, the predominant sites correspond to a heat evolution of near 250-180 kJ/mol and are in number about 3 kmol/m2. On the nonstoichiometric aluminate Zn1.5Al204.5 a similar number of strong sites is measured, but
150
100
!w
0
0
3
4
n (ad) ( p o 1 / 3 )
Figure 8. Differential heats of hexafluoro-2-propanoladsorption on aluminum and magnesium compounds (symbolsas in Figure 5).
the corresponding evolved heat is distinctly lower (205140 kJ/mol). Finally, on the stoichiometric ZnAl204 the again similar number of strong sites corresponds to even smaller evolved heats, equivalent to those of alumina. An only apparently different picture is deduced from the calorimetric measures relative to HFIP adsorption on the magnesium aluminate samples. In this case in fact integral heats at saturation decrease in the order of
Surface Basicity of Mixed Oxides magnesium content (A1203 > MgAlzOr > Mgl.lA1204.1) except for MgO that shows the highest &value. However, the Bintvalues are affected both by the energy of adsorption on each site and by the density of sites. When the density of sites is considered, as volumetric isotherms and gravimetric measurements show, a smaller number of adsorption sites per unit area are active on both MgO and Mgl.lA1204.1 (near 2 pmol/m2) than on the stoichiometric spinel and on alumina. The curves of the differential adsorption heats (Figure 8) reveal that the basicity of MgAl204 is similar to that of alumina and of ZnAlzOr. Slightly higher adsorption heats are measured for the nonstoichiometric material at least at the lower coverage region, while MgO results to be, as expected, a very strong solid base. The lower density of basic sites on MgO and on the nonstoichiometric magnesium aluminate with respect to the other compounds can be due to residual carbonate species retained by these basic materials after the relatively mild pretreatment we used. Carbonate species would poison some of basic sites on their surfaces. According also to our previous studylo and to the above considerations, other very strong basic solids, such as Tho2 show a limited basic site density after relatively mild activation.
Conclusions Although surface acidity characterization methods are today well established, few data are available in the literature about surface basicity characterization. This is in spite of the great interest for surface properties of solid basic materials well documented in the fields of heterogeneous catalysis, adsorption science, ceramic, and glass technologies.g~10~2~-27 The data reported above allow extention of our previous work concerning the surface basicity of pure metal oxides and represents a first effort for a better understanding of the surface properties of mixed oxides. The data reported here for alumina agree with those reported previously,"-' concerning a different, more crystalline alumina preparation. The only difference involves the lack of detection here of few very strong sites we had previously observed on a y,6-Al2O3material. This is likely just related to the difference in crystallinity of these materials. Alumina crystallization therefore enhances both its acidity28129 and its basicity. ZnO was not included in our previous study concerning the basicity of pure oxides. From our data it is a mediumhigh basicity material, comparable to Ti02 but weaker, as a base, than ThO2, MgO, and FezO3.l0 This result apparently contradicts those reported recently by Auroux and Ger~asini.~ Using a microcalorimetric method similar to ours but using COz as the probe molecule, these authors found ZnO to be a very strong basic material, stronger than MgO and ThO2. The discrepancy can be due to the nature of the acidic probe molecule. It is in fact known that C02 adsorption is not a selective process occurring on basic sites. In fact, acid-base pairs can be involved, with the production of polydentate carb0nates.W (23) Hattori, H. Mater. Chem. Phys. 1988,18, 533. (24) Scokart, P. 0.;Rouxhet, P. G. J . Colloid Interface Sci. 1982,86,
96.
(25) Spitz, R. N.; Barton, J. E.; Barteau, M.A.; Staley, R. H.; Sleight, A. W . J. Phys. Chem. 1986,90,4087. (26) Duffy, J. A. J. Am. Ceram. SOC.1989, 72, 2012. (27) Iwmoto, N.;Makino, Y.; Kasahara, 5.J. Non-Cryst. Solids 1984,
68.379 .-, - . - and -.- 389. - - -.
(28) Abbattieta, F.; Delmaetro, 9.; Gozzelino, G.; Mazza, D.; Vallino, M.; Buaca, G.; Lorenzelli, V.; Ramie, G. J . Catal. 1989, 117, 42. (29) Nortier, P.; Fourre, P.; Mohammad S a d , A. B.; Saur, O.;.Lavalley, J. C. Appl. Catal. 1990, 61, 141.
Langmuir, Vol. 7, No. 11, 1991 2681 Surprisingly, the basic site distributions on the two normal spinel aluminates MgA1204 and ZnAlzO4 are apparently very similar to each other and to that of A1203, being much smaller than those of the corresponding pure oxides MgO and ZnO. This can be rationalized considering the coordination state of surface oxide ions on the different structures. We can propose that the basic strength of a surface oxide ion is stronger with more coordinative unsaturations and weaker with the greater polarizing power of the cations to which it is bonded. The polarizing power of the small trivalent cation AP+ is certainly much stronger than those of the bigger bivalent ions Mg2+and Zn2+. So, strong basic sites would arise from structures where surface oxide ions bridge over two or more bivalent ions, as occurs on MgO and ZnO, while weak basic sites are expected on the surface of aluminas where oxide ions bridge A13+ cations. Oxide ions in the bulk of normal spinels are tetracoordinated to three AP+ ions and to one bivalent cation. When oxide ions are placed on the surface, they would lack one or more coordination valencies, being still bonded to one or more AP+ ions. Oxide ions bridging over two bivalent cations do not exist in the bulk of normal spinel aluminates and can consequently be forecast to be nonexistent also on their surfaces. So, strong basic sites cannot be expected on the surface of stoichiometric spinel aluminates. These sites can however be present on nonstoichiometric spinels having an excess of bivalent ions. Previous studies seem to indicate the possibility of solid solutions of the aluminate spinels with the oxide of their bivalent ion. This has been found for MgA1~04~J~ and is also thought for the parent spinel type compound Z n C r ~ 0 4 . ~ ~ In these nonstoichiometric structures, part of the bivalent cations can occupy octahedral sites and oxide ions bridging over two or more bivalent ions can occur. We can attribute the increasing basicity of metal aluminates whose bivalent metal/trivalent metal ratio exceeds 0.5 to the presence of few oxide species bridging over two bivalent ions, exposed on the surface. In any case, our data indicate that the addition of MgO and ZnO to A1203 in an equimolar ratio forming the stoichiometric aluminates does not strengthen the basic character of the surface. However, further MgO and ZnO addition, producing probably nonstoichiometric aluminates, has a more important effect. This can explain why magnesium aluminates used in the field of heterogeneous catalysis are generally nonstoichiometricmaterials, where the MgO/A1203 molar ratio largely exceeds unity.lJJ2 The behavior of these catalysts, both in the polymerization of organic compounds such as propylene oxide2 and y-prop i o l a c t ~ n eand ~ ~in the adsorption and reduction of sulfur trioxide,' is dominated by their surface basic character. Magnesium aluminates, in contrast to MgO, can be prepared having high surface area and good physical and thermal stability. The utilization of magnesium excess aluminates can allow a proper balance of the physical stability of the powders with their sufficiently high surface basic strength. A similar behavior could be expected, although with weaker basic strength, for zinc aluminates.
Acknowledgment. The authors thank the Institut Francais du PBtrol (IFP) which supplied some oxides and Dr. P. Chaumette for very helpful discussions. (30) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7,89. (31) Del Piero, G.; TrifirQ F.; Vaccari, A. J . Chem. SOC.,Chem. Commun: 1984, 656. (32) Nakatsuka,T.;Kawasaki,H.;Yamaehita,S.;Kohija,S. Bull. Chem. SOC.Jpn. 1979, 52, 2449.
