Langmuir 1994,10, 159-165
159
Characterization of Mg-A1 Mixed Oxides by Temperature-Programmed Reaction of 2-Propanol Chistopher T. Fishel and Robert J. Davis* Department of Chemical Engineering] University of Virginia] Charlottesville, Virginia 22903-2442 Received May 24,1993. In Final Form: October 2 6 , 1 9 9 P
In this work, temperature-programmeddesorption (TPD)and temperatureprogrammed reaction (TPR) of several probe moleculeswere used to investigate the surface structure of magnesia, magnesium-aluminum mixed oxides, and alumina. Results from TPD of C02 and 2-aminopropane and TPR of 2-propanol indicatethat the mixed oxides derived from hydrotalciteshave surface adsorptioncapacities and reactivities similar to those of MgO. However, these characteristics are signifcantly influenced by the hydrotalcite synthesismethod. The adsorption capacities of these materials for COz, 2-aminopropane, and 2-propanol suggest that the surfaces of calcined hydrotalcites (mixed oxides) are oxygen-terminated (111)planes with surface defects that expose metal-oxygen pairs. These defects are the likely sites for adsorption and reaction. We speculate that, as the amount of aluminum incorporated into the mixed oxide increases, more of these surface defects are formed, thus increasing the adsorption capacity of the materials.
Introduction Hydrotalcite (magnesium-aluminum hydroxycarbonate) is an anionic clay that decomposes upon hightemperature calcination to form a high-surface-area,basic, mixed oxide. The structure of hydrotalcite consists of positively-charged layers of magnesium and aluminum hydroxide octahedra containing magnesium and aluminum in ratios typically ranging from 1.7 to 4.l In naturally occurring hydrotalcites (and the hydrotalcites synthesized for this work), interstitial carbonate anions balance the layer charge. Heating of hydrotalcite results in the decomposition of the hydroxycarbonate structure, and this thermal decomposition has been described by Reichle et al.? Miyata? and Rey et ala4 Generally, interlayer water is lost at about 400-600 K, while carbonate anions decompose to C02 and the hydroxide layers dehydroxylate at about 500-750 K to give the final mixed oxides. These calcined hydrotalcites are potentially useful as basic catalysts, since their high surface areas are stable to steam treatment,5 and are active for several base-catalyzed reactions, including aldol condensation,lI6H-D exchange of certain hydrocarbons,' and polymerization of propylene oxide7 and 8-propiolactone.8 In addition, these mixed oxides are potentially useful supports for catalytically active noble m e t a l ~ . ~The J ~ synthesis, characterization, and catalytic activity of hydrotalcites were recently reviewed by Cavani et al." In a previous report, we described the synthesis, characterization, and steady-state reaction of 2-propanol
* To whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, December 15, 1993. (1) Raichle, W. T. J . Catal. 1985, 94, 547. (2) Reichle, W. T.; Kang, S. Y.; Everhardt, D. S. J . Catal. 1986,101, 352. (3) Miyata, S . Clays Clay Miner. 1980,28, 60. (4) Rey, F.; Fornes, V.; Rojo, J. M. J . Chem. Soc., Faraday Trans. 1992,88,2233. (6)Schaper, H.;Berg-Slot, J. J.; Stork, W. H. J.Appl. Catal. 1989,54, 79. (6) Suzuki, E.; Ono, Y . Bull. Chem. SOC.Jpn. 1988,61, 1008. (7) Kohjiya, S.; Sato, T.; Nakayama, T.; Yamashita,S. Makromol. Chem., Rapid Commun. 1981,2, 231.
over MgO, a series of calcined hydrotalcites, and A1203.12 We found that calcined hydrotalcites had surface reactivities similar to that of pure MgO, as determined by the activity and selectivity to propanone during the steadystate decomposition of 2-propanol to propanone and propene. Apparently, A1 at the surface of a hydrotalcite is in a local oxidic environment that does not favor the dehydration of 2-propanol to propene that occurs over pure A1203. However, the selectivity depended on the method of hydrotalcite synthesis. Hydrotalcite samples prepared with Na-containing reagents were less selective to propanone than samples prepared with K-containing reagents. These differences were attributed to an inhomogeneous distribution of A1 at the surfaces of hydrotalcites made with Na. This work describes temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) studies of magnesia and magnesium-aluminum mixed oxides. Since the densities of basic sites on the materials were measured previously by temperature-programmed desorption of C02,12 in this work the acid site densities were measured by thermogravimetric analysis (TGA) of adsorbed 2-aminopropane. The TPR of 2-propanol was also studied to probe the active sites, since 2-propanol generally dehydrogenates to form propanone over basic catalysts and dehydrates to form propene over acidic catalysts.13J4 The results of the present work indicatethat a significant restructuring of the mixed oxide surface occurs. Furthermore, this restructuring is an important factor in determining the reactivity of the mixed oxides.
Experimental Methods Preparation of Materials. Hydrotalcite samples were prepared by a coprecipitation method in which aqueous magnesium nitrate and aluminum nitrate solutions were mixed with
an aqueous solution of potassium hydroxide and potassium
carbonate. A second series of hydrotalcites was prepared using sodium hydroxide and sodium carbonate as precipitating agents. Hydrotalcites were prepared with Mg:AI molar ratios of 5:1,3:1, and 2:l. Hereafter, the hydrotalcites are identified by the Mg:A1 ratio and the metal ion in the precipitation reagent (Le., HT
(8)Nakabuka,T.;Kawasaki,H.;Yamashita,S.;Kohjiya,S.Bull.Chem. Soc. Jpn. 1979,52,2449. (9) Davis, R. J.; Derouane, E. G. Nature 1991, 349, 313. (10)Davis, R. J.; Derouane, E. G. J . Catal. 1991, 132, 269. (11)Cavani, F.; Tifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173.
0743-7463/94/2410-0159$04.50/0
(12) McKenzie, A. L.; Fishe1,C. T.; Davis,R. J. J . Catal. 1992,138,547. (13)Gervasini, A,; Aurous, A. J. Catal. 1991, 131, 190. (14) Krylov, 0. V. Catalysis by Nonmetals; Academic Press: New York, 1966.
0 1994 American Chemical Society
Fishel and Davis
160 Langmuir, Vol. 10, No. 1, 1994 Table 1. Elemental Analysis of Synthetic Hydrotalcites bulk surface bulk surface hydrotalcite Mg:Al Mg:Al hydrotalcite Mg:Al Mg:Al sample" ratiob ratioc sample" ratiob ratioc 4.40 2.7 4.57 2.4 HT51-Na HT51-K 2.01 1.6 3.09 HT 2:l-Na HT31-K HT 2:l-K 1.87 1.5 HT 2:1-NH4 1.86 a Sample designation refers to the nominal Mg:Al atomic ratio expected from thesynthesisconditionaand to the preparation method (use of reagents with K, Na, or NH,). Determined by elemental analysis.12 Determined by XPS.12 5:l-K, HT 2:l-Na). Magnesium hydroxide and aluminum hydroxide were prepared separately by the addition of aqueous KOH to solutions of Mg(NOd2 and Al(NOs)a, respectively. To allow for direct comparisons, all of these samples are the same as the ones used in our previous work.12 We also synthesized a new hydrotalcite sample using ammonium carbonate and ammonium hydroxide as the precipitating agents to eliminate any effect of residual alkali on the surface properties of the mixed oxides. In addition, an ultrafine single-crystal magnesia (usc MgO) from Ube Industries was also studied.'6 Characterization of Materials. The COz adsorption capacities and surface areas were determined previously by CO2 TPD and N2 adsorption, respectively.12 These experiments were performedwith the same atmospheric flow system used for TPR.12 Thermogravimetric analysis (TGA) of adsorbed 2-aminopropane was performed to estimate the acid site densities of the oxides. First, a sample (14-20 mg) was heated in flowing Nz (L'Air Liquide, 99.999%) at 4 K min-l to 823 K and calcined in Nz for 4 h at 823 K. The sample was then cooled to 373 K and exposed for 1 h to 2-aminopropane (Aldrich, 99%) from a saturator at room temperature. The sample was then exposed to flowing N2 at 373 K for 12 h to remove weakly adsorbed 2-aminopropane. Finally, the sample was heated from 373 to 823 K at a rate of 10 K min-l while the changes in the weight of the sample were continuously monitored with a Seteram Microbalance TG-DSC 111. Temperature-Programmed Reaction of 2-Propanol. The TPR of 2-propanolwas performed using an atmospheric pressure flow system with a quadrupole mass spectrometer. A sample was first calcined at 823 K for 4 h in He (Air Products 99.99% ), producing approximately 0.25 g of oxide. The sample was then cooled to 373 K and exposed for 30 min to 2-propanol (Fisher Scientific, HPLC Grade) from a saturator maintained at 273 K. The system was then purged with flowing He for 30 min to remove weakly adsorbed 2-propanol. The TPR occurred during heating in flowing He (37 STP cm3min-l) at a rate of 10 K min-' to a final temperature of 823 K. The desorption of unreacted 2-propanol (mass 45) and formation of propanone (mass43) and propene (mass 41) were monitored with the mass spectrometer. Mass 44 was also monitored to check for the evolution of carbon dioxide. Calibration pulses of 2-propanol were injected before the sample was heated, while pulses of propanone, propene, and COz were injected after heating. Contributions of each species to the total signals for masses 45,43, and 41 enabled quantification of the amounts of each product evolved. The TPD of the reaction product propanone was also performed by exposing a freshly calcined sample at 373 K to a He stream saturated with propanone at 273 K and then heating at 10 K min-1 to 823 K to desorb the propanone.
Results The bulk Mg:A1 ratios of the synthetic hydrotalcites (Galbraith Laboratories) and the surface concentrations (X-ray photoelectron spectroscopy (XPS)) are listed in Table 1. As noted previously, the surface regions of t h e calcined hydrotalcites are apparently enriched in aluminum.12 T h e hydrotalcites were calcined in flowing helium at 823 K and decomposed t o give materials which we refer (15) Matauura, I.; Hashimoto, Y.;
273.
Takayasu, 0. Appl. Catal. 1991,74,
Table 2. Production of COa during Hydrotalcite Decomposition COZproduction1 C02 production/ (10-8 mol) (10-8mol) catalyst" the0l.b measd catalystQ theorb measd HT51-K 510 HT21-K 941 880 534 598 700 605 581 HT31-K HT21-Nh a Catalysts calcined at 823 K for 4 h in helium. b The theoretical CO2 production calculated to be half the moles of Al (from elemental analysis) in a hydrotalcite sample. Table 3. Adsorption Capacities of Calcined Hydrotalcites, MgO, and Ala08 surface 2-aminoareabl 2-propmolc/ propanedl C02V samDle0 (m2 rl)(10-8mol m-2) (10-8mol m-2) (10-8 mol m-2) UBC Mgd 100 3.6 5.3 1.2 4.4 200 3.6 MgO HT 51-K 230 2.3 1.1 2.2 HT 31-K 2.9 210 4.5 HT 2:l-K 230 1.8 2.8 5.3 HT 51-Na 1.5 1.9 200 2.3 HT 2:l-Na 2.5 240 3.0 HT 2:1-NH4 230 2.1 5.1 290 2.2 1.4 3.1 A1203 Catalysts calcined for 4 h in He at 823 K. Determined by N2 adsorption.12 Calculated from the s u m of propanone,propene, and unreacted 2-propanol evolved during 2-propanolTPR. d Determined by TGA. e Determined by TPD.12 f Provided by Ube Industries. ~
~
~~~
~
to as mixed magnesium-aluminum oxides. Carbon dioxide evolved from interlayer carbonate over the range of 500650 K, and as a check for the completeness of the decomposition, the amount of C02 evolved was quantified with the mass spectrometer. For stoichiometric decomposition, the amount of C02 produced should be half the A1 content of the calcined hydrotalcite. As shown in Table 2, the measured amounts of CO2 account for nearly all the carbonate present in the samples. Therefore, the decomposition of the hydrotalcite was assumed to be complete above 650K, which is supported by X-ray diffraction that revealed only t h e pattern of MgO. Since we did not detect any additional crystalline phases by X-ray diffraction, we assume that the A1 is associated with the MgO. However, we cannot rule out the possibility that amorphous phases exist in our materials. A more complete description of these materials is included in our previous work.lZ The amounts of 2-propanol, 2-aminopropane, and COS that adsorbed on the oxides are listed in Table 3. The amount of 2-propanol adsorbed is the s u m of the propanone, propene, and unreacted 2-propanol evolved during 2-propanol TPR. A small amount of COZ(mass44)was detected at high temperature during TPR but accounts for 7% or less of the total 2-propanol adsorbed as determined by the sum of all Cg products. The amount of adsorbed CO2 reported in Table 3 was found by TPD of preadsorbed C02, as described in our earlier work.lZ T h e amounts of CO2 and 2-propanol adsorbed on MgO ex-hydroxide and usc MgO are nearly t h e same. Interestingly, less 2-propanol and COZadsorbed on HT 51-K and HT 51-Na than on MgO. In addition, the amount of 2-aminopropane that adsorbed on HT 5:l-K is also less t h a n the amount that adsorbed on MgO. However, the number of adsorption sites for all three probe molecules generally increases as t h e aluminum content of the hydrotalcite increases. T h e concepts of Lewis acidity and basicity as applied to metal oxide surfaces are not very well understood and continue t o be developed by many researchers (such as
Characterization of Mg-AI Mixed Oxides
Stair,18 Kawakami and Yoshida,17 and Auroux and GervasinP). In this work, the 2-aminopropane TGA and CO2 TPD results were used to rank the mixed oxides in terms of surface acidity and basicity,since 2-minopropane adsorbs to a greater extent on acidic metal oxides, such as alumina,1s22 and CO2 adsorbs to a greater exent on basic oxides, like Mg0.2s2s This is evident from the adsorption capacities of MgO and Al203for these molecules as shown in Table 3. Our results in Table 3 indicate that the mixed oxides are basic, like MgO, since the C02 adsorption site densities of these materials were greater than the 2-aminopropane site densities. The TPD and TGA results also show that the adsorption capacities of the calcined hydrotalcites depend on the preparation method. The hydrotalcites with K adsorbed more C02 and less 2-aminopropane than the corresponding hydrotalcites with Na. The coverage of 2-aminopropane on the alumina is 1.3 X 10l8 molecules m-2, which is essentially equal to the value of 1.5 X 10l8molecules m-2 on y-alumina reported by Tittensor et al.22 Upon heating, the 2-aminopropane desorbed from each oxide over a broad temperature range. A broad desorption feature was also observed over y-alumina by Tittensor et al.22and was attributed to amine interactions with Lewis acid sites. TemperatureProgrammedReaction of 2-Propanol. The desorption of 2-propanol, propanone, and propene during TPR on MgO, HT 5:l-K, and A1203 is shown in Figure 1. All of the peaks are normalized for direct comparison, as indicated by the scaling factors. The areas under the curves represent the total moles of each product desorbing from the surface. Generally, the peak for 2-propanol desorption reaches a maximum between 450 and 500 K, while the propanone and propene peaks reach maxima at 550-600 K. In addition, Figure 1 shows the decreased evolution of the compounds on HT 51-K compared to MgO. A high production of propene from 2-propanoladsorbed on alumina is also evident. The TPR profiles for the K-hydrotalcite series are shown in Figure 2, indicating the effects of different Mg:A1 ratios. As the aluminum content increased, so did the amounts of each reaction product. Figure 3 shows the production of 2-propanol, propene, and propanone on HT 5:l-K and HT 51-Na, illustrating the effects of the preparation method. The peak for propene is clearly larger for HT 5:l-Na than HT 5:l-K. These trends are quantified in Table 4 which lists the amounts of the products desorbed, and the selectivities of the oxides to propanone. The selectivities are calculated from the moles of propanone evolved divided by the total moles of propanone and propene evolved. Since 2-propanol generally dehydrogenates to form propanone over solid bases and dehydrates to form propene over solid acids, the steady-state selectivity of the 2-propanol reaction is a common measure of the relative acidity (16) Stair,P. C. J. Am. Chem. SOC.1982,104,4044. (17) Kawakami, H.; Yoshida, S. J. Chem. Soc.,Faraday Trans. 2 1984, 80, 921. (18) Auroux, A.; Gervaaini, A. J. Phys. Chem. 1990,94,6371. (19) Parrillo,D. J.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J. Appl. CataZ. 1990,67, 107. (20) Biaglow, A. 1.; Adamo, A. T.; Kokotailo, G. T.; Gorte, R. J. J. Catal. 1991, 131, 252. (21) Gricua Kofke, T. J.; Gorte, R. J.; Kokotailo, G. T. Appl. Catal. 1989,54, 177. (22) Tittamor, J. G.; Gorte, R. J.; Chapman, D. M. J. CataZ. 1992,138, 714. (23) Fukuda, Y.; Tanabe, K. Bull. Chem. SOC.Jpn. 1973,46,1616. (24) Meixner, D. L.; Arthur, D. A.; George, S. M. Surf. Sci. 1992,261, 141. (25) Choudhary, V. R.; Rane, V. H. Catal. L e t t . 1990, 4, 101.
Langmuir, Vol. 10, No. 1, 1994 161
HT5:l-K Propanone (x5) Q)
Q
Propene (x7)
-
v)
m
Propanone (x5)
560
400
600
700
800
T/K Figure 1. 2-Propanol TPR profiles for MgO, HT 51-K, and AlzOs. A heating r a b of 10 K min-' was used, and all samples were calcined at 823 K. HT5:l-K
2-Propanol
Propanone (x5) Propene (x7)
-
0 C
'wi i7j
,
Propanone (x5)
c
E 2
Propene ( x 7 )
c
0
la_ Propanone (x5)
400
500
600
700 Propene800 (x7)
T/K Figure 2. 2-Propanol TPR profiles over calcined hydrotalcites. A heating rate of 10 K min-1 was used, and all samples were calcined at 823 K.
and basicity of catalysts.l3 This work shows that the selectivities for 2-propanol conversion to propanone determined by TPR are similar to those determined by steady-state reaction at 593 K over the same materials.12 A comparison of the results from the two methods is shown in Table 5. A likely reason for the differences is that, during the steady-state experiments, the oxides were exposed to a continuouspressure of 2-propanolthroughout the catalytic cycle, while there is very little gas-phase 2-propanol present during TPR. Also, selectivities in a
Fishel and Davis
162 Langmuir, Vol. 10, No. 1, 1994
-
0
c cn
HT5:l-K
A
2-Propanol Propanone (x5)
iij
c L
HT5:I-Na
0 Z-Propanol
.. v) v)
=9 (d
/A\, Propanone (x5)
,I
i i
Propene (x7)
I
400
500
600
700
800
T/K Figure 3. 2-Propanol TPR profiles over HT 5 1 - K and HT 51Na. A heating rate of 10 K min-1 was used, and both samples were calcined at 823 K. Table 4. Results from Temperature-ProgrammedReaction of 2-Propanol samplea wc MgOC MgO HT 51-K
HT 31-K HT 2:l-K
HT 51-Na HT 2:l-Na HT 2:l-NHr alumina
reaction producta/(W mol m-2) %-propanol propanone propene 3.29 0.33 0 2.28 1.22 0.14 1.97 0.33 0.029 0.043 0.63 3.79 0.69 0.12 4.45 0.089 1.89 0.35 0.15 0.32 2.53 0.043 0.64 4.41 0.57 2.38 0.18
propanone selectivityb 1.00 0.90 0.92 0.94 0.85 0.80 0.68 0.94 0.24
a Samples were calcined at 823 K for 4 h in flowing He. During TPR, samples were heated from 373 to 823 K at 10 K min-l. b Selectivityis defined as the moles of propanone evolved divided by the sum of the moles of propanone and propene evolved. Provided by Ube Industries.
Table 5. Comparison of Selectivities to Propanone Determined by Different Methods. selectivity selectivity sampleb determined by TPRC determined at steady stated 0.90 0.97 MgO 0.92 0.94 HT 5:l-K HT 3:l-K 0.94 0.84 HT 2:l-K 0.85 0.70 HT 5:l-Na 0.80 0.58 HT 21-Na 0.68 0.53 Ala03 0.24 0.03 0 Selectivity is defined aa the moles of propanone divided by the sum of the moles of propanone and propene. Sampleswere calcined at 823 K for 4 h in flowing He. During TPR, the samples were heated from 373 to 823 K at 10 K min-1. Steady-state reaction of 2-propanol performed at 593 K.12
steady-state measurement are dependent on reaction temperature. Thus, the selectivity provided by TPR may be considered an "average" of the continuum of the selectivities from the temperature range over which propanone and propene are produced. Regardless of the differences, there are several trends in common for the selectivitiesdetermined by either method. Both show that the calcinedhydrotalcites have propanone selectivitiesthat are comparable to that of MgO and much greater than that of A1203. In addition, the hydrotalcites made with K-containing reagents are more selective to propanone than those with Na-containingreagents. It could be argued that the influence of the hydrotalcite preparation method on the selectivity may be caused by the reactions of 2-propanol occurring over the highly basic oxides of K and Na. However, even though small amounts of Na and
460 5i)O
660
760 860
T/K Figure 4. Propanone TPD profiles for MgO and HT 5:l-K. A heating rate of 10 K min-1 was used, and both samples were calcined at 823 K prior to TPD.
K were detected in the hydrotalcites,12 we believe that 2-propanolreacts primarily on the Mg-A1-0 surface sites. This conclusion is supported by the low surface impurity concentrations measured by XPS for HT 5:l-K (1.4 atom % ' K)and H T 51-Na (1.3 atom % Na).I2 Given that the concentrations of alkali metals are approximately the same, if the reactions of 2-propanol occurred primarily over the basic oxides of Na and K, then the selectivitiesare expected to be similar. In addition, we studied 2-propanol TPR on HT 2:1-NH4 and found that material to be as selective for propanone as hydrotalcites prepared with K and much more selective than the hydrotalcites with Na (Table 4). Apparently, the surface properties of the hydrotalcites synthesized with K are little affected by the alkali impurities. Instead, we attribute the differences in the surface reactivities to differences in the mixed oxide structures as previously detected by 27AlNMR spectroscopy.12 It should be noted that trace alkali impurities can significantly reduce the dehydration activity of pure aluminas,26but apparently the surface structure of the mixed oxide dominates the dehydration behavior of these materials. The profiles for the TPD of propanone from MgO and HT 5:l-K are shown in Figure 4. Both desorption peaks reached a maximum at approximately 450 K. However, the peak for desorption from MgO also has a shoulder at higher temperatures. The amounts of propanone that desorbed were 0.30 X 10-6mol m-2 on MgO and 0.16 X 10-6 mol m-2 on HT 5:l-K. Since both samples were colored dark gray after the propanone TPD experiment, we suspect that carbonaceous residues are deposited on the samples from a high-temperature decomposition of aldol condensation reaction products. Also, the amount of propanone desorbed is an order of magnitude less than the amounts of C02,2-aminopropane, or 2-propanol that adsorbed on the same materials (Table 3). Evidently, most of the propanone remains on the surface. Discussion The amounts of C02 and 2-propanol that adsorbed on the calcined samples indicate that considerably fewer adsorption sites exist on the 5:l hydrotalcites than on MgO and that the number of adsorption sites increases as the Mg:Alratio decreases to 3:l and then levels off. Similarly, the 2-aminopropane adsorption study produced the unexpected result that slightly fewer adsorption sites exist on H T 5:l-K than on MgO. One may speculate that (26) Narayanan, C. R.; Srinivasan, S.; Datye, A. K.; Gorta,R.; Biaglow, A. J. Catul. 1992,138,659.
Characterization of Mg-A1 Mixed Oxides
Figure 5. A comparison of MgO surfaces: (a) defect-free (100) plane, (b) defect-free oxygen-terminated (111)plane. Shaded circles re resent Mg2+;open circles represent 0%.Ionic radii: Os,1.4 Mg2+, 0.66 A.@
aluminum enrichment at the surface of the 5:l mixed oxides,as detected by XPS,causes the reduced adsorption capacities. However, as can be seen in Table 3, mixed oxides with larger aluminum contents adsorb more of the probe molecules, which contradicts this hypothesis. A reasonablealternative explanation for these results is based on the geometric arrangement and defect structure of the various crystallographic planes exposed by the metal oxides, and is discussed below. The most stable surface of MgO is the (100) which exposes both metal cations and oxygen anions. A comparison of the ideal MgO (100)and oxygen-terminated (111)planes is shown in Figure 5. Experiments with zinc oxide have shown that exposed metal-oxygen pairs are necessary for the chemisorption of both C0z2* and 2-propanoP*Msince neither chemisorbon an oxygen-polar ZnO face. Presumably, exposed metal-oxygen pairs are also necessary for the adsorption of these molecules on MgO. As shown in Figure 5, such pairs are abundant on the (100) plane, while metal cations are inaccessible on the oxygen-terminated (111)plane. Thus, adsorption on the (111)-0plane would occur primarily at surface defects that expose the underlying metal cations. The surfaces of both Ube MgO and MgO ex-hydroxide consist predominantly of stable (100) p l a n e ~ , las ~ *man~~ ifested by the approximately equal amounts of 2-propanol and C02 that adsorbed on the two MgO samples (Table 3). An early study of the thermal decomposition of Mg(OH)2 to MgO by Freund and Sperling found that a high-defect MgO, which retains the hexagonal brucite symmetry, is formed first and then collapses progressively to the cubic MgO structure.32 In the intermediate hexagonal MgO, as much as 50% of the anion sites of the original brucite may be u n o ~ c u p i e d . However, ~~ subsequent high-resolution TEM studies by Dahmen et aL3I showed that high-defect MgO formed from Mg(OH)2 (27) Gibeon, A.; Haydock, R.; LaFemina, J. P. J. Vac. Sci. Technol., A 1992,10, 2361. (28) Akhter, S.; Lui, K.; Kung, H. H. J. Phys. Chem. 1985,89,1958. (29) Zwicker, G.; Jacobi, K.; Cunningham, J. Int. J . MaSS Spectrom. Ion Processes 1984,60,213. (30) Vohs, J. M.; Bartem, M. A. J. Phys. Chem. 1991,94297. (31) Dahmen,U.; Kim, M.-G.;Searcy, A. W. Ultram~roscopy1987, 23,365. (32) Freund, F.; Sperling, V. Mater. Res. Boll. 1976, 11, 621.
Langmuir, Vol. 10,No. 1, 1994 163
consists of cubes aligned in a common (111) direction. This alignment explains why the hexagonal symmetry of brucite is maintained. Nonetheless, these cubes expose MgO (100) surfaces.31 Derouane et al. argued that the incorporation of A1 into the MgO structure stabilizes the oxygen-terminated MgO (111)plane.33 Specifically,the replacement of three Mg2+ by two Al3+ creates a cation vacancy, and the surface acts as a sink for the vacancy in order to optimize the lattice energy. Ultimately, one Mg2+is removed from the surface, resulting in a surplus of 02-ions. *Theoxygen-terminated (111) planes can accommodate this excess of 02-ions, provided that the oxide particles are in the nanometer size range.331~ This stabilizingeffect of A1on the MgO surfacestructure should inhibit the formation of defects. However, as the aluminum content increases, the adsorption capacity of a calcined hydrotalcite increases, suggesting that more surface defects are formed. One possible explanation for the increasing number of surface defects can be found in the description of hydrotalcite decomposition by Brindley and K i k k a ~ a . 3They ~ argued that the 02-anion formed by the decomposition of interlayer Cos2- to C02 is subsequently incorporated into the layers of the hydrot a l ~ i t e .This ~ ~ incorporation could cause distortions in the lattice structure that produce surface defects, thus exposing metal ions. As the aluminum content of the hydrotalcite increases, the number of oxygen anions to be incorporated also increases and more surface defects are formed. Thus, the incorporation of Al into the hydrotalcite has two opposite effects on the formation of surfacedefects. The stabilization of the (111) plane at low Al contents inhibits the formation of defects, while the decomposition of the COS%associated with high Al contents promotes the formation of defects. Derouane et al. did not discuss the minimum amount 3 of A1 necessary to stabilize a MgO (111)~ l a n e . ~However, Kurokawa et al. reported that the quantity of C02 adsorbed on MgO with 3 wt 9% Al was an order of magnitude less than that adsorbed on pure MgO.= It seems unlikely that the effect of aluminum on the surface basicity of MgO was responsible for this large decrease, especially since the amount of CO2 adsorbed on pure alumina in this study was more than a quarter of the amount adsorbed on MgO. An alternative explanation for this large decrease is that 3 wt 9% A1 is sufficient to stabilize a MgO (111) plane, while no COS> was present to produce surface defects. The proposed model for the hydrotalcite surface provides an explanation for our adsorption results. Decreases in the 2-propanol, 2-aminopropane, and C02 adsorption capacities on HT 5:l-K, as compared to MgO, may be due to a large fraction of the surfaceconsistingof oxygen anions with no adjacent exposed metal cations. As the amount of aluminum incorporated into the hydrotalcite increases, the number of surface defects increases, creating more adsorption sites. Although the selectivities determined by TPR differed from those determined by the steady-state reaction of 2-propanol, both results show that the selectivities to propanone of the hydrotalcites are comparable to that of MgO ex-hydroxide and far greater than that of alumina, indicating that relatively little acidity is present on the (33) Derouane,E. G.;Jullien-Lardot,V.;Davis,R. J.; Blom, N.; HajlundNielaen, P. E. loth International Congress on Catalysis, Budapest, Hungary, July 19-24,1992. (34) Hajlund-Nieleen, P. E. Nature 1977,267,822. (35) Brindley, G. W.; Kikkawa, S. Clays Clay Miner. 1980,28,87. (36) Kurokawa, H.; Kato, T.; Kuwabara, T.;Ueda, W.; Morikawa, Y.; Moro-Oka, Y.; Ikawa, T. J. Catal. 1990,126,208.
164 Langmuir, Vol. 10,No. 1, 1994
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peak, by as much as 30 K. However, over alumina, the calcined hydrotalcite surfaces. Thus, the TPR results propene peak is approximately 20 K lower than the support our previous conclusionthat AP+located primarily propanone peak. The results over MgO and the hydroin a MgO lattice is in an oxidic environment that does not talcites are consistent with the mechanism proposed by favor dehydration reactions that typically occur over pure Mazanec for propene formation over a basic oxide.43 In alumina.12 this mechanism, propene is formed from adsorbed proThe TPR results indicate that the selectivity to propanone by @-hydrogenextraction followed by a-protonapanone of MgO ex-hydroxide is 90% while no measurable tion. The extra energy required for the deprotonation propene was formed over usc MgO. A possible explanation should result in propene being formed at temperatures for this difference is that propene formation may occur higher than those for propanone formation. Dehydroprimarily over low-coordination surface sites. Matsuura genation occurring at a lower temperature than dehydraet al. reported that the surfaces of usc MgO consist tion has been observed over ZnO by Vohs and Barteau,3O predominantly of 5-fold coordination siteswith extremely Bowker et al.,44 and Chadwick and O'MallePS and few 3-fold coordination sites.15 If the formation of propene compares with the results of 2-propanol decomposition over MgO required a 3-fold coordination site, then the observed over MgO and the hydrotalcites with K in this amount formed over the usc MgO would probably be too study. Additional evidence that the mechanism for small to detect. The lack of low-coordination sites on usc 2-propanol dehydration differs over the hydrotalcites with MgO may also explain why 4 times as much propanone K and alumina is provided by the activation energies, E,, forms on MgO ex-hydroxide as on usc MgO even though that we previously reported.12 The activation energies the amounts of 2-propanol adsorbed on both are nearly for propene formation over the hydrotalcites are approxthe same. Several studies have shown that the most reactive sites on MgO are those of lowest c o ~ r d i n a t i o n . ~ ' ~ ~imately 50 kJ mol-' larger than the value for alumina, but within 15 kJ mol-' of the value for magnesia.I2 These Thus, 2-propanol may readily adsorb on the 5-fold differences in Ea further suggest that the mechanism for coordination sites of usc MgO but may not be able to react 2-propanol dehydration over the hydrotalcites with K is to form either propanone or propene. the same as that over MgO. The TPD of propanone could indicate that the TPR results are reaction-limited, instead of desorption-limited, However, 2-propanol dehydration over the hydrotalcites since propanone formation during TPR peaks at 550-600 prepared with Na apparently occurs by a different K, which is well above the TPD peak temperatures of mechanism than on MgO and the hydrotalcites prepared approximately 450 K observed for both MgO and H T 5:lwith K. On the Na-hydrotalcites, the peak maxima for K. Furthermore, during %-propanolTPR, Hz was first propene and propanone formation occur at about the same detected at the same temperature as propanone, providing temperature, which is inconsistent with the mechanism additional evidence that propanone production isreactionproposed by Mazane~.~3 More compelling evidence for a limited. A complication in the interpretation of the different mechanism is provided by the activation energies propanone TPD is that both catalysts were highly disdetermined from the steady-state reaction of 2-propan01.'~ colored after propanone TPD. This discoloration was The activation energies for propene formation over probably caused by the aldol condensation of propanone, hydrotalcites with Na are at least 30 kJ mol-' less than the forming higher molecular weight products that did not values for the hydrotalcites with K. This large difference desorb from the samples. At higher temperatures during in Ea for the different types of hydrotalcites suggests a the TPD, these species could then be deposited on the possible difference in the reaction mechanisms. We surface as an unreacted carbonaceous layer. Aldol conpreviously proposed that differences in selectivity to densation of propanone occurs over many basic catalysts, propanone between the hydrotalcites with K and those including calcined hydrotalcites.' In our case, the conwith Na were caused by an inhomogeneous distribution densation reaction probably occurs due to the high surface of aluminum at the surface of the hydrotalcites with Na.12 concentration of adsorbed propanone. Unfortunately, the An inhomogeneous distribution of aluminum may also signficant amount of reacted propanone left on the surface account for the possible difference in propene formation precluded the determination of the binding energy relative mechanisms. On the hydrotalcites with Na, dehydration to that of 2-propanol. There was no noticeable discolof 2-propanol probably occurs over an aluminum-rich oration of any sample after 2-propanol TPR, indicating region. Early work with alcohol dehydration on alumina that neglibible aldol condensation of propanone (and surfaces is consistent with a concerted mechanism resubsequent coking) occurred. During TPR, most of the quiring participation of both the intrinsic acid and base propanone was produced at higher temperatures than ~ites.~6 There is not enough information to determine if those at which propanone desorbed during TPD. Therethe mechanism for propene formation on the hydrotalcites fore, the concentration of propanone on the surface during with Na is the same as that on A1203, since alcohol TPR is probably never large enough for aldol condensation dehydration reactions on acidic oxides are postulated to to occur. occur by more than one path~ay.469~'We are currently Propene formation on the calcined hydrotalcites with exploring these pathways on the mixed oxides. K probably occurs by a different mechanism than on alumina. The main evidence for this conclusion is from Conclusions the peak temperatures for propene and propanone formation during TPR. Over MgO and the hydrotalcites, The measured differences in the amounts of C02 and the propene peak reaches amaximum after the propanone 2-propanol adsorbed on MgO and the 5:l hydrotalcites (37) Wu, M.-C.; Goodman, D. W. Catal. Lett. 1992, 15, 1. (38) Peng, X. D.; Barteau, M. A. Langmuir 1991, 7, 1426. (39) Peng, X. D.; Barteau, M. A. Catal. Lett. 1992, 12, 245.
(40)Ito, T.;Kuramoto, M.; Yoshiko, M.; Tokuda, T. J . Phys. Chem. 1983,87,4411. (41) Ito,T.; Sekino, T.; Morai, N.; Tokuda, T. J. Chem. SOC., Faraday Tram. 1 1981, 77, 2181. (42) Coluccia,S.; Boccuzzi,F.; Ghiotti, G.; Morterra, C. J . Chem. SOC., Faraday Trans. 1 1982, 78, 2111.
(43) Mazanec, T. J. J . Catal. 1986, 98,115. (44)Bowker, M.; Petta, R. W.; Waugh, K. C. J . Chem. SOC.,Faraday Trans. 1 1985,81,3073. (45) Chadwick, D.; OMalley, P. 3. R. J . Chem. SOC.,Faraday Trans. 1 1987,83, 2227. (46) Pines, H.; Manassen, J. Adu. Catal. 1966,16,49. (47) Yamaguchi, T.; Tanabe, K. Bull. Chem. Soc. Jpn. 1974,47,424. (48) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr. 1969, B25,925.
Characterization of Mg-AI Mixed Oxides
suggest that the surfaces of the calcined hydrotalcites are stabilized oxygen-terminated (111)planes in contrast to a pure MgO surface which exposes (100) planes. The adsorption sites on hydrotalcites are likely to be surface defects that expose metal-oxygen pairs, and more of these defects are formed as the aluminum content of the hydrotalcite increases. The results of %-propanolTPR, 2-aminopropane TGA, and COz TPD support our previous findings that calcined hydrotalcites are basic oxides and that the surface properties of calcined hydrotalcites depend on the synthesis method. We also believe that the mechanism for propene formation over hydrotalcites with K may differ from that over hydrotalcites with Na. This difference may be due
Langmuir, Vol. 10,No. 1,1994 165
to an inhomogeneous surface distribution of aluminum on. the hydrotalcites made from reagents with Na, as described previously. The product selectivitiesfrom 2-propanol decomposition during TPR compare very well with selectivities measured at steady state,which suggests that either method is useful for measuring the reactivity of an oxide surface. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CTS-9108206). R.J.D.acknowledges support from a NATO travel grant to perform the 2-aminopropane adsorption experiments in the laboratory of Professor E. G. Derouane, Facultes Universitaires Notre-Dame de la Paix, Namur, Belgium.
