Formation of Base Sites on Calcined Mg− Ga Hydrotalcite-like [Mg1-x

5112

J. Phys. Chem. B 1997, 101, 5112-5117

Formation of Base Sites on Calcined Mg-Ga Hydrotalcite-like [Mg1-xGax(OH)2](CO3)x/2‚mH2O Esteban Lo´ pez-Salinas,* Mayela Garcı´a-Sa´ nchez, Ma. Elena Llanos-Serrano, and Jua´ n Navarrete-Bolan˜ oz Instituto Mexicano del Petro´ leo, Subdireccio´ n de Transformacio´ n Industrial, Eje Central La´ zaro Ca´ rdenas 152, 07730 Me´ xico D. F., Me´ xico ReceiVed: January 17, 1997; In Final Form: April 3, 1997X

A series of Ga-substituted hydrotalcite-like materials (GaHTs), [Mg1-xGax(OH)2](CO3)x/2‚mH2O where x ) 0.12-0.36, were prepared in order to examine their textural and basic properties upon different Mg/Ga contents and calcination temperature. Specific surface area increases from 46 to 156 m2 g-1 (Mg/Ga ) 7.7) when calcining from 523 to 673 K as a consequence of dehydroxylation and decarbonation of the material. The total number of base sites increases upon higher calcination temperatures until reaching a maximum at 873 K and then decreases. CO2 thermal programmed desorption indicates that different Mg/Ga relative contents have a considerable effect on the amount of base sites. For instance, a GaHT with Mg/Ga ) 7.7 exhibits 2.4 times more total base sites than that of Mg/Ga ) 2.9. Additionally, in the former, the generation of medium and strong base sites is considerably higher than in the latter. By means of CDCl3 adsorption and FTIR, several types of base sites were identified. The base sites strength (pKa ) -1 to +19) and proton affinity (815-987 kJ mol-1) values were estimated considering the νCD shift of adsorbed CDCl3 and correlating it with the pKa of bases that form hydrogen-bonded complexes.

Introduction Hydrotalcite, [Mg6Al2(OH)16](CO3)‚4H2O, is one of the few naturally occurring clays capable of anion exchange.1 These materials can be thought of as comprised of brucite-like Mg(OH)2 layers, in which each Mg2+ cation is 6-fold bonded to hydroxyl groups in an octahedral arrangement. These octahedra share edges to form infinite sheets. When some Mg2+ are replaced by Al3+, the layer gains a positive charge, originating in the trivalent cation, which is neutralized by interlamellar anions. A wide variety of synthetic hydrotalcitelike materials can be obtained by the substitution of both cations in the layer and anions in the interlayer region. Thus, the empirical formula which represents hydrotalcite-like materials can be expressed as follows:

[M(II)1-xM(III)x(OH)2]x+[An-x/n]x-‚mH2O where M(II) ) Mg2+, Ni2+, Zn2+, etc., M(III) ) Al3+, Fe3+, Cr3+, etc., and An- ) [CO3]2-, Cl-, [NO3]-. The potential uses of hydrotalcite and hydrotalcite-like compounds as precursors of solid adsorbents, anion scavengers, and catalyst supports has prompted many studies reviewed by Cavani et al.2 In hydrotalcites, or layered double hydroxides as they are also referred to, the residual electropositive charge in each trivalent cation [MIII(OH)2]+ unit forces them to accommodate as far apart as possible from each other among electrically neutral [MII(OH)2] units due to electrostatic repulsion, resulting in a material with homogeneously well-mixed components. This characteristic, however, can be exploited only in a narrow range of MII-MIII compositions, typically x ) 0.170.33 (MII/MIII ) 5-2), since outside this range single MII,MIII(OH)x metal hydroxides segregate from the layered compounds.3,4 Notwithstanding, Mg-Ga hydrotalcites can be obtained free of undesired phases within a wider range of x, * Corresponding author. E-mail [email protected]. X Abstract published in AdVance ACS Abstracts, May 15, 1997.

S1089-5647(97)00255-1 CCC: $14.00

i.e., 0.07 e x e 0.35.5 The calcination of hydrotalcites above ca. 573 K brings about the collapse of their layered array which is caused by dehydroxylation of the layers and expulsion of the volatile anion, i.e., [CO3]2-, [NO3]-.6,7 The heat treatment of hydrotalcite yields mixed metal oxides with both increased basic properties and specific surface area in comparison with those of the as-synthesized hydrotalcites. For instance, Schaper et al. have reported that in Al-stabilized MgO, obtained via calcination (973 K) of Mg-Al hydrotalcite precursors, the base site strength is 159 times greater than that of pure MgO.8 Calcination at 1073 K of a synthetic hydrocalumite-like compound [Ca2Al(OH)6](NO3)‚4H2O, which has a layered structure similar to hydrotalcite, results in a material with almost exclusively strong base sites (96% at CO2 desorption temperature 1020 K) and shows high catalytic activity in the isomerization of 1-butene.9 Additionally, the surface area and pore volume in calcined (at ca. 773 K) hydrotalcite increase 6 and 1.25 times, respectively, because of pore formation caused by the expulsion of water and CO2 molecules, unaltering their crystal size and shape.6 However, the role played by Mg/Al relative contents in the number and strength of base sites in calcined hydrotalcites is still unclear. Nakatsuka et al. claimed that the amount of base sites in a series of Mg-Al hydrotalcites increased with higher Mg/Al ratios.10 According to Miyata et al., the Mg/Al ratio in hydrotalcites affects the basic strength in a rather complicated way.11 McKenzie et al. reported that the incorporation of Al3+ into the precursor hydrotalcites alters the final basicity by inhibiting the formation of strongly basic surface sites.12 We have reported recently the preparation of Ga-substituted hydrotalcite-like materials.5 In this study the basic properties of a series of Mg-Ga hydrotalcites, [Mg1-xGax(OH)2](CO3)x/2‚ mH2O, were examined as a function of the calcination temperature and the Mg/Ga molar ratio. Attempts to throw light on the local structure of the base sites were carried out by means of CDCl3 adsorption and FTIR. © 1997 American Chemical Society

Base Sites on [Mg1-xGax(OH)2](CO3)x/2‚mH2O

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TABLE 1: Composition of xGaHTs sample

Mg/Ga (M)

Ga2O3 (wt %)

MgO (wt %)

CO2 (wt %)

1.8GaHT 2.9GaHT 5.3GaHT 7.7GaHT

1.8 2.9 5.3 7.7

39.3 30.0 21.2 16.1

30.4 38.6 48.3 53.3

10.8 6.9 a 4.5

a

Not analyzed.

Experimental Section Preparation of [Mg1-xGax(OH)2](CO3)x/2‚mH2O. The preparation of the Ga-substituted hydrotalcite-like materials (hereinafter referred to as xGaHT, where x ) Mg/Ga) was carried out using a coprecipitation method in which only the amount of the reactants was varied and all other conditions were the same for all the preparations. Deionized water was used throughout all the experiments without any further treatment. For instance, the synthesis of a hydrotalcite with Mg/Ga ) 3.0 was obtained as follows: First an aqueous solution containing 11.17 g (26.7 mmol) of Ga(NO3)3‚9H2O and 20.48 g (79.9 mmol) of Mg(NO3)2‚6H2O in 100 cm3 of water was prepared. After this, 2.76 g (20 mmol) of K2CO3 and 15.19 g (270.72 mmol) of KOH in 300 cm3 of water were dissolved to make a second solution. These two solutions were added dropwise into a flask containing 200 cm3 of water at 313 K upon vigorous stirring. The rate of addition of the two solutions was controlled in order to keep a constant pH (11-12), which was monitored continuously throughout the coprecipitation procedure by means of a pH meter. After completing the addition of the solutions, the white gel obtained was immediately washed several times and separated in a centrifuge. After this, the white paste was dried in an oven in static air at 353 K for 24 h. Finally, a white solid was obtained (yield ) 76%). The composition of the samples used in this study before any calcination procedure is shown in Table 1, and all have a layered structure as reported elsewhere.5 Characterization Methods. The metal content in the solid materials was determined by means of inductively coupled atomic emission spectroscopy (AES) in a SPS 1500VR plasma spectrometer from Seiko Instruments. The solid samples were easily dissolved in aqueous HNO3 at appropriate concentrations in order to carry out the analysis by AES. The CO2 content in the solid Ga-hydrotalcites was determined in a LECO CR-12 apparatus by calcining the samples at 1045 K, and the gases generated were passed through a series of traps to remove fine particles and humidity. Finally, CO2 was quantified by means of a solid-state infrared detector. Specific surface areas using the BET method with N2 adsorption at 77 K and pore size distribution by means of N2 adsorption-desorption isotherms were measured in an ASAP2000 apparatus from Micromeritics. The thermal programmed desorption of CO2 (CO2 TPD) was used to estimate the amount and strength of base sites formed on the surface of GaHT. First, a sample of previously calcined GaHT was heated (10 K min-1) in flowing helium (Linde UHP, 99.999%) until reaching the calcination temperature and kept for 60 min. After this, the sample was allowed to cool to room temperature and exposed to flowing CO2 (Linde UHP, 99.999%) for 30 min. Later, the system was purged at 303 K for 30 min with helium in order to remove weakly adsorbed CO2. The temperature of the sample rose linearly (10 K min-1) from ambient to the indicated temperatures. CO2 concentration was monitored by means of a thermal conductivity detector. The amount of CO2 was quantified by comparing the areas under the curve in the sample with those of known amounts of CO2 injected after each run. The CO2 TPD analysis was carried out

Figure 1. Specific surface area evolution of 2.9- and 7.7GaHT upon different calcination temperatures.

in an AMI-3 apparatus from Altamira Instruments. The CO2 TPD curves were deconvoluted using an ORIGIN 3.0 software assuming Gaussian type peaks. The area under each single peak was determined in order to estimate the relative distribution of the base sites. Here, the strength of the base sites is classified considering the temperature of maximum CO2 desorption (Tmax) as follows: w ) weak (Tmax < 423 K), m ) medium (423 K e Tmax e 523 K), and s ) strong (Tmax > 523 K). The study of the nature of base sites on GaHTs was performed by a Fourier transform infrared (FTIR) analysis using Nicolet 710 equipment. The equipment was furnished with a cell of interchangeable windows in which self-supported wafers of sample were placed in order to follow in situ adsorption studies of deuteriochloroform molecules, CDCl3. Before wafer preparation, all GaHTs and Mg(OH)2 were calcined at 873 K for 5 h. Ga2O3 was obtained by calcining Ga(NO3)3‚xH2O at 873 K for 8 h. All wafers were prepared with 19 mg of sample, except in the case of Ga2O3 which required 26 mg. In all cases, samples were thermally treated in vacuum (1 × 10-6 Torr) at 623 K for 30 min in order to remove adsorbed molecules and then cooled to room temperature. The reagent grade CDCl3 was distilled and dried with anhydrous CaO before use and supplied to the measurement cell by means of a breakable capillary tube placed into the cell along with the self-supported wafer. Results and Discussion Textural Properties vs Calcination Temperature. The effect of the calcination temperature on the specific surface area of two GaHTs is shown in Figure 1. The as-synthesized 2.9GaHT and 7.7GaHT have 46 and 54 m2 g-1, respectively. When GaHTs are calcined at 673 K, the surface area increases about 3 times compared with the initial value, reaching a maximum value. Accordingly, in Mg-Al hydrotalcites dehydroxylation and decarbonation cause the formation of pores due to H2O and CO2 venting.6 Calcination temperatures above 673 K yield materials with smaller surface areas. At 673 K GaHTs transform into Ga-substituted MgO-like compound; i.e., the

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Figure 2. CO2 thermal programmed desorption patterns of 2.9GaHT calcined at (a) 673, (b) 873, and (c) 1073 K.

Figure 3. CO2 thermal programmed desorption patterns of 5.3GaHT calcined at (a) 673, (b) 873, and (c) 1073 K.

layered structure collapses below 673 K, and above 773 K a spinel type MgGa2O4 compound forms.13 The decrease in surface area above 673 K may be associated with the formation of MgGa2O4 which has a closed packed structure. It appears that Ga-rich GaHTs have a marked tendency to sintering, while surface area of Mg-rich samples withstand higher temperatures. Thus, the surface area in a 2.9GaHT decreases 56% at 1073 K, while that of 7.7GaHTs diminishes 36%, both compared with their maximum values (i.e., at 673 K). All GaHTs, regardless of the calcination temperature (473-1073 K), showed similar pore size distributions, being predominantly unimodal with maximum population centered around 400-500 Å, i.e., mostly in the mesoporous region. Basic Properties. Number of Base Sites as a Function of Mg/Ga Ratio and Calcination Temperature. The CO2 TPD patterns of three GaHTs, 2.9-, 5.3-, and 7.7GaHT, calcined at indicated temperatures, are shown in Figures 2, 3, and 4, respectively. The CO2 TPD patterns of the three samples calcined at 673 K are made up of two distinguishable peaks. The first desorption peak below 473 K arises from CO2 species bonded to weak base sites, and the second one which appears between 473 and 523 K is ascribed to base sites of medium strength. The second peak appears as a shoulder partially overlapped with the first one, but it becomes more clearly distinguishable when increasing the amount of Mg, i.e., higher Mg/Ga ratios (compare Figures 2a, 3a, and 4a). When the GaHTs are calcined at 873 K, the peak of the medium base sites becomes more prominent in comparison with the one calcined at 673 K. A comparison of Figures 2b, 3b, and 4b with that of MgO calcined at 873 K (Figure 5c) clearly indicates that Ga incorporation into hydrotalcite-like material decreases the amount of medium strength base sites in Ga-rich GaHTs (Mg/Ga ) 2.9, 5.3). In fact, pure Ga2O3 shows very few basic properties (Figure 5a,b). However, a Mg-rich GaHT, particularly 7.7GaHT, shows a considerable increase in the intensity of the second peak compared with 2.9GaHT, 5.3GaHT, and even that of MgO (Figure 5c), indicating that small amounts of Ga may promote the formation of medium and strong base sites. A small shoulder above 673 K (Figure 4b) points out the presence of a small portion of very strong base sites in 7.7GaHT. The CO2 TPD patterns of the single oxides, MgO and Ga2O3 calcined at 873 and 1073 K, are shown in Figure 5. MgO,

Figure 4. CO2 thermal programmed desorption patterns of 7.7GaHT calcined at (a) 673, (b) 873, and (c) 1073 K.

obtained after calcining Mg(OH)2, is made up mainly of weak and medium strength base sites. The maximum CO2 desorption temperature of the second peak is 513 K and is considerably lower than that of the second peak in the 7.7GaHT (Figure 4b), which is 583 K. This result indicates that Ga incorporation in 7.7GaHT increases the base strength compared with that of pure MgO. The amount of strong base sites (ca. 673 K) in MgO increases significantly when calcining at 1073 K (Figure 5d). The relative base site distribution depends on both the calcination temperature and the Mg/Ga content as shown in Table 2. In all GaHT cases higher calcination temperatures favor a greater population of medium and strong base sites, probably due to a decrease of OH groups responsible for the weak base sites. On the other hand, Mg-richer GaHTs show a greater amount of strong base sites in comparison with those of Ga-rich ones. Particularly, 7.7GaHT calcined at 873 K showed predominantly strong base site population and no medium base sites at all. The total number of base sites in GaHTs as a function of

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Figure 5. CO2 thermal programmed desorption patterns of Ga2O3 (a, b) and MgO (c, d) calcined at (a, c) 873 and (b, d) 1073 K.

TABLE 2: Effect of the Calcination Temperature on the Relative Distribution and the Number of Base Sites in GaHTs, MgO, and Ga2O3 density of base sitesb rel distribution calcination total (µmol (µmol of temp (K) w (%) m (%) s (%) of CO2 g-1) CO2 m-2) no. of base sites a

sample Ga2O3 2.9GaHT 5.3GaHT 7.7GaHT MgO

873 1073 673 873 1073 673 873 1073 673 873 1073 873 1073

33 34 8 51 25 16 70 20 9 24 15

50 31 59 30 41 36 14 0 36 64 60

17 35 33 19 34 49 16 80 55 12 25

344 155 1186 1077 503 981 874 432 851 2545 522 1659 1335

7.6 9.9 7.3

5.6 23.8 5.4

a Estimated by deconvolution of CO TPD patterns: w ) weak, m 2 ) medium, s ) strong. b Density ) (number of base sites)/(surface area in Figure 1).

Mg/Ga molar ratio and the calcination temperature is shown in Table 2. Here, the corresponding amount of base sites in Ga2O3 and MgO is also included as a comparison with that of GaHTs. 2.9GaHT and 7.7GaHT show about a 10% decrease in the number of base sites when calcining from 673 to 873 K. As a contrast, the number of base sites in 7.7GaHT increases 3-fold when calcining from 673 to 873 K. A calcination temperature of 1073 K brought about a considerable diminishment in the amount of total base sites in all GaHTs. Thus, a 53, 51, and 80% decrease in the total base site amount in 2.9-, 5.3-, and 7.7GaHTs, respectively, when calcining from 873 to 1073 K was observed. This decrease in the total population of base sites could be related to the transformation of Ga-substituted MgO into MgO and a spinel type MgGa2O4 which occurs between 873 and 1073 K.13 The total number of base sites in pure MgO calcined at 873 K is larger than that corresponding to GaHTs with Mg/Ga ) 2.9 and 5.3, pointing out an inhibiting effect of Ga in the development of base sites, since Ga2O3 contains only a small amount of base sites. However, 7.7GaHT shows 1.5 times more base sites than MgO calcined at 873 K, suggesting that other factors may be acting in the formation of

base sites. Accordingly, in calcined Mg-Al hydrotalcites the higher the Mg/Al ratio the greater the number of base sites.10 The density of base sites in GaHTs, i.e., number of sites per unit area, increases when calcining from 673 to 873 K, in spite of the fact that the surface area decreases considerably in this temperature range (see Figure 1). These results point out that the formation of base sites is not completely associated with the development of surface area. Base Strength of GaHTs by CDCl3 Adsorption and FT IR. A suitable choice of probe molecule is very important in the development of accurate methods of studying the base properties of adsorbents and catalysts surfaces. Paukshtis and Yurchenko have reported that CDCl3 can be used as a probe to characterize the type and strength of surface base sites.14 The main advantage of this method is that CDCl3 is a weak base, and its interaction with aprotic or protonic surface centers is negligible. Furthermore, the νCD bands do not overlap the bands of the surface OH groups, and therefore the complexes of the probes with the base sites can be clearly recognized. The different modes of CDCl3 interaction with surface base sites on GaHTs are represented in Figure 6. In Figure 7 the IR spectra (2300-1950 cm-1) of CDCl3 adsorbed on Ga2O3, 1.8GaHT, 7.7GaHT, and MgO are shown. The spectra of MgO and Ga2O3 are shown for comparison purposes; the former is made up mainly of two bands at 2241 and 2217 cm-1, and the latter shows two bands at 2251 and 2214 cm-1. The band at 2241 cm-1 in MgO arises from CDCl3 interacting with MgO-Mg bonds and with adjacent Mg-OH groups (site type a). The second band in MgO (2217 cm-1) may be ascribed to CDCl3 interacting with an Mg-O-Mg bond (site type b). Similar interactions can be associated to CDCl3 adsorbed on Ga2O3 (Figure 7a) where the bands at 2251 and 2214 cm-1 arise from interaction type c and d, respectively. Due to electrostatic repulsion of octahedral [Ga(OH)2]+ units in the layered structure of Mg-Ga hydrotalcites, it is unlikely for two such units to be adjacent. Moreover, in all GaHTs the Mg content is greater than that of Ga. Hence, the formation of Ga-O-Ga bonds can be ruled out, at least below 873 K. Considering this assumption, the first band in GaHTs at 2250 cm-1 is ascribed to a type e interaction. The second band at 2240-2238 cm-1 may be associated to a type a interaction. An interaction of CDCl3 with an oxygen in Mg-O-Ga or MgO-Mg, as in type f, may be responsible for the band at 22112212 cm-1. The bands at 2155 and 2143 cm-1 may be ascribed to isolated Mg-O-Ga or Mg-O-Mg bonds, i.e., absence of adjacent OH groups. Furthermore, a comparison of the spectra of the GaHTs with those of the single metal oxides shows that the νCD bands (2300-2100 cm-1) tail markedly toward the lowenergy region, indicating that the number of stronger base sites is higher in GaHTs than in the single oxides. An estimation of the base strength can be obtained for each base site taking into account their ∆νCD shift (∆νCD ) [νCD]gas - [νCD]adsorbed) and compare it with the pKa of bases which form hydrogen-bonded complexes as reported elsewhere.14,15 Table 3 shows the estimated pKa and proton affinity values for 7.7GaHT which can be as high as +19 and 987 kJ mol-1, respectively. However, from the relative intensities of the bands in spectrum c of Figure 7, it is clear that strong base sites (pKa ) +17 to +19) are a small portion of all the base sites population in 7.7GaHT. The adsorption of CDCl3 also brings about important changes in the νOH region of the GaHTs as shown in Figures 8 and 9. In the differential spectrum of 1.8GaHT (Figure 9c), the intensity of the νOH band at 3453 cm-1 decreases considerably upon CDCl3 adsorption, indicating that a part of the initial OH groups

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Figure 6. Different modes of interaction of CDCl3 with MgO, Ga2O3, and GaHTs.

Figure 7. FTIR spectra (νCD region) of CDCl3 adsorbed on (a) Ga2O3, (b) 1.8GaHT, (c) 7.7GaHT, and (d) MgO.

TABLE 3: Estimated pKa and Proton Affinity (PA) Values for 7.7GaHT Calcined at 873 K νCD (cm-1)

∆νCD (cm-1)

pKaa

PAa (kJ mol-1)

2251 2238 2212 2155 2143

14 27 53 110 122

-1 +5 +10 +17 +19

815 872 924 979 987

a Estimated from refs 14 and 15. ∆ν CD ) [νCD]gas - [νCD]adsorbed. [νCD]gas ) 2265 cm-1, PA ) (3.54 + log[∆νCD])/0.0057.

transform into other species. Additionally, the appearance of several bands at 3655, 3171, 3004, 2963, and 2687 cm-1 occur. The band at 3655 cm-1 is probably associated with OH groups interacting with Cl atoms in CDCl3 (type a, e, or g) and explains the intensity decrease of the initial νOH band. The band at 2687 cm-1 can be ascribed to νOD, indicating that a hydrogenexchange reaction between surface OH groups and CDCl3 took place. Accordingly, upon D2O deuteration on Al-pillared montmorillonite16 and TiO2-SiO2,17 a νOD band at 2699 and 2620 cm-1, respectively, has been reported. The bands at 3004 and 2693 cm-1 arise from CH vibrations in CHCl3, which confirm the hydrogen-exchange reaction. The origin of the band at 3171 cm-1 in both spectra of 1.8- and 7.7GaHT is still unclear. In 7.7GaHT the differential spectrum in Figure 9c

Figure 8. FTIR spectra (νOH region) of CDCl3 adsorbed on 1.8GaHT: (a) heat treated at 673 K, (b) after exposure to CDCl3 vapor, (c) differential spectrum (b) - (a).

shows bands at 3711, 3655, 3171, 3004, 2963, and 2687 cm-1, which arise from νOH, CH vibrations, and νOD, respectively, similar to the 1.8GaHT case. Here, the intensity decrease of the initial νOH band upon CDCl3 adsorption is not so pronounced as that in 1.8GaHT (compare Figures 8c and 9c), indicating that more OH groups interact with Cl in the Ga-rich material than in the Mg-rich one (7.7GaHT). Noteworthy, the νOH shift of the highest energy band in 1.8GaHT is larger (202 cm-1) than that in 7.7GaHT (181 cm-1), indicating that the interaction of Cl in CDCl3 with surface OH groups is stronger in the Garich material than in the Mg-poor one. Paukshtis et al. have reported that a smaller ∆νOH shift implies a greater basicity.15 Conclusions The specific surface area of Ga-substituted hydrotalcite-like GaHT increases 3-fold when calcining at 673 K in comparison with that of the as-synthesized material. Above 673 K GaHTs decrease their surface area probably due to MgGa2O4 formation. The surface area decrease in Ga-rich GaHTs is more pronounced than that in Mg-rich samples. Upon dehydroxylation and decarbonation (410-773 K), GaHTs develop basic properties associated with the formation of Mg-O-Ga and Mg-O-Mg

Base Sites on [Mg1-xGax(OH)2](CO3)x/2‚mH2O

J. Phys. Chem. B, Vol. 101, No. 26, 1997 5117 strength of 7.7GaHT calcined at 873 K, which is the highest among GaHTs, was estimated to range between pKa -1 and +19. However, the relative population of strong base sites (pKa +17 to +19) is small in comparison with those sites of pKa -1 to +10. When GaHTs were exposed to CDCl3 vapor at room temperature, IR bands ascribed to Mg(Ga)-O-D bonds (νOD ) 2687 cm-1) were found, indicating that calcined GaHTs exhibit hydrogen-exchange properties. References and Notes

Figure 9. FTIR spectra (νOH region) of CDCl3 adsorbed on 7.7GaHT: (a) heat treated at 673 K, (b) after exposure to CDCl3 vapor, (c) differential spectrum (b) - (a).

bonds. The relative Mg/Ga contents and the calcination temperature exert a strong effect on the total number and relative distribution of base sites. 7.7GaHT (Mg-rich sample) shows 3 times more base sites when calcined at 873 than that at 673 K. On the other hand, 7.7GaHT shows 2.4 times more base sites than 2.9GaHT (Ga-rich sample). In all GaHTs, at 1073 K the total number of base sites diminishes between 50 and 80% in comparison with their maximum values at 873 K. The base

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