Physicochemical Study of Nanocapsular Layered Double Hydroxides

J. Phys. Chem. C 2009, 113, 5547–5555

5547

Physicochemical Study of Nanocapsular Layered Double Hydroxides Evolution Jaime S. Valente,*,†,‡ Julia Prince,‡ Ana M. Maubert,‡ Luis Lartundo-Rojas,† Paz del Angel,† Gerardo Ferrat,† Jose G. Hernandez,† and Esteban Lopez-Salinas† Instituto Mexicano del Petroleo, Eje Central No. 152, CP 07730, Mexico, D. F., Mexico, and UniVersidad Autonoma Metropolitana-A, AV. San Pablo No. 180, CP 02200, Mexico, D. F., Mexico ReceiVed: NoVember 23, 2008; ReVised Manuscript ReceiVed: January 24, 2009

A sol-gel method for the synthesis of MgAl layered double hydroxides is presented. The sols thus obtained were submitted to several hydrothermal treatments, in presence or absence of water, in order to examine the thermal evolution of specific physicochemical properties, such as morphology, crystal size, basicity, and textural properties. The morphological evolution was followed by transmission electron microscopy (TEM) and scanning electron microscopy techniques. TEM analyses revealed unique nanocapsular morphology; the nanocapsules grow with increasing hydrothermal treatment time and aggregate in the presence of water. X-ray diffraction confirms the expected increase in crystal size. The nitrogen adsorption-desorption surface analysis (Brunauer-Emmett-Teller) of the calcined solids indicates that surface areas range from 251 to 289 m2 g-1. Furthermore, CO2 thermoprogrammed desorption and Fourier-transform infrared analyses using CDCl3 as a probe molecule indicate that basic strength is inversely proportional to crystal size. Introduction Layered double hydroxides (LDHs) are a class of naturally occurring anionic clays. The basic structure of LDHs may be derived from partial isomorphous substitution of divalent cations in a brucite lattice [Mg(OH)2] by trivalent cations, such that the layer acquires a positive charge, which is compensated for by the intercalation of anions between the layers, along with hydration water molecules. Since an ample assortment of compounds with the basic LDH structure might be prepared, they are represented by the general formula: [M2+(1-x)M3+x(OH)2] An-x/n · mH2O. M2+ and M3+ can be any divalent and trivalent metal ions capable of occupying the octahedral interstices of a brucitelike sheet (Mg2+, Zn2+, Ni2+, Al3+, Fe3+, Ga3+, etc.) and An- can be almost any organic or inorganic anion.1 LDHs have been studied as catalysts or catalyst precursors to a great extent for the past few decades. Given that calcination of LDH yields mixed oxides with basic properties, they have been successfully employed in many organic reactions catalyzed by bases.2 Moreover, LDHs have found a wide range of technological applications, such as hybrid composites, antacid agents, flame retardants, and PVC additives. Several research groups worldwide have attempted to introduce biological species and organic compounds between the layers in order to use LDHs in diverse fields as drug delivery carriers, sensing devices, etc.3 Conventionally, LDHs have been synthesized through the coprecipitation of metallic salts with an alkaline solution. However, alternative routes have been recently explored with the aim of a better control over the structural and textural properties, which are also related to the catalytic and anion exchange properties of LDHs. This has been achieved, for instance, through microwave irradiation or ultrasonic treatment in the course of the classical coprecipitation route from inorganic precursors4-7 by urea hydrolysis and via the sol-gel method.8-13 * To whom correspondence should be addressed. Phone: + 52 (55) 91758444. E-mail: [email protected]. † Instituto Mexicano del Petroleo. ‡ Universidad Autonoma Metropolitana-A.

A wide variety of studies have been conducted regarding crystal growth and evolution of basic and textural properties of LDHs prepared both by coprecipitation and urea hydrolysis, after being submitted to hydrothermal, microwave,6,7 and ultrasound treatments.6 However, to the best of the authors’ knowledge, there exist no reports on the evolution of sol-gel-synthesized LDHs, which may afford a deeper understanding and control over their physicochemical properties and crystallization mechanisms. Furthermore, a model for the generation of basic sites in MgO, originally proposed by Coluccia and Tench,14,15 suggests that in a MgO crystal, there exist several Mg-O ion pairs with different coordination number. Ion pairs of low coordination number exist at crystalline defects such as corners and edges. According to this model, different basic sites appear to correspond to the ion pairs of different coordination number. Even though this model was developed for MgO, it is thought to be valid for other basic metal oxides, such as calcined LDHs. In this sense, a smaller crystal size would necessarily implicate a larger amount of defects. Thus, this study intends to establish a relation between crystal growth of MgAl LDHs and the number and strength of Lewis basic sites in their calcination products. In a similar approach, K. P. de Jong et al. have established a correlation between lateral dimension of LDH crystallites and the number of catalytically active sites, in LDHs with OH- interlayer anions acting as Bro¨nsted basic sites; this was explained in terms of the accessibility of basic sites, not on their generation.16 Recently, the authors reported a new strategy for LDH synthesis by means of a sol-gel technique.13 The success of this approach is attributable to the control of certain key parameters during the synthesis procedure. The polarity, reactivity, and amount of the intended solvent, generally an alcohol, are essential factors in controlling the reaction’s kinetics. Also, the different reactivities of metal alkoxides employed as precursors, related to their different electronegativities, must be taken into account. To facilitate the preparation of homogeneous materials, the most reactive precursor, in this case aluminum alkoxide, must be complexed. Metal complexation with acetic

10.1021/jp810293y CCC: $40.75  2009 American Chemical Society Published on Web 03/17/2009

5548 J. Phys. Chem. C, Vol. 113, No. 14, 2009 acid, trifluoroacetic acid, and other chelating ligands has often been used as a simple way to control hydrolysis and condensation reactions, and the growth of inorganic precursors.13,17-19 Acetic acid was found suitable for this end, since it is a weak acid that does not substantially alter the reaction medium and may be easily removed upon calcination. In addition, the amount of water was maintained in a substoichiometric value to control the hydrolysis reaction. The LDHs prepared following this methodology revealed peculiar nanocapsular morphology, in contrast to the classic plateletlike morphology of LDHs. Alkoxy groups from the alcoholic solvent were found to be located in the interlayer region. The intercalation degree was found to be MgAl-ethanol > MgAl-propanol . MgAl-butanol. Particularly, the high intercalation degree attained using ethanol as solvent allowed a greater homogeneity and a large surface area.13 This work reports the sol-gel method for MgAl LDHs synthesis, using ethanol as solvent, and the evolution of morphology, crystal size, and basicity, when the sols were submitted to diverse hydrothermal treatments. Hence, direct and systematic control of specific properties should enable the application of these materials in fields as diverse as catalysis, biochemistry, and sustained release drug delivery, optics, thin films, etc. Experimental Section Synthesis Procedure. The synthesis procedure has been reported previously.13 Briefly, LDHs were prepared by dissolving aluminum tri-sec-butoxide (ATB) in ethanol at 70 °C under constant stirring for one hour. Nitric acid (3 N) was then added dropwise. After one hour, the system was taken to room temperature, and acetic acid (AA) was added to complex the aluminum alkoxide. One hour later, the temperature was lowered to 0 °C, and magnesium methoxide was added dropwise. The system was stirred for 24 h at room temperature, and finally deionized water was added. The molar ratios of reactants were ATB:EtOH ) 1:60, ATB:HNO3 ) 1:0.03, ATB:AA ) 1:1, M2+:M3+ ) 3:1, ATB:H2O ) 1:1. After the synthesis procedure was completed, the sols thus obtained were placed in a sealed digestion bomb and kept at 120 °C under autogenous solvent vapor pressure for 1-10 days. Samples will be referred to as MgAl-THx, where x stands for the number of days of thermal treatment. Sample MgAlTH1W received a 1-day treatment; the sol was set in the presence of 50% vol excess bidistilled water. Two more samples are presented for comparison: MgAl-E, a sol-gel sample with no thermal treatment, and MgAl-CP, a coprecipitated sample prepared by following a procedure reported elsewhere.20 The Mg/Al molar ratio was 2.8, 2.7, 2.6, 2.6, and 3.02 ( 0.3 for MgAl-E, MgAl-TH3, MgAl-TH10, MgAl-TH1W, and MgAlCP, respectively. Characterization Techniques. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Samples were analyzed through TEM in a JEOL JEM-2200FS with a Schottky-type field emission gun, operating at 200 kV. High-resolution (HR) TEM studies of the powder samples were performed in ultrahigh-resolution (UHR) configuration (Cs ) 0.5 mm; Cc 1.1 mm; point-to-point resolution, 0.19 nm). Samples were dispersed in ethanol before placing them on Lacey Formvar carbon coated copper grids. Digital images were obtained using Digital Micrograph Software and a camera CCD from GATAN. SEM analysis was carried out in a Philips XL30 ESEM with an acceleration voltage of 25 kV. Prior to analysis, the samples

Valente et al. were covered with gold and were subsequently mounted on aluminum stub over double-coated carbon film. X-ray Diffraction (XRD). The XRD pattern of the samples was measured in a θ-θ Bruker D-8 Advance diffractometer with Cu KR radiation, a graphite secondary-beam monochromator, and a scintillation detector. Diffraction intensity was measured between 4 and 80°, with a 2θ step of 0.02° and a counting time of 9 s per point. Thermal Gravimetric Analysis. Thermogravimetric (TG) analyses were carried out on a Netzch STA-409EP equipment which was operated under a helium flow at a heating rate of 10 °C min-1 from 25 to 1000 °C. In all determinations, 100 mg of finely powdered dried LDH samples were used. Mass Spectrometry. The evolution of the thermal decomposition products was followed in an Autosorb-1C apparatus coupled with a Prisma-QMS 200 quadrupole spectrometer. The sample was annealed from room temperature to 700 °C in a He atmosphere at a heating rate of 10 °C min-1 and a He flow rate of 60 cm3 min-1. The sample’s weight was ∼100 mg. The release of water (m/z ) 17 and 18), CO2 (m/z ) 44), and organic species fragments (m/z ) 12, 29, and 46) was monitored throughout the decomposition process. Textural Analysis. The texture properties of the calcined samples (at 550 °C for 4 h in air) was analyzed by N2 adsorption-desorption isotherms at -196 °C on an Autosorb1C apparatus. Prior to the analysis, the samples were outgassed in a vacuum (10-5 Torr) at 400 °C for 5 h. The surface areas were calculated by using the Brunauer-Emmett-Teller (BET) method, and the pore size distribution and total pore volume were determined by the Barrett-Joyner-Hallenda (BJH) method. CO2 Thermoprogrammed Desorption (TPD). The number of basic sites and base-strength distribution was determined by means of CO2 TPD in an Autosorb-1C apparatus from Quantachrome connected to a Pfeiffer Prisma-QMS-200 mass spectrometer. Before exposure to CO2, all the samples were heattreated in situ at 550 °C in a He flow for 1 h. Then, the temperature was lowered to 50 °C, and the sample was exposed to a flow of CO2 for 1 h. Afterward, CO2 desorption started with a heating rate of 10 °C min-1 up to 600 °C. IR Spectroscopy. CDCl3 was used as a basicity probe molecule by means of diffuse reflectance IR Fourier-transform spectroscopy (DRIFTS), which was carried out in a Bruker Equinox 55 spectrometer, between 4000 and 1000 cm-1 with a resolution of 4 cm-1 after 350 scans per spectrum. The equipment was furnished with a Harrick Praying Mantis chamber and CaF2 windows, allowing in situ treatments at temperatures below 500 °C. Prior to DRIFTS analysis, all samples were calcined ex situ in air, at 550 °C, 2 °C min-1 for 6 h. Once transferred into the chamber, the samples were heattreated in flowing air (6 cm3 min-1) at 460 °C for 4 h, and then evacuated at 0.13 Torr and cooled to room temperature. After this, the calcined sample spectra, just before injection of 5 µL of CDCl3, as well as its adsorption spectrum were registered, respectively. Results and Discussion Morphology and Particle Arrangement. TEM images of four different samples, MgAl-E, MgAl-TH10, MgAl-TH1W, and MgAl-CP are shown in Figure 1; the MgAl-TH3 micrograph is not shown since it is very similar to MgAl-E. MgAl-E and MgAl-TH10 illustrate apparent rings that, because the image was formed by transmitted electrons, in fact correspond to hollow capsules or shells, with diameters ranging from 10 to 40 nm for sample MgAl-E (Figure 1a). In the case of MgAl-

Nanocapsular LDH Evolution

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5549

Figure 2. SEM images of sol-gel and coprecipited LDHs.

Figure 1. TEM images of LDHs. Insert in MgAl-TH10 is a high resolution micrograph.

TH10, a growth of the nanocapsules is evident, as a result of the coalescence of former capsules. In addition, HRTEM revealed that the nanocapsules’ shells are lamellar (inset in MgAl-TH10). In contrast, MgAl-TH1W presents the classic plateletlike morphology with a highly ordered layered structure, which can be appreciated in the high-resolution micrograph; see the inset in MgAl-TH1W. This result could be explained by assuming that water enhances the interaction, aggregation, and coalescence of the capsules to form plates, and the thermal treatment provides the required energy for the ordering of the material. The coprecipitated sample MgAl-CP exhibits also plateletlike morphology that is characteristic of LDHs. In addition, these results indicate that the capsules are metastable and that their formation is favored by the alcoholic medium. Indeed, the generation of this peculiar morphology may be related to the hydrophobic organic tail of the alcohol (-R). The hydrophobicity of -R could avoid the contact with the hydroxyl groups of the layers, acting as a structure-directing agent; consequently the layers might be curved. This is a rough explanation of why the occurrence of nanocapsules is restricted to nonaqueous systems; because of this, this peculiar morphology has not been observed when different procedures were employed for LDH synthesis. The same morphology has also been observed and thoroughly analyzed during the synthesis of aluminas following a sol-gel procedure resembling the one reported here.21 SEM of the same four samples are displayed in Figure 2. Analysis of MgAl-CP sample reveals that it is made up of irregularly shaped platelets or flakelike particles, in agreement with both TEM results and previous reports. Sample MgAlTH1W also presents this flakelike morphology, but the shape, size and distribution of these flakes appears to be more uniform. In both MgAl-E and MgAl-TH10, the samples are made up by small, rounded aggregates. The growth of these aggregates with thermal treatment is even more evident than in TEM analysis. The same phenomenon was observed in sample MgAl-TH3 (data not shown). Crystalline Structure and Crystal Size. XRD patterns of all five samples are presented in Figure 3. The MgAl-CP curve shows the peaks corresponding to an LDH structure. A

Figure 3. XRD patterns of coprecipitated and sol-gel samples.

comparison of this pattern to those of sol-gel reveals some differences; mainly, a widening of the peaks, attributed to the small crystal size and unique morphology presented by this materials,13 and a slight shift of the 003 peak to a lower 2θ angle, indicating the intercalation of alkoxy groups in the interlayer region and its consequent expansion. These phenomena have been reported elsewhere when using alcohols during LDH synthesis.13,22,23 Samples MgAl-TH3 and MgAl-TH10 have essentially the same behavior as MgAl-E, although the peaks gradually become sharper, due to the crystal growth taking place during thermal treatments. However, this growth is extremely slow, when compared to the times required for full crystallization of LDHs by ultrasonication, microwave, or hydrothermal treatment, which range from a few minutes to one day.5-7 Thus, crystallization must depend on something other than time and temperature of thermal treatment. To this effect, an excess of water (50 vol %) was added and autoclaved for 24 h, obtaining sample MgAlTH1W. This XRD pattern has very narrow peaks, indicating that water greatly favors capsule coalescence and crystal growth, in accordance with the morphology observed by TEM. Unit cell parameters c and a were calculated from the 003 and 110 peak positions, respectively, assuming a hexagonal 3R packing of the layers, where c ) 3d003 and a ) 2d110. Crystal sizes were determined from the full width at half-maximum

5550 J. Phys. Chem. C, Vol. 113, No. 14, 2009

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TABLE 1: Unit Cell Parameters and Crystal Sizes unit cell parameters (Å)

crystal size (Å)

a

sample

a

c

L(003)

L(110)

MgAl-E MgAl-TH3 MgAl-TH10 MgAl-TH1W MgAl-CP

3.089 3.076 3.067 3.055 3.056

28.110 27.021 28.297 23.011 23.192

18 21 24 139 91

56 76 107 422 271

a

Determined using Scherrer’s equation.

(fwhm) of the 003 and 110 reflections using the Scherrer equation. Crystal sizes and unit cell parameters of the five samples are shown in Table 1. Unit cell parameter a, related to the average cation-cation distance within the layers, is larger in MgAl-E and decreases gradually in MgAl-TH3, MgAl-TH10, and MgAl-TH1W. This fact has two possible explanations: (i) it might be due to a progressive incorporation of Al cations into the brucitelike sheets with increasing thermal treatment time, as the occurrence of traces of Mg(OH)2 cannot be ruled out at this point; (ii) another possibility would be that the brucitelike layers become slightly curved when arranged around the nanocapsules’ shell, which will set about a separation between metal sites. In fact, as thermal treatment time increases the capsules grow, therefore, the curvature effect diminishes and so does the distance between cations as is observed in Table 1, where the MgAl-TH1W and MgAl-CP both have the same plateletlike morphology, thus their a parameters are similar and smaller than those with nanocapsular morphology. LDH sheet flexibility has been previously demonstrated; bending of the layers was accomplished by intercalation of longchain anionic surfactants. Variances on a parameters related to the deformation degree were reported.24 On another study,25 MgAl LDHs synthesized in dilute acetic acid solution were found by SEM analyses to be composed of a loose agglomeration of wavy sheets, which supports our hypothesis that -R groups can have a similar curving effect on LDH layers. Cell parameter c is significantly larger in sol-gel samples prepared in absence of water than in samples MgAl-TH1W and MgAl-CP, both of which have an interlayer distance that corresponds well to the intercalation of CO32- and H2O molecules. In MgAl-E, MgAl-TH3, and MgAl-TH10 samples, the evident expansion of interlayer region can be attributed to the intercalation of alkoxy groups from the alcohol employed as solvent.13,22,23 Crystal size presents the expected increasing tendency from MgAl-E to MgAl-TH1W, with preferential crystal growth in the 110 direction. Also, crystal size for MgAl-TH1W is nearly twice than that of the coprecipitated sample, thus confirming the conclusions drawn previously by TEM and SEM analyses. Thermal Decomposition. TG analyses (Figure 4) demonstrate that all samples have quite similar behaviors regarding weight loss intervals but varying the amount of weight loss in each. To better understand these differences, the decomposition products were recorded by mass spectroscopy, presented in Figure 5. The coprecipitated sample, MgAl-CP, presents a total weight loss of 45%, divided into three phases: (i) between room temperature and 250 °C, 18.7%, mostly of H2O + OH and a little contribution from CO2, assigned mainly to crystallization water and/or weakly bound hydroxyls and physisorbed CO2; (ii) from 250 to 450 °C, a 22.8%, which corresponds to CO2 from interlayer CO32- anions and water molecules from dehy-

droxylation of brucite-like sheets; (iii) finally, between 450 and 650 °C, a 3.5% weight loss that comes from strongly bounded anions. The only decomposition products identified by mass spectroscopy were H2O, OH, and CO2 (Figure 5). Firstderivative TGA profile is characterized by two small, endothermic, peaks centered at ∼170 and ∼370 °C (Figure 4). On the other hand, three weight loss stages were identified for MgAl-E: (i) between 26 and 200 °C, 14.83% that corresponds to the elimination of water molecules (H2O + OH) and a very small fraction of organic moieties; (ii) from 200 to 500 °C, a 23.67% loss, consequence of the removal of alkoxy groups as organic species, mainly C2H5, and dehydroxylation of the layers; (iii) from 500 to 700 °C, 1.65%. This last loss is relatively small and is ascribed to the continued elimination of strongly bounded anions and organic compounds, with a small contribution of OH groups. The elimination of organic molecules at temperatures over 200 °C, well above the alcohol’s boiling point, likely indicates that they were located as alkoxy anions in the interlayer region instead of merely physisorbed at the surface. Samples MgAl-TH3 and MgAl-TH10 behave in a similar way to MgAl-E, both presenting three weight losses in concomitant intervals. Given the resemblance of their behaviors, only MgAlTH10 is shown in Figures 4 and 5. The total weight loss is 51.63% and 50.23% for MgAl-TH3 and MgAl-TH10, respectively. In both cases, the main decomposition products above 250 °C are organic fragments CxHy, C, and CO2. Furthermore, the larger weight loss in these samples (∼50 vs 40% in MgAlE) might be indicative of a higher alkoxy intercalation degree. Analysis of the mass spectroscopy curves (Figure 5) revealed that both CO2 and C2H5 were released in larger quantities for MgAl-TH10 than for MgAl-E. However, CO2 may originate from interlamellar CO32- or from combustion of organic molecules. Water-treated MgAl-TH1W lost 44.35% in three steps: (i) 2.55% between 25 and 110 °C; (ii) 11.71% between 110 and 250 °C; (iii) 27.14% between 250 and 480 °C. The small weight loss in the first interval is significant, since it implies that most of the excess water is not physisorbed; instead, it is strongly bound to the brucitelike layers. Other than that, the sample behaves in a rather similar way to the coprecipitated sample. It presents two large endothermic peaks shifted to higher temperatures, at ∼200 and ∼390 °C (see Figure 4). As it can be observed in Figure 5, the main decomposition products are CO2, H2O, and OH groups, the contribution of organic fragments is significantly lower than that in MgAl-E, MgAl-TH3, and MgAlTH10, indicating the deintercalation of alkoxy molecules as a consequence of the presence of water. Alkoxy anions are most likely replaced in the interlayer region by carbonate anions trapped from air and bidistilled water, as LDHs are well-known for their large affinity for CO32-. Textural Properties. Textural analyses results are reported in Table 2. Pore volumes for calcined samples are in the range of 0.298 cm3 g-1 for MgAl-TH1W and 1.148 cm3 g-1 for MgAlCP. Surface area values were 251, 254, 263, 264, and 289 m2 g-1 for MgAl-TH1W, MgAl-CP, MgAl-TH3, MgAl-TH10, and MgAl-E, respectively. N2 adsorption-desorption isotherms are shown in Figure 6. All curves are type IV isotherms, according to IUPAC classification, which correspond to mesoporous solids. Multilayer adsorption processes begin at low relative pressures, with this process being continuous for MgAl-CP, while MgAl-E reveals two steps in the adsorption path, the first one at approximately P/P0 ) 0.6 and the second one at approximately 0.85. These



Figure 4. TG analyses and first-derivative weight loss from TGA for samples MgAl-E, MgAl-TH10, MgAl-TH1W, and MgAl-CP.

Figure 5. Profiles of products from thermal decomposition of LDHs followed by mass spectrometry.

variations are due to the different morphologies of the solids and can be better understood by analyzing the hysteresis loops.

Samples MgAl-CP and MgAl-TH1W show H3 and H4 type hysteresis loops according to IUPAC classification, respectively.


Valente et al.

TABLE 2: Textural Properties and Basicity of LDHs

samplea MgAl-E MgAl-TH3 MgAl-TH10 MgAl-TH1W MgAl-CP a

pore surface volume area (m2g-1) (cm3g-1) 289 263 264 251 254

0.812 0.525 0.566 0.298 1.148

pore size (Å) I

II

67 48 55 30 124

415 94 123 290 397

CO2 basic site density uptake (µmol g-1) (µmol m-2) 154 131 120 115 105

0.5329 0.4981 0.4545 0.4582 0.4134

Calcined at 550 °C for 4 h.

Both types correspond to solids formed by platelet-shaped particles, creating slit-shaped pores. The H3 loop is usually presented by solids with irregular pore size or distribution, while H4 belongs to pores with regular size and distribution.26 These results are in agreement with the morphology observed by TEM analyses and also with the pore size distribution presented as an inset in Figure 6. On the other hand, sample MgAl-E has an H1 hysteresis loop, while MgAl-TH3 and MgAl-TH10 both have H2 type loops. These are characteristic of solids conformed by spherical agglomerates, H1 corresponds to uniform pores; H2 corresponds to pores with nonuniform size and shape, as it can be appreciated by the pore size distribution (Figure 6).26 Furthermore, the two steps in the MgAl-E loop are indicative of a bimodal pore size distribution. The first step, in the range 0.6 < P/P0 < 0.85, corresponds to pores with regular geometry, attributed to intraparticle porosity. The second, in the range 0.85 < P/P0 < 1.0 corresponds to irregular meso and macropores from interparticle voids.13 Pore size distributions, displayed as insets in Figure 6, clearly show two peaks. In the case of MgAl-E, the sharpness of the first peak indicates homogeneous intraparticle pores, while the second interparticle pores have a wider distribution. Pore volume in this sample is mainly due to the first, homogeneous pore size distribution. In samples MgAlTH3 (not shown) and MgAl-TH10, the position of the first peak indicates larger intraparticle pores. Interparticle porosity is lower in average and contributes to a larger proportion of pore volume, which is indicative of irregular ordering of the spheroidal particles upon calcination. Thus, the pore size distribution appears to be more heterogeneous. Textural properties of sample MgAl-TH1W are remarkable; even though its morphology is very similar to the coprecipitated samples, pore sizes are smaller in average, and pore size distribution is more homogeneous. The smaller pore size suggests that porosity comes mainly from a “cratering” process that occurs upon calcination.27,28 Interparticle stacking and layer stacking defects are also present but in significantly smaller proportion. In contrast, porosity in the coprecipitated sample arises mainly from interparticle stacking created during the calcination treatment, resulting in a highly heterogeneous pore size distribution. Number and Strength of Basic Sites. The CO2 desorption profiles of calcined LDHs are shown in Figure 7, being very similar. Regardless of hydrothermal treatment time (in sol-gel LDHs) or preparation procedure (coprecipitation vs sol-gel) all asymmetrical curves had a CO2 desorption maximum at about 175 °C with a long tail toward higher temperature up to about 450 °C. Above this temperature, only negligible CO2 could be detected. However, in the coprecipitated sample, a hump on the tail at about 300 °C indicates that sites of high strength may be responsible for the tailing shape of the curves. Accordingly,

well crystallized MgAl LDHs show three distinguishable CO2 desorption peaks at ∼130, 170-200, and 250 °C, which correspond to weak, moderate, and strong basic sites.29 The weak basic sites correspond to CO2 adsorbed on either Mg-OH and/ or Al(OH) groups; the moderate basic sites are associated to Mg-O-Mg(Al) pairs, and the strong basic sites result from coordinatively unsaturated O2-.30-32 In our case, sol-gel LDHs are made up of small crystallites (d110 ≈ 56-107 Å, see Table 1), thus the averaging effect of the different heterogeneities of basic sites is revealed as an overlapped curve composed in fact by the three types of basic sites. The total number and density of basic sites, presented in Table 2, varied from 115 to 154 µmol CO2 g-1 in sol-gel LDHs, clearly dropping along the thermal treatment time. In sol-gel procedure, thermal treatment promotes cross-linking of colloidal LDHs particles where two M-OH groups dehydrate to form M-O-M bonds;33 thus the number of OH groups and/or accessible base sites decreases because of crystal growth. The density of base sites in sol-gel samples was between 0.454 and 0.533 µmol CO2 m-2, decreasing along the aging time. In comparison with an LDH obtained by typical coprecipitation, a sol-gel LDH (see as-synthesized MgAl-E) contains ca. 30% more base sites. The lower basic site density of MgAl-TH1W is explained by its large crystal size and extremely well ordered structure, and the consequent decrease in surface defects. Surface defects, such as corners and edges, in MgO and LDH crystals have been associated with the strength of different basic sites, given the fact that they generate oxygen atoms with lower coordination number (O2-).6,14,15 As a comparison, in a series of hydrotalcites obtained by a hydrothermal method in an alkalifree solution, the total number of basic sites in Mg/Al ≈ 3 hydrotalcites (heat-treated at 450 °C under flowing N2), as determined by CO2 TPD, was 82-91 µmol g-1, having a basic site density of 0.40-0.48 µmol m-2.29 A determination of the basic strength of the different basic sites can be obtained through adsorption of appropriate probe molecules and then monitored by IR spectroscopy.34 Paukshtis et al.35,36 have reported that CDCl3 can be used as a probe molecule to characterize the different types and strengths of basic sites. The main advantages of using CDCl3 as probe molecule35,37 are that it does not undergo chemical modification under ambient conditions and that the relevant C-D stretching vibration (νCD) does not overlap with the bands of the νOH groups. In the gas phase, CDCl3 displays an isolated νCD at 2264 cm-1, which is expected to shift to lower frequencies upon hydrogen bonding (i.e., that of basic OH groups). For instance, CDCl3 FTIR carried out on calcined MgGa LDHs (Mg/Ga molar ratio, 1.8-7.7) showed several IR peaks at 2250, 2238, 2212, 2155, and 2143, ascribed to diverse base sites with base strength of -1 to +19 pKa.38 A DRIFTS spectrum of CDCl3, in the νCD region (2300 to 2100 cm-1), adsorbed on sol-gel LDHs yielded a similar spectra among them, regardless of aging time. A representative spectrum of CDCl3 adsorbed on MgAl-E is shown in Figure 8, which can be deconvoluted into three peaks (see gray curves) at 2290, 2246, and 2224 cm-1; these last two bands account for 96% of total area under the curve. The νCD shift was determined by taking gaseous νCD ) 2265 cm-1 (from CDCl3) as a reference;39 thus, ∆νCD for these two bands was 19 and 41, having pKa values of ca. 3.5 and 7.6, respectively, which were estimated from references.35,36 When CDCl3 is adsorbed on basic materials, OH groups are able to interact with the probe molecule in different modes, perturbing the position of the OH groups bands.38 As long as



Figure 6. N2 adsorption-desorption isotherms and pore size distribution for samples MgAl-E, MgAl-TH10, MgAl-TH1W, and MgAl-CP.

Figure 8. DRIFTS νCD region of CDCl3 adsorbed on MgAl-E LDH. Figure 7. CO2 TPD profiles.

LDHs remain calcined below ca. 600 °C (that is, before their irreversible conversion to spinel MgAlO4 and MgO) their crystallites are topochemical to pristine LDH, then, remaining OH groups can be used as an indication of accessibility when interacting with probe molecules. On the other hand, when an LDH is calcined below 600 °C, most OH groups are withdrawn as water vapor, but a considerable amount of isolated OH groups remain, adjacent to and/or in the proximity of Lewis type basic Mg-O-Mg(Al) bonds, which result from LDH partial dehydroxylation. The DRIFTS νOH region of calcined MgAl-E is shown in Figure 9b and is made up of two distinguishable peaks at 3696 and 3548 cm-1 and a long tail (ca. 3475-2750 cm-1) toward lower wave numbers. This feature is an indication of a myriad of highly disordered OH groups. The peak at 3696 cm-1 has been ascribed to OH groups in Mg(OH)2; while that at 3500 cm-1, as well as the long tail, arise from Mg-(OH)-Al in

different positions or orientations of the crystallite. When CDCl3 is adsorbed on this sample, the peak at 3696 cm-1 mostly disappeared, and a band at 3650 cm-1 appeared instead (see negative peak at 3750 cm-1 in the differential spectrum). The differential spectrum remained flat between 3475-2750 cm-1, indicating that these OH groups did not interact at all with CDCl3. When CDCl3 vapor was withdrawn from the cell, the spectra was reversibly restored to the initial one. In fact, the population of the interacting OH groups (i.e., 3696 cm-1), can be modulated by means of the thermal treatment time and/or the preparation method, as shown in Figure 10: the longer the aging time the lower the number of these OH groups. Then, the band at 3696 cm-1 represents OH groups located at the edges of the crystallites. Also, it must be kept in mind that there is a possibility of the existence of Mg(OH)2 traces, as mentioned previously (see Crystal Structure and Crystal Size).


Valente et al. numerous scientific and technological fields, for instance, in catalysis, for microreactor coatings, for sensor development, in optics, etc. The larger surface area and basic site density in sol-gel LDHs may have a significant effect in many organic reactions. In addition, this particular morphology may be exploited for the entrapment of organic and biologic molecules, to develop drug delivery carriers, biocatalysts, biosensors, etc. Acknowledgment. We thank the Mexican Institute of Petroleum for the financial support and Jaime F. Jaramillo for providing technical support. J.P. gratefully acknowledges Conacyt for a graduate school scholarship. References and Notes

Figure 9. DRIFTS νOH region of sol-gel MgAl-E LDH. (a) After exposing to CDCl3. (b) Calcined MgAl-E, before exposing to CDCl3. (c) Differential spectrum of parts a and b.

Figure 10. DRIFTS νOH region of sol-gel and coprecipitated LDHs.

Then, the band at 3696 cm-1 would be assigned to Mg(OH)2 in excess, which progressively disappears when going from MgAl-E to MgAl-TH1W due to the formation of the LDH phase by incorporation of Al. The presence of Mg(OH)2 in MgAl-E, MgAl-TH3, and MgAl-TH10 could account for the higher basic sites density. Alternatively, these OH groups are fewer when using either coprecipitation and/or hydrothermal methods in comparison with those in sol-gel. Accordingly, in a series of calcined and reconstructed MgAl-LDHs, OH-bands at 3709 and 3684 cm-1 appeared when exposing LDHs to CDCl3, indicating that these OH groups are the only ones interacting with the CDCl3, thus being the only accessible ones.40 Conclusions Solids obtained by a modified sol-gel technique are made up of unique nanocapsular particles. These capsules gradually coalesce as they receive thermal treatments. Aside from treatment time and temperature, the presence of water is crucial in crystal growth. In fact, the use of substoichiometric amounts of water, along with equalization of Mg2+ and Al3+ cations hydrolysis rates, through acetic acid complexation, results in nanometric crystal sizes, suitable for thin films. Furthermore, basic site density and accessibility to edge-located basic OH groups diminishes as crystals grow, as indicated by CO2 TPD and FTIR with CDCl3 as a probe molecule, in agreement with a surface model for the generation of basic sites. The unique and easily controllable properties of these materials enable them for a wide variety of applications in

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Physicochemical Study of Nanocapsular Layered Double Hydroxides

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