Self-Assembled Mesoporous Zirconia and Sulfated Zirconia ...

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Self-Assembled Mesoporous Zirconia and Sulfated Zirconia Nanoparticles Synthesized by Triblock Copolymer as Template Swapan K. Das,† Manas K. Bhunia,† Anil K. Sinha,‡ and Asim Bhaumik*,† Department of Materials Science, Indian Association for the CultiVation of Science, JadaVpur, Kolkata, 700 032, India, and Catalytic ConVersion and Process DeVelopment DiVision, Indian Institute of Petroleum, Dehradun, 248 005, India ReceiVed: February 16, 2009

Self-assembled highly crystalline ZrO2 nanoparticles with mesoscopic ordering have been synthesized by a simple chemical process, evaporation-induced self-assembly (EISA), in the presence of an amphiphilic block copolymer, pluoronic F127, as template. The mesoscopic assembly of the ZrO2 nanoparticles and the crystallinity of the pore walls were studied by using small- and wide-angle X-ray powder diffractions and transmission electron microscopy (TEM) image analysis. N2 sorption studies and high-resolution TEM results further revealed that mesopores are formed by the regular arrangement of the ca. 7.0 nm size nanoparticles and their broad interparticle pore size distribution. This mesoporous ZrO2 nanomaterial has been sulfated by 1 N sulfuric acid, and the resulting sulfated material showed strong acidity in NH3-temperature-programmed desorption (TPD) analysis. Catalytic activity of the mesoporous sulfated zirconia material has been utilized in the Friedel-Crafts alkylation (benzylation) of mesitylene, where it showed excellent catalytic efficiency for the monoalkylated products. Introduction Mesoporous materials have attracted widespread interest in different areas of science because of their unique properties like high surface area, uniform pore size distribution, and large pore volume since the first report of M41S materials.1 Their convenient syntheses through different chemical routes, their large diversity of framework structures, and their novel properties have become a very active area of research for over a decade, and these materials can find important applications in many frontier areas such as catalysis,2 adsorption,3 biomolecular separation,4 drug delivery,5 and so on. The unique feature for the synthesis of mesoporous materials is focused on the use of supramolecular assembly of template molecules as structuredirecting agent.1 The soft templates such as triblock copolymers or surfactants, which form the self-aggregated superstructures in solution phase, play the key role in directing the formation of organized porous structures.6 This approach for the synthesis of mesoporous silica-based materials has been extended to nonsilicious oxides, which contain semicrystalline domains within their thick pore walls. Nonsiliceous mesoporous materials are often more difficult to synthesize by precipitation routes and often exhibit poor crystallinity and low thermal stability. In the search for catalytically interesting new mesoporous materials,2 a large number of synthetic strategies have been reported for the different mesoporous metal oxides. Although a generalized synthesis approach for the semicrystalline mesoporous materials was reported by Yang et al.,7 synthesis of fully crystalline networks is still a major area of interest. On the other hand, the self-organization of nanoparticles is another thrust area of research for the practical application of the nanomaterials in the fabrication of a nanodevice.8 For nanometer-sized particles, the ratio of the number of atoms in * To whom correspondence should be addressed. E-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ Indian Institute of Petroleum.

the surface to the number in the bulk is much larger than for micrometer-sized materials, and this can lead to novel properties. The unique nanostructures of materials make them potentially useful in a wide range of advanced applications, such as sensing,9 catalysis,10 magnetism,11 optoelectronics,12 and so forth. Self-assembly through noncovalent interactions like H-bonding interaction,13 electrostatic interaction,14 charge-transfer interaction,15 acid/base proton transfer,16 van der Waals forces,17 host-guest interaction,18 and so forth is an effective technique that has been proven successful in forming different nanostructured materials. Thus, understanding different self-assembly processes that direct the precise optimization of the functional properties of the nanomaterials having potential applications is very crucial.19 Nanoparticles (NPs) 1-10 nm in size have some unique properties because of their inherent large surface-to-volume ratio and quantum size effects, which differ from those of the corresponding bulk materials.20 Self-assembly of these NPs into a mesoporous material with multiscale structures is of remarkable significance because of their unique properties associated with the nanostructures. There are some reports on self-assembly of different nanoparticles forming the mesoporous structures;21-26 Chen et al.26 reported nanocrystalline zirconia hydrothermally using mixed surfactant route at 403 K. They obtained disordered nanocrystalline zirconia in mixed tetragonal and monoclinic phases exhibiting both macropore and small mesopore. However, designing the self-assembly of zirconium oxide nanoparticles with controllable pore structure using surfactant templating pathway is still challenging. Thus, it is highly desirable to design a mesoporous material with pore walls composed of crystalline nanoparticles and to tune their surface chemical and physical properties. Moreover, most of the previously reported mesoporous zirconia materials synthesized through surfactant templating pathway27 possess amorphous framework, and in many cases, these materials undergo structural collapse of their nanostructure on high-temperature crystallization/calcination,

10.1021/jp9014096 CCC: $40.75  2009 American Chemical Society Published on Web 04/22/2009

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which could greatly limit their practical applications in adsorption and catalysis. Herein, we report the synthesis of ca. 7.0 nm sized zirconia nanocrystals in an evaporation-induced selfassembly (EISA) method, using F127 as template, and their selfassembly toward mesoporous ZrO2 superstructure. Further, we have sulfated the mesoporous zirconia surface and have studied the surface acidity through NH3-temperature-programmed desorption (TPD) and liquid-phase Friedel-Crafts alkylation reactions. To the best of our knowledge, functionalization of these self-assembled mesoporous nanoparticles into a strongly acidic nanostructured material has not been explored. Compared to the sulfated mesoporous zirconia, synthesis of self-assembled zirconia nanoparticles followed by functionalization can provide an alternative way to produce functionalized mesoporous materials active in acid catalytic reactions. Experimental Section Chemicals. Pluronic F127 (Mav ) 12 600, EO106PO70EO106) and zirconium(IV) butoxide [Zr(OC4H9)4] were purchased from Sigma-Aldrich. Hydrochloric acid (HCl) and citric acid (CA) were obtained from E-Merck. All chemicals were used without further purification. Synthesis. The following synthesis procedure was employed for the preparation of self-assembled mesoporous zirconia nanoparticles. In a typical synthesis, 20 mL absolute ethanol was acidified with 1.65 g (35 wt %) hydrochloric acid. Then, 1 g pluronic F127 was added and was allowed to stir until dissolution. After dissolving, 0.630 g (0.003 M) of citric acid was added to it under vigorous stirring. This mixture was stirred at room temperature for 2 h. After that, 3.84 g (0.01 M) zirconium(IV) butoxide taken in absolute ethanol was added slowly to the above solution. The mixture was covered and was stirred at ambient temperature at 343 K for 2 days (for drying and making fine powder) to obtain the solid product by evaporation-induced self-assembly (EISA) technique. Calcination was carried out by slowly increasing the temperature to 773 K (1 K min-1 ramping rate) followed by heating at 773 K for 5 h in the presence of air to obtain template-free selfassembled mesoporous zirconia nanoparticles. Preparation of Mesoporous Sulfated Zirconia Catalyst. Mesoporous sulfated zirconia was synthesized by treating 1 g of the above prepared calcined mesoporous zirconia sample twice with 15 mL of 1 N sulfuric acid followed by calcination in air at 773 K for 3 h. Characterization Techniques. Fourier transform infrared (FT IR) spectra of these samples were recorded using a Nicolet MAGNA-FT IR 750 Spectrometer Series II. Carbon, hydrogen, and nitrogen contents were analyzed using a Perkin-Elmer 2400 Series II CHN analyzer. Powder X-ray diffraction patterns of the samples and the films were obtained with a D8 Advanced Bruker AXS diffractometer using Cu KR (λ ) 0.15406 nm) radiation. Nitrogen adsorption/desorption isotherms were obtained using a BEL Japan Inc. Belsorp-HP surface area analyzer at 77 K. Prior to gas adsorption, all the samples were degassed for 4 h at 423 K. Transmission electron microscopic images were recorded on a JEOL 2010 TEM operated at 200 kV. A Jeol JEM 6700 field emission scanning electron microscope was used for the determination of morphology of the particles. UV-visible diffuse reflectance spectra were recorded on a Shimadzu UV 2401PC with an integrating sphere attachment. BaSO4 was used as background standard. Temperatureprogrammed desorption of ammonia (TPD-NH3) was carried out in a Micromeritics Instrument Corporation ChemiSoft Unit 1. In a typical experiment for the TPD measurement, the sulfated

Figure 1. TEM image of self-assembled mesoporous zirconia seen through the direction perpendicular to the pore axis.

Figure 2. SAED pattern of the TEM image of mesoporous ZrO2.

mesoporous zirconia sample was activated under the flow of He gas at 873 K (3 h) followed by cooling at 298 K and then adsorbing ammonia through injection until complete saturation (observed by Thermal Conductivity Detector (TCD) signal). Physically adsorbed ammonia was then removed by desorbing it in He at 373 K. TPD experiment was then carried out by raising the temperature of the catalyst in programmed manner (10 deg/min). Areas under the peak were converted to equivalent ammonia per gram on the basis of injection of known volumes of ammonia at similar conditions. Results and Discussion High-Resolution Transmission Electron Microscopy (HRTEM) and Field Emission-Scanning Electron Microscope (FE-SEM). A representative TEM image of mesoporous zirconia material is shown in Figure 1. In the image, low electron density spots (pores) are seen throughout the specimen, and the particles of size ca. 7.0 nm are arranged in a regular mesoscopic order. Interparticle pores as seen in this image (low electron density spots) vary from 3.0 to 5.0 nm in the length scale. Selected Area Electron Diffraction (SAED) analysis (Figure 2) of the HRTEM image of mesoporous zirconia confirms that the pore walls of our studied materials are made up of nanocrystalline oxides that show characteristic diffuse electron diffraction rings. The d-spacings corresponding to the diffraction rings of the SAED pattern are in good agreement with the cubic phases of ZrO2 nanocrystal (JCPDS, CAS number 27-0997; cubic ZrO2 with lattice parameter of 0.509 nm). An HRTEM image shown

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Figure 3. HRTEM image of self-assembled mesoporous zirconia nanoparticles and the pore that was created by assembly of zirconia nanoparticles. Seen through the direction perpendicular to the pore axis.

Das et al.

Figure 5. Small-angle powder XRD patterns of calcined self-assembled mesoporous zirconia (a) and sulfated zirconia (b).

Figure 6. Wide-angle powder XRD pattern of self-assembled zirconia nanomaterial. Figure 4. FE-SEM image of mesoporous ZrO2.

in Figure 3 suggests the orientation of the nanoparticles within the pore walls and several nanocrystallites with well-defined lattice planes indicating high crystallinity of the sample. FE SEM image of the mesoporous zirconia sample is shown in Figure 4. As seen from the figure, this mesoporous zirconia material is composed of very tiny spherical nanoparticles. The size of the nanoparticles agrees very well with the TEM image (Figure 1). Since we sonicated the sample before the SEM measurement, these zirconia nanoparticles lost their regular self-assembled arrangement as seen in the TEM image. Powder X-ray Diffraction (XRD). The powder X-ray diffraction patterns for calcined self-assembled zirconia and sulfated self-assembled zirconia materials are shown in Figure 5. Both zirconia and sulfated zirconia samples showed a single broad peak in their respective small-angle powder XRD patterns.26,27 This result suggested that although there is some long-range periodicity among the nanoparticles as shown in the TEM image (Figure 1), in the bulk the material is more or less disordered in nature. Single peaks in their respective small-angle diffraction patterns correspond to the characteristic distribution maximum of the nearest-neighbor particle-center-to-particlecenter distance (t). The small-angle peaks at 2θ ) 0.67° and 0.78°, respectively, correspond to an internanoparticle separation of 13.2 and 11.3 nm for mesoporous zirconia and mesoporous sulfated zirconia materials. These mesoporous materials have retained their respective mesostructure during calcinations and surface functionalization through sulfonation. In Figure 6, we have shown the wide-angle powder diffraction pattern of mesoporous zirconia material. This XRD pattern of the calcined

mesoporous ZrO2 sample shows well-resolved broad peaks characteristic of the cubic phase of individual ZrO2 nanoparticles that correspond to a primary crystal size of ca. 7.1 nm (Supporting Information) as calculated by applying the Scherrer equation on the (111) diffraction peak. This result agrees quite well with the TEM results (Figure 1, average particle size ca. 7.0 nm). These six well-resolved broad peaks indicate that zirconia is the nanocrystalline and can be indexed in turn to the 111, 200, 220, 311, 222, and 400 reflections, and they correspond to the well crystallized with cubic phases (JCPDS, CAS number 27-0997; cubic ZrO2 with lattice parameter of 0.509 nm) without any indication of other crystalline byproducts.26 Thus, our self-assembled mesoporous zirconia nanocrystals have cubic lattice. Sulfated mesoporous zirconia also retained these peaks in its wide-angle XRD (not shown) suggesting the retention of the structure during rigorous surface functionalization. N2 Sorption. Brunauer-Emmett-Teller (BET) surface area, average pore diameter, and pore volume for self-assembled zirconia and sulfated zirconia materials estimated from their respective adsorption isotherms are given in Table 1. The BET surface area of the calcined self-assembled zirconia and sulfated zirconia materials were 124 and 100 m2 g-1, respectively. The N2 adsorption/desorption isotherms of mesoporous zirconia material are shown in Figure 7. This isotherm can be classified as type IV characteristic of the mesoporous materials.1-6,28 Pronounced desorption hysterisis suggested the existence of large mesopores in the sample. This hysterisis is an intermediate between typical H1 and H2-type hysteresis loop in the P/P0 range from 0.50 to 0.73 suggesting large uniform mesopores

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TABLE 1: Physico-Chemical Properties of Mesoporous Zirconia and Sulfated Zirconia sample name mesoporous zirconia mesoporous sulfated zirconia

pore pore particle volume acidity surface area (m2 g-1) width (nm) size (nm) (ccg-1) (µmolg-1) 124

4.8

8.5

0.16

27

100

5.2

7.0

0.11

147

Figure 8. UV-vis diffuse reflectance spectrum of the calcined selfassembled mesoporous zirconia (a) and sulfated self-assembled mesoporous zironia (b) materials.

Figure 7. N2 adsorption/desorption isotherms of mesoporous ZrO2. Adsorption points are marked by filled circles and desorption points are marked by empty circles. NLDFT pore size distribution is shown in the inset.

with a cagelike pore structure connected by windows with a small size.7 The broad increase in N2 uptake for adsorption was observed at P/P0 ) 0.43-0.76 clearly indicating that there is a wide range of mesopores in the sample. Pore size distribution of these samples estimated by employing the Non Local Density Functional Theory (NLDFT) method is shown in the inset of Figure 7. Peak pore width (w) observed for this sample was 4.8 nm, which agrees quite well with the interparticle pore widths obtained from the HRTEM image analysis (Figure 3). Previously reported calcined nanocrystalline zirconia materials do not show such a type of hysteresis loop or uniform pore size distribution but rather show the existence of macropore.26 Further, the average particle-center-to-particle-center distance (t) as obtained from the small-angle powder XRD patterns corresponds closely with the sum of pore width and particle diameter (t ) w + d). Sulfated zirconia sample also showed type IV isotherms suggesting the existence of mesopores. Thus, the high-temperature calcinations procedure is successful to remove the F127 template molecules from the as-synthesized material to generate the interparticle mesoporosity. UV-Vis Absorption. UV-visible spectroscopy is used for characterizing the optical absorbance of the zirconia nanocrystals forming the pore walls of the self-assembled mesoporous zirconia materials. Figure 8 shows the UV-visible diffuse reflectance spectra of calcined and sulfated mesoporous ZrO2 materials. Zirconia nanocrystals showed very strong absorption bands in the wavelength range 200-300 nm.29 The calcined as well as the sulfated samples show similar spectral features. The peak at 228 nm could be attributed to the oxygen-to-metal charge-transfer transition of Zr(IV). The UV absorption edge wavelength is very sensitive to the particle size of semiconductor nanocrystals.30,31 When the crystallite size is below 10 nm, the band gap energy increases with decreasing the crystallite size and the absorption edge of the interband transition is blueshifted. Such blue shifts of the interband transition energy (i.e., band gap) are clearly seen in the UV region of DRS for our very small zirconia particles with the appearance of an additional

Figure 9. TPD-NH3 profile over self-assembled mesoporous sulfated zirconia material.

peak at 298 nm. This spectroscopic result suggested that the ZrO2 nanocrystals compose the pore walls of the mesoporous zirconia material. However, unlike conventional mesoporous materials1-6 with continuous pore wall, the pore wall of these self-assembled mesoporous zirconia material can be considered as composed of many discrete nanodomains separated by void spaces. TPD-NH3. Temperature-programmed desorption of ammonia over mesoporous sulfated zirconia is shown in Figure 9. In this profile, one broad peak centered at 635 K is observed. Compared to the desorption peak observed for sulfated zirconia supported over mesoporous silica at 530 K,32 our self-assembled mesoporous sulfated zirconia sample showed a peak at much higher temperature (635 K). This peak (TCD signal maxima) can be assigned because of the formation of strongly bound (chemisorbed) ammonia on highly acidic sulfated zirconia surface. The total amount of desorbed ammonia calculated from this profile corresponds well with the coverage of acid sites because of the sulfate content in the solid sample. The total acidity of this mesoporous sulfated zirconia material was 147 µmolg-1. A very high temperature desorption characteristic indicates a strong interaction between the acid sites of the mesoporous sulfated zirconia with ammonia.33 Relatively lower surface area of the mesoporous sulfated zirconia vis-a`-vis sulfated zirconia supported over mesoporous silica32 could be responsible for a decrease in total acidity. However, interparticle porosity generated through the self-assembled sulfated zirconia nanoparticles

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Das et al. TABLE 2: Benzylation of Mesitylene Catalyzed by Mesoporous Sulfated Zirconia at 348 K

entry

mole ratio of mesitylene: benzyl chloride

time (h)

conversion (%)

selectivity (%)

1 2 3b

10:1 5:1 10:1

10 10 10

94.6 90.3 1.7

100 100 85.2

TONa 8.4 12.8

a Moles of mesitylene converted per mole of sulfate present in the catalyst. b Reaction was carried out in the absence of any catalyst.

Figure 10. Mechanism for formation of self-assembled ZrO2 nanoparticles with mesoscopic ordering through evaporation-induced selfassembly (EISA).

could hold the trapped gas molecules at its surface resulting in more strong acidity in this material. Mechanism of Self-Assembly Process. Our synthetic method involves a self-assembly process through the slow evaporation of ethanolic dispersion of nanoparticles generated in situ in the presence of a triblock copolymer surfactant. The block copolymer possesses polyethylene and polypropylene groups. Several factors play an important role during the nanoparticle generation and self-assembly process. In Figure 10, we propose a model for the self-assembled mesoporous ZrO2 nanoparticles. Zirconium(IV) butoxide vigorously reacts in aqueous ethanolic medium as it expands its coordination number with simultaneous fast condensation. Exposure under ambient conditions causes rapid particle growth.34 Thus, zirconium(IV) butoxide in acidic ethanolic solution generates zirconia nanoparticles and grows slowly because of the slow introduction of moisture from the ambient environment. Since the pH of the resultant sol (e0.5) is below the point-of-zero charge (pzc)35 of zirconia (pzc ∼ 4-6), the zirconia nanoparticles are positively charged. The presence of citric acid further enhances the affinity to the hydrophilic blocks of the block copolymer.35,36 Thus, citric acid functionalizes the nanoparticle surface through the carboxylic acid group, and other hydroxyl groups involve around the ethylene oxide (EO) moieties through hydrogen bonding, ZrO2-C-O...H+...EO.23a During evaporation of the solvent, the triblock-copolymer micelles (F127) induce cooperative assembly of functionalized zirconia nanoparticles to attain ordered packing with a pore width between 4 and 5 nm. This results in the formation of large mesopores with a crystalline wall consisting of the cubic phase of zirconia nanoparticles as confirmed in wide-angle X-ray powder diffraction (Figure 6). During calcination at 773 K, rearrangement of the individual nanoparticles could occur, which results in the formation of strong covalent bridges between the particles. This covalent bonding between particles could be responsible for the high thermal stability of the mesoporous ZrO2 nanoparticles. Thus, ZrO2 nanocrystallites self-assembled to form thick mesoporous walls, whose crystalline structure effectively sustains the local strain caused during the mesophase formation. Catalysis. The presence of strong acid sites in our mesoporous sulfated zirconia sample has motivated us to utilize this material in Friedel-Crafts alkylation of aromatics under liquidphase conditions. Solid acid catalysts having strong-to-moderate

Lewis acid sites are often used for benzylation reaction using benzyl chloride as alkylating agents.37-40 Results on benzylation of mesitylene over mesoporous sulfated zirconia material are given in Table 2. From these results, it is clear that a very high yield of benzylated mesitylene is formed for two different molar ratios of the substrate to reactant. In this benzylation reaction, the desired product is the monoalkylated compound. For high substrate to benzyl chloride ratio, the product formed in the benzylation is the monobenzylated one (ArCH2C6H5, Ar ) mesityl group), and there is no formation of polybenzylated products. At relatively high concentration of benzyl chloride in the reaction mixture (5:1), conversion for the monobezylated mesitylene marginally decreases. However, in this case, the monobenzylated product is solely formed. When the reaction is carried out in the absence of any catalyst, the reaction does not proceed suggesting that the acidity at the surface of mesoporous sulfated zirconia plays a crucial role in the benzylation reaction. The conversions are based on the decrease in the concentration of benzyl chloride. Strong acid sites present in the surface of mesoporous sulfated zirconia could be responsible for this high catalytic activity in these benzylation reactions. Conclusions In conclusion, we have presented the nonaqueous synthesis route for self-assembled mesoporous ZrO2 nanoparticles with an average diameter of ca. 7.0 nm and highly crystalline pore walls of mesoscopic order through evaporation-induced selfassembly (EISA) method using pluoronic F127 as the template. HRTEM image analysis suggested the formation of mesopores through the mesoscopic ordering of the ZrO2 nanoparticles. N2 sorption studies revealed large mesopores of dimension ca. 4.8 nm formed by arrangement of the nanoparticles. This mesoporous ZrO2 nanomaterial has been sulfated by dilute sulfuric acid, and the resulting sulfated material showed strong acidity in the temperature-programmed desorption of ammonia. Strong acidity at the surface of mesoporous self-assembled sulfated zirconia nanoparticles is responsible for the very high catalytic activity of this material in Friedel-Crafts benzylation of aromatics, and thus this method for the synthesis of sulfated zirconia nanomaterials has good potential to find utility in other acid-catalyzed reactions. Acknowledgment. A. B. wishes to thank DST, New Delhi, for a Ramanna Fellowship grant. This work was partly funded by the NanoScience and Technology Initiative of DST. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature (London) 1992, 359, 710–712.

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