Controlled Synthesis of Different Morphologies of MgO and Their Use

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Controlled Synthesis of Different Morphologies of MgO and Their Use as Solid Base Catalysts Narottom Sutradhar, Apurba Sinhamahapatra, Sandip Kumar Pahari, Provas Pal, Hari C. Bajaj, Indrajit Mukhopadhyay,* and Asit Baran Panda* Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSIR), G. B. Marg, Bhavnagar
364002, Gujarat, India

bS Supporting Information ABSTRACT: MgO is one of the most promising solid base catalysts and has attracted much attention because of its superior performance. The extent of catalytic properties of MgO is highly controlled by its morphology, particle size, crystalinity, and surface area. Here, the synthesis of MgO with different morphologies, such as random nanoflakes, arranged nanoflakes toward flower and house of card structure spheres, cubes, and hexagonal plates, through the calcination of magnesium carbonate hydrates (MCH) intermediate is presented. The intermediate MCH has been synthesized under hydrothermal or supercritical hydrothermal as well as solvothermal treatment of clear solution of Mg(NO)3, (NH4)2CO3, or nesquehonite rods in different pH's and amounts of free carbonate ions. A probable reduction mechanism is proposed to explain the formation of the MCH morphologies. The amount of carbonate ion has crucial role in the formation of different morphologies in hydrothermal condition. On calcinations of the synthesized MCH morphologies resulted in MgO with almost identical morphologies as parental MCH. The microstructures of calcined MgO are porous and made of MgO nanoparticle building blocks of the size 4
6 nm. The formed MgO nanoparticles consists of large number of edges and corners, step edges and step corners and numerous base sites of various strength (surface hydroxyl groups, low coordinate O2- sites) which are recognized as active basic sites in heterogeneous catalysis. The calcined MgO microstructures function as a strong solid base catalyst for the solvent-free Claisen-Schmidt condensation of benzaldehyde with acetophenone giving 99% conversion in 4 h. The MgO catalysts are easily recyclable with no significant loss in catalytic activity in the subsequent cycles.

’ INTRODUCTION In recent years, self-assembled inorganic nanostructured materials with well-controlled morphologies have attracted a lot of interest, as the physicochemical properties of most of these materials are controlled by their architecture and morphologies.1
4 Specifically, three-dimensionally (3D) assembled nanoarchitectures have gained much attention because of their vast application in catalysis, water treatment, sensor, energy, cosmetics, pigments, and so forth.5
11 Various methods have been developed for the synthesis of 3D structures. Templateassisted growth, solvothermal and hydrothermal, thermal oxidation and reduction, and orientation attachments are a few of them.5
20 However, it is a challenge to synthesize new architectures or develop suitable simple protocol for the existing architecture with improved properties. Magnesium oxides (MgO's), a typical wide band gap (7.2 eV) semiconductor, represent an important class of functional metal oxides with a broad range of properties. They also find tremendous application in catalysis, refractory industries, electronics, cosmetics, and toxic wastewater treatment remediation.21
34 To extend the application of MgO's as catalysts, it is essential to tailor their r 2011 American Chemical Society

surface chemistry as well as morphologies, as the catalytic process occurs only over the surface. MgO being a weak base, its pH in aqueous solution is highly dependent on the morphology and surface area,34 the sludge formed during the water treatment process is easier to precipitate and filter than that formed by other alkalis.21 For the water treatment process, surface architecture and morphologies are also crucial factors. So, in recent years enormous efforts have been made to synthesize MgO with enhanced surface area in varying morphologies such as rods, wires, belts, tubes, and so forth.35
39 There have also been few reports on 3D assembled nanoarchitectures.25,40
42 Generally, MgO's are obtained via calcining their various precursors.21,40
48 Magnesium carbonate hydrates (MCHs) are good precursors for the synthesis of 3D assembled nanoarchitectures, as they have diverse morphologies that vary with composition and phase structures. It is easy to obtain MgO from MCH on calcination. MCHs themselves are also an industrially important class of materials Received: March 9, 2011 Revised: April 24, 2011 Published: June 02, 2011 12308

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The Journal of Physical Chemistry C for their vast application in pharmaceuticals, cosmetic manufacturing, rubber industries, and lithographing ink. Up to now, a large number of chemical methods have been developed for the synthesis of MCH with varying morphologies.21,40
48 In our recent study,48 we have found that the addition of aqueous (NH4)2CO3 solution to an aqueous solution of Mg(NO3)2 gave a clear solution and, after continuous stirring for 2
12 h, resulted in uniform rods of nesquehonite {Mg(HCO3)OH.2H2O}. The synthesized nesquehonite rods have resulted from MgO rods in subsequent calcinations, and the MgO rods have been found to be highly active catalytically. However, there are very few reports for the controlled synthesis of a wide range of MCHs with varying morphologies and compositions, as well as of MgO's with morphologies identical to those of MCH and the effect of the architectures of MgO's on their properties. Herein, we report the synthesis of MCH with different morphologies from the clear solution of Mg(NO3)2, (NH4)2CO3, as well as from nesquehonite rods48 employing different synthetic (hydrothermal, supercritical-hydrothermal, and solvothermal) approaches. A tentative mechanism for the formation of such MCH morphologies is depicted. Calcination of the MCH resulted in the formation of MgO with shapes similar to those of their precursors. We further report the catalytic activity of the calcined MgO toward Claisen
Schmidt condensation of benzaldehyde with acetophenone.

’ EXPERIMENTAL SECTION Preparation of MCH in Hydrothermal Conditions. In a typical synthesis, aqueous (NH4)2CO3 solution (250 mL 0.65 M) was added to an aqueous Mg(NO3)2 (250 mL 0.2 M) solution, in less than 10 s with vigorous stirring. After 5 min of stirring, 33 mL of clear reaction mixture was placed in a 50 mL Teflon-lined stainless steel autoclave, sealed properly, placed in a preheated oven, and heated at 130 °C for 6 h. The remaining reaction mixture was stirred continuously. After 15
20 min of stirring, it became turbid, and stirring was continuied for another 12 h. Then 33 mL of the turbid solution was placed to a 50 mL Teflon-lined stainless steel autoclave, sealed properly, and heated at 130 °C for 6 h. The precipitates from the remaining bulk solution were collected by centrifuge. In the manuscript, the clear solution obtained from the centrifuge will be termed the “mother-liquid”. The centrifuged precipitate was washed several times with deionized water followed by final washing with acetone, and was dried in room temperature overnight. The resultant product is the nesquehonite rods.48 Then, 1.5 g of dried as-synthesized powder was taken in three separate beakers. Forty milliliters of deionized water was added into the first beaker; 40 mL 0.65 M ammonium carbonate and 40 mL saturated ammonium carbonate solution were added to the second and third beaker, respectively, and stirred for 2
3 min. Thirty-three milliliters of the resultant turbid solutions from each mixture was placed in three separate Teflon-lined stainless steel autoclaves, sealed properly, placed in a preheated oven, and heated at 130 °C for 6 h for hydrothermal treatment. After 6 h of heating, the autoclave was cooled at room temperature naturally. The resultant precipitate was recovered by centrifuge followed by throrough washing by water and acetone and drying at 60 °C for overnight. Preparation of MCH in Supercritical Hydrothermal or Solvothermal Condition. All the syntheses in supercritical conditions were performed in a pressure-resistant SUS316 vessel (inner volume 5 mL). In a typical synthesis, 4 mL of water or propanol was added to 100 mg of as-synthesized nesquehonite powder and stirred for 5 min. Then 3.5 mL of the milky

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suspension was placed into the reaction vessel, sealed properly, and heated at 400 °C for 20 min. Supercritical hydrothermal synthesis was also performed using the clear mixture of aqueous Mg(NO3)2 and (NH4)2CO3 solution. For this, 3.5 mL of the clear solution was placed in the reaction vessel, sealed properly, and heated at 400 °C for 20 min. Finally, the resultant precipitate was recovered from the cool reactor, after proper washing by water and acetone, and drying at 60 °C for overnight. Preparation of MgO from the Synthesized MCH. MgO from all the MCH morphologies was obtained by calcining at 500 °C for 6 h. Characterizations. The prepared MCH and MgO samples were characterized by scanning electron microscope (SEM), transmission electronic microscope (TEM), thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), Brunauer
Emmett
Teller (BET) surface area, Fourier transform infrared (FTIR) and CO2 temperature-programmed desorption (TPD) analysis.The SEM (Leo series 1430 VP) equipped with INCA was used to determine the morphology of samples. For SEM, powder samples were supported on aluminum stubs and then coated with gold by plasma prior to measurement. TEM images were collected using a JEOL JEM 2100 microscope, and samples were prepared by mounting an ethanol-dispersed sample on a lacey carbon Formvar coated Cu grid. TGA was performed on a Mettler-Toledo (TGA/SDTA 851e) in air at a heating rate of 10 °C/min. XRD patterns were recorded in the 2θ range of 10
70 on a Philips X’pert X-ray powder diffractometer using Cu KR (λ = 1.54178 Å
) radiation. For BET surface area measurements, the nitrogen sorption measurements were performed at 77 K by using a ASAP 2010 Micromeritics, USA, after degassing samples under vacuum (10
2 Torr) at 250 °C for 4 h. The surface area was determined by the BET equation. Pore size distributions were determined using the Barrett
Joyner
Halenda (BJH) model of cylindrical pore approximation. The FTIR measurements were carried out using a Perkin-Elmer GX spectrophotometer. The spectra were recorded in the ranges 400
4000 cm
1 and 1200
1800 cm
1, respectively, using a KBr pellet. Reaction Procedure of Claisen
Schmidt Condensation of Benzaldehyde with Acetophenone. All reactions were carried out in a 50 mL glass reactor equipped with a magnetic stirrer, water condenser, and temperature controller in solvent-free conditions under nitrogen atmosphere. For catalytic applications, calcined (500 °C) MgO morphologies were used, and prior to catalytic reaction the MgO samples were activated at 150 °C for 2 h. In a typical Claisen
Schmidt condensation reaction, mixture of acetophenone (5 mmol), benzaldehyde (5.2 mmol), and 10 wt % of catalyst was heated in an oil bath at 140 °C under constant stirring for 5 h. After the end of the reaction, the reaction mixture was diluted with dichloromethane followed by centrifugation, and reaction products were analyzed by gas chromatography
mass spectrometry (GC-MS). To monitor the progress of the reaction, a small amount of reaction mixture was withdrawn intermittently and analyzed by GC-MS. For comparison, the reaction was also carried out using commercially available MgO (C-MgO, Wako chemicals, irregular morphology, BET surface area 28 m2/g). The used catalysts were separated by simple centrifugation and washed several times with acetone. The washed catalyst was dried at 200 °C for 6 h and reused. 12309

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Figure 1. SEM images of nesquehonite rod synthesized in ambient conditions (a), different morphologies of hydromagnesite (b
f) and magnesite (g) synthesized in hydrothermal conditions at 130 °C, and nesquehonite rods in island form in solvothermal (propanol) treatment at 130 °C (h).

’ RESULTS AND DISCUSSION Effect of Hydrothermal Conditions and Protocol on Variation of Morphology of MCH. Figure 1 represents a set of

typical SEM images of the synthesized MCH in ambient and hydrothermal conditions. Simple mixing of ammonium carbonate and magnesium nitrate resulted in uniform rods in ambient conditions (Figure 1a), instead of flower-like morphology as reported by Janet et al.44 The obtained microstructure is identical to our previous study.48 Similar rod-like morphology has also been reported by Zhang et al.43 and Mitsuhashi.47 The morphology of the MCH changed drastically under hydrothermal conditions with the variation of precursor and environment of the system (Figure 1b
h). The clear precursor solution of Mg(NO3)2 and (NH4)2CO3 upon hydrothermal treatment resulted in a “house of cards” structure (assembly of nanoflakes), whereas random nanoflakes and arranged nanoflakes toward flowers and

spheres resulted in different reaction environments using MCH (nesquehonite) rods synthesized in ambient conditions as the precursor. The hydrothermal treatment of MCH rods with the mother-liquid resulted in random nanoflakes (Figure 1e). Hydrothermal treatment of dried MCH rods in deionized water produced uniform and monodispersed flower-like morphology with 8
10 μm diameters (Figure 1c,d), hereafter termed as big flowers in the manuscript. However, hydrothermal treatment for short time yielded incomplete flowers (Supporting Information), while with increased duration of hydrothermal treatment the number of incomplete flowers decreased. Hydrothermal treatment of the dried MCH rods in 0.65 M ammonium carbonate solution resulted in predominantly small flower-like morphology, with some random nanoflakes (Figure 1f). However, hydrothermal treatment of the dried MCH rods in saturated ammonium carbonate solution produced monodispersed spherical particles of 5
6 μm diameter 12310

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Figure 2. SEM images of the morphologies synthesized in supercritical hydrothermal conditions with mixed (MgCO3) and Mg(OH)2 phase (a
d) and in supercritical solvothermal conditions with mixed Mg3O(CO3)2 and Mg(OH)2 (e,f).

with smooth surface (Figure 1g). Interestingly, no such morphological change was identified upon solvothermal treatment of the rods in propanol, except the agglomeration of the rods in island morphology (Figure 1h). Effect of Supercritical Condition on Morphology of MCH. Typical SEM images of the products synthesized under different supercritical hydrothermal or solvothermal conditions are shown in Figure 2. Supercritical hydrothermal treatment of the clear precursor solution of Mg(NO3)2 and (NH4)2CO3 mainly favors the formation of rombohedra or cubical morphology (Figure 2a). Similar treatment of MCH rods with the mother-liquid produced assembled step-edged rombohedra or cubical morphology (Figure 2b). From the inset of Figure 2b, it is evident that the step edges are very prominent, and it looks like supra-molecular assembly of nearly square MCH sheets. Supercritical treatment of dried rods in deionized water resulted in hexagonal sheet like morphology (Figure 2c,d). The synthesized sheets are mainly random (Figure 2b), but sometimes stacked (inset Figure 2b). However, the reaction in supercritical propanol of dried MCH rods produced a house-of-cards-like architecture over one-dimensional rods of larger diameter in a typical reaction time of 20 min, while longer duration of solvothermal treatment to 1 h yielded random plates. The phase purity of the produced MCH was ascertained by powder XRD. Figure 3a
e represents the XRD patterns of the synthesized MCH with different morphologies. All the diffraction lines of the MCH rod (Figure 3a) can be indexed to the monoclinic nesquehonite phase {Mg(HCO3)OH 3 2H2O}. The XRD pattern

showed that the compounds obtained under hydrothermal conditions with nanoflake-type morphologies, including random nanoflakes and arranged nanoflakes toward house-of-cards and small and large flower structures, can be indexed to the monoclinic hydromagnesite phase{4MgCO3 3 Mg(OH)2 3 4H2O}(Figure 3b). XRD pattern of spherical morphology synthesized in hydrothermal conditions in saturated ammonium carbonate confirmed the trigonal magnesite phase (MgCO3) (Figure 3c). However, different MCH obtained under supercritical conditions with various morphologies showed a mixed phase of MCH and magnesium hydroxide {Mg(OH)2}. Mixed (MgCO3) and Mg(OH)2 phases are produced when the synthesis is carried out in supercritical water (Figure 3d). It may be noted that in supercritical propanol, mixed phases of magnesium oxy-carbonate (Mg3O(CO3)2 and Mg(OH)2 (Figure 3e) are obtained. The successive calcination at 500 °C of the as-synthesized MCH with different morphologies had no noticeable effect on morphologies and resulted in the formation of MgO phases with almost the same morphologies as the precursor MCH (Figure 4a
f). XRD patterns of the calcined samples can be assigned as cubic MgO phases (Figure 3f
i). The average crystallite sizes of the MgO particles calcined at 500 °C calculated from X-ray line broadening of the (200) diffractions using Scherrer’s equation were in the range of 6
9 nm depending on the morphology (Table 1). The crystallite size of the same morphology increased gradually with increased calcination temperature due to enhancement in crystallinity. The TEM image of the calcined big flower indicates that the flowers are made of nanoflakes, and the nanoflake building blocks 12311

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Figure 3. XRD patterns of synthesized MCH morphologies (a
e): nesquehonite rods (a), hydromagnesite flowers (b), magnesite spheres (c), mixed magnesite and magnesium hydroxide hexagonal sheet (d), and mixed magnesium oxy-carbonate and magnesium hydroxide houseof-cards structure (e). XRD patterns of MgO flowers (f) and spheres (g) obtained after calcination of the corresponding MCH at 500 °C, and MgO flowers calcined at 600 (h) and 700 °C (i).

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are observed to be connected with each other in the center core (Figure 4d). TEM images of the calcined samples indicate that the morphologies composed of nanoflakes and plates are porous (inset Figure 4d,f), but the spherical and cubic microstructures are too dense to reveal the porosity. TEM images at larger magnification confirm that the microstructures are made of very small MgO nanoparticle building blocks (Figure 4d) with particle sizes in the range of 4
6 nm It is worth mentioning that calcination of the synthesized microstructures generates a lot of edges, corners, step edges, and step corners on the surfaces of synthesized MgO morphologies, which are recognized as active basic sites in heterogeneous catalysis (Figure 4h,i). The distinct lattice fringes in the high-resolution TEM (HR-TEM) of the particles again supports the idea that the calcined materials are highly crystalline, and lattice fringe spacing of 0.214 nm is in good agreement with the d-spacing between the (200) plane of cubic MgO. The electron diffraction ring pattern indicates the polycrystalline nature of the MgO nanoflower due to rotational orientation of the nano-single-crystalline domains with respect to each other (Figure 4J). Figure 5 represents N2 sorption isotherms of some selected MgO morphologies obtained by calcinations at 500 °C. All the isotherms are similar to type IV, indicating that the synthesized MgO morphologies are mesoporous in nature.49 The hysteresis loops are almost H3 type (according to the IUPAC classification), generally observed with the aggregated powders.49 Table 1 represents the specific surface area calculated using the BET equation of the synthesized MgO morphologies. The specific surface area of the MgO morphologies varied in the range of 62
115 m2g
1 except for MgO cubes (33 m2g
1). Moreover, the BJH pore size distribution, calculated from the desorption wing of the isotherm (inset of Figure 5) shows that the pore size distributions are very narrow (except spheres) and diameters vary in the range of 4
8 nm, matching well with the interparticle pores that is observed in the TEM studies. In the spherical particles, a

Figure 4. SEM images (a
c) of calcined (at 500 °C) random flakes (a), house-of-cards (b), and spherical (c) structures. TEM images of calcined flowers (d,g), cubes (e), and hexagonal plates (f). HR-TEM images and electron diffraction patterns of calcined (500 °C) flowers (h,i), and the electron diffraction pattern of a nanoflower (j). 12312

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Table 1. Textural and Catalytic Properties of the Synthesized MgO with Various Morphologies sample rod

surface area (m2/g)

crystallite size (nm)

yield (%)a

115

6

99

big flowers house of cards

82 91

7 6

99 97

small flowers

87

6

94

random flakes

97

6

96

spheres

62

8

83

cubes

33

9

57

plates

75

7

89

bulk

28

27

23

a

Yield of 1,3-diphenyl propenone in Claisen
Schmidt condensation. Reaction conditions: acetophenone, 5 mmol; benzaldehyde, 5.2 mmol; catalyst, 34 mg (150 °C for 2 h); reaction temperature, 140 °C; reaction time, 4 h. The reaction was carried out under N2 atmosphere.

Figure 5. N2 sorption isotherm of MgO rods (a), big flowers (b), random flowers (c), and spheres (d) calcined at 500 °C. Inset: the corresponding pore size distributions.

bimodel pore size distribution was observed. The narrow size distribution in the range of 4 nm is most probably from the small particle assembly of inside spheres, and the broad distribution can be assigned from the interparticle pores in between big spheres. The FTIR spectrum of MCH nanorod (Figure 6a) is very similar to that of the nesquehonite phase.43,50 The characteristic bands at 852 cm
1 (ν2 mode), 1100 cm
1 (ν1 mode), and 1425, 1510 cm
1 (ν3 mode) can be attributed to the CO32
absorption bands and 1645 cm
1 and 3600
3000 cm
1 can be attributed to the O
H bending mode of water molecule of highly crystalline nesquehonite.43 The FTIR spectra of nanoflakes (Figure 6b), obtained by hydrothermal treatment of the nesquehonite rods under varied conditions, are quite different from nesquehonite rods. The bands between 3600 and 3400 cm
1 become narrower, and a new sharp band appears at 3453 cm
1 corresponding to the O
H vibration.43,51 Again the bending vibration band of the carbonate ion split into three absorption bands at 798, 853, and 882 cm
1. The observed characteristic bands of nanoflakes confirms hydromagnesite phase.43 In the

Figure 6. FTIR spectra of nesquehonite rods (a), hydromagnesite flowers (b), magnesite spheres (c), and calcined MgO (d).

spectrum of MCH nanospheres (Figure 6c), the sharp band between 3600 and 3400 cm
1 disappeared, and characteristic bands are almost identical to that of magnesite. The FTIR spectra further confirm different phases as MCH, supporting the XRD results. Upon calcination of the MCH at 500 °C, CO2 and H2O molecules evolve and thus generate numerous Lewis and Br€onsted acid and base sites on the MgO surface. The three distinct IR bands in the calcined MgO at 1640, 1447, and 1010 cm
1 (Figure 6d) are assigned to the carbonate species. However, these carbonates are chemisorbed on the surface of the MgO. It has been reported that carbonate ions in the hydromagnesite phase are strongly bridged between magnesium ions whereas on calcination, carbonate ions are chemisorbed in a monoand bidentate manner on the surface of MgO.52 The broad band at 3421 cm
1 indicates the presence of four coordinated hydroxyl groups interacting with surface O2-LC ions and the sharp band at 3707 cm
1 attributed to the multicoordinated hydroxyl groups.53 Again a shoulder band at 3728 cm
1 can be attributed to the presence of three coordinated O2- ions.53 Thus the FTIR studies confirm the presence of surface chemisorbed carbonate species, bound and free hydroxyl groups, and various O2-LC ions. The results in TGA plots of individual MCH phases (Supporting Information) also support the XRD and FTIR findings. Evaluation of Growth Mechanism. The clear precursor solution of magnesium ammonium carbonate complex was prepared by the addition of aqueous Mg(NO3)2 solution to aqueous ammonium carbonate solution. The formed soluble magnesium ammonium carbonate is metastable and produced thermodynamically stable, one-dimensional nesquehonite rods on stirring under ambient conditions. Formation of such complex was confirmed by FTIR spectra of obtained clear solutions of ammonium carbonate, magnesium ammonium carbonate complex, and the precipitates. A significant shifting of carbonate peak toward higher wavenumber and N
H peak toward lower wavenumber in magnesium ammonium carbonate solutions compared to pure ammonium carbonate solutions clearly indicated the complex formation. After stirring the clear precursor solution for 10
15 min, it became turbid, and detailed identification of the precipitate indicated the presence of small amorphous seeds, probably due to xMgCO3 3 Mg(OH)2, as reported by Mitsuhashi et al.,47 as an intermediate from the soluble magnesium ammonium carbonate complex solution in basic medium. These intermediate particles tend to assemble and crystallize as rods on prolonged stirring. On the basis of chemical bonding theory for the growth of single crystal, the intrinsic 12313

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The Journal of Physical Chemistry C crystallographic structure can predict the thermodynamically stable morphology.54,55 Therefore, the MCH phase on prolonged stirring resulted in nesquehonite, which is grown in a typical rod-type morphology, a thermodynamically stable morphology at room temperature. The formed nesquehonite rods were used as a seed for the other morphologies. From the thermodynamic point of view, these rods are not stable at higher temperature.47,56,57 However, under hydrothermal conditions, these rods can be rearranged into a more stable nanoflake-like structure by a dissolution-formation process; and depending on other environmental parameters, the nanoflakes are self-assembled to different morphologies. The crystal phase also changed from nesquehonite to hydromagnesite or magnesite phase. We presume that at first the rods are mainly converted to nanoflake like structure in hydrothermal conditions, and then the nanoflakes are rearranged to different morphologies. In order to confirm our assumption, the samples were collected in the intermediate state of hydrothermal treatment for characterization. The corresponding SEM image is shown in Figures 7a,b and S1 (Supporting Information). The SEM image showed the growth of leaf-like nanoflakes from the surface of the rods at the initial stages (Figure 7a). After the formation of nanoflakes, they are arranged in different morphologies depending on the pH and concentration of the free carbonate ion. In the neutral pH region and in the absence of additional carbonate (i.e., in deionized water), nanoflakes are arranged in a flower structure with the hydromagnesite phase (Figure 1c,d). The SEM of the intermediate state depicted the presence of some arranged flakes with random flakes (Figure 7b), confirming that the rods are transformed to flakes and then the flakes are arranged to flowers. However at pH ∼9, with varying amount of carbonate ion, the nanoflakes are arranged in a different fashion. Although the pH's of the mother-liquid, 0.65 M ammonium carbonate solution, and saturated ammonium carbonate solution are same (pH ∼ 9), the concentration of free carbonate ions increases in ascending order due to different ammonium carbonate content. With the increase in the carbonate ion population in the reaction mixture, the tendency of assembly formation of nanoflakes increased. After hydrothermal treatment of rods in the mother-liquid, no arranged structure was observed (Figure 1e), whereas in the 0.65 M ammonium carbonate solution, a short-range ordered structure was observed (Figure 1f). The composition of both of the morphologies was hydromagnesite. However, in saturated ammonium carbonate solution, probably the nanoflakes are not formed; in the presence of large amount of carbonate its composition changed to magnesite, and the morphology changed to a spherical morphology (Figure 1g).58 To confirm our assumption that the concentration of carbonate ions is the controlling factor for the assembly formation in the same pH, one separate experiment was performed at pH 9 using sodium hydroxide. The corresponding SEM image (Supporting Information) showed the flower-like morphology with the hydromagnesite phase, identical to that synthesized in neutral pH, and endorses our assumption. However, at higher pH (adjusted by NaOH), magnesium hydroxide with random morphology was formed (Supporting Information). Moreover, on hydrothermal treatment of clear precursor solution of magnesium ammonium carbonate, probably the nesquehonite rod-like particles become seeds of hydromagnesite nanoflakes and the formed nanoflakes are arranged in a house-ofcards structure. Due to the instability of nesquehonite phase in hydrothermal condition, it was difficult to capture the nesquehonite rods to confirm the predicted path of formation of the nanoflakes. The formed nanoflakes were arranged in a house-of-card structure

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Figure 7. SEM image of intermediate stages of rods during hydrothermal treatment in deionized water after 1 h (a) and 2.5 h (b). SEM image of intermediate stage during supercritical hydrothermal treatment of a clear precursor solution of Mg(NO3)2 and (NH4)2CO3 (c) and a rod (d).

at pH ∼ 9 and in the presence of a moderate amount of free carbonate ion. Therefore, on hydrothermal treatment of a clear precursor solution or a nesquehonite rod, thermodynamically stable hydromagnesite flakes were formed,47,56,57 and then a pressureinduced arrangement of flakes took place with varying amount of carbonate. However, in the presence of an excess amount of carbonate (saturated ammonium carbonate solution), a trigonal magnesite phase with spherical morphology particle was formed, most probably due to the phase-induced morphology change. In supercritical hydrothermal conditions, the cubes of mixed (MgCO3) and Mg(OH)2 phase formed from the clear precursor solution instead of the nanoflake (Figure 2a), wherein the rods are most probably the transient species. The SEM of the sample of intermediate stage (Figure 7c) depicts the assembled cubes in rodlike morphology, which partially confirms the formation of rods in the intermediate stage. However, in the supercritical hydrothermal treatment of nesquehonite rods, probably, at first, the nesquehonite rods were converted to nanoflakes, and then, the nanoflakes were changed to square or hexagonal sheets. In the intermediate state, the presence of both flake and plate supports this assumption (Figure 7d). Under basic conditions and in the presence of carbonate ion (in the mother-liquid), the step-edged cubic structure was observed (Figure 2b). Here the formation mechanism is complicated to predict, and the morphological changes are too fast to capture the intermediate state. In neutral pH and in the absence of free carbonate (in deionized water), the resultant hexagonal sheets were mainly random (Figure 2c,d); however, few stacked morphology was also observed (inset Figure 2d). In supercritical propanol, the nanoflakes formed from rods were arranged in the house-of-cards structure with mixed phase of magnesium oxycarbonate (Mg3O(CO3)2 and Mg(OH)2. Therefore, in supercritical conditions, initially temperature-induced thermodynamically stable hydromagnesite flakes were formed47,56,57 and then 12314

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Scheme 1. Schematic Representation of Stepwise Formation of MCH in Hydrothermal Conditions

Scheme 2. Schematic Representation of the Performed Claisen Condensation Reaction Figure 8. Reaction profile (conversion) obtained for the Claisen condensation of benzaldehyde and acetophenone over MgO calcined at 500 °C.

the more stable mixed phase of magnesium hydroxide, magnesium carbonate (magnesite) and magnesium oxy-carbonate was formed in the high temperature and pressure depending on the amount of carbonates and reaction medium, which forced crystal-phased induced morphology change to cubes, hexagonal sheets. From the above observation and discussions, the major chemical reactions for the formation of different morphologies and compositions in hydrothermal condition can be formulated as

Scheme 1 depicts the schematic representation of the formation steps of different morphologies in hydrothermal conditions. Catalytic Activity Study for Claisen
Schmidt Conditions Reaction. To evaluate the catalytic activity of the calcined (500 °C) MgO morphologies and to understand the relationship between the morphology and reactivity, Claisen condensation of benzaldehyde and acetophenone was performed in solvent-free conditions (Scheme 2). All the morphologies of calcined MgO provided 100% selectivity for 1,3-diphenyl propenone with varied conversion rate of acetophenone (Table 1). All the calcined (500 °C) MgO samples with different morphology showed substantially higher activity for the titled reaction than that of bulk MgO under identical reaction conditions. The conversion of acetophenone using MgO with morphologies consisting of flakes was more than 90% after 4 h; more specifically, the big flower and house-of-cards structure gave 99% and 97% conversion, respectively, whereas under identical conditions the bulk MgO gave only 22% conversion of acetophenone. The high catalytic activity can be attributed to the presence of numerous amounts of step edges, step corners, low coordinate oxide sites,59 and lattice defects60 (cation and anion vacancies). Again the morphologies constituted of nanoflakes resulted in higher activity most probably due to the high surface area, as the amount of adsorption of the reactant on the active sites of the surface of catalyst

is crucial, which is controlled by the surface area of catalyst. Moreover, the efficiency of diffusion of organic species through the thin flakes facilitates the reaction, as the flake-like building blocks are fully accessible by reactants. Figure 8 represents the respective reaction profiles of Claisen condensation of benzaldehyde and acetophenone big flowers, house-of-cards structure constructed of nanoflakes, and random flakes. As can be seen, all the calcined (500 °C) MgO catalysts with different morphologies showed a similar profile and conversion of acetophenone ranging from 85% to 95% with 100% selectivity for 1,3-diphenyl propenone in only 2 h. The bulk MgO showed a gradual increase of conversion with time: only 13% conversion was observed after 2 h. We also investigated the reusability of the MgO catalysts. After reaction, the catalyst was separated, washed with acetone several times, dried at 200 °C for 6 h, and reused. No significant change in catalytic activity was observed with respect to fresh catalyst. After the fourth cycle, the big flower MgO catalyst gave 97% conversion of acetophenone with 100% selectivity.

’ CONCLUSION We successfully synthesized 3D MgO architectures with cube and hexagonal plate, spherical, random nanoflake, and arranged nanoflakes toward flower and house-of-cards morphologies by the calcination of MCH intermediates having identical morphologies. The MCH morphologies were formed by hydrothermal or supercritical hydrothermal as well as solvothermal treatment of a clear solution mixture consisting of Mg(NO3)2, (NH4)2CO3, or MCH (nesquehonite) rods. A possible dissolution
formation growth mechanism in hydrothermal process is proposed, where the concentration of free carbonate ions is shown as one of the prime controlling factors to define various morphologies. The MgO morphologies obtained by calcining at 500 °C are made of 4
6 nm nanoparticle building blocks, and the particle size is found to increase with calcination temperature. The formed MgO nanoparticles contain a large number of edges and corners, step edges and step corners, and numerous base sites of various strength (O2-LC, surface hydroxyl groups). The synthesized morphologies exhibit good catalytic performance toward solvent-free Claisen
Schmidt condensation of benzaldehyde with acetophenone giving a maximum conversion of 99% with 100% selectivity for a definite morphology and is expected to be useful for other base-catalyzed reactions. 12315

dx.doi.org/10.1021/jp2022314 |J. Phys. Chem. C 2011, 115, 12308–12316

The Journal of Physical Chemistry C

’ ASSOCIATED CONTENT

bS

Supporting Information. The SEM images and XRD patterns of intermediate stages and DTA plots of the MCH phases. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: þ91-278-2567562. Tel: þ92-278-2567760, ext. 704. E-mail: [email protected] (A.B.P.); [email protected] (I.M.).

’ ACKNOWLEDGMENT This work was supported by Department of Science and Technology (DST), India, (SR/S1/IC-11/2008) and (SR/S1/PC-1/2010). Authors also acknowledge analytical discipline of CSMCRI for materials characterization. ’ REFERENCES (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (2) Roduner, E. Nanoscopic Materials: Size-Dependent Phenomena; RSC Publications: Cambridge, U.K., 2006. (3) Moon, H. R.; Urban, J. J.; Milliron, D. J. Angew. Chem., Int. Ed. 2009, 48, 6278. (4) Yan, C.; Nikolova, L.; Dadvand, A.; Harnagea, C.; Sarkissian, A.; Perepichka, D. F.; Xue, D.; Rosei, F. Adv. Mater. 2010, 22, 1741. (5) Cao, Y.-B.; Zhang, X.; Fan, J.-M.; Hu, P.; Bai, L. Y.; Zhang, H. B.; Yuan, F.-L.; Chen, Y. F. Crystal Growth & Design 2011, 11, 472. (6) Li, W. N.; Yuan, J. K.; Shen, X. F.; Gomez-Mower, S.; Xu, L. P.; Sithambaram, S.; Aindow, M.; Suib, S. L. Adv. Funct. Mater. 2006, 16, 1247. (7) Cao, M.; He, X.; Chen, J.; Hu, C. Cryst. Growth Des. 2007, 7, 170. (8) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 2287. (9) Yu, X.; Yu, J.; Cheng, B.; Jaroniec, M. J. Phys. Chem. C 2009, 113, 17527. (10) Cai, W.; Yu., J; Jaroniec, M. J. Mater. Chem. 2010, 20, 4587. (11) Liu, S.; Yu, J.; Jaroniec, M. J. Am. Chem. Soc. 2010, 132, 11914. (12) Cai, W.; Yu, J.; Gu, S.; Jaroniec, M. Cryst. Growth Des. 2010, 10, 3977. (13) Price, G. J.; Mahon, M. F.; Shannon, J.; Cooper, C. Cryst. Growth Des. 2011, 11, 39. (14) Zhang, J.; Wang, Y.; Lin, Z.; Huang, F. Cryst. Growth Des. 2010, 10, 4285. (15) Duan, X.; Lian, J.; Ma, J.; Kim, T.; Zheng, W. Cryst. Growth Des. 2010, 10, 4449. (16) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (17) Cao, M.; Liu, T.; Gao, S.; Sun, G.; Wu, X.; Hu, C.; Wang, Z. L. Angew. Chem., Int. Ed. 2005, 44, 4197. (18) Liu, J.; Liu, F.; Gao, K.; Wu, J.; Xue, D. J. Mater. Chem. 2009, 19, 6073. (19) Subramanian, V.; Zhu, H. W.; Vajtai, R.; Ajayan, P. M.; Wei, B. Q. J. Phys. Chem. B 2005, 109, 20207. (20) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 1964. (21) Gao, C.; Zhang, W.; Li, H.; Lang, L.; Xu, Z. Cryst. Growth Des. 2008, 8, 3785. (22) Verziu, M.; Cojocaru, B.; Hu, J.; Richards, R.; Ciuculescu, C.; Filip, P.; I. Parvulescu, V. I. Green Chem. 2008, 10, 373. (23) Stankic, S.; Sterrer, M.; Hofmann, P.; Bernardi, J.; Diwald, O.; Knozinger, E. Nano Lett. 2005, 5, 1889. (24) Choudary, B. M.; Kantam, M. L.; Ranganath, K. V. S.; Mahendar, K.; Sreedhar, B. J. Am. Chem. Soc. 2004, 126, 3396.

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