Ecofriendly Synthesis of Ceria Foam via Carboxymethylcellulose ...

Research Article pubs.acs.org/journal/ascecg

Ecofriendly Synthesis of Ceria Foam via Carboxymethylcellulose Gelation: Application for the Epoxidation of Chalcone H. R. Ramananarivo,† H. Maati,⊥ O. Amadine,⊥ K. Abdelouahdi,‡ A. Barakat,§ D. Ihiawakrim,○ O. Ersen,○ Rajender S. Varma,△ and A. Solhy*,† †

Center for Advanced Materials, Université Mohammed VI Polytechnique, Lot 660−Hay Moulay Rachid, 43150 Ben Guerir, Morocco ⊥ FST, Université Hassan II, B.P. 146, 20650 Casablanca, Morocco ‡ UATRS-CNRST, Angle Allal Fassi/FAR, B.P. 8027, Hay Riad, 10000 Rabat, Morocco § INRA, UMR 1208 (IATE) 2, place Pierre Viala, 34060 Montpellier Cedex 1, France ○ IPCMS-Groupe Surfaces & Interfaces, CNRS-UdS UMR 7504, 23 Rue du Loess, B.P. 43, 67034 Strasbourg, France △ National Risk Management Research Laboratory, Sustainable Technology Division, U.S. Environmental Protection Agency, 26 West Martin Luther King Drive, MS 443, Cincinnati, Ohio 45268, United States S Supporting Information *

ABSTRACT: A simple and innovative process is described for the ecofriendly preparation of ceria foams via carboxymethylcellulose gelation by Ce4+ cations; heat treatment of the ensuing xerogels produces ceria foams. The influence of the concentration of cerium and of the calcination temperature of the xerogels is studied. Several characterization methods have been used and the obtained results demonstrate that this technique allows the controlled growth of ceria foams. The foamy structure apparently is responsible for UV absorption, and the ceria foam is basic enough to promote the epoxidation of chalcone; comparison of the catalytic activity of the ceria foam versus ceria prepared via a coprecipitation method shows that the ceria foam is most active as it promotes epoxidation of electron-deficient alkenes with dilute aqueous hydrogen peroxide. KEYWORDS: Ceria foams, Biopolymer, Gelation, Xerogel, Epoxidation, Chalcone

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properties and varied applications in different fields.23−28 In this context, a few studies have been devoted to the preparation of the 3D porous ceria foams. Shen et al. prepared a series of oxide foams of a rare-earth element or of a lanthanide using a hydrothermal treatment of precursors in the presence of Lasparagine followed by calcination.29 Similarly, Alhumaimess et al. synthesized the gold-adorned ceria foam using L-asparagine, to catalyze the oxidation of benzyl alcohols.30 In another study, the control of Gd-doped ceria was easily achieved by an electrochemical deposition process for application in optics and magnetism fields.31 The use of glucose as a foaming agent afforded a three-dimensional macroporous mixed oxide (Sm0.2Ce0.8O1.9).32 Xing et al. were able to produce ceria foams having thin single-crystal walls, by utilizing thermal decomposition under a NH3 atmosphere of the mixed oxide of cerium and germanium.33 Furler et al. showed the application of reticulated porous ceramic foam for CO2 splitting via thermochemical redox reactions.34 Neumann et al.,35 using the

INTRODUCTION Ceria is one of the most important and functional rare earth oxides1 and has attracted significant interest because of its numerous applications in cutting-edge technology sectors such as automotive three-way catalysts, solid oxide fuel cells, UV absorption ability, gas sensors, and antioxidant therapy, among others.2−8 This is due to ceria’s unique properties, high oxygen capacity, and high oxygen mobility originating from the storage/release of O2 due to the Ce4+/Ce3+ redox couple. However, the specific properties of this oxide are influenced by several parameters including the structure, size, shape, morphology, and the porous texture.9,10 Various approaches have been developed for its preparation, especially to control its morphology and porous texture,11−17 thus culminating in several morphologic forms of cerium oxide which includes nanorods, nanotubes, hollow structures, nanocubes, flower-like structures, and nanospheres.13,18−21 Corma et al. successfully prepared hierarchically structured porous ceria with a highsurface-area via the self-assembly of individual nanoparticles of ceria in a liquid crystal phase to build an operational organicdye-free solar-cell.22 Additionally, ceria foams have attracted much attention in the recent years due to their distinctive © 2015 American Chemical Society

Received: July 10, 2015 Revised: October 1, 2015 Published: October 8, 2015 2786

DOI: 10.1021/acssuschemeng.5b00662 ACS Sustainable Chem. Eng. 2015, 3, 2786−2795

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ACS Sustainable Chemistry & Engineering

physicochemical characteristics of the prepared cerium oxide. During the calcination process, considerable swelling of the material was observed. Thus, we note these materials are a foamy powder of low density that are readily dispersible in organic or aqueous solvents. The hydrogel is formed by three-dimensional entanglement of hydrophilic polymer swollen by water (Figure 1). It has a

polyurethane as foaming agent, revealed the synthesis of homogeneous open-cell foams of the ceria and samaria possessing a bimodal pore size, which have been used to catalyze the oxidative coupling of methane. The preparation of nanomaterial from an ecodesign perspective presents itself as a new global approach that encompasses the incorporation of both, Green Chemistry and sustainable development principles. Eco-design incorporates, among other considerations, functionality and performance of materials prepared by controlling their nucleation and growth; several natural products have been exploited to develop nanomaterials such as biopolymers,36−38 fungus,39 and others. Epoxides are extremely versatile intermediates in organic synthesis and pharmaceutical chemistry.40−44 Among several traditional methods available for their synthesis,45−47 a number of environmentally friendlier approaches using heterogeneous catalysts have been developed.48,49 It is in this sense that the oxides occupied a key place in the catalysis of this reaction.50,51 As an example, W. Lueangchaichaweng et al. used gallium oxide nanorods, prepared via precipitation method, to catalyze the epoxidation of alkenes with hydrogen peroxide (H2O2).52 Among various oxidants used,46,53 H2O2 is apparently the cleaner oxidant of choice for economic and environmental reasons,46,54,55 because (i) the oxidation process gives off water which is the only byproduct of this process and (ii) H2O2 is one of the cheapest oxidants available on the market. Herein, we describe a novel, simple, and ecofriendly ultralight cellular ceria foam obtained after postsynthesis treatment of the xerogels, prepared via the gelation of carboxymethylcellulose (CMC) by Ce4+. As an application of this material, we evaluated the catalytic activity to promote the epoxidation reaction of chalcone.

Figure 1. Digital photo of hydrogel H_CMC@Ce_0.1.

yellow color likely from the reduction of Ce4+ to form bonds with the positively charged groups, as well as through the creation of hydrogen bonds. We observe a difference in the morphology among the three xerogels prepared: X_CMC@ Ce_0.1, X_CMC@Ce_0.2, X_CMC@Ce_0.3 (Figure 2).

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RESULTS AND DISCUSSION The preparation of ceria foam comprises cross-linking of CMC (Figure S1) with Ce4+ for obtaining hydrogels. These hydrogels are dried either in a conventional manner, which gives rise to xerogels, or using supercritical CO2, which avoids the appearance of the liquid/gas interface and thus producing aerogels. The key point of the strategy is the gelation of carboxymethyl cellulose by Ce4+ at a suitable concentration range; three different concentrations of cerium source were used to jellify the CMC: 0.1, 0.2, and 0.3. Thus, three xerogels were prepared after drying the corresponding hydrogels, which were prepared in accordance to adopted concentrations and were termed: H_CMC@Ce_0.1, H_CMC@Ce_0.2, and H_CMC@Ce_0.3 for the hydrogels and X_CMC@Ce_0.1, X_CMC@Ce_0.2, and X_CMC@Ce_0.3 for the xerogels. Thereafter, a heat treatment was performed to release the inorganic matrix and to obtain the cerium oxide (see details in the Supporting Information). The xerogels and the aerogels were calcined for 4 h under air wherein the aerogels gave a definite nanostructured ceria but were not foamy in nature (Figure S2). In the following section, we restrict ourselves to the results pertaining to the thermal treatment of xerogels that led to ceria foams. These samples are designated as F_CeO2_0.1, F_CeO2_0.2, and F_CeO2_0.3, according to the concentration used. The minimum calcination temperature necessary to remove organic residues was determined by thermogravimetric analysis; the details are described in the Supporting Information (Figure S3). Five different temperatures, namely, 350, 500, 700, 900, and 1100 °C, were tested in order to investigate the influence of the temperature on the

Figure 2. Digital photos of xerogels (X) and aerogels (A): X_CMC@ Ce_0.1 (a), X_CMC@Ce_0.2 (b), and X_CMC@Ce_0.3 (c); A_CMC@Ce_0.1 (d), A_CMC@Ce_0.2 (e), and A_CMC@ Ce_0.3 (f).

We also note that the xerogel X_CMC@Ce_0.1 has a circular but flattened shape. The SEM images of this material at low magnification show that this sample has a mushroom shape (Figure S4). These observations imply that the amount of water retained in the hydrogel is higher and that the molecules of the core of the bead, H_CMC@Ce_0.1, are weakly cross-linked by Ce4+ cations during the gelation process. The morphology of the X_CMC@Ce_0.2 and X_CMC@Ce_0.3 is slightly different compared to the digital images of the first case. The rounded shape suggests that drying at room temperature did not cause the collapse of the gel structure, in contrast to xerogels prepared with a concentration of 0.1 M in Ce4+. The use of higher concentrations (0.2 and 0.3 M cerium) enables to completely gel beads hydrogels of the shell toward the core. 2787


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last two samples when compared with the first sample (F_CeO2_0.1). All samples prepared in this study, xerogels, aeorgels, ceria foams, or cerium oxides, have been characterized by X-ray diffraction. The first conclusion drawn from this analysis is that the xerogels and aerogels are amorphous. Figure 4a presents the X-ray diffraction spectra of ceria foams prepared by calcinating X_MCM@Ce_0.1 for 4 h at the following set of temperatures: 500, 700, 900, and 1100 °C. The ensuing ceria foams exhibit the face-centered-cubic crystal lattice of the fluorite-type structure and the space group Fm3̅m (JCPDS 00-034-0394), whose main crystallographic planes are (111), (200), (220), (311), (222), (400), (331), and (420). We also noticed that the diffraction peaks of the sample calcined at 500 °C are large and less intense than the ones calcined at higher temperatures (700, 900, and 1100 °C), and they exhibit strong and narrow peaks. Figure 4b illustrates the X-ray diffractogram of the foam obtained by the calcinating xerogels (X_CMC@Ce_0.1) at 900 °C, with the most intense peak of atomic plane (220) which can be explained by a preferential orientation according to this facet; crystallite size was calculated by the Debye−Scherer formula. The results of this calculation are shown in Table 1 and indicate that the cubic lattice constant “a” is approximately 5.4 nm for all the above samples, with a slight increase as the calcination temperature of the sample increased. The effect of temperature is significant as the crystallite size increases from 20 nm for the sample calcined at 500 °C to 60 nm for the other sample calcined at 1100 °C. This observation has perfect correlation with the diffractograms of the samples shown in Figure 4 whose broad peaks are characteristic of nanostructured materials. Figure 4c represents the XRD spectra of the foams obtained by calcinating at 500 °C of X_CMC@Ce_0.1, X_CMC@Ce_0.2, and X_CMC@Ce_0.3; all three foams are crystallized with the same face-centered cubic structure. The crystallite sizes calculated for the three samples are 20, 21, and 24 nm, respectively, indicating the influence of the concentration on the crystallinity. The foams become more crystalline as the concentration of the cerium precursor increases. This is probably due to a possible aggregation of small nanoparticles during the calcination process of a relatively high concentration (consequently indicating a high density of these smaller nanoparticles per unit volume). We note that the nanoparticles tend to condense to form relatively large crystallites with good crystallinity (Ostwald ripening). The ceria foams prepared via the calcination of xerogels were analyzed by TEM (Figure 5) as were the nanostructured cerium oxides obtained by the calcination of the aerogels samples (Figure S6). The TEM images of the sample obtained by calcination of X_CMC@Ce_0.1 to 350 °C are shown in Figure S7 and demonstrates the shape of these nanoparticles is almost spherical with a uniform size, which varies between 7 and 10 nm. The TEM image of the ceria foam obtained by calcination of X_CMC@Ce_0.1 at 500 °C is shown in Figure 5a where the porous structure is apparent in view of this unusual nanostructure of ceria. For this sample, the dispersion is less homogeneous than the first sample (F_CeO2_0.1) and the size varies from 20 to 50 nm, which corresponds to a relatively large size dispersion. This particular assembly of nanoparticles forms the walls of ceria foam and the average width of the windows range between 25 and 50 nm. Image b in Figure 5 shows a representative TEM image of the foam obtained by calcinating X_CMC@Ce_0.1 at 700 °C. Here the dispersion is much less homogeneous than the first sample (Figure 5a), and the size

The structure of the two hydrogels at the above concentrations is rigid because of the rate of linkage of the carboxymethylcellulose chains and the high concentration of the cerium precursor solution. The conventional drying method based on evaporation of the liquid phase at atmospheric pressure and at room temperature leads to a contraction of the gel. Indeed, because of capillary forces that result from the liquid/gas interface, the contraction induces irreversible withdrawal of water. Therefore, the obtained xerogels texture is very distinct from the texture of the starting hydrogel. The digital photographs of the aerogels compared with those of xerogels show the difference between these two methods of drying (Figures 2). Figure S4 shows the digital photos of some ceria foams obtained by calcinating X_CMC@Ce_0.1 at 500, 700, 900, and 1100 °C, respectively; a swelling phenomenon was observed after the calcination of the xerogels. The SEM images in Figure 3 present the morphologies of the ceria foams obtained by calcinating X_CMC@Ce_0.1 at 350,

Figure 3. SEM images of ceria foams obtained by calcination of the X_CMC@Ce_0.1 at different temperatures: (a and b) 350, (c and d) 500, (e) 700, (f) 900, and (g) 1100 °C. SEM images of ceria foams obtained by calcinating at 500 °C of the (h) X_CMC@Ce_0.2 and (i) X_CMC@Ce_0.3.

500, 700, 900, and 1100 °C, respectively. The frothy texture remains stable and the material seems spongier, particularly in correspondence with increased temperature. The foamy texture of all samples is visible with open micrometric pores, which are interconnected through small windows. The image (g) confirms the thermal stability at elevated temperature (1100 °C) of the foamy texture. The pores enlarge and connect with the interparticle spaces. The cells are always present at high temperature; there is not any collapse of porosity noticed by this analysis. We also noted that the density of cells and their morphology changed. The cells are dense, with small disordered windows, and with strongly interconnected walls. The comparison of SEM images, d−i corresponding to the foams obtained by calcinating at 500 °C X_CMC@Ce_0.1, X_CMC@Ce_0.2, and X_CMC@Ce_0.3, reveals a difference in the shape and size of the pores of these samples. In addition, the interconnections of the pores are more accentuated in the 2788


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Figure 4. X-ray diffraction patterns of some representative samples of ceria foams: (a) X_CMC@Ce_0.1 calcined, respectively, at 500 (1), 700 (2), 900, (3) and 1100 °C (4). (b) Spectrum of the ceria foam obtained by the calcination of xerogels at 900 °C. (c) Spectra of the foams obtained by calcinating at 500 °C the corresponding xerogels: X_CMC@Ce_0.1 (1), X_CMC@Ce_0.2 (2), and X_CMC@Ce_0.3 (3). (d) Unit cell of the ceria (face-centered cubic lattice).

about 450 nm (Figure 5d) and can be explained by sintering phenomenon due to the calcination at this high temperature. Figure 5e of the sample F_CeO2_0.2, obtained by the calcination of X_CMC@Ce_0.2 at 500 °C, shows a superb nanostructure formed by small substantially spherical nanoparticles of a single layer and interconnected pores. We also note that the particles are relatively large, with an irregular geometrical shape, and in contact with one another. This structure is relatively dense compared to the structure of F_CeO2_0.1 (Figure 5a). For the ceria foam (F_CeO2_0.3) obtained by the calcination at 500 °C of the X_CMC@Ce_0.3 (Figure 5f), we note that the structure is made of more compact aggregates of large particles compared to the other structures that are obtained with the lower concentrations of 0.3 M (0.1 M and 0.2M). This difference, in comparison to the other samples (F_CeO2_0.1 and F_CeO2_0.2), can be explained by the multiplication of the density of small nanoparticles during the heat treatment; additionally, the effect of an Ostwald ripening at a low concentration is not dominant because the nanoparticles are relatively distant one from another. It should be stressed that the elemental analysis of ceria foam samples by EDS, performed while capturing TEM images, reveals the presence of the constituent elements of these materials (Ce and O), as expected (Figure S8). The HR-TEM images of foams F_CeO2_0.1 obtained by the calcination of X_CMC@Ce_0.1 at 350, 500, 700, and 900 °C, are shown in Figure 6 depicting the arrangement of nanoparticles in these nanostructures. The nanoparticles synthesized at 350 °C formed compact aggregates, whereas the particles which were obtained by calcination at other temperatures are larger and interconnected through crystallographic facets, thus creating well-defined mesopores (Figure 6).

Table 1. Lattice Parameters and Crystallite Size As a Function of the Calcination Temperature

a

temperature (°C)

500

700

900

1100

lattice parameter “a” (nm) lattice volume (nm3) crystallite size (nm)a

0.53928 0.15683 20

0.53937 0.15691 45

0.53943 0.15696 55

0.53991 0.15738 60

Calculated via Debye−Scherer.

Figure 5. TEM images of ceria foams obtained by calcinating X_CMC@Ce_0.1 at 500 (a), 700 (b), 900 (c), and 1100 °C (d). Representative TEM images of F_CeO2_0.2 (e) and F_CeO2_0.3 (f).

varies from 70 to 130 nm. Thus, in this case, we have a heterogeneous dispersion in size but a fairly homogeneous shape. For the ceria foam obtained by calcinating X_CMC@ Ce_0.1 at 900 °C (Figure 5c), the particle size is approximately 250 nm and the dispersion is very large. The sample obtained by the calcination of X_CMC@Ce_0.1 to 1100 °C, shows the appearance of a large agglomerated particle which has a size of 2789


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Figure 6. Representative HRTEM images of ceria foams F_CeO2_0.1 obtained by calcinating X_CMC@Ce_0.1 at 350 (a), 500 (b and c), 700 (d and e), and 900 °C (f).

The measured interplanard-spacing on image a, corresponding to ceria foam obtained by calcination of X_CMC_Ce_0.1 at 350 °C is 0.312 nm, which corresponds to the atomic plane (111) of the fluorite structure of ceria. The same distance was calculated in the case of the foam of the xerogels obtained by calcinating (X_CMC_Ce_0.1) at 500 °C (image b); the analysis of one more nanoparticle shown in image c, led to the identification of another atomic lattice plane whose distance is 0.270 nm, which is characteristic at (220) plane of the same fluorite structure. However, the same distances are measured in the sample obtained by calcinating xerogels X_CMC@Ce_0.1, at 700 °C [images d and e]. Alternatively, the sample obtained by the calcination of the xerogel X_CMC@Ce_0.1 at 900 °C (f), the distances corresponding to (220) family plane which may be explained by a preferential orientation according to this plane. This observation correlates well with the results from the XRD analysis and further corroborated by the SAED (Figure S10). Several typical aggregates of X_CMC@Ce_0.1 calcined at 350 and 500 °C have been analyzed by electron tomography in order to provide direct information on the 3D characteristics and the relative arrangement of ceria nanoparticles inside the aggregates. The representative results obtained for the two types of aggregates are summarized in Figure 7. Their quantification shows that the mean nanoparticles size, as deduced from 3D reconstructed volumes, is in good agreement with the values obtained from the 2D analysis. However, the main finding of the 3D analysis concerns the difference in shape of the nanoparticles in the two aggregates: for the specimen calcined at 350 °C, the nanoparticles are roughly spherical, while for the second one submitted to 500 °C the particles present well-defined facets. Related to the relative arrangement of nanoparticles, their 3D distribution in the first sample corresponds to a classical packing governed by the steric effect of several 3D individual entities. On the contrary, the aggregates of the second samples are much more compact, due to the subsequent growth of the first set of nanoparticles in

Figure 7. Electron tomographic analyses of the samples X_CMC@ Ce_0.1 annealed at 350 (left) and 500 °C (right). (a and b) 2D-TEM images at 0° tilt from the tilt series used to reconstruct the volumes of the analyzed aggregates. (c−f) Typical slices extracted from the reconstruction, showing the shape of the individual particles and the characteristics of the interface between two adjacent particles. (g and h) 3D corresponding models, allowing direct illustration of the relative arrangement of nanoparticles inside an aggregate.

the remaining volume. In addition, the particles are in contact on crystallographic facets creating well-defined interfaces between two adjacent particles. These parameters are of crucial 2790


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Figure 8. (a) XPS spectra of xerogel and foams F_CeO2_0.1 obtained by calcinating the corresponding xerogels at various temperatures. (b) XPS spectra of O 1s for structural oxygen and adsorbed oxygen. (c) UV−visible absorbance spectra of the xerogel and F_CeO2_0.1: xerogel (black curve; onset of absorption, 500 nm), foam obtained by calcinating the xerogel at 500 °C (red curve; onset of absorption, 410 nm), foam obtained by calcinating the xerogel at 700 °C (green curve; onset of absorption, 400 nm) and foam obtained by calcinating the xerogel at 900 °C (blue curve; onset of absorption, 420 nm). (d) UV−visible absorbance spectra of the foams obtained by calcinating the prepared xerogels with different concentrations at 700 °C: 0.1 (black curve), 0.2 (red curve), and 0.3 M (blue curve).

cations are dominant but the presence of Ce3+ is also noticed; peaks corresponding to Ce4+ mask the characteristic peaks of the latter. Furthermore, Figure 8b presents the XPS spectra of the oxygen 1s of the foams samples calcined at different temperatures, 500 °C, 700 and 900 °C. The strong peak at 529.6 eV for all the calcined samples is the characteristic peak of the structural oxygen contributing to the fluorite structure of cerium oxide, while the peak at 532.1 eV is specific to the oxygen absorbed by the material. The variation of the ratio of the peak intensity of adsorbed oxygen and structural oxygen shows this ratio decreases with the increase of the calcination temperature (Figure S12), which means that the sample calcined at 900 °C absorbed a higher quantity of oxygen than the other samples calcined at a lower temperature. The size of cerium oxide nanoparticles is a known key factor in oxygen absorption/storage, due to the significant presence of Ce3+ in the vicinity of the surface. This high oxygen absorption property for the xerogel sample calcined at 900 °C can be due to its preferential orientation structure. Figure 8c shows the UV−visible spectra of the X_CMC@ Ce_0.1 and corresponding foams obtained by its calcination at different temperatures: 500, 700, and 900 °C. All the samples

importance for the subsequent potential applications, as the properties of these nanocrystals depend on the type of their exposed facets and on the nature of the atoms present at their surface. The surface areas of the prepared foams are determined using a nitrogen adsorption/desorption method. According to the SEM and TEM analysis, cells that have large pores that are interconnected and cell walls that are endowed with small windows form the foams. The estimated diameter of these cells is 500 nm to 2 μm, which is too large to be analyzed by the N2 adsorption/desorption technique that is based on the physical adsorption of nitrogen on the surface of material below the pressure of the saturated vapor. We believe that the ideal technique for this type of analysis is the mercury porosimetry (Figure S11 and Table S1). Figure 8a shows the XPS spectra of the 3D electron of ceria foams prepared with 0.1 M of Ce4+ with the xerogel sample containing a large amount of Ce3+ cations and displaying two characteristic peaks at 885.9 and 904.1 eV, corresponding to the final state Ce 3d94f1O 2p6. The satellite peak at 917.1 eV is characteristic of Ce4+, which confirms the various valence states of cerium present in the xerogel. In the foams samples, Ce4+ 2791


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ACS Sustainable Chemistry & Engineering Table 2. Epoxidation of Chalcone Catalyzed by Ceria Foam Using H2O2 as an Oxidanta entry

catalyst

calcination temperature (°C)

cat [mol %]

solvent (ml)

time (h)

yield [%]b

1 2 3 4 5 6 7 8c 9d 10 11 12 13 14 15 16 17 18 19

F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.2 F_CeO2_0.2 F_CeO2_0.3 F_CeO2_0.3 CeO2 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1 F_CeO2_0.1

500 700 900 700 900 700 900 700 700 700 700 700 700 700 700 700 700 700 700

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.25 0.15 0.5 0.5 0.5

methanol (1) methanol (1) methanol (1) methanol (1) methanol (1) methanol (1) methanol (1) methanol (1) methanol (1) ethanol (1) 1-propoanol (1) methanol (2) methanol (3) methanol (5) methanol (1) methanol (1) methanol (1) methanol (1) methanol (1)

48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 24 12 6

73 85 70 69 42 57 45 50 49 58 18 65 40 27 77 68 70 30 10

a

Chalcone (0.581 mmol), methanol (1 mL), 30% H2O2 (0.35 mL), catalyst (0.5 mol %), at rt, for 48 h. bYield of isolated product. cCatalyst was prepared via the coprecipitation method. dReaction carried out using NaClO as oxidant.

absorbed in the region below 400 nm, which is characteristic of the cerium oxide, but as shown in the graph, the absorption curves are different one from another. The absorptions below 400 nm are mainly due to the charge transfer of oxygen O 2p to cerium Ce 4f,56 and the interband transition from 5d to 4f for cerium.20 Notably, the xerogel absorbed UV−visible from 550 nm, which is in agreement with the yellowish color of the sample. The significant presence of Ce3+ in the xerogel promoted the samples obtained by calcination at 500 °C to shift. Even in the samples calcined at 900 °C, the XPS analysis indirectly reveals the huge presence of Ce3+, as displayed by the presence of oxygen deficiency. The analysis of Figure 8c curves shows that in the sample obtained by calcination at 700 °C, almost all the range of the UV radiation (230 to 350 nm) is absorbed. This high amount of absorption of UV radiation is an important property of the sample obtained by calcination at 700 °C reminiscent of similar result reported by Li et al. for cerium oxide nanorods that were synthesized differently.57 Figure 8d shows the UV absorption spectra of the three foams obtained by calcinating xerogels at 700 °C that were prepared with three different concentrations: 0.1, 0.2, and 0.3 M. The variation of the concentration does not strongly affect the UV absorption of these samples. Besides the charge transfer, the interband transition, the high refractive index of the cerium oxide and the foaming texture contributed to this high UV absorbance rate that is larger than the one in all other samples in all ranges. Taking into consideration the findings of this study allowed us to focus on the mechanism of formation of the ceria foam which is similar to the mechanism acting in the formation of “popcorn”, a worldwide consumed food. At a given temperature and pressure, the grains of corn break out by the deterioration of the envelope, that is embedded in the starch. The grain can then expand about 30−50 times its original volume (Figure S13). Similarly, FTIR analysis shows the difference between xerogels and aerogels and explains why only xerogels give rise to foams (Figure S14).

Ceria foam was evaluated as a heterogeneous catalyst for the epoxidation of chalcone with aqueous hydrogen peroxide (Table 2); this transformation was chosen as a model reaction for studying the catalytic performance of cerium foam. Initially, the influence of the calcination temperature of the foams F_CeO2_0.1, F_CeO2_0.2, and F_CeO2_0.3, respectively, on their catalytic activities was examined. It became apparent immediately that F_CeO2_0.1 is the better catalyst mainly for the sample calcined at 700 °C, which is in accord with the analyses of the XPS and UV−visible because it afforded 85% yield for the epoxidation of chalcone. The comparison of this result with the one obtained when catalyzing the same reaction of ceria oxide prepared by coprecipitation (Table 2, entry 8), clearly shows the superior catalytic performance of the ceria foam. The use of NaClO as an oxidant results in 49% yield (Table 2, entry 9), which further promotes the predominance of the use of greener oxidant, H2O2. We would like to point out that the nature of the solvent (entries 2, 10, and 11), the volume of methanol (entries 2, 12, 13, and 14), and the amount of catalyst (entries 2, 15, and 16) are decisive parameters in this chemical transformation. Finally, the catalyst is easy to regenerate several times with only a modest decrease in yield in the fifth cycle (66%).

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CONCLUSIONS In summary, we have developed an innovative and eco-friendly technique to prepare ceria foams that would invoke a major interest for several technologies. Initial formation of xerogels via gelation of carboxymethylcellulose by cerium(IV) followed by a postsynthesis thermal treatment provided this foam. The direct effect of the concentration of precursor and the mode of drying/calcination temperature on the morphology, the crystallinity, and the texture of the foam was established and are highlighted in this work. Ceria foam displays a comparable catalytic activity, in the ecofriendly epoxidation of chalcone by H2O2, than the one observed with several catalysts listed in the literature. In the light of these results, this strategy could be 2792


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Catalytic Application-Epoxidation of Chalcone with H2O2. Chalcone (0.581 mmol), methanol (1 mL), and catalyst (0.5%mol) were added to a 10 mL flask. The reaction was started with the addition of aqueous hydrogen peroxide (30 wt %, 0.35 mL) at room temperature within the appropriate time under stirring. After completion of the reaction, the product was extracted by ethyl acetate (3 × 10 mL), dried over Na2SO4, concentrated by evaporation and purified using column chromatography. The obtained epoxide was characterized by 1H NMR spectroscopy. For regeneration tests, the recovered catalysts were dried before reuse.

generalized with the aim to prepare doped ceria foams and related foamy oxides for others applications.

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EXPERIMENTAL SECTION

MATERIALS. The precursor of cerium(IV) used in this study is ammonium cerium(IV) nitrate ((NH4)2Ce(NO3)2). Other cerium precursors (Ce(NO3)3,6H2O, CeCl3) were also tested in order to investigate the effect of the nature of the source of cerium. The sodium carboxymethylcellulose used in this study has a molecular weight of about 90 000 g/mol with a degree of substitution that is equal to 0.7 (the number of carboxymethyl per monomer unit). This biopolymer is generally used as a basic sodium salt. In this study, the CMC was used without purification, and the viscosity varied depending on its concentration (see rheological study Figure S15). The percentage by weight of CMC was found to be 5% from several preliminary experiments to establish the optimized percentage based on the quality of gelation. Characterization Techniques. Supercritical drying was accomplished using the Quorum Technologies Society’s (Polaron, E3100 Critical Point Dryers). The rheological study was performed at room temperature, using the MCR500 rheometer equipped with concentric cylinder geometry. Thermogravimetric analysis (TGA) was conducted under air in a TA Instrument Q500 apparatus, with a 10 °C/min ramp between 25 and 1000 °C. Fourier transform infrared (FT-IR) spectra of samples in KBr pellets was measured on a Bruker Vector 22 spectrometer. X-ray diffraction patterns were obtained at room temperature on a Bruker AXS D-8 diffractometer using Cu−Kα radiation in Bragg−Brentano geometry (θ-2θ). TEM micrographs were obtained on a Tecnai G2 microscope at 120 kV. High resolution transmission electron microscopy analysis was carried out on a Jeol 2100F microscope, equipped with a high resolution pole piece, field emission gun and operating at 200 kV. The gas adsorption data were collected using a Quantachrome Autosorb-1 automatic analyzer using N2. Prior to N2 sorption, all samples were degassed at 150 °C overnight. The specific surface areas were determined from the nitrogen adsorption/desorption isotherms (at −196 °C), using the BET (Brunauer−Emmett−Teller) method. Pore size distributions were calculated from the N2 adsorption isotherms with the “classic theory model” of Barret, Joyney and Halenda (BJH). XPS studies were carried out in a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV), operating at 150 W, using a multichannel plate and delay line detector under 1.0 × 10−9 Torr vacuum. The survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 and 20 eV, respectively. The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy of 83.95 eV. The spectrometer dispersion was adjusted to give a binding energy of 932.63 eV for metallic Cu 2p3/2. Samples were mounted in a floating mode in order to avoid differential charging; charge neutralization was required for all samples. The electronic binding energy of C 1s (284.80 eV) was used as the internal standard. These data were analyzed with commercially available software, CasaXPS. The individual peaks were fitted with a Gaussian (70%)-Lorentzian (30%) (GL30) function after Shirley type background subtraction. For the UV characterization, a spectrophotometer LAMBDA 1050 UV/vis/NIR of the company PerkinElmer was used, with a wavelength range spanning from 100 to 3300 nm. Synthesis of Ceria Foams. The synthesis of ceria foams was accomplished using the carboxymethylcellulose (CMC) gel. Five g of CMC were dissolved in 100 mL of deionized water (18 MΩ) at room temperature, forming a viscous solution. A separate solution was prepared by dissolving 5.5 g of (NH4)2Ce(NO3)2 in 100 mL of deionized water. Then, the CMC gel (5%: w/w) was added dropwise, at room temperature, to the cerium(IV) precursor solution via a syringe with a 0.8 mm diameter needle and constantly stirred for 1 h. The hydrogels obtained were washed with distilled water then dried at room temperature for 24 h or by supercritical CO2 in order to prepare xerogels and aerogels, respectively. These beads (xerogels or aerogels) were then calcined at different temperature (350, 500, 700, 900, and 1100 °C).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00662. Chemical structure of carboxymethylcellulose; schematic representation of the process; detailed experimental section; TGA with interpretation, digital photos of ceria foams; supplementary SEM images, supplementary TEM and HR-TEM SEM images; EDX analysis; SAED pattern of ceria foams; nitrogen adsorption/desorption isotherms of the ceria foam and data of specific surface area with an interpretation of result; scheme of the mechanism with an interpretation; FTIR spectra; rheological studies of aqueous solution of 5% CMC, with interpretation of result (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The U.S. Environmental Protection Agency, through its Office of Research and Development, funded and managed, or partially funded and collaborated in, the research described herein. It has been subjected to the Agency’s administrative review and has been approved for external publication. Any opinions expressed in this paper are those of the author(s) and do not necessarily reflect the views of the Agency; therefore, no official endorsement should be inferred. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial assistance of the Office Chérifien des Phosphates in the Moroccan Kingdom (OCP Group) towards this research is hereby acknowledged. This work was also supported by a grant from the OCP Foundation. The corresponding author (SA) also extend his warmest thanks to Mr. Mohamed El Kadiri, Managing Director & General Secretary of OCP Group, who gave him the opportunity to integrate the great project: UM6P.

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REFERENCES

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