Ce2IIIO3-silica mesoporous composite materials for oxidation and esterification reactions

Accepted Manuscript Catalytic activity of CeIVO2/Ce2 IIIO3-silica mesoporous composite materials for oxidation and esterification reactions Nabanita Pal, Eun-Bum Cho, Dukjoon Kim, Chamila A. Gunathilake, Mietek Jaroniec PII: DOI: Reference:

S1385-8947(14)01400-4 http://dx.doi.org/10.1016/j.cej.2014.10.068 CEJ 12811

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 July 2014 8 October 2014 18 October 2014

Please cite this article as: N. Pal, E-B. Cho, D. Kim, C.A. Gunathilake, M. Jaroniec, Catalytic activity of CeIVO2/

Ce2 IIIO3-silica mesoporous composite materials for oxidation and esterification reactions, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.10.068

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Catalytic activity of CeIVO2/Ce2IIIO3-silica mesoporous composite materials for oxidation and esterification reactions Nabanita Pal,† Eun-Bum Cho,*,‡ Dukjoon Kim*,† Chamila A. Gunathilake,§ and Mietek Jaroniec§

†

School of Chemical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746,

Korea ‡

Department of Fine Chemistry, Seoul National University of Science and Technology, Seoul

139-743, Korea §

Department of Chemistry, Kent State University, Kent, Ohio 44242, United States

*Address for correspondence: E.-B. Cho: e-mail: [email protected], Tel.: +82-2-970-6729 D. Kim: e-mail: [email protected], Tel.: +82-31-290-7250

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Abstract Bifunctional catalytic performance of various ceria-containing mesoporous silica samples have been investigated for oxidation as well as acid-catalyzed esterification reactions under mild conditions. Hexagonal (p6mn) and cubic (Ia3d) ordered mesoporous silica materials with incorporated CeIVO2 were synthesized with high Ce content and successfully reduced to Ce2IIIO3-silica species under H2 gas flow. Small angle X-ray scattering (SAXS) analysis of the samples shows ordered patterns of the materials studied. The surface area and porosity of the samples were determined from N2 adsorption isotherms. Both the oxidation states of ceria have been proved as highly efficient catalysts for liquid phase oxidation of numerous hydrocarbons like cyclohexene, styrene, benzyl alcohol, and xylene and so on at room temperature under solvent-free conditions, whereas Lewis basic role of cerium oxide has been explored for esterification reaction of various alcohols like benzyl alcohol, octanol, and decanol and so on using acetic acid under mild conditions. The surface acidic sites present on both CeIVO2-silica and Ce2 IIIO3-silica composites have been measured and compared by ammonia-TPD analysis performed at 393 K. The reusability and heterogeneity of the catalysts with minimum loss have also been examined under identical conditions. Keywords: Reduction of ceria, bifunctional catalyst, hydrocarbon oxidation, esterification of alcohol.

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1. Introduction Ceria-based nanomaterials are highly important and well known due to the versatile role of ceria in solid oxide fuel cells (SOFC) [1], optics [2], electronics [3], catalysis [4,5], adsorption [6], water purification [7] and biomedical applications [8]. Incorporation of ceria into solids with high surface area such as silica [9], alumina [10] and zeolite [11] improves not only the mechanical and thermal stability of metal oxides but it can be cost-effective too [12]. Several attempts have been made to prepare Ce-incorporated silica or alumina ordered mesostructures with high Ce content [13-15]. Nowadays, the deposition of Ce oxide on mesoporous silica supports like MCM-41 or SBA-15 attracted a lot of attention [16]. Recently, Cho et al. reported highly ordered ceria-silica composite materials with cerium loading up to 43 wt% [17]. A unique feature of ceria is high mobility of oxygen in the lattice owing to the multivalent nature of cerium, which assures an easy electron transfer between cerium(III) (Ce3+) and cerium(IV) (Ce4+) states, leading to high catalytic activity [18]. A good oxygen storage capacity of cerium oxide is attributed to the following reaction: 2CeIVO2 → Ce2 IIIO3 + 0.5O2 [19,20]. Thus, the catalytic performance of the oxygen-rich CeO2 and oxygen-poor Ce2O3 may depend on the availability of external oxygen resources. Here we present a comparative study and discussion of the catalytic activity of these two oxides under suitable reaction conditions. Direct esterification of primary and secondary alcohols using aliphatic acids is significant and popular transformation in organic chemistry [21,22]. Esterification, specifically acylation of alcohols, are generally carried out by treating alcohols with acylating agents like acetic a nhydride or acetyl chloride or alternatively acetic acid in the presence of acid or base catalyst in a suitable organic solvent [23]. Different acetates like ethyl acetate, benzyl

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acetate and their derivatives obtained from respective alcohols via acylation reaction are very essential chemicals for pharmaceutical and food industry [24]. These reactions are usually driven by common Brønsted, Lewis acids like H2SO4, metal triflates [25], and Lewis bases like pyridine, organic base like tetramethylethylenediamine (TMEDA) [26], cause a number of limitations like the requirement of large amount of alcohols and the formation of corrosive chemical wastes, etc [27]. Use of heterogeneous solid acid or base catalysts can overcome the aforementioned problems, however sometimes elevated temperatures and an excess of esterifying acid are necessary to carry out the forward reaction and get a satisfactory yield [28]. Although, Pace et al. reported acylation of methanol performed in the presence of inorganic basic oxides like CaO and MgO at room temperature and with equivalent ratio of acyl halide, but the organic solvent used bring some drawbacks and the catalyst was not reusable for the process [29]. Mediation of acidic ionic liquid is a ‘green’ process and it works well in presence of equivalent amount of acid used under solvent-free condition, but ionic liquids are too expensive thus not so suitable from industrial point of view [30]. Hence, our materials CeO2-silica and Ce2O3-silica composites [17] are much more desirable, because first, they have been proved to be highly efficient for esterification of various common alcohols like benzyl alcohol, octanol, decanol, and cyclohexanol with satisfactory yield under mild temperature conditions, secondly, a little excess like 1:3 ratio of esterifying acid is sufficient to make the reaction going on and above all these materials are also very cost effective, highly stable, recyclable thus well enough for industrial purpose. Beside the aforementioned features, these ceria-silica heterogeneous catalysts are also highly effective for solvent-free oxidation of hydrocarbons like styrene, cyclohexene, toluene, and benzyl alcohol at room temperature. Low temperature oxidation of CO over Au-Mn coprecipitate material reported by Lee et al. is also very worthy to mention here [31].

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Considering the increasing environmental concerns [32], we have used green oxidant hydrogen peroxide (H2O2) together with our catalysts in order to avoid toxic organic byproducts, though tert-butyl hydroperoxide (TBHP) gives a higher yield at similar conditions. The product benzaldehyde and phenyl acetaldehyde are very valuable and demanding chemicals widely used in laboratory, cosmetics, dyestuffs as well as intermediates for many important organic synthesis [33]. There are some recent reports on solvent-free selective oxidation of styrene, benzyl alcohol by Patel et al. [34] and cyclohexene by Cai et al. [35], but room temperature oxidation under solvent-free conditions is rarely reported [36]. Our previous report on oxidation of benzyl alcohol referred to such conditions, but a strong oxidant TBHP was used [13]. To the best of our knowledge, there is no report on bifunctional highly reusable ceria-catalyst efficient for simultaneous liquid phase oxidation of hydrocarbons in presence of non-toxic oxidant H2 O2 at room temperature under solvent-free conditions as well as on esterification of alcohol under mild conditions. Hence, our ceriasilica material being an environment friendly, reusable, heterogeneous, non-toxic, non-air sensitive catalyst with bifunctional activity [37,38] can be a promising addition to industrial chemistry. In this report, the synthesis was scaled up successfully two times in comparison to the previous report.

2. Experimental section. 2.1. Materials. All the chemicals and reagents for catalytic reactions were purchased from Sigma-Aldrich and used as received without further purification. 2.2 Synthesis of mesoporous ceria(IV)-silica materials

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Hexagonally ordered p6mm mesoporous ceria(IV)-silica composites with Ce/Si = 0.5 and 0.3 were prepared by a surfactant-assisted hydrothermal method under ammonia basic conditions. In a typical synthesis for producing HCS-50 (ceria-silica with 50% Ce loading), CTAB (3.0 g, 8.2 mmol) was completely dissolved in a mixture of 60 mL of deionized water and 91.2 mL of ethanol followed by 70 mL (1.0 mol) of NH4OH solution under vigorous stirring with a magnetic bar in a glass bottle. After stirring the solution for about 40 min, 6.0 mL (26.0 mmol) of TEOS were added to the solution and stirred well while a heavy white precipitate was formed. After further 30 min of constant stirring 2.80 g (2.6 mmol) of cerium(IV) hydroxide was added to the mixture. The final mixture was stirred for further 20 h at room temperature, followed by aging at 373 K in a convection oven for 24 h in a closed container. On next day, the mixture was cooled at room temperature, filtered, washed with an adequate amount of deionized water as well as ethanol repeatedly and finally dried in air. The surfactant was removed from the as-synthesized sample by calcination at 823 K for 5 h at 3 K min-1 under flowing air. The hexagonally ordered ceria-silica materials with 30% Ce loading named as HCS-30 was prepared similarly except the amount of Ce(OH)4 (7.8 mmol for HCS30) used. For synthesis of Ia3d cubic ceria(IV)-silica composites with 30% Ce loading (named CCS30), CTAB (2.4 g, 6.6 mmol) surfactant was dissolved completely in a mixture of 100 mL of deionized water and 50 mL of ethanol. Then, 12 mL (0.18 mol) of NH4OH solution was added to the mixture followed by constant stirring for 30-40 min at room temperature. Next, an adequate amounts of TEOS (1.94 mL, 8.4 mmol) and cerium(IV) hydroxide (0.54 g, 0.5 mmol) were added simultaneously to the solution. After stirring at a constant speed of 500 rpm for further 20 h at room temperature, the mixture was subjected to aging for 24 h at 373 K in a convection oven. The resulting product was filtered, washed with an adequate amount

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of deionized water, ethanol and finally calcined at 823 K for 5 h in air. 2.3 Synthesis of mesoporous ceria(III)-silica materials Trivalent ceria-silica composite materials were prepared by reduction of ceria(IV)-silica material under flowing a mixture of H2/N2(7 vol% of H2) gas at 1173 K temperature for 5 h in a furnace. The reduced materials obtained from HCS-50, HCS-30 and CCS-30 were named as HCS-50HR, HCS-30HR and CCS-30HR, respectively, as listed in Table 1. 2.4 Procedure for catalytic oxidation reaction Liquid-phase oxidation reactions of different hydrocarbons and alcohols over various Ce catalysts were carried out in a magnetically stirred 25 mL capacity round bottom flask closed with stopper. In a typical procedure, 0.05 g of catalyst was added to a suspension containing 1 mmol of substrate and 1 g of oxidant (aqueous 30 wt% H2O2 or tert-butyl hydroperoxide, TBHP, 5.0– 6.0 M in decane). The mixture was stirred at room temperature (296 K) for 24 h at a fixed stirring rate. Aliquots of the reaction mixtures were withdrawn periodically for analysis. From the reaction mixtures, the catalyst was separated and the reaction products with unconverted reactants were collected. 2.5 Procedure for catalytic esterification of alcohol The esterification reaction was carried out at 338 K in a 25 mL capacity two-neck roundbottom flask fitted with water condenser and placed in a magnetically stirred heating mantle. For a typical batch of reaction, alcohol (2 mmol) was mixed with acetic acid (6 mmol) and 0.05 g of Ce catalyst was added to this mixture. The mixture was stirred at the particular temperature for 24 h. The products were collected at different time interval for analysis. The obtained aliquots of all catalytic reactions were analyzed as well as identified using a 6890N GC/5975i MS (Agilent, USA) gas chromatography-mass spectrometry (GC/MS) system equipped with HP-5 capillary column. 1 µL sample was injected by a syringe with 7

split ratio of 20:1 to the injector at 553 K and He gas was used as carrier gas at a flow rate of 1 mL/min. For quantitative determination of the reactants and products the area% values for individual peaks were obtained by using system software. Ion source for mass analysis was EI with analysis range of 50~800 m/z. Percentage conversion and product selectivity were calculated as follows: conversion or total conversion (%) = [(peak area of all the products / sum of peak areas of substrates and products)  100], Product selectivity (%) = [(peak area of that particular product / sum of peak areas of all the products)  100]. 2.6 Measurements and calculations A synchrotron radiation source (E = 10.9215 keV, λ = 1.1352 Å) of a 3C beam line in Pohang Accelerator Laboratory (PAL) was used to measure small angle X-ray scattering (SAXS) patterns. Each sample was placed in a copper-alloy sample holder and secured on both sides using Kapton tape. The wide angle X-ray diffraction (XRD) measurements were performed using a PANalytical Empyrean multipurpose diffractometer with Cu-Kα radiation (λavg = 1.5418 Å) at 40 kV and 30 mA in Korea Basic Science Institute (KBSI) Daegu Center. The samples were ground manually and put on the microscope holder at room temperature. The spectra were collected versus 2 θ from 10 to 90 degrees with a scan rate of 0.04 degree/s. Nitrogen adsorption-desorption isotherms of all the samples were measured at 77 K temperature on a Micromeritics 2010 analyzer. The samples were degassed at 393 K under vacuum below 30 mmHg for at least 2 h prior each measurement. The Brunauer-EmmettTeller specific surface areas (S BET) were calculated from N2 adsorption isotherms in the relative pressure range of 0.05-0.2 using a cross sectional area of 0.162 nm2 per nitrogen molecule. The single-point pore volume (Vsp) was estimated from the amount adsorbed at a relative pressure (P/P0) of ~ 0.98. The pore size distributions (PSD) were calculated using 8

adsorption branches of nitrogen adsorption-desorption isotherms by the improved KJS method for cylindrical mesopores [39]. The pore width (Wmax) was obtained at the maximum of the PSD curve. Elemental mapping images for Si, O and Ce were recorded using a field emission SEM (JEOL JSM-4300F) equipped with embedded EDS system operated at an accelerating voltage of 15 kV at the KBSI Seoul center. Cerium weight percentage in the sample was determined by Jobin Yvon inductively coupled plasma (ICP) (ICP-OES, JY Ultima2C) analysis at the KBSI Seoul center adjusting the wavelength to 395.254 nm (for Ce atom). To evaluate the acidic sites of the hexagonal and cubic ceria-incorporated mesoporous silica samples, NH3 temperature-programmed desorption (TPD) experiments were conducted using a Micromeritics Auto Chem II Chemisorption Analyzer (Norcross, GA) that was equipped with a thermocouple detector (TCD). Approximately 30-100 mg of each sample were loaded in a quartz tube microreactor supported by quartz wool and subjected to pretreatment in the range of 303-823K for 10 min before NH3 adsorption, using a heating rate of 10 K/min in flowing helium (at a rate of 50 cm3/min). Next, the samples were cooled to selected temperature (393 K) using heating rate of 10 K/min and exposed to the flowing 5% NH3-He (50 cm3/min) for 60 minutes and finally purged in flowing helium for 30 minutes. In the NH3-TPD experiments, the samples were heated up to 823 K using a heating rate of 10 K/min and kept 60 min. The amounts of desorbed NH3 were obtained by integration of the desorption profiles at the temperature range of 700-840 K and referenced to the TCD signals calibrated for known volumes of analyzed gases.

3. Results and discussions 3.1 Structural and compositional properties of ceria-silica composites

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Fig. 1 shows the SAXS patterns of calcined mesoporous ceria-silica composite samples with Ce/Si ratios 0.3 and 0.5 indicating that both CeIV and CeIII-incorporated composite samples are highly ordered. A 2D hexagonal (p6mm) mesostructure giving three well-resolved peaks indexed as (100), (110) and (200) according to the p6mm symmetry group is observed for HCS-30, HCS-50 and their respective reduced materials (Fig. 1 left panel). The d-spacing (d100) values obtained from the most intense Bragg peak decreased slightly after reduction of CeIV to CeIII species but remained in the range of 3.81 to 3.58 nm for all four samples. The unit cell parameters (a) were evaluated by calculating 2d100/√3 from each d-spacing value and vary in the range of 4.40-4.13 nm. Similarly, the template-free bicontinuous cubic (Ia3d) ordered mesostructures CCS-30 and CCS-30HR were obtained with Ce/Si = 0.3 as shown in Fig. 1 (right panel). Both SAXS patterns show one intense Bragg peak and other seven well-resolved peaks indexed as (211), (220), (321), (400), (420), (332), (422), and (431), respectively, characteristic to

3D

bicontinuous cubic (Ia3d) mesostructure. The Bragg's spacing (d211) values are in the range of 3.65-3.43 nm for all Ia3d cubic samples. The corresponding unit cell parameters (a), calculated from the d211values by using the following equation: a = dhkl(h2 + k2 + l2)1/2 (i.e., a = √6d211), are in the range of 8.97-8.43 nm. The reduced samples showed lower characteristic distances. The SAXS patterns indicate that highly ordered MCM-41 and MCM48-type mesostructures are present in ceria(IV)-silica and ceria(III)-silica composites even with high cerium content and after reduction at high temperature. Essential structural features such as the crystalline nature and the oxidation state of Ce in the ceria-silica composites (which is equivalent to the presence of CeO2 and Ce2O3) have been confirmed by wide angle X-ray diffraction (XRD) as shown in Fig. 2. Left panel in Fig. 2

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shows a typical face-centered cubic phase of ceria: JCPDS no. 034-0394, and right panel shows the chemical formula of Ce4.667Si3O13 (or Ce9.33(SiO4)6O2) according to JCPDS reference number of 043-0441, which has an apatite-like structure with a hexagonal unit cell (a = 9.657 Å and c = 7.121 Å) [17]. X-ray diffraction patterns clearly demonstrate the oxidation number is changed from +4 to +3, which is identical as in our previous report [17]. Detailed analysis for the XRD patterns was described in our previous publication [17]. 3.2 Adsorption properties of ceria-silica composites Nitrogen adsorption-desorption isotherms measured at 77 K for HCS-30, HCS-50, CCS30, HCS-30HR, HCS-50HR, and CCS-30HR samples are shown in Fig. 3 (left panel). Hexagonal and cubic mesostructures with incorporated CeIVO2 possess small amount of micropores, which did not change much after reduction to Ce2IIIO3. Nitrogen adsorption isotherms for the samples studied are type IVc according to IUPAC classification. Capillary condensation and capillary evaporation steps for these samples coincide, which means the lack of hysteresis loop [39]. This type IVc isotherms are typical for highly ordered mesostructures with mesopores below 4. Table 1 shows the adsorption and structural parameters such as the specific surface area, single point pore volume, total pore volume and mesopore diameter for the samples studied. These parameters were calculated as briefly described in the experimental section. All samples studied possessed uniform mesopores, which is reflected by nearly vertical capillary condensation-evaporation steps and the narrow PSD curves (Fig. 3 right panel). The samples exhibited sharp capillary condensation-eva poration steps starting at a relative pressure of about 0.20-0.25. This characteristic feature is observed for both CeIVO2 –silica and Ce2IIIO3-silica mesostuctures. Changes in the shape of consecutive isotherms upon reduction are reflected by changes in the resulting parameters. For instance, the specific surface area decreased from 462 m2 g−1 for HCS-30 to

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390 m2 g−1 for HCS-30HR, and the pore width decreased from 3.5 nm for HCS-30 to 2.9 for HCS-30HR. Furthermore, adsorption isotherms (See Fig.3 left panel) do not level off at relative pressures above 0.8 but slightly rise; this rising is smaller upon reduction, which reflects some diminishment in the textural porosity (secondary disordered mesopores) possibly due to thermal treatment during reduction process. 3.3 SEM-EDS analysis The SEM-EDS images of a representative sample, HCS-50, are shown in Fig. 4. The shape of particles is irregular, but as indicated by the EDS mapping the spatial distribution of Si, O and Ce species is homogeneous. 3.4 NH3-TPD studies of acidic properties The surface acidic properties of the hexagonal and cubic ceria-incorporated mesoporous silica samples were investigated by TPD of NH3. HCS-50, HCS-30, CCS-30, HCS-50HR, HCS-30HR, and CCS-30HR materials were subjected to pretreatment using inert He gas followed by preferential NH3 chemisorption at 393 K and consequently, TPD by ramping temperature up to 823 K. The NH3-TPD profiles for the aforementioned samples studied are shown in Fig. 5, and the total amounts of chemisorbed ammonia (acidity of ceria-silica) are shown in Table 2. These profiles possess desorption peak at the range of 783-803 K which is attributed to strongly adsorbed NH3 species. This broad peak on the NH3-TPD profiles in Fig. 5 indicates the existence of a wide range of basic sites on the surface of the samples studied. At this desorption temperature range, adsorbed NH3 species are desorbed. However, the strength of acidic sites on CeIVO2-silica mesostuctures, estimated on the basis of the maximum desorption temperature of NH3, was reduced upon reduction. All the Ce2 IIIO3silica samples have less acidic sites as compared to CeIVO2 –silica, which indicates that these samples are more suitable for selective adsorption and catalysis involving acidic species. This

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result agrees with the fact that the acidity of metal oxides increases with increasing oxidation number of the metal. Adsorption capacity of NH3 that is a measure of the surface acidity decreased with increasing ceria percentage in mesoporous ceria-silica samples [40]. The NH3 uptake changes from 2.03 mmolg−1 for HCS-30 to 1.72 for HCS-50. This uptake is further lowered for HCS-50HR (1.08 mmol g-1; Table 2), indicating less acidic sites. There is no substantial change in the NH3 uptake for the cubic-ceria mesostructures as compared to the hexagonal ceria-silica counterparts. Overall NH3 adsorption capacity decreased in the following order: HCS-30>CCS-30>HCS-30HR>HCS-50>CCS-30HR>HCS-50HR manner. The amounts of desorbed NH3 were obtained by integration of the TPD profiles. 3.5 Catalytic oxidation reactions A general scheme for catalytic oxidation of various hydrocarbons like alkanes, alkenes and alcohols over ceria(IV) and ceria(III) catalysts are shown in Scheme 1. The reactions were carried out without any additional solvent in the presence of both oxidants hydrogen peroxide and tert-butyl hydroperoxide (TBHP) at room temperature (296 K). The result of conversion of different substrates over hexagonal ceria(IV)-silica (HCS-50) is shown in Table 3. A comparison of the oxidation of alkenes (e.g., styrene, cyclohexene), alkanes (e.g., ethyl benzene, toluene), and alcohols (e.g., benzyl alcohol) showed the highest conversion for cyclohexene (about 91%; Table 3, Entry 2), while benzylic substrates exhibited lower conversion. In the presence of mild oxidant H2O2 the steric factor dominates, which is reflected in the yield of various products. In order to ensure the catalytic role of the ceria in the catalysts studied blank reactions were also performed with a pure MCM-41 (Entry 6) and without any catalyst (Entry 7) at identical conditions, giving only about 7% conversion and no conversion, respectively.

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The effect of loading of cerium and also its oxidation state in silica is shown in Table 4 by carrying out the same oxidation reaction using six different ceria-silica catalysts. The oxidizing capability of the CeO2-silica catalysts is higher than that of the reduced counterparts as indicated by the poor yield of aldehyde obtained from styrene. Hexagonal Ce2O3-silica catalysts containing 50 and 30% of Ce (HCS-50HR and HCS-30HR) showed conversions ca. 22.01% and 13.01%, respectively, which are much lower than those for the respective ceria(IV) samples. A comparison of the catalytic activity of HCS-50 and HCS-50HR in the presence of H2O2 with respect to time is presented in Fig. 6. In the case of both hexagonal CeO2-silica samples the amount of oxidized products decreases (Table 4, Entries 1 and 3) with decreasing Ce loading in the material. For cubic ceria-silica samples with 30% Ce loading (CCS-30) the oxidation amount is higher than that for the respective hexagonal sample (HCS-30) with benzaldehyde as a single product. A little higher conversion of styrene over cubic mesostructures as compared to hexagonal counterparts can be attributed to higher surface area of the former one at similar ceria loadings. Data in Table 4 suggest that ceria(IV)-silica catalysts are more preferable than the counterparts for the oxidation reactions in presence of H2O2 oxidant. The effect of a strong oxidant like TBHP is shown in Table 5. In this case the yield of styrene oxidation is higher for catalysts with ceria in both oxidation states (HCS-50 and HCS-50HR) and the difference in their oxidizing ability almost vanished as compared to that observed in the presence of benign H2O2. Thus, the TBHP oxidant plays a major role as indicated by similar yield values of the products obtained by styrene oxidation (Table 5, Entries 2 and 3). A comparison of the oxidizing activity of the ceria-silica with the other conventional oxidation catalysts such as ceria-titania reported in literature [41] shows that in the latter case

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only 3.4% of toluene was partially oxidized at room temperature under flowing O2 gas [41], while the catalysts studied in this work exhibited conversion of ~39% in the presence of H2O2 at room temperature. Though a complete conversion of toluene was reported over noble metal (Au) loaded metal oxide-titania, but this process was performed in a high temperature reactor under extreme conditions [42,43]. 3.6 Esterification reaction Esterification of alcohols occurs generally in the forward direction when an excess amount of ester, in this case acetic acid, is employed. In this study, alcohol and acetic acid (in excess) were used in 1:3 molar ratio to carry out the reactions over various ceria-silica catalysts (Scheme 2). The conversions obtained for different primary and secondary alcohols to their respective products at 338 K temperature are shown in Table 6. Blank reactions were carried with pure silica (Entry 8) and without any catalyst (Entry 9), giving about 7% conversion only and a very small conversion (about 1%), respectively. This result clearly indicates the major role of cerium species in the ceria-silica composite catalysts in this esterification reaction. Highly ordered hexagonal HCS-50 showed good conversions of benzyl alcohol, its methoxy and nitro derivatives to the corresponding acetates. For long chain aliphatic alcohols such as octanol and decanol, the conversion values are lower than those benzyl-containing alcohols. The conversion decreases with increasing length of the carbon chain due to steric effects. Esterification of secondary alcohols such as cyclohexanol (Entry 6) was also performed over HCS-50 with a lower yield under identical conditions. A comparison of the catalytic activity of all ceria-catalysts for esterification of benzyl alcohol is presented in Table 7. Depending on the ceria loading in the composite catalysts from HCS30 to HCS-50, a rising trend in the percentage yield of benzyl ester is observed because the esterification reaction studied is catalyzed by the surface basic sites of CeO2 [29], the 15

concentration of which increases with increasing Ce content as evidenced by NH3-TPD data. However, a comparison of the performance of the catalysts with similar loading of ceria but with different oxidation states shows that HCS-50HR exhibits slightly lower activity than HCS-50, as observed in the case of HCS-30 vs. HCS-30HR and CCS-30 vs. CCS-30HR. The reason of this behavior can be higher surface area as well as slightly larger pore diameter in CeIVO2-silicathan in the corresponding reduced samples (Table 1). Moreover, the samples reduced in H2 under severe conditions became lightly fragile and apatite Ce4.667Si3O13 structure with hexagonal unit cell is formed, which could not compete with symmetric cubic CeO2 structure [17] in terms of the percentage of ester conversion (see data for the HCS50HR and HCS-50 samples in Table 7) [41]. Fig. 7 shows the kinetic plots for ester formation over the aforementioned two catalysts. Analogous esterification was also investigated at room temperature and the corresponding data are provided in Table 8. These data show that the conversion decreases with lowering temperature. In the case of HCS-50 and HCS-50HR, both having similar percentage of Ce and in spite of high basicity of HCS-50HR, the difference between framework ceria species plays the dominating role as revealed by the respective percentages of ester yields (Entries 1 and 2 in Table 8) [44]. The advantage of using ceria-silica over conventional Al-MCM-41 mesoporous materials reported in literature is the possibility of alcohols esterification at much lower temperature instead of high temperatures (around 423 K-523 K) applied in the case of Al-MCM-41 [4547]. Note that Al-MCM-41 is efficient for esterification of short chain alcohols, whereas conversion of alcohols with long chains such as n-octanol, n-decanol to esters is quite satisfactory (Entries 4 and 5 in Table 6). For both catalytic oxidation and esterification reactions the reusability of the catalysts was examined. The catalysts were filtered from

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reaction mixtures, washed several times with ethanol and acetone, and then dried at 373 K temperature oven for a few hours. The recovered catalysts HCS-50 and HCS-50HR showed almost the same activity towards the conversion of styrene (in the case of oxidation) and benzyl alcohol (in the case of esterification) after being reused for three times. The reusability test for oxidation of styrene using HCS-50 in the presence of H2O2 is shown in Fig. 8. The Ce content in the HCS-50 sample after catalytic runs as estimated by ICP-AES was 40 wt%. As compared to the Ce content of 43 wt% [17] before catalytic runs, the result represents ceria nanoparticles that are very stable even after catalytic reactions. Hence a very low concentration of Ce leaching (~ 3%) is observed after repeated catalytic reactions.

4. Conclusions Mesoporous CeIVO2-silica composites were reduced to Ce2IIIO3-silica counterparts using H2. Both types of composite catalysts with up to 43 wt% of Ce content showed very good performance for oxidation of hydrocarbons and hydroxycarbons in the presence of H2O2 or TBHP as well as for direct esterification of various alcohols in the presence of acetic acid under mild conditions. The NH3-TPD data show clearly that the basicity of the materials increases gradually with increasing amount of cerium in silica; this effect is reflected in the esterification results obtained for the HCS-50 and HCS-30 ceria-silica samples. A comparative study has been made to find out the efficiency of CeO2 and Ce2O3 towards the catalytic solvent-free oxidation of styrene, cyclohexene, ethyl benzene, toluene, benzyl alcohol showing that CeO2-silica is more suitable for such reactions. On the other hand, analogous studies on the direct esterification reactions of primary alcohols such as benzyl alcohol, its derivatives, long chain aliphatic alcohols like n-octanol, n-decanol and secondary alcohols viz. cyclohexanol in the presence of acetic acid show a decent conversion to

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respective esters under mild conditions. Acknowledgements E.-B. Cho acknowledges partial support under the New & Renewable Energy R&D program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) provided by the Korea Government Ministry of Trade, Industry, and Energy (No. 20113020030040) and by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2012-R1A1A-2000855). D. Kim was supported by

the

Korean

National

Research

Foundation

(NRF-2012-R1A2A1A-05026313).

Experiments at PLS were supported in part by MSIP and POSTECH.

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24

Captions for Figures Figure 1

Synchrotron SAXS patterns for (left) hexagonally (p6mm) structured and (right) cubic (Ia3d) ceria(IV)-silica and ceria(III)-silica composites particles prepared under basic conditions.

Figure 2

Wide angle X-ray diffraction patterns of mesoporous ceria-silica composites prepared under basic conditions. The sequence of the XRD patterns from (a) to (b) refer to the following samples: (a-A) HCS-30, (a-B) HCS-50, (a-C) CCS-30, (b-A) HCS-30HR, (b-B) HCS-50HR, and (b-C) CCS-30HR.

Figure 3

Nitrogen adsorption isotherms (left panel) and the PSD curves (right panel) for hexagonal and cubic Ce-Si samples; 3&4 and 5&6 isotherm pairs are shifted by 200 and 300 cm3 STP/g, respectively from 1 & 2. PSD curves are shifted by multiple of 0.6 cm3 g-1 nm-1.

Figure 4

SEM-EDS images of HCS-50 mesoporous ceria-silica composite sample.

Figure 5

NH3 TPD profiles of hexagonal and cubic ceria- silica samples at 393 K.

Figure 6

A comparative kinetic study of oxidation of styrene in presence of H2O2 using the catalysts (a) HCS-50 and (b) HCS-50HR.

Figure 7

A comparative kinetic study of esterification of benzyl alcohol in presence of acetic acid using the catalysts (a) HCS-50 and (b) HCS-50HR.

Figure 8

The reusability test up to three cycles for oxidation of styrene using HCS-50 in presence of H2O2.

25

Table 1. Physicochemical parameters determined from N2 sorption isotherms.a Sample

Vsp (cc/g)

S BET (m2/g)

Wmax (nm)

Vt (cc/g)

HCS-30

0.39

462

3.5

0.41

HCS-50

0.29

302

3.2

0.31

CCS-30

0.43

481

3.3

0.46

HCS-30HR

0.22

390

2.9

0.25

HCS-50HR

0.25

307

3.1

0.28

CCS-30HR

0.29

446

2.9

0.34

a

Notation: Vsp = Single point pore volume calculated at P/P0 = 0.98; SBET = Specific surface area calculated from adsorption data in relative pressure range 0.05-0.20; Wmax = Pore width calculated at the maximum of PSD, using improved KJS method; Vt = Total pore volume calculated by integration of the PSD curve;

26

Table 2. Amount of NH3 adsorbed by hexagonal and cubic ceria- silica samples at 393 K. a Sample name

NH3 Uptake (mmol/g)

HCS-50

1.72

HCS-50HR

1.08

HCS-30

2.03

HCS-30HR

1.73

CCS-30

1.92

CCS-30HR

1.71

a

The amounts of desorbed NH3 was obtained by integration of the desorption profiles during TPD.

27

Table 3. Results of oxidation of various substrates over hexagonal mesoporous Ce-silica catalyst.a Entry

Catalyst

Substrate

Oxidant

1

HCS-50

Styrene

H2O2

Products

Yield (%)

Benzaldehyde and phenyl

2

HCS-50

Cyclohexene

H2O2

42.24 acetaldehyde 2-Cyclohexen-1-one

91.22

Benzaldehyde and 3

HCS-50

Ethyl benzene

H2O2

4

HCS-50

Toluene

H2O2

Benzaldehyde

39.44

5

HCS-50

Benzyl alcohol

H2O2

Benzaldehyde

29.55

acetophenone

25.86

Benzaldehyde and phenyl 6

b

7c

MCM-41

-

Styrene

Styrene

H2O2

H2O2

a

7.46 acetaldehyde -

-

Reaction conditions: substrate = 1.0 mmol, oxidant = 1.0 g, catalyst = 0.05 g, T = 296 K, time = 24 h. b reaction carried out with pure MCM-41. c Blank test performed without any catalyst.

28

Table 4. Results of oxidation reactions over various mesoporous Ce-silica catalysts.a Entry

Catalyst

Substrate

Oxidant

1

HCS-50

Styrene

H2O2

Products

Yield (%)

Benzaldehyde and phenyl

2

HCS-50HR

Styrene

H2O2

42.24 acetaldehyde Benzaldehyde and phenyl

22.01

acetaldehyde Benzaldehyde and phenyl

3

HCS-30

Styrene

H2O2

28. 77 acetaldehyde Benzaldehyde and phenyl

4

HCS-30HR

Styrene

H2O2

13.01 acetaldehyde

5

CCS-30

Styrene

H2O2

Benzaldehyde

39.12

6

CCS-30HR

Styrene

H2O2

Benzaldehyde

16.94

a

Reaction conditions: substrate = 1.0 mmol, oxidant = 1.0 g, catalyst = 0.05 g, T = 296 K, time = 24 h.

29

Table 5. Effect of oxidant on the catalytic oxidation of styrene over different ceria-silica materials.a Entry

Catalyst

Substrate

Oxidant

1

HCS-50

Styrene

H2O2

Products

Yield (%)

Benzaldehyde, and phenyl 42.24 acetaldehyde Benzaldehyde, styrene 2

HCS-50

Styrene

TBHP

oxide and phenyl

67.05

acetaldehyde Benzaldehyde, styrene 3

HCS-50HR

Styrene

TBHP

oxide and phenyl

66.85

acetaldehyde a

Reaction conditions: substrate = 1.0 mmol, oxidant = 1.0 g, catalyst = 0.05 g, T = 296 K, time = 24 h.

30

Table 6. Result of catalytic esterification of various alcohols over ceria-silica material.a Entry

Catalyst

Alcohol

Product

Yield (%)

1

HCS-50

Benzyl alcohol

Benzyl acetate

79.34

2

HCS-50

4-Nitrobenzyl

4-Nitrobenzyl

32.88

alcohol

acetate

4-Methoxybenzyl

4-Methoxybenzyl

alcohol

acetate

3

HCS-50

57.91

4

HCS-50

n-Octanol

n-Octyl acetate

67.51

5

HCS-50

n-Decanol

n-Decyl acetate

56.33

6

HCS-50

Cyclohexanol

Cyclohexyl acetate

30.40

7b

HCS-50

Benzyl alcohol

Benzylpropionate

56.62

8c

MCM-41

Benzyl alcohol

Benzyl acetate

7.21

9d

-

Benzyl alcohol

Benzyl acetate

1.09

a

Reaction conditions: alcohol: acetic acid (AA) = 1:3 molar ratio, catalyst = 0.05 g, T = 338 K, time = 24 h. b reaction carried out with propionic acid. c reaction carried out with pure MCM-41. d blank test performed without any catalyst.

31

Table 7. Result of catalytic esterification of alcohols over different ceria-silica materials.a Entry

Catalyst

Alcohol

Product

Yield (%)

1

HCS-50

Benzyl alcohol

Benzyl acetate

79.34

2

HCS-50

n-Octanol

n-Octyl acetate

67.51

3

HCS-30

Benzyl alcohol

Benzyl acetate

69.40

4

CCS-30

Benzyl alcohol

Benzyl acetate

68.40

5

HCS-50HR

Benzyl alcohol

Benzyl acetate

76.18

6

HCS-50HR

n-Octanol

n-Octyl acetate

56.73

7

HCS-30HR

Benzyl alcohol

Benzyl acetate

36.10

8

CCS-30HR

Benzyl alcohol

Benzyl acetate

29.88

Reaction conditions: alcohol: acetic acid (AA) = 1:3 molar ratio, catalyst = 0.05 g, T = 338 K, time = 24 h. a

32

Table 8. Effect of temperature on the catalytic esterification of alcohols .a

a

Entry

Catalyst

Temperature (K)

Alcohol

Product

Yield (%)

1

HCS-50

296

Benzyl alcohol

Benzyl acetate

52.95

2

HCS-50HR

296

Benzyl alcohol

Benzyl acetate

35.14

3

CCS-30

296

Benzyl alcohol

Benzyl acetate

50.54

Reaction conditions: alcohol: acetic acid (AA) = 1:3 molar ratio, catalyst = 0.05 g, time = 24h.

33

Table 9. Comparative catalytic performance of reported oxidation and esterification catalysts with our materials. Reaction involved

Catalyst

Reactant

Temp (K)

Oxidant/Acid

Conversion (%)

Ref.

Ceria-titania

Toluene

RT

Flowing O2

3.4

[41]

Au-loaded ceria/titania

Toluene

Very high

Air

~100.0

[42]

CexTiO2

Toluene

Very high

O2

~100.0

[43]

Ceria-silica

Toluene

RT

H2O2

~39.0

This work

Al-MCM-41

Butanol

398

Acetic acid

~80.0

[45]

Al-MCM-41

Amyl alcohol

523

Acetic acid

100.0

[46]

Al-MCM-41

Propanol

473

Acetic acid

~85.0

[47]

Ceria-silica

Octanol

338

Acetic acid

67.51

This work

Oxidation

Esterification

34

Figure 1 (Pal et al.)

35

Figure 2 (Pal et al.)

36

Figure 3 (Pal et al.)

600

1 2 3 4 5 6

5 500

HCS-30 HCS-30HR HCS-50 HCS-50HR CCS-30 CCS-30HR

6

3 400 5

PSD (cm3 g-1 nm-1)

3

3

Volume adsorbed (cm STP/g)

6

4

300

4

2 1

3

200

1 2 100

2 1 0

0 0.0

0.2

0.4

0.6

0.8

Relative pressure

37

1.0

2

3

4

5

Pore w idth (nm)

6

Figure 4 (Pal et al.)

38

Figure 5 (Pal et al.)

TCD Signal (a.u)

1 2 3 4 5 6

HCS-30 HCS-30HR HCS-50 HCS-50HR CCS-30 CCS-30HR

1

793 K

2

3

6

5

4 400

500

600

700

Temperature (K)

39

800

Figure 6 (Pal et al.)

45

Conversion (%)

40 35

(a) HCS-50

30 25 20 (b) HCS-50HR

15 10 5 0 0

5

10

15

Time (h)

40

20

25

Figure 7 (Pal et al.)

80 (a) HCS-50

Conversion (%)

70 60

(b) HCS-50HR

50 40 30 20 10 0 0

5

10

15

Time (h)

41

20

25

Figure 8 (Pal et al.)

1st 2nd 3rd

40

Conversion (%)

35 30 25 20 15 10 5 0

0

4

8

12

16

Time (h)

42

20

24

Scheme 1 (Pal et al.)

Scheme 1: General Scheme for catalytic oxidation reaction of alkane, alkene or alcohols over ceria catalyst.

43

Scheme 2 (Pal et al.)

Scheme 2: General Scheme for esterification reaction of alcohol with acetic acid over ceriasilica sample.

44

Graphical Abstract

CeIVO2 and Ce2IIIO3-silica composite mesoporous materials are proved to be highly efficient heterogeneous, reusable catalyst for solvent-less oxidation of different hydrocarbons at room temperature as well as acylation of various aromatic and aliphatic alcohols in presence of acetic acid under mild condition.

45

Highlights




Scale-up synthesis was prepared for mesoporous silica/CeIVO2 and Ce2IIIO3-silica composites.




Ce/Si molar ratio was obtained up to 0.5 maintaining ordered mesostructure.




Esterification of various alcohols shows high performance under mild temperature conditions.




Solvent-free oxidation of hydrocarbons shows high performance at room temperature.




Acylation of various alcohols shows high performance in the presence of acetic acid.

46