Promoting effect of cerium on the characteristic and catalytic activity of Al, Zr, and Al–Zr pillared clay Academic research paper on "Chemical sciences"

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Applied Clay Science

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Research paper

Promoting effect of cerium on the characteristic and catalytic activity of Al, Zr, and Al-Zr pillared clay^

S. Mnasri-Ghnimi a,\ N. Frini-Srasra a,w

a Laboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications, Centre National des Recherches en Sciences des Matériaux, Technopole Borj Cédria, BP 95-2050 Hammam Lif, Tunisia b Départements de Chimie, Faculté des Sciences de Tunis, 1060 Tunis, Tunisia

ARTICLE INFO

ABSTRACT

Article history:

Received 9 March 2013

Received in revised form 17 September 2013

Accepted 30 October 2013

Available online 11 January 2014

Keywords: Pillared clay Single pillars Mixed pillars Acidity Cerium Acetalization

A series of pillared interlayered clays (PILCs) including Al-, Zr- and Al-Zr-PILC have been prepared and characterized by X-ray diffraction, elemental analyses, N2 adsorption, cationic exchange capacity and IR measurements after n-butylamine adsorption. Cerium introduced in the Zr4+ and/or Al3+ intercalated solution allows for an improvement of the stability and crystallinity of PILC and creates pillared clays with new properties. The resulting materials were used for the synthesis of 1,3-dioxolane. The addition of cerium has a major influence in this reaction.

© 2013 The Authors. Published by Elsevier B.V. All rights reserved.

1. Introduction

Although clays are very useful for many applications in the field of catalysis, adsorption and ion exchange, they have one main disadvantage: their lack of permanent porosity. Smectites swell upon hydration, but upon severe dehydration (heating) the layers collapse and the inter-layer surface becomes no longer accessible for chemical processes. To avoid this problem, researchers found a way to prop open the clay layers by the introduction of stable pillars in the interlayer region. Pillared clays (PILCs) are an interesting class of2-dimensional microporous materials. These materials are prepared by the intercalation of organic or inorganic compounds between the silicate layers of the clay, resulting in an increase of basal spacing, pore volume and specific surface area. Due to their high specific surface area and permanent porosity they are very attractive solids for adsorption and catalysis purposes. PILCs are not sufficiently stable and collapse with a loss of specific surface area and catalytic activity at high temperatures. To avoid sintering of the pillared clay, the thermal stability of the pillars must increase. One way to achieve this goal is to introduce mixed-oxide pillars; A1-Ga (Coelho and Poncelet, 1991), A1-Zr (Canizares et al., 1999; Occelli, 1986), A1-Fe (Bergaya et al., 1993; Lee etal., 1989; Oades, 1984), and

☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

E-mail addresses: saidamnasri@gmail.com (S. Mnasri-Ghnimi), nfrini@gmail.com (N. Frini-Srasra).

1 Tel.: +216 71 430 470; fax: +216 71 430 934.

Al-Cu (Frini et al., 1997). Another appropriate way to overcome such a problem of collapse is the addition of rate earth. Sterte (1991) found that the incorporation of lanthanide elements in the preparation of the pillaring agent resulted in materials whose basal spacing was greater than that of conventional materials. An improvement of the thermal stability of Al-pillared clays was reported by Tokarz and Shabtai (1985), who prepared pillared clay catalysts by first exchanging the clay with Ce3+ or La3+, then exchanging these clays with refluxed, partly hydro-lyzed Al3+ solutions. The migration of the protons from the interlayer to the clay octahedral sheets and the hydrolysis of structural Si-O-Al bonds by protons are largely prevented by the presence of the Ce3 + ions (Tichit et al., 1991). The addition of rate earth improves not only the thermal stability but also the adsorptive and catalytic properties of the pillared products. The presence of Ce3+ in Al-pillared clays produces a substantial increase in the conversion of n-heptane and in the selectivity towards cracking products owing to the increase in the number of acid centers (Hernando et al., 1996). Similarly, the presence of the Ce3+ ions in the Zr-pillared clays has been found to improve its catalytic activity for cyclohexanol dehydration by preserving the Bronsted acid center (Mishra and Rao, 2003).

On the other hand, acetals are an important class of compounds that have found direct applications in diverse areas in the chemical industry such as in perfumes (Bauer et al., 1990; Yang et al., 2005), flavors (Clode, 1979), solvents (Ley and Priepke, 1994), pharmaceuticals (Franchini et al., 2010; Luo et al., 2000), and polymer chemistry (Stao et al., 1990). Acetals are commonly used in protecting group chemistry for carbonyl functional groups because dimethyl acetals and 1,3-dioxolanes are stable under neutral and basic conditions (Chapuzet et al., 2001;

0169-1317/$ - see front matter © 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.10167j.clay.2013.10.030

Greene, 1981). Acetalization reaction is generally performed in the presence of protic acid catalysts commonly sulfuric acid, hydrochloric acid, phosphoric acid and p-toluenesulfonic acid (Greene, 1981; Ogata and Kawasaki, 1970). These homogenous catalysts are harmful and corrosive. In recent years, green catalysis has been found to be the method of choice for the production of acetals. Acidic clays have many advantages over other catalysts, such as facile modification of acidity via the exchange of cations in the interlayer, ease of handling, non-corrosiveness, low cost and ability to be regenerated. Moreover, they are environmentally benign. The present study aims to evaluate the promoting effect of cerium on the structural and textural properties as well as on the catalytic activity of Zr- and/or Al pillared clays. The characterization of catalyst was performed by X-ray diffraction, N2 adsorption, cationic exchange capacity, chemical analysis, and FTIR spectroscopy.

2. Experimental

2.1. Materials

The starting material for the pillaring procedure was the sodium form of a purified Tunisian interstratified illite/smectite (Na-bentonite). Its chemical composition expressed in the oxide form/100 g of the calcined sample, is SiO2 61.38, Al2O3 24.80, Fe2O3 8.03, Na2O 3.06, MgO 1.38, CaO 0.13, and K2O 1.40 with a structural formula of [Si743 Al057]IV

[Al2 .96 Fe0.73 Mg0.24] Na0.71 K0.21 Ca0.01; O22. The cation exchange capacity (CEC) of Na-bentonite is 78 meq/100 g. Its specific surface and micropore volume are respectively 107 m2 g-1 and 0.06 cm3 g-1.

2.2. Synthesis ofPILC with single and mixed oxide pillars

PILCs were synthesized by cationic exchange reaction of sodium clays with hydroxy-oligomeric solutions of simple cations (Al3+ or Zr4+) or mixed ones (Al-Zr, Ce-Zr, Ce-Al or Ce-Al-Zr). Zirconium tetrachloride (ZrCl4) is used as a source of zirconium polycations. In spite of the extensive literature reported about PILC synthesis using zirconium oxychloride ZrOCl2, and zirconyl hydroxyacetate, there is practically few papers that reported the use of zirconium tetrachloride (ZrCl4) as a source of zirconium polycations (Dominguez et al., 1998). The aluminum pillaring solution was prepared using Al (NO3)3-9H2O as the precursor salt. The source of cerium was Ce (NO3)3-6H2O. Pillaring solutions of different cations were prepared by slowly adding of a basic solution (NaOH 0.2 M) to the corresponding cationic solution (Al3+, Zr4+, Al-Zr, Ce-Zr, Ce-Al or Ce-Al-Zr) (0.1 M) under constant stirring at home temperature until a desired pH was reached (Table 1). In the case of zirconium polyoxocations, the pH was 2.8. This value of pH is the most useful in the literature (Awate et al., 2004; Dominguez et al., 1998). The obtained solution was aged under stirring at room temperature for 24 h. Then, the pillaring solution was added drop wise to a 1 mass% bentonite suspension in distilled water with a metal/clay ratio equal to 10 mmol g-1. The mixture was allowed to react at room temperature for 24 h. After intercalation, the resulting products were centrifuged, washed by dialysis with distilled water, dried at 77 °C and finally calcined for 2 h at 550 °C. The samples are labeled as a function of the nature of pillars (Zr, Al, Al-Zr, Ce-Zr, Ce-Al or Ce-Al-Zr).

2.3. Catalytic study

The acetalization reaction of 7.68 g (124 mmol) of ethylene glycol and 3.64 g (63 mmol) of acetone in the presence of 0.1 g ofPILC was

Table 1

pH used for the synthesis ofPILC with single and mixed oxide pillars.

Samples Al-G Zr-G Al-Zr-G Ce-Al-G Ce-Zr-G Ce-Al-Zr-G

pH 4.1 2.8 3.8 4.1 2.8 3.8

carried out in the autoclave (100 cm3) at 40 °C under autogenously pressure and without solvent. Then, the reaction mixture was cooled and filtered to separate the clay catalyst. The filtrate was treated with 20 ml of distilled water (3 times) to eliminate the residual ethylene gly-col. The organic phase was extracted with diethyl ether. After evaporation of the solvent (ether and acetone) the 2,2-dimethyl-1,3-dioxolane was recuperated.

2.4. Characterization methods

The X-ray diffraction (XRD) study was done in a 'PANalylitical X'Pert HighScore Plus' device, which operates with Cu Ka radiation.

N2 adsorption-desorption experiments were carried out at —196 °C on a Quantachrome, USA instrument. The N2 isotherms were used to determine the specific surface areas (SAs) using the BET equation. The micropore volume was determined using the t-plot method and the total pore volume of the samples, Vt, was calculated at P/P0 = 0.99. Before each measurement the samples were outgassed for 2 h at 130 °C.

The chemical analysis of the starting material and modified samples was determined by atomic adsorption, the spectrometer used is of the type AAS Vario.

Cation exchange capacity was determined by Kjeldhal method. Samples of 200 mg were exchanged with the ammonium acetate (1 M) three times and then washed with anhydrous methanol; a final wash was performed with deionized water three times. The amount of ammonium retained was determined using a unit Kjeldhal. The CEC is expressed as milli-equivalent per gram of the calcined sample.

Bronsted and Lewis acid centers were determined by FT-IR spectroscopy method on the basis of adsorption of butylamine. With this method 10 ml of prepared butylamine in a cyclohexane solution was added to 0.1 g of catalyst. The mixture was shaken at room temperature. After drying, each sample was calcined at different temperatures. FT-IR spectra were recorded in the region1800-400 cm-1 on a Perkin-Elmer infrared Fourier transform spectrometer using the KBr pellet technique.

For the 1H NMR and 13C NMR study, different spectra were recorded at respectively 300 MHz and 75 MHs on a Bruker AM 300 spectrometer using 5 mm outer diameter spinning sample tubes. Temperature was fixed at 25 °C using a Bruker VT 1000 variable temperature control unit, measured by a calibrated Pt-100 resistance thermometer. The chemical shifts are given in ppm with respect to external TMS reference at 0 ppm.

3. Results and discussion

3.1. Characterizations of the pillared clays 3.1.1. XRD

Fig. 1 shows the XRD patterns of samples obtained by Zr-, Al- and Al-Zr-bentonite with and without incorporation of cerium. The inter-layer spacing in the sample of Na-bentonite increases from 12 to 18.2 after Zr-pillared treatment, indicating an expansion of the clay layer during the pillaring process. This proves that the bigger Zr oligomer must be located in the interlayer space of the bentonite layer and causes structural changes. Furthermore, the interlayer spacing of Zr-G increases from 18.2 to 19.3 after cerium adding into Zr-pillared bentonite. Concerning crystallinity, it can be seen that, the Zr-G sample shows a wider and broad peak: the broadness of 001 diffraction peak mostly reported in the literature (Colin et al., 2005; Maes et al., 1997) indicates the heterogeneity in the size of the pillars and possible delamination of the clay structure due to the low pH of the Zr oligomeric solution. This is not evident for the Ce-Zr-G sample which presents a sharp (001) peak showing that the layers of the clay are homogenously spaced. The increase observed in the basal spacing value and crystallin-ity of Ce-Zr-G shows that favorable conditions are met to form zirconium pillars by conventional treatment because of the presence of cerium. This observation has been reported by Carriazo et al. (2007), Chen et al.

(2009) and Tomul (2011). The incorporation of cerium in the aluminum intercalated solution produces a basal spacing of 18.7 A, clearly stable after calcination at 550 °C indicating that the layer structure of clay materials is retained at a high temperature because of the structural rigidity imparted by the oxide pillars. This value of d001 is slightly higher than the d001 of the aluminum pillared clay (17.4 A). Nevertheless, no significant change was observed at the crystallinity. In fact the Ce-Al-G shows a similar behavior to the Al-pillared one. Mixing Zr or Al with Ce in the pillaring solution seems to increase the basal spacing value of Zr-, and Al-PILC. However, no considerable differences could be observed, in the basal spacing value, comparing the Ce-Zr-Al-PILC sample with those without addition of cerium. The spacing was 19.2 and 19.15 for the Zr-Al-PILC and Ce-Zr-Al-PILC samples respectively. But, the introduction of cerium in the mixed Zr-Al pillaring solution has a beneficial effect in the crystallinity of the resulting materials. In fact an acute peak centered at 19.15 appears in the XRD patterns and indicates the homogenous intercalation of the polymerization of the Ce-Zr-Al species. As shown in Fig. 1, it can be clearly seen that Al-, Zr- and Zr-Al-PILC show a beneficial effect of cerium on physical characteristics. It is interesting to report that in our samples no characteristic peaks of the ceria phase were observed, indicating the possibility that there was such a minor part of ceria outside the clay layers that it is undetectable by XRD. So it is suggested that most of the ceria existed in the inter-layer spacing. This observation was reported by Chen (2009), Melo et al. (2002) and Zuo et al. (2008), who reported that no visible peak attributed to the cerium oxide crystal phase can be observed in Ce-Zr-Mt and they suggested that most of the cerium oxide may be transferred into the internal layer of Zr-MMT and part of the cerium oxide exists in the form of highly dispersed particles.

3.1.2. Chemical analysis

The chemical compositions of all the samples are presented in Table 2. Pillaring of the starting bentonite by Zr-, Al- or Al-Zr pillars resulted in an increase in the ZrO2 and/or Al2O3 contents with the concomitant replacement of the interlayer cations, such as calcium and sodium. An increase of ZrO2 and/or Al2O3 contents with a corresponding decrease in the amount of exchangeable cations points to the successive replacement of the interlayer cations with Zr, Al or Al-Zr pillars. In the

case of samples modified with cerium, the introduction of cerium in the samples is revealed, showing a practically greater value in solids modified with the simple system Al-Ce (0.42%) or Zr-Ce (0.55%), while materials modified with the mixed system Zr-Al-Ce show a value of 0.30%. The same behavior was observed by Perez et al. (2008). In fact they reported that the amount of cerium in solids modified with the mixed system Al-Ce-Fe is practically lower than that in the material modified only with Al-Ce, and they explained this difference on the cerium amount introduced by mixed and simple oxide pillars to a competitive phenomenon between iron and cerium during the intercalation process, taking about the low cerium quantities introduced in the synthesis process. Much less Ce is incorporated into the pillars than originally present in the solution. Carriazo et al. (2005) observed the same behavior when pillaring a bentonite with a Ce-Al polycation and they suggested that this low amount of cerium could be explained from the structure proposed for the Al-Ce polycation, different from the simple structure for the Keggin ion (Gil et al., 2000; Hernando et al., 2001) in which a Ce atom in tetrahedral coordination is surrounded by four Ali3 units, giving the possibility of generating pillars with a low Ce content. The amount of SiO2 and Al2O3 after adding with cerium remained almost constant. This result indicates that the composition of the clay layer is preserved in the pillared bentonite samples.

3.1.3. Textural properties

Table 3 presents the textural properties of starting and modified clays. A higher increase both in the specific surface area as in the

Table 2

Chemical composition of Na-bentonite and the pillared clays.

SiÛ2 AI2O3 Fe2Û3 MgO CaO K2O Na2Û Z1O2 CeÛ3

Na-bentonite 61.38 24.80 8.03 1.38 0.13 1.40 3.06 0 0

Zr-G 52.09 20.34 7.59 1.36 0.11 1.19 0.64 16.71 0

Ce-Zr-G 51.84 20.18 7.50 1.34 0.11 1.17 0.61 16.57 0.55

Al-G 55.84 34.60 6.96 0.91 0.10 0.95 1.11 0 0

Ce-Al-G 55.75 34.11 6.91 0.89 0.07 0.92 1.09 0 0.42

Al-Zr-G 47.27 26.48 5.91 1.37 0.13 1.27 0.46 17.31 0

Ce-Al-Zr-G 47.80 26.11 5.82 1.28 1.12 1.14 0.43 16.96 0.30

Table 3

Textural properties of different samples.

SBET Sext S smp Vt Vp

(m2 g-1) (m2g-1; 1 (m2 g-1) (cm3 g-1 ) (cm3 g-1)

Na-bentonite 107 67 40 0.15 0.021

Zr-G 200 68 132 0.21 0.063

Ce-Zr-G 180 85 95 0.20 0.025

Al-G 191 65 126 0.19 0.062

Ce-Al-G 123 78 45 0.18 0.024

Al-Zr-G 162 68 94 0.18 0.046

Ce-Al-Zr-G 121 79 42 0.16 0.023

micropore volume was observed in pillared samples in relation to the initial bentonite (Na-bentonite). This is a consequence of the creation of micropores between the clay sheets in the pillaring process. This increase is greater in the sample incorporating aluminum or zirconium. It can be observed that the specific surface area of Na-bentonite increased from 107 to 200 m2 g-1 on Zr-G, 191 m2 g-1 on Al-G and 162 m2 g-1 on Al-Zr-G. The increase in specific surface area after pillaring is expected, since the process creates regular porosity. After pillaring, the micropore volume increased from 0.02 cm3 g-1 in Na-bentonite to 0.06 cm3 g-1 in Zr-G and A-G and to 0.04 in Al-Zr-G. It was clear that the BET surface area and micropore volume of the single pillared clay Zr- and Al-pillared clay are higher than those of the mixed Zr-Al pillared clay. The same observation was noted in previous works dealing with the mixed Al/Zr, Al/Fe and Al/Cr (Canizares et al., 1999) and mixed Zr-V (Bahranowski et al., 2000). Incorporating cerium resulted in a decrease in specific surface area and micropore volume. The loss of microporosity was proved that the Ce3+ with a larger radius entered into the clay layer and resulted in pore blocking (Zuo et al., 2009). It has been reported in the previous literature that the specific surface area and the pore volume of pillared clay decrease after addition of cerium (Chen et al., 2009; Huang et al., 2010; Sanabria et al., 2009; Zuo et al., 2009). However, in the case of Ce-Zr-G there is a slight decrease of the specific surface area from 200 to 180 m2 g-1. To explain this result we suggest a substitution of the Zr4+ cation with Ce3+ cation instead of pore blocking by cerium. Keeping in mind that those cations have similar ionic radius (0.087 nm for Zr4+ and 0.102 nm for Ce3+). This is not the case for Ce-Al-G. In fact the Ce3+ cation has an ionic radius different from Al3+ (0.05 nm). And the specific surface area decreases from 191 to 123 m2 g- 1 after cerium addition indicating a pore blocking.

3.1.4. CEC

The cation exchange capacity (CEC) of the parent clays as well as the pillared ones was measured by the ammonium acetate exchange according to the Kjeldhal method, in order to see the influence of the pillaring process effect. As can be seen, the CEC of all pillared samples decreases compared to the CEC of parent clay. The low values of CEC after pillaring (Table 4) suggest the irreversibility of cationic exchange; the intercalated polycations were hardly exchanged. Thus, CEC represents only the exchange of the residual interlayer cations; mostly Na+. The residual exchange capacity of Zr-G is lower than that of the Al-G which shows that zirconium poly-hydroxy-cations are more efficient in blocking negative charge than Al ones.

The incorporation of cerium leads to a slight decrease in the CEC value in all pillared samples. This decrease is more pronounced in the case of Al-PILC. In fact the CEC decreases from 43 to 35 meq/100 g for

Table 4

Cationic exchange capacities of different samples.

Na-bentonite Zr-G Ce-Zr-G Al-G Ce-Al-G Al-Zr-G Ce-Al-Zr-G

CEC 78 24 22 43 32 27 26

Al-PILC and Ce-Al-PILC respectively. The decrease in cation-exchange capacity of samples saturated with cerium was partially attributed to the migration of the cerium ion to octahedral sites of the 2:1 layers. Similar trends were reported by Poyato et al. (1987).

3.1.5. Surface acidity

The acidity is one of the important properties required from PILC. It was also investigated to evaluate the application of PILC to a commercial catalytic process. The acidity of PILC with respect to the type of pillar was observed by FTIR spectroscopy. The infrared spectra of n-butylamine adsorbed on the starting bentonite and all modified samples after evacuation, at 25 and 400 °C are shown in Fig. 2. The bands at 1445,1590 and 1623 cm-1 are mainly assigned to a Lewis acid site. The band at 1490 cm-1 is attributed to both Lewis and Bronsted acid sites. The peak at 1540 is assigned to a Bronsted acid site. At 25 °C, all of the PILCs contained an enormously high surface acidity compared to the original bentonite. This simply indicates that the acidity of the clay is enhanced by pillaring the clay. On thermal treatment at 400 °C, the band at 1540 cm-1 in the spectrum of Al-G disappeared showing that at this temperature the Bronsted sites present are not strong enough to interact with n-butylamine. Whereas, a slight reduction in the intensity of

-1-1-1-

1400 1500 1600 1700 1300

Wavenumber (cm )

Fig. 2. FT-IR spectra of n-butylamine adsorbed on Na-bentonite and modified samples (thin line, treated at 298 K; thick line, treated at 673 K).

oh Me catalyst ^O Me

+ -X + H 2 O

OH Me p. Autog.25°C ^o Me

Scheme 1.

the peaks of both Bronsted and Lewis acidities was observed in the case of samples containing Zr4+. This indicates a stronger acidity of Zr pillars than Al pillars. The weakness of Bronsted acid sites of Al pillars is already reported in the literature (Chae et al., 2001; Reddy et al., 2009). The addition of the cerium may also have an effect on the surface acidity of PILC. Thus, it is necessary to examine the effect of Ce3+ on acidity. Firstly, as shown in Fig. 2, the addition of the cerium in the pillaring solutions (Zr or/and Al solutions) leads to the apparaission, in all samples, of a new IRband at1638 cm-1 corresponding to Bronsted acid sites (Chae et al., 2001). Secondly, the bands assigned to n-butylamine coordinated onto Bronsted acid sites (1540) in the case of Ce-Al-G remain after evacuation at 400 °C. This suggests the stronger nature of the Bronsted acid sites when compared to the Al-PILC. Then, the intensities of the bands assigned to n-butylamine coordinated onto Lewis and Bronsted acid sites are slightly reduced in all samples after 400 °C. This result indicates that cerium improves the Lewis and Bronsted acidities on the surface. A similar behavior was observed by Tomul (2011).

32. Catalytic activity for dioxolane forming

On investigating the use of pillared clay, we applied these catalysts to the preparation of2,2-dimethyl1,3-dioxolane. Usually ace-tals are prepared from carbonyl compounds and alcohols, diols or orthoesters using acid catalysts like ammonium, zinc(II) or iron(III), and p-toluenesulfonic acid (Greene et al., 1981; Ogata et al., 1970). Though clays, such as natural kaolin (Ponde et al., 1996), acid activated Mt (Besbes, 2010), and Al3 + or Ce3+ exchanged-Mt (Tateiwa et al., 1995) have been used for acid-catalyst acetalyzation, and Ce-Al, and Zr-pillared clays, without solvent, have never been tested for these reactions. In this paper Ce-, Al-, and Zr-pillared clays are used as an efficient heterogeneous catalyst for acetal preparation without catalyst under mild conditions (Scheme 1).

The reaction procedure has been optimized starting from acetone and ethylene glycol as substrates. The 2, 2-dimethyl-1, 3-dioxolane is clear to identify with 1H RMN and 13C RMN. The 13C RMN spectra show a signal at 25.62 ppm relative to carbon CH3, a signal at 64.5 ppm qualified to carbon CH2 and a signal at 108.5 ppm attributed to carbon CH. 1H NMR spectra of 2,2-dimethyl-1,3-dioxolane show a signal at 1.38 (d) and 1.27 (s) ppm relative to hydrogen CH3, signals at 3.85-4.32 (m) and 3.94 (s) ppm relative to hydrogen CH2 and a signal at 4.99 (q) ppm attributed to hydrogen CH.

The yield is determined by the experimental mass per theoretical mass ratio. Experimental mass was obtained by the integration of the product intensity donated by 1HNMR spectrum data. The analysis of results donated by Fig. 3 shows that no conversion was observed in a blank run. A similar trend was reported by Thomas et al. (2011). Also, it is interesting to note that the starting Na-bentonite completely fails as a catalyst for this facile reaction. The same result has been previously reported by Ballantine et al. (1983). We suggest that the inactivity in the case of Na-bentonite is due to its insignificant Bronsted and Lewis acidities. Modified bentonites are efficient catalysts for the formation of ace-tals. However, the yield of acetal depends on the nature of the catalyst. Acetone reacted with ethylene glycol in the presence of Al-bentonite to give a 15% yield of the dimethyl acetal after 24 h of reaction. Use of catalysts rich with zirconium (Zr-G or Al-Zr-G) gives a high yield of the corresponding acetal (41%). This difference in the yield of the 2,2-dimethyl-1,3-dioxolane among the diverse catalysts doesn't correlate with the specific surface area of them. It can be explained by the strength of Lewis and Bronsted acidities. In fact, the reason for the lower activity of Al-G is due to the absence of Bronsted acidity and the incapability of the Lewis acidity to catalyze the acetalyzation reaction. Zr and Al-Zr-PILC have stronger Lewis and Bronsted sites and therefore present higher catalytic activity than Al-PILC.

The reaction between acetone and ethylene glycol in the presence of cerium was most spectacular. The catalytic behavior of solids reveals the

Fig. 3. Evolution of the yield with the time for all the catalysts.

beneficial effect of introducing cerium species into the materials. Using the Ce-Al-G catalyst leads to the doubling of the amount of acetal from 15% to 30% at 24 h. Similarly, adding cerium to Zr pillared solution increases the yield from 41% to 58% and reduces the time of reaction from 120 h to 24 h. The Ce-Al-Zr-G catalyst gives a moderate yield of the acetal equal to 33% and decreases the time of reaction from 96 h to 24 h. So Ce-, Al or/and Zr-pillared clay produces larger quantities of acetals compared to Al or/and the Zr-pillared one. Tateiwa et al. (1995) reported the acetalyzation of carbonyl compounds with methanol in the presence of different cation exchanged Fe3+-, Al3+-, Zn2+-, H+-, Na+-, Ce3+- and Zr4+-Mt. They proposed that, in the case of Ce3+-Mt, Ce3+ cation can in principle act as a Lewis acid site and thus can activate the carbonyl group by coordination, in the order of 1 kJ mol-1. Thomas et al. (2011) reported that the higher activity of the Ce, H-Y zeolite compared to the Ce, Na-Y Zeolite is due to the Bronsted acidity and not to the number of acid sites. In our case, by using a pillared clay containing cerium, the variation of activity among the various catalysts seems to be correlated with the cerium content and especially the strength of acid sites. Therefore Ce-Zr-G has the highest catalytic activity with a yield of the 2,2-dimethyl-1,3-dioxolane equal to 58%. Really the yield over the different solids is moderate; the highest yield (58%) is obtained after reaction times over 24 h. But this value of yield is good with the value reported in the literature (Arfaoui et al., 2006; Besbes et al., 2010).

Finally, It should be noticed that in all the reactions, the 2,2-dimethyl-1,3-dioxolane was obtained as the only product showing the high selectivity of the reaction.

4. Conclusion

In this work, Na-bentonite and a series of pillared clays were synthesized and used as catalyst in acetalyzation of acetone with ethylene glycol. XRD results show that a super structure was observed after the addition of cerium. An increase of the basal spacing and enhancement of the crys-tallinity are observed. Surface acidity measurements show that the cerium leads to an enhancement of the Lewis and Bronsted acid centers. Activity tests in the synthesis of 1,3-dioxolane show that Ce3+ improves the yield of acetal and reduces the time of reaction to 24 h.

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