Kinetic study of the condensation of benzaldehyde with ethylcyanoacetate in the presence of Al-enriched fluoroapatites and hydroxyapatites as catalysts

(1)

Kinetic study of the condensation of benzaldehyde with ethylcyanoacetate in the presence of Al-enriched

fluoroapatites and hydroxyapatites as catalysts

Nadia Elazarifi

^a

, Abdelaziz Ezzamarty

^a

, Jacques Leglise

^b

, Louis-Charles de Ménorval

^c

, Claude Moreau

^c^,∗

aLaboratoire de Catalyse Hétérogène, Université Hassan II, Faculté des Sciences A¨ın Chock, BP 5366, Maarif, Casablanca, Morocco

bMinistère de la Recherche, 51038 Chˆalons en Champagne, France

cLaboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR CNRS 5618, ENSCM, 8, Rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France

Received in revised form 3 March 2004; accepted 5 March 2004 Available online 21 April 2004

Abstract

The Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was performed in different solvents, at room temperature, in the presence of as-synthesized and Al-enriched fluoroapatites and hydroxyapatites as catalysts, and, for comparison, in the presence of a magnesium oxide and a mixed magnesium–aluminium oxide. The catalytic activity is significantly improved for the Al-enriched fluoroapatites and hydroxyapatites compared to their as-synthesized precursors. However, the highest activity is observed for the magnesium oxide and the mixed magnesium–aluminium oxide where the reaction is nearly complete within 15 min in the absence of diffusion limitation at the external surface. The higher activity of magnesium-based oxides can be explained by the differences in the nature of the active sites. MgO possesses principally isolated O²⁻sites, whereas the mixed oxide would possess Mg²⁺–O²⁻and Al³⁺–O²⁻acid–base pairs. The presence of Al³⁺–O²⁻pairs might also been postulated to account for the higher activity of Al-enriched hydroxyapatite and fluoroapatite, the higher activity being reinforced by the presence of superficial HPO42−basic species in the hydroxyapatite series. When the reaction is carried out in methanol as solvent, it was shown that transesterification of ethyl cyanoacetate into methyl cyanoacetate and trans-␣-ethyl-2-cyanocinnamate into trans-␣-methyl-2-cyanocinnamate may occur in the presence of strongly basic catalysts.

Keywords: Knoevenagel condensation; Apatites; Magnesium oxide; Hydrotalcite

1. Introduction

In recent papers, it has been reported that a Al-enriched hydroxyapatite containing superficial HPO₄²⁻ species was a better carrier than␥-alumina for the selective conversion of dimethyldisulfide into methanethiol and hydrodesulfur- ization of thiophene over sulfided NiMo catalysts[1–3].

The presence of such anionic species capable of acting as basic species then led us to check the activity of this Al-enriched hydroxyapatite support directly as a catalyst in a reaction known to be catalyzed by basic catalysts. The Kno-

∗Corresponding author. Tel.:+33-4-67-16-34-5;

fax:+33-4-67-16-34-70.

E-mail address: cmoreau@cit.enscm.fr (C. Moreau).

evenagel condensation of benzaldehyde with ethyl cyanoacetate (Scheme 1) was chosen since the literature is relatively abundant on the utilization of solid catalysts in this reaction, and some results are already available with apatite materials as catalysts[4,5].

2. Experimental

All the catalysts were characterized by elemental analysis, X-ray diffraction, FT-IR spectroscopy, MAS³¹P and

27Al NMR measurements. Specific surface areas and pore volumes were calculated from the isotherms of nitrogen adsorption/desorption at 77 K. The total basicity of the catalysts was measured by thermal adsorption and desorption of

doi:10.1016/j.apcata.2004.03.012

(2)

C6H5

C O H2C

CN

H CO2C2H5

C6H5

H

C C

CN

CO₂C₂H₅

H2O

Benzaldehyde Ethyl cyanoacetate Ethyl trans-α -cyanocinnamate

+ Base +

Scheme 1. Reaction scheme for condensation of benzaldehyde with ethyl cyanoacetate.

CO₂. The number of basic sites was determined by the number of moles of CO₂ adsorbed on the surface of the solids at 373 K.

2.1. Catalysts preparation and characterization

Hydroxyapatite and fluoroapatite materials were prepared by precipitation as previously reported [1,2,6]. The as-synthesized hydroxyapatite and fluoroapatite were de- noted as HAP0 and FAP0, respectively, and their Al-enriched derivatives as HAP1 and FAP1, respectively. Their chemical composition as well as their textural properties (specific surface area and pore volume) are reported inTable 1.

The mixed magnesium–aluminium oxide used in this work was KW2000 from Kyowa with an aluminium molar fraction of 0.3 and a particle size of 70␮m. Magnesium oxide was prepared from the brucite-like magnesium hydrox- ide from Strem Chemical. All the catalysts were calcined prior to use at 823 K for 6 h under air flow.

From elemental analysis (Table 1), the Ca/P atomic ratio of the HAP0 catalyst is equal to 1.58 that agrees with a calcium-deficient apatite. The Ca/P ratio of the apatite component in HAP1 is calculated as 1.57, and is similar to HAP0. By contrast, the corresponding Ca/P ratios of fluoroapatites FAP0 and FAP1 were found to be close to the stoichiometric Ca/P ratio.

As also seen inTable 1, the addition of weak aluminium amounts in the synthesis stage of fluoroapatites and hydroxyapatites results in an intimate mixture of crystalline apatite and amorphous AlPO4. This leads to an important increases in both specific surface area and pore volume.

From X-ray diffraction, it was concluded in a previous work[1–3] that the diagrams of the different solids HAP0 and HAP1 resemble that of the hydroxyapatite[7]. No vari- ation in the cell parameters was observed after addition of aluminium in fluoroapatites and hydroxyapatites.

The presence of the phosphate phase in fluoroapatites and hydroxyapatites was confirmed by MAS³¹P and²⁷Al NMR

Table 1

Elemental analyses and textural properties of as-synthesized and Al-enriched apatite catalysts

Catalysts Al (%) Ca (%) F (%) P (%) Ca/P^a SBET(m²/g) Vp(cm³/g)

HAP0 0 38.3 0 18.8 1.58 83 0.32

HAP1 1.5 36.0 0 19.4 1.57 140 0.54

FAP0 0 38.5 3.81 17.5 1.70 40 0.17

FAP1 1.4 33.7 2.21 17.3 1.66 132 0.29

a P in the apatite phase.

Table 2

Total basic strength (−Hads), number of basic sites (meq. CO2/g) Catalysts −Hads(kJ/mol CO2) TPD CO2 (meq./g)

HAP0 23.68 0.21

HAP1 55.38 0.15

FAP0 65.24 0.10

FAP1 31.60 0.19

MgOâ 69.6â 0.60â

KW2000â 60.3â 1.04â

a Data from[10].

measurements, in agreement with results reported in the literature [8]. Moreover, a better dispersion of the phosphate phase was observed by²⁷Al NMR for Al content less than 1.5%.

Finally, from FT-IR measurements, it was noticed that the solids referred to as HAP0 and HAP1 contain a band at 875 cm⁻¹ characteristic of HPO₄²⁻ groups of calcium-deficient apatite[1–3,9]. This band is not observed in the FAP0 and FAP1 infrared spectra, although they have the same Ca and P contents. As we had noticed in the HAP1 infrared spectra, there exists a band at 3795 cm⁻¹ which intensity increases with Al percentage. This band, assigned to AlOH species, was in accordance with the presence of an amorphous AlPO₄phase together with the apatite phase[7].

The basic strength of the catalysts used in this work was determined from microcalorimetric measurements. The total basicity is given by the adsorption enthalpy of CO2and the number of basic sites by the number of adsorbed CO2per g of catalyst (Table 2).

2.2. Procedure and analyses

Reactants, benzaldehyde (5 mmol) and ethyl cyanoacetate (5 mmol), and dodecane used as internal standard (5 mmol), are poured in a thermostated (298 K) 50 ml glass reactor containing 10 ml of solvent. Two hundred fifty milligram of freshly calcined catalyst are then added to this solution.

(3)

Zero time was taken when the agitation began. The agitation rate was kept at 700 rpm to avoid diffusion limitation at the external surface. Samples were periodically withdrawn and analyzed on a HP 6890 gas chromatograph equipped with a flame ionisation detector using hydrogen as carrier gas, and a HP5-capillary column (30 m ×0.32 mm, 0.25-␮m film thickness), 355 K (4 min) to 513 K (20 K min⁻¹). The main reaction product, trans-␣-ethyl-2-cyanocinnamate (mass balance over than 95%) was identified by comparison with an authentic sample and GC–MS analysis.

Initial reaction rates were deduced from the experimental concentration versus time curves by curve fitting and simulation using appropriate softwares [11] assuming all the reactions to be first order in the organic reactant or by determination of the slope at the origin. The experimental error can then be estimated to≈15%.

3. Kinetic results and discussion

3.1. Influence of agitation speed and catalyst weight

As far as the mixed magnesium–aluminium oxide KW2000 was expected a priori to exhibit high basic properties as previously shown by Corma and coworkers [12,13], the kinetic study was first carried out over this catalyst. As it will be seen later, when the reaction is carried out in methanol as solvent, it will be shown that transesterification of ethyl cyanoacetate into methyl cyanoacetate and trans-␣-ethyl-2-cyanocinnamate into trans-␣-methyl-2-cyanocinnamate may occur in the presence of strongly basic catalysts.

In order to check the influence, or not, of possible diffusion limitation at the external surface, isopropanol was then

0 0.05 0.1 0.15

0 100 200 300 400 500

Initial rate of condensation x 105 mol min-1 m-2

Catalyst weight, mg

Fig. 1. Influence of the amount of KW2000 catalyst on the initial reaction rate of condensation of benzaldehyde with ethyl cyanoacetate in isopropanol as solvent.

Table 3

Solvent effect on the initial rates of condensation of benzaldehyde with ethyl cyanoacetate (×10⁵mol min⁻¹m⁻²) (initial concentrations in reactants: 5 mmol in 10 ml of solvent, 500 mg of KW2000 catalyst, 298 K)

Solvent Initial rates

Methanol (ε=33) 0.045

Isopropanol (ε=20.3) 0.149

Tetrahydrofuran (ε=7.6) 0.118

chosen as the solvent, because of its nearly similar polarity, but a lower nucleophilicity, compared to methanol. Indeed, no transesterification reaction was observed in isopropanol.

It was first rapidly shown that the initial reaction rate of condensation of benzaldehyde with ethyl cyanoacetate was nearly constant over a speed of agitation of 700 rpm.

Then, by plotting the initial reaction rate of condensation as a function of the catalyst weight, a classical behavior was obtained, i.e. a saturation phenomenon at high catalyst amount (Fig. 1).

3.2. Solvent effect

The condensation of benzaldehyde with ethyl cyanoacetate was also performed in tetrahydrofuran (non polar and aprotic solvent) and in methanol (polar and protic solvent).

The initial reaction rates over the KW2000 catalyst are reported inTable 3.

It is immediately seen that the reaction occurs at nearly similar rates, whatever the protic or aprotic character of the solvent, and whatever the polarity of the solvent as given by its dielectric constant [14]. Such a behavior was not completely unexpected as far some results have already mentioned that tetrahydrofuran could behave as methanol

(4)

0 0.2 0.4 0.6

60 30

0 90

Benzaldehyde Ethyl cyanoacetate Ethyl cyanocinnamate Methyl cyanocinnamate Methyl cyanoacetate

Concentrations, M

Time, min

Fig. 2. Condensation of benzaldehyde with ethyl cyanoacetate over MgO as catalyst and in methanol as solvent.

due to the possibility of interaction with a catalyst surface through the electron lone pairs of the oxygen atom [15].

However, as mentioned at the beginning of this para- graph, the reaction carried out in methanol as the solvent also leads to products of transesterification. As illustrated in Fig. 2 in the case of MgO as catalyst, it is seen that trans-␣-methyl-2-cyanocinnamate is formed more rapidly than trans-␣-ethyl-2-cyanocinnamate, by a factor of about 2 in the early stage of the reaction. After 90 min of reac- tion, a 76/24 mixture of trans-␣-methyl-2-cyanocinnamate to trans-␣-ethyl-2-cyanocinnamate is obtained as the result of the relatively slow transesterification of trans-␣-ethyl-2- cyanocinnamate in a large amount of methanol. That would also means that transesterification of ethyl cyanoacetate into methyl cyanoacetate reaction must take place in the early stage of the reaction, and that methyl cyanoacetate is rapidly consumed to yield trans-␣-methyl-2-cinnamate, since methyl cyanoacetate is only detected as very low amounts in the first 15 min of reaction.

By contrast, when the mixed magnesium–aluminium oxide KW2000 is used as catalyst, the reverse situation is ob- served, i.e. trans-␣-ethyl-2-cyanocinnamate is formed more rapidly than trans-␣-methyl-2-cyanocinnamate, by a factor of about 10 in the early stage of the reaction, although the rate of disappearance of the starting reactants is only slightly less than with MgO. After 90 min of reaction, a 22/78 mixture of trans-␣-methyl-2-cyanocinnamate to trans-␣- ethyl-2-cyanocinnamate is obtained as the result of the slow transesterification of trans-␣-ethyl-2-cyanocinnamate over the KW2000 catalyst compared to magnesium oxide, in agreement with previous results reported on the transesterification of triglycerides in the presence of the same catalytic systems[16].

3.3. Kinetic results

As far as isopropanol does not give rise to transesterification reactions, the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was performed in this solvent in the presence of as-synthesized and Al-enriched fluoroapatites and hydroxyapatites as catalysts, and, for comparison, in the presence of a magnesium oxide and a commercial mixed magnesium–aluminium oxide, KW2000. The initial specific reaction rates obtained are reported inTable 4, together with the basic properties of the catalysts.

In methanol as the solvent, it was shown that the rate of disappearance of the starting reactants was only slightly less (by a factor of about 2) in the presence of KW2000 catalyst than in the presence of MgO. In isopropanol as the solvent, the reversed situation is observed, i.e. the rate of disappearance of the starting reactants is now higher (by a factor of about 4) in the presence of KW2000 catalyst than in the presence of MgO. This would corroborate the hypothesis

Table 4

Initial specific reaction rates (×10⁵mol min⁻¹m⁻²), catalysts textural and basic properties, for the condensation of benzaldehyde with ethyl cyanoacetate (initial concentrations in reactants: 5 mmol in 10 ml of isopropanol, 250 mg of catalyst, 298 K)

Catalysts −Hads

(kJ/mol CO2)

TPD CO2

(meq./g)

SBET

(m²/g)

Initial specific reaction rate

HAP0 23.68 0.21 83 0.0124

HAP1 55.38 0.15 140 0.0247

FAP0 65.24 0.10 40 0.0062

FAP1 31.60 0.19 132 0.0107

MgO 69.6^a 0.60^a 148 0.0466

KW2000 60.3^a 1.04^a 240 0.123

a Data from[10].

(5)

that methanol and tetrahydrofuran are not completely inert toward the catalyst surface.

4. Discussion

As shown inTable 4, the catalytic activity is significantly improved for the Al-enriched fluoroapatites and hydroxyapatites compared to their as-synthesized precursors. However, the highest activity is observed for the magnesium oxide and the mixed magnesium–aluminium oxide where the reaction is nearly complete within 15 min in the absence of diffusion limitation at the external surface.

On catalysis by phosphate-based catalysts, two relevant papers are worth mentioning. In the first one in 1984, by Campelo et al.[17], it has been put forward that (i) the activity for the Knoevenagel condensation reaction is affected by two major factors, the chemical nature of the surface (number and strength of the acidic and basic sites) and the textural properties of the solids, and (ii) the reaction operates by the established mechanism for Knoevenagel reactions. The application of the Hammett equation for the condensation of para-substituted benzaldehydes with ethyl cyanoacetate leads to a Hammett ρ value of 0.48. This positive value then implies an increase in the electronic density in the rate- determining step of the reaction and, as a consequence, a nucleophilic attack by an anionic species. In the first step of the reaction, one active methylene proton is abstracted by a

0 0.05 0.1 0.15

0 0.4 0.8 1.2

Initial specific activity x 105 mol min-1 m-2

Number of basic sites, meq CO

2 g^-1

KW2000

MgO

FAP0

FAP1 HAP0 HAP1

Fig. 3. Plot of the initial reaction rates of benzaldehyde conversion as a function of the number of basic sites.

O

O P OH + AlPO₄

O

O P O Al

O O P O

Scheme 2. Possible formation of Al³⁺–O²⁻acid–base pairs in Al-enriched hydroxyapatites.

base, followed, in the second step, by the nucleophilic addition of the resulting carbanion species on the carbonyl group of the aldehyde. In the second paper in 2003 by Holt and coworkers[18], it has been shown that the catalytic activity of mono-, meta- and di-phosphate complexes in the condensation of benzaldehyde with ethyl cyanoacetate in ethanol as the solvent was related to the electropositivity of the cation present in the structure and to the availability of the phosphate moiety to serve as a base.

Such a dual-site mechanism was not completely unexpected since, for magnesium-based catalysts, it is well known that the incorporation of small amounts of Al³⁺ cations to MgO increases the reaction rates as, for example, in aldol condensation[19]or alcohol elimination reactions [20], due to the formation of Lewis acid-strong base pair sites. On MgO, strong basic sites consist of isolated O²⁻

species, whereas Al surface enrichment leads to the formation of Mg²⁺–O²⁻ et Al³⁺–O²⁻ acid–base pair sites.

Although MgO and two apatite catalysts have a total basicity similar, or higher, as compared to the mixed Mg–Al oxide, the latter catalyst has a number of basic sites higher than in bulk MgO, and then is more active due to the presence of such acid–base pair sites. The correlation depicted inFig. 3(correlation coefficient=0.97) clearly illustrates the influence of the number of basic sites of the catalysts instead of the total basicity on the catalytic activity. The basic part of the catalyst activates the abstraction of an active proton of the methylene group of ethyl cyanoacetate,

(6)

the nucleophilic addition of the resulting carbanion species is facilitated by the activation of the carbonyl function of benzaldehyde over the Lewis acidic part of the catalyst.

The catalytic activity is significantly improved for the Al-enriched hydroxyapatite HAP1 compared to its as- synthesized precursor HAP0 and to the Al-enriched fluoroapatite FAP1. The presence of superficial HPO₄²⁻ species in the Al-enriched hydroxyapatite would be responsible for its higher activity due to the possibility to form Al³⁺–O²⁻ acid–base pair sites from the interaction between Al (AlPO₄) with the OH group of HPO₄²⁻as postulated inScheme 2.

5. Conclusions

As-synthesized hydroxyapatite and fluoroapatite are not particularly active in the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate in the absence of diffusion limitation at the external surface. However, their Al-enriched derivatives have been found more active due an increase in their surface area and their number of basic sites. The activity of the Al-enriched hydroxyapatite HAP1 is only half of that of MgO assumed to be due to the possibility to form Al³⁺–O²⁻acid–base pair sites of the same kind as proposed for the mixed magnesium–aluminium oxide KW2000 catalyst, by far the most active and easily recyclable catalyst.

Acknowledgements

The Comité Mixte Interuniversitaire Franco-Marocain is gratefully acknowledged for financial support within the frame of Actions Intégrées No. 97/018/SM and No.

MA/02/36.

References

[1] N. El Azarifi, A. El Ouassouli, M. Lakhdar, A. Ezzamarty, J.N.M.

van Gestel, J. Leglise, Phos. Res. Bull. 10 (1999) 430.

[2] N. El Azarifi, A. El Ouassouli, M. Lakhdar, A. Ezzamarty, L.C. de Ménorval, C. Moreau, J. Lamotte, J. Leglise, in: Proceedings of the 3rd International Meeting on Environment, Catalysis and Process Engineering, Fes, Morocco, November 2000.

[3] N. El Azarifi, A. El Ouassouli, M. Lakhdar, A. Ezzamarty, C. Moreau, A. Travert, J. Leglise, Ann. Chim. Sci. Mater. (2004), in press.

[4] A. Smahi, A. Solhi, H. El Badaoui, A. Amoukal, A. Tikad, M. Maizi, S. Sebti, Appl. Catal. A 250 (2003) 151 (and references therein).

[5] S. Sebti, R. Tahir, R. Nazih, A. Saber, S. Boulaajaj, Appl. Catal. A 228 (2002) 155.

[6] A. Laghzil, N. Elhrech, O. Britel, A. Bouhaouss, M. Ferhat, J.

Fluorine Chem. 101 (2000) 69.

[7] J.C. Elliott, Structure and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier, Amsterdam, 1994.

[8] H. Kraus, R. Prins, J. Catal. 102 (1996) 251.

[9] R.M. Wilson, J.C. Elliott, S.E.P. Dowker, J. Solid State Chem. 174 (2003) 132.

[10] E. Breysse, Doctorat Thesis, University of Montpellier, 2000.

[11] J. Joffre, P. Geneste, A. Guida, G. Szabo, C. Moreau, Stud. Phys.

Theor. Chem. 71 (1990) 409.

[12] A. Corma, V. Fornés, R.M. Mart´ın-Aranda, F. Rey, J. Catal. 134 (1992) 58.

[13] M.J. Climent, A. Corma, S. Iborra, A. Velty, J. Mol. Catal. A 182–183 (2002) 327.

[14] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, VCH, Weinheim, 1990.

[15] R.L. Augustine, R.W. Warner, R.J. Melnick, J. Org. Chem. 49 (1984) 4853.

[16] E. Leclercq, A. Finiels, C. Moreau, J.A.O.C.S. 78 (2001) 1161.

[17] J.A. Cabello, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, J.

Org. Chem. 49 (1984) 5195.

[18] J. Bennazha, M. Zahouilly, A. Boukhari, E.M. Holt, J. Mol. Catal.

A 202 (2003) 247.

[19] J.I. Di Cosimo, V.K. D´ıez, M. Xu, E. Iglesia, C.R. Apestegu´ıa, J.

Catal. 178 (1998) 499.

[20] V.K. D´ıez, C.R. Apestegu´ıa, J.I. Di Cosimo, J. Catal. 215 (2003) 220.