Selective engineering using Mg–Al calcined hydrotalcite and microwave irradiation in mono-transesterification of diethyl malonate with cyclohexanol

Chemical Engineering Journal 230 (2013) 547–557

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Selective engineering using Mg–Al calcined hydrotalcite and microwave irradiation in mono-transesterification of diethyl malonate with cyclohexanol Ganapati D. Yadav ⇑, Anup A. Kadam Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Synergism between microwave

irradiation and solid base catalysis.  Mono-transesterification of diethyl malonate with cyclohexanol Mg–Al calcined hydrotalcite.  Microwave irradiation showed more than threefold increase vis-a-vis conventional heating.  69% conversion with 98% selectivity for cyclohexyl ethyl malonate at 100 °C in 2 h.  Full catalyst characterization and theoretical analysis to develop kinetic model.

a r t i c l e

i n f o

Article history: Received 11 March 2013 Received in revised form 18 June 2013 Accepted 20 June 2013 Available online 28 June 2013 Keywords: Hydrotalcite Microwave irradiation Diethyl malonate Cyclohexanol Cyclohexyl ethyl malonate Kinetics

C2H5 O OH O C2H5

O

O

O

O

C2H5 +

Diethyl malonate

Hydrotalcite MW

Cyclohexanol

O

+

C2H5OH

O

Ethanol

Cyclohexyl ethyl malonate

a b s t r a c t Selectivity engineering aspects of synergism between microwave irradiation and solid base catalysis is investigated for a highly selective mono-transesterification of diethyl malonate with cyclohexanol using Mg–Al calcined hydrotalcite as catalyst. Microwave irradiation showed threefold increase in cyclohexanol conversion vis-a-vis conventional heating and only the mono-transesterified product was obtained. The catalyst was characterised by XRD, FT-IR, CO2-TPD, surface area and pore size analysis. Hydrotalcite with Mg:Al composition as 3:1 gave the best results. Effect of various parameters was studied on conversion and selectivity. Under optimised reaction conditions, 69% conversion with 98% selectivity for cyclohexyl ethyl malonate was obtained for catalyst loading of at 100 °C in 2 h. The catalyst showed excellent reusability. Complete theoretical and experimental analysis has been done to develop a kinetic model for the reaction. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Transesterification reaction is one of the most useful organic transformations for the synthesis of esters and various intermediates of industrial importance and it is usually catalysed by using mineral acids and Lewis acids such as titanium or aluminium

⇑ Corresponding author. Tel.: +91 22 3361 1001; fax: +91 22 3361 1002/ 1020. E-mail addresses: [email protected], [email protected] (G.D. Yadav). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.06.075

alkoxides, or bases such as alkali-metal alkoxides, 4-dimethylaminopyridine (DMAP), or other amines [1]. However, it is well known that use of traditional homogeneous catalysts causes severe pollution and corrosion problems, loss of yield and presence of deleterious impurities in the final product. A heterogeneous catalytic system serves the purpose of intensification of reaction rates, better selectivity, easy workup, lesser reactor volume and minimisation of waste [2]. Catalysis forms one of the most important principles of green chemistry and the strategies to reduce waste,

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Nomenclature A B CA CAo CAS CB CBo CBS CE CES CS CT

reactant species A, cyclohexanol (mol/cm3) reactant species B, diethyl malonate (mol/cm3) concentration of A (mol/cm3) initial concentration of A in bulk liquid phase (mol/cm3) adsorpation concentration of A on active site S (cm3/gcat) concentration of B (mol/cm3) initial concentration of B in bulk liquide phase (mol/ cm3) adsorption concentration of B on active sites (cm3/g-cat) concentration of E, cyclohexyl ethyl malonate (mol/cm3) adsorption concentration of E on active sites(cm3/g-cat) concentration of vacant sites of type S (cm3/g-cat) total concentration of vacant sites(cm3/g-cat)

improve selectivity and minimise energy consumption are therefore directed at synthesising new active and selective solid catalysts, and the processes can be assisted with different forms of energy such as microwave irradiation and ultrasound [2,3]. Amongst various kinds of heterogeneous catalysts, acid and base catalysed esterification and transesterification reactions are the most widely used reactions [4–6], which can be intensified further with ultra-sound and microwave irradiation [7]. Various solid catalysts have been investigated for transesterification, for instance, distannoxane [1], clay [8], NaIO4, KIO4, and CaCl2 [9], zeolites [10], Mg–Al–O–t–Bu hydrotalcite [11], hydrotalcite [12], alkali earth metal oxide [13], supported enzymes [14,15], microwave assisted enzyme catalysis [16]. Transesterification is routinely studied for biodiesel production using a variety of catalysts, for instance, in reactions of vegetable oils [17]. The mono-transesterification of diethyl malonate with cyclohexanol to produce cyclohexyl ethyl malonate is commercially attractive wherein control of transesterification at the mono-level is also challenging. There is no detailed account on the kinetics and process development aspects of solid base catalysis for this reaction. The earlier published work shows that a better solid base catalytic system [18] could be developed to overcome the issues of prolonged times, less conversions and poor selectivity, particularly using hydrotalcites [19]. Microwave-induced chemistry has attracted much attention in the past few years [20–22]. Microwave irradiation can be used as an efficient source for thermal energy which may lead to faster and cleaner reactions without thermal decomposition of product and minimisation of unwanted side reactions. In recent years solid base catalytic systems involving hydrotalcites and mixed oxides, have been investigated in various organic transformations and these could be intensified using microwave irradiation. Catalysis by solid acids and super acids such as zeolites [23] sulfated zirconia [24], heteropoly acids [25] has been extensively studied for industrial reactions including esterification and transesterification. However, in contrast with solid acids, comparatively a few studies have been reported on solid bases as industrial catalysts [26]. Homogeneous base catalysis finds applications in alkylation, isomerization, Michael addition, aldol, Knoevenagel and Claisen–Schmidt condensations, which are used for the production of bulk chemicals, fine chemicals, pharmaceuticals, perfumes and flavors. Over 1.5 MMTA of bulk chemicals are produced via processes catalyzed by alkalies such as NaOH, KOH and Ca(OH)2, which are neutralized with acids contributing to higher product cost associated with product separation, purification and wastewater treatment. In this regard, hydrotalcites have shown a great

CW E KA KB KE KW M rA W w XA t

concentration of W, ethanol (mol/cm3) product species E, cyclohexyl ethyl malonate adsorption equilibrium constant for A (cm3mol1) adsorption equilibrium constant for B (cm3mol1) adsorption equilibrium constant for E (cm3mol1) adsorption equilibrium constant for W (cm3mol1) molar ratio of CBo/CA0 rate of reaction (mol cm3 s1) product species W, ethanol catalyst loading (g/cm3) fractional conversion of A time (min)

promise as heterogeneous bases [19,26]. The use of metal oxide base catalysts in transesterification reaction of Camelina sativa oil along with microwave irradiation results into better yields and selectivity [27,28]. The present work highlights the synergism between microwaves and hydrotalcite derived solid base catalyst, for highly selective mono-transesterification of diethyl malonate with cyclohexanol including development of a kinetic model based on experimental data. The results are new and conclusive.

2. Experimental 2.1. Materials Magnesium nitrate hexahydrate, aluminium nitrate nonahydrate, sodium hydroxide, sodium carbonate anhydrous, cyclohexanol and diethyl malonate all of AR grade were obtained from M/s s.d. Fine chemicals Pvt. Ltd. Mumbai, India. 2.2. Catalyst preparation Mg–Al hydrotalcite catalysts , with different Mg/Al atomic ratio were prepared using co-precipitation method. Under vigorous stirring, an aqueous solution (solution A) containing Mg (NO3)26H2O and Al (NO3)39H2O (1.0 M in (Al + Mg) with an Mg/Al ratio of 1, 2, 3, 4) was slowly dropped in a solution (solution B) containing NaOH and Na2CO3. The temperature was maintained at 50 °C and the final pH was adjusted to 10 ± 0.1. The gel was aged at 50 °C for 12 h. The gel was washed with distilled water until the pH of the wash was 7. The solid so obtained was filtered, dried at 80 °C and then calcined in air at 450 °C (5.0 C/min. heating rate) for 12 h. The material obtained after calcination was called CHT (calcined hydrotalcite). The Mg:Al composition in the catalyst is varied as 1:0, 1:1, 2:1, 3:1, and 4:1. Thus, they were identified as CHT10, CHT11, CHT21, CHT31, and CHT41, respectively. 2.3. Catalyst characterisation 2.3.1. FT-IR FT-IR spectra of the samples were recorded on Perkin Elmer FTIR system (model-Spectrum GX) in the scan range of 4000– 600 cm1 at a resolution of 4 cm1. Prior to analysis, samples were dried at 100 °C for 1 h. Pellets were prepared using spectroscopy grade previously dried KBr.

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2.3.2. Thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) Thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA) of the uncalcined hydrotalcite samples were done using Perkin Elmer Pyris Diamond TG/DTA. Argon was used as sweeping gas at the flow rate of 5 cm3 min1. Weighed quantity of the sample was heated from 30 °C to 450 °C at a heating rate of 5 °C min1. 2.3.3. BET-surface area and pore size analysis The textural characteristics, such as BET surface area and pore size distribution were determined by multipoint N2 adsorption– desorption at 77 K in an ASAP-2010 (Micromeritics, USA) instrument. Prior to analysis the samples were degassed at 150 °C and 103 torr for 2 h to remove physically adsorbed moisture. 2.3.4. X-ray diffraction (XRD) Powder X-ray diffraction pattern of the calcined Mg–Al hydrotalcite sample (CHT31), which was the most effective catalyst, was obtained with Rigaku Miniflex X-ray diffractometer using Ni-filtered Cu Ka radiation (k = 0.1541 nm) at room temperature. Data were collected in the 2h range of 10–80° at a scan rate of 2° min1. Samples were dusted on the double-sided sticky tape and mounted on glass microscope slides. 2.3.5. Scanning electron microscopy (SEM) Scanning electron microscopic (SEM) images were recorded using JEOL JSM 6380LA Analytical Scanning Electron Microscope. The dried samples were mounted on the specimen studs and sputter coated with a thin film of platinum to prevent charging during analysis. The platinum coated surface was then scanned at various magnifications using scanning electron microscope. 2.3.6. CO2-Temperature programmed desorption (CO2-TPD) Basic site density was measured by temperature programmed desorption (TPD) of CO2, pre-adsorbed at room temperature in a Micromeritics-2920 TPR/TPD analyser. Samples were treated in N2 at 120 °C for 1 h, cooled to 27 °C and then exposed to 10% CO2/He stream for 1 h. Weakly adsorbed CO2 was removed by purging the samples with He at room temperature. The temperature was then increased at the rate of 10 °C min1 from 27 to 450 °C. The amount of CO2 chemisorbed and its desorption profile were then measured. 2.4. Reaction procedure 2.4.1. Conventional heating The reactor consisted of a flat-bottomed cylindrical glass vessel of 50 ml capacity equipped with four baffles, a standard turbine stirrer and a condenser, and a thermowell for measuring the temperature. The assembly was kept in a thermostatic oil bath at a known temperature and mechanically agitated with an electric motor. A typical reaction mixture consisted of 0.025 mol cyclohexanol, 0.125 mol diethyl malonate and 0.4 cm3 of n-dodecane as an internal standard. The reaction was conducted with catalyst loading of 0.03 g/cm3 at 100 °C and 1100 rpm. 2.4.2. Microwave reactor The studies were carried out in a microwave reactor (Discover, CEM-SP1245 model, CEM Corporation, USA). The reactor was a 100 cm3 capacity fully baffled, 4.5 cm i.d. cylindrical glass vessel with provision for mechanical stirring and temperature measurement using a thermowell. A temperature probe was inserted to measure the temperature of the reactor mass. A standard fourbladed pitched turbine impeller of 1.5 cm diameter was used for agitation. However, the actual reactor volume exposed to the

549

microwave irradiation was 45 ml with 5.5 cm height. The quantities of reactant and catalyst and reaction procedure were identical to those used for the conventional heating. 2.5. Analysis Samples were withdrawn periodically and analyzed by gas chromatography (Chemito Model 8510, FID detector) using SS column (3.25 mm  2 m) packed with liquid stationary phase of 10% SE-30. Conversions were based on the limiting reactant, cyclohexanol. Quantitative results were obtained by comparing results with the calibration form synthetic mixture. The product confirmation was done using GC–MS. 3. Results and discussion 3.1. Catalyst characterization 3.1.1. FT-IR The FT-IR spectra of the uncalcined samples (HT-series) are shown in Fig. 1A. It displays broad absorption band at 3500– 3600 cm1 attributed to the stretching vibration of hydrogen bonded in the OH groups located between the brucite layers. The deformation vibration of water molecules appears at 1630– 1650 cm1 and its magnitude depends on the hydration degree and nature of interlayer anions. The interlayer carbonate ion may exist either as mono or bidentate and presents characteristics antisymmetric stretching band v3, at around 1506 and 1384 cm1 [29]. The low frequency region showed bands at about 790 and 960 cm1, corresponding to the Al–O and the band at 660 cm1 is assigned to Mg–O [30]. Fig. 1B displays FT-IR spectra of calcined hydrotalcite samples. The intensity of the band at 3500– 3600 cm1 has decreased significantly due de-hydroxylation. The decrease in intensity of vibration assigned to interlayer carbonates at around 1380 cm1 was observed in the calcined hydrotalcite samples. It also shows that layered structure of hydrotalcite is not completely destroyed after calcination. 3.1.2. TGA-DTA The thermogravimetric curves for different hydrotalcite samples are given in Fig. S.1 (in the Supplementary material section). For all the samples except HT10, two prominent weight losses are observed. The first one is in the range of 100–210 °C and the second one in the range of 330–400 °C. The first weight loss in the TG curve is due to elimination of loosely bound water and interlayer water molecules. The second loss is ascribed to the removal of hydroxyl groups (dehydroxylation) in the metal hydroxide layers and removal of CO2 (decarbonation) resulting form the decomposition of CO2 present in the interlayer space 3 as charge-balancing anion [31,32]. The first stage of dehydration from 30 °C to 200 °C is comprised of three different stages. (i) A quasi-isothermal step between 30 and 70 °C – it is because of loosely structurally bounded water. (ii) A non-isothermal between 77 and 170 °C – it is because of loss of water hydrogen bonded to itself in the interlayer spaces. This is also accompanied with the partial collapse of the layers. (iii) Quasi-isothermal step between 175 and 220 °C – it is because of loss of water hydrogen bonded to the hydrotalcite hydroxyl surface [33]. 3.1.3. BET surface area and pore size analysis Nitrogen adsorption–desorption isotherms for calcined hydrotalcites are shown in Table 1. All isotherms belong to Type IV or V with small plateau at high relative pressures, characteristic of mesoporous materials. CHT31 shows a type H2 hysteresis loop, with large almost flat plateau in which significant differences in

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(d) HT41

(d) HT41

1640.57 832.41 1422.40 1650.85

853.13

1503.39 1386.77 1509.22

(c ) HT31

3450.11 1384.97 1637.55

(c) HT31 866.42 3589.38

835.34 646.88

1637.13 836.14 3503.15 1384.47

4.07

(b) HT21

3568.07

(b) HT21

1384.37

3.8 3.6 3.4 3.2 1635.84

1434.26

2.8

663.54 (a) HT11

1636.38

3.0

958.59

1524.62

(a) HT11

2.6

798.45

2.4 2.2 1384.21

%T

3449.29

2.0 3524.66

1.8

847.32 1636.38

1.6 1434.26

1.4 1635.95

1524.62

1.2 1.0

1519.55

0.8 0.6 966.78

0.4

3467.77 1383.85

00.0

3600

3200

2800

2400

2000

1800

1600

1400

790.73 659.30

1200

1000

800

600.0

3524.66

847.32

0.2 -0.11 4000.0

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800

600.0

cm-1

-1

cm

(A)

(B)

Fig. 1. FT-IR spectra of (A) uncalcined Mg–Al hydrotalcite and (B) Mg–Al calcined hydrotalcite.

Table 1 Effect of catalyst composition on surface area, pore size distribution, basicity and activity.

a

Catalyst

Mg:Al

BET surface area (m2 g1)

Avg. pore diameter ðÅÞ

CO2-TPD (lmol of CO2 g1)

Initial activitya (mol g cat1 s1)

CHT10 CHT11 CHT21 CHT31 CHT41

1:0 1:1 2:1 3:1 4:1

165 254 219 235 227

36 58 60 52 51

484 273 462 803 623

5.3  106 3.6  106 8.0  106 11.6  106 8.0  106

Cyclohexanol 0.025 mol, diethyl malonate 0.125 mol, n-dodecane 0.4 cm3, catalyst loading 0.03 g cm3, temperature 100 °C, and speed of agitation 1100 rpm.

the relative pressure and shapes of the adsorption and desorption branches are observed, indicating heterogeneous pore geometry

[34]. There was no specific relationship observed between Mg–Al ratio and average pore diameter (Table 1). Pore diameter remained

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551

Fig. 2. X-ray Diffractogram of CHT31.

almost same for all Mg–Al calcined hydrotalcite samples. All the calcined samples showed very good BET surface area between 165 and 254 m2/g. Hence mesoporous, high surface area material was obtained after calcination. 3.1.4. XRD Fig. 2 shows XRD pattern for calcined hydrotalcite with Mg–Al ratio of 3:1. The characteristic reflections observed at 2h  43 and 63° correspond to a Mg–Al–O (periclase type structure, JCPDS 87-0653). Peaks are broad, showing poorly crystallised MgO-periclase type phase. 3.1.5. Scanning electron microscopy The particle morphology obtained for CHT31 by SEM is shown in micrographs in Fig. S.2 (in the Supplementary material section). The sample morphology can be described in terms of small number of large agglomerates (70–30 lm) (Fig. S.2a and b) and a large number of smaller ones (30–10 lm) (Fig. S.2c and d). The size distribution is very broad. This may be due to, before calcining the dried samples were finely ground in mortar pastel, which resulted in broad distribution for particle size. Hence catalyst used in the actual reaction was sieved and average particle diameter was in

the range of 80–120 lm. An average of 100 lm was used to calculate the solid–liquid mass transfer coefficient. 3.1.6. CO2-temperature programmed desorption CO2-TPD was done for calcined hydrotalcite samples. Typical thermogram for CHT31 is shown in Fig. 3. It is observed that there are basic sites of different strength in calcined Mg–Al hydrotalcite. It comprises of low strength basic sites which may be due to surface hydroxyl groups and medium strength and high strength basic sites which due to surface basic oxygen atoms. Basic site density in terms of moles of CO2 adsorbed at STP is given in Table 1. It is seen that as percentage of Al increases the basic site density of calcined hydrotalcite increases, it passes through maximum for CHT31 and then again decreases. This indicates that, even if isomorphic substitution of Mg2+ by Al3+ is responsible to induce basicity in hydrotalcite like materials, optimum ratio of Mg/Al is required maximise basic site density. This is also supported by the results obtained for catalytic activity testing in studies mentioned ahead. From above characterisation results it is seen that Mg–Al calcined hydrotalcite is a high surface area, mesoporous material. The origin of basicity is surface oxygen species. It also contains basic sites of different strength, which can be tuned depending

Fig. 3. TPD of CO2 thermogram for CHT31.

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60

conversion and only microwave irradiation without catalyst did not initiate the reaction. Thus there is a definite synergism between catalysts and microwave irradiation. Hence all further studies were done using microwave irradiation as source of energy for the reaction.

50

3.3. Effect of catalyst composition

40

Effect of Mg/Al composition on reaction rate was studied under otherwise similar reaction conditions in the absence of any internal and external mass transfer resistance. In the case of Mg–Al calcined hydrotalcite, Al3+ is substituted isomorphically for Mg2+ in MgO lattice. This gives rise to O2 strong Lewis base sites in the lattice. Hence, the number of basic sites increases with an increase in the amount of Al introduced; however, at high Al content the strength of basic sites decreases [35]. In the current case, the highest catalyst activity is obtained when Mg/Al ratio is 3:1. Table 1 shows that as the amount of Al increases in MgO the initial rate of reaction increases, it passes through the maximum for CHT31 i.e. Mg/Al of 3:1 and again decreases for higher Al content. This shows that under the reaction conditions CHT31 provides optimum strength of basic sites for maximum conversion. Also the basic site density is fairly good as seen from the CO2-TPD results and the catalyst has high surface area which collectively results in higher reaction rate for this reaction. Hence catalyst with Mg/ Al ratio of 3:1 (CHT31) was selected for further studies. Scheme 1 shows the reaction.

80

Converison (%)

70

30 20 10 0 0

50

100

150

Time (min.) Fig. 4. Comparison of effect of conventional heating versus microwave irradiation on conversion cyclohexanol: diethyl malonate mole ratio1:5, catalyst loading 0.03 g cm3, n-dodecane 0.4 cm3, temperature 100 °C, speed of agitation 1100 rpm. () Conventional heating. (j) Microwave irradiation.

on method of preparation. Hence depending on requirement of the process, catalyst with basic site strength optimum for the process under study can be prepared.

3.4. Effect of speed of agitation 3.2. Conventional heating versus microwave irradiation The transesterification reactions were performed using conventional heating and controlled microwave irradiation. It was found that under otherwise similar reaction conditions, cyclohexanol conversion in conventionally heated system was 20%, but microwave irradiated system showed more than threefold increase in cyclohexanol conversion to 69% in 2 h (Fig. 4). Selectivity to mono-transesterification was the same at 96% in both the cases. This shows that under microwave irradiation rate of reaction is enhanced. Microwave irradiation leads to an instantaneous localized superheating due to dipole rotation or ionic conduction and the energy transfer occurs within 109 s with each cycle of electromagnetic energy and is faster than the rate at which the molecules can relax (105 s) which results in non-equilibrium conditions and high instantaneous temperatures. An increase in temperature causes greater movement of molecules leading to a greater number of energetic collisions and hence enhancement in reaction rate and product yield. The reactant cyclohexanol and co-product ethanol are both polar which are in the vicinity of the sites, which are heated rapidly due to material–wave interactions, leading to thermal effects (connected to dipolar and charge space polarization) and purely non-thermal (accelerating the molecular rotation) effects and this leads to improved catalyst activity. The thermal gradients and flow of heat are the reverse of those in materials heated by conventional means and this is mainly responsible for the increased activity and selectivity. Thus, microwave irradiation acts synergistically with base catalysis. In the reaction mixture, cyclohexanol and ethanol may be the good microwave absorbing liquid and its dipole may be reorienting quickly under microwave irradiation. It was found that under microwave irradiation, the reaction rate improved up to 3.5-fold and shorter period of time was needed to achieve the higher conversion as compared to that of conventional heating. The control experiments in the absence of catalyst did not show any

In order to assess the role of external mass transfer on the reaction rate, the effect of speed of agitation was studied in the range of 700–1100 rpm under otherwise similar reaction conditions (Fig. 5). It was observed that conversion of cyclohexanol was marginally less at 700 rpm, but it was practically same at 900 rpm and 1100 rpm without any change in the selectivity. Thus, it showed that external mass transfer effects did not influence the reaction rate beyond 900 rpm. On the conservative side all further reactions were done at 1100 rpm. A theoretical analysis of this assessment is provided in support of this observation. Details of this theory for general slurry reactions are given elsewhere [36–38]. For a solid–liquid slurry reaction it is important to ascertain the influence of solid–liquid mass transfer resistance, before true kinetic model is developed. This reaction involved the transfer of cyclohexanol, the limiting reactant (A), and diethyl malonate (B) form bulk liquid phase to the catalyst wherein external mass transfer of reactants to the surface of the catalyst particle, followed by intraparticle diffusion, adsorption, surface reactions, and desorption. Depending on the relative rates, different controlling mechanisms have been put forward [36–38]. The liquid phase diffusivity values of the reactants A (cyclohexanol) and B (diethyl malonate), denoted by DAB and DBA, and were calculated by using the Wilke–Chang equation [67] at 100 °C as 1.31  105 and 0.7  105 cm2 s1, respectively. The solid–liquid mass transfer coefficients for both A and B were calculated from the limiting value of Sherwood number (e.g., ShA = kSLA dp/DAB) of 2. The actual Sherwood numbers are typically higher by order of magnitude in well agitated systems but for conservative estimations a value of 2 is taken. The solid–liquid mass transfer coefficients kSLA and kSLB values were obtained as 2.18  103 and 1.17  103 cm s1, respectively. Thus, the values of mass transfer rates of A and B from the bulk liquid to the external surface of the catalysts, kSLA ap CA0 and kSLB ap CB0 were typically found to be 7.26  105 and 1.94  104 gmol cm3 s1, respectively at mole ratio A:B of 1:5. The initial observed rate of reaction was found to be 3.51  107 mol cm3 s1, which is two orders of

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G.D. Yadav, A.A. Kadam / Chemical Engineering Journal 230 (2013) 547–557 C2H5 O OH O C2H5

O

O O

O

C2H5 +

Hydrotalcite MW

Diethyl malonate

O

+

C2H5OH

O

Cyclohexanol

Ethanol

Cyclohexyl ethyl malonate Scheme 1. Reaction pathway.

80

3.5. Effect of catalyst loading

70

In the absence of mass transfer resistance, the rate of reaction is directly proportional to catalyst loading based on the entire liquid phase volume. The catalyst loading was varied over the range of 0.01–0.04 g cm3 on the basis of total volume of the reaction mixture. Fig. 6 shows the effect of catalyst loading on the conversion of cyclohexanol. The conversion was found to increase with increase in catalyst loading, which is obviously due to the proportional increase in the number of active sites. At higher catalyst loading, the rate of mass transfer is excessively high and therefore there is no more increase in the rate. Hence all further studies were done at 0.03 g cm3 catalyst loading. At steady state, the rate of external mass transfer (i.e. from the bulk liquid phase in which A and B are located with concentration [A0] and [B0] respectively) to the exterior surface of the catalyst is proportional to ap, the exterior surface area of the catalyst where the concentrations of A and B are [As] and [Bs], respectively. For a spherical particle, ap is also proportional to w (i.e., ap = 6w/qpdp), the catalyst loading per unit liquid volume. It is possible to calculate the values of [As] and [Bs]. For instance:

Conversion (%)

60 50 40 30 20 10 0 0

20

40

60

80

100

120

140

Time (min.) Fig. 5. Effect of speed of agitation on conversion. Cyclohexanol:diethyl malonate mole ratio 1:5; n-dodecane 0.4 cm3; catalyst loading 0.03 g cm3; temperature 100 °C. () 700 rpm, (j) 900 rpm and (N) 1100 rpm.

K SLA ap f½Ao  ½Asg ¼ r obs

at steady state 3

¼ 3:51667  107 mol cm

80

ð1Þ

Thus putting the appropriate values, it is seen that [As]  [Ao], similarly [Bs]  [Bo]. Thus, any further addition of catalyst is not going to be of any consequence for external mass transfer.

70 60

Conversion (%)

s1

3.6. Absence of intra particle diffusion resistance

50 40 ;

30 20 10 0 0

20

40

60

80

100

120

140

Time (min)

The average particle diameter of the catalyst used in this reaction was 0.012 cm and thus a theoretical calculation was done based on the Wiesz–Prater criterion to assess the influence of intraparticle diffusion resistance [39]. According to this, the dimensionless parameter Cwp which represents the ratio of the intrinsic reaction rate to intra-particle diffusion rate and can be evaluated from the observed rate of reaction, the particle radius (Rp), effective diffusivity of the limiting reactant (De) and concentration of the reactant at the external surface of the particle.

Fig. 6. Effect of catalyst loading on conversion. Cyclohexanol:diethyl malonate mole ratio 1:5, n-dodecane 0.4 cm3; temperature 100 °C; speed of agitation 1100 rpm. () 0.01 g cm3, (j) 0.02 g cm3, (N) 0.03 g cm3, and () 0.04 g cm3.

(i) If C wp ¼ robs qp R2p =De ½As  1, then the reaction is limited by severe internal diffusional resistance. (ii) If Cwp 1, then the reaction is intrinsically kinetically controlled.

magnitudeless than the external mass transfer rate. This indicates that the reaction rate is independent of the external mass transfer effects.

The effective diffusivity of cyclohexanol (De) inside the pores of the catalyst was obtained from the bulk diffusivity (DAB), porosity (e), and tortuosity (s) as 6.43  106 cm2 s1 where De = DAB. e/s. In the present case, the value of Cwp was calculated as

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Table 2 Effect of mole ratio on conversion and selectivity and initial activity.

120

Cyclohexanol conversion (%)a

Selectivity for cyclohexyl ethyl malonate (%)

Initial activity (mol g cat1s1)

1:5 1:3 1:1 2:1 5:1

69 43 19 37 18

98 84 75 72 58

2.58  104 1.91  104 1.55  104 1.03  104 2.64  104

100

Conversion (%)

Cyclohexanol: diethyl malonate molar ratio

Reaction conditions: n-dodecane 0.4 cm3; catalyst loading 0.03 g cm3; temperature 100 °C; and speed of agitation 1100 rpm. a Reaction duration – 120 min).

80

60

40

20

0

0.90  103 for the initial observed rate, which is much less than 1 and this signifies the absence of resistance due to intraparticle diffusion. Hence the reaction could be considered as an intrinsically kinetically controlled reaction.

0

50

100

150

Time (min) Fig. 7. Effect of temperature on conversion. Cyclohexanol:diethyl malonate 1:5, ndodecane 0.4 cm3, catalyst loading 0.03 g cm3, speed of agitation 1100 rpm. () 100 °C, (j) 105 °C, and (N) 110 °C.

3.7. Effect of mole ratio The effect of the mole ratio of the reactants was studied at cyclohexanol to diethyl malonate mole ratio of 1:5, 1:3, 1:1, 2:1 and 5:1 under otherwise similar conditions. As seen from Table 2, as the concentration of diethyl malonate was increased from its equimolar concentration, the conversion as well as initial rate of the reaction increased. Also increase in conversion and rate of reaction was observed when the concentration of cyclohexanol was increased from its equimolar concentration. This showed that rate of reaction is dependent on the concentration of both the reactants. But at higher concentration of cyclohexanol selectivity towards mono-transesterification decreases, hence all further studies were done at the mole ratio of 1:5 (cyclohexanol:diethyl malonate).

Similarly adsorption of diethyl malonate (B) on the vacant site is presented by following equation: KB

B þ S ¢ BS

ðbÞ

2. Surface reaction of AS with BS (diethyl malonate), in the vicinity of the site, leading to formation of cyclohexyl ethyl malonate (ES) on the site. K2

AS þ BS ¢ ES þ WS

ðcÞ

Desorption of cyclohexyl ethyl malonate (ES) and ethanol (WS): 1=K E

ES ¢ E þ S 1=K W

WS ¢ W þ S

3.8. Effect of temperature

ðdÞ ðeÞ

The total concentration of the sites, Ct expressed as:

The effect of temperature was studied at 100, 105 and 110 °C (Fig. 7). The conversion was found to increase substantially with increasing temperature, which suggested that the reaction was intrinsically kinetically controlled and the activation energy values should be determined, which will be discussed later.

C t ¼ C S þ C AS þ C BS þ C ES þ C WS or,

Ct ¼ CS þ K A CA CS þ K BCB CS þ K ECE CS þ K WCWCS

ð2Þ

or, the concentration of vacant sites,

Ct ð1 þ K A C A þ K B C B þ K E C E þ K W C W Þ

3.9. Reaction mechanism and kinetics

CS ¼

From the calculated values of mass transfer rates of A and B, initial observed rate, it is evident that the rate is independent of the external mass transfer effects. It is also seen from the values of activation energy, that the intra-particles diffusion resistance is absent. Thus, the reaction could be controlled by one of the following steps, namely: (a) adsorption, (b) surface reaction or (c) desorption. Therefore, for the further development of model, the actual reaction mechanism was under taken. The initial rate data can be analysed on the basis of Langmuir– Hinshelwood–Hougen–Watson (LHHW) or Eley–Rideal mechanisms. From the initial rate data, the following analysis was found to be the most appropriate. The mechanism shown in Scheme 2 can be used to arrive at the LHHW type of mechanism.

If surface reaction controls the rate of reaction, then the rate of reaction of A is given by following equation:

1. Adsorption of cyclohexanol (A) on the vacant site S is given by following equation:

¼

KA

A þ S ¢ AS

ðaÞ

rA ¼

dC A 0 ¼ k2 C AS C BS  k2 C ES C WS dt

dC A k2 fK A K B C A C B  ðK E K W C E C W Þ=K 2 gC 2t ¼ dt ð1 þ K A C A þ K B C B þ K E C E þ K W C W Þ2

ð3Þ

ð4Þ

ð5Þ

When the reaction is far away from the equilibrium:

dC A k2 C 2t K A K B C A C B ¼ P dt ð1 þ K i C i Þ2 kR2 wC A C B P ð1 þ K i C i Þ2

ð6Þ

ð7Þ

where kR2 w ¼ k2 C 2t K A K B and w is catalyst loading. If the adsorption constants are very small, then the above equation reduces to:

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G.D. Yadav, A.A. Kadam / Chemical Engineering Journal 230 (2013) 547–557

HO

H5 C2

C2 H5

O O

+ H 5C 2

O

O

C2 H5

H

O

O

O O

O

O

O

H5 C2

O

O

HO

C 2H 5 H5 C2

O

O O

O H5 C2

O

O

O

C2 H5

O

C 2H 5

H

O

O

+H H 5C 2

O

O O

C2 H5

O

+H

O

Scheme 2. Proposed reaction mechanism.

dC A ¼ kR2 C A C B w dt

ð8Þ

Let,

ð9Þ

This upon integration leads to:

C B0 ¼M C A0

ð8aÞ

lnfðM  X A Þ=Mð1  X A Þg ¼ kR2 wC A0 ðM  1Þt

M–1

ð10Þ

And

The molar ratio of diethyl malonate to cyclohexanol at t = 0. Then Eq. (8) can be written in terms of fractional conversion as:

0.6

0.4

0.4

0.2

0.2

0

0 0

10

20

30

40

50

-0.2

-0.2

-0.4

-0.4

XA ¼ kR2 wC A0 t 1  XA

M¼1

ð11Þ

1.4 1.2

ln[(M-X A)/M(1-X A)]

0.6

XA/1-XA

ln{(M-XA)/(M(1-XA))}

dX A ¼ kR2 wC A0 ð1  X A ÞðM  X A Þ dt

1 0.8 0.6 0.4 0.2 0

-0.6

Time (min.)

-0.6

Fig. 8. Validation of model for all values of mol ratio (M). The right hand side Y-axis for M = 1, the lower line for M = 0.2. (N) 5:1, () 1:1, () 1:3, and (j) 1:5.

0

10

20

30

40

50

Time (min.) Fig. 9. Validation of model for different temperatures for same value of M () 100 °C, (j) 105 °C, and (N) 110 °C.

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G.D. Yadav, A.A. Kadam / Chemical Engineering Journal 230 (2013) 547–557

surface area of the used catalyst were measured including basicity as given earlier. XRD pattern was also recorded. It was within 3–5% of original value in each case. Thus the catalyst is robust and stable.

7 6.8

4. Conclusion

6.6

The current work has investigated the synergism between solid base catalyst, Mg–Al calcined hydrotalcite and microwave irradiation in transesterification of diethyl malonate with cyclohexanol to give cyclohexyl ethyl malonate. The rates of reaction are higher by orders of magnitude that those for conventional heating. The reaction is selective mono-transesterification. The catalyst is highly active, selective and robust. A kinetic model for the reaction mechanism was successfully developed and it follows Langmuir– Hinshelwood–Hougen–Watson (LHHW) mechanism.

ln k

6.4 6.2 6 5.8 5.6 0.0026

0.00262

0.00264

0.00266

0.00268

0.0027

1/T (K-1)

Acknowledgements G.D.Y. thanks the Darbari Seth Professor Endowment, R.T. Mody Distinguished Professor Endowment, and Department of Science and Technology, Govt. of India for J.C. Bose National Fellowship. A.A.K. acknowledge Ambuja Educational Institute for awarding Senior Research fellowship. Part of the work was also supported by NMTLI programme of CSIR.

Fig. 10. Arrhenius plot.

100 90

Appendix A. Supplementary material

80 70

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2013.06.075.

60 50

References

40 30 20 10 0 Fresh use % conversion

1st reuse

2nd reuse % selectivity

Fig. 11. Catalyst usability. Cyclohexanol:diethyl malonate 1:5,n-dodecane 0.4 cm3; catalyst loading 0.03 g cm3; speed of agitation 1100 rpm, and temperature 100 °C.

Thus, a plot of ln {(M  XA)/(M (1  XA))} against time is shown in Fig. 8 for different values of M. It is seen that the data fit very well. The values of rate constants at different temperatures were obtained from the similar plots made at different temperatures (Fig. 9), which once again shows an excellent fit and the reaction confirms the proposed model. The Arrhenius plot of ln k1 versus 1/T was made to get the apparent activation energy as 25.10 kcal mol1 (Fig. 10). The high value of activation energy also suggests that the reaction was intrinsically kinetically controlled. The reaction follows second order kinetics at fixed catalyst loading. 3.10. Reusability of catalyst The reusability of Mg–Al calcined hydrotalcite was verified by employing it three times. After the each run the catalyst was washed thoroughly with methanol and dried at 60 °C. A make-up quantity of the catalyst was added to cover lost catalyst during filtration. The results are shown in Fig. 11. It is seen that the conversion for second reuse remained almost same as that of first one without any change in selectivity. Pore size distribution and

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