Nanoporous Montmorillonite Catalyzed Condensation

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Nanoporous Montmorillonite Catalyzed Condensation Reactions under Microwave Irradiation: A Green Approach S. Ramesha,b, B.S. Jai Prakasha,c and Y.S. Bhata* a

Chemistry Research laboratory, Bangalore Institute of Technology, K.R Road, Bangalore, India

b

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, South Korea

c

Director, IEHMM,V.V Pura College of Science, K.R Road, Bangalore, India Abstract: Acid treatment of montmorillonite enhanced the acidity and also nanoporosity of the virgin clay. Montmorillonite clay was treated with p-TSA and HCl under microwave irradiation (MWI) and modified materials were characterized by BET, TPD-NH3 and FT-IR techniques. The activities of clays were studied for important condensation reactions such as the crossed aldol condensation and amide synthesis without solvent under MWI. Under conventional heating, maximum conversion was observed in 300 minutes while the same conversion was achieved in just 30 minutes under MWI. The activities of p-TSA treated clay were compared with HCl treated clays. The p-TSA treated clay showed surprisingly higher activity compared to other catalysts. The activity was attributed to the dealumination of structural Al and its migration to the interlayer region of the clay. The removal of Al made the material more nanoporous due to which accessibility to the acidic sites was better than the unmodified clay. Enhanced activity was due to higher amounts of interlayer aluminium in the p-TSA treated compared to HCl-treated ones. p-TSA treated clays retained their activity even after three subsequent runs in both the modes of heating. The catalyst used in the reaction rejuvenated by a simple procedure could be used for catalyzing the reaction again.

Keywords: Aldol condensation, dealumination, microwave irradiation, montmorillonite, solvent free reactions. INTRODUCTION Condensation reactions are catalyzed not only by bases but also by acids and considered to be one of the most important carbon-carbon bond-forming reactions in organic synthesis [1]. The aldols find usage as anti-AIDS agents, cytotoxic with antiangiogenic activity, antimalarial antiinflammatory and antitumor [2]. The condensation of ketones with aldehydes is of special interest and the crossedaldol condensation is an effective pathway for carbon-carbon bond forming reactions. However, the traditional acid-base catalyzed reactions resulted in poor yield due to the reversible nature of the reaction [3] and self condensation of starting reactant molecules [4]. A general method for the formation of a carbon-carbon bond in many classes of carbonyl compounds is aldol condensation [5-7]. Due to the importance of the methylene structural unit, which is found in many naturally occurring compounds and antibiotics [8], the condensation of cycloalkanones with aldehydes, ketones and anilines with aromatic carboxylic acids are of special interest. Many researchers have reported a variety of catalysts such as zirconium compounds ZrCl4 [9] and Cp2ZrH2in combination with metal salts for the aldol condensation of cycloalkanones. Bis(pmethoxyphenyl) telluroxide [10], KF-Al2O3[11], anhydrous RuCl3 and TiCl3 (SO3CF3) have alsobeen used for this purpose *Address correspondence to this author at the Chemistry Research laboratory, Bangalore Institute of Technology, K.R Road, Bangalore, India; Tel: +918026615865; Fax: +918022426796; E-mail: [email protected] 2213-3372/15 $58.00+.00

under solvent-free conditions. Recently tin hydroxyl chloride was reported as effective catalyst for Prince Condensation reactions [12]. The use of expensive and toxic reagents, long reaction time, low yields, and the formation of a mixture of products are some of the drawbacks of the reported methods. Acid catalyzed condensation reactions are generally carried out by using strong protonic acids and Lewis acids such as H2SO4, AlCl3 and BF3. These catalysts, besides being highly corrosive, are often required to be used in more than stoichiometric amounts. They are difficult to handle, not reusable and the work up procedure generates large amounts of polluting wastes [13]. Efforts have been made to replace these acids by reusable, eco-compatible heterogeneous solid acids like zeolites [14-17], acidic clays [18-21] and hetero poly acids [22]. Acid activated montmorillonite is one of the extensively used solid acid catalysts for organic synthesis like alkylation, isomerization, acylation, dehydration and esterification [1822]. There is a demand for acid activated clays for carrying out different organic reactions. In this context it is important to develop acid treated clays with different number, strength and nature of acid sites also enhanced accessibility to the acidic sites. There are a very few studies that correlate the acid activation parameters with structural modifications and their catalytic activity for various organic transformations [23, 24]. Finding environmentally friendly catalysts to achieve solvent free condensation in short intervals of time is a challenging task for green organic synthesis. Microwave assisted © 2015 Bentham Science Publishers

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Current Organocatalysis, 2015, Vol. 2, No. 1

Ramesh et al.

organic synthesis has attracted considerable interest in recent years. It has been reported that in all types of thermally driven chemical reactions, use of microwave irradiation as a source of heat can profoundly reduce the reaction times.

therm measurements at -196 °C have been carried out in order to study the porosity and textural variation of the acid treated clays under microwave irradiation. Full adsorption/desorption cycles were determined up to the saturation vapor pressure of nitrogen at -196 °C. Porosity of the samples was determined by nitrogen adsorption/desorption isotherms. Pore size distribution was calculated from adsorption data using BJH method. The Lewis and Bronsted acidity of the modified clays were measured by IRspectroscopy using pyridine as probe molecule. All the samples were activated by degassing at 110°C for 2 h andthen saturated with pyridine. The catalyst samples were evacuated at 150°C for 2 h to remove physisorbed pyridine. IR spectra of the samples were then recorded in the range 400–4000 cm-1 using Bruker model Alpha-P IR spectrophotometer having resolution of 4 cm-1 fitted with anATR cell.

EXPERIMENTAL Materials The used montmorillonite is obtained from Bhuj area, Gujarat, India supplied by Ashapura Group of Industries, India. The clay was treated with 1 M NaCl solution to convert sodium form. The composition was found using ICP to be 42.86% SiO2, 16.64% Al2O3, 10.05% Fe2O3, (2.58% MgO, 1.91% Na2O, with idealized structural formula Si4[Al1.348Fe0.386Mg0.266]O10(OH)2Na(0.324). The chemicals p-Toluene sulphonic acid, aniline, aldehydes, cyclohexanone and cayboxylic acids were procured from S.D fine chemicals, India. Aldedydes and aniline were distilled before used in the reaction. All other chemicals were used directly without further purification.

Total acidity of samples was obtained by Temperature Programmed Desorption (TPD) ofammonia. TPD of NH 3 experiments were carried out with AMI 200 equipment (Alta Mira). Sample was first activated at 120°C (10°C /min) for two hours and cooled to 120°C in a stream of flowing helium. The sample was saturated by continuously flowing ammonia at 120°C. The temperature-programmed desorption was performed by ramping the sample temperature at 10°C /minute to 300°C monitoring the concentration of the desorbed species by built-in thermalconductivity detector.

Catalyst Preparation Clay catalyst was prepared using a method reported elsewhere [25]. In a typical procedure, 20 g of Montmorillonite and 200 ml of 2 mol L-1 aqueous solution of p-TSA or HCl were taken in a 500 ml round bottom flask. The mixture was refluxed under microwave irradiation with built in infrared temperature control and magnetic stirrer (Microwave lab station START-S) for 10 minutes. After the reaction, mixture was centrifuged and the residual solid obtained was washed thoroughly with hot water until the centrifugate has neutral pH. In case of HCl clay, the washing was carried out until negative test for chloride as tested by silver nitrate was observed. The solid obtained was dried at 120°C for 4 hours and ground to a fine powder for further studies. The modified clays were designated as 2M p-TSA clay and 2M HCl-clay.

Catalytic Activity Microwave-irradiated reactions were carried out in a microwave lab station ‘START-S’ having software that enables the on-line control of temperature of the reaction mixture with the aid of infrared sensor. Reactions studied in presence of catalyst samples were (a) aldol condensation and (b) amide synthesis. All the reactions were carried out in a 50ml glass vessel. Reactor vessel was kept in such a way that the reaction mixture was exactly in line with infrared sensor that monitors the temperature. Power up to 1000W was applied by microprocessor-controlled single magnetron to maintain the temperature of the system. Under conventional heating, reactions were carried out in stainless steel autoclave. Autoclave was heated in a hot air oven with microprocessor based temperature controller. Replicate measurements were carried out and mean of the three nearest results have been reported.

Catalyst Characterization Untreated clay and the acid treated clay samples were characterized by various techniques to compare the structural and textural features before and after acid treatment under microwave irradiation. Surface area measurements were done using Quanta Chrome Nova-1000 surface analyzer under liquid nitrogen temperature. Nitrogen adsorption iso-

a)

b) a) Amide synthesis b) Crossed aldol condensation

Nanoporous Montmorillonite Catalyzed Condensation Reactions under Microwave Irradiation

Table 1.


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Surface characters of the p-TSA and HCl treated clay samples. Catalyst

Surface Area (m2 /g)

Pore Volume (cm3 /g)

Pore Diameter (nm)

Untreated Clay

29.6

0.037

50.27

2 M p-TSA- Clay

136

0.117

35.31

2 M HCl-Clay

142

0.194

32.40

Procedure for Aldol Condensation Reaction In a typical procedure, 10 mmol of cyclohexanone, 20 mmol of aldehydes and 0.5 g of p-TSA clay catalyst were mixed in 50 ml glass vessel and the mixture was irradiated with microwaves up to 30 minutes. After completion of the reaction, ethyl acetate was added to the solidified mixture and the insoluble catalyst was separated by filtration. The filtrate was dried over anhydrous Na2SO4. The solvent was evaporated to get the solid product. Then it was subjected to GC–MS analysis to confirm the products. The structure of product obtained was further confirmed by FT-IR and 1H NMR analysis. A variety of substituted benzaldehydes were used for this reaction.

dine was assigned to 1440 cm-1 and the Brønsted acid-bound pyridinium cation to 1550 cm-1. The peak at 1490 cm-1 is assigned to Bronsted+ Lewis acid sites. p-TSAclay showed spectra with all the peaks at specified wave numbers. HCltreated clay sample, however, showed a few more minor peaks. This is possibly due to the leaching out of structural Mg and Fe (found in the leachate) resulting in different types of acid sites in the voids that interact with pyridine.

Procedure for Amide Synthesis The vessel was charged with 5 mmol aromatic acid, 10 mmol aniline, and 0.2 g p-TSA clay catalyst. The reaction mixture was irradiated with microwaves up to 1000 W power for 10 minutes at desired temperature. The reaction mixture was cooled, dissolved in 20 ml toluene and filtered to separate the catalyst. Analysis of the reaction ingredients before and after the reaction was performed by gas chromatography (Chemitomodel GC1000, FID detector) using a BP 20 capillary column (30 m × 0.32 mm) and the products were confirmed by GC-MS studies.

Fig. (1). IR spectra of pyridine adsorbed clay samples.

RESULTS AND DISCUSSION

Total Acidity by TPD-NH3

Surface area Measurements by BET

In our previous work, we had observed a clear correlation between the reduction in CEC as a consequence of the removal of structural Al by acid treatment and the increase in the interlayer Al[25]. The dislodged Al migrates into the interlayer thus causing an increase in accessible edge site hydrated aluminium ions. Total acidity as determined by TPD ammonia method for different clay catalysts is presented in Table 2. The ammonia uptake could be attributed to the increase in the interlayer hydrated aluminum that gets exposed at the edge sites of the clay platelets. In the case of p- TSA treated clays, as reported earlier, there is a good correlation between the acidity and interlayer Al with the increase in concentration of p-TSA [25]. One important difference was, in the case of p-TSA treatment the CEC values of the catalyst samples were found to be almost the same as the parent clay. However, a decrease in CEC was observed in the case of HCl treatment. The latter is due to the fact that HCl removes Fe and Mg ions responsible for the layer charge along with structural Al. As a consequence, the CEC of the treated clay decreases with the HCl treatment which explains the lower amounts of interlayer Al as compared with p-TSA treated clay samples (Table 2).

(Table 1) gives the surface areas of untreated and acid treated clays as determined by adsorption of nitrogen by the BET method. The surface area of the untreated clay is 29.6 m2 g-1 of clay and that for p-TSA and HCl treated clays are respectively 136 m2 g-1 and 142 m2g-1. A slightly higher increase in surface area in the case of HCl treatment may be due to the removal of Fe and Mg ions in addition to Al ions in the octahedral layer (Table 1). Pore volume and pore diameter clearly indicates there is an increase in nanoporosity of the clay modified by acid treatment. Acidity, Interlayer Al and CEC Interlayer cations are known to be responsible for the acidity of clay samples. The water molecules in the hydration shell of the metal ions in the clay interlayer are known to act as the Brønsted acid sites. The acidity measurements as determined by IR- ATR studies are shown in (Fig. 1). The IR absorption bands are assigned according to our previous reported results [26]. Accordingly, Lewis acid bound pyri-

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Table 2.

Ramesh et al.

Amount of interlayer Al, CEC and TPD-NH3 of p-TSA and HCl treated clays. Catalyst

TPD Ammonia Desorbed µmol/g clay

CEC meq g-1

Interlayer Al, meq/g

Catalytic Activity for Aldol Condensation

Untreated Clay

-

0.83

-

nil

p-TSA Clay

218

0.72

0.70

65

HCl Clay

125

0.59

0.45

34

Reaction conditions: cyclohexanone (10 mmol), anisaldehyde (20 mmol), 0.5 g of p-TSA clay catalyst, 30 minutes at 160 °C under microwave irradiation.

(a)

(b)

Reaction conditions (a) for aldol reaction: cyclohexanone (10 mmol), anisaldehyde (20 mmol), 0.5 g of p-TSA clay catalyst, 30 minutes at 160 °C (b) for amide reaction: benzoic acid (5 mmol) aniline (10 mmol), 0.2 g of p-TSA clay catalyst at 160°C. Error bars within ±5% yield for three results are shown

Fig. (2). Catalytic activity for the amide synthesis and aldol condensation (a) Microwave heating (b) Conventional heating.

Catalytic Activity The activity of the catalyst is also found to increase with the increase in interlayer Al. Similar reports have been published relating the catalytic activity and interlayer Al in the case of Al3+-exchange clays. [18, 25, 26] The catalytic activity for aldol condensation is shown in Table 2. In comparison with p-TSA-treated clays, HCl treated samples, showed reduced catalytic activity which is attributed to lower amounts of interlayer aluminum. It could also be seen from (Table 2) that the p-TSA clay showed higher acidity, interlayer Al and the yield of aldol. Further accessibility to active sites was much better with the removal of Al in the octahedral layer resulting in enhanced nanoporosity. The results had a similar trend in the case of of amide yield. Comparison of Activity under Microwave and Conventional Heating Effect of reaction time on the condensation reactions were studied with p-TSA treated clay catalyst. Reaction time was varied from 5 to 60 minutes under microwave heating and 30 to 300 minutes under conventional heating. Percentage yield of the product obtained as a function of reaction time under microwave and conventional heating are shown in (Fig. 2a and 2b) respectively. The results showed that yield of the product increased with increase in time in both the modes of heating. Conventional heating required more time to get the same conversion as that of microwave heating,

which is due to the different mode of heat transfer from the source to the reactants mixture in the vessel. In order to understand the effect of microwave on the conversion, time taken to reach the required temperature were measured for the both the modes of heating. It was observed that just 22 seconds is enough to reach the required temperature with microwaves, whereas conventional heating required longer time. This is due to the heating mechanism of microwaves that is, microwaves are transparent to the reaction vessel and heat the reactants directly whereas in conventional heating, vessel gets heated first and then it will transfer the heat to the reactants. Effect of Substrates on Aldol Condensation Effect of different substrate molecules in the aldol condensation was investigated in order to understand the behaviour of the active sites developed on the catalyst by the pTSA treatment. A range of substrates such as benzaldehyde, and substituted benzaldehydes were chosen for aldol condensation reaction to examine the relative effect of substituents on the reaction. The mechanism of the reaction is given in (Scheme 1). In the first step of the reaction, Brønsted acid site of the clay interacts with keto group of the cyclohexanone to give an enol. Further, the enol formed reacts with the benzaldehyde to form an addition product, which eliminates water molecule to give the intermediate product α, β unsaturated carbonyl compound. The intermediate product formed undergoes alkylation with another molecule of alde-



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Scheme 1. Plausible mechanism for the aldolcondensation reaction.

hyde to give 2, 6 - α, β unsaturated carbonyl compound as final product. In all the cases, appreciable yields of the products were obtained. The results of the reactions studied under optimal conditions are presented in Table 3. It is observed that nitro substituted benzaldehydes are produced in highest yield in comparison with benzaldehyde, methoxybenzaldehyde p-chlorobenzaldehyde, pbromobenzaldehyde and p-hydroxybenzaldehyde. This may be attributed to the electron withdrawing nature of the nitro group which activates the aldehyde group by enhancing the electrophilicity, leading to higher yields. On the other hand,hydroxy and methoxy groups deactivate the aldehyde group towards nucleophilic addition due to their electron donating nature. In the case of chloro-benzaldehyde and bromo-benzaldehyde, the low yields are attributed to the mesomeric effect. Activity of the Regenerated Catalyst The catalyst was found to be recyclable. After each cycle the used catalyst was filtered, washed twice with distilled water and activated at 120°C for about 4 hours before being used for the next cycle. There was no change in structural integrity of the clay before and after p-TSA treatment and also after using in the catalytic reaction (Fig. 3). The activity of the catalyst was the same even after three cycles which is due to enhanced nanoporosity, which increased the accessibility for the reacting molecules to approach active sites. (Table 4) Summarize results of these experiments.

Fig. (3). XRD pattern of clay samples (a) untreated (b) p-TSA treated before reaction (c) p-TSA treated after the reaction.

CONCLUSION The p-TSA dealuminated clays showed higher reactivity under microwave irradiation for the condensation reactions than HCl treated clays. This is attributed to the higher amounts of interlayer aluminium in the p-TSA clays compared to HCl-treated ones. The higher amount of interlayer Al on dealumination, in the case of p-TSA clay, is evidently due to the non-removal of structural Fe and Mg ions responsible for the layer charge. Aldol condensation with substituted aldehydes showed that the electron withdrawing groups favoured the reaction, while the electron donating groups

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Ramesh et al.

Table 3. Effect of substrates on aldol condensation.

Reaction conditions: Cyclohexanone( 10 mmol), aromatic aldehydes ( 20 mmol), 0.5 g of p-TSA clay catalyst,30 minutes at 160°C under microwave irradiation.

Table 4.

Activity of the regenerated p-TSA clay catalyst.

Catalyst

% Yield of Aldol

% Yield of Amide

Fresh catalyst

65

62

First regeneration

65

63

Second regeneration

64

62

Third regeneration

65

63

Reaction conditions for aldol synthesis: Cyclohexanone( 10 mmol), anisaldehyde(20 mmol), 0.5 g of p-TSA clay catalyst,30 minutes at 160°C under microwave irradiation. Reaction conditions for amide synthesis: Benzoic acid 5 mmol, aniline10 mmol, 0.5g of p-TSA clay, 10 minutes at 160°C.

showed the opposite effects. The used regenerated catalyst by a simple procedure could be used again in condensation reactions exhibiting the same activity of the fresh catalyst. This is attributed to enhanced nanoporosity in p-TSA treated catalyst, which increased the accessibility for the reactants to approach the active sites. The present study highlights an alternative green chemistry approach for organic reactions by microwave irradiation. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest.


ACKNOWLEDGEMENTS

[12]

The authors sincerely acknowledge the VTU, Belgaum for funding for this project. The authors also acknowledge thank the Principal and the Governing Council of Bangalore Institute of Technology for the encouragement and facilities provided. They wish to express their gratitude to Dr. B. S. Nanjundaiah and Dr. M. Peeran, P. G. Department of Chemistry, VVPSC, Bangalore for discussion.

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Received: July 11, 2014

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Revised: October 01, 2014

Accepted: October 02, 2014