Silicotungstic Acid Modified Bentonite: an Efficient Catalyst for Synthesis of Acetal Derivatives of Aldehydes and Ketones
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Journal of Analytical Sciences, Methods and Instrumentation, 2013, 3, 193-201 193 Published Online December 2013 (http://www.scirp.org/journal/jasmi) http://dx.doi.org/10.4236/jasmi.2013.34025 Silicotungstic Acid Modified Bentonite: An Efficient Catalyst for Synthesis of Acetal Derivatives of Aldehydes and Ketones Reshu Chaudhary, Monika Datta* Department of Chemistry, University of Delhi, New Delhi, India. Email: *[email protected] Received September 7th, 2013; revised October 7th, 2013; accepted October 19th, 2013 Copyright © 2013 Reshu Chaudhary, Monika Datta. This is an open access article distributed under the Creative Commons Attribu- tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Acetals and ketals are among the important materials of organic synthesis and as protecting agent of carbonyl function- ality. A milder, efficient and green synthesis of acetals and ketals has been developed using Silicotungstic acid modified Bentonite (STA-Ben) as a catalyst. STA-Ben has been synthesized and characterized by various analytical techniques. It has been found to be an efficient and reusable catalyst for the synthesis of acetyl derivatives in excellent yields. In order to elucidate the efficiency of the STA-Ben as catalyst, reaction has also been performed using various catalysts. The re- action conditions (time and amount of catalyst) have been optimized using various catalysts. The products of the vari- ous reactions have been characterized by FT-IR, NMR. Keywords: Silicotungstic Acid; Bentonite; Aldehydes; Ketones 1. Introduction Therefore, there is further scope to explore mild and ef- ficient methods for acetalization of carbonyl compounds. Acetals and ketals are one of the most important per- Solid acids offer many advantages by their nature, fume materials and industrial materials of organic syn- over soluble counterparts such as aluminium chloride and thesis. The protection of carbonyl functionality as an ace- hydrogen fluoride. The substitution of liquid acids by tal or thioacetal derivative is a common practice in orga- solids as catalysts for organic synthesis offers a potential nic synthesis owing to their stability under mild acidic for superior effectiveness and environmental integrity. and basic conditions. They are generally obtained by the Although they differ in structure from liquid acids, solid condensation of carbonyl compound with acetic anhy- acid catalysts work by the same principle. Among the dride [1], diols, dithiols and orthoformates [2-5], using solid acids, heteropoly acids (HPAs) have been exten- Lewis acids as catalysts. A variety of catalysts are em- sively studied as acid catalysts for many reactions and ployed for this purpose like CoCl2 [6], NiCl2 [2], HCl [7], HClO [8], sulfated zirconia [9] etc. But the most recent found industrial applications in several processes [15]. The 4 Keg- gin-type HPAs typically represented by the formula methods employ MMT-K10 [10], ionic liquids [11], me- 5+ tal cation exchanged MMT [12], natural kaolinite clay H8−x[XM12O40], where X is the heteroatom (e.g., P or Si4+), x is its oxidation state and M is the addenda atom [3], silica sulfuric acid [13], TaCl5-silicagel [14] as ca- 6+ 6+ talysts. However, many of these methods have some (usually Mo or W ), are the most important catalysts, drawbacks such as low yields of the products, long reac- especially H3PW12O40 (PW), H3PMo12O40 (PMo) and tion times, harsh reaction conditions, difficulties in work- H4SiW12O40 (SiW) [16]. They are more active than the up. Moreover, the main disadvantage of almost all exis- conventional catalysts, such as mineral acids, ion-ex- ting methods is that the catalysts are destroyed in the change resins, zeolites, SiO2/Al2O3 and H3PO4/SiO2 [17- work-up procedure and cannot be recovered or reused. 19]. HPAs have several advantages over liquid acid ca- *Corresponding author. talysts, including being non-corrosive and environmen- Open Access JASMI 194 Silicotungstic Acid Modified Bentonite: An Efficient Catalyst for Synthesis of Acetal Derivatives of Aldehydes and Ketones tally benign. Thus, they present fewer disposal problems stirring for 16 hours. The residue was washed several and more economically and environmentally attractive. times with double distilled water till the complete re- Heteropoly acids (HPAs) due to their strong acidity have moval of chloride ions was confirmed by AgNO3 test. attracted much interest as the catalysts mostly in homo- The residue, thus obtained was dried in an oven at 100˚C geneous systems [20,22]. It’s not easy to separate HPA is referred to as Al-Pillared Bentonite (Al-Ben) [29,30]. from reaction mixture and reuse it unless it is supported Acid Activated Bentonite (H+-Ben) on some catalyst. Clays have also been proposed as suita- 5.0 g of Na-Ben was added into a 100 ml beaker con- ble acid catalysts [23,24]. Clays are furthermore used as taining 50 ml of 3N H2SO4, this mixture was exposed to adsorbents, decoloration agents, ion exchangers, and ca- microwave radiation for 30 minutes. The residue was talysts [25]. The use of clays, as heterogeneous catalysts, washed several times with double distilled water till the 2 offers many advantages over homogeneous acid catalysts complete removal of SO4 ions was confirmed by such as ease of separation, mild reaction conditions, bet- BaCl2 test. The residue thus obtained was dried in an ter selectivity of the desired product, and elimination of oven at 100˚C to generate the Acid activated Bentonite waste disposal problems. In a large number of organic (H+-Ben) [28]. reactions clays have been used as catalysts on laboratory/ Pillared Acid Activated Bentonite (PAA-Ben) + industrial scales. The properties of clay can further be 1.0 g of H Ben, was added into a 250 ml conical flask improved by making pillared clays. Pillared clays are containing 100 ml of double distilled water. 50 ml of clays with high permanent porosity obtained by separa- pillaring solution was gradually added with vigorous ting the clay sheets by a molecular prop or pillaring agent. stirring for 16 hours. The residue was washed several These pillaring agents can be organic, organometallic, or times with double distilled water till the complete re- inorganic complexes, preferably of a high positive charge. moval of chloride ions was confirmed by AgNO3 test. Pillared clay (PILC) possesses several interesting proper- The residue thus obtained was dried in an oven at 100˚C ties, such as large surface area, high pore volume and and is referred to as Pillared Acid Activated Bentonite tunable pore size (from micropore to mesopore), high (PAA-Ben). thermal stability, strong surface acidity and catalytic ac- Silicotungstic Acid Modified Bentonite (STA-Ben) tive substrates/metal oxide pillars. These unique charac- In a 250 ml conical flask, 1.0 g Na-Ben was suspended teristics make PILC an attractive material in catalytic in 50 ml double distilled water. To this, aqueous solution reactions. It can be made either as catalyst support or of silicotungstic acid (100 mg) was added drop wise and directly used as catalyst [26-28]. then stirred for 16 hours. After this, the mixture was fil- In the present work silicotungstic acid (a heteropoly tered and washed with double distilled water to remove acid) modified bentonite have been synthesized to ex- the excess of silicotungstic acid. The product was dried plore the properties of both clay and silicotungstic acid (a at 100˚C. heteropoly acid). This makes it possible to carry out the catalytic process at a lower catalyst concentration and/or 2.2. General Procedure for the Synthesis of at a lower temperature. Further, catalysis of heteropoly Acetal Derivative of Aldehydes and Ketones acid lacks side reactions. In order to elucidate the role of the STA-Ben as catalyst, a controlled reaction was carried out using STA, ben- 2. Experimental tonite, Al-Ben, H+-Ben, PAA-Ben and STA-Ben as cata- 2.1. Catalyst Preparation lyst with benzaldehyde as reactant. The best results were obtained with STA-Ben in microwave. Na-Bentonite A number of aromatic/aliphatic aldehydes or ketones 1.0 g of Bentonite clay was added into a 250 ml coni- (10 mmol) and ethylene glycol (10 mmol) were mixed cal flask containing 50 ml of 1.0 M NaCl this clay sus- with the STA-Ben (50 mg). The mixture was exposed to pension was stirred for 16 hours. The residue after cen- microwave raditions. After irradiation of the mixture for trifugation was washed several times with double dis- a specified period, the contents were gradually cooled to tilled water till complete removal of chloride ions. The room temperature. The completion of the reaction was residue thus obtained after above procedure was dried in checked by TLC. After confirming the completion of the an oven at 100˚C to generate the Na form of the Ben- reaction, the catalyst was recovered by filtration. The tonite (Na-Ben). catalyst thus separated by filtration was washed 2 - 3 Al-Pillared Bentonite (Al-Ben) times with absolute ethanol to remove organic matter. In a 250 ml conical flask containing 1.0 g of Na-Ben The residue obtained after evaporation of the solvent in 100 ml of double distilled water, 50 ml of the pillaring from the filtrate was purified using TLC/column chro- solution (keggin ion) was gradually added with vigorous matography with CHCl3:MeOH with increasing polarity. Open Access JASMI Silicotungstic Acid Modified Bentonite: An Efficient Catalyst for Synthesis of 195 Acetal Derivatives of Aldehydes and Ketones Optimization of Reaction Conditions Table 3. Products synthesized. For the optimization of reaction conditions 10 mmol of S. No. Reactant Product Time, Sec Yield, % ethylene glycol and 10 mmol of Benzaldehyde have been O O selected as reactants. O Optimization of the Catalysts Amount 01 180 82 Reactions have been performed using varying amount of different catalysts under uniform conditions (Table 1).