Cerium (IV) Phosphomolybdate as a Bronsted Acid Catalyst in the

 Synthesis of Propyl Acetate

 

A. P. Apsara1 and B. Beena2

1Department of Chemistry, FMN College, Kollam

2Department of Chemistry, D B College, Sasthamcotta

*Corresponding Author E-mail: apsaraap@gmail.com

 

ABSTRACT:

Crystalline Cerium (IV) Phosphomolybdate (CPM) an inorganic ion exchanger has been prepared and characterized by elemental analysis, XRD, thermal analysis (TGA) and spectral analysis (FTIR). The protons contained in the structural hydroxyl groups of the material are the active sites.  The material shows good potential as cation exchanger and Bronsted acid catalyst. Na+ exchange capacity was found to be 7.00 meq/g. Catalytic activity has been studied using esterification of propanol as a model reaction wherein propyl acetate has been prepared. The study reveals the promising use of CPM as an eco-friendly solid acid catalyst.

 

KEYWORDS: Inorganic ion exchanger, Bronsted acid catalyst, esterification.

 


 

INTRODUCTION:

Tetravalent Metal Acid (TMA) salts are cation exchangers obtainable in the crystalline and amorphous forms. They have the general formula M(IV) (HXO4)2. nH2O where M(IV) = Zr, Ti, Sn, Ce, Th etc and X = P, Mo, W, As, Sb etc. These compounds have structural hydroxyl groups, the H of –OH being exchangeable sites. A number of cations can be exchanged with H+ due to which the materials possess cation exchange properties 1-3.  Ion exchangers offer all the advantages of solid acid catalysts. They possess high catalytic acitvity, selectivity and do not corrode the reaction vessels. They can be easily separated from the reaction medium and reused 4,5. The interest in using inorganic exchangers with rare earths is due to the ability of these ions to increase the acid sites in the structure of the catalyst and improve adsorption.  It is interesting to prepare cerium compounds in the Ce(IV) valence state which is often the catalytically active state 6. The purpose of the present work is to study the catalytic activity of an inorganic ion exchanger Cerium(IV) Phosphomolybdate (CPM ) based on cerium(IV) by selecting esterfication of propanol with acetic acid as a model reaction.

 

Experimental procedure:

For the preparation of CPM, equimolar solutions of ceric sulphate, ammonium hepta molybdate and disodium hydrogen phosphate were mixed in the volume ratio1:1:2 with slow and continuous stirring at a pH~ 2.  The gel was kept overnight, filtered, washed with conductivity water and dried at 400C.  The dried material was brought to desired particle size (30-60 mesh) by grinding and sieving and finally converted to acid form by immersing in 1M HCl, the acid being replaced intermittently.  It was then washed with conductivity water till free of chloride and again dried at 400C.

 

For the preparation of propyl acetate, acetic acid and alcohol was combined in refluxing assembly. The catalyst was then added in required amount. The mixture was heated and the ester formed was distilled over slowly. The amount of ester formed was monitored on a gas chromatograph. Reactions were carried out by varying the amount of catalyst, mole ratio of propanol and acetic acid, particle size, temperature and reaction time.

Characterization

 

Elemental analysis of the prepared material was carried out by ICP-AES. Chemical stability of the material was assessed in mineral acids, alkalies and organic solvents. X-Ray diffractograms were taken from Bruker D8 Advance difffractometer using Cu Kα radiation.TG was recorded on a Perkin-Elmer thermal analyzer at a heating rate of 100C/min. FTIR was recorded on a Perkin-Elmer IR spectrometer. Ion exchange capacity was determined by column method7.

 

RESULTS AND DISCUSSION:

CPM was obtained as bright yellow powder. The material showed no change in colour or form on heating with water. It was found to be stable in mineral acids like HCl, H2SO4 and HNO3 upto 7 M concentrations and in organic media namely ethanol, propanol, diethyl ether and acetic acid as evidenced by no change in colour, form or weight of samples used.

 

Table 1- Analysis of XRD of CPM

FWHM

%  Relative intensity

d- spacing(A0)

hkl

21.602

0.244

20

4.110

100

26.458

0.310

100

3.366

110

30.666

0.232

40

2.913

110

36.125

0.234

25

2.484

210

55.806

0.193

20

1.646

211

 

 

CPM was found to be unstable in higher concentrations of bases like NaOH and KOH (above 6M) where the material turned white in colour, may be due to hydrolysis.

 

The strong peaks observed in the diffractogram of CPM shows its crystalline nature8.  The analysis of X-ray diffractogram of CPM is presented in Table 1. The material belongs to simple cubic system as evident from hkl values.

CPM was found to contain 5.88% of cerium, 3.99% of phosphorous and 41.7% of molybdenum. The thermogram of CPM shows sharp change in weight around 1000C corresponding to the loss of external water molecules and a small weight loss between 200 - 4000C due to the removal of co-ordinated water molecules after which a gradual loss in weight is observed till 8000C. This may be due to the condensation of structural hydroxyl groups, which is the usual behaviour of inorganic ion exchangers 9.

 

FTIR spectrum of CPM shows broad bands in the region ~3400 cm-1 attributed to asymmetric and symmetric O-H stretches. A medium band at ~1400 cm-1 is due to O-H bending vibration. An intense band at 1064 cm-1 is attributed to P-OH vibrations while small bands around 860-450 cm-1 may be due metal-oxygen vibrations.

The Na+ exchange capacity of CPM was found to be 7.00 meq/g which is comparable to organic resins.

 

Table 2 - Percentage yield of propyl acetate without using solvent

Amount of catalyst (g)

Acetic acid   :  Propanol (Mole ratio)

% Yield of PA

0.5

1 : 1

18.6

1.0

1 : 1

63.6

0.5

2 : 1

58.3

1.0

2 : 1

80.9

1.0

1 : 2

74.5

 

The concern for developing environment friendly procedures has led to vigorous research activities to circumvent the use of solvents in organic synthesis that are a major cause of pollution 10. Therefore, it was thought worthwhile to analyze the catalytic activity of the material, in the absence of solvent. The esterification of carboxylic acid is a reaction subject to general Bronsted acid catalysis. The reaction of propanol with acetic acid was selected as model reaction to study the catalytic acivity of CPM.

 

It was observed that the yield of ester increased with increase in mole fraction of acid and decreased with increase in mole fraction of alcohol (Table 2). This may be due to the preferential adsorption of alcohol on the catalyst, which results in blocking of active sites. It is also evident from Table 2 that, with increasing amount of catalyst, there is an increase in percentage yield of ester. As the catalyst amount increases there may be a propotional increase in the number of active sites. However, higher yields were obtained when the catalyst concentration was 1 g and acid to alcohol ratio was 2:1. It was also observed that in the formation of ester, change in particle size, reaction temperature or reaction time did not effect the yield of ester obtained. The ester obtained in each was clear without catalyst contamination.

 

During the course of the reaction there is a change in colour of the catalyst from yellow to brown. This is an indication of the adsorption of reactant molecules on the surface of the catalyst. The adsorption is weak and so the catalyst regains its original colour when treated with dilute HCl, during regeneration. There was not much change in the percentage yield of ester with the regenerated catalyst. Thus, the material CPM has several advantages as solid acid catalyst for liquid phase reactions, since it is insoluble, easily reusable and thermally stable. The ester formed can be simply distilled over and there is no catalyst contamination.

 

In conventional methods where H2SO4 is used as catalyst for preparing esters, the yields were high but traces of acid are difficult to be removed. The water eliminated during the process of conversion dilutes the acid so that large quatities of acid are required. The use of solid acids thus eliminates the problem of dilution. The esters formed can be simply distilled over and there is no catalyst contamination. The above results establish the use of inorganic ion exchangers in Bronsted acid catalysis.

 

CONCLUSION:

The ion exchanger CPM exhibit good ion exchange capacity comparable to organic resins. The material shows thermal stability besides exhibiting stability in different acidic, basic and organic media. Propyl acetate is obtained in high yields using CPM as catalyst. The main advantages of the using the material are ease of operation, simplicity in work which involves mere filtration of the catalyst and reusability. The use of CPM as solid acid catalyst also eliminates the problem of acid pollution in esterification reactions where sulphuric acid is the conventional catalyst.

 

ACKNOWLEDGEMENTS:

This work was supported by UGC ( FIP Plan XI).

 

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Received on 17.11.2011         Modified on 12.12.2011

Accepted on 21.12.2011         © AJRC All right reserved

Asian J. Research Chem. 5(1):  January 2012; Page 50-52