Catalytic esterification of benzyl alcohol with acetic...

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Indian Journal of Chemical Technology Vol. 8, September 2001, pp. 362-367 Catalytic esterification of benzyl alcohol with acetic acid by zeolites and their modified forms K R Sharath, Sreenivasan Vijayashree & N Nagaraju* Department of Chemistry, St. Joseph's College P.G. Centre, 46, Langford Road, Bangalore 560 027, India Received 26 June 2000; revised 12 April 2001; accepted 23 May 2001 An attempt has been made to synthesise benzyl acetate from benzyl alcohol and acetic acid using Na-(3, Na-Y, and Na- ZSM5 zeolites and their ion exchanged forms as catalysts. All the catalysts are characterised for their surface acidity, surface area and crystallinity by n-butyl amine titration method, BET and XRD respectively. The esterification reaction has been carried out in liquid phase taking the reactants in different acid to alcohol molar ratios (1 :1, 1:2, 2:1 }. The amount of the est er formed has been estimated indirectly by titrating the unreacted acid with a standard base. The selectivity was found to be 100% with reasonably good yield (44-83%). The catalysts are also checked for their reusability. Zeolites have shown a great potential to act as selective catalysts in petrochemical processesu and organic synthesis of intermediates and fine chemicals 3 . The catalytic activity of these zeolites is associated with the presence of acid centres in their intracrystalline surface. To be suitable in a particular reaction as catalyst, the acidic properties of the zeolites can be altered by pre and post synthesis modifications such as varying the gel composition, cation exchange etc,. The main advantages of zeolites over conventional catalysts are due to the varied acid strength they can possess and their great adaptability to all types of catalysis 4 . The esterification of carboxylic acids and the acyl-ation of alcohols are fundamental reactions in organic chemistry. A direct reaction of carboxylic acids with alcohols is generally avoided because the equilibrium that is established between the reagents and the products requires the use of excess reagents or the elimination of water from the reaction mixture to lead the process to completion. Commercially benzyl acetate is synthesized by the esterification of benzyl chloride with acetic acid in the presence of a lewis acid. This reaction is of great importance in perfume industries as benzyl acetate is used as a base for many of the perfumes 5 . Previous studies have shown that zeolites 6 · 7 and zirconium oxide 8 catalyse esterification reaction involving different alcohols. In view of the industrial importance of the above mentioned reaction and the potential application of zeolites with different pore *For correspondence (E-mail: [email protected]) sizes in esterification reactions as catalysts 9 , authors were interested in investigating the catalytic activity of zeolites and their modified forms in the synthesis of benzyl acetate and optimise the reaction conditions to get good selectivity and yield of the product. In this paper the applicability of zeolites Na-f3, Na-Y, Na- ZSM5 and their ion exchanged forms in the liquid phase esterification of benzyl alcohol with acetic acid has been reported. The reusability of these catalysts has also been investigated. Experimental Procedure Preparation of the catalyst Zeolites f3, Y and ZSM-5 were obtained in their sodium form from United Catalysts India Ltd. These zeolites were modified by ion exchanging the Na+ ions in the parent zeo li te by H+, Cu 2 +. The zeolites were calcined at 823K overnight before exchanging the Na form with other cations. (a) Protonated forms of the Na-zeolites were prepared following the standard method reported in the literatures 10 - 12 20g of the zeolite ca lcined at 823K were immersed in 250cm 3 of ammonium nitrate solution containing 20g of the salt (1M). The mixture was stirred overnight, fi ltered washed with deionised water. The solid was dried at 393K in an air oven overnight and calcined at 823K for 12 h to get their protonated forms viz. H-f3, H-Y and H-ZSM5. (b) Copper exchanged forms of the zeolites were prepared by immersing 20g of the protonated form of the zeo lite calcined at 823K in 250 cm 3 of 0.5M

Transcript of Catalytic esterification of benzyl alcohol with acetic...

Indian Journal of Chemical Technology Vol. 8, September 2001, pp. 362-367

Catalytic esterification of benzyl alcohol with acetic acid by zeolites and their modified forms

K R Sharath, Sreenivasan Vijayashree & N Nagaraju*

Department of Chemistry, St. Joseph's College P.G. Centre, 46, Langford Road, Bangalore 560 027, India

Received 26 June 2000; revised 12 April 2001; accepted 23 May 2001

An attempt has been made to synthesise benzyl acetate from benzyl alcohol and acetic acid using Na-(3, Na-Y, and Na­ZSM5 zeolites and their ion exchanged forms as catalysts. All the catalysts are characterised for their surface acidity, surface area and crystallinity by n-butyl amine titration method, BET and XRD respectively. The esterification reaction has been carried out in liquid phase taking the reactants in different acid to alcohol molar ratios (1 :1, 1:2, 2:1 }. The amount of the ester formed has been estimated indirectly by titrating the unreacted acid with a standard base. The selectivity was found to be 100% with reasonably good yield (44-83%). The catalysts are also checked for their reusability.

Zeolites have shown a great potential to act as selective catalysts in petrochemical processesu and organic synthesis of intermediates and fine chemicals3.

The catalytic activity of these zeolites is associated with the presence of acid centres in their intracrystalline surface. To be suitable in a particular reaction as catalyst, the acidic properties of the zeolites can be altered by pre and post synthesis modifications such as varying the gel composition, cation exchange etc, . The main advantages of zeolites over conventional catalysts are due to the varied acid strength they can possess and their great adaptability to all types of catalysis4 . The esterification of carboxylic acids and the acyl-ation of alcohols are fundamental reactions in organic chemistry. A direct reaction of carboxylic acids with alcohols is generally avoided because the equi librium that is established between the reagents and the products requires the use of excess reagents or the elimination of water from the reaction mixture to lead the process to completion.

Commercially benzyl acetate is synthesized by the esterification of benzyl chloride with acetic acid in the presence of a lewis acid. This reaction is of great importance in perfume industries as benzyl acetate is used as a base for many of the perfumes5. Previous studies have shown that zeolites6·7 and zirconium oxide8 catalyse esterification reaction involving different alcohols. In view of the industrial importance of the above mentioned reaction and the potential application of zeolites with different pore

*For correspondence (E-mail: [email protected])

sizes in esterification reactions as catalysts9, authors were interested in investigating the catalytic activity of zeolites and their modified forms in the synthesis of benzyl acetate and optimise the reaction conditions to get good selectivity and yield of the product. In this paper the applicability of zeolites Na-f3, Na-Y, Na­ZSM5 and their ion exchanged forms in the liquid phase esterification of benzyl alcohol with acetic acid has been reported. The reusability of these catalysts has also been investigated.

Experimental Procedure

Preparation of the catalyst Zeolites f3, Y and ZSM-5 were obtained in their

sodium form from United Catalysts India Ltd. These zeolites were modified by ion exchanging the Na+ ions in the parent zeolite by H+, Cu2+. The zeolites were calcined at 823K overnight before exchanging the Na form with other cations.

(a) Protonated forms of the Na-zeolites were prepared following the standard method reported in the literatures 10- 12• 20g of the zeolite calcined at 823K were immersed in 250cm3 of ammonium nitrate solution containing 20g of the salt (1M). The mixture was stirred overnight, fi ltered washed with deionised water. The solid was dried at 393K in an air oven overnight and calcined at 823K for 12 h to get their protonated forms viz. H-f3, H-Y and H-ZSM5.

(b) Copper exchanged forms of the zeolites were prepared by immersing 20g of the protonated form of the zeolite calcined at 823K in 250 cm3 of 0.5M

SHARATH et al.: CATALYTIC ESTERIFICATION OF BENZYL ALCOHOL WITH ACETIC ACID 363

Table !-Surface acidity and surface area of zeolites and their modified forms

Catalyst Wt %ofCu Surface Surface area acidity (m2/g)

(mmol/g)

H-13 1.353 448.65

Na-13 1.203 437.65

Cu-13 1.10 1.242 298.49

H-Y 0.8590 281.10 Na-Y 0.6460 289.42 Cu-Y 4.10 0.6995 145.42

H-ZSM5 1.373 275.44 Na-ZSM5 1.246 280.04 Cu-ZSM5 0.26 1.281 270.67

copper nitrate (Loba chem) solution. The mixture was stirred overnight, filtered and washed with deionised water. The solid was first dried at 393K in an air oven overnight and then calcined at 823K for 12 h.

Characterisation of the catalyst All the catalysts prepared have been characterised

for their total surface acidity, crystallinity and surface area. The total surface acidity of the zeolites and their modified forms was estimated by n-butyl amine back titration method 13"14 using bromothymol blue as the indicator. The X-ray powder diffraction patterns of all the samples were collected on JEOL JDX-SP instrument using CoKa. radiation over a 28 range of 3° to 50°. The BET surface area of the samples was measured using nitrogen as the adsorbent in a NOV A 1000 High speed Gas Sorption Analyser Version 3.70.

Copper in the ion exchanged form of the zeolite was extracted into the solution by digesting a known amount of the zeolite with cone. HCI and estimated spectrophotometrically at 630 nm wave length following the standard procedure 15•

Catalytic activity studies The catalytic activity of the zeolites and their

modified forms was determined in the esterification of benzyl alcohol with acetic acid in liquid phase.

In a typical reaction 0.5g of freshly calcined zeolite catalyst was added to the reaction mixture containing different molar ratios of acetic acid and benzyl alcohol (I: l, l :2, 2:1) keeping the total volume constant at 16 cm3 in a 100 cm3 R.B. flask and refluxed for different intervals of time at 403K. Ice

1.4

1.2

g 0 E .s "' "" 0 .6 "0 ·a "' 8 0.6 "' 5 (/)

!§ 0.4 0 1-

0.2

0 Zeolite Y Zeolite B Zeolile ZSM5

Fig. 1-Bar graph depicting the dependence of the surface acidity on the type of zeolite and the cations exchanged

cold water was circulated in the condenser during the course of the reaction. After the stipulated time the reaction mixture was cooled to room temperature and the catalyst was allowed to settle. The unreacted acetic acid in the reaction mixture was estimated by titrating it against a standard base. The reaction mixture was also analysed by G.C using Chemito 2865 G.C (with FID and 2m ss column with 10% AT-1200+ 1% H3P04 on chromosorb W-HP as the stationary phase). The nature of the components was confirmed with the authentic sample. The catalysts separated from the reaction mixture were washed with deionised water several times, dried and calcined at 873K. These catalysts were again used in the esterification reaction. The catalysts were reused several times and their catalytic activity was studied. The esterification reactions were also carried out in the absence of any catalyst.

Results and Discussion

Surface acidity and Surface area All the samples were analysed for their surface area

and total surface acidity after calcination at 823K. The results are shown in Table 1. It is a well-established fact that surface acidity depends on the type of zeolite i.e. their Si/AI ratio. This property of zeol ites is revealed in this investigation as the different zeolites showed di fferent surface acidity. These data are presented as bar graphs in Fig. 1. Zeolite-Y exhibited lowest surface acidity of all the samples. Zeolite­ZSM5 was found to have the highest surface acidity. The surface acidity of the ion exchanged forms of the zeolites used were found to be in the order, H-zeo > Cu-zeo > Na-zeo (where zeo represents zeol ites B. Y or ZSM-5). These results show that the protonated

364 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

Table 2-Percentage yield of benzyl acetate in the presence of zeolite ZSMS and its modified forms as catalysts under different reaction conditions

Cataly st (O.Sg)

Molar ratio %yield of benzyl acetate at different reflux times

Blank

H­ZSMS

Na­ZSMS

Cu­ZSMS

(Acid:Alcoh ol)

I : I 1:2 2: I

I : I 1:2 2:1

1:1 1:2 2:1

1:1 1:2 2: I

lh

26 29 32

38 43 58

38 39 43

36 38 50

2h

37 46 52

47 59 71

44 52 59

45 53 62

4h

47 53 64

52 66 83

50 57 68

53 60 70

Sh

49 57 66

56 72 85

54 62 74

55 66 76

6h

51 61 69

60 81 87

58 68 82

58 71 84

8h

54 66 74

65 90 94

64 78 86

61 80 87

ISh

59 72 79

73 95 96

70 83 88

74 86 90

Table 3-Percentage yield of benzyl acetate in the presence of zeolite 13 and its modified forms as catalysts under different reaction conditions

Catalys t

(0.5g)

Molar ratio % yield of benzyl acetate at different reflux times

H-13

Na-13

Cu-13

(Acid:Alcoh ol)

I: I 1:2 2:1

I: I 1:2 2:1

I: I 1:2 2:1

lh

35 39 48

34 36 44

32 34 38

2h

44 49 55

45 48 53

44 50 53

forms · of zeolites generally show higher acidity compared to other ion exchanged forms. Surface area val.ues of Na- forins of zeolites are in good agreement with those reported in the literature10' 16'22• Surface area of zeolite-~ was found to be the highest among the samples analysed. It was also observed that the surface area of the Cu forms of the zeolites-~ and Y was less compared to their Na- and H- forms. This

4h

52 57 63

47 56 63

50 56 65

Sh

54 64 69

53 60 69

52 62 69

6h

57 68 75

56 64 75

55 70 73

8h

60 73 81

59 71 80

59 76 81

ISh

63 79 83

61 74 83

63 78 83

may be due to blocking of pores by the deposition of copper as its hydroxide/oxide on them. The extent of copper exchanged/ deposited on the zeolites varies inversely with their surface acidity. As the surface acidity of the zeolites decreases copper gets deposited as copper hydroxide blocking the pores thereby decreasing the surface area of these samples. Zeolite­y which showed the lowest surface acidity had the

SHARATH et at.: CATALYTIC ESTERIFICATION OF BENZYL ALCOHOL WITH ACETIC ACID 365

Table 4-Percentage yield of benzyl acetate in the presence of zeolite Y and its modified forms as catalysts under different reaction conditions

Catalyst Molar ratio %yield of benzyl acetate at different reflux times (0.5g) (Acid:Aicohol) lh 2h 4h

H-Y 1:1 38 45 52 1:2 41 59 66 2:1 46 58 73

Na-Y

Cu-Y

100

80

u ., E .2

60 " ;;; Q; 0

"'

',1.~· ;j

~ c 40 "' .0

0 #

20

1:1 32 43 1:2 38 48 2:1 40 60

1:1 31 42 1:2 42 55 2:1 45 58

-·-- -- ---- -- - - - -,.-­,,.... . . ...

..... Lr··"' --- ·• <> .. Bianlr.

_._ H-ZSMS

-•- H·B -e-H-Y.

0------~----~----~,-----~----~ a · 3 6 9 12 15

Time of reflux (h)

Fig. 2- Variation of the percentage of ester formed with reflux time over proton exchanged zeolites

52 56 69

51 61 69

highest amount of copper exchanged/ deposited. The effect of these parameters on the catalytic activity of the zeolites is discussed.

X-ray diffraction studies The X-ray powder diffraction pattern of the

zeolites is characterised by a combination of sharp and broad reflections. The diffraction pattern of all the zeolites is in good agreement with X-ray diffractograms published in the literature17•18• This indicates that the zeolite structure remains intact on ion exchange. Thus one may conclude that the structures of the ion-exchanged forms of the zeolites are largely -preserved.

5h 6h 8h 15h

58 63 68 72

71 75 80 82

77 81 84 86

54 57 60 66 64 70 78 81 75 79 81 84

53 56 59 66

65 69 72 78 74 78 82 84

100~------------------------------~

BO

'0

§ ~ 60 !'! B .. >. N

~ 40

20

··· · ···.

· · · g · ·Blank

. _,__ Na-ZSM5

-•- Na-B -e--Na-Y

0._-----.-----.,-----,-----~----~

0 3 6 9 12 15

Time of reflux (h)

Fig. 3-Variation of the percentage of ester formed with reflux time over Na form of the zeolites

Catalytic activity studies Gas chromatographic analysis of the product

mixture of the esterification reaction between benzyl alcohol and acetic acid carried out in the presence of different catalysts clearly indicated the presence of only three components. A comparison with the authentic samples confirmed the presence of acetic acid, benzyl alcohol and benzyl acetate in the reaction mixture. This shows that all the zeolites exhibit 100% selectivity towards benzyl acetate formation . The amount of ester formed (estimated titrimetrically) in the presence of different zeolites as catalysts, the effect of refluxing time and the mole ratios of the

366 INDIAN J. CHEM. TECHNOL., SEPTEMBER 2001

100

80 . .. .... . ..

u .. E l1 "

60 · · n ··Blank lii

_._Cu-ZSMS ~ - •- Cu-B "5.

-<>-Cu·Y ... <: 40 " .. 0 ;!!.

20

0

0 3 6 12 15

Time of reflux (h)

Fig. 4--Variation of the percentage of ester formed with reflux time over Cu exchanged zeolites

reactants is given in Tables 2-4. Plots of % yield of the ester when the yields are maximum i.e. when the acid: alcohol molar ratio is 2: I as a function of time in the presence of various catalysts and in their absence is presented in Figs 2-4.

At the first instance it can be inferred from the results that all the samples i.e. zeolites-~, -Y and -ZSM5 and their ion exchanged forms catalyse the reaction and hence possess active sites for esterification reactions. The amount of the ester formed was found to vary with the type of zeolites and their ion-exchanged forms and also with the molar ratios of the reactants. Even though it has been reported that acidity data from indicator methods cannot be used directly to predict the catalytic behaviour19, it has been observed that the sample with the highest surface acidity i.e. H-ZSM-5 showed highest catalytic activity. This is evident from the % of ester formed in the reaction. It is not only the strength of the acid sites associated with the zeolites but also their amount that plays an important role in catalysing a particular reaction20'21 • This property is observed in the case of zeolites -~, -Y. The Na forms of both these catalysts have a lower surface acidity value compared to their Cu forms, but their catalytic activity is found to be greater than the Cu forms. This is probably because the number of the acid sites responsible for the esterification activity is Jess in the case of the Cu forms. These samples are also found to have lesser surface area compared to their Na- forms. This was not observed in the case of ZSM-5 catalysts. It may be because the % of copper exchanged in the case of ZSM-5 was very low compared to the other

two samples. Though ~ zeolites have high surface acidity as well as high surface area, the yield was not proportionally high. This may be because of the deactivation of the active centres on the zeolites by the formation of polycyclic aromatic compounds. The zeolites that provide enough space for the synthesis of polycyclic aromatics inside the pores would be deactivated fast22, as in the case of~ z-eolites.

As expected the yield of the ester was found to be higher in the reaction mixtures having one of the reactants in excess (i.e. molar ratios 2: I or 1 :2). As can be predicted in an acid catalysed reaction such as esterification reaction the yield was found to be greatest in cases where acid was present in excess. Decrease in the reflux time Jed to the decrease in the percentage of the ester formed. This may be due to the reduction in the time the reactants are in contact with each other and with the catalysts. There was no considerable increase in the yield of the ester when the reflux time was increased from 8h to 15h implying that the reaction has attained the equilibrium state. Though this reaction is reported to be an autocatalysis reaction23 the present study shows that the y.ield in this reaction can be increased using suitable zeolites as catalysts. It was found that the yield was less in reactions carried out in the absence of any catalyst. The catalysts were active even when they were reused several times. The yield of the ester decreases only marginally (1-2%) on reusing them several times. This shows that the characteristic properties of these catalysts don't change during the course of the reaction.

Conclusion In general, the zeolites ~. Y and ZSM-5 and their

ion exchanged forms showed good catalytic activity in the esterification reaction between benzyl alcohol and acetic acid. Though selec tivity towards e~ter

formation is 100%. The yield of the ester depends on the type of zeolite and the concentration of the acid sites available to catalyse the reaction. These zeolites can also be reused several times without any appreciable decrease in the yield implying that the use of the catalysts are cost effective and Eco-friendly. Thus it can be concluded that zeolites can be modified suitably to exhibit better catalytic activity and higher yields in the esterification reactions.

Acknowledgement The authors gratefully acknowledge the help

received from Dr B. S. Jayprakash Bangalore Institute

SHARATH et al.: CATALYTIC ESTERIFICATION OF BENZYL ALCOHOL WITH ACETIC ACID 367

of Technology in the surface area analysis of the samples and liSe authorities for lending their library facilities. Financial support for this project and the JRF fellowship to SKR by the DST, New Delhi, India, is also gratefully acknowledged.

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