Indian Journal of Chemi suy Vol. 43A, October 2004, pp. 2060-2065

Esterification of isoamyl alcohol using solid acid catalysts

N Nagaraju* & S Z Mohamed Shamshuddin

St. Joseph's College PG Center, 46, Langford Road, Shanthinagar, Bangalore 560 027 , India Email : nagarajun @yahoo.com

Received 26 April 2004; revised 5 August 2004

Catalytic ac ti vity o f different solid ac ids obtained by calc ination of hydrox ides of AI, Si , Zr and Ti as well as the ir sul fa ted forms has been in ves tiga ted in liquid phase esteri ficat ion of isoamyl alcoho l with propionic acid. The catalysts have been characterized fo r the ir tex tural properties, surface ac idi ty. surface area and sulfur content. The major product of the catalyti c react ion has been found to be isoamyl propionate. The yield (%) of the ester has been found to depend on the textural properties, tota l surface acidity and surface area of the catalysts. In the case o f sulfated zirconia which exhibited highest ac idity, isoamyl ether has also been formed to an ex tent o f 0 .6-4.5%. Effects of amount of catalyst, durati on of the reacti on and the molar rati os of the reac tants on the y ie ld o f ester have been studied. A pl ausible reaction mechan ism for the formati on of isoamyl propi onate is described.

IPC Code: Int. Cl 7 C07C 67/00; 80 11 2 1/00

So lid ac ids have been used under heterogeneous conditions and evaluated fo r their cata lytic activity in a number o f industri a l processes involving the manufacture of organi c fin e chemicals 1.2_ Such heterogeneous cata lytic reactio ns have several advantages over homogeno us reactio ns, vi z., ease of separation of catalyst from reaction mixture, regenerati on and reutili zati on of the catalyst, etc. Efforts are constantl y being made to replace the corros ive conventio nal ac id cata lysts with the environment-fri endly so lid ac id catalys ts in ac id catalyzed syntheti c organ ic reacti o ns.

Metal oxides and zeolites are so lid ac ids that have been reported to be effic ient catalysts in the synthes ts of several organic fine chemi ca ls3

. Earlier we have made several attempts to in ves tiga te the catalyti c ac ti vity of oxides, zeolites and their modi fied fo rms in es teri f ication reactio n involving di fferent carboxy lic ac ids and alcohols4-7. The catalyti c acti vity of these materials has been found to depend on several factors which are interdependent.

T he structural and cata lyti c properties of sulfated metal oxides and hence the nature of the reactio n products have been fo und to be affec ted by the metal oxide support, amount of sulfati o n, the method of preparation of sul fa ted metal ox ide and heat treatment8

In the present in vestigation the cata lyti c perfo rmance of di ffe rent sul fa ted metal oxides, viz., So}-/A l20 3, SO/'!Si02, SO/-!ZrO" and S04

2-/Ti02,

has been compared in the sy nthesis of isoamyl propionate which has a pleasant odour of pineapple. The effect of vari ati on o f the amount of sulfate io n and the reacti on parameters such as molar ratio of reactants, amount of catalyst, duratio n of reaction etc ., on the yield (%) of the product and selectivity has a lso been evaluated . An attempt has also been made to con-elate the phys ico-chemical properties of the materi a ls with the catalytic acti vity and propose a poss ible reactio n mechani sm fo r the es teri f ication reacti on over the se lected catalyti c materi als.

Materials and Methods Preparation of catalyst materials

Thi s step in volves two stages: Preparatio n of supports as hydrox ides and impregnat ion of sulfate ions on the supports dried at 120°C.

(a) Preparation of supports: Al(O H)3 and Zr(OH )4

gels were prepared by the precipitation method. The precipi tatio n o f a luminum and zirco nium hydroxides was can ied out separately by the addition of I : 1 ammoni a to a hot (- 80°C) I M aq ueous solution of Al (N03)3.9HzO and Z rOCI 2.8HzO res pective ly . The precipi tates of hydrox ides o f AI and Zr were filtered, washed thoroughly with deioni sed water and dried at

120°C for 12h in an a ir oven. Si02 gel was prepared by precipitatio n fro m a hot

(- 80°C) I M aq ueous solution of Na2Si03 with 1:1 HN03 (ref. 9) . The gel was fil tered , washed thoroughly with deio ni sed water and dried at 120°C for 12 h in an air oven.

NAGARAJU & SHAMSHUDDIN: ESTERIFICATION OF ISOAMYL ALCHOL USING SOLID ACID CATALYSTS 206 I

These supports were dried at 120°C and designated as hydrated oxides. Ti02 was a commercial product.

(b) Preparation of sulfate ion impregnated supports: All the supports, hydrated alumina, zirconia and silica as well as titania were impregnated with sulfate ions. For this purpose, 1.5 mL of 2 M H2S04

was added to 3 g of the support and mixed thoroughly to get a homogenous paste. The paste was dried in an air oven at I 20°C for 12 h (ref. 5).

S042

- ion containing supports are abbreviated as SA (sulfated alumina), SS (sulfated silica), SZ (sulfated zirconia) and ST (sulfated titania). Hydrated Zr02

containing different amounts of sulfur was also prepared by impregnating 3 g of the support with 1.5 mL of 0.5-6.0 M H2S04 solution. The mixture was thoroughly mixed to get a paste and dried at 120°C for 12 h in an air oven. These samples were designated as SZ-1- SZ-6.

The supports and their sulfated forms were ground to get a fine powder. The fine powders were calcined at 550°C for 5h in a muffle furnace just before using them as catalysts in the title reaction.

Catalyst characterization All the catalysts were characterized for their

powder XRD, total surface acidity, surface area, and sulfur content.

The amorphous or crystalline nature of the catalysts was determined by recording their powder XRD patterns on a Seimens-05005 X-ray diffractometer with a Ni filtered Cu-Ka radiation ( 1.5418 A). The total surface acidity was estimated by volumetric method using a standard solution of n-butyl amine prepared in benzene, in the presence of bromothymol blue as an indicator. The surface area of the catalysts was measured by BET method using NOV A-1000 Version 3.70 instrument. The amount of sulfur associated with sulfated supports was obtained by conducting elemental analysis using Elementar Varia EL III Carlo Erba 1108 instrument.

Catalytic activity The catalytic activity of different supports and their

sulfate modified forms were determined by the liquid phase esterification of isoamyl alcohol (IAA) with propionic acid (PA) . The esterification reaction was carried out at the refluxing temperature region over a mantle, by taking the reactants, isoamyl alcohol and propionic acid directly in a RB flask along with the

catalyst. The total volume of the reactants was kept at 15 mL. The reaction mixture after a definite period of time was cooled to room temperature and the catalyst was separated by filtration. The components of the filtrate were analyzed by gas chromatograph (column: 20% SE-30, chromosorb W-AW, 801100 mesh, 3m x 118"). The nature of the products was confirmed with GC-MS (Varian). Blank reactions were conducted without using any catalyst. The experiments were carried out by varying the amount of catalyst, molar ratio of the reactants and time of reflux, to check the effect of these parameters on the selectivity and yield of the product. The conversion of isoamyl alcohol was calculated based on the GC analysis using the expression 10 given elsewhere.

Results and Discussion Powder XRD patterns

PXRD patterns of all the calcined and uncalcined oxides and sulfated oxides were recorded. Among the oxides, hydrated Al20 3 and Si02 were found to be amorphous both in their calcined and uncalcined forms as well as sulfated forms.

On the other hand, the uncalcined sample of Zr02

was amorphous but when calcined, exhibited crystalline nature both in its sulfate ion treated and untreated forms. The commercial sample of Ti02 was crystalline and did not show any change in its textural properties either on sulfate ion incorporation or heat treatment to 550°C. Further, it was noticed in the case of SZ, the peaks at (28 = 30.3, 35.3, 50.7) due to tetragonal phase of the catalyst increased with increase in the sulfur content whereas the monoclinic peak (28 = 24.7, 28.4, 31 .6) decreased (Fig. 1). Since the increase in surface acidity in the case of SZ is attributed to the increase in sulfur content'', it may be said that the change of phase from monoclinic to tetragonal was facilitated by so4 2- ions and this phase change may be taken as an indication of a change in surface acidity in sulfated zirconia.

Total surface acidity

Total surface acidity values in mmollg as estimated by n-butylamine titration method, for the oxides and sulfated oxides are included in Table l. At the outset, the acidity values of the plain supports were found to increase in the following order: Al20 3 < Zr02 < Ti02

< Si02 Hydrated alumina being an amphoteric oxide exhibited the lowest and silica an acidic oxide exhibited highest total surface acidity.

2062 INDIAN J CHEM, SEC A, OCTOBER 2004

Table 1- Physico-chemical characteristics of catalysts calcined at 550°C

~ <IJ c: 4> :g

Catalyst

SA ss ST sz

I = 0.0% sulfur

186 129 056 073

Surface area (m2/g) 2 3

202 216 181 206 069 086 102 149

2 = 0.9% sulfur 3 = 3.5% sulfur

T

T

2theta

Fig. !- Powder XRD patterns of some calcined sulfated zirconia samples. [T- Tetragonal, M-Monoclinic].

Total surface acidity (mmol/g) 4 2 3 4

232 0.38 0.41 0.45 0.49 218 0.45 0.50 0.69 0.73 094 0.39 0.41 0.47 0.54 198 0.47 0.65 0.96 I. II

4 = 6.3% sulfur

The total surface acidity of the plain supports increased in general, on treatment with sulfate ions. However, the increasing order changed to SA < ST < SS < SZ. The increase in acidity on sulfate ion treatment was remarkably high in the case of zirconia support.

Interestingly, correl ation between the textural and acidic properties surface acidity of the materials has been clearly noticed. The amorphous solids such as hydrated Ah03 and Si02 did not show much increase in their surface acidity whereas SZ which undergoes crystallization on calcination to 550°C exhibited a substantial increase in surface acidity. This observation is further confirmed by the fact that uncalcined zirconia which was amorphous also did not show high acidity. Ti02 which was crystalline before sulfate ion treatment did not show much increase in its surface acidity. Thus, sulfation of crystalline oxides does not result in a super acid or any increase in the number of acid sites. These observations are in accordance with the discussion made on similar solids by Arata12

. This clearly indicates that crystallization of the sulfated material during calcination is one of the desirable conditions to get higher acidity .

SZ, which exhibited highest acid site concentration, was incorporated with different amounts of sulfate ions and total acidity of the resulting samples was estimated. The results indicate that an increase in sulfate ion loading increased the total surface acidity of hydrated zirconia (Fig. 2). It was further noticed that the increase in surface acidity with sulfate loading till 1.9% sulfur, was very rapid. However, since the method followed in the present investigation for the determination of surface acidity, was applicable only for the total number of acidic sites irrespective of their strength, it is not possible to infer here that increase in total acidity on increase in sulfur content is also associated with the generation of new

NAGARAJU & SHAMSHUDDIN: ESTERIFICATION OF ISOAMYL ALCHOL USING SOLID ACID CATALYSTS 2063

1.2 -----550 °C

1.1 ----120 °C

----------. g 1.0

/ . 0

E • .s 0.9 ./ £ / -o 0.8 (.)

C<l / Q) 0.7 (.)

~ • ~ 0.6 I ·- · en

-----·---·-------~ 0.5 ·- · 0

/ f-

0.4

0 2 3 4 5 6 7

Sulfur(%)

Fig. 2-Effect of sulfur loading on the total surface ac idity of calcined and uncalcined SZ.

acid s ites with hi gher acid strength as reported in literature that sulfation of zirconia makes it a 'super acid.J 2. 13 .

Surface area

The BET surface area of the calcined plain supports and their sulfate modified forms are g iven in Table 1. The following increasi ng o rder of surface area of the plain supports was noticed: Ti02 < Zr02 < Si02 < Ab03.

The higher surface area of Ab03 a nd Si02 may be attributed to the amorphous nature of these materi a ls, particles of which are smaller in s ize tha n the crystalline Ti02 and Zr02. On inco rporation of sulfate ions with these supports, the surface area was found to increase to an extent of I 0-30% but re mained in the same o rder of varia tion . Arata 12 has observed under SEM that these oxides on sulfate ion treatment crack down in to fine particles in compari son with those of the particles without sulfate treatment. The increase in surface area may thus be att ributed to the decrease in particle s ize upon su lfate ion treatment. It is also interes ting to note that the increase in surface area of SZ shows a linear re lat ionship w ith inc rease in the sulfate ion/s ul fur concentration. Thus, the sulfate ion concentration seems to be the parameter responsible in cracking down the zi rconi a parti c les.

Sulfur content of the sulfated supports

The percent age of sul fur associated wi th different hydrated metal ox ides is shown in Table I. The amount of the su lfur in the fo rm of SO/ reta ined on

the surface of various hydrated oxides was found to be in good agreement within experimental en-or (6%) with that associated with 1.5 mL of different molar sulfuric acid used for impregnation . It indicates most of the sulfuric acid, which was used as a source of so/·, was retained on these hydroxides. Further, the percentage of sulfur on calcined supports was found to be higher than that on the uncalcined samples .

This is mainly due to the Joss of hydroxyl groups and the water molecules on calcination from the surface of the support without affecting the sol species . Hence the relative wt% of sulfur increases on calcination . This process can be clearly understood from Clearfield model proposed fo r the generation of Lewis and Bry;nsted acid s ites on SZ

13.

Catalytic activity Esterification of IAA with PA was canied out 111

liquid phase at refluxing temperature to evaluate the catalytic properties of hydrated Ab03, Si02, Ti02 and Zr02 and their sulfated form s. Analysis of the reaction products by gas chromatograph showed the formatio n of only two products. The product identification by GC-MS indicated isoamyl pro pionate (lAP) as the major and isoamy l ether (fAE) as the minor products . In the presence of all plain supports and SA, SS and ST, the only product fo rmed was lAP. These materials may thus said to be 100% selective fo r ester formati on. SZ, on the other ha nd, also formed IAE to an extent 0.6- 4.5 % .

Whe n the esterification reactions were cani ed out in the absence of any catalyst, conversion was observed , but only to an ex te nt of II % even when the reaction mixture was refluxed for 12 h. Thi s res idual activity may be attributed to the intrinsic acidity of propi oni c acid used as one o f the reactants.

The results of the catalytic studi es conducted in the presence of di ffe re nt amounts of the plain supports and durations of the reacti on are presen ted in Table 2. The pe rcentage of lAP was found to vary between 30 a nd 72. Thi s increased activity is mainly due to the avail ability of the catalytically act ive sites for the reaction to occur o n the surface of the supports. It was no ti ced that an increase in the amo unt of the catalyst o r the duration of reaction indicated a marginal increase in the percentage of ester formation. Further, the difference in the cata lytic ac ti v ity of different supports was not very s ignifi cant. This indicates that al l the plai n su ppo rts possess almost the same concentra ti o n of catal yti ca ll y acti ve s ites fo r the es terifi cati on react ion and the difference in the

2064 INDIAN J CHEM, SEC A, OCTOBER 2004

Table 2-Effect of amount of catalyst and reaction time on the conversion (%) of lA A [Catalyst calcination temp. = 550 °C; molar naio of IAA:PA = I :2; reaction temp= refluxing temp]

Catalyst 1h O.lg 0.5g I.Og 0.1g

Simple oxide catalysts AI20 3 30.1 33.6 38.9 46.9 Si02 34.4 39.7 43.5 57.1 Ti02 30.3 36.1 42.9 48.0 Zr02 30.9 34.3 40.1 47.8

Sulfated oxides containing 0.9% sulfur SA-l 31.9 35.6 41.3 47.6 SS-1 37.1 41.2 47.6 55.6 ST-1 35.9 40.2 46.3 54.9 SZ-1 39.7 43.6 50.0 58.6

Sulfated oxides containing 3.5% sulfur SA-2 32.6 36.3 42.7 49.1 SS-2 39.0 44.9 49.9 56.0 ST-2 38.4 41.3 50.9 55.3 SZ-4 47.1 52.6 59.3 69.9

textural properties seems to have no effect on thei r catalytic activity.

On modification of the plain oxides by incorporating sulfate ions (0.9% sulfur), the catalytic activity for ester formation was increased to a small extent (Table 2). It was also noticed in general, with all the sulfated oxides the increase in % conversion of IAA to lAP when the duration of the reaction increased from 1 to 6 h was more when compared to the increase from 6 to 12 h. Refluxing the reaction mixture beyond 12 h did not show any noticeable change in the amount of ester. This may be due to the fact that esterification being an equilibrium reaction attains a state of saturation when a certain amount of ester was formed in the reaction mixture.

When the amount of sulfur associated with the supports was increased to 3.5 %, though SA, SS and ST did not exhibit much increase in ester formation activity, that of SZ went up to 92% (Table 2) . It is to be noted here that when the percentage of sulfur on the surface of zirconia was low (0.9%), there was no marked increase in its catalytic activity for ester formation. However, when the amount of sulfur increased to 3.5%, the catalytic activity of SZ increased to a noticeable extent. This clearly indicates that sulfation increases the concentration of the catalytically active acid sites on zirconia but not on other oxides . Another important observation made with respect to SZ was the formation of IAE as the by-product. Ether formation from alcohols requires stronger acid sites 10

. It is known that acid sites with higher strength are generated on sulfation of

6h 12h 0.5g LOg 0.1g 0.5g I.Og

50.6 56.1 50.2 54.3 63.6 62.9 66.9 60.9 69.1 72.4 53.1 57.9 55.6 60.1 68.4 52.8 56.8 52.9 59.3 64.9

53.1 56.2 49.9 59.6 64.4 57.9 61.6 59.6 64.1 68.9 57.1 60.0 59.1 62.3 65.1 65.1 67.3 61.6 70.0 76.8

54.2 57.8 53.6 60.4 69.1 58.3 66.5 60.3 65 .6 70.9 61.5 62.6 60.1 65.0 69.6 79.9 82.3 74.4 86.6 91.6

zirconia 12• Accordingly, formation of ether as one of

the products confirms further the generation of stronger acid sites on sulfation of zirconia.

A systematic study on the effect of increase in the sulfur content in the range 0 .0-6.3 %, on the catalytic activity of SZ revealed that the esterification activity increased with increase in sulfur content on SZ. It was also noticed that activity for ether formation also increased with increase in sulfur content on SZ indicating an increase in acid site concentration with higher strengths. Sulfated zirconia showed an increase in the intensity of the peak due to tetragonal phase as well as an increase in the total acidity with the concentration of sulfur. From these results it can be inferred that tetragonal phase of SZ is associated with acid sites that are catalytically active in bringing about esterification of isoamyl alcohol and propionic acid as well as ether formation. Work is in progress to further confirm the active phase of SZ (whether monoclinic or tetragonal), which is responsible for its enhanced catalytic activity.

Influence of molar ratio of alcohol to acid was investigated using SZ-4 (3.5 % sulfur). The reactions were carried out for 6h using 0.5g of the catalyst. An increase in the concentration of the acid in the reaction mixture showed an increased percentage of ester in the product mixture (Table 3). A slight increase in the selectivity for lAP was noticed with concomitant decrease in the selectivity for IAE. The enhanced ester formation activity is due to the combined effect of increased concentration of the acid sites on the catalysts available as well as the intrinsic

NAGARAJU & SHAMSHUDDIN : ESTERIFICATION OF ISOAMYL ALCHOL USING SOLID ACID CATALYSTS 2065

~f1;ToH

OH

OH -1-J/111/IIIIJJ

<;:sH11

I

a8 -I-

""""""

<;:sH11

lr;

o0 -1-111111111111

<;:sH11

I 0 @o-) Q-01-\!

C,H110H ~+oe ~ ~-toe -HzO ~~~ c)_H OH 0

o0 00) OH -1- - 1-Will lit Ill -1- 111111111111

111111111111

Scheme 1

Table 3--Effect of molar ratio of lAA and PA on yield(%) of lAP.

[Catalyst = 0.5 g SZ-4 ; Refluxing time = 6 h ; Reaction temp = refluxing temp]

Molar ratio Yield(%) of Molar ratio Yield(%) of (IAA:PA) lAP (IAA:PA) lAP

1:1 61.1 2:1 56.6 1:2 79.9 3:1 52.4 I :3 86 .6 4:1 48.4 1:4 91.4 5: I 44.5 I :5 95.3

acidity of the carboxylic acid. It is also interesting to note that when the alcohol concentration was increased in the reaction mixture the activity for ether formation showed an increased trend. Ether formation may be thus accounted for an increased possibility of intramolecular dehydration of alcohol when its concentration was more.

It is to be noted here that catalytic activity determinations were conducted initially using both SZ catalysts calcined at 550°C as well as uncalcined samples. The conversion (%) was very low in the case of uncalcined samples. Further, with these catalysts, lAP was the only product and IAE was not noticed at all. Formation of lAP may only be due to the dil. sulfuric acid that was used to incorporate sulfate ions on zirconia as well as due to the carboxylic acid used as one of the reactants. Thus, higher concentration of catalytically active stronger acid sites is generated only when the catalysts are activated by calcination at 550°C.

The mechanism of the esterification of propionic acid with isoamyl alcohol over protonic acid sites (Scheme 1) is similar to the conventional mechanism,

which involves the nucleophilic attack by isoamyl alcohol on the carboxyl atom 14

• The resulting loss of water would give the ester. The role of an acid catalyst here is to facilitate the formation of the carbocation, and to help remove OR from the carboxylic group.

Acknowledgement The authors gratefully acknowledge the help

received from Dr. B S Jaiprakash, Bangalore Institute of Technology in the surface area analysis of the samples, Mr. George Kuriacose, Prof. R.A.P. Rao, Department of Chemistry, St. Joseph's College, Bangalore, for discussions.

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