Indian Journal of Chemical Technology Vol. 19, September 2012, pp. 342-350

Esterification of lower aliphatic alcohols with acetic acid in presence of different acid catalysts

Shilpa Khire, Priya Vikram Bhagwat, Martina Fernandes, Prakash Babu Gangundi* & Hitesh Vadalia Monarch Catalyst Pvt. Ltd., A-94-F1/2-MIDC Phase 1,

Dombivli 421 203, Thane, India

Received 5 May 2011; accepted 21 June 2012

The preparation of sulfated zirconia (SZ-H) and silica supported sulfated zirconia (SZ/silica-H) catalysts using H2SO4 as sulfating agent has been reported. These catalysts are then evaluated and compared with other acid catalysts, viz. para- toluene sulfonic acid (PTSA), Amberlyst-15, H-ZSM5 and γ-alumina for the esterification of ethanol and n-butanol using acetic acid. Among the solid acid catalysts, SZ/silica-H shows superior esterification activity for both the alcohols. However, during the reusability of SZ/silica-H catalyst for 5 recycles, the observed gradual decrease in esterification activity (62.26 - 43.65%) is attributed to the partial leaching (35.9% after 5 cycles) of sulfated zirconia content from the silica support. To establish the leaching of sulfated zirconia, the fresh SZ/silica-H, SZ/silica-N and SZ/silica-Cl catalysts prepared by using different sulfating agents like H2SO4, (NH4)2SO4 and ClSO3H respectively have been given the water wash. The analysis of water washed catalysts shows the partial leaching (50-75% of the deposited quantity) of sulfated zirconia. The water washed catalysts containing the residual or trapped fraction of sulfated zirconia in the pores of silica show negligible esterification activity. These results indicate that the active sites on silica surface responsible for esterification activity are effectively leached out in aqueous medium.

Keywords: Acetic acid, Amberlyst-15, n-butanol, Ethanol, Esterification, H-ZSM5, Silica supported sulfated, Solid acid catalyst

The esters of low molecular weight such as ethyl acetate and butyl acetate usually have pleasant and fruity odor. They are added to a wide range of consumer products such as soft drinks, chewing gum, candy, ice cream, cheese, and baked food products. Also large volumes of esters are used as solvents. By 2004, due to increased demand for ethyl acetate an estimated 1.3 million tons were produced worldwide1. Commercially the ethyl acetate and butyl acetate are manufactured by either a batch or a continuous synthetic process using acid catalysts2-5. According to a recent review of industrial acid–base catalysis, out of the >120 processes identified, over 90% are solid acid-catalyzed6. It is a clear indication of the importance of solid acid catalysts and the scope of their commercial application. The use of conventional liquid acids such as PTSA, H2SO4, HCl, HF, AlCl3, BF3, ZnCl2 and SbF5, involves risks in handling, containment, disposal and regeneration due to their toxic and corrosive nature. There is a need to eliminate the ecologically harmful mineral acids to carry out a large number of acid-catalyzed industrial

processes. This can be achieved by the development of strong solid acid catalysts that are stable, reusable and active at moderate temperatures.

Therefore, the solid acid catalysts are gaining much attention. They have the proven advantage of heterogeneous catalysts, like simplified product isolation, mild reaction conditions, high selectivity, ease in recovery and reuse of the catalysts as well as reduction in the generation of wasteful byproducts7-9. Most of the recent work on esterification reactions is reported on use of solid acid catalysts such as sulfated zirconia10,11 and tetravalent metal acid salts12.

In literature13-15, the method used for the preparation of sulfated zirconia catalyst involves simple steps such as hydrolysis of zirconium oxychloride using ammonia to form zirconium hydroxide. Thus, formed zirconium hydroxide is washed, dried and treated with sulfuric acid and calcined at high temperature (> 500°C) to get sulfated zirconia catalyst. These sulfated zirconia catalysts are highly active in acid catalyzed organic reactions but deactivation of activity occurs by the loss of sulfate ions (-SO3H or SO4

2-), thereby recycling of the catalyst is limited. Few authors in their studies modified the preparation method by supporting

——————— *Corresponding author. E-mail: babu@monarchcatalyst.com

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sulfated zirconia on supports like silica16-18, HMS19, and carbon molecular sieve20 to minimize the loss of acidic group during the reaction, thereby improving the performance and reusability of the catalyst. However, most of the attempts show little success in preventing the loss of sulfate ions during organic reactions. In spite of such drawbacks, over the past few years the preparation and characterization of sulfated zirconia catalysts has been receiving much attention, due to the fact that the fresh catalysts show superior catalytic activity for variety of hydrocarbon conversions21-23.

To establish leaching of Zr and S it is relevant to conduct experiments with the supported sulfated zirconia catalyst, where the analysis of the support will show the difference in percentage of Zr and S before (fresh) and after (used) the reaction. Also, it is imperative to maintain constant mass of the fresh and used catalyst during the esterification activity and then compare the difference in activity with respect to change in catalyst properties.

In this paper we report the methods for preparation of sulfated zirconia catalyst with and without the silica support and characterization by various analytical methods such as particle size distribution, BET and XRF. The catalytic activity of sulfated zirconia catalysts is evaluated for esterification of ethanol and n-butanol using acetic acid by varying the alcohol/acetic acid ratio and catalyst dosage. For comparison purpose, other acid catalysts such as PTSA, Amberlyst-15, HZSM5 and γ-alumina are also evaluated under similar reaction conditions. The reusability of silica supported sulfated zirconia (SZ/silica-H) catalyst is studied for 5 recycles by replenishing the marginal loss in catalyst mass and then the catalyst is analyzed for loss in Zr and S content to correlate with the observed decrease in esterification activity. The different sulfating agents such as H2SO4, (NH4)2SO4 and ClSO3H are used to sulfate the unsupported zirconium hydroxide and silica supported zirconium hydroxide precipitates and analyzed for the extent of leaching of Zr and S content in water. Experimental Procedure Preparation of catalysts

Sulfated zirconia catalysts were prepared by following the similar procedure as reported24-26 earlier. Ammonia solution was added to zirconium oxychloride to attain a pH of 10. The zirconium

hydroxide precipitate thus obtained was aged for 2 h and washed with distilled water till the filtrate shows absence of chloride ions. The wet precipitate was dried in the hot air oven at 110ºC for 16 h and then the dried powder was made into three equal parts. The three parts are sulfated separately using different sulfating agents, viz. H2SO4, (NH4)2SO4 and ClSO3H. The amount of H2SO4 added sulfation was 15 mL g-1 of 0.5 M followed by drying at 110ºC. Similarly, the other two parts were treated separately with aqueous solutions of (NH4)2SO4 and ClSO3H having concentration equal to the moles of dried Zr(OH)4 precipitate and dried at 110ºC. The obtained three dried sulfated powders were then further calcined at 550ºC. The sulfated zirconia catalysts prepared by using H2SO4, (NH4)2SO4, and ClSO3H were referred as SZ-H, SZ-N and SZ-Cl respectively. It is important to note that there is no washing step involved after sulfation.

The silica supported sulfated zirconia catalysts were prepared by using the silica gel powder treated with alkali. The silica gel powder (100 g) was soaked in 200 mL 1% KOH for 30 min at room temperature. The alkali solution was filtered and the silica gel was washed, dried at 110°C and then calcined at 550°C for 1 h. The zirconium oxychloride solution was mixed with the above silica gel powder and calculated volume of water so that wetting of gel particles is complete. After 1 hour, the silica gel was removed and kept on a filter paper for 5–10 min and then transferred to a crucible for drying at 110°C. The zirconium oxychloride loaded silica gel powder was converted into zirconium hydroxide by adding ammonia solution at pH 10. The Zr(OH)4 precipitate thus formed was allowed to stay in the vessel for 2 h and then filtered. The wet precipitate was dried at 110°C and then made into three equal parts. The sulfation and calcination of these three parts were done as per the procedure described above for SZ catalysts. The three sulfated zirconia/silica catalysts obtained by using different sulfating agents such as H2SO4, (NH4)2SO4, and ClSO3H were referred as SZ/silica-H, SZ/silica-N and SZ/silica-Cl respectively.

HZSM5 catalyst was prepared in the autoclave by mixing tetrapropyl ammonium bromide, sodium aluminate, sodium silicate and H2SO4 for pH adjustment. The gel pH was adjusted between 11.0 and 11.5, and then autoclave was heated to 180°C. After 48 h, the autoclave was quenched in water and

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zeolite powder slurry was filtered, washed, dried and calcined at 540°C. The resulting NaZSM5 powder was ion-exchanged with 10% NH4Cl solution at 60°C for three times for 3 h. Finally, NH4ZSM5 was decomposed to HZSM5 at 500°C for 3 h. The catalysts γ-Alumina (Ga200, BASF), Amberlyst-15 (Fluka) and PTSA (Thomas Baker) and other chemicals H2SO4, (NH4)2SO4, and ClSO3H were procured and used for the studies. Characterization

The silica gel powder with and without alkali treatment was analyzed for particle size using accusizer (PSS NICOMP, Particle Sizing Systems, Santa Barbara CA 93117). The BET method was used for measuring surface area and pore volume by Autosorb -1 (Quantachrome, USA) using N2 for adsorption at –196°C. The Si, Zr and S content of the silica supported sulfated zirconia catalysts was analyzed by using XRF instrument Spectro Xepos-3 (Spectro Analytical Instruments GmbH, Germany) by standardizing the instrument with catalysts of known composition. Esterification of ethanol and n-butanol with acetic acid

A 500 mL 4-neck round bottomed flask was kept in a water bath. The reactants were charged into the flask by maintaining the different ratios of ethanol/acetic acid between 0.5 and 2.0. The known amount of catalyst was added to the reactants at 2-10% by weight of total reactants. During the reaction between ethanol and acetic acid under reflux conditions with continuous stirring the ethyl acetate and water were formed. A simple water condenser was used instead of Dean & Stark apparatus and the reaction was carried out at lower temperature (80–85°C). The product samples were then taken out after 2.5 h and analyzed by GC using capillary column (RTX-1) of 30 m length, N2 gas flow rate of 2 mL/min and with heating program of 50-200°C. The esterification of n-butanol was carried out by following the method described above.

The reaction was carried out under reflux conditions as well as using Dean-Stark trap at 115-120°C. Results and Discussion Catalyst characterization

The surface area/pore volume of silica gel powder before and after alkali treatment are listed in Table 1. After treating with 1% KOH, the surface area of silica gel decreases from 474.6 m2/g to 155.9 m2/g, the pore volume decreases marginally from 0.56 cc/g to 0.51 cc/g and pore size increases from 47.65 Å to 131.3 Å (pore diam). Significant drop in surface area indicates that most of the micro pores that contribute to high surface area and negligible towards pore volume are converted into large pores by dissolution of pore wall between them during alkali treatment.

The SZ/silica-H shows surface area of 143.6 m2/g and pore size of 142.4 Å (pore diam) similar to the properties of silica support. The expected surface area increase due to deposition of porous sulfated zirconia on porous silica might have balanced by the pore blockage of silica during the deposition. The unsupported SZ-H catalyst is comparatively less porous (surface area = 98.7 m2/g) and its pore size is 51.87 Å (diam). For HZSM5 (surface area = 365.1 m2/g) and γ-alumina (surface area = 238.7 m2/g) catalysts high surface area values obtained indicate that they are highly porous in nature and known to contain large population of micropores. It has been reported27 that the Amberlyst-15 is a sulfonated organic polymer catalyst and it is less porous (surface area = 52.16 m2/g) as compared to SZ/silica-H.

Particle size (d50 values) of silica gel, SZ-H, SZ/silica-H, HZSM5 and alumina powders are in the range of 17–25 microns. Only the Amberlyst-15 catalyst particles are in the range of 600–800 microns, i.e. almost 30 times larger than the size of the other solid acid catalyst particles.

Table 1—Characterization of the catalysts Catalyst d50 Surface area Pore volume Av. pore diam microns m2/g cc/g Å Silica gel 19.1 474.6 0.565 47.62 Silica gel (KOH treated) 17.14 155.9 0.511 131.11 SZ-H 17.14 98.7 0.128 51.87 SZ/silica-H 20.16 143.6 0.511 142.34 H-ZSM5 15.39 365.1 0.265 NR γ-alumina 25.10 238.7 0.985 165.06 Amberlyst-15 600 – 800 52.16 0.412 315.95

Av. pore diam (average pore diameter) = [(2 × pore volume) / surface area)] × 2 × 10000 NR (not relevant) = Av. pore diam calculation is not relevant for crystalline H-ZSM5 sample (5.3-5.6 Å, diam).

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Esterification of ethyl alcohol Different grades of acid catalysts were tested for

esterification of ethanol under simple reflux condition by maintaining ethanol/acetic acid mole ratio of 1.13, temperature of 80–85°C and time of 2.5 h (Table 2). The PTSA catalyst is found to be soluble and present in liquid state in the reaction medium, whereas other catalysts (γ-alumina, H-ZSM5, Amberlyst-15, SZ-H and SZ/silica-H) remain as solid in reaction medium. The loading of PTSA catalyst is found to be 1.07% by weight of the total reactants and all other solid catalysts loading is 10.7% by weight. Even with such low catalyst dosage, PTSA gives maximum conversion of ethanol to ethyl acetate (66.98%). Among the solid acid catalysts, Amberlyst-15, SZ/silica-H and SZ-H catalysts show 66.10%, 65.26% and 63.98% ethyl acetate respectively which is higher than that obtained with HZSM5 and γ-alumina catalysts.

It is important to know that a contrast performance of these catalysts is observed in industry, e.g. HZSM5 and γ-alumina are highly active catalysts at high temperature vapour phase (300–500°C) reactions28,29 as compared to sulfated catalysts (SZ-H, PTSA and Amberlyst-15). In other words, the protons of Bronsted acid sites in sulfated catalysts are labile and

available easily for the reactions at low temperatures and in liquid phase reactions30. Whereas, the protons of Bronsted acid sites in HZSM5 and γ-alumina are less labile at < 100°C and they become more labile at high temperature and in vapour phase reactions. Therefore, in liquid phase esterification of ethanol at low temperatures (< 150°C) both HZSM5 and γ-alumina catalysts are less effective as compared to those of sulfated catalysts. A similar observation of inferior activity of HSZM5 with respect to Amberlyst-15 and SZ-H catalysts was made by Peters et al.10 and Namba et al.31 during esterification of i-butanol and n-butanol respectively with acetic acid. They claimed that the reason for inferior activity is that the esterification occurs only on the external surface of the HZSM5 catalyst. They also mentioned that the number of active sites per unit weight of the catalyst is less than that observed with Amberlyst-15 and SZ-H catalysts.

Surprisingly the ethyl acetate formation is found to be the least (4.88%) with γ-alumina, which is less than that obtained from the reaction carried out without any catalyst (9.09%). In the absence of catalyst, the reactant acetic acid acts as mild autocatalyst for esterification of ethanol which is similar to the observation made by Ma et al.32

Table 2—Reaction conditions used for esterification of ethanol with acetic acid (AA) Catalyst Ethanol:AA Ethanol+AA Catalyst Sulfur Temp. Cycles Ethyl acetate (grade) (mole ratio) g g m moles °C % yield PTSA 1.13:1.00 140 1.50 7.875 80-85 Fresh 66.98 SZ/silica-H 1.13:1.00 140 15.00 8.934 80-85 Fresh 65.26 Amberlyst-15 1.13:1.00 140 15.00 69.000 80-85 Fresh 66.10 SZ-H 1.13:1.00 140 15.00 105.350 80-85 Fresh 63.98 HZSM5 1.13:1.00 140 15.00 NA 80-85 Fresh 15.09 γ-alumina 1.13:1.00 140 15.00 NA 80-85 Fresh 4.88 Nil 1.13:1.00 140 Nil NA 80-85 NA 9.09 PTSA 1.13:1.00 140 0.20 1.050 80-85 Fresh 47.44 PTSA 1.13:1.00 140 0.50 2.625 80-85 Fresh 61.86 SZ-H 1.13:1.00 140 2.00 14.040 80-85 Fresh 47.36 SZ-H 1.13:1.00 140 5.00 35.100 80-85 Fresh 62.06 SZ-H 1.13:1.00 140 5.00 NA 80-85 Reuse-5 40.12 SZ/silica-H 1.13:1.00 140 2.00 1.191 80-85 Fresh 32.43 SZ/silica-H 1.13:1.00 140 5.00 2.978 80-85 Fresh 62.26 SZ/silica-H 1.13:1.00 140 5.00 NA 80-85 Reuse-5 43.65 Amberlyst-15 1.13:1.00 140 2.00 9.200 80-85 Fresh 44.70 Amberlyst-15 1.13:1.00 140 5.00 23.000 80-85 Fresh 63.81 Amberlyst-15 1.13:1.00 140 5.00 NA 80-85 Reuse-5 53.16 SZ/silica-H 0.50:1.00 140 15.00 8.934 80-85 Fresh 50.37 SZ/silica-H 0.66:1.00 140 15.00 8.934 80-85 Fresh 56.36 SZ/silica-H 0.86:1.00 140 15.00 8.934 80-85 Fresh 66.04 SZ/silica-H 1.13:1.00 140 15.00 8.934 80-85 Fresh 65.26 SZ/silica-H 1.50:1.00 140 15.00 8.934 80-85 Fresh 63.26 SZ/silica-H 2.00:1.00 140 15.00 8.934 80-85 Fresh 55.24

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However, the observed very low level of esterification with γ-alumina indicates existence of strong adsorption of one of the reactants or it might be catalyzing the hydrolysis of ester to give back the reactants. Further studies are essential with γ-alumina to establish the reason for such low esterification activity as compared to many other solid acid catalysts.

The four acid catalysts, viz. PTSA, SZ/silica-H, Amberlyst-15 and SZ-H showing almost equilibrium conversion of ethanol to ethyl acetate, have been chosen for further studies which involve optimization of dosage for all the four catalysts and reusability test for only three solid acid catalysts. Varying amount of PTSA, SZ/silica-H, Amberlyst -15 and SZ-H catalysts was used during the esterification by maintaining identical reaction conditions (ethanol/acetic acid mole ratio 1.13, temperature 80–85°C, total reactants 140 g and time 2.5 h). The minimum dosage required to achieve maximum (equilibrium) conversion is found to be different for these four catalysts. On the mass basis, comparatively PTSA catalyst is found to be the most active in esterification reaction and it gives maximum conversion at 0.70 g (0.50%) catalyst dosage (by wt.). The SZ/silica-H, Amberlyst-15 and SZ-H catalysts are less active and are used at 7.5 g (5.35%), 7.2 g (5.14%) and 7.3 g (5.21%) respectively to achieve maximum esterification (to ethyl acetate) under identical reaction conditions (Fig. 1).

Generally, esterification of alcohol with acetic acid is known30-33 to be catalyzed by the protons associated with -SO3H groups of PTSA and Amberlyst-15 and SO4

2- groups of sulfated catalysts (SZ/silica-H, and SZ-H). The concentration of such -SO3H or -SO4

2-

groups in these catalysts is different for a unit mass of the respective catalysts. Therefore, esterification on the basis of dosage of sulfur (-SO3H or -SO4

2-) has been analyzed instead of the total amount of catalyst mass added in the reactants and plotted in Fig. 2. The maximum ethyl acetate formation (equilibrium) is observed with dosage of 3.67 m moles, 4.50 m moles, 33.12 m moles and 51.24 m moles of -SO3H or -SO4

2- from PTSA, SZ/silica-H, Amberlyst-15 and SZ-H catalysts respectively for 140 g reactants at 1.13 mole ratio of ethanol/acetic acid.

The better performance of PTSA catalyst for esterification is due to the fact that it is in the liquid form in the reactant mixture, and hence the dispersion and accessibility of protons of -SO3H groups to the reactant molecules are expected to be higher as compared to that of solid catalysts. Among the three solid acid catalysts, the SZ/silica-H shows superior performance which is attributed not only to better dispersion of sulfated zirconia on porous silica support but also to the higher surface area and larger size pores that could enable easy accessibility of reactant molecules to the active sites. In case of unsupported SZ-H catalyst34 and Amberlyst-15, the higher concentration of active sites (SO4

2- and -SO3H) and comparatively lower surface area result into inferior dispersion of active sites.

In the reaction medium for a constant catalyst mass dosage, the catalyst with larger size particles will have less number of particles, and hence less number of contacts with reactant molecules as compared to those catalysts having smaller size particles. Therefore, Amberlyst-15 catalyst with particle size of 600-800 microns shows comparatively lower esterification activity than the SZ/silica-H catalyst with particle size

Fig. 1—Esterification of ethanol with acetic acid using different quantities of PTSA, SZ-H, SZ/silica-H and Amberlyst-15 (Amb) catalysts [reactants 140 g, temp. 80-85°C and ethanol/acetic acid 1.13 mole ratio]

Fig. 2—Esterification of ethanol with acetic acid using different -SO3H or SO4

2- contents of PTSA, SZ-H, SZ/silica-H and Amberlyst-15 (Amb) catalysts [reactants 140 g, temp. 80-85°C and ethanol/acetic acid 1.13 mole ratio]

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of 18-20 microns. A similar dependence of benzene acylation activity on particle size of the sulfated zirconia catalyst has been well established by Yadav and Pujari24. That is, sulfated zirconia with particle size of 75-90 microns, 125-150 microns and 160-200 microns shows acylation activity of 85, 75 and 50% respectively. Based on the concentration of the active sites and esterification activity data of the above four sulfated catalysts, we can conclude that only 8-10% of total active sites in SZ-H and Amberlyst-15 are accessible to the reactants as compared to that availability of active sites from SZ/silica-H catalyst.

Separation of dissolved PTSA catalyst from the liquid ester product is found to be tedious, whereas SZ/silica-H, Amberlyst-15 and SZ-H solid catalysts can be separated from the liquid ester product by simple filtration method. The loss in catalyst mass during the filtration is replenished by adding fresh catalyst to ensure similar esterification activity with similar mass of the catalyst under identical reaction conditions. This procedure is repeated for 5 cycles to test the esterification activity with equal mass of catalyst maintained during each cycle. The Amberlyst-15 catalyst shows marginal loss in activity after fifth recycle, i.e. from 63.81% to 53.16% (Table 2). Under the similar esterification conditions, the SZ/silica-H and SZ-H catalysts show significant loss in activity between fresh and 5th reuse of catalysts, i.e.

from 62.26% to 43.65% and from 62.06% to 40.12% respectively. A similar deactivation of catalyst activity has also been reported18-24 between fresh and 2nd reuse of sulfated zirconia and HMS supported sulfated zirconia catalysts in case of acylation (from 82% to 25%) and alkylation (from 45% to 28%) reactions respectively. The authors further reported that the deactivation of catalyst activity is due to the loss in catalyst particles (about 20%) during the filtration step before the next reuse of the catalyst. However, in our studies the constant catalyst mass is maintained during each recycle, and therefore the activity loss needs to be established with other experimental evidences.

A systematic study on loss in catalyst mass and variation in catalyst composition if any has been conducted using fresh and reused SZ-H and SZ/silica-H catalysts after each recycle. The unsupported SZ-H shows about 5-6% loss in catalyst mass during each recycle and it is replenished with fresh catalyst before the next recycle (Table 3). By the end of 5th recycle the catalyst mass loss amounts to about 28% (i.e. loss of 1.4 g out of 5.0 g).

The analysis of silica supported sulfated zirconia (SZ/silica-H) catalyst shows only a marginal loss in the catalyst mass during each recycle and at the end of 5th recycle it reaches to 2.4% mass loss (i.e. loss of 0.12 out of 5.0 g catalyst). Also, the XRF analysis shows that as compared to the fresh catalyst, the used

Table 3—Analysis of fresh and reused catalys Cycle By XRF Catalyst, g % Ethyl acetate %SiO2 %ZrO2 %-SO3H (Reused + Fresh)

SZ — H Catalyst Fresh Nil 41.74 56.19 0.00 + 5.00 62.06 1st reuse Nil NA NA 4.78 + 0.22 53.30 2nd reuse Nil NA NA 4.72 + 0.28 42.52 3rd reuse Nil NA NA 4.69 + 0.31 40.77 4th reuse Nil NA NA 4.68 + 0.32 40.50 5th reuse Nil 40.44 53.35 4.73 + 0.27 40.12

Total fresh catalyst (g) replenished during the 5 recycles = 1.40 SZ/Silica — H Catalyst

Fresh* 83.55 1.92 4.76 0.00 + 5.00 62.26 1st reuse NA NA NA 4.97 + 0.03 54.69 2nd reuse NA NA NA 4.98 + 0.02 52.84 3rd reuse NA NA NA 4.98 + 0.02 52.23 4th reuse NA NA NA 4.97 + 0.03 50.57 5th reuse 83.46 1.66 2.87 4.98 + 0.02 43.65

Total fresh catalyst (g) replenished during the 5 recycles = 0.12

The % - OH and - H contents of the catalyst are not shown in Table. NA — Not analyzed. *100 g fresh SZ/silica-H catalyst contains 6.68 g sulfated zirconia.

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catalyst contains lower concentration of Zr (shown as ZrO2) and S (shown as -SO3H ) on silica support. In case of unsupported catalyst (SZ-H) the loss in both Zr and S contents shows only decrease in mass but not in the composition.

To confirm such variation in catalyst composition and loss in catalyst mass, we extended the studies with sulfated zirconia catalysts prepared by using different sulfating agents. The fresh SZ-H, SZ-N SZ-Cl, SZ/silica-H, SZ/silica-N and SZ/silica-Cl catalysts were washed with 1000 mL hot water for 10 g catalyst and analyzed for mass loss as well as variation in composition, if any. Table 4 shows that the unsupported SZ-H and SZ-Cl catalysts are present in the form of water soluble salts (sulfated zirconia / zirconium sulfate, solubility = 52.5 g in 100 mL water at 20°C), thereby > 95% of the catalyst mass is lost during the water wash. The hydroxide (solubility = 0.02g in 100 mL water) and oxide (solubility < 0.01 g in 100 mL water) of zirconium are water insoluble compounds, and hence it can be inferred that H2SO4, and ClSO3H are effective sulfating agents and converts most of zirconium hydroxide into zirconium sulfate which is soluble in water. However, the use of (NH4)2SO4 as sulfating agent is found to be less effective in converting quantitatively zirconium hydroxide into zirconium sulfate/sulfated zirconia. Therefore during calcination the fraction of

un-reacted (not sulfated) zirconium hydroxide becomes zirconia which is water insoluble. Also, the mass loss of unsupported SZ-N catalyst after the water treatment is found to be comparatively lower, i.e. 65%.

In case of silica supported sulfated zirconia catalysts, irrespective of the sulfating agent used, the content of sulfated zirconia is found to leach out partially from silica surface (from the initial 6.68 – 10.17% to the final 2.39–3.56%) after the water wash experiments. Such partial loss of sulfated zirconia content in silica supported catalyst systems is in contrast to the almost complete loss observed in unsupported SZ-H and SZ-Cl catalyst systems. It is possible that a certain fraction of sulfated zirconia is strongly adsorbed or trapped in the pores of silica even after the water wash. Further, it is interesting to note that when the equal masses of fresh and water washed catalysts are tested, a large difference in the esterification is observed (Table 4). That is, the fresh SZ/silica-H, SZ/silica-N and SZ/silica-Cl catalysts show 62.26, 65.73 and 55.96% ethyl acetate formation. The fresh catalysts after the water wash show 9.33, 9.22 and 9.96% ethyl acetate formation which is comparable to that obtained without using any catalyst (9.09%). In other words, the esterification activity contribution from the water washed silica supported sulfated zirconia catalysts is negligible

Table 4—Analysis of fresh and water washed catalyst Catalyst By XRF Catalyst, g % Ethyl acetate

%SiO2 %ZrO2 %-SO3H (at 5 g)a SZ-H

Nil 41.74 56.19 10.00 62.06 Fresh washed Nil NA NA 0.26 NA SZ-N

Nil 39.10 58.04 10.00 62.94 Fresh Washed Nil NA NA 3.54 NA SZ-Cl

Nil 27.56 67.53 10.00 53.30 Fresh Washed Nil NA NA 0.02 NA SZ/silica-H

83.55 1.92 4.76 10.00 62.26 Fresh Washed 83.46 1.55 2.01 9.68 9.33 SZ/silica-N

81.06 3.96 6.21 10.00 65.73 Fresh Washed 81.66 1.16 1.23 9.22 9.92 SZ/silica-Cl

81.97 1.12 4.97 10.00 55.96 Fresh Washed 82.03 0.41 1.53 9.54 9.96

The % - OH and - H contents of the catalyst was not shown in Table. NA — Not analyzed due to loss in catalyst mass. aFor each catalyst.

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(< 1.0%). These results clearly indicate that during the water washing the active sites responsible for catalyzing the esterification are either preferentially washed away from the surface of SZ/silica-H, SZ/silica-N and SZ/silica-Cl catalysts or the active sites remaining after the water wash are located in pores of silica support which are not accessible to the reactant molecules.

Using SZ/silica-H catalyst the esterification reaction conducted at varying mole ratio of ethanol/ acetic acid from 0.55 to 2.00 shows a maximum conversion between 0.85 and 1.10 mole ratios. However, more experimental data is needed to arrive at exact mole ratio that gives highest ethyl acetate during the above reaction conditions. Esterification of n-butanol

A few selected experiments on esterification of n-butanol with acetic acid were conducted to confirm the observations made in earlier sections using acid catalysts for ethanol esterification.

A first set of n-butanol esterification was carried out using the PTSA catalyst (liquid in reaction mixture) and different grades of solid acid catalysts under simple reflux conditions by maintaining n-butanol /acetic acid mole ratio 0.86, temperature 115–120°C and time 2.5 h. The loading of PTSA catalyst was 1.03% of the total reactants against all other solid catalysts loaded at 10.3%. As observed earlier with ethanol esterification even in this case of n-butanol esterification the PTSA, SZ/silica-H Amberlyst-15 and SZ-H catalysts are found to show higher activity (72.81%, 66.29% 68.52% and 65.43% butyl ester respectively) than HZSM5 and γ-alumina catalyst. The butyl ester formation is found to be the least i.e. 30.30% with γ-alumina, which is even less than that observed without any catalyst

(34.06%). This observation is also similar to that observed during the ethanol esterification using γ-alumina catalyst.

The second set of esterification of n-butanol carried out using Dean & Stark apparatus shows higher activity than that observed under simple reflux condition (Table 5). The observed higher activity is attributed to the effective removal of water during the reaction, thereby promoting forward esterification reaction. Even the esterification carried out in the absence of catalyst with n-butanol and acetic acid shows 47.86% butyl acetate. However, the activity trend of these catalysts is found to be almost similar both in presence of Dean & Stark apparatus and using simple reflux conditions. Such high conversion with n-butanol in the absence of catalyst is due to higher reaction temperature 115-120°C as well as the product butyl acetate is least soluble in byproduct water (0.7 g in 100 mL water at 20°C). The least solubility of butyl acetate might favour the forward reaction due to heterogeneous phases of two products. The water washed SZ/silica-H shows loss in n-butanol esterification activity from 66.29% to 35.80% during reflux conditions and from 92.58% to 49.38% while using Dean & Stark apparatus. The observed low esterification activity from the water washed catalyst is almost equal to the activity obtained in the absence of catalyst. Such loss in activity is possible only in case of leaching of active sites responsible for esterification of n-butanol. Conclusion

The PTSA (as liquid acid catalyst) shows higher activity as compared to those of the solid acid catalysts, such as Amberlyst-15, SZ/silica-H, SZ-H, HZSM5 and γ-alumina for esterification of ethanol and n-butanol using acetic acid. Among the sulfated

Table 5—Reaction conditions used for esterification of n-butanol with acetic acid (AA) Catalyst n-butanol:AA n-butanol+AA Catalyst Sulfur Temp. Cycles Butyl acetate (grade) (mole ratio) g g m moles °C % yield Refl D&S PTSA 0.86:1.00 155 1.50 7.875 115-120 Fresh 72.81 (93.21) SZ/silica-H 0.86:1.00 155 15.00 8.934 115-120 Fresh 66.29 (92.58) Amberlyst-15 0.86:1.00 155 15.00 69.000 115-120 Fresh 68.52 (93.57) SZ-H 0.86:1.00 155 15.00 105.350 115-120 Fresh 65.43 (90.67) HZSM5 0.86:1.00 155 15.00 NA 115-120 Fresh 46.39 (86.44) γ-alumina 0.86:1.00 155 15.00 NA 115-120 Fresh 30.30 (39.28) Nil 0.86:1.00 155 Nil NA 115-120 NA 34.06 (47.86) SZ/silica-H 0.86:1.00 155 15.00 8.934 115-120 water wash 35.80 (49.38)

Refl — Reflux conditions used. D&S—Dean & Stark apparatus used.

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acid catalysts, on the basis of dosage of S (-SO3H or SO4

2-) content, the silica supported sulfated zirconia (SZ/silica-H) catalyst is found to be superior to unsupported SZ-H and Amberlyst-15 catalysts for esterification of both the alcohols. This observation suggests that high surface area, wider pores and better dispersion of sulfated zirconia in SZ/silica-H catalyst are responsible for the observed higher esterification activity. However, during the reusability of SZ/silica-H catalyst for 5 recycles the observed gradual decrease in esterification activity is attributed to the leaching of Zr (zirconia) as well as S (-SO3H or SO4

2-) content from the silica support. The Amberlyst-15 catalyst shows only a marginal loss in esterification activity even after 5 recycles which could be due to the negligible leaching of -SO3H content from the organic polymer support or disintegration of resin particles during the recycles.

During the study of water wash effects on fresh catalysts, the unsupported catalysts, viz. SZ-H and SZ-Cl, are found to show >95% solubility in water. The silica supported catalysts, viz. SZ/silica-H, SZ/silica-N and SZ/silica-Cl, show the leaching of 50–75% of the deposited sulfated zirconia from the silica surface. After the water wash, the residual or trapped fraction of sulfated zirconia in the pores of silica are found to show negligible (almost nil) esterification activity. References

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