264950 The Acetylation of Glycerol Over Solid Acid Catalysts: Catalytic Characteristics and Parameters Optimization

Thursday, November 1, 2012: 10:10 AM
320 (Convention Center )
Adesina Adesoji A. Sr., School of Chemical Sciences and Engineering, School of Chemical Sciences and Engineering, The University of New South Wales, Sydney 2035, Australia, Sydney, Australia and Zhou Limin, State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, 330013, PR China, Nanchang, China

The acetylation of glycerol over solid acid catalysts: catalytic characteristics and parameters optimization

Limin Zhou, Essam Al-Zaini, Adesoji A. Adesina*


School of Chemical Sciences and Engineering, The University of New South Wales, Sydney 2035, Australia


The acetylation of glycerol over different solid acid catalysts were investigated. Amberlyst-15 showed high activity and selectivity towards diacetin (DAG) and triacetin (TAG). The different catalytic activities between various catalysts were explained in terms of acid sites, pore diameter, and hydrophilic properties. The consecutive reaction of glycerol acetylation and the “steric effect” for the products was confirmed. Both the molar ratio of acetic acid to glycerol (AA/G ratio) and the temperature had positive effects on the glycerol conversion and the selectivity towards higher esters. However, the AA/G ratio was the more influential factor than the temperature, especially for improving the combined selectivity of diacetin and triacetin At the optimal operating conditions (AA/G ratio 9/1 and 110 oC), the glycerol conversion predicted by the nonlinear models is 98.47%, and the corresponding selectivity is 8.65%, 46.56%, 44.79%, and 91.35% towards MAG, DAG, TAG, and DAG+TAG, respectively, all of these values are close to the experimental data.

Keywords: Glycerol; Acetylation; Reaction characteristic; Parameters optimization

*Corresponding author: Tel.: + 61-2-9385-5268; fax: + 61-2-9385-5966.

E-mail address: a.adesina@unsw.edu.au (Adesoji A. Adesina) ; minglzh@sohu.com (L. Zhou).


1. Introduction

   The acetylation of glycerol can be a good alternative for the glycerol produced from biodiesel. Compared to homogeneous catalysts, solid acid catalysts are easy to remove from the products, and also demonstrate better selectivity towards the desired products [1]. In this work, the acetylation of glycerol was studied over different solid acid catalysts. The effect of conditions (acetic acid /glycerol molar ratio and temperature) was investigated. The parameters were optimized by following factorial design and response surface methodology.

2. Experimental

2.1. Reaction procedure and product analysis

The reactor was charged with a measured amount of glycerol and acetic acid (total volume 150 mL ). When the reacting mixture reached the desired temperature, the measured amount of catalysts was added to the mixture in the reactor. Samples were taken after filtration through the 0.45 µm pore-diameter filter and analysed by GC (Shimadzu GC-17A).

3. Results and discussion

3.1 Comparison of different solid catalysts

    In the blank test, the acetylation of glycerol led to MAG as the main product with the selectivity. However, the conversion of glycerol can still reach more than 70 % after 70 min even without a catalyst (Fig.1). These results indicated that the acetylation of glycerol had self-catalyzed ability. In comparison with the product distribution in the presence of Amberlyst-15 (Table 1), in this case the esterification led to DAG and TAG as the main product with the combined selectivity of 81-92 %. Therefore, the main function of the catalysts in this reaction is assumed to increase the selectivity of the desired products (DAG and TAG).        

The highest activity of Amberlyst-15 can be explained by its high acid sites. However, the low conversion and selectivity to the DAG and TAG, observed for the HZSM-5 and HUSY zeolites might also be attributed to diffusion problems.

For A-15-1 catalysts the experimental results indicate the existence of a dominant “steric effect” [2]. The amount of 2-MAG and 1,2-DAG was very small compared to 1- MAG and 1,3- DAG, despite over 12 % of the selectivity towards 2-MAG in the final products for the blank experiment.

Table 1

Characteristics and activity of catalysts


Area (m2/g)


Water content


Si/Al molar ratio



Average pore



Catalyst loading

(mmol H+/L)

Activation temperaturec




S d (%)



Amberlyst-15 (A-15-1)



1410b -4700a






Amberlyst-15 (A-15-2)








195 b









286 b






a Data obtained from supplier.

b Estimated from NH3-TPD.

c Rate = 10 oC/min; time in activation temperature = 2 h.

dXG,Glycerol conversion; S, Selectivity. Obtained at 110oC and AA/G ratio 9:1

Fig.1 Conversion of glycerol and selectivity to the products with time (without catalysts; temperature=110 oC; AA/G molar ratio = 9/1)

3.2. Influence of initial AA/G ratio

Fig.2 show that higher AA/G ratio (lower glycerol concentration) delays the product distribution equilibrium and prolongs the equilibrium time, with that of 200, 240, and >300 min at AA/G 3/1, 6/1, and 9/1, respectively. Both the glycerol conversion as well as the combined selectivity of DAG and TAG increased with increasing the AA/G ratio. The consecutive reaction characteristic of glycerol acetylation could be confirmed from the product distribution as shown in Fig.2. The selectivity of MAG decreased with the reaction time, while that of DAG passed a maximum value before its decrease.

Fig. 2. Influence of the acetic acid to glycerol molar ratio: (a) 9:1; (b) 6:1; (c) 3:1 on the product distribution of glycerol acetylation (Catalyst A-15-1: 82.86 mmol H+/L or 2.645 g in 150 mL reactants; temperature 95 oC)

3.3. Influence of temperature

     Fig.3 showed that with increasing temperature the reaction rate increases, and thus the production distribution reaches equilibrium in a shorter time. The reaction time for the conversion of MAG to DAG, and DAG to TAG also became earlier. The increase in the temperature improved the reaction rate significantly and also facilitated the distribution of the esters toward higher substituted species. Moreover, both the glycerol conversion and the selectivity of TAG increase with temperature. The most appropriate temperature for DAG formation in the final products was 80 oC (AA/G molar ratio 9/1) with 60.5 % of the DAG selectivity, but with increasing temperature the selectivity of DAG in the final products decreased.

Fig. 3. Influence of different temperature: (a) 110 oC; (b) 95 oC; (c) 80 oC on the kinetics of glycerol acetylation (AA/G molar ratio 9:1; Catalyst A-15-1: 82.86 mmol H+/L or 2.645 g in 150 mL reactants)

3.4. Parameters optimization

The experimental design applied to this study was a full 32 design. The statistical models are listed in Table 2. Statistical analysis identifies the AA/G molar ratio as the most important factor in the glycerol conversion response. The second factor in importance is temperature followed by the molar ratio–temperature interaction. All of them have a positive effect on the glycerol conversion (Eq. (1) and (6)). Fig. 4 clearly shows an enhanced conversion of glycerol at high temperatures and AA/G molar ratios.

The selectivity towards DAG+ TAG followed the opposite tendency to the selectivity towards MAG, increasing consequently with AA/G molar ratio and reaching maximum values as a function of temperature at a high AA/G molar ratio (9/1). However, it demonstrated that the AA/G molar ratio was the more influential factor on the selectivity towards DAG and TAG than the temperature (Eq. (5) and (10)).

At optimized conditions (AA/G 9:1, 110 oC), the glycerol conversion predicted by the nonlinear models is 98.47%, and the corresponding selectivity is 8.65%, 46.56%, 44.79%, and 91.35% towards MAG, DAG, TAG, and DAG+TAG (Eq.(2)-(5) or Eq. (7)- (10)), respectively, all of these values are very close to the experimental data.

Table 2

Predictive equations obtained by design of experiments


Statistical models (Equations in terms of coded factors):


XG =81.01+ 5.31IT +6.82IMR +1.82 ITIMR -0.49IT2+4.00I MR 2   R2 = 0.9784


SMAG =33.10-3.67IT -26.88IMR +1.82ITIMR +1.93IT2+2.35I MR 2     R2 = 0.9985


SDAG =47.41-2.34IT +11.18IMR-4.17ITIMR -0.43IT2-5.11 I MR 2     R2 = 0.9870


STAG =19.47+6.00 IT +15.70IMR+2.35ITIMR-1.50IT2+2.77I MR 2     R2 = 0.9899


SDAG+TAG=66.88+3.66+26.88IMR-1.82ITIMR-1.93IT-2.34 I MR 2     R2 = 0.9962


Technological models (Equations in terms of actual factors):


XG =53.14+ 0.53T -6.90MR+0.04 T MR –2.17×10-3 T 2+0.44 MR 2   R2 = 0.9604


SMAG =219.98-2.12 T -15.93 MR +0.040 T MR R +8.58×10-3 T 2+0.26 MR 2   R2 = 0.9941


SDAG =-50.66+0.76 T +19.34 MR -0.093 T MR -1.91×10-3 T 2-0.57 MR 2   R2 = 0.9542


STAG =-69.29+1.35I T -3.42 MR +0.052 T MR -6.67×10-3 T 2+0.31 MR 2   R2 = 0.9814


SDAG+TAG=-119.95+2.11 T +15.92 MR -0.041 T MR -8.58×10-3 T 2-0.26 MR 2      R2 = 0.9826


Note: IT and IMR, coded factors(1,0,or -1) ; T, temperature (80,95, or 110 oC); MR, acetic acid/glycerol molar ratio(3:1, 6:1 or 9:1); I, coded value; XG, conversion of glycerol; S, selectivity towards the different products (MAG, DAG, and TAG).

Fig. 4. Response surface of glycerol conversion. (Catalysts A-15-1: 82.86 mmol H+/L or 2.645 g in 150 mL reactants; reaction time 4.5 h)

Fig. 5. Response surfaces of (a) selctivity to DAG, (b) selctivity to TAG, and (c) selctivity to DAG+TAG (Catalysts A-15-1: 82.86 mmol H+/L or 2.645 g in 150 mL reactants; reaction time 4.5 h)




[1] Rahmat N, Abdullah AZ, Mohamed AR. Recent progress on innovative and potential technologies for glycerol transformation into fuel additives: A critical review. Renew Sust Energ Rev 2010;14: 987–1000.

[2] Liao X, Zhu Y, Wang S, Chen H, Li Y. Theoretical elucidation of acetylating glycerol with acetic acid and acetic anhydride. Appl Catal B: Environ 2010;94:64–70.

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