The catalytic ability of the sulfonic acid functionalized PMO material has been explored for an esterification reaction, i. e., the glycerol acetylation reaction (Figure 4). The activity of EP-(CH2)3-SO3H is compared with a commercially available catalyst Amberlyst-15 and moreover the catalysts’ reusability is explored.

In this study the esterification of glycerol is probed due to its economic importance. Glycerol is an important by-product of first generation bio­diesel and is produced in a relative large quantity [50]. This overproduc­tion of glycerol can be used in order to develop second generation biodies­el which uses glycerol as a raw product. As carboxylic acid, acetic acid is probed as shown in the general reaction (Figure 4). Three products may in principle be obtained from this reaction: glycerol monoacetate (MAG),
glycerol diacetate (DAG) and glycerol triacetate (TAG). However, experi­mentally, only MAG (~94%) and DAG (~6%) are formed using the spe­cific catalytic conditions described in the experimental part [51].

The catalytic activity of EP-(CH2)3-SO3H for the esterification of ace­tic acid with glycerol is presented in Figure 5 where the total acetylation yield is shown as a function of time. The total acetylation yield is defined according to the equation below:

Yield (%) = ([P]/[HAC]0) (Ohac/V x 100

where [P]t and [HAc]0 represent the product and acetic acid concentra­tion at a certain reaction time and at t = 0, respectively. Furthermore, uHAc and up represent the stoichiometric coefficients of the acetic acid and the ester formed, i. e., 1 for mono-substituted, 2 for di-substituted and 3 for fully substituted products, respectively. Also, as acetic acid contains acid protons which can induce a self-catalyzed process, the reaction in absence of any solid catalyst was monitored. Corresponding data are shown in Figure 5. It is clear that the sulfonated PMO possesses a significant cata­lytic activity with a yield of almost 80% for this reaction after ~300 min; whereas the blank test (without any solid catalyst involved) yielded only ~50% of esters after ~300 min. The conversion of Amberlyst-15 is shown in the same figure. Comparing the two materials clearly shows that EP — (CH2)3-SO3H exhibits a similar catalytic activity as Amberlyst-15, which is a well-performing catalyst in this type of reaction.

FIGURE 5: The total acetylation yield for the catalytic reaction with EP-(CH2)3-SO3H and Amberlyst-15. Also the blank reaction is represented for clarity. A catalyst loading of 0.25 g per 40 mL of glycerol was used. The lines are intended as visual aids only.

Total acetylation yield / %

FIGURE 7: Recyclability experiment for EP-(CH2)3-SO3H: a comparison between the catalytic activity of the pristine material and the third catalytic run. The blank reaction is presented for clarity. The lines are intended as visual aids only.

FIGURE 8: The total acetylation yield for the EP-(CH2)3-SO3H, the second and third run. After 60 min (represented by the vertical line) the liquid is separated from the catalyst and the catalytic activity of the liquid phase of run 2 and 3 is further followed in function of time (open square and triangle). The blank reaction is presented for clarity. The dotted lines are intended as visual aids only.

Furthermore, the recyclability of the sulfonic acid containing PMO material is studied for three consecutive runs. First, the initial rate of the catalytic reaction is studied for each run by focusing on the first hour of the acetylation (Figure 6). These experiments are all performed in the same catalytic set-up as the standard catalytic experiment. The solid is filtered after 1 hour and re-used without any further treatment in the subsequent run with a fresh reaction medium (run 2); this being performed again for two additional consecutive runs (runs 3 and 4). As one can see from the figure, the material still possesses catalytic activity for the acetylation af­ter four runs. However, recycling of the material in the consecutive runs results in a slight decrease of the initial reaction rate.

After the last catalytic run, i. e., run 4, the total acetylation yield is mon­itored for approximately 10 h in order to compare it with the acetylation yield of the pristine sulfonated PMO material, EP-(CH2)3-SO3H (Figure 7). Although a decrease in the initial reaction rate was observed as already mentioned, at the third consecutive run, the material still reaches the equi­librium after approximately 5 h and finally results in the same acetylation level as the fresh pristine material.

Moreover, additional tests are performed to evaluate the actual heteroge­neous character of the observed catalytic activity. Therefore, the solids are removed from the liquid after 1 h along the first and second runs, and the corresponding recovered solutions are kept under the catalytic conditions to follow the occurrence of a further evolution of the total acetylation yield without solid catalyst in the system anymore. As can be seen from Figure 8, it is clear that the acetylation occurs much slower, i. e., in the range of the blank, when the catalysts are removed of the reaction media, than when the catalyst is maintained in the reactor for the whole test duration.