Greener route to 4-vinyl-1-cyclohexane 1,2-epoxide synthesis using batch and continuous reactors

3.1.1 Effect of reaction temperature

Alkene epoxidation with alkyl hydroperoxides require a thorough screening of the reaction temperature in order to achieve high conversion of the oxidant and high product selectivity. Hence, experiments were conducted to study the effect of different reaction temperatures on the yield of epoxide. Reaction temperatures used for this study were 60°C, 70°C, 80°C and 90°C. As expected, higher reaction temperatures gave a higher yield of 4-VCH 1,2-epoxide at a fixed reaction time. It can be seen from Figure 1 that the reaction reached equilibrium within the first 5 min for experiments carried out at 80°C due to the distinct exothermic effect observed during the first 5 min of the reaction, i.e. the temperature of the reaction mixture overshoots the desired temperature, which was controlled immediately to maintain the reaction mixture at 80°C. This effect, however, was not significant when the experiments were conducted at 60°C and 70°C. The experiments were replicated twice for all three temperatures, i.e. 60°C, 70°C and 80°C and the same behavior was observed in all cases. When the epoxidation reaction was conducted at 90°C, it was difficult to collect samples because of the uncontrollable exothermic nature of this reaction and difficulty in maintaining a constant reaction temperature. As a result, the experiment conducted at 90°C was not reported. By contrast, the trends for the yield of 4-VCH 1,2-epoxide for experiments conducted at 60°C and 70°C were quite similar. A significantly higher yield of 4-VCH 1,2-epoxide was obtained at 80°C as compared to the experiments conducted at 60°C and 70°C. The yield of 4-VCH 1,2-epoxide after 5 min at 60°C, 70°C and 80°C was ∼18%, ∼28% and ∼94%, respectively. By contrast, the yield of 4-VCH 1,2-epoxide at 260 min was ∼95% for all three temperature ranges. Ambroziak et al. [14] observed similar exothermic effects while studying the effect of reaction temperature on cyclohexene epoxidation with TBHP catalyzed by polymer-supported Mo(VI) complex. It is evident from Figure 1 that ∼95% yield of 4-VCH 1,2-epoxide was achieved at 10 min for reactions at 80°C. Since it was not possible to conduct the epoxidation experiment beyond 80°C in a controlled manner, 80°C was considered to be the optimum reaction temperature for this process.

Figure 1

Effect of reaction temperature on the conversion of tert-butyl hydroperoxide (TBHP) to 4-vinyl-1-cyclohexane 1,2-epoxide (4-VCH 1,2-epoxide) for epoxidation of 4-VCH with TBHP catalyzed by polystyrene 2-(aminomethyl)pyridine supported Mo(VI) complex (Ps.AMP.Mo) catalyst using a batch reactor at feed molar ratio (FMR) of 4-VCH to TBHP: 5:1; catalyst loading: 0.3 mol% Mo; stirrer speed: 400 rpm.

3.1.2 Effect of FMR

The efficiency in the use of oxidant and its conversion to the desired product is a major criterion for assessing the proficiency of alkene epoxidation with alkyl hydroperoxide. Batch experiments were carried out at 2.5:1, 5:1 and 10:1 molar ratio of 4-VCH to TBHP to study the effect of different FMRs of 4-VCH to TBHP on the yield of 4-VCH 1,2-epoxide. No significant effect in the rate of formation of epoxides was found when the FMR of 4-VCH to TBHP was increased beyond 2.5:1, as shown in Figure 2. For instance, the yield of 4-VCH 1,2-epoxide obtained at 260 min for 2.5:1, 5:1 and 10:1 FMR of 4-VCH to TBHP was ∼92%, ∼96% and ∼98%, respectively. However, the reaction reached equilibrium at 80 min for an FMR of 4-VCH to TBHP of 10:1, and this is considered as the optimum FMR for this study.

Figure 2

Effect of feed molar ratio (FMR) on the conversion of tert-butyl hydroperoxide (TBHP) to 4-vinyl-1-cyclohexane 1,2-epoxide (4-VCH 1,2-epoxide) for epoxidation of 4-VCH with TBHP catalyzed by polystyrene 2-(aminomethyl)pyridine supported Mo(VI) complex (Ps.AMP.Mo) catalyst using a batch reactor at reaction temperature: 70°C; catalyst loading: 0.3 mol% Mo; stirrer speed: 400 rpm.

3.1.3 Effect of catalyst loading

An increase in catalyst loading increases the number of active sites per unit volume of reactor, leading to an increase in the yield of 4-VCH 1,2-epoxide. The effect of catalyst loading was investigated by conducting batch experiments using 0.15 mol% Mo, 0.3 mol% Mo and 0.6 mol% Mo loading. In this research, catalyst loading was defined based on the active Mo component instead of total mass of Ps.AMP.Mo catalyst, due to the slight differences in Mo content obtained from different batches of the prepared Ps.AMP.Mo catalyst. However, all batch and continuous studies in this work were conducted from the same batch of the prepared catalyst. The rate of epoxide formation for experiments carried out with 0.6 mol% Mo recorded a significantly high yield of 4-VCH 1,2-epoxide compared to 0.15 mol% Mo and 0.3 mol% Mo as shown in Figure 3. By contrast, despite the equilibrium being reached within the first 20 min for catalyst loading of 0.6 mol% Mo and within 100 min for 0.3 mol% Mo catalyst loading, the yield of 4-VCH 1,2-epoxide at 260 min for catalyst loading of 0.3 mol% Mo and 0.6 mol% Mo was found to be similar, i.e., ∼95%. It should be noted that 92% yield of epoxide was achieved at 20 min for catalyst loading 0.6 mol% Mo and this is regarded as the preferred catalyst loading for epoxidation of 4-VCH with TBHP using the Ps.AMP.Mo catalyst.

Figure 3

Effect of catalyst loading on the conversion of tert-butyl hydroperoxide (TBHP) to 4-vinyl-1-cyclohexane 1,2-epoxide (4-VCH 1,2-epoxide) for epoxidation of 4-VCH with TBHP catalyzed by polystyrene 2-(aminomethyl)pyridine supported Mo(VI) complex (Ps.AMP.Mo) catalyst using a batch reactor at a reaction temperature: 70°C; feed molar ratio (FMR) of 4-VCH to TBHP: 5:1; stirrer speed: 400 rpm.

3.1.4 Catalyst reusability and supernatant studies

The reusability potentials of Ps.AMP.Mo catalyst were evaluated by recycling the catalyst several times for batch experiments. In this study, fresh Ps.AMP.Mo catalyst was used in epoxidation experiments and plotted as Run 1 (see Figure 4A). At the end of the experiment, i.e. Run 1, the catalyst particles were filtered from the reaction mixture, washed carefully with 1,2-dichloroethane and stored in a vacuum oven at 40°C, while the filtrate obtained was vacuum distilled to recover the residue of the catalyst particles. The obtained residue from Run 1 was used as a potential catalyst for epoxidation known as supernatant studies and plotted as ‘After Run 1’ (see Figure 4B), to evaluate the extent of Mo leaching from the polymer supported catalyst. The stored Ps.AMP.Mo catalyst particles were reused in the subsequent experiment and plotted as Run 2 and the residue obtained from Run 2 was employed as the catalyst for epoxidation reaction and plotted as ‘After Run 2’. This procedure was repeated for the successive catalyst reusability and supernatant studies experiments.

Figure 4

(A) Catalyst reusability studies of polystyrene 2-(aminomethyl)pyridine supported Mo(VI) complex (Ps.AMP.Mo) catalyst for 4-vinyl-1-cyclohexane (4-VCH) epoxidation with tert-butyl hydroperoxide (TBHP) using a batch reactor at reaction temperature: 70°C; catalyst loading: 0.3 mol%; feed molar ratio (FMR) of 4-VCH to TBHP: 5:1; stirrer speed: 400 rpm; (B) supernatant studies of 4-VCH epoxidation with TBHP catalyzed by residues isolated from supernatant solutions of Ps.AMP.Mo catalyst using a batch reactor at reaction temperature: 70°C; FMR of 4-VCH to TBHP: 5:1; stirrer speed: 400 rpm.

Figure 4A shows that a high rate of epoxide formation was observed in Run 1 as compared to the subsequent runs. This is due to the availability of sufficient active sites in the fresh catalyst that are available for adsorption by the reacting species as compared to subsequent runs. The yields of 4-VCH 1,2-epoxide for Run 2 and Run 3 were similar, as both reached equilibrium at 160 min. However, a gradual decrease in the rate of epoxidation was observed in the subsequent experimental runs (Runs 4–6). The yields of 4-VCH 1,2-epoxide for Runs 4, 5 and 6 at 260 min were ∼90%, ∼81% and ∼74%, respectively. Supernatant studies of Ps.AMP.Mo catalyst revealed that some catalytically active Mo species were present in the residues isolated from Run 1 to Run 3, but became negligible ‘After Run 4’ (Figure 4B). The leaching of Mo could either be soluble leached complexes or as traces of Mo containing microgel released as a result of mechanical attrition of the beads or both.

3.2.1 Effect of reaction temperature

The effect of reaction temperature on the conversion of TBHP and the yield of 4-VCH 1,2-epoxide was investigated at 60°C, 70°C and 80°C. Experiments were carried out using 1:1 stoichiometric ratio of 4-VCH to TBHP for all the temperature ranges, while the feed flow rate was maintained at 0.1 ml/min. As shown in Figure 5A, an increase in reaction temperature resulted in higher conversion of TBHP and higher yield of 4-VCH 1,2-epoxide. For instance, ∼50% conversion of TBHP was achieved at steady state for reaction conducted at 80°C, whilst for the same period of time reactions carried out at 60°C and 70°C achieved ∼43% and ∼30% conversion of TBHP, respectively. Hence, it can be concluded that 80°C is the optimum reaction temperature for both batch and continuous epoxidation of 4-VCH with TBHP in the presence of Ps.AMP.Mo catalyst.

Figure 5

Conversion of tert-butyl hydroperoxide (TBHP) and the yield of 4-vinyl-1-cyclohexane 1,2-epoxide (4-VCH 1,2-epoxide) at steady state for continuous epoxidation of 4-VCH with TBHP using a FlowSyn continuous flow reactor in the presence of polystyrene 2-(aminomethyl)pyridine supported Mo(VI) complex (Ps.AMP.Mo) catalyst (∼1.5 g): (A) effect of reaction temperature at feed flow rate: 0.1 ml/min; feed molar ratio (FMR) of 4-VCH to TBHP: 1:1; (B) effect of feed flow rate at reaction temperature: 80°C; FMR of 4-VCH to TBHP: 1:1. (C) effect of FMR of 4-VCH to TBHP at reaction temperature: 80°C; feed flow rate: 0.1 ml/min.

3.2.2 Effect of feed flow rate

The effect of feed flow rate was investigated at feed flow rates of 0.1 ml/min, 0.13 ml/min and 0.16 ml/min. Continuous epoxidation experiments were carried out at 80°C (i.e. the optimum reaction temperature) and at stoichiometric FMR of 4-VCH to TBHP of 1:1. Experiments carried out at a flow rate of 0.16 ml/min, which corresponds to feed residence time of ∼3 min in the reactor, gave ∼36% conversion of TBHP and ∼29% yield of 4-VCH 1,2-epoxide at steady state as shown in Figure 5B. The residence time of the feed was increased to ∼4 min by reducing the flow rate to 0.13 ml/min yielding ∼40% conversion of TBHP and ∼36% yield of 4-VCH 1,2-epoxide. However, TBHP conversion and yield of 4-VCH 1,2-epoxide increased to ∼50% and ∼43%, respectively, when the flow rate was further reduced to 0.1 ml/min (residence time of ∼5 min). It can be concluded that an increase in feed flow rate caused reduction in feed residence time in the reaction zone (packed column) that consequently led to decrease in both the conversion of TBHP and the yield of 4-VCH 1,2-epoxide. Hence, feed flow rate of 0.1 ml/min can be considered as an optimum for continuous synthesis of 4-VCH 1,2-epoxide using Ps.AMP.Mo catalyst in a FlowSyn reactor.

3.2.3 Effect of FMR

In most of the catalyzed epoxidation processes, an excess amount of alkene is employed in order to suppress side reactions and enhance the conversion of the oxidant [20]. The effect of FMR was studied at 1:1, 2.5:1 and 5:1 molar ratios of 4-VCH to TBHP and at temperature and flow rate of 80°C and 0.1 ml/min, respectively. Figure 5C illustrates that the experiment conducted at 5:1 molar ratio resulted in ∼82% yield of epoxide and 94% conversion of TBHP as compared to the experiments carried out at 2.5:1 and 1:1 molar ratios, which gave 70% and 43% yield of epoxide, respectively. By contrast, the residence time required for ∼80% yield for continuous flow reaction conducted at 70°C is ∼5 min, whereas it takes >40 min (Figure 2) to arrive at a similar yield of epoxide in a batch reactor. Based on the results obtained from this study, a continuous flow reactor for epoxidation of 4-VCH can be considered to be more efficient than the batch reactor and 5:1 FMR of alkene to TBHP can be regarded as the optimum for continuous epoxidation of 4-VCH with TBHP in the presence of Ps.AMP.Mo catalyst.

3.2.4 Reusability studies of Ps.AMP.Mo catalyst

The reusability studies of Ps.AMP.Mo catalyst were conducted for continuous epoxidation of 4-VCH with TBHP in a fixed bed reactor using the same catalyst packing for four consecutive runs and each experiment lasted for 6 h. Therefore, the catalyst packing was used continuously for a period of 24 h. The reusability studies confirmed that Ps.AMP.Mo catalyst had negligible loss in catalytic activity when used for 24 h. TBHP conversion and the yield of 4-VCH 1,2-epoxide at steady state for each 6 h experiment was found to be in the range of 95±4% and 82±4%, respectively. It could be concluded that the problem of attrition of catalyst particles and Mo leaching observed using the batch reactor was eliminated in the continuous reactor. Hence, Ps.AMP.Mo catalyst could be reused several times in a continuous reactor for epoxidation of 4-VCH with TBHP.