Optimization was carried out using catalyst 1a. The reaction was monitored at 3 h intervals up to 9 h (Fig. 3.2). The temperature was kept constant at 80 °C in acetonitrile (MeCN) where the molar catalyst:styrene ratio used was 1:100, whilst the styrene:TBHP molar ratio was 1:2.5. The conversion increases with time, however, the yield to benzaldehyde and styrene oxide decreases slightly from 6 to 9 h. After 24 h, a 98% conversion was found, with a lower yield to benzaldehyde (19%) and styrene oxide (4%). These results are consistent with the fact that at high temperatures, vinyl C-H bonds of styrene are highly active.8,27,28 All further reactions were stopped after 9 h. Deeper oxidation products, like benzoic acid and carbon dioxide were also observed and obtained in higher yield at longer reaction times (> 9 h). This is also noted with other cobalt systems.22 Unlike other reported studies done on the oxidation of styrene, benzene was formed, which increases over time (7% (3 h) to 12% (9 h)).
Figure 3.2. Conversion of styrene and yield to benzaldehyde and styrene oxide.
Conditions: Catalyst:Styrene (1:100); Styrene:TBHP (1:2.5); Temperature: 80 °C; Solvent: MeCN;
Catalyst 1a.
0 20 40 60 80 100
0 5 10 15 20 25 30
3 6 9
% Conversion
% Yield
Time/ h
Benzaldehyde Styrene oxide Conversion
A lower styrene:TBHP molar ratio (1:1.5) under the same conditions mentioned above (catalyst:styrene (1:100); temperature 80 °C; and MeCN as the solvent) was also investigated and after 9 h, the conversion was 82% and the yield to benzaldehyde and styrene oxide was 28% and 15% respectively. However, when the catalyst:styrene ratio was decreased from 1:100 to 1:50 with the styrene:TBHP ratio maintained at 1:2.5 at 80 °C, a 87% conversion was obtained, with lower yields to benzaldehyde (24%) and styrene oxide (10%) due to over oxidation.
The reaction was also carried out at room temperature (RT) and 50 °C where the styrene:TBHP ratio used was 1:2.5 and catalyst:styrene ratio used was 1:100. At 50 °C, using TBHP as the oxidant, a 45% conversion was observed with a 29% yield to benzaldehyde and 2% yield to styrene oxide. At RT no conversion was noted. When the oxidant was changed to hydrogen peroxide, with the same ratios mentioned above at both 50 and 80 °C, the conversion increases, with a higher yield to benzaldehyde at 80 °C (Table 3.1). However, better yields and conversion at these temperatures using TBHP were obtained. The activation energy at these respective temperatures was also calculated. The activation energy for the reaction using TBHP (21 kJ mol-1) as the oxidant was lower than when H2O2 (35 kJ mol-1) was used. This explains the higher conversion for the TBHP system. When using H2O2, in some reported cases, reactions fail to activate the peroxide or, in others, there is less selectivity to the desired products, such as the epoxides and a greater formation to unwanted and cleaved products.7,29,30,33 The oxidant, N-methyl morpholine (NMO), was also investigated at 80 °C and no conversion was observed. When m-CPBA (meta- chloroperoxybenzoic acid) was used as an oxidant, rapid catalyst decomposition occurred.
Table 3.1. Oxidation of styrene using H2O2 at 50 °C and 80 °C.
Temperature/ °C Conversion Yield
Benzaldehyde Styrene oxide
50 6 4 1
80 41 33 1
Conditions: Styrene:TBHP ratio (1:2.5) and catalyst:styrene (1:100); Time 9 h; Catalyst 1a. Other products observed benzene and benzoic acid.
Lastly, the effect of solvent on these reactions was investigated. Using 1,2-dichloroethane (DCE) as the solvent, with TBHP as the oxidant (styrene:TBHP; 1:2.5) at 80 °C, the reaction was monitored up to 9 h (Fig. 3.3). The conversion and yield to benzaldehyde increases up to 3 h and stabilizes thereafter. After 3 h the conversion reached 95% with the yield to benzaldehyde reaching a maximum (25%). The yield to styrene oxide decreased over time
(13% at 3 h to 7% at 9 h), however, the yield to phenylacetaldehyde increase from 3% (3 h) to 7% (9 h). Styrene diol was also observed after 2 and 3 h (2% yield). The increase in conversion could be attributed to the improved solubility of the oxidant in the solvent. When a similar study was carried out using supported vanadium catalysts with DCE as the oxidant, the yield to both benzaldehyde and styrene oxide increased as compared to when MeCN was used.31 Also with iridium catalysts an increase in reaction rate was found using a chlorinated solvent (tetrachloroethane (TCE)), due to the solvent radicals generated from the reaction of TBHP and TCE.1 A blank reaction (with no catalyst) was carried out, and after 3 h, a 20%
conversion was observed, with benzoic acid forming as the main product with no yield to styrene oxide and a 1% yield to benzaldehyde.
Figure 3.3. Conversion of styrene over time and yield to benzaldehyde and styrene oxide.
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C; Solvent: DCE;
Catalyst 1a.
To investigate the effect of having an ethylene spacer group between the phosphorous and nitrogen atom (catalyst 2a), versus no spacer atom (catalyst 1a), the reactions were compared at 1 h intervals for 3 h (Fig. 3.4). The activities of both catalysts increase over time, where the catalyst with the more rigid ligand backbone (1a) is slightly more active than the catalyst with the flexible backbone (2a). The yield to benzaldehyde and styrene oxide increases over time, with the yield of both these products greater for the first two hours over catalyst 2a. The rigidity of complex 1 probably results in the slow formation of the active species. The yield to styrene oxide drops slightly after 2 h for catalyst 2a due to the formation of phenylacetaldehyde (3% at 3 h). A slightly higher benzene formation after 3 h for catalysts 2a (7%) in comparison to 1a (6%) accounts for the slightly lower yield to benzaldehyde over catalyst 2a.
0 20 40 60 80 100
0 5 10 15 20 25 30
1 2 3 6 9
% Conversion
% Yield
Time/ h
Benzaldehyde Styrene oxide Conversion
Figure 3.4. Conversion of styrene over time and yield to benzaldehyde and styrene oxide over catalysts 1a and 2a.
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C; Solvent: DCE
The catalysts with the different substituents on the nitrogen atom were then screened under the optimized conditions (Table 3.2). Catalysts 1 are more active than catalysts 2 and a greater yield to benzaldehyde and styrene oxide is found with catalysts 1 than catalysts 2.
This is also seen with the higher turnover number (TON) towards benzaldehyde and styrene oxide of catalysts 1. The catalysts bearing the cyclohexyl substituent (1a and 2a) on the nitrogen atom are least active. The activity of catalyst 1 bearing the pentyl substituent (1c) is highest, with greater TONs towards benzaldehyde and styrene oxide. The TONs towards benzaldehyde of catalysts 2 are comparable.
Table 3.2. Screening of catalysts 1 and 2 at optimized conditions.
Catalyst % Conversion % Yielda TON
Benzaldehyde Styrene oxide Benzaldehyde Styrene oxide
1a 92 25 13 23 13
1b 96 22 7 26 9
1c 96 23 11 28 14
2a 84 22 8 24 9
2b 94 19 6 23 7
2c 95 19 5 24 6
Conditions: Catalyst:styrene (1:100); Styrene: TBHP (1:2.5); Temperature: 80 °C; Solvent: DCE; Time 3 h.
aOther products: benzoic acid, benzene, phenylacetaldehyde.
0 20 40 60 80 100
0 5 10 15 20 25 30
1 2 3
% Conversion
% Yield
Time/ h
Benzaldehyde 1a Benzaldehyde 2a Styrene oxide 1a Styrene oxide 2a Conversion 1a Conversion 2a
In comparison, other cobalt-based systems are reported to be less efficient. Studies performed using cobalt zeolites gave no yield to benzaldehyde and low conversions and low TONs towards styrene oxide (0.1 – 15).30 Li et al. obtained a 63.3% conversion using cobalt- encapsulated zeolite Y with higher selectivity to benzaldehyde than styrene oxide.25 With Schiff based polymer-cobalt complexes, 20% selectivity to benzaldehyde was observed.32 When cobalt oxide was used as a catalyst, a 0.1 % selectivity to benzaldehyde was reported,33 and when molecular sieves containing cobalt were used as catalysts no yield to benzaldehyde was reported.30,34
Scheme 3.1. Proposed mechanism for the oxidation of styrene by catalysts 1 and 2, (L=Ligand).
To elucidate the mechanism of this study, benzaldehyde and styrene oxide were used as substrates under optimum catalytic conditions using catalysts 1 and 2. When the substrate was benzaldehyde, benzene and benzoic acid formed. However, when styrene oxide was used as
CoIIL
CoIIIO2L
CoIIIO2L
O O CoIIIL
O O
O
O
+ other products t-BuOOH
+ other products A
B
D
C
the substrate, highest yield to phenylacetaldehyde is obtained. On the basis of this experimental work and information obtained from literature, we propose the mechanism shown in Scheme 3.1. The Co(II) can bind to the oxygen from the TBHP (t-BuOOH) to form a Co(III) super oxo species (A).25,30,35 The oxygen bound cobalt species (Co(III)_(O2-)) reacts with styrene to form an active oxygen intermediate species (B) which is responsible for the epoxidation reaction.30,35 Rearrangement of intermediate B to C and generation of the cobalt catalyst then occurs. The formation of the epoxide and benzaldehyde via two different pathways occurs through intermediate D.16,25,30,35