**2. Supported ruthenium catalysts for epoxidation of alkenes**

In 1998 [6], Mesoporous MCM-41 molecular sieves are used for the immobilization of a ruthenium complex of meso-tetrakis (2,6-dichlorophenyl)porphyrin (**Figure 2**), [RuII(TDCPP)(CO)(EtOH)]. The supported Ru catalyst can affect highly selective heterogeneous alkene epoxidations with the terminal oxidant of choice being 2,6-dichloropyridine N-oxide in the presence of CH2Cl2. Conversion of aromatic and aliphatic alkenes to their epoxides can be successfully undertaken, with satisfactory amounts and selectivity, while the epoxidation of cis-alkenes (e.g. cis-stilbene) can be undertaken in a stereospecific manner as shown in **Table 1**. The leaching and/or deactivation of the catalyst may be the reason why activity is lost.

Another study in 2002 observed that poly(ethylene glycol) (PEG) binds to ruthenium porphyrin through a covalent etheric bond. What characterizes these catalysts is that they are highly reactive and selective for epoxidation of alkenes with 2,6-dichloropyridine N-oxide as terminal oxidant [7].

Ethene was subjected to electrochemical epoxidation with 0.3 M chloride ion concentration by employing nanocrystalline RuO2 and Co-doped RuO2 deposited electrodes in aqueous acidic media [8]. The by-products of this reaction were oxirane and 2-chloroethanol. Epoxide formation was achieved based on a threemembered transition state involving the binding of ethylene to an oxygen on the surface of RuO2. Furthermore, a single-step sol gel process (SSSG) facilitated the synthesis of RuO2-loaded meso-porous assembled TiO2 nanocrystals with high selectivity and recyclability for the purpose of liquid-phase cyclohexene epoxidation employing H2O2 [9]. In this context, it was observed that the temperature of calcination influenced both the catalytic activity and the catalyst selectivity. In the case of SSSG, the optimal calcination temperature was established to be 450 °C, which yielded maximal epoxide selectivity of up to 80%. The epoxide selectivity and conversion related to the calcined SSSG did not alter even on the third sequential run.

**Figure 2.** *Meso-Tetrakis(2,6-dichlorophenyl)porphyrin (TDCPP).*


*Reaction conditions: alkene (1 mmol), Cl2pyNO (1.1 mmol), 0.4 wt % Ru/M-41(m) (0.195* μ*mol of Ru), HCl (*∼*0.3 mmol), CH2Cl2 (5 mL), 40 °C under an Ar atmosphere. <sup>a</sup>*

*Yields are based on the number of substrates consumed; products were identified and quantified by either GLC or 1H NMR.*

*b Trace amounts of benzaldehyde and phenylacetaldehyde were also detected.*

*c 2-Cyclohexen-1-ol (14%) and 2-cyclohexen-1-one (11%) were formed. TOF were determined by monitoring the reactions using GC within the first 2 h of the reactions.*

#### **Table 1.**

*Epoxidations of different alkenes using a MCM-41-supported ruthenium porphyrin complexes [6].*

In one synthetic approach that has been proposed, an amino acid L-valine was affixed to styrene-divinylbenzene co-polymer beads with 8% and 6% cross-linking. The formation of the metal complex on the support was achieved by applying a ruthenium (III) chloride solution to polymeric ligands that included bidentate N,O donor sites. The supported catalysts were employed to investigate the catalytic epoxidation of styrene, Norbornylene, cyclooctene and cyclohexene, with *tert*-butyl hydroperoxide as the terminal oxidant. In the case of Norbornylene and cis-cyclooctene, it was noted that epoxides formed selectively and with increase the reaction temperature from 28 °C to 45 °C there is an increase in the epoxide yield at similar levels of catalyst concentration. Meanwhile, the equivalent epoxide, benzaldehyde and acetophenone were derived for styrene. Although the catalyst could be reused, repeated recycling caused the metal to leach gradually from the support, with negative implications for its use [10].

One study investigated alkene epoxidation with ruthenium(III) salophen chloride [Ru(salophen)Cl] based on support of functionalized chloromethylated polystyrene (PS). 1,4-diaminobenzene, 4-aminophenol and 4-aminothiophenol were used for PS modification, while axial ligation facilitated the binding of [Ru(salophen)Cl] to the supports. Alkene epoxidation with sodium periodate (NaIO4) as a best oxidant at ambient temperature was successfully performed with the employed catalysts, satisfactory activity being noted. It was possible to use the heterogeneous catalysts again in the reactions, and they were recycled repeatedly. They also observed that the benefits provided by these catalysts include the fact that they are uncomplicated to prepare and handle, the support is available on the market, and the supported catalysts can be readily recovered and reused [11].

Another study showed preparation of ruthenium-doped H-Montmorillonite (H-Mont) and Ti-pillared clay (PILC) was undertaken to investigate cyclohexene oxidation, with the chosen source of oxygen being *tert*-butyl hydroperoxide (TBHP). Ru/Ti-PILC showed better effective catalytic activity than Ru/H-Mont. The use of 5% Ru/Ti-PILC as catalyst resulted in conversion of cyclohexene in proportion of 59%, selectivity of 87% and 13% respectively for 2-cyclohexene-1-one and 2-cyclohexene-1-ol at a temperature of 700 °C for six hours, without formation of epoxide. It was observed that the oxidation of cyclohexene was heterogeneous, and no leaching of ruthenium was observed [12, 13].

In our previous work, supported ruthenium catalysts (1%Ru/TiO2) have been utilized for the epoxidation of 1-decene under solvent-free conditions. The reaction was continued for 24 h at 90 °C in atmospheric air with very small amount of *tetra*-butyl hydroperoxide (TBHP). 1% Ru/TiO2 catalyst was prepared using two different preparation methods called sol-immobilization and wet-impregnation. For preparation of sol-immobilization, in brief, an appropriate quantity of RuCl3. xH2O was added to deionized water (800 mL) with continuous stirring. To protect and stabilize the Ru nanoparticles, a freshly prepared 1 wt.% solution of poly (vinyl alcohol) (m.w. = 10 000, 80% hydrolyzed) was added (PVA/Ru (by wt) = 0.65). After a further 15 min of stirring, a dark brown sol was generated by the addition of a freshly prepared solution of sodium borohydride (0.2 M, molar ratio NaBH4/ Ru = 5). The sol was stirred for a further 30 min with dropwise addition of H2SO4 to adjust the acidity to pH = 2. The TiO2 support (~1.98 g) was then added and the mixture stirred for 2 h prior to wash thoroughly with deionized water (2 L) and dried at 110 °C for 16 h. In the wet-impregnation method, catalyst (1 g) was prepared by dissolving an appropriate quantity of RuCl3·xH2O in deionized water and added an appropriate amount of TiO2 support and allowed water to evaporate with continuous stirring at 80 °C. The obtained paste was dried for 16 h at 110 °C and grounded prior to calcination for 3 h in static air at 300 °C (heating rate = 20 °C/min).

For the preparation of 1 g of catalyst via the wet impregnation technique, a suitable amount of RuCl3·xH2O was dissolved in deionized water, after which a suitable quantity of TiO2 support was added, and water evaporation was permitted with constant stirring at 80 °C. This process yielded a paste that was left to dry for 16 hours at 110 °C and was ground before being calcined for 180 minutes in static air at 300 °C, with a heating rate of 20 °C min−1. The standard reaction of epoxidation involved addition of 0.1 g catalyst to 1-decene (53 mmol, 10 mL) in a glass flask with a round bottom and 50 cm3 volume affixed with a reflux condenser. Following the addition of 0.01 mL *ter-*butyl hydroperoxide (TBHP) as the radical initiator, the reaction mixture was placed on a hotplate to heat to 90 °C with magnetic stirring. When the established reaction time ended, the mixture was allowed to reach ambient temperature and was subjected to filtration before being analyzed via gas chromatography (GC) [14].

1-Decene epoxidation under solvent free conditions was investigated based on 1%Ru/TiO2. The reaction was performed for 24 hours at 90 °C in air with a catalytic quantity of TBHP. The initial step involved assessing the blank reaction with solely TBHP present, which revealed poor epoxidation reaction activity, with 2% conversion, and 10% selectivity to 1,2-epoxydecane (**Figure 3**). Subsequent assessment of TiO2 displayed poor 1-decene conversion (4%) and epoxide selectivity (16%) as well. However, when 1% Ru/TiO2 synthesized via sol-immobilization was used, the epoxide yield improved substantially. This was reflected in the increase in 1-decen conversion from 4 to 16% and in the increase in epoxide selectivity from 16 to 37% (**Figure 3**) [14].

High epoxide selectivity continues to pose difficulties in the context of the epoxidation reaction. This has led to the identification and quantification of several additional by-products, which have been comprehensively detailed in earlier studies *Ruthenium Catalyst for Epoxidation Reaction DOI: http://dx.doi.org/10.5772/intechopen.96466*

#### **Figure 3.**

*Effect of TiO2 and 1% Ru/TiO2 on 1-decene epoxidation. Reaction conditions: Catalyst (0.1 g), 1-decene (53 mmol, 10 mL), TBHP (0.064 mmol, 0.01 mL), 90 °C, atmospheric pressure air, reaction time 24 h, rate of stirring 900 rpm. Error bars indicate range of data based on three repeat experiments.*

[15, 16]. When the reaction runs for 24 hours, it results in the formation of substantial amounts of allylic products. Therefore, since the formation of certain products may accompany the oxidation of other products, it is useful to examine the product profile as the reaction unfolds. Time online studies were undertaken for 96 hours to gain insight into the reaction profile of 1-decene epoxidation with 1% Ru/TiO2 catalyst. The increase in the reaction time from 4 to 96 hours determined an equivalent increase in 1-decene conversion from 1 to 35%, without an interval of induction (**Figure 4**). This means that the lengthier the reaction run, the higher the 1-decene conversion is likely to be. Furthermore, at the start of the reaction, epoxide selectivity was poor, with the main products being allylic compounds. However, as the reaction time increased up to 48 hours, so did the epoxide selectivity. On the other hand, epoxide selectivity gradually declined to 18% at 96 hours when the reaction time exceeded 48 hours, which is usually explained in terms of the ring opening reaction that occurs between epoxide and water as a by-product of condensation reactions or the breakdown of hydroperoxyl intermediate to the allylic ketone and water [16]. After formation of water, epoxide is immediately hydrolyzed to diol. Earlier studies also found that increase in reaction time improved alkene conversion and epoxide selectivity in the case of the terminal alkenes 1-decene, 1-hexene, and 1-octene. Additionally, it was observed that cracking of heptanoic, octanoic, and nonanoic acids was accompanied by enhanced selectivity as well [16].

The technique of synthesis is a major determinant of catalytic activity [17]. By contrast to the wet-impregnation technique, preparation of 1% Ru/TiO2 catalyst via sol-immobilization displays greater activity for 1-decene epoxidation (**Figure 5**). More specifically, 1% Ru/TiO2 is associated with 15% conversion and 37% epoxide selectivity when prepared via sol-immobilization and 10% conversion and 29% epoxide selectivity when prepared via wet impregnation.

The 1% Ru/TiO2 catalyst synthesized via sol-immobilization was employed in excess amount to conduct the above reaction and thus evaluate reusability [14]. The procedures that followed reaction termination included catalyst filtration, washing with acetone, and 16-hour oven drying at 110 °C. Subsequently, the amount

#### **Figure 4.**

*Effect of reaction time on conversion and selectivity. Reaction conditions: 1% Ru/TiO2 (0.1 g), 1-decene (53 mmol, 10 mL), TBHP (0.064 mmol, 0.01 mL), 90 °C, atmospheric pressure air, rate of stirring 900 rpm. Allylic products =* ∑ *(1-decen-3-one, 1-decen-3-ol, 2-decenal, 2-decen-1-ol). Others =* ∑ *(C7 + C8 + C9 acids, C8 + C9 aldehyde, C7 + C8 alcohols, 3-nonen-1-ol, 3-nonanone, cyclododecane, 2-decenoic acid). Error bars indicate range of data based on three repeat experiments.*

#### **Figure 5.**

*Effect of the catalyst preparation method on 1-decene oxidation [14]. Reaction conditions: 1% Ru/TiO2 (0.1 g), 1-decene (53 mmol, 10 mL), TBHP (0.064 mmol, 0.01 mL), 90 °C, atmospheric pressure air, rate of stirring 900 rpm. Error bars indicate range of data based on three repeat experiments.*

of catalyst necessary for regular reaction was extracted to be used again. **Table 2** provides the activity data related to both the fresh and reused catalysts. Thus, by contrast to the fresh catalyst, which allowed 15% conversion of 1-decene and 37% epoxide selectivity, the reused catalyst subjected to drying with no previous washing showed suboptimal activity, so reuse was unsuccessful. One reason for this could be the fact that the adsorbed reaction products that were present caused the catalyst


*atmospheric pressure air, reaction time 24 h, rate of stirring 900 rpm.*

**Table 2.**

*Catalyst reusability study for epoxidation of 1-decene: 1% Ru/TiO2.*

to deactivate. On the other hand, better conversion (10%) and epoxide selectivity (24%) were exhibited by the catalyst washed with acetone before being reused, but even so, the activity was still poorer compared to that of fresh catalyst, most likely owing to the fact that carbon inhibited active sites. Active component leaching into the solution is major problem of heterogeneous catalysts, particularly in the liquid phase. However, ICP analysis suggested that ruthenium did not leach.
