Selective oxidation of cumene into 2-phenyl-2-propanol and acetophenone over activated carbon supported Co_1.5PW₁₂O₄₀ material

12-Tungstophosphoric acid H₃PW₁₂O₄₀.13H₂O was prepared according to a well-known method (Rocchiccioli-Deltcheff et al 1983).

Co_1.5PW₁₂O₄₀ (abbreviated CoPW) was prepared by proton substitution of the H₃PW₁₂O₄₀ heteropolyacid. This substitution is as follows. To an aqueous solution of H₃PW₁₂O₄₀, a required amount of Ba(OH)₂.8H₂O (to neutralize the 3 protons) is added slowly. Then, the required amount of Ba(OH)₂.8H₂O was added, resulting in the formation of insoluble barium salt, which was removed by filtration. The filtrate is allowed to stand for a few days at 4 °C to allow precipitation of the CoPW salt which is then recovered from the solution by filtration.

The attachment of the CoPW heteropolyanions onto the activated carbon (AC) support necessitates the creation of oxygenated groups on the support, and what was done by oxidation was created by oxidation with concentrated nitric acid.

The preparation of the supported catalyst was achieved as follow:100 mg sample of AC was dispersed in 100 ml of nitric acid (65%) and heated for 5 h at 80 °C with stirring. The desired amount of carbon support was dispersed in 100 ml of nitric acid (65%) and heated for 5 h at 80 °C with stirring. The mixture containing the activated carbon is allowed to cool to room temperature, then filtered and washed with deionized water at pH 7 and dried at 100 °C overnight. A desired amount oxidized AC was then added to the desired amount of CoPW already dissolved in acetone and the mixture was kept under stirring for 30 min. Then after it was heated at about 60 °C to remove the excess acetone and dried overnight in an oven at 80 °C. the supported catalyst was denoted AC-CoPW.

Figure 1 shows the FT-IR spectra of the oxidized AC, CoPW and AC-CoPW catalysts. The spectrum of the oxidized AC (figure 1(b)) shows bands at 1346, 1730, and 1536 cm⁻¹ which were assigned to C—O, C=O, and C=C groups, respectively. The peak at around 1730 cm⁻¹ correspond to the C=O strength vibration in the COOH group (Fanning and Vannice 1993, Zhuang et al 1994, Shanmugharaj et al 2007, Yang et al 2017). This band could be due to either lactone groups (Fanning and Vannice 1993) or to carboxyl groups in aromatic compounds (Zhuang et al 1994, Yang et al 2017). Thus, these results show that the oxidation treatments have created oxygen groups, such as carboxylic and hydroxyl groups, on the carbon surface.

Zoom In Zoom Out Reset image size

The spectrum of CoPW (figure 1(a)) shows bands at 1080.2, 982.9, 883.2 and 766.8 cm⁻¹. These bands which are assigned to the stretching modes vas (P–O_d), vas (W–O_d), vas (W–O_b–W), and vas (W–Oc–W), (Oa, oxygen atom bound to 3W atoms and the central P atom; O_b and Oc, bridging oxygen atoms; O_d, terminal oxygen) are characteristic of the keggin structure (Rocchiccioli-Deltcheff et al 1983). As for the FT-IR spectrum of AC-CoPW, it can be seen in figure 1(b), that besides the bands related to the carbon, the four characteristic bands of keggin structure are present. These latter indiquent that CoPW loaded on the AC has preserved its Keggin structure.

Figure 2 the x-ray diffraction patterns of the CoPW and AC-CoPW. The reflections at 9.8°, 21.0°, 26.2°, and 34.9°, observed in the patterns of CoPW (figure 2(a)), indicated that this latter has both a Keggin structure (Fournier et al 1992, Yoshimune et al 2002). Indeed, the x-ray diffractions for the keggin structure show peaks in each of the four ranges of 2θ, namely 7°–10°, 16 –23°, 25°–30° and 31°–38°. The reflections observed at 26.1°, 43.1°, 53.5° and 78.7° 2θ for AC-CoPW (figure 2(b)) correspond to (002), (100), (004) and (110) diffractions respectively of hexagonal graphite (Zhang et al 2002, Sun et al 2005, Delidovich and Palkovits 2016). These reflections indicated that the graphitic structure of AC was preserved after oxidation treatment. Besides the reflections assigned to AC, characteristic peaks of CoPW are also observed. These results indicated that Keggin structure was preserved after loading on AC support. Thus, both XRD and FTIR characterizations showed that AC-CoPW catalyst was successfully synthesized.

Zoom In Zoom Out Reset image size

Elemental analysis of CoPW was done by ICP and the results were reported in table 1(a). The results which were determined by considering 1 atom of phosphorous per Keggin unit were found in a good agreement with the expected ones for tungsten and counter ion. Analysis of CoPW loaded on AC support was done by UV-visible in aqueous solution. It has been reported that CoPW in aqueous solution showed an absorbance at 254 nm (Eid et al 2013, Grama et al 2014). The amount of CoPW loaded on AC support was determined by considering the intensity of this band. The results (table 1(b)) showed that the experimental amount of CoPW (0.315 g/0.100g) was very close to the nominal one loaded on AC support (0.350 g/0.100g)

The surface morphology of the activated carbon before and after treatment with HNO₃ was analyzed by SEM. The results obtained (figure 3) showed that the activated carbon before treatment consisted of particles of variable sizes forming spaces between them (pores) whereas, for the activated carbon treated with nitric acid, the particles are agglomerated and the surface appeared as a porous mass. These results are confirmed by the surface area measurement. In fact, the activated carbon before treatment has a surface area of 1019.66 m² g⁻¹, whereas the acid treated activated carbon has a surface area of 628.69 m²/g. The treatment of the activated carbon with HNO₃ reduces considerably the specific surface area. These results are corroborated by those reported in the literature. In fact, many studies reported that nitric acid treatment reduces the specific surface area (Soudani et al 2013, Bernal et al 2018, Rehman et al 2019). Soudani et al 2013 reported that this reduction depends on the concentration of HNO₃. In the opinion of the author, this reduction is also due to the oxygen groups created by the treatment with HNO₃ which are probably fixed at the entrance of the micropores leading to a reduction in the pore volume.

Zoom In Zoom Out Reset image size

The effect of the oxidizing system composition on the conversion and selectivities was examined by oxidizing cumene by CO₂ alone, H₂O₂ alone and H₂O₂/CO₂ mixture. The results (table 3) show that in the case of oxidation by CO₂ alone and H₂O₂ alone, the conversions obtained are 3.53% and 12.9% respectively. Interestingly when cumene was oxidized by H₂O₂/CO₂ mixture, the conversion increased significantly (31.7%). The improvement in the oxidation of cumene by the H₂O₂/CO₂ oxidizing system is the result of two factors. The first is the effective activation of H₂O₂ by Co_1.5PW₁₂ and the second is the formation of a percarbonate species, a strong oxidant formed by the reaction of H₂O₂ with CO₂. For the activation of H₂O₂ by Co_1.5PW₁₂, it has been reported that in the presence of tungstate-based systems, oxidation by H₂O₂ leads to high reactivities and low activities for the non-productive decomposition of H₂O₂ (Mizuno et al 2008). Among the tungstate-based systems, the Peroxotungstate [PO₄(WO(O₂)₂)₄]³⁻, also called Venturelo complex has been postulated as being a catalytically active species in the oxidation of various reactions (Ishii et al 1988, Duncan et al 1995, Qi et al 2011). Venturello et al 1983 reported that in aqueous H₂O₂, [PW₁₂O₄₀]³⁻ led to the formation of the complex [PO₄{W(O)(O₂)₂}₄]³⁻. This complex which is composed of two species [W₂O₂(O₂)₄] attached to the PO₄³⁻ anion- reacts with the olefin to give an intermediate which subsequently loses a molecule of water to lead to the reaction products and to regeneration of the catalyst.

$$\begin{eqnarray}&&\begin{array}{l}{\left\{{{\rm{PO}}}_{4}{\left[{{\rm{WO}}}_{3}\right]}_{12}\right\}}^{3-}+24\,{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {\left\{{{\rm{PO}}}_{4}{\left[{\rm{WO}}{\left({{\rm{O}}}_{2}\right)}_{2}\right]}_{4}\right\}}^{3-}\\ \,\,+\,4{\left[{{\rm{W}}}_{2}{{\rm{O}}}_{3}{\left({{\rm{O}}}_{2}\right)}_{4}{\left({{\rm{H}}}_{2}{\rm{O}}\right)}_{2}\right]}^{2-}+8{{\rm{H}}}^{+}+12{{\rm{H}}}_{2}{\rm{O}}\end{array}\end{eqnarray} \tag{ 1 }$$

As for the effect of the percarbonate entity (${{\rm{HCO}}}_{4}^{-}$) on the oxidation, it has been reported (Richardson et al 2000) that this entity which is formed from hydrogen peroxide and sodium carbonate can oxidize alkene into epoxide. Beckman also postulated the formation of ${{\rm{HCO}}}_{4}^{-}$ by the reaction of aqueous CO₂ and hydrogen peroxide (Beckman 2003). In the opinion of the author, the percarbonate species (HCO₄⁻) can be formed by the reaction of H₂O₂ with aqueous bicarbonate (${{\rm{HCO}}}_{3}^{-}$) and this entity can epoxidize alkenes. It has also been reported by Hâncu et al 2002 that a biphasic aqueous H₂O₂/CO₂ mixture is an efficient epoxidizing system, where ${{\rm{HCO}}}_{4}^{-}$ is formed through various reactions of water, CO₂, and H₂O₂ and transfers of oxygen to alkenes (equations (2) and (3))

$$\begin{eqnarray}&&{{\rm{CO}}}_{2}+{{\rm{H}}}_{2}{\rm{O}}\rightleftharpoons {{\rm{H}}}_{2}{{\rm{CO}}}_{3}\rightleftharpoons {{\rm{HCO}}}_{3}^{-}+{{\rm{H}}}^{+}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{HCO}}}_{3}^{-}\to {{\rm{H}}}_{2}{\rm{O}}+{{\rm{HCO}}}_{4}^{-}\end{eqnarray} \tag{ 3 }$$

Truzzi et al 2019 mentioned that the acceleration of oxidation by ${{\rm{HCO}}}_{3}^{-}$/CO₂ on oxidations mediated by H2O2 are due to the percarbonate species (${{\rm{HCO}}}_{4}^{-}$), an oxidant with two electrons stronger than H₂O₂ which is present in the H₂O₂/${{\rm{HCO}}}_{3}^{-}$/CO₂ mixture. It was noted that small concentrations of ${{\rm{HCO}}}_{4}^{-}$ present in the mixtures are sufficient to significantly increase oxidation.

The effect of co-catalyst amount on cumene oxidation was investigated in the mass fraction ranging from 0.16 to 0.34. The results depicted in figure 4 showed that increasing the mass fraction from 0.16 to 0.28 increased the conversion from 9.5% to 31.7%. Beyond 0.28, there is no significant change. The selectivity of Ph-PrOH presented the same trend as the conversion. That is to say it increases when the mass fraction increases from 0.16 to 0.28 and then remains unchanged above 0.28. The increase in the selectivity of Ph-PrOH is to the detriment of that of Ph = O and Ph-OH which clearly indicate that the co-catalyst prevents the deep oxidation but favors the weak oxidation (formation of Ph-PrOH alcohol). Given the above results, it can be deduced that an appropriate amount of the co-catalyst contributes to the activation of CO₂. Therefore, the optimal mass fraction 0.29 was selected for all subsequent investigations.

Zoom In Zoom Out Reset image size

The effect of the reaction temperature (R_T) on the conversion and selectivities is depicted in figure 5. The results showed that increasing reaction temperature to 75 °C increased the conversion. Further increase in reaction temperature up to 85 °C caused a decrease in conversion. The conversion decay observed for temperatures above 75 °C could be attributed to the decomposition of H₂O_2. The selectivity of Ph = O increased slightly, whereas that of Ph-PrOH and Ph-OH decreased slightly throughout the temperature range.

Zoom In Zoom Out Reset image size

Heteropolyatungstates are among the most suitable catalysts for H₂O₂ oxidation reactions. Indeed,the keggin heteropolyoxotungstates have 12 peripheral tungsten atoms (W) and each W atom has a large positive charge. This positive charge makes the heteropolytungstates capable of accepting electron pairs in vacant W orbitals to form a stable complex with H₂O₂. This makes the oxygen atoms of H₂O₂ more active and, therefore, the H₂O₂ activity for the oxidation reactions can be improved (Donoeva et al 2009). This tempted us to examine the effect of H₂O₂ on cumene oxidation. Experiments with various proportions of H₂O₂ were carried out. The results (figure 6) show that increasing H₂O₂/cumene volume ratio increased the conversion to a maximum value of 46.4% for a volume ratio of 2 and then gradually decrease to a value of 20.3% for a volume ratio of 3.5. The decrease in conversion was due to the gradual decrease in cumene solubility in the polar medium which increases with the amount of H₂O₂. As for the selectivities of the reaction products, it can be noticed that increasing the concentration of H₂O₂ decreased, the selectivity of Ph-PrOH and PhOH in favor of Ph = O. This is expected since cumene is insoluble in the aqueous phase, it is the alcohols that are oxidized instead.

Zoom In Zoom Out Reset image size

The effect of catalyst amount on cumene reaction was studied in the range 0.50–1.25g and the results are presented in figure 7. As expected, the conversion increases with the increase in the amount of the catalyst. On the other hand, the selectivity of the products depends on the conversion. It is well known that the increase in conversion increases the consecutive reactions. This is in agreement with our results. In fact, increasing the amount of catalyst increased both the conversion and Ph = O selectivity but decreased the selectivity of Ph-PrOH and phenol PhOH. This results indicated that Ph = O might be formed directly from cumene and from Ph-PrOH and/or phenol PhOH by consecutive reactions.

Zoom In Zoom Out Reset image size

The variation in conversion and selectivity as a function of time is shown in figure 8. The results obtained show that the conversion increases linearly with the reaction time. With regard to the selectivity, it is noted that the selectivity for both Ph = O and Ph-PrOH has slightly decreased in favor of that of Ph-OH. This increase in the Ph-OH selectivity as a function of reaction time indicates the progress of the cracking reaction. This is explained by the decrease in the concentration of H₂O₂, and therefore it is the effect of CO₂ that remains predominant (constant CO₂ pressure) thus leading to cracking reactions. This interpretation is supported by the increase in the selectivities of the products resulting from the cracking reactions (toluene, ethyl benzene, benzene and toluene) obtained.

Zoom In Zoom Out Reset image size

The effect of CO₂ on the course of the reaction was studied by varying the CO₂ pressure from 0M Pa to 0.55 MPa. The results obtained are illustrated in figure 9. It is noted from these results that the conversion increases rapidly with the pressure of CO₂. On the other hand, and what is very interesting is the increase of the selectivity of Ph-PrOH to the detriment of Ph = O and Ph-OH (products resulting from deep oxidation reaction and cracking). This result can be explained by the action of the percarbonate entity (HCO₄⁻) which is responsible for oxidation by oxygen transfer. This result is very interesting because we observe an increase in conversion and selectivity at the same time and this is not the case in the majority of oxidation reactions.

Zoom In Zoom Out Reset image size