Samikannu Ajaikumar

Catalytic Upgrading of Extractives to Chemicals: Monoterpenes to "EXICALS".

ion. Thus, the Ru activity was significantly improved when small volumes of an initiator were added, and the recoverability of the catalyst was demonstrated. Makgwane et al. also studied the application of vanadium phosphate oxide catalysts, obtaining 26 wt % tert-ρ-cymene hydroperoxide (TCHP) in a 4-h on-stream experiment. The improved surface area of (VO)2P2O7 resulted in 85 wt % selectivity toward TCHP. The high selectivity was also explained by the slow catalytic decomposition of TCHP, especially at substrate conversions of 35 wt % or less. In contrast, the catalyst contributed significantly to the TCHP decomposition under an oxygen atmosphere. Vetrivel and Pandurangan studied the oxidation of ρ-cymene over mesoporous Siand Al-MCM-41 molecular sieves impregnated with manganese. The reaction yielded 38 wt % PMA. The catalysts were synthesized by a hydrothermal method, and XRD measurements showed the formation of well-ordered mesoporous structures. Mn-MCM-41 had the finest dispersion of nonframework manganese oxide particles, hence giving rise to the highest activity. 4-Isopropylbenzaldehyde, 1,2-epoxyisopropylbenzaldehyde, and 4-methylstyrene were the main byproducts identified. Previously, Martins et al. obtained 51 wt % 4-isopropylbenzoic acid catalyzed by Mn(III) porphyrin complexes. Their catalyst was an exceptionally active one, whic, h, even at room temperature was capable of converting up to 74 wt % ρcymene. ρ-Cymenol, ρ-isopropylbenzyl alcohol, thymoquinone, and cumic aldehyde were other reaction products observed. Much earlier, the method of direct oxidation of α-pinene to TPA involving V2O5 in a trickle-bed column was proposed at an elevated temperature of 370 °C and a large air flow of 600 L/h. Moreover, significant amounts of the catalyst were used: Only 9 mL of α-pinene passed through 43 cm of the catalytic bed per hour. Furthermore, extraction with benzene was required to purify the reaction products. The mentioned setup gave only a 20 wt % yield of TPA (Table 11). Nevertheless, this experiment was apparently the only reported successful synthesis of terephthalic acid over a heterogeneous catalyst. For the highest ρ-cymene conversion, Vetrivel and Pandurangan recommended a temperature range of 350−400 °C because of the intensive decomposition of the intermediate ρ-cymene hydroperoxides. They reported that significant coke formation was initiated at temperatures above 400 °C, causing a gradual decline in catalytic activity. However, Makgwane and co-workers managed to demonstrate the conversion of ρ-cymene at ca. 100 °C, with ρ-cymene hydroperoxides as the main reaction products. This observation supports the intermediate role of ρ-cymene hydroperoxides in the described reaction mechanism (Scheme 7). Similarly to the case of continuous flow-through operations, intensive coking was also reported at prolonged times on stream. The ρ-cymene hydroperoxide decomposition step could be the major source of coke. As is generally observed, ρ-cymene is stable in heterogeneously catalyzed oxidations (Figure 7). However, the low conversion of ρ-cymene is compensated by the substantial selectivity of the process and is further combined with low amounts and numbers of byproducts, in comparison to corresponding reactions of α-pinene or limonene substrates. A prolonged reaction time should decrease the selectivity while increasing the conversion of stable ρcymene. The initiation of the autoxidation reactions by activation of the CH bonds is hampered in the noncatalyzed ρ-cymene oxidation; hence, longer induction periods are required in the absence of a catalyst (Figure 7). For heterogeneously catalyzed ρ-cymene oxidation, both batch and continuous operating conditions could be recommended. The effectiveness of Co, Mn, Br, and V with bromic or acetic acids as oxidation promoters has been highlighted. Nair et al. noted a significant bromine influence in the catalytic oxidation of aromatic hydrocarbons. Nevertheless, the synthesis of terephthalic acid over heterogeneous catalysts remains a challenge.