3.3.2 Isomerization Reactions


Similar to the isomerization of xylenes and also other aliphatic substrates, the isomerization the α-pinene and α-pinene oxide have the right to be catalyzed by zeolites, and various zeolites are at this time being to work (Scheme 3). Both processes are highly crucial for the fine chemicals industries, as all the isomerized assets are high-value substrates (Scheme 3) in the production of various fragrances, cosmetics, and also pharmaceuticals.

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Traditionally, TiO2 catalysts are used for the isomerization that α-pinene at atmospheric pressure and also a temperature of 200 °C, but really slow isomerization prices are a concern in this process (304). Among the miscellaneous isomerization products from α-pinene (as shown in system 3), camphene is the most preferable product. Camphene is the precursor substrate because that camphor, which has actually several industrial and other applications; there are explosives-related, culinary, and medicinal provides of this link (Scheme 3) (310). In order come achieve much better reactivity at a reasonable isomerization rate, along with higher selectivity, miscellaneous zeolites have been employed in this procedure over the years. Commonly boron-, titanium-, or vanadium-modified beta-zeolites provide greater camphene selectivity 보다 TiO2-based timeless catalytic equipment at 0 °C in the liquid-phase isomerization the α-pinene (304,311,312). Together modified zeolites (also ZSM-5, mordenite, MCM-41) v a Si/Al proportion of 27–33 room reported come exhibit greater catalytic task and better selectivity towards the desired commodities than TiO2-based traditional catalytic equipment for such isomerization reaction (304,311).

Zinc halides (ZnX2, X = Cl, Br) are taken into consideration to be the state-of-the-art catalysts because that the isomerization the α-pinene oxide (313). Industrially, campholenic aldehyde is the most essential product the the isomerization that α-pinene oxide (Scheme 3A) (310). The campholenic aldehyde is a well-known fine chemistry intermediate for the manufacturing of a terpenoid with a sandalwood-like smell, i m sorry is a vital component in a range of fragrances, perfumes, and also even pharmaceuticals (310,313). However, the zinc halide-catalyzed isomerization procedure also suffers from several innate limitations, including quick catalyst deactivation, short turnover number (implying high catalyst loading), and generation of large quantities of rubbish containing hefty metals (313). To overcome these drawbacks, zeolites are gradually getting popularity for the isomerization that α-pinene oxide. Among various zeolite catalysts because that the isomerization the α-pinene oxide, zeolite USY is famed for its high conversions and selectivity (up to 75%) toward campholenic aldehyde within 24 h in ~ 0 °C using toluene as solvent (310,314). The well-dispersed Lewis mountain sites are thought to be the crucial behind the success the this process. Various other zeolites and also mesoporous materials, such as zeolite Ti-beta, H-Y, and also Zn-MCM-41, are likewise reported to be reliable catalysts because that the isomerization of α-pinene oxide (313,315).

Another important course of reactions that deserve to be catalyzed by zeolites is a subcategory the isomerizations, namely carbohydrate isomerization and also epimerization (316–318). These species of change are key reactions in the health and also food industry and rely on enzymes as catalysts. Among these transformations, the isomerization the glucose right into fructose constitutes one of the biggest biocatalytic processes in the USA (316). This particular revolution is likewise becoming necessary in the field of biomass conversion, together it is one intermediate step from biomass to chemicals and also fuels. The Lewis-acidic large-pore Sn-beta zeolite to be proven to it is in a an extremely efficient catalyst for the isomerization the glucose to fructose, v high activity and selectivity as reported through Moliner et al. (316). The large-pore zeolites are crucial for this transformation, as medium-pore zeolites are completely ineffective. Also, an easy zeolites might degrade glucose. The solid and nondestructive interaction in between the tin and also the hydroxyl-/carbonyl-moieties of hexoses is presumably the reason for this catalyst's reactivity (Scheme 3B) (319). The an essential role of tin in this isomerization is noticeable from the negative performances of various other catalysts of comparable type, such as Ti-beta, Sn-MCM-41, and also TS-1 (316). Later, this methodology was prolonged to synthesize hydroxymethylfurfural (HMF) directly from glucose using a “one-pot” biphasic water/THF reactor system (Scheme 3B) (317). Moreover, Nikolla et al., attempting come synthesize HMF in the “one-pot” fashion, completed high counter of glucose by utilizing Sn-beta zeolite and an additional acid catalyst (HCl), in the above-mentioned biphasic reaction mixture (317). The authors proposed that tin atoms act as isolated, Lewis-acidic catalytic centers for the early stage isomerization of glucose to furustos (317). The exact role of Sn-beta in the final dehydration action is not yet clear. The robustness of Sn-beta zeolites in these processes opens up up a variety of possibilities in carbohydrate chemistry. Because that instance, Gunter et al.

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reported the Sn-beta is also an reliable catalyst for the selective epimerization of various aldoses (e.g., glucose, xylose, or arabinose) in the visibility of an additional inorganic borate salt in aqueous media (Scheme 3B) (318).