Alkene Epoxidation and Thioether Oxidation with Hydrogen Peroxide Catalyzed by Mesoporous Zirconium-Silicates

Abstract

Mesoporous zirconium-silicates have been prepared using two different methodologies, evaporation-induced self-assembly and solventless organometallic precursor dry impregnation of commercial SiO2. The samples were characterized by elemental analysis, XRD, N2 adsorption, TEM, DRS UV–vis and Raman spectroscopic techniques. The catalytic performance of the Zr-Si catalysts was assessed in the epoxidation of three representative alkenes, cyclohexene, cyclooctene and caryophyllene, as well as in the oxidation of methyl phenyl sulfide using aqueous hydrogen peroxide as a green oxidant, with special attention drawn to the structure/activity relationship and catalyst stability issues. The key factors which affect substrate conversion and epoxide selectivity have been defined. The catalysts with larger contents of oligomeric ZrO2 species revealed higher activity. The nature of alkene and, in particular, its molecular hindrance is crucial, since the adsorption of the epoxide product is the main factor leading to fast catalyst deactivation. In fact, bulky epoxides do not show this effect. After optimization, the oxidation of caryophyllene gave endocyclic monoepoxide with 77% selectivity at 87% alkene conversion. Methyl phenyl sulfoxide afforded 37% of sulfoxide and 63% of sulfone at 57% sulfide conversion. The nature of catalysis was truly heterogeneous and no Zr leaching was observed.

Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H2O2 Activation Pathways: Unusual Effect of Base

“…69 The addition of catalytic amounts of protons accelerated the reaction and enhanced selectivity toward heterolytic oxidation products. 69 A similar effect of protons was observed for alkene oxidation over some heterogeneous Zr catalysts, specifically zirconium-silicates 21 and Zr-MOFs. 24,25 What is more, acid additives could increase the oxidant utilization efficiency up to >99%.…”

“…The addition of catalytic amounts of protons accelerated the reaction and enhanced selectivity toward heterolytic oxidation products . A similar effect of protons was observed for alkene oxidation over some heterogeneous Zr catalysts, specifically zirconium-silicates and Zr-MOFs. , What is more, acid additives could increase the oxidant utilization efficiency up to >99% . Note that the results acquired for the Zr-substituted Lindqvist POM differed significantly from those reported for Zr-substituted Keggin POMs, which suggested (i) nucleophilic character of active Zr-peroxo species and (ii) that protonation favors H 2 O 2 decomposition and homolytic oxidation routes …”

“…5−10 The use of Zr catalysts for activation of H 2 O 2 has been documented in the literature; 11−21 however, they are scarcely employed in selective oxidations mainly because of their significant activity in H 2 O 2 unproductive degradation, which leads to predomination of homolytic oxidation pathways. 12,17,21,22 Zr-MCF prepared by impregnation of a mesostructured silica support with Zr(IV) isopropoxide catalyzed oxidation of α-pinene to a mixture of verbenol and verbenone 19 while Zr-based metal− organic frameworks (Zr-MOFs) favored oxidation of cyclohexene (CyH) to allylic oxidation products. 23−26 On the other hand, some Zr-based catalysts could catalyze CyH oxidation with predominant formation of the corresponding epoxide and/or diol.…”

“…Hydrogen peroxide is one of the most convenient and affordable oxidants, which has 47% atom efficiency and produces water as the only byproduct, thus satisfying most of the principles of green chemistry. − Since H 2 O 2 is inert toward the majority of organic substrates, its application in the oxidation reactions requires catalysts. − The use of Zr catalysts for activation of H 2 O 2 has been documented in the literature; − however, they are scarcely employed in selective oxidations mainly because of their significant activity in H 2 O 2 unproductive degradation, which leads to predomination of homolytic oxidation pathways. ,,, Zr-MCF prepared by impregnation of a mesostructured silica support with Zr­(IV) isopropoxide catalyzed oxidation of α-pinene to a mixture of verbenol and verbenone while Zr-based metal–organic frameworks (Zr-MOFs) favored oxidation of cyclohexene (CyH) to allylic oxidation products. − On the other hand, some Zr-based catalysts could catalyze CyH oxidation with predominant formation of the corresponding epoxide and/or diol. ,,, Thus far, the factors that govern the activity and selectivity of Zr catalysts in the activation of H 2 O 2 remain poorly understood.…”

The Control of the Coordination Chemistry for the Genesis of Heterogeneous Catalytically Active Sites in Oxidation Reactions**

“…The solventless impregnation approach was also applied for the preparation of mesoporous zirconium-silicate catalysts starting from zirconocene dichloride (Zr(Cp) 2 Cl 2 ). [132] In contrast to Zr-and Nb-based catalysts, zirconium counterparts are prone to rapid deactivation due to the adsorption of the epoxide product on the Zr sites. Nevertheless, when bulky electron-rich alkenes are used as substrates, high yields can be achieved (e.g.…”