Periodic Trends in Highly Dispersed Groups IV and V Supported Metal Oxide Catalysts for Alkene Epoxidation with H₂O₂

Abstract

Supported metal oxides are important catalysts for selective oxidation processes, such as alkene epoxidation with H 2 O 2 . The reactivity of these catalysts is dependent on both identity and oxide structure. The dependence of the latter on synthesis method can confound attempts at comparative studies across the periodic table. Here, SiO 2 -supported metal oxide catalysts of Ti(IV), Zr(IV), Hf(IV), V(V), Nb(V) and Ta(V) (all of Groups IV and V) were synthesized by grafting a series of related calixarene coordination complexes at surface densities less than ~0.25 nm -2 . Select catalysts were investigated by solid state NMR, UV-visible, and Xray absorption near edge spectroscopies. As-synthesized and calcined materials were examined for the epoxidation of cyclohexene and styrene (1.0 M) with H 2 O 2 (0.10 M) at 45°C and 65°C.Nb catalysts emerge as high-performing materials, with calcined Nb-SiO 2 proceeding at a cyclohexene turnover frequency of 2.4 min -1 (>2x faster than Ti-SiO 2 ) and with ~85% selectivity towards direct (non-radical) epoxidation pathways. As-synthesized Zr, Hf and Ta catalysts have improved direct pathway selectivities compared to their calcined versions, particularly evident for Ta-SiO 2 . Finally, when the materials are synthesized from these precursors, but not simple metal chlorides, the direct pathway reaction rate correlates with Pauling electronegativities of the metals, demonstrating clear periodic trends in intrinsic Lewis acid catalytic behavior.

Kinetic and spectroscopic evidence for reaction pathways and intermediates for olefin epoxidation on Nb in *BEA

“…Optical band gaps (E g ) were calculated from the x-intercept of the linear portion of a Tauc plot ( Fig. S3) [20,41]. Table 1 summarizes the optical band edges of Nb-β, Nb 10 -SiO 2 , bulk Nb 2 O 5 9 [42], and calixarene-assisted Nb grafted onto silica (Calix-Nb 1.7 -SiO 2 ) [20].…”

“…Isolated metal centers and small oligomers of early transition metal oxides (e.g., Al III [14,15], Ti IV [7,[16][17][18], Zr IV [19,20], Ta V [21][22][23], and Nb V [24,25]) catalyze the epoxidation of olefins with H 2 O 2 . Isolated Nb atoms grafted onto silica give greater rates and selectivities for olefin epoxidation compared to isolated Ti centers as well those of other group IV and V metals (i.e., Zr, Hf, V, and Ta) on silica, which was attributed to differences between the Lewis acidity (estimated from the ionic character of the M-O bond) of the different metal atoms [20]. Zeolite frameworks can stabilize relatively high loadings of isolated transition metal atoms [26][27][28][29][30], which in some cases, are believed to possess higher turnover rates ((mol product)(mol active site·s) -1 ) and selectivities than their oligomeric-and bulk-oxide counterparts.…”

Ionic Liquid Stabilized Niobium Oxoclusters Catalyzing Oxidation of Sulfides with Exceptional Activity

“…70 % conversion (entries 5–7). Only tiny amounts of cyclohexenol or cyclohexenone were detected in the epoxidation of cyclohexene (entry 6), which meant that the reaction did not proceed by a single‐electron transfer mechanism . Some byproducts were detected in styrene epoxidation mainly as benzaldehyde (entry 7).…”

“…Only tiny amounts of cyclohexenol or cyclohexenone wered etected in the epoxidation of cyclohexene (entry 6), which meantt hat the reaction did not proceedb yasingle-electron transfer mechanism. [31] Some byproducts were detected in styrene epoxidation mainly as benzaldehyde (entry 7). Furthermore,t he epoxidation of 1-octene, and internal olefins also produced the corresponding epoxide very selectively and the configurations around the C=Cm oiety were retainedi nt he corresponding epoxides, although the conversion was slightly lower likely owningt os teric hindrance (entries 8and 9).…”

Sn-based catalysts for Baeyer-Villiger oxidations by using hydrogen peroxide as oxidant

“…These materials have been used in the isomerization of alkanes and other petrochemical processes such as alkylation of aromatics and hydrocarbon cracking [105]. Monometallic oxides such as MgO [106], SnO2 [107], WO3 [108], ZrO2 [109], TiO2 [110], Al2O3 [111], Nb2O5 [112], Ta2O5 [113], and bimetallic oxides such as Nb-Ta oxides [114,115], Mg-Ta oxides [116], Nb-W oxides [117], Sn-W oxides [118], Zr-W oxides [119], and WO3-TiO2 [120,121] have been extensively explored.…”