Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO 2 ) irreversibly activate hydrogen peroxide (H 2 O 2 ) to form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η 2 -O 2 )) species for alkene epoxidation. The product distributions from reactions with Zstilbene, in combination with time-resolved UV−vis spectra of the reaction between H 2 O 2 -activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η 2 -O 2 ) moieties epoxidize substrates on the Nb-and Ta-containing materials. Kinetic measurements of styrene (C 8 H 8 ) epoxidation reveal that these materials first adsorb and then irreversibly activate H 2 O 2 to form pools of interconverting M-OOH and M-(η 2 -O 2 ) intermediates, which then react with styrene or H 2 O 2 to form either styrene oxide or H 2 O 2 decomposition products, respectively. Activation enthalpies (ΔH ⧧ ) for C 8 H 8 epoxidation and H 2 O 2 decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C 8 H 8 epoxidation. Values of ΔH ⧧ for C 8 H 8 epoxidation and H 2 O 2 decomposition also decrease linearly with the ligand-to-metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H 2 O 2 decomposition rates, which shows that more electrophilic M-OOH and M-(η 2 -O 2 ) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of ΔH ⧧ for C 8 H 8 epoxidation and adsorption enthalpies for C 8 H 8 within the pores of *BEA and SiO 2 reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C 8 H 8 epoxidation with respect to the 5.4 nm pores of M-SiO 2 , while H 2 O 2 decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H 2 O 2 . Thus, the differences in reactivity and selectivity between M-BEA and M-SiO 2 materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways.

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.

Reductive catalytic fractionation (RCF) is a promising approach to fractionate lignocellulose and convert lignin to a narrow product slate. To guide research towards commercialization, cost and sustainability must be considered....

Many industrially significant selective oxidation reactions are catalyzed by supported and bulk transition metal oxides. Catalysts for the synthesis of oxygenates, and especially for epoxidation, have predominantly focused on TiO x supported on or co-condensed with SiO2, whereas much of the rest of Groups 4 and 5 have been less studied. We have recently demonstrated through periodic trends using a uniform molecular precursor that niobium­(V)-silica catalysts reveal the highest activity and selectivity for efficient utilization of H2O2 for epoxidation across all of Groups 4 and 5. In this work, we graft a wide range of Nb­(V) precursors, spanning surface densities of 0.07–1.6 Nb groups nm–2 on mesoporous silica, and we characterize these materials with UV–visible spectroscopy and Nb K-edge XANES. Further, we apply in situ chemical titration with phenylphosphonic acid (PPA) in the epoxidation of cis-cyclooctene by H2O2 to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO2 catalysts. By this method, the fraction of kinetically relevant NbO x species ranges from ∼15% to ∼65%, which correlates with spectroscopic evaluation of the NbO x sites. This titration leads to a single value for the average turnover frequency, on a per active site basis rather than a per Nb atom basis, of 1.4 ± 0.52 min–1 across the 21 materials in the series. These quantitative maps of structural properties and kinetic consequences link key catalyst descriptors of supported Nb-SiO2 to enable rational design for next-generation oxidation catalysts.

Lignin solvolysis from the plant cell wall is the critical first step in lignin depolymerization processes involving whole biomass feedstocks. However, little is known about the coupled reaction kinetics and transport phenomena that govern the effective rates of lignin extraction. Here, we report a validated simulation framework that determines intrinsic, transport‐independent kinetic parameters for the solvolysis of lignin, hemicellulose, and cellulose upon incorporation of feedstock characteristics for the methanol‐based extraction of poplar as an example fractionation process. Lignin fragment diffusion is predicted to compete on the same time and length scales as reactions of lignin within cell walls and longitudinal pores of typical milled particle sizes, and mass transfer resistances are predicted to dominate the solvolysis of poplar particles that exceed approximately 2 mm in length. Beyond the approximately 2 mm threshold, effectiveness factors are predicted to be below 0.25, which implies that pore diffusion resistances may attenuate observable kinetic rate measurements by at least 75 % in such cases. Thus, researchers are recommended to conduct kinetic evaluations of lignin‐first catalysts using biomass particles smaller than approximately 0.2 mm in length to avoid feedstock‐specific mass transfer limitations in lignin conversion studies. Overall, this work highlights opportunities to improve lignin solvolysis by genetic engineering and provides actionable kinetic information to guide the design and scale‐up of emerging biorefinery strategies.