Kinetics and Mechanism of Olefin Epoxidation with Aqueous H₂O₂ and a Highly Selective Surface-Modified TaSBA15 Heterogeneous Catalyst

The reaction kinetics of cyclohexene epoxidation using aqueous H2O2 oxidant and the highly selective epoxidation catalyst BucapTaSBA15 were studied. The reaction was determined to be first-order in Ta(V) surface coverage. The reaction rate exhibited saturation with respect to increasing concentrations of cyclohexene and H2O2. An Eley−Rideal mechanism and rate equation may be used to describe the epoxidation kinetics, which are similar to those for Ti(IV)SiO2-catalyzed epoxidations. The observed kinetics may also be modeled by a double-displacement mechanism typically associated with saturation enzyme catalysts. In addition, 1H NMR spectroscopy was employed to investigate H2O2 decomposition by BucapTaSBA15 and the unmodified TaSBA15 catalysts. Little decomposition occurred over the surface-modified material, but the unmodified material catalyzed a 30% conversion of H2O2 after 6 h. UV−visible absorbance and diffuse reflectance UV−visible (DRUV−vis) spectroscopy were used to investigate the structure of the Ta centers on the TaSBA15 catalysts. DRUV−vis spectroscopy was also used to identify a Ta(V)-based epoxidation intermediate, proposed to be a Ta(V)(η2-O2) species, which forms upon reaction of the TaSBA15 and BucapTaSBA15 materials with H2O2

Structure–Function Relationships for Electrocatalytic Water Oxidation by Molecular [Mn₁₂O₁₂] Clusters

A series of Mn₁₂O₁₂(OAc)_{16–<i>x</i>}L_<i>x</i>(H₂O)₄ molecular clusters (L = acetate, benzoate, benzenesulfonate, diphenylphosphonate, dichloroacetate) were electrocatalytically investigated as water oxidation electrocatalysts on a fluorine-doped tin oxide glass electrode. Four of the [Mn₁₂O₁₂] compounds demonstrated water oxidation activity at pH 7.0 at varying overpotentials (640–820 mV at 0.2 mA/cm²) and with high Faradaic efficiency (85–93%). For the most active complex, more than 200 turnovers were observed after 5 min. Two structure–function relationships for these complexes were developed. First, these complexes must undergo at least one-electron oxidation to become active catalysts, and complexes that cannot be oxidized in this potential window were inactive. Second, a greater degree of distortion at Mn1 and Mn3 centers correlated with higher catalytic activity. From this distortion analysis, either or both of these two Mn centers are proposed to be the catalytically active site

Simultaneous Dehydration of Glucose and Xylose Present in a Process-Relevant Biorefinery Hydrolysate to Furfurals Using Heterogeneous Solid Acid Catalysts

In this work, we report the simultaneous dehydration of glucose and xylose present in a process-relevant biorefinery hydrolysate to furfural and 5-hydroxymethylfurfural (HMF) using heterogeneous solid acid catalysts in a microwave reactor. Initially, several solid acid catalysts with varied Brønsted and Lewis acidity were screened to evaluate their activity and selectivity in dehydration of pure glucose to HMF. A noticeable improvement in HMF yield from dehydration of 8 wt % glucose was obtained by combining an acidic ion-exchange resin (Purolite CT-275DR) with an amorphous silica-alumina catalyst (Davicat-3115) resulting in HMF yields of 27–33% using a homogeneous solvent system of aqueous dioxane (dioxane/water, 2:1 v/v) at 195 °C in 5 min. Under the same reaction conditions, catalysts, and solvent system but with the addition of NaCl in catalytic amounts (33–100 mM), a more than 2-fold increase in HMF yields (66–70%) was achieved for the dehydration of 8 wt % glucose, whereas furfural yields approaching 95% were achieved for the dehydration of 6 wt % xylose, when conducted separately. Notably, using the same catalyst and solvent system while slightly modifying the reaction conditions to 197 °C and 5 min, simultaneous dehydration of 4 wt % xylose and 9 wt % glucose present in a process-relevant corn stover hydrolysate resulted in furfural and HMF yields of 96 and 74%, respectively, resulting in a combined furfural yield of 80%. The results further showed that the pH of the reaction solution played an important role in maximizing product yields. A pH < 2 resulted in low HMF yields due to the increased formation of HMF degradation products, whereas a pH 2–3 gave high HMF yields by possibly stabilizing the reaction intermediates and product, suppressing the occurrence of side reactions

Size and Bandgap Control in the Solution-Phase Synthesis of Near-Infrared-Emitting Germanium Nanocrystals

We present a novel colloidal synthesis of alkyl-terminated Ge nanocrystals based on the reduction of GeI4/GeI2 mixtures. The size of the nanocrystals (2.3−11.3 nm) was controlled by adjusting both the Ge(IV)/Ge(II) ratio and the temperature ramp rate following reductant injection. The near-infrared absorption (1.6−0.70 eV) and corresponding band-edge emission demonstrate the highly tunable quantum confinement effects in Ge nanocrystals prepared using this mixed-valence precursor method. A mechanism is proposed for the observed size control, which relies upon the difference in reduction temperatures for Ge(II) versus Ge(IV)

The Overlooked Potential of Sulfated Zirconia: Reexamining Solid Superacidity Toward the Controlled Depolymerization of Polyolefins

Closed-loop recycling via an efficient chemical process can help alleviate the global plastic waste crisis. However, conventional depolymerization methods for polyolefins, which compose more than 50% of plastics, demand high temperatures and pressures, employ precious noble metals, and/or yield complex mixtures of products limited to single-use fuels or oils. Superacidic forms of sulfated zirconia (SZrO) with Hammet Acidity Functions (H0) ≤ – 12 (i.e., stronger than 100% H2SO4) are industrially deployed heterogeneous catalysts capable of activating hydrocarbons under mild conditions and are shown to decompose polyolefins at temperatures near 200 °C and ambient pressure. Additionally, confinement of active sites in porous supports is known to radically increase selectivity, coking and sintering resistance, and acid site activity, presenting a possible approach to low-energy polyolefin depolymerization. However, a critical examination of the literature on SZrO led us to a surprising conclusion: despite 40 years of catalytic study, engineering, and industrial use, the surface chemistry of SZrO is poorly understood. Ostensibly spurred by SZrO’s impressive catalytic activity, the application-driven study of SZrO has resulted in deleterious ambiguity in requisite synthetic conditions for superacidity and insufficient characterization of acidity, porosity, and active site structure. This ambiguity has produced significant knowledge gaps surrounding the synthesis, structure, and mechanisms of hydrocarbon activation for optimized SZrO, stunting the potential of this catalyst in olefin cracking and other industrially relevant reactions, such as isomerization, esterification, and alkylation. Toward mitigating these long extant issues, we herein identify and highlight these current shortcomings and knowledge gaps, propose explicit guidelines for characterization of and reporting on characterization of solid acidity, and discuss the potential of pore-confined superacids in the efficient and selective depolymerization of polyolefins

Surface Chemistry Exchange of Alloyed Germanium Nanocrystals: A Pathway Toward Conductive Group IV Nanocrystal Films

We present an expansion of the mixed-valence iodide reduction method for the synthesis of Ge nanocrystals (NCs) to incorporate low levels (∼1 mol %) of groups III, IV, and V elements to yield main-group element-alloyed Ge NCs (Ge_1–<i>x</i>E_<i>x</i> NCs). Nearly every main-group element (E) that surrounds Ge on the periodic table (Al, P, Ga, As, In, Sn, and Sb) may be incorporated into Ge_1–<i>x</i>E_<i>x</i> NCs with remarkably high E incorporation into the product (>45% of E added to the reaction). Importantly, surface chemistry modification via ligand exchange allowed conductive films of Ge_1–<i>x</i>E_<i>x</i> NCs to be prepared, which exhibit conductivities over large distances (25 μm) relevant to optoelectronic device development of group IV NC thin films

Direct Conversion of Renewable CO₂‑Rich Syngas to High-Octane Hydrocarbons in a Single Reactor

The synthesis of branched hydrocarbons for high-octane gasoline and sustainable aviation fuel directly from CO2-rich syngas in a single reactor holds potential to decrease capital and operating costs and increase overall energy and carbon efficiencies in a biorefinery. Here, we report the cascade chemistry of syngas to hydrocarbons under mild reaction conditions in a single reactor with C4+ single-pass yields of 13.7–44.9%, depending on the relative catalyst composition employing our dimethyl ether homologation catalyst, Cu/BEA zeolite. With co-fed CO2 at a concentration representative of biomass-derived syngas, 2.5:1:0.9 for H2:CO:CO2, a hydrocarbon yield of 12.2% was observed with similar selectivity to C4+ products compared to the CO2-free feed. Definitive evidence of CO2 incorporation into the hydrocarbon products was demonstrated with isotopically labeled 13CO2 co-feed experiments, where mass spectrometry confirmed the propagation of 13C into the C4+ hydrocarbons, highlighting the feasibility to co-convert CO and CO2 in this single reactor approach

Activating Molybdenum Carbide Nanoparticle Catalysts under Mild Conditions Using Thermally Labile Ligands

Transition-metal carbides are promising low-cost materials for various catalytic transformations due to their multifunctionality and noble-metal-like behavior. Nanostructuring transition-metal carbides offers advantages resulting from the large surface-area-to-volume ratios inherent in colloidal nanoparticle catalysts; however, a barrier for their utilization is removal of the long-chain aliphatic ligands on their surface to access active sites. Annealing procedures to remove these ligands require temperatures greater than the catalyst synthesis and catalytic reaction temperatures and may further result in coking or particle sintering that can reduce catalytic performance. One way to circumvent this problem is by replacing the long-chain aliphatic ligands with smaller ligands that can be easily removed through low-temperature thermolytic decomposition. Here, we present the exchange of native oleylamine ligands on colloidal α-MoC1–x nanoparticles for thermally labile tert-butylamine ligands. Analyses of the ligand exchange reaction by solution 1H NMR spectroscopy, FT-IR spectroscopy, and thermogravimetric analysis–mass spectrometry (TGA-MS) confirm the displacement of 60% of the native oleylamine ligands for the thermally labile tert-butylamine, which can be removed with a mild activation step at 250 °C. Catalytic site densities were determined by carbon monoxide (CO) chemisorption, demonstrating that the mild thermal treatment at 250 °C activates ca. 25% of the total binding sites, while the native oleylamine-terminated MoC1–x nanoparticles showed no available surface binding sites after this low-temperature treatment. The mild pretreatment at 250 °C also shows distinctly different initial activities and postinduction period selectivities in the CO2 hydrogenation reaction for the ligand exchanged MoC1–x nanoparticle catalysts and the as-prepared material

Femtosecond Measurements Of Size-Dependent Spin Crossover In Fe^II(pyz)Pt(CN)₄ Nanocrystals

We report a femtosecond time-resolved spectroscopic study of size-dependent dynamics in nanocrystals (NCs) of Fe­(pyz)­Pt­(CN)₄. We observe that smaller NCs (123 or 78 nm cross section and <25 nm thickness) exhibit signatures of spin crossover (SCO) with time constants of ∼5–10 ps whereas larger NCs with 375 nm cross section and 43 nm thickness exhibit a weaker SCO signature accompanied by strong spectral shifting on a ∼20 ps time scale. For the small NCs, the fast dynamics appear to result from thermal promotion of residual low-spin states to high-spin states following nonradiative decay, and the size dependence is postulated to arise from differing high-spin vs low-spin fractions in domains residing in strained surface regions. The SCO is less efficient in larger NCs owing to their larger size and hence lower residual LS/HS fractions. Our results suggest that size-dependent dynamics can be controlled by tuning surface energy in NCs with dimensions below ∼25 nm for use in energy harvesting, spin switching, and other applications

Site-Isolated Pt-SBA15 Materials from Tris(<i>tert</i>-butoxy)siloxy Complexes of Pt(II) and Pt(IV)

Two novel tris(tert-butoxy)siloxy complexes of Pt(II) and Pt(IV) were prepared in high yields, (cod)Pt[OSi(OtBu)3]2 (1; 87%; cod = 1,5-cyclooctadiene) and Me3Pt(tmeda)[OSi(OtBu)3] (2; 81%; tmeda = N,N,N′,N′-tetramethylethylenediamine). The structures of these compounds were determined by multinuclear NMR spectroscopy and by single-crystal X-ray analysis. The thermolytic chemistry of 1 and 2 in the solid state was studied by thermogravimetric analysis. The thermal decomposition of these complexes resulted in the formation of Pt metal, with the elimination of HOSi(OtBu)3. Precursors 1 and 2 react with the surface Si−OH groups of mesoporous SBA15 silica to generate surface-supported Pt centers. The coordination environments of the supported Pt centers in these new materials, termed Pt(II)SBA15 and Pt(IV)SBA15, were investigated using Fourier-transform infrared spectroscopy, X-ray absorption near-edge spectroscopy, and extended X-ray absorption fine structure analysis. These materials were also characterized using N2 porosimetry, powder X-ray diffraction and transmission electron microscopy. Comparisons with the molecular precursors 1 and 2 revealed many similarities, and the results are indicative of isolated Pt(II) and Pt(IV) centers. In addition, isolated Pt centers proved to be robust in inert atmosphere to 150−200 °C, which is similar to the decomposition temperatures of 1 and 2