Rhodium(0) Nanoparticles Supported on Nanocrystalline Hydroxyapatite: Highly Effective Catalytic System for the Solvent-Free Hydrogenation of Aromatics at Room Temperature

The hydrogenation of aromatics under mild conditions remains a challenge in the fields of synthetic and petroleum chemistry. Described herein is a new catalytic material that shows excellent catalytic performance in terms of activity, selectivity, and reusability in the hydrogenation of aromatics in solvent-free systems under mild conditions. The catalyst, consisting of rhodium nanoparticles supported on nanocrystalline hydroxyapatite, can quantitatively hydrogenate neat benzene to cyclohexane with exceptionally high rates (initial TOF > 10³ h^–1) at 298 K and 3 bars of initial H₂ pressure. This new material maintains its inherent catalytic activity after several reuses. Importantly, catalyst preparation does not require elaborate procedures because the active metal nanoparticles are readily formed from the in situ reduction of Rh³⁺-exchanged hydroxyapatite while submerged in the aromatic solvent at room temperature under 3 bars of H₂ pressure

Heterogeneous Epoxide Carbonylation by Cooperative Ion-Pair Catalysis in Co(CO)₄^–‑Incorporated Cr-MIL-101

Despite the commercial desirability of epoxide carbonylation to β-lactones, the reliance of this process on homogeneous catalysts makes its industrial application challenging. Here we report the preparation and use of a Co­(CO)₄^–-incorporated Cr-MIL-101 (Co­(CO)₄⊂Cr-MIL-101, Cr-MIL-101 = Cr₃O­(BDC)₃F, H₂BDC = 1,4-benzenedicarboxylic acid) heterogeneous catalyst for the ring-expansion carbonylation of epoxides, whose activity, selectivity, and substrate scope are on par with those of the reported homogeneous catalysts. We ascribe the observed performance to the unique cooperativity between the postsynthetically introduced Co­(CO)₄^– and the site-isolated Lewis acidic Cr­(III) centers in the metal–organic framework (MOF). The heterogeneous nature of Co­(CO)₄⊂Cr-MIL-101 allows the first demonstration of gas-phase continuous-flow production of β-lactones from epoxides, attesting to the potential applicability of the heterogeneous epoxide carbonylation strategy

Solvent Polarity and Framework Hydrophobicity of Hf-BEA Zeolites Influence Aldol Addition Rates in Organic Media

Solvent identity and pore polarity are known to influence Lewis acidic catalysis in zeolite pores for a variety of liquid-phase chemistries. We investigated how these parameters alter the rates of self-aldol addition of ethyl pyruvate (EP), a model biomass-derived compound, over hydrophobic and hydrophilic Hf-BEA zeolites in both toluene and acetonitrile solvents. Aldol addition rates are of first order across the entire EP activity range (0.02–0.4) for all four systems, consistent with the nucleophilic attack by the enolate as the rate-determining step and a single adsorbed EP as the most abundant reactive intermediate. Apparent first-order rate constants span 2 orders of magnitude across the four systems; at 363 K, the highest rates were observed over hydrophobic Hf-BEA-F in toluene (kapp = 0.36 (mmol) (mmol closed Hf)−1 (s)−1), while the lowest rates were observed in hydrophilic Hf-BEA-OH in an acetonitrile solvent (kapp = 0.0026 (mmol) (mmol closed Hf)−1 (s)−1). Apparent reaction enthalpies and entropies for each system, estimated using non-ideal transition-state theory, revealed that despite the substantial rate constant variation across the four systems, apparent enthalpies for Hf-BEA-F in both solvents and Hf-BEA-OH in acetonitrile were within the error of each other (∼70 kJ mol–1). Reactions performed using Hf-BEA-OH with toluene featured a higher apparent enthalpic barrier of 83.8 kJ mol–1. The differences between the systems are attributed to hydrogen-bonding interactions between the EP molecules and polar silanol nests during catalysis in toluene using Hf-BEA-OH, which hinder EP adsorption to the active site in the hydrophilic framework. These hydrogen-bonding interactions are not present when acetonitrile is used as the solvent, as acetonitrile itself binds to and blocks silanol groups. Equilibrium EP absorption measurements indicate that while both toluene and acetonitrile are present in pores during catalysis, neither solvent forms a tight solvation shell around EP in the pores that must be disrupted prior to EP adsorption. These findings show that aldol addition kinetics are not significantly modified by solvent polarity in hydrophobic frameworks beyond site-blocking effects; however, silanol nests in hydrophilic frameworks significantly alter substrate adsorption to the active site

Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole

Vapor-phase hydrodeoxygenation (HDO) of anisole was investigated at 593 K and H₂ pressures of ≤1 bar over supported MoO₃/ZrO₂ catalysts with MoO₃ loadings ranging from 1 to 36 wt % (i.e., 0.5–23.8 Mo/nm²). Reactivity studies showed that HDO activity increased proportionally with MoO₃ coverage up to a monolayer coverage (∼15 wt %) over the ZrO₂ surface. Specific rates declined for catalysts with high loadings exceeding the monolayer coverage, because of a decreasing amount of redox-active species, as confirmed by oxygen chemisorption experiments. For low catalyst loadings (1 and 5 wt %), the selectivities toward fully deoxygenated aromatics were 13 and 24% on a C-mol basis, respectively, while at intermediate and high loadings (10–36 wt %), the selectivity was ∼40%. Post-reaction characterization of the spent catalysts using X-ray diffraction and X-ray photoelectron spectroscopy showed that the catalysts with 25 and 36 wt % MoO₃ loadings were over-reduced, as evidenced by the prevalence of Mo⁴⁺ and Mo³⁺ oxidation states summing to 54 and 67%, respectively. In contrast, catalysts with low and intermediate Mo loadings exhibited a prevalence of Mo⁶⁺ species (∼60%). We hypothesize that Mo⁵⁺ species are more easily stabilized in oligomeric and isolated forms over the zirconia support. The catalysts with intermediate loadings feature HDO and alkylation rates higher than those of catalysts with low loadings because the latter feature a higher proportion of isolated species. Once the monolayer coverage is exceeded, MoO₃ crystallites are formed, which can undergo facile reduction to less reactive MoO₂

Structural Properties and Reactivity Trends of Molybdenum Oxide Catalysts Supported on Zirconia for the Hydrodeoxygenation of Anisole

Vapor-phase hydrodeoxygenation (HDO) of anisole was investigated at 593 K and H₂ pressures of ≤1 bar over supported MoO₃/ZrO₂ catalysts with MoO₃ loadings ranging from 1 to 36 wt % (i.e., 0.5–23.8 Mo/nm²). Reactivity studies showed that HDO activity increased proportionally with MoO₃ coverage up to a monolayer coverage (∼15 wt %) over the ZrO₂ surface. Specific rates declined for catalysts with high loadings exceeding the monolayer coverage, because of a decreasing amount of redox-active species, as confirmed by oxygen chemisorption experiments. For low catalyst loadings (1 and 5 wt %), the selectivities toward fully deoxygenated aromatics were 13 and 24% on a C-mol basis, respectively, while at intermediate and high loadings (10–36 wt %), the selectivity was ∼40%. Post-reaction characterization of the spent catalysts using X-ray diffraction and X-ray photoelectron spectroscopy showed that the catalysts with 25 and 36 wt % MoO₃ loadings were over-reduced, as evidenced by the prevalence of Mo⁴⁺ and Mo³⁺ oxidation states summing to 54 and 67%, respectively. In contrast, catalysts with low and intermediate Mo loadings exhibited a prevalence of Mo⁶⁺ species (∼60%). We hypothesize that Mo⁵⁺ species are more easily stabilized in oligomeric and isolated forms over the zirconia support. The catalysts with intermediate loadings feature HDO and alkylation rates higher than those of catalysts with low loadings because the latter feature a higher proportion of isolated species. Once the monolayer coverage is exceeded, MoO₃ crystallites are formed, which can undergo facile reduction to less reactive MoO₂

SSZ-13 Crystallization by Particle Attachment and Deterministic Pathways to Crystal Size Control

Many synthetic and natural crystalline materials are either known or postulated to grow via nonclassical pathways involving the initial self-assembly of precursors that serve as putative growth units for crystallization. Elucidating the pathway(s) by which precursors attach to crystal surfaces and structurally rearrange (postattachment) to incorporate into the underlying crystalline lattice is an active and expanding area of research comprising many unanswered fundamental questions. Here, we examine the crystallization of SSZ-13, which is an aluminosilicate zeolite that possesses exceptional physicochemical properties for applications in separations and catalysis (e.g., methanol upgrading to chemicals and the environmental remediation of NO_<i>x</i>). We show that SSZ-13 grows by two concerted mechanisms: nonclassical growth involving the attachment of amorphous aluminosilicate particles to crystal surfaces and classical layer-by-layer growth via the incorporation of molecules to advancing steps on the crystal surface. A facile, commercially viable method of tailoring SSZ-13 crystal size and morphology is introduced wherein growth modifiers are used to mediate precursor aggregation and attachment to crystal surfaces. We demonstrate that small quantities of polymers can be used to tune crystal size over 3 orders of magnitude (0.1–20 μm), alter crystal shape, and introduce mesoporosity. Given the ubiquitous presence of amorphous precursors in a wide variety of microporous crystals, insight of the SSZ-13 growth mechanism may prove to be broadly applicable to other materials. Moreover, the ability to selectively tailor the physical properties of SSZ-13 crystals through molecular design offers new routes to optimize their performance in a wide range of commercial applications

Computational Investigation on Hydrodeoxygenation (HDO) of Acetone to Propylene on α‑MoO₃ (010) Surface

Density functional theory (DFT) calculations were performed on the multistep hydrodeoxygenation (HDO) of acetone (CH₃COCH₃) to propylene (CH₃CHCH₂) on a molybdenum oxide (α-MoO₃) catalyst following an oxygen vacancy-driven pathway. First, a perfect O-terminated α-MoO₃ (010) surface based on a 4 × 2 × 4 supercell is reduced by molecular hydrogen (H₂) to generate a terminal oxygen (O_t) defect site. This process occurs via a dissociative chemisorption of H₂ on adjacent surface oxygen atoms, followed by an H transfer to form a water molecule (H₂O). Next, adsorption of CH₃COCH₃ on the oxygen-deficient Mo site forms an O–Mo bond and then the chemisorbed CH₃COCH₃ forms CH₃COCH₂ by transfer of an H atom to an adjacent O_t site. The surface bound hydroxyl (OH) then transfers the H atom to the immobilized O atom to form surface-bound enol, CH₃CHOCH₂. The next step releases CH₃CHCH₂ into the gas phase, while simultaneously oxidizes the surface back to a perfect O-terminated α-MoO₃ (010) surface. The adsorption of H₂, and the formation of a terminal oxygen (O_t) vacancy, moves the conduction band minimum (CBM) from 1.2 eV to 0 and 0.3 eV, respectively. Climbing image-nudged elastic band (CI-NEB) calculations using a Perdew–Burke–Ernzerhof (PBE) functional in combination with double-ζ valence (DZV) basis sets indicate that the dissociative adsorption of H₂ is the rate-limiting step for the catalytic cycle with a barrier of 1.70 eV. Furthermore, the lower barrier for surface-mediated H transfer from primary-to-secondary carbon atom (0.63 eV) compared to that of a concerted direct H transfer to the secondary C atom with simultaneous desorption (2.02 eV) emphasizes the key role played by the surface in H transfer for effective deoxygenation

Cascade Reactions for the Continuous and Selective Production of Isobutene from Bioderived Acetic Acid Over Zinc-Zirconia Catalysts

Bio-oil (obtained from biomass fast pyrolysis) contains a high concentration of acetic acid, which causes problems related to its storage and handling. Acetic acid was upgraded directly to isobutene over a Zn_<i>x</i>Zr_<i>y</i>O_<i>z</i> binary metal oxide. The reaction proceeds via a three-step cascade involving ketonization, aldol condensation, and C–C hydrolytic bond cleavage reactions, which was corroborated by isotopic labeling studies. Separately, ZnO and ZrO₂ are incapable of producing isobutene from either acetic acid or acetone. In contrast, under optimal conditions, a Zn₂Zr₈O_<i>z</i> catalyst generates a ca. 50% isobutene yield, which corresponds to 75% of the theoretical maximum. Spectroscopic investigations revealed that a balanced concentration of acid and base sites is required to maximize isobutene yields

Alloying Tungsten Carbide Nanoparticles with Tantalum: Impact on Electrochemical Oxidation Resistance and Hydrogen Evolution Activity

Metal-terminated bimetallic carbide nanoparticles (NPs) of tungsten and tantalum are synthesized in a monodisperse particle size distribution of 2–3 nm. The bimetallic particles feature enhanced electrocatalytic behavior with respect to the monometallic composition. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements indicate that the Ta_0.3W_0.7C NPs consist of a well-mixed random alloy featuring a compressed lattice that favorably impacts stability and catalytic activity. Electrochemical testing shows that the incorporation of 30% tantalum into the tungsten carbide lattice increases the electrochemical oxidation resistance of the NPs. The onset of surface passivation in 0.5 M H₂SO₄ shifted from +0.2 V vs RHE to +0.45 V vs RHE, and the maximum surface oxidation current shifted from +0.4 to +0.75 V vs RHE. The activity toward hydrogen evolution (HER) of the carbon-supported Ta_0.3W_0.7C NPs is preserved relative to the activity of unmodified carbon-supported WC NPs. The increase in electrochemical oxidation resistance is attributed to the presence of surface Ta moieties as determined by X-ray photoelectron spectroscopy (XPS) while the preservation of the HER activity is attributed to the observed lattice compression

Natural Gas and Cellulosic Biomass: A Clean Fuel Combination? Determining the Natural Gas Blending Wall in Biofuel Production

Natural gas has the potential to increase the biofuel production output by combining gas- and biomass-to-liquids (GBTL) processes followed by naphtha and diesel fuel synthesis via Fischer–Tropsch (FT). This study reflects on the use of commercial-ready configurations of GBTL technologies and the environmental impact of enhancing biofuels with natural gas. The autothermal and steam-methane reforming processes for natural gas conversion and the gasification of biomass for FT fuel synthesis are modeled to estimate system well-to-wheel emissions and compare them to limits established by U.S. renewable fuel mandates. We show that natural gas can enhance FT biofuel production by reducing the need for water–gas shift (WGS) of biomass-derived syngas to achieve appropriate H₂/CO ratios. Specifically, fuel yields are increased from less than 60 gallons per ton to over 100 gallons per ton with increasing natural gas input. However, GBTL facilities would need to limit natural gas use to less than 19.1% on a LHV energy basis (7.83 wt %) to avoid exceeding the emissions limits established by the Renewable Fuels Standard (RFS2) for clean, advanced biofuels. This effectively constitutes a <i>blending</i> limit that constrains the use of natural gas for enhancing the biomass-to-liquids (BTL) process