Ammonia Titration Methods To Quantify Brønsted Acid Sites in Zeolites Substituted with Aluminum and Boron Heteroatoms

Ammonia titration methods were developed to discriminate and quantify Brønsted acid sites of different strength that compensate aluminum and boron heteroatoms incorporated within zeolite frameworks. Borosilicate and boroaluminosilicate MFI zeolites (B-Al-MFI) were synthesized with different Al contents and crystallite sizes, which are typically correlated structural properties in aluminosilicates synthesized hydrothermally, but independently varied here by incorporating boron as a second framework heteroatom and using ethylenediamine as a structure directing agent. Temperature-programmed desorption (TPD) of ammonia from B-Al-MFI samples saturated via liquid-phase NH₄NO₃ ion exchange resulted in quantifying the total number of Al and B heteroatoms. In contrast, TPD performed after NH₄-form B-Al-MFI samples were purged in flowing helium (433 K), or after gas-phase NH₃ adsorption (433 K) onto H-form B-Al-MFI samples, quantified only protons charge-compensating framework Al heteroatoms. Turnover rates for methanol dehydration to dimethyl ether, when measured in zero-order kinetic regimes that are sensitive predominantly to Brønsted acid strength, are dependent only on the number of protons compensating framework Al atoms in B-Al-MFI zeolites. The NH₃ titration methods developed here are useful in rigorously normalizing turnover rates of Brønsted acid-catalyzed reactions in boroaluminosilicate zeolites, which have been recognized previously to be dependent solely on Al content. The incorporation of B heteroatoms into zeolite frameworks, which generate protons that are essentially unreactive, provides a strategy to influence crystallite sizes independently of Al content, especially relevant in cases where catalytic behavior is influenced by intracrystalline transport phenomena

Structure Determination of a Surface Tetragonal Pt₁Sb₁ Phase on Pt Nanoparticles

Structure Determination of a Surface Tetragonal Pt₁Sb₁ Phase on Pt Nanoparticle

Correlating Heat of Adsorption of CO to Reaction Selectivity: Geometric Effects vs Electronic Effects in Neopentane Isomerization over Pt and Pd Catalysts

Silica-supported Pt and Pd nanoparticles from 1 to 10 nm in diameter were evaluated for neopentane conversion (hydrogenolysis and isomerization). Characterization of the catalysts was conducted utilizing scanning transmission electron microscopy (STEM), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, X-ray absorption spectroscopy (XAS), and isothermal calorimetry of CO adsorption to determine how geometric or electronic structure effects can explain changes in reactivity. Isomerization selectivity of Pt was much higher than Pd for all particle sizes. There is a pronounced effect of particle size on selectivity, with the highest isomerization selectivity achieved over catalysts containing the largest particle size for both Pt (57%) and Pd (26%) catalysts. For both Pd and Pt catalysts, DRIFTS showed a decrease in the ratio of linear-to-bridge bonded CO with particle size, while isothermal calorimetry of CO adsorption shows that both Pt and Pd enthalpies of adsorption decrease with increasing particle size. The isomerization selectivity was found to correlate inversely with the strength of CO adsorption for all catalysts suggesting that the chemisorption energy and not the particle size, coordination geometry, or ensemble size is the most important factor for increasing the isomerization selectivity

Size-Selective Synthesis and Stabilization of Small Silver Nanoparticles on TiO₂ Partially Masked by SiO₂

Controlling metal nanoparticle size is one of the principle challenges in developing new supported catalysts. Typical methods where a metal salt is deposited and reduced can result in a polydisperse mixture of metal nanoparticles, especially at higher loading. Polydispersity can exacerbate the already significant challenge of controlling sintering at high temperatures, which decreases catalytic surface area. Here, we demonstrate the size-selective photoreduction of Ag nanoparticles on TiO₂ whose surface has been partially masked with a thin SiO₂ layer. To synthesize this layered oxide material, TiO₂ particles are grafted with <i>tert</i>-butylcalix­[4]­arene molecular templates (∼2 nm in diameter) at surface densities of 0.05–0.17 templates.nm^–2, overcoated with ∼2 nm of SiO₂ through repeated condensation cycles of limiting amounts of tetraethoxysilane (TEOS), and the templates are removed oxidatively. Ag photodeposition results in uniform nanoparticle diameters ≤ 3.5 nm (by transmission electron microscopy (TEM)) on the partially masked TiO₂, whereas Ag nanoparticles deposited on the unmodified TiO₂ are larger and more polydisperse (4.7 ± 2.7 nm by TEM). Furthermore, Ag nanoparticles on the partially masked TiO₂ do not sinter after heating at 450 °C for 3 h, while nanoparticles on the control surfaces sinter and grow by at least 30%, as is typical. Overall, this new synthesis approach controls metal nanoparticle dispersion and enhances thermal stability, and this facile synthesis procedure is generalizable to other TiO₂-supported nanoparticles and sizes and may find use in the synthesis of new catalytic materials

Influence of the Metal (Al, Cr, and Co) and Substituents of the Porphyrin in Controlling Reactions Involved in Copolymerization of Propylene Oxide and Carbon Dioxide by Porphyrin Metal(III) Complexes. 3. Cobalt Chemistry

A series of cobalt­(III) complexes LCoX, where L = 5,10,15,20-tetraphenylporphyrin (TPP), 5,10,15,20-tetrakis­(pentafluorophenyl)­porphyrin (TFPP), and 2,3,7,8,12,13,17,18-octaethylporphyirn (OEP) and X = Cl or acetate, has been investigated for homopolymerization of propylene oxide (PO) and copolymerization of PO and CO₂ to yield polypropylene oxide (PPO) and polypropylene carbonate (PPC) or propylene carbonate (PC), respectively. These reactions were carried out both with and without the presence of a cocatalyst, namely, 4-dimethylaminopyridine (DMAP) or PPN⁺Cl^– (bis­(triphenylphosphine)­iminium chloride). The PO/CO₂ copolymerization process is notably faster than PO homopolymerization. With ionic PPN⁺Cl^– cocatalyst the TPPCoOAc catalyst system grows two chains per Co center and the presence of excess [Cl^–] facilitates formation of PC by two different backbiting mechanisms during copolymerization. Formation of PPC is dependent on both [Cl^–] and the CO₂ pressure employed (1–50 bar). TPPCoCl and PO react to form TPPCo­(II) and ClCH₂CH­(Me)­OH, while with DMAP, TPPCoCl yields TPPCo­(DMAP)₂⁺Cl^–. The reactions and their polymers and other products have been monitored by various methods including react-IR, FT-IR, GPC, ESI, MALDI TOF, EXAFS, and NMR (¹H, ¹³C­{¹H}) spectroscopy. Notable differences are seen in these reactions with previous studies of (porphyrin)­M­(III) complexes (M = Al, Cr) and of the (salen)­M­(III) complexes where M = Cr, Co

Labile Cu(I) Catalyst/Spectator Cu(II) Species in Copper-Catalyzed C–C Coupling Reaction: Operando IR, in Situ XANES/EXAFS Evidence and Kinetic Investigations

Insights toward the Cu-catalyzed C–C coupling reaction were investigated through operando IR and in situ X-ray absorption near-edge structure/extended X-ray absorption fine structure. It was found that the Cu­(I) complex formed from the reaction of CuI with β-diketone nucleophile was liable under the cross-coupling conditions, which is usually considered as active catalytic species. This labile Cu­(I) complex could rapidly disproportionate to the spectator Cu­(II) and Cu(0) species under the reaction conditions, which was an off-cycle process. In this copper-catalyzed C–C coupling reaction, β-diketone might act both as the substrate and the ligand

Selective Dimerization of Propylene with Ni-MFU‑4<i>l</i>

We report the selective dimerization of propylene to branched hexenes using Ni-MFU-4<i>l</i>, a solid catalyst prepared by cation exchange. Analysis of the resulting product distribution demonstrates that the selectivity arises from 2,1-insertion and slow product reinsertion, mechanistic features reproduced by a molecular nickel tris-pyrazolylborate catalyst. Characterization of Ni-MFU-4<i>l</i> by X-ray absorption spectroscopy provides evidence for discrete, tris-pyrazolylborate-like coordination of nickel, underscoring the small-molecule analogy that can be made at metal–organic framework nodes

Insights into Nitrate Reduction over Indium-Decorated Palladium Nanoparticle Catalysts

Nitrate (NO₃⁻) is an ubiquitous groundwater contaminant and is detrimental to human health. Bimetallic palladium-based catalysts have been found to be promising for treating nitrate (and nitrite, NO₂⁻) contaminated waters. Those containing indium (In) are unusually active, but the mechanistic explanation for catalyst performance remains largely unproven. We report that In deposited on Pd nanoparticles (NPs) (“In-on-Pd NPs”) shows room-temperature nitrate catalytic reduction activity that varies with volcano-shape dependence on In surface coverage. The most active catalyst had an In surface coverage of 40%, with a pseudo-first order normalized rate constant of <i>k</i>_cat ∼ 7.6 L g_{surface-metal}⁻¹ min⁻¹, whereas monometallic Pd NPs and In₂O₃ have nondetectible activity for nitrate reduction. X-ray absorption spectroscopy (XAS) results indicated that In is in oxidized form in the as-synthesized catalyst; it reduces to zerovalent metal in the presence of H₂ and reoxidizes following NO₃⁻ contact. Selectivity in excess of 95% to nontoxic N₂ was observed for all the catalysts. Density functional theory (DFT) simulations suggest that submonolayer coverage amounts of metallic In provide strong binding sites for nitrate adsorption and they lower the activation barrier for the nitrate-to-nitrite reduction step. This improved understanding of the In active site expands the prospects of improved denitrification using metal-on-metal catalysts

Conversion of Dimethyl Ether to 2,2,3-Trimethylbutane over a Cu/BEA Catalyst: Role of Cu Sites in Hydrogen Incorporation

Recently, it has been demonstrated that methanol and/or dimethyl ether can be converted into branched alkanes at low temperatures and pressures over large-pore acidic zeolites such as H-BEA. This process achieves high selectivity to branched C₄ (e.g., isobutane) and C₇ (e.g., 2,2,3-trimethylbutane) hydrocarbons. However, the direct homologation of methanol or dimethyl ether into alkanes and water is hydrogen-deficient, resulting in the formation of unsaturated alkylated aromatic residues, which reduce yield and can contribute to catalyst deactivation. In this paper we describe a Cu-modified H-BEA catalyst that is able to incorporate hydrogen from gas-phase H₂ cofed with dimethyl ether into the desired branched alkane products while maintaining the high C₄ and C₇ carbon selectivity of the parent H-BEA. This hydrogen incorporation is achieved through the combination of metallic Cu nanoparticles present on the external surface of the zeolite, which perform H₂ activation and olefin hydrogenation, and Lewis acidic ion-exchanged cationic Cu present within the H-BEA pores, which promotes hydrogen transfer. With cofed H₂, this multifunctional catalyst achieved a 2-fold increase in hydrocarbon productivity in comparison to H-BEA and shifted selectivity toward products favored by the olefin catalytic cycle over the aromatic catalytic cycle

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Pd–Ag alloy catalysts with very dilute amounts of Pd were synthesized. EXAFS results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H₂ dissociation. In addition, for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein molecule for facile hydrogenation of the aldehyde functionality