Mechanism and Kinetics of Ethanol Coupling to Butanol over Hydroxyapatite

The mechanism and kinetics for ethanol coupling to <i>n</i>-butanol over hydroxyapatite (HAP) were investigated at 573–613 K. <i>In situ</i> titration experiments show that the active sites for acetaldehyde and butanol formation are different. In combination with FTIR studies, it was found that ethanol dehydrogenation is catalyzed by Ca–O sites, whereas condensation of acetaldehyde is catalyzed by CaO/PO₄^3– pairs. Measurements of the reaction kinetics at various ethanol (3.5–9.4 kPa) and acetaldehyde (0.055–0.12 kPa) partial pressures reveal that direct condensation involving two ethanol molecules does not play a significant role in butanol formation; instead, <i>n</i>-butanol is formed via a Guerbet pathway. At a constant acetaldehyde pressure, enolate formation is rate-limiting, and ethanol inhibits acetaldehyde condensation rates by competitive adsorption. A model of the reaction kinetics consistent with all experimental observations is developed

Factors Influencing the Activity, Selectivity, and Stability of Rh-Based Supported Ionic Liquid Phase (SILP) Catalysts for Hydroformylation of Propene

An investigation has been carried out on the effects of catalyst preparation on the activity and stability of supported ionic liquid phase (SILP) catalysts for propene hydroformylation. Catalyst activity and stability were found to be strongly influenced by ligand and ionic liquid composition, ligand-to-rhodium ratio, and the surface density of silanol groups on the silica support. Highest activity was achieved using rhodium sulfoxantphos (SX) complexes in the presence of [bmim]­[OctSO₄]. In situ FT-IR and solid-state ³¹P and ²⁹Si MAS NMR characterization suggest that active Rh centers are not present as homogeneous complexes dissolved in an ionic liquid film, instead are present as HRh­(CO)₂SX complexes bound to the support by interactions of the sulfonate groups of SX with silanol groups of the support. The function of the ionic liquid is to inhibit undesired interactions of SX ligands, since such interactions render the phosphine groups unavailable for interaction with the Rh⁺ cations. Catalyst deactivation is attributed mainly to the formation of catalytically inactive [Rh­(CO)­(μ-CO)­SX]₂ or HRh­(SX)₂ complexes when the SX/Rh ratio is too low or high, respectively

Propene Oligomerization using Alkali Metal- and Nickel-Exchanged Mesoporous Aluminosilicate Catalysts

A series of alkali metal- and nickel-exchanged Al-MCM-41 catalysts were prepared via aqueous ion exchange and then investigated for gas-phase oligomerization of propene at 453 K and near ambient pressures. All catalysts were active and produced oligomers with >98% selectivity. The highest activities per Ni²⁺ cation were observed when the cations were highly dispersed as a consequence of either lowering the Ni loading for a fixed MCM-41 Si/Al ratio or by decreasing the concentration of exchangeable sites within the material by increasing the MCM-41 Si/Al ratio at a fixed Ni loading. The identity of the alkali metal cation had no significant effect on the catalytic activity or degree of dimer branching, except for the sample containing Cs⁺ cations, where the decreased pore volume resulted in a lower catalyst activity and slightly more linear dimer products. Comparison of Ni-MCM-41 prepared with and without Na⁺ cations showed that a higher yield of oligomers could be achieved when Na⁺ cations are present because of partial removal of strong Brønsted acid sites. For the same reaction conditions, Ni-Na-MCM-41 was more than twice as active as smaller-pored Ni-Na-X zeolites, demonstrating that the activity of Ni²⁺ cations increases with the increasing free volume near the site. This effect of free volume on the activity of Ni²⁺ cations was further confirmed by comparing the activities of Ni-Na-X, Ni-Na-MCM-41, Ni-Na-MCM-48, and Ni-Na-SBA-15 with respect to pore size

In Situ Formation of Wilkinson-Type Hydroformylation Catalysts: Insights into the Structure, Stability, and Kinetics of Triphenylphosphine- and Xantphos-Modified Rh/SiO₂

An investigation has been carried out to identify the effects of catalyst preparation on the activity, selectivity, and stability of phosphine-modified rhodium/silica catalysts (Rh/SiO₂) for propene hydroformylation. High selectivity to aldehydes was achieved, without the formation of propane or butanol. Catalyst activity and selectivity was found to depend strongly on the nature and concentration of the phosphine ligands and the amount of rhodium dispersed on the silica support. Screening of different ligands showed that a bidentate xantphos (X) ligand was ∼2-fold more active than the monodentate phosphine ligand (PPh₃) screened at a ligand-to-rhodium ratio of 15:1. Investigation of the effects of reaction temperature, reactant partial pressures, and phosphine-to-rhodium ratio indicates that the kinetics of propene hydroformylation over X-promoted Rh/SiO₂ is nearly identical to those for sulfoxantphos-modified rhodium-containing supported ionic liquid phase (SX-Rh SILP) catalysts. In-situ FTIR and solid-state ³¹P MAS NMR characterization provide evidence for the formation of HRh­(CO)_<i>n</i>(PPh₃)_4–<i>n</i> species on PPh₃-modified Rh/SiO₂, and HRh­(CO)₂(X) species on xantphos-modified Rh/SiO₂. The high catalytic activity observed over rhodium-containing silica catalysts is attributed to formation of Rh^(I)(CO)₂ species by the process of corrosive chemisorption of Rh nanoparticles by CO and the subsequent ligation of phosphine ligands to the dicarbonyl species. Evidence is also presented suggesting that the active form of the catalyst resides on the surface of the Rh nanoparticles

Experimental and Theoretical Study of <i>n</i>‑Butanal Self-Condensation over Ti Species Supported on Silica

The effects of the coordination environment and connectivity of Ti on the rate of <i>n</i>-butanal self-condensation over Ti-silica catalysts were investigated. Ti was introduced in two ways, either during the synthesis of mesoporous SBA-15 or via grafting onto amorphous silica with a disordered pore structure. The connectivity of Ti was then characterized by XANES, UV–vis, and Raman spectroscopy. For the lowest Ti loadings, the Ti is found to be predominantly in isolated monomeric species, irrespective of the manner of sample preparation, and as the Ti loading is increased, a progressively larger fraction of Ti is present in oligomeric species and anatase nanoparticles. The turnover frequency for butanal condensation decreased monotonically with increasing Ti loading, and the apparent activation energy increased from 60 kJ mol^–1 for monomeric species to 120 kJ mol^–1 for oligomeric species. A kinetic H/D isotope effect was observed over isolated titanol and Ti dimer catalysts suggesting that α-H abstraction is the rate-determining step. This conclusion is supported by theoretical analysis of the reaction mechanism. In agreement with experimental results, the calculated activation barrier for alkanal condensation over a Ti dimer is roughly two times greater than that over Ti-OH sites. The cause for this difference was explained by energy decomposition analysis of the enolate formation step which showed that there is a large energetic penalty for the substrate to distort over the Ti–O–Ti dimer than the Ti-OH monomer

Effects of Composition and Structure of Mg/Al Oxides on Their Activity and Selectivity for the Condensation of Methyl Ketones

The effects of chemical composition and pretreatment on Mg–Al hydrotalcites and alumina-supported MgO were evaluated for the gas-phase, self-condensation reaction of C₃–C₅ biomass-derived methyl ketones. We show that the selectivity toward the acyclic dimer enone and the cyclic enone trimer can be tuned by controlling the temperature of hydrotalcite calcination. Methyl ketone cyclization is promoted by Lewis acidic sites present on the hydrotalcite catalysts. XRD and thermal decomposition analysis reveal that the formation of periclase MgO starts above 623 K accompanied by complete disappearance of the hydrotalcite structure and is accompanied by an increase in hydroxyl condensation as the formation of well-crystallized periclase. ²⁷Al MQMAS and ²⁵Mg MAS NMR show that at progressively higher temperatures, Al³⁺ cations diffuses out of the octahedral brucite layers and incorporate into the tetrahedral and octahedral sites of the MgO matrix thereby creating defects to compensate the excess positive charge generated. The oxygen anions adjacent to the Mg²⁺/Al³⁺ defects become coordinatively unsaturated, leading to the formation of new basic sites. A kinetic isotope effect, <i>k</i>_H/<i>k</i>_D = 0.96, is observed at 473 K for the reaction of (CH₃)₂CO versus (CD₃)₂CO, which suggests that carbon–carbon bond formation leading to the dimer aldol product is the rate-determining step in the condensation reaction of methyl ketones. We also show that acid–base catalysts having similar reactivity and higher hydrothermal stability to that of calcined hydrotalcites can be achieved by creating defects in MgO crystallites supported alumina as a consequence of the diffusion of Al³⁺ cations into MgO. The physical properties of these materials are shown to be very similar to those of hydrotalcite calcined at 823 K