Cascade Reaction of Ethanol to Butadiene over Ag-Promoted, Silica- or Zeolite-Supported Ta, Y, Pr, or La Oxide Catalysts

Ethanol converts to 1,3-butadiene in the presence of suitable multifunctional catalysts. In this work, Lewis acid cations Ta, Y, Pr, and La were dispersed on amorphous silica or β zeolite, and after physically mixing with silica-supported Ag nanoparticles, were tested in the cascade reaction of ethanol to butadiene at 573 K. The Lewis acid catalysts were characterized by X-ray fluorescence, N2 physisorption, scanning transmission electron microscopy (STEM), X-ray diffraction, and diffuse reflectance (DR) UV–vis and X-ray photoelectron spectroscopies. High-resolution STEM images confirmed the small oxide cluster size on the silica support. Results from DR UV–vis spectroscopy showed that zeolite-supported Ta and Pr catalysts had a smaller metal oxide cluster size, relative to their SiO2 counterparts. X-ray photoelectron spectroscopy confirmed that the oxidation state of the cations supported on the zeolite remained the same as that of their SiO2-supported analogues. The selectivity of the C4 coupling products toward butadiene relative to butanol correlated with the acid strength of the Lewis acid cations, as evaluated by the 2-propanol decomposition reaction to propene and acetone, with Ta being the most selective. The rate of C–C coupling over the zeolite-supported cations was enhanced by an order of magnitude compared to those cations supported on amorphous SiO2

Surface Raman Spectroscopy of the Interface of Tris-(8-hydroxyquinoline) Aluminum with Mg

Surface Raman spectroscopy in ultrahigh vacuum is used to interrogate interfaces formed between tris-(8-hydroxyquinoline) aluminum (Alq3) and vapor-deposited Mg. The Raman spectral results for deposition of Mg mass thicknesses between 5 and 20 Å indicate formation of a complex interfacial region composed primarily of Mg−Alq3 adducts and small-grained amorphous or nanocrystalline graphite, the presence of which may have a significant effect on the electronic properties of this metal−organic interface. The observed shifts in νring, ν(C−N), ν(Al−N), and ν(Al−O) modes along with the appearance of ν(Mg−C) and ν(Mg−O) modes suggest a structure for the Mg−Alq3 adduct in which Mg is bound to the O and C atoms of Alq3. In addition, several intense, broad modes are observed that are consistent with partial graphitization of the Alq3 film

Investigation of the Interfaces of Tris-(8-hydroxyquinoline) Aluminum with Ag and Al Using Surface Raman Spectroscopy

Surface Raman spectroscopy in ultrahigh vacuum is used to interrogate interfaces formed between tris-(8-hydroxyquinoline) aluminum (Alq3) and vapor-deposited Ag and Al. Vapor deposition of Ag onto Alq3 films results in preservation of the Alq3 structure with evolution of simple surface enhancement of Alq3 spectral intensities. Changes in the relative intensities of ring breathing and ring stretching modes of Alq3 upon deposition of Ag are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq3 films results in the appearance of new vibrational modes, but only for Al coverages >10 Å mass thickness, consistent with the formation of an Al−Alq3 adduct. At these coverages, slight surface enhancement of spectral intensities is also observed along with growth of several broad modes consistent with partial graphitization of the organic film

Cascade Reaction of Ethanol to Butadiene over Multifunctional Silica-Supported Ag and ZrO₂ Catalysts

Although butadiene is currently a byproduct of naphtha cracking, interest in producing butadiene from biobased ethanol has increased because of the lower environmental impact of the ethanol to butadiene reaction. This work explores a multifunctional catalyst system composed of silica-supported Ag and ZrO2 used for the cascade reaction of ethanol to butadiene at 573 K. The Ag and ZrO2 components were synthesized on separate support particles, enabling the characterization of each component without interference from the other. High selectivity to butadiene (65%) at high ethanol conversion (75%) was achieved with an appropriate ratio of Ag and ZrO2 in the reactor. Silver catalyzed the initial dehydrogenation of ethanol to acetaldehyde, while ZrO2 catalyzed the C–C coupling and subsequent dehydration reactions. The silica-supported ZrO2 exhibited superior selectivity relative to bulk ZrO2 in the Ag-promoted ethanol to butadiene reaction. Results from Zr K-edge X-ray absorption spectroscopy and UV–vis spectroscopy showed that ZrO2 was highly dispersed on the silica support over a range of loadings. Infrared spectroscopy of adsorbed pyridine, CO, and CO2, and kinetics of probe reactions 1-butene double bond isomerization, 2-propanol decomposition, and ethanol hydrogenation of acetone were used to compare the acid–base nature and chemical reactivity of silica-supported ZrO2 to bulk ZrO2

Anhydrous and Water-Assisted Proton Mobility in Phosphotungstic Acid

Nonlocal gradient-corrected density functional theoretical calculations were used to determine the energetics associated with proton migration in phosphotungstic acid. The activation energy for anhydrous proton hopping between two oxygen atoms on the exterior of the molecular Keggin unit was calculated to be 103.3 kJ mol-1. The quantum-tunneling effect on the rate of proton movement was determined using semiclassical transition-state theory and was found to be a major contributor to the overall rate of proton movement at temperatures below approximately 350 K. The adsorption of water on an acidic proton decreases the activation barrier for hopping to 11.2 kJ mol-1 by facilitating proton transfer along hydrogen bonds. The overall rate constant for proton hopping was determined as a function of temperature and water partial pressure. Small amounts of water greatly enhance the overall rate of proton movement

Classical-to-Topological Transmission Line Couplers

Recent advances in topologically robust waveguiding for electromagnetic systems have presented opportunities for improving practical photonic and microwave devices. To bring this rich area of physics within the reach of application, it is critical for such systems to be interfaced with classical, continuous waveguiding and transmission line technology. This Letter presents a compact, highly efficient transition from a classical metallic transmission line to a topologically nontrivial line wave emulating the quantum spin Hall effect. A zero-gap antipodal slot line is used as the starting transmission line, which is then coupled to the topological metasurface via a field matching procedure. Additional modifications to the interface between the two structures to eliminate unwanted edge coupling improves transmission further. A simulated loss analysis isolates the effect of the transitions from the rest of the structure, showing a loss contribution of only 2.1% per classical-to-topological conversion. Using the transition, a quantitative characterization of the robustness of common topologically protected devices is presented. This design lays the foundation to integrate topologically robust metasurface transmission lines to traditional systems, opening the door to future uses of such structures in systems

Aldol Condensation of Acetaldehyde over Titania, Hydroxyapatite, and Magnesia

The kinetics of aldol condensation of acetaldehyde were studied over anatase titania (TiO₂), hydroxyapatite (HAP), and magnesia (MgO). Reactions were carried out in a fixed-bed reactor with a total system pressure of 220 kPa at temperatures between 533 and 633 K and acetaldehyde partial pressures between 0.05 and 50 kPa. Crotonaldehyde was the only product observed over all three catalysts, and severe catalyst deactivation occurred at acetaldehyde partial pressures of 5 kPa or greater. The aldol condensation reaction over all three catalysts was first order at low acetaldehyde partial pressure and approached zero order at high acetaldehyde partial pressure. No kinetic isotope effect (KIE) was observed with fully deuterated acetaldehyde reacting over TiO₂ or HAP, implying that C–H bond activation is not kinetically relevant. These measurements are consistent with a mechanism in which adsorption and desorption steps are kinetically significant during the reaction. Characterization of the catalysts by adsorption microcalorimetry of acetaldehyde and ethanol and diffuse reflectance Fourier transform infrared spectroscopy of adsorbed acetaldehyde, crotonaldehyde, and acetic acid revealed a very high reactivity of these catalysts, even at low temperatures

A Quantum Chemical Study of the Decomposition of Keggin-Structured Heteropolyacids

Heterpolyacids (HPAs) demonstrate catalytic activity for oxidative and acid-catalyzed hydrocarbon conversion processes. Deactivation and thermal instability, however, have prevented their widespread use. Herein, ab initio density functional theory is used to study the thermal decomposition of the Keggin molecular HPA structure through the desorption of constitutional water molecules. The overall reaction energy and activation barrier are computed for the overall reaction HnXM12O40 → Hn-2XM12O39 + H2O. and subsequently used to predict the effect of HPA composition on thermal stability. For example, the desorption of a constitutional water molecule is found to be increasingly endothermic in the order silicomolybdic acid (H4SiMo12O40) < phosphomolybdic acid (H3PMo12O40) 4SiW12O40) 3PW12O40), in agreement with the experimental ordering of their thermal stability. The presence of an adjacent Keggin unit may stabilize the structural defect created by the water desorption, thus suggesting that constitutional water loss is an initial step toward the decomposition into a bulk mixed oxide. The equilibrium concentration of defective Keggin units is determined as a function of temperature and water partial pressure. It is concluded that the loss of constitutional water molecules is a plausible deactivation mechanism of the acid catalyst. The intermediate structures along the decomposition path are proposed as possible active sites for oxidation catalysis. The results presented herein provide molecular level insight into the dynamic nature of the heteropolyacid catalyst structure

Multiproduct Steady-State Isotopic Transient Kinetic Analysis of the Ethanol Coupling Reaction over Hydroxyapatite and Magnesia

The Guerbet coupling of ethanol into butanol was investigated using multiproduct steady-state isotopic transient kinetic analysis (SSITKA) in a comparative study between stoichiometric hydroxyapatite (HAP) and magnesia (MgO) catalysts at 613 and 653 K, respectively. The steady-state catalytic reactions were conducted in a gas-phase, fixed-bed, differential reactor at 1.3 atm total system pressure. Multiproduct SSITKA results showed that the mean surface residence time of reactive intermediates leading to acetaldehyde was significantly shorter than that of intermediates leading to butanol on both HAP and MgO. This finding may suggest that the dehydrogenation of ethanol to acetaldehyde is fast on these surfaces compared with C–C bond formation. If adsorbed acetaldehyde is a key reaction intermediate in the Guerbet coupling of ethanol into butanol, then SSITKA revealed that the majority of adsorbed acetaldehyde produced on the surface of MgO desorbs into the gas-phase, whereas the majority of adsorbed acetaldehyde on HAP likely undergoes sequential aldol-type reactions required for butanol formation. Adsorption microcalorimetry of triethylamine and CO₂ showed a significantly higher number of acid and base sites on the surface of HAP compared with those on MgO. Diffuse reflectance infrared Fourier transform spectroscopy of adsorbed ethanol followed by stepwise temperature-programmed desorption revealed that ethoxide is more weakly bound to the HAP surface compared with MgO. A high surface density of acid–base site pairs along with a weak binding affinity for ethanol on HAP may provide a possible explanation for the increased activity and high butanol selectivity observed with HAP compared with MgO catalysts in the ethanol coupling reaction

DRIFTS of Probe Molecules Adsorbed on Magnesia, Zirconia, and Hydroxyapatite Catalysts

Acid sites, base sites, and acid–base site pairs on zirconia, magnesia, and hydroxyapatite were investigated using diffuse reflectance FT-IR spectroscopy (DRIFTS) to evaluate the interaction of various adsorbed probe molecules with their surfaces. The DRIFTS spectra were recorded under continuous flow conditions at atmospheric total pressure during a temperature-programmed thermal ramp. Lewis acidity was assessed by observing the various pyridine ring mode conformations and the peak shift associated with adsorption of CO. Basicity was probed by the adsorption of CO₂ to form carbonates and bicarbonates on the samples. The acid–base bifunctional nature of the oxides was explored by adsorption of acetylene and glycine. As expected, zirconia exposed the strongest Lewis acid sites of the three samples, whereas magnesia exhibited the strongest basic sites. In contrast, hydroxyapatite had a poor affinity for all probe molecules used in this study based on temperature-programmed desorption experiments, indicating the presence of only weak acid and base sites on the surface, which might account for its high catalytic activity and unique selectivity in the Guerbet coupling of ethanol to butanol