Ethoxy Species Research Articles

Computational insights into the zeolite-supported gold nanocluster-catalyzed ethanol dehydrogenation to acetaldehyde.

Zeolite-supported gold nanoclusters play increasingly important roles in heterogeneous catalysis and exhibit unique catalytic properties for ethanol dehydrogenation to acetaldehyde. Nevertheless, the reaction mechanism and potential roles of the zeolite-encapsulated gold nanoclusters during the catalytic process remain unclear. Herein, computational studies were carried out to gain mechanistic insights into ethanol dehydrogenation to acetaldehyde under both aerobic and anaerobic conditions catalyzed by a silicalite-1 zeolite-encapsulated Au3 cluster cation (Au3+-S1). The presence of O2 can significantly promote the ethanol dehydrogenation catalyzed by Au3+-S1. A feasible mechanistic pathway could be initiated via the O2 induced H-atom transfer (HAT) step from the hydrogen of the hydroxyl group to afford ethoxy and OOH radical species. Subsequently, the OOH induced second HAT from α-C-H of the ethoxy intermediate could follow to afford the acetaldehyde product. Moreover, the possible confinement and stabilization effect of the zeolite channels on the ethanol dehydrogenation reaction was discussed.

Observation of reactive intermediates in the initial stage during ethene conversion over acidic zeolites

The surface ethoxy species were identified as key intermediates to equilibrate with ethene and contribute to the formation of the initial C–C bond during the ethene conversion on acidic zeolites.

Shape-Controlled Pathways in the Hydrogen Production from Ethanol Steam Reforming over Ceria Nanoparticles.

The ethanol surface reaction over CeO2 nanooctahedra (NO) and nanocubes (NC), which mainly expose (111) and (100) surfaces, respectively, was studied by means of infrared spectroscopy (TPSR-IR), mass spectrometry (TPSR-MS), and density functional theory (DFT) calculations. TPSR-MS results show that the production of H2 is 2.4 times higher on CeO2-NC than on CeO2-NO, which is rationalized starting from the different types of adsorbed ethoxy species controlled by the shape of the ceria particles. Over the CeO2(111) surface, monodentate type I and II ethoxy species with the alkyl chain perpendicular or parallel to the surface, respectively, were identified. Meanwhile, on the CeO2(100) surface, bidentate and monodentate type III ethoxy species on the checkerboard O-terminated surface and on a pyramid of the reconstructed (100) surface, respectively, are found. The more labile surface ethoxy species on each ceria nanoshape, which are the monodentate type I or III ethoxy on CeO2-NO and CeO2-NC, respectively, react on the surface to give acetate species that decompose to CO2 and CH4, while H2 is formed via the recombination of hydroxyl species. In addition, the more stable monodentate type II and bidentate ethoxy species on CeO2-NO and CeO2-NC, respectively, give an ethylenedioxy intermediate, the binding of which is facet-dependent. On the (111) facet, the less strongly bound ethylenedioxy desorbs as ethylene, whereas on the (100) facet, the more strongly bound intermediate also produces CO2 and H2 via formate species. Thus, on the (100) facet, an additional pathway toward H2 formation is found. ESR activity measurements show an enhanced H2 production on the nanocubes.

Redox-mediated C–C bond scission in alcohols adsorbed on CeO2− x thin films

The decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water.

The effect of lanthanum in Cu/La(-Zr)-Si oxide catalysts for aqueous ethanol conversion into 1,3-butadiene

This paper presents the results of an investigation of three (Cu/La-Si and Cu/Zr-Si) and four (Cu/La-Zr-Si) component oxide systems in aqueous ethanol conversion into 1,3-butadiene, with special accent to the effect of lanthanum in the catalyst composition. The catalysts were characterized by HRTEM, SEM, DR UV−vis, in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), FTIR with pyridine, and TPD-CO2. The high (≥65%) 1,3-butadiene selectivity is achieved during the conversion of both rectified and aqueous ethanol over the studied catalysts. Catalyst activity of the four-component oxide systems depends on an introduction order of the components, which determines acid-base characteristics of the surface. A modification of the Cu/Zr-Si catalyst with lanthanum increases their activity in aqueous ethanol conversion into 1,3-butadiene. The positive effect of lanthanum addition into Cu/Zr-Si catalyst can be explained by the formation of lanthanum-containing base sites like [La–Ö–Si] and [La–Ö–H] being tolerant to the action of water. The differences in the interaction of reaction intermediates with surface Zr- and La-containing sites are found out by C 1 s NAP-XPS measurements, and the effect of H2O on adsorption of carbon coke, ethoxy species and acetaldehyde over the catalyst surface is shown.

Promoting the Selectivity of Pt/m-ZrO2 Ethanol Steam Reforming Catalysts with K and Rb Dopants.

The ethanol steam reforming reaction (ESR) was investigated on unpromoted and potassium- and rubidium-promoted monoclinic zirconia-supported platinum (Pt/m-ZrO2) catalysts. Evidence from in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization indicates that ethanol dissociates to ethoxy species, which undergo oxidative dehydrogenation to acetate followed by acetate decomposition. The acetate decomposition pathway depends on catalyst composition. The decarboxylation pathway tends to produce higher overall hydrogen selectivity and is the most favored route at high alkali loading (2.55 wt.% K and higher or 4.25 wt.% Rb and higher). On the other hand, decarbonylation is a significant route for the undoped catalyst or when a low alkali loading (e.g., 0.85% K or 0.93% Rb) is used, thus lowering the overall H2 selectivity of the process. Results of in situ DRIFTS and the temperature-programmed reaction of ESR show that alkali doping promotes forward acetate decomposition while exposed metallic sites tend to facilitate decarbonylation. In previous work, 1.8 wt.% Na was found to hinder decarbonylation completely. Due to the fact that 1.8 wt.% Na is atomically equivalent to 3.1 wt.% K and 6.7 wt.% Rb, the results show that less K (2.55% K) or Rb (4.25% Rb) is needed to suppress decarbonylation; that is, more basic cations are more efficient promoters for improving the overall hydrogen selectivity of the ESR process.

The first carbon-carbon bond formation mechanism in methanol-to-hydrocarbons process over chabazite zeolite

<h2>Summary</h2> The first C–C bond formation mechanism in the methanol-to-hydrocarbons (MTH) process has always been a highly controversial issue. Using solid-state NMR spectroscopy, the initial C1 reactants and C1 intermediates were observed on the HSSZ-13 zeolite surface. In particular, for the first time, the highly reactive ethene precursor, surface ethoxy species, was directly captured under a real MTH reaction. These findings and <i>in situ</i> captured activated C1 reactants linked C1 species and initially generated ethene, in which C1 intermediates activate methanol/DME to form the first C–C bond. For such a difficult-to-detect and extremely initial reaction process, the advanced <i>ab initio</i> molecular dynamics technology simulated the complete process from C1 reactants to product. The visualized reaction process vividly shows methanol/DME in the gas phase approaching SMS/TMO on zeolite, being activated via effective collision and methylation to generate an ethene precursor. The zeolite catalyst and active C1 intermediates contribute together to the first C–C bond formation in a synergistic manner.

Effect of sodium loading on Pt/ZrO2 during ethanol steam reforming

Ethanol steam reforming (ESR) was investigated on unpromoted and several sodium promoted Pt/ZrO2 catalysts. From DRIFTS experiments, the following steps during ESR were inferred: dissociation of ethanol to produce ethoxy species; oxidative dehydrogenation of ethoxy species to acetate; and acetate decomposition. Acetate decomposition depends on the catalyst formulation. Decarboxylation is the most favored route at high sodium loading (2.5 and 5 wt.%); acetate decomposes in the forward direction to CH4 and a carbonate, which further decomposes to CO2. In contrast, decarbonylation is prevalent for the unpromoted catalyst or catalysts having low sodium loading. Acetate likely decomposes to CH3OH and CO. Adsorbed methanol may undergo further steam reforming by oxidative dehydrogenation to formate species, which decarbonylates via reverse decomposition to CO and H2O. Temperature programmed desorption/reaction and activity data confirmed that alkali promotion, especially at 1.8 %Na and higher loading, facilitates the forward acetate decomposition step, favoring decarboxylation over decarbonylation.

Influence of the support in aqueous phase oxidation of ethanol on gold/metal oxide catalysts studied by ATR-IR spectroscopy under working conditions

Effects of supports (Co3O4, CeO2, NiO) in the gold-catalyzed aqueous phase oxidation of ethanol (EtOH) to acetic acid (AcOH) were examined. ATR-IR spectroscopy under working conditions of the catalysts uncovered on gold particles bidentate ethoxy, and on supports monodentate ethoxy species, multi-layered ethanol, and acetate. Preferential formation of bidentate ethoxy species and adsorbed EtOH were identified as key factors for high activity and selectivity to AcOH. These requirements were best matched with Au/Co3O4. On NiO, monodentate ethoxy species on the support deteriorated the catalytic performance due to consecutive esterification of AcOH and/or acetate species with EtOH producing undesirable ethyl acetate.

Controlled selectivity for ethanol steam reforming reaction over doped CeO2 surfaces: The role of gallium

The ethanol steam reforming reaction, together with the adsorption and decomposition of ethanol was studied on CeO2 and gallium-doped ceria (CeGaOx) by a combined experimental and theoretical approach using infrared spectroscopy (IR), mass spectrometry (MS) and density functional theory (DFT) calculations. At 100 °C, different types of monodentate ethoxy species were identified as standing-up (SU) on Ce4+ and lying-down (LD) on Ce4+ and Ga3+, with the alkyl chain more perpendicular or parallel to the surface, respectively. It is suggested that the incorporation of Ga into the ceria lattice changes the decomposition pathway of LD species, which converts to acetate instead of ethylene, attributed to the increased lattice oxygen lability in the CeOGa interface upon doping and the propensity to form GaH surface species. Under ethanol steam reforming conditions, Ga doping of ceria-based materials has a drastic effect by improving the H2:CO2 ratio, changing the product distribution and reducing coke formation.

Detailed Mechanism of Ethanol Transformation into Syngas on Catalysts Based on Mesoporous MgAl2O4 Support Loaded with Ru + Ni/(PrCeZrO or MnCr2O4) Active Components

Mechanism of ethanol partial oxidation into syngas over catalysts based on mesoporous MgAl2O4 spinel loaded with fluorite PrCeZrO or spinel MnCr2O4 oxides and promoted by Ru + Ni was studied by in situ FTIRS and 18O SSITKA. Surface species (ethoxy, adsorbed ethanol, acetaldehyde, acetate, etc.) were identified and their thermal stability and reactivity were estimated. Analysis of kinetics of the 18O transfer into reaction products (CO, CO2, CH3CHO) allowed to estimate the rates of steps and present a scheme of the reaction mechanism including (1) fast CH3CHO formation on mixed metal oxide sites; (2) rate-limiting stage of surface oxygen species incorporation into acetaldehyde or ethoxy species with C–C bond rupture yielding CO and CO2 along with H2 and H2O; (3) water gas shift reaction by redox mechanism affecting CO/CO2 ratio and their oxygen isotope fraction. Strong interaction of PrCeZrO or MnCr2O4 oxides with MgAl2O4 support results in decreasing constants of main reaction steps in comparison with those for catalysts based on bulk fluorite and spinel oxides, correlating with a higher surface oxygen bonding strength and its low coverage revealed by pulse microcalorimetry. DFT analysis confirmed a low energy barrier of the step of Ru–O oxygen incorporation into C–C bond of ethoxy species with its rupture explaining a higher syngas selectivity for Ru-doped catalysts.

Observation of an oxonium ion intermediate in ethanol dehydration to ethene on zeolite

Zeolite-catalyzed dehydration of ethanol offers promising perspectives for the sustainable production of ethene. Complex parallel-consecutive pathways are proposed to be involved in the reaction network of ethanol dehydration on zeolites, where the initial step of ethanol dehydration is still unclear particularly for the favorable production of ethene at lower temperature. Here we report the observation of a triethyloxonium ion (TEO) in the dehydration of ethanol on zeolite H-ZSM-5 by using ex situ and in situ solid-state NMR spectroscopy. TEO is identified as a stable surface species on the working catalyst, which shows high reactivity during reaction. Ethylation of the zeolite by TEO occurs at lower temperature, leading to the formation of surface ethoxy species and then ethene. The TEO-ethoxide pathway is found to be energetically preferable for the dehydration of ethanol to ethene in the initial stage, which is also supported by theoretical calculations.

Interaction of intermediates with transition metal surfaces in the dehydrogenation of ethanol to ethyl acetate: A theoretical investigation

By employing a combined approach of the unity bond index–quadratic exponential potential method and density functional theory within the generalized gradient approximation, we have studied the interaction of intermediates in the ethanol dehydrogenation process to ethyl acetate on Cu, Ag, Ni, Pd, Pt, Co, Au and Ir(111) transition metal surfaces. Binding energies and geometries were optimized for the main intermediates of this process. Electronic structures were computed for some intermediates/transition metal systems. We also calculated the activation energies for the elementary steps of the reactions. The results show that amid the studied surfaces, Cu(111) stabilizes ethoxy and acetyl species, preventing their dissociation. Inducing the η2 binding mode of acetaldehyde by alloying Cu with Ni, Co, Pd, Pt or Ir can enhance the catalytic proprieties of the Cu(111) clean surface.

Room-Temperature Ethene Hydrogenation Activity of Transition-Metal-Free HY Zeolites

The conversion of C2H4 in the absence and presence of H2 was examined over dealuminated HY30, HY15, and HY2.6 zeolites with different Si/Al ratios under ambient conditions (1 atm, 25 °C). Only in the presence of H2 did these materials catalyze the conversion of C2H4 into C2H6 with measurable rates and no sign of C2H4 dimerization. The C2H4 hydrogenation activity of these materials increases with the Si/Al ratio, implying that in this case, the strength of Bronsted acid sites (i.e., Si–OH–Al) is an important factor for the hydrogenation process. The adsorption of both C2H4 and H2 over Bronsted acid sites was calculated to be exothermic without activating the H2 molecule, while the conversion of C2H4 into ethoxy species is slightly endothermic. Heterolytic dissociation of H2 is strongly unfavorable over Al/O or Si/O sites of the zeolite framework, but it is exothermic over the extraframework Al species, indicating that cooperation of Bronsted acid sites with extraframework Al species is indeed required for ...

Selective hydrogenation of acetic acid to ethanol on Cu-In catalyst supported by SBA-15

The supported copper based catalysts could be potential candidate for the selective catalytic hydrogenation of acetic acid to produce ethanol instead of noble metals. In this work, a series of highly efficient bimetallic Cu-In/SBA-15 catalysts were designed and prepared via the deposition-precipitation method. Detailed characterizations were carried out to investigate the textural properties, dispersion and chemical state of the metals, the interactions between copper and indium species, the adsorption and activation of acetic acid on the catalysts surface. The results clearly demonstrated the coexistence of Cu-In alloy and metallic Cu under the reductive reaction atmosphere. The strong synergistic effect inside Cu-In alloy not only accelerated the dissociation of acetic acid to acetate, but also hindered the combination of acetyl and ethoxy species to form ethyl acetate, thus promoting the catalytic hydrogenation performance significantly. It was also found that the concentration of adsorbed H atoms on catalysts was one of the indispensable factors affecting the ethanol selectivity, which was positively correlated to the exposed Cu surface area and H2 partial pressure. The optimal catalytic hydrogenation activity was obtained for 9Cu1In/SBA-15 under the reaction conditions of 2.5 MPa and 623 K, the conversion of acetic acid and selectivity of ethanol reached 99.1% and 90.9%, respectively. These results might provide an inspiration in the rational design of novel copper based catalysts for carboxylic acid hydrogenation.

Ethanol aerobic and anaerobic oxidation with FeVO4 and V2O5 catalysts

This study compares the aerobic and anaerobic transformation of ethanol using FeVO4 and V2O5 catalysts. Despite their different structure, the two oxides showed very similar catalytic performances and their main product was acetaldehyde. However, in the absence of oxygen, the catalysts produced an equimolar amount of ethane and acetaldehyde, and this aspect has been little studied in the literature. In-situ XPS and DRIFT spectroscopy studies showed that the active species for the disproportionation of the alcohol into ethane and aldehyde was the reduced V3+ ion; nevertheless, the Fe in the FeVO4 catalysts was responsible for directing the reduction of metals toward the formation of a Fe-V-O spinel phase which was homogeneous and more stable than V2O5. Moreover, an in-situ DRIFT spectroscopy study showed that ethanol adsorbs in different ways on the surface of the catalysts during the reduction of samples (anaerobic reaction), forming H-bonded and dissociated ethoxy species, and giving rise to new surface OH groups that participate in the aldehyde/alkane formation. To conclude, a new mechanism of hydrogen transfer for the anaerobic ethanol disproportionation into ethane and acetaldehyde is proposed. This research completes the picture about ethanol oxidation to acetaldehyde on V-based catalysts, demonstrating that the catalytic behavior is mainly affected by the oxidation degree of the vanadium species which, in turn, depends on the reaction environment, and not on the structure itself.

Kinetic modeling of polymer-grade ethylene production by diluted ethanol dehydration over H-ZSM-5 for industrial design

The production of ethylene from bioethanol dehydration is considered one of the most promising processes for industry. In this work, H-ZSM-5 zeolites with different Si/Al ratios (Si/Al = 23, 30, and 280) were evaluated as catalysts during diluted bioethanol dehydration. In situ DRIFTS ethanol adsorption experiments showed that ethanol is adsorbed on both the SiOHAl Brønsted acid sites and SiOH silanol Lewis species, preferentially over the latter in the HZSM-5 with a Si/Al of 280 at temperatures lower than 300 °C. This process likely leads to the formation of CO ethoxy species, which are further turned into ethylene. Activity tests showed total conversion of ethanol and selectivity of 99% to ethylene using a H-ZSM-5 with a Si/Al = 280 at 300 °C and a space velocity (W/F) of 0.72 gcat h gEtOH−1. This ethylene production was maintained during 72 h of time on stream (TOS). A Langmuir-Hinshelwood-based kinetic expression was proposed for the diluted ethanol dehydration in the range of 200–300 °C using H-ZSM-5 with a Si/Al ratio of 280. This reactor equation was used to propose a conceptual design of an industrial plant for producing polymer-grade ethylene from diluted bioethanol. Capex of 1 million USD (72 USD/ton-year ethylene) and Opex of 4.7 million USD/year (338 USD/ton-year) were calculated for a plant that produces 13.9 Mton year−1 ethylene from bioethanol. These costs are comparable to those from the oil industry (Capex of 1100 USD/ton-year and Opex of 324 UDS/ton-year), showing the large opportunity for sustainable production of ethylene from bioethanol.

Extra-Framework Aluminum-Assisted Initial C-C Bond Formation in Methanol-to-Olefins Conversion on Zeolite H-ZSM-5.

Surface methoxy species bound to an extra-framework Al (SMS-EFAL) was unambiguously identified by advanced 13 C-{27 Al} double-resonance solid-state NMR technique in the methanol-to-olefins reaction on H-ZSM-5 zeolite. The high reactivity of the SMS-EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C-C bond formation in ethene was proposed and corroborated by density functional theory calculations.

Extra‐Framework Aluminum‐Assisted Initial C−C Bond Formation in Methanol‐to‐Olefins Conversion on Zeolite H‐ZSM‐5

AbstractSurface methoxy species bound to an extra‐framework Al (SMS‐EFAL) was unambiguously identified by advanced 13C‐{27Al} double‐resonance solid‐state NMR technique in the methanol‐to‐olefins reaction on H‐ZSM‐5 zeolite. The high reactivity of the SMS‐EFAL leads to the formation of surface ethoxy species and ethanol as the key intermediates for ethene generation in the early reaction stage. A direct route for the initial C−C bond formation in ethene was proposed and corroborated by density functional theory calculations.

Ethanol and diethyl ether catalytic conversion over commercial alumina and lanthanum-doped alumina: Reaction paths, catalyst structure and coking

Commercial high-pore-volume alumina and La-doped aluminas have been characterized and tested as catalysts for ethanol conversion to ethylene and diethyl ether and for diethyl ether cracking. In order to go deeper on reaction paths and mechanisms, steady state, TPSR and static experiments in an IR cell were performed. It is established that ethylene forms from ethanol by two parallel ways: i) cracking of ethoxy groups that occurs already at low temperature, and ii) the parallel synthesis and cracking of DEE at intermediate temperatures. Coordination of diethyl ether on Lewis sites represents the first step in its decomposition path. Lewis bonded DEE first cracks to ethoxy species and ethylene gas, while ethoxy species in part crack to a second step to another ethylene gas molecule and in part (only at low temperature) can desorb as gaseous ethanol. Commercial low loading lanthanum-doped alumina contain dispersed La3+-O2− species mainly interacting with the most reactive defect, edge and corner sites of alumina nanocrystals. At higher loading (4 wt% La2O3) very small LaxOy clusters also appear. Lanthanum doping slightly reduces the number of active sites for ethanol dehydration as well as for DEE cracking, thus reducing catalytic activity, but does not modify significantly selectivities and ethylene yields at high temperature. However, it also considerably reduces the amount of carbonaceous residues formed upon both reactions over the catalyst. Thus, La-doping is proposed as a way to improve the alumina catalyst stability in the process. Catalytic cracking of DEE at 673 K does not represent a good way to remove odorous and dense DEE vapours from air, due to the coproduction of small amounts of acetaldehyde together with ethylene.