Vinyl Acetate Formation Research Articles

Mechanistic Insights on Coverage-Dependent Selectivity Limitations in Vinyl Acetate Synthesis.

Developing improved catalysts for sustainable chemical processes often involves understanding atomistic origins of catalytic activity, selectivity, and stability. Using density functional theory and steady-state kinetic analyses, we probe the elementary steps that form decomposition products that limit selectivity in vinyl acetate (VA) synthesis on Pd surfaces covered with acetate species. Acetate formation and coupling with ethylene control the VA formation catalytic cycle and steady-state coverage, but acetate and ethylene can separately decompose to form CO2. Both decompositions involve initial C-H activations at acetate vacancies, followed by additional C-H activations and eventual C-O formations and C-C cleavages involving reactions with molecular oxygen. Acetate decomposition paths with non-oxidative kinetically-relevant steps exhibit similar free energy barriers to oxidative paths. In contrast, the non-oxidative ethylene path involving an ethylidyne intermediate exhibits a much lower barrier than paths with oxidative kinetically-relevant steps. Ethylene decomposition is very facile at low coverages but is more coverage-sensitive, leading to similar decomposition and VA formation barriers at coverages accessible at steady state, which is consistent with moderate VA selectivity in measurements and ethylene vs. acetate decomposition contributions assessed from regressed kinetic parameters. These insights provide a detailed framework for describing VA synthesis rates and selectivity on metallic catalyst surfaces.

Synthesis of Vinyl Acetate Monomer Over PdCu Alloys: The Role of Surface Oxygenation in the Reaction Path

AbstractThis work uses sol–gel and sonochemical methods to prepare bimetallic PdCu catalysts supported on modified ZrO2 with Ti and Al. The catalysts are tested for vinyl acetate synthesis from ethylene, acetic acid, and oxygen, varying the reaction temperature for two catalysts prepared, which show higher activity. Catalysts characterization results show bimetallic species and distort alloys with a dispersed distribution of active metal onto the support. The in situ reaction by DRIFT‐MS identifies the surface formation of main intermediates like palladium acetate monomers and mono and bidentate intermediates, associated to vinyl acetate formation, like vinyl hydrogenated species over PdCu. Finally, this behavior will be attributed to the bimetallic distortions of PdCu samples, provided by interaction effects between PdCu and supports, indicating a more exposition of PdCu species suitable for the reaction, according to high‐resolution transmission microscopy electron microscopy results for PdCu/ZrTi sample. Thus, this sample exhibits a minimal formation of sub‐products and catalytic stability for 18 hh. These results evidence the participation of hydroxyls and oxygen vacancies of the catalyst in the catalytic reaction measured in situ, monitoring the products formed at the reactor outlet. Finally, it is proposed a reaction pathway as a function of reaction conditions.

Mathematical modeling of ethylene oxidative acetylation process

In the article, the kinetic laws, kinetics, and mechanism of the oxidative acetylation reaction of ethylene in the vapor phase were studied in detail in the 0.4%Rd+4%Cu+7%CH3 COOK/YuKTs catalyst. As a result of the research, the following optimal conditions for the reaction were selected: in the middle zone of the reactor at a temperature of 165°C, a volume velocity of 2000 h-1, a pressure of 4 atm, a ratio of ethylene to acetic acid of 4:1 and an oxygen content of 7% by volume. Under these optimal conditions, the activation energies of the reactions of vinyl acetate formation and ethylene oxidation are respectively equal to: Ea(VA) = 8.17 kcal/(mol∙K) and Ea(SO2) = 19.61 kcal/(mol∙ K). It was found that the rate of oxidation of ethylene is higher than the rate of formation of vinyl acetate when the temperature exceeds 220°C. The reaction mechanism of vinylacetate formation from ethylene and acetic acid in the presence of a palladium catalyst was proposed.

Mechanistic aspects of the Pd(OAc)n (n = 1–3) catalyzed ethylene acetoxylation: A density functional theory study

Vinyl acetate is a valuable industrial material used in the production of various polymers. The homogeneous Pd(OAc)n (n = 1–3) can catalyze ethylene acetoxylation to produce vinyl acetate. However, the reaction mechanism of Pd‐catalyzed formation of vinyl acetate remains unclear, especially in recognizing the active valence states of the Pd center. The full catalytic cycle of Pd‐catalyzed vinyl acetate formation includes ethylene‐acetate coupling, β‐H elimination, and catalyst regeneration. Based on the Wacker mechanism, two pathways were proposed for the ethylene‐acetate coupling, including a six‐membered ring mechanism through the attack of carbonyl oxygen (O2) of acetate, and a four‐membered ring mechanism through the attack of acetoxy oxygen (O1) of acetate on the coordinated ethylene. Our density functional theory (DFT) calculation shows that PdOAc cannot catalyze the acetoxylation of ethylene to form vinyl acetate due to the presence of a high energetic span. Pd(OAc)2 catalyzed reaction via the six‐membered mechanism is preferred, and the β‐H elimination is the rate‐determining step. For Pd(OAc)3, the six‐membered mechanism is favored as well, and the ethylene‐acetate coupling is the rate‐determining step. Overall, Pd(OAc)2 showed high activity (path a) toward the formation of vinyl acetate with a calculated TOF of 349 h−1.

Simulation and improvement of the separation process of synthesizing vinyl acetate by acetylene gas-phase method

Abstract Vinyl acetate, as an essential organic chemical raw material, can be used to produce polyvinyl acetate, polyester vinyl alcohol, and other products. The existing classical vinyl acetate production process has the problems of low product purity and excessive heat load. In this study, in the classical design of the process, acetylene is separated first, and then acetaldehyde is removed with the formation of an azeotrope between ethylene acetate and water. Meanwhile, considering the solubility of acetaldehyde in water and insolubility of vinyl acetate in water, the process was optimized to separate acetic acid after removing acetylene, so as to avoid the azeotrope formation of vinyl acetate and water. The nonrandom two-liquid-Hayden–O’Connell thermodynamic hybrid model was used to simulate the classical process and improved process (IP). Finally, the reflux ratio and theoretical tray number of the main separation equipment of IP were optimized to get the better parameters. The simulated results show that the purity of vinyl acetate increased from 99.1% to 99.8%, the cooling energy consumption was reduced by 16.83%, and the thermal energy consumption was reduced by 6.18%. At the same time, the equipment investment was also decreased.

Molecular insights into the vinyl acetate synthesis on Pd/Au (1 1 1) Surface: Effects of potassium ion

The surface structure of bimetallic Pd/Au catalyst, accompanied by the presence of potassium acetate (KOAc) are critical for synthesis of vinyl acetate (VAc). In this study, different configurations of the Pd/Au (111) surface were constructed. The density functional theory (DFT) was used to investigate the adsorption of ethylene and acetate on the Pd/Au (111) surface. The reaction mechanism of VAc formation was studied by following the Samanos pathway. The existence of potassium ion (K+) enhanced the adsorption of ethylene and acetate on the Pd/Au (111) surface, and promoted the dehydrogenation of acetic acid. The Pd/Au (111) surface configuration, with two adjacent palladium atoms embedded on the gold surface, is the most favorable surface structure for VAc synthesis. The introduction of K+ was obstructive to the formation of ethyl acetate-like intermediates, but facilitated β-H elimination during the formation of VAc. This work provides a deeper mechanistic understanding of VAc synthesis on the Pd/Au (111) surface, which could shed some light on the catalysts design and the industrial control of VAc synthesis.

Влияние количества активных компонентов катализатора на выход продукта при синтезе винилацетата из этилена и уксусной кислоты

The development of active and selective catalysts for the synthesis of vinyl acetate from ethylene and acetic acid, the effect of the amount and ratio of individual components of the catalyst to control the catalytic properties of the introduced components were studied. In vinyl acetate synthesis, the effects of each component were studied to describe the effect of catalyst constituents on its properties, and empirical one-factor mathematical dependencies were obtained. Their generalized mathematical relationships of the activities of the catalysts, defined as the rates of the reactions of the formation of vinyl acetate and CO2, have been proposed. As a result of the research, a mathematical model was proposed to select the most optimal composition of the catalyst. The activity of the catalyst containing 0.4%Pd+4%Cu+7%CH3COOK/HSZ averaged 700 g VA/(l×cat×h) at 93–97% selectivity of ethylene-vinyl acetate formation.

Structure of Pd/Au Alloy Nanoparticles from a Density Functional Theory-Based Embedded-Atom Potential

On the basis of DFT results for a set of representative bulk, surface, and cluster systems, we determined a new embedded-atom potential for Pd/Au alloys. This embedded-atom approach accurately reproduces DFT properties of such alloy systems. We applied this potential in Monte Carlo simulations to study the effects of temperature and composition on the structure and bonding of Pd/Au alloy nanoparticles up to 5 nm in diameter. We characterized the structure of those particles by evaluating the gold concentration at the surface and in the interior, the coordination numbers of atoms, the nature of Pd entities at the surface, and the number of suggested active sites for vinyl-acetate formation.

Vinyl acetate formation in the reaction of acetylene with acetic acid catalyzed by zinc acetate supported on porous carbon spheres

A kind of porous carbon spheres (PCS) was prepared by the carbonization of poly(vinylidene chloride) synthesized by suspension polymerization. Structure analyses revealed the existence of bumps and holes on the surface of PCS. The PCS, with the pore size between 0.8–1.2 nm, could be used as the support of zinc acetate because of the regular shape, high specific surface area, and good mechanical strength. Vinyl acetate was produced from acetylene and acetic acid using the PCS-supported zinc acetate (PCS-Zn) under mild conditions. In a single-pass operation performed at 220°C, the conversions of acetic acid and acetylene reached 22.6 and 5.3% respectively while the activity of vinyl acetate formation was above 1000 g mol−1 h−1.

Promotional Effects of Au in Pd-Au Catalysts for Vinyl Acetate Synthesis

Silica supported Pd-Au bimetallic catalysts are highly selective for the acetoxylation of ethylene to vinyl acetate (VA). In this study we have used model catalysts consisting of planer surfaces and supported nanoparticles to investigate the promotional effects of Au in Pd-Au bimetallic catalysts. Low energy ion scattering spectroscopy, low energy electron diffraction, X-ray photoelectron spectroscopy, infrared reflection adsorption spectroscopy, and temperature-programmed desorption et al, were used to characterize the model systems. The catalytic performance for acetoxylation of ethylene to VA was examined for these model surfaces. In this paper, we summarize the current understanding of the promotional effects of Au in Pd-Au bimetallic catalysts for VA synthesis. The key results are that Au atoms break contiguous Pd atom ensembles at the surface into isolated Pd monomers. The absence of contiguous Pd sites significantly reduces the formation of combustion by-products and suppresses the poison effects of CO, thus enhancing the VA formation selectivity and activity.

Vinyl Acetate Synthesis over Model Pd−Sn Bimetallic Catalysts

Pd−Sn bimetallic model catalysts were prepared as alloy films on a Rh(100) substrate via physical vapor deposition. The surface composition, structure, and chemisorption properties were studied by low energy ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), infrared reflection adsorption spectroscopy (IRAS), and temperature programmed desorption (TPD). The ordered surface alloy of c(2 × 2) was formed after annealing the Pd−Sn mixtures to 700 K as evidenced by LEED and LEIS showing a 50% surface concentration of Pd. This ordered surface arrangement was further confirmed by IRAS, LEED, and TPD studies using CO as a probe molecule, in which the surface Pd atoms are completely isolated by Sn atoms. The surface Pd composition was determined to be 0.5 monolayers (ML). The catalytic properties of this Pd−Sn surface were tested with respect to vinyl acetate (VA) synthesis by ethylene acetoxylation that showed a maximum in the VA formation rate at a surface Pd coverage of 0.5 ML, i.e., a c(2 × 2) surface arrangement. This is consistent with our previous proposal that a pair of suitably spaced, isolated Pd monomers is the more efficient site for VA synthesis.

Atomic-Scale Assembly of a Heterogeneous Catalytic Site

The distance between surface Pd atoms has been shown to control the catalytic formation of vinyl acetate from ethylene and acetic acid by AuPd catalysts. Here, we use the bulk alloy's thermodynamic properties, as well as the surface lattice spacing of a AuPd(100) alloy single-crystal model catalyst to control and optimize the concentration of the active site (Pd atom pairs at a specific Pd-Pd distance with Au nearest-neighbors). Scanning tunneling microscopy reveals that sample annealing has a direct effect on the surface Pd arrangements: short-range order preferentially forms Pd pairs located in the c(2 x 2) sites, which are known to be optimal for vinyl acetate synthesis. This effect could be harnessed for future industrial catalyst design.

Direct utilization of carbon dioxide in chemical synthesis: Vinyl acetate via methane carboxylation

The use of CO2 as a reactant for chemical synthesis generally is limited by the unfavorable equilibrium that results from the large, negative free energy of formation of CO2. Here, we report the direct catalytic synthesis of vinyl acetate from an equimolar gaseous mixture of CO2 + CH4 + C2H2 at 1 atm and temperatures between 200 and 400 °C. We are not aware of any previous reports of this reaction, which appears to proceed in two steps: (1) acetic acid is formed by the reaction of CO2 and CH4; (2) acetic acid then reacts with acetylene to form vinyl acetate. The formation of acetic acid from CO2 and CH4 is very unfavorable thermodynamically. However, the reaction of acetic acid with acetylene shifts the overall equilibrium to the right. Although 5% Pt/Al2O3 catalyzes the formation of vinyl acetate from the mixture of CO2 + CH4 + C2H2, the most effective catalyst system is an admixture of 5% Pt/Al2O3, which has been shown to catalyze the direct formation of acetic acid from CH4 + CO2, and Zn acetate/carbon, which is known to catalyze the formation of vinyl acetate from acetic acid and acetylene.

Probing reaction pathways on model catalyst surfaces: Vinyl acetate synthesis and olefin metathesis

The reaction pathway for the palladium-catalyzed synthesis of vinyl acetate from acetic acid, ethylene and oxygen is investigated using reflection–absorption infrared spectroscopy by monitoring the rate of acetate titration by gas-phase ethylene. This reveals that acetate species are removed by reaction with gas-phase ethylene resulting in vinyl acetate formation. Reaction with C 2D 4 reveals a large (∼6) isotope effect indicating that hydrogen is involved in the rate-limiting step. This also results in the appearance of an infrared feature that is assigned to an acetoxyethyl-palladium intermediate. Acetate reaction rates are different for the isotopomers, CH 2CD 2 and CHDCHD. These observations are consistent with a reaction pathway first proposed by Samanos in which ethylene reacts with an acetate species to form an acetoxyethyl-palladium intermediate, which then reacts to form vinyl acetate by a β-hydride elimination reaction. The Hérisson–Chauvin pathway for olefin metathesis, which proposes that reaction occurs between a surface carbene and alkene to form a C 3 metallacycle, which decompose by the reverse of this pathway to form metathesis products, is tested on a MoAl alloy formed by reactions with Mo(CO) 6 with an alumina thin film grown on a refractory metal substrate. This is the accepted pathway in homogeneous phase, and is demonstrated to occur on the alloy surface by grafting intermediates using iodine-containing precursors.

Synthesis of vinyl acetate on Pd-based catalysts

Vinyl acetate (VA) synthesis over Pd–Au catalysts is an important industrial reaction that has been studied extensively; however, there is no consensus regarding the reaction mechanism, the active site, the key intermediates, and the role of Au. Recent results from our laboratories using a combination of surface science and kinetic methods on technical and model catalytic systems have established that the VA synthesis reaction is structure sensitive, including being dependent on the Pd–Au particle size. The role of Au is to isolate surface Pd atoms into Pd monomeric sites thereby enhancing the VA formation rate and selectivity. This paper reviews the current understanding of this reaction on Pd, Pd–Au, and Pd–Sn catalysts.

The nature of the active site for vinyl acetate synthesis over Pd–Au

The surface composition of a Pd–Au alloy was determined using low energy ion scattering spectroscopy (LEISS). A stable surface composition was found between 700 and 1000 K with substantial enrichment in Au compared to the bulk. Infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) were used to investigate the surface adsorption sites and ensembles. The isolated Pd sites, PdAu 6 for Pd–Au on Mo(1 1 0) and for Pd/Au(1 1 1), and PdAu 4 for Pd/Au(1 0 0), were observed by controlling the Pd amount and the annealing temperature. Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of Au in a Pd–Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two, non-contiguous, suitably spaced Pd monomers. The results show that the role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product while inhibiting the formation of undesirable reaction by-products.

The Promotional Effect of Gold in Catalysis by Palladium-Gold

Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.

Particle size effects in vinyl acetate synthesis over Pd/SiO 2

Pd catalysts with varying particle size supported on a high-surface-area (600 m 2/g) SiO 2 were prepared by wet impregnation. The kinetics of vinyl acetate (VA) synthesis at atmospheric pressure was investigated in a fix-bed microreactor. With transmission electron microscopy (TEM), the average diameter of the Pd particles was estimated to be 4.0 ± 0.2 and 2.5 ± 0.1 nm over the reduced Pd(5.0 wt%)/SiO 2 and Pd(1.0 wt%)/SiO 2 catalysts, respectively; limited sintering of the Pd particles during the reaction was observed for both catalysts. X-ray diffraction (XRD) showed primarily metallic Pd crystallites on the reduced catalyst and the formation of Pd carbide (PdC x ) on the reacted catalyst. The Pd kinetics shows that the apparent activation energy increases with a decrease in the Pd particle size, varying from 17.0 kJ/mol over Pd(5.0 wt%)/SiO 2 to 39.0 kJ/mol over Pd(1.0 wt%)/SiO 2, and the rate of VA formation was significantly enhanced. On the other hand, the reaction orders (i.e., negative order with respect to ethylene and positive order with respect to oxygen) showed very little dependence on the Pd particle size.

Vinyl Acetate Formation by the Reaction of Ethylene with Acetate Species on Oxygen-Covered Pd(111)

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.

Polyaniline/PVAc blends: Variation with time of structure and conductivity of films cast from aqueous dispersions

Recently, a polymerization process of Anilinium-Dodecyl Benzene Sulfonic Acid (DBSA) complex in an aqueous dispersion was developed in our laboratories. Simple mechanical mixing of the aqueous PANI–DBSA dispersion with a PVAc aqueous latex leads to highly conductive blends at low PANI–DBSA contents. The percolation threshold of the dried films is extremely low (∼0.5 wt %). The combined aqueous PVAc/PANI–DBSA dispersions exhibit a gradually decreasing electrical conductivity accompanied by a gradually increasing viscosity, with the storage time. However, an aged cast film from these blends maintains its electrical conductivity with time. These phenomena are associated with acidic hydrolytic reactions of the ester group, resulting in the formation of vinyl acetate–vinyl alcohol copolymer and evolution of acetic acid, and also the interaction of the DBSA surfactant with the PVAc, causing swelling and disintegration of PVAc particles. A chemical structural model describing these changes with storage time is suggested. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 760–766, 2001