Shouying Huang

Catalytic Oxidative Carbonylation over Cu2O Nanoclusters Supported on Carbon Materials: The Role of the Carbon Support

Stable catalysts prepared by dispersing Cu2O nanoparticles on activated carbon were investigated in the oxidative carbonylation of methanol to dimethyl carbonate. The effect of the surface oxygen containing groups (OCGs) on the rate of dimethyl carbonate formation and the selectivities of the catalyst for dimethyl carbonate and the byproduct methyl formate were determined. The carbon support surface OCGs played a key role in the oxidative carbonylation. For carbon supports with the same amount of OCGs, the highest catalytic activity was achieved at a certain level of Cu loading. The optimal Cu loading as well as catalytic activity increased linearly with the amount of OCGs. The active sites of the catalysts were the Cu2O nanoparticles that coordinated to the OCGs on the carbon surface.

Elucidating the nature and role of Cu species in enhanced catalytic carbonylation of dimethyl ether over Cu/H-MOR

To investigate the role of Cu species in dimethyl ether (DME) carbonylation over Cu/H-MOR catalysts, ion-exchange with copper ammonia solution (Cu/H-M(x)) and solid state ion-exchange with CuCl (SSIE Cu/H-M(x)) methods were applied to prepare a series of samples with different Cu loadings. Compared to H-MOR, the reduced Cu/H-M(x) samples dramatically facilitated the conversion of DME, in which Cu+ and Cu0 species as well as Bronsted acid sites coexisted. Physical adsorption, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) were carried out to prove the negligible influence of the preparation processes on the textural and morphological properties of MOR. Fourier transform infrared (FTIR) spectroscopy, adsorption of pyridine, CO temperature programmed desorption (CO-TPD), and X-ray photoelectron spectroscopy (XPS) were employed to qualitatively and quantitatively explore the variation of both Bronsted acid sites and Cu species in 8-membered ring (8-MR) and 12-membered ring (12-MR) channels. With an increase of the Cu dopant, the amount of Cu0 increased gradually while Cu+ had no obvious regularity. The relationship between Cu0 and catalyst activity was established for Cu/H-M(x) catalysts. In addition, the formation of methyl acetate (MA) over SSIE Cu/H-M(x) catalysts decreased sharply with increasing Cu+ loading, which further excluded the promoting effect of Cu+ species present in MOR.

Reaction mechanism of dimethyl carbonate synthesis on Cu/β zeolites: DFT and AIM investigations

The mechanism of dimethyl carbonate (DMC) synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set. The adsorption energy (ΔE) and decomposition activation energy (Ea) for O2 are −1.84 and 1.72 eV, respectively, suggesting that the decomposition of O2 occurs readily under reaction conditions on the Cu site. The formed O atom further reacts with methanol to form surface-bound (CH3O)(OH)–Cu(I)/β, in which CH3O and OH were coadsorbed on the Cu+ of the catalyst; this process proceeds without an activation barrier and with an energy release of 1.23 eV. The (CH3O)(OH)–Cu(I)/β species then reacts with another methanol molecule and carbon monoxide to produce DMC through two different reaction pathways. In path I, insertion of carbon monoxide into the (CH3O)(OH)–Cu(I)/β leads to the formation of monomethyl carbonate species (CH3OCOOH), which then reacts with methanol to produce DMC and H2O. The activation energies for both steps are 0.97 and 0.65 eV, respectively. In path II, (CH3O)(OH)–Cu(I)/β reacts with methanol first to produce a dimethoxide species ((CH3O)(CH3O)–Cu(I)/β), and the formation of DMC is via the insertion of carbon monoxide into the (CH3O)(CH3O)–Cu(I)/β. The activation energies for these elementary reactions are 0.65 and 0.70 eV, respectively. The topological properties of electron density distributions for all the related stationary points involved in this reaction have also been examined using the atoms in molecule (AIM) theory for the illustration of the bond paths and weak interactions of all the stationary points in the reaction path.

Modifying the acidity of H-MOR and its catalytic carbonylation of dimethyl ether

Among the reactions catalyzed by zeolites there are some that exhibit high selectivity due to the spatial confinement effect of the zeolite framework. Tailoring the acidity, particularly the distribution and location of the Bronsted acid sites in the zeolite is effective for making it a better catalyst for these reactions. We prepared a series of H-mordenite(H-MOR) samples by varying the composition of the sol-gel, using different structure directing agents and post-treatment. NH 3 -TPD and IR characterization of adsorbed pyridine were employed to determine the amount of Bronsted acid sites in the 8-membered ring and 12-membered ring channels. It was shown that controlled synthesis was a promising approach to improve the concentration of Bronsted acid sites in MOR, even with a low Al content. Using an appropriate composition of Si and Al in the sol-gel favored a higher proportion of Bronsted acid sites in the 8-membered ring channels. HMI as a structure-direct agent gave an obvious enrichment of Bronsted acid sites in the 8-membered ring. Carbonylation of dimethyl ether was used as a probe reaction to examine the modification of the acid properties, especially the Bronsted acid sites in the 8-membered ring channels. There was a linear relationship between methyl acetate formation and the number of Bronsted acid sites in the 8-membered ring channels, demonstrating the successful modification of acid properties. Our results provide information for the rational design and modification of zeolites with spatial constraints.

Synthesis of Dimethyl Carbonate through Vapor‐Phase Carbonylation Catalyzed by Pd‐Doped Zeolites: Interaction of Lewis Acidic Sites and Pd Species

Palladium on solid acidic supports has been used as a catalyst in various industrial processes and is becoming increasingly important in modern organic reactions. It has been reported that the catalytic performances of these Pd-based catalysts are strongly affected by the support materials and additives, with particular emphasis on the nature of the acidic sites. For instance, in nucleophilic reactions catalyzed by transition metals, the reaction rate was improved and higher conversions and selectivities were achieved by the introduction of Lewis acidic sites; for alkane hydroconversion reactions, the enhanced catalytic performance was attributed to an increase in the Lewis acidity of the supports. In green chemistry terms, dialkyl carbonates have obtained widespread attention in industry because of their excellent physical and chemical properties. Vapor-phase carbonylation of alkyl nitrite over supported Pd-based catalysts is considered to be a promising route to produce dialkyl carbonates. 4] Pddoped zeolites, prepared by a conventional ion-exchange (CIE) technique with aqueous solutions of Pd(NH3)4 2+ ions, exhibit better catalytic performances and higher stability in this process. 5] It was reported that the activities of the catalysts were influenced by several factors, including the local environment and concentration of Pd 6] and the type of acidic sites of the zeolite support. It is widely accepted that Brønsted acids, formed in the preparation of Pd-doped zeolite catalysts, could accelerate the decomposition of alkyl nitrite, which would lead to poor activities. 5] Thus, Brønsted acids were avoided to the greatest extent. The effect of Lewis acidic sites on the vapor-phase carbonylation of alkyl nitrite has not been clarified. Therefore, this paper attempts to address the specific role of Lewis acids in Pd/zeolitecatalyzed vapor-phase carbonylation reactions. The nature of Lewis acidic sites in zeolites strongly depends on the composition of the zeolite framework (e.g. , SiO2/Al2O3 ratio). Therefore, we first prepared Pd-doped zeolites with various SiO2/Al2O3 molar ratios (i.e. , tuning surface acidity) by employing the CIE method. The Pd-doped zeolites were then tested in the vapor-phase carbonylation of methyl nitrite (MN) to dimethyl carbonate (DMC), which we used as a probe reaction. Among microporous materials, Faujasite (FAU) zeolites have been frequently employed as supports for vapor-phase

Monodisperse nano-Fe3O4 on α-Al2O3 Catalysts for Fischer-Tropsch Synthesis to Lower Olefins: Promoter and Size Effects

The Fischer–Tropsch synthesis to lower olefins (FTO) is a desirable nonpetroleum‐based route to produce basic chemicals. A novel two‐step method was applied to synthesize iron‐based supported catalysts, which is to prepare nano‐Fe3O4 first by thermal decomposition method and sequentially load them on α‐Al2O3 by impregnation. TEM and XRD results manifested that the controllable, uniform Fe3O4 nanoparticles are monodispersed on the surface of α‐Al2O3. H2‐TPR demonstrated that the reduction of Fe species was facilitated because of the weak interaction between Fe species and the support. These superior properties contribute to an enhanced catalytic activity and stability compared with the catalyst prepared by directly impregnating ammonium iron citrate on α‐Al2O3. Then, the effect of promoters was investigated at the same Fe loading and nanoparticle size. The appropriate addition of K could enhance the catalytic activity and suppress the secondary hydrogenation. On the contrary, S has a negative impact on CO conversion and strongly decreases C5+ selectivity. Particularly, the combination of K and S could obtain more pronounced CO conversion and higher lower olefins selectivity (≈40 %). Furthermore, size effects were explored by precisely tailoring the iron oxide particle size with the Fe loading kept constant. It was found that 12.0 nm nano‐Fe3O4 on α‐Al2O3 with or without K plus S promoters showed the best catalytic activity among the catalysts with different particle size.

In situ DRIFTS study on the oxidative carbonylation of methanol to dimethyl carbonate over Cuβ catalyst

Abstract The mechanism of oxidative carbonylation of methanol to dimethyl carbonate (DMC) over Cuβ catalyst was investigated by using in situ DRIFTS; the adsorption of single methanol, carbon monoxide and DMC as well as their mixtures on the Cuβ catalyst were considered. The results indicated that methoxide species are formed when methanol is adsorbed on the catalyst due to presence of CuOx. Only one type of active sites that are located in the six-membered ring of β zeolite is found, over which adsorbed methanol can be oxidized to methoxide and water. DMC is adsorbed on the catalyst through the contact of the oxygen atom in carbonyl group with the active sites. There were two pathways for the oxidative carbonylation: by the mono-methoxide pathway, carbon monoxide can react with mono-methoxide species to form monomethyl carbonate (MMC) and MMC then reacts with methoxide to form DMC, or by the di-methoxide pathway, DMC is formed through inserting of carbon monoxide in the di-methoxide species; latter one is more favorable over the Cuβ catalyst.

Clarification of copper species over Cu-SAPO-34 catalyst by DRIFTS and DFT study of CO adsorption

In this work, the nature, location and evolution of Cu+ ions in Cu-SAPO-34 are investigated by diffuse reflectance infrared Fourier transform spectrum (DRIFTS) of CO adsorption and density functional theory (DFT) calculation. By combination with DFT results, characteristic Cu+–CO bands located at 2154 and 2136 cm−1 are attributed to CO adsorbed on Cu+ ions located at sites I (in the plane of six-membered ring connected to the large cages) and site II (in the eight-membered ring cages near the tilted four membered ring) in the framework of H-SAPO-34 zeolite. Subsequently, both the influences of Cu loading and preparation method are considered and discussed. By varying the Cu loading, the site-occupation preference of Cu+ ions on site I is confirmed, especially at low Cu loadings. Through elevating the desorption temperature, migration of Cu+ ions is revealed because of the adsorption-induced effect. Furthermore, a facile and more efficient approach to introduce Cu+ ions into CHA zeolite, compared with solid-state ion exchange with CuCl and conventional ion exchange in aqueous solution, and the different preparation methods also result in different occupations of Cu+ ions.

Enhanced CuCl dispersion by regulating acidity of MCM-41 for catalytic oxycarbonylation of ethanol to diethyl carbonate

CuCl supported on molecular sieves has attracted increasing attention in catalyzing oxidative carbonylation of ethanol to diethyl carbonate. Mesoporous MCM-41 has been widely used as catalyst support due to its large surface area and well defined mesoporous structure. Considering its intrinsic weak acidity, MCM-41 was modified by a simple impregnation method to incorporate Al. The incorporation of Al components resulted in the high dispersion of Cu species and the increase of acid sites without changing the mesoporous structure of MCM-41, and thus enhanceed the catalytic activity of CuCl/MCM-41for diethyl carbonate synthesis.

Fabrication of Fe2C Embedded in Hollow Carbon Spheres: a High-Performance and Stable Catalyst for Fischer-Tropsch Synthesis

The Fischer−Tropsch synthesis (FTS) is a non‐petroleum‐based alternative route, which directly produces fuels and value‐added chemicals (e. g. lower olefins) from coal‐, biomass‐ or natural gas‐derived syngas. The ϵ‐iron carbide, such as Fe2C, has been predicted to be active but not stable under high‐temperature FTS conditions. In this work, we have fabricated a novel catalyst with Fe2C embedded in hollow carbon spheres (HCS) by pyrolyzing the coated polymer and Fe(NO3)3 on silica spheres and then etching the hard template. XRD, XPS, TEM and N2 physical adsorption were employed to characterize the evolution and properties of as‐prepared catalysts, which significantly depend on pyrolysis temperature. Under FTS conditions, the obtained catalysts exhibit good dispersion, robustness of geometric construction, and resistance to sintering. More importantly, Fe2C was confirmed as the dominant and stable iron carbide species. The unique chemical surrounding and confinement effect provided by carbon matrix contribute to these peculiarities that are responsible for superior activity and stability in FTS. Furthermore, we found that the products distribution could be manipulated by changing the geometric diameters of HCS, due to the tunable CO/H2 ratio.