Formation Of Dimethoxymethane Research Articles

A detailed kinetic model for the methanol oxidative dehydrogenation on vanadia-based catalysts: Aggregation state role and active site requirements

A kinetic model was derived for the methanol oxidative dehydrogenation to formaldehyde, methyl formate and dimethoxymethane on sub-monolayer and multilayer V2O5/TiO2 catalysts. Considering a hemiacetal intermediate, the model combines Langmuir-Hinshelwood/Eley-Rideal/Mars-van Krevelen mechanisms, accounting for lattice oxygen redox sites and Brønsted acid sites. The fitted model depends on 4 thermodynamically consistent parameters, adequately simulating yield trends with temperature, residence time, and partial pressure of methanol, oxygen and added water. Oxidation steps follow a Mars-van Krevelen redox cycle, while the dimethoxymethane formation is reversible, as the water addition favored the hemiacetal intermediate. The catalyst with higher vanadia surface density was intrinsically more active (lower activation energy of the rate limiting step), attributed to its higher reducibility, while showing weaker methanol adsorption (lower enthalpy of chemisorption). The increase in reducibility correlates with the transition from monomers to polymers to crystalline vanadia when vanadium content increases, as observed by XRD, Raman spectroscopy and DRS-UV–vis.

Selective Methanol Oxidation to Green Oxygenates—Catalyst Screening, Reaction Kinetics and Performance in Fixed-Bed and Membrane Reactors

Experimental and simulation-based investigations are carried out for the selective oxidation of green methanol to the oxygenates dimethoxymethane (DMM) and methyl formate (MF), including an initial catalyst screening, the derivation of a reaction kinetic model, and a feasibility study of a fixed-bed and a membrane reactor with oxygen distribution. The catalyst screening of different supports and loading of vanadium revealed a 6.6 wt.-% VOx/TiO2 catalyst offering the highest potential to the formation for the target products. Kinetic experiments performed in a broad range of operation conditions, e.g., residence time, temperature, and oxygen concentration, are used for the postulation of a reaction network, providing the basis for mathematical modeling of the individual five reaction rates with a reduced mechanistic approach. A simulation study based on the derived reaction kinetics and parameters revealed the high potential of a distributed oxygen dosing at high residence times, outperforming the conventional fixed-bed reactor by up to 6% in the yield of DMM and up to 19% in the yield of MF. The formation of DMM is favored at low temperatures, whereas the formation of MF is supported by high temperatures.

The mechanism of dimethyl carbonate synthesis on Cu-exchanged zeolite Y

The mechanism of dimethyl carbonate (DMC) synthesis from oxidative carbonylation of methanol over Cu-exchanged Y zeolite has been investigated using in situ infrared spectroscopy and mass spectrometry under transient-response conditions. The formation of DMC is initiated by reaction of molecularly adsorbed methanol with oxygen to form either mono- or di-methoxide species bound to Cu + cations. Reaction of the mono-methoxide species with CO produces monomethyl carbonate (MMC) species. DMC is formed via two distinct reaction pathways—CO addition to di-methoxide species or by reaction of methanol with MMC. The rate-limiting step in DMC synthesis is found to be the reaction of CO with mono-methoxide or di-methoxide species. The first of these reactions produces MMC, which then reacts rapidly with methanol to produce DMC, whereas the second of these reactions produces DMC directly. Formaldehyde was identified as an intermediate in the formation of dimethoxy methane (DMM) and methyl formate (MF). Both byproducts are thought to form via a hemiacetal intermediate produced by the reaction of methanol with adsorbed formaldehyde at a Cu + site.

The Local Environment of Cu+in Cu−Y Zeolite and Its Relationship to the Synthesis of Dimethyl Carbonate

Cu-exchanged Y zeolite was investigated in order to determine the location of the copper cations relative to the zeolite framework and to determine which Cu cations are active for the oxidative carbonylation of methanol to dimethyl carbonate (DMC). Cu-Y zeolite was prepared by vapor-phase exchange of H-Y with CuCl. The oxidation state, local coordination, and bond distances of Al and Cu were determined using Al K-edge and Cu K-edge X-ray absorption spectroscopy (XAS). Complimentary information was obtained by H2 temperature-programmed reduction and by in-situ infrared spectroscopy. Cu-Y has a Cu/Al ratio of unity and very little occluded CuCl. The average Al-O and Al-Cu bond distances are 1.67 angstroms and 2.79 angstroms, respectively, and the average Cu-O and Cu-Si(Al) bond distances are 1.99 angstroms and 3.13 angstroms, respectively. All of the Cu exchanged is present as Cu+ in sites I', II, and III'. Cu-Y is active for the oxidative carbonylation of methanol, and at low reactant contact time produces DMC as the primary product. With increasing reactant contact time, DMC formation decreases in preference to the formation of dimethoxy methane (DMM) and methylformate (MF). The formation of DMM and MF is attributed to the hydrogenation of DMC and the hydrogenolysis of DMM, respectively. Observation of the catalyst under reaction conditions reveals that most of the copper cations remain as Cu+, but some oxidation of Cu+ to Cu2+ does occur. It is also concluded that only those copper cations present in site II and III' positions are accessible to the reactants, and hence are catalytically active. The dominant adsorbed species on the surface are methoxy groups, and adsorbed CO is present as a minority species. The relationship of these observations to the kinetics of DMC synthesis is discussed.

Designing the activity/selectivity of surface acidic, basic and redox active sites in the supported K 2O–V 2O 5/Al 2O 3 catalytic system

The supported K 2O–V 2O 5/Al 2O 3 catalytic system was designed to create surfaces that were 100% acidic, 100% basic, 100% redox, mixed redox-acidic and mixed redox-basic. The resulting nature of the surface sites was controlled by the impregnation of the specific additives (K-basic or V-redox/acidic), their order of impregnation and their surface coverage. The exact locations of the surface methoxy intermediates (Al OCH 3, K OCH 3 or V OCH 3) on the mixed oxide catalyst surfaces during methanol oxidation were determined with in situ Raman spectroscopy. The surface chemistry of the various surface sites and their surface reaction intermediates were chemically probed by CH 3OH oxidation steady-state and temperature programmed surface reaction (TPSR) spectroscopy studies. The specific reactivity order and the product selectivity of the various surface sites were found to be: V OCH 3 (HCHO) ∼ Al OCH 3 (CH 3OCH 3) ≫ K OCH 3 (primarily CO 2 and minor amounts of HCHO). Formation of dimethoxy methane, (CH 3O) 2CH 2, required the presence of dual surface redox-acidic sites surface redox sites to yield H 2CO and surface acidic sites to insert the surface methoxy into H 2CO to form dimethoxy methane, (CH 3O) 2CH 2. The addition of basic surface potassium oxide to Al 2O 3 possessing surface acid sites completely suppressed reactions from the surface acidic sites and formed a surface with only basic characteristics. The addition of redox surface vanadia to the supported K 2O/Al 2O 3 catalyst was able to completely suppress reactions from surface basic sites and formed a surface with only redox characteristics. These studies demonstrate that it is possible to determine the specific surface site requirements for each reaction pathway for methanol oxidation to products, and that this informative approach should also be applicable to other reactant molecules.

Site titration with organic bases during catalysis: selectivity modifier and structural probe in methanol oxidation on Keggin clusters.

The selective and permanent titration of protons with organic bases (2,6‐di‐tert‐butylpyridine or pyridine) provides an accurate measure of the dispersion of Keggin structures during catalytic oxidation of methanol. Titration allows the systematic control of the redox and acid properties of H3+nPVnMo12−nO40 (n=0–4) Keggin clusters and leads to unprecedented selectivity in the formation of dimethoxymethane (>80 %) and high yields.