Site Titration with Organic Bases During Catalysis: Selectivity Modifier and Structural Probe in Methanol Oxidation on Keggin Clusters

Abstract

Catalytic reactions often require several types of sites with distinct functions, and the relative abundance of these sites influences the rate and selectivity of desired reactions. Heteropolyacid clusters with Keggin structures, which contain acid and redox functions, [1,2] have recently emerged as interesting catalysts for organic reactions involving bifunctional pathways.We recently discovered a selective one-step synthesis of dimethoxymethane (CH 3 OCH 2 OCH 3 , DMM, methylal) by oxidation of methanol at low temperatures (453-493 K) on unsupported and SiO 2 -supported H 3+n PV n Mo 12Àn O 40 (n = 0-4) Keggin clusters.[3] The yields and selectivities (based on the absence of dimethyl ether) of DMM resemble those obtained using supported ReO x catalysts, the only catalysts that enable the formation of DMM in substantial yields.[4] Redox and Brønsted acid sites are required for DMM synthesis, and the reaction involves oxidative dehydrogenation of CH 3 OH to formaldehyde (HCHO), acid-catalyzed acetalization of CH 3 OH/HCHO mixtures, and condensation of hemiacetal or methoxymethanol intermediates (formed in acetalization reactions) [5,6] with CH 3 OH to form DMM. Brönsted acidity is required to complete the synthesis of DMM, but reaction rates are predominately controlled by the initial formation of HCHO on redox sites, the density and reactivity of which were varied in our studies by changing the dispersion and V/Mo ratio of the Keggin structures, with consequent changes in the rates of DMM synthesis. These compositional changes led to concurrent changes in the number of acid sites, because of the stoichiometry required to balance the charge. DMM selectivity is decreased by side reactions involving CH 3 OH dehydration. These reactions are catalyzed by strongly acidic protons in H 3+n PV n Mo 12Àn O 40 and lead to the undesired formation of dimethyl ether (DME). The latter product ultimately converts into HCHO and DMM products, and can even re-form CH 3 OH, but forms DMM more slowly than CH 3 OH.[3] Thus, the formation of DME through these side reactions necessitates longer residence times to achieve high yields of DMM from CH 3 OH.We report here the selective titration of protons with organic bases to control the densities of acid sites in Keggin clusters and to measure their dispersion; in both cases we do this during the catalytic reaction, a requirement imposed by the dynamic changes in accessibility that arise from reactions of polar molecules on Keggin clusters. [1,2] In this manner we are able to measure turnover rates (per exposed Keggin unit; KU) and to control the redox and acid properties independently for a given composition of Keggin cluster. This approach has led to unprecedented DMM selectivities (> 80 %) and to a family of stable organic-inorganic composites that provide effective bifunctional catalysts for broad classes of redox-acid bifunctional reactions.The dispersion of Keggin structures was measured by titration of Brønsted acid sites with a sterically hindered pyridine (2,6-di-tert-butylpyri...

Catalytic Consequences of Protons in Methanol Oxidative Dehydrogenation on Molybdenum-Based Polyoxometalate Clusters

“…In situ chemical titration with pyridine and DTBP titrants differentiates the rate contributions arising from the Brønsted and Lewis sites. − Pyridine with a kinetic diameter of 5.7 Å binds to both Brønsted and Lewis sites, whereas DTBP with a kinetic diameter of 10.5 Å binds only to BAS. , For the case of H 3 PMo- fresh clusters, pyridine incorporation into the methanol-O 2 feed stream at 433 K almost completely depressed the STYs of DME and DMM but decreased those of MF to ∼60% of their initial values, as shown in Figure c. The STYs did not recover upon pyridine removal, thus the site occupation is irreversible.…”

On the Spatial Design of Co-Fed Amines for Selective Dehydration of Methyl Lactate to Acrylates

“…where is the van der Waals diameter and is the calculated van der Waals volume. The kinetic diameter ( ) can be calculated from the critical volume ( ) or critical temperature ( ) and pressure ( ) of a fluid, 68 = 0.841 1/3 (16) = 2.44( ) 1/3 (17) where is in cm 3 mol -1 , is in K, and is in atm. However, data for the critical properties are not readily available for all amines in this study.…”

VOx Modified H-Beta Zeolite for Dimethyl Ether Direct Oxidation to Polyoxymethylene Dimethyl Ethers

“…In addition, new synthesis routes of DMMx were put forward including the direct oxidation of dimethyl ether (DME, CH 3 -O-CH 3 ) [10][11][12]. DME can be synthesized via a one-step process at low cost from syngas generated from coal, biomass and natural gas, and it has been known as a clean fuel and a potential non-petroleum oil route chemical synthesis material [13,14].…”