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...

Preparation, characterization, and catalytic activity of H5PMo10V2O40 immobilized on nitrogen-containing mesoporous carbon (PMo10V2/N-MC) for selective conversion of methanol to dimethoxymethane

“…4 shows the reaction pathway for selective conversion of methanol to DMM over bifunctional oxidation and acid catalyst. It is known that formaldehyde (FA) is produced by oxidation catalysis of HPA, while dimethyl ether (DME) is produced by acid catalysis of HPA in the primary reaction of methanol [10][11][12]. In the secondary reaction, dimethoxymethane (DMM) is produced via acidcatalyzed reaction of methoxymethanol (MM) intermediate with methanol.…”

“…produced via consecutive bifunctional oxidation-acid-oxidation catalysis [10][11][12]. This implies that both well-controlled oxidation and acid catalytic functions are required for the selective conversion of methanol to DMM.…”

“…For the direct and selective conversion of methanol to DMM, RuO 2 [7], ReO x -based catalyst [8], V 2 O 5 /TiO 2 [9], and supported heteropolyacid (HPA) catalyst [10] have been investigated. In particular, a supported HPA catalyst [11] and a modified HPA catalyst [12] have been reported to show high DMM selectivity due to their favorable bifunctional acid and redox functions. The selective conversion of methanol to DMM has also been investigated as a model reaction to probe the acid and redox catalytic properties of HPA catalysts [10][11][12].…”

“…In particular, a supported HPA catalyst [11] and a modified HPA catalyst [12] have been reported to show high DMM selectivity due to their favorable bifunctional acid and redox functions. The selective conversion of methanol to DMM has also been investigated as a model reaction to probe the acid and redox catalytic properties of HPA catalysts [10][11][12].…”

Preparation of Mesoporous V–Ce–Ti–O for the Selective Oxidation of Methanol to Dimethoxymethane

“…It has been reported that the selective oxidation of methanol to DMM might involve two steps: (1) oxidation of methanol to formaldehyde on redox sites and (2) condensation of formaldehyde produced with additional methanol to DMM on acidic sites. Thus, the strengths of surface acidity and redox ability of a catalyst were two important aspects in determining the selective oxidation of methanol to DMM [36][37][38][39][40].…”