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

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, 2] have recently emerged as interesting catalysts for organic reactions involving bifunctional pathways. We recently discovered a selective one-step synthesis of dimethoxymethane (CH3OCH2OCH3, DMM, methylal) by oxidation of methanol at low temperatures (453–493 K) on unsupported and SiO2-supported H3+nPVnMo12 nO40 (n= 0– 4) Keggin clusters. The yields and selectivities (based on the absence of dimethyl ether) of DMM resemble those obtained using supported ReOx catalysts, the only catalysts that enable the formation of DMM in substantial yields. Redox and Brønsted acid sites are required for DMM synthesis, and the reaction involves oxidative dehydrogenation of CH3OH to formaldehyde (HCHO), acid-catalyzed acetalization of CH3OH/HCHO mixtures, and condensation of hemiacetal or methoxymethanol intermediates (formed in acetalization reactions) with CH3OH to form DMM. Br8nsted 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 CH3OH dehydration. These reactions are catalyzed by strongly acidic protons in H3+nPVnMo12 nO40 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 CH3OH, but forms DMM more slowly than CH3OH. [3] Thus, the formation of DME through these side reactions necessitates longer residence times to achieve high yields of DMM from CH3OH. 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. 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-butylpyridine) during catalytic reactions of mixtures of CH3OH and O2. This 2,6-di-tert-butylpyridine titrant can protonate Brønsted acid sites, but it cannot interact with Lewis acid sites because of steric constraints near the N atom. Its essentially hydrophobic character also prevents its dissolution and migration into secondary structures of Keggin clusters. This result is in contrast with more polar pyridine titrants, which dissolve and penetrate into these secondary structures. Thus, uptake of 2,6-di-tert-butylpyridine during CH3OH reactions (per KU) reflects the number of accessible protons, and for a given H3+nPVnMo12 nO40 stoichiometry, the fraction of the Keggin structures accessible at external surfaces in supported and unsupported secondary structures. We note that such titrations must be carried out during the reaction, because of the known ability of various reactants to solvate and expose internal regions within secondary packing structures of Keggin clusters to varying degrees. The number of 2,6-di-tert-butylpyridine molecules adsorbed during reactions of CH3OH and O2 at 453 K on H5PV2Mo10O40/SiO2 (0.28 KUnm 2 surface density on SiO2) increased with time and reached saturation at 1.2 H per KU after about 12 C 10 s (Figure 1). This value corresponds to a nominal fractional dispersion of 0.24, on the basis of the expected H/KU stoichiometry; we note, however, that some of the protons in the stoichiometric starting cluster may have been removed during condensation reactions of OH groups in solvated Keggin clusters with silanols on anchoring to SiO2. H/KU ratios decreased from 1.6/1 to 0.7/1, which corresponds to a decrease in fractional KU dispersion from 0.32 to 0.15, as the KU surface densities increased from 0.1 to 0.65 KUnm 2 on H5PV2Mo10O40/SiO2 samples. This value was 0.02 H per KU for bulk H5PV2Mo10O40. The rates of DMM synthesis (per KU) decreased in parallel with this decrease in fractional KU dispersion as the surface density increased from 0.1 to 0.65 KUnm 2 (Figure 2). This excellent correlation between rates and titrant uptake for all samples, including an unsupported version of this Keggin composition, indicates that 2,6-di-tert-butylpyridine predominately titrates those Keggin structures that participate in bifunctional DMM [*] Prof. Dr. E. Iglesia, Dr. H. Liu, N. Bayat Department of Chemical Engineering University of California at Berkeley, and Chemical Sciences Division, E.O. Lawrence Berkeley National Laboratory Berkeley, CA 94720 (USA) Fax: (+1)510-642-4778 E-mail: [email protected]