Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ∼15% Cu and ∼85% Cu. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu to Cu. The products, methanol, dimethyl ether, and CO, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45-60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

Organic structure-directing agents (OSDAs) have been widely used for the synthesis of zeolites. In most cases, OSDAs are occluded in zeolites as an isolated cation or molecule geometrically fitted within the zeolite cavities. This is not the case for zeolite beta synthesized by using tetraethylammonium (TEA(+)) cation as an OSDA, in which a cluster/aggregate of ca. six TEA(+) cations is occluded intact in the cavity (i.e., the channel intersection) of zeolite beta. The structure direction of TEA(+) in such a nontypical, clustered mode has remained elusive. Here, zeolite beta was hydrothermally synthesized using TEA(+) in the absence of other alkali metal cations in order to focus on the structure-directing behaviors of TEA(+) alone. The solid products formed throughout the hydrothermal synthesis were analyzed by an array of characterization techniques including argon adsorption-desorption, high-energy X-ray total scattering, Raman and solid-state NMR spectroscopy, and high-resolution transmission electron microscopy. It was revealed that the formation of amorphous TEA(+)-aluminosilicate composites and their structural, chemical, and textural evolution toward the amorphous zeolite beta-like structure during the induction period is vital for the formation of zeolite beta. A comprehensive scheme of the formation of zeolite beta is proposed paying attention to the clustered behavior of TEA(+) as follows: (i) the formation of the TEA(+)-aluminosilicate composites after heating, (ii) the reorganization of aluminosilicates together with the conformational rearrangement of TEA(+), yielding the formation of the amorphous TEA(+)-aluminosilicate composites with the zeolite beta-like structure, (iii) the formation of zeolite beta nuclei by solid-state reorganization of such zeolite beta-like, TEA(+)-aluminosilicate composites, and (iv) the subsequent crystal growth. It is anticipated that these findings can provide a basis for broadening the utilization of OSDAs in the clustered mode of structure direction in more effective ways.

No abstract

Cu-exchanged zeolites are known to be active in the selective oxidation of methane to methanol at moderate temperatures. Among them, Cu-exchanged mordenite (MOR) is the system that has so far shown the highest methanol yield per Cu atom. This high efficiency is attributed to the ability of MOR to selectively stabilize an active tricopper cluster with a [Cu3(μ-O)3]2+ structure when activated in the presence of O2 at high temperatures. In this study, we investigate the elementary steps in the formation of [Cu3(μ-O)3]2+ by in situ X-ray absorption spectroscopy and ultraviolet–visible spectroscopy. We demonstrate that the Cu cations undergo a series of thermally driven steps during activation that precede the formation of the active oxidizing species. We hypothesize that the thermal formation of highly mobile Cu+ species by autoreduction of Cu2+ in an inert gas is essential to enable the reorganization of Cu ions in MOR, which is necessary for the formation of a reduced precursor of [Cu3(μ-O)3]2+. Such a precursor can be oxidized in the presence of strong oxidantssuch as O2 and N2Oto form active [Cu3(μ-O)3]2+ at temperatures as low as 50 °C.

A versatile method for the formation of monodisperse, bridged silsesquioxane nanoparticles with hollow interiors and porous shells has been developed using silica nanospheres as templates. Tunable size and shell thickness, as well as high surface areas and large pore volumes of the hollow particles, allow for practical application of these nanoparticles in many fields.

Cu-zeolites are able to directly convert methane to methanol via a three-step process using O 2 as oxidant. Among the different zeolite topologies, Cu-exchanged mordenite (MOR) shows the highest methanol yields, attributed to a preferential formation of active Cu–oxo species in its 8-MR pores. The presence of extra-framework or partially detached Al species entrained in the micropores of MOR leads to the formation of nearly homotopic redox active Cu–Al–oxo nanoclusters with the ability to activate CH 4 . Studies of the activity of these sites together with characterization by 27 Al NMR and IR spectroscopy leads to the conclusion that the active species are located in the 8-MR side pockets of MOR, and it consists of two Cu ions and one Al linked by O. This Cu–Al–oxo cluster shows an activity per Cu in methane oxidation significantly higher than of any previously reported active Cu–oxo species. In order to determine unambiguously the structure of the active Cu–Al–oxo cluster, we combine experimental XANES of Cu K- and L-edges, Cu K-edge HERFD-XANES, and Cu K-edge EXAFS with TDDFT and AIMD-assisted simulations. Our results provide evidence of a [Cu 2 AlO 3 ] 2+ cluster exchanged on MOR Al pairs that is able to oxidize up to two methane molecules per cluster at ambient pressure.

No abstract

A facile approach is demonstrated for the preparation of hollow mesoporous silica nanoparticles with large pore volume and high dispersibility. Core–shell nanoparticles consisting of iron oxide (α-Fe2O3) core and mesostructured silica–surfactant composite shell are treated with hexamethyldisiloxane under acidic conditions to achieve capping of silanol groups and removal of both surfactants and the Fe2O3 cores in a single step.