In-situ Gas Dosed Powder Diffraction Under Industrially Relevant Conditions

The adsorption of small molecules onto functionalized, high surface area microporous materials is important for the advancement of industrial and environmental processes ranging from catalysis and chemical separations, to CO2 sequestration and energy storage. Over the past several years we have focused our research efforts on understanding the molecular interactions of these small molecules with a variety of microporous materials using in-situ powder diffraction methods to correlate structure with chemical properties. Background will be given on the design of gas dosing apparatus for in-situ diffraction studies at synchrotron X-ray and neutron powder beamlines. The result is that accurate doses can be made per quantity of interest (moles of cations, per unit cell, per pore, etc.), or under high pressures (100 bar), and/or chemical reactions can be followed versus temperature/pressure. Several of our recent investigations of CO2/N2/CH4 sorption in cation-exchange zeolites including Zeolite A (5A) and CHA are presented. While many industrial processes use zeolites to carry out these functions, more emphasis has been placed on metal-organic frameworks (MOFs) on late since their properties can be tuned by varying the synthetic components. A number of studies on an isostructural series, M-MOF-74, have been considered investigating why certain functionalization leads to increased specificity for applications such as CO2, O2, CO, and hydrocarbon separations. The ultimate goal is to use the knowledge gained to improve the design of new MOF materials.

Synthesis, structure and water sorption in Zr metal-organic frameworks

Metal-organic frameworks (MOFs) based on zirconium secondary building units (SBUs) have proven to have great thermal and chemical stability,[1,2] which make them ideal for their use in different applications. We have prepared a series of six new MOFs made from the Zr₆O₄(OH)₄(-CO₂)_n secondary building units (n = 6, 8, 10, or 12) and variously shaped carboxyl organic linkers to make extended porous frameworks, with the aim of studying their performance as water adsorbents. Thus, we have evaluated the water adsorption properties of these new MOFs and other reported porous materials to identify the compounds with the most promising materials for use in applications such as thermal batteries or delivery of drinking water in remote areas. An X-ray single-crystal and a powder neutron diffraction study reveal the position of the water adsorption sites in one of the best performing materials, and highlight the importance of the intermolecular interactions between adsorbed water molecules within the pores.

Unprecedented adsorption of molecular hydrogen in the porous hydride framework

"Recently the first porous hydride, gamma-Mg(BH4)2, featuring so-called ""borohydride framework"" and capable to store reversibly guest species was discovered [1]. This example clearly shows that the covalently bound hydride anions, such as borohydride, can act as directional ligands, capable to form molecular and polynuclear complexes, as well as framework structures typically occurring in classical coordination chemistry. Various small molecules are reversibly absorbed in gamma-Mg(BH4)2. In this work we show that molecular hydrogen and nitrogen have different adsorption sites in gamma-Mg(BH4)2, leading to different capacities on saturation and to different H2 and N2 BET areas. Only up to 0.66 N2 molecules are adsorbed per Mg atom, but the saturation capacity is double for the smaller hydrogen molecule. Moreover, at higher pressures, the second hydride phase forms with unprecedented hydrogen content of ~22 weight % (!). The density of hydrogen adsorbed into the pores is much higher than in liquid hydrogen, having no analogues among other porous systems. On the technical side, we will illustrate how in-situ diffraction at neutron and synchrotron sources allows to follow adsorption isobars, aiming for extraction of isosteric heats of adsorption directly from diffraction data, as well as for clarifying the microscopic mechanisms in terms of guest-host and guest-guest interactions."