David C. Calabro

1H MAS NMR (Magic-Angle Spinning Nuclear Magnetic Resonance) Techniques for the Quantitative Determination of Hydrogen Types in Solid Catalysts and Supports

Distinct hydrogen species are present in important inorganic solids such as zeolites, silicoaluminophosphates (SAPOs), mesoporous materials, amorphous silicas, and aluminas. These H species include hydrogens associated with acidic sites such as Al(OH)Si, non-framework aluminum sites, silanols, and surface functionalities. Direct and quantitative methodology to identify, measure, and monitor these hydrogen species are key to monitoring catalyst activity, optimizing synthesis conditions, tracking post-synthesis structural modifications, and in the preparation of novel catalytic materials. Many workers have developed several techniques to address these issues, including 1H MAS NMR (magic-angle spinning nuclear magnetic resonance). 1H MAS NMR offers many potential advantages over other techniques, but care is needed in recognizing experimental limitations and developing sample handling and NMR methodology to obtain quantitatively reliable data. A simplified approach is described that permits vacuum dehydration of multiple samples simultaneously and directly in the MAS rotor without the need for epoxy, flame sealing, or extensive glovebox use. We have found that careful optimization of important NMR conditions, such as magnetic field homogeneity and magic angle setting are necessary to acquire quantitative, high-resolution spectra that accurately measure the concentrations of the different hydrogen species present. Details of this 1H MAS NMR methodology with representative applications to zeolites, SAPOs, M41S, and silicas as a function of synthesis conditions and post-synthesis treatments (i.e., steaming, thermal dehydroxylation, and functionalization) are presented.

The effects of pyridine on the structure of B-COFs and the underlying mechanism

Previous work demonstrated the ability of a trace amount of pyridine to stabilize Covalent Organic Frameworks (COF)-5 and -10 in humid air. Pyridine was found to form a mixture of Lewis and Bronsted B[4] Py–B complexes in addition to the un-complexed B[3] sites in the framework structures. Further research has shown that higher doses of pyridine convert all remaining B[3] in COF-5/-10 to Lewis B[4] and bring about the total and irreversible structural decomposition of COF-5 and COF-10. The results suggest that the accumulated strain in the five-member rings of COF-5/-10 resulting from the formation of tetrahedrally-distorted B[4] sites at high pyridine loadings, may explain the decomposition of these structures. Alternatively, COF-1 is unstable to exposure to humid air at all pyridine loadings tried, but is not unstable to high doses of pyridine. Whereas the same tetrahedrally-distorted B[4] sites are formed in COF-1, in this case the six-membered B3O3 ring can accommodate the accumulated ring strain and retain an ordered structure. A thorough solid state NMR and molecular dynamics investigation has led to a new proposed stabilization mechanism in humid air based on the formation of Bronsted B[4].

Core-Shell Crystals of Porous Organic Cages

Abstract The first examples of core–shell porous molecular crystals are described. The physical properties of the core–shell crystals, such as surface hydrophobicity, CO2 /CH4 selectivity, are controlled by the chemical composition of the shell. This shows that porous core–shell molecular crystals can exhibit synergistic properties that out‐perform materials built from the individual, constituent molecules.