Guest–host interactions of a rigid organic molecule in porous silica frameworks

Abstract

Molecular-level interactions at organic-inorganic interfaces play crucial roles in many fields including catalysis, drug delivery, and geological mineral precipitation in the presence of organic matter. To seek insights into organic-inorganic interactions in porous framework materials, we investigated the phase evolution and energetics of confinement of a rigid organic guest, N,N,N-trimethyl-1-adamantammonium iodide (TMAAI), in inorganic porous silica frameworks as a function of pore size (0.8 nm to 20.0 nm). We used hydrofluoric acid solution calorimetry to obtain the enthalpies of interaction between silica framework materials and TMAAI, and the values range from −56 to −177 kJ per mole of TMAAI. The phase evolution as a function of pore size was investigated by X-ray diffraction, IR, thermogravimetric differential scanning calorimetry, and solid-state NMR. The results suggest the existence of three types of inclusion depending on the pore size of the framework: single-molecule confinement in a small pore, multiple-molecule confinement/adsorption of an amorphous and possibly mobile assemblage of molecules near the pore walls, and nanocrystal confinement in the pore interior. These changes in structure probably represent equilibrium and minimize the free energy of the system for each pore size, as indicated by trends in the enthalpy of interaction and differential scanning calorimetry profiles, as well as the reversible changes in structure and mobility seen by variable temperature NMR.mesoporous silica | thermodynamics | porous materials K nowing both the structure and molecular mobility of guest matter in nanosized pores and channels, which often differ from those in the bulk unconfined material or solution, is essential for fundamental understanding of processes in both science and technology, with applications including natural processes such as biomineralization (1-3) and membrane transport (4, 5), engineering processes such as oil recovery (6-8), CO 2 sequestration (9-11), catalysis (12-14), and biomedical processes including diagnostics and drug delivery (15-17). Most of the pioneering research has used soft matter as guests, including gas and liquid phases, low-melting point organic solids, and longchain polymers (18)(19)(20).In our earlier studies, various calorimetric methods have been designed to investigate guest-host interactions. Piccione et al.(21) developed a novel system for hydrofluoric acid (HF) solution calorimetry to study the interactions of four different silica zeolite frameworks with several quaternary ammonium structure-directing agents (SDAs). The enthalpies of interaction were measured to be −32.0 to −181.0 kJ per mole of SDA. Slightly stronger interactions were found by Trofymluk et al. (22) for mesoporous silica phases containing long-chain molecules. Recently, Wu et al. (23) measured the enthalpy of interaction of various small molecules with mesoprous silicas using immersion calorimetry. The hydration enthalpies of a series of cation exchanged aluminosilicate or gallosilicate ze...

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