Development of an isolated flow variable-temperature magic-angle spinning (MAS) nuclear magnetic resonance (NMR) probe for heterogeneous catalysis studies and high-temperature high-speed 19F MAS NMR techniques applied to fluoropolymers.

摘要

Commercially available fluoroelastomers are often based on copolymers of vinylidene fluoride (VF2)/hexafluoropropylene (HFP), VF 2/tetrafluoroethylene (TFE), or terpolymers containing all three monomer units. These fluoroelastomers have remarkable resistance to flames, solvents, heat, and oxidation, but are often insoluble or intractable, making analysis of microstructure and monomer composition difficult. To analyze these compounds variable-temperature (250°C) solid-state 19F NMR spectroscopy with high-speed magic angle spinning (25 kHz MAS) was used to study the microstructure and monomer composition for copolymers containing VF2/HFP, VF 2/TFE and terpolymers containing all three monomers.; We investigated the dynamics of hydrocarbons produced from the conversion of methanol to gasoline (MTG) over zeolite HZSM-5 through interactions with the catalysts. Using two-dimensional exchange NMR and rotational resonance. Simulations have shown that for silicalite and HZSM-5 linear alkanes are relatively free to move in all channels of the zeolite, while branched alkanes are located primarily in the intersections of the zeolite channels. These computer simulation results suggest that most of the time the branched alkane molecule is located in the intersection of the channels but occasionally hops from one intersection to another. We present NMR data which indicates that these simulations apply to systems where several species are present under actual reaction conditions.; We have developed an isolated flow MAS NMR probe to enable simultaneous observation of events occurring on a catalytic surface with characterization of the effluent gas using an external analytical instrument. The probe design is unique in that it uses ceramic ball bearings to support the rotor. The spinning and spectroscopic capabilities of the probe were proven by obtaining a 13C CP/MAS NMR spectrum of hexamethylbenzene at MAS rates of 2 kHz. The flow capabilities of the probe were demonstrated by observing methanol adsorption onto zeolite HZSM-5 and by studying conformational changes of a vapochromic sensor material when it was exposed to volatile organic compounds. Variable-temperature capabilities >300°C were achieved for the probe. The reaction of methanol to dimethyl ether on zeolite HZSM-5 was used to demonstrate the ability of the probe to study heterogeneous catalysis reactions in situ.