NMR probe for heterogeneous catalysis with isolated reagent flow and magic-angle spinning

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. The variable-temperature capabilities were shown by monitoring the change in the ²⁰⁷Pb chemical shift of Pb(NO₃)₂ as a function of temperature. Temperatures >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. The observed products varied depending on whether flow vs. batch conditions were used. Under flow conditions only dimethyl ether was observed. In previous studies under sealed (i.e. batch) conditions, an equilibrium was observed between methanol and dimethyl ether.