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 obs ervation of events occurring on a catalytic surface with characterization o f the effluent gas using an external analytical instrument. The probe desig n is unique in that it uses ceramic ball bearings to support the rotor. The spinning and spectroscopic capabilities of the probe were proven by obtain ing a C-13 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 compoun ds. The variable-temperature capabilities were shown by monitoring the chan ge in the Pb-207 chemical shift of Pb(NO3)(2) as a function of temperature. Temperatures >300 degrees C were achieved for the probe. The reaction of m ethanol to dimethyl ether on zeolite HZSM-5 was used to demonstrate the abi lity of the probe to study heterogeneous catalysis reactions in situ. The o bserved 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 b etween methanol and dimethyl ether. (C) 1999 Elsevier Science B.V. All righ ts reserved.