TRANSIENT METHODS FOR IN-SITU NMR OF REACTIONS ON SOLID CATALYSTS USING TEMPERATURE JUMPS

Two methods for carrying out magic angle spinning (MAS) NMR experiments with temperature jumps were evaluated for their suitability for studies of rapid chemical reactions in situ. Temperature profiles, both spatial and temporal, were determined through the use of Pb-207 chemical shift thermometry and melting transitions, The useful range of the previously reported lead nitrate shift thermometer was extended using standard heated or cooled gas methods to 123-548 K with reasonable linearity and a slope of 0.775 +/- 0.007 ppm/K. As shown previously, high temperatures and appreciable temperature jumps could be achieved by focusing 10.6-mu m radiation from a CO2 laser onto the outside of a quartz MAS rotor; however, the spatial temperature distribution observed with this method was large, The second method of achieving temperature jumps was based on radio frequency inductive heating of platinum metal coatings on quartz or zirconia MAS rotors. This method, demonstrated previously for static heating of solution samples, shows considerable potential for static heating and temperature jumps in MAS studies of solids. Our inductive heating experiments were performed using an unmodified double resonance, single coil MAS probe by injecting a continuous signal 130 kHz off-resonance from the H-1 frequency into the decoupler channel. While the spectrometer used was generally capable of performing continuous wave (CW) heating experiments with 100 W, the standard probe design limited CW heating power to similar to 20 W; thus, the results reported here understate the potential of the method. Since the volume of the rotor efficiently inductively heated was several times that in the laser experiment, the spatial temperature gradients were significantly smaller for the former compared to the latter. For example, with static heating at 373 K, Pb-207 thermometry revealed a distribution of +/-8 K with inductive heating vs > +/-50 K with the laser heater, even though the sample size was smaller in the laser experiment, Using a combination of thermometry techniques, it was possible to establish 30-60-s heating regiments for in situ 1H and C-13 NMR studies of the reactions of methanol on the acidic zeolite catlyst HZSM-5. Application of 18 W at 199.7 MHz to a 5-mm-o.d. Pt-coated zirconia rotor heated the samples from an initial value of 298 K to a final state fully equilibrated at 623 K in 30 s or less. Single-shot H-1 spectra acquired every 3 s during the dynamic part of the experiment and C-13 spectra acquired after quenches to 298 K were consistent with established features of the catalytic reaction mechanism.