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

Two methods for carrying out magic angle spinning (MAS) NMR experiment s with temperature jumps were evaluated for their suitability for stud ies of rapid chemical reactions in situ. Temperature profiles, both sp atial and temporal, were determined through the use of Pb-207 chemical shift thermometry and melting transitions, The useful range of the pr eviously reported lead nitrate shift thermometer was extended using st andard heated or cooled gas methods to 123-548 K with reasonable linea rity and a slope of 0.775 +/- 0.007 ppm/K. As shown previously, high t emperatures and appreciable temperature jumps could be achieved by foc using 10.6-mu m radiation from a CO2 laser onto the outside of a quart z MAS rotor; however, the spatial temperature distribution observed wi th this method was large, The second method of achieving temperature j umps was based on radio frequency inductive heating of platinum metal coatings on quartz or zirconia MAS rotors. This method, demonstrated p reviously for static heating of solution samples, shows considerable p otential for static heating and temperature jumps in MAS studies of so lids. Our inductive heating experiments were performed using an unmodi fied double resonance, single coil MAS probe by injecting a continuous signal 130 kHz off-resonance from the H-1 frequency into the decouple r channel. While the spectrometer used was generally capable of perfor ming continuous wave (CW) heating experiments with 100 W, the standard probe design limited CW heating power to similar to 20 W; thus, the r esults 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 s ignificantly smaller for the former compared to the latter. For exampl e, with static heating at 373 K, Pb-207 thermometry revealed a distrib ution of +/-8 K with inductive heating vs > +/-50 K with the laser hea ter, even though the sample size was smaller in the laser experiment, Using a combination of thermometry techniques, it was possible to esta blish 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. Appli cation of 18 W at 199.7 MHz to a 5-mm-o.d. Pt-coated zirconia rotor he ated 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 acquir ed every 3 s during the dynamic part of the experiment and C-13 spectr a acquired after quenches to 298 K were consistent with established fe atures of the catalytic reaction mechanism.