TRANSIENT METHODS FOR IN-SITU NMR OF REACTIONS ON SOLID CATALYSTS USING TEMPERATURE JUMPS
TRANSIENT METHODS FOR IN-SITU NMR OF REACTIONS ON SOLID CATALYSTS USING TEMPERATURE JUMPS
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DOI：
10.1021/ac00114a034
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发表时间：
1995-09-15
影响因子：
7.4
通讯作者：
HAW, JF
中科院分区：
文献类型：
--
作者：
FERGUSON, DB;HAW, JF
评估了两种进行温度跃变魔角旋转 (MAS) NMR 实验的方法是否适用于原位快速化学反应研究。空间和时间的温度分布是通过使用 Pb-207 化学位移测温法和熔化转变来确定的。之前报道的硝酸铅位移温度计的有用范围使用标准加热或冷却气体方法扩展到 123-548 K，具有合理的线性度和 0.775 +/- 0.007 ppm/K 的斜率。如前所述，通过将 CO2 激光器发出的 10.6μm 辐射聚焦到石英 MAS 转子的外部，可以实现高温和明显的温度跳跃；然而，用这种方法观察到的空间温度分布很大。实现温度跳跃的第二种方法是基于石英或氧化锆 MAS 转子上的铂金属涂层的射频感应加热。该方法先前已针对溶液样品的静态加热进行了演示，在固体的 MAS 研究中显示出静态加热和温度跳跃的巨大潜力。我们的感应加热实验是使用未经修改的双谐振、单线圈 MAS 探头进行的，将来自 H-1 频率的连续 130 kHz 非谐振信号注入解耦器通道。虽然所使用的光谱仪通常能够执行 100 W 的连续波 (CW) 加热实验，但标准探头设计将 CW 加热功率限制为类似于 20 W；因此，这里报告的结果低估了该方法的潜力。由于有效感应加热的转子体积是激光实验中的几倍，因此前者的空间温度梯度明显小于后者。例如，在 373 K 下静态加热时，Pb-207 测温显示感应加热的分布为 +/-8 K，而激光加热器的分布为 > +/-50 K，即使激光实验中的样品尺寸较小，使用测温技术的组合，可以建立 30-60 秒的加热组，用于甲醇在酸性沸石催化剂上的反应的原位 1H 和 C-13 NMR 研究HZSM-5。将 199.7 MHz 下的 18 W 功率应用至 5 毫米外径。铂涂层氧化锆转子在 30 秒或更短的时间内将样品从初始值 298 K 加热到完全平衡的最终状态 623 K。在实验的动态部分每 3 s 采集一次单次 H-1 光谱，淬火至 298 K 后采集的 C-13 光谱与催化反应机理的既定特征一致。
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.