(768f) NMR of Chemical Reactions at Elevated Process Conditions

Nuclear magnetic resonance (NMR) can be used to study chemical reactions in-situ and in real time. This is not common practice at conditions of high temperature and high pressure as the design of an NMR compatible reactor is not straightforward. In this study, we report data from oxidation, hydration, hydrogenation and hydrocracking. We use sealed glass capillaries (ID = 2.6 mm, OD = 7 mm) to hold the sample plus the required oxygen or hydrogen and any catalyst. To seal the capillaries, they are submerged in liquid nitrogen meaning that any sample with a vapor pressure below 0.1 MPa (1 bar) at 77 K can be loaded and sealed. In addition to adding molecules that either freeze or condense at 77K (including oxygen), hydrogen gas can be added by using a metal hydride that decomposes to release hydrogen at reaction conditions. This means that it is possible to study oxidation, hydration, hydrogenation, hydrogenolysis and isomerization reactions at process conditions with NMR.

The experiments discussed are performed using a low field, permanent magnet with a field strength of 1 T (43 MHz proton frequency). The magnet has x, y, z, and z² shims which produce a field homogeneity of approximately 3 Hz (0.07 ppm), full width half maximum (FWHM). Though this low field strength is not typically used for high resolution NMR, it is possible to track formation of new species as new peaks appear when the reaction progresses. Lower magnetic fields are readily available using permanent magnets or electromagnets which are better suited to online process monitoring than the more typical high-field, high resolution superconducting magnets.

We report, as an example, the common hydration reaction of cyclohexene and water reacting to produce cyclohexanol. Initially, the spectrum will show three peaks for cyclohexene and one peak for water. When cyclohexanol appears, a new peak for the H^* of CH^*OH is present, well separated from cyclohexene and water peaks. As the sample is heated to drive the reaction, spectra are acquired sequentially to track the formation of new peaks which indicate the formation of cyclohexanol. This gives insight into the degree and rate of reaction for a given set of conditions: reactant concentration, catalyst, temperature and pressure.

Acquiring spectra at process conditions yields insight into the rate of reaction in real time. There is no need to cool the reaction or sample portions of the material to measure the products at a given time. As the experiments can be performed at process conditions, it may be possible to detect intermediates that will not be present after a sample is cooled because they condense, evaporate or further react.

The idea of using NMR to study chemical reactions is certainly not a new idea; however, the ability to do this at high temperature and high pressure process conditions offers the possibility of new information about a variety of industrially important reactions. For a wide range of refinery and commodity reactions, the low-field of 1 T provides sufficient resolution to monitor the reactions. The low field magnet brings us one step closer to the ultimate goal of online process monitoring.