(179k) Probe Design for NMR of Chemical Reactions at High Temperature and Pressure

We report here nuclear magnetic resonance (NMR) apparatus for the study of chemical reactions at elevated temperature and pressure in situ and in real time. NMR is a measurement that is frequently used to evaluate the chemical structure of a sample. In addition to chemical structure, other parameters are also available such as diffusion and molecular relaxation. An interesting application for NMR is to follow a chemical reaction from changes in the NMR spectrum. To do this, elevated temperatures and pressures are often required. This presentation will discuss two probe designs that allow us to follow reactions at elevated conditions, opening a window to refinery and commodity chemical processing reactions.

The first probe is a titanium alloy vessel with an internal free-piston. The liquid sample is loaded into a ceramic sample tube with the free-piston sealed to the end of the ceramic. The free piston equalizes the pressure inside and outside of the ceramic so the pressure is held only by the titanium vessel and not the ceramic tube. The titanium holds the argon pressurizing gas and is non-magnetic. The radio frequency (rf) coil is inside the titanium, wrapped directly around the sample holder for maximum signal. There are also two heaters inside the titanium vessel, to heat the ceramic vessel and sample. This probe has been tested to a temperature of 600 °C and a pressure of 40 MPa.

An advantage of the titanium probe is independent control of temperature and pressure. The probe is pressurized with argon from a high pressure argon cylinder. The heaters are controlled by a temperature regulator regulating at a thermocouple sealed inside the titanium vessel. With this design, the sample must be loaded as a liquid at ambient conditions.

In the second probe, the sample is held in a sealed glass capillary. The current capillaries are 7 mm OD, 2.6 mm ID. The sample is heated with forced air flowing over a wire-wound heater situated below the sample. The temperature is controlled by a regulator sensing a thermocouple located at the top of the heater. This probe can run up to 400 °C and the capillaries are routinely able to hold up to 7 MPa. The pressure is determined by the temperature, as the sealed samples are at constant density. An advantage of the glass capillary is simplicity. All of the pressure-holding task is the “responsibility” of the sealed capillary. Additionally, the sealed capillary approach provides the option to prepare samples that are both liquid and gas at atmospheric conditions. For example, we can load liquid oxygen or liquid carbon monoxide using liquid nitrogen to cool the tube while it is flame-sealed. Hydrogen gas can be provided from metal-hydrides that release hydrogen at reaction temperatures. Thus, liquids as well as normally-gaseous samples (O₂, CO or H₂) can be loaded.

Considering a two phase sample, liquid and gas, at room temperature, NMR experiments can be acquired on either the liquid or on the vapor by moving the sample tube within the rf coil of the probe.

NMR measurements of the chemical spectra at increasing temperatures and/or pressures can show formation of new chemical species. Measurements of the diffusion coefficient can give insight into the phase of the fluid or solution, with a much faster diffusion coefficient for gas than liquid. Using these measurements, the phase transition at different values of temperature and pressure can be plotted. The T₁ relaxation of a sample can also be measured in this vessel and will, again, indicate the phase of the sample and can also give insight into molecular dynamics. All of these measurements can be acquired in liquid, gas or above the critical point, in the supercritical region.

These probes yield information about a reaction at process conditions, both high temperature and pressure. It is difficult or impossible to acquire these data with external analysis when sampling from the reaction vessel where cooling and depressurization of the sample are required before analysis of the products. Phase fractionation upon return to ambient conditions can result in solids precipitating and gases escaping from the external analysis. In addition, the in situ NMR measurements may capture transient chemical intermediates. In this presentation, the design of both probes and the types of data that can be acquired will be discussed.