High-Temperature and High-Pressure In situ Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Spectroscopic analysis of samples is an analytical tool used to gain valuable information about materials of interest such as their chemical state, structure, or reactivity. In a simplistic view, nuclear magnetic resonance (NMR) is one such technique that utilizes a strong magnetic field to manipulate the spin state of atomic nuclei to better understand the chemical environment of the species of interest. The nuclear spin state refers to the relative direction of the magnetic moment induced by the motion of the spinning nucleus, a positively charged particle. In the absence of a magnetic field, the nuclear spins are randomly oriented but in the presence of a magnetic field, nuclear spins preferentially align with the external field of the magnet in a low energy spin state. This splitting of spin states to discrete energy values is known as the Zeeman effect. The difference between these energy levels (ΔE) is modeled by Equation 1:

where h is Plank’s constant, B₀ is the strength of the external magnetic field and γ is the gyromagnetic ratio of the nucleus. The chemical environment of these spins also applies slight perturbations to these energy levels. Radio waves of corresponding frequencies can be used to excite the nuclei, which generates a transverse magnetization due to spins gaining phase coherence as longitudinal magnetization (based on the population of spins in parallel and anti-parallel states) is decreased. As the nuclei continue precessing about the axis of the magnetic field, the rotating magnetic movement creates a magnetic field that is also rotating and generating an electric field. This field modulates the electrons in the NMR detection coil, generating the NMR signal. Slight differences in the chemical environment of the nuclei in the sample affect the frequencies detected in the coil.

NMR analysis of solid samples introduces complexities not found in fluids. In fluids, the molecules tumble at fast rates, averaging the chemical environment spatially around the nuclei. In solid samples, no such averaging effect occurs, introducing an orientation-dependent chemical environment and broad spectral lines in the NMR signal. To mitigate these challenges, a technique known as magic angle spinning (MAS) is employed^1,2. In MAS NMR, the samples are quickly rotated (several kilohertz) at an angle of 54.7356° with respect to the external magnetic field using an external spinning mechanism to address the orientation-dependent (anisotropic) interactions of NMR. This substantially narrows the NMR features and enhances the spectral resolution by averaging the orientation-dependent terms of the chemical shift anisotropy, dipolar interactions, and quadrupolar interactions. Two notable exceptions do hinder the line narrowing abilities of MAS NMR. The first is strong homonuclear coupling sometimes present in ¹H NMR that requires high spinning speeds (~70 kHz) to remove. However, the significantly elevated temperatures of the high temperature applications will greatly suppress the ¹H homonuclear interaction by imparting enhanced thermal motion such that a much reduced sample spinning rate can be utilized for significantly enhanced spectral resolution. Furthermore, with the technology continuously evolving, rotors with smaller diameters can now be fabricated to achieve spinning rates far exceeding 5 kHz, which helps to further suppress the ¹H homonuclear dipolar interactions. The second exception is residual second-order quadrupolar interactions for nuclei with spin that exceeds one-half since only the first order term is eliminated at the magic angle, leaving more complex lineshapes that can only be improved by stronger external magnetic fields. It should be emphasized that 2D MQMAS techniques can be readily incorporated into the current technology so that a true isotropic chemical shift spectrum can be obtained in a similar way as to the standard MQMAS experiments³.

MAS NMR has enabled detailed characterization of solid materials, strengthening the quality of observations. However, the necessity of spinning the samples in NMR rotors (the sample holder) at high rates also imposes challenges in conducting experiments at elevated temperatures and pressures which may be more relevant to the conditions of interest. It may, at times, be desirable to examine materials under conditions that are relatively harsh for NMR rotors. A number of efforts have successfully adapted liquid-state NMR technologies to conduct high-temperature, high-pressure NMR^4,5,6,7; however, commercial rotor caps used for solid-state MAS NMR may be expelled from the rotor at high pressures, causing significant damage to the equipment. Such effects may be compounded by examining a decomposition reaction that greatly increases the pressure in the sample holder. As such, new designs are required to effectively and safely conduct in situ NMR experiments. For example, the rotor must adhere to several qualities for effective use in MAS NMR, namely non-magnetic, lightweight, durable, temperature resistant, low NMR background material, sealable, high-strength, and chemical resistant. The pressures the rotor must withstand are quite large. Not only must the rotor withstand the pressure of the sample contained within (e.g., high-pressure gas), the rotation of the device imparts centrifugal force which has its own contribution to the total system pressure⁸, P_T, by equation 2:

R_I and R_O are the inner and outer rotor radii, respectively, ω is the rotational frequency in radians per second, and P_s is the sample pressure.

A number of strategies have been developed to address these concerns⁹. Early examples resembled flame-sealed tubes^10,11,12 or polymer inserts^13,14, which were insufficient for extended, fine-controlled operation at elevated temperatures and pressures. Iterations to rotor designs have suffered from limitations in the maximum operating temperature imparted by the use of epoxy or sample volume reductions from ceramic inserts^8,15,16. A recent technology reduces unit production costs by employing simple snap-in features in a commercial rotor sleeve, but offers relatively less control over the conditions with which it can operate¹⁷. The design employed herein is an all-zirconia, cavern-style rotor sleeve milled with a threaded top¹⁸. A cap is also threaded to allow for a secure seal. Reverse threading prevents sample rotation from loosening the zirconia cap and an O-ring constitutes the sealing surfaces. This rotor design is visible in Figure 1 and similar rotors and instructions to make them have been patented¹⁹. Such a strategy enables high mechanical strength, chemical resistance, and temperature tolerance.

These designs are suitable for temperatures and pressures of at least 250 °C and 100 bar, limited in temperature by readily-available NMR probe technology. When combined with specialized sample preparation equipment, it represents a truly powerful technique that has been employed for far-reaching applications as carbon sequestration, catalysis, energy storage, and biomedicine²⁰. Such equipment includes a way to pretreat the solid materials to remove unwanted surface species such as water. A furnace is often employed for this step. A dry box is typically used to load the solid samples into the NMR rotor. From there, the rotor is transferred into an exposure device which enables the rotor to be opened under a tightly controlled atmosphere to load a desired gas or mixture into the rotor. Such a device is depicted in Figure 2.