Monitoring Crystallization Processes in Confined Porous Materials by Dynamic Nuclear Polarization Solid-State Nuclear Magnetic Resonance

Abstract

Establishing mechanistic understanding of crystallization processes at the molecular level is challenging, as it requires both the detection of transient solid phases and monitoring the evolution of both liquid and solid phases as a function of time. Here, we demonstrate the application of dynamic nuclear polarization (DNP) enhanced NMR spectroscopy to study crystallization under nanoscopic confinement, revealing a viable approach to interrogate different stages of crystallization processes. We focus on crystallization of glycine within the nanometric pores (7-8 nm) of a tailored mesoporous SBA-15 silica material with wall-embedded TEMPO radicals. The results show that the early stages of crystallization, characterized by the transition from the solution phase to the first crystalline phase, are straightforwardly observed using this experimental strategy. Importantly, the NMR sensitivity enhancement provided by DNP allows the detection of intermediate phases that would not be observable using standard solid-state NMR experiments. Our results also show that the metastable β polymorph of glycine, which has only transient existence under bulk crystallization conditions, remains trapped within the pores of the mesoporous SBA-15 silica material for more than 200 days.

Figure 1

Schematic representation of crystallization of glycine within the pores of mesoporous SBA-15 silica material. (a) Tailored mesoporous silica material containing wall-embedded TEMPO radicals (represented in yellow and defined in the box below). (b) Full impregnation of the pores of the mesoporous silica material with an aqueous solution of glycine (shown in light blue). (c) Evaporation of solvent (water) leads to crystallization of glycine within the pores of the material.

Figure 2

Schematic of the experimental DNP NMR strategy to probe the evolution of crystallization processes.

Figure 3

¹³C CPMAS (blue and orange) and T_1ρ-filtered ¹³C CPMAS (green) NMR spectra recorded for mesoporous SBA-15 silica materials with wall-embedded TEMPO radicals and impregnated at room temperature with an aqueous solution of [1-¹³C] glycine (7.7% w/w). The samples were quenched after crystallization times of 15 min (a–c), 25 h (d–f), 48 h (g–i), and 72 h (j–l). Blue spectra were recorded without microwave irradiation and at 98 K; orange and green spectra were recorded with microwave irradiation and at 110 K (the difference in temperature is due to the heating effect of the microwave irradiation). Window (n) shows the superposition of spectra (b) and (c). Window (m) shows the superposition of spectra (e) and (f). In (n) and (m), the intensities are scaled to correspond to the same absolute intensity.

Figure 4

¹³C CPMAS NMR spectrum recorded at 100 K with microwave irradiation for the mesoporous material impregnated with aqueous glycine solution 224 days after impregnation. The signal denoted “Am” is due to the amorphous water/glycine frozen solution. DNP enhancements: ε_DNP = 1 for the amorphous phase and α polymorph; ε_DNP = 1.5 for the β polymorph.

Figure 5

Standard ¹³C CPMAS NMR spectra recorded (at room temperature and with no polarizing agent present) under the following conditions: (a) solid sample recovered by filtration after 3 days of crystallization in a bulk aqueous solution of [1-¹³C] glycine (7.7% w/w) and (b) 3 days after impregnating nonporous silica with an aqueous solution of [1-¹³C] glycine (7.7% w/w). The signal denoted “Ad” is due to glycine adsorbed on the silica surface.^,

Figure 6

Pulse sequence for recording T_1ρ filtered ¹³C CPMAS NMR spectra. A spinlock is applied on the ¹H channel before the cross-polarization (CP) pulse sequence.