In Situ Multinuclear Magic-Angle Spinning NMR: Monitoring Crystallization of Molecular Sieve AlPO4-11 in Real Time

Abstract

Molecular sieves are crystalline three-dimensional frameworks with well-defined channels and cavities. They have been widely used in industry for many applications such as gas separation/purification, ion exchange, and catalysis. Obviously, understanding the formation mechanisms is fundamentally important. High-resolution solid-state NMR spectroscopy is a powerful method for the study of molecular sieves. However, due to technical challenges, the vast majority of the high-resolution solid-state NMR studies on molecular sieve crystallization are ex situ. In the present work, using a new commercially available NMR rotor that can withhold high pressure and high temperature, we examined the formation of molecular sieve AlPO₄-11 under dry gel conversion conditions by in situ multinuclear (¹H, ²⁷Al, ³¹P, and ¹³C) magic-angle spinning (MAS) solid-state NMR. In situ high-resolution NMR spectra obtained as a function of heating time provide much insights underlying the crystallization mechanism of AlPO₄-11. Specifically, in situ ²⁷Al and ³¹P MAS NMR along with ¹H → ³¹P cross-polarization (CP) MAS NMR were used to monitor the evolution of the local environments of framework Al and P, in situ ¹H → ¹³C CP MAS NMR to follow the behavior of the organic structure directing agent, and in situ ¹H MAS NMR to unveil the effect of water content on crystallization kinetics. The in situ MAS NMR results lead to a better understanding of the formation of AlPO₄-11.

Scheme 1. (a) Framework of AlPO₄-11, (b) ab Plane Viewed along the c-Axis, and (c) Projection of AlPO₄-11 Structure along the c-Direction with Three Crystallographically Non-Equivalent P Sites Shown

Figure 1

Powder XRD patterns of selected dry gel samples crystallized at 200 °C inside sealed-glass tubes. The PXRD patterns were obtained using Cu Kα radiation (λ = 1.5418 Å). (*) Asterisks denote the signals from the sample holder.

Figure 2

In situ NMR spectra of dry gel samples (1.0 g) mixed with additional 0.4 mL of water: (a) ²⁷Al MAS, (b) ³¹P MAS, (c) ¹H → ³¹P CP MAS, and (d) ¹H MAS spectra. The spinning rate is 4.7 kHz. (*) Asterisks denote the spinning sidebands. Top inset: intensity scaled liquid water peak (4.0 ppm) in ¹H MAS NMR spectra. Bottom inset: vertically enlarged ¹H MAS NMR spectra at 20, 80, and 140 °C showing the peak corresponding to protonated DPA (8.4 ppm).

Figure 3

Powder XRD patterns of the samples recovered after the in situ crystallization: (a) with additional 0.4 mL of water; (b) with additional 0.1 mL of water; and (c) simulated AlPO₄-11. The PXRD patterns were obtained using Cu Kα radiation (λ = 1.5418 Å) except for that in (b) which was obtained with Co Kα radiation (λ = 1.7902 Å). The 2θ values were converted to those corresponding to Cu Kα radiation for comparison.

Figure 4

In situ NMR spectra of dry gel samples (1.0 g) mixed with additional 0.1 mL of water: (a) ²⁷Al MAS; (b) ³¹P MAS; (c) ¹H → ³¹P CPMAS; and (d) ¹H MAS NMR spectra. The spinning rate is 4.7 kHz. (*) Asterisks denote the spinning sidebands. Top inset: ¹H MAS NMR spectra acquired at 200 °C on a wider scale. Bottom inset: vertically enlarged ¹H MAS NMR spectra acquired at 20, 80, and 140 °C showing the peak corresponding to protonated DPA (8.4 ppm).

Figure 5

¹H MAS NMR spectra of selected AlPO₄-11 gel samples: (a) ex situ: initial dry gel mixed with 0.4 mL of additional water and crystallized for 1 day at 200 °C. The spectrum was taken immediately after cooling to room temperature; (b) in situ: initial dry gel mixed with 0.4 mL of additional water. The spectrum was taken inside the rotor at 20 °C right before heating for in situ study; and (c) in situ: initial dry gel mixed with 0.4 mL of additional water. The in situ spectrum was taken at 200 °C after heating for 15 h.

Figure 6

In situ ¹H → ¹³C CP MAS NMR spectra of the dry gel samples (1.0 g) mixed with additional 0.4 mL of water. The spinning rate is 4.7 kHz.

Scheme 2. Schematic Representation of the Proposed Mechanism for Formation of the AlPO₄-11 Framework