Investigation of the Dynamic Behaviour of H2 and D2 in a Kinetic Quantum Sieving System

Porous organic cages (POCs) are nanoporous materials composed of discrete molecular units that have uniformly distributed functional pores. The intrinsic porosity of these structures can be tuned accurately at the nanoscale by altering the size of the porous molecules, particularly to an optimal size of 3.6 Å, to harness the kinetic quantum sieving effect. Previous research on POCs for isotope separation has predominantly centered on differences in the quantities of adsorbed isotopes. However, nuclear quantum effects also contribute significantly to the dynamics of the sorption process, offering additional opportunities for separating H2 and D2 at practical operational temperatures. In this study, our investigations into H2 and D2 sorption on POC samples revealed a higher uptake of D2 compared to that of H2 under identical conditions. We employed quasi-elastic neutron scattering to study the diffusion processes of D2 and H2 in the POCs across various temperature and pressure ranges. Additionally, neutron Compton scattering was utilized to measure the values of the nuclear zero-point energy of individual isotopic species in D2 and H2. The results indicate that the diffusion coefficient of D2 is approximately one-sixth that of H2 in the POC due to the nuclear quantum effect. Furthermore, the results reveal that at 77 K, D2 has longer residence times compared to H2 when moving from pore to pore. Consequently, using the kinetic difference of H2 and D2 in a porous POC system enables hydrogen isotope separation using a temperature or pressure swing system at around liquid nitrogen temperatures.

2. Synthesis of RCC3.The imine cage CC3-R (926 mg, 0.83 mmol) was dissolved in a CHCl 3 / methanol mixture (50 mL each) under stirring.Once the solution became clear, sodium borohydride (NaBH 4 , 1.00 g, 26.5 mmol) was added, and the reaction was stirred for a further 12 hours at room temperature before water (2 mL) was added.The reaction was then stirred for a further 12 hours.The solvent was then removed under vacuum, resulting in a milkylike solid, which was extracted with chloroform (2 × 50 mL).The combined organic phase was washed with water (2 × 100 mL), and the CHCl 3 phase was dried using anhydrous MgSO 4 before being removed under vacuum, yielding around 92% crude RCC3.
3. Purification of RCC3.Taking advantage of the aminal reversibility of AT-RCC3 (AT stands for acetone) in solution, RCC3 crystals were dissolved in acetone (around 100 mg RCC3 in 10 -15 mL acetone).The solution was then covered and left to stand.Crystals started appearing on the wall and bottom of the vials shortly.The crystals (AT-RCC3) were collected after one day by filtration.AT-RCC3 was then dissolved in a CHCl 3 / CH 3 OH mixture (1:1 v/v) by stirring.After 12 hours, several drops of distilled water (2 mL) were added to the solution, and the mixture was stirred for another 12 h.After removing the solvents, pure RCC3 was collected (approximate yield of 60%) 4. Synthesis of 6ET-RCC3.Acetaldehyde (200 mg,4.55 mmol) was dissolved in MeOH (10 mL) and stirred at 0 ˚C.RCC3 (500 mg, 0.438 mmol) in MeOH (20 mL) was added to the solution.A white precipitate appeared upon the addition of RCC3.The reaction was stirred for a further 2 h at room temperature and collected by filtration.6ET-RCC3 (around 75%) was obtained after washing the product with MeOH (3 x 10 mL) and drying the sample, giving 6ET-RCC3 an 82% yield.-0.5 -0.4 -0.

FigureFigure S3 :
Figure S1: a) N 2 sorption at 77 K and CO 2 sorption at 273 K on 6ET-RCC3.b) H 2 sorption results at 77 K on 6ET-RCC3 just after synthesis and after neutron experiments.c) Pore surface distribution analysed with DFT performed on N 2 sorption isotherm measured at 77 K. d) 1 H-NMR spectra at room temperature for starting cage CC3, parent cage RCC3 and 6ET-RCC3 in CDCl 3.
Figure S5: Forward Compton scattering for 6ET-RCC3 with D 2 loading: black curve: corrected scattering output; purple curve: D recoil peak; green curve: H peak, deep blue curve: C peak, light blue: N peak, pink curve: Al peak (sample holder).

Figure S8 :Figure S9 :
FigureS8: Fitted spectra for H 2 at 77 K at different Q levels and pressures (black for total fitting; green for delta; pink for Gaussian and light pink for Lorentzian.).

Figure S11 :
Figure S11: Fitted spectra for D 2 at 50 K with different Q levels and pressures (green for delta/ resolution).

Table S1 :
Self-diffusivity (D)and residence time (τ) of H 2 and D 2 at 50 K and 77 K from Lorentzian peak fitting.

Table S2 :
Self-diffusion length (L) and residence time (τ e ) of H 2 underwent quantum effect at 50 K and 77 K from Gaussian peak fitting.