Molecular Simulations Probing the Adsorption and Diffusion of Ammonia, Nitrogen, Hydrogen, and Their Mixtures in Bulk MFI Zeolite and MFI Nanosheets at High Temperature and Pressure

Recent advances in the synthesis
of MFI zeolite nanosheets have
led to highly selective membranes that are promising candidates for
small-scale ammonia separation from ammonia/nitrogen/hydrogen mixtures
in distributed green ammonia production plants. Using force-field-based
molecular simulations, we evaluate the performance of bulk all-silica
MFI zeolite and a 3 nm thick all-silica MFI nanosheet for ammonia/nitrogen/hydrogen
separations at moderate temperature and pressure conditions (T = 373 K, p = 5 bar) as well as conditions
relevant for a membrane-based reactor–separator process (T = 523 or 623 K, p = 80 bar). Isobaric–isothermal
Gibbs ensemble Monte Carlo (GEMC) simulations were carried out to
understand the selective adsorption behavior for these mixtures. Molecular
dynamics simulations in the canonical ensemble were performed to examine
the transport behavior of ammonia in a mixture with nitrogen or hydrogen
at loadings obtained from the GEMC simulations. Our results show that
both bulk MFI and the nanosheet with explicit surface silanols are
highly selective toward ammonia adsorption, but the adsorption selectivity
decreases by factors of 4 and 10 for ammonia/nitrogen and ammonia/hydrogen
mixtures as the temperature is increased from 373 to 623 K. Conversely,
the diffusion selectivities toward ammonia are more favorable at process-relevant
temperatures and for the MFI nanosheet. At T = 523
K and p = 80 bar, our simulations predict overall
separation factors of 3.8 ± 0.2 and 4.5 ± 0.3 for ammonia/nitrogen
and ammonia/hydrogen mixtures, respectively, in the bulk MFI zeolite,
and separation factors of 8.6 ± 0.4 and 12.5 ± 0.8, respectively,
for the MFI nanosheet.