(641a) Enhancement of Adsorbed Natural Gas Storage in Defect-Engineered MOFs during Long-Term Cycling

Vehicular adsorbed
natural gas (ANG) technology is urged by the demand to balance the increasing
development of vehicle industry and the environmental sustainability. Metal-organic
frameworks (MOFs) have been considered as one of the most promising materials
for ANG applications. However, MOF-based ANG is hampered by two challenges,
i.e. the impurities from natural gas not only accumulate in MOFs, but also
cause structural degradation of the materials (see Figure 1a) during
long-term cycling, leading to significant reduction of ANG capacity
(deliverable energy reduces up to 50% after 200 cycles). To overcome these challenges,
combining molecular simulation, process modeling and experimental procedures, (1)
we designed a class of Zr-based dual-ligand MOFs (see Figure 1b) by well-tuning
the distribution (short-range order) of labile ligand-B in the structures, which
were attacked and split preferentially to create stable and defect-controllable
(defect-engineered) MOFs during ANG cycling, increases the MOF porosity and
working capacity (deliverable capacity increase about 20% compared to
single-ligand MOFs, as denoted in Figure 1c), (2) developed a
mathematical ANG model to quantitatively optimize the regeneration strategy for
the materials, which recovers the storage capacity (up to 99% after 673 K/2 h/vacuum
treatment) and lengthens the working life time period compared to the single-ligand
MOFs.

This work focuses on (1) investigating
the mechanism (transition states) of metal -ligand connection breaking by the
impurity attack using DFT simulations, and select a serial of appropriate ligand
pairs to construct dual-ligand MOFs; (2) optimizing the ligand composition and
distribution in the dual-ligand MOFs to control the defects and maintain
structural stability; (3) developing a ANG process model to predict the
performance of the materials, and optimize the regeneration period,
temperatures and pressures to resume their ANG performance. The results of this
work provide a novel strategy to design MOFs in overcoming the drawback of ANG capacity
degeneration, facilitating the popularization of vehicular ANG technology in
practical applications.

Figure 1. (a) Impurities randomly attack single-ligand MOFs and cause structural
collapse, (b) impurities selectively attack the labile ligand (ligand B) in the
dual-ligand MOFs and create controllable defect materials (Defect-engineered
MOFs), (c) natural gas working capacity of defect-engineered MOFs and
single-ligand MOFs.