Separations & Purifications

The maximum utilization of raw materials would be the priority for any Industrial plant. For instance, the waste gas from ammonia plant contains carbon monoxide and methane. If carbon monoxide could be effectively recovered from the waste gases, it can be used as a raw material for producing chemicals like methanol, acetic acid etc. Additionally, unsaturated hydrocarbons such as ethylene and propylene are basic raw materials in synthetic chemistry. These are produced from naphtha/natural gas cracking or by dehydrogenation of paraffin. Invariably these are obtained as mixtures necessitating separation before their use. Again, atmospheric industrial gases like nitrogen, oxygen and argon are commercially produced from air employing separation techniques. Whilst there are variations in process details, reflecting desired product mix and other factors, all air and flue gas separation techniques employ one of two types of process technology cryogenic or non-cryogenic such as adsorptive separations. The adsorptive separation techniques separate gaseous products using near-ambient-temperature based on differences in properties such as molecular structure, size and mass to generate relatively pure gases.

An innovative material design in this field will make this process more feasible to industrial non-cryogenic separation technology. For instance, our recent work on new class of ethane selective adsorbent for ethylene purifications is a good example for the innovative thinking in this area. Also, We reported an unprecedented capture of N2 for both natural gas upgrading and air separation using a mesoporous Metal-Organic Framework (MOF) material containing accessible Cr(III) sites. A combination of advanced experimental and computational tools revealed that the separation mechanism for both N2/CH4 and N2/O2 gas mixtures is driven by the presence of these unsaturated Cr(III) that allows a much stronger binding of N2 vs CH4 and O2. This concept opens new horizons to address several challenges in chemistry, such as the removal of nitrogen or the design of heterogeneous biomimetic catalysts through nitrogen fixation.

Natural gas must be dehydrated before it can be transported and used, but conventional drying agents such as activated alumina or inorganic molecular sieves require an energy-intensive desiccant-regeneration step. We reported a hydrolytically stable fluorinated metal-organic framework, AlFFIVEH2O-1-Ni, with a periodic array of open metal coordination sites and fluorine moieties within the contracted square-shaped one-dimensional channel. This material selectively removed water vapor from gas streams containing CO2, N2, CH4, and higher hydrocarbons typical of natural gas, as well as selectively removed both H2O and CO2 in N2 containing streams. The complete desorption of the adsorbed water molecules contained by the AlFFIVEH2O-1-Ni sorbent requires relatively moderate temperature (~105 °C) and approximately half the energy input for commonly used desiccants. Nevertheless, there are enormous quest to develop suitable adsorbents/desiccants to leverage the non-cryogenic separation techniques in the industrial separation and purifications.

Separation of chiral molecules is important for the production of enantiopure molecules in the pharmaceutical, agricultural, and fragrance industries. Chiral MOFs are promising materials for these separations, and their crystalline nature may greatly improve our understanding of chiral recognition.