Flexible Rack System
Figure 1. A molecular framework anchored to a surface selectively separates enantiomers.
Source: KIT-IFG."In this way, the molecular layers are assembled one after the other comparable to a rack system," explains Fischer. "The new procedure developed by us in principle is suitable for any type of surfaces, including inner walls of tubes, etc. Since the process basically consists of repeated immersions, it is also relatively cheap," Wöll adds.
Chiral organic molecules that serve as the links or struts of the rack systems separate the enantiomers. Their enantiopure structure provides excellent separation efficiencies for a single enantiomer, making the process ideal for pharmaceutical applications, say the researchers. More details can be found in a recent article in Angewandte Chemie.
Further work to increase the mesh width of the porous structures to test the method for larger molecules used as pharmaceuticals is under way. "Pharmaceutical substances are two or more nanometers in size and, hence, larger than hexanediol [which already has been separated in the laboratory]. The development of surface-attached networks with such large structures is a big challenge," explains Wöll.
He expects these larger molecules will take more time, but for medium-size molecules (diameters of up to 4 nm) he anticipates major progress by the end of the year.
"In principle, MOFs are a very large class of materials, due to the almost countless number of combinations between different ligands and metal-oxo nodes. …. Certainly, it will be difficult to handle really large enantiomers with sizes exceeding, say, 20 nm," Wöll admits.
Another challenge, he says, is optimizing the stability of the SURMOFs for typical solvents used in pharmaceutical manufacturing processes. "Reaching a sufficiently high level of stability will be one of the key issues with regard to industrial applications," Wöll notes.
To tailor the selectivity of the SURMOFs for specific enantiomers, "it will be crucial to add functionalities to the ligands used to construct the MOFs, possibly by using a post-synthesis modification process," says Wöll. "Presently, we are investigating the possibility [of attaching] short peptides to the ligands used in the SURMOF construction process in order to make this tailoring more straightforward. …. We clearly see the chance to have a base material [that] at a later point in time can be optimized to separate a given target enantiomer," he adds.
Wöll believes it's possible to get close to 100% enantioselectivity by optimizing the ligands for a given enantiomer.
Outside of the pharmaceuticals industry, Wöll says there's huge potential of MOFs for sensing and electrochemical applications.
"I think it's rather clear that due to the countless number of different MOFs which can be fabricated, the possible applications which these materials may be used for has not been fully realized yet. … Our SURMOF process, which allows using MOF materials to prepare homogeneous rigid coatings on materials, also has a number of applications ranging from chromatography to high selectivity membranes, electrochemical applications and enantiomeric separations… Scientists looking for solutions with regard to technological challenges should consider MOF materials and in particular homogeneous MOF coatings for developing novel solutions," Wöll concludes.
