Welcome to Chapter 6! So far, we have dealt mostly
with rather dense crystal structures. For example, you have analyzed the crystal structure of benzene. Here, the molecules are packed tightly.
There is no open space. One ring of benzene lies
close to the next one. The distance between this ring and the one
on top of it is about four Angstroms. And this is very typical for solids.
Usually the space is completely filled. This week we take a look at structures, in which empty space plays an important role. This compound is different. It is a so-called metal-organic
framework or short M-O-F: MOF. We also see aromatic rings like the ones in benzene. But this time the structure is much less dense. There even is some empty space.
Look here. The distance from
this aromatic ring to this other one is
about twelve Angstroms. That is three times more
than in the benzene crystal. The difference between the
two structures lies here. Obviously there is something
holding the benzene rings apart. Compounds like these are porous. You can think of them as
sponges with very tiny holes. In most MOFs these holes are typically
five to 30 Angstroms in diameter. And it’s this existence of holes
that makes MOFs special. Because if we consider the
surface of such a MOF crystal we not only have to take the
outer surface into account but also the inner surface,
generated by their pores. The specific surface areas – this
means surface per mass unit – of MOFs can be extremely large. It is not uncommon that a
single gram of MOF crystals – like this little pile
that you can see here – has a specific surface area of
several thousand square meters. For comparison: A Basketball court is 420 m² large. And a soccer or football field is about 7000 m² large. Typical values for MOFs lie
somewhere in between those values. And again: that is per gram. This feature along with their
structural diversity makes them very interesting candidates for many
applications, such as gas storage. A very big chemical company
manufactures them on a large scale and examines their properties as a
storage material for natural gas. There is an excellent video by Chris Wilmer about this matter available
on YouTube. We find it very instructive, So, take a look at it!
https://www.youtube.com/watch?v=QaKSekjAnqY Explaining all the possible applications of MOFs would constitute a MOOC on its own. But of course we will provide some information on them in the references and resources tab. In this chapter we will explain some
general structural features of MOFs. We will start the next
unit with the question: Why are they even called metal-organic frameworks?