A seminar by Dr. Jeffrey Reimer on Metal Organic Frameworks

10.22.19

After class one day I attended a departmental seminar by Dr. Jeffrey Reimer, who visited us from the University of California, Berkeley. I had seen a student of his present at Rice University's Gulf Coast Undergraduate Research Symposium, and was really interested in his work on carbon capture. Dr. Reimer's group does NMR characterization of absorbaphores for carbon capture. So to begin, he has a database full of 325,000 known possible metal organic frameworks, and their henry's law coefficients at conditions that mimic carbon capture conditions. They then plot the CO₂/N₂ selectivity and plot K_HCO₂ vs. K₂H₂O. This is in units of mol/(kg^-1bar^-1). They then selected 35 isorectangular metal organic frameworks (MOFs), and they attempted to synthesize all of them. They then focused in on a Al₂(OH)₂TBAPy MOF, which cycles very well with adsorbing and desorbing carbon dioxide.

He then spoke about how we can understand MOFs using the concept of apportment from biology. There are four cattegories, large clusters, small clusters, random, and alternative. We can't observe the apportment directly so we look at a redor curve, that gives us the average distance compared to the actual distance between the organic clusters. He then can used Monte Carlo calculations to adjust their interaction parameters (E_i,j) for the MOF substituents. Using these fitting parameters we can make the simulation and NMR experimental data match. We then looked at calculating self-diffusion coefficients form Pulse Field Gradient experiments. The process is as follows, we load a liquid sample into the NMR and apply spatial and temporal lags in the magnetic fields to elucidate the diffusion time of difference species within the MOF. If there's diffusion then there's a vector sum of how far everything has moved and you can transform this vector into an exponential. And they load large crystals into the NMR so that they're not finding grain boundary diffusion rates. What they find is that the CO₂ diffuses perpendicular to the MOF tubes. This doesn't make much sense unless you realize that this just means that there are holes and other defects present in the tubes. So they need to realize pinhole free MOFs.

In the next part Dr. Reimer spoke about synthesis. They want to be able to synthesize large uniform crystals. To measure how large the crystals are for a given mass of sample they want to measure the surface area. This principle works off of the fact that the solvent when added to the crystal sees two different environments. It's either intercrystalline or intracrystalline. As they add solvent to the sample (while it's in an nmr machine) the two peaks that represent intra/inter solvent location change in ratio. In the beginning everything absorbs into the crystal and then after time when the crystal is fully saturated the solvent goes into the intracrystal system.