Gas-sorption Flashcards
what is a gas-sorption isotherm?
a measurement of quantity of gas adsorbed onto the sample as a function of pressure, at a constant temp
relative pressure on the x axis, quantity adsorbed on the y axis
what are the pressures at which the different pore sizes are filled?
micro at low relative pressure
meso at medium relative pressure
macro at atmospheric pressure (approaching 1)
why are both H2 and N2 used for gas-sorption?
N2 has more vdw so more heat of adsorption so adsorbs more strongly than H2 in bigger pores
H2 fills the small pores so N2 won’t be adsorbed in the pores
what does the sorption of gases depend on?
- the porosity of the sample = the amount of available surface for adsorption and if the gas molecule can fit through the pores
- the strength of interaction between the gas and the surface
- the temperature - the rigidity/flexibility
- more pore channels = they’re more accessible to nitrogen for higher adsorption - MUST BE accessible porosity though (internal)
how does crystal packing affect different sorption behaviour?
the different crystal packing - a higher density polymorph (connected voids) vs a lower density (non-connected voids) give different sorptions
How do crystallisation conditions affect the porosity?
rapid, forced crystal growth forms imperfect crystals and defects which can actually increase the SA as there’s more accessible surface area, pore volume, and less mass
what is the benefit of co-crystallising cages?
gives lots of unique cages with each cage having unique gas-sorption properties
allows for control of properties such as surface area and pore size
however it’s only possible bc cages are soluble small molecules and a driving force is needed
what is a driving force of co-crystallisation?
if the cages have helical chirality and are flexible they can switch between the isomers and form a racemic mixture
if the cages are rigid then a homochiral cage can be made
the opposite chiralities of cages may fit together better (more favourable packing) and be a driving force for co-crystallisation e.g. R completely surrounded by S
give an example of bimodal pore size
narrow pores run through the inside of the cages
wider pores run between the pillars of tubular cages that separate the layers of tetrahedral cages
How can controlling precipitation conditions control the sorption properties?
controlling temp and rate of addition can control size, shape, and crystallinity of the crystals
can also be used to grow crystals inside other support materials (beads/films)
applications of porous organic cages for shape separations?
can be packed into separation columns or coated inside gas chromatography columns, since they’re soluble
can then separate isomers and similar molecules that can’t normally be separated by just distillation
applications of porous organic cages for size separations?
Xenon gas fits favourably in a porous organic cage (CC3) and a packed column of the cage was able to separate it from the other components of air
SF6 can also be separated
applications of porous organic cages for porous liquids?
1) adsorbing a gas (Xe) or trapping a molecule in the cage
2) cages dissolved in a solvent that won’t fit through the windows
3) then a fluid to adsorb/compete with guests to release the gas from the cage cavities
4) can be pumped around a system for filtration
