sulfur hexafluoride ( sf6 ) is a much more potent greenhouse 
 gas than co2 , with an estimated atmospheric 
 lifetime of 8003200 years .
the 
 intergovernmental panel on climate change found sf6 to 
 be the most potent greenhouse gas that it evaluated , with a global 
 warming potential 23,900 times higher than co2 .
sf6 is also a good thermo - acoustic 
 insulator for windows , a contrast agent in medical applications , and 
 a plasma etchant in the semiconductor industry .
there is much current interest in finding effective materials 
 for the separation of sf6 from gas mixtures to prevent 
 its release into the atmosphere and to allow economically viable capture 
 and reuse .
pressure swing adsorption / desorption 
 processes using a suitable porous material offer considerable energy 
 savings over liquefaction , but this requires high selectivity for 
 sf6 adsorption over n2 adsorption .
various porous solids have been tested for sf6 adsorption or separation such 
 as carbons , zeolites , metal  organic 
 frameworks ( mofs ) , and pillared clays . until now ,
the most selective of these materials 
 have been zeolite 13x and a series of 
 isostructural mofs with a high density of unsaturated metal sites .
the success of these materials was attributed 
 to their pore diameters ( 10 and 11  , respectively ) , which were 
 identified by monte carlo simulations as close to ideal .
all microporous solids tested for sf6 separation so 
 far have been insoluble , extended networks or macromolecules .
nitschke 
 et al . demonstrated that a metal  organic capsule could capture 
 and release sf6 in solution , but such molecular separation of sf6 by a cage molecule 
 has not yet been shown in the gas phase .
these cages contain an internal void that is accessible via well - defined 
 windows , and the rigid structure of the cages prevents collapse , thus 
 providing porosity for guest molecules .
porous organic cages have 
 been synthesized from imines , boronic esters , and by direct carbon 
teller 
 ( bet ) surface areas for nitrogen as high as 3758 m g have been achieved .
since 
 these cages are discrete molecules , as opposed to frameworks , they 
 are soluble in common organic solvents and can be processed into support 
 materials and precipitated or crystallized into the solid state as 
 required .
we previously reported a class of [ 4 + 6 ] cycloimine cage 
 compounds that show gas uptakes and physicochemical stabilities that 
 are remarkable for molecular organic crystals .
the gas sorption 
 properties of these cages depends both on their molecular structure , 
 and on their crystal packing , and many of these cages can exist as 
 multiple polymorphs .
these cages are excellent candidates for the molecular separation 
 of noble gases , chiral molecules , and hydrocarbon isomers . in this study
, 
 a series of these organic cages ( figure 1 ) was investigated for their potential in 
 the separation of sf6 .
one cage , cc3 , shows 
 remarkably high sf6 selectivity , and this is related to 
 the flexible nature of the molecular cage crystal .
( a ) synthesis and structure of cages cc1 , cc2 , cc5 , and cc13 .
( b ) simplified structural 
 representations of the packing and porosity of these cages as derived 
 from single crystal structures .
cc2 exhibits one - dimensional 
 pore channels ( yellow ) in addition to internal cage cavities ( orange ) , 
 which may also be accessible depending on the size of the guest .
cc13 packs with the same diamondoid pore network as cc3 ( running through the inside of the cages , shown 
 in yellow ) , but with an additional , narrower interpenetrating diamondoid 
 pore network between the cages ( cyan ) .
cc5 packs 
 in the same window - to - window fashion , but the cages and hence the 
 pore channels ( yellow ) are larger .
1,3,5-triformylbenzene ( tfb ) 
 was purchased from manchester organics , uk and used as received .
powder samples were degassed 
 offline at 100 c for 15 h under dynamic vacuum ( 10 bar ) before analysis , followed by degassing on the analysis port 
 under vacuum , also at 100 c .
isotherms were measured using a 
 micromeritics 3flex surface characterization analyzer , equipped with 
 a cold - edge technologies liquid helium cryostat chiller unit for temperature 
 control . evacuated prism shaped 
 single crystals of cc3
the crystals of were transferred to a 
 sample vial and after 28 h a single crystal data collection was recorded .
single crystal x - ray data for cc3-s(sf6)2.5(h2o)3 was measured at beamline i19 , diamond light source , didcot , uk using 
 silicon double crystal monochromated synchrotron radiation (  
 = 0.6889  , kappa 4-circle goniometer , rigaku saturn724 + detector ) .
empirical absorption corrections , using equivalent 
 reflections , were applied by the program sadabs .
the structure was solved by shelxd , and refined by full - matrix least - squares on |f| by shelxl , interfaced 
 through the program olex2 .
absolute configuration 
 was determined using a priori knowledge of the cage 
 chirality . for full refinement details , see the supporting information ( si ) .
supplementary single crystal 
 xrd data , including structure factors , is available free of charge 
 from the cambridge crystallographic data centre ( ccdc ) via www.ccdc.cam.ac.uk/data_request/cif .
crystal data for cc3-s(sf6)2.5(h2o)3 , ccdc number 
 ccdc 1437443 : formula c144h174f15n24o3s2.5 ; m = 2654.40 gmol ; triclinic space group p1 , colorless 
 prism shaped crystal ; a = 17.385(5 ) , b = 17.425(6 ) , c = 17.479(6 )  ;  = 60.384(3 ) , 
  = 60.308(3 ) ,  = 60.168(3) ; v = 3768(2 )  ;  = 1.170 gcm ;  = 0.108 mm ; f ( 000 ) 
 = 1405 ; crystal size = 0.13  0.07  0.07 mm ; t = 100(0 ) k ; 52  288 reflections measured ( 1.376 
 <  < 25.503 ) , 28  131 unique ( rint = 0.0448 ) , 22  028 ( i > 2(i ) ) ; r1 = 0.0894 for observed 
 and r1 = 0.1091 for all reflections ; wr2 = 0.2549 for all reflections ; max / min difference 
 electron density = 0.801 and 0.923 e ; data / restraints / parameters = 28  131/363/1786 ; gof = 1.037 . 
 flack parameter 0.14(4 )
. the structure was refined with the twinlaw 
 [ 100 001 010 ] and basf refined to 0.227(2 ) .
powder x - ray diffraction 
 ( pxrd ) data were collected in transmission mode on loose powder samples 
 held on thin mylar film in aluminum well plates on a panalytical xpert 
 pro mpd equipped with a high throughput screening ( hts ) xyz stage , 
 x - ray focusing mirror , and pixcel detector , using cu k radiation . 

data were measured over the range 450 in 0.013 
 steps over 60 min . in situ powder diffraction 
 data under an sf6 atmosphere
were collected at beamline 
 i11 at diamond light source using the low pressure capillary gas cell .
a finely ground sample of cc13 
 was packed in a 0.7 mm diameter borosilicate capillary and mounted 
 on the low - pressure capillary gas cell .
samples were activated by 
 heating to 350 k using an oxford cryostream plus under dynamic vacuum 
 ( approximately 10 mbar ) .
data were collected using 
 the mythen - ii position sensitive detector ( psd ) at 230 k. an initial 
 powder diffraction profile of guest - free cc13 
 was collected under dynamic vacuum .
gas was dosed 
 into the system , initially to 2.7 bar and then 3 bar .
samples were 
 allowed to equilibrate at both pressures for approximately of 45 min 
 after gas was dosed into the cell .
the sample was then evacuated 
 at 373 k under dynamic vacuum and the powder profile collected to 
 confirm removal of the guest from the pore structure .
a 100 ns well - tempered metadynamics 
 simulation was performed with dl_poly2.20 and plumed2 .
the opls - aa force field 
 parameters , and the leapfrog verlet algorithm with a time step of 0.5 fs was used .
the nose - hoover 
 thermostat was used to keep the temperature 
 fixed at 300 k and no interactions were applied between periodic images 
 in a cubic system with cell length 39  .
a time step of 0.5 fs 
 with sampling step of 1 ps was chosen and full molecular motion was 
 allowed throughout the simulation .
the collective variable along which 
 the metadynamics bias was accumulated measured the distance between 
 the center of mass of the fully flexible cc3 and the 
 sulfur atom of the sf6 .
gaussian hills with a width of 
 0.15 nm and an initial height of 1.2 kj mol were 
 added every 500 md steps and the so - called well - tempered factor was 
 set equal to 10 .
an additional well - tempered metadynamics simulation using 
 two collective variables ( the distance and a torsion angle ) was performed 
 to analyze the mechanism further ; see the si for further details .
breakthrough curves were measured 
 for a fixed bed of cc3 or zeolite 13x at 298 k 
 using a 90:10 ( v / v ) n2/sf6 gas mixture .
the 
 breakthrough curves were measured using an automated breakthrough 
 analyzer ( manufactured by hiden isochema , warrington , u.k . ) .
cc3 was made into small pellets ( 500700 m ) , 
 which were packed into an adsorption bed for the breakthrough experiment . 

the materials were activated in situ by heating to appropriate activation 
 temperatures and flowing helium through the column .
desorption of 
 n2/sf6 was performed by flowing helium through 
 the bed at the same flow rate as the breakthrough experiment .
gas adsorption analyses revealed that all four cages were porous 
 to sf6 , but with marked differences in terms of adsorption 
 capacities that relate to their structures ( figure 2 ) .
it is interesting that sf6 is 
 adsorbed by the smaller cages at all : from a simple static representation 
 of their crystal structures , sf6 would not be expected 
 to be able to diffuse through the pore channels of cc2 , cc3 , and cc13. however , 
 these organic crystals are somewhat flexible .
they are composed of discrete molecules held together by weak intermolecular 
 dispersion forces , rather than covalently bonded frameworks , and this 
 has been shown previously to allow 
hence cc2 , cc3 , and cc13 can adsorb sf6 , despite the fact that 
 this gas is larger than the static window diameter in the cages .
the 
 kinetic diameter of sf6 , is 5.5  , and thus one might not expect it to diffuse into the smaller 
 imine cages ( cc2 , cc3 , cc13 ) since their window diameters are 3.6  .
indeed , previous unbiased md simulations for cc3 demonstrated that sf6 did not escape 
 the cage cavity in which it was originally positioned over a 10 ns 
 simulation using a force field tailored to describe the flexibility 
 in imine cages .
hence , to analyze the 
 sf6 diffusion mechanism for these smaller cages and to 
 calculate the energetic barrier to this event , we carried out well - tempered 
 metadynamics simulations of sf6 and a single cc3 molecule ( see the si for full simulation 
 details ) . to understand the diffusion mechanism
, we can think of the 
 sf6 molecule as two connected triangular faces , rather 
 than a sphere that describes the widest possible diameter .
as can 
 be seen in figure 3 and movie s1 , sf6 exits the 
 cage by first aligning its outermost triangular face with the triangular 
 cage window .
there is then a rotation of 60 ( figure 3a  d ) such 
 that the second face can align with the cage window and thus pass 
 through .
the barrier to the sf6 diffusion through the flexible 
 cage window is calculated to be at most 40 kj mol , and the configurations near the saddle point have the center of 
 the sf6 traversing the window . while crystal packing effects 
 would likely influence this barrier to a small extent ,
it is of similar 
 magnitude to that previously calculated for the allowed para - xylene diffusion in the same host system .
it is therefore reasonable that the sf6 diffuses , albeit 
 slowly , in this system .
a comparison of a single empty cc3 window diameter distribution with the measured window diameter during 
 the sf6 passage ( figure 3e ) reveals the cooperative character of the mechanism . 

clearly , the cage window size increases when the sf6 is 
 inside the window .
gas sorption isotherms for the uptake of sf6 in the 
 various cages ; adsorption curves shown as filled symbols , desorption 
 curves as unfilled symbols .
( a ) cc2 , ( b ) cc3 , ( c ) cc5 , and ( d ) cc13. ( e ) this plot shows the uptake expressed in terms of the 
 number of sf6 molecules per cage molecule at 230 k. ( f ) 
 heat of adsorption of sf6 for the various cages .
( a ) free energy surface for a single sf6 molecule diffusing 
 through the window of a single cc3 cage as a function 
 of the distance between the centers of mass of cc3 and 
 sf6 . ( b ) the 2d free energy as a function of the distance 
 used for ( a ) and the angle of rotation as shown in part ( c ) . ( c )
cc3 and sf6 configurations corresponding to the 
 positions marked 1 ( left ) and 2 ( right ) on the plots .
structure 1 
 corresponds to the global minimum orientation with sf6 in 
 the cc3 cavity , and structure 2 represents the structure 
 at highest point of the free energy surface , when sf6 is 
 placed exactly in the center of the window .
the angle used for the y - axis in panel ( b ) is indicated on these configurations . 

this angle is small when the sf6 begins to escape from 
 the cage and increases as the sf6 reaches position 2 .
( e ) comparison of the pore envelope of the cc3 window , for an empty cage ( black ) , for cc3 with sf6 occupying the cage cavity ( green ) , and for cage where sf6 is positioned in the window ( red ) .
cc3 shows the highest affinity for sf6 of the four cages tested , and a steep type i isotherm that saturates 
 at low pressures ( figure 2b ) .
this is manifested in the highest heat of adsorption of 
 3540 kj mol ( figure 2f ) .
the saturation uptake corresponds 
 to around 2.9 sf6 molecules per cc3 cage ( figure 2e ) .
this is consistent 
 with one sf6 molecule in the internal cage cavity , plus 
 one sf6 molecule shared in each of the four window 
window 
 sites surrounding every cage , equating to a maximum theoretical loading 
 of 3 sf6 molecules per cc3 cage .
cc2 shows the lowest sf6 uptake and the lowest heat 
 of adsorption ( figure 2a , f ) .
the 1d pore channels in cc2 , which run between the hexagonally arrayed cages , 
 are wide enough to accommodate sf6 .
the uptake of 1.4 sf6 molecules per cage ( figure 2e ) confirms that these 1d channels must be at least 
 partially occupied .
cc5 displays the highest sf6 adsorption capacity , of 10.2 sf6 per cage , in 
 line with its larger internal void size and its higher surface area 
 and pore volume .
however , the heat of 
 adsorption is much lower than for cc3 , and hence cc5 adsorbs less sf6 at lower pressures .
cc13 gives a lower heat of adsorption than cc3 , despite having a similar pore structure .
the difference 
 between the high temperature ( 298 k ) and low temperature ( 230 k ) sf6 uptakes for cc13 is significant . at 230 
 k , cc13 adsorbs around 6 sf6 molecules 
 per cage  that is , twice as much gas as cc3. 
 this
can be rationalized only if the sf6 molecules are 
 located in the intercage sites , of which there are three per cc13 ( figure 1b ; blue nodes ) , as well as in the cage cavities and the window
 window 
 sites . while these intercage sites were shown previously to be accessible 
 to nitrogen , the size of the nodes relative 
 to sf6 suggested to us that significant rearrangement would 
 be required to accommodate this larger gas . to explore this ,
the adsorption 
 of sf6 in cc13 was monitored by pxrd 
 ( figure 4 ) .
the pxrd 
 pattern changes significantly as the structure is loaded with sf6 , losing intensity in the high angle range .
this is consistent 
 with the preservation of the long - range packing of the cage modules , 
 but a more disordered local structure , which could allow the large 
 sf6 guest to diffuse through the crystal .
this reorganization 
 of the cc13 structure allows the incorporation of such 
 a large quantity of sf6 ( 6 per cage ) , which can not not 
 be rationalized by the original empty structure .
it is remarkable 
 that the crystallinity is restored completely to its initial state 
 after removal of the sf6 , even after multiple cycles ( figure s1 ) .
this behavior is allowed by the relatively 
 weak dispersion forces between the cages , which enable reorganization 
 in response to guests .
the sf6 isotherms for cc3 suggested 
 excellent potential for sf6 separation from nitrogen , and 
 therefore the nitrogen isotherms were measured at equivalent temperatures 
 ( figure s2 ) to allow the calculation of 
 ideal adsorbed solution theory ( iast ) predicted selectivity ( figures 5 and s3 ) .
the industrially 
 relevant composition for separation of an sf6/n2 mixture 
 is sf6:n2 = 10%:90% . at 1 bar pressure
, cc3 gives a selectivity of 178 at 273 k and 74 at 298 
 k. this surpasses the most promising candidate material previously 
 reported , uio-66-zr ( selectivity = 74 , at 1 bar , 293 k ) and other similar framework materials such as 
 zn - mof-74 ( selectivity = 46 , at 1 bar , 298 k ) , ca - a zeolite ( 28 at 1 bar , 298 k ) , and 
 zeolite-13x ( 44 at 1 bar , 298 k ) . the 
 total capacity of cc3 for sf6 is higher 
 than that of many of the frameworks reported ( e.g. , uio-66-zr is 1.5 
 mmol , at 293 k and 1 bar ) , but lower than that of the highest ( mg - mof-74 
 = 6.42 mmol g at 1 bar , 298 k ) .
powder x - ray diffraction data for in situ sf6 loading 
 of cc13 at 230 k. under excess pressures of sf6 , the cc13 structure becomes more disordered 
 at short - range , indicated by loss of high angle diffraction intensity . 

iast selectivity plots for sf6 over n2 for cc3 at sf6:n2 ratios of 50:50 and 10:90 
 at ( a ) 273 k and ( b ) 298 k. a crystallographic study allowed for the location of sf6 in cc3 to be accurately determined ( figures 6 and s4 ) .
well - ordered sf6 molecules were 
 located , with full site occupancies , in the intrinsic cc3 cavities ( figure s4 ) .
cc3 provides an ideal fit for sf6 in terms of both size and 
 geometry of the cage cavity ( figure 6 ) , which explains the high heat of adsorption that 
 we observe .
the ordering of sf6 molecules in the cc3 cavities transforms the crystallographic symmetry from f4132 , as determined for the empty cc3 host , to p1 , but this does not alter the 
 crystal packing of the cc3 molecules .
( a ) resolved position of sf6 in the cc3 cavity determined from the single crystal structure ;
iast calculations are useful to suggest selectivity in a separation 
 of two or more gases for an adsorptive separation process , but iast 
 does not accurately represent a gas mixture flowing through a packed 
 bed of material . in particular , iast 
 calculations say little about separation kinetics since the calculations 
 are derived from single - component gas sorption isotherms that are 
 collected at thermodynamic equilibrium .
therefore , breakthrough experiments 
 were carried out to establish the practical potential of cc3 for n2/sf6 separations , and in particular 
 to demonstrate that the sf6 diffusion kinetics are sufficiently 
 fast for real separations .
all breakthrough experiments were performed 
 at 298 k with a 90:10 ( v / v ) n2/sf6 mixture . 

the breakthrough curves and desorption curves for n2 and 
 sf6 flowing through a bed of cc3 are 
 shown in figure 7 . 

the selectivity determined from the relative uptake capacities is 
 76.5 , in good agreement with the iast calculations .
nitrogen breaks 
 through the column within 1 min , whereas sf6 does not start 
 to break through until after 6.5 min and does not completely break 
 through until approximately 20 min . the large difference in breakthrough 
 time between n2 and sf6 reflects the much higher 
 affinity of cc3 toward sf6 . the breakthrough 
 curve for n2
shows the characteristic  roll - up  
 as the concentration at the outlet is temporarily higher than at the 
 inlet as the sf6 is preferentially adsorbed and displaces 
 the n2 .
the desorption was performed by flowing helium 
 through the column under the same conditions as for breakthrough . 

n2 is desorbed quickly from the column , with 97% of the 
 gas being desorbed within the first minute .
as expected , sf6 is desorbed more slowly , with full desorption taking about 20 min , 
 approximately the same time as for full breakthrough .
breakthrough and desorption curves for cc3 
 and zeolite 13x at three different gas flow rates are compared in figures s5 and s6 . in each case
, the desorption 
 of sf6 from cc3 takes approximately the same 
 time as the full breakthrough .
however , for zeolite 13x with a 25 
 ml min flow rate , sf6 desorption takes 
 twice as long as it does to breakthrough ( figure s6 ) .
the more rapid desorption efficiency of cc3 therefore gives it a potential advantage over zeolite 13x 
 as a material for n2/sf6 separation .
n2/sf6 ( 90:10 ) breakthrough curve for cc3 at 298 k. total flow rate was 25 ml min , and pressure was 1 bar .
desorption was performed by flowing helium 
 through the bed at the same flow rate and pressure .
intrinsically porous molecules , as opposed to framework or network 
 materials , have been investigated for sf6 uptake separation . 
 despite the pore limiting diameters in cc3 being 
 considerably narrower than the 11
 recommended 
 as the optimum pore size by simulation , this cage shows unprecedented selectivity for sf6 over 
 n2 .
our simulations suggest this may be a result of the 
 flexibility of the cc3 molecular crystal , which 
 allows sf6 to diffuse by cooperative effects , before the 
 structure relaxes back to produce a closer , near - ideal interaction 
 with the sf6 guest .
this hypothesis is further supported 
 by research reported by camp and sholl while this article was being 
 written . in that study ,
transition state 
 theory methods were used to simulate the diffusion of various gases 
 in cc3 , and it was found for sf6 that 
 no diffusion would be expected in a static system , while diffusion 
 should be possible if the flexibility of the host is taken into account . 

the iast selectivity of cc3 for an industrially 
 relevant mixture of 10:90 sf6:n2 at 298 k and 
 1 bar is higher than that of other reported materials .
breakthrough 
 experiments confirmed that cc3 is effective for 
 separation of n2 and sf6 .
also , desorption curves 
 show that sf6 is more efficiently desorbed from cc3 than from zeolite 13x .
similarly , 
 the flexibility of molecular crystals , such as cc3 , 
 allows for a stronger gas binding with sf6 , which gives 
 these materials potential for practical gas separations under flow .
 a series of porous organic cages is examined for the selective 
 adsorption of sulfur hexafluoride ( sf6 ) over nitrogen . 
 despite lacking any metal sites , 
 a porous cage , cc3 , 
 shows the highest sf6/n2 selectivity reported 
 for any material at ambient temperature and pressure , which translates 
 to real separations in a gas breakthrough column . 
 the sf6 uptake of these materials is considerably higher than would be expected 
 from the static pore structures . 
 the location of sf6 within 
 these materials is elucidated by x - ray crystallography , and it is 
 shown that cooperative diffusion and structural rearrangements in 
 these molecular crystals can rationalize their superior sf6/n2 selectivity . 
Introduction
Methods
Results and Discussion
Summary