in recent years , natural 
 gas , comprised primarily of methane , ch4 ( 8796% ) , has become an 
 important economic and technological target as a bridging fuel to 
 a low carbon future . although not renewable , large reserves ( approximately 
 two - thirds of available fossil fuel reserves ) and advances in recovery 
 make natural gas an incredibly abundant energy source . this high availability , 
 coupled with the lowest co2 emissions of any hydrocarbon 
 fuel and a low sulfur content , ideally place natural gas as a replacement for 
 coal and oil . currently the widespread use of natural gas is restricted 
 because under ambient conditions , gaseous ch4 has a low 
 energy density and therefore requires constant access to a pipeline . the energy density of bulk ch4 can be increased by liquefaction 
 or , to a lesser extent , compression , using either complex cryogenic 
 tanks or heavy - walled high - pressure vessels with expensive compressors , 
 precluding their use in mass - market applications . an alternative approach to increasing the storage 
 density of ch4 is by adsorption on the surface of a porous 
 material , reducing 
 the intermolecular distance and therefore increasing the packing density . metal  organic framework ( mof ) materials have been widely investigated 
 for their ability to store significantly higher volumes of gas than 
 a conventional tank under the same conditions . this has pronounced 
 effects in a number of materials , which have been reported to adsorb 
 gas species ( upon saturation uptakes ) at densities comparable to their 
 respective liquid phase . to date , this effect has been observed primarily 
 in mofs showing high co2 or h2 adsorption under 
 extreme conditions , typically low temperatures , and/or high pressures . high volumetric ch4 uptakes [ v(stp)/v ] have been reported 
 for a number of state - of - the - art mofs at ambient temperature and 35 
 bar . mof-74-ni , hkust-1 , pcn-14 , mof-519 
 , and nott-220   all have significantly higher capacity than 
 bulk ch4   under the same conditions . it is worth 
 noting that , throughout this article , the volumetric uptake is derived 
 from the bulk material density on the basis of single crystals , and 
 when the efficiency of powder packing is considered , the uptakes will 
 be reduced accordingly . variation of 
 metal ions / clusters and organic ligands enables a 
 multitude of design possibilities for mof materials allowing incorporation 
 of functional groups and/or desirable metal binding 
 sites for specific guest binding interactions . to design 
 future materials with improved gas adsorption properties , it is vital 
 to understand the intermolecular contacts , location , and binding interactions 
 of adsorbed guest molecules within the cavity of the host materials . determination of the binding of molecular h2 and ch4 within porous materials is a very challenging proposition 
 due the short scattering length of hydrogen in x - ray experiments . 
 in addition , adsorbed h2 and ch4 molecules have 
 low enthalpies of adsorption , resulting in them being highly disordered 
 within the pores of mofs . indeed , h2 and ch4 have been shown to behave as a  free - rotor  in the 
 solid state when condensed at cryogenic temperatures . to date , determination 
 of the location of adsorbed h2 and ch4 molecules 
 has only been achieved in exceptional cases , typically within mof systems incorporating strong binding sites 
 such as open metal sites or geometrically confined 
 pores in which the movement ( translational motion ) of adsorbed gas 
 molecules is highly constrained . the 
 nature of the potential interaction(s ) of pendant functional groups 
 with adsorbed ch4 within pores at a molecular level remains 
 unclear . herein , we report the high volumetric uptake of ch4 in an in(iii)-tetracarboxylate material , mfm-300(in ) [ 202 v / v at 
 298 k and 35 bar ; 328 v / v at 110 k and 0.6 bar ] . the material also 
 shows high volumetric uptake for h2 ( 488 v / v at 77 k and 
 20 bar ) . direct observation and quantification of adsorbed h2 and ch4 molecules within mfm-300(in ) have been achieved 
 using a combination of neutron diffraction and scattering techniques 
 coupled with computational investigations . these complementary results 
 suggest that the adsorbed ch4 molecules form specific interactions 
 with metal - bound hydroxyl groups within the pore , supplemented by 
 intermolecular dipole interactions between adsorbed ch4 molecules and the phenyl - ring lined walls of the pore . these cooperative 
 supramolecular interactions allow packing of ch4 to very 
 high density upon saturation ( 0.37 g cm ) reminiscent 
 of liquid ch4 ( 0.42 g cm at 111 k ) and 71% of that of the solid ( 0.52 g cm at 20.4 k ) . interestingly , to our 
 best knowledge , the h4ch  o interaction reported here represents the first example 
 of such molecular contact in mofs , and contrasts with the structures 
 observed in ch4/water clathrate materials in which the 
 ch4 molecule is completely orientationally disordered and 
 not observed to interact with the hoh cage - forming 
 linkage . h4l ( biphenyl-3,3,5,5-tetracarboxylic 
 acid 330 mg , 1.00 mmol ) and in(no3)35h2o ( 585 mg , 1.50 mmol ) were mixed and dispersed in a dmf / mecn 
 mixture ( 30 ml , 2:1 v / v ) in a 250 ml glass pressure reactor . the white 
 slurry was acidified with conc . nitric acid ( 65% 1.0 ml ) , and the 
 vessel sealed and heated at 80 c for 48 h. the resultant flaky 
 white precipitate was washed with dmf and dried in air . yield : 347 
 mg ( 42% yield based upon solvent content from microanalysis ) calcd for [ in2(oh)2(c16h6o8)]1.28h2o2.87(c3h7no ) : c , 35.92 ; h , 3.75 ; n , 4.88 . found : c , 35.92 ; h , 3.53 ; 
 n , 4.88 . the acetone - exchanged material was prepared by suspending 
 the as - synthesized sample in an excess of acetone for 5 days with 
 frequent exchange of solvent . calcd for [ in2(oh)2(c16h6o8)]1.10h2o1.76(c3h6o ) : c , 35.91 ; h , 2.94 ; 
 n , 0.00 . found : c , 35.91 ; h , 2.94 ; n , 0.00 . selected ir(atr ) : /cm = 3501 ( br ) o  h , 1705 ( s ) , 1610 ( s ) , 1550 
 ( br ) , 1418 ( br ) , 1359 ( s ) , 1220 ( s ) , 1089 ( w ) , 782 ( s ) , 748 ( s ) . gravimetric isotherms ( 020 
 bar ) were recorded at 77 and 87 k under liquid nitrogen and argon , 
 respectively , for h2 , and at 110 , 215 , and 245 k ( liquid 
 nitrogen cryo - furnace ) , 195 k ( dry ice / acetone ) , and 273 , 283 , 293 , 
 303 , and 308 k ( temperature controlled water - bath ) for ch4 . data were collected using an iga-003 system ( hiden isochema , warrington , 
 uk ) equipped with a turbomolecular pumping system . acetone exchanged 
 samples were loaded into the system and degassed at 120 c and 
 1  10 mbar for 20 h to give a dry , desolvated 
 material of typical mass ca . 99.995+% grade h2 and 
 ch4 were purchased from air liquide and used as received . high pressure ( 050 bar ) gravimetric gas adsorption and desorption 
 data were collected using a xemis system ( hiden isochema , warrington , 
 uk ) equipped with a turbomolecular pumping system backed by a diaphragm 
 pump . ch4 isotherms ( 050 bar ) were recorded at 
 273 , 283 , 293 , 298 , and 303 k by a temperature controlled water bath . neutron powder diffraction 
 experiments were undertaken at the wish diffractometer at the isis 
 facility . mfm-300(in ) was loaded into a 6 mm diameter vanadium sample 
 can and outgassed at 1  10 mbar and 100 
 c for 1 day . the sample was loaded into a liquid helium cryostat 
 and cooled to 7 k for data collection . d2 and cd4 gas were introduced by warming the samples to 50 and 150 k , respectively , 
 and the gas was dosed volumetrically from a calibrated volume . the 
 gas - loaded sample was then cooled to 7 k over a period of 2 h to ensure 
 good mobility of adsorbed d2 and cd4 within 
 the crystalline structure of mfm-300(in ) . the sample was kept at 7 
 k for an additional half an hour before date collection to ensure 
 the thermal equilibrium . inelastic 
 neutron scattering 
 ( ins ) experiments were undertaken using the tosca spectrometer at 
 the isis facility . mfm-300(in ) was loaded into an 11 mm diameter vanadium 
 sample can and outgassed at 1  10 mbar and 
 100 c for 1 day . the sample was loaded into a helium closed 
 cycle refrigerator ( ccr ) cryostat and cooled to 11 k for data collection . h2 and ch4 gas were introduced by warming the 
 sample to 50 and 150 k , respectively , and the gas was dosed volumetrically 
 from a calibrated volume . the gas - loaded sample was then cooled to 
 7 k over a period of 2 h to ensure good mobility of adsorbed d2 and cd4 within the crystalline structure of mfm-300(in ) . the sample was kept at 7 k for an additional half an hour before date 
 collection to ensure the thermal equilibrium . three types of calculations were performed : 
 geometry optimization ( go ) , lattice dynamics ( ld ) , and molecular dynamics 
 ( md ) . go is used to determine the ground - state structure , binding 
 energies of adsorbed ch4 molecules , and rotational potentials 
 for ch4 molecules when combined with single point energy 
 ( spe ) calculations . ld was used to determine the molecular vibrations 
 and thus the hydrogen vibrational density of states for comparison 
 with the ins data . physisorbed ch4 is particularly problematic 
 for ld methods due to the weak intermolecular interactions . md simulations 
 were , therefore , performed to determine the vibrational density of 
 states from the velocity autocorrelation function . all calculations used the gga - pbe functional , and some tests were performed with semiempirical 
 dispersive corrections ( dft - d ) using the grimme parametrization . the calculations were performed on a single 
 unit cell of size 15  15  12  and therefore used a k - point set including 
 only the gamma point . ultrasoft pseudopotentials were used to separate 
 the core and ( pseudo ) valence electrons ( 1 for h , 4 for c , 6 for o , 
 and 13 for in were treated explicitly in the calculations ) , and the 
 plane wave cutoff was set to 340 ev for all calculations except md , 
 for which a lower cutoff of 260 ev was used to reduce computational 
 time . md simulations were performed in the nvt ensemble using a time 
 step of 1 fs and ran for 10  000 steps , therefore covering a 
 time frame of 10 ps . for go calculations , energy convergence was set 
 to 10 ev / atom , and the maximum residual force 
 per atom was typically 0.02 ev / atom . the solvated 
 framework complex [ in2(oh)2(c16h6o8)]1.28h2o2.87(dmf ) [ mfm-300(in)-solvate ] 
 was prepared via solvothermal reaction of h4l ( h4l = biphenyl-3,3,5,5-tetracarboxylic acid ) and in(no3)35h2o in a 2:1 ratio mixture 
 of dimethylformamide ( dmf ) and acetonitrile and was isolated as a 
 microcrystalline powder . structure solution by synchrotron powder 
 x - ray diffraction confirmed that this material crystallizes in the 
 tetragonal space group i4122 , and is isostructural 
 to the related mfm-300(al ) . we note that this mof 
 has been previously reported in a different preparation , which yields 
 large single crystals . in our hands , 
 these two forms of mfm-300(in ) are equivalent despite the difference 
 in morphology . mfm-300(in ) exhibits an open structure comprising chains 
 of [ ino4(oh)2 ] moieties bridged by tetracarboxylate 
 ligands l to afford a porous structure with channels 
 formed by corner - sharing [ ino4(oh)2 ] octahedra 
 linked via two mutually cis-2-oh 
 groups . the [ ino4(oh)2] chain 
 in mfm-300(in ) is similar to that found in mil-53(in ) , although the latter features trans-2-oh groups , resulting in pronounced framework 
 flexibility . the cis-2-oh groups 
 in mfm-300(in ) tie the framework into a rigid  wine - rack  
 arrangement . desolvated mfm-300(in ) displays a surface area of 1071 
 m g and a total pore volume of 0.419 
 cc g as determined from the n2 isotherm 
 at 77 k. these values are consistent with the porosity derived from 
 the single - crystal structure , and confirm the complete activation 
 and phase purity of the material . h2 and ch4 sorption isotherms confirm that 
 mfm-300(in ) displays completely reversible uptake over a wide range 
 of temperatures ( 77308 k ) and pressures ( figure 1 ) . the h2 adsorption 
 properties of this material are commensurate with the measured pore 
 volume and bet surface area , reaching 275 v / v at 77 k and 1 bar , rising 
 to 488 v / v at 20 bar , comparable with other mof materials with similar 
 bet surface areas . the heat of adsorption at zero 
 loading is estimated to be 6.8 kj mol , within 
 the range ( 48 kj mol ) typically observed 
 for mof materials with comparable uptakes . ( a , b ) 
 adsorption isotherms of h2 and ch4 in 
 mfm-300(in ) , respectively . ( c ) variation of the isosteric heat ( qst ) and entropy ( s ) 
 of adsorption of ch4 in mfm-300(in ) . ch4 adsorption in mfm-300(in ) shows a maximum 
 uptake 
 of 328 v / v at 110 k and 0.6 bar , dropping gradually with increasing 
 temperature , with uptakes at 20 bar of 266 , 224 , and 198 v / v at 195 , 
 245 , and 273 k , respectively . high pressure ch4 adsorption 
 isotherms revealed uptake capacities of 220 and 202 v / v at 273 and 
 298 k , respectively , at 35 bar , rising to 228 and 213 v / v , respectively , 
 at 50 bar . grand canonical monte carlo ( gcmc ) simulations show excellent 
 agreement with these experimental data ( figure 1b ) . the ch4 uptake of mfm-300(in ) 
 at 35 bar and 298 k is one of the highest values reported for a mof 
 to date , surpassed only by mof-74(ni , co ) and hkust-1 ( table 1 ) . the isosteric heat of adsorption of ch4 was estimated using the vant hoff isochore to be 
 16 kj mol , which gradually rises to 20 kj mol with increasing ch4 loading ( figure 1 ) , comparable to 
 that of other mofs with similar uptake capacities ( 1221 kj 
 mol ) . the steady increase 
 in the heat of adsorption reflects the presence of intermolecular 
 interactions between adsorbed ch4 molecules within the 
 pores . langmuir model , 250 bar , 298 k. the density of adsorbed ch4 in mfm-300(in ) 
 can be estimated 
 by using the total pore volume ( 0.419 cc g ) derived 
 from the n2 isotherm at 77 k ( assuming no distortion of 
 the pore structure ) . at 110 k and 0.6 bar , the density of adsorbed 
 ch4 in mfm-300(in ) was calculated to be 0.37 g cm , comparable to that of liquid ch4 ( 0.42 g cm at 111 k ) , suggesting an ultraclose packing of the adsorbed ch4 molecules in the pore , which could be a result of strong 
 geometric confinement and/or specific host binding . significantly , 
 to our best knowledge , this density ( 0.37 g cm ) found in mfm-300(in ) represents the highest observed value in a 
 porous mof ( table 1 ) . the density of adsorbed ch4 in mfm-300(in ) at 298 k 
 and 35 bar is estimated as 0.231 g cc ( 55% of 
 the liquid density ) . similarly , the density of adsorbed h2 approaches that of the liquid , 0.069 g cc at 
 77 k and 20 bar ( liquid hydrogen density = 0.0708 g cc at 20 k ) . the high density packing of the adsorbed gas molecules 
 indicates that adsorbed ch4 and h2 molecules 
 in mfm-300(in ) pack very efficiently , resulting in a very high storage 
 density in a given pore volume . this led us to study the packing of 
 ch4 and h2 molecules within mfm-300(in ) at crystallographic 
 resolution and investigate vibrational and rotational motions within 
 the pore to determine the intermolecular interactions , which account 
 for this observation . the locations of adsorbed h2 and ch4 molecules within mfm-300(in ) were determined 
 by in situ neutron powder diffraction ( npd ) as a 
 function of gas loading ( figure 2 ) . npd patterns were recorded at 7 k for the desolvated 
 material and at loadings of 1.0 , 1.5 , 3.0 , 4.5 , and 6.0 d2/in and 1.0 , 2.0 , and 3.0 cd4/in . fourier difference map 
 analysis of the npd data of the desolvated mof indicates no residual 
 nuclear density peaks within the pore , thus confirming the complete 
 activation and structural stability of the desolvated sample . the 
 bare material was then dosed volumetrically to the desired loading 
 of d2 at 50 k and cd4 at 150 k , accompanied 
 by significant changes in the relative peak intensities of the diffraction 
 patterns . sequential fourier difference map analysis of the npd patterns 
 revealed the position of the center of gravity of the adsorbed d2 and cd4 molecules , which were further developed 
 by rietveld refinement of these data . view of the crystal structure of h2/ch4 loaded 
 mfm-300(in ) showing ( a , b ) view of the corner - sharing extended [ ino4(oh)2 ] chain highlighting interactions between 
 the framework structure with d2 and cd4 molecules , 
 respectively ; and ( c , d ) view of the c - crystallographic 
 axis showing the positions of the adsorbed d2 and cd4 molecules , respectively . d2 molecules in ( a ) and 
 ( c ) are colored according to the scheme : site i , green ; site ii , orange ; 
 site iii , yellow ; site iv , purple ; site v , blue ; site vi , gray ; site 
 vii , light blue . cd4 molecules in ( b ) and ( d ) are colored 
 according to the following scheme : site i , green ; site ii , orange ; 
 site iii , yellow ; site iv , purple ; site v , blue ; site vi , gray ; site 
 vii , light blue . at the first loading 
 of d2 , two independent binding 
 sites ( d2 and d2 ) were 
 located within the one - dimensional pore of mfm-300(in)(d2)1.80 . the initial binding site , d2 , is located near the bridging hydroxyl group of the [ ino4(oh)2 ] backbone [ d2h  o 
 = 2.54(1 )  ] with an occupancy of 0.73 . d2 is located interstitially between two phenyl rings of neighboring 
 ligand molecules [ d2phenyl 
 ring = 3.78(1 )  ] . d2 is 3.54(2 )  
 from the edge of the pore channel , with an occupancy of 0.17 , with 
 an intermolecular distance between d2 and d2 of 5.20(2 )  . upon the second loading , 
 mfm-300(in)(d2)3.20 , the occupancy of 
 d2 increases to 0.91 , accompanied by the occupancy 
 of d2 tripling from 0.17 to 0.50 and moving 
 closer to the edge of the pore channel [ d2phenyl ring = 2.78(1 )  ] . additional adsorbed 
 d2 molecules now fill a third site ( d2 ) at an occupancy of 0.19 , sandwiched between two adjacent d2 sites [ d2d2 = 2.50(1 )  ] . on additional d2 dosing to mfm-300(in)(d2)5.98 , the occupancy of d2 remains 
 at 0.92 , and the occupancy of d2 increases 
 to 0.76 and shifts closer to the edge of the pore channel [ d2phenyl ring = 2.59(2 )  ] . site 
 iii ( d2 ) remains in the same position , with 
 an increase of occupancy from 0.19 to 0.55 . an additional d2 binding site d2 is observed interstitial 
 between a d2 and d2 , with an occupancy of 0.80 between sites d2 and d2 [ d2d2 d2 = 3.21(1 ) and 4.06(2 )  , respectively ] . upon the fourth gas 
 dosing ( 4.8 d2/in ) , the positions of the previously determined 
 d2 binding sites are unchanged , with the addition of two 
 further adsorption sites . the first new site , d2 , is located toward the center of the pore , 2.05(1 )  from d2 and 3.46(1 )  from the central carbon atom 
 of the organic ligand with an occupancy of 0.75 . the second new site , 
 d2 , is located between sites d2 and d2 with a distance of 2.72(1 ) 
  for d2d2 , with an occupancy of 0.36 . at the final loading of 
 d2 , mfm-300(in)(d2)12.10 , the 
 occupancies of d2 increase further 
 toward saturation ( table 2 ) . the position of the newly observed d2 site is shifted considerably toward the center of the pore channel 
 at this loading , increasing the distances to d2 to 3.13(4 )  , accompanied by an increase in occupancy to 1.40 
 ( 2 ) . at this saturation loading , a further adsorption site , d2 , interstitial between d2 and d2 is observed [ d2d2 = 2.95(3 )  ] at 
 an occupancy of 0.12 . at this final loading , mfm-300(in ) is saturated 
 with d2 , with a gas loading corresponding to an uptake 
 of 689 cc cc . this high uptake capacity indicates 
 very close packing of molecular d2 at 7 k. quantum effects 
 may also account for the high saturation uptake for deuterium in this 
 mof material . d2 is 
 refined as a single 
 atom with occupancy 2.0 because the orientation could not be determined 
 from the npd data . after taking into account the site multiplicity , the occupancies are shown as d2 molecule per indium ion . upon loading the regenerated 
 material with methane , two independent 
 cd4 binding sites ( cd4 and cd4 ) were located in mfm-300(in)(cd4)2.18 ( table 3 ) . cd4 is located near 
 the bridging hydroxyl group [ ch  o 
 = 3.881(12 )  ] with an occupancy of 0.796(4 ) ; cd4 is located interstitially between the phenyl rings from 
 two ligand molecules [ cphenyl ring 
 = 3.424(8 )  ] and is 3.060(8 )  from the edge of the 1d 
 channel with a lower occupancy of 0.292(5 ) . the intermolecular distance 
 between cd4 and cd4 is 5.284(3 )  . on further loading to mfm-300(in)(cd4)4.18 , the occupancy for cd4 increases slightly to 0.872(4 ) , and its distance to the bridging 
 hydroxyl group is also increased [ ch  o 
 = 4.040(8 )  ] . the location of cd4 remains 
 the same [ cd4cd4 = 5.248(2 )  ] , and the occupancy doubles from 
 0.292(5 ) to 0.596(6 ) . additional adsorbed cd4 molecules 
 now fill a third site ( cd4 ) , which is sandwiched 
 by sites cd4 and cd4 [ cc and cc distances of 3.327(8 ) and 5.089(8 ) 
  , respectively ] and 4.123(8 )  from the edge of the 1d 
 channel with an occupancy of 0.284(4 ) . in the final loading , mfm-300(in)(cd4)6.22 , the occupancy of cd4 increases to saturation with a refined occupancy of 1.008(12 ) [ ch  o = 3.989(8 )  ] , while cd4 is unchanged . the occupancy of cd4 increases to 0.478(6 ) , and its location moves slightly 
 toward the pore center [ 4.629(10 )  from the edge of the 1d channel ; 
 cc distance of 5.649(11 ) 
  ] . interestingly , a fourth site ( cd4 ) appears to accommodate the remaining cd4 molecules . cd4 is sandwiched between two cd4 sites with an intermolecular separation of 3.984(11 ) 
  for cc and an 
 occupancy of 0.564(14 ) . thus , the adsorbed molecules cd4 form a 1d alternating molecular chain in the 
 1d channels of mfm-300(in ) via intermolecular interactions ( figure 2b ) , consistent with 
 the steady increase in the heat of adsorption on loading of ch4 . after taking into account the site 
 multiplicity , the occupancies are shown as cd4 molecule 
 per in ion . in mfm-300(in)(cd4)6.22 , the material is saturated with a total cd4 occupancy corresponding to 
 an uptake of 340 v / v and with an adsorbed cd4 density of 
 0.63 g cm , 92% of that of solid deuterated ch4 ( 0.68 g cm ) . this result is in excellent agreement with the measured ch4 isotherm uptake ( 328 v / v ) at 110 k. the slightly higher value observed 
 for cd4 adsorption is probably due to a quantum effect 
 as observed in h2/d2 adsorption studies . comparison of the densities of adsorbed and 
 solid cd4 indicates a very efficient intermolecular packing 
 of ch4 molecules in mfm-300(in ) , considering that the density 
 of adsorbed cd4 may be underestimated by the use of the 
 pore volume derived from n2 isotherm at 77 k , as the total 
 void space of mfm-300(in ) as measured and determined by the n2 bet experiment may not be entirely accessible to the bulkier 
 ch4 molecules . the high packing efficiency is also evidenced 
 by the observation of the comparable intermolecular distance between 
 the adsorbed cd4 molecules at different sites when compared 
 to that in solid ch4 ( 3.954.28  ) . this npd study confirms that the free bridging hydroxyl group 
 within 
 the pore of mfm-300(in ) is the primary binding site for adsorbed h2 and ch4 molecules , leading to high occupancy at 
 low gas loadings . this suggests that adsorbed h2 and ch4 molecules populate at site d2/cd4 at low surface coverage as a direct result of 
 a cxdyh  o binding interaction . d2 forms a direct binding interaction with 
 the bridging hydroxyl group in the pore , and the binding distance 
 is slightly longer than that observed in mofs with open metal sites , consistent with the nature of the different binding 
 mechanisms . upon higher loadings of d2 , multiple sites 
 appear at the center of the pore , forming a 3d intermolecular binding 
 network similar to that observed in solid h2 . at higher 
 loadings of cd4 , the occupancy of this initial site cd4 increases in occupancy from 0.796(4 ) to 0.872(4 ) , 
 reaching a maximum of 1.008(12 ) from the first to the third loading . this suggests that adsorbed ch4 molecules populate site 
 cd4 at low surface coverage as a direct result 
 of d4ch  o binding . with increased ch4 loading , site cd4 reaches saturation rapidly with residual ch4 molecules filling other sites stabilized , in principle , by 
 intermolecular interaction between adsorbed ch4 molecules . the d4ch  o distance for the first adsorption site does not vary significantly 
 with loading [ 3.881(12)4.040(8 )  ] , indicating that the 
 strength of the d4ch  o interaction is constant regardless of the presence of 
 additional cd4cd4 interactions . this distance is very similar to that observed in ch4/water 
 clathrates ( co = 
 3.773.92  ) . closer examination 
 reveals an interesting structural difference between the d4ch  o binding 
 in mfm-300(in ) and that in ch4/water clathrates . in the 
 latter case , water molecules form two different types of cages via 
 intermolecular hydrogen bonds : ( i ) six large cages having 12 pentagonal 
 and 2 hexagonal faces ( denoted by 56 ) formed 
 by 24 water molecules and ( ii ) two small cages having 12 pentagonal 
 faces ( denoted by 5 ) formed by 20 water molecules . the 
 large and small cages have an average cavity size of 4.33 and 3.95 
  , respectively , and host one ch4 molecule per cage , 
 giving a stoichiometry of ( ch4)0.174h2o ( figure 3 ) . 5  , and this 
 is very similar to the intermolecular distance observed for adsorbed 
 cd4 molecules in mfm-300(in ) , confirming the high packing 
 efficiency in the mof . the stoichiometries for adsorbed cd4 molecules in mfm-300(in ) with respect to the ho - m group are ( cd4)1.09ho - m , ( cd4)2.09ho - m , and ( cd4)3.11ho - m for the 
 first , second , and third loadings , respectively . although these numbers 
 are much higher than that observed for clathrates , it is worth noting 
 that the nature of the host  guest complexes ( clathrate and 
 mof ) is not strictly comparable . indeed , this result suggests that , 
 in addition to the ho - m group , the geometrical confinement due to 
 the phenyl rings in the pore also plays a significant role in trapping 
 ch4 in the pore . on both cages of the clathrate , the protons 
 on water molecules all point toward each other , assembling cage  edges  
 with a fully disordered ch4 molecule at the center . thus , 
 the dominating binding interaction in clathrates will be h3c  ho  h . in contrast , adsorbed cd4 molecules bind to the hydroxyl 
 groups along the h  o bond in mfm-300(in ) , and thus the dominating 
 binding interaction is h3c indeed , the position of the proton on the hydroxyl group 
 was refined in the npd data , and no distortion of the ho - m group was 
 observed upon cd4 inclusion , as is also evidenced by further 
 neutron spectroscopic studies and corresponding modeling ( see below ) . thus , this study may open new investigations of binding interactions 
 in clathrate materials when strong space confinement is considered . views 
 of ( a ) the cage structure and h3c  ho interaction of ch4 in 
 a clathrate ; and ( b ) the channel and h4ch  o interaction 
 of ch4 in mfm-300(in ) . distances do not take into account 
 van der waals radii ; the equivalent distances taking this into account 
 are 4.12 and 7.5  for the ch4water clathrate 
 and mfm-300(in ) , respectively . the absolute orientation 
 for all of the adsorbed d2 and cd4 molecules 
 can not be unambiguously determined from the npd data due to the thermal 
 motion of the host  guest system , despite data collection at 
 7 k. it is widely accepted that both of these adsorbates ( h2 and ch4 ) act as a  free rotor  in bulk 
 phase , but studies on the rotational motions and 
 dynamics of adsorbed h2 are still in their infancy and 
 the dynamics of ch4 molecules in mofs remain unexplored . 
 to gain insights into these properties , inelastic neutron scattering 
 ( ins ) spectra were recorded for bare mfm-300(in ) as a function of 
 h2 and ch4 loading to investigate the rotational 
 and vibrational properties of the host  guest system . ins is 
 a very powerful neutron spectroscopic technique used to study the 
 dynamics of h - containing systems , enabled by the very large scattering 
 cross section of hydrogen ( 12 orders of magnitude ) in comparison 
 to most other elements . ins spectra of the bare mfm-300(in ) and ch4 loaded material at loadings of 1.0 , 2.0 , 4.0 , and 6.0 h2/in and 0.5 and 1.0 ch4/in were recorded at 11 
 k ( figures 4 , 5 , and s41s45 ) . the ins spectrum for the bare mfm-300(in ) is very similar to that 
 of the iso - structural mfm-300(al ) because 
 metal ions ( in and al ) have a negligible effect on the inelastic neutron 
 scattering . comparison of the ins spectra for measurements 
 of mfm-300(in ) at 
 tosca , calculations by md , and calculations by ld ; ( bottom ) view of 
 the structural model showing the almost free rotation of ch4 molecules at the bridging hydroxyl group ( 1 thz = 4.14 
 mev = 33.4 cm ) . additional ins spectra are shown 
 in the supporting information . upon loading this material with para - h2 , there is no distortion of the vibrational modes arising 
 from the 
 internal modes of the bare material , confirming the absence of structural 
 change , consistent with the npd analysis and the low enthalpy of adsorption . 
 at a low h2 loading ( 1.0 h2/in ) ( figure 4 ) , a new peak was observed 
 at 9.5 mev , lower than that of the solid h2 ( 14.7 mev ) , 
 corresponding to the rotational motion of the adsorbed h2 . the appearance of this 
 recoil motion indicates that there is little interaction between the 
 adsorbed h2 and host material . the almost - free rotation 
 of adsorbed h2 within this material is consistent with 
 the crystal structure of pure solid h2 , in which the molecular 
 positions can be discerned , but the atoms are spherically disordered , 
 even at temperatures as low as 2 k. at 
 a higher h2 loading of 2.0 h2/in , there is an 
 increase in the area of the peaks relating to the rotational and recoil 
 motion of the adsorbed h2 , consistent with the h2 binding at the low loading . further increasing the h2 loading to 4.0 and 6.0 h2/in results in the consistent 
 growth of the rotational and recoil peaks , accompanied by an additional 
 peak at 14.7 mev , consistent with the formation of  solid - hydrogen - like  
 material within the pores of the mof . this observation is in excellent 
 agreement with the insight gained from the in situ crystallographic study , which reveals that at this loading , the 
 intermolecular bond length between adsorbed h2 molecules 
 ( 35  ) is of an order similar to that observed in solid 
 h2 ( 3.75  ) . ins spectra of ch4-loaded mfm-300(in ) show a considerable 
 increase in intensity with a broad peak at low energy transfer ( below 
 40 mev ) as a result of adsorbed ch4 molecules ( figures s44 ) . difference ins spectra exhibit 
 an intense and broad peak at 1030 mev as the dominating signal . no significant change to the vibrational peaks arising from the internal 
 modes of the bare mof was observed , indicating that there is no strong 
 distortion of the local h modes from the porous material upon ch4 adsorption . this result suggests that adsorbed ch4 molecules in mfm-300(in ) have a wide amplitude , weakly hindered 
 rotational motion , even at a temperature that is well below its melting 
 point ( 90.7 k ) . this is consistent with the structure of solid ch4-i that is shown to have well - defined molecular positions , 
 but spherically disordered protons . geometry optimization ( go ) calculations were performed with both 
 fixed and variable cell parameters , with an observed increase of the 
 unit cell dimensions of 23% . however , we found this did not 
 affect the calculated binding energies , rotational potentials , or 
 vibrational spectra . binding energies were calculated for the two 
 most stable sites ( cd4 and cd4 ) , which are populated with adsorbed ch4 molecules 
 at low surface coverage . density functional theory ( dft ) calculations 
 determine the binding energies to be 20.5 and 19.7 kj mol for cd4 and cd4 , 
 respectively , in excellent agreement with that determined experimentally 
 from the adsorption isotherms ( 1620 kj mol ) . the similarity of the calculated binding energies at two different 
 sites also explains the absence of any marked steps in the adsorption 
 profiles ( even in logarithmic view as shown in figure s12 ) that would be indicative of the presence of adsorption 
 sites of significantly different stabilities . rotating the ch4 molecule on the most stable site ( cd4 ) about a c  h bond aligned with the o h 
 group ( figure 5 ) and 
 performing a series of single point energy ( spe ) calculations enabled 
 the rotational potential of adsorbed ch4 molecules to be 
 determined . in this case , the rotational barrier is 5 mev ( 0.48 kj 
 mol ) , and therefore the rotation of adsorbed ch4 molecules is almost free , consistent with the observed strong 
 recoil motion at low energy transfer in the ins spectra . lattice 
 dynamics ( ld ) calculations of the bare mfm-300(in ) enable 
 the hydrogen density of states to be determined , which can be compared 
 to the experimental spectrum . above 18 thz or 74 mev , there are pronounced 
 vibrational peaks , which show a good match between ld calculations 
 of mfm-300(in ) and the measured spectrum ( figure 5 ) . the vibrational spectrum is a fingerprint 
 of the solid - state structure , and reproducing the measured spectrum 
 from the experimental structure using a high level of theory ( dft ) 
 validates the structural model . molecular dynamics ( md ) simulations 
 were performed at temperatures 
 ranging from 50 to 250 k. the simulation at 250 k gave the most appropriate 
 sampling of the potential energy surface ( pes ) . the hydrogen vibrational 
 density of states from this run was in good agreement with the ld 
 calculation and ins experiment ( figure 5 ) . md simulations of ch4-loaded mfm-300(in ) 
 allowed the vibrational density of states of the ch4 molecules 
 to be determined , and this calculated spectrum for ch4 was 
 compared to the experimental difference spectrum ( figure s43 ) . in the low frequency part of the spectrum , where 
 signals are related to whole molecule vibrations and low frequency 
 liberational / rotational modes , the spectra are in good agreement . 
 at higher frequency , the calculated spectrum shows the internal modes 
 ( bending and stretching ) of ch4 , but these are not observed 
 in the experimental ins spectrum . waller factor ( exp(qu ) ) , which is 
 stronger for ch4 than for the mof host , revealing a higher 
 mean square displacement ( u ) for the 
 ch4 because it is relatively weakly bound and can undergo 
 almost free rotations as suggested from spe calculations . thus , 
 this neutron spectroscopic study of mfm-300(in ) has investigated 
 the rotational modes and packing of adsorbed h2 and ch4 within this material . ins spectra of h2 loaded 
 mfm-300(in ) confirm the appearance of two types of adsorbed h2 : one with reduced rotational energy and one with almost - free  
 rotations ; the latter is a reminiscent of solid h2 . this 
 study further confirms that the bridging hydroxyl is the strongest 
 binding site for methane within mfm-300(in ) , forming h3c hh  o binding interactions , with the adsorbed ch4 molecules 
 having wide amplitude , weakly hindered rotational motion in the confined 
 cavity of the mof , even at 11 k. to improve the ch4 storage 
 capacity , further efforts are required to  immobilize  
 adsorbed ch4 molecules in the pore by increasing the binding 
 energies and reducing their rotational motion via introduction of 
 more specific host  guest interactions . several key insights into the binding of h2 and ch4 within the hydroxyl - decorated material mfm-300(in ) have been 
 revealed from the direct visualization and quantification of the host this has implications for the design and discovery of 
 future materials capable of storing h2 and ch4 at high capacity . the binding of h2 within this material 
 is mediated by a combination of a number of weak intermolecular interactions 
 at low loadings and a combination of adsorbate  adsorbent and 
 adsorbent  adsorbent interactions at high loadings . the very 
 high density of adsorbed ch4 molecules observed in mfm-300(in ) the 
 phenyl rings on the linker play a further role in providing space 
 confinement for the adsorbed ch4 molecules , thereby inducing 
 a change from the h3c  ho  h binding mode observed in clathrates . these multiple 
 relatively weak binding interactions are aligned in the pore with 
 an optimal geometry , offering a highly cooperative binding environment 
 for the adsorbed h2 and ch4 molecules . as a 
 result , the adsorption and binding modes of h2 and ch4 within this material have been observed and rationalized 
 by both experimental and theoretical investigations . we hope that 
 the strategy developed here will help the development of improved 
 materials with optimized pore environment for natural gas storage 
 and separation .
the key requirement for a portable 
 store of natural gas is to maximize 
 the amount of gas within the smallest possible space . 
 the packing 
 of methane ( ch4 ) in a given storage medium at the highest 
 possible density is , therefore , a highly desirable but challenging 
 target . 
 we report a microporous hydroxyl - decorated material , mfm-300(in ) 
 ( mfm = manchester framework material , replacing the nott designation ) , 
 which displays a high volumetric uptake of 202 v / v at 298 k and 35 
 bar for ch4 and 488 v / v at 77 k and 20 bar for h2 . direct observation and quantification of the location , binding , 
 and rotational modes of adsorbed ch4 and h2 molecules 
 within this host 
 have been achieved , using neutron diffraction and 
 inelastic neutron scattering experiments , coupled with density functional 
 theory ( dft ) modeling . 
 these complementary techniques reveal a very 
 efficient packing of h2 and ch4 molecules within 
 mfm-300(in ) , reminiscent of the condensed gas in pure component crystalline 
 solids . 
 we also report here , for the first time , the experimental 
 observation of a direct binding interaction between adsorbed ch4 molecules and the hydroxyl groups within the pore of a material . 
 this is different from the arrangement found in ch4/water 
 clathrates , the ch4 store of nature .