Flexible Yttrium Coordination Geometry Inhibits “Bare-Metal” Guest Interactions in the Metal-Organic Framework Y(btc)

: Y(btc) (btc = 1,3,5-benzenetricarboxylate) is a metal-organic framework that exhibits signiﬁcant adsorption of industrially-relevant gases such as H 2 , CH 4 , and O 2 . Previous studies have noted a surprising lack of close interactions between the adsorbed guest molecules and Y, despite the apparent availability of a “bare-metal” binding site. We have extended our previous work in a detailed investigation of the adsorption behaviours of CO 2 , CD 4 , and O 2 in Y(btc) over a range of concentrations using in situ neutron powder diffraction methods. The O–Y–O bond angles enclosing the bare-metal site are found to change considerably depending on the type and quantity of guest molecules present. Multiple binding sites are found for each guest species, and the largest changes in O–Y–O angles are accompanied by changes in the ﬁlling sequences of the binding sites, pointing to an important interplay between guest-induced framework distortions and binding site accessibility. These results suggest the potential for coordinatively ﬂexible rare-earth metal centres to promote guest-selective binding in metal-organic frameworks.


Introduction
The atomic-scale understanding of gas-sorption mechanisms in porous solid sorbents has become a focus of major research efforts in recent times, driven in particular by the need for efficient gas separators in many energy-related applications.Porous crystalline materials such as metal-organic frameworks (MOFs) are often targeted for these applications due to their favourable properties: good gas selectivity and capacity, ease of handling compared to liquid sorbents, as well as adequate mechanical, thermal, and chemical stabilities.Importantly, these materials are highly tuneable, and significant opportunities exist for engineering their chemistries toward particular sorbent applications.Key framework features which improve adsorption properties need to be identified and optimised for the development of better future materials.For example, many studies have shown that the presence of coordinative-unsaturated metal centres tends to enhance total uptake of gases such as hydrogen [1][2][3], methane [4] and ammonia [5], among others, and can also contribute to guest selectivity in the presence of gas mixtures [6].These "bare" metal sites interact strongly with many guest molecules, yielding higher binding energies and often resulting in denser arrangements of guests Energies 2016, 9, 836 2 of 11 in the pores of the sorbent material.Unfortunately, the operative stability of bare metal-containing MOFs is often compromised by their ability to readily adsorb guests from the air, such as water and gaseous species, which can lead to permanent degradation of their crystallinity and adsorptive function [7][8][9].By contrast, the excellent moisture stability of the well-known Zr 6 O 4 (OH) 4 (BDC) 6 (where BDC = benzene-1,4-dicarboxylate), also known as UiO-66(Zr) material, has been partially attributed to the absence of coordinatively unsaturated Zr in the Zr 6 O 4 (OH) 4 (CO 2 ) 12 oxide cluster, preventing hydrolytic attack by water molecules [8].
Bare-metal sites are typically created in MOFs by the removal of coordinated solvent molecules from the material following its initial synthesis.In these cases, the preferred coordination geometry of the metal centre and the fixed network topology of the linker molecules both serve to maintain the overall structure of the MOF without significant changes to the framework geometry in the vicinity of the metal centre.However, MOFs containing rare-earth metals may display more flexibility in their coordination geometry upon removal of the solvent due to the greater number of satisfactory geometries adopted by these large metal ions [10][11][12].One such example is Y(btc) (btc = 1,3,5-benzenetricarboxylate), a MOF which displays appreciable uptake of H 2 [13], CH 4 and O 2 [14].Y(btc) is a structural analogue of Tb(btc) (also known as MOF-76 [15]) and consists of parallel 4 1 -type helices of Y atoms linked by the bridged carboxylate groups of three btc ligands, resulting in square channels which extend along the c axis in the tetragonal space group P4 3 22 (Figure 1).The distance between corresponding atoms on opposite walls of the square channels is ~10.3Å, equivalent to the a parameter of the unit cell.Structural studies have demonstrated significant relaxation of the linker geometry around the Y centre upon removal of the coordinated water-of-crystallisation, yielding a bare-metal site which may be less accessible to guest molecules [14].In the same study, CD 4 was found to display no significant interaction with the Y centre at 1 CD 4 :Y loading, while O 2 interacted weakly with Y only at the least populated of its three observed binding sites.Similarly, no interaction with the bare-metal site was reported for any of the four observed binding locations of D 2 in the material [13].
Energies 2016, 9, 836 2 of 11 arrangements of guests in the pores of the sorbent material.Unfortunately, the operative stability of bare metal-containing MOFs is often compromised by their ability to readily adsorb guests from the air, such as water and gaseous species, which can lead to permanent degradation of their crystallinity and adsorptive function [7][8][9].By contrast, the excellent moisture stability of the well-known Zr6O4(OH)4(BDC)6 (where BDC = benzene-1,4-dicarboxylate, also known as UiO-66(Zr) material has been partially attributed to the absence of coordinatively unsaturated Zr in the Zr6O4(OH)4(CO2)12 oxide cluster, preventing hydrolytic attack by water molecules [8].
Bare-metal sites are typically created in MOFs by the removal of coordinated solvent molecules from the material following its initial synthesis.In these cases, the preferred coordination geometry of the metal centre and the fixed network topology of the linker molecules both serve to maintain the overall structure of the MOF without significant changes to the framework geometry in the vicinity of the metal centre.However, MOFs containing rare-earth metals may display more flexibility in their coordination geometry upon removal of the solvent due to the greater number of satisfactory geometries adopted by these large metal ions [10][11][12].One such example is Y(btc) (btc = 1,3,5-benzenetricarboxylate), a MOF which displays appreciable uptake of H2 [13], CH4 and O2 [14].Y(btc) is a structural analogue of Tb(btc) (also known as MOF-76 [15]) and consists of parallel 41-type helices of Y atoms linked by the bridged carboxylate groups of three btc ligands, resulting in square channels which extend along the c axis in the tetragonal space group P4322 (Figure 1).The distance between corresponding atoms on opposite walls of the square channels is ~10.3Å , equivalent to the a parameter of the unit cell.Structural studies have demonstrated significant relaxation of the linker geometry around the Y centre upon removal of the coordinated water-of-crystallisation, yielding a bare-metal site which may be less accessible to guest molecules [14].In the same study, CD4 was found to display no significant interaction with the Y centre at 1 CD4:Y loading, while O2 interacted weakly with Y only at the least populated of its three observed binding sites.Similarly, no interaction with the bare-metal site was reported for any of the four observed binding locations of D2 in the material [13].The linear CO2 molecule has a smaller minimum diameter than CH4 and is more polarisable than either H2 or O2, giving it the opportunity for better access to, and more favourable interactions with, the restricted Y bare-metal site in Y(btc).Furthermore, the capture of CO2 by porous sorbents is of special relevance to the energy industry, due to the drive to minimise carbon dioxide emissions from the waste streams of existing fossil-fuel technologies such as coal plants [16,17], as well as The linear CO 2 molecule has a smaller minimum diameter than CH 4 and is more polarisable than either H 2 or O 2 , giving it the opportunity for better access to, and more favourable interactions with, the restricted Y bare-metal site in Y(btc).Furthermore, the capture of CO 2 by porous sorbents is of special relevance to the energy industry, due to the drive to minimise carbon dioxide emissions from the waste streams of existing fossil-fuel technologies such as coal plants [16,17], as well as ensuring Energies 2016, 9, 836 3 of 11 the viability of natural gas reserves.We have therefore undertaken a detailed investigation of the adsorption behaviour of CO 2 in Y(btc), extending a previous investigation of CH 4 and O 2 adsorption in the material by exploring the guest concentration effects on both the binding and the framework structure.As with the previous investigations of gas adsorption in Y(btc), neutron scattering methods were used for structural characterisation of the host-guest system, as scattering from the heavy yttrium dominates the X-ray data and inhibits the location of lighter framework and guest atoms.In the context of this new work and of results previously reported for O 2 , CH 4 , and H 2 [13,14], we present an analysis of the Y coordination environment in Y(btc) and its behaviour in the presence of different guest species and concentrations, with a view to understanding how MOF sorbent functionality is influenced by the use of coordinatively flexible rare-earth metal centres.

Adsorption Isotherms
Gas adsorption isotherms measured for Y(btc) at 298 K demonstrated good uptake of CO 2 , the measured value of around 5 mmol•g −1 at 10 bar being almost twice the uptake of CH 4 and four times that of O 2 or N 2 at the same pressure and temperature (Figure 2).Uptake kinetics for all gases were very rapid, with equilibration achieved within ~2 min after each dose was applied to the sample.Water adsorption in Y(btc) was found to be fully reversible and the isothermal curve was reproduced after a second adsorption cycle, highlighting the relatively-good stability of the material to moist environments (Figure S1).
Energies 2016, 9, 836 3 of 11 ensuring the viability of natural gas reserves.We have therefore undertaken a detailed investigation of the adsorption behaviour of CO2 in Y(btc), extending a previous investigation of CH4 and O2 adsorption in the material by exploring the guest concentration effects on both the binding and the framework structure.As with the previous investigations of gas adsorption in Y(btc), neutron scattering methods were used for structural characterisation of the host-guest system, as scattering from the heavy yttrium dominates the X-ray data and inhibits the location of lighter framework and guest atoms.In the context of this new work and of results previously reported for O2, CH4, and H2 [13,14], we present an analysis of the Y coordination environment in Y(btc) and its behaviour in the presence of different guest species and concentrations, with a view to understanding how MOF sorbent functionality is influenced by the use of coordinatively flexible rare-earth metal centres.

Adsorption Isotherms
Gas adsorption isotherms measured for Y(btc) at 298 K demonstrated good uptake of CO2, the measured value of around 5 mmol•g −1 at 10 bar being almost twice the uptake of CH4 and four times that of O2 or N2 at the same pressure and temperature (Figure 2).Uptake kinetics for all gases were very rapid, with equilibration achieved within ~2 min after each dose was applied to the sample.Water adsorption in Y(btc) was found to be fully reversible and the isothermal curve was reproduced after a second adsorption cycle, highlighting the relatively-good stability of the material to moist environments (Figure S1).

Binding Site Locations
The structure of the empty Y(btc) framework was initially refined against neutron powder diffraction (NPD) data collected for the CO2-loaded material.Three crystallographically-distinct CO2 adsorption sites were determined, all of which were observed to be partially occupied after the first dose of 1 CO2:Y (that is, one mole of CO2 per mole of Y(btc)) (Table 1).Y(btc)•CO2 refinement results can be found in the Supporting Information (Tables S1 and S2).

Binding Site Locations
The structure of the empty Y(btc) framework was initially refined against neutron powder diffraction (NPD) data collected for the CO 2 -loaded material.Three crystallographically-distinct CO 2 adsorption sites were determined, all of which were observed to be partially occupied after the first dose of 1 CO 2 :Y (that is, one mole of CO 2 per mole of Y(btc)) (Table 1).Y(btc)•CO 2 refinement results can be found in the Supporting Information (Tables S1 and S2). Figure 3 shows the arrangement and orientation of all symmetry-generated locations for each of the three CO 2 adsorption sites within a single Y(btc) unit cell.The molecules located at sites B CO 2 and C CO 2 lie along the [y, 0, 1/4] and [0, y, 1/2] 2-fold axes, respectively, and therefore occur with half the multiplicity of site A CO 2 .Site occupancy restrictions arising from close interactions between these adsorption sites are discussed in Section 2.1.3.The binding site with the highest CO 2 population, A CO 2 , is located approximately 3.1 Å above one carboxylate-functionalised arm of the btc ligand and 4.1 Å from the nearest available bare-metal site.It is interesting to note that one of the shortest host-guest distances is between the A CO 2 O atom nearest to the Y bare-metal site and the carboxylate O atom coordinated to the same Y atom.Although electrostatic repulsion is generally expected between two oxygen atoms, it appears likely that the coordinatively unsaturated Y atom draws electron density more strongly from the coordinated atoms, decreasing their partial negative charges to the point where a weak O-O distance of 3.14 (10) Å is permitted between them.The other nearest-neighbour distances between A CO 2 and Y(btc) are attributed to typical electrostatic interactions between electron-deficient and electron-rich atoms, with observed distances in the intermediate-to-weak range of 2.86-3.42Å.
The second-most populated CO 2 adsorption site, B CO 2 , is located approximately 2 Å above the pseudo-plane of two adjacent btc ligands and interacts symmetrically with four carboxylate functional groups.As a result of the relative positioning and distance from each of these groups, the observed host-guest interactions are attributed to electrostatic interactions dominated by the quadrupolar nature of the CO 2 guest.The O atoms of the CO 2 molecule are positioned 3.17(9) Å from the C atom of the nearest carboxylate group, and 2.94(8) Å from the adjacent C atom on the 6-membered ring.
Energies 2016, 9, 836 5 of 11 These electron-deficient C atoms provide a favourable environment with which the electron-rich O guest atoms can interact.Similarly, O atoms from two of the nearby carboxylate groups appear to sandwich the electron-deficient C atom of CO 2 , forming a ring of favourable electrostatic host-guest interactions below the guest molecule.Nevertheless, while site B CO 2 appears more favourable based on the quantity of electrostatic host-guest interactions, the observed occupancy factor of 0.199(17) CO 2 :Y at a loading of 1 CO 2 :Y is less than half that of site A CO 2 .
The third observed adsorption site, C CO 2 , displays the longest host-guest distances and the lowest site occupancies after both CO 2 doses.This site is located 3.628(10) Å and 4.21(3) Å from the nearest and next-nearest btc carboxylate groups, respectively.These distances are generally considered too large to result from significant host-guest interactions, so it is proposed that this site arises from the formation of a bilayer with previously adsorbed CO 2 molecules.When the gas-loaded system is viewed along the c axis, it can be seen that site C CO 2 is closest to the pore centre (Figure 3b), further supporting this hypothesis.
It should be noted that none of the three CO 2 adsorption sites display any significant interaction with the available Y bare-metal sites, with the closest interaction distances occurring for sites A CO 2 and C CO 2 at 4.16(9) and 4.640(10) Å, respectively.It is typically expected that the bare-metal sites a coordination framework should provide guest molecules, particularly quadrupolar CO 2 molecules, with a favourable interaction site.This result will be discussed further below.
The total refined occupancies across the observed CO 2 sites after both doses of CO 2 equated to 80%-85% of the amount of CO 2 dosed, indicating that up to 20% of guests were disordered within the framework and could not be located crystallographically.The relative occupancies of each site remained largely unchanged between the first and second doses, with all occupancy factors increasing by around 100% when the total CO 2 dosage was doubled.

Intermolecular Interactions and Occupancy Restrictions
As seen in Figure 4, several of the possible CO 2 binding sites lie in close proximity to one another, and this is expected to impose some occupancy restrictions.The shortest guest-guest distance of 2.063(1) Å occurs between two symmetrically-equivalent C CO 2 sites, restricting the maximum occupancy of this site to 50% (0.5 CO 2 :Y).The highest observed C CO 2 occupancy factor, ~26%, lies well within this upper bound.The nearest intermolecular distance between sites A CO 2 and C CO 2 (2.10(8) Å) also restricts the co-occupation of these sites to 50%.As the multiplicity of site A CO 2 is 8, the maximum 50% occupancy of this site corresponds to 1 CO 2 :Y.This value is approached after the second dose of CO 2 .
6-membered ring.These electron-deficient C atoms provide a favourable environment with which the electron-rich O guest atoms can interact.Similarly, O atoms from two of the nearby carboxylate groups appear to sandwich the electron-deficient C atom of CO2, forming a ring of favourable electrostatic host-guest interactions below the guest molecule.Nevertheless, while site BCO2 appears more favourable based on the quantity of electrostatic host-guest interactions, the observed occupancy factor of 0.199 (17) CO2:Y at a loading of 1 CO2:Y is less than half that of site ACO2.
The third observed adsorption site, CCO 2 , displays the longest host-guest distances and the lowest site occupancies after both CO2 doses.This site is located 3.628(10) Å and 4.21(3) Å from the nearest and next-nearest btc carboxylate groups, respectively.These distances are generally considered too large to result from significant host-guest interactions, so it is proposed that this site arises from the formation of a bilayer with previously adsorbed CO2 molecules.When the gas-loaded system is viewed along the c axis, it can be seen that site CCO 2 is closest to the pore centre (Figure 3b), further supporting this hypothesis.
It should be noted that none of the three CO2 adsorption sites display any significant interaction with the available Y bare-metal sites, with the closest interaction distances occurring for sites ACO 2 and CCO 2 at 4.16(9) and 4.640(10) Å , respectively.It is typically expected that the bare-metal sites of a coordination framework should provide guest molecules, particularly quadrupolar CO2 molecules, with a favourable interaction site.This result will be discussed further below.
The total refined occupancies across the observed CO2 sites after both doses of CO2 equated to 80%-85% of the amount of CO2 dosed, indicating that up to 20% of guests were disordered within the framework and could not be located crystallographically.The relative occupancies of each site remained largely unchanged between the first and second doses, with all occupancy factors increasing by around 100% when the total CO2 dosage was doubled.

Intermolecular Interactions and Occupancy Restrictions
As seen in Figure 4, several of the possible CO2 binding sites lie in close proximity to one another, and this is expected to impose some occupancy restrictions.The shortest guest-guest distance of 2.063(1) Å occurs between two symmetrically-equivalent CCO 2 sites, restricting the maximum occupancy of this site to 50% (0.5 CO2:Y).The highest observed CCO 2 occupancy factor, ~26%, lies well within this upper bound.The nearest intermolecular distance between sites ACO 2 and CCO 2 (2.10(8) Å ) also restricts the co-occupation of these sites to 50%.As the multiplicity of site ACO 2 is 8, the maximum 50% occupancy of this site corresponds to 1 CO2:Y.This value is approached after the second dose of CO2.The nearest interactions between sites A CO 2 and B CO 2 occur at a distance of 2.35(2) Å.These two sites are aligned in an offset-parallel orientation relative to one another that takes advantage of the electrostatic interactions between the slightly positively-charged C and slightly negatively-charged O atoms.Although an end-to-side interaction (similar to that observed in solid CO 2 ) would more effectively exploit the quadrupolar nature of the CO 2 molecule, the observed arrangement appears to represent an optimal guest-guest configuration given the space constraints inside the Y(btc) pore and the presence of additional host-guest electrostatic interactions.

Methane and Oxygen Adsorption at High Dosage
Each NPD measurement described in [14] for CD 4 and O 2 at 1 guest:Y dosing immediately followed by a second dose of 1 guest:Y applied to the same sample, for a total dose amount of 2 guest:Y.NPD data were also collected after the second dose of each gas.The fractional occupancies at all binding sites previously identified for CD 4 and O 2 increased after the second dose, but no new binding sites were identified for either gas (Table 2).Full refinement results for the guest-loaded systems with 2 guest:Y loading can be found in the Supporting Information (Tables S3 and S4).A reversal of binding site preference upon increased loading was observed for CD 4 , with site B CD 4 (interacting with the carboxylate groups and first aryl carbon atom) displaying a higher occupancy than site A CD 4 (interacting with carboxylate only) after the second dose.Similarly, the preference for guest binding at sites B O 2 (near carboxyl and aryl carbons) and C O 2 (Y bare metal site) was reversed after the second O 2 dose, though site A O 2 (carboxylate groups and first aryl carbon) remained the dominant binding site, containing more than half of the adsorbed O 2 .
Adsorption of all guests results in a slight contraction of the unit cell of Y(btc) (Table 2).The a parameter decreases slowly and uniformly with increased loading of CO 2 , CD 4 and O 2 , except for a sharp decrease of ~0.25% between the first and second CO 2 dose.The c parameter also decreases uniformly by a similar magnitude, with the largest overall change occurring for O 2 (~0.15% decrease after the second dose).

The Yttrium Coordination Sphere
The possible region for guest binding between the Y atom and the guest-accessible pore space is enclosed by carboxylate O atoms belonging to four different btc ligands.The opposite pairs of these O atoms are crystallographically equivalent and correspond to sites designated O1 and O3.The angles formed by these atom pairs and the Y centre change considerably upon removal of the water of crystallisation from the as-synthesised framework, with the btc ligands closing in around the vacated binding site (Table 3).The subsequent addition of other guest molecules causes a partial re-opening of the O1-Y-O1 angle, even though the direct interactions between the guests and Y at this location are minimal [14].Interestingly, the angle opening becomes more pronounced at higher loadings of CD 4 and O 2 , but does not change significantly with increased CO 2 loading.The O3-Y-O3 angle, by contrast, contracts further upon the addition of guests to the empty framework.The only exception occurs at the 2O 2 :Y loading where the angle becomes slightly larger than that of the empty framework, probably due to the small amount of O 2 binding to the bare Y site at high loadings [14].A graphical representation of the changing YO 6 coordination environment is shown in Figure 5. exception occurs at the 2O2:Y loading where the angle becomes slightly larger than that of the empty framework, probably due to the small amount of O2 binding to the bare Y site at high loadings [14].
A graphical representation of the changing YO6 coordination environment is shown in Figure 5.

Discussion
The four guest species D2, CO2, CD4, and O2 display markedly different adsorption behaviour in Y(btc).In the absence of direct binding to the bare-metal site, the polar carboxylate groups tend to offer the most favourable adsorption sites and dominate guest occupation at low dosage amounts.However, the precise locations and orientations of these binding sites are unique to each guest, and the order of subsequent site filling also varies among the different species.
D2 occupies four distinct binding sites, and is the only guest for which these sites are filled in a sequential fashion.Luo et al. [13] found that Site A, which lies near the carboxylate C atoms, is more than 65% occupied before any significant population is observed at Site B, which is closer to the aryl rings.Following the addition of further D2, Site A saturates and Site C near the carboxylate O atoms populates almost to the level of Site B; B and C then co-fill to saturation.Site D appears to interact primarily with the carboxylate O atoms and other bound D2 molecules, and is only slightly populated at loadings higher than 4 D2:Y.

Discussion
The four guest species D 2 , CO 2 , CD 4 , and O 2 display markedly different adsorption behaviour in Y(btc).In the absence of direct binding to the bare-metal site, the polar carboxylate groups tend to offer the most favourable adsorption sites and dominate guest occupation at low dosage amounts.However, the precise locations and orientations of these binding sites are unique to each guest, and the order of subsequent site filling also varies among the different species.D 2 occupies four distinct binding sites, and is the only guest for which these sites are filled in a sequential fashion.Luo et al. [13] found that Site A, which lies near the carboxylate C atoms, is more than 65% occupied before any significant population is observed at Site B, which is closer to the aryl rings.Following the addition of further D 2 , Site A saturates and Site C near the carboxylate O atoms populates almost to the level of Site B; B and C then co-fill to saturation.Site D appears to interact primarily with the carboxylate O atoms and other bound D 2 molecules, and is only slightly populated at loadings higher than 4 D 2 :Y. the CO 2 adsorption sites.The CO 2 guests were modelled as complete molecules with variable site occupancies in subsequent refinement cycles.

Figure 1 .
Figure 1.The square channel of Y(btc) viewed along the c axis (vanishing perspective is used).Shown are Y (light blue), C (dark grey), O (red) and H (light grey).

Figure 1 .
Figure 1.The square channel of Y(btc) viewed along the c axis (vanishing perspective is used).Shown are Y (light blue), C (dark grey), O (red) and H (light grey).

Figure 3 Figure 3 .
Figure 3 shows the arrangement and orientation of all symmetry-generated locations for each of the three CO2 adsorption sites within a single Y(btc) unit cell.The molecules located at sites BCO 2 and CCO 2 lie along the [y, 0, 1/4] and [0, y, 1/2] 2-fold axes, respectively, and therefore occur with half the multiplicity of site ACO 2 .Site occupancy restrictions arising from close interactions between these adsorption sites are discussed in Section 2.1.3.

Figure 3 .
Figure 3. Depiction of all symmetry-generated CO 2 adsorption sites within the Y(btc) unit cell as determined by Rietveld refinement.Adsorption locations are viewed along the (a) b axis and (b) c axis.Shown are Site A CO 2 (green), Site B CO 2 (orange), Site C CO 2 (blue), and framework atoms Y (light blue), C (dark grey), and O (red).Framework H atoms are omitted for clarity.

Figure 4 .
Figure 4. Intermolecular interactions of CO2 molecules at a dose of 1 CO2:Y, as viewed along the a axis.Shown are CO2 adsorption-site ACO 2 (green), site BCO 2 (orange) and site CCO 2 (blue), and framework atoms Y (light blue), C (dark grey), and O (red).Framework H atoms are omitted for clarity.

Figure 4 .
Figure 4. Intermolecular interactions of CO 2 molecules at a dose of 1 CO 2 :Y, as viewed along the a axis.Shown are CO 2 adsorption-site A CO 2 (green), site B CO 2 (orange) and site C CO 2 (blue), and framework atoms Y (light blue), C (dark grey), and O (red).Framework H atoms are omitted for clarity.

Figure 5 .
Figure 5. Graphical representation of the YO6 coordination geometry site in Y(btc) under various guest loading conditions.Y is represented by light blue spheres.The O1, O2 and O3 positions are depicted for the hydrated structure (light grey), empty structure (red), and guest-dosed structures at 1 guest:Y (dark blue) and 2 guest:Y (orange) loading concentrations.

Figure 5 .
Figure 5. Graphical representation of the YO 6 coordination geometry site in Y(btc) under various guest loading conditions.Y is represented by light blue spheres.The O1, O2 and O3 positions are depicted for the hydrated structure (light grey), empty structure (red), and guest-dosed structures at 1 guest:Y (dark blue) and 2 guest:Y (orange) loading concentrations.

Table 1 .
Crystallographic details for CO 2 sites within Y(btc) at a dose of 1 CO 2 :Y.

Table 1 .
Crystallographic details for CO2 sites within Y(btc) at a dose of 1 CO2:Y.

Table 2 .
Unit-cell parameters and site-occupancy factors determined using Rietveld analysis of NPD data for empty and guest-loaded Y(btc).

Table 3 .
[13,14]g O-Y-O angles which enclose the bare-metal Y site in the hydrated, dehydrated and guest-loaded Y(btc) materials.O1 and O3 refer to the relevant framework O atoms[13,14].

Table 3 .
[13,14]g O-Y-O angles which enclose the bare-metal Y site in the hydrated, dehydrated and guest-loaded Y(btc) materials.O1 and O3 refer to the relevant framework O atoms[13,14].