High Resolution Inelastic Neutron Scattering and Neutron Powder Diffraction
Study of the Adsorption of Dihydrogen by the Cu(II) Metal-Organic
Framework Material HKUST-1
Samantha K. Callear,1 Anibal J. Ramirez-Cuesta,1 William I.F. David,1 Franck Millange,2 and
Richard I. Walton*3
1. ISIS Facility, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot,
OX11 0QX, UK
2. Institut Lavoisier (CNRS UMR 8180), Université de Versailles, 78035 Versailles, France
3. Department of Chemistry, University of Warwick, Coventry, CV4 7AL, UK
*author for correspondence: email r.i.walton@warwick.ac.uk
Abstract
We present new high-resolution inelastic neutron scattering (INS) spectra
(measured using the TOSCA and MARI instruments at ISIS) and powder neutron diffraction
data (measured on the diffractometer WISH at ISIS) from the interaction of the prototypical
metal-organic framework HKUST-1 with various dosages of di-hydrogen gas. The INS spectra
show direct evidence for the sequential occupation of various distinct sites for dihydrogen
in the metal-oganic framework, whose population is adjusted during increasing loading of
the guest. The superior resolution of TOSCA reveals new subtle features in the spectra, not
previously reported, including evidence for split signals, while complemetary spectra
recorded on MARI present the full information in energy and momentum transfer. The
analysis of the powder neutron patterns using the Rietveld method shows a consistent
picture, allowing the crystallographic indentication of binding sites for dihydrogen, thus
building a complete picture of the interaction of the guest with the nanoporous host.
For special issue of Chemical Physics, Proceedings of the "Advances and Frontiers in
Chemical Spectroscopy with Neutrons” Symposium.
1
Introduction
The adsorption of hydrogen by porous metal-organic framework (MOF) materials has
attracted a good deal of attention over the past few years, with the challenging goal of
discovering novel materials for the potential use in the storage and transport of hydrogen
for energy applications [1-6]. This body of work has resulted in some important correlations
between structure and storage capacity that offer the prospect of tuning the properties of
the materials towards hydrogen uptakes for practical uses [3]; in particular, a strong linear
relationship between specific surface area and hydrogen uptake at 77 K has been observed
for a wide variety of MOFs [7, 8], which has lead to the recent development of ultra high
porosity materials [9, 10]. In order to enhance the H2 binding energy in such materials,
control of the nature of porosity has also been considered, using tactics such as the
formation of very narrow pores or the formation of interpenetrated networks [11-13] [14].
Another important structural feature of MOFs with respect to sorption of guest molecules,
is the presence of strong binding sites, especially open metal sites that are coordinatively
unsaturated, where hydrogen may interact through direct coordination to the metal [1521].
Among the wide diversity of metal-organic framework materials that continue to be
reported in the literature, the material known as HKUST-1, a copper(II) framework
constructed from dimeric, paddle-wheel units, linked by 1,3,5-benzene tricarboxylate
ligands [22], has emerged as a model system for the study of the properties of metalorganic frameworks. Although this may be in part due to it being one the earliest such
materials reported in the literature and its ease and reproducibilty of synthesis [23], the
material has also been shown to possess a range of properties in the solid-state, including
cooperative magnetism [24-26], catalysis [27-30], negative thermal expansivity [31, 32], and
its crystallisation from clear solutions has been used as a model for understanding the
assembly of MOFs under solvothermal conditions [33-37]. In terms of structure, the
tridentate ligand results in a three-dimensionally connected network that has a trimodal
pore structure, with each pore system distinct, having dimensions ranging from ~ 5 Å to 12
Å in diameter [38]. The Cu(II) centres in HKUST-1 are each five coordinate with respect to
oxygen donor atoms of the ligands, with a sixth site occupied by a solvent molecule in its asmade state. Upon thermal or vacuum activation this solvent maybe removed to leave
2
coordinatively unsaturated metal centres. The combination of complex porosity and the
potential for open metal sites thus provides an interesting system for study of gas sorption,
and indeed the uptake of a variety of small molecules on HKUST-1 has been performed [39],
including water [40, 41], carbon dioxide [42, 43], ammonia [44] and hydrocarbons [45-50].
Many studies of hydrogen uptake by HKUST-1 have been reported using a range of
methods from gravimetric adsorption through to diffraction and spectroscopy [7, 51-60] and
these have shown that the material shows 2-3 weight % uptake of hydrogen at 77 K with a
maximum uptake of ~ 6 weight % at 25 K, but the precise values may depend on the method
of sample activation. Some detailed studies of the location of H2 (or D2) as a function of
loading using both spectroscopic and diffraction methods have been made by Peterson and
co-workers, who have deduced the presence of nine distinct sites for D2 binding (seen using
neutron diffraction and hence the use of a deuterated guest molecule), with an overall
expansion of the structure upon increased loading [54, 56, 57, 59]. The purpose of the
present work is to illustrate the use of the highest possible resolution neutron diffraction
and scattering instruments to shed further light on this complex system, which provides a
model for understanding guest location and their interaction with framework atoms in
porous MOFs. We have used high incident neutron flux, time-of-flight techniques at ISIS, the
UK’s neutron spallation source: the combination of spectroscopy and diffraction methods
allows new insights into the molecular level interaction of a complex porous host with the
simplest of guest molecules.
Experimental Section
Synthesis The HKUST-1 sample was prepared following a procedure previously described
[53]. Benzene-1,3,5-tricarboxylic acid (2.10 g, 10 mmol, Aldrich) and copper(II) nitrate
hemipentahydrate (2.41 g, 10 mmol, Aldrich) were stirred for 15 min in 50 mL of solvent
consisting of equal parts of N,N'-dimethylformamide (Fluka), ethanol (Fluka), and deionized
water in a 250 mL volume Teflon container. The solution was refluxed for 12 h to yield a
blue polycrystalline powered sample of the desired phase. The as-synthesised material was
washed with N,N'-dimethylformamide at room temperature to remove unreacted reagents.
After this pre-treatment, the material was finally treated in situ at ISIS prior to diffraction
3
and spectroscopy studies, under vacuum at 180 °C to give the purple activated (i.e., free of
solvent) material prior to the neutron diffraction and inelastic measurements.
Inelastic Neutron Scattering (INS) Studies INS spectra were measured using the instrument
TOSCA at the UK neutron spallation source, ISIS [61]. TOSCA is a crystal-analyser inversegeometry spectrometer, where the final neutron energy is selected by two sets of pyrolytic
graphite crystals placed in forward scattering (at around 42.6° with respect to the incident
beam) and in backscattering (at about 137.7° with respect to the incident beam). This
arrangement sets the nominal scattered neutron energy to E1 = 3.35 meV (forward
scattering) and to E1 = 3.32 meV (backscattering). Higher-order Bragg reflections are filtered
out by 120 mm-thick beryllium rods, wrapped in cadmium and cooled down to a
temperature lower than 30 K. The incident neutron energy, E0, spans a broad range allowing
to cover an extended energy transfer (E = E0 − E1) region: 3 meV < E < 500 meV. Around 1 g
of accurately weighed HKUST-1 was loaded ina quartz ampoule which was held under
vacuum at 180 oC overnight to remove completely any water, leaving the coordinatively
unsaturated Cu(II) sites. The sample was transferred to a pre-dried aluminium sample
holder in a dry box before being sealed and transferred to the spectrometer. The INS
spectra of molecular para-hydrogen was measured at the following coverages: 0.5, 1.0, 1.5,
2.0, 3.0, 4.0 and 5.7 pH2:Cu atom.
Gasses were dosed using standard gas dosing Sievert apparatus available at ISIS, and the
precision of the amounts of dosed gas is 5%.
MARI was used to measure the INS spectra with momentum transfer resolution, i.e the
𝑆(𝑄, 𝜔) map. By studying the Q dependence of the rotational and vibrational features we
can determine the nature of the motion associated with the spectroscopic features.
Neutron Diffraction High resolution neutron powder diffraction measurements were made
using the WISH diffractometer at ISIS [62]. After heat treatment at 180 °C under vacuum,
the activated HKUST-1 sample was then loaded onto the diffractometer and data collected
at 10 K for the evacuated material. The material was then warmed to 35K and dosed with
volumes of hydrogen equivalent to 0.5D2/Cu atom, 1D2/Cu atom, 1.5D2/Cu atom, 2D2/Cu
atom, 3D2/Cu atom, 4D2/Cu atom, 5D2/Cu atom and 6D2/Cu atom. After each dosing the
MOF was cooled to 10 K for the collection of neutron diffraction data. Data analysis was
4
performed using the Pawley and Rietveld refinement codes within the TOPAS academic
suite [63]. A Pawley fit was used to extract the intensities for each reflection; the extracted
intensities and their errors were then used, together with the calculated structure factors
for the HKUST-1 framework (obtained from the refinement of the previous loading) and the
correlation weighting for (near-)overlapping reflections, to optimise the maximum entropy
of the fourier difference map as per the David and Sivia method published elsewhere [64].
This resulted in a Fourier difference map showing locations of the deuterium molecules as
clouds of neutron density.
Results and Discussion
First we consider the INS results, which reveal information about vibrational, librational and
rotational modes of D2 trapped within the HKUST-1 structure, as has been illustrated by
other studies on MOF materials [65, 66] [56, 57, 67]. The advantage of TOSCA for these
studies, compared with other neutron spectrometers, is its superior resolution
(Δ𝜔⁄𝜔 ~1.5%) and dynamical range (3-1000 meV). Figure 1 shows the INS spectra at
various loadings with the background of pure, activated HKUST-1 that was subtracted as an
inset.
Figure 1: INS spectra of parahydrogen adsorbed on CuBTC for a series of coverages, n
pH2:Cu with n=0.5,1,1.5,2,3,4 and 5.6. Peak 3 is split into 3a and 3b after the load exceeds
5
n=3. The peaks in the box are due to the combination of rotational and transational
motion, see below.
Figure 2: Is it better to offset these rather than overlay them?
The first distinct peak in the INS to appear upon introduction of hydrogen at ~ 9 meV (Peak
1 on Figure 1) has associated with it a set of higher energy loss peaks that initially grow at
the same time with further increases of hydrogen loading. Peak 1 may be assigned as a
rotational peak, and the higher energy partners, occuring at ~18.2, 26.0 and 32.7 meV, are
due to combinations of the rotational line plus a translational energy. At the highest
loadings (4 or more equivalents of hydrogen per copper) the higher energy loss peaks
disappear into a broad background. This would be consistent with the presence of glass-like
hydrogen at the highest pore fillings giving largely a background of recoil. One possible
explanation is that crowding of hydrogen on neighbouring sites removes the well-defined
translational contribution giving rises to the higher energy loss bands.
The splitting of Peak 1 with increasing loading, Figure 2, is a significant new
observation, not previously seen with lower resolution spectrometers [56, 57]. One
explanation for this is that there is some lateral interaction between neighboring
dihydrogen molecules at this site. In the previous studies, by Kepert and co-workers the
6
motion of the strongly bound hydrogen (i.e. giving peak 1, the first significant feature to
observe at low loadings) was considered to be strongly two-dimensional and thus a series of
rotational levels of hydrogen were calculated. The authors then proposed that the ~9 meV
peak can be assigned as a transition from the J = 0, M = 0 ground state to J = 1 M ± 1 excited
state and furthermore that the J = 1, M= 0 state was not observed because it lies very high
in energy, and that the second rotation level (J = 2, M ± 2) should fall at ~ 36 meV. In fact our
spectra measured over a range of energy transfers up to 75 meV, using the MARI
spectrometer, Figure 3, reveal that this cannot be the case, there is no transition with that
energy transfer and the right momentum transfer. In Figure 3 (bottom) it can be seen that
the maximum in the Q cuts shifts for the energy transfers above 14.7 meV.
The shift of the Q dependence spectroscopic features, to higher momentum transfers is
characteristic of the presence of combined rotational and translational quantum events. In
particular, the peaks 9 and 12.5 meV correspond to pure rotational excitation of the
hydrogen molecule, the peaks at 19 and 24 meV correspond to an excitation of a rotational
plus a translational mode (R+𝜔), where the translational mode is the vibration of the
molecule against the adsorption site, see reference [Ramirez-Cuesta, A. J.; Mitchell, P. C. H.;
Ross, D. K.; Georgiev, P. A.; Anderson, P. A.; Langmi, H. W.; Book, D. Journal of Materials Chemistry
2007, 17, 2533.]. The peak at around 26 meV corresponds to a pure rotation plus an first
overtone (R+2𝜔) whereas the peak at 33 meV corresponds to a pure rotation plus a second
overtone (R+3𝜔). It can be seen, from figure 3 (top) that there are further features that
keep displacing their intensity maximum to higher momentum transfer as the energy of the
transition increases. There is no evidence of the splitting mentioned in [57] around ~36
meV.
7
3
0
E
(
m
e
V
)
6
0
2
0
4
0
1
0
2
0
0
2
4
6
8
1
|Q
|(
Å)
1
0
Figure 3 (Top) the 𝑆(𝑄, 𝜔) map of hydrogen on 1pH2:Cu in CuBTC, for an incident
energy of 75 meV. (Bottom) Series of cuts along momentum transfer (Q) at a
series of energy transfers corresponding to the peaks shown in the right panel,
that is the integrated intensity for along energy transfer for the 𝑆(𝑄, 𝜔) map
shown above; the arrows are a guide to the eye to see the energy at which the
cuts have been made and the position of the maxima. The maximum of the
curves is located around 2.7 Å-1 for ω = 9 meV, however for ω = 19 meV the
maximum appear around 3.3 Å-1, for ω = 26 meV appears at 4.1 Å-1 and for ω
= 33 meV at 4.8 Å-1, the assignments of the origin of the transitions is on the left
hand side of the figure, R stands for pure rotational transitions, 𝑅 + 𝑛𝜔
corresponds to a simultaneous excitation of a rotation and a translational mode
of order n.
8
Upon increasing hydrogen loading other sharp, and distinct features appear in the
INS labelled as Peaks 2-4 on Figure 1. This would be consistent with the occupation of
various distinct sites for dihydrogen as dosing of the gas is increased. Integrating the peaks
gives a plot of site occupancy vs loading, as shown in Figure 4. Note that although the total
area of peaks increases linearly, Figure 4b, consistent with the sequential increase of
hydrogen in the system, each individual site is filled at a different time, and the occupation
of a particular site varies with time.
Figure 4: (top) Integrated area under the main peaks as function of coverage, peak 3
correspond to the total integrated areas of peak 3a and 3b.
With the resolution of our INS spectra, two important observations can be made.
First, the occupancy of site reaches a maximum at 3 equivalent loadings of hydrogen, and
9
then decays at the expense of sites 3 and 4 whose occupancy increases at this point. Thus
there is some competitive occupation of various sites as loading is increased.
High resolution powder neutron diffraction patterns, Figure 5, were used to provide
complementary evidence for the location of dihydrogen sites in the material. Using the coordinates of the areas of significant neutron density obtained from the MaxEnt Fourier
difference maps, the positions of potential di-deuterium molecules were inputted into a
Rietveld refinement and their occupancies allowed to refine. Figure 6 shows an example of
the Rietveld refinement and fit to the data. The co-ordinates of the di-deuterium molecules
were refined, together with the isotropic thermal parameters of all the di-deuterium
molecules except D1 for which the thermal parameters were refined anisotropically.
Importantly this means that the deuterium sites from one loading to another were not fixed
as the same but instead are inputted each time from the MaxEnt map and then allowed to
move to enable the best modelling of the deuterium molecules inside the pore space. The
total deuterium content calculated from the occupancies and multiplicities of each
deuterium site are in agreement with the values loaded. During the refinement, the
flexibility of the framework and the thermal parameters of the framework atoms were also
optimised (the thermal parameters of the framework atoms were refined parametrically by
element; isotropic parameters were used for the framework atoms). The deuterium sites
and their refined occupancies are in Tables 1 and 2, together with the refinement statistics
(see ESI for further information).
11
10
Intenisity (arb. units)
9
6D2/Cu
8
4D2/Cu
7
6
2D2/Cu
5
4
1D2/Cu
3
0.5D2/Cu
2
1
0D2/Cu
0
2
4
6
d-spacing (Angstrom)
10
Figure 5 Stack plot of the neutron powder diffraction data (90° bank) collected on WISH for
each D2 loading.
Yobs
Ycalc
Difference
(a)
1.5
Intensity
1.0
0.5
0.0
1
2
3
4
5
6
7
d-spacing / Angstrom
Yobs
Ycalc
Difference
(b)
1.5
Intensity
1.0
0.5
0.0
1
2
3
4
5
6
7
d-spacing / Angstrom
Figure 6 Typical (a) Pawley and (b) Rietveld refinement for selected dataset 4D2/Cu (90°
bank).
D2 site
Approx. co-ordinate
multiplicity
11
Wyckoff
position
D1
0.15, 0.15, 0
y,y,0
48
h
D2
0.25, 0.25, 0.30
¼, ¼, x
48
g
D3
0.16, 0.16, 0.16
x,x,x
32
f
D3b
0.14,0.14,0.14
x,x,x
32
f
D4a
0.5, 0.37, -0.37
½, y,-y
48
i
D4b
0, 0.30, 0.08
0,y,z
96
j
D5
0.79, 0.79, -0.79
x,x,-x
32
f
D6
0.071, 0.27, 0
y,z,0
96
j
D7a
0.031, 0.21, 0.06
x,y,z
192
l
D7b
0.06, 0.21, 0.06
x,z,x
96
k
D8
0.58, -0.58, 0.58
x,-x,x
32
f
Table 1 Approximate co-ordinates for D2 sites (averaged over loadings to 2 decimal places);
not all sites are present at the same time, see Table 2. The co-ordinates for each site present
in each loading are in Table S1, ESI.
0
0
0
0
0
0
0
0
0
0
Observed
loading
D2/Cu
0
0.5
0.57465
0
0.0484
0
0
0
0
0
0
1
0.7467
0
0.1295
0
0.0827
0.0399
0
0
2
0.8956
0.2085
0.357133
0
0.4108
0.2515
0
0
Loading
D2/Cu
D1
D5
D2
D6
D3
D4
D7
D8
D9
a/
Angstrom
26.2879(3)
2.449
0.6
26.3033(3)
2.369
0
1.0
26.3074(3)
2.286
0
2.1
26.3034(3)
2.282
26.2971(2)
1.840
26.3142(3)
2.524
4
0.91845
0.2469
0.626233
0.40295
0.6898
0.142333
0.6717
0.2158
0
3.9
6
0.90525
0
0.602433
0
1.7435
0.5885
0
1.4898
0.654833
6.0
Table 2 Number of deuterium molecules per Cu atom for each deuterium molecule site,
lattice parameter and refinement statistics for each loading.
The cubic lattice parameter of the MOF changes on loading (Figure 7) but only by a
maximum of 0.0025 Å. Although this is only a small amount compared to other more flexible
MOFs, where changes of several Ångströms have been observed [68, 69], the benzene tricarboxylate ligand is inherently flexible. A search of the Cambridge Structural Database (CSD
5.34) using Mogul (Mogul 1.5, Build RC5) yields a wide range of torsion angles between the
plane of the carboxylic group and the benzene ring with ~75% of structures on the CSD
deviating from planar by up to +/- 10° and a further 23% of structure deviating from planar
by +/- 10-25°.<ref programs> In the pure and hydrated forms of benzene tri-carboxylic acid
12
Rwp
(CSD refcodes BTCOAC and FONHEW respectively)<ref>, torsion angles of up to 9° are
observed across the carbon atoms of the aromatic ring and the carboxylic acid groups. The
benzene tri-carboxylate ligand thus flexes not only via the twist of the carboxylate groups
with respect to the plane of the benzene ring, but also across the C-C bonds connecting the
carboxylate groups to the benzene ring. Indeed recently published work by Peterson et al.
discusses the flexibility of the framework in HKUST-1 [59], and our data in Figure 7 are very
similar to those reported.
axis length (Angstrom)
26.32
26.315
26.31
26.305
26.3
26.295
26.29
26.285
0
2
4
D2 molecules loaded per Cu
6
Figure 7 Lattice parameter of HKUST-1 as a function of D2 loading per Cu.
The structure of HKUST-1 contains three pores, Figure 8, the smallest with a diameter of ~10
Å, another with a diameter of 16 Å and the largest pore has a diameter of 18 Å {Chui, 1999
#19}. On loading the material with di-deuterium gas, the first adsorption site to be filled, D1,
is situated directly over the co-ordinatively unsaturated copper atom and is thus positioned
in the large pore. The high resolution neutron powder diffraction data enables the precise
determination of the Cu···D2 distance as 2.4633(4) Å for 0.5D2/Cu. On increasing the loading,
the Cu···D2 distance increases to 2.4893(4) Å at a loading of 2D2/Cu but then decreases on
further loading to 2.3542(4) Å at a loading of 6D2/Cu The quality of the data also enables
anisotropic refinement of thermal parameters. The shape of the resulting ellipsoid is
representative of the motion of the spherical deuterium molecule, showing the motion to
be perpendicular to the direction of the Cu-Cu bond (see Figure8x). This corroborates with
DFT calculations made by Peterson et al. from inelastic neutron scattering data {Peterson,
2011 #20}. On increasing the loading up to 4D2/Cu, the ellipsoid becomes more disc shaped
13
(see Table xx ESI), indicating a reduction in the motion along the direction between the
medium pores, and an increase in motion towards the window between the large and small
pores. This may be due to the clustering of additional deuterium molecule sites close to site
D1 thus inhibiting its movement (see later). On further increasing the loading to 6D2/Cu, the
motion returns to that observed in the 0.5 D2/Cu loading.
a
b
c
Figure 8 Different pores of HKUST-1 (framework shown): (a) small, (b) medium and (c) large.
a
b
c
d
Figure 8x Shape of the deuterium molecule ellipsoid for site D1 for 0.5D2/Cu loading viewed
(a) orthogonal to the Cu-Cu bond and (b) along the Cu-Cu bond. (c) and (d) show the shape
of the ellipsoid orthogonal to the Cu-Cu bond and along the Cu-Cu bond respectively for
4D2/Cu loading. Ellipsoids shown at 50% probability.
The next site found to be occupied is D3, sitting in the window of the small pore, which
yields a D2···O distance of 3.855 Å to 4.003 Å on increasing loading from 0.5D2/Cu to
6D2/Cu, indicating interaction of the deuterium molecule with the carboxylate groups
(Figure 9a). On increasing loading, the site moves away from the centre of the small pore,
thus slightly increasing the length of the weak D2···O framework interactions. The third site
to be filled at 1D2/Cu loading is D5 which is situated behind D3, within the small pore (Figure
9b). On increasing the loading to 2D2/Cu, site D2 is observed within the small pore. This site
is the furthest from the carboxylate groups and positioned closest to the centre of the small
14
pore (Figure 9c). The positioning of sites D2 within the small pore suggests interaction of the
deuterium molecules with the delocalised π-electrons from the aromatic rings which form
the walls of the small pore. On further increasing the loading to 4D 2/Cu, The D3 site is best
modelled as being split into two sites, D3a and D3b, thus resulting in an arc of D2 molecule
sites forming around the carboxylate groups within the small pore. However, on increasing
the loading further to 6D2/Cu, site D3b and site D2 are no longer observed, although the
total number of deuterium molecules within the small pore remains approximately the
same as the 4D2/Cu loading.
a
b
c
d
15
e
Figure 9 The D2 molecule distribution around the carboxylate groups within the small pore –
a cut through the small pore is shown for clarity; cream is site D1, yellow is site D3 (darker
yellow is site D3b), orange is site D5 and dark red is site D2 for loadings (a) 0.5D2/Cu, (b)
1D2/Cu, (c) 2D2/Cu, (d) 4D2/Cu and (e) 6D2/Cu. All atoms and deuterium molecules are
shown as spheres with 50% probability except D1 which is shown as a thermal ellipsoid with
50% probability.
At low loadings the medium pore is also started to be occupied with D4b located in the
window between the medium and the large pores, thus yielding D 2···O distances of 3.731 Å
with the adjacent carboxylate groups, again indicative of interactions between the
deuterium molecules and the carboxylate groups (Figure 10a). At a loading of 2D2/Cu, site
D4b is seen to move closer to the carboxylate groups, resulting in a shorter D2···O distance
of 3.422 Å. Furthermore a second site, D4a, is observed adjacent to the carboxylate groups
(Figure 10b). As this site is positioned more centrally across the carboxylate group, the
D2...O distance is longer at 3.773 Å. On further increasing the loading to 4D2/Cu, site D4b
moves closer to the carboxylate groups (D2···O distances of 3.379 Å) and a further two
deuterium sites are observed resulting in a semi-circle of deuterium molecules surrounding
the carboxylate groups in the medium pore (Figure 10c). On increasing the loading further
to 6D2/Cu, the distribution of the deuterium across the wall of the medium pore is observed
to change. The D6 site is no longer observed, but instead site D8 is observed, located on the
wall of the pore, adjacent to the centre of the benzene molecule (Figure 10d). The distance
of 3.183 Å between D8 and the centroid of the benzene molecule suggests the presence of
pi interactions between the D2 molecule and the benzene ring.<ref>
16
a
b
c
d
Figure 10 D2 molecule distribution around the carboxylate groups in the medium pore;
cream is site D1, blue is site D4b, dark blue is site D4a, light blue is site D6, purple is site D7,
for loadings (a) 1D2/Cu, (b) 2D2/Cu, (c) 4D2/Cu and (d) 6D2/Cu. The HKUST-1 framework is
represented as balls and sticks, deuterium molecules are shown as spheres with 50%
probability except D1 which is shown as a thermal ellipsoid. In 4D2/Cu, site D7 is positioned
closest to site D1 over the Cu atom; D1 shows reduced motion in the direction of the D7
sites compared to the other loadings.
From the refined occupancies of the deuterium molecules at each site, it is possible to
calculate the number of D2 molecules per Cu atom that are contributed by each site (Figure
11a) from the occupancies of each deuterium site obtained from the Rietveld refinement
(Figure 11b). These data show site D1 filled first and reaches maximum occupancy most
quickly. Site D3 is the second to be filled, and also reaches maximum occupancy most
17
quickly, although due to the multiplicity of the site, does not contribute as highly to the total
number of deuterium molecules in the unit cell. The competitive adsorption relationship
between sites D4a, D4b and D6 can also be observed, with the occupancy of site D4a
dropping as the occupancy of site D4b increases. Similarly, as the occupancy of site D4b
increases, the occupancy of site D6 also decreases. On loading with 6D2/Cu rearrangement
of the deuterium molecules is observed with the addition of new site D8 and the loss of
sites D6 and D7.
2
number of D2 molecules per Cu observed for
each site
1.8
1.6
D1
1.4
D3
D4b
1.2
D5
1
D2
0.8
D4a
0.6
D6
0.4
D7
0.2
D8
0
0
1
2
3
4
D2 molecules loaded per Cu
5
6
D2 site occupancy (D atoms)
2.5
D1
2
D3
D4b
1.5
D5
1
D2
D4a
0.5
D6
D7
0
0
1
2
3
4
5
D2 molecules loaded per Cu
18
6
7
D8
number of D2 molecules per Cu
observed for each site combination
4.5
4
3.5
3
D1
2.5
D2+D5
2
D3
1.5
D4+D6+D7+D8
1
0.5
0
0
2
4
6
D2 molecules loaded per Cu
8
Figure 11 (a) The number of deuterium molecules per Cu atom for each site at each loading;
(b) the occupancy (number of D atoms) for each deuterium site at each loading; (c) the
number of deuterium molecules per Cu atom for each group of sites at each loading
Note that in (c) saturation is achieved at lower loading that seen by INS: is this the
difference between H2 and D2? The grouping of diffraction sites allows a comparison with
the features seen in INS: Figure 11c should be compared with Figure 4a. The correlation is
not exact but the qualitative agreement shows that we are observing a consistent picture by
spectroscopy and diffraction of sequential filing of various sites. The site filling, together
with the short contact distances for each of the sites, enables the peaks observed in the INS
data (replotted in Figure 12 to allow comparison with the diffraction analysis) to be related
to the sites observed in the NPD data. The peak that is filled first and most quickly in the INS
data, peak 1 at 8meV, can therefore be attributed to site D1 where the D 2 molecule
interacts with the Cu atom. On increasing the loading, peaks begin to form at 12.3 and 14.7
meV. From previous data it is known that the interaction of deuterium molecules with
benzene rings results in only a weak interaction with only a slight change in the rotational
line of hydrogen observed for hydrogen adsorption on graphite.<refs> This suggests that the
peak at 14.7meV is due to the interaction of the deuterium with the pi electrons associated
with the aromatic rings that surround the small pore (i.e. site D2 and then D2 and D5 on
higher loading).
19
Figure 12 Relative peak areas for each of the peaks in the INS data
???????????
peak 1 = D1
peak 3 = D5
peak 2
peak 6 = D4a, D4b
peak 5 = D2
peak 7 = D7
peak 4 = D6
D8 = ?
???????????????
Site comparison with Peterson data: (I will do some re-ordering of the site labels to be
consistent with her data and the site filling order.)
Peterson site
Our site
D1
D1
D2, D3
D3
D4a,b
D4a,b
D5
~D5
comment
We use a different special
20
position, but it’s approx the
same place
D6
D6
D7
~D7
D8
D8
Applicable for some loadings
Our results are similar – some of the sites are slightly different positions (understandably so
as the smearing of nuclear density will likely be different across the pores) and also the
loadings at which our sites appear and disappear and move to are sometimes different. I
don’t know if you want a thorough comparison of our results with theirs – I think it would be
hard to say either are right or wrong, just a different sample, loading, cooling, instrument
etc...
21
Conclusions
The superior resolution of the INS spectra measured using the time-of-flight TOSCA
instrument at ISIS has allowed fine detail concerning the interaction of dihydrogen with a
porous metal organic framework material. By measurement of complementary neutron
powder diffraction data we are able to build a consistent picture of the binding of
dihydrogen in one of the prototypicla metal-organic framework structures that provide new
reference data for understanding the adsorption of an important gaseous molecule with a
high surface area host.
Acknowledgements
We thank the STFC for provision of beamtime at ISIS.
22
Electronic Supplementary Information for
High Resolution Inelastic Neutron Scattering and Neutron Powder Diffraction Study of the
Adsorption of Deuterium in HKUST-1.
Figure S1 Slices of MARI data
Figure S2 Comparison of MARI+TOSCA data
23
loading
D2/Cu
D1
D5
D2
D2
D6
D3(y)
D4
D7
y,y,0
¼, ¼, x
x,x,x
x,x,x
½, y, -y
0,y,z
x,x,-x
y,z,0
D8
4D2/Cu:
x,y,z
6D2/Cu:
x,z,x
D9
x,-x,x
0
0.5
0.15024
1 0.14993
0.16572
0.16913
2
0.14973
0.29094
0.15812
4
0.14997
0.31055
0.17123
6
0.15352
0.37657
0.15003
0.37608
0.14091
0.27805, 0.78843
0.05006
0.34565, 0.82237
0.09706
0.33435, 0.79274
0.09513
0.31883, 0.79411
0.09132
0.07069, 0.03209,
0.027425 0.21352,
0.05484
0.06077,
0.20665
0.58494
Table S1 Deuterium molecule sites for each loading; the co-ordinates for each site are
shown with the variable parameter noted according to the site symmetry.
Loading
D2/Cu
0.5
U11, U22
U33
U12
U13
U23
0.1569(53)
0.1019(63)
-0.1219(55)
0
0
1
0.1452(39)
0.11300(50)
-0.1004(41)
0
0
2
0.1521(37)
0.1426(50)
-0.1266(38)
0
0
4
0.1576(38)
0.1612(45)
-0.0951(41)
0
0
6
0.1514(37)
0.0900(39)
-0.1142 (38)
0
0
Table S2 Anisotropic displacement parameters for site D1.
24
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