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Supplementary information:
1. Additional information on figure 7:
The Raman spectra obtained in figure 7 does not allow us to state whether the hydrates
from the binary or ternary mixtures show multiple cage occupancies in the small cage.
Generally, a peak doublet in spectra of the guests in hydrate phases arises from guest
occupancy in small and large cages respectively, as in the case of methane hydrate39 and
nitrogen hydrate46. However, for this binary hydrate all the large cages are occupied by
CO2 therefore it is not possible that one of the peaks in the doublet shown in figure 7 is
from H2 in the small cages and the other from H2 in the large cages. However, it is
possible that one of the peaks in the doublet is due to partial occupancy of small cages
occupied by bimolecular H2. In order to investigate this more detailed study was carried
out. It was found that the ratio of the peak intensity at 4120 cm-1 to that at 4126 cm-1 is
independent of the hydrate synthesis pressure. This eliminates the possibility that one of
the peaks is due to doubly occupied H2 in the small cages, as on application of higher
pressure more doubly occupied cages are expected. The peak ratio changes upon storage
in liquid nitrogen27 with the higher frequency peak intensity increasing with respect to the
lower frequency peak. Based on this result we can conclude that upon storage at liquid
nitrogen temperatures, H2 gas present in the hydrate phase converts from ortho- hydrogen
to para- hydrogen thus changing the peak intensities of the doublet.
2. Cage occupancy calculation:
Calculation of cage occupancy values are based on the fact that in a unit cell of structure I
hydrate there are 6 large cages, 2 small cages and 46 water molecules. For CO 2 hydrate
with all the large cages occupied by CO2 and all the small cages being empty, NMR
result should show ~0.24 grams of CO2 /per gram of hydrate. Mass % of H2 in the
hydrate synthesized for this study is very small (0.4 mass %) and for practical
calculations can be ignored.
Mass from 6 CO2 molecules in the large cages (6x44) = 264 units
Mass from 0 CO2 molecule in the small cages (0x44) = 0 units
Mass from 46 water molecule (46x18) = 828 units
Therefore CO2 mass fraction of this hydrate = 264/(264+828) = 0.241
2
Since, NMR results suggest that the synthesized hydrate has 0.21 (±0.02) grams of CO2
per gram of hydrate, all the CO2 in the hydrate phase has to be in the large cages (for a
stable hydrate, 100% of the large cages are assumed to be filled).
From gas chromatography results we know that for every 100 moles of gas
coming out of hydrate after decomposition there is 92 moles of CO2 and 8 moles of H2. In
structure I for 100% occupied large cages (by CO2) and 100% occupied small cages (by
H2) the ratio should have been 92 moles (of CO2) and 31 moles (of H2) (as large cage to
small cage ratio in structure I is equal to 3:1). Instead we see just 8 moles of H 2, which
suggests that only 25% of the small cages are occupied by H2. Also from NMR we see
that integral peak intensity for doubly occupied small cage is 3 times as big as singly
occupied small cages and given the fact that doubly occupied cage (due to two hydrogen
molecules) would give twice the area of singly occupied cage, the effective ratio of
doubly occupied cages to singly occupied cages are 3:2. Hence these two equations can
be solved for H2 occupancy values in small cages.
2x + y = 25
x / y = 3/2
x= % of bi molecular hydrogen in the small cages
y= % of single hydrogen in the small cages
x= 9.3 & y= 6.2
Similarly, in structure II hydrate there are 8 large cages, 16 small cages and 136 water
molecule. Fortunately molecular mass of C3H8 and CO2 are same, hence the calculation
of cage occupancy becomes easier.
Mass from 8 C3H8/CO2 molecules in the large cages (8x44) = 352 units
Mass from 16 CO2 molecules in the small cages (16x44) = 704 units
Mass from 136 water molecules (136x18) = 2448 units
Mass fraction of 100% occupied large cages = 352/(704+352+2448) = 0.100
NMR results suggest 0.05 grams of C3H8 per grams of hydrate in the hydrate phase.
Therefore, for first approximation 50% of the large cages are occupied by C 3H8 and rest
3
by CO2 (0.05 grams of CO2). Mass fraction of the 100% occupied small cages is equal to
0.200, therefore 0.075 grams of remaining CO2 (which does not occupy the large cages)
should give 37.5% occupancy of small cages. Regressing the calculation (for the second
time using 37.5% occupied small cages for calculating the overall mass of the hydrate)
we converge for these numbers for cage occupancy (shown in table 2). Again the mass
fraction of H2 has been ignored from the calculation.