REGLAS DE VUELO Y OPERACIÓN GENERAL SUBPARTE D - OPERACIONES DE VUELO ESPECIALES

Of the 17 materials which were scaled up for N2 and CO2 uptake measurements, 5 are discussed here in more detail as they either showed promise as porous materials, or a single crystal structure were obtained. CA-

R/AM-L, CA-E/AM-M, SA-J/AMO and SA-B/AM-P all showed either a high

SABET, or an impressive uptake of CO2 (Table 4). SA-J/AM-O and SA-J/AM-

M also had a single crystal structures, which provides more information with

respect to structure-property relationships observed. The isotherms collected from the other chosen hits can be found in Chapter 5.

Table 4 SABET and CO2 uptake for 5 selected co-crystals.

Precursors SABET, N2 (m2 _g-1₎ N2 Uptake (mmol g-1₎ N2 Uptake (cm3 _g-1₎ CO2 Uptake (mmol g-1₎ CO2 Uptake (cm3 _g-1₎ CA-R/AM-L 40.69 1.30705 31.3692 0.08379 2.01 CA-E/AM-M 4.54 0.41524 9.96576 0.11791 24.70 CA-J/AM-M 3.36 0.14848 3.56352 0.08875 2.13 SA-J/AM-O 10.16 0.71338 17.12112 0.52693 12.65 SA-B/AM-P 73.49 1.77752 42.66048 0.10146 2.43 5.4.1 Co-crystal CA-R/AM-L

CA-R/AM-L was identified from the initial screen as being potentially

porous, with a T of 6.86197 °C. Single crystals of CA-R/AM-L did not grow, however the powder diffraction patterns implied that it was a semi-crystalline powder, with the pattern varying significantly from the starting materials. In

Figure 39, the isotherm shows a linear trend, however at higher pressure the

gas uptake of CA-R/AM-L increases.

If assuming that isotherm corresponds to BET adsorption, which assume a multi-layer coverage, the isotherm shape would imply a macroporous solid. However, this is highly unlikely, therefore the isotherm can be better described by a Langmuir-Freundlich adsorption, which assumes there is only mono-layer coverage.100 _{A linear trend implies there is weak} adsorption by the co-crystal CA-R/AM-L.

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CA-R/AM-L showed a maximum gas uptake of 1.308 mmol g-1 _(31.37 cm3 _g-1_{) N2 at 1 bar, and the SABET of CA-R/AM-L was determined to be} 40.6941 m2_g-1_{, although this is not as high as the SA of the CPOS by Xing et} al., this still shows promise for the HT methodology. CO2 uptake was low for

CA-R/AM-L, implying that the material did show porosity, but the VdW radii for

CO2 is 2.32 Å at its widest, whereas N2 is only 1.55 Å, leading to the conclusion that there are likely to be pores within the system, however these may be too small for CO2 to access (Figure 41).

Figure 41 Linear adsorption isotherm of N2 for CA-R/AM-L, and the isotherm of CO2.

Figure 42 compares the powder patterns for the co-formers and

potential co-crystal. It is evident there is a different, less crystalline phase formed with no resemblance to the co-formers CA-R and AM-L.

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Figure 42 Powder patterns of CA-R/AM-L, CA-R and AM-L.

5.4.2 Co-crystal CA-E/AM-M

CA-E/AM-M showed the best CO2 uptake of all the materials which were measured, with a maximum uptake of 24.696 cm3_g-1_{, despite the SABET} only measuring at 4.5391 m2_g-1 _{(Figure 43). It is unlikely that the CO2 uptake} arises from pores in the structure, but rather interactions with the diamine AM-

M.101 _{Other materials have been shown to undergo improved CO2 uptake when} functionalised with amines. For example, Long et al. showed that incorporating

N,N’-dimethylethylenediamine into a MOF resulted in increased CO2 uptake, making it one of the best MOFs for CO2 uptake and selectivity.102 _Amine scrubbing has been used since the 1930s for the effective separation of CO2 from other natural gases and hydrogen.103 _{This process uses amines to bind} CO2, and selectively remove them in low concentrations from a mixture of gases. This process of binding is the most likely explanation for the exceptional CO2 uptake in CA-E/AM-M.

Figure 43 Type II isotherm for N2, showing a non-porous material, and the gas uptake for CO2 in CA-E/AM-M.

Figure 44 shows the powder patterns for CA-E/AM-M and its co-

formers. The powder patterns shown for CA-E/AM-M both pre- and post- sorption are very similar to AM-M, implying there was no co-crystallisation between the two starting materials. The HTIR screen didn’t show any potential for CO2 uptake when testing using just AM-M, however we see impressive CO2 uptake for CA-E/AM-M.

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Figure 44 Powder patterns for CA-E/AM-M, CA-E and AM-M, both pre- and

post-sorption.

5.4.3 Co-crystal SA-J/AM-M

Co-crystals of SA-J/AM-M were grown through a layered crystallisation, with the crystallisation occurring at the interface between the two immiscible solvents, EtOAc for the diamine AM-M, and water for the sulfonic acid SA-J. The displacement ellipsoid plot is shown in Figure 45.

Figure 45 Displacement ellipsoid plot for the asymmetric unit of SA-J/AM-M,

ellipsoids shown at 50 % probability.

The co-crystals had a 2:1 ratio of the sulfonic acid:diamine. The crystals grew forming a close-packed crystal structure, with both the diamines forming an ammonium ion. Disordered solvent was found in the structure, which was

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bound to the sulfonic acid, however it was removed from the image in Figure

46.

Figure 46 Crystal packing along the a-, b- and c-axes, showing the close-

packed crystal structure.

The isotherms shown in Figure 47 shows that despite a T of 2.43 °C, the structure was non-porous to either N2 or CO2. The N2 type II isotherm is typical for either macroporous or non-porous materials.104 _{The maximum} uptake for CO2 at 1 bar was 0.08875 mmol g-1_{, which is particularly low.}

Figure 47 N2 isotherm for the gas uptake from 0 – 1 bar of SA-J/AM-M, showing no evidence of porosity.

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Powder patterns, shown in Figure 48, were collected comparing the precursors, the co-crystals both pre- and post- sorption, and the simulated powder pattern from the single crystal structure. There seems to be good agreement between the simulated powder pattern and the diffraction after the HT sorption measurements, with a shift in the value of 2 most likely corresponding to the solvent in the structure. The lack of any evidence of porosity, alongside the close packed structure implies the fast recrystallisation had the same results as single crystal growth, and is the same phase.

Figure 48 Powder patterns for SA-J/AM-M, SA-J and AM-M, both pre- and

post-sorption.

5.4.4 Co-crystal SA-J/AM-O

Co-crystals of SA-J/AM-O were grown through a layered recrystallisation, SA-J was dissolved in water, and the triamine AM-O in ethyl acetate. At the immiscible solvent interface, needle crystals were grew over a period of one week. The displacement ellipsoid plot is displayed in Figure 49. The disordered EtOAc was modelled using an EADP restraint, with a 50% occupancy for all except for O5 and O5A which were modelled with a 25% occupancy.

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Figure 49 Displacement ellipsoid plot of SA-J/AM-O, with disordered EtOAc.

Ellipsoids displayed at 50% probability.

Figure 50 shows the crystal packing of SA-J/AM-O along the a-, b- and

c-axes, with the solvent removed from the figures. When viewed along the c-

axis, the solvent occupied the void space and SA-J/AM-O had a T of 6.90 °C in the HTIR kit, implying the material was potentially porous. However, when recording complete isotherms the material was shown to in fact be non-porous. The sulfonic acid was disordered and modelled across two positions, with SO3 split across two positions. S1 was modelled with 44.1% occupancy, and S1A 55.9%. The oxygen atoms were also disordered and modelled with varying occupancies, O1 65.7%, O2 80.9% and O3 49.3%.

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Figure 50 Crystal packing of SA-J/AM-O, along the a-, b- and c-axes.

The crystal showed a salt had formed between the triamine and sulfonic acid, proven by the formation of an ammonium salts in one position of the triamine. Prior to all gas uptake measurements, the crystals were placed under vacuum for 24 hours, both at 50 °C and at room temperature, ensuring full removal of solvent from the void space. Despite the formation of the salt, the resulting structure was non-porous, with a type II isotherm for N2 adsorption. The CO2 uptake by SA-J/AM-O was 0.52693 mmolg-1_{, which converts to 12.65} cm3 _g-1 _{(Figure 51).}

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Figure 51 Type II isotherm for SA-J/AM-O, showing limited N2 uptake at 1 bar, and the CO2 uptake for SA-J/AM-O.

The powder diffraction patterns shown in Figure 52 compares the two co-formers, SA-J and AM-O, the co-crystal SA-J/AM-O post before and after HT sorption and the simulated powder pattern from the single crystal structure. Powder patterns for the co-crystals are slightly different to the simulated pattern, with a shift in 2 which could correspond to the solvent in the structure. The co-crystals show some similarities, however don’t appear to be highly crystalline. This difference could arise from either exposure to the higher temperatures in the vacuum oven, or the faster recrystallisation method being less effective.

The SABET of SA-J/AM-O was only 10.16 m2_g-1 _{using N2 as the probe} gas, however the CO2 uptake was 12.65 cm3_g-1_{, which is comparable to a [3+2]} propeller cage synthesised by Zhang et al., which had a selective CO2 uptake of ~ 9 cm3 _g-1_.105 _{This cage however, had a diameter of 10.4 Å at it’s largest} and a higher SABET, therefore the CO2 uptake could potentially also be a result of both the amine binding effect.

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Figure 52 Powder diffraction patterns for SA-J/AM-O, SA-J and AM-O,

showing no change between the structure after HT sorption, and also indicating an alternative co-crystal has been obtained.

5.4.5 Co-crystal SA-B/AM-P

SA-B/AM-P was identified as a hit in the initial HT screening process,

with a T of 6.72 °C. This is a significant change in temperature, with CC3, a known porous material showing a T of < 2.4 °C. Such a significant change in temperature could arise from two things, either a significant uptake of CO2, or binding to the amines in AM-P. Powder patterns were assessed both prior to and following the HT gas sorption, which showed no change, implying if the structure has formed a porous network or framework this has been maintained after exposure to both vacuum and gas . Co-crystals were grown on a larger scale for effective gas adsorption, using both CO2 and N2 (Figure 53).

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Figure 53 Powder diffraction patterns for SA-B/AM-P, SA-B and AM-P,

showing no structural change after gas sorption, and indicating the formation of a new material based on the starting materials.

The SABET when using N2 was found to be 73.49 m2_g-1_{, and N2 uptake} of 1.78 mmolg-1 _{(31.37 cm}3 _g-1_{). The isotherm for SA-B/AM-P was a Langmuir-} type isotherm, whereby the isotherm has a linear correlation. The gas uptake continually increases with the uptake of N2.This isotherm type is typical of a material where all sites have equal energy at all sorbent concentrations. Compared with the N2 uptake, the CO2 uptake was very low, only 2.44 cm3_g-1_, which shows selectivity for N2, or pores which are not wide enough for the diffusion of CO2 (Figure 55).

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Figure 55 Adsorption isotherms for N2 and CO2 sorption for SA-B/AM-P.

5.5 Starting Material Hits

We have discussed in detail the effect of amine scrubbing, i.e. the impact of CO2 binding in the presence of amines. We have seen that certain co-formers were responsive to the HT CO2 sorption, however the most responsive are shown in Figure 56. The fact these co-formers showed potential porosity meant some were excluded from being carried forward. Despite the low SABET some of the co-crystals showed, there was impressive CO2 uptake. This could be a result of this binding, however it is unlikely it was the amines alone responsible, otherwise this would have been identified earlier during the screening process.