Performance evaluation of CuBTC composites for room temperature oxygen storage

Oxygen is commonly separated from air using cryogenic liquefaction. The inherent energy penalties of phase change inspire the search for energy-efficient separation processes. Here, an alternative approach is presented, where we determine whether it is possible to utilise simpler, stable materials in the right process to achieve overall energy efficiency. Adsorption and release by Metal–Organic Frameworks (MOFs) are an attractive alternative due to their high adsorption and storage capacity at ambient conditions. Cu-BTC/MgFe2O4 composites were prepared, and magnetic induction swing adsorption (MISA) used to release adsorbed oxygen quickly and efficiently. The 3 wt% MgFe2O4 composites exhibited an oxygen uptake capacity of 0.34 mmol g−1 at 298 K and when exposed to a magnetic field of 31 mT, attained a temperature rise of 86 °C and released 100% of adsorbed oxygen. This water vapor stable pelletized system, can be filled and emptied within 10 minutes requiring around 5.6 MJ kg−1 of energy.


Powder X-Ray Diffraction and SEM on MgFe 2 O 4 nanoparticles:
As shown in Fig. S2, a), the XRD pattern of the synthesised MgFe 2 O 4 nanoparticles with diffraction peaks of planes (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0), for a cubic spinel MgFe 2 O 4 phase, matches the standard powder diffraction data (ICSD #00-036-0398) of the MgFe 2 O 4 phase from literature 3 . The mean crystallite size of 10.2 nm was calculated from the peak broadening of the diffraction planes and used in the Scherrer's equation, d = Kλ/(β Cosθ), where d is the crystallite size, λ is the wavelength of the X-ray radiation (Co-1.78897 Å), K is a shape factor with values in between 0.9-1,  is the Braggs peak in degrees and β is the line broadening (full width at half maximum). Scanning electron microscope (SEM) analysis on the samples was carried out on a JEOL JSM-7001F microscope and, as shown in Fig. S2, b), the images show uniform, spherical, and agglomerated MgFe 2 O 4 nanospheres with a diameter of about 150-160 nm.

Thermo-magneto gravimetric analysis
Curie point is a temperature at which magnetic materials undergo loss of their magnetic properties and become paramagnetic. This sharp change in magnetic properties is measured by measuring the samples' weight changes as a function of temperature. The curie temperature of the MgFe 2 O 4 nanoparticles was determined using the Thermo-magneto gravimetric analysis, which is a very efficient technique to ascertain the magnetic transitions in the nanoparticles. A weighed amount of MgFe 2 O 4 nanoparticles were heated from room temperature to 800 o C in a nitrogen atmosphere using a heating rate of 40 o C /min, and the Curie temperature was found to be Tc=566 o C.

Specific Absorption Rate (SAR):
The rate at which the magnetic nanoparticles will absorb the magnetic energy and convert it into thermal energy can be predicted using a Specific adsorption rate (SAR). Apart from the diameter, shape, and composition of the nanoparticles, SAR is strongly governed by the frequency of the applied magnetic field. High SAR values are preferred because it means higher heating efficiency. Magnetic nanoparticles with higher SAR values are ideal for an efficient MISA process. SAR is calculated by dispersing the particles in a liquid medium and the measuring: Where Cs is the specific heat capacity of suspension, dT/dt at t =0 is the initial gradient of the heating curve, ms, and mm are the specific masses of the suspension and magnetic particles, respectively. The units of SAR are watts per kilogram (W/kg). The SAR of the synthesised MgFe 2 O 4 nanoparticles was calculated using the temperature rise profile by dispersing 10 mg of the MgFe 2 O 4 nanoparticles in 1ml of water and exposing it to a 25 mT magnetic field. As seen from figure S4-b, the temperature stabilised at 74 0 C, and the SAR was calculated to be 130.6 W/g. The concentration of nanoparticles in the composite significantly influences its adsorption and desorption properties. Composites with higher magnetic content can lower its adsorption capacities but provide higher heating rates to achieve desorption whereas composites with lower magnetic content can demonstrate high adsorption capacities but insufficient heating abilities. Using 3 wt.% binder for pelletising the composites, the effect of varying magnetic contents on the surface area properties of the MFC were studied.

Thermal Stability and Atmospheric Stability of the composites:
Thermal stabilities of the samples were studied by thermal gravimetric analysis (TGA) using weighed samples that were heated from 25 o C to 800 o C using a heating rate of 10 o C min -1 . As observed in Figure S5

Isosteric Heat of Adsorption:
The isosteric heats of adsorption, Qst, reveals the extent of interaction between the adsorbed molecules and the adsorbate under constant loading conditions. In the case of CuBTC MOF, this interaction that is dependant on the reactions at the exposed cationic Cu2+ sites and adsorption at the windows sites of the octahedral CuBTC cage is calculated to be -15.3 kJ mol-1 ( Figure S6-b). This near-constant Qst curve was plotted using the adsorption data measured at 204 K, 273, and 298 K ( Figure S6 is reported as the sum of the energy required to heat the adsorbent to the desorption temperature, and the energy required to desorb bound gas species from the adsorbent, according to the equation: Where, Cp = specific heat capacity of adsorbent (jg -1 K -1 ) msorbent = mass of adsorbent (g) ΔT = Temperature difference between adsorption and desorption conditions (K) Δh = heat of adsorption (kJmol-1) Δq = working capacity, it can be defined as the difference between the O 2 loadings at adsorption and O 2 loadings at the end of desorption.
mass of oxygen adsorbed at that pressure (g) 2 = Applying this equation to the values in Table S1, the energy required to regenerate the amount of oxygen captured at 200 mbar, 400 bar, 800 mbar and 1000 mbar is calculated, and the values are: To establish the energy efficiency of the MISA process, power calculations were done using a power meter to measure the power required by the induction coil at varying magnetic fields of 25 mT, 31 mT, and 33 mT. To determine the power required to regenerate a weighed amount of sample, these calculations were done without exposing the samples to the magnetic fields and with samples being exposed to the magnetic fields ( Figure S11-a). The power required was calculated from the output readings from the power meter, the time required for the samples to desorb all the adsorbed molecules, and the mass of the desorbed molecules.  Based on these calculations it was determined that using a magnetic field strength of 31 mT, that triggered a temperature rise of 86 o C in the 0.6gms of 3 wt.% CuBTC-MgFe 2 O 4 composite, 0.65 kWh/kg O2 power was consumed to desorb 0.3 mmol/g of adsorbed oxygen molecules. To understand how the weight of the sample influences the regeneration energies, using a magnetic field strength of 31 mT, the calculations were done using two samples weighing 0.33 gms and 0.64 gms each, and for 0.33 gms sample 5.1 MJ kg -1 is required at 1000 mbar, and for 0.64 gms of the same sample, 5.6 MJ kg -1 of energy is required at 1000 mbar( Figure S11-b).