Adsorbed Natural Gas
Group 12-20
The first problem to be solved is whether Metal Organic Framework (MOF) is a better adsorbent than activated carbon in terms of methane storage.
Activated carbon has a specific surface area of 2500~3000 m2/g wherease the specific surface area of MOF is mostly larger than 3500 m2/g. This means that MOF is more advantaged due to its larger Specific Surface Area (SSA) for adsorption. By comparing the volumetric uptake, it is obvious that the uptake from the graph shown (1 bar = 0.1MPa), it is obvious that the uptake of the activated carbon is limited around 200 cm3/cm3. On the other hand, MIL-101, for example, has an uptake of 350 cm3/cm3, which is a lot more efficient than activated carbon. In conclusion, Metal Organic Framework is a beter adsorbent than activated carbon in terms of methane adsorption.
Problem 1 solved.
To solve problem 2, Specific Surface Area (SSA) is compared in the first place, then the total pore volume. Since MOFs are porous, SSA and pore volume are crucial for gas adsorption, including methane. In this chart, NU-111 and MIL-101 both have SSA above 3000 m2/g and total pore volumes larger than 1.00 cm3/g. Hence, their physical properties are statistically better than the rest.
It is also important to investigate on the volumetric uptake of methane adsorption. Generally, the higher the volumetric uptake is, the more effective an adsorbent becomes. However, the volumetric uptake can only roughly compare the effectiveness of MOFs. Here the adsorption curves of these MOFs are very close, thus we have to introduce the predominant factor, delivery capacity, to take real limitations into consideration.
Delivery capacity is the difference between the volumetric uptakes of an adsobent at two givven pressures. Accrding to the graph, the delivery capacity capacities of some MOFs can be calculated, as shown here. Among them, MIL-101, HKUST-1 and Aluminium MOF-519 are outstanding. (as shown in the chart below)
To add on, the applicability of Metal Organic Framework (MOF) in the area of methane storage also depends on the the cost of chemicals and the complexity of synthesis. Research on the last section of data analysis is being carried out recently.
In conclusion, based on the analysis of the experimental data collected, considering the various factors, MOF is a better adsorbent than the activated carbon for methane adsorption. Until the Semi-Finals, the most applicable MOFs for methane storage are MIL-101, HKUST-1, and Aluminium MOF-519.
SEMI-FINALS:
FINALS:
For the Final round, factors including the cost of chemicals, complexity of the synthesis processes and stability of the products are taken into consideration.
Complexity of synthesis process:
1. MIL-101
Chemicals needed: Chromic nitrate (4 g), terephthalic acid (1.66 g), (12 M) hydrochloric acid (1 mL), deionized water (50 ml), dimethylformamide (DMF).
Mix the chemicals in a teflon-lined stainless steel autoclave reactor and kept at 200 °C for 12 hours. Then the autoclave reactor is slowly cooled down to room temperature. After cooling, the reaction mixture is mixed with dimethylformamide (DMF) and heated at 80 °C to remove the unreacted terephthalic acid. The green crystals of MIL-101 (Cr) are recovered by centrifugation. Further, a solvothermal treatment of the product has to be employed by using anhydrous ethanol in an autoclave at 80 °C for 12 hours to remove the DMF or any free terephthalic acid remaining in the MIL-101 pores. The resulting solid was cooled down, centrifuged and finally dried overnight at 80 °C.
Time needed: 24 hours
2. HKUST-1
Small scale method (can be scaled up to 20 times of moles and concentration):
To a solution of Cu(OAc)2·H2O (0.06g, 0.3 mmol) in 6 ml water, add rapidly into a 0.126g suspension of H3BTC (0.6 mmol) in 6 ml water with strong stirring (1200 rpm) for 60 min at room temperature. The products were collected by repeated sonication with ethanol and centrifugation (9000 rpm, 12 min) for three times, and dried at 100 °C under vacuum overnight. The percentage of yield is approximately 90.0 %.
Time needed: 12 hours
3. Al MOF-519
The sample is prepared by heating a mixture containing aluminum nitrate, H3BTB, nitric acid, and N,N-dimethylformamide (DMF) at 150 °C for 4 days.
[A modified synthesis with higher concentration of nitric acid resulted in lower yield but afforded a single crystal.]
Time needed: 4 days
4. Ni MOF-74
In a 400 mL jar, 0.5 g of 2,5-dihydroxyterephthalic acid (DHTA) and 1.5 g of Co(NO3)2 6H2O are dissolved in 70 mL of dimethylformamide (DMF), 70 mL of ethanol, and 70 mL of water with sonication. The jar was capped tightly and placed in a 100 °C oven for 2.75 days. After cooling to room temperature, the mother liquor was gradually poured and the products are washed with methanol, and immersed in methanol. The methanol solvent is decanted and replaced once per day over the next three days. The products are then evacuated to dryness and heated under vacuum to 250 °C. After 24 h, the sample cooled to room temperature for storage.
Time needed: 5 days
IN CONCLUSION, in terms of complexity of synthesis, the one that require the simplest steps is HKUST-1.
Cost of chemicals:
1. MIL-101
Cr3(NO3)3 : SGD 8000 / t
Terephthalic acid [C6H4(CO2H)2] : SGD 1400 / t
HCl (12M) : SGD 280 / t
DMF (Dimethylformamide) : SGD 5 / 500ml
2. HKUST-1
H3BTC [BTC: Benzenetricarboxylate] : SGD 27000~36000 / t [ Expensive ]
Cu(OAc)2 [OAc-: Acetate/Ethanoate] : SGD 200 / t
3. Al MOF-519
H3BTB [BTB: Benzenetriyltribenzoic acid] : SGD 488 / g [ VERY expensive ]
Al(NO3)3 : SGD 8000 / t
HNO3 : SGD 560 / t
DMF (Dimethylformamide) : SGD 5 / 500ml
4. NU-111
Hexacarboxylic acid : SGD 562/ g [ VERY expensive ]
Cu3(BHEI) [BHEI ion: 5,5′,5″-(benzene-1,3,5-triyl-tris(buta-1,3-diyne-4,1-diyl))triisophthalate] : price cannot be found, too rare.
5.Ni MOF-74
2,5-dihydroxyterephthalic acid : SGD 49 / g
Ni(NO3)2 : SGD 11200 / t
DMF (Dimethylformamide) : SGD 5 / 500ml
IN CONCLUSION, in terms of cost of chemicals, the most economic choice is MIL-101.
Thermal stability:
HKUST-1 (270 °C ) < MIL-101 (350 °C )
The temperature of structure crack-down is not found for Al MOF-519.
IN CONCLUSION, in terms of thermal stability and endurance, the one that can withstand the highest temperature is MIL-101.
IN FINAL CONCLUSION, the best material for future methane storage is MIL-101.