Methane and hydrogen are "clean energy" to reduce carbon emissions, but high-pressure compression is required for storage and application, which is often unsafe and expensive. The development of new adsorbents is the ultimate goal for safe and economical storage of methane and hydrogen, and metal-organic framework (MOFs) materials are an ideal adsorbent. MOFs synthesized by the team of world-renowned experts Professor Farha in this field can safely and effectively store methane and hydrogen, thus becoming the most ideal candidate adsorbent material.

With the development of society and economy, the CO2 emissions produced by using petroleum as the main fuel have attracted worldwide attention, prompting people to search for cleaner energy to replace petroleum. Among them, methane and hydrogen are relatively good choices, and both can be used to replace petroleum as fuel. Methane is regarded as a transitional fuel. Although CO2 is also produced during the combustion process, its emissions are much smaller than those of gasoline. Hydrogen can ensure zero CO2 emissions during the combustion process, so hydrogen is considered to be "future fuel". However, high-pressure compression is required for storage and application, and the application standard can only be achieved when H2-700 bar and CH4-250 bar are achieved, but this is often unsafe and expensive .

In recent years, led by the United States Department of Defense Energy Corporation (DOE), it has established indicators specifically for the development of new energy storage and transportation systems, pointing out that the development of new adsorbents is the ultimate goal to achieve safe and economical storage of methane and hydrogen . Especially porous materials with a specific surface area of up to 2000 m2 / g, such as metal-organic frameworks (MOFs) , porous carbon, covalent organic frameworks and porous organic polymers have been widely used as candidates for on-board storage of clean energy gases One of the adsorbents. The characteristics of these adsorbents allow gas-laden vehicles to withstand extreme loading pressures lower than those required by current methane storage systems. At the same time, among these adsorbents, MOFs composed of inorganic and organic linkers have become important materials for gas storage due to their controlled chemical properties, porosity, and clear structure-property relationship. In addition, in practical applications, the volume and weight of the material must be considered at the same time, which is essential for the development of methane and hydrogen storage.

Recently, a world-renowned expert in the field of metal-organic frameworks (MOFs) , Professor OmarK. Farha (corresponding author) of Northwestern University in the United States has developed an ultra-high porosity between weight and volume surface area to solve the huge challenges of CO2 emissions today. The balance of adsorbent materials makes it easier to store hydrogen and methane to replace traditional fossil fuels. Related papers were published on Science on April 17, 2020, entitled " Balancing volumetric and gravimetric uptake in highly porous materials for clean energy "

This paper reports the simulated synthesis of ultraporous metal-organic frameworks (MOFs) based on metal trinuclear clusters , namely NU-1501-M (M = Al or Fe), compared with the previously studied ultraporous MOF, NU-1501- Al has both a high-weight BET area of 7310 m2 g-1 and a high-volume BET area of 2060 m2 cm-3, while meeting four BET conformance standards. Therefore, the high porosity and surface area of this MOF have excellent hydrogen and methane weight and volume storage performance : NU-1501-Al exceeds the 0.5 g g-1 methane storage capacity set by the US Department of Energy at 0.66 g g-1 , And in the case of temperature and pressure changes, show the best storage capacity of hydrogen.

Figure 1. The design and synthesis process of NU-1501. (AC) Schematic diagram of NU-1501-M (M = Fe and Al) and 6-c acs network; (D) Optical image of NU-1501 single crystal.

Figure 2. Molecular simulation predicts the tradeoff between weight and volume. (A) Superposition of VSA and GSA composed of two colors; (B) Superposition of GSA and VSA and porosity; (C) Plotted relationship between GSA and VSA and maximum pore size of MOF; (D) Volumetric storage capacity of hydrogen (VDC) and weight delivery capacity (GDC); (E) methane VDC and GDC at 270K; (F) methane VDC and GDC at 296K.

Figure 3. Porosity test of NU-1501. (A) NU-1501 and NU-1500-Al experimental and simulated N2 adsorption isotherms (77K); (B) NU-1501 experimental and simulated Ar adsorption isotherms (87K); (C) N2 (77K) adsorption DFT pore size distribution of NU-1501-Al under isothermal curve.

Figure 4. NU-1500-Al and NU-1501-Al high-pressure hydrogen and methane adsorption properties. (A, B) Experimental and simulated hydrogen absorption of NU-1501-Al and NU-1500-Al at 77, 160 and 296K; (C, D) NU-1501-Al and NU-1500-Al at 270 and 296K Experimental and simulated adsorption of methane at the time; (E, F) NU-1501-Al and NU-1500-Al calculated by the crystal density of the volumetric absorption of hydrogen and methane.

Figure 5. Trade-offs in weight and volume properties based on experimental adsorption studies. (A) The trade-off between the weight and volume BET area of the selected ultra-high porous material; (B) According to the capacity to store hydrogen under the action of temperature and pressure; (CF) MOFs at room temperature and near freezing temperature 5 to The weight and volume of 80bar and 5 to 100bar can store methane.

In short, the author reasonably designed a series of microporous NU-1501MOFs with mesopores and a pore size of less than 2.5nm. The advantage is that the synthesized MOFs can balance the BET between the weight and the volume, so that they can safely and effectively store methane and hydrogen, thus becoming the most ideal candidate adsorbent material . In particular, NU-1501 has a medium pore volume compared with traditional ultraporous materials such as MOF-210, NU-110 and DUT-60 after meeting all four BET conformance standards. NU-1501 has Impressive volume BET.

The combination of experiments and molecular simulations shows that in the actual environment, NU-1501 achieves both excellent weight and volume storage capacity of methane and hydrogen, and capacity that can be further promoted, thus making these materials a new type of promising The MOF adsorbent is used to store and transport methane and hydrogen clean energy . Finally, the clear structure-performance relationship derived from the material‘s performance, high-throughput computational modeling and experimental results will point the way to the design and synthesis of next-generation ultraporous adsorbents. (Text: Caspar)

Source: Materials Science and Engineering

Thesis link

https://science.sciencemag.org/content/368/6488/297