A new breakthrough in the ability to separate and store gas-the birth of a new material with high porosity

With the continuous progress of industrial civilization and human society, global energy demand continues to rise. In 2019, the total global primary energy consumption reached 583.9EJ, and the International Energy Agency predicts that by 2040, global energy demand will still increase by 25%. The massive burning of traditional fossil energy such as oil and coal has caused environmental problems such as the greenhouse effect, acid rain, and photochemical smog.
On the one hand, as a clean energy carrier, gas has both high calorific value and low carbon emissions, which has attracted more and more attention. On the other hand, some gases are also important raw materials in the chemical industry. For example, ethylene is a raw material for many chemical products, with a global output of 200 million tons. However, the production process of these chemical raw materials consumes a lot of energy, and the energy consumption for producing one ton of ethylene is 26×10 9 J. As far as the entire chemical energy consumption is concerned, the energy consumed for gas separation and purification in the production of these basic chemical raw materials accounts for 40% of the total. Therefore, the development of high-efficiency, energy-saving, and economical gas separation technology has become a hot spot pursued by scientists.
Metal-organic framework compounds (MOFs) are porous materials with a three-dimensional topological structure formed by the self-assembly of metal ions and organic ligands. Compared with traditional porous materials such as charcoal and zeolite, it has a unique pore structure, ultra-high specific surface area and porosity. In addition, due to the diverse assembly units, this material has structural adjustability and functional design, and is easy to functionalize. Based on this structural feature, MOFs can not only be used as adsorbents to achieve high-density storage of clean fuel gas, but also can produce differential interactions for different gas molecules, thereby achieving economic and energy-saving separation of gases. Therefore, It is widely used in the field of gas storage and separation, and has made a series of breakthroughs.
Gas storage
The earliest gas storage application of MOFs can be traced back to 1997, when methane became the first storage gas tested, and then storage tests for hydrogen, ethylene, etc. were successively launched.
Natural gas is the main fuel gas, and its main component is methane, so methane storage has become an important area of ​​gas storage.
The U.S. Department of Energy has set the methane storage capacity evaluation index as 350cm 3 (STP)/cm 3 (a cubic centimeter of material absorbs 350 cubic centimeters of gas in a standard state). The classic MOFs material HKUST-1 can adsorb methane up to 267cm 3 (STP)/cm 3 under the conditions of room temperature and pressure of 6.5MPa, which meets the standards of storage technology. A large number of studies have proved that suitable pore size and favorable binding sites are the key to improving the storage performance of MOFs for methane. But under high pressure, pore volume is the ultimate factor in determining storage capacity.
The material with the highest hydrogen storage capacity reported so far is a metal organic frame material with nickel as the metal center. The hydrogen storage capacity can reach 23.0g/L 3 between -75°C and 25°C. In 2020, the U.S. Department of Energy announced the hydrogen storage indicators for light fuel cell vehicles, requiring the mass capacity of the hydrogen storage medium to be above 4.5%, and the volume capacity to be no less than 30g/L, and the storage temperature to be -40℃～60℃ . The nature of hydrogen itself makes it unable to produce a strong interaction force with porous materials, which directly weakens the storage capacity of the material. The currently reported MOFs cannot yet meet the above storage standards. However, studies have proved that the presence of unsaturated metal sites can enhance the storage capacity of MOFs for hydrogen under low pressure conditions.
Acetylene, which is an important raw material for chemical products and electrical materials, will explode when the pressure exceeds 0.2Mpa, so the storage of this gas is facing great challenges. In the study of MOFs applied to acetylene storage, suitable pore structure and unsaturated metal sites are considered to be the key points to enhance the interaction between acetylene and MOFs structure. In addition, modification of pyridyl, amino and other functional groups on organic ligands can also promote the adsorption of acetylene by such materials. According to reports, the storage capacity of a cadmium metal organic frame material for acetylene is 98 times that of high-pressure gas cylinders.
Gas separation
Carbon dioxide capture and separation
The burning of fossil fuels releases a large amount of carbon dioxide, which leads to the greenhouse effect. Therefore, many researches are devoted to the use of MOFs for carbon dioxide capture.
There are two main situations for the separation of carbon dioxide: one is flue gas, which separates carbon dioxide from nitrogen; the other is natural gas, which separates carbon dioxide from methane. The introduction of Lewis base sites such as unsaturated metal sites and amine groups is beneficial to improve the selective adsorption of carbon dioxide by MOFs. However, these scenarios often require the presence of water vapor, so the water stability of the MOFs material is very important for the capture of carbon dioxide.
Toxic gas capture
In recent years, the capture of toxic gases by MOFs has attracted increasing attention. These toxic gases usually include sulfur dioxide, hydrogen sulfide, chlorine, nitrogen oxides, ammonia, etc., most of which are produced by industrial waste gas emissions, which are serious for the environment and human health. harm. However, these toxic gases often produce strong interactions with MOFs materials, resulting in that once they are adsorbed and cannot be desorbed, the recycling of the materials is low, which reduces the practical application value of the materials; on the other hand, the trapping environment of these gases is often In the presence of water vapor, the water stability of MOFs has become another focus of attention.
Therefore, recent research has paid more attention to the development of toxic gas traps that can be recycled and have water stability. At present, the research team of the University of Manchester has made an important breakthrough in sulfur dioxide capture. The MOFs material can not only achieve the desorption of sulfur dioxide under relatively mild conditions, but also maintain the capture performance of sulfur dioxide in the presence of water vapor.
Separation of carbon dioxide and acetylene
Acetylene is not only an important chemical raw material, but also an important energy gas. Its production sources are mainly methane partial combustion and petroleum hydrocarbon cracking. Carbon dioxide is often symbiosis with the production process of acetylene, so the separation of carbon dioxide from acetylene is of great significance for obtaining high-purity acetylene. However, the molecular size, boiling point and other physical and chemical properties of the two are very similar, which makes the separation of the two gases extremely difficult. The introduction of functional sites such as unsaturated metal sites enhances the affinity between acetylene molecules and the MOFs framework, so that most MOFs used in the separation can selectively adsorb acetylene. From an application point of view, reverse adsorption-that is, selective adsorption of carbon dioxide is more practical, so this reverse adsorption of MOFs has become a research hotspot in recent years.
Separation of olefins and alkanes
Ethylene is an important raw material for petrochemical products. Traditional separation methods often use energy-consuming cryogenic distillation to separate it from ethane. The separation of these two gases by MOFs materials can be divided into two categories: one is the preferential and selective adsorption of ethylene; the other is the preferential and selective adsorption of ethane. The adsorption mechanism of the former is mainly based on the relatively strong interaction between ethylene and MOFs and the relatively small molecular size of ethylene; hydrogen bonds, electrostatic attraction, and van der Waals forces are the design guidelines for the latter selective materials.
The separation of propylene and propane is a very important and challenging industrial process. Inserting unsaturated metal sites in the structure of MOFs can promote the separation of propylene from propane by using the strong interaction between it and olefins. Based on the principle of molecular sieve, the researchers designed a yttrium metal-organic framework with high hydrothermal stability. It can produce up to 99.5% propylene during the selective separation of propylene and propane, which is considered to be truly useful Adsorbent for propylene/propane separation.
Separation of alkenes and alkynes
Industrial separation of acetylene and ethylene mainly uses partial hydrogenation and solvent extraction. The difficulty of MOFs materials in this application is how to maintain high selectivity to acetylene while taking into account high adsorption capacity. The SIFSIX series of MOFs using anionic ligands are considered to have outstanding performance for this separation application.
Future trend
MOFs materials have been widely used in gas storage and separation in the past 30 years due to their high specific surface, high porosity, and adjustable structure. They have great potential in the field of gas storage and separation. In addition to the types of gas storage and separation described above, some more valuable and more difficult gas applications, such as the separation of rare gases, the separation of isotopic gases, and the separation of N 2 /O 2 have begun to receive more attention.
Flexible MOFs are considered to have great potential in the field of gas separation due to their unique gating effect. Combining functional sites on the basis of flexible MOFs to control the gating effect is the development direction of this type of material.
In addition, the evaluation of gas storage capacity mainly includes mass capacity, volume capacity, working capacity, etc., but various studies have not yet unified standards, so there are obstacles in performance comparison; in gas separation, gas component ratio, temperature, pressure, etc. Will affect the separation performance of MOFs materials. Therefore, the establishment of a unified and in line with the actual industry test standards and test conditions is not only conducive to the horizontal comparison between materials, but also conducive to judging its practical application value.
Gas is widely used as an energy material in industry and life. The development of economical and energy-saving porous materials for storing and separating gases is not only of great significance from a scientific and industrial point of view, but also an important aspect of energy chemistry and materials research. In the future development, MOFs materials will have broader application prospects in the field of gas storage and separation.