How can the "membrane" method material technology be obtained without carbon peak and carbon neutrality?

In December 2020, the Central Economic Work Conference listed carbon peaking and carbon neutrality as one of the eight key tasks in 2021
.
China has continuously improved its goals in tackling climate change, and has also made positive contributions to the global response to climate change
.
The introduction of carbon peak and carbon neutral goals is also China's internal demand for achieving sustainable development and the only way to a "beautiful China"
.
At this stage, China is facing complex challenges such as large total carbon emissions, short carbon emission reduction time, economic transformation and upgrading challenges, and difficult energy system transformation
.
The main components of flue gas discharged from coal combustion for power generation after purification by processes such as desulfurization and denitrification are carbon dioxide and nitrogen
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The capture of CO2 in flue gas is an important link in the realization of carbon capture, utilization and storage, and is of great significance for reducing greenhouse gas emissions and reducing the occurrence of extreme climates
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? Membrane separation technology is an energy-saving CO2 separation technology that is developing rapidly in the world today.
It is a relatively new physical separation method without phase change.
It has simple equipment, small footprint, convenient operation, and high separation efficiency.
The advantages of low energy consumption, environmental friendliness, and ease of integration with other methods have made the research and development of this technology a hotspot for competition in the high-tech field by countries all over the world
.
? Membrane CO2 capture is a process in which the pressure difference on both sides of the membrane is used as the driving force, and the gas separation process is realized according to the different permeation rates of the components in the membrane
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The mass transfer mechanism of gas molecules in the membrane can generally be divided into dissolution-diffusion, transfer promotion and molecular sieving mechanisms
.
The separation capability of a gas separation membrane depends on the structure of the membrane material and the selectivity of the membrane material to different gases
.
1.
Membrane separation technology type? Membrane separation technology, as the name implies, separates, purifies or concentrates each component in a solution through a membrane
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The operating conditions of membrane filtration are relatively mild, and the entire operating process can be mechanized and automated, and the separation efficiency is very high
.
From the molecular level, that is, the pore size, separation membranes can be divided into microfiltration membranes (MF), ultrafiltration membranes (UF), nanofiltration membranes (NF), and reverse osmosis membranes (RO).
Their respective pore size ranges, separation mechanisms, See Table 1 for driving force and intercepted substances
.
? (Image source: Qiu Xiaoman: Membrane separation technology and its application in the fermented condiment industry)? Although there are many membrane materials currently studied, their separation principles are basically the same, that is, the use of selective permeability of the membrane and the use of driving force , Mass transfer between membrane modules to achieve the separation of different components, the driving force is generally concentration difference, pressure difference, electric potential difference or temperature difference
.
2.
Membrane material? Early membranes used for CO2 separation were mostly organic membranes, such as polyoxyethylene, polysulfone, polyester, etc.
, but due to poor separation performance and stability of membrane materials, the application was limited
.
With the advancement of materials science, the separation performance and stability of membrane materials have been continuously improved.
At the same time, inorganic membranes (such as metal, zeolite, carbon membranes, etc.
) and mixed matrix membranes have been developed, which broaden the application fields
.
Hybrid matrix membranes can complement the advantages of organic and inorganic materials, and have great potential in CO2 separation, and are considered to be one of the most important development directions in the field of separation membranes in the future
.
2.
1 Organic polymer membrane materials? The preparation process of organic polymer membranes has the advantages of relatively simple, low energy consumption, easy expansion, etc.
, and is suitable for large-scale manufacturing
.
Organic polymer membrane materials can be divided into two categories: glassy state and rubbery state according to the form of the polymer
.
The glassy polymer has a lower chain migration ability and a more stable structure than the rubbery polymer.
Therefore, the glassy polymer has better selectivity, but its disadvantage is poor permeability
.
Rubbery polymers have good permeability, but are easy to swell and deform under high pressure
.
?? Commonly used polymer membrane materials for gas separation are cellulose acetate, polyimide, polyaluminum and polyetherimide.
They all have good gas selectivity, but their gas permeability coefficients are low.
Polydimethylsiloxane, polytrimethylene silane propyne, etc.
have high gas transmission coefficients
.
2.
2 Inorganic polymer membrane material? Inorganic membrane is a membrane made of inorganic materials, because of its high temperature resistance, corrosion resistance, good chemical stability, high separation efficiency, easy cleaning, easy disinfection and long service life of the membrane, etc.
Advantages, the development and application of inorganic membranes has become a major research and development hotspot in the field of membrane technology
.
Inorganic membranes can be divided into ceramic membranes, metal membranes, alloy membranes, molecular sieve composite membranes, zeolite membranes and glass membranes, among which ceramic membranes are the most used
.
Ceramic separation membrane is a separation material made of porous ceramics as a carrier.
It is mainly based on the "sieving" theory, using pressure differences to achieve the separation of mixed materials, and is generally used for microfiltration and ultrafiltration
.
The ceramic membrane materials currently developed include titanium dioxide (TiO2), aluminum oxide (Al2O3), cobalt oxide (CoO), zinc oxide (ZnO), silicon dioxide (SiO2), silicon carbide (SiC), carbon nanotubes (CNT) and Graphene oxide (GO) and so on
.
Among them, GO is a new type of nanomaterials, which has shown great development prospects in the development of anti-fouling nanocomposite membranes
.
? 2.
3 Mixed matrix membranes? Hybrid particles (dispersed particle phase) are filled in high molecular polymers (polymer phase), and mixed matrix membranes (MMMs) are prepared through the interaction between inorganic fillers and high molecular polymers
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The mixed matrix film not only has the advantages of high film-forming ability of the polymer film and not easy to be broken, but also optimizes the arrangement of the polymer chain due to the introduction of inorganic materials in the polymer matrix
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Under ideal circumstances, the mixed matrix membrane combines the advantages of the two phases of polymer phase and dispersed particle phase, that is, it has the processing and mechanical properties of the polymer and the special transport properties of molecular sieves, making it suitable for CO2? separation.
Has potential advantages
.
Due to the compatibility problem of the polymer and the inorganic phase in the mixed matrix film, only an appropriate amount of inorganic material can be allowed to load, and the best dispersion and interface contact can be obtained
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For hybrid membranes filled with porous filler particles, the screening effect of the filler is the most critical factor in improving performance
.
For inorganic particle zeolite, the whisker-like structure formed on the surface provides additional roughness for the linkage between the polymer chain and the zeolite
.
? 3.
Mixed matrix film filler? For the formation of mixed matrix film, good adhesion between the polymer matrix and the inorganic filler is very important, especially when the polymer has a high Tg (glass transition temperature), This is especially true for good mechanical properties and stability of properties over time
.
The structural characteristics and interaction of fillers and polymers are also facing great challenges
.
These different fillers also have different effects on the separation performance of mixed matrix membranes
.
? 3.
1 Inorganic particle filler? Combined with the polymer matrix, the most studied fillers are zeolite, carbon mesoporous silica and metal organic framework (MOF)
.
Inorganic particles used for hybridization generally include carbon nanotubes, molecular sieves, silica and the like
.
Inorganic fillers mainly control the separation performance of membranes through their structure
.
However, due to the poor compatibility of the inorganic filler with the polymer membrane matrix, the membrane tends to agglomerate, which reduces the separation performance of the membrane
.
Discovering more inorganic particles with high compatibility with polymer membranes is an urgent problem to be solved
.
? 3.
2 Organic filler? The advantage of organic filler structure is that it is controllable and has good flexibility, and has excellent compatibility with the polymer matrix, but it has poor solvent resistance and corrosion resistance and cannot maintain good under harsh operating conditions.
The gas separation performance
.
Common organic fillers include porous organic polymers (POPs), covalent organic frameworks (COFs) and so on
.
3.
3 Metal-organic framework fillers Metal-organic framework materials (MOFs) are a kind of porous network materials, which are self-assembled by organic ligands and certain metal cations through coordination bonds
.
Most MOFs have uniform pore size and present a three-dimensional network conformation
.
In addition, compared with traditional porous materials (zeolite molecular sieves, activated carbon, etc.
), this material has the advantages of diverse structures and controllability
.
?? Because the research of MOFs effectively combines the advantages of inorganic fillers and organic fillers, filling the polymer matrix with MOFs can effectively improve the gas separation performance of the membrane
.
The MOFs material has an excellent affinity with the polymer matrix, which can effectively avoid problems such as non-selective gaps between the two phases, and greatly improve the separation performance of the membrane
.
A type of zeolite imidazole framework material (ZIF) in MOFs as filler particles can effectively improve the separation performance of the matrix membrane for CO2 gas
.