Research progress on the removal of VOCs by molecular sieve adsorption

Volatile organic compounds (VOCs) are a general term for a class of organic compounds, which refer to organic compounds with saturated vapor pressure> 70 Pa at normal temperature and 50-260 ℃ boiling point at normal pressure. Such compounds have a great impact on human health. Great harm. The types of VOCs include ketones, hydrocarbons, alcohols and lipids. Man-made emissions of VOCs mainly come from industrial activities that use organic solvents, fuel combustion, and transportation projects. According to statistics, the emission of VOCs in my country is very large, much higher than the emission of pollutants such as dust and sulfur dioxide. From the current point of view, my country's technology for controlling VOCs emissions is still immature, and there are many problems, which bring greater difficulties to the control of VOCs emissions, and are not conducive to environmental governance and improvement. Therefore, it is urgent to carry out research on the governance of VOCs.



In recent years, the research of VOCs adsorbent has attracted the interest of many scholars. The reports on this system are mostly activated carbon and molecular sieves. Activated carbon molecular sieve has disadvantages such as difficulty in regeneration, poor hydrophobicity, and flammability, resulting in its application effect in industry is not obvious.



Therefore, it is particularly important to develop new adsorbents to improve the control of VOCs. Zeolite is a material with a crystal structure and regular pore size. The uniform pore size prevents molecules larger than a certain size from entering the crystal lattice and has the function of molecular sieve. Among them, hydrophobic zeolite has attracted wide attention due to its good recyclability, hydrophobicity and thermal stability.



1 Molecular sieve adsorption technology to treat VOCs



Molecular sieve is not only a kind of catalytic material, but also has good adsorption and separation functions. It plays a huge role in petrochemical and environmental chemical industries. So far, there are 213 kinds of molecular sieves with known structures, and from the perspective of the diversity of component elements and framework structures, there is still a lot of room for development. The reported molecular sieve adsorbents mainly include A-type, X-type, Y-type, and ZSM-type molecular sieves. Molecular sieve materials are used for the separation and purification of VOCs and have great application prospects.



1.1 Type A molecular sieve



Among many types of zeolite molecular sieves, type A molecular sieves are the more commonly used ones. Its structure is similar to NaCl and belongs to the cubic crystal system. This type of molecular sieve has strong adsorption capacity and ion exchange capacity. It is often used as a desiccant and adsorbent and is widely used in chemical, petroleum, and pharmaceutical fields. The adsorption of different VOCs by type A molecular sieve is shown in Table 1.



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Among them, Liu Zhiping found that the maximum adsorption capacity of the three n-alkanes in the mesoporous 5A zeolite is greater than the maximum adsorption capacity of each in the microporous 5A. It is further verified that the adsorption and microporous diffusion of n-alkanes on the binder-free 5A molecular sieve have a great influence, and the adsorption and perforation diffusion on the binder-free 5A molecular sieve are directly related. At the same time, the adsorption and diffusion of various alkanes with multiple carbon chains on 5A zeolite molecular sieve were investigated under certain temperature and pressure conditions, and the data was correlated with the Langmuir model to explore the diffusion coefficient and carbon of different types of multiple carbon chains. The relationship between the number of chains and the heat of adsorption.



Ruthven et al. found that the activation energy of diffusion increases with the increase of carbon number, and the initial adsorption heat has a linear relationship with carbon number.



Carlos A et al. studied the adsorption and diffusion of propylene and propane in a 5A zeolite molecular sieve in the temperature range of 323 to 423 K, and obtained the corresponding adsorption data and equilibrium kinetic data (see Table 1). At 100 KPa, The adsorption selectivity of propylene increases with increasing temperature.



Modification of type A zeolite molecular sieve to prepare molecular sieve composite material, so that the adsorption performance is improved. The modified composite material has different adsorption capacities for VOCs of different polarities. The greater the polarity, the stronger the adsorption capacity of toluene and ethyl acetate memory acetone.



Zhang Hope et al. prepared A-type molecular sieve/activated carbon fiber composite material, which significantly improved the problem of poor adhesion of molecular sieve on the surface of activated carbon fiber. The specific surface area, pore volume and micropore volume of the composite material were significantly improved compared to pure molecular sieve. The composite material exhibits an excellent adsorption effect on methylene chloride.



In summary, the addition of binder to A-type molecular sieve blocks part of the effective pores of the molecular sieve, resulting in lower adsorption capacity and



Lower heat of adsorption, this is mainly because the adsorbate molecules show stronger adsorption after the addition of the binder; the activation energy of diffusion and the initial heat of adsorption are linearly related to the carbon content of the adsorbate molecules; type A molecular sieve The preparation of the composite material can significantly improve its adsorption performance.



1.2 X-type molecular sieve



X-type zeolite is a microporous material that can not only perform ion exchange, but also exhibit its catalytic and adsorption functions. The X-type zeolite molecular sieve belongs to the equiaxed crystal system and has a cubic faujasite structure (FAU), and its silica and alumina framework are the same as natural faujasite.



Table 2 shows the adsorption of different VOCs by X-type zeolite.



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Comparing different types of molecular sieves can clearly and intuitively obtain the molecular sieve with the best adsorption performance.



Through comparison, Wang Guoqing et al. found that the 10X zeolite exhibited strong adsorption capacity when adsorbing formaldehyde, indicating that the pore size of zeolite and the cations in the zeolite framework can effectively enhance the adsorption capacity of formaldehyde.



Ki-joong Kim et al. compared the adsorption of mordenite and faujasite on various molecular sieves at 25 ℃ and found that the adsorption capacity of faujasite is better than that of mordenite, and the adsorption capacity of VOCs is positively correlated with the volume of mesopores. For a kind of molecular sieve, the optimal adsorption conditions can be obtained through single factor experiments.



The new composite materials prepared from X-type molecular sieve and other materials also have good adsorption capacity.



Atsuo et al. prepared a TiO2-X zeolite porous glass composite material, and the results showed that when propylamine was adsorbed (3×10-)4, compared with only TiO2 coated glass, the composite X-type zeolite had higher zeolite The specific surface area, the adsorption rate of propylamine increased by 30%. In addition, the molecular sieve can also be modified to change its adsorption capacity. See Table 3 for the adsorption of VOCs by modified X-type molecular sieve.



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Oumaima et al. found that the modified material has an increased adsorption capacity for acetone and 1-x propanol. Structural characterization shows that the modified 13X molecular sieve has a high specific surface area and large pores. The adsorption capacity is not only related to the structural parameters of the adsorbent material. It is related and also related to the configuration and polarizability of each organic molecule.



Sun Jianping used the aqueous ion exchange method to modify the 13X zeolite molecules with Ca2+ exchange. The reason is that after the modification, the Ca2+ charge-to-diameter ratio in the CaX molecular sieve is stronger than the Na+ charge-to-diameter ratio in the 13X molecular sieve, which can generate a stronger electric field and increase polarization. , The hydroxyl polar group of the formaldehyde molecule is easy to be polarized, so the ability to adsorb formaldehyde is stronger.



1.3 Y-type molecular sieve



The Y-type molecular sieve has been paid attention to because of its good ion exchange performance and strong stability, and the research on its adsorption performance has been gradually increased.



Different zeolite molecular sieves and activated carbon are used as experimental objects, and penetration time and adsorption capacity are used as reference quantities. By observing the adsorption capacity of different adsorbents, we can study the corresponding different adsorption performances of VOCs at different intake concentrations and space velocity.



Zhou Chunhe verified through experiments that NaY has the strongest adsorption capacity for toluene, which is almost equal to that of AC.



Wang Zhizhen and others also conducted related experiments and also verified that the adsorption capacity of NaY is very strong, but due to the poor hydrophobicity of NaY molecular sieve, when the environmental humidity is 50%, its adsorption performance is very low. The hydrophilicity of Y-type molecular sieve hinders the process of further industrialization. Generally, industrial exhaust gas has high humidity. Therefore, in the experiment to explore the performance of molecular sieve adsorption of VOCs, relative humidity conditions are indispensable.



Zhou Ying et al. investigated the competitive adsorption of benzene and water on the surface of the Y molecular sieve by controlling the ratio of the three-channel gas flow, controlling the concentration and relative humidity.



Lu Hanfeng et al. obtained ultra-stable Y-type molecular sieve by high-temperature hydrothermal method, put it in a fixed-bed reactor, observed and recorded its adsorption of toluene over a period of time, and concluded that the organic adsorption on the surface of low-silicon NaY molecular sieve The molecules will be replaced by water molecules, and the silicon-to-aluminum ratio increases, and its adsorption performance is enhanced.



1.4 ZSM molecular sieve



ZSM-5 molecular sieve has generally high hydrophobicity and hydrothermal stability, and has good ion exchange performance, so it is very popular in the removal of VOCs.



According to the relevant process conditions, the influence of factors such as air humidity, VOCs initial concentration, and air specific velocity on the adsorption of ZSM molecular sieve was investigated.



Guo Haoqian et al. found through experiments that ZSM-5 molecular sieve has the best adsorption performance at 25 ℃, with an adsorption capacity of 4.26 mg/g.



After ion exchange and changing the ratio of silicon to aluminum, the adsorption performance and hydrophobic performance of the new molecular sieve will change.



Gu Yongyi et al. compared the cation-exchanged H-ZSM-5 molecular sieve with Na-ZSM-5 molecular sieve and concluded that the pore size is not the main factor that causes the difference in the adsorption capacity of the two molecular sieves, but the pore volume is the main factor determining the adsorption capacity. ; Na-ZSM-5 molecular sieve (with a silicon-to-aluminum ratio of 300) was selected as the best molecular sieve; Na-ZSM-5 molecular sieve has a good adsorption effect on small molecule VOCs (<0.6 nm), and the molecular size is close or larger Molecular sieve channels (≥0.6 nm) have poor adsorption effect.



In summary, under the coexistence of VOCs and water vapor, different silicon-to-aluminum ratios affect the hydrophobicity of molecular sieves and obtain different adsorption properties; the pore volume of molecular sieves is also the key to the adsorption of VOCs; VOCs small molecules have the same groups, The relative molecular mass becomes larger, the molecular diameter and polarity increase correspondingly, and the adsorption performance of the molecular sieve becomes stronger. At this time, the temperature needs to be increased to complete the thermal desorption.



2 Industrial application technology of molecular sieve adsorption treatment of VOCs



2.1 Molecular sieve runner concentration technology



The concept of making adsorbents into a honeycomb structured runner and used in the separation process was first proposed by the Swedes and applied for a patent in 1974.



With the advancement of technology and the need for environmental governance, molecular sieve runners have been widely used in industrial production. The system is composed of molecular sieve runner and drive motor, runner box, processing zone, regeneration zone, cooling zone, regeneration heater, blower, etc.



The organic volatile gas discharged from the factory is filtered and dusted and sent to the adsorption zone of the molecular sieve runner through a blower. After being adsorbed by the molecular sieve, the purified air is discharged out. The adsorption runner rotates slowly under the driving of the drive motor. After desorption, the runner is regenerated; after regeneration, the runner returns to the adsorption treatment zone through the cooling zone to complete the adsorption desorption cooling cycle. The regeneration air volume of the system is 1/10 to 1/30 of the processing air volume, and the concentration of VOCs at the inlet of the concentrated processing air is 10 to 30 times.



Weili Jin has developed 3 kinds of commonly used VOCs honeycomb molecular sieve runners, covering the purification of VOCs in various fields such as automobile manufacturing, printing industry, furniture manufacturing, liquid crystal manufacturing, etc., and for adsorbing runner substrate materials and impregnated coating glue The ratio is optimized, and the purification efficiency of the improved runner can be increased by 2 to 6 percentage points.



2.2 Molecular sieve fixed bed adsorption technology



Wang Jianli and others applied the fixed-bed adsorption process to the adsorption of VOCs. The process uses molecular sieve as the adsorption layer and honeycomb cordierite as a whole to make a monolithic molecular sieve adsorbent. The prepared product is used in a motorcycle production spraying workshop in Guangdong to treat exhaust gas and work 9 hours a day. The adsorption efficiency of the monolithic molecular sieve has been maintained above 90% for a long time, and the outlet concentration meets the emission standard. The regeneration effect is good, and it has good hydrophobic properties. The humidity has almost no effect on the adsorption and desorption of the adsorbent, which is better than the activated carbon adsorption device of the same volume.



Zhang Baoping used fly ash from thermal power plants as the matrix, modified it with acid-base solutions of different concentrations, and prepared X molecular sieves at different temperatures and times to study the effect of acid-base solutions on the physical and chemical properties of fly ash. And simulated the factory fixed bed adsorption test for toluene.



According to the results of the adsorption experiment, compared with industrial activated carbon, the synthesized molecular sieve is feasible for industrial production, and it also realizes the reuse of fly ash and reduces the industrial cost.



Mays S Hussein et al. studied the fixed bed adsorption and intermittent adsorption of benzene and toluene in aromatics by 5A zeolite. The analysis shows that compared with benzene, zeolite has a greater adsorption capacity for toluene, and the strong adsorption of toluene replaces the weak adsorption of benzene, and both adsorbents reach equilibrium within 120 minutes. The industrial application of molecular sieve fixed bed adsorption has considerable market prospects. Its adsorption efficiency, hydrophobic performance, recycling regeneration and cost control are better than activated carbon adsorbents, and have been applied in many industrial productions.



3 Conclusion



In recent years, zeolite molecular sieves have made some progress in the removal of VOCs, and a series of excellent adsorption materials have been discovered. Studies have shown: (1) Zeolite pores, silicon-to-aluminum ratio, surface properties, balance cations, etc. have a greater impact on the adsorption of zeolite molecular sieves. In addition, the type and polarity of VOCs also have a certain impact on the adsorption performance. Modification treatment, structural adjustment, composite materials and other methods can obtain molecular sieve adsorption materials with a large amount of VOCs adsorption; (2) The adsorption effect of molecular sieve is significant after loading Pt, Pd and other precious metals, but the price of precious metals is relatively expensive, and direct use will A large amount of economic cost is generated, so it needs to be scientifically designed to increase the composition of metal oxides as much as possible, thereby reducing costs and obtaining the optimal adsorbent.



At present, although some results have been achieved in the removal of VOCs by molecular sieve adsorption method, there are still problems and challenges such as the following aspects as a whole: (1) Currently, molecular sieves adsorb more benzene, toluene, and formaldehyde, while The adsorption of acids, esters, alcohols, ketones and other VOCs is relatively small. In the future, it is necessary to study the effects of zeolite and supported zeolite.