Rare earth has been widely used in different catalytic reactions due to its unique properties, such as catalytic hydrogenation. Rare earth hydrogenation catalysts can not only simplify the process flow, but also improve the catalytic reaction activity, selectivity and stability. At present, catalysts containing precious metals have important applications in heterogeneous catalytic hydrogenation, but their large-scale industrial applications are limited because of their limited reserves and high prices. Therefore, using rare earth elements to replace precious metals, or developing supported rare earth containing catalysts with nano mesoporous and skeleton structures, and applying them to catalytic hydrogenation reactions, is expected to solve the problems of low selectivity, high cost, low stability, high pollution and difficult recovery of catalysts in traditional heterogeneous catalysis, which is conducive to the realization of green chemical production processes.
Synthesis of rare earth doped molecular sieve catalysts
The synthesis of rare earth doped molecular sieves is mainly to introduce rare earth elements directly into the framework of molecular sieves or exchange with non framework elements to enter the structure of molecular sieves. According to its ion doping process, it can be divided into hydrothermal method, immersion method, ion exchange method and precipitation method.
Hydrothermal method uses water as solvent and reaction medium to synthesize or modify rare earth doped molecular sieves under certain temperature and pressure. Singha et al. Prepared MgO and CeO2 promoted Ni nanoparticles supported ZnO catalysts by hydrothermal method. It is found that the high alkalinity of magnesium oxide and the excellent redox performance of cerium oxide increase the dispersion of Ni, generate strong metal support interaction, and effectively reduce the carbon on the surface area of the catalyst.
The impregnation method mainly relies on the adsorption sites on the surface to adsorb rare earth ions in the solution. In order to improve the performance of the hydrogenation catalyst for aromatic nitro compounds, Nie et al. Successfully introduced rare earth into the single copper catalyst by immersion method with SiO2 as the carrier to prepare cu-re2o2 / SiO2 (re = CE, SM, Ho, Yb) rare earth catalysts. The differential reactor was used to evaluate the catalytic hydrogenation of nitrobenzene. Among them, the Cu-CeO2 / SiO2 catalyst had the best effect, The load of primary life (without regeneration) exceeds 121.28% of the Cu / SiO2 Catalyst widely used in industrial production.
The principle of the ion exchange method is that the ions on the surface of the carrier can be exchanged with the active components, which has high selectivity. The precipitation method is easy to obtain higher dispersion and uniformity, but the selectivity is poor. Liu et al. Prepared rare earth yttrium modified bimetallic catalysts by ion exchange method. The hydrogenation activity of tetrahydronaphthalene was investigated in a high pressure fixed bed catalytic reactor. The results showed that except for 2wt% y catalyst, the conversion of tetrahydronaphthalene could be adjusted by changing the content of yttrium in the reaction, and the relationship between the two was inverse linear. Zhao et al. Prepared cu-re-al2o3 (re = y, La, CE, Dy or HO) catalysts by precipitation method for CO2 hydrogenation to methanol, and found that cu-la-al2o3 rare earth catalysts have high hydrothermal stability and methanol selectivity.
Effect of doping rare earth elements on the structure and properties of hydrogenation catalysts
2.1 effect of rare earth doping on structural stability of hydrogenation catalyst
Rare earth can improve the surface acidity of hydrogenation catalyst, enhance the chemisorption ability of catalyst, and increase the collapse temperature of molecular sieve, which has an important impact on its structural stability. In addition, different rare earth ions can coordinate to form relatively stable binuclear octahedral complexes, thus inhibiting the dealumination of the skeleton and improving the stability of the molecular sieve structure.
Liu et al. Studied the modification of catalytic cracking catalysts by CE and La. The results showed that CE was more conducive to the formation of strong acid sites. The FCC (catalytic cracking) catalysts modified with appropriate CE had the advantages of high selectivity for gasoline and low coke. Compared with La, the molecular sieve structure stability effect of CE at low content was better than LA.
The introduction of rare earth elements can also enhance the interaction between the mesoporous framework and the metal active center. Xu et al. Prepared a series of rare earth (LA, CE, SM and PR) - Doped Nickel Based mesoporous materials by evaporation induced self-assembly strategy and used them to catalyze the hydrogenation of CO2 to methane. It was found that rare earth can effectively inhibit the thermal sintering of metal Ni nanoparticles, enhance the strong interaction between the mesoporous framework and the Ni active center, effectively stabilize metal Ni nanoparticles, and improve the structural stability of the catalyst.
2.2 effect of rare earth doping on selectivity of hydrogenation catalyst
The doping of rare earth elements can improve the selectivity of hydrogenation reaction. Su Chunyan et al. Prepared a supported fe-cu-k-ce rare earth catalyst, with a CO2 hydrogenation conversion of more than 60% and a low olefin selectivity of about 20%. When Deng Guocai's research group studied the rare earth catalyzed hydrogenation of carbon dioxide to low-carbon olefins, it found that when the composition ratio of fe-co-mn-k in the catalyst was 100 ∶ 25 ∶ 5 ∶ 5, neodymium in the rare earth element had a better effect. The addition of 2.5% neodymium could make the total selective production of ethylene and propylene reach 6% under normal pressure.
Park et al reported that the CO2 conversion of cuzno-zro2 / SAPO-34 composite catalyst was 33.9%, and the final yield of low-carbon olefins was less than 10%; In contrast, Rong et al. Used cuo-zno-zro2 (CZZ) and rare earth (y, La, CE) modified SAPO-34 as a composite catalyst, and found that the selectivity of low-carbon olefins increased significantly, and the selectivity of alkanes decreased significantly. CZZ / la-sapo-34 showed good selectivity (54.5%) and yield (27.1%).
Table 1 catalytic performance of different catalysts for CO2 hydrogenation
Rare earth doping can modify the surface of metal oxide, increase the number of acid sites adsorbed by raw materials, and facilitate the selective hydrogenation reaction. Hou et al. Studied the liquid-phase selective hydrogenation of crotonaldehyde to butanol catalyzed by la2o2co3 supported Pt and Pt co nanocatalysts, and found that Pt atoms formed strong chemical interactions with the surface of la2o2co3, resulting in positively charged Pt atoms and oxygen vacancies. After deposition, Pt atoms tended to anchor on the surface of lanthanum, thus promoting the polarization and hydrogenation of carbonyl oxygen. At the PT La interface, they hydrogenated with activated hydrogen to finally produce butanol, The addition of CO can destroy the PT La interface and form Pt Co particles, thus significantly improving the hydrogenation selectivity of the catalyst for Crotonyl alcohol.
Ma et al. Prepared ZrO2 doped with rare earth metals such as La, PR, Nd, CE by surfactant assisted coprecipitation / hydrothermal crystallization method, and used CO as catalyst to catalyze the hydrogenation of furfural in water to furfural alcohol. Using CO / zrla0.2ox as raw material, under the condition of 40 ℃, 2MPa H2, the yield of furfural in water was 95 mol% in 10 hours. The results show that the doping of rare earth can modify the surface of ZrO2, increase the number of acid centers adsorbed by carbonyl raw materials, and there is a strong interaction between the support and C = O bonds, as well as between CO clusters and C = O bonds, which is conducive to the hydrogenation of carbonyl groups, thus improving the selectivity of carbonyl hydrogenation.
Li et al. Prepared supported Ni by impregnation/ γ- Al2O3 catalyst and rare earth modified Ni/ γ- Al2O3 catalyst. The synthesis of 2-methylfuran by gas-phase hydrogenation of furfural was used as a probe reaction. It was found that the addition of rare earth Nd or EU could increase Ni/ γ- The selectivity of Al2O3 to 2-methylfuran increased from 95.3% to 96.2% or 97.4%.
2.3 effect of rare earth doping on hydrogenation catalyst activity
There is a division of labor and synergy between metals and rare earth ions, which can increase the number of isolated metal ions, inhibit the decrease of reaction acid density and improve the catalyst activity while improving the dispersion of metal species on the catalyst surface. In addition, rare earth elements can be used as electronic modifiers. After doping, the apparent activation energy of reactive molecules can be significantly reduced, which is conducive to the activation of reactive molecules, thus improving their low-temperature catalytic activity, as shown in Fig. 1.
Figure 1 zrla0.2ox supported cobalt nanoclusters for hydrogenation of furfural to furfuryl alcohol in water
Yang et al. Prepared Cu ZnO Al2O3 (CZA) catalyst modified by rare earth elements La, CE, PR and ND for the hydrogenation of CO2 to methanol. It was found that CE / CZA catalyst has strong adsorption capacity for reactant molecules. In addition, the strong interaction between Cu and ZnO improves the catalytic activity, which makes the methanol yield of CE / CZA catalyst increase by 43.6% compared with CZA catalyst.
Shi et al. Prepared CE / SBA-15 (C-X) composite molecular sieve catalyst by ion exchange method. It was found that CE loading not only did not destroy the original structure of Y / SBA-15, but also increased the active center of the composite molecular sieve. In addition, CE increases the migration probability of the skeleton and hydroxyl silicon aluminum hydroxyl groups through polarization and entrainment, thus improving the B acid strength of C-X and making it more active when used in heavy oil processing.
3. effect of rare earth doping on catalyst stability
The supported metal oxide catalyst has the advantages that the active components are not easy to be lost, the hydrothermal stability is high, and it can be used for high-temperature and liquid-phase reactions, but the disadvantages are that the acid strength is relatively weak, and the carbon deposits generated in the catalytic hydrogenation reaction will cover the surface of the molecular sieve, resulting in the deactivation of the molecular sieve catalyst. Modifying these problems is the main direction of developing hydrogenation catalysts at present, and rare earth element doping has an obvious effect. For example, Yan et al. Found that rare earth metal oxides can improve the stability of catalysts, and the doping of rare earth elements can significantly enhance the anti coking performance, greatly improve the stability and reducibility (low temperature) of catalysts, and prolong the life of catalysts.
Luo et al. Used rare earth metals (LA, CE, SM, PR, y, Nd, EU, Gd, ER) to modify HZSM-5 molecular sieves for stability and deactivation regeneration research. It was found that 13% LA / HZSM-5 molecular sieves and 13% nd / HZSM-5 molecular sieves can inhibit the generation of carbon deposits and improve the stability of catalysts due to the disappearance of strong acid sites. After the catalyst was regenerated many times, the catalytic effect still remained the original activity and stability.
Guo et al. Prepared Cu Cr Mo LA / SiO2 composite hydrogenation catalyst by sol-gel method and used it for gas phase hydrogenation of nitrobenzene to aniline. It was found that the particle size and dispersion of the catalyst were better, and the addition of Rare Earth La reduced the grain size of CuO, which made Cu / SiO2 have better structural stability and anti coking ability.
4. effect of the type and content of rare earth elements doped on the performance of hydrogenation catalyst
The sodium type molecular sieves of catalytic cracking catalysts generally have no catalytic activity. They need other cations (NH4 +, Ca2 +, Mg2 +, Mn2 + and rare earth ions) to replace Na + before they have catalytic cracking activity. Among them, the molecular sieves exchanged with rare earth ions have high activity and good stability.
The introduction of different rare earth elements can affect the performance of hydrogenation catalysts. Kondo et al. Synthesized Co / MgAl2O4 catalyst containing rare earth by impregnation method with spinel with alumina content greater than 18mol%, and studied the effect of rare earth (y, La, CE, PR, Nd, Gd, TB, Dy, Ho, er, Yb) on the catalytic performance of CO / MgAl2O4 catalyst for CO hydrogenation to C5 Hydrocarbons. The results show that the Co / MgAl2O4 catalyst with 25% Ce content has better activity. Except for Dy, other rare earth elements can improve CO conversion and C5 hydrocarbon yield.
The mixing ratio of rare earth elements can affect the activity and physical properties of catalysts. Liu et al. Synthesized a nickel based catalyst by co precipitation method for oil hydrogenation. During the preparation process, rare earth elements, lanthanum and cerium nitrate were doped, and the hydrogenation of palm oil was taken as a model reaction. When the doping ratio of La ∶ CE is 1 ∶ 4, the grain size and crystal habit remain unchanged, but the specific surface area and the dispersion of active components of the catalyst increase, and the pore volume and particle size decrease.
Table 2 Catalytic Performance of catalysts containing different rare earth elements
5. recovery of rare earth hydrogenation catalyst
In some molecular sieves modified by rare earth doping, it contains 4% ~ 5% of rare earth oxides, which is equivalent to the content of rare earth in rare earth ores. Therefore, it is of great value to recycle the rare earth in waste molecular sieves, and it is imperative to turn waste into treasure and make comprehensive utilization.
The leaching behavior of Al and re follows the shrinking core model, and the surface chemical reaction affects the whole leaching process. Wang et al. Proposed a new hydrometallurgical process for recovering rare earth and aluminum from catalytic cracking residue. The effects of leaching temperature, hydrochloric acid concentration and leaching time on metal extraction were studied. It was found that the leaching rates of La, CE and Al could reach 91.0%, 92.2% and 94.2% respectively.
Guo et al. Studied the particle size of 20 μ m. The recovery rate of rare earth in the heavily polluted waste catalyst that is not suitable for regeneration. The results show that the total leaching rate of rare earth is 91.3% ~ 94.5% with kerosene as extractant under the conditions of leaching temperature 60 ℃, hydrochloric acid concentration 3.33mol/l and leaching time 3H. With 50% p-507 (2-ethylhexyl phosphonic acid Mono-2-ethylhexyl ester) + 50% kerosene as extractant, the extraction rate can reach 94.5% at the extraction temperature of 25 ℃, the ratio of 2:1, and the extraction time of 30min. Under the ratio of 1:3, the recovery rate of lanthanum and cerium is 98.1%. Industrial products of lanthanum and cerium can be prepared by precipitation roasting.
Wen et al. Studied the two-step recovery of lanthanum and cerium from spent FCC catalyst by nitric acid leaching and di (2-ethylhexyl) phosphoric acid extraction. The results show that the dissolution rates of lanthanum and cerium are about 93% and 42% respectively, and the dissolution rate of Al is only 11% when the solution is leached with 2mol / L (126 g / L) HNO3 at 80 ℃. In the subsequent solvent extraction step, it was found that the extraction effect of n-decane was ideal, and no pH adjustment was required. In this case, the yields of La (III) and Ce (III) can reach 60% and 74% respectively in the same stage.
Since the industrialization of heterogeneous catalysis technology, it has become an important means of catalytic hydrogenation. Noble metals have good performance when applied to heterogeneous catalysts, but their reserves and prices limit their large-scale industrial application in heterogeneous catalysis. Doping different rare earth ions into molecular sieves by different methods can effectively improve the activity, structural stability and anti poisoning ability of molecular sieves, and is conducive to promoting the high-value application of rare earth resources. The current new concept of environmental governance puts forward many requirements for traditional heterogeneous catalysts. The supported rare earth containing catalysts with nano mesoporous and skeleton structures are expected to solve the problems of low selectivity, high cost, low stability, high pollution and difficult recovery of catalysts in traditional multiphase hydrogenation catalysis, which is conducive to the realization of green chemical production processes. In order to realize industrialization, the field of waste rare earth catalyst recovery should also focus on the study of new leaching mechanism, improvement of leaching and separation conditions, and optimization of separation methods.
Fund Project: National Natural Science Foundation of China (217630112162012); Jiangxi outstanding young talents training plan (20192bcb23015); Young Jinggang scholar of Jiangxi Province ( 57); Ganzhou Innovative Talents Project in 2020; China Postdoctoral Science Foundation (2018m642597); 2019 Jiangxi Post Doctoral Fund Project; Qingjiang top talent plan project of Jiangxi University of Technology (jxustqjyx2017006)
Industrial catalysis, network launch time: March 11, 2022
Authors: Zeng Li, Zhang Huan, Zhu Lihua