Full Length ArticleHydroformylation for reducing the olefin content in the FCC light gasoline with magnetic rhodium-catalysts
Graphical abstract
Introduction
With the increase of worldwide automobile consumption year by year, the automobile exhaust emissions have brought serious pollutions. According to the VI standard of vehicle commercial gasoline, the olefin content should not be higher than 15%–18% [1]. When olefin is burned in the engine, it is easy to form carbon deposit which will affect the combustion efficiency of the engine and increase the pollutants emission [2], [3], [4]. The fact is that more than three-quarters of commercial gasoline for automobiles in China is FCC gasoline, in which the olefin content is usually over 40%. At present, etherification, alkylation and hydrogenation are the major methods to reduce olefin content in gasoline [3], [5], [6], but a decreased octane number of gasoline usually accompanies with the conversion of olefins. Therefore, how to reduce the olefin content in FCC gasoline without losing its octane number has become the main challenge for refineries.
Liu et al. [7] used 5A zeolites to adsorb and separate the normal paraffins and normal olefins in FCC gasoline, with an increase in research octane number (RON). Another alternative method to utilize olefins is hydroformylation, in which an olefin will react with syngas under the catalysis of a Rh-P or Co-P complex to form an aldehyde or alcohol [8], [9], [10], [11], [12], [13], [14]. Because the oxygenates usually have a large octane number [15], [16], it might be a potential choice to use the hydroformylation to upgrade the gasoline. Since discovered by Otto Roelen in 1938, hydroformylation has become the most widely used homogeneous catalytic reaction in industry [17]. The development of the catalysts has experienced carbonyl cobalt catalysts and carbonyl rhodium catalysts. At present, the homogeneous catalytic system consisting of carbonyl rhodium catalyst and triphenylphosphine analogues is mainly used for light olefins hydroformylation [18], [19], [20], [21]. The higher price of rhodium together with the higher energy consumption, however, makes the separation between catalyst and product in the traditional homogeneous hydroformylation the major problem affecting its industrial application. Thus, supported catalysts [10], [22], [23], [24], [25], [26], [27] as well as water/oil two phase catalytic system [28], [29], [30], [31] have been extensively studied. The supports include SiO2, molecular sieves, organic polymers, carbon materials and magnetic iron oxides [32], [33], [34], [35], [36], [37]. Among them, the magnetic nanoparticles (MNP) display such an advantage that the catalyst can be simply and effectively separated by an external magnetic field, with the high activity of the catalyst still maintained [38], [39], [40], [41], [42]. Duanmu et al. [22] synthesized a new ligand from 4-(diphenylphosphino)benzoic acid and dopamine, which was anchored to the surface of MNP. The magnetic ligand was then used together with Rh(OAC)2 as a catalyst precursor for the hydroformylation of styrene. Although with an excellent catalytic result, the side reactions after the second run was greatly increased because the content of the ligand was too low and the rhodium complex was not stable enough. In order to make the surface of the MNP easier to modify, the support is usually coated with a layer of SiO2 to avoid unnecessary contact between the active center and the magnetic center, and the silanol groups present on the surface make it easier to functionalize. Abu-reziq et al. [1] grafted a polyamide-type dendrimer (PAMAM) on Fe3O4@SiO2, which was more stable with a higher solubility in organic solvents. The phosphorous atoms on the dendrimer could complex with [Rh(COD)Cl]2 and showed excellent hydroformylation catalysis performance. Garcia et al. [32] also synthesized a magnetic dendritic support Fe3O4@SiO2-N(CH2PPh2)2, which was applied in the 1-octene hydroformylation. The catalyst showed a performance of 96% conversion, 82% chemical selectivity, and a linear-to-branched isomer ratio of 2.28. Different from the grafting technology, Ma et al. [43] prepared Rh/Fe3O4 and Co-Rh/Fe3O4 by coprecipitation. The doping of the bimetal could generate more active sites, like Co-Rh-COx-(PPh3)y. The two catalysts were used together with triphenylphosphine to catalyze the hydroformylation of dicyclopentadiene, and the latter showed a better catalytic effect, because the bimetallic loading could easily produce more active sites.
Although hydroformylation is a famous method to utilize olefins, as far as we know, few studies were focused on the hydroformylation of FCC light gasoline, not to mention the use of magnetic Rh-catalysts. In order to investigate the performance of Rh-catalysts in the hydroformylation of gasoline, homogeneous Rh(acac)(CO)2 (acac = acetylacetonate) catalyst was applied and proved with considerable activity. Four magnetic Rh-based catalysts were further synthesized by surface modification of Fe3O4 nanoparticles, and then employed in the hydroformylation of FCC gasoline. The results in the catalysis capability and the recycling performance show that hydroformylation is a prospective way for gasoline upgrading.
Section snippets
Materials and equipment
The FCC light gasoline was obtained from Lanzhou Petrochemical Company, which was the light components separated from FCC gasoline after selective hydrogenation. The syngas (VH2:VCO = 1:1) was purchased from Beijing Beiwen Qiti Company. RhCl3·3H2O and Rh(acac)(CO)2 (>99%) were purchased from Beijing Persisted Technology Company Limited. Ferric chloride hexahydrate (A.R., A.R. = Analytical Reagent), ferrous chloride tetrahydrate (A.R.), aqueous ammonia (A.R.), methanol (A.R.), alcohol (A.R.),
Catalyst characterization
The crystal structures of the prepared catalysts were analyzed by X-ray diffractometer (XRD), as shown in Fig. S1 in the Supplementary Material. All the four catalysts exhibit characteristic diffraction peaks of Fe3O4, located at 30.1°, 35.5°, 43.0°, 53.4°, 57.1°, and 62.7°, respectively, which corresponds to the crystal planes of the cubic crystal phases (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) of Fe3O4, respectively. No diffraction peaks of Rh were observed, indicating that Rh was well
Conclusion
In this paper, the upgrading of FCC light gasoline was conducted through hydroformylation under the catalysis of homogeneous Rh-catalyst as well as magnetic Rh-catalysts. The experimental results indicated that though the olefins in the gasoline were mainly internal and branched, the homogeneous Rh(acac)(CO)2 was effective in converting olefins to aldehydes, leading to a reduced olefin content and a higher RON. The prepared four magnetic catalysts also displayed a good catalysis performance,
CRediT authorship contribution statement
Weili Jiang: Methodology, Project administration, Supervision. Yaqi Chen: Investigation, Formal analysis. Lijie Gao: Investigation, Data curation. Manying Sun: Writing - review & editing. Xiaosheng Wang: Writing - original draft. Zhengxi Li: Formal analysis. Xing Ji: Data curation. Guanglin Zhou: Methodology. Hongjun Zhou: Methodology, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Scientific Research and Technological Development Project of CNPC (No. LH-17-08-53-02) and Key Project of Special Funds for Cooperation between Municipality, college and Institute in Dongying (No. 2018-02-05-02) for their supports.
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