Ind. Eng. Chem. Res. 2009, 48, 10359–10363
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Study on Several Mesoporous Materials Catalysts Applied to the Removal of Trace Olefins from Aromatics and Commercial Sidestream Tests Chang-wei Chen, Wen-juan Wu, Xian-song Zeng, Zhen-hong Jiang, and Li Shi* The State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China
Deep removal of trace olefins from aromatics using mesoporous materials and modified mesoporous materials was studied with regard to the removal of trace olefins from aromatic hydrocarbons. The mesoporous materials and the modified mesoporous molecular sieve materials were characterized by X-ray diffraction and Fourier transform infrared (FT-IR) spectroscopy. Under the commercial sidestream test, the effects of four types of mesoporous materials, with regard to the removal of olefins from the aromatics, were evaluated. The result showed that the mesoporous materials that had surface areas larger than 300 m2/g and modified with AlCl3 were best, with regard to the effect on the removal of olefins from aromatics. The FT-IR results showed that the modifier could increase the concentration of weak Lewis acid of mesoporous materials and obviously enhance the effect of removing olefins and prolonging the reaction time. 1. Introduction In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking. These aromatic streams also contain undesirable olefin impurities that are very harmful to the followed technological processes and applications of aromatics. Therefore, the impurities must be removed with suitable treatment technologies.1 Two processes have been discovered for the removal of trace olefins from aromatics (i.e., particulate clay treatment and catalytic hydrogenation treatment). However, these two processes have some drawbacks; for example, the clays have very limited lifetimes. A 450 000 t/a PX device (Sinopec Zhenhai Refining and Chemical Company) was employed, using the Eluxyl technology of the IFP Company, France. Simulated moving bed technology was used to separate and produce PX with a highly selective patent adsorbent (SPX3000). The bromine index (BI) was ∼600-1100. To protect the adsorbent, clay was used to remove trace olefins from the aromatic hydrocarbons before the aromatic hydrocarbons enter into the adsorption tower. However, the clay has a very limited lifetime. With increasing environmental awareness and demands on the process of healthy economic development, some type of catalyst that protects the environment and improves economic efficiency must be applied in this field.2 Research at East China University of Science and Technology has developed a series of mesoporous material catalysts, and we have conducted commercial sidestream tests beside the industry clay tower of the Zhenhai Refining and Chemical Company. The test provides a basis for future industrialization. 2. Experimental Section 2.1. Materials. The aromatic hydrocarbons were obtained from the bottom of the naphtha reforming column at the Sinopec Zhenhai Refining and Chemical Company. The BI was ∼1400. In the commercial sidestream tests, the aromatic hydrocarbons were obtained from the bottom of the naphtha reforming column at the Shanghai Petrochemical Co., Ltd. (the BI is ∼700-1100, and the components are shown in Table 1). * To whom correspondence should be addressed. Tel.: 02164252274. E-mail:
[email protected].
Catalyst A had mesoporous material with surface areas of >300 m2/g; catalyst C had mesoporous material with surface areas of >200 m2/g; catalyst B was catalyst A that had been modified by AlCl3; and catalyst D was catalyst C that had been modified by AlCl3. 2.2. Catalysts Preparation. The mesoporous materials and different zeolites were mixed with adhesive γ-Al2O3, blended with the proper amount of a 10% HNO3 solution, which were added to modify/alter the AlCl3 content, and the materials were squeezed into the form of strips to make a series of modified catalysts. These were calcined in a oven, in an air atmosphere at 723 K for 3 h, crushed, and screened to 20-40 mesh for use. 2.3. Catalytic Tests in Laboratory. The catalytic activity tests were performed in a fixed-bed tubular microreactor that was equipped with flow controllers and a heating system. Two milliliters of the synthesized catalyst was placed between two quartz sands (40-60 mesh) and inserted into the reactor. The reaction was carried out under the following conditions: reaction temperature, 448 K; reaction pressure, 1 MPa; and weight hourly space velocity (WHSV), 30 h-1. Inlet and effluent liquids to and from the reactor were analyzed using a BI analyzer. In the commercial sidestream test, the reaction was heated by steam and the temperature was ∼433-441 K. The reaction pressure was 2.0-2.1 MPa, and the volume space velocity was 2 h-1. 2.4. Commercial Sidestream Tests: Process Flow and Catalyst Loading. Figure 1 shows the flow scheme of the commercial sidestream test process. The commercial sidestream tests were conducted beside the industrial clay tower. Aromatics enter into the reactor from the top, and flow out from the bottom of the reactor after being treated in the reactor. The operating temperature of the reactor was controlled by regulating the steam Table 1. Aromatic Hydrocarbon Components component
content (wt %)
nonaromatics toluene ethylbenzene p-xylene m-xylene o-xylene C9 aromatics C9+ aromatics
200 m2/g, and catalyst D was catalyst C that had been modified with AlCl3. These samples were also dried at 393 K for 6 h and finally calcined at 823 K for 3 h. The results are shown in Figure 5. Figure 5 shows the catalytic activities of two samples were the same initial olefin conversions. However, after 2 h, the activity of catalyst C decayed quickly. The olefin conversions of sample C was less than that of sample D. As can be seen in this figure, the catalytic activity of sample C has been improved by modification with AlCl3. 3.4. XRD Analyses of Four Samples. Figure 6 shows XRD patterns of four catalysts. The XRD analyses indicate that there is no peak of impurities or other obvious changes for the catalyst A samples, indicating that the framework structure of catalyst A was barely damaged and well-maintained after AlCl3 modification. There is no characteristic peak of AlCl3, because it is present in a small quantity and highly dispersed.3 AlCl3 was not detected in the XRD spectra of the catalyst D sample. The results also showed that the catalyst D sample presented peaks
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Figure 7. FT-IR spectra of four catalysts (A, B, C, and D) are compared at 473 and 723 K.
that were similar to those of the catalyst C sample, which demonstrated that the crystal structure of catalyst D was same as that of catalyst C. The mesoporous materials have large surface areas and a very small amount of AlCl3, so that AlCl3 can be highly dispersed on the surface of catalyst C mesoporous materials. 3.5. Pyridine FT-IR Spectra of Four Samples. To investigate the number of the surface acidic sites, FT-IR spectra for the adsorption of pyridine at 473 and 723 K were obtained. The spectrum displayed many bands in the wavenumbers in the range of 1400-1600 cm-1, which attributed to the interaction of pyridine with Lewis (L) and Brønsted (B) acid sites on the sample surfaces. As shown in Figure 7, the spectra present bands of adsorption at 1450 and 1490 cm-1, which is typical of adsorbed pyridine.4 There is no band that is observed at 1540 cm-1, as can be seen in Figure 7, so the amount of total B acid could not be calculated. We mainly observe the band at ∼1450 cm-1, which is due to the 19b ν(C-C) vibration of pyridine adsorbed at Lewis acid sites.5 It was shown that the amount of total L acid sites, strong L acid sites, and weak L acid sites all increased. However, compared to the strong L acid sites, the number of weak L acid sites increases fast. As we can see from four samples, the changes in the total L acid sites were influenced mainly by the weak L acid sites. Therefore, increasing the amount of the weak L acid sites contributed to the increase in the activity of the
Figure 8. Plot showing the conversion of aromatics of four samples (and Lin-an clay) at a commercial sidestream test.
catalyst, thus improving the capacity of removing trace olefins from aromatics. 3.6. Commercial Sidestream Test Analyses. Figure 8 shows conversion of aromatics of four catalyst samples and Lin-an clay. The conversions of olefins with Lin-an clay remained above 50% for four days. The reaction times of the four samples
Ind. Eng. Chem. Res., Vol. 48, No. 23, 2009 a
Table 3. Acidic Properties of Four Samples
Acidity (×10-4mol/g) sample
total L acid sites
strong L acid sites
weak L acid sites
A B C D
6.21 21.90 2.44 5.64
5.14 14.34 0.86 1.32
1.07 7.56 1.58 4.32
a
The term “L acid” denotes a Lewis acid.
are all longer than that of Lin-an clay. The evaluation showed that catalyst B has the best reaction time for 18 days and catalyst C had the worst reaction time for 11 days. Compared to Lin-an clay, the reaction time of catalyst B increases by a factor of 4.5. Thus, catalyst B is the optimal catalyst: using catalyst B can reduce heavy environmental pollution, complex operations, and cost, and it protects the environment and improves economic efficiency. 4. Conclusions (1) With regard to the physical and chemical properties of zeolites, the pore size and surface acid strength are the most important parameters that have an effect on the capacity of removing trace olefins from aromatics. (2) Through modification, the activity and capacity of removing trace olefins from the aromatics of mesoporous materials have greatly improved. (3) The commercial sidestream test showed that the lifetime of the four catalyst samples (A, B, C, and D) have greatly
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improved, compared to that of Lin-an clay. The result showed that catalyst B mesoporous material was the best, with regard to the effective removal of olefins in aromatics, and catalyst C was the worst. (4) The FT-IR analyses of the four catalyst samples showed that the amount of total L acid sites, strong L acid sites, and weak L acid sites all increased, especially that of weak L acid sites. Thus, the activity of the catalysts and the capacity of removing trace olefins from aromatics have improved. Literature Cited (1) Sachtler, J. W. A.; Barger, P. T. Removal of trace olefins from aromatic hydrocarbons. U.S. Patent 4,795,550, 1989. (2) Iyer, P. S.; Scherzer, J. ACS Symp. Ser. 1988, 368, 48. (3) Kasztelan, S.; Moffat, J. B. The oxidation of methane on heteropolyoxometalates. III. Effect of the addition of cesium on silica-supported 12molybdophosphoric acid, molybdena, vanadia, and iron oxide. J. Catal. 1988, 112 (1), 54,zlpg > 65. (4) Hoang, V.-T.; Qinglin, H. Adrian Ungureanu Effect of the acid properties on the diffusion of C7 hydrocarbons in UL-ZSM-5 materials. Microporous Mesoporous Mater. 2006, 92, 117–128. (5) Pranjal, K.; Narendra, M.; Gupta, R. K. Synergistic role of acid sites in the Ce-enhanced activity of meso-porous Ce-Al-MCM-41 catalysts in alkylation reactions: FTIR and TPD-ammonia studies. J. Catal. 2007, 245, 338–347.
ReceiVed for reView July 6, 2009 ReVised manuscript receiVed September 1, 2009 Accepted September 2, 2009 IE901062C
