Energy Fuels 2010, 24, 3764–3771 Published on Web 02/16/2010
: DOI:10.1021/ef901368w
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Novel Micro- and Mesoporous Composite Molecular Sieve Assembled by Zeolite L Nanocrystal and Its Performance for the Hydrodesulfurization (HDS) of Fluid Catalytic Cracking (FCC) Gasoline† Quan Huo,‡,§ Yanjun Gong,‡,§ Tao Dou,*,‡,§ Zhen Zhao,*,‡,§ Huifang Pan,§ and Feng Deng
State Key Laboratory of Heavy Oil Processing, and §The Key Laboratory of Catalysis, China National Petroleum Corporation, China University of Petroleum, Beijing 102249, People’s Republic of China, and Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, People’s Republic of China )
‡
Received November 14, 2009. Revised Manuscript Received February 3, 2010
A novel micro- and mesoporous composite molecular sieve (denoted as LMC) was synthesized by using the nanocrystal clusters of zeolite L as the precursor and a cation surfactant cetyltrimethylammonium bromide (CTAB) as the template. The physicochemical properties of samples were characterized by means of X-ray diffraction (XRD), nitrogen-adsorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), Fourier transform infrared spectroscopy (FTIR), and Fourier transform infrared spectroscopy of pyridine adsorption (Py-FTIR). The results showed that composite molecular sieve LMC was synthesized by the self-assembly of zeolite L nanocrystal clusters under the template effect of CTAB. In addition, surface area, pore volume, and pore size of LMC significantly increased in comparison to those of conventional microporous zeolite L. The results measured by Py-FTIR showed that LMC had an appropriate acid amount and acid distribution. For evaluation of fluid catalytic cracking (FCC) gasoline hydrodesulfurization (HDS), the catalyst that introduced material LMC exhibited the excellent performances of desulfurization, isomerization, aromatization, olefin retention, and preserving the research octane number (RON) value compared to the catalyst that introduced ordinary microporous zeolite L or mesoporous Al-MCM-41 and used bare alumina as the support. The excellent catalytic performances of the catalyst should be attributed to the appropriate acidity distribution and open pore structure of material LMC. literature4,5 show that the addition of the acidic component of zeolites in catalysts can promote the desulfurization activity of the HDS catalyst. Similar to a member of microporous zeolites, zeolite L is also considered as the additives of the support for such a catalyst system because of its selective hydrogenation performance6 and excellent activities for HDS,7 isomerization, and aromatization.8 However, the major drawback of zeolite L, resembling some other microporous zeolites, is the limited size of the pore channels (LMC2>mechanical mixture > MCM-41. All of the above results indicted that LMC2 possessed an appropriate acid amount and acid distribution. The surface acid amounts and the types of studied catalysts are listed in Table 6. The results showed that the total acid amounts of the catalysts followed the order: catalyst 1> catalyst 2 > catalyst 4 > catalyst 0 > catalyst 3. In addition, catalyst 2 (introduced LMC) possessed the highest L/B value in all studied catalysts. The L/B values of the studied catalysts were in the range of about 2-4. In comparison to catalyst 3 (introduced MCM-41), the medium and strong Brønsted acid sites still existed in catalyst 2. It demonstrated that the introduction of LMC made an improvement in the acid amount and acid strength of the catalyst. Moreover, in comparison to catalyst 4 (introduced the mechanical mixture of microporous zeolite L and MCM-41), catalyst 2 possessed a relatively low amount of the medium and strong Brønsted acid sites, which is favorable for avoiding the excessive cracking of hydrocarbons. 3.8. Catalytic Activity Results. In the present work, the HDS catalysts of catalyst 1, catalyst 2, catalyst 3, and catalyst 4 were prepared using zeolite L, LMC, MCM-41, and the mechanical mixture of zeolite L and MCM-41 as additives, respectively, to partially substitute the γ-alumina support. For comparison, the catalyst using bare γ-alumina as the support (catalyst 0) was also included. The catalytic performances including HDS efficiency and the ability to preserve the RON of the studied catalysts were investigated. As shown in Figure 12, the sulfur content in hydrogenated gasoline over catalyst 2 containing LMC was much lower than those over catalyst 1, catalyst 3, and catalyst 4. In other words, the HDS efficiency of catalyst 2 was much higher than those of catalyst 1, catalyst 3, and catalyst 4. Catalyst 2 and catalyst 0 (used bare γ-Al2O3 as the support) had a very close level of HDS efficiency, while the olefin and aromatic contents over catalyst 2 were much greater than those over catalyst 0 (see Table 7). In comparison to the feedstock, the hydrogenated gasoline over catalyst 2 had the higher isoparaffin and aromatic contents. In addition, both the content of isoparaffins and RON of hydrogenated gasoline over catalyst 2 were the highest in all studied catalysts, demonstrating that material LMC possesses excellent performances for selective hydrogenation and isomerization. The characters for the performances of the catalysts containing LMC are favorable for maintaining the relatively high contents of olefins, isoparaffins, and aromatics, which are important for keeping a high RON value. Furthermore, in comparison to catalyst 4, catalyst 2 had much better catalytic performances. It may be due to the presence of much more open pore structures and composite phases of micro- and mesopores. When the physicochemical properties and the catalytic performances of the studied samples were correlated, the following four reasons could be found for the catalyst 2 to give the best catalytic performances including HDS efficiency and the preservation of the RON value among the studied catalysts. First, the large pore size of LMC improves
Figure 11. Py-FTIR spectra of the protonated samples degassed at 200 and 350 °C: (a) L, (b) LMC2, (c) MCM-41, and (d) mechanical mixture of zeolite L and MCM-41. Table 5. Surface Acid Amount and the Types of Studied Additives acid amounta (200 °C) (μmol g-1)
acid amountb (350 °C) (μmol g-1)
additive
B
L
BþL
L/B
B
L
BþL
L/B
L LMC2 MCM-41 mixture
683.2 299.6 186.1 309.3
164.9 140.3 68.2 76.5
848.1 439.9 254.3 385.8
0.24 0.47 0.37 0.25
173.2 64.2 8.7 73.2
70.3 52.2 20.4 41.9
243.5 116.4 29.1 115.1
0.41 0.81 2.34 0.57
a Total acid amounts determined at the degassed temperature of 200 °C by py-FTIR. b Medium þ strong acid amounts degassed at the desorption temperature of 350 °C by py-FTIR.
3.7. Acidity Measurement. The surface acid amounts and types are in relation to the physicochemical properties of samples, and they can be determined by Py-FTIR. Py-FTIR spectra of the protonated samples were measured in the region of 1700-1400 cm-1. It can be seen from Figure 11 that the IR characteristic band of pyridine adsorbed on the Brønsted acid site was at ∼1540 cm-1 and pyridine bound to the Lewis acid site was at ∼1450 cm-1. However, an IR band at 1490 cm-1, which can be assigned to pyridine co-adsorbed on both Brønsted and Lewis acid sites, appeared.36,37 The results showed that both Brønsted and Lewis acid sites coexisted on all of the samples. When the desorption temperature rose to 350 °C, IR bands at 1450 and 1490 cm-1 were still detected in all of the samples but the peak at 1540 cm-1 representing the Brønsted acid site nearly disappeared in the MCM-41 curve. The analysis results are also listed in Table 5. In this case, the Brønsted acid amount for mesoporous MCM-41 was only 8.7 μmol g-1. In contrast, in comparison to MCM-41, LMC2 still retained the larger amount of Brønsted acid sites, 64.2 μmol g-1. Different from the mechanical mixture, LMC2 exhibited the higher Lewis acid (36) Hughes, T. R.; White, H. M. J. Phys. Chem. 1967, 71, 2192–2201. (37) Zhang, X.; Wang, J.; Zhong, J.; Liu, A.; Gao, J. Microporous Mesoporous Mater. 2008, 108, 13–21.
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Energy Fuels 2010, 24, 3764–3771
: DOI:10.1021/ef901368w
Huo et al.
Table 6. Surface Acid Amount, Acid Types, and Specific Surface Area of the Catalysts That Introduced Different Additives acid amount (200 °C) (μmol g-1)
acid amount (350 °C) (μmol g-1)
catalyst
B
L
BþL
L/B
B
L
BþL
L/B
catalyst 0 catalyst 1 catalyst 2 catalyst 3 catalyst 4
0 121.3 73.2 51.3 90.2
243.1 257.2 248.2 136.6 167.2
243.1 378.5 321.4 187.9 257.4
B/L = 0 2.1 3.4 2.7 1.9
0 39.3 16.2 0 23.4
92.5 84.9 59.6 42.9 57.3
92.5 124.2 75.8 42.9 80.7
B/L = 0 2.2 3.7 B/L = 0 2.4
weak Brønsted acid sites is helpful for the isomerization reaction. Fourth, the appropriate ratio of the L/B value (in the present work, the appropriate L/B value is in the range of 2-4) favors the aromatization reaction.41 Especially, the aromatization reaction of hydrocarbons can provide obvious contributions to enhance the RON value of hydrogenated gasoline. 4. Conclusions (1) A kind of micro- and mesoporous composite molecular sieve was synthesized by the self-assembly of zeolite L nanocrystal clusters in the presence of template CTAB. The results demonstrated that these materials possessed the composite pore structures with micro- and mesopores, appropriate acid properties, and excellent hydrothermal stability. (2) In comparison to the catalysts containing conventional microporous zeolite L or mesoporous MCM-41 or used bare alumina as the support, the catalyst introducing the micro- and mesoporous composites showed excellent multifunctional performances in the HDS reaction of full-range FCC gasoline. The multifunctional performances included desulfurization, isomerization, aromatization, and preserving olefin and the RON value. The excellent catalytic performances of the catalysts should be attributed to appropriate acidity distribution and the open pore structure of the composite material. As a result, the super characters and the multifunctional performances of selective hydrogenation, isomerization, and aromatization over the catalysts that introduced such micro- and mesoporous materials should play an important role in preserving the RON value of hydrogenated gasoline.
Figure 12. Comparison charts of the sulfur content and RON of gasoline obtained over different HDS catalysts. The evaluation conditions of the hydrogenation reaction are 270 °C, 2.0 MPa, 2.0 h-1 (WHSV), and 300 (H2/oil), respectively. (/) W(S) is the sulfur content of hydrogenated gasoline. Table 7. Group Compositions of Gasoline Obtained by Catalysts That Introduced Different Additives group compositions of gasoline (wt %) sample
n-paraffin
i-paraffin
olefin
naphthene
aromatics
feed catalyst 0 catalyst 1 catalyst 2 catalyst 3 catalyst 4
4.61 18.27 11.98 8.43 12.39 11.79
32.39 42.10 43.79 45.28 42.28 43.21
33.90 5.15 8.02 12.15 10.02 9.54
8.16 12.19 12.70 11.23 12.06 12.43
20.94 22.29 23.51 22.91 23.25 23.03
the diffusion ability of large molecules of reactants and products. Second, the relatively large amount of acid sites enhances the cracking of the C-S bond, thus improving the HDS efficiency.38-40 Third, a relatively large amount of
Acknowledgment. This work was supported by the National Natural Science Foundation of China (20833011, 20773163, and 20876174) and the Ministry of Science and Technology of PetroChina Company Limited (2008A-3801).
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