Physicochemical Properties and Catalytic Performance of a Novel Aluminosilicate Composite Zeolite for Hydrocarbon Cracking Reaction

Transcription

1 Catalyst Research Physicochemical Properties and Catalytic Performance of a Novel Aluminosilicate Composite Zeolite for Hydrocarbon Cracking Reaction Qi Jian 1 ; Zhao Tianbo 1 ; Xu Xin 1 ; Li Fengyan 2 ; Sun Guida 2 (1. Institute for Chemical Physics, Beijing Institute of Technology, Beijing ; 2. Department of Applied Chemistry, Beijing Institute of Petrochemical Technology) Abstract: A novel micro-micro/mesoporous aluminosilicate ZSM-5-Y/MCM-41 composite zeolite with a MCM-41 type structure was synthesized through a novel process of the self-assembly of CTAB surfactant micellae with silicaalumina source originated from alkaline treatment of ZSM-5 zeolite. The physical properties of the ZSM-5-Y/MCM- 41 composite zeolite were characterized by XRD, Py-FTIR and N 2 adsorption-desorption techniques. Different kinds of molecular sieves including ZSM-5, Y zeolite, Al-MCM-41, ZSM-5/MCM-41 and ZSM-5-Y/MCM-41 as cracking catalysts were investigated, using 1,3,5-triisopropylbenzene (1,3,5-TIPB) as the probe molecule. Catalytic tests showed that the ZSM-5-Y/MCM-41 composite zeolite exhibited higher catalytic activity compared with the microporous ZSM- 5 zeolite, Y zeolite, mesoporous Al-MCM-41 molecular sieve and ZSM-5/MCM-41 composite zeolite under the same conditions. The remarkable catalytic activity was mainly attributed to the presence of the hierarchical pore structure and proper acidity in the ZSM-5-Y/MCM-41 composite catalyst. Meanwhile, a carbenium ion mechanism was put forward for the cracking of 1,3,5-TIPB. Key words: physicochemical properties; aluminosilicate composite zeolite; catalytic cracking; large molecule 1 Introduction Nowadays, along with the increasing supply of heavy crude resources all over the world, catalytic cracking technology has already become an important method for producing light distillates from crude and pyrolysis of residuum in petroleum processing [1]. But how to improve the conversion of residuum during catalytic cracking and the selectivity of small olefins is currently the key subject in the study of catalytic cracking technology. As we all know, micro-microporous composite zeolite have excellent performance in shape-selective catalysis, rationally distributed acidity and good hydrothermal stability, and these zeolites have good potential for transformation of heavy feedstock into light oils [2]. Shen et al [3] synthesized micro-microporous ZSM-5/Y composite zeolite by a novel one-pot procedure coupled with a two-step crystallization method. The heavy oil MAT results showed that, compared to a catalyst consisting of a mechanical mixture of the ZSM-5 and Y zeolite, the composite zeolite catalyst afforded an increased gasoline yield along with a decrease in light cycle oil and coke yields. However, some large molecules in heavy oil cannot diffuse through the inner pores of the ZSM-5 and Y zeolite due to their small pores. Therefore catalytic reactions could only occur on the external surface of microporous zeolite and result in low conversion of heavy feed. Subsequently, micro-mesoporous composite zeolites such as mesoporous ZSM-5 zeolite [4], mesoporous Y zeolite [5] with dual channel structure and dual acidity have been synthesized and are considered to have potential impact on heterogeneous catalysis. But so far, micro-micro/mesoporous composite zeolite, which has the combined characteristics of both micromicroporous zeolites and micro-mesoporous zeolites, has not been reported yet. Herein we report the synthesis of a novel micro-micro/mesoporous ZSM-5-Y/MCM-41 composite zeolite through a novel self-assembly process of CTAB surfactant micellae with silica-alumina source originated from the alkaline treatment of ZSM-5 zeolite. 2 Experimental 2.1 Catalyst preparation Firstly, ZSM-5 (SiO 2/Al 2O 3 ratio=50) zeolite was treated with a sodium hydroxide solution. Typically 12.0 g of ZSM-5 powder was treated in 60 ml of 2.0 mol/l Corresponding author: Prof. Zhao Tianbo, Telephone: ; Fax: ; zhaotb@bit.edu.cn 17

2 NaOH solution for 30 min. After 120 ml of 10% cetyltrimethylammonium bromide (CTAB) aqueous solution was added to the above solution, the resultant mixture was stirred at 60 for 30 min to mix homogeneously, and then was transferred to a Teflonlined stainless steel autoclave to carry out crystallization at 110 for 24 h. After 24 h of crystallization, the crystallized solution was divided into four parts, and then the ph value was adjusted to 8.5, 9.5, 10.5 and 11.5, respectively, by a 2 mol/l HCl solution. Meanwhile, 3.5 g of Y zeolite was added to every solution and stirred for another 30 min, and was then transferred to the Teflon-lined stainless steel autoclave for crystallization at 110 for another 24 h. The ZSM-5/MCM-41 composite zeolite was synthesized according to the above-mentioned method, and the difference was that the Y zeolite was not added to the synthetic solution. The Y zeolite was synthesized by the literature method [6]. The mesoporous Al-MCM-41 molecular sieve was prepared from NaAlO 2, NaOH, H 2O, CTAB and tetraethylorthosilicate (TEOS). The final molar composition of the synthesized solution was expressed as follows: 1Al 2 O 3 : 2.7Na 2 O: 2010H 2 O: 3.6CTAB: 50SiO 2. The crystallization process was carried out at 110 for 48 h. Finally, all samples were calcined in air at 550 for 6 h after being filtered, washed and dried. Subsequently, the samples were ion-exchanged with an 1.0 mol/l NH 4 Cl solution at 90 for 1 h, followed by calcination at 550 for 5 h. 2.2 Physico-chemical characterization composite catalysts were used to evaluate the catalytic performance and were compared with the ZSM-5-Y/MCM- 41 catalyst. The analyses of the catalytic products were carried out using the GC-5890II chromatograph (Hewlett- Packard Co.) equipped with a FID detector. Catalytic cracking of 1,3,5-TIPB was carried out by using the pulsation method. The catalytic testing was performed according to the following standard conditions: the catalyst amount was 60 mg, and the reaction temperature was μl of 1,3,5-TIPB was injected into the catalyst bed with nitrogen carrier gas at a flow rate of 50 ml/min. 3 Results and Discussion 3.1 X-ray diffraction (XRD) The powder XRD patterns of the ZSM-5-Y/MCM-41 samples synthesized with different ph values are displayed in Figure 1 showing three types of peaks which are obtained from the microporous ZSM-5 zeolite [7] [(101), (020), (501), (151) and (303)], Y zeolite [8] [(111), (220), (311), (331), (511), (440), (642), (733), (660), (555), (840), (842), (931)] and from the hexagonally symmetrical mesoporous MCM-41 molecular sieve [9] [(100), (110) and (200)]. It is also observed that the peak intensity of hexagonal MCM-41 increased with an increasing ph value, and the (100), (110) and (200) reflections were shifted to low diffraction angles. Meanwhile, the d 100 spacing increased gradually, and the diffraction peaks corresponding to d 100 spacing were 37.9, 40.4, 40.7 and 41.1 Å, respectively (with a 0=43.7, 46.6, 47.0 and 47.4 Å, and the unit cell parameter a 0=2d 100/ 3 ) [10]. X-ray diffraction (XRD) patterns were recorded on a XRD-7000 diffractometer (made by Shimadzu, Japan) using Cu-Kα radiation (λ=0.154 nm) to detect the crystal structure of the samples. A nitrogen adsorption-desorption isotherm for the sample was measured at 77 K on an automatic adsorption-measurement system (Autosorb-1-MP, USA). An FT-IR spectroscopy (Nicolet- 750, USA) was used in the pyridine adsorption method. 2.3 Catalytic tests The microporous ZSM-5 zeolite, Y zeolite, mesoporous Al-MCM-41 molecular sieve and ZSM-5/MCM-41 2θ,( ) Figure 1 XRD characterization of ZSM-5-Y/MCM-41 samples formed at different ph values: 8.5 (a), 9.5 (b), 10.5 (c) and 11.5 (d) 18

3 We consider that more and more parent ZSM-5 zeolites were dissolved to form aluminosilicate nanoclusters with an increasing ph value, and the nanoclusters were combined with CTA + cations to transform more mesoporous MCM-41 phase, so the peak intensity of MCM-41 phase increased, showing its typical crystal transformation characteristics. Table 2 Textural properties of different samples Samples HZSM-5 HY HAl-MCM-41 HZSM-5/MCM-41 HZSM-5-Y/MCM-41 S BET, S MIC, S MES+EXT, D MIC, D MES, m 2 /g m 2 /g m 2 /g nm nm V Total, cm 3 /g Acidity (Py-FTIR) The differences in acid-site distribution among HAl- MCM-41, HZSM-5/MCM-41 and HZSM-5-Y/MCM-41 composite molecular sieve are shown in Table 1. When the temperature was 200, it is observed that HAl- MCM-41 molecular sieve exhibited a much higher concentration of Lewis acidic sites than those of HZSM-5/ MCM-41 molecular sieve and HZSM-5-Y/MCM-41 composite molecular sieve (with L HAl-MCM-41>L HZSM-5/MCM- 41>L HZSM-5-Y/MCM-41 ). However, the HAl-MCM-41 molecular sieve exhibited a much lower concentration of Brönsted acidic sites than those of HZSM-5/MCM-41 zeolite and HZSM-5-Y/MCM-41 composite molecular sieve (with B HAl-MCM-41 <B HZSM-5/MCM-41 <B HZSM-5-Y/MCM-41 ). When the temperature was 350, it was identified that L HAl-MCM-41 >L HZSM-5-Y/MCM-41 >L HZSM-5/MCM-41, and B HAl-MCM-41 <B HZSM-5-Y/MCM-41 <B HZSM-5/MCM-41. The reasons for this phenomenon might be ascribed to their different frameworks and elemental compositions. (with a surface area of 435 m 2 /g). Similarly, the pore diameters of microporous ZSM-5 (0.48 nm) and Y (0.51 nm) zeolites were larger than those of ZSM-5/MCM-41 molecular sieve (0.47 nm) and ZSM-5-Y/MCM-41 composite molecular sieve (0.47 nm). The pore size distribution showed similar mesoporous size of nm for Al-MCM-41 molecular sieve, HZSM-5/MCM-41 molecular sieve and HZSM-5-Y/MCM-41 composite molecular sieve. Judging from the property analysis data, it is obvious that the ZSM-5-Y/MCM-41 composite molecular sieve contained a bimodal mesopore system (MCM-41) as well as a microporous structure of the ZSM-5 zeolite and Y zeolite. The unique pore system could greatly enhance the accessibility of the catalytically active sites in ZSM-5-Y/MCM-41 composite molecular sieve to larger reactant molecules, accelerate the diffusion of products, and reduce secondary reactions [11]. 3.4 Initial catalytic activity of different catalysts for cracking of 1,3,5-TIPB 3.3 Nitrogen adsorption-desorption isotherm The catalytic performance of the HZSM-5-Y/MCM-41 catalyst was tested for cracking 1,3,5-TIPB at 400 and Some properties of five samples are presented in Table 2. compared with that of the microporous HZSM-5 zeolite, It is identified that the BET surface area of composite HY zeolite, mesoporous HAl-MCM-41 zeolite and molecular sieve (474 cm 2 /g for ZSM-5/MCM-41 molecular sieve, and 499 cm 2 /g for ZSM-5-Y/MCM-41 compos- HZSM-5/MCM-41 catalysts. The volume of 1,3,5-TIPB was 1.0 μl and the catalysts dosages were 60 mg for all ite molecular sieve) were between those of microporous cases. The catalyst evaluation results are listed in Table 3. zeolite (545 m 2 /g for ZSM-5 zeolite, and 529 m 2 /g for Y zeolite) and mesoporous molecular sieve Al-MCM-41 According to the product distributing data in Table 3, we can postulate that catalytic Table 1 Acidic properties of HAl-MCM-41, HZSM-5/MCM-41 and cracking of 1,3,5-TIPB leads to HZSM-5-Y/MCM-41 samples a prevalent reaction network Samples Total, Total Medium and with three dominating in-series B acid, au L acid,aub acid, au L acid, au au (L)/(B) strong (L)/(B) reactions [12], i. e.: (i) HAl-MCM dealkylation of 1,3,5-TIPB to HZSM-5/MCM form MIPB and propylene, (ii) HZSM-5-Y/MCM dealkylation of MIPB to give 19

4 Table 3 Initial catalytic activity of different catalysts for cracking of 1, 3, 5-TIPB Selectivity, % Catalyst Conversion, % Propylene Benzene IPB MIPB PIPB X Others HZSM HY HAl-MCM-41 HZSM-5/MCM-41 HZSM-5-Y/MCM Note: IPB, MIPB, and PIPB denote isopropylbenzene, m-diisopropylbenzene, and p- diisopropylbenzene, respectively. X denotes distillate products with molecules larger than feed molecules. IPB and propylene, and (iii) dealkylation of IPB to form benzene and propylene. While the above-mentioned steps appear as the dominant ones, there are other reactions such as disproportionation, isomerization and condensation that may affect gas phase product distribution and polymer formation [13]. It is observed that the ability of catalysts for converting aromatic hydrocarbon molecules decreased in the following order: HZSM-5-Y/ MCM-41>HZSM-5/MCM-41>HAl-MCM-41>HY> HZSM-5. Microporous zeolite catalysts (HZSM-5 and HY) exhibited lower conversion during catalytic cracking of 1,3,5-TIPB. The result suggests that HZSM-5 and HY zeolites are not favourable for catalytic cracking of large molecules of 1,3,5-TIPB, because the microporous ZSM-5 zeolite has 10-ring pores with a diameter no larger than 0.56 nm and the microporous Y zeolite has 12-ring pores with a diameter no greater than 0.74 nm. In this work, the pore diameter of ZSM-5 zeolite and Y zeolite was 0.48 and 0.51 nm (Table 2), respectively. However, 1,3,5-TIPB has a dynamic diameter of 0.95 nm [12], and cannot diffuse through the inner pores of the microporous ZSM-5 zeolite and Y zeolite. Therefore catalytic reaction could only occur on the external surface of ZSM-5 and Y zeolite crystals [14]. For microporous ZSM-5 and Y catalysts, the main products were propylene and MIPB. Meanwhile, it is observed that microporous catalysts demonstrated an unexpected behavior given that the products contained a few polymers formation arising from condensation reactions of 1,3,5- TIPB hydrocarbon molecules, polymerization of intermediate products or secondary reactions, because of constraints on outward diffusion of the product molecules [12]. Moreover, benzene was not detected in products formed over the Y zeolite catalyst, and the main cause might be attributed to alkylation of benzene with propylene that took place over the Y zeolite catalyst under optimum conditions [15], and benzene molecules were exhausted completely upon being converted into IPB. In contrast, mesoporous type HAl-MCM-41, HZSM-5/MCM-41 and HZSM-5-Y/MCM-41 catalysts gave high conversion and did not cause polymer formation thanks to their larger pores (~2.8 nm, as shown in Table 2). 1,3,5-TIPB molecules could diffuse through the inner pores and came into contact with inner surface acid sites of the catalysts and overcame the diffusion limitations. Additionally, the conversion of 1,3,5-TIPB molecules on HZSM-5- Y/MCM-41 composite molecular sieve (92.6%) was much higher than HAl-MCM-41 (62.5%) and HZSM-5/MCM- 41 (88.6%), which might be reasonably attributed to the stronger Brönsted acidic sites on the HZSM-5-Y/MCM- 41 composite molecular sieve compared to the HAl-MCM- 41 zeolite and the HZSM-5/MCM-41 molecular sieve. Meanwhile, among the three kinds of mesoporous catalysts, it was found that the selectivities of propylene and benzene increased in the following order: HAl- MCM-41<HZSM-5/MCM-41<HZSM-5-Y/MCM-41, with decreasing selectivities of IPB, MIPB and PIPB at the same time. The result indicated that the cracking degree of 1,3,5-TIPB on the HZSM-5-Y/MCM-41 catalyst was deeper and more complete than HAl-MCM-41 and HZSM-5/MCM-41 catalysts. 3.5 The mechanism for catalytic cracking of 1, 3, 5- TIPB To study the 1,3,5-TIPB cracking reactions, according to the distribution of cracking products (Table 3), a possible carbenium ion mechanism is put forward as shown in Figure 2. Firstly, 1,3,5-TIPB molecules are adsorbed on the acidic centers and form carbenium ion [TIPB] +. Subsequently, the carbenium ion [TIPB] + undergoes dealkylation and forms the isopropyl carbenium ion and MIPB, then the isopropyl carbenium ion undergoes dehydrogenation to form propylene. Similarly, MIPB molecules may form carbenium ion [MIPB] +, then [MIPB] + 20

5 Figure 2 Mechanism for catalytic cracking of 1,3,5-TIPB undergoes dealkylation and forms IPB. IPB is adsorbed on the acidic centers of the catalyst and forms carbenium ion [IPB] +, then [IPB] + undergoes dealkylation and forms the final product benzene. On the other hand, during the catalytic reaction process the carbenium ion [IPB] + is attacked by a free or weakly adsorbed IPB and forms carbenium ion [IPB IPB] +, afterwards [IPB IPB] + undergoes disproportionation reaction and yields PIPB and benzene. 4 Conclusions A novel aluminosilicate ZSM-5-Y/MCM-41 composite molecular sieve with two microporous phases and a mesoporous phase was synthesized through a novel selfassembly process. The ph value had played a very important role in the formation of mesoporous MCM-41 phase. The ZSM-5-Y/MCM-41 composite catalyst exhibited higher catalytic activity than the conventional microporous ZSM-5 zeolite, Y zeolite, mesoporous Al- MCM-41 molecular sieve and ZSM-5/MCM-41 catalysts. The remarkable catalytic reactivity on 1,3,5- TIPB by the HZSM-5-Y/MCM-41 composite molecular sieve catalyst was mainly attributed to the presence of the hierarchical pore structure and proper acidity in the ZSM-5-Y/MCM-41 composite zeolite. These exciting results suggest that the present ZSM-5-Y/MCM-41 composite zeolite catalyst may find wide applications in catalytic cracking of large hydrocarbon molecules. Acknowledgements: This work was supported by the 973 plan item under Grants (2003CB615802) References [1] Gao Z, He M Y, Dai Y Y. Zeolite Catalysis and Separation Technology[M]. Beijing: China Petrochem Press, 1999: (in Chinese). [2] Du J, Wang Y, Meng S M, et al. Synthesis, characterization and catalytic property of Y/beta micro-microporous composite molecular sieve[j]. J Shanxi Datong University, (Nat. Sci.), 2009, 25(2): (in Chinese) [3] Chen H L, Shen B J, Pan H F. In situ formation of ZSM-5 in NaY gel and characterization of ZSM-5/Y composite zeolite[j]. Chem Lett, 2003, 32(8): [4] Liu Y, Zhang W Z, Pinnavaia T J. Steam-stable MSU-S aluminosilicate mesostructures assembled from zeolite ZSM-5 and zeolite beta seeds[j]. Angew Chem Int Ed, 2001, 40(7): [5] Schomburg C, Wöhrle D, Schulz-Ekloff G. In situ synthesis of azo dyes in mesoporous Y zeolites[j]. Zeolites, 1996, 17(3): [6] Shen Y L. Synthesis and characterization of the Y zeolite with hierarchical pores[d]. Beijing: Beijing Institute of Technology, 2007 (in Chinese). [7] Yeong Y F, Abdullah A Z, Ahmad A, et al. Synthesis, structure and acid characteristics of partially crystalline silicalite-1 based materials[j]. Micropor Mesopor Mater, 2009, 123(1-3): [8] Wang X J. Study on the formation mechanism of NaY zeolite prepared from metakaolin by hydrothermal synthesis[d]. Beijing: University of Science and Technology Beijing, 2006 (in Chinese). [9] Beck J S, Vartuli J C, Roth W J, et al. A new family of mesoporous molecular sieves prepared with liquid crystal 21

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