Synthesis and Catalytic Performance of ZSM5/MCM4

Authors: Jindan Na Guozhu Liu Tianyou Zhou Guochang Ding Shenlin Hu Li Wang

Publish Date: 2013/01/29

Volume: 143, Issue: 3, Pages: 267-275

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Abstract

ZSM5/MCM41 zeolite composites with varying mesopore sizes were prepared through alkalidesilication and surfactantdirected recrystallization with different chain length of alkyltrimethylammonium bromide XRD and TEM revealed that the composites possessed the characteristics of both ZSM5 and mesoporous MCM41 with hexagonal symmetry N2adsorption–desorption 27Al MAS NMR NH3TPD and in situ FTIR results suggested that the sizes of the mesopores and the accessibility of acid sites could be affected greatly by the chain length of alkyltrimethylammonium bromide used Catalytic cracking of ndodecane over ZSM5/MCM41 composites was studied in the form of coatings on the inside of a tubular reactor at 550 °C and 4 MPa The conversion of ndodecane increased gradually with increasing the mesopore sizes of ZSM5/MCM41 composites implying a positive effect of the mesopores on the diffusion inside pore channels and the accessibility of acid sites by the reactantsRecently an interest in the catalytic cracking of hydrocarbon fuels is motivated by the desire to enhance engine performance over the entire spectrum of flight regimes For hypersonic flight hydrocarbon fuels can serve as not only source of heat through combustion but also coolant through the cracking reaction to remove waste heat from aircraft systems 1 2 3 4 5 Huang et al 1 Sobel and Spadaccini 6 and Sicard et al 7 investigated the cracking of several fuels including conventional kerosenebase jet fuels JP7 JP8 and JP8+100 nheptane and Norpar 12 a blended normal paraffin in a zeolitecoated reactor under the simulated highspeed flight conditions 650 °C and 24 MPa Fan et al 8 studied the catalytic cracking of China No 3 aviation kerosene over wallcoated HZSM5 zeolite under conditions similar to the practical scramjet applications 777 °C and 70 MPa Liu et al 9 10 studied the effects of the Si/Al ratio and crystal size nano and microscale of ZSM5 zeolites coated on the wall of a flowing tubular reactor on the cracking of ndodecane at 550 °C and 40 MPa The results indicated that HZSM5 zeolite is an ideal candidate for the cracking of hydrocarbon fuels because of its relatively low coke deposition However the relatively small pore size of ZSM5 zeolite limits the diffusion of bulky molecules from the surface to active sites located in the pores which makes its catalytic activity being not fully exhibited To overcome this limitation one way is to shorten the diffusion length and another way is to improve the effective diffusivity of the reactant molecules in the zeolite pores For the latter strategy micro/mesopore composite zeolites with the strong acidity of micropore zeolites and the mesopores in the typical range of 2–15 nm have been intensively developed 11PérezRamírez et al 11 Čejka and Mintova 12 and Prokešová et al 13 systematically summarized the development in the remarkably growing field of the synthesis characterization and application of zeolitebased micro/mesoporous composites as catalysts in acidcatalyzed reactions oxidation reactions and discussed the advantages and disadvantages of the synthesis approaches for the preparation of the micro/mesoporous composites in their review and research articles Desilication by alkali treatment reported for the first time by Groen et al 14 15 16 17 is an important and widely used methodology for the preparation of hierarchical zeolites of micro and mesopores Matsukata and coworkers 18 investigated the effects of alkali treatment parameters on the morphology and acidity of the ZSM5 zeolite and the conversion of the cumene cracking and found that mesopores created during desilication improved the adsorption and diffusion properties of cumene resulting in the increase in the conversion of the cumene cracking Seo and coworkers 19 modified ZSM5 zeolites by a similar alkali treatment process and employed them as catalysts in the catalytic cracking of noctane The results showed that the low acidity and short residence time induced by alkali treatment resulted in the reduction of cracking conversion but enhanced the selectivity to primary cracking products Shen and coworkers 20 studied the relationship between the SiO2/Al2O3 ratios and the catalytic activity of alkalitreated ZSM5 zeolites and found that the zeolite with the low SiO2/Al2O3 ratios preserved the relatively complete crystal structure after alkali treatment and gave a higher yield of light olefins for the heavy oil cracking compared to the untreated zeoliteA major problem often encountered during the creation of mesopores in micropore zeolite crystals by alkali treatment is the loss of crystallinity and acid sites which causes the decrease in the catalytic performance PérezRamírez et al 21 developed a novel desilication method involving NaOH treatment of ZSM5 in the presence of quaternary ammonium cations which act as a poregrowth mediator protecting the zeolite crystal during the demetallation process The authors proposed that the category of the hierarchical porous zeolites could be quantitatively described using a parameter namely hierarchy factor HF which is independent of the type and synthesis method and gave a correlation between the catalytic activity in benzene alkylation and HF of the hierarchical porous ZSM5Another alternative approach is recrystallization of the dissolved species in the presence of surfactants after desilication Goto et al 22 synthesized ZSM5/MCM41 composite zeolites using hexadecyltrimethylammonium chloride as templates in NaOH solution The products showed strong acid sites like ZSM5 and an increase in the cracking conversion of nhexane Qi et al 23 studied the effect of recrystallization time on the morphology of the products The parent ZSM5 zeolite was dissolved and its structure was destroyed completely with prolonging crystallization time up to 72 h The sample recrystallized for 12 h showed a high catalytic activity in 135triisopropylbenzene cracking Ying and coworkers 24 and Zhao and coworkers 25 synthesized ZSM5mesoporous silica composite zeolites using cetyltrimethylammonium bromide as templates under the alkaline reaction conditions The composite materials possessed a core–shell structure and showed a higher cracking conversion of ndodecane than the untreated HZSM5 The authors concluded that the reason for high catalytic activity was due to the existence of mesoporous silica shells which can passivate the acid sites on the external surface of zeolites thereby alleviating the formation of cokes Stein et al 26 synthesized ZSM5 zeolites with mesopores through desilication and reassembly processes They found that the physicochemical properties of the treated zeolites depended on the Si/Al ratios the hydroxide concentration and the presence of surfactantsThe aim of this paper is to better understand the relationship between the catalytic activity and the pore structure of zeolites Firstly ZSM5/MCM41 composite zeolites were synthesized by sequential alkalidesilication and surfactantdirected recrystallization with alkyltrimethylammonium bromide as templates The mesopore sizes of the composite zeolites were manipulated by changing the chain length of alkyltrimethylammonium bromide CnTAB n = 10 12 14 and 16 Secondly physicochemical properties of the ZSM5/MCM41 composite zeolites were characterized by Xray powder diffraction XRD nitrogen adsorption–desorption measurements transmission electron microscopy pyridineadsorbed Fourier transforminfrared spectroscopy ammonia temperatureprogrammed desorption inductively coupled plasma atomic emission spectroscopy and 27Al solidstate magicanglespinning nuclear magnetic resonance spectra Finally the catalytic cracking of supercritical ndodecane was carried out in a flowing tubular reactor with wallcoated by ZSM5/MCM41 composite zeolites at 550 °C and 4 MPaThe HZSM5 zeolite SiO2/Al2O3 molar ratio of 107 with the average crystal sizes of 3–4 μm was purchased from the Novel Chemical Corporation Shanghai China Decyltrimethylammonium bromide C10TAB dodecyltrimethylammonium bromide C12TAB tetradecyltrimethylammonium bromide C14TAB and cetyltrimethylammonium bromide C16TAB were purchased from JK Chemical Ltd without further purification nDodecane with 995  purity was obtained from Sinopharm Chemical Reagent Co Ltd Shanghai ChinaZSM5/MCM41 composite zeolites were prepared via the surfactantdirected recrystallization process using alkyltrimethylammonium bromide with various chain lengths CnTAB n = 10 12 14 and 16 as templates In a typical run 05 g HZSM5 was added to 15 M sodium hydroxide aqueous solution 5 mL and then the mixture was stirred for 30 min at room temperature 10 g aqueous solution of alkyltrimethylammonium 10 wt was added to the suspension followed by an additional stirring for 30 min Then the mixture was transferred to a Teflonlined autoclave and hydrothermally treated at 120 °C for 12 h After cooling to room temperature the pH value of the reaction mixture was adjusted to 85 by dropwise addition of 20 M HCl under vigorous stirring The mixture was transferred into the Teflonlined autoclave and heated at 120 °C for 24 h Finally the product was recovered by centrifugation washed repeatedly with deionized water and dried at 100 °C overnight The resulting powder was calcined at 550 °C for 6 h to remove the surfactant

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