Exploring Meso-/Microporous Composite Molecular Sieves with Core–Shell Structures

Chemistry - A European Journal, Volume 18, Issue 3, pages 931–939, January 16, 2012

Exploring Meso-/Microporous Composite Molecular Sieves with Core–Shell Structures

Xufang F. Qian¹,

Bin Li¹,

Yuanyuan Y. Hu¹,

Prof. Dr. Guoxing X. Niu¹,

Prof. Dr. D. Yahong H. Zhang¹,

Prof. Dr. Renchao C. Che¹,

Prof. Dr. Yi Tang¹,

Prof. Dr. Dangsheng S. Su^2,3,

Prof. Dr. Abdullah M. Asiri⁴,

Prof. Dr. Dongyuan Y. Zhao^1,4,*

dyzhao@fudan.edu.cn

¹ Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Laboratory of Advanced Materials, Fudan University, 200433 Shanghai (P.R. China), Fax: (+86) 21-5163-0205

² Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016 (P.R. China)

³ Fritz Haber Institute of the Max Planck Society, Faradayweg 4–6, 14195 Berlin (Germany)

⁴ Chemistry Department and the Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah (Saudi Arabia)

Abstract

A series of core–shell-structured composite molecular sieves comprising zeolite single crystals (ZSM-5) as a core and ordered mesoporous silica as a shell were synthesized by means of a surfactant-directed sol–gel process in basic medium.  This was accomplished by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethylorthosilicate (TEOS) as silica precursor.

Using this coating method, uniform mesoporous silica shells closely grow around the anisotropic zeolite single crystals.  Shell thickness can easily be tuned in the range of 15–100 nm by changing the ratio of TEOS/zeolite. The resulting composite molecular sieves have compact meso-/micropore junctions that form a hierarchical pore structure from ordered mesopore channels (2.4–3.0 nm in diameter) to zeolite micropores (≈0.51 nm).  When Al species are introduced during the coating process, the core–shell composite molecular sieves show a graded acidity distribution from weak acidity of mesopores (predominant Lewis acid sites) to accessible strong acidity of zeolite cores. The probe catalytic cracking reaction of n-dodecane demonstrates the superiority of the unique core–shell structure over pristine ZSM-5.  The core–shell composite structure with hierarchical pore and graded acidity distribution show great potential for petroleum catalytic processes.

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