Synthesis, characterization and hydroisomerization catalytic performance of nanosize SAPO-11 molecular sieves
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Catalysis Letters, Vol. 118, Nos. 1–2, October 2007 (Ó 2007) DOI: 10.1007/s10562-007-9138-1
Synthesis, characterization and hydroisomerization catalytic performance of nanosize SAPO-11 molecular sieves Shengzhen Zhang, Sheng-Li Chen,* Peng Dong, Zhiyong Ji, Junying Zhao, and Keqi Xu State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum, Beijing 102249, P. R. China
Received 15 April 2007; accepted 15 May 2007
A new static hydrothermal method was developed for synthesizing nanosize SAPO-11 molecular sieve.The new method involves aging pretreatment step followed by crystallization process of a precursor gel. The effect of the aging pretreatment on morphology and size of SAPO-11 molecular sieves was investigated. The results of XRD and SEM showed that the aging pretreatment changed the size and morphology of the SAPO-11 dramatically, and the size could be controlled by varying the time for aging. Nanocrystals of SAPO-11 molecular sieves were synthesized by aging the precursor gel at 150 °C for 135 min and then crystallizing the gel at 190 °C for 24 h. Compared with the SAPO-11 samples that exhibit the pseudo-spherical aggregates ranging from 3 to 15 lm synthesized by conventional method, the SAPO-11 samples synthesized by the new method not only possessed much smaller cubic single crystals of about 400–500 nm, but were more uniform in size. Moreover, the SAPO-11 samples synthesized by the new method had larger specific surface area and larger external specific surface area. The results of n)hexadecane hydroisomerization showed that the Pt/SAPO-11 prepared with the new method exhibited higher catalytic activity and better hydroisomerization selectivity than the Pt/SAPO-11 synthesized by conventional hydrothermal method. KEY WORDS: aging treatment; hydroisomerization; nanocrystals; n)hexadecane; SAPO-11 molecular sieves; static hydrothermal synthesis.
1. Introduction Silicoaluminophosphate molecular sieves denoted as SAPO were synthesized by Lok et al. [1] and composed of strictly alternating AlO4, PO4 and SiO4 tetrahedra. Among of these SAPO materials, SAPO-11 has the AlPO4-11 (AEL) topology, comprising of unidirectional, non-intersecting, 10-membered ring channels with elliptical pore apertures of 0.39 nm 0.63 nm. Recently, SAPO-11 has been found to be highly active and selective for the skeletal isomerization of n)butene [2], especially for the hydroisomerization of long-chain n)paraffins [3–5]. Owing to the medium acidity and suitable pore size of SAPO-11, Pd or Pt/SAPO-11 catalyst shows excellent performance in iso-dewaxing to produce high quality lubricant with low pour point and high viscosity index. For long-chain n-paraffin, which is components of wax in petroleum products, the hydroconversion takes place mainly in the pore mouth and the external surface of the catalyst [6]. Thus, for the SAPO-11 used for hydroisomerization of heavy n)alkanes, only the active sites near pore mouths and on external surface are effective. In comparison with microm
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