Synthesis of hierarchical MeAPO-5 molecular sieves – Catalysts for the oxidation of hydrocarbons with efficient mass transport Lipeng Zhou a , Tianliang Lu a , Jiaolong Xu a , Minzhu Chen a , Chaofeng Zhang a , Chen Chen b , Xiaomei Yang a,⇑ , Jie Xu b,⇑ a The College of Chemistry and Molecular Engineering, Zhengzhou University, 100 Kexue Road, Zhengzhou 450001, PR China b State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China article info Article history: Received 7 January 2012 Received in revised form 25 March 2012 Accepted 30 April 2012 Available online 26 May 2012 Keywords: Aluminophosphate molecular sieves Hierarchical zeolites Aerobic oxidation Hydrocarbons abstract Hierarchical molecular sieves possess the characters of both microporous molecular sieves and meso- and/or macro-porous materials, and have potential application in adsorption and separation of macro- molecules and diffusion limited catalytic reactions. In this work, hierarchical MeAPO-5 (MeAPO-5-meso, Me = Co, Mn, Fe, Mg and Ti) molecular sieves were synthesized directly using glucose as mesopore tem- plate. The synthesized MeAPO-5-meso molecular sieves were characterized by X-ray diffraction, X-ray fluorescence, N 2 physisorption, thermogravimetric analysis and scanning electron microscopy. It was proved that mesopores with the pore size distribution of 5–30 nm were introduced to MeAPO-5-meso. The improvement effect of the introduced mesopores on the catalytic performance of MeAPO-5-meso (Me = Mn, Fe and Co) was investigated in the oxidation of various hydrocarbons with different molecular dimensions including cyclohexene, ethylbenzene, indan, tetralin, diphenylenemethane and fluorene. For comparison, these oxidation reactions were also performed over the ordinary MeAPO-5. The results indi- cated that mesopores benefited the diffusion of the reactants and the products, so the conversion of the reactants was improved while the selectivity of the products was slightly improved or maintained at higher conversion. In addition, the promotion effect is dependent on the dimension of the substrates with respect to the micropore size of molecular sieves. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Zeolites are widely used in adsorption, separation, and hetero- geneous catalysis fields due to their uniform, small pore size, high internal surface area, flexible frameworks, and controlled chemis- try [1]. The well-defined micropores endow zeolites with molecu- lar sieve effect, so, as heterogeneous catalysts, the shape-selective catalysis can occur on the basis of the complete exclusion, or on the strongly hindered diffusion, of certain reactants, intermediates, or products in the zeolite micropores [2,3]. However, from another point of view, the sole presence of micropores also imposes diffu- sion limitations on the reactions involving reactants or products whose size is comparable to the diameter of micropore. In these cases, mass transport to and from the active sites located within the micropores is slow, which limits the performance of the zeolite catalysts [4]. To circumvent the diffusion limitations imposed by the zeolite micropores, several strategies have been developed. One possibility is the synthesis of zeolites/zeotypes containing meso- and/or macropores. Numerous methods for preparing hier- archical zeolites have been reported in the literatures, and the tem- plating approach is one of the most effective methods. In a typical procedure, a mesopore template, such as carbon particles [5,6], carbon nanotube [7], mesoporous carbon [8,9], carbon derived from sucrose [10], starch gel [11], polyurethane foams [12], and polymer [13], is mixed with a zeolite precursor mixture after which a hydrothermal treatment is carried out to grow the crystals. In the following calcination step, the templates are removed result- ing in meso- and/or macropores in the zeolite crystals. Aluminophosphate molecular sieves (AlPO-n) have attracted much interest since the discovery of this class of microporous materials in the early 1980s [14], and AlPO-5 with AFI structure is one of the most frequently studied members of this family. Mes- opores can also be generated in aluminophosphate molecular sieves like that in zeolites. For example, mesoporous AlPO-5 has been synthesized in fluoride media using carbon as mesopore tem- plate by Christensen et al. [15]. Recently, our group reported the synthesis of hierarchical AlPO-5 (AlPO-5-meso) using glucose as mesopore template. It was disclosed that glucose was incorporated into AlPO-5-meso through the interaction with the surface nega- tive charges. After crystallization, the glucose embedded in the 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.04.058 ⇑ Corresponding authors. Tel.: +86 371 67781780; fax: +86 371 67766076. E-mail addresses: yangxiaomei@zzu.edu.cn (X. Yang), xujie@dicp.ac.cn (J. Xu). Microporous and Mesoporous Materials 161 (2012) 76–83 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso