The b-axis-oriented ZSM-5 zeolite is regarded as a highly desired material for catalytic reactions and membrane separation. Here, the tailored synthesis of ultrathin ZSM-5 nanosheets with controllable b-axis thickness and aspect ratio is achieved by adjusting the amount and aging time of silicalite-1 seed suspension in the fluoride medium for crystallization in a short time. On the basis of synthesis and scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 19F MAS NMR analysis, the seed suspension overcomes the difficulty of nucleation in a near-neutral solution, and its [010] crystal facet facilitates the adsorption of fluoride, which promotes the b-axis-oriented growth of nanosheets. This work highlights the important role of seed suspension and fluoride in the growth process. The thinner nanosheets exhibit better catalytic performance (conversion, lifetime, and resistance to deactivation) in the cracking of n-heptane, which can be attributed to their larger surface area and shorter straight channel length. It is anticipated that ZSM-5 nanosheets can be widely used in catalytic reactions that are more sensitive to diffusion limitation.

ZSM-5 nanosheets are promising catalysts for the catalytic reactions controlled by diffusion limitations. This study reveals its significant application in the alkylation of benzene with methanol. The b-axis-oriented ZSM-5 nanosheets with similar acid property but varied thicknesses of about 30, 90, and 300 nm were prepared to investigate the effect of thickness on their catalytic properties for alkylation reactions. Comparative results demonstrate that the sample with a thickness of 30 nm exhibits higher benzene conversion, xylene selectivity, and methyl selectivity (up to 97%), accompanied by an ultralong lifetime (up to 1000 h, 10 times longer than that of the sample with a thickness of 300 nm) and lower byproduct ethylbenzene selectivity. This is ascribed to the shortened straight channel length, increased specific surface area, and enlarged mesopore volume that significantly facilitate the diffusion of reactants and products, increase the accessibility of acid sites, and decrease the coke formation. Moreover, compared with conventional ZSM-5 nanocrystals, ZSM-5 nanosheets deliver a substantially extended lifetime due to fewer framework defects. Most significantly, this study unravels the diffusion effect on ethylbenzene selectivity over ZSM-5 nanosheets with different thicknesses and illustrates the role of strong Brønsted acid sites in the dynamic changes of ethylbenzene selectivity. In light of the above analysis, we developed a precoking strategy and an introducing-heteroatom strategy to precisely tailor the catalyst acidity, further suppressing the ethylbenzene formation (<0.3%) while maintaining long-term stable operation (>300 h).

Social annotations on a Web document are highly generalized description of topics contained in that page. Their tagged frequency indicates the user attentions with various degrees. This makes annotations a good resource for summarizing multiple topics in a Web page. In this paper, we present a tag-oriented Web document summarization approach by using both document content and the tags annotated on that document. To improve summarization performance, a new tag ranking algorithm named EigenTag is proposed in this paper to reduce noise in tags. Meanwhile, association mining technique is employed to expand tag set to tackle the sparsity problem. Experimental results show our tag-oriented summarization has a significant improvement over those not using tags.

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