| C4～C7isomerization process is a conversion of normal paraffin feedstock into corresponding iso-paraffins, which is one of the most effective means of producing gasoline with high octane number. Currently, mordenite (MOR) with mono-dimensional straight pore channels is a usual support for isomerization catalysts. However, Molecules cannot pass each other in the channels so a molecule located at the pore mouth blocks the entrance or escape of other molecules. The transport in the pores is, therefore, strongly hindered by this one-dimensional diffusion, resulting in the decrease of effectiveness and stability of the catalyst. Additionally, another high-silica zeolite with mono-dimensional straight pore channels, i.e. ZSM-12, is reported to be a promising support for the isomerization catalysts. However, similar to MOR, the micropore accessibility is an important issue in the use of ZSM-12.An effective method to improve the mass transport in zeolites is to introduce mesopores into the microporous zeolite. In this thesis, two posttreatments including the acid treatment followed by the alkaline leaching as well as the sole alkaline leaching were employed on MOR and ZSM-12, respectively, to improve their mass transfer properties. Isomerization catalysts with excellent performance were obtained by loading platinum on the modified zeolites. This thesis mainly consists of two parts as follows:1. Mordenite with Si/Al ratio of10(Zeolyst) was employed as the parent zeolite. Firstly, the MOR was treated by an acid solution to improve the Si/Al ratio. Subsequently, the acid-treated zeolite was treated by an alkaline leaching followed by an acid rinsing with a diluted acid solution. The effects of the treatments on the crystallinity, textural properties, and acidity of the MOR were investigated in detail. The results showed that the combined acid-alkaline treatment was an effective method to introduce mesopores in mordenite with low Si/Al ratio while preserving the microporosity. The external specific surface area of the treated MOR could be as high as126m2·g-1and the diameter of the generated mesopores was about9nm, also confirmed by the TEM observations. Afterwards, the posttreated MOR zeolites were loaded with platinum by the ion-exchanged method. The adsorption of n-hexane in the Pt/MOR catalysts revealed that the uptake rate of the catalyst prepared with meso-MOR by the combined acid-alkaline treatment increased significantly. The hydroisomerization reaction results indicated that the mesoporous catalyst presented an excellent performance in terms of activity, selectivity for isomers, and stability.2. Lab-made ZSM-12with Si/Al ratio of60was employed as the parent and treated by the alkaline-leaching to create mesopores in ZSM-12crystals. Three modes of the alkaline-leaching were investigated for comparison. a) template-free ZSM-12was treated by a NaOH solution; b) template-free ZSM-12was treated by a mixture of NaOH and TEAOH solution; c) as-synthesized ZSM-12was treated by a NaOH solution. XRD, N2adsorption-desorption at-196℃, NH3-TPD measurements etc. were applied to characterize the crystallinity, the textural properties and the acidity of posttreated samples. The results indicated that the alkaline treated with NaOH solution on the as-synthesized ZSM-12could create mesopores (about12nm) in the zeolite crystals while preserving the crystallinity and micropore structures. Finally, the mesoporous ZSM-12was loaded with platinum by the ion-exchanged method. The obtained Pt/ZSM-12catalyst exhibited excellent performance in the hydroisomerization in terms of activity, selectivity for dibranched isomers, and stability. |
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