Catalysts, shape selective silica-alumina

Although cracking also occurs on chlorine-treated clays and amorphous silica-aluminas, the application of zeolites has resulted in a significant improvement in gasoline yield. The finite size of the zeolite micropores prohibits the formation of large condensed aromatic molecules. This beneficial shape-selectivity improves the carbon efficiency of the process and also the lifetime of the catalyst. 

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The present paper describes the phenomena of shape-selective polymerization involved in converting C--C olefins over HZSM-5 to higher boiling olefins and the similarities and differences compared to amorphous silica-alumina. The channel systems of ZSM-5 (8), Figure 1, impose shape-selective constraints on the shape of the large molecules accounting for the differences with amorphous catalysts. 

Shape selectivity. Crystalline zeolites form a special class of heterogeneous catalysts. They are oxides, generally combinations of silica and alumina, having open-framework... 

Many reactions in the fine-chemical industry are performed with organic liquids or in organic solvents. When the surface of a solid-acid catalyst is hydrophilic, the presence of small amounts of water can completely block the surface of the catalyst. Carefully dried catalysts and reagents must then be employed. It may be noted that zeolites with a high silica-to-alumina ratio are hydrophobic. The (internal) surface of the zeolite is, therefore, not readily poisoned by accumulation of (traces of) water. With zeolites, moreover, use can be made of the uniform size of the pores to perform shape-selective reactions after passivation of the catalytic sites on the external surface of the zeolite crystallites. In the bulk-chemical industry several interesting reactions have been developed with zeolite catalysts [27] (cf. the review of Tanabe and Hdlderich [28]). 

Metals or metal oxides are usually supported on the supports such as silica and alumina when they are used as catalysts. The deposition of metal species on the supports often results in the improvement of catalytic activity and selectivity, and/or in the inhibition of their sintering at high temperatures due to the chemical interaction between the metal species and the supports. Because the supported metal or metal oxides are prepared conventionally with an impregnation method, the metal species are mainly supported on the surface of supports. On the other hand, we have prepared the supported metal catalysts by using microemulsion systems [1,2]. The preparation methods can produce the metal or metal oxide particles uniformly covered with silica layers. Thus, the metal species interact strongly with silica. In addition, because metals or metal oxides which work as active sites for the catalytic reactions are covered with silica layers with porous structures, new functions such as shape selectivity could be expected in the catalytic reaction over them. 

The alkylation of phenol with propylene over several solid acid catalysts such as HZSM-5 with different silica to alumina ratios, H-Beta, H-USY and Y-AI2O3 has been studied. It has been found that zeolite structure has great influence on product distribution. Apart from shape selectivity taking effect in phenol alkylation with propylene over HZSM-5 zeolites, acidic properties (i.e. acid strength and acid density) also influence product distribution. It has been found that H-ZSM-5 exchanged with different alkali metal ions, such as Na and Cs could apparently enhance the selectivity for para-iso-propylphenol due to the change of acidic properties. The acidic properties of the zeolites were characterized by NH3-TPD. 

The acidic function of hydrocracking catalysts can be provided by various solid acids such as amorphous silica-alumina, zeolites and doped alumina. The type of application dictates the nature of the acidic component. Hydrocracking catalysts designed for the production of naphtha require strong acidity, provided by zeolites (Y-zeolite). For special applications, such as dewaxing, where shape-selective reactions must be promoted, zeolites such as ZSM-5 and mordenite are used. 

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