## 2 Amorphous Oxide Modified Zeolite Composite Catalysts  
### Amorphous Silica Decorated Zeolite Composite Catalysts  
Toluene disproportionation and transalkylation is an important process that converts low-value toluene and heavy aromatics into high value mixed xylenes. In the traditional toluene disproportionation process, an equilibrium mixture of xylenes is produced after which the _para_-xylene product has to be recovered by either crystallization or selective adsorption, and the remaining xylenes are recycled through the xylene isomerization process. In order to raise _para_-xylene selectivity, a new process known as selective toluene disproportionation has been developed recently which produces _para_-xylene with selectivitity greater than 90%, and the product is obtained directly without needing a subsequent isomerization process. It is known that the inner micropore channels of zeolites have shape-selective properties, while the external surface active sites of the zeolite crystallites are none shape-selective. When toluene molecules enter the pore channels of the zeolite, they disproportionate to benzene and an equilibrium mix of three xylene isomers (see Figure 1a). Because of their greater diffusivity, benzene and _para_-xylene leave the pores more rapidly and the remaining _ortho_- and _meta_-isomers re-equilibrate to form more _para_-xylene (shape-selective property). While on external surface, if there are some acid sites, re-isomerization of the _para_-xylene may occur rapidly, which results in a decrease in the final _para_-xylene selectivity of the catalyst. Thus, in order to improve the _para_-xylene selectivity, it is necessary to deactivate the acidic sites on the external surface of zeolite to suppress xylene isomerization. By modification, in addition to the decrease of the overall number of acidic sites present on the zeolite, the size of the pore entrances on the external surface is also decreased to increase the relative diffusivity of _para_-xylene by several orders of magnitude compared to the larger _ortho_- and _meta_- isomers. Therefore, the _para_-xylene selectivity will increase. However, the activity of the catalyst will be reduced to some extent, so zeolite modification should be implemented carefully to achieve the best balance between zeolite diffusivity and external surface activity. A variety of modification methods have been adopted to improve the catalytic selectivity of ZSM-5 for toluene disproportionation or alkylation reactions such as impregnation of metallic or non-metallic compounds, chemical vapor deposition of silica (SiO\({}_{2}\)-CVD), pre-coking [64, 65, 66] and chemical liquid deposition of silica (SiO\({}_{2}\)-CLD) modification. For the catalysts prepared by these methods, supporting metallic or non-metallic compounds, such as H\({}_{3}\)PO\({}_{4}\), CaO, MgO and Fe\({}_{2}\)O\({}_{3}\), on ZSM-5 could eliminate the external acidic sites of zeolite, but they also greatly decrease the acidic sites in channels [67, 68, 69, 70]. The on-line pre-coking modification of zeolite is often difficult to carry out, and this modification has to be conducted again after regeneration of zeolite catalyst [71, 72], so SiO\({}_{2}\)-CLD modification was adopted instead to prepare a commercial ZSM-5 catalyst for shape-selective disproportionation of toluene to _para_-xylene [73]. It was reported in Mobil's patents that in methylation of toluene or toluene disproportionation, high selectivity for _para_-xylene (up to 90%) may be reached by adjusting the CVD or CLD conditions and selecting polysilaxone as SiO\({}_{2}\) precursor [74, 75, 76]. CVD is commonly used in the laboratory, however, CLD is more easily transferred to large-scale industrial preparation.