## 2 Amorphous Oxide Modified Zeolite Composite Catalysts
### Phosphorus Oxide Modified Zeolite Composite Catalysts  
Propylene is an important basic chemical intermediates which is produced primarily as a byproduct from petroleum refining or from ethylene production by steam cracking of hydrocarbon feedstocks. The petrochemical industry is presently facing a major squeeze in propylene availability as a result of the increasing demand for propylene derivatives (e.g., polypropylene). The traditional methods for propylene production are not satisfactory to meet this increasing demand. Recently, the catalytic cracking of mixed C\({}_{4}\)- or C\({}_{5}\)-olefins for producing propylene and ethylene over various zeolite catalysts, such as ZSM-5 [80, 81, 82, 83, 84], ZSM-48 [85], ZSM-23 [85], and MCM-22 [86] has been a research focus. The zeolites were found to have high selectivity and yield of propylene in the C\({}_{4}\)-olefin cracking reaction, but because of the strong polymerization tendency of the olefin-containing reactants and/or products on the acidic catalysts, one of the most critical problems for such reaction is catalyst deactivation. Phosphorus oxide modified zeolite composite catalysts have been researched for dozens of years [88, 89, 90, 91, 92], and it has been proved that phosphorus modification would be an effective method to elevate the hydrothermal stability of ZSM-5. The role of phosphorus in the catalytic cracking over ZSM-5 catalyst, however, has seldomly been reported so far. Most studies believe that the hydrothermally stable acid sites of the modified HZSM-5 originate from the framework aluminum being protected from dealumination. The changes in the structure of the modified acid sites during hydrothermal treatment have scarcely been investigated, and few convincing results on the phosphorous status under hydrothermal conditions in the modified HZSM-5 have been elucidated to date. It seems that the details of the interaction between P and HZSM-5 and the nature of the acid sites thus induced are worthy of further clarification because they are closely related to the anti-coke deposition as well as the catalytic behavior of the catalyst. Recently, we prepared a series of catalysts for C\({}_{4}\)-olefin cracking by impregnating the HZSM-5 with different amounts of phosphoric acid, followed by calcination in air at high temperature [93]. XRD patterns showed that the structure of zeolite framework was scarcely damaged after P modification. NH\({}_{3}\)-TPD and IR spectra indicated decreased acidity in the P-modified HZSM-5. The modification also resulted in dealumination of the HZSM-5 framework. The introduction of a suitable amount of phosphorus into HZSM-5 zeolite can improve its catalytic performance in the cracking of C\({}_{4}\)-olefins. Both coke deposition and dealumination were the crucial causes of HZSM-5 deactivation. It was found that P modification could enhance the catalyst's anticoking ability in C\({}_{4}\)-olefin cracking reaction, which is mainly due to the fact that P modification, had caused dealumination and thus eliminated part of the strong acid sites in HZSM-5. During the C\({}_{4}\)-olefin cracking reaction process the tetrahedral aluminum framework of the P-modified HZSM-5 was changed mainly to distorted tetrahedral aluminum or pentacoordinated aluminum species which still could act as stable active centers and hence, the P-modified HZSM-5 showed excellent stability. The dimerization-\(\beta\)-scission mechanism was a dominant process for C\({}_{4}\)-olefin cracking to produce propylene. HZSM-5 modified with an appropriate amount of P (1.5 wt%) was proven to be the optimal catalyst for C\({}_{4}\)-olefin cracking to produce propylene (lifetime exceeding 800 h on a single pass, and it could probably be applied to an industrial process). New kind of acid sites created in phosphorus-modified HZSM-5 through treatment in 100% steam at 1,073 K show much better hydrothermal stability and significantly modified catalytic performance for C\({}_{4}\)-olefin cracking.