## 2 Amorphous Oxide Modified Zeolite Composite Catalysts
### Phosphorus Oxide Modified Zeolite Composite Catalysts
Our investigation using D\({}_{2}\)/OH exchange and solid-state \({}^{31}\)P- and \({}^{27}\)Al-NMR measurements show that the new acid sites seem to be related to the phosphorus entering into the zeolitic position left by dealumination and stabilized by some extra-lattice aluminum [94]. The entry of phosphorus into the zeolitic framework protects the residual framework from further dealumination. The integrated effect causes the hydrothermally stable acid sites in P-modified HZSM-5 and enhances the catalytic performance for C\({}_{4}\)-olefin cracking after steam treatment at high temperature. At present, it is difficult to describe the exact structure of the new acids formed in P/HZSM-5 during steam treatment. As early as 1986, [PERSON] _et al._[97] proposed a model to describe the interaction between the bridged hydroxyls of ZSM-5 and orthophosphoric acid. [PERSON] _et al._ recently summarized the models appearing in the literature describing the interaction between HZSM-5 and phosphorous species for elucidation of the status of phosphorus species and hydroxyl groups [95, 96, 97, 98, 99]. One widely stated view is that the framework aluminum, or a part of the framework aluminum, is stabilized via the interaction with phosphorus species. This has been convincingly supported by \({}^{27}\)Al-NMR measurements. However, these models cannot explain the changes in the number of OH groups in a series of P/HZSM-5 catalysts. Apparently, a model to account for the structure of the modified acid sites has not yet been proposed. To summarize the current results, Figure 2 shows the schematic mechanisms for the interaction of phosphorus with HZSM-5 (Figure 2e), along with other previously proposed models (Figure 2a, 2b, 2c, and 2d). It seems that these four models cannot account for the change in the number of OH groups. According to the model we proposed in Figure 2e, two zeolitic hydroxyls condense with one phosphate molecule before the steam treatment but calcined in dry air, leading to a decrease in the number of OH groups on P modification [Figure 2e(2)], which was in agreement with the results of [PERSON] _et al._[98]. During the steam treatment (dealumination), some zeolitic OH groups, (SiO)\({}_{4}\)Al-H, were substituted by phosphorous hydroxyls (P-OH), stabilized by nonframework Al, which are hydrothermally stable [Figure 2e(3)]. More recently, [PERSON] _et al._[98] proposed a new model concerning the interaction between orthophosphoric acid and bridged hydroxyls before steam treatment. They proposed that the phosphorous stabilized framework aluminum pairs (Figure 2c and 2d). Our current results support this proposal. However, the situation changed due to steam treatment at high temperatures. A new kind of OH group was formed. We believe that the phosphorus entered the framework by occupying the vacancies left by dealumination during steam treatment. The framework P stabilized by nonframework Al resulted in a new kind of acid site that is hydrothermally stable. The mechanism reported herein should be very important in zeolite applications for hydrocarbon reactions.