## 2 Amorphous Oxide Modified Zeolite Composite Catalysts
### Metal Oxide Decorated Zeolite Composite Catalysts  
Based on our previous research on phosphorus oxide modified zeolite composite catalysts for C\({}_{4}\)-olefin cracking, we investigated the introduction of tungsten into phosphorus-modified HZSM-5 (P/HZSM-5) [100]. As we know, among heterogeneous acid catalysts under investigation, the heteropoly acids (HPAs) are strong Bronsted acids. It is unlikely that H\({}_{3}\)PW\({}_{12}\)O\({}_{40}\) (HPW) would be formed in the zeolite channels by the introduction of tungsten into phosphorus-modified HZSM-5, but some interaction between tungsten and phosphorus species should be expected by doping tungsten species to P/HZSM-5. Furthermore, it has been documented that supported tungsten oxides are good  
Figure 2: Models proposed for the interaction of phosphorus with the Bronsted acid sites of HZSM-5 prepared by impregnation with phosphorus modifiers and calcinations. **(a)** proposed by [PERSON] _et al._[95] and [PERSON] _et al._[96]; **(b)** proposed by [PERSON] _et al._[97], **(c)** and **(d)** proposed by [PERSON] _et al._[98]; **(e)** proposed in this work [94].  
catalysts for isomerization of hydrocarbons, e.g., 1-butene, _n_-decane and C7+ paraffins [101, 102, 103, 104, 105]. Considering these properties of tungsten and the anticipated interaction between phosphorus and tungsten, we concluded that doping tungsten into P/HZSM-5 would further promote its catalytic performance for 1-butene cracking.  
As expected, we found a synergistic effect among tungsten, phosphorus and HZSM-5 in 1-butene cracking to propylene and ethylene, which was demonstrated by catalytic tests [100]. The conversion rates of 1-butene cracking over the catalysts were calculated relative to OH density (shown in Figure 3a). It is clear that the tungsten-doped P/HZSM-5 (W-P/HZSM-5) shows better catalytic performance than P/HZSM-5; and the highest specific conversion rate is observed on the steamed W-P/HZSM-5 with 1% P. In comparison with P/HZSM-5, the enhancement in catalytic performance of W-P/HZSM-5 surely reflects the contribution of the doped tungsten. The conversion rate of 1-butene over W/HZSM-5, however, is just slightly affected by the tungsten doping. Hence, a synergistic effect between the tungsten and phosphorous on the acid sites of zeolite can be infered. The structure similar to the HPW species might be formed in and out of zeolitic channels and is responsible for the enhanced conversion rate. The new acid sites for the interaction of P and W are also deduced by D2/OH exchange and NH3 adsorption microcalorimetry. After steam treatment at 1073 K for 4 h, the steamed W-P/HZSM-5 catalysts show more distinct cracking enhancement, as shown in Figure 3a. This result leads to the conclusion that the synergistic effect of P and W is very stable in steam at high temperature. Figure 3b shows the specific activity of 1-butene cracking over steamed P/HZSM-5 and steamed W-P/HZSM-5 with time on stream. It can be seen that the specific activity of steamed W-P/HZSM-5 sample is much higher than that of steamed P/HZSM-5 for 1-butene cracking on the basis of OH groups. A similar deactivation is shown but the higher activity even with low acid density in W-P/HZSM-5 catalyst is quite attractive. Besides, according to the selectivity to propylene, W-P/HZSM-5 showed higher propylene selectivity than P/HZSM-5.  
Figure 3: **(a)** List of the conversion rate of 1-butene over a series of catalysts at the same reaction conditions; **(b)** relative catalytic performance of steamed P/HZSM-5 (containing 1%P) and steamed W–P/HZSM-5 (containing 1%P) samples as a function of time on stream (temperature: 803 K; WHSV = 13 h\({}^{-1}\); N2/1-butene = 3). Adapted from Reference [100].  
The enhanced performance of the catalyst for 1-butene cracking to propylene and ethylene is related to the synergistic effect between the doped tungsten and phosphorous on the reaction network of the cracking process. The W-P/HZSM-5 is a promising catalyst for the cracking of 1-butene to propylene and ethylene.