## 3 Composite Zeolites by Co-Crystallization or Overgrowth
This demonstrates that the MCM-49 and ZSM-35 analogues in co-crystallized zeolites may be stacked much closer than in the physical mixture, and some parts of intergrowth may be formed due to the partially similar basic structure of MCM-49 and ZSM-35. In order to test the catalytic performance of the MCM-22/ZSM-35 composite catalysts, pulse reactions were carried out to study the initial activity of butene aromatization with the products formed directly detected by an on-line multi-channel MAS [129]. To our surprise, the synthesized MCM-22/ZSM-35 composite catalysts demonstrated relative higher aromatization ability than the pure phase of either MCM-22 or ZSM-35. Besides, with regard to their catalytic performance for the aromatization of olefins in LPG/FCC gasoline, the coexistence of MCM-22 and ZSM-35 in the composite (MCM-22/ZSM-35 = 45/55 wt/wt) was also observed to exert a notable synergistic effect on the aromatization ability for butene conversion and FCC gasoline updating, which is possibly due to the intergrowth of some MCM-22 and ZSM-35 layers [129]. The second composite zeolite we investigated was BEA/MOR (a co-crystalline zeolite of beta and mordenite) zeolite composite which was synthesized using tetraethylammonium fluoride as composite template [132]. The crystallization process of the BEA/MOR composite zeolite had been systematically investigated by XRD, ICP, SEM, TGA, and nitrogen adsorption characterizations. It was found that the beta and mordenite phases did not appear simultaneously in the crystallization process when the crystallization was going on. The beta phase was the favored product at the beginning of crystallization, thereafter the mordernite phase emerged, and the BEA/MOR composite zeolite was gradually formed. The morphology of obtained particles changed a bit during the crystallization process, but the granularity distribution range broadened. Asymmetry of BEA/MOR composite zeolite particles led to uneven distribution of the mesopores. The acid properties of BEA/MOR samples were characterized by temperature programmed desorption of NH\({}_{3}\) (NH\({}_{3}\)-TPD) [133]. There is a volcano-type relationship between the higher NH\({}_{3}\)-TPD peak temperature and the mordenite content of the BEA/MOR samples, and the NH\({}_{3}\)-TPD peak temperature reached the maximum at mordenite content of about 65% (Figure 4a). In contrast, there is a linear relationship between the higher NH\({}_{3}\)-TPD peak temperature and the mordenite content of the mechanical mixtures of zeolite beta and mordenite. The NH\({}_{3}\)-TPD peak temperature represents the acid strength of the zeolite catalysts. The catalytic activity measurements of all the samples for methanol dehydration were carried out in a fixed bed reactor under atmospheric pressure, temperature of 180\({}^{\circ}\)C, and methanol space velocity of 2 h\({}^{-1}\). Dimethyl ether and water were the only products for each catalyst. No coke formation and catalyst deactivation were observed in a 6 h reaction. There is a linear relationship between the methanol conversion and the mordenite content of the mechanical mixtures. The BEA/MOR samples with mordenite content higher than 40% showed higher catalytic activity than mordenite, and the BEA/MOR with 65% mordenite showed the highest methanol conversion (Figure 4b). The catalytic activity of BEA/MOR also exhibited a volcano-typebehavior. Compared with the mechanical mixtures, the BEA/MOR samples displayed higher catalytic activity, which should be closely related to the acid strength of the zeolite catalysts. Another composite zeolite we investigated was a MOR/MFI core/shell composite [134]. It was successfully fabricated by the overgrowth method, through which mordenite cores were firstly pre-treated with an amine solution and then ZSM-5 shells overgrown on the surface of the mordenite crystallites (Figure 5). It was found that the first pre-treatment step was crucial for the fabrication of MOR/MFI overgrowth structures.