Poly-quaternary Ammonium Salts As Templates For Synthesis Of Hierarchical Porous Zeolites

Microporous crystalline aluminosilicate zeolites are widely used in petrochemistry and fine-chemical synthesis because of their large surface area, high adsorption capacity, high thermal and hydrothermal stabilities, and strong acid sites within their defined micropores. However, the sole micropores of zeolites strongly hinders mass-transfer in catalytic reaction, resulting in decrease of catalytic activities. To overcome this limitation, integrating mesopores into microporous zeolites to synthesize mesoporous or hierarchical zeolites is regarded to be a good choice, becuase the mesoporous zeolites have combined the advantages of both mesostructured materials (fast diffusion and accessible for bulky molecules) and microporous zeolites (strong acidity and high hydrothermal stability).Conventionally, the synthesis of mesoporous zeolites is high-cost, because the expensive template is required, such as surfactant derivatives. In this work, we demonstrate for the first time that a non-surfactant cationic polymer can act as a sole dual-function template to synthesize mesoporous zeolite single crystals with large BET surface area, high mesoporosity and hydrothermal stability. For example, mesoporous aluminosilicate zeolite Beta was successfully synthesized from a commercial cationic polymer, which acts as a single template to generate zeolitic micropores and mesopores simultaneously. Mesopore structure and crystal nature of mesoporous Beta zeolite, dual-function template effect of cationic polymer, catalytic activity, hydrothermal stability of the mesoporous Beta and crystallization mechanism in synthesis were investigated. The main results are achieved as follows:(1) This is the first demonstration of a single non-surfactant polymer acting as such a dual-function template for synthesizing mesoporous Beta zeolite (Beta-MS). Using high-resolution electron microscopy and tomography, we discovered that Beta-MS has abundant and highly interconnected mesopores. More importantly, we demonstrated using a three-dimensional electron diffraction technique that each Beta-MS particle is a single crystal. The use of non-surfactant templates is essential to gaining single crystalline mesoporous zeolites. The single crystalline nature endows Beta-MS with better hydrothermal stability, compared with surfactant-derived mesoporous zeolite Beta. Beta-MS also exhibited remarkably higher catalytic activity than did conventional zeolite Beta in acid-catalyzed reactions involving large molecules.(2) To understand the formation process of Beta-MS, we investigated the intermediates at different crystallization times by means of XRD, N2 sorption, SEM, and TEM techniques. The results indicate that:PDADMA consociated with the aluminosilicate species at the initial stage of the synthesis; Small crystalline grains embedded in the amorphous matrix were observed in the sample crystallized for 36 h and with prolonged crystallization, they gradually grew into large crystals along with the disappearance of the amorphous particles. Taken together, these results suggest that despite the inclusion of polymers, the formation of Beta-MS single crystals still follows the classical "nucleation and growth" model of crystallization. The mesopore size of Beta-MS can be tuned in the range of 4 to 10 nm by simply varying the molecular weight of PDADMA. Moreover, we found that another non-surfactant quaternary ammonium cationic polymer, polydiallyldiethylammonium chloride, is also capable of directing the synthesis of mesoporous single-crystalline zeolite Beta. These results suggest that the present method may be generalized for preparing mesoporous zeolite single crystals of other topologies by rationally designing the functional groups on the non-surfactant polymer. To enlarge the synthesis of Beta-MS, we carefully adjusted the composition of the start gels for gaining mesoporous Beta, as well as synthetic temperature and the silicon sources.(3) We demonstrated one-step syntheses of hierarchical Beta, ZSM-11, ZSM-23 and EU-1 zeolites using a class of simple non-surfactant poly-quaternary ammonium salt containing-(CH2)n-N+(CH3)2- group as template. These hierarchical zeolites assemblied by nanosized zeolites. We found that the structure of zeolite is sensitive with the length of alkyl chain of organic poly-quaternary ammonium. For example, when n=6 in the alkyl chain, we can obtain hierarchical Beta or ZSM-11 under different Si/Al conditions; when the organic template contained both n=3 and n=4 group, hierarchical EU-1 can be obtained; when the organic template contained only n=3 group, hierarchical zeolite ZSM-23 can be synthesized. Finally, we evaluated catalytic activities of these hierarchical Beta, ZSM-11 and ZSM-23 zeolites using the model reaction of LDPE pyrolysis. The catalytic results indicate that these hierarchical zeolites exhibited more excellent catalytic properties than the conventional ones, because of the large external surface and mesopore volume, which are helpful for the mass-transfer during the reactions.