Tailoring The Structure Of Hierarchically Porous Zeolite Through Modified Orientated Attachment Growth

Zeolite is a family of crystalline aluminosilicates possessing ordered, uniform micropores (<2nm). Guest molecules can be adsorbed into the micro-channels selectively with respect to their shape and size. Zeolites are widely used in the fields such as adsorption, separation and heterogeneous catalysis. With their superior shape-selective properties, zeolites are importance in the field of petrochemistry. Most zeolites catalysed reactions are limited by constraints in mass-transport, it is henceforth desirable to reduce the diffusion path of reactants and products in the zeolite. This quest has driven numerous activities in academia and industry in the past decade to design hierarchically porous zeolites. Usually, cost-and time-intensive templating strategies or difficult postsynthetic modifications have been applied to achieve mesoporosity in their preparation.In this thesis, an industrially viable protocol to generate mesoporous zeolites by combining the organosilane inhibition method to a dry gel conversion synthesis is developed.The thesis starts from the growth mechanism of Beta zeolite under high-supersaturation in a dry gel system. By monitoring the zeolite growth process, the crystallization of zeolite Beta in a dry gel system is found to follow the orientated attachment growth route. The initial giant precursor gel decomposes into amorphous particular aggregates. Subsequently, the amorphous aggregates are converted into protozeolite species and nucleation starts. Further growth undergoes via orientated attachment mechanism by fusion of small crystallites into bulky ones.Based on the above formation mechanism, a new strategy to generate hierarchically porous zeolite Beta is proposed. Organosilane modifications are employed to interrupt the growth of zeolite Beta after a precrystallization process. A resumed crystallization afford a hierarchically porous Beta zeolite with good pore-connectivity. To probe the particle size, pore-connectivity as well as the surface acidity of the hierarchically porous zeolite Beta, XRD, N2physisorption, SEM, TEM,27Al MAS NMR,29Si MAS NMR, FT-IR and NH3-TPD have been undertaken. Two organosilanes, hexadecyltrimethoxysilane (HTS) and1,8-bis(triethoxysilyl)octane (BTO), are found to have distinct impact on the structure of the final products. HTS has little effect on the Bronsted/Lewis acidity, whereas BTO causes substantial reduction of strong Br(?)nsted acid sites. Arbitrary choice of organosilanes is found to be effective to tailor the particle size, pore-connectivity as well as acidity of the hierarchically porous zeolite Beta. In hydroisomerization of n-heptane, Pt supported HPZ beta derived from this route exhibits superior catalytic performances with respect to the conventional counterpart.In addition, the thesis also discuss about the feasibility of extrapolating the method to hierarchically porous ZSM-5and mordenite zeolite in structure directing agent free system.