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The evolution of zeolites has been followed by University of Minnesota chemical engineer Michael Tsapatsis and colleagues using microscopy and X-ray diffraction. Their study could lead to a new approach to designing and synthesizing novel variations on the zeolite theme for use as molecular sieves, catalysts, and sensors. Zeolites are commonly used in chemical filters and as ion-exchange agents in advanced detergents. But, they are also critical to petroleum processing and industrial separation and catalysis on a large scale. Nevertheless, a route to designer zeolites for specific applications remains elusive. The ultimate goal would be to make a zeolite or analogue of controlled pore size and shape and overall structure. Such materials could become the next generation of highly specific catalysts, hydrogen-storage media for fuel cell applications, and highly selective membranes for chemical separations. "Controlling the growth of a certain crystal structure is difficult because it is done by trial-and-error, or what some critics may call a 'mix, wait and see' approach," explains Tsapatsis. Chemists need a clearer understanding of crystal nucleation and growth processes in the formation of zeolites and related organic-inorganic structures in order to produce such materials by design rather than this ad hoc approach. Tsapatisis, Tracy Davis, and their colleagues have spent more than a year monitoring the growth of zeolites in a laboratory setting, where they could watch the crystal growth process using high-resolution transmission electron microscopy and small angle X-ray scattering. "These are complex structures containing hundreds of atoms per unit cell and their formation is determined largely by kinetics," Tsapatsis says, "Our approach is to slow down the kinetics and exhaustively study the evolution by all techniques available to us." Their study revealed that zeolites are formed in a step-by-step, hierarchical, fashion, with silicon-oxygen nanoparticles forming first. These then form aggregates with larger, more complex structures, incorporating other atoms and molecules as they grow but leaving substantial pores and tunnels. The team has now created a mathematical model of this process, which they hope will shed new light on how to manipulate the chemical conditions necessary for zeolite growth so that different functional groups can be made to line different sized pores at will. "There are essentially unlimited opportunities for these crystals if we can control their pore structure and crystal shape, tailoring designs to specific applications ranging from catalysts to bio-implants," Tsapatsis adds. Related links:
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Nanoparticles aggregate heirarchically to form a zeolite |
