(387h) Navigating the Dimensionless Map of Mass Transfer in LTA Zeolite Crystallization Using Boundary Layer Diffusion Modeling and Microfluidic Reactor Design

Over the last century, kinetic models have attempted to mathematically explain the molecular behavior of crystal nucleation and growth through classical crystallization theory. Early models, such as Avrami transformations, empirically fit parameters that lacked physical meaning. These generalized models were heavily limited by restrictive assumptions concerning the synthesis chemistry, such as constant nucleation and growth rates, that clearly did not reflect experimental observations. Within the last thirty years, the development of sophisticated scattering and microscopy instrumentation has provided increased resolution and expanded collective understanding in favor of a non-traditional crystallization mechanism involving nanoparticles of countless shapes, sizes, and compositions.

Despite technological advances, contradictory characterization results still arise from insufficient attention to the design of synthesis vessels. Constantly changing conditions within traditional autoclaves make collecting reliable kinetic data extremely difficult. Accurate kinetic measurements vitally depend on highly precise reaction conditions, and microfluidic reactor designs can deliver the required level of engineering control. Truly intrinsic measurements are motivated by the need for crystallization rate to be kinetically limited instead of diffusion limited. Therefore, this goal is accomplished through the minimization of the crystallization Damköhler number, the ratio of diffusion and reaction times.

In stagnant mixtures a reduction in reactor volume decreases the length required for building block precursors to travel from solution to crystal surface. Alternatively, increasing free velocity over the crystal surface effectively reduces the boundary layer through which precursors must diffuse. In both scenarios, mass transfer is the rate-limiting step for crystal growth. Strategic reactor design can tune mixture conditions to increase the overall mass transfer rate thereby shifting the rate-determining step from boundary layer diffusion to the surface growth reaction.

Carrying out syntheses in traditional autoclave batch reactors and micro-batch droplets demonstrates the impact of shrinking the length scale on mass transfer and observed reaction rates. Crystallinity was characterized using ex situ x-ray diffraction and Raman spectroscopy. Comparing the crystallinity curves, a clear reduction in induction time is observed in micro-batch systems as both characteristic LTA reflections and framework vibrations appear at shorter synthesis times. Engineering controls afforded by microfluidics have the potential to consistently measure the intrinsic kinetics of zeolite growth and elucidate the contested crystallization mechanism.