(90e) Reactor Design Criteria for Reliable Kinetics in Zeolite Synthesis Via Micro-Scale Crystallization

Hydrothermal zeolite synthesis simultaneously combines classical and non-classical crystallization mechanisms. Thermodynamic phase transformations, kinetic chemical condensations, three-phase mass transfer, and spatial-temporal thermal gradients become convoluted leading to an apparent deficiency in our quantitative understanding of the transport phenomena that dominate the syntheses pathways. This work proposes a set of theoretically and experimentally derived design guidelines that outline how to achieve the kinetically-controlled, scalable conditions necessary for elucidating the underlying crystallization mechanism.

Unbalanced external convection and internal conduction can result in heat transfer limitations within the crystallizer leading to internal spatial and temporal gradients. This is especially prominent as reactor volume increases, generating a range of dynamic environments that drive nonuniform reaction rates. To overcome these heterogeneities, ultrafast heating using a segmented microdroplet crystallizer ensures a well-controlled isothermal environment that creates more steady driving forces throughout the crystallization.

Theoretically, classical batch reactor heat up is characterized by the time constants for internal conduction (τ_cond) and external convection (τ_conv), the ratio of which is the Biot number (Bi) whose magnitude reflects the presence of internal temperature gradients. Analogous to the Damköhler number for mass transfer, the ratio of crystallization rate to heat transfer rate were calculated as τ_heat ∕ τ_cryst. A systematic reaction engineering evaluation of crystallizer designs for the hydrothermal sol-gel synthesis of LTA revealed substantial external and internal heat transfer limitations, particularly during the initial induction period.

Designs spanning from conventional batch to novel acoustically generated microbatch droplets, heat transfer regimes were carefully mapped, and specific criteria were established for overcoming thermal limitations. In scenarios where internal heat transfer cannot occur sufficiently fast, caution is advised against using ultrafast external heating modes as internal gradients can induce poor uniformity of the crystallization environment, yielding unpredictable results. For this reason, it is suggested that when τ_heat ∕ τ_cryst < 1 cannot be met, slower heating be used such that Bi < 1.