Zeolites have enhanced properties for adsorption and activity resulting from synergistic interactions between the solid framework and the reacting species. Depending on the extent of confinement imposed by the pore and channel aperture, shape selectivity governs the diffusion of reactants, intermediates, and products. If such confinement is severe, diffusion limitations dictates the relative rates in which reaction occurs. One way to compare these diffusion limited systems is using the reaction-transport parameter Thiele modulus. However, the variables used in the estimation of such parameter are sometimes unavailable.<br/>Here, we have combined an experimental method with a reaction-transport model to quantify the extent of mass-transfer limitations. Titration of active sites (by pyridine adsorption and Na exchange) and measurement of the consequent rate decays are combined to estimate Thiele moduli. Liquid phase self/cross aldol-condensation of acetone and cyclopentanone was performed in a batch reactor over commercial samples of HZSM5 and HY.<br/>Due the small pore sizes of MFI, the slow diffusion of both ketones significantly impacts reactivity. Moreover, a higher Thiele modulus is found for cyclopentanone than acetone both at low and high acid site concentrations. In zeolite Y, the higher acetone conversion rates can be attributed to its faster diffusion. When both ketones are cofed a change in consumption rate highlights the preference for cyclopentanone activation, [x] = [C] or [A] (see Figure). As site titration progresses, a shift in selectivity occurs. While [x]A products are favored in more diffusion limited regimes, [x]C products are preferentially formed with low site density, i.e. kinetically controlled regimes. Both titration methods result in similar shift in selectivity. However, Na-titration was more effective in determining the desired site density and Thiele modulus for both self- and cross-aldol condensation. While in-situ titration with pyridine is effective in controlling product selectivity, being less reliable to obtain Thiele moduli.

Turning cross-condensation selectivity by controlling mass-transfer limitations in zeolites

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