Molecular Modifiers Suppress Nonclassical Pathways of Zeolite Crystallization

There has been significant interest in the area of zeolite synthesis, and more broadly in the development of microporous catalysts, to establish methods of reducing crystal size. A promising approach to tailor crystal habit is the use of organic modifiers, which are molecules capable of altering the anisotropic rates of crystal growth to generate crystalline materials with well-defined size and shape. Here, we examine the putative mechanism by which a modifier alters the growth of silicalite-1, a siliceous zeolite that is often used as a model for fundamental studies of crystallization. The modifier selected for this study is the amino acid arginine, which decomposes in situ to ornithine-lactam with the release of acid. A concomitant reduction in solution pH provides a unique opportunity to switch the growth medium from a solution of soluble silicate species (monomers) to one comprising amorphous silica nanoparticle precursors. These distinct growth units operate by disparate modes of action: monomer addition is a classical process of growth, whereas nanoparticle addition is a nonclassical pathway. We show that the latter mechanism leads to silicalite-1 crystals with longer dimensions along the principal direction of internal diffusion. Our findings also reveal that the growth modifier, ornithine-lactam, induces a reduction in crystal size, not through the conventional mechanism of binding to preferential crystal surfaces, but rather through its ability to impede nanoparticle addition. A combination of bulk crystallization experiments and molecular modeling is used to elucidate the mechanism of zeolite growth modification and to demonstrate that an ability to switch the dominant mode of growth from nonclassical to classical pathways offers advantages of tailoring crystallization to achieve small particle sizes with reduced internal mass transport limitations