In many catalytic applications of zeolites, diffusion limitations and pore blockage are serious limitations to their efficient use. Mesopores are therefore introduced, but typically in a poorly controlled way. Recently, several strategies have been proposed to synthesize zeolites with a controlled mesoporosity. However, reliable and reproducible methods to create truly meso-structured zeolites remain a challenge. A crystallization method has been used to convert SBA-15 into a zeolitic material, but the collapse of the meso-structure is hard to avoid. Another possibility to introduce mesoporosity into zeolites is to use mesoporous carbon as a template. Here, we explore a new approach based on a combination of a carbon templating method and a crystallization method.

Three different synthesis routes were followed, each with the objective to obtain SBA-15/silicalite-1 composites. The materials were characterized by XRD, N2 adsorption/desorption, and TEM. In this presentation, we will especially focus on a method that involves SBA-15 with a well-developed carbon structure in its mesopores. This method shows promise, as the carbon template may improve the structural stability and preserve the mesoporosity during the crystallization process. However, TEM typically shows a fine mixture of a zeolite phase and a mesoporous phase, rather than an embedded, hierarchical structure. Despite some claims in the literature, we will explain why we question the possibility to synthesize such an embedded SBA-15/zeolite structure. Nevertheless, for catalytic purposes, the latter may not be needed. We discuss the diffusion properties of the new materials, as well as the potential to use SBA-15/zeolite composites for catalysis.