(509cn) Integrating Nanostraws in Porous Catalyst Supports to Enhance Molecular Transport to Catalytic Sites

In many porous catalyst supports, the accessibility of interior catalytic sites to reactant species could be restricted due to limitations of reactant transport through pores comparable to reactant dimensions. The interplay between reaction and diffusion in porous catalysts is defined through the Thiele modulus and the effectiveness factor, with diffusional restrictions leading to high Thiele Moduli and reduced effectiveness factors and a reduction in the observed reaction rate. We demonstrate a method to integrate ceramic nanostraws into the interior of the ordered mesoporous silica MCM-41to mitigate diffusional restrictions. The nanostraws are the natural aluminosilicate tubular clay minerals known as halloysite. Such halloysite nanotubes (HNT) have a lumen diameter 15-40 nm which is significantly larger than the 2-4 nm pores of MCM-14, thus facilitating entry and egress of larger molecules to the interior of the pellet. The method of integrating HNT nanostraws into MCM-41 is through a ship-in-a-bottle approach of synthesizing MCM-41 in the confined volume of an aerosol droplet that contains halloysite nanotubes. The concept is applied to a system in which microcrystallites of Ni@ZSM-5 are incorporated into MCM-41. Using the liquid phase reduction of nitrophenol as a model reaction catalyzed by the Ni@ZSM-5, we show that the insertion of HNT nanostraws into this composite leads to a 50% increase in the effectiveness factor. The process of integrating nanostraws into MCM-41 through the aerosol-assisted method is a one-step facile method that complements traditional catalyst preparation techniques. This reproducible and scalable synthesis technique to the mitigation of diffusional restrictions has implications to catalyst and separation technologies.