Synthesis and catalytic testing of Lewis acidic nano-MFI zeolites for the epoxide ring opening reaction with alcohol

Abstract

Zeolites have well-defined pores that can impart shape selectivity, but can also cause diffusion limitations, reducing catalytic performance. While several strategies have been investigated to overcome diffusion limitations for Brønsted acidic zeolites, limited research has investigated modifying material properties for Lewis acidic zeolites. In this work, diffusion limitations for catalytic reactions are overcome through the low temperature synthesis of nanozeolites containing Lewis acidic tin sites. The growth of the tin-substituted zeolites is monitored using dynamic light scattering, showing that including tin as a heteroatom at a 200:1 (Si:Sn) in the zeolite synthesis gel does not impact the zeolite growth rate. While similar growth rates are observed for pure silica and tin-substituted MFI, the low temperature synthesis methods result in low tin incorporation that can be overcome through adding more tin precursor to the synthesis gel while still producing nanoparticles. The nano-sized zeolites are demonstrated to overcome apparent diffusion limitations that conventional Sn-substituted zeolite MFI encounter for the epoxide ring opening reaction with alcohol. Indeed, conventional Sn-MFI materials can catalyze the ring opening of 1,2-epoxyhexane with methanol and achieve only 55% conversion after 24 h; the nano Sn-MFI zeolites achieve >95% conversion in the same amount of time. Catalyst reuse experiments demonstrate that the nano zeolite does not leach Sn, but that organic accumulation in the pores causes loss in apparent catalytic activity. The organic can be removed through calcination, enabling recovery of the catalytic activity. Overall, engineering of zeolites with reduced crystal size provides an efficient route to overcome diffusion limitations of bulky molecules.