(296c) A Commercially-Viable One-Step Synthesis Method to Prepare MWW Zeolite Nanosheets
Zeolites are shape-selective microporous crystals that have been widely used as heterogeneous catalysts in petroleum and chemical industries.1 However, pore dimensions (4-7 Å) of typical zeolites often encounter diffusion limitations, especially in applications that involve bulky molecules. MCM-22 (MWW type), one of the most investigated two-dimensional zeolites, exhibits properties that can alleviate mass transport-related issues by providing strong accessible acid sites on its exterior surface.2 Conventional layered MCM-22 precursors, or MCM-22(P), result in the direct condensation of MWW layers during calcination, sacrificing significant accessibility to surface acid sites. In this talk, we will discuss an alternative method to avoid layer condensation.
The ability to generate layered MCM-22 traditionally requires post-synthesis treatment with an exfoliating agent, such as surfactants, to disorientate layers of MCM-22(P).3 In this talk, we will describe how we have modified the synthesis of MCM-22 to directly yield MWW nanosheets via a single step, as determined by a combination of powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and gas adsorption measurements. This modified synthesis method increases the external surface area to 257 m2/g as compared to conventional MCM-22 layered materials (106 m2/g). In addition, the coordination environment of Al isotopes was investigated by 27Al magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy after calcination, revealing that the amount of extra-framework Al species in disordered MWW is 7 mol% less than the ones of ordered MWW, thus suggesting the optimized one-step method helps to prevent dealumination. To confirm the advantages of creating higher surface area nanosheets, we performed a cumene cracking reaction to evaluate the catalytic activity of the delaminated MWW, which has a longer lifetime than the ordered one. Collectively, our studies reveal an innovative way to prepare delaminated MWW catalysts for commercial applications in the (petro)chemical industries.
1. Davis, M. E.; Nature. 417 (2002) 813-821
2. Roth, W. J.; Nachtigall, P.; Morris, R. E.; Äejka, J.; Chem. Rev. 114 (2014) 4807-4837
3. Xu, L. et al.; Chem. Mater. 28 (2016) 4512-4521