Abstract
Germanosilicate zeolites with extra-large-/multidimensional pore systems have a high potential for catalytic applications. However, their insufficient hydrothermal stability, high cost, and lack of strong acid sites limit their use.
This work presents a synthetic approach involving post-synthesis degermanation/germanium recycling and remetalation steps for the cost-efficient preparation of Bronsted and Lewis acid zeolite catalysts. Optimization of degermanation conditions (i.e., pH and duration of the leaching treatment) allowed to recover up to 78-94% germanium from ITH, IWW, and UTL zeolites.
Further metalation of hydrolyzed IWW zeolites resulted in a set of Al-, Ti-, and Sn-substituted catalysts showing enhanced activity in model acid-catalyzed reactions, such as 1-hexanol tetrahydropyranylation, 1-octene epoxidation, and Baeyer-Villiger oxidation of cyclohexanone. Noticeably, the phase selectivity of zeolite formation upon germanium recycling strongly depended on the method for parent zeolite separation from the leaching solution.
In contrast to microfiltration, which produces a versatile source of germanium for the preparation of various zeolites, filtration leads to the formation of germanosilicates with the topology of the parent zeolite regardless of recycling conditions. Such a "memory effect" was rationalized based on the characterization of the germanium source and crystallization products using a combination of techniques (e.g., X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy).