Double Four Ring Units-Containing Zeolites: Synthesis, Structural Modification and Catalytic Applications

In zeolite frameworks, double four-ring (d4r) configurations are among the most frequent composite building units. The composition variations in d4r units greatly influence the energy and structural modifiability of the zeolitic framework. The introduction of germanium, with a larger ionic radius than silicon or aluminum, not only reduces the energy constraints of d4r in the nucleation and crystal growth of zeolites, but also opens a new window for constructing novel crystalline structures, especially with large or extra-large pores and channels. Ge-enriched d4r units endow germanosilicates with structure diversity readily for post treatments. Promising catalytic materials have been gradually developed and increasingly studied by direct synthesis or post-synthetic isomorphous substitution for Ge. This review focuses on the recent progress in the synthesis, modification, and catalytic application of d4r-containing zeolites, including germanosilicates, aluminosilicates, and silicates.