Direct Synthesis of Highly Siliceous ZnO-FAU Zeolite with Enhanced Performance in Hydrocarbon Cracking Reactions

Abstract

The hydrothermal stability and catalytic activity of zeolite Y (faujasite, FAU) is highly dependent on its composition. High silicon content is often desirable for catalytic applications; however, direct synthesis of faujasite with high silicon content (Si/Al > 2.5) is nontrivial. Here, we present an organic-free synthesis of FAU-type zeolite with Si/Al = 3.4 using zinc oxide as a modifier. A combination of spectroscopy and microscopy techniques confirms that ZnO is well-distributed within zeolite pores as extra-framework species, and the nature of these species differs from bulk ZnO and framework zinc in Zn-FAU crystals. We demonstrate that the increased Si/Al ratio leads to improved hydrothermal stability, while catalytic cracking of 1-hexene and cumene show that ZnO-FAU exhibits a significantly longer lifetime compared to in-house and commercial zeolite Y. Collectively, this study presents a facile and efficient method to prepare more siliceous FAU with enhanced catalytic performance.

Acetate Ions Facilitated Immobilization of Highly Dispersed Transition Metal Oxide Nanoclusters in Mesoporous Silica

“…When compared to reference TMO synthesized using the precipitation method (Figures S18 and S21), the observed changes in optical properties of the obtained TMO nanoclusters/SBA-15 materials primarily occurred in the visible region (Figure S17G). These changes included variations in absorption strength, red shift of characteristic peaks induced by the quantum confinement effect, and the appearance of surface hydroxy signals, all of which suggest the successful immobilization of hydroxylated TMO nanoclusters within the mesopores. − This immobilization of various TMO nanoclusters led to the generation of acidic sites on the treated mesoporous silica (Figure S17H). XPS spectra (Figure S17I,L) confirmed the presence of dispersed TMO nanoclusters with valence states such as Zn­(II), Mn­(II/III/IV), Co­(II/III), and Ni­(II).…”

The Current Understanding of Mechanistic Pathways in Zeolite Crystallization

“…The addition or substitution of heteroatoms into zeolite growth mixtures has led to disparate effects on synthesis mixtures, which include altering crystallization kinetics, ,, crystal size, ,, phase selection, ,, and other phenomena such as IZT. These effects are in addition to that which is generally of greatest interest and the reason why heteroatoms are typically added to growth mixtures: the ability of heteroatoms to alter acid properties, including density, strength, zoning, or siting. ,− Similar to the effects of Si and Al source selection, the choice of heteroatom reagent has the potential to direct growth pathways from classical to nonclassical (or vice versa) . Investigation of these systems has made it clear that the effects of heteroatoms at a molecular level are manifold, influencing electrostatics, oligomerization, nanoparticle stability, metallosilicate speciation, ,,− bond angles, , OH – consumption, and precipitation of impurity phases, ,,, among others.…”