Few‐Unit‐Cell MFI Zeolite Synthesized using a Simple Di‐quaternary Ammonium Structure‐Directing Agent

Abstract

Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively.

What Is the Smallest Zeolite That Could Be Synthesized?**

“…We find that T m ( D ) of these zeolites decreases slowly as the particle size diminishes to about 20 nm, consistent with the experimental finding that the framework energy of zeolites larger than 40 nm is essentially the same as in the bulk crystal [14] . The GT equation predicts that the stability decreases slowly as D decreases from ≈20 to ≈5 nm, capturing well the experimental melting of ≈7 nm silicalite‐1 particles [4] (blue star in Figure 1a).…”

“…The high ratio of area to volume in nanoparticles results in higher solubilities of nanoscopic zeolites compared to their bulk counterparts [6] and a decrease in the temperature of crystal to amorphous melting transition. The latter is evident in the comparison of the ≈1100 °C at which bulk silicalite‐1 loses crystallinity [7] to the ≈750 °C at which that occurs for its ≈7 nm crystallites [4] . What is the particle size at which the zeolite would be less stable than the amorphous precursor at the 60 to 180 °C temperatures typical of hydrothermal synthesis?…”

An environment-friendly and acid-degradable polymer templated synthesis of single-crystal hierarchical zeolites

“…Hierarchical zeolites with multi-modal pore structures have been a subject of great interest for nearly 30 years in both scientic and industrial communities worldwide. [1][2][3][4][5][6][7][8][9] Hierarchical pore architecture in zeolites can reduce the diffusion limitations for converting bulky molecules, enhance the mass transfer of reactants and products, and increase the accessibility of active sites, and therefore endows hierarchical zeolites with dramatically enhanced activity, selectivity and stability. [10][11][12][13][14][15][16] To date, various strategies have been developed for creating meso-/macro-pores in microporous zeolites, including destructive top-down approaches via dealumination and/or desilication, [17][18][19] constructive bottom-up ones by using multifunctional hard 20 /so [21][22][23][24][25][26][27][28][29] macro-/meso-porogens, and seed-directed methods.…”

Coarse-Grained Model for the Hydrothermal Synthesis of Zeolites

“…In the next sections, we present the parameterization of the model and show that it reproduces the experimental speciation of the silicates a function of degree of condensation in the polymerization of silica 32 and is able to spontaneously nucleate and grow a zeolite from the hydrated amorphous phase, as observed in experiments. 14,24,36 The computational efficiency of the model presented here makes it appropriate to investigate the mechanisms involved in the multiple steps involved in the hydrothermal synthesis of zeolites: the formation of NPs, the evolution of these NPs to produce nanozeolites, and their growth by oriented attachment.…”

“…The experimental characterization of these processes along the synthesis of zeolites is challenging. The early stages of silica polymerization are mainly followed by 29 Si NMR and, occasionally, by mass spectrometry. − Furthermore, recent advances in transmission electron microscopy (TEM), cryo-high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy have resulted in unprecedented resolution of the size and shape of the condensed aggregates that result from the polymerization process. ,,− However, these ex situ studies cannot yet follow the evolution of individual amorphous nanoparticles (NPs) into zeolite crystals, and the high temperature and elevated pH of the HS of zeolites make it difficult to follow these processes in situ. Despite the enormous advances in the study of the synthesis of zeolites using advanced imaging and spectroscopies, ,,,− a spatially resolved, molecular-level understanding of the formation of the amorphous phase and its transformation into zeolite crystals is still lacking.…”