A Review on the Effects of Organic Structure-Directing Agents on the Hydrothermal Synthesis and Physicochemical Properties of Zeolites

Abstract

The study on the synthesis of zeolites, including both the development of novel techniques of synthesis and the discovery of new zeolitic frameworks, has a background of several decades. In this context, the application of organic structure-directing agents (SDAs) is one of the key factors having an important role in the formation of porous zeolitic networks as well as the crystallization process of zeolites. There are various elements that are needed to be explored for elucidating the effects of organic SDAs on the final physicochemical properties of zeolites. Although SDAs were firstly used as pore generators in the synthesis of high-silica zeolites, further studies proved their multiple roles during the synthesis of zeolites, such as their influences on the crystallization evolution of zeolite, the size of the crystal and the chemical composition, which is beyond their porogen properties. The aim of this mini review is to present and briefly summarize these features as well as the advances in the synthesis of new SDAs during the last decades.

Synthetic Control of the Defect Structure and Hierarchical Extra-Large-/Small-Pore Microporosity in Aluminosilicate Zeolite SWY

“…OSDA_AlOH, prepared in a solution of (diDABCO-C8)-(OH) 2 , contained crystals in the 0.3−1.0 μm range, while TPA_AliPr and TPA_AlOH, prepared in a TPAOH solution, led to larger crystals (0.8−1.4 μm). Large amounts of organic molecules in the solution have been shown to lead to smaller crystal sizes in other zeotypes, 31,32 but here, the TPA type gels had larger amounts of organic reagents overall (TPA + + diDABCO-C8 2+ ) than the OSDA_AlOH gel. Thus, the decrease in crystal size is likely impacted by the more elongated shape of the diDABCO-C8 2+ cation that is present in higher quantities in the OSDA_AlOH gel, similar to what has been previously observed in MFI crystals synthesized with TPA + or longer derivatives of TPA + .…”

A Comparative Review of Binder-Containing Extrusion and Alternative Shaping Techniques for Structuring of Zeolites into Different Geometrical Bodies

“…Zeolites as crystalline metallosilicates, conventionally in the form of aluminosilicates, have well-defined microporous networks containing regular cavities and channels [5]. Up to now, various excellent features have been reported for zeolites, such as molecular sieving, shape selectivity, high surface area, pore size tuning, high thermal and hydrothermal stability, tunable acidity, crystallinity, and ion-exchange property [6][7][8][9][10][11][12]. However, despite the outstanding features exhibited by zeolites, they are inherently microporous materials (d pore < 1.5 nm) and, thus, suffer from intense diffusion limitations imposed by their micropores, particularly, in contact with bulky molecules [12].…”

Analysis and Augmentation of Guest–Host Interaction Energy Models as CHA and AEI Zeolite Crystallization Phase Predictors

“…As the OSDA increases in size but maintains its overall shape and symmetry, the void space within the zeolite also increases and roughly maintains its structure and connectivity. As the OSDA becomes more asymmetric, with elongated substituents, topologies with pores and channels are formed. − …”