Continuous flow synthesis of ZSM-5 zeolite on the order of seconds

Abstract

The hydrothermal synthesis of zeolites carried out in batch reactors takes a time so long (typically, on the order of days) that the crystallization of zeolites has long been believed to be very slow in nature. We herein present a synthetic process for ZSM-5, an industrially important zeolite, on the order of seconds in a continuous flow reactor using pressurized hot water as a heating medium. Direct mixing of a well-tuned precursor (90°C) with the pressurized water preheated to extremely high temperature (370°C) in the millimeter-sized continuous flow reactor resulted in immediate heating to high temperatures (240-300°C); consequently, the crystallization of ZSM-5 in a seed-free system proceeded to completion within tens of or even several seconds. These results indicate that the crystallization of zeolites can complete in a period on the order of seconds. The subtle design combining a continuous flow reactor with pressurized hot water can greatly facilitate the mass production of zeolites in the future. Z eolites have typically been synthesized via hydrothermal treatment, a process designed to artificially mimic the geological formation conditions of natural zeolites (1, 2). Depending on the targeted framework and composition, several hours to days (or even several weeks) are required to complete the synthesis of a crystalline product (3). As a great improvement over the hundreds or thousands of years required for the formation of natural zeolites, artificial synthesis on the order of days allows us more opportunities to obtain zeolites with tunable properties (4, 5). Nevertheless, the hydrothermal synthesis of zeolites has remained a time-consuming process. Long periods of hydrothermal treatment, for one thing, cause a burden on both energy efficiency and operational costs. Furthermore, pressure-tight vessels used for the hydrothermal treatment as well as harsh conditions therein make mechanistic studies using in situ techniques difficult to perform (6, 7). The complexity of crystallization, in turn, hinders the attempts at shortening the synthesis period. This dilemma explains to some extent the reason behind the long-held belief that crystallization of zeolites is extremely slow in nature.The other major hurdle for the widespread application of zeolites is the difficulty in mass production. Batchwise synthesis using autoclaves is convenient for basic laboratory investigations but poses several challenges--in terms of economic efficiency, quality control, and operation flexibility--when large-scale manufacturing is considered (8). In contrast, continuous flow process has proved to be beneficial in the production of various chemicals (9, 10). Due to the time-consuming hydrothermal treatment used for the synthesis of zeolites, however, making a direct switch from batchwise synthesis to continuous flow manufacturing is not easy. To realize continuous flow production, shortening the synthesis period is therefore an issue of the highest priority. Recently, we have developed an ultrafast route to synthesize indust...

Synthesis of high-performance polycrystalline metal–organic framework membranes at room temperature in a few minutes

“…Another possible route for further reducing the growth duration is by making use of the accelerated growth kinetics at an elevated temperature similar to what was demonstrated recently for the crystallization of the zeolite ZSM-5. 56…”

Continuous flow synthesis of zeolite FAU in an oscillatory baffled reactor

“…(a) An effective seeding to avoid aging stage and heterogeneous nucleation, (b) a well‐tuned synthesis precursor to enable fast crystal growth, (c) fast heating to avoid decomposition of the organic structure‐directing agent, and (d) high temperature to accelerate the crystal growth rate of the target structure . Another approach was proposed by the same group using pressurized hot water (370°C) to increase the reaction mixture temperature (240°C‐300°C). They demonstrated, for the first time, the crystallization of ZSM‐5 zeolite in the order of seconds.…”

“…Continuous flow processes having high solid contents are susceptible to hydrodynamic failures caused by constriction, bridging, and random detachment of solid particles . Liu et al proposed a compressed‐air‐driven vibrator to minimize precipitation or blockage problems. Continuous oscillatory baffled reactors/crystallizers (COBR/COBC) have been used in processes where a crystallization step is involved .…”

Ultrafast synthesis of AFX-Type zeolite with enhanced activity in the selective catalytic reduction of NOx and hydrothermal stability

“…This has led to innovative studies that report efficient methods to produce zeolites in short period of time. [28][29][30][31] Temperature, a signicant factor in zeolite syntheses that govern the rate of nucleation and crystal growth is crucial for shortening the synthesis time of zeolites, which would otherwise take several days to even weeks. Hitherto, the complexity surrounding the zeolite formation mechanism remains unexplained and many studies have suggested that in fact, it is the nucleation that consumes much time rather than the crystal growth.…”

“…Hitherto, the complexity surrounding the zeolite formation mechanism remains unexplained and many studies have suggested that in fact, it is the nucleation that consumes much time rather than the crystal growth. [30][31][32][33][34] Recently, the ultrafast synthesis techniques that involves tubular reactor with the assistance of seed that reduces the synthesis time of zeo-type and zeolites viz., AFI, CHA, MFI, ERI, and MOR down to several minutes were reported by us. 30,[35][36][37][38][39][40][41][42] Table S1 † compares the experimental conditions of these zeolites with the current study.…”