Pioneering In Situ Recrystallization during Bead Milling: A Top-down Approach to Prepare Zeolite A Nanocrystals

Top-down approach has been viewed as an efficient and straightforward method to prepare nanosized zeolites. Yet, the mechanical breaking of zeolite causes amorphization, which usually requires a post-milling recrystallization to obtain fully crystalline nanoparticles. Herein we present a facile methodology to prepare zeolite nanocrystals, where milling and recrystallization can be performed in situ. A milling apparatus specially designed to work under conditions of high alkalinity and temperature enables the in situ recrystallization during milling. Taking zeolite A as an example, we demonstrate its size reduction from ~3 μm to 66 nm in 30 min, which is quite faster than previous methods reported. Three functions, viz., miniaturization, amorphization and recrystallization were found to take effect concurrently during this one-pot process. The dynamic balance between these three functions was achieved by adjusting the milling period and temperature, which lead to the tuning of zeolite A particle size. Particle size and crystallinity of the zeolite A nanocrystals were confirmed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and water adsorption-desorption. This work presents a pioneering advancement in this field of nanosized zeolites, and will facilitate the mass production as well as boost the wide applications of nanosized zeolites.


Supplementary
Comparison of various established techniques with in situ recrystallization methodology.

Table S2
Influence of temperature, time and external recrystallization on crystallinity, average particle size, and yield of zeolite A samples prepared by in situ recrystallization during bead milling.

Figure S1
Schematic representation of the in situ recrystallization during bead milling.

Figure S2
Influence of time and external recrystallization on zeolite A particle size and crystallinity at 30 °C: a, Powder x-ray diffraction patterns with crystallinity data for raw, 30°C_ymin and 30°C_ymin_2h zeolite A samples. b, SEM images of 30°C_ymin zeolite A samples.

Figure S3
SEM images of 30°C_ymin_2h zeolite A samples.

Figure S5
Influence of time on zeolite A particle size and crystallinity at 45 °C: a, Powder x-ray diffraction patterns with crystallinity data for raw and 45°C_ymin zeolite A samples. b, SEM images of raw and 45°C_ymin zeolite A samples.

Figure S6
Influence of 2 h external recrystallization on zeolite A particle size and crystallinity of 45°C_ymin_2h zeolite A samples: a, Powder x-ray diffraction patterns with crystallinity data for raw and 45°C_ymin_2h zeolite A. b, SEM images of raw and 45°C_ymin_2h zeolite A samples.

Figure S7
Influence of time on zeolite A particle size and crystallinity at 60 °C: a, Powder x-ray diffraction patterns with crystallinity data for raw and 60°C_ymin zeolite A samples. b, SEM images of raw and 60°C_ymin zeolite A samples.

Figure S8
Influence of 2 h external recrystallization on zeolite A particle size and crystallinity of 60°C_ymin_2h zeolite A samples: a, Powder x-ray diffraction patterns with crystallinity data for raw and 60°C_ymin_2h zeolite A. b, SEM images of raw and 60°C_ymin_2h zeolite A samples. Si/Al = 1.0, cation: Na + ) and NaOH was purchased from Wako Pure Chemical Industries, Ltd.
All chemicals were used as received without further purification. Deionized water from a Millipore water purification system was used in all experiments.  We calculated the average particle size from SEM images of samples by measuring the individual sizes of a few hundred particles and taking the average of the same. The yield of the final products were calculated using the following equation: where, 'Y' -Yield; 'X' -total weight of zeolite present in initial slurry; 'X1' -weight of final sample after recovery by washing and drying; 'Z' -total weight of the initial slurry; 'Z1' -total weight of withdrawn slurry. h. Aliquots were collected at intervals of 30 min, 60 min, and 120 min during each in situ run, leading to a total of nine samples. Each sample was divided into two equal parts: one was processed for sample recovery and the other was subjected to external recrystallization for 2 h at a temperature matching the temperature at which the sample was drawn during the experiment.
Thus, a total of 18 samples were obtained from the two aforementioned stages. In the process of recovering from the final slurry, samples were washed thrice with pure water to remove NaOH and dried overnight at 80 °C. Samples from the in situ stage were labelled as x°C_ymin, where x denotes the in situ system temperature in °C and y denotes the in situ system time in min.
Similarly, samples from the second stage were denominated x°C_ymin_2h, where x and y retain their meanings used in the labelling system for the first stage and 2h represents the 2-h period of the external recrystallization performed on the samples after they were collected from the in situ system. For example, 30°C_30min denotes the zeolite A sample recovered directly after the in situ recrystallization during bead milling at 30°C for 30 min, whereas 45°C_60m_2h represents the zeolite A sample first treated by in situ recrystallization during bead milling at 45 °C for 60 min. This is followed by 2 h of external recrystallization at 45 °C after it was collected from the in situ system. Furthermore, an ex situ post-milling recrystallization was performed at 30°C using 30 min milled zeolite A powder for extended period of time with 2M NaOH.  a) Derived from XRD peak fitting results and calculated according to Equation (S1); b) Derived from high-magnification SEM images Figure S1. Schematic representation of in situ recrystallization during bead milling. Figure S2. Influence of time and external recrystallization on zeolite A particle size and crystallinity at 30 °C: a, Powder x-ray diffraction patterns with crystallinity data for raw, 30°C_ymin, and 30°C_ymin_2h zeolite A samples. b, SEM images of 30°C_ymin zeolite A samples. Figure S2a shows a decrease in the crystallinity of the 30°C_ymin and 30°C_ymin_2h samples as time is increased from 30 to 120 min. Figure S2b shows the influence of time on the particle size of the 30°C_ymin samples. Figure S3. SEM images showing the influence of external recrystallization on the particle size of 30°C_ymin_2h zeolite A samples. Figure S3 confirms that the morphology and particle size of the 30°C_ymin_2h samples are not