(217b) Effect of Processing on Physicochemical Properties and Powder Flow Performance of Zeolites

Zeolites are hydrated aluminosilicates composed of a crystalline structure of joined silicon oxide (SiO₄) and aluminium oxide (AlO₄) tetrahedra in a three-dimensional network. Thanks to their unique structure of regularly spaced cages and channels of precise sizes and shapes, zeolites can absorb and interact with various elements based on their size, shape, and chemical characteristics. Their high surface area, acidity and excellent thermal stability make them indispensable materials in a wide range of applications in adsorption, ion exchange, and catalysis ^[1]. Zeolites are produced in batches, and the manufacturing process involves several steps (Figure 1). The zeolite powder, obtained after drying, undergoes high-temperature calcination to create the necessary porosity for its various applications. This process causes a significant change in the physical and chemical properties of the zeolites, which ultimately affects the flow behaviour of the powder ^[2].

Despite the availability of many techniques for characterizing the physical and chemical properties of zeolites, the comprehensive understanding needed for optimizing manufacturing design and resource utilization during scale-up is limited by the absence of a feedback loop between single particle properties and bulk properties. Although research primarily applicable to pharmaceutical powders has examined how processing affects particle properties like surface (roughness, surface energy, surface area), size and shape ^[3-6], there remains a gap in similar investigations applied to zeolites, posing significant challenges in predicting the feasibility of utilizing small-scale prepared zeolites on a large scale. In this study, by using zeolites with specific properties, we aim to develop a linkage of various micro-/single properties of zeolites to their bulk powder behaviour under thermal processing (drying and calcination).

Two commercial and eight Johnson Matthey synthesized ZSM-5 zeolites; four after drying and four after calcination (Figure 1), were studied for their physical and chemical properties, including surface energy, surface area, surface chemistry, particle size, density, morphology, and crystal structure. A range of different characterisation methods was employed, such as X-ray techniques (XRD, XPS, XRF), N₂ adsorption, helium pycnometry, imaging techniques (SEM) and particle size analysis to examine the samples before (as made) and after the calcination (calcined) stage. The dispersive surface energy of the samples was determined using inverse gas chromatography (IGC) through the injection of a sequence of non-polar alkane probes (C₅-C₈). Additionally, flow measurements of the zeolite powders were conducted to understand their flow behaviour under dynamic and shear conditions. A stability and variable flowrate test was performed using an FT4 powder rheometer to determine the basic flowability energy (BFE) of the samples at varying tip speeds during the downward movement of the blade through the powder bed. For the shear testing, an Anton Paar shear cell was used to determine the flow function coefficient (ff_c) of the samples composed of 5 points (1, 2, 3, 4 and 5 kPa). Shear steps were taken at three applied normal stresses, at 30%, 50%, and 70% of the preshear normal stress, to determine every yield locus.

Regardless of the preparation history of the samples, the process of calcination resulted in a significant increase in their surface area and porosity. The average surface area of as-made samples was 54 m²/g, which increased to 410 m²/g after calcination. Similarly, porosity increased by ̴ 20% with this process. This was in line with the expected result of the organic template being removed from the zeolite structure. The surface energy of the samples also increased after calcination by an average of ̴ 98 mJ/m². The bulk and surface properties of zeolites 79A and 79B appeared to be significantly influenced by the source of aluminium used during production. For sample 79A, Al(NO₃)₃•9H₂O was used as the Al source, whereas sample 79B was synthesised with Al(OH)₃. Both materials differed in terms of particle size distribution, surface area, and surface energy yet exhibited the same gel SAR (~90) and percent crystallinity (100%) (Figure 2). N₂ adsorption analysis of the zeolites before and after calcination showed that 79B had a greater external surface area, surpassing 79A by a margin of 143% pre-calcination and 26% post-calcination (Figure 2), while maintaining a closely similar microporous area. This suggests that agglomeration evolved during calcination, which was more prominent for zeolite 79B than 79A. This was accompanied by a sharper increase in its surface energy post-calcination increasing by 149% in contrast to 15% for sample 79A.

Powder flow measurements of the materials indicated that samples T840 and 82A as made exhibited the best flow performance with average BFE values of 43 mJ and 60 mJ, respectively. This was attributable to their larger particle sizes (D50 ~10µm) in comparison to the other samples. 79A and 79B as made demonstrated the poorest flow (BFEs of 264 mJ and 330 mJ), which is in line with their small particle size (D50 ~1µm). Examining the powder flow behaviour using shear testing suggested comparable trends (Figure 3). Except for T840 and 82A which exhibited a free flowing flow (ff_c ̴ 10), most samples fall into the very cohesive region (1<ff_c<2) (Figure 3). Calcination had a significant effect on flow behaviour, leading to decreased BFEs, or in other words an increase in the ff_c and enhanced powder flow. The initial findings indicate a correlation (r=-0.8267, p=0.0114) between the enhancement in powder flowability and the increase in zeolite porosity through calcination. This was further supported by the decrease in bulk density observed after calcination, which positively correlated with the BFE (r=0.8447, p=0.0083), leading to improved flow. Additional experiments are underway to validate the observed differences by exploring binary zeolite mixtures under high-temperature conditions.

In summary, this work highlights the effect of particle handling methods and processing, mainly calcination, on zeolite properties during manufacturing. Analysis of zeolites with different processing histories and forms showed that surface energy, surface area, and porosity increased after calcination. Zeolites 79A & 79B prepared with different Al sources exhibited differences in surface properties. Particle properties exerted an impact on the flow behaviour under various conditions, emphasizing the critical role of physicochemical characteristics in determining or even predicting powder performance at an industrial scale.

References

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