(38e) Investigating the Impact of AlOx Atomic Layer Deposition on the Properties and Flow Behaviour of ZSM-5 Zeolites Toward Improved Powder Processing

Zeolites are a class of microporous crystalline materials with regularly arranged pore structures and tunable chemical functionalities. These exceptional properties make them extremely valuable for various industrial applications, including catalysis, separation, and adsorption ^[1][2]. Throughout their production process, the properties of zeolite undergo significant transformations influenced by various particle handling techniques, including grinding, calcination, drying, storage, discharge, and transport, alongside specific processing parameters ^[3]. Understanding the flow behaviour of zeolites becomes imperative for optimizing these procedures, thus enhancing the quality of the final product, and maximizing process efficiency. While the influence of particle properties like particle size and shape on powder flow is well-studied, identifying the interplay between different factors governing powder behaviour remains elusive. In this aspect, the surface attributes of particulate solids, such as chemical composition and surface roughness, play a profound role in powder flow behaviour ^[4-6]. Nevertheless, there is a lack of comprehensive understanding regarding how surface modification and the resulting alteration of material properties can influence interparticle interactions within zeolite powders, with limited prior research ever dedicated to this aspect.

Over the past few years, Atomic Layer Deposition (ALD) has emerged as a powerful tool for modifying the surface characteristics of catalytic substances like zeolites ^[7]. ALD is a self-limiting method for atomic-level deposition of nanoparticles and film growth. Depending on the desired application ALD is often applied to regulate zeolite properties such as their surface acidity, defect concentration, and hydrophilicity. In this work, we aim to investigate how the ALD process of AlO_x on ZSM-5 zeolites affects the particle properties and its consequent effect on powder flow behaviour to leverage the understanding of the multifaceted factors governing this process.

AlO_x was deposited on the parent ZSM-5 to create four samples of increased layering (1, 2, 5 and 10 layers) and the materials were examined and referenced to the bare zeolite using a variety of advanced characterisation techniques. Inverse gas chromatography (IGC) was employed to determine the surface energetics of the samples through the injection of a series of non-polar (pentane, hexane, heptane) and polar (dichloromethane and ethyl acetate) probes ^[8]. The moisture sorption characteristics of the samples were examined at relative humidities ranging up to 90%, using dynamic vapour sorption (DVS) ^[9]. A surface elemental analysis was performed using X-ray photoelectron spectroscopy (XPS), and the results were compared with the bulk composition of the materials using X-ray fluorescence (XRF). N₂ adsorption for BET surface area and pore volume determination, true and bulk density measurements, particle size analysis, and scanning electron microscopy (SEM) were also utilised to comprehend the alterations in physicochemical characteristics and surface chemistry induced by the ALD process. Moreover, the flowability of the five powders was examined using an Anton Paar shear cell for consolidation stresses ranging from 1 kPa to 5kPa. The experiment obtained stress data for normal and shear stress at different incipient failure points and performed shear tests at three different applied normal stresses - 30%, 50%, and 70% of the pre-shear normal stress - to determine the yield locus of each powder sample. This enabled the calculation of the flow function coefficient (ff_c) at different applied pressures. A statistical analysis was carried out using JMP software to examine the correlations between the physicochemical and flow properties of the samples.

It was found that the surface chemistry modification resulting from the ALD significantly affected the bulk and surface properties of the samples, leading to an improvement in powder flow with the increase in AlO_x layering (Figure 1). The ALD treatment caused a decrease in the total surface area, the micropore area and the pore volume of the zeolites (Figure 2). The parent sample possessed a surface area of 407 m²/g, which was reduced by 13% with the 10^th layer coating. Similarly, micropore area and pore volume decreased by 11% and 17%, respectively. This change was also reflected in the reduction of the moisture uptake of the samples with the increase of AlO_x layers. Furthermore, the ALD modifications led to a substantial reduction in both surface silica to alumina ratio (SAR) and bulk SAR, with a notable decrease of 97% and 56%, respectively, following the addition of the 10th layer (Figure 3). While primary particle size and particle morphology seemed to remain unaffected after the ALD, the introduction of AlO_x appeared to weaken interparticle cohesion, as evidenced by variations in the particle size distribution among the samples. These findings prompt ongoing investigation through surface energy studies to provide deeper insights into the observed effects.

It was found that there is a strong positive relationship (r=0.88) between the number of AlO_x layers and the flow function coefficient (ff_c). Strong correlations have also been established between various zeolite surface properties, and their powder and bulk characteristics (Figure 4). For instance, the surface area negatively correlated with the ff_c at 1kPa of consolidation stress (r=-0.88). The decrease in surface area decreased the contact area between particles, hence reducing interparticle friction and facilitating flow. Similarly, the pore volume and moisture uptake at 90% RH developed a negative trend with the ff_c (r=-0.83 and r=-0.94). These results suggest that decreasing the pore volume of zeolites and, as a result, their water sorption capacity potentially lessened the available space for water molecules to accumulate, minimizing the capillary forces among particles and improving the flow. Further experiments are currently being conducted to confirm the observed trends.

In summary, this research underscores the role of the ALD modification on the zeolite’s particle properties and in determining powder flowability, which can play a pivotal role in optimizing industrial processes and improving product quality. Increasing the ALD coating resulted in a significant alteration of the physicochemical attributes of zeolites, thereby enhancing their powder flowability.

References

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