Complex Crystallization Kinetics of a Mg–Al Hydrotalcite and Their Practical Implications from the Process Point of View

Abstract

Hydrotalcites are an important class of layered materials, displaying ion-exchange, adsorption, and base catalytic properties. The crystallization kinetics for hydrotalcites are however hardly available. Nevertheless, as their reconstruction from the oxides (also called as “memory effect”) is considered a synthesis route, this can be used to study the crystallization phenomena. This note looks at the reconstruction kinetics of a Mg–Al hydrotalcite using previously reported kinetic expressions. It was found that high temperature is beneficial if the process is controlled by nucleation. The temperature effect is less obvious when the process reaches a diffusional control regime. For example, temperature is beneficial to shorten the processing time in a nucleation-regime conversion (e.g., 40%). However, to achieve a high conversion (e.g., 98–99%), an intermediate temperature shows the optimal condition, i.e., shortest processing time. The work shows useful side effects of diffusional control. It also highlights the importance of obtaining the kinetics over the entire range for process optimization and, finally, emphasizes that both parameters in the Avrami–Erofe’ev model impact the time required to achieve a given conversion. Directions for further studies to understand the kinetic-process relationships have been highlighted.

Hydrotalcites as Versatile Functional Materials for Renewable Feedstock Upgrading to Value-added Chemicals

“…For the synthesis of hydrotalcites, inorganic and organic salts can be utilized to assist the formation of lamellar structures as they can release anions. Nevertheless, the crystallization of hydrotalcites does not necessarily require the presence of organic salts such as those used in many zeolites (e.g., alkyl ammonium salts or hydroxides) and so they can be obtained with less complicated synthesis procedures under milder reaction conditions compared to zeolites [8,9] . In addition, they are commonly inexpensive and benign solid base materials.…”