An exploratory study of the mechanochemical synthesis of layered double hydroxides

Abstract

Layered double hydroxides (LDHs) are clay-like minerals commonly referred to as anionic clays" with a wide range of physical and chemical properties. LDHs often find application in pharmaceuticals, as polymer additives, as additives in cosmetics, as nanomaterial's and in catalysis. This is due to having variable layer charge density, reactive interlayer space, ion exchange capabilities, a wide range of chemical compositions and rheological properties (Forano et al., 2006).

Various techniques exist for the synthesis of layered double hydroxides. These include co-precipitation, the urea method, induced hydrolysis, sol-gel and hydrothermal methods. Many of these produce environmentally unfriendly effluents or by-products, are energy intensive, make use of metallic salts or require inert synthesis environments (Rives, 2001). Limitations associated with these existing processes make LDH synthesis at an industrial level expensive or difficult to achieve. The need for 'green', affordable and repeatable synthesis methods are therefore often sought after.

Recently the use of mechanochemistry as an alternative synthesis technique has gained wide-spread attention. Mechanochemistry involves the breaking and forming of chemical bonds due to an induced mechanical force. Various mechanochemical techniques for the synthesis of LDH materials exist or have been explored. These include methods such as single-step, two-step and mechano-hydrothermal grinding techniques. Grinding methods can be conducted dry, wet or collectively (Qu, Zhang, et al., 2015a). Mechanochemistry has further been used in conjunction with micro-wave energy and ultrasonic irradiation. The use of mechanochemistry as a synthesis method has proven to be promising with successful and unique LDHs produced. Intercalation of unique or complex anions within the interlayer has further been proven possible. The versatility and robust nature of this synthesis method makes it ideal for industrial application.

Although many studies exist it was noted that limited research has been conducted on single-step wet grinding for LDH synthesis and warrants further investigation (Qu, Zhang, et al., 2015a) (Iwasaki,Yoshii, et al., 2012). This was due to factors such as incomplete conversion, difficulties associated with grinding and morphological imperfections. Single step wet milling could be benifi cial as a synthesis procedure as it eliminates hazards associated with dry powder, contains less process steps and is therefore possibly more cost effective and can be conducted batch, semi-batch or continuously due to fluid flow. Throughout the literature research conducted it was further noted that not many different milling devices have been explored. Ball mills, mixer mills and manual grinding were the most common methods used to supply mechanical energy to a system. The study therefore aims to expand on single-step wet synthesis of LDH materials by making use of a different milling device, namely a Netzsch LME 1 horizontal bead mill. The selected mill is designed for wet grinding application and can easily be up-scaled to a commercial batch, semi-batch or continuous process. Raw materials selected were a combination of oxides, hydroxides and basic carbonates. This would eliminate hazardous salt by-products and effluent, promoting 'green' synthesis of LDH materials. It was noted that the synthesis of LDH with the use of these materials have previously proven to be challenging (Qu, Zhang, et al., 2015a).

The study was divided up into two sections namely a 'parameter study' and a 'versatility study'. The 'parameter study' involved exploring the in influence of milling and experimental parameters, such as rotational speed, retention time, solids loading, bead size and jacket water temperature, on the synthesis of Mg-Al LDH. The raw materials selected were MgO and Al(OH)3 combined at a divalent to trivalent cationic ratio of 2:1. The parameters were individually investigated, with the exception of jacket water temperature as it was varied with a change in retention time and a change in rotational speed. Unless stated otherwise or under investigation, parameters were investigated at a set speed of 2000 rpm, jacket water temperature of 30 °C, solids loading of 10 %, retention time of 1 h and with 2 mm yttrium stabilised zirconia beads. Therefore when investigating a specific c parameter, the others remained as stated above.

Comparatively the 'versatility' study further explores the synthesis of Mg-Al, Ca-Al, Cu-Al and Zn-Al LDH by adapting optimal synthesis conditions, derived from existing mechanochemical techniques and methods, to the selected process. These were related to the divalent to trivalent cationic ratio and selected starting materials. Ageing of the samples obtained through the 'versatility study' were further explored to determine if the potential for a two-step commercial process exists. The study was investigated at a set speed of 2000 rpm, jacket water temperature of 30 °C, solids loading of 10 %, retention time of 1 h and with 2 mm yttrium stabilised zirconia beads. Half of the sample collected was subjected to ageing at 80 °C for 24 h under atmospheric conditions.