A Effect of Synthetic Parameters on Crystallinity of Hydrotalcite-Like Anionic Clays with Elucidation and Identification through X-Ray Diffraction Analysis

Abstract

Hydrotalcite-like anionic clays (HTs) which are referred to as Layered double hydroxides (LDHs) are an important class of ionic lamellar solids. Their structure consists of positively charged brucite [Mg(OH)₂]-like layers along with charge compensating anions and water molecules in the interlayer space (fig. 1). These materials provide numerous advantages such as economical and simple synthetic methods, easy separation from the reaction mixture, recycling potential, according to the Green Chemistry point of view. HTs/LDHs also possessed various unique features such as anion exchange capability, self-healing properties, large surface area, Bronsted-Lewis acid, and base sites etc. The XRD technique remains an essential tool in the characterization of many materials including HTs/LDHs. The XRD characterization revealed the hexagonal structure of HTs/LDHs with characteristic peaks as sharp and intense lines for their well-ordered crystalline nature. Generally, XRD diffractograms for HTs show sharper, intensive and symmetric lines at low 2θ values while less intense and usually asymmetric lines are observed at higher 2θ values. Also, the distinctive point of an XRD diffractogram for HTs is a doublet peak at a 2θ value of 60° which indicated an ordered layered material. The d spacing which is known as the interlayer spacing, formed from the metal hydroxide sheet (≅ 4.8 Å), and the gallery region that contains the intercalated anions (≅ 2.8 Å) [1]. However, a number of factors play important role in the crystallinity of HTs like aging time, temperature, pH, nature and composition of divalent and trivalent metal cations and anions etc (fig. 1). During the synthesis of these materials, the crystallinity is smaller and amorphous content larger at the beginning of the reaction. Hydrothermal aging affected the crystallinity of HTs/LDHs and showed high crystallinity with increased aging time. The aging time of HTs/LDHs improved the crystallinity along with their catalytic activities. Besides, microwave irradiation also enhanced the crystalline nature of HTs/LDHs. Additionally, the crystallinity of HTs/LDHs also observed well-ordered when synthesized in alkaline pH conditions (usually between 8-10).

Basal spacing, crystallite size changed with change in anions (CO₃, NO₃, MoO₄, VO₄) as well as the ratio of divalent and trivalent cations [2]. The decrease in metal ion ratio that is the increase in the content of trivalent ion, the crystalline peaks become sharper and more intense in hydrotalcite phase which is interpreted as a greater crystalline perfection. The pH and ratio of metal ion are most considerable factors to produce good crystalline HTs/LDHs with desirable morphology [3]. Moreover, the calcination temperature also affects the crystallinity phase and degree of HTs/LDHs and transformed to oxides between the temperature range 300-500℃. The reconstruction of oxides into respective layered structure of HTs/LDHs in the presence of anions and water known memory effect or self-healing property. While, the calcination at higher temperature leads to permanent decomposition and converted into mixed metal oxides without self-healing properties [4]. The crystallinity and other properties of HTs/LDHs such as morphology, surface area, adsorption capability, pore size distribution etc. are important to determine their performances in intercalations process and various applications including catalysis, adsorption, electrochemical performance, packaging composite material and delivery agent of bioactive and agrochemicals etc. This study highlighted and summarizing the various research reports related to the effect of various factors on the crystallinity and basal spacing and identification through XRD analysis intended to study and understand the crystallinity of HTs/LDHs.