Synthesis of layered double hydroxides (LDHs) with varying pH: A valuable contribution to the study of Mg/Al LDH formation mechanism

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Abstract

Very few papers deal with comprehension of mechanisms leading to the formation of layered double hydroxide (LDH) compounds which are usually obtained by coprecipitation at constant pH value. In this paper, we study the influence of pH values of coprecipitation on the formation of LDH phases, their crystallinity and chemical composition, and then discuss about LDH formation mechanisms, using a synthesis route with varying pH. We clearly show that LDH phases can be obtained by such a method, at pH values from 10 to 13.2. A rich aluminium specie is firstly obtained which tends to continuously incorporate Mg until LDH species with the targeted Mg/Al ratio are obtained.

Introduction

Due to their interest for catalysis and environmental applications, layered double hydroxides (LDHs) are the focus of many studies [1], [2], [3], [4], [5], [6]. These compounds have the general formula: [M(II)1−xM(III)x(OH)2][An]x/n·mH2O where M(II) are bivalent cations such as Mg2+, Fe2+, Zn2+, etc., M(III) are trivalent cations such as Al3+, Fe3+, Cr3+, etc. and An are anions [1]. Anions are localised in the interlayer space of LDHs to counterbalance the positive charge induced through the substitution of a part of bivalent cations by trivalent cations in the hydroxide layers. Even if LDHs, also known as anionic clays, are the focus of many synthesis studies [1], [2], [3], [4], [5], [6], very few papers deal with comprehension of the mechanisms leading to the formation of LDH compounds [7], [8]. Moreover, among the different synthesis routes available, coprecipitation at constant pH value is mainly used [1], [2], [3], [4], [6]. Very few papers deal with LDH synthesis using variable pH [5], [6].

In this paper, we study the influence of pH values of coprecipitation on the formation of LDH phases, their crystallinity and chemical composition and we discuss about LDHs formation mechanisms. Using a synthesis route with varying pH, we have studied the formation mechanism of LDHs, and distinguished several reactions occurring at different pH values.

Section snippets

Materials and methods

All the syntheses have been realised using deionised water. A first aqueous solution, containing 0.1 mol of aluminium and 0.3 mol of magnesium, has been made using aluminium nitrate (Prolabo, 98% purity) and magnesium nitrate (Prolabo, 96% purity). A second solution has been made introducing 280 ml of a 3.5 M NaOH solution (NaOH solid from Panreac (98%)) in 1000 ml of a 1 M Na2CO3 solution (Na2CO3 solid from Labosi).

Syntheses have been performed using the varying pH method, introducing the solution

Results and discussion

LDH compounds are considered to be issued from a phenomenon of coprecipitation of trivalent and divalent cations at high pH values [1], [9]. In most of the studies, the synthesis of these compounds is realised at a fixed pH value of 10 [1]. This method generally allows obtaining well-crystallised LDHs but does not allow understanding the formation mechanism of these compounds. Using a synthesis route with varying pH allows studying the evolution of both mother liquor and solids formed and thus

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