Title : Alkali-Free NiAl hydrothermally reconstructed layer double hydroxide
Abstract:
ecent developments in heterogeneous hydrotalcite (HT) synthesis have led to a proliferation of studies on various preparation steps and its application in many ways. Nowadays, transition metals such as Ni are receiving great interest from researchers due to the promising advantages they possess such as good conductivity, eco-friendliness and inexpensive catalysts, and having the ability to produce various types of host structure with high proton mobility. In terms of catalytic performance, Ni catalysts are preferable due to their active phase especially in hydrogenation reactions, steam reforming, depolymerisation of lignin and their ability to be applied as electrochemical capacitors and highperformance supercapacitors. Nevertheless, the catalytic activity and lifetime of Ni catalysts could vary depending on their physical and chemical properties. Nano-sized NiAl Layered Double Hydroxides (LDH) has gained great attention and been applied in a variety of applications due to its speciality to reconstruct the lamellar structure after calcinationrehydration process. NiAl LDH has been predicted to be hard and challenging to reconstruct due to the rigid formation of mixed oxide after the calcination process. Due to that, a dearth of studies into the reconstruction of NiAl LDH has been reported compared to its mixed oxide which has better stability exhibited from the Lewis basic site. Nevertheless, calcined mixed oxide LDH has a drawback which attributes that include lower catalytic activity. Rehydration alternatively exhibits Bronsted base OH-1 species which are highly favourable for base catalysed reactions. Generally, reconstruction of NiAl LDH is possible yet needs a special condition such as controlling the calcination temperature or the reconstruction time. In addition to the above reasons, it is important to find an alternative to synthesise and rehydrate NiAl LDH to obtain good morphologies, structural properties and at the same time to improve the catalytic performance. In this study, NiAl LDH has been prepared and reconstructed under hydrothermal conditions. A family of NiAl HT catalysts were synthesised with different Ni:Al atomic ratios ranging from 1.5:1 to 4:1 (pH 9.5) via the green free-alkali hydrothermal reconstructed method. NiAl LDH underwent calcination at 350 °C followed by reconstruction under a hydrothermal process. The successful synthesis has been confirmed via a range of characterisation techniques. Catalyst later has been tested in the transesterification reaction involving lower to bulkier TAG (C4 -C18). The catalytic activity of reconstructed NiAl LDH has been studied and thoroughly investigated.
