Abstract
NiAl layered double hydroxides (LDHs) are promising bifunctional catalysts comprising tunable redox and Lewis acidic sites. However, most studies of NiAl LDH employ alkali hydroxide carbonate precipitants which may contaminate the final LDH catalyst and leach into reaction media. Here, we report an alkali-free route to prepare NixAl LDHs with a composition range x = 1.7 to 4.1 using (NH4)2CO3 and NH4OH as precipitants. Activation of LDHs by calcination–rehydration protocols reveal NixAl LDHs can be reconstructed under mild hydrothermal treatment (110 °C for 12 h), with the degree of reconstruction increasing with Ni content. Catalyst activity for tributyrin transesterification with methanol was found to increase with Ni content and corresponding base site loadings; TOFs also increased, suggesting that base sites in the reconstructed LDH are more effective for transesterification. Hydrothermally reconstructed Ni4.1Al LDH was active for the transesterification of C4–C12 triglycerides with methanol and was stable towards leaching during transesterification.
Highlights
IntroductionAre interesting materials for photo-, electro-, and thermocatalysis [1,2] and electronic applications [3–5]
Layered double hydroxides (LDH) of general formula [M2+ (1−x) Al3+ x (OH)2 ]x+are interesting materials for photo, electro, and thermocatalysis [1,2] and electronic applications [3–5]
The successful synthesis of alkali-free NiAl LDH materials was verified by XRD, Scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy detector (EDS)
Summary
Are interesting materials for photo-, electro-, and thermocatalysis [1,2] and electronic applications [3–5] They typically adopt three-dimensional, lamellar structures of mixed metal oxyhydroxide platelets, with the interlayer voids providing nanoporous chemical reactors [6]. Activation of LDH materials is often required to increase their catalytic utility [16] This commonly involves calcination to decompose the precipitated LDH by removing charge compensating anions (accompanied by dehydroxylation) to form an amorphous mixed oxide. Reconstruction of these mixed oxides into lamellar structures is observed on subsequent hydration, termed the ‘memory effect’, which introduces vacancies into the metal oxyhydroxide layers, due to the replacement of interlayer CO3 2− with. We explore the hydrothermal reconstruction and catalytic activity of a family of NiAl LDHs for the transesterification of model (C4-C12) triglycerides to fatty acid methyl esters (FAME)
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