Zn-Al layered double hydroxides (LDHs) of general formula [Zn 2+ (1−x) Al 3+ x (OH) 2 ] x+ (CO 3 2− ) x/2 ·yH 2 O are promising solid base catalysts for the transesterification of lipids to biofuels. However, conventional synthetic routes employ alkali hydroxide/carbonate precipitants which may contaminate the final LDH catalyst and biofuel. The use of (NH 3 ) 2 CO 3 and NH 3 OH as precipitants affords alkali-free Zn-Al-LDHs spanning a wide composition range. The hydrothermal reconstruction of calcined Zn-Al-LDHs offers superior solid basicity and catalytic activity for the transesterification of C 4 -C 18 triglycerides with methanol, compared with cold liquid phase or vapour phase reconstruction. Hydrothermally activated Zn 3.3 -Al-LDH was stable towards leaching during transesterification. Abstract: Zn-Al layered double hydroxides (LDHs) of general formula [Zn 2+ (1x)Al 3+ x(OH)2] x+ (CO3 2− )x/2·yH2O are promising solid base catalysts for the transesterification of lipids to biofuels. However, conventional synthetic routes employ alkali hydroxide/carbonate precipitants which may contaminate the final LDH catalyst and biofuel. The use of (NH3)2CO3 and NH3OH as precipitants affords alkali-free Zn-Al-LDHs spanning a wide composition range. The hydrothermal reconstruction of calcined Zn-Al-LDHs offers superior solid basicity and catalytic activity for the transesterification of C4-C18 triglycerides with methanol, compared with cold liquid phase or vapour phase reconstruction. Hydrothermally activated Zn3.3-Al-LDH was stable towards leaching during transesterification.Scheme 1. Transesterification of triglyceride to biodiesel and glycerol by-product. Scheme 1. Transesterification of triglyceride to biodiesel and glycerol by-product.

Key indicatorsSingle-crystal X-ray study T = 298 K Mean (C-C) = 0.003 Å R factor = 0.045 wR factor = 0.132 Data-to-parameter ratio = 13.4For details of how these key indicators were automatically derived from the article, see

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A facile, rapid, and noninvasive method for reconstructing ZnAl layered double hydroxide (LDH) is reported. ZnAl LDH series were synthesized at different Zn2+/Al3+ atomic ratio (1.5-4) via an alkali-free method and reconstructed under hydrothermal route (HTM) for the first time. Fresh Zn/Al LDHs were activated at 300°C and reconstructed under hydrothermal process. A better insight and correlation study between the physiochemical properties of reconstructed ZnAl LDH in terms of their crystallinity, surface area and basicity also will be gained here. BET surface area of rehydrated samples increased up to 355 m2/g (Zn:Al ratio 3:1). CO2-TPD probed high number of basic sites density (0.1 mmol/g).

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.