Abstract
Zn–Al layered double hydroxides (LDHs) of general formula [Zn2+(1−x)Al3+x(OH)2]x+(CO32−)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.
Highlights
IntroductionBiodiesel is an attractive renewable liquid transportation fuel when derived from non-edible planBt i[o1]doiersaellgiasl aonilsa[t2tr]a, catnivime arlefnaetws [a3b],leorliwquaisdtetcraonoskpinogrtoaitlio[4n],faunedl wcahnenbeduesreivdeads farostmanndoanlo-nedeifbuleel polranbtle[1n]doerdawlgiatlhopilest[r2o]l,eaunmim-daelrfiavtesd[3d],ieosrewl [a5s–t7e]c. oCookminmg eorilci[a4l],raonudtecsatnobbeioudseiedseals, awshtaicnhdacolomneprfiuseels ofratbtlyenadceidd wmitehthpyetlroelsetuemrs-d(eFrAivMedEds)i,eseeml [p5–lo7y]
Diffraction patterns characteristic of the hexagonal unit cell of layered double hydroxides (LDHs) [38] were observed in all cases (Figure 1), with sharp, intense reflections at 2θ = 11◦, 23◦, 35◦, 39◦, 47◦, 60◦, and 62◦ corresponding to
Zn4Al2(OH)10(CO3)·xH2O is reportedly the most stable LDH composition formed by hydrothermal aging of as-synthesised Znx–Al-LDHs, regardless of the initial composition [36,40], which might suggest complete reconstruction would be favoured by Zn–Al-LDHs with Zn:Al ratios ≤ 2:1
Summary
Biodiesel is an attractive renewable liquid transportation fuel when derived from non-edible planBt i[o1]doiersaellgiasl aonilsa[t2tr]a, catnivime arlefnaetws [a3b],leorliwquaisdtetcraonoskpinogrtoaitlio[4n],faunedl wcahnenbeduesreivdeads farostmanndoanlo-nedeifbuleel polranbtle[1n]doerdawlgiatlhopilest[r2o]l,eaunmim-daelrfiavtesd[3d],ieosrewl [a5s–t7e]c. oCookminmg eorilci[a4l],raonudtecsatnobbeioudseiedseals, awshtaicnhdacolomneprfiuseels ofratbtlyenadceidd wmitehthpyetlroelsetuemrs-d(eFrAivMedEds)i,eseeml [p5–lo7y]. Mg–Al-LDHs (usually termed hydrotalcites) have proven highly active and tunable catalysts for triglyceride transesterification, with alkali-free co-precipitation from nitrate precursors using NH4CO3 and NH3OH as pH regulators reported [25,26]. Urea hydrolysis offers an alternative route to LDHs [28], such as [Zn0.67Al0.33(OH)2](CO3)0.165·0.5H2O, but requires post-modification with Na2CO3 to produce a pure LDH phase [22,29], which again risks complications from entrained alkali. It is, surprising that the use of NH4CO3 and NH3OH precipitants has not been extended to other M2+:Al3+ combinations utilised in LDH synthesis [25]. Liquid and vapour phase water reconstruction is known to influence the structure and performance of calcined Mg–Al-LDHs, no such comparative studies exist for Zn–Al-LDHs for transesterification. Hydrothermal reconstruction generates the most active catalysts for the transesterification of model C4–C18 triglycerides with methanol, with base site loadings and activity increasing with Zn content
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