Influence of acid–base properties of calcined MgAl and CaAl layered double hydroxides on the catalytic glycerol etherification to short-chain polyglycerols

• One-pot etherification reaction of glycerol towards short-chain polyglycerols.
• Calcined MgAl-LDH and calcined CaAl-LDH were used as catalysts.
• Basicity and acidity of the catalysts influenced the catalytic results.
• Stronger basic sites and lower acidity favoured short-chain polyglycerols.
• Higher temperature of calcination favoured a higher degree of polymerisation.

Several MgAl-LDHs (HT) and CaAl-LDHs (HC) were synthesized in different conditions and later calcined at 723 K, resulting in catalysts (cHT and cHC), with different acid–base properties. Calcined samples were tested in the etherification reaction of glycerol towards short-chain oligomers (di- to pentaglycerol). All catalysts were characterised by XRPD, ICP, N2 physisorption and TEM techniques. The basicity was evaluated using the CO2-TPD technique and the acidity by cyclohexylamine adsorption. Catalytic results were correlated with catalysts properties. Catalysts with higher acidity showed higher conversion (96% for a cHT and 75% for a cHC) but also higher selectivity to other products, with main contribution of acrolein (88% for a cHT and 58% for a cHC). In contrast, catalysts with lower acidity resulted in lower conversion (24% for a cHT and 40% for a cHC) but higher selectivity towards di- and triglycerol (100% and 64%, respectively). The formation of triglycerol and other low-weight polyglycerols (tetra- and pentaglycerol) was favoured in catalysts with low acidity and strong basic sites. When the temperature of calcination was raised to 1073 K for one cHC, the acidity decreased and the number of strong basic sites increased, resulting in higher selectivity to triglycerol (20%), the formation of tetra- and pentaglycerol (15% and 6%, respectively) and the decrease of the acrolein amount.

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