Facile synthesis of NiAl-LDHs with tunable establishment of acid-base activity sites

NiAl-LDHs with tunable acid-base sites were fabricated by hexamethylenetetramine(HMT) hydrolysis method, and acid-base sites were probed in synthesis of furfural diethyl acetal from furfural and ethanol. XRD, FT-IR, XPS, EXAFS, PALS, NH3-TPD, Pyridine-IR, ICP/AES and CHN element analysis were used to investigate the composition, structure, and origination of the acid-base activity sites of NiAl-LDHs. The result showed that nitrate intercalated LDH (marked as NiAl-NO3-LDH) and carbonate intercalated LDH (marked as NiAl-CO3-LDH) could be tunably fabricated under different synthetic pH. The acid-base sites of NiAl-CO3-LDH and NiAl-NO3-LDH presented significant difference. NiAl-NO3-LDH expressed preferable acid catalytic property with fine acid activity sites. However, NiAl-CO3-LDH presented inert activity in the acid catalysis reaction due to the absence of appropriate acid activity sites. Further investigations proved that the appearance of proper oxygen vacancy arising from low coordination Ni2+ in NiAl-NO3-LDH contributed to the establishment of Lewis acid sites. Accordingly, the intrinsic law of synthetic pH orienting the establishment of acid-base activity sites was revealed.