Structure and surface properties of Ni–Al hydrotalcite-like compounds and their catalytic application in highly selective acetalization at room temperature
Ni–Al hydrotalcite-like compounds (Ni–Al HTLCs) containing nitrate anions were synthesized and applied in the acetalization of p-anisaldehyde with ethylene glycol under mild reaction conditions. The Ni–Al HTLCs showed good selectivity for acetalization and achieved almost complete conversion of p-anisaldehyde. The Ni–Al HTLCs as water-tolerant Lewis acids were proved to be an efficient and recyclable catalyst for the acetalization. The oxygen vacancies play an important role in the catalytic activity of Lewis acidic sites on the surface of Ni–Al HTLCs. The NO3− anions coordinate to the matrix Ni2+ cations through oxygen atoms to form [Ni(OH)5NO3] units in the distorted octahedral crystal field, and the most oxygen vacancies in freshly-prepared Ni–Al HTLCs result from the strongly distorted [Ni(OH)5NO3] units. The gradual conversion of distorted [Ni(OH)5NO3] units into regular [Ni(OH)6] units will lead to a loss in catalytic activity of Ni–Al HTLCs.