Preparation of tungstophosphoric acid intercalated MgAl layered double hydroxides with a tunable interlayer spacing and their catalytic esterification performance in the deacidification of crude oil

This study demonstrates the synthesis of MgAl layered double hydroxides (LDHs) intercalated with tungstophosphoric acid (H₃PW₁₂O₄₀, HPW) by an ion exchange method, and different interlayer spacings (d₀₀₃) are obtained by adjusting the ion exchange temperature and time. The crystalline structures, molecular structures, atomic compositions, acidity, and specific surface areas of the LDH samples are rigorously characterized. A relatively high ion exchange temperature is demonstrated to be favorable for the formation of a large d₀₀₃ value of around 1.46 nm, while a long exchange time is favorable for the formation of a small d₀₀₃ value of around 1.05 nm. The different values of d₀₀₃ are the result of different orientations of HPW anions within the interlayer space. Here, d₀₀₃=1.46 nm is obtained when P₂W₁₈O₆₂^6- and PW₁₁O₃₉^7- anions are arranged in the interlayer with their C₂ axes respectively tilted toward and perpendicular to the LDH layer planes. In contrast, d₀₀₃=1.05 nm is obtained when PW₁₂O₄₀^3- anions are grafted onto the LDH layers with their C₂ axis perpendicular to the layer planes. Furthermore, the catalytic esterification performance of the samples is investigated for the deacidification of a model crude oil. Compared with PW₁₂O₄₀^3- anions, the presence of P₂W₁₈O₆₂^6- and PW₁₁O₃₉^7- anions in the interlayer produce a higher proportion of sites with intermediate acidity that function as catalytic sites. Moreover, a large value of d₀₀₃ facilitates the diffusion of reactants into the interlayer, which enhances their contact with the catalytic sites, and thereby increases the catalytic esterification performance of the LDHs in the deacidification of crude oil.