Aldehyde self-condensation catalysis by aluminum aminoterephthalate metal-organic frameworks modified with aluminum isopropoxide

Porous materials based on aluminum(III) 2-aminoterephthalate metal organic frameworks (MOFs NH₂MIL101(Al) and NH₂MIL53(Al)) and their composites with aluminum isopropoxide (Al-i-Pro) are studied as sorbents of vapors of volatile aldehydes and catalysts of acetaldehyde dimerization to ethyl acetate via the Tischenko reaction. MOF/Al-i-Pro composites obtained by simple impregnation of the MOFs with hydrocarbon solutions of Al-i-Pro are stable due to the formation of bonds between the MOF carbonyls and Al-i-Pro. The specific BET surface areas of the MOFs NH₂MIL101(Al) and NH ₂MIL53(Al) ranged from 1650 to 1980 and 670-780 m²/g, respectively, and were lowered 6-12-fold by impregnation of Al-i-Pro into the MOF pores. However, the acetaldehyde and acrolein uptake by the MOF/Al-i-Pro composites from saturated vapor atmosphere is comparable to that of their respective parent MOFs and exceeds the aldehyde uptake of activated carbon or molecular sieves. Due of the propensity of the Al-i-Pro to catalyze dimerization of acetaldehyde to ethyl acetate, the latter is the main product of the reaction between acetaldehyde and MOF/Al-i-Pro materials, whereas crotonaldehyde is found in the products of the acetaldehyde self-condensation on the parent MOF NH₂MIL101(Al). The kinetics of acetaldehyde dimerization into ethyl acetate catalyzed by NH₂MIL101(Al)/Al-i-Pro in deuterated benzene at room temperature are measured over three consecutive cycles. The apparent second-order reaction rate is 5.2 × 10^-5 M ^-1s^-1, which is of the same order as in the analogous reaction catalyzed by a homogeneous solution of Al-i-Pro. The MOF/Al-i-Pro materials are proven to be recyclable heterogeneous catalysts.