In Situ Growth of Highly Active MgAl Layered Double Hydroxide on η-Al2O3 for Catalytic Hydrolysis of Urea in Wastewater

Magnesium aluminum layered double hydroxide (MgAl–LDH) was first successfully deposited in situ on η-Al2O3 applied to catalyze urea hydrolysis. Characterization by AAS, XRD, SEM, EDS, DSC, FT-IR, N2-BET, determination of base strength and basicity and experiments on hydrolysis activities of in situ MgAl–LDH catalysts show that the crystallinity and coverage degree of MgAl–LDH on η-Al2O3 gradually increase with increasing initial c(Mg2+). When initial c(Mg2+) = 2 mol L−1, η-Al2O3 is completely covered and the smallest particle size, the highest crystallinity, basicity and urea degradation rate of in situ MgAl–LDH are obtained. The basicity is mainly from Mg–OH at the edges of MgAl–LDH on η-Al2O3. The highest basicity corresponds to the highest urea degradation rate, Mg–OH is the active sites for catalytic hydrolysis of urea. The specific surface area, basicity and catalytic activity of MgAl–LDH by in situ synthesis are evidently higher than MgO and MgAl–LDH purchased. The urea concentration is 8.162 mg L−1 at 165 °C after 120 min with in situ MgAl–LDH (c(Mg2+) = 2 mol L−1) as catalyst, which is below the effluent standard. In situ MgAl–LDH also shows excellent recycling ability and the slight loss of activity should be due to the reduction of basicity, which is caused by hydrothermal conditions. In addition, a possible catalytic mechanism of urea hydrolysis over MgAl–LDH is proposed.