Though great efforts have devoted to prepare planar Al-13 [Al-13(OH)(24)(H2O)(24)(15+), M-Al-13], the lack of knowledge of its stability and hydrolysis behavior in aqueous solution limits its practical application as coagulant. In this study, the stability, morphological transformation and coagulation property of M-Al-13 were thoroughly investigated. The Al-27-NMR spectra revealed temperature-dependent morphological transformation from M-Al-13 to K-Al-13 in aqueous solution. This transformation was completely inhabited below 5 degrees C, but was accelerated with increased temperature from 5 to 70 degrees C. Moreover, higher temperature and longer aging time led to further transformation of K-Al-13 to the aluminum species with higher degree of polymerization such as Al-30. Thus, low temperature was favorable for the morphological stability of M Al-13. Due to the maintenance of M-Al-13 structure, all as-prepared M-Al-13 crystal, M-Al-13 solution (0 degrees C) and freeze dried M-Al-13 showed excellent and comparable flocculation property in the terms of floc growth rate, floc size and floc strength. Interestingly, the flocculability dramatically decreased after M Al-13 solution aged at 25 degrees C or 40 degrees C for 120 min with 29% and 62% of M-Al-13 transformed to K-Al-13, respectively. These results implied that M-Al-13 was an important flocculant with better performance than the well-known and efficient flocculant K-Al-13. Our study about the stability and hydrolysis behavior of M Al-13 might prompt its application as a highly efficient flocculent for water purification. (C) 2017 Elsevier B.V. All rights reserved.