Optimization of the Ni/Al layered double hidroxide synthesis by factorial design

LDH materials are based on a metallic M2+/M3+ hidroxide network. These are layered materials in which the interlayer space is occupied by anions to compensate the interlayer positive charge hence being anion exchangers. This property allows inserting anionic homogenous catalysts between layers giving rise to heterogenous catalysts.1 To accomplish the liquid phase catalysis the layered structure must exhibit thermal stability as high as possible, low carbonate content (for the ease of ion exchange), cristallinity and high surface area. Ni/Al LDHs were synthesized applying the varying pH approach. For optimization it was employed a 24 factorial desing: the varied parameters were those of Table 1. Samples were characterized by XRD, FTIR and TGA. From XRD patterns it was calculated the mean crystal size using the Scherrer formula.2 These data permited to evidence which conditions set afforded the most crystalline sample (D = 7.1 nm): precipitation pH = 11, no washing, Ni:Al 3:1 and isolation by filtration. However this XRD pattern presented 2nd phase peaks. Similar condition sets were inspected and that applying pH = 8 and washing presented same crystallinity level (D = 5.2 nm). In addition FTIR evidenced this sample has the lowest carbonate content (bands at 1400 and 1360 cm-1)3 and TGA showed it is one of the most thermal stable. ICP-OES, BET and SEM characterizations are underway.