An infrared emission spectroscopic study of the thermal transformation mechanisms in Al-13-pillared clay catalysts with and without tetrahedral substitutions

Pillaring of smectites with Al-13 was studied in situ by infrared emission spectroscopy. Exchange of montmorillonite with Al-13 gave Al-OH and Al-H2O OH-stretching modes of Al-13 at 3682 and 3538 cm(-1) and at 3641 and 3334 c m(-1) for Al-13-hectorite. Al-13 conversion resulted in removal of the AI-O H mode >400 degrees C. The Al-H2O band was replaced by bands at 3574 and 35 05 cm(-1) due to structural rearrangement within Al-13. The intensities dim inished, but are still observed at 800 degrees C, suggesting that the pilla r structure incompletely converted to "Al2O3." In hectorite the Al-13 compl etely converted upon calcination. Below 1750 cm(-1) the Al-13-montmorilloni te displays bands at 642, 1008, 1321, 1402, and 1512 cm(-1). The 1512 cm(-1 ) band disappeared at 500 degrees C, followed by the other bands above 600 degrees C, The 642 cm(-1) band intensity diminished but is still observed a t 800 degrees C. At 700 degrees C a new band is observed at 722 cm(-1) due to Al-O bond formation. The Al-13-hectorite displays bands at 1227, 1315, 1 395, and 1510 cm(-1). The last three band intensities diminished but remain ed visible. The 1227 cm(-1) band disappeared above 500 degrees C. Expansion of saponite with Al-13 gave bands at 3626 and 3244 cm(-1), which disappear ed at 700 degrees C, indicating that the transition to "Al2O3" is complete. The Mg-OH band is not influenced by calcination, indicating that the pilla ring does not involve the octahedral clay layer. Below 1750 cm(-1) Al-13-sa ponite shows bands at 1316, 1424, and 1521 cm(-1). Calcination resulted in a disappearance of the 1521 and 1316 cm(-1) bands at 600 degrees C, while t he 1424 cm(-1) band disappeared at 400 degrees C. The 1316 cm(-1) band is r eplaced by bands at 1269, 1314, and 1388 cm(-1) above 600 degrees C, due to formation of M-[IV]-O(H)-Al-[VI] linkages between pillars and tetrahedral clay layers. (C) 1999 Academic Press.