572 Volume 53, Number 5, 1999 APPLIED SPECTROSCOPY 0003-7028 / 99 / 5305-0572$2.00 / 0 q 1999 Society for Applied Spectroscopy Dehydroxylation of Aluminum (Oxo)hydroxides Using Infrared Emission Spectroscopy. Part II: Boehmite RAY L. FROST, * J. THEO KLOPROGGE, SHANE C. RUSSELL, and JENNIFER SZETU Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Q 4001, Australia The dehydroxylation of boehmite has been studied by the applica- tion of infrared emission spectroscopy over the 200 to 750 8C tem- perature range. The dehydroxylation is followed by the loss of in- tensity of the hydroxyl stretching frequencies observed at 3478, 3319, and 3129 cm 21 and by the loss of intensity of the hydroxyl deformation modes at 1140 and 1057 cm 21 . Dehydroxylation starts at 250 8C and is complete by 450 8C. No difference was found be- tween the synthetic and natural boehmite dehydroxylation. The hy- droxyl stretching frequencies show a pronounced blue shift, while the hydroxyl deformation modes show a pronounced red shift. In- frared absorption bands were observed at 3413, 3283, and 3096 cm 21 for the hydroxyl stretching frequencies and at 1161 and 1071 cm 21 for the hydroxyl deformation frequencies. Low-frequency in- frared absorption bands are observed at 749, 635, and 542 cm 21 and infrared emission bands at 811, 716, 611, and 456 cm 21 . The infrared emission low-frequency bands moved to higher frequencies upon thermal treatment. Spectral changes in the low-frequency bands con®rm that dehydroxylation commenced at 250 8C. Infrared emission spectroscopy allows the phase changes of the Al 2 O 3 ±H 2 O alumina system to be studied in situ at the elevated temperatures. Index Headings: Bauxite; Boehmite; Dehydroxylation; Infrared emission; FT-IR. INTRODUCTION Bauxite deposits form the major source of aluminum in our industrial world. Australia has major deposits near Weipa, Queensland. In this deposit, the two major alu- mina phases are gibbsite and boehmite. Boehmite is AlO(OH) or sometimes written as Al 2 O 3 ´H 2 O and re- ferred to as g-alumina monohydrate. The major alumi- num phases recognized in bauxites and laterites are gibb- site, also known as hydrargillite [ g-Al(OH) 3 ], boehmite [g-AlO(OH)], and diaspore [a-AlO(OH)]. In addition, other commonly observed phases are kaolinite, allo- phane, quartz, goethite, and haematite. 1±3 Gibbsite is the principal component in tropical bauxites formed in areas characterized by a hot rainy climate with alternating dry periods (monsoon). The bauxite deposits at Weipa, Queensland contain approximately 33% boehmite. Baux- ites with mainly boehmite seem to be more restricted to the subtropical areas (Mediterranean-type bauxite). Ther- mal action or low-grade metamorphism mostly favors di- aspore formation. Furthermore, diaspore is found as a minor constituent in many bauxites in addition to gibbsite and boehmite. 4±6 Boehmite has the same structure as lepidocrocite [ g- FeO(OH)]. The structure of boehmite consists of double layers of oxygen octahedra partially ®lled with Al cat- Received 3 November 1998; accepted 4 January 1999. * Author to whom correspondence should be sent. ions. The stacking arrangement of the three oxygen layers is such that the double octahedral layer is in cubic closed packing. 7 Within the double layer, one can discriminate between two different types of oxygen. Each oxygen in the middle of the double layer is shared by four other octahedra, while the oxygens on the outside are shared only by two octahedra. These outer oxygens are hydro- gen-bonded to two other similarly coordinated oxygens in the neighboring double layers above and below. The stacking of the layers is such that the hydroxyl groups of one layer are located over the depression between the hydroxyl groups in the adjacent layer. The IR spectrum of boehmite has a characteristic OH stretching band with two equally strong maxima at 3297 and 3090 cm 21 ac- cording to Ryskin. 8 Van der Marel and Beutelspacher, however, reported a very strong maximum at 3280±3287 cm 21 and a very strong maximum at 3090 cm 21 . 9 The enormous splitting has been ascribed to the presence of a direct bonding between the equivalent hydroxyls and to the high structural regularity of the structure. In the OH bending region, boehmite is characterized by two vi- brations at 1160 and 1080 cm 21 . The vibration at 755 cm 21 involves the hydrogen vibrations according to Fri- piat et al. 10 Van der Marel and Beutelspacher report an additional vibration at 636 cm 21 . 9 While there has been a great deal published on alumi- num oxyhydroxides and their thermal transformations, there are many contradictory ®ndings. Much of this dis- crepancy may be due to differing experimental techniques and conditions used to determine these thermal transfor- mations. The dehydroxylation of boehmite and gibbsite has been described as deceptively simple. 11,12 The most stable member of the Al 2 O 3 ±H 2 O series is a-alumina. Di- aspore easily dehydrates to a-alumina; boehmite (as well as gibbsite) does not. The reason may be attributed to the packing of a-alumina as close packed, and diaspore also is hexagonally close packed, whereas gibbsite and boehm- ite are cubic. 13 There is much disagreement about the de- hydroxylation (dehydration) pathways of boehmite. Lippens and co-workers make the point that most of the confusion arises from insuf®cient information about the reaction conditions. 14 The use of thermal techniques to study the dehydration and dehydroxylation of gibbsite has been widely documented. 11 It is clear that many variables must be taken into account when using techniques such as dif- ferential thermal analyses (DTA), differential scanning calorimetry, (DSC), thermogravimetric analyses (TGA), cathode ray tube analyses (CRTA), and quasi-isothermal TGA and isobaric TGA. 14±17 Such variables include heat- ing rate, external pressure, water vapor pressure, sample