(62c) Effect of Peptizing Agents on Pre-Extruded Pseudoboehmite Powder

Most heterogeneous catalysts used by the petrochemical and petroleum industry typically consist of metals supported on gamma-alumina pellets or spheres. The gamma-alumina support is formed by calcination of pseudoboehmite (alpha-alumina monohydrate, AlOOH) at 500-550oC. To increase the porosity of the support, the pellets are initially obtained by extrusion of a paste resulting from the mulling of the monohydrate with pure water and, in many cases, with addition of a peptizing agent (e. g. nitric or other acid). Mulling is performed on several different industrial mixers (e.g. Eirich, Simpson or Littleford) which add “work” to the materials by different degrees of shear and grinding. During the mulling operation (also known as granulation), a paste is formed with sufficient plasticity to be extruded. Further mulling tends to decrease this plasticity. Generally, testing of the mixture is done by “feel” to qualitatively determine the apparent “cohesiveness” that leads to the proper plasticity. The endpoint is usually determined by the operators “feel”, based on experience. Less often, a test with a Carver press is used, generally only in scoping.

In previous work in this project, we have utilized several different characterization methods to follow the chemical and physical changes occurring during the granulation step: X-ray diffraction, UV-VIS spectroscopy, TGA, nitrogen porosimetry (PSD, SA), and particle size distribution by light scattering and SEM. For these methods, we have observed at best only minor effects.

The focus of this talk is the investigation of the chemical and physical modification of the pre-extruded AlOOH powder mixtures upon the addition of water and various peptizing agents at different concentrations and mixing times by infrared spectroscopy (ATR). Specifically, we have investigated the chemical changes that occur through two specific absorbance features for the pseudo boehmite: Al-OH asymmetric bending mode at 1077 cm-1 and OH deformation mode at 900 cm-1. These modes allow a direct measurement of the hydrogen bonding between the octahedral structure inside the boehmite particle (bulk mode) and the H-bonding between water molecules and AlOOH (surface mode). We find that the mixture of dry AlOOH monohydrate mixed with pure water or/and HNO3 (concentration < 1%) exhibit the same Al-OH bulk and surface mode area, independent of the mixing time: no change in the crystalline structure is observed suggesting that the addition of nitric acid has no effect on the crystallinity of the ultimate 4.5nm crystallites. But the intensity of the surface mode declines with mixing time, suggesting increased interaction of the crystallites. Finally, when some metal chloride salts (Na, Ca, La) are added, a clear decrease in the surface mode with mixing time is observed, indicating that the metal chlorides strongly interact with the surface of the AlOOH particles by displacing hydrogen bonding of the surface to water molecules. For most salts, the area of the Al-OH bulk mode is relatively unchanged except for the addition of a high concentration of LaCl3, which may induce a reorganization of the bulk crystalline structure.