Conversion of Boehmite Precursor to γ-Alumina Membranes

Thin supported γ‑alumina membranes have traditionally been used for isotope separation. More recently they have been used as intermediate layers for amorphous silica membranes and as model systems for nano-filtration of dilute salt solutions. Mesoporous γ‑alumina membranes are typically formed by colloidal deposition of Boehmite (AlOOH) precursor on macroporous α‑alumina supports, followed by thermal conversion. Boehmite colloids are synthesized by hydrolysis and condensation of (C₄H₉O)₃Al. Polyvinyl Alcohol (PVA) is added to the colloid to promote film formation, and is removed in the early stages of thermal treatment. γ‑alumina membranes on polymer supports are considered for use as a scaffold to make cost-effective CO₂-selective membranes. For that application, thermal conversion of the Boehmite precursor must be done such that the polymer support is not degraded. This led us to a detailed study of the thermal conversion of Boehmite precursor with PVA to the desired γ‑alumina structure.

Thin Boehmite precursor layers on porous alumina supports were subjected to a series of successive thermal treatments at gradually increasing temperatures, using either Conventional Thermal Processing (CTP) or Rapid Thermal Processing (RTP). CTP was done in a furnace with heating rates of <2°C/min; RTP was done with infra-red radiation and heating rates of <1000°C/min. Baseline membranes, for comparison, were made with CTP at 600°C or RTP at 700°C. After every treatment step, the samples were subjected to non-destructive testing: Spectroscopic ellipsometry (SE) was used to obtain the thickness and refractive index. Information on layer porosity was obtained by interpretation of the refractive index, and the time needed for a drop of methanol to permeate into the porous structure. Removal of PVA was derived from color changes. In parallel, sorption studies were performed on bulk Boehmite samples, subjected at subsequent CTP steps. The results were used to establish optimized thermal processing conditions for the formation of mesoporous membranes on several support types.