Response surface statistical optimisation of zeolite-X/silica by hydrothermal synthesis

A hydrothermal alkaline synthesis of self-supporting zeolites from co-generation boiler sugar cane bagasse ash (SCBA) was measured by X-ray powder diffraction (XRD) scan yields (scan area percentages) method. A factorial design and a response surface statistical method were used to optimise the synthesis method. Temperature, NaOH concentration and aluminium/silica (Al/Si) ratio were determined to be the most influential factors in controlling zeolite-X yields, and these three variables were included in a response surface model (RSM) with a central composite design (CCD). The RSM model indicates that optimal zeolite-X formation conditions are 72.5 °C, 5 M NaOH and an Al/Si ratio of 3:5. The RSM/CCD matrix established an efficient statistical modelling of zeolite synthesis optimisation with the fewest possible number of experiments. Scanning electron microscopy examination shows that SCBA particles (20–100 µm) are covered with zeolite crystallites (0.3–0.8 µm in size) producing a self-supporting structure. XRD analyses show a dominance of zeolite-X (33.6%), with zeolite-A (4.7%), and an average Al/Si ratio of 4:5 that is close to published values. The Brunauer–Emmett–Teller (BET) apparent specific surface area measured 228 m² g⁻¹ (P/Po = 0.045), and ≈ 90% of the micro-porosity distribution is associated with ≈ 7 Å internal micropore, which is typical of zeolite-X. The self-supporting, composite nature and large effective grain size of the zeolites reported in this work opens a number of uses for the materials produced.