Rapid genesis of active phase during calcination of promoted sulfated zirconia catalysts

Amorphous sulfated zirconium hydroxide was promoted with iron or manganese via the incipient wetness technique to give 0.5–3.5 wt% promoter in the final catalyst. An exothermic reaction during the calcination temperature ramp (to 823 or 923 K) leads to a rapid overheating (“glow”) of the sample. X-Ray diffraction shows that crystallization starts before and progresses during the overheating. The surface area shrinks during the glow, and its final size (85–120 m2 g−1) and the porosity appear to be largely determined by the glow event. Manganese and iron ions prevent the coalescence of particles and lead to high surface areas. Variation of the batch volume (2.2, 8.4, or 17.1 ml) for calcination produced different catalysts from the same precursor. For both promoters, samples calcined in large batches exhibited the highest surface areas, interconnected mesopores (2.2–3.8 nm), and the highest maximum rate in n-butane isomerization (338 K, 1 kPa n-butane). Long-term performance was independent of surface area and morphology. The concentration of the active sites depended on the calcination batch size, indicating that the active phase is formed in kinetically controlled reactions during the rapid overheating. The catalytic and textural properties of sulfated zirconia catalysts can be reproduced by controlling the batch size used for calcination.