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

Abstract

Amorphous sulfated zirconium hydroxide was promoted with iron or manganese via the incipient wetness technique, to give 0.5-3.5 weight% 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. XRD shows that crystallization starts before and progresses during the overheating. The surface area shrinks during the glow, and its final size (85-120 m 2 ·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 through controlling the batch size used for calcination.

Oxo‐Anion Modified Oxides

“…Crystallization starts before overheating is observed. Surface area and diffractograms of a sample that was cooled down directly after the glow largely correspond to those of samples that underwent the complete heating program, indicating that structure and morphology are determined during the overheating [205]. During the temperature overshoot, the desired maximum temperature may be exceeded, which allows for a chemistry that is not possible within the planned program.…”

“…Additives of any kind shift the glow event to higher temperature [189,190,196,917,204,205] and subdue or suppress it [193,195], for sulfate the exotherm temperature shifts linearly with content [190]. Estimation of the expected temperature overshoot from the enthalpy changes due to crystallization or surface area loss, under the assumption of quasi-adiabatic conditions, shows that both processes can individually account for the phenomenon [205].…”

“…The data show that variations in the bed size and packing during calcination can produce different materials from a single precursor batch [204,205]. Although the nominal conditions (heating ramp, holding temperature and time, atmosphere) are identical in these calcinations, the samples experience quite different actual conditions.…”

The Balance Between Reactivity and Stability of Modified Oxide Surfaces Illustrated by the Behavior of Sulfated Zirconia Catalysts

“…These precursor materials were dried for 21 h at 383 K. Samples promoted with 0.5 or 2.0 wt% iron or manganese were prepared from dried XZO 682 according to the -incipient wetness‖ impregnation method, using aqueous solutions of either Mn(NO 3 ) 2 ·4H 2 O or Fe(NO 3 ) 3 ·9H 2 O (both Merck, p.a.). After impregnation the samples were dried at room temperature for at least 24 h. An amount of 20-25 g of raw material was calcined in a 17.1 ml quartz boat under a 200 ml min -1 synthetic air flow, applying a nominal heating rate of 3 K min -1 and a holding time of 3 h [37,38]. Pure and sulfated zirconia were calcined at 823 K and are designated as -Z‖ and -SZ‖.…”

“…While this sensitive behavior presents a challenge in investigating these materials and probably contributes to the reported problem of poor reproducibility [36][37][38], it also presents an opportunity to identify properties essential to the catalytic activity.…”