Temperature imposed textural and surface synergism affecting the isomerization activity of sulfated zirconia catalysts

Abstract

Using sulfuric acid as the sulfating agent, two catalyst series were obtained from hydroxide and nitrate precursor with a sulfate loading identical to commercial sulfated hydroxide, i.e., 4.2 mass %. After calcination at 500, 600 and 700 °C, all nine samples had various contents of residual sulfates depending on the origin of the catalyst. Accordingly, their surface properties were different, which, together with various textural properties, govern the formation of the active phase and their catalytic activity in the n-hexane isomerization reaction. The dominant activity and yield of mainly mono-branched isomers were attained in reaction at 200 °C with a commercially sulfated zirconia catalyst calcined at 500 °C. Among the SZ catalyst series synthesized from hydroxide and nitrate, the second according to its activity profile was similar to that of the commercially sulfated one, while samples originating from hydroxide showed some activity only after calcination at 600 °C. This is due to the poorer textural properties of the hydroxide series, necessitating a higher calcination temperature in order to promote the simultaneous decomposition of S-containing species and their re-adsorption into the zirconia matrix following interaction and active phase formation. It seems that the tetragonal zirconia phase was not responsible for the catalytic activity but a synergistic effect of the textural properties of the samples and the sulfate loadings, which determine different acid strengths on the catalyst surface.

Synthesis of zirconia-based solid acid nanoparticles for fuel cell application

“…Zirconia modified with sulphates exhibits superior catalytic activity. The presence of sulphates increas- es the stability of zirconia as well as the content of the tetragonal crystal phase, which is the one believed to be catalytically active (Zarubica et al, 2009). It has been reported that S-ZrO 2 exhibited a Hammett acid strength H 0 of -16.03, whereas for 100% sulphuric acid it is only -11.99 and shows higher strength (Yadav & Nair, 1999).…”

A comparative study of n-hexane isomerization over solid acids catalysts: Sulfated and phosphated zirconia

“…In order to achieve the surface acidity required for sufficient catalytic activity in isomerization reactions, zirconia may be promoted with acid functions such as sulfates [12] or phosphates. It has been established that zirconia can exist in a number of polymorphs at atmospheric pressure: monoclinic, tetragonal, cubic and orthorhombic [13].…”

“…Further debate concerned different reactivity of sulfated zirconia catalysts according to the preparation method and activation conditions [16]. Additionally, acid functions have been reported to stabilize the desired tetragonal phase in zirconia [12,13,17,18] for reasons not yet fully understood. Possible explanation ascribes the stabilization to crystal size effect, additionally connected with the fact that the value of surface energy difference between tetragonal and monoclinic phase surpasses the bulk energy difference between the two phases due to such small size of the particular crystal [10,19,20].…”

Surface properties and memory effect: Key factors in determining the catalytic efficiency of sulfated zirconia

“…Nešto niža aktivnost, ali uporediva, katalizatora na bazi alkoksida i sintetisanog pri nižoj pH vrednosti, rezultat je memorijskog efekta koji potiče od prekursora i njegovih svojstava, kao i nešto drugačijih finalnih fizičko-hemijskih svojstava primenjenog katalizatora. [16][17][18][19][20][21][22][23][24][25] …”

“…It is known that the properties of sulfated zirconia (SZ) may highly depend on the preparation method, the activation procedure and the precursor type [14][15][16]. In addition, it is believed that sulfate groups increase the surface acidity and the stability of zirconia [17][18][19][20][21]. Namely, sulfate acid functions have been related to the stabilization of the SZ tetragonal crystal phase, but the explanations are still not totally defined [17,19,20,22].…”