Effect of Aluminum Addition and Surface Moisture Content on the Catalytic Activity of Sulfated Zirconia in n-Butane Isomerization

Abstract

The effects of aluminum addition and surface moisture on the surface-structure properties and catalytic activity of sulfated zirconia were investigated in n-butane isomerization. Adding 4 mol % Al into zirconia support with coprecipitation method greatly improves the catalytic activity. This is attributed to the monolayer coverage of sulfate species and the maximum of Brønsted acid sites, resulting from the strongly stabilized sulfate species on the tetragonal zirconia (t-ZrO2) surface. The in situ DRIFTS investigations and density functional theory (DFT) calculations show that the dehydration temperature affects both the density and the strength of Brønsted acid sites, and there is a balance between them. The optimal catalytic activity is reached with complete desorption of physical adsorbed water and partial desorption of surface hydroxyl groups. The strongly stabilized sulfate species on appropriately dehydrated surface of t-ZrO2 provide the superacidity and the strong oxidizing ability via a strong electron-withdrawing effect of SO bonds.

Alkane isomerization over sulfated zirconia solid acid system

“…Lately, our density functional theory (DFT) calculations showed that the deprotonation energies (DPE) values of Zr─OH on the SZ surface were depended on their surrounding geometry and hydration degree . The DPE values for terminal and bridging Zr─OH groups on a highly dehydrated SZ surface were 1087 and 1206 kJ/mol, which is well in correspondence with the values estimated by lglesia et al (1110‐1165 kJ/mol) .…”

“…At high water coverage, however, the adsorption of water molecules, including the chemical and physical adsorbed ones, decreases the strength of acid sites, resulting from the weakened electron‐withdrawing effect of sulfate species. On the other hand, the DFT calculations by our group highlighted that the DPE values of Zr─OH groups tightly depended on the hydration degree of surface . Namely, with the increase of water coverage, the DPE values of Zr─OH groups increased, implying a weakening in the strength of Brønsted acid sites.…”

“…An activation pretreatment at higher temperature is required to obtain high n ‐alkane isomerization activity over strong hydrophilic SZ catalysts, because of its surface complexity under different hydration degrees. The temperature rather than the atmosphere during activation remarkably influences isomerization activity of SZ . Klose et al found that more than 95% of water adsorbed on the SZ catalyst was removed after activation at 550°C in aerobic or inert atmosphere by in situ IR studies, corresponding with the great promotion of isomerization activity .…”

“…On one hand, Brønsted and Lewis acid sites can reversibly interconvert under different dehydration degrees. The number of Brønsted sites decreases during the activation because of the dehydroxylation of Zr─OH, which explains the detrimental effect of activation at high temperature (higher than 500°C) on the isomerization activity . Simultaneously, more unsaturated Zr 4+ ions expose on the surface and act as Lewis sites that are active in the dissociative adsorption of water molecules, generating Zr─OH groups with high Brønsted acid strength.…”

“…For example, the in situ Fourier transform infrared (FTIR) investigations conducted by several groups unequivocally confirmed the possibility of oxidative activation of n-alkane on SZ catalysts. [5][6][7][8] Additionally, the advanced theoretical calculations give a new approach to reveal the surface structure and the nature of active sites. 5,[9][10][11] Haase and Sauer found that the most stable configurations of sulfur species on SZ surface were tridentate sulfate anions or tridentate SO 3 complexes on the (1 0 1) and (0 1 1) surface of tetragonal zirconia.…”