Sintering‐induced aromatization during n‐heptane reforming on Pt/Al₂O₃ catalysts

Abstract

A platinum/alumina catalyst was sintered in oxygen and hydrogen atmospheres using two metal loadings of the catalyst: 0.3% Pt and 0.6% Pt. After sintering, the aromatization selectivity was investigated with the reforming of n-heptane as the model reaction at a temperature of 500• C and a pressure of 391.8 kPa. The primary products of n-heptane reforming on the fresh platinum catalysts were methane and toluene, with subsequent conversion of benzene from toluene demethylation. To induce sintering, the catalysts were treated with oxygen at a flow rate of 60 mL min −1 , pressure of 195.9 kPa and temperatures between 500 and 800• C. The 0.3% Pt/Al 2 O 3 catalyst exhibited enhanced aromatization selectivity at various sintering temperatures while the 0.6% Pt/Al 2 O 3 catalyst was inherently hydrogenolytic. The fact that aromatization was absent on the 0.6% Pt/Al 2 O 3 catalyst was attributed to the presence of surface structures with dimensionality between two and three as opposed to essentially 2-D structures on the 0.3% Pt/Al 2 O 3 catalyst surface. On the 0.3% Pt/Al 2 O 3 catalyst, the reaction product ranged from only toluene at a 500• C sintering temperature to predominantly cracked product at a sintering temperature of 650 • C and no reaction at 800• C. For sintering at about 650 • C, subsequent conversion of n-heptane was complete and dropped thereafter. The turnover number was observed to change from 0.07 to 2.26 s −1 as the dispersion changed from 0.33 to 0.09. The Koros-Nowark (K-N) test was used to check for the presence of internal diffusional incursions and Boudart's criterion was used for structural sensitivity determination. The K-N test indicated the absence of diffusional resistances while n-heptane reforming was found to be structure sensitive on the Pt/Al 2 O 3 catalyst.

Metal Redispersion Strategies for Recycling of Supported Metal Catalysts: A Perspective

“…Using a cyclic oxidative/reductive process, it was found that at all studied temperatures (500–800 °C), the particle sizes of Pt were smaller after the oxidation step (2.21–9.56 nm) compared with the Pt particle size after the reduction (3.03–11.11 nm), although the particle size even after reduction was still smaller than that observed for the fresh catalyst . Similar trends were noted in other studies of the same monometallic Pt catalyst . It should, however, be noted that this was quite a prolonged reduction step (16 h at 150 °C and 2 h at 250 °C) in pure hydrogen.…”

“…16 Similar trends were noted in other studies of the same monometallic Pt catalyst. 85 It should, however, be noted that this was quite a prolonged reduction step (16 h at 150 °C and 2 h at 250 °C), in pure hydrogen. Consequently, the fact that some sintering occurred during this reduction step is not surprising.…”

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

“…It is posited that the sintered bimetallic catalyst contains a favourable structure for aromatic selectivity -a cuboctahedral structure of 6-atom (111) planes. 16,51 …”

“…15 A second paper explored these effects for two different Pt loadings on Al 2 O 3 . 16 These studies were carried out in a microcatalytic reactor where the reforming of n-heptane was chosen as the model reaction.…”