Fs Modica

Platinum/zeolite catalyst for reforming n-hexane: Kinetic and mechanistic considerations

Abstract A platinum/L-zeolite-reforming catalyst exhibits higher activity and selectivity for converting n-hexane into benzene than other Pt catalyst. The reaction pathways indicate that for all catalysts, e.g., Pt/K L or Pt/K Y, benzene is formed as a primary product by one-six-ring closure and methylcyclopentane is formed as a primary product via one-five-ring closure. The ratio for one-six to one-five-ring closure, however, is about two times greater for the Pt/K L than for the Pt/K Y, or other platinum catalysts. The preference for the one-six-ring closure in L zeolite appears to be related to the optimum pore size of the L zeolite. In addition to an increased selectivity for one-sixring closure, the Pt/K L-zeolite catalyst also displays increased reactivity. For example, the turnover frequency of the Pt/K L-zeolite catalyst is 10 times higher for formation of benzene and 3.3 times higher for formation of methylcyclopentane compared with the Pt/K Y-zeolite catalyst. Although the Pt/K L is more reactive than Pt/K Y, the apparent activation energies, 54 kcal/mol for one-sixring closure and 39 kcal/mol for one-five-ring closure, are the same for both catalysts. Differences in reactivity are associated with an increase in the preexponential term for the Pt/K L catalyst. The increased aromatics selectivity for Pt/K L is consistent with the confinement model which proposes that n-hexane is adsorbed as a six-ring pseudo-cycle resembling the transition state for one-sixring closure.

Sulfur Poisoning of a Pt/BaK-LTL Catalyst: A Catalytic and Structural Study Using Hydrogen Chemisorption and X-ray Absorption Spectroscopy

The sulfur poisoning of a Pt/BaK-LTL catalyst has been studied with X-ray absorption spectroscopy and hydrogen chemisorption. The fresh catalyst contained highly dispersed platinum inside the zeolite pores. EXAFS analysis determined a Pt-Pt coordination number of 3.7, suggesting an average platinum cluster size of 5 or 6 atoms, consistent with the TEM and chemisorption data (H/Pt = 1.4). The catalyst was poisoned with H2S until the dehydrocyclization activity of n-hexane decreased to 30% of fresh activity. The first-shell PtPt coordination number increased to 5.5, indicating a growth of the average platinum cluster size to 13 atoms. Hydrogen chemisorption measurements of the poisoned catalyst show a decrease in the H/Pt value to 1.0. The EXAFS data also provide evidence for the presence of sulfur adsorbed on the surface of the platinum particles with a PtS bond distance of 2.27 A. The high sensitivity of the Pt/LTL catalyst to poisoning by very low levels of sulfur is attributed to the loss of active platinum surface by adsorption of sulfur and the growth of the platinum clusters. Much of the available platinum surface was found to be capable of chemisorbing hydrogen, but with no activity for dehydrocyclization. Growth of the platinum particle was sufficient to block the pore. In the sulfur-poisoned catalyst, only the sulfur-free platinum atoms exposed through the pore windows remain active. The evidence suggests the location of the sulfur was at or near the metal-zeolite interface. Since both high activity and selectivity require extremely small platinum particles, regeneration of sulfur-poisoned catalysts will require removal of the adsorpted sulfur and restoration of the original particle size.

Pt clusters in BaKL zeolite : characterization by transmission electron microscopy, hydrogen chemisorption, and X-ray absorption spectroscopy

Platinum supported on BaKL zeolite was characterized by Transmission Electron Microscopy (TEM), hydrogen chemisorption, and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The results of all three techniques indicate the presence of highly dispersed platinum in the zeolite pores. There is no evidence of platinum outside the zeolite pores. The EXAFS data determine a Pt-Pt coordination number of 3.7, suggesting that the average platinum cluster in the zeolite consists of 5 or 6 atoms, consistent with the TEM and chemisorption data. The EXAFS data also provide evidence of the platinum-zeolite interface, indicated by Pt-O contributions at 2.14 and 2.70 Å, and a Pt-Ba contribution at 3.8 Å. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.

The Relation between Catalytic and Electronic Properties of Supported Platinum Catalysts: The Local Density of States as Determined by X-Ray Absorption Spectroscopy.

Abstract The intensity of the white line of the LII and LIII X-ray absorption edge spectra of small platinum particles increases with decreasing particle size. The combined white line intensity of the LII and LIII X-ray absorption spectra for platinum catalysts with comparable average particle size is higher when dispersed on acidic than on neutral supports. This indicates that platinum particles are more electron deficient on acidic than on neutral supports. The propane hydrogenolysis TOF for platinum supported on γ-Al2O3 or H-LTL is found to be more than an order of magnitude higher than for platinum supported on a non-acidic K-LTL zeolite. The differences in catalytic behavior are related to differences in the d-band density of states.

The Influence of Metal-Support Interactions on the Whiteline Intensity

The whiteline intensity of Pt/K-LTL catalysts reduced at 300, 450, and 600°C decreases with increasing reduction temperature. This change in whiteline intensity was ascribed to the removal of hydrogen from the metal-support interface by reduction at higher temperature.

The influence of hydrogen pretreatment on the structural and catalytic properties of a Pt/K-LTL catalyst

ABSTRACT A platinum, non–acidic K–LTL catalyst, reduced at 300, 450, and 600°C, was characterized by extended X–ray absorption spectroscopy (EXAFS), hydrogen chemisorption, hydrogen temperature programmed desorption (H2–TPD) and methylcyclopentane hydrogenolysis. Reduction at 300°C produces small platinum crystallites with an interfacial layer of hydrogen. Reduction at 450°C increases the particle size, releasing interfacial hydrogen. This irreversible hydrogen desorption is observed in the TPD around 300°C. Reduction at 600°C results in further growth of the platinum cluster with the loss of the remaining interfacial hydrogen. In the TPD, a second high temperature H2 desorption is observed at around 610°C. Because of the confined space within the zeolite pore, as the platinum particle size approaches the pore size, there is a reduction in hydrogen chemisorption capacity and catalytic activity compared to a particle of equivalent size on an amorphous support.