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Experimental evaluations of nonacidic supported platinum catalysts for the conversion of n -hexane to benzene demonstrate that the support geometry plays a major role in controlling the primary amount of benzene formed relative to methylcyclopentane, or the 1–6 to 1–5 ring closure selectivity. For most catalysts including non-acidic zeolite and amorphous supports, the 1–6 to 1–5 ring closure selectivity is about 0.5. However, Pt/K–LTL catalysts exhibit a ring closure selectivity greater than 1.0. In addition, increasing the alkali level above the ion-exchange capacity of the zeolite increases the ring closure selectivity slightly. Although the C 1 –C 5 hydrogenolysis selectivity is determined primarily by the inherent properties of the metal, a small excess of alkali was also important for optimum reduction of the hydrogenolysis selectivity. The catalyst selectivities are discussed with respect to the models proposed in the literature which account for the high benzene yield of Pt/K–LTL compared to other catalysts. The experimental results are most consistent with the preorganization model, in which the reactant molecule is adsorbed in the zeolite channel in a conformation which is similar to the transition state, leading to a perference for 1–6 ring closure. The significance of 1–6 to 1–5 ring closure selectivity was demonstrated via computer simulations of the reaction pathway of n -hexane to benzene. The simulations demonstrate that a high 1–6 to 1–5 ring closure selectivity leads to a higer apparent activity at high n -hexane conversions. In addition, at high conversion a catalyst with high ring closure selectivity gives almost double the benzene yield at constant C 1 –C 5 yield. These differences are important in the selection of a catalyst for commercial application and make the Pt/K–LTL catalyst with small platimum particles and a slight excess of alkali the clear choice.

The ring opening of methylcyclopentane (MCP) over well-characterized Pt/SiO{2}(EUROPT-1), Pt/@c-Al{2}O{3}, and Pt/K-LTL catalysts was studied as a function of hydrogen partial pressure and reduction temperature. The MCP ring opening selectivity did not change in the range of H{2}:MCP ratios studied (8-200). The turnover frequency (TOF) went through a maximum as the H{2}:MCP ratio increased. The maximum TOF of the Pt/@c-Al{2}O{3} after reduction at 450}o{C is about three times higher than the maximum specific activity of ttle Pt/K-LTL and Pt/SiO{2} catalysts. The H{2}:MCP partial pressure ratio at which maximum activity is obtained increases in the. series Pt/K-LTL < Pt/@c-Al{2}O{3} < Pt/SiO{2}. This sequence is rationalized using reported adsorption energies of H{2} and assuming a decreased adsorption energy of MCP on Pt/K-LTL. The data can be described with a reaction mechanism that includes the cleavage of a C-C bond as the rate-determining step. Kinetic analysis of the changes in specific reaction rate as a function of the H{2}:MCP ratio showed that the reaction proceeds through multiple adsorbed MCP species. The surface reaction rate is more than an order of magnitude higher for the Pt/@c-Al{2}O{3} catalyst than for the Pt/K-LTL catalyst, but decreases with increasing reduction temperature for both catalysts.

Platinum supported on H-LTL, K-LTL, and H-MAZ was reduced at temperatures from 573 to 873 K and the structure of the metal-support interface was determined by EXAFS. In all samples, the platinum was highly dispersed, with metal particle sizes from 5-1l atoms. After low temperature reduction (LTR, 573 K), the distance between the platinum atoms and the oxide atoms of the support (Pt-O distance) is 2.7 Å, which is significantly longer than the Pt-O distance of 2.2 Å observed after high temperature reduction (HTR, 873 K). At intermediate reduction temperatures both the long and the short Pt-O distances are observed. The shortening of the Pt-O distance, combined with the decrease in both the Debye-Waller factor and the inner potential shift, implies a stronger interaction between the platinum atoms and the oxide support. A structural model is proposed wherein the longer Pt-O distance results from the presence of hydrogen in the interfacial layer between the metal particle and the support. During high temperature reduction, the hydrogen is released from the interface, leaving platinum in direct contact with the support. The shortening of the Pt-O interfacial distance is accompanied by a decrease in the hydrogen chemisorption capacity, and may additionally be related to changes in the catalytic properties.

Infrared spectroscopy of adsorbed carbon monoxide was investigated on a series of platinum (or palladium) zeolite catalysts plus platinum on silica. The platinum catalysts all showed a strong absorbance band around 2070 [plus minus] 20 cm[sup -1] due to linear-bonded CO and a weaker band around 1850 [plus minus] 20 cm[sup -1] due to bridge-bonded CO. Four platinum catalysts, Pt/K[sup +] LTL, Pt/K[sup +] MFI, Pt/K[sup +] MTW, and Pt/K[sup +] SiO[sub 2], had an additional absorbance near 1970 cm[sup -1]. The additional band is not observed for Pt/silica, Pt/La[sup +3] LTL, or platinum supported on H[sup +] LTL or H[sup +] MFI, i.e., neutral and acidic supports. The additional band on the alkalized catalysts is speculated to result from an electrostatic attraction of linear-bonded CO with an alkali cation when Pt is on a nonacidic support. The electrostatic attraction increases the CO adsorption strength and lowers the stretching frequency. The additional linear CO absorption band was also observed on Pd/K[sup +] LTL. The additional infrared band suggests a [open quotes]new type of metal (catalytic) site,[close quotes] no relation was found between the catalytic activity or selectivity for n-hexane dehydrocyclization and the additional CO absorbance band. 49 refs., 6 figs.,more » 6 tabs.« less

Hydrogen temperature-programmed desorption (TPD) of supported platinum catalysts, Pt/KLTL, Pt/H-LTL, Pt/K-MAZ, Pt/H-MAZ, Pt/γ-Al 2 O 3 , and Pt/SiO 2 , was performed after hydrogen reduction at 300, 450, or 650°C. For all catalysts, reversible desorption of chemisorbed hydrogen occurred at approximately 175°C. In addition to chemisarbed H 2 , at least three peaks corresponding to higher temperature, irreversibly desorbed H 2 , were observed and assigned to spillover hydrogen. The quantity of spillover hydrogen is related to the number of support hydroxyl groups and the maximum reduction temperature of the catalyst. It is suggested that the irreversibility of the spillover hydrogen results from dehydroxylation of the support at high temperature. The structure of Pt/K-LTL and Pt/H-LTL catalysts has been examined by EXAFS following reduction at 300, 450, and 600°C. After low-temperature reduction, it is proposed that a layer of spillover hydrogen is present between the platinum particle and the support. The distance between the platinum and the support (oxide) surface is ca. 2.6-2.7 A. During high-temperature reduction, or during TPD, the interfacial hydrogen is irreversibly lost, and the platinum-oxygen distance decreases to 2.2 A. The catalysts′ turnover frequency (TOF) for propane hydrogenolysis was dependent on both the reduction temperature and the type of support. Platinum on acidic supports and reduced at the lowest temperature displayed the highest TOF. Qualitatively, the catalysts′ hydrogenolysis activity increased with increasing amounts of spillover hydrogen.

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.

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.

The relation between the catalytic activity, electronic properties, morphology, and structure of the metal-support interface was studied for Pt/@c-Al{2}O{3}after low (300}o{C, LTR) and high temperature reduction (450}o{C, HTR). EXAFS revealed that after LTR the platinum particles were three-dimensional, contained 11 Pt atoms on average, and were at a distance of 2.7 A from the support oxygen. During HTR, the morphology of the platinum particles changed from three-dimensional to rafts with a structure similar to Pt(100), as indicated by a decrease in the Pt-Pt coordination number and the absence of the third coordination shell in the EXAFS spectrum. After HTR the Pt-O distance was shortened to 2.2 A due to the desorption of hydrogen from the metal-support interface. The shortening of the Pt-O distance upon HTR treatment agrees with previous studies on zeolite supported platinum. However, zeolite supported platinum particles retained their three-dimensional structure upon HTR. The changes in the structure of the catalyst affected the catalytic, chemisorption, and electronic properties. After HTR the selectivity for hydrogenolysis of both neopentane and methylcyclopentane to methane decreased. At the same time the specific activity for neopentane isomerization and methylcyclopentane ring opening increased. The hydrogen chemisorption capacity after HTR was lower than after LTR. HTR shifted the asymmetric linear CO infrared absorption from 2063 to 2066 cm}-{}1{. Comparison of the Pt L{I}{I}{I}and L{I}{I}X-ray absorption edge intensities after LTR and HTR revealed that the number of holes in thed-band of the platinum atoms increased by 9.5% during HTR. It was suggested that the decrease in hydrogen chemisorption capacity, hydrogenolysis selectivity, and number of holes in thed-band are related to the change in the structure of the metal-support interface. The increase in specific activity for isomerization of neopentane and the shift in the CO infrared absorption band with a raise in reduction temperature agreed with reported results for single crystals and was attributed to the higher concentration of atoms with Pt(100) symmetry in the catalyst after HTR.

A series of Pt/LTL zeolite catalysts with varying Pt loading and support acidity was studied by hydrogen temperature-programmed desorption (TPD) and reaction of neopentane and propane. Specific desorption peaks in the TPD were associated with hydrogen spilled-over onto the support. The ability of the spilled-over hydrogen to migrate on the support surface was limited to approximately 35 nm from the platinum cluster. Saturation capacity for spilled-over hydrogen was related to the support acidity/basicity, with a maximum of about 1018 atoms/m2 for the most acidic support studied. Spilled-over hydrogen was inert to oxygen treatment at 100°C, and did not contribute to catalytic activity for neopentane or propane conversion. Instead, hydrogenolysis TOF and the amount of spilledover hydrogen were mutually dependent on support acidity/basicity. Both declined with increasing K/Al ratio of the support, and continued to decline even for basic supports with K/Al ratios greater than 1. The continued decline in activity for K/Al > 1 indicates than the metal-support interaction is not dependent on acidic hydroxyls.

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.

The adsorption of carbon monoxide on a non-acidic Pt/K-LTL catalyst has been studied by diffuse reflectance and transmission IR spectroscopy. The CO spectrum is strongly dependent on the experimental conditions. Adsorption on the small Pt clusters in the presence of water gives linear-CO bands between 2060 and 1990 cm−1 and a bridging-CO band around 1800 cm−1. In the absence of water, the linear bands are red shifted to about 1940 and 1720 cm−1, respectively. The frequency shift is attributed to an ion-dipole interaction between adsorbed CO and support cations. The ion-dipole interaction is screened by the adsorbed water leading to a smaller red shift in the CO stretching frequency.

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 A, and a Pt-Ba contribution at 3.8 A. The Pt/BaKL zeolite is one of the most highly dispersed supported platinum samples and one of the most structurally uniform supported metal catalysts.