Zeolite-supported Platinum Catalysts Research Articles

Hydrophobic modification of zeolite-supported platinum catalysts for complete oxidation of toluene

Zeolite-supported noble metal nanoparticles have been reported as active catalysts for complete oxidation of volatile organic compounds (VOCs) due to the combination of advantages of zeolite supports and noble metal species, where introducing more Al species (reduction of SiO2/Al2O3 ratios, SAR) and increasing hydrophobicity for zeolite supports are favorable for the complete oxidation of VOCs. Unfortunately, introduction of more Al species in the zeolite frameworks normally reduces the hydrophobicity of zeolite crystals. To overcome this problem, we rationally synthesize hydrophobic zeolites with low SAR by anchoring methyl group, exhibiting exceptionally catalytic performances in the complete oxidation of toluene at very low temperature (T90 at 144 °C) after loading Pt species (1 wt%). Characterizations of the catalysts show a vital role of hydrophobic zeolites for adsorption of more toluene and fast desorption of water. The strategy for preparation of hydrophobic zeolite-supported noble metal catalysts might offer an alternative way to develop highly efficient heterogeneous catalysts for the complete oxidation of VOCs in the future.

Superior performance in catalytic combustion of toluene over mesoporous ZSM-5 zeolite supported platinum catalyst

Catalytic combustion over zeolite supported platinum catalysts is regarded as one of the efficient methods for removal of volatile organic compounds (VOCs). In this work, we show a preparation of Pt loaded mesoporous ZSM-5 zeolite (Pt/Meso-ZSM-5) with various cations. The sample N2 sorption isotherms and TEM image show abundant mesoporosity existing in the zeolite crystals. The catalytic toluene combustion tests show that the Pt/Meso-ZSM-5 catalysts have higher activities than conventional ZSM-5 supported Pt catalysts (Pt/ZSM-5), which is related to their difference in the concentration of active sites Pt0 and the rate of mass transfer. Particularly, after reduction with hydrogen at 300°C, the Pt nanoparticles supported on Meso-ZSM-5 with K+ and Cs+ cations (Pt-R/Meso-KZSM-5 and Pt-R/Meso-CsZSM-5) exhibit much higher activities than those on Meso-ZSM-5 with Na+ and H+ cations, which is well consistent with their order of the Pt0 concentration. For example, Pt-R/Meso-KZSM-5 gives the temperature for conversion of 98% toluene (T98) at only 175°C, while Pt-R/Meso-HZSM-5 shows the T98 at 190°C. More importantly, Pt-R/Meso-KZSM-5 has a long life, and its activity cannot be influenced by the water and CO2. These features are critically important for practical applications of these catalysts in the future.

Zeolite supported platinum catalysts for benzene hydrogenation and naphthene isomerization

Benzene hydrogenation over zeolite supported platinum catalysts; particularly the effect of catalyst preparation procedures was studied. It was found that Pt/zeolite prepared with wet solution ion exchange procedure using cationic platinum precursors showed the best catalytic performances. The potential of Pt/beta in catalyzing benzene hydrogenation–naphthene isomerization cascaded reaction was demonstrated, showing the highest methyl-cyclopentane to cyclohexane product ratio comparable to the thermodynamic equilibrium value, the lowest cracking gas yield and a maximum octane barrel number.

Hydroisomerization of a hydrocarbon feed containing n-hexane, n-heptane and cyclohexane on zeolite-supported platinum catalysts

Bifunctional catalysts consisting of platinum supported on various types of zeolites (viz. beta, mordenite, USY, ZSM-5 and ferrierite) were prepared by using an incipient wetness impregnation method to obtain a 0.5 wt.% metal load. A beta zeolite sample was dealuminated with nitric or oxalic acid to obtain solids that were used as catalytic supports. The catalysts thus obtained were characterized by using various techniques. Analyses included specific surface area and acid properties; the latter were determined either by adsorption of pyridine or by temperature-programmed desorption of the same chemical. The metal phase was characterized by temperature-programmed desorption of hydrogen and by hydrogen chemisorption, which allowed the dispersion of the metal on the support surface to be determined. The catalysts obtained were assessed for activity and selectivity in the isomerization of a mixture consisting of n-hexane (65 wt.%), n-heptane (15 wt.%) and cyclohexane (20 wt.%). Both parameters were correlated with the acid and metal properties of the catalysts, as well as with the topology and pore geometry of the zeolites. The effect of each feed component on catalytic activity was also studied, using binary mixtures of the three hydrocarbons.

Platinum Clusters Supported in Zeolite LTL: Influence of Catalyst Morphology on Performance inn-Hexane Reforming

KLTL zeolite-supported platinum catalysts were synthesized from aqueous tetraammineplatinum(II) nitrate solutions and nonacidic KLTL zeolite crystallites, including some with dimensions as little as 300×500 Å. The zeolite crystallites had various morphologies, some being predominantly disk-shaped particles and some predominantly mosaics of rod-like domains with a range ofc-dimension lengths. The activity and selectivity of each catalyst were evaluated for dehydrocyclization ofn-hexane in the presence of H2to form predominantly benzene at conversions of typically 45–90%. The data presented here provide a detailed characterization of the deactivation of such catalysts in the absence of sulfur. EXAFS data show that the platinum in each catalyst was present in clusters of about 20 atoms each, on average. Electron micrographs show that the platinum clusters were nearly evenly dispersed on the surfaces of the zeolite crystallites, including the intracrystalline and extracrystalline surfaces. The catalytic performance was virtually independent of the zeolite channel length, but activity, selectivity, and resistance to deactivation were found to be correlated with the ratio of the surface area external to the crystallite domains to that within the intracrystalline pores. The catalyst performance is dependent on this ratio (which is related to the zeolite morphology) as follows: in comparison with the others, the catalysts with the relatively low fractions of platinum outside the intracrystalline pores are more active, more selective for benzene formation, and more resistant to deactivation. One well-prepared catalyst, for example, gave greater than 90% selectivity for benzene and no measurable deactivation over 140 h of operation in a flow reactor at 420°C and atmospheric pressure. These data match those characterizing the most selective catalysts reported. Consistent with the interpretation of E. Iglesia and J. E. Baumgartner (in“New Frontiers in Catalysis” (L. Guczi, F. Solymosi, and P. Tetenyi, Eds.), p. 993,Studies in Surface Science and Catalysis,Vol. 75. Elsevier, Amsterdam, 1993), catalyst deactivation is associated with platinum outside the zeolite pores; it is hypothesized that coke formation outside the pores is relatively rapid and that a distinguishing characteristic of the best catalysts is the presence of most of the platinum in the intracrystalline pores, where coke formation may be inhibited by the constraints of the pores.

Electronic effects and effects of particle morphology in n-hexane conversion over zeolite-supported platinum catalysts

n-Hexane conversion over zeolite-supported platinum catalysts has been studied at 360°C in a packed bed reactor. In the absence of sulfur impurities, the deactivation appears to be due to two types of coke formation. The propensity for terminal hydrogenolysis is primarily determined by the particle morphology, i.e., the type of surface sites exposed. The increase in benzene selectivity correlates with a shift to lower stretching frequencies of the CO absorption bands, indicating that an increase in the electron density at the surface metal atoms results in higher benzene selectivity. The effect of the support in the high activity and aromatization selectivity of a monofunctional platinum catalyst for n-hexane conversion is observed to be twofold, i.e., the stabilization of extremely small metal particles of specific morphology under reaction conditions and metal-support interaction, resulting in an increased electron density over the metal particles.

Influence of Platinum Ammine Complex Ion Species on the Preparation of Pre-Pelletized Zeolite Supported Platinum Catalysts.

Influence of Platinum Ammine Complex Ion Species on the Preparation of Pre-Pelletized Zeolite Supported Platinum Catalysts.

Microcalorimetric Study of Silica- and Zeolite-Supported Platinum Catalysts

Microcalorimetric measurements of the differential heats of hydrogen and carbon monoxide adsorption versus adsorbate coverage were made at 403 K for platinum supported on silica, magnesia/alumina, L-zeolite, Y-zeolite, and ZSM-5. The differential heats at zero coverage for hydrogen and carbon monoxide adsorption were 90 and 140 kJ/mol, respectively. for platinum supported on silica and nonacidic zeolites. The differential heats were larger by approximately 20 kJ/mol for hydrogen and carbon monoxide adsorption on platinum particles supported on basic supports such as potassium/silica, magnesia/alumina, and zeolites containing basic cations (K+, Ba2+) exchanged in excess of the zeolite framework aluminum content. The microcalorimetric results suggest that the high paraffin aromatization activity and selectivity observed for L-zeolite-supported platinum catalysts do not appear to be caused solely by changes in the adsorptive properties of the cluster-sized platinum particles located within the zeolite.

Characterization of montmorillonites pillared by cationic silicon species using 129Xe-NMR spectroscopy

The l”‘Xe-NMR technique has recently been extensively used to probe the porosity of zeolites, the location of metal clusters and cluster precursors as a function of calcination conditions in NaY zeolite-supported platinum catalysts, as well as the arrangement of organic molecules within the zeolite Y supercages [l--4]. It is of interest to investigate whether xenon can be used to characterize other microporous systems. The aim of this work was to study the microporosity of a new class of materials, pillared interlayered clays (PILC ). We have found that 12’Xe-NMR is sensitive to the microporosity between the pillars and to some extent to the partial crystallinity of these materials.

Metal Catalysts Supported on Zeolite. II. Dehydrogenation of Cyclohexane and Hydrodemethylation of Toluene over Zeolite-Supported Platinum Catalysts

Abstract Correlations between the amount of hydrogen adsorption and the catalytic activities of the zeolite-supported platinum catalysts were interpreted in terms of the dispersion state of platinum particles. The activities for dehydrogenation of cyclohexane and hydrodemethylation of toluene were directly proprotional to the amount of hydrogen adsorption. The tiny platinum particles with diameters of less than 20 Å, which had little ability to chemisorb hydrogen, showed a poor activity in both reactions.