Z. Paál

The mechanism of aromatization on platinum black catalyst; dehydrocyclization of hexadienes and hexatrienes

Abstract The aromatization of hexadiene isomers as well as trans - and cis -1,3,5-hexatriene on Pt-black catalyst has been studied in the presence of helium and hydrogenhelium mixtures. The extent of benzene formation from all of the hydrocarbons was of the same order of magnitude, and the activity of the catalyst rapidly decreased when subsequent pulses of hydrocarbons were introduced without regeneration. The only exception was cis -triene where the benzene yield was much higher and deactivation slower. The presence of hydrogen even at small partial pressure considerably slowed down the deactivation. The role of hexatrienes as intermediates of aromatization has been evaluated in the case of different hexadiene isomers. A stepwise scheme of aromatization involving steps of dehydrogenation and cis-trans isomerization has been proposed. The importance of these two processes is considered to be about the same in the overall reaction rate, whereas the ring closure step is regarded as very fast. The precursors of coke formation on the catalyst surface are supposedly trans -polyenes. The relative importance of benzene and coke formation are determined by the presence of hydrogen in the gas phase.

Characterisation and activity in n-hexane rearrangement reactions of metallic phases on Pt–Sn/Al2O3 catalysts of different preparations

The platinum–tin interactions in Pt–Sn/Al2O3 catalysts were followed through several characterisation methods and modified by using two preparation procedures (1.5 wt% Pt, Sn:Pt=1:1): conventional coimpregnation with H2PtCl6 and SnCl4 (T sample) or by use of the bimetallic precursor [Pt(NH3)4]SnCl6, which was synthesised in the support porosity (N sample). The effects of these interactions on catalytic properties were displayed by the activity and selectivity in n-hexane rearrangement reactions. For both samples platinum and tin are reduced, but they have very different platinum dispersions which are related to different temperature-programmed reduction profiles: 52% for sample T and 4% for sample N. Insitu tin Mossbauer spectroscopy confirms that the majority of tin is reduced, and a minority remains as SnII; air treatment leads to a partial reoxidation of SnII to SnIV, sample N retaining more tin as alloy. X-Ray diffraction displays the simultaneous presence of PtSn, Pt3Sn and Pt with more alloys on sample N; the co-impregnated sample, which has a greater platinum phase, shows a better dispersion of tin (XPS data), in accordance with a high interaction with alumina. The catalytic activity was controlled by the platinum phase; for sample T, the influence of the addition of tin is restricted, whereas the catalyst prepared from the bimetallic precursor exhibits particular properties, attributable to the stabilisation of platinum in smaller ensembles, and the modifying effect of tin was clearly evidenced. The catalytic properties are explained by the distribution and morphology of Pt ensembles present on various faces of Pt–Sn alloys. The lower amount of alloys in sample T can be related to a higher initial activity in C5 ring closure whereas the higher amount of these phases on catalyst N is in accord with a higher turnover frequency, and a good selectivity for the formation of olefins which are transformed into C6 saturated skeletal isomers in longer runs. The results are supplemented by thermodynamic data on the reduction of tin oxides and by the geometric properties of the low-index faces of PtSn and Pt3Sn alloys.

Metal-Catalyzed Cyclization Reactions of Hydrocarbons

Publisher Summary This chapter emphasizes on the metal-catalyzed cyclization reactions, with reference to oxide and dual-function catalysts. Product cycles may contain five or six carbon atoms. A common feature of any cyclization reaction is that a new intramolecular C-C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called “simple” cyclization processes, although their mechanism is, as a rule, complex. As few as six different kinds of adsorption are proposed in the chapter as being responsible for a great variety of hydrocarbon transformations over metal catalysts. This approach is fully accepted that the character of primary adsorption determines the structure of the product. One of the main points that is stressed is that very different reactions may often be concealed behind the expression “cyclization.” An attempt is made in the chapter to correlate primary adsorption (consequently the reactions expected) with two main factors: the nature of the metal and the amount of hydrogen available during the catalytic process. The latter may be of paramount importance: the amount of surface hydrogen may govern which type of chemisorbed species is formed and, by doing so, determine catalytic selectivity.

Activation of unsupported CoMo catalysts in thiophene hydrodesulfurization

Abstract A series of unsupported oxidic CoMo catalysts with different mole fractions r = Co (Co + Mo) was prepared by coprecipitation of solutions of (NH 4 ) 6 Mo 7 O 24 and Co(NO 3 ) 2 . The calcined catalysts contain a-CoMoO 4 ( r = 0.5) mixed with MoO 3 ( r 3 O 4 ( r > 0.5) according to X-ray diffraction (XRD) and electron diffraction. The a-CoMoO 4 transforms partially into b-CoMoO 4 upon grinding. The higher the cobalt ( r ) and b-CoMoO 4 contents, the higher are the surface area increase and the degree of reduction of calcined catalysts during hydrogen treatment at 673 K. Electron microscopy (EM) data agree well with the surface area increase observed after reduction. X-ray photoelectron spectroscopy (XPS) shows the reduction of molybdenum rather than that of cobalt. Reduced crystalline phases cannot be identified by these techniques. Sulfidation with a mixture of H 2 /thiophene following reduction caused a drastic drop in surface area but the particle size seen by EM does not increase. Weak oxythiomolybdate XRD bands appeared after slight sulfidation, most XRD signals disappeared after massive sulfidation of samples with r = 0.5. Cobalt promotes sulfidation of molybdenum in the bulk, but the maximum sulfidation degree was about half of the stoichiometric value. XPS shows surface cobalt enrichment, XRD and EM traces of Co 9 S 8 in samples with r = 0.38 and 0.50. A pronounced maximum was observed in initial hydrodesulfurization (HDS) activity and hydrogenation (HYD) selectivity at medium Co content. On used catalysts, this synergism disappeared. We attribute the highest HDS activity of short-living cobalt-oxythiomolybdate(s) formed initially during sulfidation. HYD was promoted by sulfided molybdenum and by less surface Cobalt.

Photoelectron spectroscopy of polycrystalline platinum catalysts

Pt black catalysts have been characterized by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The spectra measured after standard purification (O2 and H2 at 600 K) compared well with those of a purified reference Pt foil. All samples exhibited pronounced Fermi-edge intensities in UPS although only 60–70% Pt was detected on their surfaces by XPS, the remainder being C and O. Line analysis of the C 1s XPS, region showed the presence of partly oxidized graphite and hydrocarbon polymer, likely in three-dimensional islands. OH/H2O species attached to the metallic Pt sites were detected by UPS bands, in agreement with O 1s XPS line analysis. Similar spectral features are recorded at 600 K. Carbon could not be removed entirely by O2 up to 850 K; hydrogen did not remove surface oxygen even up to 750 K. UPS features of C on Pt used in hydrocarbon reactions were similar to those reported for amorphous hydrogenated carbon overlayers. Consequences of the present findings for the catalytic properties of Pt in n-hexane reactions and the quantification of H2–O2 titration are discussed briefly.

Sulfur adsorbed on Pt catalyst: Its chemical state and effect on catalytic properties as studied by electron spectroscopy and n-hexane test reactions

Abstract Pt black samples in a fresh state, deactivated by coking and treated with H2S were studied with electron spectroscopy. Their catalytic properties were tested in skeletal reactions of n-hexane. The Pt 4f XPS peaks show that Pt is predominantly in the metallic state after the treatments. After sulfidation, a minor amount of PtS can be detected in the difference spectrum (Pt unsulfided - Pt sulfided) only. The growth of the O 1s intensity after sulfidation can partly be attributed to the sulfate component. C 1s shows, as a rule, residual carbon in low amounts on sulfided samples. The S 2p band shows 4–8 at.-% S with respect to the total surface in two valence states: sulfate and sulfide, the latter including also minor amounts of organic sulfur which could arise from the sulfidation of hydrocarbonaceous residues. UPS demonstrated chemical Pt—S interaction even after O2/H2 regeneration when the Fermi-edge Pt intensity rose to a height almost equal to that of a regenerated sulfur-free Pt. ISS shows that sulfur — as opposed to carbon and oxygen — is a surface component. Sputtering effect of He+ ions used in ISS can remove sulfate while a large part of sulfide is removed during O2/H2 regeneration and/or during n-hexane test runs. Both sulfidation and carbonization strongly decrease the overall catalytic activity. The selectivity of Pt with S present mainly as sulfate was similar to that of carbonized Pt, mainly producing hexenes. Pt mainly containing sulfide, produced initially mainly methylcyclopentane, as reported also for the sulfided single crystal catalyst. Carbonized Pt could be fully regenerated by O2/H2 treatment at 600 K but the same treatment — even repeated — restored the overall activity of a sulfided Pt only partially. We believe that sulfide acts mainly as a bonding modifier whereas sulfate can be a structural modifier influencing the selectivity similarly as coke or coke precursors.

Comparative study of various platinum catalysts in skeletal reactions of C6-hydrocarbons

Four platinum catalysts, Pt-black, PtC, PtSiO2, and PtAl2O3, have been compared in skeletal reactions of 3-methylpentane (3MP) and methylcyclopentane (MCP), in the presence of various partial pressures of hydrogen. Analogous hydrogen effects (defined as primary and secondary) have been observed for each catalyst. The selectivities could be explained almost exclusively by the influence of hydrogen. The selectivity of isomerization plus C5-cyclization vs fragmentation increased with increasing hydrogen pressure as did the ratio of saturated vs unsaturated C6 products. The 2-methylpentane vs n-hexane ratio (2MP/n-H) from both 3MP and MCP exhibited a strong hydrogen dependence (permitting separation of bond shift and C5-cyclic isomerization as a function of the hydrogen pressure), and significant crystallite size effects were also observed here.

Catalytic transformations of cyclohexanol on Group VIII metal catalysts

Abstract Transformations of cyclohexanol and cyclohexanone have been studied over various Group VIII metals as catalysts. For each metal the predominant reaction of cyclohexanol was dehydrogenation to cyclohexanone. Two main groups of metals can be distinguished. Selectively dehydrogenating metals are those where dehydrogenation stops at the stage of cyclohexanone (Os, Co, Fe, Re, Ru). Aromatizing metals catalyze also the further dehydrogenation of cyclohexanone to aromatics (Pd, Pt, Ni). Rh and Ir occupy an intermediate position: they dehydrogenate in nitrogen and aromatize in hydrogen. Radiotracer methods show that cyclohexanone is the intermediate of phenol formation, except for Pt and Pd where there is a “direct” route of phenol formation from cyclohexanol. Benzene is the product of the hydrogenolytic splitting of the phenolic OH group. Dehydration of cyclohexanol to cyclohexene is not important, although it occurs over some dehydrogenating metals. Ru is the only metal where there is considerable additional formation of benzene via cyclohexene. Hydrogenolysis of the alcoholic OH group was not observed. Hydrogenolysis of the CC bond of the ring is favoured by hydrogen carrier gas; it is considerable over dehydrogenating metals as well as Rh and Ir. The enhanced reactivity of cyclohexanol as compared with cyclohexane is due to the presence of the OH group facilitating the interaction of the molecule with the surface. Knors model of localized/free-electron interplay as well as the number of unpaired d electrons could be used to interpret the different activity of various metals.

Transformation of n-hexane over EUROPT-1: fragments and C6 products on fresh and partially deactivated catalyst

The reactions of n-hexane have been studied on 6.3% Pt/SiO2(EUROPT-1) at different hydrogen and n-hexane pressures, and at 543–633 K, over fresh catalyst and over catalysts deactivated by long runs. Turnover numbers are compared with literature data: the differences are attributed to hydrogen pressure effects. Deactivation influences first of all, selectivity. In addition, the ‘depth’ and ‘pattern’ of hydrogenolysis have been determined. At low temperature multiple splitting seems to be favoured. Isomerization gives predominantly 3-methylpentane. At medium temperatures, isomerization, C5-cyclization and internal splitting prevail; their ratio is controlled by the hydrogen pressure. The ratio of 2-methylpentane to 3-methylpentane is related to the ratio of internal to terminal rupture. Terminal splitting prevails at highest temperature. Aromatization increases with temperature but seems to be independent of the other reactions. The results are interpreted in terms of three different surface states. These correspond to Pt—H, Pt—C—H and Pt—C under increasing severity of conditions.

Recently published work on EUROPT-1, a 6% Pt/SiO2 reference catalyst

Abstract EUROPT-1 is a 6% Pt/SiO 2 catalyst which has been thoroughly characterised with respect to its composition, structure, chemisorption behaviour and catalytic properties. Extensive further work on this catalyst has been published and this is now reviewed. Debye function analysis of X-ray diffraction spectra suggest platinum particles to be cubooctahedral, although preliminary extended X-ray absorption fine structure results do not accord with this model. Further isotherms for carbon monoxide, hydrogen and oxygen chemisorption have been reported and enthalpies of adsorption for the last two gases measured. Limited results on structure-insensitive reactions such as exchange of methane with deuterium and alkene hydrogenation are available. Methane homologation as well as skeletal reactions of alkanes in the presence of hydrogen represent structure-sensitive reactions. Some further work on ethane, propane and n-butane is reported; detailed results on n-hexane and methylcyclopentane include changes in rates and selectivities as a function of hydrogen and hydrocarbon pressure. They are interpreted in terms of availability of surface hydrogen and hydrogen effects on the formation of carbonaceous deposits of various degree of dehydrogenation. Effects of sintering have also been studied, and active centres for various reactions have been tentatively suggested. On the whole, EUROPT-1 shows behaviour which is characteristic of platinum catalysts containing small platinum particles but in some limited respects it shows unusual and unexpected properties. Reactions of some oxygenated compounds are described briefly. Catalytic properties of EUROPT-1 can be modified by metals, oxides, sulphur and nitrogen; treatment with cinchonidine converts it into an enantioselective catalyst for the hydrogenation of methyl pyruvate. Finally the importance of selecting appropriate reference supported metal catalyst (s) is stressed and the future role for EUROPT-1 as a possible candidate is pointed out; results must however be compared under carefully defined conditions if they are to have real value.