Search

The discussion is devoted to some controversial points related to the aromatization of paraffins at high temperature (around 500°C), and low hydrogen pressure (1–8 bar) over non-acidic catalysts, in particular platinum plus a second component on a neutral(ized) support. Special attention is paid to those points which relate to the question why the second component enhances the stability of the dehydrocyclization (DHC) activity of supported platinum. We first consider the valence state of the second component. On the basis of, among other factors, the total hydrogen uptake we argue that at least part of the second component is present in the reduced state, while TPR points to alloy formation. A comparison of the catalytic and chemisorptive behaviour of bulk alloys with that of supported bimetallics indicates that modification (alloying) of the gas phase-exposed platinum surface is an essential action of the second component. This, in turn, suggests that the superior stability of bimetallic DHC catalysts is related to the catalytic properties of the alloys. Considering the changes in selectivity observed upon alloying we will argue that the most important one is the enhanced selectivity towards dehydrogenation of paraffins (olefins) to the corresponding olefins (poly-olefins), which leads to the question: “Are bimetallics under DHC conditions merely selective dehydrogenation catalysts?” From the invariance in TON for dehydrogenation per surface-exposed platinum atom up to very high dilution of platinum in gold (< 1% at Pt in Au), we infer that only one platinum atom is involved in the rate-determining step. We will extrapolate the aforementioned results to actual DHC catalysts and consider the proposition that the active site in PtSn, PtRe (S), Pt (S) and deactivated Pt (i.e. Pt (C)) DHC catalysts consists of one platinum atom embedded in an “inactive” matrix. Finally we will list some points deserving further study, amongst them the interpretation of the results of techniques such as TPR, the relative importance of different DHC pathways, the chemisorptive properties of alloys and the causes underlying the low hydrogen uptake of “overreduced” supported platinum catalysts.

Abrupt changes in the reaction ambient from 12 CO H 2 to 13 CO H 2 are utilized to obtain transientkinetic information regarding the causes of activity decline in the methanation over Raney nickel. From the responses we infer that the methanation proceeds via at least two parallel pathways. The relative contribution of the two pathways changes with catalyst aging. The gradual development of a less reactive carbonaceous overlayer alone is insufficient to explain the observations. We conclude that aging affects the nickel catalyst itself.

Exchange of silicate anions into hydrotalcite-type materials leads to the ready formation of well-ordered two-dimensional silicate sheets. The combined evidence from diffraction, NMR, and IR allows for a global identification of the reaction product. Its structure probably consists of positively charged brucitelike layers intercalated with puckered, silicic-acid-type macroanions.

Chemisorption, ir, and XPS data indicate that rhenium atoms covered with chemisorbed sulfur atoms divide the metal surface of platinum-containing catalysts into ensembles consisting of a small number of contiguous platinum atoms. Data on the catalytic conversion of n-hexane show that it is the combined action of rhenium and sulfur that converts platinum into a catalyst of very high selectivity for mild dehydrogenation. It thus appears that the beneficial effects on catalyst selectivity and stability which are brought about by adding widely different catalyst modifiers (gold, tin, sulfur, carbon) to platinum catalysts are largely due to one common cause, namely, the division of the platinum surface into very small ensembles of platinum atoms.

The rates of dehydrogenation of propane to propene over platinum and very dilute platinumin-gold alloys have been measured. In the composition range of 0.5–14.0 atom % platinum, the rates per unit surface area of the alloy powders vary linearly with the bulk platinum concentration in the alloys. From this it is concluded that only one platinum atom is involved in the rate-determining step. For both platinum and the alloys, activation energies of some 29 ± 2 kcal/mole were measured. The reaction rate order in hydrogen, however, is different. It is proposed that propane dehydrogenation over platinum and over the alloys occurs via the same reaction mechanism, namely, dissociative chemisorption of propane on a single platinum atom to which two adsorption sites are associated, one of which carries a hydrogen atom. The subsequent conversion of the propyl radical into π-bonded propene via β-hydrogen elimination appears to be rate determining. The last step, desorption of π-bonded propene, has a comparatively low activation energy. The difference in negative reaction order, with respect to hydrogen, between platinum and its diluted alloys reflects a lower steady-state θH on Pt atoms surrounded by Au atoms vis-a-vis Pt atoms on a Pt surface.

Precipitation of Al3+ at pH = 10 in excess Li2CO3 leads to an anion exchanging compound, [Al2Li(OH)6]+2CO2−3. This compound exhibits, compared to [Mg3Al(OH)8]+2CO2−3, a higher degree of size selectivity in anion exchange. The structure of the [Al2Li(OH)6]+ layers is gibbsite-like, with a (110) diffraction feature at d = 4.35 A indicating a pronounced Al3+ ordering. As claimed originally by Serna et al., the structure is [ Al 2 Li(OH) 6 ] + A z− 1 z rather than [Al 2 (OH) 6 ]Li + A z− 1 z , with the Li+ coordinated in the octahedral positions left vacant by Al3+. This emerges from the details of a lithium-leaching process, which proposedly leads to a novel compound, [ Al 2 H(OH) 6 ] + A z− 1 z .

Stepwise switches from a 12 CO H 2 to a 13 CO H 2 feed and vice versa have been utilized to obtain transient kinetic information without disturbing the steady-state catalysis. In the context of methanation and Fischer-Tropsch catalysis it emerges that of the large carbidic overlayer which develops only a small part belongs to reaction intermediates proper. Under the prevailing conditions the fractional coverage in growing chains is low and, consequently, the chain growth relatively fast i.e., not rate-determining. Details in the relaxation behavior are suggestive of intermediates that are carbidic in nature.

An abrupt change of the reaction environment from 12COH2 to 13COH2 leads to a gradual ingrowth of 13C in hydrocarbons. This rate of ingrowth can be modeled in a manner which is essentially independent of the details of the reaction mechanism. It is calculated that the rate of CC bond formation varies from ≥1 s-1 (ruthenium) to ≤0. 1 s-1 (cobalt). Further findings include the experimental observation of surface heterogeneity and the relatively short residence times for the C1 monomers involved in chain growth.

Distinct selectivity changes have been observed in the conversion of n-hexane over Pt upon its being alloyed with Sn. In systems consisting of PtSn on nonacidic supports, the presence of true alloys can be confirmed. Addition of Sn causes a drastic lowering of the rates of those reactions which produce catalyst poisons and hence gives an increase in stability. The change in selectivity can be interpreted in terms of a change in adsorption properties due to differences in the number of adjacent platinum atoms required for the various reaction pathways (ensemble effect). It is assumed that with these catalysts and at temperatures above 500 °C, secondary reactions in the gas phase become important. Hexatrienes formed on the catalyst surface might then contribute to the benzene production and radicals might initiate hydrogenolysis. The present results allow some speculations on the role of the metal component in bimetallic dual-functional Pt catalyst systems.

An X-ray photoelectron spectroscopy study of supported tungsten oxide has indicated that calcination of WO 3 γ- Al 2 O 3 leads to formation of tungstate. Proof has been obtained that WO 3 γ- Al 2 O 3 cannot be appreciably reduced in hydrogen at 550 °C, whereas WO 3 SiO 2 is reduced to lower valence states and unsupported WO 3 is completely reduced to tungsten metal under these conditions.

The accessibility of the interlayer space in layered double-metal hydroxides is being reflected by their sorption of nitrogen and hydrocarbons. The results indicate that the interlayer space of dehydrated materials intercalated with [Fe 11 (CN) 6 ] 4− and [Fe III (CN) 6 ] 3− anions is accessible to molecules as large as methyl eyelohexane whereas even nitrogen is excluded from the interlayer space in the material intercalated with NO 3 − anions. These findings confirm that their microporosity can be widely changed by varying the size and charge of the intercalating anions.

Experimental work is continuing on four separate projects related to coal conversion processes. The direct digital control of exothermic multiphase reactions is being studied in an experimental adiabatic flow reactor. The existence of two stable steady states for the Fischer-Tropsch reaction network at the same temperature and feed condition has been verified and quantified. Various absorbents for SO/sub 2/ and NO/sub X/ are being studied. The absorption of NO/sub 2/ by methanol and N-cyclohexyl-2-pyrrolidone has been extensively examined. Preliminary data have been obtained with triethylene-tetraamine. Hindered amines will be studied next. Procedures for the preparation of liquid membranes have been tested and the incorporation of hindered amines in them will now be examined. Isotopic switching is being used to study the way in which promoters affect supported metal catalysts. With improved resolution from the mass spectrometer, early quantitative results give indications of three different surface species and of non-statistical ingrowth of /sup 13/C into the product molecules. A program for the study of the extraction of coal and oil shale using supercritical fluids is being carried out. The effect of the presence of piperidine on the amount of toluene solubles produced by supercritical extraction of coal with toluene/piperidine mixture hasmore » been determined. A new kinetic model for the extraction/liquefaction of coal by supercritical toluene and THF has been developed and proven satisfactory. Bruceton coal and Hi Na lignite have been extracted with supercritical water. 3 references, 7 figures, 6 tables.« less

Transient-kinetic experiments reveal that the surface intermediates on platinum exhibit lifetimes and abundances which are two orders of magnitude lower than on nickel.