Gordon Kelly

Competitive reactions in alkyne hydrogenation

Abstract Competitive hydrogenation reactions between alkynes have been studied over a palladium/carbon catalyst. The competitive reactions of 1-pentyne, phenyl acetylene, 2-pentyne, and 1-phenyl-1-propyne have been examined. The results show that in general, the competitive reaction results in a reduction of the hydrogenation rate for both alkynes. However, the 1-pentyne/2-pentyne couple revealed a rate enhancement for both alkynes. This is thought to be due to enhanced hydrogen transfer on the surface. Terminal alkynes reduced the rate of hydrogenation of a competing alkyne more effectively than internal alkynes. Phenyl acetylene was very sensitive to the presence of the second alkyne with its hydrogenation rate being reduced proportionately more than the aliphatic alkyne. This may be related to a disruption of its π–π stacking ability. Competitive hydrogenation increased selectivity to the respective alkenes but had no effect on the cis : trans ratio.

Stable amorphous georgeite as a precursor to a high-activity catalyst

Copper and zinc form an important group of hydroxycarbonate minerals that include zincian malachite, aurichalcite, rosasite and the exceptionally rare and unstable—and hence little known and largely ignored—georgeite. The first three of these minerals are widely used as catalyst precursors for the industrially important methanol-synthesis and low-temperature water–gas shift (LTS) reactions, with the choice of precursor phase strongly influencing the activity of the final catalyst. The preferred phase is usually zincian malachite. This is prepared by a co-precipitation method that involves the transient formation of georgeite; with few exceptions it uses sodium carbonate as the carbonate source, but this also introduces sodium ions—a potential catalyst poison. Here we show that supercritical antisolvent (SAS) precipitation using carbon dioxide (refs 13, 14), a process that exploits the high diffusion rates and solvation power of supercritical carbon dioxide to rapidly expand and supersaturate solutions, can be used to prepare copper/zinc hydroxycarbonate precursors with low sodium content. These include stable georgeite, which we find to be a precursor to highly active methanol-synthesis and superior LTS catalysts. Our findings highlight the value of advanced synthesis methods in accessing unusual mineral phases, and show that there is room for exploring improvements to established industrial catalysts.

The Hydrogenation of c-5 Alkynes over Palladium Catalysts

The hydrogenation of 1-pentyne and 2-pentyne was studied over palladium catalysts. The internal triple bond hydrogenated faster than the terminal and a particle size effect was observed on the rate of hydrogenation and isomerisation.

Methanol decomposition over copper/silica: effect of temperature and co-reactant on carbon deposition

The interaction of methanol with a copper/silica catalyst at 373 and 523K under reducing, oxidising and inert carrier gas flows has been studied. Under all conditions there is retained material associated solely with the copper. In general the retained species is adsorbed methanol/methoxy; only over an oxidised catalyst after treatment at 523K is there no evidence for adsorbed methanol/methoxy. Desorption of carbon dioxide is associated with an up-take in dioxygen indicating oxidation of a surface species, probably formate. After laydown under reducing or inert gas flow, the copper does not re-oxidise under the TPO gas flow, even at temperatures >673K indicating that material is still retained by the copper. Bulk re-oxidation of the reduced catalyst in the absence of retained species is rapid at 293K. Under oxidising conditions at 523K there is no evidence for adsorbed methanol/methoxy on the surface of the copper; in this case the retained species may be more akin to a carbonate.

Aldol condensation reactions of acetone and formaldehyde over vanadium phosphate catalysts: Comments on the acid–base properties

The acid–base properties of vanadium phosphate catalysts are investigated using the aldol condensation of acetone and the reactions of 2-methylbut-3-yn-2-ol (MBOH). Three well characterised samples of VOHPO4·0.5H2O were prepared using the reaction of V2O5 and H3PO4 with aqueous hydrochloric acid or isobutanol as reducing agents, or from the reaction of VOPO4.2H2O with isobutanol. (VO)2P2O7, prepared by heating VOHPO4·0.5H2O in He (8 h, 750 °C), before and following partial oxidation in air or butane/air, and αI-VOPO4 were also investigated. The reaction of MBOH was used to probe the nature of the acid–base properties of the vanadium phosphates. The V4+ phases (VOHPO4·0.5H2O and (VO)2P2O7) exhibited only acidic active sites, whereas the V5+ phases (αI-VOPO4 and oxidised (VO)2P2O7) exhibited some basic sites in addition to the acid sites. For the aldol condensation reactions of acetone, the V4+ phases were found to be selective for the formation of isophorone from acetone alone and methyl vinyl ketone from the reaction of acetone and formaldehyde. In contrast, vanadium phosphate catalysts containing V5+ phases are not selective to these products and only form hydrocarbons (typically isobutane and isobutene). For all these reactions, the catalyst activity is short lived and the deactivation that is observed is due to the surface becoming fouled by the adsorption of products of polymerisation of the reaction products. However, the catalyst reactivity can be restored by a simple oxidation treatment. The nature of active sites in n-butane oxidation to maleic anhydride is also discussed and it is concluded that basic sites are required in addition to acidic surface sites for the selective formation of maleic anhydride. For the reaction of MBOH, the data are found to give a linear relationship for a Cremer–Constable plot and this is discussed in terms of the enthalpy of adsorption of MBOH.

FTIR Studies of the Adsorption of Acetic Acid on Silicas

A range of cesium doped silica samples (0.15 – 0.90 mmol g−1) have been characterized using a combination of deuterium exchange and adsorption of acetic acid. FTIR results show a strong correlation between the carboxylate formation on acetic acid adsorption and catalyst activity. Cesium loadings of > 0.3 mmol g−1 do not give any increased acetic acid adsorption, suggesting a saturation of the silica exchange capacity. Deuterium exchange studies demonstrate that 35% of the surface silanol groups on the fresh silica are rapidly exchanged with deuterated water at room temperature.

Supported nickel catalysts: Hydrogenolysis of ethane, propane, n-butane and iso-butane over alumina-, molybdena-, and silica-supported nickel catalysts

The catalytic properties of three supported nickel catalysts, 0.97 wt.% Ni/SiO2, 0.95 wt.% Ni/Al2O3 and 0.54 wt.% Ni/MoO3, are reported for the hydrogenolysis of ethane, propane, n-butane and iso-butane. The reactions were carried out in a flow reactor at atmospheric pressure. The three nickel catalysts had the following order of hydrogenolysis activity: Ni/SiO2>Ni/Al2O3>Ni/MoO3. The active site for the hydrogenolysis reactions over the three nickel catalysts is formed insitu and is likely to contain a carbonaceous component. Any carbonaceous component that is formed will not necessarily be the same in each catalyst as the laydown will be a function of the characteristics of the fresh catalysts (nickel dispersion, support etc.). The Ni/SiO2 catalyst showed the highest activity for the hydrogenolysis reactions of the alkanes tested. The higher specific rate of hydrogenolysis of the Ni/SiO2 catalyst is likely to be an effect not only of the small particle size of the nickel but also the manner in which carbonaceous matter builds up on these particles. The Ni/MoO3 catalyst had a lower activity than would be expected from its nickel dispersion. The suppression activity on the Ni/MoO3 sample may be related to a strong interaction between the metal and the support. The selectivity behaviour shown during hydrogenolysis by the Ni/SiO2 and Ni/Al2O3 catalysts was typical of that expected for nickel catalysts (multiple hydrogenolysis, demethylation, low isomerisation). These selectivity features can be accounted for by a reverse Fischer–Tropsch synthesis mechanism that involves 1,1,2 adsorbed alkylidene intermediates on the catalyst surface. The Ni/MoO3 catalyst demonstrated uncharacteristic isomerization activity during the hydrogenolysis of n- and iso-butane. This can be accounted for by a bifunctional mechanism involving acid sites on the MoO3 support.

Supported nickel catalysts: Preparation and characterisation of alumina-, molybdena-, and silica-supported nickel, and the identification of reactive oxygen on these catalysts by exchange with isotopically labelled carbon dioxide

Nickel catalysts, supported on alumina, silica, and molybdena, have been prepared by impregnation and co-crystallization. In the precursor state the catalysts were characterised by UV–visible spectroscopy, thermogravimetric analysis/differential thermal analysis (TGA/DTA), and X-ray photoelectron spectroscopy (XPS). The nickel was principally in the 2+ oxidation state with an octahedral coordination. However, the ligand sphere surrounding the nickel ion was sensitive to the support, indicating that the species on the different supports were not identical thus suggesting a metal complex–support interaction. Reduction was followed by temperature programmed reduction (TPR) and TGA, the results of which indicated that reduction and decomposition of nickel nitrate occurred simultaneously. X-ray diffraction (XRD) analysis revealed that, with the Ni/MoO3 sample, no hydrogen bronze was formed on reduction. The reduced catalysts were characterised by carbon monoxide chemisorption, carbon dioxide chemisorption, and by reaction of buta-1,3-diene with dihydrogen. In the absence of a dihydrogen stream it was found that the catalysts adsorbed no carbon monoxide due the presence of sub-monolayer quantities of surface oxygen. The extent of the oxygen was quantified by exchange with isotopically labelled carbon dioxide. Differences in the electronic nature of the nickel between the Ni/MoO3 sample and the other catalysts wererevealedbytheirbehaviourtowardsbuta-1,3- diene hydrogenation.

The effect of Mg location on Co-Mg-Ru/γ-Al2O3 Fischer-Tropsch catalysts.

The effectiveness of Mg as a promoter of Co-Ru/γ-Al2O3 Fischer–Tropsch catalysts depends on how and when the Mg is added. When the Mg is impregnated into the support before the Co and Ru addition, some Mg is incorporated into the support in the form of MgxAl2O3+x if the material is calcined at 550°C or 800°C after the impregnation, while the remainder is present as amorphous MgO/MgCO3 phases. After subsequent Co-Ru impregnation MgxCo3−xO4 is formed which decomposes on reduction, leading to Co(0) particles intimately mixed with Mg, as shown by high-resolution transmission electron microscopy. The process of impregnating Co into an Mg-modified support results in dissolution of the amorphous Mg, and it is this Mg which is then incorporated into MgxCo3−xO4. Acid washing or higher temperature calcination after Mg impregnation can remove most of this amorphous Mg, resulting in lower values of x in MgxCo3−xO4. Catalytic testing of these materials reveals that Mg incorporation into the Co oxide phase is severely detrimental to the site-time yield, while Mg incorporation into the support may provide some enhancement of activity at high temperature.

The formation of acrylonitrile over solid base catalysts

The reaction of methanol and acetonitrile to form acrylonitrile has been studied over solid base catalysts derived from MgO. High surface area MgO produced the best yields of acrylonitrile and propionitrile, but higher specific activity was achieved by impregnating the MgO support with chromium oxide.