Site Time Yields Research Articles

Reactivity and stability investigation of supported molybdenum oxide catalysts for the hydrodeoxygenation (HDO) of m-cresol

The vapor-phase hydrodeoxygenation (HDO) of m-cresol is investigated at 593K and H2 pressures ⩽1bar for supported catalysts comprised of 10wt% MoO3 dispersed over SiO2, γAl2O3, TiO2, ZrO2, and CeO2. Reactivity data show that all catalysts selectively cleave CO bonds without saturating the aromatic ring, thus effectively transforming m-cresol into toluene at moderate to high conversions. MoO3/ZrO2 and MoO3/TiO2 feature the highest initial site time yields (23.4 and 13.9h−1, respectively) and lowest first-order deactivation rate constants (0.013 and 0.006h−1, respectively) of all catalysts tested after ca. 100h on stream. Characterization studies demonstrate that the supports play an important role in stabilizing partially reduced, coordinatively unsaturated (CU) sites in surface oligomeric Mo moieties. Post-reaction X-ray photoelectron spectroscopy shows that the catalysts with higher activity feature larger proportions of intermediate oxidation species (Mo5+ and Mo3+). In contrast, the catalysts with lower reactivity show different oxidation states: bulk MoO3 features mostly Mo4+ and metallic Mo species, while MoO3/CeO2 features a high proportion of Mo6+ species. An inverse correlation is established between the catalyst activity and both the maximum hydrogen consumption temperature obtained during temperature programmed reduction, and the support cation electronegativity (with the exception of MoO3/CeO2).

Reprint of “Ethanol to higher hydrocarbons over Ni, Ga, Fe-modified ZSM-5: Effect of metal content”

The effect of metal content on catalyst properties was studied by comparing unmodified HZSM-5 and 0.5–7wt.% Ga, Fe and Ni modified HZSM-5 in the ethanol conversion to hydrocarbons at 623K by combining detailed catalyst characterization (XRD, TEM, N2 adsorption, H2-TPR and NH3-TPD) and catalytic testing. Low metal amounts (<1wt.%) were found to have a positive effect on the production of light olefins, C2–C4 paraffins, aromatics and C5+ hydrocarbons. Increasing the amount of metal leads to a decreased production of these hydrocarbons, which is attributed to bulky metal clusters formation. These clusters decrease the accessibility of the acid sites due to pore blockage. For the first time, catalyst performance in ethanol conversion have been assessed at similar conditions, i.e. same conversion, showing that the selectivity towards the various product classes was not altered by the metal introduction. The site time yields of the metal modified HZSM-5 catalysts, based on the total concentration of accessible acid sites, exhibit a decreasing trend as function of metal content. This can be attributed to a lower acid strength of the accessible acid sites.

Ethanol to higher hydrocarbons over Ni, Ga, Fe-modified ZSM-5: Effect of metal content

The effect of metal content on catalyst properties was studied by comparing unmodified HZSM-5 and 0.5–7wt.% Ga, Fe and Ni modified HZSM-5 in the ethanol conversion to hydrocarbons at 623K by combining detailed catalyst characterization (XRD, TEM, N2 adsorption, H2-TPR and NH3-TPD) and catalytic testing. Low metal amounts (<1wt.%) were found to have a positive effect on the production of light olefins, C2–C4 paraffins, aromatics and C5+ hydrocarbons. Increasing the amount of metal leads to a decreased production of these hydrocarbons, which is attributed to bulky metal clusters formation. These clusters decrease the accessibility of the acid sites due to pore blockage. For the first time, catalyst performance in ethanol conversion have been assessed at similar conditions, i.e. same conversion, showing that the selectivity towards the various product classes was not altered by the metal introduction. The site time yields of the metal modified HZSM-5 catalysts, based on the total concentration of accessible acid sites, exhibit a decreasing trend as function of metal content. This can be attributed to a lower acid strength of the accessible acid sites.

Structural and catalytic differences in the effect of Co and Mo as promoters for Pt-based aqueous phase reforming catalysts

Reaction rates (site time yields, normalized to CO chemisorption sites) and product selectivity were determined for PtCo, PtMo, and Pt supported on multi-walled carbon nanotubes for aqueous phase reforming of glycerol. The bimetallic PtCo and PtMo catalysts had 4.6× and 5.4× higher glycerol consumption rates than Pt, and 3.9× and 0.6× rates in H2 formation compared to Pt. Hydrogen generation selectivity was similar on Pt and PtCo, but PtMo had an increase in selectivity to CO cleavage products, which reduced H2 yield at conversions over 60% (85–90% hydrogen yield for Pt and PtCo, 65% for PtMo). X-ray absorption spectroscopy and scanning transmission electron microscopy results indicate that PtCo adopts multiple mono- and bimetallic structures (Pt shell/Co core, well-mixed alloy, Pt only), which maintain surface Pt sites that are selective to hydrogen generation while adding a promotional metal that increases reaction rates. This is in contrast to a previously characterized PtMo catalyst which introduced surface sites capable of deoxygenation reactions (in the form of PtMo metallic or PtMoO/OH acid pairs) which result in the observed decrease in selectivity.

Effect of Co Loading on the Activity and Selectivity of PtCo Aqueous Phase Reforming Catalysts

The reaction site time yields (STYs, normalized to CO chemisorption sites) and product selectivity were measured for a series of bimetallic, multiwalled carbon nanotube supported PtCo catalysts with varying Pt/Co ratios for aqueous phase glycerol reforming. The STYs for all products increased by factors of around 2 for PtCo 1:0.5 and 1:1, and a factor of 4 for PtCo 1:5 relative to a monometallic Pt catalyst. The PtCo catalysts had similar hydrogen selectivity (>85%) at glycerol conversions up to 60%. X-ray absorption spectroscopy and scanning transmission electron microscopy characterization revealed that PtCo catalysts adopt monometallic Pt, mixed PtCo alloy, and Pt shell/Co core particle configurations. A linear correlation between the fraction of mixed PtCo alloy particles and the STY was found, indicating that higher Co loading resulted in a higher fraction of mixed PtCo alloy particles (the promoted phase) that provided the STY increase.

Hydroconversion of n-hexadecane on Pt/silica-alumina catalysts: Effect of metal loading and support acidity on bifunctional and hydrogenolytic activity

Bifunctional catalysts based on platinum and amorphous silica–alumina were studied in the hydroconversion of n-hexadecane. The influence of platinum loading and support acidity on activity and selectivity were assessed. The contribution of hydrogenolysis reactions on top of bifunctional hydrocracking was shown to depend not only on metal loading, but also on the effect of support acidity on the intrinsic activity of the platinum sites. The yield of cracking products, and their linear alkane fraction, increased with metal loading, while the isomerization yield was practically independent of the metal content. On a support of high Brønsted acidity, the rate of formation of methane was proportional to the platinum surface area, indicating that demethylation occurred by metal-catalyzed hydrogenolysis. On the other hand, the methane site-time yield was one order of magnitude lower on a catalyst with negligible Brønsted acidity. Pt-catalyzed hydrogenolysis was also investigated during selective poisoning of acid sites by co-feeding pyridine and comparing the effect of hydrogen partial pressure on reaction rates. In the presence of pyridine, total hydroconversion activity was reduced by one order of magnitude while rates to methane and linear cracking products remained relatively high. These observations indicate that acid sites do not intervene in the mechanism, but that support acidity affects the hydrogenolytic activity of platinum sites.

Fischer–Tropsch Synthesis Over CNT Supported Cobalt Catalysts: Role of Metal Nanoparticle Size on Catalyst Activity and Products Selectivity

Nanoparticle size effects on catalyst activity and products selectivity of CNT supported cobalt catalysts in Fischer–Tropsch synthesis are studied. CNT supported cobalt catalysts prepared by two different methods (microemulsion and impregnation) are employed to yield higher hydrocarbons from synthesis gas. To achieve a series of catalysts with different Co particle size, water-to-surfactant molar ratio varied from 4 to 12. The results showed that the average particle sizes depended linearly upon the respective ratio and with decreasing the water content in microemulsion method, prepared cobalt particles to control a narrow nanoparticle size distribution with different water to surfactant ratios. The results show that cobalt-time yields increased substantially (from 3.5 to 2.9 × 10−3 mol CO/(mol(Co)s)), the site-time yields were decreased (from 26.9 to 37.0 × 10−3 s−1), C5+ selectivity and chain growth probability for hydrocarbons decreased when metal particle size reduced.

Fischer–Tropsch synthesis over TiO2 supported cobalt catalyst: Effect of TiO2 crystal phase and metal ion loading

Five kinds of cobalt-loaded TiO2 (Co/TiO2) catalysts, which were prepared from TiO2 samples with different crystal phase and specific surface area by an impregnation method, were studied for Fischer–Tropsch (FT) synthesis with the stirred slurry phase tank reactor. Co/TiO2 catalysts prepared from TiO2 with more than 15% of rutile phase exhibited about four times higher CO conversion rate than those from pure anatase TiO2 did. We further examined eleven kinds of metal ion as the additives of the TiO2 support. Co/TiO2 catalysts loaded with alkali earth elements showed high CO conversion rate, while Mn- and V-loaded catalysts showed high C5+ selectivity and low CH4 selectivity. Among the prepared catalysts, Co/TiO2 loaded with small amount (0.8wt%) of Ca ions showed the highest CO conversion rate and site-time yield of C5+ products, which were 1.7 and 1.9 times higher than those over the bare Co/TiO2 catalyst. The activity of Co/TiO2 catalysts proportionally increased with increasing the surface area of cobalt metal, showing that the reaction would be structure-insensitive and depend on the number of the exposed Co metal sites on the catalyst surface.

Fischer–Tropsch Synthesis on SiC-Supported Cobalt Catalysts

Seven SiC supports provided by SICAT with different surface areas and pore volumes were impregnated with 12,5 wt% Co. H2-chemisorption, N2-adsorption, temperature programmed reduction and Fischer–Tropsch synthesis in a fixed-bed reactor at 483 K, 20 bar and H2/CO = 2.1 were performed in order to characterize and test the samples. The performances were compared with well characterized Co/Al2O3 and Co-Re/Al2O3 reference catalysts. The selectivity towards heavier hydrocarbons (C5+) was found to be moderately higher for the SiC supported catalysts while the site-time yields was 20 to 66 % lower than the Co-Re/Al2O3 catalyst. Elemental analysis showed the presence of several impurities in the SiC material. Alkali and alkaline earth elements, such as Na, K and Ca, are all known to lower the catalytic activity and also to influence the selectivity. It is proposed that these impurities in addition to sulfur and phosphorus known to be present in SiC, are responsible for the significantly lower catalytic activity of the SiC supported catalysts.

Partial Hydrogenation of C2H2 on Ag-Doped Pt Nanoparticles

Pt nanoparticles (NPs) with submonolayer Ag coverage were synthesized and catalytically tested to investigate the effects of surface Ag and NP shape on C2H2 hydrogenation activity and selectivity. Various NP shapes (cubes, cuboctahedra, and octahedra) in the 6–8 nm range were synthesized using colloidal methods with PVP (polyvinylpyrrolidone) as a capping ligand and Ag+ for structure direction. Solid-state 13C NMR of PVP–NP interactions, as well as electrochemical measurements of oxygen evolution reaction (OER) activity, revealed that submonolayer levels of Ag significantly modify both the physical and electronic structure of the Pt NP surface. Octahedral NP catalysts with 0.5 monolayers of Ag were found to be highly selective for C2H2-to-C2H4 hydrogenation (C2H4/C2H6 >8) at reaction rates comparable to total hydrogenation on pure Pt. Traditionally prepared PtAg catalysts synthesized via incipient wetness showed analogously high selectivities but lower site time yields compared to NPs. In contrast, total ...

Microkinetic analysis and mechanism of the water gas shift reaction over copper catalysts

Using microkinetic modeling, we have determined the closed catalytic cycle that describes the water gas shift reaction on copper catalysts. Eight elementary reactions constitute the cycle. Dissociation of water and the recombination of surface hydrogen result in dihydrogen. While surface hydroxyl reacts with surface CO to give a surface carboxyl intermediate, which, in turn, reacts with another surface hydroxyl to give water and carbon dioxide to complete the cycle. The kinetic model further predicts that the most abundant surface species is a formate species formed via the hydrogenation of carbon dioxide. This reaction is not part of the catalytic cycle, and surface formate is a spectator species which increases substantially at high pressure and low temperature. Adsorption–desorption of dihydrogen is equilibrated; while water dissociation is kinetically significant with a high degree of irreversibility. And, depending on the reaction conditions, carboxyl formation also becomes kinetically important. The model identifies the binding energies of surface H, HCOO, and OH to be important parameters and allows us to explain the difference in the site time yields of various copper catalysts.

Light alkane oxidation over Ru supported on ZnAl 2O 4, CeO 2 and Al 2O 3

Zinc aluminate, alumina and ceria supported ruthenium catalysts were tested in the total oxidation of propane, n-butane and iso-butane. Structure of the catalysts was characterized by means of XRD, TEM, BET and H 2 chemisorption techniques. The Ru dispersion changes from 0.67 for 5% Ru/CeO 2, 0.56 for 4.5% Ru/ZnAl 2O 4 and to 0.28 for 4.7% Ru/γ-Al 2O 3 catalyst. TEM and XRD results confirmed high dispersion of the samples. All oxidation reactions occur at much lower temperatures over Ru/CeO 2 compared to those over Ru/γ-Al 2O 3 and Ru/ZnAl 2O 4. Also, the site-time yields of all catalysed oxidation reactions follows the order Ru/CeO 2 > Ru/γ-Al 2O 3 > Ru/ZnAl 2O 4. Activity results cannot be explained by the differences in the Ru dispersion. The redox species of ruthenium on CeO 2 oxide, easily reacted with the lattice oxygen of CeO 2, are responsible for the enhanced activity of the Ru/CeO 2 catalyst in the VOCs oxidation.

Effect of Alkali Metal Impurities on Co–Re Catalysts for Fischer–Tropsch Synthesis from Biomass-Derived Syngas

The effect of alkali impurities on Co-based Fischer–Tropsch (FT) catalysts is important for processing biomass-derived synthesis gas containing inorganic ash impurities. The effects of Na, K, Li, and Ca impurities on γ-Al2O3-supported Co–Re powder catalysts were studied at impurity loadings between 25 and 1,000 ppm. Impurity addition did not have any effect on H2 chemisorption, but the catalyst activity decreased during FT synthesis experiments. The impurities were also found to slightly increase the reduction temperatures of Co. Carbon selectivities to CH4 decreased with increasing impurity loading, while CO2 and C5+ hydrocarbon selectivities increased. Catalyst behavior was attributed mostly to electronic effects from the alkali addition, leading to decreased surface H concentrations and increased CO adsorption and dissociation. Site-time yield (STY) as a function of metal impurity loading at constant conversion of CO.

Catalytic Cracking of Methylcyclohexane on FAU, MFI, and Bimodal Porous Materials: Influence of Acid Properties and Pore Topology

Catalytic cracking of methylcyclohexane has been studied on eight commercially available zeolites, five FAUs and three MFIs, and on two newly developed zeotype materials with bimodal porous structure, BIPOMs. Both BIPOMs are composed of an MFI ultramicropore (<1 nm) network and a different supermicropore (1.5−2.0 nm) network. Site time yields obtained on FAU and MFI zeolites with varying acid properties are in the same range, showing that mass transfer limitations inside the pores of both zeolite frameworks are absent. Site time yields obtained on BIPOM3 are comparable to those on commercial MFI with similar Si/Al ratio, while BIPOM1 is significantly less active. Within a given framework type, the zeolite acid properties determine its activity in methylcyclohexane cracking, while the pore topology controls its selectivity. On FAU, methylcyclohexane isomerization, followed by ring opening and subsequent cracking, is the main reaction pathway, while on MFI, protolytic scission, followed by cracking, is pred...

Catalytic Cracking of 2,2,4-Trimethylpentane on FAU, MFI, and Bimodal Porous Materials: Influence of Acid Properties and Pore Topology

Cracking experiments using 2,2,4-trimethylpentane as a model component have been performed on five FAU and three MFI zeolites. In addition to these eight commercially available catalysts, two newly developed zeotype materials with bimodal pore structure, BIPOMs, have been investigated. Both BIPOMs possess an MFI ultramicropore (<1 nm) network but a different ordered supermicropore (1.5−2.0 nm) network. Site time yields are lower on MFI than on FAU because of the slower diffusion of the reactant inside the pores. The site time yield obtained on the BIPOMs is comparable to commercial MFI with similar Al content. Within one framework type, the zeolite acid properties determine its activity in catalytic cracking of 2,2,4-trimethylpentane, while the framework topology controls its selectivity. The main reaction route on FAU is hydride transfer followed by β-scission leading to mainly C4 species, while on MFI protolytic scission is responsible for the formation of high amounts of C1−C3 species. This points to t...

Liquid-phase synthesis of ethyl tert-butyl ether over acid cation-exchange inorganic–organic resins

In this study the possibility to increase the productivity of acid cation-exchange resin catalysts active sites in ETBE synthesis was elucidated. For this purpose the inorganic–organic sulfonated cation-exchange resins with different polymer loadings (1.5–9.2 wt.%) have been synthesized. Such polymer/carrier composites as well as commercial acidic microporous gel-type ion-exchange resin KU-2-8 and macroporous Amberlyst 15 have been studied in the ETBE synthesis. Acid properties of all solids were determined by quasi-equilibrium thermogravimetry of ammonia desorption. We have found out that polymer loading and acid capacity affect oppositely on the ETBE productivity. As a result, there is no direct correspondence between the ETBE productivity of the inorganic–organic sulfonated cation-exchange resins per gram of catalyst on their acidity. However, there is a linear relation between the productivity per gram of polymeric phase and acidity. It was established that due to a better accessibility of acid sites for reactants loaded sulfonated cation-exchange resins exceeds the commercial catalysts by the site time yield values.

Fischer–Tropsch synthesis over un-promoted and Re-promoted γ-Al 2O 3 supported cobalt catalysts with different pore sizes

The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al 2O 3 supported cobalt catalysts was investigated in fixed-bed reactors at T = 483 K, P = 20 bar, and H 2/CO = 2.0. Catalysts containing 20 wt.% cobalt and 0 or 0.5 wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al 2O 3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al 2O 3 supports had very different pore characteristics. The post-calcination Co 3O 4 crystallite size was predominantly controlled by the γ-Al 2O 3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co 3O 4 was reduced in two steps to Co 0 with CoO as intermediate species. However, while reduction of Co 3O 4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co 0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100 h on stream. Positive correlations were found between the catalyst pore diameter and the C 5+ selectivity and between the cobalt particle size and the C 5+ selectivity. Re had a consistent positive, albeit small, effect on the C 5+ selectivity.

Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer–Tropsch Synthesis Catalysts

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of γ-Al2O3 supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co3O4 crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co3O4 crystallite size. On all three γ-Al2O3 supports, Co3O4 was present as aggregates of many crystallites (20–270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer–Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H2/CO = 2.1). Although the cobalt-time yields varied significantly (4.6–6.7 × 10−3 mol CO/mol Co s), the site-time yields were constant (63–68 × 10−3 s−1) for the three samples. The C5+ selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the γ-Al2O3 support.

Fischer–Tropsch synthesis: Cobalt particle size and support effects on intrinsic activity and product distribution

The influence of cobalt particle size in the range 3 to 18 nm on the Fischer–Tropsch synthesis intrinsic activity and product distribution was investigated over 26 alumina ( γ-Al 2O 3 and α-Al 2O 3) supported catalysts. A volcano-like curve was obtained for the γ-Al 2O 3 based catalysts when the C 5+ selectivity was plotted as a function of the particle size. The maximum selectivity was located at 7–8 nm. This is the first time an optimum particle size has been identified. An apparent optimum size was also discovered for the series of α-Al 2O 3 based catalysts. Interestingly, the C 5+ selectivity of the α-Al 2O 3 based catalysts was higher than the selectivity of the γ-Al 2O 3 based catalysts at all particle sizes. Thus, the selectivity is related both to the particle size and the support. No relation was observed between the cobalt particle size and the cobalt site-time yield.

Fischer–Tropsch synthesis over γ-alumina-supported cobalt catalysts: Effect of support variables

A systematic study of the effect of γ-Al 2O 3 support variables on Fischer–Tropsch synthesis activity and selectivity was carried out at industrially relevant conditions ( T = 483 K , P = 20 bar , H 2 / CO = 2.1 ). A total of 13 catalysts were prepared by incipient wetness impregnation of γ-Al 2O 3 supports of varying pore characteristics and chemical purities with aqueous solutions containing the required amounts of cobalt and rhenium precursor to give nominal loadings of 20 and 0.5 wt%, respectively. The catalysts were analysed for cobalt, rhenium, nitrogen, and sodium and characterised by nitrogen adsorption/desorption, mercury intrusion, X-ray diffraction, hydrogen chemisorption, temperature-programmed reduction, and oxygen titration. The size of the Co 3O 4 cobalt particles was controlled by the support pore size, with small particles formed in narrow pores and large particles formed in wide pores. The degree of reduction increased with increasing catalyst pore size and Co 3O 4 particle size. The catalysts contained different amounts of sodium (20–113 ppm) and the site-time yield decreased with increasing sodium content. Positive correlations were found between cobalt particle size and C 5+ selectivity and between catalyst pore size and C 5+ selectivity. The results indicate that the C 5+ selectivity is controlled by the size and appearance of cobalt particles.