Trends in hydrodesulfurization (HDS) activity are investigated on the basis of surface properties calculated by density functional theory for a series of HDS catalysts. It is shown that approximately linear correlations exist between HS group binding energies and activation barriers of key elementary reactions in HDS of thiophene. These linear correlations are used to develop a simple kinetic model, which qualitatively describes experimental trends in activity. The kinetic model identifies the HS-binding energy as a descriptor of HDS activity. This insight contributes to understanding the effect of promotion and structure–activity relationships.

Sterical hindrance of large S-containing molecules is known to be a significant cause of low hydrodesulfurization activity. To shed light on the atomistic aspects of such steric effects, we use atom-resolved scanning tunneling microscopy (STM) to probe the detailed adsorption configurations of dibenzothiophene (DBT) and alkyl-substituted DBT molecules on both unpromoted and Co-promoted MoS2 nanoclusters. We show that the unsubstituted DBT molecule can adsorb directly through its sulfur atom at sulfur vacancies on the nanocluster. However, only vacancies at corner sites are found to be accessible for DBT adsorption. In a similar way, DBT is found to adsorb on the Co–Mo–S sites exposed at the corners of the Co-promoted nanoclusters. In contrast, the 4,6-DMDBT molecule adsorbs in a flat configuration on the brim sites of the MoS2 clusters without the requirement of a nearby vacancy. Our findings support a model where hydrogenation and sulfur extrusion may take place on different sites.

A revamp has made possible the large-scale production of clean diesel with a significant percentage of renewable content.

From an interplay of Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) experiments, and density functional theory (DFT) calculations, we investigate the fundamental effect of the use of organosulfur compounds for the sulfidation of MoS2 nanoclusters in the hydrotreating catalyst. Our results reveal that incorporation of carbon in MoS2-based hydrotreating catalysts as carbide-type phase is not favorable when synthesized with or exposed to dimethyl disulfide (DMDS) or dimethyl sulfide (DMS). These results suggest that substitution of sulfur with carbon on the predominant type of edge structures in MoS2 nanoclusters is thermodynamically unfavorable, a result that is confirmed by DFT to be valid for all edge structures of MoS2 exposed under catalytically relevant conditions. However, the results show that the choice of the sulfiding agent can strongly influence the morphology and dispersion of the sulfided phase, and such effects are therefore expected to be of major importance for the activity of the freshly sulfided catalyst.

As the oil reserves are depleting the need of an alternative fuel source is becoming increasingly apparent. One prospective method for producing fuels in the future is conversion of biomass into bio-oil and then upgrading the bio-oil over a catalyst, this method is the focus of this review article. Bio-oil production can be facilitated through flash pyrolysis, which has been identified as one of the most feasible routes. The bio-oil has a high oxygen content and therefore low stability over time and a low heating value. Upgrading is desirable to remove the oxygen and in this way make it resemble crude oil. Two general routes for bio-oil upgrading have been considered: hydrodeoxygenation (HDO) and zeolite cracking. HDO is a high pressure operation where hydrogen is used to exclude oxygen from the bio-oil, giving a high grade oil product equivalent to crude oil. Catalysts for the reaction are traditional hydrodesulphurization (HDS) catalysts, such as Co–MoS2/Al2O3, or metal catalysts, as for example Pd/C. However, catalyst lifetimes of much more than 200 h have not been achieved with any current catalyst due to carbon deposition. Zeolite cracking is an alternative path, where zeolites, e.g. HZSM-5, are used as catalysts for the deoxygenation reaction. In these systems hydrogen is not a requirement, so operation is performed at atmospheric pressure. However, extensive carbon deposition results in very short catalyst lifetimes. Furthermore a general restriction in the hydrogen content of the bio-oil results in a low H/C ratio of the oil product as no additional hydrogen is supplied. Overall, oil from zeolite cracking is of a low grade, with heating values approximately 25% lower than that of crude oil. Of the two mentioned routes, HDO appears to have the best potential, as zeolite cracking cannot produce fuels of acceptable grade for the current infrastructure. HDO is evaluated as being a path to fuels in a grade and at a price equivalent to present fossil fuels, but several tasks still have to be addressed within this process. Catalyst development, understanding of the carbon forming mechanisms, understanding of the kinetics, elucidation of sulphur as a source of deactivation, evaluation of the requirement for high pressure, and sustainable sources for hydrogen are all areas which have to be elucidated before commercialisation of the process.

Infrared (IR) spectroscopy using NO as a probe molecule has been one of the important methods for characterizing hydrotreating catalysts, since this technique provides information on the nature and quantity of active edge sites of these catalysts. However, due to the strong adsorption of NO, which may lead to significant edge reconstructions, it has not been clear, how the characteristics of the adsorption complexes may reflect the nature of the original edge sites. By combining IR spectroscopy measurements with scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, we present new atomic-scale insight into the nature of NO adsorption on MoS2 and Co–Mo–S nanoclusters. The DFT calculations and STM experiments show that NO does not adsorb at fully sulfided MoS2 edges not containing hydrogen. However, typical sulfided catalysts will have hydrogen present at the edge in the form of S–H groups. For such samples, the results indicate a "push–pull" type mechanism involving simultaneous vacancy creation, NO adsorption and H2S release. This mechanism is observed to dominate in the IR experiments. In STM experiments, stable vacancies can be generated by dosing atomic hydrogen, and these vacancies are observed to adsorb NO dimers. The detailed nature of the adsorption is revealed by DFT. IR measurements recorded during temperature-programmed desorption (TPD) show the presence of several NO adsorption complexes and the assignment to specific species is achieved by comparison to calculated frequencies and adsorption energies obtained from DFT. The results show that mononitrosyl species dominate at the Mo-edges, whereas stable dinitrosyl species are found at both the unpromoted and the Co-promoted S-edges. Thus, based on the present results, it is possible to use NO as a probe molecule to obtain detailed atomic-scale information on hydrotreating catalysts and the origins of activity differences.

The promotion of the activity of MoS2-based hydrotreating catalysts by various first-row transition metals exhibits a typical variation referred to as a volcano plot. Co and Ni are seen to substantially promote the catalytic activity of MoS2, whereas the neighboring first-row metals promote reactivity to a much lesser extent, or not at all. In order to provide a better atomistic understanding of the catalytic synergies, we perform here a comparative scanning tunneling microscopy (STM) analysis of the atomic-scale structure and morphology of MoS2 nanoclusters doped with the first-row transition metals: Fe, Co, Ni, and Cu. We reveal that addition of all four dopant metals results in the formation of mixed-metal "Co-Mo-S"-type structures shaped as single-layer hexagonally truncated triangular MoS2-like nanoclusters. The modification of the preferred nanocluster equilibrium morphology is explained as a direct consequence of a favored substitution of dopant metal atoms into the S-edges of MoS2. The degree of truncation and the edge dispersion are, however, found to depend greatly on the type of dopant atom since the relative length of the dopant-stabilized edges decreases with the number of valence shell electrons of the dopant transition metal. A comparison of the observed atomic structure and morphology with the hydrotreating activity measured for industrial-style prepared Me-Mo-S catalysts (Me = Fe, Co, Ni, and Cu) supported on carbon reveals that two parameters are relevant to describe the promotional behavior: (i) a geometric parameter, which relates to the relative number of promoted and unpromoted sites in the Me-Mo-S nanoclusters, and (ii) a more conventional parameter relating to bonding and adsorption strength, i.e., describing the intrinsic activity of the particular Me-doped S-edge. (C) 2010 Elsevier Inc. All rights reserved.

Basic nitrogen-containing compounds such as pyridine are well known to be inhibitors of the hydrodesulfurization (HDS) reaction for the MoS2-based catalysts. From an interplay of scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations, atomic-scale insight into pyridine adsorption on MoS2 is obtained. In agreement with previous IR-spectroscopy and DFT studies, the STM results show that the pyridine molecule itself interacts weakly or not at all with the MoS2 nanoclusters. However, in the presence of hydrogen at the MoS2 edges, adsorbed species are revealed by STM also at the edges. The calculated DFT energies and simulated STM images allowed us to conclude that these species are pyridinium ions located at the catalytically active brim sites. Furthermore, the DFT results for the vibrational modes of the adsorbed pyridinium species agree well with those observed in earlier IR experiments on high surface alumina-supported MoS2 catalyst. The adsorption sites appear to be very similar to the brim sites involved in hydrogenation reactions in HDS. Thus, the combined STM and DFT results provide new atomic-scale insight into the inhibition effect of basic N-compounds in HDS and the first direct observation of the adsorption mode of basic N-compounds on the catalytically active MoS2 edges. Our results lend further support to previously reported correlations between inhibiting strength and proton affinity for the N-containing compounds. (C) 2010 Elsevier Inc. All rights reserved.

To produce diesel fuel from renewable organic material such as vegetable oils, it has for a number of years been known that triglycerides can be hydrogenated into linear alkanes in a refinery hydrotreating unit over conventional sulfided hydrodesulfurization catalysts. A number of new reactions occur in the hydrotreater, when a biological component is introduced, and experiments were conducted to obtain a more detailed understanding of these mechanisms. The reaction pathways were studied both in model compound tests and in real feed tests with mixtures of straight-run gas oil and rapeseed oil. In both sets of experiments, the hydrogenation of the oxygen containing compounds was observed to proceed either via a hydrodeoxygenation (HDO) route or via a decarboxylation route. The detailed pathway of the HDO route was further illuminated by studying the hydroprocessing of methyl laurate into n-dodecane. The observed reaction intermediates did not support a simple stepwise hydrogenation of the aldehyde formed after hydrogenation of the connecting oxygen in the ester. Instead, it is proposed that the aldehyde formed is enolized before further hydrogenation. The existence of an enol intermediate was further corroborated by the observation that a ketone lacking α-hydrogen (that cannot be directly enolized) had a much lower reactivity than a corresponding ketone with α-hydrogen. In real feed tests, the complete conversion of rapeseed oil into linear alkanes at mild hydrotreating conditions was demonstrated. From the gas and liquid yields, the relative rates of HDO and decarboxylation were calculated in good agreement with the observed distribution of the n-C17/n-C18 and n-C21/n-C22 formed. The hydrogen consumption associated with each route is deduced, and it was shown that hydrogen consumed in the water-gas-shift and methanization reactions may add significant hydrogen consumption to the decarboxylation route. The products formed exhibited high cetane values and low densities. The challenges of introducing triglycerides in conventional hydrotreating units are discussed. It is concluded that hydrotreating offers a robust and flexible process for converting a wide variety of alternative feedstocks into a green diesel fuel that is directly compatible with existing fuel infrastructure and engine technology.