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Thisis an author version of the contribution published on:Questa è la versione dell'autore dell'opera: By Federico Cesano, Serena Bertarione, Andrea Piovano, Giovanni Agostini, Mohammed Mastabur Rahman, Elena Groppo, Francesca Bonino, Domenica Scarano, Carlo Lamberti, Silvia Bordiga, Luciano Montanari, Lucia Bonoldi, Roberto Millini and XRPD, HRTEM, Raman and UV-Vis characterization methods have been applied to obtain information on the morphology and the structure of the catalysts (including degree of staking and structural disorder) as well as on the vibrational and spectroscopic properties. It is shown that, when compared with HRTEM results, XRPD, Raman and UV-Vis data give a realistic information on the staking degree, on the particle size distribution and on the heterogeneity of supported MoS 2 particles on the various supports. EXAFS and XANES spectroscopies have been also used to set up the best sulfidation procedure on the MoS 2 /SiO 2 system, and the obtained recipe has been adopted for the MoS 2 /γ-Al 2 O 3 and MoS 2 /MgO systems. UV-vis analysis under controlled atmosphere has been performed to understand the effect of reductive and sulfiding treatments on the presence of sulfur vacancies and on the valence state of Mo ions associated with them. To explore the structure of coordinatively unsaturated Mo sites after reducing or sulfiding treatments (with CS 2 or, occasionally, with H 2 S), in situ FTIR of CO adsorbed at low-temperature has been performed on all samples. It is demonstrated that CO is a sensitive probe for coordinatively unsaturated sites and that the formation of sulfur vacancies on the MoS 2 surface upon reduction in pure H 2 at 673 K is accompanied by an increase of the coordinative unsaturation and a decrease of the valence state of a fraction of surface Mo cations, mainly located on corner and edge sites. Considering the non planarity of a consistent fraction of lammelae (as revealed in particular by HRTEM of MoS 2 on Al 2 O 3 and MgO), the presence of reducible Mo ions, located in defective positions on basal planes, cannot be excluded. Furthermore, it is demonstrated that this process can be reversed upon interaction with the sulfiding agent and that this reversible behavior is really mimicking some of the elementary acts occurring in the HDS process. Comparing the results obtained by all the adopted characterization techniques, it is concluded that the reductive effect of H 2 preferentially affects the particles characterized by the lowest staking degree. The complexity of the IR results suggests that the adopted reduction procedure in pure H 2 at 673 K induces the formation of several type of sulfur vacancies, presumably located in different crystallographic positions of the MoS 2 particles. In conclusion, the IR results obtained with the CO probe fully support the idea that the adopted reduction procedure in pure H 2 at 673 K is not only inducing the formation of several families of sulfur vacancies but that a continuity exists between the stages associated with the reducti...

The features of molecular structure of seven n-C7 asphaltenes isolated from different feedstocks (i.e., vacuum residue, atmospheric residue and crude oil) have been investigated by pyrolysisgas chromatography/mass spectrometry (Py-GC/MS), and their average structural parameters have been estimated using a mathematical optimization method. The results of structural analysis pointed out substantial differences among the asphaltenes investigated. The size of aromatic fused ring systems ranged from 5.1 to 14.9, while a substantial fraction of peripheral carbon in hydroaromatic/aromatic sheets (0.28-0.40) is substituted with aliphatic chains whose average length was between 2.8 and 6.3. A significant fraction of aliphatic moiety (0.15-0.48) was found to be present as naphthenic structures. In all cases, the main classes of compounds identified during pyrolysis tests were homologous series of alkanes (up to C 25-30 ), 1-alkenes (up to C 25-30 ), branched paraffins, and alkyl-substituted aromatic compounds. The formation of pyrolysis products was explained on the basis of literature data regarding thermolysis reactions of model compounds mimicking the structural features of asphaltenes. The results of this study are consistent with the view that asphaltenes are a complex polydisperse mixture of molecules made up of polyaromatic and hydroaromatic units joined by aliphatic bridges and substituted with aliphatic chains containing up to 25-30 carbon atoms. However, both NMR data and results of structural analysis indicate that the major part of these chains (more than 80%) is formed by short alkyl groups (C1-C4).

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The molecular weight distribution of the asphaltene fractions of two types of crude oils from two different Italian fields (samples 1 and 2) was investigated. The analytical tools used to perform these analyses were matrix assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) mass spectrometry. After observing that the use of the matrix (as well as the addition of Ag+) did not improve the quality of the data compared to that obtained in LDI conditions, all further measurements were performed with the latter technique. Operating under usual conditions of laser power and delay time, a very low resolution was observed, showing only macroscopic differences between the two samples in the molecular weight distribution of the different components. An accurate study on the possible reasons of this undesirable behavior indicates that it can originate from space charge phenomena occurring either in the ion source region or during the flight. A valid parameterization of the delay time and the laser power allowed higher quality spectra to be obtained. Surface-enhanced laser desorption ionization (SELDI) measurements were also performed using normal phase (silica) as the sample holder surface. Under these conditions, better results are obtained, proving that the sample-surface interaction is important to achieve, by means of laser irradiation, a homogeneous set of product ions. Both asphaltene samples were fractionated in five subfractions by gel-permeation chromatography (GPC) to obtain a better separation of the molecular weight distributions; the related spectra confirmed these findings. By using different approaches, relevant and reproducible differences between the asphaltene fractions of the two oil samples were observed.

In the conversion of heavy oil into valuable fuels by thermal processes, one of the main problems is the formation of soft coke-like substances that can cause equipment fouling and catalyst deactivation. Asphaltenes are present in large quantity in heavy crude oil and are known to be coke precursors. The objective of the present study was to obtain chemical structure information about the asphaltene molecules when they undergo a thermal treatment and to investigate the mechanism of coke formation. Asphaltenes were separated from two industrial thermal treated heavy crude oils and characterized by elemental analysis, NMR, FTIR, and FT-ICR MS. Analytical data were then compared with that obtained from asphaltene samples after thermal treatment at 400 °C. From complementary and comparative use of the different analytical techniques, we demonstrated that asphaltenes thermally treated at 400 °C tend to aromatize to form coke precursors. We found that the species rich in saturated rings and/or alkyl chains are less stable than the one containing aromatic rings; the main reaction in thermal treatment is the intramolecular cyclization/aromatization and not the cleavage of residual aliphatic chains. Moreover, asphaltene classes containing sulfur atoms present a lower stability than the other species.

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