Characterization of Asphaltenes in Solution and Inside the Pores of

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Characterization of asphaltenes in solution and inside the pores of catalysts by 1H NMR relaxometry Didier Espinat, Florine Gaulier, Francoise Norrant, Jérémie Barbier, Bertrand Guichard, Mickael Rivallan, and Pierre Levitz Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Characterization of asphaltenes in solution and inside the pores of catalysts by 1H NMR relaxometry

D. Espinat1, F. Gaulier1, F. Norrant2, J. Barbier1, B. Guichard1, M. Rivallan1, P. Levitz3 1. IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP3, 69360 Solaize, France 2. IFP Energies nouvelles, 1 et 4 avenue de Bois Préau, 92852 Rueil-Malmaison, France 3. Laboratoire PHENIX, Université Pierre et Marie Curie, Case Courrier 51 – 4 place Jussieu – 75252 Paris Cedex 5

Abstract The upgrading of heavy petroleum fractions needs the development of more and more efficient heterogeneous catalysts. One of the major issues of these processes is the diffusion of asphaltenes to the active site through the porous network of the alumina support. The catalytic efficiency is deeply impacted by the transport phenomena and the interfacial interactions. The aim of this work is to capture the extent to which low-field 2D 1H NMR relaxation time correlations can contribute to a better understanding of the dynamics of asphaltene in solution and within the pores of catalyst supports. 2D T1-T2 maps for asphaltenes in solution in toluene exhibit several T1-T2 contributions, varying with the asphaltene concentration and the size of the asphaltenic fractions obtained by ultrafiltration separation. According to the nanoaggregate structure proposed by the Yen-Mullins aggregation model of asphaltenes, it was possible to unravel the different asphaltenic proton relaxation behaviors. By the use of NMR relaxometry, we have confirmed the stronger interaction of water with alumina than the one of the toluene. The presence of macropores in catalyst clearly boosts the toluene mobility through the porous network. 2D T1-T2 maps for asphaltenes inside the pores show various types of protons, all of them with a severe constrained dynamics. Asphaltene nano-aggregates and clusters can be seen as large entities jammed into the pores, slowly mobile and affecting the solvent (toluene) mobility. When macroporosity exists in the support, the asphaltene overcrowding is less sensitive, enabling a faster dynamics of asphaltenes and toluene.

1. Introduction The conversion of heavy petroleum fractions into more valuable products, as for instance gasoline or diesel fuel, still remains an important target of the petroleum industry1. This stake involves need for developing more efficient catalysts to perform hydrocracking (HCK), hydrodesulphurization (HDS) and/or hydrodemetallization (HDM). All those catalysts consist industrially in sulphide heterogeneous catalysts. They usually consist of a combination of an active phase, made by molybdenum and cobalt or nickel sulphide, deposited on an alumina

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support which is nanoporous (average size of mesopores 10 nm), exhibiting a high specific surface area, usually over 200 m2/g. It is well known that the main industrial issues depend on the presence of asphaltenes. These large macromolecules consist of a complex mixture of molecules, having different sizes and chemical composition2. They are defined as the insoluble fraction in alkane, as for instance nheptane (AsC7). During the latest thirty years, a very large number of contributions enabled to make great strides on the description of asphaltenes3,4,5. From the chemical point of view, asphaltenes are mainly constituted by carbon and hydrogen but some heteroatoms are inserted in the structure as, sulfur, nitrogen and metals, nickel and vanadium. On the basis of the results provided by several analytical methods2-10 (fluorescence, infrared spectroscopies, NMR, EXAFS/XANES, GC-pyrolyis,…) some contributions try to propose an average chemical structure. Two main representations are classically proposed11-15: the first one corresponds to the “continental” type, in which asphaltenes exhibit a large central aromatic region surrounded with some alkyls chains with variable lengths, and the second one, named ”archipelago” type, where the size of the aromatic regions is much smaller, these aromatic nuclei being linked together by alkyl chains. The advances of mass spectrometry (Petroleomics16,17), with the high mass resolving power and the various methods for desorption and ionization18-21 confirm an average asphaltene molecular weight close to 750 daltons for the smallest entities. The main property of these various molecules is related to the self-assembling of small entities giving rise to aggregates characterized by a large polydispersity. The aggregation process of asphaltenes was peculiarly investigated by scattering techniques (X-ray (SAXS), neutron (SANS) or light) 22-28. One of the most recent contribution used complementary neutron and X-ray small angle scattering data in order to give a prominent description of the asphaltene nano-aggregate 27. It appears as a disk-like particle of total radius 3.2 nm with 30% polydispersity and a thickness close to 0.67 nm. Several nano-aggregates can further aggregate themselves, forming larger clusters exhibiting extended polydispersity. An average mass fractal cluster can be described by the aggregation of roughly 12 nano-aggregates with a radius of gyration close to 7 nm. Results provided by a large pool of techniques confirm this macrostructure description nicely summarized by the Yen-Mullins model29. We know that this macrostructure is strongly affected by an increase of the temperature30. The average cluster size decreases with temperature and if the number of nana-oggregates tends to decrease it was demonstrated that an aggregation of small molecules by π-π interaction, which means stacking of the aromatic domains, is still present even at very high temperature (300°C)28. The asphaltene transformation, during the refining process, still remains not completely understood. We previously insisted on the sizes of the aforementioned nano-aggregates and clusters which are for the first ones, a little bit smaller than the pore sizes of the catalyst, and for the second ones of the same range of size and even larger. The heavy fractions upgrading being processed at high temperature (370°C – 400°C) is in favor of the de-aggregation behavior1. Nevertheless, we have no precise information on the aggregation state of the asphaltene at this temperature. These simple observations suggest that the catalytic process will be decidedly affected by the molecular hindrance diffusion, through the catalytic pore


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network. We know, from industrial experience, that the contact time (Liquid Hourly Space Velocity – LHSV), can reach several hours1 in order to achieve a good conversion level. Likewise, it was clearly established that catalysts characterized by a bimodal or multimodal distribution of pores are more active than those having a monomodal porosity 31. All these observations have paved the way for an investigation of the asphaltene diffusion and adsorption through the catalytic pore network. From the experimental point of view, two main targets were identified: -

The first one consists in the following of the process diffusion, putting in contact various asphaltene solutions with the catalysts 32-36. The aim of these experiments is to follow the asphaltene penetration into the catalytic support present as beads (1-2 mm of diameter) or model extrudates (cylinders: 3 mm of diameter and between 6 to 10 mm length). The first experimental approach for succeeding is straightforward: it consists in measuring the evolution of the asphaltene solution concentration versus time and to observe the penetration front in the support by optical microscopy (the catalytic support is a white alumina and the asphaltene solution is black). From these experiments, diffusion models can be applied in order to extract key parameters governing the diffusion and adsorption process 32-35, especially the asphaltene diffusion coefficient in the porosity, the thermodynamic Langmuir coefficients of asphaltene and solvent (Langmuir law for adsorption isotherm), the saturation concentration and the asphaltene average molecular weight. The fit of such model to experimental data is made difficult because of the complexity of the system in particular due to the size-polydispersity of asphaltenes and the multimodal distribution of pores.

-

The second one consists in the characterization of the diffusion of the various entities (asphaltenes and solvent) in the porosity. By this way, we aim at measuring some parameters previously mentioned (diffusion coefficient, tortuosity,…) and included in the models and get information on the dynamics of the molecules. Several contributions on the literature dealt with the membrane diffusion measurements37-41. Various additional information can also be deduced from these experiments: (i) the diffusion coefficient when narrow size range fractions prepared by size exclusion chromatography are concerned, (ii) the modification of the initial asphaltene size polydispersity and the chemical composition versus time. In support to these features, NMR technique can clearly provide information on the mobility of molecules in the pores.

Virtually, we can distinguish in the literature two types of approaches, using high or low field NMR experiments. 1H and 13C high field NMR spectroscopies for asphaltene solutions are routinely used for the determination of their structural motifs10,42. Solid-state 13C NMR and 2D HSQC-NMR spectroscopies were applied for a comprehensive characterization of various asphaltenes43,44. Pulsed field gradient NMR has been extensively used for diffusion coefficient determination of various systems, and especially asphaltenes45-47. The diffusion-


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ordered two dimensional NMR spectroscopy (DOSY) 48,49, which can be considered as a high-resolution version of the PFG sequence, was applied for asphaltene solutions investigation of different origins and at various concentrations50,51. PFG-NMR spectroscopy was extensively used to probe liquid molecules trapped into nanoporous material. Many contributions dealt with the measurement of self-diffusion of molecules in zeolites52. For the improvement of liquid-phase heterogeneous catalysts, the accessibility of the reactants to the active surface site must be as fast as possible. Consequently, many investigations using PFGNMR technique were launched in order to better characterize this accessibility, peculiarly, by the diffusion coefficients of the molecules in the bulk pore space and on the surface of the catalysts53-58. Very different catalytic supports have been investigated (γ-Al2O3, TiO2, SiO2), displaying various surface area, chemical surface properties and metallic active phase (Au, Pd, Ag). PFG-NMR gives access to the tortuosity of the support, based on the measurement of the effective self-diffusivity of the molecules inside the pores (Deff) and the free bulk liquid self-diffusivity (D0): ߬=

‫ܦ‬଴ ‫ܦ‬௘௙௙

Nevertheless, Mantle et al.58 have shown that the tortuosity deduced from the previous equation is meaningful when the solvent used has no chemical interaction with the porous network. Virtually, this is the case when alkanes are considered. D’Agostino et al.57 have observed a singular behavior of different solvents within the porous medium, following the ‫ܦ‬ compared to the one of a series of alkanes. Molecules, containing carbonyl ratio ଴൘‫ܦ‬ ௘௙௙ functions, show a reduced self-diffusivity compared to alkanes. On the contrary, polyols show an enhanced self-diffusivity in the pores. The authors suggest the disappearance of the intermolecular hydrogen bonding network for polyols, giving rise to an enhanced translation and tumbling motion, in comparison with compounds for which the porous medium doesn’t affect the hydrogen bonding network. Weber et al.54 have determined the bulk pore and pore surface diffusion coefficients of 1-octene diffusing in nanoporous 1 wt % Pd/θ-Al2O3 catalyst. On the base of a two site exchange model, the respective values obtained were the following: 1.3.10-9 m2.s-1 for bulk and 1.7.10-11 m2.s-1 for the surface and the mean residence time of the molecule on the pore surface of 150 ms. NMR relaxation times measurement is another way to observe dynamics of molecules in colloidal systems, especially petroleum systems59-63, or in porous media43,55-57,64-72. The longitudinal T1 and transverse T2 relaxation times are strongly dependent on the molecular dynamics and the ratio surface-to-volume when the liquid is trapped into the porous system. When the mobility of the spin of the molecule is reduced, both relaxation times tend to decrease. That is the situation when a molecule has an interaction with the surface. An analytical model of the relaxometry T1 and T2 was proposed, based on a biphasic fast exchange between the bulk and the surface64-66:


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1 1 ߣܵ 1 = + . ܶଵ,ଶ ܶ஻,ଵ,ଶ ܸ ܶௌ,ଵ,ଶ

ሾ1ሿ

Where T1,2, TB,1,2, TS,1,2 are respectively the longitudinal (1) and transverse (2) relaxation times measured, in the bulk solvent (B) and at the surface (S). For complex systems that contain many different species present in various environments, the relaxation behavior can’t be described by one relaxation time. We usually consider a multiexponential behavior and we can have access to the distribution function of relaxation times by inverse-Laplace transformation62,68. The relaxation times depend on the Larmor frequency (ν=ω/2π) and a characteristic or correlation time (τc). According to the physical model proposed by Bloembergen, Purcell and Pound (BPP model73), the following equations in the situation of homonuclear dipolar coupling were obtained74,75: 1 2τୡ 8τୡ = 2K ൤ + ൨ ଶ ଶ Tଵ 1 + ω τୡ 1 + 4ωଶ τଶୡ

ሾ2ሿ

10τୡ 4τୡ 1 = K ൤6τୡ + + ൨ ଶ ଶ Tଶ 1 + ω τୡ 1 + 4ωଶ τଶୡ ఓ

௛

‫ ( = ܭ‬బ )ଶ ߛ ସ ( )ଶ ଶగ

ଶగ

ூ(ூାଵ) ସ଴.௥ ల

ሾ3ሿ

[4]

Where, µ0 is the vacuum permeability, I is the spin number (1/2 for protons), γ is the gyromagnetic ratio, h the Planck’s constant and r is the internuclear distance. τc can be defined as the time taken by one molecule to rotate of 1 rad. Figure 1 traduces the evolution of T1 and T2 as a function of τc for two frequencies (25 and 300 MHz). For very low values of τc, T1=T2, and for higher values, T1 goes up instead of T2 plummeting. A frequency variation (25 or 300 MHz) does not produce any variation of T2 relaxation time. If the frequency specific of the motion (1/τc) is larger than the Larmor frequency (ν), molecules behave in the fast motion regime43. Thus, the ratio T1/T2 or the two-dimensional T1-T2 relaxation correlation experiments can provide an insight in the mobility of the molecule and the strength of the interactions with the surface43,71. T1 and T2 dependence of the Larmor frequency (NMR fast field cycling (FFC)) has been nicely used for a better understanding of (i) the influence of the pore structure on various types of molecules present during some catalytic transformations70, and (ii) the structure of crude oils with different asphaltene concentrations in relation with the viscosity59,60. The final aim of this work is to capture the extent to which low-field 2D 1H NMR relaxation time correlations can contribute to a better understanding of the dynamics of asphaltene solution within the pores of catalyst supports. At first, to give a straightforward answer to this question, we have targeted to analyze asphaltenes in solution in toluene for different concentrations. Two different asphaltene fractions, obtained by nanofiltration, are studied, in order to address the asphaltene size-polydispersity and to discuss the non-fractionated


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asphaltenes data. Finally, NMR relaxation times are measured for asphaltene solutions within the porous network. 2. Experimental section 2.1 Samples 2.1.1 Asphaltenes Asphaltenes have been extracted by n-heptane precipitation from a Safaniya vacuum residue according to the NF T60-115 method. Asphaltenes characteristics are listed in Table 1. Nheptane was purchased from Aldrich with 99 % grade. For our purpose, the asphaltenes were first used as obtained here before. In order to reduce the asphaltene size-polydispersity and be able to study some selected asphaltenes according to their size, an ultrafiltration separation was performed. The experimental set-up used for this fractionation consists of a small dead-end filtration cell housing flat membranes sheets. A polyethersulfone porous membrane with a 20 kDa molecular weight cut-off was used. A 2 wt % asphaltene solution in toluene (Aldrich 99.5 % grade) was submitted to the membrane filtration at 25°C and for a 10 bar feed pressure. Retained and filtrated asphaltene fractions, respectively collected in the feed compartment of the cell and at the end of the filtration experiment, were recovered and the toluene was further eliminated (see75 for more information on this experimental development). All these asphaltene samples were characterized from the elemental composition and size polydispersity points of view. Table 1 gives the elemental composition. Size-Exclusion Chromatography (SEC) was used for a size-polydispersity investigation76 (Figure 2). The molecular weight is given in equivalent polystyrene molecular mass. In practice, fractions are enriched in small (further named “Small Asphaltenes”) or large entities (further named “Big Asphaltenes”) depending the one considered. 2.1.2 Porous media Two different alumina (γ-Al2O3) supports were investigated. Each one can be representative of the classical supports used for hydrotreatments. The first one (M14nm) exhibits a meso single-mode pore size distribution close to 14 nm. The second one (B14nm), is characterized by a bi-modal porous distribution, the meso-porosity being centered to 14 nm and the macroporosity reaching pore diameters near 1000 nm. The characteristics of these two alumina supports are summarized in table 2. Both supports are cylindrical extrudates with 3 mm diameter and an average 10 mm length. 2.2 NMR methods The NMR experiments have been performed on a Maran Ultra proton spectrometer from an Oxford Instrument having a proton Larmor frequency of 23.7 MHz. Free induction decay (FID) and transverse magnetization CPMG decay curves were


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measured. Two-dimensional T1-T2 maps were obtained by an inversion recovery sequence followed by a CPMG sequence: P180 – τ1 – P90 – (τ – P180 – echo - )n [5] where τ1 is respectively equal to 40 values spaced logarithmically and the number of echo n is chosen in order to reach zero signal (noise level). Thus, the magnetization signal depends on two times, τ1 and nτ, which can be later plotted as a T1-T2 2D map computed with an home-made 2D inverse Laplace transform software76. For the different experiments, the resolution for the relaxation time determination is close to 50 µs. We used one NMR probe of diameter 10 mm and 10 mm height. The detection of “solid like protons” needs very short probe dead time, especially when analyzing alumina supports, with very short relaxation times. For the 10 mm probe, the dead time is down to 5 µs. This allows short 90° pulse duration to reach a larger frequency bandwidth when detecting solid protons. The filter dead time can also be reduced significantly using a 1 MHz bandwidth, at the expense of the signal-to-noise ratio68. The main characteristics of the NMR Instrument are summarized in table 3. For the solution experiments, asphaltenes were dissolved in perdeuterated toluene (99.55% D), supplied by Eurisotop, in order to remove most of the protons coming from the solvent (toluene). The solutions were prepared two weeks before any use. The asphaltene solutions were introduced into small flaks of 6 mm diameter, filled up and thoroughly sealed in order to avoid any evaporation of the solvent. Each flask was further put inside the NMR probe for experimental purpose. For the alumina support experiments, the extrudates were previously dried at 100°C during 6 hours in order to remove adsorbed water. Then, they were soaked in water, toluene or asphaltene solutions for a given concentration. The time in contact with the solvent (water or toluene) was fixed to 24 hours, in order to allow the solvent to diffuse through the entire porous network. For the asphaltene solutions, the presence of large asphaltenic entities, impose a much longer contact time, fixed to 6 weeks in order to reach equilibrium of diffusion and adsorption within the pores36. After these immersion times, the impregnated supports were collected, wiped on filter paper and introduced into the NMR probe. NMR experiments were performed at a controlled temperature at 30°C. 3. Results and Discussion 3.1 Asphaltene solutions Three non-fractionated asphaltene solutions with concentrations of 2, 3 and 5 wt % and the two fractions of asphaltenes (“Small” and “Big”) for a 2 wt % concentration were analyzed by low-field NMR. 2D T1-T2 maps are presented in figure 3. For all these maps, we can clearly distinguish six peaks which are unambiguously detected,


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but more or less intense. The values of relaxation times, measured on top of the peak, are summarized in table 4. The ratio T1/T2 was also calculated. The first one, on the top right corresponds to T1 and T2 values close to 2.0 - 2.2 s. This signal comes from the small part of non-deuterated toluene molecules. Even if the range of T2 values is wider than the range of T1, we can clearly attribute this peak to free toluene; this means that solvent molecules are present in an environment similar to the one found for pure toluene. We have checked that the relaxation times (T1 and T2) for pure toluene are in the same range, equal to 2.66 s, close to 2.0 - 2.2 s. We can observe a clear tendency to a decrease of both solvent relaxation times as the asphaltene concentration increases, the ratio T1/T2 being quite constant (1) for 2 and 3 wt % asphaltene concentration, but slightly increasing for a 5 wt % concentration (2.2) (see table 3). This observation fits with previous results of the literature78,59-61, dealing with, either asphaltene solutions or pure crude oils. Considering the biphasic fast exchange model (see the equation [1]), asphaltene nano-aggregates and clusters can be seen as a smooth surface where some toluene molecules can be in tight interaction, corresponding to a shorter surface relaxation time (TS,1,2). Consequently, the measured relaxation times (T1,2) will be shorter as asphaltene concentration increases78. A more concentrated asphaltene solution will induce higher viscosity and a general slowing of the motions of the toluene entities. The spot number 1 (figure 3) spans a rather wide range of relaxation times, for T1 (200 – 300 ms) and T2 (80 – 160 ms). The ratio T1/T2 is roughly constant as a function of the concentration, but shows a slight increase for big asphaltenes. Peaks number 2 to 5 exhibit rather similar T1 relaxation time in the range 45 to 160 ms, but very different T2 (0.1 – 30 ms) relaxation time. This behavior witness molecular (or spins) motions with characteristic inverse correlation times similar or slower than the Larmor frequency61. Moreover, the ratio T1/T2 rises sharply for peaks 3 to 5, proving a huge decrease of the molecular mobility. It’s important to notice that spots 3,4 and 5 are not seen for small asphaltenes solution. We have plotted (figure 4) the evolution of the T2 relaxation time distributions for asphaltene solutions at three different concentrations (2, 3 and 5 wt %). The distributions were rescaled considering the same mass of asphaltene solution analyzed by NMR. Moreover, the distribution of the relaxation times of the empty NMR cell was measured and used for the correction of the data carried out for the asphaltene solutions. We can see (figure 4) that the rise of the asphaltene concentration is followed by a shift of the distribution towards much smaller T2 values. This observation props up the previous discussion, focusing on an overall severe reduction of the molecular mobility when the asphaltene concentration goes up. In low field 1H-NMR experiments, we can observe all the protons of the sample. For the asphaltene solutions, these protons come from asphaltenes but also from the non-


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deuterated fraction of the toluene used as solvent. This is one limitation of the technique which can prevent an accurate discussion of the 2D T1-T2 maps. In that way, the good resolution of high field 1H-NMR is clearly more efficient allowing the separation of the different types of proton and their relaxation time acquisition43. Nonetheless, we have tried to estimate the number of protons coming respectively from non-deuterated toluene and asphaltenes at rather low concentration (some few wt %). It is a non-trivial issue, because we don’t know exactly what the different hydrogenated impurities of the deuterated toluene are. We have made a simple hypothesis among which, the hydrogen atoms are located on the methyl group of the partly hydrogenated toluene (-CH3). According to the elemental analysis, we know the hydrogen asphaltene concentration (see table 1). Thus, a simple calculation provides an assessment of the ratio H from asphaltenes to H from the toluene (table 5). We can conclude that the number of protons, brought by the solvent, remains relatively small in comparison with the asphaltene hydrogen atoms. If we look at the solvent contribution on the 2D T1-T2 maps, we can highlight a certain decrease of its intensity as the asphaltene concentration rises, due to fewer protons from toluene and more from asphaltene molecules. Consequently, the asphaltene protons contribute mainly to peak intensities 1 to 5, but we can’t rule out some contribution of the hydrogen of the toluene. As discussed previously, some solvent molecules in interaction with nanoaggregates or asphaltenic clusters may show a reduced mobility or like trapped in a sort of network made by the asphaltene species, especially for a 5 wt % concentration. This behavior was discussed by Korb et al.57,58 characterizing the dynamics of small molecules of crude oils near asphaltene aggregates. Toluene molecules, as light hydrocarbons of crudes, follow a pathway marked, first by translation diffusion in a more or less continuous phase (solvent) surrounded by the asphaltenic network, and secondly a more or less long adsorption or trapping within nano-aggregates or clusters. To assert this description, a simple calculation allows assessing the average distance between nano-aggregates. Considering the average molecular weight (16000 Da) given by Eyssautier et al.27, we have found distances in between 8 and 11 nm regarding the experimental concentrations (2 to 5 wt %). Having in mind that the asphaltenic nano-aggregate described by Eyssautier et al.27, is a disk-like particle with a diameter of 3.2 nm and a thickness close to 0.7 nm, we see that the previous calculated inter-nano-aggregate distance is in the same range of size of this aggregate. In fact, the formation of polydispersed clusters according to the Yen-Mullins model29, made by the aggregation of several nano-aggregates27,79, conveys the existence of regions enriched in asphaltenes in the solution and consequently surrounding regions where the solvent is in majority. This concentration heterogeneity of the solution was already observed by Small Angle X-ray Scattering (SAXS) and by Cryo-Scanning Electron Microscopy23. We can suggest that the ratio T1/T2 for H-toluene molecules trapped into the asphaltene dense regions will be high. The different asphaltenic proton relaxation behaviors observed on the 2D T1-T2 maps can be unraveled considering the nano-aggregate structure proposed by Eyssautier et al. 27,28. A sketch of this nanostructure is presented on figure 5. As aforementioned,


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this nano-aggregate appears as a disk of radius close to 3.2 nm, a thickness of near 0.7 nm and an average molecular weight of 16 kDa as described on figure 5. The main driving force for the building of this nanostructure is π-π interaction, of around two or three aromatic sheets. The whole nano-aggregate contains between 15 and 20 elementary molecules. Coupling SAXS and SANS techniques27,, it was clearly figured out that no solvent can diffuse into the nano-aggregate core, contrary of what can occur inside the shell of the aggregate where the solvent can be present surrounding aliphatic or alicyclic chains. In this harsh environment, we can easily suggest that the protons mobility will be more or less affected. The spin of the proton of jammed molecules at the center of the nano-aggregate will relax very differently from the spin of protons attached to aliphatic chains (-CH2 or –CH3) outside the aggregate. We have proposed an illustration for the assigning of the five peaks seen on 2D T1-T2 maps on figure 5. The observation of the 2D T1-T2 maps for small and big asphaltenes (figure 3) remarkably props up the previous attribution of the different spots of the maps for pure asphaltenes. Small asphaltenes two dimensional representation only exhibits two main contributions, the first one depending on the solvent and the second one due to protons of asphaltenes and toluene, in a smaller proportion. In that way, two peaks (1 and 2, the most intense) can be distinguished with a rather small T1/T2 ratio in between 1.5 and 2 (see table 4). These small entities are mainly represented by elementary asphaltene molecules, as illustrated by the modified Yen-Mullins model29, and the contribution of nano-aggregates in this fraction is clearly very weak. Subsequently, we can suggest that the mobility of the asphaltene protons is much less affected that in non-fractionated asphaltenes where the aggregation state is predominant. This interpretation is asserted by the observation of the big asphaltenes 2D T1-T2 map. In addition to the free toluene contribution, we find four peaks (1 to 4 – figure 3) again similar to those previously observed for a 5 wt % pure asphaltene concentration. The intensities of these peaks are much lower that the intensities measured for 5 wt % asphaltene concentration, but we can perceive a fifth intense peak (5), characterized by a very low T2 (1.2 10-1 ms) and then a very high T1/T2 ratio (625). This last contribution has been previously detected (see in particular the 5 wt %. asphaltene concentration). It suggests the occurrence of a large number of protons with a significant reduction of the mobility. Several papers of the literature22-30,76 have shown that the asphaltene aggregation state is the main characteristic of this big asphaltene solution (see figure 2). We can forecast that a large population of asphaltene or toluene protons will get into a tangle and their motions strongly reduced. All these observations are in good agreement with the work of Dutta Majumbar et al. 43 (see figure 4 in ref. 43). High field NMR experiments (300 MHz) provide of course high resolution for the assignment of the different chemical groups of asphaltenes (CH, CH2, CH3 – see figure 3 in ref 43), which is a significant advantage on low field NMR. The values measured for the relaxation time T2 are in the range 24.4-86.9 ms, in reasonably good agreement with our data. For peaks 1 and 2, T2 varies between 20 to 160 ms (Figure 3 and table 3). The authors clearly distinguish aliphatic CH3 or CH2 groups that are located on the inside of the cluster and show a restricted motion.


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Conversely, CH3 or CH2 groups on the periphery are more mobile and consequently have shorter T2’s. From the T1/T2 ratio and according to the homonuclear dipolar coupling model (see formulae 2 and 3 and figure 1), it is possible to assess the characteristic time τc. Values of the T1/T2 ratio in the range of 2 to 300 correspond to values of τc between 5 and 10 ns. These values are significantly higher than the typical values for small molecules nearby a few picoseconds. An assessment of the correlation time τc for various sizes of molecules can be obtained using the Einstein-Stokes equation considering molecules as spheres of diameter d moving in a fluid of viscosity η: ߬௖ =

ߨ݀ଷ ߟ 6݇ܶ

where T is the temperature and k the Boltzmann constant. In a very poor approximation, if we consider the disk-like nano-aggregate as a sphere of diameter 6.4 nm, moving in toluene (η=0.59 10-3 Pa.s), τc is found close to 20 ns in reasonable good agreement with values deduced from the NMR experiments. 3.2 Asphaltenes within the porous network 3.2.1 2D T1-T2 map for the dry alumina support. The two dimensional relaxation map for the dry alumina support M14nm (see table 3) is presented in figure 6. An intense signal is observed corresponding to T1 and T2 respectively equal to 4.0 and 0.3 ms. The dispersion of the values of the relaxation time T2 is narrower than the one of T1. A very weak and diffused signal for T2 close to 40µs, at the limit of detection, is due to the presence of hydroxyls on the support. It has been already observed for kaolinite and smectite68, but for T2 value close to 60 µs. The T1/T2 ratio of the main contribution is close to 13, a good indication of the restricted mobility of the protons. Consequently, it can be straightforwardly attributed to some remaining water molecules not completely eliminated during the drying. The NMR measurement of the magnetization decay, the extrapolation to time zero and the comparison with a fixed mass of water in similar experimental conditions, has provided an estimation of the water content of the support equal to 2 wt %. This latest estimation is rather coherent taking into account that no-dried alumina support, like the one we have been using, contains water between 5 to 10 wt % at room temperature. 3.2.2 2D T1-T2 map of the solvent inside the porous network Figure 7 shows the 2D T1-T2 maps for the two alumina supports M14nm and B14nm. The supports were soaked into two different solvents, water and toluene. T1 and T2 values are listed in table 6. Both values are lower than the relaxation times of free solvents, respectively equal to 2.66 s for toluene and 2.7 s for water, conveying, as expected, the confinement of the liquids into the pores.


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Two different behaviors of toluene and water inside the porosity are clearly distinguished. For water, T2 is much smaller than T1, when for toluene system, T1 and T2 are in the same order of magnitude. This confirms a classical observation of the literature for protic and aprotic liquids in contact with porous material64,65,70. Unlike toluene, water can easily exchange protons with the surface of the support. T1 and T2 relaxation times of protons of the toluene significantly increase when the liquid wets the B14nm support. This is due to the macroporosity of the support, boosting the molecular mobility through the porous matrix. This observation is also in agreement with the general trend of the increase of the relaxation times with the pore diameter64,65 observed by Korb et al. on calibrated granular packing, The relaxation times of water confined into mesoporous (M14nm) or macroporous (B14nm) supports are similar, with a ratio T1/T2 respectively equal to 12 and 7 in good agreement with the literature69,70. This latest observation suggests that relaxation times are mainly governed by the water molecules interactions with the solid, especially the proton exchange as previously mentioned, and not by the porous network properties, as for instance the pore size distribution. In a similar sense concerning porosity characterization, D’Agostino et al.55 have shown that the true tortuosity of the porous network can be truly determined with small probe molecules which don’t exhibit any chemical functions being liable to interact with the surface (alkanes). Moreover, according to D’Agostino et al.71, the ratio T1/T2 is related to the surface adsorption energy. The determination of the porosity was possible thanks to the magnetization measurement at initial time proportional to the spin number and consequently the mass or volume of toluene inside the pores. Knowing the mass of the dried support and the structural density (see table 2) we can have access to the NMR porosity equal to 69% in good agreement with the porosity given by mercury porosimetry. 3.2.3 2D T1-T2 map of asphaltenes solution in toluene trapped into the pores For these experiments, two supports M14nm and B14nm were put in contact, at room temperature, with 2 wt % asphaltene solutions in deuterated toluene during five or six weeks, in fact a rather long contact time, but strictly necessary for a satisfactory penetration of asphaltene entities inside the porosity36. After 5 weeks of waiting time, low field NMR experiments were performed and the 2D T1-T2 maps are shown in figures 8c (M14nm) and 8d (B14nm). After 6 weeks contact time, some impregnated extrudates of the M14nm support were recovered and dried at 100°C during six hours in order to evaporate the major part of the deuterated toluene present in the pores. 2D T1T2 results are presented in figure 8b. Just for clarity of the discussion, the 2D T1-T2 map for the non-impregnated and dried support M14nm is recalled in figure 8a. In order to pick up a good signal to noise ratio, the acquisition time of each 2D map was long (12 hours). Relaxation times measured from the 2D maps are listed in table 7. A maximum of four peaks are detected from the T1-T2 maps. For the M14nm support impregnated with asphaltenes and dried, three main signals appear, characterized by low T1 (20 ms) and T2 (0.3 ms) and high T1/T2 ratio (6 to 200). For the supports, still


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wetted by toluene, we found more or less the same characteristics of the dried impregnated support, except for a signal 4, more intense and spread out a large range of T2, for the multimodal catalytic support (B14nm). Looking at the map of the impregnated and dried support (M14nm – figure 8b), the major contribution (peak 3) fits with a T2 close to 40 µs, in limit of the T2 instrumental resolution, lower than the peak detected for non-impregnated and dried support (figure 8a). We have previously attributed a diffuse and weak signal (T2=40 µs) on M14nm support to hydroxyls (see section 3.2.1). With the presence of asphaltenes on the support, the intense peak 3 (figure 8b) can’t be only attributed to hydroxyls. In order to address this point, we can appraise the ratio of the number of protons of asphaltenes to the number of protons belonging to hydroxylic groups (NH,a/NH,OH). The calculation is based on the classical value for alumina80,81 of 7 OH/nm2. We have measured the amount of asphaltene on the support after 6 weeks in contact with the solution, equal to 92 mg/g(of support) or 8.4 wt %. Consequently, the ratio NH,a/NH,OH is close to 4.4 and the contribution of asphaltene protons to the intensity of the peak 3 is high, clearly showing a very restricted mobility of asphaltene entities. On the other hand, two additional peaks (1 and 2 –figure 8b) have relaxation times not very far from the contribution of the clean support M14nm (figure 8a). We have previously attributed this last contribution to water molecules adsorbed at the pore surface. Even if the support has been carefully dried before putting it into contact with the asphaltene solution, we can guess that a small quantity of water could still be trapped on the support, especially during the transfer between the oven and the solution. We have figured out the ratio of the number of protons from asphaltene to the number of protons brought by adsorbed water. The water quantity adsorbed on M14nm is close to 2 wt %, as estimated by NMR measurements (see section 3.2.1). With an asphaltene concentration of 8.4 wt %, a simple calculation shows that the number of protons from the asphaltene is twice higher that the protons coming from the adsorbed water molecules. Due to the complex chemical composition, asphaltenes molecules are known for their amphiphilic properties82. Thus, an adsorption competition between water and asphaltene entities may occur at the alumina surface. Consequently, a contribution of asphaltenic protons to peak 1 and 2 can’t be totally excluded, corresponding to protons having certain mobility. 2D maps of supports soaked with toluene and asphaltenes (figure 8 c and d) look like the map observed in figure 8b. For the M14nm support, the peak 3 confirms the presence of protons with restricted motions probably from adsorbed asphaltenes and residual water or hydrogenated toluene molecules in strong interaction with the surface or the asphaltene deposit. Spots 1 and 4, with a rather low ratio T1/T2 (table 7), are probably linked to relatively mobile toluene in the pores. The diffusion and adsorption process of asphaltene aggregates through the porosity of meso or macroporous alumina supports is very slow36,83,84. After soaking during 5 weeks, we are not very far from a stationary regime for which the concentration of the asphaltene solution surrounding the extrudates of alumina support doesn’t vary, probably far from the thermodynamic equilibrium. The amount of asphaltene inside the porosity is close to 8.4 wt %. According to the dried asphaltene density measured


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by mercury porosimetry (da=1.22±0.12), the volume occupied by asphaltene species represents more or less 10 % of the total porous volume (M14nm). The asphaltene deposit along the radius of the extrudate is not homogeneous, more concentrated on the edges of the cylindrical extrudate83,84. The pore diameter distribution (M14nm) during the asphaltene diffusion and adsorption process exhibits a loss of largest pores (15 – 20 nm). In summary, asphaltene nano-aggregates and clusters can be sketched as large entities jammed into the pores, with very slow mobility. This congestion of the pore texture also affects the solvent (toluene) mobility, strongly reduced in comparison with solvent alone inside the porous network. The B14nm 2D T1-T2 map supports this behavior, and the peak 4 with a wide T2 distribution and a quite low T1/T2 ratio is the sign of the macroporosity. In fact, we can propose that the asphaltene overcrowding in large pores will be less sensitive causing a faster dynamics of asphaltenes and toluene protons.

4. Conclusion In this contribution, we have shown that H-NMR relaxation times measurements can contribute to the investigation of the dynamics of molecules especially when they are diffusing through a porous network. 2D T1-T2 maps of asphaltene solutions for various concentrations can be interpreted using the aggregation model proposed by Eyssautier et al.27 or the Yen-Mullins model29. The asphaltene nano-aggregate exhibits several kinds of protons; those trapped inside the heart of the nano-aggregate have a clear reduction of their mobility when those located on the edges of this nano-aggregate are more mobile, probably belonging to aliphatic chains. As the asphaltene concentration increases, we have observed a decrease of the solvent molecules (toluene) dynamics. The investigation of asphaltenic fractions, of low and high molecular weights, made us assert the 2D T1-T2 map peak identification. We have discussed the presence of protons coming from the non-deuterated toluene which can influence the peak identification as the asphaltene concentration becomes very low. The correlation time τc was estimated from T1/T2 ratio and in good agreement with the average size of the asphaltene nano-aggregate. The next step will be to use high field NMR sequences for relaxation time measurements in order to get access to a higher NMR resolution anabling the observation of the different types of protons present in the asphaltenic solution43. We have emphasized on the BPP model used for T1 and T2 evolution regarding the correlation time (τc). Let’s recall that this model is based on the hypothesis that the time correlation function has an exponential form73,74. We can suspect that this model is not perfectly true when molecules and thus protons are confined into small pores. In that way, NMR relaxometry experiments varying temperature could be interesting. According to the solvent and asphaltenes behavior into the alumina supports, we first investigated the dynamics of the solvent. We have confirmed the stronger interaction


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of water with alumina than the one of the toluene. The effect of the presence of macropores is clearly observed, boosting the toluene mobility through the porous network. 2D T1-T2 maps for asphaltenes inside the pores show various types of protons, all of them with a severe constrained dynamics. Thus, asphaltenes nanoaggregates and clusters in the porosity are characterized by a very slow mobility, in complete agreement with the very long times of diffusion of these entities through the pore network of alumina beads or extrudates used as industrial catalysts36,83,84.


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Asphaltenes

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Retained Asphaltenes Big asphaltenes

Filtrated asphaltenes Small Asphaltenes

Ratio H/C

1,10 +/- 0,05

1,10 +/- 0,03

1,14 +/- 0,03

Carbone

82,3 +/- 0,4 wt %

81,9 +/- 0,4 wt %

81,0 +/- 0,4 wt %

Hydrogen

7,5 +/- 0,1 wt %

7,4 +/-0,1 wt %

7,3 +/- 0,1 wt %

Sulfur

8 +/- 2 wt %

8 +/- 2 wt %

8 +/- 2 wt %

Vanadium

550 +/- 50 ppm

555 +/- 30 ppm

271 +/- 15 ppm

Nickel

190 +/- 30 ppm

185 +/- 7 ppm

68 +/- 2 ppm

Table 1: Elemental composition of (i) asphaltenes extracted from a Safaniya vacuum residue and (ii) retained (Big Asphaltenes) and filtrated asphaltene (Small Asphaltenes) fractions obtained by ultrafiltration process76.


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M14nm

B14nm

Specific surface area, SBET (m2/g)

194 ± 10

165 ± 10

Parameter C from the BET

113 ± 6

98 ± 5

Mesoporous volume, VBJH (mL/g)

0.80 ± 0.04

0.60 ± 0.04

Macroporous volume, VBJH (mL/g)

/

0,35 ± 0.02

Mean pore diameter, dmean (nm)

14.5 ± 0.7

14.3 ± 0.7 (mesopores)

Structural density

3.22 ± 0.32

3.05 ± 0.32

71 ± 4

75 ± 4

Porosity (%)

Table 2: A porous characteristic of alumina supports M14nm and B14nm.


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Probe diameter (mm)

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Probe dead time (µs)

P90 (µs)

Filter dead time 100 kHz/1 MHz (µs)

τ spacing (µs)

1st FID point (µs)

5

2

15

25

21

5

15

11

10

Table 3: Main characteristics of the NMR Instrument (P90 is the 90° pulse duration).


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Signal T1 (ms) Asphaltene Solution _ 2 wt % 2.2 .103 solvent toluene 3.0 .102 n°1 8.0 .101 n°2 6.0 .101 n°3 6.0 .101 n°4 3.2 .101 n°5 Asphaltene Solution _ 3 wt % 2.0 .103 solvent toluene 2.0 .102 n°1 9.0 .101 n°2 1.2 .102 n°3 7.0 .101 n°4 8.0 .101 n°5 Asphaltene Solution _ 5 wt % 1.6 .103 solvent toluene 2,8 .102 n°1 7.4 .101 n°2 7.0 .101 n°3 8.0 .101 n°4 4.5 .101 n°5 Small asphaltene _ 2 wt % 2.2 .103 solvent toluene 3.0 .102 n°1 4.5 .101 n°2 Big asphaltenes _ 2 wt % 2.2 .103 solvent toluene 3.0 .102 n°1 6.0 .101 n°2-3 7.5 .101 n°4 7.5 .101 n°5

T2 (ms)

T1/T2

2.2 .103

1

1.5 .102 3.0 .101 6.0 .100 1.0 .100 3.0 .10-1

2 2.7 10 60 107

2.0 .103

1

9.0 .101 2.0 .101 3.0 .100 8.0 .10-1 3.0 .10-1

2.2 5 40 88 267

7.0 .102

2.2

1,6 .102 2.0 .101 5.0 .100 1.0 .100 1.5 .10-1

2 4 14 80 300

2.2 .103

1

1.5 .102 3.0 .101

2 1.5

2.2 103

1

8.0 .101 9.5 .100 1.5 .100 1.2 .10-1

4 6 50 625 (à verifier)

Table 4: Relaxation times and ration T1/T2 measured from 2D T1-T2 maps of asphaltene solutions in deuterated toluene (see figure 3).


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Asphaltene concentration (wt %)

NH,A/NH,T

2 3 5

4.2 6.3 10.7

Table 5: Ratio number of protons from asphaltenes to number of protons from deuterated toluene for different asphaltene concentrations.


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Solvent

Support

T1 (ms)

T2 (ms)

T1/T2

M14nm

7.2.101

6.0

12

B14nm

6.1.101

8.4

7

M14nm

1.7.102

1.1.102

1.6

B14nm

5.8.102

4.8.102

1.2

Water

Toluene

Table 6: Relaxation times T1 and T2 (ms) for water and toluene inside the porosity of two alumina supports M14nm and B14nm (see table 2 and figure 7).


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Signal

T1 (ms)

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T2 (ms)

T1/T2

3.0 .10-1

12

Support M14nm dried n°1

4.0 .100

Support M14nm impregnated with asphaltenes (2 wt %) and dried n°1 n°2 n°3

1.7 .100 1.8 .101 2.0 .101

3.0 .10-1 3.0 .10-1 4.0 .10-2

6 60 200

Support M14nm impregnated with asphaltenes (2 wt %) n°1 n°2 n°3 n°4

2.0 .100 2.0 .101 2.0 .101 30 to 100

3.0 .10-1 2.0 .10-1 5 .10-2 1 to 20

7 100 400 5 to 50

Support B14nm impregnated with asphaltenes (2 wt %) n°1 n°2-3 n°4

1.0 .100 4.0 .101 20 to 100

1.7 .10-1 1.0 .10-1 0,3 to 13

6 400 6 to 60

Table 7: Relaxation times T1 and T2 (ms) and ratio T1/T2 deduced from 2D maps (see figure 8)


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References 1. Catalysis by transition metal sulphides – From Molecular Theory to Industrial Application; Toulhoat, H., Raybaud, P., Eds.; Editions Technip: Paris, 2013. 2. Merdrignac, I., Espinat, D., Hénaut, I., Argillier, J. F. Properties and Composition. In Heavy Crude Oils – from Geology to Upgrading – An Overview; Huc A. Y., Eds.; Editions Technip: Paris, 2011; Chapter 4, p. 34. 3. Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. 4. Asphaltenes, Heavy Oils, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G., Eds.; Springer; New York, 2007. 5. Mojelsky, T. W., Ignasiak, T. M., Frakman, Z., McIntyre, D. D., Lown, E. M., Montgomery, D. S., Strausz, O. P. Structural Features of Alberta Oil Sand Bitumen and Heavy oil Asphaltenes. Energy Fuels 1992, 6, 83-96. 6. Waldo, G. S., Mullins, O. C., Penner-Hahn, J. E., Cramer, S. P. Determination of the chemical environment of sulphur in petroleum asphaltenes by X-ray absorption spectroscopy. Fuel 1992, 71, 53. 7. Speight, J. G., Pancirov R. J. Structural types in petroleum asphaltenes as deduced from pyrolysis/gas chromatography/mass spectrometry. Liquid Fuels Technology, 1984,2(3), 287-305 . 8. Mitra-Kirtley, S., Mullins, O. C., van Elp, J., George, S. J., Chen, J., Cramer, S. P. Determination of the nitrogen chemical structures in petroleum asphaltenes using XANES spectroscopy. J. Am. Chem. Soc. 1993, 115, 252-258. 9. Goulon, J., Retournard, A., Friant, P., Goulon-Ginet, C., Berthe, C., Muller, J. F., Poncet, J. L., Guilard, R., Escalier, J. C., Neff, B., Structural characterization by X-ray Absorption Spectroscopy (EXAFS/XANES) of the Vanadium Chemical Environment in Boscan asphaltenes. J. Chem. Soc. Dalton Trans. 1984, 1095-1103. 10. Bouquet, M., Bailleul, A. Routine method for quantitative 13C n.m.r editing and providing structural patterns. Fuel 1986, 65, 1240-1246. 11. Murgich, J. Molecular Simulation and the Aggregation of the Heavy fractions in Crude Oils. Molecular Simulations 2003, 29, 451-461. 12. Bunger, J. W., Li, N. C. « Chemistry of asphaltenes » Advances in Chemistry Series 195, 1981, American Chemical Society. 13. Strausz, O. P., Mojelsky, T. W., Mown, E. M. The molecular structure of asphaltene: an unfolding story. Fuel 1992, 71, 1355-1363. 14. Pacheco-Sanchez, J. H., Alvarez-Ramirez, F., Martinez-Magadan, J. M. Morphology of Aggregated Asphaltene Structural Models. Energy Fuels 2004, 18, 1676-1686. 15. Acevedo, S., Castro, A., Negrin, J. G., Fernandez, A., Escobar, G., Piscitelli, V. , Delolme, F., Dessalces, G. Relations between Asphaltene Structures and Their Physical and Chemical Properties: The Rosary-type structure. Energy Fuels 2007, 21, 2165-2175.


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Characterization of Asphaltenes in Solution and Inside the Pores of

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