Effects of resin I on the catalytic oxidation of n-C₇ asphaltenes in the presence of silica-based nanoparticles

Abstract

This study aims to evaluate the effects of resin I on the n-C₇ asphaltene thermal decomposition under an oxidative atmosphere in the presence of hybrid nanoparticles (SNi1Pd1) of NiO and PdO supported over fumed silica nanoparticles. Resin I and n-C₇ asphaltenes were characterized by elemental analyses, thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR). The adsorption of resin I and n-C₇ asphaltenes was evaluated using heavy oil model solutions through a combined method of thermogravimetric analysis and softening point measurements. Adsorption isotherms were measured for individual resin I and n-C₇ asphaltene samples as well as for different n-C₇ asphaltene to resin I ratios of 7 : 3, 1 : 1 and 7 : 3. For the first time, competitive adsorption of n-C₇ asphaltene and resin I on functionalized nanoparticles with NiO and PdO is assessed. The oxidation tests were carried out in an air atmosphere for a specific n-C₇ asphaltene loading in each sample (ca. 0.20 ± 0.02 mg m⁻²). In this order, for samples adsorbed from different A : R ratios of 7 : 3, 1 : 1 and 3 : 7, the amounts of resin I adsorbed were 0.06, 0.10 and 0.20 ± 0.01 mg m⁻², respectively. Hence, the A : R ratios in the adsorbed phase were 10 : 3, 2 : 1 and 1 : 1. The catalytic effect was measured through thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, which evaluated the effluent gases of the catalytic oxidation process. The adsorption isotherms were modeled using the solid-liquid-equilibrium (SLE) model, and the effective activation energies for the oxidation process of the adsorbate were calculated through the non-linear integral method of Vyazovkin (NLN). As a result, it was observed that the temperature of n-C₇ asphaltene decomposition did not vary significantly with the inclusion of resin I in the system. Rate of mass loss curves showed that the main peak temperatures of n-C₇ asphaltenes and resin I decreased drastically from approximately 500 °C to 250, 260 and 270 °C for resin I loadings over SNi1Pd1 nanoparticles of 0.20, 0.10 and 0.06 mg m⁻², respectively. However, the catalytic effect of the nanoparticles was indeed affected, as revealed by the increase in the estimated effective activation energy as the amount of resin I in the system increased. It is expected that this work opens a better outlook about the use of catalytic nanoparticles in the oil and gas industry, mainly in improved (IOR) or enhanced oil recovery (EOR) processes for heavy and extra-heavy oil upgrading.

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1. Introduction

Asphaltenes and resins are refractory and high-molecular-weight molecules that directly impact the physicochemical properties of crude oil, leading to expensive operations in the production, transport and refining processes.^1,2 In fact, asphaltenes and resins are chemical compounds with similar chemical structures containing polar and non-polar groups.^1–3 The chemical compositions of asphaltenes and resins may consist of various amounts of sulfur, oxygen, hydrogen, nitrogen, carbon, and heavy metals such as nickel and vanadium.^1,2 These molecules are highly complex and portray self-association behavior that results in aggregate formation and subsequent deposition in porous media, which consequently causes formation damage.^1,2,4 Both asphaltenes and resins are mainly composed of a polyaromatic hydrocarbon core surrounded by aliphatic chains, but compared with asphaltenes, resins have longer aliphatic chains.^2,3,5 Nevertheless, asphaltenes have a higher polarity than resins due to the increased number of heteroatoms and the way they are distributed in the asphaltene structure. Generally, oxygen and nitrogen atoms are located close to each other in the asphaltene structure, which results in a high dipole moment in comparison with resins.⁵ In addition, asphaltenes and resins have different solubilities. Asphaltenes are widely defined as a class of materials that are soluble in light aromatic hydrocarbons such as toluene, benzene or pyridine but insoluble in low molecular weight paraffins. Conversely, resins are soluble in n-paraffins such as n-pentane and n-heptane.² Actually, some studies have concluded that resins could favor the solubility of asphaltenes in crude oil,⁶ mainly in the light oil where asphaltenes are most likely to be unstable.^7,8 For the case of improved (IOR) or enhanced oil recovery (EOR) of heavy oil (HO) and extra heavy oil (EHO), resins can promote the stabilization of asphaltenes and consequently inhibit possible formation damage.⁹ Furthermore, it is believed that resins can also impact the thermal cracking processes of HO and EHO.¹⁰

Recently, due to their unique properties, metal and metal oxide nanoparticles have attracted much attention in various applications, including the inhibition of formation damage^7,11–13 and EOR/IOR processes.^14–20 Several authors^{5,7,12,18,19,21–33} have studied asphaltene sorption and subsequent catalytic decomposition using nanoparticle catalysts. Nanoparticles have the capability to rapidly adsorb asphaltenes within minutes from an oil matrix with a relatively small dose, which makes their application time- and cost-effective. Studies have also shown that nanocatalysts can considerably reduce the decomposition temperature of the asphaltenes as well as the effective activation energy, confirming their catalytic activity toward asphaltene decomposition.^22,23,27,28 More recently, our research group has demonstrated that multi-metallic nanoparticles composed of a nanoparticulated support functionalized with mono- or bimetallic metal oxides over its surface enhance the adsorptive capacity of the nanoparticles and greatly reduce the thermal decomposition temperature of asphaltenes.^19,22,23,25 It was observed that the presence of NiO and PdO nanocrystals over different supports such as alumina, titania and silica considerably reduced the thermal decomposition temperature of asphaltenes and inhibit coke formation over the support surface.^18,19,25

It is believed that the presence of resins in the oil matrix could impact asphaltene adsorption and thermal decomposition. However, the effects of resins on the catalytic activity of nanoparticles towards asphaltenes is still unclear. To understand the role that resins play on the mechanisms of upgrading HO and EHO, the interactions between asphaltene–asphaltene, asphaltene–resin and resin–resin in the presence of a nanoparticulated catalyst need to be fully understood. The importance of the effects of resins on the adsorption/catalysis is because resins may compete for adsorption and get adsorbed over the catalyst surface as individual components or be adsorbed over the asphaltene surface and act as a peptizing agent.⁵ Also, due to the easy decomposition of alkyl chains and aromatization of resins, in thermal processes it may lead to different reaction mechanisms that would result in different addition reactions that could impact the nanoparticles catalytic activity for the in situ upgrading of HO and EHO.^34,35 During resins decomposition the main products are asphaltenes, aromatics and volatiles.³⁶ In fact, it is well accepted in literature that the cracking of resins during thermal processes will produce asphaltene in majority,^34,37 mainly in air injection processes.^36,38

In a previous study,⁵ the effects of resin I on n-C₇ asphaltene adsorption over silica and hematite nanoparticles were addressed. It was found that at a concentration < 3000 mg L⁻¹, resin I did not have a significant effect on the adsorption of n-C₇ asphaltenes, as resin I has a solvent-like behavior in that concentration range. Hence, this work is a continuation of our previous study and assesses for the first time the effects of resins on the catalytic thermal decomposition of asphaltenes under an oxidative atmosphere in the presence of NiO and/or PdO-supported-on-silica nanoparticles. Here, simultaneous adsorption isotherms of resins I and n-C₇ asphaltene over functionalized nanoparticles are obtained for the first time. The oxidation process was evaluated using thermogravimetric analysis (TGA) coupled with FTIR spectroscopy for monitoring the effluent gases. Catalytic activity of functionalized nanoparticles towards resin I and n-C₇ asphaltene is corroborated through the estimation of the effective activation energies using the non-linear integral method of Vyazovkin (NLN). This study provides additional insights toward understanding the mechanism of interactions between resins/asphaltenes/nanoparticles during thermal EOR/IOR processes based on the oxidation of asphaltenes and resins assisted by nanocatalysts for HO and EHO in situ upgrading.

2. Materials and methods

2.1. Nanoparticles

Fumed silica (S) nanoparticles functionalized with 1 wt% each of NiO and PdO (SNi1Pd1) were used as the adsorbent and catalyst for n-C₇ asphaltene. The fumed silica (S) support nanoparticles were purchased from Sigma-Aldrich (USA). Distilled water and salt precursors of Ni(NO₃)₂·6H₂O (Merck KGaA, Germany) and Pd(NO₃)₂ (Merck KGaA, Germany) were used for nanoparticle functionalization. The functionalized nanoparticles were prepared following the procedure developed in a previous study.²² The S nanoparticles mean crystallite size was estimated to be 7.1 ± 0.2 nm with a S_BET of 389.00 ± 0.01 mg m⁻². In addition, the Brunauer, Emmett and Teller surface area (S_BET) of SNi1Pd1 nanoparticles was 201.50 ± 0.01 mg m⁻² with NiO and PdO mean crystallite sizes of 1.3 and 2.2 ± 0.2 nm, respectively. Additional information about nanoparticle characterizations can be found in previous studies.^19,22,39

2.2. n-C₇ asphaltenes and resin I

A Colombian extra-heavy crude oil (EHO) sample obtained from a reservoir located in the central region of Colombia was used as the source of resin I and n-C₇ asphaltenes. The EHO has 6.2° API and viscosity of 3.47 × 10⁶ cP. Average contents of saturates, aromatics, resins, and asphaltenes (SARA) were determined using a Iatroscan MK-6 thin layer chromatograph following the IP 469 method,⁴⁰ with values of 19.2, 16.3, 52.0 and 12.5 wt%, respectively. For extracting the n-C₇ asphaltenes from the EHO sample a Soxhlet setup is used. A mixture of n-heptane (99%, Sigma-Aldrich, St. Louis, MO) and crude oil is prepared in a ratio of 40 mL g⁻¹. Resin I was obtained from the Soxhlet liquid by distillation, and n-C₇ asphaltenes were obtained from the Soxhlet chamber.^5,41 More details about resin I and n-C₇ asphaltene extraction can be found in our previous study.⁵ Solutions with different n-C₇ asphaltenes to resin I (A : R) mass ratios of 7 : 3, 1 : 1 and 3 : 7 as well as different concentrations of the n-C₇ asphaltene–resin mixture from 500 mg L⁻¹ to 5000 mg L⁻¹ in toluene are prepared.

n-C₇ asphaltenes and resin I were characterized through elemental analysis, determination of the molecular weight and Fourier transform infrared spectroscopy (FTIR). The molecular weight of a samples was determined using a K-7000 vapor pressure osmometer (Knauer, Wissenschaftliche Gerätebau, Germany). The FTIR spectra of resin I and n-C₇ asphaltenes were obtained using an IRAffinity spectrophotometer (Shimadzu, Japan). For FTIR analysis, 300 mg of KBr pellets were added to 13 mg of sample.⁴² For the detection of the samples, a KCl cell with a 0.25 mm spacer was used and placed in the FTIR scanning path at room temperature. Sixteen scans per minute were taken for each sample in a range from 4000 to 400 cm⁻¹ at a resolution of 2 cm⁻¹. The obtained FTIR spectra were analyzed using the Origin 2015 software (OriginLab Corporation, USA), and different indices were calculated to provide further qualitative information about the resin I and n-C₇ asphaltene molecules. Each measurement was performed in triplicate to ensure the reproducibility of the measurements. All indices were calculated based on the related band areas from valley to valley and applying a deconvolution technique in order to avoid overlapping of the selected bands.^43–45 For example, the area calculated from the band located at 1600 cm⁻¹ is denoted as A₆₀₀. A brief description of the calculated indices is shown in Table 1.

Table 1 Defined indices of FTIR transmittance bands for the structural characterization of resin I and n-C₇ asphaltenes.^43,46,47

Index	Equation ∑A = A1700 + A1600 + A1460 + A1376 + A1030 + A864 + A814 + A743 + A724 + A(2953,2923,2862)	Description
Aromaticity index	A1600/∑A (aromatic structures)	The higher the index, the higher the aromaticity
Car/Har index	A1750–1520/A3000–2800	The index is related to the aromaticity of the sample. The higher the index, the higher the number of aromatic carbons relative to the aliphatic hydrogens
Aliphatic index	A1460 + A1376/∑A	The higher the index, the higher the aliphaticity
Aromatic ring index	A970 + A912 + A742/∑A	The index related to the number of aromatic rings. The higher the index, the higher the number of aromatic rings
Long chains index	A724/(A1460 + A1376) (aliphatic structures)	The higher the index, the larger the aliphatic chains
CH2/CH3 ratio	A2922/A2952	The higher the index, the larger the aliphatic chains
Carbonyl index	A1700/∑A	The higher the index, the higher the amount of carbonyl groups

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2.3. Methods

2.3.1. Adsorption experiments. Batch-mode adsorption experiments were carried out at 25 °C using a constant ratio of 10 : 1 of nanoparticles to solution volume. Adsorption isotherms were obtained for different A : R mass ratios of 7 : 3, 1 : 1 and 3 : 7 as well as individual n-C₇ asphaltenes and resin I.²⁴ The amount of n-C₇ asphaltenes and/or resin I adsorbed over the selected nanoparticles was measured by combined thermogravimetric analysis (TGA) and softening point (SP) measurements for the simultaneous construction of adsorption isotherms of n-C₇ asphaltenes and resins as explained in our previous study.⁵ Adsorption isotherms were modeled using the solid–liquid equilibrium (SLE) model, which provides valuable information about adsorbate–adsorbate and adsorbate–adsorbent interactions for self-associative molecules.⁴⁸ Full information about SLE model can be found in our previous works.⁴⁸

2.3.2. n-C₇ asphaltenes and/or resin I oxidation in the presence and absence of nanoparticles. Thermogravimetric analyses have been widely used for determining the efficiency of nanocatalysts on the reduction of the decomposition temperature of n-C₇ asphaltenes.^19,23,32 The oxidation of n-C₇ asphaltenes and/or resin I in the presence and absence of the selected nanoparticles was carried out by heating from 30 °C to 800 °C with an air flow rate of 100 mL min⁻¹ at three different heating rates of 10, 20, and 30°C min⁻¹ using a Q50 thermogravimetric analyzer (TA Instruments, Inc., New Castle, DE). The samples with n-C₇ asphaltenes and/or resin I adsorbed were selected for the thermal analysis with an adsorbed amount of n-C₇ asphaltenes of 0.20 ± 0.02 mg m⁻². Effluent gases were tested in an IRAffinity FTIR spectrophotometer (Shimadzu, Japan), which operates at a resolution of 2 cm⁻¹ with 16 scans per minute in the range of 4000 to 400 cm⁻¹. The coupled TGA-FTIR method has been widely used for analysis of gases produced from oxidation, pyrolysis and gasification reactions.^19,23,32 The CO, CO₂, CH₄ and other light hydrocarbons were analyzed in this case. The highest signal in the FTIR spectra that corresponded to CO₂ was used for normalizing the gas production profiles.²³ All experiments were performed in triplicate for confirming the reproducibility.

3. Oxidation kinetics of asphaltenes/resins in the presence and absence of nanoparticles

For a linear heating rate, the non-isothermal overall rate of an oxidation reaction for a condensed phase can be described by the differential form as follows:⁴⁹where R (J mol⁻¹ K⁻¹) is the ideal gas constant, T (K) is the reaction temperature, E_α (kJ mol⁻¹) is the effective activation energy for a constant conversion, A_α (1/s) is the pre-exponential factor, β = dT/dt is the heating rate, α is the reaction conversion ranging between 0 and 1.0, and f(α) is the reaction mechanism function dependent on the particular reaction model. α can be calculated from thermogravimetric analysis as follows:where m_f is the final mass of the sample, m₀ is the initial mass of the sample and m_t is the mass at a given temperature.

The kinetic parameters in eqn (1) can be estimated by a number of isoconversional methods.⁴⁹ In this case, the non-linear integral method of Vyazovkin (NLN) was used to estimate the effective activation energies, assuming that, for a constant reaction conversion, the reaction rate is a function of the state and the temperature.^31,49 Accordingly, after integrating and rearranging eqn (1), eqn (3) is obtained:

Eqn (3) can be solved by direct numerical integration.^50,51 Therefore,

The main assumptions of the isoconversional method are that g(α) is independent of β and J[E_α,T_i(t_α)] is equal for all experiments at given value of α. Then, the effective activation energy can be obtained by finding a value of E_α that minimizes eqn (5) for a particular value of α.⁵²

with

The right side of eqn (6) is evaluated by numerical methods for small time segments in a set of different heating programs. The estimation of E_α is performed by the minimization of the NLN method with a tolerance of 10⁻⁶ using a home-made MATLAB (version 7.6.0 R2008a) routine.⁵²

4. Results and discussion

The results are divided into four main sections, namely: (1) characterization of resin I and n-C₇ asphaltenes; (2) adsorption isotherms of n-C₇ asphaltenes or resin I over selected nanoparticles constructed independently for each compound; (3) determination of the competitive adsorption between n-C₇ asphaltenes and resin I for mixtures with different A : R ratios over the selected nanoparticles; and (4) estimation of the influence of resin I on the catalytic thermal oxidation of n-C₇ asphaltenes on nanoparticles through TGA as well as the estimation of the effective activation energies of the reactions.

4.1. Characterization of n-C₇ asphaltenes and resin I

Resin I and n-C₇ asphaltenes were characterized by elemental analysis of CHNSO and FTIR. Fig. 1 shows the obtained FTIR spectra of resin I and n-C₇ asphaltenes. In addition, Table 3 displays the results of the elemental analyses and the values of the calculated indices according to Table 1. As observed in Fig. 1, the FTIR spectra are similar for both of the studied components. This is not surprising as resins and asphaltenes may contain the same heteroatoms and similar chemical structures. In this case, as seen in Table 2, the studied resin I and n-C₇ asphaltenes are S-, N- and O-containing molecules. The main bands associated with aromatic compounds in resin I and n-C₇ asphaltenes are the ν C–H of aromatics at 3050 cm⁻¹, ν C=C of aromatic systems at 1600 cm⁻¹ and C–H as well as C–H in condensed aromatic systems in the range 1640–1600 cm⁻¹ or between 1300 and 1100 cm⁻¹.^42,53–55 Regarding the aliphatic moieties, for the aliphatic ν C–H for CH₂ and CH₃ groups, the related bands are found at 2872, 2962, 2953 and 2926 cm⁻¹, δ_as C–H of CH₃ and δ_s C–H scissoring of CH₂ at 1460 cm⁻¹, δ_s C–H scissoring of CH₃ at 1375 cm⁻¹, ω and τ of the CH₂ group at 1350–1150 cm⁻¹ and ρ CH₂ oscillations in phase at 724 cm⁻¹.⁵⁶ In addition, the bands related to the heteroatoms confirm the presence of different functional groups and are located at 3500–3000 cm⁻¹ for ν O–H and functional groups that are able to form intermolecular hydrogen bonds. The ν C=O of O-containing functional groups are found at 1744 and 1700 cm⁻¹, ν S=O of sulfoxide groups are found at a band centered at approximately 3456 cm⁻¹, and bands related to free NH group stretching and symmetric (medium) nitro compounds as well as C–N single bonds are found at 1380 and 1020 cm⁻¹, respectively.^42,53,55 Generally, thiophene, sulfidic and sulfoxide are the S-related functional groups; hydroxyl, carbonyl and carboxyl are the O-containing functional groups; and pyrrolic, pyridine and quinoline are the N-related groups.^56,57

Fig. 1 FTIR spectra of resin I and n-C₇ asphaltenes considered in this study.

Table 2 Elemental analyses and values of the structural indices obtained by FTIR for n-C₇ asphaltenes and resin I

Parameter	Resin I	n-C7 asphaltenes
a By difference.
Elemental analyses
C (wt%)	82.10	81.70
H (wt%)	11.00	7.80
N (wt%)	0.54	<0.5%
S (wt%)	6.10	6.61
O^a (wt%)	0.26	3.56
H/C ratio	1.61	1.14
O/C ratio	0.002	0.033
(O + N)/C ratio – bulk polarity	0.008	0.036
Molecular weight (g mol^−1)	2868	907
Proposed molecular formula	C196H315S5NO	C62H71S2NO2
Structural indices
Aromaticity index ± 0.03	0.47	0.74
Car/Har index ± 0.02	0.11	3.19
Aliphatic index ± 0.04	0.53	0.26
Aromatic rings index ± 0.01	0.06	0.03
Long chains index ± 0.01	0.20	0.19
CH2/CH3 ratio ± 0.01	0.92	0.83
Carbonyl index ± 0.01	0.02	0.16

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It is clear from Table 2 that the aforementioned groups are prone to be in the resin I or n-C₇ asphaltene structures. Nevertheless, the main differences between both structures can be found in the oxygen content, bulk polarity and H/C ratio. As seen, n-C₇ asphaltenes contain more oxygen than resin I, which is directly associated with the polarity of the molecule and is in agreement with the (O + N)/C ratio. The polarity of n-C₇ asphaltenes is higher than of resin I not only due to a larger amount of oxygen but also due to the positioning of N and O heteroatoms in the molecular structure.⁵ Calculated structural indices are also listed in Table 2. As expected and in agreement with the H/C ratio, the aromaticity index was higher for n-C₇ asphaltenes than for resin I, and this result is corroborated by the C_ar/H_ar index, which indicates that the ratio of aromatic carbons relative to the aliphatic hydrogens is higher for n-C₇ asphaltenes. Nevertheless, the aromatic ring index and the CH₂/CH₃ ratio were higher for resin I, indicating that the number of aromatic rings is higher and that the aliphatic chains are larger in the resin I molecules. This could indicate that the size of the resin molecules is larger than that of the n-C₇ asphaltenes, and this result is corroborated by the molecular weights of both samples that resulted in a higher value for resin I in comparison to that of the n-C₇ asphaltenes. The carbonyl index was higher for n-C₇ asphaltenes than for resin I and could be a strong indicator of the higher polarity of the n-C₇ asphaltene molecules in comparison to the resin I molecules.

4.2. Individual adsorption of n-C₇ asphaltenes and resin I

Panels (a) and (b) from Fig. 2 show the individual experimental adsorption isotherm data for (a) n-C₇ asphaltenes and (b) resin I on SNi1Pd1 and silica nanoparticles at 25 °C together with the SLE model fitting. It can be observed from Fig. 2a that the n-C₇ asphaltene adsorption isotherm shows type I behavior according to the last International Union of Pure and Applied Chemistry (IUPAC) classification.⁵⁸ Additionally, from Fig. 2 it is clear that the SNi1Pd1 nanoparticles have a higher adsorption capacity towards n-C₇ asphaltenes than the silica nanoparticles. These results are in agreement with Franco et al.^22,25 who evaluated the adsorption of n-C₇ asphaltenes extracted from Capella crude oil over NiO and/or PdO-supported-on-γ-Al₂O₃ or TiO₂ (ref. 25) and SiO₂ nanoparticles.²² Asphaltene aggregation in solution is a factor to be considered in adsorption studies, and it could be envisioned that in this case, a lower adsorption could be expected when compared with resins where no such aggregation is present. This could be more significant at low sample concentrations where the presence of the “free” n-C₇ asphaltene molecules is relatively high.⁵⁹ These arguments are consistent with the isotherms shown in Fig. 2.

Fig. 2 Adsorption isotherms of (a) n-C₇ asphaltenes and (b) resin I over silica and SNi1Pd1 nanoparticles at 25 °C, constructed separately for each component. The symbols are experimental data, and the solid lines are from the SLE model.

From Fig. 2b, it can also be observed that SNi1Pd1 nanoparticles have a higher adsorption capacity of resin I than the silica support, confirming the synergistic effect of the hybrid material towards heavy hydrocarbon uptake. As expected, the presence of transition metal oxides increases the adsorption due to the bonding of functional groups acting as ligands to Ni and Pd.⁶⁰ However, both nanomaterials evaluated showed a lower adsorption affinity for resin I than for n-C₇ asphaltenes. Adsorption isotherms of resin I over silica and SNi1Pd1 nanoparticles showed type III and type V behaviors according to the IUPAC,⁵⁸ respectively, over the range of resin I concentrations evaluated. It is expected that the individual selectivities of the silica support, NiO and PdO towards different heteroatom-containing functional groups would be unified in the SNi1Pd1 material.²⁵ These two isotherm shapes are indicative of multilayer adsorption, even if the resin–resin interactions in the bulk phase are low,^3,61 and once adsorbed over the SNi1Pd1 nanoparticle surface, the polar character of the resins suggests that the resin–resin interactions over the solid surface are not negligible and result in self-association around the active sites leading to the formation of multilayers.³ However, for the type V isotherm, the adsorption could be associated with the low affinity between the adsorbate-adsorbent in Henry's region⁴⁸ and the subsequent saturation of the nanoparticle oxide active sites for C_E > 200 mg L⁻¹, as it has been shown that silica-supported metal oxides are capable of inhibiting the self-association of heavy polar hydrocarbons.

The adsorption of polar hydrocarbons such as resin I and n-C₇ asphaltenes over SNi1Pd1 nanoparticles is controlled by the Brønsted and Lewis acid sites that increase the electron withdrawing.^18,19 Brønsted acid sites release surface protons and are generally those attributed to the Si–OH functional groups, while the Lewis acid sites are stronger sites related to Si–OSi and Si–OM bonds that are more prone to accept electron pairs from nucleophiles.^62–64 The SLE model showed a good fit towards the experimental results as revealed by the R² and x² values presented in Table 3. According to the H parameter, the adsorption affinity is higher for the hybrid material than for the silica support due to the increased active sites of the SNi1Pd1 nanoparticles. In addition, for both n-C₇ asphaltenes and resin I, it is observed that the K parameter is lower for the SNi1Pd1 nanoparticles, which is indicative of the inhibition of the adsorbate self-association around the nanoparticle active sites.¹⁹

Table 3 SLE model parameters for n-C₇ asphaltenes and/or resin I adsorption over silica and SNi1Pd1 nanoparticles at 25 °C

Material	A : R	n-C7 asphaltenes					Resin I
		H (mg g^−1)	K (g g^−1)	qm (mg m^−2)	x^2	R^2	H (mg g^−1)	K (g g^−1)	qm (mg m^−2)	x^2	R^2
Silica	Individual components	1.31	5.13 × 10^−4	1.89	0.84	0.99	126.70	637.77	2.01	0.30	0.99
SNi1Pd1	Individual components	0.28	1.97 × 10^−4	4.44	0.63	0.99	80.78	455.18	2.35	0.97	0.99
	7 : 3	3.22	2.30 × 10^−4	4.16	0.72	0.99	2.59	0.96	0.24	1.20	0.95
	1 : 1	6.35	4.83 × 10^−4	4.02	0.66	0.99	6.07	0.98	0.58	0.98	0.98
	3 : 7	10.98	5.51 × 10^−4	2.19	0.81	0.99	7.05	1.00	0.96	1.01	0.97

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4.3. Competitive adsorption between n-C₇ asphaltenes and resin I

Adsorption isotherms of n-C₇ asphaltenes and resin I were constructed simultaneously at different A : R values of 7 : 3, 1 : 1 and 3 : 7 at 25 °C. Panels (a) and (b) from Fig. 3 show the experimental adsorption isotherms together with the SLE model fitting obtained for the competitive adsorption of (a) n-C₇ asphaltenes and (b) resin I over the surfaces of the SiNi1Pd1 nanoparticles. The isotherms of adsorption for the silica support have been evaluated in a previous work, and they showed type I and type II behaviors.⁵ For both n-C₇ asphaltene and resin I adsorption as well as for all of the A : R ratios evaluated, the shapes of the adsorption isotherms follow a clear type I behavior similar to n-C₇ asphaltenes in the absence of resin I. For the case of n-C₇ asphaltenes, these results indicate that for a higher A : R ratio, the decrease in the amount adsorbed could be more closely associated with the restrictions in the migration from the bulk phase to the SNi1Pd1 nanoparticle surface. Asphaltene–resin interactions are more likely to occur than those of asphaltene–asphaltene or resin–resin when both adsorbates coexist.⁶¹ In fact, in the presence of resin, the colloidal state of asphaltenes changes due to peptization,⁶ with the resin acting as terminator molecules⁶⁵ for asphaltene self-association or even by the formation of a micelle-type polar asphaltene core surrounded by resin.⁶⁶ When comparing the adsorption isotherms for n-C₇ asphaltenes with those of resin I, it can be observed that the amount of n-C₇ asphaltenes adsorbed is always higher than that for resin I, indicating that when the adsorbate molecule is more polar, the amount adsorbed over the SNi1Pd1 nanoparticles increases. Adsorption isotherms of resin I over SNi1Pd1 nanoparticles for A : R ratios of 1 : 1 and 3 : 7 differ from those obtained for silica nanoparticles in our previous study,³ where adsorption isotherms showed type II behavior due to the increased number of active sites over the surfaces of the nanoparticles that leads to a more homogenous adsorption. Type II or sigmoidal adsorption implies adsorption near a site already occupied in the otherwise clean surface. Thus, aside from the dilution factor, the presence of n-C₇ asphaltenes at the nanoparticle surface changes the adsorption isotherm of the resin. Although accounting for this requires research, it seems reasonable to suggest that this is related to the way asphaltenes accommodate themselves at the nanoparticle surface. Contrary to surfaces corresponding to bulk solids, where n-C₇ asphaltenes lie flat on the surface, asphaltenes should adsorb onto the nanoparticle site head on. The adsorption of resins is inhibited by bulky asphaltenes forcing them back into solution or forming surface aggregates with them.

Fig. 3 Adsorption isotherms of (a) n-C₇ asphaltenes and (b) resin I over SNi1Pd1 nanoparticles at 25 °C constructed simultaneously for different n-C₇ asphaltene to resin I ratios of 7 : 3, 1 : 1 and 3 : 7. The symbols are experimental data, and the solid lines are from the SLE model.

For both n-C₇ asphaltene and resin I adsorption, the SLE model showed good agreement with the experimental findings. As expected, for the n-C₇ asphaltene adsorption, the value of the H parameter increased as the amount of n-C₇ asphaltenes in the bulk solution decreased (Table 3), indicating a higher adsorption affinity between the n-C₇ asphaltene-nanoparticle couple. As expected, the same trend is observed in the case of resin I, where the affinity could be related to n-C₇ asphaltene–resin I interactions and the presence of the NiO and PdO nanocrystals. In addition, observations about the solvent-like behavior of resin I were corroborated by predicting the amount of adsorbed n-C₇ asphaltenes for the different A : R ratios employed. Additional information about the prediction of the amount of n-C₇ asphaltenes can be found in the ESI document.† It was found that the adsorption of n-C₇ asphaltenes depends mainly on the asphaltene concentration in the bulk solution.

4.4. Effects of resin I on the n-C₇ asphaltene catalytic thermo-oxidative decomposition process

4.4.1. Virgin n-C₇ asphaltenes, virgin resin I and different A : R ratios. Catalytic thermal oxidation of asphaltenes was performed to obtain insight into the effects of the selected nanoparticles under thermogravimetric conditions and analyze the influence of resin I on the process. The oxidation tests were carried out in an air atmosphere for a specific n-C₇ asphaltene loading in each sample (ca. 0.20 ± 0.02 mg m⁻²). In this order, for samples adsorbed from different A : R ratios of 7 : 3, 1 : 1 and 3 : 7, the amounts of resin I adsorbed were 0.06, 0.10 and 0.20 ± 0.01 mg m⁻², respectively. Hence, the A : R ratios in the adsorbed phase were 10 : 3, 2 : 1 and 1 : 1. For this reason, the same A : R ratios in the absence of nanoparticles were evaluated by TGA. Panels (a) and (b) from Fig. 3 show the (a) conversion α and (b) rate of mass loss of n-C₇ asphaltenes and resin I for the different A : R ratios evaluated. As observed in Fig. 4 and for temperatures lower than 380 °C, the degree of material conversion increases in the order resin I > 1 : 1 > 2 : 1 > 10 : 3 > n-C₇ asphaltenes, indicating that higher conversion is reached as the amount of resin I in the mixture increases. This could be due to the resin having a larger content of aliphatic compounds that decompose at relatively low temperatures.

Fig. 4 (a) Conversion and (b) plot of the rate of mass loss as a function of temperature for virgin n-C₇ asphaltenes, virgin resin I and different A : R ratios of 10 : 3, 2 : 1 and 1 : 1.

However, after 450 °C the trend changes to n-C₇ asphaltenes > resin I > 1 : 1 > 2 : 1 > 10 : 1. These results indicate that the presence of resin I in the mixture could improve the formation of heavier products from addition reactions due to the increased availability of polycyclic aromatic hydrocarbons (PAH) after the decomposition of the alkyl chains. These results are in agreement with the structural indices of resin I in comparison to n-C₇ asphaltenes (see Table 2).²⁵ This is corroborated in Fig. 4b, where two main peaks in the plot of the rate of mass loss can be observed for all A : R ratios evaluated at approximately 400 and 500 °C. In addition, all samples except virgin n-C₇ asphaltenes show a small shoulder in the range between 200 and 300 °C, which is due to the oxidation of lighter compounds present in the resin I molecules. In this region, it is evident that the rate of mass loss increases as the amount of resin I in the mixture increase. Additionally, in comparison with virgin n-C₇ asphaltenes, samples containing resin I show a second peak larger than the first one at 494, 497, 499 and 513 °C for A : R ratios of 10 : 3, 2 : 1, 1 : 1 and virgin resin I, respectively, indicating that as the amount of resin I in the sample increases, the temperature of oxidative decomposition increases.

4.4.2. Thermal oxidation of n-C₇ asphaltenes and resin I in presence of SNi1Pd1 nanoparticles. Panels (a) and (b) from Fig. 5 shows the (a) conversion and (b) rate of mass loss as a function of temperature for n-C₇ asphaltene and resin I thermal oxidation in presence of the selected nanoparticles. In general, it is observed that for both resin I and n-C₇ asphaltenes, the catalytic activity of SNi1Pd1 nanoparticles is higher than that for S nanoparticles. It can be seen in Fig. 5a that generally for a fixed temperature, the conversion percentages are higher for SNi1Pd1 nanoparticles. In addition, the main peak of the rate of mass loss is found at lower temperatures for the bimetallic nanoparticles. Below 440 °C, the conversions of resin I and n-C₇ asphaltenes in the presence of SNi1Pd1 nanoparticles are higher than those of the virgin compounds. For example, at approximately 300 °C, the conversion of adsorbed n-C₇ asphaltenes is approximately 40% higher than that of the virgin compound. A similar situation is observed for resin I, where the conversion at 300 °C is higher by 30%.

Fig. 5 (a) Conversion and (b) plot of the rate of mass loss as a function of temperature for n-C₇ asphaltenes and resin I in presence of S and SNi1Pd1 nanoparticles.

From Fig. 4b, it is observed that the higher rates of mass loss for both n-C₇ asphaltenes and resin I in the presence of SNi1Pd1 nanoparticles occur at approximately 250 °C, with reductions at 200 and 260 °C regarding the virgin materials, respectively. Nevertheless, not all of the asphaltenes adsorbed become gases at this temperature, and they continue to undergo significant oxidation until a temperature of approximately 650 °C is reached. Similarly, virgin resin I completes its oxidation process at approximately 540 °C, and for resin I adsorbed over SNi1Pd1 nanoparticles, the oxidation is enhanced by lowering the temperature at which the process starts, i.e., 220 °C.

4.4.3. Catalytic oxidation of n-C₇ asphaltenes with different loadings of resin I in the presence of SNi1Pd1 nanoparticles. Panels (a–d) from Fig. 6 show (a and b) conversion and (c and d) rate of mass loss of n-C₇ asphaltenes and resin I adsorbed from solutions with different A : R ratios over S and SNi1Pd1 nanoparticles, respectively. The amount of n-C₇ asphaltenes adsorbed was maintained at 0.20 ± 0.02 mg m⁻², and the amounts of resin I adsorbed was 0.06, 0.10 and 0.20 ± 0.01 mg m⁻² for samples adsorbed from different A : R ratios of 7 : 3, 1 : 1 and 3 : 7, which resulted in A : R ratios in the adsorbed phase of 10 : 3, 2 : 1 and 1 : 1, respectively.

Fig. 6 Conversion and plot of the rate of mass loss as a function of temperature for different A : R ratios of 10 : 3, 2 : 1 and 1 : 1 in the presence of (a and c) S and (b and d) SNi1Pd1 nanoparticles.

As observed in Fig. 6a and b, the conversion of the adsorbed material is higher than for the different A : R mixtures at temperatures lower than 500 °C, thus corroborating the catalytic effect of the nanoparticles. For the S nanoparticle support, it is observed that the degree of conversion increases as the amount of n-C₇ asphaltenes in the system decreases. Additionally, when comparing conversion curves for n-C₇ asphaltene and resin I mixtures in the presence of SNi1Pd1 nanoparticles (Fig. 6b), no significant effect of the resin I loading can be observed, indicating that the catalytic effect of nanoparticles over n-C₇ asphaltene and resin I decomposition is maintained even for high amounts of adsorbed resin I. The same can be inferred from the plots of the rate of mass loss in Fig. 6d. As can be observed, the peak temperatures for the rate of mass loss of n-C₇ asphaltenes and resin I for different A : R ratios decreased drastically from approximately 500 °C to 250, 260 and 270 °C for resin I loadings of 0.20, 0.10 and 0.06 mg m⁻², respectively, indicating that as the amount of resin I in the system increases, the maximum rate of mass loss shifts to the left. This could be due to that resins have intermediate polarity in comparison to asphaltenes based on the FTIR and elemental analyses, and the chemical structure of resins differs from the chemical structure of the asphaltenes mainly in the reduced number of aromatic carbon relative to the aliphatic hydrogen species as well as the length of the alkyl chains, resulting in a higher H/C ratio.^3,57 In the case of the rate of mass loss for S nanoparticles, the main peak is found at approximately 540 °C, corroborating the catalytic effect of the NiO and PdO nanocrystals. Additionally, it can be observed that as the amount of resin I increases, the height of the first peak increases and could be due to the early decomposition of resin I alkyl chains.

The catalytic effect of nanoparticles was further confirmed by the determination of the produced gases during the thermal decomposition of n-C₇ asphaltenes and resin I. Fig. 7 shows the profile of gas production as a function of temperature for the system with 0.20 mg m⁻² of resin I adsorbed as well as for the A : R ratio of 10 : 3 in the absence of nanoparticles. As seen, for both cases the gas profile has a similar shape to that of the rate of mass loss, indicating that for a higher rate of decomposition, a higher production of effluent gases occurred. The same trend was obtained for the other A : R ratios evaluated.

Fig. 7 The evolution profiles of the production of CO, CH₄, CO₂ and other light hydrocarbons for the systems with (a) 0.20 and 0.20 mg m⁻² of resin I and n-C₇ asphaltenes adsorbed and (b) an A : R ratio of 10 : 3 in the absence of SNi1Pd1 nanoparticles.

4.5. Estimation of the effective activation energies

The effective activation energies (E_α) were calculated using the NLN isoconversional method to corroborate the reaction mechanism and the catalytic effect of the selected nanoparticles as well as for evaluating the effect of resin I on the n-C₇ asphaltene decomposition. Fig. 8 shows the estimated E_α as a function of the degree of conversion for individual resin I and n-C₇ asphaltenes in the presence and absence of SNi1Pd1 nanoparticles estimated through the NLN method. As is observed in Fig. 8, the effective activation energies for resin I in the absence and presence of SNi1Pd1 nanoparticles is higher than that required for n-C₇ asphaltene decomposition. For the systems without nanoparticles, this could be due to the formation of more refractory compounds by resin I than for n-C₇ asphaltenes during the thermal decomposition process and is corroborated by the position of the second maximum in the rate of mass loss for each compound (see Fig. 4). Addition reactions occur due to the reaction mechanism of free radical chains; hence, one would expect that in presence of resin I, these addition reactions will occur at lower temperatures due to the early decomposition of the molecule.²³ However, in presence of nanoparticles, this could be due to higher interactions between the n-C₇ asphaltenes and the nanoparticles, as revealed by the increased adsorption affinity in comparison to resin I (see Table 3). The results are in agreement with previous studies on n-C₇ asphaltene adsorption and subsequent catalytic oxidation, in which it was observed that by increasing the adsorption affinity, the E_α is reduced.^19,23

Fig. 8 Effective activation energies, estimated by the non-linear integral method of Vyazovkin, as a function of the conversion for resin I and n-C₇ asphaltenes in the absence and presence of SNi1Pd1 nanoparticles.

Fig. 9 shows the values of E_α estimated through the NLN method for the oxidation of different mixtures with different A : R ratios in the absence and presence of the selected nanoparticles. As seen in Fig. 8, the values of E_α for all of the A : R ratios evaluated are lower than those for virgin resin I. Hence, it can be said that resin I impact the oxidation process due to as the amount of resin I increases, addition reactions would be promoted at lower temperatures. In the presence of the SNi1Pd1 nanoparticles, the effective activation energies until a certain degree of conversion are lower than those for systems in the absence of nanoparticles. In general, the E_α for resin I and n-C₇ asphaltene decomposition follows the order 10 : 3 > 2 : 1 > 1 : 1, indicating that resin I directly impacts the energy required to achieve the oxidation.

Fig. 9 Effective activation energies by the non-linear integral method of Vyazovkin as a function of the conversion for different A : R ratios of 10 : 3, 2 : 1 and 1 : 1 in the absence and presence of SNi1Pd1 nanoparticles.

5. Conclusions

In this study, the competitive adsorption of n-C₇ asphaltene and resin I over SNi1Pd1 nanoparticles is reported for the first time. It was observed that for all of the A : R ratios evaluated, the amount of adsorbed asphaltenes or resin I was higher for SNi1Pd1 nanoparticles than for the fumed silica support. In addition, the role of resin I on the catalytic oxidation of n-C₇ asphaltenes was evaluated through TGA and by estimating the effective activation energies by the non-linear integral method of the Vyazovkin (NLN) method. Resin I oxidation in the absence of nanoparticles showed that the decomposition starts before that of n-C₇ asphaltenes, probably due to the rupture of aliphatic chains in the molecular structure. Additionally, it was found that the decomposition of resin I finishes at higher temperatures than for n-C₇ asphaltenes, and this could be due to the formation of refractory compounds during the oxidation process. Both S and SNi1Pd1 showed a decrease in the temperature of resin I and n-C₇ asphaltene decomposition, with better results for the hybrid material. The conversion of the different A : R mixtures was higher in presence of nanoparticles mainly at temperatures lower than 500 °C, thus corroborating the catalytic effect of the nanoparticles. For S nanoparticles it was observed that the degree of conversion or a fixed value of temperature increases as the amount of resin I in the system increases. However, the higher catalytic activity of SNi1Pd1 nanoparticles was evidenced as TGA results revealed that the temperature of n-C₇ asphaltenes is not significantly altered by the presence of resin I. In this case the values of the main peak in the rate of mass loss plot were 250, 260 and 270 °C for a fixed n-C₇ asphaltene loading of 0.20 mg m⁻² and loadings of resin I of 0.20, 0.10 and 0.06 mg m⁻², indicating an average reduction of 240 °C regarding to the A : R ratios in absence of SNi1Pd1 nanoparticles. Nevertheless, the effect of resin I on the catalytic activity of the SNi1Pd1 nanoparticles was observed as the effective activation energies needed for the decomposition of n-C₇ asphaltenes and resin I increased as the amount of resin I in the adsorbed phase increased. This study should give a wider landscape on the use of nanoparticle technology for heavy oil enhanced recovery and catalytic upgrading, which could be a viable alternate green technology.

Acknowledgements

The authors acknowledge the Universidad Nacional de Colombia for logistical and financial support and COLCIENCIAS as well as ECOPETROL for the support provided in agreement 264 of 2013. The authors are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), the Department of Chemical and Petroleum Engineering at the Schulich School of Engineering at the University of Calgary.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17554f

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