Novel Deep Eutectic Solvent-Dissolved Molybdenum Oxide Catalystfor the Upgrading of Heavy Crude OilS. M. Shuwa, R. S. Al-Hajri,* B. Y. Jibril, and Y. M. Al-Waheibi

Petroleum and Chemical Engineering Department, Sultan Qaboos University, P.O. Box 33, PC 123 Muscat, Oman

*S Supporting Information

ABSTRACT: A MoO3/deep eutectic solvent (DES) catalyst precursor solution was prepared by dissolving molybdenumtrioxide in a DES based on choline chloride/urea. The catalyst precursor solution was characterized for its physical and chemicalproperties. The characterization results showed almost no change in the DES properties after the addition of MoO3. The solutionwas used in the catalytic upgrading reaction of heavy crude oil. The performance of the catalyst was analyzed by gaschromatography−mass spectrometry, Fourier transform infrared, and viscosity measurements of the heavy oil before and afterthe reaction. The use of the catalyst in the catalytic aquathermolysis showed an increase in the oil viscosity. In the presence ofhydrogen and catalyst, the results showed a 43% reduction in the heavy oil viscosity, a 2.5° increase in the API gravity, and 32 wt% sulfur reduction.

1. INTRODUCTION

Heavy oil and bitumen require energy-intensive operations fortheir production, upgrading, and transportation to refineries forsubsequent production operations. Therefore, reducing theviscosity of the heavy oil simultaneously with thermal enhancedoil recovery has attracted a lot of attention in the past few years.Moore et al. and Weissman et al. were among the first topropose the concept of in situ catalytic upgrading of heavy oilduring in situ combustion.1−3 Tian et al.4 used water-solubleammonium heptamolybdate and NiNO3 in upgrading residualoil from the Petronas Refinery in Malaysia. The upgradingreactions were conducted in a batch mode under a pressure of 7MPa of H2 and 340 °C using a 300 mL high-pressure stirredreactor. The catalyst increased the hydrogen-to-carbon ratiofrom 1.46 to 1.86 (27.9% increment) with 62.2% viscosityreduction. Molybdenum acetylacetonate and iron alkylhex-anoate based oil-soluble catalysts offered about a three-foldreduction in the viscosity, a 9° increase in the API gravity, and46 wt % sulfur reduction.5 The experiments were conducted inbatch (Parr Instrument Company) stirred reactor of 1800 mLcapacity at a temperature of 400 °C in a hydrogen atmosphereof a final pressure of 10.8 MPa, at a stirring speed of 1000 rpm,catalyst loadings of 10 wt % with respect to oil, and a residencetime of 24 h.The process in which the viscosity of heavy oil is reduced

with the aid of water is called aquathermolysis. Not all of thecatalysts reported having good catalytic activity on the heavyoils. In some cases, the viscosities of reacted heavy oilsregressed rapidly after reaction.6,7 This is the reason whyresearchers applied hydrogen or hydrogen-donor compoundsin heavy oil upgrading. The process that involves hydrogen inaquathermolysis is called hydrothermolysis, which is aterminology used to distinguish it from aquathermolysis.8

Chao et al. used a bifunctional catalyst, alkyl ester sulfonatecopper, which has not only a catalytic center but also ahydrogen precursor structure. The catalyst showed goodactivity in aquathermolysis of heavy oil in both field and

laboratory tests. The laboratory results showed 90.72%reduction in the viscosity using 0.3 wt % catalyst at 240 °Cand 24 h, with 10.12% conversion of heavy content to lightcontent.6 Mohammed and Mamora9 carried out an exper-imental study on the in situ upgrading of a local Venezuelan(Jobo) heavy oil under steam injection at a temperature of 273°C and a pressure of 500 psig using an oil-soluble organo-metallic catalyst at a concentration of 750 ppm and tetralin asthe hydrogen donor. The results showed an increase in oilrecovery by 15% by adding 5 wt % tetralin above that of puresteam injection. When the oil was mixed with tetralin, a catalystsolution, and sand, about 20% higher oil recovery than that ofpure steam injection was observed. In another work,7 the effectof a hydrogen-donor additive on the viscosity of heavy oilduring steam stimulation was investigated. The results showedthat the incorporation of tetralin (0.8%) as a hydrogen donorled to a viscosity reduction of 80% after 24 h of reaction time ata temperature of 240 °C.The shortcomings of water-soluble catalysts are a low surface

area-to-volume ratio, which leads to inefficient contact betweenthe catalyst and feed, and the production of particles of biggersizes, which can lead to formation damage in the reservoir. Oil-soluble catalysts are expensive, which prompted researchers toseek other alternatives. Recently, some researchers haveemployed the services of ionic liquids to upgrade and enhancethe recovery of heavy crude oils. Nares et al. conducted a batchreactor study of upgrading a heavy crude oil from the Gulf ofMexico using ionic liquids elaborated with iron andmolybdenum. The upgrading experiments were conductedusing ionic liquids based on iron (10 wt %) and molybdenum(2 wt %) compounds, in a liquid phase homogeneously mixedwith heavy crude oil in a batch reactor of 500 mL, at 400 °C

Received: December 28, 2014Revised: March 14, 2015Accepted: March 23, 2015Published: March 23, 2015

Article

pubs.acs.org/IECR

© 2015 American Chemical Society 3589 DOI: 10.1021/ie5050082Ind. Eng. Chem. Res. 2015, 54, 3589−3601

under a 10.8 MPa total pressure of hydrogen with residencetimes of 24, 48, and 72 h. The oil was successfully upgradedbecause there was substantial reduction in the viscosity andsulfur content and increase in the API gravity.10 Fan et al.investigated the potential of metal-modified ionic liquids inupgrading Liaohe heavy crude oil, resulting in a good viscosityreduction property and possibly leading to high oil recovery.11

Deep eutectic solvents (DESs) are types of solvents thatbelong to the family of ionic liquids but with a special propertycomposition of two or three cheap and safe components thatare capable of self-association, often through hydrogen-bondinginteractions, to form a eutectic mixture with a melting pointlower than that of each individual component.12 DESs aregenerally liquid at temperatures lower than 100 °C and belongto a category of green solvents that have vast applications incatalysis, organic synthesis, electrochemistry, and materialchemistry.12,13 They are another form of ionic liquids that arecheaper, greener, and easier to prepare compared with typicalionic liquids.Molybdenum is the basic constituent of the most active

hydroprocessing catalysts.14−17 The major source of molybde-num is the mineral molybdenite (crystalline molybdenumsulfide, MoS2), which is roasted to produce molybdenum oxideand purified by dissolution in aqueous ammonia to producemolybdates such as ammonium heptamolybdate and ammo-nium dimolybdate, which are further purified by fractioncrystallization and flash evaporation at 100 °C, respectively, toobtain the water-soluble catalyst precursors. Thus, ammoniumheptamolybdate and ammonium dimolybdate are among themajor sources of water-soluble molybdenum catalysts.Metal oxides can be introduced into the structure of some

DESs like choline chloride/urea through dissolution to formcatalyst precursor compounds. An active dispersed catalystprecursor produced from this material in a simple process couldhave low cost compared to oil-soluble catalyst precursors. Itcould also solve problems associated with water-solubleprecursor compounds such as the generation of large catalystparticle sizes, and the use of excess amounts of water in thefeedstock will be avoided, which is undesired in hydrocracking.The main purpose of this work was to investigate theperformance of new catalysts based on DESs in the upgradingof a heavy crude oil. The catalyst is expected to lead to areduction in the viscosity and sulfur content and an increase inthe API gravity of the original oil. The operation conditions forthe reaction were chosen to model typical reservoir conditions.To our knowledge, the use of DES/molybdenum trioxide in

the aquathermolysis/hydrocracking reactions has not beenreported. It is the objective of this work to investigate itspotential in upgrading Omani heavy oils.

2. EXPERIMENTAL PROCEDURES2.1. Catalyst Synthesis and Characterization. A sample

of DES was synthesized using choline chloride (ChCl) and urea(molar ratio of 1:2). The synthesis was conducted in anincubator shaker at 80 °C. Appropriate quantities of the saltswere weighed, thoroughly mixed, put into the shaker, andallowed to completely melt to a homogeneous colorless liquid.Details of the synthesis have been reported elsewhere.18 Inorder to form a solution of the DES and molybdenum trioxide(MoO3), a known quantity of the oxide (to give 1000 ppm ofmolybdenum with respect to the crude oil) was measured, putin a sample vile containing the DES (10 wt % with respect tothe oil), and placed in the thermoshaker operated at 80 °C for

24 h. The pure DES, DES + H2O mixture (the mass ratio ofDES to water is 2:1), DES/MoO3 solution, and DES/MoO3 +H2O (mass ratio of DES/MoO3 to water is 2:1) aqueousmixtures were formed and characterized at different temper-atures for density, viscosity, conductivity, and surface tension.The density and API gravity measurements were performedusing an Anton Paar vibrating tube densimeter (DMA 4500).The equipment measures periods of harmonic oscillation of abuilt-in U-tube made of borosilicate glass containing thesample. The temperature of the measuring cell is controlled byan integrated Peltier thermostat. Before each measurement, thecell was rinsed with deionized water and slowly evacuated toavoid any trapped air in the system. Then the inlet valve wasopened and the sample introduced into the cell; at least 10 minwas allowed for the temperature to reach stability, and thenmeasurements were taken. The API gravity at 15 °C wasmeasured using the API method developed in the instrument.The instrument computed the API gravity automatically fromthe specific gravity values at 15 °C. The viscosity measurementswere carried out with an Anton Paar rheometer (Rheolab QC)at a constant shear rate of 300 s−1. The instrument had a built-in temperature sensor, and an external water bath was used fortemperature control. For every measurement, about 14 mL ofthe sample was put into the sample cell with its spindles andconnected to the instrument. About 15 min was allowed fortemperature equilibration before measurements were taken.The electrical conductivity was measured using a Metler

Toledo conductivity meter, which operates with an alternatingcurrent of 60 Hz frequency. The conductivity meter wascalibrated using a KCl reference solution. The instruments hada built-in temperature sensor, and variation of the temperaturewas achieved with a water bath. For each measurement, 5 mL ofsample was used, the conductivity sensor was immersed in theglass vials containing the samples, and the conductivity valueswere displayed on the instrument’s digital screen. After everymeasurement, the conductivity cell was washed with deionizedwater and acetone to remove any adhering sample and driedbefore using it in the next measurement.The surface tension measurements were carried out using a

Kruss digital tensiometer (K10ST) by the Du Nouy ringmethod.19 An external water bath was connected to thetensiometer for temperature control. The platinum−iridiumring was cleaned by flaming, and the glassware was rinsedconsecutively with acetone and distilled water before eachmeasurement. The equipment calibration was determined bymeasuring the surface tension of pure water. After eachmeasurement, the glassware was cleaned thoroughly with waterand acetone before the next measurement.Prior to these measurements, a solubility test of the metal

oxide in the DES was conducted. Amounts of 0.08−0.2 g ofMoO3 at 0.02 g intervals were dissolved in 10 mL of DES andshaken in the thermoshaker for 24 h at 80 °C. The mixtureswere then observed for the formation of clear homogeneoussolution of the oxide and DES. A clear homogeneous solution isan indication of complete dissolution of the metal oxide in theDES.Thermal and structural studies were undertaken using

Fourier transform infrared (FTIR) and thermogravimetricanalysis (TGA) to see if there was any change caused to theDES after the incorporation of MoO3 into the DES structure.TGA of the DES and DES/MoO3 was conducted on asimultaneous thermal analyzer (PerkinElmer, STA 6000). Thesamples (10−20 mg) were heated from 30 to 500 °C at a

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heating rate of 10 °C/min under a nitrogen atmosphere (30mL/min flow rate) at a pressure of 3 bar.2.2. Heavy Oil Upgrading Experiments. To investigate

the activity of the proposed MoO3/DES system, a heavy oilsample from an Omani field that has a viscosity of 13800 cPwas utilized.Upgrading experiments of the heavy crude oil were carried

out using a 500 mL capacity batch-type laboratory reactor(4575 series, Parr Instrument Company). The reactor vesselwas equipped with a magnetic stirrer for mixing samples in thevessel, a dip tube for liquid and gas sample withdrawal from thevessel, and a thermowell. The thermocouple was positionedinside the reactor through the thermowell to monitor thetemperature of the samples inside the vessel. The temperatureand stirring speed are controlled automatically by a digitalcontroller (4848 series, Parr Instrument Company), while thepressure was monitored with a pressure gauge mounted on thereactor head connected to the reactor vessel. The followingconstant reaction conditions were utilized: 300 °C reactiontemperature, 11 bar initial hydrogen pressure, 24 h reactiontime, and 750 rpm stirring speed. The hydrocrackingexperiments were conducted under a hydrogen atmosphereby heating 100 g of a heavy oil sample in the reactor vessel withand without catalysts, while the aquathermolysis runs wereconducted under a nitrogen atmosphere with a heavy oilsample containing 30 wt % water (oil-to-water mass ratio of70:30), i.e., 100 g of oil to 43 g of water.The catalyst loading used was 1000 ppm (0.1 wt %

molybdenum) and 10 wt % DES with respect to the heavyoil. Table 1 gives the breakdown of all of the runs and

conditions employed for the reactions. Naphthalene was usedas the model oil along with water under a nitrogen atmosphereto check for any thermal activity. Less than 4% naphthaleneconversion was noticed after 24 h and 300 °C operatingtemperature. The oil plus the catalytic precursor was thenmounted on the reactor assembly and the reactor purgedseveral times with nitrogen to remove air from the vessel.Heating was then started from room temperature to the desiredreaction temperature. The reaction began when the desiredtemperature was reached and stirring was initiated. Thetemperature and pressure were monitored and recordedcontinuously during the reaction.After the reaction was completed, the reaction mixture was

cooled to around 70 °C. For tests containing water, the waterwas decanted first and the liquid products from the runs wererecovered and analyzed for the density and API gravity using anAnton Paar vibrating tube density meter and the viscosity usingan Anton Paar rheometer. The elemental sulfur content of theruns was evaluated with an energy-dispersive X-ray fluorimeter(NEX QC+ Rigaku) using 32 mm single sample cup with a thin

film of prolene. The FTIR spectrum was obtained by placing adrop of the samples between two sodium chloride cells andthen spreading on the surface of the cells. The cells were thenmounted on the equipment and the settings were as follows; 14scans, at a resolution of 4 cm−1, and wavenumbers from 400 to4000 cm−1. The GC−MS spectrum was obtained using a Clarus600 chromatograph (PerkinElmer) with a RTX-5MS column(30 m × 250 μm × 0.25 μm). For nonvolatile polarcomponents of oil (asphaltenes and resins) removal, solventprecipitation and column separation were first conducted. Thisis to prevent the components from damaging the column.The solid precipitate recovered from run S3 was further

characterized using X-ray diffraction (XRD) and scanningelectron microscopy−energy-dispersive spectroscopy (SEM−EDS) techniques. The diffractogram was obtained with aPanAnalytical (XpertPro) XRD machine. The SEM image andelemental analysis of the samples were obtained with a JEOLelectron microscope with built-in EDS technology forelemental determination.

3. RESULTS AND DISCUSSION3.1. Characterization of Catalyst Precursor Solutions.

Physical properties like density, viscosity, refractive index, andsurface tension are important characteristics of DESs. To date,few works are available in the open literature on this area ofinterest.13,18,20−25 It is imperative to know the characteristics ofthe DES and its aqueous mixtures in order for them to beapplied in industrial and chemical processes. Because the DESwas mixed with molybdenum oxide and the fact that water isone of the major components found in oil reservoirs, the DES(pure) and DESs dissolved in MoO3 and diluted with waterwere formulated and characterized. These characteristics couldprovide information on the purity of the precursor solutionsand molecular interaction in the liquid.23 Table 2 gives the

results of these characteristics with experimental errors atdifferent temperatures. The result of the solubility test showedMoO3 dissolved in the DES, with 0.16 g of MoO3 soluble in 10mL of DES; this is equivalent to 16000 ppm. A solution of18000 ppm (0.18 g of MoO3 in 10 mL of DES) did not form aclear homogeneous phase, which is an indication of the limitedsolubility of the oxide in the DES at that condition. The 16000ppm solution formed a clear homogeneous mixture, whichindicates complete dissolution of the oxide in the DES.Figure 1 is a plot of the density as a function of the

temperature for all of the samples. From the graph, a decreasein the density with temperature can be seen, as expected with alinear relationship. However, the incorporation of molybdenumoxide into the DES structure did not increase its density, butrather a slight decrease was observed. The values are in

Table 1. Experimental Runs and Conditions Employed

sample H2 N2 H2Ocatalyst (DES/

Mo) comments

S1 control sample: fresh heavyoil

S2 √S3 √ √S4 √ √S5 √ √ √S6 √ √S7 √ √ √

Table 2. Retention Time of Standard ParaffinicHydrocarbons (C8−C33)

C8 2.658 C17 13.283 C26 21.201C9 3.049 C18 14.328 C27 21.912C10 4.249 C19 15.324 C28 22.607C11 5.65 C20 16.274 C29 23.287C12 7.09 C21 17.185 C30 24.052C13 8.471 C22 18.055 C31 24.938C14 9.781 C23 18.89 C32 25.998C15 10.942 C24 19.686 C33 27.199C16 12.177 C25 20.456

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agreement with the values reported in the literature.22,23 Thereduction in the density due to the addition of water to pureDES and DES/MoO3 at all temperatures was found to beproportional.Viscosity is an important property of DES and is a strong

temperature-dependent variable. Figure 2 displays the vis-

cosities of the various solutions as a function of thetemperature. The incorporation of MoO3 in the structure ofthe DES led to an increase in the viscosity. This is due to anincrease in the strength of hydrogen bonding betweencomponents in the structure because the viscosity is knownto be associated with hydrogen-bonding interactions. A 2:1eutectic mixture of urea with ChCl has ionic components ofcholine cations and chloride anions while having hydrogenbonding of the urea molecules. MoO3 binds to the ureamolecules or to the chloride anion. There is also the possibilityof forming an anion/metal/urea complex. The presence of astrong coordinating anion is needed for the complexation ofmetal oxide to form soluble species with a ChCl/ureamixture.18 The addition of water led to a reduction in theviscosity as expected because the viscosity is also a function ofthe composition. This may be associated with distortion ofextensive hydrogen bonding between the components in the

structure, which led to an increase in the mobility of freespecies within the DES.Figure 3 shows variation of the surface tension as a function

of the temperature. Almost similar behavior and trend were

observed with viscosity. The surface tension also depends onthe strength of intermolecular forces between the species in thestructure and, as a result, strongly correlates with the viscosity.The addition of water to the DES led to a drastic reduction inthe surface tension. Conductivity is also an important propertyof DESs. Because ChCl/urea consists of ionic species that aredissociated in the liquid and are free to move independently,26

it is expected to have some degree of conductivity. Figure 4

displays this behavior. When water was added to the DES andDES/MoO3 solutions, the conductivity increased as observed.This can be attributed to the increase in the mobility of theionic species in the system, especially the chloride anion (beingthe principle migrating species), which is more facile than thecholine cation or the urea anion. It can also be observed fromFigure 4 that the introduction of MoO3 into the DES led to aslight decrease in the conductivity. It has been reported that

Figure 1. Densities of catalyst carriers (DES) and catalyst precursorswith and without water as a function of the temperature.

Figure 2. Viscosities of catalyst carriers (DES) and catalyst precursorswith and without water as a function of the temperature.

Figure 3. Surface tensions of catalyst carriers (DES) and catalystprecursors with and without water as a function of the temperature.

Figure 4. Conductivities of catalyst carriers (DES) and catalystprecursors with and without water as a function of the temperature.

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DESs with high viscosity have low conductivity.12 Because theviscosity correlates inversely with the conductivity, the increasein the viscosity observed after adsorption of MoO3 into theDES structure led to a reduction in the conductivity.TGA for both pure DES and DES/MoO3 is shown in Figure

5. It is evident from the thermograms that the DES and DES/

MoO3 are thermally stable up to around 200 °C. This agreeswith the observation reported in the literature.27,28 Themaximum decomposition temperature was found to be 295°C for the two samples analyzed. The presence of MoO3 in theDES structure did not change the thermal stability of thesolvent or the maximum decomposition temperature, with onlya small difference in the amount of residue left afterdecompositions of the compounds at 295 °C.The IR spectrum was acquired to see if there was any change

in the structure of the DES after the introduction ofmolybdenum oxide. Figure 6 shows the IR spectrum of thepure DES and DES/MoO3. Upon comparion of the two spectrain the figure, it can be seen that two new transmission bandsappeared at 3825 and 3771 cm−1 in the spectrum of DES/MoO3. This was not observed in the spectrum of pure DES,indicating the interaction of pure DES with dissolved

molybdenum oxide. This peak could be assigned to thehydrogen bond N−H of urea in the DES, with thechlorometalate complex species anion expected to have beenformed because of the presence of the metal oxide.29,30

It is assumed that there were hydrogen-bonding interactionsbetween MoO3 and the DES, as shown in the FTIR spectra.The transmission bands at 3771 and 3825 cm−1 are attributedto hydrogen bonding between urea and the complex anionexpected to have been formed due to dissolution of the metaloxide in the DES. Although there is no work in the openliterature carried out on dissolving MoO3 in ChCl/urea DES,Abbot et al.18 dissolved V2O5 and CrO3 in ChCl/urea DES.These two metals/metal oxides exhibit some commonsimilarities with MoO3. Molybdenum and chromium are inthe same group in the periodic table with many similar chemicalproperties. They have high oxidation states (6+) in the twooxides and, as a result, possess good catalytic activity. V2O5 alsoexists in high oxidation state (5+) and possesses strong catalyticactivities like MoO3 and CrO3. All three metal oxides areoxidizing agents. Considering the above-mentioned similaritiescommon to the three metal oxides, MoO3 dissolved in ChCl/urea DES is expected to exhibit behavior similar to that of CrO3and V2O5 in ChCl/urea DES. The FTIR spectra confirmed thisassertion by not showing any new bands apart from thoseattributed to hydrogen-bonding interactions between the ureaand the complex anion species. It can be speculated thatdissolution of MoO3 in the DES led to the formation of acomplex anion species such as [MoO2Cl3]

− similar to[CrO2Cl3]

− and [VO2Cl2]− obtained when CrO3 and V2O5

were dissolved in ChCl/urea DES, respectively.18 Theoxidation state of molybdenum in the complex anion is 6+,which means no reduction after dissolution. There is apossibility that the oxide could be reduced in the dissolutionprocess; however, this was not observed because the solution ofthe oxide in the DES remained colorless even after dissolution,which is a characteristic of the metal in its “6+” oxidation state.A similar characteristic was observed when CrO3 and V2O5were dissolved in ChCl/urea DES18 and V2O5 in ionic liquid31

in spite of the fact that the two metal oxides are strongeroxidizing agents than MoO3. Many metal oxides do notundergo reduction after dissolution in ChCl/urea DES.32 Thispoints out that the major outcomes of dissolving metal oxide in

Figure 5. Thermogram of the samples.

Figure 6. FTIR spectrum of DES before and after the incorporation of MoO3.

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ionic liquids and DESs are the formation of a complex anion insitu, and the mechanism of interaction between the solutes andsolvents is hydrogen bonding.18,29,31,33,34 On the basis of theforegoing discussions, it is expected that the oxide will bereduced in situ by reacting with the sulfur compounds in the oilto form molybdenum disulfide (MoS2), which is the active formof the catalysts. The presence of the complex anion containingmolybdenum in the 6+ oxidation state and hydrogen will pavethe way for reduction of molybdenum from the 6+ to 4+oxidation state as in MoS2.Also, the absorption bands at 2161 and 2331 cm−1, which

appeared in the spectrum of pure DES, shifted to broader andhigher frequency bands at 2190 and 2336 cm−1, respectively.

This may be attributed to the formation of more hydrogenbonds between urea and ChCl because of the presence ofMoO3.

35,36 Because the viscosities of ionic liquids and ionicliquid analogues are governed essentially by hydrogenbonding,37 it is reasonable to assume that an increase in thehydrogen-bonding interaction and its strength is the reasonbehind the increase in the viscosity when MoO3 was dissolvedin the ChCl/urea DES.

3.2. Compositional Changes. The GC−MS study wasundertaken to evaluate changes in the hydrocarbon compo-nents in the oil samples before and after reactions under thestudied conditions. The results are presented in Figures 7−9.The amounts of saturated hydrocarbons C8−C33 (retention

Figure 7. GC−MS spectra of samples S2 and S3 (hydrocracking without water) in comparison with the fresh sample S1.

Figure 8. GC−MS spectra of samples S4 and S5 (hydrocracking with water) in comparison with the fresh sample S1.

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time shown in Table 2) increased for all of the samplescompared with the fresh oil sample (S1).A remarkable behavior is displayed by S2 and S3 by showing

high amounts of saturates compared to other samples. S3contains the highest amounts of saturates, followed by S2,which has the amounts of hydrocarbons nearly equal to thoseof S3 (Figure 7). See Table 3 for the total yields of thesaturates.

The presence of a high partial pressure of hydrogen in S2 andS3 aided the conversion of unsaturates present in the freshsample to saturated hydrocarbons (hydrogen addition) withmore of C−C cleavage (carbon rejection or cracking) takingplace. The small increase in saturated hydrocarbons observed inthe other samples (S4−S7) must be due to the cracking ofhigh-molecular-weight hydrocarbons in the fresh sample toform saturates and light hydrocarbon gases. S3 showed higheramounts of saturates compared to other samples due to theconversion of bigger molecules to small hydrocarbons and evento gases that were not monitored in the course of the reaction.This is due to the presence of the catalysts in the reactionmedium. Although S4−S7 displayed higher quantities ofsaturates than S1, hydrocarbons of higher carbon numberwere observed, as shown in Figure 9. In addition to the GCchromatograms, Figure S1 in the Supporting Information (SI),depicting columns showing peak areas of all of the reacted oilsamples from the runs, was plotted. A comparison between theruns (S1−S7) in terms of the peak areas from the raw total ionchromatogram (TIC) can be easily made from the figure.

This must have been as a result of polymerization reaction ofthe molecules in the fresh oil. The formation of high-molecular-weight hydrocarbons can be explained through free-radicalchain reactions caused by the absence or presence of low partialpressure of hydrogen in the reaction. Thermal cracking ofasphaltenes and resins can lead to the formation of reactivespecies that can react with each other through polymerizationand condensation reactions to form bigger molecules.38 This isthe reason why there is a big difference in the saturate contentsbetween S4 and S2 in which water was the only differencebetween the two starting reactive mixtures. Hydrogen presentin S2 was able to stabilize the reactive free radicals generated atthat condition, whereas the high partial pressures of water in S4inhibit the free-radical stabilization. Because polymerization andcondensation reactions are proceeded by free-radical mecha-nisms, this can lead to the production of components heavierthan the feed, and as a result, low saturates will be formedcompared with S2.In summary, S1 and S2 showed the significant presence of

saturates because of the presence of hydrogen and catalyst (inS3), which is the result of hydrogenation and crackingreactions. S3 and S5 showed lower amounts of higher saturatedhydrocarbons (C23−C33) because of their conversion to low-molecular-weight hydrocarbons as a result of the presence ofcatalyst and hydrogen. S4 displayed amounts of hydrocarbonsC28−C33 higher than those of the fresh oil because ofcondensation and polymerization reactions taking place inwhich the presence of water aided the reaction. For S6 and S7,even though there were low amounts of C28−C33, hydro-carbons of higher carbon number were detected from theproduct of the reaction. This can be attributed to the presenceof water, as in S4, and the absence of hydrogen, which is knownto inhibit polymerization reactions.The FTIR spectra of the oil samples before and after

upgrading reactions were acquired to compliment GC−MSanalysis. The results of FTIR analysis are represented in Figures10 and 11. The transmission band at 3278 cm−1, which isassigned to alkynes and aromatic C−H stretching vibra-

Figure 9. GC−MS spectra of samples S6 and S7 (aquathermolysis) in comparison with the fresh sample S1.

Table 3. Total Yield of Saturated Hydrocarbons (C8−C33)for All of the Samples

sample yield (wt %) sample yield (wt %)

S1 2.9 S5 4.2S2 8.6 S6 4.8S3 9.7 S7 5.4S4 3.5

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tions39,40 in unsaturated hydrocarbons, lost intensity andbecame weaker in S2, S3, and S5. This is an indication of theconversion of unsaturates to saturated hydrocarbons in thepresence of hydrogen. The bands at 2927 cm−1 assigned to theC−H stretching vibration6,39−41 found in saturated hydro-carbons obviously gained intensity and became stronger in S2and S3 and lost intensity to become weaker in S4 and S7. Also,the absorption bands at 1460 and 1376 cm−1 attributed to C−H bending vibrations39,41 found in alkanes became stronger inS2 and S3 and weaker in S4 and S7. This implies that thesaturate content in S2 and S3 increased as a result of thehydrogenation of unsaturates and pyrolysis of long-chainalkanes, while the weakening of the bands in S4 and S7illustrates a reduction in the content of saturates due tocracking to light hydrocarbon gases and/or condensationreactions to form bigger hydrocarbons. The stretching vibration

bands of a conjugate polyene CC (1600 cm−1), whichsignifies the presence of an aromatic ring, became weaker in S2and S3.6,39−41 This may be due to hydrogenation, open-cycle,and breaking reactions42

3.3. Change in the Physical Properties of Oil Samplesafter Upgrading Reactions. The density, viscosity, and APIgravity measurements for the samples before and after reactionare shown in Table 4. The change in the viscosity and APIgravity was calculated according to the following equation:

Δ =−

×MM M

M100%0

0

where ΔM is the change in the property [viscosity (mPa s, 30°C) or API gravity (at 15 °C)], M0 is the initial property(viscosity or API gravity), and M is the property of the sampleafter reaction.

Figure 10. FTIR spectra of oil samples S2−S4 (hydrogen process) with fresh S1.

Figure 11. FTIR spectra of oil samples S5−S7 (aquathermolysis) with fresh S1.

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In general, the results agreed well with those obtained usingGC−MS and FTIR. The presence of hydrogen during S2 andS3 resulted in a higher C8−C33 content compared to thatpresented in the fresh sample and, hence, a lower viscosity. Theaddition of the catalyst enhanced hydrogenation and hydro-cracking reaction and therefore resulted in an even lowerviscosity, 43%. The addition of water to samples S4−S7resulted in an increase in the viscosity, which corresponds wellwith the relatively higher C28−C33 content and other heavierproducts formed because of free-radical addition reactions inthose samples. The liquid viscosity is reported to correlate wellwith the molecular mass or apparent molecular mass due toaggregation.43 Upon a comparison of S2, S4, and S5, it is clearthat the addition of water caused a reduction in thehydrogenation/hydrocracking reactions with acceleration inthe reactions that led to the formation of much heaviermolecules. The addition of the catalyst in S5 enhanced thehydrogenation reaction but with an end result of almost nooverall change in the oil viscosity. A similar trend can beobserved by comparing runs S6 and S7.3.4. Sulfur Reduction. The elemental sulfur results of the

samples are represented in Table 5. The results showed a trend

similar to that of viscosity reduction. A 32% reduction wasachieved with S3, followed by S2 with 15% desulfurization. Thehydrogen addition process without water displayed the highestdesulfurization compared with the aquathermolysis processwithout catalyst, 8% for S6, and with catalyst, 4% for S7. Uponcomparison of the hydrogen addition runs S2−S4, it is evidentthat the presence of water inhibits the desulfurization process.Water is known to inhibit desulfurization reactions duringupgrading processes.44 The difference in the percent desulfur-ization of the two runs S2 (15%) and S4 (2%) shows a clearinhibition of the process by water. When a catalyst was used inthe presence of water in run S5, the desulfurization wasimproved from 2% (S4) to 6% (S5). The aquathermolysis runsS6 and S7 displayed low desulfurization with 4.2% for S6 and8% for S7, indicating an improvement of about 4% when acatalyst was added to run S6, although there is no meaningfuldifferences for the aquathermolysis runs within experimental

error. This can be explained with the fact that theaquathermolysis process involves breaking of the weak C−Sbond, which results in sulfur reduction.6,8,41,45,46 Although theprocess can lead to desulfurization, it is not effective like thehydrocracking process. Viscosity reduction normally observedin the process was not observed in this case.It is clear from the runs that the presence of water in both

hydrocracking and aquathermolysis led to the formation ofcomponents in reacted oil samples heavier than the originalsample, as demonstrated in S4−S7. In addition to the productsof condensation/polymerization reactions, it can be presumedthat water hydrolyzes the sulfur and nitrogen compounds in theoil such as sulfoxides, aliphatic sulfides, and theophenicaromatic sulfides. The products of this hydrolysis reactionthat still retain the sulfur in their molecules are normallysulfones, sulfoxides, and oximes. This may be the reason why asmall reduction in sulfur was observed in runs S4−S7 comparedto S2 and S3; because the elemental sulfur analyzer (XRF) useddetermines the sulfur content in the reacted liquid oil samples,any sulfur compound other than H2S will remain in thesamples. The presence of these groups of compounds in thereacted oil samples is expected to result in a highly structured,higher viscosity oil.43

Water is also capable of hydrolyzing the aromatics in the oilto phenols. The appearance of a weakly intense band at 1600cm−1 assigned to the stretching vibration band of conjugatepolyene CC, which indicates the presence of an aromaticring (higher intensity in S4−S6), supports that argument. Thiscan be speculated based on the FTIR spectra of the oil samplesS4−S6 compared to the fresh oil sample S1. Because thesecompounds formed from hydrolysis of the sulfur- and nitrogen-containing species in the oil are more polar than the startingmaterials (aliphatic sulfides and naphthenic aromatic sulfidessuch as dibenzotheophene), they cannot be detected by GC−MS analysis. The reason is because all polar compounds wereremoved by passing the dichloromethane-dissolved oil samplesthrough silica gel, which, in turn, separates them from othernonpolar components of the oil samples. It can be observed inFigure S1 in the SI that the total saturate contents (C8−C33)of samples S2 and S3 is higher than that of S4−S7, asdemonstrated by their peak areas. Hydrocracking is known forproducing liquid distillates and as such not many heavymolecules and gaseous products. The aquathermolysis runsproduced probably heavier components and other productsthat can be linked to the hydrolysis reactions because of thepresence of water. As a result of all of these, the liquid distillateproducts (represented by C8−C33 in this case) of thehydrocracking runs is expected to be higher than theaquathermolysis runs, as demonstrated by the peak areas inthe GC−MC results (Figure S2 in the SI).For the polymerization/condensation reactions, the presence

of water under the condition studied can be presumed to favorfree-radical addition reactions to free-radical cracking reactions.Because polymerization reactions proceed by free-radicalmechanisms as in ethylene to polyethylene plastics, this maylead to the formation of heavier molecules in the reacted oilsamples. It is the free-radical cracking reactions that normallylead to desulfurization and a reduction in the viscosity in theaquathermolysis process; although sulfur reduction cannot berule out in this case (because a small reduction in sulfur and anincrease in the saturate contents observed support that), free-radical addition reactions are more pronounced. In short, the

Table 4. Physical Properties of the Oil Samples from theRuns and the Fresh Crude Oil

sampleviscosity,

cPchange in theviscosity, %

change in the APIgravity

density,g/cm3

S1 13800 NA 12.75 0.9800S2 10683 −22.59 +1.7 0.9769S3 7889 −42.83 +2.4 0.9761S4 17098 +23.94 −0.85 0.9837S5 14492 +5.02 +0.40 0.9783S6 17062 +23.64 −0.47 0.9837S7 14973 +8.5 −1.09 0.9827

Table 5. Results of Sulfur Analysis

sample sulfur amount, wt % percent desulfurization, %

S1 4.49 NAS2 3.81 15.14S3 3.05 32.07S4 4.39 2.23S5 4.20 6.46S6 4.13 4.23S7 4.30 8.02

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loss in desulfurization is more likely due to the formation ofheavier products by radical addition reactions.In summary, the runs with hydrogen showed higher

desulfurization than the aquathermolysis process. The hydro-gen addition processes (like hydrodesulfurization) are the

known conventional desulfurization processes used in therefineries for removal of sulfur in crude oil fractions. Sulfur canbe removed from thiophenes and mercaptans present in crudeoils in the form of hydrogen sulfide, especially when catalystsare used. This is unlikely to take place in aquathermolysis

Figure 12. XRD patterns of the solid precipitate recovered from the reaction of S3.

Figure 13. High-resolution SEM image of the solid precipitate recovered from reaction S3 at different magnifications: (a) 350× (10 μm); (b) 1000×(10 μm); (c) 6000× (1 μm); (d) 37000× (100 nm).

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because of the lack of hydrogen. Although hydrogen can beproduced in situ in aquathermolysis via a water−gas shiftreaction, it is not present in large amounts to lead tohydrodesulfurization.3.5. Solid Precipitate Characterization. SEM−EDS and

XRD were carried out on the solid precipitate recovered fromreaction sample S3. The yield of the solid product recoveredwas 7.6 wt %. SEM is useful in exploring the morphology andsize of the catalytic particles, while EDS, being a semi-quantitative technique, can be used for chemical analysis. XRDcan be used for phase identification and crystal structureelucidation.Figure 12 shows the X-ray diffractogram of the solid particles.

Despite the noise observed in the spectrum, broader peaks canbe clearly seen, which indicates small crystal sizes. Althoughbroader peaks mean small crystal sizes, the XRD patternindicates the presence of stacking layers of slabs of MoS2crystallites, which may indicate a high interplanar distance (dspacing). The peaks at 2θ = 33.02°, 40.6097°, 58.5848°, and58.7203° corresponding to the intensity counts of 149.66,24.49, 143.26, and 108.43, respectively, are attributed to thepresence of MoS2.

47,48 The peak at 2θ = 23° indicates thepresence of MoO2, which might have been formed from thereduction of MoO3.

49 MoS2 is known to be an active form of amolybdenum-based hydrotreating catalyst. Its presence in theproduced solid samples after the upgrading process is anindication of its generation in situ. Many researchers havereported that MoS2 can be transformed in situ from relevantmolybdenum precursors under conditions similar to those usedin this study, in sulfur-containing crude oils.16,48,50−52 Thesulfide was generated after decomposition of DES/MoO3 togive dispersed and reactive MoO3, which was subsequentlysulfided by the sulfur in the oil. The precise mechanism of thistransformation is still unclear, but direct reduction of the salt byhydrogen sulfide seems probable.14,17,51,52 The presence of fourdifferent reflections attributable to MoS2 versus one for MoO2with no characteristic peak of MoO3 is an indication of thegood sulfiding behavior of the precursor. The identification ofone reflection is not enough to confirm the presence of MoO2because the reduction of MoO3 to MoO2 requires severeoperating conditions and high partial pressures of hydrogen.Figures 13 and S2 and S3 in the SI show the SEM images

and results of chemical analysis from EDS. Figure 13a showsthe SEM image of the particles (350× magnification), whileFigure S3b in the SI was acquired at higher magnification(1000×). When a smaller image (1 μm) was obtained at 6000×magnification, a spherical type particle was observed. Figure13d shows the image (100 nm) at a magnification of 37000×,in which an estimation of the size of the particle was made. 1.37μm was found to be the average size of the particles of theagglomerate formed after the reaction. The EDS results, images,and chemical analysis are shown in Figures S2 and S3 in the SI.The SEM image in Figure S2 in the SI shows five differentspectra (1−5) in which spectra 1 and 5 were chosen forchemical analysis using the EDS software. The reason is due tothe presence of a high concentration of metals in those zones.The chemical analysis and image of spectrum 1 can be seen inFigure S3 in the SI. The tables in the two figures represent theresults of chemical analysis. Carbon, chlorine, and oxygendisplay high signals observed from the spectra. Oxygen andnitrogen are originally present in the heavy crude oil in theform of heteroatoms in constituents like asphaltenes. Oxygencan also be attributed to the presence of oxides, especially metal

oxides, because many metals were also identified from the EDSresults. Some of the oxides like silica are inherent to thereservoir. Another element with a high signal is nitrogen, whichis also present in the original crude oil because heavycomponents contain nitrogen. This indicates that the upgradingreactions conducted on the oil samples led to heteroatomremoval (oxygen and nitrogen). The DES (ChCl/urea) used isalso another source of nitrogen because of the urea present inthe reaction medium. Decomposition of the DES during thereaction might lead to the formation of nitrogen-containingcompounds. The high signal of chlorine observed in the solidprecipitate from chemical analysis can also be traced to ChCl ofthe DES. Some dissolved salts in crude oils contain chlorine inthe form of metal salts of chlorine. Iron and nickel can beexplained as originally present in the crude oil. Their presencein recovered solid indicates a reduction of the heavy metalcontent after the upgrading reactions. The molybdenum andsulfur signals were also observed as expected. The overlappingof the signals for the two observed in both zones (spectra 1 and5) gives an indication of the interaction of the two as in MoS2.Almost the same amounts of molybdenum were found in thetwo regions (spectra 1 and 5), which is an indication of theuniform distribution of the catalyst in the system.

4. CONCLUSIONCatalytic upgrading of an Omani heavy crude oil was conductedunder different conditions simulating hydrocracking andaquathermolysis processes. Some runs of the upgradingreactions were carried out in the presence of catalyst formulatedby dissolving MoO3 in DES based on ChCl/urea. Results fromthe catalyst characterization study show that the incorporationof MoO3 in the DES did not significantly change the structureof the DES, but slight changes in some physicochemicalproperties were observed. The best of all of the runs is thehydrogenation run with catalyst in which water was not added.It gave the best performance in terms of sulfur, viscosity, andheavy metal reduction. XRD and SEM−EDS analysis of thesolid recovered from this run showed the presence of MoS2, anactive form of the catalyst, suggestive of its in situ generation.The presence of water in the reaction medium led to anincrease rather than a decrease in the viscosity. This negativeeffect of water in the reaction medium involving this heavy oil,in particular, made the aquathermolysis process (in thepresence of water) an undesired route in upgrading thiscrude oil under the condition studied. Overall, the catalyst iseffective in upgrading the crude oil via the hydrogenation route;also the hydrocracking runs performed better than theaquathermolysis runs.

■ ASSOCIATED CONTENT*S Supporting InformationResults of physical property measurements and experimentalerrors, bar chart comparison of the GC−MS results betweenthe seven reacted oil samples and standards of saturatedhydrocarbons, and SEM−EDS results of recovered solids. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +968 2414 2557. Fax: +968 2414 1354. E-mail:rashidh@squ.edu.om.

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

The authors thank The Research Council of Oman forproviding financial support to this work through ResearchGrant RC/ENG/PCED/11/01. The authors are also grateful toSultan Qaboos University Oman for allowing use of theirfacilities and other support offered.

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