Green synthesis of iron oxide nanorods from deciduous Omani mango tree leaves for heavy oil viscosity treatment

1 Introduction

Heavy oil represents an enormous part of the worlds’ unproduced hydrocarbon reserves. Heavy oil is characterized by low API gravity, between 10° and 20°, and high viscosity values <10,000 cP (Alboudwarej and Schlumberger, 2006). This is because of the presence of asphaltenes, which are long chain hydrocarbons with high molecular weight that tends to aggregate. Oil viscosity has been shown to proportionally increase with rich asphaltene content (Ovalles et al., 2011). At ambient temperatures, crude oil and bitumen are resistant to flow through reservoir rock because of their high viscosities. Consequently, the energy expended to produce and upgrade a barrel of oil can be as high as 40% of the total energy available from the crude oil resource. The physicochemical properties of crude oil, bitumen or oil sands influence the choice of extraction technology. To this end, thermal enhanced oil recovery techniques such as steam assisted recovery and also in situ combustion are employed. In situ combustion methods involve combustion of a small portion of the oil in place to mobilize the rest through the heat released from the combustion process, which reduces the viscosity of the remaining oil and allows it to flow to a producer well. All these recovery techniques are expensive, complicated and need periodic maintenance; therefore, direct and cheap solutions are the main targets for high in situ upgrading. The radiation energy from different sources varies widely. The energy available from radiation is in orders of magnitude higher than the activation energy needed for breaking a carbon–carbon, carbon–hydrogen, carbon–sulfur or carbon–nitrogen bonds. Microwave heating has an incontestable place in analytical and organic laboratory practices as a very effective and non-polluting method of activation and heating. Microwaves were utilized in the petrochemical industry (Mutyala et al., 2010; Kustov and Sinev, 2010) and the early studies by Wan and Kriz (1985) used microwave absorbing additives to describe the microwave-assisted catalytic hydrogenation of alkenes. Fanson et al. (2006) described the microwave assisted cracking of hydrocarbons in the presence of a precious metal impregnated, metal oxide catalyst, with high conversion efficiency in the presence of carbon at atmospheric pressure. Britten et al. (2005) observed that this radiation causes induced reactions within the compounds, which results in chain scission and recombination. In essence, there will be sites of instantaneous localized superheating where reactions will take place much faster than in the bulk. This explains the rate acceleration seen when performing reactions under microwave irradiation (Nicholas and Rashid, 2008). Ultra-dispersed in situ catalytic upgrading has been reported to outperform the augmented catalytic upgrading achieved by incorporating pelleted refinery catalyst to the horizontal production well (Hart et al., 2015; Hashemi et al., 2014; Krishnamoorti, 2006). For instance, iron oxide nanoparticles (IONPs) with high specific surface area and small diffusion path length improve interaction with macromolecules and cracking reaction. Also, the nanoparticles (NPs) experience lower inter-particle distances that increase the probability of active phase interaction with the hydrocarbon molecules. IONPs are chemical compounds composed of iron and oxygen which are naturally occurring either in the form of the soil or by deposition in rocks and mountains. The most common pure iron oxide is maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) particles. Iron oxide (IO) nanoparticles are considered as one of the most multipurpose and safe nanoparticles used in medical applications because of their magnetic properties and non-toxicity (WiseGEEK; Ling and Hyeon, 2012). Both IONPs and zero-valent iron (Huber, 2005; Sun et al., 2006) can be synthesized in both chemical and physical methods. The most common methods for synthesizing high quality IONPs include co-precipitation, thermal decomposition, hydrothermal synthesis, micro-emulsion (Wu et al., 2008). For instance, the chemical co-precipitation method was utilized to produce iron oxide nanoparticles by mixing Fe²⁺ and Fe³⁺ aqueous salt solutions with a hydroxide base (Wu et al., 2008). It can also be fabricated by adding uni-molar concentration of ferric chloride and urea into a hydrothermal cell (Rahman et al., 2011). Green synthesis is a bottom-up approach where the atoms and molecules assemble to form a nano-sized structure. Also, it’s a single step process with no addition polymers or reducing agent, and it only uses green tea polyphenols. The green synthesis of IONPs and/or iron oxide nanorods (IONRs) has been recently proposed as a cost effective, environmental friendly method since a different material from bio-renewable natural sources can be utilized (Njagi et al., 2011; Smuleac et al., 2011). The green synthesis of iron oxide nanoparticles requires the existence of polyphenols, which can be found in coffee, tea, proteins, vitamins and wine (Shahwan et al., 2011; Hoag et al., 2009). Consequently, these components have emerged as a substitute for the established chemical fabrication of iron oxide nanoparticles. Furthermore, these component extracts are non-toxic, biodegradable and the green material acts as both dispersive and capping agents, helping to reduce the oxidation and agglomeration of iron oxide nanoparticles (Smuleac et al., 2011). The fabrication of iron oxide nanorods from tea polyphenols has been tested in the context of in vitro biocompatibility, and used for degrading bromothymol blue by Fenton oxidation (Hoag et al., 2009). Even different kinds of tea extracts (Kuang et al., 2013) namely green tea, black tea and oolong tea were utilized to synthesize iron nanoparticles. Also, in many studies iron oxides nanoparticles are produced from different plant extracts such as Eucalyptus globulus leaf (Madhavi et al., 2013; Wang et al., 2014), pomegranate leaf (Rao et al., 2013), and banana peel ash (Thakur and Karak, 2014) and tridax procumbens leaves (Masibo and He, 2008). Therefore, using polyphenols from tea and plant leaves extract is a green way to synthesize IONRs. Oman’s third production crop is mango and many mango trees lose their leaves due to its natural season plant habit. These waste leaves were found as a rich source of various polyphenol compounds in their tissues (Barreto et al., 2008). Therefore it’s a good candidate for green chemical synthesis of very small size IONRs. In this study we synthesized IONRs from mango leaves by green chemical synthesis method to utilize them in crude oil viscosity reduction. The produced iron nanoparticles were characterized using SEM/EDX, TEM, XRD and viscometer.

2 Experimental setup

The synthesis of IONRs using green tea extracts has been described previously (Shahwan et al., 2011; Hoag et al., 2009). Here Omani mango tree leaves were chosen as an alternative extract solution.

2.1

2.1 Syntheses of iron oxide nanorods

Ferrous sulfate solution FeSO₄ (500 g, s.d. fine-chem ltd.) and NaOH powder were utilized in the experiments as main sources of iron oxide. First, green mango leaves (genus: Mangifera indica L., cultivar code: Alkhokh, area: Al-Dakhiliyah Governorate, Izki) were dried and grinded well to form green powder. Then an extract solution was prepared by dissolving 12.00 g of dry leaves powder in 200 ml of distilled water and then heated for 60 min at 70 °C by using ultrasonic bath. A solution extract was obtained by filtration. Then the remaining extract was isolated in separate test tube as depicted in Fig. 1(A). The iron solution was prepared by dissolving 1.51 g of FeSO₄ powder in 100 ml of distilled water and then heated for 10 min at 40 °C. Also, an amount of 1.19 g of NaOH powder was dissolved in 100 ml of distilled water and heated at 50 °C for 10 min, see Fig. 1(B). Then, 100 ml of the prepared solution of FeSO₄ was mixed with 50 ml of prepared solution of NaOH to balance the average solution pH, which is important for the formation of solid IONRs. After that, 100 ml of FeSO₄⋅NaOH was added to 50 ml of mango leaves extract (1:2 volume ratios) and heated for 60 min at temperature of 25 °C as shown in Fig. 1(C). Then final solution was heated in a microwave at 140 °C for 2 min and then dried in an oven overnight. The obtained black solution of IONRs was taken for characterizations. Morphology and structural analysis were done by field-emission scanning electron microscopy (FESEM), energy-dispersed X-ray spectroscopy (EDX), X-ray diffraction (XRD) and transmission electron microscope (TEM). Oil viscosity tests were carried out by viscometer device.

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Figure 1

Schematic illustration of chemical co-precipitation synthesis steps of IONRs.

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3 Results and discussions

Iron oxide nanorods were obtained when reactant mixed solutions turned from brown to black color during the growth process. Fig. 2(A) and (B) shows FESEM images at different magnification of iron oxide big clusters. EDX analysis in Fig. 2(C) reveals elemental composition of the resultant clusters indicating the presence of iron, oxygen, carbon, sulfur, platinum and potassium. The existence of carbon and platinum signals in the EDX spectrum is due to the FESEM sample preparation, where synthesized iron oxide powder was distributed over conductive carbon tape and then sputtered with platinum to reduce charging effect to get clearer image, while sulfur may came from the starting material iron (II) sulfate (FeSO₄). The only element that contaminates the sample was potassium with very low percentage and it seems to be an overlap of lower energy transitions of the existence elements given by EDX program.

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Figure 2

FESEM images (A and B) of iron oxide micro-clusters, (C) EDX spectrum taken for FESEM image (B).

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The big cluster formation is due to aggregation and accumulation of tiny building blocks, especially when synthesis process undergoes between chemical solutions. Therefore, to separate rods, resultant iron oxide nanorods (IONRs) were mixed with acetone and centrifuged for 7 h. Then a small drop on a TEM Cu grid was prepared for the investigation. Small black sticks in Fig. 3(a)–(c) indicate rod like nanostructures of iron oxide. The IONRs were randomly distributed with different concentrations. The HRTEM image shows resolved lattice fringes of α-Fe₂O₃ (1 1 0) and γ-Fe₂O₃ (2 2 0) planes with a spacing of ca. 0.29 and 0.38 nm respectively, as in Fig. 3(d). Fig. 3(d) also reveals the inset selected area electron diffraction (SAED) pattern taken from the marked part in Fig. 3(c), which can be indexed to a rhombohedral hexagonal phase (space group R3c) with lattice constants a = 0.5128 and c = 1.3856 nm. In addition, IONRs’ size differs from each other, but they were partly short and thin. Five nanorods were selected and each nanorods’ diameter and length were measured 20 times separately using the imageJ program (Schneider et al., 2012). The average length and diameter of nanorods are 15 ± 2 nm and 3.0 ± 0.2 nm respectively. The distribution histograms of iron oxide nanorods diameters and lengths are shown in Fig. 4.

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Figure 3

HRTEM images of iron oxide nano-rods with low magnification (a and b) and high magnification (c and d). An inset image of SAED pattern in (d) is taken from marked rectangle in (c).

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Figure 4

Distribution histograms of (A) diameters and (B) lengths of iron oxide nanorods.

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Comparing our IONRs with previous published studies, we conclude that our nanorods were small in diameter and length and were repeated in uniform way. These smaller nanorods are important to increase chemical reactivity and heat conductivity with hydrocarbons when heated from external source of radiation. Heavy oil viscosity treatment depends on molecular long chain cracking, where IONRs should be locally placed between C–C chemical bonds and in situ heated. X-rays with Cu Kα source were utilized to examine IONRs and the resulted XRD spectrum declares the poly-crystalline nature of these rods as shown in Fig. 5. XRD peaks show the existence of two iron oxide essential compounds: alpha phase, hematite (α-Fe₂O₃) JCPDS 87-1164 and gamma phase, maghemite (γ-Fe₂O₃) JCPDS 39-1346. γ-Fe₂O₃ appears at diffracted planes (black color) of (3 1 1) and (2 2 0) where α-Fe₂O₃ appears (in blue) at (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4) and (1 1 6). Also, hydroniumjarosite, (H₃O)Fe₃(SO₄)₂(OH)₆ (CJPDS 31-0650) and melanterite, FeSO₄(H₂O)₇ (CJPDS 76-0657) crystals were detected, due to the starting solution of FeSO₄. Overall, XRD proves that IONRs contain iron oxide nanoparticles with the type of Fe₂O₃. From HRTEM results as shown in Fig. 3 the rods mainly consist of γ-Fe₂O₃ and α-Fe₂O₃ after removing any extra solutions due to centrifuging process.

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Figure 5

XRD patterns of iron oxide nanorods, diffraction planes indicate the existence of γ-Fe₂O₃, α-Fe₂O₃,(H₃O)Fe₃(SO₄)₂(OH)₆ and FeSO₄(H₂O)₇.

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Now to highlight on the green chemical synthesis of the resulted iron oxide nanorods, five oxidation reactions occur as follows:

As (1) and (2) combine together and react with the extract of green leaves, it speeds up reaction (3):

From (4), FeOOH will dissociate to the formation of Fe₂O₃ nuclei, where it starts to build ID IONRs at different places within the mixed solution. The reaction time and temperature play an important tool to control the size and morphology of evolved rods.

At the end of chemical reactions, Fe₂O₃ is produced. After a period of time, Fe₂O₃ dissociates to its γ and α types. The initially evolved Fe₂O₃ nuclei works as building blocks to form the final nanorods. With reaction time under suitable conditions of solution concentrations, sonication and heating temperatures, the Fe₂O₃ nuclei undergoes continues enlargements until it reached its final shape, nanorods shape. It is believed that mango leaves extract influenced the formation of the IONRs’ final structures and size. This is similar to the effect of green tea extract (Smuleac et al., 2011; Shahwan et al., 2011).

3.1

3.1 Viscosity test of heavy oil after adding iron oxide nanorods

Applying electromagnetic energy at microwave frequencies is an effective way to heat non-conducting materials because the energy is transferred directly to the molecule of the material. The material’s molecules become stimulated and rotate millions of time a second in response to the electromagnetic field. This rotation quickly generates heat within the material in a manner similar to friction (Metaxas and Meredith, 1993). The graph of oil viscosities versus temperature for oil before and after mixed with iron oxide nanorods at different weight percentages is revealed in Fig. 6.

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Figure 6

(A) Oil viscosity Vs. temperature for heavy oil before and after adding IONRs with different wt%. (B) An inset column chart shows viscosity reduction % at each temperature and additives wt%.

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In Fig. 6 generally dynamic oil viscosity is inversely proportional to the microwave heating temperature, as temperature increases oil viscosity decreases and this is due to rapid motion of hydrocarbon molecules at higher temperatures. But at low temperature range of 30–50 °C, the effect of IONRs additives of (02–08 g/L) in the viscosity reduction is noticeable. For instance, the oil viscosity is reduced by 10% when an amount of 0.2 g IONRs is added to 1 liter of heavy oil at T = 30 °C. This reduction increases up to 38% and 49% when 0.4 and 0.6 g/L additives were added respectively at the same temperature, see inset column chart within Fig. 6. The resulted oil viscosity reduction is varied due to IONRs’ additives and heating temperature. But when more additives of IONRs (0.8 g/L) are added the viscosity is reduced with 50%, which is only 1% increment than 0.6 g/L trail. And this is repeated at various heating temperatures as shown in Fig 6. That may reveals the saturation limit condition of the heavy oil mixture, where extra IONRs may not affect the viscosity reduction rather than increasing it. Even the oil become diluted but after a period of time, about 10 min, oil viscosity starts to increase again and returns to its initial values, experiment results not shown here. Due to water–oil content 17%, normally a direct heating microwave temperature will allow the hydrocarbons molecules to slide past each other with greater ease. As a result, the viscosity decreases. When iron oxide nanorods were added to heavy oil while heating, a reduction in viscosity was noticed, and this means more molecules bonds were broken in a faster time. Energy transferred and conversion depend on the microwave energy absorption coefficient of IONRs (Kruger et al., 1999). Also, it was found that microwaves-heat conversion in mixed water and dispersed (10 nm in diameter) iron oxide nanoparticles is better compared to oil with only water content (Keho, 1969). To understand hydrocarbons (asphaltene)-microwave interactions a schematic drawing is shown in Fig. 7.

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Figure 7

Schematic drawing of heavy oil cracking principle by IONRs under microwave irradiation.

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First of all, Heavy oil is a water-oil emulsion, which is a dielectric materials and when irradiated with microwaves, a rapid increase in temperature accrues in the dielectric emulsion and that reduces its viscosity. Also, it increases the molecular rotation, which neutralizes the Zeta potential because of the rearrangement of electrical charges surrounding the water droplets (Miadonye and Nwankwor, 2014). This resultant heat will dissipate within emulsion and to increase further heat transfer, iron oxide nanorods were utilized.

As the diameter of iron oxide become smaller, more nanorods penetrate and are distributed within the content of heavy oil. More local heat spots will result after microwave radiations or varying electromagnetic field hits the impregnated IONRs with permanent dipoles. This amount of generated heat breaks hydrocarbons bonds in big molecules such as asphaltene. This cracking effect is due to an extra heat generated by IONRs to the content rather than directly by rods themselves. As a result long chain of carbons will break at different locations forming less viscous oil. These cracked hydrocarbon chains may recombine after short period of time (∼5 to 10 min) if the heating source is removed. Therefore, finding terminated chain ends after the completion heating process is an important step to increase the lasting time of the produced diluted oil.

4 Conclusion

Green chemical syntheses of iron oxide nanoparticles using extracts of Omani mango leaves were successfully produced. The poly-crystalline nanorods with (15 ± 2) nm in average length and (3.0 ± 0.2) nm in average diameter were utilized for heavy oil viscosity treatment. The relative smaller size, distribution, heat conversion make the obtained nanorods as a good candidate for heavy oil cracking process. Oil viscosity is reduced by 10%, 38% and 49% when an amount of 0.2, 0.4 and 0.6 g/L IONRs is added to 1 liter of heavy oil at T = 30 °C. An illustration of oil hydrocarbon cracking principle is proposed.