Gum Formation in Gasoline and Its Blends: A Review - Energy & Fuels

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Gum Formation in Gasoline and Its Blends: A Review Florian Pradelle,*,† Sergio L. Braga,† Ana Rosa F. A. Martins,‡ Franck Turkovics,§ and Renata N. C. Pradelle§ †

Departamento de Engenharia Mecânica (DEM), and ‡Departamento de Engenharia Química e de Materiais (DEQM), Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Rio de Janeiro 22453-900, Brazil § Peugeot Citroën do Brasil Automóveis Ltda, Porto Real, Rio de Janeiro 27570-000, Brazil ABSTRACT: Gasoline is a volatile mixture of hydrocarbons that is used in spark-ignition (SI) engines. It is a complex mixture composed of olefinic, paraffinic, naphthenic, and aromatic hydrocarbons (C4−C12), among other substances in a smaller concentration. In several countries, such as Brazil, ethanol is used pure as a renewable fuel for SI engines, especially in flex fuel engines, and/or an additive to improve the octane number of gasoline. During storage, some classes of hydrocarbons in gasoline blends, particularly olefins and diolefins, are able to slowly react, at ambient temperatures, with the oxygen in the air. The formed oxidation products are responsible for the formation of an insoluble solid, commonly called deposits or gums, which sticks to the metal surfaces along the vehicle-fuel system, from the tank to the combustion chamber. Accumulation of these products can cause engine wear and can have adverse effects on engine efficiency, performance, emission, and durability. Consequently, it is necessary to predict gasoline blend behavior and prevent gum formation, improving gasoline quality and using additives. Even if the number of publications dedicated to gum formation in gasoline blends is reduced, results available in the literature for other fuels can be applied to the gasoline issue. This review intends to define more precise fuel stability concepts and what is considered a gum. It also aims to present the oxidation mechanism involved in gum formation, determine the main parameters influencing gum deposition in gasoline blends, and describe the experimental tools available to measure gum content.

1. INTRODUCTION Automotive gasoline is a fuel extensively used in spark-ignition (SI) engines. It is a complex mixture of olefinic, paraffinic, naphthenic, and aromatic hydrocarbons in the C4−C12 range and with boiling points between 30 and 220 °C. This volatile and inflammable mixture also presents low contents of oxygenates and traces of sulfur, nitrogen, and metals, which introduce instability to the product. The quantity of these hydrocarbons, oxygenates, and other trace compounds determines the physicochemical properties of the fuel and has a great influence on engine performance.1−4 All around the world, private and governmental programs have been established to develop alternative and ecologically friendly fuels. In several countries, ethanol is used pure as an alternative fuel, especially in flex fuel engines, and/or an additive to improve the octane number of the gasoline. Moreover, ethanol is a renewable biofuel that can be produced from agricultural feedstocks, participating in the reduction of greenhouse effect gas emissions. With the National Alcohol Program (ProÁ lcool), launched in 1975, and the development of flexible-fuel vehicles, Brazil was the first country to implement a large-scale program for the use of alcohol as an automotive fuel, by either the use of pure ethanol or the addition of anhydrous ethanol (in the range of 18−27%, v/v) in gasoline blends. The ethanol concentration in gasoline is specified within that range by the country’s Petroleum National Agency (ANP). Three different gasolines are available in Brazilian gas stations: regular, additivated, and premium. Regular gasoline is the cheaper and most common fuel consumed by the Brazilian automotive fleet. Additivated gasoline contains detergent components to increase gasoline © 2015 American Chemical Society

performance and stability, and premium gasoline is a fuel with a high octane number and additives.1,2,5 During storage, some hydrocarbons present in gasoline react with absorbed atmospheric oxygen and with each other, forming resinous, polymeric, and non-volatile materials with a high molar mass that are commonly called gum. This implies changes in the physicochemical characteristics of the blend. The gum formation leads to an increase of the fuel density, distillation temperatures, aromatics, and oxygen concentration and a decrease of the concentration of olefins. Consequently, as the gum content increases, the air/fuel mixture formation and combustion processes become non-optimal, the fuel combustion is incomplete, the engine efficiency declines, and the amount of noxious substances in the exhaust gas increases.2,3 Even if the number of publications on this subject is reduced, gum formation is a pre-occupation since the late 1920, with the problem of stability in cracked gasoline.6−17 The present work intends to review all information available in the literature on the formation of gum in gasoline blends. First, stability concepts, gum formation, and a general structure of gum are defined. Second, the oxidation mechanism involved in gum formation is presented, and kinetics aspects are discussed. Then, the main parameters influencing gum deposition in gasoline blends are listed. Finally, the experimental tools available to measure gum content are described, and their impact on the chemistry of gum formation is discussed. In some cases, the authors consider publications dealing with other fuels (mainly diesel oil and jet fuel) to complete the Received: August 19, 2015 Revised: October 30, 2015 Published: November 3, 2015 7753

DOI: 10.1021/acs.energyfuels.5b01894 Energy Fuels 2015, 29, 7753−7770

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Figure 1. Overall process of fuel stability.19

Table 1. Glossary of the Vocabulary Relative to Gum Formation75 term existent gum insoluble gum potential gum precipitate residue soluble gum total potential residue total unwashed gum unwashed gum wall-adhered insoluble gum washed gum

definition evaporation residue of aviation fuels, without any further treatment73 deposit adhering to the glass sample container after removal of the aged fuel, precipitate, and soluble gum (measured as the increase in weight of the glass sample container)74 sum of the soluble and insoluble gum74 sediment and suspended material physically separated from a solution75 material remaining after evaporation, distillation, filtration, or extraction75 non-volatile residue by evaporating the aged fuel and the toluene−acetone washings from the glass sample container74 sum of the potential gum and the precipitate74 deterioration products collected from the aged fuel and from the toluene−acetone-soluble portion of the deposit on the glass sample container74 evaporation residue of motor gasoline consisting of existent gum and non-volatile additive components73 toluene−acetone-soluble portion of the deposit on the glass sample container75 heptane-insoluble portion of the evaporation residue of motor gasoline (existent gum)73

highly reactive reagents, the concentration of olefins decreases and the concentration of aromatics and oxygen increases. The products form deposits along the vehicle fuel system, from the tank to the combustion chamber. The deposition in large quantity of gum in the carburetor and the admission valves prevents good work of the parts in the fuel line and results in an inappropriate air/fuel mixture and deficient fuel burn. After some time of use, the deposit volume causes starting and lowgear problems, and it is necessary to clean the carburetor and the injection nozzles and plugs in electronic injection cars. Gum deposits in the injection system and combustion chambers affect (i) drivability (engine choking and hesitation), (ii) engine performance (power loss, reduced acceleration, increased fuel consumption, and detonation), and (iii) exhaust gas emissions (emissions of CO, NO, etc. as a result of incomplete combustion).1−4,21 2.2. Nomenclature and Legislation Aspect. Gum is a general word referring to a large kind of species, and a specific terminology has been established (Table 1), mainly as a function of experimental characterization. This diversity of vocabulary shows that the gum formation issue can be treated by different means. According to Mayo et al.,22 soluble gum is always formed before deposits or insoluble gum and usually in greater quantity. They provided good evidence that deposits come from soluble gum, and they suspected that hard deposits on hot engine parts arise mostly from gum in solution in the fuel. Among the various approaches to evaluate gum formation, the washed gum content in gasoline and the induction period are the most important parameters to evaluate the fuel quality. Brazilian ANP legislation regarding gasoline quality standards establishes that the maximum gum content should be inferior to 5.0 mg/100 mL of gasoline and the induction period at 100

understanding of the issue. Similar works has been published on this issue in the early 1990s, with a particular emphasis in shale oil,18 diesel oil,19 and jet fuel.20

2. GUM FORMATION INTO FUEL STABILITY MECHANISMS 2.1. Fuel Stability Concept. The term fuel stability implies all of the phenomena related to the general resistance of a fuel to change, and the overall process of fuel stability is presented in Figure 1. Two expressions are frequently met in the literature: storage stability and thermal stability. The first term refers to the ability of a fuel, stored over extended periods of time, to remain unchanged or without appreciable deterioration under ambient conditions. Thermal stability is defined as the ability of a fuel to suffer relatively high-temperature stress for short periods of time, without appreciable deterioration. Such changes or degradations include color change, development of soluble and/or insoluble gum, development of particulate matter followed by sediment/deposit, development of coke and fouling materials, change in global physicochemical properties, modification of fuel composition, combustion properties, and compatibility with other fuels.19 Gasoline aging also causes evaporation of light components, which can affect engine performance and, mainly, exhaust emissions.1 Gum formation results from the slow reaction between hydrocarbons present in fuel at ambient temperatures with atmospheric oxygen and with each other, promoting changes in their physicochemical characteristics. Thus, gum formation is synonymous with the storage stability process, which starts from production and continues throughout transport and storage. By formation of high-molecular-weight polymeric materials, this process increases the fuel density and the distillation temperatures. Because olefins and diolefins are 7754


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Energy & Fuels °C should be superior to 360 min.5 Expensive antioxidizers23−26 and metal deactivators27 are added to gasoline to delay the process. However, after a period of approximately 2 months, the fuel is no longer guaranteed by the distributors to meet the specifications for engine use.1,2,21 2.3. Gum Composition. Schwartz et al.28 showed that gums are highly aromatic compounds, with a molecular weight range from 200 to 500 Da. On the basis of nuclear magnetic resonance (NMR) studies and infrared (IR) spectra, it was observed that the aromatic constituents of the gum were highly substituted. In the 1960s, elemental analysis of gums always showed the presence of carbon, hydrogen, sulfur, nitrogen, and oxygen. There was a high degree of concentration of sulfur and nitrogen compounds evidenced in the gums, especially in the earlier stages of the gum formation. 28 According to Ackermann,29 elemental analysis of the gums proved that sulfur accumulated in the gum had a concentration 6 times higher than the original ratio on gasoline. It was also stated that most of the sulfur present was in the form of thioethers; hardly any disulfide, mercaptan, sulfoxide, or sulfone was present.30 Kawahara also showed that gums contained thioether, dialkyl peroxide, and ether groups, as well as ester, carbonyl, acid, hydroperoxide, and hydroxyl groups. He also observed that gum solubility is a function of the percentages of oxygen, nitrogen, and sulfur compounds present on the gum. Nevertheless, the severe reduction of sulfur compounds in current gasoline composition strongly modified the nature of gums.30 In 2005, Pereira et al.3 evaluated the influence of anhydrous ethanol and copper contents in the formation and composition of gums in Brazilian ethanol−gasoline blends. According to them, the potential gums obtained in various conditions [gasoline (G), gasoline + alcohol (GAL), and gasoline + alcohol spiked with copper (GCu)] were macromolecules with high carbon and oxygen contents (Table 2). The oxygen

3. MECHANISMS AND KINETIC ASPECTS 3.1. General Considerations. The exact mechanisms of the reactions are not fully known, but it is generally agreed that they involve a series of free radical chain processes.31 The literature reports that many olefins react partially or completely through the addition of peroxide radicals to the double bond. At low temperatures, the reaction that takes place is the addition of peroxide to the double bond (kinetic control). However, when the reaction occurs at very high temperatures and a low peroxide concentration, the remotion of allylic hydrogen takes place (thermodynamic control).21 Such reactions may be promoted by the presence of transition metal ions, water, microorganisms, and light, catalyzing the decomposition of hydroperoxides into radicalar species.3,4 Even if the initial reaction leading to hydroperoxide formation is extremely slow, the consumption of oxygen and the consequent formation of oxygenates becomes fairly fast after the accumulation of a sufficient concentration of hydroperoxide. Once the chain reaction is established, an additional source of alkyl radicals can be the decomposition of hydroperoxide and peroxy radicals.24,32,33 These radicals can react via hydrogen abstraction or degenerate branching to form oxygenated products, such as ketones and aldehydes.34 The general mechanism usually quoted in the literature is RH → R• + H• •

a

gum sample

G

GAL

GCu

74.20 ± 0.10 7.40 ± 0.06 1.30 ± 0.04 17.10 0.23

73.20 ± 0.10 7.10 ± 0.10 4.40 ± 0.80 15.40 0.21

75.20 ± 0.10 7.60 ± 0.04 1.60 ± 0.10 15.70 0.21

R + O2 → ROO

(2)

ROO• + RH → ROOH + R•

(3)

R• + R• → R−R

(4)

ROO• + R• → ROOR

(5)

ROO• + ROO• → products

(6)

where RH is an unsaturated organic compound, such as olefins, R• is a free radical, ROO• is the associated peroxy radical, and ROOH is the associated hydroperoxide. In reaction 1, a free radical is formed from the unsaturated hydrocarbon during the initiation reaction. In the propagation reactions 2 and 3, the free radical reacts with oxygen to yield a free peroxide radical, which, in turn, reacts with an unsaturated hydrocarbon molecule, whereby a further free radical is produced besides a hydroperoxide. Repetition of these two reactions carries on oxidation until the chain is terminated with one of the reactions 4, 5, or 6. Beaver et al.35 developed an alternative model for diesel fuel to improve the understanding of the first phase of the degradation process in which oxygen is incorporated into reactive fuel molecules. They proposed that a reaction can occur between electron-rich molecules with low oxidation potentials and oxygen via a mechanism that is not consistent with the operation of the well-known peroxyl radical-chain process used for gasoline and jet fuel. They designated this reaction as electron-transfer-initiated oxygenation (ETIO). They illustrated this theory with the examples of the oxidation of 1,2,3,4-tetrahydrocarbazole (THC) and 3-methylindole (3MI) as oxygen scavenger additives and, later, with 2,5dimethylpyrrole (DMP),36 triphenylphosphine (TPP),37 1,2,5trimethylpyrrole (TMP),38 and dicyclohexylphenylphosphine (DCP).39 For all of the compounds, they analyzed product distributions, considered kinetic issues, and proposed a detailed mechanism.

Table 2. Elemental Analysis of the Different Gum Samples3 C content (%) H content (%) N content (%) O contenta (%) ratio O/C

(1) •

Oxygen is obtained by difference.

present in the structure could be associated with ketone carbonyl groups and O−H groups of alcohols (Figure 2), confirming the observations by Kawahara.30 The absorption spectres in the IR region showed that some bands were present in all samples, such as the large bands of O−H stretching of alcohols with a hydrogen bond (≈3400 cm−1), which indicated polymeric associations, as shown in Figure 2a. The bands associated with C−H stretching (≈2962, 2930, and 2870 cm−1) were also important. In Figure 2b, the bands at 1655 and 1500 cm−1 indicated an axial deformation of C−C bonds within the ring and the bands at 1457 and 1378 cm−1 were symmetrical angular deformations β-CH2 and β-CH3, respectively. The stretching band of carbonyl at 1708 ± 10 cm−1 and bands 1167 and 1000 cm−1 observed in Figure 2b indicated asymmetrical axial deformation of C−O−C. The relation between the chemical structure and mechanism of gum formation was discussed in the next section. 7755


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Figure 2. (a) Absorption spectrum in the IR region of gums formed in GAL, GCu, and G, by KBr pellet and (b) enlargement in the IR region (1900−600 cm−1).3

the mechanisms of middle distillate fuel oxidative degradation.45−48 3.2. Proposed Detailed Mechanisms. Pereira et al.21 suggested a complete oxidation mechanism for 2,4-hexadiene (Figure 3), pointing out that the more stable the allylic radical, the longer its life and the greater the probability of oxidation occurring. Consequently, the driving force of this mechanism is the radical stability. Therefore, the smaller the induction period of gasoline, the higher the potential of fuel to form gum. Using gas chromatography coupled with mass spectroscopy (GC−MS) analysis, Dantas24 proposed a mechanism for gum formation from naphtha, modeled by 2-octenal, in the presence of an antioxidant. The formation of a diolefinic compound results from the reaction between a radical belonging to an olefinic hydroperoxide compound and 2-octenal. The possibilities of reactions with atmospheric oxygen are proportional to hydrocarbon unsaturations present in the molecules. Double bonds stabilized with some heteroatoms hardly react in oxidative processes. The formation of triolefin followed the same reaction scheme as the formation of the diolefinic compound. Dantas suggested the fragmentation of highmolecular-weight compounds, such as triolefin, to generate

The oxidation reactions lead, after several steps, to polar and high-molecular-weight products, which have a low solubility in the nonpolar hydrocarbon matrix and, consequently, can precipitate, forming insoluble gum (or deposit).40 The analysis of thermally stressed jet fuel by electrospray ionization mass spectrometry (ESI−MS), under different experimental conditions, showed the presence of a broad molecular mass band of polar compounds in the range of 250−400 Da. Aggregation processes of these species, through polymerization or clustering reactions (600−900 Da), could be responsible for the formation of insoluble compounds, which ultimately lead to the formation of the oxidative deposits after precipitation.41 On the basis of the review of the literature on mechanisms involved in jet fuel oxidative deposit formation, Beaver et al.42 suggested that, for middle distillates, a similar chemistry is involved in deposit formation for both storage and thermal oxidative degradation. They specified that this similarity only applies to the gross mechanistic details: autoxidation, followed by coupling reactions forming soluble macromolecular oxidatively reactive species (SMORS), followed by further coupling to form oxidative deposits. This model, first described by Hardy and Wechter,43,44 is coherent with later literature on 7756


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single compound RH. The initiation step was an undescribed reaction leading to R• formation, whereas the propagation steps were the same as formerly described. The termination step was reduced to the recombination of ROO•.31

formation of R•

(9)

R• + O2 → ROO• •

(10) •

ROO + RH → ROOH + R

(11)

ROO• + ROO• → products

(12)

Reaction 10 does not have activation energy, whereas reaction 11 has a significant activation energy for most hydrocarbons. Consequently, reaction 11 is the slow step of the mechanism, and ROO• is the major radical species. The termination reaction has been written as the recombination of ROO• rather than R•. Moreover, if reaction 11 has a small activation energy, the hydrocarbon is easily oxidized. In term of the chemical structure, this requires a weak carbon−hydrogen bond (see section 4.2). Using the stationary-state hypothesis (i.e., concentrations of R• and ROO• radicals are constant in time), the concentration of radicals can be calculated through eqs 13−15, where Ri is the rate of formation of radicals in reaction 9. d[R•] = R i − k10[R•][O2 ] + k11[ROO•][RH] = 0 dt

(13)

•

d[ROO ] = k10[R•][O2 ] − k11[ROO•][RH] dt

Figure 3. Possible oxidation mechanism and formation reactions of allylic radicals of 2,4-hexadiene and their respective resonance structures.21

− 2k12[ROO•]2 = 0

[ROO•] =

new compound radicals, thus giving other chain reactions. With the fragmentation of the triolefinic compound, the species become soluble again in naphtha.23,24 3.3. Kinetic Aspect. No kinetic model of gum formation for gasoline blends was found in the literature. However, some papers dealing with diesel49 and jet fuel blends31 are available. Because papers use a general oxidation mechanism, it is acceptable to consider that the same conclusion can be extrapolated for gasoline blends. In 2007, Grishina et al.49 studied the kinetic of the deposition of gums formed during diesel long-term storage (up to 100 months). In their work, the process of gum formation in diesel fuel is represented by a simple global reaction, in which n molecules of low-molecular-mass compounds A react with each other to yield a high-molecular-mass gum particle An. nA → A n (7)

2

(14)

R i /2k12

(15) •

Using the concentration of ROO from eq 15 and the steadystate equation 13 and assuming that Ri is negligible when compared to the rate of formation of hydroperoxide radicals, the rate of oxygen consumption can be written as −

d[O2 ] = k10[R•][O2 ] = R i + k11[ROO•][RH] dt Ri ≈ k11 2 [RH] 2k12

(16)

Equation 16 showed that the peroxidation of hydrocarbons through the proposed mechanism is not dependent upon the oxygen concentration. This conclusion is true only for situations in which sufficient oxygen is available for the reaction, such as in the accelerated aging test. In flowing tests, the amount of oxygen available is limited to the oxygen dissolved and absorbed in the fuel at the beginning of the transit, and consequently, there is an oxygen dependence in these cases (see section 4.3). In this case, as the oxygen is removed, ROO• can no longer be the major radical.

Consequently, a kinetic equation for the calculation of the gum formation rate was proposed

d[gum] = k[gum]−n (8) dt where [gum] is the concentration of gums at a given time, k is a constant, and n is the rate order of the reaction. The value of −2 for the global rate order found experimentally indicates that the gum embodies processes of self-retardation oxidation, owing to the natural stabilizer (antioxidant) properties of gums. In 1994, Heneghan et al. proposed a kinetic study of jet fuel oxidation based on a four-reaction mechanism considering a

4. PARAMETERS THAT INFLUENCE GUM FORMATION 4.1. Storage Conditions: Temperature and Aging Period. It is observed in all studies that gum formation is accelerated by an increase of the temperature and storage time. Pradelle et al.50 studied the behavior of Brazilian gasoline, regular and additivated, blended with ethanol (gasohol) during different prolonged storages at several temperatures and ethanol concentrations. The authors quantified the influence 7757


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Energy & Fuels of these variables in relation to the gum formation issue and observed a higher gum content when the temperature and aging period increased. Nagpal et al.51 studied the effect of the aging period on gum formation tendencies of some pure olefins under accelerated test conditions. In Figure 4, the gum content values of 4-

Figure 5. Effect of the temperature on gum formation for various pure olefins.51

Figure 4. Effect of the aging time on potential gum and peroxide number for 4-vinyl-1-cyclohexene.51

vinylcyclohexene quickly increase with aging duration. However, the peroxide number initially increases when the aging duration is increased up to 4 h, and it is quasi-constant when the aging duration is further increased to 6 h. This indicates a faster conversion of peroxides into sediments with an increased aging duration. The dependence of the temperature on gum formation tendencies of three olefins seems to respect an Arrhenius law, as showed in Figure 5. It can be observed that the augmentation is higher in the case of 4-vinyl-1-cyclohexene than 1-octene and 2,4,4-trimethylpentene. 4.2. Gasoline Composition and Origin. The formation of gums in gasoline stored for a very long time and under normal temperature conditions mainly depends upon the gasoline constituents, origin of the petroleum, and type of refining. The cuts come from different processes, such as reforming, fluid catalytic cracking (FCC), isomerization, oxygenates, straightrun distillation, alkylation, or thermal processes, and consequently, they differ in their contributions to the potential of gum formation. With respect to the refining, de la Puente et al.33 showed that isomerate and reformate streams have a low incidence but FCC naphtha and cuts derived from thermal processes have a strong impact on this issue. Rahimi et al.52 also observed the positive impact of hydrotreatment on naphtha jet and gas oil fractions of the hydrocracked distillates from western Canadian bitumen atmospheric residue. In their work, Nagpal et al.53,54 plotted the gum formed during 3 months of storage of various naphtha samples against time (Figure 6) and showed that high olefinic FCC naphthas, with larger amounts of diolefins, formed more gums than the naphthas from thermal cracking. As in 2004, 70% of the Brazilian gasoline was provided by FCC streams. The FCC operative parameters have

Figure 6. Stability test for 3 months at 43 °C with cracked naphthas, such as FCC naphthas, visbreaking (VB) naphthas, and coker naphtha.53

to be optimized to reduce the concentration of gasoline gum precursor, such as dienes, formed by thermal cracking and partially consumed in the FCC catalytic reactions.55 With respect to hydrocarbon composition, a widely held view was that small proportions of reactive nitrogen- and sulfurcontaining compounds are largely responsible for the formation of gum and deposits.22 Bhan et al.56 suggested that oxidation of neutral compounds to polar intermediates may be a major pathway for sediment formation and darkening of marine diesel fuels. The majority of polar compounds (carboxylic, sulfonic, and phenolic species) in the fuel passively participate in sediment and gum formation via various types of chemical bonding, including hydrogen bonding. Within a compound class, compounds with a higher molecular weight and higher aromaticity were observed to be the most active in sediment formation. Nevertheless, in their study with middle distillate fuels, Wechter and Hardy did not manage to correlate the concentration of acidic species naturally present or formed during accelerated aging with the amounts of insoluble 7758


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Figure 7. Reaction of (a) thiophenol and (b) 1-hexanethiol with various hydrocarbons.59

sediments.57 Bauserman et al. investigated the relation between 21 middle distillate fuels with their organic nitrogen content. They showed that there was not a specific organic nitrogen compound (among indoles, carbazoles, pyrroles, pyridines, quinolines, and tetrahydroquinolines) responsible for instability. However, an imbalance in either the basic or non-basic organic nitrogen compounds probably caused a shift in equilibrium, resulting in sediment or gum formation.58 Later, Epping et al. doped surrogate fuels with representative compounds containing oxygen (2,6-dimethylphenol and 2naphthol), nitrogen (2,5-dimethylpyrrole and 2-methylindole), and sulfur (dibenzothiophene and pentamethylene sulfide). In general, the tested nitrogen compounds and the phenols tended to form oxidized oligomers, whereas the sulfur compounds led to sulfoxides and sulfones.40 Schwartz et al.59 studied, in the early 1970s, the reactivity of certain sulfur compounds and hydrocarbons to form gum. Their work showed that thiols were the most reactive sulfur compounds (Figure 7). Sulfur compounds are essential to gum formation and appeared to be selective toward olefins. Disulfides and trisulfides appear to be active promoters.30,60 Szmant and his collaborators published an extensive work on the mechanism of the thiol−olefin co-oxidation (TOCO), reactions between a thiol, an olefin, and oxygen, leading to the formation of β-hydroxysulfoxide, between 1967 and 1987.61−69 They studied, in particular, the effect of several parameters, such as the temperature, solvation,66,67 and thiol substituent structure,61 on the distribution (stereochemistry) and kinetic behavior of products to determine a self-consistent mechanism under radicalar63 and non-radicalar68 inducing conditions. However, the severe control of the sulfur content to improve

the fuel quality reduced the influence of such species on gum formation. Nagpal et al.54 and Zanier4 showed that the stability of catalytic oxidation decreases in the order of paraffins, naphthenes, isoparaffins, aromatics, monoolefins, aromatic olefins, and diolefins. According to de la Puente et al.,33 cyclic and branched olefins are the main gum producers. Also, very reactive, conjugated diene compounds are considered promoters of these reactions, and a synergistic effect with monoolefins may result. Nagpal et al.51 studied the gum-forming tendencies of different types of olefins in a known surrogate matrix through potential gum measurements under accelerated aging. Considering similar straight-chain olefin (the double bond is always in the first carbon atom), there was an increase in the gumforming tendency with the increase in the carbon number. Moreover, branching generally increased the gum formation, but the position of branching in the iso-olefins played an important role besides the double bond. Cyclic and dicyclic structures had been found to have a high contribution to gum formation (Figure 8). A synergistic effect of dienes with straight-chain and branched olefins was also studied for the addition of 1, 2, and 3% of conjugated dienes (Figure 9). The addition of 1% of conjugated diene had a relatively low impact, while the addition of 2−3% of the diolefin substantially increased the gum content. On the other hand, in the case of blends containing 2-methyl-2-butene, even the addition of 1% of the diolefin has substantially increased the potential gum. Later, Pereira et al.21 observed through the variation of density that not all olefins contributed to the oxidation of gasolines. Different behaviors were observed as a function of the types of allyl radicals formed. Those forming more stable 7759


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Figure 8. Variation of total potential gum for various concentrations of pure olefins after 4 h of aging.51

intermediates as secondary allylic radicals (cyclohexene and 2,4hexadiene) had high participation to the formation of gum, whereas the olefins that form primary allylic radicals (1-hexene, 1-octene, and 1,5-hexadiene) did not contribute to the formation of gum (Figure 10). This showed that the formulation of gasolines with high stability requires a minimum addition of cyclic and conjugated olefins and the use of fractions containing linear and non-conjugated olefins. The presence of an antioxidant additive in the blend strongly influences the gum formation. Pradelle et al.50 showed in their experimental study that the presence of such an additive reduced the measured washed gum content in ethanol− gasoline blends. They also observed in their predictive mathematical model that additivation reduced the relative influence of ethanol addition in comparison to thermodynamic parameters (temperature and aging period). 4.3. Absorption of Oxygen in the Air. It is commonly accepted that the rate of existent gum concentration change during storage depends upon to the oxygen content absorbed by gasoline. The greater this proportion, the higher the

oxidation rate and the faster the existent gum concentration attains the maximally permissible value (5 mg/100 mL). In a study about jet fuel oxidation and deposit formation, Heneghan found the same dependence upon the oxygen content for the formation of deposit in jet fuels: the amount of deposit formed decreased significantly if oxygen is removed from the fuel.31 Beaver et al. studied the initial rate of the oxygenation of 2,5dimethylpyrrole (DMP) under different experimental conditions. In nitrobenzene at 50−70 °C and in dodecane or toluene at 70−120 °C, they found that this reaction is secondorder overall and first-order in both DMP and O2. The relevance of the model obtained for DMP oxygenation with the real fuel degradation is confirmed by investigating the effect of oxygen overpressure on peroxide formation in two jet fuels, under simulated oxidative degradation.36 Commodo et al. studied the chemical reactions occurring in the thermally stressed commercial Jet A-1 by ultraviolet−visible light absorption and one-dimensional (1D) fluorescence spectroscopy70 and three-dimensional (3D) fluorescence analysis.71 They observed an increase in the absorption and emission 7760


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where P1 and P2 were the pressures in the recipient before the start of the experiments and after its cooling (kPa), respectively, and T1 and T2 were the temperatures of the surrounding air before heating and after cooling (°C), respectively. Storage test results were used to determine standard PAO values. Gasolines with PAO inferior to 25% are storable for a prolonged period (3 years and longer), with PAO between 25 and 85%, and for a short time (up to 1 year), with PAO superior to 85%, for immediate use. 4.4. Transition Metal Contamination. Traces of transition metals strongly accelerate gasoline oxidation, catalyzing the decomposition of hydroperoxides into radicalar species, in particular those suffering the transference of one electron, such as copper, iron, cobalt, and manganese ions, which are the most effective. The presence of metallic species in automotive fuels is generally undesirable because it is associated with corrosion, metal deposition on engine parts, and poor fuel performance as a result of oxidative decomposition reactions. Thus, metallic species may be introduced by corrosion of equipment during fuel production (especially the cracking process), processing, transportation vehicles, storage, and/or fuel adulteration, as well as the use of ethanol, where corrosion problems are intrinsically more severe. During the past decade, ethanol added to gasoline was produced in copper fermenting tanks and/or distillers, and consequently, copper was one of the main contamination sources that contributed to the formation of gum in Brazilian gasoline. For this reason, Brazilian literature on the effect of copper was numerous in the 2000s.1−3,32,82,83 Moreover, as a result of chemical differences between oilderived fuels and alcohols, corrosion problems are intrinsically more severe when alcohols are present in the fuel, liberating metal ions in the fuel, promoting gum formation. Teixeira et al.2 and Souza et al.82 investigated the influence of different transition metals (zinc, nickel, copper, lead, and iron) on gum formation in gasoline−ethanol blends, considering different metal contamination levels and aging periods (Figures 11 and 12). On one hand, results showed that copper and iron strongly increased the rate of gum formation. For any aging period, the rate of gum formation increased with the increase of these metal contaminations (A’reff32 in gasoline and Beaver et al. in diesel fuel35 observed the same behavior for these two metals). On the other hand, nickel and zinc had a weaker influence on the rate of gum formation, while the effect of lead was almost negligible. It can be noticed that the effect of zinc and nickel was independent of the aging period. Nery et al.83 studied the effect on the content of washed gum in Brazilian gasoline−ethanol blends as a function of the concentration of copper and aging period (until 47 days). Regardless of the aging period, it was observed that the uncontaminated samples remain with gum content below the maximum limit established by the ANP.5 However, for the contaminated samples, the gum content increases substantially with the amount of copper added and the aging period. The addition of only 0.6 mg/L copper was sufficient to exceed the gum content specifications after only 7 days of aging. For 3.7 mg/L copper, the washed gum content after storage of 47 days was 20 times higher than for the sample without contamination. Pereira et al.3 studied the stability of the gasoline samples with different copper contents, measuring the washed gum content and induction period. As observed by Nery et al.,83 Figure 13 shows that the gum content increased as more copper is added in the blends. For copper values close to 0.07 mg/kg, there was a small variation in the gum content;

Figure 9. Synergistic effect of dienes on branched and straight-chain olefins for total potential gum.51

wavelengths. Such a shift in the emission spectra was not observable if oxygen was removed from the fuel. This is coherent with the formation of high-molecular-mass oxygenated compounds, in particular when polymerization reactions are activated by increased temperature and the amount of dissolved oxygen. The dependency of gum formation with the oxygen concentration is commonly used in accelerated aging methods, such as the induction period,72 washed gum,73 and potential residue74 method tests, and some methods to evaluate gum formation behavior of gasoline,75−78 distillate fuel,79 and jet fuel.70,71,80,81 Moreover, Stavinoha et al.75 presented a table to correlate accelerated test conditions with various temperatures and various oxygen concentrations for a given time of storage using two different mathematical relations. For instance, Shalatov et al.78 proposed a method of determination of chemical stability of 50 ± 1 mL of automobile gasolines based on the accelerated oxidation (3 h) under a high pressure of oxygen (800 ± 10 kPa) and high temperature (120 ± 1 °C), followed by 180 min of cooling. The chemical stability of the automobile gasolines was calculated from the proportion of absorbed oxygen (PAO), defined as ⎡ P (T + 273) ⎤ PAO (%) = 100⎢1 − 2 1 ⎥ P1(T2 + 273) ⎦ ⎣

(17) 7761


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Figure 10. Gasoline density variation with different olefin contents, after the addition of (a) 1-hexene, (b) 1-octene, (c) 1,5-hexadiene, (d) cyclohexene, and (e) 2,4-hexadiene before and after aging for 4 h at 110.8 °C.21

Figure 11. Washed gum content as a function of the metal concentration after 7 days of aging.82

Figure 12. Washed gum content as a function of the aging period for 1.0 mg/L metal.82

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Figure 13. Gum content variation as a function of the copper content in anhydrous alcohol: (a) low-content and (b) high-content regions.3

increase in the rate of formation of free radicals and catalyzing the beginning of peroxidation.3,27,30,35,84 Another mechanism proposed for the oxidation process is the formation of a complex between copper and oxygen present in gasoline. It is immediately followed by the peroxidation process and the consequent attack of olefins.3,27

however, for values superior to 0.09 mg/kg, the gum content increased rapidly. Figure 14 showed that the induction period

Cu n + + O2 → [Cu n +···O•−O•] n+

•

•

[Cu ···O −O ] + RH → Cu

n+

(21) •

•

+ HOO + R

(22)

4.5. Surface Interaction. According to Watkinson and Wilson,85 two paths of physicochemical processes lead to deposit formation: direct formation on the wall surface from precursor species or adhesion/precipitation of bulk insoluble material (Figure 15). In both cases, the deposition processes

Figure 14. Variation of the induction period as a function of the copper content present in anhydrous ethyl alcohol.3

decreased significantly as the copper content increased, confirming that copper decreases its stability. Alcohol samples with a copper content of 0.03−0.09 mg/kg presented an induction period superior to 360 min, the maximum value authorized by the ANP.5 In a mechanistic point of view, the copper ions added to gasoline accelerate its peroxidation, catalyzing the decomposition of the hydroperoxides. The equations usually quoted in the literature are ROOH + Cu+ → RO• + Cu 2 + + HO−

(18)

ROOH + Cu 2 + → ROO• + Cu+ + H+

(19)

Figure 15. General multi-step chemical reaction fouling mechanism.85

depend upon the transfer of species and the characteristics of the metal surface (for instance, composition and surface finishing). Nevertheless, the chemical and physical effects of the surface on auto-oxidative deposition are not well understood.86 Superficial catalytic effects (see previous section) can influence surface reactions by significantly modifying the deposition rate and deposit characteristics.20 For example, the deposition formation rate within stainless-steel tubes treated by adding a silica-based layer was slower than untreated stainlesssteel tubes, because the silica-based layer prevents direct contact of fuel and nickel containing stainless steel.87 Altin and Eser reported that the amount of insoluble materials formed on the surface is higher on stainless-steel walls than on a glass insert.88 On the basis of studies performed with jet fuel, the

where the global equation is 2ROOH → ROO• + RO• + H 2O

(20)

A metal−hydroperoxide complex is probably formed in these oxidation−reduction reactions, followed by an electron transfer to the free radical. In this case, copper behaves as an oxidizing agent and a reducing agent, resulting into a considerable 7763


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Pereira et al. 3 found that the higher the ethanol concentration in gasoline, the higher the fuel density and the lower the gum content. Figure 16 shows that the gum content

deposition rate can also be associated with the physical nature of the surface, because smoother surfaces tend to inhibit deposition.89,90 As the properties of the surface in contact with the fuel are modified by the deposit accumulation, the prediction of the surface−fuel interaction is quite complex. A low initial deposition rate is often seen in treated, clean, and smooth surfaces. However, as the test duration increases, the overall effect of different metal surfaces becomes less substantial. In addition, wall reactions are increasingly common in recently developed chemistry models. Therefore, knowing the species and temperature distributions is essential in accurately modeling the thermal oxidative deposition process.86 Moreover, Kinoshita et al. showed that the superficial temperature is the most important parameter to control deposition. The first option is to maintain the nozzle temperature lower than 90% distillation temperature of the fuel. Above this temperature, the residual fuel in the nozzle hole remained in a liquid state and the deposit precursor is easily washed away by the fuel injection.91 The second option solution to prevent deposit formation is to raise the surface temperature to levels high enough to gasify the deposits and/or its precursors. 92 Observations of deposit thickness by ellipsometry in aviation fuel tended to confirm this dependency with the temperature.93 Such behavior was also observed in gasoline direct injection (GDI) engines. 4.6. Addition of Solvent and Ethanol. The adulteration of gasoline by the addition of hydrocarbons, such as kerosene, paint thinner, turpentine oil, and diesel oil, had been often observed in Brazil in early 2000, before the implementation of a more severe inspection policy. In addition to the undeserved market advantage, this also causes serious damage to the consumer and to the environment, modifying the fuel properties and performance and the tendency for gum formation. Dos Santos et al.94 analyzed the influence of some commercially available hydrocarbons on the tendency for gum formation of gasoline. Among the solvents tested, raffinate showed the more accentuated tendency of gum formation in gasoline (Table 3).

Figure 16. Variation of experimental and theoretical (dilution law) potential gum content as a function of alcohol.3

decreased as the alcohol content increased. This demonstrated that the gum content was only due to the gasoline present in the fuel blends and that ethanol has a dilution effect of the gum derived from gasoline. They built a curve for the theoretical gum content, considering a dilution law based on the measured gum content in a sample with 100% gasoline. It was observed that the experimental and theoretical curves were very close, with a maximum gum content variation of 1.5 mg/100 mL. This confirms that alcohol did not behave as a catalyst or inhibitor of the oxidation reaction of the olefins present and did not suffer oxidation reactions that affected the gum content. Considering only the gum formation issue, the addition of alcohol to gasoline is beneficial, because it reduces the gum deposit in the engine, per liter of fuel consumed. One of the possible explanations for the absence of impact in the formation of gum is the fact that the oxidation mechanism of ethanol into aldehyde and, later, to carboxylic acid is not radicalar, in contrast to the oxidation process of olefins (Figure 17).

Table 3. Washed Gum Content in Gasoline for Blends with Various Solvents94 solvent gasoline gasoline gasoline gasoline gasoline gasoline

+ turpentine oil + paint thinner + raffinate + diesel oil + kerosene (reference)

concentration of solvent (%, v/v)

washed gum content (mg/100 mL)

20 20 5 4 7 0

0.5 0.0 9.5 2.5 0.5 0.5

Figure 17. Oxidation of ethyl alcohol to carboxylic acid.3

In 2001, D’Ornellas95 studied several parameters relative to gasoline instability in gasohol blends with approximately 20% (v/v) anhydrous ethanol. She observed an increase in potential gum and washed gum after storage at ambient temperature and 43 °C after 8, 16, and 24 weeks, in the presence of anhydrous ethanol. However, filterable precipitate insolubles, wall-adhered insolubles, and, consequently, the insoluble portion are reduced when ethanol is added. She concluded that ethanol has a solubility effect on insoluble gums. Nevertheless, such behavior on washed gum was not observed in later literature.3,51 As described in section 4.4, an eventual contamination of ethanol with copper could explain the results obtained by D’Ornellas.

In their study, Pradelle et al. also observed an apparent dilution effect of ethanol in gum formation in regular and additivated gasohol blends. Aqueous ethanol was added in the range of 0−100% in initial Brazilian gasoline with 20% (v/v), while the temperature and aging period varied from 20 to 40 °C and from 0 to 150 days, respectively.50 Moreover, Pereira et al. showed that functional groups determined by absorption spectroscopy in the IR region (Figure 2, already discussed in section 2.3) are basically the same for all studied samples (G, GAL, and GCu). They also studied these potential gums by thermogravimetry (TG) and differential thermogravimetry (DTG) in two different atmos7764


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Figure 18. TG and DTG curves in dynamic (a) N2 and (b) air atmospheres, using a heating rate of 10 °C/min for samples of potential insoluble gum from GAL and GCu mixtures and G.3

hydroperoxides, to initiate the instability reaction sequence.4 Different laboratory tests exist to determine the oxidation of gasoline. Some of them are quite extensive and inconvenient for routine control, and until today, none of them may give the resistance to oxidation during storage, particularly because the storage conditions are extremely variable.26 The main laboratory tests are listed in this section. 5.1. ASTM D525: Induction Period Method. This test method covers the determination of the stability of gasoline under accelerated oxidation conditions. The sample is oxidized in a recipient initially filled at 15−25 °C with oxygen pressure at 690−705 kPa and heated at a temperature between 98 and 102 °C. The time required for the sample to reach the breaking point is the observed induction period at the temperature of the test, from which the induction period at 100 °C can be calculated.72 The induction period may be used as an indication of the tendency of gasoline to form gum in storage, especially evaluating the ability to form free radicals. Nevertheless, no clear correlation can be performed with the formation of gum in natural storage as a result of the different storage conditions and the different gasoline compositions.72 5.2. ASTM D381: Gum Content in Fuels by Jet Evaporation. This test method covers the determination of the existent gum content of aviation fuels, gasolines, or other volatile distillates (including those containing alcohols and

pheres (nitrogen and air). The results of the TG and DTG curves showed that gums derived from gasoline added with alcohol presented larger thermal stability in both atmospheres. In a N2 atmosphere, GAL and GCu presented several overlapping events, in which there were mass losses of ≈60− 81% (Figure 18a) up to 600 °C. The analysis of the TG and DTG curves obtained in a dynamic air atmosphere showed that mass loss occurred throughout the entire analysis (Figure 18b), but two significant events were detected at 220 and 600 °C. The GCu sample presented two thermal events with an onset temperature close to that of the G sample. In addition, at temperatures superior to 600 °C, gum degradation is practically complete. Because the automotive engine does not reach such a temperature, clogging problems of the combustion chamber and admission valves can occur.3

5. PRACTICAL MEASUREMENT OF STORAGE STABILITY Because, at ambient temperature, reactions leading to gum are very slow, it is necessary for characterization to develop accelerated tests, generally at elevated temperatures, often combined with air or oxygen flow. Assuming that the mechanism remains unchanged over a limited temperature range, accelerated tests give an indication of predicting storage stability. In fact, the two deterioration processes are similar because both depend upon an active oxygen species, i.e., 7765


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Energy & Fuels ethers). For automotive gasoline, 50 ± 0.5 mL of fuel is evaporated under controlled conditions of the temperature and flow of air. The residue is weighed before and after extraction with heptane, and the results are reported as milligrams per 100 mL.73 The true significance of this test method for determining gum in motor gasoline is not firmly established. It has been proven that high gum can cause induction system deposits and sticking of intake valves, and in most instances, it can be assumed that low gum will ensure the absence of induction system difficulties. However, the test method is not correlative to induction system deposits. The primary purpose is the measurement of the oxidation products formed in the sample prior to or during the accelerated oxidation procedure.73 5.3. ASTM D873 Modified: Potential Residue Method. The initial ASTM D873 test method covers the determination of the tendency of aviation fuels to form gum and deposits under accelerated aging conditions. Nevertheless, adaptations of this norm are encountered in the literature to be used with gasoline fuels.51,75 A total of 100 ± 1 mL of fuel is oxidized at 100 ± 0.1 °C in a pressure vessel filled with oxygen (690−705 kPa) during X h aging time. The amounts of soluble gum, insoluble gum, and precipitate formed are extracted and weighed.74 The results give an indication of storage stability of these fuels. The tendency of fuels to form gum and deposits in these tests has not been correlated with field performance with the formation of gum and deposits under non-accelerated storage conditions.74 5.4. Accelerated Oxidation Conditions versus Ambient Aging: Limitations. Only one study dealt with this aspect in cracked naphtha, but some additional publications can be found for diesel blends and jet fuel blends. As in section 3.3, it is acceptable to consider that the same conclusion can be extrapolated for gasoline blends. In his work, Kawahara compared gum formed in gasoline under natural and accelerated aging in terms of elemental analysis, moiety composition, and oxygen distribution in gums. He observed that the proportions of total oxygen present as carbonyl, acid, and ester are approximately the same in both gums, whereas the distributions of hydroxyl, ether, and dialkyl peroxides are different. The oxygen content was mainly in ether form in accelerated aged gum and through alcohol function in naturally aged gum. He suggested that this difference is due to the addition and abstraction reactions of alkoxy radicals, which are generated from hydroperoxides, peroxides, and alkylperoxy radicals.30 Hazlett et al. analyzed by pyrolysis/field ionization mass spectrometry (FIMS) solids isolated from several different diesel fuels under a variety of conditions of aging. On the basis of FIMS analysis, they found that composition of filterable and adherent insolubles for different stress regimes appeared very similar to ambient storage gum.96 Venkataraman and Eser made the same conclusion based on the similarities of chemical and morphological properties of deposits from high-pressure diesel injectors (HPDIs) formed during engine operation and from the thermal oxidative degradation of a model compound in short-duration experiments.97 Nevertheless, the amplitude of these differences in gum structure strongly depends upon the accelerated aging conditions. Power studied IR spectra of soluble and filterable insoluble gums and showed that gums formed under two accelerated

aging conditions, an inert atmosphere and ambient storage for 1 year, are not the same.98 Figure 19 shows the spectra of the soluble gums recovered from the aged fuel samples. All samples present bands at the

Figure 19. IR spectra of residual soluble gums from diesel fuel aged by (a) method 1 (1 year and ambient), (b) method 2 (O2 sparge, 95 °C, and 16 h), (c) method 3 (O2 atmosphere, 100 °C, 168 h, and 610 kPa), and (d) method 3 (N2 atmosphere, 100 °C, 168 h, and 610 kPa).98

characteristic frequency of C−H, CO, CC, and C−O bonds. Moreover, the analysis points out that ether bonds cross-link aromatic chains in an oxidized polyaromatic network [Ar CC(−O)]. This figure also indicates that accelerated oxidation yielded soluble gums, which contained higher proportions of CO, Ar CC(−O), and C−O (esters and 7766


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than in the ambient-aged insoluble gum. This situation is the reverse of the observed tendency for soluble gums. However, under accelerated oxidation experiments, the spectra of soluble and insoluble gums (Figures 19b and 20b) are quite similar, indicating that these gums may have been formed by similar mechanisms.98 There are evident differences in the spectra of the soluble (Figure 19a) and insoluble (Figure 20a) gums isolated from the ambient aged fuels after storage for 1 year. The spectra suggested that a incorporation of precursors that contain Ar CC(−O) moieties is a major contributor to the formation of insoluble gum, whereas the soluble gum incorporated only a minor amount of these species and a higher relative concentration of non-aromatic hydrocarbons.98 These observations are coherent with the relation between the polarity, molecular weight, and deposition process. After examination of the micrographs of the deposits formed during accelerated aging of jet fuel samples, distinct morphological differences clearly exist between deposits produced by nitrogen-sparged fuels and those produced by fuels with dissolved oxygen. The dissimilarities in the structure indicate that there are differences in the chemical mechanisms that are working to form these deposits. Nevertheless, more investigations are required to understand better the difference in deposit formation mechanisms in fuels with different dissolved oxygen levels.34

acids) moieties than during ambient storage as a result of the severity of oxidation. The oxidation of the fuel under an inert atmosphere was presumably due to the presence of dissolved oxygen already in the sample. Nevertheless, the oxidative degradation process can be considered similar under an inert atmosphere and at ambient storage, because the initial concentrations of dissolved oxygen and gum precursors in the fuel are the same.98 The spectra of filterable insoluble gum, which formed during the ambient and accelerated aging, are shown in Figure 20. The

6. CONCLUSION AND PERSPECTIVE The present review contextualized gum formation on fuel stability concepts and gave some elements to sketch out a general macromolecular character of the gum structure. Second, a general oxidation mechanism involved in gum formation followed by two applied examples was presented. Even if the exact mechanisms of the reactions were not fully known, it was generally agreed that they involve a series of free radical chain processes. These processes were mainly dependent upon gasoline composition (hydrocarbon groups, dissolved oxygen content, transition metal contamination, and addition of solvent), related to petrol origin, type of refinement, and storage conditions (duration, temperature, and surface interaction). Consequently, expensive antioxidizers and metal deactivators were added to gasoline to delay the process. Moreover, considering only the gum formation issue, it was even possible to state that the addition of alcohol to gasoline seemed to be beneficial for this issue, because it reduced the gum deposit in the engine by the dilution effect. Finally, the experimental tools available to measure the gum content, such as the ASTM standard, were described, showing that accelerated oxidation conditions were used to give an indication of the behavior of the blend. The limitations of these tests were also discussed by a critical analysis of their influence on the chemistry of gum formation. If influencing factors and consequences on the engine of gum formation are well-known at the macroscopic scale, the microscopic mechanisms, detailed kinetics, and exact gum composition and structure remain poorly understood. More studies have to be performed on mechanistic and kinetic aspects to determine how gum precursors act. Among them, contribution of ethanol at a low concentration [parts per million (ppm)] needs to be better investigated because the oxygen available could contribute to oxidation of unsaturated hydrocarbons. All this information would allow for an adjustment of the gasoline production policy. Nevertheless, as

Figure 20. IR spectra of filterable insoluble gums isolated from diesel fuel aged by (a) method 1 (1 year and ambient) and (b) method 2 (O2 sparge, 95 °C, and 16 h).98

spectra of the gums for all of the fuels were similar and present CO, Ar CC(−O), C−O, and C−H moieties. They also showed a broad absorption associated with the bonded O−H or N−H moieties. The Ar CC(−O) and characteristic aromatic ether C−O moieties are the major functional groups in the gum, indicating the oxidation of carbon atoms. Consequently, carbonyl stretching has a relatively low intensity. These data indicate that the insoluble gum may have contained aromatic nuclei extensively cross-linked by ether bridges. Such products could be formed by oxidative coupling reactions of trace phenolic constituents in the fuel.98 The spectral shapes of filterable insoluble gum formed during the accelerated test are similar to the ambient aging sample. However, the bands for Ar CC(−O) and C−O moieties have a much lower intensity than C−H stretching. This indicates that the overall degree of oxidation and the proportion of oxygenated aromatic species in the insoluble gum formed in accelerated conditions were substantially lower 7767


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(15) Morrell, J. C.; Dryer, C. G.; Lowry, C. D.; Egloff, G. Gum in Cracked Gasoline: Formation and Composition. Ind. Eng. Chem. 1936, 28, 465−469. (16) Yabroff, D. L.; Walters, E. L. Gum Formation in Cracked Gasolines. Ind. Eng. Chem. 1940, 32, 83−88. (17) Walters, E. L.; Minor, H. B.; Yabroff, D. L. Chemistry of Gum Formation in Cracked Gasoline. Ind. Eng. Chem. 1949, 41, 1723−1729. (18) Fathoni, A. Z.; Batts, B. D. A literature review of fuel stability studies with a particular emphasis on shale oil. Energy Fuels 1992, 6, 681−693. (19) Batts, B. D.; Fathoni, A. Z. A literature review on fuel stability studies with particular emphasis on diesel oil. Energy Fuels 1991, 5, 2− 21. (20) Hazlett, R. N. MONO1: Thermal Oxidation Stability of Aviation Turbine Fuels; ASTM International: West Conshohocken, PA, 1991; Petroleum Products, Lubricants, and Fossil Fuels Collection, DOI: 10.1520/MONO1-EB. (21) Pereira, R. C.; Pasa, V. M. Effect of mono-olefins and diolefins on the stability of automotive gasoline. Fuel 2006, 85, 1860−1865. (22) Mayo, F. R.; Lan, B. Y. Gum and deposit formation from jet turbine and diesel fuels at 130°C. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 333−348. (23) Alves, J. Estudo do Desempenho Antioxidativo de um Novo Composto Derivado do Cardanol Hidrogenado Aplicado á Gasolina ́ Automotiva; Departamento de Engenharia Quimica, Universidade Federal do Rio Grande do Norte: Natal, Brazil, 2006; Research Monograph PRH 14/ANP. (24) Dantas, M. Obtençaõ de Novos Derivados de β-Naftol e Cardanol Hidrogenado e Avaliaçaõ dos seus Efeitos Antioxidativos em Gasolina Automotiva. D.Sc. Thesis, Pós-Graduaçaõ em Engenharia ́ Quimica, Universidade Federal do Rio Grande do Norte, Natal, Brazil, 2005. ́ (25) Queiroz, L. D. Sintese e Estudo da Eficiência Antioxidante de ́ Novos Aditivos Aplicados a Combustiveis; Departamento de Engenharia ́ Quimica, Universidade Federal do Rio Grande do Norte: Natal, Brazil, 2004; Research Monograph PRH 14/ANP. (26) Gonçalo, A. C. Estudo da Eficiência de Novos Aditivos na Gasolina Através da Cromatografia Gasosa; Departamento de Engenharia ́ Quimica, Universidade Federal do Rio Grande do Norte: Natal, Brazil, 2004; Research Monograph PRH 14/ANP. (27) Waynick, J. A. The Development and Use of Metal Deactivators in the Petroleum Industry: A Review. Energy Fuels 2001, 15, 1325− 1340. (28) Schwartz, F. G.; Whisman, M.; Allbright, C.; Ward, C. Storage Stability of GasolineFundamentals of Gum Formation Including a Discussion of Radiotracer Techniques; U.S. Bureau of Mines, U.S. Department of the Interior: Washington, D.C., 1964; Bulletin 626. (29) Ackermann, L. Gum Formation in Cracked Gasolines Polymerization of Cyclopentadiene. Period. Polytech., Chem. Eng. 1969, 13, 29−39. (30) Kawahara, F. K. Composition of Gum in Cracked Naphtha. Ind. Eng. Chem. Prod. Res. Dev. 1965, 4, 7−9. (31) Heneghan, S. P.; Zabarnick, S. Oxidation of jet fuels and the formation of deposit. Fuel 1994, 73, 35−43. (32) A’reff, H. Effect of the Copper and Iron Concentration on Gum Formation in Gas Oil of Baiji Refiner. J. Chem. Chem. Eng. 2011, 5, 652−656. (33) de la Puente, G.; Sedran, U. Formation of Gum Precursors in FCC Naphthas. Energy Fuels 2004, 18, 460−464. (34) Roan, M. A.; Boehman, A. L. The Effect of Fuel Composition and Dissolved Oxygen on Deposit Formation from Potential JP-900 Basestocks. Energy Fuels 2004, 18, 835−843. (35) Beaver, B.; Gilmore, C.; Veloski, G.; Sharief, V. Role of indoles in the oxidative degradation of unstable diesel fuels. The ferric and cupric ion catalyzed oxidation of 1,2,3,4-tetrahydrocarbazole as a model compound study. Energy Fuels 1991, 5, 274−280. (36) Beaver, B. D.; Treaster, E.; Kehlbeck, J. D.; Martin, G. S.; Black, B. H. Kinetic Study of the Oxygenation of 2,5-Dimethylpyrrole. A Model Compound Study Designed To Probe Initiation of the

a result of the high complexity of the gasoline mixture, this requires a large analytical work to determine the exact structure of gum and residual gasoline, such as GC−MS24,99 and pyrolysis/FIMS,96 already used to characterize auto-oxidation products. Another option encountered in the literature to study the kinetics is the use of a chemometrical model based on middle range IR (MIR) spectroscopy.100 Moreover, it is necessary to improve the correlation of gum formation using available experimental analyses, such as ASTM standards, to the different storage conditions and different gasolines. Development of new accelerated tests and measurement of new parameters, such as absorbed oxygen content, can be a part of the solution.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +55-21-3527-1708. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are indebted to CNPq/MCT, CAPES, FAPERJ, and FINEP for the financial support to DEM at PUC-Rio. The authors thank Peugeot Citroën do Brasil Automóveis Ltda for the financial support to this paper.

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REFERENCES

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