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Distribution of Total Sulfur in Acidic, Basic and Neutral Fractions on Brazilian Asphalt Cements and its Relationship to the Aging Process Leandro M. de Carvalho, Paulo C. do Nascimento, Denise Bohrer, Luis E. Claussen, Luis Ferraz, Carla Grassmann, Hugo T. S. Braibante, Daiane Dias, Margareth Cravo, and Leni F. Mathias Leite Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5025643 • Publication Date (Web): 24 Feb 2015 Downloaded from http://pubs.acs.org on February 28, 2015

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Energy & Fuels

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Distribution of Total Sulfur in Acidic, Basic and Neutral Fractions on

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Brazilian Asphalt Cements and its Relationship to the Aging Process

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Leandro M. de Carvalho*a, Paulo C. do Nascimentoa, Denise Bohrera, Luís E. Claussena, Luis Ferraza,

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Carla Grassmann a, Hugo T. S. Braibantea, Daiane Diasb, Margareth Cravoc and Leni F. M. Leitec

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a

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Brazil

9

b

10

c

11

PETROBRAS, Rio de Janeiro, RJ, Brazil

Departamento de Química, Universidade Federal de Santa Maria, 97110-970, CP 5051, Santa Maria, RS,

Departamento de Química, Universidade Federal do Rio Grande, Rio Grande, RS, Brazil Centro de Pesquisas e Desenvolvimento “Leopoldo Américo Miguez de Mello” (CENPES),

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AUTHOR INFORMATION:

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Phone/FAX: +55-32208870. E-mail:[email protected]

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ACS Paragon Plus Environment

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ABSTRACT: In this work, studies were performed in Brazilian asphaltic cements by

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separating fractions of maltenes and asphaltenes using non-aqueous ion exchange liquid

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chromatography (NIELC). Initially, three different aging processes (RTFOT, PAV and

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SUNTEST) were applied to 23 different asphaltic cements for evaluating the changing of

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asphaltenic (12.7±2.5 %) and maltenic

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process was studied in a comparative way for 5 asphalt cements samples, since it caused the more

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pronounced changes in maltenes and asphaltenes among the asphaltic cements supplied by

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different Brazilian refineries. Asphaltenes presented the highest amount of polar compounds in all

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the studied samples, herein attributed to compounds of acidic and basic character that contain

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sulfur. It was also found that the aging of asphalt cement leads to an increase of the more reactive

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acidic (9.5−18.9%) and basic (14.6−30.9%) fraction in both asphaltenes and maltenes. As a result,

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a reactivity index (Ir) for asphaltic cements was calculated and proposed for classifying the studied

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samples regarding their aging susceptibility. It was also observed that the less reactive asphaltic

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cement presented the highest amount (>80%) of neutral sulfur compounds in maltenic fraction.

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Lastly, it was concluded that a higher content of total sulfur in asphalts does not imply necessarily

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in a higher chemical reactivity (susceptibility to aging) of the cement, as observed for 5 studied

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samples having total sulfur ranging from 1.8 to 3.4% (w/w). The presence of functional groups

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related to their specific heteroatoms, such as sulfur, suggest a possible way to explain a greater or

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lesser susceptibility of asphalts to aging.

(90.6±5.0 %) compounds. The RTFOT+SUNTEST

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Keywords: Asphalt cements; total sulfur distribution; aging processes, SUNTEST, reactivity

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index.

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Energy & Fuels

1.

INTRODUCTION

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The speciation of sulfur in asphalt cements has becoming an important issue from

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scientific and technological viewpoints, since sulfur compounds are considered precursors of

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asphalt oxidation reactions. In this context, it is already known that changes in the

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physicochemical and rheological properties of asphalt cements are caused mainly by alteration in

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the chemistry of sulfured functional groups, the viscosity and the balance of polar/nonpolar

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groups.1 Among the processes involved in the asphalt aging, oxidation is considered the most

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significant one, since it takes place during the application of the wearing course and continues

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slowly during its pavement service.2 Beyond the formation of oxygenated compounds, the polar

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groups present in the asphalt cement tend to associate, forming micelles and agglomerates of high

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molecular weight and higher viscosity (asphaltenes).

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It is well known that oxidation processes in asphalts involve mainly the heteroatom sulfur

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by oxidation of e.g. sulfides to sulfoxides.1,2 Beyond oxidation processes involving sulfur

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compounds, carbonyl formation associated to e.g. naphthenes are involved in the chemical

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transformations taking place during asphalt aging.1 Chemical transformation of polar fractions in

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asphalts involve also the formation of ketones and alcohols concurrently with sulfoxides, being

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alcohols the main contributors to oxidative aging of asphalts with high content of sulfur.3

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Furthermore, ketones are formed primarily from the oxidation of benzyl carbons in side chains

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attached to highly condensed aromatic ring systems, which exist largely in the polar aromatics

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fraction of asphalts.4,5 Beyond the organic compounds containing basically sulfur, nitrogen and

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oxygen as heteroatoms, metallorganic compounds, such as vanadyl porphyrins, have been also

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described for asphaltic matrices and are also involved in the aging processes.6,7

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In this context, chemical transformations involving specific classes of sulfur compounds,

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such as thiophenes, sulfides, sulfoxides and sulfones of acidic, basic and neutral character were

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studied by Green, Payzant and co-workers.8−13 According to these work, there is a distribution of

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species according to their acid-base character in each respective class of sulfur compounds. These

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species characterize the more polar and, consequently, more reactive fraction of asphalt cements

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and can be directly involved in the aging mechanism. However, the understanding of the

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mechanisms associated with specific sulfur classes and their transformations during the aging has

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been investigated so little depth up to date.

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Speciation studies of sulfur compounds in asphalt have been described in the literature for

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asphaltic matrices produced in different countries.8−13 Although Brazil is one of the important

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producers of petroleum and petroleum-derived products worldwide, systematic studies focused on

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the physicochemical characterization of the asphalt cements produced in the refineries spread over

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the country have not been performed in depth up to date. Furthermore, the knowledge of sulfur

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species as markers of aging and quality indicators of asphalt cement is highly relevant, since it can

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contribute to establishing of criteria for the production of more resistant asphaltic mixtures and

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their additives as well.

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The aging process of asphalt cements have been evaluated by different analytical methods,

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which use techniques merely for specification studies or elucidation/understanding of the asphalt

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cement behavior. A typical method used for the characterization of asphalt cements is the liquid

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chromatography using silica columns (IATROSCAN), which identifies and quantifies the more

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generic chemical fractions (SARA: Saturates, Aromatics, Resins and Asphaltenes). Gel

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permeation chromatography (GPC) is used for separating the compounds according to the

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apparent molecular weight and the infrared spectroscopy (FTIR) identifies some functional groups

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(e.g. carbonyls and sulfoxides) formed after oxidation.14−17 Atomic force microscopy (AFM) has


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been recently applied to studies of physical properties of asphalts submitted to aging processes.18,

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19

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In this work, studies were performed in asphalt cements by separating different fractions in

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order to understand their formation, especially regarding sulfur compounds. Initially, the

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separation of virgin asphalt cements was carried out for asphaltenes and maltenes by using the

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method described in ASTM D 4124.20 The next step consisted in a second fractionation, where

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both maltenes as asphaltenes were separated into acidic, basic and neutral fractions. For this

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purpose, the method described by Green and co-workers9 was adapted and applied to samples of

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Brazilian asphalt cements. The separations based on both the mentioned methods were made

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comparatively for virgin and aged asphalt cements. Thus, total sulfur was determined in the acidic,

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basic, and neutral asphalt fractions submitted to the different aging processes. Thus, the

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distribution of the compound classes in asphaltenic and maltenic fractions was obtained. The

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distribution profile for these compound classes was observed in asphaltenes and maltenes, which

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gave information on chemical transformations occurring during the aging process of asphalt

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cements. Considering the studied total sulfur distribution, a reactivity index (Ir) for 5 different

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asphaltic cements was calculated and proposed for classifying the studied samples regarding their

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aging susceptibility.

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2.

EXPERIMENTAL SECTION

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2.1 Instrumentation and Apparatus

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All asphalt cement samples were heated at 100 ° C for 60 minutes in an oven provided of

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air circulation (Q314 M - Quimis) before sampling the matrix for separations in order to keep the


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sample in a homogeneous form. The sample was then weighted for applying in the separation

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systems by NIELC.9

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2.2 Reagents

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Ion-exchange resins MP-1 (anionic) and PM-50 (cationic) were obtained from Biorad

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Laboratories (Richmond, CA, USA) with particle sizes of 200-400 mesh (37-75 µm). These resins

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were used for separation of acidic, basic and neutral sulfur compounds by non-aqueous ion

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exchange liquid chromatography (NIELC).9 Methanol, isooctane, benzene, cyclohexane, ethyl

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ether, propanol, toluene, n-pentane, sulfuric acid, dichloromethane, sodium hydroxide were

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obtained from Merck (Damrstadt, Germany). Propylamine was obtained from Sigma-Aldrich (St.

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Louis, MO, USA).

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2.3 Samples

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A total of 23 samples of asphaltic cements were analyzed in this work for asphaltenes and

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maltenes content. Five samples of asphalt cements were studied before and after the three aging

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processes (described below) and analyzed in a comparative way for speciation purposes. The

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samples were supplied by five different refineries in Brazil and analyzed as received.

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2.4 Analytical procedures

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2.4.1 Separation of asphaltenes and maltenes

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The separation of asphaltenic and maltenic fractions was carried out according to the

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ASTM D4124.20 Briefly, the sample was weighted and refluxed in isooctane (100 mL for each

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gram of sample). The mixture was heated and stirred once the reflux was initiated. The

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temperature was maintained at 99 °C by stirring for 1-2 h, until no observation of adherence of the

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asphalt matrix at the glassware. Once the asphalt cement was dissolved, the solution was stirred

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for 1 h longer. The solution was cooled and stirred for 2 h and the condensator was washed from

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top to bottom with 10-20 mL isooctane. After, this step, the solution was kept at resting for 2 h

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before filtration. The solution was filtered using vacuum and the residues of asphaltenes adhered

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to the glassware was washed with 100 mL isooctane by refluxing for 30 min. The filtration cake

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was washed with isooctane until it became colorless. The filtrate represents the soluble fraction

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(maltenes) and the residue of the funnel represents the insoluble fraction (asphaltenes). The

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obtained filtrate (maltenes) was evaporated in order to eliminate the solvent residue. Both the solid

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fractions (asphaltenes and maltenes) were weighted for calculating the mass balance of the

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process.

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2.4.2 Separation of acidic, basic and neutral compounds by NIELC

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The fractionation of asphalt matrix in acidic, basic and neutral compounds was carried out

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by using the NIELC method developed by Green et al.9 Briefly, 2.5 g of virgin asphalt cement was

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dissolved in ciclohexane and applied to a separation system containing two columns composed of

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a cationic and an anionic ion-exchange stationary phase connected in series. The columns were


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previously cleaned and activated according to the procedure. After applying the sample to the two

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column systems, ciclohexane (100 mL) was added and the neutral fraction was obtained. The

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columns were disconnected and treated separately. 10 mL of methanol was added to each

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disconnected columns and the aliquots were collected separately for using in the respective

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extraction procedure. The column containing the cation exchange resin was extracted in a Soxhlet

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system for 10 h with benzene and formic acid (10 mL) in order to obtain the acidic fraction. The

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column containing the anionic exchange resin was extracted in another Soxhlet system for 10 h

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with benzene and propylamine (10 mL) in order to obtain the basic fraction. After the extraction

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processes, the products were evaporated and weighted for determining the mass of the fraction.

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2.4.3 Aging process of asphaltic cements

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Three different procedures were used for simulating the aging process of asphalt cements.

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The Rolling Thin Film Oven Test (RTFOT) simulates the short term aging of asphalt during the

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hot mix, truck and silo storage, spray and compaction process, which takes place during its

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application. For this purpose, the assay is carried out at high temperatures and in the presence of

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oxygen (ASTM D 2872).21 The Pressure Aging Vessel (PAV) test was developed by the Strategic

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Highway Research Program (SHRP) and simulates the aging of the asphaltic binder in situ, i.e.,

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during its use as a road pavement at service temperatures. In the long term procedure, the asphalt

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sample is normally first conditioned using the RTFOT method. Thus, the residue from the RTFOT

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is placed in standard stainless steel pans and aged at the specified conditioning temperature for 20

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h in a vessel pressurized with air to 2.10 MPa. The conditioning temperature is selected according

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to the grade of asphalt cement. The residue is lastly vacuum degassed (ASTM D 6521).22 The

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SUNTEST aging procedure simulates a natural weathering of the asphalt cements under influence


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of UV radiation in pavement service.2 Herein, asphalt cements were also first conditioned using

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the RTFOT method and then submitted to UV radiation test (48 hours at 60ºC).

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2.4.4 Determination of total sulfur

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In order to approach the possible role of sulfur compounds present in acidic, basic and

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neutral asphalt fractions, the determination of total sulfur in the obtained fractions were carried out

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by combustion of the samples based on the determination of SO2 performed by infrared

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spectroscopy (ASTM D1552).23 All the samples of asphalt cements and its fractions were

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dissolved in toluene prior to the analysis.

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3. RESULTS AND DISCUSSION

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The chemical characterization of asphalt cements is a very important issue, since it is

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directly related to the physical performance of asphalts. However, the chemical speciation of

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sulfur compounds in maltenic and asphaltenic fractions and its relationship with the mechanisms

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that take place during the aging of asphalts has been little explored up to date. Thus, the

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distribution of sulfur compounds in different samples of asphalt cements produced by different

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Brazilian refineries was investigated. In a first step, 23 samples were characterized according to

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the distribution of asphaltenes and maltenes. In a second step, five samples were randomly

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selected for speciation studies of sulfur compounds and its relationship with the aging processes.

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The 23 samples investigated in this work showed a distribution of asphaltenes and

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maltenes with a significant higher amount of maltenes (90.6±5.0%) compared to asphaltenes

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(12.7±2.5%), as expected and previously reported for other asphaltic products.1,15,24,25 Thus, after


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characterizing the samples according to their asphaltenic and maltenic fractions, the speciation

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studies were conducted in order to elucidate the distribution of sulfur compounds within these

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fractions.

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3.1 Speciation of acidic, basic and neutral sulfur compounds in asphaltenes and maltenes

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According to Green,8−11 the fractionation of asphalt cements by ion exchange liquid

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chromatography

in

non-aqueous

medium

(NIELC)

is

based

on

the

obtaining

of

224

positively/negatively charged and neutral compounds using cationic and anionic exchange resins.

225

According to Payzant,12,13 the fractionation of dissolved asphalt cements in the ion exchange

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resins lead to the obtaining of acidic, basic and neutral fractions, which consist of the following

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majority sulfur compound classes: sulfoxides, sulfones, sulfides, thiophenes, mercaptans, and

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sulfonic acids. Even being present in a variety of structures, the conditions during formation and

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maturation of a petroleum reservoir can lead to the existence of some preferential forms. All these

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species can also be present as aliphatic or aromatic forms. Regardless the form they are present,

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sulfur compounds can be associated with three different oxidation states in asphalts: S(−2):

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sulfides, disulfides, thiols and thiophenes; S(+2): sulfoxides; and S(+4): sulfones and sulfonic

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acids. Therefore, the chemical speciation of sulfur in asphalt cements can involve both their

234

oxidation states (redox speciation) and their binding forms (surrounding chemical environment).

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The sulfur charged structures separated in the ion exchange resins9 may be defined as the

236

most reactive fraction, which is more susceptible to undergo oxidation, agglomeration, and

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condensation reactions.15,24,25-28 As shown in Table 1, both the asphaltenic and maltenic fractions

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in Brazilian asphalts are composed mostly of neutral compounds containing sulfur. After

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fractionation of maltenes and asphaltenes into acidic, basic and neutral subfractions, we observed

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a similar pattern in the total sulfur distribution. Despite the observed measurement deviations, the


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neutral fraction was found to be the majority fraction in maltenes and asphaltenes. This behavior

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was observed for a total of 23 samples analyzed, considering merely the gravimetric determination

243

of the solid products obtained after fractionating the acids, bases and neutral species.

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TABLE 1

246 247

The acidic, basic and neutral compounds containing sulfur present in asphalts allows us to

248

establish that: (1) most of the compounds present in asphalt cements (in both asphaltenes and

249

maltenes) are of neutral character (less reactive species); (2) acidic and basic compounds (more

250

reactive species) represent a minority fraction of asphalt cements. Furthermore, asphaltenes

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presented the highest amount of polar compounds (25.3−50.9%) in the studied samples, herein

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attributed to acidic and basic compounds that contain sulfur.

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Considering a mechanistic approach based on the established sulfur organic chemistry, the

254

compounds of acidic and basic character found in asphaltenes and maltenes may have in their

255

structures the heteroatoms sulfur, nitrogen and oxygen. Possible structures of sulfur-containing

256

acidic compounds are associated mainly to aliphatic and aromatic positively charged species.

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Besides sulfur, the presence of nitrogen in these structures may also contribute to the acidic

258

character of these compounds as a protonated group.29 Regarding the compounds of basic

259

character, possible structures may be associated mainly to aliphatic and aromatic negatively

260

charged species. Besides sulfur, the presence of nitrogen and oxygen in these structures should

261

also contribute to the basic character of the compounds. Thus, the basic character may be linked

262

directly to the sulfur heteroatom (e.g. sulfoxides, sulfonates, thiols and mercaptans) as well as the

263

concomitant presence of sulfur and nitrogen/oxygen heteroatoms in the structure of each

264

individual speciated compound.8,30−32


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By making a rationalization from the sulfur organic chemistry and previous work reporting

266

data from other asphalts,8, 30−32 one can postulate that sulfur and nitrogen containing compounds of

267

amphoteric character may also be present in both separated acidic and basic forms, depending on

268

the apparent pH of the media and pKa of each species. As hypothetical example, sulfur structures

269

such as sulfilimines and sulfoximines29 may be formed depending on the environmental

270

conditions. Likewise, compounds of acidic and basic character can be protonated or deprotonated

271

and, consequently, acquire the majority neutral form depending on the medium conditions. All

272

these possible acid-base equilibriums occurring in the medium of dissolved asphalt cement may

273

explain why sulfur functionalities defined as e.g. sulfides, thiophenes, and sulfoxides are almost

274

omnipresent in acidic, basic and neutral fractions.8 Additionally, acidic and basic funcionalities

275

such as carboxylic acids, phenols, indoles, carbazoles, amides, pyrroles, pyrimidines, pyridines,

276

quinolines, acridines, and phenazines may be present in these fractions30−32 separated by NIELC.

277

Moreover, the knowledge of the chemical speciation associated with sulfur and its surrounding

278

heteroatoms (N and O) is well recognized as the key to understanding the processes of conversion

279

and migration that take place during the aging of asphalts.9,10,26

280 281

3.2

Aging of asphalt cements and the speciation of sulfur compounds

282 283

The aging process of asphalt cements occurs mainly due to loss of mass by volatilization and

284

oxidation.1,9 These processes can significantly vary depending on the different sources of asphalt

285

cements. Other factors such as temperature, light effect, chemical reactions with aggregates may

286

also contribute to this process.1 According to some authors,8,24 asphalts should have a balanced

287

amount of polar and nonpolar molecules, which would lead to the homogeneity of the binder.

288

Furthermore, the balance of these compound classes plays a key role in the aging susceptibility

289

and performance of the asphalt binder, so that any chemical imbalance in this mixture may


12

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influence on its final quality.1

291

It is also well known that the aging of asphalts influences and alters the composition of

292

asphaltenes and maltenes in different ways.1,9,14,15,26 So an increased asphaltenic fraction and a

293

decreased maltenic fraction are expected after aging. Thus, the distribution of asphaltenic and

294

maltenic fractions in 5 virgin asphalt samples submitted to three aging processes (RTFOT,

295

RTFOT+PAV and RTFOT+SUNTEST) was investigated, which were lately studied for the

296

speciation involving sulfur compounds. As expected (figure 1), all the studied aging processes

297

lead to an increase in the asphaltenic fraction (figure 1A) in all the 5 samples of asphaltic cements.

298

These changes involve the formation of compounds high apparent molecular weight from

299

processes such as agglomeration and condensation.1,28 Furthermore, these new high apparent

300

molecular weight compounds may contain sulfur mainly in their higher oxidation states (+II and

301

+IV) forming sulfoxides and sulfones.9,33 For maltenes (figure 1B), it was observed that the

302

behavior was almost constant in the RTFOT aging, causing a small decrease in the RTFOT+PAV

303

aging and a more pronounced decrease in the RTFOT+SUNTEST process.

304 305

FIGURE 1

306 307

The subsequent separation of acidic (AF), basic (BF) and neutral (NF) fractions was

308

performed in maltenes and asphaltenes in order to identify the variations involving these classes

309

during the aging process. Taking into account that the RTFOT+SUNTEST process lead to the

310

more pronounced alteration in asphalt cements (figure 1), the 5 investigated samples were

311

analyzed in a comparative way for total sulfur distribution considering this process. It should be

312

also pointed out that the RTFOT+SUNTEST process takes into consideration also the effect of

313

sunlight in the asphalt aging, what is relevant for asphaltic cements used in pavements of tropical

314

countries. Figure 2 shows the behavior of sulfur compounds in virgin and RTFOT+SUNTEST


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aged samples in a comparative way for asphaltenes. As can be seen, the average percentage of

316

acidic, basic and neutral fractions showed an increase of acid (AF) and basic (BF) sulfur

317

compounds, while the neutral compounds (NF) decreased.

318 319

FIGURE 2

320 321

Furthermore, an overall increase of about 22% of the most reactive fractions (AF + BF) in

322

asphaltenes after aging is probably related to the oxidation, condensation and agglomeration

323

reaction of sulfur compounds, which may also partially migrate from maltenes. In contrast, the

324

neutral fraction (FN) decreased by about 23%. In the case of maltenes (figure 3), the behavior of

325

sulfur compounds after aging is similar to that shown for asphaltenes. As can be seen, the average

326

percentage of acidic, basic and neutral fractions for 5 samples showed an overall increase of ~32%

327

in the more reactive fractions, while the neutral fraction (NF) decreased ~30% after aging by the

328

RTFOT+SUNTEST process.

329 330

FIGURE 3

331 332

The loss of neutral compounds in both the asphaltenic and maltenic fractions may be also

333

associated with volatilization, agglomeration and condensation reactions forming sulfur, carbon

334

and nitrogen compounds of higher polarity.14,28 These reactions can lead also to the transformation

335

of aromatics into resins and the resins into asphaltenes, resulting in a continuing increase of heavy

336

fractions14,28 and the consequent hardening of the asphalt cement. However, the aging of asphalts

337

according to these well-known majority processes was not yet specifically explained in terms of

338

distribution of sulfur compounds of acid-base character in asphaltenes and maltenes. Furthermore,

339

the observed transformations of neutral aromatics into polar compounds (resins) may be related to


14

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Energy & Fuels

340

ring-open reactions observed for thiophenic fractions within the studied samples. For supporting

341

this hypothesis, a dominant process was observed in this work involving thiophenes separated by

342

the method proposed by Payzant and co-workers.13 After performing these experiments in 5

343

different asphaltic cements, a decrease of thiophenic compounds in the range 24.3−48.2% after

344

aging by RTFOT+PAV process22 was observed.

345 346

3.3 Speciation of sulfur compounds and the aging susceptibility of asphalt cements

347 348

The aforementioned and discussed results for acidic, basic and neutral fractions of

349

maltenes and asphaltenes may also be approached in terms of chemical reactivity of the sample.

350

Herein, polar compounds may confer a higher reactivity to the asphalt involving their more active

351

sites (acidic and basic functionalities). Therefore, the sum of the most reactive fractions (AF + BF)

352

divided by the less reactive fraction (NF) is proposed as a calculation index for reactivity of the

353

asphaltic cements. Thus, the sample that presents the highest reactive fraction will be the one,

354

which is more susceptible to aging. This calculation can be expressed according to the equation:

355 356 357

Ir = AFa + BFa + AFm + BFm NFa + NFm

358 359

where Ir is the reactivity index, AFa and BFa the basic and acidic fractions of asphaltenes;

360

AFm and FBm are the acidic and basic fractions of maltenes; and NFm and NFa the neutral fractions

361

of asphaltenes and maltenes, respectively. This calculation considers the sum of the more reactive

362

fractions divided by the sum of the less reactive fractions of the whole asphalt (maltenes +

363

asphaltenes). Table 2 summarizes the results in terms of reactivity of the asphalt cement compared

364

to the content of total sulfur and asphaltenes for the 5 studied virgin and aged asphalt samples. The

365

reactivity index can be also calculated specifically to each fraction of asphaltenes and maltenes


15

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Page 16 of 28

366

containing AF, BF and NF. Herein, a similar distribution of the samples was achieved concerning

367

their reactivity and aging susceptibility.

368

As can be seen, a higher concentration of total sulfur in asphalts does not imply necessarily

369

in a higher reactivity (susceptibility to aging) of the cement. Therefore, the chemical speciation

370

regarding the classes of sulfur compounds in the asphalt structure play a more important role and

371

may count lastly for a higher or lower resistance of the asphalt to the aging. Herein, compounds of

372

acidic and basic character seem to be crucial for understanding these processes and for quantifying

373

the chemical stability of the asphalt cement according to chemical parameters as well. So these

374

results suggest a classification of asphalt cements in terms of their susceptibility to aging

375

(reactivity) based on the chemical speciation of sulfur compounds, which is normally based on

376

physical parameters. Furthermore, this knowledge would allow the production of more stable

377

binder mixtures.

378 379

The classification of the studied samples considering their susceptibility to aging based on the chemical reactivity (calculated as Ir) can be expressed as follows:

380 381 382

Asphalt cement samples:

A1 < A3 < A2 < A4 < A5 Reactivity

383 384

According to the reactivity classification regarding the total sulfur distribution and

385

asphaltenes in virgin asphalt samples, there is a high similarity between asphaltene composition

386

and reactivity based on sulfur compounds. So sample A1 presented a lower reactivity and lower

387

content of asphaltenes, whereas sample A5 is the one with higher reactivity and higher content of

388

asphaltenes (table 2). However, it can vary from sample to sample. This situation suggests that a

389

higher content of asphaltenes may be related, but not necessarily imply in a higher reactivity of the

390

asphalt cement, as can be observed for samples A4 and A5 (table 2). This would depend on the

391

sample, which has specific functional groups related to their heteroatoms (S, N and O),


16

Page 17 of 28

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Energy & Fuels

392

characterizing a greater or lesser susceptibility of the cement to aging. Furthermore, it was

393

observed that the less reactive sample (A1) presented the highest amount (>80%) of neutral sulfur

394

compounds in maltenic fraction of asphalt. Considering that this sample suffered the higher

395

variation of Ir after aging (table 2), one can infers that the most important chemical transformation

396

in the asphaltic cement occurred in maltenes involving all neutral compounds. Considering that

397

oxidative aging is also a time-dependent process, asphalt cements having neutral compounds as a

398

majority class seem to be more resistant to aging by RTFOT+SUNTEST process. Additionally,

399

the neutral fraction of maltenes has probably amphoteric compounds that may be involved in the

400

chemical transformations taking place during aging. Furthermore, sample A1 presented the higher

401

variation of asphaltene content (+3.0%) after aging (table 2). Since this sample presented the

402

lower asphaltene content (virgin sample), the higher variation of Ir after aging is related mostly to

403

the increasing of acidic and basic sulfur compounds in this fraction, which may partially migrate

404

from maltenes as oxidized compounds (e.g. sulfoxides and sulfones). As can be also observed in

405

table 2, sample A5 suffered less pronounced transformations in asphaltenic fraction (+0.1%), what

406

is probably related to a high initial concentration of polar compounds (Ir = 1.18) present as

407

asphaltenes in this cement. Beyond compounds containing sulfur, ketone formation has been also

408

identified as a major factor that lead to asphaltene formation, being asphaltenes the primary

409

responsible for viscosity increase on asphalt aging.34,35 In addition, it is already known that the

410

formation of ketone functional groups changes the polarity and solubility of the associated

411

condensed aromatic ring compounds, so that they can agglomerate and become part of the

412

asphaltene fraction.3

413 414 415 416


17

Energy & Fuels

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417

4.

Page 18 of 28

CONCLUSIONS

418 419

Asphalt cement is a matrix of highly complex constitution, which depends also on their

420

origin. The results discussed in this work were important for deepening the knowledge about the

421

influence of sulfur chemical speciation on the asphalt quality. Thus, the RTFOT+SUNTEST

422

process was studied in a comparative way for 5 asphalt cements samples, since it was the process

423

that caused the more pronounced changes in maltenes and asphaltenes among 23 different samples

424

supplied by different refineries. It was found that the aging of asphalt cement leads to an increase

425

of the more reactive fractions of sulfur (acidic and basic) in both asphaltenes and maltenes. In

426

addition, a decrease in the less reactive fraction (neutral) was observed in both asphaltenes and

427

maltenes. This study also showed the importance of other heteroatoms, such as nitrogen and

428

oxygen, bound to sulfur (or surrounding it) in the asphalt chemical structure, what may influence

429

on the susceptibility of the asphalt cement to aging. It was observed that sulfur appears in larger

430

amounts in the less reactive fraction (neutral) in both maltenes and asphaltenes. Herein, a

431

classification of the studied samples was proposed taking into account the reactivity calculated in

432

terms of acidic, basic and neutral compounds. It was also observed that the less reactive sample

433

(A1) presented the highest amount (>80%) of neutral sulfur compounds in maltenic fraction of

434

asphalt. Therefore, the presence of functional groups related to their specific heteroatoms seemed

435

to be definitive to explain a greater or lesser susceptibility of asphalts to aging.

436

Lastly, this study contributed to a greater understanding on the aging processes in asphalt

437

cements regarding the speciation of sulfur compounds. This understanding is necessary to improve

438

the quality of the final product, allowing the production of cements that could be more resistant to

439

aging as well as the development of processes to retard it. Moreover, the establishment of specific

440

markers within the sulfur compounds (e.g. aryl/alkyl sulfidic or tiophenic acids and bases) that


18

Page 19 of 28

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Energy & Fuels

441

could define an asphalt binder of greater or lesser aging resistance might be studied from this

442

approach on.

443 444

ACKNOWLEDGEMENTS

445 446

The authors acknowledge the financial support provided by the foundations CNPq and

447

CAPES. The authors also greatly acknowledge the financial support from PETROBRAS through

448

the Center of Research and Development “Leopoldo Américo Miguez de Mello” (CENPES).

449 450 451

REFERENCES

452 453

(1)

Siddiqui, M. N; Ali, M. F. Studies on the aging behavior of the Arabian asphalts. Fuel 1999, 78, 1005−1015.

454 455

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Whiteoak, D. SHELL Bitumen handbook. SHELL, England, 1990.

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Petersen, J. C.; Glaser, R. Asphalt oxidation mechanisms and the role of oxidation products on age hardening revisited. Road Mat. Pav. Design 2011, 12, 795−819

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(4)

asphalts. Anal. Chem. 1974, 46, 2242−2246.

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Petersen, J. C. A dual, sequential mechanism for the oxidation of petroleum asphalts. Petrol. Sci. Technol. 1998, 16, 1023−1051.

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Dorrence, S. M.; Barbour, F. A.; Petersen, J. C. Direct evidence for ketones in oxidized

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Aleshin, G. N.; Altukhova, Z. P.; Antipenko, V. R.; Marchenko, S. P.; Kam’yanov, V. F.

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Distribution of vanadium and vanadyl porphyrins in petroleum distillates of different

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chemical types. Petrol. Chem. U.S.S.R. 1984, 24, 191-195.


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Zhao, X.; Liu, Y.; Xu, C.; Yan, Y.; Zhang, Y.; Zhang, Q.; Zhao, S.; Chung , K.; Gray, M. R.;

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Shi, Q. Separation and characterization of vanadyl porphyrins in Venezuela Orinoco heavy

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crude oil. Energ. Fuels 2013, 27, 2874–2882.

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Types in Asphalt. Energ. Fuel 1993, 7, 119−126.

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Green, J. B.; Yu, S. K. T.; Pearson, C. D.; Reynolds, J. W. Analysis of Sulfur Compound

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Green, J. B.; Hoff, J.; Woodward, P. W.; Stevens, L. L. Separation of liquid fossil fuels

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into acid, base and neutral concentrates: 1. An improved nonaqueous ion exchange method.

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Fuel 1984, 63,1290− −1301.

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Green, J. A.; Green, J. B.; Grigsby, R. D.; Pearson, C. D.; Reynolds, J. W.; Shay, J. Y.;

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Sturm Jr., G. P.; Thomson, J. S.; Vogh, J.W.; Vrana, R. P.; Yu, S. K-T.; Diehl, B. H.;

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Grizzle, P. L.; Hirsch, D. E.; Hornung, K. W.; Tang, S-Y.; Carbognani, L.; Hazos, M.;

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Sanchez, V. Analysis of Heavy Oils: Method Development and Application to Cerro Negro

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Heavy Petroleum. Topical Report NIPER-452, NTIS No. DE90000201; NTIS: Springfield,

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VA, 1989; Chapter 4.

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Green, J. A.; Green, J. B.; Grigsby, R. D.; Pearson, C. D.; Reynolds, J. W.; Shay, J. Y.;

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Sturm Jr., G. P.; Thomson, J. S.; Vogh, J. W.; Vrana, R. P.; Yu, S. K-T.; Diehl, B. H.;

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Grizzle, P. L.; Hirsch, D. E.; Hornung, K. W.; Tang, S-Y.; Carbognani, L.; Hazos, M.;

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Sanchez, V. Analysis of Heavy Oils: Method Development and Application to Cerro Negro

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Heavy Petroleum. Topical Report NIPER-452, NTIS No. DE90000200; NTIS: Springfield,

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VA, 1989; Chapter 3.

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1986, 9, 357−369.

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Payzant, J. D.; Montgomery, D. S.; Strausz O. P.. Sulfides in petroleum. Org. Geochem.

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Payzant, J. D.; Mojelsky, T. W.; Strausz, O. P. Improved Methods for the Selective

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Isolation of the Sulfide and Thiophenic Classes of Compounds from Petroleum. Energ.

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Fuels 1989, 3, 449−454.


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Mat. 2002, 16, 15−22.

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Lu, X.; Isacsson, U. Effect of ageing on bitumen chemistry and reology. Constr. Build.

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Qin, Q.; Schabron, J. F.; Boysen, R. B.; Farrar, M. J. Field aging effect on chemistry and

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rheology of asphalt binders and rheological predictions for field aging. Fuel 2014, 121,

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of environmental aging on Colombian asphalts. Fuel 2014, 115, 321–328.

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Lee, S. J.; Amirkhanian, S. N.; Shatanawi, K.; Kim, K. W. Short-term aging

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characterization of asphalt binders using gel permeation chromatography and selected

499

Superpave binder tests. Constr. Build. Mater. 2008, 22, 2220−2227.

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Introducing two new techniques. Fuel 2014, 118, 365–368.

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Fischer, H. R.; Dillingh, E. C. On the investigation of the bulk microstructure of bitumen –

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Rebelo, L. M.; de Sousa, J. S.; Abreu, A. S.; Baroni, M. P. M. A.; Alencar, A. E. V.;

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Soares, S.A.; Mendes Filho, J.; Soares, J. B. Aging of asphaltic binders investigated with

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atomic force microscopy. Fuel 2014, 117, 15–25.

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ASTM D4124-09. Standard Test Method for Separation of Asphalt into Four Fractions.

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ASTM D2872. Standard Test Method for Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin Film Oven Test).

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Pressurized AgingVessel (PAV).

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ASTM D1552. Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method).

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ASTM D6521-08. Standard Pratice for Accelerated Aging of Asphalt Binder Using a

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Zhang, F.; Yu, J.; Han, J. Effects of thermal oxidative aging on dynamic viscosity,

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TG/DTG, DTA and FTIR of SBS- and SBS/sulfur-modified asphalts. Constr. Build. Mater.

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Mastrofini, D.; Scarsella, M. The application of rheology to the evaluation of bitumen ageing. Fuel 2000, 79, 1005-1015.

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during the oxidative aging of two selected asphalts from different sources. Petroleum &

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Coal 2008, 50, 1−10.

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asphaltenes from heavy petroleum feedstocks. Fuel 1984, 63, 141−146.

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chemical reactivity of two kinds of polymer modified asphalts during thermal degradation.

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Chem. Eng. J. 2011, 176-177, 231−36.

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York, 1977, p. 232−602.

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Dutta, P. K.; Holland, R. J. Acid-base characteristics of petroleum asphaltenes as studied by non-aqueous potentiometric titrations. Fuel 1984, 63, 197−201.

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Oae, S. Organic Chemistry of Sulfur. University of Tsukuba, Japan. Plenum Press, New

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Okuno, I.; Latham, D. R.; Haines, W. E. Type Analysis of Nitrogen in Petroleum Using

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Nonaqueous Potentiometric Titration and Lithium Aluminum Hydride Reduction. Anal.

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Chem. 1965, 37, 54−57.

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asphaltite and oil shale pyrolysis products. J. Anal. Appl. Pyrol. 1990, 17, 227−235.

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Önen, A.; Sarac, A. S. Nonaqueous potentiometry for analyses of nitrogen bases from

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Waldo, G. S.; Mullins, O. C.; Penner-Hahn, J. E.; Cramer, S. P.; Determination of the

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chemical environment of sulphur in petroleum asphaltenes by X-ray absorption

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spectroscopy. Fuel 1992, 71, 53−57.

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(34) Petersen, J. C. A review of the fundamentals of asphalt oxidation – chemical,

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22

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Lin, M. S.; Linsford, K. M.; Glover, C. J.; Davidson, R. R.; Bullin, J. A. The effects of

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asphaltenes on the chemical and physical characteristics of asphalts, Asphaltenes:

542

Fundamentals and Applications, E.Y. Shen and O.C. Mullins, eds., Plenum Press, N.Y.,

543

1995, p. 155-176.

544 545 546

Captions to the figures

547 548

Figure 1 – Distribution pattern (n = 3) of asphaltenes (A) and maltenes (B) in asphalts submitted

549

to the aging processes RTFOT, RTFOT+PAV and RTFOT+SUNTEST compared to a virgin

550

cement sample.

551 552

Figure 2 – Distribution pattern (n = 3) of sulfur in acidic, basic and neutral compounds in

553

asphaltenes of virgin and aged asphalts (N = 5). AF: Acidic fraction; BF: Basic fraction; NF:

554

Neutral fraction. All the results are expressed as the average values related to 5 different asphalt

555

cements (described in section 2.3).

556 557

Figure 3 – Distribution pattern (n = 3) of sulfur in acidic, basic and neutral compounds in

558

maltenes of virgin and aged asphalts (N = 5). AF: Acidic fraction; BF: Basic fraction; NF: Neutral

559

fraction. All the results are expressed as the average values related to 5 different asphalt cements

560

(described in section 2.3).


23

Energy & Fuels

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561

Page 24 of 28

Table 1. Total sulfur distribution regarding acidic, basic and neutral compounds in asphalt fractions (maltenes and asphaltenes) Sulfur distribution (%) a

Sample Acidic compounds b

Basic compounds c

Neutral compounds d

Asphaltenes

Maltenes

Asphaltenes

Maltenes

Asphaltenes

Maltenes

A1

14.6

6.6

22.1

5.1

63.3

88.3

A2

10.8

24.1

14.5

22.4

74.7

53.5

A3

12.3

13.9

15.8

24.6

71.9

61.5

A4

23.5

15.7

24.1

20.5

52.4

63.8

A5

17.9

26.6

33.0

30.4

49.1

43.0

562

a

Mean values (n=3); b RSD (Relative standard deviation; n=3): 21.8−28.3%; c RSD (Relative standard deviation; n=3): 25.0−31.0%;

563

d

RSD (Relative standard deviation; n=3): 18.9−30.8%

564

24 ACS Paragon Plus Environment

Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

565

Table 2. Characterization of virgin and aged asphaltic cements regarding the content of total sulfur, reactivity index (Ir) and asphaltenes.

566

Sample

Total S (%) a

567

Ir

Asphaltene content (%)

Virgin asphalt

Aged asphalt b

Virgin asphalt

Aged asphalt b

568

A1

2.2

0.32

2.23

8.6

11.6

569

A2

3.4

0.56

2.09

10.2

12.7

570

A3

3.1

0.50

1.46

11.0

13.8

571

A4

1.8

0.72

1.27

14.5

15.0

572

A5

3.1

1.18

1.72

13.4

13.5

573

a

RSD (Relative standard deviation; n=3): 8.0−15.0%

574

b

Asphalt cements aged by RTFOT+SUNTEST methods


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575 576 577

Figure 1

578

579 580


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Energy & Fuels

581 582 583

Figure 2

584

585 586 587


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588 589 590 591

Figure 3

592

593


Distribution of Total Sulfur in Acidic, Basic, and ... - ACS Publications

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