Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Full Length Article Study on the separation and thin film deposition of tarry aromatics mixtures (soot extract and naphthalene pitch) by high-vacuum heating B. Apicella a, ⁎ , A. Tregrossi a , C. Popa b , V. Mennella b , A. Ciajolo a , C. Russo a a Istituto di Ricerche sulla Combustione, IRC-CNR, P.le Tecchio 80, 80125 Napoli, Italy b Osservatorio Astronomico di Capodimonte – INAF, Napoli, Italy ARTICLE INFO Keywords: Soot extract Naphthalene pitch Tarry materials Structural characterization Thin film PAH ABSTRACT Tarry mixtures derived from coal and heavy fuel processing, organic synthesis or formed in combustion systems are complex mixtures of organic, mainly aromatic, species with molecular weight spanning from few hundreds up to thousands of mass units. Many different diagnostics are required for their characterization, nevertheless the speciation of the huge number of aromatic molecules is only partially effective, avoiding reaching the complete map of the tar composition. Understanding of composition and structures of tars from different sources and processes would lead to a greater comprehension of their possible transformation in carbons relevant in combustion and environmental fields as well as for material production. To this regard, the pre-separation in lighter and heavier fractions simplifies the further characterization of tar composition. In the present work a fractionation method based on moderate heating in high-vacuum conditions (10 -6 mbar) was tested on a synthetic naphthalene pitch and on a flame-formed tar, typically extracted from soot (soot extract), getting information on the components distribution and characteristics by means of chro- matography, mass spectrometry and spectroscopy. Although the fractionation of light and heavy PAH compo- nents was not thorough even in the case of soot extract, the method appeared successful in separating tars in classes with narrower MW distribution, allowing to get further insights on their characterization. The lighter components obtained by condensation/deposition as thin films were analyzed by polarized light microscopy to infer their self-organization in cluster assembly and/or crystal forms. 1. Introduction The speciation of the huge number of aromatic components of tars and pitches derived from coal and heavy fuel processing or formed in fuel-rich combustion, is not straightforward because of the tar compo- sitional complexity as well as the rather wide and high molecular weight (MW) range spanning from few hundreds up to thousands of mass units. Indeed, the tars structure and composition determine their transformation in carbons relevant in many scientific and technological fields so that the determination of tarry materials composition is an analytical problem shared by scientific communities involved in dif- ferent research fields like combustion, environment, material science etc [1–8]. Gas chromatography–mass spectrometry (GC–MS), conventionally used for the qualitative and quantitative analysis of polycyclic aromatic hydrocarbons (PAHs) content, allows detecting and identifying at most 50 wt% of tars typically formed in combustion systems and often ac- companying soot formation and emission [9]. It can be supposed that the lack of identification is due to the presence of high-MW aromatic species, not analysable with the conventional chromatographic and mass spectrometric techniques. FT-IR absorption showed the presence of aliphatic hydrogen in spite of the very few alkyl-substituted PAHs and the complete absence of aliphatic species as found by GC–MS analysis. Therefore, the high-MW species were supposed to be con- stituted of large aromatic ring systems interconnected by aliphatic bridges, as also suggested by fluorescence emission and UV–Visible spectroscopy [10,11]. Beside to be itself less amenable to be analyzed, the heavier fraction of tars is especially difficult to be analyzed when in admixture with light components. Hence, the pre-separation of tar in lighter and hea- vier fractions should simplify the further characterization of its com- position. The choice of the pre-separation method has to take into ac- count for the scarce solubility and thermal degradability of heavier components. Actually, many methods as solvent extraction, thin layer or size exclusion chromatography did not succeed a complete separa- tion of PAHs from the matrix, presumed to be constituted of an ali- phatic-aromatic material [12–18]. In the present work, the moderate heating in high-vacuum http://dx.doi.org/10.1016/j.fuel.2017.08.014 Received 19 March 2017; Received in revised form 25 July 2017; Accepted 2 August 2017 ⁎ Corresponding author. E-mail address: apicella@irc.cnr.it (B. Apicella). Fuel 209 (2017) 795–801 Available online 07 August 2017 0016-2361/ © 2017 Elsevier Ltd. All rights reserved. MARK