Article pubs.acs.org/EF
Structural Level Characterization of Base Oils Using Advanced Analytical Techniques Nadim Hourani,*,† Hendrik Muller,‡ Frederick M. Adam,‡ Saroj K. Panda,‡ Matthias Witt,§ Adnan A. Al-Hajji,‡ and S. Mani Sarathy*,† †
Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Jeddah 23955-6900, Saudi Arabia ‡ Research and Development Center, Saudi Aramco, P. O. Box 62, Dhahran 31311, Saudi Arabia § Bruker Daltonik GmbH, Fahrenheitstrasse 4, 28359 Bremen, Germany S Supporting Information *
ABSTRACT: Base oils, blended for finished lubricant formulations, are classified by the American Petroleum Institute into five groups, viz., groups I−V. Groups I−III consist of petroleum based hydrocarbons whereas groups IV and V are made of synthetic polymers. In the present study, five base oil samples belonging to groups I and III were extensively characterized using high performance liquid chromatography (HPLC), comprehensive two-dimensional gas chromatography (GC×GC), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) equipped with atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) sources. First, the capabilities and limitations of each analytical technique were evaluated, and then the availed information was combined to reveal compositional details on the base oil samples studied. HPLC showed the overwhelming presence of saturated over aromatic compounds in all five base oils. A similar trend was further corroborated using GC×GC, which yielded semiquantitative information on the compound classes present in the samples and provided further details on the carbon number distributions within these classes. In addition to chromatography methods, FT-ICR MS supplemented the compositional information on the base oil samples by resolving the aromatics compounds into alkyl- and naphtheno-subtituted families. APCI proved more effective for the ionization of the highly saturated base oil components compared to APPI. Furthermore, for the detailed information on hydrocarbon molecules FT-ICR MS revealed the presence of saturated and aromatic sulfur species in all base oil samples. The results presented herein offer a unique perspective into the detailed molecular structure of base oils typically used to formulate lubricants.
1. INTRODUCTION Lubricant oils are essential for operating machinery in the transportation and industrial sectors where they are used to protect, seal, cool, and clean automotive engine and gear oils as aviation and marine engine oil and as transmission and hydraulic fluids. An estimated 1.2% of the total petroleum is used as lubricant oil.1 The increasing demand for higher capacity lubricants is driven by expanding industrial plants that require optimum friction reductions. Typically finished engine lubricants are composed of a base oil (90 vol %) with synthetic additives (10 vol %). The base oil is essentially a complex mixture of saturated hydrocarbon molecules (n-paraffins, isoparaffins, and naphthenes) that are generally refined from heavy petroleum fractions such as vacuum gas oils (VGOs) via solvent extraction, hydrotreating, and hydroisomerization plus less abundant aromatic hydrocarbon molecules. In addition to hydrocarbon molecules (molecules consisting of only carbon and hydrogen atoms), heteroatom (sulfur and nitrogen)containing species may be present.2 Base oils are classified into five groups by the American Petroleum Institute (API).3 Group I base oils are composed of fractionally distilled petroleum that is further refined using solvent extraction processes while group II base oils distillates are refined via hydrocracking and hydroisomerization. Group III base oils have been further hydrocracked and isomerized to © XXXX American Chemical Society
produce a mixture with a higher viscosity index (VI) which is a measure of how viscosity changes with temperature. Higher VI base oils remain stable and retain their viscous properties across a wider range of operating temperatures. Groups IV and V such as polyalphaolefins (PAOs) and polyalkylene glycols (PAG oils), are fully synthetic lubricants. The finished lubricant oil used in engines is formulated by mixing various grades of base oils with a specific set of additives to achieve specific performance targets whereby the properties and performance of the finished lubricants are highly dependent on the base oil selection. Lubricant degradation caused by oxidation and thermal decomposition at increased temperatures alters the original content. The influence of a base oil’s composition on its oxidative stability and other physical properties were discussed by Sharma et al.4 For example, the presence of nitrogen-containing compounds or polyaromatic hydrocarbons has a negative impact on the base oil’s stability whereas the presence of a sulfur-containing class is considered a factor that inhibits oxidation.5 Understanding the complex molecular composition of these hydrocarbon mixtures by mapping their distribution in various crude oil fractions can help tune refinery Received: January 8, 2015 Revised: March 25, 2015
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DOI: 10.1021/acs.energyfuels.5b00038 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Typical Bulk Content Assay of the Base Oil Samples group
samples
sulfur, %
nitrogen, %
aromatics, %
saturates, %
GI G III
BO-1, BO-2, BO-3 BO-4, BO-5
