Fractionation by liquid chromatography combined with comprehensive two-dimensional gas chromatography–mass spectrometry for analysis of cyclics in oligomerisation products of Fischer–Tropsch derived light alkenes

Kinetic Properties Study of H Atom Abstraction by CH3Ȯ2 Radicals from Fuel Molecules with Different Functional Groups

The detailed kinetic properties of hydrogen atom abstraction by methylperoxy (CH₃Ȯ₂) radicals from alkanes, alkenes, dienes, alkynes, ethers, and ketones are systematically studied in this work. Geometry optimization, frequency analysis, and zero-point energy corrections were performed for all species at the M06-2X/6-311++G(d,p) level of theory. The intrinsic reaction coordinate calculation was consistently performed to ensure that the transition state connects the correct reactants and products, and one-dimensional hindered rotor scanning results were performed at the M06-2X/6-31G level of theory. The single-point energies of all reactants, transition states, and products were obtained at the QCISD(T)/CBS level of theory. High-pressure-limit rate constants of 61 reaction channels were calculated using conventional transition state theory with asymmetric Eckart tunneling corrections over the temperature range of 298.15-2000 K. Reaction rate rules for H atom abstraction by CH₃Ȯ₂ radicals from fuel molecules with different functional groups are constructed, which can be used in the development of combustion models of these fuels and fuel types. In addition, the influence of the functional groups on the internal rotation of the hindered rotor is also discussed.

Profiling Olefins in Gasoline by Bromination Using GC×GC-TOFMS Followed by Discovery-Based Comparative Analysis

An analytical workflow for the analysis of olefins in gasoline that combines selective bromination and comprehensive two-dimensional (2D) gas chromatography time-of-flight mass spectrometry (GC×GC-TOFMS) with discovery-based analysis is reported. First, a standard mix containing n-alkanes, 1-alkenes, and aromatic species was brominated and quantified using % reacted as a metric for each compound class, defined as the difference in the total peak area between the brominated and original samples normalized to the original sample. The average % reacted (1 s.d.) values were -1.45% (2.8%) for the alkanes, 99.5% (0.4%) for the alkenes, and 6.7% (11.6%) for the aromatics, demonstrating excellent selectivity toward the alkenes with only minor aromatic bromination. The bromination chemistry was then applied to gasoline, followed by GC×GC-TOFMS analysis of the original and brominated gasoline. This GC×GC-TOFMS data set was then submitted to the supervised discovery tool tile-based F-ratio analysis (FRA), which reduced the large data set to only the chromatographic regions which distinguish between the original and brominated gasoline samples. FRA discovered 314 hits, 56 of which were determined statistically significant using combinatorial null distribution analysis (CNDA), a permutation-based significance test. Since the brominated olefins elute in an uncrowded region of the 2D chromatogram and have no signal in the original sample, their discoverability was greatly increased relative to the original olefins. By combining the information gained from brominated olefin standards and the structured patterns of the GC×GC separations, the top hits were identified as the dibromoalkane products of mono-olefins, with five C5 mono-olefins identified on a species level. The analytical workflow has broad implications for using selective reaction chemistries to facilitate supervised discovery by targeting desired compound classes.

Insights into Dodecenes Produced from Olefin Oligomerization Based on Two-Dimensional Gas Chromatography-Photoionization-Time of Flight Mass Spectrometry and Multivariate Statistics

Catalyzed light olefin oligomerization is widely used in petrochemical industries to produce fuels and chemicals. Light olefins such as propene and butenes are commonly selected as feedstocks. Solid phosphoric acid (SPA) and zeolite are representative acidic catalysts. Both the feedstocks and catalysts have an impact on the product composition. In this study, state-of-the-art instrumentation two-dimensional gas chromatography (GC × GC) coupled photoionization-time of flight mass spectrometry was employed to investigate the composition of dodecene products produced from olefin oligomerization. Information such as the olefin congener distribution, dodecene structural subgroup distribution, and individual dodecene isomers was obtained and utilized in the statistical analyses. By using specific data sets of the product composition, the distinguishment between SPA and zeolite catalysts as well as among the feedstocks was achieved by applying the unsupervised screening approaches (principal component analysis and hierarchical clustering analysis). The potential indicators of catalysts and feedstocks were selected by the feature selection methods (univariate analysis: analysis of variance and multivariate analysis: partial least squares-discriminant analysis).

Unraveling the Complex Olefin Isomer Mixture Using Two-Dimensional Gas Chromatography-Photoionization-Time of Flight Mass Spectrometry

Commercial dodecenes are a complex chemical mixture with a majority of C₁₂ olefins and minority of C_8-18 olefins. Structurally, dodecene products may consist of straight-chain alkenes, branched alkenes, as well as cyclic hydrocarbons. Due to the difference of feeds and catalysts used in the oligomerization reaction, the composition of the dodecenes is complex and their properties are very different. Knowing the complex composition of dodecenes can help tune the production process and select the appropriate products according to their end use. To reveal the complex profile of dodecenes, an analytical method using two-dimensional gas chromatography (GC×GC) coupled photoionization (PI) - time of flight mass spectrometry (TOFMS) was developed in this study. A conventional (nonpolar × polar) column combination (non-polar column as 1^st dimension and mid-polar column as 2^nd dimension) was selected. The analytical condition of GC was optimized using fractional factorial experimental design (DoE). Olefin congener grouping by carbon chain length and double bond equivalent (DBE) was achieved based on the detection of molecular ions by PI-TOFMS. Grouping of dodecenes by linear, mono-branched, di- and tri-branched subgroups was achieved based on branching index (BI) under the assumption of no retention time (RT) overlap among subgroups. Certain dodecene isomers were identified by retention index (RI) and further confirmed by PI mass spectra. The information altogether provided a multimodal characterization possibility to be used with statistical tools. Principal component analysis (PCA) and hierarchical clustering analysis (HCA) of seventeen dodecene samples explained the composition variance between catalysts solid phosphoric acid and zeolite, as well as between feeds with C₄ and without C₄.

The pre-separation of oxygen containing compounds in oxidised heavy paraffinic fractions and their identification by GC-MS with supersonic molecular beams

The heavy petroleum fractions produced during refining processes need to be upgraded to useable products to increase their value. Hydrogenated heavy paraffinic fractions can be oxidised to produce high value products that contain a variety of oxygenates. These heavy oxygenated paraffinic fractions need to be characterised to enable the control of oxidation processes and to understand product properties. The accurate identification of the oxygenates present in these fractions by electron ionisation (EI) mass spectrometry is challenging due to the complexity of these heavy fractions. Adding to this challenge is the limited applicability of EI mass spectral libraries due to the absence of molecular ions from the EI mass spectra of many oxygenates. The separation of oxygenates from the complex hydrocarbon matrix prior to high temperature GC-MS (HT-GC-MS) analysis reduces the complexity of these fractions and assists in the accurate identification of these oxygenates. Solid phase extraction (SPE) and supercritical fluid chromatography (SFC) were employed as prefractionation techniques. GC-MS with supersonic molecular beams (SMBs) (also named GC-MS with cold-EI) utilises a SMB interface with which EI is done with vibrationally cold sample compounds in a fly-through ion source (cold-EI) resulting in a substantial increase in the molecular ion signal intensity in the mass spectrum. This greatly enhances the accurate identification of the oxygenates in these fractions. This study investigated the ionisation behaviour of oxygenated compounds using cold-EI. The prefractionation by SPE and SFC and the subsequent analysis with GC-MS with cold-EI were applied to an oxygenated heavy paraffinic fraction.