Article pubs.acs.org/JPCA Modeling Oil Shale Pyrolysis: High-Temperature Unimolecular Decomposition Pathways for Thiophene AnGayle K. Vasiliou,*,† Hui Hu,‡ Thomas W. Cowell,† Jared C. Whitman,† Jessica Porterfield,∥,§ and Carol A. Parish‡ † Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, United States ‡ Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, Virginia 23713, United States § Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States ABSTRACT: The thermal decomposition mechanism of thiophene has been investigated both experimentally and theoretically. Thermal decomposition experiments were done using a 1 mm × 3 cm pulsed silicon carbide microtubular Δ → reactor, C4H4S+ Products. Unlike previous studies these experiments were able to identify the initial thiophene decomposition products. Thiophene was entrained in either Ar, Ne, or He carrier gas, passed through a heated (300−1700 K) SiC microtubular reactor (roughly ≤100 μs residence time), and exited into a vacuum chamber. The resultant molecular beam was probed by photoionization mass spec- troscopy and IR spectroscopy. The pyrolysis mechanisms of thiophene were also investigated with the CBS-QB3 method using UB3LYP/6-311++G(2d,p) optimized geometries. In particular, these electronic structure methods were used to explore pathways for the formation of elemental sulfur as well as for fi the formation of H2S and 1,3-butadiyne. Thiophene was found to undergo unimolecular decomposition by ve pathways: C4H4S → − − (1) S C CH2 + HCCH, (2) CS + HCCCH3, (3) HCS + HCCCH2, (4) H2S + HCC CCH, and (5) S + HCC CH fi CH2. The experimental and theoretical ndings are in excellent agreement. 1. INTRODUCTION levels should not exceed 75 ppb.10 To comply with these regulations, sulfur must be removed from fuels during the Energy production is one of the most challenging issues of fi modern times. Fossil fuels are still the most widely used source re ning process. The most commonly used industrial process of energy in the world, accounting for 81% of the world energy to reduce sulfur content in petroleum is hydrodesulfurization 1 15 ff share, half of which comes from petroleum. As global energy (HDS). Aromatic organosulfur compounds are less e ectively demand is expected to rise 35% by 2035, the utilization of catalyzed under HDS conditions and often require higher alternate forms of energy, such as shale oil, will also need to pressures and temperatures to be effectively removed, making increase.1 In order to obtain fuels that will burn cleanly, sulfur them a nuisance to the refining process.16 compounds must be removed from gaseous, liquid, and solid Thiophene is one of the more abundant aromatic organo- products.2 Shale oils are typically rich in both sulfur and organic sulfur contaminants found in fuels such as petroleum and shale compounds, as sulfates accumulate in sedimentary basins along gas.3,16 Thiophene is also the simplest molecule in the class of with oil producing planktons. Organic matter in shale oil can be thiophenic compounds, one of the four main chemical motifs up to 3.1% sulfur by weight, while mineral components can be fi 3 that make up organosulfur fuel contaminants (sul des, up to 2.6% sulfur by weight. disulfides, thiols, and thiophenic). As a result, understanding fl Combustion emissions of sulfur oxides strongly in uence the the thermal decomposition of thiophene is especially important chemistry of the atmosphere, which adversely affects air quality − as thiophene is not only present in fuels but also acts as a model and human health.4 8 Air pollution in the United States is compound for an entire class of thiophene derivatives. Notably, regulated by the Clean Air Act, which enables the EPA to set air thiophene is an aromatic organosulfur compound of high innate quality standards for six criteria of air pollutants, one of which is 4 stability, making it more difficult to remove during typical sulfur dioxide, SO2. Human exposure to the airborne pollutant ff desulfurization methods, such as hydrodesulfurization.15 For all SO2 has unfavorable e ects on immune capable cells and airway responsiveness. The health effects from sulfurous air pollution of these reasons, it is important to better understand thiophene − have been well documented.9 14 All population subgroups are ff a ected by SO2, including the most vulnerable: children, Received: July 31, 2017 adolescents, cardiac- and respiratory-compromised individuals, Revised: September 13, 2017 and asthmatics.5 As such the EPA maintains that sulfur dioxide Published: September 14, 2017 © 2017 American Chemical Society 7655 DOI: 10.1021/acs.jpca.7b07582 J. Phys. Chem. A 2017, 121, 7655−7666 The Journal of Physical Chemistry A Article Figure 1. Schematic of the pulsed microtubular reactor used for thiophene thermal cracking. Samples were collected in a 5 K cryogenic matrix for analysis by infrared absorption spectroscopy or photoionized with fixed frequency vacuum ultraviolet (VUV) light. and its decomposition mechanism, as it is expected to play a Torr. Upon shock heating, acetylene was found to be the major critical role in the combustion and processing of fuels. product at all temperatures; however, ethene, ethanethiol, Despite the important role of thiophene in industrial hydrogen sulfide, carbon disulfide, and several oligomers were processes and combustion, a detailed pyrolysis mechanism is also detected.19 Based on the experimental findings, Ur still unclear. To our knowledge, the current mechanism for the Rahman Memon et al. suggested the thermal decomposition thermal decomposition of thiophene is based on end point of thiophene was initiated by the homolytic cleavage of the C− chemistry, meaning that the reaction mechanisms have been S bond followed by ring opening and isomerization. Hore et al. proposed without direct evidence of reactive intermediates. conducted a laser pyrolysis study on thiophene and other five- 20 This knowledge gap hinders any progress in desulfurization membered ring compounds. They suggested that a 1,2 methodology since current efforts for improving thiophene hydrogen transfer is the most probable initiation step in 20 removal technologies are done with an incomplete picture of thiophene decomposition. the molecular level chemistry. The results from this study can In this study, the thermal decomposition of thiophene was be used in engineering simulations in order to model and investigated using a pulsed microtubular reactor and two ff predict the success of new or improved methodologies for di erent methods of detection. The products of the reaction desulfurization. were monitored using 118.2 nm (10.487 eV) vacuum One of the earliest studies of the decomposition of thiophene ultraviolet photoionization mass spectrometry (PIMS) and was conducted by Wynberg and Bantjer in a continuous flow matrix isolation IR spectroscopy. Within the microtubular reactor. A Vycor glass tube was heated to 1073−1123 K, and a reactor, thiophene seeded in an inert carrier gas is rapidly 17 heated to 1000−1300 K and decomposition is initiated. The stream of thiophene was passed through at a rate of 5 mL/h. − μ From this they identified three isomeric dithiophenes as well as relatively short residence time in the reactor (50 100 s) carbon disulfide, free carbon, hydrogen sulfide, and other ensures that the observed chemistry emphasizes the unim- hydrocarbons. Although this experiment identified many of the olecular processes excluding all but the most rapid bimolecular potential products formed from the decomposition of chemistry. High-level ab initio methods were used to identify and thiophene, the bimolecularity inherent to the liquid phase fi allows for the formation of many additional compounds, con rm decomposition pathways associated with the observed making it difficult to determine the initial decomposition experimental spectra. products. More recent continuous flow reactor studies by Winkler et al. carried out the pyrolysis of thiophene in a quartz 2. EXPERIMENTAL AND COMPUTATIONAL METHODS continuous flow reactor heated to 1373 K at atmospheric A high-temperature pulsed microtubular reactor (or hyper- pressure with a reactor residence of 20 s. Under these thermal nozzle) was used to decompose thiophene. The conditions a variety of products were identified, including microtubular reactor is a version of the Chen-Ellison reactor methane, benzene, hydrogen sulfide, and hydrogen.18 Winkler that has been used for several years to produce reactive − et al. concluded that the thermal decomposition of thiophene intermediates.21 28 The hyperthermal nozzle features a (1 mm was initiated by a C−H bond cleavage. ID × 3 cm long) SiC tube that can be heated up to 1700 K, Thiophene decomposition has also been studied using shock with the temperature monitored by a type C thermocouple tube methods. Ur Rahman Memon et al. used a single pulse mounted to the outer wall of the SiC tube. A benefit of the stainless steel shock tube filled with 50 Torr of 0.5% thiophene microtubular reactor is the short residence time (50−100 μs).29 in argon separated from the helium driver gas at 1598−2022 This short residence time eliminates problems one might face 7656 DOI: 10.1021/acs.jpca.7b07582 J. Phys. Chem. A 2017, 121, 7655−7666 The Journal of Physical Chemistry A Article in a typical vacuum pyrolysis experiment, including secondary were confirmed to have all real frequencies while transition − reactions.30 36 states were confirmed