Theoretical and Experimental Studies on the Thermal Cracking of

VOLUME 21, NUMBER 1

JANUARY/FEBRUARY 2007

© Copyright 2007 American Chemical Society

Articles Theoretical and Experimental Studies on the Thermal Cracking of Tetrahydrothiophene Daohong Xia,*,† Yongliang Tian,† Genquan Zhu,† Yuzhi Xiang,† Liwen Luo,† and Thomas T.-S. Huang‡ College of Chemistry and Chemical Engineering, State Key Laboratory of HeaVy Oil, China UniVersity of Petroleum, 257061 Dongying, China, and Department of Chemistry, East Tennessee State UniVersity, Johnson City, Tennessee 37614-0695 ReceiVed March 1, 2006. ReVised Manuscript ReceiVed October 17, 2006

As a representative compound of cyclic sulfides, tetrahydrothiophene in different solvents was thermally decomposed under several high temperatures in a microreactor. The observed sulfur-containing product distribution reveals that the solvent used has a large effect on the sulfur distribution. When benzene was the solvent, dihydrothiophene, thiophene, and hydrogen sulfide were the main sulfur compounds produced in the decomposition process. However, only hydrogen sulfide was the main product when tetrahydrothiophene in tetralin was thermally decomposed. 1,3-Butadiene is the significant component among all of the hydrocarbons produced. Plausible decomposition pathways leading to the observed products were investigated by quantum mechanical calculations using the B3LYP density functional theory for geometry optimization and QCISD(T) for energy evaluation. On the basis of the calculated results, favorable decomposition pathways were proposed.

Introduction There is a great increase in the production of heavier crude oils that contain a significant amount of fuel-bound sulfur and nitrogen in recent years. The combustion of petroleum products containing sulfur and nitrogen produces SOx and NOx, which ultimately lead to environmental pollution. High sulfur content also causes a problem in the refinery processes such as sulfurrelated equipment corrosion. It has been an important subject of research in the petroleum industry to prevent these problems. Even though thermal cracking has been in use for many years in the production of light oil and special chemicals and, in some refineries, the process has been replaced or combined with other processes, it still plays important roles in refineries. For hydrocarbons, there have been many studies of the mechanisms, kinetics, and product distributions in thermal cracking.1,2 Aribike * To whom correspondence should be addressed. Telephone: 8605468395783. Fax: 8605468391971. E-mail: [email protected]. † China University of Petroleum. ‡ East Tennessee State University.

and Susu researched the thermal cracking of n-butane and a light hydrocarbon mixture at temperatures of 700-800 °C and space times of 0.5-0.87 s and found that the pyrolysis of n-butane at high conversions was first-order kinetics.3 It is wellknown now that hydrocarbons are mainly transformed via freeradical mechanisms. However, to the best of our knowledge, the mechanistic and kinetic details of the reactions involving non-hydrocarbon product formation in petroleum processing are less well-known. The knowledge of how sulfur- and nitrogencontaining compounds are produced in the thermal-cracking process is very important, because these reactions directly affect the quality of petroleum products such as the sulfur and nitrogen contents and their distribution.4,5 (1) Albright, L. F.; Crynes, B. L.; Cororan, W. H. Pyrolysis: Theory and Industrial Practice; Academic Press: New York, 1983. (2) Nand, S.; Mann, R. S.; Sarkar, M. K. Kinetics of thermal cracking of a hydrocarbon mixture by an empty pulsed microreactor. Chem. Eng. J. 1980, 19, 251-253. (3) Aribike, D. S.; Susu, A. A. Thermal cracking of n-butane and a light hydrocarbon mixture. J. Anal. Appl. Pyrolysis 1988, 14, 37-48.

10.1021/ef060095b CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006

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Figure 3. Gas chromatogram of hydrocarbons produced from the thermal cracking of tetrahydrothiophene at 475 °C. 1, methane; 2, propylene; 3, trans-2-butene; 4, 1-butene; 5, cis-2-butene; 6, 1,3butadiene. Figure 1. Fixed-bed cracking microreactor.

Figure 2. Temperature dependence of the sulfur-containing compound distribution in thermal cracking of tetrahydrothiophene in benzene.

The thermal stabilities of sulfur-containing organic compounds, such as thiols, sulfides, and thiophenic analogues, have been researched earlier;6,7 up to now, however, there have been only a few papers on thermal conversions of these compounds. Winkler et al.8 investigated the continuous flow pyrolysis of thiophene, benzothiophene, and dibenzothiophene in the temperature range of 500-1100 °C. They found that temperature has a great effect on the pyrolysis conversion pathways of these aromatic sulfur compounds, which mainly included the molecular skeleton remaining and only C-H bond cleavage below 800 °C and skeleton fragmentation above 800 °C. The conversions of dibenzothiophene were also researched in the closed pyrolysis system by Dartiguelongue et al.9 recently, and they suggested the thermal-cracking mechanisms including the C-S (4) Birch, S. F.; Cullum, T. V.; Dean, R. A.; Denyer, R. L. Sulfur compounds in kerosine boiling range of Middle East crudes. Ind. Eng. Chem. 1955, 47, 240. (5) Nishioka, M.; Tomich, R. S. Isolation of aliphatic sulfur compounds in a crude oil by a non-reactive procedure. Fuel 1993, 72, 1007. (6) Faragher, W. F.; Morrell, J. C.; Comay, S. Thermal decomposition of organic sulfur compounds. Ind. Eng. Chem. 1928, 20, 527. (7) Morris, J. C.; Lanum, M. J.; Helm, R. V.; Haines, W. E.; Cook, G. L.; Ball, J. S. Purification and properties of ten organic sulfur compounds. J. Chem. Eng. Data 1960, 5, 112. (8) Winkler, J. K.; Karow, W.; Rademacher, P. Gas-phase pyrolysis of heterocyclic compounds, Part 1 and 2: Flow pyrolysis and annulation reactions of some sulfur heterocycles: Thiophene, benzo[b]thiophene, and dibenzothiophene. A product-oriented study. J. Anal. Appl. Pyrolysis 2002, 62, 123-141. (9) Dartiguelongue, C.; Behar, F.; Budzinski, H.; Scacchi, G.; Marquaire, P. M. Thermal stability of dibenzothiophene in closed system pyrolysis: Experimental study and kinetic modeling. Org. Geochem. 2006, 37, 98116.

Figure 4. Temperature dependence of the sulfur-containing compound distribution in thermal cracking of tetrahydrothiophene in tetralin.

bond cleavage leading to the degradation of the molecular skeleton and oligomerization of two dibenzothiophenes or dibenzothiophene with other aromatics produced in the process. Memon et al.10 reported the pyrolysis of tetrahydrothiophene in a shock tube in 2004 and found that hydrogen sulfide, carbon disulfide, thiophene, and ethyl mercaptan were the sulfurcontaining compounds produced. Clark et al.11 studied the hydrolysis and thermolysis of tetrahydrothiophene in relation to steam-stimulation processes and found that hydrogen sulfide is the main sulfur-containing compound. However, they all did not give the pyrolysis mechanism of tetrahydrothiophene at the molecular level. Recently, we focused on investigating the thermal decomposition and fluidized catalytic cracking mechanisms and product distribution of alkyl and cyclic sulfides. The results of the experimental and theoretical investigations of thermal decomposition of tetrahydrothiophene are reported in this paper. Experimental Section The thermal decomposition of tetrahydrothiophene was carried out in a fixed-bed cracking microreactor as illustrated in Figure 1. The reaction tube, which was made of stainless steel with 8 cm (10) Memon, H. U. R.; Williams, A.; Williams, P. T. A shock tube study of pyrolysis of tetrahydrothiophene at elevated temperatures. Int. J. Energy Res. 2004, 28, 581-595. (11) Clark, P. D.; Hyne, J. B.; Tyrer, J. D. Chemistry of organosulphur compound types occurring in heavy oil sands. 1. High temperature hydrolysis and thermolysis of tetrahydrothiophene in relation to steam stimulation processes. Fuel 1983, 62, 959-962.

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Scheme 1. Thermal Decomposition Pathways of Tetrahydrothiophene Leading to 1,3-Butadiene and Hydrogen Sulfide

Scheme 2. Thermal Decomposition Pathways of Tetrahydrothiophene Leading to Dihydrothiophene and Thiophene

long and 1.5 cm in inside diameter, was filled with about 2 g of 100-300 mesh spherical neutral Al2O3 as a supportor. The longer reaction tube with a thinner layer of neutral Al2O3 in the heater can generally maintain the reaction region with isothermal. The temperature of the reaction system can be controlled automatically with a thermoelectric pile, generally at the range of 450-500 °C, and the reaction pressure was about 1 atm with the reactor linked to the gas bag under atmosphere. Under this condition, the reactant was gasified immediately as soon as it was introduced into the reactor. After nitrogen was introduced into the reactor at the rate of 10 mL/min for 30 min for pulling out the air in the reactor, the reaction mixtures containing 0.5 wt % tetrahydrothiophene in benzene or tetralin were pumped into the reaction tube using a plunger pump at a rate of 0.9 mL/min. After thermal cracking of tetrahydrothiophene in the reaction tube takes place at about 2-3 s, water and more nitrogen were pumped into the reactor at the rate of 0.2 and 40 mL/min, respectively, for 30 min. Then, only nitrogen at the same rate was introduced for another 30 min to pull out all gaseous products. The purposes of the operations are that water vapor can steam the liquid products on the supporter and that the nitrogen, as the gas carrier, can pull out all of the gaseous mixtures. Because water was introduced into the reactor after the thermal reactions took place, it has no affect on the reaction and product distributions. For the analysis of the reaction products, the produced H2S, which was absorbed with 3 m % sodium hydroxide solution, was analyzed by the potential titration method. To identify the hydrocarbons in the gas product, gas samples collected in the gas bag were analyzed by Varian Star gas chromatography/flame ionization detector (GC/FID). The liquid products were collected in the ice bath and analyzed for sulfur compounds and total sulfur content with Beifen3420 gas chroma-

tography/flame photometric detector (GC/FPD), VG7070E gas chromatography/mass spectrometry (GC/MS), and the WK3 oxidative microcoulomb. All of the GC identifications of the reaction products were based on their retention times in comparison with that of the known standard substances. The chemicals used in the experiments were all of AR purity. The sulfur distribution was calculated as the ratio of the sulfur content of each compound to the total sulfur content in the feed. Most of the mass balance of sulfur in all experiments were greater than 95 m %, and others were greater than 90 m %. To elucidate mechanisms of the thermal decomposition reactions, quantum mechanical calculations were carried out. All of the equilibrium- and transition-state structures along the proposed reaction channels were optimized by Becke’s three-parameter hybrid density functional theory using Lee-Yang-Parr’s correlation functional (B3LYP).12 The 6-31G** basis set was used in geometry optimizations. Vibrational analysis was carried out for each structure to make sure that it has the desired number of imaginary vibrational frequencies. Zero-point vibrational energy (ZPE) was approximated by one-half of the sum of B3LYP/6-31G** vibrational frequencies. Relative energies of the reaction intermediates and transition states were evaluated by QCISD(T)/cc-pVDZ. All of the calculations were performed using the Gaussian94 program package.13

Results and Discussion To determine the distribution of reaction products in tetrahydrothiophene pyrolysis and the influence of temperature on the (12) Liu, R.; Huang, T. T.-S.; Tittle, J.; Xia, D. A theoretical investigation of the decomposition mechanism of pyridyl radicals. J. Phys. Chem. A 2000, 104, 8368.

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Figure 5. Prominent structural features of the transition states and intermediates of the decomposition channels in Scheme 1. The bond lengths are given in angstroms, and the bond angles are given in degrees.

rate of product formation, a series of runs were carried out covering the temperature range of 450-500 °C. Figure 2 shows the distribution of sulfur-containing products for the decomposition of tetrahydrothiophene in benzene at different temperatures. It can be seen that hydrogen sulfide, dihydrothiophene, and thiophene are the main products containing sulfur. With the increase of the cracking temperature, both the proportion of tetrahydrothiophene in the reaction mixture and the production of dihydrothiophene decrease, whereas the production of hydrogen sulfide and thiophene increase. The results demonstrate that more tetrahydrothiophene was converted to other sulfur-containing compounds at higher temperatures and that dihydrothiophene is probably an intermediate of tetrahydrothiophene decomposition. With the conversion data obtained under different temperatures, the average activation energy for the overall pyrolysis in benzene was calculated as 31.8 kJ/mol. To determine the hydrocarbon products produced in the decomposition process, the gas products obtained at 475 °C were analyzed with GC/FID. A typical chromatogram is shown in Figure 3. From Figure 3, it can be seen that 1,3-butadiene is the main product among all of the hydrocarbons produced. Figure 4 shows the sulfur product distribution of the thermal reaction of tetrahydrothiophene in tetralin. It can be seen from Figure 4 that hydrogen sulfide is the only product containing sulfur. With an increase of the cracking temperature, the proportion of hydrogen sulfide in the reaction mixture is increasing. The calculated average activation energy for the overall pyrolysis in tetralin was 61.1 kJ/mol. With the comparison of the average activation energy for the pyrolysis obtained with different solvents and the results in Figures 2 (13) Zhei, L.; Zhou, X.; Liu, R. A theoretical study of pyrolysis mechanisms of pyrrole. J. Phys. Chem. A 1999, 103, 3917.

and 4, it can be concluded that the solvent used in the experiments has a great affect on the thermal decomposition and its products. Because tetralin has a stronger hydrogendonating ability, which limited the dehydrogenation of tetrahydrothiophene, the dihydrothiophene and thiophene were not observed in the reaction. According to the experimental results, the main thermal reaction channels of tetrahydrothiophene leading to the observed products 1,3-butadiene and hydrogen sulfide and dihydrothiophene and thiophene can probably be described in Schemes 1 and 2. To investigate the feasibility and elucidate mechanistic details of the proposed thermal decomposition mechanisms, we carried out some quantum mechanical calculations. Structural features of the proposed transition states and decomposition intermediates in Schemes 1 and 2 were optimized by the B3LYP density functional theory and the 6-31G** basis set. The results are presented in Figures 5 and 6. In these figures, bond lengths are given in angstroms and bond angles are given in degrees. Energies of these structures were calculated relative to that of tetrahydrothiephene (S0), and the energy profiles of the tetrahydrothiephene decomposition pathways leading to 1,3-butadiene and thiophene investigated in this study are presented in Figure 7. Energies in this figure are those of QCISD(T)/cc-pVDZ with ZPE correction relative to that of tetrahydrothiophene in kcal/mol. In Scheme 1, there are two reaction channels from S0 to S4. The energy needed for breaking the carbon-sulfur bond to form the diradicals (S1) is 66.3 kcal/mol (Figure 7). After the carbonsulfur bond is broken, only a small additional amount of energy is needed to form S4 through a concerted hydrogen transfer transition state S2. When this pathway is compared to the one-

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Figure 6. Prominent structural features of the transition states and intermediates of the decomposition channels in Scheme 2. The bond lengths are given in angstroms, and the bond angles are given in degrees.

Figure 7. Energy profiles of the decomposition channels of tetrahydrothiophene in Schemes 1 and 2. The energies are given in kcal/mol relative to that of tetrahydrothiophene.

step concerted reaction through hydrogen transfer from carbon to sulfur and the carbon-carbon double-bond formation to produce S4 via S3, the former has a lower activation barrier. From S4 to S9, the concerted reaction through S8 is more reasonable than that via S5 and S7, because S8 has a relative energy of 85.9 kcal/mol and S5 and S7 are 5.5 and 45.7 kcal/mol higher in energy than S8, respectively. Thus, on the basis of the calculated activation energies, the thermal decomposition of tetrahydrothiophene leading to the observed products 1,3-butadiene and hydrogen sulfide is probably by the pathway S0 f S1 f S2 f S4 f S8 f S9. In Scheme 2, the dehydrogenation of tetrahydrothiophene to produce thiophene by the simultaneous loss of two hydrogen molecules (S13) should be more difficult than the stepwise loss of hydrogen molecules, because the activation barrier of the

simultaneous loss of two hydrogen molecules is expected to be higher than the transition state of either S11 or S14. In reality, we failed to optimize the structure of S13 because the optimization points to a structure where the SCF procedure failed to converge. As an intermediate, the dihydrothiophene (S12) was detected in the decomposition products. Its energy is relatively lower (35.2 kcal/mol) than S0 (Figure 7), which means that dihydrothiophene was relatively stable in the decomposition process. From the calculated high activation barriers of 121.7 and 150.8 kcal/mol of the transition states S11 and S14, respectively, it can be reasonably expected that the transition state S13 would have an even higher energy because four hydrogen atoms are cleaved simultaneously from the molecule. The energy calculation shows that the more favorable decomposition pathway of tetrahydrothiophene producing thiophene

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is the one via dihydrothiophene. The overall activation barrier is 150.8 kcal/mol, and the product is 51.0 kcal/mol higher in energy than tetrahydrothiophene. Conclusions Thermal decomposition of tetrahydrothiophene was investigated in the temperature range of 450-500 °C, and quantum mechanical calculations were carried out in an effort to elucidate the decomposition mechanisms leading to the observed products. On the basis of the experimental and calculated results, the favorable thermal decomposition pathways for tetrahydrothiophene

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leading to hydrogen sulfide and 1,3-butadiene are involving C-S bond cleavage and hydrogen transfer; however, the formation of thiophene in the thermal process is generated by dehydrogenation of tetrahydrothiophene via the intermediate dihydrothiophene. Acknowledgment. This research was supported by the Excellent Young Teachers Program of MOE, People’s Republic of China. Dr. Xia thanks Dr. Ruifeng Liu (previously at East Tennessee State University) for hosting his visit to the U.S., where some of the quantum mechanical calculations were carried out. EF060095B