Energy & Fuels 2007, 21, 19-22
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Hydrodesulfurization of Benzothiophene and Hydrogenation of Cyclohexene, Biphenyl, and Quinoline, Assisted by Ultrasound, Using Formic Acid as Hydrogen Precursor Jacqueline Grobas, Carmelo Bolivar, and Carlos E. Scott* Centro de Cata´ lisis Petro´ leo y Petroquı´mica, Escuela de Quı´mica, Facultad de Ciencias, UniVersidad Central de Venezuela, Apartado Postal 47102 Los Chaguaramos, Caracas, Venezuela ReceiVed August 18, 2006. ReVised Manuscript ReceiVed October 20, 2006
The study of the chemical effects of ultrasound is a rapidly growing research area, and its use in heavy crude oil upgrading has been explored. In this work, hydrogenation of cyclohexene, biphenyl, and quinoline and hydrodesulfurization of benzothiophene in the presence of formic acid (a hydrogen precursor), a Pd/C catalyst, and ultrasound irradiation is investigated. It was found that the use of formic acid in the presence of ultrasonic irradiation was an effective system for promoting the hydrogenation of cyclohexene (98%) and biphenyl (21%), the desulfurization of benzothiophene (18%), and the hydrogenation of quinoline (19%) at very mild conditions (i.e., ambient temperature and pressure).
Introduction Hydroprocessing has become increasingly difficult, on the one hand because of stringent environmental legislation regarding the maximum content of sulfur, nitrogen, and aromatic compounds in fuels, and on the other hand because of the gradual decline of conventional petroleum reserves, which have increased the need for processing heavier petroleum fractions, such as heavy crude oils and oil sands derived from bitumen, with higher quantities of contaminants. When dealing with heavy feedstocks, high temperatures and pressures (around 673 K and g9.0 MPa) are required for their hydroprocessing. Thus, the development of new processes to improve refining technology is desirable. It is in this sense that there has recently been an increased interest in the use of ultrasound in petroleum processing. It is well-known that the chemical effects of ultrasound derive primarily from acoustic cavitation1 (the formation, growth, and collapse of bubbles). Bubble collapse in liquids results in an enormous concentration of energy from the conversion of the kinetic energy of the liquid motion into heating of the contents of the bubble, which produces intense local heating and high pressures. It has been estimated that these hot spots have temperatures of ∼5000 K, heating and cooling rates above 1 × 1010 K s-1, and about 101 MPa of pressure.2 The high local temperatures and pressures, combined with extraordinarily rapid cooling, provide a unique means for driving chemical reactions under extreme conditions. The use of ultrasonic irradiation for heavy crude upgrading,3,4 Athabasca bitumen visbreaking,5 asphaltenes,6 tar sands,7 and heavy gas oil upgrading8 has been explored.
Even though the results seem to be promising, a better understanding of the chemical steps involved in the ultrasonic treatment of the type of compounds present in heavy crude oil feeds seems to be needed. Thus, Tu and Yen9 studied the radical induced demetalization of porphyrins by ultrasound and found that, under optimal conditions, 90% of the metalloporphyrins could be decomposed through an oxidative process. Gopinath et al.8 studied the ultrasound-assisted denitrogenation of acridine, carbazole, and 9-ethyl carbazole. Low conversions were obtained (e9%), and basic nitrogen compounds were more easily decomposed compared to the nonbasic ones. Also, the ultrasound-assisted oxidation of 4,6-dimethyldibenzothiophene has been studied,10 and high conversions were obtained, as well as the oxidation of benzothiophene in aqueous solution.11 In a previous paper,12 ultrasonic irradiation of thiophene, benzothiophene, or dibenzothiophene dissolved in a water/ ethanol mixture in the presence of Ni/Al or Ni/Zn catalysts at 333 K was used to produce desulfurization of the aforementioned compounds. It was found that the system could be effective for thiophene desulfurization, but to a much lesser extent for benzothiophene and dibenzothiophene. It was proposed that the role of ultrasonic irradiation was to increase the reaction rate, change catalyst morphology, producing a cleaner surface and smaller particle size, and dissociate water and ethanol (in the presence of the catalyst) to produce, in situ, the hydrogen needed for the HDS reactions. In this work, formic acid, another source of in situ hydrogen, and a Pd/C catalyst were used in the presence of ultrasonic
* Corresponding author. E-mail:
[email protected]. (1) Suslick, K. S.; Price, G. J. Annu. ReV. Mater. Sci. 1999, 29, 295. (2) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1397. (3) Lee, A. S.; Xu, X. W.; Yen, T. F. Preprints 4th UNITAR/UNPD International Conference on HeaVy Crude and Tar Sands; UNITAR/ UNPD: Edmondton, Alberta, Canada, 1988; Vol. II, paper 135. (4) Sadeghi, K. M.; Lin, J. R.; Yen, T. F. Energy Sources 1994, 16, 439. (5) Chakma, A; Berruti, F. Energy Process. 1991, 84, 16.
(6) Lin, J. R.; Yen, T. F. Energy Fuels 1993, 7, 111. (7) Sadeghi, K. M.; Sadegui, M. A.; Kuo, J. F.; Jang, L. K.; Yen, T. F. Energy Sources 1990, 12, 147. (8) Gopinath, R.; Dalai, A. K.; Adjaye, J. Energy Fuels 2006, 20, 271. (9) Pin, S.; Yen, T. F. Energy Fuels 2000, 14, 1168. (10) Deshpande, A.; Bassi, A.; Prakash, A. Energy Fuels 2005, 19, 28. (11) Kim, I.; Jung, O. Bull. Korean Chem. Soc. 2002, 23, 990. (12) Scott, C. E.; Altafulla, J. R.; Bolivar, C.; Urbina de Navarro, C. Stud. Surf. Sci. Catal. 2003, 145, 343.
10.1021/ef0603939 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006
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Grobas et al. Table 1. Conversion and Selectivity for Ultrasound-Assisted Hydrogenation of Cyclohexene in an Ultrasonic Bath time (min)
conversion (%)
selectivity (mol %)
20
98
cyclohexane (98), benzene (2)
chromatographic peaks, respectively, that could not be identified; they are referred to as other products (OP). A response factor of 1 was assumed for these compounds. Similarly, selectivities were figured out from chromatographic area ratios, using the appropriate response factors. All reactions except those for HYD of cyclohexene were carried out twice. Products concentrations were obtained from the calculated conversions assuming that Ca ) Ca0(1 - x) where Ca ) concentration, Ca0 ) initial concentration, and x ) conversion. Figure 1. Reactor for ultrasonic bath tests.
irradiation to see if the conversion for more refractory molecules, such as benzothiophene, quinoline, and biphenyl, could be enhanced. 2. Experimental Section 2.1. Chemicals. Biphenyl (Aldrich 99.5%), benzothiophene (Aldrich 99%), palladium on carbon catalyst (Aldrich 5 wt %), cyclohexene (Riedel-de Hae¨n g99.5%), ethanol (Riedel-de Hae¨n g99.5%), formic acid (Riedel-de Hae¨n g98%), and quinoline (Riedel-de Hae¨n 98%) were used. 2.2. Reactions. Reactions were carried out in two different systems. The first conducted reactions in a 50 kHz ultrasonic bath (ULTRAsonik 104 X), whereas the second used a 20 kHz, ≈80 W/cm2 ultrasonic processor (VCX-600 from Sonics & Materials Ltd). Cyclohexene, biphenyl, quinoline, and benzothiophene were used as model compounds, and the procedure involved adding the compounds (already dissolved in ethanol, 2% w/w), the catalyst (0.250 g.), and formic acid (10% v/v) to the reactor. The mixture was then purged with nitrogen and immediately sonicated. For the ultrasonic bath, the reactions were carried out in a glass reactor (Figure 1) for 10, 30, and 45 min at 301 K and atmospheric pressure. It should be noted, however, that some pressure developed during the tests that was due to hydrogen production. A set of reactions were also performed at 288 and 318 K for 30 min. With the ultrasonic probe, all the reactions were carried out for 10 min in a stainless steel, 50 mL capacity, sealed atmosphere treatment chamber (Sonics & Materials Ltd.). Ports located above the sample level permit purging with nitrogen and capturing samples of released gases. An integral cooling jacket, through which cool water was circulated, inhibits heat buildup during operation and maintains the reaction temperature. Mechanical stirring was never used in any of the tests. In all cases, once the samples were sonicated, the reactor was open, the reaction mixture filtered off, and the liquid analyzed by gas chromatography using a Perkin-Elmer Autosystem XL chromatograph with a flame ionization detector (FID) and a capillary column (PE-5 Phenyl Methyl Siloxane, 30 m). In some experiments, a gaseous sample was also withdrawn and analyzed by a gas chromatograph using a TCD with a similar Perkin-Elmer instrument but equipped with a Mol Sieve 5 Plot column (30 m). Conversions were determined from chromatograms by adding up the area of the products and dividing between the areas of the products plus the area of the substrate. A response factor of 1 was used for hydrocarbons, 0.70 for benzothiophene, 0.90 for biphenyl, and 0.82 for quinoline and 1,2,3,4,-tetrahydroquinoline. For the reactions with quinoline and benzothiophene, there were 2 and 3
3. Results and Discussion 3.1. Cyclohexene Hydrogenation. Analysis of gaseous samples after the sonication showed the presence of large quantities of hydrogen. Moreover, the evolution of hydrogen was evident after the addition of formic acid to the catalystliquid feed slurry and sonication. Conversions and selectivity obtained for reactions carried out with cyclohexene, using the ultrasonic bath, are illustrated in Table 1. It is observed that cyclohexene is effectively hydrogenated, which is in agreement with reports from Boudjouk and Han,13 who found that formic acid/palladium-on-carbon coupled with ultrasonic irradiation was an effective hydrogenating system for olefins. Also, a small quantity of dehydrogenated product (i.e., benzene) is obtained, which is not surprising, because it is commonly observed that catalytic hydrogenation of cyclohexene also produces dehydrogenation.14 Very small amounts (93%) is 1,2,3,4-tetrahydroquinoline (Table 3). It has been reported that when quinoline is treated with a CoMo or a NiMo catalyst at hydrotreating conditions, it is rapidly hydrogenated to 1,2,3,4-tetrahydroquinoline, and an equilibrium between the two is reached.20,21 The equilibrium constant was found to be dependent on hydrogen pressure. It is possible that, under the conditions used in this work, this equilibrium is also rapidly attained at a conversion of approximately 17-19%. It also implies that the system is efficient to hydrogenate, but does not denitrogenate the quinoline molecule (at least significantly). Even though poisoning of the catalyst could also be a possible explanation for this observation, it is considered to be less likely, because it has been reported22,23 that when a liquid-solid interface is subjected to high-intensity ultrasound, the acoustic cavitation generates high-speed jets of liquid directed at the surface, producing a cleaning effect that reduces the quantities of adsorbed molecules that can act as poisons. The hydrogenation of biphenyl produces a maximum conversion of 21% for 45 min of sonication, and only a single benzene ring is saturated. Even though the conversion seems to be low, it is worth noting for a system working at atmospheric pressure and temperature. With respect to the desulfurization of benzothiophene, it is observed that conversion is also low. The main product is (20) Satterfield, C. N.; Modell, M.; Hites, R. A.; Declerck, C. J. Ind. Eng. Chem. Process Des. DeV. 1978, 17, 141. (21) Kim, S. C.; Simons, J.; Massoth, F. E. J. Catal. 2002, 212, 201. (22) Suslick, K. S.; Johson, R. E. J. Am. Chem. Soc. 1984, 106, 6856. (23) Suslick, K. S.; Casadonte, J. J. Am. Chem. Soc. 1987, 109, 3459.
Energy & Fuels, Vol. 21, No. 1, 2007 21
ethylcyclohexane, with ethylbenzene in smaller quantities. The presence of these two products implies that the reaction network is similar to the one reported for the hydrodesulfurization of dibenzothiophene with hydrotreating catalysts at high temperatures and pressures.24 From the open literature, the first step in the HDS of BT is the C-S hydrogenolysis to produce styrene, which is later hydrogenated to form ethylbenzene.24 Cyclohexene conversion to cyclohexane is more difficult to achieve than the hydrogenation of styrene to ethylbenzene, and this study demonstrated 98% conversion of cyclohexene to cyclohexane. Therefore, it can be concluded that styrene would be completely hydrogenated under these conditions. Ethylbenzene undergoes further hydrogenation to produce ethylcyclohexane. In this reaction network, the limiting reaction step is the C-S hydrogenolysis.24 After an extensive literature review, no similar reports for hydrogenating biphenyl or for desulfurization of benzothiophene in a similar system were found. The data from Figure 2 were tested for first- and secondorder reactions with poor correlation factors (e0.9644), particularly for the experiments for dibenzothiophene desulfurization (e0.8936). In the experiments, it was visually observed that after the first few minutes of sonication, hydrogen evolution ceased. It is possible that during this time, all formic acid was consumed and the hydrogen produced from its decomposition, which did not react with the substrate, remained as molecular hydrogen in the gaseous phase. In fact, molecular hydrogen was observed in the gaseous products, and there was an observed pressure build-up in the reactor. This molecular hydrogen can still react with substrates to give hydrogenated products but, as has been shown for other hydrotreating reactions,25 molecular hydrogen would be several times less effective than nascent hydrogen produced from the formic acid decomposition. 3.2.2. Reaction Temperature. Figure 3 shows the evolution of conversion versus temperature for samples sonicated for 30 min in an ultrasonic bath. For biphenyl only phenylcyclohexane was obtained, whereas for quinoline, the main product was 1,2,3,4-tetrahydroquinoline. For the desulfurization of benzothiophene, the main products were ethylcyclohexane (8289%) and ethylbenzene (6-10%). The conversion reaches a maximum for the hydrogenation of biphenyl and for the decomposition of quinoline as the reaction temperature is increased from 288 to 301 K. Beyond this temperature, no further increase in conversion is observed. A similar behavior has been reported for the reaction of benzaldehyde and n-heptyl bromide catalyzed by lithium (the Barbier reaction) carried out in the presence of ultrasound at different temperatures.26 In ultrasound, any increase in temperature will raise the vapor pressure of the medium, thereby leading to easier cavitation and less violent collapse. However, at higher temperatures approaching the solvent boiling point, a large number of cavitation bubbles are generated concurrently, which act as a barrier to sound transmission and dampen the effective ultrasonic energy from the source that enters the liquid medium.27 In fact, the temperature of 318 K is not far from the normal boiling point of ethanol (351 K), which is the solvent used in this work. (24) Daly, F. P. J. Catal. 1978, 51, 221. (25) Ng, F. T. T.; Milad, I. K. Appl. Catal., A 2000, 200, 243. (26) de Souza-Barboza, J. C.; Pe´trier, C.; Luche, J. L. J. Org. Chem. 1988, 53, 1212. (27) Mason, J. T. Sonochemistry; Oxford University Press: New York, 1999; p 13.
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Grobas et al.
Table 2. Conversion and Selectivity for Ultrasound-Assisted Reaction Using an Ultrasonic Bath substrate
time (min)
conversion ( 1 (%)
selectivity (mol %)
10 30 45 10 30 45 10 30 45
7 12 21 17 18 19 5 7 9
phenylcyclohexane (100) phenylcyclohexane (100) phenylcyclohexane (100) tetrahydroquinoline (96), other products (4) tetrahydroquinoline (95), other products (5) tetrahydroquinoline (93), other products (7) ethylcyclohexane (86), ethylbenzene (9), other products (5) ethylcyclohexane (87), ethylbenzene (6), other products (7) ethylcyclohexane (89), ethylbenzene (4), other products (7)
biphenyl quinoline benzothiophene
Table 3. Conversion and Reaction Products for Ultrasound-Assisted Reaction Using an Ultrasonic Probe (reaction time 10 min and T ) 301 K) substrate
conversion ( 1 (%)
reaction products
biphenyl quinoline benzothiophene
9 17 16
phenylcyclohexane and bicyclohexyl tetrahydroquinoline ethylcyclohexane and ethylbenzene
For the desulfurization of benzothiophene, a different trend is observed. The conversion increased with temperature (a temperature that was raised from 288 K to 318 K) but no maximum was reached. Increasing the temperature beyond 318 K could result in discovering the conversion limit, but this
action would increase the temperature to the proximity of the solvent boiling point. 3.2.3. Ultrasonic Processor. When an ultrasonic probe was used, conversion for biphenyl hydrogenation increased from 7 to 9%, as was expected. The most significant finding however, is the observed increase of benzothiophene HDS from 5 to 16%. Quinoline conversion remained constant because of equilibrium being reached, as explained before. Even though biphenyl conversion only marginally increased, there was a difference in product distribution. It was found that the second aromatic ring was hydrogenated to produce a small quantity of bicyclohexyl. The reaction products from the HDS of benzothiophene with an ultrasonic processor were the same as those produced using an ultrasonic bath. Conclusions The use of formic acid in the presence of ultrasonic irradiation and a Pd/C catalyst was shown to be an effective system for promoting the hydrogenation of cyclohexene (98%) and biphenyl (21%), the desulfurization of benzothiophene (9%), and the hydrogenation of quinoline (19%) in an ultrasonic bath at very mild conditions (i.e., ambient temperature and pressure). A 3-fold increase in conversion for benzothiophene HDS was obtained with the used of an ultrasonic probe.
Figure 3. Conversion of model compounds vs temperature for samples sonicated for 30 min in an ultrasonic bath. [, BP; 9, Q; b, BT.
Acknowledgment. We thank FONACIT, Project CONIPET 97003783, for its financial support, and John Thompson for editing the manuscript. EF0603939
