Spectroscopic Study of Low-Pressure Water Plasmas and Their

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Spectroscopic Study of Low-Pressure Water Plasmas and Their Reactions with Liquid Hydrocarbons Gloria Gambu´s, Pedro Patin˜o,* and Juan Navea Escuela de Quı´mica, Facultad de Ciencias, Universidad Central de Venezuela, P.O. Box 47102, Caracas 1041A, Venezuela Received July 9, 2001. Revised Manuscript Received September 18, 2001

Characterization of a water plasma was carried out in a range of water vapor flow rate of 0.1 to 2.2 mmol/h in a radio frequency discharge, in order to study its properties as either an oxidizing or a reducing agent in heterogeneous reactions with different liquid hydrocarbons. The variation of relative population of the species in the plasma, namely ‚H, O(3P), and ‚OH, was studied by optical emission spectroscopy by varying the flow rate, at a fixed radio frequency power of 100 W. Reactions with liquid hydrocarbons, such as fuel residuals and organic compounds with similar functionalities, were performed. Hydrogenation products were the most abundant at low flow rates, whereas oxidation was the prevalent process at moderate-to-high flow rates, alcohols, carbonyl compounds, epoxides and phenols being the products, depending on the starting substances. The cetane number of a light gas oil, containing 89.75% of alkanes, 5.57% of aromatics, and 4.68% of olefins, was improved by 59% after 8 h treatment at 0.071 mmol of water per hour.

1. Introduction The process of fuel production generates a substantial amount of residual materials with high boiling points. Transforming part of these materials or “adding value” to them has been one of the biggest challenges of the oil industry, because of the high demands of fuel in the world and the decrease of light crude oil supplies. This, in turn, has increased the proportion of residuals generated at the refineries. For that reason, the search for new technologies to upgrade the quality of those residuals has recently begun. The methods used for this purpose until now had been either catalytic or thermal, so alternative processing methods would be, without doubt, highly desirable. In addition, it is well-known that, in the fuel production process, the catalytic and thermal cracking conversion processes generate a high amount of byproducts that contain a significant percentage of olefins, conjugated diolefins, and aromatics. These species, especially the conjugated diolefins, restrain those residuals from being directly incorporated in the fuel formulation because they produce rubber, increase particulate emissions, and decrease the efficiency of fuel combustion, among other problems. To minimize these difficulties, a severe process such as hydro-treatment must be used. Among others, the objectives here are the following: the removal of sulfur, nitrogen, and metals; the conversion of heavy fractions into others with lower boiling points; the conversion of olefins into paraffins; and the saturation of polyaromatic compounds. On the other hand, it has been shown that the use of oxygenate compounds as additives in gasoline and diesel formulations improves the combustion process because they diminish particulate emissions, non * Corresponding author. E-mail: [email protected].

burnt hydrocarbons, carbon monoxide, and exhaust emissions.1 Fifteen years of investigations in our laboratory have shown that it is possible to achieve the chemical transformation of low-vapor-pressure liquid materials using nonequilibrium plasmas. Recently, the oxidation of 13 liquid hydrocarbons, both alkanes, alkenes, and arenes, and a light fuel oil has been studied by means of an oxygen plasma.2 The most important result has been the increase of the cetane number of the fuel due to the insertion of oxygenated groups into the structures of its components. Following that work, the purpose of this research is the use of water vapor plasmas to improve the properties of petroleum industry residuals by employing either the hydrogenation or the oxidation reactions that can be produced with such a tool. 2. Experimental Section Spectroscopic Techniques. The experimental setup utilized to perform the spectroscopic study of the water vapor plasma is basically the optical emission system previously employed to study an oxygen plasma.2,3 The radiation of the plasma was focalized by means of a 10 cm focal length convergent quartz lens, on the entrance slit of a 0.5-m CzernyTurner monochromator equipped with a 1276 lines/mm diffraction grating. The emission signal was detected by a Hamamatsu R955 photomultiplier that fed a SR-250 Boxcar data collector (Stanford Research Instruments) and was finally plotted on a Cole Parmer recorder. (1) Karas, I. J.; Kesling, H. S., Jr.; Liotta, F. J.; Nandi, M. K. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (2), 316-321. (2) Gambu´s, G.; Patin˜o, P. B.; Sifontes, A.; Navea, J.; Martı´n, P.; Taylor, P. Energy Fuels 2001, 15, 881-886. (3) Patin˜o, P.; Herna´ndez, F. E.; Rondo´n, S. Plasma Chem. Plasma Process. 1995, 15, 159-171.

10.1021/ef010157s CCC: $22.00 © 2002 American Chemical Society Published on Web 10/18/2001

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Figure 1. Experimental system. F: Radio frequency generator. W: Wattmeter. S: Matching Box. G: Water container. R: Reactor. B: Cooling bath. P: Pressure meter. T: Trap for volatile compounds. V: Vacuum pump. Several lines and bands were identified in the spectrum of the plasma, and water vapor flow rate was varied in order to observe the effect on the intensity of three particular signals: the Balmer series Hβ line of ‚H, located at 486.5 nm;4 the ‚OH band situated at 306.8 nm, corresponding to the v′ ) 0 f v′′ ) 0 transition of the system A2Σ+-X2Πi;5 and the O I lines at 844.6 and 844.7 nm, corresponding to the 3p 3P f 3s 3S transition.4 Plasma-Hydrocarbon Interactions. The reactions were carried out by making the active species of the water vapor plasma impinge on the surface of three liquid model hydrocarbons under study, namely cyclohexene, squalene, and toluene, and LCGO, a light gas oil. A radio frequency (rf) system was employed to produce the plasma, using a Branson IPC 100 generator that works at 13.4 MHz and variable power from 0 to 500 W. To get an efficient coupling between the source and the electric discharge, a matching box was used to connect the generator to the copper coil. This is placed externally around the glass reactor of 3 cm diameter, at 16 cm from the bottom as shown in Figure 1. The experiments were carried out by keeping constant the water vapor flow rate for squalene and toluene, and by varying it for the cyclohexene and LCGO reactions. In each case, the reactor was plunged in a cooling bath to avoid gas-phase reactions. The temperature and the substrate volume were kept constant and the reaction time was changed between 2 and 8 h. The product mixtures were analyzed by means of GCMS and 1H NMR with a Varian 3800 GC/Saturn 2000 MS system and a JEOL ECLIPSE 270 MHz NMR spectrometer, respectively.

Figure 2. Relative abundance of active species in the water plasma.

hydrogen atom showed an expected behavior,6 that is, for low flow rates the energy of the discharge is enough to dissociate a relatively high quantity of water molecules; however, when the flow rate is increased, the recombination rate of H radicals to produce hydrogen molecules progressively equals their production. The oxygen atom density showed a behavior similar to that of ‚H, as expected from a previous work with a high voltage plasma with oxygen as source gas.3 In the case being analyzed, the maximum signal of O I has been observed at about 0.3 mmol/h. Finally, the diatomic species ‚OH showed a drastic density increase starting from ca. 0.1 mmol/h, reaching a wide range of water pressure for maximum production and, then, decreasing after 2 mmol/h. The analysis of Figure 2 has led us to find out not only the appropriate flow rates to carry out specific reactions between the water plasma and liquid hydrocarbons. It is also possible to determine the chemical mechanisms involved in the plasma.

H2O + e- f H‚ + ‚OH + eH2O + e- f H2O* H2O* f H‚ + ‚OH

3. Results and Discussion

‚OH + e- f H‚ + O + e-

Optical Emission Spectroscopy. Behaviors of the identified species in the water plasma are shown in Figure 2, for different flow rates of water vapor and a radio frequency power of 100 W. In recording the optical emission signal for each species, the amplification of the photomultiplier had to be adjusted according to the intensity of the respective transition. Hence, each curve only shows the relative population of a particular species. Obviously, the flow rate is a crucial factor to determine dissociation and recombination reactions. The

‚OH + e- f ‚OH*

(4) Harrison, G. Wavelength Tables. Massachusetts Institute of Technology; The MIT Press: Cambridge, MA, 1969. (5) Rosen, B. Tables of Constants and Numerical Data. Spectroscopic Data Relative to Diatomic Molecules; Pergamon Press: Oxford, 1970.

‚OH* f H‚ + O On the basis of this sequence of reactions it is possible to explain the characteristic behavior of the different species in the plasma, as shown in Figure 2. That is the case for the maximum value of the atomic hydrogen concentration at low flow rates, which is generated through four different sources. However, when the flow rate is increased, it is necessary to bear in mind that (6) Avtaeva, S. V.; Mamytbekov, M. Z.; Otorbaev, D. K. Diagnostic of Magnetically-Enhanced RF Discharge Plasmas in Methane: Absolute Density of Hydrogen Atoms. 12th International Symposium on Plasma Chemistry, Minneapolis, MN, 1995; pp 409-414.

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Table 1. Products Distribution Obtained from the Reactions of the Water Plasma with Cyclohexene (Temperature was -100 °C for 3.0 mL of liquid; power was 100 W.) product

flow: 0.071 mmol/h t: 300 min

cyclohexane epoxycyclohexane cyclohexanone cyclohexanol cyclohexadione bicyclohexyl total conversion (%)

50.94 3.16 9.62 8.11 19.78 2.03 11.50

flow: 0.290 mmol/h t: 120 min 8.51 29.41 18.14 26.92 3.95

the sources for hydrogen atom generation are twice the number of the others. This means that hydrogen atoms are the main contributing agents to the system entropy and their recombination is hence possible:

H‚ + ‚H + W f H2 + W H‚ + H‚ + M f H2 + M H‚ + ‚OH + W f H2O + W H‚ + ‚OH + M f H2O + M O + O + W f O2 + W O + O + M f O2 + M H‚ + O + W f ‚OH + W H‚ + O + M f ‚OH + M H‚ + H2O f H2 + ‚OH where W is a wall of the reactor system and M symbolizes other particles in the plasma. Clearly, the hydrogen atom shows higher tendency than other species to recombine. Furthermore, it exhibits recombination with atomic oxygen and water to produce ‚OH. This may be an explanation to the maximum shown by the latter throughout a wide range of water flow rate. Plasma-Hydrocarbons Reactions. To verify the reactivity of the active species present at low and high flow rates in the water vapor plasma, two olefins and one alkylbenzene were utilized. They have in common the feasibility of undergoing both hydrogenation and oxidation reactions. Finally, a light gas oil was treated under similar conditions with the aim of testing the upgrading capability of the water plasma. Cyclohexene. This is a rather simple olefin that allows a hydrogenation versus oxidation test to be performed. Results from the reactions of 3 mL of liquid cyclohexene with water plasmas at two flow rates, namely 0.071 and 0.290 mmol/h, respectively, are shown in Table 1. A strong dependence of the reaction with the flow rate was clearly observed, hydrogenation being slightly favored at low water input, while oxidation was the unique process that took place at moderate flow rate. These results are in good agreement with those of the optical emission study of the plasma. Epoxicyclohexane, cyclohexanone, cyclohexadione, and cyclohexanol were the products from the reactions with O(3P), while cyclohexane and bicyclohexyl were produced through the reactions with ‚H.

Figure 3. NMR 1H spectra of pure squalene (a), the sample treated for 3 h with the water plasma at 0.071 mmol/h (b), and treated for 2 h with the water plasma at 0.8 mmol/h (c).

Squalene. This is 2,6,10,15,19,23-hexamethyltetracosahexaene, a high-boiling-point C30 hydrocarbon with six double bonds in its molecular structure. Figure 3 shows the 1H NMR spectra of the original (a) and the treated sample (b) after 3 h at low flow rate (0.071 mmol/h), respectively, where the appearance of four new groups of signals can clearly be seen. The first one, located between 0.7 and 1.0 ppm, corresponds to methyl groups of aliphatic chains. This is the first indication that hydrogenation of CdC bonds has taken place for all methyl groups that were originally attached to them appeared at 1.4-1.7 ppm. The second, situated between 2.3 and 3.0 ppm, corresponds to methylene protons R to carbonyl groups and protons R to C-O. Here, the respective ketones and epoxides are products of the oxidation of the olefinic fraction by O(3P).2,7 The signals located between 4.5 and 4.9 ppm are readily assigned to olefinic protons, apart from the original signal between 5.1 and 5.3 ppm. The new signals are shifted to higher fields due to the elimination of CdC bonds in the neighborhood, thus confirming the partial hydrogenation of the squalene molecule. When a -HCdCHfragment of a molecule has some others CdC nearby, (7) Patin˜o, P.; Sa´nchez, N.; Suhr, H.; Herna´ndez, N. Plasma Chem. Plasma Process. 1999, 19, 241-254.

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Table 2. Results from the Reaction of Squalene with the Water Plasma (Temperature was 10 °C for 3 mL of liquid.) H2O flow rate (mmol/h)

reaction time (hours)

fraction of double bond hydrogenated (%)

0.071 0.80

3 2

26.0 10.4

and part of these are hydrogenated, the protons of the former will exhibit smaller downfield shift. From these premises, the fraction of double bonds that have been hydrogenated can be calculated by dividing the integration area of the new signals by that of the 5.1-5.3 ppm signal. The result here is 26.0% hydrogenation. The forth group consists of two weak signals between 9.2 and 9.7 ppm that correspond to aldehydic protons. This suggests that partial oxidation of methyl groups has taken place as observed on saturated hydrocarbons when exposed to oxygen plasmas for long times. For the squalene sample treated at moderate flow rate (0.80 mmol/h) and 2 h of reaction, just the new group of signals between 4.4 and 4.9 ppm can be seen in the 1H NMR spectrum shown in Figure 3c. It corresponds to olefinic protons as observed in the reactions at low flow rate. This again indicates the partial hydrogenation of the molecule. In this case, the fraction of double bonds that have been hydrogenated only amounts to 10.4%. These results indicate that ‚OH seems to exhibit a low reactivity under the conditions of this study. In fact, despite the fact that this species shows its maximum density at ca. 0.80 mmol/h, no oxygenated product has been observed in the 1H RMN spectrum. The experimental conditions and the results are listed in Table 2. Toluene. The reactions of this model compound with the water plasma were carried out at -90 °C and 0.071 mmol/h of water vapor with 3 mL of liquid during 3 to 8 h of treatment. The GC-MS analysis of the treated toluene samples showed no significant variation when the reaction time was increased, except for the appearance of two new peaks of low intensity (less than 1% in weight) corresponding to o- and p-cresol. This confirms the low reactivity of aromatic hydrocarbons with plasmas as had been observed in previous works.8 LCGO. The stream used for experiments was a Light Coker Gas Oil (LCGO) supplied by a Venezuelan refinery. It was obtained by thermal decomposition of heavy fractions that remained after processes such as vacuum distillation and fluid coking (FLC). Its composition, measured by NMR, was of 89.75% alkanes, 5.57% aromatics, and 4.68% alkenes, mainly R-olefins. Samples of 8 mL of the liquid were treated at -40 °C, just 2° above its freezing point, applied power being 100 W. Figure 4 shows the 1H NMR spectra of the original gas oil (a), and the treated samples, at water flow rates of 0.071 mmol/h (b), and 0.80 mmol/h (c), respectively. Spectrum (b) shows the following features: The signals of the olefinic protons (4.4-6.0 ppm) highly decreased; the intensity of the aliphatic region (0.7-3.5 ppm) increased; those of CH3 and CH2 at R positions to aromatic carbons, between 2.0 and 2.5 ppm, became weaker; weak signals around 3 ppm, due to alcohols and epoxides, were observed; that of the aldehydic protons (8) Patin˜o, P.; Ropero, M.; Iacocca, D. Plasma Chem. Plasma Process. 1996, 16, 563-575.

Figure 4. NMR 1H spectra of pure LCGO (a) and the sample treated for 8 h with the water vapor plasma at 0.071 mmol/h water flow (b) and 0.80 mmol/h for 12 h (c).

weakly appeared at 9.1 ppm in the spectrum. This indicates that the dominant reaction at this flow rate seems to be hydrogenation, partial oxidation of the olefinic fraction having taken place but just in low proportion. In spectrum (c), corresponding to the higher flow rate, the signals assigned to oxygenated groups can hardly be observed, while the olefinic and aromatic regions are weakened, thus indicating that hydrogenation is the most important reaction as observed for squalene under the same conditions. The conversion index for every fraction was calculated by dividing its corresponding integrated area by the total area. The values are shown in Table 3. The quality performance of both LCGO samples, original and treated, was tested by measuring the cetane number, according to the ASTM method D-613.9 This is a parameter that represents the autoignition capability of a fuel when it is subjected to compression. The values were 37.7 units for the first and 59.8 for the (9) ASTM Annual Book, 1950; p 19103.

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Table 3. Effects of the Water Plasma Treatment on the Properties of 8 mL of LCGO at -40 °C parameter

origina sample

treated samplea

treated sampleb

alkane index (%) aromatic index (%) olefin index (%) carbonyl index (%) oxygen content (p/p %) H/C atomic ratio cetane number

89.75 5.57 4.68 0.0 0.0 1.71 37.7

94.48 4.27 1.00 0.25 3.87 1.75 59.8

95.24 4.22 0.54 1.76 1.77 52.3

a

8 h reaction at 0.071 mmol-H2O/h. b 12 h at 0.80 mmol/h.

treated sample for 8 h at 0.071 mmol water per hour, i.e., an increase of 59%. These results clearly indicate the improvement of LCGO after treatment. This can be associated with the hydrogenation process, based on the variation of the cetane number of different kinds of hydrocarbons in a diesel fuel as a function of the number of carbon atoms.10 Nevertheless all oxygenated organic molecules do not increase cetane, the contribution of these species, mainly epoxides, product of partial oxidation of the olefinic fraction of the fuel, might be taken into account, too. The results at moderate water flow rate confirm these comments, the increase of cetane number being lower in this case. Recently, the oxidation of several liquid, long-chain hydrocarbons (alkanes, alkenes, arenes) and LCGO has been studied by using a low-pressure, high-voltage oxygen plasma. Particularly for the light fuel oil, at 120 min of treatment, the olefinic fraction was eliminated and the cetane number was improved by 66% without any additional treatment.2 A preliminary evaluation of the efficiency of the oxidizing bench system utilized in that work indicated some advantages with respect to the classical oxidation process based on potassium permanganate.11-13 In both cases, only operation costs were considered. The radio frequency generator utilized to produce the water vapor plasma of this research consumes about 3.8 times more power than the high voltage system. It is nevertheless comparable to the permanganate system. Additionally, for safety reasons water is a more convenient reagent compared to oxygen and inorganic oxidizers. The elemental analysis of original and treated LCGO is shown in Table 3. After 8 h of reaction, at low flow rate of water, the fuel sample contained 3.9% of oxygen, carbon and hydrogen percentages having decreased consequently. This result is encouraging if a comparison (10) Suchanek, A. J. Reduction of Aromatics in Diesel Fuels. NPRA Annual Meeting, 1989. (11) Venkat, Ch.; Bellemead, N. J.; Walsh, D. E.; Richboro, P. U.S. Pat. 4,494,961, 1985. (12) Taylor, W. F.; Mountainside, N. J. U.S. Pat. 4,723,963, 1988. (13) Chertkov, A.; Kunina, E. A.; Kirsanova, T. I. J. Appl. Chem. URSS 1980, 53 (7), 1245-1250.

with current industrial procedures is made. In fact, the upgrading of diesel fuels is being done by adding oxygenated compounds such as tripropylene glycol monomethyl ether, dipropylene glycol monomethyl ether, dibutylmaleate, 1-methoxy-2-propanol, 2-ethoxyethyl acetate, 2-ethoxyethyl ether, and diethyladipate.14,15 By these means 7 wt % oxygen has been enough to achieve 28 to 42% reduction of exhaust emissions. This can readily be accomplished by increasing the time of direct reaction of the water plasma with the fuel. Scaling experiments to improve efficiency of the plasma process are currently being performed. The LCGO treated samples were stored at room temperatures, in amber bottles, under air, for several months. No change in its chemical composition was observed. No instability was observed even when performing the cetane number tests. 4. Conclusions Low-pressure water vapor plasmas have proved to be selective in hydrogenating olefins at low flow rates and oxidizing pure hydrocarbons and mixtures of them at relatively medium to high flow rates. Products are alkanes and mixtures of epoxides, carbonyl compounds, alcohols, and phenols, respectively, depending on the starting material. Observed reactions are in good agreement with the relative population of the active species in the plasma. ‚H presented its maximum abundance at low flow rates; O(3P) was relevant at medium values, and ‚OH showed a wider range for optimum concentration. From the point of view of improving the quality of fuel oils it looks convenient to treat these with water vapor at low flow rates, particularly when the starting material contains a certain amount of olefins. These species can be either hydrogenated to produce alkanes or oxidized to produce epoxides, carbonyl compounds, and alcohols. In both cases the cetane number of the sample is improved in a better manner compared to classical wet oxidation.11-13 Acknowledgment. Funding for this research has been provided by CONICIT and CDCH-UCV through Grants 97003740 and 03-12-4083-99, respectively. EF010157S (14) Natarajan, M.; Frame, E. A.; Naegeli, D. W.; Asmus, T.; Clark, W.; Garbak, J.; Gonza´lez-D. M. A.; Liney, E.; Piel, W.; Wallace, J. P., III. Society of Automotive Engineers, Inc., Paper Nο. 01FL-576, 2000. (15) Gonza´lez-D., M. A.; Piel, W.; Asmus, T.; Clark, W.; Garbak, J.; Liney, E.; Natarajan, M.; Naegeli, D. W.; Yost, D.; Frame, E. A.; Wallace, J. P., III. Society of Automotive Engineers, Inc., Paper Nο. 01FL-577, 2000.