Article pubs.acs.org/EF
Supporting Tungsten Oxide on Zirconia by Hydrothermal and Impregnation Methods and Its Use as a Catalyst To Reduce the Viscosity of Heavy Crude Oil Hao Wang,* Yan Wu, Li He, and Zhenwei Liu School of Chemistry and Chemical Engineering, South West Petroleum University, Chengdu 610500, People’s Republic of China ABSTRACT: Tungsten oxide was supported on zirconia via hydrothermal and conventional impregnation methods and used as a catalyst to reduce the viscosity of heavy crude oil without the addition of water. Their properties and viscosity reduction activity were compared. X-ray diffraction and the Rietveld method were used to characterize the crystal structure and determine the phase composition. The properties of catalysts were also measured by N2 adsorption−desorption, differential scanning calorimetry, a Hammett indicator, and ammonia temperature-programmed desorption. Compared with the impregnation method, the hydrothermal method facilitates the formation of strong tungsten−zirconia interaction and promotes the diffusion of tungstate on zirconia, hence leading to better-dispersed tungsten oxide, as well as more tetragonal zirconia and acid sites. Under the reaction conditions of 220 °C, 6 h, and 2 MPa, the catalysts prepared via the hydrothermal method exhibit higher viscosity reduction activity than those prepared by impregnation. It is found the catalytic activity mainly depends on the acidity of the catalyst. The catalyst containing 20 wt % tungsten prepared by the hydrothermal method can attain a viscosity reduction ratio of 82.2% and reduce the viscosity of oil from 5.74 Pa s (50 °C) to 1.02 Pa s. Different from the widely used aquathermolysis technique, the presence of water is not required and the viscosity of the treated oil will not regress, even over long periods of time.
1. INTRODUCTION Crude oil with an American Petroleum Institute (API) gravity smaller than 20 is called heavy oil.1 Heavy oil accounts for a large proportion of the proven oil reserves. With the increasing demand for oil and the shortfall of conventional oil, the production of heavy oil has been gradually increasing in recent years.2 However, heavy oil is difficult to explore and transport, because of its high viscosity and low flow property;3 therefore, the key factor to handling heavy oil is reducing the viscosity. Thermal recovery is the most widely used way to produce heavy oil, wherein heat is injected into the formation to reduce the viscosity of the oil and enable heavy oil to flow through the porous media of reservoirs. Hot fluid injection, in situ combustion, and steam stimulation are the thermal recovery methods.4 During the steam injection process, besides the effect of high temperature, it is believed that the chemical reaction between steam and oil also makes a great contribution to viscosity reduction by breaking large molecules into smaller ones. Hyne et al.5 first called this reaction aquathermolysis. The principal aspect of aquathermolysis is cleavage of the C−S bond in asphaltene, because of the injected superheated water, which causes a decrease in resins and asphaltenes and an increase in saturates and aromatics. Many researchers have reported that the presence of a catalyst could accelerate the aquathermolysis reaction significantly, as well as further upgrade the quality of the heavy oil.6−10 Recently, progress in catalytic aquathermolysis was well-reviewed by Maity et al.,11 who classified the catalysts as mineral, water-soluble, oil-soluble, and dispersed catalysts. Most catalysts contain salts of metals, such as Fe, Ni, V, Mo, Co, Ru, and Al, which may strongly interact with sulfides in heavy oil to improve the rupture of the C−S bond or © 2012 American Chemical Society
react with hot water to provide hydrocracking activity by producing an acidic environment. In general, the catalytic aquathermolysis was performed under the following conditions: reaction temperature, 160−280 °C; pressure, 3−25 MPa; time, 12−240 h; and water content, 2−40 wt %. The viscosity of heavy oil can be reduced by 40%−90%; moreover, if the hydrogen donor is present, the viscosity reduction ratio can reach above 90%.11 Therefore, catalytic aquathermolysis for producing heavy oil is becoming an important research area. Indeed, some heavy oils are waterless or have a lower water content; therefore, it is hoped that the viscosity can be reduced without adding water. In addition, to greatly decrease the consumption of light oil used to dilute the waterless heavy oil, an idea is generated: use the catalyst to reduce the viscosity of heavy oil at lower temperature in an oil gathering station to produce less-viscous oil and then return it to the well to dilute the heavy oil. However, less attention has been paid on the catalytic viscosity reduction of waterless heavy oil. It is accepted that the C−C bond in larger hydrocarbons can be broken down at temperatures above 300 °C, regardless of the presence of water,11 which causes the cracking of heavy hydrocarbons and, consequently, the decrease in viscosity. Introducing catalyst can decrease the reaction temperature. Solid acids are widely used catalysts to promote the cracking of hydrocarbons. Therefore, it is hoped that solid acids can be used as catalysts to reduce the viscosity of heavy oil via mild cracking at the relatively lower temperatures. Solid superacid refers to an acid with a strength corresponding to 100% H2SO4.12 Zirconia promoted by sulfate Received: April 28, 2012 Revised: September 25, 2012 Published: September 26, 2012 6518
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the IM method, the support was soaked by an ammonium metatungstate solution whose pH was adjusted to 6 by an ammonia solution. This pH was chosen according to Yori’s finding:26,27 such a pH value is lower than the isoelectric point (IEP) of Zr(OH)4 and can make large-sized metatungstate species (W12O39)6− evolve into smallsized (HW6O21)5−. The slurry was kept at room temperature for 12 h, dried at 110 °C for 4 h, and calcined at 450−750 °C for 4 h. The corresponding catalyst was designated as x-W/Zr-IM-y, where x and y refer to the tungsten content and the calcination temperature, respectively. For the HD method, zirconia and ammonium metatungstate solution with a liquid-to-solid ratio of 5 mL/g were added into a Teflon-lined stainless steel autoclave, and then heated to 110−180 °C in a roller oven for 12 h. Prior to addition, the pH of ammonium metatungstate solution was also adjusted to 6. After cooling to room temperature, the product underwent the same treatment as W/Zr-IM. This series of catalyst was similarly denoted as x-W/Zr-HD-y. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns were recorded on an X’Pert Pro diffractometer (PANalytical, The Netherlands) equipped with an X’Celerator detector and Cu Kα radiation, operating at 40 kV and 40 mA. The diffraction data was collected in the scanning angle (2θ) range of 10°−80° with a step size of 0.02° and counting time of 12 s at each step. For the quantitative phase analysis, X’Pert HighScore plus software (version 2.2d) with Rietveld structural models based on the ICSD database was applied.28,29 The crystallite sizes of tetragonal and monoclinic zirconia were calculated using the Scherrer equation. The specific surface area, pore volume, and pore size of the sample were determined with a Quadrasorb SI analyzer (Quantachrome, USA) at −195 °C, using liquid N2. Before analysis, the sample was degassed at 300 °C for 6 h under vacuum. Differential scanning calorimetry (DSC) of the noncalcined sample was carried out on a STA-449F3 (Netzsch, Germany) thermal analyzer with a ramp rate of 10 °C/min in an air flow. The acid strength of the catalysts was examined by Hammett indicator method, using nitrobenzene (pKa = −12.4), 2,4-dinitrofluorobenzene (pKa = −14.5), and 1,3,5-trinitrobenzene (pKa = −16.1) as an indicator, respectively.30 Prior to measurement, the sample was dried under vacuum at 120 °C for 2 h. The amount of acid sites was measured by temperature-programmed desorption (TPD) of ammonia on an Autochem II 2920 chemisorption analyzer (Micromeritics, USA) equipped for thermal conductivity detector (TCD). The measurement process is according to the literature.22 The total amount of ammonia desorbed from the sample was estimated by the standard curve obtained from chromatographic analysis,31 and the percentages of weak, intermediate, and strong acids were calculated by Gaussian deconvolution of the TPD curve.32 2.3. Catalytic Activity Assessment. Catalysts W/Zr were used to reduce the viscosity of the heavy crude oil. The reaction was carried out in a stainless steel autoclave (0.5 L) that was equipped with a stirring paddle and a thermocouple. First, the powdered catalyst (0.4 g) was well-mixed with a heavy crude oil (200 g) under vigorous stirring. Second, the mixture was transferred into the autoclave and nitrogen was injected to make the initial pressure 2 MPa. Third, the temperature was increased to 220 °C and maintained for 6 h at the given stirring speed. Finally, the autoclave was cooled to room temperature and the oil product was collected and, simultaneously, the product in gas was collected and analyzed using a gas chromatography (Model GC-9790, Fuli, China) that was equipped with a TCD detector and a flame photometric detector (FPD). The viscosity of oil was measured on a rotation viscometer (Model DJ-8SN, Jingke, China) equipped with a water bath apparatus to keep the sample at 50 °C. Heavy crude oil used here is provided by the Liaohe oilfield with a viscosity of 5.74 Pa s at 50 °C, a density of 0.9589 g/mL at 20 °C, and containing no water. The activity of catalyst is expressed in terms of the viscosity reduction ratio, which is most widely used for the viscosity reduction of heavy crude oil.6−11 Saturates, aromatics, resins, and asphaltenes (SARA) fractions in oil were analyzed according to the industrial standard of China Petroleum Chemical Industry (No. SH/T-0509). The composition of elemental
groups or metal oxides are commonly used solid superacids. Because of the strong acidity, zirconia-based superacids even can catalyze the alkane isomerization at near room temperature.13 Accordingly, it is considered that superacids can be used as catalysts to reduce the viscosity of heavy oil. Jing et al.14 used SO42−/ZrO2 superacid doped with Ni2+ or Sn2+ as a catalyst to reduce the viscosity of Shengli oil without adding water, and they found that Ni2+- or Sn2+-modified SO42−/ZrO2 decreased the viscosity of oil from 0.316 Pa s (50 °C) to 0.135 Pa s and 0.163 Pa s, respectively. Tungsten oxide supported on zirconia also behaves as a solid superacid that is more stable than sulfate-modified zirconia.15 However, little literature is available about the application of zirconia-supported tungsten oxide for the viscosity reduction of heavy oil. The activity of tungsten−zirconia is greatly influenced by the preparation method.16 Many synthetic techniques, such as conventional precipitation, impregnation, microemulsion, and sol−gel, have been employed for the preparation of tungstenpromoted zirconia.17 The hydrothermal method is also used to prepare tungsten−zirconia or zirconia. Armendáriz et al.18 prepared WO3−ZrO2 via the hydrothermal treatment of amorphous mixed zirconium−tungsten hydroxide gels. Cortés-Jácome et al.19 synthesized WO3−ZrO2 by precipitating the solutions of zirconium oxynitrate and ammonium metatungstate with ammonium hydroxide via three precipitate treatment routes: aging, refluxing, and hydrothermal conditions. They found that the hydrothermally treated sample exhibited the highest tungsten dispersion, which caused strong structural deformation of the tetragonal ZrO2, which was responsible for the strongest surface acidity. Pan et al.20 and Song et al.21,22 used hydrothermally synthesized zirconia as a support to prepare Pt-SO42−/ZrO2 and Pt/WO3−ZrO2 via the conventional impregnation method. They found that crystalline zirconia was formed at higher hydrothermal temperatures and the catalysts had more acid sites and a stable pore structure. Moreover, recent research indicates that, because of the low viscosity of water at high temperature and pressure, the hydrothermal environment can promote the diffusion of active species onto the surface and into the pore of support; therefore, the hydrothermal method not only can act as a method to synthesize metal oxides, but also can be applied to load active species onto the support, leading to the formation of highly dispersed active species.23−25 The hydrothermal method had been used to prepare W/Al2O323 and NiW/Al2O324,25 catalysts with high tungsten dispersion. However, to our knowledge, using the hydrothermal method to support tungsten oxide on zirconia has not yet been reported. In the present work, tungsten oxide was supported on zirconia via the hydrothermal and impregnation methods, and their properties were characterized and compared; moreover, they were used as catalysts to reduce the viscosity of heavy oil without water.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Hydrous zirconia support was synthesized through coprecipitation by adding a 14.7 M ammonia solution slowly into a 0.5 M zirconyl chloride (ZrOCl2·8H2O) solution with stirring until the pH of the mother liquor reached 9. The slurry was further aged statically at room temperature for 6 h and then filtered, washed, and finally dried at 110 °C for 12 h. A series of zirconia-supported tungsten oxide (W/Zr) containing various tungsten contents and calcined at different temperatures were prepared via the impregnation (IM) and hydrothermal (HD) methods, which were referenced as W/Zr-IM and W/Zr-HD, respectively. For 6519
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C, H, S, and N in the oil was measured by an elemental analyzer (Model Vario EL-III, Elementar, Germany). To evaluate the variation of boiling range distribution of oil before and after reaction, true boiling point (TBP) stimulated distillation was carried out on a gas chromatography (Model GC-9890B, Renhua, China) that was equipped with a CP-SimDist High Temp capillary column (Varian, USA), according to ASTM D5307. The amount of coke formed during the reaction was obtained by measuring the content of carbon deposited on the recovered catalyst through the above elemental analyzer. The catalyst was recovered via the following steps: first, the fresh catalyst was tabletted; second, after reaction, gasoline was added to dilute oil and then the used catalyst was collected by settlement and filtration; third, the catalyst was washed by gasoline and toluene in turn, until the washed liquid became colorless, to verify the complete removal of the residual oil and gasoline; finally, the catalyst was dried at 120 °C for 12 h to remove toluene. Coke yield during the reaction was calculated by the relationship33 coke yield =
Table 1. Quantitative Phase Analysis Results and Specific Surface Areas of the 15-W/Zr-HD-650 Sample Prepared at Different Hydrothermal Temperaturesa composition (%) hydrothermal temperature (°C)
ZrO2(t)
ZrO2(m)
110 130 150 180
76.5 86.6 91.3 86.0
23.5 13.4 8.7 14.0
WO3(t)
specific surface area (m2 g−1) 58.2 80.2 76.0 107.5
a
ZrO2(m), ZrO2(t), and WO3(t) represent monoclinic (m) zirconia, tetragonal (t) zirconia, and triclinic (t) tungsten oxide, respectively.
content of coke in catalyst × mass of catalyst mass of crude oil
3. RESULTS AND DISCUSSION 3.1. W/Zr-HD Prepared at Various Hydrothermal Temperatures. To investigate the effect of hydrothermal temperature on the structure and property of W/Zr-HD, four 15-W/Zr-HD-650 samples were prepared at temperatures of 110, 130, 150, and 180 °C, respectively. The XRD patterns are shown in Figure 1; the quantitative phase composition results
Figure 2. Rietveld refinement plot of the 15-W/Zr-HD-650 sample prepared at a hydrothermal temperature of 150 °C. Black continuous line corresponds to the experimental data, and the blue cross line corresponds to the calculated data. The difference between the experimental data and the calculated data is also shown, as a continuous red line.
as a support. The hydrothermal environment characterized by lower viscosity and high temperature promotes the collision of hydrated zirconia colloids and thus forms a gel network via oxolation to stabilize the zirconia structure and finally retain a high amount of tetragonal zirconia.20 With the increasing hydrothermal temperature, the surface area of the 15-W/ZrHD-650 sample increases and exceeds 100 m2 g−1 at 180 °C, because of the developed pore structure that formed during hydrothermal treatment.20 Pan et al.20 and Song et al.21 reported that, before calcination, crystalline zirconia was formed in the hydrothermally synthesized hydrous zirconia at hydrothermal temperatures above 130 °C. However, in the present investigation, the noncalcined 15-W/Zr-HD samples prepared at various hydrothermal temperatures were all X-ray amorphous (see Figure 3). The transformation of amorphous zirconia to crystallized zirconia is supposed to be similar to the conversion of γalumina into well-crystallized boehmite upon hydrothermal treatment. Stanislaus et al.34 suggested that the dissociative adsorption of water onto the anion vacancies of alumina and the subsequent diffusion of the hydroxyl groups led to the rehydration of alumina and, thus, the formation and recrystallization of boehmite under hydrothermal conditions. However, the coexistence of tungstate and zirconia may cause some tungstate ions to occupy the anion vacancies and interact with hydroxyl groups on zirconia, thus inhibiting the crystallization of amorphous zirconia, which is close to the
Figure 1. XRD patterns of 15-W/Zr-HD-650 prepared at different hydrothermal temperatures (110 °C (spectrum a), 130 °C (spectrum b), 150 °C (spectrum c), and 180 °C (spectrum d)). Herein, t and m indicate tetragonal and monoclinic zirconia, respectively.
determined via the Rietveld method and the specific surface areas are given in Table 1. Figure 2 shows a typical plot of the Rietveld refinement corresponding to the 15-W/Zr-HD-650 sample prepared at a hydrothermal temperature of 150 °C. As shown in Figure 1, all the samples appear to be a twophase mixture of tetragonal zirconia (International Centre for Diffraction Data (ICDD) PDF Card No. 79-1771) and monoclinic zirconia (ICDD PDF Card No. 83-0942), wherein the tetragonal phase is dominant. There are no free WO3 crystallites detected by XRD on any of the samples. It indicates that the WO3 species are highly dispersed on zirconia and the hydrothermal temperature has little influence on tungsten dispersion. The percentage of tetragonal zirconia increases gradually with the increasing hydrothermal temperature and reaches a maximum at 150 °C, and then decreases (see Table 1). Song et al.21 also found a similar trend when Pt/WO3− ZrO2 was prepared using a hydrothermally synthesized zirconia 6520
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Figure 3. XRD patterns of noncalcined 15-W/Zr-HD samples prepared at different hydrothermal temperatures: 110 °C (spectrum a), 130 °C (spectrum b), 150 °C (spectrum c), and 180 °C (spectrum d).
restraining effect of F or P on the recrystallization of alumina to boehmite.34 Armendáriz et al.18 found that pure zirconium oxyhydroxide gel yielded a mixture of tetragonal and monoclinic zirconia when hydrothermally treated at 145 °C without calcination, whereas, when tungsten−zirconium oxides (15 wt % WO3) were synthesized by hydrothermal coprecipitation at 145 °C, only amorphous zirconia was detected, because the crystallization and phase transformation of zirconia were retarded by tungsten. Nevertheless, they found that tungsten−zirconium oxides became progressively more crystalline with increasing hydrothermal temperature (165 and 225 °C), since high hydrothermal temperatures favored the crystallization of amorphous zirconia. Cortés-Jácome et al.19 also reported the formation of well-crystallized WO3−ZrO2 synthesized via hydrothermal coprecipitation. However, crystalline zirconia is absent on the 15-W/Zr-HD samples, even when the hydrothermal temperature exceeds 150 °C. Consequently, this observation suggests that the HD method may improve the effect of tungsten on delaying the phase transformation of amorphous zirconia. 3.2. W/Zr-IM and W/Zr-HD Samples Calcined at Different Temperatures. A series of W/Zr samples loaded with 15 wt % tungsten and calcined at different temperatures were prepared via the IM and HD methods, respectively, wherein W/Zr-HD samples were hydrothermally treated at 150 °C to obtain more tetragonal zirconia. The XRD patterns are shown in Figure 4, and quantitative phase analysis results and specific surface areas are given in Table 2. The characteristic diffraction peaks corresponding to triclinic WO3 (ICDD PDF Card No. 20-1323) are observed on the W/ Zr-IM samples that have been calcined at 450−750 °C; moreover, the content of free WO3 increases with calcination temperature, because of the poor tungsten oxide dispersion.35 Sohn et al.12 also observed triclinic WO3 in the W/Zr samples loaded with 13 wt % tungsten prepared via the IM method and calcined at 400−1100 °C. The diffraction peaks of WO3 are absent in the W/Zr-HD samples that have been calcined at 450−650 °C, while present at 750 °C. With the increasing calcination temperature, the surface tungsten oxide bonds strongly with zirconia; therefore, some W atoms are trapped in the zirconia lattice, effectively inhibiting the sintering of zirconia, thus causing an increase in tetragonal zirconia. Nevertheless, at higher temperature, the considerable sintering
Figure 4. XRD patterns of the W/Zr-IM sample (spectrum a) and W/ Zr-HD sample (spectrum b) calcined at different temperatures. Herein, the symbol “w” denotes triclinic WO3.
of zirconia will cause the tetragonal zirconia structure to partially collapse and segregate WO3 crystallites.36 Compared to the W/Zr-IM samples, the better tungsten dispersion and greater amount of tetragonal zirconia observed in the W/Zr-HD samples may be attributed to the stronger tungsten−zirconia interaction caused by the reaction of tungsten with hydroxyl groups of zirconia under the hydrothermal conditions. Moreover, the greatly decreased viscosity of aqueous solution under the hydrothermal conditions can promote the diffusion of the tungstate species on the support surface and into pore channels effectively, resulting in improved tungsten dispersion23 and, hence, a better antisintering effect. The crystallite sizes of tetragonal and monoclinic zirconia are listed in Table 2. As the calcination temperature increases, the crystallite sizes increase slightly but a rapid increase appears at temperatures above 650 °C, because of the serious sintering of zirconia. The crystallite sizes of both tetragonal and monoclinic zirconia in the W/Zr-HD samples are smaller than those in the W/Zr-IM samples at the same calcination temperature, confirming the better antisintering effect of tungsten in the former. As the calcination temperature increases, the surface areas of the W/Zr samples decrease considerably. Below 750 °C, the surface areas of the W/Zr-HD samples are much higher than those of the W/Zr-IM samples, because of the better tungsten dispersion, more tetragonal zirconia, and smaller zirconia crystallites. Cortés-Jácome et al.19 synthesized WO3−ZrO2 via the hydrothermal coprecipitation method and found that it had a higher tungsten dispersion and tetragonal zirconia concentration than those coprecipitated at room temperature or with reflux aging. To compare the modification effects of the 6521
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Table 2. Quantitative Phase Analysis Results, Zirconia Crystallite Sizes, and Specific Surface Areas of W/Zr Samples Calcined at Different Temperatures composition (%)
crystallite size (nm)
sample
ZrO2(t)
ZrO2(m)
WO3(t)
ZrO2(t)
ZrO2(m)
specific surface area (m2 g−1)
15-W/Zr-IM-450 15-W/Zr-IM-550 15-W/Zr-IM-650 15-W/Zr-IM-750 15-W/Zr-HD-450 15-W/Zr-HD-550 15-W/Zr-HD-650 15-W/Zr-HD-750 15-W/Zr-HCP-650
70.8 72.1 75.3 42.4 79.3 83.5 91.3 48.5 74.6
28.3 26.8 23.2 52.8 20.7 16.5 8.7 47.9 25.0
0.9 1.1 1.5 4.8
16.4 16.9 16.6 20.6 14.9 14.9 15.2 20.3 15.2
16.2 16.3 17.6 21.2 13.7 14.4 14.7 20.1 16.5
90.4 72.1 55.4 35.5 106.7 87.3 76.0 38.3 55.4
3.6 0.4
slightly higher than that in the 15-W/Zr-HD-650 sample (0), but lower than that in the 15-W/Zr-IM-650 sample (1.5%). In addition, the 15-W/Zr-HCP-650 sample exhibits a much lower surface area than that of the 15-W/Zr-HD-650 sample but is similar to that of the 15-W/Zr-IM-650 sample (see Table 2). The research groups led by Armendáriz18 and CortésJácome19 also found that WO3−ZrO2 synthesized via hydrothermal coprecipitation shows similar surface areas with that synthesized via conventional coprecipitation methods. The average pore sizes of the 15-W/Zr-IM-650, 15-W/Zr-HD-650, and 15-W/Zr-HCP-650 samples are 6.87, 7.02, and 20.50 nm, respectively. It suggests that HCP leads to a more-remarkable pore-enlarging effect caused by the recrystallization of zirconia during hydrothermal treatment, and also confirms the lesser amount of recrystallization occurring in the HD process, which is attributable to the better inhibition effect of tungsten. The main difference between HCP and HD is the pH value of initial solution. In HCP, the adsorption of tungstate and precipitation of zirconyl chloride take place in a basic environment (pH 9−10), whereas, in HD, the adsorption of tungstate on zirconia occurs in an acidic environment (pH 6). Tungstate anions require a positively charged support surface to enable the adsorption. Zirconia derived from zirconyl chloride has an IEP between 6 and 7.27 At pH values above 9, although tungsten-containing anions are dominantly present in smaller-sized WO42− species, the surface of the hydrous zirconia is negatively charged and an electrostatic repulsion will exist between the surface and the tungstate, so the adsorption of tungstate on zirconia may be greatly inhibited and the antisintering effect of tungsten is weakened.17 However, at pH values near and lower than 6, (HW6O21)5− species
