ARTICLE pubs.acs.org/IECR
Kinetic Study and Optimization of Oxidative Desulfurization of Benzothiophene Using Mesoporous Titanium Silicate-1 Catalyst Aryav Sengupta, Prashant D. Kamble, Jayanta Kumar Basu, and Sonali Sengupta* Department of Chemical Engineering, Indian Institute of Technology, Kharagpur India 721302 ABSTRACT: The oxidative desulfurization (ODS) of benzothiophene (BT) in isooctane as a model fuel with 30% aqueous H2O2 was studied using three different titanium silicate (TS) zeolites, synthesized mesoporous TS-1, synthesized mesoporous titanium beta, and commercial TS-1 catalyst, which were found to give 85.6, 45.74, and 25.31% conversions, respectively. Therefore, mesoporous TS-1 was selected as the catalyst for ODS of BT. Reaction time, temperature, catalyst loading, and molar ratio of H2O2:S were selected as the pertinent parameters for the optimization of conversion based on the BoxBehnken design. The predicted maximum conversion was observed to be 89.9% at a temperature of 60 °C, catalyst loading of 0.064 g, and mole ratio of BT and H2O2 of 0.209. An empirical kinetic model was used to fit the rate data. The activation energy was found to be 25.20 kJ/mol.
1. INTRODUCTION The presence of organosulfur compounds such as thiophene (Th), benzothiophene (BT), dibenzothiophene (DBT), and their alkyl derivatives are the major unwanted species present in crude oil fractions. These lead to corrosion in refinery equipment and engines of automobiles, to poisoning of catalysts used in secondary treatment in refineries, and to formation of sulfur oxides after combustion of fuel that cause severe environmental pollution such as acid rain, depletion of the ozone layer, and smog generation.1 Hence it is very essential to reduce the sulfur content in sulfur-bearing petroleum fractions by suitable techniques which are both technologically and economically feasible. Legislation in Japan and Europe have limited the sulfur content in light oil to a maximum of 50 ppm. The U.S. Environmental Protection Agency (EPA) issued new sulfur standards of 30 ppm by 2004 and of 15 ppm by 2006 in diesel fuels and gasoline.2 The Indian government also issued a notification to introduce the EURO IV standards and a sulfur level of 50 ppm in fuel.3 At present, there are several methods which are available for the removal of sulfur compounds from hydrocarbon fuels such as selective adsorption, extractive separation, biodegradation, hydrodesulfurization (HDS), and oxidative desulfurization (ODS). Currently, catalytic HDS is the most popular method for reducing sulfur content in petroleum fractions. HDS is a high severity process which accompanies large operating and capital costs. Moreover, it is difficult to remove polyaromatic sulfur compounds such as BT, DBT, and their derivatives because of very low reactivity.4 By contrast, oxidative desulfurization (ODS) was considered as one of the most effective and alternative methods to produce fuels with very low sulfur content.2,57 ODS is advantageous over HDS because the former process can be carried out at near-ambient conditions such as 50 °C and atmospheric pressure in the liquid phase. The mechanism of the ODS reaction is the electrophilic addition of oxygen atom to divalent sulfur with the formation of unstable sulfoxides (1-oxides) and then sulfones (1,1-dioxides) in the heterocyclic thiophene ring.8 The difference between the physicochemical properties of the sulfones and those of the hydrocarbons present in fuel oil makes easy separation of sulfones from the fuel oil by solvent extraction, distillation, adsorption, etc. r 2011 American Chemical Society
The selective oxidation of thiophene and its alkyl derivatives in n-octane solvent using titanium silicate-1 (TS-1) as catalyst, H2O2 as oxidant, and tert-butyl alcohol or water as solvent was reported.9 Also, ODS of Th, BT, DBT, and their derivatives with tert-butyl hydroperoxide (t-BuOOH) as oxidant were studied using catalysts such as titanium zeolites, MoAl2O3, and cobalt aluminum phosphate.10,11 Improved reactivities of aromatic sulfur compounds were observed at mild conditions using V2O5 Al2O3 and V2O5TiO2 catalysts, when the oxidant H2O2 was added slowly.12 The use of peroxy acids such as performic acid, pertrifluoroacetic acid, and a mixture of formic acid and H2O2 as efficient oxidants was reported for selectively oxidizing sulfur compounds in fuel oil.10 The oxidation of tetrahydrothiophene, diphenyl sulfide, 2-acetylthiophene, and 2,5-dimethylthiophene with H2O2 as oxidant and molecular sieve catalysts such as TS-1, titanium-beta, and TiHMS has been studied.1The use of H2O2 as oxidant over catalysts such as formic acid, polyoxometalate, and molecular sieve zeolites for selective oxidation of thiophenebased compounds has also been reported.13 Among these catalysts, titanium silicates were found to give the maximum conversions under mild conditions. The use of complexes as catalysts, containing transition metals such as Ti, Mo, Fe, Ru, and Re, with H2O2, O2, and O3 as oxidants was investigated for oxidizing sulfur compounds in fuel oil.11 The use of an ultrasound-assisted ODS process was found to enhance the reaction rate of sulfur compounds greatly compared to that of ODS without ultrasound.14 The two-phase ODS reactions faced decreased reaction rate and low conversions because the reaction occurs only at the interface. The use of specific phase-transfer catalysts (PTC) in ODS reactions was proved to be of great advantage compared to ordinary catalysts as PTC transport reactants from one phase to another and hence improve the reaction rate. Hence the application of PTC is of great importance to the industrial use of ODS processes. The use of quaternary ammonium salts such as tetramethylammonium Received: June 24, 2011 Accepted: November 23, 2011 Published: November 23, 2011 147
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Table 1. Properties of Catalytic Materials Ti/(Ti + Si) (mole ratio)
surface area (m2/g)
average pore width (Å)
pore volume (cm3/g)
Ti-beta (mesoporous)
0.059
134.88
128.5
0.418
TS-1 (commercial) (microporous)
0.022
442.64
14.16
0.378
TS-1 (mesoporous)
0.034
514.16
29.91
0.3708
catalyst
bromide, tetraethylammonium bromide, tetrapropylammonium bromide, and tetrabutylammonium bromide as PTC with formic acid as the oxidizing agent under ambient conditions has been reported, mentioning high conversions and faster reaction rates without the use of expensive solvents.14 The use of coordinated ionic liquid (CIL) catalysts formed by the reaction of urea and tetraethylammonium chloride as PTC was also reported for the ODS of fuel oil in the presence of a mixture of acetic acid and H2O2.15 The use of phosphotungstic acid as catalyst, tetraoctylammonium bromide (TOAB) as PTC, and H2O2 as oxidant was reported for ultradeep desulfurization of diesel fuel.16 Titanium silicate zeolites such as TS-1, TS-2, and titaniumbeta with Ti ions in various locations were found to give high conversions in the catalytic oxidation of alkenes, alcohols, and aromatics using H2O2 as oxidant under mild conditions.17 TS-1 was used for removing sulfur compounds such as thiophene in noctane solvent using H2O/tert-butyl alcohol as solvent.9,18 The Ag loaded TS-1 catalyst was used to remove sulfur compounds from FCC gasoline using H2O2 as an oxidizing agent and water as a polar solvent to achieve 86% conversion after 4 h.19 Commercial microporous TS-1 catalyst and 30% aqueous H2O2 solution were used as catalyst and oxidizing agent respectively for oxidation and simultaneous extraction of thiophene from n-dodecane using a CSTR with methanol as the solvent. The oxidation activity was found to increase with the increase in solvent/oil ratio in the mixture.20 In our present study a large pore titaniumbeta zeolite was synthesized by the DGC (dry gel conversion) method using tetraethylammonium hydroxide (TEAOH) as an organic template/structure-directing agent. The catalyst gave high conversions in reactions using either H2O2 or tert-butyl hydroperoxide as oxidant.21 Because of the larger pore size of the DGC synthesized Ti-beta catalyst compared to commercial microporous TS-1, the former gave higher conversions in the case of oxidation of organic sulfur compounds with large molecular structures such as BT, DBT, 2,5-dimethylthiophene, and 4,6-dimethyl-BT. Therefore, to overcome the difficulties associated with microporous TS-1 for the reaction of large structured sulfur compounds, mesoporous TS-1 was synthesized by the DGC method using activated carbon black as the structure-directing agent.22 In our study, we have chosen carbon black as template instead of other better templates such as CMK-3, triethanolamine, and organic silane surfactants such as [3-(trimethoxysilylpropyl)]octadecyldimethylammonium chloride (TPOAC). This is because activated charcoal is cheap and readily available.22 The mesoporous TS-1 catalyst synthesized by the DGC method was used to desulfurize sulfur compounds such as benzothiophene dissolved in isooctane as a synthetic mixture considered as a model fuel. A comparative study for oxidative desulfurization of different sulfur compounds such as thiophene, benzothiophene, and dibenzothiophene over mesoporous TS-1 catalyst is also done. A set of experiments were conducted to study the effects of agitation speed, reaction time, temperature, catalyst loading, and mole ratio of benzothiophene and H2O2 on the conversion of the given sulfur compound. Response surface
Figure 1. (a) Reaction mechanism of ODS of benzothiophene. (b) Reaction stoichiometry of ODS of BT.
methodology (RSM) technique was applied to determine the optimum conversion and the corresponding values of three major process parameters, namely, reaction temperature, molar ratio of benzothiophene and H2O2 in the reaction mixture, and catalyst loading. A kinetic rate equation was also proposed.
2. MATERIALS AND METHODS 2.1. Materials. Benzothiophene was procured from Himedia, India. Dibenzothiophene, hydrogen peroxide solution (30%, v/v), isooctane, tetrapropylammonium hydroxide (40% solution in water), tetra-n-butyl orthotitanate, tetraethyl orthosilicate, sodium hydroxide, and 1-butanol were procured from Merck Specialties Pvt. Ltd. Thiophene and tetraethylammonium hydroxide (25% aqueous solution) was procured from Spectrochem Pvt. Ltd., India. Commercial titanium silicate (TS-1) (1.5 mm extrudate) was supplied by Sud Chemie, India. Sodium aluminate was obtained from Standard Products Pvt. Ltd., India, as a gift chemical. Activated charcoal was obtained from Ranbaxy, India. Concentrated sulfuric acid (assay 9799%) was procured from Qualigens, India. Fumed silica was procured from SigmaAldrich, Laborchemikalien. 2.2. Methods. 2.2.1. Preparation of Titanium-beta and Titanium Silicate-1 by DGC Method. In order to carry out the oxidative desulfurization (ODS) reaction of various thiophene compounds in isooctane, two catalysts in the mesoporous range were synthesized: titanium-beta, prepared by the dry gel conversion (DGC) method using TEAOH as the structure-directing agent,21 and titanium silicate-1, prepared by the DGC method by using a different template, activated carbon.22 TS-1 catalyst 148
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Table 2. Mass Balance of the ODS of BT under Optimized Conditions basis
Table 3. Range and Levels of Process Parameters range and levels
1 mol of H2O2 independent variables
BT + 2H2O2 f BTS + 2H2O
1
0
1
temperature, X1 (°C)
20
40
60
BT
28.1283 g
mole ratio, X2 (S:H2O2)
0.06
0.23
0.4
H2O2
34 g
catalyst loading, X3 (g)
0
0.0375
0.075
H2O
42 g (from 30% H2O2)
total
104.1283 g
final mass (after reaction) BT
2.8409 g
BTS
31.3171 g
H2O2
21.1867 g
H2O
42 + 6.7835 = 48.7835 g
total
104.1282 g
initial mass (before reaction)
with pore width in the microporous range was commercially obtained. The catalysts were characterized by BET apparatus (Quantachrome Autosorb-1, Model AS 1 MP/Chemi-LP) to determine surface area, pore volume, and average pore diameter, and the particle size analysis was done by a Mastersizer 2000E Ver 5.20 (Malvern Instruments Ltd., U.K.). A comparative study of the composition and physical properties of the three different catalysts used in the ODS of BT is shown in Table 1. The particle size analysis of those three catalysts is shown in Figure 4. 2.2.2. Catalytic Experiments. The ODS reaction was performed in a three-necked glass batch reactor (150 cm3) with 5.5 cm i.d. equipped with a glass stirrer, a thermometer for measuring the exact reaction temperature, and a reflux condenser, kept in a water bath whose temperature was maintained at a constant value with an accuracy of (1 °C by using a digital controller cum indicator. The catalytic experiments were carried out in a laboratory scale batch reactor using the three different titanium silicate zeolites, namely, commercial TS-1, laboratorysynthesized Ti-beta, and the TS-1. The model fuel was prepared by dissolving the required amount of sulfur compound (Th/BT/ DBT) in 40 mL of isooctane. It was then added to the batch reactor followed by the addition of specific amounts of different titanium silicate catalysts. The reactor was then heated to a desired reaction temperature. A typical experimental run was carried out at optimum conditions such as maintaining of the temperature at 60 °C, catalyst loading of 0.064 g, and H2O2 to S mole ratio of 4.77 in the reactor. The mixture was then stirred at a desired revolutions per minute up to a reaction time of 3 h, during which samples were withdrawn and analyzed periodically using high performance liquid chromatography (HPLC). The experiment was repeated by varying different process parameters, namely, catalyst loading, reaction temperature, stirrer speed, and mole ratio of H2O2 and sulfur compound, and also the catalyst type and the type of sulfur compound. The analyzer was equipped with an Agilent SB C-18 column (length, 250 mm; diameter, 4.6 mm; packing size, 5 μm). The pump and detector used were a Perkin-Elmer Series 200 pump and a Perkin-Elmer Series 200 UV/vis detector. The mobile phase used was a mixture of methanol and water (9:1 v/v) with a flow rate of 1 mL/min, and detection of the organic sulfur compounds was performed at a wavelength of 254 nm. Parts a and b of Figure 1 show the detailed mechanism and stoichiometry of the BT oxidation reaction by H2O2, respectively. The reaction is a series reaction where BT is first oxidized to
Figure 2. Influence of catalyst type on BT conversion. Stirring speed, 900 rpm; reaction time, 180 min; reaction temperature, 50 °C; H2O2:S, 10:1; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2.
benzothiophene sulfoxide (BTO) and water which is subsequently oxidized by another H2O2 molecule to form benzothiophene sulfone (BTS) and water. The mass balance of the ODS of BT under optimized conditions is described in Table 2. 2.2.3. Experimental Design. Response surface methodology (RSM) was applied to optimize the process parameters for catalytic oxidative desulfurization of benzothiophene with TS-1 as a catalyst and hydrogen peroxide as an oxidant. The reaction temperature, X1 (2060 °C), moles of benzothiophene per mole of hydrogen peroxide, X2 (1:2.5 to 1:15), and amount of catalyst loading, X3 (00.075 g) were used as independent variables to optimize the benzothiophene conversion. The coded and uncoded levels of the independent parameters are shown in Table 3. RSM can be used to define the relationships between the dependent variable called response(s) and the independent process variables by statistical analysis along with the contribution of the effect of the independent variables, alone or in combination, in the processes. In addition to analyzing the effects of the independent variables, this experimental methodology also generates a mathematical model to predict the response. The range of the process variables is predetermined on the basis of some selective trial experimental runs depending on the special criteria and the selection of experimental points. Working with a statistical experimental design not only randomizes the experimental error to each experimental point but also equals the distribution of experimental points in the investigated range of independent variables. These increase the accuracy of the model equation. The main objective of the RSM is to determine the optimum operational conditions for the system and to determine the region that satisfies the operating specifications. Response surface methodology (RSM) is a very efficient tool for optimizing different significant process parameters, and the prediction of a model 149
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Figure 3. (a) Pore size distribution curves for (a) synthesized Ti-beta catalyst, (b) commercial TS-1 catalyst, and (c) synthesized TS-1 catalyst.
generated by them can yield three-dimensional (3D) surface plots and contour plots. Design of experiment (DOE) is a structured, organized optimization technique that is used to determine the relationship between the different factors (X) affecting a process (inputs) and the output of that process termed as response (Y). The response surface methodology coupled with factorial design is an empirical optimization technique used to evaluate the relationship between a set of controllable experimental factors and observed results, and it involves three major steps: (i) performing statistically designed experiments, (ii) estimation of the coefficients in a mathematical model, and (iii) predicting the response and checking the adequacy of the model. Design Expert 7.0 was used as the statistical analysis software for the design. The following quadratic equation was used for the optimization process. Y ðxÞ ¼ a0 þ
effects, respectively. The optimum response (Yopt) and also the corresponding process parameters were also determined. The statistical significance of the model and the coefficients were analyzed by means of the F-test and t test, respectively.23
3. RESULTS AND DISCUSSION 3.1. Selection of Catalyst. The ODS reaction of BT is carried out with three different titanium silicate catalysts, namely, commercial TS-1, Ti-beta, and mesoporous TS-1, under the same reaction conditions. The variation of substrate conversion with time as a function of the catalyst type is illustrated in Figure 2. Figure 2 shows that, after a reaction time of 180 min, mesoporous TS-1 gives the highest conversion among the three different titanium silicate catalysts. Mesoporous TS-1 is found to give a conversion of 85.6%, while Ti-beta and commercial microporous TS-1 give conversions of 45.74 and 25.31%, respectively, under the same reaction conditions.
∑ ai xi þ ∑ aii xi 2 þ ∑ aijxi xj
where Y(x) is the response (substrate conversion) and a0, ai, aii, and aij are the coefficients of the intercept, linear, square, and interaction 150
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Figure 5. Influence of stirrer speed on BT conversion. Reaction time, 180 min; reaction temperature, 50 °C; H2O2:S, 10:1; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; catalyst, mesoporous TS-1.
indicates that the majority of the pores present in the catalyst have a pore width/diameter greater than 20 Å but less than 500 Å, suggesting the sample is of highly mesoporous type. Figure 3b, showing the pore size distribution curve for commercial TS-1 catalyst, proves that the majority of the pores present in the catalyst have a pore width/diameter less than 20 Å, suggesting the commercial variety of TS-1 catalyst is of highly microporous type. Figure 3c, the pore size distribution curve for DGC synthesized TS-1 catalyst, shows an appreciable number of pores with the pore diameter lying within the range from 20 to 500 Å, although some of the pores have a pore width below 20 Å. The reason behind the higher conversion of BT for mesoporous TS-1 compared to synthesized Ti-beta is the higher intrinsic catalytic activity of Ti atoms present in the former catalyst.1 Therefore, in our experimental study we have chosen DGC synthesized mesoporous TS-1 as a catalyst for the ODS of BT. The effects of various process parameters affecting the substrate conversion have also been illustrated. Prior to optimization of the process, all external mass transfer effects were minimized to a sufficiently possible extent through stirrer speed variation. The internal mass transfer effect is assumed to be negligible for all size ranges of catalyst particles because of their very small sizes. The particle size distributions of the three catalysts determined with the help of a particle size analyzer are shown in Figure 4, where it is proved that 90% of the particle sizes of the catalysts, commercial TS-1, mesoporous TS-1, and Ti-beta are 170, 31.2, and 69 μm, respectively. In our earlier work24 the internal resistance to mass transfer for the Ti catalysts was found to be negligible in the size range 845215 μm. Hence, further experiments to determine the internal diffusional effect for this system were not done. 3.2. Effect of Variation of Different Process Parameters on Reactant Conversion/Kinetic Study. The stirrer speed, temperature, catalyst loading, and mole ratio of H2O2 to S are the main process parameters that affect the substrate conversion. The effects of these parameters on reactant conversion are illustrated in Figures 58. 3.2.1. Elimination of External Mass Transfer. The effect of stirrer speed on oxidation of BT was studied by varying the stirrer speed from 300 to 1100 rpm. This was done in order to observe the optimum stirrer speed to overcome the influence of external mass transfer resistance on conversion. Figure 5 illustrates the effect of different stirrer speeds, namely, 300, 500, 700, 900, and 1100 rpm, on reactant conversion. All other process parameters
Figure 4. (a) Particle size distribution for commercial TS-1 catalyst powder: d(0.1) = 4.777 μm; d(0.5) = 38.805 μm; d(0.9) = 170.075 μm. (b) Particle size distribution for mesoporous TS-1 catalyst powder: d(0.1) = 5.191 μm; d(0.5) = 13.288 μm; d(0.9) = 31.233 μm. (c) Particle size distribution for Ti-beta catalyst powder: d(0.1) = 5.977 μm; d(0.5) = 30.812 μm; d(0.9) = 69.049 μm.
The low conversion of BT in the case of commercial TS-1 is because of the large size of bulky BT molecules that could hardly penetrate the small pores of the catalyst1 as the average catalyst pore width lies in the microporous range (14.16 Å). Moreover, the total pore volume of TS-1 commercial is 0.378 cm3/g; of that, the micropore volume is 0.2544 cm3/g, and the micropore surface area is 319.7 m2/g. However, in the cases of Ti-beta and mesoporous TS-1, the BT conversion is sufficiently higher, probably due to their large pore size and mesopore volume. Mesopore volumes of Ti-beta and TS-1 (mesoporous) are 0.4185 and 0.1978 cm3/g and mesopore surface areas are 134.3 and 329.3 m2/g, respectively. The titanium silicate catalysts (Ti-beta and TS-1) prepared by the DGC method have the average catalyst pore width lying in the mesoporous range. Figure 3 shows the pore size distribution curves for the three catalysts. Figure 3a, showing the pore size distribution curve for Ti-beta, 151
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Figure 6. Influence of reaction temperature on BT conversion. Stirring speed, 900 rpm; reaction time, 180 min; H2O2:S, 10:1; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; catalyst, mesoporous TS-1.
Figure 8. Influence of mole ratio of hydrogen peroxide to S on BT conversion. Stirring speed, 900 rpm; reaction time, 180 min; reaction temperature, 60 °C; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; catalyst, mesoporous TS-1.
loading on the reactant conversion, with or without catalyst. The absence of any catalyst in the reaction medium shows a very low conversion of BT. It is noted that the substrate conversion increases by increasing the catalyst amount from no catalyst to 0.075 g of catalyst from 0.69 to 95% at 3 h point of time. Therefore, this result reflects a very prominent catalyst effect on conversion. With increase in catalyst loading from 0.01 to 0.075 g, the increase in conversion is 82%. The bulk preparation of DGC synthesized mesoporous TS-1 is costly. Therefore, in order to economize the production of TS-1 catalyst, we consider 0.05 g as the catalyst amount in most of our experiments because the conversions for 0.05 and 0.075 g are close. 3.2.4. Influence of H2O2 and BT (S) Mole Ratio on the Reaction. The effect of the mole ratio of H2O2:S on reactant conversion was studied by varying it from 2.5:1 to 15:1 and by keeping the stirrer speed constant at 900 rpm, the reaction temperature at 60 °C, and the catalyst loading at 0.05 g. Figure 8 illustrates the effect of mole ratio on reactant conversion. It is noted that with an increase in the molar ratio of H2O2 and BT, the reactant conversion increases markedly until the ratio reaches 10:1. Beyond this ratio there is no appreciable increase in the conversion of BT. Hence the mole ratio is considered to be 10:1 for all subsequent reactions. 3.2.5. Effect of Reaction Time on the Conversion of BT. In order to choose the optimum process time, BT conversion was observed by varying the reaction time from 15 to 240 min, in trial experiments, keeping all other process parameters constant. This is represented in Figure 9. The BT conversion increases from 53.2% to about 89.07% from 15 to 240 min time at 60 °C, stirrer speed of 900 rpm, and catalyst loading of 0.05 g. However, it is found that at 180 min the conversion is 88.79%; hence, there is very little increase in conversion from 180 to 240 min. Therefore, further increase in the reaction time is not significant in increasing the conversion. However, H2O2 is highly unstable and with an increase in reaction time more and more H2O2 decomposes into water and oxygen, which is not desirable for the reaction.13 Therefore, 180 min was chosen as the optimum reaction time. 3.3. Effect of TS-1 Catalyst on the Conversion of Various Thiophene Derivatives. Thiophene (Th), benzothiophene (BT), and dibenzothiophene (DBT) are the major thiophenic sulfur compounds present in gasoline. Three separate reactions
Figure 7. Influence of catalyst loading on BT conversion. Stirring speed, 900 rpm; reaction time, 180 min; reaction temperature, 60 °C; H2O2:S, 10:1; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; catalyst, mesoporous TS-1.
such as reaction temperature, catalyst loading, and H2O2 to S mole ratio were kept fixed. It was found that the substrate conversion was highly influenced by the external mass transfer at or below 900 rpm and above that value of revolutions per minute all external mass transfer effects are negligible. Hence, all the successive experiments were carried out at 900 rpm in order to save power consumption for agitating the reactant mixture. 3.2.2. Temperature Dependency of ODS Reaction. The effect of temperature on reactant conversion was studied by varying the temperature from 20 to 70 °C at a stirrer speed of 900 rpm, catalyst loading of 0.05 g, and mole ratio of H2O2 to S at 10:1, and the results are illustrated in Figure 6. The temperature has a direct effect on the kinetic rate constant (k), thereby increasing the rate of reaction and hence the substrate conversion. It was found that the conversions at 60 and 70 °C are 89 and 90.1%, respectively, at 3 h. Hence 60 °C was chosen as the optimum temperature. 3.2.3. Variation of Catalyst Loading. The effect of catalyst loading on reactant conversion was studied by varying the catalyst amount from 0.01 to 0.075 g and by keeping the stirrer speed constant at 900 rpm, the reaction temperature at 60 °C, and the H2O2:S mole ratio at 10:1. The same reaction was carried out without catalyst, also. Figure 7 illustrates the effect of catalyst 152
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Table 4. BoxBehnken Design Matrix coded values observation
Figure 9. Effect of reaction time on conversion of BT. Stirring speed, 900 rpm; reaction temperature, 60 °C; n(H2O2):n(S),10:1; catalyst amount, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2; reaction time, 240 min.
Figure 10. Conversions of different thiophenic compounds on mesoporous TS-1 (180 min of reaction time). Stirring speed, 900 rpm; reaction temperature, 50 °C; H2O2:S, 10:1; catalyst loading, 0.05 g; volume of isooctane, 40 mL, with 2780 ppm BT; oxidizing agent, 30% aqueous H2O2.
X1
X2
X3
actual values X1
X2
X3
conversion Yexpt
Ypred residual
1
1
0
1 60 0.23 0.075
85.62
87.63 2.01
2
1
1
0 60 0.06 0.038
89.37
86.18
3.19
3
1
0
1 20 0.23 0.075
44.19
40.57
3.62
4
0
0
0 40 0.23 0.0375 58.36
58.36
0.0
5
0
1
1 40 0.06 0.075
86.57 1.2
1 20 0.23 0.000
86.37
6
1
0
7
0
1
8 9
1 1
0 1
10
0
0
0 40 0.23 0.038
58.36
58.36
0.0
11
0
0
0 40 0.23 0.0375 58.36
58.36
0.0
12
1
1
0 20 0.06 0.0375 36.39
38.80 2.41
13
1
1
0 60 0.4
14
0
1
1 40 0.4
15
0
1
1 40 0.4
0.075
0.850 1.15 38.23
2.0
38.65 0.42
1 60 0.23 0.000 3.570 7.19 3.62 0 20 0.4 0.0375 28.72 31.92 3.2
39.94
2.41
0.0000
0.0375 42.35 1.13
0.07
1.2
1 40 0.06 0.0000
4.57
4.13
0.44
Figure 11. Comparison between experimental and predicted responses (Y).
parameters (+1, 0, 1), their actual values, and the corresponding responses (predicted values). The residual conversion is determined by the difference between the experimental and predicted conversions. The plot of actual versus predicted conversion of BT is shown in Figure 11. The best fit curve for the comparative study represents a 45° line passing through the origin and having a regression coefficient (R2) value of 0.994. The following quadratic model was obtained to describe the mathematical relationship between the three independent process parameters (X1, X2, X3) and the dependent response (Y):
were carried out taking those sulfur compounds singly, under similar reaction conditions, which include a stirrer speed of 900 rpm, a reaction temperature of 50 °C, a catalyst loading of 0.05 g, and a H2O2:S mole ratio of 10:1. With the use of mesoporous TS-1 as the catalyst, it was found that thiophene and its derivatives were oxidized at different rates, thereby leading to different conversions at the end of 180 min. The more condensed aromatic thiophenes, namely, DBT and BT, were oxidized much faster than the thiophene.1 The effect of DGC synthesized mesoporous TS-1 on the conversion of different thiophenic sulfur components is illustrated in Figure 10. The reaction was carried out at a temperature of 50 °C, a stirrer speed of 900 rpm, a catalyst loading of 0.05 g, and H2O2:S at 10:1. Mesoporous TS-1 catalyst was found to give 51.01% conversion in the case of thiophene, 85.6% conversion in the case of BT, and a maximum of 89.64% conversion in the case of a more condensed DBT. 3.4. Optimization by BoxBehnken Design. The Box Behnken design was chosen in this study for optimization of the conversion of BT substrate (Y) in the ODS reaction. In our study 15 experimental observations were taken at random for the optimization process. Table 4 shows the data resulting from the 15 random experiments which involve the coded values of the
Y ðXÞ ¼ 58:36 þ 13:85X1 13:28X2 þ 30:54X3 4:08X1 2 5:07X2 2 20:72X3 2 9:84X1 X2 þ 9:68X1 X3 11:18X2 X3 ð1Þ The results of the analysis of variance (ANOVA) shown in Table 5 present the successful validation of the experimental data to the predicted model. The F-value for the model was found to be 106.38, which is much higher than the tabulated F-value (F95,0.05) of 4.77. Such a large value of F for all models indicates that the predicted second-order polynomial is highly significant to a 95% confidence level. The corresponding P-values in Table 5 153
dx.doi.org/10.1021/ie2024068 |Ind. Eng. Chem. Res. 2012, 51, 147–157
Industrial & Engineering Chemistry Research
ARTICLE
X2X3, X1X3, X22, and X32 are significant model parameters. The P-value for the term X12 was found to be slightly higher than 0.05. This signifies that the term is of relatively less significance. Other terms such as X13, X23, and X33 with very large P-values are neglected from the predicted equation to improve the accuracy, and the correlation coefficient was found to be 0.9974. This signifies that the predicted response tallies well with the experimental data. A very high R2 (R-Sq) value of 0.9948 indicates that the predicted polynomial model is reasonably well fitted with the data. The predicted R2 (Pred R-Sq) value of 0.9169 is in reasonable agreement with the adjusted R2 (Adj R-Sq) value of 0.9855. Response surfaces can be visualized as 3D plots that represent the variation of the response with two parameters, keeping the other parameters fixed. The resulting 3D surface response plots for the BT conversion as a function of (a) temperature and moles of BT per mole of H2O2, (b) temperature and catalyst loading, and (c) catalyst loading and moles of BT per mole of H2O2 are shown in Figure 12, respectively. The response surfaces of mutual interactions between the parameters were found to be elliptical in nature. The coordinates of the central point within the maximum contour levels in each of the given figures represent the optimum values of the respective parameters. The maximum
are used to analyze the F-statistics to explain the statistical significance between the predicted response and actual response. A P-value lower than 0.05 signifies that the model is statistically significant. Therefore, in this case the terms X1, X2, X3, X1X2, Table 5. Analysis of Variance (ANOVA) for Conversion DF
Seq SS
Adj SS
Adj MS
F
P
model
9
3314.48
3314.48
1479.39
106.38
