Kinetics of Catalytic Oxidation of Gas Oils - American Chemical Society

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Ind. Eng. Chem. Res. 2009, 48, 5642–5655

Selective Homogeneous Oxidation System for Producing Hydroperoxides Concentrate: Kinetics of Catalytic Oxidation of Gas Oils Syed Mumtaz Danish Naqvi and Fasihullah Khan* Department of Applied Chemistry & Chemical Technology, and Department of Chemical Technology/ Engineering, UniVersity of Karachi, Karachi-75270, Pakistan

A selective and mild (55 °C and 50 psig) homogeneous oxidation system has been described to produce hydroperoxides from a narrow cut made from typical gas oil. During this process oil, dissolved in wet acetic acid, is treated with air in the presence of the redox couple Co(III)/Co(II) (partially oxidized catalyst, POC). A straightforward route to POC consists of oxidizing Co(II) in wet acetic acid with calculated aqueous potassium chlorate. Initiation is the manifestation of the potent oxidizing ability of Co(III) toward generating alkyl radicals from hydrocarbons in the oil. Classical propagation then leads to assorted hydroperoxides, simultaneously decomposed by POC. Kinetics of the simultaneous formation and decomposition of hydroperoxides, under varying sets of process and composition variables, was studied in an isothermal, mechanically agitated, semibatch reactor, operating in the pure kinetic regime. A unique kinetic model, based on a rare irreversible consecutive reaction network (pseudo-first-order formation of hydroperoxides followed by their second-order decomposition), has been found to reasonably describe the kinetic data (R2 ) 0.9214). This kinetic model allowed the evaluation of the formation and decomposition rate constants that permitted the estimation of conversion of oil and selectivity to hydroperoxides. On average 15.12 ( 3.23% conversion was achieved in 9.3 ( 4.0 min with 99.86 ( 0.23, 92.88 ( 7.95, and 28.77 ( 11.67% selectivities for 0.10, 1.00, and 10.0% conversions, respectively. 1. Introduction Gas oils (rich source of intermediate molecular weight paraffins) could be used as raw material for cheap production of hydroperoxides (ROOH). Hydroperoxides produced in this manner would be of utmost interest for industries based on polymerization and oxidation processes since it provides them with an economical source of the fundamental constituent, which form the basis for such processes. The present research was focused to devise a homogeneous oxidation system for practical conversion of gas oils into high grade paraffinic hydroperoxides with exceptional selectivities. Hydroperoxides are organic compounds containing peroxy (-O-O-) groups that can easily break, forming free radicals. This property makes pure hydroperoxides, or cheaper mixtures thereof, vital constituents for polymerization and oxidation processes. The plastic and rubber industries are the biggest users where such compounds are used as accelerators, activators, catalysts, cross-linking agents, curing agents, hardeners, initiators, and promoters. The second major consumers are industries based on oxidation processes where hydroperoxides are used as efficient oxidants due to fewer handling risks than hydrogen peroxide, less corrosiveness, fair thermal stability, better selectivity, excellent solubility in nonpolar solvents, neutral nature, and easy separation from the coproduct (corresponding alcohols). Minor uses include catalyst components for olefins epoxidation etc.1-3 Being an essential raw material, cheap production of hydroperoxides is highly desired. While pure hydroperoxides are usually prepared from the corresponding hydrocarbons,4-8 refinery streams such as gas oils can directly be oxidized to a mixture of hydroperoxides.3 The processes available for production of hydroperoxides3-8 usually make use of heterogeneous catalysis under harsh conditions at which hydroperoxides are * To whom correspondence should be addressed. E-mail: [email protected].

inherently unstable, resulting in poor selectivities. Moreover heterogeneous catalysis is expensive. Although high selectivity and economical operation is possible under mild conditions, but such conditions in association with heterogeneous catalysis results in an uneconomical slow reaction rate. Additionally the use of pure hydrocarbons4-8 would be more expensive as compared to refinery streams.3 Therefore a compromise between high selectivity and low cost is possible through homogeneous operation by virtue of its permitting mild reaction conditions and simplified operation9 and using refinery streams which require none or minimal pretreatment. Primarily the present study aimed at developing an environment friendly homogeneous process that allows the use of cheap raw materials (gas oils as substrate, air as oxidant, and acetic acid as solvent) for production of hydroperoxides concentrate. The reaction has been shown to be promoted by cobalt based complex ions of the type Co(III) ) [Co3O(OAc)6(HOAc)3]+ + [Co3O(OAc)5(OH)(HOAc)3]+.10-12 Homogeneous operation in conjunction with such ions actually permits the use of mild reaction conditions, a prerequisite for improved selectivity, and economical operation as is achieved during present work. A simple and cheap source of Co(III) has been described during this work as a chemically generated redox couple Co(III)/ Co(II) (partially oxidized catalyst, POC), synthesized by oxidizing cobalt(II) acetate tetrahydrate in wet acetic acid with aqueous potassium chlorate. This novel POC has been revealed in the following sections to offer a modified, simple, yet inexpensive way of producing hydroperoxides concentrate in practical time periods. The process ensures high selectivity to hydroperoxides during the initial stage and can be fine-tuned by varying process and composition variables. The major disadvantage of homogeneous catalysis is usually the separation of the product from the reaction mixture.9 However, in the present study, it is found that the hydroperoxides concentrate may very easily be separated from the reaction mixture by diluting it with excess water followed by

10.1021/ie900364w CCC: $40.75  2009 American Chemical Society Published on Web 05/11/2009

Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009

extraction with a medium polarity, low boiling point organic solvent such as chloroform. Water readily reduces any unstable Co(III) to stable cobalt(II) aquo-complex that has almost zero solubility in the extracting solvent. The solvent may then be removed using vacuum distillation yielding hydroperoxides concentrate, a readymade dilute solution of hydroperoxides in the remaining unreacted gas oil along with some decomposition products such as carbonyls, etc. The resulting preparation needs not be further processed before marketing. Another remarkable feature of the study consists of successful modeling of the kinetic data, using an irreversible consecutive reaction network consisting of pseudo-first-order formation of hydroperoxides followed by their second order decomposition. This is a unique example of a consecutive reaction that is almost scant in the literature. 2. Theoretical Details Partial oxidation of hydrocarbon (RH) substrates with air is one of the most important unit processes.13,14 Generally this reaction proceeds by a free-radical chain process involving alkylperoxy (RO2•) radicals, and the primary product is the corresponding hydroperoxide. For most such substrates in solution, in the presence of sufficiently high dissolved oxygen concentration, the rate controlling propagation step of the chain involves attack on the substrate by alkylperoxy radicals. In these cases, the chain reaction may be represented by15-19 k1

RH + O2 98 R• + HO2• k2

R• + O2 98 RO2• k3

RH + RO2• 98 ROOH + R•

(1)

(2)

(3)

At high oxygen concentration k2 . k3, therefore, termination occurs through following bimolecular process:15-17 k4

2RO2• 98 products + O2

(4)

This process is characterized by a long induction period caused by slow initiation (1).14 To get rid of this, cobalt ions are used, which have the ability to shorten the induction period through decomposition of hydroperoxides which have either been added as initiator or formed in situ:1,13,14,18 ROOH + Co(II) f Co(III) + RO• + OH-

(5)

ROOH + Co(III) f Co(II) + RO2• + H+

(6)

2,13,14,17,20

Several workers pointed out that during these reactions cobalt ion cycles between two oxidation states, to produce a large number of free radicals; thus, it plays the role of a catalyst for such decomposition, and the overall stoichiometry of these reactions therefore constitutes a bimolecular process that does not involve metal ions: Co(III)/Co(II), k5

2ROOH 98 RO• + RO2• + H2O

(7)

Apart from this there is an entirely different class of hydrocarbon oxidations wherein induction period is eliminated by using the potent oxidizing agent Co(III), even when hydroperoxides have initially been excluded carefully. In these

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cases initiation is caused by direct electron transfer from C-H bonds to Co(III).21-36 However, mechanisms for such initiations cannot be similar for saturates (paraffins and naphthenes)21-26 and aromatics:27-36 Saturates RH + Co(III) f R• + Co(II) + H+

(8)

Aromatics RH + Co(III) T RH+ + Co(II)

(9)

+

•

+

RH f R + H (10) The use of either pure Co(III) or a POC of some kind could therefore promote initiation. There are examples for such POC applications; however, the ways that the POC is employed are different: e.g. preoxidation of part of Co(II) acetate with ozone,26 use of a mixture of Co(III) and Co(II) acetates,30 or in situ electrochemical generation35,36 are reported. Laborious preparation of Co(III) acetate37 restricts easy handling while electrochemical assistance naturally requires additional costs. This work describes a straightforward and cheap chemical route to POC where cobalt(II) acetate tetrahydrate in wet acetic acid is treated with aqueous chlorate. As a result pink Co(II) ) Co(HOAc)n(H2O)m(OAc)2 oxidizes to olive green Co(III):12 H2O, 85 - 90oC, ≈ 15min

ClO3- + 6H+ + 6Co(II) 98 6Co(III) + Cl- + 3H2O

(11)

This technique offers the flexibility of varying the amount of Co(III) in the POC mixture by altering the amount of chlorate (Figure 1). Although Co(III) is a powerful oxidizing agent in the thermodynamic sense but the electron transfer between highspin Co(II) and low-spin Co(III) is slow,38 hence sluggishness (taking ≈15 min for apparent completion) consistent with the present observations reveals that materials involved are certainly high-spin Co(II) and low-spin Co(III). This novel POC system has been applied during present work for production of hydroperoxides concentrate from air oxidation of substrate “oil” isolated from high speed diesel, HSD (Scheme 1). Since suggested POC always contains some Co(III), initiation (reactions 8-10) would result that does not require the addition of any free-radical initiator and is associated with a negligible induction period. Propagation (reactions 2 and 3) will then result in assorted hydroperoxides (paraffinic + naphthenic + aromatic) whose yield attains a maximum value in time domain. This maximum yield will be proportional to the amount of Co(III) initially present. Since propagation is a moderate process15-17 while initiation involving electron transfer from substrate to Co(III) is a slow process,38 hence rate of formation of hydroperoxides during the initial stage of the reaction will be governed by the initiation. In the early stage in particular both reactions 5 and 6 will be responsible for decomposition. However, during the later stage, in the presence of chloride ions and dissolved oxygen, reactions 27 and 29 in addition to reaction 5 will maintain an equilibrium concentration of Co(III) responsible for decomposition via reaction 6. Since shuttling Co(III) S Co(II) is a slow process,38 the overall decomposition (reaction 7) is expected to be much more gradual. As the reaction progresses and concentration of oil decreases, competition between formation and decomposition will persist up to the point where decomposition will become significant. Thus during the initial stage, the process will result in a sigmoidal change in the yield of hydroperoxides in the time domain.

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Figure 1. Illustrative example of POC material balance, based on equations given on the right-hand side written for (CH3COO)2Co · 4H2O according to stoichiometry of reaction 11. (data) COil ) 0.01795 mol/L, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%, VR ) 200 mL, VC ) 30 mL, concentration of stock solutions (CCoAc ) 0.1 mol/L, CKClO3 ) 0.25 mol/L, CKCl ) 0.50 mol/L), molar masses (MCoAc ) 249.08 g/mol, MKClO3 ) 122.55 g/mol, MKCl ) 74.56 g/mol, MH2O ) 18.015 g/mol, MOil ) 227.16 g/mol equivalent to average molar mass of middle distillate range saturates54), densities @ 25 °C (FSKClO3 ) 1.0156 g/mL, FSKCl ) 1.0198 g/mL, FH2O ) 0.99704 g/mL, FOil ) 0.8155 g/mL).

Scheme 1. Isolation of Substrate Oil from HSD

Free radicals produced as a result of the simultaneous decomposition of hydroperoxides (reaction 7), undergo C-C bond cleavages leading to carbonyls that are further oxidized to corresponding acids.13 Another route for disappearance of hydroperoxides could be the formation of peracetic acid, similar to a reversible reaction between acetic acid and hydrogen peroxide which requires either ion-exchange resin39 or sulfuric acid40-42 as catalyst and where the formation of peracetic acid is favored by the use of excess hydrogen peroxide.41,42 In the present study, no sulfuric acid or resin was used and further the concentration of oil in the reaction mixture was small (0.003-0.03 mol/L) leading to even smaller concentrations of hydroperoxides. Therefore, formation of peracetic acid would be highly improbable. In this manner, prolonged oxidation of oil can lead to a variety of compounds. However, the present study was aimed at the production of hydroperoxides, and therefore, only those steps leading to hydroperoxides were considered. 2.1. Kinetic Models. Essentially the proposed process will produce hydroperoxides as intermediates. Therefore, the process may be envisioned as a sequence where the formation of hydroperoxides is followed by their decomposition. This requires the reactor to be accordingly designed for the optimum hydroperoxides yield and selectivity that is necessary for economical operation. The most important question in this respect would be, at which time will the amount of hydroperoxides be at its maximum in the reactor? This question will be unanswered unless a rational mathematical description of the process is available that describes the experimental dynamic response. On the basis of the nature of the reaction system, this mathematical model could be represented by an irreversible consecutive reaction network. 2.1.1. Model “A”. In a complex hydrocarbon mixture, as the one employed during the present study, initiation in the presence of POC would occur through reactions 8-10 and probably through much easier aromatics (reactions 9 and 10). Thus, the presence of a small amount of aromatics is beneficial


from an initiation viewpoint. Furthermore, a mechanically agitated semibatch reactor, as used during present work, ensures intimate mixing of gas and liquid phases, resulting in saturation of the reaction mixture with oxygen.43 Net production of hydroperoxides, under these conditions, may be assumed to follow a mechanism consisting of reactions 2, 3, 7 and 4, where RH ) Oil. The rates of consumption of oil and accumulation of total hydroperoxides may therefore be written as

2

( (

))

τ I1(2√κτ) - β′(2K1(2√κτ)) 100 κ I (2√κτ) + β′(2K (2√κτ)) 0 0

Yˆ′ ) -

(18)

where

(12)

dCROOH ) k3CRO2•COil - 2k5CROOH2 (13) dt These rates may differently be expressed by such equations that do not involve the unknown concentration of the alkylperoxy radical. These expressions of the rates would be based on an irreversible consecutive reaction network where pseudofirst-order formation of hydroperoxides is followed by their second order decomposition under the following assumptions: kF ) k3CRO

Where IR and KR are modified Bessel functions involving real arguments. Therefore eq 17 may be rewritten as

β′ )

dCOil ) -k3CRO2•COil dt

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I1(2√κ) 2K1(2√κ)

2.1.2. Model “B”. In order to ascertain whether or not model A, eq 18, adequately describes the experimental time course yield data, a variant to model A can be constructed where the decomposition of hydroperoxides may assumed to be pseudofirst-order despite having a bimolecular stoichiometry similar to reaction 7. In such a case, the consecutive sequence would become k′ k′ F D Oil 98 ROOH 98 products

(19)

•

Oil 98 ROOH

The rate of accumulation of total hydroperoxide in this case will become

kD ) 2k5

2ROOH 98 products

(14)

These assumptions further simplify the situation, and the rate eqs 12 and 13 may simply be written as

dCROOH ) kF′ COil - kD′ CROOH (20) dt Equation 20 is a first order linear differential equation in CROOH. Using the approach of Chien,44 the solution to eq 20 would be

dCOil ) -kFCOil dt

τ′κ′ - τ′ Yˆ″ ) 100 1 - κ′

dCROOH ) kFCOil - kDCROOH2 (16) dt Equation 16 is a first-order nonlinear differential equation in CROOH. Chien theoretically discussed a similar general consecutive reaction.44 Following his treatment the solution to eq 16 may be written in terms of Bessel functions. Accordingly, the simulated yield of hydroperoxides in time domain may be expressed by Yˆ )

( )

CROOH 100 ) COil

( (

))

(1) τ iJ1(2i√κτ) - βH1 (2i√κτ) 100 κ J (2i√κτ) - iβH(1)(2i√κτ) 0 0 (17)

where β) τ ) e-kFt κ )

COilkD kF

iJ1(2i√κ) H(1) 1 (2i√κ)

H(1) R (2i√κτ) ) JR(2i√κτ) + iYR(2i√κτ)

R ) 0, 1 In eq 17 JR and YR are Bessel functions of first and second kind and H(1) R is a Hankel function of first kind. As eq 17 involves imaginary arguments, a more convenient form, from a computational point of view, may be obtained through following transformations:45 IR(2√κτ) ) i-RJR(2i√κτ) KR(2√κτ) )

π R+1 (1) i HR (2i√κτ) 2

(

(15)

)

(21)

where ′

τ′ ) e-kFt kD′ κ′ ) kF′ 2.1.3. Conversion and Selectivity. The conversion of oil χA and χB for models A and B, respectively, will then be very easily defined as χA )

(

COil - COilt COil

)

100 ) (1 - τ)100

(22)

χB ) (1 - τ′)100

(23)

And selectivity to hydroperoxides SA and SB for models A and B would, respectively, be SA )

(

)

(

)

kDCOil(Yˆ′/100)2 dCROOH /dt 100 ) 1 100 -dCOil /dt kF

(

SB ) 1 -

)

kD′ (Yˆ″/100) 100 kF′

(24) (25)

3. Experimental Section 3.1. Materials. HSD was supplied by Shell, Pakistan. Substrate oil was isolated from HSD (Scheme 1). The Fourier transform infrared (FTIR) spectrum of oil was acquired on Bruker VECTOR-22 spectrophotometer. GC/MS analysis was performed on Agilent 6890N gas chromatograph coupled to a Jeol JMS-600 MSroute mass spectrometer (70 eV, m/z ) 50-450). Separation (1.0 µL, split ratio ) 1:35) was achieved

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Figure 2. (a) Schematic of the indigenously designed and fabricated experimental rig where the core reactor is a 316-stainless steel autoclave. The process constitutes a semiautomatic, isothermal, mechanically agitated, semibatch operation. (b) Schematic of the analytical rig for dead-stop-end-point biamperometric determination of hydroperoxides, a modification of the rig previously defined.49

on an HP-5 column (30 m × 0.32 mm, 0.25 µm film) with helium as the carrier gas (1.8 mL/min). The gas chromatograph was temperature programmed (injector ) 250 °C, oven ) 50 °C for 2 min; 5 °C/min to 250 °C; hold for 30 min, detector ) 250 °C). Chemical reagents (cobalt(II) acetate tetrahydrate, potassium chlorate, potassium chloride, acetic acid, 2-propanol, potassium iodate, potassium iodide, sodium thiosulfate, sulfuric acid, sodium hydroxide, magnesium sulfate, basic alumina 90, silica gel 60, and petroleum spirit) were of extra pure or analytical grade, purchased commercially, and were used in general without any further treatment. However, alumina (400 °C, 16 h) and silica gel (200 °C, 2 h) were activated prior to use. Nitrogen (99.999%) was obtained from BOC Pakistan. Wet acetic acid was selected as solvent because it provides the essentially inert reaction medium in which both POC and oil are soluble forming homogeneous mixture. 3.2. Safety. Potassium chlorate and hydroperoxides are strong oxidizing agents, and contact with reducible material may cause explosion or fire. Further self-decomposition of hydroperoxides, at temperatures above ambient may also lead to explosion or fire. Additionally, potassium chlorate is a potential irritant whereas hydroperoxides may be toxic or corrosive. Although dilute neutral aqueous solution of potassium chlorate is safe, POC mixture should be prepared in Pyrex or stainless steel vessels. As chlorate is completely reduced to chloride (reaction 11), it will not eventually create any environmental hazard. Likewise, cobalt(II) acetate tetrahydrate falls under the irritant category and is a likely mutagen while acetic acid is corrosive. After separation of the hydroperoxides and the expensive cobalt catalyst, waste liquor will contain acetic acid, water, and potassium chloride, making the process environmentally benign. Air was selected as oxidant because of its low cost and relative safety as compared to pure oxygen or mixtures thereof and ozone. Due to the overall corrosive nature of the raw materials

and products, the materials of construction for experimental and analytical rigs were appropriately selected as 316-stainless steel or Pyrex. 3.3. Chemically Generated POC System: Material Balance and Synthesis. In the illustrated POC material balance (Figure 1), point x ) 0.00 corresponds to zero chlorate added; therefore, the entire system is in the form of Co(II). At x ) 1.00, moles of Co(III) become equal to the moles of Co(II) taken initially, along with Cl- present at all but x ) 0.00. Between these two extremes, various proportions of POC are possible. Since nKClIni ) f(nCo, x), the POC system is fortified at all but x ) 1.00, with nKClAdd moles of potassium chloride to get to the same level of Cl- in every POC mixture. For elucidation of the effect of excess Cl- on the oxidation of oil, excess may be added in the form of potassium chloride. On the whole, the shape of the material balance is independent of nCo and VR. At a certain nCo value, nH2OCoAc remains constant, whereas nH2OKClO3 ) f(x). As water is expected to affect the proportion of POC,46,47 so nH2OC moles of water must be added so that each POC mixture is produced at the same water level nH2OMax. During the present study, it was selected as 1.4 mL, sufficient for CCo up to 0.01 mol/L and VR ) 200 mL. As the POC mixture is fortified with aqueous potassium chloride that is another source of water, so the reaction mixture is prepared with a certain known water level nH2O by adding additional water VH2OR. The volume of water VH2O can be varied to study the effect of water concentration upon oxidation of oil. During synthesis, POC mixture ingredients (Figure 1) were taken in stoppered test tubes and refluxed at 85-90 °C for 15 min. After cooling, calculated aqueous potassium chloride was added. POC mixtures were kept stoppered in a refrigerator until used. 3.4. Oxidation of Oil. Oxidation of oil in wet acetic acid was performed on the experimental rig (Figure 2a). The exhaust gas absorber contained 10% aqueous sodium hydroxide. Reaction mixture ingredients (Figure 1) along with the POC mixture


Figure 3. Precision analysis for the experimental technique. (reaction conditions) t ) 36 min, T ) 55 °C, P ) 50 psig, QAir ) 2.0 L/min, x ) 0.50, CCo ) 0.005 mol/L, CH2O ) 2.300 mol/L, PKClExcess ) 50%, COil ) 0.02872 mol/L, VR ) 200 mL.

were mixed in the reactor which was immediately positioned in the rig. Instant purging with nitrogen (0.5 bar, 1 min) was done by opening ball valve 1 (BV-1), needle valve 1 (NV-1), NV-3, and NV-4 while BV-2, BV-3, BV-4, and NV-2 were closed. At the same time, heating was started. After purging, BV-1, NV-1, and NV-4 were closed, BV-2, BV-3, BV-4, and NV-2 were opened, and the pressure microregulator (PMR) was adjusted for required air pressure. When the required temperature was attained, the solenoid valve (S) triggered the air supply. NV-2 and NV-4 were manually adjusted for air flow being monitored on the rotameter (FI). As soon as the preset air bubbling time was up, S terminated the air supply. NV-4 was adjusted to depressurize the reactor at approximately 2.0 L/min. 3.5. Analysis of Total Hydroperoxides. The amount of total hydroperoxides present in the reactor at any time was estimated using iodometric titration described by Kokatnur et al.48 The visual determination of the end point in their method was replaced by a much more sensitive dead-stop-end-point biamperometric method which is again a modification of the technique explained by Montgomery et al.49 In the modified technique, the reaction mixture was taken along with a titration solvent in a tall form beaker and refluxed (85-90 °C, 10 min) in order to reduce any Co(III) and to exclude oxygen which may both interfere. Potassium iodide was then added and further refluxed (5 min) with occasional swirling. Liberated iodine was biamperometrically titrated against standard sodium thiosulfate (prepared in a 1:1 water-2-propanol mixture) until the current dropped to 0.1-0.3 µA (Figure 2b). The experimental yield of hydroperoxides Y was calculated using the following equation: Y)

(

CNa2S2O3(VA - VB) 2COilVROOH

)

100

(26)

3.6. Analysis of Precision. Precision analysis for the experimental technique used herein is contained in Figure 3. It is evident from the descriptive statistics that, under the specified reaction conditions and at 95% confidence level, the random measurements of hydroperoxides do not differ by more than the computed uncertainty which is favorably low. This implies that the experimental technique is associated with a good precision which is also apparent from the accompanied histogram where random measurements are normally distributed around the mean. Similar precision may be assumed at all other combinations of the process and composition variables at the same confidence level.

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3.7. Analysis of Dynamic Yield Data. An error-compensation algorithm based on nonlinear least-squares has been used that offers an efficient means for balancing the effects of process and composition variables which are hard to strictly control during experimentation. The present algorithm consists of taking rate constants kF or kF′ and kD or kD′ as the parameters to be determined, simulating the yield profile (eqs 18 or 21) to obtain the parameters when the residual is minimal. Values of rate constants corresponding to the simulated yield profile would be the most likely rate constants for formation and decomposition of hydroperoxides. However, such an algorithm requires an initial estimate of the parameters to be determined. To lessen the restrictions on initial parameter estimation, the LevenbergMarquardt method50 was applied. In every case (Figures 6a-10a), yield profiles go through a maximum, equivalent to the maximum amount of hydroperoxides in the reactor. Physically, the maxima correspond to time tmax where net decomposition of hydroperoxides starts. The conjugate-gradient method51 was used to estimate tmax that permits the calculation of selectivities (eqs 24 or 25) up to tmax. Kinetic and statistical analyses were performed using Mathcad Professional (MathSoft, Inc.) and Microsoft Excel (Microsoft Corporation) on an Intel Core Duo based machine. 4. Results and Discussion 4.1. Characterization of Substrate. The FTIR spectrum of oil presents a spectroscopic overlap of hundreds of compounds in the mixture (Figure 4c). Although the overall profile reflects characteristic spectrum of saturates (2955.8, 2924.9, 2855.8, 1461.2, 1394.1, 1376.8 cm-1), features related to aromatics (3041.2, 2000-1600, 1596.3, 1523.4, 810.9, 724.0, 723.1 cm-1) may well be noticed despite having weak intensities in the spectrum. However, the presence of olefins is not indicated due to the absence of any strong signal in the 1000-675 cm-1 region. On the other hand, no broad signal was noted in the 3700-3300 cm-1 region, indicating that oil is free from hydroperoxides. GC/MS analysis indicates a central baseline hump between C9 and C42 (Figure 4a). This hump, characteristically observed for middle distillates, is defined in terms of UCM (unidentified complex mixture) which results from chromatographic overlap of hundreds of isoparaffins, naphthenes, and aromatics in the C10-C40 range. Only compounds at high concentrations produce distinct peaks on top of the hump, but even these peaks are likely to consist of multiple overlapping compounds.52 Regardless of removal of substantial isoparaffins and aromatics (Scheme 1), there remain hundreds of naphthenes creating typical UCM hump. Discrete peaks on top of the hump may be recognized as paraffins because the mass spectral pattern for most of these peaks were very much similar to paraffins, for example Figure 4b. It was concluded that the substrate oil contained mostly paraffins with naphthenes as the second largest components. Although some aromatics were present, the oil was virtually free from olefins and hydroperoxides. Therefore, the substrate could characteristically be considered as saturates. Consequently, the hydroperoxides concentrate obtained during air oxidation would consist of saturated hydroperoxides as the major constituents. 4.2. Promoting Factors. 4.2.1. Air Flow Rate. The reactor used during present study was a mechanically agitated, semibatch reactor in which air was continuously bubbled through the reaction mixture under constant pressure. This arrangement ensures saturation of the liquid phase with

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Figure 4. Spectral analysis of substrate oil.

Figure 5. Effect of air flow rate and oil concentration. (reaction conditions) t ) 45 min, T ) 55°C, P ) 50 psig, x ) 0.50, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%, VR ) 200 mL. 43

oxygen, and under these conditions, the reaction leading to hydroperoxides may be assumed to follow the stated mechanism (section 2.1.1.). In order to study the dynamic response, the reaction should be studied in pure kinetic regime which requires that the effect of mass transfer must be eliminated. Despite having mechanical agitation yield of hydroperoxides increased markedly when air flow rate was increased from 0.0 to 1.0 L/min (Figure 5). Since hydroperoxides are not produced by direct air oxidation of hydrocarbons but are the result of propagation (reactions 2 and 3) that follows the promoted initiation by the powerful oxidant Co(III) (reactions 8-10), therefore almost 80% of the maximum yield was obtained with 0.0 L/min air, where the only way oxygen was dissolved in the reaction mixture was by agitation. Presumably the observed increase in the yield of hydroperoxides with increasing air flow rate from 0.0 to

1.0 L/min under constant reaction conditions was due to the effect of mass transfer. To make certain that the effect of mass transfer had been eliminated, the air flow rate was further increased from 1.0 to 2.0 L/min, and it was observed that almost the same yield was obtained. It was concluded that in order to study the dynamic response of the reaction system, the air flow rate should be between 1.0 to 2.0 L/min, as at these flow rates effect of mass transfer was no longer be able to cause any hindrance. 4.2.2. Fraction of Co(III) in POC System. For the reaction system consisting of POC having x ) 0.00, neither an initial supply of Co(III) was present nor was any hydroperoxide added for initiation through reactions 8-10 or 5 and 6, respectively, despite the fact that the induction period was less than 1 min (Figure 6a). This observation may be described in terms of the oxidizing ability of dissolved oxygen and chloride ions toward likely production of Co(III) from Co(II) in wet acetic acid via the following reactions: 1/2O2 + 2H+ + 2Co(II) S 2Co(III) + H2O -

1/2O2 + Cl S OCl

-

(27) (28)

OCl- + 2H+ + Co(II) S Co(III) + 1/2Cl2 + H2O (29) Co(III) thus produced then initiated the reaction, resulting in hydroperoxides (reactions 2 and 3). The hydroperoxide yield soon attained a mean limiting value of about 0.2%. At this point, a steady state was reached between hydroperoxides formation and decomposition (Figure 6a). This behavior was in agreement with the usual theory of catalysis of oxidation of hydrocarbons with Co(II) in acetic acid.53 In this way although a bidirectional possibility was available (reactions 27 and 29) for the production of hydroperoxides even in the absence of an initial supply of Co(III), however, the yield was extremely poor. Therefore, the reaction system consisting of POC having x ) 0.00 would not


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Figure 6. Effect of fraction of Co(III) ions in POC. (a) Plot of experimental data and modeled yield profiles. (b) Multiple conversion-yield-selectivity chart. (c) Relation between fraction of Co(III) ions in POC and Ymax. (reaction conditions) T ) 55 °C, P ) 50 psig, QAir ) 1.0 L/min, COil ) 0.01795 mol/L, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%, VR ) 200 mL.

make an economically favorable choice for production of hydroperoxides concentrate from oxidation of diluted gas oils. On the other hand, the reaction systems, consisting of POC having x > 0.00, always had an initial supply of Co(III) (Figure 1), which had the ability to initiate the process (reactions 8-10), resulting in a buildup of hydroperoxides during the initial stage (Figure 6a). Simultaneously decomposition of hydroperoxides started (reaction 7). Thus, the hydroperoxide yield started declining after passing through a maximum which was responsible for the expected hump shaped yield profiles as was argued in the theoretical section, 2. Multiple conversion-yield-selectivity profiles are plotted up to tmax (Figure 6b). The appearance of hydroperoxides in the reactor without any appreciable induction period has clearly shown that, under operating conditions, oil directly interacts with Co(III) and the effect intensifies with increasing x. The maximum level of hydroperoxides can be seen to be directly proportional to the amount of Co(III) initially present, and in fact, this dependence has been found to follow a linear relation between Ymax and x (Figure 6c). This implies that with increasing x more oil is converted into hydroperoxides because a higher proportion of Co(III) will increase the probability of initiation through direct electron transfer (reactions 8-10). It was therefore concluded that the formation of hydroperoxides is stoichiometric in nature. Chemically generated POC systems have shown high selectivities to hydroperoxides at low conversions (Figure 6b and Table 1). This indicates that during the initial stage almost none of the oil will convert to nonselective products such as olefins via: (30) R• + Co(III) f olefin + Co(II) + H+ However, for reaction systems consisting of POC having x ) 0.00 selectivity falls rapidly to 0% for less than 2.5% conversion,

again showing the inefficiency of such a system. Selectivities estimated for various conversions (Table 1) clearly show that, for low conversions, chemically generated POC acts as an efficient reagent for producing hydroperoxides from air oxidation of diluted gas oils. 4.2.3. Concentration of Cobalt. The cobalt concentration in the reaction mixture has also shown an equally significant effect upon oxidation of oil (Figure 7 and Table 1). Hydroperoxides appear with a negligible induction period, and Ymax increases linearly with CCo (Figure 7c), which again supports the stoichiometric nature of the system toward formation of hydroperoxides. This effect may be described in terms of the ratio xCCo/COil. At constant x and COil the greater the CCo, the larger the ratio (higher amount of Co(III) in the reaction mixture) resulting in increased probability of the oil molecules to directly interact with Co(III) thereby increasing the yield of hydroperoxides. Once again high selectivities to hydroperoxides can be observed at low conversions (Figure 7b and Table 1). 4.3. Inhibiting Factors. 4.3.1. Concentration of Oil. The rise in the initial concentration of oil had decreased the yield of hydroperoxides (Figure 5). However, the decrease was not linear and followed a power law of the form Y ) RCOil-b. This inhibiting effect can also be described in terms of the ratio xCCo/ COil. At constant x and CCo, the lower the number of molecules of oil (smaller COil), the larger the ratio and the greater the probability that a large proportion of the oil molecules will interact with Co(III) resulting in higher yield of hydroperoxides. This suggests that lower COil favors higher yields. During the present work COil ) 0.01795 mol/L was selected and kept constant during the entire study. 4.3.2. Concentration of Water. The concentration of water has shown a fairly strong inhibiting effect above 1.717 mol/L

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Table 1. Kinetic Analyses of Experimental Yield Dataa model A

model B

χ (%) 0.01

factor

kF × 104 kD (1/s) (L/(mol s))

R2

tmax

χmax (s) (%)

12.2381 20.8809 8.5744 6.5195 0.8901

0.5466 2506.0 2.29 0.64 0.5054 0.9719 181.7 10.20 3.77 3.2827 0.9960 187.1 17.75 7.47 5.9634 0.9831 181.9 21.30 9.41 7.7426 0.9154 580.4 33.83 17.16 6.2024

1.2785 2.5541 5.3632 3.6293

4.8022 9.5797 2.7726 1.4649

0.9280 0.9008 0.9267 0.8342

819.9 9.95 3.65 410.6 9.96 3.66 439.0 20.98 9.23 716.4 22.89 10.32

5.2559 2.5856 14.8460 5.3478

6.5626 5.0464 23.6991 19.6587

0.9366 0.9248 0.8957 0.9413

313.6 531.0 100.4 197.8

15.19 12.83 13.85 10.04

6.15 4.99 5.48 3.69

4.9307 4.1662 3.3197 5.8402

9.9263 11.8060 8.9367 13.4007

0.9141 0.8431 0.6489 0.7968

274.9 282.7 362.3 220.0

12.68 11.11 11.33 12.06

4.92 4.18 4.28 4.62

2.6373 3.8436 1.6639 1.1801

0.7332 1.8611 4.4346 3.9097

0.9585 0.9586 0.9910 0.9592 0.9840

313.7 417.3 393.0 358.8 855.8

1.57 12.80 20.89 24.25 41.19

0.26 3.59 6.96 8.56 18.50

99.976396.3211 99.9994 99.9999 99.9999 100.00099.9682

97.665468.7442 99.9368 99.9853 99.9911 99.997899.6827

17.27283.0531 93.9323 98.5405 99.1166 99.775796.8718

10.2137 33.0193 47.5776 80.735173.0689

1.1181 2.0182 4.0013 3.0551

2.4691 4.6242 3.1004 2.0515

0.8560 1312.9 13.65 3.91 99.9993 0.7828 708.1 13.32 3.78 99.9993 0.8529 758.3 26.17 9.53 99.9999 0.6840 1090.7 28.34 10.67 99.9999

99.9326 99.9327 99.9907 99.9928

93.5519 93.5607 99.0778 99.2790

9.9519 9.9574 46.3039 53.5427

3.3929 2.1964 7.1980 2.9514

4.4425 3.3804 12.5943 7.2631

0.9057 0.9287 0.8943 0.9307

626.9 864.9 241.0 459.7

19.16 17.30 15.93 12.69

6.17 5.37 4.80 3.55

99.9998 99.9996 99.9997 99.9993

99.9776 99.9650 99.9714 99.9340

97.7920 96.5775 97.1888 93.6833

22.8120 15.1688 18.1573 10.0361

3.1483 2.6422 2.1394 3.1566

5.4396 5.3591 4.2444 5.8736

0.8666 0.8359 0.5396 0.8452

556.0 590.7 741.3 526.0

16.06 14.45 14.66 15.30

4.86 4.22 4.30 4.55

99.9996 99.9995 99.9995 99.9996

99.9639 99.9492 99.9517 99.9588

96.4724 95.0822 95.3204 95.9926

14.7731 11.4604 11.8429 13.2844

99.9950 99.9913 99.9522 99.9406

99.5026 99.1359 95.3655 94.2829

63.7856 48.2326 11.9218 10.5102

x ) 0.50, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%

T (°C)

a

18.9438 7.9724 6.7828 6.8509 1.9714

T ) 55°C, x ) 0.50, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L

PKClExcess(%)

25 40 70 85

10.0

T ) 55°C, x ) 0.50, CCo ) 0.005 mol/L, PKClExcess ) 50%

CH2O(mol/L)

0 25 75 100

tmax ′

Ymax ′ (%)

T ) 55°C, x ) 0.50, CH2O ) 1.725 mol/L, PKClExcess ) 50%

CCo(mol/L)

0.500 1.113 2.913 4.100

R 2′

χmax ′ (s) (%)

T ) 55°C, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%

x 0.00 (non-POC) 0.0926 0.25 5.9238 0.50 10.4420 0.75 13.1680 1.00(non-POC) 7.1139

0.001 0.003 0.007 0.009

Ymax kF′ × 104 kD′ × 103 (%) (1/s) (1/s)

0.10 1.00 S (%) subscripts are selectivities predicted by model B

0.8626 1143.6 26.04 12.17 2.1412 0.9277 628.9 21.47 9.51 3.3911 0.8149 725.9 11.38 4.30 1.3331 0.8658 935.6 10.45 3.88 1.0212

1.1471 2.5057 2.5200 2.1431

0.7650 1799.1 31.97 12.70 100.000 0.8031 923.1 26.88 9.90 99.9999 0.6102 1231.5 15.14 4.49 99.9995 0.7249 1491.3 14.13 4.09 99.9994

P ) 50 psig, QAir ) 1.0 L/min, COil ) 0.01795 mol/L, VR ) 200 mL.

Figure 7. Effect of the concentration of cobalt in the reaction mixture. (reaction conditions) T ) 55°C, P ) 50 psig, QAir ) 1.0 L/min, COil ) 0.01795 mol/L, x ) 0.50, CH2O ) 1.725 mol/L, PKClExcess ) 50%, VR ) 200 mL.

of water (Figure 8c and Table 1). This water concentration can be taken as the critical CH2O. A similar inhibiting effect of water has already been discussed by other authors.30,46,47 For example Kamiya et al.30 discussed that the presence of a small amount of water probably increases the concentration of more active monohydroxide than triacetate of Co(III). This

is responsible for much efficient initiation (reactions 8-10), resulting in increasing hydroperoxides yield. This effect persists up to critical CH2O. However, when the concentration of water becomes high enough, the deactivation effect of water (reverse of reaction 27) may become significant. Therefore for a favorable response, the water level must be


5651

Figure 8. Effect of the concentration of water in the reaction mixture. (reaction conditions) T ) 55°C, P ) 50 psig, QAir ) 1.0 L/min, COil ) 0.01795 mol/L, x ) 0.50, CCo ) 0.005 mol/L, PKClExcess ) 50%, VR ) 200 mL.

Figure 9. Effect of excess chloride ions in the reaction mixture. (reaction conditions) T ) 55 °C, P ) 50 psig, QAir ) 1.0 L/min, COil ) 0.01795 mol/L, x ) 0.50, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, VR ) 200 mL.

around the critical water concentration. Again, low conversions of oil favor high selectivity to hydroperoxides, and it decreases almost linearly with increasing CH2O at each conversion (Table 1). 4.3.3. Excess Chloride Ions. Excess chloride ions also exhibit a faint inhibiting effect (Figure 9c and Table 1). This

might be due to the reactions 28 and 29 whereby excess chloride ions will adjust the shuttling Co(II) S Co(III) equilibrium, resulting in a somewhat diminished Co(III) level. This implies that the rate of depletion of Co(III) will be slightly greater than the rate of production. Accordingly, there will be rather decreased interaction of oil molecules with

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Figure 10. Effect of temperature. (reaction conditions) P ) 50 psig, QAir ) 1.0 L/min, x ) 0.50, COil ) 0.01795 mol/L, CCo ) 0.005 mol/L, CH2O ) 1.725 mol/L, PKClExcess ) 50%, VR ) 200 mL. Table 2. Reduction Rate of Co(III) in Acetic Acid solution46 (Modified from inverse hours to inverse seconds) k × 104 (1/s) T (°C)

in acetic acid

in 5% water-95% acetic acid

in 10% water-90% acetic acid

90 100 110 120 130

0.0224 0.0672 1.0389 3.5278 13.6111

0.0853 0.2464 0.6611 2.0167 7.6111

0.1231 0.2689 0.8333 2.4056 6.3611

Co(III) that will reduce the ultimate yield of hydroperoxides resulting in the observed faint inhibiting effect. The direct consequence of this effect is that practically the conversion-yieldselectivity response of the system can be tuned by changing the quantity of chloride ions in the reaction mixture (Table 1). 4.3.4. Temperature. The influence of temperature was tested in the range 25-85 °C (Figure 10 and Table 1). A strong linear negative correlation was obtained between Ymax and temperature (Figure 10c). This indicates that higher temperatures markedly decrease the hydroperoxide yield. At the same time, an almost linear dropoff in the selectivity to hydroperoxides is observed (Figure 10b and Table 1). The effect of temperature upon oxidation of oil may be attributed toward the Co(III) level which is highly sensitive against temperature changes. In wet acetic acid Co(III) reduces to Co(II) via reverse reaction 27 instead of being consumed by initiation reactions 8-10. This side reduction has been shown by Nakaoka et al.46 to have accelerating rates at higher temperatures (Table 2). Since Co(III) is the main agent for initiation, and for higher hydroperoxide yields a high Co(III) level is required, therefore the proposed reaction should be carried out at temperatures as low as possible to get a favorable response. 4.4. Results of Kinetic Analysis. A compilation of kinetic model predictions along with experimental yields of hydroperoxides is presented in Figure 11. The bold dashed line of

Figure 11. Comparison of the kinetic models and the ideal response of the proposed oxidation system for production of hydroperoxide concentrate from POC promoted air oxidation of diluted gas oils.

best fit (R2 ) 0.9214) for model A almost overlaps the ideal behavior (predicted yield of hydroperoxides ) experimental yield of hydroperoxides) shown by the solid line. On the other hand, the bold dotted line (R2 ) 0.8953) for model B substantially deviates with increasing hydroperoxide yield. Additionally, the scatter in the points predicted (against experimental points) by model B is larger than that predicted by model A. This suggests that model B is not a realistic representation of the process under consideration. This is also supported by the individual coefficients of determination (R2) contained in Table 1. The values of R2 in most (≈90%) of the cases for model A are greater than those for model B. The only noticeable exceptions are for systems consisting of POC having x ) 0.00 and x ) 1.00 both of which cannot be considered as true POC systems. Therefore, the most probable mechanism for the production of hydroperoxides concentrate from POC pro-

Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009 Table 3. Rate Constants for Decomposition of Hydroperoxides in Chlorobenzene20 T (°C) 25.0 22.7 45.0 45.0 23.5 45.0 45.0

R in ROOH t-butyl

n-butyl R-cumyl

catalyst

k (L/(mol s))

cobalt(II) 2-ethylhexanoate cobalt(III) stearate

23.2 ( 1.5 1.66 21.9 ( 1.1 17.1 ( 1.2 5.2 3.1 ( 0.2 21

cobalt(II) stearate cobalt(II) 2-ethylhexanoate cobalt(III) stearate cobalt(II) 2-ethylhexanoate

moted air oxidation of diluted gas oils would be an irreversible consecutive reaction network where pseudo-first-order conversion of oil is followed by second-order decomposition of hydroperoxides which is a direct consequence of the mechanism proposed in section 2.1.1. The magnitudes of first-order rate constants for formation of hydroperoxides (Table 1) approximately match the magnitudes of rate constants for the reduction of Co(III) ions in wet acetic acid, as reported by Nakaoka et al.46 (Table 2). The trivial differences in the magnitudes may be attributed to different reaction conditions. Since propagation is a moderate process15-17 while initiation involving electron transfer from substrate to Co(III) is a slow process,38 hence the rate of formation of hydroperoxides during the initial stage of the reaction is governed by initiation. The approximate compliances between the rate constants for formation of hydroperoxides and rate constants for the reduction of Co(III) ions in wet acetic acid, therefore, support the argument that the formation of hydroperoxides is a pseudo-first-order reaction during which oil directly interacts with Co(III) present in the POC. Similarly values for second-order rate constants for model A (Table 1) are comparable to rate constants for catalytic decomposition of hydroperoxides in chlorobenzene, reported by Hiatt et al.20 (Table 3). Conclusively, the net decomposition of hydroperoxides can be considered as a second order reaction which is a conformity proof for the mechanism proposed in section 2.1.1. 4.5. Variation in Specific Rates. Variation in specific rates for formation and decomposition of hydroperoxides as a function of process and composition variables can be seen in Figure 12. This is a very complicated picture that shows the simultaneous influence of the experimental variables on the specific rates. However, within this figure, the effect of chloride ions needs some discussion. As can be seen, PKClExcess has an almost negligible effect on the specific rates for formation and decomposition of hydroperoxides (bold lines of best fit). The bold solid line has an almost zero slope (4 × 10-7) which means

5653

that chloride ions will have no participation either in the initiation or propagation steps both of which are responsible for formation of hydroperoxides. On the other hand, the bold dashed line (slope ) 0.0163) shows an extremely weak positive influence on the decomposition of hydroperoxides. Since the slope for the decomposition is far greater than that for formation, the system shows the net faint inhibiting (Figure 9c) effect of PKClExcess on the production of hydroperoxides during oxidation of diluted gas oils in the presence of POC. 5. Conclusion The proposed process permits direct use of gas oils, a cheap source of hydrocarbons, for manufacture of hydroperoxides under mild conditions. The technique provides a simplified procedure for producing POC mixture, avoiding arduous preparation and characterization of the heterogeneous catalyst systems proposed in the literature for oxidation of hydrocarbons leading to hydroperoxides.3-8 Chemically generated POC (high x and CCo, at low T with CH2O taken around its critical value and in the presence of some chloride ions) can be a modified, simple, yet inexpensive way of producing hydroperoxides concentrate from air oxidation of diluted gas oils in practical time periods (15.12 ( 3.23% conversion in 9.3 ( 4.0 min) as compared to processes (0.80 ( 1.05% conversion in 60 min) claimed elsewhere.3 It has been found that the process follows an irreversible consecutive reaction network consisting of pseudo-first-order formation of hydroperoxides followed by second-order decomposition. This is a distinctive example of an irreversible consecutive reaction network, and such examples are almost scarce in the literature. The present process ensures high selectivity to hydroperoxides (99.86 ( 0.23, 92.88 ( 7.95, and 28.77 ( 11.67% for 0.10, 1.00, and 10.0% conversion) during initial stages. Furthermore, the process can be fine-tuned by varying process and composition variables in such a manner to get the optimum response with respect to conversion of oil and selectivity to hydroperoxides. In this way, the proposed process will permit mild conditions and enhanced selectivity, the economic goal usually sought within the industrial community. Acknowledgment This research was jointly funded by the Ministry of Science and Technology and the Higher Education Commission, Government of Pakistan. One of the authors (S.M.D.N.) expresses his deep gratefulness to the University of Karachi for granting study leave. Indebtedness is extended to HEJ Research Institute of Chemistry, University of Karachi, for analytical expertise. Nomenclature

Figure 12. Specific rates for formation and decomposition of hydroperoxides as function of composition and process variables.

t ) air bubbling time, s CCo ) concentration of cobalt in reaction mixture, mol/L CCoAc ) concentration of cobalt(II) acetate tetrahydrate stock solution, mol/L CROOH ) concentration of hydroperoxides, mol/L COil ) concentration of oil at time t ) 0, mol/L COilt ) concentration of oil at time t, mol/L CKClO3 ) concentration of potassium chlorate stock solution, mol/L CKCl ) concentration of potassium chloride stock solution, mol/L CNa2S2O3 ) concentration of sodium thiosulfate, mol/L CH2O ) concentration of water in reaction mixture, mol/L PKClExcess ) Excess chloride ions in reaction mixture, % QAir ) flow rate of air through the reactor, L/min

5654


x ) fraction of Co(III) ions in POC MCoAc ) molar mass of cobalt(II) acetate tetrahydrate, g/mol MOil ) molar mass of oil, g/mol MKClO3 ) molar mass of potassium chlorate, g/mol MKCl ) molar mass of potassium chloride, g/mol MH2O ) molar mass of water, g/mol nKClO3 ) moles of chlorate to produce nCo3+ moles of Co(III), mol nKClAdd ) moles of chloride ions added to reaction mixture to maintain same level for all x, mol nKClExcess ) moles of chloride ions added to study effect of excess chloride ions, mol nKClIni ) moles of chloride ions produced as a result of reaction 11, mol nKClMax ) moles of chloride ions produced for x ) 1, mol nCo2+ ) moles of Co(II) in reaction mixture, mol nCo3+ ) moles of Co(III) in reaction mixture, mol nCo ) moles of cobalt ions in reaction mixture, mol R2 ) coefficient of determination P ) operating pressure, psig T ) operating temperature, °C ki ) rate constants, 1/s or L/(mol s) SA ) selectivity to hydroperoxide estimated using model A, % SB ) selectivity to hydroperoxide estimated using model B, % tmax ) time corresponding to the maxima of yield profiles, s VH2OCoAc ) volume of water supplied from cobalt(II) acetate tetrahydrate, mL CO2 ) concentration of dissolved oxygen in the reaction mixture, mol/L VAcOHC ) volume of acetic acid to make up catalyst mixture, mL VAcOH ) volume of acetic acid to make up reaction mixture, mL VC ) volume of catalyst mixture, mL VKClO3 ) volume of potassium chlorate stock solution required to produce nCo3+ moles of Co(III), mL VKCl ) volume of potassium chloride stock solution containing required moles of chloride ions, mL VROOH ) volume of reaction mixture taken for analysis, mL VR ) volume of reaction mixture, mL VOil ) volume of required oil in the reaction mixture, mL VA ) volume of standard sodium thiosulfate consumed during titration of sample, mL VB ) volume of standard sodium thiosulfate consumed during titration of blank, mL VCoAc ) volume of stock solution containing nCo moles of cobalt ions, mL VH2OC ) volume of water added to maintain same concentration of water in catalyst mixture, mL VH2OR ) volume of water added to maintain same concentration of water in reaction mixture, mL VH2OKClO3 ) volume of water supplied from potassium chlorate stock solution, mL VH2OKCl ) volume of water supplied from potassium chloride stock solution, mL Ymax ) yield of hydroperoxides at tmax, % Y ) experimental yield of hydroperoxide, % Yˆ′ ) predicted yield of hydroperoxides in the case of model A, % Yˆ′′ ) predicted yield of hydroperoxides in case of model B, % Greek Symbols χA ) conversion of oil estimated using model A, % χB ) conversion of oil estimated using model B, % χmax ) conversion of oil at tmax, % FSKCl ) density of aqueous potassium chloride stock solution, g/mL FSKClO3 ) density of aqueous potassium chlorate stock solution, g/mL

FOil ) density of oil, g/mL FH2O ) density of water, g/mL

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ReceiVed for reView September 29, 2008 ReVised manuscript receiVed April 17, 2009 Accepted April 22, 2009 IE900364W

Kinetics of Catalytic Oxidation of Gas Oils - American Chemical Society

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