Co-solvent and pressure effect on the thermal decomposition of 2,2′ azobis(isobutyronitrile) in supercritical CO2 using UV–Vis spectroscopy

Journal of Supercritical Fluids 21 (2001) 227– 232 www.elsevier.com/locate/supflu

Co-solvent and pressure effect on the thermal decomposition of 2,2% azobis(isobutyronitrile) in supercritical CO2 using UV–Vis spectroscopy Hongping Li a, Jun Liu a, Haifei Zhang a, Shougang Wang a, Buxing Han a,*, Fang Fang Liu b b

a Center for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Department of Chemical Engineering, Hebei Uni6ersity of Science and Technology, Shijiazhuang 050018, China

Received 15 January 2001; received in revised form 20 June 2001; accepted 6 July 2001

Abstract The thermal decomposition of 2,2%-azobis(isobutyronitrile) (AIBN) in supercritical CO2 was studied at 335.15 K, and at 12 and 14 MPa by using UV/Vis spectroscopic method. The effect of co-solvents methanol and cyclohexane on the decomposition rate was also investigated. The co-solvents, especially methanol, can accelerate the decomposition considerably, which is attributable to local composition enhancement. The local concentration of the co-solvents around the reaction species was estimated, and the results showed that the local concentration of the cosolvent can be 20 times of that in the bulk, which depends on pressure and co-solvent concentration. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Supercritical CO2; Co-solvent; 2,2%-azobis(isobutyronitrile); Thermal decomposition; Pressure effect

1. Introduction Supercritical Fluids (SCFs) have already been used in some industrial processes [1 – 4]. Over the past decade there has been a growing interest in using SCFs as reaction media. There are some unique advantages to conduct chemical reactions in SCFs. For example, reaction rates, yields, and

* Corresponding author. Tel.: + 86-10-6256-2821; fax: + 86-10-6255-9373. E-mail address: [email protected] (B. Han).

selectivity can be tuned by varying pressure or small amount of co-solvents. SCFs (such as CO2, H2O) can be used to replace environmentally undesirable solvents or avoid undesirable byproducts; mass transfer is improved for heterogeneous reactions; and simultaneous reaction and separation may be accomplished for some reactions. It is not surprising that in recent years the use of SCFs, especially supercritical (SC) CO2 and H2O, as solvents for chemical reaction media is receiving much attention, and many reactions have been studied, including polymerizations [5,6], and this topic has been reviewed recently [6–14].

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H. Li et al. / J. of Supercritical Fluids 21 (2001) 227–232

In a polymerization process, the properties of the products, such as molecular weight and molecular weight distribution, are closely related with the decomposition rate of the initiator [15]. It is no doubt that study on the decomposition rate of initiator in SCFs is of importance for controlling the properties of the polymers. 2,2%-Azobis(isobutyronitrile) (AIBN), a commonly used initiator, plays a very important role in free radical polymerization in supercritical CO2. Guan et al. [16] have studied the decomposition rate of AIBN in SC CO2 at three temperatures (332.55, 342.45, and 352.55 K) in the pressure range of 10– 35 MPa, and they found that the decomposition rate of AIBN in SC CO2 is ca. 2.5 times lower than that observed in benzene at 332.55 K and ambient pressure. They also gave the activation energy (134.6 kJ/mol) for the thermal decomposition of AIBN in SC CO2. It is well known that small amount of co-solvents in SCFs can improve the properties of the fluids significantly. In this work, we studied the effect of co-solvents methanol and cyclohexane on the thermal decomposition rate of AIBN, and the local density enhancement of the co-solvents around the reaction species was also studied.

stainless steel body and two optical quartz windows. Outside of the cell body was wrapped with electrical heating wire and heat insulation material. The structure of the sample cell is shown in the section B of Fig. 1. The temperature of the cell was measured and controlled by a platinum resistance thermometer and XS/A-1 temperature controller (Beijing Tianchen Company), which was accurate to 9 0.1 K. The path length and the internal volume of the cell was 1.1 cm and 1.885× 10 − 3 l, respectively. The pressure gauge was composed of a transducer (FOXBORO/ICT, Model 93) and an indicator, which was accurate to 0.025 MPa in the pressure range of 0–20 MPa. Before experiments, the sample cell was washed thoroughly by different solvents and dried by vacuum. In a typical experiment, 3.8 mg AIBN were loaded into the sample cell. The cell was then purged slowly with CO2 for ca.10 min, then the desired amount of co-solvent was injected into the cell. Valve 8 was closed and the cell was installed into the sample chamber of the UV/Vis spectrophotometer as shown in Fig. 1. The temperature of sample cell was maintained at 335.15 K. The air in the pipe was removed by vacuum. CO2 was charged into the sample cell by the

2. Experimental Materials: Carbon dioxide (99.995% purity) was supplied by Beijing Analytical Instrument Factory. Methanol and cyclohexane (Analytical grade, \ 99.5%) were provided by Beijing Chemical Reagent Plant. AIBN was A. R. grade supplied by Beijing Chemical Reagent Plant and was recrystallized twice from methanol prior to use. Apparatus and procedures for UV spectra determining: The schematic diagram of the experimental setup is shown in Fig. 1. It consisted mainly of a gas cylinder (1), a high-pressure pump (3), a pressure gauge (5), an UV– Vis spectrometer (9), a high-pressure and temperature-controllable UV sample cell (10), temperature controller (11), vacuum pump (12), and valves and fittings. UV/Vis spectrophotometer was produced by Beijing General Instrument Company (model TU-1201, resolution: 0.5 nm). The sample cell consisted of a

Fig. 1. Schematic diagram of the experimental setup. Section A: 1. Gas cylinder, 3. High pressure pump, 5. Pressure gauge, 9. UV– Vis spectrometer, 10. High pressure and temperaturecontrollable UV sample cell, 11. Temperature controller, 12. Vacuum pump, 2, 4, 6, 7, 8 Valves. Section B: High pressure and temperature-controllable UV sample cell.

H. Li et al. / J. of Supercritical Fluids 21 (2001) 227–232

Fig. 2. UV spectra for the thermal decomposition of AIBN in CO2 –methanol (0.26 mol/l) at 335.15 K and 14 MPa.

high-pressure pump to the desired pressure. Then the UV spectra were recorded by a computer at different times after the sample in the cell reached thermal equilibrium. The method was similar to that reported by Guan et al. [16]. In this work, we determined the decomposition rate of AIBN in SC CO2 at 335.15 K, and at 12 and 14 MPa, respectively. The effect of co-solvents methanol and cyclohexane on the decomposition rate was also studied. In the experiments the concentration of AIBN was 1.23× 10 − 2 mol/ l. The concentrations of co-solvents (CC) range from 0 to 0.66 mol/l (0.13, 0.26, 0.39 mol/l for cyclohexane, 0.13, 0.26, 0.66 mol/l for methanol). All the experiments were carried out in single phase region, i.e. the AIBN and the co-solvents could be dissolved in SC CO2 at our experimental conditions, which could seen clearly from the windows of the sample cell.

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of the change of the absorption with time. In order to determine the reaction rate quantitatively from the spectra, the absorbance (A) of AIBN is required. Since the peaks of the above two compounds overlap to some extent, a deconvolution method was developed. The spectra curves of the two compounds were assumed to be Gaussian. A computer routine was used to optimize the locations, heights, and variance of the two peaks, so that the sum of the deconvolved peaks most accurately fit the experimental spectra. Fig. 3 shows some typical graphs of − ln A (absorbance at 347 nm) versus time for thermal decomposition of AIBN in SC CO2 at 14 MPa with and without co-solvents. There exists a linear relationship between − ln A and the reaction time. At other conditions − ln A vs time curves are also linear. This indicates that the decomposition follows the first-order kinetics, and the decomposition rate constants (Kd) can be calculated from the slope of the curves, which is discussed briefly in the following. For the first-order kinetics − d[CAN]/dt = Kd[CAN],

(1)

where CAN is the concentration of AIBN at reaction time t. From Eq. (1), we can get ln CAN = p1 − Kd × t.

(2)

According to Lamber–Beer law,

3. Results and discussion As an example, Fig. 2 shows the UV spectra for the thermal decomposition process of AIBN in SC CO2 with methanol as co-solvent at 335.15 K and 14 MPa. The absorption maximum at 347 and 290 nm are assigned to the absorption of AIBN and ketenimine (temporary adduct), respectively [16]. The decreasing of the absorption at 347 nm corresponds to the loss of AIBN. The decomposition rate can be calculated on the basis

Fig. 3. Graph of −ln A (absorbance at 347 nm) vs time for AIBN thermal decomposition at 335.15 K in pure CO2, CO2 with 0.13 mol/l cyclohexane, and CO2 with 0.13 mol/l methanol at 14 MPa.

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Fig. 4. Dependence of the decomposition rate of AIBN in CO2 –methanol and in CO2 –cyclohexane as a function of pressure and co-solvent concentration at 335.15K.

A = mAN × CAN ×L.

(3)

From the Eq. (2) and Eq. (3), one can obtain the following equation ln A = p2 − Kd × t,

(4)

where A and mAN stand for the absorbance and mole absorption coefficient of AIBN, repectively. L is the optical path length, and p1 and p2 are constants. Fig. 4 illustrates Kd at various conditions. The data in Fig. 4 show that Kd in pure CO2 at 14 MPa is larger than that at 12 MPa because the dielectric constant of CO2 increases with pressure..The Kd in CO2 – methanol and CO2 – cyclohexane mixtures is larger than that in pure CO2, and the Kd increases with the concentrations of the co-solvents. Furthermore, the effect of methanol on the rate constant is much more significant than cyclohexane. Guan et al. determined the rate constant of the thermal decomposition of AIBN in SC CO2 at 332.6 K and 20.7 MPa with 5 vol% tetrahydrofuran as co-solvent [16], and the rate constant is about 1.6 times as large as that in pure CO2 at the same temperature and pressure. This means that Kd increases with increasing solvent polarity, indicating that a dipolar interaction between the reactant’s transition state and the solvent medium exists. The lower decomposition rate in CO2 – cyclohexane is attributable to the lower dielectric constant of mixture comparing with CO2 – methanol mixture.

Comparing with some other reactions, such as ionic reactions, radical reactions are not very solvent sensitive with rates that usually span less than an order of magnitude [17]. The decomposition rate of AIBN in liquid solvents shows a solvent dependence with an overall variation in the rate constant of a factor up to 4 when the solvents change from nonpolar to strong polar [16]. Fig. 4 shows that the increase in Kd can be as large as two times when a small amount of methanol is added to SC CO2. This change is very large considering the fact that the Kd of radical reactions is relatively less sensitive to the polarity of solvent. According to transition state theory, the quantitative effect of pressure (at constant temperature) on an individual rate constant can be expressed as

  llnk lP

T

=−

DV " , RT

(5)

where DV " is the activation volume, and k is the reaction rate constant (expressed in pressure-independent concentration units) [16]. Normally, three factors determine the activation volume: (i) the intrinsic size of the reacting species as determined by its van der Waals radius; (ii) the interaction of the species with the solvent to cause electrostriction; (iii) the interaction of the species with all the solute species, including itself [18]. For dilute solution, (iii) is negligible and to a first approximation the observed activation volume can be regarded as the sum of an intrinsic and a solvational component as [19] " DV " = DV " solvation + DV intr

(6)

The intrinsic activation volume (DV " intr) is generally considered to be independent of solvent, the solvation activation volume (DV " solvation) represents all volume changed associated with changes in polarity, electrostriction, and dipole interactions during the course of the reaction. Using a linear least squares treatment, the Kd for AIBN can be expressed as a function of dielectric constant (m) and intrinsic activation volume DV " intrby the following equation [16]

lnKd = −a+ b

  



H. Li et al. / J. of Supercritical Fluids 21 (2001) 227–232

m− 1 DV − 2m + 1 RT

" intr

P,

where a and b are constant, P and T pressure and temperature, respectively, the gas constant. The dielectric constant of pure CO2 K and 12 and 14 MPa were calculated (8) [20], in which the density (d) of calculated from Huang’s equation [21] (1/d)(m − 1)/(m +2)=0.007676.

(7) stand for and R is at 335.15 from Eq. CO2 was (8)

As DV " intr is generally considered to be independent of solvent, we assume that the DV " intr is +13.1×10 − 3 l/mol as reported by Brandrup et al. [22]. Then from the Kd in pure CO2 we can get the two constants a and b in Eq. (7), which are 12.41 and 3.56, respectively. ln Kd as a function of Kirkwood parameter (m-1)/(2m +1) are shown in Fig. 5 (the lines), which should be applicable to all the solvents. Wesch et al. [23] measured the dielectric constant of CO2 –ethanol mixture up to 30 MPa. They found that the dielectric constant of the mixture (mm) was very close to that calculated from the model proposed by Looyenga [24] (Eq. (9)), especially at lower ethanol concentrations.

Fig. 5. The dependence of ln Kd on the Kirkwood parameter of the solvents. The lines are theoretical prediction with Eq. (7) in which a =12.41, b =3.56 and DVint r = 13.1×10 − 3 l/mol. The solid line is at 12 MPa, and the dot line is at 14 Mpa, C6 denotes cyclohexane.

1/3 3 mm = [(1− ƒ2)m 1/3 1 + ƒ2m 2 ] ,

231

(9)

where m1 and m2 are dielectric constant of component 1 and 2, respectively, and ƒ2 denotes the volume fraction of component 2, which is calculated by ƒ2 = y2V2/Vm,

(10)

where y2 is mole fraction of component 2, V2 and Vm stand for the molar volumes of component 2 and the mixture, respectively. In this work the dielectric constants of the CO2 –methanol and CO2 –cyclohexane mixtures are also calculated from Eq. (9). The experimental ln Kd as a function of Kirkwood parameter (m-1)/(2m +1) is also shown in Fig. 5. It can be seen that the experimental Kd in SC CO2 with the co-solvents is much larger than that predicted from the Eq. (7). The large discrepancy can be explained as following. A number of experimental and theoretical studies show that the local density of the co-solvents around solute molecules can be significantly greater than the bulk density [25– 28]. In our reaction system, the co-solvents around the reaction species are higher than that in the bulk, and therefore the local dielectric constant is larger than those calculated from the bulk (apparent) concentration (CA) of the co-solvents because the dielectric constants of the co-solvents are higher than CO2 (mMeOH = 26.06, mcyclohexane = 1.96) [29]. The local density enhancement is not considered when we calculate the Kirkwood parameter (m-1)/(2m + 1) of the CO2/co-solvents mixtures in Fig. 5. It is the local dielectric constant that governs the Kd. Thus the experimental Kd is much higher than that calculated from Eq. (7) with CA. From the experimental Kd, we can calculate the local dielectric constant from Eq. (7), and then we can get the local concentrations (CL) of the co-solvents from Eq. (9) and Eq. (10). Fig. 6 shows the variation of CL/CA with CA. The data in the figure illustrate that the local concentration of the co-solvents can be 20 times higher than that in the bulk. As expected, CL/CA decreases with the apparent concentration of the co-solvents.

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[10]

[11] [12] [13]

[14] [15] [16] Fig. 6. The ratio of local and apparent co-solvent concentrations. C6 denotes cyclohexane.

Acknowledgements

[17] [18]

This work was financially supported by National Key Basic Research Project (G2000048010) and National Natural Science Foundation of China (29725308).

[19] [20]

[21]

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