Kinetics investigation on the hydrogenation of acrylonitrile-butadiene rubber latex by using new catalytic reaction system

    Kinetics investigation on the hydrogenation of acrylonitrile-butadiene rubber latex by using new catalytic reaction system Haoming Ou, Yang Wang, Wei Zhou, Xiaohong Peng PII: DOI: Reference:

S1566-7367(16)30091-7 doi: 10.1016/j.catcom.2016.03.006 CATCOM 4608

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

12 November 2015 5 March 2016 15 March 2016

Please cite this article as: Haoming Ou, Yang Wang, Wei Zhou, Xiaohong Peng, Kinetics investigation on the hydrogenation of acrylonitrile-butadiene rubber latex by using new catalytic reaction system, Catalysis Communications (2016), doi: 10.1016/j.catcom.2016.03.006

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Kinetics Investigation on the Hydrogenation of

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Catalytic Reaction System

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Acrylonitrile-Butadiene Rubber Latex by Using New

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*Corresponding author: Tel/fax: +86-020-87114799. E-mail address: [email protected].

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Haoming Ou, Yang Wang, Wei Zhou, Xiaohong Peng* School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China

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Abstract: The kinetics of hydrogenation of acrylonitrile-butadiene rubber latex with hydrazine hydrate / hydrogen peroxide / selenium catalytic reaction system was investigated by calculating the initial reaction rate constants. The reaction rate constants of the reaction system were obtained by initial reaction measurements of different reactant dosages, such as selenium, hydrazine hydrate, and hydrogen peroxide. Hydrogenation rate constants and reaction order corresponding to the various factors were obtained from the hydrogenation kinetic data. The kinetic equation of this reaction system between 50~70°C and the reaction activation energy were derived.

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Keywords: Hydrogenation; Acrylonitrile-butadiene rubber latex; Kinetics

1. Introduction

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The residual unsaturated bonds in diene-based polymer backbones are susceptible to aging degradation over time. Catalytic hydrogenation of diene-based polymers are important process as the hydrogenated products can be high performance elastomers. Hydrogenated acrylonitrilebutadiene rubber (HNBR) is a modified product of acrylonitrile-butadiene rubber (NBR) with most of the butadiene chain segments saturated by hydrogenation. HNBR can be used in many harsh environments, meeting the high-performance use in various industrial areas due to its powerful performances such as chemical resistance, heat resistance and various excellent mechanical properties.1,2 Currently, solution method is the main industrial process of hydrogenation of NBR to commercialize HNBR which has been classically performed using homogeneous transition metal catalysts and high-pressure hydrogen in a process that suffers from cost associated with the dissolution of NBR. With the advantages of not using noble metal catalysts, less pollution, simple and safe process, a breakthrough method performed by diimide (diazene, NH=NH) for the direct conversion of NBR latex to HNBR latex was developed.3,4 Previous studies showed that diimide is extremely efficient in the reduction of the carbon-carbon double bonds in NBR, even in aqueous solutions including the latex originally synthesized in the industrial production of NBR.5-7 But a detailed kinetic analysis of this catalyst system was very sparse, and no kinetic equation has appeared in the literature. In this work, the kinetics of the process of hydrogenation NBR using hydrazine hydrate / hydrogen peroxide / selenium catalytic reaction system was systematically studied. Based on the 1

ACCEPTED MANUSCRIPT experimental results, the kinetic equation was derived and the reaction order has been estimated. The kinetic equation fits well with the experimental results and furthers our understanding of the mechanism of hydrogenation NBR latex with diimide catalytic system.

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2. Experimental sections

cBD 100 cBD  cHBD

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2.1 Materials NBR latex, with an acrylonitrile content of 26 % and solid content of 43 %, was obtained from Wuxi Yatai Synthetic Adhesive Co.,LTD (Wuxi, China). Hydrazine hydrate (100 %, hydrazine 64 %) and selenium powder (200 mesh, 99.5 %), were obtained from Acros Oganics. Hydrogen peroxide (30 %) was received from the standard sources. 2.2 Hydrogenation of NBR latex The NBR latex hydrogenation runs were performed in a 0.15 L mechanically stirred glass reactor with three entries, two of which were equipped with a condenser and an addition funnel. 15ml NBR latex, defined amounts of selenium powder and hydrazine hydrate were added. Then a specific volume of hydrogen peroxide was introduced dropwise over 3 hours. The mixture was stirred at 60°C for 4h. Arithmetic average method and analysis of experiments were applied to investigate the main effects based on the average degree of hydrogenation sampling in same reaction time points, all experiments were repeated three times under the same conditions. 2.3 Characterization 2.3.1 The hydrogenation degree obtained in each experiment was determined by infrared spectroscopy (IR) using an Brucker model VERTEX 70 spectrometer operating in a range of 4000-600 cm-1. 2.3.2 Determination of the degree of hydrogenation The unsaturation of the products was analyzed using attenuated total reflection infrared spectrometry8 according to SHT 1762-2008 “Rubber-Determination of Residual Unsaturation of Hydrogenated Nitrile Rubber by Infrared Spectroscopy”.9 The unsaturated percentage of NBR and HNBR (abbreviated UNBR and UHNBR, respectively) can be calculated as follow: (1)

Where cBD and cHBD are concentrations of butadiene and hydrogenated butadiene calculated by the corrected absorbance of the characteristic absorption bands. And thus hydrogenation degree (HD%) of hydrogenated acrylonitrile-butadiene rubber can be calculated as follow:

HD%  (1 

U HNBR ) 100 U NBR

(2)

3. Results and discussion The hydrogenation reaction (3) of carbon-carbon double bonds to form hydrogenated polymer in the presence of hydrazine, hydrogen peroxide, and selenium is attributed to the action of diimide, formed in the reaction between hydrazine and hydrogen peroxide. This mechanism has been proposed based on the reactivity of selenium in the decomposition of hydrazine-generating diimide:10 The selenium has been shown to be very active in the formation of diimide, oxidizing hydrazine, and forming the active species, N2H2 and Se2H2, as shown in (4) and (5). 2

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C

Se

+ NH2NH2 + H2O2

C

C H

H Se Se

Se

Se

+

H2O2

+

C

+ N2 + 2H2O

(3)

2Se + 2H2O

H

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H

N

H

N

H H

N

C

C

C

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N

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H

H

H

H

+ N2

H

(4)-(5)

d [cC C ]  kcCC cSeβ cNγ 2 H4 cHδ 2O2 dt

(6)

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The kinetics of hydrogenation NBR latex with hydrazine hydrate / hydrogen peroxide / selenium catalytic reaction system was studied by calculating the initial reaction rate constants. The initial reaction rate constants can be achieved by determining the initial hydrogenation degree of different concentrations of reactants (such as selenium, hydrazine hydrate, and hydrogen peroxide). For low hydrogenation degree, reaction rate -dx/dt can be estimated by -Δx/Δt, which is the basic assumption11 of studying the kinetics by the initial apparent rate constants. The kinetic equation of catalytic hydrogenation reaction system can be written as:

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Where k is the reaction rate constant, and α, β, γ, and δ are the corresponding reaction orders. Reaction conditions are set as follows: mol ratio of selenium powder and carbon-carbon double bonds is 0.010:1, mol ratio of hydrazine hydrate and carbon-carbon double bonds is 2.0:1, and mol ratio of hydrogen peroxide and carbon-carbon double bonds is 2.0:1, then the kinetic study is proceed by appropriate changes of each component on the above basis setting. 3.1 Effect of NBR latex concentration on the hydrogenation reaction rate To simplify the kinetic equation (6), we change the concentrations of NBR latex (carbon-carbon double bond) at 60°C while the others remain unchanged. It can be assumed that:

kc  kcSeβ cNγ 2 H4 cHδ 2O2

(7)

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Since kc is a constant, now that kinetic equation (6) can be abbreviated as:

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d [cC C ]  kc cCα C dt

(8)

Assuming α = 1, then (8) can be integrand as:

 ln(1  x)  k t

(9)

Where x is the hydrogenation degree. NBR latex concentrations of 1.96 mol/L, 2.52 mol/L, 2.93 mol/L were prepared to hydrogenated, the hydrogenation degrees of samples of different reaction times were determined (Fig.1a) and -ln(1-x)~t curve was made (Fig.1b). Fig.1 Fig.1a shows that the higher the concentration of NBR latex is, the early reaction rate of hydrogenation is faster. The hydrogenation degree has risen sharply in the early reaction and leveled off after about 150 min of reaction time, the appropriate NBR latex concentration of the 3

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catalytic hydrogenation is 2.93 mol/L. Fig.1b shows that -ln(1-x) and t is a linear relationship, and the slopes of the lines are the reaction rate constants. This means that in the above reaction conditions, the hydrogenation rate and the concentration of NBR latex are first order relationship, which shows that assuming α = 1 is correct. 3.2 Effect of selenium amount on the hydrogenation reaction rate Similarly, we just change the concentrations of selenium at 60°C. Then formula (9) can be changed as follows:

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 ln(1  x)  kSe t 

Where k Se is constant,

(11)

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 kSe  kcSeβ cNγ 2 H4 cHδ 2O2

(10)

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Different amounts of selenium were added to latex hydrogenation reaction while the others remain unchanged. Samples of various hydrogenation degrees in different reaction times were determined (Fig.2a) and -ln(1-x)~t curve was made (Supporting information, Fig.S1). Fig.2 Fig.2a shows the effects of different mol ratios of [Se]/[C=C] on the hydrogenation rate of NBR latex. It can be seen that for each level of selenium/double bond mol ratio, there is an increase in the hydrogenation degree with the reaction time and 92% hydrogenation degree can be reached. The hydrogenation rate is found to be steeply dependent on the catalyst loading. The higher level of catalyst loading will result in a faster reaction rate in some extent. The k′ Se can be estimated to -1 -1 -1 be 0.0055min , 0.0063min , and 0.0073min at the selenium/double bond mol ratios of 0.006, 0.008, and 0.010, respectively from the curves in the Fig.S1. To make formula (11) both side logarithmic forms as follows:

ln kSe  ln[kcNγ 2 H4 cHδ 2O2 ]  β ln cSe

(12)

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Based on the natural logarithm maps division into subtraction:

ln cSe / C C = ln cSe - ln cC C

(13)

Where cC=C is a constant (2.93mol/L), so the relationship between ln cSe and ln cSe / C C is 

linear function in (13). To build curve of ln kSe ~ ln cSe / C C (Fig.2b), and the slope of the line is the reaction order. It can be calculated that: β = 0.55. 3.3 Effect of hydrazine hydrate dosage on the hydrogenation reaction rate To change the concentrations of hydrazine hydrate at 60°C while the others remain unchanged. Then formula (9) can be changed as follows:

 ln(1  x)  kN 2 H4 t

(14)

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Since k N2 H 4 is constant,

kN 2 H4  kcSeβ cNγ 2 H4 cH 2O2

(15)

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increasing mol ratios of [N2H4]/[C=C], the reaction rate constant k N2 H 4 also follows larger. This

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is because with increase in the amount of hydrazine hydrate, the number of active reaction center diimide (N2H2) arising from hydrazine reaction becomes larger, that provide the more

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opportunities to react with NBR latex particle. The k N2 H 4 for it by calculating the slopes of the

ln kN 2 H4  ln[kcSe cHδ 2O2 ]   ln cN2 H4 disposal

way to

the selenium

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In a similar

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-ln(1-x)~t curves (Fig.S2), can be estimated to be 0.0034min-1, 0.0051min-1, and 0.0063min-1 when the mol ratios of [N2H4]/[C=C] were 1.50, 1.80, and 2.00, respectively. To make equation (15) both side logarithmic forms as follows:

concentration.

(16) To build

curve of

ln kN 2 H4 ~ ln cN2 H4 /C=C (Fig.3b), and the slope of the line is the reaction order. It can be

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calculated that: γ = 2.15.

Fig.3

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3.4 Effect of hydrogen peroxide dosage on the hydrogenation reaction rate Assuming that the dosages of hydrogen peroxide change at 60°C, while the others remain unchanged. Then formula (9) can be changed as follows:

 ln(1  x)  kH2O2 t 

(17)

Where k H 2O2 is constant,

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kH 2O2  kcSe cN 2 H4 cH 2O2

(18)

Different dosages of hydrogen peroxide were added to latex hydrogenation reaction. Samples of hydrogenation degrees in different reaction times were determined (Fig.4a) and -ln(1-x)~t curve was made (Supporting information, Fig.S3). The quantitative hydrogenation degree of NBR latex at different mol ratios of [H2O2]/[C=C] as a function of reaction time is shown in Fig.4a. For each mol ratio of [H2O2]/[C=C], the hydrogenation degree is observed to increase with the reaction time. When the mol ratio of [H2O2]/[C=C] increases in some extent, the slope of the curve becomes larger. That also means the hydrogenation latex rate becomes faster. This is because that the hydrogen peroxide is an oxidant which can react with selenium, the latter may be bridged with two hydrogen atoms to form water and selenium catalyst, as shown in (4). The concentration of active centers is increased, and the reaction rate becomes larger. 

The k H 2O2 can be estimated to be 0.0043min-1, 0.0054min-1, and 0.0063min-1 at the hydrogen peroxide / double bond mol ratios of 1.50, 1.80, and 2.00, respectively from the curves in Fig.S3. To make formula (18) both side logarithmic forms as follows: 5

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(19)

Using similar treatment method as the selenium concentration. To build curve of

ln kH 2O2 ~ ln cH2O2 /C=C (Fig.4b), and the slope of the line is the reaction order. It can be

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calculated that: δ = 1.38.

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Fig.4 3.5 Effect of reaction temperature on the hydrogenation reaction rate To calculate the reaction activation energy, we change the reaction temperatures while the others remain unchanged. Then formula (9) can be changed as follows:

 ln(1  x)  kT t 

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Where kT is constant,

(20)

(21)

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kT  kcSe cN 2 H4 cH 2O2

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The reaction temperatures were 50°C, 60°C, and 70°C, respectively in the NBR latex hydrogenation reaction. Hydrogenation degrees of samples for different times were determined (Fig.5a) and –ln(1-x)~t curve was made (Supporting information, Fig.S4). Fig.5

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The kT can be estimated to be 0.0055 min-1 at 50°C, 0.0063 min-1 at 60°C, and 0.0074 min-1

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at 70°C by calculating the slopes of the curves in the Fig.S4. To make Arrhenius equation (22) both side logarithmic forms as follows:

k   A  exp(

(22)

Ea RT

(23)

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ln k   ln A 

Ea ) RT

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Ink′ is a function of temperature. It can also be expressed as ln kT . To build curve of

ln kT ~

E 1 (Fig.5b), and the slope of the line is  a , then activation energy (Ea) of the R T

hydrogenation reaction can be calculated as Ea = 13.40 kJ/mol, the value is significantly lower than value reported in the current literature to be similar hydrogenation of acrylonitrile-butadiene rubber latex with Wilkinson′s catalyst.12

4. Conclusions The kinetics of NBR latex hydrogenation with hydrazine hydrate / hydrogen peroxide / selenium catalytic hydrogenation reaction system was systematically investigated by calculating the initial reaction rate constants. The kinetic equation of this reaction system between 50~70 °C is derived as follow:



d [cC C ] 0.55 2.15  kcC1 C cSe cN2 H4 cH1.38 2 O2 dt

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Acknowledgment

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This work was supported financially by the National Natural Science Foundation of China (Project No. 51273071)

References

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1. X. Jun, World Rubber Industry, 33(2006)44. 2. H. Wang, G. L. Rempel, J. Ind. Eng. Chem., 25(2015)29-34. 3. S.-Y. Li, MS Dissertation: Studies on Gel Mechanism and Hydrogenation Process of Nitrile Rubber Latex, Beijing University of Chemical Technology, Beijing, China, 2005. 4. X.-Z. Wang, L.-Q. Zhang, Y. Han, X.-K. Shi, W.-M. Wang, D.-M. Yue, J. Appl. Polym. Sci., 127(2013)4764-4768. 5. X.-W. Lin, Q.-M. Pan, G. L. Rempel, Appl. Catal. A-Gen., 276(2004)123-128. 6. S.-Q. Zhou, H.-D. Bai, J.-G. Wang, J. Appl. Polym. Sci., 91(2004)2072-2078. 7. G. A. S. Schulz, E. Comin, R. F. de Souza, J. Appl. Polym. Sci., 115(2010)1390-1394. 8. H.-J. Zhu. Journal of Capital Normal University(Natural Science Edition), 32(2011)41-43. 9. SHT 1762-2008, Rubber-Determination of Residual Unsaturation of Hydrogenated Nitrile Rubber by Infrared Spectroscopy, China Petrochemical Press, Beijing, China, 2008. 10. K. Kondo, S. Murai, N. Sonoda, Tetrahedron Lett., 42(1977)3727-3730. 11. G. A. S. Schulz, E. Comin, R. F. de Souza, J. Appl. Polym. Sci., 123(2012)3605-3609. 12. H. Wang, L.-J. Yang, S. Scott, Q.-M. Pan, G. L. Rempel, J. Polym. Sci., Part A: Polym. Chem., 50(2012)4612-4627.

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Fig.1 (a)Hydrogenation degree of different [C=C] concentrations in various stages; (b)Curves of versus time at different [C=C] concentrations. Fig.2 (a)Hydrogenation degree of different mol ratios of [Se]/[C=C] in various stages; (b)Curve of

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ln kSe versus ln cSe / C C .

Fig.3 (a)Hydrogenation degree of different mol ratios of [N2H4]/[C=C] in various stages; (b)Curve 

of ln k N2 H4 versus ln cN2 H4 /C=C . Fig.4 (a)Hydrogenation degree of different mol ratios of [H2O2]/[C=C] in various stages; (b)Curve 

of ln k H

2 O2

versus ln cH 2O2 /C=C .

Fig.5 (a)Hydrogenation degree of different reaction temperatures in various stages; (b)Curve of

ln kT versus

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Figure 5

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Graphical abstract

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Highlights

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•·A new catalytic reaction system of hydrazine/ hydrogen peroxide/

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selenium with NBR rubber latex is investigated by varying reaction parameters.

•·Diimine species react rapidly, reducing the carbon–carbon double

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bonds of NBR resulting in the formation of HNBR rubber.

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reaction system are derived.

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•·The kinetic equation and hydrogenation activation energy of this

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