Iodometric Determination of Hydroperoxides in Hydrocarbon

Subscriber access provided by Kaohsiung Medical University

Applied Chemistry

Iodometric Determination of Hydroperoxides in Hydrocarbon Autoxidation Reactions Using Triphenylphosphine Solution as a Titrant: A New Protocol Hossein Roohi, and Mehrdad Rajabi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05403 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Iodometric Determination of Hydroperoxides in Hydrocarbon Autoxidation Reactions Using Triphenylphosphine Solution as a Titrant: A New Protocol Hossein Roohi† and Mehrdad Rajabi∗,‡,¶ †Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran, Postal code: 419 383 3697 ‡Department of Chemistry, University Campus 2, University of Guilan, Rasht, Iran ¶Research Department, National Petrochemical Co., Research & Technology, Arak, Iran, Postal code: 399 163 5868 E-mail: [email protected] Phone: +98 91 2289 6761. Fax: +98 13 3336 7066

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hydroperoxides are important oxidants in epoxidation processes and play a central role in the industrial hydrocarbon autoxidation. This paper presents a new and waterfree/low-water iodometric titration technique that can be utilized to quantify organic hydroperoxides. In this protocol, the aqueous thiosulfate titrant used in the conventional iodometric titration method was replaced with a new titrant (triphenylphosphine solution) in order to prevent the need for aqueous solutions. In addition, a comparison of two methods showed that titration time in this new method is shorter than that of thiosulfate titrant based one. It is predicted that a colorless charge-transfer complex is formed from interaction between triphenylphosphine and iodine. The proposed protocol can be utilized in the industrial processes that go through hydroperoxide intermediates.

Keywords Organic peroxides, Triphenylphosphine, 1-Phenyl ethyl hydroperoxide, Iodometric titration.

2


Page 2 of 33

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction The liquid phase air oxidation reaction of alkylbenzenes to oxygenated products through their hydroperoxide intermediates is extensively practiced in the industry worldwide. 1–4 Although organic hydroperoxides (ROOHs) play a central role in these chemical industries, the knowledge for its determination by volumetric methods is limited. 5–8 ROOHs are also important oxidants in epoxidation processes. As an interesting example, the reaction of ethylbenzene (EB) with air to produce 1-phenyl ethyl hydroperoxide (PEHP), with more than 5×106 tons per year in the world, acts as an oxygenating agent in the propylene epoxidation reaction to produce simultaneously styrene and propylene oxide. 4 Mixtures containing hydroperoxides are generally difficult to analyze by means of conventional techniques. The commercial production of hydroperoxides, as an intermediate or final product, requires a fast method to determine the purity of the hydroperoxides often in the presence of by-products that may be interfering. In addition, there is a general need for the new methods of quantifying ROOH concentrations, especially in purely organic media in industrial applications that go through ROOH intermediates. Numerous instrumental and analytical techniques such as High-Performance Liquid Chromatography (HPLC), 9 GC-Mass spectrometry, 10 Near-Infrared spectroscopy, 11 Gas Chromatography (GC), 12 polarography, 13 and Ferrous Oxidation-Xylenol orange (FOX) assay, 14 were developed to determine ROOHs. In the most established volumetric methods, ROOHs are allowed to oxidize iodide ions to iodine which is then determined by colorimetric titration with sodium thiosulfate solution using starch as an indicator (e.g. ASTM D3703, 15 ASTM E298, 16 ). These methods are time-consuming, expensive or require strict control of ambient oxygen levels. There are also some introduced methods 17–20 in which ROOH is quantified using the reaction of ROOH with triphenylphosphine (TPP) as a reactant (eq 1) which leads to the formation of the corresponding alcohol (ROH) and triphenylphosphine oxide (TPPO):

3



ROOH + TPP −−→ ROH + TPPO

Page 4 of 33

(1)

The measurement of the produced alcohol or TPPO allows identification of the original ROOH. Measurement of the alcohol or TPPO by GC 19,21,22 or GC-Mass spectrometric methods 10 requires instrumental analysis and tend to be more expensive and time-consuming than the conventional iodometric titration. 16 Gravimetric measurement of the product TPPO 23 is performed only at a relatively high peroxide level and tends to be prone to poor reproducibility at low ROOH levels. In the traditional iodometric titration procedure for quantification of ROOHs, sodium iodide-isopropyl alcohol (NaI-IPA) systems are used and the standardized sodium thiosulfate (TS) solution is utilized as titrant. 5 For example, iodometric titrations using TS have also been applied to measure the amount of PEHP. 8,24 On the other hand, the TPP forms charge-transfer (CT) complexes with iodine in organic solutions and the reaction of TPP with I2 leads to disproportionation of iodine and the formation of [TPPI]+ and I3 − . 25 Zhang et al., 26 postulated that at high concentration ratios of TPP/I2 , a colorless complex [TPPI]+ [I]− is formed and at low ratios the yellow color complexe [TPPI]+ [I3 ]− is formed. We found that an organic TPP solution could be used as titrant for iodometric titration because TPP can form colorless CT complex with iodine liberated from the reaction of ROOH with iodide ion. This can be used for endpoint detection. However, water-free/low-water titrations of ROOHs are of general interest in studying liquid phase oxidations and, as pointed out, conventional standard methods, 15,16 introduce significant water. Besides, measuring in aqueous solutions can be problematic in titrating non-polar organic mixtures. The main goal of this study was to propose a new methodology for measuring the ROOH concentrations in water-free/low-water hydrocarbon autoxidation (purely organic) mixtures. In our new approach, the organic TPP solution was used as a new titrant for iodine liberated 4


Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from the reaction of ROOH with iodide ion. As a case study, the PEHP produced from the autoxidation reaction of EB was considered in detail. Quantitative values of PEHP using TPP dissolved in EB as titrant were compared with the values obtained by standard iodometric methods using TS aqueous solution. This protocol makes possible the determination of the PEHP in mixtures containing also acetophenone, α-phenyl ethanol, and EB. Various important reaction factors were all investigated for titration with TPP in EB solutions in comparison to iodometric titration with TS solution.

Experimental Reagents. Potassium iodide (KI), sodium iodide (NaI), glacial acetic acid, sodium acetate, absolute ethanol (Abs-EtOH), and sodium thiosulfate pentahydrate (TS) were of analytical grade and purchased from Merck Inc. These reagents were used without further purification. Triphenylphosphine (TPP) was purchased from Fluka and recrystallized from hot ethanol 27 and dried in the nitrogen atmosphere before use (purity was checked by its melting point at 80 ◦ C and thin layer chromatography). The TPP exists as relatively air stable, colorless crystals at room temperature, which undergoes slow oxidation by air to give TPPO:

2 TPP + O2 −−→ 2 TPPO

(2)

This impurity removed by recrystallization of TPP from hot ethanol. 27 The TPP titrant solutions (0.08 molar in EB) were conveniently prepared by adding 2.10 g of purified and dried TPP and 0.01 g of antioxidant (2,6-di-t-butyl-p-cresol), to prevent air oxidation, into a 100 mL volumetric flask and filling the flask with freshly distilled EB under the nitrogen atmosphere. The solution was standardized against the known hydrogen peroxide concentration according to the known standard iodometric method. 16 However, if the iodometric titration with TPP is performed immediately after preparation of the TPP solution or titration completely carried out in the nitrogen atmosphere, then there is no need to add antioxidant. 5



Isopropyl alcohol (IPA), reagent grade (Merck Co.), was de-oxygenated by passing nitrogen bubbles through it for 15 minutes at a temperature near the IPA boiling point. As mentioned in material safety data sheet (MSDS), IPA is susceptible to autoxidation and classified as peroxidizable. When using this chemical it should be avoided from light, ignition sources, excess heat and exposure to moist air or water and also should be periodically tested for peroxide formation on long-term storage. EB was provided by the pars petrochemical company in Iran. It had 0.35 weight% of PEHP (from standard ASTM E298 method, 16 ) which was purified after distillation under the nitrogen atmosphere. Actually, according to our recently published paper, 28 the reaction of TPP with ROOH (eq 1) helps to determine the presence or absence of ROOH in a sample qualitatively. The PEHP was obtained from the liquid phase air oxidation of EB at 148 ◦ C and three atm pressure. 28 The reaction mixture had acetophenone and α-phenyl ethanol as main side products (quantified by the GC analysis, 28 ). The 0.1 N standardized TS solutions were conveniently prepared by dissolving 12.40 g of Na2 S2 O3 · 5 H2 O in 500 mL of freshly distilled water and were standardized according to the literature. 29

Modified iodometric method A method described by Mair et al., 5 was modified in this study by using the Abs-EtOH and solid KI or NaI instead of 99% IPA and a saturated solution of KI in IPA as follows: Sufficient Abs-EtOH (usually 20-25 mL) was added to the 0.5 g of KI/NaI in a 100 mL volumetric flask, followed by 0.5 mL of acetic acid-sodium acetate buffer. The mixture was heated almost to boiling point (65 − 70 ◦ C), kept at incipient boiling for 3-5 min with slow magnetic stirring. Preheating was for producing a saturated solution of iodide ion in alcohol. The main purpose of heating the reaction after ROOH addition was to complete the sluggish iodine liberation and complete the reaction between ROOH and iodide ion to produce iodine. 6


Page 6 of 33

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Then, 0.1-0.3 g of the sample containing ROOH was added without cooling. The mixture was heated for further 5 to 10 minutes (the reaction mixture was turned to bright yellow to yellow-reddish color based on the ROOH level in the sample). The liberated iodine was titrated with 0.1 N TS (standardized against potassium dichromate, 29 ) until the yellow color of the iodine disappeared. Because of the aqueous media, starch could be used for better detection of the endpoint. Blank analysis performed in parallel. Although in this method, oxygen error is almost zero, we recommend having a nitrogen blanket during analysis. The ROOH (wt%) concentration in the samples was calculated by the following equation: 8

%ROOH =

(S − B)N F Mw × 100 2000G

(3)

where S (mL) and B (mL) are, respectively, the volumes of 0.1 N standard TS solution used for the sample and blank titration, respectively. The N, G (g), Mw , and F are, respectively, the normality of TS solution, the sample weight, hydroperoxide molecular weight and the correction factor for the proper concentration of TS solutions, respectively. This formula previously used by Alcantara et al.. 8

Triphenylphosphine iodometric method The iodine liberated from the reaction of ROOH with iodide ions was titrated with a standardized TPP titrant solution (0.08 molar solutions in EB) until the yellow color of the iodine disappeared. The ROOH (wt%) concentration in the samples was calculated by the following formula:

%ROOH =

(S − B)CMw × 100 1000G

(4)

where, S (mL) and B (mL) are the volumes of standard TPP solution used for the sample and blank titration, respectively. The C (molL – 1 ), G (g), and Mw are TPP molarity concentration 7



in a titrant, the sample weight and hydroperoxide molecular weight, respectively. Our solvent system as well as our new titrant (TPP in EB), all were water-free/lowwater reagents. Therefore, starch cannot be used in this non-aqueous media. However, in standard methods such as ASTM E298, 16 and ASTM D3703, 15 the iodine liberated from the reaction of ROOH with iodide ion is then titrated with standard aqueous sodium thiosulfate solution which starch can be used for better detection of the titration endpoint. In ASTM E299 method, 30 the endpoint detection has been done by UV-visible spectrometry. The absorbance of the solution is measured at 470 nm and the amount of active oxygen present in the sample is determined by reference to a calibration curve prepared from iodine. Hence, there was no need to use starch indicator. According to proposed protocol, the solvent mixture including solid metal iodide was first heated in the presence of acetic acid for three to five min at a temperature of 65 − 70 ◦ C to get a saturated solution of iodide ions. Then, without cooling, the sample containing the ROOH was added and was kept at a temperature of 65 − 70 ◦ C for 10 to 15 min. At this stage, the reaction of ROOH with iodide ions was completed and iodine was released. The amount of iodine released is equal to the stoichiometric amount of ROOH present in the sample. Then, the released iodine was titrated with TPP solution. Therefore, giving enough time to complete the reaction of ROOH with iodide ion, in none of the stages of titration, TPP with ROOH was not possible to react, because before adding the TPP, all the ROOH reacted with iodide ion. However, the reaction of ROOH and iodide ions is relatively slow and after adding ROOH and remaining 10 to 15 minutes at 65 − 70 ◦ C, a little amount of ROOH might remain in the solution. On the other hand, the reaction between ROOH and TPP is faster than the reaction between ROOH and iodide ions. So, when the TPP solution is used as a titrant, initially, a rapid reaction is performed between the remaining ROOH and TPP and then TPP reacts with iodine forming a colorless TPP-I2 complex. The proposed new titration method had a clear endpoint and the type of endpoint detection was based on the visible changes of the solution color.

8


Page 8 of 33

Page 9 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Results and Discussion The hydrocarbon air oxidation reaction proceeds via a free radical chain reaction mechanism, which leads to oxygenated hydrocarbon derivatives such as alcohols, ketones, and carboxylic acid derivatives via radical terminations pathways. 31 Therefore, the oxidation reaction mixture of hydrocarbons usually has these chemicals in addition to ROOH. To control the industrial production process of ROOHs, it is necessary to know its concentration during the oxidation process of hydrocarbons. Currently, the determination of ROOH concentration is done by the one-established system, that of iodide oxidation to iodine/triiodide with ROOH, which is usually titrated with aqueous thiosulfate. During the titration reaction, reduction of one-mole ROOH releases one-mole I2 :

ROOH + 2 H+ + 2 I− −−→ I2 + ROH + H2 O (Clear)

(5)

(Yellow)

The produced iodine can be titrated with standardized TS solutions according to following equation:

I2 + 2 Na2 S2 O3 −−→ Na2 S4 O6 + 2 NaI (Yellow)

(6)

(Clear)

After consuming all of the iodine, the solution lost its color, indicating the end point of the reaction. In the case of TPP, it is predicted that a colorless charge-transfer (CT) complex [TPPI]+ [I]− is formed from titration of the produced iodine with TPP as given below.

I2 + TPP −−→ [TPPI]+ [I]− (Yellow)

(7)

(Clear)

In fact, when an equimolar amount of TPP and released I2 was reacted, no free iodine present in the titration and the solution remained colorless. If the reaction of ROOH with iodide

9



was not complete, the free iodine would be accumulated and the solution would be turned to yellow. The reaction between organic ROOHs and iodide ions, especially in aqueous solutions, is generally slow and difficult to complete. Therefore, after adding the sample containing the ROOH, the solution was kept at a temperature of 65 − 70 ◦ C for 10 to 15 min. Figure 1 shows the color change of solution during CT complex formation of iodine with TPP in 1:1 solvent mixture of Abs-EtOH and EB at room temperature. Iodine was added manually, not by the reaction of iodide ions with ROOH or any other method. Just shows the color change sequence during TPP−I2 complex formation. Actually, a few seconds after TPP addition the yellow color was disappeared and Figure 1 shows a clear solution after one minute to verify that the formed CT complex was stable and colorless. There was not any delay in observing a change in color after adding the TPP solutions and the endpoint detection was sharp and stable. Anyway, since the iodometric titration with TPP was carried out at 65 − 70 ◦ C, so, at the time of titration, there was no need to wait one minute between every addition to observing the color change. In fact, as Zhang et al. 26 have stated, the rate of the TPP−I2 complex formation is so high that it can be considered instantaneous, especially at high temperature.

Figure 1: I2 -TPP Charge-Transfer (CT) complexes in 1:1 solvent mixture of Abs.EtOH and EB. From left to right: (1) I2 dissolved in solvent mixture - no CT complex.(2) A few seconds after adding stoichiometric amounts of TPP, CT complex is formed and the solution began to get colorless (3) One minute after adding TPP, the CT complex [TPPI]+ [I]− has been formed and the solution remains colorless.

Since the reaction of iodine and TPP is stoichiometric and proceeds with the color change, 10


Page 10 of 33

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


it was used for organic ROOH quantification as endpoint detection in the iodometric titration. In solution, triiodide ions appear yellow in low concentration and brown at higher concentration. Therefore, the yellow solution, before adding TPP, is only due to the presence of triiodide ions. After adding TPP, the [TPPI]+ [I]− complex was formed which is colorless. From the data presented by Zhang et al., 26 when the TPP/I2 concentration ratio changes from one to 10, the triiodide signal at 360 nm in the UV-visible spectrophotometer decreased and, in turn, disappeared at a ratio of 10. When I2 levels were very traced, this ratio is achieved with a small amount (a drop) of TPP solution to the stoichiometric amount of I2 . However, this does suggest another reason for slightly high measurements using this method. The titration also can be followed by UV-visible spectrophotometry to better distinguish the endpoint by following the triiodide signal at 360 nm. This signal is decreased as the triiodide concentration decreased at the endpoint.

Methods Comparison Iodometric titration methods for the determination of ROOHs are simple, rapid, and need only simple laboratory equipment. In the most established methods, 5 iodide ions are oxidized to iodine by ROOH; 2 I – + 2 e – −−→ I2 , iodine is then quantified by titration with a standardized TS solution (reaction 6), using starch as an indicator. 15,16 In spite of the above-mentioned advantages for iodometric quantification of ROOHs, there are many inherent disadvantages about current iodometric titration, especially for organic ROOHs, due to one or more of the following comparison factors: - The standard ASTM iodometric methods 15,16 are somewhat time-consuming and it is not suitable for rapid process-control work. Actually, as described in the summary section of these test methods, a sample is dissolved in a mixture of methylene chloride or 1,1,2-trichloro1,2,2-trifluoroethane and acetic acid. A saturated aqueous solution of NaI or KI is added and the mixture is allowed to react at room temperature for at least 15 min. The liberated iodine is then titrated with a standard TS solution. It should be noted that preparation 11



of standardized TS solution and starch indicator solution, in turn, is time-consuming. It is recommended that the saturated aqueous solution of NaI and starch indicator solution should be prepared just prior to use, which in turn takes time. However, the time duration of our method was about 20 min per analysis, without the need for preparing additional sensitive reagents. - Although the ASTM E298 directly mentioned that this test method covers the assay of organic peroxides that are easily reduced, the TPP and ASTM E298 methods both could be used for quantification of organic and non-easily reduced ROOHs such as PEHP. However, the ASTM E298 method is a time-consuming method which needs to handle several rather sensitive reagent solutions. Actually, the analysis time duration of ASTM E298 method for PEHP quantification was above 30 min, while for TPP method was about 20 min per analysis, without the need for preparing additional sensitive reagents. - TS method stated by Alcantara et al., 8 for PEHP quantification requires the use of 99% isopropyl alcohol (IPA) as the reaction medium. IPA (99%) is relatively expensive. As mentioned in the material safety data sheet (MSDS), IPA is susceptible to autoxidation and classified as peroxidizable. It is readily autoxidized in contact with oxygen or air, forming acetone and hydrogen peroxide. When using this chemical it should be avoided from light, ignition sources, excess heat, exposure to moist air or water and also should be periodically tested for peroxide formation on long-term storage. It means that there is needed a blank titration in addition to using the nitrogen blanket during ROOH measurement. - TS method needs to use a standard solution of TS, which is not so stable against heat, light, and atmospheric oxygen. 32 - The oxygen atoms in peroxide ion have an oxidation state of -1, which make it unstable. When an R group in peroxide structure (R-O-O-R) is an alkyl or aryl group, its steric hindrance and electronegativity affect the reactivity of peroxo (O-O) group. Therefore, noneasily reduced peroxide is the one that has large R group bonded to the peroxo group. If some electronegative functional group exists, the resulting peroxide becomes more easily reduced

12


Page 12 of 33

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peroxide. The PEHP has two bulky phenyl ethyl groups and is an example of non-easily reduced peroxide. Since the PEHP is a non-easily reduced hydroperoxide, the titration with TS method involved problematic endpoint detection, while titration with TPP method did not have any problem in detecting the endpoint. It could be correct for other non-easily reduced hydroperoxides - In TS method the reaction is highly sensitive to oxygen, so accurate and reproducible results require scrupulous oxygen removal. 16 - TPP method was water-free and was more suitable for organic ROOH quantification than TS method, which works with aqueous solutions. - Iodine released from the reaction of iodide ions with ROOH was completely soluble in an organic media and formed a charge transfer complex with TPP in an organic solvent. 26 The titration with TS for organic ROOHs was done in two phases (aqueous and organic) and the water retarding effect existed with TS titration. 5 Consequently, it is recommended that TS method is primarily used to test relevant samples or determining the evolution of easily reduced ROOHs in the same samples over time. Examples of relevant samples are those that relate to a particular experiment and at different times. The reaction between iodine and TPP was instantaneous in Abs-EtOH solvent system. 25 Using excess iodine source a complex of triiodide was formed, 5 which was not volatile and prevented the loss of liberated iodine, through evaporation, during the titration. Comparison to additional methods. Ferrous oxidation-xylenol orange (FOX) method, which is based on the oxidation of ferrous (Fe2+ ) to ferric (Fe3+ ) ions by ROOHs with the subsequent binding of the Fe3+ ion to the ferric-sensitive dye xylenol orange is sensitive (nanomole to micromole levels of ROOHs), cheap, and not affected by ambient oxygen concentrations, 6 but samples with more than micromole levels of ROOHs must be diluted before analysis. Due to large extinction coefficients, the concentration must be low for analysis of the color solution to not saturate the detector. Hence, if the concentration of ROOH in the sample was higher than micromole, the FOX method is not appropriate because

13



it must be diluted before analysis. Since in industrial hydrocarbon oxidation processes, the approximate concentration is about percent by weight, a serial dilution from percent to micromole is a tedious work with an accumulation of errors. Also, the FOX method is inconvenient due to the need for spectrophotometric analysis, which is time-consuming and costly. Fundamentally the FOX method is convenient for determination of ROOH in micromole level. 14 Spectrophotometric and HPLC hydroperoxide determination using TPP has been done by other authors, 9,10,20 in which after hydroperoxide reduction with TPP, the produced TPPO is determined by HPLC in combination with an ultraviolet detector. However, these spectroscopic methods are time-consuming and more costly than the volumetric methods and are not suitable for controlling industrial process that requires rapid methods. Currently, the traditional iodometric methods, 5 have been used in industrial processes. In this regard, we decided to suggest the TPP method, which is inexpensive, simple, and with good precision, that can be used in industrial processes. In addition, the reaction of ROOHs with TPP at room temperature is sufficiently fast as studied by Hiatt et al.. 33 The reaction (1) is rapid, stoichiometric, and irreversible and so used extensively for the analysis of hydrocarbon autoxidation products. 18,19 Measurement of the product alcohol or TPPO allows identification of the original hydroperoxide, and some other researchers have chosen to originally quantify the TPPO product by gravimetric or titration, 23 but later by the use of GC, 12 HPLC, 9 or GC-MS. 10 If an excess amount of TPP is used, then the amount of TPP consumed can be calculated by subtracting, which requires two analyzes and tends to be poorly repeatable especially at low ROOH levels. Usually, the resulting alcohol is not so informative for ROOH determination, because the hydrocarbon autoxidation reactions proceed with producing alcohol as a by-product, which can be interfered with alcohol originated from the reaction (1) and became a source for error. However, in this work, we wish to measure the ROOH, not the product alcohol or TPPO. The reaction of ROOH with iodide is relatively slow. Therefore, after adding ROOH and

14


Page 14 of 33

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


remaining 10 to 15 min at 65 − 70 ◦ C, a few ROOH may remain in the solution. On the other hand, the reaction (1) is faster than the reaction (5) and therefore, when the TPP solution was used as a titrant, initially, a rapid reaction is performed between the remaining ROOH and TPP and then the reaction of TPP with iodine released from the reaction of (5) forms a colorless TPP−I2 complex. For example, in the case of PEHP, using the TPP solution as the titrant, a rapid reaction between the residual PEHP and TPP is initially performed and then the reaction of TPP with iodine liberated from a relatively slow reaction between iodide and PEHP will be done. It should be noted that, according to reactions (1), (5), and (7), one-mole TPP reacts with one-mole ROOH and one-mole ROOH produces one mole of I2 , and one-mole TPP reacts with one mole of I2 , respectively. A comparison of results for PEHP quantification with both modified TS and TPP iodometric methods using the IPA or Abs-EtOH as solvent and NaI or KI as an iodine source at different reaction times is given in Table 1. The selected samples as ROOH source were from the reaction mixture of EB air oxidation. 28 This mixture also had acetophenone and αphenyl ethanol as by-products. In each case, 0.10 g sample with a concentration around the 10% by weight was added to the titration flask holding at near boiling temperature (68 ◦ C) for 5, 10, and 15 min and the hot solutions were titrated with 0.1 N aqueous TS solution or 0.08 molar TPP in EB solution. As reference method, we used the ASTM E298 standard method. 16 The data presented in Table 1 are the average of at least three determinations per value with standard deviations included near the measurement values. The use of Abs-EtOH instead of 99% IPA as the reaction medium offered several advantages and gave more accurate results. The accuracy of Alcantara (TS titration method), 8 and TPP titration method (presented in this paper) was evaluated by comparing the results of PEHP quantification with ASTM E298 method. 16 As can be seen in Table 1, the accuracy of TPP method had a good agreement with results obtained by the ASTM E298 method, while, the peroxide values were slightly higher with TPP titration than with TS method. As previously stated by Zhang et al., 26 an equilibrium is suggested for the reaction of

15



Page 16 of 33

Table 1: PEHP quantification by different ingredients and methods Run

Number of replicates

1 2 3 4 5 6 7 8 9 10 11 12

3 4 5 4 5 3 5 5 4 4 3 5

Solvent/ Iodine Source IPA/KI IPA/KI IPA/KI Abs-EtOH/KI Abs-EtOH/KI Abs-EtOH/KI IPA/NaI IPA/NaI IPA/NaI Abs-EtOH/NaI Abs-EtOH/NaI Abs-EtOH/NaI

a

%PEHPb

Time (min)

ASTM E298

5 10 15 5 10 15 5 10 15 5 10 15

10.72±0.12 11.31±0.09 11.50±0.08 10.31±0.14 10.12±0.11 11.24±0.12 11.49±0.07 11.32±0.06 11.28±0.04 11.03±0.08 10.34±0.07 10.38±0.07

TS 9.33±1.42 9.67±1.52 9.43±1.32 10.11±1.52 10.46±1.47 10.58±1.35 9.55±0.98 9.73±1.03 10.30±0.95 10.98±0.87 10.21±0.76 10.28±0.71

TPP 10.62±0.25 11.39±0.26 11.72±0.22 10.23±0.12 10.07±0.11 11.33±0.11 11.56±0.15 11.32±0.14 11.40±0.15 11.29±0.11 10.32±0.10 10.23±0.13

a Reaction conditions: Temp.=68 ◦ C, Solvent=25 mL, Acetic acid buffer=0.5 mL, Iodine source=0.5 g, Sample size=0.10 g, TS=Standardized 0.1 N sodium thiosulfate, TPP=0.08 molar triphenylphosphine in EB. b These data are average of at least three determination per value.

TPP with I2 , and with yellow color present until TPP:I2 ratios were almost 10:1. When I2 levels are very traced a 10:1 ratio is achieved with a small amount of TPP. In addition, as verified by UV-visible spectroscopy, 26 as the TPP:I2 concentration ratio increased from 1 to 10 (TPP in excess), the I3 – signal (yellow color) at 360 nm decreased and was absent at a ratio of 10. This could be a reason for slightly high measurements using TPP method. EB oxidation reaction is done in the presence of oxygen and heat (near EB boiling point). Therefore, regardless of how much oxygen dissolved in the solvent, it is released by heating the solution. It means that the solubility of oxygen in Abs-EtOH decreases with rising temperature. For this reason, before adding sample containing ROOH, the solution was warmed up to near the boiling point for at least three min. So, the possibility of oxidation of EB in ethanol at 65 − 70 ◦ C is nearly zero. However, the reaction of IPA with atmospheric oxygen can result in the production of acetone and hydrogen peroxide. In this regard, hydrogen peroxide has an unwanted effect on the iodometric titration, making the measured ROOH content more than the actual amount of ROOH in the sample. For this reason, blank titration was required in the titrations using the IPA solvent, but not for Abs-EtOH.

16


Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reaction Time. In current methods for PEHP quantification by iodometric titration, it is recommended only 1-5 min reaction time for the complete reaction between PEHP and iodide. 8 Considering the data in Table 1, it can be concluded that the five min reaction time is not enough for all three proposed methods, except for the Abs-EtOH/NaI systems for which the 5 min seems to be enough. Due to the more solubility of NaI than KI in AbsEtOH, when using NaI, the time needed to get a complete reaction was less in this solvent. Anyway, using any selected iodide source at a solvent incipient boiling point temperature, enough iodide ions could be produced in solution for ROOHs reduction. In fact, the PEHP reduction reaction with iodide ions to produce iodine was relatively slow and a long time was needed to complete the reaction. It seems that, in addition to the non-easily reduced feature of PEHP, the solubility of NaI/KI plays an important role in PEHP quantification with different methods shown in Table 1. At the same reaction time, using NaI gave higher peroxide values compared to KI, which was validated, for example, by comparing run 4 and 10, Table 1. This was presumed to be due to the greater solubility of NaI than KI in all selected solvent systems, resulting in a greater concentration of iodide ions in the saturated solution. At a temperature of 65−70 ◦ C, the solubility of NaI/KI was high enough and the reaction between PEHP and iodide ions was accelerated. In addition, since the PEHP samples were prepared from EB autoxidation at various reaction time, they had polar impurities such as acetophenone and α-phenyl ethanol, with different concentrations. These impurities can increase the polarity of solvent and lead to better iodide source (NaI/KI) dissolution and increase the iodide concentration. The reaction rate between PEHP and iodide ions would be increased by increasing the iodide concentration. When using excess iodide source, the iodine molecules liberated from the reaction between PEHP and iodide ions all turned to I3 – , which are less volatile than I2 . However, as shown in Figure 2, giving longer reaction time at 65 − 70 ◦ C resulted in the removal of some amounts of I3 – by evaporation and measured PEHP concentration reduced. This was verified by maintaining the reaction solution at 30,

17



45 and 60 min and then measuring the PEHP concentration. Additionally, the loss of iodine at higher temperatures resulted in greater measurement uncertainty. Hence, as affirmed by Mair and co-worker, 5 to prevent the iodine evaporation, a reflux system was required. 11

PEHP Concentration (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

10

9 %PEHP with TS method

%PEHP with TPP method 8 15

30

45

60

Time (min)

Figure 2:

PEHP concentration at various reaction times. Reaction conditions: Temp.=68 ◦ C, solvent

(Abs-EtOH)= 25 mL, acetic acid buffer=0.5 mL, KI= 0.5 g, Sample size=0.1 g, TS= Standardized 0.1 N sodium thiosulfate, TPP= 0.08 molar TPP in EB.

When the ROOH samples have some amounts of water, from any sources (humidity of the air, side products from hydrocarbon oxidation), and where the iodine liberation was slow, it is advisable to heat the solution at a longer time to make sure that the reaction of iodide ions with ROOH is completed before the titration. However, when the reaction of ROOH and iodide ions was relatively slow, as for PEHP, after adding ROOH and remaining 10-15 min at 65 − 70 ◦ C, a few ROOH might remain in the solution. Since the reaction between ROOH and TPP is faster than the reaction between ROOH and iodide ions, when the TPP solution was used as a titrant, a rapid reaction between the remaining ROOH and TPP was initially performed and then the reaction of TPP with iodine released from a relatively slow reaction of ROOH with iodide ions was done and a colorless TPP−I2 complex was formed. Therefore, when iodine liberation was slow, it was suggested to heat the solution up to 15 min (not more). After this time the remaining peroxides were consumed by the first addition of TPP and then the reaction of TPP with iodine resulted in the colorless TPP-I2 complexes as endpoint detection. From these experimental results, we concluded that the reaction time of 10 min in relatively dry condition with Abs-EtOH-NaI/KI system was the best for PEHP quantification. 18


Page 18 of 33

To confirm the accuracy of the proposed methods, we used different PEHP concentrations, which were prepared from the EB oxidation reaction. 28 The results are shown in Figure 3 for TPP method and Figure 4 for TS method. 4

RSD Conc.

10

3

8 6

2

4 1

y = 1.034x - 0.0519 R² = 0.9995

2

RSD (%)

Measured Concentration (wt %)

0

0 0

2

4

6

8

10

PEHP Concentration (wt %) by TPP method

Figure 3: Comparison of theoretical and measured concentrations of PEHP in EB autoxidation mixtures by TPP method. The correlation (R2 ) is very good, and the relative standard deviation (RSD) lies within about 1.82% for this dataset where each measured concentration is the average of three independent determinations on separate samples. The concentration was calculated from the original purity of the PEHP by ASTM E298 method (10.3%).

10

10 Conc. RSD

8

8

RSD (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Measured Concentration (%)

Page 19 of 33

6 4

6 y = 1.0266x - 0.048 R² = 0.9985

2 0

4 0

2

4

6

8

10

PEHP Concentration (wt %) by TS method

Figure 4: Comparison of theoretical and measured concentrations of PEHP in EB autoxidation mixtures by TS method. The correlation (R2 ) is very good, and the relative standard deviation (RSD) lies within about 3.12% for this dataset where each measured concentration is the average of three independent determinations on separate samples. The concentration was calculated from the original purity of the PEHP by ASTM E298 method (10.3%).

Our selected solvent system for iodometric titration was Abs-EOH/NaI (run 11, Table 1). Here it was clear that at least in the case of PEHP containing solutions, the correlation between the calculated and the measured hydroperoxide concentration was very good. The error (shown here as the relative standard deviation, i.e. the ratio of standard deviation 19



to mean value) appears to be non-systematic and random, and within about 1.82% (from 2.04% to 3.86%) for TPP method and 3.12% (from 6.09% to 9.21%) for TS method. The concentration was calculated from the original purity of the PEHP by ASTM E298 method (10.3%). It means within about 1.82% (from 2.04% to 3.86%) for TPP method and within about 3.12 (from 6.09% to 9.21%) for TS method. Reaction temperature. The selected temperature for titration was incipient boiling point temperature of Abs-EtOH (65 − 70 ◦ C). At these temperature ranges, both the solubility of NaI / KI in the reaction media and the reaction rate of PEHP with iodide to liberate iodine were high. Using excess iodide, the liberated iodine was changed to triiodide ions, which are not as volatile as iodine ions and the removing iodine due to evaporation was prevented at these relatively high temperatures. 14 The thermal decomposition of PEHP at 65 − 70 ◦ C during up to 15 min was almost zero. In order to verify this, a solution with a known concentration of PEHP, measured by the standard ASTM E298 method, 16 was placed at a temperature of 70 ◦ C, and sampling was done at a certain time intervals. The concentration of PEHP in the samples was almost constant during the first 30 min. Therefore titration at this temperature was possible without thermal decomposition of PEHP. Antioxidant effect. We performed some experiments to test the effects of air oxidation on the TPP titrant solution using a TPP titrant solution in EB with and without antioxidant (Table 2). The titration was performed under air and nitrogen atmosphere. The sample was 6.7% PEHP, prepared from the air oxidation reaction of EB, 28 which its concentration was measured with ASTM E298 standard method. 16 As pointed out in the experimental section, the TPP is known as relatively air stable, colorless crystals at room temperature. However, TPP can be oxidized in the air slowly and produce TPPO (eq 2). This impurity can be removed by recrystallization of TPP from either hot ethanol or hot IPA. 27 Hence, before preparing the TPP solution in EB the solid TPP was purified by recrystallization with hot ethanol. As shown in Table 2, when the TPP titrant solution exposed to the atmospheric oxygen for a long time, then it should be used

20


Page 20 of 33

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2: Effect of air oxidation on the titrant solution using 0.08 molar TPP solution in EB as titrant.a Run

Titrant

Atmosphere

preparation condition

%PEHP

1 2 3 4 5

TPP in EB TPP in EB TPP in EB (TPP+ antioxidant) in EB (TPP+antioxidant) in EB

Air Air N2 Air Air

Freshly prepared After one day in open air Freshly prepared Freshly prepared After one day in open air

7.11 7.31 6.72 6.71 6.62

a

Reaction conditions: Solvent(Abs.EtOH)=25 mL, Iodine source (KI)=0.5 g, Temperature=68 ◦ C, Reaction time=10 min., acetic acid buffer=0.5 mL, Sample size=0.2 g, Sample concentration by ASTM E298=6.71%

antioxidant, but when the rapid titration is performed, there was no need for antioxidant, if using the nitrogen blanket. Adding a little amount of antioxidant, such as 2,6-di-t-butyl-pcresol, to the TPP titrant solution prevents oxidation of TPP with air. Since the PEHP is obtained from the liquid phase air oxidation of EB at 148 ◦ C and three atm pressure, 28 during the titration at 65 − 70 ◦ C in atmospheric pressure the EB autoxidation is relatively impossible. However, there are some published methods, 17–20 in which ROOHs are quantified using the reaction of ROOH with TPP as a reactant, forming the corresponding alcohol (ROH) and (TPPO). It could be concluded that during the titration at 65 − 70 ◦ C in atmospheric pressure the EB autoxidation is relatively impossible, but the reaction of ROOH with TPP is possible. The white powder of TPPO in the reaction flask when adding TPP solution to the sample containing PEHP was also observed. Solvent and Iodide source compositions. The proper solvent should be non-toxic, inexpensive, do not react with other materials in the titration solution, and all components of the reaction, including peroxide, have good solubility in it. Among the existing solvents, Abs-EtOH was more suitable than other solvents, especially the 99% IPA, because it had all the above features. The solubility of reaction ingredients in Abs-EtOH at near its boiling point temperature was more than IPA. The IPA can be easily oxidized to acetone and hydrogen peroxide in the presence of oxygen in the air. Therefore, when using the IPA as a solvent, the blank correction was necessary at the beginning of the titration. Abs-EtOH does

21



not easily oxidize in the air and the blank titration was not necessary. Therefore, Abs-EtOH was our selected solvent. The main function of solvent was to dissolve hydroperoxide and other reaction ingredients, so the quantity of the solvent was not an important factor. Using high volumes of solvent in the titration makes it difficult to recognize the color change at the endpoint because the color is distributed in a large volume of solvent. In fact, the proper amount of solvent should be so that to effectively dissolve all the reaction ingredients and resulted in a clear solution. Additional solvent not only increases the cost of the analysis but also increases the clarity of the color solution and affects the endpoint detection. In this work, KI and NaI were used as a solid powder, because both were sufficiently soluble in hot Abs-EtOH and the reaction solution was kept in the saturated state during titration. The volatility of iodine is a source of error for the iodometric titration, this was effectively prevented by using an excess iodide and cooling the titration mixture. 5 However, using excess iodide in an acidic environment (acidic triiodide solution) resulted in the formation of a I3 – complex that reacted similarly to free iodine. This strategy shifts the equilibrium; I2 + I – ←−→ I3 – away from iodine. Although the volatility of triiodide ions are not as much as I2 , they could escape either from boiling the reaction mixture or from purging the reaction flask with nitrogen gas. Therefore, at a temperature of 65 − 70 ◦ C, some amounts of triiodide ions can leave the reaction mixture by evaporation and the measured peroxide value became less than the actual value. In order to ensure that the solution was saturated with iodide ions, the presence of small amounts of solid NaI/KI in the solution was needed. The reactions of ROOH-iodide (reaction 5) and iodine-TPP (reaction 7) tend to consume iodide ions, therefore using excess NaI/KI, the needed iodide ions produced immediately and the concentration of iodide ions in the solution remains constant. The least iodide ions in the solution should be such that the solution was saturated with iodide ions at the reaction temperature. Sample sizes and concentrations. The sample sizes depend on the approximate concen-

22


Page 22 of 33

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tration of ROOH in the samples and were in the range of 0.05 to 0.3 g for 25% to 0.1% PEHP in the samples, respectively. When more than 5 mL of titrant (TS or TPP solution) was consumed, for more accurately determine the consumed volume of the titrant, the samples were diluted and when less than one mL of titrant was consumed, 0.01 N TS or 0.008 molar TPP was used. It was related to the volume/size of the burette used. As shown in Table 3, the sample size had not an important role in PEHP quantification. Table 3: Effect of sample size on PEHP value measurement using different methods.a Run

Sample Sizes (g)

% PEHP with 0.1 N TS

%PEHP with 0.08 molar TPP

1 2 3 4 5

0.1 0.2 0.3 0.5 1.0

10.16 10.15 10.00 10.16 10.27

10.17 10.59 10.72 10.27 10.40

a

Reaction conditions: Solvent(Abs.EtOH)=25 mL, Iodine source Temperature=68 ◦ C, Reaction time=10 min., acetic acid buffer=0.5 mL.

(KI)=0.5

g,

In order to test the dilution effect on the accuracy of PEHP measurement with TPP and modified TS methods, the standard ASTM E298 test method 16 was selected as an accurate comparison method. The samples were prepared from the EB oxidation reaction as described in our earlier paper, 28 and the dilution was performed with freshly distilled EB (Table 4). Table 4: Effect of dilution on the accuracy of PEHP measurement with TPP and modified TS methods compared with the standard ASTM E298 method.a Run 1 2 3 4 5 6 7

%PEHP with ASTM E298 25.32 10.20 8.03 6.08 4.14 2.10 0.12

%PEHP with 0.1 N TS 25.06 10.13 7.98 6.01 3.89 1.95 0.08

a

%PEHP with 0.08 molar TPP 25.53 10.17 8.09 6.10 4.17 2.10 0.15

Reaction conditions: Solvent(Abs.EtOH)=25 mL, Iodine source (KI)=0.5 g, Temperature=68 ◦ C, Reaction time=10 min, acetic acid buffer=0.5 mL, Sample size=0.1 g for run 1-3 and 0.2 g for run 4-5 and 0.3 g for run 6-7.

23



Page 24 of 33

As can be seen from the data in Table 4, for the EtOH-KI system and titration with TPP solution, dilution had not an important effect on PEHP quantification. Oxygen and water effects. Regardless of how much atmospheric oxygen dissolved in water, IPA or ethanol, the dissolved oxygen is released by heating the solution. For this reason, before adding sample containing ROOH, the solution was warmed up to near the boiling point for at least three min. However, the important issue is the reaction of oxygen with solvent. The dissolution of oxygen in an aqueous medium does not lead to a specific reaction with water, but oxygen can react with ethanol and IPA. The reaction of ethanol with molecular oxygen is relatively slow and leads to the production of acetaldehyde and then acetic acid, which does not have an unwanted effect in the proposed titration reaction. However, the reaction of IPA with atmospheric oxygen results in the production of acetone and hydrogen peroxide. In this regard, hydrogen peroxide has an unwanted effect on the iodometric titration, making the measured ROOH content more than the actual amount in the sample. For this reason, blank titration is required in the titrations using the IPA solvent, but not for Abs-EtOH. To avoid interference with oxygen, the Abs-EtOH was de-oxygenated at a temperature of 65−70 ◦ C for at least three min before to adding the iodide source and the ROOH containing samples. The solution was also kept under a dry nitrogen blanket to prevent any oxygen diffusion. However, such precaution was unnecessary as it was approved through testing (Table 5), maybe because titration was performed at a temperature close to the solvent boiling point, and in this condition, the reaction mixture was always refluxed. Table 5: Titrant used (mL) by TPP method for blank tests in various iodometric solvent systems in open air.a Time(hr.)

IPA-KI

IPA-NaI

EtOH-KI

EtOH-NaI

1 12 24 48

0.11 0.15 0.18 0.25

0.14 0.18 0.20 0.26

0.00 0.05 0.10 0.15

0.00 0.00 0.00 0.25

a Reaction conditions: Temperature=25 ◦ C, Solvent=25 mL, glacial acetic acid=0.5 mL, Iodine source=0.5 g, Sample size= 0.0 g

24


Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


From the data in Table 5, the oxidation reaction rate of the alcohols at 25 ◦ Cwas relatively low, but longer reaction time increased the diffusion of oxygen into the solutions and resulted in higher peroxide value because of the slow reaction of hydroperoxide with iodide changed to a fast reaction of oxygen with iodide. When a solution containing Abs-EtOH, NaI/KI and acetic acid (blank solution) were heated up to 68 ◦ C for 2 hours, no yellow color was detected. Therefore, when Abs-EtOH was used as a solvent and the above instructions were followed, there was no need to correct the titrant volume for the blank solution. In the absence of PEHP, the blank solution remained colorless. After a day, the solution was yellowed by reaction with diffused atmospheric oxygen. Thus, the "oxygen error" was eliminated for relatively rapid titration at 68 ◦ C. As previously reported by Mair et al., 5 the reaction between iodide ion and ROOHs in the presence of water and in acidic solutions is slowed down. This result was valid for titration with TS solution. When the titer is small, due to the slowness of reaction between iodine and TS, some amounts of water must be present at the titration endpoint to avoid over titration. Indeed, in the titration with TS, the ROOH was dissolved in a saturated solution of KI/NaI in 99% IPA in acidic medium and the water concentration was low. When the titer was small (low concentration of PEHP), the water from thiosulfate solution and the concentration of iodide, both were low (the solubility of KI in IPA=0.138 g per 100 ml of solution at 25 ◦ C), so due to the greater solubility of KI in water (102.8 g per 100 ml of solution at 25 ◦ C), it was needed to add water at near the titration endpoint to dissolve KI and gave a greater iodide concentration. By increasing iodide concentration, the rate of the reaction between iodide and ROOH increased, and the slowness of the reaction between iodine and TS was compensated by producing more iodine. However, as shown in Table 6, in the case of PEHP and titration with TPP solution, the water had no important effect on the hydroperoxide reduction by iodide. The iodine released from the reaction of iodide with PEHP has better solubility in organic than in

25



aqueous solutions. This was the reason that we preferred to use the TPP dissolved in EB as titrant. In the TPP method, the water content of the samples was very low compared to necessary levels in ASTM E298 method. So, the water retarding effect does not seem like it would be a major contributor to TPP method. Table 6: The water effect on PEHP quantification with different methods.a Run

Water added (mL)

% PEHP by 0.1 N TS

%PEHP by 0.08 molar TPP

1 2 3 4

0 2 5 100

10.54 10.17 9.88 9.53

10.76 10.58 10.36 10.52

a

Reaction conditions: Solvent (Abs-EtOH)=25 mL, Iodine source (KI)=0.5 g, glacial acetic acid=0.5 mL, Temperature=68 ◦ C, Sample size=0.10 g.

Anyway, PEHP determination was done in relatively dry condition. The starch indicator cannot be used in dry conditions. However, if the water is added to the solution, a starch indicator can be used, but this caused the two-phase system and was not necessary.

Conclusions A new technique to determine organic ROOH concentrations in organic mixtures was successfully developed in which the iodine liberated from the reaction of iodide with ROOH was titrated with a new titrant (TPP dissolved in organic solution), instead of a solution of aqueous TS in the iodometric titration, to prevent the need for aqueous solutions. As a case, the quantification of PEHP in EB oxidation reaction mixtures studied in detail. The TPP iodometric method for hydroperoxide quantification was clearly superior to TS titrations already established, especially for non-easily reduced organic hydroperoxides in relatively dry conditions. This method was effective, simple and inexpensive with good precision. Experiments with different PEHP concentrations gave at hand that the method has a determination error of less than 4%. There was good agreement between the iodometric PEHP quantification with TPP and TS solutions as the titrant, compared with standard ASTM E298 method. 26


Page 26 of 33

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


However, the iodometric titration with TPP solution was able to measure PEHP at various concentrations. Using this new TPP iodometric procedure, all ingredients were relatively soluble at 65−70 ◦ C and a saturated saturated solution of iodide ions was achieved during the titration. If adhered to the instruction described in this work, no blank titration is required, the endpoint is easily visible to within a drop or two of TPP solution, the reaction of iodine with TPP is fast during the titration, and the possibility of passing through the endpoint is very low. This method can be used for many types of organic peroxides and hydroperoxides, both liquids, and solids; provided that they are soluble in Abs-EtOH.

Acknowledgement This work was supported by the National Petrochemical Company, Research, and Technology in Iran. The authors thank members of the chemistry department of the University of Guilan in Iran for their scientific support through the years.

27



References (1) Weber-Manfred; Weber-Markus; Kleine-Boymann, M. Ullmann’s Encyclopedia of Industrial Chemistry Wiley-VCH Verlag GmbH & Co. KGaA Weinheim Germany 2012, 26, 503. (2) Zhang, P.; Nguyen, V.; Frost, J. W. Synthesis of Terephthalic acid From Methane. ACS Sustainable Chem. Eng. 2016, 4(11), 5998. (3) Fadzil, N. A. M.; Rahim, M. H. A.; Maniam, G. P. A brief review of para-xylene oxidation to terephthalic acid as a model of primary C-H bond activation. Chin. J. Catal. 2014, 35, 1641. (4) Buijink, J. K. F.; Vlaanderen, J. J. M. V.; Crocker, M.; Niele, F. G. M. Propylene epoxidation over titanium-on-silica catalyst-the heart of the SMPO process. Catal. Today 2004, 93-95, 199. (5) Mair, R. D.; Graupner, A. J. Determination of Organic Peroxides by Iodine Liberation Procedures. Anal. Chem. 1964, 36, 194. (6) Kokatnur, V. R.; Jelling, M. Iodometric determination of peroxygen in organic compound. J. Am. Chem. Soc. 1941, 63, 1432. (7) Wagner, C. D.; Smith, R. H.; Peters, E. D. Determination of organic peroxides; Evaluation of Modified Iodometric Method. Anal. Chem. 1947, 19(12), 976. (8) Alcantara, R.; Canoira, L.; Joao, P. G.; Santos, J. M.; Vazquez, I. Ethylbenzene oxidation with air catalysed by bis(acetylacetonate) nickel(II) and tetra-nbutylammonium tetrafluoroborate. Appl. Catal., A 2000, 203, 259. (9) Baj, S. Quantitative determination of organic peroxides. Fresenius J. Anal. Chem. 1994, 350, 159.

28


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10) Wiklund, P. Karlsson, C.; Levin, M. Determination of Hydroperoxide Content in Complex Hydrocarbon Mixtures by Gas Chromatography/Mass Spectrometry. Anal. Sci. 2009, 25, 431. (11) Tavassoli-kafrani, M. H.; Curtis, J. M.; Van de Voort, F. R. A Primary Method for the Determination of Hydroxyl value of Polyols by Fourier transform Mid-Infrared Spectroscopy. J. Am. Oil Chem. Soc. 2014, 91, 925. (12) Hong, J.; Maguhn, J.; Freitag, D.; Kettrup, A. Determination of Organic Peroxides by Gas Chromatography with Cold On-Column Injection. J. High Resolut. Chromatogr., short Communications 1999, 22(8), 475. (13) Molnar, S.; Peter, F. Polarographic investigations of organic peroxides. J. Electroanal. Chem. 1969, 22, 63. (14) Delong, J. M.; Prange, R. K.; Hodges, D. M.; Forney, C. F.; Bishop, M. C.; Quilliam, M. Using a Modified Ferrous Oxidation-Xylenol Orange (FOX) Assay for Detection of Lipid Hydroperoxides in Plant Tissue. J. Agric. Food Chem. 2002, 50, 248. (15) ASTM D 3703, "Standard Test Method for Peroxide Number of Aviation Turbine Fuels," ASTM International. (16) ASTM E 298, "Standard Test Method for Assay of Organbic Peroxides," ASTM International. (17) Wang, N.; Ma, T.; Yu, X.; Xu, L.; Zhang, R. Determination of Peroxide values of Edible Oils by Ultraviolet Spectrometric Method. Food Anal. Methods 2016, 9, 1412. (18) Nakamura, T.; Maeda, H. A Simple Assay for Lipid Hydroperoxides Based on Triphenylphosphine Oxidation and High Performance Liquid Chromatography. Lipids 1991, 26, 765.

29



(19) West, Z. J.; Zabarnick, S.; Striebich, R. C. Determination of Hydroperoxides in Jet Fuel via Reaction with Triphenylphosphine. Ind. Eng. Chem. Res. 2005, 44, 3377. (20) Pinkernell, U.; Effkemann, S.; Karst, U. Simultaneous HPLC Determination of Peroxyacetic Acid and Hydrogen Peroxide. Anal. Chem. 1997, 69(17), 3623. (21) Perkel, A. L.; Krutskaya, L. V.; Freidin, B. G. Application of Triphenylphosphine to the Gas Chromatographic Determination of Peroxides in the Oxidation Products of Organic Compounds. Zh. Anal. Khim. 1994, 49, 768. (22) Barnard, D.; Wong, K. C. The determination of small amounts of organic hydroperoxides with triphenylphosphine. Anal. Chim. Acta, 1976, 84, 355. (23) Dulog, L.; Burg, K. H. Bestimmung Organischer Peroxide. Z. Anal. Chem. 1964, 203, 184. (24) Toribio, P. P.; Campos-Martin, J. M.; Fierro, J. L. G. Liquid-phase ethylbenzene oxidation to hydroperoxide with barium catalysts. J. Mol. Catal. A: Chem. 2005, 227, 101. (25) Cotton, F. A.; Kibala, P. A. Reactions of Iodine with Triphenylphosphine and Triphenylarsine. J. Am. Chem. Soc. 1987, 109, 3308. (26) Zhang, Y. R.; Aronson, S.; Mennitt, P. G. Ionic dissociation in complexes of iodine with triphenylphosphine and triphenylamine. Can. J. Chem. 1992, 70, 2394. (27) Armarego, W. L. F.; Perrin, D. D.; Perrin, D. R. Purification of Laboratory Chemicals (2nd ed.); New York: Pergamon, 1980. (28) Roohi, H.; Rajabi, M. Noncatalytic Liquid Phase Air Oxidation of Ethylbenzene to 1-Phenyl Ethyl Hydroperoxide in Low Oxygen Volume Fraction Org. Process Res. Dev. 2018, 22(2), 136.

30


Page 30 of 33

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Asakai, T.; Hioki, A. Reliability in standardization of sodium thiosulfate with potassium dichromate. Microchem. J. 2015, 123, 9. (30) ASTM E 299, "Standard test method for Assay of Organbic Peroxides," ASTM International. (31) Toribio, P. P.; Gimeno-Gargallo, A.; Capel-Sanchez, M. C.; Frutos, M. P.; CamposMartin, J. M.; Fierro, J. L. G. Ethylbenzene oxidation to its hydroperoxide in the presence of N-hydroxyimides and minute amounts of sodium hydroxide. Appl. Catal., A 2009, 363, 32. (32) Kilpatrick, M. Jr.; Kilpatrick, M. L. The Stability of Sodium Thiosulfate Solutions. J. Am. Chem. Soc., 1923, 45 (9), 2132. (33) Hiatt, R.; Smythe, R. J.; McColeman, C. The Reaction of Hydroperoxides with Triphenylphosphine. Can. J. Chem. 1971, 49, 1707.

31



Graphical TOC Entry KI + EtOH + Acid

ROOH containing solution

Thiosulfate solution

2Na2S2O3 + I2  Na2S4O6 + 2NaI

68 °C

68 °C (5 min)

ROOH + H+ + 2II2 + ROH + H2O

Triphenylphosphine solution

68 °C

(15 min)

32


TPP + I2  TPP-I2

68 °C

Page 32 of 33

ROOH KI + EtOH Page 33 of containing 33 + Acid solution

1 268 °C 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Thiosulfate solution

2Na2S2O3 + I2  Na2S4O6 + 2NaI

68 °C (5 min)

ROOH + H+ + 2II2 + ROH + H2O

Triphenylphosphine solution

68 °C


TPP + I2  TPP-I2

68 °C

(15 min)


Iodometric Determination of Hydroperoxides in Hydrocarbon

Recommend Documents