EFFECT OF OXYGENATED COMPOUNDS ON 1,3-BUTADIENE POLYMERIZATIONS PERFORMED WITH NEODYMIUM VERSATATE. PART I: ALCOHOLS, ALDEHYDES, KETONES, AND WATER
ABSTRACT
1,3-butadiene (1,3-BD) is a building block produced mainly as a byproduct of the ethylene steam cracking process. However, due to the growing interest in sustainable technologies, there is also growing interest in manufacturing 1,3-BD from ethanol. For this reason, taking into account that the ethanol-derived 1,3-BD can contain oxygenated contaminants that are difficult to remove, the present manuscript investigates for the first time how the presence of low concentrations of some oxygenates (acetaldehyde, crotonaldehyde, 3-hydroxybutyraldehyde, acetone, water, ethanol, 1,3-butanodiol, 3-buten2-ol, crotyl alcohol, and 1-butanol) in the 1,3-BD monomer can affect polymerization reactions performed with the neodymium versatate catalyst and modify the characteristics of the obtained polybutadiene products. It is shown that the presence of oxygenated compounds can cause inhibitory effects on the course of the polymerization and modify the molar mass distributions and flow properties of the final products, although all analyzed samples presented the characteristic high-cis character of polybutadienes produced with the neodymium versatate catalyst.
INTRODUCTION
1,3-butadiene (1,3-BD), worldwide demand for which is higher than 9 million tons per year, is used mostly as a raw material for manufacture of synthetic rubbers, including polybutadiene (BR) and styrene-butadiene rubber (SBR), intended mainly for applications in the tire industry.1,2 Owing to the growing interest in producing polymers from renewable sources, there also is growing interest in investigating the manufacture of renewable green monomers,3,4 including green butadienes. According to the literature, two routes have been widely explored to produce green butadienes, known as the Ostromisslensky and the Lebedev routes. The Ostromisslensky route for manufacture of 1,3-BD, commercialized in the United States in 1940s with CuO-Ta2O5/SiO2 as catalyst,5 involves a two-step process that makes use of a mixture of ethanol and acetaldehyde as raw material. On the other hand, the Lebedev route for manufacture of 1,3-BD, commercialized in Russia in 1940s,6 converts ethanol directly into butadiene.7,8 Both processes are represented in Figure 1.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Despite these processes, 95% of the 1,3-BD manufactured worldwide is still produced as byproduct of the ethylene steam cracking process.2,9,10 It is important to note that recent discoveries of large shale gas reserves, especially in the United States, may gradually impose the replacement of steam cracking units used for production of ethylene,8 since shale gas is a more profitable raw material than naphtha, which can cause a direct impact on the supply of 1,3-BD. The increase of the worldwide demand for 1,3-BD, coupled with the possible and simultaneous reduction of the global supply of this monomer, has led researchers to seek new technologies for the production of this important commodity from renewable sources throughout the world.8,11,12 In addition, the growing environmental concerns and the stricter environmental regulations are encouraging the search for monomers and polymers that can be obtained from renewable sources.13,14
In this scenario, several studies have been investigating the manufacture of 1,3-BD from ethanol with help of homogeneous and heterogeneous catalysts, including MgO/SiO2/Cr2O5, Al2O3/ZnO, Mn/sepiolites, NiO/MgO/SiO2, K2O, Na2O, CuO-MgO/SiO2 prepared and used at different conditions.15–19 Additionally, CuO-Ta2O5/SiO2 was used commercially as catalyst for more than 20 yr to produce 1,3-BD from ethanol. For instance, in 320,000 metric tons of 1,3-BD were manufactured through this process.5,6 These catalysts can indeed provide 1,3-BD from ethanol, but also produce complex mixtures of several other compounds with smaller selectivities, including ethane, ethene, propane, propene, butenes, acetone, diethyl ether, pentenes, hexenes, ethyl acetate, butanol, acetaldehyde, crotonaldehyde, 3-hydroxybutyraldehyde, among others.17 From a certain point of view, the manufacture of this long list of byproducts is unavoidable due to the existing thermodynamic constraints.
As a matter of fact, few papers discuss the distribution of byproducts generated through the conversion of ethanol into 1,3-BD. Despite that, acetaldehyde is recognized as a major byproduct in most published articles, reaching as much as 5 mol% of the product stream.19 For this reason, acetaldehyde can be separated and recycled back to the process.19–21 The presence of crotonaldehyde, 3-hydroxybutiraldehyde, water, and acetone in small concentrations (below 1 mol% of the product stream) has also been reported frequently.2,11,19,22–24 However, to the best of our knowledge, none of the available studies explored the effects of these oxygenated byproducts on 1,3-BD polymerizations. Considering that the removal of oxygenated contaminants from the product 1,3-BD stream can be difficult and expensive, investigating the effects associated with the presence of low concentrations of these oxygenates in the monomer stream on the course of 1,3-BD polymerizations and properties of the obtained products constitutes an important technological problem.
Currently, neodymium versatate (NdV) is the Ziegler-Natta catalyst used most often to perform 1,3-BD polymerizations.25 Particularly, the NdV catalyst is highly specific and sensitive to modifications of the reaction operation conditions and composition of the reaction medium, producing essentially linear polybutadienes with very high-cis contents through solution 1,3-BD polymerization processes.25 In principle, the presence of oxygenates in the reaction medium can affect the site formation mechanism and modify the kinetic features of the catalyst.26,27
Based on the previous paragraphs, the main objective of the present study is to analyze for the first time the effects of adding small amounts of acetaldehyde, crotonaldehyde, 3-hydroxybutyraldehyde, acetone, water, ethanol, 1,3-butanodiol, 3-buten2-ol, crotyl alcohol, and 1-butanol on 1,3-BD polymerizations performed with the NdV catalyst in solution. In order to do that, experiments were carried out to indicate how the presence of these compounds in the monomer feed stream can affect the rates of the polymerization and some properties of the produced rubber. For this reason, experiments were performed in batch and observed reaction temperature profiles, pressure profiles, polymer microstructure (cis, trans, and vinyl level) and molecular weight distributions were compared with the respective data obtained in standard 1,3-BD polymerization experiments carried out with the pure monomer.
MATERIALS AND METHODS
Materials
Toluene, n-hexane, diisobutylaluminium hydride (DIBAH, provided as a 1 M solution in n-hexane), and tert-butyl chloride (CTB) were supplied as analytical grades by Sigma Aldrich (Rio de Janeiro, Brazil). Neodymium versatate (provided as 40 wt% solution in n-hexane) was supplied by Rhodia Rare-Earths and Gallium (La Défense, France). Premier nitrogen, with minimum purity of 99.99 mol%, was supplied by Air Products (Rio de Janeiro, Brazil). Irganox-I1520® (2-methyl-4,6-bis(octylsulfanylmethyl)phenol) was supplied by Arlanxeo Brasil (Rio de Janeiro, Brazil). 1,3-butadiene (polymerization grade with minimum purity of 99 wt% and not containing stabilizers) was provided by Braskem (São Paulo, Brazil), stored in typical cylinders and kept under refrigeration at −17 °C. Ethanol, n-butanol, 1,3-butanediol, 3-buten-2-ol, crotyl alcohol, acetone, 3-hydroxybutyraldehyde, acetaldehyde, and crotonaldehyde were used as contaminants and supplied as analytical grades by Sigma Aldrich. All chemicals were used as received without further purification. Water was distilled and deionized prior to use.
Catalyst synthesis
The catalyst was synthesized in a glass reactor, kept under nitrogen atmosphere and refrigeration of 10 °C, with molar ratio Cl/Nd = 3 (ref 28). Reagents were added dropwise in the desired amounts in the following order: (i) 15 mL of the DIBAH solution in n-hexane (2 wt%); (ii) 1.15 mL of the original neodymium versatate solution in n-hexane (40 wt%); (iii) 1.15 mL of the CTB solution in n-hexane (30 wt%). After addition of the reagents, the mixture was kept under continuous magnetic stirring for 4 h. Finally, the catalyst was stored in amber glass flasks at 10 °C.
Polymerization procedures
Polymerizations were carried out in a 1-L high-pressure stainless-steel reactor. Alumina beds were used to dry the n-hexane before use in reaction tests or preparation of the contaminant solutions. After drying, the n-hexane and the contaminant solution were introduced into the reactor under nitrogen atmosphere. The monomer feed was prepared in solution of n-hexane to reach the final solid content of 9 wt%. The 1,3-BD monomer was added directly into the dry solvent with the help of a 50-mL stainless-steel container, and then the reactor was heated to 70 °C under continuous agitation. Finally, the catalyst solution (10 wt% in respect to the 1,3-BD feed) was fed into the reactor with a syringe, after aging for 10 min at room temperature. Polymerization runs were performed for 2 h. In these reaction trials, 25 ± 3 g of 1,3-BD and catalyst solution stored at 10 °C were used. All reactions were terminated through addition of a mixture of ethanol (150 mL) and Irganox-I1520® (1.5 mL). The resulting polymer solution was coagulated in deionized water at 90 °C under mild stirring. The polymer mass was then collected and dried in an air recirculation oven at room temperature. Product manipulation was always performed under nitrogen atmosphere to minimize problems associated with oxidization of polymer samples.
CHARACTERIZATION
Gel permeation chromatography
The molar mass distributions of polymer products were determined by size exclusion chromatography (SEC) using an Agilent (Santa Clara, CA, USA) 1200 series high performance liquid chromatography (HPLC) equipped with a Phenomenex linear Phenogel column (Tokyo, Japan). Calibration was carried out with polystyrene standards. Chloroform (HPLC grade from Aldrich, St. Louis, MO, USA) was used as a solvent at flow rate of 1 mL/min. Analyses were performed at room temperature. In order to determine the number-average molar mass (Mn) and the weight-average molar mass (Mw), it was considered that the height (intensity) of the chromatogram was proportional to the mass fraction of the respective sample.29
Fourier transform infrared sprectroscopy
The microstructure of BR samples was characterized by Fourier transform infrared spectroscopy using a Nicolet-6700 spectrometer (Waltham, MA, USA) in the wavenumber range from 500 to 4000 cm−1 using a diffuse reflectance accessory. The resolution was equal to 4 cm−1, and spectra were stored as averages of 128 scans. The isomer contents were calculated using the absorbances at 725 cm−1 (cis), 910 cm−1 (vinyl), and 967 cm−1 (trans). Equations 1–4 were used to estimate the BR microstructure in the form:30
where
and A1, A2, and A3 are the absorbance areas of the characteristic peaks for 1,4-cis, 1,4-trans, and 1,2-vinyl, respectively. The constants ε1, ε2, and ε3 are the absorptivities of the respective peaks, whose values are equal to 0.175, 0.647, and 1.000 (ref 30).
Nuclear magnetic resonance
Nuclear magnetic resonance (1H-NMR) measurements were carried out in a Bruker (San Jose, CA, USA) AV-400 NMR spectrometer operating at 400 MHz and temperature of 25 °C. Samples (25 mg) were dissolved in chloroform-D1 (5 mL) in 5 mm NMR tubes at room temperature prior to analyses.
RPA 2000 analysis
The processing characteristics of the samples were characterized on a RPA 2000 Rubber Processing Analyzer (Alpha Technologies, Hudson, OH, USA). The tan delta was measured as a function of frequency between 0.1 and 100 Hz at 100 °C.
RESULTS AND DISCUSSION
In principle, the presence of oxygenates in the reaction medium can cause the interaction with the NdV Ziegler-Natta catalyst through at least three distinct manners: (i) interaction between the oxygenated compound and the catalyst active center, modifying the nature of the active site; (ii) incorporation of the contaminant into the polymer chain, so that the contaminant can behave as a comonomer or a chain modifier; (iii) the contaminant can behave as an inert compound, not exerting any significant effect on the course of the reaction or the final properties of the product.
First, the effect of alcoholic contaminants (ethanol, n-butanol, 1,3-butanediol, 3-buten-2-ol, and crotyl alcohol) was analyzed. Alcohols are expected to be produced as byproducts in different chemical routes used to convert ethanol into 1,3-BD.2,11,19,22–24 In particular, the presence of ethanol in the 1,3-BD stream can constitute a major technological challenge, since ethanol is normally used to halt 1,3-BD polymerization reactions.25,31 Obtained results are presented in Table I, and the alcohols used as contaminants are presented in Figure 2.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Figures 3 and 4 show the temperature and standardized pressure profiles for reactions performed in the presence of 200 ppm of different alcohols, indicating the occurrence of significant inhibitory effects. It is important to observe the level of 200 ppm of contamination was selected here as reference because this usually is the maximum concentration of contaminants present in commercial 1,3-BD grades, purified through distillation. Therefore, for practical purposes the contaminant can be regarded as inert if inhibitory effects are absent for this level of contamination. However, this is not the case shown in Table I and Figures 3 and 4, since addition of any of the analyzed alcohols caused the reduction of the rates of polymerization, reflected both in terms of the temperature increase (due to lower rates of heat released by reaction) and the pressure drop (due to lower rates of monomer consumption).
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
One must observe that the formation of the catalytic complex was carried out before the polymerization trials, when the neodymium pre-catalyst and the cocatalysts (DIBAH and CTB) reacted to form the catalytic complex. Additionally, the contaminant concentrations were always small, so that the possible reaction of contaminants with the excess of DIBAH present in the catalyst solution should not be expected to generate high amounts of heat or cause significant variations of the system pressure. Finally, blank experiments performed in absence of monomer did not indicate any detectable variations of the reactor temperature and reactor pressure. For all these reasons, temperature and pressure variations over the reaction time are trustable indicators of rates of polymerization. Particularly, ethanol caused the strongest inhibition among the analyzed alcohols, as might already be expected, since ethanol is generally used for termination of 1,3-BD polymerization reactions performed with neodymium catalysts.31 According to Figures 3 and 4, inhibitory effects can be ordered in the form 1,3-butanediol < 3-buten-2-ol < n-butanol < crotyl alcohol < ethanol, so that the ethanol concentration can possibly be selected as the key indication of alcoholic contamination in 1,3-BD grades produced from renewable raw materials. A possible explanation for this more significant inhibitory effect of ethanol is the fact that the molar concentration of ethanol in the reaction medium is approximately two times higher than for the remaining analyzed alcoholic contaminants, when the contamination level of 200 ppm is considered. It must also be noted in Figure 4 that reactions performed with 1,3-butanediol, 3-buten-2-ol, and n-butanol caused the reduction of approximately 20% of the monomer conversion, while the reduction of monomer conversion was equal to 50% in presence of crotyl alcohol and almost 100% in presence of ethanol.
Given the strong inhibitory effect induced by ethanol, 1,3-BD polymerizations performed in presence of different concentrations of ethanol were also investigated, as shown in Table I and Figures 5 and 6. Figures 5 and 6 show that the variation of the ethanol concentration induced the development of very intriguing nonlinear phenomena, as inhibition was strong above contamination levels of 80 ppm, but some sort of catalyst activation could be observed below the contamination level of 50 ppm. Therefore, it seems clear that ethanol interacts strongly with the active catalyst centers and can significantly modify the catalyst performance. (To the best of our knowledge, this activation effect had never been described before.) Figure 6 shows that the higher rates of reaction (and faster increase of temperature) were followed by higher monomer conversions when the ethanol contamination contents were equal to 50 and 10 ppm.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Figure 7 and Table I show that the contamination effects on the obtained molar mass distributions of obtained BR samples were not very significant and possibly related to the temperature variations, indicating that the presence of less than 200 ppm of 1,3-butanediol, 3-buten-2-ol, n-butanol, and crotyl alcohol on the 1,3-BD feed are not expected to affect significantly the course of the polymerization and the properties of the final products. On the other hand, Figure 8 and Table I indicate that the effects of ethanol on the molar mass distributions of obtained BR samples were remarkable and could not be assigned to observed temperature variations. First, when contamination of 50 ppm of ethanol is considered, observed temperature and pressure profiles were very similar to the ones obtained for the standard reaction performed in absence of contaminants, although the obtained molar mass distributions of BR samples were very different and shifted toward lower molar masses in presence of ethanol, leading to lower polydispersities. Then, when contamination of 10 ppm of ethanol is considered, the molar mass distribution of the obtained BR sample was very broad and very different from the previous ones. The interesting point is the fact that additional shifting toward lower molar masses could be observed in this case, while the simultaneous production of chains of high molar masses, resembling the molar mass distribution of the standard BR sample prepared in absence of contaminants, could also be observed, possibly indicating the existence of a mixture of the original catalyst sites and sites modified by the interaction with ethanol. From a practical point of view, the molar mass distributions of BR samples prepared in presence of ethanol can be regarded as worse than the one of the standard product (because average molar masses were smaller and polydispersity was higher), meaning that the apparent activation effect on catalyst activity caused by ethanol is not necessarily positive, as this is accompanied by negative effects on the molar mass distribution of the product. Therefore, the ethanol contamination level of the 1,3-BD feed obtained from renewable resources should be controlled very strictly.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Regarding the microstructural properties of the obtained BR samples, the cis, trans, and vinyl contents of the final polymer materials were not very sensitive to the presence of the analyzed contaminants, which can be regarded as a positive factor and suggests that the stereochemistry of the active catalyst sites was not affected significantly by the analyzed compounds. Consequently, inhibitory effects on catalyst activities can possibly be explained by reactions with the hydroxyl groups of the contaminants, which can lead to permanent hindering of the catalyst sites and prevent the occurrence of the propagation reaction with 1,3-BD. Given the much smaller size of the ethanol molecule, permanent hindering of the catalyst sites may not occur at extreme low concentrations (<50 ppm), leading to formation of distinct catalyst sites.
Figure 9 presents the 1H-NMR and 13C-NMR spectra of BR samples prepared in presence of the unsaturated contaminants crotyl alcohol and 3-buten-2-ol. As a matter of fact, 1H-NMR spectra of all samples were always very similar, given the very low contaminant concentration levels considered here, and since it is impossible to detect any significant modification of the chain structure caused by incorporation of the contaminants through NMR analyses. However, addition of crotyl alcohol and 3-buten-2-ol into the reaction medium induced the appearance of a band placed at 1.60 ppm.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
In the cases of crotyl alcohol and 3-buten-2-ol, two basic hypotheses can be formulated to explain the incorporation of these molecules into the polymer chain. First, incorporation can occur through the hydroxyl groups, leading to formation of a characteristic peak placed at 1.60 ppm in the 1H-NMR spectrum, as shown in Figure 9a, and close to 80 ppm in the 13C-NMR spectrum, as shown in Figure 9b. However, in addition to the low concentration of contaminants and the expected low intensity of these peaks, the characteristic 13C-NMR peak coincides with the characteristic peak of the deuterated solvent (chloroform) used to perform the analyses, rendering the unambiguous identification of this peak impossible. Second, incorporation can occur through the double bond unsaturation, generating a tertiary carbon atom, a characteristic peak placed at 1.60 ppm in the 1H-NMR spectrum, and a characteristic peak placed at 30 ppm in the 13C-NMR spectrum. However, identification of this characteristic 13C-NMR peak can also be difficult, because the characteristic peaks of secondary carbon atoms are also located around 30 ppm and are present in much higher concentrations. Therefore, despite the evidence of incorporation of these contaminants, more fundamental studies regarding this subject should be performed in the near future for presentation of more definitive conclusions.32
Figure 10 presents the RPA analyses of BR samples prepared in presence of the unsaturated contaminants crotyl alcohol and 3-buten-2-ol. The RPA analyses indicated that the BR samples produced in presence of the unsaturated alcohols presented very similar mechanical behavior, which were different from the behavior of the standard BR sample. As a matter of fact, the BR samples prepared in presence of the unsaturated alcohols presented more “rubbery” performances than the reference BR samples, suggesting the possible incorporation of the contaminants and modification of the molecular architecture. In addition, it is normally expected that samples with higher Mw values should also present smaller tan delta values;33 however, the standard BR sample presented smaller tan delta and Mw values simultaneously, when compared with the BR sample prepared in presence of crotonaldehyde, implying that the sample prepared in presence of crotyl alcohol or 3-buten-2-ol could present better processability than the standard sample and a higher degree of branching.34 This can indeed open room for more detailed investigations regarding the use of these unsaturated alcohols as polar additives for manufacture of rubbers with improved properties in the near future.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
In addition to the analyzed alcohols, other contaminants can be present in 1,3-BD streams produced from renewable resources, including 3-hydroxybutyraldehyde, crotonaldehyde, acetone, acetaldehyde, and water.2,11,19,22–24 As described before, obtained results are presented in Table II, and the analyzed contaminants are presented in Figure 11.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Figures 12 and 13 show the temperature and standardized pressure profiles for reactions performed in the presence of 200 ppm of different contaminants, indicating once more the occurrence of significant inhibitory and activation effects. Surprisingly, the presence of water did not prevent the occurrence of the polymerization and accelerated the reaction rates, leading to higher temperature peaks and pressure drops (monomer conversions) than observed previously with ethanol. This was unexpected, since water has been usually treated as a powerful inhibitor for 1,3-BD polymerizations performed in solution with the NdV catalyst, being carefully removed from the 1,3-BD feed during pre-treatment of the main reactants.35
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Comparing Figures 12 and 13 with Figures 5 and 6, it becomes possible to conclude that inhibitory effects caused by alcoholic components were on average more intense than observed with the other contaminants, with the exception of 3-hydroxybutyraldehyde, which also contains a hydroxyl function. Therefore, it seems clear the presence of the hydroxyl group enhances the chemical interactions that lead to inhibition of the active site. This can also explain the effect of water, given the presence of hydroxyl groups and the much smaller size of the molecule, when compared to the other contaminants, which can allow the interaction of water molecules with the active centers of the NdV catalyst and the modification of the active sites without preventing the 1,3-BD incorporation. It is important to report that additional polymerization reactions performed with water contents above 1000 ppm did not produce any significant amounts of polymer material, confirming the well-known inhibitory effect of water in the system. This can possibly indicate that multiple water molecules can interact with the active centers of the NdV catalyst, preventing the occurrence of the 1,3-BD propagation reaction.
According to Figure 13, inhibitory effects can be ordered in the form water < acetaldehyde < crotonaldehyde < acetone < 3-hydroxybutyraldehyde, although the ethanol concentration can still be regarded as the key indication of oxygenates contamination in 1,3-BD grades produced from renewable raw materials, given the much more intense inhibitory effects induced by ethanol. Despite that, the presence of acetone and crotonaldehyde in the feed can also induce the significant reduction of reaction rates. Accordingly, in the case of crotonaldehyde, despite presenting an exothermic profile at the reaction beginning, low reaction consumption and the product obtained a high fraction with lower molar masses, indicating a reduction in reaction rates and the inhibition caused by the compound.
As shown in Figures 14 and 15 and in Table II, the molar mass distributions of BR samples prepared in presence of 3-hydroxybutyraldehyde and acetone were very similar to the molar mass distribution of the standard BR sample, although the molar mass distributions of BR samples prepared in presence of acetaldehyde, crotonaldehyde, and water were broader (water and crotonaldehyde) and shifted toward lower molar masses. This reinforces the assumption that the size of the molecule and the presence of unsaturated double bonds can also affect the interaction (and consequent inhibitory effects) of the contaminant with the active centers of the NdV catalyst. In particular, as also observed previously for ethanol, the molar mass distributions obtained in presence of water and crotonaldehyde suggest the production of a mixture of the original catalyst sites (which give birth to longer polymer chains) and additional modified catalyst centers. As addition of acetaldehyde did not cause significant inhibitory effects, but caused significant shifting of the molar mass distributions toward lower molar masses, it seems plausible to assume that this molecule can behave as a typical chain transfer agent.36
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Regarding the microstructural properties of the obtained BR samples, once more the cis, trans, and vinyl contents of the final polymer materials were not very sensitive to the presence of the analyzed contaminants, which can be regarded as a positive factor and suggests that the stereochemistry of the active catalyst sites was not affected significantly by the analyzed compounds, as already discussed. This reinforces that inhibitory effects on catalyst activities can possibly be explained by interactions with the oxygenated groups of the contaminants, which can lead to permanent hindering of the catalyst sites and prevent the occurrence of the propagation reaction with 1,3-BD. As a matter of fact, the most significant observed changes regarded the yields and the average molar masses, which reflect macroscopic aspects of the polymerization reaction. If formation of distinct catalytic sites had occurred, one would be likely to observe changes of the microscopic characteristics of the polymer material, including changes of the cis, trans, and vinyl contents of the rubber.
Figure 16 presents the (a) 1H-NMR and (b) 13C-NMR spectra of BR samples prepared in presence of acetaldehyde, crotonaldehyde, and water. As one can see, the addition of acetaldehyde and water into the reaction medium led to significant changes of the 1H-NMR spectra of obtained BR samples. As discussed previously, addition of acetaldehyde also induced the appearance of a band placed at 1.60 ppm. In this case, only a hypothesis for incorporation into the polymer chain arises due to the structure of the molecule of acetaldehyde, because it only has a carbonyl group. If the reaction is done via carbonyl group, the structure formed would also be an ether that has a characteristic peak close to 1.60 ppm in 1H-NMR, as seen in Figure 16a, and close to 80 ppm in 13C-NMR, as seen in Figure 9b. Again, it is impossible to distinguish this peak, because this characteristic peak in 13C-NMR coincides with the characteristic peak of the deuterated solvent. Therefore, despite there being evidence of incorporation, a more focused study on each of the contaminants is necessary to reach more definitive conclusions.32
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
Figure 17 presents the RPA analyses of BR samples prepared in presence and absence of 3-hydroxybutyraldehyde, which caused a strong inhibitory effect on the polymerization system. The RPA analyses indicated that the BR sample produced in presence of 3-hydroxybutyraldehyde presented a behavior that was very different from the behavior of the standard BR sample. The remaining RPA analyses are not shown because the molar mass distributions of the distinct samples were very different and, for this reason, useless for comparative purposes. As also observed in Figure 10, the BR sample prepared in presence of 3-hydroxybutyraldehyde presented more “rubbery” performance than the reference BR sample, suggesting the possible incorporation of the contaminant and modification of the molecular architecture. In addition, the standard BR sample presented smaller tan delta values than the BR sample prepared in presence of 3-hydroxybutyraldehyde, implying that the sample prepared in presence of the aldehyde could present better processability than the standard sample. As observed previously, this can open room for more detailed investigations regarding the use of this contaminant as polar additive for manufacture of rubbers with improved properties in the near future.
Citation: Rubber Chemistry and Technology 95, 1; 10.5254/rct.21.78979
CONCLUSIONS
The present work investigated the influence of oxygenated compounds (acetaldehyde, crotonaldehyde, 3-hydroxybutyraldehyde, acetone, water, ethanol, 1,3-butanodiol, 3-buten2-ol, crotyl alcohol, and 1-butanol) present as possible contaminants of 1,3-butadiene (1,3-BD) feed streams produced from renewable materials, on the course of 1,3-BD polymerizations performed in solution using neodymium versatate (NdV) as catalyst. According to the obtained results, inhibitory effects can be ordered in the form water < acetaldehyde < crotonaldehyde < acetone < 1,3-butanediol < 3-buten-2-ol < n-butanol < crotyl alcohol < 3-hydroxybutyraldehyde < ethanol, so that the ethanol concentration can possibly be selected as the key indication of oxygenates contamination in 1,3-BD grades produced from renewable raw materials. Obtained results also showed that contamination effects on the obtained molar mass distributions of obtained polybutadiene (BR) samples were not significant in presence of less than 200 ppm of acetone, 1,3-butanediol, 3-buten-2-ol, n-butanol, crotyl alcohol, and 3-hydroxybutyraldehyde. On the other hand, the effects of water, acetaldehyde, crotonaldehyde, and ethanol on the molar mass distributions of obtained BR samples were remarkable and suggested that smaller molecules can interact and modify the active centers of the NdV catalyst and simultaneously allow the incorporation of 1,3-BD into the polymer chain. In particular, addition of small amounts of water and ethanol into the reaction system caused significant acceleration of reaction rates, although producing BR samples with broad molar mass distributions, suggesting the formation of mixtures of original and modified active catalyst sites. In addition, acetaldehyde acted as a conventional chain transfer agent, not affecting the reaction rates and shifting the molar mass distributions toward lower molar masses. Moreover, the cis, trans, and vinyl contents of the final polymer materials were not very sensitive to the presence of the analyzed contaminants, although the trans and vinyl contents of the products tended to increase little in presence of the contaminants. Finally, 1H-NMR and RPA analyses indicated that the BR samples prepared in presence of the contaminants can present distinct molecular architectures, due to incorporation of contaminant molecules into the chain and chain transfer reactions, leading to more “rubbery” behavior when compared to BR samples prepared in absence of contaminants.
The direct and the two-step processes normally used to manufacture green 1,3-butadiene.
Chemical structure of analyzed alcoholic contaminants: (1) ethanol, (2) n-butanol, (3) 1,3-butanediol, (4) 3-buten-2-ol, and (5) crotyl alcohol.
Temperature profiles for 1,3-BD polymerization reactions performed in presence of 200 ppm of different alcohols.
Standardized pressure profiles for 1,3-BD polymerization reactions performed in presence of 200 ppm of different alcohols.
Temperature profiles for 1,3-BD polymerization reactions performed in presence of different amounts of ethanol.
Standardized pressure profiles for 1,3-BD polymerization reactions performed in presence of different amounts of ethanol.
Molar mass distributions of BR samples prepared in presence of 200 ppm of different alcohols.
Molar mass distributions of BR samples prepared in presence of different amounts of ethanol.
1H-NMR (a) and 13C-NMR (b) spectra of BR samples prepared in presence of unsaturated alcohols.
RPA analyses of BR samples prepared in presence of unsaturated alcohols.
Chemical structure of analyzed contaminants: (1) 3-hydroxybutyraldehyde, (2) crotonaldehyde, (3) acetone, (4) acetaldehyde, and (5) water.
Temperature profiles for 1,3-BD polymerization reactions performed in presence of different oxygenated contaminants.
Standardized pressure profiles for 1,3-BD polymerization reactions performed in presence of different oxygenated contaminants.
Molar mass distributions of BR samples prepared in presence of 200 ppm of acetaldehyde, acetone, and 3-hydroxybutyraldehyde.
Molar mass distributions of BR samples prepared in presence of 200 ppm of crotonaldehyde and water.
(a) 1H-NMR and (b) 13C-NMR spectra of BR samples prepared in presence of acetaldehyde, crotonaldehyde, and water.
RPA analyses of BR samples prepared in presence and absence of 3-hydroxybutyraldehyde.
Contributor Notes