Hydrogenation of natural rubber in the presence of OsHCL(CO)(O2)(PCY3)2: Kinetics and mechanism

The homogeneous catalyst precursor, OsHCl(CO)(O₂)(PCy₃)₂, was utilized for the hydrogenation of natural rubber to convert the unsaturated structure to a saturated form, providing an alternating ethylene‐propylene copolymer. A detailed kinetic investigation was carried out by monitoring the amount of hydrogen consumption during the reaction using a gas‐uptake apparatus. ¹H NMR spectroscopy was used to determine the final olefin conversion to the hydrogenated product. Kinetic data, collected according to a statistical design framework, defined the influence of catalyst and polymer concentration, hydrogen pressure, and reaction temperature on the catalytic activity. The kinetic results indicated that the hydrogenation rate exhibited a first‐ shifted to zero‐order dependence on hydrogen at lower hydrogen pressure, which then decreased toward an inverse behavior at pressures higher than 41.4 bar. The hydrogenation was also observed to be first‐order with respect to catalyst concentration, and an apparent inverse dependence on rubber concentration was observed due to the impurities in the rubber. The hydrogenation rate was dependent on reaction temperature, and the apparent activation energy over the temperature range of 125–145°C was found to be 122.76 kJ/mol. Mechanistic aspects of the hydrogenation of natural rubber in the presence of OsHCl(CO)(O₂)(PCy₃)₂ were proposed on the basis of the observed kinetic results. The addition of some acids and certain nitrogen containing materials showed an effect on the hydrogenation rate. The thermal properties of hydrogenated natural rubber indicated that the thermal stability increased with increasing % hydrogenation of the rubber. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4499–4514, 2006     

Hydrogenation of synthetic cis‐1,4‐polyisoprene and natural rubber catalyzed by [Ir(COD)py(PCy3)]PF6

In the presence of chlorinated solvents, the catalytic complex [Ir(COD)py(PCy₃)]PF₆ (where COD is 1,5‐cyclooctadiene and py is pyridine) was an active catalyst for the hydrogenation of synthetic cis‐1,4‐polyisoprene and natural rubber. Detailed kinetic and mechanistic studies for homogeneous hydrogenation were carried out through the monitoring of the amount of hydrogen consumed during the reaction. The final degree of olefin conversion, measured with a computer‐controlled gas‐uptake apparatus, was confirmed by Fourier transform infrared spectroscopy and ¹H‐NMR spectroscopy. Synthetic cis‐1,4‐polyisoprene was used as a model polymer for natural rubber without impurities to study the influence of the catalyst loading, polymer concentration, hydrogen pressure, and reaction temperature with a statistical design framework. The kinetic results for the hydrogenation of both synthetic cis‐1,4‐polyisoprene and natural rubber indicated that the hydrogenation rate exhibited a first‐order dependence on the catalyst concentration and hydrogen pressure. Because of impurities inside the natural rubber, the hydrogenation of natural rubber showed an inverse behavior dependence on the rubber concentration, whereas the hydrogenation rate of synthetic rubber, that is, cis‐1,4‐polyisoprene, remained constant when the rubber concentration increased. The hydrogenation rate was also dependent on the reaction temperature. The apparent activation energies for the hydrogenation of synthetic cis‐1,4‐polyisoprene and natural rubber were evaluated to be 79.8 and 75.6 kJ/mol, respectively. The mechanistic aspects of these catalytic processes were discussed on the basis of observed kinetic results. The addition of some acids showed an effect on the hydrogenation rate of both rubbers. The thermal properties of hydrogenated rubber samples were determined and indicated that hydrogenation increased the thermal stability of the hydrogenated rubber but did not affect the inherent glass‐transition temperature. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4219–4233, 2006     

Selective hydrogenation of nitrile‐butadiene rubber catalyzed by thermoregulated phase transfer phosphine rhodium complex

Hydrogenation of polymer having C?C double bond can be carried out with the metal–organic complex as catalyst, which has the property of themoregulated phase transfer. In this study, a new complex RhCl[PPh[(OCH₂CH₂)_5≤n≤6CH₃]₂]₃ (Rh/AEOPP) was synthesized with a good yield, which was further used as catalyst to selectively hydrogenated nitrile‐butadiene rubber (HNBR). This is the first time that Rh/AEOPP complex was synthesized and applied in nitrile‐butadiene rubber (NBR) hydrogenation. The result shows that hydrogenation degree of product (HNBR) can be extended to 90%, when the condition is [Cat] = 3% (based the weight of NBR), L₂: Cat (Weight Ratio) = 2, [NBR] = 5% (based on the weight of xylene solution), P (H₂) = 1.5 MPa, T = 155°C, and t = 8 h. Also, by adjusting temperature, the catalyst could be easily separated from products with 89% catalyst complex recovery. In addition, ¹H‐NMR and infrared (IR) spectra showed that C?C double bonds in NBR was successfully hydrogenated without causing reduction of the CN group. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Catalytic Oxidation of Ethene on Polycrystalline Palladium: Influence of the Oxidation State of the Surface

¹³C DEPT-MRI is used to provide the first spatial mapping of alkene isomerisation and hydrogenation during an alkene hydrogenation reaction occurring within a trickle-bed reactor. The implementation of a pulse sequence combining the spatial resolution of a magnetic resonance imaging (MRI) pulse sequence with a ¹³C DEPT magnetic resonance spectroscopy pulse sequence enables spatially resolved ¹³C spectra to be recorded of natural abundance ¹³C species. Observation of the ¹³C nucleus, which has a much larger chemical shift range than the ¹H nucleus, provides spectra from which direct identification of the products of isomerisation and hydrogenation is achieved. This technique is illustrated with respect to the hydrogenation of 1-octene over a 1 wt% Pd/Al₂O₃ catalyst. In this preliminary study we demonstrate the ability of this technique to identify the effect of changing the hydrogen flow rate on the evolution of isomerisation and hydrogenation processes occurring along the length of the bed.     

Kinetics and parameters affecting degradation of purified natural rubber

Purified natural rubber (PNR) was obtained by treatment of high ammonia NR latex with proteinase enzyme for 24 h, followed by double centrifugation. The PNR was later redispersed into latex form with 0.5% (w/v) sodium dodecyl sulfate. The degradation of PNR was performed in latex form by using a combination of the radical initiator potassium persulfate (K₂S₂O₈) and propanal. The intrinsic viscosity [η] of the degraded rubber or liquid rubber that was obtained depended on various parameters such as the initiator concentration, amount of propanal, dry rubber content, reaction time, and temperature. It was found that the [η] of the rubber can be reduced from 4.31 to 0.19 for the PNR after a 25‐h reaction time using 5% dry rubber content PNR latex, 1 part per hundred rubber (phr) of K₂S₂O₈, and 32 phr of propanal at 80°C. The kinetics of the degradation reaction were investigated. The highest rate constant found was 11.33 × 10^?2 s^?1. The activation energy of the degradation reaction was 76.56 kJ mol^?1. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3546–3555, 2003     

Low methane selectivity using Co/MnO catalysts for the Fischer-Tropsch reaction: Effect of increasing pressure and Co-feeding ethene

CO hydrogenation using cobalt/ manganese oxide catalysts is described and discussed. These catalysts are known to give low methane selectivity with high selectivity to C₃ hydrocarbons at moderate reaction conditions (GHSV < 500 h^–1, < 600 kPa). In this study the effect of reaction conditions more appropriate to industrial operation are investigated. CO hydrogenation at 1–2 MPa using catalyst formulations with Co/Mn = 0.5 and 1.0 gives selectivities to methane that are comparable to those observed at lower pressures. At the higher pressure the catalyst rapidly deactivates, a feature that is not observed at lower pressures. However, prior to deactivation rates of CO + CO₂ conversion > 8 mol/1-catalyst h can be observed. Co-feeding ethene during CO hydrogenation is investigated by the reaction of¹³C0-¹²C₂H₄-H₂ mixtures and a significant decrease in methane selectivity is observed but the hydrogenation of ethene is also a dominant reaction. The results show that the co-fed ethene can be molecularly incorporated but in addition it can generate a C, species that can react further to form methane and higher hydrocarbons.     

Catalytic hydrogenation of nitrile rubber using palladium and ruthenium complexes

The catalytic hydrogenation of acrylonitrile‐butadiene copolymer (nitrile rubber, NBR) using Pd(OAc)₂ or RuCl₂(PPh₃)₃ catalysts has been investigated in order to produce a totally saturated nitrile rubber. The hydrogenation of NBR is effective with both catalysts and achieved total conversion under the appropriate reaction conditions. In the case of palladium the effects of reaction parameters such as reaction temperature, pressure, time, catalyst concentration, and NBR concentration have been investigated. Even though both ruthenium‐ and palladium‐based catalysts are effective in the production of HNBR, the former requires harsh reaction conditions and has the drawback of gel formation under high conversion, motivating the migration to RuCl₂ (PPh₃)₃ as an alternative catalyst. The degree of hydrogenation was determined by IR and NMR spectroscopy. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Investigations on stability and reusability of [Pd(2-pymo)2]n as hydrogenation catalyst

The MOF [Pd(2-pymo)₂]_n (2-pymo = 2-pyrimidinolate) was used in the liquid-phase hydrogenation of 1-octene and cyclododecene. Up to a reaction time of 4 h, reactant shape selectivity could be observed, i.e. 1-octene was hydrogenated, but not the more bulky molecule cyclododecene. However, if the catalyst was exposed to hydrogen for more than 4 h, also the hydrogenation of cyclododecene took place. Visually a color change of the catalyst was observed and by XRD it was found that Pd⁰ nanoparticles were formed and that the MOF [Pd(2-pymo)₂]_n was transformed into a new crystalline phase, where the Pd⁰ seemed to be accessible. Additionally, Pd⁰ was leached out and was available for hydrogenation. When reusing the MOF after reaction times less than 1 h, the reaction rate increased with every reuse. We assume that even in the first 4 h Pd⁰ was formed, but could not be detected by XRD. In-situ XRD measurements exposing the MOF to hydrogen in the gas phase showed that the MOF was much more stable in gas phase hydrogenation, because the MOF structure remained intact even when Pd⁰ was formed.     

Kinetics of the hydrogenation of NBR by hydrazine and oxygen,using selenium as a catalyst

The kinetics of hydrogenation of the acrylonitrile‐polybutadiene (NBR) rubber by the action of hydrazine in the presence of selenium and oxygen was studied by varying reaction parameters such as latex and catalyst concentrations. The method of initial rates gives a reaction order of 0.91 and a rate constant of 3.2 × 10¹ L mol^?1 h^?1 in relation to the NBR latex concentration, and an order of 0.86 and a rate constant of 3.3 × 10¹ L mol^?1 h^?1 in relation to the catalyst concentration. Based on these values, a first‐order mechanism with the formation of a diimide intermediate is suggested, which is formed through the oxidation of hydrazine in an oxygen atmosphere in the presence of selenium catalyst. This diimine species reacts rapidly, reducing the carbon–carbon double bonds of NBR resulting in the formation of HNBR rubber. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and characterization of natural rubber-based telechelic oligomers via olefin metathesis

Metathesis degradation and functionalization of natural rubber (NR) were conducted with 1-hexene, 1-octene, 1-decene, 1-dodecene, trans-stilbene, and 4,4′-dibromo-trans-stilbene as chain transfer agents (CTAs) in presence of Grubbs 2nd generation catalyst to generate NR-based telechelic oligomers that had been a long-lasting challenge due to the structure and compositions of NR with various impurities. Orthogonal experiments were applied and the effects of the CTA type, CTA concentration, catalyst concentration, reaction time, and reaction temperature on the formation of telechelic oligomers were studied, indicating that the catalyst concentration was the major factor influencing the number average molecular weights (M_n) and polymer dispersity index (PDI) of telechelic oligomers. The structures of the oligomers were characterized using ¹H NMR, ¹³C NMR, and MALDI-TOF-MS, which confirmed the formation of the designed terminal groups. The results showed that well-defined telechelic oligomers with a M_n of a few thousand and a PDI around 1.6 were obtained, with potential applications in binder, lubricant and many other fields.     

Thermoplastic elastomeric hydrogenated styrene-butadiene elastomer: Optimization of reaction conditions,thermodynamics, and kinetics

Thermoplastic elastomeric hydrogenated styrene—butadiene (HSBR) elastomer was prepared by diimide reduction of styrene-butadiene rubber in the latex stage. The products were characterized by infrared, ¹H-NMR, ¹³C-NMR spectroscopy, and differential scanning calorimetry (DSC). The standard free energy change, ΔG⁰ at 298°K is −44.7 × 10⁴ kJ/mol, indicating that the formation of HSBR is thermodynamically feasible. The value of heat change of the reaction at constant volume, ΔU_T is −41.6 × 10⁴ kJ/mol. The effect of different reaction parameters on the level of hydrogenation, calculated from nuclear magnetic resonance spectroscopy, was also investigated. The degree of hydrogenation increases with the increase in reaction time, temperature, the concentration of reactants and catalyst. A maximum of 94% hydrogenation was obtained under the following conditions: time, 4 h; temperature, 45 ± 2°C; pH, 9.36; cupric sulphate (CuSO₄ · 5H₂O) catalyst concentration, 0.0064 mmol; hydrazine concentration, 0.20 mol; and hydrogen peroxide concentration, 0.26 mol. The diimide reduction of SBR is first-order with respect to olefinic substrate, and the apparent activation energy is 9.5 kJ/mol. The glass transition temperature increases with the increase in saturation level due to development of crystalline segments. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1151–1162, 1997     

Hydrogenation of natural rubber latex in the presence of OsHCl(CO)(O2)(PCy3)2

Hydrogenation is an important method of chemical modification, which improves the physical, chemical, and thermal properties of diene elastomers. Natural rubber latex (NRL) can be quantitatively hydrogenated to provide a strictly alternating ethylene–propylene copolymer using a homogeneous osmium catalyst OsHCl(CO)(O₂)(PCy₃)₂. A detailed kinetic investigation was carried out by monitoring the amount of hydrogen consumption during the reaction using a gas‐uptake apparatus. The kinetic results of NRL hydrogenation indicated that this system had a second‐order dependence of the hydrogenation rate on hydrogen pressure and then decreased toward a zero‐order dependence for hydrogen pressures above 13.8 bar. The hydrogenation was also observed to be first‐order with respect to catalyst concentration and inverse first‐order on rubber concentration due to impurities present in the rubber latex. Additions of a controlled amount of acid demonstrated a beneficial effect on the hydrogenation rate of NRL. The temperature dependence of the hydrogenation rate was investigated and an apparent activation energy (over the range of 120–160°C) was calculated as 57.6 kJ/mol. Mechanistic aspects of this catalytic process are discussed on the basis of kinetic results. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 640–655, 2006     

Kinetics of hydrogenation of 4‐chloro‐2‐nitrophenol catalyzed by Pt/carbon catalyst

Hydrogenation of 4‐chloro‐2‐nitrophenol (CNP) was carried out at moderate hydrogen pressures, 7–28 atm, and temperatures in the range 298–313 K using Pt/carbon and Pd/γ‐Al₂O₃ as catalysts in a stirred pressure reactor. Hydrogenation of CNP under the above conditions gave 4‐chloro‐2‐aminophenol (CAP). Dechlorination to form 2‐aminophenol and 2‐nitrophenol is observed when hydrogenation of CNP is carried out above 338 K, particularly with Pd/γ‐Al₂O₃ catalyst. Among the catalysts tested, 1%Pt/C was found to be an effective catalyst for the hydrogenation of CNP to form CAP, exclusively. To confirm the absence of gas–liquid mass transfer effects on the reaction, the effect of stirring speed (200–1000 rpm) and catalyst loading (0.02–0.16 g) on the initial reaction rate at maximum temperature 310 K and substrate concentration (0.25 mole) were thoroughly studied. The kinetics of hydrogenation of CNP carried out using 1%Pt/C indicated that the initial rates of hydrogenation had first order dependence with respect to substrate, catalyst and hydrogen pressure in the range of concentrations varied. From the Arrhenius plot of ln rate vs 1000/T, an apparent activation energy of 22 kJ mol^?1 was estimated. © 2001 Society of Chemical Industry     

On the mechanism of the methanol synthesis involving a catalyst based on zirconia support

The nature of the pivotal intermediate during the synthesis of methanol from CO₂/H₂, in the presence of ZnO/ZrO₂ aerogel catalyst is envisaged. The kinetic studies performed using in situ FTIR spectroscopy of the species formed on the surface of the catalyst in the absence and in the presence of hydrogen show that the initial reactive adsorbed species formed from C0₂ gas is the unidentate carbonate species. Its hydrogenation into the formate species is much faster than the hydrogenation of the formate species into methoxyl species. The comparison is based on a quantitative measurement of the rate constant of the hydrogenation of the various species. The results explain that during the C0₂/H₂ reaction only formate and methoxyl species are observed.     

Green and Simple Method for Catalytic Hydrogenation of Diene-Based Polymers

Catalytic hydrogenation of diene-based polymers is investigated in bulk form with different types of homogeneous and heterogeneous catalysts. Among these catalysts, we found that RhCl(PPh₃)₃, which could be promoted by its co-catalyst ligand (PPh₃), was able to diffuse into the bulk polymer. It was shown that a required high conversion (95?mol?%) was achieved within a few hours for the hydrogenation of acrylonitrile-butadiene rubber, styrene-butadiene rubber, and polybutadiene rubber using this catalyst. As an example, the hydrogenation of NBR in bulk form was investigated with respect to the effects of reaction temperature, pressure, and catalyst loading in an attempt to understand the hydrogenation of the bulk polymer.     

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ have been studied in aqueous acidic medium over Pd/SiO₂ catalyst in presence of different halide ions (viz. F⁻, Cl⁻ and Br⁻). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H₂O₂ formation selectivity in the H₂ to H₂O₂ oxidation over the catalyst. The H₂O₂ destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br⁻ was found to selectivity promote the conversion of H₂ to H₂O₂ by significantly reducing the H₂O₂ decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H₂O₂ formation by promoting the H₂ combustion to water and/or by increasing the rate of decomposition/hydrogenation of H₂O₂ over the catalyst. An optimum concentration of Br⁻ ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H₂O₂ yield in the direct synthesis.     

Palladium Supported on an Acidic Resin: A Unique Bifunctional Catalyst for the Continuous Catalytic Hydrogenation of Organic Compounds in Supercritical Carbon Dioxide

1% Palladium‐doped acidic resin (Amberlyst^® 15; styrene‐divinylbenzene matrix with sulfonic acid groups) is shown to be a highly active catalyst for the continuous catalytic hydrogenation of CC bonds in supercritical carbon dioxide (scCO₂) without affecting CO bonds. This 1% Pd/Amberlyst‐15 catalyst promotes the industrially important selective formation of 2‐ethylhexanal from crotonaldehyde in a “one‐pot” pathway involving hydrogenation and aldol condensation with a number of merits. The selectivity behavior of 1% Pd/Amberlyst‐15 is strikingly different compared to that of 1% Pd/C and 1% Pd/Al₂O₃ due to its prominent bifunctional nature based on sulfonic acid groups adjacent to metallic Pd sites. Hybrid “[Pd_n–H]⁺” sites are suggested to act as both metal and acid sites promoting the bifunctional catalysis.     

Hydrogenation of Low Molecular Weight Polymers in Ionic Liquids and the Effects of Added Salt

The biphasic hydrogenations of a number of polymeric materials, polybutadiene (PBD), nitrile‐butadiene rubber (NBR) and styrene‐butadiene rubber (SBR), were investigated in a toluene/N,N′‐butylmethylimidazolium tetrafluoroborate, (BMI⁺BF₄⁻) system with a water‐soluble analogue of Wilkinson's catalyst, RhCl(TPPTS)₃ (TPPTS=triphenylphosphine, trisulfonated) at 100 °C and 3.1 MPa. The catalyst shows reasonable activity within ionic liquids with PBD although it was necessary in the case of NBR and SBR to add water as a co‐solvent to solubilize the catalyst within the ionic media. Both the extent of hydrogenation and the ratio of the internal 1,4‐olefins to vinyl 1,2‐olefins were monitored. A clear preference for the external olefins was observed even within NBR and SBR where functionalization on the polymer might enhance the degree of hydrogenation of the internal olefins. The effect of adding NaCl to the PBD system was also investigated. The addition of salt decreases the activity of the catalyst but has no effect on the preference for the hydrogenation of vinyl olefins.     

Hydrogenation of cis‐1,4‐poly(isoprene) catalyzed by OsHCl(CO)(O2)(PCy3)2

The quantitative hydrogenation of cis‐1,4‐poly(isoprene) (CPIP) provides an easy entry to the alternating copolymer of ethylene–propylene, which is difficult to prepare by conventional polymerization. The homogeneous hydrogenation of CPIP, in the presence of OsHCl(CO)(O₂)(PCy₃)₂ as catalyst, has been studied by monitoring the amount of hydrogen consumed during the reaction. The final degree of olefin conversion measured by computer‐controlled gas uptake apparatus was confirmed by infrared spectroscopy and ¹H nuclear magnetic resonance analysis. Kinetic experiments for CPIP hydrogenation in toluene solvent indicate that the hydrogenation rate is first order with respect to catalyst and carbon–carbon double bond concentration. A second‐order dependence on hydrogen concentration for low values and a zero‐order dependence for higher values of the hydrogen concentration was observed. The apparent activation energy for the hydrogenation of CPIP over the temperature range of 115–140°C was 109.3 kJ/mole. Mechanistic aspects of this catalytic process are discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 142–152, 2003     

Liquid‐Phase Catalytic Hydrogenation of p‐Chlorobenzophenone to p‐Chlorobenzhydrol over a 5 % Pd/C Catalyst

The kinetics of the liquid‐phase catalytic hydrogenation of p‐chlorobenzophenone have been investigated over a 5 % Pd/C catalyst. The effects of hydrogen partial pressure (800–2200 kPa), catalyst loading (0.4–1.6 gm dm^–3), p‐chlorobenzophenone concentration (0.37–1.5 mol dm^–3), and temperature (303–313 K) were studied. A stirring speed > 20 rps has no effect on the initial rate of reaction. Effects of various catalysts (Pd/C, Pd/BaSO₄, Pd/CaCO₃, Pt/C, Raney nickel) and solvents (2‐propanol, methanol, dimethylformamide, toluene, xylene, hexane) on the hydrogenation of p‐chlorobenzophenone were also investigated. The reaction was found to be first order with respect to hydrogen partial pressure and catalyst loading, and zero order with respect to p‐chlorobenzophenone concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to a model involving reaction between adsorbed p‐chlorobenzophenone and hydrogen in the liquid phase.