Heterogeneous Catalysts For Solution Hydrogenation Of Nitrile Butadiene Rubber

Hydrogenated nitrile butadiene rubber (HNBR) is an excellent elastomer obtained by the selective hydrogenation of C=C bonds in nitrile butadiene rubber (NBR) chain. The HNBR not only maintains excellent resistance to oil, but also has outstanding mechanical properties, resistance to ozonolytic aging and oxidation, even at high temperatures. Due to its excellent properties, HNBR is widely applied in auto industry, petrochemical industry and aerospace. Generally, HNBR can be obtained by hydrogenation of NBR in heterogeneous or homogeneous processes. A homogeneous catalyst has two commonly recognized advantages:high selectivity and reactivity. But, the recovery of expensive Rh catalyst is difficult, resulting in the relatively high production cost. A heterogeneous catalytic system is highly appraised for its much easier handling of the catalyst recovery. Nevertheless, the catalyst reactivity decays gradually with the proceeding of the reaction. In order to improve the activity of recovered heterogeneous catalyst for hydrogenation of NBR, several methods were proposed, including the exploitation of a new ligand, support surface modification and optimal structure design of the supports. The main contents in this paper are summarized as follows:An Rh-based complex, T-Rh-PPh3, was developed through a facile one-step process. In this method, tannin acts as a stabilizer and a ligand, and RhCl3 · 3H2O is used as a precursor. FTIR,1HNMR and 31PNMR showed that the tannin and triphenylphosphine were successfully incorporated into the framework of the T-Rh-PPh3. The T-Rh-PPh3 was used for the hydrogenation of NBR, exhibiting satisfactory catalytic activity, excellent selectivity, and good air stability. The enhanced air stability is the phenolic hydroxyls in tannin protect the active species Rh+ from oxidation. The hydrogenation results showed that under optimum conditions, a degree of hydrogenation of above 96% was achieved.Then, a recyclable heterogeneous Rh catalyst (MTS-T-Rh) was prepared by loading Rh species onto an amino-silica support (MTS) grafted by tannin, an intermediate linker and stabilizer, and fully characterized by XRD, EDS, FIRT, SEM, and XPS. Also, the preparation condition of the MTS-T-Rh was optimized. The importance of tannin in improving the activity and stability of the MTS-T-Rh was discussed. The catalytic hydrogenation of NBR was evaluated in solution. Compared with Rh/SiO2, the MTS-T-Rh exhibited considerably improved reusability, which suffered about 11% deactivation for hydrogenation of NBR, while the Rh/SiO2 was 53%. The reason is the tannin is a stabilizer for chelating with the Rh active sites through chemical bond, which enhanced the interactions between Rh nanoparticles and silica. Above 96% conversion and 100% selectivity to carbon-carbon double bond were obtained at optimal NBR hydrogenation condition over MTS-T-Rh.The surface of SiO2 was chemically modified with the silane coupling agent APTS, and Rh nanoparticles (NPs) were deposited on MSiO2 surface to fabricate an Rh-loaded supported catalyst, Rh(Ⅲ)/MSiO2. FTIR, SEM, XRD, XPS and BET demonstrated that the incorporation of amine groups on the surface of the modified SiO2. In addition, the Rh(Ⅲ)/MSiO2 showed a higher catalytic activity and stability in hydrogenation of NBR than Rh(Ⅲ)/SiO2. Factors that influence the degree of hydrogenation were also fully investigated and the optimum reaction conditions were as follows:mass fraction of NBR solution was 3 wt%, mass fraction of supported catalyst was 2 wt%, mass fraction of PPh3 was 1 wt%, temperature of 140℃, H2 pressure of 3.0 MPa, and reaction time of 8 h, a degree of hydrogenation above 98%was obtained. Also, the Rh(Ⅲ)/MSiO2 could be recovered by a simple centrifugation and recycled for three consecutive runs with less loss of activity. After treatment of the recovered catalyst with chlorobenzene and acetone, the catalytic activity of the catalyst restored from 67.61% to 81.66%. The enhanced interaction between the metal ions and the support, and the increase of the amount of metal species as well as their dispersion could be the reasons for the high activity of Rh(Ⅲ)/MSiO2.The properties of graphene such as high thermal, chemical, and mechanical stability as well as high surface area provide an excellent catalyst support for Rh NPs to fabricate heterogeneous catalysts for hydrogenation of NBR. The Rh NPs supported on reduced graphene nanosheets (Rh/RGO) was synthesized by chemical reduction of aqueous solutions of graphene oxide and Rh ions with the use of reducing agent NaBH4 or N2H4 · H2O. The catalysts were fully characterized by a variety of spectroscopic techniques including XRD, Raman, TGA, SEM, TEM, and XPS. Compare with other supported catalysts, such as Rh/AC, Rh/CaCO3 and Rh/CNT, Rh/RGO revealed excellent activity and selective in hydrogenation of NBR. The Rh/RGO.HH demonstrated better catalytic activity for the NBR hydrogenation reaction assisted with ligand PPh3 than the Rh/RGO-NaBH4. The remarkable activity of the Rh/RGO-HH is attributed to the high degree of the dispersion of Rh NPs with smaller particle size. The hydrogenation of NBR was investigated with respect to the temperature, pressure, catalyst loading, concentration of NBR solution and amount of hydrazine hydrate in an attempt to achieve a high degree of hydrogenation. The Rh/RGO-HH can be recycled for 3 times with partial deactivation. However, the degree of hydrogenation of the recycled catalyst can be recovered to above 96% by adding few fresh catalysts. The deactivation mechanism of Rh/RGO-HH is attributed to a small quantity of Rh NPs leaches into the reaction solution and occupation of the catalytic active centers by the hydrogenated polymer coils. Finally, other polymer substrates including NR, SBR and IR were selected to evaluate the universality application of Rh/RGO-HH in solution hydrogenation diene polymer.Bimetallic catalysts RGO/Rh-Ru and RGO/Rh-Pd are synthesized and evaluated in the hydrogenation of NBR, which is proved to be superior over single component catalysts due to the synergistic effects. Above 96% conversion to C=C is obtained at optimal NBR hydrogenation condition over RGO/Rh-Ru.Enhanced mechanical and electrical RGO/HNBR composites have been prepared by one-step hydrogenation of NBR with graphene-based Rh catalyst. The mechanical or electrical properties of RGO/HNBR composites over a range of weight fractions of RGO are carried out. The as-prepared HNBR/RGO composites exhibit better mechanical properties compared with that of HNBR, which can be attributed to the good RGO dispersion and RGO-Rubber interaction. But, the electrical properties of RGO/HNBR are poor, which is likely to the defect of the chemical reduction of GO.