Authors: Anisha Chakrabarti
Source: Lehigh Preserve (article, 2018-01-01)
Field: Chemistry — Organic Chemistry

Catalytic Olefin Polymerization and Metathesis: Molecular Structure-Activity Relationships Olefin chemistry has a long and significant history in the catalysis literature. The polymerization and metathesis reactions were discovered around the same time in the early 1950s. The polyolefin industry has now grown to a multibillion dollar industry. The three main classes of olefin polymerization catalysts are (i) Phillips-type catalysts (CrOx/SiO2); (ii) Ziegler-Natta catalysts (transition metal compound with an activator); and (iii) singlesite homogeneous catalysts or supported homogeneous catalysts (i.e. metallocene). The Phillips-type heterogeneous supported CrOx/SiO2 catalysts are one of the most widely studied catalysts. It was discovered in the early 1950s at Phillips Petroleum Company, when J.P. Hogan and R.L. Banks determined that ethylene could be converted to high-density polyethylene (HDPE) by supported Cr/SiO2. This catalyst is now responsible for over half of the production of HDPE sold globally. The reason for the widespread use of the Phillips catalyst lies in its ability to synthesize over 50 different types of HDPE and linear low-density polyethylene (LLDPE), without the use of additional activators, which simplifies the catalyst preparation and production process. The process is also important because HDPE is produced at lower temperatures (65-180 °C) and atmospheric pressure. A supported CrOx/SiO2 catalyst was synthesized and characterized using time-resolved operando and in situ molecular spectroscopy both before and during ethylene polymerization reaction conditions to investigate the structure-activity relationships for this important industrial catalytic reaction. Metal oxides (AlOx, TiOx, and ZrOx) were used as promoter oxides. A combination of spectroscopic techniques (Raman, UV-vis, XAS, DRIFTS, and TPSR) during ethylene polymerization allows for the first time to monitor 2 the molecular events taking place during activation of supported CrOx/MOx/SiO2 catalysts by ethylene to establish the structure-activity relationships for this reaction. During reaction, the initial surface Cr+6Ox sites reduce to Cr+3 sites to form Cr-(CH2)2CH=CH2 and Cr-CH=CH2 reaction intermediates, whose activities depend on the promoter oxide (ZrOx ~ TiOx &gt;&gt; CrOx ~ AlOx). Olefin metathesis is also quite significant in industry and was commercialized in the late 1960s to produce ethylene and 2-butene from propylene in the Phillips Triolefin Process. There is a current global propylene shortage caused by the shift to lighter feedstocks derived from shale gas fracking, and due to the complete reversibility of the metathesis reaction, the reverse reaction can be used to counteract the propylene shortage. Heterogeneous supported MoOx/Al2O3 catalysts are one type of commercial catalyst employed, used in industrial processes such as the Shell Higher Olefin Process (SHOP) and operate between room temperature and ~200 °C. Supported MoOx/Al2O3 catalysts were synthesized and characterized with in situ Raman, UV-vis, DRIFTS, and TPSR, both before and during propylene metathesis reaction conditions. Three distinct MoOx species on the Al2O3 support were identified: isolated surface dioxo (O=)2MoO2, anchored to the basic HO-μ1-AlIV sites (&lt;1 Mo atom/nm2 ), oligomeric surface mono-oxo O=MoO4/5 anchored to more acidic HO-μ1-AlV/VI sites (1- 4.6 Mo atoms/nm2 ), and crystalline MoO3 nanoparticles also present above monolayer coverage (&gt;4.6 Mo atoms/nm2 ). The surface oligomeric mono-oxo O=MoO4/5 species easily activate at mild temperatures 25-200 °C while the isolated surface dioxo (O=)2MoO2 species require very high temperatures for activation (&gt;400 °C). The crystalline MoO3 nanoparticles decrease the number of accessible activated surface MoOx sites by their 3 physical blocking. For the first time, the structure-reactivity relationship is established for olefin metathesis by supported MoOx/Al2O3 catalysts and demonstrates the significant role that the anchoring surface hydroxyl sites on alumina have on the reactivity of surface MoOx species. Catalysis ; Polymerization ; Polyethylene ; Coordination polymerization ; Polyolefin ; Ethylene ; Olefin fiber ; Catalyst support ; Homogeneous catalysis Synthetic Organic Chemistry Methods ; Organometallic Complex Synthesis and Catalysis ; Marine Biology and Environmental Chemistry