Search Results (7)

Silica-supported chromium oxide catalysts, also named Phillips chromium catalysts (PCCs), provide more than half of the world’s production of high- and medium-density polyethylenes. PCCs are usually prepared in the Cr(VI)/SiO₂ form, which is subjected to reductive activation. It has been explicitly proven that CO reduces Cr(VI) to Cr(II) species that initiate ethylene polymerization; ethylene activates Cr(VI) sites as well, but the nature of the catalytic species is complicated by the presence of the ethylene oxidation products. It is widely accepted that the catalytic species are of a Cr(III)–alkyl nature, but this common assumption faces the challenge of “extra” hydrogen: the formation of similar species under the action of even-electron reducing agents requires an additional H atom. Relatively recently, it was found that saturated hydrocarbons can also activate CrO_x/SiO₂, and alkyl fragments turn out to be bonded with a polyethylene chain. In recent years, there have been numerous experimental and theoretical studies of the structure and chemistry of PCCs at the different stages of preparation and activation. The use of modern spectral methods (such as extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), and others); operando IR, UV–vis, EPR, and XAS spectroscopies; and theoretical approaches (DFT modeling, machine learning) clarified many essential aspects of the mechanisms of CrO_x/SiO₂ activation and catalytic behavior. Overall, the Cosse–Arlman mechanism of polymerization on Cr(III)–alkyl centers is confirmed in many works, but its theoretical support required the development of nontrivial and contentious mechanistic concepts of Cr(VI)/SiO₂ or Cr(II)/SiO₂ activation. On the other hand, conflicting experimental data continue to be obtained, and certain mechanistic concepts are being developed with the use of outdated models. Strictly speaking, the main question of what type of catalytic species, Cr(II), Cr(III), or Cr(IV), comes into polymerization still has not received an unambiguous answer. The role of the chemical nature of the support—through the prism of the nature, geometry, and distribution of the active sites—is also not clear in depth. In the present review, we endeavored to summarize and discuss the recent studies in the field of the preparation, activation, and action of PCCs, with a focus on existing contradictions in the interpretation of the experimental and theoretical results. Full article

In this study, a mathematical model for the time evolution of molecular weight distribution (MWD) was developed. This temporal model is based on the well-known Ziegler–Natta polymerization mechanism and reaction kinetics by the parametric solving of related differential equations. However, due to the generality of the reactions involved, the model can be extended to the other type of catalysts, such as metallocenes, Phillips, etc. The superiority of this model lies in providing the possibility of a more precise prediction over the active sites and kinetic parameters using a simple mathematical equation, which leads to improved reactor design in large-scale production. The model uses a function to develop a methodology for MWD calculations. In this way, the transient response is limited to the first few minutes of the reaction; however, it is important as it demonstrates the establishment of the final MWD. According to the results, almost for practical conditions with negligible transfer resistances, the time dependency of the MWD has a transient interval, depending on the kinetic constants of polymerization reactions. Increasing the time to infinity results in an increase in MW and a widening in MWD, which confirms the experimental plots well. In short, the main advantage of our proposed model over the previous ones is its ability to predict the MWD even before the completion of the polymerization reaction. The results of the present model match well with those of the well-known Schulz–Flory distribution, which only predicts the final molecular weight distribution, thus confirming that the model is reliable and generalizable. Full article

Both metallocene and Phillips chromium catalysts are used in the commercial manufacture of polyethylene. Unlike most other commercial metallocene systems, the Chevron Phillips Chemical (CPC) platform does not use methylaluminoxane or fluoroorganic boranes. Instead, the support itself serves to activate (ionize) the metallocenes, which then polymerize ethylene at high activity. Most of these solid acid supports can also be used to anchor Cr to make a Phillips catalyst. This provides an interesting opportunity to compare the polymerization responses by these two disparate systems, Phillips Cr and CPC metallocene, when supported on the same solid acid carriers. In this study, both chromium oxide and several metallocenes were deposited onto a variety of solid oxides, under a variety of conditions, and the resulting support effects were observed and compared. Although using seemingly different chemistries, the two catalyst systems exhibited a surprising number of similarities, which can be attributed to the acidity and porosity of these diverse supports. Full article

The rapid growth of negative consequences from climate changes provokes divergent effects in all economic sectors. The experts proved that a core catalyst which bootstrapped the climate changes was greenhouse gas emission. This has led to a range of social, economic, and ecological issues. Such issues could be solved by extending innovation and information technology. This paper aimed to check the hypothesis that innovation and information technology allowed for a reduction of greenhouse gas emissions. The author used such methodology as OLS, fully modified OLS (FMOLS), dynamic OLS (DMOLS), Dicky-Fuller and Phillips-Perron tests. The research is informed by the report of the World Economic Forum, World Data Bank, Eurostat for the Visegrád countries (Hungary, Poland, Check Republic, Slovakia) for the period of 2000–2019. The findings were confirmed in models without control variables, and an increase of 1% of patents led to reducing greenhouse gas (GHG) emissions by 0.28% for Poland, 0.28% for Hungary, 0.38% for the Slovak Republic and 0.46% for the Czech Republic. At the same time, for the models with control variables, only Hungary experienced a statistically significant impact. There, an increase of patents by 1% led to reduction of GHG emissions by 0.22%. The variable R&D expenditure was statistically significant for all countries and all types of models (with and without control variables). The increase of R&D expenditure provoked a decline of GHG emissions by 0.29% (without control variables) and 0.11% (with control variables) for Poland, by 0.26% (without control variables) and 0.41% (with control variables) for Hungary, by 0.3% (without control variables) and 0.23% (with control variables) for the Slovak Republic and by 0.54% (without control variables) and 0.38% (with control variables) for the Czech Republic. Full article

In the present work, the effect of basic components on the energy pathway of ethylene oligomerization using the landmark Chevron-Phillips catalyst has been explored in detail, using density functional theory (DFT). Studied factors were chosen considering the main components of the Chevron-Phillips catalyst, i.e., ligand, cocatalyst, and halocarbon compounds, comprising (i) the type of alkyl substituents in pyrrole ligand, i.e., methyl, iso-propyl, tert-butyl, and phenyl, as well as the simple hydrogen and the electron withdrawing fluoro and trifluoromethyl; (ii) the number of Cl atoms in Al compounds (as AlMe₂Cl, AlMeCl₂ and AlCl₃), which indicate the halocarbon level, and (iii) cocatalyst type, i.e., alkylboron, alkylaluminium, or alkylgallium. Besides the main ingredients, the solvent effect (using toluene or methylcyclohexane) on the oligomerization pathway was also explored. In this regard, the full catalytic cycles for the main product (1-hexene) formation, as well as side reactions, i.e., 1-butene release and chromacyclononane formation, were calculated on the basis of the metallacycle-based mechanism. According to the obtained results, a modification on the Chevron-Phillips catalyst system, which demonstrates higher 1-hexene selectivity and activity, is suggested. Full article

This paper reports the principal discoveries which have played a major role in the polyolefin field and have positioned polyolefins as the most produced plastics. The early development of polyolefins covering the production of LDPE (Low density polyethylene) at ICI (Imperial Chemical Industries) and the discovery of Phillips or Ziegler-Natta catalysts are highlighted in the first section. In the second part, the impact of the implementation of molecular catalysts on the research in polyolefins is discussed together with the most recent advances leading to high-performance tailor-made resins. Full article

Why, in the midst of public debates related to religion, are unrepresentative orthodox perspectives often positioned as illustrative of a religious tradition? How can more representative voices be encouraged? Political theorist Anne Phillips (2007) suggests that facilitating multi-voiced individual engagements effectively dismantles the monopolies of the most conservative that tend to privilege maleness. In this paper, with reference to the 2003–2005 faith-based arbitration debate in Ontario, Canada, I show how, in practice, Phillips’ approach is unwieldy and does not work well in a sound-bite-necessitating culture. Instead, I argue that the “Sharia Debate” served as a catalyst for mainstream conservative Muslim groups in Ontario to develop public relations apparatuses that better facilitate the perspectives of everyday religious conservatives in the public sphere. Full article