Examination of boran phosphonium catalysts using spectroscopy

In a study from ACS . CatalystIn this article, the researchers developed a series of new boran phosphonium catalysts and examined them using Raman spectroscopy in the context of carbon dioxide.2/epoxide copolymerization. This paper provides new insights into the borane/phosphonium cooperation. Leads to the development of low-cost, non-toxic, high-activity tertiary phosphonium catalysts that can be produced sustainably Polyalklein Carbonate, a low carbon plastic.

Stady: Phosphonium and bifunctional quaternary catalysts for carbon dioxide/epoxide copolymerization: a mechanistic investigation using in situ Raman spectroscopy.. Image Credit: IU Liquid and water photo / Shutterstock.com

Copolymerization and polyalkaline carbonate

Copolymerization is the formation of polymers of two or more monomers. The copolymerization of carbon dioxide and epoxides offers an interesting possibility of carbon dioxide conversion2 Waste streams in polyalkaline carbonate.

Polyalkylene carbonate is a unique, environmentally friendly polymer that requires far fewer petrochemicals than traditional elastomers. Carbon dioxide contributes to approximately 50% of the raw feedstock, which greatly reduces the use of petrochemicals.

This technique could enable the sequestration of greenhouse gases in the form of carbon dioxide.

The role of catalysts in copolymerization

Polyalkaline carbonate is produced in the presence of suitable catalysts. In this context, metal-based catalysts have been extensively explored, including Cr, Zn and Al systems.

organic

Organic organelles have gained considerable research and are emerging as effective catalysts in many polymerization processes due to their low cost and non-toxicity. For example, onion salts and Et3Bi-catalyst can enhance carbon dioxide2/epoxide copolymerization.

phosphonium boran

In the past, metal-based binary catalyst systems have used phosphonium salts. This is because phosphonium cations can react with anions by opening them σ*(pp.) orbitals due to Lewis acidity.

Positively charged H atoms are located on the – and α-carbons and the interactions are more dispersed due to the higher radius of phosphorous than that of nitrogen. In addition, due to not specifying the shipping location, HB and ha They are superior donors of H bonds compared to their ammonium equivalents.

These characteristic properties of phosphonium cations can be used to stabilize critical transition states and media in the CO catalytic cycle.2/epoxide copolymerization.

The importance of using Raman spectroscopy in bifunctional stimulation

Raman is a method based on light scattering in which substrate concentrations and signal response are linearly related. This concept monitors the complete reaction profile and determines the concentrations of substrates at any time.

Raman spectroscopy is highly sensitive to structural disturbances in molecules. In addition, the fast data collection time makes this spectroscopic approach ideal for high-quality kinetic investigations in the reactor.

Use of borane phosphonium catalysts for CO2/Epoxide polymerization

For CO2The copolymerization of epoxide, quaternary and tertiary phosphoniumborane were selected as catalysts. The structures of the catalyst have been carefully altered to gain insight into the complex interaction between structure and activity.

Co-polymerization processes have been observed by Raman spectroscopy, allowing calculation of polymerization rate constants and enabling quantitative and comprehensive comparison of different catalysts.

The researchers devised a non-thermal kinetic approach that allowed direct mapping of the polymerization rate constant (k).s) as a function of polymerization temperature to effectively analyze the catalysts. This technique also extracted the enthalpy of activation, entropy, and ideal temperature for polymerization in one continuous experiment.

Epoxides are polymerized with carbon dioxide2 Using phosphoniumborane catalysts to produce polyalkaline carbonate with chemical selectivity greater than 95%.

From the perspective of developing an industrially applicable homogeneous solution polymerization method, the following qualities were targeted:

  1. high thermal stability
  2. Molecular weight and chain-end control
  3. Rapid polymerization rate

Important study results

The results of isothermal kinematic operation at 80 °C indicated the following:

  1. For all stimuli, CHO depletion follows first-order kinetic properties.
  2. Cord length and substituents in borane and phosphonium significantly affect the first-order rate constant.
  3. The larger isotope boran is the fastest catalyst at 80 °C.

The non-thermal kinetic method made it possible to construct a direct curve of the entropy and enthalpy of activation of the Eyring equation and to estimate the optimum polymerization temperature.

In-depth examinations show complex relationships between Lewis acidity, entropy, and enthalpy of activation with respect to structural changes. These results are consistent with the mechanistic hypothesis where carbonate species is the resting state, and the opening of the epoxide ring is the rate-limiting step.

DFT calculations reveal that non-covalent fixations generated by phosphonium moieties are essential. This paper identifies the third-order isotopes of phosphoniumborane with the acidic phosphonium proton as the most effective catalysts based on the polymerization rate constant and the optimum polymerization temperature.

This research highlights the bifunctional catalysis between a Lewis acid and a non-localized cation, which can extend to organic or metallic systems.

reference

Jonathan Schaefer, Hua Zhou, Erin Li, Nicolas S. Lambek, Jorso Kolko, Matthew W. Holtkamp, ​​Francis C. Rex, Tzu Bean Lane. (2022) Bifunctional phosphonium and tetraborane catalysts for carbon dioxide/epoxide copolymerization: a mechanistic investigation using in situ Raman spectroscopy. ACS . Catalyst. https://pubs.acs.org/doi/10.1021/acscatal.2c03843

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