A Route to Industrial Carbon Dioxide Fixation: Isocyanate Free Polyurethanes.

[NNadir]

The paper I'll discuss in this post is this one: One-Pot Synthesis of Dimethyl Hexane-1,6-diyldicarbamate from CO2, Methanol, and Diamine over CeO2 Catalysts: A Route to an Isocyanate-Free Feedstock for Polyurethanes (Meng et al, ACS Sustainable Chem. Eng. 2019 7 12 10708-10715.

It is now widely understood that plastics are representing a huge intractable problem in the environment, but the bulk of this problem is mostly represented by single use plastics, wrappers, cups, stirrers, plastic bags, etc., things most of us see and use every day without much thought.

However some polymers are designed for long term use, and to the extent that they can be made using materials other than dangerous fossil fuel based materials (which do represent the source of the materials from which the bulk of plastics are made) they actually sequester carbon in a form other than proposed waste dumps, which are not being built and never will be built.


The polyurethanes are often utilized in this way; they are strong, long lived, and far less sensitive to high temperatures than other plastics. They are used as lightweight wear resistant parts in complex machinery, as structural materials and in coatings such as varnishes. Some forms of "super glues" are polyurethanes, polymerized in situ where needed to repair products.

They are made by the condensation diisocyanates and dialcohols.

Isocyanates themselves are made by the use of phosgene, COCl2, originally developed as a war gas, and widely utilized in World War I to kill people, and amines with the elimination of HCl, hydrochloric acid.

Early in my career, I worked with phosgene in relatively large (for lab scale) processes and I have personally seen chemical reactors which handle phosgene on a ton scale in my career.

The reason that phosgene is a toxic gas is that it is highly reactive, and therefore highly useful despite being potentially dangerous.

Because of my familiarity with phosgene, coupled with my interest in removing carbon dioxide from the air and transforming it into useful products that effectively sequester carbon dioxide - the general area of industrial science called "CCU," carbon capture and utilization - this paper caught my eye.

From the paper's introduction:

Organic carbamates are widely used as environmentally benign compounds and unique intermediates of versatile chemical products, including herbicides, pesticides, biologically active compounds, and various kinds of pharmaceutical agents.1?4 Additionally, carbamates play great roles as linkers in organic chemistry and amino groups’ protectors in peptide chemistry. 5,6 Dicarbamates can be decomposed into diisocyanates used in polyurethane production.7,8 This way eliminates hazards of the phosgene-based synthesis of diisocyanates. Moreover, dicarbamates can be directly used to prepare polyurethanes.9,10 Therefore, they may serve as isocyanate-free reagents for polyurethane preparation.

Up to now, several methods to obtain dicarbamates were reported. For example, oxidative carbonylation of diamines,11 the reaction of diamines with dimethylcarbonate (DMC)12?18 or carbamates,19?21 of diamines with urea and alcohol,22?25 of polyureas with dialkylcarbonates,26 of aniline with DMC and subsequent condensation using formaldehyde.27,28

CO2 is a recyclable and naturally plentiful carbon source for various chemical feedstocks and the emissions of CO2 have significantly increased and contributed to global warming.29?31 Thus, the utilization of CO2 has attracted attention in the last decades.32?34

The authors have developed cerium dioxide based catalysts to replace phosgene (the penultimate source of dimethyl carbonate) with carbon dioxide.

The graphics in the paper are largely concerned with the morphology of the catalyst. They are here with their captions.

Figure 1. TEM images of (a) commercial CeO2 nanospheres, (b) CeO2(c), and (c) CeO2nanorods.

Figure 2. (a,b) HRTEM images and (c) SAED pattern of CeO2 nanorods.

Figure 3. XRD patterns of commercial CeO2 nanospheres, CeO2(c), and CeO2 nanorods.

Scheme 1. Side Reactions Leading to the Formation of PU and DMC

DMC, dimethyl carbonate, is useful as a fuel, and or a solvent, or, as stated, can be utilized to manufacture isocyanates, and the production of DMC by substituting carbon dioxide for phosgene is another topic widely discussed in the literature.

Figure 4. Product composition vs reaction time for the reaction CO2 + CH3OH + HDA over the CeO2 nanorods catalyst. Reaction conditions: NMP 20 mL, HDA:CH3OH = 5 mmol:500 mmol, CeO2 nanorods catalyst 0.20 g, CO2 5.0 MPa, 423 K.

Figure 5. Logarithmic plot for the influence of CO2 pressure on HDC average formation rate. Reaction conditions: NMP 20 mL, HDA:CH3OH = 5 mmol:500 mmol, CeO2nanorods catalyst 0.20 g, 423 K, 2 h.

Figure 6. (a,b) HRTEM images and (c) SAED pattern of the third regenerated CeO2 nanorods catalyst. The scales for (a) and (b) are 20 and 5 nm, respectively

Here is a description of the process of isocyanate synthesis in this process:

Catalytic Performance Measurement. The catalytic reaction of HDC formation from CO2, methanol, and HDA was conducted in a 50 mL autoclave with high speed mechanical stirring. Typically, 0.20 g catalyst, 500 mmol methanol, 5 mmol HDA, and 20 mL NMP were put into the autoclave. CO2 was purged into the reactor and released from it for several times to ensure oxygen-free reaction environment. Reactor temperature and the initial pressure of CO2 were set to predetermined values and the reactions were carried out for selected durations. The resultant reaction mixtures were filtered through PES membrane (pore size 0.45 ?m). HDA conversion and HDC productivity were calculated from the results of gas chromatography (Tianmei GC-7900II chromatograph equipped with a flame ionization detector and TM-Wax column) and high-performance liquid chromatography (Shimadzu SPD-M20A HPLC equipped with UV?vis detector and Shimadzu RP C18 (250 mm × 4.6 mm, 5 ?m) column), respectively.

Note that this process requires energy, and if the energy comes from dangerous fossil fuels, the process is useless, and no, trashing the desert with stupid "solar thermal reactors" will not work on an economically viable scale.

Cerium is the most abundant of the lanthanide elements, and is readily available from used nuclear fuels as well meaning that the element can by synthesized from essentially infinitely abundant uranium.

The chemistry of cerium catalysts represents a critical tool in addressing climate change, a subject in which we have essentially no interest left and right, the right being in denial, and the left being dogmatically attached - to the point of deliberate ignorance - in hyping so called "renewable energy" which hasn't worked, isn't working, and won't work to address climate change, and demonizing energy systems that have demonstrably produced far greater results in addressing climate change, nuclear energy.

From the conclusion of the paper:

In conclusion, an effective way for the one-pot synthesis of dimethyl hexane-1,6-diyldicarbamate from CO2, methanol, and 1,6-hexanediamine over the CeO2 nanorods catalyst was developed. Under the optimized reaction conditions, dimethyl hexane-1,6-diyldicarbamate was successfully synthesized with around 80% isolated yields. The CeO2 nanorods catalyst can be reused for several runs with slight deactivation. The studies on the influence of different reagents on reaction outcome imply that DMC might be an intermediate in HDC formation and PU alcoholysis is not the main reaction route. This work is the first example of one-pot dicarbamate synthesis from CO2, alcohol, and diamine, being cheap and easily available reagents.

I hope you are enjoying your weekend.