Displacing phosgene with carbon dioxide for the synthesis of urethanes and isocyanates.
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 (Xiao et al, ACS Sustainable Chem. Eng 2019 7 12 10708-10715)
Polyurethanes are hard, durable plastics widely utilized as varnishes, to make mechanical parts such as wheels and gears, furniture and carpets - albeit containing environmentally problematic flame retardants - seals, gaskets and a variety of other multiple use polymers. As such they represent, to the extent that they can be made from products derived from carbon dioxide - sequestered carbon, although the current technology utilizes dangerous fossil fuel sources almost exclusively.
Chemically they are prepared from a diol - a molecule having two alcoholic functions - and an diisocyanate. Isocyanates are in turn generally are made by the elimination of hydrochloric acid from chlorocarbamates, which in turn are made from amines and the war gas phosgene.
I worked quite a bit with phosgene when I was a kid, early in my career, which was kind of fun, since it was thought to be dangerous, although something is truly dangerous when it injures or kills people. I have personally met hundreds of people who have worked with phosgene, none of whom were killed by the gas, although it must be said that during World War I - which was the worst war in history until World War II, more or less a continuation of WWI, came along. Nobody was killed by phosgene in WWII; even though tens of millions of people were routinely being killed, the use of phosgene as a weapon was too scary.
Go figure.
Anyway, since I made highly specialized urethanes using phosgene, I am always interested in phosgene replacements, particularly when the replacement is carbon dioxide, since I believe our expressed contempt for all future generations - while we wait Godot like for the grand so called "renewable energy" nirvana that has not come, is not here, and will not come - will require them to remove this gas from the atmosphere and sequester it, and to the extent this can be done using economically viable materials, as opposed to carbon dioxide dumps that have not worked, are not working and will not work, this is desirable.
Hence my interest in this paper.
From the introduction:
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 more and more attention in the last decades.32−34
Previously, dicarbamates were synthesized in one step from CO2 and diamines by the reaction with dialkyltin dimethoxides35 or titanium alkoxides.4 In these methods, the alkoxides must be used in stochiometric or excess quantities with respect to diamines.
It was published that carbamates can be obtained by one-pot synthesis from CO2, amines, and alcohols, or silicate esters.36−44 Synthesis of cyclic carbamates from CO2 and amino alcohols was also performed.45 One-pot synthesis of N-substituted dicarbamates from CO2, alcohols and diamines might be an efficient approach as it uses easily available, cheap, and relatively safe starting reagents and excludes synthesis and isolation of intermediate compounds.
The DMC route is lab scale to the best of my knowledge; in any case DMC is often made from phosgene and methanol.
Anyway, the paper goes into some detail about the formation of 1,6 hexane dimethoxycarbamates. Methanol can and sometimes is made from carbon dioxide, although more commonly it is made by the partial oxidation of dangerous natural gas, dangerous natural gas being the unsustainable and extremely dangerous compound that is being fronted by the wind and solar industry as it is utilized to destroy the planetary atmosphere. These carbamates can be dehydrated to make isocyanates.
Much of the paper is focused on the preparation and use of the cerium dioxide catalyst. Cerium is the most common lanthanide - Chinese chemists often write about it since the world supply of lanthanides is centered in China - and is an extremely useful element owing to its two oxidation states. I have written about it as a catalyst for the thermochemical splitting of both water and carbon dioxide in this space.
It is a relatively common fission product as well, and could be in theory recovered from used nuclear fuels for use.
Some pictures from the paper. First the cartoon graphic showing what is going on structurally:
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 CeO2nanospheres, CeO2(c), and CeO2 nanorods.
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 CeO2nanorods catalyst. The scales for (a) and (b) are 20 and 5 nm, respectively
Scheme 1. Side Reactions Leading to the Formation of PU and DMC.
It's an interesting little paper. Things like this may prove important to future generations who we've been screwing while we dream uselessly of our Tesla car/wind/solar fantasies.
By the way, we can make any chemical now obtained industrially from dangerous petroleum, dangerous natural gas, or dangerous coal from carbon dioxide and hydrogen, although in many cases, processed biomass would work equally as well. The only requirement is to have clean energy capable of sustaining, continuously, high temperatures.
I wish you a pleasant Sunday.
= new reply since forum marked as read
Urethane skateboard wheels were the miracle product when I was a kid. They were vastly superior to steel or clay composite wheels.
After graduating from Annandale High School in Northern Virginia in 1969, Nasworthy attended Virginia Tech for a year. Back with his family for the summer of 1970, he visited a plastics factory in Purcellville called Creative Urethane, owned by a friend’s father. The factory had experimented with a polyurethane roller skate wheel that was sold to Roller Sports Inc., which supplied wheels for rental skates at roller rinks. The rationale was that a softer wheel with improved grip would help novice roller skaters, but the wheel was largely rejected by roller skaters who favored the hard steel wheels that allowed for faster speeds on the wooden floors of the roller rinks.
Up to this point, skateboards had also been manufactured with either the same steel wheels as rollerskates, or out of a clay composite – a combination of plastic, paper, and finely ground walnut shells. These wheels wore out far too quickly, in as little as seven or eight hours.
https://en.wikipedia.org/wiki/Frank_Nasworthy
Atomic Skateboard and Scooter Wheels™ would be made from atmospheric carbon dioxide using nuclear power and cerium catalysts extracted from nuclear waste, maybe even Caesium-135 produced in certain molten salt and molten metal reactor designs as a decay product of Xenon-135.
How's that for a business plan?
Sometimes it's the seemingly trivial things that sell a much bigger idea.
