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PFAS lock down

26 October 2019 - The environment

The Netherlands are already in a reactive nitrogen lock down with 18,000 building and construction projects halted indefinitely because reactive nitrogen emissions from these projects destroy biodiversity. One possible solution considered by policy makers is the reduction of the agro industry (taking out a lot of ammonia) which of course got the farming community in an uproar. But reactive nitrogen is not the only threat to the Dutch enterprise, 2019 is also the year of perfluorinated alkylated substances or PFAS (TheGuardian article). It is a collection of around 6000 man-made chemicals. An example is perfluorooctanesulfonic acid and all compounds having to do the GenX chemical process are also involved notably Hexafluoropropylene oxide dimer acid (HFPO?DA). According to a Volkskrant article (link) new legislation prohibits the handling of soil (a typical thing to see in construction work) if it contains more than 0.1 microgram per kilogram PFAS. The article (we cry a little that the author describes PFAS as miniscule chemical particles) several builders are interviewed complaining that this basically means not a single scoop of dirt in the country can be moved because even virgin soil exceeds this limit.

But how did this legislation come about? In June the European Chemicals Agency decided to label HFPO-DA as a substance of very high concern (link) honoring a Dutch proposal. The compound is bad for your health and for the environment and annoying will last forever because it lacks biodegradability. The current list of candidates (link) for designation as problematic has around 200 compounds of which at least 12 contain fluorine. Pentadecafluorooctanoic acid is also on the list and a candidate since 2013. The Dutch government then came up with the 0.1 mg safety limit (link) and will wait for more research on the harmful effects (better safe than sorry). The origin of the 0.1 mg is vague but possibly based on three times the detection limit.

And why is the country so interested in PFAS? It is host a GenX Du Pont plant (Chemours) in the city of Dordrecht with a long history of chemical accidents (release of perfluorisobuteen is an example) and pollution (link). Locals have for years been exposed to GenX related compounds. A cunning plan to get rid of this plant now appears to backfire.

A hexagonal planar complex

21 October 2019 - Inorganic chemistry

A hexagonal planar transition metal complex is reported from the Crimmin Group (Marti Garçon et al. DOI). In textbook chemistry any metal with 6 ligands either adopts a Trigonal prismatic molecular geometry or a Octahedral molecular geometry geometry so this arrangement is unusual. The new compound has a central palladium atom with three hydride ligands and three magnesium ligands in a planar alternating fashion. It was synthesized in two steps. In step 1 CpPd(cinamyl) which is cyclopentadienyl allyl palladium with the allyl group replaced by a cinnamyl group was reacted with an aluminum hydride with a nacnac ligand forming (PdAl(H)2nacnac)2 (for simplicity the nacnac ligand is called nacnac). This intermediate was then reacted with MgH(nacnac), another magnesium compound with bridging hydrogen atoms. Single-crystal X-ray diffraction of the resulting hexagonal product PdH3(Mg(nacnac))3 showed that the maximum deviation from the plane was only 10 degrees and that the Mg-H distance was too large for bond formation. Hydrogen ligands are electron donating and sigma donation is more efficient in a equatorial fashion that an axial fashion. As this is not compatible with the high oxidation state of palladium the authors correctly suspected that alternating sigma-donors (hydrogen is expected to have a negative charge) and sigma-acceptors (Mg with a positive change) could do the trick.

New in SLA

20 October 2019 - 3D printing

Unusual to see the three authors of a Science article also listed as the CEO, CTO and chairman of a commercial business. The article is about 3D printing (DOI) and the company is 3D printing company Azul3D. Okay, Walker, Hedrick and Mirkin also work at Northwestern University.

Stereolithography (SLA) is around since the eighties: a laser illuminates a 2D pattern at the bottom of a tank of photopolymerizable liquid through a window and as the resulting solid object is step-wise dragged out of the tank, 3D printing takes place. The inventor filed a patent and founded 3D Systems (600 million dollar in annual revenues).

In 2014 , also as an article in Science (DOI) a variation was introduced called continuous liquid interface printing (CLIP), the associated patents are now commercialized by company carbon3d.com (raised 260 million dollar in funding this year link) CLIP, according to the inventors is truly continuous which allows for faster production and industrial-scale production. To prevent clogging up the window at the bottom with polymer, a oxygen-permeable membrane creates a dead-zone just above the window with oxygen acting as a polymerization inhibitor.

The new Northwestern work promises even higher throughput as we go from SLA and CLIP to HARP ((high-area rapid printing). The dead zone at the bottom of the tank is replaced by a flow of fluorinated oil with the key additional advantage that it can remove the generated heat. Heat generation from the polymerization process is now a bottleneck. The oil is recycled with the heat removed in a heat exchanger and filtered to get rid of interfering debris. The new system does not require a oxygen-depleted zone and therefore tolerates more photopolymerizable resins. A polyurethane resin is reported with a vertical speed of 12 cm per second. Funding was provided by a bunch of non-profits and inevitably the military.

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