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Urea in your Diesel

22 September 2015 - VW emissions scandal

volkswagen emissions.PNGDo Volkswagens have secret urea compartments? The car company has been found cheating at NOx emission tests in the USA which makes their cars (diesels more specifically) look much more environmentally friendly than they really are. But what does the "defeat device" look like? In many reports it is just a piece of software that kicks in when the car senses it is part of an emission test. The Guardian initially reported it was al down to an injection of urea into the engine at the time of testing. That would mean a secret urea compartment?

Time to catch up with the specialists here, here and here. The summary: all modern Volkswagen diesels use so-called BlueTec technology for emissions control. It is basically an urea injection system to the exhaust. It works by selective catalytic reduction and the basic reaction taking place is 4NO + 2(NH2)2CO + O2 -> 4N2 + 4H2O + 2CO2. Get rid of NOx and have nitrogen, water and carbon dioxide returned.

The problem is the diesel exhaust fluid, a diesel needs a lot of it (1 gallon for 1500 miles?) and it is expensive (NYT). From a consumer point of view what money is saved in terms of fuel is spent just as easily on exhaust fluid. The Volkswagen solution: kill the urea injection unless at the emissions control testing facility. That is where the software comes in. And how does a Volkswagen know it is in such a facility in the first place? By registering a non-zero speed at zero displacement.

The LA Times in 2008 jokingly suggested urine should replace urea to make the technology economically feasible, If only VW had listened (link)

The microscrubbers

17 September 2015 - Making It Move XVI

micromotors Uygun 2015.PNGIn our continuing coverage of Things-That-Move (Making-It-Move) see previous episode, we have a first contestant with an actual usability claim! Carbon dioxide sequestration no-less. In a recent Angewandte Uygun et al. (DOI) describe so-called microscrubbers that in the near-future should patrol the oceans scrubbing it of CO2. Here is how it works.

Nature's way to get rid of carbon dioxide is to convert it to solid calcium carbonate. This is what happens in the planet's oceans but only slowly. The enzyme carbonic anhydrase (CA) is able to catalyse the hydration of carbon dioxide, which happens to be the rate-limiting step. But the enzyme on it's own is too unstable and still too slow. In the new microscrubber the enzyme is attached to a micromotor. It gives the enzyme stability and because the solid support moves around it's efficiency is improved by a combination of self-mixing, convection and inhibition of sedimentation.

The micromotor construction manual describes the following steps: take a polycarbonate membrane with 5 micrometer conical shaped micropores, sputter gold on one side, electropolymerize a polypyrrole-COOH/PEDOT inner layer, then add a platinum layer by electrodeposition, then remove the gold layer by hand polishing and then dissolve membrane in methylene chloride to release the microtubes. Then link the terminal carboxylic acid groups on the surface of the microtubes to CA enzyme using ENSI and N-Hydroxysuccinimide NHS as coupling agents.

The swimming arena is plain seawater but with hydrogen peroxide and sodium cholate added for micromotor propulsion. And the results? The microtubes can be observed swirling around while pooping out solid clumps of calcium carbonate. Average speed: 100 micrometer per second, payload 600 microgram of CA per three million microtubes and 90% yield in just 5 minutes.

In their conclusion the authors have promised that their next-generation scrubber would have the peroxide fuel replaced with something more realistic.

Increasing decreasing complexity we are not sure

12 September 2015 - OrgOrgChem

increasing complexity or decsreasing.PNGLi and Eastgate in Organic & Biomolecular Chemistry introduce a complexity index for molecules and for synthetic schemes (DOI). Nothing fancy to do with computer algorithms or graph theory but simply inviting a troupe of experienced chemists in the flesh to rank molecules or reactions by complexity. The interesting find is that these chemists to a fair degree agree on complexity not only in terms of molecules but also in terms of reactions. Case study: the epic strychnine total synthesis with 13 routes published thus far and counting. According to the expert panel the Vanderwal effort of 2011 beats that of the effort by God-King Woodward of 1954. All nice and well, there is just one nagging annoyance with this publication: why rank along a complexity index with the highest complexity the value of 1? That should be the other way around? It makes all graphs (see for example inset) confusing. This blog calls for a less complex definition of a complexity index.

Amine basicity and ionic liquids

11 September 2015 - PhysOrgChem

Mao basicity ionic liquids 2015.PNGMao et al. in a recent JOC (DOI) took it on themselves to measure the basicity of 21 amines in an ionic liquid. The basicity range of amines in water or acetonitrile is well established. But what about IL's? The Mao crew think highly of themselves: 'The standard deviation was much superior to that sparsely reported elsewhere'. Competition, be ashamed! So now for the results. The ionic liquid was BmimNTf2 (a cousin of BMIM-PF6) and in this solvent the basicity was consistently lower compared to acetonitrile by a pKa difference of 3.6. The report summarises the results as H2O about equal to DMSO < ionic liquid < acetonitrile. The relationship with the relative permittivity (for unknown reasons the authors opt to call it 'polarity index') only goes that far. For water this value is 78, for acetonitrile 37 but for the IL it is only 12. The beta Kamlet-Taft parameter then? Does not help either, the ionic liquid is again at the bottom of the rankings. So with compliments for the accurate measurements, what is the main driver for amine basicity? Researchers research!