all blogs

Carbon dioxide plus methane

22 August 2015 - And hot electrons

carbon dioxide reforming and hot electrons.PNGNews from the field of carbon dioxide reforming, the experimental process of taking carbon dioxide and methane directly to syngas (carbon monoxide and hydrogen). Methane is already used commercially in syngas production via steam reforming so a modification would get rid of a lot of greenhouse gas carbon dioxide in the same effort. Huimin Liu et al report on a new catalyst system in the Angewandte DOI involving photochemistry and notably hot electrons!.

Catalyst recipe: take a poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (P123) triblock copolyer, dissolve in water, add HCl and add tetraethyl orthosilicate. Stir for 24 hours. Then add chloroauric acid to water solution of urea (forces the gold to precipitate), stir 4 hours at 80°C and then calcinate at 400°C for another 4 hours. This process gives you mesoporous silica (SBA-15) a well-known support for nanoparticles, in this case gold nanoparticles. To this configuration can be added rhodium nanoparticles by adding rhodium chloride in water to suspension of Au/SBA-15 with stirring and again calcination. Rhodium loading is 1% and gold loading is between 1% and 5%.

The catalyst was tested by exposing 5 mg in a alumina cell to a methane and carbon dioxide flow in 1:1 molar ratio at 500°C and with visible-light irradiation. GC was the monitoring tool. Result 1: without light Au/SBA-15 did nothing but identical catalytic activity was observed for Rh/SBA-15 and Rh-Au/SBA-15. Conclusion 1: gold does not have anything to contribute. Result 2: with irradiation Rh-Au/SBA-15 outperformed Rh/SBA-15 although increase is less than twofold. Conclusion 2: with irradiation gold does something.

At the selected temperature range photocatalysis alone cannot explain the observed reactivity but the report offers a rationale based on a concept called surface plasmon resonance that alone the gold particles can provide. This resonance creates the "hot electrons" required for the reaction taking place. How? For gold this resonance takes place in the visible range (the UV range for rhodium) creating a plasma of ions and electrons. This creates a local magnetic field which in turn accelerate (?) the electrons into "hot electrons". In simulations local magnetic fields can be observed between the different surface-bound nanoparticles. The hot electrons then are able to polarise the otherwise inert carbon dioxide and methane molecules and hence the enhanced catalytic activity. Hot electrons!

Cellulose to glucose

19 August 2015 - Catalysis

cellulose to glucose Katz 2015.PNG Anh The To et al. of the Alexander Katz group at Berkeley report on a faster way to hydrolyze crystalline cellulose to glucose. (DOI) This process is of interest to biofuels and the synthesis of chemicals from biological sources. Synthetic catalysts have an efficiency of 20% and for carbon catalysts the efficiency drops to 4%. The improved method is bioinspired. As enzymes have weak-acid sites for the activation of the glycosidic bonds so a carbon source is fitted with those weak-acid sites as well. Result: 70% yield of which 96% glucose.

The recipe: MSC-30 is a mesoporous carbon (more precise petroleum coke activated with molten
potassium hydroxide). This material was treated with sodium hypochlorite in water (RT, several hours). The pH was brought back from 12 to 5 by HCl treatment. It leaves surface-bound carboxylic acid groups as well as lactones and phenols, all weak acids.

The adsorption of cellulose on the carbon particles was tested by immersing them in a HCl solution of cellulose. Does it come as a surprise that compared to untreated carbon, the adsorption affinity for cellulose of the treated carbon is much reduced? In a real live hydrolysis reaction 1.5 mg of cellulose in 1 mL of water was treated with 20 mg of the carbon compound (that much?) at 150°C for 24 hours. The carbon varieties best equipped for adsorbing cellulose were now the worst performers. Top results were obtained for hypochlorite treated carbon at pH 5.

Nevertheless it seems a lot of work to hydrolyse cellulose this way. The efficiency of enzymatic hydrolysis compared to this work was not discussed. This blog attempted to get some data from the literature but although a mountain of articles have been written on the topic the simple question of crystalline cellulose + cellulase = ? conversion @temperature and @hours proved to be an impossible one to answer.


Tianjin chemistry

18 August 2015 - Chemical disasters

tianjin calcium carbide.PNGThe death toll of the 2015 Tianjin explosions currently stands over a hundred. As expected the media debate the cause. Everyone agrees the following chemicals were present in the explosive mix calcium carbide, potassium nitrate and sodium cyanide (BBC News, Der Spiegel). A dozen Youtube clips demonstrate how any combination of calcium carbide, water and an open-flame will set of an ignition (example here). The local fire department is blamed for using water to fight the fire. The release of the gas phosphine (from phosphorus contaminations) may present a longer-term problem (cdc.gov). The media also report that the ammonium nitrate could have been the source of the secondary explosion. See this clip for a very stark example of ammonium nitrate combustion.

The sodium cyanide is a current concern, but how? It does not just form hydrogen cyanide, the lethal gas, just like that. It requires an acid. The website cdc.gov is helpful (here) with the information that carbon dioxide from the air is sufficiently acidic to help liberate it. The NYT by the way is certain hydrogen cyanide is a liquid (here). More details: a molten wheel hub here to give an indication of the heat developed in the explosion.

Update: The Guardian in 16 August reports the amount of sodium cyanide originally stored at the disaster site may be several hundreds of tons (Link). It also reports fear that potential rain will react with remaining calcium carbide. It is unclear what the report means with "and creating a potentially toxic airborne gas". Acetylene as flammable but toxic or does it refer to the phosphine gas?

Target aryne

14 August 2015 - Image of the month

aryne AFM pavlicek 2015.PNGArynes are well established reactive intermediates in organic chemistry but if you are one of those people who believe something only when they see something you had to wait until 2015 when Avlivek et al. actually took an (AFM) image of one of them as reported in Nature Chemistry (DOI).

A specific naphtoperylene with two iodo substituents was selected as a precursor to an aryne. This molecule was then deposited on a layer of NACl on copper. A STM scan revealed the precise location of one molecule and then both iodine atoms were stripped from it by hovering the STM tip over them at a precise voltage. The report displays AFM images showing the perylene core with the two discarded iodine atoms at a distance. So what is to do next is comparing the AFM images of the aryne with a reference perylene that does not have the iodine substituents. The authors note that the aryne hexagon appears curved and that based on bond lengths and scan intensity (height) the resonance structure best representing the aryne is a cumulene rather than an alkyne or a diradical. Sanity check: the same AFM tip can reposition the iodine atoms on the perylene core proving that it is indeed an aryne.