First-generation phosphoric acid

Various types of fuel cells have been developed to generate power according to the applications and load requirements (Chaurasia, 2000). There are several types of electrolyte, which plays a key role in the different types of fuel cells. It must permit only the appropriate ions to pass between the anode and cathode. The main electrolyte types are alkali, molten carbonate, phosphoric acid, proton exchange membrane (PEM), and solid oxide. The first three are liquid electrolytes, the last two are solids. 

The shikimate pathway begins with a coupling of phosphoenolpyruvate (PEP) and D-erythrose 4-phosphate to give the seven-carbon 3-deoxy-D-arabino-heptulo-sonic acid 7-phosphate (DAHP) through an aldol-type condensation. Elimination of phosphoric acid from DAHP, followed by an intramolecular aldol reaction, generates the first carbocyclic intermediate, 3-dehydroquinic acid. Shikimic acid (394) is... 

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

The reaction of acetic anhydride with phosphoric acid will generate the same electrophile and offers the advantage that only the monoacyl produci results. Acylation of the first ring deactivates the second and the concentration of CH3C+=0 from the phosphoric ucid reaction is too small to produce the diacyl product. 

According to the proposed mechanism (Scheme 6.20), the first step of this cascade reaction is protonation of the substituted pyridine by CPA to generate the pyridinium salt 51. Then, the reduction of 51 by 1,4-hydride transfer from the Hantzsch ester gives the enamine intermediate 52, which isomeri-zes to iminium 53 in the presence of CPA. The subsequent AFC reaction will afford the desired product and release the chiral phosphoric acid. 

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