SENSITIZED REDUCTION OF CARBON DIOXIDE 93 



a higher alcohol is provided which is capable of binding carbon dioxide 

 in a substituted carbonic acid ester: 



(4.28) CO, + ROH > RHCO, 



In other words, carbon dioxide must be bound to a "lipophilic" organic 



molecule, and thus held in the nonaqueous phase. Whether this is 



O 



II 

 achieved by the formation of an esterified carboxyl group, Ri — C — OR2, 



O 



II 

 or by the formation of a carbonic acid ester, HO — C — OR, is unimportant; 



in both cases, if R is sufficiently large, the product is lipophilic and does 

 not pass into the aqueous phase. (The absence of free carboxyl reduces 

 the affinity to water.) In the first case, the oxidant can also be the 

 sensitizer; in the second case, a separate sensitizer must be added. 

 Baur, Gloor and Kunzler used acetate silk, colored with "cibacet" or 

 "celliton" dyes, coated with cetyl alcohol and suspended in aqueous 

 methylene blue solution, which also contained suspended calcium 

 carbonate. From all these experiments, Baur concluded that the 

 prerequisite of artificial photosynthesis is a two-phase system, Avith the 

 sensitizer and oxidant in a nonaqueous phase, and the reductant and an 

 "auxiliary oxidation-reduction system'' in the aqueous phase. 



In 1943 Baur and Niggli announced that two-phase systems contain- 

 ing chlorophyll in geraniol or phjrtol {e. g., 50 mg. chlorophyll in 25 ml. 

 geraniol), and methylene blue in dry glycerol {e. g., 30 mg. in 50 ml.), 

 produced steadily from circulating carbon dioxide gas both oxygen and 

 formaldehyde, at a rate of about 5 mg. per 24 hours, which corresponds 

 to about 5% of the saturation yield produced by the same quantity of 

 pigment in a living plant. 



Bukatsch (1939) thought that ascorbic acid (cf. Chapter 10) may play the part of 

 the "auxiliary system." He therefore compounded two-phase mixtures containing 

 ascorbic acid (chlorophyll in colophony or lecithin; ascorbic acid in water), and obtained 

 positive formaldehyde tests (with Schiff's reagent) after illuminating these systems for 

 5-15 hours with 25,000 lux. 



Our opinion of Baur's work was stated at the beginning of this 

 discussion. Despite the unnecessary complication introduced by the 

 "electrophotochemical" terminology, the general idea of the research 

 was sound. The provision of complete oxidation-reduction systems, 

 the substitution of less reluctant oxidants and reductants for carbon 

 dioxide and water, the attempts to unite the reactants in molecular 

 complexes, and to separate the products by the provision of two phases — 

 all these were reasonable steps towards the reproduction of the essential 



