Generation of Syngas from Formaldehyde or Paraformaldehyde

Of particular interest for the in situ generation of syngas are formaldehyde and paraformaldehyde. Commercial solutions of formaldehyde in water are called formal or formalin. A saturated water solution of about 40vol% or 37 mass% formaldehyde is commercialized as 100% formalin. Formaldehyde is commonly produced by the metal-catalyzed oxidation of methanol (Formox process) in several million tons per year [10]. Paraformaldehyde is the solid polymerization product (mp = 120 C) of formaldehyde with an average degree of polymerization of 8-100 units. It is depolymerized to formaldehyde upon heating. Up to now, the trimerization product of formaldehyde, 1,3,6-trioxane, does not play a role in hydroformylation. 

Syngas generated in this manner can be used in a second catalytic cycle for the hydroformylation of an olefin. As an alternative to this intermolecular process, hydroformylation can occur without the liberation of H2 and CO in an intramolecular maimer by transfer of the formyl group to the olefin in the coordination sphere of the transient metal—acyl complex. Both mechanisms can probably compete and are dependent on the reaction conditions [12]. 

Under these conditions, RhCfiPPhjjj or Ru(CO)3(PPhj) was virtually inactive. By increasing the temperature from 120 to 150 C, the yield of the aldehydes decreased and the formation of alcohols and esters became dominant. 

Rosales etal. [15] discovered that, in contrast to the hydroformylation with syngas, the reaction with paraformaldehyde proceeds best with rhodium complexes hosting two diphosphine ligands. Apparently, small chelate rings with rhodium are advantageous, since the reaction rate with complexes of the type [Rh(diphos)2]  

Moreover, the activity of such bis-ligand complexes was higher than that of the type HRh(CO)2(diphos). The latter were more active in the hydroformylation with syngas. 

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