BASE-CATALYZED DECOMPOSITION OF METHYL FORMATE TO CARBON-MONOXIDE AND METHANOL OVER ZEOLITE CATALYSTS

Sodium-13X and calcium-5A zeolites, and samples of the zeolites doped with NaOH or KOH, were characterized and tested with regard to their performance as basic catalysts in the decomposition of methyl formate (MF) to CO and methanol. Carbon dioxide chemisorption per m2 of the samples increases exponentially with NaOH loading within the range 0-7.8 wt.%, but catalytic activity correlates more closely to the basic strength than to the total number of basic centers. The activity of Na-13X for this reaction is moderate and stable, while lightly doped 13X samples (3.8 wt.% NaOH) have a high initial activity, which decays to a low, stable value within an hour. A more heavily doped sample (7.8 wt.% NaOH), which is actually composed of NaAlSiO4 and Na2O . Al2O3 . SiO2, gave a low but stable activity. Calcium-5A zeolite with 3.3 wt.% residual Na has a relatively high initial activity, which decays gradually. Basic strengths of the chemical entities involved were estimated by the partial charge of their oxygen anions. From the calculated partial charges and activity measurements, it was inferred that free Na2O and NaOH are highly active and responsible for the high initial activity, which decays rapidly owing to the formation of HCOONa. Sodium zeolites and aluminosilicates are moderately active and responsible for the low, stable activity. Water is a strong poison that probably blocks the strongly basic O2- sites with its H+ ion or H2O dipole and also catalyzes the formation of HCOONa. But the relatively weak basic sites, responsible for the low but stable activity, are not blocked. The lag of the CO evolution behind methanol under TPD conditions and the simultaneous evolution of CO and methanol under continuous-flow or frontal conditions, are even more conspicuous here than in the case of NaOH/MgO. Explicit schemes for two concurrent mechanisms of methyl formate decomposition on the basic catalysts are proposed.