Promotion effect of Fe on the selectivity of Pd zeolite-X in methanol synthesis

Abstract

Here evidence is provided for the promotion effect of iron on methanol formation on PdX zeolite catalysts. The high activity and selectivity for methanol formation are markedly superior to those observed either on non-promoted Pd or on Fe zeolite. The activation energy obtained for the optimum promoted zeolite catalyst is close to those observed on other oxide supports. It is suggested that the zeolite framework does not primarily influence the reaction path. The promotion effect is attributed to the presence of PdFe bimetallic particles located mainly inside the cages and their migration is hindered by Fe²⁺ ions. This is the first report on the formation of iron-containing bimetallic particles inside zeolite cages. The optimum atomic ratio for promotion is found at Fe : Pd = 0.07, and the catalyst with Fe : Pd = 0.2 still displays excellent catalytic performance. Above Fe : Pd = 0.3 formation of a separate α-iron phase is suggested, since the activity and selectivity for methanol drops and higher hydrocarbons are formed.