Adsorption of 1,3-butadiene on supported and promoted silver catalysts

After exposure to 1,3-butadiene, unpromoted Ag/α-Al2O3 and Ag/α-Al2O3 promoted with Cs and Cl were characterized using DRIFTS. Weakly adsorbed molecular butadiene was the dominant species for all samples regardless of the adsorption temperature. After purging alumina with He, no IR absorption features remained with alumina, whereas spectra showed that butadiene adsorbed irreversibly at 300 K via one of its CC bonds on either the reduced or O-covered unpromoted Ag surface which led to a surface species resembling 1,2-butadiene, and on the O-covered Ag surface, crotonaldehyde and carbonaceous species were also present. The formation of these carbonaceous species on an O-covered Ag catalyst implied that chemisorbed oxygen facilitates additional CH bond breaking. On Cs-promoted Ag/α-Al2O3 covered with a monolayer of oxygen following reduction at 673 K, a dehydrogenated 1,2-butadiene-type surface species appeared to exist at 300 K in addition to other carbonaceous species. Isotope studies revealed that 1,3-butadiene adsorbs through both CC bonds on a Cs-promoted Ag catalyst and may exist as an oxametallacycle intermediate requiring two types of sites, i.e., an unpromoted Ag site and a Cs-promoted Ag site containing surface oxygen. In the presence of Cl, 1,3-butadiene was weakly adsorbed on the Ag surface at 300 K through a single CC bond regardless of the presence or absence of chemisorbed oxygen. DRIFT spectra obtained under epoxidation reaction conditions at 473 K and 1 atm indicated that 1,3-butadiene dehydrogenated on alumina alone to give carbonaceous species, whereas carboxylate compounds were the principal species on both unpromoted and promoted Ag catalysts, although crotonaldehyde was also observed. No epoxybutene was detected.