This Article is focused on the study of the effect of oxygenate impurities like esters and acids in the mixed methanol and ethanol feed stream on the conversion of methanol and ethanol to light olefins. The conversion of such components at 450°C over ZSM-5 was investigated. Furthermore, the effect on conversion and selectivity of adding yttrium oxide to the catalyst was studied. It was shown that the presence of relatively small amounts of esters in the alcohol feed stream has a significant effect on catalyst lifetime. These esters will decompose on the zeolite surface leading to additional methanol or ethanol (depending on the type of ester) and to a carboxylic acid. The additional methanol or ethanol will react according to the alcohol to olefins mechanism, leading to either an increase or a decrease in the propylene/ethylene (P/E) selectivity ratio, respectively. The acid that is released upon decomposition of the ester is responsible for the decrease in lifetime of the zeolite by adsorption on the zeolite active sites. Addition of metal oxides, specifically yttria, to ZSM-5 limits this decrease in catalyst lifetime. The proposed pathway is by a stronger adsorption of the acid on yttria than on the active site of the zeolite. Removal of the acid by yttria can in some cases (typically when the ester conversion is not 100% without the yttria present) lead to a higher conversion of the ester and thereby indirectly to an effect on the P/E selectivity ratio due to the additional methanol or ethanol released. ■ INTRODUCTIONWorldwide, due to the continued rise in oil price, significant research efforts are undertaken to deliver technology capable of producing ethylene and propylene from a wide variety of alternative feedstocks, such as biomass, natural gas, and coal. 1 To this end, syngas is produced by the gasification of the alternative feedstock. Syngas can be converted to ethylene and/ or propylene via methanol by methanol to olefins (MTO) and methanol to propylene (MTP) technologies or via higher alcohols by dehydration. 2 For MTO, both UOP/Hydro and DICP's (Dalian Institute of Chemical Physics) DMTO (partner with ABB Lummus) are currently available for license and are based on a fluidized bed reactor using a SAPO-34 catalyst. 3 For MTP, Lurgi's MTP process and Tsinghua University's FMTP are available for license. 4,5 Lurgi's process is based on a fixed bed reactor system using a ZSM-5 catalyst, while the FMTP uses a SAPO-34/SAPO-18 mixed catalyst in a fluidized bed reactor similar to that of MTO. 6 Substantial research on the catalytic conversion of syngas to mixed alcohols has been conducted since the beginning of the 20th century. 1,7−9 The Dow Chemical Co. developed CoMoS catalysts for mixed alcohols synthesis from syngas in the early 1980s. 10,11 Recently, Dow restarted the research of syngas to mixed alcohols and the production of ethylene and propylene via mixed alcohols, that is, methanol and ethanol over zeolite catalysts. It was observed that very small amounts of esters and acids were produced with ...