Catalytic Performance and Mechanistic Insights into the Synthesis of Polyoxymethylene Dimethyl Ethers from Dimethoxymethane and Trioxymethylene over ZSM-5 Zeolite

Abstract

Acidic zeolites are considered a kind of efficient catalysts in the synthesis of polyoxymethylene dimethyl ethers (PODEn) with the fomula of CH3O–(CH2O)n–CH3, a promising green diesel fuel or additive from methanol derivatives; however, the crucial parameters that determine PODEn yield and reaction mechanism over zeolite catalyst remains rather ambiguous. In this work, the relationship between the catalytic performance of HZSM-5 zeolite and Si/Al ratio and reaction temperature was revealed through PODEn synthesis from dimethoxymethane (DMM) and trioxymethylene (TOM). The results indicate that the Si/Al ratio and reaction temperature are interdependent in determining the PODEn yield; High Al HZSM-5 zeolites( Si/Al ≤ 50) show the highest PODE2-8 selectivity and yield only at 70–90 °C, whereas low Al zeolites (Si/Al ≥ 109) possess the better activity to PODE2–8 at 70–150 °C. The reaction mechanism was then investigated through in-situ IR characterization and DFT calculation. The important intermediates ZO-CH2OCH2OCH2OH, ZOCH2OCH3 and ZO(CH2O)nCH3 (ZOH: HZSM-5 zeolite) were detected by In situ IR spectra during the exposure of HZSM-5 to TOM, DMM and co-feed. DFT calculations further verified above reaction intermediates and elucidated the reaction elementary steps. Reaction mechanism was proposed as follows: the Bronsted acidic sites provided by Al–OH of HZSM-5 zeolite promote the dissociation of TOM to monomer CH2O and transform DMM to ZOCH2OCH3 and CH3OH; Next, the CH2O species react with short chain ZO(CH2O)nCH3, forming long chain ZO(CH2O)nCH3; After that, the PODEn products are formed by sealing ZO(CH2)nOCH3 with CH3OH. Ultimately, the difference of catalytic performance resulting from the effect of Si/Al ratio and reaction temperature, based on the proposed mechanisms, was given a logical explanation. The detailed reaction mechanism in the synthesis of polyoxymethylene dimethyl ethers from dimethoxymethane and trioxymethylene over HZSM-5 zeolite were investigated through a series of in-situ IR characterization and DFT calculation. Subsequently, the difference of catalytic performance resulting from the effect of Si/Al ratio and reaction temperature, was given a possible explanation.