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

• Published in: Catalysis letters 2021-03, Vol.151 (3), p.670-684
• Main Authors: Wu, Jianbing, Sun, Zeping, Wei, Zhihong, Qin, Zhangfeng, Zhao, Yongxiang
• Format: Article
• Language: English
• Subjects: Aluminum; Catalysis; Catalysts; Chemical synthesis; Chemistry; Chemistry and Materials Science; Diesel fuels; Ethers; Formaldehyde; Industrial Chemistry/Chemical Engineering; Infrared spectroscopy; Mathematical analysis; Organometallic Chemistry; Physical Chemistry; Reaction mechanisms; Selectivity; Silicon; Zeolites
• ISSN: 1011-372X; 1572-879X
• DOI: 10.1007/s10562-020-03345-2

Acidic zeolites are considered a kind of efficient catalysts in the synthesis of polyoxymethylene dimethyl ethers (PODE n ) with the fomula of CH 3 O–(CH 2 O) n –CH 3 , a promising green diesel fuel or additive from methanol derivatives; however, the crucial parameters that determine PODE n 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 PODE n synthesis from dimethoxymethane (DMM) and trioxymethylene (TOM). The results indicate that the Si/Al ratio and reaction temperature are interdependent in determining the PODE n yield; High Al HZSM-5 zeolites( Si/Al ≤ 50) show the highest PODE 2-8 selectivity and yield only at 70–90 °C, whereas low Al zeolites (Si/Al ≥ 109) possess the better activity to PODE 2–8 at 70–150 °C. The reaction mechanism was then investigated through in-situ IR characterization and DFT calculation. The important intermediates ZO-CH 2 OCH 2 OCH 2 OH, ZOCH 2 OCH 3 and ZO(CH 2 O) n CH 3 (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 Brönsted acidic sites provided by Al–OH of HZSM-5 zeolite promote the dissociation of TOM to monomer CH 2 O and transform DMM to ZOCH 2 OCH 3 and CH 3 OH; Next, the CH 2 O species react with short chain ZO(CH 2 O) n CH 3 , forming long chain ZO(CH 2 O) n CH 3 ; After that, the PODE n products are formed by sealing ZO(CH 2 ) n OCH 3 with CH 3 OH. 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. Graphic Abstract 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.