Enhancement of the Stability of Mordenite for use in Dimethyl Ether Carbonylation

  • Author / Creator
    Reule, Allen A.C.
  • Recently, considerable research has been conducted into solid-acid catalyzed carbonylation of dimethyl ether (DME) to methyl acetate (MeOAc), which can be further used for the iodine-free production of ethanol or acetic acid. The zeolite mordenite (H-MOR) is known as a potential catalyst but is subject to a rapid deactivation that so far hinders the process commercialization. The objective of the current study is to find a simple and effective means by which H-MOR can be stabilized for DME carbonylation. The bimetallic liquid-based ion-exchange (IE) of Cu2+ and Zn2+ onto MOR was used to enhance its stability. Compared to the original H-MOR (Si/Al ratio of 6.5), 1Cu-4Zn/H-MOR (Cu:Zn ratio of 0.25) had 3 times the lifetime and produced 4 times the total MeOAc before deactivation at 438 K. High selectivity to MeOAc was also maintained on catalyst deactivation. Cu and Zn occupied around 55% of the acid sites on MOR but there was no decrease in activity compared to the H-MOR. Despite Cu being a known carbonylation catalyst, it did not enhance the catalyst activity. It was determined that, due to the competitive IE of Cu2+ and Zn2+ over MOR, the two metals were forced into blocking different unselective acid sites that would normally have contributed to coking reactions. This was shown by quantum chemical modeling of the potential IE locations for Cu2+ and Zn2+, which was in agreement with catalyst characterization results. Specifically, competitive IE at the T1 acid site was responsible for the unique behaviour of the 1Cu-4Zn/H-MOR. The use of Zn also stabilized Cu in its monovalent state and prevented any sintering from occurring. Thus, it is shown that the selectivity and stability of H-MOR can be substantially improved by selective poisoning of acid sites. This has important implications for Cu/H-MOR catalysts that have found increasing use, such as in methane-to-methanol processes. Dealumination of MOR via acid leaching was also used in an attempt to increase its stability and to understand the relative contributions of different acid sites. Gradual dealumination from the original Si/Al ratio of 6.5 resulted in activity loss, but also increased H-MOR’s selectivity to MeOAc during deactivation. At a Si/Al ratio of 15.4, the H-MOR was substantially deactivated by the dealumination. The catalyst characterization showed that the acid leaching was preferably removing the T3 acid site. This acid site had been previously theorized to be the only site at which DME carbonylation selectively occurred. This work provided substantial experimental evidence supporting this theory. Mild dealumination to a Si/Al ratio of 8.6 did improve H-MOR performance and it was determined that, while the other acid sites may contribute to coking, they were not solely responsible for the catalyst deactivation. Too high an acid site density near to the T3 acid site is also detrimental to the performance of the catalyst. Applying the principle of selective site poisoning derived from the bimetallic Cu2+-Zn2+ IE study, Fe2+ was placed onto MOR via oxidative solid-state IE (Fe(II)/H-MOR). The resultant catalyst had 2 times the lifetime and produced over 3 times the MeOAc compared to acidic MOR. High selectivity to MeOAc was maintained even with catalyst deactivation. The use of monometallic Fe2+ on MOR is preferable to the use of monometallic Cu2+ or Zn2+ placed onto mordenite via IE. When combined with Zn2+, the bimetallic 3Fe(II)-1Zn/H- MOR catalyst (Fe:Zn ratio of 3) had very similar performance to the bimetallic 1Cu-4Zn/H-MOR catalyst. Thus, three potential catalysts were identified for possible use in industrial DME carbonylation: Fe(II)/H-MOR, 1Cu-4Zn/H-MOR, and 3Fe(II)-1Zn/H-MOR. All of these catalysts have high peak activity levels and maintain high selectivity to MeOAc for the entirety of the catalyst lifetime, and are significant improvements over H-MOR alone. It is still required that the reaction conditions be optimized as well as suitable regeneration procedures put in place to restore the catalysts after deactivation.

  • Subjects / Keywords
  • Graduation date
    Fall 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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