Improvement of Lifetime Using Transition Metal-Incorporated SAPO-34 Catalysts in Conversion of Dimethyl Ether to Light Olefins

Hyo-Sub Kim,Jin-Bae Lee,Su-Gyung Lee,Young-Ho Kim,Dong-Hee Lee,Chu-Sik Park

doi:10.1155/2013/679758

Abstract

Transition metal (Mn, Fe, or Ni) incorporated SAPO-34 (MeAPSO-34) nanocatalysts were synthesized using a hydrothermal method to improve the catalytic lifetime in the conversion of dimethyl ether to light olefins (DTO). The structures of the synthesized catalysts were characterized using several methods including XRD, SEM, BET,29Si-MAS NMR, and NH3-TPD techniques. Although the structure of the MeAPSO-34 catalysts was similar to that of the SAPO-34 catalyst, the amount of weak acid sites in all MeAPSO-34 catalysts was markedly increased and accompanied by differences in crystallinity and structural arrangement. The amount of weak acid sites decreased in the following order: NiAPSO-34 > FeAPSO-34 > MnAPSO-34 > SAPO-34 catalyst. The MeAPSO-34 catalysts, when used in the DTO reaction, maintained DME conversion above 90% for a longer time than the SAPO-34 catalyst, while also maintaining the total selectivity above 95% for light olefins. In addition, the NiAPSO-34 catalyst showed the longest catalytic lifetime; the lifetime was extended approximately 2-fold relative to the SAPO-34 catalyst. Therefore, the increase in the catalytic lifetime is related to the amount of weak acidic sites, and these sites are increased in number by incorporating transition metals into the SAPO-34 catalyst.

Highlights

As a microporous material, aluminophosphates (AlPO) nanomolecular sieves have been used in many processes as a shape-selective catalyst [1,2,3,4,5]
The small changes in the crystal sizes may be attributed to the differences in the sizes of the metal ions substituted with Al ion in the SAPO-34 framework [27]
From the results of dimethyl ether to light olefins (DTO) reaction, it was observed that the MeAPSO-34 catalysts show long catalytic lifetimes and high selectivities toward light olefins compared with the SAPO-34 catalyst

Summary

Introduction

Aluminophosphates (AlPO) nanomolecular sieves have been used in many processes as a shape-selective catalyst [1,2,3,4,5]. AlPO molecular sieves are electrically neutral due to their structure, in addition to completely lacking acidic sites and catalytic properties. To enhance the catalytic lifetime, various transition metals are incorporated into the SAPO-34 framework via isomorphous substitution to form MeAPSO-34 molecular sieves [23,24,25,26,27,28]. The incorporation of transition metals influences the arrangement of the Si atoms, which can only be substituted at the P(4Al) sites [29] It induces a difference in acidity and catalytic activity compared with SAPO-34 because the Si atom plays an important role during acidic site formation in SAPO-34. The variations in the structure and acidity of molecular sieves were characterized using 29Si magic-angle spinning (MAS) NMR and ammonia temperature-programmed desorption (NH3-TPD) At this point, we have focused on the acidic site of catalyst that influences the efficiency of DTO process. DTO reactions were conducted using the MeAPSO-34 catalysts to evaluate the catalytic performance and lifetime, as well as the selectivity for light olefins

Experimental

Results and Discussion

Conclusions