Synthesis and In Situ Modification of Hierarchical SAPO-34 by PEG with Different Molecular Weights; Application in MTO Process

TL;DR

This study develops modified SAPO-34 catalysts with tailored mesopores using PEG of different molecular weights, enhancing methanol-to-olefins (MTO) performance through structural and physicochemical optimization.

Contribution

It introduces a novel synthesis method using PEG to modify SAPO-34, controlling mesoporosity and acidity for improved MTO catalytic stability.

Findings

PEG MW of 6000 produces RHO and CHA frameworks.

PEG MW of 4000 yields pure SAPO-34 phase.

SAPO-34 with PEG MW 4000 and PEG/Al 0.0125 shows superior stability.

Abstract

Modified structures of SAPO-34 were prepared using polyethylene glycol as the mesopores generating agent. The synthesized catalysts were applied in methanol-to-olefins (MTO) process. All modified synthesized catalysts were characterized via XRD, XRF, FESEM, FTIR, N2 adsorption-desorption techniques, and temperature-programmed NH3 desorption and they were compared with conventional microporous SAPO-34. Introduction of non-ionic PEG capping agent affected the degree of homogeneity and integrity of the synthesis media and thus reduced the number of nuclei and order of coordination structures resulting in larger and less crystalline particles compared with the conventional sample. During the calcination process, decomposition of absorbed PEG moieties among the piled up SAPO patches formed a great portion of tuned mesopores into the microporous matrix. These tailored mesopores were served as auxiliary diffusion pathways in MTO reaction. The effects of molecular weight of PEG and PEG/Al molar ratio on the properties of the synthesized materials were investigated in order to optimize their MTO reaction performance. It was revealed that both of these two parameters can significantly change the structural composition and physicochemical properties of resultant products. Using PEG with MW of 6000 has led to the formation of RHO and CHA structural frameworks i.e. DNL-6 and SAPO-34, simultaneously, while addition of PEG with MW of 4000 resulted the formation of pure SAPO-34 phase. Altering the PEG/Al molar ratio in the precursor significantly influenced the porosity and acidity of the synthesized silicoaluminophosphate products. SAPO-34 impregnated with PEG molecular weight of 4000 and PEG/Al molar ratio of 0.0125 showed superior catalytic stability in MTO reaction because of the tuned bi-modal porosity and tailored acidity pattern.