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