Structural and Mechanistic Investigation of a Phosphate-Modified HZSM-5 Catalyst for Methanol Conversion

Abstract

Phosphorus modification of a HZSM-5 (MFI) zeolite by wet impregnation has long been known to decrease aromatic formation in methanol conversion chemistry. We prepared and studied a catalyst modified by introducing trimethylphosphine under reaction conditions followed by oxidation. Magic-angle spinning (MAS) NMR shows that extensive dealumination occurs, resulting in a catalyst with a much higher framework SiO2/Al2O3 ratio, as well as extraframework aluminum and approximately 1.4 equiv of entrained phosphoric acid (under working conditions) per aluminum. Upon dehydration or regeneration, the phosphoric acid is converted, reversibly, to entrained P4O10. The aromatic selectivity of the modified catalyst is significantly lower than that of an unmodified zeolite with a similar, increased framework SiO2/Al2O3 ratio. By comparing the rates of H/D exchange in propene under conditions similar to those in methanol conversion chemistry, we determined that the acid site strength is indistinguishable on modified and unmodified zeolites, and this is consistent with theoretical modeling. On the phosphorus-modified zeolite, the rate of propene oligomerization is greatly suppressed, suggesting that entrained phosphate is an impediment to sterically demanding reactions.

Local silico-aluminophosphate interfaces within phosphated H-ZSM-5 zeolites

“…12a. 9 The appearance of the broad band in FT-IR between 3520 cm À1 and 3200 cm À1 , which indicates intramolecular bonds between framework protons and oxygen, supports this. 40 Furthermore, OFAl species that can coordinate with three water molecules, appear to be able to coordinate with phosphorus as well.…”

“…29 Haw and co-workers suggested that phosphorus interacts with framework aluminium through hydrogen bonds. 9 This was supported by an early paper of Lischke et al in which the reversible character of the phosphorus framework interaction was described, as phosphorus could be eluted using a hot water washing step. After a hydrothermal treatment phosphorus could not be washed out, which was attributed to the formation of insoluble extra-framework AlPO 4 species.…”

“…5,6 The selectivity towards propylene is increased even more with the introduction of phosphorus. [6][7][8][9][10] Usually, the improved selectivity towards propylene is attributed to the decrease in the acid site strength that is so often observed in phosphated H-ZSM-5, although steric factors have also been suggested to be responsible. [6][7][8][9][10][11][12] Even though there is still some debate on the effect of acid site strength on the MTO product distribution, the promoting effect of phosphorus on the hydrothermal stability of H-ZSM-5 is undisputed.…”

“…[6][7][8][9][10][11][12] Even though there is still some debate on the effect of acid site strength on the MTO product distribution, the promoting effect of phosphorus on the hydrothermal stability of H-ZSM-5 is undisputed. 4,9,[13][14][15][16][17][18][19][20][21][22][23][24][25] Zeolite H-ZSM-5 catalysts that contain phosphorus are more stable under hydrothermal conditions i.e. they retain more framework aluminium species and strong acid sites than their non-phosphated counterparts.…”

Conversion of Methanol to Hydrocarbons over Phosphorus-modified ZSM-5/ZSM-11 Intergrowth Zeolites

“…High selectivity of propylene has been obtained on modified ZSM-5 zeolites [2,3]. Phosphorusmodified ZSM-5 has been extensively investigated in methanol conversion [4][5][6][7][8][9][10][11][12][13][14], and catalytic cracking reaction [15][16][17][18][19][20]. The structure of p-modified ZSM-5 has been characterized by many research groups [21][22][23][24][25][26][27].…”