TY - JOUR
T1 - Silicon redistribution, acid site loss and the formation of a core-shell texture upon steaming SAPO-34 and their impact on catalytic performance in the methanol-to-olefins (MTO) reaction
AU - Minova, Ivalina B.
AU - Barrow, Nathan S.
AU - Sauerwein, Andrea C.
AU - Naden, Aaron B.
AU - Cordes, David B.
AU - Slawin, Alexandra M.Z.
AU - Schuyten, Stephen J.
AU - Wright, Paul A.
N1 - IBM has received funding from the Engineering and Physical Sciences Research Council (EPSRC, Centre for Doctoral Training in Critical Resource Catalysis, EP/I017008/1) and Scotland's Chemistry departments (ScotCHEM). IBM also received a scholarship from the SCI and Santander. Johnson Matthey is thanked for in-kind contributions and hosting IBM in their R&D labs. ABN gratefully acknowledges support from the EPSRC (grants EP/L017008/1 and EP/R023751/1).
The research data supporting this publication can be accessed at: https://doi.org/10.17630/09ddc03e-f121-4e79-9b55-674f64d9c8c4 [62].
PY - 2021/3
Y1 - 2021/3
N2 - SAPO-34 is a commercially-implemented silicoaluminophosphate catalyst for selective high yield production of ethene and propene from methanol, but high temperature regeneration in the presence of steam leads to its deactivation. A comprehensive investigation of the effect of prolonged hydrothermal treatment on the structure and properties of SAPO 34 explains the changes in its catalytic methanol-to-olefins (MTO) performance. Microcrystalline powdered SAPO-34 (ca. 3 µm crystals, Al17.1P15.6Si3.3O72) and two batches of larger single crystals of SAPO-34 of different Si concentration (20-100 µm; Al17.3P14.7Si4.0O72 and Al17.7P12.3Si5.9O72 ) were steamed (pH2O = 0.95 atm) at 873–1023 K for up to 240 h. The acidity (NH3-TPD), crystallinity (PXRD), framework cation environment (solid-state 27Al, 29Si and 31P MAS NMR) and porosity were followed for all materials; larger crystals were amenable to single crystal X-ray diffraction, FIB-SEM and synchrotron IR microspectroscopy, including operando study during methanol and dimethyl ether conversions. Some level of steaming improved the lifetime of all SAPO-34 materials in MTO catalysis without affecting their olefin selectivity, although more severe conditions led to the formation of core-shell structures, microporosity loss and eventually at 1023 K, recrystallization to a dense phase. All these irreversible changes occurred faster in crystals with higher Si contents. The initial increase in catalytic lifetime results from an activated reduction in acid site density (Eact = 146(18) kJ mol⁻1), a result of redistribution of Si within the SAPO framework without porosity loss. Operando IR with online product analysis during methanol conversion suggests similar reaction pathways in calcined and steamed crystals, but with greatly reduced methoxy group densities in the latter. The gradual development of optically dark crystal cores upon progressive steaming was shown by FIB-SEM to be due to the formation of regions with meso- and macropores, and these were shown by IR mapping to possess low hydroxyl densities.
AB - SAPO-34 is a commercially-implemented silicoaluminophosphate catalyst for selective high yield production of ethene and propene from methanol, but high temperature regeneration in the presence of steam leads to its deactivation. A comprehensive investigation of the effect of prolonged hydrothermal treatment on the structure and properties of SAPO 34 explains the changes in its catalytic methanol-to-olefins (MTO) performance. Microcrystalline powdered SAPO-34 (ca. 3 µm crystals, Al17.1P15.6Si3.3O72) and two batches of larger single crystals of SAPO-34 of different Si concentration (20-100 µm; Al17.3P14.7Si4.0O72 and Al17.7P12.3Si5.9O72 ) were steamed (pH2O = 0.95 atm) at 873–1023 K for up to 240 h. The acidity (NH3-TPD), crystallinity (PXRD), framework cation environment (solid-state 27Al, 29Si and 31P MAS NMR) and porosity were followed for all materials; larger crystals were amenable to single crystal X-ray diffraction, FIB-SEM and synchrotron IR microspectroscopy, including operando study during methanol and dimethyl ether conversions. Some level of steaming improved the lifetime of all SAPO-34 materials in MTO catalysis without affecting their olefin selectivity, although more severe conditions led to the formation of core-shell structures, microporosity loss and eventually at 1023 K, recrystallization to a dense phase. All these irreversible changes occurred faster in crystals with higher Si contents. The initial increase in catalytic lifetime results from an activated reduction in acid site density (Eact = 146(18) kJ mol⁻1), a result of redistribution of Si within the SAPO framework without porosity loss. Operando IR with online product analysis during methanol conversion suggests similar reaction pathways in calcined and steamed crystals, but with greatly reduced methoxy group densities in the latter. The gradual development of optically dark crystal cores upon progressive steaming was shown by FIB-SEM to be due to the formation of regions with meso- and macropores, and these were shown by IR mapping to possess low hydroxyl densities.
KW - Steamed SAPO 34
KW - Methanol-to-olefins
KW - IR spectroscopy and microscopy
KW - FIB SEM
U2 - 10.1016/j.jcat.2021.01.012
DO - 10.1016/j.jcat.2021.01.012
M3 - Article
AN - SCOPUS:85101206598
SN - 0021-9517
VL - 395
SP - 425
EP - 444
JO - Journal of Catalysis
JF - Journal of Catalysis
ER -