TY - JOUR
T1 - Consideration of Acid Strength of a Single OH Group on Zeolites by Isotope Exchange Reaction with Ethane at High Temperatures
AU - Kubota, Takafumi
AU - Osuga, Ryota
AU - Yokoi, Toshiyuki
AU - Kondo, Junko N.
N1 - Funding Information:
Acknowledgements This research is partly supported by a PRESTO (Hyper-nano-space design toward innovative functionality) of JST.
Publisher Copyright:
© 2017, Springer Science+Business Media, LLC.
PY - 2017/12/1
Y1 - 2017/12/1
N2 - Infrared probe (IR) methods to estimate the acid strength of OH groups on H-form zeolites were compared on mordenites (HM90, Si/Al = 45 and HM20, Si/Al = 10), ZSM-5 (Z5-90H, Si/Al = 45) and Y (HY5.6, Si/Al = 2.8). Heat of adsorption as well as the internal frequency of adsorbed CO was not sensitive enough to evaluate the acid strength of different zeolites. Thus, H/D isotope exchange reaction of acidic OD groups with a small hydrocarbon, C2H6, was employed. For the estimation of intrinsic activation energy, heat of adsorption of C2H6 was first measured. However, C2H6 molecules could not enter the 8 membered ring (MR) of HM20, and the secondary interaction of C2H6 with pore walls of Z5-90H increased the heat of adsorption, both due to the observation at low temperatures. This prohibited the comparison of heat of adsorption on a single Brønsted acid site in different spaces of zeolite with different topologies. Considering that the structural restrictions of such small molecules as C2H6 become negligible at high temperatures as a result of the vibrational excitation of pore mouth-breathing modes of zeolites, and that the topology difference of a single Brønsted acid site in heat of adsorption is relatively small, we proposed that the apparent activation energy for H/D isotope exchange reaction would reflect the acid strength of single sites. The present results are compared and discussed with conclusions led by other methods.
AB - Infrared probe (IR) methods to estimate the acid strength of OH groups on H-form zeolites were compared on mordenites (HM90, Si/Al = 45 and HM20, Si/Al = 10), ZSM-5 (Z5-90H, Si/Al = 45) and Y (HY5.6, Si/Al = 2.8). Heat of adsorption as well as the internal frequency of adsorbed CO was not sensitive enough to evaluate the acid strength of different zeolites. Thus, H/D isotope exchange reaction of acidic OD groups with a small hydrocarbon, C2H6, was employed. For the estimation of intrinsic activation energy, heat of adsorption of C2H6 was first measured. However, C2H6 molecules could not enter the 8 membered ring (MR) of HM20, and the secondary interaction of C2H6 with pore walls of Z5-90H increased the heat of adsorption, both due to the observation at low temperatures. This prohibited the comparison of heat of adsorption on a single Brønsted acid site in different spaces of zeolite with different topologies. Considering that the structural restrictions of such small molecules as C2H6 become negligible at high temperatures as a result of the vibrational excitation of pore mouth-breathing modes of zeolites, and that the topology difference of a single Brønsted acid site in heat of adsorption is relatively small, we proposed that the apparent activation energy for H/D isotope exchange reaction would reflect the acid strength of single sites. The present results are compared and discussed with conclusions led by other methods.
KW - Acid strength
KW - High temperature
KW - IR
KW - Zeolites
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U2 - 10.1007/s11244-017-0834-9
DO - 10.1007/s11244-017-0834-9
M3 - Article
AN - SCOPUS:85027886397
VL - 60
SP - 1496
EP - 1505
JO - Topics in Catalysis
JF - Topics in Catalysis
SN - 1022-5528
IS - 19-20
ER -