Consideration of Acid Strength of a Single OH Group on Zeolites by Isotope Exchange Reaction with Ethane at High Temperatures

Takafumi Kubota, Ryota Osuga, Toshiyuki Yokoi, Junko N. Kondo

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)1496-1505
Number of pages10
JournalTopics in Catalysis
Volume60
Issue number19-20
DOIs
Publication statusPublished - 2017 Dec 1
Externally publishedYes

Keywords

  • Acid strength
  • High temperature
  • IR
  • Zeolites

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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