Methanol diffusion dynamics in microporous materials as a function of acid site density by quasi-elastic neutron scattering (QENS)

Microporous materials SAPO-34 and ZSM-5 play a crucial role in various petrochemical and environmental processes such as methanol to hydrocarbons (MTH) which enables replacement of fossil fuels with carbon neutral renewable methanol feedstock to produce gasoline, olefins and aromatics [1-3]. Thus, the fundamental studies to unravel the underlying MTH reaction and deactivation mechanisms are renewed to design and develop efficient catalysts [2,3]. However, studies on the transportation of gas phase molecules to and from the active¬†Br√łnsted¬†acid site located within the zeolite pores, which is crucial for an efficient process, are limited [4,5]. We report methanol diffusion dynamics in microporous SAPO-34 and H-ZSM-5 pores as a function of Si/Al ratio, which determines the¬†Br√łnsted¬†acid site density, and of temperature, by quasi-elastic neutron scattering(QENS). To this end, SAPO-34 and H-ZSM-5 with varied Si/Al ratios were systematically studied at different temperatures (8, 298, 323, 348 and 373 K) by QENS at ISIS pulsed neutron and muon source, Harwell. QENS shows (within the instrumental resolution) that methanol mobility is restricted even at 373 K in zeolites with a higher acid site density than that with a lower [6,7]. The methanol mobility increases with increasing Si/Al ratio (i.e., decreasing the acid site density) of the zeolites, which is also true with increasing measurement temperature from 298 to 373 K. The temperature dependent mobile fraction is observed across all the ratios of the zeolites. The effect of acid site density on the nature of methanol diffusion dynamics and mobile fraction will be discussed.

References:

  1. Chang, C.D et al. The conversion of methanol and other O-compounds to hydrocarbons over zeolite catalysts. J. Catalysis 47, 249-259 (1977).
  2. Van Speybroeck, Catlow, C.R.A et al. Advances in theory and their application within the field of zeolite chemistry. Chemical Society Reviews 44, 7044-7111 (2015). 
  3. Minova, I, Matam, S.K, Catlow, R.A, Wright, P, Howe, R. et al., Elementary steps in the formation of hydrocarbons from surface methoxy groups in HZSM5 seen by synchrotron infrared microscopy. ACS Catal., 9, 6564-6570 (2019). 
  4. Jobic, H, Theodorou, D.N. Quasi-elastic neutron scattering and molecular dynamics simulation as complementary techniques for studying diffusion in zeolites. Micro. Meso. Materials, 102, 21-50 (2007). 
  5. Matam, S.K, Silverwood, I, Boudjema, L, O’Malley, A, Catlow, R.A, Phil. Trans. R. Soc. A, 381, 20220335 (2023).
  6. Matam, S.K, Howe, R, Thetford, A, Catlow, R.A, Room temperature methoxylation in zeolite H-ZSM-5: an operando DRIFTS/mass spectrometric study, Chem. Commun. 54, 12875-12878 (2018).
  7. Matam, S.K, S.A. Nastase, A. Logsdail, A, Catlow, R.A, Methanol loading dependent methoxylation in zeolite H-ZSM-5, Chem. Sci. 11, 6805-6814 (2020).

Biography:

Santhosh Matam photo

Santhosh Kumar Matam is a research associate at Cardiff University with Prof. Sir C. Richard A. Catlow, FRS and is based at the UK Catalysis hub, STFC Rutherford Appleton Laboratories, Harwell. He earned his PhD in a German Research Foundation project from Humboldt University of Berlin, Germany. The Research Council of Norway fellowship and Swiss National Foundation grants helped him to pursue his research. Santhosh’s research activities are primarily centred on in situ/operando spectroscopy for deriving catalyst structure-activity relationships. He employs Neutrons, X-rays and Laser based techniques and closely collaborates with computational chemists with an aim to design and develop inorganic solid materials for energy and environmental applications, which include carbon neutral renewable energy and exhaust after-treatment technologies. He is also interested in operando reactors that allow real operation of chemical processes without intrinsic limitations.

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