A Quasi-elastic neutron scattering (QENS) study of methanol diffusion dynamics in H-ZSM-5 as a function of Si/Al ratio Santhosh Matam 1 , A.O’Malley 2,3* , I. Silverwood 2,4 , C. Richard A. Catlow 1,2,5 1 Cardiff University, UK, 2 Rutherford Appleton Laboratory, UK, 3 University of Bath, UK, 4 ISIS Facility, UK, 5 University College London, UK Zeolite ZSM-5 is crucial in many petrochemical processes [1-3], including methanol to gasoline (MTG) that can potentially meet the global demand for gasoline with carbon neutral renewable methanol feedstock and can help to achieve net carbon zero emissions. Extensive research to understand the underlying MTG reaction mechanism using both experimental and computational tools has been reported [2,3] however, studies on methanol diffusion dynamics in ZSM-5 are scarce. Transportation of methanol molecules to and from the active Brønsted acid site located within the zeolite pores with distinct pore architecture involves adsorption and diffusion. Also, after the reaction products desorb and diffuse back through the zeolite pores into the gas-phase. Therefore, diffusion is one of the key steps of the MTG reaction that needs to be better understood and quantified in order to develop efficient processes [4,5]. Here we report methanol diffusion dynamics in H-ZSM-5 pores as a function of Si/Al ratio, which determines the Brønsted active site density. For this, ZSM-5 with varied Si/Al ratios (11, 25, 40 and 140) are methodically studied at different temperatures (298, 323, 348 and 373 K) by in situ quasi-elastic neutron scattering (QENS) at ISIS, UK. In situ QENS data show that, within the instrumental resolution, methanol mobility is greatly restricted (≈78%) even at 373 K in ZSM-5-11 (Si/Al=11) with the highest acid site density, while the mobility is significantly better (<;30% immobile fraction) in ZSM-5-140 (Si/Al=140) with the lowest acid site density studied in the series of zeolites. It is evident that the nature of methanol diffusion dynamics is dependent on acid site density. At 373 K, ZSM-5-11 shows isotropic methanol rotation while, translational diffusion within a confined pore volume is observed in ZSM-5-25 with Si/Al ratios of 25 [5], 36 [6] and 40 [7]. Differently, long-range translational diffusion is detected in ZSM-5-140 at 373 K [7]. It is noteworthy that the methanol diffusional dynamics have switched from isotropic rotation at 298 K to translational diffusion at 373 K in ZSM-5-25 [5], indicating that the methanol dynamics are temperature dependent, too. The immobile methanol fraction in H-ZSM-5 is attributed to the occurrence of hydrogen bonded methanol and methoxy species as evident from infrared spectroscopy [8]. References 1. Chang, C.D et al. J. Catalysis 1977 , 47 , 249. Van Speybroeck; Catlow, C.R.A et al. Chemical Society Reviews 2015 , 44 , 7044. 2. Minova, I.; Matam, S.K.:Catlow, R.A.; Wright, P.; Howe, R. et al. ACS Catal., 2019 , 9 , 6564. 3. Jobic, H.; Theodorou, D.N.; Micro. Meso. Materials 2007 , 102 , 21-50. 4. Matam, S.K; Catlow, C. R. A.; Silverwood, I.; O’Malley, A. Topics in Catalysis 2021 , 64 , 69. 5. Omojola, T.; Silverwood, I.; O’Malley, A. Catal. Sci. Technol., 2020 , 10 , 4305. 6. Matam, S.K; Catlow, C. R. A et al. to be submitted, 2023.S.K. Matam, R.F. Howe, A. Thetford, C. Richard A. Catlow, Chem. Commun., 54 , 12875 (2018).
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