Influence of the Si/Al ratio and Al distribution on the H-ZSM-5 lattice and Brønsted acid site characteristics

• H-ZSM-5 Brønsted Al fraction active for isopropylamine cracking decreases with Si/Al.
• Consistent predicted and experimental H-ZSM-5 lattice trends with varying Si/Al.
• H-ZSM-5 Brønsted site and lattice properties depend on Al density and distribution.

An experimentally-guided theoretical study was conducted to investigate H-ZSM-5 aluminosilicate lattice intrinsic Brønsted acid site properties and reactive functionalities. The variation of intrinsic parameters, including Si/Al ratio and Al distribution, were probed with VASP fully-minimized periodic atomic models, and compared to characterization results. Isopropylamine (IPA) and pentane adsorption and protonation were predicted at selected Brønsted acid sites in these H-ZSM-5 configurations. The following trends were observed with decreasing Si/Al ratio: The predicted H-ZSM-5 lattice parameters increased, consistent with H-ZSM-5 X-ray diffraction measurement trends. The Brønsted acid site O–H distance was predicted to increase and the proton charge to decrease. This trend was explained by increased lattice covalency in the predicted electronic density of states. Temperature programmed desorption–mass spectrometry showed that the H-ZSM-5 IPA cracking reactivity increased with Brønsted acid site density, utilizing a reduced fraction of total Al sites. The IPA adsorption with spontaneous protonation was predicted to increase in favorability. Pentane physisorption electron transfer and favorability was also predicted to increase. However, the favorability of activated pentane protonation decreased, due to poor charge accommodation in the increasingly covalent lattices. Thermochemical analyses predicted decreased H-ZSM-5 deprotonation energies, DPEH-ZSM-5, but also decreased stabilization interactions of deprotonated H-ZSM-5 with protonated products. On the other hand, Al substitution in next–next nearest-neighbor T-sites at a low Si/Al ratio was shown to increase lattice and Brønsted acid site polarization, facilitating both proton transfer and protonated product stabilization. This configuration was predicted to reduce both the DPEH-ZSM-5 and increase the stabilization interaction for protonated adsorbates.

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