(541c) Reaction of Bio-Alcohols in H-FAU, H-Mor, H-ZSM-5 and H-ZSM-22

Cuong M. Nguyen, Marie-Francoise Reyniers and Guy B. Marin

Laboratory for Chemical Technology, Ghent University, Krijgslaan 281 S5, B-9000 Gent, Belgium.

Of recent interest is the green production of bio-olefins from renewable resources with lower energy consumption and lower carbon dioxide emission. This work systematically investigates the mechanism and kinetics of the conversion of ethanol in H-FAU, H-MOR, H-ZSM-5 and H-ZSM-22 at industrially relevant temperatures (400-800 K) by combining periodic PBE-D//PBE-D density functional theory calculations with statistical thermodynamics [1,2]. To probe the effect of the alcohol structure, the reaction of primary, secondary and tertiary butanols is also sampled in H-FAU.

The conversion of alcohol into olefin proceeds through a step-wise mechanism: (i) dehydration into the alkoxide intermediate and water and (ii) decomposition of the alkoxide to the olefin. In the four zeolites, the total conversion rate of ethanol is governed by the initial dehydration step (E_a = 129-165 kJ mol^-1) while significantly lower activation energies are found for the second step (E_a = 92-97 kJ mol^-1). Also, the medium-pore zeolites (H-ZSM-5 and H-ZSM-22) stabilize the transition states to a greater extent than the large-pore zeolites (H-FAU and H-MOR) leading to a decrease in activation energy of up to 36 kJ mol^-1[2]. For H-MOR, the activation energy in the eight-membered ring side pocket is about 10 kJ mol^-1higher than that in the twelve-membered ring channel, in agreement with experimental work [3]. At 400 K, the dehydration of ethanol is 2 to 3 orders of magnitude faster in the medium-pore zeolites than in the large-pore ones. However, the difference in reactivity between the two zeolite groups becomes less pronounced with increasing temperature.

Whereas the conversion of primary and secondary butanols proceeds via the alkoxide intermediate, tertiary butanol is initially dehydrated into the tertiary carbenium ion. The existence of tertiary carbenium ion as a minimum is attributed to the stabilization effect of the concurrent water molecule. Without water, the tertiary carbenium ion could not be located in H-FAU [2]. The difference in activation energy between secondary/tertiary butanol and primary butanol amounts to 40 kJ mol^-1.

References

[1] C. M. Nguyen, M. F. Reyniers, G. B. Marin, J. Phys. Chem. C, 2011, 115, 8658.

[2] C. M. Nguyen, B. A. De Moor, M. F. Reyniers, G. B. Marin, submitted to J. Phys. Chem. C.

[3] H. Chiang and A. Bhan, J. Catal. 2010, 271, 251.