(270e) Inhibition of Xylene Isomerization in the Production of Renewable Aromatic Chemicals from Biomass-Derived Furans

High selectivities
(>90%) have been achieved with an H-BEA catalyst in the Diels-Alder
cycloaddition, and subsequent dehydration, of 2,5-dimethylfuran (DMF) and
ethylene to produce p-xylene.^1-4 This success is unexpected
due to the ability of xylene to isomerize in the presence of zeolite acid
catalysts at similar conditions. In this work, three possible mechanisms describing
the conversion of 2,5-dimethylfuran to p-xylene were evaluated, where the
dominant mechanism was indicated as the homogeneous, uncatalyzed cycloaddition
of DMF and ethylene, followed by an acid catalyzed dehydration (Figure 1a).⁵
The inhibition of xylene isomerization was attributed to large active site
coverage of 2,5-hexanedione, the ring opened product of DMF. This is supported
by the similar adsorption enthalpies of 2,5-hexanedione and the cycloadduct
versus the less favorable adsorption enthalpy of p-xylene (Figure 1b). Though
all reaction components were independently examined, only the presence of 2,5-dimethylfuran
and 2,5-hexanedione, even at 1/50^th the concentration of xylene, inhibited
transalkylation and methyl shift reactions of p-xylene on H-Y (Si/Al
2.6) and H-BEA (Si/Al 12.5) zeolites. A combination of experimental techniques
(thermogravimetric analysis, TGA, diffuse reflectance infrared Fourier transform
spectroscopy, DRIFTS, and ²⁷Al NMR) and ONIOM computational method
allowed for the investigation of adsorption energetics of relevant species on
zeolite acid active sites.

Figure
1. Proposed mechanisms of reaction of 2,5-dimethylfuran and ethylene to p-xylene
and adsorption enthalpies of compounds in reaction mixture. A) Proposed catalytic
cycles converting dimethylfuran to p-xylene. Brønsted acid sites within
H-BEA and H-Y zeolite catalyze conversion of DMF and ethylene to p-xylene
and water.
B) Heat
of adsorption versus proton affinity in H-Y zeolite. The heat of adsorption
correlates linearly with proton affinity above 200 kcal mol⁻¹. Deviation from
the linear proton affinity prediction exists below 200 kcal mol⁻¹.

[1] Williams,
C. L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas, S.; Vlachos, D. G.; Lobo,
R. F.; Fan, W.; Dauenhauer, P. J. ACS Catal., 2012, 2, 935-939.

[2] Chang,
C.-C.; Green, S. K.; Williams, C. L.; Dauenhauer, P. J.; Fan, W. Green Chem.
2014, 16, 585-588.

[3] Patet,
R. E.; Nikbin, N.; Williams, C. L.; Green, S. K.; Chang, C.-C.; Fan, W.;
Caratzoulas, S.; Dauenhauer, P. J.; Vlachos, D. G. ACS Catal., 2015, 5,
2367-2375.

[4] Williams,
C. L.; Vinter, K. P.; Chang, C.-C.; Xiong, R.; Green, S. K.; Sandler, S. I.;
Vlachos, D. G.; Fan, W.; Dauenhauer, P. J. Catal. Sci. Tech., 6,
178-187.

[5] Williams,
C. L.; Vinter, K. P.; Patet, R. E.; Chang, C.-C.; Nikbin, N.; Feng, S.;
Wiatrowski, M. R.; Caratzoulas, S.; Fan, W.; Vlachos, D. G.; Dauenhauer, P. J. ACS
Catal., 2016, 6, 2076-2088.