Conversion of Isobutanol to Olefins and Aromatics over HZSM-5-Based Catalysts: Tuning of Product Selectivity

Abstract

Isobutanol derived from biomass can serve as a potential renewable candidate for the production of light olefins and aromatics, which are the building blocks to manufacture fuels and chemicals in the petrochemical industry. Dependent upon the product requirements, the product selectivity toward olefins or aromatics can be tuned by adjusting the reaction conditions and the properties of HZSM-5 catalysts. On bare HZSM-5, mild conditions (low temperature, low space time, and low acidity) favored the production of olefins and harsh conditions (high temperature, high space time, and high acidity) were more likely to produce aromatics. Ga was impregnated onto HZSM-5 to enhance the production of aromatics. Unexpectedly, Ga introduction only induced the intertransformation between olefins and aromatics without significantly impacting the selectivity of low-value paraffins. The exchanged Ga sites functioning together with Brønsted acid were identified as active Ga species and strengthened the direct dehydrogenation of C6–C8 olefins. The in situ-generated water from isobutanol dehydration affected the formation of Ga species. This resulted in different promotional effects of Ga on the transformation of isobutanol and isobutene. The reaction pathways were developed on the basis of the catalyst evaluation results, in tandem with an in situ diffuse reflectance infrared Fourier transform spectroscopy study on the intermediates formed during isobutanol transformation. Selectivity of olefins and aromatics from isobutanol transformation can be tuned by adjusting reaction conditions and properties of HZSM-5 catalysts.

Bioisobutanol as a Promising Feedstock for Production of “Green” Hydrocarbons and Petrochemicals (A Review)

“…5а, 5b). Du et al [93] have shown that, when performing the isobutanol conversion on the 2% Ga 2 O 3 /SiO 2 catalyst for a long time, the cis/trans-2-butene ratio increases; this trend is attributed to a decrease in the yield of trans-butene due to catalyst deactivation.…”

“…2а-2c). Du et al [93] attribute high content of ethylene and propylene in products of isobutanol transformations on HZSM-5 to the cracking of oligomers formed by oligomerization of isobutene as a primary product of isobutanol dehydration.…”

“…Du et al [93] studied transformation of isobutanol into olefins and aromatic hydrocarbons on HZSM-5 zeolite and suggested the product formation scheme shown in Fig. 5.…”

“…The isobutene and butenes formed by dehydration and isomerization dimerize to form С 8 olefins, which can subsequently transform into С 2 -С 5 and С 6 -С 8 olefins by reversible cracking and oligomerization reactions. Du et al [93] believe that direct cyclization and dehydrogenation of С 6 -С 8 olefins occur to a minor extent and make a minimal contribution to the formation of aromatic compounds. They assume that aromatic hydrocarbons and low-molecular-mass С 2 -С 5 alkanes are formed by hydrogen transfer reactions between С 2 -С 5 and С 6 -С 8 olefins.…”

“…Among aromatic compounds, С 8 hydrocarbons (ethylbenzene and xylenes) prevail; they are formed by the reaction of two butene/ isobutene molecules and dehydrocyclization of dimers. On the other hand, Du et al [93] believe that benzene and toluene are formed by secondary processes such as cracking of С 8 olefins and oligomerization of С 2 -С 5 olefins. The authors note that introduction of gallium did not prevent the hydrogen transfer reactions, because the selectivity of formation of С 2 -С 5 alkanes remained virtually the same.…”