Silica-supported heteropoly acids as catalysts for low-temperature dehydration of 1-butanol in the gas phase

Introduction:
The dehydration of 1-butanol (BuOH) to linear butene isomers is an important industrial reaction because butenes are key feedstocks for the production of polymers, synthetic rubber, and high-performance fuels. Conventional zeolite catalysts, such as H-ZSM-5, H-Mordenite, and HY, are widely used for this transformation; however, they often suffer from deactivation due to coke formation at elevated temperatures. Keggin-type heteropoly acids (HPAs), particularly H₃PW₁₂O₄₀ (HPW) and H₄SiW₁₂O₄₀ (HSiW), exhibit stronger Brønsted acidity than zeolites and have emerged as promising solid acid catalysts for low-temperature alcohol dehydration. In this work, silica-supported HPA catalysts were investigated for the gas-phase dehydration of 1-butanol and benchmarked against conventional zeolite catalysts under comparable conditions.

Results and Discussion:
The silica-supported catalysts, 25% HPW/SiO₂ and 25% HSiW/SiO₂, showed significantly higher catalytic activity than zeolite catalysts. At 130°C and a BuOH partial pressure of 2.0 kPa, both catalysts achieved 99% BuOH conversion with 99% total butene selectivity. The distribution of n-butene isomers followed the order: 1-butene (8.4%) < cis-2-butene (30.3%) < trans-2-butene (61.3%). In contrast, zeolite catalysts required higher temperatures (200–220°C) to reach similar conversion levels. The HPA catalysts also demonstrated excellent stability, maintaining performance for at least 24 h on stream with only minor coke formation (2–3%). Kinetic analysis indicated that the reaction follows a Langmuir-type adsorption mechanism and becomes zero-order in BuOH at partial pressures above 1 kPa. Furthermore, a clear positive correlation was observed between Brønsted acid strength and turnover frequency (TOF). Catalyst activity increased with HPA loading up to approximately 40%, after which it reached a plateau, suggesting a surface-type catalytic mechanism.

Conclusion:
Silica-supported Keggin-type HPA catalysts are highly efficient for the low-temperature gas-phase dehydration of 1-butanol, offering superior activity, selectivity, and stability compared with conventional zeolites. These results highlight their strong potential for sustainable and energy-efficient production of n-butene isomers from bio-butanol.