Key Takeaways & Executive Findings
- •• • Zeolite type dictates dodecane conversion: SAPO-34 (0.38 nm pores) yields only 24.33% conversion, whereas ZSM-5-38 and USY achieve near-complete conversion, underscoring pore size as a primary selectivity filter. • • ZSM-5-38, with moderate acidity, delivers the highest light olefin yield (18.40%) among different zeolite types, while minimizing coke (0.18%), indicating an optimal balance between cracking and hydrogen transfer. • • USY, with higher acid density, over-promotes hydrogen transfer and coking, producing light alkanes and 12.90% coke, demonstrating that excessive acidity is detrimental to olefin yield. • • Reducing acid site density in ZSM-5 (ZSM-5-200) boosts total light olefin yield to 41.93% (ethylene 11.11%, propylene 20.33%, butenes 10.49%) at 97.79% conversion, with coke remaining low at 0.43%, offering a clear strategy for process optimization.
Abstract
Catalytic cracking of gasoline and diesel to light olefins is a pivotal route for high-value utilization of surplus fuels, typically employing zeolite catalysts. This study systematically investigates the effects of zeolite type and acidic properties on the catalytic cracking of dodecane, a diesel model compound, using SAPO-34, ZSM-5 with SiO2/Al2O3 ratios of 38, 85, and 200, and USY. Catalysts were characterized by XRD, SEM, N2 physisorption, NH3-TPD, and pyridine-FTIR, and evaluated in a fixed-bed reactor. Results demonstrate that zeolite type is the primary determinant of conversion and product distribution. SAPO-34, with 0.38 nm pores, achieved only 24.33% conversion and negligible BTX yield, with severe external coking. ZSM-5-38 and USY, with larger pores, achieved near-complete conversion; however, ZSM-5-38, possessing moderate acidity, yielded the highest light olefins (18.40%) and minimal coke (0.18%), while USY, with higher acidity, promoted hydrogen transfer and coking (12.90% coke). Within ZSM-5 series, lower acid site density (ZSM-5-200) proved optimal, achieving 97.79% conversion and a total light olefin yield of 41.93% (ethylene 11.11%, propylene 20.33%, butenes 10.49%) with low coke (0.43%). The study proposes reaction pathways and regulatory mechanisms, highlighting that zeolite type and acidity govern the relative rates of cracking, hydrogen transfer, oligomerization, aromatization, and coking, thereby dictating performance. These findings provide a rational basis for optimizing zeolite catalysts in commercial gasoline/diesel cracking processes.
1. Introduction
Commercial catalytic cracking of gasoline and diesel fractions to light olefins is constrained by the trade-off between conversion and selectivity. Conventional zeolite catalysts, such as USY, exhibit high activity but suffer from excessive hydrogen transfer and coking, leading to poor olefin yields and rapid deactivation. Conversely, small-pore SAPO-34 offers high olefin selectivity but suffers from severe mass transport limitations and low conversion of larger feed molecules like dodecane. The lack of systematic understanding of how zeolite topology and acidity independently influence the complex reaction network has hindered rational catalyst design.
This study addresses this bottleneck by systematically varying zeolite type (SAPO-34, ZSM-5, USY) and acidity (via SiO2/Al2O3 ratio) while keeping reaction conditions constant. By correlating detailed characterization (XRD, SEM, N2 physisorption, NH3-TPD, pyridine-FTIR) with catalytic performance in a fixed-bed reactor, the authors delineate the individual roles of pore architecture and acid site density. The identification of ZSM-5-200 as an optimal catalyst, achieving 41.93% light olefin yield at 97.79% conversion, provides a concrete benchmark for developing next-generation catalysts for diesel cracking.
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MA Wenshuo, YANG Zhihao, CHEN Mengxin, WANG Jingxian, TIAN Yuanyu, QIAO Yingyun (2026). Effects of Zeolite Type and Acidic Properties on the Catalytic Cracking Performance of Dodecane. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0034
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Frequently Asked Questions
What is the impact of pore size on dodecane conversion and product distribution across different zeolite frameworks?
Pore size critically determines accessibility and shape selectivity. SAPO-34 (0.38 nm) restricts dodecane diffusion, resulting in only 24.33% conversion and negligible BTX yield, with severe external coking. ZSM-5 (0.55 nm) and USY (0.73 nm) allow near-complete conversion, but USY's larger pores facilitate hydrogen transfer and coke formation (12.90% coke), whereas ZSM-5's moderate pores favor olefin production (18.40% light olefins).
How does the SiO2/Al2O3 ratio in ZSM-5 influence acid site density and catalytic performance?
Increasing SiO2/Al2O3 ratio from 38 to 200 reduces total acid site density, particularly Brønsted acid sites. ZSM-5-200, with the lowest acidity, achieves 97.79% conversion and the highest light olefin yield (41.93%) among ZSM-5 samples, while maintaining low coke (0.43%). This indicates that lower acid site density suppresses hydrogen transfer and secondary reactions, enhancing olefin selectivity.
What are the main deactivation mechanisms observed, and how do they correlate with zeolite properties?
Coking is the primary deactivation route. SAPO-34 suffers severe external coking due to its small pores, leading to pore blockage. USY, with large pores and high acidity, promotes hydrogen transfer and coke formation (12.90% coke yield). In contrast, ZSM-5-38 and ZSM-5-200 exhibit minimal coke (0.18% and 0.43%, respectively), attributed to moderate pore size and reduced acid site density, which limit coke precursors.
Can the findings be scaled to industrial diesel cracking processes, and what are the potential operational advantages?
The identification of ZSM-5-200 as an optimal catalyst suggests that tailoring acidity can significantly improve light olefin yield (41.93%) at high conversion (97.79%) with low coke, potentially extending catalyst lifetime and reducing regeneration frequency. However, industrial validation is needed to assess long-term stability, mechanical strength, and performance with real diesel feeds containing sulfur and nitrogen compounds.
What is the proposed reaction pathway for dodecane cracking over these zeolites, and how does acidity modulate it?
The pathway involves initial cracking to lighter alkanes and olefins, followed by secondary reactions: hydrogen transfer, oligomerization, aromatization, and coking. High acidity (USY) accelerates hydrogen transfer and coking, yielding alkanes and coke. Moderate acidity (ZSM-5-38) balances cracking and hydrogen transfer, favoring olefins. Low acidity (ZSM-5-200) further suppresses hydrogen transfer, maximizing olefin yield while maintaining high conversion.
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