Density Functional Theory Study of Mixed Aldol Condensation Catalyzed by Acidic Zeolites HZSM-5 and HY

详细信息    查看全文

• 作者：Angela N. Migues ; S. Vaitheeswaran ; Scott M. Auerbach
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：September 4, 2014
• 年：2014
• 卷：118
• 期：35
• 页码：20283-20290
• 全文大小：390K
• ISSN：1932-7455

文摘

We applied density functional theory to investigate the mixed aldol condensation of acetone and formaldehyde in acid zeolites HZSM-5 and HY, as a prototypical bond-forming reaction in biofuel production. We modeled the acid-catalyzed reaction in HZSM-5 and HY in two steps: keto鈥揺nol tautomerization of acetone and bimolecular condensation between formaldehyde and the acetone enol. For both acid zeolites, the keto鈥揺nol tautomerization of acetone was found to be the rate-determining step, consistent with the accepted mechanism in homogeneous acid-catalysis. Convergence studies of the activation energy for keto鈥揺nol tautomerization, with respect to cluster sizes of HZSM-5 and HY, exhibit rather different convergence properties for the two zeolites. The keto鈥揺nol activation energy was found to converge in HY to 鈭?0 kcal/mol for a cluster with 11 tetrahedral atoms (11T cluster), which does not complete the HY supercage. In contrast, the activation energy for HZSM-5 reaches an initial plateau at a value of 鈭?8 kcal/mol for clusters smaller than 20T and then converges to 鈭?0 kcal/mol for clusters of size 26T or greater, well beyond the completion of the HZSM-5 pore. As such, completing a zeolite pore surrounding a Br酶nsted acid site may be insufficient to converge activation energies; instead, we recommend an approach based on converging active-site charge.