Aldol Condensation and Esterification over Ti-Substituted *BEA Zeolite: Mechanisms and Effects of Pore Hydrophobicity

Journal Article · Mon Jan 10 00:00:00 EST 2022 · ACS Catalysis

DOI:https://doi.org/10.1021/acscatal.1c04518· OSTI ID:1902709

Zhang, Zhongyao  ^[1]; Berdugo-Díaz, Claudia E.  ^[2]; Bregante, Daniel T.  ^[2]; Zhang, Hongbo  ^[2]; Flaherty, David W.  ^[1]

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1. DOE Center for Advanced Bioenergy and Bioproducts Innovation, Urbana, IL (United States); Univ. of Illinois at Urbana-Champaign, IL (United States)
2. Univ. of Illinois at Urbana-Champaign, IL (United States)

Aldol condensation and esterification reactions provide paths to upgrade ethanol and acetaldehyde to higher- value molecules useful as fuels or intermediates for the synthesis of polymers. Transition-metal-substituted BEA zeolites (M-BEA) catalyze these reactions; however, the mechanisms for these processes in M-BEA and the effects of incidental or purposefully included silanol groups are not reported. Here, we combine kinetic and spectroscopic measurements obtained during catalytic reactions of acetaldehyde (CH₃CHO), ethanol (C₂H₅OH), and hydrogen (H₂) mixtures over a series of Ti-BEA catalysts that possess a known range of silanol group densities to examine the kinetic relevance of intervening steps and the impact of silanol groups on catalytic rates. Across all Ti-BEA, rates for aldol condensation and esterification increase with the pressure of CH₃CHO; however, C₂H₅OH and H₂O weakly inhibit the rates of these reactions. The substitution of CD₃CDO for CH₃CHO decreases aldol condensation rates slightly (~10%) but leads to greater esterification rates (2- to 5-fold). The kinetic isotope effects together with the measured dependence of rates on reactant pressures suggest that aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps. CH₃CHO deprotonates irreversibly, and the kinetically relevant nucleophilic attack of the enolate to CH₃CHO* (i.e., adsorbed CH₃CHO on Ti sites) leads to aldol products, while the nucleophilic attack of the enolate to C₂H₅OH* gives esters. Selectivities toward aldol condensation increase with the ratio of CH₃CHO to C₂H₅OH pressure and with increases in the silanol density of the as-synthesized Ti-BEA. During catalysis, in situ infrared spectroscopy demonstrates that these silanol groups react with C₂H₅OH to form ethoxysilane groups (i.e., SiOC₂H₅) that modify the polarity of the environment near Ti active sites. As initial silanol densities increase, steady-state turnover rates for aldol condensation and esterification increase by factors of 5 and 2, respectively. The changes in rates and selectivities among Ti-BEA catalysts likely reflect changes in excess free energies of transition states for enolization and nucleophilic attack of the enolate to adsorbed coreactants. The differences in excess stability report on the interactions among reactive intermediates at framework Ti atoms and the ethoxysilane and remaining silanol groups present. Furthermore, the in situ modification of these pore environments confers changes in the stability of reactive species in a manner that contradicts intuition when considering the initial state of the catalyst but can be reconciled after accounting for the formation of persistent alkoxy surface moieties in the pores.

• Research Organization: Univ. of Illinois at Urbana-Champaign, IL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Biological and Environmental Research (BER)
• Grant/Contract Number: SC0018420
• OSTI ID: 1902709
• Journal Information: ACS Catalysis, Vol. 12, Issue 2; ISSN 2155-5435
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
silanol
alkoxysilane
mechanism
hydrophobicity
kinetic isotope effect
in situ
aldol reactions
catalysts
materials
organic reactions
oxides