Catalytic routes for oxygenate conversion on solid acids and supported metal nanoparticles

Berdugo Diaz, Claudia Eugenia

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https://hdl.handle.net/2142/121184 Copy

Description

Title

Catalytic routes for oxygenate conversion on solid acids and supported metal nanoparticles

Author(s)

Berdugo Diaz, Claudia Eugenia

Issue Date

2022-09-29

Director of Research (if dissertation) or Advisor (if thesis)

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Fout, Alison R
• Guironnet, Damien S
• Kenis, Paul J. A.

Department of Study

Chemical & Biomolecular Engr

Discipline

Chemical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

hydrogenation, hydrogenolysis, hemiacetal, bifunctional catalysis

Abstract

As an abundant and renewable carbon-based resource, biomass provides a promising feedstock to produce value-added chemicals and fuels. However, biomass contains a significant amount of oxygen, and consequently catalytic deoxygenation of bioderived feedstocks provides a promising strategy to increase the energy density and increase hydrothermal stability of bio-derived products. Thus, a fundamental understanding on how oxygenates adsorb, activate, and react upon solid surfaces in essential for catalyst design and reaction engineering. Here, we use a combination of experimental synthesis, kinetic, spectroscopic, and thermodynamic tools and analyses to develop quantitative structure-activity relationships for two catalytic routes: aldol condensation and ester reduction using solid acids and supported metal nanoparticles. Chapter 1 discusses the relevance of studying deoxygenation of bioderived feedstocks and the importance of understanding reaction mechanisms, catalyst properties and structure-function relationships to improve the activity and selectivity of processes involving heterogeneous catalysts. We discussed the current understanding of aldol condensation and esterification reaction networks on Lewis acid catalysts and highlight how framework-substituted zeolites emerge as promising catalysts for the upgrading of abundant bioderived ethanol. Chapter 1 also illustrates advances in knowledge of the reaction networks and mechanisms, key reactive intermediates, and active site motifs that allow for the selective production of desired compounds by reduction of esters over heterogeneous catalysts. These insights draw from the mechanistic understanding gained from comparisons between kinetic measurements, in situ characterization of catalysts and organic intermediates, and ab initio calculations. Chapter 2 presents our examination on aldol condensation and esterification reactions that 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. 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. Chapter 3 discusses the pathways for reactions of propyl acetate (PA) in the presence of hydrogen (H2) over Pd nanoparticles supported on high surface area Nb2O5. Over Pd-Nb2O5, PA reacts by three competing primary pathways: hydrogenation to form ethyl propyl ether (EPE) by apparent C=O bond rupture, hydrogenolysis to form acetaldehyde and propanol (Cacyl–O bond rupture), and hydrolysis to form acetic acid and propanol. Secondary reactions yield other alcohols, esters, ethers, and hydrocarbons. Hydrogenation and hydrogenolysis rates do not change with the pressure of PA and increase with a sublinear dependence on H2 pressure. Furthermore, these dependencies and apparent activation enthalpies remain similar for Pd nanoparticles with different mean diameters (4–22 nm), which shows the extent of undercoordination of Pd does not significantly affect the mechanism or kinetics for C–O bond rupture steps. Ether formation rates remain constant when D2 replaces H2 as the reductant, which together with the rate dependence on H2 suggests ethers form by kinetically relevant C–O bond cleavage in a partially hydrogenated intermediate (e.g., hemiacetal). Ex situ titration of Brønsted acid sites by exchange with K+ ions suppresses mass-averaged rates of EPE formation by seven-fold, and physical mixtures of Pd-SiO2 and Nb2O5 give rates more than ten times lower than Pd-Nb2O5. These results demonstrate that C5H12O formation requires Brønsted acid sites that reside in close proximity to Pd nanoparticles. Collectively, these observations suggest a reaction mechanism for the reduction of esters that hydrogenates the carbonyl by stepwise addition of H*-atoms to form a hemiacetal that dehydrates at proximal Brønsted acid sites or cleaves the Cacyl–O bond to form lower carbon number products. These findings reveal a pathway to convert renewable oxygenates, such as carboxylic acid derivatives, into value-added chemicals useful as surfactants and solvents. Chapter 4 shows that esters reduce to form ethers and alcohols on contact with metal nanoparticles supported on Brønsted acidic faujasite (M-FAU) that cleave C-O bonds by hydrogenation and hydrogenolysis pathways. Rates and selectivities for each pathway depend on the metal identity (M = Co, Ni, Cu, Ru, Rh, Pd, and Pt). Pt-FAU gives propyl acetate consumption rates up to 100-times greater than other M-FAU catalysts and provides ethyl propyl ether selectivities of 34%. Measured formation rates, kinetic isotope effects, and site titrations suggest that ester reduction involves a bifunctional mechanism that involves the stepwise addition of H* atoms to the carbonyl to form hemiacetals on the metal sites, followed by hemiacetal diffusion to nearby Brønsted acid site to dehydrate to ethers or decompose to alcohol and aldehyde. The rates of reduction of propyl acetate appear to be determined by the H* addition to the carbonyl bond and by the C-O cleavage of hemiacetal species. Chapter 5 evaluates the effect of metal-to-acid ratio and Brønsted acid sites density on rates and selectivities for reduction of PA with H2 over platinum nanoparticles supported on Brønsted acidic faujasite zeolite (Pt-FAU) catalysts prepared by incipient wetness impregnation. Infrared spectroscopy of CO show that Pt species exist in the metallic state and the electronic properties of exposed Pt sites (Pts) do not vary significantly with metal-to-acid ratio. Temperature-programmed reaction of amines adsorbed into Brønsted acid sites demonstrate that accessible Brønsted acid sites (H+) decrease with increasing Pt density. CO Hydrogenation formation rates increase by 13-fold, and hydrogenolysis formation rates increase by 33-fold when the atomic ratio of Pts to H+ increases from 0.08 to 0.6. Hydrogenation and hydrogenolysis formation rates that increase with Pts density and show no correlation with H+ density indicate kinetically relevant steps on Pt sites. These elementary steps likely involve the formation of hydrogenated intermediates (i.e., hemiacetal) than communicate with Brønsted acid sites by diffusion. The ratio of hydrogenation to hydrogenolysis turnover rates decreases with increasing the atomic ratio of Pts to H+ and subsequently become constant. Thus, differences in selectivities show that C-O cleavage steps are also kinetically relevant and become more significant with increasing atomic ratio of Pts to H+. These correlations confirm that ester reduction rates and selectivities depend sensitively on the metal-to-acid ratio and proximity between these functions, like metal-acid bifunctional catalysts used in processes industrially established (i.e., hydroisomerization and hydrocracking). Chapter 6 summarizes the findings from Chapter 2-5 on the catalytic routes for oxygenate conversion on solid acids and supported metal nanoparticles. Chapter 6 also includes the proposed future research directions to further the understanding of direct reduction of esters to ethers and the design of selective catalysts. Collectively, this dissertation demonstrates structure-function relationships for alcohol aldol condensation and reduction of carboxylic acid derivatives. Fundamental understanding of this structure-function relationships will enable the rational design of catalysts for biomass upgrading.

Graduation Semester

2022-12

Type of Resource

Thesis

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