Effects of functional groups on physicochemical properties of carbon and zeolites

Zhang, Zhongyao

Permalink

https://hdl.handle.net/2142/115500 Copy

Description

Title

Effects of functional groups on physicochemical properties of carbon and zeolites

Author(s)

Zhang, Zhongyao

Issue Date

2021-12-16

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

Flaherty, David W

Doctoral Committee Chair(s)

Flaherty, David W

Committee Member(s)

• Seebauer, Edmund G
• Guironnet, Damien S
• Fout, Alison R

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)

Surface function, catalysis, carbon, zeolites

Abstract

Surface functional groups can affect the physical and catalytical properties of materials. Surface functions on carbon materials can influence their mechanical strength and catalytical activities but characterizing surface functions is challenging due to light-absorbance property. Zeolitic materials are also featured with silanol groups that influence hydrophobicity of zeolites. Here, we develop methods to identify and quantify surface functions on carbon materials and investigate the influence of surface functions on mechanical strength of carbon fiber composites. For zeolite materials, Ti atoms are incorporated into different frameworks with varying densities of silanol groups. Aldol condensation of ethanol is investigated on Ti-substituted zeolites to elucidate the effects of silanol groups and topologies on reaction rates, selectivities and mechanisms. CHAPTER 1 discusses about the importance of surface functions on carbon and zeolite materials, and how these surface functions can potentially affect physical and chemical properties. Physically, the surface functions on carbon materials can interact with each other at interfaces of materials and form new chemical bonding that contributes to the shear strength. Chemically, functions within zeolite materials can affect the stability of reactive intermediates that are confined within small pores, and such interaction further influences rates and selectivities of ethanol upgrading through aldol condensation reactions. In CHAPTER 2 and 3, we develop titration methods for characterizing surface functions on carbon materials. Titration methodologies, specifically potentiometric and Boehm titrations, classify the types of acidic functions present on carbon materials into broad categories described as carboxylic, lactonic, and phenolic functions based upon the measured or estimated acid dissociation constants (pKa). The accuracy of these methods depends, however, on the estimates for pKa values and suffers when multiple possible species possess similar pKa values. Here, we improve upon past methods by combining potentiometric and chemical titrations, which involves identifying acidic functions by pKa values but also examining how functional groups react with alkaline titrants. Specifically, Brønsted acids like carboxylic and phenolic groups deprotonate reversibly when contacted with alkaline titrant solutions; however, lactonic functions undergo hydrolysis to form carboxylic acids and hydroxyls during identical treatments. Consequently, comparisons between sequential potentiometric titrations can be used to discriminate between functional groups by examining their pKa values but also their reactivity. Similar concept is extended to nitrogen functions on carbon materials combining chemical reactivity and pKa, which works well for amines, pyridinic nitrogen and amides on carbon materials. In CHAPTER 4, we investigate the effect of surface functions on mechanical strength of carbon fiber composites. The morphology and chemical function of surfaces strongly influence adhesion at interfaces within carbon fiber composites (i.e., carbon fiber reinforced polymer) and the mechanical strength of the composite materials. Reactive plasma discharges modify these properties of composite surfaces and can improve adhesion. However, the outcome depends upon the specific composite adhesive used, which is likely due to subtle difference in the formulation of these components. Here, we select a model system widely used for aerospace applications (T300 composites formed by the combination of T300 fiber, Cycom 934 epoxy resin, and FM377 adhesives) to investigate the influence of oxygen (O2) plasma treatments on the topology and chemistry of composite surfaces and the differences in mechanical strengths correlated to these changes. Lap shear tests show that shear strengths increase by 10% because of low-pressure O2 plasma treatments. These plasma treatments increase surface roughness by two-fold, which may improve mechanical modes of interlocking at composite-adhesive interfaces. Chemical analysis of composite surfaces by X-ray photoelectron spectroscopy (XPS) shows that treated composites expose a greater number of imide moieties (N1s binding energies near 400.7 eV), and reactive titrations with fluorescent probe molecules corroborate these results. The differences in the surface coverage of imide groups strongly correlates with the shear strength of these composites, and both increase with the power and time of the O2 plasma treatment, which is presumably due to greater cumulative exposure to reactive oxygen species. The imide functions that promote adhesion likely form by the oxidation of aniline derivatives present in the T300 composite, and we propose these functions form covalent bonds with epoxides within the adhesives to improve interfacial chemical bonding. These findings suggest that O2 plasmas increase both mechanical interlocking and interfacial chemical bonding, and thus, provide greater shear strengths for carbon fiber composites. In CHAPTER 5 and 6, we study aldol condensation of ethanol and acetaldehyde on Ti-substituted zeolites (Ti-zeolite). 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, but the mechanisms remain unclear. 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 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 aldol condensation and esterification occur on unoccupied Ti sites and involve multiple kinetically relevant steps. First, CH3CHO deprotonates irreversibly, and subsequently, kinetically relevant nucleophilic attack of the enolate to CH3CHO* (i.e., adsorbed CH3CHO on Ti sites) leads to aldol products while 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 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 confer 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. Similar mechanistic study is also performed on other Ti-silicates (MFI, FAU and mesopore SiO2) to examine the mechanism of aldol condensation and esterification. Measured pressure dependences of reaction rates of aldol condensation and esterification on CH3CHO and C2H5OH are the same across all Ti-silicates, which suggests the proposed mechanism of aldol condensation and esterification remains the same for all Ti-silicates. Cascade aldol condensation is investigated on physical mixture of Cu catalysts and Ti-silicates with C2H5OH as the only carbon source. As mean pore diameter increases from MFI to BEA, FAU and SiO2, selectivity of primary C4 products tend to decrease at similar conversions, which confirms the confinement effect on product distribution. On Ti-MFI materials, lower temperature tends to show larger selectivity to primary C4 aldol condensation products, while product distribution on Ti-SiO2 is almost independent of temperature changes. Collectively, these results clarify the mechanism of aldol condensation on Ti-zeolites and the effects of micropores, which assists the rational design of selective catalysts for aldol condensation of ethanol. CHAPTER 7 shows the future directions proposed to achieve a better understanding of ethanol upgrading reactions through aldol condensation reactions by enhancing the interactions among reactive intermediates, solvents and zeolite pores. Proper use of solvent can increase reaction rates by orders of magnitude (e.g., epoxidation of 1-octene), and these motivate us to investigate how solvation effect can be enhanced for aldol condensation and how rate and selectivities of aldol condensation can be affected. We seek to explore pore condensation conditions of ethanol aiming to have ethanol molecules condensed within micropores of zeolites to create liquid-like environment to enhance the interactions between solvent and reactive intermediates. Future work will focus on how the mechanism, rates and selectivities of aldol condensation can be affected at pore condensation conditions. Another future direction focuses on interactions between zeolite pores and reactive intermediates. Smaller pores can destabilized the large transition states that are responsible for the formation of bulky products, and further tune the selectivities of aldol condensation products. Currently MFI is the zeolite with smallest mean pore diameter (0.55 nm) among the silicates tested, and we plan to incorporate Ti atoms into frameworks of CHA with smaller pores (0.38 nm) to strengthen the dispersive interaction between pore walls and reactive intermediates. Collectively, these results demonstrate how surface functions on carbon and zeolites can affect the physical and chemical properties of the materials. Proper design of surface functions can facilitate rational choice of materials for applications in structural materials and catalysts.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2021 Zhongyao Zhang

Owning Collections