Investigation of the Effects of Solid-State Treatments on the Structure and Mobility of Copper in Zeolites

Zeolites are microporous, aluminosilicate catalysts that play an important role in industrial applications as well as studies for the fundamental understanding of catalysts for emerging reactions of interest. The introduction of aluminum into the zeolite lattice introduces a negative charge on the framework that can be balanced with extra-framework cations. The control of the aluminum distribution and the choice of charge balancing cations allows for the ability to tailor the active sites to facilitate a desired reaction. This research focuses on studying copper active sites in zeolites. Copper oxide was used as a copper precursor to introduce copper ions in zeolites through solid-state ion-exchange (SSIE). Solid-state ion-exchange was studied using both dry air and wet air treatments at elevated temperatures. Three different zeolite topologies were studied: CHA (small pore), ZSM-5 (medium pore), and MOR (large pore). After SSIE, the copper-zeolites were characterized with atomic absorption spectroscopy (AAS) after sodium back exchange to quantify the number of ionic copper species, and temperature programmed desorption (TPD). These characterization techniques were used to understand how many copper ions were mobilized into the zeolites, which are potential active sites in zeolites. Based on current experimental data on Cu-MOR, SSIE using a wet air treatment has a greater impact for mobilizing copper in zeolites compared to a dry air treatment. The same trend is expected to follow on other zeolite topologies, ZSM-5 and CHA, that are still being studied

Synthesis and characterization of microporous and mesoporous catalysts for shale gas upgrading

The petroleum industry is gradually shifting from using naphtha to ethane as its feedstock and new ethane steam crackers are being built. Using ethane as a feedstock produces more ethene and less co-products such as propene, aromatics and C4 alkenes. Thus, the shift in feedstock will reduce the availability of these co-products. Developing methods for the interconversion of alkenes, and specifically using ethene as a reactant, can solve this problem. This project focuses on synthesis and characterization of zeolites with the BEA framework topology, and their nickel exchanged versions as potential ethene dimerization catalysts. In future work, these materials will be used to carry out catalytic studies for ethene dimerization. Nickel exchanges were carried out using nickel nitrate Ni(NO3)2 solution on commercial aluminosilicate samples from Clariant (HCZB-25 Si/Al=12.5) and Zeolyst (CBV Si/Al=12.5) and an in-house synthesized aluminosilicate (Si/Al=13). Nickel exchange was determined to reach equilibrium by 16 hours at 75oC. Nickel exchanges were performed at this equilibration time with varying nickel nitrate molarities (0.005-0.1M), but keeping all other factors constant (temperature at 75oC, stirring at 300 R.P.M), to obtain ion-exchange isotherms that can be used to estimate the fraction of framework metal exchange sites. Zincosilicate molecular sieve CIT-6 samples were also synthesized with different Si/Zn molar ratios (20, 33, 40, 70, 100), in which changes in the Si/Zn ratio in the reactant gel led to changes in the time for crystallization

Hydrophobic Zeolites for Applications in Adsorption and Catalysis

Lewis acidic zeolites such as Sn-Beta are commonly studied for use as selective catalysts for glucose isomerization to fructose in liquid water. Glucose to fructose isomerization is a critical reaction for lignocellulosic biomass upgrading, which converts abundant and renewable feedstocks into commercially desirable fuels and chemicals. Industrial applications require catalysts that maintain optimal reactivity over long time scales, yet at typical reaction temperatures, Lewis acidic Beta zeolites are known to deactivate in liquid water through poorly understood mechanisms. Recent work in our group has shown that interactions between water and Sn-Beta zeolites can cause leaching of active Sn sites from the zeolite framework, and can also hydrolyze siloxane linkages to form silanol groups that also lower catalytic isomerization rates. This study investigates how water exposure and treatment affects the structure of Sn-Beta, by systematically changing the water contact time and analyzing the resulting glucose isomerization rates, in order to determine the catalysts and treatments to maintain optimal reactivity. Functionalization procedures were performed on Sn-Beta using hydrophobic alkylsilane precursors to form an external hydrophobic shell and minimize water diffusion into the zeolite pores. Limiting water contact may slow the deactivation rates caused by active Sn site leaching or silanol group formation. Functionalized zeolites showed a substantial decrease in initial glucose isomerization rates compared to that of untreated zeolites, however, reaction rates remain constant for longer time scales after functionalization, suggesting that silylation treatments may improve time-on-stream stability of Sn-Beta catalysts in liquid water

Identification of Proximal and Isolated Aluminum Heteroatoms in Zeolites by Infrared Spectroscopy

High demand for energy production and limited fossil fuel reserves are two factors that motivate intense research for new alternative energy resources. While we are still far from completely moving to renewable energy solutions, a new solution that replaces crude oil and coal with shale gas is currently under investigation. For this modern technology, new zeolite catalysts need to be developed for the conversion of light hydrocarbons gases to liquid transportation fuels. These catalysts are of special interest in the production of liquid fuels since they exhibit high reaction rates, molecular sieving properties and selectivity behavior. In this work, the effect of sequential ion exchange on the K/Al ratio of ZSM-5, CHA and FER zeolites was investigated. This was done using a 0.5M KNO3 solution to exchange NH4 ions with K ions on the zeolite framework. CHA zeolites used in this work were synthesized and characterized using X-ray diffraction. On the other hand, atomic adsorption spectroscopy was used to determine the K/Al ratio on ZSM-5, FER and CHA zeolites. Our results show that the potassium uptake on the zeolite does not change significantly with sequential ion exchange

Determining Glucose Isomerization Mechanisms on Lewis Acidic Beta Zeolites Using Isotropic Tracer Studies and 1H NMR

Biofuels synthesized from biomass sources are becoming necessary for sustainable production due to their significantly lower net CO2 production than fuels synthesized from fossil-based carbon sources such as petroleum. Catalytic pathways for the primary biomass-to-biofuels reaction pathway include the isomerization of glucose to fructose, which can be catalyzed by either Lewis acids or bases. Isolated metal atoms and metal oxide particles on Beta zeolites serve as active sites that catalyze this reaction through a Lewis acid 1,2-intramolecular hydride shift or by a Lewis base proton transfer mechanism, respectively. The Lewis acid mechanism has proven to have higher fructose selectivity than the Lewis base mechanism. Determining the glucose-fructose isomerization mechanism provides critical information about the active site placement in catalysts prepared by different methods, making it an ideal test of quality control for new material syntheses. Using glucose reactants deuterated at the second carbon, catalytic reaction mechanisms could be determined by tracing the location of the deuterium atom in the sugar products using 1H NMR spectroscopy. Comparison of fructose product spectra with an unlabeled fructose standard was used to show that glucose isomerization to fructose followed the Lewis acidic pathway on the samples in this study. The outcomes of these isotopic labeling studies provide insight into the placement of Lewis acid metals in zeolite frameworks and help to further understand this important step in biomass conversion to biofuels

Quantification and Characterization of Aluminum Distributions in Commercial Beta and Mordenite Zeolites by Cobalt Exchange

The aluminum distribution throughout the zeolite framework determines the structural, ion-exchange and catalytic properties of the zeolite. Several methods have been proposed to control the Al distribution, but in order to accurately assess these methods a procedure is needed to quantify Al distribution in various zeolite frameworks. Co2+ ions exchange onto the zeolite framework at Al pairs, and atomic absorbance spectroscopy (AAS) can be used to quantify the number of exchanged Co2+ ions and, in turn, the overall number of Al pairs. Each framework exhibits differences in pore size and channel configuration which affect the equilibrium conditions needed for saturation of all paired Al sites with Co2+ ions. In order to achieve saturation of the Co2+ ions, a reproducible exchange procedure must be developed for each framework of interest. Commercial beta (BEA) and mordenite (MOR) zeolites were subjected to liquid-phase cobalt ion exchange with varying exchange solution molarity, temperature, number of repetitions and time of exchange. The zeolites were then washed and treated in an oxidizing environment at high temperatures before undergoing AAS analysis to determine Co2+ concentration and diffuse reflectance UV-Vis spectroscopy (DRUV-VIS) to ensure only bare Co2+ ions were present. The BEA framework was found to achieve saturation at the following conditions: 0.50 M Co(NO3)2 exchange solution, ambient temperature, 1 repetition and 12 hour exchange time. The exchange procedure for MOR zeolites requires a 0.05 M Co(NO3)2 solution, ambient temperature, 24 hour exchange time and 1 repetition. These procedures will aid in the creation of an accurate catalog of the Al distribution in various commercially available BEA and MOR zeolites, as well as aiding in further synthesis studies to control the Al distribution in BEA and MOR zeolites

Modeling the Aqueous-Phase Copper Ion-Exchange Behavior onto SSZ-13 Zeolites

Copper-exchanged zeolites are utilized as catalysts for the selective catalytic reduction of nitrogen oxides, which are atmospheric pollutants found in diesel engine exhaust. The total amount of copper ions and the types of copper species (Cu(II) or Cu(II)OH) exchanged onto a zeolite can be varied. Copper is exchanged onto SSZ-13 (an aluminosilicate zeolite with the chabazite topology) during a process known as aqueous ion exchange, where the zeolite is mixed in a copper-containing solution. The distribution of copper on SSZ-13 is influenced by exchange conditions, including the molarity, temperature, and pH of the copper solution. The effect of exchange conditions on the amount and type of copper exchanged onto SSZ-13 has not been thoroughly investigated. In order to study these effects, ion exchange experiments were performed with solutions containing different copper concentrations and pHs. The copper loading (wt%) of each SSZ-13 sample was determined by atomic absorption spectroscopy (AAS). Data from AAS shows that SSZ-13 samples exchanged in solutions with higher copper molarities have higher copper loadings. Further exchanges are being done to test the effects of pH on the amount and type of copper species exchanged onto SSZ-13 through characterization by AAS and temperature programmed desorption (TPD). Using the collected data, a model will be developed to predict the amount and distribution of copper on SSZ-13 based on the exchange conditions

Initiating a Research-Focused Academic Career in Chemical Engineering: Perspectives from Faculty at Different Career Stages

Each fall, eager young researchers participate in the Meet the Faculty Candidates poster session at the AIChE Annual Meeting, and many more apply to tenure‐track faculty openings at academic institutions across the United States and throughout the world. These individuals embark on this journey with the eventual goal of becoming full professors. The process of initiating an academic career and developing a successful independent research program is an arduous journey that involves multiple stages. These stages include being hired into a faculty position, building an independent research program that involves recruiting students and can include constructing a laboratory, identifying research areas and specific problems to investigate, and establishing oneself as a scientific leader of a particular subject matter area within a broader community. Faculty candidates commonly seek advice from mentors or peers who have recently navigated the faculty interview and hiring processes to successfully obtain an academic position. Additionally, they often review the wealth of resources that are available on the Internet and in print. However, it can be daunting to sieve through this collective knowledge base to identify relevant information, as it invariably contains conflicting viewpoints and advice that may be subjective, generic to any research‐focused faculty position, or highly field specific

Initiating a Research-Focused Academic Career in Chemical Engineering: Perspectives from Faculty at Different Career Stages

Each fall, eager young researchers participate in the Meet the Faculty Candidates poster session at the AIChE Annual Meeting, and many more apply to tenure‐track faculty openings at academic institutions across the United States and throughout the world. These individuals embark on this journey with the eventual goal of becoming full professors. The process of initiating an academic career and developing a successful independent research program is an arduous journey that involves multiple stages. These stages include being hired into a faculty position, building an independent research program that involves recruiting students and can include constructing a laboratory, identifying research areas and specific problems to investigate, and establishing oneself as a scientific leader of a particular subject matter area within a broader community. Faculty candidates commonly seek advice from mentors or peers who have recently navigated the faculty interview and hiring processes to successfully obtain an academic position. Additionally, they often review the wealth of resources that are available on the Internet and in print. However, it can be daunting to sieve through this collective knowledge base to identify relevant information, as it invariably contains conflicting viewpoints and advice that may be subjective, generic to any research‐focused faculty position, or highly field specific

Framework and Extraframework Tin Sites in Zeolite Beta React Glucose Differently

Here, we show that framework tin sites in pure silica zeolite Beta (Sn-Beta) can isomerize glucose to fructose by a Lewis acid-mediated intramolecular hydride shift in aqueous solvent, but not in methanol solvent. Mechanistic studies using isotopically labeled (^(2)H, ^(13)C) glucose reactants show that in methanol, Sn-Beta instead epimerizes glucose to mannose by a Lewis acid-mediated intramolecular carbon shift mechanism known as the Bilik reaction. We also provide evidence that extraframework tin sites located within the hydrophobic channels of zeolite Beta can isomerize glucose to fructose in both water and methanol solvent, but through a base-catalyzed proton-transfer mechanism. SnO_2 particles located at external zeolite crystal surfaces or supported on amorphous silica catalyze isomerization in methanol but not in water, suggesting that contact with bulk water inhibits isomerization at SnO_2 surfaces. ^(119)Sn MAS NMR spectroscopy was used to unambiguously identify framework Sn sites, which give resonances for octahedral Sn (−685 to −700 ppm) in hydrated Sn-Beta that disappear upon dehydration, with the concomitant appearance of resonances for tetrahedral Sn (−425 to −445 ppm). In sharp contrast, spectra of hydrated samples containing extraframework SnO_2 show resonances for octahedral Sn centered at −604 ppm that do not change upon dehydration. These findings demonstrate that aldose–ketose isomerization reactivity on Sn-zeolite samples cannot be ascribed to the presence of framework Sn sites in the absence of isotopic labeling studies. They also indicate that any Sn-zeolite samples that initially convert glucose to fructose, instead of mannose, in methanol solvent contain Sn species that are structurally different from framework Sn centers