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The PhD thesis concerns the synthesis of novel zeolite materials, investigation of their properties and their possible use in catalytic application. The work was focused on the two- dimensional zeolites. The thesis was worked out at the Department of Synthesis and Catalysis at J. Heyrovský Institute of Physical Chemistry, AS CR. Germanosilicate UTL (Si/Ge molar ratio 4.0-6.5) was found to undergo unique structural changes in the neutral or acid environment leading to transformation of its three-dimensional framework into two-dimensional layered material denoted IPC-1P. The UTL degradation, so called top-down synthesis, was enabled due to a presence of double-four-units (D4Rs), which can be seen as supporting units/pillars between the rigid layers. The preferential location of Ge in D4Rs makes the units an ideal target for their selective degradation. The interlayer space in lamellar IPC-1P was modified by swelling with long-organic chain surfactant (material IPC-1SW). To keep the interlayer space permanently expanded (up to 3.3 nm) the silica amorphous pillars were subsequently introduced (material IPC-1PI). The integrity of the layers and their preserved UTL character was confirmed in all members of IPC-1 family by HRTEM and electron diffraction measuring. The layers of IPC-1P were condensed back...

<p>Catalysis has a responsibility to provide solutions to the growing grand challenge of sustainability in the fuels and chemical industry to help combat climate change. These changes; however, cannot be realized without a more fundamental understanding of the active sites that catalyze chemical reactions, and how they can be tuned to control rates and selectivities. Four specific examples of active site modification will be considered in this work: the speciation of isolated metals in zeolite frameworks, solvent thermodynamics and structure at defects in zeolite frameworks, the electronic modification of platinum through alloying in well-defined intermetallic nanoparticles, and the mobility and shape of gold nanoparticles in zeolite channels. Each will highlight how quantum chemistry calculations can provide a fundamental understanding of how these active site modifications influence the kinetics of chemical reactions, and how they can be controlled to pursue solutions to the reduction of carbon through sustainable utilization of shale gas as well as renewable chemicals production through biomass upgrading.</p> <p>Zeolites exchanged with metal heteroatoms can behave as solid Lewis or Br<a>ø</a>nsted acids depending on heteroatom identity. Lewis acid heteroatoms can adsorb water and hydrolyze to speciate into “open sites” which have been shown to differ in their ability to catalyze reactions such as glucose isomerization as compared to “closed sites” which are fully coordinated to the zeolite framework. The structure and catalytic properties of these sites are interrogated by a gas phase reaction, ethanol dehydration, in Sn-Beta by a combined Density Functional Theory (DFT) and experimental study. DFT is used to map the possible reaction mechanisms for ethanol dehydration, including the speciation of Sn sites into hydrolyzed configurations from water or ethanol. A microkinetic model for ethanol dehydration including unselective and inhibitory intermediates is constructed. This microkinetic model predicts the population of reactants and products on the catalyst surface as well as the sensitivity of individual elementary steps to the total rates. Powerful anharmonic entropy methods using <i>ab-initio </i>molecular dynamics (AIMD) is used to capture the entropy of confined reactive intermediates, which is shown to be necessary to compare with experiment. Results on closed and hydrolyzed open zeolite sites can then be compared with ethanol dehydration on “defect open” sites which were shown experimentally to occur at material stacking faults. A grain boundary model is constructed of zeolite Beta, where unique sites have similar ligand identity as hydrolyzed open sites. These defect open sites are found to not contribute to the observed reaction rate as they cannot stabilize the same transition state structures that were observed in internal Beta sites. </p> <p>Intuition about the ethanol dehydration reaction in Sn-Beta was then used to map a more expansive and diverse chemical network, the synthesis of butadiene from acetaldehyde and ethanol. For elementary reactions in this mechanism, which included aldol condensation, MPV reduction, and crotyl alcohol dehydration in addition to ethanol dehydration, the hydrolyzed open sites were found to be crucial reactive intermediates. Hydrolyzed sites were necessary to stabilize favorable transition states, which requires reconstruction of the local framework environment. Methods to preferentially stabilize hydrolyzed sites were then explored, using a screening algorithm developed to consider all possible sites in each zeolite framework. It was found that the stability of these hydrolyzed sites could be correlated to the local strain exerted by the surrounding silica matrix. This provides a new descriptor that stabilizes intermediates relevant to the synthesis of butadiene and ethanol dehydration.</p> <p>Next, the structure and thermodynamic stability of water networks around Sn-Beta defects and heteroatom active sites was considered using AIMD. As many biomass reactions occur in the presence of water, the interactions of water with hydrophobic and hydrophilic functionalized defects dictate how the stability of reactive intermediates and transition states is affected by a solvating environment. Locally stable and strongly nucleated clusters of water were observed to form at Sn defects, with less densely packed water structures stable at hydrophilic defects. This is in comparison to defect-free siliceous Beta, where significantly less water uptake is observed. These local clusters are in equilibrium with the less dense liquid-like phase that extends between defects. These results motivate localized cluster models around active sites in Lewis acids, as well as advance the fundamental understanding of hydrophobic/hydrophilic interactions in microporous materials. The local cluster models are then applied to the ethanol dehydration reaction in protonated aluminum Beta zeolites where experimentally observed non-unity coefficient ratios are rationalized by quantifying a different degree of solvation for the ethanol reactant state as opposed to the transition state, validated by a thermodynamic phase diagram.</p> <p>Changes in the electronic energy levels of <i>d</i> electrons upon alloying was studied in conjunction with a new spectroscopic technique being performed at Argonne National Laboratory to develop new descriptors to predict the degree of coking for different alloys. Resonant Inelastic X-ray Scattering (RIXS) simultaneously probes the occupied and unoccupied valence states of platinum in nanoparticles at ambient conditions. The specific excitation process of this spectroscopy is particularly amendable to DFT modeling, which was used to provide richer chemical insight into how changes in observed RIXS signature related to the electronic structure changes of platinum upon alloying. From a suite of multiple 3d alloy promoter catalysts synthesized, a quantitative comparison with DFT modeled spectroscopy was developed. The stability of DFT calculated coke precursors, relevant to dehydrogenation catalysts to convert light alkanes into olefins, was then correlated to DFT modeled RIXS spectra, which is a better descriptor for adsorption of unsaturated chemical intermediates that used previously, as well as being a descriptor accessible to direct experimental usage.</p> <p>Finally, the diffusion of gold nanoparticles in the TS-1 catalyst was studied using AIMD to help understand what structural motifs of gold are present under reaction conditions and how the shape and binding sites of gold is strongly influenced by deformation by the zeolite framework. This is used to help predict new zeolites for use in direct propylene epoxidation using molecular oxygen and hydrogen. The optimization of this catalyst is environmentally relevant to reduce the usage of inorganics and reduce the cost associated with production of hydrogen peroxide. Following these discussions, the role of computation in the prediction of active site structures and kinetics in conjunction with experiment was included. The broader impact of these findings will also be considered, which span beyond these specific reactions and materials.</p>

School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa August 2017 / Global warming, resulting from emission of greenhouse gases (GHGs), is the cause of drastic climate changes that threatens the economy and living conditions on the planet. Currently, recovery and mitigation of these greenhouse gases remains a technological and scientific challenge. Various recovery processes for the mitigation of GHGs have been reported including among others carbon capture and storage (CCS). The most mature and applied technology in CCS process involves the absorption of carbon dioxide on amine based solvents. However, studies have shown that this process has several drawbacks that include low stability and high energy required to strip off the absorbed CO2 and regenerate the solvent. This presents an opportunity for the development of new materials for CO2 capture such as zeolite membranes. Previous studies have shown that the separation of CO2 can be achieved with high selectivity at low temperatures using thin-film SAPO-34 membranes (thin layers on supports). This is because CO2 adsorbs strongly on the membranes compared to other gases found in flue gas. In the thin-film membranes supported on ceramic or sintered stainless steel, thermal expansion mismatch may occur at higher operating temperatures resulting in loss of membrane selectivity due to the formation of cracks. A new method is required to overcome the aforementioned problems, thereby enhancing the separation application of the membranes at higher temperatures. The effective separation and capture of CO2 from the coal-fired power plant flue gas is an essential part in the CCS process (Figueroa et al., 2016; Yang et al., 2008). Currently, the capture stage is a huge contributor to the overall cost of CCS (Yang et al., 2008). This is due to the high-energy intensity and inefficient thermal processes applied in the separation and capture in various industrial applications (Yang et al., 2008). This work presents the use of nanocomposite SAPO-34 zeolite membranes synthesized via the pore-plugging hydrothermal method for the separation of CO2 during post-combustion CO2 capture. The SAPO-34 membranes used were supported on asymmetric α-alumina as membrane supports. The membranes were characterized with a combination of dynamic and static physicochemical techniques such as Basic Desorption Quality Test (BDQT), X-ray diffraction (XRD) spectroscopy, Scanning Electron Microscopy (SEM), Fourier Transform Infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The characteristic peaks at 2θ = 21°, 26°, and 32° on the XRD pattern confirmed the presence of SAPO-34 with a rhombohedral crystalline structure. The SEM images showed the formation of the cubic crystalline which were consistent with the reported morphology of SAPO-34. FTIR spectra showed the presence of the essential double-6 membered rings (D6R) and TO4 structural groups in surface chemistry of crystalline materials further confirming the presence SAPO-34. The TGA confirmed that the membranes possessed high thermal stability. To assess the feasibility of the synthesis process, the nanocomposite zeolites were grown within the tubular supports. The SEM images of the cross-section of the membrane confirmed the presence of the zeolites within the pores of the support confirming the fabrication of nanocomposite membranes by the pore-plugging synthesis method. The permeation tests used a dead-end filtration mode to measure the single gas permeance and the ideal selectivity of CO2 and N2 were calculated. The BDQT was essential in the study of the quality of the as-synthesized nanocomposite membranes. The quality of the membranes increased with an increase in the synthesis layers of the membranes. However, with an increase in synthesis layers, the membrane thickness also increases. The membrane thickness affected the gas permeance for CO2 and N2 significantly. The permeance of the N2 gas decreased from 10.73 x10-7 mol.s-1.m2Pa-1 after the first synthesis to 0.31 x10-7 mol.s-1.m2Pa-1 after seven synthesis layers. Alternatively, the more adsorbing gas CO2 decreased from 12.85 x10-7 mol.s-1.m2Pa-1 to 2.44 x10-7 mol.s-1.m2Pa-1. The performance of these zeolite membranes depends significantly on the operating conditions. Hence, we studied extensively the influence of the various operating conditions such as temperature, feed pressure and feed flowrate in this work. Results indicated that the membrane separation performance in this study is largely dependent on the temperature. In addition, the ideal selectivity decreased significantly with an increase in temperature. High temperatures results in less adsorption of the highly adsorbing CO2 gas, the permeance reduces significantly, while the permeance of the less adsorbing N2 increased slightly. The feed flow rate has less effect on the adsorbing gas while the non-absorbing gas increased resulting in a decrease in the ideal selectivity as well. The nanocomposite membranes in this study have a low flux compared to their thin film counterparts. An increase in feed pressure significantly increased the flux significantly as well as the ideal selectivity. Maxwell-Stefan model simulation was done in this study to describe the permeance of pure CO2 single gas permeance as a function of temperature. This model considered explicitly the adsorption-diffusion mechanism, which is the transport phenomenon, involved in the transport of CO2 through the zeolite membrane. The description of the support material was included in the model as well. However, the model was only applied to the CO2 gas permeation well within the experimental data. We then compared the model was with the experimental results and a good correlation was observed. In conclusion, SAPO-34 nanocomposite zeolite membranes were obtained at low temperatures (150 °C) with a short synthesis time (6 h). In addition, the high thermal stability of the as-synthesized SAPO-34 membranes makes them ideal for high temperature CO2 separation such as the intended post-combustion carbon capture. The BDQT revealed that the quality of the membranes was related to the thickness of the membranes. Therefore, better membrane quality was obtained with relatively thicker membranes. The separation performance evaluation was conducted on the membrane with the greatest quality. Our findings demonstrate that the performance of the membranes depends extensively on the operating conditions. / MT2018

Dehydroaromatization of methane (MDA) reaction was investigated over platinum modified Mo/H-ZSM-5 catalysts which were pre-carbided at 750 oC. The influence of platinum on the catalytic performance and product selectivity of Mo/H-ZSM-5 catalysts for the MDA reaction at 700 oC was studied. The presence of platinum led to a slight decrease in methane conversion. As the platinum loading increased, the methane conversion decreased further and the catalytic stability increased with time-on-stream (TOS) during the MDA reaction. Aromatic selectivities above 90% were obtained with catalysts containing low platinum loadings (0.5 and 1.0 wt.%), with benzene being the most prominent product. A decrease in coke selectivity and coke deposits was noted with the platinum modified Mo/H-ZSM-5 zeolite catalysts. A comparative study was performed to compare platinum, palladium and ruthenium promoted Mo/H-ZSM-5 zeolite catalysts with un-promoted Mo/H-ZSM-5. The ruthenium promoted catalyst proved to be superior in catalytic performance, with a higher methane conversion obtained than found for platinum promoted and palladium promoted Mo/H-ZSM-5 catalysts. Benzene selectivity of about 60% was obtained for ruthenium and palladium promoted Mo/HZSM- 5 catalysts and the total aromatic selectivity was maintained at 90%. TGA results showed a total reduction of 50% by weight of carbon deposited on the promoted Mo/H-ZSM-5 catalyst. Dehydroaromatization of methane was studied over tin modified Pt/Mo/HZSM-5 catalysts and compared to Pt/Mo/H-ZSM-5 catalyst at 700 oC. Addition of tin decreased the activity towards methane aromatization. However, the formation of aromatic compounds was favoured. The CO FT-IR adsorption and CO chemisorption techniques showed that the catalyst preparation method had an effect on the catalytic performance of tin modified Pt/Mo/H-ZSM-5 catalysts. High aromatic selectivity and low coke selectivity were obtained with co-impregnated and sequentially impregnated Pt/Sn catalysts. While a decrease in the formation rate of carbonaceous deposits is mainly dependent on the availability of platinum sites for the hydrogenation of carbon. The order of sequentially loading platinum and tin has an effect on the electronic and structural properties of platinum as shown by XPS and FT-IR studies. CO chemisorption and the FT-IR adsorption studies showed that addition of tin decreased the adsorption capacity of the platinum surface atoms. Catalyst preparation methods and successive calcination treatments affected the location of both tin and platinum atoms in the catalyst. Catalysts prepared by the coimpregnation method showed a good platinum dispersion, better than found for the sequentially impregnated catalysts. The MDA reaction was carried out at 800 oC over manganese modified H-ZSM-5 zeolite catalysts prepared by the incipient wetness impregnation method. The effect of a number of parameters on the catalytic performance and product selectivity was investigated, such as reaction temperature, manganese precursor-type, tungsten as promoter, manganese loading and use of noble metals. The study of the effect of reaction temperature showed that the methane conversion increased linearly with increase in reaction temperature from 700 to 850 oC. The selectivity towards aromatic compounds (of about 65%) was attained for the reactions performed at 750 and 800 oC. Formation rate of carbonaceous deposits increased linearly with increase in reaction temperature. The use of different manganese precursors to prepare Mn/H-ZSM-5 catalysts had an effect on both the catalytic behaviour and the product distribution. High catalytic activities were obtained for the catalysts prepared from Mn(NO3)2 and MnCl2 salts. However, the product distribution was significantly different, with the Mn(NO3)2 catalyst being more selective towards aromatic compounds while the MnCl2 catalyst was more selective toward coke. The effect of manganese loading was studied at 800 oC and an optimum catalyst activity was obtained at 2 and 4 wt.% manganese loadings. The aromatic selectivity above 70% and coke selectivity of 20% were obtained for a 2 wt.% loaded catalyst. Addition of tungsten as a promoter onto the 2 wt.% loaded catalyst (2Mn/H-ZSM-5) lowered the catalytic activity but the catalyst remained fairly stable with increase in TOS. Tungsten modified catalysts favoured the formation of carbonaceous deposits over aromatic compounds. TGA results showed a coke deposit of 164 mg/g.cat, an 88% increase in coke deposit when tungsten was used a promoter. Noble metals were added to reduce the total amount of coke on the tungsten modified Mn/H-ZSM-5 catalysts. The presence of a noble metal favoured the formation of aromatic compounds and suppressed the formation of coke. Platinum and ruthenium promoted catalysts were the active catalysts and aromatic selectivity increased from 12% to 55% and 46% respectively. A reduction in the total amount of coke deposit on the platinum promoted catalyst (42%) and the ruthenium promoted catalyst (31%) was noted.