Understanding Grafted Cations in Controlled Environments for Heterogeneous Catalysis

Catalytic processes occurring on the surfaces of heterogeneous catalysts are controlled by the molecular structures of active sites where these reactions occur. These active sites can be broadly thought to consist of an active center, where bond making and breaking events occur, surrounding by the surface of the support. These are the inner-sphere (i.e. active center) and outer-sphere (i.e. surface) environments of the active site. Catalyst design typically focuses on the choice of the optimal inner-sphere environment, while surfaces are often regarded as inert oxide supports onto which active sites are dispersed to facilitate catalyst recovery. In this thesis, I demonstrate that the outer-sphere surface environment is, in fact, an essential element for controlling the structure and reactivity of active sites supported on silicates.The theoretical concepts, silicate supports, and synthetic approaches that are used in this thesis are introduced in Chapter 1. Given the importance of silanol groups as grafting sites in synthetic approaches used in this thesis, I begin by providing a detailed study of silanol speciation across zeotypes and amorphous supports, in Chapter 2. Subsequently, I provide an example of how silanol environments control grafting processes and how crystalline silicates provide stable support environments for FeIII cations, in Chapter 3. I then introduce an approach to synthesizing well-defined active sites by controlling the structure of a grafted cation using an organic ligand, applied to calix[4]arene-TiIV complexes grafted on amorphous SiO2 as epoxidation catalysts, in Chapter 4. Having established the structure of silicates and approaches to synthesize well-defined active sites on their surface, I present three studies where this enables the study of structure and catalytic properties. In Chapter 5, I demonstrate how this approach enables the unambiguous deconvolution of the effect of support outer-sphere on epoxidation catalysis. In Chapter 6, I investigate how the support outer-sphere can also control the conformation and structure of grafted complexes, while providing insight into adsorption processes occurring on surfaces. Finally, in Chapter 7, I provide a detailed mechanistic study of how partially confining outer-sphere environments impact catalytic reactivity for olefin epoxidation. Taken together, this work provides fresh insights into the structure of silicate supports and their ability to control catalysis, providing an additional and important avenue to the design of heterogeneous catalysts

Zeolites and ordered porous solids: fundamentals and applications

Pérez Pariente, J.; Martínez Sánchez, MC. (2011). Zeolites and ordered porous solids: fundamentals and applications. Editorial Universitat Politècnica de València. http://hdl.handle.net/10251/11205Archivo delegad

Organic transformations utilising in-situ hydrogen peroxide synthesis: An experimental and theoretical study

The research presented within this thesis focused on the utilisation of hydrogen peroxide for a subsequent oxidation reaction. Hydrogen peroxide (H2O2) is considered a ‘green’ oxidant as it can be easily degraded to form water. To complete the utilisation, the H2O2 was made via the direct synthesis route, in a one-pot synthesis reaction. Furthermore, this in-situ technique decreases the complexity of the simulated manufacturing process as only one reactor is required. The first part of this thesis focused on experimentally investigating the utilisation of H2O2 for the ammoximation reaction. Cyclohexanone was used as the target molecule for this ammoximation reaction. Cyclohexanone oxime is an important precursor to the manufacturing process of the plastic nylon-6. The tandem-catalyst system used comprised of a supported gold-palladium (AuPd) catalyst, with a low loading (0.66 wt%, 1:1), and a Titanium Silicalite-1 (TS-1) molecular sieve. The first catalyst synthesised the H2O2 needed to form hydroxylamine (NH2OH), which was synthesised using the TS-1. In a non-catalytical reaction at 353.15 K, NH2OH reacted with cyclohexanone. In the present work, a series of the H2O2 synthesis catalysts were made and investigated for both the direct H2O2 synthesis reaction, and the cyclohexanone ammoximation reaction utilising in-situ direct H2O2 synthesis. The second part of this thesis focused on computationally investigating the utilisation of H2O2 for the epoxidation reaction. The key advantage of this method of investigation is that catalytic performance can be estimated to a good degree of accuracy, which in turn can provide information on how to synthesise a highly active and selective catalyst. Propene was used as the target molecule as it was a computationally simple molecule to use, and propene oxide is an important precursor to numerous manufacturing processes, such as the production of polyurethane and propene glycol. Using Density Functional Theory, surfaces of Au and Pd were used to represent the facets of those catalysts. The surfaces were cut at Miller indices (111) and (100). The reactions studied and presented in this work were the direct H2O2 synthesis reaction and the propene epoxidation utilising in-situ direct H2O2 synthesis. Both parts of the research in this thesis could be used to inform future work on catalytic in-situ H2O2 synthesis and its subsequent utilisation

Synthesis and Characterization Of Fe-modified Imogolite Nanotubes

During the past decades, and after introducing the most famous carbon nanotubes, the main role in these fields has been playing by the single- and multi-wall carbon nanotubes which have received tremendous research interest due to their superior mechanical, chemical, electrical and thermal properties. However, several problems in carbon nanotube technology, such as high-temperature process with low yield product, imprecise control over nanotube dimensions and chirality, limitations of chemical composition, intrinsic color of nanotubes which limits their application as nanofiller in transparent hybrid materials, low compatibility of carbon nanotubes with human body in bio-applications and the most important, in recent years, effect of carbon nanotubes on human health and environment because of their potential toxic nature, encouraged the research for analogue structure among inorganic materials and the possibility of applying them in other fields, such as catalysis and ion adsorption.Inorganic nanotubes like MoS2 and WS2 were the first inorganic nanotubes that succeeded the discovery of carbon nanotubes and in the following years, other metal oxide/hydroxide nanotube structures have been reported. Among the inorganic nanotubes, which have been developed in recent years, imogolite (IMO, chemical formula (OH)3Al2O3Si(OH)), a natural alumino-silicate clay mineral, recently entered this nanotechnologic scenario. It was discovered for the first time 1962 in Japanese soils of volcanic origins but its structure was determined ten years later. Being an analogue material to carbon nanotubes, IMO type materials can represent an intriguing different source for new technological potential applications as anions/cations retention from water, catalysis, gas adsorption, separation and storage, scaffold for biomedical applications and inorganic nanofiller for polymer matrixes. The main features of imogolite is the crystalline organization of its walls, folded to create a nanotube with two distinct surfaces: an inner surface presenting free silanol groups and an outer surface characterized by hydroxyl groups bridged between two octahedral aluminum atoms. Since, variables such as purity, composition, reproducibility, and specifically designed features can be often better controlled in synthetic procedures rather than using natural clay specimens (which typically contain impurities and are often not easily available), synthetic single walled imogolite nanotunes with high monodispersity in diameter was obtained for the first time in 1972 by a sol-gel synthesis in acid environment. Present PhD project is going to describe the developments obtained in imogolite research field, concerning the synthesis and characterization of a new kind of modified imogolite nanotubes by iron inclution in nanotubes outer surface, either by direct or post-synthesis reactions, as compared to unmodified synthetic IMO. A series of samples of iron-doped IMO, with a range of iron content, were obtained. To investigate the role of Fe ions in nanotube formation and the effect of Fe structural position on nanotubes textural properties, they were characterized by means of low angles X-ray Diffraction (XRD); IR spectroscopy (FT-IR); Transmission Electron Microscopy (HR-TEM); energy dispersive spectroscopy (EDS) and N2 sorption isotherms at -196 °C, indicating the limited level of Fe inclusion into nanotube structure, about up to 1 %wt Fe. The higher amount of Fe will hinder tube formation. The obtained modified samples by direct synthesis show more closely packing in bundles and presents slightly larger inner pores. Then, their physico-chemical properties were compared to those of proper IMO. Several experimental results are reported of nature and structural positions of Fe species in the samples obtained by direct (Fex-IMO) or post synthesis method (Fex-loaded-IMO) by Diffuse Reflectance (DR) UV-Vis pectroscopy, Raman spectroscopy, magnetic test and Electron Paramagnetic Resonance (EPR) spectroscopy which indicate the preferential isomorphic substitution of Fe for Al in the sample prepared by direct synthesis (Fex-IMO) and of the preferential formation of Fe2O3 clusters in that obtained by post-synthesis doping (Fex-loaded-IMO). Same as other nanoporous aluminosilicate materials, IMO nanotubes materials contain considerable amount of water in their pores in ambient condition that influences and governs their properties. Therefore, hydration/dehydration behavior of bare and Fe-modified nanotubes was of paramount importance in dictating the operating conditions for any application requiring a surface interaction like catalytic activity or ion adsorption. Accordingly, the behavior of hydrated Imogolite markedly depends on thermal pre-treatments, which has been investigated by several complementary methods (XRD, IR spectroscopy, TG/DT analysis and N2 sorption isotherms at -196 °C). Obtained results show that, modification of imogolite nanotubes by iron either by direct or post synthesis, accelerate dehydroxylation and decrease their thermal stability, by likely forming some structural defects, able to catalyze silanols condensation. Such defects may be ascribed to those Fe ions substituting for Al ions that likely form also by post-synthesis procedure. After removing of water present at both the external and internal surfaces at 250 oC under residual vacuum equal to 10-3 mbar, inner ≡SiOH groups will be accessible to probes. Surface acidic properties and accessibility of surface groups were investigated in gas and water media. In gas phase, surface acidic properties investigated by combination of IR spectroscopy and interaction with probe molecules (CO and NH3). Moreover a catalytic reaction: epoxidation of propylene by O2 over the obtained samples as catalyst was carried out to provide more information about surface acidity of Fe-modified samples in gas phase. According to the results, with both modified samples, when Fe substitutes for Al, at the outer surface Fe(OH)Al groups occur, the intrinsic acidity of which is only marginally different from that of Al(OH)Al. Fe(OH)Al groups likely act as crystallization centres for the growth of Fe2O3 nanoclusters, bearing less acidic OH groups. The acidity of modified samples were studied in water, as shown by -potential measurements and interaction with (acid orange 7, an organic sodium salt with formula C16H11N2NaO4S) AO7 molecules, for two important reaction: adsorption of AO7 molecules and catalytic reaction azo-dye molecule degradation by H2O2. In water, Fe(OH)Al bridged groups, which are slightly less acidic than Al(OH)Al groups, but provide accessible Fe3+ sites that may be accessible to species able to coordinate iron, as observed in the case of AO7-, leading to a higher efficiency towards the retention of such moiety and, more generally, of anions in aqueous solution. Finally we can conclude that Interaction with AO7- in water solution occurs in different ways, as documented by the observed pH changes: i) with proper IMO, AO7- anions preferentially adsorb via H-bonding; ii) with Fe1.4-IMO, Fe3+ cations of Fe(OH)Al groups act coordination centers for N atoms in the AO7- moiety; iii) with Fe1.4-loaded-IMO, Fe2O3 nanoclusters likely hinder AO7- adsorption. The best condition for degradation if AO7 was observed in the presence of IMO sample as catalyst, by formation of very active OOH groups and then carries out an intermolecular rearrangement with the neighboring adsorbed AO7 molecules to achieve the degradation. In this case higher acidity of Al(OH)Al groups in water provide reacitve sites. Fe(OH)Al groups which are more basic in water environment seems to be weaker in H2O2 decomposition. Therefore IMO with more than 95% of degradation just in a few minutes could be proposed as a new candidate for waste water treatment

Polymeric catalytic membrane reactors : application to propylene and propyne hydrogenations

Tese de doutoramento. Engenharia Química. Faculdade de Engenharia. Universidade do Porto. 200

Preparation and characterisation of gold and palladium based catalysts for the direct synthesis of hydrogen peroxide

I The research presented in this thesis describes the direct synthesis of hydrogen peroxide from H2 and O2 using supported gold-palladium based catalysts. The direct synthesis process offers a green and sustainable approach compared to the anthraquinone autoxidation (AO) process, which is currently used on an industrial scale to produce >95% H2O2 worldwide. The work presented in this thesis is an attempt to examine the direct synthesis process in terms of determining optimum catalyst compositions for potential scale-up in the near-future. The primary aim of this investigation is centred on catalyst design and characterisation. The first part of this work is a catalyst optimisation study for 2.5 wt% Au-2.5 wt% Pd/TiO2, and involved changing the amount of water used in the catalyst preparation, in this case wet impregnation. It was found that the addition of small amounts of water resulted in approximately 100% enhancement in activity for TiO2-supported catalysts but not for carbonsupported Au-Pd catalysts. The rate of Au/Pd uptake was contrasted and it was determined that the isoelectric point of the support was highly influential. While the activity can be enhanced for TiO2-supported catalysts, both catalyst nanostructure and stability were detrimentally affected by the addition of water during the impregnation step. The second part of this work is focussed on understanding the precise nature of the acid pre-treatment effect, where treatment of a carbon support in dilute nitric acid prior to the impregnation of Au and Pd precursors can result in the complete switching-off of sequential H2O2 hydrogenation activity over the catalyst. Characterisation and heat treatment studies gave an improved understanding of the relationship between Au/Pd and the carbon support. The next part of the study addresses the use of a colloidal immobilisation method to pre-fabricate Au-Pd ‘designer’ nanoparticles onto supports and is accompanied by extensive advanced aberration corrected electron microscopy studies. The effect of acid pre-treating silica based supports is then considered for catalysts prepared by wet-impregnation, specifically the fact that acid pre-treatment of silica is required to induce synergy between Au and Pd metals for the direct synthesis of hydrogen peroxide. The final part of this work considers the effect of introducing a third metal into the catalyst design, specifically the addition of Pt to Au/Pd compositions. An extensive catalyst screening study is undertaken for Au-Pd-Pt/CeO2 catalysts