Dissertationen zum Thema „Light Olefines“

L'augmentation de la concentration de CO2 atmosphérique présente des défis environnementaux significatifs et souligne l'urgence de développer des procédés chimiques durables. Une approche prometteuse pour aborder ces problèmes est la conversion catalytique du CO2 en produits chimiques à valeur ajoutée, tels que le méthanol et les oléfines légères. Cette thèse se concentre sur le développement de catalyseurs pour la synthèse de méthanol et la synthèse d'oléfines légères à partir de CO2 par médiation de méthanol. De plus, l'hydrogénation du CO médiée par le méthanol en oléfines légères est également étudiée: le CO peut être considéré comme une alternative au CO2, car il peut être produit par la réaction du gaz à l'eau inverse. Les travaux rapportés dans cette thèse fournissent de nouvelles perspectives sur la conception de catalyseurs pour l'hydrogénation de COx en méthanol ou en oléfines légères, en suggérant de nouvelles stratégies pour améliorer la sélectivité des produits. De plus, la thèse fait progresser la compréhension des aspects mécanistiques de ces réactions. Pour l'hydrogénation du CO2 en méthanol, le catalyseur commercial CuO-ZnO-Al2O3 a été promu avec des halogènes (Br, Cl, I) pour améliorer la sélectivité au méthanol. Il a été observé que le Br permettait d'améliorer la sélectivité de 10 % par rapport au catalyseur initial. Une analyse cinétique a montré que le Br entraînait la suppression de la réaction de conversion inverse de l'eau en gaz et de la réaction de décomposition du méthanol, toutes deux responsables de la production parallèle de CO. Pour l'hydrogénation du CO2 en oléfines légères médiée par le méthanol, une série de catalyseurs bifonctionnels basés sur des oxydes de Zn, In, Mn, Cr ou Ga et différents zéolithes SAPO-34 a été étudiée. L'analyse des corrélations sélectivité-conversion a permis d'élucider les fonctions de chaque composant du catalyseur. Il a été découvert que la sélectivité aux oléfines légères au sein des fractions d'hydrocarbures dépendait finalement du composant zéolithique et diminuait en fonction du rendement en hydrocarbures. Le composant catalyseur à base d'oxyde métallique était responsable de la conversion du CO2, de la sélectivité globale aux hydrocarbures et au CO. La morphologie et l'acidité du SAPO-34 ont été identifiées comme des descripteurs majeurs de la sélectivité aux oléfines légères sans CO dans l'hydrogénation du CO2 sur des catalyseurs bifonctionnels. Enfin, pour la synthèse d'oléfines légères médiée par le méthanol à partir de gaz de synthèse, ce travail a étudié l'activité d'un catalyseur bifonctionnel composé de nanoparticules d'argent supportées mélangées à la zéolithe SAPO-34. Les catalyseurs résultants ont montré une sélectivité plus élevée aux oléfines légères par rapport à un catalyseur conventionnel oxyde-zéolithe. Il a été observé que la réaction est sensible à la structure, et la taille des particules d'argent influence la sélectivité aux oléfines légères
The increasing concentration of atmospheric CO2 presents significant environmental challenges and emphasizes the urgency for sustainable chemical processes. One promising approach to address this issues is the catalytic conversion of CO2 into value-added chemicals, such as methanol and light olefins. This thesis focuses on the catalyst development for the methanol synthesis and the methanol-mediated light olefins synthesis from CO2. Moreover, the methanol-mediated CO hydrogenation to light olefins is also studied: CO can be considered as an alternative to CO2, as it can be produced by the Reverse Water Gas Shift reaction. The work reported in this thesis provides new insights into catalyst design for the COx hydrogenation to methanol or light olefins, suggesting new strategies to improve product selectivity. Additionally, the thesis advances the understanding of mechanistic aspects of these reactions. For the CO2 hydrogenation to methanol, the commercial CuO-ZnO-Al2O3 catalyst was promoted with halogens (Br, Cl, I), to improve selectivity to methanol. It was observed that Br allowed to improve the selectivity of 10 % compared to the pristine catalyst. A kinetic analysis showed that Br caused the suppression of the Reverse Water Gas Shift reaction and of the methanol decomposition reaction, both responsible of the parallel production of CO. For the methanol-mediated CO2 hydrogenation to light olefins, a series of bifunctional catalysts based on oxides of Zn, In, Mn, Cr, or Ga and different SAPO-34 zeolites were studied. The analysis of the selectivity-conversion correlations allowed to elucidate the functions of each catalyst component. It was uncovered that the selectivity to LO within hydrocarbon fractions depended ultimately on the zeolite component and decreased as a function of hydrocarbon yield. The metal-oxide catalyst component was responsible for the CO2 conversion, overall hydrocarbon and CO selectivity. The SAPO-34 morphology and acidity were identified as major descriptors of the CO-free LO selectivity in the CO2 hydrogenation over bifunctional catalysts. Finally, for the methanol-mediated synthesis of light olefins from syngas, this work studied the activity of a bifunctional catalyst composed by supported silver nanoparticles mixed with SAPO-34 zeolite. The resulting catalysts exhibited higher selectivity to light olefins compared to a conventional oxide-zeolite catalyst. It was observed that the reaction is structure-sensitive, and the silver particle size influences the selectivity to light olefins

The influence of catalyst characteristics, i.e., acidity and porosity on the product distribution in the cracking of triglyceride-rich biomass under fluid catalytic cracking (FCC) conditions is reported. It has found that the degradation degree of triglyceride molecules is strongly dependent on the catalysts’ acidity. The higher density of acid sites enhances the conversion of triglycerides to lighter products such as gaseous products and gasoline-range hydrocarbons. The formation of gasolinerange aromatics and light olefins (propene and ethene) is favored in the medium pore channel of H-ZSM-5. On the other hand, heavier olefins such as gasoline-range and C4 olefins are formed preferentially in the large pore structure of zeolite Y based FCC catalyst (Midas-BSR). With both catalysts, triglyceride molecules are mainly converted to a mixture of hydrocarbons, which can be used as liquid fuels and platform chemicals. Hence, the utilization of the existing FCC units in conventional petroleum refineries for processing of triglyceride based feedstock, in particular waste cooking oil may open the way for production of renewable liquid fuels and chemicals in the near future
Bài báo trình bày kết quả nghiên cứu khả năng tích hợp sản xuất nhiên liệu sinh học và hóa phẩm từ nguồn nguyên liệu tái tạo sinh khối giầu triglyceride bằng công nghệ cracking xúc tác tấng sôi (FCC) trong nhà máy lọc dầu. Kết quả nghiên cứu cho thấy xúc tác có ảnh hưởng mạnh đến hiệu quả chuyển hóa triglyceride thành hydrocarbon. Tính acid của xúc tác càng mạnh thì độ chuyển hóa càng cao và thu được nhiều sản phẩm nhẹ hơn như xăng và các olefin nhẹ. Xúc tác vi mao quản trung bình như H-ZSM-5 có độ chọn lọc cao với hợp chất vòng thơm thuộc phân đoạn xăng và olefin nhẹ như propylen và ethylen. Với kích thước vi mao quản lớn, xúc tác công nghiệp FCC dựa trên zeolite Y ưu tiên hình thành C4 olefins và các olefin trong phân đoạn xăng. Ở điều kiện phản ứng của quá trình FCC, triglyceride chuyển hóa hiệu quả thành hydrocarbon mà có thể sử dụng làm xăng sinh học cho động cơ và olefin nhẹ làm nguyên liệu cho tổng hợp hóa dầu

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)
It was studied the catalytic performance of ZSM-12 zeolites modified by desilication in the cracking of naphthenic hydrocarbons. ZSM-12 zeolites with nominal a SiO2/Al2O3 ratio were synthesized during 96 or 144 h under hydrothermal conditions and the obtained zeolites were treated with NaOH solutions under different conditions. The samples were characterized by thermogravimetric analysis (TA), X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 physisorption, temperature programmed desorption of ammonia (NH3-TPD), 27Al MAS NMR spectroscopy, energy-dispersive X-ray spectroscopy (EDX) and infrared spectroscopy (Py-FTIR). The treatment with NaOH solutions under mild conditions was more effective for the ZSM-12 zeolite synthesized during 96 h. For the HZ12-96-0,2 sample, obtained by treatment at 35 °C with 0.2 mol.L-1 NaOH solution during 15 min, it was verified an increase in the external surface area and the formation of mesopores in the range of 2 to 14 nm. On that zeolite occurred a higher yields to light olefins during the cracking of cyclohexane at 400 °C and also in the cracking of methyl and ethylcyclohexane at 450 °C. This result was mainly related to the higher density of acid sites exhibited by the HZ12-96-0,2 zeolite compared with the parent ZSM-12 one, as consequence of the applied alkaline treatment. It was also verified that the ZSM-12 zeolites modified by more severe alkaline treatment (0.5 or 1.0 mol.L-1 NaOH solution at 80 °C for 30 min) presented significant increase of the external surface area and mesopores volume. The catalytic cracking of cyclohexane, methylcyclohexane and ethylcyclohexane at 500 °C, as well as the physicochemical characteristics of the MZ12-96-0,5 zeolite enhanced the formation of light olefins. The highest yield to light olefins was obtained on that zeolite during the cracking of ethylcyclohexane, which increased 9% when compared with the yield obtained on the not modified HZSM-12 zeolite. The selectivity to light olefins on the studied HZSM-12 zeolites was strongly influenced by presence of a side chain in the naphthenic ring (methyl or ethyl), as well as by the employed cracking operating conditions.
Estudou-se o desempenho de zeólitas ZSM-12 modificadas por dessilicalização no craqueamento de hidrocarbonetos naftênicos. Zeólitas ZSM-12 com razão SiO2/Al2O3 igual a 80 foram sintetizadas em 96 ou 144 h sob condições hidrotérmicas. As zeólitas ZSM-12 obtidas foram modificadas sob diferentes condições de tratamento alcalino com soluções de NaOH e posteriormente caracterizadas por termogravimetria, difratometria de raios X, microscopia eletrônica de varredura, fisissorção de nitrogênio, dessorção de amônia à temperatura programada, ressonância magnética nuclear do 27Al, espectroscopia de energia dispersiva de raios X e espectroscopia na região do infravermelho com adsorção de piridina. O tratamento alcalino, sob condições mais brandas, foi mais efetivo para a zeólita ZSM-12 sintetizada em menor tempo de cristalização (96 h). Para a zeólita HZ12-96-0,2, obtida por tratamento com solução de NaOH 0,2 mol.L-1 a 35 °C por 15 min, verificou-se um aumento na área superficial externa e distribuição de tamanho de mesoporos entre 2 e 14 nm. Nessa zeólita, ocorreu um maior rendimento a olefinas leves no craqueamento de cicloexano a 400 °C e, também, no craqueamento de metil- e etil-cicloexano realizado a 450 °C. Esse resultado foi relacionado, principalmente, com a maior concentração de sítios ácidos na zeólita HZ12- 96-0,2, em relação à zeólita HZSM-12 precursora, como consequência do tratamento alcalino. Verificou-se, também, que as zeólitas ZSM-12 modificadas por tratamento alcalino, sob condições mais severas (solução de NaOH 0,5 ou 1,0 mol.L-1 a 80 °C por 30 min), apresentaram aumento significativo de área superficial externa e volume de mesoporos. O craqueamento de cicloexano, metil- e etil-cicloexano realizado a 500 °C, assim como as características físicas e químicas da zeólita MZ12-96-0,5, favoreceram a formação de olefinas leves. O maior rendimento a olefinas leves ocorreu durante o craqueamento do etil-cicloexano sobre essa zeólita, com aumento desse rendimento em torno de 9% quando comparado ao rendimento obtido sobre a zeólita HZSM-12 não modificada. A seletividade a olefinas leves sobre as zeólitas HZSM-12 preparadas neste estudo sofreu forte influência da presença da cadeia lateral no anel naftênico (metil ou etil), assim como também das condições operacionais de craqueamento empregadas.

>Magister Scientiae - MSc
Ethylene and propylene are greatly used for their importance as feedstocks for producing useful materials. Due to rise in prices and the demand of ethylene and propylene, the need to increase the selective production of these light olefins is necessary. To achieve this, zeolites, specifically ZSM-5 has been used to investigate catalytic cracking of several types of hydrocarbons for the production of these light olefins. This study focuses on developing hierarchical macro and/or mesoporous ZSM-5 zeolites with variable Si/Al ratios. The synthesized materials were then evaluated on their performance via catalytic cracking of hexane, dodecane and tyre derived oil [TDO] to produce light olefins, particularly ethylene and propylene.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The petrochemical industry is currently strongly based on the production of light olefins ethylene and propylene, which are mainly produced by hydrocarbons from oil. Due to the environmental appeal and unstable oil market scenario, alternative routes to the production of these olefins are being developed, meanly regarding the use of alcohol as raw material. In this context, ethanol is highlighted with Ethanol to Olefins Process (ETO), in which there is catalytic conversion via reactions of dehydration, oligomerization, cracking, isomerization, among others. This work aims to obtain ethylene and propylene from ethanol using ZSM-5 zeolite as catalyst in its acid form. The synthesis of this material was performed using non-conventional sources of silicon and aluminum, kaolin and silica extracted from rice husk ash, and in the absence of nitrogenous templates. The use of seeds was employed together with ethanol, which acts as cotemplate of the zeolitic structure, in different quantities, and in different silica/alumina ratios and crystallisation times. The influence of each of these variables was evaluated with the support of a full factorial experimental design on the final characteristics of the synthesized samples, such as relative crystallinity, chemical composition and textural properties. All samples presented the characteristic crystal structure of ZSM-5 as verified by both X-ray diffractograms and infrared spectra. However, samples with small amounts of seed and ethanol added to short periods of crystallization presented lower crystallinities and specific areas in comparison to other samples. On the other hand, the use of high quantities of seed can lead to the formation of quartz when the crystallization time is extended. With the catalytic tests of ethanol conversion into olefins it was possible to evaluate the influence of synthesis variables, the residence time and the concentration of ethanol in feed, proving the importance of all synthesis variables in the distribution of the reaction products The total conversion of ethanol was observed in all tests made, evidencing the high activity of hZSM-5 in the dehydration of ethanol to ethylene, being the propylene yield strongly influenced by the reaction temperature and characteristics of the catalitic material, with a maximum yield of 27% at 500 °C. The HZSM-5 presented high stability under reaction conditions while maintaining the production of ethylene for more than 40 hours of reaction, whereas the coke formation drastically reduces the production of propylene still in the early hours of reaction.
A indústria petroquímica atualmente está fortemente baseada na produção das olefinas leves eteno e propeno, que são principalmente produzidas através de hidrocarbonetos oriundos do petróleo. Em virtude do apelo ambiental e do instável cenário do mercado de petróleo, rotas alternativas para a produção destas olefinas estão sendo desenvolvidas, principalmente no que tange a utilização de álcoois como matéria-prima. Neste contexto, o etanol ganha destaque com o processo Ethanol to Olefins (ETO), no qual se tem conversão catalítica via reações de desidratação, oligomerização, craqueamento, isomerização, entre outras. O presente trabalho tem por objetivo a obtenção de eteno e propeno através de etanol empregando como catalisador a zeólita do tipo ZSM-5 na forma ácida. A síntese deste material foi realizada utilizando fontes não convencionais de silício e alumínio, o caulim e a sílica extraída da cinza da casca de arroz, e na ausência de direcionadores de estrutura nitrogenados. O emprego de sementes foi adotado juntamente com etanol, que atua como codirecionador da estrutura zeolítica, em diferentes quantidades, assim como em diferentes razões sílica/alumina e tempos de cristalização. A influência de cada uma destas variáveis foi avaliada com o auxílio do planejamento experimental fatorial completo nas características finais das amostras sintetizadas, como cristalinidade relativa, composição química e propriedades texturais. Todas as amostras obtidas apresentaram estrutura cristalina característica da ZSM-5, comprovada tanto nos difratogramas de Raios-X como nos espectros de absorção na região do Infravermelho. Entretanto, as amostras com pequenas quantidades de sementes e de etanol somadas a curtos períodos de cristalização apresentaram cristalinidades e áreas específicas reduzidas em relação às demais amostras. Em contrapartida, o emprego de elevadas quantidades de sementes pode levar a formação de quartzo quando o tempo de cristalização é prolongado. Com os testes catalíticos de conversão de etanol em olefinas foi possível avaliar a influência das variáveis de síntese, do tempo de residência e a concentração de etanol na alimentação, comprovando a importância de todas as variáveis de síntese na distribuição dos produtos da reação. A conversão total de etanol foi observada em todos os testes realizados, evidenciando a elevada atividade da HZSM-5 na desidratação de etanol a eteno, sendo o rendimento a propeno fortemente influenciado pela temperatura de reação e características do material catalítico, com máximo rendimento igual a 27% na temperatura de 500°C. A HZSM-5 apresentou elevada estabilidade nas condições de reação, mantendo a produção de eteno por mais de 40 horas de reação, enquanto que a formação de coque reduz drasticamente a produção de propeno ainda nas primeiras horas de reação.

The influence of catalyst characteristics, i.e., acidity and porosity on the product distribution in the cracking of triglyceride-rich biomass under fluid catalytic cracking (FCC) conditions is reported. It has found that the degradation degree of triglyceride molecules is strongly dependent on the catalysts’ acidity. The higher density of acid sites enhances the conversion of triglycerides to lighter products such as gaseous products and gasoline-range hydrocarbons. The formation of gasolinerange aromatics and light olefins (propene and ethene) is favored in the medium pore channel of H-ZSM-5. On the other hand, heavier olefins such as gasoline-range and C4 olefins are formed preferentially in the large pore structure of zeolite Y based FCC catalyst (Midas-BSR). With both catalysts, triglyceride molecules are mainly converted to a mixture of hydrocarbons, which can be used as liquid fuels and platform chemicals. Hence, the utilization of the existing FCC units in conventional petroleum refineries for processing of triglyceride based feedstock, in particular waste cooking oil may open the way for production of renewable liquid fuels and chemicals in the near future.
Bài báo trình bày kết quả nghiên cứu khả năng tích hợp sản xuất nhiên liệu sinh học và hóa phẩm từ nguồn nguyên liệu tái tạo sinh khối giầu triglyceride bằng công nghệ cracking xúc tác tấng sôi (FCC) trong nhà máy lọc dầu. Kết quả nghiên cứu cho thấy xúc tác có ảnh hưởng mạnh đến hiệu quả chuyển hóa triglyceride thành hydrocarbon. Tính acid của xúc tác càng mạnh thì độ chuyển hóa càng cao và thu được nhiều sản phẩm nhẹ hơn như xăng và các olefin nhẹ. Xúc tác vi mao quản trung bình như H-ZSM-5 có độ chọn lọc cao với hợp chất vòng thơm thuộc phân đoạn xăng và olefin nhẹ như propylen và ethylen. Với kích thước vi mao quản lớn, xúc tác công nghiệp FCC dựa trên zeolite Y ưu tiên hình thành C4 olefins và các olefin trong phân đoạn xăng. Ở điều kiện phản ứng của quá trình FCC, triglyceride chuyển hóa hiệu quả thành hydrocarbon mà có thể sử dụng làm xăng sinh học cho động cơ và olefin nhẹ làm nguyên liệu cho tổng hợp hóa dầu.

Mestrado em Engenharia Química
Nos últimos anos tem-se observado um aumento da procura de diesel, comparativamente com a gasolina. A produção de gasolina aumentou à custa do aparecimento das unidades de FCC. Deparando com este facto, a produção de diesel tem de acompanhar a sua crescente procura, e essa reposta encontra-se precisamente nestas unidades de FCC. Aquando a formação de gasolina nestas unidades, um dos subprodutos gerados em maior quantidade é a corrente de olefinas leves. As olefinas, na presença de um catalisador, e sujeitas a alta pressão e temperatura formam produtos de elevado valor comercial na gama do diesel. Nesta dissertação foi estudada, precisamente, a oligomerização de olefinas leves através de ensaios catalíticos. O processo consiste na combinação no mesmo reator, de um catalisador zeolítico a 200 com uma alimentação de buteno, acompanhado de um caudal de inerte para diluição do reagente. A oligomerização do 1-buteno permite obter produtos na gama diesel C10 a C20. A instalação experimental foi montada no início da dissertação. Antes da sua utilização, sucessivas correcções a nível de fugas, durante vários ciclos de aquecimento, tiveram de ser efectuadas de modo a deixá-la operacional. Foi utilizada para activação do catalisador, calibração do GC e para a realização da oligomerização de 1-buteno. Foi utilizado o catalisador zeolítico H-ZSM-5 comercial (Zeolyst CBV 3024E com uma razão Si/Al=15). Este catalisador devido à sua microporosidade e estrutura permite a ocorrência de selectividade de forma, que favorece a formação de produtos lineares. A instalação foi testada e foram efectuadas experiências a alta pressão (30 bar), tendo sido possível obter produtos na gama do diesel. Estes produtos foram identificados por cromatografia gasosa com um detector FID acoplado. Um modelo de equilíbrio e cinética foi estudado e programado de modo a prever o comportamento da reacção através da variação do tempo adimensional de reacção, pressão, temperatura e da alimentação.
In past years it has been observed an increase demand of diesel compared to gasoline. The production of gasoline has increased significantly after the installation of FCC units. During gasoline production, light olefins are obtained as side product. These light olefins, in the presence of a catalyst and submitted to high temperature and pressure, form high commercial products in diesel range. In this work, 1-butene oligomerization via zeolite catalysis was studied. The process can be conducted in a reactor with an acid catalyst at 200 with 1-butene diluted in nitrogen (feed) to form products in diesel range (C10-C20). The experimental set-up was assembled at the beginning of the thesis. Before use, successive leak tests, consisting of heating-cooling cycles, have been performed to leave the equipment operational. The installation is able to carry out the catalyst activation and 1-butene oligomerization. With respect to the catalyst, commercial H-ZSM-5 (Zeolyst CBV 3024E, Si/Al=15) has been used. This catalyst due its microporosity and its structure provides shape selectivity, which favours the formation of more linear products. The installation was tested and several runs were performed at high pressure (30 bar), which allowed to obtain diesel range products. Their identification was accomplished by gas chromatography with FID detector. The modeling of literature data was studied in order to predict the reaction behaviour for distinct sets of reaction time, pressure, temperature and feed concentration.

In this work, the reaction scheme of the MTO process was written in terms of elementary steps and generated by means of a computer algorithm characterizing the various species by vectors and Boolean relation matrices. The number of rate parameters is very large. To reduce this number the rate parameters related to the steps on the acid sites of the catalyst were modeled in terms of transition state theory and statistical thermodynamics. Use was made of the single event concept to account for the effect of structure of reactant and activated complex on the frequency factor of the rate coefficient of an elementary step. The Evans-Polanyi relation was also utilized to account for the effect of the structure on the change in enthalpy. The structure was determined by means of quantum chemical software. The number of rate parameters of the complete reaction scheme to be determined from experimental data is thus reduced from 726 to 30. Their values were obtained from the experimental data of Abraha by means of a genetic algorithm involving the Levenberg-Marquardt algorithm and combined with sequential quadratic programming. The retained model yields an excellent fit of the experimental data. All the parameters satisfy the statistical tests as well as the rules of carbenium ion chemistry. The kinetic model also reproduces the experimental data of Marchi and Froment, also obtained on SAPO-34. Another set of their data was used to introduce the deactivation of the catalyst into the kinetic equations. This detailed kinetic model was used to investigate the influence of the operating conditions on the product distribution in a multi-bed adiabatic reactor with plug flow. It was further inserted into riser and fluidized bed reactor models to study the conceptual design of an MTO reactor, accounting for the strong exothermicity of the process. Multi-bed adiabatic and fluidized bed technologies show good potential for the industrial process for the conversion of methanol into olefins.

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Nowadays, the Brazilian petroleum is extracted from very deep fields and possesses a high naphthenic hydrocarbons composition, which imposes new challenges to refineries and specially to the catalytic cracking process. In that process, the catalyst must act maximizing the production of the highly demanded gasoline, diesel and light olefins from heavy fractions. Taking into consideration the above discussed context, this work aimed to evaluate the effect of basic or acid treatments applied on ZSM-5 zeolites (Si/Al=12 or 23) in the activity to cyclohexane or methylcyclohexane transformation. XRD and 27Al-NMR showed that the dealuminated zeolites presented an increase in their crystallinity due to the extra-framework aluminum lixiviation. On the other hand, in the desilicated zeolites occurred a decrease in their crystallinity as a consequence of the extra-framework aluminum generation. MEV images do not evidence any morphological change that could have been produced by the acid or basic treatments, however, the desilicated ZSM-5 zeolites treated under harder conditions presented significant textural modifications. As expected, the chemical ICP analyses showed a decrease in the Si/Al ratio in the desilicated zeolites and an increase of that ratio for those dealuminated ones, being the last variation more significative in the external surface of the zeolite crystals, as was evidenced by XPS analyses. Data from NH3-TPD showed that the acid treatment resulted in a higher ratio of strong acid sites, which suffered more deactivation during reaction. N2 fisisorption analyses of the ZSM-5 zeolites, showed that the desilication done at higher temperature was more efficient to mesopore generation. In the cyclohexane and methylcyclohexane transformation, the dealuminated zeolites were less active due to their lower aluminum content, nevertheless were more stable and presented a small increase to light olefins selectivity. The desilicated ZSM-5 zeolites presented higher activity and higher yield to light olefins that were supported by their lower Si/Al ratio and mainly by the presence of mesoporosity that enhanced the reagents and products internal diffusivity.
A produção nacional de petróleo, extraído de jazidas cada vez mais profundas, possui um elevado teor de hidrocarbonetos naftênicos, o que impõe novos desafios às refinarias brasileiras e, em particular, ao processo de craqueamento catalítico. Nesse processo, o catalisador deve maximizar a transformação das frações pesadas em produtos de alta demanda como gasolina, diesel e olefinas leves. Nesse contexto, esta dissertação objetivou avaliar o efeito de tratamentos de lixiviação ácida ou básica em zeólitas ZSM-5 (Si/Al=12 ou 23), na atividade para a transformação de cicloexano ou metilcicloexano. Dados de DRX e 27Al-RMN mostraram que as zeólitas desaluminizadas apresentaram um aumento da sua cristalinidade devido à remoção de átomos de alumínio extra-rede, por outro lado, nas zeólitas dessilicalizadas ocorreu uma redução da cristalinidade devido à geração de alumínio extra rede. As micrografias de MEV não evidenciaram modificação morfológica devido aos tratamentos, entretanto nas amostras dessilicalizadas sob condições mais severas, houve significativa mudança das propriedades texturais. Como esperado, as análises químicas por ICP mostraram uma redução na razão Si/Al para as amostras dessilicalizadas e um aumento dessa razão para as zeólitas desaluminizadas, sendo essa variação mais significativa na superfície externa dos cristais, como mostraram resultados de XPS. As análises de DTP-NH3 mostraram que o tratamento ácido resultou numa maior proporção de sítios ácidos fortes, os quais sofreram maior desativação durante a reação. Dados de fisissorção de N2 das zeólitas mostraram que a dessilicalização em temperatura mais elevada foi mais eficiente na geração de mesoporos. Na transformação do cicloexano e do metilcicloexano, as zeólitas desaluminizadas apresentaram menor conversão como resultado da diminuição do teor de alumínio, entretanto tiveram maior estabilidade e apresentaram um ligeiro aumento na seletividade a olefinas leves. As amostras dessilicalizadas apresentaram maiores conversões e rendimentos a olefinas leves, que se justificaram em função da diminuição da razão Si/Al, mas principalmente, como resultado da presença de mesoporosidade, que melhorou a difusão interna de reagentes e produtos.

The global demand and sustainability concerns for producing light olefins encouraged researchers to look for an alternative and sustainable feedstock. Alkenes, such as ethene, propene and butene, are known as light olefins. Olefins are the backbone of the chemical industry because they serve as the chemical building blocks for the manufacture of polymers, fibers, and numerous organic chemicals. Feedstocks such as naphtha, natural gas and liquefied petroleum gas (LPG) are currently used for producing light olefins, but they are non-renewable and hence unsustainable. In contrast, biomass as a potential feedstock for the production of fuels and chemicals is renewable. Microalgae, in particular, are a promising resource due to their fast growth rate and ability to act as a CO2 sink. The objective of my research was to assess the potential of thermochemical production of the light olefins ethene, propene, and butene from the marine microalga Nannochloris oculata in the absence and presence of catalysts and study the effect of catalyst to cell mass ratio on the production of these chemicals. Thermal cracking was conducted using two catalysts, aluminosilicate (Si/Al) and H-ß zeolite at 400-650 °C in a semi-batch reactor system and gas analysis was performed using mass spectrometry. Cracking of N. oculata by the aluminosilicate catalyst was studied in more detail at catalyst-to-algae mass ratios of zero, 5:1, 10:1 and 20:1 using (Si/Al) catalyst and a comparative study was performed at catalyst-to-algae mass ratio of 10:1 using (Si/Al) and H-ß zeolite catalyst. The formation of light olefins ethene, propene, and butene was quantified. Higher temperature and catalyst to algae ratio led to an increase in the yield of all olefins, although a diminishing effect was observed above 600 °C and a ratio of 5:1. Although ethene was the most significant product, the concentration of all olefins increased significantly, when catalysts were employed in the cracking reaction. Moreover, the comparative study revealed that ethene was the most significant product when (Si/Al) was used and propene was the most significant product when H-ß zeolite was used.

The use of organic bases in the synthesis of zeolites can lead to the crystallisation of several completely original structures. Zeolite ZSM-5 is produced using tetrapropylammonium cations. This cationic material can be replaced by amines or diamines, although in this case it becomes more difficult to synthesise the zeolite. ZSM-5 has a three dimensional channel system, having apertures which are slightly larger than 0,5 nm. This means that during reactions, there is a control over the size of the molecules that can enter or exit from the pores of the zeolite: this process is called "shape selective11 catalysis. The conversion of methanol over ZSM-5 proceeds via dehydration to give firstly dimethyl ether and then light olefins. The olefins may subsequently undergo dehydrocyclisation to produce a mixture of light paraffins and aromatics, which are restricted to monocyclics and do not contain more than ten carbon atoms. As a result of these properties, ZSM-5 is an ideal catalyst for several industrial processes. Methanol can be converted directly to synthetic gasoline, or by restricting the conversion, olefins can be obtained. Naturally olefins themselves, as well as alkanes, can be converted to aromatics. An entirely different reaction has been found by using a high pressure and relative low temperature, where olefins can be oligomerised to produce diesel.

En vue de promouvoir l’économie circulaire, de nombreux procédés chimiques sont actuellement réexaminés afin de développer des variantes plus durables. Cela a mené à une forte augmentation de production au cours des 60 dernières années, entraînant une production totale de 367 millions de tonnes en 2020.La méthodologie a ensuite été généralisée à l’aide d’apprentissage automatique non supervisé, ce qui a permis de dépasser les trois dimensions et de réduire le besoin d’intervention humaine. L’espace des descripteurs généré à partir de données microcinétiques (catalyseurs virtuels) est exploré en utilisant la méthode systématique du regroupement (clustering) et du classement (labelling) non supervisés. L’espace de la performance du catalyseur est regroupé en clusters et le nombre minimale de clusters est identifié. Chaque catalyseur virtuel (représenté par une certaine combinaison de descripteurs) est identifié du point de vue du cluster auquel il appartient. Il est ainsi possible d’obtenir l’étendue des valeurs des descripteurs dans le cluster ayant le meilleur rendement d’alcènes légers. Il est observé que les valeurs obtenues sont conformes à celles du catalyseur virtuel optimal identifié dans l’inspection visuelle précédente. On peut donc conclure qu’une méthode combinant la microcinétique et l’apprentissage automatique a été présentée pour le développement des catalyseurs et pour l’investigation détaillée de leurs propriétés, tout en diminuant le besoin d’intervention humaine.Finalement, la méthode d’apprentissage automatique a été étendue dans l’intention de pouvoir réaliser des prédictions de plusieurs sélectivités en se concentrant sur la production des alcènes légers à différentes conditions opérationnelles. Afin d’atteindre cet objectif, 4 modèles d’apprentissage automatique alternatifs ont été employés, i.e. la méthode lasso (lasso regression), la méthode des k plus proches voisins (k nearest neighbor regression ou KNN), la méthode de la machine à vecteurs de support (support vector machine regression ou SVR) et le réseau de neurones artificiels (Artificial Neural Network ou ANN). Les capacités de ces techniques sont évaluées par rapport à la reproduction du comportement linéaire de la conversion et la sélectivité en fonction des variables du procédé comme cela a été simulé par le modèle SEMK. Il est constaté que les modèles à base d’un réseau de neurones artificiels correspondent le plus aux résultats de référence du modèle SEMK. Une analyse supplémentaire utilisant la technique d’interprétation de la valeur SHAP a été appliquée aux modèles à base d’un réseau de neurones artificiels ayant la meilleure performance, en vue de mieux expliquer le fonctionnement des modèles.L’ensemble de l’étude a rapporté des connaissances essentielles, telles que les descripteurs de catalyseur optimales : les enthalpies de chimisorption atomique de l’hydrogène (QH ≈ 234 kJ/mol), du carbone (QC ≈ 622 kJ/mol) et de l’oxygène (QO ≈ 575 kJ/mol), pour la conception de catalyseurs ayant une sélectivité en alcènes légers élevée en utilisant un modèle SEMK mécaniste. De plus, l’étendue des conditions opérationnelles menant à la meilleure sélectivité en alcènes légers a été déterminée en adoptant plusieurs stratégies de modélisation (le concept des SEMK et l’apprentissage automatique). Il a été constaté que les effets de la température (580-620K) et la pression (1-2 bar) étaient les plus importants. Ensuite, une investigation est réalisée dans le but d’évaluer à quel point les résultats des modèles d’apprentissage automatique correspondent à ceux du modèle SEMK. Par exemple, une analyse préliminaire a pu être réalisée en utilisant un modèle d’apprentissage automatique pour l’analyse des données obtenues à l’aide d’expérimentation à haut débit. Ensuite, le modèle mécaniste a permis d’acquérir une compréhension chimique approfondie
Striving towards a circular economy has led to the re-investigation of many existing processes, with the target of developing more sustainable variants. In our present economy, plastics form an important and omnipresent material affecting our daily lives. They are inexpensive, durable, corrosion resistant, and light weight leading to their use in a wide variety of applications.Within the plastic chemical recycling scheme, Fischer-Tropsch synthesis (FTS) could play a key role as the syngas feedstock that is converted in it, can be generated via the gasification of the considered plastics. This syngas is then chemo-catalytically converted into hydrocarbons such as paraffins and light olefins. Typical FTS catalysts are based on supported cobalt or iron species.Among the mechanistic kinetic models, the comprehensive variant based on the Single Event MicroKinetics (SEMK) concept has been widely applied in the field of oligomerization, autoxidative curing, etc. and has proven to be a versatile tool to simulate Fischer-Tropsch synthesis. However, developing mechanistic models for every chemical engineering challenge is not always feasible due to their complexity and the in-depth knowledge required to build such models.A detailed evaluation on the potential of using machine learning approaches to match the performance of results obtained using the Single-Event MicroKinetic model was carried out. Initially, the focus was on a single dominant output scenario (methane selective catalyst). The current work thus shows that more widely applied techniques in data science can now be applied for systematic analysis and interpretation of kinetic data. Similar analysis using experimental data can also help experimenters in their preliminary analysis, to detect hidden trends in the data, and thus to identify importance features. After gaining confidence on the investigated interpretation techniques, for the FTS reaction with single dominant output, a similar investigation on the potential of iron based catalysts with enhanced light olefin selectivity is carried out next

L'hydrogénation du CO et du CO2 sont une voie intéressante de conversion des matières premières non pétrolières et renouvelables tels que la biomasse, le plastique et les déchets organiques, en carburant et en produits chimiques. L'activité, la sélectivité vers la production d’oléfines légères et la stabilité sont des défis majeurs de ces réactions sur les catalyseurs à base de fer. Dans cette thèse, nous avons synthétisé différents catalyseurs à base de fer pour l'hydrogénation du CO et du CO2 afin d'obtenir des catalyseurs hautement sélectifs, actifs et stables. Pour l'hydrogénation du CO, SiO2 a été utilisée comme support tandis que pour la réaction d'hydrogénation du CO2, les catalyseurs supportés par de la ZrO2 ont présenté les résultats les plus encourageants. Les résultats sont appuyés sur l'expérimentation à haut débit (EHD) pour identifier parmi 27 promoteurs les plus efficaces pour la synthèse de FT en évaluant également les différentes tendances de sélectivité en la réaction FT. Les tests EHD nous ont permis d'identifier clairement Sn, Sb, Bi et Pb comme les promoteurs les plus prometteurs afin d'obtenir des catalyseurs de Fe avec une plus grande activité. Après, nous nous sommes concentrés sur l'étude des promoteurs Sb et Sn, sur la performance catalytique des catalyseurs à base de fer supportés sur SiO2, en utilisant une combinaison de techniques avancées et in-situ. Les images MET du catalyseur FeSn/SiO2 activé ont montré des nanoparticules de Sn hautement dispersées sur le support de silice. D'autre part, le catalyseur FeSb/SiO2 activé a montré une morphologie coeur-coquille. Plus petite quantité de dépôt de carbone détectée est cruciale pour une meilleure stabilité des catalyseurs promus par Sn- et Sb dans la réaction FT. Finalement, nous nous sommes concentrés sur l'identification des promoteurs pour les catalyseurs de fer supportés sur ZrO2 pour la réaction d’hydrogénation du CO2. Nous avons observé une nette augmentation de la vitesse de réaction pour les catalyseurs promus par le K et le Cs. L’EHD a clairement montré que la présence de K est essentielle pour obtenir une plus grande sélectivité en oléfines légères. En plus, le Mo, Cu, Cs, Ce et Ga ont été identifiés comme des promoteurs capables d’augmenter encore la sélectivité en oléfines. Le travail effectué au cours de cette thèse a permis de concevoir de nouveaux catalyseurs pour la réaction d'hydrogénation du CO et du CO2 qui pourraient être facilement mis en oeuvre au niveau industriel. Les catalyseurs étudiés pour les deux réactions ont montré une amélioration de trois aspects clés : l'activité, la sélectivité et la stabilité
CO and CO2 Hydrogenation are an attractive way to convert non-petroleum and renewable feedstocks such as biomass, plastic and organic waste into fuels and chemicals. Activity, selectivity to light olefins and stability are major challenges of these reactions over Fe catalysts. In this thesis, we synthesized different iron-based catalysts for both CO and CO2 hydrogenation in order to get highly selective, active and stable catalysts. For CO hydrogenation SiO2 was used as support while for CO2 hydrogenation reaction ZrO2 supported catalysts presented the most encouraging results. We relied on High Throughput Experimentation (HTE) to identify among 27 promoters the most efficient ones for FT synthesis at the same time that different selectivity trends were evaluated. HTE tests allowed us to clearly identify Sn, Sb, Bi and Pb as the most promising promoters in order to obtain Fe catalysts with higher activity in FT synthesis. Then, we focused on studying the strong promoting effects of Sb and Sn on the catalytic performance of SiO2 supported iron Fischer Tropsch catalysts using a combination of advanced and in-situ techniques. TEM in the activated FeSn/SiO2 catalyst showed highly dispersed Sn nanoparticles on the silica support. On the other hand, activated FeSb/SiO2 catalyst showed a core-shell morphology. Additionally, smaller amount of carbon deposition detected is crucial for better stability of the Sn- and Sb-promoted catalysts in FT reaction. Finally, we focused on the identification of efficient promoters for ZrO2 supported iron catalysts in CO2 hydrogenation reaction. We observed the most pronounced increase in the reaction rate for the K and Cs promoted catalysts. HTE clearly showed that the presence of K was essential to achieve higher light olefin selectivity. Additionally, Mo, Cu, Cs, Ce and Ga were identified as possible promoters to further increase the selectivity of CO2 hydrogenation to this fraction. The work performed during this thesis allowed to design new catalysts for CO and CO2 hydrogenation reaction that could be easily implemented at industrial level. Catalysts studied for both reactions showed improvement three key aspects: activity, selectivity, and stability

A new type of solid catalyst for use in alkylation of isobutene has been developed using a mesoporous support, MCM-41 loaded with an organic superacid, trifluoro methanesulfonic acid (triflic acid). The state of the acid in the pores, catalytic activity and characterization has been studied for this catalytic system. The organic acid has been found to exist in an adsorbed state within the support as well as in a "free" form without a significant association with the surface. The activity of the catalyst during alkylation reactions has been found to be much improved over traditional solid acid zeolite catalysts. The mechanism by which the reaction proceeds is consistent with liquid phase reactions invoking carbocation based mechanisms and a relatively new type of reaction, self alkylation, appears to be occurring in the reactions. Isobutene delivers much higher conversion levels compared to the linear butenes due to carbocation stability considerations. 1-butene however yields higher than expected selectivites for octanes compared to 2-butene. This phenomenon has been related to the self alkylation regime. The catalyst possesses very high surface areas but exhibits some structural collapse as a result of the acid loading. Thermal analysis data has also shown that significant coke deposits begin at reaction temperatures above 200C̕.

With the global growth in fuel demand for transportation and increasing environmental concerns, the oligomerization of light olefins obtainable from fossil or renewable sources and refinery streams, represents a promising route for producing clean synthetic fuels with low aromatics and sulphur contents, and other added-value chemicals. This thesis deals with the oligomerization of 1-butene to produce diesel range products, under high pressure and continuous operation, in the presence of heterogeneous acid catalysts. The oligomerization of light olefins is a complex reaction system. The yields and characteristics of the products are governed by the properties of the catalytic materials and the operating conditions. These aspects were investigated with the practical goal of producing clean diesel range products, using porous inorganic acid catalysts based on silicon and aluminium oxides. The materials were prepared via different methodologies and characterized by complementary techniques, with special attention given to morphological, textural and acid properties. The catalytic performances were evaluated in terms of activity, selectivity to clean diesel type products and stability, based on experimental studies and multivariate statistical tools. The characteristics of the catalytic reaction products were studied based on comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC-ToFMS) and nuclear magnetic resonance (NMR) spectroscopy. In the search for promising catalysts, the research work evolved from eco-friendly mesoporous aluminosilicate of the type TUD-1 prepared via one-pot or stepwise approaches, and a composite material comprising BEA nanocrystallites embedded in a TUD-1 siliceous matrix, to micro/mesoporous zeotypes possessing different topologies (BEA, MFI) prepared via bottom-up or top-down approaches. The catalysts were benchmarked with commercially available zeolites and a catalyst based on the MFI topology which was developed for commercial oligomerization processes, namely COD-9. The micro/mesoporous zeotypes outperformed the commercial zeolites (Beta, ZSM-5, COD-9), leading to conversions of butenes of up to 86 % and selectivity to diesel ranged products of up to 71 wt.%, at 200 ºC, 30 bar and 2.2 g gcat-1 h-1. Based on principal component analysis (PCA), structure-activity relationships were established that pointed to the importance of good compromises between textural and acid properties for maximizing the yields of clean diesel range products - intermediate concentrations of acid sites and enhanced mesoporosity resulted in superior catalytic performances. One of the best-performing catalysts was MZS-0.4-Cl prepared via top-down approach from commercial ZSM-5. Optimization studies were carried out for 1-butene oligomerization over this type of catalyst. The optimization was based on a Box-Behnken design of experiments (DoE) and response surface methodology (RSM), contemplating the yields of the diesel range products, as well as the product quality (reduced aromatics content). These studies indicated that the favourable operating conditions were in the ranges 220-250 ºC of reaction temperature, 30-40 bar and 2.5-3.5 g1C4 gcat-1 h-1 weight hourly space velocity. Finally, PCA studies were conducted for all materials studied in this thesis, to show the complex interplay of material properties influencing the catalytic performances.
Com o aumento global do consumo de combustíveis para o setor dos transportes e as crescentes preocupações ambientais, a oligomerização de alcenos leves provenientes de fontes fósseis ou renováveis, ou de efluentes de refinarias, representa uma via de valorização promissora para produzir combustíveis limpos com reduzidos teores de compostos aromáticos e enxofre, e outros produtos químicos de valor acrescentado. Esta tese incide na oligomerização do 1-buteno em produtos do tipo diesel sintético, em reator contínuo, a alta pressão, usando catalisadores heterogéneos ácidos. A oligomerização de alcenos leves envolve mecanismos reacionais complexos. Os rendimentos e as características dos produtos dependem das propriedades dos materiais catalíticos e das condições de operação. Estes aspetos foram investigados com o objetivo prático de produzir diesel limpo, usando catalisadores ácidos inorgânicos porosos à base de óxidos de silício e alumínio. Os materiais foram preparados por diversas metodologias e caracterizados por técnicas complementares, com especial atenção dada às propriedades morfológicas, texturais e ácidas. Os desempenhos catalíticos foram avaliados em termos de atividade, seletividade para produtos do tipo diesel e estabilidade, com base em estudos experimentais e o recurso a ferramentas estatísticas de análise multivariada. As misturas de produtos reacionais foram caracterizadas com base em cromatografia de gás bidimensional abrangente acoplada a espectrometria de massa com analisador por tempo de voo (GC×GC-ToFMS) e espectroscopia por ressonância magnética nuclear (RMN). À descoberta de catalisadores promissores, o trabalho de investigação evoluiu de aluminossilicatos mesoporosos do tipo TUD-1 sintetizados por metodologias relativamente limpas (sem agentes tensoativos) e um compósito de nanocristais de zeólito Beta dispersos numa matriz do tipo TUD-1, até zeótipos micro- e mesoporosos possuindo diferentes topologias (BEA, MFI) e preparados por estratégias bottom-up (não destrutivas) ou top-down. Os desempenhos dos catalisadores preparados foram comparados com zeólitos comerciais e um catalisador que foi desenvolvido para processos comerciais de oligomerização, nomeadamente o COD-9 (baseado na topologia MFI). Os zeótipos micro/mesoporosos apresentaram melhores desempenhos do que os zeólitos comerciais (Beta, ZSM-5, COD-9), obtendo-se conversões de butenos até 86 % e seletividades para produtos do tipo diesel até 71 % (m/m), a 200 ºC, 30 bar e 2.2 g gcat-1 h-1. Com base em análise estatística de componentes principais (PCA) foram estabelecidas relações de atividade-estrutura que apontaram para a necessidade de haver compromissos entre as propriedades texturais e ácidas para maximizar os rendimentos em diesel limpo – concentrações intermédias de centros ácidos e elevada mesoporosidade resultaram em melhores desempenhos catalíticos. Um dos catalisadores mais promissores foi o MZS-0.4-Cl preparado pela abordagem top-down a partir do zeólito comercial ZSM-5. Foram realizados estudos de otimização para a oligomerização do 1-buteno usando este tipo de catalisador. A otimização baseou-se no desenho fatorial de experiências (DoE, com uma matriz Box-Behnken) e a metodologia da superfície de resposta (RSM), contemplando os rendimentos em produtos do tipo diesel, assim como aspetos da qualidade dos produtos (teor de compostos aromáticos). Estes estudos indicaram as seguintes gamas de condições de operação mais favoráveis: 220-250 ºC de temperatura de reação, 30-40 bar de pressão e 2.5-3.5 g1C4 gcat-1 h-1 de velocidade espacial por unidade de massa de catalisador. Por fim, estudos de PCA com todos os materiais estudados nesta tese mostraram que a influência das propriedades dos materiais nos desempenhos não é trivial.
Programa Doutoral em Engenharia Química

Solid phosphoric acid is a catalyst used for the upgrading of light olefins into fuels. To delve into the mechanism of olefin dimerisation over the catalyst, the oligomerisation of 1- hexene was investigated over a wide range of operating conditions. The reaction progression of 1-hexene dimerisation over solid phosphoric acid was interpreted by means of kinetic experiments for both a linear hexene (1-hexene) and a branched hexene (2,3-dimethylbutene). The reaction rate for both reagents was described by using an elementary kinetic model. From the experimental data it was shown that the rate of dimerisation of branched hexenes was faster than the rate observed for linear hexene dimerisation. To correlate the two sets of kinetic data, the reaction network was expanded to incorporate skeletal isomerisation of 1-hexene with dimerisation only taking place by the co-dimerisation of linear and branched hexenes and the dimerisation of branched hexenes. The fit of the kinetic equation demonstrated that the reaction rate of 1-hexene is essentially controlled by the rate of skeletal isomerisation. Due to the large activation energy for skeletal isomerisation, low reaction temperatures favoured the co-dimerisation of linear and branched hexenes whereas at higher temperatures, the reaction rate was dominated by the dimerisation of branched hexenes. The product distribution indicated that, because of the fast rates of both cracking and secondary dimerisation (dimerisation of cracked products), the product distribution instantaneously reached a pseudo equilibrium after the dimerisation of hexenes. Therefore the carbon distribution was found to depend only on the reaction temperature, not on the residence time in the reactor. Solid phosphoric acid is a supported liquid phosphoric acid where the condensed state of the acid, e.g. ortho phosphoric acid (H3PO4) and pyro phosphoric acid (H4P2O7), is dependent on the quantity of water present in the reaction mixture. With a decrease in water content, the distribution of acid shifts and the ortho phosphoric acid becomes more condensed (H4P2O7, H5P3O9 etc.), i.e. high water content → low acid strength, low water content → high acid strength. The experiments completed at various degrees of catalyst hydration and free acid loading showed that the rate of reaction over solid phosphoric acid was dependent on the acid strength of the catalyst. The effect of acid strength on the reaction rate was integrated into the rate constants by means of an exponential dependency on acid strength. It was also shown that both the product distribution and the degree of branching remained unaffected by acid strength. The constant product indicates that the rate of cracking is limited by the rate of oligomerisation of hexenes, irrespective of the acid strength of the catalyst. Since the product from the dimerisation of 1-hexene could be used as fuel, the quality of the desired fuel would therefore depend solely on the reaction temperature, not on the hydration of the catalyst. The work performed in this thesis has been published in two peer-review articles: 1. Schwarzer R.B., Du Toit E. and Nicol W. (2008) Kinetic model for the dimerisation of 1-hexene over a solid phosphoric acid catalyst, Applied Catalysis A: General, 340, 119-124. 2. Schwarzer R.B., Du Toit E. and Nicol W. (2009) Solid phosphoric acid catalysts: The effect of free acid composition on selectivity and activity for 1-hexene dimerisation, Applied Catalysis A: General, 369, 83-89.
Thesis (PhD(Eng))--University of Pretoria, 2012.
Chemical Engineering
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This dissertation presents the conception, synthesis, optimization and evaluation of a novel class of catalysts used in aromatization of short chain hydrocarbons. It was found that when an acidic H-ZSM-5 zeolite with Si/Al atomic ratio equal to about 40, was mixed with a ZnO based co-catalyst, aromatization selectivity followed the principles of synergy. For a feed made up of mostly ethylene, as generated in a bench scale propane steam-cracker, ZnO precipitate in the amount of 5wt.-% or ZnO/Al$\sb2$O$\sb3$ co-precipitate in the amounts of 5-20wt.-%, provided optimum catalytic activity. Furthermore, it was established that for the co-precipitate, a Zn/Al atomic ratio equal to or greater than 1.0 conferred the best performance. The distance between acid (aromatizing) major component and oxide minor component, was evaluated as being greater than 1$\mu$m. The increased aromatization selectivity found in hybrid systems was explained by the migration of hydrogen adsorbed species across surface boundaries. A Hydrogen Back-Spillover model was devised. Sink and scavenging actions on co-catalyst surfaces, resulting in the formation of more important amounts of molecular hydrogen or ethane, were investigated. The differences in intrinsic selectivities of ZnO precipitate and ZnO/alumina co-precipitate (Zn/Al = 1.0) were explained. The nature of the active sites was deduced from physico-chemical characterization data correlated with catalytic activity experiments. The influence of preparation techniques as well as starting reagents was studied. The disordered nature of the co-precipitate, which is favorable to surface restructuration during induction, was found to be a pre-requisite. Kinetic studies of ethylene aromatization on a pure H-ZSM-5 zeolite, yielded a negative apparent activation energy, indicative of intracrystallite diffusional limitations. Thermogravimetric studies of used catalysts indicated that soluble coke was trapped inside the zeolite pores when the catalyst was used in its pure form at high temperatures. Migration of C$\sb9\sp+$ aromatic coke precursors out of ZSM-5 channels is induced by the co-catalyst, which gives rise to synergy in hybrid systems. A model based on transport barriers was proposed to explain hydrogen and coke migration.

Membrane separations provide a potentially attractive technology over conventional processes due to their advantages, such as low capital cost and energy consumption. The goal of this thesis is to design hybrid membranes that facilitate specific gas separations, especially olefin/paraffin separations. This thesis focuses on the designing dendrimer-based hybrid membranes on mesoporous alumina for reverse-selective separations, synthesizing Cu(I)-dendrimer hybrid membrane to facilitate olefin/paraffin separations, particularly ethylene/methane separation, and investigating the influence of solvent, stabilizing ligands on facilitated transport membrane. Reverse-selective gas separations have attracted considerable attention in removing the heavier/larger molecules from gas mixtures. In this study, dendrimer-based chemistry was proved to be an effective method by altering dendrimer structures and generations. G6-PIP, G4-AMP and G3-XDA are capable to fill the alumina mesopores and slight selectivity are observed. Facilitated transport membranes were made to increase the olefin/paraffin selectivity based on their chemical interaction with olefin molecules. Two approaches were explored, the first was to combine facilitator Cu(I) with dendrimer hybrid membrane to increase olefin permeance and olefin/paraffin selectivity simultaneously, and second was to facilitate transport membrane functionality by altering solvents and stabilizing ligands. Promising results were found by these two approaches, which were: 1) olefin/paraffin selectivity slightly increased by introducing facilitator Cu(I), 2) the interaction between Cu(I) and dendrimer functional groups are better known.

The increase in shale gas exploitation has motivated the studies towards new processes for converting light alkanes into higher valuable chemicals, including fuels. The works in this dissertation focuses on two processes: propane dehydrogenation and ethylene oligomerization. The former involves the conversion of propane into propylene and hydrogen, while the latter converts light alkenes into higher molecular weight products, such as butylene and hexene.

The thesis project focuses on understanding the effect of geometric effects of Pt alloy catalysts for propane dehydrogenation and the methodologies for their characterization. Pt-Co bimetallic catalysts were synthesized with increasing Co loadings, characterized and evaluated for its propane dehydrogenation performance. In-situ synchrotron X-Ray Powder Diffraction (XRD) and X-Ray Absorption (XAS) were used to identify and differentiate between the intermetallic compound phases in the nanoparticle surface and core. Difference spectra between oxidized and reduced catalysts suggested that, despite the increase in Co loading, the catalytic surface remained the same, Pt₃Co in a Au₃Cu structure, while the core became richer in Co, changing from a monometallic Pt fcc core at the lowest Co loading to a PtCo phase in a AuCu structure at the highest loading. Co^II single sites were also observed on the surface, due to non-reduced Co species. The catalytic performance towards propane dehydrogenation reinforced this structure, as propylene selectivity was around 96% for all catalysts, albeit the difference in composition. The Turnover Rate (TOR) of these catalysts was also similar to that of monometallic Pt catalysts, around 0.9 s^-1, suggesting Pt was the active site, while Co atoms behaved as non-active, despite both atoms being active in their monometallic counterparts.

In the second project, a single site Co^II catalyst supported on SiO₂ was evaluated for ethylene oligomerization activity. The catalyst was synthesized, evaluated for propane dehydrogenation, propylene hydrogenation and ethylene oligomerization activities and characterized in-situ by XAS and EXAFS and H₂/D₂ exchange experiments. The catalysts have shown negligible conversion at 250^oC for ethylene oligomerization, while a benchmark Ni/SiO₂ catalyst had about 20% conversion and TOR of 2.3x10^-1 s^-1. However, as the temperature increased to above 300^oC, ethylene conversion increased significantly, reaching about 98% above 425^oC. In-situ XANES and EXAFS characterization suggested that H₂ uptake under pure H₂ increased in about two-fold from 200^oC to 500^oC, due to the loss of coordination of Co-O bonds and formation of Co-H bonds. This was further confirmed by H₂/D₂ experiments with a two-fold increase in HD formation per mole of Co. In-situ XAS characterization was also performed with pure C­₂H₄ at 200^oC showed a similar trend in Co-O bond loss, suggesting the formation of Co-alkyl, similarly to that of Co-H. The in-situ XANES spectra showed that the oxidation state remained stable as a Co²⁺ despite the change in the coordination environment, suggesting that the reactions occurs through a non-redox mechanism. These combined results allowed the proposition of a reaction pathway for dehydrogenation and oligomerization reactions, which undergo a similar reaction intermediate, a Metal-alkyl or Metal-Hydride intermediates, activating C-H bonds at high temperatures.

Light olefins such as ethylene and propylene, are considered the backbone of the petrochemical industry. They are the precursors of numerous plastic materials, synthetic fibers and rubbers. Commercially proven light olefin production technologies such as Steam Cracking (SC), Fluid Catalytic Cracking (FCC), and Deep Catalytic Cracking (DCC) are believed to have reached their full potential and cannot accommodate current demands of the petrochemical industry. The market demand for ethylene and propylene is projected to be about 140 and 90 million tons by year 2010, respectively. These current technologies cannot respond sufficiently to the rapidly growing demand for propylene, since propylene is only produced as a co-product of ethylene production. In addition, the high-energy consumption and the high GHG emissions are major setbacks for SC, which is regarded as the main light olefin technology. Thus, it is imperative that a new alternative should be developed in order to improve the production of light olefins. Thermo-Catalytic Cracking (TCC) has been recognized as a promising alternative route for light olefins production. Although, this process is still in the development stage, preliminary results show that the TCC offers several major advantages when compared to conventional SC: higher combined yields of light olefins, and significant energy savings. In this dissertation, the TCC activities, kinetic study, and structural-textural-surface properties of different catalyst formulations, which have been investigated thoroughly for their potential use in the TCC process, will be discussed. We report on our efforts to date to develop a suitable and an efficient catalyst that is characterized by high activity, high selectivity to light olefins, and high stability. A particular formulation studied was the hybrid catalyst configuration in which two components, microporous (zeolite) and mesoporous co-catalyst (supported metal oxide (i.e. MoO 3 -CeO), were firmly bound to each other within a clay binder, such that a "pore continuum" effect was developed. Another version was the mesoporous supported bi-oxide catalyst, which is based on MoO-CeO 2 supported on high surface area-metal oxide. Explicitly, it was found that supported bi-oxide catalysts are quite active, stable and selective to light olefins in the Thermo-Catalytic Cracking of n-hexane, which was used as a model molecule for petroleum light naphtha. Furthermore, it was observed that the physicochemical properties and subsequently the catalytic performance of these catalysts were influenced by many factors. Yttria stabilized alumina aerogel, which was prepared via sol-gel synthesis using super critical drying techniques, was considerably more effective as a catalyst support. Our results showed unambiguously that yttria stabilized alumina aerogel did not only possess a high surface area, but also was thermally and hydrothermally stable. In addition, it demonstrated a high ability of inducing homogenous distributions of impregnated metal oxides at high calcination temperature. The latter has resulted in significant improvements in the dispersion degree of Mo, Ce and MoCe species, and the retardation of sintering and sublimation of Mo species. More significantly, it was found that the on-stream-long term stability and the selectivity to light olefins over aromatics were increased upon the addition of CeO 2 into the supported mono-oxide MoO 3 catalyst

ABSTRACT Mixed Petroleum Hydrocarbons and Biomass Derived Compounds Used in the Thermal Catalytic Steam Cracking (TCSC) Process for the Production of Light Olefins HaiTao Yan Light olefins and diolefins such as ethylene, propylene, butenes and 1,3-butadiene are considered as the backbone of the petrochemical industry as they are precursors of numerous plastic materials, synthetic fibers, and rubbers. The most prevalent technologies for producing these precursors are steam cracking and fluid catalytic cracking using petroleum-based feedstock like light naphtha and gas oil. However, petroleum based feeds have several problems in terms of limited reserves, environmental pollution and economic and geopolitical problems. Therefore, it is imperative to find an alternative source, which may be able to overcome the limitation of petroleum oil. In the current work, hydrocarbons-alcohol mixed feeds have been used in the Thermal-Catalytic/Steam-Cracking (TCSC) process for the production of propylene and ethylene. Alcohols like methanol and ethanol can be obtained from biomass, a potential sustainable and renewable source, through gasification and/or fermentation, and they can also be produced from natural gas and coal which are longer lasting fossil fuels than petroleum. The results from on-stream cracking of mixed feedstocks indicated difference in behaviors of ethanol and methanol. While ethanol undergoes predominantly dehydration into ethylene, methanol predominantly intervenes directly on reactions involving hydrocarbons (reactants and their intermediates). Moreover, the addition of methanol to hydrocarbons feedstock significantly increased the product yield of C2-C4 olefins, particularly that of ethylene and propylene. However, there was a maximum limit of efficiency for the methanol content in the mixed feed. Over 25wt% of methanol, the beneficial effect was not as important as expected. In addition, the increasing presence of methanol in the feed significantly accelerated the kinetics of the catalytic cracking. The gradual and significant decrease of the apparent activation energy with increasing methanol concentration in the mixed feed was attributed to the effect of intensive interactions between the hydrocarbons and methanol. These results demonstrated the possibility of partial replacement of petroleum based feedstocks by methanol for the production of propylene and ethylene. In the last part of this work, co-processing biomass derived glycerol with hydrocarbon feedstock over TCSC process was studied. It was found that glycerol as an additive to hydrocarbon feed, can be beneficial till a content of 30 wt%. However, the main concern is the rapid catalyst decay caused by formation of coke. Therefore, there is a need for a more advanced hybrid catalyst having higher hydrogen spillover activity.

This thesis describes two separate projects. The first part of the thesis involves the synthesis of materials for enabling thermochemical water splitting at temperatures below 1000ºC, while the second part focuses on the structure-property relationships for the catalytic conversion of methanol-to-olefins.

In the first project, metal oxide clusters are impregnated in a silica support and tested as catalysts for the thermochemical splitting of water at temperatures below 1000ºC. These supported catalysts are able to do either the oxidation or reduction half cycle, but not both. Thermodynamic analyses reveal that for most common metal / metal oxide pairs, a thermochemical water splitting cycle can not occur in two steps below 1000ºC. Thus, a multi-step thermochemical cycle is developed using Mn₃O₄ and Na₂CO₃. This new cycle can be closed at temperatures that do not exceed 850ºC. Additionally, three metal spinels (Mn, Fe and Co based) are investigated with three alkali metal carbonates (Li, Na and K) for both thermochemical water splitting and thermochemical CO₂ reduction. The manganese, sodium system is found to be the optimal combination for water splitting.

The second project explores the effects that zeolite structure has on the product selectivity in the methanol-to-olefins (MTO) reaction. After first ensuring that literature results on the more common MTO catalysts could be reproduced, the effects of several zeolite structure features on product selectivity are elucidated. Structural features such as channel width and channel eccentricity, cage size (tested on the LEV, CHA and AFX frameworks) and framework composition (tested on the LEV, AEI, CHA and AFX frameworks) are explored. It is found that the product selectivity does not have a strong correlation with channel width and eccentricity. Ethylene selectivity did increase with a reduction in cage size, while propylene selectivity is a maximum with the CHA cage. The most consistent theme noted is that between the aluminosilicates (zeolites) and the silicoaluminophosphates (SAPOs), the effect of temperature on the C₃₌/C₂₌ is the same (if the aluminosilicate shows an increase in C₃₌/C₂₌ with an increase in temperature, so does the silicoaluminophosphate, etc.).