Investigação das propriedades catalíticas de zeólitas ZSM-5 em reações do glicerol
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Data
2020-09-18
Autores
Catuzo, Gabriel Liscia [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
A crescente produção do biodiesel, combustível renovável complementar ao diesel de petróleo, tem levado a um excedente de glicerol, que é o coproduto da síntese. Isso torna a competitividade do biodiesel frente ao diesel de petróleo diminuída, já que o valor de mercado do glicerol tem diminuído ao longo dos anos. Nesse contexto, duas reações catalíticas de transformação do glicerol em produtos de alto valor agregado foram estudadas nesta tese. A primeira foi a reação de cetalização do glicerol com acetona para a formação de solketal, um aditivo oxigenado de combustíveis. A segunda é a reação de desidratação, para a produção de acroleína, um intermediário da produção de uma série de compostos, como o ácido acrílico. A zeólita ZSM-5 foi aplicada nessas reações por possuir uma série de características que a torna a zeólita mais aplicada na indústria, como: alta estabilidade térmica, porosidade que permite seletividade de forma a uma série de reações e uma acidez bastante variável. Portanto, é bastante vantajoso explorar as suas propriedades em reações do glicerol. Ambas as reações apresentam desafios durante o processo catalítico. A reação em fase líquida, de cetalização, por exemplo, ocorre em um sistema trifásico, com o catalisador preferencialmente junto ao glicerol (sistema em duas fases), e com menor contato com a acetona (terceira fase). As zeólitas ZSM-5 de várias razões molares Si/Al foram aplicadas. Como as zeólitas de maior razão Si/Al e com ligações hidrofóbicas ≡Si-O-Si≡ abundantes possuem menos afinidade com o glicerol, há uma melhora na miscibilidade interfacial do glicerol/acetona nas proximidades dos sítios ativos. Os resultados foram aperfeiçoados utilizando etanol como cossolvente e aprimorando a hidrofobicidade com a troca de parte dos sítios ácidos com cátions n-hexilamônio. Adicionalmente, a água, um subproduto da reação, adsorve nos sítios ácidos, diminuindo a conversão catalítica do glicerol. Isso ocorre preferivelmente nas zeólitas mais hidrofílicas; essa tendência foi intensificada ao usar zeólitas saturadas com água. Em relação à reação de desidratação do glicerol, em fase gás, há a desativação dos catalisadores, devido a formação de coque, acarretando a diminuição da acessibilidade aos sítios ácidos. Dois tipos de coque foram formados: poliéteres, na superfície externa e aromáticos, nos poros, que representam a maioria (~ 98%, em massa) dos compostos carbonáceos. A formação de coque ocorre quase totalmente nos primeiros minutos da reação, mas ele envelhece com o tempo, tornando-se mais volumoso e desidrogenado, acarretando em alterações na estrutura cristalina da zeólita e perda de acessibilidade. Para as zeólitas com menor razão Si/Al, formou-se coque proporcionalmente ao volume de microporos. Entretanto, para a zeólita menos ácida, a formação de coque é minimizada, devido a pequena densidade de sítios ácidos. Além disso, o grau de desenvolvimento do coque também se mostrou dependente da distância entre os sítios ácidos, já que as etapas de ciclização do coque são favorecidas quando os sítios ácidos estão próximos. Esse estudo da desativação pode fornecer ferramentas importantes para síntese de catalisadores com densidade de sítios ácidos apropriados, visando uma minimização da formação de coque ou facilitando a regeneração do catalisador.
The growing production of biodiesel, a renewable fuel complementary to petroleum diesel, has led to an excess of glycerol, which is the synthesis co-product. It makes biodiesel competitiveness compared to diesel from petrochemical sources reduced since glycerol's market value has decreased over the years. In this context, two catalytic reactions for transforming glycerol into high added value products were studied in this thesis. The first one was the glycerol ketalization reaction with acetone to form solketal, an oxygenated fuel additive. The second is the dehydration reaction, for the production of acrolein, an intermediate in producing a series of compounds, such as acrylic acid. The zeolite ZSM-5 was applied in these reactions because it has a series of characteristics that make it the most applied zeolite in the industry, such as high thermal stability, porosity that allows shape selectivity in a series of reactions, and a highly variable acidity. Therefore, it is quite advantageous to explore its properties in the reactions of glycerol. Both reactions present challenges during the catalytic process. The ketalization liquidphase reaction, for example, occurs in a three-phase system, with the catalyst preferably next to glycerol (two-phase system), and with minor contact with acetone (third phase). ZSM-5 zeolites of various molar ratios Si/Al and were applied. As zeolites with a higher Si/Al ratio and with abundant ≡Si-O-Si≡ hydrophobic bonds have less affinity for glycerol, there is an improvement in glycerol/acetone's interfacial miscibility in the vicinity of active sites. The results were improved using ethanol as a co-solvent and by increasing hydrophobicity by exchanging part of the acidic sites with n-hexylammonium cations. Additionally, water, a by-product of the reaction, adsorbs on acidic sites, decreasing glycerol's catalytic conversion. It occurs preferably in the most hydrophilic zeolites; this trend was intensified when using water-saturated zeolites. In relation to the glycerol dehydration gas-phase reaction, the catalysts were deactivated due to coke formation, resulting in decreased accessibility to acidic sites. Two types of coke were formed: polyethers, on the outer surface, and aromatics, in the pores, representing the majority (~ 98 wt.%) of the carbonaceous compounds. The formation of coke occurs almost entirely in the first minutes of the reaction, but it ages over time, becoming bulkier and more dehydrogenated, resulting in changes in the crystalline structure of the zeolite and loss of accessibility. For zeolites with a lower Si/Al ratio, coke was formed in proportion to micropores' volume. However, for MFI-140 zeolite, coke formation is minimized due to the low density of acidic sites. Besides, the coke's degree of development was also shown to depend on the distance between the acidic sites, since the stages of coke cyclization are favored when the acidic sites are close. This deactivation study can provide important tools for the synthesis of catalysts with an appropriate density of acid sites, aiming to minimize coke formation or facilitate the regeneration of the catalyst.
The growing production of biodiesel, a renewable fuel complementary to petroleum diesel, has led to an excess of glycerol, which is the synthesis co-product. It makes biodiesel competitiveness compared to diesel from petrochemical sources reduced since glycerol's market value has decreased over the years. In this context, two catalytic reactions for transforming glycerol into high added value products were studied in this thesis. The first one was the glycerol ketalization reaction with acetone to form solketal, an oxygenated fuel additive. The second is the dehydration reaction, for the production of acrolein, an intermediate in producing a series of compounds, such as acrylic acid. The zeolite ZSM-5 was applied in these reactions because it has a series of characteristics that make it the most applied zeolite in the industry, such as high thermal stability, porosity that allows shape selectivity in a series of reactions, and a highly variable acidity. Therefore, it is quite advantageous to explore its properties in the reactions of glycerol. Both reactions present challenges during the catalytic process. The ketalization liquidphase reaction, for example, occurs in a three-phase system, with the catalyst preferably next to glycerol (two-phase system), and with minor contact with acetone (third phase). ZSM-5 zeolites of various molar ratios Si/Al and were applied. As zeolites with a higher Si/Al ratio and with abundant ≡Si-O-Si≡ hydrophobic bonds have less affinity for glycerol, there is an improvement in glycerol/acetone's interfacial miscibility in the vicinity of active sites. The results were improved using ethanol as a co-solvent and by increasing hydrophobicity by exchanging part of the acidic sites with n-hexylammonium cations. Additionally, water, a by-product of the reaction, adsorbs on acidic sites, decreasing glycerol's catalytic conversion. It occurs preferably in the most hydrophilic zeolites; this trend was intensified when using water-saturated zeolites. In relation to the glycerol dehydration gas-phase reaction, the catalysts were deactivated due to coke formation, resulting in decreased accessibility to acidic sites. Two types of coke were formed: polyethers, on the outer surface, and aromatics, in the pores, representing the majority (~ 98 wt.%) of the carbonaceous compounds. The formation of coke occurs almost entirely in the first minutes of the reaction, but it ages over time, becoming bulkier and more dehydrogenated, resulting in changes in the crystalline structure of the zeolite and loss of accessibility. For zeolites with a lower Si/Al ratio, coke was formed in proportion to micropores' volume. However, for MFI-140 zeolite, coke formation is minimized due to the low density of acidic sites. Besides, the coke's degree of development was also shown to depend on the distance between the acidic sites, since the stages of coke cyclization are favored when the acidic sites are close. This deactivation study can provide important tools for the synthesis of catalysts with an appropriate density of acid sites, aiming to minimize coke formation or facilitate the regeneration of the catalyst.
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Palavras-chave
Zeólitas, Acidez, Hidrofobicidade, Desativação, Glicerol