Combinação de direcionadores estruturas e de poros na síntese de zeólitas hierárquicas MCM-22 para a valorização do glicerol
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Data
2018-09-28
Autores
Rodrigues, Mariana Veiga [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
O emprego de zeólitas com distribuição hierárquica de poros em reações que envolvam moléculas volumosas tem ganhado destaque, principalmente pela acessibilidade aprimorada desses materiais aos reagentes e produtos, que supera uma das maiores desvantagens das zeólitas no cenário atual que são as restrições impostas pelos microporos. Nessa tendência, este trabalho propôs a síntese de zeólitas hierárquicas de estrutura MWW pela combinação de direcionadores de estrutura, primeiramente com o uso de organossilanos como direcionador de mesoporos por método direto e, também com o uso do processo de esfoliação pós-síntese, com a síntese da zeólita ITQ-2 esfoliada. Organossilanos de cadeias com até 18 átomos de carbono foram aplicados na síntese das zeólitas MWW hierárquicas. Os organossilanos de cadeia carbônica até 12 átomos de carbono apresentaram maiores modificações estruturais do que organossilanos de cadeia maiores que 12 átomos de carbono, esses organossilanos apresentarem baixa incorporação na rede zeolítica, assim, não gerando a porosidade esperada. Os materiais sintetizados com organossilanos de até 12 átomos de carbono foram aplicados como catalisadores na reação de condensação do glicerol com a acetona em fase líquida. As zeólitas apresentaram melhores desempenhos frente a conversão do glicerol e a seletividade ao principal produto, o solketal, devido a acessibilidade aprimorada obtida com a inserção de mesoporos e do caráter mais hidrofóbico desses materiais, causada pela diminuição do alumínio na rede zeolítica. As zeólitas sintetizadas com o método direto na síntese e com o processo pós-síntese foram aplicadas na desidratação do glicerol em fase gás para o estudo da influência da acessibilidade aos sítios ativos na reação. Estudos in-situ foram realizados com acompanhamento por espectroscopia na região do infravermelho para a determinação de intermediários e com o objetivo de encontrar as condições reacionais para melhorar o desempenho dos catalisadores. A zeólita esfoliada teve um rendimento alto a acroleína comparada aos outros materiais, entretanto como seus sítios ácidos de Brønsted mais fracos, a seletividade à acroleína ainda não foi baixa. Neste trabalho também se abordou o uso de sementes nas sínteses de zeólitas MWW, buscando a aplicação futura deste método como facilitador na síntese direta dos catalisadores na forma esfoliada.
The application of zeolites with a hierarchical pore distribution in reactions with bulky molecules has been highlighted, mostly caused by the improved accessibility to the inner active sites, overcoming the main drawback in zeolites in the current scenario, the constraints imposed by the micropores. Following this view, this work proposed the synthesis of hierarchical MWW zeolites by the combination of templates, firstly with the use of organosilanes as a mesopore directing agent by a bottom-up method and a top-down method, the delamination process to the exfoliated ITQ-2. Organosilanes with chains sizes up to 18 carbons were applied in synthesis of hierarchical MWW zeolites. Increasing the organosilane chain sizes, the framework changes become more severe, however chain sizes larger than 12 carbons, the organosilane incorporation in the zeolite framework was very low, than these materials did not show the desired mesoporosity. The synthesized zeolites with organossilanes up to 12 carbons were applied in the glycerol condensation to solketal. The hierarchical zeolites showed better performances toward the glycerol conversion and solketal selectivity, due to the improved accessibility obtained with the mesopores and the hydrophobic character of these materials, caused by the decreased aluminum content in the zeolitic structure. The synthesized zeolites in the bottom-up and the delaminated MWW derivative ITQ-2 synthesized by top-down method were applied in the glycerol dehydration to investigate the accessibility to the active sites in the reaction. In-situ studies were realized with the infrared spectroscopy to determine the reaction intermediates, aiming the improvement of the catalysts performance, seeking for the best design to these. The delaminated zeolite showed high acrolein yield and glycerol conversion compared to the other materials, though as its Brønsted acid sites were weaker than the MCM-22 ones, the microporous zeolite showed an improved yield even better than the ITQ-2 and then it is neccessary to glycerol dehydration a catalyst with improved accessibility and strong Brønsted acid sites. This work approached the seeding method to synthesize MWW zeolites either, seeking for a future application of this method as a facilitator in a direct synthesis to delaminated catalysts.
The application of zeolites with a hierarchical pore distribution in reactions with bulky molecules has been highlighted, mostly caused by the improved accessibility to the inner active sites, overcoming the main drawback in zeolites in the current scenario, the constraints imposed by the micropores. Following this view, this work proposed the synthesis of hierarchical MWW zeolites by the combination of templates, firstly with the use of organosilanes as a mesopore directing agent by a bottom-up method and a top-down method, the delamination process to the exfoliated ITQ-2. Organosilanes with chains sizes up to 18 carbons were applied in synthesis of hierarchical MWW zeolites. Increasing the organosilane chain sizes, the framework changes become more severe, however chain sizes larger than 12 carbons, the organosilane incorporation in the zeolite framework was very low, than these materials did not show the desired mesoporosity. The synthesized zeolites with organossilanes up to 12 carbons were applied in the glycerol condensation to solketal. The hierarchical zeolites showed better performances toward the glycerol conversion and solketal selectivity, due to the improved accessibility obtained with the mesopores and the hydrophobic character of these materials, caused by the decreased aluminum content in the zeolitic structure. The synthesized zeolites in the bottom-up and the delaminated MWW derivative ITQ-2 synthesized by top-down method were applied in the glycerol dehydration to investigate the accessibility to the active sites in the reaction. In-situ studies were realized with the infrared spectroscopy to determine the reaction intermediates, aiming the improvement of the catalysts performance, seeking for the best design to these. The delaminated zeolite showed high acrolein yield and glycerol conversion compared to the other materials, though as its Brønsted acid sites were weaker than the MCM-22 ones, the microporous zeolite showed an improved yield even better than the ITQ-2 and then it is neccessary to glycerol dehydration a catalyst with improved accessibility and strong Brønsted acid sites. This work approached the seeding method to synthesize MWW zeolites either, seeking for a future application of this method as a facilitator in a direct synthesis to delaminated catalysts.
Descrição
Palavras-chave
Zeólitas MWW, Condensação do glicerol, Desidratação do glicerol, Estudos no infravermelho in situ, Zeólitas hierárquicas, Esfoliação, Catálise heterogênea, Materiais porosos, Delamination, Hierarchical zeolites, Glycerol dehydration, Glycerol condensation