Aplicação de silicatos porosos impregnados com óxido de vanádio como catalisadores heterogêneos em reação de desidrogenação do propano e acetalização do glicerol
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Data
2022-03-11
Autores
França, Adriana Maneira
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Universidade Estadual Paulista (Unesp)
Resumo
No intuito de minimizar algumas limitações que os catalisadores a base de vanádio suportado apresentam, nas últimas décadas inúmeros esforços vêm sendo feitos para aprimorar este tipo de material. Sendo assim, é de suma importância conhecer as características físicas e químicas destes e os principais fatores que influenciam seu desempenho catalítico. Neste trabalho foram aplicados três tipos de silicatos porosos, incluindo a silicalita-1 microporosa, a SBA-15 mesoporosa e a sílica pirolítica macroporosa, para demonstrar como a textura do suporte influencia a formação de espécies de VOx. Esses suportes foram impregnados com 5% de vanádio. As propriedades químicas e estruturais dos catalisadores foram caracterizadas por SAXS, DRX, MEV, EDX, Fisissorção de N2, TGA, 29Si RMN, IRFT, espectroscopia RD-UV-Vis, RTP-H2 e TPD-NH3. Diferentes espécies de vanádio, tal como VOx tetraédricas, quadrado piramidal e octaédricas, e graus de polimerização destas foram encontrados de acordo com cada suporte. Os catalisadores impregnados com vanádio foram testados na reação de desidrogenação direta de propano (PDH) para obtenção de propeno e na reação de acetalização de glicerol com acetona para obtenção de Solketal. Os testes catalíticos para PDH ocorreram à 580 oC e revelaram que o catalisador mesoporoso apresentou a melhor porcentagem de conversão inicial (32%), seletividade do propeno acima de 80% e boa estabilidade catalítica durante 6 h de reação. Este resultado está relacionado à sua alta área específica que favorece as espécies muito ativas de VOx altamente polimerizadas sem formar V2O5 cristalino. Diante deste resultado, foram preparados e testados novos catalisadores impregnando o suporte mesoporoso (SBA-15) com diferentes porcentagens de vanádio. E foi notado que o desempenho catalítico varia com o aumento na porcentagem de vanádio, devido às diferentes espécies que se formaram durante a impregnação. No intuito de melhorar a seletividade do propeno e estabilidade catalítica as amostras foram submetidas à um pré-tratamento de redução com H2 que levou à formação de V-OH na superfície do catalisador. O teste catalítico na reação de acetalização do glicerol ocorreu à 60 oC e mostrou que o catalisador mesoporoso suportado com vanádio teve o melhor desempenho catalítico em 120 min de reação, apresentando valores crescentes para conversão de glicerol e seletividade para o produto de interesse, o solketal acima de 80 %, ao longo da reação. Apesar do bom desempenho catalítico os catalisdores sofreram desativação devido à lixiviação das espécies de vanádio, provocada pela H2O produzida durante a reação. No entanto, ao analisar a superfície do catalisador sólido após diferentes tempos de reação, notou-se que as espécies de vanádio quadrado piramidal foram as que mais sofrem lixiviação e após 40 min de reação voltaram a se depositar na superfície do suporte. Estas não são as principais espécies ativas da reação, pois enquanto elas sofrem lixiviação a atividade catalítica é crescente. Diante dos resultados obtidos, foi observado então, que o suporte mesoporoso impregnado com 5% de vanádio é um promissor catalisador para as reações de desidrogenação do propano e acetalização do glicerol, pois apresentou estabilidade, atividade e seletividade para os produtos de interesse.
In order to minimize some limitations that supported vanadium-based catalysts present, in the last decades numerous efforts have been made to improve this type of material. Therefore, it is very important to know their physical and chemical characteristics and the main factors that influence their catalytic performance. In this work, three types of porous silicates were applied, including microporous silicalite-1, mesoporous SBA-15 and macroporous pyrolytic silica, to demonstrate how the texture of the support influences the formation of VOx species. These supports were impregnated with 5% vanadium. The chemical and structural properties of the catalysts were characterized by SAXS, XRD, SEM, EDX, N2 Physisorption, TGA, 29Si NMR, IRFT, DRS-UV-Vis, TPR-H2 and TPD-NH3. Different vanadium species, such as tetrahedral, square pyramidal and octahedral VOx, and different degrees of polymerization of these species were found according to each support. The vanadium impregnated catalysts were tested in the direct dehydrogenation reaction of propane (PDH) to obtain propene and in the reaction of glycerol acetalization with acetone to obtain Solketal. The catalytic tests for PDH took place at 580 oC and revealed that the mesoporous catalyst presented the best percentage of initial conversion (32%), propene selectivity above 80% and good catalytic stability during 6 h of reaction. This result is related to its high specific area that favors highly polymerized VOx species with high activity, without forming crystalline V2O5. In view of this result, new catalysts were prepared and tested impregnating the mesoporous support (SBA-15) with different percentages of vanadium. And it was noticed that the catalytic performance varies with the increase in the percentage of vanadium, due to the different vanadium species that were formed during impregnation. In order to improve the propene selectivity and catalytic stability, the samples were submitted to a pre-treatment of reduction with H2 that led to the formation of V-OH on the catalyst surface. The catalytic test in the glycerol acetalization reaction occurred at 60 oC and showed that the vanadium supported mesoporous catalyst had the best catalytic performance in 120 min of reaction, showing increasing values for glycerol conversion and selectivity for the interest product, the solketal above 80% throughout the reaction. Despite the good catalytic performance, the catalysts suffered deactivation due to the leaching of vanadium species, caused by the H2O produced during the reaction. However, when the surface of the solid catalyst after different reaction times were analyzed, it was noticed that the pyramidal square vanadium species were the ones that suffered the most leaching and after 40 min of reaction, they returned to deposit on the surface of the support. These are not the main active species in the reaction, as while they undergo leaching the catalytic activity is increasing. In view of the results obtained, it was observed that the mesoporous support impregnated with 5% vanadium is a promising catalyst for the reactions of propane dehydrogenation and glycerol acetalization, as it presented stability, activity and selectivity for the products of interest.
In order to minimize some limitations that supported vanadium-based catalysts present, in the last decades numerous efforts have been made to improve this type of material. Therefore, it is very important to know their physical and chemical characteristics and the main factors that influence their catalytic performance. In this work, three types of porous silicates were applied, including microporous silicalite-1, mesoporous SBA-15 and macroporous pyrolytic silica, to demonstrate how the texture of the support influences the formation of VOx species. These supports were impregnated with 5% vanadium. The chemical and structural properties of the catalysts were characterized by SAXS, XRD, SEM, EDX, N2 Physisorption, TGA, 29Si NMR, IRFT, DRS-UV-Vis, TPR-H2 and TPD-NH3. Different vanadium species, such as tetrahedral, square pyramidal and octahedral VOx, and different degrees of polymerization of these species were found according to each support. The vanadium impregnated catalysts were tested in the direct dehydrogenation reaction of propane (PDH) to obtain propene and in the reaction of glycerol acetalization with acetone to obtain Solketal. The catalytic tests for PDH took place at 580 oC and revealed that the mesoporous catalyst presented the best percentage of initial conversion (32%), propene selectivity above 80% and good catalytic stability during 6 h of reaction. This result is related to its high specific area that favors highly polymerized VOx species with high activity, without forming crystalline V2O5. In view of this result, new catalysts were prepared and tested impregnating the mesoporous support (SBA-15) with different percentages of vanadium. And it was noticed that the catalytic performance varies with the increase in the percentage of vanadium, due to the different vanadium species that were formed during impregnation. In order to improve the propene selectivity and catalytic stability, the samples were submitted to a pre-treatment of reduction with H2 that led to the formation of V-OH on the catalyst surface. The catalytic test in the glycerol acetalization reaction occurred at 60 oC and showed that the vanadium supported mesoporous catalyst had the best catalytic performance in 120 min of reaction, showing increasing values for glycerol conversion and selectivity for the interest product, the solketal above 80% throughout the reaction. Despite the good catalytic performance, the catalysts suffered deactivation due to the leaching of vanadium species, caused by the H2O produced during the reaction. However, when the surface of the solid catalyst after different reaction times were analyzed, it was noticed that the pyramidal square vanadium species were the ones that suffered the most leaching and after 40 min of reaction, they returned to deposit on the surface of the support. These are not the main active species in the reaction, as while they undergo leaching the catalytic activity is increasing. In view of the results obtained, it was observed that the mesoporous support impregnated with 5% vanadium is a promising catalyst for the reactions of propane dehydrogenation and glycerol acetalization, as it presented stability, activity and selectivity for the products of interest.
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Palavras-chave
Catalisadores a base de óxido de vanádio, Silicatos porosos, VOx impregnado, Desidrogenação do propano, Acetalização do glicerol