Conversão de metanol em olefinas catalisada por silicoaluminofosfatos do tipo SAPO-34 e SAPO-5

• Autor(a) principal: Alves, Ingryd de Carvalho
• Data de Publicação: 2017
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da UERJ
• Texto Completo: http://www.bdtd.uerj.br/handle/1/12047
• topic: SAPO-34; SAPO-5; SAPO-34 / SAPO-5 with hierarchical structure; MTO; SAPO-34; SAPO-5; SAPO-34/SAPO-5 com estrutura hierárquica; MTO; CNPQ::ENGENHARIAS::ENGENHARIA QUIMICA

SAPO-34 is a silicoaluminophosphate with similar structure of chabazite zeolite (CHA). It has pores with a diameter of approximately 4 Å, being considered a microporous molecular sieve, with moderate acidity. The conversion process of methanol into olefins (MTO) is particularly interesting using materials with these properties. However, the porous structure of SAPO-34 entails the rapid deactivation by blocking the porous structure by the formation of coke. Due to this limitation, it has been studied in the literature the synthesis of catalysts with hierarchical structure of pores, which aims to generate meso/macroporosity in catalysts that have diffusion problems due to microporous structure. In this work, the hierarchical catalysts were synthesized using the cetyl trimethyl ammonium bromide (CTAB) as mesoporous mold. These samples were characterized in order to determine their physical and chemical properties, and their catalytic behavior was evaluated in methanol conversion reactions in olefins (MTO). According to the analysis by X-ray diffraction and by scanning electron microscopy of the prepared samples, it was possible to verify a mixture of AFI and CHA phases. AFI refers to SAPO-5 which has larger pores. It was possible to verify the generation of mesopores for the samples synthesized with the mold. However, the present work was divided in two stages:in the first step, a comparison between commercial samples of SAPO-34, named SAPO-34 (1) and SAPO-34 (2), synthesized SAPO-34, named SAPO-34 (3) and SAPO-5was made; the second step consisted in evaluating the catalytic performance of the samples synthesized with the mixture of SAPO-34 and SAPO-5 phases in two different crystallization times, 20 h and 72 h. In the first step, the XRD and SEM results for SAPO-34 samples revealed the presence of different particle sizes and confirmed a typical crystalline phase of these materials (CHA). SAPO-34 (1) presented crystals with lower proportions than the SAPO-34 (2) and SAPO-34 (3) samples. The acid characteristics of the samples were determined by the NH3 TPD, the results showed that the SAPO-34 (1) and SAPO-5 samples had similar total densities of acid sites, but SAPO-34 (1) had a higher number of strong acid sites. The results of the catalytic tests (Treaction = 450 °C, WHSV = 1.5 h-1 and MeOH partial pressure = 0.04 atm) indicated that the SAPO-34 (1) has a higher stability compared to the SAPO- 34 (3), which is due to the lower density of acid sites and the formation of smaller crystals. It is concluded that samples of SAPO-34 formed by small crystals and total density of sites with intermediate value present showed better results for the conversion of methanol to olefins. In the second stage of the present work, the results of the catalytic tests and the physical chemical properties were influenced by the proportions of the phases of each of the samples. Samples with the highest proportion of the SAPO-5 phase presented similar results to SAPO-5 pure sample, with a slight improvement in the selectivity that can be attributed to the presence of the SAPO-34 phase. For the sample with the highest proportion of the SAPO-34 phase, the catalytic test was very similar to the SAPO-34 pure sample and the small proportion of the SAPO-5 phase did not influence its catalytic performance