Síntese de polímeros de ácido lático utilizando catalisadores suportados em sílica, alumina e sílica-alumina

Dissertação (mestrado)—Universidade de Brasília, Instituto de Química, Programa de Pós-Graduação em Química, 2020.O poliácido lático (PLA) é um polímero bastante versátil principalmente por ser um biopolímero biodegradável e biocompatível, isto é, ele pode se degradar em moléculas facilmente metabolizadas no corpo humano ou no ambiente. O PLA pode ser sintetizado via duas rotas principais: polimerização por abertura de anel (ROP) ou policondensação. A ROP produz polímeros de massas molares elevadas e propriedades mecânicas melhores do que a policondensação, com a desvantagem de utilizar catalisadores a base de estanho(II) que podem apresentar certa toxicidade quando sobrarem resíduos no PLA, principalmente, para fins médicos. Em contrapartida, os polímeros obtidos por policondensação costumam apresentar propriedades mecânicas limitadas e menores massas molares, mas, demandam condições de síntese mais amenas, além de precursores de mais baixo custo. O uso de catalisadores se faz necessário em ambas as rotas, principalmente no que concerne à produção de um polímero com elevada massa molar e estéreo regularidade, que pode lhe conferir importantes propriedades físicas e mecânicas, possibilitando seu uso em variadas aplicações. Assim, o presente trabalho teve como pauta a aplicação de catalisadores heterogêneos de ácido 12-tungstofosfórico (HPW) suportados em sílica, alumina e sílica- alumina na reação de policondensação do ácido L-lático e do ácido D,L-lático. Os polímeros obtidos utilizando o ácido D,L-lático como precursor apresentaram resultados interessantes, como massa molar intermediária para os catalisadores de 20% HPW/sílica e 20% HPW/alumina, com valores acima de 10.000 g mol-1 . Os resultados de DRX e polarimetria indicam excesso do enantiômero L na composição do polímero, resultado praticamente não reportado na literatura para PLA obtido a partir de misturas racêmicas, o que sugere que os catalisadores foram enantiosseletivos. O PLA obtido a partir do ácido L-lático e catalisado pela sílica-alumina apresentou massa molar acima de 18.000 g mol-1 além de também ter a formação majoritária do isômero L.Polylactic acid (PLA) is a very versatile polymer mainly because it is biodegradable and biocompatible biopolymer, i.e., it is degraded in molecules easily metabolized in the human body or in the environment. PLA can be synthesized through two main routes: ring opening polymerization (ROP) or polycondensation. ROP produces higher molar mass polymers with better mechanical properties than those produced by polycondensation, with the disadvantage of using tin(II) based catalysts, which may have certain toxicity when residues are left in the PLA, mainly for medical purposes. On the other hand, polymers obtained by polycondensation usually have limited mechanical properties and smaller molar masses, but it demands smoother synthesis conditions, as well as low cost precursors. The use of catalysts is necessary in both routes, mainly with respect to the production of a polymer with high molar mass and stereoregularity, which can impart important physical and mechanical properties, allowing its use in various applications. Thus, the present work focused on the application of heterogeneous 12-tungstophosforic acid (HPW) catalysts supported on silica, alumina and silica-alumina in the polycondensation reaction of L-lactic acid and D,L-lactic acid. The polymers obtained using D,L-lactic acid as precursor presented interesting results, such as intermediate molar mass for the 20% HPW/Silica and 20% HPW/Alumina catalysts, with values above 10,000 g mol-1. The results of XRD and polarimetry indicated an excess of the L-enantiomer in the polymer composition. These results were practically not reported in the literature for PLA obtained from racemic mixtures, which suggested that the catalysts were enantioselective. The PLA obtained from L-lactic acid and catalyzed by silica-alumina had molar mass above 18,000 g mol-1, as well as presented the majority formation of the L-enantiomer

Polymers based on PLA from synthesis using D,L Lactic acid (or racemic lactide) and some biomedical applications : a short review

Poly(lactic acid) (PLA) is an important polymer that is based on renewable biomass resources. Because of environmental issues, more renewable sources for polymers synthesis have been sought for industrial purposes. In this sense, cheaper monomers should be used to facilitate better utilization of less valuable chemicals and therefore granting more sustainable processes. Some points are raised about the need to study the total degradability of any PLA, which may require specific composting conditions (e.g., temperature, type of microorganism, adequate humidity and aerobic environment). Polymerization processes to produce PLA are presented with an emphasis on D,L-lactic acid (or rac-lactide) as the reactant monomer. The syntheses involving homogeneous and heterogeneous catalytic processes to produce poly(D,L-Lactic acid) (PDLLA) are also addressed. Additionally, the production of blends, copolymers, and composites with PDLLA are also presented exemplifying different preparation methods. Some general applications of these materials mostly dedicated to the biomedical area over the last 10–15 years will be pointed out

Liquid Phase Calorimetric Method as a Tool for Acid Strength Measurements and Application to a Variety of Sustainable Catalysts

It has been about 36 years since the first published paper about the calorimetry and adsorption (Cal-Ad) method by Prof. Drago. These separated methods are very old and important characterization tools for different molecules and materials, as recognized in chemistry. The idea of coupling these two techniques arose from the need to have more information about the thermodynamic parameters of a catalyst. The Cal-Ad method provides a measure of the magnitude (Ki), strength (−∆Hi), and quantity (ni) of sites present in a catalyst. The original idea is based on the application of the donor-acceptor concept using the Electrostatic Covalent Model, ECW in the areas of catalysis and material chemistry. Particularly, enthalpy measurements of acidity are directly related to the activity of various catalysts in a variety of reactions. Currently, more than twenty-five catalysts have been carefully characterized by this method in addition to spectroscopic and other thermal methods. Thus, this review seeks to present the fundamentals of the method and show different applications of the characterized catalysts for a variety of reactions in order to use these data as an alternative to choose a catalyst for an acid-dependent type reaction

Brazilian Flora 2020: Leveraging the power of a collaborative scientific network

International audienceThe shortage of reliable primary taxonomic data limits the description of biological taxa and the understanding of biodiversity patterns and processes, complicating biogeographical, ecological, and evolutionary studies. This deficit creates a significant taxonomic impediment to biodiversity research and conservation planning. The taxonomic impediment and the biodiversity crisis are widely recognized, highlighting the urgent need for reliable taxonomic data. Over the past decade, numerous countries worldwide have devoted considerable effort to Target 1 of the Global Strategy for Plant Conservation (GSPC), which called for the preparation of a working list of all known plant species by 2010 and an online world Flora by 2020. Brazil is a megadiverse country, home to more of the world's known plant species than any other country. Despite that, Flora Brasiliensis, concluded in 1906, was the last comprehensive treatment of the Brazilian flora. The lack of accurate estimates of the number of species of algae, fungi, and plants occurring in Brazil contributes to the prevailing taxonomic impediment and delays progress towards the GSPC targets. Over the past 12 years, a legion of taxonomists motivated to meet Target 1 of the GSPC, worked together to gather and integrate knowledge on the algal, plant, and fungal diversity of Brazil. Overall, a team of about 980 taxonomists joined efforts in a highly collaborative project that used cybertaxonomy to prepare an updated Flora of Brazil, showing the power of scientific collaboration to reach ambitious goals. This paper presents an overview of the Brazilian Flora 2020 and provides taxonomic and spatial updates on the algae, fungi, and plants found in one of the world's most biodiverse countries. We further identify collection gaps and summarize future goals that extend beyond 2020. Our results show that Brazil is home to 46,975 native species of algae, fungi, and plants, of which 19,669 are endemic to the country. The data compiled to date suggests that the Atlantic Rainforest might be the most diverse Brazilian domain for all plant groups except gymnosperms, which are most diverse in the Amazon. However, scientific knowledge of Brazilian diversity is still unequally distributed, with the Atlantic Rainforest and the Cerrado being the most intensively sampled and studied biomes in the country. In times of “scientific reductionism”, with botanical and mycological sciences suffering pervasive depreciation in recent decades, the first online Flora of Brazil 2020 significantly enhanced the quality and quantity of taxonomic data available for algae, fungi, and plants from Brazil. This project also made all the information freely available online, providing a firm foundation for future research and for the management, conservation, and sustainable use of the Brazilian funga and flora