Orientador: Nelson Henrique Morgon / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-07T03:03:20Z (GMT). No. of bitstreams: 1 Sanvido_CibelledeSouza_M.pdf: 3049772 bytes, checksum: 93473c5a86f022408435d3d8d4bfadd4 (MD5) Previous issue date: 2006 / Mestrado / Físico-Química / Mestre em Química

Lignocellulosic biomass is a significant pool of energy resource, which can be harnessed to supplement or replace the dwindling fossil fuel reserves. This requires development of economically viable means to efficiently convert biomass to biofuels. A major requirement in biofuel industry is to develop highly active, selective and stable catalysts. Zeolites are an important class of micro-porous crystalline solids, and have proven to be effective and stable acid catalysts for a variety of petrochemical and fine-chemical processes. Nitrided zeolites -- i.e., those with Si-O-Si and Si-OH-Al groups substituted by Si-NH-Si and Si-NH2-Al -- have shown promise as shape-selective basic catalysts, and are potential candidates for biofuel production catalysts. In the first part of this dissertation, the stability and base characteristics of nitrided zeolites have been explored. The nitridation mechanism in HY and silicate type zeolites is computed by first time implementation of embedded-cluster procedure with nudged-elastic-band method of finding elusive transition states. The stability of nitrided sites is investigated by modeling the kinetics of nitridation in reverse, going back to untreated zeolite plus ammonia. Our calculations suggest that nitrided silicalite and HY zeolites require high temperatures to form, but once formed, they remain relatively stable, auguring well for their use as shape-selective base catalysts. In addition, a systematic study of base strength versus aluminium content or alkali cation of nitrided zeolites is also performed. Our studies suggest that K-N-Y (Si:Al = 11) optimizes the balance of activity, stability and cost. Pyrolysis of lignocellulosic biomass is a burgeoning technology to obtain renewable fuels. Commercializing pyrolysis would require efficient process design, especially reactors as they are one of the most energy intensive units in the whole process. This would in turn require detailed understanding of complex pyrolysis chemistries. Biomass is mainly composed of the biopolymer cellulose; therefore, understanding cellulose pyrolysis chemistries is important for efficiently modeling and optimizing pyrolysis reactors. In the second part of this dissertation, the mechanism(s) of conversion of crystalline cellulose to precursors of major products in cellulose pyrolysis have been explored. As the first step, the transformation of cellulose Iβ to a high-temperature (550 K) structure is modeled by computing infrared (IR) spectra as a probe of hydrogen bonding using constant-pressure classical molecular-dynamics simulations. To assist in the analysis of IR spectra, a novel synthesis of normal mode analysis and power spectrum methods is developed to assign the O-H stretches. Simulated IR spectra at elevated temperatures suggests a structural transformation above 450 K, a result in agreement with experimental IR results. The low-temperature (300-400 K) structure is found to be dominated by intrachain hydrogen bonds, whereas in the high-temperature structure (450- 550 K), many of these intrachain hydrogen bonds transform to longer, weaker interchain hydrogen bonds. Next, the subsequent decomposition of cellulose is modeled at 600 and 873 K using Car-Parrinello molecular- dynamics simulations and the metadynamics method. The computed nascent processes can explain the formation of precursors to major products observed during cellulose pyrolysis such as levoglucosan (LGA), hydroxy-methylfurfurral (HMF) and fragmentation products such as formic acid. LGA is found to be kinetically and thermodynamically favorable in comparison to other products, which explains why LGA is the major product observed during cellulose pyrolysis. The molecular insights presented in this part of the study will be helpful in developing detailed kinetic models for optimizing pyrolysis reactors.

Theoretical methods can be used to describe surface chemical reactions in detail and with sufficient accuracy. Advances, especially in density functional theory (DFT) method, enable to compare computational results with experiments. Quantum chemical calculations employing ONIOM DFT/B3LYP/6-31G**-MM/UFF cluster method provided in Gaussian 03 are conducted to investigate water adsorption on rutile (110), and water and ammonia adsorption on anatase (001) surfaces of titanium dioxide. Water and ammonia adsorption on anatase (001) surface is studied by also performing PW:DFT-GGA-PW91 periodic DFT method by using VASP code and the results are compared with the results of ONIOM method. The results obtained by means of ONIOM method indicate that dissociative water adsorption on rutile (110) surface is not favorable due to high activation barrier, whereas on anatase (001) surface, it is favorable since molecular and dissociative water adsorption energies are calculated to be -23.9 kcal/mol and -58.12 kcal/mol. Moreover, on anatase (001) surface, dissociative ammonia adsorption is found energetically more favorable than molecular one (-37.17 kcal/mol vs. -23.28 kcal/mol). Thermodynamic functions at specific experimental temperatures for water and ammonia adsorption reactions on anatase (001) surface are also evaluated. The results obtained using periodic DFT method concerning water adsorption on anatase (001) surface indicate that dissociative adsorption is more favorable than molecular one (-32.28 kcal/mol vs. -14.62 kcal/mol) as in ONIOM method. On the same surface molecular ammonia adsorption energy is computed as -25.44 kcal/mol. The vibration frequencies are also computed for optimized geometries of adsorbed molecules. Finally, computed adsorption energy and vibration frequency values are found comparable with the values reported in literature.

The discovery of M (II)-α-diimine (M = Ni, Pd) catalysts has been promoting a revolution in industrial production of polyolefins, once these can produce polymers with different topologies only varying the reaction conditions, using only ethylene as monomer. This ability has been explored by many researchers in recent decades, experimental and theoretical levels. In this study, we developed theoretical calculations on the behavior of different catalytic systems in ethylene polymerization and dimerization reactions. In the first study, we employ the Hartree-Fock (HF) Method to evaluate energies and structures involved in steps representing the ethylene dimerization by Ni (II)-bis [(pyrazolyl) ethyl] amine cationic bulky complex, discussing the behavior of complex through the dimerization steps, π-complex formation, isomerization and low probability for trimerization during dimerization process. In the last two papers we describe a theoretical investigation of ethylene polymerization reaction catalyzed by cationic complexes bulky Ni (II)-α-diimine. We employ a combination of Density Functional Theory (DFT) and molecular mechanics (MM) contained in the ONIOM approach to evaluate structures and energies of representing steps of ethylene polymerization mechanism catalyzed by Brookhart and Guan systems, discussing electronic influences and stereo environment imposed by ligands around the active site in each stage of the process, highlighting the olefin coordination angles and isomerization process of growing polymer chain. / Conselho Nacional de Desenvolvimento Científico e Tecnológico / A descoberta de catalisadores do tipo M(II)-α-diimina (M=Ni, Pd) vem promovendo uma revolução na produção industrial de poliolefinas, pois estes podem produzir polímeros com diferentes topologias variando apenas as condições reacionais, usando apenas o eteno como monômero. Esta habilidade foi explorada por diversos pesquisadores nas últimas décadas, em nível experimental e teórico. Neste estudo, desenvolvemos cálculos teóricos sobre o comportamento de diferentes sistemas catalíticos nas reações de polimerização e dimerização do eteno. No primeiro estudo, empregamos o Método Hartree-Fock (HF) para avaliar energias e estruturas envolvidas nos passos representativos da dimerização do eteno via complexo catiônico volumoso do tipo Ni(II)-bis[(pirazolil)etil]amina, discutindo o comportamento do complexo ao longo dos passos da dimerização, formação de complexos-π, isomerização da cadeia e baixa probabilidade para trimerização durante o processo de imerização.Nos dois últimos trabalhos descrevemos uma investigação teórica da reação de polimerização do eteno catalisada por complexos catiônicos volumosos Ni(II)-α-diimina. Empregamos a combinação da Teoria do Funcional de Densidade (DFT) e Mecânica Molecular (MM), contida na aproximação ONIOM, para avaliar estruturas e energias dos passos representativos do mecanismo de polimerização do eteno catalisado pelos sistemas de Brookhart e Guan discutindo as influências eletrônicas e do ambiente estéreo imposto pelos ligantes em torno do sítio ativo em cada estágio do processo, enfatizando principalmente ângulos de coordenação da olefina e processo de isomerização da cadeia polimérica em crescimento.