Preparação de heteroestruturas formadas por hidróxidos duplos lamelares recobertos com sílica mesoporosa

Sólidos inorgânicos porosos são materiais que apresentam grande área superficial e são normalmente utilizados em aplicações envolvidas com separação, adsorção, catálise e liberação modificada de substâncias, que são processos que se beneficiam de tal propriedade. Os hidróxidos duplos lamelares (HDL) e as sílicas mesoporosas (SM) são exemplos de sólidos inorgânicos porosos que despertam muito interesse científico e tecnológico, não só pela grande área superficial que possuem, mas devido as suas propriedades estruturais, texturais e químicas. Neste estudo, os HDL do tipo hidrotalcita [Mg4Al2(OH)12]CO3·xH2O e [Zn4Al2(OH)12]CO3·xH2O foram combinados à SM visando produzir heteroestruturas compostas de um núcleo de HDL envolto por uma camada externa de SM. Diferentes condições sintéticas foram testadas visando produzir um recobrimento de sílica uniforme e com porosidade ordenada, mas utilizando quantidades menores de direcionador estrutural usado para formação dos poros na sílica em relação aos métodos reportados na literatura, além do uso do HDL com Zn na composição, ainda não reportado na literatura. A difratometria de raios X e a espectroscopia vibracional confirmaram a presença das fases de sílica e HDL. A distribuição e o arranjo entre essas fases foram determinados por microscopias eletrônicas de varredura e transmissão. A microscopia de varredura mostrou uma grande influência da razão molar entre os reagentes e dos reagentes utilizados na etapa do recobrimento no hábito da partícula resultante. A microscopia de transmissão indicou que o recobrimento das partículas de HDL foi efetivo quando usado menores concentrações de direcionador estrutural e sem etanol como cosolvente, aparentemente, uniforme e sem a formação de fases segregadas. Entretanto, o processo de calcinação para remoção do molde levou à desestruturação do HDL, fato que não ocorreu quando o molde foi removido por extração. A reconstrução parcial do arranjo lamelar do HDL na heteroestrutura calcinada foi conseguida através do efeito memória, que algumas das fases HDL apresentam. A calcinação também afetou a porosidade das heteroestruturas, que somadas ao efeito de reconstrução parcial da fase de HDL observado, podem indicar a ocorrência de uma possível reação entre a sílica e o HDL, formando uma outra fase não porosa, fato que ainda precisa ser investigado em estudos futuros. Os resultados mais importantes deste estudo foram: o desenvolvimento de um procedimento sintético que utilizou menor concentração do direcionador estrutural, que possibilitou a remoção posterior por extração de forma mais eficiente; a confecção de heteroestruturas inéditas com o HDL Zn2Al-CO3; a avaliação do efeito memória para reconstrução da estrutura lamelar do HDL na heteroestrutura após o processo de calcinação para remoção do direcionador estrutural usado na formação do recobrimento de sílica.Inorganic porous solids are materials with large surface area, commonly used in applications such as separation, adsorption, catalysis and modified release of substances, which are processes benefited from such property. Layered double hydroxides (LDH) and mesoporous silica (MS) are examples of inorganic solids that have such characteristics and are currently of great scientific and technological interest not only for their large surface area but also for their structural, textural and chemical properties. In this study, the hydrotalcite-like LDH [Mg4Al2(OH)12]CO3•xH2O and [Zn4Al2(OH)12]CO3•xH+O were combined to MS to produce heterostructures composed of an LDH core surrounded by an MS shell (MII 2Al-CO3@mSiO2). Different synthetic conditions were tested in order to obtain a uniform and ordered porous silica coating but using smaller amounts of structural directing agent compared to the methods reported in the literature, besides the use of the Zn2Al-CO3 LDH not yet already reported in the literature. X-ray diffraction and vibrational spectroscopy confirmed the presence of silica and LDH phases in the produced heterostructures. The distribution and arrangement between these phases were determined by scanning and transmission electron microscopy. Scanning microscopy showed a great influence of molar composition among the reagents used in the coating step in the habit of the resulting particle. Transmission microscopy indicated that the LDH particle coating was effective and apparently uniform without the formation of segregated phases. However, the calcination process for template removal led to the destruction of the LDH, which did not occur when the template was removed by extraction. Partial reconstruction of the lamellar arrangement of LDH after calcination was achieved through the memory effect. Calcination also affected the porosity of the heterostructures, which in addition to the partial reconstruction effect of the observed LDH phase, could indicate the occurrence of a possible reaction between silica and LDH, forming another non-porous phase, a fact that has must be investigated in future studies. The most important results of this study were: the development of a synthetic procedure that used a lower concentration of the structural directing agent; the production of heterostructures containing Zn2Al-CO3 as the core; the evaluation of the memory effect to reconstruction the LDH core after calcination to remove the template.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)CAPES: 1744920FAPESP: 2016/50317-9FAPESP: 2019/05467-0

Evaluation of the Structural Integrity of Layered Double Hydroxides and Mesoporous Silica During the Preparation of Heterostructures

International audienceHeterostructures constructed with mesoporous silica and layered double hydroxides are interesting for catalytic and drug delivery applications. Different arrangements between these phases are possible. In this study, we prepared heterostructures by embedding layered double hydroxides phases [M4Al2(OH)12](CO3) (M = Mg2+ or Zn2+) within the MCM41 mesoporous silica type phase. According to our results, the most critical step of this preparation is the removal of the organic template used to create the mesopores in the silica phase, which can be done by calcination or extraction with solvent, normally a mixture of ethanol and a mineral acid. The results reported in this study demonstrate that both can cause structural changes in the components of the heterostructure at different extensions. Calcination promoted the collapse of the layered hydroxide phases. Attempts made for their reconstruction through rehydration, which is a quite known process, were not completely effective and also dependent on the chemical composition of the layered phase. The complete template removal with preservation of the layered phases was possible using the extraction method but by replacing the mineral acid with NH4Cl. However, some discrete structural changes were identified possibly due to a partial lixiviation of Al3+ from the double hydroxide layers

Evaluation of the Structural Integrity of Layered Double Hydroxides and Mesoporous Silica During the Preparation of Heterostructures

Heterostructures constructed with mesoporous silica and layered double hydroxides are interesting for catalytic and drug delivery applications. Different arrangements between these phases are possible. In this study, we prepared heterostructures by embedding layered double hydroxides phases [M4Al2(OH)12](CO3) (M = Mg2+ or Zn2+) within the MCM41 mesoporous silica type phase. According to our results, the most critical step of this preparation is the removal of the organic template used to create the mesopores in the silica phase, which can be done by calcination or extraction with solvent, normally a mixture of ethanol and a mineral acid. The results reported in this study demonstrate that both can cause structural changes in the components of the heterostructure at different extensions. Calcination promoted the collapse of the layered hydroxide phases. Attempts made for their reconstruction through rehydration, which is a quite known process, were not completely effective and also dependent on the chemical composition of the layered phase. The complete template removal with preservation of the layered phases was possible using the extraction method but by replacing the mineral acid with NH4Cl. However, some discrete structural changes were identified possibly due to a partial lixiviation of Al3+ from the double hydroxide layers.</div

Localized 4f states and dynamic Jahn-Teller effect in PrO2

Neutron spectroscopic measurements of the magnetic excitations in PrO2 reveal (1) sharp peaks characteristic of transitions between levels of the 4f(1) configuration of Pr4+ split by the cubic crystal field, and (2) broad bands of scattering centered near 30 and 160 meV. We present a simple model based on a vibronic Hamiltonian that accounts for these contrasting features of the data. The analysis shows that 90% +/- 10% of the Pr ions have a localized 4f(1) configuration and provides strong evidence for a dynamic Jahn-Teller effect in the Gamma (8) electronic ground state