Layered double hydroxides (LDHs) – carbon nanostructure (CNS) composites: synthesis and application in base-catalysed condensation reactions
File(s)
Author(s)
Morales Vega, Nicolas
Type
Thesis
Abstract
The activity and selectivity produced by novel Layered Double Hydroxides (LDH) supported in both multi-walled carbon nanotubes (MWNT) and graphene oxide (GO) was investigated for a set of base-catalysed aldol condensation reactions. The MWNT/LDH composites were studied in the self-condensation of acetone and retroaldolization of diacetone alcohol (DAA), while the GO/LDH materials were used in the latter reaction, as well as the benzaldehyde-acetone condensation, where the material was activated in-situ. The effect of carbon content was investigated with the aim to compare the performance of the solids during reaction.
Adding the LDH into either carbon support increases dispersion and generally decreases LDH particle size, which produces an increment in both surface area and the availability of basic sites in the hybrids. As a result, activity in all three condensation processes is enhanced by 3 to 4 times, in comparison to the unsupported LDH. Similarly, stability during reaction is also increased.
However, problems regarding the use of LDH and the composites were identified. Activated unsupported meixnerite-type LDH (OH- interlayer anion) has high initial activity, but tends to deactivate faster. This effect can be reduced by using the nanocarbon supports. The use of MWNT as support allows increased rates to be achieved compared to GO, but high contents of this less volume efficient support are necessary. The pre-treatment stage of MWNT is also extremely lengthy, while care should be taken during MWNT purification before composite synthesis, as carbon debris could hinder activity greatly, especially at higher loadings. Conversely, very low amounts of GO material lead to the best activity in this study, probably related to the coherence between the natural charge and geometry between the LDH and GO layers. Nevertheless, both types of composites tend to adsorb reaction products at long reaction times, which possibly reduces the availability of basic sites of the catalysts.
The self-condensation of acetone is an equilibrium limited process where the highest conversion is achieved at 273 K. This hinders the ability to compare the basic properties of the materials appropriately. The problem is avoided by studying the catalytic effect of the composites in the DAA retroaldolization, which requires low amounts of catalyst and rates can be compared at mild conditions. The in-situ activation study of benzaldehyde-acetone condensation showed that higher Mg content and the use of the carbon supports increases both activity and selectivity towards the desired product, benzalacetone.
Adding the LDH into either carbon support increases dispersion and generally decreases LDH particle size, which produces an increment in both surface area and the availability of basic sites in the hybrids. As a result, activity in all three condensation processes is enhanced by 3 to 4 times, in comparison to the unsupported LDH. Similarly, stability during reaction is also increased.
However, problems regarding the use of LDH and the composites were identified. Activated unsupported meixnerite-type LDH (OH- interlayer anion) has high initial activity, but tends to deactivate faster. This effect can be reduced by using the nanocarbon supports. The use of MWNT as support allows increased rates to be achieved compared to GO, but high contents of this less volume efficient support are necessary. The pre-treatment stage of MWNT is also extremely lengthy, while care should be taken during MWNT purification before composite synthesis, as carbon debris could hinder activity greatly, especially at higher loadings. Conversely, very low amounts of GO material lead to the best activity in this study, probably related to the coherence between the natural charge and geometry between the LDH and GO layers. Nevertheless, both types of composites tend to adsorb reaction products at long reaction times, which possibly reduces the availability of basic sites of the catalysts.
The self-condensation of acetone is an equilibrium limited process where the highest conversion is achieved at 273 K. This hinders the ability to compare the basic properties of the materials appropriately. The problem is avoided by studying the catalytic effect of the composites in the DAA retroaldolization, which requires low amounts of catalyst and rates can be compared at mild conditions. The in-situ activation study of benzaldehyde-acetone condensation showed that higher Mg content and the use of the carbon supports increases both activity and selectivity towards the desired product, benzalacetone.
Version
Open Access
Date Issued
2017-07
Date Awarded
2018-02
Advisor
Chadwick, David
Sponsor
Consejo Nacional de Ciencia y Tecnologia (Mexico)
Bio Nano Consulting (Firm)
Coahuila (Mexico : State). Secretaría de Educación Pública
Publisher Department
Chemical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)