Resumen de Microwave-assisted synthesis of supported nanocatalysts: a route from nanoparticles to nanoclusters, from batch to continuous
The main drawbacks of the traditional batch chemical processes are the excessive energy consumption, the variable product quality, the limited scaled‐up and the poor efficiency. The European Training Network for Continuous Sonication and Microwave Reactors (ETN‐COSMIC) aims to support the transition of the chemical industry from batch to continuous flow technologies with the investigation of alternative no contact energy sources, such as ultrasounds and microwaves.
The current PhD work targets the development of high efficient nanocatalysts suitable for heterogeneous catalytic reactions, focusing on the effects of microwaves and continuous flow reactors.
This thesis covers the following aspects to successfully design the synthesis reactor and the final high efficient nanocatalysts: ‐ Development of a method to accurately control the temperature in microwave-heated continuous flow microreactor for the synthesis of metallic nanoparticles.
‐ Definition of the optimum heating patterns for the synthesis of metallic nanoclusters.
‐ Development of a synthesis procedure, based on microwave heating, for the in-situ synthesis of metallic nanoclusters on catalytic supports.
‐ Testing of catalytic activity.
The thesis is structured in five experimental chapters.
In chapter 2, it is introduced the microwave‐heated continuous flow reactor used in this PhD research, benchmarking its efficiency with a common silver nanoparticles procedure that is carried out in a batch‐type reactor comparing a conventional heating mode, such as an oil bath, and the alternative electromagnetic heating. An accurate investigation of the temperature mapping, performed by integrating simulated and experimental results, confirmed that microwaves guarantee a higher heating rate and consequently higher synthesis yield. Furthermore, the different heating profile counteracts the wall fouling and then, improves the quality of the final product.
In chapter 3, the optimization of the heating pattern for the production of ultra-small nanocatalysts was conducted in a batch‐type process, evidencing the effects of the nucleation rate on the size distribution of the resulting nanoparticles. A detailed analysis of the temperature profile evidenced that the quality of the final product may increase by adopting a rapid selective heating rate, function of the microwave irradiation power. The quality of the produced nanomaterials was remarkable not only for their high activity in the tested conditions but also for their long‐term stability which is more than 18‐months. The nanoparticles produced were deposited into mesoporous SBA‐15, obtaining the first catalyst B-AgNPs@SBA-15, and its activity was tested using the hydrogenation of 4‐Nitrophenol with sodium borohydride, comparing the developed nanosystem with literature results.
In chapter 4, the process was switched to continuous flow, including a rapid quenching step. The nanoclusters were uniformly supported over the mesoporous channels of SBA‐15, and the catalyst C-AgNCs@SBA-15 was tested for alkynes’ hydrogenation. The high density of uncoordinated Ag atoms was responsible for the high activity observed, confirming that supported nanoclusters may represent a bridge between low active nanoparticles and unrecoverable silver salts.
In chapter 5, an alternative reactor for in‐situ nucleation of silver nanoclusters was introduced with the purpose of increasing the loading yield, lowering the metal loss. The clusters size was still reduced and the synthesis yield was higher than 90%. The higher metal loading may play a crucial role in accelerate some catalytic reaction, as demonstrated by the hydrogenation of 4‐Nitrophenol. Furthermore, the nanocatalyst synthesized confirmed its usefulness for a wide range of C≡C cyclization and its morphological and catalytic stability was proved after one year of storage. To conclude, the scalability of the batch method was evaluated moving from 100 mg to 1 g of catalyst, in 2 minutes synthesis time.
Finally, chapter 6 is focused on the production of bimetallic nanoclusters, which may present interesting and fascinating properties. The continuous flow reactor was properly modified to allow a dual‐step reducing process, and bimetallic structures were synthesized, investigating the effects of residence time and temperature profile. The bimetallic nanoclusters were directly synthesized on the carbon support in the continuous flow, maximizing the synthesis yield and optimizing clusters distribution.
