No abstract

This paper provides a comprehensive description of the design and commissioning of a dual-stage flow reactor for hydrothermal synthesis, notably heterogeneous nanomaterials such as core−shell particles or nanocomposites. The design is based on the hypothesis that the next frontier of studies within continuous, hydrothermal synthesis lies as much with scalability as it does with the materials properties and performance in applications. Therefore, this reactor belongs to the up-scaled end of a laboratory system with a synthesis capacity of up to 50 g/h. Commissioning was accomplished with TiO 2 nanoparticles as a model material. Results comply with earlier ones obtained from single-stage reactors. Dual-stage synthesis of a TiO 2 @SnO 2 nanocomposite was performed by adding a SnCl 4 solution to newly formed 9 nm TiO 2 nanoparticles, yielding deposition of 2 nm rutile SnO 2 . Synthesis of pure SnO 2 produced much larger nanocrystals, indicating that TiO 2 nanoparticles provide the nucleation sites for SnO 2 and impede the growth beyond 2 nm.

Microwave-assisted heterogeneous catalysis (MHC) is gaining attention due to its exciting prospects related to selective catalyst heating, enhanced energy-efficiency, and partial inhibition of detrimental side gas-phase reactions. The induced temperature difference between the catalyst and the comparatively colder surrounding reactive atmosphere is pointed as the main factor of the process selectivity enhancement towards the products of interest in a number of hydrocarbon conversion processes. However, MHC is traditionally restricted to catalytic reactions in the absence of catalyst coking. As excellent MW-susceptors, carbon deposits represent an enormous drawback of the MHC technology, being main responsible of long-term process malfunctions. This work addresses the potentials and limitations of MHC for such processes affected by coking (MHCC). It also intends to evaluate the use of different catalyst and reactor configurations to overcome heating stability problems derived from the undesired coke deposits. The concept of long-term MHCC operation has been experimentally tested/applied to for the methane non-oxidative coupling reaction at 700 °C on Mo/ZSM-5@SiC structured catalysts. Preliminary process scalability tests suggest that a 6-fold power input increases the processing of methane flow by 150 times under the same controlled temperature and spatial velocity conditions. This finding paves the way for the implementation of high-capacity MHCC processes at up-scaled facilities.

γ-Fe2O3@SiO2, TiO2@SiO2, and α-Fe2O3@SiO2 core-shell nanoparticles were synthesized by a surfactant-free hydrothermal method in a continuous flow dual-stage reactor. Uniform silica shells of 2 nm thickness were obtained when grown on γ-Fe2O3 and TiO2 nanoparticles while thicker and more irregular shells were deposited on α-Fe2O3 due to low pH.

TiO2@SnO2 hybrid nanocomposites were successfully prepared in gram scale using a dual-stage hydrothermal continuous-flow reactor. Temperature and pH in the secondary reactor were found to selectively direct nucleation and growth of the secondary material into either heterogeneous nanocomposites or separate intermixed nanoparticles. At low pH, 2 nm rutile SnO2 nanoparticles were deposited on 9 nm anatase TiO2 particles; the presence of TiO2 was found to suppress formation of larger SnO2 particles. At high pH SnO2 formed separate particles and no deposition on TiO2 was observed. Ball-milling of TiO2 and SnO2 produced no TiO2@SnO2 composites. This verifies that the composite particles must be formed by nucleation and growth of the secondary precursor on the TiO2 . High concentration of secondary precursor led to formation of TiO2 particles embedded in aggregates of SnO2 nanoparticles. The results demonstrate how nanocomposites may be produced in high yield by green chemistry.