Metal oxide-containing SBA-15-supported gold catalysts for base-free aerobic homocoupling of phenylboronic acid in water

Systematic Incorporation of Gold Nanoparticles onto Mesoporous Titanium Oxide Particles for Green Catalysts

“…These kinetic patterns allowed for the calculation of activation energy (Ea) for the composite particles, which was derived from Arrhenius plots by the relationship between the logarithm of the rate constant (k) and the inverse temperature (1/T). The apparent Ea of the reaction was calculated to be ~53.4 kJ/mol for the 20AuMPT particles and ~50.7 kJ/mol for the 50AuMPT particles, which was comparable to the reported value range (27-61 kJ/mol) [13,27,48]. Although this marginally lower Ea implied that the homocoupling of phenylboronic acid over 50AuMPT is slightly easier than that over 20AuMPT under our reaction conditions, this marginal difference does not clearly provide distinct relationships between the structural features of the incorporated AuNPs (e.g., size and distribution) and Ea.…”

“…This additional experiment clearly indicated that applying slightly increased amounts of gold salts (i.e., 20AuMPT) onto the MPT particles resulted in a high-yield reaction in a short period of time under a mild reaction temperature. The use of a green organic solvent (e.g., EtOH) contributed to the improvement of the mass transfer of organic molecules (i.e., phenylboronic acid and biphenyl) to induce effective interactions with the surfaces of the integrated AuNPs onto the MPT particles, which has been explained by the collision theory [10,13,47,48]. It is also important to remember that EtOH as a solvent did not show detectable formation of common phenol byproducts, unlike the same reaction in water [28].…”

“…As a control experiment, nonporous TiO 2 and mesoporous SiO 2 particles treated with 20 wt% gold salts were tested as catalysts under the same reaction but exhibited relatively low yields, possibly due to the poor loading and relatively large size of the loaded AuNPs. Although elucidating the exact catalytically active species in AuNP-based catalysts is still a challenging task, the efficient homocoupling reaction of arylboronic acid molecules could be catalyzed by primary Au 0 and minor Au δ+ around the AuNPs onto MPT particles, which was also explained by other similar systems [12,13,24,49]. Our ongoing work involves the use of additional oxide-based materials possessing mesoporous structures (e.g., SiO 2 , CeO 2 , and Al 2 O 3 ) to examine the effects of supports in the catalytic reaction [12,19,50].…”

“…One attractive candidate is a colloidal form of nanoscale metal particles that can be prepared in green solution media (e.g., water and EtOH) and still show catalytic reactivity and recyclability [2,[6][7][8]. Among many metal nanoparticles, gold nanoparticles (AuNPs) have shown ideal characteristics due to their biocompatible nature and well-established preparation strategies for diverse structures [2,[9][10][11][12][13]. In this sense, bare and surface-modified AuNPs have been tested as catalysts in numerous chemical reactions, including dye degradation, reduction, oxidation, and coupling reactions [1,5,8,9,12,14,15].…”

Comparative Catalytic Properties of Supported and Encapsulated Gold Nanoparticles in Homocoupling Reactions

“…The activation energy (E a ) was then derived from Arrhenius plots using these three slopes where the apparent E a of the reaction was ∼45 kJ/mol for the supported AuNPs and ∼43 kJ/mol for the encapsulated AuNPs. The size of both integrated AuNPs was relatively large compared to other reactive AuNP-based catalysts (≤ ∼5 nm in diameter), but their abundant bare surfaces that act as catalytically active sites might play an important role to display the comparable E a values within the reported range (27-61 kJ/mol) (Wang et al, 2013;Karanjit et al, 2015;Liu et al, 2016). One can also speculate about the size-dependent E a for the supported and encapsulated AuNPs (Sharma et al, 2003;Fenger et al, 2012;Murzin, 2019), but the comparable E a values, even with notably different sizes and distributions of the AuNPs (e.g., uniform 7 nm vs. polydisperse 20 nm), were possibly due to the compromise effect between the reactivity and dispersity in solution (e.g., higher reactivity but poorer dispersity for supported AuNPs vs. lower reactivity but better dispersity).…”