Enhancing Cooperativity in Bifunctional Acid–Pd Catalysts with Carboxylic Acid-Functionalized Organic Monolayers

Abstract

Cooperative catalysts containing a combination of noble metal hydrogenation sites and Brønsted acid sites are critical for many reactions, including the deoxygenation (DO) of biomass-derived oxygenates in the upgrading of pyrolysis oil. One route toward the design of cooperative catalysts is to tether two different catalytically active functions so that they are in close proximity while avoiding undesirable interactions that can block active sites. Here, we deposited carboxylic acid (CA)-functionalized organophosphonate monolayers onto Al2O3-supported Pd nanoparticle catalysts to prepare bifunctional catalysts containing both Brønsted acid and metal sites. Modification with phosphonic acids (PAs) improved activity and selectivity for gas-phase DO reactions, but the degree of improvement was highly sensitive to both the presence and positioning of the CA group, suggesting a significant contribution from both the PA and CA sites. Short spacer lengths of 1–2 methylene groups between the phosphonate head and CA tail were found to yield the best DO rates and selectivities, whereas longer chains performed similarly to self-assembled monolayers having alkyl tails. Results from a combination of density functional theory and Fourier transform infrared spectroscopy suggested that the enhanced catalyst performance on the optimally positioned CAs was due to the generation of strong acid sites on the Al2O3 support adjacent to the metal. Furthermore, the high activity of these sites was found to result from a hydrogen-bonded cyclic structure involving cooperativity between the phosphonate head group and CA tail function. More broadly, these results indicate that functional groups tethered to supports via organic ligands can influence catalytic chemistry on metal nanoparticles.

Highly selective conversion of phenol to cyclohexanol over Ru/Nb2O5-nC18PA catalysts with increased acidity in a biphasic system under mild conditions

“…The P-CH 3 stretching vibration exists at 1465 cm −1 , indicating that the functionality of the deposited phosphonic acid remains in the organic monolayer. It also shows that the phosphate ester exists in the tail of the alkyl group, 31,44 and the relatively high frequency of asymmetric -CH 2 bands indicates a considerable degree of disorder. The disorder of high-frequency asymmetric -CH 2 has been proved in the literature to be consistent with a lack of coordination between phosphonic acid and the support, indicating that the head group of C18PA has Brønsted acidity.…”

“…Interestingly, the study by J. Will Medlin et al [31][32][33] demonstrated that the introduced Brønsted acidity is related to the activity of deoxygenation (DO) when C18PA is deposited on the surface of Pd/Al 2 O 3 . This is consistent with the negative correlation between Brønsted acid sites and cyclohexanol selectivity.…”

“…In this case, hydrophobic surface modification of the support can be performed by introducing organic ligands, which not only stabilizes the catalyst structure, inhibiting catalyst deactivation, but also modifies the support surface in a controlled manner while maintaining the intrinsic structure of the support. 31 Among these approaches, the introduction of alkyl phosphonates is promising. The application of alkyl phosphonates in metal oxide supports has been reported to enhance the catalyst activity and improve the structural stability of the catalyst introducing acidic sites.…”

Effects of metal oxide surface doping with phosphonic acid monolayers on alcohol dehydration activity and selectivity

“…These results were in line with previous studies of reactions of alcohols on phosphonic acid functionalized TiO2-and Al2O3-based catalysts, which showed no loss of functionality after exposure to reaction conditions. [9,23,24] We noted that there were subtle shifts in peak frequency and intensity after the thermal treatment; as shown in Figure S1, for example, the peak intensity for methylene stretching in some cases increased after heating. Such changes have previously been interpreted in terms of reorganization of the monolayers to adopt different orientations (e.g., with different degrees of tilt) during annealing.…”

“…Inductively coupled plasma (ICP) techniques have been used to estimate the coverage of several phosphonic acids on Al2O3 to be approximately 3-4 molecules nm -2 , and analysis of benzyl phosphonic acids on TiO2 have indicated a similar coverage. [9,13,24] Attempts to accurately measure coverage changes across different oxides with ICP in this study were hampered by the low surface areas of many of the tested materials. In addition to the possibility of changes in surface coverage, it is important to note that different oxides have been reported to yield different modes of phosphonate attachment.…”