EFFECT OF THE PREPARATION OF THE CATALYST AND PROTECTIVE AGENT IN LIQUID PHASE HYDROGENATION REACTIONS

Abstract In this Ph.D. project, the effective parameters in the sol-immobilization method which can affect catalytic reactivity including: capping agent, solvent of synthesis, and support were studied. The synthesized catalysts were employed in the liquid phase hydrogenation reactions of two biomass derived molecules namely furfural and vanillin. In this regard, the role of protective agents, which are used to stabilize colloidal nanoparticles (NPs) in solutions, for Pd NPs supported on carbon support in the liquid phase hydrogenation of furfural was explored. The use of Pd as a hydrogenation catalyst is well documented, as hydrogenation reactions can only be performed by metals that can easily chemisorb hydrogen. Pd is one of such metals capable of dissociate hydrogen even at room temperature. The capping agent adsorbed on the surface of the NPs can alter their activity and selectivity, by modifying the particle size, size distribution, morphology, and stability against leaching and agglomeration. Besides, the effect of the amount of protective agent and the synthesis solvents have been investigated, allowing better insight into the metal-support interaction in Pd/TiO2 catalysts for the hydrogenation of furfural. Then, the effect of using different carbonaceous supports with various chemical-physical properties on the activity and selectivity towards the hydrodeoxygenation of vanillin as a lignin model compound was explored. To better understand the role of the capping agents in controlling the activity and the selectivity of the furfural hydrogenation, a series of carbon-supported Pd nanoparticles were prepared through controlled sol-immobilization method using different capping agents, including polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and poly(diallyldimethylammonium) chloride (PDDA) containing oxygen and/or nitrogen groups. The catalysts were characterized by different techniques. UV-Vis and Fourier-transform infrared (FTIR) spectroscopy were used to evaluate the interaction between capping agents and Pd precursor, while transmission electron microscopy (TEM) was performed to study the final morphology of the catalyst. Finally, X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemistry of the carbon support, the chemical state, and the exposure of supported palladium species. The UV-Vis spectra of the system composed of Pd precursor plus capping agent demonstrated the interaction of the metal with PVA and PVP. In the case of PDDA, changes to the precursor salt complex were noticed through shifts in [PdCl4]2- absorbance bands. The disappearance of the peaks associated with Pd2+ and the observed scattering indicated a complete reduction to Pd0 and the formation of metal nanoparticles. The interaction of the capping agents with the Pd precursor was studied by FTIR spectroscopy. For PVA, the peaks corresponding to the stretching vibrations of C–O–C linkage were observed, which suggested the presence of cross-linked PVA molecules, and the slight modification observed for the peaks in the spectrum of PVA + Na2PdCl4 suggested a weak interaction between the metal precursor and the –OH groups present in PVA. In the case of PVP, a decrease in the intensity of the peak after the addition of Pd was observed which confirmed that both O and N groups are present in PVP interaction with the metal precursor. For PDDA, after the addition of Pd, the intensity of the peaks decreased, which indicated that the activity of the vibrational modes is modified in the mixture, probably due to PDDA–Pd interaction. The morphological characteristics of the synthesized catalysts were evaluated by HRTEM. We noticed that the capping agent has a major effect on the size and distribution of Pd NPs when using activated carbon as the support. All catalysts had an average particle size of 3–4 nm, with the presence of isolated larger particles in the case of PdPDDA/C. The XPS survey data revealed that only Pd, C, N, and O species were present on the surface of the catalysts. Depending on the capping agent used, substantial differences were observed in the relative amount of Pd on the surface: PdPVA/C (1.30 %) > PdPDDA/C (0.76 %) > PdPVP/C (0.50 %). The data also showed a different oxygen content in the samples. PdPVA/C displayed the highest relative amount of O (14.7 % compared to PdPDDA/C (9.70 %) and PdPVP/C (9.10 %)), respectively. The highest oxygen content on the surface of PdPVA/C catalyst is probably due to the presence of –OH groups in PVA. PdPVP/C exhibited the highest N content (2.95 %), higher than PdPDDA/C (1.86 %) due to the presence of a pyrrole-type N species in PVP and dimethyl-ammonium groups in PDDA, while in the PdPVA/C N was not detected on the surface, as expected. The performance of the prepared catalysts was examined for the liquid-phase hydrogenation of furfural (furfural 0.3 M; furfural/metal ratio 500 mol/mol, 5 bar H2, temperature range of 25–75 °C) with 2-propanol as solvent. The catalytic results revealed that the activity of catalysts is correlated to the relative amount of Pd at the surface: PdPVA/C (1.3%) > PdPDDA/C (0.76%) > PdPVP/C (0.50%). At 75 °C the catalysts exhibit similar reactivity. We ascribed this effect to the partial removal of a capping agent during the reaction, thereby exposing a higher number of active sites to the reactant. In the next step, since the best results were obtained using PVA, this latter was chosen as the protective agent, focusing on the effects that its amount and the change of the solvent of synthesis to methanol-water mixtures might have on the preparation of Pd/TiO2 catalysts. This new approach indicates the necessity of using the capping agent, and results also show that the acidification step, which lowers the isoelectric point (IEP) to afford anchoring of the NPs to the surface of the support, can be eliminated while still maintaining the same degree of Pd immobilization (≥ 96%) and particle size control (< 2 nm). These samples were evaluated for furfural hydrogenation; there was an improvement in selectivity towards furfuryl alcohol and tetrahydrofurfuryl alcohol, whereas ether by-products were suppressed. Supported Pd NPs were prepared in accordance with the standard sol-immobilization method, in which the temperature of the chemical reduction was maintained at 1 °C. This temperature was chosen as earlier research in our workgroup reviled the formation of smaller nanoparticles when lower temperatures were used. UV-Vis spectroscopy was performed to characterize the precursor salt and colloidal solutions during NPs preparation. Changes to the precursor salt complex were noticed through shifts in [PdCl4]2- absorbance bands. Catalysts prepared with increasing volumes of MeOH showed shifts in the distinctive ligand to metal charge transfer (LMCT) and d-d transitions. This confirms a change to the Pd metal precursor complex when it is solvated in either the MeOH/H2O mixture or in pure MeOH. One reason for this is the exchange of ligands between the Pd salt complex and the solvent synthesis. The Pd % loading of each catalyst was measured using microwave plasma – atomic emission spectroscopy (MP-AES). All catalysts were characterized using TEM, in order to get information of their average NP sizes and particle size distributions/dispersions. An initial comparison of catalysts, showed that an acidified immobilization step increases the average NP size. The bigger NP size and dispersion was found for the sample prepared by H2O as the solvent, PVA, and acidification step, which it can be ascribed to the interaction of the stabilizer agent (PVA) with acid during the immobilization step. XAFS was performed to investigate both changes to Pd oxidation state (XANES) and the local structural environment with respect to Pd (EXAFS). The results confirmed that by using PVA in the solvent system, the Pd oxidation state contains a greater quantity of Pd2+. Interestingly, by removing the acidification step during the synthesis, an increase in the oxidized Pd state was observed. This was consistent with the TEM data indicating this increase in Pd2+ is related to the observed particle sizes. Pd surfaces are known to form passivating oxide layers when exposed to air with an increase in the temperature needed to form a bulk oxide structure. Therefore, the Pd2+ fraction refers to the quantity of the available Pd surface and hence the particle size. Fitting of the 1st shell Pd K edge EXAFS data was consistent with the trends observed through TEM and XANES characterization. The presence of small Pd NPs is confirmed by the decreased magnitude of Pd-Pd scattering, signified by smaller CNPd-Pd. All catalysts were tested for the hydrogenation of furfural at 50°C. Although the XANES, EXAFS, and TEM data have all confirmed that sample prepared by the pure MeOH contains smaller NPs than the sample prepared by pure H2O, the initial catalytic activity showed that the sample synthesized by H2O has a significant increase in initial TOF h-1. In addition, furfural hydrogenation performed over the bare TiO2 support displayed 98.4 % selectivity to the acetal product (2-(diisopropoxymethyl) furan), at isoconversion, while sample prepared by H2O revealed less selectivity to the acetal (18.8%). To understand these differences in activity/selectivity we performed further HRTEM investigations of the fresh samples. These studies have identified a 'halo' of PVA around NPs produced using MeOH (e.g. PdMeP) and at the interface between NPs and support. We suggest that the poor solubility of PVA in methanol is responsible for this increase in PVA clustering on the catalytic surface/support interface identified with HRTEM. We also propose that the active sites on TiO2 are formed through the spillover of hydrogen onto the support and accounts for the superior yield of acid catalysed products for Pd/TiO2 compared to TiO2 alone. In the case of Pd/TiO2, the catalysts prepared using MeOH result in aggregation of PVA at the Pd-TiO2 interface, which restrict spillover effects and decreases the amount of acid catalysed products. For the catalysts prepared without the addition of PVA, we observed a higher proportion of ethers than for the analogous catalysts prepared with PVA. This further supports our hypothesis that PVA at the metal-support interface limits the spillover of hydrogen. Catalyst recycle tests were carried out over six consecutive hydrogenation cycles. The catalysts prepared without PVA quickly deactivated with almost negligible performance by the sixth consecutive run. The TEM analysis of the spent sample confirmed that when PVA is not present the samples quickly agglomerate and effective surface area of Pd rapidly diminishes. Finally, the effect of support on both activity and selectivity of the catalyst in the hydrogenation of vanillin to vanillyl alcohol and the subsequent hydrodeoxygenation (HDO) to creosol was studied. Four types of carbonaceous materials (three activated carbon: Norit, KB, G60, and a carbon nanofiber: HHT) were used as support for Pd nanoparticles. Catalysts were synthesized with sol immobilisation method using PVA as the capping agent, and tested in the hydrodeoxygenation reaction of vanillin under mild reaction conditions (50 °C, 5 bar H2 and isopropanol as solvent). Catalysts have been extensively characterized by TEM, XPS and BET in order to correlate the surface properties with the catalytic behaviour and selectivity to the target products. In the end, recycle tests were carried out on the most active catalyst to determine the reusability of the material used. BET analysis was conducted on Pd-supported catalysts. Pd/Norit catalyst has the highest surface area (2000 m2/g), whereas Pd/HHT has the lowest surface area (40 m2/g). The average pore radius of all samples was in the range of 2 nm for Pd/KB to 25,4 nm for Pd/HHT. XPS analysis were performed on the synthesized catalysts to determine the oxidation state of the Pd, the graphitization order and the presence and abundance of oxygen functionality. Pd/Norit, Pd/KB and Pd/G60 had a similar amount of C1s species (84.4 %, 87.3 % and 91.1 %, respectively), while Pd/HHT had the highest number of C 1s (98.8 %). Evaluation of O1s species revealed that the Pd/HHT catalyst has the lowest amount of functionalisation (0.9 %), while Pd/Norit the highest one (14.3 %). The results showed that the Pd/HHT catalyst has the highest amount of C=C (82.1 %), therefore the support can be considered highly graphitised. TEM analysis were performed to determine the mean particle size and size distribution. In all samples, the nanoparticles are homogeneously distributed on the support. Pd/Norit and Pd/KB had similar mean Pd particle sizes (2.5 and 2.7 nm respectively). Whereas, Pd/G60 and Pd/HHT displayed higher mean Pd nanoparticle diameters (3.5 and 3.9 nm, respectively). The catalysts were tested in the vanillin HDO reaction. The reaction profile shows the features of a typical consecutive reaction, with vanillyl alcohol as the intermediate product, and creosol as the final product. Interesting, an additional product was detected in the reaction mixture, namely vanillyl isopropyl ether. The ether is produced by reaction of vanillyl alcohol with a molecule of solvent (isopropanol) and it is consumed with time since it is in equilibrium with the alcohol. The results revealed that the rate of conversion of vanillin enhances with increasing degree of graphitisation. These results can be explained by the strong interaction between the graphitic plane of the support and the aromatic ring of the substrate that allows a better interaction with the active metal nanoparticles. At the same time, the activity in the vanillin hydrogenation reaction decreases with an increase in oxygen content at the carbon surface. The support-substrate interaction is responsible for the change in activity; the increase in oxygen functionality actually disrupts the graphical plane structure of the support. The support affected the selectivity at the lower conversion. In fact, at 50 % of conversion, vanillyl alcohol was the main product of Pd/Norit, Pd/HHT and Pd/KB (selectivity in the 73-82 %), while Pd/G60 provided high levels of both vanillyl isopropyl ether and creosol (23 and 21 %, respectively). The formation of ether was associated with the amount of carboxylic support functionality (COOH groups). The recycling tests were carried out on the most active catalyst: Pd/HHT. After five consecutive reactions, the conversion did not significantly decrease, demonstrating the high stability of the catalyst. Comparably, the selectivity of vanillyl alcohol remained almost unchanged.