Abstract
A new Schiff base ligand (E)-4-(((quinolin-2-ylmethylene) amino) methyl) phenol (L1) along with its reduced counterpart 4-(((quinolin-2-ylmethyl) amino) methyl) phenol (L2), were synthesized through Schiff base condensation and NaBH4 reduction reactions respectively. All ligands were characterized using nuclear magnetic resonance spectroscopy 1H NMR, 13C{1H} NMR, infrared (FT-IR) spectroscopy, high resolution electrospray ionisation mass spectrometry (HR-ESI-MS), melting point determination, CHN elemental analysis and Single crystal X-ray diffraction (SCXRD) in the case of L1. Complexation reactions of these ligands with [Ru(p-cymene) Cl2]2, [PdCl2(CH3CN)2], or [Cp*IrCl2]2 gave the corresponding bidentate N^N Ru(II), Pd(II) and Ir(III) complexes (C1 - C6). These complexes were also characterized using the aforementioned analytical and spectroscopic techniques.
Mesoporous silica (SBA-15) was synthesized and functionalized using 3-iodopropyltrimethoxysilane. Complexes C4 - C6 were immobilized onto the support through the interaction between the siloxane functionalized SBA-15 (SBA-15-IPTMS) and the hydroxyl group present in the complexes. The synthesised SBA-15, SBA-15-IPTMS and all the heterogenized catalysts (C4-SBA-15, C5-SBA-15, and C6-SBA-15) were characterized using various solid state techniques infrared (FT-IR) spectroscopy, powder X-ray diffraction (XRD), nitrogen adsorption/desorption (BET) surface analysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). The results showed that the complexes were successfully immobilized onto the SBA-15 support. The TEM chromatographs revealed that the structural integrity of the SBA-15 support did not collapse upon immobilization of the complexes. From the EDX and XPS spectra we were able to confirm the presence of the complexes in the solid support and also confirm that the metals were not reduced to zero oxidation states during the immobilization process. Lastly, ICP-OES quantitatively determined the metal loading of 5.72% Ru, 9.20% Pd and 11.1% Ir in the immobilized catalysts C4-SBA-15, C5-SBA-15 and C6-SBA-15 respectively.
Both the molecular and immobilized catalysts were evaluated in the hydrodeoxygenation of bioderived compound δ-furfurylidenelevulinic acid (FDLA) to various products. When complexes C1 – C6 were evaluated as catalysts at a temperature and pressure of 110 °C and
x
10 bar H2 gas, respectively it was observed that all the catalysts showed 100% selectivity towards the olefin hydrogenated product A (Scheme 3.3). However, when the temperature was raised to 170 °C, the amine complexes C4 – C6 showed higher activity as catalysts with catalysts C5 and C6 exhibiting 100% substrate conversion. Also, catalysts C4 – C6 were able to yield the fully deoxygenated product (decane) without the aid of acid catalyst. This activity exhibited by the catalysts is superior to what has been reported in literature for the same substrate.
The immobilized catalysts showed lower activity compared to their molecular counterparts. This trend was evident from the decrease in FDLA conversion observed in C4-SBA-15 catalyst from 88% to 54% and decrease in decane yield from 6% to 4% compared to the molecular catalyst C4. This may be attributed to diffusion obstacles experienced when the reactants try to access the active metal centre or when the products have to be released from the pores of the SBA-15 support. A hot filtration test showed that all the immobilized catalysts catalysed the hydrodeoxygenation reaction following a heterogenous pathway. Moreover, all the immobilized catalysts were re-usable at least once without any loss in activity. Notably, immobilized catalyst C6-SBA-15 was re-used four times with loss of activity only observed after the fourth run by the decrease in substrate conversion. Characterization of the spent catalyst after the fourth cycle suggests that the metal leached off the solid support, this may have occurred during the catalyst recovery and washing process.