Pd(II) and Pt(II) catalysed selective synthesis of furfuryl alcohol: Solvent effects and insights into the mechanism

Highlights

• Pd(II) and Pt(II) complexes were synthesized and characterized by spectroscopic and analytical techniques.

• All the complexes were active pre-catalysts in hydrogenation of furfural to furfuryl alcohol using formic acid hydrogen carrier.

• A plausible reaction mechanism for furfural to furfuryl alcohol reaction was proposed and the active catalyst was identified.

Abstract

Selective conversion of furfural to furfural alcohol, a bio-based commodity chemical with applications in the agriculture, metal casting and polymer industries is presented. Pd(II) and Pt(II) complexes were synthesized and characterized using multi-nuclear magnetic resonance spectroscopy, infrared spectroscopy, high resolution electrospray ionisation mass spectrometry as well as elemental analysis. These complexes were found to serve as excellent pre-catalysts for furfural alcohol synthesis with excellent conversion and selectivity (>99%), more so under solvent-free conditions via transfer hydrogenation method. Catalysts C1 and C5 also showed enhanced activity and selectivity as pre-catalysts for the hydrogenation of furfural using formic acid as a hydrogen carrier. The platinum catalyst C5 was recycled twice (with consistent activity and selectivity), and proved to function as a molecular catalyst as evidenced by mercury poisoning experiments. In situ NMR studies were performed using C5 culminated in the proposition of plausible reaction mechanism, and a likely catalytically active species has been identified with the aid of ¹H and ³¹P{¹H} NMR.

Introduction

The world’s utilization of fossil fuels and petrochemical products is increasing rapidly, whereas the reserved resources for obtaining these products are decreasing [1,2]. Global liquid fuels for transportation are heavily reliant on fossil resources (such as oil, natural gas and coal) [[3], [4], [5], [6], [7]]. The Key World Energy Statistics Report shows that energy demand is expected to increase across many regions [8]. With the depletion of fossil fuel reserves and increase in energy demand, great efforts are being made to convert renewable biomass into valuable fuels and chemicals [9,10]. In recent years, the transformation of biomass as an alternative environmentally friendly, widely abundant and inexpensive renewable resource has attracted attention in both scientific and industrial communities [[11], [12], [13], [14], [15]].

A promising renewable resource for the production of liquid fuels and chemicals is plant biomass, which contains lignocellulose. The constituents of lignocellulosic biomass are lignin (15–20%), cellulose (40–50%), and hemicellulose (25–35%) [[16], [17], [18], [19]]. The hemicellulose component has been converted to C₅ industrial sugars that can further be converted into furfural, as has been reported on demonstration scale using the Xylex®Technology [15]. The C₅ sugars can readily be dehydrated to furfural (FF) using acid catalysts [11,[20], [21], [22], [23], [24]], of which furfural is a key platform molecule which can be converted to a variety of furan based compounds, alcohols, esters and alkanes [[25], [26], [27]]. Furfuryl alcohol (FA), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran (MF), γ-valerolactone (γ-GVL), methyl/ethyl levulinate (EL), 1,5-pentanediol (1,5-PDO) and methyl furan are amongst some of the chemicals derived from furfural Scheme 1 [28,29]. These have significant application as renewable chemicals intermediates, solvents and fuels [30]. The hydrogenation of heterocyclic compounds is one of the most important catalytic transformations and it has a wide range of applications in the pharmaceutical and fine chemical industries [31,32]. Among these compounds, FA and THFA, which are products of furfural hydrogenation, are particularly attractive.

Approximately 62% of the FF produced worldwide is converted into FA [20,33,34], and this is used a polymer monomer, as a solvent, as a constituent in foundry sand binders in the metal casting industry and in the separation of hydrocarbons [[35], [36], [37]]. For example, in the petrochemical industry, paraffinic and naphthenic components can be easily separated from undesired aromatic and olefinic components using furfuryl alcohol [38].

FA is currently manufactured industrially by hydrogenation of FF in a liquid or gas phase over copper chromite catalyst at high temperatures using molecular hydrogen. This industrial process employs an environmentally toxic chromium-based catalyst, therefore there is need to develop new Cr-free catalysts to selectively convert FF to FA [[39], [40], [41]]. Several reports on the hydrogenation of FF to FA over various heterogeneous catalysts mainly Ru, Ir, Pt, Cu and Pd noble metals have been reported in literature [36,[42], [43], [44]]. Sitthisa et al. proposed a catalytic strategy for the hydrogenation of FF using 10 bar of H₂ over 5%Pd/SiO₂ catalyst. In this report 74% conversion of FF was achieved with 65% yield of FA at 230 °C [45], while O’Driscoll et al. achieved 45% FF conversion and 8% selectivity to FA in ethanol, using 20 bar of H₂ at 100 °C [46]. In another report, Thompson et al. reported the hydrogenation of FF to FA over [Pd(NH₃)₄][ReO₄]₂ on Al₂O₃ which exhibited remarkably high activity, resulting in 87% conversion of FF with 73% selectivity to FA using 10 bar H₂ at 150 °C [44].

Hronec et al. reported the application of 5% Pt/C as catalyst using 80 bar H₂ at 175 °C in just 0.5 h, to give 99% conversion, with 48% selectivity to FA in n-butanol [29]. A look at the effect of solvent and metal (Ni, Pt, Pd, or Ru) on the hydrogenation selectivity, showed that Pt was more selective to produce FA than other metals and n-butanol proved to be the best solvent [29]. Bhogeswararao and co-workers found 95% yield of FA with 96% conversion of FF over Pt/Al₂O₃ catalyst at 120 °C over 1 h [46,47]. Very few studies have been devoted to the hydrogenation of furfural using homogenous metal complexes. Their high selectivity to the desired product with minimum side products formation often seen with homogenous catalysts, under mild conditions, makes them preferable over heterogeneous catalysts [32]. Platinum Group Metal complexes containing metals such as Rh, Ru and Ir have been reported in literature [32,[48], [49], [50], [51]]. In 1994, Burk et al., reported on a rhodium(I) complex, [Rh(COD)(DiPFc)]⁺OTf⁻, which was successfully used to reduce the furfural without catalyst deactivation via decarbonylation and with 100% conversions in less than 3 h under 2–4 bar H₂. However, the reaction required the use of degassed methanol since in solution, there was a tendency for the catalyst to react with oxygen, leading to deactivation of the catalyst [50]. Apart from the noble metals that have been used in the conversion of furfural to furfuryl alcohol, Gorgas et al., recently synthesized complexes using earth-abundant iron and used these as catalysts for the hydrogenation of various aldehydes to their respective alcohols, including the hydrogenation of furfural to furfuryl alcohol [52]. However, to the best of our knowledge, no reports on the use of homogenous platinum and palladium catalysts exist in the literature for the hydrogenation of FF to FA, and this sparked our interest to develop and study these systems. This may not only provide mild reaction avenues and chromium-free catalytic systems, but can also allow for probing reaction pathways, so as to understand product and/or by-products formation. Herein, we report on the synthesis, characterization and evaluation of new Pt(II) and Pd(II) complexes as catalyst precursors for the hydrogenation of FF.