Bridging homogeneous and single-site heterogeneous catalysts for the Guerbet reaction

The catalytic Guerbet reaction is one of the archetypal approaches used for ethanol to n-butanol transformation, which can be catalysed by employing either homogeneous or heterogeneous catalysts, under mild reaction conditions (150 oC, 1 bar Ar, batch reactor and solventless liquid phase, typical Guerbet reaction conditions). Poor catalysis performance (low ethanol conversion and catalytic activity) of both types of catalysts is observed under the reaction conditions. Hence, the application of new stable and active catalysts for Guerbet reaction is required. In this thesis, single-site catalyst, showing both the distinct nuclearity of a homogenous catalyst and the recyclability of a heterogeneous catalyst, is studied and screened for upgrading ethanol to n-butanol.

In Chapter 3, a heterogeneous single-site catalyst comprised of an atomically dispersed Ru(bipy) complex was incorporated into a polyphenylene polymer support and applied to the Guerbet reaction. The catalytic performance was studied, showing ethanol upgrading exceeding that of contemporary homogeneous and heterogeneous catalysts, with a TON of 3565 Ru-1 and 82% selectivity to n-butanol over 16 h. Catalyst deactivation was observed after 16 h in a time-on-line study, which can be attributed to inhibition by the water byproduct and structural transformation of the catalyst. The catalyst could be partially reactivated through removal of water from the reaction, via recharging the reactor with fresh ethanol, or by addition of 3Å zeolite molecular sieves to remove water in-situ. The PPhen-[bipy-RuIIICl4] (Ru PPhenbipy) sample showed greater stability in the presence of the molecular sieve compared to the homogeneous catalyst, so achieving higher butanol yields. No leaching of Ru within the framework was observed after reaction via XRF analysis. Characterisation of the fresh and used catalyst via TEM, XAFS and XPS showed structural transformation of the single atom Ru(bipy) into small nanoparticles of Ru(0), which grow during the reaction. While the used 16 h catalyst had moderate recyclability (TON of 1000 Ru-1) the highly active species seen in the fresh catalyst were not stable during reaction. It is unclear if these highly active species are the initial single atom Ru(bipy) species or Ru(0) clusters formed during the initial 2h of reaction.

Chapter 4 investigated the catalytic performance of blank PPhenbipy, which was shown to comprise of 0.36 wt% Pd impurities. It provided a TON of 15919 Pd-1 over 16 h and time-on-line study showed that PPhenbipy was still active even running the catalysis up to 48 h. Catalyst deactivation was however observed with water spiking and addition of 3Å molecular sieves did not improve the catalytic activity. A 3.43 wt% PPhen-[bipy-PdIICl2] (Pd PPhenbipy) single site catalyst was then synthesised. The performance of this catalyst was studied and gave 2155 Pd-1 TON and similar n-butanol selectivity after 16 h catalysis. Palladium K-edge EXAFS showed the formation of small nanoparticles of Pd(0), suggesting the catalyst had changed structure during the reaction.

Bicyclic diphosphane is one type of phosphine ligand commonly used as a bridging ligand in coordination chemistry with transition metals. However, there has been a noticeable lack of investigation into screening this ligand under bridged homogeneous catalytic Guerbet reaction conditions. In Chapter 5, the in situ formed homogeneous catalysts from mixtures of [RuCl2(η6-p-cymene)]2 precursor and bicyclic diphosphane ligands, (P-P(R)) [R = NMe2, OMe and H], were evaluated for ethanol upgrading. The results suggests that the binuclear complex is an active catalyst in catalytic reaction. The preformed dimeric [RuIICl2(η6-p-cymene)]2(P-P(R)) complexes exhibited similar catalytic activity with monomeric [RuIICl(η6-p-cymene)(dppm)]Cl. Catalyst deactivation was observed after 2 h in a time-on-line study, which can be attributed to the decomposition of homogeneous catalyst into Ru(0) metal. Analysing the [RuCl2(η6-p-cymene)]2/dppm postreaction solutions and filter solid precipitate with ruthenium K-edge EXAFS demonstrate the formation of nanoparticle Ru(0) after catalysis, matching the plateaued ethanol conversion after 2 h reaction.