Catalytic upgrading of carboxylic acids and esters to bio fuels and bio chemicals

The research activity was focused on the transformation of methyl propionate (MP) into methyl methacrylate (MMA), avoiding the use of formaldehyde (FAL) thanks to a one-pot strategy involving in situ methanol (MeOH) dehydrogenation over the same catalytic bed were the hydroxy-methylation/dehydration of MP with FAL occurs. The relevance of such research line is related to the availability of cheap renewable bio-glycerol from biodiesel production, from which MP can be obtained via a series of simple catalytic reactions. Moreover, the conventional MMA synthesis (Lucite process) suffers from safety issues related to the direct use of carcinogenic FAL and depends on non-renewable MP. During preliminary studies, ketonization of carboxylic acids and esters has been recognized as a detrimental reaction which hinders the selective synthesis of MMA at low temperature, together with H-transfer hydrogenation with FAL or MeOH as the H-donor at higher temperatures. Therefore, ketonization of propionic acid (PA) and MP was investigated over several catalysts (metal oxides and metal phosphates), to obtain a better understanding of the structure-activity relationship governing the reaction and to design a catalyst for MMA synthesis capable to promote the desired reaction while minimizing ketonization and H-transfer. However, ketonization possesses scientific and industrial value itself and represents a strategy for the upgrade of bio oils from fast pyrolysis of lignocellulosic materials, a robust and versatile technology capable to transform the most abundant biomass into liquid biofuels. The catalysts screening showed that ZrO2 and La2O3 are the best catalysts, while MgO possesses low ketonization activity, but still, H-transfer parasitic hydrogenation of MMA reduces its yield over all catalysts. Such study resulted in the design of Mg/Ga mixed oxides that showed enhanced dehydrogenating activity towards MeOH at low temperatures. It was found that the introduction of Ga not only minimize ketonization, but also modulates catalyst basicity reducing H-transfer hydrogenations.

Asymmetric synthesis of benzylic and heterobenzylic amines

C2-Symmetrical, enantiopure 2,6-di[1-(1-aziridinyl)alkyl]pyridines (DIAZAPs) were prepared by a high-yielding, three-step sequence starting from 2,6-pyridinedicarbaldehyde and (S)-valinol or (S)-phenylglycinol. The new compounds were tested as ligands in palladium-catalyzed allylation of carbanions in different solvents. Almost quantitative yield and up to 99% enantiomeric excess were obtained in the reactions of the enolates derived from malonate, phenyl- and benzylmalonate dimethyl esters with 1,3-diphenyl-2-propenyl ethyl carbonate. Asymmetric synthesis of 2-(2-pyridyl)aziridines from chiral 2-pyridineimines bearing a stereogenic center at the nitrogen atom was development. The envisioned route involves the addition of chloromethyllithium to the imine derived from 2-pyridinealdehyde and (S)-valinol, protected as O-trimethylsilyl ether. The analogous reaction performed on the imine derived from (S)-valine methyl ester gave the product containing the aziridine ring as well as the α-chloro ketone group coming from the attack of chloromethyllithium to the ester function. Other stereogenic alkyl substituents at nitrogen gave less satisfactory results. Moreover, the aziridination protocol did not work on other aromatic imines, e.g. 3-pyridineimine and benzaldimine, which are not capable of bidentate chelation. The N-substituent could not be removed, but aziridine underwent ring-opening by attack of nitrogen, sulfur, and oxygen nucleophiles. Complete or prevalent regioselectivity was obtained using cerium trichloride heptahydrate as a catalyst. In some cases, the N-substituent could be removed by an oxidative protocol. The addition of organometallic (lithium, magnesium, zinc) reagents to 2-pyrroleimines derived from (S)-valinol and (S)-phenylglycinol gave the N-substituted-1-(2-pyrrolyl)alkylamines with high yields and diastereoselectivities. The (S,S)-diastereomers were useful intermediates for the preparation of enantiopure 1-[1-(2-pyrrolyl)alkyl]aziridines by routine cyclization of the β-aminoalcohol moiety and of (S)-N-benzoyl 1-[1-(2-pyrrolyl)alkyl]amines and their N-substituted derivatives by oxidative cleavage of the chiral auxiliary. 1-Allyl-2-pyrroleimines obtained from (S)-phenylglycinol and (S)-valinol underwent highly diastereoselective addition of allylmetal reagents, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to 1-propenyl isomerisation of the 1-pyrrole substituent. Protection of the 2-aminoalcohol moiety as oxazolidinone, amide or Boc derivate followed by ring closing metathesis of the alkene groups gave the unsaturated bicyclic compound, whose hydrogenation afforded the indolizidine derivative as a mixture of separable diastereomers. The absolute configuration of the main diastereomer was assessed by X-ray crystallographic analysis.

Upgrading Bio-Platform Molecules in the Gas-Phase: From Levulinic Acid to Bio-Chemicals

Levulinic acid (LA) is a polyfunctional molecule obtained from biomass. Because of its structure, the United States Department of energy classified LA as one of the top 12 building block chemicals. Typically, it is valorized through chemical reduction to γ-valerolactone (GVL). It is usually done with H2 in batch systems with high H2 pressures and noble metal catalysts, making it expensive and less applicable. Therefore, alternative approaches such as catalytic transfer hydrogenation (CTH) through the Meerwein–Ponndorf–Verley (MPV) reaction over heterogeneous catalysts have been studied. This uses organic molecules (alcohols) which act as a hydride transfer agent (H-donor), to reduce molecules containing carbonyl groups. Given the stability of the intermediate, reports have shown the batch liquid-phase CTH of levulinate esters with secondary alcohols, and remarkable results (GVL yield) have been obtained over ZrO2, given the need of a Lewis acid (LASites) and base pair for CTH. However, there were no reports of the continuous gas-phase CTH of levulinate esters. Therefore, high surface area ZrO2 was tested for gas-phase CTH of methyl levulinate (ML) using ethanol, methanol and isopropanol as H-donors. Under optimized conditions with ethanol (250 ℃), the reaction is selective towards GVL (yield 70%). However, heavy carbonaceous materials over the catalyst surface progressively blocked LASites changing the chemoselectivity. The in situ regeneration of the catalyst permitted a partial recovery of the LASites and an almost total recovery of the initial catalytic behavior, proving the deactivation reversible. Tests with methanol were not promising (ML conversion 35%, GVL yield 4%). As expected, using isopropanol provided complete conversion and a GVL yield of 80%. The reaction was also tested using bioethanol derived from agricultural waste. In addition, a preliminary study was performed for the hydrogenolysis of polyols to produce bioethanol, were Pd-Fe catalyst promoted the ethanol selective (37%) hydrogenolysis of glycerol.

Catalytic reduction of levulinic acid derivatives to bio-fuel components

Levulinic Acid and its esters are polyfunctional molecules obtained by biomass conversion. The most investigated strategy for the valorization of LA is its hydrogenation towards fuel additives, solvents and other added-value bio-based chemicals and, in this context, heterogeneous and homogeneous catalysts are widely used. Most commonly, it is typically performed with molecular hydrogen (H2) in batch systems, with high H2 pressures and noble metal catalysts. Several works reported the batch liquid-phase hydrogenation of LA and its esters by heterogenous catalysts which contained support with Brønsted acidity in order to obtain valeric acid and its esters. Furthermore, bimetallic and monometallic systems composed by both a metal for hydrogen activation and a promoter were demonstrated to be suitable catalysts for reduction of carboxylic group. However, there were no studies in the literature reporting the hydrogenation of alkyl levulinates to 1-pentanol (1-PAO). Therefore, bimetallic and monometallic catalysts were tested for one-pot hydrogenation of methyl levulinate to 1-PAO. Re-based catalysts were investigated, this way proving the crucial role of the support for promoting the ring-opening of GVL and its consecutive reduction to valeric compounds. All the reactions were performed in neat without the need of any additional solvents. In these conditions, bimetallic Re-Ru-O/HZSM-5 afforded methyl valerate and valeric acid (VA) with a productivity of 512 mmol gmetal-1 h-1, one of the highest reported in literature to date. Rhenium can also promote the reduction of valeric acid/esters to PV through the formation of 1-pentanol and its efficient esterification/transesterification with the starting material. However, it was proved that Re-based catalysts may undergo leaching of active phase in presence of carboxylic acids, especially by working in neat with VA. Furthermore, the over-reduction of rhenium affects catalytic performance, suggesting not only that a pre-reduction step is unnecessary but also that it could be detrimental for catalyst’s activity.