Valorisation of plastic waste via metal-catalysed depolymerisation.

Metal-catalysed depolymerisation of plastics to reusable building blocks, including monomers, oligomers or added-value chemicals, is an attractive tool for the recycling and valorisation of these materials. The present manuscript shortly reviews the most significant contributions that appeared in the field within the period January 2010-January 2020 describing selective depolymerisation methods of plastics. Achievements are broken down according to the plastic material, namely polyolefins, polyesters, polycarbonates and polyamides. The focus is on recent advancements targeting sustainable and environmentally friendly processes. Biocatalytic or unselective processes, acid-base treatments as well as the production of fuels are not discussed, nor are the methods for the further upgrade of the depolymerisation products.

Biomass-derived chemical substitutes for bisphenol A: recent advancements in catalytic synthesis.

Bisphenol A is an oil-derived, large market volume chemical with a wide spectrum of applications in plastics, adhesives and thermal papers. However, bisphenol A is not considered safe due to its endocrine disrupting properties and reproductive toxicity. Several functional substitutes of bisphenol A have been proposed in the literature, produced from plant biomass. Unless otherwise specified, the present review covers the most significant contributions that appeared in the time span January 2015-August 2019, describing the sustainable catalytic synthesis of rigid diols from biomass derivatives. The focus is thereupon on heterogeneous catalysis, use of green solvents and mild conditions, cascade processes in one-pot, and continuous flow setups. More than 500 up-to-date references describe the various substitutes proposed and the catalytic methods for their manufacture, broken down according to the main biomass types from which they originate.

Continuous-Flow Oxidation of HMF to FDCA by Resin-Supported Platinum Catalysts in Neat Water.

The oxidation reaction of 5-hydroxymethylfurfural to the bioplastic monomer 2,5-furandicarboxylic acid over heterogenous resin-supported Pt catalysts was investigated in detail, under continuous flow, base-free conditions, and in neat water. The product was continuously obtained in 99 % yield by running the reaction at 120 °C, 303 s residence time, 1.2 mL min-1 O2 flow rate, and 7.7 bar O2 pressure. The product was isolated with a high space-time-yield of 46.0 g L-1 h-1 without purifications or acid/base treatments.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts.

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H2 O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Continuous-flow processes for the catalytic partial hydrogenation reaction of alkynes.

The catalytic partial hydrogenation of substituted alkynes to alkenes is a process of high importance in the manufacture of several market chemicals. The present paper shortly reviews the heterogeneous catalytic systems engineered for this reaction under continuous flow and in the liquid phase. The main contributions appeared in the literature from 1997 up to August 2016 are discussed in terms of reactor design. A comparison with batch and industrial processes is provided whenever possible.

Selective hydrogenation over Pd nanoparticles supported on a pore-flow-through silica monolith microreactor with hierarchical porosity.

Well-dispersed Pd nanoparticles have been synthesized inside the mesoporosity of a silica monolith featuring hierarchical porosity of homogeneous interconnected macropores (4 microns) and mesopores (11 nm). These monoliths have been implemented as microreactors for selective hydrogenation reactions. Conversion and selectivity can be tuned by adjusting the flow rates of hydrogen and substrates. In the selective hydrogenation of cyclooctadiene, a conversion of 95% and a selectivity of 90% in the monohydrogenated product, constant over a period of 70 h, have been reached. These figures correspond to a productivity of 4.2 mmol s(-1) g(-1)(MonoSil) (or 0.32 mol s(-1) g(-1)(Pd)). In the stereoselective hydrogenation of 3-hexyn-1-ol a constant conversion of 85% was observed, with however moderate selectivity into the cis isomer, over a test period of 7 h. These results open the route to the synthesis of important chemicals and intermediates via safe and green processes.

Green production of polymer-supported PdNPs: application to the environmentally benign catalyzed synthesis of cis-3-hexen-1-ol under flow conditions.

Pd nanoparticles generated on gel type ion-exchange resins under catalytic conditions show high activity, selectivity and durability in partial hydrogenation reactions under mild conditions, thus providing a green, low-cost option for fine-chemicals production. The application to the continuous-flow synthesis of the leaf alcohol fragrance cis-3-hexen-1-ol is demonstrated.

Iodine activation of coordinated white phosphorus: formation and transformation of 1,3-dihydride-2-iodidecyclotetraphosphane.

Double stabilization: Previously unknown polyphosphorus compounds are obtained by activation of white phosphorus (P(4)) coordinated between two CpRu(PPh(3))(2) moieties with iodine, and subsequent hydrolysis. The polyphosphorus compounds (P(4) H(2) I, P(4) H(2), P(3) H(5); see scheme, Cp=cyclopentadienyl) are all stabilized by coordination to two ruthenium centers.

Emerging strategies in sustainable fine-chemical synthesis: asymmetric catalysis by metal nanoparticles.

The combination of chiral- and nano-technologies is potentially one of the most promising strategies for "green" catalytic conversions aimed at fine-chemical production. This paper reviews the most representative examples and highlights recent achievements in the field of enantioselective processes catalyzed by metal nanoparticles (MNPs), with a view to the potential of the methodology and future perspectives. Applications are described according to the nanocatalyst type used, either liquid-phase suspensions or solid-supported systems. The focus is on the design of the catalysts, their preparation and characterization, applications in catalysis and mechanistic studies.

Getting a clue to the hydrolytic activation of white phosphorus: the generation and stabilization of P(OH)2PHPHPH(OH) at ruthenium centers.

The bimetallic compound [{CpRu(PPh(3))(2)}(2)(mu,eta(1:1)-P(4))][CF(3)SO(3)](2), in which the tetrahedral P(4) is bound to two CpRu(PPh(3))(2) fragments, slowly reacts under mild conditions with a moderate excess of water (1:20) to yield a mixture of compounds. Among the hydrolysis products, the new, remarkably stable complexes [{CpRu(PPh(3))}{CpRu(PPh(3))(2)}{mu(1,4:3),eta(2:1)-P(OH)(2)PHPHPH(OH)}](CF(3)SO(3))(2) (2) and [{CpRu(PPh(3))(2)}{CpRu(PPh(3)){P(OH)(3))}(mu,eta(1:1)-P(2)H(4))](CF(3)SO(3))(2) (3) have been isolated. In the former, the previously unknown 1,1,4-tris(hydroxy)tetraphosphane molecule, P(OH)(2)PHPHPH(OH), is 1,4- and, respectively, 3-coordinated to the CpRu(PPh(3)) and the CpRu(PPh(3))(2) moieties; in the latter, the diphosphane P(2)H(4) is stabilized through coordination to two different metal fragments. All of the compounds were characterized by elemental analyses and IR and NMR spectroscopy. The crystal structure of 2 was determined by X-ray analysis. The formation of the hydroxytetraphosphane, containing the tetraphosphorus entity, provides a clue to the hydrolytic activation of the P(4) molecule.

NMR studies on the novel heterobimetallic complexes [M(dppm)(Ph(2)PCH(2)PPh(2)PPPP) {Pt(PPh3)2}]OTf (M = Rh, Ir) derived from the stepwise activation of white phosphorus.

The solution structures of the novel heterobimetallic complexes [Ir(dppm)(Ph(2)PCH(2)PPh(2)PPPP){Pt(PPh(3))2}]OTf and [Rh(dppm)(Ph(2)PCH(2)PPh(2)PPPP){Pt(PPh(3))(2)}]OTf derived from the reaction of Rh and Ir--P(5) precursors with [Pt(C2H4)(PPh3)2] have been unambiguously assigned on the basis of 1H NMR and 31P{1H} NMR data. The results are in agreement with the regio-selective insertion of the {Pt(PPh3)2} moiety resulting in a new pentaphosphorus topology which agrees with the formal formation of a unique phosphonium(+)-tetraphosphabutadienide(2-) ligand.

Collective headgroup conformational transition in twisted micellar superstructures.

Predictions on amphiphilic self-assemblies traditionally rely on considerations on molecular shape and charge of the surfactant. In the case of functional surfactants a more sophisticated toolbox becomes necessary to design amphiphiles encoding chemical functionalities that provide additional responsive properties to their self-assemblies. Here we report on a comprehensive and combined structural-spectroscopic characterization of 1,2-dilauroyl-phosphatidyl-adenosine (DLPA) micelles in phosphate buffer. The temperature dependence, more precisely the thermal history of the sample, is explicitly taken into account. The experimental data, supplemented with MD simulations, indicate the presence of two possible states at room temperature, characterized by distinctly different structural properties that depend on the thermal history of the sample. The twisted superstructures, produced by aging DLPA micelles through intermicellar assembly of locally cylindrical aggregates at room temperature, collapse upon warming at 35 °C, yielding aligned filaments and/or wormlike structures. The initial superstructures cannot be recovered by thermal inversion. The reason for this behaviour is that the thermal activation causes a redistribution of syn-anti conformations of adenosine headgroups, as indicated by spectroscopic results (NMR, CD, FTIR), which is then collectively frozen thanks to molecular constraints present in the aggregate.

Hydrolysis of dinuclear ruthenium complexes [{CpRu(PPh3)2}2(micro,eta(1:1)-L)][CF3SO3]2 (L=P4, P4S3): simple access to metal complexes of P2H4 and PH2SH.

The reaction of [CpRu(PPh(3))(2)Cl] (1) with half an equivalent of P(4) or P(4)S(3) in the presence of AgCF(3)SO(3) as chloride scavenger affords the stable dimetal complexes [{CpRu(PPh(3))(2)}(2)(micro,eta(1:1)-P(4))][CF(3)SO(3)](2).3 CH(2)Cl(2) (2) and [{CpRu(PPh(3))(2)}(2)(micro,eta(1:1)-P(apical)-P(basal)-P(4)S(3))][CF(3)SO(3)](2).0.5 C(7)H(8) (3), in which the tetrahedral P(4) and mixed-cage P(4)S(3) molecules are respectively bound to two CpRu(PPh(3))(2) fragments through two phosphorus atoms. The coordinated cage molecules, at variance with the free ligands, readily react with an excess of water in THF under mild conditions. Among the hydrolysis products, the new, remarkably stable complexes [{CpRu(PPh(3))(2)}(2)(micro,eta(1:1)-P(2)H(4))][CF(3)SO(3)](2) (4) and [CpRu(PPh(3))(2)(eta(1)-PH(2)SH)]CF(3)SO(3) (8) were isolated. In the former, diphosphane, P(2)H(4), is coordinated to two CpRu(PPh(3))(2) fragments, and in the latter thiophosphinous acid, H(2)PSH, is coordinated to the metal centre through the phosphorus atom. All compounds were characterised by elemental analyses and IR and NMR spectroscopy. The crystal structures of 2, 3, 4 and 8 were determined by X-ray diffraction.

Hydrogenation of arenes over silica-supported catalysts that combine a grafted rhodium complex and palladium nanoparticles: evidence for substrate activation on Rh(single-site)-Pd(metal) moieties.

The complex Rh(cod)(sulfos) (Rh(I); sulfos = (-)O(3)S(C(6)H(4))CH(2)C(CH(2)PPh(2))(3); cod = cycloocta-1,5-diene), either free or supported on silica, does not catalyze the hydrogenation of benzene in either homogeneous or heterogeneous phase. However, when silica contains supported Pd metal nanoparticles (Pd(0)/SiO(2)), a hybrid catalyst (Rh(I)-Pd(0)/SiO(2)) is formed that hydrogenates benzene 4 times faster than does Pd(0)/SiO(2) alone. EXAFS and DRIFT measurements of in situ and ex situ prepared samples, batch catalytic reactions under different conditions, deuterium labeling experiments, and model organometallic studies, taken together, have shown that the rhodium single sites and the palladium nanoparticles cooperate with each other in promoting the hydrogenation of benzene through the formation of a unique entity throughout the catalytic cycle. Besides decreasing the extent of cyclohexa-1,3-diene disproportionation at palladium, the combined action of the two metals activates the arene so as to allow the rhodium sites to enter the catalytic cycle and speed up the overall hydrogenation process by rapidly reducing benzene to cyclohexa-1,3-diene.

Recycling asymmetric hydrogenation catalysts by their immobilization onto ion-exchange resins.

Ion-exchange resins can be used as supports for the preparation of single-site, heterogenised asymmetric hydrogenation catalysts. The immobilised catalysts obtained can be efficiently and conveniently recovered and recycled. This article reviews the significant contributions in the field including the main concepts behind the design and the applications of this type of catalyst.

Recycling asymmetric hydrogenation catalysts by their immobilisation onto ion-exchange resins.

New systems based on cationic chiral phosphine-rhodium complexes anchored to a commercial cation-exchange gel-type resin showed high efficiency and easy recycling in the asymmetric hydrogenation of prochiral olefins.