Hydrothermal Synthesis of Ruthenium Nanoparticles with a Metallic Core and a Ruthenium Carbide Shell for Low-Temperature Activation of CO2 to Methane.

Ruthenium nanoparticles with a core-shell structure formed by a core of metallic ruthenium and a shell of ruthenium carbide have been synthesized by a mild and easy hydrothermal treatment. The dual structure and composition of the nanoparticles have been determined by synchrotron X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) analysis, and transmission electron microscopy (TEM) imaging. According to depth profile synchrotron XPS and X-ray diffraction (XRD) analysis, metallic ruthenium species predominate in the inner layers of the material, ruthenium carbide species being located on the upper surface layers. The ruthenium carbon catalysts presented herein are able to activate both CO2 and H2, exhibiting exceptional high activity for CO2 hydrogenation at low temperatures (160-200 °C) with 100% selectivity to methane, surpassing by far the most active Ru catalysts reported up to now. On the basis of catalytic studies and isotopic 13CO/12CO2/H2 experiments, the active sites responsible for this unprecedented activity can be associated with surface ruthenium carbide (RuC) species, which enable CO2 activation and transformation to methane via a direct CO2 hydrogenation mechanism. Both the high activity and the absence of CO in the gas effluent confer relevance to these catalysts for the Sabatier reaction, a chemical process with renewed interest for storing surplus renewable energy in the form of methane.

Noble-Metal-Free Carbon Encapsulated CoNi Alloy Catalyst for the Hydrogenation of 5-(Hydroxymethyl) Furfural to Tetrahydrofurandiol in Aqueous Media.

The selective hydrogenation of 5-(hydroxymethyl)furfural (HMF) into 2,5-bis-(hydroxymethyl)tetrahydrofuran (BHMTHF) in flow reactor using water as a green solvent, has been achieved on a non-noble metal catalyst based on monodispersed CoNi alloy nanoparticles covered by a thin carbon layer. The alloyed catalyst containing CoNi (molar ratio 1 : 1) was prepared in a one-step synthesis following a hydrothermal method. Total conversion of HMF with 91 % selectivity to BHMTHF was achieved. The reaction network has been stablished, in which the carbonyl group of HMF is first reduced to alcohol giving the 2,5-bis-(hydroxymethyl)furan (BHMF) with an apparent activation energy of 25 KJ/mol, and then the double bonds of the furan ring are hydrogenated (apparent Ea=31 KJ/mol). Formation of byproducts, mainly proceed from furan ring opening and ring rearrangement processes of BHMF, promoted by water. BHMTHF resulted a compound highly stable under reaction conditions. The fixed bed flow reactor was maintained operational for 65 h without observing any loss of catalytic activity and selectivity.

Synthetic Routes for Designing Furanic and Non Furanic Biobased Surfactants from 5-Hydroxymethylfurfural.

5-hydroxymethylfurfural (HMF) is one of the most valuable biomass platform molecules, enabling the construction of a plethora of high value-added furanic compounds. In particular, in the last decade, HMF has been considered as a starting material for designing biobased surfactants, not only because of its renewability and carbon footprint, but also because of its enhanced biodegradability. This Review presents recent examples of the different approaches to link the hydrophilic and lipophilic moieties into the hydrophobic furan (and tetrahydrofuran) ring, giving a variety of biobased surfactants that have been classified here according to the charge of the head polar group. Moreover, strategies for the synthesis of different non-furanic structures surfactant molecules (such as levulinic acid, cyclopentanols, and aromatics) derived from HMF are described. The new HMF-based amphiphilic molecules presented here cover a wide range of hydrophilic-lipophilic balance values and have suitable surfactant properties such as surface tension activity and critical micelle concentration, to be an important alternative for the replacement of non-sustainable surfactants.

Selective Conversion of HMF into 3-Hydroxymethylcyclopentylamine through a One-Pot Cascade Process in Aqueous Phase over Bimetallic NiCo Nanoparticles as Catalyst.

5-hydroxymethylfurfural (HMF) has been successfully valorized into 3-hydroxymethylcyclopentylamine through a one-pot cascade process in aqueous phase by coupling the hydrogenative ring-rearrangement of HMF into 3-hydroxymethylcyclopentanone (HCPN) with a subsequent reductive amination with ammonia. Mono- (Ni@C, Co@C) and bimetallic (NiCo@C) nanoparticles with different Ni/Co ratios partially covered by a thin carbon layer were prepared and characterized. Results showed that a NiCo catalyst, (molar ratio Ni/Co=1, Ni0.5 Co0.5 @C), displayed excellent performance in the hydrogenative ring-rearrangement of HMF into HCPN (>90 % yield). The high selectivity of the catalyst was attributed to the formation of NiCo alloy structures as hydrogenating sites that limited competitive reactions such as the hydrogenation of furan ring and the over-reduction of the formed HPCN. The subsequent reductive amination of HPCN with aqueous ammonia was performed giving the target cyclopentylaminoalcohol in 97 % yield. Moreover, the catalyst exhibited high stability maintaining its activity and selectivity for repeated reaction cycles.

Enzymatic and chemo-enzymatic strategies to produce highly valuable chiral amines from biomass with ω-transaminases on 2D zeolites.

Amino transaminases (ATAs) have been supported on a 2D ITQ-2 zeolite through electrostatic interactions, resulting in a highly stable active biocatalyst to obtain a variety of valuable chiral amines starting from prochiral ketones derived from biomass. We have extended the biocatalyst applications by designing a chemo-enzymatic process that allows, as the first step, prochiral ketones to be obtained from biomass-derived compounds through an aldol condensation-reduction step using a bifunctional metal/base catalyst. The prochiral ketone is subsequently converted into the chiral amine using the immobilized ATA. We show that it is feasible to couple both steps in a semi-continuous process to produce industrially relevant chiral amines with yields of >95% and ∼100% enantiomer excess.

A recyclable bifunctional acid-base organocatalyst with ionic liquid character. The role of site separation and spatial configuration on different condensation reactions.

A series of bifunctional organic catalysts containing acid and basic sites with ionic liquid characteristics have been prepared and their catalytic activity and reaction coordinate for aldol and Knoevenagel condensations have been compared. While the only factor controlling catalyst activity for the Knoevenagel condensation was the distance between the acid and base sites, the spatial orientation of the organocatalyst is also key to achieve high activity and selectivity in the Claisen-Schmidt condensation. Mechanistic studies based on theoretical DFT calculations show that the acid-base bifunctional organocatalyst follows a mechanism inspired in natural aldolases for the synthesis of trans-chalcones, being able to produce a large variety of these compounds of industrial interest. The combination of the acid-base pairs within the proper geometry and the ionic liquid nature makes this catalyst active, selective and recyclable.

Bifunctional acid-base ionic liquid organocatalysts with a controlled distance between acid and base sites.

Bifunctional acid-base ionic liquid organocatalysts with different distances between the two sites have been synthesised, and their activity for the Knoevenagel condensation has been tested. As has been found to be the case with enzymes, the distance between the acidic and basic sites determines the activity of the bifunctional organocatalyst, and at the optimal distance the reaction rate increases by two orders of magnitude with respect to the purely acidic or basic counterpart organocatalysts. The experimental results have been rationalised through the study of the reaction mechanism of the Knoevenagel condensation between malononitrile and benzaldehyde by means of DFT calculations. It has been found that it consists of two consecutive steps. First, deprotonation of malononitrile on the basic site to obtain a methylene carbanion intermediate takes place, and second, co-adsorption and activation of benzaldehyde on the acid centre of this intermediate followed by the C-C bond-formation reaction. The calculations and the kinetic study indicate that there is an inversion of the rate-controlling step when the distance between the acidic and the basic sites is modified, with a direct implication on the reaction rate.

Transforming Methyl Levulinate into Biosurfactants and Biolubricants by Chemoselective Reductive Etherification with Fatty Alcohols.

Biomass-derived surfactants with very good surface tension and critical micellar concentration properties were obtained by conversion of methyl levulinate into methyl 4-alkoxypentanoates through reductive etherification with aliphatic alcohols. Among different bifunctional acid/metal catalysts best results were obtained with Pd on carbon bearing acid sites. The reaction occurred through the formation of an enol ether intermediate followed by hydrogenation. Pd in high-density planes was the active hydrogenation species, and an optimum crystal size was found to be approximately 10 nm. The reductive etherification with aliphatic alcohols was extended to other aliphatic and cyclic ketones and aldehydes obtained from biomass, and excellent results were obtained on supported Pd catalysts with the reaction route and experimental conditions described in this work.

Chemoenzymatic Synthesis of 5-Hydroxymethylfurfural (HMF)-Derived Plasticizers by Coupling HMF Reduction with Enzymatic Esterification.

Biobased plasticizers, as substitutes for phthalates, have been synthesized from 5-hydroxymethylfurfural (HMF) and carboxylic acids (or esters) through a chemoenzymatic cascade process that involves as its first step the reduction of 5-hydroxymethylfurfural into 2,5-bis(hydroxymethyl)furan (BHMF), followed by the esterification of BHMF with carboxylic acids (or esters) by using a supported lipase (Novozym 435). The reduction of HMF into BHMF is performed by using monodisperse metallic Co nanoparticles with a thin carbon shell (Co@C) with high activity and selectivity. After optimization of reaction conditions (temperature, hydrogen pressure, and solvent), it is possible to achieve 97 % conversion of HMF with 99 % selectivity to BHMF after 2 h reaction time. The reduction of HMF and esterification of BHMF using carboxylic acids or vinyl esters as acyl donors by lipase are optimized separately in batch and in fixed-bed continuous reactors. The coupling of two flow reactors (for reduction and subsequent esterification) working under optimized reaction conditions affords the diesters of BHMF in roughly 90 % yield with no loss of activity during 60 h of operation.

Molecular Oxygen Lignin Depolymerization: An Insight into the Stability of Phenolic Monomers.

During oxidative depolymerization of lignin in aqueous alkaline medium using molecular oxygen as oxidant, the highly functionalized primary phenolic monomers are not stable products, owing to various not fully identified secondary reaction mechanisms. However, better understanding of the mechanisms responsible for the instability of the main part of the products of interest derived from lignin is of much interest. Evaluation of their individual reactivities under oxidative conditions should significantly help to find a better way to valorize the lignin polymer and to maximize the yields of target value-added products. Consequently, the main objective of this study is to assess the individual stabilities of some selected lignin-based phenolic compounds, such as vanillin, vanillic acid, and acetovanillone, together with some other pure chemical compounds such as phenol and anisole to give an insight into the mechanisms responsible for the simultaneous formation and repolymerization of those products and the influence of the oxidation conditions. Various complementary strategies of stabilization are proposed, discussed, and applied for the oxidative depolymerization reactions of a technical lignin extracted from pinewood with a high content of ß-O-4 interconnecting bonds to try to obtain enhanced yields of value-added products.

Direct synthesis of the organic and Ge free Al containing BOG zeolite (ITQ-47) and its application for transformation of biomass derived molecules.

Aluminosilicate boggsite (Si/Al-BOG) has been hydrothermally synthesized without adding organic structure-directing agents (OSDAs) in the synthesis gel using the borosilicogermanium ITQ-47 (Si/B-ITQ-47) zeolite as seeds. The introduction of the costly and environmentally less benign phosphazene organic structure-directing agent is not required to grow the zeolite. Physicochemical characterization experiments show that Si/Al-BOG has good crystallinity, high surface area, tetrahedral Al3+ species, and acid sites. In order to test the catalytic performance of the zeolite, the synthesis of l,l-lactide from l-lactic acid was performed. Si/Al-BOG exhibits 88.2% conversion of l-lactic acid and 83.8% l,l-lactide selectivity, which are better than those of other zeolites studied up to now.

Correction: Direct synthesis of the organic and Ge free Al containing BOG zeolite (ITQ-47) and its application for transformation of biomass derived molecules.

[This corrects the article DOI: 10.1039/D0SC04044D.].

Multisite solid catalyst for cascade reactions: the direct synthesis of benzodiazepines from nitro compounds.

Substituted 1,5-benzodiazepines are selectively synthesized in one pot from substituted nitroaromatics and ketones. The reaction is performed in the presence of hydrogen and in the absence of solvent by using a bifunctional solid catalyst with a chemoselective hydrogenation functional group capable of reducing the nitro group to a diamino group and an acid functional group, which catalyzes the cyclocondensation of the amino group with the ketone.

One-Pot Synthesis of Biomass-Derived Surfactants by Reacting Hydroxymethylfurfural, Glycerol, and Fatty Alcohols on Solid Acid Catalysts.

A new type of biomass-derived non-ionic surfactants has been obtained by reacting hydroxymethylfurfural (HMF), glycerol, and fatty alcohols. For instance, 5-(octyloxymethyl)furfural glyceryl acetal can be obtained in a one-pot process by etherification of HMF with fatty alcohols followed by acetalization with glycerol. For a successful solid catalyst, acidity and polarity have to be optimized to improve conversion, selectivity, and catalyst deactivation owing to the different adsorption characteristics of the reactant molecules. Accordingly, Beta zeolite with a high Si/Al ratio and practically free of connectivity defects showed good results when dealing with these biomass derivatives, which include a highly polar reactant such as glycerol. The scope of the reaction is good and a variety of new stable surfactant molecules can be obtained that present hydrophilic-lipophilic balance (HLB ) values in the range 4.9 to 6.6, which are of interest for water in oil emulsions.

One-Pot Selective Catalytic Synthesis of Pyrrolidone Derivatives from Ethyl Levulinate and Nitro Compounds.

N-substituted 5-methyl-2-pyrrolidones were prepared in a one-pot process starting from ethyl levulinate and nitro compounds in the presence of a nanosized Pt-based catalyst. Pt supported on TiO2 nanotubes (Pt/TiO2 -NT) catalyzed the synthesis of N-substituted 5-methyl-2-pyrrolidones through a cascade process involving the reduction of nitro compounds, formation of the intermediary imine, hydrogenation, and subsequent cyclization. A bifunctional metal-acid system was a suitable catalyst for the process. Pt supported on TiO2 showed lower catalytic activity than Pt/TiO2 -NT owing to the strong adsorption of nitro compounds during the first reaction step that poisoned the acidic sites and strongly decreased the rate of amination and cyclization. However, Pt/TiO2 -NT with milder acid sites was less affected by the adsorption of nitro compounds and the full cascade process could proceed. The results indicate that the prepared Pt/TiO2 -NT is a chemoselective and reusable catalyst that can be applied to the synthesis of a variety of N-substituted 5-methyl-2-pyrrolidones starting from nitro compounds with excellent yields in absence of an additional organic solvent under mild reaction conditions.

Transformation of Cellulose into Nonionic Surfactants Using a One-Pot Catalytic Process.

Alkyl glucosides surfactants are synthesized by a cascade process that involves the methanolysis of cellulose into methyl glucosides followed by the transacetalization with n-octanol. The first step was performed using methanol as solvent and acid catalysts (such as, inorganic acids, heteropolyacids, ionexchange resins, or modified carbon materials). Subsequently, long-chain alkyl glucosides are obtained in the second step by transacetalization, which involves the reaction of methyl glucosides with a fatty alcohol using the same acid catalyst. The overall process was performed under mild conditions. Amorphous sulfonated carbon catalyst achieved the best results for the complete conversion of cellulose in methanol at 200 °C with methyl α,ß-glucopyranosides yields higher than 80 %. Moreover, this material containing -SO3 H groups is ideal to perform the second step to obtain octyl and decyl glucosides in yields higher than 73 % at 120 °C. In addition, the sulfonated carbon catalyst (C-SO3 H) can be reused with only a slightly decrease of its activity after four consecutive cycles.

Postsynthesis-Treated Iron-Based Metal-Organic Frameworks as Selective Catalysts for the Sustainable Synthesis of Nitriles.

The dehydration of aldoximes to the corresponding nitriles can be performed with excellent activity and selectivity by using iron trimesate as a homogeneous catalyst. Iron trimesate has been heterogenized by synthesizing metal-organic frameworks (MOFs) from iron trimesate, that is, Fe(BTC), and MIL-100 (Fe). These materials were active and selective aldoxime dehydration catalysts, and postsynthesis-treated MIL-100 (Fe) produced the desired nitriles with 100 % conversion and selectivities >90 % under mild reaction conditions and in short reaction times. X-ray photoelectron spectroscopy showed the presence of different Fe species in the catalyst, and in situ IR spectroscopy combined with catalytic results indicates that the catalytic activity is associated with Fe framework species. The postsynthesis-treated MIL-100 (Fe)-NH4 F can be recycled several times and has an excellent reaction scope, which gives better catalytic results than other solid acid or base catalysts.

Solid catalysts for multistep reactions: one-pot synthesis of 2,3-dihydro-1,5-benzothiazepines with solid acid and base catalysts.

1,5-Benzothiazepines derivatives were obtained first by starting from 1,3-diphenylpropenone derivatives (chalcones) and 2-aminothiophenol by using aluminosilicate solid catalysts. However, diffusional limitations and the strong adsorption of products on the catalyst are deleterious for catalyst activity and life. Then a structured amorphous mesoporous catalyst with large pores and mild acidity that works at higher temperatures allowed us to obtain high conversions (99 %) and selectivities (98 %) of the desired product. A one-pot synthesis of 1,5-benzothiazepines that starts from benzaldehyde, acetophenone, and 2-aminothiophenol with 95 % yield was performed by combining optimized solid base and acid catalysts in batch mode as well as in a continuous-flow reactor system. Much better conversion and selectivity as well as process intensification has been achieved with the structured mesoporous materials by avoiding intermediate and final neutralization and purification steps required in the synthesis reported previously that uses homogeneous catalysts.

Biomass-derived chemicals: synthesis of biodegradable surfactant ether molecules from hydroxymethylfurfural.

A new class of biodegradable anionic surfactants with structures based on 5-alkoxymethylfuroate was prepared starting from 5-hydroxymethylfurfural (HMF), through a one-pot-two-steps process which involves the selective etherification of HMF with fatty alcohols using heterogeneous solid acid, followed by a highly selective oxidation of the formyl group with a gold catalyst. The etherification step was optimized using aluminosilicates as acid catalysts with different pore topologies (H-Beta, HY, Mordenite, ZSM-5, ITQ-2, and MCM-41), different active sites (Bronsted or Lewis) and different adsorption properties. It was shown that highly hydrophobic defect-free H-Beta zeolites with Si/Al ratios higher than 25 are excellent acid catalysts to perform the selective etherification of HMF with fatty alcohols, avoiding the competitive self-etherification of HMF. Moreover, the 5-alkoxymethylfurfural derivatives obtained can be selectively oxidized to the corresponding furoic salts in excellent yield using Au/CeO2 as catalyst and air as oxidant, at moderated temperatures. Both H-Beta zeolite and Au/CeO2 could be reused several times without loss of activity.