Synthesis of hybrid materials combining two or more components with different properties is a commonly used strategy to obtain high-performance materials for a wide range of different applications, including catalytic reactions with a complex reaction network. In this work, two synthesis approaches are investigated to prepare hybrid materials containing two well-defined phases, i.e. H-ZSM-5 and CeO2. By dispersing nano-sized H-ZSM-5 on CeO2, and by synthesizing a core-shell catalyst with a nano H-ZSM-5 core and a ceria shell, a high intimacy between H-ZSM-5 and CeO2 was obtained, while keeping the characteristic properties of each phase. The dehydration of 1,3-butanediol is used as a test reaction to explore the potential of these materials. A ratio between the selectivities of the desired product and intermediates, i.e. 1,3-butadiene (BD), 3-buten-2-ol (3B2OL), and 2-buten-1-ol (2B1OL) to the undesired product propylene (PP) was defined: (BD+3B2OL+2B1OL)/PP. Dispersing H-ZSM-5 zeolites on a CeO2 surface was the optimal synthesis route to enhance the 1,3-butadiene selectivity, and a (BD+3B2OL+2B1OL)/PP selectivity ratio was obtained (3.9) compared to its parent material (2.1).

Reconstructed hydrotalcites serve as powerful catalysts for the aldolization of alpha-hydroxyketones, exemplified here by the glycerol-derived dihydroxyacetone (DHA), toward the formation of branched hexoses (dendroketoses). Due to the multichemical functionality of these hydroxyketones, various competitive reactions such as keto-aldehyde isomerization (e.g., dihydroxyacetone (DHA)/glyceraldehyde (GLA) equilibrium) are possible, reducing the branching selectivity of the aldolization reaction. This study reveals that the nature of the basic sites (as determined via CDCl3 probe FT-IR and CO2-TPD experiments) strongly affects the branching selectivity of the condensation reaction as well as the stability of the hydrotalcite catalyst. For instance, strong basic sites not only allow the undesired keto-aldehyde isomerization but also promote the Cannizzaro reactions toward the formation of organic carboxylic acids leading to the catalyst leaching and waste generation. Thus, subsequent chemical transformations of branched sugars cannot occur without prior purification. For instance, the hydrogenation toward biobased branched polyol, which is the industrial target in casu base condensation of DHA. Tuning the basic properties of the reconstructed hydrotalcite based on this knowledge ultimately led to an active, selective, and stable catalyst with improved regeneration possibility.

Zeolites are the most widely reported solid materials that are used in primary amine-containing postcombustion CO2 capture processes for quicker solvent regeneration at a lower energy consumption. Catalytic solvolysis of the carbamate intermediate, assisted by the Bronsted acid sites (BAS), is commonly accepted as an explanation. However, there is little, if any, attention given to the regeneration of BAS in such basic amine-rich solution. Herein, we revisit the role of zeolite for CO2 desorption in primary amine solution at room to moderately elevated temperature ranges. We noticed indeed an accelerating effect on the CO2 desorption rate in the presence of BAS. Both their numbers as well as their accessibility for the amine-CO2 adduct (i.e., carbamate) (direct) or amine (indirect pathway) are important. However, we also noticed, using spectroscopic techniques and by zeolite reuse, a very strong chemical interaction between BAS and the regenerated amine. This suggests that BAS recovery to close the catalytic cycle is difficult and that zeolites can hardly be considered as genuine catalysts, but rather, this study concludes a merely stoichiometric effect of the zeolites for the overall desorption process, and this is in contrast to reference oxides such as TiO(OH)(2).

We disclose the mechanism of catalystpoisoning by organicsulfur species that are present in commercial 5-HMF feedstock duringits hydroconversion. The transformation of 5-hydroxymethylfurfural (HMF) intoring-saturatedfuranics is a vital step in carbohydrate valorization. In this work,we report on the remarkable catalyst poisoning effect of numeroussulfur species for HMF hydroconversion. The presence of minor amountsof dimethyl sulfoxide (DMSO) affects ring-saturated product selectivityfor the metal-catalyzed reactions using molecular hydrogen, whereasit fully deactivates catalytic transfer hydrogenation (CTH) in 2-propanol.The degree of poisoning correlates with the thermodynamic favorabilityof the metal sulfide formation. Reduced sulfur species (sulfide orthiol) are the ultimate metal poisoning agent. Their easy formationfrom more oxidized sulfur compounds explains the observed poisoningeffect for such species. Here, the metal's oxophilicity determinesthe catalysts' behavior in the presence of oxidized sulfurspecies by forming (or not) poisoning sulfur-metal interactions.To overcome the sulfur poisoning, we propose DMSO removal with organicsolvent extraction and catalyst oxidation post-treatment. These findingspinpoint the crucial, though overlooked, role of the biobased HMFpurity for reductive catalytic studies. We provide a deeper understandingof the noble metal poisoning by sulfur from different origins andoxidation states that may be present during HMF hydroconversion.

5-hydroxymethylfurfural (HMF) is an important bio-derived platform molecule that is generally obtained from hexoses via acid-catalyzed dehydration. It can be effectively transformed into a variety of value-added derivatives, thus being an ideal candidate for fossil replacement. Both HMF oxidation and hydrogenation processes enable the synthesis of numerous chemicals, monomers for polymerization, and biofuel precursors. This Review summarizes the most recent advances in heterogeneous catalytic hydroconversion of HMF into valuable chemicals with strong focus on 2,5-bishydroxymethyl furan (BHMF), 2,5-bishydroxymethyltetrahydrofuran (BHMTHF), and 2,5-dimethyltetrahydrofuran (DMTHF). In addition, multifunctional catalytic systems that enable a tunable production of various HMF derived intermediates are discussed. Within this chemistry, the surprising impact of HMF purity on the catalytic performance, such as selectivity and activity, during its upgrading is highlighted. Lastly, the remaining challenges in the field of HMF hydroconversion to the mentioned chemicals are summarized and discussed, taking into account the knowledge gain of catalyst properties and feedstock purity.

Methyl lactate (ML) conversion to methyl acrylate is studied in the gaseous phase over ZSM-5 zeolite catalysts. High acrylate selectivity and catalyst service time were achieved using the K-ZSM-5 catalyst with low content of Bronsted acid sites (below 1 mu mol g-1) and an overall K-to-Al atom ratio of unity. Feeding of ML in methanol containing 5 to 25 vol % of water improves catalyst stability. As such, up to 80% acrylate yield at complete ML conversion, along with minor deactivation after days-on-stream and fully recoverable catalysis, is presented.

Selective conversion of 5-hydroxymethylfurfural (HMF) to liquid fuels is an important step in the valorization of carbohydrates. Although not paid much attention to in the scientific community, we discovered an enormous impact of the HMF purity on the product selectivity during its hydroconversion in the presence of Ru/C. The presence of dimethyl sulfoxide (DMSO) showed the most dramatic effect on the aromatic hydrogenation activity, with almost full selectivity for either 2,5-dimethylfuran (DMF) or 2,5-dimethyltetrahydrofuran (DMTHF). The sulfur to surface ruthenium molar ratio predicts the selectivity outcome. A similar selective poisoning effect of DMSO was observed while using 2,5-bis(hydroxymethyl)furan (BHMF) as feedstock. These findings provide an approach to modify the selectivity of HMF conversion using the Ru/C catalyst and draw attention to the utmost importance of biobased reagent purity for catalytic studies to avoid erroneous conclusions about catalyst properties.

Heterogeneous catalysis is a promising technology for the valorization of renewable biomass to sustainable advanced fuels and fine chemicals. Porosity and nanostructure are the most versatile features of heterogeneous solid catalysts, which can greatly determine the accessibility of specific active sites, reaction mechanisms, and the selectivity of desirable products. Hence, the precise tuning of porosity and nanostructure has been a potential strategy towards developing novel solid catalysts with indispensable characteristics for efficient biomass valorization. Herein, we present a timely and comprehensive review of the recent advances in catalytic biomass conversions over microporous zeolites, mesoporous silicas, and nanostructured metals/metal oxides. This review covers the catalytic processing of both edible (lipids and starch) and non-edible (lignocellulose) biomass as well as their derived compounds, along with a systematic evaluation of catalyst reusability/kinetic/mechanistic aspects in the relevant processes. The key parameters essential for tailoring particle size, morphology, porosity, acid-base, and redox properties of solid catalysts are emphasized, while discussing the ensuing catalytic effects towards the selective conversion of biomass into desirable chemicals. Special attention has been drawn to understand the role of water in liquid phase biomass conversions as well as the hydrothermal stability and the deactivation of nanoporous catalysts. We believe this comprehensive review will provide new insights towards developing state-of-the-art solid catalysts with well-defined porosity and nanoscale properties for viable biomass conversion.

Different Ti-Si catalysts, viz. TiO2 supported on amorphous SiO2 or Si-MCM-41, TiO2-SiO2 xerogels, and Ti zeolites (TS-1 and Ti-beta), were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88-92% at conversions below 50%. From DR UV-vis spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO4 sites is higher than those of polymerized TiO5 or the octahedral TiO6 counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO4 sites may be attributed to an increased number of vacant sites (e.g., two for TiO4, zero for TiO6). Lactide productivity thus highly benefits from an increased dispersion of Ti sites on the catalyst surface and could be increased by a factor of 2.5 (up to 10 g(LD) g(cat)(-1) h(-1)) when TiO2 is dispersed on a Si-MCM-41 support, with higher surface areas in comparison to amorphous SiO2 gels.

A new route to lactide, which is a key building block of the bioplastic polylactic acid, is proposed involving a continuous catalytic gas-phase transesterification of renewable alkyl lactates in a scalable fixed-bed setup. Supported TiO2 /SiO2 catalysts are highly selective to lactide, with only minimal lactide racemization. The solvent-free process allows for easy product separation and recycling of unconverted alkyl lactates and recyclable lactyl intermediates. The catalytic activity of TiO2 /SiO2 catalysts was strongly correlated to their optical properties by DR UV/Vis spectroscopy. Catalysts with high band-gap energy of the supported TiO2 phase, indicative of a high surface spreading of isolated Ti centers, show the highest turnover frequency per Ti site.

The production of drop-in chemicals from bio-based renewable sources is gaining a lot of momentum due to proven negative impact of fossil-based economy on environment and society. In this Review, various bio-derived platform molecules are assessed as renewable alternatives to fossil resources for the catalytic production of acrylates. Acrylic acid and its esters are key building blocks of a large number of high-value oligomers and polymers in the current industry. In spite of the encouraging successes reported on gram or lab-scale, real implementation of bio-based examples remain scarce mainly due to the current high cost and limited availability of the bio-based substrates. As lactic acid and their derivatives are one of the most promising feedstocks for bio-acrylate production, they are the main focus of this Review.

Selective dealkylation of alkylphenols, the opposite reaction of the more commonly studied phenol alkylation, may represent an important reaction in the production of base chemicals like phenol and olefins from fossilized and raw lignocellulosic matter. This study reports the first thermodynamics and kinetics studies of the vapor-phase conversion of ethylphenol (EP) over acidic gamma-Al2O3, amorphous (ASA) and crystalline aluminosilicates (like ferrierite, ZSM-22, ZSM-5, beta, and USY) in the absence of hydrogen and noble metals, as a way to produce phenol and ethylene. The reaction was studied deliberately in presence of steam to get stable time on stream catalysis. The thermodynamic analysis shows an endothermal EP conversion to phenol and ethylene, favoured at high reaction temperature, while isomerisation, disproportionation and transalkylation are thermodynamically preferred at low temperature. The kinetic study examines the role of the catalytically active sites; it reveals the importance of site constraint on the activity, selectivity and stability, and shows the complex temperature dependency of the dealkylation. Both Bronsted and Lewis acid sites are active, but multifactor dependency (such as acid strength and site accessibility) complicates the establishment of simple quantitative relationships with the acid type. EP does not enter the micropores of ferrierite and ZSM-22, as suggested by adsorption experiments. Kinetics without significant diffusion limitations were obtained with ZSM-5, beta and USY. Thus, in absence of intracrystalline diffusion limitation (as verified by calculations using reported effective diffusivities, and substantiated by a comparably high apparent activation energy for all zeolites), the increased reaction turnover rate with increasing pore size from medium to large pore zeolites is largely explained by a change in reaction pathway (from monolecular to bimolecular) to convert EP to phenol and ethylene. A pathway proceeding through fast thermodynamically favourable bimolecular reactions occurs in the spatially non-constrained pores and crystal surface, whereas monomolecular reactions take place in the micropores of ZSM-5. Despite the lower rate, the selectivity over ZSM-5 strongly benefits from active site confinement, being responsible to achieve quantitative formation of phenol and ethylene from ethylphenol. The excellent performance of ZSM-5 thus accords with its shape selective property that avoids undesired side reactions such as the sterically demanding bimolecular reactions like disproportionation, transalkylation and C-C cracking, and severe cokes formation.

This contribution studies the steam-assisted dealkylation of 4-npropylphenol (4-n-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (e.g., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (i) analyzing products distribution under varying temperature and contact time conditions, (ii) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (iii) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed to a pore confinement, avoiding disproportionation/transalkylation as a result of a transition state shape selectivity. The presence of water maintains a surprisingly stable catalysis, especially for ZSM-5 with low acid density. The working hypothesis of this stabilization is that water precludes diphenyl ether(s) formation in the pores by reducing the lifetime of the phenolics at the active site due to the high heat of adsorption of water on H-ZSM-5, besides counteracting the equilibrium of the phenolics condensation reaction. The water effect is unique for the combination of (alkyl)phenols and ZSM-5.

A catalytic process to produce glycolide, the cyclic dimer of glycolic acid (GA), is proposed. Glycolide is the key building block of the biodegradable plastic polyglycolic acid. Instead of the current industrial two-step route, which involves the polycondensation of GA and a subsequent backbiting reaction, a new route based on the gas-phase transesterification of methyl glycolate (MGA) over a fixed catalyst bed is presented. With specific supported TiO2 catalysts, a high glycolide selectivity of 75-78% can be achieved at the thermodynamically-limited equilibrium conversion of MGA (54% at 300 degrees C, 5.6 vol% MGA, 1 atm). The absence of solvent and the continuous nature of the process should allow for easy product separation and recycling of unconverted esters, while the few side-products, i.e. linear alkyl glycolate dimers and trimers seem recoverable via methanolysis. The reaction is compared to the cyclization of other alpha-hydroxy esters, such as methyl lactate to lactide, over the same catalysts, in terms of kinetics and thermodynamics. The absence of a methyl substitution on the alpha-carbon seems to lead to faster cyclization kinetics of MGA when compared to methyl lactate or the double-substituted methyl-2-hydroxy-isobutyrate. Contrarily, glycolide production is less favored thermodynamically compared to lactide. The absence of glycolide decomposition at temperatures up to 300 degrees C however allows to increase equilibrium conversion by taking the endergonic reaction to higher temperatures.

Increasing demand for renewable feedstock-based chemicals is driving the interest of both academic and industrial research to substitute petrochemicals with renewable chemicals from biomass-derived resources. The search towards novel platform chemicals is challenging and rewarding, but the main research activities are concentrated on finding efficient pathways to produce familiar drop-in chemicals and polymer building blocks. A diversity of industrially important monomers like alkenes, conjugated dienes, unsaturated carboxylic acids and aromatic compounds are thus targeted from renewable feedstock. In this context, on-purpose production of 1,3-butadiene from biomass-derived feedstock is an interesting example as its production is under pressure by uncertainty of the conventional fossil feedstock. Ethanol, obtained via fermentation or (biomass-generated) syngas, can be converted to butadiene, although there is no large commercial activity today. Though practised on a large scale in the beginning of the 20th century, there is a growing worldwide renewed interest in the butadiene-from-ethanol route. An alternative route to produce butadiene from biomass is through direct carbohydrate and gas fermentation or indirectly via the dehydration of butanediols. This review starts with a brief discussion on the different feedstock possibilities to produce butadiene, followed by a comprehensive summary of the current state of knowledge regarding advances and achievements in the field of the chemocatalytic conversion of ethanol and butanediols to butadiene, including thermodynamics and kinetic aspects of the reactions with discussions on the reaction pathways and the type of catalysts developed.

Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by (29) Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2 , temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1-2 wt% on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg−O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.

Upcoming bio-refineries will be at the heart of the manufacture of future transportation fuels, chemicals and materials. A narrow number of platform molecules are envisioned to bridge nature's abundant polysaccharide feedstock to the production of added-value chemicals and intermediate building blocks. Such platform molecules are well-chosen to lie at the base of a large product assortment, while their formation should be straightforward from the refined biomass, practical and energy efficient, without unnecessary loss of carbon atoms. Lactic acid has been identified as one such high potential platform. Despite its established fermentation route, sustainability issues - like gypsum waste and cost factors due to multi-step purification and separation requirements - will arise as soon as the necessary orders of magnitude larger volumes are needed. Innovative production routes to lactic acid and its esters are therefore under development, converting sugars and glycerol in the presence of chemocatalysts. Moreover, catalysis is one of the fundamental routes to convert lactic acid into a range of useful chemicals in a platform approach. This contribution attempts a critical overview of all advances in the field of homogeneous and heterogeneous catalysis and recognises a great potential of some of these chemocatalytic approaches to produce and transform lactic acid as well as some other promising alpha-hydroxy acids.

Direct synthesis of 1,3-butadiene (BD) from ethanol has been studied using magnesia–silica catalysts doped with transition metal(oxide)s. The effects of Mg/Si ratio, the synthesis procedure, and the dopant concentration were studied. It was demonstrated that modification of the magnesia–silica binary system using a consecutive impregnation step significantly increases the ethanol conversion rate and BD yield. The BD yield higher than 55 mol.% was obtained at full ethanol conversion for materials containing Cu and Ag modifiers. The influence of the reaction temperature and the ethanol concentration in the feed was also investigated. This investigation led to high BD productivity (>0.15 gBD gcat−1 h−1) and high BD concentration in the product stream (>10,000 Vppm).