Giacomo M. Lari

Deactivation mechanisms of tin-zeolites in biomass conversions

Metal-containing zeolites comprise outstandingly active and selective catalysts for a multitude of Lewis-acid and redox catalysed reactions that valorise renewable substrates into chemicals. Herein, focussing on tin-zeolites applied to the isomerisation of dihydroxyacetone (DHA) and xylose, we systematically study the influence of the framework type (MFI, MOR, BEA and FAU), preparation method (hydrothermal synthesis or alkaline-assisted metallation), hydrophobicity and nature of the solvent (water or methanol) on their intrinsic activity and, especially, on their stability. BEA and FAU zeolites were more active than MFI and MOR stannosilicates in aqueous and, particularly, in methanol-based tests owing to the larger relative amount of tetrahedral tin sites and/or superior mass transfer properties. Hydrothermally-prepared zeolites generally exhibited higher turnover frequencies than those obtained by a post-synthetic approach in view of their higher tin quality and less hydrophilic character. Remarkably, continuous-flow tests in a fixed-bed reactor indicated that exposure to reaction conditions for 24 hours could provoke dramatic changes in performance. Mainly due to amorphisation and modification of the tin structure (from tetra- to hexacoordination), the activity and selectivity of MOR, BEA and FAU zeolites were substantially, if not fully, depleted in the aqueous isomerisation of DHA, while MFI zeolites, especially the post-synthetically stannated sample, better preserved their initial performance. These deactivation phenomena were alleviated through the use of methanol, an industrially more amenable solvent, but, due to the greater retention of activity, fouling was more pronounced, particularly for MFI zeolites. In contrast, more extensive metal leaching was detected owing to the higher solubility of tin in the alcohol rather than water, where it hydrolyses into insoluble hydroxide species. In xylose isomerisation, hydrothermally-prepared MFI and BEA stannosilicates displayed reasonably stable operation. Deactivation mechanisms partly resembled those of the aqueous DHA isomerisation, but the chelating properties of the substrate played a greater detrimental role in tin loss.

Hybrid Palladium Nanoparticles for Direct Hydrogen Peroxide Synthesis: The Key Role of the Ligand

Ligand-modified palladium nanoparticles deposited on a carbon carrier efficiently catalyze the direct synthesis of H2 O2 and the unique performance is due to their hybrid nanostructure. Catalytic testing demonstrated that the selectivity increases with the HHDMA ligand content from 10 % for naked nanoparticles up to 80 %, rivalling that obtained with state-of-the-art bimetallic catalysts (HHDMA=C20 H46 NO5 P). Furthermore, it remains stable over five consecutive reaction runs owing to the high resistance towards leaching of the organic moiety, arising from its bond with the metal surface. As rationalized by density functional theory, this behavior is attributed to the adsorption mode of the reaction intermediates on the metal surface. Whereas they lie flat in the absence of the organic shell, their electrostatic interaction with the ligand result in a unique vertical configuration which prevents further dissociation and over-hydrogenation. These findings demonstrate the importance of understanding substrate-ligand interactions in capped nanoparticles to develop smart catalysts for the sustainable manufacture of hydrogen peroxide.

Glycerol oxidehydration to pyruvaldehyde over silver-based catalysts for improved lactic acid production

Sustainable technologies for the valorisation of the burgeoning amounts of glycerol (GLY) obtained as waste in the production of biodiesel are increasingly sought after. Its conversion into lactic acid (LA) is appealing due to the versatility of this platform chemical and its high added value. Here, we introduce Ag-based catalysts for the oxidehydration of GLY to pyruvaldehyde (PAl) and demonstrate the superiority of this compound in comparison to dihydroxyacetone (DHA) as the intermediate of an alternative two-step GLY-to-LA process. Evaluation of various metals and carriers identified Ag/Al2O3 as the best performer for PAl production. This was rationalised based on the optimal redox potential of the metal and the high concentration of Lewis-acid sites and the limited Bronsted acidity of the support. At 623 K and O2/GLY = 0.5, a PAl yield of 80% was attained, which remained stable for 24 h. Characterisation of the used catalyst indicated that the surface of the silver nanoparticles was partially oxidised upon reaction. Density functional theory (DFT) modelling revealed that the oxidation of acetol obtained from GLY after the initial dehydration step is kinetically and thermodynamically favoured on a partially oxidised silver surface (AgOx/Ag) compared to metallic (Ag) or fully oxidic (Ag2O) ones. Finally, we show that PAl can be isomerised into LA and methyl lactate over Sn-containing zeolites with the same rates as DHA but at a 40 K lower temperature (343 vs. 383 K). This not only allows for energy savings but also for a remarkably increased catalyst stability.

Hierarchical NaY Zeolites for Lactic Acid Dehydration to Acrylic Acid

The industrial viability of heterogeneous catalysts for the gas‐phase conversion of lactic acid to acrylic acid will strongly depend on their selectivity and durability. Here, we initially screened various aluminum‐rich zeolites confirming that NaY is the most efficient catalyst for this reaction. This material was modified by sequential dealumination and alkaline treatment attaining a solid featuring a hierarchical distribution of micro‐ and mesopores, reduced Lewis acidity, and increased basicity owing to the presence of well‐dispersed sodium ions interacting with external siloxy groups, as evidenced by in‐depth characterization. These properties were crucial for determining higher selectivity (75 %) and minimizing the activity loss in a 6 h run. Remarkably, the catalyst gained in selectivity and stability upon reuse in consecutive cycles. This was ascribed to the depletion of the stronger basic sites and the clustering of sodium into oxidic particles upon the intermediate calcination.

Design of a technical Mg–Al mixed oxide catalyst for the continuous manufacture of glycerol carbonate

The availability of a heterogeneous catalyst, which contains cheap and abundant elements, has a scalable synthesis, is highly active and stable, retains its performance upon shaping into a technical form and can be operated in continuous mode, would pave the way for a more ecological and economical production of glycerol carbonate from glycerol and urea. Here, we show that a mixed oxide of Mg and Al is a promising active phase for this reaction. The solid comprises widely available and non-toxic metals, is easily obtained through the thermal decomposition of a hydrotalcite-like material and can almost match the product yield of state-of-the-art Zn-based catalysts, while displaying an outstanding resistance against leaching, which causes the rapid dissolution of the latter. In-depth characterisation uncovered that Lewis-basic centres are crucial to activate glycerol through dehydrogenation. Their concentration was maximised by optimising the composition and calcination temperature of the precursor, thus reaching up to 60% glycerol carbonate yield. Millimeter-sized extrudates featuring comparable basic properties to the powder sample, a well-developed meso- and macroporosity and high mechanical stability are obtained using a natural clay, bentonite, as a binder and thermally activating the hydrotalcite only after shaping. Upon testing in a continuous reactor under tuned conditions of temperature and pressure and in the presence of an aprotic solvent, the system attains the same glycerol yield as in the batch tests. During 100 h on stream, its activity decreases by 20% due to fouling, but can be fully restored upon burning-off of the carbonaceous deposits. This work discloses the development of a green material that exhibits high efficacy in a sustainable transformation, highlighting key parameters that should be generally taken into account in the design of an industrially relevant chemocatalytic technology.

Bifunctional Hierarchical Zeolite-Supported Silver Catalysts for the Conversion of Glycerol to Allyl Alcohol

The establishment of suitable processes for the conversion of glycerol into allyl alcohol is hindered by the fast deactivation of solid acids in the dehydration of the substrate to acrolein and by the requirement of hydrogen donors to enhance the selectivity of the subsequent reduction step. In this work, silver nanoparticles deposited onto a hierarchical ZSM‐5 zeolite are proved to be an effective bifunctional catalyst to conduct the two reactions in the gas phase and in the presence of hydrogen by using a continuous fixed‐bed reactor. The acidic function was accomplished by using a ZSM‐5 zeolite modified by facile alkaline and acid treatments, which decreased the amount of Lewis acid centers while preserving the amount of Brønsted acid centers, and introduced an auxiliary network of intracrystalline mesopores, thus boosting the selectivity to acrolein (62 %) and the resistance to coking. Upon screening of various metals supported on the aluminosilicate, silver was identified as a superior hydrogenation catalyst, enabling a relatively high activity with >50 % allyl alcohol selectivity. Tuning of the metal loading, temperature, pressure, and contact time led to 15 % yield of allyl alcohol, thus approaching the state‐of‐the‐art transfer hydrogenation systems, and stable behavior for 100 h on stream. Our results highlight the advantage of conducting the two transformations over a bifunctional material rather than over two separate single‐function solids.

Catalyst and Process Design for the Continuous Manufacture of Rare Sugar Alcohols by Epimerization-Hydrogenation of Aldoses.

Sugar alcohols are applied in the food, pharmaceutical, polymer, and fuel industries and are commonly obtained by reduction of the corresponding saccharides. In view of the rarity of some sugar substrates, epimerization of a readily available monosaccharide has been proposed as a solution, but an efficient catalytic system has not yet been identified. Herein, a molybdenum heteropolyacid-based catalyst is developed to transform glucose, arabinose, and xylose into less-abundant mannose, ribose, and lyxose, respectively. Adsorption of molybdic acid onto activated carbon followed by ion exchange to the cesium form limits leaching of the active phase, which greatly improves the catalyst stability over 24 h on stream. The hydrogenation of mixtures of epimers is studied over ruthenium catalysts, and it is found that the precursor to the desired polyol is advantageously converted with faster kinetics. This is explained by density functional theory on the basis of its more favorable adsorption on the metal surface and the lower energy barrier for the addition of a hydrogen atom to the primary carbon atom. Finally, different designs for a continuous process for the conversion of glucose into mannitol are studied, and it is uncovered that two reactors in series with one containing the epimerization catalyst and the other containing a mixture of the epimerization and hydrogenation catalysts increases the mannitol/sorbitol ratio to 1.5 from 1 for a single mixed-bed reactor. This opens a prospective route to the efficient valorization of renewables to added-value chemicals.

Selective dehydrogenation of bioethanol to acetaldehyde over basic USY zeolites

Zeolites featuring basic sites have displayed outstanding performance in the valorisation of biobased substrates through condensation and coupling reactions. Herein, we present the continuous-flow, gas-phase dehydrogenation of ethanol to acetaldehyde as a novel application of the recently developed alkali-activated high-silica USY zeolites. Evaluation of the hydroxides of Group 1 metals in the catalyst preparation by alkaline treatment identified sodium hydroxide as the most suitable base. This is due to the stronger character and the higher number of mildly basic sites formed coupled with the minimal impact on the pristine porous properties of the zeolite, which resulted in superior catalytic performance. Compared to existing basic catalysts such as MgO and hydroxyapatites, no C4-condensation products were observed and the occurrence of ethanol dehydration to ethylene was marginal due to the negligible acidity of the material. At the optimal reaction temperature, acetaldehyde was attained with a ca. 15-fold higher yield (50%) and with high selectivity (80%) when adding oxygen to the ethanol feed. The origin of this peculiar positive behaviour was unraveled through kinetic and in situ infrared spectroscopic studies as well as by the temperature-programmed surface reaction of ethanol. These investigations revealed the exclusive presence of ethoxide species at the catalyst surface, which (i) are formed upon adsorption of ethanol at basic siloxy groups, (ii) are stabilised through hydrogen bonding with vicinal silanols and (iii) react in a rate-limiting step with gas-phase oxygen through an Eley–Rideal mechanism to form acetaldehyde, thus releasing water. In contrast to the rapid deactivation shown by noble metal based catalysts, the material exhibited stable performance over 24 h on stream.

Towards sustainable manufacture of epichlorohydrin from glycerol using hydrotalcite-derived basic oxides

Commercial two-step processes to convert glycerol into epichlorohydrin are more benign compared to the predominant industrial route starting from propene in terms of materials requirements and CO2 emissions. Still, the use of alkali hydroxides in stoichiometric amounts in the second reaction, i.e., the dehydrochlorination of the dichloropropanol intermediate, leads to the formation of large amounts of salt wastes, thus limiting the greenness of the technology. Here, we show for the first time that the latter transformation can be selectively conducted in the gas phase in the presence of a heterogeneous hydrotalcite-derived mixed oxide of Al and Mg. Upon reaction, the lamellar solid is rehydrated to a hydrotalcite-like compound, which can effectively activate the alcoholic group of dichloropropanol owing to its strong Bronsted basic character and moderately high surface area. In-depth characterisation of the porous, compositional, structural and acid/base properties demonstrates that the HCl formed during the reaction causes the progressive exchange of interlayer OH groups by Cl atoms, thus gradually diminishing the reactivity of the material. Facile calcination restores the original mixed oxide structure and is shown to enable three equivalent consecutive reaction runs. Since the HCl evolved along with water upon regeneration can be recycled in the first step of the process, i.e., glycerol hydrochlorination, our approach paves the way for a waste-free and more atom efficient biobased epichlorohydrin production process.

Environmental and economical perspectives of a glycerol biorefinery

Glycerol conversion into chemicals and fuel additives is pursued to valorise a burgeoning by-product in the bioenergy sector. To this aim, heterogeneous catalysts have been developed that enable, in many cases, efficient and green transformations. Still, the evaluation of the environmental and economic footprint that would be associated with their large-scale application has often been neglected, limiting their commercial attractiveness. Furthermore, the impact of integrating different glycerol upgrading routes within a biorefinery, which is highly instrumental to determine the effective sustainability and profitability of biodiesel production from vegetable oils, has not been assessed. Here, the manufacture of the most relevant chemical derivatives of glycerol is considered, i.e., lactic acid, acrylic acid, glycerol carbonate, propanediols, epichlorohydrin and allyl alcohol. State-of-the-art catalysts for each upgrading route are briefly reviewed. Based on their performances, processes are rigorously modelled and relevant indicators, the global warming potential, the cumulative energy demand and the operating costs, quantified by life-cycle analysis. Glycerol-based processes are generally found more attractive than the conventional technologies nowadays applied for the production of the same chemicals, among which the paths to lactic acid and glycerol carbonate are particularly promising. In addition, the process variables mostly contributing to the environmental and cost metrics are identified. Accordingly, future studies should target further optimisation mainly in relation to selectivity, solvent volatility, reactants ratio and catalyst stability. Finally, the processes are integrated simulating a prospective glycerol biorefinery and the advantages deriving from the exchange of heat between the different routes quantified. If the glycerol feed is split equally among all routes the CO2 emissions and energy requirements are decreased by 15 and 32%, respectively, and the profit is increased by 5% as compared to the sum of the individual glycerol-based processes. In order to minimise the ecological impact of the biorefinery, glycerol should be rather divided in an 80 : 20 mass ratio among 1,2-propanediol and glycerol carbonate production, which are expected to have a significant market size. The innovative approach outlined in this work holds potential to guide both fundamental chemical research and process design in the development of CO2 and other bio-refineries.