Cecilia Mondelli

Indium Oxide as a Superior Catalyst for Methanol Synthesis by CO2 Hydrogenation

Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2 O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu-ZnO-Al2 O3 catalyst, which is unselective and experiences rapid deactivation. In-depth characterization of the In2 O3 -based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co-feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.

Sustainable chlorine recycling via catalysed HCl oxidation: from fundamentals to implementation

The heterogeneously catalysed oxidation of HCl to Cl2 comprises a sustainable route to recover chlorine from HCl-containing streams in the chemical industry. Conceived by Henry Deacon in 1868, this process has been rejuvenated in the last decade due to increased chlorine demand and the growing excess of by-product HCl from chlorination processes. This reaction suffered from many sterile attempts in the past two centuries to obtain sufficiently active and durable catalysts. Intense research efforts have culminated in the recent industrial implementation of RuO2-based catalysts for HCl oxidation. This paper reviews the new generation of technologies for chlorine recycling under the umbrella of Catalysis Engineering, that is, tackling the microlevel (catalyst design), mesolevel (reactor design), and macrolevel (process design). Key steps in the development are emphasised, including lab-scale catalyst screening, advanced catalyst characterisation, mechanistic and kinetic studies over model and real systems, strategies for large-scale catalyst production, mini-plant tests with a technical catalyst, and reactor design. Future perspectives, challenges, and needs in the field of catalysed Cl2 production are discussed. Scenarios motivating the choice between catalysed HCl oxidation and HCl electrolysis or their integration for optimal chlorine recycling technology are put forward.

Highly Selective Lewis Acid Sites in Desilicated MFI Zeolites for Dihydroxyacetone Isomerization to Lactic Acid

Desilication of commercial MFI-type (ZSM-5) zeolites in solutions of alkali metal hydroxides is demonstrated to generate highly selective heterogeneous catalysts for the aqueous-phase isomerization of biobased dihydroxyacetone (DHA) to lactic acid (LA). The best hierarchical ZSM-5 sample attains a LA selectivity exceeding 90 %, which is comparable to that of the state-of-the-art catalyst (i.e., the Sn-beta zeolite); this optimized hierarchical catalyst is recyclable over three runs. The Lewis acid sites, which are created through desilication along with the introduction of mesoporosity, are shown to play a crucial role in the formation of the desired product; these cannot be achieved by using other post-synthetic methods, such as steaming or impregnation of aluminum species. Desilication of other metallosilicates, such as Ga-MFI, also leads to high LA selectivity. In the presence of a soluble aluminum source, such as aluminum nitrate, alkaline-assisted alumination can introduce these unique Lewis acid centers in all-silica MFI zeolites. These findings highlight the potential of zeolites in the field of biomass-to-chemical conversion, and expand the applicability of desilication for the generation of selective catalytic centers.

Environmental and economic assessment of lactic acid production from glycerol using cascade bio- and chemocatalysis

Recently, lactic acid has emerged as one of the most relevant platform molecules for the preparation of bio-chemicals. Due to the limited productivity of sugar fermentation, the dominant industrial technology practiced for its manufacture, new chemocatalytic processes are being developed in order to meet the expected demand for this intermediate. The Lewis-acid catalysed isomerisation of dihydroxyacetone has attracted particular interest. If the reaction is performed in water, lactic acid is attained directly, while if alcohol is used as the solvent, the desired product can be obtained upon subsequent hydrolysis of the alkyl lactates formed. Herein, we (i) demonstrate tin-containing MFI zeolites prepared by scalable methods as highly active, selective and recyclable catalysts able to operate in concentrated dihydroxyacetone aqueous and methanolic solutions, and (ii) reveal by life cycle analysis that a process comprising the enzymatic production of dihydroxyacetone from crude glycerol and its chemocatalytic isomerisation in methanol is advantageous for the production of lactic acid compared to glucose fermentation in terms of both sustainability and operating costs. In particular, we demonstrate that the reduced energy requirements and CO2 emissions of the cascade process originate from the valorisation of a waste feedstock and from the high performance and recyclability of the zeolite catalyst and that the economic advantage is strongly determined by the comparably low market price of glycerol. It is also shown that the bio-/chemocatalytic route remains ecologically and economically more attractive even if the purity of glycerol is as low as 38%.

Shaped RuO2/SnO2–Al2O3 Catalyst for Large-Scale Stable Cl2 Production by HCl Oxidation

The gas-phase catalytic oxidation of hydrogen chloride to chlorine (Deacon process, 4 HCl+O2

A delafossite-based copper catalyst for sustainable Cl2 production by HCl oxidation

2 Cl2+2 H2O) is an eco-efficient route to recover Cl2 from HCl-containing waste streams in the chemical industry. For a long time, the HCl oxidation process suffered from a lack of suitable catalysts, as common systems based on copper (the Deacon catalyst) and chromium (Mitsui–Toatsu) exhibited low activity and were prone to volatilization and, eventually, corrosion in the plant. Ruthenium-based catalysts were first introduced by Shell in the 1960s using SiO2 as the support. [3] The remarkable Deacon performance exhibited by this metal has led the way to a wider scope for industrialization of the hydrochloric acid oxidation process. Sumitomo Chemicals brought a TiO2 (rutile)-supported RuO2 catalyst to market, which was optimized for use in a fixed-bed tube bundle reactor. Bayer MaterialScience AG and Bayer Technology Services recently patented an alternative Rubased catalyst using a SnO2 (cassiterite) support optimized for application in a single adiabatic reactor cascade. Despite the benefits introduced, further improvements are needed to address another critical aspect for a robust HCl oxidation catalyst, which is the long-term stability of the RuO2 phase under Deacon conditions. The origin of the catalyst deactivation has not been extensively investigated, but pretreatments of the support to favor the epitaxial growth of RuO2 as a film on top of TiO2 (rutile) or SnO2 (cassiterite) due to lattice matching of both the active phase and the carrier have been reported as the main “trick” to attain improved catalytic properties. We have found that this tactic is not sufficient to avoid deactivation, at least in the case of the catalyst supported on SnO2. Herein we present a novel shaped Deacon catalyst with high potential for large-scale implementation due to its high activity and remarkable longevity in pilot test for 7000 h. In addition to the appropriate choice of active phase (RuO2) and carrier (cassiterite), a binder/stabilizer (g-Al2O3) has been introduced. The latter compound is shown to minimize agglomeration of the ruthenium phase under reaction conditions, thus perpetuating stable behavior. The three main components of the catalyst have been designed as follows. Concerning the active constituent, ruthenium remained the most suitable option due to its unrivalled activity in HCl oxidation at low temperatures with respect to other metals. The commercial SnO2-cassiterite powder was calcined at 1273 K prior to use to ensure the formation of the rutile structure also at a surface level and therefore to allow the epitaxial growth of RuO2 onto the support. The procedure is effective in short times, leading to a material with a surface area (SBET) equal to 9 m 2 g . A good coating of the support by the active phase not only aims at preventing structural alterations of the RuO2 phase, but also at protecting SnO2 from eventual chlorination under Deacon conditions and consequent volatilization as SnCl4. The catalytically active phase was deposited through impregnation of RuCl3 over a SnO2–Al2O3 composite shaped in 2 mm spherical pellets (see Experimental section). Low-temperature calcination (523 K) was employed for the latter step to minimize sintering of the ruthenium phase, thus achieving a higher dispersion. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) of a pellet cross-section indicated that the active phase is uniformly distributed within the pellet (see the Supporting Information, Figure SI 1). The binder matrix included in the catalyst formulation was g-Al2O3 [particle size distribution (by TEM) = 5–20 nm, SBET = 200 m g ] , the amount of which corresponded to 10 wt. % in the final catalyst. The primary function of this component is to improve the textural properties of RuO2/SnO2 to allow shaping of the material in order to derive a technical catalyst. Most importantly, the intimate contact of the alumina binder with the RuO2-coated SnO2 grains inhibits agglomeration of the active phase under Deacon conditions, which is the main factor responsible for catalyst deactivation (see below). The corresponding alumina-free RuO2/SnO2 catalyst was synthesized in powder form according to the same method to serve as a ref-

A continuous process for glyoxal valorisation using tailored Lewis-acid zeolite catalysts

A copper catalyst based on a delafossite precursor (CuAlO(2)) displays high activity and extraordinary lifetime in the gas-phase oxidation of HCl to Cl(2), representing a cost-effective alternative to RuO(2)-based catalysts for chlorine recycling.

Deactivation mechanisms of tin-zeolites in biomass conversions

The aqueous-phase heterogeneously catalysed isomerisation of bio-oil derived glyoxal is herein introduced as a novel route for the sustainable production of glycolic acid. While commercial ultra-stable Y zeolites displayed only moderate performance, their evaluation enabled us to highlight the crucial role of Lewis acidity in the reaction. Gallium incorporation into these zeolites boosted the glycolic acid yield, although the best catalytic results were obtained over tin-containing MFI-type zeolites, reaching 91% yield of the desired product at full conversion. These materials comprised hydrothermally-synthesised Sn-MFI as well as a novel catalyst obtained by the introduction of tin into silicalite-1 by means of a simpler and more scalable method, i.e. alkaline-assisted metallation. In-depth spectroscopic characterisation of these systems uncovered a substantial similarity of the tin centres obtained by the top-down and bottom-up synthetic approaches. NMR spectroscopic studies gave evidence that the reaction follows a 1,2-hydride shift mechanism solely catalysed by Lewis-acid sites. The Sn-MFI analogue could be reused in 5 cycles without the need for intermediate calcination, did not evidence any tin leaching, and demonstrated suitability for utilisation under continuous-flow operation. The tin-based zeolites exhibited remarkable performance also in alcoholic solvents, leading to the one-pot production of relevant alkyl glycolates.

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production

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.

Production of bio-derived ethyl lactate on GaUSY zeolites prepared by post-synthetic galliation

CuCrO2 Delafossite: A Stable Copper Catalyst for Chlorine Production With time comes wisdom : Since the implementation of CuCl2 for HCl oxidation by Deacon in 1868, the search for stable copper catalysts has been futile. Cuprous delafossite, CuCrO2 (see picture), is shown to have unprecedented stability against chlorination, allowing for durable Cl2 production with no metal loss. Based on this, a highly active CuCrO2-CeO2 composite was developed, a cost-effective Cl2 recovery method. Angewandte Chemie