Ag- and Cu-promoted mesoporous Ta-SiO2 catalysts prepared by non-hydrolytic sol-gel for the conversion of ethanol to butadiene

The direct catalytic conversion of bioethanol to butadiene, also known as the Lebedev process, is one of the most promising solution to replace the petro-based production of this important bulk chemical. Considering the intricate reaction mechanism – where a combination of acid-catalyzed dehydration reactions and metal-catalyzed dehydrogenation have to take place simultaneously – tailor-made bifunctional catalysts are required. We propose to use non-hydrolytic sol-gel (NHSG) chemistry to prepare mesoporous Ta-SiO2 materials which are further promoted by Ag via impregnation. An acetamide elimination route is presented, starting from silicon teraacetate and pentakis(dimethylamido)tantalum(V), in the presence of a pluronic surfactant. The catalysts display advantageous texture, with specific surface area in the 600-1000 m².g-1 range, large pore volume (0.6-.1.0 cm³.g-1), an average pore diameter of 4 nm and only a small contribution from micropores. Using an array of characterization techniques, we show that NHSG allows achieved a high degree of dispersion of tantalum, mainly incorporation as single sites in the silica matrix. The presence of these monomeric TaOx active sites is responsible for the much higher dehydration ability, as compared to the corresponding catalyst prepared by impregnation of Ta onto a pristine silica support. We attempt to optimize the butadiene yield by changing the relative proportion of Ta and Ag and by tuning the space velocity. We also demonstrate that Ag or Cu can be introduced directly in one step, during the NHSG process. Copper doping is shown to be much more efficient than silver to guide the reaction towards the production of butadiene

Innovative sol-gel routes to mesoporous bifunctional catalysts for the upgrading of bioethanol to butadiene

Bioethanol production has been promoted by strong political and incentives and is technologically sound. In parallel, butadiene is one of the compounds that is expected to suffer future shortage due to the shift towards shale gas from the traditional steam cracking of naphtha. Therefore, intensive research is currently conducted towards the direct catalytic conversion of ethanol to butadiene. The reaction is long known, and consists of a complex network of dehydration and dehydrogenation reactions catalysed respectively by acid and redox active sites that must operate in a controlled and balanced fashion to maximize the butadiene yield.1 In the highly versatile toolbox of sol-gel chemistry, non-hydrolytic sol-gel (NHSG) is particularly effective to synthesize mesostructured materials with tailored properties (texture, homogeneity, surface chemistry) and has already shown its potential to obtain effective catalysts.2-4 Here, NHSG was used to prepare bifunctional Ta-Cu-SiO2 catalysts via the acetamide elimination route (“Ac”) and compare them with similar compositions obtained through the ether route (“Et”). Both pathways yielded mesoporous materials (Figure 1A; SSA≈600 m2 g–1; Vp≈0.1 mL g–1; Dp≈4.0 nm for the acetamide elimination route, SSA≈700 m2 g–1; Vp≈0.02 mL g–1; Dp≈7.0 nm for the ether route), with similar CuO crystallite size of around 20 nm. IR spectroscopy and XPS pointed to a successful Ta incorporation inside the silica matrix. The catalysts obtained through the acetamide elimination route perform systematically better than those synthesized by the ether route (Figure 1B). After optimization of the Cu and Ta loading, the best formulation (Ac-2Ta4Cu) reached an ethanol conversion of 75 % and a butadiene selectivity of 50 %

Surface-functionalized mesoporous gallosilicate catalysts for the efficient and sustainable upgrading of glycerol to solketal

Two series of functionalized mesoporous Ga silicates were prepared in a straightforward and sustainable one-pot procedure using different alkyl silanes. The efficacy of the adopted co-synthetic approach based on aerosol processing has been proved by 29Si solid-state NMR experiments revealing a degree of functionalization close to the theoretical value. The successful incorporation of gallium as single sites within the silica framework was confirmed via71Ga solid-state magic-angle-spinning NMR measurements. These materials were tested as catalysts for the synthesis of solketal from glycerol at low temperature and under solventless conditions. A systematic study evidenced the importance of a careful tuning of surface polarity, achievable with surface functionalization as well as with different thermal treatments. The solids functionalized with a low degree of methyl groups (5%) displayed enhanced performances compared to the non-functionalized analogues, highlighting the highly beneficial role of surface hydrophobicity as well as the importance of the careful tuning of the hydrophilic/hydrophobic balance. The best functionalized catalysts proved to be easily reusable for multiple catalytic runs. With such a high-performance catalyst in hand, we propose a process which shows a favorable E-factor, indicating that the production of solketal can be envisaged in a sustainable way

Hafnium-doped silica nanotubes for the upgrading of glycerol into solketal: Enhanced performances and in-depth structure-activity correlation

An unprecedented type of Hf-doped silica nanotubes was synthesized using a straightforward one-pot sol–gel procedure. The well-defined nanotubes with a diameter of 14–20 nm exhibited high specific surface area and a widely open texture. The method – involving a key pH adjustment step – allowed a quantitative insertion of hafnium in the materials (Si/Hf = 74) and favored the insertion of Hf as dispersed species. Depending on the synthesis parameters, the chemical environment around Hf was modified, as evidenced by XPS, NH3-TPD and NH3-IR. Hf-doped silica nanotubes showed excellent activity in the conversion of glycerol to solketal, a reaction of high relevance in the context of biorefineries. Importantly, the turnover frequency and the acidity were unambiguously correlated with the insertion of Hf in the silica matrix. The best catalyst was proven to be stable and recyclable, and this sustainable reaction was also amenable to further catalytic enhancement upon optimized reaction conditions

Non-hydrolytic Sol–Gel Routes to Bifunctional Cu–Ta–SiO2 Catalysts for the Upgrading of Ethanol to Butadiene

The one-step catalytic conversion of bio-based ethanol to 1,3-butadiene is an attractive way to produce this important C4 building block, to be exploited as a sustainable drop-in chemical in the tire and nylon industry. For this catalytic process, bifunctional catalysts combining both redox and acidic properties are required. Here, we leverage non-hydrolytic sol–gel (NHSG) chemistry to prepare tailored Cu–Ta–SiO2 catalysts featuring an open texture, dispersed acidic Ta sites, and small Cu nanoparticles. In the ether route, silicon tetrachloride and tantalum pentachloride undergo polycondensation reactions with diisopropyl ether as the oxygen donor. In the acetamide elimination route, silicon tetraacetate reacts with pentakis(dimethylamido)tantalum(V). In both routes, copper(II) acetylacetonate is added and trapped in a tantalosilicate matrix. Upon calcination, CuO nanoparticles form and the resulting bifunctional material develop a mesoporous texture with specific surface areas in the 650–950 m2 g–1 range, pore volumes between 0.75 and 0.90 cm3 g–1, and average pore diameters above 3 nm. With the help of NH3-TPD, FTIR, CO- and pyridine-adsorbed FTIR, XRD, XPS, and STEM-EDS, we demonstrate that the catalysts made via the acetamide elimination route show higher performance in the ethanol-to-butadiene reaction, with low selectivity in dehydration byproducts, owing to moderate Lewis acidity, smaller Cu nanoparticles, and higher active site proximity. After optimization of the Ta and Cu loadings, a butadiene productivity as high as 0.38 gBD gcat–1 h–1 is obtained, surpassing state-of-the-art catalysts with similar formulations and tested under similar reaction conditions

Innovative sol-gel routes to mesoporous bifunctional catalysts for the upgrading of bioethanol to butadiene

Here, NHSG was used to prepare bifunctional Ta-Cu-SiO2 catalysts via the acetamide elimination route (“Ac”) and compare them with similar compositions obtained through the ether route (“Et”). Both pathways yielded mesoporous materials (SSA≈600 m2 g–1; Vp≈0.1 mL g–1; Dp≈4.0 nm for the acetamide elimination route, SSA≈700 m2 g–1; Vp≈0.1 mL g–1; Dp≈7.0 nm for the ether route). IR spectroscopy and XPS pointed to a successful Ta incorporation inside the silica matrix. For a given catalyst composition, the butadiene yield is systematically better with the catalysts obtained through the acetamide elimination route (Figure 1A). After optimization of the Cu and Ta loadings to notably maximize the selectivity of the oxidation step, the best formulation (Ac-2Ta4Cu) reached an ethanol conversion of 75 % and a butadiene yield of 36 %. The overperformance of the acetamide route is attributed to a better active sites dispersion, availability and proximity, as shown in STEM-EDS images (Figure 1), as well as XPS and IR. Cu nanoparticles are closer to more available Ta isolated sites, allowing for the long reaction mechanism to take place in a facilitated way and limiting unwanted by-products like ethylene

Aerosol-assisted sol–gel synthesis of mesoporous Ag–Ta–SiO₂ catalysts for the direct upgrading of ethanol to butadiene

The Lebedev process, or the direct catalytic conversion of bioethanol to butadiene, offers an up-and-coming sustainable alternative to the petrochemical route toward this high-demand C4 hydrocarbon. Since the reaction mechanism involves a cascade of dehydrogenation, hydrogen transfer and dehydration steps, a bifunctional catalyst combining both redox (for the dehydrogenation reaction) and acid (for hydrogen transfer and dehydration reactions) functionalities is required. Multi-step preparation methods are typically implemented to obtain tailored bifunctional catalysts, and one of the challenges is to balance the two functions to maximize the BD yield. Here, we disclose a straightforward, one-step, and continuous preparation method of Ta-doped SiO2 loaded with Ag nanoparticles by coupling sol–gel chemistry with aerosol processing. Combining tantalum ethoxide, silver nitrate, hydrolysed tetraethyl orthosilicate and pluronic F127 as templating agent in the aerosol process leads to mesoporous bifunctional catalysts featuring a specific surface area between 310–370 m2 g−1, a pore volume of ca. 0.5 mL g−1 and an average pore diameter of 5 nm. As attested by a variety of characterization techniques, the method leads to the homogeneous incorporation of highly dispersed tantalum species in the silica matrix, thereby creating the required acidic sites. These new catalysts have higher dehydration activity, as compared to the corresponding reference catalysts prepared by classical impregnation. Concomitantly, relatively small silver nanoparticles are stabilized (∼15 nm). The relative Ta and Ag loading can be tuned easily. In the ethanol to butadiene reaction, these aerosol-made catalysts achieve a butadiene yield of ca. 25% by optimizing the relative loadings of Ta and Ag, outcompeting the corresponding formulations prepared by impregnation

The high activity of mesoporous Ga-SiO2 catalysts in the upgrading of glycerol to solketal explained by in-depth characterization

Mesoporous Ga-silicates were prepared using a sustainable and continuous aerosol synthesis procedure. The novel solids showed record activity in the synthesis of solketal from glycerol and acetone. Three different Si/Ga ratios of 34, 74 and 148 were investigated and all materials displayed favorable features for catalytic applications such as high surface area and calibrated mesoporosity. An in-depth characterization allowed to elucidate the remarkable catalytic performance. The insertion of Ga predominantly as single site in tetrahedral coordination in the silica framework was highlighted by X-ray photoelectron spectroscopy (XPS), using the Auger parameter of Ga in a Wagner plot representation. The speciation of Ga was further clarified using solid state 71Ga NMR spectroscopy, confirming the formation of mainly isolated Ga species. Consistently, the aerosol-made Ga-silicates displayed outstanding turnover frequencies (up to 677 h−1) and selectivity, markedly outcompeting other reference metallosilicate catalysts reported in literature. Moreover, the most active catalyst was successfully reused in multiple catalytic cycles thus proving its stability under the selected reaction conditions