Cu–Ni nanoalloy phase diagram – Prediction and experiment

The Cu-Ni nanoalloy phase diagram respecting the nanoparticle size as an extra variable was calculated by the CALPHAD method. The samples of the Cu-Ni nanoalloys were prepared by the solvothermal synthesis from metal precursors. The samples were characterized by means of dynamic light scattering (DLS), infrared spectroscopy (IR), inductively coupled plasma optical emission spectroscopy (ICP/OES), transmission electron microscopy (TEM, HRTEM), and differential scanning calorimetry (DSC). The nanoparticle size, chemical composition, and Cu-Ni nanoparticles melting temperature depression were obtained. The experimental temperatures of melting of nanoparticles were in good agreement with the theoretical CALPHAD predictions considering surface energy.Fázový diagram nanoslitiny Cu-Ni respektující velikost nanočástic jako další proměnné byl vypočten metodou CALPHAD. Vzorky Cu-Ni nanoslitin byly připraveny solvotermální syntézou z prekurzorů kovů. Tyto vzorky byly charakterizovány pomocí dynamického rozptylu světla (DLS), infračervené spektroskopie (IR) s indukčně vázanou plazmou a optickou emisní spektroskopií (ICP / OES), transmisní elektronovou mikroskopií (TEM, HRTEM) a diferenciální skenovací kalorimetrií (DSC). Velikost nanočástic, chemické složení a Cu-Ni deprese teploty tání nanočástic byly získány experimentálně a v dobré shodě s teoretickou předpovědí metodou CALPHAD s uvážením povrchové energie nanočástic

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. </p

Non-hydrolytic sol–gel as a versatile route for the preparation of hybrid heterogeneous catalysts

The tools of sol–gel chemistry allow synthesizing a plethora of functional materials in a controlled bottom-up fashion. In the field of heterogeneous catalysis, scientists utilize sol–gel routes to design solids with tailored composition, texture, surface chemistry, morphology, dispersion, etc. A field of investigation which shows great promises is that of hybrid heterogeneous catalysts. Examples are flourishing to show that catalysts featuring a combination of inorganic and organic components often display improved activity, selectivity, or even chemical stability as compared to the purely inorganic counterparts. Yet, classic sol–gel methods face some well-known limitations—related to the different reactivity of the precursors and to the high surface tension of water—which complicate the tasks of chemists, specifically for the synthesis of hybrid catalysts. Non-hydrolytic sol–gel (NHSG) chemistry appears as a pertinent alternative. Being realized in the absence of water, NHSG routes allow reaching an excellent control over the solid properties and on the distribution of the organic and inorganic components at the nano- and microscales. In this review, we briefly recapitulate the main types of non-hydrolytic sol–gel routes and we present the modified protocols to hybrid materials. Then, we present an overview of the non-hydrolytic sol–gel approaches that have been proposed to synthesize hybrid heterogeneous catalysts. For both Class I and Class II hybrids, we discuss how the NHSG technique has allowed tailoring the key properties that command catalytic performance. From this panorama, we argue that NHSG has a prominent role to play for the development of advanced hybrid heterogeneous catalysts

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 %

Mildly Acidic Aluminosilicate Catalysts for Ethanol Dehydration

Herein we present synthesis, characterization, and catalytic application of highly homogeneous amorphous aluminosilicates with medium strength acid sites. These catalysts were prepared by non-hydrolytic sol-gel (NHSG), an unconventional synthetic method. The dispersion of aluminum is very homogeneous (XPS, ToF-SIMS) within the mesoporous silicate matrix (N2 physisorption). Catalysts are amorphous and possess high amount of medium strength acid sites (NH3-TPD). Decisively, this NHSG preparation technique also allows for the direct and homogeneous incorporation of organic groups using organosilane precursors. Both aliphatic and aromatic as well as terminal and bridging groups were used (IR and SS NMR spectroscopy) with the intention to increase the hydrophobicity of the surface (H2O physisorption) and thereby boosting the performance and stability of the catalysts during the dehydration reaction

Mildly Acidic Aluminosilicate Catalysts for Stable Performance in Ethanol Dehydration

Ethanol dehydration is effectively catalyzed by strongly acidic zeolite catalysts which are known, however, to exhibit poor time on stream stability. Alumina and silica-alumina on the other hand are relatively stable but reach only low activity levels. Here, a series of aluminosilicate catalysts (Si:Al ratio = 16) was prepared by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of activity, between the HZSM-5 zeolite and a commercial silica-alumina. Importantly, the best samples, were very stable with time on stream. Unlike HZSM-5, which also catalyzes ethylene oligomerization due to its strong acid sites and is therefore prone to coking, NHSG prepared catalysts did not produce any traces of ethylene oligomers and did not show any trace of coke formation. Characterization (ICP-OES, N2 physisorption, TEM, XPS, IR coupled with pyridine adsorption, Raman spectroscopy, solid state NMR spectroscopy) reveal that the unconventional synthetic method presented here allowed to prepare mesoporous aluminosilicate materials with a remarkable degree of homogeneity. It is this thorough dispersion of Al in the amorphous silicate matrix which is responsible for the formation of acid sites which are intermediate (in terms of strength and nature) between those of commercial silica-alumina and HZSM-5 zeolite. The texture of the best NHSG catalyst – mainly mesoporous with a high specific surface area (800 m² g−1) and pore volume (0.5 cm³ g−1) – was also unaffected after reaction. To overcome deactivation issues in ethanol dehydration, this study suggests to target amorphous aluminosilicate catalysts with open mesoporosity and with an intimate mixing of Al and Si

Ethanol dehydration over hybrid aluminosilicate catalysts prepared by non-hydrolytic sol-gel

The dehydration of (bio)ethanol to ethylene is an essential catalytic reaction in the perspective of the development of bio-based industry.[1] Traditional catalysts employed in this reaction are fully inorganic: alumina, silica-alumina, and HZSM-5.[2] Each of these systems come with their limitations: only moderate activity in the case of Al2O3 and silica-alumina, and rapid deactivation by coking in the case of zeolite catalysts. Recently, we have shown, that non-hydrolytic sol-gel (NHSG) provides highly homogeneous and porous aluminosilicate materials exhibiting superior activity and long-term stability in ethanol dehydration.[3] The ethylene selectivity was improved by one-pot incorporation of organic groups.[4] In this follow-up study, the best performing NHSG-prepared aluminosilicate catalysts (fully inorganic) were post-synthetically modified by grafting with trimethysilyl groups. In such a way, the Brønsted acid sites (≡Si−O(H)∙∙∙Al≡) were transformed into Lewis acid sites (≡Si−OSiMe3 and Al≡). The number of reacted ≡Si−OH moieties and thus the trimethylsilyl groups loading and Brønsted/Lewis ratio was controlled via a temperature-vacuum pretreatment of aluminosilicate samples. Structure, porosity, acidity, and hydrophobicity of NHSG-prepared catalysts were closely followed by MAS NMR studies, N2 physisorption, IR-pyridine analyses, and water adsorption. Moreover, aluminosilicates were tested in a gas-phase fixed-bed catalytic reactor in ethanol dehydration. Lewis acid Al sites were found to exhibit a high ethylene selectivity even at relatively low temperatures (200−300 °C). These tailored NHSG-prepared aluminosilicate catalysts exhibited high ethylene productivities, markedly outperforming commercial silica-alumina

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 is required, combining both redox (for the dehydrogenation reaction) and acid (for hydrogen transfer and dehydration reactions) functionalities. Multi-step preparation methods are typically implemented to obtain tailored bifunctional catalysts, and a challenge is to balance the two functions to maximize the BD yield. Here, we disclose a straightforward, one-step, and continuous preparation 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

Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration

Ethanol dehydration is effectively catalyzed by strongly acidic zeolites which, however, exhibit poor time-on-stream stability. For example, HZSM-5 features strong acid sites, catalyzes ethylene oligomerization, and is prone to coking. Alumina and silica-alumina on the other hand have lower acidity, are relatively stable, but reach only low activity. Here, a series of mesoporous aluminosilicate catalysts was prepared by non-hydrolytic sol-gel (NHSG) and are shown to feature an intermediate level of acidity (both in strength and nature), resulting in intermediate catalytic performance. Importantly, the best NHSG-made samples were very stable with time on stream, did not produce any traces of ethylene oligomers, did not show any trace of coke formation, and their texture was unaffected. Characterization (ICP-OES, N2-physisorption, TEM, XPS, IR-pyridine, Raman and solid state NMR spectroscopies) revealed that this behavior must be correlated with the remarkable degree of homogeneity in the NHSG-made aluminosilicate (only tetrahedrally coordinated Al species)