Ethanol dehydration with aerosol-made mesoporous aluminosilicates featuring dispersed active sites

Density Functional Theory Investigations of Zeolite and Intermetallic Alloy Active Site Structures for Kinetics of Heterogeneous Catalysis

Organic-inorganic hybrid materials are nowadays intensely studied for potential applications in heterogeneous catalysis because their properties and catalytic behavior differ from pristine inorganic counterparts. The organic groups at the catalyst surface can modify not only its hydrophilicity, but also acidity, hydrothermal stability, porosity, etc. In some cases, such properties alteration leads to improved catalytic performance in terms of activity, selectivity, or stability. However, the choice of organic groups stays relatively narrow, as most reports focus on pendant methyl groups. Here, a series of mesoporous hybrid aluminosilicate materials containing various organic groups was prepared in one pot by non-hydrolytic sol-gel (NHSG). Both aromatic and aliphatic, pendant and bridging organic groups were incorporated. The presence of the organic groups in the bulk and at the outermost surface of the materials was verified by solid-state NMR and ToF-SIMS, respectively. Aluminum is mostly incorporated in tetrahedral coordination in the hybrid silica matrix. The organically modified mesoporous aluminosilicate samples were tested as catalysts in the gas phase ethanol dehydration (which relies on solid acids) and most of them outperformed the purely inorganic catalyst benchmark. While a direct influence of surface hydrophilicity or hydrophobicity (as probed by water sorption and water contact angle measurements) appeared unlikely, characterization of acidity (IR-pyridine) revealed that the improved performance for hybrid catalysts can be correlated with a modification of the acidic properties. In turn, acidity is determined by the quality of the dispersion of Al centers in the form of isolated sites in the hybrid silica matrix. All in all, this study establishes a "ranking" for a variety of organic groups in terms of their effect on gas-phase ethanol dehydration to ethylene; ethylene yield decreases in this order: bridging xylylene ≈ pendant methyl > pendant benzyl > bridging methylene ≈ inorganic benchmark (no organic groups) > bridging ethylene.

Ethanol dehydration is effectively catalyzed by solid acids, such as HZSM-5, alumina, or silica-alumina. In these catalysts, the amount, nature, and strength of acid sites is believed to determine catalyst activity and stability. However, surface hydrophilicity or hydrophobicity can be suggested as another decisive catalyst property that can directly influence performance. For example, a more hydrophobic surface might be beneficial in repelling the co-product of the reaction, water. However, these aspects have been studied only scarcely in the context of alcohol dehydration. Here, a series of mesoporous hybrid aluminosilicate catalysts containing CH3Si groups was prepared in one pot by non-hydrolytic sol-gel (NHSG). The presence of the methyl groups was verified by IR, solid-state NMR, and ToF-SIMS. Aluminum is mostly incorporated in tetrahedral coordination in the hybrid silica matrix. Two parameters were varied: (i) the Si:Al ratio and (ii) the Si:MeSi ratio. On the one hand, changing the Si:Al ratio had a marked impact on hydrophilicity, as attested by water sorption measurements. On the other hand, unexpectedly, the introduction of methyl groups had no clear influence on sample hydrophilicity. Nevertheless, some of the methylated aluminosilicate catalysts markedly outperformed the purely inorganic catalysts and a commercial silica-alumina benchmark. While a direct influence of surface hydrophilicity or hydrophobicity could be excluded, characterization of acidity (IR-pyridine) revealed that these improved performances are correlated with a modification of the acidic properties in the hybrid catalysts caused by the presence of methyl groups. A decisive role of acidity in ethanol dehydration was confirmed by an experiment with delayed addition of the Al precursor in the NHSG synthesis. This led to a higher Al surface concentration, marked acid sites number increase, and better catalytic performance, even competing with HZSM-5 in terms of activity.

Hierarchical leaf-like alumina-carbon nanosheets with ammonia water modification for ethanol dehydration to ethylene

Structure and Solvation of Confined Water and Alkanols in Zeolite Acid Catalysis

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.

This study aims to convert ethanol to higher value-added products, particularly diethyl ether and ethylene using the catalytic dehydration of ethanol. Hence, the gas-phase dehydration of ethanol over Al2O3-HAP catalysts as such and modified by addition of palladium (Pd) in a microreactor was evaluated. The commercial Al2O3–HAP catalyst was first prepared by the physical mixing method, and then, the optimal ratio of the Al2O3–HAP catalyst (2:8 by wt %) was impregnated with Pd to develop a new functional catalyst to alter surface acidity. Based on the results, the combination of Al2O3 and HAP catalysts generated significant quantities of weak acid sites which demonstrates an enhancement in catalytic activity. In addition, Pd modification in the optimal composition ratio of the Al2O3–HAP catalyst extremely increased the amount of weak acid sites as well as weak acid density due to the synergistic effect between the Pd and Al2O3–HAP catalyst that are supposed to suggest the active sites in the reaction. Among all catalysts, the Al20-HAP80-Pd catalyst displayed brilliant catalytic performance in the course of diethyl ether yield (ca. 51.0%) at a reaction temperature of 350 °C and ethylene yield (ca. 75.0%) at a reaction temperature of 400 °C having an outstanding stability under time-on-stream for 10 h. This is recognized to the combination of the effects of weak acid sites (Lewis acidity), small amount of strong acid sites, and structural characteristics of the catalytic materials used.

In this study, mesoporous nanocomposite silicotungstic acid (STA) incorporated MCM-41 and mesoporous aluminosilicate catalysts with narrow pore size distributions, in the range of 2.5–3.5 nm, were successfully synthesized following different impregnation procedures. Results showed that the catalyst preparation procedure had significant influence on its activity as well as the product distribution in ethanol dehydration. STAMCM41C catalyst, which was prepared by the impregnation of STA into calcined MCM-41 containing a W/Si ratio of 0.24, and STAMAS catalyst, which was prepared by the impregnation of STA into mesoporous aluminosilicate, showed very high activities in dehydration of ethanol. Ethylene yield showed an increasing trend with temperature, reaching to about 100% above 250 °C. In contrast to ethylene, DEE was formed at lower temperatures, reaching to a yield value of about 70% at 180 °C with STAMCM41C. DEE formation at lower temperatures was concluded to be due to the presence of Bronsted acid sites of this catalyst.

Cost-effective atomically dispersed Fe-N-P-C complex catalysts are promising to catalyze the oxygen reduction reaction (ORR) and replace Pt catalysts in fuel cells and metal-air batteries. However, it remains a challenge to increase the number of atomically dispersed active sites on these catalysts. Here we report a highly efficient impregnation-pyrolysis method to prepare effective ORR electrocatalysts with large amount of atomically dispersed Fe active sites from biomass. Two types of active catalyst centers were identified, namely atomically dispersed Fe sites and FexP particles. The ORR rate of the atomically dispersed Fe sites is three orders of magnitude higher than it of FexP particles. A linear correlation between the amount of the atomically dispersed Fe and the ORR activity was obtained, revealing the major contribution of the atomically dispersed Fe to the ORR activity. The number of atomically dispersed Fe increases as the Fe loading increased and reaching the maximum at 1.86 wt% Fe, resulting in the maximum ORR rate. Optimized Fe-N-P-C complex catalyst was used as the cathode catalyst in a homemade Zn-air battery and good performance of an energy density of 771 Wh kgZn−1, a power density of 92.9 mW cm−2 at 137 mA cm−2 and an excellent durability were exhibited.

Mildly acidic aluminosilicate catalysts for stable performance in ethanol dehydration

A ball milling method for highly dispersed Ni atoms on g-C3N4 to boost CO2 photoreduction

Ethylene production from ethanol dehydration over mesoporous SBA-15 catalyst derived from palm oil clinker waste

Characterization of the active sites on the surface of Al 2O 3 ethanol dehydration catalysts by EPR using spin probes

Thorough understanding of reconstructed active sites evolving from their initial states is crucial for a variety of catalytic reactions, as it not only promotes an atomic-level comprehension of the catalytic mechanism but also guides the design of practically usable catalysts. In particular under electrochemical operation, the reconstruction phenomenon of surface sites during reactions. After reconstruction, the active site is no longer the original pristine structure but rather the newly formed surface site. Exploring surface reconstruction under Operando catalytic conditions can aid in a better understanding of the active sites and catalytic pathways, helping designing more efficient catalyst systems.Atomically dispersed catalysts (ADCs), specifically the ones based on carbon, show tremendous potential in critical electrochemical reactions, including the hydrazine oxidation reaction (HzOR), which exhibits zero-carbon emissions and possesses a high energy density, presents considerable promise for the application of hydrazine fuel cells with improved power density. The dynamic reconstruction of atomically dispersed catalytic sites during electrocatalytic processes has emerged as a subject of intensive investigation. On one hand, knowledge of the reconstruction process is crucial, as it provides deep insights into the atomically dispersed active site and catalytic mechanism, laying a firm basis for material innovation. On the other hand, atomically dispersed catalytic sites have clear and uniform coordination structures, making them ideal quasi-model catalysts for revealing reconstruction mechanisms of active sites. However, discovering these reconstruction behaviors has focused primarily on metallic atomically dispersed catalytic sites so far, leaving non-metallic ones much unexplored, despite the fact that their performance in catalysis is comparable to or even superior to that of costly noble metals. An accurate vision of active sites during reactions is vital in developing non-metallic atomically dispersed catalysts; from a scientific perspective, it is essential to extract information about possible dynamic reconstruction processes in non-metallic atomically dispersed catalytic sites, similar to that of metallic ones.Combining ex situ high resolution electron microscopy and in situ X-ray adsorption spectroscopy to monitor the structurally uniform and well-defined single atomic site of atomically dispersed catalyst as an ideal model system could provide valuable atomic insights to active sites and the corresponding catalytic reaction mechanism. In situ techniques, particularly operando near and extended X-ray absorption fine structure (EXAFS and XANES) spectroscopies, in which the probe parameters such as the edge intensity, “white line”, and its energy position are affected by spectator species adsorbed on the electrode surface and intermediates formed upon reaction, may deliver chemical fingerprints that capture evolution of active sites during reactionIn this study, we synthesized a carbonaceous, non-metallic atomically dispersed selenium (Se) catalyst and confirmed the atomically dispersed configuration of Se in a SeC4 configuration. The as-prepared Se ADCs was found active in the hydrazine oxidation reaction (HzOR) in alkaline media (-114 mV (vs. reversible hydrogen electrode) at a current density of 1 mA cm-2), even surpassing that of noble-metal platinum (Pt) catalysts. Using the Se ADCs as a quasi-model catalyst, in situ X-ray absorption spectroscopy and Fourier-Transform infrared spectroscopy were used to trace the dynamic reconstruction process of atomically dispersed Se catalytic sites. We found that pristine SeC4 will pre-adsorb an *OH ligand in alkaline aqueous solutions, and the electrochemical oxidation of the N2H4 molecule will subsequently occur on the opposite side of the OH-SeC4. Theoretical calculations suggest that the pre-adsorbed *OH group pulls electrons from the Se site, resulting in a more positively charged Se and a higher polarity of Se-C bonds, and consequently higher surface reactivity toward HzOR. Our findings provide valuable insights into the reconstruction mechanism of atomically dispersed semimetallic catalytic sites and promote their practical use in a wide range of energy systems. Figure 1

1. The dehydrogenation of ethanol over a phosphate catalyst proceeds in two directions:formation of ethylene and formation of ether. 2. When the catalyst is treated with alkali its activity for the formation of ethylene falls linearly with the amount of alkali adsorbed. This indicates the acidic nature of the catalyst and the homogeneity of its active sites. The increase in the yield of ether over a catalyst poisoned with alkali is probably due to the kinetic peculiarities of the various stages of the process. 3. The amounts of alkali required for the complete poisoning of phosphate and aluminum silicate catalysts in the reaction of ethylene formation are in almost the same ratio as the catalytic activities of the catalysts, which indicates the identical activities of their active sites.