Catalytic conversion of furanic compounds over Ga-modified ZSM-5 zeolites as a route to biomass-derived aromatics

Herein we report a mechanistic study of aromatization of furanics, as model compounds for cellulosic biomass, over (Ga)HZSM-5 catalysts. Applying combined gas chromatography and mass-spectrometry product analysis we were able to analyse conversion and selectivity reaction profiles with high temporal resolution. The thorough analysis of the product distribution allowed us to resolve the deoxygenation pathways of the furan molecules. We found that depending on the methyl substitution oxygen is removed either as water or COx, effecting the carbon efficiency of the process. While unsubstituted furan undergoes decarbonylation to form COx, methylated furans are deoxygenated by dehydration, resulting in a much higher carbon-efficiency. Furthermore, using in situ IR spectroscopy, we found that promotion of HZMS-5 with Ga in addition to enhanced aromatic selectivity influences the deactivation pathway leading to the preferential formation of proton-deficient polycyclic aromatic compounds

石炭の塩素化

種別:卒業論文東京帝国大学工学

Aromatization of ethylene over zeolite-based catalysts

Light aromatic compounds (BTX: benzene, toluene and xylenes) represent an important class of building blocks in the chemical industry. Currently, light aromatics are obtained exclusively from fossil feedstock, whose utilization is associated with serious environmental concerns. Developing new routes for a more sustainable BTX production is, therefore, of high importance. In this work, aromatization of ethylene over well-defined metal-modified HZSM-5 zeolite catalysts is examined. The results show that modification of zeolite with gallium, zinc and silver leads to a significant increase in aromatics production. Metal species are responsible for catalysing dehydrogenation pathways with Ga being the most efficient for BTX production. Increasing temperature and ethylene partial pressure facilitate ethylene aromatization. Employing a combination of isotope labelling with a thorough characterization of zeolite-entrapped species by means of IR and MAS NMR spectroscopy provides evidence for the involvement of intra-zeolite aromatic hydrocarbon species in the catalytic cycle

Tuning the reactivity of molybdenum (oxy)carbide catalysts by the carburization degree: CO2 reduction and anisole hydrodeoxygenation

Molybdenum (oxy)carbide catalysts supported on activated carbon were prepared by a carbothermal hydrogen reduction method without passivation step. Four carburization temperatures (500 °C, 600 °C, 700 °C and 800 °C) were selected to control the catalyst carburization degree based on studies of catalyst precursor carburization process by TGA-MS and in situ XANES. Quasi in situ XRD, XAS and XPS revealed that two types of material were produced – molybdenum oxycarbide (500 °C and 600 °C) and molybdenum carbide (700 °C and 800 °C) catalysts. The oxycarbide catalysts are rich in Mo-oxide and Mo-oxycarbide species (MoO2 and MoOxCy) and the carbide catalysts rich in Mo-carbide species (α-MoC1−x and β-Mo2C) with a certain remaining oxygen atoms. The carbidic and oxophilic Mo sites in the catalysts were respectively probed by CO and N2O chemisorption. The structure–performance relationships of these catalysts in CO2 hydrogenation and anisole hydrodeoxygenation (HDO) were studied. For CO2 hydrogenation, the carbide catalysts were much active than the oxycarbide catalysts and CO was the main product in all the catalysts. In contrast, the oxycarbide and carbide catalysts displayed comparable activity towards anisole conversion and the main products shifted from a mixture of phenol and benzene to only benzene upon increasing the carburization temperature from 600 °C to 700 °C. These catalytic results demonstrate that the catalytic performance of molybdenum (oxy)carbide material can be effectively tuned by varying the carburization degree and such tuning effect depends on the nature of reactant molecules: the carbidic Mo sites in Mo-carbide species are associated with CO2 and anisole-to-benzene conversions, and the oxophilic Mo sites in Mo-oxycarbide species are related to anisole-to-phenol conversion

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation

Different mechanisms of ethane aromatization over Mo/ZSM-5 and Ga/ZSM-5 catalysts

Aromatization of light hydrocarbons can contribute to a secure supply of aromatics for the chemical industry. In this work, we investigate the influence of modification of zeolite ZSM-5 with Ga and Mo on the reaction mechanism underlying the activation and aromatization of ethane. Well-defined Mo/ZSM-5 and Ga/ZSM-5 zeolites efficiently promote ethane aromatization to benzene-toluene-xylene mixtures. Both catalysts suffer from coke formation, which leads to rapid deactivation. From catalytic tests, temperature-programmed surface reaction and pulsed reaction experiments, we infer that ethane conversion on Ga/ZSM-5 follows a conventional sequential dehydrogenation-oligomerization-aromatization mechanism, while the reaction over Mo/ZSM-5 involves reactive surface carbon (hydrocarbon pool) species

Gallium-promoted HZSM-5 zeolites as efficient catalysts for the aromatization of biomass-derived furans

The development of novel technologies to convert renewable biomass feedstocks to fuels and chemicals is of increasing interest for making our chemical industry more sustainable. Plant biomass or its biomass-derived platform molecules are typically over-functionalized, requiring substantial modification to produce the chemicals currently demanded by industry. Furanic compounds are intermediates in the catalytic fast pyrolysis of lignocellulosic biomass or sugar dehydration and can in principle be further converted to aromatics. While upgrading of furanics by zeolite-catalysed aromatization typically results in a large loss of carbon due to coke deposition, carbon laydown can be mitigated by the addition of ethylene and by the modification of the zeolite with Lewis acid Ga sites. Here, we investigate the influence of the Ga loading on the physicochemical properties of Ga-modified HZSM-5 zeolite and its performance in the gas-phase aromatization of 2,5-dimethylfuran with ethylene. Characterization of the morphological, textural and acidic properties were carried out to understand the role of Brønsted and Lewis acid sites on the catalytic reaction. We demonstrate a crucial role of the dispersion of Ga-species and the resulting Lewis acidity of the Ga/ZSM-5 catalysts; and show means how to control both parameters by adjusting the synthesis method

Gallium-promoted HZSM-5 zeolites as efficient catalysts for the aromatization of biomass-derived furans

The development of novel technologies to convert renewable biomass feedstocks to fuels and chemicals is of increasing interest for making our chemical industry more sustainable. Plant biomass or its biomass-derived platform molecules are typically over-functionalized, requiring substantial modification to produce the chemicals currently demanded by industry. Furanic compounds are intermediates in the catalytic fast pyrolysis of lignocellulosic biomass or sugar dehydration and can in principle be further converted to aromatics. While upgrading of furanics by zeolite-catalysed aromatization typically results in a large loss of carbon due to coke deposition, carbon laydown can be mitigated by the addition of ethylene and by the modification of the zeolite with Lewis acid Ga sites. Here, we investigate the influence of the Ga loading on the physicochemical properties of Ga-modified HZSM-5 zeolite and its performance in the gas-phase aromatization of 2,5-dimethylfuran with ethylene. Characterization of the morphological, textural and acidic properties were carried out to understand the role of Brønsted and Lewis acid sites on the catalytic reaction. We demonstrate a crucial role of the dispersion of Ga-species and the resulting Lewis acidity of the Ga/ZSM-5 catalysts; and show means how to control both parameters by adjusting the synthesis method

Reversible nature of coke formation on Mo/ZSM-5 methane dehydroaromatization catalysts

Non-oxidative dehydroaromatization of methane over Mo/ZSM-5 zeolite catalysts is a promising reaction for the direct conversion of abundant natural gas into liquid aromatics. Rapid coking deactivation hinders the practical implementation of this technology. Herein, we show that catalyst productivity can be improved by nearly an order of magnitude by raising the reaction pressure to 15 bar. The beneficial effect of pressure was found for different Mo/ZSM-5 catalysts and a wide range of reaction temperatures and space velocities. High-pressure operando X-ray absorption spectroscopy demonstrated that the structure of the active Mo-phase was not affected by operation at elevated pressure. Isotope labeling experiments, supported by mass-spectrometry and 13 C nuclear magnetic resonance spectroscopy, indicated the reversible nature of coke formation. The improved performance can be attributed to faster coke hydrogenation at increased pressure, overall resulting in a lower coke selectivity and better utilization of the zeolite micropore space