Identifying the Role of Brønsted and Lewis Acid Sites in the Diels-Alder Cycloaddition of 2,5-DMF and Ethylene

The role of Lewis and Brønsted acid sites in the Diels-Alder cycloaddition (DAC) of ethylene to 2,5-dimethylfuran (2,5-DMF) to p-xylene was investigated. Amorphous silica catalysts containing Al3+ (ASA), Ga3+ (ASG), and In3+ (ASI) were prepared via homogeneous deposition-precipitation. Silica modified with Zr4+ (ASZ) was prepared by impregnation. Their acidic properties were characterized by various IR and NMR spectroscopic techniques. Measurements using pyridine as a probe molecule highlighted the presence of mostly Lewis acid sites (LAS) in all materials. Using CO as a probe, in contrast, demonstrated the existence of Brønsted acid sites (BAS) in ASA and ASG, which were nearly absent in ASI and ASZ. Differences in basic strength can explain the contrast in results observed between the two probe molecules. The highest p-xylene yield (~20%) in the DAC reaction, could be achieved with ASA and ASG. The lack of BAS in ASI and ASZ resulted in inferior performance in the DAC, with p-xylene yields below 5%. These results indicate the importance of BAS for the DAC reaction. Several other heterogeneous and homogeneous catalysts were explored for the DAC reaction to show the generality of our conclusion that BAS play a critical role in obtaining p-xylene from 2,5-DMF and ethylene

Al Promotion of In2O3 for CO2 Hydrogenation to Methanol

In2O3 is a promising catalyst for the hydrogenation of CO2 to methanol, relevant to renewable energy storage in chemicals. Herein, we investigated the promoting role of Al on In2O3 using flame spray pyrolysis to prepare a series of In2O3−Al2O3 samples in a single step (0−20 mol % Al). Al promoted the methanol yield, with an optimum being observed at an Al content of 5 mol %. Extensive characterization showed that Al can dope into the In2O3 lattice (maximum ∼ 1.2 mol %), leading to the formation of more oxygen vacancies involved in CO2 adsorption and methanol formation. The rest of Al is present as small Al2O3 domains at the In2O3 surface, blocking the active sites for CO2 hydrogenation and contributing to higher CO selectivity. At higher Al content (≥10 mol %Al), the particle size of In2O3 decreases due to the stabilizing effect ofAl2O3. Nevertheless, these smaller particles are prone to sintering duringCO2 hydrogenation since they appear to be more easily reduced. These findings show subtle effects of a structural promoter such asAl on the reducibility and texture of In2O3 as a CO2 hydrogenation catalyst

Improving the performance of ASA in the DAC of 2,5-DMF and ethylene

A variety of methods are employed to synthesize amorphous silica-alumina (ASA) to resolve the role of Al speciation and surface area in the catalytic performance in the Diels-Alder cycloaddition reaction of 2,5-dimethylfuran and ethylene to p-xylene. ASA was prepared by homogeneous deposition-precipitation (HDP) of Al 3+ on ordered mesoporous silica, i.e., SBA-15 and OMS prepared under hydrothermal synthesis conditions using an imidazole-based template, and one-step flame spray pyrolysis (FSP). IR spectroscopy and 27Al MAS NMR showed that the resulting ASA represented a set of materials with distinct textural and acidic properties. ASA prepared by grafting Al to ordered mesoporous silica led to a much higher concentration of Brønsted acid sites (BAS). These samples performed much better in the DAC reaction, with p-xylene yields higher than those obtained with a HBeta zeolite benchmark. Materials with Al partially in the bulk of silica (OMS, FSP) and containing significant alumina domains are less acidic and exhibit much lower p-xylene yields. These findings point to the importance of Brønsted acidity for p-xylene formation. This study shows that careful design of the Al speciation can lead to amorphous silica-alumina with similar DAC performance to microporous zeolites.</p

Improving the performance of ASA in the DAC of 2,5-DMF and ethylene

A variety of methods are employed to synthesize amorphous silica-alumina (ASA) to resolve the role of Al speciation and surface area in the catalytic performance in the Diels-Alder cycloaddition reaction of 2,5-dimethylfuran and ethylene to p-xylene. ASA was prepared by homogeneous deposition-precipitation (HDP) of Al 3+ on ordered mesoporous silica, i.e., SBA-15 and OMS prepared under hydrothermal synthesis conditions using an imidazole-based template, and one-step flame spray pyrolysis (FSP). IR spectroscopy and 27Al MAS NMR showed that the resulting ASA represented a set of materials with distinct textural and acidic properties. ASA prepared by grafting Al to ordered mesoporous silica led to a much higher concentration of Brønsted acid sites (BAS). These samples performed much better in the DAC reaction, with p-xylene yields higher than those obtained with a HBeta zeolite benchmark. Materials with Al partially in the bulk of silica (OMS, FSP) and containing significant alumina domains are less acidic and exhibit much lower p-xylene yields. These findings point to the importance of Brønsted acidity for p-xylene formation. This study shows that careful design of the Al speciation can lead to amorphous silica-alumina with similar DAC performance to microporous zeolites.</p

Structure–performance descriptors and the role of Lewis acidity in the methanol-to-propylene process

The combination of well-defined acid sites, shape-selective properties and outstanding stability places zeolites among the most practically relevant heterogeneous catalysts. The development of structure-performance descriptors for processes that they catalyse has been a matter of intense debate, both in industry and academia, and the direct conversion of methanol to olefins is a prototypical system in which various catalytic functions contribute to the overall performance. Propylene selectivity and resistance to coking are the two most important parameters in developing new methanol-to-olefin catalysts. Here, we present a systematic investigation on the effect of acidity on the performance of the zeolite 'ZSM-5' for the production of propylene. Our results demonstrate that the isolation of Bronsted acid sites is key to the selective formation of propylene. Also, the introduction of Lewis acid sites prevents the formation of coke, hence drastically increasing catalyst lifetime

One-pot synthesis of nano-crystalline MCM-22

[EN] Nano-crystalline MCM-22 zeolite was synthesized in a one-pot procedure by the use of an organosilane (dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride, TPOAC) in the zeolite synthesis gel. This crystal growth inhibition procedure introduced mesopores in the MCM-22 crystallites. The lower mechanical stability of the nano-crystalline MCM-22 zeolite compared with bulk MCM-22 can be countered to some extent by pillaring. The increased external surface of the microporous zeolite domains resulted in increased accessibility of the Bronsted acid sites, as followed from the better performance in liquid-phase benzene alkylation with propylene as compared with bulk MCM-22. The increased accessibility of the internal acid sites in Mo-loaded hierarchical MCM-22 was also evident from the improved benzene selectivity during methane aromatization. Silylation of hierarchical Mo/MCM-22 was detrimental for the catalytic performance in MDA. The nano-crystalline MCM-22 has physico-chemical and catalytic properties intermediate between those of MCM-22 and ITQ-2 with the benefit over ITQ-2 that it can be synthesized in a single step. (C) 2015 Elsevier Inc. All rights reserved.Funding from the 7th Framework Program of the European Commission through the Collaborative Project Next-GTL (agreement no 229183) and financial support by the Spanish Government-MINECO through "Severo Ochoa" (SEV 2012-0267), Consolider Ingenio 2010-Multicat (CSD2009-00050) and MAT2012-31657 are acknowledged. Marta E. Martinez Armero thanks MINECO for economical support through pre-doctoral fellowship for doctors training (BES-2013-066800). The authors thank B. Esparcia for technical assistance.Tempelman, CHL.; Portilla Ovejero, MT.; Martínez Armero, ME.; Mezari, B.; De Caluwe, NGR.; Martínez, C.; Hensen, EJM. (2016). One-pot synthesis of nano-crystalline MCM-22. Microporous and Mesoporous Materials. 220:28-38. https://doi.org/10.1016/j.micromeso.2015.08.018S283822

Hydrothermal synthesis and characterization of a layered zirconium silicate

A layered zirconosilicate composed of SiO4 tetrahedra and ZrO6 octahedra was hydrothermally synthesized in the presence of tetramethylammonium hydroxide. The layered material with a nominal composition Na2ZrSi4O11·xH2O was further characterized by XRD, SEM, thermogravimetric analysis, UV-Raman spectroscopy and single- and triple-quantum 23Na and 29Si MAS NMR spectroscopy. There are no Zr–O–Zr chains in the framework structure implying that the ZrO6 octahedra are isolated by SiO4 tetrahedra. 23Na MAS NMR indicates at least two different Na sites in the framework of SZS and the Na located in the interlayer can be exchanged by TMAOH, leading to an increase of the interlayer spacing. The layered zirconosilicate SZS has a high activity in the isomerization of glucose to fructose in water and can be reused.status: publishe

A versatile mono-quaternary ammonium salt as a mesoporogen for the synthesis of hierarchical zeolites

Here we report a versatile method to synthesize hierarchically porous zeolites with FER, CHA and MFI topologies by using inexpensive mono-quaternary ammonium N-cetyl-N-methylpyrrolidinium (C 16NMP) as a mesoporogen. Extensive characterization revealed that the mesoporous zeolites are crystalline, possess a high mesopore volume and exhibit comparable Brønsted acidity to their bulk counterparts. Due to the improved accessibility of the microporous domains, these hierarchical zeolites display enhanced performance as catalysts in various reactions such as the dehydration-isomerization of 1-butanol to iso-butene (FER) and methanol-to-hydrocarbons reaction (CHA and MFI)