Lewis Acid Sites In Catalysts Research Articles

Resolving the Acid Site Distribution in Zn-Exchanged ZSM-5 with Stimulated Raman Scattering Microscopy

Zeolites are widely used acid catalysts in research and in industrial processes. The catalytic performance of these materials is affected by the nature and concentration of Brønsted and Lewis acid sites. The balance between these types of active sites—and thus the activity and selectivity of the zeolite—can be altered by the introduction of metal species, e.g., by ion exchange. Although the acidic properties of zeolites are routinely characterized by bulk-scale techniques, this ensemble-averaged approach neglects the local variations in the material. Insights into the distribution of active sites at the single-particle level are thus critical to better understand the impact of post-synthetic modifications on the zeolite acidity. In this contribution, we spatially resolve Brønsted and Lewis acid sites in protonated and Zn-exchanged ZSM-5 crystals. To this end, the vibrational modes of pyridine chemisorbed on active sites are mapped with stimulated Raman scattering (SRS) microscopy. The SRS images reveal sharp inter- and intra-particle heterogeneities in the distribution of Lewis acid sites introduced upon ion exchange, ascribed to local variations in the Al content. Besides assessing the impact of Zn exchange on the active site distribution in ZSM-5 crystals, this approach enables uniquely to map the distribution of Lewis acid sites in catalysts at the single-particle level.

Effect of acid sites on catalytic destruction of trichloroethylene over solid acid catalysts

The catalytic destruction of trichloroethylene (TCE) over several solid acid catalysts (HZSM-5, γ-Al2O3 and SBA-15/P) was evaluated under dry conditions. The activity order was found to be: HZSM-5>SBA-15/P>γ-Al2O3. It was reported that Bronsted and Lewis acid sites of catalysts both played an important role on TCE catalytic destruction, while the Bronsted acid sites were more decisive. Additionally, the formation of the polychlorinated by-product (tetrachloroethylene, PCE) over HZSM-5 and γ-Al2O3 catalysts was observed and attributed to the presence of Lewis acid sites and basic O2−, and PCE was not detected over SBA-15/P catalyst due to the presence of only Bronsted acid sites. The TCE/O2-TPSR studies demonstrated that the main oxidation products during TCE catalytic destruction are CO, CO2 and Cl2, and the carbon in TCE was firstly converted to CO and then further oxidized into CO2 by gas phase O2.

Fabrication of hydrophobic polymer foams with double acid sites on surface of macropore for conversion of carbohydrate

Herein we reported a simple and novel synthetic strategy for the fabrication of two kinds of hydrophobic polymer foam catalysts (i.e. Cr3+-HPFs-1-H+ and HPFs-1-H+) with hierarchical porous structure, inhomogeneous acidic composition and Lewis–Brønsted double acid sites distributed on the surface, which was used to one-pot conversion of carbohydrate (such as cellulose, glucose and fructose) to a key chemical platform (i.e. 5-hydroxymethylfurfural, HMF). The water-in-oil (W/O) high internal phase emulsions (HIPEs), stabilized by both Span 80 and acidic prepolymers as analogous particles offered the acidic actives, were used as the template for simultaneous polymerization of oil phase in the presence of divinylbenzene (DVB) and styrene (St). After subsequent ion-exchange process, Lewis and Brønsted acid sites derived from exchanged Cr3+ and H+ ion were both fixed on the surface of cell of the catalysts. The HPFs-1-H+ and Cr3+-HPFs-1-H+ had similar hierarchical porous, hydrophobic surface and acid sites (HPFs-1-H+ with macropores ranging from 0.1μm to 20μm, uniform mesopores in 14.4nm, water contact angle of 122° and 0.614mmolg−1 of Brønsted acid sites, as well as Cr3+-HPFs-1-H+ with macropores ranging from 0.1μm to 20μm, uniform mesopores in 13.3nm, water contact angle of 136° and 0.638mmolg−1 of Lewis–Brønsted acid sites). It was confirmed that Lewis acid sites of catalyst had a slight influence on the HMF yield of fructose came from the function of Brønsted acid sites, and Lewis acid sites were in favor of improving the HMF yield from cellulose and glucose. This work opens up a simple and novel route to synthesize multifunctional polymeric catalysts for efficient one-pot conversion of carbohydrate to HMF.

Mechanism of Hydrodesulfurization of dibenzothiophenes on unsupported NiMoW catalyst

Hydrodesulfurization (HDS) of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) on unsupported NiMoW catalyst was studied. Moreover, mechanisms and reaction networks were revealed on the basis of GC and GC-MS analyses of the reaction products. The result shows that, the HDS rate of dibenzothiophenes is in the order of 4,6-DMDBT≈4-MDBT<DBT for the spatial restraining effect of the methyl group. Unsupported NiMoW catalyst has high hydrogenation activity in aromatics saturation, it favours the hydrogenation of alkyl-substituted aromatic ring, weakens the spatial restraining effect, leads to the effectively removing of alkyl-substituted dibenzothiophenes. The hydrodesulfurization products of DBT could be hydrogenated further and its distribution is similar to the hydrogenation products of biphenyl(BP). The hydrogenation route of the hydrodesulfurization of 4-MDBT have two reaction pathways, the electron donor induction of the methyl group could promote the hydrogenation of the adjacent phenyl. On the Lewis acid sites of catalyst, part of 4-MDBT and 4,6-DMDBT transformed into DBT through the demethylation reaction in their hydrodesulfurization process.

Mechanism of 1,2-dichloroethane dehydrochlorination on the acid sites of oxide catalysts as studied by IR spectroscopy

The adsorption of 1,2-dichloroethane on zeolite HZSM-5 and γ-Al2O3 at temperatures from 25 to 400°C was studied by Fourier transform IR spectroscopy. The forms of adsorbed 1,2-dichloroethane and the products of its conversion at the Bronsted and Lewis acid sites of catalysts were identified. The kinetics of 1,2-dichloroethane conversion on the surface of catalysts was studied by in situ Fourier transform IR spectroscopy. It was demonstrated that 1,2-dichloroethane was dehydrochlorinated at the Lewis sites of γ-Al2O3 even at 100°C, whereas the reaction came into play at the Bronsted sites of zeolite HZSM-5 only at 200°C. It was found that, at the Lewis acid sites of catalysts, the resulting vinyl chloride underwent oligomerization with the intermediate formation of a dimer (1,3-dichloro-2-butene), whereas the formation of 1,3-dichloro-2-butene at the Bronsted sites of zeolite HZSM-5 was not observed. The mechanisms of 1,2-dichloroethane conversions at Lewis and Bronsted acid sites were proposed.

Preparation, characterization and hydrotreating performances of ZrO 2–Al 2O 3-supported NiMo catalysts

A series of Al 2O 3–ZrO 2 composite supported NiMo catalysts with various ZrO 2 contents were prepared. Several techniques including XRD, SEM, N 2 physisorption, H 2-TPR, and UV–vis DRS were used for typical physico-chemical properties characterization of the ZrO 2–Al 2O 3 composite supports and their NiMo/ZrO 2–Al 2O 3 catalysts. The test results showed that the composite supports prepared by the chemical precipitation method existed as amorphous phase in the samples with insufficient contents of ZrO 2, and the incorporation of ZrO 2 into supports provided a better dispersion of NiMo species, which made their reductions become easier. The pyridine-adsorbed FT-IR results indicated that the Lewis acid sites of catalysts increased significantly by the introduction of ZrO 2 into the supports. The activities of these catalysts for diesel oil hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) were evaluated in a high pressure micro-reactor system. The results showed that the ZrO 2–Al 2O 3-supported NiMo catalysts with suitable ZrO 2 contents exhibited much higher catalytic activities than that of Al 2O 3-supported one, and when the ZrO 2 contents were 15% and 5%, the NiMo/Al 2O 3–ZrO 2 catalysts presented the highest HDS and HDN activities, respectively.

MnOx-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures

A series of manganese-cerium oxide catalysts were prepared by co-precipitation method and used for low temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess O2. These catalysts were characterized by X-ray diffraction (XRD), surface area measurement and FTIR. The experimental results showed that the best Mn-Ce mixed-oxide catalyst yielded 95% NO conversion at 150°C at a space velocity of 42,000h−1. As the manganese content was increased from 0 to 40% (i.e. the molar ratio of Mn/(Mn+Ce)), NO conversion increased significantly, but decreased at higher manganese contents. The most active catalyst was obtained with a molar Mn/(Mn+Ce) ratio of 0.4. Only N2 rather than N2O was found in the product when the temperature was below 150°C. At higher temperatures, trace amounts of N2O were detected. A mechanistic pathway for this reaction was proposed based on earlier findings and FTIR results obtained in this work. The initial step was the adsorption of NH3 on Lewis acid sites of catalyst, followed by reaction with nitrite species to produce N2 and H2O. Possible intermediates are proposed and all the intermediates could transform into NH2NO, which could further react to produce N2 and H2O.