Presence Of Bronsted Acid Sites Research Articles

Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small-Pore Zeolites.

The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined zeolite pores can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments, and pulse‐response temporal analysis of products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π–H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations.

Bifunctional Sulfonated MoO3–ZrO2 Binary Oxide Catalysts for the One-Step Synthesis of 2,5-Diformylfuran from Fructose

Sulfonated MoO3–ZrO2 binary oxides (MZS) with different Mo/Zr ratios were synthesized and applied as bifunctional catalysts for the simple one-pot transformation of fructose to 2,5-diformylfuran (DFF). The presence of Bronsted acid sites and the molybdenum oxide species in the catalysts is responsible for the high efficiency and good activity of the catalysts. The former contributes to a high yield of 5-hydroxymethylfurfural (HMF) in the fructose dehydration, and the latter has the role of catalyzing the selective aerobic oxidation of the resulted HMF into DFF. In optimized reaction conditions, DFF yield of 74% with fructose of 100% can be achieved in a one-step reaction. The catalyst can be separated, simply regenerated, and reused without any significant loss in activity, indicating its great potential for the industrial mass production of DFF from fructose.

Removal of Sulfur and Nitrogen Compounds from Diesel Oil by Adsorption Using Clays as Adsorbents

Stringent specifications for sulfur compounds content and the need to use oils with large amount of nitrogen compounds are challenges for fuel producers. Adsorption is an alternative process to remove sulfur and nitrogen compounds and clays are promising adsorbents for this removal. In this work, the adsorption performance of different commercial clays—Clay A (attapulgite), Clay B (bentonite), and Clay C (bentonite)—for the removal of sulfur and nitrogen compounds from a real diesel stream was studied through kinetic and isothermal experiments. The bentonite clays showed the best adsorptive capacity for the removal of sulfur and nitrogen compounds, probably due to the presence of Bronsted acid sites. The highest adsorption capacity was observed for Clay B: 0.174 mol kg–1 for sulfur compounds and 0.127 mol kg–1 for nitrogen compounds. Clay A was more selective to the removal of nitrogen compounds. Equilibrium data showed that adsorbate–absorbate and adsorbate–surface interactions predominate for sulfur and...

Selective ethanol synthesis via multi-step reactions from syngas: Ferrierite-based catalysts and fluidized-bed reactor application

Selective ethanol synthesis from syngas through cascade multi-step reactions via methyl acetate (MA) intermediate route was investigated using a highly crystalline ferrierite (FER) zeolite and hybridized bifunctional Cu-ZnO-Al2O3/FER (CZA/FER). The extent of crystallinity of the FER zeolites played crucial roles for catalytic activities of syngas conversion to dimethyl ether (DME), DME carbonylation to MA and MA hydrogenation to ethanol. With an increase of crystallinity of FER, DME productivity by CO2 (or CO) hydrogenation was largely enhanced due to a fast dehydration rate of methanol to DME on the CZA/FER having a large surface area of metallic copper nanoparticles. The bifunctional CZA/FER was utilized to convert MA to ethanol up to the similar equilibrium yield of ∼42 mol%. In addition, highly crystalline of FER@FER, which was synthesized by using a FER seed, showed a higher DME conversion to MA and stability due to the abundant presence of Bronsted acid sites with less defect sites. To verify the possible integration of two series cascade reactions such as direct syngas conversion to DME and its consecutive carbonylation to MA, the segregated double-layer formation in one bubbling fluidized-bed reactor was simulated. Two catalytic reaction regions with a light and dense-phase at similar reaction conditions were completely segregated at optimal linear velocity ranges (minimum fluidizing velocity, Umf) by using two different catalyst particle sizes having an average size of 112.5 and 82.5μm in a bottom and upper reaction zone, respectively.

Inelastic Neutron Scattering Study on the Location of Brønsted Acid Sites in High Silica LTA Zeolite

The presence of Bronsted acid sites (SiOHAl) is crucial in many of the applications of zeolites in petroleum refining and related industries. This acidity relies on the number and location of acid sites, but the latter is difficult to know accurately in spite of a large number of experimental techniques available. Zeolites usually contain many types of acid sites depending on the crystallographic different O-sites and their chemical environment. In the present work we show how inelastic neutron scattering (INS) can be employed for an accurate determination of the location of Bronsted acid sites in zeolites. This is achieved using a sample of LTA zeolite with few defects of connectivity, few (O1H, O2H, O3H) crystallographically different oxygens, and high Si/Al ratio. The narrow and intense peaks corresponding to SiOH bending modes ca. 1100 cm–1 have been shown, using periodic DFT calculations, to strongly depend on the type of oxygen to which the proton is bonded. A comparison between calculations and INS...

Synthesis and Characterization of Tin (IV) Tungstate Nanoparticles – A Solid Acid Catalyst

Tin (IV) tungstate, a tetravalent metal acid salt was synthesized in the nanoform by chemical coprecipitation method using EDTA as capping agent. The material was found to be stable in mineral acids, bases and organic solvents except in HF and aquaregia. The material was characterized using EDS, TG/DTA, FTIR, XRD, SEM, HRTEM and BET surface area measurement. The molecular formula of the compound is 2SnO2 3WO3.5H2O determined from elemental analysis using TG/DTA. Surface morphology and particle size were obtained using SEM and HRTEM. The surface area was found to be 205-225m2/g. The Na+ exchange capacity found to be 3.8 meq/g, indicates the presence of surface hydroxyl group and hence the presence of Bronsted acid sites. The catalytic activity of the material was tested by using esterification and oxidation as model reactions. For the esterification of different alcohols, the percentage yield was found to be high for n-alcohol compared to isomeric alcohols. Oxidation of benzyl alcohol gives benzaldehyde and benzoic acid as the only products. © 2012 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Hierarchically Porous ZSM-5 Synthesized by Nonionic- and Cationic-Templating Routes and Their Catalytic Activity in Liquid-Phase Esterification

Hierarchically porous MFI zeolites (ZSM-5) have been synthesized by hydrothermal treatment in the presence of trialkoxysilylated-derivatives of nonionic poly(oxyethylene) alkyl ether or alkyl quaternary ammonium cation as mesopore-generating agent, along with tetrapropylammonium cation as zeolite structure-directing agent. Powder X-ray diffraction revealed that zeolites have been crystallized, and scanning electron microscopy showed rugged surface morphology that was quite different from conventional ZSM-5. The mesoporosity was confirmed by nitrogen adsorption-desorption measurement showing type IV isotherms with narrow distribution of mesopore diameters. The catalytic activity of these mesoporous ZSM-5 was tested in liquid-phase esterification of benzyl alcohol with hexanoic acid. The conversion of benzyl alcohol on mesoporous ZSM-5 prepared via cationic-templating route was almost 100%, being much higher than on mesoporous ZSM-5 prepared with silylated nonionic surfactant as well as on conventional ZSM-5 with no mesopores. The presence of Bronsted acid sites, together with the mesopores, was responsible for this catalytic conversion, as confirmed by pyridine adsorption monitored by in situ infrared and 27 Al magic angle spinning nuclear

Sulfated Zirconia Modified SBA-15 Catalysts for Cellobiose Hydrolysis

Zirconia modified SBA-15 becomes a very active catalyst for the selective hydrolysis of cellobiose to glucose after sulfation. Spectroscopic investigations indicate the presence of Bronsted acid sites with similar properties to those present in conventional sulfated zirconia. Indications are found that the sulfate groups attached to zirconia interact with silanol groups of SBA-15. The catalytic activity in cellobiose hydrolysis correlates well with results for temperature-programmed decomposition of i-propylamine for a range of sulfated ZrO2/SBA-15 catalysts. A glucose yield of 60% during cellobiose hydrolysis at a reaction time of 90 min at 160 °C is obtained.

Characterization and acidic properties of Al-SBA-3 mesoporous material

SBA-3 mesostructured silica containing framework aluminum is reported here. This material was successfully synthesized using sodium aluminate via post synthesis method. The structural properties of this novel material were studied by X-ray diffraction and FTIR. The Al insertion was determined by 27Al-NMR and the acidity was monitored by infrared spectroscopy of Pyridine adsorption/desorption. The 27Al NMR results indicate that Al has been effectively incorporated into the framework of SBA-3 material by post-synthesis method. The bridging hydroxyl (Si 3O–Al–OH) acid sites are observed after the adsorption of basic probe molecules, such as pyridine; this fact strongly supports the presence of Bronsted acid sites. The alumination of Si-SBA-3 produces stronger Brønsted acidity than the conventional mesoporous aluminosilicates.

Hydrodealkylation of C9 + Heavy Aromatics to BTX over Zeolite-Supported Nickel Oxide and Molybdenum Oxide Catalysts

Two series of catalysts with nickel oxide or molybdenum oxide as the active component and with different zeolites as supports were prepared by incipient wetness impregnation method, and their structural properties and acidity were characterized by means of XRD, H2-TPR, N2 adsorption and desorption, FTIR spectrum of adsorbed pyridine and NH3-TPD. The catalytic activity of the prepared materials was investigated for the hydrodealkylation of commercial C9+ heavy aromatics to BTX. The results show that the catalytic performance is greatly influenced by the specific surface area, the acidity and the interaction between zeolite and metal oxide, and little affected by the pore diameter of catalysts. For NiO catalysts, the samples with the moderate interaction between zeolite and NiO exhibit the relatively high selectivity of BTX. And for MoO3 catalysts, the presence of the molybdenum species associated with Bronsted acid site causes the remarkable reduction of selectivity. The presence of Bronsted acid sites, the growth of the strength of Lewis acid sites and the increase of acid amount and the specific surface area can all enhance the conversion of C9+ aromatics. Finally, compared with other zeolites, the HMCM-56 catalysts show the excellent overall catalytic performance with the yield of BTX more than 60 mol%, whether using NiO or MoO3 as the active component.

Silicotungstic Acid Impregnated MCM-41-like Mesoporous Solid Acid Catalysts for Dehydration of Ethanol

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.

The role of acidity in the decomposition of 1,2-dichlorobenzene over TiO2-based V2O5/WO3 catalysts

Abstract The influence of Lewis and Bronsted acid sites on the performance of V2O5/TiO2 and V2O5–WO3/TiO2 catalysts in the total oxidation of o-dichlorobenzene was investigated. Catalytic activity of these materials resulted strongly affected by their acidic properties. The presence of Bronsted acid sites significantly increases the o-DCB conversion but also leads to the uncompleted degradation of chlorinated compounds, promoting the formation of partial oxidation products, as dichloromaleic anhydride. On the contrary, Lewis acid sites, acting as absorbing sites, promote the further oxidation of intermediates to CO and CO2, without any by-products desorption. Furthermore, the presence of water in the feed-stream was proven to decrease o-DCB conversion but also to play a positive role on process selectivity, increasing COx production. Plausible reasons for this effect are the reduction of Bronsted acid sites and the hydrolysis of anhydride during wet tests.

Cobalt-exchanged mordenites: preparation, characterization and catalytic activity for the abatement of NO with CH4 in the presence of excess O2

The abatement of NO with methane in the presence of oxygen was studied on various commercial MOR in the Na-form (Na-MOR) and H-form (H-MOR), or exchanged to various extents with cobalt (Co-MOR). The sodium and cobalt contents were determined by atomic absorption. Samples were characterized by FTIR and volumetric measurements of CO adsorption. Chemical analysis indicated that one cobalt species replaced two Bronsted acid sites in H-MOR and two Na+ ions in Na-MOR. The IR analysis of the OH stretching region, evidencing an unexpected presence of Bronsted acid sites (band at 3610 cm−1) in Co-MOR, indicated that the exchange process had a more complex stoichiometry. The adsorption of CO at RT on Co-MOR, in addition to the bands of the corresponding H-MOR and Na-MOR matrices, yielded two types of CoII-carbonyls, the first type occupied the␣mordenite main channels, and the second one the mordenite smaller channels. Bronsted acid sites in mordenites were active for the selective catalytic reduction of NO with CH4. Co-MOR samples were far more active than Na-MOR and H-MOR samples, showing that acid protons play a negligible role when Co is present. Co-MOR catalysts showing the highest activity had the largest amount of CoII-carbonyls in the main channels. This result strongly suggests that CoII in the main channels of MOR are the active sites for the CH4 + NO + O2 reaction.

Structure of the Active Sites on H3PO4/ZrO2 Catalysts for Dimethyl Carbonate Synthesis from Methanol and Carbon Dioxide

The catalytic properties of H3PO4/ZrO2 catalysts for dimethyl carbonate (DMC) synthesis from CH3OH and CO2 were investigated. The modification of ZrO2 with H3PO4 promoted the activity for selective DMC formation. The characterization of H3PO4/ZrO2 catalysts was performed by means of BET, XRD, 31P MAS NMR, LRS, DRIFT, and NH3-TPD. From the characterization results, it was determined that ZrO2 had Lewis acid sites and did not have Bronsted acid sites. The presence of Bronsted acid sites as well as Lewis acid sites was observed on H3PO4/ZrO2 (P/Zr = 0.05). The acidity of the Bronsted sites is weak, and the site's structure is a bridged OH. Furthermore, this Bronsted site is formed by the interaction between tetragonal Zr and P. In the reaction scheme of DMC formation, the Bronsted acid site can contribute to CH3OH activation and to the enhancement of the catalytic activity.

Systematic Comparison of a Saponite Clay Pillared with Al and Zr Metal Oxides

Alumina and zirconia pillared Ballarat saponites have been prepared by cation exchange of the saponite in aluminum chlorhydrate and zirconyl chloride solutions, respectively. The nature of the pillaring species and the method of preparation have been varied in order to examine their effect on the amount of occluded oxide, as well as the thermal stability, surface area, and acidic properties of the resulting pillared clays. Higher reaction (cation exchange) temperature (80 °C) enhances the degree of polymerization, increases the mobility of the pillaring species, and results in greater incorporation between the sheets of the saponite. In general, it is observed that the alumina-pillared saponites have a higher thermal stability than their zirconia-pillared analogues and retain an ordered layered structure even after calcination at 700 °C. Infrared spectroscopy of adsorbed pyridine suggests the presence of Bronsted acid sites and two types of Lewis acid sites (of different strength) for the alumina-pillared materials. Only single types of Bronsted and Lewis acid sites, however, were detected for the zirconia-pillared saponites. The thermal desorption of cyclohexylamine and dehydration of pentan-1-ol were used as further probes to monitor the acidity of the different pillared saponites.

Acid activated montmorillonite and vermiculite clays as dehydration and cracking catalysts

Treatment of montmorillonite and vermiculite with hydrochloric acid results in an overall increase in acidity and acidic sites. IR and DSC studies of base adsorbed catalysts have revealed the heterogeneity of surface acidity on the activated catalysts. Clay catalysts have been found to dehydrate 1-butanol to 1-butene, which directly isomerizes tocis- andtrans-2-butenes, confirming the presence of Lewis and Bronsted acid sites. Cumene cracking studies using the clay catalyst also confirm the presence of Bronsted acid sites. The temperature and concentration of acid have been found to have a significant influence on the catalytic activity.

Evidence of Brønsted acidity on sulfided promoted and unpromoted [formula omitted] catalysts

It is uncertain what effect acidity, especially protonic (Bronsted) acidity, has on typical hydrotreating reactions over molybdenum-containing, sulfided catalysts. In a study of the hydrogenation of 1-hexene, small amounts of propylene were found together with the major product, hexane. The amount of the former increased with increase in the H{sub 2}S partial pressure, leading to the supposition that H{sub 2}S increased the Bronsted acidity of the sulfided catalyst. The hydrodenitrogenation of quinoline was also found to be promoted by H{sub 2}S, which was attributed to an increase in the number of Bronsted acid sites. However, no direct evidence for the presence of Bronsted acid sites was obtained. One of the advantages of using pyridine as the probe molecule for monitoring acidity is that it can adsorb both as coordinated and protonated pyridine on Lewis and Bronsted acid sites, respectively. These adsorbed pyridine species can be easily distinguished by infrared spectroscopy. Bronsted and Lewis acid sites have been detected for oxidic promoted and unpromoted Mo/Al{sub 2}/O{sub 3} catalysts but only Lewis acidity has been found on the corresponding sulfided catalysts. It should be pointed out that most of the previous IR studies have been carried out with pyridine adsorption at relativelymore » low temperatures (below 423 K). It occurred to the authors that since the Bronsted acidity, if it exists, must be weak, higher temperatures may be required to produce the pyridinium ion. The present not reports IR evidence of Bronsted acidity at elevated temperatures corresponding to those typically employed under hydroprocessing reactions.« less

Catalytic desulfurization. Part 16. Infrared spectroscopic investigation on surface acidity of Ni/HNaY catalyst and adsorption state of thiophene.

The nature of surface hydroxyl groups formed by reduction of NiNaY zeolite, and behavior of adsorbed thiophene on reduced NiNaY (Ni/HNaY) were investigated by means of infrared spectroscopy. The newly formed surface hydroxyl groups on Ni/HNaY were more stable to thermal treatment than those originally existed on NiNaY. Those surface hydroxyl groups formed on Ni/HNaY showed Bronsted acidity in the i.r. spectra of pyridine adsorbed on Ni/HNaY. The strong interaction between surface acidic hydroxyl groups and adsorbed thiophene, and the formation of surface species having a C=S double bond were observed in the i.r. spectra of thiophene adsorbed on Ni/HNaY. It was concluded that the presence of Bronsted acid sites on Ni/HNaY surface is important for adsorption of thiophene and cleavage of the C-S bond of adsorbed thiophene. In the light of catalyst active sites, the reaction mechanism for hydrodesulfurization of thiophene over Ni/HNaY zeolite catalyst was proposed.