Number Of Bronsted Acid Sites Research Articles

Mechanistic insights into the interaction between binders and Y-type zeolites in fluid catalytic cracking

Binders play essential roles in fluid catalytic cracking catalysts, and their interaction mechanism with zeolites remains to be explored. The effects of three binders on REUSY zeolite were investigated. The alumina sol most notably blocked the pore channels and destructed the framework of zeolites by removing the framework Al. The silica sol diffused into the silicon hydroxyl nests and form new Si-O tetrahedrons, which enhanced the structural stability and increased the number of Bronsted acid sites in zeolites, rendering the samples’ better hydrothermal stability and higher micro-reaction activity. The influence of the colloidal sol of pseudo boehmite on zeolites was between the two.

On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

Promoting the Effect of Au on the Selective Hydrogenolysis of Glycerol to 1,3-Propanediol over the Pt/WOx/Al2O3 Catalyst

The selective hydrogenolysis of glycerol to 1,3-propanediol (1,3-PDO) is an important yet challenging reaction because it requires the selective cleavage of the sterically hindered secondary C–O bond of glycerol. In this work, we have developed a new catalyst, 0.1Au-2Pt/7.5W/Al, which affords both high glycerol conversion (77.5%) and 1,3-PDO selectivity (54.8%), which are significantly higher than the Au-free counterpart. HAADF-STEM reveals that the structure of 0.1Au-2Pt/7.5W/Al is characterized with ∼2 nm Pt NPs decorated with Au-WOₓ at the periphery, where the Au additive exists most likely as single atoms. Chemisorption and XPS show that the electron transfer from W to Au can help to weaken the strong metal support interaction (SMSI) between Pt and WOₓ, and thus greatly enlarges the exposed Pt surface and increases the H-spillover capacity. The 2-butanol dehydration in H₂ reveals that the H-spillover capacity is proportional to the number of Bronsted acid sites, which, in conjunction with the only water desorption in H₂-TPD results, strongly suggests that the active sites for breaking the secondary C–O bond of glycerol should be Bronsted acid sites generated by the H-spillover at the interface between Pt and WOₓ, and the modification by the Au additive increases the number of interfacial sites, and thus enhances the 1,3-PDO yield.

Promotional effect of ion-exchanged K on the low-temperature hydrothermal stability of Cu/SAPO-34 and its synergic application with Fe/Beta catalysts

Kions were introduced onto Cu/SAPO-34 catalysts via the ion-exchange process in order to improve their stability under low-temperature hydrothermal aging. The changes in structure and copper-species contents of these catalysts upon hydrothermal aging were probed in order to investigate their effects on selective catalytic reduction (SCR) activity. For the fresh Cu/SAPO-34 catalysts, K ions had little influence on the chabazite framework but effected their acidities by exchanging with acid sites. After hydrothermal aging, the structural integrity and amount of active sites decreased on pure Cu/SAPO-34. While the K-loaded catalysts showed improved chabazite structure, acidity, and active site conservation with increasing K loading. However, although the 0.7 wt% K catalyst maintained the same crystallinity, active site abundance, and low-temperature SCR activity as the fresh catalyst upon aging, an apparent decrease in SCR activity at high temperature was observed because of the inevitable decrease in the number of Bronsted acid sites. To compensate for the activity disadvantage of K-loaded Cu/SAPO-34 at high temperature, Fe/Beta catalysts were co-employed with K-loaded Cu/SAPO-34, and a wide active temperature window of SCR activity was obtained. Thus, our study reveals that a combined system comprising Fe/Beta and K-loaded Cu/SAPO-34 catalysts shows promise for the elimination of NOx in real-world applications.

Features of the phase composition, acidity, and isomerization activity of Pt/WO3/ZrO2 catalysts

Textural characteristics, phase composition, and acidic properties of Pt/WO3/ZrO2 catalysts for C7-alkanes isomerization were studied. It was shown that the most active and selective catalysts are samples with WO3 content of 15–25 wt.% with a fraction of the active tetragonal phase of ZrO2 of 73–94 wt.%. It was found that an increase in the number of Bronsted acid sites contributes to an increase in the activity of tungstated zirconia catalysts in the isomerization of n-heptane.

Tin, niobium and tin-niobium oxides obtained by the Pechini method using glycerol as a polyol: Synthesis, characterization and use as a catalyst in fructose conversion

The selective and efficient conversion of fructose into chemicals has been extensively explored, especially from the perspective of a more sustainable industry based on renewable inputs together with a more environmentally friendly process. In this study, mixed metal oxides based on tin and niobium were synthesized by the Pechini method, replacing ethylene glycol with glycerol, a renewable chemical. This replacement allows the use of pure or mixed oxides exhibiting high specific surface areas but did not lead to significant differences in the amount and nature of the acid sites present in each case. Furthermore, the mixed oxides had increased numbers of Lewis acid sites, while the number of Bronsted acid sites decreased. Catalytic tests performed at 150 °C demonstrated that the use of these materials leads to conversion results higher than those observed for pure oxides (SnO2 (EG) = 48.3 %, Nb2O5 (EG) = 51.6 % and SnO2 (G) = 56.8 %, Nb2O5 (G) = 59.6 %) because 78.5 and 75.7 % consumption of fructose for SnNb (EG) and SnNb (G) was attained at 2 h, respectively. In addition, the catalysts show promising 5-hydroxymethylfurfural (5-HMF) selectivity, and the presence of lactic acid was also detected. In addition, recycling experiments were performed, and stability was observed for 4 cycles without structural change in the catalysts.

Double Catalyst-Catalyzed: An Environmentally Friendly Sustainable Process to Produce Methallyl Alcohol

A green, sustainable process route to produce methallyl alcohol via tandem reaction concluding α-H oxidation, esterification and hydrolysis reaction is discussed. Pd–Au bimetallic nanocatalysts over SiO2 mixed oxides incorporating K2+ and Al3+ like promoters were prepared by equivalent volume impregnation, which is using catalyzed the tandem reaction. Homo-dispered ZnO–S2O82− nanoparticles supported on ZSM-5 were prepared by ultrasonic adsorption method, which is using catalyzed hydrolysis reaction. Characterization by transmission electron microscopy confirmed that alloyed Pd/Au nanoparticles with a mean diameter of 0.5 to 1.0 nm were formed. The results of X-ray diffraction confirmed that the crystal structure were Pd/Au(111). The electronic effect and geometric effect of Pd/Au significantly improved the catalytic performance. The effects of reaction pressure, temperature, and ratio of material were discussed in details. The catalytic activity of ZnO–S2O82−/ZSM-5 exhibited more excellent than H2SO4 and other reported catalysts with 100% conversion and selectivity. The excellent catalytic activity of ZnO–S2O82−/ZSM-5 may be attributed to the synergistic effect from the nano-effect of ZnO–S2O82− nanoparticles and the mesostructure of ZSM-5, and the pore size, number of Bronsted acid sites. The reaction temperature are critical factors affecting the catalytic activity for this system. Based on the results, a supposed mechanism of the tandem and hydrolysis reaction was proposed. Atom economic green reaction: acetyl oxidation and hydrolysis reaction.

Influence of Silanol Defects of ZSM-5 Zeolites on Trioxane Synthesis from Formaldehyde

The silanol defects in ZSM-5 zeolite have been recognized as an important factor in catalytic activity. Here, ZSM-5 zeolites with different amounts of silanol defect sites were synthesized from synthetic gels containing fluoride medium and were applied as catalysts for trioxane synthesis. The results of XRD, SEM, NH3-TPD, Py-IR, OH-IR, 27Al MAS NMR, 1H MAS NMR, and TG indicated that all ZSM-5 zeolites showed similar crystal size, relative crystallinity, porosity, and the number of Bronsted acid sites. However, the silanol defects reduced obviously and the number of Lewis acid sites reduced correspondingly when a little NH4F was added in the synthesis gels and both of them decreased slightly with increase of F/Si ratio. Compared with ZSM-5 zeolite prepared in hydroxide medium, ZSM-5 zeolite prepared in fluoride medium displayed higher selectivity to trioxane and increased gradually with increase of F/Si ratio. Moreover, the lifetime of ZSM-5 zeolite prepared in fluoride medium was longer than that of prepared in hydroxide medium. Thus, ZSM-5 zeolite prepared in fluoride medium which contained the few silanol defects and low Lewis acid sites is an efficient catalyst for trioxane synthesis.

Study on iron-based adsorbents for alternating removal of H2S and O2 from natural gas and biogas

This study deals with the development of iron oxide/hydroxide adsorbents for the alternating removal of H2S and O2 from natural gas and biogas. The considered procedure initially implies the chemisorption of H2S. In the second step, O2 is removed by reaction with the H2S-treated substrate accompanied by regeneration of the adsorbent. 35 different iron-based samples were physical-chemically characterized and tested towards H2S/O2 break through behavior and adsorption capacity to evaluate structure-performance correlations. It was derived that high BET surface area, high proportion of meso and micro pores, small average pore diameter, low crystallinity and high number of Bronsted acid sites are crucial properties of the adsorbents. As a consequence, Fe(OH)3 and α-FeOOH revealed highest efficiency for the successive adsorption of H2S and O2, while α-Fe2O3 provided rather low performance.Mechanistic investigations were carried out with α -FeOOH and α -Fe2O3 indicating rapid dissociation of H2S on α -FeOOH, while slow molecular adsorption prevailed on α -Fe2O3. With proceeding H2S exposure, the adsorbents were partially transformed into sulfide and sulfate species with some formation of elemental sulfur. In subsequent reaction with O2, the H2S-exposed samples were re-oxidized under partial restructuring, whereas elemental sulfur and sulfate entities were additionally produced.Finally, the concept of the alternating adsorption of H2S and O2 on best α -FeOOH substrate was technically evaluated in a biogas pilot plant. The investigations showed efficient conversion of H2S for 40–100 h, while subsequent O2 conversion was achieved for ca. 5.5 h.

Impact of Brønsted acid sites in MWW zeolites modified with cesium and amine species on Knoevenagel condensation

Abstract Layered zeolites of MWW family, MCM-22 and pillared MCM-36, were used as supports for base modifiers: cesium species (introduced via cation exchange or impregnation) and 3-aminopropyl trimethoxysilane (AP). The obtained materials were characterized by different methods (ICP-OES, N2 adsorption, XRD, XPS, TG/DTA, FTIR combined with pyridine adsorption, 2-propanol decomposition) for evaluation of chemical, structural and surface properties. All materials obtained were subjected to the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate and ethyl acetoacetate. The effect of the zeolite structure, the stability of the catalysts as well as acid-base properties of zeolites on the activity in Knoevenagel condensation were considered. Of particular interest was the role of Bronsted acid sites (BAS). The nature of basic sites and BAS played different roles depending on the methylene compound used in the Knoevenagel condensation. AP-modified zeolites were the most active in the condensation between benzaldehyde and ethyl cyanoacetate, in which, in the first step of the reaction AP abstracted hydrogen from methylene carbon in ethyl cyanoacetate. A different reaction pathway was postulated for the condensation with ethyl acetoacetate on the basis of the highest activity of unmodified zeolites and the relationship between benzaldehyde conversion and the number of BAS. For this reaction protonation of benzaldehyde was postulated as the initial step of the reaction.

Preparation of Cu-Mn and Cu-Mn-Ce Oxide/Mesoporous Silica via Silicate Exfoliation for Removal of NO and Hg0

ABSTRACT Cu-Mn and Cu-Mn-Ce oxide-incorporated mesoporous silica was formed by hydrothermally exfoliating silicate, and the physicochemical properties and NO/Hg0 removal efficiency were investigated. The exfoliation induced structural reformation, resulting in a large specific surface area and the uniform dispersion of metal oxides on the surface. The transfer of valences between Cu2+ and Mn3+ in the Cu-Mn silica contributed to the single reduction peak displayed in the H2 temperature-programmed reduction profiles and the high Mn4+/Mn and Cu+/Cu ratios observed via X-ray photoelectron spectroscopy (XPS). The high oxygen lability of the Cu-Mn silica may have inhibited its ability to remove NO. By contrast, when SO2 was present, incorporating Ce enhanced the NO removal efficiency due to the increased number of Bronsted acid sites. Hg0 removal tests indicated that adsorption was the primary removal mechanism for both the Cu-Mn and the Cu-Mn-Ce silica samples. Cu2Mn8 exhibited the highest Hg removal efficiency, suggesting that Ce’s enhancing effect on Hg0 adsorption was diminished when a large amount of Mn was present. Of the gaseous components, the adsorbed HCl was mainly responsible for the oxidation and subsequent adsorption of Hg0. Furthermore, with the addition of SO2, the competitive adsorption of SO2 and the resulting HgCl2 did not decrease the Cu-Mn silica’s efficiency in oxidizing Hg0, but the oxidized Hg was less adsorptive.

An Alternative to the Cymenes Production from Scrap Tire Rubber Using Heteropolyacid Catalysts

The catalytic reforming of pyrolytic oil comes from scrap tires rubber (STR) was studied using as catalyst three different heteropolyacids (HPA) supported on commercial silica (CARiACT Q-10); H3PW12O40 (HPW), H3PMo12O40 (HPMo) and H4PMo11VO40 (HPMoV). A specify attention was given to influence of acidity of catalysts (i.e. strength and acid site types i.e. Lewis and/or Bronsted) on the concentration of cymenes in the pyrolytic oil. The catalytic performance was evaluated in a fixed bed reactor at laboratory-scale equipped with two independent thermal zones; in the first, the STR pyrolysis takes place to produce volatile compounds which, consequently, were transformed in a catalytic zone. The resulting pyrolysis oil in each test was collected in two cooling traps and characterized by GC-MS and GC-FID for the identification and quantification of all aromatic compounds. The results showed a decrease on the pyrolytic oil yield when HPA based catalysts were used, which seems to be promoted by an higher number of Bronsted sites. It has reached an increase in the aromatic concentration, compared to the test without catalyst, of up to 140% for the Molybdenum-based catalysts and close to 89% for the Tungsten-based catalyst. The major compounds obtained were cymenes and its production was closely associated to the type and strength of acid sites. Thus, the highest amount of cymenes (mainly p-cymene) was obtained with the highest number of Lewis sites and the highest ratio of Lewis/Bronsted acid sites, i.e. Molybdenum-based catalysts. On the contrary, the lowest yields were obtained when the Tungsten-based catalyst was used associated to its higher number of Bronsted acid sites and its strong acidity that promote the cracking reactions.

Study of the alkali metal poisoning resistance of a Co-modified Mn/Ni foam catalyst in low-temperature flue gas SCR DeNO x

A Co–Mn/Ni foam catalyst and alkali metal K-poisoned catalyst were prepared by an impregnation method using foamed Ni metal as a carrier. Mn was the active component, and Co was a trace auxiliary. The effects of different levels of alkali metal K poisoning and different concentrations of Co auxiliaries on the K poisoning resistance of the catalyst were studied. The results showed that the number of Bronsted acid sites on the surface of the catalyst, the reduction capacity and the number of unsaturated Ni atoms decreased after the addition of K. The interaction between the active Mn and the metallic Ni decreased, and the activity of the catalyst decreased. As the addition amount of added K increased, the catalyst alkali metal poisoning phenomenon became more significant. The kinetic studies showed that the activation energy increased after the addition of K, whereas the activation energy of K0.3Co0.5/Fresh catalyst decreased after Co modification. The amounts of Bronsted acid and Lewis acid sites on the surface of the K-poisoned catalyst were improved dramatically by Co modification of the Mn/Ni foam catalyst. K destroys the extremely important Bronsted acid sites that adsorb and activate NH3, reducing the performance to less than half. Additionally, NH3-TPD and in situ DRIFT analyses showed that Co, K, Mn and Ni synergistically improved the acid sites. The number of Bronsted acid sites and Lewis acid sites on the catalyst surface were greatly increased in the K0.3Co0.5/Fresh catalyst, whose reduction performance increased substantially compared with the K0.3/Fresh catalyst.

Understanding the Active Sites of Ag/Zeolites and Deactivation Mechanism of Ethylene Catalytic Oxidation at Room Temperature

Microporous Ag/zeolite (ZSM-5, Beta, Y, and Mordenite) catalysts were found to be potentially good catalytic materials for ethylene oxidation at room temperature. These catalysts were evaluated under both dry and humid atmospheres, achieving 100% conversion of 100 ppm of ethylene mineralized into CO2 at 25 °C. Moreover, the zeolite framework type and relative humidity had a significant effect on catalytic stability. Pyridine Fourier transform infrared spectra (FTIR) and solid-state hydrogen-1 (1H) magic angle spinning nuclear magnetic resonance (MAS NMR) studies revealed that Bronsted acid sites were the active sites in Ag/zeolites; the deactivated Ag/zeolites had no available Bronsted acid sites. When the number of Bronsted acid sites of Ag/ZSM-5-humid and Ag/Beta-humid were reduced, the catalytic activities of Ag/ZSM-5-humid and Ag/Beta-humid decreased. Temperature-programmed oxidation mass spectrometry (TPO-MS) and water adsorption/desorption results indicated that H2O adsorbed on Bronsted acid sites l...

Determination of Acid Site Location in Dealuminated MCM-68 by 27Al MQMAS NMR and FT-IR Spectroscopy with Probe Molecules

A series of MCM-68 zeolites with different Si/Al ratios were prepared by treatment with nitric acid and compared with beta zeolites. Speciation of aluminum and location of acid sites changed depending on the Si/Al ratio. The location of Bronsted acid sites in MCM-68 samples was able to be classified by FT-IR measurements with pyridine and 2,6-di-tert-butylpyridine as probe molecules, and the number of Bronsted acid sites was quantified according to the locations. For high-aluminum MCM-68, Bronsted acid sites were broadly distributed in both the 12-ring channel and 10-ring windows as well as inside the supercage. The Bronsted acid sites in the 12-ring channel were easily removed by the acid treatment, and consequently, high-silica MCM-68 had Bronsted acid sites predominantly in the 10-ring windows and inside the supercage. 27Al MQMAS NMR spectra of high-silica MCM-68 showed two specific peaks assignable to T6 and T7 sites, which did not face the 12-ring channel, forming the Bronsted acid sites highly toler...

Cadmium Modified HZSM-5: A Highly Efficient Catalyst for Selective Transformation of Methanol to Aromatics

Cadmium modified HZSM-5 catalyst (Cd/ZSM-5) prepared by incipient wetness impregnation shows markedly high selectivity of aromatics and low selectivity of light alkanes and C5–C9 nonaromatics in the methanol-to-aromatics (MTA) transformation. The majority of the introduced Cd cations can exchange with protons of the bridged hydroxyl groups of HZSM-5, leading to a reduced number of Bronsted acid sites while generating new Lewis acid sites on the catalysts. The selectivity of aromatics with the Cd/ZSM-5 is found to be linearly correlated with the concentration of the strong Lewis acid sites. Compared to Zn/ZSM-5, the most extensively investigated catalyst thus far for the MTA, Cd/ZSM-5 is found to be more effective in promoting the aromatics formation. The different catalytic behavior observed for Cd/ZSM-5 and Zn/ZSM-5 catalysts is most likely due to the difference in ionic radius of Cd and Zn.

Direct activation of CH4 to oxygenates and unsaturated hydrocarbons using N2O on Fe-modified zeolites

Direct activation of CH4 to oxygenates and unsaturated light hydrocarbons was investigated using Fe-modified ZSM-5 and Ferrierite (FER) for a partial oxidation of CH4 with N2O oxidant. The amount of active α-oxygen sites and number of Bronsted acid sites on the Fe-modified zeolites were well correlated with CH4 conversion rate and product distributions. The amount of α-oxygen sites was largely changed according to preparation method such as wet impregnation or ion-exchange of iron precursor and types of zeolites. A large number of Bronsted acid sites and α-oxygen sites on the Fe-modified FER revealed a higher oxygenate formation such as methanol and dimethyl ether (DME) with COx, and a larger number of strong acid sites on Fe-modified ZSM-5 was also responsible for a higher selectivity to light hydrocarbons by a successive dehydration of oxygenates formed. The different catalytic performances were verified through proper measurements of the amount and type of acidic sites as well as the α-oxygen sites measured by N2O pulse chemisorption. The Fe-modified FER prepared by impregnation method possessed a larger amount of α-oxygen sites due to abundant Bronsted acid sites, which was responsible for a higher rate of CH4 conversion to oxygenates with the help of N2O decomposition on the α-oxygen sites originated from iron oxide nanoparticles.

Evaluation of the acidic properties of the B2O3–Al2O3 and Pt/B2O3–Al2O3 systems by spin probe EPR spectroscopy and their correlation with the occurrence of the joint hydroisomerization of heptane and benzene

The acidity of a series of borate-containing aluminum oxide samples with the concentrations of B2O3 from 0.9 to 27.5 wt % and Pt-containing catalysts obtained on their basis was studied by spin probe EPR spectroscopy. With the use of anthracene and the TEMPO nitroxide radical as probe molecules, it was established that an increase in the B2O3 content of the B2O3–Al2O3 and Pt/B2O3–Al2O3 systems leads to a symbatic increase in the number of medium-strength Bronsted acid sites (BASs). In this case, the yields of target hydroisomerization products (isoheptanes and methylcyclopentane) correlate with the number of BASs.

Effect of Fe on the activity of Au/FeO x -TiO2 catalysts for CO oxidation

A thorough characterisation of Au/TiO2 and Au/FeOx-TiO2 catalysts was conducted in order to get a better understanding of the effect of Fe on the Au/TiO2 catalysts and link it to differences observed in their activities for CO oxidation. Several techniques including HRTEM, temperature-programmed reduction (TPR), UV-Vis and temperature-programmed desorption (TPD) of isopropyl amine, CO and CO2 were used to characterise the catalysts. Au/FeOx-TiO2 (300 °C), which was found to be the most active catalyst for CO oxidation, had the highest number of Bronsted acid sites, although its concentration of Lewis acid sites was similar to all other tested catalyst systems. X-ray photoelectron spectroscopy (XPS) revealed that the Fe species on the Au/FeOx-TiO2 catalyst is Fe2+ with a very small amount of Fe3+. Fe2+ is comprised of both FeO and Fe3O4 species, and according to TPR, the ratio (FeO/Fe3O4) between these two species increases with increasing calcination temperature. The presence of Fe on the Au/TiO2 catalyst seems to stabilise the Au nanoparticles from agglomeration. The activation energy of desorption (Ed) of CO from Au/FeOx-TiO2 (300 °C) was 82.7 kJ/mol, whilst on Au/TiO2, the Ed for CO was 108.8 kJ/mol. On both catalysts, the Ed for CO2 was ca. 99 kJ/mol.

Synthesis of N-methylaniline over molecular sieve catalysts

The preparation of N-methylaniline in two various processes: alkylation of aniline with methanol and hydroalkylation of nitrobenzene with methanol has been studied. Bifunctional molecular sieves containing metal copper as the hydrogenating component and zeolites BEA, MOR, MFI, FAU (Y) and mesoporous material MCM-41 as an alkylating component were used as catalysts. It has been shown that the modification of samples with copper nitrate increases the number of Lewis acid sites and decreases the number of Bronsted acid sites as copper embeds into cation exchange positions. It has been determined that the catalyst modification with copper increases the activity and enhances the selectivity towards N-alkylated products.