Strong Bronsted Acid Sites Research Articles

Preparation and characterization of SO4 2−/TiO2 and S2O8 2−/TiO2 catalysts

Nanosized solid superacids SO4 2−/TiO2 and S2O8 2−/TiO2, as well as MCM-41-supported SO4 2−/ZrO2, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N2 adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO4 2−/TiO2 and S2O8 2−/TiO2 possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm2/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO4 2−/ZrO2/MCM-41, SO4 2−/TiO2 and S2O8 2−/TiO2 exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO4 2−/TiO2 promoted with H2SO4, while Lewis acid sites on S2O8 2−/TiO2 even stronger than those on SO4 2−/ZrO2/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO4 2−/TiO2 and S2O8 2−/TiO2 exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO4 2−/ZrO2/MCM-41. It can be concluded that the existence of more Brønsted acid sites was favorable for proton participation in the cyclization reaction.

Mesoporous aluminosilicate catalysts from FAU precursor under mild acidic conditions and with Al in totally tetrahedral coordination

Al-SBA-15 mesoporous catalyst with strong Bronsted acid sites and Al stabilized in a totally tetrahedral coordination was synthesized from the addition of hydrothermally aged zeolite Y precursor to SBA-15 synthesis mixture under mildly acidic condition of pH 5.5. The materials possessed surface areas between 690 and 850 m2/g, pore sizes ranging from 5.6 to 7.5 nm and pore volumes up to 1.03 cm3, which were comparable to parent SBA-15 synthesized under similar conditions. As much as 2 wt.% Al was present␣in the most aluminated sample that was investigated, and the Al remained stable in totally tetrahedral coordination, even after calcination at 550 °C. Calcined Al-SBA-15 showed high hydrothermal stability when treated with steam (20% v/v in nitrogen) at 650 °C for 2 h. Textural characteristics are maintained on steam treatment, and very little or no conversion of Al from tetrahedral to octahedral coordination resulted. The Al-SBA-15 mesoporous catalyst showed significant catalytic activity for cumene dealkylation, and activity increased as the amount of zeolite precursor added to the SBA-15 mixture was increased. The catalyst’s activity was not affected by the aging time of the precursor for up to the 24 h aging time investigated. This method of introducing Al and maintaining it in a total tetrahedral coordination is very effective, in comparison to other direct and post synthesis alumination methods reported.

Mechanism for the reduction of NOx on Rh/sulfated titanias

Titanias of different surface areas have been sulfated and used as supports of Rh oxide for the selective catalytic reduction of nitrogen oxides. Only the sulfation of TiO2 of large surface areas gives strong Bronsted acid sites, retaining pyridine up to 773 K. Low surface area anatase is unable to retain sulfates. The catalytic activities measured at 523 K, increase with the number of acid sites, and then reach a plateau, showing the intervention of acidity in the SCR. This is true only for Rh but not for Pt. The investigation of the elemental steps on Rh/sulfated TiO2 by in situ diffuse reflectance spectroscopy permits to clarify a few points: the oxidation of propene, presumably to acetaldehyde, occurs by the reaction with nitrates adsorbed on the support. The further oxidation of this intermediate by NO2 yields an isocyanate, which can be hydrolysed to ammonia.

Dehydration of Ethanol into Ethylene over Solid Acid Catalysts

The dehydration of ethanol into ethylene was investigated over various solid acid catalysts, such as zeolites and silica–alumina, at temperatures ranging 453–573 K under atmospheric pressure. Ethylene was produced via diethyl ether during the dehydration process. H-mordenites were the most active for the dehydration. It was suggested that the catalyst activity could be correlated with the number of strong Bronsted acid sites in the catalyst. Further, the H-mordenite was more stable with a SiO2/Al2O3 ratio of 90 than with a SiO2/Al2O3 ratio of 20.

Multitechnique Characterization of Coke Produced during Commercial Resid FCC Operation

The characterization of industrial coked resid fluid catalytic cracking (RFCC) catalysts is reported. The aim is to provide insight into the coke deposition on commercial resid fluid catalytic cracking catalysts sampled after the stripper of a commercial RFCC unit and to relate it to the potential process chemistry. Physicochemical techniques were used to characterize the used catalysts and the deposited coke. 95% of the coke was insoluble in CH2Cl2. This coke was located in the mesopores of the catalyst matrix. The results suggest the existence of domains of polyaromatic and heterogeneously distributed coke, in which saturated hydrocarbons are trapped. IR spectroscopy of adsorbed pyridine shows that the largest fraction of strong Bronsted acid sites is free after the catalyst has passed the stripper. The results indicate that for RFCC not the local deactivation of the acid sites but rather blocking of domains of the catalyst is the most important mode of deactivation after passing through the riser reactor.

Kinetic Study of n-Butane Isomerization over Pt−H-Mordenite

The kinetics of n-butane isomerization over bifunctional Pt−H-mordenite was studied by varying reactant partial pressure and temperature. The main products were isobutane, propane, and pentanes. Yields of other products, especially methane, ethane, ethene, and propene, were very low, which suggests that protolytic cracking and hydrogenolysis were present only to a minor extent. Especially the protolytic cracking was inhibited, since after introduction of Pt into H-mordenite all strong Bronsted acid sites disappeared and the total density of acid sites decreased. The reaction rate showed complex dependence on the reactant partial pressure, suggesting that two reaction mechanisms on the acid sites, i.e., monomolecular or bimolecular, for isobutane formation are present and the prevailing mechanism depends on the reaction conditions. Three kinetic models were developed based on the current understanding of the reaction mechanisms including hydrogenation and dehydrogenation steps on the metal sites, skeletal ...

Common Mechanistic Aspects of Liquid and Solid Acid Catalyzed Alkylation of Isobutane with N‐butene

AbstractFor Abstract see ChemInform Abstract in Full Text.

Effects of binder, coking and regeneration on acid properties of H-mordenite during TDP reaction

The effects of binder, coking and regeneration on the acid properties of H-mordenite zeolite during toluene disproportionation reaction (TDP) have been investigated by solid-state 31P-MAS-NMR of various adsorbed phosphorous probe molecules in conjunction with elemental analysis by ICP-MS technique. A series of fresh, spent and regenerated mordenite-based commercial catalysts were examined and the results were also compared with binder-free H-mordenite zeolite and unformulated γ-alumina binder. It is found that parent H-mordenite zeolite possessed only Bronsted acidity, which is responsible for the observed catalytic activity. In contrast, the γ-Al2O3 binder exhibited only Lewis acidity and plays a minor role during the catalytic reaction. While the amount of strong Bronsted acid sites decreased rapidly during initial coking, it reached a plateau at a total coke content of ca. 7 wt%, corresponding to ca. 80% decrease in total acidity. That the catalyst remained active even under deep coke deposition (>7 wt%) condition indicated catalytic activity may be invoked by subsequent coking taking place on the external surface rather than intracrystalline channels of the zeolite catalyst. Furthermore, upon catalyst regeneration treatment, ca. 75% of the total acidity could be effectively recovered.

Low‐Temperature Modification of Mesoporous MCM‐41 Material with Sublimated Aluminum Chloride in Vacuum.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Remarkable Improvement on the Methane Aromatization Reaction: A Highly Selective and Coking-Resistant Catalyst

An effective way (without addition of any oxidative gases to the reactant) to suppress coke formation (by modification of the Mo/HZSM-5 catalyst) and thus to significantly improve the aromatic selectivity in the methane aromatization reaction is presented in this study. A greater yield of benzene and an enhanced durability of the catalyst when compared with the conventional Mo/HZSM-5 catalysts are observed. Multinuclear solid-state NMR, TPD, TPO, TG, and UV-Raman methods are applied to characterize and rationalize the structure-property relationship of the improved catalytic performance of the modified catalysts. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane aromatization reaction, while the superfluous strong Bronsted acid sites, which can be removed upon steaming treatment, are shown to be related with the aromatic carbonaceous deposits on the catalysts.

Stable and Selective Dehydrocondensation of Methane Towards Benzene on Modified Mo/HMCM-22 Catalyst by the Dealumination Treatment

A modified Mo/HMCM-22 catalyst by the dealumination treatment (Mo/HMCM-22-D) exhibited remarkable performance for the catalytic dehydrocondensation of methane with a higher selectivity of benzene and a lower selectivity of coke, in comparison with the same Mo catalyst supported on parent HMCM-22 (Mo/HMCM-22). Excellent catalytic stability as well as a high benzene formation rate of 1500 nmol/(g-cat·s) was obtained on a 6%Mo/HMCM-22-D catalyst at 1023 K, 3 atm and 2700 ml/(g·h) owing to the efficient suppression of coke formation. Dealumination of the HMCM-22 zeolite was characterized by XRD, 27Al and 1H MAS NMR and NH3-TPD techniques. It was found that the dealumination treatment of HMCM-22 zeolite resulted in an effective suppression of acid sites, particularly the Bronsted acid sites (proton form in Al--O--Si) owing to the removal of tetrahedral framework aluminum, while the microporous structure and the zeolite framework remained unchanged. It was suggested that the stable and selective dehydrocondensation of methane towards benzene is based on the suppression of coke formation owing to the effective decrease of strong Bronsted acid sites by the dealumination treatment of the HMCM-22 zeolite.

Surface Acidic Properties of A HMCM-22 Zeolite: Collidine Poisoning and Hydrocarbon Adsorption Studies

The total and external surface acidic properties of a HMCM-22 (SiO2/Al2O3 = 33) zeolite have been studied by temperature-programmed ammonia desorption (TPAD) and temperature-programmed collidine desorption (TPCD), respectively. Ammonia TPD results indicate that the Bronsted acid sites in HMCM-22(33) have a relatively uniform acid strength, and most of them are strong. Collidine TPD shows that the Bronsted acid sites on the external surface accessible to collidine accounts for 12% of the total Bronsted acid sites. Strong Bronsted acid sites are present on the zeolite surface as indicated by the collidine desorption at high temperature. The two distinct desorption regions in the low- and high-temperature ranges shown in the collidine TPD curve are ascribed to collidine physisorption and chemisorption, respectively. Chemisorbed collidine on the zeolite surface does not influence the sorption rate of 3-methylpentane, indicating that it does not block 10-ring pore mouth openings. This means that collidine can ...

Hydroconversion of a Model Mixture and Fluid Catalytic Cracking Gasoline for Octane Enhancement. Main Reaction Pathways over Monofunctional HZSM5(x)−Alumina Catalysts

The changes in RON and liquid yield during the hydroconversion of an n-heptane−benzene−toluene mixture were evaluated in a continuous high-pressure flow reactor using HZSM5(x)−alumina catalysts with variable contents of zeolite. The results from these experiments were compared with those obtained with a real fluid catalytic cracking (FCC) hydrotreated gasoline as the feedstock. An increase in the zeolite concentration altered the acid properties of the catalysts that showed a gradual increase in the intensity of the 3612 cm-1 IR band, associated with the internal strong Bronsted acid sites of the zeolite. The hydroconversion results show that increasing the zeolite content in the catalyst leads to a gradual increase in both the RON number of the liquid product and the amount of light hydrocarbons (≤C4), promoted mainly by secondary cracking and dealkylation reactions. Because of this opposite trend of RON and liquid yield, zeolite contents higher than 10 wt % led to an almost constant barrel-octane (RON ×...

Benzene alkylation with 1-dodecene over H-mordenite zeolite

The alkylation of benzene with 1-dodecene was studied over solid-acid zeolites, such as H-mordenite, H-USY, HY, and H-ZSM-5, in a batch reactor at 140°C and 10 atm. H-mordenite showed the highest selectivity of 78.2% for 2-phenyldodecane with 100% conversion. Depending on the catalyst amount (from 0.5 to 1.0 g) and the molar ratio of benzene to 1-dodecene (from 7 to 10), the conversion of 1-dodecene varied in the range from 63.8 to 100%. Furthermore, modification of H-mordenite by dealumination using nitric acid and by solid ion exchange with Mg2+ and Fe3+ ions led to the improvement of the selectivity for 2-phenyldodecane, and simultaneously reduction of the conversion of 1-dodecene is observed. In addition, it was found by pyridine adsorption FT-IR study that both strong Bronsted and Lewis acid sites are closely related to the conversion.

NMR Spectroscopy Applied to Zeolite Catalysis: Progress and Prospects

Recent advances in NMR spectroscopy methods for studying zeolites and their catalyzed reactions are briefly reviewed. In-situ NMR spectroscopy, with its controlled atmosphere conditions, is used to characterize the active surface sites of zeolites (highly acidic micro-porous molecular sieves) and to elucidate the hydrocarbon reactions they catalyze. The strong Bronsted acid sites of these catalysts, SiO(H)Al, are accessible to adsorbed molecules via well defined pore systems (0.3 to 1.2 nm). Substantial progress has been made in: obtaining high resolution NMR spectra under conditions (pressure, temperature, reagent flow) similar to real reactor conditions, developing new NMR methods for characterizing quadrupolar nuclei in solids, understanding acidity in solids and evidencing the details (intermediates, pathways, kinetics) of zeolite catalyzed chemical reactions. Some of the practical aspects and limitations in performing these studies are mentioned along with future prospects. Given its high versatility and adeptness for determining: the local structure around selected nuclei (2H, 17O, 27Al, 29Si), and the structure and dynamics of organic molecules (1H, 13C), NMR spectroscopy is destined to attain evermore important and far-reaching application in this field of study.

Catalytic Properties of ZrO2 Systems in the Reduction of Nitrogen Oxides by Hydrocarbons

The catalytic properties of zirconia prepared by different methods with impregnated variable-valence metals (chromium, cobalt, and cerium) as well as rhodium-promoted samples were studied in the reduction of nitrogen oxides by C1 and C3–C4 hydrocarbons and also in the presence of sulfur dioxide. TPDA and IR spectroscopy showed that the different catalytic behavior of the MexOy/ZrO2 samples is related to their acid properties. The more active samples had strong Bronsted acid sites.

A study of the influence of the synthesis conditions upon the catalytic properties of silicoaluminophosphate molecular sieves

The skeletal isomerization of 1‐butene was performed over a series of silicoaluminophosphate molecular sieves with AEL structure (SAPO‐11). The results were compared with those obtained over an aluminophosphate molecular sieve (AlPO4‐11). The three SAPO‐11 samples, with different acidic properties, were synthesized by either varying the chemical composition of the synthesis gel or by varying the preparation time of the aforementioned gel. The catalytic results indicate that irrespective of how the acidity of the SAPO‐11 samples is changed (viz., independently of the method elected to modify the acidic properties of the samples), a close parallelism between the selectivity towards the skeletal isomerization and the number of (moderate + strong) Bronsted acid sites (sites retaining pyridine above 623 K) was observed for the SAPO‐11 solids. These results, definitively, indicate the participation of these acid sites in the skeletal isomerization process.

Laser-Induced Fluorescence of Base Adsorbed on Surface Sites in Solid Catalysts

The interaction of quinoline with the surface of Nafion-H and Nafion-H (13 wt%)/silica gel composites, silica gel itself, HY zeolite, dealuminated or not, and two types of alumina is studied by laser-induced fluorescence. Strong Bronsted acid sites interact with quinoline to form the quinolinum, that ion being revealed by a band peaking at 390 nm. When Lewis acid sites are present, a complex formed between these sites and quinoline is observed. A dimeric species and the diffusion of the adsorbates on the surface of the Nafion-H polymer are discussed.

Structure, acidity and catalytic properties of dealuminated SAPO-11 molecular sieves

A SAPO-11 molecular sieve was dealuminated in H 4EDTA solution to different Al content. Dealuminated samples were characterized by catalytic transformation of butene, o-xylene and cumene, IR analysis of adsorbed pyridine and 31 P -, 29 Si -MAS-NMR. Dealumination resulted in formation of strong Bronsted acid sites around Si domains. Amorphous AlPO 4 formed by deep dealumination of SAPO-11, led to a decrease of Bronsted acidity. The Lewis acidity was reduced by dealumination. The acidity of dealuminated SAPO-11 was measured and used to elucidate the catalytic properties. Butene isomerization was proceeded on Bronsted acid sites of medium strength. The selectivity of toluene disproportionation was favored on Lewis acid sites by low temperature and space velocity. Dealumination of SAPO-11 samples improved cumene cracking, especially at high temperatures. The reaction kinetics and mechanism were discussed.

Mechanism of formic acid decomposition on P−Mo heteropolyacid

Hydrated (undecomposed) form of heteropolyacid H3PMo12O40/SiO2 exhibits a higher activity in the formic acid decomposition than the corresponding dehydrated sample. The formic acid decomposition takes place on strong Bronsted acid sites of the heteropolyacid.Ab initio SCF MO LCAO method was used for the calculation of the electronic state of two surface complexes of HCOOH molecule (S1 and S2) coordinated to a proton H+. The S1 complex is formed by proton addition to the carbonyl oxygen, whereas the S2 complex is formed proton addition to the oxygen atom of the C−O−H fragment of HCOOH. The selective weakening of the C−O bond and localization of the positive charge on the (O=C−H) fragment in the protonated complex S2 are favorable for the decomposition of formic acid to CO and H2O.