Strength Of Bronsted Acid Sites Research Articles

A density functional theory study on the mechanism of Dimethyl ether carbonylation over heteropolyacids catalyst

Dimethyl ether (DME) carbonylation is considered as a key step for a promising route to produce ethanol from syngas. Heteropolyacids (HPAs) are proved to be efficient catalysts for DME carbonylation. In this work, the reaction mechanism of DME carbonylation was studied theoretically by using density functional theory calculations on two typical HPA models (HPW, HSiW). The whole process consists of three stages: DME dissociative adsorption, insertion of CO into methoxyl group and formation of product methyl acetate. The activation barriers of all possible elementary steps, especially two possible paths for CO insertion were calculated to obtain the most favorable reaction mechanism and rate-limiting step. Furthermore, the effect of the acid strength of Bronsted acid sites on reactivity was studied by comparing the activation barriers over HPW and HSiW with different acid strength, which was determined by calculating the deprotonation energy, Mulliken population analyses and adsorption energies of pyridine.

Acetone Conversion to Isobutylene over Magnesium-Containing Micro-Mesoporous MOR Zeolites

Isobutylene synthesis from acetone in the presence of micro–mesoporous MOR zeolites obtained by hydrothermal recrystallization and modified with magnesium acetate by incipient wetness impregnation has been studied. A kinetic study of the reaction has been conducted, and product formation routes have been determined. It has been found that an increase in the mesopore content leads to an increase in the catalyst activity owing to the removal of diffusion restrictions. It has been shown that isobutylene formation occurs on both Lewis and Bronsted acid sites. The modification of the initial aluminosilicates with magnesium has been found to decrease the number and strength of Bronsted acid sites and thereby lead to a decrease in the number of side reactions and an increase in the time-on-stream stability of the catalysts.

Tailoring selectivity in the liquid-phase isomerization of α-pinene on dealuminated ferrierite-type zeolites

Abstract Ferrierite-type zeolite (Zeolyst Intern.) with Si/Al = 10 was dealuminated sequentially using solutions of hydrochloric acid of different concentrations. The course of dealumination was followed by XRD, XPS, sorption of nitrogen, scanning electron microscopy, FTIR, and solid-state 29Si, 27Al and 1H MAS NMR. The dealuminated samples retained high crystallinity and revealed good sorption properties. The tetrahedrally coordinated aluminium in ferrierite was distributed between TB sites located in the main channels and TA ones siting in hidden positions. It was shown that dealumination removed aluminium from ferrierite in a non-random manner; up to 60% of aluminium was removed from TB and up to 19-% from TA sites. The amount and strength of Bronsted acid sites decreased with dealumination. The samples were tested in the liquid-phase isomerization α-pinene at 348–363 K; camphene and limonene were the main products. The initial reaction rates and turnover frequencies have been estimated and compared to other zeolite catalysts. The selectivity to camphene and limonene was in the range of 77–98.9 %.

Room-Temperature Ethene Hydrogenation Activity of Transition-Metal-Free HY Zeolites

The conversion of C2H4 in the absence and presence of H2 was examined over dealuminated HY30, HY15, and HY2.6 zeolites with different Si/Al ratios under ambient conditions (1 atm, 25 °C). Only in the presence of H2 did these materials catalyze the conversion of C2H4 into C2H6 with measurable rates and no sign of C2H4 dimerization. The C2H4 hydrogenation activity of these materials increases with the Si/Al ratio, implying that in this case, the strength of Bronsted acid sites (i.e., Si–OH–Al) is an important factor for the hydrogenation process. The adsorption of both C2H4 and H2 over Bronsted acid sites was calculated to be exothermic without activating the H2 molecule, while the conversion of C2H4 into ethoxy species is slightly endothermic. Heterolytic dissociation of H2 is strongly unfavorable over Al/O or Si/O sites of the zeolite framework, but it is exothermic over the extraframework Al species, indicating that cooperation of Bronsted acid sites with extraframework Al species is indeed required for ...

Synthesis, Characterization and Catalytic Activity of Nanocrystalline Ce2(MoO4)3/SiO2 as a Novel Catalyst for the Selective Production of Anhydrous Formaldehyde from Methanol

In this paper, a series of catalysts of (0.5–10 wt%) Ce2(MoO4)3 supported on SiO2 gel were synthesized via a precipitation assisted impregnation method. The original and calcined catalysts were extensively characterized by TG-DTA, TEM, XRD, FTIR, N2 sorption analysis and acidity measurements. The catalytic activity successfully tested for the direct dehydrogenation of methanol into formaldehyde. Results revealed that, the complete conversion of methanol, with selectively of 100%, to formaldehyde was achieved at 350 °C and about 95% conversion obtained at 325 °C. In addition, a strong correlation between the catalytic activity and acidity was observed. Moreover, the moderate strength of Bronsted acid sites created on the surface of catalyst plays the main role in the production of formaldehyde. Finally, the catalyst exhibited a unique stability towards anhydrous formaldehyde formation up to a duration time of 200 h.

Gas-Phase Carbonylation of Dimethoxymethane to Methyl Methoxyacetate on Solid Acids: The Effect of Acidity on the Catalytic Activity

The gas-phase carbonylation reaction of dimethoxymethane (DMM) to methyl methoxyacetate on different solid acids was studied. It was established that this reaction was accompanied by the occurrence of a side reaction of DMM disproportionation into dimethyl ether and methyl formate. It was shown that the activity of solid acids in both of the reactions depends on the strength of Bronsted acid sites according to an equation like the Bronsted–Evans–Polanyi–Semenov correlations.

Determining the Strength of Brønsted Acid Sites for Hydrodewaxing over Shape-Selective Catalysts

This paper reports a method for directly determining the effective strength of those acid sites on a molecular sieve catalyst that are actually involved in reactions such as hydrodewaxing. The method is here applied to the conversion of a waxy paraffin over a catalyst containing platinum and a SAPO-11 sieve of high isomerization selectivity, as well as a catalyst containing platinum and a H-ZSM-5 zeolite (SiO2/Al2O3 = 150) of high cracking selectivity. Results are compared against the measured Bronsted acid site strength on these molecular sieves determined from temperature-programmed desorption of ammonia and Fourier transform infrared analysis of adsorbed pyridine. Characterization of acid sites showed both concentration and apparent acid strength for H-ZSM-5 higher than those for SAPO-11, with the latter having a finite but smaller amount of strong acid sites that adsorbed pyridine at 573 and 673 K. Reactions with n-hexadecane gave significantly higher conversion rates on H-ZSM-5, and it is shown that ...

Characteristics of catalysts for enhanced green production of distillates and chemicals in bio-oil from catalytic dehydration of bio-ethanol

The dehydration of ethanol to gasoline and C6–C8 hydrocarbons has been made possible by using a HZSM-5 zeolite due to its proper acid property and shape selectivity. HZSM-5 had been also used as a support of phosphorous- (P), antimony- (Sb), and bismuth- (Bi) oxides for ethanol dehydration, which mainly gave high gasoline in oil. Since the moderate pore size of HZSM-5 limits the production of heavier biofuels in the distillate ranges (namely, kerosene and gas oil), therefore, in this work, the zeolites, HY and HBeta, with a larger pore size were employed, aiming to produce larger hydrocarbons. HZSM-5 was also used for comparison. Moreover, Group 5A oxides (P-, Sb-, and Bi oxides) were also doped onto the zeolites at 5 wt.% of elemental loading, expectedly to further enhance the production of distillate-range products. The results from the experiments carried out at 500 °C showed that, regardless of the type of loaded oxides, the distillate production was primarily governed by the channel opening size of zeolites, but the yield also depended on other parameters such as acid strength, and the complexity of the pore channel. HZSM-5 tended to produce high contents of large hydrocarbons ( $${\text{C}}_{9}^{ + }$$ ) whereas HBeta and HY tended to produce high amounts of C6–C8 hydrocarbons, especially benzene, and toluene. HBeta and HY gave high selectivity of p-xylene in mixed xylenes. Furthermore, oxygenates were produced in a trace amount using the parent zeolites, but their formation was significantly enhanced with using oxide-promoted catalysts, especially when HY was used as the support. In addition, Group 5A oxides doped on HBeta and HY can increase the oil yield. The content of distillates in bio-oil was also more greatly enhanced by Sb x O y and Bi x O y than by P x O y , promoted on HBeta and HY, possibly due to the promoter loading that created new stronger Bronsted acid sites. Generally speaking, high production of distillates in bio-oil preferably requires a support that possesses high strength of Bronsted acid sites in a large channel opening with low tortuosity of channel structure. Furthermore, the selected promoter should offer a new type of acid sites on the catalyst.

Esterification of acetic acid with methanol to methyl acetate on Pd-modified zeolites: effect of Brønsted acid site strength on activity

The effect of the amount of Bronsted acid sites with their strengths on bare and palladium-modified H-ZSM5 and H-Ferrierite on the catalytic activity in the esterification of acetic acid with methanol was investigated by comparing the variation of Bronsted acid site strength. The strong acid sites on zeolites are responsible for the low reaction rate of esterification and low selectivity to methyl acetate by the strong adsorption properties of reactants. The stronger Bronsted acid site strengths, which are more abundant on bare H-Ferrierite than H-ZSM5, were properly modified with Pd promoter without significant loss of total acid sites. The esterification rates were largely increased with a concomitant increase of methyl acetate selectivity after Pd modification. The strongly adsorbed methoxy species by forming surface dimeric alcohols can be attributed to the suppressed rate of esterification. The roles of Bronsted acid sites and their strengths to catalytic activity were verified through NH3-TPD and adsorbed pyridine or methanol through FT-IR analyses.

IR absorption intensities as a new spectral criterion for the activation of adsorbed molecules in heterogeneous acid catalysis

The criterion conventionally used to judge bond activation in molecules adsorbed on active sites of heterogeneous catalysts is the bathochromic shift of the corresponding IR absorption bands. The intensity of these bands, which characterizes bond polariability, can be used as an additional bond activation criterion. This new spectral criterion is particularly promising for acid and acid-base catalysis, in which the activation of adsorbed molecules is due to their polarization by active sites. Examples are provided here to illustrate the fruitfulness of the new approach. These examples include judging the strength of Bronsted acid sites from the intensity of the OH stretching band and an analysis of the dissociative adsorption and dehydrogenation of light paraffins on metal cation forms of zeolites and of proton transfer from Bronsted acid sites to adsorbed paraffin molecules.

Synthesis and Characterization of Ta2O5−ZrO2Systems: Structure, Surface Acidity, and Catalytic Properties.

The preparation of binary materials based on Ta2O5−ZrO2 by a sol−gel method and with a wide range of compositions is reported (Ta2O5 nominal content from 4 to 74% w/w). Materials were characterized by X-ray diffraction, and infrared and Raman spectroscopy. Their acidic characteristics were evaluated from the model reaction of 2-propanol dehydration and the adsorption of CO at 77 K and 2,6-dimethylpyridine at 373 K, both followed by infrared spectroscopy. Increasing tantalum content brings about a development of a surface TaOx phase followed by a progressive formation of aggregates of amorphous Ta2O5. Increased stabilization of tetragonal zirconia was also observed with increasing Ta loading. The surface area, number and strength of Bronsted acid sites, and the catalytic activity in the 2-propanol dehydration increase with the increase in tantalum content up to ca. 50% w/w Ta2O5. A surface area of 103 m2 g-1 was obtained for a solid with a Ta2O5 content of 53% w/w after calcination at 873 K. The abundance ...

Acid properties of semicrystalline zeolitic mesoporous UL-ZSM-5 materials

UL-ZSM-5 materials have been prepared by templated solid-state crystallization of zeolites starting from the amorphous mesostructured aluminosilicate Al-Meso. Microcalorimetry and FTIR have been employed to characterize their surface acidity. In good agreement with 27Al MAS NMR data, UL-ZSM-5 displayed an improved density and strength of Bronsted acid sites, as compared to Al-Meso, owing to the incorporation of aluminium in a tetrahedral environment similar to that of zeolite ZSM-5. Moreover, they showed an enhanced Bronsted/Lewis relative acid ratio. However, Al-Meso showed the highest concentration of strong Lewis acid sites due to its largest amount of aluminium in extraframework positions.

Zeolites-catalyzed Alkylation of Isobutane with 2-butene: Influence of Acidic Properties

The influence of acidity properties of zeolites on the alkylation of isobutane with 2-butene was studied using ultrastable Y (USY) zeolites. Most zeolites possessed almost comparable hydrogen transfer activity except for USY-30, which has the lowest acid site density. On the other hand, the strength of acid sites increased with degrees of dealumination. During the alkylation, a maximum in catalytic activity for certain Si/Al ratios were observed. This behavior is believed to be a result of a balanced level of acidic properties between the concentration and strength of Bronsted acid sites. It can prohibit the formation of heavy carbonaceous materials that deactivate the zeolites.

Surface Acidity and Basicity of La2O3, LaOCl, and LaCl3 Characterized by IR Spectroscopy, TPD, and DFT Calculations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Surface Acidity and Basicity of La2O3, LaOCl, and LaCl3 Characterized by IR Spectroscopy, TPD, and DFT Calculations

Adsorption sites of La2O3, LaOCl, and LaCl3 catalysts were characterized with probe molecules using infrared spectroscopy, temperature-programmed desorption (TPD), and density-functional theory (DFT) calculations. Surface acid sites were probed with CO, pyridine, and 2,6-dimethylpyridine (DMP), and basic sites were probed with CO2. Shifts of the CO vibrational frequency at low surface coverage at 77 K suggest that the strength of Lewis acid sites increases with the concentration of Cl in the material; i.e., La2O3 < LaOCl < LaCl3. DFT estimates for CO adsorption energies and LUMO energies were consistent with this ranking. On the basis of a downward shift of the surface OH stretching bands interacting with CO, pyridine, and DMP spectra at room temperature (RT) and TPD results, and confirmed by DFT calculations, the strength of Bronsted acid sites was concluded to increase in the same order. Additional DFT calculations with a frequency analysis were used to elucidate CO2 adsorption modes. DFT calculations a...

Role of mordenite acid properties in silver cluster stabilization

Abstract In the present work, the role of SiO 2 /Al 2 O 3 molar ratio in silver cluster stabilization inside mordenite pores was studied. It was found that SiO 2 /Al 2 O 3 molar ratio regulates silver cluster stabilization by means of change of mordenite acid properties. The high concentration and high strength of Bronsted acid sites favor the stabilization of silver in the form of clusters inside the pores. In contrast, low concentration and low strength of Bronsted acid sites and high concentration of Lewis sites inside the channels can hinder stabilization of silver clusters. In this case, large particles on the external surface of zeolite are preferably formed. It was revealed that silver cluster introduction in its turn influences acid properties of mordenites, changing the concentration and structure of acid sites. As a result of silver incorporation, four types of Lewis acid sites presented in H-mordenites disappear and a new type of Lewis acid site appears. For the majority of samples, it is accompanied by enhancement of concentration of Lewis sites so that all Al becomes ‘IR visible’.

Modification of mordenite acidity by isomorphous substitution of trivalent cations in the framework sites using the atom-planting method

Aluminum, gallium and antimony cations were incorporated into the framework of highly siliceous mordenite to modify the acid strength of Bronsted acid sites by the ‘atom-planting method’ with the corresponding metal chloride vapors at elevated temperatures. The framework incorporation of metal cations was confirmed by IR, MAS NMR and catalytic reactions. The optimum temperature to achieve a maximum incorporation of metal cations into the mordenite framework was demonstrated to be 873 K. Tetrahedral Ga cations exhibited an IR band owing to acidic bridging Si(OH)Ga groups at 3620cm −1 and a characteristic band at 159 ppm in the 71Ga MAS NMR spectrum. The IR band owing to bridging Si(OH)Sb groups was observed at 3663 cm −1. The acid strength of Si(OH)Me (Me = Al, Ga and Sb) groups estimated from their IR frequency, IR spectra of adsorbed pyridine and the activity for toluene disproportionation and cumene alkylation with 2-propanol was in the order Si(OH)Al>Si(OH)Ga> > Si(OH)Sb. The weakest acid sites, Si(OH)Sb, inactive for the toluene disproportionation, showed a significant activity for the cumene alkylation with higher para-selectivity.

Quantum-Chemical Justification of the Zeolite Acid Strength Measurement by Infrared Spectroscopy

The quantitative measurement of the strength of Bronsted acid sites in zeolites would be very useful in catalyst development. One possible method is the measurement of the shift in the stretching frequency of Bronsted hydroxyls on adsorption of a weak base. An empirical correlation for deprotonation energy versus infrared shift has been suggested in the literature, but it cannot be directly experimentally tested. We have performed quantum-chemical calculations of the variation of the infrared shift with Bronsted acid strength. The three probe bases considered are as follows: ethene, carbon monoxide, and nitrogen. The calculations were carried out at the MP2 level with a basis set close to 6-31G** and with explicit consideration of anharmonicity of the OH stretching potential. The results show that the experimental correlation for carbon monoxide gives both absolute acid site deprotonation energies and differences between them to within 10%, supporting the quantitative use of this correlation.

Acidity study of Nb-containing MCM-41 mesoporous materials. Comparison with that of Al-MCM-41

The acidity of Nb-MCM-41 molecular sieves as well as their hydrogen forms was studied using FTIR spectroscopy with pyridine and 2,6-dimethylpyridine as probe molecules and in test reactions (isopropanol and cumene decomposition). The number of Lewis acid sites (LAS) and Bronsted acid sites (BAS) was estimated from the intensity of IR bands at 1450 and 1549 cm-1 respectively. The acidity of niobium-containing materials was compared with that of Al related molecular sieves. The parent Al- and Nb-MCM-41 mesoporous materials studied in this work did not show Bronsted acidity which only appeared after ammonium exchange followed by calcination. The number and the strength of BAS was much higher on H,Al-containing material than that on H,Nb-containing molecular sieves. H,Nb-MCM-41 exhibited a higher concentration of LAS than H,Al related material.

AlPO4–Al2O3catalysts with low alumina content. Part IV.—Effect of fluoride ion addition on texture, surface acidity and catalytic performance in cyclohexene and cumene conversions

Fluoride-modified (2.5 wt.% F–) AlPO4–Al2O3(APAl–A; 5–25 wt.% Al2O3) catalysts have been prepared and compared for their catalytic activity/selectivity in cyclohexene conversion and cumene cracking. The characterization of catalysts modified with F– was performed using X-ray diffraction (XRD), 27Al and 31P NMR and diffuse reflectance FTIR (DRIFT) spectroscopies and N2 adsorption methods, and by the measurement of the surface acid character by chemisorption of pyridine (PY) and 2,6-dimethylpyridine (DMPY). Fluoride addition shifted the transition of AlPO4 from an amorphous to a crystalline (α-cristobalite) phase at a lower temperature. Fluoride-treated APAl-A catalysts contained both tetrahedral and octahedral aluminium (their ratio varied with anion type) while phosphorus always remained in P(OAl)4 environments. Moreover, two types of Al surface hydroxy groups at 3786 and 3738 cm–1, and only one type of P surface hydroxy group at 3670 cm–1, are found in the DRIFT spectra of the APAl-A catalysts. The DRIFT measurements also revealed that fluoride ion substitutes hydroxy groups on tetrahedrally coordinated Al and P centres in the surface. The PY and DMPY adsorption (pulse chemisorption and DRIFT spectroscopy) indicates that the addition of fluoride ion appreciably modifies the surface acidity of the AlPO4–Al2O3 catalyst by increasing the number and strength of Bronsted acid sites. Furthermore, the addition of fluoride increases remarkably the activity for both processes and, for cyclohexene conversion, in contrast to pure AlPO4–Al2O3 catalysts, where skeletal isomerization is the only catalysed reaction, bimolecular hydrogen transfer also takes place. The hydrogen transfer/skeletal isomerization ratio decreases as the Al2O3 loading increases although with all APAI-A-FM (fluoride-modified) catalysts the skeletal isomerization always predominates. The catalytic activities were correlated with both the acidity and the acid strength of the catalyst. Fluoride addition transformed a low activity cracking/dehydrogenation catalyst to one selective for cracking and exhibiting much greater catalytic activity.