Concentration Of Bronsted Acid Sites Research Articles

Continuous-flow reductive etherification of furfural over CuAlOx catalyst combined with HZSM-5-Al2O3 composite

Furfuryl alkyl ethers (FAEs) are considered attractive biofuels or gasoline additives. Reductive etherification of bio-based furfural (FF) is a promising method for production of these compounds. In present work, we study the catalytic behavior of Cu-Al mixed oxide in combination with HZSM-5-Al2O3 composites for the syntheses of FAEs in a flow reactor using H2 as a reducing agent. The Cu-based catalyst ensures the hydrogenation of FF to furfuryl alcohol, which then reacted with alcohol in the presence of the acid catalyst. It was found that the reaction of FF with 2-propanol over catalytic system including HZSM-5(Si/Al = 40)-Al2O3 leads to the formation of 2-(isopropoxymethyl)furan (IMF) in excellent yield due to the optimal concentration of strong Bronsted acid sites in the composite. Lower selectivity to IMF was observed in the presence of HZSM-5 with Si/Al ratio of 15, 25 and 140. In addition, the use of primary alcohols instead of 2-PrOH gives a significantly lower yield of FAEs.

Supported Ionic Liquid Catalysts for the Oxidation of S- and N-Containing Compounds—The Effect of Bronsted Sites and Heteropolyacid Concentration

In this article, a series of effective catalysts based on betaine and sulfuric or phosphomolybdic acids was obtained. These compositions were characterized by various physicochemical methods and tested in the oxidation of sulfur- and nitrogenous-containing compounds by H2O2. An increase in the amount of heteropolyacid (HPA) leads to a non-linear change in acidity, and the degree of removal of sulfur-containing compounds correlates with the concentration of Bronsted acid sites on the surface. On the contrary, the degree of pyridine removal is determined primarily by the content of heteropolyacids in the catalyst.

Structure–performance correlations in the hybrid oxide-supported copper–zinc SAPO-34 catalysts for direct synthesis of dimethyl ether from CO2

Growing CO2 emissions lead to global warming, which is currently one of the most challenging environmental phenomena. Direct catalytic hydrogenation to dimethyl ether over hybrid catalysts enables CO2 utilization, hydrogen and energy storage and produces sustainable fuels and an important platform molecule. In this paper, we evaluated structure–performance correlations in the bifunctional hybrid copper–zinc SAPO-34 catalysts for direct synthesis of dimethyl ether via CO2 prepared using zirconia, alumina and ceria used as oxide carriers. Higher copper dispersion and higher CO2 conversion rate were uncovered over the alumina and zirconia supported catalysts followed by ceria supported counterpart. The CO2 hydrogenation seems to be principally favoured by higher copper dispersion and to a lesser extent depends on the concentration of Bronsted acid sites in the studied catalysts. Because of lower reverse water gas-shift activity, the alumina supported catalyst exhibited a higher dimethyl ether yield compared to the zirconia and ceria supported counterparts.

Total Oxidation of Toluene and Propane over Supported Co3O4 Catalysts: Effect of Structure/Acidity of MWW Zeolite and Cobalt Loading

A set of supported Co3O4 catalysts have been designed and prepared to study the effect of textural characteristics and Brønsted acid sites concentration of MWW zeolite support, as well as cobalt loading on catalyst activity. Detailed characterization of the catalysts with a thorough study on their performance in the total oxidation of toluene and propane revealed that MCM-22 is the optimal support and that increasing Si/Al and decreasing external surface of MCM-22 positively affect the activity of supported Co3O4 catalysts, which is determined by their low-temperature reducibility. The activity of the Co/MCM-22 catalysts increased with cobalt content (5-20 wt %), consistent with enhancing the amount of low-temperature reducible Co3O4. The optimized catalyst containing 20% Co supported on dealuminated MCM-22 presented high turnover frequency (TOF) values in both toluene (2.6 × 10-5 s-1 at 270 °C) and propane (3.9 × 10-5 s-1 at 215 °C) oxidation and was characterized by outstanding cycling stability, long-term durability, water tolerance, and sintering resistance.

Alkylation of Toluene with Isopropanol on Lantanum Modified ZSM-5 Zeolite

The effect of lanthanum concentration on the physicochemical and catalytic properties of HZSM-5 zeolite in a reaction of toluene alkylation with isopropanol was studied in a temperature range of 250–350°C. Based on the data obtained by X-ray diffraction analysis, low-temperature nitrogen adsorption (BET method), and IR spectroscopy, it was shown that an increase in lanthanum concentration in HZSM-5 from 1.0 wt % to 7.0 wt % reduces both the specific surface area and the pore volume of the zeolite despite the retention of its crystalline structure. This also results in redistribution of acid sites, specifically decreases the concentration of strong Bronsted acid sites (B) and increases that of moderate Lewis acid sites (L), thus decreasing the B/L ratio from 3.53 to 0.20. All these occurrences have a crucial effect on the selectivity of reaction products and, in particular, on the p-isopropyl toluene (4-IPT) selectivity. Maximum selectivity, making up 72.4%, is achieved by a zeolite containing 5.0 wt % lanthanum at a B/L ratio of 0.25.

Acid-Base Properties of Cobalt Ferrite Surface Examined by Different Physicoсhemical Methods

The acid-base properties of cobalt ferrite synthesized by the mechanochemical method with different ratios of the main components Fe2O3 : Co3O4 have been investigated using the temperature-programmed desorption of ammonia, IR spectroscopy using adsorbed pyridine with ammonia, and the Hammett indicator method. A comparison of the results for the total content of Lewis and Bronsted acid sites by three methods has shown that all three samples were quite similar, with a probability of 0.5%. The introduction of cobalt oxide into the composition of cobalt ferrite leads to an increase in the concentration of Bronsted and Lewis centers with a simultaneous increase in the number of strong basic centers determined by the temperature-programmed desorption of ammonia. The indicator method is the most informative method for a quantitative study of the acid-base properties of the cobalt ferrite surface, since it allows one to assess the concentration and strength of Lewis and Bronsted acid sites and basic sites.

Carbonylation of dimethyl ether on ferrierite zeolite: Effects of crystallinity to coke distribution and deactivation

Abstract Different distributions and locations of coke precursors on the highly crystalline seed-derived ferrierite (FER) altered the catalytic stability during carbonylation of dimethyl ether (DME) in a gas-phase. The larger amounts of heavy cokes on external Bronsted acidic sites of the FER were responsible for a fast deactivation, and their distributions on the active internal Bronsted acid sites such as 8-membered ring (8-MR) channels were largely affected by their crystallinity. Although the surface cokes on the FERs were inevitably formed, the FER having an optimal Bronsted acid site concentration in its 8-MR channels which were formed by recrystallization of Lewis acidic extra-framework Al species during a seed-assisted OSDA-free synthesis step, revealed slower deactivation rate at the proper seed content in the range of 7–15 wt%. The slow deactivation phenomena on the seed-derived crystalline FERs, especially on the optimal FER(S15) and FER(S7) having the proper amount of crystallization sites were originated by minimizing the coke depositions on the external surfaces.

Surface Acidity, Adsorption Capacity, and Photocatalytic Activity of SiO2 Supported on TiO2 Nanotubes for Rhodamine B Degradation

The surface acidity, adsorption capacity, and photocatalytic performance of nanotubes (NTs) superficially modified with SiO2 were analyzed. The NTs were prepared by the hydrothermal method. Photocatalysts with 1, 3 and 5 wt% of SiO2 were used to promote the adsorption and efficient removal of rhodamine B (Rh B) from water. The photocatalysts were characterized by SBET, XRD, HR-TEM, Zeta potential, FT-IR surface acidity, Raman, and UV–Vis spectroscopies. It was found that the addition of 3 wt% of SiO2 improved remarkably the photocatalytic performance and adsorptive properties of the titania nanotubes. The loading of 3 wt% of SiO2 enhanced the surface area and promoted the formation of Bronsted acid sites on the surface of the titania nanotubes. The adsorption of OH groups increased as the concentration of Bronsted acid sites increased. The OH groups triggered the formation of ·OH radicals, which promoted the oxidation pathway reaction of Rh B to CO2 and water. It was found that the Bronsted acid sites along with the electronic properties of the semiconductor materials played a major role in the photocatalytic process.

Zeolites interactions with microwaves during methane non-oxidative coupling

Microwave-zeolite interactions during direct non-oxidative reactions of methane were studied using three H-ZSM-5 zeolites with different concentrations of Bronsted acid sites. The formation of C2 and aromatic hydrocarbons increased with increasing surface acidity. When compared to conventional heating, the microwave reaction produced less carbon with higher thermal stability. The increase in diffraction shifts and the decrease in surface area of microwave-reacted zeolite suggested that the carbon started depositing inside the micropores rather than on the external surfaces. Inhomogeneous carbon deposits with filamentous carbon whiskers were observed under microwave compared to homogeneous spherical carbon deposits on the conventionally reacted sample. The dielectric properties and Raman analysis suggested that the graphitic nature and the thermal stability of deposited carbon increased with zeolite surface acidity under microwave. We believe that this is a very important study for designing microwave active zeolite-based catalysts with a low affinity towards carbon formation.

Effects of nanosheet catalysts on synthesis of aromatics and light hydrocarbons from acetylene

Abstract A nanosheet zeolite ITQ-2 formed by delamination of MCM-22 zeolite precursor was used and tested as a catalyst for the aromatization of acetylene. Along with MCM-22 (calcined form of MCM-22 zeolite precursor) zeolites, the resulting materials were successfully prepared and characterized in terms of pore structure and surface property. The performance of aromatization of ITQ-2 zeolites was comparable to that of MCM-22 zeolites. Due to the isolated and thin-layer structure, ITQ-2 zeolites had a significant fraction of external active sites. This feature seemed to result in relatively higher amount of C3–C8 aliphatic compounds rather than the C6–C8 aromatic compounds because of the shortened residence and, thus, reaction time. We first found that the cumulative C3–C8 aliphatic products in the case of ITQ-2 zeolites increased with the amounts of external surface area, while no clear relation was observed for the case of MCM-22 zeolites. Second, the initial selectivity towards aromatics was found to clearly show a linear relation with the concentration of Bronsted acid sites regardless of the used catalyst. In addition, the nanosheet ITQ-2 zeolites showed a much slower deactivation unlike the traditional microporous MCM-22 zeolites. Finally, to investigate the difference in the catalytic performance, we rebuilt the reaction pathways from acetylene to the aromatics and aliphatics.

Untangling the role of the organosilane functional groups in the synthesis of hierarchical ZSM-5 zeolite by crystallization of silanized protozeolitic units

Abstract Crystallization of hierarchical ZSM-5 zeolite has been performed under hydrothermal conditions from silanized protozeolitic units. The role of amine and phenyl groups present in the organosilanes, as well as the use of dipodal silanization agents, on the creation of mesoporosity in the zeolite samples has been investigated. The presence of amine groups and dipodal moieties in the organosilane increases their interactions with the protozeolitic nanounits in the gel, which results in hierarchical zeolites showing enhanced secondary porosity and a higher degree of modification of the textural properties. Nevertheless, dipodal organosilanes hinder strongly the aggregation of the nanounits, leading to non-completely crystalline materials. In contrast, the combination of phenyl and amine groups into a single organosilane has shown to be effective for the generation of more uniform mesopores. Thus, N-(2-N-Benzylaminoethyl)-3-aminopropyltrimethoxysilane (Ph-2A) has been found as the best silanization agent, affording a hierarchical zeolite with a narrow mesopore size distribution and a high concentration of strong Bronsted acid sites. Moreover, during LDPE catalytic cracking in TGA tests, the latter material allowed the conversion of the polymer to occur in a narrow temperature range as a consequence of its more uniform acid and textural properties. Likewise, this sample exhibited a high activity in LDPE cracking at 340 °C using a laboratory scale reaction system to produce mainly gasoline-range hydrocarbons and light olefins.

High activity of Ga-containing nanosponge MTW zeolites in acylation of p-xylene

Abstract Aluminosilicate and gallosilicate MTW zeolites were prepared in the bulk and nanosponge form by direct hydrothermal synthesis. Designed materials were tested in acylation of p-xylene with benzoyl chloride and compared with large pore zeolite *BEA and medium pore zeolite MFI. Nanosponge MTW exhibited generally better performance than bulk MTW zeolites due to higher accessibility of the catalyst´s active sites in nanosponge zeolite. Selective transformation of benzoyl chloride towards target 2,5-dimethylbenzophenone was achievable over nanosponge MTW zeolites. The gallosilicate nanosponge MTW exhibited significantly higher conversion (61%) of benzoyl chloride than its aluminosilicate counterpart (27%) with the same concentration of Bronsted acid sites (0.04 mmol/g). This result is clearly caused by the suitable strength of acid centres in Ga-MTW, which enables the sufficient activation of BzCl molecules as well as efficient desorption of formed polar products. Additionally, the gallosilicate MTW outperformed both conventional zeolites *BEA and MFI used as reference materials.

Procedure for the synthesis of AlSBA-15 with high aluminium content: Characterization and catalytic activity

Abstract In this study, AlSBA-15 materials with low nSi/nAl ratio were hydrothermally prepared by using three different acids, i.e. HCl, HNO3 and H2SO4, aiming to investigate the effect of the synthesis environment on their physicochemical properties. The resulting aluminium substituted molecular sieves were used as a supports for Pt catalysts. The AlSBA-15 materials and the Pt/AlSBA-15 catalysts were characterised by different techniques (N2 sorption, XRD, Py–FTIR, 27Al MAS NMR, CO chemisorption, TEM and SEM) and tested in n-heptane hydroconversion. It was found that the crystallisation of AlSBA-15 structure under the use of HNO3 led to the formation of AlSBA-15 with the highest amount of Al atoms in tetrahedral coordination. Depending on the AlSBA-15 preparation conditions, the Pt/AlSBA-15 catalysts displayed different physicochemical features, especially in term of acidity. As a consequence of the highest substitution of Al in tetrahedral positions in AlSBA-15 sample synthesised under the use of HNO3, the corresponding Pt catalyst exhibited the highest Bronsted acid sites concentration. The results of n-heptane hydroconversion showed that, this catalyst provided the highest yield of the most desired high-octane number products, i.e. multibranched heptane isomers. At the same time it was less selective towards hydrocarbons displaying the boiling temperatures higher than this of n-heptane (they are produced in cyclisation and aromatization) which are the precursors of carbonaceous deposits.

Influence of Localization of Acid Sites on Deactivation of Zeolite MFI in Oligomerization Process of Light Alkenes

A series of samples of zeolite MFI with the same morphology, but different concentrations of Bronsted acid sites, have been obtained by means of ion exchange. Methods of temperature-programmed desorption of ammonia (TPD NH3) and IR spectroscopy of absorbed 2,6-di-tert-butylpyridine have demonstrated that the samples differ in the ratio of acid sites in the bulk of zeolite and on the external surface. Stability of the samples against deactivation has been determined in the oligomerization reaction of butylenes using the accelerated deactivation test under severe conditions. It has been shown that the main contribution to deactivation is made by acid sites located on the external surface of zeolite crystals.

Preparation of mesoporous Beta zeolite by fluoride treatment in liquid phase. Textural, acid and catalytic properties

Abstract Beta zeolite (nominal Si/Al = 19) has been treated with HF/NH4F solutions under different HF concentration, temperature and reaction time. It has been found that the crystallinity of the resulting materials is >80% for HF concentration up to 0.5 M, but it drops below this value for the most severe condition (HF = 1 M and 40 °C). The treatment dissolves selectively aluminum, in such a way that the Si/Al increases to 34 for the most attacked material. As a consequence of these treatments, the mesopore volume increases by up to 80% respect to the parent Beta for samples retaining high crystallinity, corresponding to newly created mesopores in the range of 5–20 nm. Detailed STEM-HAADF studies reveal that the chemical attack of the Beta crystals by the acidic fluoride solution progresses slowly along the crystals, leaving behind highly corroded and porous “sponge-like” crystals where the structure of the zeolite is nevertheless retained. Pyridine adsorption/desorption experiments show that a fraction of the extraframework Al present in the starting Beta is reinserted in framework sites, a process that would be mediated by aluminum-fluoride soluble species. As a consequence of this process, the Bronsted/Lewis ratio of the strong acid sites increases from 1 in the starting Beta to close to 3. The concentration of total and strong Bronsted acid sites associated to framework Al increases with the treatment. However, the activity of the samples in the isomerization/disproportionation of m-xylene is lower than that of the parent Beta zeolite, a result that could be due to the higher B/L ratio.

Generation of hierarchical porosity in beta zeolite by post-synthesis treatment with the cetyltrimethylammonium cationic surfactant under alkaline conditions

Abstract Commercial Beta zeolite (Si/Al = 19) was treated with aqueous solutions of hexadecyltrimethylammonium (CTA) in the presence of NaOH or NH4OH to generate hierarchical porosity in the material. The combination of a basic media and surfactant (CTA) allows generation of mesoporosity while preserving the crystallinity and acidity of the parent Beta zeolite. In addition, characterization by XRD and TEM gave no evidence for the formation of segregated mesoporous phases. TEM and N2 adsorption isotherms suggest the presence of substantial intracrystalline mesoporosity (average pore size of 3 nm) in the Beta zeolite crystals, which was more pronounced for samples treated with CTA/NaOH solutions. The concentration of total Bronsted acid sites was determined by pyridine adsorption, and it increased in the treated samples as compared with the parent Beta zeolite. The strongest Bronsted acid sites slightly increased for the CTA/NH4OH sample, while they decreased when ammonium hydroxide was replaced by NaOH. All these results indicate that the basic character (i.e. strength) of the aqueous solution affects the degree of the zeolite mesostructuring process. The activity of the treated samples in the isomerization-disproportionation (I-D) of m-xylene was related to the concentration of the strongest Bronsted acid sites. The treatment with sodium hydroxide or ammonia aqueous solutions led to an I/D ratio lower than that of the parent Beta zeolite, evidencing the benefits of the additional mesoporosity by enhancing the formation of the bulkiest bimolecular intermediate required to form disproportionation products.

Direct Measurement of Zeolite Brønsted Acidity by FTIR Spectroscopy: Solid-State 1H MAS NMR Approach for Reliable Determination of the Integrated Molar Absorption Coefficients

Fourier transform infrared (FTIR) spectroscopy is broadly applied nowadays for probing the concentration and strength of acid sites in zeolite catalysts. Accuracy of direct determination of the quantity of different hydroxyl groups by FTIR method suffers from uncertainty of the integrated molar absorption coefficients, e. The values of e reported by different authors might differ by an order of magnitude. This paper provides an approach for reliable determination of the integrated molar absorption coefficients by combining 1H magic-angle spinning (MAS) NMR and FTIR spectroscopy techniques. The concentration of Bronsted acid sites for the series of H-ZSM-5 and H-ZSM-23 zeolite samples with different Si/Al ratio has been reliably established with 1H MAS NMR using methane and benzene as internal standards adsorbed on the studied samples. The data on the obtained concentration were further used to analyze same zeolite samples with FTIR spectroscopy and derive the information on the values of the integrated mo...

Concretes Using Modified Mineral Extenders

The possibility of modification of mineral extenders with a specific surface of 350 m2/kg with ultraviolet irradiation is considered. Infrared spectroscopy confirmed that under the influence of ultraviolet irradiation, the bonds of OH-groups with SiO2 tetrahedra are weakened, that contributes to the formation of extenders of the acid centers of Lewis and Bronsted on the surface. The optimum time of ultraviolet exposure is determined, at which the concentration of active Lewis and Bronsted acid sites on the extenders surface increases by more than 100%. It was found that replacing 10% of cement mass with a modified extenders makes it possible to produce concretes with strength values 20-30% higher than for composites without extenders. Obtaining equal strength concrete is possible with replacing 20% of cement with modified extenders.

Biomass catalytic fast pyrolysis over hierarchical ZSM-5 and Beta zeolites modified with Mg and Zn oxides

Hierarchical ZSM-5 and Beta zeolites, loaded with MgO and ZnO, have been explored for the catalytic fast-pyrolysis of eucalyptus woodchips. These materials exhibit a high dispersion of the MgO or ZnO phases, which is probably due to the presence of a hierarchical porosity with both micro- and mesopores in the zeolitic supports. The incorporation of these metal oxides led to a significant reduction in the textural properties and to changes in the acidic properties of the parent zeolites. Thus, a decrease in the concentration of Bronsted acid sites was observed, which was accompanied by the generation of additional Lewis acid sites with medium strength. In addition, the incorporation of metal oxide promotes the formation of significant amount of basic sites, especially for the samples loaded with MgO. Catalytic fast pyrolysis experiments of eucalyptus woodchips were performed in a fixed bed reactor at 500 °C and atmospheric pressure under a nitrogen flow. In comparison with non-catalytic fast pyrolysis, the use of zeolitic catalysts caused a decrease in the bio-oil* (water free basis bio-oil) production due to enhanced formation of gases, as well as coke deposition on the catalyst. However, the quality of the bio-oil* was enhanced since the catalysts were able to decrease its oxygen content. In this way, h-ZSM-5-based catalysts showed a clearly deeper deoxygenation degree compared to those having h-Beta as support, with very low content of anhydro sugars and the formation of a significant amount of aromatics. Regarding the effect of the metal oxide phase, MgO-loaded samples provided bio-oil* with enhanced energy yields and lower oxygen content, probably due to the adequate balance of Lewis acid and basic sites. Likewise, significant differences were observed among the catalysts regarding the deoxygenation pathways and the compounds families present in the bio-oil*.

Dimerization of Bicyclo[2.2.1.]hepta-2,5-diene Over Various Zeolite Catalysts

The activities of HY, Hbeta, HZSM-5 and mesoporous Al-MCM-41 catalysts were evaluated during the dimerization of bicyclo[2.2.1]hepta-2,5-diene (norbornadiene, NBD). This study focuses on the effects of the pore structure and acidity of the catalysts on the NBD dimerization reaction. When conducting the NBD dimerization reaction over the HY, Hbeta, HZSM-5 and Al-MCM-41 catalysts, it was confirmed that NBD dimer (NBDD) synthesis could be facilitated at a temperature of 250 °C, whereas NBD scarcely reacts through a thermal reaction without a catalyst. Although the NBD conversion rates of the four catalysts are nearly identical after a 12-h reaction, the HY zeolites showed much higher selectivity to NBDD than those with the Hbeta, HZSM-5 and Al-MCM-41 catalysts. The HY catalyst showed the highest yield, reaching 41.5 wt% after a 12-h reaction. The higher selectivity of the HY catalyst during the dimerization of NBD could be explained in terms of its appropriate pore structure and by the higher concentration of Bronsted acid sites. The present study reveals that the HY zeolite is a potential heterogeneous catalyst for NBDD synthesis through the dimerization of NBD at a high yield.