Bronsted Sites Research Articles

ChemInform Abstract: Synthesis, Characterization, and Catalytic Properties of Clean and Oxygen-Modified Tungsten Carbides

Abstract High surface area WC and β-W 2 C powders (30–100 m 2 g −1 ) were prepared by direct isothermal carburization of WO 3 and W 2 N in CH 4 -H 2 mixtures. After surface cleaning with H 2 , their surfaces are equilibrated with bulk stoichiometric carbides and free of polymeric carbon; they chemisorb 0.2–0.4 monolayers of CO and H. These carbides catalyze neopentane hydrogenolysis with high selectivity. Chemisorbed oxygen also inhibits hydrogenolysis rates and introduces surface sites for neopentane isomerization, a reaction that occurs only on Pt, Ir, and Au metals. Chemisorbed oxygen also inhibits hydrogenolysis of n-hexane and n-heptane on tungsten carbides and introduces surface sites that lead to high isomerization selectivity (70–99%). Kinetic and isotopic tracer studies of n-heptane, 3,3 dimethylpentane, methylcyclohexane, propylene, and methanol reactions show that dehydrogenation reactions and methyl-shifts of unsaturated intermediates occur on oxygen-modified WC powders. Carbidic sites (WC x ) catalyze C-H activation reactions; chemisorbed oxygen titrates such WC x sites and introduces Bronsted acid surface sites (WO x ). Thus, these materials catalyze both dehydrogenation and carbenium-ion reactions, reflecting the bifunctional nature of oxygen-modified transition metal carbide surfaces.

Proton Transfer Mediated by Water: Experimental Evidence by Neutron Diffraction

Neutron Rietveld refinement of a synthetic low silica ferrierite (Si/Al = 8.5) in deuterium form rehydrated with D2O vapor (RD-FER) was performed in the Immm space group. Three Bronsted acid sites were identified. The first (D1) was on O4 framework oxygen, occupied to 18%; the second (D2) was on O6, occupied to 8%; the latter (D3) was on O1, occupied to 35%. This Bronsted site distribution strongly differs from that found in the nonrehydrated deuterium form (D-FER) in the same ferrierite sample (Martucci, A.; Alberti, A.; Cruciani, G.; Radaelli, P.; Ciambelli, P.; Rapacciuolo, M. Microporous Mesoporous Mater. 1999, 30, 95.), where D3 site is absent and D1 and D2 are occupied to 14 and 16%, respectively. The discrepancy between the Bronsted sites found in RD-FER and D-FER indicated remarkable acidic proton mobility and was explained by a proton transfer mechanism where water molecules or water clusters act as carriers of protons.

Influence of sulfation conditions on the acidic and catalytic properties of sulfated alumina in isobutane alkylation with butylenes and n-pentane isomerization

The influence of the sulfation parameters (the source and concentration of sulfate ions) and the calcination temperature on the acidic and catalytic properties of sulfated alumina in the alkylation of isobutane with butylenes and n-pentane isomerization was studied. IR spectroscopy of adsorbed probe molecules and temperature-programmed desorption of ammonia were used to characterize the acidic properties of the catalysts. An increase in the content of sulfate groups to the value corresponding to a formal value of the monolayer capacity increases the activity of alkylation and the concentration of strong Bronsted sites. The dependence of the stability of activity in alkylation on the sulfate group concentration is extreme with a maximum at the concentration close to the monolayer capacity. It was concluded from the IR spectroscopic data that the decrease in the stability of activity with the further increase in the content of sulfate groups is due to an increase in the concentration of strong Lewis sites and/or an increase in the surface density of strong Bronsted sites. The absence of the correlation between the catalytic behavior of sulfated alumina samples in n-pentane isomerization and acidity indicates that paraffin activation on these samples occurs via the non-acidic mechanism.

NO x Reduction by Ethanol on Pd/Sulphated Zirconia

The NO reduction by ethanol was studied on palladium catalyst supported on sulphated zirconia. Temperature programmed desorption of NO and ethanol (TPD) and temperature programmed surface reaction (TPSR) analyses as well as catalytic tests in reducing and oxidizing conditions (O2 presence), besides diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed the formation of intermediate species during the reaction, such as ethoxy species that reacted forming ethylene. Besides dehydrogenate formed adsorbed acetate species, which than decompose and/or react with hydroxyls of the support. The sulphated zirconia support increased the acid sites with the formation of strong Bronsted sites, favoring the formation of ethoxy species. Acetate species also react with NO adsorbed on Pd forming N2, N2O, CO and CO2. The excess of O2 favored ethanol oxidation to CO2, consequently less ethanol was available to react with NOx.

Comparing Al-SBA-15 Support and Pt/Al-SBA-15 Catalyst: Changes in Al Speciation and Acidic Properties Induced by the Introduction of Pt via Aqueous Medium

Differences between the acidic properties of silicoaluminic Pt-containing catalysts and those assessed on their parent supports have been reported in the literature and attributed to the presence of the metal nanoparticles and to their influence on the acid sites. It is shown here that for mesoporous materials containing various types of Al species, an alternative explanation can be proposed. 27Al NMR spectroscopy, FTIR of adsorbed CO and probe catalytic tests suggest the redistribution of aluminium atoms upon contact of the parent support with the aqueous solution containing the Pt precursor. Upon contact with water and thermal treatment, strong and mild Bronsted sites (Si–O(H)–Al) transform into strong Lewis sites (isolated tetracoordinated Al atoms). As a consequence, it may not be straightforward to deduce the acidic properties of metal-containing catalysts supported on Al-containing mesoporous materials from those of the bare support, because the surface species may differ significantly.

Nanostructured Al-based Fluoride-Oxide Materials with a Core-Shell Morphology

The investigation of the thermal behavior of β-AlF2.6OH0.4 compound which adopts the hexagonal tungsten bronze network shows that this structure is stable up to 873 K. As revealed by a detailed analysis combining high resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and 19F and 27Al MAS NMR, the sample annealed at 873 K exhibits an unusual core−shell like morphology. The shell which is amorphous contains some AlO6, AlO5, and AlO4 species which have been identified and quantified by 27Al NMR. Contrarily, the core part of the material is crystallized and has been identified as a phase closely related to the hydroxyl free compound β-AlF3 in agreement with the dehydroxylation process. The annealing treatment lowers the Lewis acidity, but strong Bronsted acid sites have been created as evidenced by pyridine adsorption. These strong Bronsted sites arise from the reaction between water and strongly under-coordinated species formed upon annealing. It should ...

Configuration of MFI-type zeolite interacting with the probe molecule P(CH3)3 – a theoretical study

With P(CH3)3 as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Bronsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH3)3]+ species can be stabilized by the first or second Bronsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH3)3]+ species from the first to the second Bronsted acidic site.

Hydrogen Storage in Chabazite Zeolite Frameworks.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mesoporous Molecular Sieves Modified with Carbonaceous Deposits

There were investigated aluminosilicate MCM-41 samples in the as-prepared form and those modified by the deposition of carbonaceous compounds during conversion of cyclohexene for 12 h. The amount of the deposits decreased with the rising reaction temperature and increased with the Al content of the samples. The cyclohexene conversion followed mainly two mechanisms: cyclohexene skeletal isomerization and hydrogen transfer. The products with 6 carbon atoms in a molecule prevailed in all cases. The process of conversion, proceeding on the Bronsted acid sites, resulted in formation of both coke deposits and volatile products. The creation of coke caused a decrease in the effective concentration of both the Bronsted and the Lewis acid sites. Thermodesorption of pyridine showed that (i) the concentration of these sites before and after the conversion differed only slightly and (ii) the acidic strength of the Bronsted sites was practically independent of their concentration and the sample Si/Al ratio. The chemical composition of the deposits was insignificantly affected by the Al content of the materials and depended mostly on the temperature and duration of the reaction. At relatively low temperatures, both aliphatic and aromatic compounds were formed, being rather weakly bound to the surface of the material. After a longer (55 h) reaction period, some deposits appeared that were strongly bound to the surface. Isotherms of adsorption of water, benzene, and nitrogen were determined, from which a mechanism of this process was derived. It included most probably multilayer adsorption at lower relative pressures, followed by capillary condensation. The sorption capacities of the uncoked samples for benzene and nitrogen were relatively high and independent of the sample Al content. In the case of water, however, an observed reduction in the sorption capacity with the increasing Al content suggested that clusters of the adsorbed molecules formed around the Al centers and caused partial clogging of the material pores. The deposited coke strongly decreased both the surface area and the sorption capacity of the materials.

Synthesis and Characterization of Pillared High Layer Charged Synthetic Saponite

Alumina pillared clays were prepared using synthetic saponites with different layer charges, and their cracking activities were characterized by cumene conversion reaction. It was found that the pillared saponite with super high layer charges (2.0) exhibited stable cracking activity in the thermal treatment temperature range from 500 to 700°C. The BET surface areas of the pillared saponites (calcined at 500°C for 2 h) with layer charges of 1.0, 1.5 and 2.0 were 360, 280 and 310 m2/g respectively, according to the N2 adsorption-desorption determinations at 77 K. A significant layer charge effect was found on the catalytic cracking activity of the pillared clays according to the model reaction, indicating that substitutions in the tetrahedral sheets played an important role in the development of surface Bronsted acidic sites of pillared clay. Because of strong Bronsted sites of pillared saponites, peaks centered at 432°C were found in the spectra of the temperature programmed desorption of ammonia. The structure of pillared saponite with 2.0 layer charges, being the best solid acid catalyst in this study, was also characterized by 27Al MAS NMR. A new peak centered at 1.9 ppm after pillaring was observed, which was ascribed mainly to the octahedral aluminum in the pillar.

A Periodic Structure Density Functional Theory Study of Propylene Chemisorption in Acidic Chabazite: Effect of Zeolite Structure Relaxation

Density functional theory (DFT) periodic structure calculations have been employed to investigate the interaction of propylene within the acidic chabazite zeolite (Si/Al = 11). In agreement with previous studies, it is found that secondary alkoxy formation is preferred over primary alkoxy formation. Steric constraints appear not to affect the course of the reaction. Analysis of the radial distribution of the zeolitic atoms with respect to the Bronsted site aluminum atom allows further insight into the reaction. Changes in the zeolite oxygen atom position are more significant than those of the zeolite silicon atoms. Relaxation of unit cell size and shape has a dramatic influence on energetic parameters of the reaction path.

The characterization of tungsten-oxide-modified zirconia supports for dual functional catalysis

The results of catalytic titration measurements indicated that WOx/ZrO2 catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WOx/ZrO2 contains ∼0.004 meq-H+/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Bronsted acid sites on the surface of these catalysts. The concentration of strong Bronsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Bronsted–Lewis site. Pt/WOx/ZrO2 undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WOx/ZrO2 (∼16% W) catalyst, consistent with the presence of bulk WO3 in this material as observed by TPR.

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

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

Acid Sites in HZSM-5 Upon Copper Exchange by FTIR and DSC Using Ammonia

The acidity of the parent HZSM-5 zeolite and Cu-HZSM-5 catalyst has been examined by FTIR and DSC using ammonia desorption. Sites of different strength were found in HZSM-5, desorbing ammonia at a relative temperature difference of about 100 °C. Upon copper exchange a fraction of Bronsted sites were transformed to Lewis sites, but the acid strength of the remaining Bronsted sites was increased. Aside from Lewis sites originating from copper exchange, there might be some additional sites formed from precipitated copper. This could explain the quantity of adsorbed NH3 on Cu-ZSM-5 which is higher than theoretically expected. While changing their nature, acid sites of higher strength keep their location, which is manifested by some diffusion effect towards ammonia.

NMR and Theoretical Study of Acidity Probes on Sulfated Zirconia Catalysts

The measurement of the type and number of acid sites on sulfated zirconia catalysts using the 31P NMR spectrum of adsorbed P(CH3)3 has been vexed by spectral assignment controversies. Using a combination of NMR experiments and theoretical methods, including chemical shift calculations at the GIAO-MP2 level, we show that a previously observed 31P resonance at +27 ppm is due to P(CH3)4+, formed in a reaction that consumes a Bronsted site. The coproduct of this reaction, PH(CH3)2, is protonated on the surface to yield a 31P resonance in the region expected for P(CH3)3 on a Lewis site. Further complications result from a signal due to OP(CH3)3, formed by oxidizing sites on the surface, complexed to unidentified acid sites. As an alternative, we show that carefully designed 15N experiments using the less reactive and less basic probe pyridine-15N provide more easily interpreted measurements of Bronsted and Lewis sites on sulfated zirconias of diverse composition, preparation, and treatment. Quantitative studie...

Acidic properties of sulfated zirconia

IR spectroscopy of adsorbed probe molecules (CO, pyridine) is used to characterize the acidic properties of sulfated zirconia derived from zirconium oxide and hydroxide. Their acidic properties are found to be similar. The strength of the Lewis and Bronsted site measured by the frequency shift of adsorbed CO is lower than that in zeolites. It is concluded that sulfated zirconia have no superacid Bronsted and Lewis sites. Bronsted sites capable of protonating pyridine vanish when calcining the catalysts at temperature above 773 K, but the strength and concentration of the Lewis acid sites (LAS) do not change.

Acetonitrile as probe molecule for an integrated 1H NMR and FTIR study of zeolitic Brønsted acidity: Interaction with zeolites H-ferrierite and H-beta

In order to investigate the potentialities of an integrated IR and 1H NMR study of the acidity of Bronsted sites, a spectroscopic study of the interaction of CD3CN (at different coverages) with zeolites H-ferrierite and H-beta is reported. The formation of H-bonded CD3CN adducts of different strength with silanols and with strong Bronsted sites is described. It is shown that an integrated spectroscopic study gives complementary information on the observed H-bond adducts. While the presence of isosbestic points in IR spectra allows a clear demonstration of the existence of equilibria between species, 1H NMR spectra give information about dynamic adsorbate exchange phenomena.

Introduction of Zn, Ga, and Fe into HZSM-5 Cavities by Sublimation: Identification of Acid Sites

M/ZSM-5 zeolites with M = Ga, Fe, or Zn were prepared by subliming volatile compounds onto HZSM-5. They were characterized by XRD, H2-TPR, CO-TPR, FTIR, adsorption of pyridine and acetonitrile-d3 and, in the case of Fe/ZSM-5, also by ESR spectroscopy. Immediately after sublimation, the concentration of Bronsted acid sites is low, but with Ga/ZSM-5 and Fe/ZSM-5, it rises again after hydrolysis. This dissipation of multipositive ions to monopositive ions, each located near the negative countercharge in the zeolite matrix, is favored by a considerable gain in Coulomb energy. In Ga/ZSM-5, some of the regenerated Bronsted sites are stronger than those in the original HZSM-5. In Fe/ZSM-5 and Zn/ZSM-5, such Bronsted sites are weaker, as evidenced by CD3CN adsorption. However strong Lewis acid sites are formed, they are characterized by NH3-TPD, and pyridine and CD3CN adsorption. Zeolites with a high Si/Al ratio contain lattice defects with internal silanol groups. These defects disappear when metal ions are adde...

Suppression of radical-cationic benzene oligomerization on sulfated zirconia

The effect of catalyst activation conditions on transformations of benzene cation radicals on sulfated zirconia has been studied by in situ ESR spectroscopy. After the catalyst activation at 300°C there was no oligomerization of the cation radicals to biphenyl and heavier products, which is observed after catalyst calcination at 500°C. It has been suggested that the oligomerization is suppressed by the presence of strong Bronsted sites on the surface.

Acid centers in sulfated, phosphated and/or aluminated zirconias

On sulfated ZrO2, the comparison of the effects of adsorbing water or ammonia on the infrared bands between 1400 and 1000 cm−1 suggests that besides structural Lewis sites on the surface of ZrO2, strong Lewis sites are made from chemisorbed SO3. Upon adsorption of water, SO3 is converted, partially, into a surface sulfated species which may act as strong Bronsted sites. At moderate surface hydration, both types of sites may coexist. The catalytic activity in the isomerization of isobutane is a function of the overall nominal surface density in SO4. The acid sites on the surface of phosphated mesoporous zirconia are attributable to surface P–OH groups working as weak Bronsted sites. On both sulfated and phosphated zirconia, surface coating of alumina stabilizes the porosity, but it does not modify the nature of their acid centers.