Broensted Acid Sites Research Articles

Estimating the efficiency of catalysts for catalytic pyrolysis of polyethylene

The paper is devoted to investigation of the catalytic pyrolysis of high-density polyethylene (PE) in the presence of HBEA, HZSM-5 and HFER catalysts and natural clay. Catalytic pyrolysis of plastic materials is a promising method for treatment of secondary raw materials because it allows converting polymers into chemical compounds, which further serve as a source for chemical industry. Physicochemical parameters of the catalysts were estimated using various methods: IR Fourier spectroscopy, X-ray diffraction analysis, physical adsorption of N2, thermogravimetric analysis, and pyrolytic gas chromatography. Temperature dependences of PE destruction were obtained as well as the dependence of chemical composition of the catalytic pyrolysis products on the catalyst type. Two main factors were shown to determine the efficiency of cracking and the qualitative composition of products – structural and acidic parameters of the catalyst. The presence of Broensted acid sites in zeolites promoted the cracking and aromatization reactions. The possibility of using the clay sample for thermal decomposition of PE was estimated.

SiO2- and Al2O3-Supported Sulfated Zirconiа Catalysts for Hexane Isomerization. Effect of the Support Nature

The effect of the support nature on the performance of hexane isomerization reaction was studied for the sulfated zirconiа catalysts supported on SiO2 and Al2O3, which have different textural characteristics. It was shown that a higher conversion of hexane is reached over sulfated zirconiа catalysts supported on aluminum oxides. IR spectroscopy of adsorbed CO revealed that the concentration of Broensted acid sites (BAS), which are characterized by adsorbed CO with the absorption band (a.b.) at 2170 cm–1, and strong Lewis acid sites (LAS), which are characterized by adsorbed CO with a.b. at 2210 and 2224 cm–1, is higher in the Al2O3-supported catalysts compared to the SiO2-supported ones. In the Al2O3-supported catalysts with different textural characteristics, an increase in the contribution of LAS to the total acidity essentially increases the yield of high-octane 2,2-dimethylbutane and the depth of hexane isomerization.

Simultaneous Isomerization of Hexane and Heptane in the Presence of Pt-Containing Tungstated Zirconia Catalysts

Isomerization of heptane and hexane and their mixtures in the presence of Pt/WO3-ZrO2 catalysts with the tungsten oxide content of 10–35 wt.% was studied. The best characteristics of isomerization of the С6–С7 alkanes mixture were observed on the catalysts with the WO3 content of 15–20 wt.%. According to ammonia thermodesorption data, the number of acid sites typically increases in this range of tungsten oxide content; according to IR spectroscopy of adsorbed CO, this occurs mostly with an increase in the number of Broensted acid sites.

Acidic and catalytic co-functionalization for tuning the sensitivity of sulfated tin oxide modified by ruthenium oxide to ammonia gas

This work is aimed at elaborating a route for co-modification of acidic and catalytic sites on the surface of nanocrystalline tin oxide with the aim to enhance the sensitivity to ammonia gas. Sulfated tin oxide with controlled amount 0–10 wt.% of SO42− as the acidic modifier was synthesized by wet chemical deposition. It was modified by the catalytic additive of ruthenium oxide (0–5 wt.% Ru). Surface acidity was evaluated by temperature-programmed desorption and in situ infrared spectroscopy of adsorbed ammonia. The concentration of Lewis and Broensted acid sites was strongly dependent on sulfate content. Increasing amount of ruthenium oxide depressed surface acidity of catalytically-modified materials. Sensor response to 2–100 ppm NH3 in air was measured depending on the materials composition. At Ru-loading lower 0.5 wt.% the sensor signal was detected at temperature 200–300 °C and its value was monotonously dependent on SO42− content. The highest ammonia sensitivity was detected at lower temperature of 140–150 °C for co-functionalized tin oxide with comparable contents of Ru and SO42− (1–3 wt.%). From the sensing tests and in situ infrared spectroscopy data it was deduced that the improved sensing of ammonia gas was provided by equivalent impacts from the processes of target molecules adsorption on sulfate-promoted acid sites and NH3 oxidation on the catalytic clusters of ruthenium oxide.

Hydrogenolysis of glycerol to propanediols over heteropolyacids promoted AgCu/Al2O3 catalysts

Various amounts and different types of heteropolyacids promoted 5Ag15Cu/Al2O3 catalysts were prepared by impregnation method and analyzed through many techniques. The synthesized catalysts were evaluated for hydrogenolysis of glycerol to propanediols. The results demonstrated that heteropolyacids loading facilitated the reduction, promoted the dispersion, enhanced the acidity, and increased Broensted acid sites of the AgCu catalysts, which benefited the generation of 1,3-propanediol. Compared with phosphotungstic acid and phosphomolybdic acid, silicotungstic acid promoted AgCu catalyst had a better performance for 1,3-propanediol due to the better Cu dispersion and higher Broensted acidity. In addition, when the reaction was performed at 220 °C under 3.5 MPa for 8 h, the chosen 5Ag15Cu-10HSiW/Al2O3 achieved a 69.6% glycerol conversion with 35.6% 1,2-propanediol selectivity and 21.5% 1,3-propanediol selectivity.

Continuous liquid phase acylation of toluene over HBEA zeolite: Solvent effects and origin of the deactivation

The continuous liquid phase Friedel–Crafts acylation of toluene (T) by acetic anhydride (AA) over HBEA zeolite was carried out in a fixed bed reactor, with acetic acid (AC) as a solvent. 4-Methylacetophenone (4-MAP) was selectively formed in the initial reaction stage. However, a rapid catalyst deactivation occurred with a sharp decrease of the conversion of acetic anhydride, and this was mainly caused by 4-MAP and heavy compounds (‘coke’) existing in the zeolite pore, which poisoned the active sites of the catalyst. The use of excess toluene and moderate acetic acid enhanced catalyst activity and stability to some extent as it limited both the retention of 4-MAP and the formation of ‘coke’. Moreover, a considerable reduction of Broensted acid sites after deactivation revealed that the toluene acylation is primarily a Broensted acid catalyzed reaction.

Active Sites on Nanocrystalline Tin Dioxide Surface: Effect of Palladium and Ruthenium Oxides Clusters

Active sites of nanocrystalline tin dioxide materials with variable particle size, surface area, and catalytic modifiers were studied. Effect of palladium and ruthenium oxides clusters on the activity and concentration of tin dioxide surface centers was evaluated by temperature-programmed desorption techniques using probe molecules, FTIR spectroscopy, EPR, and thermogravimetric methods. The surface site concentration decrease was observed with an increase of SnO2 particle size and BET area decrease. The active sites of SnO2 were found to be selectively promoted by the additives. Accumulation of surface OH groups including hydroxyl spin centers and Broensted acid sites was characteristic for SnO2/PdOx nanocomposites as a result of water chemisorption enhancement due to proposed electronic clusters–support interaction. Ruthenium oxide was shown to increase the concentration of chemisorbed oxygen species via oxygen spillover route.

Seeds-induced fluoride media synthesis of nanosized zeolite Beta crystals

This study reports on the seeded synthesis of zeolite Beta in fluoride media. Crystal growth kinetics of the material as a function of seed content, crystallization temperature and Si/Al ratio of the initial gel is studied. After the optimization of crystallization conditions a highly crystalline material with crystal size 100–300nm is synthesized. A counterpart zeolite Beta sample with similar characteristics is obtained in hydroxyl media. The physicochemical properties of fluoride and hydroxyl media syntheiszed zeolites are compared as particular attention is paid to the nature, number and distribution of their active sites. Zeolite Beta sample synthesized in fluoride media exhibits larger number of Broensted acid sites, while the concentration of Lewis acid sites is higher in the zeolite synthesized in hydroxyl media. The two materials are tested in m-xylene transformation, where zeolite Beta synthesized in fluoride medium shows superior performance. The set of experimental data unambiguously proves that seeded synthesis of zeolites in fluoride medium is a promising alternative of the conventional zeolite synthesis in hydroxyl medium.

ChemInform Abstract: Toward Green and Sustainable Chemical Glycosylation: Enhanced Lewis Acidity of Recyclable Solid Super Acid Catalyst, SO4/ZrO2 by CaCl2 Doping.

AbstractThe enhanced glycosylation efficacy concerning the calcium chloride‐doped sulfated zirconia significantly depends on the ion radius of the calcium cation and how calcium chloride coordinates to the oxygen atom of sulfated zirconia to convert a Broensted acid site into a much more functional Lewis acid site.

Effect of extra-framework Al formed by successive steaming and acid leaching of zeolite MCM-22 on its structure and catalytic performance

Dealuminated MCM-22 samples have been prepared by a two-step dealumination procedure. Detailed assessment of the properties of the materials obtained at each one of the successive stages, i.e. steaming (at 500°C, 700°C and 800°C) and acid reflux (HCl and oxalic), has been made by XRD, N2 adsorption–desorption, m-xylene adsorption, 27Al MAS NMR and FT-IR of pyridine adsorption. It was found that steaming generates extra-framework aluminum (EFAl) species and the majority of them cannot be extracted by the consecutive acid leaching. These extra-lattice entities block the zeolite micropores which makes the remaining Broensted acid sites isolated and inefficient. It is shown that the presence of such species vastly affects the catalytic performance of zeolite MCM-22 in the reaction of m-xylene conversion. The consequences are reduced adsorption capacity and catalytic activity, modified reaction products distribution, enhanced p-xylene selectivity, as well as altered mode of coke formation and composition of the coke precursors.

ChemInform Abstract: Transformation of m-Xylene Over Al-Pillared Clays and Ultrastable Zeolite Y.

The reaction of m-xylene has been carried out at 623 K over Al-pillared beidellite and montmorillonite, and over ultrastable zeolite Y (USY) as a reference catalyst. These microporous acidic solids exhibit similar open porosities and, therefore, the activity toward disporportionation and/or isomerization is not primarily ruled by geometrical limitations but merely reflects the relative amount and strength of the acid sites, strong Broensted acid sites being required for disproportionation and weaker ones for isomerization. In terms of overall conversion and disproportionation selectivity, USY is more active than the pillared clays. This is due to a higher content of strong acid sites and to the greater micropore volume of USY. Al-pillared beidellite is more active than Al-pillared montmorillonite, which is interpreted in terms of framework substitution differences between the two clays. The OH/Al molar ratio of the starting pillaring solution does not confer to the pillared materials any significant effect on their catalytic performances. Deactivation is observed for all the catalysts, and there is some evidence that the stronger sites would deactivate first. 40 refs., 3 tabs., 2 tabs.

Synthesis and Characterization of Task‐Specific Ionic Liquids Possessing Two Broensted Acid Sites.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Transalkylation of toluene with cumene over zeolites Y dealuminated in solid-state. Part I. Effect of the alteration of Broensted acidity

Transalkylation of toluene with cumene was studied over a series of Y zeolites with framework Si/Al ratio of 2.5, 5.6, 12.3 and 21.2 at 453 and 493 K. Samples were prepared by progressive dealumination of the parent NH 4Y(Si/Al=2.5) zeolite in solid-state with crystalline (NH 4) 2[SiF 6]. It was proved by IR spectroscopy that the replacement of framework aluminum by silicon resulted in an increase in the strength and the thermal stability of the acid sites of Broensted-type. These samples do not contain extra-framework aluminum species. The parent Y zeolite possessing the highest amount of weak Broensted acid sites showed low activity in the transalkylation reaction, and, in addition a considerable amount of reactants and products was found to adsorb on this sample at 453 K. The highest cumene conversion was achieved on the sample with Si/Al ratio of 5.6. The balance between the number and strength of the acid centers that assures an optimal activity appears to be disadvantageous in respect to the selectivity. It was demonstrated that the reaction system is very sensitive towards these characteristics influencing not only the activity but also the catalysts’ selectivity and stability.

Transalkylation of toluene with cumene over zeolites Y dealuminated in solid-state: Part II. Effect of the introduced Lewis acid sites

The effects of the introduced Lewis acid sites of different kind (InO + or AlO +) and the variation of InO + concentration on the catalytic behavior of dealuminated in solid-state HY zeolites in the reaction of cumene–toluene transalkylation have been studied. The catalysts, indium modified HY(3.7) and ultra stable zeolite USY(3.4), containing essentially equivalent amounts of framework aluminum and Lewis acid sites (InO + and AlO +, respectively), have been compared to the initial HY(3.7), not containing any extra-framework aluminum (EFAl). Strictly controlled conditions were used for the formation of Lewis acid sites: through reductive solid-state ion exchange (RSSIE) for InO + or by steaming in the case of AlO +. The ratio between the Lewis and Broensted acid sites was varied by progressive replacement of the protons by InO + cations in HY(5.6) zeolite. The zeolites modified by monovalent (InO + and AlO +) have, as a result, an enhanced catalytic activity in comparison with HY(3.7). This effect is mainly due to intense side reactions of dealkylation, oligomerization, cracking and re-alkylation at the expense of the cymenes formation. The data for the distribution of the reaction products suggest a highly preferred mechanism of dealkylation/alkylation with the increase of the Lewis/Broensted acid site ratio. The presence of cation-connected Lewis acid sites is supposed to be responsible for the fast samples’ deactivation.

An FT-IR study on Diels–Alder reactions catalysed by heteropoly acid containing sol–gel silica

Following the general trend of using heterogeneous catalysts whenever possible, the replacement of traditional homogeneous Lewis acid catalysts for the Diels–Alder reaction is currently considered in the literature. In this contribution, we report the successful application of sol–gel derived tungstophosphoric acid–silica composites as catalysts in the reaction of 1,3-cyclohexadiene and 2-propenal. The reaction was monitored by in situ FT-IR spectroscopy following the changes in the fine structure of the ν(CO) band. The catalytic activity of the sol–gel immobilised heteropoly acids seems to be comparable with that of more conventional sol–gel silica–aluminas and their selectivity is even better because Broensted acid sites are destroyed upon calcination of the dry gel.

Infrared spectroscopic studies on the surface chemistry of bimetallic zeolite systems. Acidity of Pt,Co- and Pt,CuZSM-5 zeolites

Acid sites on Co-, Cu- and PtZSM-5 as well as on Pt,Co- and Pt,CuZSM-5 samples were investigated by means of pyridine adsorption monitored by IR spectroscopy. After reduction in hydrogen a significant amount of acidity was observed on each zeolite sample. The concentration of the Broensted acid centers was substantially diminished after treating these samples in NaCl solution. When NaBH4 was used as the reducing agent much less, in some cases a negligible amount of Broensted acid sites was detected, which did not significantly increase after reduction with hydrogen. Creation of the Lewis acid sites proved to be more complex indicated by several IR bands of coordinatively bound pyridine. As a result of this study, an applicable procedure is suggested for preparation of bimetallic zeolites of predetermined acidity.

Role of Zinc in Zn-Loaded ZSM-5 Zeolites in the Aromatization of n-Hexane

Aromatization of n-hexane over zinc-modified ZSM-5 zeolites was investigated. It was shown that incorporation of zinc by ion exchange into cationic positions of NH4-ZSM-5 zeolite causes acid-site strength redistribution and generation of new relatively strong Lewis acid sites in zeolite increasing the selectivity of n-hexane aromatization in comparison with the parent NH4-ZSM-5 zeolite. Simultaneous presence of Lewis and Broensted acid sites in ZSM-5 zeolite does not affect the strength of Broensted acid sites in zeolite. For the activity/selectivity of aromatization of n-hexane on Zn-modified ZSM-5 zeolites, the amount of Zn and its localization in the cationic positions are decisive. The reaction of n-hexane can be also initiated by the Zn species alone in the cationic positions. ZnO species alone as an extraframework phase was found inactive in the catalyst for aromatization of n-hexane. The influence of ZnO addition on the performance of pure ammonium forms of ZSM-5 zeolites in n-hexane conversion is a result of partial migration of zinc into cationic positions of zeolite by solid-state ion exchange.

Theoretical study of then-heptane-HZSM-5 ring structure model interaction

We present a theoretical analysis for the interaction of the n-heptane molecule with an HZSM-5 zeolite, modeled as a ring structure. The Turbomole program, which is a density functional theory based method, was used. Quantum mechanical (QM) calculations were all-electron using the gradient-corrected BLYP approach. We employed orbital basis sets of DZP quality for all atoms. Two coordination modes were studied for the n-heptane–zeolite interaction: a reference structure with the n-heptane moiety located at the center of the ring, and a structure where n-heptane is close either to a Broensted acid site (the region around the Al atom) or to a Lewis acid site. Although the chosen ring represents a minimal model for a zeolitic cavity, the obtained results give insight about the formation of the carbocationic species, proposed as intermediaries during the catalytic cracking reactions. The key electronic effects such as charge transfers and frontier molecular orbitals, involved in the adsorption of n-heptane over the inner surface of the HZSM-5 cavity are presented and discussed for the representative coordination modes studied. The Mulliken and the Roby-Davidson population analysis were done. They are very useful, particularly the second one, to identify the catalytic sites, nucleophilic and/or electrophilic centers, as well as to locate the possible intermediates or transition states with a carbocation character, which are of considerable importance in the hydrocarbon catalytic cracking chemistry. Lastly, we have studied some of the effects that the surroundings produce on the chosen rings–hydrocarbon systems. This was accomplished through the use of a QM/MM(molecular mechanics) methodology. In this way, the embedded QM region representing the cavity it is described more properly for these host–guest interactions. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 725–740, 1999

ChemInform Abstract: Broensted Acid Sites in Zeolites Characterized by Multinuclear Solid‐State NMR Spectroscopy

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Characterization and Catalytic Activity of WNiMo/Al2O3 Catalyst for Hydrodenitrogenation of Pyridine

Various transition-metal catalysts supported on {gamma}-alumina were prepared and applied for the hydrodenitrogenation (HDN) of pyridine. The NiMo/Al{sub 2}O{sub 3} catalyst showed higher activity than the Mo/Al{sub 2}O{sub 3} catalyst. On the basis of the results of reaction experiments and measurements of IR and TPR, the enhancement in the HDN activity with nickel addition could be attributed more to the improvement in the reducibility of molybdenum than to the effect of Broensted acid site. Tungsten impregnated on the NiMo/Al{sub 2}O{sub 3} catalyst as a secondary promoter enhanced the catalytic activity. From the results of various characterization analyses, the positive evidences supporting the HDN activity promotion were obtained. These are the formation of more Broensted acid sites (FT-IR), the enhanced reducibility of Mo (TPR), the increase of octahedrally coordinated Mo{sup 6+} species (DRS), and the creation of more anion vacancy sites (ESR and NO adsorption). Apparently, all of these features are combined to give the promotional effect of tungsten in the NiMo/Al{sub 2}O{sub 3} catalyst for the HDN activity.