Dehydrogenation of propane over natural clinoptilolite zeolites

ArticleinMicroporous and Mesoporous Materials 61(1):189-198 · July 2003with 90 Reads
Abstract
Clinoptilolite is one of the most commonly used natural zeolites in environmental and industrial applications. In this study, the activity and selectivity of two different clinoptilolite tuffs from Thrace region of Greece were investigated using the model reaction of propane dehydrogenation and were compared to those of synthetic proton and metal-modified ZSM-5 zeolite catalysts. The clinoptilolite bearing tuff from Metaxades (MCH) showed appreciable activity for propane dehydrogenation, similar to that of a steamed Cr-modified ZSM-5 zeolite and considerably higher compared to the second natural clinoptilolite tuff (SF) from the area of Pentalofos and to the H-ZSM-5 zeolite. The total propylene yield for the MCH sample at 540 °C was 8.4%, compared to 3.4% for SF, 5.3% for steamed H-ZSM-5 and 8.3% for steamed Cr-modified ZSM-5. The proton-exchanged forms of both natural zeolites showed improved conversion of propane but resulted in lower propylene selectivity due to subsequent hydrogen transfer, cracking and cyclo-oligomerization reactions induced by the higher number of framework Brönsted acid sites.

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    ... These results corroborate previous studies suggesting that kinetic adsorption of azo dyes on clay matches the pseudo-second order isotherm ( Wang & Li, 2005;Li et al., 2009). As expected and reported in the N 2 adsorption- desorption isotherms study, the treated kaolins (SDBa and Fe-SDB) display greater uptake values resulting from the partial destruction of kaolinite, and modification of the clay-surface charge due to acid activation ( Gonzalez-Pradas et al., 1998) or Fe impregnation (Katranas et al., 2003). capacity achieved with modified SDB may be attributed to surface-protonation phenomena for SDBa, and to the charge reversal on the surface and the formation of new hydroxyl groups by the incorporation of Fe 3+ for Fe-SDB. ...
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  • ... These results are in good agreement with previous studies on the zeolitic tuffs of the Gourounorema location (approximately 1 km east of Xerovouni) which report an average amount of 58 wt.% in the HEU-type zeolite (e.g. Tsirambides et al., 1993; Filippidis, 1993; Misaelides et al., 1994 Misaelides et al., , 1995a Koutles et al., 1995; Symeopoulos et al., 1996; Haidouti, 1997; Tserveni-Gousi et al., 1997; Yannakopoulos et al., 1998; Vlessidis et al., 2001; Katranas et al., 2003; Papadopoulos et al., 2004; Kantiranis et al., 2006). Other mineralogical phases determined were: cristobalite (3À26 wt.%), plagioclase (3À21 wt.%), alkali feldspar (3À19 wt.%), quartz (3À26 wt.%), smectite (3À11 wt.%) and mica (3À6 wt.%). ...
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  • ... 9). Οι τελικές αυτές τιμές του pH και των ποιοτικών παραμέτρων στο διαυγές νερό, είναι μικρότερες από το ανώτατο επιτρεπόμενο όριο των πρότυπων νερών, για διάθεση σε φυσικό αποδέκτη, για άρδευση, κολύμβηση και et al. 1995b, 1998, 2002, Vlessidis et al. 2001 60 HEU - , 1997, Symeopoulos et al. 1996, Katranas et al. 2003 ωση ψαριών. Η ζεολάσπη είναι άοσμη, συνεκτική και κατάλληλη για ασφαλή απόθεση, εξαιτίας της καθήλωσης των επιβλαβών συστατικών στους μίκρο-μέσοκαι μάκρο-πόρους του ΕΛΦΥΖΕ. ...
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    The influence of alumina binder on the catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation was studied. Solid-state 27Al MAS NMR spectroscopy, temperature-programmed desorption of ammonia (TPDA), temperature-programmed oxidation (TPO), catalytic grain intensity, hydrogen chemisorption, BET surface area, and pore size distribution measurements were used to characterize the catalysts. It was found that the addition of binder results in a decrease in the surface area of the catalyst but an obvious improvement in its particle intensity. Some soluble Al species from the binder might migrate into the ZSM-5 zeolite framework during calcination so as to produce some acid sites of moderate intensity, which consequently increases the acid amount of the catalyst. When propane dehydrogenation is carried out under the same space velocity by changing the weight of the corresponding catalyst, the acid amount is the dominant factor influencing the catalytic properties. When this reaction is performed under the same catalyst weight, a small amount of binder is seen to have a positive effect, increasing the catalytic activity as a result of the enhanced metal dispersion and acid amount. However, a negative effect is observed when the binder amount continues to increase. The function of stabilizing the tin species can be strengthened by alumina addition, which might facilitate the transport of the carbon deposits from the active sites to the carrier. TPO profiles of the corresponding catalysts under different reaction conditions represent dissimilar behaviors, which is attributed to an increase in acid amount and change in metal dispersion in the presence of binder. Finally, a model for the influence of the alumina binder on the catalytic performance of PtSnNa/ZSM-5 catalyst for propane dehydrogenation is proposed.
  • Article
    The kinetic behavior of a modified clinoptilolite zeolite for the methanol to dimethylether dehydration reaction has been investigated using a differential fixed bed reactor. It was observed that at high partial pressures, MeOH plays an inhibition role. A novel Langmuir–Hinshelwood type reaction mechanism has been developed that predicts the latter effect.Graphical abstractThe kinetic behavior of a modified clinoptilolite zeolite for the methanol to dimethylether dehydration reaction has been investigated using a differential fixed bed reactor. It was observed that at high partial pressures, MeOH plays an inhibition role. A novel Langmuir–Hinshelwood type reaction mechanism has been developed that predicts the latter effect. The modified clinoptilolite zeolite subject of this study might be of high industrial interest because of the relative lower activation energy (ca. 60 kJ mol−1) compared to other reported zeolitic and non-zeolitic catalysts.
  • Article
    Full-text available
    Treatment of urban wastewaters (pHinitial 8.2), with 6 g of Hellenic Natural Zeolite (HENAZE) of an grain-size < 1.5 mm, gave overflowed clear water of pH 7.5, free of odors and improved quality parameters by 87% for the suspended particles, 88% for the color, 91% for the P2O5 content, 93% for the chemical oxygen demand (COD) and 97% for the NH4 content. Compared to previous ex-periment, the decrease of the amount of HENAZE by 1.5 g (20%) resulted to the purification worsening only by 1-2%. The HENAZE comes from Ntrista stream of Petrota village of Trigono Municipality of Evros Prefecture, Northeastern Greece. HENAZE contains 89 wt.% HEU-type zeolite and exhibit an ammonia ion exchange capacity (sorption ability) of 226 meq/100g. The mineralogical composition and the unique physico-chemical properties, make the HENAZE suit-able material for numerous environmental, industrial, agricultural and aquacultural applications, such as: Animal nutrition, soil amendment for agriculture, pH soil regulation, greenhouse and flowers substrates, durability and health improvement of lawn, purification of industrial and urban wastewaters, treatment of sewage sludge, odor control, fishery and fish breeding, gas purification and drying systems, oxygen enrichment of aquatic ecosystems, improvement of drinking water quality, constructed wetlands and wastewater treatment units. The treatment gave as precipitate an odorless and cohesive zeo-sewage sludge, suitable for safe deposition and also for the reclamation of agricultural soils. The zeo-sewage sludge, produced either from the urban wastewater treatment or from the mixing of HENAZE and sewage sludge, can be used for the reclamation of agricultural soils. The presence of HENAZE in the agricultural soils, increases the crops yield by 29-57% and improves the quality of agricultural products by 4-46%, reduces the use of fertilizers by 55-100%, reduces the usage of irrigation water by 33-67%, prevents the seepage of dangerous species into the water environment (e.g., NO3- by 55-57%), protecting thus the quality of surface and under-ground waters. The usage of HENAZE in vivarium units and in the animal nutrition, increases the production and improves the quality of the relevant products.
  • Article
    PtSnNa/ZSM-5 catalysts with different amounts of Na were prepared by the sequential impregnation method on HZSM-5 zeolites, and their catalytic activities and stabilities in propane dehydrogenation were investigated. The effect of the addition of sodium on the physicochemical properties was studied by means of XRD (X-ray diffraction), IR spectrum of adsorbed pyridine, NH3-TPD, H2 chemisorption, TPR (temperature-programmed reduction), etc. The results indicated that the addition of Na had an obvious impact on the catalytic performance of the PtSn/ZSM-5 catalyst. A suitable concentration of Na could not only reduce the Brönsted acid sites and moderate/strong Lewis acid sites, thus preventing the catalyst from coking and improving the catalytic stability, but also increase the fraction of bare metallic Pt on the surface of the catalyst, thus increasing the catalytic activity. It was found that the propene selectivity and yield could reach the maximum, when the Na content was 1.0%. After reaction for 30 h, the propane conversion was still high (36.4%). However, the continuous addition of Na increased the weak Lewis acid sites and promoted the tin species to be reduced to metallic tin. In this instance, the cracking and hydrogenolysis of propane could be carried out easily, which was disadvantageous to the dehydrogenation.
  • Article
    Research in the field of NOx abatement has grown significantly in the past two decades. The general trend has been to develop new catalysts with complex materials in order to meet the stringent environmental regulations. This review discusses briefly about the different sources of NOx and its adverse effect on the ecosystem. The main portion of the review discusses the progress and development of various catalysts for NOx removal from exhaust by NO decomposition, NO reduction by CO or H2 or NH3 or hydrocarbons. The importance of understanding the mechanism of NO decomposition and reduction in presence of metal ion substituted catalysts is emphasized. Some conclusions are made on the various catalytic approaches to NOx abatement.
  • Article
    PtSn/ZSM-5 catalyst with different amounts of Sn was prepared for propane dehydrogenation. It was found that the addition of Sn not only had “geometric effect”, thus decreasing the size of the surface Pt ensembles, but also changed the interfacial character between metal and support. The presence of Sn could facilitate the transfer of the carbon deposits from the active sites to the carrier, which in consequence improved the catalytic stability. Suitable concentration of Sn on PtSn/ZSM-5 catalyst was preferable for the reaction. With the continuous addition of Sn, more amounts of Sn0 species could be formed, which was disadvantageous to the reaction. Compared to PtSn/γ-Al2O3, the capacity of the catalyst that supported on ZSM-5 zeolite to accommodate the coke was much better. The possible reason may attribute to the larger surface area and the particular character of the channels of ZSM-5 zeolite.
  • Article
    Clay minerals are well known for their ability to breakdown heavy oils by catalytic cracking, but not much is known of the potential of natural zeolites for oilsands bitumen cracking. Our previous work shows that natural zeolites can effectively crack oilsands bitumen into lighter, less viscous products, while producing fewer residues than analogous thermal cracking reactions. In this follow up study we have investigated the potential of both ion-exchanged and untreated forms of natural zeolites as solid acid agents for petroleum cracking. We compared the acid properties of various treated and untreated natural zeolites (particularly chabazites and clinoptiolites from different deposits) with the commercial petroleum cracking catalyst, zeolite Y. Energy dispersive X-ray analysis, inductively coupled Argon plasma-atomic emission spectrometry, acid–base titration using Hammett indicators and temperature programmed desorption of ammonia revealed that natural zeolites, both treated and untreated, have higher or comparable strength and density of acid sites compared to commercial zeolite Y. We propose that higher acid strength and site density, combined with a favorable morphology and a high fraction of accessible surface area contribute to the unique cracking ability of these extraordinary minerals.
  • Article
    Many factors may affect the heavy metals sorption on natural zeolites among them the temperature, for this reason in this paper the cadmium retention behavior on Mexican zeolitic rich tuff as a function of temperature is considered. The kinetic and the isotherms were determined at 303, 318, and 333 K, the remaining cadmium in the solution samples was analyzed by atomic absorption spectrometry. The pseudo-second order rate constant, k, as well as the apparent diffusion coefficients were calculated from the cadmium uptake by the zeolitic rock as a function of the contact time and temperature, the highest amounts were found for the experiments done at 333 K. The maximum cadmium adsorption capacity by the zeolitic material was 12.2 mg Cd2+/g at 318 K corresponding to 20% of the effective ion exchange capacity of the Chihuahua zeolitic rock. In order to explain the cadmium sorption behavior different kinetics and isotherm models were considered.
  • Article
    PtSnNaLa/ZSM-5 catalysts with different amounts of lanthanum were prepared by sequential impregnation method, and characterized by XRD, nitrogen adsorption, NH3-TPD, FT-IR, TPR, H2-TPD, XPS and TPO techniques. It was found that the La species were well dispersed on the internal and external surfaces of ZSM-5 zeolite. With a little addition of La, the catalyst acidity decreased slightly and the active sites of the catalyst were stabilized, which resulted in the increased catalytic selectivity and stability. In PtSnNaLa/ZSM-5 catalyst containing 1.4% La, the stability of catalyst reached the best state and the catalytic selectivity was at the maximum. The average yield of propene was about 35.2% over 80h for the reaction of propane dehydrogenation at 590°C. However, as the La content increased, the character of Pt active sites changed obviously and the amount of Lewis acid sites increased, which in consequence decreased the catalytic activity and selectivity. Results from XPS analysis indicated that the addition of La could inhibit the reduction of tin species. Temperature-programmed oxidation (TPO) experiments showed that suitable loading of La could prevent the catalyst from coking effectively, whereas the opposite effect was observed when the content of La was excess.
  • Article
    Catalytic materials prepared from natural clinoptilolite by ion exchange and acid treatment have been successfully used in the gas phase conversion of acetaldehyde and formaldehyde into acrolein. The modifications made on the raw clinoptilolite have generated important changes of the texture and catalytic sites. The results in catalysis showed that the nature and the amount of condensation products strongly depend on the acid–base properties of catalysts. The highest stability, activity and selectivity to acrolein have been proved by the catalysts having high number and strength of basic sites.
  • Article
    A series of iron containing zeolites with varying Si/Al ratios (11.5–140) and low iron content (∼0.9 wt.% Fe) have been synthesised by solid-state ion exchange with commercially available zeolites and tested, for the first time, in the oxidative dehydrogenation of propane (ODHP) with N2O. The samples were characterised by XRD, N2-Adsorption, NH3-TPD and DR-UV–vis spectroscopy. The acidity of the Fe-ZSM-5 can be controlled by high temperature and steam treatments and Si/Al ratio. The selectivity and yield of propene were found to be the highest over Fe-ZSM-5 with low Al contents and reduced acidity. The initial propene yield over Fe-ZSM-5 was significantly higher than that of Fe–SiO2 since the presence of weak and/or medium acid sites together with oligonuclear iron species and iron oxides on the ZSM-5 are found to enhance the N2O activation. The coking of Fe-ZSM-5 catalysts could also be controlled by reduction of the surface acidity of ZSM-5 and by the use of O2 in addition to N2O as the oxidant. Fe-ZSM-5 zeolites prepared with solid-state method have been shown to have comparable activity and better stability towards coking compared with Fe-ZSM-5 zeolites prepared by liquid ion exchange and hydrothermal synthesis methods.
  • Article
    In this paper, a batch system was applied to study the adsorption behavior of congo red (CR) on raw and modified clinoptilolites. Raw clinoptilolite (Raw-CL) was treated with Fe(NO3)3 in ethanol to obtain its iron-grafted form (Fe-CL). Adsorbents were characterized by X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), energy dispersive X-ray spectroscopy (EDX), thermogravimetric/differential thermal analysis (TG/DTA), zeta-potential measurement and N2 gas adsorption-desorption techniques. Effects of the experimental parameters (initial pH, dye concentration, temperature and adsorption time) were investigated to find optimum conditions that result in highest adsorption capacity for CR removal. The obtained results suggest that the solution pH appears to be a key factor of the CR adsorption process. The maximum dye adsorption was achieved with Fe-CL adsorbent at pH ∼6.3 and the corresponding adsorption capacity was found to be 36.7mg/g, which is higher than that of its raw counterpart (16.9mg/g). A significant decrease in CR removal was given by Fe-CL between pH 7 and 11 opposite to Raw-CL which has nearly constant qe in the same pH range. The Fe(3+)-grafting increased the zeta potential of raw clinoptilolite, leading to a higher adsorption capacity compared to that of unfunctionalized adsorbent. Also, temperature change was found to have a significant effect on the adsorption process.
  • Article
    Full-text available
    Se(IV) and Se(VI) anions are the dominant species of Se existing in aqueous systems. In this study, the iron(III)-modified natural zeolitic tuff (Fe-CLI) from the Serbian deposit Zlatokop has been investigated as an adsorbent for the Se oxyanions. Fe-CLI shows adsorption activity for both Se(IV) and Se(VI) which decreases with increasing pH. The adsorption capacity of Fe-CLI is found to be higher for Se(IV) than for Se(VI). Kinetics data follow the pseudo-second-order model and the obtained parameters k indicate that the rates of adsorption and desorption are higher for Se(VI). Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) analyses reveal that Se is bound at the zeolite surface forming not only the Se-O-Fe but also Se-O-Si bonds. The adsorption mechanism depends of the type of oxo ions. Samples of zeolitic tuff which contain Se were tested as soil supplements for the cultivation of Pleurotus ostreatus mushrooms. The fungus adsorbed the inorganic Se from zeolitic tuff transforming it to a more valuable organically bound form. (C) 2014 Published by Elsevier Inc.
  • Article
    BackgroundZSM-5 zeolite has been used as a common support for PtSn-based catalyst in the propane dehydrogenation reaction. But due to its large acidities, the coke accommodation and the stability of the catalyst would not be satisfied. Here, in order to improve these performances, the new hierarchical-porous ZSM-5 (abbreviated henceforth as ZQ) has been synthesized.ResultsBy the appearance of hierarchical-pores in ZQ, the pore structure was changed, the acid content was decreased, and the surface properties were promoted. Besides, these modifications on zeolite greatly improved the capacity of catalysts to accommodate coke. Moreover, the coke deposit on PtSnNa/ZQ would migrate from active metal to the support. As expected, for the catalyst of PtSnNa/ZQ, the catalytic performance of propane dehydrogenation to propene and the stability of the catalyst were significantly promoted.Conclusions The new hierarchical-porous ZSM-5 support (ZQ) exhibits the enhanced capacity to accommodate coke, and PtSnNa/ZQ possessed better propane conversion and stability of catalyst.
  • Article
    Solid-state grinding is a simple and effective method to introduce guest species into the channels of microporous materials through filling. The structure and the surface acidity of the materials were obtained from BET isotherms and NHj-TPD, respectively. XRD, UV-vis, UV diffuse-reflectance, and TEM were used to characterize the phases, and the morphology, respectively. The clustered layers of MgO-Al2O3 phases were formed in the internal pore surface and were highly dispersed inside the channels of the ZSM-5 host. So the volume of Mg0-Al203/ZSM-5 composite was larger than the ZSM-5 zeolite itself and some mesoporous channels appeared when Mg/Al species entered the channels. Meanwhile, new acid sites emerged in MgO-Al2O3/ZSM-5 composite and the acid amount of the sample changed. The improved Pt dispersion and the increased acid content would cause the increase of propane conversion and the modification of selectivity during the reaction.
  • Article
    PtSnSr/HZSM-5 catalysts with different amounts of strontium were prepared by sequential impregnation method, and characterized by BET analysis, TEM, NH3-TPD, H2-TPR, TPO and TG techniques. The results showed that the addition of strontium could modify the characteristics and properties of both acid function and metal function of Pt-Sn-based catalysts. In this case, PtSnSr/HZSM-5 catalyst with an appropriate amount of Sr (1.2%) showed higher catalytic activity and lower amount of coke deposits than PtSn/HZSM-5 catalyst. However, excessive loading of Sr could facilitate the reduction of Sn, which was unfavorable to the reaction. Afterwards, 1.0 m% of Na was added into the PtSnSr(1.2%)/HZSM-5 catalyst to improve the catalytic performance in propane dehydrogenation, and this catalyst displayed the best catalytic performance during our experiments. After having been subjected to reaction for 5 h, the PtSnNa(1.0%)Sr(1.2%)/HZSM-5 catalyst had achieved a higher than 95% selectivity towards propene along with a corresponding propane conversion rate of 32.2%.
  • Article
    The porous material ATZ with micro-mesopore hierarchical porosity was prepared by alkali treatment of parent HZSM-5 zeolite and applied for propane dehydrogenation. The zeolite samples were characterized by XRD, N 2-physisorption, and NH3-TPD analysis. The results showed that the alkali treatment can modify the physicochemical properties of HZSM-5 zeolite. In this case, the porous material ATZ showed larger external surface area with less acid sites as compared to the HZSM-5 zeolite. It was found out that the alkali treatment of HZSM-5 zeolite could promote the catalytic performance of PtSn/ATZ catalyst. The possible reason was ascribed to the low acidity of ATZ. Furthermore, the presence of mesopores could reduce the carbon deposits on the metallic surface, which was also favorable for the dehydrogenation reaction.
  • Article
    In this study, we sought to optimize the performance of catalytic membrane reactors for the production of hydrogen through the dehydrogenation of methylcyclohexane. Finite element method was used to simulate the radial and axial distributions of velocity, temperature, and concentrations. We examined a number of design parameters and their effects on reactor performance, including the feed flow rate of methylcyclohexane, the mass of catalysts, and pressure on the permeation side of the hydrogen-selective membrane. Dimensionless analysis using the Damköhler number and Péclet number was also employed in the optimization of the reactor. The catalytic membrane reactor optimized in this work achieved a hydrogen production rate more than five times higher than that of existing systems based on the same reactor volume. Simulations at the microscopic scale were also performed to investigate the effects of the pore size and the porosity of the catalytic layer on hydrogen production.
  • Article
    In this study we investigated the catalytic degradation of polyethylene (PE) in a fixed-bed reactor composed of a melter and a catalytic cracking bed. Solid acid catalysts, silica–alumina and zeolites [HZSM-5, natural zeolite (NZ), Y zeolite, etc.] were screened for polyethylene degradation in the range of 450–500°C. The degradation products of PE, especially a liquid fraction, formed over solid acid catalysts, were analyzed by gas chromatography–mass spectroscopy (GC–MS). Compared with those obtained by thermal degradation, the catalytic degradation products are distributed in a narrow range of carbon numbers. The liquid fraction contained large amounts of iso-paraffins and aromatics, as are present in the gasoline fraction of petroleum. The NZ catalyst (clinoptilolite structure, occurring in the Youngil area of Korea) was an efficient catalyst for the PE degradation. The acidity and characteristic pore structure of zeolites appear to be responsible for the good performance. The effects of temperature, contact time, and the presence of nickel metal on the product distribution in PE degradation are also discussed in this work.
  • Article
    Full-text available
    The natural history of the patients of ductal carcinoma in situ(DCIS) with microinvasion is poorly defined, and the clinical management of these patients with particular reference to management of the axilla, has been controversial. Previous studies of this lesion have used and/or arbitrary criteria for the evaluation of microinvasion.
  • Article
    The direct catalytic conversion of alkanes into aromatics has found potentially important industrial applications. Initially only alkanes with 6 and more carbon atoms in the chain were concerned. Supported platinum catalysts were found active for the aromatization of alkanes; the drawbacks of these catalysts were their deactivation with time on stream and the existence of simultaneous parallel reactions. Much discussion has been published on the aromatization of C6+ alkanes. A bifunctional mechanism which involves both the metal and the acid sites of the support and a monofunctional mechanism involving only the metallic sites operate over, respectively, Pt supported on acidic support and Pt supported on nonacidic support. In the present review the mechanisms proposed for the aromatization of alkanes are described. Over monofunctional Pt catalysts two possible mechanisms prevail: 1,6 ring closure on the Pt surface involving primary and secondary C-H bond rupture, followed by dehydrogenation of the cycloalkanes into aromatics (1,5 ring closure to a lesser extent also contributes to aromatic production); or dehydrogenation of the alkanes into olefins, dienes, and trienes followed by thermal ring closure. Zeolites were found most suitable as support for preparing catalysts more active and more selective in the alkane aromatization. In addition catalysts based on noble metals supported on zeolite appeared more resistant against deactivation by coke. In this review the aromatization of hexane, heptane, and octane over Pt-zeolite catalysts is discussed in detail. Comparisons between different zeolite structures and different dehydrogenation sites are given. In particular a critical analysis of the results and interpretation concerning Pt-KL catalysts strongly suggests that the exceptional high selectivity towards aromatization of n-hexane exhibited by Pt-KL could not be explained by only the nest or constraint effect exerted by the channel dimension and morphology, not by only the terminal cracking properties, not by only the partial electron transfer from the zeolite support to the Pt particles, and not by only the Pt particle size. Zeolite structure also affects the aromatic product distribution, in particular when the alkane contains more than 7 carbon atoms. It is shown how Pt on medium-pore zeolites such as In-ZSM-5, silicalites will favor the aromatization of C8 alkane isomers into ethylbenzene-styrene with respect to other C8 aromatics. Aromatization of light alkanes, C2-C5, requires the increase of the hydrocarbon chain length up to 6 carbon atoms and higher, followed by cyclization reaction. Recently new processes to convert C2-C5 alkanes into aromatics have been disclosed, M2-forming from Mobil, Cyclar from BP-UOP, and Aroforming from IFP-Saluted. In general these processes use bifunctional catalysts possessing a dehydrogenating and an acid function. The catalysts consist of a metal ion or metal oxide supported on a microporous acid solid. In this review we analyze the results concerning mainly platinum supported on pentasil-type zeolite. It is shown that althoug Pt has better dehydrogenating properties as compared with gallium and zinc, the efficiency of catalysts based on Pt-ZSM-5 for light alkane aromatization is less because undersirable reactions such as hydrogenolysis and ethene (olefins) hydrogenation occur on the platinum surface, resulting in the production of unreactive alkanes, CH2, C2H6. These drawbacks could be partially suppressed by alloying Pt and by increasing the reaction temperature.
  • Article
    The proton form of a natural zeolite, clinoptilolite, was found to be an efficient catalyst for the skeletal isomerization of n-butenes to isobutene. The acidity and characteristic pore structure of this zeolite appeared to be responsible for the good performance. The effects of process variables were studied. When reaction conditions were chosen such that the dimerization reaction, the major side reaction, was suppressed, high selectivities to isobutene were obtained.
  • Article
    The present paper reports on the results of a systematic investigation of the conversion, at temperature around 500 C and atmospheric pressure, of propane and n.butane on a ZSM-5 catalyst loaded with Pt or Pt and Re, presulfided or not by hydrogen sulfide. When presulfided the bimetallic catalyst leads to a high BTX selectivity, low ethane and methane selectivities and low coke formation. These are attractive features for a commercial process.
  • Article
    Influence of O{sub 2} and H{sub 2} pretreatments (at 600{degrees}C for 10 h) on the initial activity and product selectivity and their variation with time on stream (0-180 min) of H-gallosilicate (H- GaMFI), H-galloaluminosilicate (H-GaAlMFI), and Ga/H-ZSM-5 catalysts (with bulk Si/(Al + Ga) = 34.5 {+-} 2.5) in the propane aromatization (at 600{degrees}C) has been investigated, using a microreactor operated in both the pulse and continuous reaction modes. The catalysts have been characterized for their acidity (measured in terms of pyridine chemisorbed at 400{degrees}C), framework and nonframework Ga and Al, and surface Si/Ga ratio. The initial propane conversion/aromatization activity and aromatics selectivity of the H-GaMFI are decreased but that of the H-GaAlMFI and Ga/H-ZSM-5 are increased significantly due to the H{sub 2} pretreatment for all the Ga-modified zeolite catalysts. The variation of the activity and product selectivity of all the three catalysts with the time on stream is found to be strongly dependent upon the pretreatment. Among the catalysts, H-GaAlMFI shows superior performance in the propane aromatization. 28 refs., 7 figs., 2 tabs.
  • Article
    The kinetics of the liquid-phase isomerization of α-pinene over a natural zeolite-clinoptilolite has been studied at 100–180°C and nitrogen pressures at 1–20 bar. Up to 80–85 wt% of conversion selectivity toward reaction products was constant with camphene and limonene as the main products. Only at relatively high conversions were other monocyclic and tricyclic products formed. The consumption rate of α-pinene followed first-order kinetics. Temperature dependence of the first-order kinetic constant obeyed Arrhenius dependence with the activation energy equal to 80.9 kJ/mol. Selectivities at particular conversions were seen to be independent of temperature and pressure. A reaction network and mechanism are advanced and the corresponding kinetic equations are derived. The kinetic model gave a good correlation between the theoretical and the experimental data.
  • Article
    The effects of oxidizing and reducing treatments over Ga/H-ZSM-5 aromatization catalysts are studied. The respective roles of the dehydrogenation function and of the acid function are discussed and it appears that the acid function is depressed through the reducing treatment. To explain the role of gallium in propane activation, the concept of the dual-site mechanism is preferred to the concept of the "porthole" mechanism.
  • Article
    A range of Pd-containing catalysts on the basis of mordenite–clinoptilolite rocks of different starting phase compositions is synthesized. Final phase composition and acidity of rocks change as the conditions of their dealumination are changed. The efficiency of catalysts synthesized is weakly influenced by change of mordenite component contents within 50–80%, and it is not inferior to the efficiency of the catalysts on the synthetic mordenite basis. The nature of active sites as trifunction ones and isomerization mechanism are proposed.
  • Article
    The kinetic modelling of propane aromatization over a Ga/HMFI catalyst and the comparison of Ga2O3-HMFI mixtures with different compositions has allowed us to specify the respective roles of dehydrogenating species and of protonic sites. Propane aromatization over Ga/HMFI catalyst occurs mainly through a conventional bifunctional reaction scheme (with desorbed intermediates) in which the dehydrogenating gallium species are responsible for dehydrogenation of propane, of oligomers and of cyclic olefins and diolefins while protonic sites are responsible for oligomerization, cyclization and cracking. This bifunctional transformation is kinetically limited by propane dehydrogenation. Unfortunately, gallium species also catalyze alkane cracking and hydrogenolysis as well as alkene hydrogenation with formation of the undesired products methane and ethane. From the comparison of the catalytic properties of MFI gallosilicates calcined under air flow at various temperatures, it can be concluded that tetrahedral gallium is inactive in dehydrogenation reactions. Dehydrogenation occurs through a bifunctional mechanism involving the joint participation of gallium oxide (extra framework species) and of protonic sites. Coke formed during propane aromatization causes a blockage of the access to the protonic sites of the zeolite which is responsible for deactivation. Ga decreases the coking/aromatization rate ratio of HMFI and facilitates the coke removal through oxidative treatment, both effects being more pronounced after Ga/HMFI pretreatment at high temperature under hydrogen because of the better dispersion of the Ga species.
  • Article
    IR measurements were carried out with Cd-MFI and Cd-clinoptilolite at 25°C. Samples of different water content were contacted with acetylene at different pressures. Adsorbed acetaldehyde was formed from acetylene and water both adsorbed on similar Cd2+Z−2 sites. The reaction between the two adsorbed reactants reveals the validity of Langmuir-Hinshelwood kinetics. Decrease of water content or increase of acetylene pressure facilitated the formation of carbonaceous deposits especially in the case of Cd-MFI. Deposits formed from acetylene displace adsorbed acetylene and acetaldehyde.
  • Article
    The ways of preparation of the linear hexane hydroisomerization catalysts on the base of mordenite–clinoptilolite rocks of Ukrainian deposits are described: under the strictly defined conditions of thermic and acidic treatments of parent rocks, followed by palladium (0.5wt%) introduction, the samples of catalysts were synthesized which are well comparable with the catalysts on the base of synthetic mordenite. The high efficiency of the samples synthesized was achieved due to the appropriate degrees of the residual crystallinity of the mordenite–clinoptilolite rocks modified as well as due to the corresponding strong acidic sites content of their zeolitic components. The isomerization mechanism through the hexane molecules protonation resulting in the formation of non-classic carbonium ions, which include the pentacoordinated carbon atoms, with subsequent dehydrogenation of these structures over Lewis acidic sites is proposed – the role of Lewis acids play probably palladium and extraframework aluminum. Based on the consideration of the different proton affinity of various C6H14 isomers, the plausible pathways to the formation of definite hexane isomers are discussed. These routes give such sets of individual branched hexanes which consist both the thermodynamically predicted and really found isomerizate compositions.
  • Article
    The effect of varying the zeolite structure on the activity and selectivity of propane conversion via dehydrogenation and cracking was examined together with the effect of the exchange level within a given zeolite type. It was observed that the overall activity increased with the acid strength on the one hand while, on the other hand, the selectivity towards dehydrogenation decreased. The data obtained in this work well fitted the soft-soft and hard-hard acid-base interaction model as leading to dehydrogenation and cracking respectively.
  • Article
    Low and high activity states in the carbon dioxide vs. inlet temperature transient curves in the catalytic combustion of m-xylenes over Pd2Ni50Nb48 amorphous alloy ribbons (I), Pd/stainless steel flakes (II) and Pd/clinoptilolite (III) samples have been investigated. In addition, in both states hysteresis, in high activity state self-sustained oscillation in the CO2 vs. time at full conversion and damped oscillation at low conversion were observed over samples (II) and (III). It was established that hysteresis is a general phenomenon in both states and is not affected by the catalyst structure, but it can be ascribed to Pd as active component. Transition between the low and high activity state may be due to change in the PdO structure as was indicated by XPS studies. Oscillation is indicative of the partial accumulation of carbonaceous deposits over the catalysts at full conversion, while damped oscillation might be a result of the change of local concentration due to overheating the catalyst sample during combustion.
  • Article
    A series of Mg-ZSM-5 zeolite catalysts with different content of magnesium were prepared via a solid-state reaction of HZSM-5 with magnesium chloride at 327°C. X-ray determinations demonstrated that the structure and crystallinity of the catalysts were not changed as compared with HZSM-5 zeolite. The acidity of the catalysts was characterized by temperature programmed desorption of ammonia and infrared spectra of pyridine adsorption. The studies indicated that their Brønsted acid sites decreased and Lewis acid sites slightly increased with increasing magnesium amount in the zeolites. The catalytic properties of the catalysts have been examined by choosing the alkylation of toluene with methanol as probe reaction. The results showed that the modified zeolite catalysts significantly improved the initial activity and lifetime. Para-selectivity also increases to a level of 80–90%. These results indicate that the solid-state reaction is an effective and convenient route for modification of the zeolite.
  • Article
    Vapour-phase hydration of 1-butene at atmospheric pressure above 373 K shows similar features on both H-clinoptilolite and sulphonic acid ion-exchange resin: the initial reaction rate linearly increases with the partial pressure of 1 -butene and passes through a maximum as a function of water pressure. Kinetics was evaluated accordingly. A decrease in the 1-butene isomerization rate indicated that under hydration conditions more than 99.9% of the acidic sites are hydrated. For the hydration, however, hydroxonium ions are the active centres where an adsorbed water molecule reacts with gasphase 1-butene in the rate determining step. The equilibrated desorption of secondary butanol ends the reaction. The kinetic inhibition by butanol can be suppressed with water. Above ca. 0.2 bar water pressure inactive polyhydrated hydroxonium ions are formed.
  • Article
    This paper describes a simple preparation method using a solid state reaction for Zn-, Mo-, and Cr-ZSM-5 catalysts for propane aromatization. NH3-temperature-programmed desorption, infrared, temperature-programmed reduction and electron spin resonance techniques were used to characterize the interaction of H-ZSM-5 with ZnO, MoCl5 and CrO3, respectively, which leads to introduction of cations into the channels of the zeolite. A solid-exchange mechanism is suggested here. Zn-ZSM-5 is the most active catalyst for propane conversion and gives a better benzene, toluene and xylenes selectivity. Over Mo-ZSM-5, propane mainly undergoes cracking to methane and ethane, and the loading ZSM-5 with Cr5+ enhances the propane dehydrogenation to propene.
  • Article
    Zinc promoted H-ZSM-5 catalysts were prepared by ion exchange (Zne) and by impregnation (Zni). The acidity of these samples was characterized by the TPD of ammonia and by FTIR spectroscopy, recording the OH vibration spectra and the spectra of adsorbed pyridine and ammonia. The method of preparation had minor influence on the activity in the conversion of propane over samples containing less than 1.5 wt.-% Zn. However, Zne samples with Zn contents of 1.5 wt.-% and more were much more active than Zni samples. The former displayed an increased number of Lewis sites and a decreased number of Brønsted sites. Otherwise, partial pore blocking with increasing zinc loading seems to limit the promoting effect in case of Zni samples. The results demonstrate that ionic zinc species determine the catalytic properties, acting as strongly acidic Lewis sites.
  • Article
    The influence of zeolite factors, such as Ga loading (0–3%), Si/Al ratio (35–132), degree of H+ exchange (2–98%) and calcination temperature (600–800°C), affecting zeolitic acidity on the activity and selectivity of Ga/H–ZSM-5 in the aromatization of propane has been investigated. The activity and selectivity in propane-to-aromatics conversion are found to be influenced strongly by the above zeolite factors. The aromatization activity and selectivity of the zeolite shows a close relationship with its strong acidity (measured in terms of pyridine chemisorbed at 400°C). The propane aromatization activity and selectivity increases sharply with the increase in the strong acid sites of the Ga/H–ZSM-5.
  • Article
    Fe-exchanged zeolites were prepared and studied as catalysts for selective catalytic oxidation (SCO) of ammonia to nitrogen in the presence of excess oxygen. Fe–ZSM-5, Fe–mordenite (MOR), and Fe–clinoptilolite (HEU) showed excellent SCO performance at a very high space velocity (GHSV=2.3×105 h−1). Greater than 97% NH3 conversion and nearly 100% N2 selectivity were obtained. Among different zeolites, the catalytic performance decreased in the sequence of Fe–ZSM-5, Fe–MOR, Fe–HEU > Fe–Y, Fe–Beta > Fe–ferrierite, Fe–chabazite. High iron content, low Si/Al ratio, and low ammonia concentration were favorable for high activities for NH3 oxidation to N2. For the Fe-exchanged zeolites, there existed a good correlation between the N2 selectivity for the SCO reaction and the activity for the SCR (selective catalytic reduction) of NO with ammonia; i.e., the higher the SCR activity, the higher the N2 selectivity in SCO. The variable valence of iron cations might be beneficial to oxygen activation and thus result in the high SCO performance for the Fe-exchanged zeolites. A kinetic study indicated that the reaction was first order with respect to NH3 and the apparent reaction activation energy (Ea) was 88 kJ/mol.
  • Article
    A way to increase the value of LPG cut from petroleum feedstocks is its direct transformation to H2 and aromatic products; these aromatic products, BTX—essentially benzene (B), toluene (T), and C, -aromatics (X)—can be used as raw material for the petrochemical industry or as a blending mixture to enhance the octane number of gasoline. However, these transformations require high temperatures. Thermodynamic data show that the conversion of paraffins into aromatics is favored by increasing the length of the chain, and that aromatics are favored in relation to olefins (Table 1) [1,2]. Whereas aromatization of propane and higher paraffins can be carried out at temperatures lower than 500°C, transformation of ethane, and especially that of methane, requires much higher temperatures. This is experimentally supported by the transformation of various hydrocarbons, at constant temperature and space velocity. For instance, over H-[All-ZSM-5, butane and isobutane react four times faster than propane and 100 times faster than ethane [3].
  • Article
    The as-synthesized alkali-free TPA, MeNH3-[Ga,Al]-ZSM-5 zeolites were steamed at different steam pressures (68kPa≤P≤92kPa), reaction times (0h≤t≤9h), and temperatures (530°C≤T≤800°C). The solids obtained were characterized by XRD, FT-IR, N2 adsorption, 27Al and 71Ga MAS-NMR. Acidity of the solids was characterized using n-heptane cracking (T=350°C, PN2/PC7=4, P=100kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, PC3=100kPa) as a model reaction. Steaming of these solids leads, even at low temperatures (530°C), to a profound and progressive demetalization (degalliation and/or dealumination). Catalysts obtained under mild steaming conditions show propane aromatization properties similar to those of catalysts prepared by means of direct calcination under dry air at higher temperatures (T>700°C), or by conventional impregnation methods. A clear synergistic effect is observed between acidic sites in the structure and dehydrogenating species composed by extraframework gallium species (EFGS). Linear dependence of initial activity in propane aromatization per protonic acidic site (A0/H+), as a function of EFGS/H+ relation, shows that this aromatization reaction should be kinetically limited by the initial dehydrogenation step produced on EFGS, which demonstrates bifunctionality of this reaction. Finally, inactivation of acidic sites of the structure of the solid (H+) is produced by EFGS, and the numeric ratio between them shows that EFGS present on galloaluminosilicates, under reaction conditions, should be species with a positive charge equal to 1.
  • Article
    ZSM-5 catalysts without (H) and with added metals (Ga, PtGa, Pt) have been studied for propane conversion and aromatization at 530°C, with the contact time varied by three orders of magnitude. From initial rate and selectivity determinations, the role of the modifying metals has been ascertained. On metal-free HZSM-5 propane yields equimolar methane and ethylene as well as propylene. Adding metals increases the rate of propylene formation, as a result of their dehydrogenation activity, which acts additively rather than synergistically. The intrinsic dehydrogenation activity, observed at low conversion and time on stream, increases in the order Ga < PtGa < Pt. At higher conversion and/or longer time on stream, the reverse order is found for the formation of aromatics. It is shown that Pt but not Ga undergoes rapid coke deactivation under aromatization conditions. 34 refs., 4 figs., 3 tabs.
  • Article
    Full-text available
    The conditions of alteration of the upper Eocene zeolite-bearing volcaniclastic sediments at Metaxades, Thrace, Greece, were examined using a variety of petrographic and mineralogical techniques. A fine silt- to clay-size matrix, containing abundant altered glass shards, predominates. The primary minerals include quartz, K-feldspar, plagioclase, micas (especially biotite) and some opaque minerals. The diagenetic phases, clinoptilolite, smectite and cristobalite, occur as microcrystalline aggregates within the matrix or as replacements of glass shards. The abundances of cristobalite and clinoptilolite are related chiefly to the original composition of the rocks. The abundance of cristobalite increases and that of clinoptilolite + smectite decreases as the abundance of the matrix increases. The formation of clinoptilolite and cristobalite took place either at near-surface temperatures or at slightly elevated temperatures (lower than 70°C). The smectite content of the altered volcaniclastics decreases with increasing depth in the 15 m thick exposed section. No other systematic variation in mineral assemblages with depth was observed. However, the ratio clinoptilolite/smectite increases systematically with depth suggesting to us that smectite may have formed subsequent to the formation of clinoptilolite and as an alteration product of clinoptilolite. The initial composition of the volcanic material and to a less extent the interstitial fluids, greatly affected the diagenetic mineral assemblages of the final alteration products.
  • Article
    Surface acidity and propane transformation on In-and Ga-containing ZSM-5 type zeolites were investigated to find relation between the acidic character of catalysts prepared by conventional or solid state ion-exchange and the catalytic activity in alkane aromatization. On the basis of the concentration of Brénsted and Lewis acid sites measured by IR spectroscopy the In and Ga containing samples may be ordered into three main groups. The presence of Brénsted acid sites results in cracking of the hydrocarbon molecules, whereas the aromatization activity is attributed to Lewis acidity associated with the intimate contact of the oxide/zeolite assemblies.
  • Article
    Much attention has been paid to the transformation of lower alkanes such as propane and butanes into aromatic hydrocarbons from both industrial and academic points of view. The aromatic hydrocarbons can be utilized as a booster for high octane number gasoline and are fundamental raw chemicals in petroleum chemistry. The activation of lower alkanes is an intriguing subject. In early work, Csicsery [1–5] described dehydrocyclodimerization of lower alkanes over bifunctional catalysts such as platinum on alumina and Cr2O3 on alumina.
  • Article
    The dehydrogenation of propane was studied over chromium-modified MFI-type zeolite catalysts. Three different chromium-modified ZSM-5 zeolites were synthesized following typical hydrothermal synthesis and impregnation procedures and were finally treated hydrothermally at relatively severe steaming conditions. The prepared catalyst samples were characterized by XRD, SEM-EDS, N2 adsorption/desorption, Ammonia-TPD and 27Al MAS NMR. The catalytic results showed that when chromium species are randomly dispersed within the zeolitic particles act synergistically with the remaining acidic framework Al inducing improved dehydrogenation activity and propylene selectivity.
  • Article
    The rate of adsorption of pure CO2, O2, N2, and CH4 on natural untreated clinoptilolite-rich volcanic tuff (Cp) from Tehuacan (Puebla, Mexico), and on cation-exchanged clinoptilolite samples (Ca−Cp, K−Cp, and Na−Cp) has been measured at 20 °C using a glass high-vacuum volumetric apparatus. The X-ray diffraction pattern of Cp showed that the main crystalline phases correspond to clinoptilolite-heulandite and minor amounts of mordenite, cristobalite, feldspar (albite), quartz, smectite, and opal. The adsorption rates of gases in the initial period (t < 180 s) were measured with a custom acquisition data card capable of registering pressure and time data five times per second, simultaneously. The influence of cation exchange on adsorption kinetics of the gases depended on the gas-adsorbent contact time (t). In the initial period, the adsorption rate of all gases on all samples decreased in the order Ca−Cp > K−Cp > Cp > Na−Cp, and the affinity decreased in the order CO2 N2 > O2 > CH4, whereas at equilibrium (t → ∞ s) the adsorption uptake decreased in the sequence CO2 CH4 > N2 > O2. The slow adsorption of methane in Na−Cp was probably due to diffusional difficulties as a result of channel blockage by Na+ cations. By cation exchange of Cp an adsorbent can be tailored for the separation of N2/O2, N2/CH4, and CO2/CH4 mixtures.
  • Effective aromatization processes for converting light hydrocarbons to BTX aromatics could improve the availability of aromatics as chemicals and high octane gasoline-blending components. To this end, M2 forming offers a potential new route. The process can convert olefins and paraffins to aromatics. The aromatic yields are only limited by the stoichiometric constraint imposed by the hydrogen content of the feed and the products. The heart of this process is the HZSM-5-type catalyst. The process operates in the cyclic mode at about 538 /sup 0/C and 1-20 atm. The reactions involved in the M2 forming process are complex, consecutive, acid-catalyzed reactions, including (1) conversion of olefinic and paraffinic molecules to small olefins via acidic cracking and hydrogen-transfer reactions, (2) formation of C/sub 2/-C/sub 10/ olefins via transmutation, oligomerization, cracking, and isomerization reactions, and (3) aromatic formation via cyclization and hydrogen transfer.
  • Article
    Cation exchanged natural zeolite catalysts for the reduction of NO with NH3 were developed. The synthetic mordenite containing Cu, V, and Cr cations showed high activity for the reduction of NO with NH3. Natural Izukalite containing mainly mordenite exhibits high activity for the selective reduction of NO when they were anchored with Cu, Mn, Co, and V cations. Furthermore, the anchored pellet catalysts prepared from Zeo 3S (clinoptilolite) and Izukalite exchanged with vanadium ions and mixed with TiO2 exhibited high activity for the reduction of NO with NH3. The honeycomb catalysts prepared from V-Izukalite were found to be especially effective for stationary diesel engine exhaust.
  • Article
    The proton form of the natural clinoptilolite zeolite (HNZ) was modified by treatment with various acids. Effects of modification on catalytic performance were investigated for the skeletal isomerization of n-butenes to isobutene. Among HNZs treated with various acids, only those modified with boric acid (B-HNZ) showed an improved catalytic performance of skeletal isomerization in terms of activity and stability. Furthermore, B-HNZ exhibited higher isobutene selectivity compared to unmodified HNZ at the same conversion of n-butenes. The performance was comparable to that of a widely known ferrierite catalyst at the same reaction conditions. The main effect of boric acid treatment on HNZ was the selective increase in the number of acid sites of moderate strength required for efficient catalysts in skeletal isomerization without collapse of its framework. These acid sites of moderate strength were considered to be selective and stable reaction sites for skeletal isomerization of n-butenes. Coke deposition poisoned non-selective strong acid sites and imposed a spatial restriction on zeolite channels, leading to suppression of undesired dimerization reactions. Factors affecting selectivity to isobutene are discussed.
  • Article
    Full-text available
    The conversion of C3-C9 paraffins to small olefins over ZSM-5 zeolite is investigated. The small olefins are primary products and are usually converted into other more stable secondary products such as aromatics on the ZSM-5 zeolites. Thermally treated HZSM-5, K/HZSM-5 and Ba/HZSM-5 catalysts were developed and favourable oxidative conditions were introduced for the conversion process to maximize selective conversion of light paraffins to small olefins at the relatively low temperature of 873 K. The role of K and Ba is to minimize bimolecular hydrogen transfer reactions and enhance the dehydrogenation activity of the catalysts. Meanwhile, the oxygen in the gas phase is effective to improve the olefin selectivity and yield. C2-C4 olefin selectivities of 70.4 and 66.8% have been obtained for propane andn-hexane feed-stocks, respectively, at a temperature of 873 K.
  • Article
    The as-synthesized alkali-free TPA, MeNH3-[Ga,Al]-ZSM-5 zeolites were steamed at different steam pressures (68 kPa≤P≤92 kPa), reaction times (0 h≤t≤9 h), and temperatures (530°C≤T≤800°C). The solids obtained were characterized by XRD, FT-IR, N2 adsorption, and MAS-NMR. Acidity of the solids was characterized using n-heptane cracking (T=350°C, PN2/PC7=4, P=100 kPa) as a model reaction. Acidic or bifunctional behavior of these catalysts was determined using propane aromatization (530°C, PC3=100 kPa) as a model reaction. Steaming of these solids leads, even at low temperatures (530°C), to a profound and progressive demetalization (degalliation and/or dealumination). Catalysts obtained under mild steaming conditions show propane aromatization properties similar to those of catalysts prepared by means of direct calcination under dry air at higher temperatures (T>700°C), or by conventional impregnation methods. A clear synergistic effect is observed between acidic sites in the structure and dehydrogenating species composed by extraframework gallium species (EFGS). Linear dependence of initial activity in propane aromatization per protonic acidic site (A0/H+), as a function of EFGS/H+ relation, shows that this aromatization reaction should be kinetically limited by the initial dehydrogenation step produced on EFGS, which demonstrates bifunctionality of this reaction. Finally, inactivation of acidic sites of the structure of the solid (H+) is produced by EFGS, and the numeric ratio between them shows that EFGS present on galloaluminosilicates, under reaction conditions, should be species with a positive charge equal to 1.
  • Article
    Aromatization of butane or propane was conducted with a series of ZSM-5 catalysts. A small amount of oxygen in the feed promoted butane conversion to aromatic hydrocarbons on H-ZSM-5. It suggests that if hydrogen atoms on the zeolite surface are removed effectively, H-ZSM-5 exhibits a high selectivity for aromatic hydrocarbons. A hybrid catalyst composed of the physical mixture of Ga/SiO2 and H-ZSM-5 showed comparable activity and aromatic selectivity to those of the Ga-supported ZSM-5 zeolite, whereas the Ga/SiO2 itself exhibited little catalytic activity for the paraffin conversion and olefin aromatization. The excellent promotional effect of Ga/SiO2for aromatics formation was observed only when it was intimately contacted with H-ZSM-5. These results suggest that the supported Ga promotes the zeolite-catalyzed aromatization of lower paraffins by promoting hydrogen desorption via the ‘reverse spillover’ effects.
  • Article
    Natural clinoptilolite zeolite HNZ has been studied as a catalyst in the degradation of polystyrene (PS) at 400 °C. Its performance on the degradation was compared with HZSM-5, silica-alumina, and also with thermal degradation. HNZ was as effective as HZSM-5 for the production of liquid oils with carbon numbers of C5–C12, without any severe deactivation. The main product of PS degradation was styrene for both thermal and catalytic degradation. Among the catalysts tested, silica-alumina showed the highest yield of ethylbenzene and the lowest one of styrene due to its mesopores. Increase in degradation temperature for HNZ resulted in a decrease of selectivity towards ethylbenzene and propylbenzene, but an increase of styrene selectivity with lower yield of styrene dimers. A kinetic analysis by thermogravimetric analysis was also carried out using a dynamic model. It offered reliable kinetic parameters for the catalytic degradation of PS over HNZ. The apparent activation energy was found to be 360 kJ/mol, and the catalytic degradation of PS followed first-order kinetics.
  • Article
    Full-text available
    The thorium and uranium uptake from their aqueous solutions by unpretreated and NaCl-pretreated zeolite-bearing volcanoclastic rock samples from Metaxades (Thrace, Greece) has been studied using a batch-type method. The concentration of the solutions varied between 50 and 20 000 mg/l. The NaCl pretreatment of the materials improved the thorium but not the uranium uptake. The absolute thorium uptake by the pretreated material, determined using neutron activation and X-ray fluorescence techniques, reached 12.41 mg/g, whereas the uranium uptake by the raw material was 8.70 mg/g. The uptake distribution coefficients (Kd) indicated that the relative thorium and uranium uptake is higher for initial concentrations below 250 mg/l. The zeolitic materials showed exceptional resistance to the initial low pH of the solutions used. The pHin was significantly increased due to the simultaneous hydrogen-ion uptake. The thorium and uranium uptake is a rather complicated phenomenon related to the aqueous chemistry of the elements, the nature of the constituent minerals and the properties of the zeoliferous rock specimens. The various metal species are bound through different uptake processes such as ion-exchange, adsorption and surface precipitation. Microporous minerals (zeolites, phyllosilicates) are mainly responsible for the considerable uptake ability of the rock samples studied.
  • Article
    Kinetics of the liquid-phase isomerization of enantioisomers of α-pinene using natural clinoptilolite was investigated. The experimental ranges of temperature and pressure (N2) were T = 120–160°C and P = 1–10 bars, respectively. Initially, mainly camphene and limonene were produced; the ratio of enantiomers corresponded to the ratio of +/− isomers in α-pinene. At relatively high conversions other monocyclic (terpinene and terpinolene) and tricyclic products (tricyclene) were formed. A kinetic model is developed which described the experimental data well.
  • Article
    The surface acidic properties of various solid catalysts (i.e., clay montmorillonite, acid modified smectite clay, HY zeolite, sulphate-promoted ZrO2 (SO2−4/ZrO2) and Cs-exchanged dodecatungstophosphoric acid (H0.5Cs2.5PW12O40)) have been comparatively evaluated by temperature programmed desorption (TPD) measurements of adsorbed NH3, C5H5N and C6H6. Mathematical analysis of TPD spectra of adsorbed ammonia indicates the presence of weak, medium and strong acid sites in all the catalysts. On the basis of the TPD patterns of NH3, C5H5N and C6H6, the nature (Brønsted and Lewis) of the acid sites has been ascertained. Catalytic measurements in CH3OH dehydration and benzene alkylation indicate that besides the acidic properties, the diffusional phenomena also play a key role on the reactivity of solid acid catalysts. The superior catalytic performance of the H0.5Cs2.5PW12O40 system in the above reactions has been explained in the light of a pseudo-liquid behaviour.
  • Article
    A series of H-ZSM-5 zeolites with different framework Si/Al ratios were prepared by hydrothermal synthesis and post-synthesis dealumination by various methods, i.e., HCl, steaming, steaming/HCl and ammonium hexafluorosilicate (AHFS). The degree of framework dealumination and the amount/type of the extra-framework phases formed were greatly dependent on the method of dealumination and the severity of the experimental conditions. The hydrothermal treatment of a parent ZSM-5 sample with Si/Al∼27 was very effective and produced significant amounts of Si–Al extra-framework phases, while the reaction with HCl in relatively strongly acidic environments resulted in very low dealumination. Treatment with AHFS was proven to be an appropriate method for preparing moderately dealuminated H-ZSM-5 samples which are free from extra-framework Al. The relative crystallinity and the microporosity of all dealuminated H-ZSM-5 zeolites were retained to a high degree (>80%), even for the severely steamed samples (∼96% dealumination). The morphology of the crystals/particles was not changed significantly; however, each dealumination method had a different effect on the partial breakdown of the crystals and on the formation of extra-framework/amorphous phases with a mesoporous character. The number of acid sites that corresponded to the high-temperature desorption peak of the ammomia-temperature programmed desorption spectra was found to be equivalent to the framework Al (FAl) content of all H-ZSM-5 samples, irrespective of the degree of dealumination and the amount of extra-framework phases. Theses phases had a low capability of adsorbing ammonia compared to the FAl atoms and were the main source of acidity for the severely steamed samples which had almost no FAl.
  • Article
    The conversion of ethane, propane, n-butane, isobutane and of the corresponding alkenes was studied at 530°C on a H-ZSM-5 zeolite (Si/Al=40). Butanes transform about 4 times more rapidly than propane and 100 times more than ethane. From propane and butanes the primary products result from dehydrogenation and cracking, cracking being more rapid than dehydrogenation. Ethane undergoes mainly dehydrogenation and leads slowly to methane. It is suggested than these reactions occur through scission of carbonium ions formed by alkane protonation. The more stable the carbonium ion the higher the reaction rate. The olefinic primary products undergo very rapid reactions through carbenium ion intermediates: oligomerization, cyclization of the oligomers followed by aromatic formation through hydrogen transfer from naphthenes to light alkenes. Whatever the reactant, the limiting step of aromatization is the initial formation of alkenes.
  • Article
    The catalytic behavior in propane oxidative dehydrogenation to propylene of V-containing silicalite-1, silicalite-2, ZSM-5 and boralite, prepared either by direct hydrothermal synthesis or by consecutive impregnation with ammonium vanadate of the zeolites in the acid form is reported. Results show that selectivity to propylene depends on the V content and is comparatively higher in samples prepared by direct hydrothermal synthesis than in those prepared by impregnation, but the concentration of propane and the propane to oxygen ratio considerably affect the results. At higher reaction temperatures the presence of weak acid sites due to vanadium ions induces the formation of aromatics. The formation of the latter is enhanced in V-containing boralite and ZSM-5 samples. Maximum selectivities to propylene up to 80-85% are obtained, but only at low propane or oxygen conversion (around 5%).
  • Article
    The correlation between electrophysiologic parameters and recurrence in 239 patients who underwent successful catheter ablation for an accessory pathway mediated tachycardia was analysed. Pathway conduction recurred in 15 patients (6.3%) after a mean follow-up of 7.3 months. Recurrence was most common in patients with right free wall pathways (17.1%). Accessory pathway conduction resumed in 18.9% of patients with decremental ventriculo-atrial (VA) conduction post-ablation compared with those patients without VA conduction (3.4%) (P < 0.001). Recurrence was common also in patients with repeated return of accessory pathway conduction during the ablation procedure (40% vs. 1.3%, P < 0.01). The patients in whom these observations are made should be monitored closely for recurrence of accessory pathway conduction post-ablation.
  • Article
    Clinoptilolite tuffs from areas in Thrace region of Greece are compared with synthetic zeolites NaY and NH4Y for the uptake of N4-ethyl-N4-(2-methansulphonamidoethyl)-2-methyl-1,4-phenylenediamin (sesquisulphate, monohydrate) with the trade name CD-3 for the purpose to be used for clean-up and recycling photo-finishing and photo-developing washwaters. The cation-exchange capacity is found to be 6.15-11.1 mg/g for zeoliferous tuffs at equilibrium concentration of 50 ppm CD-3 in aqueous solution compared to 65.0 mg/g of NaY and 48.2 mg/g for NH4Y synthetic zeolites corresponding to the removal of CD-3 from 120 to 2001 of 50 ppm aqueous solution per kg of natural zeoliferous tuff; this capacity is only 6-10 times lower than type-Y synthetic zeolite. Initial rates of uptake are 20.8 mg/l/min for natural and 38.5 mg/l/min for synthetic zeolites. Regeneration levels of 55, 23, 35, and 33% are obtained for MCH, SF, NaY, and NH4Y, respectively. The rapid and almost complete uptake of CD-3 from its aqueous solutions at low CD-3 concentrations by the natural zeolites is promising for such an application.
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