Gaseous ozone decomposition over high silica zeolitic frameworks

For several decades, it has been known that ozone emissions are harmful to humans, plants, and animals. Heterogeneous catalytic decomposition is an efficient process for removing ozone from air. This study examines the effect of the zeolite's framework and pore width on efficiency for decomposing gaseous ozone. Four highly hydrophobic zeolites are used: a large cavity zeolite (Faujasite/H‐FAU), a medium pore zeolite with parallel channel (Mordenite/H‐MOR), and two medium pore zeolites with interconnected channels (H‐ZSM‐5/H‐MFI and Na‐ZSM‐5/Na‐MFI). Experiments were conducted in fixed‐bed flow reactors loaded with zeolite at ambient conditions (20 °C and 101 kPa). Zeolite surfaces were analyzed during the experiments in order to understand the influence of physical and chemical surface properties on the ozone decomposition mechanism. A higher amount of ozone is eliminated using H‐MOR, compared with the zeolite samples H‐FAU, H‐MFI, and Na‐MFI. Pore width and micropore framework size distribution (channel and cages) appear to be key factors. A narrow channel or cage, slightly larger than the ozone molecule size, seems to promote ozone interactions with Lewis acid sites. Fourier transform infrared spectroscopy shows that Lewis acid sites (LAS), located on the walls of zeolite pores, decompose ozone. This leads to the formation of atomic oxygen species that could react with another ozone molecule to form dioxygen. Hence, LAS are regenerated, ready to decompose another ozone molecule once more.

     

Mesoporous H‐ZSM‐5 for the Conversion of Dimethyl Ether to Hydrocarbons

The potential of hierarchical H‐ZSM‐5 zeolites was studied for the conversion of DME to fuel‐compatible hydrocarbons. For this purpose, hierarchical H‐ZSM‐5 zeolites have been prepared from commercial H‐ZSM‐5 by desilication and organosilane‐directed hydrothermal synthesis. The zeolites were characterized by X‐ray diffraction, NH₃‐TPD, DRIFTS, and N₂ physisorption measurements. The catalysts have been tested in a tube reactor (1 bar, 648 K). The results indicate important structural changes in framework and acidic sites, which are significant for the synthesis of gasoline‐range hydrocarbons.     

Effect of Phosphate Modification on the Brønsted Acidity and Methanol‐to‐Olefin Conversion Activity of Zeolite ZSM‐5

Phosphate‐modified ZSM‐5 zeolites were studied by standard characterization techniques and solid‐state nuclear magnetic resonance spectroscopy, and in the methanol‐to‐olefin (MTO) conversion. Considering the physicochemical properties of the ZSM‐5 zeolites, the most important effects of the phosphate modification are a deposition of polyphosphates in the pore system and a decrease of the acid site density, but not of the acid site strength. The significant increase in the selectivity to C₂ – C₄ alkenes and the decrease of C₅₊ formation in the MTO reaction for a phosphate coverage of about 5 wt % are explained by extra‐framework phosphate species near the crossing intersections of the ZSM‐5 pore system, which hinder the formation of large intermediates and reaction products.     

Transferring polypropylene into carbon nanotubes via combustion of PP/zeolites (H‐ZSM‐5 or H‐beta)/Ni2O3

In a simple device, two kinds of zeolites were successfully used as synergistic additive to promote formation of the multi‐walled carbon nanotubes (MWNTs) from polypropylene (PP) via combustion. More importantly, this kind of process may potentially act as a new approach for recycling plastic wastes, because it could effectively transfer polyolefin wastes into valuable carbon materials. Experimental results demonstrated that the higher quality of MWNTs can be obtained from the mixture (PP/H‐Beta/Ni₂O₃) than that from the mixture (PP/H‐ZSM‐5/Ni₂O₃). At the same time, the yield of MWNTs from PP/H‐ZSM‐5/Ni₂O₃ system is much lower than that from PP/H‐Beta/Ni₂O₃ under the same condition. The reason for the different effects of both types of zeolites on the morphology and the yield of the MWNTs was analyzed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Studies on the catalytic dechlorination and abatement of chlorided VOC: the cases of 2-chloropropane, 1,2-dichloropropane and trichloroethylene

The conversion of monochloropropanes and dichloropropanes over acid catalysts has been investigated in the presence of oxygen. In the temperature range of 450–550 K, dehydrochlorination of monochloropropanes to propene and HCl occurs selectively over silica–alumina, while significant formation of chlorinated by-products is observed over ZSM5 zeolite catalyst even at higher temperatures. Dichloropropanes conversion over silica–alumina catalyst gives rise mainly to chloropropenes in the temperature range 500–700 K. CO_x are predominant products only at the highest reaction temperatures (just above 700 K). Water vapor in the feed only slightly affects conversions and selectivities. Deactivation processes occur upon dichloropropane conversion, mainly due to coke deposition.

The conversion of highly chlorinated compounds, such as trichloroethylene (TCE) has been tested over silica–alumina and over HY zeolite in the presence of water vapor in the so-called “steam reforming” conditions (HVOC:water=1:2). With diluted feed (1200 ppm) on HY, reaction occurs above 800 K and formation of chlorinated by-products is minimized, CO_x being the main reaction products. At higher HVOC concentrations conversion is obtained at even lower temperature (600 K), but no more negligible by-products formation has been detected. In our conditions zeolite catalyst is more effective in TCE total conversion than silica–alumina.     

Evaluation of H-type zeolites in the destructive oxidation of chlorinated volatile organic compounds

The deep oxidation of 1,2-dichloroethane (DCE) and trichloroethylene (TCE) over H-type zeolites (H-Y and H-ZSM-5) was evaluated. Experiments were performed at conditions of lean hydrocarbon concentration (around 1000 ppmv) in dry air, between 200 and 550°C in a conventional fixed-bed reactor. H-ZSM-5 zeolite resulted more active than H-Y zeolite in the decomposition of both chlorinated volatile organic compounds. It was established that Brønsted acidity plays an important role in controlling the catalytic behaviour of the H-type zeolites. DCE was completely decomposed at 400°C, whereas TCE required higher temperatures (550°C). Vinyl chloride was identified as an intermediate in the DCE oxidation in the range of 250–400°C. When vinyl chloride is destroyed at higher temperatures, both zeolite catalysts show a high selectivity (>90%) towards HCl formation. Trace amounts of tetrachloroethylene were detected in the TCE oxidation, which peaked at 500°C. CO was promoted in quantity in the destruction of both DCE and TCE reflecting the difficulty of carbon monoxide oxidation over H-type zeolites.     

Comparative study of the oxidative decomposition of trichloroethylene over H-type zeolites under dry and humid conditions

H-type zeolites (H-Y, H-ZSM-5, and H-MOR) have been investigated for the catalytic oxidative decomposition of trichloroethylene under dry and humid (1000, 7500 and 15,000 ppmv of water) conditions, between 200 and 550°C. The activity order was found to be: H-MOR>H-ZSM-5>H-Y. Temperature-programmed desorption of ammonia and diffuse reflectance FT-IR measurements of adsorbed pyridine revealed that strong Brønsted acidity plays an important role in controlling the oxidative activity of the H-type zeolites. The main oxidation products were CO, CO₂, HCl and Cl₂. Small amounts of tetrachloroethylene were also detected.

The addition of water to the feed stream did not alter the activity order of the zeolites observed in dry conditions. Better conversions were anticipated over H-MOR and H-ZSM-5 at low temperatures and excess of water concluding that both oxygen and water accounted for the TCE destruction. In contrast, H-Y showed a low activity in humid air due to its strong hydrophilic character. It was found that water promoted the formation of CO₂ and limited the generation of undesired chlorinated by-products (Cl₂ and C₂Cl₄). XRD analysis indicated that H-ZSM-5 was the most resistant zeolite to dealumination by chlorination.     

The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal‐Exchanged Zeolite Catalysts

An overview is given of the selective catalytic reduction of NO_x by ammonia (NH₃‐SCR) over metal‐exchanged zeolites. The review gives a comprehensive overview of NH₃‐SCR chemistry, including undesired side‐reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the preparation method, the exchange metal, the exchange degree, and the zeolite topology. A comparison of Fe‐ZSM‐5 catalysts prepared by different methods and research groups shows that the preparation method is not a decisive factor in determining catalytic activity. It seems that decreased turnover frequency (TOF) is an oft‐neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain production methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH₃‐SCR reaction over Fe‐ZSM‐5, with isolated species being the most active.     

Selective Catalytic Reduction of NO by Ammonia over Raney‐Ni Supported Cu‐ZSM–5 I. Catalyst Activity and Stability

Composite materials containing Raney Ni and Cu‐ZSM‐5 are highly active catalysts for the selective catalytic reduction (SCR) of NO by NH₃. Their catalytic properties were studied with particular attention to the influence of moisture and SO₂ in the feed, and to effects of catalyst shaping operations. Composite materials (16–20 wt‐% zeolite) were prepared by mixing the components, with different degree of segregation in the resulting pressed particles, or by growing ZSM‐5 crystallites on the surface of leached Raney Ni, which were then exchanged with Cu ions. Catalytic tests were performed with 1000 ppm NO, 1000 ppm NH₃, 2 % O₂ in He, at 3–6.5 · 10⁵ h^–1 (related to zeolite component). With physical mixtures, the catalytic behaviour strongly depended on the mixing strategy, particles containing both Ni and zeolite being inferior to mixed Ni‐only and zeolite‐only particles. The SCR activity was promoted by 2 % H₂O in the feed, SO₂ (200 ppm) was a moderate poison at low temperatures, but indifferent or slightly promoting at high temperatures. A catalyst prepared from ZSM‐5 grown on Raney Ni, which was ranked intermediate in dry feed, was promoted to excellent performance in H₂O and SO₂ containing feed at T > 700 K and was stable for 38 h at 845 K. The results suggest that SCR catalysts containing highly active zeolites should be produced avoiding shaping operations e.g. by use of zeolite crystallites grown on wire packings.     

The Kinetics and Equilibrium of Ethanol Adsorption from Aqueous Phase Using Calcined (Na‐1,6‐Hexanediol)‐ZSM‐5

Na‐ZSM‐5, synthesized using 1,6‐hexanediol as the structure directing agent, has been found to have a suitable selectivity and adsorbing capacity for ethanol adsorption from aqueous solutions. In contrast to quaternary ammonium based Na‐ZSM‐5 zeolites, this Na‐ZSM‐5 did not show any catalytic interaction with ethanol during thermal desorption. The dynamics of the adsorption process were investigated using the finite volume “uptake rate” route. The data predicted from the governing relations of the process were correlated with experimental results. The micro‐diffusion coefficient was determined as a function of temperature.The macropore diffusional mass transfer contribution for biporous zeolite particles was found to be negligible.     

Low-temperature catalytic destruction of chlorinated VOCs over cerium oxide

Cerium oxide was used as the catalyst for the catalytic destruction of chlorinated VOCs, and trichloroethylene catalytic destruction as a model reaction was investigated in detail. It was found that cerium oxide has a high catalytic activity for the low-temperature catalytic destruction of various chlorinated VOCs. The main products in the catalytic destruction of trichloroethylene are HCl, Cl₂, CO₂, and trace CO, and C₂HCl is a reaction intermediate, and the presence of an excess of water (30,000 ppm) in the feed stream would inhibit the catalytic performance of CeO₂ catalyst, but improve the selectivity of HCl.     

Creatinine adsorption capacity of electrospun polyacrylonitrile (PAN)‐zeolite nanofiber membranes for potential artificial kidney applications

Innovative dialysis membranes are needed for dialysis, which is the primary treatment for patients with end stage renal disease. In this study, we developed a polyacrylonitrile zeolite nanofiber composite membrane using an electrospinning process to adsorb uremic toxins through molecular sieve mechanism. Scanning electron microscope images revealed that the average diameter of the fiber fabricated with 10 wt % polyacrylonitrile was 673 nm and that of polyacrilonitirle‐zeolite membranes were 277?419 nm. The creatinine adsorption behavior of 500‐KOA (L), 720‐KOA (Farrierite), 840‐NHA (ZSM‐5), and 940‐HOA (Beta) zeolite powders were investigated. Among all the zeolites, 940‐HOA zeolites showed the best performance. The creatinine adsorption capacity of 940‐zeolite powders increased from 2234 µg/g in 50 µmol/L creatinine solution to 25423 µg/g in 625 µmol/L creatinine solution. The speed of adsorption was very quick; 0.025 g of 940‐zeolite powders can eliminate 91% of 2 µmol creatinine in 5 min. The zeolites incorporated inside the membrane had higher creatinine adsorption capacity than free zeolites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42418.     

Preparation,characterization, and electrical properties of ZSM‐5/PEG composite particles

ZSM‐5/PEG composites were synthesized by a simple solution method with polyethylene glycol (PEG) and H‐ZSM‐5 zeolite (Si/Al = 11.4). The obtained composites were characterized using X‐ray powder diffraction and Fourier transform infrared spectroscopy. The obtained results indicated that the ZSM‐5 was physically combined with PEG. The thermal properties and thermal stability were investigated by thermogravimetric and differential thermal analyses. In situ electrical conductivity was used to follow‐up the changes in the electrical conductance during the heating of the ZSM‐5/PEG composite. It was found that ZSM‐5 is able to effectively enhance the electrical conductivity of PEG. The results showed that the obtained weight loss during the composite decomposition to charcoal is accompanied by a decrease in the electrical conductivity. Moreover, the removal of the formed charcoal is associated with an electrical conductivity increase. Calcining the ZSM‐5/PEG composite having a content of 30% results in many effects on the structural, textural, and electrical properties of the obtained products. POLYM. COMPOS., 35:1160–1168, 2014. © 2013 Society of Plastics Engineers     

Comparison of the pervaporation separation of a water–acetonitrile mixture with zeolite‐filled sodium alginate and poly(vinyl alcohol)–polyaniline semi‐interpenetrating polymer network membranes

The pervaporation (PV) separation performance of ZSM‐5‐ and Na‐Y‐type zeolite‐filled sodium alginate (NaAlg) membranes were compared with those of pure NaAlg and semi‐interpenetrating polymer network (semi‐IPN) membranes of poly(vinyl alcohol) (PVA) with polyaniline (PANI) for the dehydration of acetonitrile. The PV separation characteristics of the zeolite‐filled membranes showed a dependence on the nature of the zeolites. The variation of the acidity function of the ZSM‐5 zeolite had an influence on the flux and selectivity of the membranes when compared to unfilled membranes. The crosslinked membranes were characterized by differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. Increasing the PANI content of the semi‐IPN network increased the separation selectivity. Among the NaAlg membranes, the plain NaAlg membrane showed the highest selectivity of 414 at 30 mass % water in the feed mixture, whereas the Na‐Y‐ and ZSM‐5 (40)‐filled NaAlg membranes exhibited much lower values of selectivity, that is, 7.3 and 4.3, respectively for 30 mass % water in the feed. When the flux and selectivity data of ZSM‐5 (250)‐filled NaAlg membranes were compared with that of Na‐Y‐ or ZSM‐5 (40)‐filled NaAlg membranes, a noticeable increase in the selectivity for the ZSM‐5 (250)‐filled NaAlg membrane was observed, but a somewhat comparable flux was observed compared to the plain NaAlg membrane. For the first time, PANI was polymerized with PVA to yield a semi‐IPN. The total flux and water flux increased systematically, whereas the selectivity decreased greatly from 251.87 to 5.95 with increasing amounts of water in the feed. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1968–1978, 2005     

Dealumination of mordenites with acetic acid and their catalytic activity in the alkylation of cumene

A mordenite (MOR) zeolite (Si/Al = 5) was treated with acetic acid to reduce acid site concentration without a significant change in its pore structure. The acid-treated MOR zeolites employed in the alkylation of cumene with 2-propanol showed longer catalyst life with retaining similar level of para-selectivity. The slight enlargement of micropores of MOR zeolite due to the acid treatment did not lower the para-selectivity, while the reduction of acid site concentration improved the stability of MOR zeolite by suppressing carbon deposit. The catalytic performance of the acid-treated MOR zeolites was discussed relating to their pore size distributions and acidities.     

Production of olefins from ethanol by Fe and/or P‐modified H‐ZSM‐5 zeolite catalysts

BACKGROUND: Much attention has been paid to the catalytic conversion of ethanol to olefins, since biomass resources such as ethanol are carbon‐neutral and renewable, and olefins are useful as both fuels and chemicals. It has been reported that zeolite H‐ZSM‐5 is effective for converting ethanol to hydrocarbons, with the chief products being aromatic compounds. RESULTS: Successive addition of Fe and P to the H‐ZSM‐5 improved the initial selectivity for propylene, while the sole addition of Fe or P and co‐addition of Fe and P showed medium initial selectivity. In general, catalysts showing higher initial selectivity for propylene exhibited a steeper decrease in propylene selectivity with time on‐stream. The cause of the change in product selectivity may be carbon deposition during reaction. Addition of Fe and P can improve catalytic stability when processing both neat and aqueous ethanol. The catalytic performance was regenerated by calcination in flowing air. CONCLUSION: Fe‐ and/or P‐modified H‐ZSM‐5 zeolite catalysts efficiently produced olefins (especially propylene) from ethanol. Effective catalyst regeneration was achieved by calcination in flowing air. Copyright © 2010 Society of Chemical Industry     

A Combined Selective Adsorption and Ozonation Process for VOCs Removal from Air

A new hybrid process combining adsorption and ozonation is examined as an alternative treatment for Volatile Organic Compounds (VOC). The physical adsorption of ozone was clarified on a ZSM‐5 zeolite. Oxidation of two industrial solvents was achieved on two zeolites and followed with temperature profiles and sub‐products detection. Total regeneration of the fixed bed was achieved with a complete mineralization of organic compounds. Detection and identification of sub‐products traces allowed us to suspect oxidation reactive species. A mass balance on oxygen showed that all the inlet ozone was used for organic compounds oxidation. A selective oxidation was achieved, allowing the separation and the recovery of minor compound.     

Gas permeation and separation properties of polyimide/ZSM‐5 zeolite composite membranes containing liquid sulfolane

The preparation, characterization, and gas permeation properties of novel composite membranes containing polyimide (PI), liquid sulfolane (SF), and zeolite (ZSM‐5) were investigated to address the interface defects between the PI and the zeolite. The free‐standing composite membranes were prepared by the solvent casting method. The gas permeability of the PI+ZSM‐5 membrane was higher than that of PI, whereas its gas selectivity was significantly reduced, suggesting that these results are attributed to the interface defects. The CO₂ selectivity of PI+ZSM‐5+SF was higher than those of the PI+ZSM‐5 membranes because of the introduction of liquid SF into the interface defects. Furthermore, liquid SF enhanced the CO₂/H₂ selectivity near the recent upper bound. Therefore, the use of liquid SF could be an effective approach to preventing interface defects and increasing the CO₂ selectivity, particularly for CO₂/H₂. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

One‐pot synthesis of sheet‐like MFI as high‐performance catalyst for toluene disproportionation

In this study, one‐pot hydrothermal synthesis of sheet‐like ZSM‐5 as a high‐performance catalyst for toluene disproportionation was carried out using binary surfactants. In the dual template, tetraethylammonium hydroxide was used to construct the microporous structure of ZSM‐5, and cationic surfactant (e.g., octadecyltrimethylammonium chloride (C₁₈TMAC), hexadecyltrimethyl ammonium bromide (C₁₆TMAB), and tetradecyltrimethylammonium bromide, (C₁₄TMAB)) can change the growth habits of the ZSM‐5 crystals by hindering the regular stacking of zeolite layers from their longer hydrophobic chain. From the XRD pattern of the as‐synthesized samples which were hydrothermally treated for different time, it was found that a lamellar mesostructured intermediate gradually transformed into the sheet‐like ZSM‐5 during hydrothermal process. With a proper amount of cationic surfactant, the thickness of the sheet‐like ZSM‐5 could be controlled to less than 30 nm. Concerning the catalyst application, the toluene disproportionation performance over the sheet‐like ZSM‐5 is 1.5 times higher than that of the commercial ZSM‐5. The higher conversion is ascribed to the faster diffusion amount due to the sheet‐like ZSM‐5.     

Long‐term study of CO2 absorption by PVDF/ZSM‐5 hollow fiber mixed matrix membrane in gas–liquid contacting process

In order to fabricate hollow fiber mixed matrix membrane (HFMMM) for long‐term CO₂ absorption process, ZSM‐5 (Zeolite Socony Mobil–5) zeolite was modified using hexadecyltrichlorosilane for increasing hydrophobicity and then added to the polyvinylidene fluoride (PVDF) spinning dope. The in‐house made HFMMMs were characterized in terms of gas permeance, overall porosity, average pore size, effective surface porosity, surface roughness, mechanical stability, and wetting resistance. The morphology of the HFMMMs was studied using SEM. The cross‐sectional SEM images indicated that the membrane structure has changed from sponge‐like to finger‐like by ZSM‐5 loading. The surface roughness increased by increasing ZSM‐5 concentration in the spinning dope. The HFMMM spun from the spinning dope with 0.5 wt % of ZSM‐5 zeolite showed that the CO₂ absorption flux decreased 18.9% in the initial 115 h of the operation and then the absorption flux remained constant until the end of the operation. For plain PVDF HFM the absorption flux decreased 36% from the initial value in the first 15 h of the experiment. Thus it could be concluded that the long term stability of HFM was improved by the incorporation of ZSM‐5. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44606.