Preparation of bipolar zeolite exchangers by gas-phase modification using NOx

Zeolites with varying Si/Al ratio, were synthesized using silica and alumina powder and by matching the XRD patterns with those given in the JCPDS files their chemical formula and crystal structure were determined. In order to make these into bipolar zeolite exchangers, they were modified (called Z₂ zeolite) using a gas-phase reaction with NO_x at 225°C and the formation of the oxynitride groups was confirmed with FTIR, ESCA and the elemental analysis. The XRD patterns of the modified zeolites were found to have changed and did not match with any of the patterns given in the JCPDS files. For these modified zeolites, we have determined the crystal structures and these were found to change from cubic for the unmodified to orthorhombic for the modified zeolites with Si/Al ?1.2 and to the tetragonal for Si/Al >1.2.The oxynitride covalent bond in Z₂ zeolites could be reduced by reacting with hydrazine hydrate to give bipolar zeolite exchangers (denoted as Z₃ zeolite) containing imine groups, thus making the zeolite bipolar in nature. The cation exchange capacities were determined and found to have the same order of magnitude as reported in the literature. The elemental analysis of the modified zeolites was carried out and the experimentally determined nitrogen value of gives 0.9-1.2 oxynitride group per formula unit of the material. The experimentally determined anion exchange capacity of the Z₃ zeolites ( for sample A1) suggests that for this value of exchange capacity, about two out of three formula units have an imine group. The difference between these two independent experiments suggests that not all imine groups participate in the anion exchange phenomena due to their non-availability. We found that Z₃ zeolite responds to organic reactions, and can be reacted with dichloroethane and subsequently quarternized with trimethylamine. Their anion exchange capacity is found to increase significantly.     

Secondary shape selectivity in zeolite catalysis

Over zeolite SSZ-16, n-hexane inhibits the cracking of n-hexadecane under hydrocracking conditions due to shape selectivity. We call this specific phenomenon secondary shape selectivity. The reason for this term is that the cracking selectivity of the zeolite is affected by one of the reactants. Whenever feed mixtures are reacted over zeolite catalysts, this type of shape selectivity can affect reaction selectivity.     

Synthesis and characterization of acidic properties of Al-SBA-15 materials with varying Si/Al ratios

Al-SBA-15 of varying Si/Al ratios in the range 11.4–78.4 was synthesized using tri-block copolymer P123. The calcined materials were examined by XRD, pore size distribution, surface area, ²⁷Al NMR spectroscopy. The acidity and acid strength distribution were studied using microcalorimetric adsorption of NH₃. The acidic properties were also examined by cumene cracking reaction as a function of Si/Al ratios. Systematic variation of acidity and activity was observed as a function of Si/Al ratio. The initial heats of NH₃ adsorption correlated well with activity indicate that acid sites with ΔH > 100 kJ/mole is responsible for cumene cracking activity. Linear correlations were obtained with total acidity and cumene cracking activities. The tetrahedral aluminum was found to be responsible for the observed acidities and catalytic activities.     

Phenol alkylation with methanol: effect of sodium content and ammonia selective poisoning of an HY zeolite

Sodium exchange and ammonia selective poisoning of the acid sites of an HY zeolite (Si/Al=20) were carried out and their effects on the catalytic properties for the alkylation of phenol with methanol (200C, 1 atm and N₂/reactants molar ratio of 4) were evaluated. Results show that the reaction is highly sensitive to the number and strength of the acid sites of the catalyst. A decrease in the number of acid sites by sodium exchange of the protons or by ammonia selective poisoning produces important changes in the selectivity of the reaction. In fact, a high increase in the anisole/cresol ratio is observed as the percentage of exchanged sodium in the zeolite increases, while the ammonia selective poisoning shows that at low desorption temperatures (250C) only anisole is formed while at higher desorption temperatures both anisole and cresols were observed. These results show that anisole formation requires sites with lower acid strength compared to those necessary for cresol formation.     

Synthesis of EMT-rich faujasite in the presence of organic template of low-cost polyquaternium-6

EMT-rich faujasite zeolite has been successfully synthesized from an organic template of low-cost polyquaternium-6 in the absence of crown ether (18-crown-6). Additionally, low-cost water glass is used as a silica source taking the place of silica sol generally used in the conventional synthesis of EMT zeolite. X-ray diffraction (XRD) patterns show that the samples exhibit obvious peaks associated with EMT structure in addition to the peaks assigned to Y zeolite, indicating the presence of EMT and Y phase, and the purity of EMT zeolite reaches 70–80%. Scanning electron micrographs (SEM) show that partial crystals of the sample are typically hexagonal symmetry, confirming the presence of zeolite EMT crystals. Thermogravimetric and differential thermal analysis (TG-DTA) show that there are obvious peaks associated with combustion of organic template, suggesting that polyquaternium-6 plays an important role for the formation of zeolite EMT phase. Furthermore, temperature programmed desorption of ammonia (NH₃-TPD) shows that the acidic amount of EMT-rich faujasite is slightly more than that of Y zeolite, and catalytic cracking of cumene shows that EMT-rich faujasite has higher activity than zeolite Y. Polyquaternium-6 is aqueous solutions of cationic homopolymer of diallyl dimethyl ammonium chloride. Total solids are near 39–41wt%, pH-stability is ranged from 1 to 14, and molecular weight is near 1.5 × 10⁵. The chemical structure of polyquaternium-6 is in the following:      

Synthesis and characterization of microporous silica–alumina membranes

The development of microporous ceramic thin layers is of prime interest for sensors or gas separation membranes working at high temperature. Microporous silica membranes can be easily prepared by the sol–gel process. However the microporosity of pure silica is rapidly modified by steam at high temperature. One way to improve hydrothermal stability is to use mixed-oxide membranes. In this work, microporous silica–alumina membranes were prepared by a simple and robust sol–gel method. Tetraethoxysilane was mixed with an acidic alumina hydrosol. Urea was added for preparing the alumina hydrosol, for controlling the mixed-oxide network polycondensation and also as porogen agent. FTIR and ²⁷Al NMR spectroscopic analyses showed that for Si/Al molar ratios up to 6/1, homogeneous mixed oxides were obtained with a random distribution of Al and Si atoms in the oxide lattice based on tetrahedral units. The derived supported layers were crack-free as demonstrated by scanning electron microscopy (SEM) observations. Their microporosity was investigated using ellipsoporosimetry (EP) with films supported on flat dense substrates. He, N₂ and CO₂ permeance measurements were performed for membranes deposited on porous tubular substrates. The measured values of He/N₂ and He/CO₂ ideal selectivities are in agreement with the microporous nature of the prepared layers.     

Acidity and catalytic activity of the mesoporous aluminosilicate molecular sieve MCM-41

The acidity and catalytic properties of aluminosilicate mesoporous molecular sieves with the MCM-41 structure and bulk Si/Al ratios in the 10–60 range have been investigated. The incorporation of 4-coordinate aluminium into the structure of MCM-41 generates both BrØnsted and Lewis acid sites in amounts increasing with the degree of incorporation. However, the BrØnsted/Lewis acid population ratio is independent of the content of aluminium. The number and strength of acid sites generated are comparable to those of a pillared acid-activated clay and lower than in zeolite H-Y with Si/Al=3.65. Aluminosilicate MCM-41 is a moderate catalyst for the conversion of cumene which proceeds predominantly via catalytic cracking to propene and benzene. The sample of MCM-41 with the highest content of framework aluminium (Si/Al=10) has the largest number of BrØnsted acid sites and exhibits highest catalytic activity.     

Effect of Al content on the assembly of Al-MSU-S mesostructures: zeolite seed structure change from zeolite LZY to LTA with increasing Al content

Al-MSU-S mesoporous molecular sieve catalysts with Al contents ranging from 2.5 to 50 mol% have been prepared from “zeolite seed” solutions and C₁₆ TMABr templates. Hexagonal mesoporous structures are formed that exhibit significantly higher amounts of tetrahedrally coordinated Al than analogous Al-MCM-41 catalysts. The Al-MSU-S catalysts also possess smaller pores than corresponding Al-MCM-41 materials. Catalytic cumene cracking activity is very high over the low Al content materials (2.5 mol%), approaching that of zeolite ZSM-5, but the catalytic activity decreases with increasing Al. As the Al content is increased, the Al atoms remain tetrahedrally coordinated but become less accessible to the cumene reagent. This and knowledge of zeolite synthesis suggest the formation of zeolite seeds other than the large pore LZY, such as the small pore LTA structure.     

高硅ZSM-5在辛烷值助剂中的应用

研究了不同硅铝比ZSM-5分子筛的催化裂化反应性能,结果表明,高硅铝比ZSM-5分子筛能实现提高汽油辛烷值的同时控制液化气产率增幅较小。考察不同硅铝比的高硅ZSM-5分子筛的反应性能,高硅ZSM-5助剂在ACE装置上的评价结果表明,助剂能使液化气和汽油辛烷值小幅增加,同时也能增加汽油中的芳烃含量。随着ZSM-5分子筛硅铝比的增加,助剂控制液化气的性能逐渐增强,但同时提高汽油辛烷值的性能逐渐减弱。在实际应用中,适宜的ZSM-5分子筛硅铝比应根据目标用户的实际情况和要求灵活选择。     

The synthesis and testing of thin film ZSM-5 catalysts

The synthesis and catalytic testing of thin ZSM-5 films on glass and alumina beads is described. The thickness of the ZSM-5 films was controlled to 150, 350, 800 and . The samples were characterised by SEM, gas adsorption and p-xylene isomerisation and 1,3,5-tri-isopropyl benzene cracking test reactions. A reaction-diffusion model adequately described the p-xylene isomerisation data. Estimates of model parameters were obtained by fitting the model to the experimental data. In both cases, the reaction rate constant increased with increasing film thickness. The xylene reaction data showed that secondary reaction products increased as expected with increasing diffusion limitations, but the increase was less than that predicted by the variation of thickness only. The trends in the reaction data could be explained by more defects in the thicker films and/or partial poisoning of the zeolite by mobile support cations in thinner films and/or orientation effects.     

Influence of initial Si/Al ratios on the structural,acidic and catalytic properties of nanosized-ZSM-5/SBA-15 analog composites prepared from ZSM-5 precursors

Nano-ZSM-5/SBA-15 analog composites (ZSC) were prepared in a two-step process from ZSM-5 precursors with different Si/Al molar ratios (10–50) via high-temperature synthesis in mildly acidic media (200 °C, pH 3.5) aiming to evaluate the influence of the initial Si/Al ratio on their structural, acidic and catalytic properties. The resulting materials were characterized by SAXS, XRD, FTIR, TEM, N₂ sorption, ²⁷Al solid state-NMR, NH₃-TPD, FTIR spectroscopy of adsorbed pyridine, AAS and ICP-AES. Under the applied synthesis conditions, a ZSC material with controlled distribution of nano-ZSM-5 and SBA-15 analog phases can be prepared from ZSM-5 precursors by adjusting the initial Si/Al ratio in the range of 20–30. Increasing the initial Si/Al ratio to 50, only ZSM-5 nanocrystals were obtained whereas reducing the initial Si/Al ratio to 10 led to the formation of a disordered mesoporous SBA-15 analog. The total acidity increases with the crystallinity of the ZSM-5 phase as varying the Si/Al ratio from 10 to 30 despite the decreased amount of incorporated aluminum. However, the acidity declines slightly when raising the Si/Al ratio to 50 because of the low incorporated aluminum. The catalytic performance of the ZSC materials compared to the reference materials, i.e. purely mesoporous Al-SBA-15 and purely microporous H-ZSM-5 was assessed in the gas phase cracking of cumene and 1,3,5-tri-isopropylbenzene (TIPB) as test reactions. The results show that a balanced ratio of nano-ZSM-5 and SBA-15 analog phases obtained by tuning the initial Si/Al ratio is crucial to achieve superior catalytic performance of the ZSC materials in the cracking of both cumene and TIPB.     

Deposition and characterisation of vanadium oxide thin films: Linking single crystal and supported catalyst

Thin films can make a useful link between single crystal and supported vanadium oxide. The deposition of vanadium oxide thin films with physical vapour deposition techniques ensures clean and highly controllable synthesis. The resulting material is easily accessed with surface sensitive techniques. On flat TiO₂ anatase substrates, XPS–XPD and UPS indicated that the vanadia deposition was epitaxial, and fully oxidised if performed in situ. A step closer to typical industrial catalysts was achieved by sputter deposition onto sub-millimetre inert particles. In addition to surface characterisation, these model particle catalysts allow use in reactors for catalytic testing under relevant process conditions. On both silica and titania supports, sputter deposited vanadia of varying thickness proved to be equally well dispersed. Oxidative dehydrogenation (ODH) activity was higher over vanadia/titania (anatase) than over vanadia/silica, demonstrating the synergetic interaction between anatase and vanadia. Highest activity in alkane ODH was observed for vanadia a few monolayers thick, supported on titania-coated particles.     

Nitrous oxide as an oxidant for ethane oxydehydrogenation

Waste nitrous oxide was used as an oxidant for ethane oxydehydrogenation performed at the range of temperature from 350 to 450 °C over iron modified zeolite catalysts. Different zeolite matrices (zeolite ZSM-5 of different Si/Al ratio, H-Y, mordenite) modified with iron cations introduced into zeolite by means of ionic exchange were applied as catalysts for the reaction under study. Additionally, amorphous silica and alumina silica as well as silicalite of MFI structure were also used as a matrix for iron ions accommodation and they were tested for oxydehydrogenation reaction. It was found that only iron modified zeolites showed activity for reaction under study. Amorphous oxide supports and crystalline neutral silicalite modified with iron cations by means of impregnation were completely inactive for oxydehydrogenation reaction. The best catalytic performance was found on iron modified zeolites of MFI structure. The Si/Al ratio of the ZSM-5 matrix influenced the activity for ethane oxydehydrogenation reaction insignificantly. N₂O oxidant was partly utilized for ethane oxidation (towards ethene or carbon oxides), while some part of the oxidant was decomposed to nitrogen and oxygen. Performing the reaction at 450 °C resulted in a high ethene yield and complete N₂O removal.     

In situ MAS NMR investigations of molecular sieves and zeolite-catalyzed reactions

Solid-state MAS NMR is a powerful technique to study heterogeneous catalysts and the way by which they operate. In situ MAS NMR has been demonstrated to be a powerful method to understand reaction mechanisms, to study the nature, dynamics and reactivity of surface intermediates and active sites, and to characterize structural modifications in the catalyst itself, in particular when using ¹³C strategically labelled substrates. In this paper, three examples selected from our own work are used to illustrate the potential of in situ MAS NMR. They are the formation of cumene and its isomerization to n-propylbenzene on zeolite H-ZSM-11, the activation of propane at low temperature and the alkylation of benzene with propane on zeolite H-ZSM-5, and the characterization of the aluminophosphate molecular sieve VPI-5 structure with temperature. Studies of the alkylation of benzene with propene confirmed that cumene was the primary reaction product. The undesired n-propylbenzene by-product results from the intermolecular reaction between cumene and benzene, enhanced by molecular shape-selective effects in medium pore size zeolites (e.g., H-ZSM-11). It explains why large pore zeolites, e.g., zeolite Beta, are used commercially today for this process. Propane can be activated at low temperature (ca. 573 K) on bifunctional medium pore size zeolites possessing intimately related acidic Brønsted sites and a dehydrogenation function provided by Ga or Zn species. In Ga/H-ZSM-5 catalysts, at 573 K, the activation of propane was shown to occur via a protonated pseudocyclopropane (PPCP) intermediate (or transition state). The latter evolves in a manner that can be formally described by the formation of CH ₃ ⁺ , C₂H ₂ ⁺ , and C₃H ₇ ⁺ carbenium ion intermediates. These species can react with olefins, alkanes, or other electron-rich molecules such as benzene. The primary reaction products of the reaction of propane with benzene are n-propylbenzene (in small amount), ethylbenzene and toluene. Their subsequent reactions lead eventually to toluene and xylenes as the final products. In the structural characterization of VPI-5, ²⁷Al, ³¹P, and ²⁷Al nutation MAS NMR spectra show that, at 294 K, fully hydrated VPI-5 contains three equally populated Al and P crystallographic sites and that one-third of Al is 6-coordinate. The VPI-5 structure then belongs to the P6₃ space group. Above 353 K, VPI-5, fully or partially hydrated, undergoes a structural transformation to a higher framework symmetry, i.e., the P6₃cm space group. The transformation occurs at nearly the same temperature in both cases, indicating that the breakdown of the hydrogen-bonded helical water structure inside the VPI-5 pores is not a factor in the process.     

Interaction of methane with surfaces of silica,aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (, and ) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactive ₁ band (2917 cm^–1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm^–1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, -alumina and HZSM-5 zeolite could exchange with CD₄ at temperatures higher than 773K, while those on -alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al₂O₃ (both and ) at temperatures higher than 473 K. The formate species would decompose to CO₂, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, -Al₂O₃ did not adsorb or react with methane in any case.     

Rey-zeolite and silica-alumina mixed support for hydrotreating heavy gas oil

Rare earth exchanged Y-type zeolite (REY-zeolite) was dispersed in a silica-alumina gel to prepare catalyst supports with better hydrogenolysis activity. Such support material showed improved hydrotreating properties compared to commercial catalysts, especially for heavy gas oils. Statistical experimental designs used to optimize the composition of such mixed supports suggested a composition of 10 wt.% silica, 25 wt.% zeolite and 65 wt.% alumina as optimum for hydrotreating a heavy gas oil (343°C to 525°C fraction) obtained from hydrocracking of Athabasca bitumen. The kinetic parameters were then evolved for the optimum catalyst.     

Morphology,order, light transmittance,and water vapor permeability of aluminum‐coated polypropylene zeolite composite films

In this study, the polypropylene–zeolite composite films having 2–6 wt % natural zeolite were coated with a thin film of aluminum (Al) by magnetron sputtering, and the contribution of the Al coating on film properties was investigated. The samples were characterized by EDX, X‐ray diffraction, SEM, AFM, UV–visible spectroscopy, and water vapor permeation analyses. The surface of the films coated with a smooth Al film having 98–131 nm thickness. EDX revealed that Al percentage on the surface appeared to be as 8–10 wt % indicating contribution of polymer surface under Al film to analysis. XRD analysis showed that the grain size of Al at the surface was 22–29 nm. The surface roughness increased after Al‐coating process. The transmission of coated films was very low for both UV and visible regions of the light spectrum. Permeation analysis indicated that water vapor permeation was lower for Al‐coated material. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Electrochemical studies of kerosene-pyrolysedcarbon films

Polycrystalline thin films of conducting carbon are deposited on alumina substrates by the pyrolysis of kerosene vapour at 1000C for 2h in argon atmosphere. Preliminary structural analysis is done by XRD, laser-Raman, FTIR and SEM studies. The electrochemical behaviour of as-grown conducting carbon films was investigated in various electrolytes at different pH and the performance was compared with that of platinum and glassy electrodes. The electrochemical window of the kerosene carbon electrode in 100mm H2SO4 was found to be 2.91V which is greater than that of glassy carbon (2.79V) and platinum (2.02V). Cyclic voltammetry reveals that Pt electrode has almost an equal tendency towards hydrogen and oxygen evolution, whereas glassy carbon favours hydrogen evolution and kerosene carbon favours oxygen evolution. It is suggested that the kerosene carbon electrode can be used as an oxygen electrode more efficiently. Unlike diamond films or glassy electrodes, kerosene carbon thin films are of low cost and good stability; they are also easy to grow on various ceramic substrates of any size. Moreover, these electrodes are very economical and promising for application in chlor-alkali industry.     

Catalytic conversion of organosilanes. 1. Disproportionation of alkylsilanes over solid acid and base catalysts

Catalytic conversions of diethylsilane (E2), triethylsilane (E3) and diethyldimethylsilane (E2M2) were examined at 373–573 K in a closed recirculation reactor by using various solid acid and base catalysts. Basically two types of reaction were found: decomposition and disproportionation. Strongly acidic catalysts such as silica-alumina (SA), alumina and sulfated ZrO₂ (SO₃/ZrO₂) exhibited high disproportionation activity, while weakly acidic and basic catalysts showed low catalytic activity and gave mainly cracking products. The order of disproportionation reactivity of three silanes tested were E2M2 > E3 > E2 over SA and SO₃/ZrO₂, while it was E2E3 > E2M2 over an alumina catalyst.     

Zeolite Beta nanosized assemblies

Nanosized zeolite Beta assemblies are prepared by a steam assisted conversion (SAC) method from micron-sized porous amorphous silica grains soaked in clear solutions containing the alumina source and organic template. The zeolite Beta assemblies are built of closely packed uniform nanocrystals (100 nm) and retain the size and morphological features of the primary silica grains. The crystallinity and the phase purity depend strongly on the temperature and time of SAC treatment as well as on the initial aluminum content. For comparison, colloidal zeolite Beta samples with similar Si/Al ratio were prepared by a hydrothermal treatment (HT). The Raman and NMR spectroscopic data reveal that the method of preparation (SAC or HT) does not affect the local structure of Al-rich samples, while for high-silica samples the degree of disorder is higher in the ones obtained via the SAC approach. The adsorption/desorption isotherms of zeolite Beta assemblies and colloidal Beta powders indicate the presence of both micro- and mesoporosity. The catalytic behavior of the zeolite Beta assemblies and colloidal Beta powders in pentane hydroisomerization reaction is studied.