Adsorptive removal of thiophene and benzothiophene over zeolites from Mae Moh coal fly ash

Zeolites have been hydrothermally synthesized using Thai coal fly ash from Mae Moh Power Plant as silica and alumina sources. The synthesis conditions, i.e., SiO₂/Al₂O₃ ratio, amount of water, amount of base, and aging temperature, were varied to prepare different topologies of zeolitic products. The zeolites attained were sodalite (SOD), gismondine (GIS), and cancrinite (CAN). The zeolites have been applied to adsorption of thiophene and benzothiophene in n-hexane solution. It was found that GIS with higher specific surface area and average pore volume had superior performance to other synthesized materials. Adsorption capacity of our developed zeolites was compared to those of commercial zeolites, i.e. NaY, HUSY, beta, and ZSM-5 obtained via the conventional synthesis methods. The results suggested a potential of zeolites derived from Mae Moh coal fly ash for removal of refractory sulfur compounds, such as benzothiophene.     

SSZ-13沸石粒径及硅铝比对甲醇制烯烃反应性能的影响

通过调控合成配方,以水热合成法制备一系列不同粒径及硅铝比的SSZ-13沸石,考察沸石孔道结构、酸性质与催化性能之间的关系。采用XRD、SEM、XRF、N₂吸附-脱附以及²⁷Al MAS NMR对沸石样品进行详细的表征,并评价其甲醇制烯烃反应(MTO)催化性能。结果表明,小晶粒SSZ-13沸石相对结晶度高、晶粒尺寸均一;在MTO中,小晶粒SSZ-13沸石的MTO催化寿命大幅高于大晶粒SSZ-13沸石;减小晶粒尺寸可改善分子扩散效率,在此基础上提高沸石的酸密度可增加其活性稳定性。     

Liquid-phase adsorption and isomerization of C<Subscript>5</Subscript>-C<Subscript>8</Subscript><Emphasis Type="Italic">n</Emphasis>-paraffins

The liquid-phase adsorption of n-pentane, n-hexane, n-heptane, and n-octane from natural gasoline on zeolite CaA and their catalytic isomerization has been investigated experimentally and theoretically with the aim of increasing the octane number of a low-octane gasoline. An integrated process flowsheet combining processes in an adsorber and in an isomerization reactor has been developed. The basic results are as follows: the ultimate activity of CaA with respect to n-pentane, n-hexane, n-heptane, and n-octane in the case of their simultaneous adsorption at 25.0°C is 4.2, 4.7, 5.1, and 6.3 kg/100 kg, respectively. Kinetic and outlet adsorption data are also presented. The maximum yield of C₅, C₆, C₇, and C₈ iso-paraffins is 62.0, 70.0, 66.0, and 47.0%, respectively. A mathematical model of the processes has been developed, and their parameters have been calculated. Calculated and experimental data are in satisfactory agreement.     

Adsorption of CO<Subscript>2</Subscript> and CO on H-zeolites with different framework topologies and chemical compositions and a correlation to probing protonic sites using NH<Subscript>3</Subscript> adsorption

Adsorption of CO₂ and CO at 25 °C has been conducted using commercially-available (Y, ZSM-5) and laboratory-synthesized (SSZ-13, SAPO-34) H-zeolites with different framework topologies and chemical compositions, and their textual and surface properties have been characterized by N₂ sorption and NH₃ adsorption techniques. All the zeolites were microporous, although ZSM-5 and SSZ-13 apparently showed a mesoporous sorption behavior due to the interparticle spaces. The zeolites had Si/Al values in the order of SSZ-13 (16.44) > ZSM-5 (16.08) ? Y (2.82) ? SAPO-34 (0.19). Regardless, high CO₂ adsorption capacity was obtained for SSZ-13 and SAPO-34 with a CHA framework. The FAU zeolite Y with the highest micropore volume showed less CO₂ adsorption than the CHA zeolites and the MFI-type ZSM-5 yielded the poorest performance. Probing acid sites in the H-form zeolites using NH₃ disclosed that these all contain both weak and strong acid sites with significant dependence of their strengths and amounts on the topology. The acid strength of the weak acid sites in the CHA zeolites was the weakest, which might allow a stronger interaction with CO₂. The H-zeolites gave CO₂/CO selectivity factors that were in the range of 4.61–11.0, depending on the framework topology.     

Adsorption of Carbon Dioxide on High-Silica Zeolites with Different Framework Topology

Adsorption isotherms of carbon dioxide were measured on six high-silica zeolites TNU-9, IM-5, SSZ-74, ferrierite, ZSM-5 and ZSM-11 comprising three-dimensional 10-ring (8-ring for ferrierite) at 273, 293, 313 and 333 K. Based on the known temperature dependence of CO₂ adsorption, isosteric heats of adsorption were calculated. The obtained adsorption capacities and isosteric adsorption heats related to the amount of CO₂ adsorbed have provided detailed insight into the carbon dioxide interaction with zeolites of different framework topology. The zeolites TNU-9 and ferrierite are characterized by pronounced energetic heterogeneity whereas due to the location of Na⁺ cations in the same positions the isosteric adsorption heats of CO₂ adsorption on IM-5, ZSM-5 and ZSM-11 zeolites are rather constant for molecular ratio CO₂/Na⁺ < 1. As IM-5 zeolite has a maximum adsorption capacity, it appears to have optimum properties for carbon dioxide separation.     

Catalytic behavior of hybrid Co/SiO2-(medium-pore) zeolite catalysts during the one-stage conversion of syngas to gasoline

The catalytic properties of 10-MR (membered ring) zeolites (ZSM-5, MCM-22, IM-5, ITQ-2, all with a similar Si/Al ratio of ca. 15) in hybrid Co/SiO₂-zeolite catalysts for the direct conversion of syngas to mainly high-octane gasoline-range hydrocarbons has been studied under typical Fischer-Tropsch (FT) conditions: 250 °C, 2.0 MPa, and H₂/CO = 2. Special emphasis has been given to the deactivation behavior and the characterization of the amount and nature of the carbonaceous deposits formed by a combination of techniques (elemental analysis, TGA (thermogravimetric analyses), GC–MS, and DR (diffuse reflectance) UV–vis spectroscopy). The presence of the medium-pore zeolite increases the gasoline yield by about 20–50%, depending on the particular zeolite, and enhances the formation of branched products with respect to the base Co/SiO₂ catalyst, which is explained by the promotion of isomerization and cracking of long-chain (C₁₃₊) n-paraffins formed on the FT component. The initial zeolite activity is mostly determined by the surface acidity rather than by the total amount of Brønsted acid sites, pointing out to the existence of limitations for the diffusion of the long-chain n-paraffins through the 10-MR channels under FT conditions. Thus, ITQ-2 bearing the largest surface area presents the highest initial yield of branched gasoline-range products, followed by ZSM-5, IM-5, and MCM-22. All zeolites experience a loss of activity with TOS, particularly during the initial reaction stages. This deactivation is governed by the morphological and structural properties of the zeolite, which finally determine the amount and location of the coke species, and not by the acidity.     

Porosity and surface properties of mesoporous silicas and their carbon replicas investigated with quasi-equlibrated thermodesorption of <Emphasis Type="Italic">n</Emphasis>-hexane and <Emphasis Type="Italic">n</Emphasis>-nonane

Adsorption of n-hexane and n-nonane on mesoporous micelle-templated silicas (SBA-15 and MLV—multilayer vesicle) and on their carbon replicas (CMK-3 and OCM—onion-like carbonaceous material) was studied by means of quasi-equilibrated thermodesorption technique, utilising a standard TPD setup with a chromatographic detector. Analysis of n-hexane adsorption isobars, determined from the thermodesorption profiles, revealed substantial heterogeneity of the adsorption sites present in the carbonaceous materials. The pore size distributions calculated from the thermodesorption profiles of n-nonane for both siliceous and carbonaceous materials were in agreement with those obtained from the low-temperature nitrogen adsorption isotherms. They confirm an uniform structure of mesopores in SBA-15, CMK-3 and OCM as well as a more complex structure of mesopores in the MLV materials.     

Condensation enthalpies of <Emphasis Type="Italic">n</Emphasis>-hexane in micelle-templated mesoporous silicas

The isosteric enthalpies of adsorption of n-hexane on ordered mesoporous silica of pore diameter between 3 and 10 nm have been measured. The heat of capillary condensation increases when mesopores are smaller. Capillary condensation of n-hexane in 3 nm mesopores is 20% more exothermic than the condensation on a flat liquid surface. The results are in good agreement with a model which takes into account the energetic contribution of the interface between the adsorbed layer and the vapour phase.     

Alkali-treatment of ZSM-5 zeolites with different SiO2/Al2O3 ratios and light olefin production by heavy oil cracking

Four kinds of ZSM-5 zeolites with different SiO₂/Al₂O₃ ratios are alkali-treated in 0.2 M NaOH solution for 300 min at 363 K. Changes to the compositions, morphologies, pore sizes, and distributions of the zeolites are compared before and after alkali-treatment. The changes observed are largely influenced by the SiO₂/Al₂O₃ ratios with which the zeolites are synthesized. A possible mechanism of desilication during alkali-treatment is proposed. The SiO₂/Al₂O₃ ratio of zeolites is found to influence the yield of light olefins that use heavy oil as feedstock. Alkali-treated ZSM-5 zeolites produce higher yields of light olefins compared to either untreated zeolites or the industry catalyst CEP-1. It is believed that alkali-treatment introduces mesopores to the zeolites and improves their catalytic cracking ability. ZSM-5 zeolites with SiO₂/Al₂O₃ ratios of 50 also present superior selectivity toward light olefins because of their optimized hierarchical pores.     

Evaluation of standard thermodynamic parameters for the sorption of n-hexane and methanol in zeolites

Adsorption isotherms for n-hexane and methanol in mordenite and ZSM5 in the temperature range from 323 K to 473 K were found to be of type 1. The adsorbate-adsorbent interaction (C₁) and adsorbate-adsorbate interaction (C₂) wee computed according to Bradley and Wilkins model, whereas the Rees and Williams model predicts the adsorption characteristics. Finally, the thermodynamic parameters such as ΔG^o, ΔS^o, ΔH^o, K, K⁺ have been evaluated for the sorption of n-hexane in mordenite and ZSM5.     

Correlation between activity and acidity on zeolites: a high temperature infrared study of adsorbed acetonitrile

The influence of the temperature on the protonation of acetonitrile by acidic zeolites was studied by infrared spectroscopy. Acidity at room or low temperature was not correlated with the protonation temperature, but the zeolitic structure played an important role. A new technique is presented for the study of the acidity of solids under reaction conditions. A good correlation was obtained in a series of various zeolites between the catalytic activity in the cracking of n-hexane and the protonation temperature of acetonitrile. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Effect of Zeolite Structure on the Activity and Selectivity in p-Xylene Alkylation with Isopropyl Alcohol

The effect of zeolite architecture and channel dimensionality on p-xylene conversion and selectivity to 1-isopropyl-2,5-dimethyl-benzene was investigated in p-xylene alkylation with isopropyl alcohol over novel zeolites SSZ-33 and SSZ-35. Catalytic behavior of these zeolites was compared with those of zeolites Beta, mordenite, ZSM-11 and ZSM-5. It was found that p-xylene conversion increases with increasing pore size and connectivity of the channel system of individual zeolites with the exception of SSZ-35, which possesses a system of one-dimensional 10-ring channels that periodically open into wide, shallow cavities circumscribed by 18-rings. SSZ-35 exhibited the highest conversion among all zeolites at the reaction temperature of 150 °C and also the highest selectivity to 1-isopropyl-2,5-dimethyl-benzene. Molecular modeling confirmed the dimensions of the 18-ring cages are optimal for the formation of this alkylation product.     

Passive NO Adsorption on Hydrothermally Aged Pd-Based Small-Pore Zeolites

Pd-based small-pore zeolites with different framework structures (AEI, CHA and RTH) were synthesized by a facile incipient wetness impregnation method. The zeolites were utilized as low-temperature passive NO_x adsorbers (PNA) for NO_x storage before and after hydrothermal aging. It was found that 1 wt% Pd/AEI showed better PNA behavior than the 1 wt% Pd/CHA and Pd/RTH samples, regardless of hydrothermal aging at 750 and 800 ℃. The AEI zeolite has a three-dimensional but tortuous pore channel structure, which accelerated Pd dispersion at 750 ℃ but inhibited Pd aggregation at 800 ℃ when subjected to hydrothermal aging. Pd/CHA showed a slight increase in Pd dispersion at 750 ℃, but extensive Pd aggregation at 800 ℃ due to its straight and unhindered three-dimensional pore structure. The Pd species in Pd/RTH zeolite were prone to accumulation during hydrothermal aging due to the two-dimensional pore structure. Therefore, Pd/AEI can be utilized as an efficient and stable PNA after activation by mild hydrothermal aging treatment.

     

Mesoporous Material from Zeolite

Zeolite/mesoporous molecular sieve composites (ZMC) are made using zeolites as silica-alumina source. XRD patterns of the ZMC show the peaks for both zeolite and mesoporous molecular sieve with hexagonal symmetry. Morphology of the ZMC is different from that of a physical mixture of the two components. Unlike Al-containing mesoporous molecular sieves, the ZMC shows a strong acidity like zeolites. The activity of cracking of n-hexane for ZMC was higher than that of the zeolite. These products are expected to be useful for catalysts and adsorbents for their acidity and pore size.     

Catalytic cracking and skeletal isomerization of n-hexenes on HY zeolite

The catalytic cracking and skeletal isomerization of n-hexenes on 80/100 mesh HY zeolite has been studied in the temperature range 350–405°C, and compared with results previously obtained on ZSM-5 zeolites. Species with less than three carbon atoms were not observed as primary cracking products, with traces of ethylene formed only as a secondary product. Although propylene and propane may be formed partially by monomolecular cracking of n-hexenes, the dominant cracking process is bimolecular. Dimerization, followed by disproportionation gives stable C₃, C₄ and C₅ species, in addition to C₉, C₈ and C₇ fragments. The probability of these larger fragments undergoing further cracking before desorption increases with temperature, but is significantly less than found on ZSM-5 zeolites. The main products of skeletal isomerization were monomethylpentenes, with those isomers which can originate from tertiary carbonium ions dominant. Dimethylbutenes were formed mainly as secondary products. The rate of the dimerization-cracking process in which two n-hexene species participate is ? six times slower than the reaction involving a monomethylpentene species and a linear hexene. The increase in the rate of the latter process with respect to the former is reduced on ZSM-5, and this can be attributed to the narrower pore size within this zeolite. In contrast to ZSM-5, there is significant formation of alicyclics, paraffins and aromatic species. The residual coke was found to be considerably poorer in hydrogen content than comparable material formed on ZSM-5.     

n-Hexane Isomerization over NiO/SO2− 4/ZrO2 Catalysts. Effect of Nickel Loading

The effect of nickel addition to the sulfated ZrO₂ catalysts for n-hexane isomerization was investigated. A series of catalysts with different nickel concentration (from 0 to 9.6 wt%) were synthesized by supporting nickel on sulfated zirconia by incipient wetness method. After a calcination procedure the catalysts were characterized by X-ray diffraction, nitrogen physisorption (BET method) and its acidic properties were determined by NH₃ adsorption. (TPD) and FT-IR of pyridine adsorption. The n-hexane isomerization reaction was carried out in a fixed bed microreactor at 673 K under 345 kPa of total pressure. Results showed that nickel content between 1.5 and 4.5 wt% improved the catalytic activity and favored the formation 2,3-dimethylbutane, which is a highly desired product. X-ray diffraction results showed no evidence of any NiO phase at NiO concentration below 9.6 wt% suggesting a NiO phase highly dispersed on the surface of ZrO₂–SO^2? ₄ with crystallite sizes lesser than 3nm. Catalytic activity and 2,3-dimethylbutane selectivity seems to be correlated with the NiO well dispersed phase rather than with their acidic properties.     

Influences of Mesoporosity Generation in ZSM-5 and Zeolite Beta on Catalytic Performance During <Emphasis Type="Italic">n</Emphasis>-Hexane Isomerization

Abstract  

Catalytic performance of Pt impregnated parent and desilicated nano-crystalline zeolites, ZSM-5 and Beta for n-hexane isomerization was studied. Difference in channel systems of the zeolites and absence/presence of mesopores therein were found to be reflected in product distributions. ZSM-5 was desilicated by NaOH and zeolite Beta with tetramethylammonium hydroxide (TMAOH.) Desilication was found to afford comparable catalytic performance to that of the parent counterpart at reaction temperature lower by 25 °C. Observed product distributions could be substantiated with characterizations such as ammonia TPD, surface area determination and SEM. Desilicated zeolite Beta was seen to be less prone to coking as deduced from the TGA study. Location of Pt with reference to proton sites within the channels and that inside the pores viz a viz external surface also have been discussed briefly.     

Conversion of light paraffins for preparing small olefins over ZSM-5 zeolites

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.     

Dual pore-size sieving in a novel oxygenate-pillared microporous adsorbent for C6 alkane isomers separation

Adsorbents executing molecular sieving mechanisms for the efficient separation of n-hexane (C6) alkane isomers require delicate pore size control, but afford unsatisfactory single-component separations according to their branch degrees. Herein, we report a novel oxygenate-pillared microporous adsorbent, MoOFOUR-Co-tpb, ([Co(tpb)₂MoO₄], tpb = 1,2,4,5-tetra(pyridin-4-yl) benzene), with three gourd-shaped channels for dual pore-size sieving of C6 isomers. In particular, MoOFOUR-Co-tpb excludes 2,2-dimethylbutane, while 3-methylpentane (3MP) can enter one channel showing a high uptake of 82.6 mg g⁻¹, contrasting to the n-hexane (nHEX) adsorption by two channels (136 mg g⁻¹). This dual pore-size control strategy renders a record high equilibrium–kinetic combined selectivity for nHEX/3MP (30.2). Moreover, three- and five-component breakthrough experiments confirm the practical separation performances and cycling stability. Multiple theoretical simulations reveal the separation mechanism and adsorption sites.     

Development of zeolite-containing reforming catalysts

Copper-bearing zeolite catalysts are studied to develop a catalyst free of noble metals for obtaining high-octane components of motor fuels. The activity of the catalysts prepared on the basis of ZHM, ZHS-III-895, ZHS-III-889, and ZSM-5 zeolites with the different contents of copper is tested in the conversion of a model hydrocarbon (n-hexane). It is found that the reactions of isomerization and aromatization of n-hexane can lead to the formation of high-octane hydrocarbons in nonstandard conditions of conversion of n-alkanes: without feeding a hydrogen-bearing gas into the reaction medium at atmospheric pressure on copper-bearing high-silica zeolites, while an industrial process involving platinum catalysts is carried out under increased hydrogen pressure. The use of the recommended conditions for the conversion of hydrocarbons should lead to a reduction in the cost of final products.