Effects of steaming-made changes in physicochemical properties of Y-zeolite on cracking of bulky 1,3,5-triisopropylbenzene and coke formation

The effects of acidic properties and structural changes of Y zeolite, produced by steaming, on the zeolite cracking activity, coking tendency and distribution of various products during catalytic conversion of bulky 1,3,5-triisopropylbenzene (TIPB) are reported. NaY zeolite with framework Si/Al ratio of 2.4 was synthesized by a hydrothermal method and ammonium exchanged. The zeolite was dealuminated by a temperature-programmed steaming to form USY1 and USY2 zeolites with framework Si/Al ratio of 8.1 and 12.3 respectively. The catalysts were characterized by XRD, XRF, SEM, AAS, NH₃–TPD and N₂ adsorption–desorption techniques. The samples were in-situ activated at 748 K and evaluated by TIPB cracking at 623 K. The coke content of the catalyst beds was estimated by TPO using an FT-IR gas cell. The results of activity measurements reveal that the dealuminated zeolites lead to lower cracking activity initially; while, they exhibit higher activity at longer times. In addition, a slight modification of the window diameter of Y zeolite, as revealed by pore size distribution analyses, alters the diffusion limitation of the reactant and products through the pores of the zeolite and significantly affects the adsorbent–adsorbate interactions. TPO experiments show that compared to the precursor zeolite, lower amount of coke is formed on the dealuminated catalysts possessing lower density of acid sites. However, the coke formed on USY samples is heavier than that formed on its precursor Y zeolite. This may be attributed to the larger pores shaped in the dealuminated catalysts which in turn provide suitable places for coke formation and growth.     

Computational study on IM-5 zeolite: What is its preferential location of Al and proton siting?

The topological structure of IM-5 zeolite has remained a mystery for nearly 10 years. Stimulated by the recently structural solution of IM-5, we firstly report the computational study on the Al locations, acid sites and acid strength, which are important to understand the catalytic mechanism of IM-5. At the B3LYP/6-31+G(d,p) level, the 8T models were applied. The substitutions of Si by Al atom at 24 inequivalent tetrahedral crystallographic sites and the corresponding H proton localizations were examined by calculating the Al, H substitution energies, proton affinities, the atomic charges on proton and hydroxyl stretching vibrational frequencies. Based on the calculated results it was predicted that the most favorable sites for Al atom substitution in IM-5 were T4, T5, T14, T15 and T19 sites, whereas the least favorable sites were T1, T3, T8, T11, and T16 sites. There are about 40 preferable Al, H locations with relatively high acidity, including the nine strongest acid sites Al19H43 > Al14H18 > Al5H13 > Al4H8 > Al10H26, Al15H26, Al15H37, Al22H45 and Al24H47. The last five sites have equivalent proton affinity values. The numerous Al, H-sites with high acidity may be responsible for the high catalytic ability of IM-5. The calculated results should be helpful for understanding the chemistry of IM-5, the most complicated zeolite material known up to now.     

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.     

Synthesis and properties of a zeolite LEV analogue from the system—Na2O–Al2O3–SiO2–N,N-dimethylpiperidine chloride–H2O

A new structure-directing agent (SDA) was firstly reported for the synthesis of a zeolite LEV analogue. N,N-dimethyl piperidine performed the SDA function, and induced the synthesis of products from a zeolite MOR with 12-ring channels to a zeolite LEV analogue with only 8-ring channels. The zeolite LEV analogue was synthesized from gels with initial compositions (5.0–6.0)Na₂O–Al₂O₃–(10–200)SiO₂–(4.0–8.0)N,N-dimethyl piperidine–400H₂O at 150 °C. The ²⁹Si NMR spectra showed that the relative intensities of the first line at −115 ppm for low Si/Al ratios were lower than that at high Si/Al ratios. Varying ion exchanges led to different acidities in the zeolite LEV analogue, with the acidity of H-LEV-HCl higher than that of H-LEV-NH₃·H₂O. Zeolite H-LEV in hydration of propene showed a higher selectivity of 1-propanol.     

苯与重芳烃烷基转移反应工艺研究

在FTH-2催化剂上,考察反应原料物质的量比、反应温度和液体原料空速对苯与二异丙苯及以三异丙苯为主的重芳烃烷基转移反应的影响。结果表明,苯与二异丙苯烷基转移反应较适宜的条件为:n(苯)∶n(二异丙苯)=10,反应温度215℃,反应压力3.0 MPa,空速5 h-1,此条件下,二异丙苯转化率和异丙苯选择性分别约为94%和98%;苯与重芳烃混合液烷基转移反应较适宜的条件为:n(苯)∶n(三异丙苯)=18,反应温度280℃,反应压力3.0 MPa,空速5 h-1,此条件下,三异丙苯转化率、异丙苯选择性和二异丙苯选择性分别约为75%、64%和31%。     

High activity in catalytic cracking of large molecules over a novel micro-micro/mesoporous silicoaluminophosphate

Abstract  

A novel micro-micro/mesoporous silicoaluminophosphate ZSM-5-SAPO-5/MCM-41 (define as MZS-5) composite material with regular spherical morphology was synthesized through a novel process of the self-assembly of CTAB surfactant micelles with silica-alumina source which originated from the alkaline treatment of ZSM-5 zeolite. The physical properties of the MZS-5 composite material were characterized by XRD, FT-IR, Nitrogen adsorption–desorption, SEM and Py-FTIR techniques. Catalytic tests showed that the MZS-5 composite catalyst exhibited higher catalytic activity compared with the conventional microporous ZSM-5, SAPO-5 zeolite and mesoporous Al-MCM-41 molecular sieve for catalytic cracking of 1,3,5-triisopropylbenzene (TIPB). The remarkable catalytic reactivity of TIPB molecules was mainly attributed to the presence of the hierarchical zeolite structure. In the MZS-5 structure, the mesopores provided pathways for transportation of larger molecules and the microporous ZSM-5 and SAPO-5 zeolite provided acidic sites for catalytic activity.     

Enhanced propene/ethene selectivity for methanol conversion over pure silica zeolite: Role of hydrogen-bonded silanol groups

Pure silica zeolite with MEL structure (Si-ZSM-11) was firstly reported as an efficient Methanol-to-Propene (MTP) catalyst in methanol conversion, with higher propene yield (14.0 wt.%) and propene/ethene ratio (5.9) than H-ZSM-11 zeolite with a Si/Al ratio of 26 (7.4 wt.% and 1.9, respectively). Hydrogen-bonded silanol groups in Si-ZSM-11 are weakly acidic and act as active sites in methanol conversion, predominantly promoting propene production and inhibiting side reactions.     

Synthesis and characterization of hollow zeolite microspheres with a mesoporous shell by O/W/O emulsion and vapor-phase transport method

Hollow microspheres of ZSM-5 with a mesoporous shell have been synthesized through formation of amorphous hollow SiO₂/Al₂O₃ microspheres by sol–gel process in multiple oil–water–oil emulsions and transformation of the amorphous species into zeolite by water–organic vapor-phase transport treatment at 160 °C for 8 days. The morphology of the amorphous and zeolite spheres observed by scanning electron microscopy shows no significant change whereas the molar ratio of Si/Al increases from 6 to 20 during the transformation. The structural feature of zeolite was characterized by X-ray diffraction and ²⁹Si and ²⁷Al magic-angle spinning nuclear magnetic resonance. Transmission electron microscopy and N₂ adsorption–desorption isotherms indicate that uniform mesopores in the shell of zeolite spheres arise from the interstices among zeolite crystallites.     

Enhancement of the photocatalytic reactivity of TiO2 nano-particles by a simple mechanical blending with hydrophobic mordenite (MOR) zeolite

The photocatalytic oxidation of gaseous acetaldehyde with O₂ on commercial TiO₂ nano-particles could be successfully enhanced by a simple mechanical blending with a high-silica mordenite (MOR) zeolite, the surface of which showed high hydrophobic properties. When the TiO₂ nano-particles of ca. 5–20 wt% were mixed with the MOR zeolite powders in an agate mortar for only 5 min, the blended TiO₂/MOR samples showed higher photocatalytic reactivity as compared to the pure TiO₂ nano-particles. Since the high-silica zeolite powders are highly transparent in UV light regions, the incident UV light is effectively irradiated onto the whole part of the TiO₂ nano-particles without any loss of light intensity. Furthermore, the siliceous MOR zeolite powders effectively adsorb the gaseous acetaldehyde molecules and supply them onto the surfaces of the blended TiO₂ nano-particles, resulting in an enhancement of the photocatalytic reactivity.     

Preparation and characterization of Pt/HMFI–SBA-15 hybrid catalyst for tetralin transformation

A Pt catalyst supported on a hybrid material, HMFI–SBA-15, was prepared. Both, support and catalyst (Pt/HMFI–SBA-15) were characterized by nitrogen physisorption, small and wide (2θ) angle XRD patterns, FT-IR, SEM and HRTEM. The acidic properties of the hybrid material were studied by cumene dealkylation and those of the catalyst were studied by FT-IR of adsorbed pyridine. The catalyst, Pt/HMFI–SBA-15, was tested for tetralin transformation at various reaction temperatures 498, 523, 548, 573, 585 and 598 K. Wide-angle XRD and FT-IR in the skeletal region indicate the presence of MFI zeolite fragments incorporated onto SBA-15. The characterization of the acid sites on the support by cumene dealkylation and FT-IR pyridine adsorption revealed the presence of Brönsted acid sites related to the HMFI zeolite fragments in the hybrid materials. For the catalyst, a homogeneous distribution of Pt clusters was found by HRTEM. In the transformation of tetralin, at all the reaction temperatures, the main products were trans + cis-decalins. However, at high reaction temperature ring contraction to spirodecane and dehydrogenation to naphthalene were observed. At 598 K, a maximum of 8% of ring contraction products was obtained.     

Disproportionation and Methylation of Toluene with Methanol Over Zeolite Catalysts

Zeolites TNU-9, SSZ-33, mordenite (MOR) and ZSM-5 were evaluated for their activities in toluene disproportionation and methylation reaction of toluene with methanol. The medium-pore zeolite TNU-9 was found to possess the highest conversion in toluene disproportionation as compared with SSZ-33, mordenite and ZSM-5 based catalyst. Zeolite mordenite with the highest Si/Al ratio of 135.9 (the lowest concentration of active sites) exhibited the highest toluene conversion and maximum xylene yield in toluene methylation. On mordenite, the presence of channel intersections allows more reaction space for the formation of bulky intermediates and/or products and the 12-ring channels on the other hand, allow diffusion without trapping, since the channel diameter is large enough. In toluene methylation, xylene selectivity at the same conversion level follows the order: ZSM-5 > TNU-9 > MOR > SSZ-33, which implies that xylene selectivity is directly related to the size of channels from medium to large pore zeolites. The medium-pore zeolite ZSM-5, with 10-ring channels, shows the lowest reactivity for further alkylation of xylene, while mordenite with 12-ring channel shows the highest reactivity for further alkylation of xylene.     

Microwave-assisted versus conventional synthesis of zeolite A from metakaolinite

This is a comparative study of the synthesis of zeolite A from metakaolinite using both conventional and microwave-assisted heating. The effects of reaction conditions on the rate of formation, crystallinity and actual % yield of zeolite A were investigated. Reaction parameters such as different NaOH molarities (1.0–5.0 M), temperatures (70 and 80 °C), durations (1–8 h) as well as the effect of seeding percentage (1–4%) were tested. The rate of zeolite A formation was found to increase by 2–3 times in microwave treated samples with a notable enhancement in the product crystallinity and % yield whether seeded or un-seeded.     

Mesoporous beta zeolite obtained by desilication

Zeolite beta crystals (Si/Al = 35) synthesized in fluoride medium were treated in aqueous 0.2 M NaOH solution for mesopore formation by selective extraction of framework silicon. A 16 parallel-batch reactor was used to study the influence of the treatment time and temperature on the physico-chemical properties of the zeolites, which were characterized by ICP-OES, XRD, N₂ adsorption at 77 K, SEM, TEM, DRIFTS, and in situ ATR-IR. Alkaline treatment of H-beta within the optimal window of Si/Al ratios identified for other zeolite families leads to extensive silicon extraction at mild treatment conditions. This originates substantial mesoporosity and presumably improved transport, but negatively impacts on the microporous and acidic properties of the resulting sample. Consequently, the alkaline-treated beta zeolites show lower catalytic activity in the acid-catalyzed liquid-phase benzene alkylation than the purely microporous parent material. The relatively low stability of framework aluminum in BEA, compared to MFI and MOR, is detrimental for the controlled mesopore formation by Si dissolution, since aluminum cannot optimally exert its pore-directing role.     

Do Cu(II) ions need Al atoms in their environment to make CuSiBEA active in the SCR of NO by ethanol or propane? A spectroscopy and catalysis study

The two-step postsynthesis method (creation of vacant T-sites and associated SiOH groups by dealumination of BEA zeolite with nitric acid followed by incorporation of copper in the resulting SiBEA by impregnation with an aqueous solution of copper nitrate) allows to obtain a CuSiBEA zeolite which contains 0.8 Cu wt%. The incorporation of Cu(II) into the lattice of SiBEA is evidenced by XRD while the concomitant consumption of SiOH groups is monitored by FTIR. The presence of mainly isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry is evidenced by diffuse reflectance UV–vis-NIR, EXAFS and XANES. The CuSiBEA zeolite is active in the selective catalytic reduction (SCR) of NO with ethanol or propane with maximum NO conversion of 40 and 20% and selectivity toward N₂ close to 80–90 and 90–100%, respectively. These results suggest that the SCR process occurs on isolated mononuclear Cu(II) in D_2d-distorted tetrahedral symmetry after Al atoms have been removed from the zeolite structure. Thus, Cu(II) ions do not need Al atoms in their environment to be catalytically active. The lack of correlation between the SCR activity in presence of ethanol and the oxidation of NO to NO₂ suggests that the two reactions are more competitive than sequential. The higher activity of CuSiBEA with ethanol than with propane may be due to different activation energies and/or reaction mechanisms.     

Dimethyl ether synthesis via methanol and syngas over rare earth metals modified zeolite Y and dual Cu–Mn–Zn catalysts

A series of zeolite Y modified with La, Ce, Pr, Nd, Sm and Eu were prepared via ion-exchange, and characterized by XRD, FT-IR and NH₃-TPD. It was found that these rare earth metals were encapsulated in the supercage of zeolite Y and resulted in its enhanced acidity. Among them, La-, Ce-, Pr- and Nd-modified zeolite Y exhibited higher activity and stability (than pure HY) for methanol dehydration to dimethyl ether (DME). For DME synthesis directly from CO hydrogenation using the dual Cu–Mn–Zn/modified-Y catalysts, it was found that Cu–Mn–Zn/La–Y and Cu–Mn–Zn/Ce–Y were more active than Cu–Mn–Zn/pure-HY. The conversion of CO on Cu–Mn–Zn/Ce–HY achieved 77.1% in an isothermal fixed bed reactor at 245 °C, 2.0 MPa, H₂/CO = 3/2 and 1500 h⁻¹.     

The action of vanadium over Y zeolite in oxidant and dry atmosphere

Y zeolite was impregnated with vanadium naphtenate and thermally treated at 720 °C for 5 h in dry air. Calcinated samples were characterized by micropore volume, UV–vis-DRS, cracking of n-butane and isopropylamine decomposition followed by simultaneous DSC–TGA. Results show that in dry air the zeolite is stable even at high vanadium loadings but the metal is able to move on the surface of the catalyst and neutralize acid sites. n-Butane conversion increases with vanadium concentration but this is a result of the metal's dehydrogenation ability. Selectivity to protolytic primary cracking (PPC) products decreases linearly with vanadium concentration as a result of the neutralization of Brønsted acid sites quantified by the isopropylamine test. Rate of cracking vs. number of acid sites shows that on a unit of time (second) approximately only one in seven thousand acid sites measured by the isopropylamine technique is able to crack n-butane by monomolecular protolytic cracking mechanism. Selectivity to hydride transfer (SHT) products remains constant with vanadium concentration except at high loadings. Vanadium is able to move on surface and neutralize preferentially acid sites where more energy is released in the reaction with isopropylamine; acid sites strong enough to crack n-butane. Neutralization of acid sites is probably the first step in Y zeolite destruction under conditions predominant in the regenerator of an FCC plant.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

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.     

Study on alkylation of benzene with propylene over MCM-22 zeolite catalyst by in situ IR

For the alkylation reaction of benzene with propylene over MCM-22 zeolite catalyst, two completely different results can be obtained for two different operating procedures. If the benzene is pumped into a reactor then followed by propylene (operation 1), the alkylation reaction proceeds normally, while the reaction can not occur if the propylene is introduced first followed by benzene (operation 2). In situ IR technology was used to investigate two modes designed to simulate the above operating processes. The mechanisms of the two operations are as follows: in operation 1, benzene molecules first fully were adsorbed on the acidic sites of MCM-22 zeolite, when propylene was introduced, propylene molecules seized the acidic sites by repelling benzene, at the same times propylene molecules were polarized and reacted with absorbed benzene molecules. This is synchronous reaction mechanism; in operation 2, propylene introduced was molecularly absorbed on acidic sites strongly resulting in producing carbonium ions of CH₃–C⁺H–CH₃, then the carbonium ions reacts with other propylene molecules further to form polymeric species. The polymers blocked the pores and covered the acidic sites so that the alkylation reaction with benzene can not take place. This process is carbonium ions reaction mechanism of propylene itself.     

Highly selective olefin hydrogenation: Refinery oil upgrading over bifunctional PdOx/H-ZSM-5 catalyst

A novel strategy is reported in this study to design a novel bifunctional PdO_x/H-ZSM-5 catalyst for selective hydrogenation of olefin. PdO_x species (basic oxides) anchored on the strong acidic sites (Brönsted acid sites) of zeolite support could not only generate new acidic sites with moderate acidic strength but also exhibit good capability for H₂ activation and dissociation. The conversion of 1-decene is 100% and total selectivity of methyl-nonane is 98.4 ± 0.4% at 350 °C. Our research might provide a valuable approach to design a highly efficient bifunctional catalyst for olefin upgrading.