A high‐resolution solid‐state NMR study on nano‐structured HZSM‐5 zeolite

Variations in the structure and acidity properties of HZSM‐5 zeolites with reduction in crystal sizes down to nanoscale (less than 100 nm) have been investigated by XRD, TEM and solid‐state NMR with a system capable of in situ sample pretreatment. As evidenced by a combination of ²⁷Al MAS NMR, ²⁹Si MAS, CP/MAS NMR and ¹H MAS NMR techniques, the downsize of the zeolite crystal leads to an obvious line broadening of the ²⁷Al, ²⁹Si MAS NMR spectrum, an increasing of the silanol concentration on the external surface, and a pronounced alteration of the acidity distribution between the external and internal surfaces of the zeolite. In a HZSM‐5 zeolite with an average size at about 70 nm, the nonacidic hydroxyl groups (silanols) are about 14% with respect to the total amount of Si, while only 4% of such hydroxyl groups exist in the same kind of zeolite at 1000 nm crystal size. The result of ¹H MAS NMR obtained using Fluorinert^® FC‐43 (perfluorotributyl amine) as a probe molecule demonstrates that most of the silanols are located on the external surface of the zeolite. Moreover, the concentration of Brønsted acid sites on the external surface of the nano‐structured zeolite appears to be distinctly higher than that of the microsized zeolite.     

Effect of crystal size and surface modification of ZSM-5 zeolites on conversion of ethanol to propylene

The conversion of ethanol to propylene was carried out over ZSM-5 zeolites (Si/Al ratio ≈ 20) with a small crystal size of ca. 30 nm. Catalyst deactivation was significantly suppressed over the nanometer-sized ZSM-5 zeolite, indicating that the small crystal was more tolerant to coking. On the other hand, the selectivity of this zeolite to propylene was lower than that of conventional ZSM-5 zeolites (ca. 2 μm). It was suggested that the large external surface area of the nanometer-sized ZSM-5 zeolite catalyzed undesired reactions. To elucidate the reason for the decreased selectivity, the external surfaces of the nanometer-sized crystals were covered with a very thin pure-silica ZSM-5 layer by a hydrothermal synthesis. The obtained crystal maintained the same crystal size and had a silica-rich surface (Si/Al ratio ≈ 50). After the surface modification, the selectivity to propylene was improved without any decrease in the catalyst life.     

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.     

Methanol to olefins conversion over metal containing MFI-type zeolites

A series of metal containing MFI-type zeolites were synthesized by the rapid crystallization method with in two hours. Al(III), Cr(III), Cu (II), and Fe(III) metal ions were incorporated in the structure. Pure siliceous (Al free) MFI structured zeolite was also prepared by the same procedure as used for the metallosilicate synthesis. Si/M ratio of the synthesis gel was 100 for metallosilicates aiming for high-silica zeolites. The products have been characterized by elemental analysis, specific surface area determination, scanning electron microscopy with EDX analysis, electron spin resonance, and thermal analysis. Catalytic property of the zeolites was measured for methanol to olefin conversion reaction. The results of XRD measurement showed the only crystalline phase present was that of MFI-type zeolite. E.s.r spectroscopy has been used to study the isomorphic substitution of the metal ions incorporated in the structure. The results show that Fe(III) and Cu (II) were mainly incorporated while, Cr(III) was observed to be present in non-framework sites. The amount of template occluded into the structure was determined by thermal analysis and found to be the same for metallosilicates and highly crystalline aluminum free zeolite. The results of catalytic activity showed that about 85% selectivity (based on gaseous hydrocarbon) for C₂⁼–C₄⁼ olefin was achieved in the case of iron containing synthesized high-silica zeolite as compared with 58% in the case of Al-containing zeolite catalyst.     

Fractal Extra-Framework Species in De-aluminated LaY Zeolites and Their Catalytic Activity

Lanthanum-containing Y (LaY) zeolites were prepared by ionic exchange from NaY parent zeolite. The LaY zeolites were de-aluminated by steaming. De-aluminated zeolites presented different Si/Al ratio. The physicochemical properties of these catalysts were characterized by X-ray diffraction, pyridine and xenon adsorption, infrared spectroscopy and ²⁹Si, ²⁷Al, ¹²⁹Xe, ¹³⁹La solid-state nuclear magnetic resonance spectroscopy. Furthermore, a fractal geometry approach was adopted to describe the evolution in the texture as a consequence of de-alumination. The catalytic properties of materials were evaluated in the n-hexane cracking reaction. The catalyst with the highest catalytic activity was the zeolite highest de-aluminated (Si/Al ratio of 3.7). Such performance was attributed on the one hand, to active extra-framework aluminum species hosted in the large cavities of zeolites and, on the other hand to redistribution of lanthanum species into the zeolite as a consequence of de-alumination.     

微波辐射加热金属离子改性Hβ分子筛催化合成蒽醌

在微波辐射加热下用不同金属离子(Al3+、Ti4+、Fe3+)对Hβ分子筛进行改性,并将其用于催化苯与苯酐合成蒽醌。采用XRD、NH3-TPD和吡啶-IR对分子筛进行了表征。结果表明,微波辐射加热能提高分子筛的改性效率,改善其催化性能,且对分子筛晶体结构没有破坏作用。不同金属离子改性分子筛的催化效果差别很大,其中,0.5 mol/L的Al2(SO4)3溶液改性的Hβ分子筛催化效果最好,蒽醌产率38.87%,选择性92.78%。分子筛的表面酸性质对其催化性能有较大影响,催化剂表面的中强度酸是催化活性中心,L酸量与B酸量的比值对分子筛的催化性能有重要影响。     

Effects of the Nitridation of Y and USY Zeolites on their Catalytic Activity for the Base Catalyzed Knoevenagel Condensation

This contribution reports on the preparation, physicochemical characterization and catalytic performances of nitrided zeolites in the Knoevenagel condensation reaction. These basic materials were prepared by subjecting one Y zeolite (Si/Al ratio of 2.6) and two ultrastable Y zeolites (Si/Al ratio of 13 and 37) to nitridation, i.e., treatment with ammonia at high temperature. Both the amount and the chemical nature of incorporated nitrogen species were controlled by the nitridation temperature. Namely, an increase of the temperature induces an increase of the nitrogen content and the appearance of nitrogenous species in the following order of increasing temperature: NH₄ ⁺, adsorbed NH₃, –NH₂, >NH and >N–. The nitridation occurred practically in the same manner whatever the Si/Al ratio of the starting material. However, from a catalytic point of view different behavior was observed. No direct correlation was found with the nitrogen content of the samples. Nitrided zeolites were found to exhibit catalytic activity as long as the zeolitic framework was maintained.     

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.     

Xylene Isomerization in the Liquid Phase Using Large‐Pore Zeolites

Three large‐pore zeolites, Beta with Si/Al ratios of 25 and 35 and Mordenite with an Si/Al ratio of 30, were studied in the conversion of o‐xylene at 493 K. Maximum conversion was achieved by the catalyst with the highest Si/Al ratio due to faster diffusion of the isomer inside the zeolite channels because of the lower acidity of the solid even with larger crystal size. A kinetic study was then carried out over this catalyst between 473 and 513 K in a batch reactor in the liquid phase. The activation energies obtained do not indicate the presence of diffusional constraints towards any isomer. Finally, the kinetic model obtained was simulated in a fixed‐bed reactor and compared to ZSM‐5 in the temperature range from 493 to 533 K. An increment in p‐xylene production of 20 % on average was obtained.     

Statistical analysis of Al distributions and metal ion pairing probabilities in zeolites

The distributions of Al tetrahedral sites are studied as a function of Si : Al ratio (from 11 to 95) in three zeolites: ZSM-5, ferrierite, and mordenite. Al configurations obeying Loewenstein’s rule are generated by a Monte Carlo procedure and analyzed in terms of accessibility to specific channels. The distribution of pairs of Al sites accessible to the same channel segregates into distinct nearest-neighbor and ring-bridging types. Such pairs provide favorable binding environments for extraframework MO_xM²⁺ cations in the metal-ion-exchanged forms of the zeolites. In all three zeolites, the probability of a given Al site having at least one suitable partner for pairing within 8.5 Å is close to unity for typical Si : Al ratios. The probability at shorter distances is more sensitive to the zeolite structure, reflecting the larger number of highly puckered five- and six-membered rings in ZSM-5. This sensitivity, and its implications for CuO_xCu²⁺ formation in the three Cu-exchanged forms, may explain why Cu-ZSM-5 is the most active of the three for catalytically decomposing NO.     

Carbonation of C–S–H and C–A–S–H samples studied by 13C, 27Al and 29Si MAS NMR spectroscopy

Synthesized calcium silicate hydrate (C–S–H) samples with Ca/Si ratios of 0.66, 1.0, and 1.5 have been exposed to atmospheric CO₂ at room temperature and high relative humidity and studied after one to 12 weeks. ²⁹Si NMR reveals that the decomposition of C–S–H caused by carbonation involves two steps and that the decomposition rate decreases with increasing Ca/Si ratio. The first step is a gradual decalcification of the C–S–H where calcium is removed from the interlayer and defect sites in the silicate chains until Ca/Si = 0.67 is reached, ideally corresponding to infinite silicate chains. In the seconds step, calcium from the principal layers is consumed, resulting in the final decomposition of the C–S–H and the formation of an amorphous silica phase composed of Q³ and Q⁴ silicate tetrahedra. The amount of solid carbonates and of carbonate ions in a hydrous environment increases with increasing Ca/Si ratio for the C–S–H, as shown by ¹³C NMR. For C–A–S–H samples with Ca/Si = 1.0 and 1.5, ²⁷Al NMR demonstrates that all aluminium sites associated with the C–S–H are consumed during the carbonation reactions and incorporated mainly as tetrahedral Al(–OSi)₄ units in the amorphous silica phase. A small amount of penta-coordinated Al sites has also been identified in the silica phase.     

碱改性介-微孔HZSM-5分子筛催化乙醇脱水制乙烯性能

对两种不同硅铝比的HZSM-5分子筛进行碱处理,制备介-微孔复合HZSM-5分子筛,研究乙醇脱水制乙烯的催化性能,并考察碱溶液浓度和处理温度对HZSM-5分子筛孔结构和表面酸性的影响。结果表明,适宜的碱处理条件有利于分子筛发生骨架脱硅和脱铝,从而形成介孔。碱处理对硅铝比低的HZSM-5分子筛酸性质影响明显,而硅铝比高的HZSM-5分子筛在碱处理过程中酸性质变化不明显,更易发生脱硅和脱铝而形成更多介孔。碱改性介-微孔HZSM-5分子筛催化剂使乙醇脱水制乙烯催化性能得到改善,尤其低温催化活性提高,这主要归功于碱处理中介孔的形成和表面酸性的调变。     

Accessibility of lithium cations in high-silica zeolites investigated using the NMR paramagnetic shift effect of adsorbed oxygen

The accessibility of lithium cations in the high-silica zeolites ZSM-5 and beta was investigated using ⁷Li MAS NMR spectroscopy. The samples are investigated in vacuum and in the presence of adsorbed oxygen molecules. The paramagnetic shift effect due to the oxygen molecules was exploited to determine the fractions of accessible lithium cations. Approximately 85% of lithium cations in Al-ZSM-5 (Si/Al=50) are accessible to oxygen molecules, while none of the cation sites in zeolite Al-beta (Si/Al=18 and 8) and only a small fraction in B-beta (Si/B=15) were available for adsorption. This difference indicates a very distinct coordination environment for Li⁺ in zeolite beta as compared to ZSM-5.     

Esterification of oleic acid in soybean oil on zeolite catalysts with different acidity

The catalytic activity of zeolites was studied in the esterification of oleic acid with methanol in soybean oil. The influences of acidity and pore structure of the zeolites were investigated in relation to conversion of the oleic acid on the zeolite catalysts. H⁺ ion exchanged ZSM-5 (HMFI) and mordenite (HMOR) zeolites conducted with different Si/Al ratio were employed to examine an influence of acidity in the reaction. Conversion of oleic acid was about 80% on HMOR zeolites and HMFI(25) zeolite. The conversion of oleic acid was improved as the amount of acid site increased. The amount of acid site of the zeolites affected significantly the catalytic activity in the esterification of oleic acid.     

Temperature-programmed desorption study of molecular oxygen adsorbed on MFI-type zeolites

The adsorption of molecular oxygen at room temperature on the proton form of MFI zeolites with different Si/Al ratios has been investigated by temperature-programmed desorption (TPD). The internal Si-OH defects (silanol groups) in these zeolites are found to serve as O₂ adsorption sites. The apparent activation energies (17.5–21.8 kcal-mol^-1) of desorption determined from O₂ TPD measurements reveal that the extent of interactions between the Si-OH defects and the adsorbed O₂ molecules becomes weaker with decreasing Al content in the zeolite.     

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.     

Gas‐ versus liquid‐phase α-pinene transformation over faujasite‐like zeolites and an amorphous silicoaluminate

The transformation of α-pinene was studied in both gas and liquid phase over a series of faujasite‐like zeolites having different framework composition. A comparison with an amorphous silicoaluminate having 13 wt% of Al₂O₃ (SA) was made. Camphene + limonene were the major products observed in liquid phase at 393 K at conversion levels lower than 90%. At 473 K, where the reaction in gas phase took place, limonene suffered an ulterior transformation mainly into terpinenes and terpinolenes and a faster deactivation of the zeolite catalysts as compared to the one in liquid phase was observed. This behaviour contrasts with the high stability shown by SA in gas phase. Over the wider pore system of SA undesired heavy products were formed in liquid phase at the expenses of the terpene isomers. However, in gas phase this ulterior transformation of monoterpenes was minimised. A maximum activity for the zeolite catalysts was observed when the number of Al^IV per unit cell reached about 17, a value where the population of Si(0Al) sites has been shown to reach the maximum. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Modified Beta Zeolites with Acid on Anisole Acetylation in a Fixed Bed Reactor

A beta zeolite sample (Si/Al = 30) was modified by citric acid, tartaric acid, and hydrochloric acid. The catalytic performances of these zeolite together with another beta zeolite (Si/Al = 12.5) were tested in the anisole acetylation in a fixed-bed configuration. The zeolites were characterized by powder X-ray diffraction, FT-IR of adsorbed pyridine, and temperature programmed desorption of ammonia. The ratios of Si/Al, the specific surface areas, and the pore sizes were also measured. The results showed that the most active and stable behavior was achieved on the HCl treated zeolite. The strong acid sites as many as possible but with an appropriate concentration and strength were needed in order to obtain an optimum activity and stability.     

The effect of hierarchical single-crystal ZSM-5 zeolites with different Si/Al ratios on its pore structure and catalytic performance

Hierarchical single-crystal ZSM-5 zeolites with different Si/Al ratios(Hier-ZSM-5-x,where x=50,100,150 and 200)were synthesized using an ordered mesoporous carbon-silica composite as hard template.Hier-ZSM-5-x exhibits improved mass transport properties,excellent mechanical and hydrothermal stability,and higher catalytic activity than commercial bulk zeolites in the benzyl alcohol self-etherification reaction.Results show that a decrease in the Si/Al ratio in hierarchical single-crystal ZSM-5 zeolites leads to a significant increase in the acidity and the density of micropores,which increases the final catalytic conversion.The effect of porous hierarchy on the diffusion of active sites and the final catalytic activity was also studied by comparing the catalytic conversion after selectively designed poisoned acid sites.These poisoned Hier-ZSM-5-x shows much higher catalytic conversion than the poisoned commercial ZSM-5 zeolite,which indicates that the numerous intracrystalline mesopores significantly reduce the diffusion path of the reactant,leading to the faster diffusion inside the zeolite to contact with the acid sites in the micropores predominating in ZSM-5 zeolites.This study can be extended to develop a series of hierarchical single-crystal zeolites with expected catalytic performance.     

Crystallinity of large single crystals of FAU-type zeolites with a wide range of Si/Al ratios

Large colorless single crystals of FAU-type zeolites were synthesized from gels with the composition xSiO₂ : 2.0NaAlO₂ : 7.5NaOH : 454H₂O : 5.0TEA, where x = 2.0–6.0. FAU-type zeolite with Si/Al = 1.26(4) was nearly pure and the maximum size of the single crystals was ca. 150 μm. In case of FAU-type zeolites with Si/Al = 1.54(5), the maximum size of single crystals was ca. 200 μm and the ratio of FAU/impurity was 0.07. The framework Si/Al ratio of the as-synthesized FAU-type zeolite tended to increase with the Si/Al ratio of gel composition. All of the large single crystals had good crystallinities for single-crystal X-ray diffraction, leading to enough numbers of significant reflections which have strong intensity. The structure of a single crystal of dehydrated zeolite Na-X (Si/Al = 1.41(4)) with composition |Na₈₀|[Si₁₁₂Al₈₀O₃₈₄]-FAU per unit cell was determined by X-ray diffraction methods in the cubic space group \( Fd \bar{3} m; \) a = 24.9434(6) Å at 294 K. The structure was refined by using all intensities to the final error indices (using only the 771 reflections for which F _o > 4σ(F _o)), R ₁ = 0.048 (based on F) and R ₂ = 0.188 (based on F ²). In the crystallographic studies, the Si/Al ratio of the synthetic FAU-type zeolite is 1.41(4) which is quite consistent with the SEM–EDS analysis.