Synthesis, characterization, and catalytic properties of a hydrothermally stable Beta/MCM-41 composite from well-crystallized zeolite Beta

A Beta/MCM-41 composite has been synthesized with a new method by using well-crystallized zeolite Beta as silica and aluminum source. The prepared composite was characterized by XRD, FTIR, N₂ adsorption/desorption at 77 K, FE-SEM, DTG, ²⁹Si MAS NMR spectral techniques. It was shown that the composite consisted of a highly ordered mesoporous MCM-41 phase and a zeolite Beta phase. Its hexagonal mesoporous structure was still retained after statically treated for 120 h in boiling water. In contrast, the structure of generally synthesized Al-MCM-41 nearly completely collapsed. This might be attributed to the assembly of the dissolved fragments such as the first and/or secondary structural units of zeolite Beta into the mesoporous structure around surfactant micelles. This is supported by the catalytic result that the prepared composite showed higher activity and selectivity for medium fraction in hydrocracking of Daqing vacuum residue than the parent zeolite Beta, the Al-MCM-41 as well as the mechanical mixture of these two materials.     

Contribution to the hydrothermal synthesis of zeolite beta and its modifications with gallium

The zeolite beta is a crystalline synthetic aluminosilicate with wide-pore size (6 to 9 Å) and a Si/Al ratio from 5 to 100. In this study, we have investigated the influence of several synthesis parameters on the physicochemical properties of the materials. The synthesis of zeolite beta has been performed by hydrothermal method, the aluminum source and gallium content in the structure being variable. The structural characterization was carried out by means of X-ray diffraction, SEM and ²⁷Al and ⁷¹Ga MAS NMR spectroscopy. When the aluminum sulfate is used as aluminium source, zeolite beta with higher crystallinity is generated. All the samples showed a XRD pattern similar to crystalline zeolite beta and the ²⁷Al and ⁷¹Ga MAS NMR spectrum demonstrated that the incorporation of both heteroatoms was efficient.     

PERFORMANCE OF ACIDIC MCM-LIKE ALUMINOSILICATE CATALYSTS IN PYROLYSIS OF POLYPROPYLENE

Mesoporous aluminosilicate catalysts having different Al/Si ratios were synthesized following a hydrothermal synthesis route and using different aluminum sources, such as aluminum nitrate and aluminum isopropoxide. These mesoporous materials have high surface areas, in the range of 520–1001 m²/g, and exhibit Type IV nitrogen adsorption isotherms. EDS and ²⁷Al MAS NMR results showed that aluminum was incorporated more effectively into the structure of the catalyst forming a tetrahedral framework when aluminum nitrate was used as the aluminum source. The activities of these catalysts in the polypropylene pyrolysis reaction were tested in a TGA apparatus. Results showed a marked reduction in the degradation temperature in the presence of aluminosilicate catalysts. The activation energy of degradation was 172 kJ/mole without any catalyst. However, using the mesoporous aluminosilicate catalysts synthesized by using aluminum nitrate as the aluminum source, activation energy of the degradation reaction decreased to values of about 24–28 kJ/mole.     

Facile tailoring of hierarchical mesoporous AlSBA-15 by ionic liquid and their applications in heterogeneous catalysis

Significant enhancement in the performance of incorporation of high content of aluminum within hierarchical mesoporous SBA-15 has been achieved by direct route using urea tetrachloroaluminate ionic liquid as novel aluminum source. The fabricated materials were fully characterized by N₂ sorption isotherms, powder X-ray diffraction (XRD), FT-IR, ²⁷Al MAS NMR, XRF, HRTEM and FESEM. The acidic properties of these materials have been examined using NH₃-TPD. The catalytic performance was investigated using cumene cracking and the esterification reaction as a probe molecules to estimate the acidity of the material. It is worth noting that the ionic liquid with accomplished Al–O–Si bonds is an efficient precursor to synthesis AlSBA-15 with high aluminum content (nSi/mAl?=?7) without destroying the structural order of the material in acidic medium. ²⁷Al MAS NMR spectra of AlSBA-15 show that all aluminum species were incorporated into the SBA-15 framework with nSi/mAl ratio up to 7. Overall, this work emphasize that the AlSBA-15 materials contain Bronsted and Lewis acid sites with medium acidity which makes them adequate to be adopted as acid catalysts in heterogeneous catalysis.     

Synthesis of mesoporous molecular sieves: influence of aluminum source on Al incorporation in MCM-41

Three series of mesoporous aluminosilicate molecular sieves, MCM-41, with various Si/Al ratios were synthesized using different aluminum sources (aluminum isopropoxide, pseudo boehmite and aluminum sulfate). XRD analysis, temperature programmed desorption ofn-butylamine,²⁷Al and²⁹Si MAS NMR, and catalytic alkylation test indicated that aluminum isopropoxide is a better source for incorporating aluminum in the framework of MCM-41 type molecular sieves with better crystallinity and acid characteristics.     

Composite SBA-15/MFI Type Materials: Preparation, Characterization and Catalytic Performance

SBA-15 type mesoporous molecular sieve was prepared and used as a parent material for synthesis of a SBA-15/zeolite composite. Thus, after addition of an aluminum source into the pristine sample, it was subjected to recrystallization in the water-vapor phase to give the SBA-15/MFI composites. The samples obtained were characterized by XRD, nitrogen sorption, scanning and transmission electron microscopies, ²⁷Al MAS NMR and FT IR. The materials after recrystallization retained their mesoporous character. Characteristics of the material revealed that all the aluminum added was inserted into the final hybrid. As shown by FT IR and NMR, at least some of aluminum was introduced into the nanoparticles of MFI and adopted four-fold coordination, typical for zeolites. Formation of Brønsted acid centres was directly confirmed by IR studies. The catalytic activity was screened in the liquid-phase isomerization of α-pinene. The main products of α-pinene transformations were camphene, limonene and γ-terpinene. The overall selectivity toward camphene + limonene was ca. 90%. Taking into account the low amount of the MFI phase present in a composite sample, calculated initial reaction rate was comparable with other catalytic systems explored in the isomerization of α-pinene.     

干气中烷烃、烯烃新型分离吸附剂的研究进展

综述了二十年来烯烃烷烃新型分离吸附剂的研究进展,与传统的π络合吸附剂相比,优先吸附烯烃的吸附剂包括金属有机材料（MOF）、Engelhard Titanosilicate （ETS）、高硅分子筛和介孔分子筛,主要是利用氢键作用、孔径大小、分子扩散速率差异或π络合作用,将烷烃烯烃分离;而优先吸附烷烃的吸附剂包括AlMePO-α、ZIF-7和凹凸棒黏土,主要利用吸附剂上甲基与烷烃的相互作用。MOF对烯烃吸附量大,但选择性较差;高硅分子筛对烯烃的动力学分离效果良好;介孔分子筛经过渡金属改性后,对烯烃有优良的选择性;ETS类对烯烃有较高的吸附量和选择性,且稳定性强,有很好的应用前景。     

The versatile synthesis method for hierarchical micro‐ and mesoporous zeolite: An embedded nanocarbon cluster approach

In this work, we are reporting for the first time the synthesis of hierarchical micro‐ and mesoporous zeolite using silica–carbon (SiO₂/C) composites prepared by pyrolysis of carbonaceous gases in the presence of silica gel. The pyrolysis effectively yielded carbon deposited onto the raw silica material. The obtained SiO₂/C composites were utilised as a bifunctional material, mesoporous template and silica source, for the zeolite synthesis. Tetrapropylammonium hydroxide (TPAOH) was used as a microporous template. The combination of the obtained composites and the TPAOH for the hydrothermal synthesis resulted in the formation of hierarchical micro‐ and mesoporous ZSM‐5. The results from the SEM, TEM, and N₂ adsorption/desorption isotherms, and ²⁷Al MAS NMR characterisations of the synthesised samples obtained after the removal of the templates confirmed the successful formation of the micro‐ and mesoporous zeolites. The mesoporosity of the zeolites could be controlled by adjusting the carbon content in the SiO₂/C composites while the carbon content could be controlled by varying the deposition time and the concentration of the carbonaceous gases used. This controllable and efficient synthesis method is considered to be a promising method for creating hierarchical micro‐ and mesoporous zeolites. © 2011 Canadian Society for Chemical Engineering     

Isomerization of α-pinene over dealuminated ferrierite-type zeolites

Isomerization of α-pinene was performed on a series of dealuminated ferrierite (FER)-type zeolites in liquid phase at 363 K using a batch reactor. The course of zeolite dealumination was followed in detail using ²⁹Si, ²⁷Al, ¹H MAS NMR, XRD, FTIR, and sorption of nitrogen. The ammonium form of FER was dealuminated with aqueous solutions of HCl. While retaining the crystallinity of the zeolite particles, the treatments removed up to 53% of the tetrahedrally coordinated aluminum atoms from the FER framework. According to ²⁹Si MAS NMR studies, the framework aluminum atoms located at the 10-membered rings in the main channels of FER (T_B sites) were depleted preferentially from their positions. Even relatively mild dealumination of FER led to an active catalyst containing both Brønsted and Lewis centers, yielding up to 97% conversion of α-pinene at 363 K, in contrast to the 72% observed for the parent hydrogen form. Such catalytic behavior was discussed in terms of the conversion of a reactant inside micropores of the zeolite catalyst, on Brønsted acid centers with enhanced strength located probably in the vicinity of Lewis sites. The selectivity toward camphene and limonene changed smoothly with the dealumination level; thus, a higher selectivity toward limonene was observed at the expense of camphene formation with increasing the n_Si/n_Al ratio of the catalysts. The selectivity toward camphene and limonene was close to 85% for all of the materials studied. The initial rates of α-pinene transformations over FER-type materials exceeded those observed for other catalytic systems, heteropoly acid/SiO₂ and H₂SO₄/ZrO₂. This study demonstrates the successful application of a medium-pore zeolite for the catalytic transformation of α-pinene in liquid phase.     

Silica Nanoboxes as New Nano-structured Materials: Their Secondary Synthesis from Alumina-Rich Zeolites

Novel silica nanoboxes were prepared by controlled dealumination of Na-X and Ca-A type zeolites using ammonium hexafluorosilicate (AHFS). The silica-richer the parent zeolite, the smaller the average pore size produced and the narrower the pore size distribution obtained. This was due to the specific reactivity of the extracting agent with the zeolite framework aluminum atoms. High temperature calcination of the dealuminated X-zeolite (ammonium form) resulted in mesoporous materials exhibiting an ink-bottle shape, a quite high surface area (330 m²/g, no micropores), an average pore diameter of 4.5 nm with a quite narrow pore size distribution (± 1.0 nm) and finally, a pore opening diameter of 3.9 nm. The latter was determined by using the nitrogen sorption isotherms (BET technique) and related pore volume data. The sorption behavior also suggested the interconnecting character of the newly created nanoboxes. The periodicity of these nanoboxes throughout the mesoporous material was clearly shown by X-ray powder diffraction at very small angles. These materials, herein called monomodal nanoboxes because of the absence of micropores in the structure, were also thermally stable. Incorporation of orthosilicate into the obtained silica nanoboxes, in accordance with the recently developed technique for pore size engineering in zeolites, led to materials with smaller pore openings but having almost the same textural properties. Solid superacidic materials were prepared by incorporating a liquid superacid (triflic acid or trifluoromethanesulfonic acid) into the silica nanoboxes using the wet impregnation technique. The maximum triflic acid loading which did not significantly affect the mesoporous framework of the materials was 24 wt%. As a reference, the maximum loading of less acidic sulfuric acid was slightly lower. All this showed the high chemical stability of the silica nanoboxes for supporting very acidic species. Temperature-programmed desorption using a combined DTA/TGA system allowed the identification of the bound phases and some liquid phase of the loaded triflic acid.     

NMR Study on the Acidity of TS-1 Zeolite

The acidic properties of TS-1 and Silicalite-1 zeolites have been investigated by the solid-state MAS NMR technique capable of in situ sample pretreatment. As shown by a combination of the ³¹P MAS NMR and ¹H MAS NMR techniques with trimethylphosphine, not only Brønsted acid sites but also Lewis acid sites exist in the TS-1 zeolites. Moreover, TS-1 zeolite is more acidic compared with Silicalite-1. The ¹H, ²⁹Si MAS NMR spectra and the resonance related to Brønsted acid species in the ³¹P MAS NMR spectra demonstrate clearly that the presence of titanium in the framework results in the formation of a new hydroxy group, titanols, which is more acidic than silanols of Silicalite-1. The ³¹P MAS NMR measurements also illustrate convincingly the existence of at least two different Lewis acid species on the TS-1 zeolites. The conversion of propylene oxide into methoxypropanol catalyzed by TS-1 or Silicalite-1 zeolite in methanol solution as a test reaction has also been described. With the increase of titanium in zeolite, TS-1 appears to have a higher activity during the reaction of propylene oxide to methoxypropanol.     

Realumination of zeolite Y under acidic conditions

Dealuminated zeolites Y were treated with aqueous solutions of various acids and ammonium salts to investigate the realumination behavior under acidic conditions. From the results of ²⁷Al MAS NMR, ²⁹Si MAS NMR and FT-IR measurements, it was found that a part of non-framework aluminum species in the dealuminated zeolite Y is effectively reinserted into the zeolite framework in CH₃COONH₄ and C₆H₅COONH₄ aqueous solutions. Pyridine adsorption experiments also revealed that most of incorporated aluminum species generate tetrahedrally coordinated framework aluminum species, namely Brönsted acid sites. Although the realumination also proceeded in H₂SO₄ and CH₃COOH aqueous solutions, large amounts of incorporated aluminum species were not necessarily responsible for generation of Brönsted acid sites. Framework connected aluminum species, presumably as 3-fold-coordinated Lewis acidic framework aluminum species, were mainly generated. In the TEM image of the realuminated zeolite Y, needle-like crystals with ca. 25–80 nm in length were observed, which are probably due to AlOOH generated from non-framework aluminum species.     

Extraction of nano-porous silica from hydrosodalite produced via modification of low-grade kaolin for removal of methylene blue from wastewater

BACKGROUND: This work employs low-grade kaolin as raw material in the economical production of nano-porous silica with extended surface area via the hydrothermal acidification of intermediate zeolite composite to remove methylene blue from wastewater. Hydrosodalite fabricated through the modification of meta-kaolin (MK) with sodium aluminate (NaAlO₂) was used as a precursor in the hydrothermal acidification step. RESULTS: The direct conversion of MK to zeolite composite generating a low crystalline phase, containing quartz which was eliminated in the acidification step, exhibited a strong affinity towards dye adsorption. It was indicated that the NaAlO₂/MK ratio could influence the hydrosodalite crystallization as well as adsorption efficiency of the nano-porous silica obtained from acidification of zeolite composite. The nano-porous silica formation involved the ion extraction from hydrosodalite cabbage-like particles, depending on crystallinity of intermediate zeolite composite. CONCLUSION: The mesoporous silica represented the appropriate adsorptive behavior when produced from precursor modified with lower NaAlO₂ content, NaAlO₂/MK = 0.24, attributed to superior specific surface area, 420 m² g⁻¹, with average pore diameter about 3.3 nm. The equilibrium data was better correlated by Redlich–Peterson isotherm. Compared to the surface area of silica powders fabricated from high grade kaolins, Al₂O₃/SiO₂, > 0.83, by the acid-etched strategy, the mesoporous silica produced from the low-grade kaolin by the template free method indicates higher value. © 2020 Society of Chemical Industry     

13C CP/MAS and2H NMR study of tert-butyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of tert-butyl cation and tert-butyl silyl ether intermediates

The dehydration reaction of tert-butyl alcohol, selectively labelled with¹³C in CH₃ or C-O groups (t-BuOH[2–¹³C₂] andt–BuOH[1-¹³C]), as well as selectively deuterated in methyl groups (t-BuOH[2-²H₉]), was studied on H-ZSM-5 zeolite simultaneously with¹³ C CP/MAS and²H solid state NMR. When adsorbed and dehydrated on zeolite at 296 K,t-BuOH[2–¹³C₁] andt-BuOH[1–¹³C] give rise to identical₁₃C CP/MAS NMR spectra of oligomeric aliphatic products. This is explained in terms of the fast isomerization of the tert-butyl hydrocarbon skeleton via the formation of tert-butyl cation as the key reaction intermediate. An alkoxide species, most probably tert-butyl silyl ether (t-BuSE), was also detected as the side reaction intermediate. This intermediate was stable within the temperature range 296–373 K and decomposed at 448 K to produce additional amounts of final reaction products, i.e. butene oligomers. NMR data point to the existence of equilibria between the initial tert-butyl alcohol, tert-butyl cation and butene that is formed from the intermediate carbocation.     

Method for preparing polyaluminocarbosilane

Polyaluminocarbosilane (PACS) was synthesized directly by the one‐pot reaction of polydimethylsilane (PDMS) with aluminum acetylacetonate [Al(acac)₃] in an autoclave. In this closed system, all the aluminum in Al(acac)₃ was converted into PACS. Therefore, the content of aluminum could be readily controlled quantitatively. On the basis of Fourier transform infrared, ¹H‐NMR, ¹³C‐NMR, ²⁹Si‐NMR, and ²⁷Al magic‐angle spinning NMR analysis, the reaction mechanism was proposed as follows: PDMS dissociated during pyrolysis to generate silicon free radicals, and then they reacted with Al(acac)₃ to yield PACS containing (Si? O)_nAl groups (n = 4, 5, or 6). Meanwhile, these reactions resulted in the cleavage of O? C and/or O?C bonds in Al(acac)₃. Some of the free‐radical fragments generated by this cleavage continued to react with the silicon free radicals and were incorporated into the structural units of PACS; the rest of them may have been converted into small oxygen‐containing compounds, which were removed in the subsequent processing after the reactions. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mesoporous MEL – Type Zeolite Single Crystal Catalysts

New mesoporous silicalite-2, ZSM-11, and titanosilicalite-2 zeolite single crystals were synthesized by use of carbon particles as a mesopore template. After removal of the carbon particles by combustion, zeolite single crystals with intracrystalline mesopore volumes between 0.31 and 0.44 cm³/g were isolated. All samples were characterized by XRD, SEM and adsorption measurements. Isomerization and cracking of n-hexadecane, and epoxidation of oct-1-ene and styrene were chosen as model test reactions. The mesoporous zeolite single catalysts were generally found to exhibit high activity in these model reactions. The advantage of introducing an intracrystalline mesopore system into zeolite catalysts is attributed to the resulting improvements in the mass transport to and from the active sites. This is particularly important for more bulky reactants, intermediates and products and especially under experimental conditions giving high reaction rates.     

柠檬酸复合改性对多环芳烃选择性开环铂/Beta催化剂性能的影响

通过水热处理、柠檬酸处理及其复合处理对Beta分子筛进行后改性,并以改性后的载体制得铂/Beta催化剂。采用X射线衍射（XRD）、X射线荧光光谱仪（XRF）、程序升温脱附（NH₃-TPD）、红外吡啶吸附（Py-IR）、骨架铝核磁共振技术（²⁷Al MAS NMR）及骨架硅核磁共振技术（²⁹Si MAS NMR）等表征了改性前后Beta分子筛的物化性质,并考察了改性前后铂/Beta的多环芳烃选择性开环性能。结果表明,Beta分子筛在柠檬酸处理过程中可同时发生络合脱铝与骨架补铝,实现骨架铝的再分布;Beta分子筛在水热处理过程中优先脱除稳定性相对较低的Si（2 Al）处骨架铝,产生骨架缺陷的同时生成一定比例的二次介孔结构;水热-柠檬酸复合处理影响Beta分子筛骨架补铝及骨架铝再分布的效果,水热处理后Beta分子筛中存在更多的骨架缺陷,促进活性Al（OH）₂⁺物种的骨架补铝作用。当Beta分子筛采用水热-柠檬酸复合处理顺序时,骨架补铝及骨架铝再分布效果显著,样品以中强酸为主,且具有较高的B酸量与L酸量的比值,所制备催化剂的多环芳烃选择性开环活性及稳定性最优。     

Synthesis and characterization of colloidal TBA-silicalite-2

Colloidal TBA-silicalite-2 (MEL type) was hydrothermally synthesized in the reactant system TBAOH–TEOS–H₂O. The influences of reactant composition, reaction temperature, and crystallization period on particle size of the zeolite were investigated. The colloidal silicalite-2 was characterized by XRD, SEM, TEM, FT-IR, ²⁹Si MAS NMR, ¹³C CP MAS NMR, thermogravimetric analysis and N₂ adsorption. The as-synthesized colloidal zeolite with a particle size of 100 nm is an agglomerate composed of nano-sized crystals of about 20 nm, displaying unique structural characteristics such as high content of silanol defects in the framework, splitting of the double-ring vibration in FT-IR spectrum and an unusual signal for methyl groups of TBA in ¹³C CP MAS NMR spectrum.     

Nature, Environment and Quantification of Titanium Species in TiSiBEA Zeolites Investigated by XRD, NMR, DR UV–Vis and XPS

Ti _x SiBEA zeolites (x = 1.5, 3.2 and 5.8 Ti wt%) are prepared by a two-step postsynthesis method involving (1) dealumination of TEABEA zeolite by nitric acid leading to nests of SiO–H groups in vacant T-atom sites and (2) incorporation of Ti into SiBEA zeolite by reacting TiCl₄ vapor with those silanol groups. The incorporation of Ti into the SiBEA framework is evidenced by XRD, ²⁹Si MAS NMR, ¹H–²⁹Si CP MAS NMR and ¹H MAS NMR. DR UV–vis and XPS investigations show that, for low Ti content, mainly framework tetrahedral Ti(IV) ions are present in Ti _x SiBEA zeolites. On the basis of XPS data, the quantification of framework tetrahedral Ti(IV) and framework and/or extra-framework octahedral Ti(IV) ions is followed as a function of Ti content.     

The function of zeolite on Pt autoreduction and dispersion in Pt/L and Pt/β catalysts

TPR, CO-FTIR and¹²⁹Xe NMR spectroscopic techniques were used to measure the distribution of platinum species after the calcination of Pt/L and Pt/ zeolites. Autoreduction which occurred in Pt/ zeolite was avoided in the channel of L zeolite. Pt particles dispersed well and exhibited excellent reactivity for the aromatization ofn-hexane in L zeolite.