ZSM-5 zeolite single crystals with b-axis-aligned mesoporous channels as an efficient catalyst for conversion of bulky organic molecules

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with b-axis-aligned mesopores, synthesized using a designed cationicamphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the b-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The b-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications.     

Aromatization ofn-butane over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica, fiber

The reaction ofn-butane aromatization was carried out over Ni-ZSM-5 and Cu-ZSM-5 zeolite catalysts prepared by using Ni and Cu impregnated silica fiber during the process of ZSM-5 zeolite synthesis. The catalysts prepared were characterized by X-ray powder diffraction, nitrogen adsorption and X-ray fluorescency. The acidic properties of the catalysts were investigated by temperature-programmed desorption of ammonia using a mass spectrometer equipped with a QTMD detector. The effect of catalyst pretreatment, reaction temperature, and time on stream on the reaction ofn-butane to aromatic hydrocarbons were investigated. The modification of ZSM-5 by Ni and Cu increased the selectivity to aromatic hydrocarbons. The state of Ni and Cu and their stabilization in the ZSM-5 structure was highly influenced by the mode of catalyst pretreatments.     

Availability and interconnectivity of pores in mesostructured ZSM-5 zeolites by the adsorption and diffusion of mesitylene

Probing the mesopore architecture in mesoporous zeolites is of importance for large scale applications of the materials. In this work, the adsorption and diffusion of mesitylene with larger molecule size in mesoporous ZSM-5 zeolites were carried out, in order to acquaint the availability and interconnectivity of mesopores in zeolite crystals. The comparisons of the shape of adsorption isotherms and the mesopore volume calculated from mesitylene and N₂ adsorption in mesoporous ZSM-5 zeolites with different mesoporosities can be used to discriminate two cases of mesopores: accessible mesopores connected to external surface of the zeolite crystals and non-accessible meso-voids that are occluded in the microporous matrix. Furthermore, the effective diffusivity and activation energy of mesitylene in mesoporous ZSM-5 extracted from ZLC desorption curves as a function of mesopore volume calculated from mesitylene adsorption reveal the enhancement of mesopore interconnectivity to molecule diffusion in zeolite crystals.     

Localization of the copper-containing component in the pore volume of zeolite ZSM-5

The distribution of the copper-containing component in the pore volume of zeolite ZSM-5 has been investigated by H₂ and N₂ adsorption at 77 K and IR spectroscopy. Samples were synthesized by ion exchange and incipient wetness impregnation. Copper-containing clusters are mostly located on the surface of the mesopores formed by packed zeolite nanocrystallites. This causes partial blocking of the volume of microporous channels for N₂ molecules, but these channels remain accessible for H₂ molecules. It has been deduced that no considerable amount of copper located in the structural channels of the zeolite. According to IR spectroscopic data, the sorption of copper ions in the Cu/ZSM-5 catalysts takes place on extraframe-work aluminum, which forms Al-OH-Al bridges and terminal Al-OH groups, and on terminal Si-OH groups located on the zeolite crystal surface.     

Fabrication of a nano-sized ZSM-5 zeolite with intercrystalline mesopores for conversion of methanol to gasoline

Carbon deposition during methanol to hydrocarbons leads to the quick deactivation of ZSM-5 catalyst and it is one of the major problems for this technology. Decreasing the crystal size or introducing mesopores into ZSM-5 zeolites can improve its diffusion property and decrease the coke formation. In this paper, nano-sized ZSM-5 zeolite with intercrystalline mesopores combining the mesoporous and nanosized structure was fabricated. For comparison, the mesoporous ZSM-5 and nano-sized ZSM-5 were also prepared. These catalyst samples were characterized by XRD, BET, NH3-TPD, TEM, Py-IR and TG techniques and used on the conversion of methanol to gasoline in a fixed-bed reactor at T = 405 °C, WHSV = 4.74 h-1and P = 1.0 MPa. It was found that the external surface area of the nano-sized ZSM-5 zeolite with intercrystalline mesopores reached 104 m2/g, larger than that of mesoporous ZSM-5(66 m2/g) and nanosized ZSM-5(76 m2/g). Catalytic lifetime of the nano-sized ZSM-5 zeolite with intercrystalline mesopores was 93 h, which was only longer than that of mesoporous ZSM-5(86 h), but shorter than that of nanosized ZSM-5(104 h). Strong acidity promoted the coke formation and thus decreased the catalytic lifetime of the nano-sized ZSM-5 zeolite with intercrystalline mesopores though it presented large external surface that could improve the diffusion property. The special zeolite catalyst was further dealuminated to decrease the strong acidity. After this, its coke formation rate was slowed and catalytic lifetime was prolonged to 106 h because of the large external surface area and decreased weak acidity. This special structural zeolite is a potential catalyst for methanol to gasoline reaction.     

Selective conversion of methane to aromatic hydrocarbons on large crystallite zeolite catalysts with mesoporous structure

Large crystallite mesoporous MFI (ZSM-5) zeolite was synthesized by using carbon nano-powder as a secondary template. The surface properties, morphological and phase composition of the synthesized material and of the commercial ZSM-5 (Zeolyst) zeolite were studied by nitrogen porosimetry, XRD and scanning electron microscopy. The results showed that the volume of mesopores volume increases with development of a secondary mesoporosity in the structure of zeolite. The obtained zeolite supports were used to prepare molybdenum-containing catalysts for the methane aromatization by solid phase preparation technique. Based on the XPS data, molybdenum particles in these catalysts are characterized by more uniform size distribution. The formation of a secondary pore structure restrains the carbon deposit formation as well as increases the methane conversion and the yield of the aromatic compounds.     

State of the copper-containing component and the catalytic properties of Cu/ZSM-5 in selective NO reduction with propane

Supported Cu/ZSM-5 catalysts have been synthesized by ion exchange and impregnation using aqueous ammonia solutions of copper nitrate containing orbital-ordered copper ions. The state of the copper-containing component in the pore space of the catalysts and copper sorption in the catalysts have been investigated by a complex of physicochemical methods. The catalytic properties of Cu/ZSM-5 in the selective catalytic reduction of NO with propane are reported. The catalytic properties depend on the copper precursor structure and deposition method, which determine the size of the copper oxide clusters on the outer surface of zeolite crystals.     

Preparation of Amorphous Silica-Alumina Using the Sol–Gel Method and its Reactivity for a Matrix in Catalytic Cracking

Amorphous silica-aluminas were prepared by the sol–gel method using organic templates such as carboxylic acid and the gel skeletal reinforcement method. Their reactivities as a matrix for the catalytic cracking were investigated. Malic acid (MA) was used as a catalyst for the sol–gel method, an organic template and a reagent for the dispersion of Al. When the ratio of MA/TEOS (tetraethoxysilane) increased from 0.22 to 1.22, surface area, pore volume and pore diameter increased and the mesopore was formed at 1.22. Their average BET pore diameters for 0.22 and 1.22 of MA/TEOS were 2.0 to 5.1 nm, respectively. Although conversions of n-dodecane were around 20% or less with single amorphous silica-aluminas, both single beta-zeolite and the mixed catalysts of zeolite and amorphous silica-aluminas showed much higher activity. Further, the mixed catalyst using silica-alumina with mesopore (MAT(MA122-5)) exhibited the higher ratio of multi-branched paraffin to single branched paraffin in the gasoline franction of products (C5–C11) than the mixed catalysts using silica-alumina with only micropore and silica with mesopore or single zeolite. In the gel skeletal reinforcement method, tetraethoxy orthosilicate (TEOS) was used as not only a precursor of silica but also an agent which reinforces the skeleton of silica-gel to prepare an aerogel and extremely large mesopores were formed for resultant silicas and silica-aluminas. When silica aerogel was reinforced by TEOS solution, the pore diameter and pore volume reached 30 nm and 3.1 cm³/g, respectively, in the N₂ adsorption measurement by the BJH method, indicating that most of pores for this silica consisted of mesopores. In catalytic clacking reaction of n-dodecane, the mixed catalyst prepared by beta-zeolite and silica-alumina with large mesopore exhibited not only the comparable activity to that for single zeolite but also the unique selectivity where large amounts of branched products were formed. When the catalyst beds of silica-alumina and zeolite were separated, the reference silica-alumina (ref.SA) → zeolite system exhibited the higher activity and the product selectivity close to those for MAT(ref.SA). It is likely that the primary cracking of n-dodecane on silica-alumina would occur to produce the primary cracked product which effectively reacted with zeolite and inhibited the coke formation by overcracking.     

Dissolution Behavior and Varied Mesoporosity of Zeolites by NH4F Etching

The mesopores formation in zeolite crystals has long been considered to occur through the stochastic hydrolysis and removal of framework atoms. Here, we investigate the NH₄F etching of representative small, medium, and large pore zeolites and show that the zeolite dissolution behavior, therefore the mesopore formation probability, is dominated by zeolite architecture at both nano- and sub-nano scales. At the nano-scale, the hidden mosaics of zeolite structure predetermine the spatio-temporal dissolution of the framework, hence the size, shape, location, and orientation of the mesopores. At the sub-nano scale, the intrinsic micropore size and connectivity jointly determine the diffusivity of reactant and dissolved products. As a result, the dissolution propensity varies from removing small framework fragments to consuming nanodomains and up to full digestion of the outmost part of zeolite crystals. The new knowledge will lead to new understanding of zeolite dissolution behavior and new adapted strategies for tailoring hierarchical zeolites.     

Butene catalytic cracking to ethylene and propylene on mesoporous ZSM-5 by desilication

Mesopore ZSM-5 was prepared by alkali treatment of parent ZSM-5 zeolite and applied for butene catalytic cracking. The zeolite was treated in the NaOH solutions with different concentrations at 85 °C. XRD showed that the intrinsic MFI structure of ZSM-5 zeolite was preserved and corresponding crystallinity remained unchangeableness when ZSM-5 was treated with low concentration NaOH solutions. However, excessive NaOH led to the destruction of zeolite structure. The BET surface area increased obviously after desilication, and the N₂ adsorption/desorption curves indicated a number of mesopores generation. The experiment of butene catalytic cracking was carried out in a fixed-bed to investigate the influence of mesopores. The results showed that catalytic performances can be greatly improved through introducing the mesopores into parent ZSM-5 by alkali treatment. Highest yield of ethylene plus propylene were obtained when the treated concentration of NaOH solution is 0.1–0.2 M.     

Preparation of highly selective zeolite ZSM-5 membranes by a post-synthetic coking treatment

Zeolite ZSM-5 membranes with high n-butane:isobutane selectivities, e.g., 322 at 185°C, are obtained by a selective deposition of coke into non-zeolitic pores. The zeolite membranes are prepared by in situ crystallization on either bare porous α-Al₂O₃ support disks or disks that are pretreated to include a diffusion barrier. The post-synthetic coking treatment is accomplished by impregnating these membranes with liquid 1,3,5-triisopropylbenzene (TIPB) for 24 h at room temperature and then calcining them in air at 500°C for 2 h. Calcination at 500°C for up to 30 h does not destroy the high n-butane:isobutane selectivity. Thermogravimetric analysis (TGA) experiments on two model pore systems ZSM-5 (5.5 Å) and Vycor glass (40–50 Å) suggest that micro-defects are selectively eliminated by the TIPB coking treatment while the intracrystalline pore space of the ZSM-5 is not affected. The elimination of non-zeolitic pores results in a large increase of n-butane:isobutane pure gas flux ratio (45 vs. 320 at 185°C) accompanied by a fourfold reduction of the n-butane flux. The permeation experiments reveal that the n-butane flux increases nonlinearly with the partial pressure in the feed while the n-butane:isobutane pure gas flux ratio remains relatively unchanged.     

Exploring meso-/microporous composite molecular sieves with core-shell structures

A series of core–shell‐structured composite molecular sieves comprising zeolite single crystals (i.e., ZSM‐5) as a core and ordered mesoporous silica as a shell were synthesized by means of a surfactant‐directed sol–gel process in basic medium by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethylorthosilicate (TEOS) as silica precursor. Through this coating method, uniform mesoporous silica shells closely grow around the anisotropic zeolite single crystals, the shell thickness of which can easily be tuned in the range of 15–100 nm by changing the ratio of TEOS/zeolite. The obtained composite molecular sieves have compact meso‐/micropore junctions that form a hierarchical pore structure from ordered mesopore channels (2.4–3.0 nm in diameter) to zeolite micropores (≈0.51 nm). The short‐time kinetic diffusion efficiency of benzene molecules within pristine ZSM‐5 (≈7.88×10^?19 m² s^?1) is almost retainable after covering with 75 nm‐thick mesoporous silica shells (≈7.25×10^?19 m² s^?1), which reflects the greatly opened junctions between closely connected mesopores (shell) and micropores (core). The core–shell composite shows greatly enhanced adsorption capacity (≈1.35 mmol g^?1) for large molecules such as 1,3,5‐triisopropylbenzene relative to that of pristine ZSM‐5 (≈0.4 mmol g^?1) owing to the mesoporous silica shells. When Al species are introduced during the coating process, the core–shell composite molecular sieves demonstrate a graded acidity distribution from weak acidity of mesopores (predominant Lewis acid sites) to accessible strong acidity of zeolite cores (Lewis and Brønsted acid sites). The probe catalytic cracking reaction of n‐dodecane shows the superiority of the unique core–shell structure over pristine ZSM‐5. Insight into the core–shell composite structure with hierarchical pore and graded acidity distribution show great potential for petroleum catalytic processes.     

ZSM-5沸石催化剂外表面的化学修饰

通过离子改性、水热处理、表面毒化及预积炭等方法均可提高ZSM-5沸石的择形催化性能.上述方法在改变沸石孔道结构时也明显地改变了沸石的表面酸性质.近年来,采用化学修饰的方法对沸石表面和孔口进行调变的研究已有一些报导.杨儒等用硅烷或SiCl_4等进行气相硅烷化,除缩小孔口尺寸外,还产生了骨架脱铝补硅作用,导致表面酸性质发生变化.王清遐等用正硅酸乙醇浸渍(ZSM-5沸石,随着硅的载入量增加,催化剂酸量和反应活性均降低,而对位选择性增加.Murakami用正硅酸甲     

Impact of the Spatial Organization of Bifunctional Metal–Zeolite Catalysts on the Hydroisomerization of Light Alkanes

Improving product selectivity by controlling the spatial organization of functional sites at the nanoscale is a critical challenge in bifunctional catalysis. We present a series of composite bifunctional catalysts consisting of one-dimensional zeolites (ZSM-22 and mordenite) and a γ-alumina binder, with platinum particles controllably deposited either on the alumina binder or inside the zeolite crystals. The hydroisomerization of n-heptane demonstrates that the catalysts with platinum particles on the binder, which separates platinum and acid sites at the nanoscale, leads to a higher yield of desired isomers than catalysts with platinum particles inside the zeolite crystals. Platinum particles within the zeolite crystals impose pronounced diffusion limitations on reaction intermediates, which leads to secondary cracking reactions, especially for catalysts with narrow micropores or large zeolite crystals. These findings extend the understanding of the “intimacy criterion” for the rational design of bifunctional catalysts for the conversion of low-molecular-weight reactants.     

Effect of Alkali Treatment on the Structure and Catalytic Properties of ZSM-5 Zeolite

Catalytic properties of ZSM-5 zeolite samples pretreated with NaOH solution have been investigated. The samples are characterized by XRD, SEM, chemical analysis, and N2 adsorption.The results indicate that mesopores are created in ZSM-5 crystals under alkali treatment without change the microporous structure and acidic strength of the zeolite, but the crystallinity is greatly decreased under severe treatment. IR indicates that the concentration of silanol is greatly enriched by alkali treatment. The etherification activities of ZSM-5 zeolites are greatly increased byalkali-treatment. The noticeably improved catalytic activity of treated samples is ascribed to the formation of mesopores and greatly enriched silanol group.     

Diffusion of n-alkanes in mesoporous 5A zeolites by ZLC method

Diffusion properties of mesostructured zeolite 5A were investigated by employing n-alkanes as probe molecules using the zero length column (ZLC) method. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (D _eff/R ²) increased with mesoporosity in the zeolites between 308 K and 393 K, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance.     

Development of VPO catalysts supported on mesoporous modified material based on an aerosil gel

Supports modified with different organic substances and vanadium-phosphorus oxide catalysts on their basis were developed with the use of the barothermal treatment of aerosil. It was found that the anchoring of a modifier to the surface of a support occurred with the participation of OH groups. The modifier content found from differential thermal analysis data depended on the pore structure of the support and the nature of the modifier. It was demonstrated that, with the use of a modified support in the catalysts, a vanadyl hydrogen phosphate hemihydrate phase—the precursor of a catalyst for the selective oxidation of hydrocarbons—was formed; under reaction conditions, this precursor was converted into vanadyl pyrophosphate. On supporting, the phase grew from the micropore bulk and covered only part of the support surface. The synthesized catalysts exhibited high activity and selectivity in the reaction of n-butane oxidation under hydrocarbon-rich conditions and also in the oxidative dehydrogenation of ethane.     

Synthesis and catalytic performance of ZSM-5/MCM-41 composite molecular sieve from palygorskite

ZSM-5/MCM-41 composite molecular sieve has been hydrothermally synthesized through a two-step crystallization process using palygorskite (PAL) as silicon and aluminum source. The products were characterized by various means and their catalytic properties for acetalization of cyclohexanone and esterification of acetic acid and n-butanol were also investigated. In the first step ZSM-5 zeolite could be formed from the acid-treated PAL after hydrothermal treatment using tetrapropylammonium bromide as template. XRD patterns, N₂ adsorption and desorption data, and TEM images show that the composite obtained in the secondary step had a well-ordered mesoporous MCM-41 phase and a microporous ZSM-5 zeolite phase. Compared with ZSM-5, ZSM-5/MCM-41 composite possessed more total acid amount, weak acid sites and large pore structure due to the formation of MCM-41 and exhibited higher catalytic activity for the acetalization and esterification reaction.     

Catalyst optimization for enhanced propylene formation in the methanol-to-olefins reaction

ZSM-5 zeolites possessing different chemical compositions, acidities and crystal sizes, were synthesized and characterized by XRD, SEM and FTIR. Those as-prepared acid catalysts were tested in the conversion of methanol into light olefins at 673 K under atmospheric pressure. Propylene-to-ethylene ratios above 5 were achieved over ZSM-5 zeolite catalyst prepared via the fluoride route. This promising methanol-to-propylene (MTP) catalyst was designed both at the molecular scale, exhibiting low Brønsted acid site density, and at the microscopic level since large crystals (25 μm) with few defects were obtained. Aiming in a possible industrial application, the synthesis medium has to be seriously modified, thus avoiding harmful hydrofluoric acid use. Hydrochloric and phosphoric acids were therefore chosen as alternatives and resulted in the same ZSM-5 crystal morphologies and similar catalytic performance.     

Effect of copper precursors on the catalytic activity of Cu/ZSM-5 catalysts for selective catalytic reduction of NO by NH<Subscript>3</Subscript>

The Cu/ZSM-5 catalysts prepared by different copper precursors were used for the selective catalytic reduction (SCR) of NO _x with NH₃. The Cu/ZSM-5 catalyst prepared by the copper nitrate (Cu/ZSM-5-N) presented the best performance among the Cu/ZSM-5 catalysts and showed above 90 % NO _x conversion at 225–405 °C. The average particle size of CuO was 5.82, 9.20, and 11.01 nm over Cu/ZSM-5-N, Cu/ZSM-5-S (prepared by copper sulfate), and Cu/ZSM-5-C (prepared by copper chloride), respectively. The Cu/ZSM-5-N catalyst showed the highly dispersed copper species, the strong surface acidity, and the excellent redox ability compared with the Cu/ZSM-5-C and Cu/ZSM-5-S catalysts. The Cu⁺ and Cu²⁺ existed in the Cu/ZSM-5 catalysts and the abundant Cu⁺ over Cu/ZSM-5-N might be responsible for the superior SCR activity.