ZSM-5 Layer Research Articles

Strategy of Development on Zeolite Membrane for Forward Osmosis Operation; Effects of Structural Parameter of Support Layer and Si/Al Ratio of ZSM-5 Layer on Water Flux

Strategy of Development on Zeolite Membrane for Forward Osmosis Operation; Effects of Structural Parameter of Support Layer and Si/Al Ratio of ZSM-5 Layer on Water Flux

Rational Design of Zinc/Zeolite Catalyst: Selective Formation of p-Xylene from Methanol to Aromatics Reaction.

The production of p-xylene from the methanol to aromatics (MTA) reaction is challenging. The catalytic stability, which is inversely proportional to the particle size of the zeolite, is not always compatible with p-xylene selectivity, which is inversely proportional to the external acid sites. In this study, based on a nano-sized zeolite, we designed hollow triple-shelled Zn/MFI single crystals using the ultra-dilute liquid-phase growth technique. The obtained composites possessed one ZSM-5 layer (≈30 nm) in the middle and two silicalite-1 layers (≈20 nm) epitaxially grown on two sides of ZSM-5, which exhibited a considerably long lifetime (100 % methanol conversion >40 h) as well as an enhanced shape selectivity of p-xylene (>35 %) with a p-xylene/xylene ratio of ≈90 %. Importantly, using this sandwich-like zeolite structure, we directly imaged the Zn species in the micropores of only the ZSM-5 layer and further determined the specific structure and anchor location of the Zn species.

Selective conversion of methanol to aromatics with superior catalytic stability by relay catalysis over quadruple ZSM-5 sequence beds with gradient-increasing acidity

Methanol to aromatics (MTA) process is an attractive non-petroleum route for producing high-valued aromatics, but simultaneously achieving high catalytic stability and high aromatics selectivity remains a challenging research target. Herein, we present a highly efficient MTA process by quadruple tandem relay catalysis containing four layers of nanosized ZSM-5 with different acidity features in series. Specifically, four ZSM-5 catalysts with gradient-increasing acid density were sequentially packed in the tubular fixed bed reactor for reaction. Among them, ZSM-5 with a high SiO2/Al2O3 ratio as 480 was packed in top layer to convert methanol into light alkanes and alkenes intermediates, then these reaction intermediates and a small amount of unreacted methanol were transported on other two ZSM-5 layers with lower SiO2/Al2O3 ratio as 240 and 120 respectively for stepwise converting to aromatics. In particular, Zn doped ZSM-5 with a low SiO2/Al2O3 ratio of 60 was packed in the bottommost layer which further promoted aromatization of upstream light hydrocarbons. Accordingly, aromatics selectivity was significantly increased to 32.6% using the sequence beds, much higher than 17.6% for the widely recognized two-step MTA. Based on deliberate analysis of methanol conversion and coke formation behavior in different beds, we demonstrate that a methanol concentration gradient existed along the sequence beds which remarkably decreased alkylation of aromatics induced by methanol. So, the catalyst stability of the ZSM-5 located in the bottommost layer was significantly enhanced to 238 h, which was 3-folds compared with the traditional two-step methanol conversion. The present work offers a promising strategy for MTA via tandem ZSM-5 catalysis systems, which has an active impact on the solving of the energy and environmental problems existing in the production process of aromatics.

Mechanical stability of ZSM-5 zeolite washcoated cordierite monoliths

ZSM-5 zeolites are washcoated on cordierite substrates by the slurry-coating and in-situ synthesis methods, respectively. A statistical approach is used to describe the mechanical stability of the prepared ZSM-5/cordierite monoliths. It is shown that both adhesive and cohesive failure take place for the slurry-coated ZSM-5/cordierite. However, for the in-situ synthesized ZSM-5/cordierite, the formation of an oriented, compact and homogeneous ZSM-5 layer leads to a very little cohesive failure, and the chemical bonding of the ZSM-5 layer to the cordierite substrate reduces the amount of adhesive failure significantly. The in-situ synthesized ZSM-5/cordierite, therefore, exhibits much higher resistances to ultrasonic vibration and thermal shock than the slurry-coated ZSM-5/cordierite. Furthermore, the in-situ synthesized ZSM-5/cordierite has high ZSM-5 loading, high surface area, high pore volume and hierarchical porosity and thus it is a good catalyst support for active species. In this study, copper and manganese are introduced by impregnation, and the prepared Cu–Mn catalyst supported on the in-situ synthesized ZSM-5/cordierite shows a satisfactory catalytic performance in o-xylene oxidation.

Fischer-tropsch synthesis in monolith catalysts coated with hierarchical ZSM-5

Fischer-Tropsch synthesis (FTS) is explored as a candidate one-step process for the selective conversion of synthesis gas to short and branched hydrocarbons. Novel structured catalysts for in-situ upgrading of FTS products to gasoline-range hydrocarbons are synthesized by coating controlled-thickness ZSM-5 films on the surface of Co-Al2O3/monolith substrates. FTS catalysts with coating of microporous and mesoporous ZSM-5 are synthesized, characterized and tested. These catalysts leverage the benefits of monolith reactors as efficient mass and heat transfer structures, with the ZSM-5 layer as a hydrocarbon cracking and isomerization agent. Hierarchical ZSM-5 co-catalysts are shown to relax mass transfer limitations through the zeolite film, thus improving conversion and selectivity. The proposed co-catalysts exhibit high CO conversion and high gasoline selectivity with high octane number. Loss of catalyst activity was observed and attributed to ZSM-5 pore plugging by C13+ saturated hydrocarbons. In-situ regeneration of the catalysts enabled stability in FTS performance.

Gasoline selective Fischer-Tropsch synthesis in structured bifunctional catalysts

The need for energy independence and security has refocused research on Fischer-Tropsch Synthesis (FTS) and its capability to selectively produce short and branched hydrocarbons. In this work, FTS selectivity to gasoline-range products was explored as a catalysis and process engineering problem. Novel structured catalysts for in situ upgrading of FTS products to gasoline-range hydrocarbons were successfully synthesized by coating controlled-thickness ZSM-5 films on the surface of Co-Al2O3/monolith substrates. These catalysts were synthesized on the premise that the monolith serves as an efficient mass and heat transfer structure, while the ZSM-5 layer functions as a hydrocarbon cracking and isomerization agent. Compared to traditional diesel-selective FTS, considerably higher quantity and quality of gasoline-range products were achieved with these bi-layered/bifunctional catalysts. Oil products with gasoline yield higher than 65 wt.% and selectivity to isomers and olefins greater than 70 wt.% were formed at relatively high temperature (230 °C) and intermediate pressure (12 bar), without compromising CO conversion, which remained as high as 79%.

MFI zeolite coating with intrazeolitic aluminum (acidic) gradient supported on SiC foams to improve the methanol-to-propylene (MTP) reaction

To hinder the deactivation and improve the propylene selectivity in the methanol-to-propylene (MTP) reaction, MFI coating with the intrazeolitic aluminum (acidic) gradient supported on SiC foam support (G-MFI/SiC foam) was proposed. The solid polycrystalline silicon was used in the synthesis of G-MFI/SiC foam catalyst provided a prolonged release of silica nutrient in the liquid phase and suppressed the precipitation phenomena. The resulting MFI coating showed the aluminum gradient along the surface normal direction of SiC foams with ZSM-5 layer (about 20 μm) near the SiC surface followed by the silicalite-1 layer (about 10 μm). The alumina (acidic) gradient in the MFI coating renders a passive outer layer of silicalite-1 with fairly large amount of weak and medium acid sites prevented the coke formation as well as promoted the selectivity to propylene in the MTP reaction. Compared to the conventional ZSM-5/SiC foam catalyst, the G-MFI/SiC foam catalyst showed excellent performance in the MTP reaction with good catalytic longevity (8 h vs. 76 h for >95% methanol conversion) and low coke deposition (6.7 × 10−3 wt.% h−1vs. 0.26 wt.% h−1), as well as high propylene selectivity (ca. 36% vs. 46%).

Effects of aluminum sources on synthesis and catalytic performance of b-oriented ZSM-5 coatings

Highly b-oriented ZSM-5 zeolite coatings were synthesized on the inner surface of stainless steel tubes (2 mm i.d. and 400 mm length) precoated with a TiO2 sublayer by in situ crystallization. Four aluminum sources (aluminum nitrate, pseudo boehmite, aluminum isopropoxide and aluminum diacetate) were used and their effects on the structure and catalytic performance of the as-synthesized coatings have been investigated. The characterizations of XRD, SEM, 27Al MAS NMR and NH3−TPD revealed that the orientation and average size of crystals, continuity of zeolite layers, as well as their Si/Al ratio and acid properties, greatly affected by aluminum sources. The ZSM-5 layers synthesized with aluminum nitrate and pseudo boehmite (ANN-C and PB-C) had high b-orientation and almost full coverage, whereas those obtained with organic aluminum sources gave the opposite results. Compared with PB-C, ANN-C presented the small average size of crystals, low Si/Al ratio and large amount of acid sites. Catalytic cracking of n-dodecane at 500 °C and 4 MPa showed that ANN-C exhibited the most rapid cracking rate of n-dodecane and best stability. This is due to its highest b-orientation, which provides increased diffusion of n-dodecane and its cracking product molecules inside pore channels of ZSM-5 coatings.

Synthesis of micro-mesoporous aluminosilicates on the basis of ZSM-5 zeolite using dual-functional templates at presence of micellar and molecular templates

Micro-mesoporous aluminosilicates consisting of agglomerates of the ZSM-5 nanoparticles were obtained using dual-functional templates [C6H13–N+(CH3)2–C6H12–N+(CH3)2–C6H13](Br−)2 (C6–6–6Br2), [C8H17–N+(CH3)2–C6H12–N+(CH3)2–C8H17](Br−)2 (C8–6–8Br2). Aluminosilicates with randomly oriented flake-like particles built from ZSM-5 layers were obtained using [C16H33–N+(CH3)2–C6H12–N+(CH3)2–C6H13](Br−)2 (C16–6–6Br2). Use of С8–6–8Br2 and additive of cetyltrimethylammonium bromide CTAB (CTAB concentration is lower than the first critical micelle concentration, CMC1) leads to an increase of the total specific surface area, mesopore surface area and the mesopore size uniformity in the product, as well as the concentration of Brønsted acid sites accessible for bulk molecules. It is assumed that CTAB role is to limit the zeolite crystals growth. Addition of CTAB to С16–6–6Br2 (CTAB concentration is also lower than CMC1) promotes the formation of micro-mesoporous aluminosilicate possessing disordered lamellar mesostructure stable upon calcination which is characterized by a higher total specific surface area and better mesopores uniformity than ZSM-5 obtained in the presence of С16–6–6Br2. The additive of a molecular template tetrapropylammonium hydroxide (TPAOH) shifts the structure formation toward zeolite and leads to an increase in the ratio of concentrations of Brønsted and Lewis acid sites for the obtained aluminosilicates. The addition of small amounts of TPAOH into the reaction mixture (TPAOH/C16–6–6Br2 = 6.5·10−3) leads to formation of self-pillared aluminosilicate. In this material the nanoparticles act as “pillars” between the layers of ZSM-5 and thus contribute to retention of low-ordered lamellar mesostructure after calcination, leading to increase of the total specific surface area and improving mesopores uniformity.

Tubular dual-layer MFI zeolite membrane reactor for hydrogen production via the WGS reaction: Experimental and modeling studies

Water–gas shift (WGS) reaction is an important intermediate step in converting fossil fuels to hydrogen (H2) for chemical production or power generation. Catalytic membrane reactor with a H2 perm-selective membrane can improve WGS reaction conversion and separate H2 from carbon dioxide (CO2) simultaneously. In this work, experimental work and modeling analysis were performed on WGS in a tubular ZSM-5/silicalite bilayer membrane composed of a 3μm ZSM-5 layer, a 8μm silicalite base layer and a 2μm YSZ barrier layer supported on α-alumina substrate. The experimental and modeling studies demonstrated that temperature, H2O/CO ratio, gas hourly space velocity (GHSV) and feed pressure are key factors that determine the WGS performance in the tubular zeolite membrane reactor. At 500°C and under 5atm with the H2O/CO ratio of 3.0 and GHSV of 72,000h−1, the CO conversion and H2 recovery reached 89.8% and 28.5%, respectively. Appropriate temperature, pressure, H2O/CO ratio and GHSV are crucial to obtain high reaction performance. Modeling analysis coupled with experimental data identifies the optimum operation conditions (550°C, feed pressure of 20atm, H2O/CO ratio of 2.0, GHSV of 60,000h−1) under which one can achieve both high CO conversion (>95%) and H2 recovery (>90%) for WGS in this zeolite membrane reactor.

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.

Highly stable bilayer MFI zeolite membranes for high temperature hydrogen separation

Stable inorganic membranes perm-selective to hydrogen will find applications in a number of important industrial processes including water gas shift reaction for hydrogen production. This paper reports a highly stable bilayer MFI zeolite membrane with good hydrogen separation characteristics. The membrane consists of a thin (2μm) ZSM-5 layer on a thick (8μm) silicalite base layer supported on macroporous α-alumina with a yttria stabilized zirconia intermediate barrier layer. The zeolitic pores of the thin ZSM-5 layer were narrowed by catalytic cracking deposition (CCD). At 500°C, the bilayer zeolite membrane exhibits H2 permeance of about 1.2×10−7molm−2s−1Pa−1, with H2 to CO2, CO and H2O vapor selectivity respectively of about 23, 28 and 180. The membrane shows slightly improved separation properties (H2 permeance, H2/CO and H2/CO2 selectivity) as the feed side pressure increases during the test of separation of industrial relevant simulated gas mixture (25% H2: 25% CO: 25% H2O: 25% CO2, with 400ppm H2S) at 500°C for 24 days. Membrane reactor made of the bilayer zeolite membrane shows stable performance under water gas shift reaction conditions (500°C, H2O/CO=3, GHSV=60,000h−1, with a ceria doped iron oxide catalyst) in terms of CO conversion, H2 recovery, and H2 permeance and selectivity of the zeolite membrane. The unprecedented thermal and chemical stability and good separation properties of the bilayer zeolite membrane are related to its unique bilayer structure and synthesis methods.

Synthesis of MFI/EMT zeolite bi-layer films for molecular decontamination

A bi-layer film composed of a thick layer of ZSM-5 zeolite (MFI-structure type) and a thin layer of EMC-2 zeolite (EMT-structure type) was synthesized on aluminum alloys that are used widely in aerospace industry. The bottom MFI layer was obtained by direct hydrothermal synthesis while secondary growth method was used for the top EMC-2 layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that both layers are highly crystallized and uniform with a total film thickness around 22μm. Nitrogen sorption measurements indicated that ZSM-5 and EMC-1 layers present an accessible porosity. The high micropore volumes lead to considerably good adsorption properties towards pollutants. Such a result was confirmed by the n-hexane sorption capacity of the bi-layer film which is identical to what is expected for a corresponding powder mixture. Adhesion of zeolite films on aluminum substrate was investigated by scratch tests which highlighted a good adhesion between the bottom layer and the support as well as between the two zeolite layers. This bi-layer film is promising for technological uses in molecular decontamination.

Zeolite hybrid films for space decontamination

A bi-layer film composed of a thick layer of ZSM-5 zeolite (MFI-structure type) and a thin layer of EMC-1 zeolite (FAU-structure type) was synthesized on aluminum alloys widely used in aerospace industry. The bottom MFI layer was obtained by direct hydrothermal synthesis while secondary growth method was used for the top FAU layer. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that both layers are highly crystallized, uniform and well-intergrown with a total film thickness around 10μm. Nitrogen sorption measurements indicated that ZSM-5 and EMC-1 layers present an accessible porosity. The high micropore volumes lead to considerably good adsorption properties towards pollutants. Such a result was confirmed by the n-hexane sorption capacity of the hybrid film which is identical to what is expected for a corresponding powder mixture. Adhesion of zeolite films on aluminum substrate was investigated by scratch tests which highlighted a good adhesion between the bottom layer and the support as well as between the two zeolite layers. The process described in this paper for the formation of a MFI/FAU bi-layer film can be easily extended to other hybrid coating film.

Laminated mordenite/ZSM-5 hybrid membranes by one-step synthesis: Preparation, membrane microstructure and pervaporation performance

Laminated mordenite/ZSM-5 hybrid zeolite membranes were fabricated on seeded porous α-alumina tubes with ZSM-5 crystals by one-step hydrothermal treatment for dehydration of industrial solutions. The microstructure was clarified with observations by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometer (XRD) analysis on the membranes synthesized as a function of crystallization time. The synthesized hybrid membranes were composed by two layers. The upper mordenite layer overgrew with 2μm thick above the ZSM-5 lower layer with 3μm thick. The formation sequence was estimated to be that (1) the ZSM-5 layer crystallized in the early stage of 4h; (2) then the mordenite crystals nucleated and grew on the lower ZSM-5 layer; (3) elongated mordenite grains crystallized for 4–8h to form the tightly packed layer.The potential pervaporation (PV) performance of dehydration was investigated for industrial solvents of 2-propanol (IPA) and acetic acid (AAc). In the results of PV at 75°C for a mixture of water (50wt.%)/IPA (50wt.%), the membrane exhibited higher permeation flux up to 6.4kgm−2h−1 and separation factors (α) with 1800. In the results of PV at 80°C for a mixture of water (50wt.%)/AAc (50wt.%), a higher permeation flux with 5.6kgm−2h−1 and a small amount of leakage of AAc with 0.68wt.% into the permeates or the separation factor α=150 were observed. These higher separating features in the laminated mordenite/ZSM-5 hybrid membrane could be caused by tightly assemblage of mordenite crystals that were formed uppermost layer of the membrane.

Preparation of ZSM-5 zeolite coatings within capillary microchannels

A method is described to incorporate a zeolite ZSM-5 layer as a potential catalytic coating or membrane into tubular supports by the in situ crystallization of a previously deposited silica layer. The silica layer simultaneously provides nucleation sites and functions as a nutrient source giving coatings with excellent uniformity and controllable coating thickness. The ability to control the simultaneous dissolution of the silica layer in conjunction with nucleation and growth processes was found important in controlling the morphology of the zeolite crystals contained in the final zeolite coating. The formation of a ±4 μm coating containing ±300 nm crystals was obtained at high silica dissolution and nucleation rates. A continuous intergrown zeolite layer of ±7.6 μm was obtainable at high silica dissolution rates in the presence of sodium.

The assembly of ZSM-5 layer on Mg–Li alloy by hot-pressing using silane coupling agent as bond and its corrosion resistance

The synthesis of ZSM-5 (Zeolite Socony Mobile-Five) layer on Mg–Li alloy has so far proved difficult. Here we report an effective strategy to assemble ZSM-5 layer on Mg–Li alloy by hot-pressing, using silane coupling as a bonding agent. The high crystalline ZSM-5 showed that the hexagon crystals appeared to be attached firmly to the support. The corrosion behaviour, assessed by electrochemical measurements, showed that ZSM-5 layer reduced the corrosion activity of Mg–Li alloy. The hot-pressing served as an environmentally friendly, corrosion-resistant coating for Mg–Li alloy. The method can potentially provide a general route to synthesize various, uniform layers on other active metals.

Electrochemical Behavior of Glassy Carbon Electrode Modified With Copper-ZSM5

The electrochemical response of modified electrode with Cu-ZSM5 deposited on glassy carbon surface has been studied using cyclic voltammetry and open circuit potential (OCP) measurements. The i-E characteristics showed two cathodic processes at c.a. 0.03V and -0.2V/SCE attributed to the formation of Cu+ and Cu0, respectively. The progressive spread of electro active species on the surface of the zeolite was observed by multi sweep cyclic voltammetry experiment. The diffusion of Cu2+ in the ZSM5 layers depends on the cation nature and increases in the order Li+>K+>NH4 +>Na+. The evaluation of reversible behavior indicated that the electrochemical process is affected by adsorption/desorption phenomena at the interface. It was found that the redox processes are proportional to the amount of Cu species in the zeolite structure.

High-flux ZSM-5 membranes with an additional non-zeolite pore system by alcohol addition to the synthesis batch and their evaluation in the 1-butene/i-butene separation

ZSM-5 membranes were crystallized on tubular TiO 2 supports and evaluated in the permeation of a 50%/50% 1-butene/i-butene mixture. If some of the water in the standard recipe 90 SiO 2:0.225 Al 2O 3:1 Na 2O:3.6 TPAOH:1.8 TPABr:1800 H 2O was substituted by the same molar amount of short-chain length alcohol, the fluxes and permeances increased remarkably but the 1-butene/i-butene shape selectivity decreased only slightly. This experimental finding is attributed to additional non-zeolite micropores which are formed by the presence of alcohols in the synthesis batch. Since the formation of this additional pore system is linked to a decrease of the crystal size in the membrane layer, the increased length of grain boundaries could represent the structural origin of these additional non-zeolite pores. In a membrane preparation if 25% of the molar amount of water was substituted by ethanol, the 1-butene flux increased by the factor of two, but the mixture separation factor decreased by only 30%. Therefore, relatively thick ZSM-5 layers of about 30 μm gave 1-butene permeances of the order of 1 m 3(STP) m −2 h −1 bar −1 with permselectivities of 20 and mixture separation factor of 6. The testing of the ZSM-5 membranes took place under practice-relevant conditions, i.e. with undiluted feeds and without applying sweep gases or reduced pressure on the permeate side.

A catalytic ZSM-5 membrane sandwiched with silicalite-1 layers for highly selective toluene disproportionation

A catalytic zeolite membrane composed of silicalite-1, ZSM-5 and silicalite-1 layers supported on a porous stainless support was prepared by hydrothermal synthesis (silicalite-1/ZSM-5/silicalite-1 membrane). The permeance of hydrogen, argon, nitrogen, n-butane and isobutane for silicalite-1/ZSM-5/silicalite-1 membrane was measured from 353 to 513 K. The permeance of these gases were from 5 to 25 × 10 −8 mol/(m 2 s Pa) the same range as typical MFI-type membranes and the ratio of the permeance of n-butane to isobutane was 2 or less. The toluene disproportionation was examined at 673 K with silicalite-1/ZSM-5/silicalite-1 membrane as a membrane reactor as well as ZSM-5/silicalite-1 membrane and ZSM-5 powder. The products detected were benzene, p-, m- and o-xylene. The toluene conversion with two membrane reactors was less than 3%. At lower than 1% of toluene conversion, the selectivity of p-xylene for silicalite-1/ZSM-5/silicalite-1 was above 96% whereas that for ZSM-5/silicalite-1 membrane was below 89%. The silicalite-1 layers sandwiching the ZSM-5 layer probably suppressed the surface reaction on both ZSM-5 layers, which would achieve the significantly high selectivity of p-xylene in toluene disproportionation.