Nano-sized ZSM-5 Research Articles

The Aromatization of Propene Via Nano-Size HZSM-5

Zeolite (ZSM) catalysts are known to convert small-size alkenes, e.g., propene, into aromatic hydrocarbons, specifically benzene, toluene and xylenes (BTX), with both high efficiency and specificity. The efficiency of conventional and hierarchical nano-size ZSM-5 for propene aromatization was compared in this study using a Design of Experiments (DOE) approach combined with detailed product analysis. Contrary to our expectations, the former showed a significantly greater BTX yield than the latter. Analysis of the obtained data by DOE and additional experiments with soybean oil cracking using both catalyst types indicated that a reason for the observed reduced activity of nano-scale zeolites may be tenacious water adsorption, which may reduce the catalyst active site availability to the substrate.

Strategies to Enhance the Catalytic Performance of ZSM-5 Zeolite in Hydrocarbon Cracking: A Review

ZSM-5 zeolite is widely used in catalytic cracking of hydrocarbon, but the conventional ZSM-5 zeolite deactivates quickly due to its simple microporous and long diffusion pathway. Many studies have been done to overcome these disadvantages recently. In this review, four main approaches for enhancing the catalytic performance, namely synthesis of ZSM-5 zeolite with special morphology, hierarchical ZSM-5 zeolite, nano-sized ZSM-5 zeolite and optimization of acid properties, are discussed. ZSM-5 with special morphology such as hollow, composite and nanosheet structure can effectively increase the diffusion efficiency and accessibility of acid sites, giving high catalytic activity. The accessibility of acid sites and diffusion efficiency can also be enhanced by introducing additional mesopores or macropores. By decreasing the crystal size to nanoscale, the diffusion length can be shortened. The catalytic activity increases and the amount of carbon deposition decreases with the decrease of crystal size. By regulating the acid properties of ZSM-5 with element or compound modification, the overreaction of reactants and formation of carbon deposition could be suppressed, thus enhancing the catalytic activity and light alkene selectivity. Besides, some future needs and perspectives of ZSM-5 with excellent cracking activity are addressed for researchers’ consideration.

Fluoride-mediated nano-sized high-silica ZSM-5 as an ultrastable catalyst for methanol conversion to propylene

Fluoride mediated nano-sized ZSM-5 (ZSM-5-F) with a high Si/Al ratio of 181 was fabricated using a seed-induction method and evaluated the catalysis of the methanol to propylene (MTP) reaction. High propylene selectivity (45%) was similar to ZSM-5-OH synthesized via a hydroxide route. However, ZSM-5-F showed much longer lifetime (305 h) compared with ZSM-5-OH (157 h) in spite of similar crystal size and aluminum content. Characterization by NH3-TPD, Py-IR, OH-IR, SEM, TG-DTA, XRD and 1H MAS NMR techniques indicated that the enhanced catalytic performance of ZSM-5-F is attributed to the fewer structural defects in the form of internal silanol groups and silanol nests.

Creating intraparticle mesopores inside ZSM-5 nanocrystals under OSDA-free conditions and achievement of high activity in LDPE degradation

This work presents a simple method for creating intraparticle mesopores inside ZSM-5 nanocrystals using a polymer/seed co-assisted approach under OSDA (organic structural directing agent)-free conditions. The effects of anionic polyacrylamide (PAM), and some derived synthesis parameters, including PAM concentration, seed concentration, sodium hydroxide concentration, and crystallization temperature on the textural properties of the final product, and especially on the intraparticle mesopore formation mechanism were systematically investigated by XRD, N2-adsorption, UV-Raman, SEM, TEM, solid state 27Al MAS NMR, and 29Si MAS NMR analyses. The proposed mechanism includes (1) Introducing a seed solution to guarantee sufficient nucleation; (2) PAM acceleration of phase separation by “immobilizing” the generated ultrafine MFI structure building units, and so playing the role of a flocculating agent in the induction period; (3) Creation of intraparticle and interparticle mesopores after removal of PAM species by calcination, which can be pre-designed by simply optimizing the synthesis parameters. The synthesis involves severe conditions and as a consequence the resulting nano-sized ZSM-5 zeolite simultaneously possesses very high crystallinity and considerable intraparticle mesoporosity. Because of the existence of intraparticle mesopores, the produced ZSM-5 nanocrystal agglomerates exhibit excellent catalytic performance in the LDPE degradation reaction. The temperature ascribed to the maximum degradation rate was found to be 22 K lower than that measured for the production of common nano-sized ZSM-5 aggregates that do not exhibit intraparticle mesoporosity.

Aggregates of nano-sized ZSM-5 crystals synthesized with template-free and alkali-treated seeds for improving the catalytic performance in MTP reaction

A seed-induced method was developed to synthesize aggregates of nano-sized ZSM-5 crystals by template-free MFI seeds and cetyltrimethyl ammonium bromide (CTAB). Various MFI zeolites after calcination, including: nano-sized silicate-1, nano-sized ZSM-5 aggregates and commercial bulk ZSM-5 crystals were used as seeds. A trace of CTAB could accelerate the crystallization and pure MFI structure was obtained in the presence of CTAB. More importantly, the aggregates of nano-sized ZSM-5 crystals were dramatically synthesized by transforming the bulk seeds into smaller sizes with NaOH treatment before crystallization. The resulting solid exhibited an excellent catalytic performance in the methanol to propylene reaction.

Facile synthesis of nano-sized hollow ZSM-5 zeolites with rich mesopores in shell

A hollow nano-ZSM-5 zeolites with mesoporous shell was successfully synthesized by a two-step alkaline treatment method. Firstly, nano-ZSM-5 zeolite was alkali treated with NaOH solution to dissolve internal silica of crystals and then the hollow structure was formed. After that, obvious mesopores were introduced in the shell of the hollow zeolites by a NaOH&TPAOH leaching based on the protective desilication. As a result, the external surface area was increased from 98 to 203 m2g-1, more than twice that of the untreated nano-ZSM-5. The thin shell and rich mesopores would improve its catalytic performance in many diffusion-limited reactions.

Correlation between H-ZSM-5 crystal size and catalytic performance in the methanol-to-aromatics reaction

The porosity of H-ZSM-5 zeolite is known to facilitate the diffusion of molecules in the methanol-to-aromatics (MTA) reaction. The activity and selectivity of the H-ZSM-5 catalyst in the MTA reaction has been studied as a function of crystal size. ZSM-5 zeolites with different crystal sizes were successfully synthesized by conventional hydrothermal methods. Tailoring ZSM-5 particle size was easily controlled by changes to the sol-gel composition, and in particular, the deionized water to tetrapropylammonium hydroxide ratio, and crystallization time. The structure of the H-ZSM-5 zeolites were characterized by X-ray diffraction and the morphology of the zeolite particles was determined by scanning electron microscopy. N2 adsorption-desorption measurements established changes to the textural properties, and compositional properties were characterized by X-ray fluorescence spectroscopy. Acidity measurements of the catalysts were measured by pyridine-adsorbed Fourier transform infrared spectroscopy and the temperature-programmed desorption of ammonia. After subjecting the catalysts to the MTA reaction, the total amount of coke formed on the spent deactivated catalysts was determined by thermal gravimetric analysis. The results show that the SiO2/Al2O3 molar ratios and acidic properties of the H-ZSM-5 samples are similar, however, the nano-sized hierarchical ZSM-5 zeolite with an additional level of auxiliary pores possesses a higher surface area, larger mesopore volume and larger macropore volume, resulting in more effective mass transportation properties. The H-ZSM-5 zeolite catalysts were evaluated for their activity towards the MTA reaction as a function of crystal size. The nano-sized H-ZSM-5 catalyst having shorter diffusion path lengths, substantial mesoporosity and a high external surface area showed excellent selectivity toward aromatics (average > 42%) and benzene, toluene and xylenes (37% at 180 min). Furthermore, lower coking levels were observed on the nano-sized H-ZSM-5 catalyst when compared with the H-ZSM-5 catalysts possessing larger particle sizes and is attributed to a reduction in polyaromatic hydrocarbons being deposited within the internal surface area.

A study on the effect of SiO2/Al2O3 ratio on the structure and performance of nano-sized ZSM-5 in methanol to propylene conversion

The effect of SiO2/Al2O3 ratio on high silica content nano-sized HZSM-5 stability and selectivity in conversion of methanol to propylene (MTP) has been investigated in a continuous flow isotherm fixed-bed reactor. Highly crystalline HZSM-5 zeolite samples with SiO2/Al2O3 ratios ranging 150–200 have been prepared and characterized using XRD, ICP-AES analytical techniques, NH3-TPD, BET surface area and SEM imaging for studying their physical and chemical behavior. The reaction was conducted at 480 °C and 1bar with WHSV = 1 h−1 and a solution with 1:1 wt% of methanol and water as feed. Over thoroughly monitoring the catalyst synthesis conditions, the nanocrystal ZSM-5 zeolite samples were fabricated with fairly close microstructural properties and crystal sizes in order to be able to study the effect of SiO2/Al2O3 ratio on the ZSM-5’s performance in MTP. Results show that although the crystal size does not considerably affect the elementary structure of the zeolite active sites, it seems to be an influential factor in bond strength of Si–O versus Al–O. The results have also showed that increase of SiO2/Al2O3 ratio leads to higher selectivity and stability of catalyst.

Direct synthesis of monolithic nano-sized ZSM-5 aggregates possessing ordered mesoporosity by controlling arrangement of nanoparticles

This work presents a facile method for synthesizing monolithic ZSM-5 nanoparticle aggregates (ZNA) by directly converting the ordered mesoporous material Al-SBA-15 under low solvent content conditions. The effects of crystallization temperature, time, and the structure-directing agents (SDAs) content on the textural properties and on the crystallinity of the final product were investigated. The crystal growth was physically confined to a nano-scale space surrounded by chemically inert units previously formed through the controllable decomposition of P123 micelles. The resulting nanoparticles exhibit uniform size in the range of 20–40 nm. Moreover, ZNA has a monolithic morphology of several micrometers in size, closely resembling that of raw Al-SBA-15, allowing the avoidance of a high-speed centrifugal separation process. The location of nanoparticles inside ZNA bulk particles can be mainly restricted to replicating the Al-SBA-15 walls by slowing down the inorganic species consumption process, and consequently, forming a visibly ordered mesostructure.

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 nano-sized 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=4.74h−1 and P=1.0MPa. It was found that the external surface area of the nano-sized ZSM-5 zeolite with intercrystalline mesopores reached 104m2/g, larger than that of mesoporous ZSM-5 (66m2/g) and nano-sized ZSM-5 (76m2/g). Catalytic lifetime of the nano-sized ZSM-5 zeolite with intercrystalline mesopores was 93h, which was only longer than that of mesoporous ZSM-5 (86h), but shorter than that of nano-sized ZSM-5 (104h). 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 106h because of the large external surface area and decreased weak acidity. This special structural zeolite is a potential catalyst for methanol to gasoline reaction.

Fabrication and characterization of composites comprising (CHA)SAPO-34 with (MFI)ZSM-5 topologies and their catalytic performances on MTO reaction

The promising composites comprising of (CHA)SAPO-34 with (MFI)ZSM-5 structures were successfully fabricated by microwave-assisted hydrothermal synthesis and characterized to further investigate their catalytic performance on methanol to olefins (MTO) reaction. The physical mixture, synthesized nanosized ZSM-5 and microsized SAPO-34 were employed as references for exploring the structure-activity correlation. These obtained products were extensively characterized by XRD, SEM, FT-IR, NH3-TPD and N2 adsorption-desorption techniques to investigate their crystalline phase, framework, morphology, acidic sites and pore structure. The SAPO-34/ZSM-5 and ZSM-5/SAPO-34 composites exhibited significantly different frameworks, morphologies, acidic properties, and micro- and mesopores ratios in spite of their similar crystalline phase compositions. The pre-treatment of microscale SAPO-34 impregnated with tetrapropylammonium hydroxide was a determining procedure to obtain well-defined SAPO-34/ZSM-5 composite without phase detachment. The catalytic test results showed that SAPO-34/ZSM-5 composite exhibited excellent catalytic performances with methanol conversion of 98.9% and light olefins selectivity of 91.93% during 60 h continuous reaction at 400 °C as well as induction period of 0.5 h. O2-TPO experimental results showed that low- and high-temperature coke deposition coexisted in SAPO-34/ZSM-5 composite even after 60 h by suppression on diffusion restriction. A synergetic effect of interface phase together with predominant weak acid sites and distinct pore structure is assigned to make essential contributions to enhanced catalytic activity to methanol conversion, superior shape selectivity to light olefins and improved stability for SAPO-34/ZSM-5 composites at mild temperatures of MTO reaction.

Aerosol-assisted synthesis of nano-sized ZSM-5 aggregates

An aerosol-assisted method is used to synthesize nano-sized ZSM-5 aggregates with good texture properties and Al species distribution.

Conversion of methanol to propylene over nano-sized ZSM-5 zeolite aggregates synthesized by a modified seed-induced method with CTAB

Nano-sized ZSM-5 zeolite aggregates with mesopores and high catalytic activity were prepared by a modified seed-induced method using silicate-1 as seeds without additional templates in the presence of a trace amount of CTAB.

Direct synthesis of high-silica nano ZSM-5 aggregates with controllable mesoporosity and enhanced catalytic properties

In the absence of additional mesoporous template, high-silica nano-sized ZSM-5 aggregates have been rapidly synthesized by solid-like state conversion.

Impact of external surface passivation of nano-ZSM-5 zeolites in the methanol-to-olefins reaction

Surfaces of nano-sized ZSM-5 zeolites have been passivated by a chemical liquid deposition (CLD) technique using tetraethylorthosilicate (TEOS). The impact of this post-synthesis treatment has been evaluated in the methanol-to-olefins (MTO) reaction.A two-step surface passivation led to enhanced selectivity towards C2–C4 light olefins, between 10 and 30% in relative, at nearly complete methanol conversion when compared with pristine zeolites. However, these post-treated zeolites exhibited a significant loss in stability.To rationalize these results and establish structure-activity relationships, zeolite catalysts have been thoroughly characterized by nitrogen adsorption–desorption, SEM, XRF, XRD and FTIR techniques. ZSM-5 zeolite (Si/Al=27) in the form of well-arranged rectangular nanocrystals behave in MTO after silylation as a commercial ZSM-5 zeolite having a Si/Al=52.

Exploring suitable ZSM-5/MCM-41 zeolites for catalytic cracking of n-dodecane: Effect of initial particle size and Si/Al ratio

In this study, various ZSM-5/MCM-41 micro/mesoporous zeolite composites have been prepared by alkali-desilication and surfactant-directed recrystallization of ZSM-5. The effects of particle size and Si/Al ratio of initial ZSM-5 zeolites on the structure and catalytic performance of ZSM-5/MCM-41 composites are studied. The results of XRD, TEM N2-adsorption–desorption, NH3-TPD and in situ FT-IR revealed that ordered hexagonal MCM-41 mesopores with 3–4nm pore size were formed around ZSM-5 crystals, and the specific surface area and mesopore volume of composites increased with increasing the Si/Al ratio of initial ZSM-5. Catalytic cracking of n-dodecane (550°C, 4MPa) showed that the ZSM-5/MCM-41 composites obtained from the high Si/Al ratio and nano-sized initial ZSM-5 zeolites exhibited superior catalytic performance, with the improvement higher than 87% in the catalytic activities and 21% in the deactivation rate compared with untreated zeolites. This could be ascribed to their suitable pore structure, which enhanced the diffusion of reactant molecules in pores of catalysts.

Differences between individual ZSM-5 crystals in forming hollow single crystals and mesopores during base leaching.

After base treatment of ZSM-5 crystals below 100 nm in size, TEM shows hollow single crystals with a 10 nm shell. SEM images confirm that the shell is well- preserved even after prolonged treatment. Determination of the Si/Al ratios with AAS and XPS in combination with argon sputtering reveals aluminum zoning of the parent zeolite, and the total pore volume increases in the first two hours of base treatment. In corresponding TEM images, the amount of hollow crystals are observed to increase during the first two hours of base treatment, and intact crystals are visible even after 10 h of leaching; these observations indicate different dissolution rates between individual crystals. TEM of large, commercially available ZSM-5 crystals shows inhomogeneous distribution of mesopores among different crystals, which points to the existence of structural differences between individual crystals. Only tetrahedrally coordinated aluminum is detected with (27) Al MAS NMR after the base leaching of nano-sized ZSM-5.

ZSM-5 Zeolite with Micro-Mesoporous Structures Synthesized Using Different Templates for Methanol to Propylene Reaction

ZSM-5 zeolites with different pore structures were synthesized using different templates(tetrapropyl ammonium hydroxide(TPAOH), cetyltrimethylammonium bromide(CTAB) and C18-6-6Br2). The obtained nanosized(NZ), mesoporous(MZ), and nanosheets(NSZ) ZSM-5 samples were compared with conventional microporous ZSM-5 zeolite(CZ). The physicochemical properties of these samples were characterized using X-ray diffraction(XRD), scanning electron microscopy(SEM), N2 adsorption-desorption,and temperature-programmed desorption of ammonia(NH3-TPD). The results showed that the mesopore volumes and surface areas of the four samples increased in the order NSZ MZ NZ CZ, and the ratio of strong/weak acidity increased in the order CZ MZ NZ NSZ. In the methanol to propylene(MTP) reaction,the catalyst porosity played an important role on the product selectivity and catalytic stability. The selectivities for propylene and total olefins improved with increasing mesoporosity; NSZ, with the largest mesopore volume, gave the highest propylene selectivity, i.e., 47.5%, and 78.4% total olefins. Meanwhile, the introduction of mesopores into the ZSM-5 zeolite extended the catalytic lifetime. The NZ sample displayed reliable MTP catalytic activity for 200 h, which was predominately attributed to its optimal combination of acidity and porosity.

Dielectric and ethanol sensing studies on synthesized nano-ZSM-5 zeolite

Nano-sized ZSM-5 zeolite crystals are synthesized without organic template using microwave assisted hydrothermal technique. The synthesized zeolite is then characterized by various techniques like X-ray diffraction, Fourier transform infrared spectroscopy and thermo gravimetric analysis. X-ray diffraction profile confirms that the synthesized zeolite structure is of ZSM-5 and grain size of synthesized zeolites is found to be about 30 nm. Fourier transform infrared spectroscopy shows the presence of T–O–T link of zeolite structure. The thermal stability of ZSM-5 is studied by using simultaneous thermo gravimetric analyzer. Screen printed thick films are prepared using the synthesized nano-ZSM-5 zeolite as a functional material. Atomic force microscopy reveals the topography of thick film. Study on ethanol sensing characteristics of zeolite thick films has shown that the film can be used as an ethanol sensor at an operating temperature of 135 °C with a quick response of 150 s. Saturation limit for the film is observed to be very low.

Synthesis of NiMo hydrodesulfurization catalyst supported on a composite of nano-sized ZSM-5 zeolite enwrapped with mesoporous KIT-6 material and its high isomerization selectivity

A novel composite material ZSM-5/KIT-6 (ZK-W) was synthesized by enwrapping nano-sized ZSM-5 zeolite crystals with mesoporous KIT-6 silica. This composite was used as catalyst support for NiMo sulfide in the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT). The NiMo/ZK-W catalyst showed the highest HDS activity and its 4,6-DMDBT conversion was about double that of a conventional NiMo/Al2O3 catalyst at 320°C, 4.0MPa, and LHSV of 150h−1 and on the basis of sulfur. The superior catalytic performance of NiMo/ZK-W is related to the synergistic effect of the excellent diffusion through the hierarchical porous structure and the suitable Brønsted acid sites. 3,6-DMDBT and 3,7-DMDBT, two isomers of 4,6-DMDBT, were identified simultaneously for the first time in the HDS products of 4,6-DMDBT over the NiMo/ZK-W catalyst. The NiMo/ZK-W catalyst exhibited a superior isomerization ability, and 4,4′-dimethylbiphenyl was the main product, indicating that the isomerization pathway is the main reaction route. A new reaction network of 4,6-DMDBT HDS over NiMo/ZK-W catalyst was proposed.