ZSM-5 Crystals Research Articles

Directional Transport in Hierarchically Aligned ZSM-5 Zeolites with High Catalytic Activity.

Zeolites, the most technically important crystalline microporous materials, are indispensable cornerstones of chemical engineering because of their remarkable catalytic properties and adsorption capabilities. Numerous studies have demonstrated that the hierarchical engineering of zeolites can maximize accessible active sites and improve mass transport, which significantly decreases the internal diffusion limits to achieve the desired performance. However, the construction of hierarchical zeolites with ordered alignments and size-controlled substructures in a convenient way is highly challenging. Herein, we develop a facile procedure using two common structure-directing agents, tetrapropylammonium hydroxide (TPAOH) and tetraethylammonium hydroxide (TEAOH), to synthesize hierarchically aligned ZSM-5 (Hie-ZSM-5) crystals with a-axis alignment substructures of controllable size. The control of the substructure size (α) in the range of 10-60 nm and the corresponding similarity (r = α/β, where β is the size of Hie-ZSM-5) ranging from 0.004 to 0.033 can be tuned by varying the Si/Al ratios (40-120). A systematic investigation of the overall crystallization process, using time-dependent XRD, SEM, TEM, and solid-state magic-angle spinning NMR (13C, 27Al, 29Si) methods, enable us to construct a solid mechanism for the generation of Hie-ZSM-5. Most importantly, directional transport in the unique structures of Hie-ZSM-5 efficiently enhances mass diffusion, as well as catalytic activity and stability. These findings improve our understanding of the zeolite crystallization process and inspire novel methods for the rational design of hierarchical zeolites.

Non-linear kinetic model of coke deactivation in methanol-to-gasoline conversion: Effects of nano and micro ZSM-5 crystals

Methanol-to-gasoline conversion over ZSM-5 catalysts, a non-petroleum carbon fuel process, is significantly impacted by coke deposition, which deactivates the catalyst by blocking zeolite pores during the reaction and diffusion process. This study investigated the differences in coke species development and deactivation using a non-linear kinetic model, comparing microcrystalline ZSM-5 with longer pore lengths to nanocrystalline ZSM-5 with shorter pores, both tested at 350°C. Catalyst and coke were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed oxidation (TPO), thermogravimetric analysis (TGA), and an elemental analyzer. Micro ZSM-5 initially achieved 99 % conversion but rapidly dropped to 61 % due to coke buildup, while nano ZSM-5 maintained over 98 % conversion for 24 hours before declining, highlighting the superior catalytic durability of shorter pores. Micro ZSM-5 showed faster coking, accumulating 7.19 % coke with higher temperature-resistant graphitic coke, whereas nano ZSM-5 exhibited a lower coke buildup of 4.56 %, forming primarily amorphous coke. A two-step non-linear kinetic model for soft and hard coke oxidation was applied, with the second-order model providing the highest correlation coefficient. The activation energy for coke decomposition was influenced by pore length, revealing distinct kinetic behaviors for hydrogen-deficient graphitic coke and amorphous coke. This study underscores the importance of zeolite pore structure on coke formation, deposition patterns, and catalytic durability, offering deeper insights into the methanol-to-gasoline conversion process.

Synchrotron Infrared Microspectroscopy of Zeolite Catalysts

This article reviews recent work undertaken at the beamline B22 of the Diamond Light Source using infrared (IR) microspectroscopy to characterise zeolite catalysts and to study their reactivity in real time. The advantage of vibrational microspectroscopic analysis when linked to the brightness and spectral bandwidth of synchrotron IR light are illustrated. The high spatial resolution means that the uniformity of acid site concentrations within individual large crystals of zeolites and between different crystals can be readily checked and changes to acid site concentrations within crystals resulting from steam treatment mapped. When an in situ reaction cell is coupled with mass spectrometric analysis of evolved gases the rapid time response of the method has provided new insight into the initial stages of the conversion of methanol to hydrocarbons over ZSM-5 and SAPO-34 single crystals. Future prospects for applying the method to other types of zeolite catalysed reactions with improved reaction cell design are also discussed.

Facile synthesis and mechanistic insights into ZSM-5 aggregates via a mesoporogen-free route

Developing hierarchical zeolites is highly important for catalytic applications. In this study, micro- and submicron-sized hierarchical ZSM-5 zeolites, composed of aggregated nanosized ZSM-5 crystals, were facilely synthesized without any mesoporogen. The effects of the gel composition in a mesoporogen-free system on the formation of ZSM-5 aggregates were studied. The results revealed that the Na2O/SiO2 molar ratio and the aluminum source exerted significant impacts. Furthermore, we demonstrated that the morphology and size of the ZSM-5 aggregate could be easily adjusted by varying the gel composition. Mechanistic insights indicated that the formation of hierarchical ZSM-5 aggregates in the mesoporogen-free system likely followed a space-confined growth mechanism.

Pyrolysis of hyphaene thebaica shell over ceramic tile dust-derived catalysts and assessment of the produced bio-oil

The purpose of this study is to demonstrate the potential of ceramic tile dust (CTD)-derived ZSM-5 zeolite (CZ) and its monodispersed composite with metal oxides (MgO and Fe2O3) in the catalytic pyrolysis of hyphaene thebaica shell (HTS). The HTS was pyrolysed in a Fixed-bed reactor at 400–600 °C, and 100–300 mL/min N2 flowrate. The maximum bio-oil production of 32 % was obtained at 500 °C and 150 mL/min N2 flowrate, with bio-oils containing 50 % acid and octadecenoic acids, as well as esters, phenols, aldehydes, ketones, ethers, aromatics, and hydrocarbons. A carboxymethyl cellulose templating agent was employed for the mesoporous zeolite synthesis from CTD. This resulted in the mesoporous zeolite with a predominant ZSM-5 crystal phase, exhibiting pore diameters ranging from 1.8-6 nm, 229 m2/g surface area and 1145 μmol/g total acidity. The catalytic pyrolysis of HTS was conducted using the ZSM-5 zeolite (CZ) and metal-oxide (MgO, Fe2O3, and Fe2O3/FeO) modified CZ, as monodispersed composite catalysts. Under best thermal pyrolysis conditions, CZ-Fe2O3, CZ-MgO, and CZ-Fe2O3/MgO demonstrated 22–23 % bio-oil yields. Notably, the CZ-Fe2O3/MgO catalyst exhibited the highest hydrocarbon yield at 16 %, while the CZ-MgO favoured the production of phenolics, esters, and alcohols. CZ-MgO also displayed the highest coking level at 7.5 %, indicating faster deactivation than the other catalysts. The synthesised catalysts exhibited remarkable catalytic activity, resulting in a notable improvement in the quality of bio-oils obtained from the intermediate pyrolysis of hyphaene thebaica shells in a fixed-bed reactor.

Role of silicon in the growth of ZSM-5 synthesized with coal fly ash as raw materials

In this work, single-phase ZSM-5 zeolite was synthesized using solid waste coal fly ash (CFA) as raw material, providing a potential alternative for creating high-value-added products from this waste. X-ray diffraction (XRD), scanning electron microscopy, X-ray fluorescence, Fourier transform infrared spectroscopy, N2 physisorption, and other techniques were used to evaluate the crystallization and morphology of the ZSM-5 obtained. The results indicated that ZSM-5 can be synthesized by the simple method of mixing, stirring, and heating. A well-defined hexagonal shape of ZSM-5 with a specific surface area of 162 m2/g and mesoporous structure was obtained at the temperature of 160 °C during hydrothermal treatment for 72 h. Theoretically, the mechanism of crystal growth of ZSM-5, i.e. the liquid phase nucleation and step growth models, was clarified. These results open the possibility of exploring the crystallization of ZSM-5 and the utilization of CFA. Simultaneously, the critical roles of silicon in framework formation and crystallization control during the formation and growth of zeolites were elucidated.

Evaluation of Zeolite Composites by IR and NMR Spectroscopy.

In this study, we assessed the quantity, strength, and acidity of zeolite composites comprising Silicalite-1 grown on ZSM-5 crystals using a combination of infrared (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy. The composites were created through the direct growth of Silicalite-1 crystals on ZSM-5 (P_ZSM-5), either with or without the organic structure-directing agent (OSDA) introduced into the ZSM-5 channels (samples: H_ZSM-5_Sil1 and TPA_ZSM-5_Sil1). The results revealed that Silicalite-1 grew differently when the ZSM-5 core was in the H+ form (empty pores) compared to when the OSDA was still present in the sample. This distinction was evident in the textural properties, with a decrease in the micropore surface area and an increase in the external surface area in the H_ZSM-5_Sil1 compared to the parent sample. The TPA_ZSM-5_Sil1 composite exhibited characteristics similar to the parent zeolite. These findings were further supported by 29Si NMR, which revealed a comparable local order for the parent (P_ZSM-5) and TPA_ZSM-5_Sil1 samples, along with a broadening of the Q4 peak for the H_ZSM-5_Sil1 composite. Additionally, the acid sites were preserved in the TPA_ZSM-5_Sil1 composite, while in the H+-form core, the concentration of Brønsted acid sites significantly decreased. This reduction in isolated Brønsted acid sites was further corroborated by 1H NMR.

Simple and sustainable synthesis of loosely stacked nano-H-ZSM-5 aggregates from kaolin and catalytic studies

The conversion of natural clay to crystalline zeolites has been the subject of considerable interest from both academic and industrial circles. We present an effective strategy for converting kaolin to ZSM-5 zeolite, addressing the issue of reduced mesopore formation in conventional nano-ZSM-5 aggregates due to close-packing. This strategy utilizes kaolin as the sole source of silicon and aluminum, and by decoupling the nucleation and growth of ZSM-5 crystals, loosely stacked nano-H-ZSM-5 aggregates with high crystallinity crystals can be synthesized in a solid-like system by employing a tiny amount of TPAOH (TPAOH/SiO2 = 0.064). The resulting nano-H-ZSM-5 aggregates exhibited high specific surface area (405.66 m2/g), high mesopore volume (0.64 cm3/g), and superior catalytic activity. This strategy offers a novel approach to the cost-effective synthesis of nano-H-ZSM-5 aggregates suitable for industrial applications.

Enhancing the catalytic performance of H-ZSM-5 in bio n-butanol dehydration by zeolite crystal engineering

NH4F etching is used as an unbiased chemical etching technique to hierarchize commercially available nano-sized ZSM-5 crystals. A structure-function relationship is reported, describing how the introduction of mesoporosity in a microporous ZSM-5 zeolite significantly increases its catalytic performance in (aqueous) n-butanol dehydration. Three parent samples are etched under two severity modes: mild (5 minutes of etching) and harsh (40 minutes of etching). Intrinsic features of the parent ZSM-5’s, such as their silanol populations, influence the outcome of the etching procedure. For all materials, crystallinity is unaffected, total pore volume increases while micropore volume is almost constant, and active site accessibility increases. For all materials, etching increases the catalytic activity for n-butanol dehydration. Cofeeding water to n-butanol further increases the stability of all catalysts. Microkinetic modeling reveals that the increased activity is related to a decreased stability of dibutyl ether (DBE), the most abundant surface intermediate. A decreased DBE adsorption energy, due to the increased accessibility of the acid sites, facilitates its desorption. NH4F etching, a flexible and easy technique to engineer zeolites' textural properties, does not affect the intrinsic nature and quantity of their active sites, leading to superior catalytic performances. Our results highlight the possibility to further increase the efficiency of zeolite catalysis in biomass conversion processes.

Synthesis of ZSM-5 From Natural Zeolite Lampung (ZAL) and Rice Husk by Seeding Method using Microwaves

ZSM-5 can be synthesized from Lampung Natural Zeolite (ZAL) and rice husks as a source of additional silica. To speed up the synthesis time, ZSM-5 was synthesized using microwaves. Based on this, the synthesis of ZSM-5 was carried out using microwaves at a temperature of 140 oC with a power of 540 watts with time variations of 10, 15 and 30 minutes with the seeding method as a template replacement of 20%. Amorphous silica with a SiO2 content of 95.324% was extracted from rice husks with a suitable alkali solution. The influence of synthesis time on the formation of ZSM-5 has been carried out. The resulting product was characterized using X-ray Diffraction (XRD), and FTIR (Fourier-Transform-Infrared), Scanning Electron Microscopy (SEM) and BET (Brunaur, Emmet and Teller). The results of the analysis show that the results of the research show that the synthesis of ZSM-5 has been successfully carried out by changing the amorphous phase from natural Lampung zeolite and rice husks into ZSM-5 crystals using microwaves which can shorten the synthesis time 216 - 72 times faster than with heating. conventional, produces high crystallinity, provides small particle sizes with larger surface area, pore volume and pore diameter, and microwave radiation does not damage the Si-O bonds. The best sample was obtained at a synthesis time of 30 minutes with a crystallinity percentage of 96.76% and a specific surface area of 138.616 m2 /g.

Constructing hierarchical ZSM-5 coated with small ZSM-5 crystals via oriented-attachment and in situ assembly for methanol-to-aromatics reaction

Constructing hierarchical ZSM-5 coated with small ZSM-5 crystals via oriented-attachment and in situ assembly for methanol-to-aromatics reaction

Amine-free synthesis of high-silica ZSM-5 assisted with calcined silicalite-1 and ethanol with the investigation of mechanism

Amine-free synthesis of high silica ZSM-5 with high crystallinity and well-defined morphology was challenging. In this work, ZSM-5 with the fed silica-alumina ratio (SAR) of 100 was rapidly synthesized with the assistance of the calcined silicalite-1 seed and ethanol, and their crystal structures, morphology and microporous properties were characterized by XRD, SEM, FT-IR, ICP and N2 adsorption-desorption and so on. Firstly, the crystallization temperature was optimized between 140 and 180 °C at a certain alkalinity and ethanol content, and 160 °C was selected to not only provide high crystallization rate but also ensure the stability of high-silica skeleton. Secondly, the influences of the EtOH/SiO2 ratio, Na2O/SiO2 ratio and crystallization time on the crystallization of high-silica ZSM-5 were investigated at 160 °C. The results showed well-crystalline high-silica ZSM-5 could be synthesized within a wide range of the EtOH/SiO2 ratios (0.5–2.0) and the increase of ethanol content slightly inhibited the growth of ZSM-5 along b-axial of hexagonal prisms due to the preferential adsorption of excess ethanol on the {010} crystal plane and the formation of Si–O–C2H5; high alkalinity was favorable for the preparation of a small-sized and well-crystalline ZSM-5 due to the rapid depolymerization of calcined seed and the contribution of mass nucleation sites; meanwhile, excellent micropore characteristic of high-silica ZSM-5 further proved that the synthetic strategy of seed + ethanol was feasible. Finally, the synergistic action mechanism of calcined seed and ethanol for the synthesis of high-silica ZSM-5 was revealed that silicalite-1 with MFI structure unit combined with Na+(H2O/EtOH) with larger radius were very important for rapidly assembly of a large number of silica species into MFI framework.

Recycling of Spent FCC Catalysts: Conversion of Leached Residues to Zeolite ZSM-5.

The commercial recycling of spent FCC catalyst typically focuses on recovering only 1-3 % of rare-earth elements, with the remaining residues often disposed of in landfills. Here, we present a novel method to close a recycling loop for spent FCC catalyst. The method involves a series of leaching steps: Firstly, the spent catalyst material is leached with HNO3 to remove rare-earth elements such as La; second, solvothermal leaching with HCl removes most of Al and impurities like Fe, Ni and V; finally, a third leaching with H2SO4 removes Ti. The solid residues are then used to synthesize ZSM-5 without the addition of any extra silicon or aluminum sources after mild activation. The impurities in the synthesis gel strongly modify the properties of the zeolite, with ZSM-5 crystals containing higher levels of impurities exhibiting lower crystallinities, surface areas, acidities, cracking activities, as well as larger particle sizes.

Ethanol steam reforming over Ni/ZSM-5 nanosheet for hydrogen production

Ethanol steam reforming over Ni/ZSM-5 nanosheet for hydrogen production

Nonionic polymer and amino acid-assisted synthesis of ZSM-5 nanocrystals and their catalytic application in the alkylation of 2-methylnaphthalene.

The synthesis of nanosized ZSM-5 zeolites with high crystallinity and suitable acidity is very significant for their great potential in various catalytic applications. Herein, a series of zeolite ZSM-5 crystals with different particle sizes and SiO2/Al2O3 ratios (10-30) were synthesized by a temperature-varying two-step crystallization method in a concentrated gel system containing L-lysine and/or polyvinylpyrrolidone (PVP) additives. By optimizing the addition amounts of the two additives, the crystal size of the ZSM-5 zeolite could be reduced to less than 100 nm. Meanwhile, relatively high crystallinity and framework Al incorporation rates could be achieved, resulting in the generation of high-quality ZSM-5 nanocrystals. The nanosized H-form ZSM-5 zeolite with a SiO2/Al2O3 ratio of 20 showed enhanced catalytic efficiency and stability for the alkylation of 2-methylnaphthalene (2-MN) with methanol to produce an important intermediate, 2,6-dimethylnaphthalene (2,6-DMN). A relatively high and steady yield of 2,6-DMN (above 7.2%) could be achieved during 20 h time-on-stream at 400 °C. The smaller crystal size, higher crystallinity and framework Al content could provide more accessible Brønsted acid sites in the 10-membered ring channel of the ZSM-5 nanocrystals, which are the main active sites responsible for the shape-selectivity of the targeted product of 2,6-DMN. As a result, the formation of other side products like 1-MN and poly-MN could be effectively inhibited, thus leading to an improved 2,6-DMN yield and coke resistance over the nanosized ZSM-5 catalyst.

Influence of ZSM-5 Crystal Size on Methanol-to-Olefin (MTO) vs. Ethanol-to-Aromatics (ETA) Conversion.

Crystal size is a key parameter of zeolites applied as catalysts. Herein, ZSM-5 crystals with similar physicochemical and acid properties, few defects, and aluminum exclusively in tetrahedral coordination are synthesized and the influence of the crystal size on the MTO and ETA conversion is investigated. Short olefins are the main products of the MTO conversion, whereas larger olefins and aromatics dominate the products after ETA conversion. In the case of both feeds, an increased crystal size decreases the catalyst's lifetime. The MTO conversion over larger ZSM-5 altered the product distribution, which was not the case for the ETA conversion. The reason is that the instantly available aromatics during ETA conversion lead to fast coking and zeolite crystals only active in the outer layers. Thus, the different reactivity of different-sized ZSM-5 is direct proof of a different conversion mechanism for both alcohols.

Low-temperature synthesis of three-pore system hierarchical ZSM-5 zeolite for converting palm oil to high octane green gasoline

Microporous zeolite commonly suffers from a slow diffusion rate due to its restricted micropore channel, especially for bulky molecules. In this case, hierarchical zeolite has emerged as an essential functional material because of its excellence in alleviating the diffusion issue and improving the catalytic performances. In this research, hierarchical ZSM-5 zeolite with a three-interconnected-pore system (3 PS, micro-meso-macro) has been synthesized at a low temperature (90 °C) in the presence of CaCO3 nanoparticles. The low-temperature condition leads to the formation of the highly crystalline ZSM-5 particles with intercrystalline mesopores having a pore size distribution ranging from 3 to 6 nm. Moreover, the resulting zeolites also exhibit void spaces in a macroscale (20 nm–80 nm) created from the removal of CaCO3 nanoparticles. Moreover, the catalytic tests on the catalytic cracking of palm oil demonstrate the superior activity of the trimodal porous ZSM-5 in producing high octane gasoline compared to the ZSM-5 with two pore system (2 PS), and one-pore system (1 PS). A three-interconnected-pore system enhances the accessibility of reactant molecules to and the product molecules out of the active sites within the ZSM-5 crystals. As a result, the 3 PS catalyst produces a remarkable gasoline yield (25–26%) with exceptional aromatic content (>90%) and a research octane number (RON) of 114 with a less amount of cokes deposited during the reaction. Ultimately, the 3 PS catalyst also possesses the highest stability as it can maintain the catalytic performance up to the third cycle.

Deconvoluting the roles of polyolefin branching and unsaturation on depolymerization reactions over acid catalysts

Developing a circular economy for plastics is a societal priority with a range of stakeholders engaged in developing pathways to mechanical and chemical recycling. Acid catalysis has been proposed as an attractive strategy to chemically recycle or upcycle plastic waste. In principle, the energetics that govern polyolefin decomposition reactions over Brønsted acid sites should be the same ones known for alkane cracking. However, the nature of polymers as feedstock imposes new challenges for these well-known processes, e.g., strong mass transfer limitations. To better understand the role of polymer structure and diffusion on cracking rates, the effect of tertiary carbons, chain unsaturation, and catalyst active sites distribution were studied. Our findings confirm that higher concentrations of tertiary carbons will govern decomposition reactions, regardless of diffusion limitations. Also, we unveil that LLDPE reactions mostly take place on the external surface of ZSM-5 crystals while HDPE conversion takes place within the zeolite pores.

Catalytic cracking of heavy atmospheric gas oil to light olefins over ZSM-5 zeolite: Effect of crystal size in the absence/presence of steam

The catalytic cracking of heavy atmospheric gas oil (AGO) has been investigated to study the effect of ZSM-5 zeolite crystal size on the selectivity to light olefin in the absence and presence of steam. Dodecane (used as model compound) and heavy AGO feed streams were utilized to study the influence of changing pore shape selectivity when comparing cracking performance by nano and micro ZSM-5 zeolites. The zeolites were characterized using BET, XRD NMR, NH3-TPD and Py-FTIR. In the presence of steam, the short pores length nano ZSM-5 crystals showed enhanced catalytic cracking activity and selectivity to light olefins. The nano ZSM-5 catalyst promoted higher selectivity to light olefins through β-scission while reducing the extent of hydrogenation reactions towards the formation of liquefied petroleum gas components (LPG). In contrast, the long pores length of micro ZSM-5 tended to favor the production of more naphtha, kerosene and LPG through hydrogen transfer routes, resulting in lower selectivity to light olefins. The resulting propylene/ethylene (P/E) ratios were altered by the type of feed used, cracking reaction temperature, influence of steam and catalyst crystal size. Specifically, higher P/E ratio was achieved over nano ZSM-5 zeolite, especially with the cracking of heavy AGO feed in the presence of steam, due to an increase of monomolecular reactions through β-scission cracking mechanism.

Modulating hierarchical ZSM-5 crystals from etching-assisted growth with substituted phenols

The rational modulation of zeolite microstructures tailoring crystal morphology and hierarchy, is an effective way to promote their mass transport and catalytic performance. However, practical realization remains challenging. We recently demonstrated a novel etching-assisted crystallization process that allows fine control over zeolite architecture by eliminating intermediate defective nanocrystals with active proton-bearing organic molecules such as phenol. Here, we investigated in detail the phenolic etching effect on the crystallization of ZSM-5 zeolites using a combination of techniques including diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and 2D 27Al multiple quantum magic angle spinning nuclear magnetic resonance (MQ MAS NMR) and systematically elucidated the structure modulation with a homologous series of phenolic compounds. Time-series characterizations confirmed the initial seed formation, intermediate dissolution, and final perfected crystallization stages during ZSM-5 synthesis in the presence of phenolic compounds. Since deprotonation to phenolate anion in-situ is required during alkaline zeolite synthesis, the electron-withdrawing (EW) groups on the phenol para-position lead to bigger ZSM-5 crystals with more mesopores, while the phenolic molecules with para- electron-donating (ED) groups exhibit less significant effect. Under the same initial pH, the phenolic compounds show unparalleled compatibility with zeolite crystallization by stabilizing the system pH values, whereas inorganic acids invariably fail the zeolite crystallization. Given the remarkable kinetic control over zeolite growth through the simultaneous but opposite nucleophilic etching process, our observation provides a versatile way to manipulate zeolite crystallization with high tunability.