Zeolite Crystal Size Research Articles

Influence of Synthesis Conditions on the Mesopore Distribution and Morphology Control of Hierarchical ZSM‐5 Zeolites Synthesized by Double‐Acyloxy Organosilane Surfactants

AbstractA novel type of double‐acyloxy amphiphilic organosilanes with different alkyl chain lengths were synthesized as mesoporous templating agents to prepare mesoporous ZSM‐5 zeolites. The zeolites obtained had sponge‐like surfaces and variable morphology; mesopores of 4, 13, and 33 nm were constructed in the crystals without damaging their crystallinity. The H2O/Si ratio, Si/Algel ratio, and organosilane template content all had an influence on zeolite crystallinity and surface properties as indicated by X‐ray diffraction (XRD), nitrogen physisorption analysis, and scanning electron microscopy (SEM). In addition, variation of the alkyl chain length of the organosilanes resulted in different zeolite crystal size, morphology, and pore‐size distribution.

On the remarkable resistance to coke formation of nanometer-sized and hierarchical MFI zeolites during ethanol to hydrocarbons transformation

The impact of the textural properties of H-ZSM-5 zeolites in the conversion of ethanol to hydrocarbons at 623K and under 3.0MPa pressure is investigated. We highlight that the lifetime of the catalysts is not correlated to the coking rate but to the number of pore mouths. The addition of macropores (fluoride leaching) or mesopores (alkaline leaching) to micron-sized zeolites is a simple approach to increase the number of pore mouths and reduce the diffusion path length of molecules in the micropores. However, the most efficient way is to reduce the zeolite crystal size to nanometers. The longest catalyst lifetime (>100h) is obtained with a hierarchical nanometer-sized zeolite even though most of its acid sites are poisoned. The important impact of the nature of coke on the catalysts regeneration is also highlighted.

Albemarle to unveil new leading-edge HC-PT catalyst as part of expanding STARS portfolio

This work systematically studies on adding alcohols to alter the crystal growth behavior of ZSM-12, using 1-propanol (1P), 2-propanol (2P), ethylene glycol (EG), 1,3-propanediol (PD), 1,6-hexanediol (HD), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TTEG) as co-solvents. The effects of co-solvent content, temperature, and excess mineralizer content were studied as well. Except 2P, all other co-solvents are observed to tune the crystal morphology into a discrete round-shape. Except DEG and TTEG, all other co-solvents promote the crystallization rate of MTW phase, verified by larger zeolite crystal size in SEM images. The observation is corroborated with the formation of cristobalite, possibly via Ostwald ripening mechanism. The finding enlightens us using a lower synthesis temperature to attain high purity MTW phase, with an adequate amount of co-solvents. It is noteworthy that using HD as co-solvent facilitate the anisotropic growth of MTW crystals along the b-axis (2θ of 8.5°), possibly due to the effect of co-solvent’s hydrophobicity as well as the backbone length. The blue shift of symmetric and asymmetric Si-O vibration modes in FTIR spectra implies that TEG and HD are incorporated within the ZSM-12 pore structure. This work provides potential synthetic route to form anisotropic ZSM-12 membranes with preferred orientation.

Mitigating coking during methylcyclohexane transformation on HZSM-5 zeolites with additional porosity

The effects of two different hierarchization procedures (alkaline and fluoride leaching) on the performances of ZSM-5 catalysts in the transformation of methylcyclohexane at 723K are highlighted and discussed in relation to their porosities. The hierarchical catalysts exhibit different porosities; namely, the fluoride treatment leads to a zeolite combining micropores and macropores, while alkaline leaching adds mesopores interconnected with the native micropores. While the initial activities and selectivities of catalysts derived from the three zeolites are very similar in the conversion of methylcyclohexane, the presence of mesopores (alkaline leaching), close to the active sites, greatly improves the stability of such a hierarchical catalyst by favoring the desorption of products. This behavior is similar to a reduction in zeolite crystal size. This increased stability is not due to a decrease of the coke toxicity, but rather to an inhibition of the growth of coke precursors, in turn related to the shorter diffusion paths of reactants and products. Two types of coke are present on the meso-/micro-porous zeolite: (i) a “light coke” composed of alkylbenzenes strongly adsorbed on Lewis acid sites and silanols, (ii) a “heavy coke” (alkylphenanthrenes and alkylpyrenes) trapped at the intersection of the zeolite channels. While the light coke has no impact on the catalyst stability, the heavy coke poisons active sites, most probably remote from the mesopores.

Controlling naphtha cracking using nanosized TON zeolite synthesized in the presence of polyoxyethylene surfactant

Valorization of naphtha, a derived intermediate from crude oil is still a crucial subject. Here we report an effort to control catalytic cracking of naphtha using nanosized TON zeolite, which was synthesized by microwave assisted hydrothermal synthesis (MAHyS) in the presence of polyoxyethylene-based surfactant. The effects polyoxyethylene (POE) surfactant on the crystal size, morphology and the crystal aspect-ratio were studied. The crystals with lengths in the range of 90–300nm were obtained. The addition of polyoxyethylene surfactant reduced the agglomeration rate of ZSM-22 zeolite crystals. The polyoxyethylene surfactant was added with POE/Al from 2.5 to 10. Effect of Si/Al in the presence of POE surfactant was studied by changing the Si/Al ratio from 45 to 80. Naphtha was simulated using a model compound, n-hexane. Excellent selectivity toward propylene was obtained over ZSM-22 zeolite crystals in n-hexane cracking. High selectivity to propylene was maintained over different size of ZSM-22 zeolite crystals. Decreasing crystal length from 300 to 90nm considerably increased the catalytic activity of ZSM-22 crystals.

Effects of adding alcohols on ZSM-12 synthesis

This work systematically studies on adding alcohols to alter the crystal growth behavior of ZSM-12, using 1-propanol (1P), 2-propanol (2P), ethylene glycol (EG), 1,3-propanediol (PD), 1,6-hexanediol (HD), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TTEG) as co-solvents. The effects of co-solvent content, temperature, and excess mineralizer content were studied as well. Except 2P, all other co-solvents are observed to tune the crystal morphology into a discrete round-shape. Except DEG and TTEG, all other co-solvents promote the crystallization rate of MTW phase, verified by larger zeolite crystal size in SEM images. The observation is corroborated with the formation of cristobalite, possibly via Ostwald ripening mechanism. The finding enlightens us using a lower synthesis temperature to attain high purity MTW phase, with an adequate amount of co-solvents. It is noteworthy that using HD as co-solvent facilitate the anisotropic growth of MTW crystals along the b-axis (2θ of 8.5°), possibly due to the effect of co-solvent’s hydrophobicity as well as the backbone length. The blue shift of symmetric and asymmetric Si-O vibration modes in FTIR spectra implies that TEG and HD are incorporated within the ZSM-12 pore structure. This work provides potential synthetic route to form anisotropic ZSM-12 membranes with preferred orientation.

Preparation of high silica chabazite with controllable particle size

In this work synthesis procedures for controlling particle size distributions of high silica chabazite (Si/Al=100) zeolites between 0.3 and 4.5μm is described. The chabzite zeolite crystallization and crystal growth have been monitored using XRD, SEM and DLS. Hydrothermal treatment parameters such as time, temperature and agitation and gel composition parameters such as amounts of water and structure directing agent (SDA) have been correlated with chabazite zeolite crystal size, crystal shape, synthesis yield and crystallinity. From this data a chabazite zeolite formation and crystal growth mechanism based on experimental studies and previous studies has been proposed. It was also found that large aggregates formed in the chabazite zeolite gel before any hydrothermal treatment (1μm large when using zeolite gel with composition 1Al2O3:100SiO2:60TMAdaOH:3100H2O). The size controlling parameter and size controlling synthesis step of the chabazite zeolite (below 1μm) has been identified as the primary fractal aggregation of the silicium/aluminium/SDA species.

Synthesis of nanocrystalline zeolite NaY by hydrothermal method and investigation of its structure and morphology

Zeolite FAU type-Y is one of the most studied framework of all zeolites, and has been used as catalysts for number of reactions in the refinery and petrochemical industry. In this research, nanocrystalline zeolite Y was synthesized by hydrothermal method. The crystal size of zeolite Y is influenced by temperature, aging time, alkalinity, and water content. The synthesized zeolite NaY is identified and characterized by Brunauer-Emmett-Tell (BET), Fourier Transmission Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM). The mean crystallites size was measured by transmission electron microscopy (TEM) and it was determined also by Rietveld's method using Scherrer's equation; with similar results estimated was about 50 nm.

Organotemplate-free Hydrothermal Synthesis of SUZ-4 Zeolite: Influence of Synthesis Conditions

Various conditions were investigated in detail for the novel organic template-free static hydrothermal synthesis of SUZ-4 zeolite in the presence of seeds. The obtained samples were characterized by XRD (X-ray diffraction), SEM (scanning electron microscope), TG (thermal gravimetric analysis), ICP (inductively coupling plasma) elemental analysis, nitrogen sorption isotherm and surface area. The results show that pure SUZ-4 zeolites with high crystallinity are obtained in a broad window of synthesis conditions: seed mass concentration 0.2%–2%, SiO2/Al2O3 molar ratio 21–25, KOH/SiO2 molar ratio 0.33–0.43, H2O/SiO2 molar ratio 7.14–38.1, aging time 24 h, crystallization temperature 160 °C, and crystallization time 6–10 d. Also, crystallinity and size of the rod-like SUZ-4 zeolite crystals are found to alter with the conditions.

MFI-type zeolite nanosheets for gas-phase aromatics chlorination: a strategy to overcome mass transfer limitations

Mass transfer limitations and catalytic activity were studied for various ZSM-5 zeolite crystal sizes in the chlorination of deactivated arenes. An estimation of the quantity of mild acidic external silanol groups of zeolite nanosheets was made.

Synthesis and characterization of silicalite-1 membrane prepared on a novel support by the pore plugging method

The effect of the support pore size on the membrane morphology was investigated for zeolite silicalite-1 membranes synthesized by pore plugging method on supports with zirconium oxide and/or titanium oxide active layer. Parameters including surface coverage, zeolite layer thickness, crystal size and shape, zeolite penetration depth were used to quantify the membrane morphology. Five supports with different pore sizes for their active layer in the range of 0.14–1.4 μm were investigated. The X-ray diffraction (XRD) analysis showed a typical silicalite-1 zeolite structure with a high internal crystalline order grown inside the pores as well as on top of all supports. The XRD results also showed that the silicalite-1 crystals in the synthesized membranes are not randomly oriented. The crystallographic preferred orientation (CPO) analysis revealed that the degree of orientation toward either the a-axis or b-axis perpendicular to the support surface, increased by decreasing the pore size of the support. The 0.45 μm support had the most preferably oriented zeolite layer for access of molecules entering into the membrane structure with the highest number of crystals oriented with the b-axis (the one with straight channels) perpendicular to the support surface. The scanning electron micrographs (SEM) analysis of the membranes revealed a dense and continuous surface morphology with the highest crystal size of silicalite-1 around 1.5 μm on the surface of the support with the 0.45 μm pore size. SEM micrographs also showed a continuous layer grown over four supports out of five supports with different pore sizes that were investigated, with no layer observed on the 1.4 μm pore size support. The average thickness of the zeolite layer was in the range of 0.7–1.4 μm, depending on the pore size of the support. The supports with 0.2 and 0.45 μm pore sizes had the most uniform zeolite layer thickness while the support with 0.8 μm pore size active layer had the least uniform zeolite layer thickness. The electron diffraction spectrometer (EDS) analysis confirmed the formation of pure silicalite-1 layer at the surface as well as inside the pores of all supports. The highest silicalite-1 crystal penetration was for the supports with 0.45 and 1.4 μm pore sizes. Single gas permeation experiments with He and N2 gases at 293 K illustrated that regardless of the pore size of the support, the He and N2 permeances were constant despite the change of the pressure across the membranes. The highest permeances were observed for the membrane prepared using the 0.45 μm pore size support, while the lowest permeances were for the membrane prepared using the 1.4 μm pore size support. These results confirmed the selective properties of the prepared membranes. No matter what is the pore size of the support or the feed pressure, N2 permeances were around three times higher than those for He.

Synthesis of 5A zeolite nanocrystals using kaolin via nanoemulsion-ultrasonic technique and study of its sorption using a known kerosene cut

In this research, 5A zeolite nanocrystals were synthesized and the effects of the controlling parameters such as, temperature, composition of the reactants, and effects of ultrasound on morphological and crystal size modifications were investigated. Nanoemulsion-ultrasonic technique was used in preparation of 4A (Na-A) zeolite which is the precursor of 5A (Ca-A) zeolite. Kaolin and/or aluminate derived from kaolin were utilized as the main sources of Al. X-ray fluorescence (XRF) were used to identify SiO2/Al2O3 ratio in kaolin and X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), were used to confirm zeolite crystallinity, crystal size and morphology, and phase purity. The optimal conditions were SiO2/Al2O3 ratio of 2.2, temperature 75°C, heating time of 8h, and ultrasound rate 4000rpm (ca 67Hz) for 15min. Then, the sample that was prepared at optimal conditions and possessed the highest crystallinity and an average crystal size of about 260nm was subject to dead-end batch sorption tests using a special cut of kerosene composed of C10–C13 linear and branched paraffinic hydrocarbons and sorption and sieving properties of the zeolite were studied by Gas Chromatography Mass Spectroscopy (GC–MS). The results show that the micro/nanoemulsion technique induces rapid growth, and formation of 4A zeolite goes to completion within 2h of the synthesis time, compared to 8–24h for the samples that were not prepared by this technique. Since 4A zeolite is the precursor for 5A zeolite, reduced synthesis time of the precursor would accordingly lead into decreased preparation time course of the final desired product 5A zeolite. In addition, the results show that the application of ultrasound plays a great role in reducing the crystal size, and could reduce the crystal size by as much as 62%. Nanoemulsion-ultrasonic method produces smaller, more uniform, and purer zeolite nanocrystals with greater ion exchange capacity than the conventional synthesis methods. Furthermore, the sorption property of the 5A zeolite is improved on average about 15%.

N-Hexadecane hydroisomerization over BTMACl/TEABr/MTEABr templated ZSM-12

The crystallization of molecular sieve ZSM-12 has been investigated by employing benzyltrimethylammonium chloride (BTMACl), tetraethylammonium bromide (TEABr) and methyltriethylammonium bromide (MTEABr) as templates. The crystallization of pure ZSM-12 framework was accomplished at Si/Al ratio of 90. The pure ZSM-12 samples so obtained were judged with respect to the differences in X-ray diffraction patterns, morphology, crystal size and total acidity to understand the role of template during crystallization. Interestingly, crystallization of ZSM-12 phase with rice, cubic and elongated cubic shaped morphology was favored in the presence of BTMACl, TEABr, and MTEABr, respectively. Finally, the effect of zeolite crystal size and morphology on hydroisomerization reaction was investigated by employing ZSM-12 (Si/Al=90) based hydroisomerization catalysts. The performance of the prepared catalysts was judged in terms of their activity, selectivity and product distribution for the hydroisomerization reaction using n-hexadecane as a model feed. Based on the obtained results, the possible advantages of using a ZSM-12 sample of specific morphological features and crystal size for hydroisomerization of long chain n-paraffins are discussed.

Designing MFI-based catalysts with improved catalyst life for [formula omitted] oligomerization to high-quality liquid fuels

Light olefin oligomerization is an important alternative for the production of clean liquid automotive fuels and can be performed in the presence of solid acid catalysts. Among these, medium-pore zeolites, and especially ZSM-5, have been widely described in the open literature. In this work, the relative importance of intracrystalline diffusion path lengths for the product molecules (depending on the zeolite crystal size and the presence of mesopores in the crystallites) and Brønsted acid site density are discussed for two different olefins, propene and 1-pentene. Thus, ZSM-5 samples with (a) the same crystallite size and different acid site density, (b) the same density of acid sites and different crystallite size, (c) postsynthesis generation of mesopores by different desilication severities, and (d) samples with similar crystal size, mesoporosity, and acid site density, but with a different ratio of external to internal acid sites have been prepared and studied for oligomerization of propene and 1-pentene. The results obtained suggest that the properties required for a best-performing catalyst (maximum conversion and lowest deactivation rate) are different for these two alkenes. Whereas Brønsted acid site density is determinant for propene oligomerization when intracrystalline diffusion path lengths are below a certain critical value, the presence of a large number of Brønsted acid sites is not sufficient in the case of 1-pentene, and additional mesoporosity becomes crucial. Thus, mesoporous ZSM-5 samples prepared by postsynthesis desilication treatments present a greater improvement in initial conversion and catalyst life for 1-pentene oligomerization than for conversion of propene.

Rapid tuning of ZSM-5 crystal size by using polyethylene glycol or colloidal silicalite-1 seed

Two methods were used to control the crystal size of ZSM-5 synthesized with commercial silica sol. One is use of organic additives; the other is use of colloidal silicalite-1 seed. Addition of polyethylene glycol (PEG) to the synthesis gel favors the formation of relatively small uniform crystals because of promotion of nucleation and somehow hindering the growth of crystals, although it has little influence on the structure, crystallinity, acidity, and crystal shape. The prepared catalyst shows higher catalytic stability than the sample synthesized with the PEG-free gel in the methanol-to-hydrocarbon (MTH) reaction. Finely controllable synthesis of different-sized ZSM-5 crystals was realized by epitaxially growing ZSM-5 crystals around colloidal silicalite-1 seed. The crystal size could be tuned from 200 to 1000nm by every 100nm through adjusting the crystallization time and the amount of the added silicalite-1 seed. This makes it possible to systematically study the effect of zeolite crystal size on its catalytic performance and tune product distribution. A significant increase in the catalytic stability was observed with decreasing crystal size for ZSM-5 catalyzing MTH reaction. The crystals with a size of about 270nm showed a catalytic life of about 6 and 3 times as long as those of the samples synthesized in the absence and presence of PEG respectively.

Size-Controlled Synthesis of Nano-Zeolites and Their Application to Light Olefin Synthesis

For the application of zeolites as heterogeneous catalysts, low diffusion resistance for hydrocarbons within the micropore is essential for improving product selectivity and catalyst lifetime. This problem has been overcome by reducing the crystal size. This review introduces size-controlled preparation of nano-sized zeolites via hydrothermal synthesis in water/surfactant/organic solvent (emulsion method) and their application to heterogeneous catalysts. The ionicity of the hydrophilic group in surfactant molecules and the concentration of the Si source affected the crystallinity and morphology of zeolites prepared using the emulsion method. When using a non-ionic surfactant, mono-dispersed silicalite-1 nanocrystals ~60 nm in diameter were successfully prepared. Nano- and macro-ZSM-5 zeolites with crystal sizes of ~150–200 nm and 1.5 μm, respectively, were prepared and applied to n-hexane cracking and acetone-to-olefin reactions to investigate the effect of zeolite crystal size on catalytic stability and light olefin yield. Application of nano-zeolite to light olefin production was effective in achieving faster mass transfer of hydrocarbon molecules within the micropore, which led to improvements in olefin yields and catalyst lifetime.

Facile synthesis of uniform FeZSM-5 crystals with controlled size and their application to N2O decomposition

FeZSM-5 zeolites with crystal sizes ranging from 70nm to 20μm were synthesized by adjusting the alkalinity of the synthesis system and optimizing the crystallization conditions. After steam-activated treatment, these zeolites were used as catalysts in the direct decomposition of N2O, showing that the larger the zeolite crystal size, the higher the reaction temperature required for the complete decomposition of N2O. Moreover, N2O decomposition over the FeZSM-5 catalysts with crystal sizes larger than 5μm at high temperatures shows that the conversion of N2O decreases with increasing the zeolite crystal size, implying that the internal mass transfer limitations are available. However, under the applied conditions, the diffusion limitations are absent and the observed reaction rates are controlled by the intrinsic kinetics of N2O decomposition over the FeZSM-5 catalysts with crystal sizes smaller than 2μm.

Preparation and characterization of ITQ-29/polysulfone mixed-matrix membranes for gas separation: Effect of zeolite composition and crystal size

This work concerns the preparation and characterization of ITQ-29 zeolite crystals with high Si/Ge ratio (100–∞) and different particle size for obtaining mixed matrix membranes. The Si/Ge molar ratio and seeding content of the synthesis gel appeared to have an effect on the final crystal size, and particles of 2.5μm were obtained for pure silica composition with good crystallinity. These were introduced, at 4, 8 and 12wt% loadings, into a commercial polysulfone matrix to prepare mixed matrix membranes that showed promising results in the separation of H2/CH4 mixtures (highest H2 permeability 21.9 Barrer and a separation factor of 118 for the 4wt% ITQ-29/polysulfone membrane). The thermal treatment of the membranes and the type of solvent were also optimized to provide good interaction between the zeolite and the polymer, limiting aggregation of the particles in the matrix, and removing all residual solvent that hinders gas permeation performance.

Nucleation promotion of zeolite crystals by in situ formation of silver-based heterogeneous nuclei

The size of zeolite crystals can be controlled by modifying the nucleation frequency by the addition of silver-based promoters. Nanoparticles of silver oxide can be formed in the alkaline solutions used in zeolite synthesis and can act as effective heterogeneous nuclei for the crystallization of zeolites in a wide range of topology and composition (zeolites LSX, X, Y, A, L, and beta). The presence of silver oxide does not affect the normal nucleation mechanism of zeolites from gels and provides a number of additional seeds proportional to the amount of silver oxide precursors introduced in the synthesis systems. The yield of zeolite crystallization is not significantly affected by the presence of the nucleation promoters. The volume of the crystals formed can be decreased by a factor 20 in the appropriate conditions. The effectiveness of the nucleation promotion is not affected by the presence of organic cations but is largely reduced in extremely alkaline media, in which silver oxides are significantly soluble.

In Situ Infrared Molecular Detection Using Palladium-Containing Zeolite Films

In situ IR detection of carbon monoxide in the presence of hydrocarbons (methanol and pentane) using Pd-containing zeolite thin films is reported. The thin films are prepared by spin coating deposition of nanosized LTL and BEA type zeolites suspensions; the palladium clusters are introduced in the nanosized zeolites by ion exchange followed by γ radiolysis of the coating suspensions. The Pd-containing zeolite films with a thickness of 200 nm are exposed to a single gas (either CO or hydrocarbons) or gas mixtures in the presence of water (100 ppm), and the IR spectra are collected continuously at 25, 75, and 100 °C. The fast recognition of very low concentrations of CO (2-100 ppm) in the presence of highly concentrated vapors of methanol or pentane (400-4000 ppm) with the Pd-containing zeolite films is demonstrated. The detection of CO and hydrocarbons is instant, which is a function of the low thickness of the films, small size of the individual zeolite crystals, and regular size and high stability of the Pd clusters in the zeolite films. The heat of adsorption for all experiments is similar (15 kJ.mol(-1)), which is explained with weak interactions between the carbon monoxide and palladium clusters in the zeolite films at temperatures below 100 °C. The nanosized zeolites with homogeneously distributed Pd clusters deposited in thin films demonstrate high molecular recognition capacity toward low concentrations of carbon monoxide under real environmental conditions, i.e., in the presence of water and hydrocarbons.