ZSM-5 Crystals Research Articles

Kinetic study of the crystallization of ZSM-5 under organic template-free conditions

Here, the ZSM-5 zeolite was prepared under organic template-free conditions at different reaction temperatures and initial gel compositions using a hydrothermal process. A kinetic study of the synthesis of Zeolite was performed based on crystal growth. The crystallinity degree of the prepared samples was calculated from the PXRD and FT-IR results and then the crystallization curves were obtained. From these curves, the kinetic parameters were computed. The obtained results showed that by increasing the Na+ content of the initial gel composition, the activation energy for the crystal growth was reduced and as a result, the crystallization rate of ZSM-5 zeolite was enhanced due to the structure-directing role of Na+ cation. The activation energy of the induction step, transition step and crystal growth step are 122.8, 46.2, and 48.5 kJ/mol respectively for the molar ratios of 29Na2O:63SiO2:Al2O3:2400H2O:16H2SO4 and 141.0, 70.4, and 80.8 kJ/mol respectively for the molar ratios of 16Na2O:31SiO2:Al2O3:2400H2O:16H2SO4.

Ni nanoparticles encapsulated within H-type ZSM-5 crystals for upgrading palmitic acid to diesel-like fuels

Meso-Ni@HZSM-5 bi-functional catalysts were successfully post-encapsulated with about 3–7 nm Ni nanoparticles within HZSM-5 crystals, which exhibited significantly efficient conversion activity (67.4 g[palmitic acid] g[Ni]−1 h−1) of palmitic acid and 100% selectivity of hydrocarbons with the outstanding stability during recycling application, compared to the impregnated Ni/HZSM-5 catalyst (14.0 g[palmitic acid] g[Ni]−1 h−1).

Microstructural evolution of condensed aggregates during the crystallization of ZSM-5 from a heterogeneous system

The microstructural evolution of precursors of ZSM-5 zeolite crystallized from a heterogeneous system using fumed silica, sodium aluminate and tetrapropylammonium ions as reagents is investigated. Entities previously described by Ren et al. (Chem. Mater. 2012, 24, 10, 1726-1737) as condensed aggregates, were extensively studied using scanning electron microscopy, and energy dispersive spectroscopy. It was observed that the condensed aggregates first comprise a core of nanocrystals that is enveloped by a shell of amorphous gel phase. During crystallization, the amorphous shell surrounding the core is converted into ZSM-5 crystals that grow to a film surrounding the core. The crystals in the film grow competitively with nutrients provided by the liquid phase from the surroundings, while the nanocrystals in the core show little or no signs of growth.

WITHDRAWN: Effects of synthesis parameter on zeolite ZSM-5 properties

Different aspects of ZSM-5 crystallization can be influenced by crystallization time, which can lead to changes in final product properties. Various crystallization times were used to examine the crystallinity of nanosized ZSM-5 zeolite. The samples were examined using XRD, XRF AFM, and BET surface area. The orthorhombic-shaped crystals formed in the as-synthesised ZSM-5 sample after 120 h of crystallization time. The best crystallization time is shown to be 120 h, yielding a substance with 52.2 nm crystal size, 103 percent crystallinity, and 325.14 m2/g surface area.

Versatile Hollow ZSM-5 Nanoreactors Loaded with Tailorable Metal Catalysts for Selective Hydrogenation Reactions.

Zeolites are one of the most commonly used materials in the chemical industry, acting as catalysts or catalyst supports in different applications. Recently, the synthesis and functionalization of hollow zeolites have attracted many research interests, owing to the unique advantages of their hollow morphology. In the development of more sustainable processes, the hollow zeolites are often endowed with additional stability, selectivity, and activity. Herein, we present a stepwise synthetic protocol to prepare a range of complex hollow ZSM-5 catalysts with catalytic nanoparticles. Solid ZSM-5 crystals were first synthesized from Stöber silica spheres. This solid ZSM-5 sample was then loaded with transition metals via the impregnation method. A subsequent hollowing process was carried out in hydrothermal conditions in which hollow ZSM-5 crystals with confined transition metals inside were synthesized. More specifically, after the encapsulation of transition metals inside hollow ZSM-5, two different approaches have been further devised to allow the deposition of noble metals into the interior cavities or onto the exterior surfaces of the hollow ZSM-5. The deposition of Pt on the exterior surface was carried out by mixing the hollow ZSM-5 sample with presynthesized Pt nanoparticles. Loading of Pd in the interior was achieved by the galvanic replacement reaction between the Pd ions and embedded transition metals inside the hollow ZSM-5 sample. The catalytic performance of these reactor-like nanocatalysts has been evaluated with hydrogenation reactions in both liquid and gas phases, and their compositional and structural merits have been illustrated. For the hollow ZSM-5 sample with Pd loaded inside, liquid-phase selective hydrogenation of styrene over 4-vinylbiphenyl has been achieved with the ZSM-5 shell acting as a molecular sieve. The deposition of Pt on the exterior has improved the C2-C4 product yield when tested for the gas-phase CO2 hydrogenation reaction.

Fast and Selective Solvent-Free Branching of Unsaturated Fatty Acids with Hierarchical ZSM-5

Branched fatty acids are of great interest in the oleochemical industry for many high-end applications due to their interesting physicochemical properties. In the last decades, research has focused mainly on the use of ferrierite (FER) as catalyst for the isomerization of fatty acids into these branched fatty acids. With the use of FER, high yields of branched fatty acids can be obtained. FER is however expensive, and additives are necessary to obtain a high selectivity toward branched fatty acids. In this letter, we focus on the use of ZSM-5, a much cheaper zeolite, which has been extensively studied in the past for other reactions. Five commercial ZSM-5 samples and one hierarchical ZSM-5 sample were tested for the isomerization of fatty acids into branched fatty acids. The results clearly show that the accessibility of Bronsted acid sites is of utmost importance for their catalytic activity. Thanks to the introduction of mesopores in the zeolite crystals of ZSM-5, creating a hierarchical zeolite, a superior catalyst was obtained with a multifold activity increase, without compromising in branched fatty acid selectivity, that is capable of competing against the state-of-the-art FER zeolite.

Anionic Tuning of Zeolite Crystallization

Zeolites are of great industrial relevance as catalysts, adsorbents, and ion-exchangers and typically synthesized under hydrothermal conditions. Rational regulation of their crystallization process...

Hierarchical hollow Al-rich nano ZSM-5 crystals for highly selective production of light olefins from naphthenes

An Al-distribution regulated one-step desilication approach is developed to fabricate hierarchical hollow Al-rich nano ZSM-5 crystals (Si/Al molar ratio = 12.5) for highly selective production of light olefins from naphthenes.

Resolving the Acid Site Distribution in Zn-Exchanged ZSM-5 with Stimulated Raman Scattering Microscopy

Zeolites are widely used acid catalysts in research and in industrial processes. The catalytic performance of these materials is affected by the nature and concentration of Brønsted and Lewis acid sites. The balance between these types of active sites—and thus the activity and selectivity of the zeolite—can be altered by the introduction of metal species, e.g., by ion exchange. Although the acidic properties of zeolites are routinely characterized by bulk-scale techniques, this ensemble-averaged approach neglects the local variations in the material. Insights into the distribution of active sites at the single-particle level are thus critical to better understand the impact of post-synthetic modifications on the zeolite acidity. In this contribution, we spatially resolve Brønsted and Lewis acid sites in protonated and Zn-exchanged ZSM-5 crystals. To this end, the vibrational modes of pyridine chemisorbed on active sites are mapped with stimulated Raman scattering (SRS) microscopy. The SRS images reveal sharp inter- and intra-particle heterogeneities in the distribution of Lewis acid sites introduced upon ion exchange, ascribed to local variations in the Al content. Besides assessing the impact of Zn exchange on the active site distribution in ZSM-5 crystals, this approach enables uniquely to map the distribution of Lewis acid sites in catalysts at the single-particle level.

Fluoride-free synthesis of mesoporous [Al]-[B]-ZSM-5 using cetyltrimethylammonium bromide and methanol-to-olefin activity with high propene selectivity

Large ZSM-5 crystals containing [B]-sites and mesopores with 2.5 nm pore sizes are synthesized in a fluoride-free, low OH− medium using cethyltrimethylammonium bromide as mesoporogen and 1,6-diaminohexane as structure directing agent. Absence of extra-framework Al sites and low concentrations of silanol sites are evidenced by 27Al MAS-NMR and in-situ FTIR analysis. The low-defect containing [Al]-[B]-ZSM-5 (Mes-[Al]-[B]-ZSM-5) is evaluated for catalyzing the methanol-to-olefins reaction and found to result in low coke formation (4.1 mg g-1 h-1) and very low alkane and aromatic formation rates. The propene to ethene ratio of 4.3 observed using Mes-[Al]-[B]-ZSM-5 is found to be superior to that (1.9) of a nano-sized [Al]-[B]-ZSM-5 (crystal sizes around 36 nm). Small particle sizes for nano-sized [Al]-[B]-ZSM-5 is found to be effective for supplying a higher coke tolerance, but insignificant for olefin product selectivity.

Effects of boron modification on the activity of HZSM-5 toward MTP

BHZSM-5 zeolite was synthesized using one-step hydrothermal crystallization by changing SiO2/Al2O3 (100 and 200) with B2O3/Al2O3=1. The catalytic activity for methanol to propylene (MTP) was studied. Boron modification led to increased propylene selectivity and improved stability. Boron modification reduced the amount of the strong Brønsted (B) acid sites. Subjected to hydrothermal treatment at 480°C, BHZSM-5 remained 50% of the amount of the strong B acid sites, higher than that of HZSM-5, showing enhanced hydrothermal stability. The distribution of the framework Al also changed. The Al located in the straight and the sinusoidal channels of the ZSM-5 crystal was stable whereas that at the channel intersections was easy to be removed, favoring the MTP activity via the olefin cycle mechanism. As the hydrothermal treatment velocity was increased from 1 h−1 to 9 h−1, the B acid sites amount of the BHZSM-5 further decreased and more Al at the intersections was removed.

Sequential mechanochemical and recrystallization methods for synthesizing hierarchically porous ZSM-5 zeolites

The development of hierarchical porosity is crucial to overcome the diffusion issue within the zeolite frameworks. Herein, we have successfully developed a sequential mechanochemical treatment and recrystallization in the presence of cetyltrimethylammonium molecules for synthesizing the hierarchically porous ZSM-5 zeolite. The former transforms the large, micron-sized ZSM-5 crystals into nanosized particles with a notable decline in crystallinity due to the high mechanical force. In contrast, the latter converts the nanosized particles into highly crystalline ZSM-5 with preserved size. The products are shown to be aggregates of nanosized ZSM-5 with abundant intercrystalline mesopores. Cetyltrimethylammonium molecules play an inevitable role in preventing excessive coalescence and crystal growth and assemble the nanosized zeolites. In the absence of cetyltrimethylammonium, the recrystallization merely results in the micron-sized ZSM-5 crystals. The hierarchical porosity obtained in this study is proven to relieve the diffusion constraint and improve the catalytic performance in LDPE pyrolysis.

Effects of Desilication in NaOH/Piperidine Medium and Phosphorus Modification on the Catalytic Activity of HZSM-5 Catalyst in Methanol to Propylene Conversion.

Propylene is one of the main petrochemical building blocks applied as a feedstock for various chemical and polymer intermediates. The methanol-to-propylene (MTP) processes are reliable options for propylene production from non-petroleum resources. The highsilica ZSM-5 zeolite is found to be a reliable candidate for the methanol to propylene catalysis. In this study, the mesoporosity was first introduced into a high silica ZSM-5 zeolite via an alkaline treatment by NaOH solution with piperidine to decrease the diffusion limitation, and then the structure of zeolite was stabilized by phosphorus modification to improve the acidic properties and to enhance the catalyst stability. High-silica H-ZSM-5 catalysts (Si/Al = 200) were successfully prepared through microwave-assisted hydrothermal technique in the presence of tetrapropyl ammonium hydroxide (TPAOH) structure-directing agent. The mesoporosity was efficiently introduced into the ZSM-5 crystals via desilication derived from alkaline NaOH/piperidine solution. Then, the acidity of the desilicated ZSM-5 samples was improved using phosphorus modification. The catalysts were subjected to XRD, ICP-OES, FE-SEM, BET, TGA, FT-IR and NH3-TPD analysis. The catalytic performance of the prepared catalysts in the methanol to propylene (MTP) reaction was examined in a fixed-bed reactor at 475 °C, atmospheric pressure and methanol WHSV of 0.9 h-1. The results showed that the alkaline treatment in NaOH/piperidine solution created uniform mesoporosity with no severe damage in the crystal structure. Similarly, phosphorus modification developed the acidic features and led to the optimal catalytic efficiency in terms of the maximum propylene selectivity (49.16%) and P/E ratio (5.97) as well as the catalyst lifetime. The results showed an excellent catalytic activity in terms of 99.21% methanol conversion, good propylene selectivity up to 49.16%, a high ratio of P/E of 5.97 and a low selectivity to C5 + hydrocarbons of 11.57% for ZS-D-PI-P sample.

Intra-crystalline mesoporous ZSM-5 zeolite by grinding synthesis method

Abstract ZSM-5 zeolite with intra-crystalline mesopores was synthesized via grinding synthesis method (GSM) without any mesoporous templates. It was found that the formation of the mesopores in the ZSM-5 crystals is due to the difference between the speed of crystallization and dissolution processes occurring during the GSM synthesis. The alkalinity of the precursor gels tailored the formation of the mesoporous structure and the crystal size of the ZSM-5 zeolite. The most regular mesopores in the ZSM-5 crystals were formed under grinding synthesis of the precursor gel with a defined concentration of alkali cations (Na2O/SiO2 = 0.113). The size of the mesopores in the ZSM-5 crystals varied from 3 nm to 25 nm, while the size of the individual ZSM-5 crystallites is in the range of 50–300 nm which depend mainly on the amount of NaOH in the precursor gel.

Studies on the synthesis of ZSM-5 by interzeolite transformation from zeolite Y without using organic structure directing agents

Abstract In this work, a study of the interzeolite transformation from a parent zeolite Y (FAU topology) into zeolite ZSM-5 (MFI topology) was carried out, without using organic structure directing agents, by addition of SiO2 to adjust the composition of the starting gel. Depending on the SiO2/Al2O3 molar ratio, zeolite Y can be present in amounts equivalent to those of seed-assisted syntheses. Zeolite Y was successfully transformed in ZSM-5, but a rigorous control of OH−/SiO2 and Na+/SiO2 molar ratios is necessary to avoid incomplete transformation and/or contamination by layered silicates, such as magadiite and kenyaite. For hydrothermal treatment duration lesser than 12 h, the phase dissolution of the Y zeolite is observed, resulting in amorphous phase after 24 h. From 48 h, nucleation and growth of ZSM-5 crystals, with increasing crystallinity up to 120 h, is observed. During the interzeolite transformation, FTIR spectra show the disappearance of the band at 568 cm−1 and the appearance of a band at 545 cm−1, assigned to the 5-membered double rings of ZSM-5. Scanning electron microscopy images suggest that the crystallites of zeolite Y condense and absorb silicate species from solution to form larger aggregates of ZSM-5 zeolite. These structural and morphological changes are accompanied by textural changes detected in nitrogen adsorption isotherms. These results suggested that a combined solution mediated and solid-state transformation mechanism is probably occurring. Since these zeolites do not possess any other structural features in common, RBU approach seems to be more adequate to describe this specific interzeolite transformation.

Synergistic effect between organic structure-directing agent and crystal seed toward controlled morphology, and bimodal pore structure of aggregated nanosized ZSM-5

Abstract Hierarchical ZSM-5 zeolite has been a growing interest due to their wide applications in petrochemical process. Developing a facile and low cost strategy to synthesize hierarchical ZSM-5 is still a challenge. In this work, aggregated nanosized ZSM-5 with bimodal pore structure was synthesized via the synergistic effect of n-Butylamin (NBA) and ZSM-5 crystal seeds. In the synthesis process, The NBA molecules were transformed into RNH3+ that would adsorbed on the surface of nuclei by electrostatic interaction in aging process. The RNH3+ was subsequently transformed into tetrahedron configuration BuNH+ in crystallization process working as structure directing agent and promoting the aggregation of primary crystals. At low content of NBA, only microscale single crystals tended to aggregate together to form large particles. Crystal seeds were decomposed into small species, which can act as nuclei to direct the formation of primary ZSM-5 crystals and support the surface for the adsorption of RNH3+ assisting the formation of aggregated nanosized ZSM-5. With no addition of seed, the obtained sample had poor structural properties, such as small specific and external surface area. By simply adjusting the content of seed and NBA, aggregated nanosized ZSM-5 zeolite with large surface area, external surface area was obtained.

Optimization of Hydrothermal Temperature and Time Parameters in the Synthesis of Hierarchical ZSM-5 from Kaolin by Taguchi Method

Hierarchical ZSM-5 was successfully synthesized by hydrothermal crystallization of kaolin clay. A statistical experimental design Taguchi method with analysis of variance (ANOVA) with an L9 Orthogonal array (OA) was used to optimize the formation of high mesoporosity in hierarchical ZSM-5. Samples from kaolin mixtures crystallized at 80-175°C for 12, 24 and 48 h, respectively using hydrothermal followed by addition of mesoporogen cetyltrimethylammonium bromide (CTABr) surfactant. The presence of pentasil zeolite groups and large mesopores surface area and pore volume has observed on the samples by FTIR and N2 adsorption-desorption technique.The optimum synthesis conditions studied by Taguchi analysis based on S/N ratio for temperature and time of hydrothermal. The optimum condition was 127.5°C and 48 h giving the highest mesoporosity. Data analysis of interaction plot for S/N ratio showed there was interaction between temperature and time in the crystallization of hierarchical ZSM-5. Hydrothermal temperature was the most influencing factors for the formation of hierarchical ZSM-5 with a percent contribution of 83%.

Postsynthesis of hierarchical core/shell ZSM-5 as an efficient catalyst in ketalation and acetalization reactions

Hierarchical core/shell Zeolite Socony Mobil-five (ZSM-5) zeolite was hydrothermally postsythesized in the solution of NaOH and diammonium surfactant via a dissolution-reassembly strategy. The silica and alumina species were firstly dissolved partially from the bulky ZSM-5 crystals and then were in situ reassembled into the MFI-type nanosheets with the structure-directing effect of diammonium surfactant, attaching to the out-surface of ZSM-5 core crystals. The mesopores thus were generated in both the core and shell part, giving rise to a micropore/mesopore composite material. The micropore volume and the acidity of the resultant hybrid were well-preserved during this in situ recrystallization process. Possessing the multiple mesopores and enlarged external surface area, the core/shell ZSM-5 zeolite exhibited higher activity in the ketalation and acetalization reactions involving bulky molecules in comparison to the pristine ZSM-5.

Effects of crystal size on methanol to hydrocarbon conversion over single crystals of ZSM-5 studied by synchrotron infrared microspectroscopy.

Operando synchrotron infrared microspectroscopy (OIMS) was used to study the conversion of methanol over coffin-shaped HZSM-5 crystals of different sizes: large (∼250 × 80 × 85 μm3), medium (∼160 × 60 × 60 μm3) and small (∼55 × 30 × 30 μm3). The induction period, for direct alkene formation by deprotonation of surface methoxy groups, was found to decrease with decreasing crystal size and with increasing reaction temperature. Experiments with a continuous flow of dimethylether showed that evolution of the hydrocarbon pool and indirect alkene formation is also strongly dependent on crystal size. These measurements suggest that the hydrocarbon pool formation and indirect alkene generation should be almost instantaneous at reaction temperatures used in practical catalysis with crystal sizes typically ∼1 μm3.

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy.

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (ρ) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm-1 are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm-1 is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm-1 can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm-1 is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Brønsted acid sites. The intensity of the characteristic mode at 3611 cm-1 reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure.