Understanding the enhancement of catalytic performance for olefin cracking: Hydrothermally stable acids in P/HZSM-5

Conversion of Methanol to Para-Xylene over ZSM-5 Zeolites Modified by Zinc and Phosphorus

In this work, the influence of different phosphorus sources and the modification of zinc and phosphorus on the performance of the conversion of methanol to aromatics (MTA) was investigated. The results showed that the phosphorus source had a significant impact on the selectivity of para-xylene (PX) in xylene and catalyst stability. The introduction of P resulted in the covering of the active acid sites and the narrowing of the pore of the ZSM-5 zeolite, which improved the shape-selectivity for PX in the methanol conversion reaction. Compared with the modifiers of H₃PO₄ and (NH₄)₃PO₄, the ZSM-5 zeolite modified by (NH₄)₂HPO₄ exhibited better catalyst stability and PX-selectivity due to its larger specific surface area, pore volume and suitable acidity. When the ZSM-5 zeolite was modified by Zn and P, the effect of Zn and P on the selectivity to aromatics and PX in xylene was almost opposite. With the increase in P-loading, the selectivity of PX in xylene gradually increased but at the cost of decreasing the aromatic-selectivity. On the other hand, the loading of Zn introduced Zn-Lewis acid sites to provide aromatization active centers and improved the aromatic-selectivity. However, excessive Zn reduced the selectivity of PX in xylene. The catalyst activity and aromatic-selectivity could be improved to some extent with an appropriate ratio of Zn and P, while maintaining or increasing the para-selectivity of xylene. Compared with 5% P/ZSM-5 catalyst modified with only (NH₄)₂HPO₄, the PX selectivity in xylene over the Zn-P/ZSM-5 catalyst modified with 5% Zn and 1% P improved from 86.6% to 90.1%, and the PX yield increased by 59%.

Synergetic Regulation of the Microstructure and Acidity of HZSM-5/MCM-41 for Efficient Catalytic Cracking of n-Decane

Alkane catalytic cracking is regarded as one of the most significant processes for light olefin production; however, it suffers from serve catalyst deactivation due to coke formation. Herein, HZSM-5/MCM-41 composites with different Si/Al₂ ratios were first prepared by the hydrothermal method. The physicochemical properties of the prepared catalysts were analyzed by a series of bulk and surface characterization methods, and the catalytic performance was tested in n-decane catalytic cracking. It was found that HZSM-5/MCM-41 showed a higher selectivity to light olefins and a lower deactivation rate compared with the parent HZSM-5 due to an enhanced diffusion rate and decreased acid density. Moreover, the structure-reactivity relationship revealed that conversion, light olefin selectivity, and the deactivation rate strongly depended on the total acid density. Furthermore, HZSM-5/MCM-41 was further extruded with γ-Al₂O₃ to obtain the catalyst pellet, which showed an even higher selectivity to light olefins (∼48%) resulting from the synergy effect of the fast diffusion rate and passivation of external acid density.

Reaction Pathway of 1-Decene Cracking to Produce Light Olefins over H-ZSM-5 at Ultrahigh Temperature

The effect of reaction temperature and weight hourly space velocity (WHSV) on the reaction of 1-decene cracking to ethylene and propylene over H-ZSM-5 zeolite was investigated. Also, the thermal cracking reaction of 1-decene was studied by cracking over quartz sand as blank. It was observed that 1-decene undergoes a significant thermal cracking reaction above 600 °C over quartz sand. In the range of 500-750 °C, the conversion remained above 99% for 1-decene cracking over H-ZSM-5, and the catalytic cracking dominated even at 750 °C. With the increase in temperature, the yields of ethylene and propylene gradually increased, and the yields of alkanes and aromatics also increased. The low WHSV was favorable for the yield of light olefins. With the increase of the WHSV, the yields of ethylene and propylene decrease. However, at low WHSV, secondary reactions were accelerated, and the yields of alkanes and aromatics increased significantly. In addition, the possible main and side reaction routes of the 1-decene cracking reaction were proposed based on product distribution.

Modification of Clinoptilolite as a Robust Adsorbent for Highly-Efficient Removal of Thorium (IV) from Aqueous Solutions

The natural zeolite has been modified with sulphate and phosphate. The adsorption of thorium from the aqueous solutions by using the natural and modified zeolites has been investigated via a batch method. The adsorbent samples were characterized by X-ray Diffraction (XRD), N₂ adsorption-desorption (BET), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDX). Modification of natural zeolite with sulphate and phosphate was found to increase its adsorption capacity of thorium but reduced its specific surface area (S_BET). The adsorption experiments were expressed by Langmuir, Freundlich and Dubinin-Radushkevitch (D-R) isotherm models and the results of adsorption demonstrated that the adsorption of thorium onto the natural and modified zeolites correlated better with the Langmuir isotherm model than with the Freundlich isotherm model. The maximum adsorption capacity (Qo) was determined using the Langmuir isotherm model at 25 °C and was found to be 17.27, 13.83, and 10.21 mg/g for phosphate-modified zeolite, sulfate-modified zeolite, and natural zeolite, respectively. The findings of this study indicate that phosphate-modified zeolite can be utilized as an effective and low-cost adsorbent material for the removal of thorium from aqueous solutions.

Enhanced light olefins production via n-pentane cracking using modified MFI catalysts

n-pentane catalytic cracking was studied over a series of MFI zeolites with varying SiO₂/Al₂O₃ ratios (30, 80, 280, 500, and 1500) using a fixed-bed reactor operated at temperature 550–650 °C. Other MFI zeolites (SiO₂/Al₂O₃ = 280) with various crystal morphology and size (such as large crystal and nano size) were also synthesized and tested for n-pentane cracking. The effects of MFI zeolite modification with ammonia and phosphorus on its physiochemical properties and catalytic activity were investigated. Among the parent MFI zeolites, MFI (280) demonstrated high selectivity (51%) towards light olefins (C₃⁼/C₂⁼ = 0.7) at 650 °C with undesired C₁–C₄ alkanes (38%). Surface modified MFI (280) zeolites of different crystal size and morphology showed improvement towards propylene selectivity by suppressing undesired reactions. Phosphorous-modified MFI zeolite with a large crystal size was found to improve light olefin selectivity (52.2%) with C₃⁼/C₂⁼ = ∼1.3 and reduce undesired C₁–C₄ alkanes (8%) formation due to suppressed strong acidic sites. The characterization and evaluation results for the modified MFI (280) revealed that the incorporation of phosphorous created moderate acidic sites, which were stabilized by some non-framework aluminum species, thereby leading to suppressing the formation of undesired C₁–C₄ alkanes with improved light olefins selectivity.

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n-pentane cracking over MFI zeolites with various Si₂O₃/Al₂O₃ ratios (30, 80, 280, 500, and 1500) was investigated.

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MFI (280) zeolite at 650 °C showed 51% selectivity for light olefins (C₃⁼/C₂⁼ = 0.7) with 23.8% undesirable C₂-C₄ alkanes.

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Phosphorous-modified large crystal MFI catalyst with moderate acid sites inhibited hydrogen transfer reaction.

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P/HZ280-LC provide improved light olefins production (selectivity 52.2%, C₃⁼/C₂⁼ = 1.3).

Chirality in Organic and Mineral Systems: A Review of Reactivity and Alteration Processes Relevant to Prebiotic Chemistry and Life Detection Missions

Chirality is a central feature in the evolution of biological systems, but the reason for biology’s strong preference for specific chiralities of amino acids, sugars, and other molecules remains a controversial and unanswered question in origins of life research. Biological polymers tend toward homochiral systems, which favor the incorporation of a single enantiomer (molecules with a specific chiral configuration) over the other. There have been numerous investigations into the processes that preferentially enrich one enantiomer to understand the evolution of an early, racemic, prebiotic organic world. Chirality can also be a property of minerals; their interaction with chiral organics is important for assessing how post-depositional alteration processes could affect the stereochemical configuration of simple and complex organic molecules. In this paper, we review the properties of organic compounds and minerals as well as the physical, chemical, and geological processes that affect organic and mineral chirality during the preservation and detection of organic compounds. We provide perspectives and discussions on the reactions and analytical techniques that can be performed in the laboratory, and comment on the state of knowledge of flight-capable technologies in current and future planetary missions, with a focus on organics analysis and life detection.

Effect of hydrothermal aging temperature on a Cu-SSZ-13/H-SAPO-34 composite for the selective catalytic reduction of NO x by NH3

Cu-SSZ-13 suffers activity loss after hydrothermal treatment at high temperatures, particularly above 850 °C. The stability of Cu-SSZ-13 can be enhanced by compositing with H-SAPO-34. This work investigates the effect of aging temperature on the composites. For the structure, the extra-framework P in H-SAPO-34 migrates and interacts with the Al in Cu-SSZ-13, forming a new framework P-Al bond. This interaction is enhanced with the increment of the aging temperature. For the cupric sites, the aging at 750 °C results in the agglomeration of Cu²⁺ ions to CuO. However, the sample aged at 800 °C exhibits higher activities than that aged at 750 °C, which might be attributed to the increased formation of framework P-Al bonds promoting the redispersion of CuO to Cu²⁺ ions. The composite suffers severe deactivation due to the significant loss of Cu²⁺ ions after aging at 850 °C.

Understanding the Catalytic Activity of Microporous and Mesoporous Zeolites in Cracking by Experiments and Simulations

Porous zeolite catalysts have been widely used in the industry for the conversion of fuel-range molecules for decades. They have the advantages of higher surface area, better hydrothermal stability, and superior shape selectivity, which make them ideal catalysts for hydrocarbon cracking in the petrochemical industry. However, the catalytic activity and selectivity of zeolites for hydrocarbon cracking are significantly affected by the zeolite topology and composition. The aim of this review is to survey recent investigations on hydrocarbon cracking and secondary reactions in micro- and mesoporous zeolites, with the emphasis on the studies of the effects of different porous environments and active site structures on alkane adsorption and activation at the molecular level. The pros and cons of different computational methods used for zeolite simulations are also discussed in this review.

Activation and conversion of alkanes in the confined space of zeolite-type materials

Microporous zeolite-type materials, with crystalline porous structures formed by well-defined channels and cages of molecular dimensions, have been widely employed as heterogeneous catalysts since the early 1960s, due to their wide variety of framework topologies, compositional flexibility and hydrothermal stability. The possible selection of the microporous structure and of the elements located in framework and extraframework positions enables the design of highly selective catalysts with well-defined active sites of acidic, basic or redox character, opening the path to their application in a wide range of catalytic processes. This versatility and high catalytic efficiency is the key factor enabling their use in the activation and conversion of different alkanes, ranging from methane to long chain n-paraffins. Alkanes are highly stable molecules, but their abundance and low cost have been two main driving forces for the development of processes directed to their upgrading over the last 50 years. However, the availability of advanced characterization tools combined with molecular modelling has enabled a more fundamental approach to the activation and conversion of alkanes, with most of the recent research being focused on the functionalization of methane and light alkanes, where their selective transformation at reasonable conversions remains, even nowadays, an important challenge. In this review, we will cover the use of microporous zeolite-type materials as components of mono- and bifunctional catalysts in the catalytic activation and conversion of C₁⁺ alkanes under non-oxidative or oxidative conditions. In each case, the alkane activation will be approached from a fundamental perspective, with the aim of understanding, at the molecular level, the role of the active sites involved in the activation and transformation of the different molecules and the contribution of shape-selective or confinement effects imposed by the microporous structure.

Recent progress in the improvement of hydrothermal stability of zeolites

Zeolites have been successfully employed in many catalytic reactions of industrial relevance. The severe conditions required in some processes, where high temperatures are frequently combined with the presence of steam, highlight the need of considering the evolution of the catalyst structure during the reaction. This review attempts to summarize the recently developed strategies to improve the hydrothermal framework stability of zeolites.

P-Site Structural Diversity and Evolution in a Zeosil Catalyst

Phosphorus-modified siliceous zeolites, or P-zeosils, catalyze the selective dehydration of biomass derivatives to platform chemicals such as p-xylene and 1,3-butadiene. Water generated during these reactions is a critical factor in catalytic activity, but the effects of hydrolysis on the structure, acidity, and distribution of the active sites are largely unknown. In this study, the P-sites in an all-silica self-pillared pentasil (P-SPP) with a low P-loading (Si/P = 27) were identified by solid-state ³¹P NMR using frequency-selective detection. This technique resolves overlapping signals for P-sites that are covalently bound to the solid phase, as well as oligomers confined in the zeolite but not attached to the zeolite. Dynamic Nuclear Polarization provides the sensitivity necessary to conduct ²⁹Si-filtered ³¹P detection and ³¹P-³¹P correlation experiments. The aforementioned techniques allow us to distinguish sites with P-O-Si linkages from those with P-O-P linkages. The spectra reveal a previously unappreciated diversity of P-sites, including evidence for surface-bound oligomers. In the dry P-zeosil, essentially all P-sites are anchored to the solid phase, including mononuclear sites and dinuclear sites containing the [Si-O-P-O-P-O-Si] motif. The fully-condensed sites evolve rapidly when exposed to humidity, even at room temperature. Partially hydrolyzed species have a wide range of acidities, inferred from their calculated LUMO energies. Initial cleavage of some P-O-Si linkages results in an evolving mixture of surface-bound mono- and oligonuclear P-sites with increased acidity. Subsequent P-O-P cleavage leads to a decrease in acidity as the P-sites are eventually converted to H₃PO₄. The ability to identify acidic sites in P-zeosils and to describe their structure and stability will play an important role in controlling the activity of microporous catalysts by regulating their water content.

Transformation of LPG to light olefins on composite HZSM-5/SAPO-5

A relationship between acidity and catalytic performance is established to promote the production of propylene and the P/E ratio.

High-quality synthesis of a nanosized CHA zeolite by a combination of a starting FAU zeolite and aluminum sources

A chabazite (CHA zeolite) was synthesized using high-silica faujasite (FAU) zeolites with a Si/Al ratio of 93, an additional alumina source (aluminum hydroxide) combined with seed crystals, and N,N,N-trimethyl-1-adamantammonium hydroxide. We compared the crystallization behavior of the starting material (HSY + Al) with that of other combinations of silica/alumina sources (high-silica and low-silica FAU, fumed silica, and aluminum hydroxide). HSY + Al rapidly yielded nanosized CHA zeolites with a crystal size of approximately 70 nm, exhibited high product crystallinity and high yield and offered a wide synthesis window. A combination of analytic experiments using electrospray-ionization mass spectrometry and nuclear magnetic resonance (NMR) suggested that in the early stage, the pre-introduced CHA seeds provide a crystal nucleus and the FAU zeolites decompose to form oligomer species in the liquid phase. Meanwhile, aluminum hydroxide retains its solid phase. Subsequent crystallization of the zeolites is accelerated by the liquid silicate oligomer and solid aluminate sources, resulting in a high yield and rapid synthesis of nanosized CHA zeolites. We observed that phosphorus-modified CHA zeolites synthesized using HSY + Al perform well as a catalyst for ethanol conversion reactions. Controlled Si/Al ratios and additional phosphorus modifications improve catalytic durability, thereby exhibiting a higher propylene yield from the reaction within the zeolite pore system.

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

BACKGROUND

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.

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Insight into interactions among P, Zn and ZSM-5 during bi-component modification on ZSM-5

The formation of Zn₃(PO₄)₂ and Zn₂SiO₄ in Zn/P/ZSM-5 during the hydrothermal procedure deteriorates the stability of the framework and the aromatization capability.

Tandem Reactions of CO2 Reduction and Ethane Aromatization

Aromatization of light alkanes is of great interest because this can expand the raw materials used to produce aromatics to include fractions of natural gas that are readily available and inexpensive. Combining CO₂ reduction with ethane dehydrogenation and aromatization can also mitigate CO₂ emissions. A one-step process that can produce liquid aromatics from the reactions of CO₂ and ethane using phosphorus (P)- and gallium (Ga)-modified ZSM-5 has been evaluated at 873 K and atmospheric pressure. The addition of P improves the hydrothermal stability of Ga/ZSM-5, reduces coke formation on the catalyst surface, and allows the formation of more liquid aromatics through the tandem reactions of CO₂-assisted oxidative dehydrogenation of ethane and subsequent aromatization. Density functional theory calculations provide insights into the effect of Ga- and P- modification on ethane dehydrogenation to ethylene as well as the role of CO₂ on the production of aromatics.

Selective production of para-xylene and light olefins from methanol over the mesostructured Zn–Mg–P/ZSM-5 catalyst

The mesostructured Zn–Mg–P/ZSM-5 catalyst is efficient in converting methanol to p-xylene and light olefins, with the additional mesoporosity compensating for the activity loss during modification.

Revealing long- and short-range structural modifications within phosphorus-treated HZSM-5 zeolites by atom probe tomography, nuclear magnetic resonance and powder X-ray diffraction

The average and the local structure of phosphorus-treated HZSM-5 zeolites were investigated by means of atom probe tomography, powder X-ray diffraction (at ambient and cryogenic temperatures) and 1H, 29Si, 27Al, and 31P magic angle spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy. Phosphatation to yield a product with P/Al ≤ 1 followed by thermal treatment leads to breaking of the Si-OH-Al bridging groups, and subsequent partial dealumination of the zeolite framework, as shown by the contraction of the orthorhombic unit-cell volume and by the loss of tetrahedral framework Al, as observed in the 27Al Multiple Quantum (MQ) MAS NMR spectrum. Most of the framework Al is present in an electronic environment distorted by the presence of phosphorus and appears not to be involved in classic Si-OH-Al Brønsted acid sites. The 31P MAS NMR signals indicate that phosphorus interacts with the zeolitic framework to locally form silico-aluminophosphate (SAPO) domains and the presence of a new kind of acidic site was confirmed by the resonance at ∼8.6 ppm in the 1H MAS NMR spectra, attributed to P-OH groups. Increasing the phosphorus loading (P/Al ≫ 1) promotes further dealumination of the framework and cross-dehydroxylation between P-OH and Si-OH species, leading to the formation of a crystalline silicon orthophosphate phase. With decreasing Al content, the monoclinic HZSM-5 structure becomes preferred, especially at 85 K where the strain relaxation is higher. However, the presence of a higher amount of silicophosphate impurities hinders the low-temperature strain release of the framework, indicating that some of these species are localized in the zeolite pores and contribute to the strain build up.

Fluorine-enhanced Pt/ZSM-5 catalysts for low-temperature oxidation of ethylene

Fluorine treatment enhances the catalytic performance of Pt/ZSM-5 for ethylene low-temperature oxidation due to its increased Brønsted acidity and water tolerance.

Facile synthesis of a high-performance titanosilicate catalyst with controllable defective Ti(OSi)3OH sites

A facile strategy was carried out to construct a high-performance titanosilicate oxidation catalyst with controllable defective Ti(OSi)₃OH sites.