Hexane cracking over steamed phosphated zeolite H-ZSM-5: promotional effect on catalyst performance and stability

Structure of Hydrothermally Stable Acid Sites and their Catalytic Role in P-Modified ZSM-5 Zeolite Revealed by Solid-State NMR Spectroscopy

The ZSM-5 zeolite is the key active component in high-severity fluid catalytic cracking (FCC) catalysts and is routinely activated by phosphorus compounds in industrial production. To date, however, the detailed structure and function of the introduced phosphorus still remain ambiguous, which hampers the rational design of highly efficient catalysts. In this work, using advanced solid-state NMR techniques, we have quantitatively identified a total of seven types of P-containing complexes in P-modified ZSM-5 zeolite and clearly revealed their structure, location, and catalytic role. The experimental findings indicate that the introduced phosphorus can stabilize a portion of the aluminum atoms in the lattice by forming an energetically favorable framework-bound Si-OH-Al-O-P structure inside the zeolite channels, preserving more acidic bridging hydroxyl groups under the working conditions of the catalyst. Besides, two other types of hydrothermally stable phosphoric acid species (H3PO4 and H4P2O7) captured by neighboring silanol groups (H3PO4···HO-Si(SiO4)3 and H4P2O7···HO-Si(SiO4)3) are identified. For the FCC process, the framework-bound Si-OH-Al-O-P structure is proven to be the active site in the conversion of the FCC reactant, while the two phosphoric acid species can promote the yield of C2-C4 olefins.

Enhancing the selectivity for light olefins through catalytic cracking of n-hexane by phosphorus doping on lanthanum-modified ZSM-5

Naphtha, as the primary raw material in the production of light olefins, could well accommodate their increasing demand through the energy-efficient process of catalytic cracking with ZSM-5. In the current work, different amounts of lanthanum and phosphorous were loaded on ZSM-5 using the wet impregnation method to tune the acidic properties of ZSM-5 for selective catalytic cracking of n-hexane to produce light olefins. Various characterization techniques such as X-ray diffraction (XRD), Al nuclear magnetic resonance (NMR), temperature-programmed desorption of NH3 (NH3-TPD), Py-Fourier transform infra-red (Py-FTIR), inductively coupled plasma optical emission spectroscopy (ICP-OES), N2 adsorption-desorption, X-ray photoelectron spectra (XPS), and scanning electron microscopy were adopted to investigate the modified zeolites. It was found that adding La to ZSM-5 (0.25 wt% to 1 wt%) improved the catalytic life and increased the n-hexane conversion (to 99.7%), while the further addition had a negative impact, reducing the conversion rate and deviating the product selectivity towards a substantial, undesired benzene, toluene, and xylene (BTX) fraction (33%). On the other hand, a 64% selectivity for light olefins was achieved on phosphorous-doped ZSM-5 (at a loading amount of 1 wt%) while reducing the BTX fraction (2.3%) and converting 69% of the n-hexane. A dual metal-modified ZSM-5 with optimal loading amount, 1P0.25LaZ5 (phosphorus 1 wt% and La 0.25 wt%), helped boost the light olefin selectivity to 62% in the tuned Lewis acid sites at an n-hexane conversion of about 77% while decreasing the undesired BTX selectivity to 3% by reducing the number of Brønsted sites. Thus, the current study reveals that tuning the acidic sites of ZMS-5 by dual metal augmentation with P.La is an effective way of controlling the amount of undesirable BTX produced at a stable n-hexane conversion rate and substantial olefin selectivity.

Improving the Hydrothermal Stability of ZSM-5 Zeolites in 1-Octene Aromatization by Sequential Alkali Treatment and Phosphorus Modification

For the aromatization of olefins (in Fischer–Tropsch synthetic oil), especially for the fluidized bed reaction with steam as the fluidized medium, improving the catalytic and hydrothermal stabilities of ZSM-5 catalysts is a research focus because of both fundamental research interests and potential commercial applications. In this work, sequential alkali treatment and phosphorus modification were carried out for ZSM-5 samples. The results obtained from characterization and reaction evaluation show that the introduction of mesopores facilitates the dispersion of phosphorus species into the pores and improves the reaction stability for 1-octene aromatization. After the hydrothermal treatment, the P-0.5A-Z5-ST sample treated with an appropriate concentration of alkali retains the most acid sites and shows the highest aromatic selectivity (22.5%) at TOS = 660 min. Therefore, a moderate distribution of mesopores in a zeolite plays an important role in the diffusions of reactants and products, as well as in the distribution of phosphorus species.

Emerging Analytical Methods to Characterize Zeolite-Based Materials

Zeolites and zeolitic materials are, through their use in numerous conventional and sustainable applications, very important to our daily lives, including to foster the necessary transition to a more circular society. The characterization of zeolite-based materials has a tremendous history and a great number of applications and properties of these materials have been discovered in the past decades. This review focuses on recently developed novel as well as more conventional techniques applied with the aim of better understanding zeolite-based materials. Recently explored analytical methods, e.g. atom probe tomography, scanning transmission X-ray microscopy, confocal fluorescence microscopy and photo-induced force microscopy, are discussed on their important contributions to the better understanding of zeolites as they mainly focus on the micro- to nanoscale chemical imaging and the revelation of structure–composition–performance relationships. Some other techniques have a long and established history, e.g. nuclear magnetic resonance, infrared, neutron scattering, electron microscopy and X-ray diffraction techniques, and have gone through increasing developments allowing the techniques to discover new and important features in zeolite-based materials. Additional to the increasing application of these methods, multiple techniques are nowadays used to study zeolites under working conditions (i.e. the in situ/operando mode of analysis) providing new insights in reaction and deactivation mechanisms.

Phosphoric Acid Modification of Hβ Zeolite for Guaiacol Hydrodeoxygenation

Regulating the acid property of zeolite is an effective strategy to improve dehydration of intermediate alcohol, which is the rate-determining step in hydrodeoxygenation of lignin-based phenolic compounds. Herein, a commercial Hβ (SiO2/Al2O3 = 25) was modified by phosphoric acid, and evaluated in the catalytic performance of guaiacol to cyclohexane, combined with Ni/SiO2 prepared by the ammonia evaporation hydrothermal (AEH) method. Incorporating a small amount of phosphorus had little impact on the morphology, texture properties of Hβ, but led to dramatic variations in acid property, including the amount of acid sites and the ratio of Brønsted acid sites to Lewis acid sites, as confirmed by NH3-TPD, Py-IR, FT-IR and 27Al MAS NMR. Phosphorus modification on Hβ could effectively balance competitive adsorption of guaiacol on Lewis acid sites and intermediate alcohol dehydration on Brønsted acid sites, and then enhanced the catalytic performance of guaiacol hydrodeoxygenation to cyclohexane. By comparison, Hβ containing 2 wt.% phosphorus reached the highest activity and cyclohexane selectivity.

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.

Nano BEA zeolite catalysts for the selective catalytic cracking of n-dodecane to light olefins

Nano BEA zeolite catalysts were synthesized and modified by desilication and then ion-exchanged with Co. The desilication was carried out using 0.1 M of NaOH. The synthesized and modified nano BEA catalysts were characterized via different characterization techniques. Ammonia temperature program desorption (NH₃-TPD) and the pyridine Fourier transform infrared (pyridine-FTIR) were utilized to investigate the acidity of catalysts. X-ray diffraction (XRD), ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy techniques were used to examine the structure of the catalysts. The XRD patterns of the as-synthesized nano BEA catalysts were identical to that of the reference, while the NMR analysis revealed the distribution of silicon and aluminum in the BEA structure. The scanning electron microscope (SEM) analysis confirmed that the fabricated catalysts were less than 100 nm. The desilication and Co ion-exchange altered the acidity of the catalyst. The catalysts were evaluated in the cracking of sssssss to light olefins in the temperature range from 400 °C to 600 °C. The conversion increased with the increase in the reaction temperature for both catalysts; the conversion was above 90% for the Co-BEA catalyst at a temperature above 450 °C. The yield of light olefins also increased at higher temperatures for both catalysts, while at a lower temperature the yield to light olefins was ca. 40% over that of Co-BEA.

Nano BEA zeolite catalysts were synthesized and modified by desilication and then ion-exchanged with Co. The desilication was carried out using 0.1 M of NaOH.

Counteracting Rapid Catalyst Deactivation by Concomitant Temperature Increase during Catalytic Upgrading of Biomass Pyrolysis Vapors Using Solid Acid Catalysts

The treatment of biomass-derived fast pyrolysis vapors with solid acid catalysts (in particular HZSM-5 zeolite) improves the quality of liquid bio-oils. However, due to the highly reactive nature of the oxygenates, the catalysts deactivate rapidly due to coking. Within this study, the deactivation and product yields using steam-treated phosphorus-modified HZSM-5/γ-Al2O3 and bare γ-Al2O3 was studied with analytical Py-GC. While at a fixed catalyst temperature of 450 °C, a rapid breakthrough of oxygenates was observed with increased biomass feeding, this breakthrough was delayed and slower at higher catalyst temperatures (600 °C). Nevertheless, at all (constant) temperatures, there was a continuous decrease in the yield of oxygen-free hydrocarbons with increased biomass feeding. Raising the reaction temperature during the vapor treatment could successfully compensate for the loss in activity and allowed a more stable production of oxygen-free hydrocarbons. Since more biomass could be fed over the same amount of catalyst while maintaining good deoxygenation performance, this strategy reduces the frequency of regeneration in parallel fixed bed applications and provides a more stable product yield. The approach appears particularly interesting for catalysts that are robust under hydrothermal conditions and warrants further investigations at larger scales for the collection and analysis of liquid bio-oil.

Methanol to aromatics: isolated zinc phosphate groups on HZSM-5 zeolite enhance BTX selectivity and catalytic stability

HZSM-5 zeolite combined with unique zinc and phosphorus species, yields excellent selectivity (∼85%) to BTX (benzene, toluene, xylenes) in aromatic products. It was found that both zinc and phosphorus species were highly distributed in the pores of the zeolite channel network to form isolated zinc phosphate groups, which directly bond to the surface of zeolite, leading to a strong Lewis acidic center and an optimized surface acidity distribution favorable for BTX formation and the hydrothermal stability of the catalyst.

HZSM-5 zeolite combined with unique zinc and phosphorus species, yields excellent selectivity (∼85%) to BTX (benzene, toluene, xylenes) in aromatic products.

Probing the Location and Speciation of Elements in Zeolites with Correlated Atom Probe Tomography and Scanning Transmission X-Ray Microscopy

Characterizing materials at nanoscale resolution to provide new insights into structure property performance relationships continues to be a challenging research target due to the inherently low signal from small sample volumes, and is even more difficult for nonconductive materials, such as zeolites. Herein, we present the characterization of a single Cu-exchanged zeolite crystal, namely Cu-SSZ-13, used for NOX reduction in automotive emissions, that was subject to a simulated 135,000-mile aging. By correlating Atom Probe Tomography (APT), a single atom microscopy method, and Scanning Transmission X-ray Microscopy (STXM), which produces high spatial resolution X-ray Absorption Near Edge Spectroscopy (XANES) maps, we show that a spatially non-uniform proportion of the Al was removed from the zeolite framework. The techniques reveal that this degradation is heterogeneous at length scales from micrometers to tens of nanometers, providing complementary insight into the long-term deactivation of this catalyst system.

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.

Identifying the effective phosphorous species over modified P-ZSM-5 zeolite: a theoretical study

In this work, a density functional theory (DFT) study was carried out to address the fundamental description of the effective phosphorous species that could improve the framework stability and reduce the coke deposition formation on the P-ZSM-5 zeolite.

Visualizing Dealumination of a Single Zeolite Domain in a Real-Life Catalytic Cracking Particle

Fluid catalytic cracking (FCC) catalysts play a central role in the chemical conversion of crude oil fractions. Using scanning transmission X-ray microscopy (STXM) we investigate the chemistry of one fresh and two industrially deactivated (ECAT) FCC catalysts at the single zeolite domain level. Spectro-microscopic data at the Fe L3 , La M5 , and Al K X-ray absorption edges reveal differing levels of deposited Fe on the ECAT catalysts corresponding with an overall loss in tetrahedral Al within the zeolite domains. Using La as a localization marker, we have developed a novel methodology to map the changing Al distribution of single zeolite domains within real-life FCC catalysts. It was found that significant changes in the zeolite domain size distributions as well as the loss of Al from the zeolite framework occur. Furthermore, inter- and intraparticle heterogeneities in the dealumination process were observed, revealing the complex interplay between metal-mediated pore accessibility loss and zeolite dealumination.

Enhancement in activity and shape selectivity of zeolite BEA by phosphate treatment for 2-methoxynaphthalene acylation

Selective and green synthesis of 2-acetyl-6-methoxynaphthalene (2,6MNAC) is studied. Phosphate impregnation in smaller quantity regulates the pore of zeolite BEA. High selectivity to 2,6MNAC over BEA is due to the shape-selective property enhanced by P modification.

Fluid catalytic cracking: recent developments on the grand old lady of zeolite catalysis

Fluid catalytic cracking (FCC) is one of the major conversion technologies in the oil refinery industry, and the largest commercial catalytic process that uses zeolite materials.

Fluid catalytic cracking (FCC) is one of the major conversion technologies in the oil refinery industry. FCC currently produces the majority of the world's gasoline, as well as an important fraction of propylene for the polymer industry. In this critical review, we give an overview of the latest trends in this field of research. These trends include ways to make it possible to process either very heavy or very light crude oil fractions as well as to co-process biomass-based oxygenates with regular crude oil fractions, and convert these more complex feedstocks in an increasing amount of propylene and diesel-range fuels. After providing some general background of the FCC process, including a short history as well as details on the process, reactor design, chemical reactions involved and catalyst material, we will discuss several trends in FCC catalysis research by focusing on ways to improve the zeolite structure stability, propylene selectivity and the overall catalyst accessibility by (a) the addition of rare earth elements and phosphorus, (b) constructing hierarchical pores systems and (c) the introduction of new zeolite structures. In addition, we present an overview of the state-of-the-art micro-spectroscopy methods for characterizing FCC catalysts at the single particle level. These new characterization tools are able to explain the influence of the harsh FCC processing conditions (e.g. steam) and the presence of various metal poisons (e.g. V, Fe and Ni) in the crude oil feedstocks on the 3-D structure and accessibility of FCC catalyst materials.

Phosphorus promotion and poisoning in zeolite-based materials: synthesis, characterisation and catalysis

Complex interactions between phosphorus and zeolites are related to several promotional and poisoning effects in zeolite catalysis.

Phosphorus and microporous aluminosilicates, better known as zeolites, have a unique but poorly understood relationship. For example, phosphatation of the industrially important zeolite H-ZSM-5 is a well-known, relatively inexpensive and seemingly straightforward post-synthetic modification applied by the chemical industry not only to alter its hydrothermal stability and acidity, but also to increase its selectivity towards light olefins in hydrocarbon catalysis. On the other hand, phosphorus poisoning of zeolite-based catalysts, which are used for removing nitrogen oxides from exhaust fuels, poses a problem for their use in diesel engine catalysts. Despite the wide impact of phosphorus–zeolite chemistry, the exact physicochemical processes that take place require a more profound understanding. This review article provides the reader with a comprehensive and state-of-the-art overview of the academic literature, from the first reports in the late 1970s until the most recent studies. In the first part an in-depth analysis is undertaken, which will reveal universal physicochemical and structural effects of phosphorus–zeolite chemistry on the framework structure, accessibility, and strength of acid sites. The second part discusses the hydrothermal stability of zeolites and clarifies the promotional role that phosphorus plays. The third part of the review paper links the structural and physicochemical effects of phosphorus on zeolite materials with their catalytic performance in a variety of catalytic processes, including alkylation of aromatics, catalytic cracking, methanol-to-hydrocarbon processing, dehydration of bioalcohol, and ammonia selective catalytic reduction (SCR) of NO_x. Based on these insights, we discuss potential applications and important directions for further research.