Zeolite Channels Research Articles

Constructing PtCe cluster catalysts by regulating metal-support interaction via Al in zeolite for propane dehydrogenation

Pt-based catalysts as one of the most used industrial catalysts in heterogeneous catalysis, it is essential for the on-purpose catalytic dehydrogenation of propane to propylene (PDH). Here, the highly dispersed PtCe clusters are constructed by high-silica Beta zeolite as support to improve the catalytic stability and propylene selectivity in the PDH reaction. The defect sites derived from silanol nests are responsible for dispersing PtCe clusters inside the channels of the high-silica Beta-557 zeolite, while the trace Al sites play an important role in further stabilizing the active site structure by regulating the metal-support interaction. The closely associated Pt and Ce species are critical to embody the outstanding catalytic activity (propane conversion of ∼42.2%, propylene selectivity of ∼91.9%) with excellent stability (deactivation constant: 0.013 h−1) at 550 °C. This work proposed an efficient strategy for designing stable PtCe bimetallic clusters by utilizing silanol nests and Al sites to regulate the metal-support interaction synergistically.

Recent advances in shape selectivity of MFI zeolite and its effect on the catalytic performance

MFI zeolite characterized by uniform pore size, adjustable acidity, and high-temperature resistance has a broad application prospect in catalytic reactions. However, controlling the product distribution of zeolite as a catalyst is still confronting great challenges and applications. It is considered as an effective way to control the product distribution by developing and improving new zeolites to modulate their shape selective effect. In recent years, researchers have achieved remarkable successes in investigating the shape selective modulation of zeolites on catalytic reaction and molecular diffusion. The microporous channels of MFI zeolite are the main places for the entry and exit of reactants or product molecules. This review provides the research progress of the shape-selective modulation of MFI zeolite channels and its influence on a series of catalytic performances in recent years. The shape-selective modulation of microporous channels of zeolite, encapsulation of micropores to metals, catalysis of mesoporous zeolite, and the distribution of framework Al were all systematically discussed. The development of advanced catalysts still faces great challenges and potential applications. Finally, we discussed the problems to be addressed urgently in the field of zeolite catalysts in the future.

Shape selectivity of zeolite for hydroisomerization of long-chain alkanes

The matching degree between the zeolite channel size and the isomer size determines the product distribution of dodecane hydroisomerization.

Differential heats, isotherm and entropies of n-pentane adsorption on LiZSM-5 and CsZSM-5 zeolites

Artificially synthesised zeolites are widely used in water purification devices as adsorbents, ion exchangers, molecular sieves; they are used as electron donors and acceptors. Also, zeolites are currently the most important catalysts for the processing of various hydrocarbon raw materials. Synthetic zeolites ZSM-5 are highly efficient catalysts of these processes. For o characterisation of zeolite channels and estimation of sorption volume npentane are the most suitable. n-Pentane fills all sorption space and adsorbed with higher energy. This paper presents the results of the basic thermodynamic characteristics and isotherms of n-pentane adsorption in Cs3,17ZSM- 5 and Li3,37ZSM-5 zeolites at 303 K. A system consisting of a universal high-vacuum adsorption unit and a Tian-Calvet type differential modified microcalorimeter, DAC-1-1A, coupled to it was used to measure isotherms and differential heats of adsorption. The correlation between adsorption-energy characteristics was found and the molecular mechanism of n-pentane adsorption in CsZSM-5 and LiZSM-5 zeolites in the whole filling region was revealed.

Application progress of small-pore zeolites in purifying NOx from motor vehicle exhaust

• Small-pore SSZ-13 and SSZ-39 zeolites have unique advantages in purifying NOx. • Pd/zeolites as PNAs present the superior low-temperature NOx adsorption activity. • Non-precious metal Co/zeolite is an ideal substitute for Pd/PNAs to reduce the cost. • Cu/zeolites possess the excellent high-temperature NH3-SCR catalytic activity. • Fe/Mn/Ce doped Cu/zeolites can broaden the NH3-SCR active temperature window. Lean-burn mode results in the high NOx emission of diesel vehicles despite its high fuel economy. NOx can be effectively removed by the SCR unit during normal operation, however, elimination during the cold start period needs to be addressed soon. Passive NOx adsorber (PNA) was proposed to adsorb and store NOx emitted during the cold start period. With the increase of exhaust gas temperature during normal operation, the adsorbed NOx will be quickly released and efficiently converted to N 2 when flowing through the downstream SCR catalyst. After years’ investigation, small-pore zeolites such as CHA and AEI have been found to possess superior hydrothermal stability and NOx selectivity compared with medium and large pore zeolites, and the former’s eight-membered ring structure effectively exclude large hydrocarbons from entering the zeolite channels, making them play a prominent role in NOx purification via both PNA and SCR units. SSZ-13 and SSZ-39 zeolites, which belong to CHA and AEI topologies respectively, has been confirmed to exhibit excellent low-temperature NOx adsorption performance and high-temperature SCR denitrification performance. In the field of purifying NOx from the diesel vehicle exhaust, the review simultaneously focused on the application of SSZ-13 and SSZ-39 zeolites in NOx purification from two aspects of PNA and SCR in recent decades. In addition, the unique advantages of their modified form of loading Pd/Co as PNAs, or loading Cu doped with Fe/Mn/Ce as SCR catalysts were systematically clarified, in order to provide some advanced guidance for the researchers in the purification of NOx from the diesel engine exhaust.

Low-cost zeolite-based sorbents prepared from industrial perlite by-product material for Zn2+ and Ni2+ removal from aqueous solutions: synthesis, properties and sorption efficiency

The conversion of waste/by-product materials into efficient sorbents is at the forefront of innovative remediation techniques. In the present study, the relationships among the synthesis conditions, physicochemical properties of synthesized sorbents and Zn2+ and Ni2+ removal efficiencies were studied in detail. Zeolite X, zeolite P, phillipsite, analcime, sodalite and cancrinite were synthesized from industrial perlite by-product material. The zeolite content in the synthesized sorbents and zeolite framework topology (dimensions, numbers and spatial configuration of channels) were the key factors affecting the removal of Zn2+ and Ni2+ from aqueous solutions. Zeolite X-based sorbent exhibited the best sorption performance mainly due to the large zeolite channel dimensions, low Si/Al ratio, high cation exchange capacity and high specific surface area. Nevertheless, the efficiency and stability of this sorbent need to be tested under field conditions prior to its application for remediation technologies.

Engineering spatial locations of Pt in hierarchically porous KL zeolite by atomic layer deposition with enhanced n-heptane aromatization

Atomic layer deposition (ALD) is an efficient method to confine the Pt particles in the channels of KL zeolite to improve the durability of catalysts. However, the diffusion of metal precursors is impeded by the size of micropore. Herein, using polyethylene glycol (PEG) as zeolite growth modifier to limit the growth of KL zeolite in [001] direction, a unique KL zeolite with trimodal porosity (native and larger micropores, mesopores) was successfully synthesized. Hierarchically porous structure engineered the spatial location of Pt precursor during ALD process. The obtained micro/mesoporous structure favored the diffusion of the Pt precursor into the pores of KL zeolite, and led to the formation of highly dispersed Pt clusters in micropores, mesopores, and intersection of micro/mesopores of KL zeolite. The resultant Pt/KLP2 catalyst showed the enhanced n-heptane aromatization performances operated at low temperature (420 °C). Systematic characterizations reveal the synergetic effect of spatial environment of hierarchical porous KL zeolite promoted the n-heptane aromatization and molecular diffusion. Namely, micropores enhanced the n-heptane dehydrocyclization, and the mesopores with shortened diffusion length facilitated the mass transport of aromatics and inhibited secondary hydrogenolysis efficiently. The present work broadens the fundamental understanding of the role of zeolite channels for alkane reforming reactions.

Increased Light Olefin Production by Sequential Dehydrogenation and Cracking Reactions

In this study, a sequential reaction using selected metal oxides, followed by ZSM-5-based catalysts, was employed to demonstrate a promising route for enhancing light olefin production in the catalytic cracking of naphtha. The rationale for the reaction is based on the induction of alkenes into hydrocarbon feeds prior to cracking. The optimum olefin induction was achieved by carefully optimizing the dehydrogenation active sites Mo/Al2O3 catalyst. The formed alkenes have a lower activation energy for C-H/C-C bond breaking compared to alkanes. This could accelerate the formation of carbenium ions, thus promoting the conversion of n-octane to produce light olefins. Detailed product distribution and DFT calculation indicated a remarkable increase in ethylene and propylene production in the final product through a modified reaction pathway. Compared with the common metal-promoted zeolite catalysts, the new route could avoid the block of zeolite channels and corresponding decreased catalytic cracking activity. The feasibility of the proposed route was confirmed with different ratios of dehydrogenation catalyst to the reactant. The highest yields of ethylene and propylene reached 13.22% and 33.12% with ratios of Mo/Al2O3 and ZSM-5-based catalyst to n-octane both 10:1 at 600 °C. Stability tests showed that the catalytic activity of the double-bed system was stable over 10 cycles.

Mg enhances the catalytic selectivity and stability of mordenite for carbonylation of dimethyl ether

Mg species were introduced into the MOR zeolite to adjust its acid sites distribution, which has a great impact on the reaction of DME carbonylation. The introduction of Mg had a vital effect on the MOR structure, as it decreased specific surface area and pore volume. When Mg ion exchanged into both the 12MR channel and 8MR channel of MOR zeolite, the strong acid sites dramatically decreased, especially for the Brønsted acid sites in the 12MR channel. Mg modification inhibited most of active sites for side reactions, whereas decreasing a few active sites for carbonylation. This caused a little loss of activity, but the higher selectivity to methyl acetate (MA) product. Furthermore, Mg-MOR exhibited superior stability in the reaction of DME carbonylation. As a result, the MA yield on Mg-MOR was 3.8 times as higher as that on H-MOR.

Direct dehydrogenation of propane over Pd nanoparticles encapsulated within IPC zeolites with tunable pore sizes

Propane dehydrogenation (PDH) yields propene, a valuable feedstock in increasing global demand. Yet, despite recent advances in supported metal nanoparticles (NPs) for such catalytic applications, preventing nanoparticle agglomeration remains a challenge. In this study, we prepared well-dispersed Pd nanoparticles and encapsulated them within IPC-2 and IPC-4 zeolites using the Assembly, Disassembly, Organization, and Reassembly (ADOR) process based on the 3D-2D-3D transformation. By structural and textural analysis, we confirmed the synthesis of two ‘ADORable’ zeolites incorporated with Pd nanoparticles, namely Pd@IPC-2 and Pd@IPC-4. In the direct dehydrogenation of propane, Pd NPs encapsulated within IPC-2 and IPC-4 zeolites outperformed their impregnated counterparts (Pd/IPC-2 and Pd/IPC-4), with Pd@IPC-2 showing a higher catalytic activity than Pd@IPC-4. Accordingly, in addition to the number of surface Pd atoms, the size of the zeolite channels and the structure of the framework strongly affect the catalytic activity of encapsulated Pd. Moreover, confining Pd NPs inside zeolite channels prevented their sintering and agglomeration during the reaction as Pd NPs in impregnated catalysts expanded during the reaction. However, the structure of the zeolite encapsulated with Pd catalysts partly collapsed due to the harsh conditions of the dehydrogenation reaction, hindering access to Pd NPs, as observed in IR spectra. Therefore, palladium NPs are stable within zeolites and do not sinter, but their catalytic activity gradually decreases with the formation of carbon deposits. Although these deposites are removable by calcination, reactivation does not completely restore the original activity due to framework disruption and limited access to the active species.

Insight into induction period of methanol conversion reaction: Reactivity of ethene-precursors over H-ZSM-5 zeolite is independent of Brϕnsted acid site density

A series of H-ZSM-5 (TZ-m, m represents different Si/Al ratios) has been prepared and their Methanol to Olefin (MTO) performance are investigated. By adjusting the catalyst weight of H-ZSM-5 loaded in the pulse micro-reactor, the total Brϕnsted acid sites (BAS) amount of TZ-m catalysts is unified. The resultant similar methanol conversion (∼24 %) over different TZ-m zeolites confirms that the MTO reaction is dominantly catalyzed by BAS. Nevertheless, the BAS density considerably influences the reaction pathways; more condensed BAS enhances the relative propagation of aromatic-based cycle to alkene-based cycle, and vice versa. However, isotopic co-feeding and switching experiment reveals that the reactivity of aromatic hydrocarbon pool (HCP) species, which has been considered as main ethene precursors, is almost independent of acid site density. Although large amounts of aromatics are generated via hydrogen transfer reactions on the sample with high acid site density, most of the confined aromatics function as coke precursors instead of active HCPs. The accumulation of such confined aromatics in zeolite channels further limits olefins mobility and their access to BAS, leading to decreased reactivity of alkene HCPs on high acid site-density samples.

The role of the spatial arrangement of single rhodium sites on ZSM-5 in the oxidative methane carbonylation to acetic acid

Single atom rhodium catalysts based on microporous ZSM-5 zeolite (SiO2/Al2O3 = 30) were studied in the reaction of oxidative methane carbonylation to acetic acid. For highly dispersed rhodium distribution on the zeolite surface, nitrogen-containing polymers, namely chitosan hydrochloride, polyethyleneimine, and polyvinylpyrrolidone, were used. Using XAS spectroscopy, all rhodium was found to be as single atom metal sites in the zeolite structure, which contributes to the acetic acid formation. The catalytic properties of rhodium zeolite catalysts in the oxidative methane carbonylation to acetic acid were revealed to depend not only on the single atom rhodium distribution and the catalyst acidity, but also on the spatial rhodium atom arrangement in the zeolite, which was established by the XPS. According to DFT calculations in combination with EXAFS modeling, rhodium anchored at the zeolite channel intersections was shown to be coordinated with four zeolite oxygen atoms and one hydroxyl group oxygen atom and be the dominant catalytic species in the acetic acid formation.

Selective enrichment of Brønsted acid site in 8-membered ring channels of MOR zeolite to enhance the catalytic reactivity of dimethyl ether carbonylation

Directional adjusting of the distribution of Brønsted acid site (BAS) in zeolite to control its catalytic performance is highly attractive but remains challenging. This study aims to tailor the distribution of BAS in 8-membered ring (B8-MR) of H-Mordenite (HMOR) by adding aniline or p-phenylenediamine as additives to the sol‐gel. The results reveal that the number of B8-MR is significantly increased but with a slight change in the number of BAS in 12-membered ring channels. When using dimethyl ether (DME) carbonylation as the probe reaction, the conversion of DME to methyl acetate over the synthesized HMOR zeolite is increased by nearly 2 times. Our finding not only provides a facile strategy to regulate the acid distribution in different channels of the HMOR zeolite but also offers a promising candidate for the catalytic DME carbonylation, which is a critical intermediate step for ethanol synthesis from syngas.

Rapid and low-cost synthesis of ZSM-5 zeolite nanosheet assemblies for conversion of methanol to gasoline

Nanoscale zeolites with short diffusion paths show significantly enhanced mass transportation and easier accessibility for active sites in catalysis compared to microsized zeolites. However, the high cost and complex process involved in existing synthetic methods limit their application, so the development of a rapid and low-cost synthesis technology is of great significance. Herein, ZSM-5 zeolite nanosheet assemblies (Z5-NS-t) are rapidly synthesized by a microwave irradiation (MW) assisted seed-induced method using low-cost polyhexamethylene biguanide hydrochloride (PHMB) as a crystal growth inhibitor of the b-axis. The crystallization kinetics of the resultant materials and the formation mechanism of ZSM-5 zeolite nanosheets, as well as the influence of physico-chemical properties on their catalytic performance for the methanol-to-gasoline (MTG) reaction, are systematically investigated. Compared with the ZSM-5 nanoparticle assemblies (Z5-NP-60), which are synthesized by using the same method except for the addition of PHMB, Z5-NS-180 nanosheets with an approximate density of strong Brønsted acid sites exhibit excellent catalytic performance in the MTG reaction, giving rise to a high gasoline selectivity of 73.2% at a methanol conversion of 99.9% and a large amount of external carbon deposits. This is attributed to the synergy of accessibility of acid sites located in the pore intersections and the shorter diffusion paths for coke precursors out of the zeolite channel. This study provides an effective strategy for the synthesis of ZSM-5 nanosheets as catalysts for the MTG reaction, and it is expected to be applied for the synthesis of other zeolite nanosheets for important catalytic processes.

Understanding the structural properties of zeolites for isobutane alkylation based on adsorption/diffusion behaviors

Although zeolites are well-accepted as environmentally friendly catalysts for the isobutane alkylation, the structure-performance relationship between C4 reactants (isobutane and butene) and zeolites is less reported. Herein, the adsorption and diffusion behaviors of C4 reactants on zeolites with different topologies were investigated using configuration bias Monte Carlo (CBMC) and molecular dynamics (MD) simulations under experimental conditions in 348 K and 2.5 MPa. It is revealed that surface area, largest cavity diameter, and pore limiting diameter are the most relevant structural parameters for zeolites to determine the adsorption and diffusion of C4 reactants. In addition, the dimensions of the zeolitic channels can also affect the selectivity of isobutane to butene and the diffusion of C4 reactants, thereby affecting the process of isobutane alkylation reaction. This work can provide a good reference for the design of novel zeolites for isobutane alkylation.

Advanced Transmission Electron Microscopy for Identification of Atomic‐Scale Configurations of Zeolite‐Supported Metal Catalysts

AbstractZeolite‐supported metal catalysts have important applications in various catalytic reactions, but unveiling their atomic‐scale structures is always a grand challenge. The developed transmission electron microscopy (TEM) characterization techniques, e.g., integrated differential‐phase contrast (iDPC) scanning transmission electron microscopy (STEM) is emerging as a powerful tool for visualizing the electron beam‐sensitive materials at atomic resolution, thus, providing new opportunities for imaging the exact configurations of metal species in the various channels of zeolites. This emerging topic summarizes recent progress utilizing iDPC‐STEM‐related technique for determination of the structures of metal species in zeolites that should offer insights into the structure‐property relationships of zeolite‐supported metal catalysts. A brief perspective about the new TEM technique is also presented, showing prospects in this field that will stimulate further scientific research in zeolite‐supported metal catalysts.

Effects of Rare Earth Modifications on Structural Stability of HZSM-5 Zeolite

HZSM-5 zeolite modified by rare earth was prepared with ion exchange method in the study. The effects of different types of rare earth elements, modification temperatures, pH, and rare earth contents on structural stability of the as-prepared zeolite were investigated. The as-prepared zeolite was characterized by XRD, SEM and FT-IR spectroscopy. The characterization results showed that the crystal structure of the zeolite was not changed by the modification of rare earth, which was evenly distributed on the surface or in the pore channel of zeolite. Sm/HZSM-5 zeolite with the highest structural stability was obtained under the following modification conditions: 2% rare earth Sm, modification temperature of 70 °C, 2 h of ion exchange, and pH 3.

Production of valuable chemicals through the catalytic pyrolysis of harmful oil sludge over metal-loaded HZSM-5 catalysts

This research studied the catalytic pyrolysis of oil sludge (OS) over metal-loaded HZSM-5 catalysts, an eco-friendly and cost-effective technology to produce value-added aromatics such as benzene, toluene, ethylbenzene, and xylene (BTEXs). In particular, it evaluated the respective effects of the experimental parameters: the type and amount of the metal loaded, the reaction temperature, and the OS/catalyst ratio, on the BTEXs yield sequentially to achieve optimum conditions. This evaluation showed that the highest yields of the BTEXs (6.61 wt%) and other aromatics were achieved when Ni was incorporated into the HZSM-5 (Ni/HZSM-5) followed by the corresponding yields of Ga/HZSM-5 and Fe/HZSM-5, due to a better distribution of Ni on the support surface and an enhanced acidity strength of this catalyst. Further, increase in Ni loading (up to 10 wt% Ni/HZSM-5) increased the BTEXs yield to 13.48 wt%. However, the excessive Ni loading (15 wt% Ni/HZSM-5) resulted in a reduced BTEXs yield due to the blockage of the zeolite channels. Next, an increase in the reaction temperature from 500 °C to 600 °C increased the yield of the BTEXs and other aromatics. However, a further increase in the reaction temperature to 650 °C decreased slightly their yield because of the stimulating secondary reactions at high temperatures. The increase of catalyst amount (OS/catalyst of 1/3) also maximized the BTEXs yield (30.50 wt%).

Diffusion behavior of gas molecules in the one-dimensional channel of AlPO4-5 molecular sieves

Understanding of molecular diffusion mechanisms in zeolite plays a crucial role in the processes of adsorption and separations and can greatly facilitate the design of zeolite at molecular level. However, compared with the case of free gas phase, the diffusion behaviors in channel of zeolite are much more complicated due to the confined pore environments. To dig more detailed information about the diffusivity mechanism of gas molecules in the zeolite pore, diffusion behaviors of H 2 , He, N 2 , CH 4 and CO 2 molecules in the one-dimensional (1-D) channel of AlPO 4 -5 (AFI) zeolite-like materials were investigated using a combined classic molecular dynamics (MD) simulations and density functional theory (DFT) calculations. It is found that the diffusion behavior for light molecules (H 2 and He) at low loading in AlPO 4 -5 channel is more freedom because of their small molecular diameter and low energy variation (ΔE) in the longitudinal direction of the pore. Then, transport of light gas molecules can be described as Knudsen diffusion and the molecular mass plays a key role in this case. Transports of N 2 , CH 4 or CO 2 , with larger molecular diameter and higher ΔE value, were proposed to compete with surface diffusion and their diffusivity should be controlled by the adsorption energy between gas molecule and pore wall. Therefore, the diffusion degree of gas molecules at low loading is decreased as follows: H 2 > He > N 2 > CH 4 > CO 2 . When the gas molecule loading increases, the gas diffusion freedom will be limited due to the increased gas-gas and gas-porous media collisions. Thus, the diffusion of these gas molecules with higher concentration is dominated by the gas-gas and the gas-AFI interactions. DFT calculation results demonstrate that both gas-gas and the gas-AFI interaction energies depend on their van der Waals diameter. As a result, the diffusion of these gas molecules at higher loading in AlPO 4 -5 channel is decreased as: He > H 2 > N 2 > CH 4 > CO 2 . Our results confirmed that the diffusivity of gas molecules within the 1-D channel of AlPO 4 -5 is controlled by the molecular mass or gas-wall interactions at low loading, while is dominated by the gas-gas/gas-wall interactions at high loading. • DFT calculation results confirmed that AlPO 4 -5 unit cell with P 1 symmetry is much more stable than that with P 6 cc symmetry. • Transport of light gas molecules (H 2 and He) with small d m and low ΔE at low loading in AlPO 4 -5 channel can be described as Knudsen diffusion and the molecular mass plays a key role in this case. • Transports of N 2 , CH 4 and CO 2 , with larger molecular diameter and higher ΔE value, at low loading in AlPO 4 -5 channel were considered as surface diffusion and their diffusivity should be controlled by the gas-AFI interactions. • The diffusion of these gas molecules in AlPO 4 -5 channel with higher concentration is dominated by the gas-gas and the gas-AFI interactions.

A theoretical insight into diffusion mechanism of benzene-methanol alkylation reaction in ZSM-5 zeolite

Alkylation reaction of benzene and methanol produces desirable product p-xylene, thus understanding the structure-performance relationship in alkylation reaction is significant. Herein, the diffusion behavior of representative components ( i.e. benzene, methanol, olefin and alkylbenzene) involved in the alkylation reaction catalyzed by Al-substituting ZSM-5 (H-ZSM-5) zeolite at 673 K was studied based on molecular dynamics simulations. As a result, the self-diffusion coefficients (D s ) of methanol and benzene at 673 K in H-ZSM-5 zeolite are 2.97 × 10 −8 and 6.50 × 10 −12 m 2 s −1 , respectively; while in CH 3 -ZSM-5 zeolite, the presence of CH 3 O- intermediates does favor to the diffusion of benzene and alkylbenzene, especially p-xylene. Furthermore, due to the distinguished dynamic diameters compared with pore sizes, the reaction species exhibit anisotropic diffusion along x-, y- and z- direction of zeolite channel. This work may not only provide an insight into the diffusion mechanism for alkylation reaction of benzene and methanol, but also help guide the design of zeolite catalysts. A theoretical insight into diffusion dependence of benzene-methanol alkylation reaction in ZSM-5 zeolite. • Diffusion-reaction relationship in alkylation reaction has been investigated. • Molecular dynamics associated with density functional theory is employed. • Both size and electrostatic effects induce the selectivity towards desirable p-xylene.