KL Zeolite Research Articles

Fully-Exposed Pt Cluster@Zeolite catalysts for efficient n-Hexane aromatization

The exploration of Pt cluster catalysts with minimal or fully exposed Pt atoms for specific reactions is meaningful but challenging, aiming to maximize Pt atomic utilization. To answer the above question, we design 4Pt cluster catalysts within a KL zeolite (Ptn@KL, n represents Pt atom numbers) for an n-hexane aromatization (n-HAM) reaction. Pt species in Ptn@KL are identified as Pt1, and Pt4, Pt9, and Pt13 clusters, respectively; the corresponding Pt–Pt coordination number ranges from 0, 2, 3.3 to 4.5. When tested in n-HAM at 440 °C, Pt4@KL shows the highest benzene yield of 85.7 % with ∼ 100 % conversion and superior durability up to 600 h. In-situ characterizations and theoretical calculations demonstrate that the optimal Ptn@KL CTs towards n-HAM is a fully exposed Pt cluster with CNPt–Pt of 2.

Zinc-assisted nanometric Pt cluster stabilized on KL zeolite via atomic layer deposition for the n-heptane aromatization

Nanometric Pt clusters show promising applications in alkane reforming. However, stabilizing the nanometric Pt clusters on the zeolite at high temperature is still challenging. Herein, we fabricate the zinc-assisted nanometric Pt cluster stabilized on the KL zeolite using atomic layer deposition technology. The deposition of Zn significantly enhances the stability of nanometric Pt cluster and, therefore, makes PtZn3/KL as a robust n-heptane dehydrocyclization catalyst. Systematic characterizations combined with DFT calculations disclose that the deposited Zn species donate electrons to Pt and thus promote the cleavage of C-H bond of n-heptane and facilitate the dehydrocyclization of alkene-like intermediates (σC7=) to aromatics. In addition, an easier desorption of toluene benefits the high reactivity of PtZn3/KL catalyst by preventing the dehydrogenation and cleavage of C-C bonds to form the by-products or coke deposition. This method for enhancing the stability of nanometric Pt clusters paves a way for designing high-efficient dehydrocyclization catalysts.

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.

Controllable synthesis of mesoporous KL zeolites via a simple one-step strategy

A simple strategy with additional KF in the synthesis gel was developed to fabricate mesoporous KL zeolites with tunable morphology, hierarchical structure and catalytic selectivity. By modulating the molar ratio of KF/Al2O3 from 0 to 10, the morphology of resultant KL zeolites changed from cylinder, to gyro shape, and to loosen stacking nanocrystal aggregates. The value of BET surface and pore volume for synthesized KL zeolites increased with increasing the KF, indicating the hierarchical structure of mesoporous KL zeolites can be systematically tailored by tuning the concentrations of KF. The effect of KF on the crystallization process of KL zeolites was also investigated, it revealed that the K+ played a crucial role to produce a large number of nucleation centers as the structure directing agent, which not only significantly shortened the crystallization time but also generated the defective growth units by influencing the electric balance between K+ with negative aluminosilicate species, leading to the formation of mesopores. Besides, the introduction of mesopores in KL zeolites can greatly improve the selectivity of bulk products in the alkylation between phenol with tert-butyl alcohol owing to the opened mesoporosity.

Tailoring Locations and Electronic States of Rh Nanoparticles in KL Zeolite by Varying the Reduction Temperature for Selective Phenol Hydrogenation

Tailoring Locations and Electronic States of Rh Nanoparticles in KL Zeolite by Varying the Reduction Temperature for Selective Phenol Hydrogenation

Highly Dispersed Single-Atom Pt and Pt Clusters in the Fe-Modified KL Zeolite with Enhanced Selectivity for n-Heptane Aromatization.

Conversion of straight-chain paraffins into aromatics is particularly attractive but extremely challenging in the oil refining industry. Constructing the Pt-supported catalysts with high aromatic selectivity is vital. Here, we report a strategy to use Fe-modified KL zeolites to improve the Pt atom utilization efficiency and anchor them inside KL zeolite channels via atomic-layer deposition technique. A combination of highly dispersed single-atom Pt and electron-rich Pt clusters is fabricated on the KL zeolite through the creation of proper nucleation sites. The resulted catalyst (PtFe-1/KL) exhibits excellent performance for the n-heptane aromatization (90.1% aromatic selectivity) with an apparent activation energy of 131 kJ/mol and much enhanced stability at a relatively lower temperature (420 °C). Experimental analysis and density functional theory calculation demonstrate that the single-atom Pt might play a key role in the initial dehydrogenation of n-heptane to 1-heptene, and the superior stable Pt clusters encapsulated inside Fe-decorated KL zeolite channels accelerate the 1-heptene dehydrocyclization to aromatics. The synergetic interaction between single-atom Pt and Pt clusters enables the PtFe-1/KL catalyst to be one of the most effective n-heptane aromatization catalysts reported to date.

Tailoring Pt locations in KL zeolite by improved atomic layer deposition for excellent performance in n-heptane aromatization

New evidences have been collected to demonstrate that Pt localization plays a crucial role in modulating the catalytic performance of Pt/KL catalyst for n-heptane aromatization. Here, an improved atomic layer deposition (ALD) technique was applied to deliberately tune the locations of Pt nanoparticles (NPs) in the Pt/KL catalysts. Both scanning transmission electron microscopy (STEM) and CO diffuse reflectance infrared Fourier transform (CO-DRIFT) spectra suggest that Pt NPs can be gradually introduced into the depths of KL zeolite channels by the increment of exposure time of Pt precursor from 1/12 to 20 min in one ALD cycle. Moreover, once KL channels are blocked by some encapsulated species (AlOx as example), Pt NPs can only be localized on the orifices or external surface and grow larger. As a result, surprisingly high aromatics selectivity (84.3%) in n-heptane aromatization is performed for a Pt/KL catalyst with Pt dominantly localized in the depth of zeolite channels even at a low temperature of 420 °C. Comparatively, the aromatics selectivity decreases to 31.5% and heptene (mainly 1-C7=) selectivity is up to 51.3% for a Pt/KL catalyst with some of Pt deposited on the orifices or external surface of KL. A suggested mechanism of n-heptane cyclization based on density-functional theory (DFT) calculations was provided to delineate the optional reaction pathways on different Pt localizations. Pt sites inside the KL zeolite channels are more preferable for heptene cyclization than those near the zeolite orifices both from thermodynamically and kinetically. A heptene molecule absorbed on Pt inside zeolite cage was found to curve itself, reducing the reaction barrier for the breakage of CH bond to be only 1.37 eV, therefore activating the 6th carbon atom (the rate-determining step). The barrier comparison for that on Pt near the orifices is 1.84 eV. The calculated mechanism broadens the fundamental understanding of the role of zeolite channels for alkane reforming reactions.

Catalytic performance of KL zeolite-supported iron and cobalt catalysts for the Fischer–Tropsch synthesis

In this work, Fe and Co were introduced into a KL zeolite support by incipient wetness impregnation to obtain catalysts for the Fischer–Tropsch synthesis (FTS). The samples were analyzed by several techniques including atomic absorption spectroscopy (AAS), N2 adsorption, X-ray diffraction (XRD), H2-temperature programmed reduction (TPR) and transmission electron microscopy (TEM). The catalytic performance for the FTS was tested with a fixed bed reactor. The particle size distribution of the samples was estimated by TEM microphotographs and showed that the diameters of the iron and cobalt particles were ~ 6 nm and 8–20 nm, respectively. The catalyst with the higher percentage of iron promoted the highest CO conversion, and the distribution of the products was shifted to light fractions. The iron-based catalysts were more active than cobalt in the CO conversion, but the latter yielded a higher content of hydrocarbons with 5 and more carbons.

Design and Properties of Confined Nanocatalysts by Atomic Layer Deposition.

Governing the process and outcome of chemical reactions is the most important aim of catalytic chemistry. The confinement of active sites inside nanosized spaces provides a powerful strategy to achieve this goal. Reacting molecules (reactants, intermediates, and products of a reaction) and nanomaterials (metal/metal-oxide nanoparticles) confined inside nanoreactors have been observed to exhibit modified behaviors and properties with respect to their unconfined counterparts. Typically, catalysts confined in zeolites, mesoporous materials, metal-organic frameworks, and nanotubes are obtained by traditional liquid-phase methods. However, excess metals or undesired solvents and other reagents must be removed. It is also difficult to precisely regulate the confined nanostructures and assemble multifunctional sites in the confined nanospaces. Atomic layer deposition (ALD) provides a controllable method to fabricate confined catalysts due to its outstanding advantages. In this Account, we describe our progress in the design and properties of confined nanocatalysts by ALD. ALD is an elegant method to directly deposit highly dispersed metal or oxide species into porous materials, including zeolites and mesoporous materials. We deposited Pt nanoclusters in the micropores of a KL zeolite with precisely controlled size by ALD. We also introduced CoOx nanoclusters into mesoporous SBA-15. We have reported pioneering works on the synthesis of confined nanoparticles with metal-in-nanotube structures by a template-assisted ALD method. Confined Cu nanoparticles were prepared by reducing CuO nanowires coated with Al2O3, TiO2, or alucone layers by ALD. Confined Cu and Au nanoparticles were also prepared starting from the corresponding metal nanowires with the assistance of sacrificial layers produced by ALD. In a more facile strategy, Au nanoparticles confined in Al2O3 nanotubes were produced using a sacrificial template by ALD. Furthermore, we synthesized a multiply confined Ni-based nanocatalyst through a template-assisted ALD method. We assembled multiple interfaces (Ni/Al2O3 and Pt/TiO2) in a confined nanospace for tandem reactions by template-assisted ALD. The synergistic effect of two interfaces enhanced the tandem reaction, and the confined nanospace favored the instant transfer of intermediates between the two interfaces. In addition, porous TiO2 nanotubes with spatially separated Pt and CoOx cocatalysts were also produced by ALD. The confined catalysts can be further treated by ALD. We used ALD to modify the mesoporous SBA-15 support to precisely tune the active species-support interaction. In addition to the support, the confined metal nanoparticles can also be coated with an ultrathin oxide layer by ALD to further improve their catalytic activities. Moreover, the structure and size of the confined nanospace can be tuned precisely by ALD. Overall, ALD has exhibited noteworthy applications in and will provide new opportunities for the design and synthesis of highly effective confined catalysts.

Acidic Zeolite L as a Highly Efficient Catalyst for Dehydration of Fructose to 5-Hydroxymethylfurfural in Ionic Liquid.

Zeolite L was synthesized by the hydrothermal method and post-treated by NH4 exchange to adjust its acidity. The samples were systematic characterized by various techniques including XRD, X-ray fluorescence spectroscopy, N2 adsorption-desorption, scanning electron microscopy, pyridine IR spectroscopy, and NH3 temperature-programmed desorption. The results demonstrated that the NH4 -exchange post-treatment increased the surface area, micropore volume, and acidity of zeolite L. The catalytic performance of the samples was tested in the dehydration of fructose to 5-hydroxymethylfurfural (HMF) in ionic liquid (1-butyl-3-methylimidazolium bromide, [bmim]Br). 99.1 % yield of HMF was obtained when the KL-80 °C-1 h sample (KL zeolite treated with 1 m NH4 NO3 solution at 80 °C for 1 h) was used. The high efficiency could be attributed to the appropriate acid properties of the catalyst. The zeolite catalyst could be reused four times without significant decrease in activity.

Controllable deposition of Pt nanoparticles into a KL zeolite by atomic layer deposition for highly efficient reforming of n-heptane to aromatics

Small-sized Pt particles inside KL zeolite channels are supposed to facilitate the dehydrogenation and cyclization of n-heptane.

Hierarchically micro-/mesoporous Pt/KL for alkane aromatization: Synergistic combination of high catalytic activity and suppressed hydrogenolysis

Pt/KL is a highly active and selective monofunctional catalyst for aromatization of n-alkanes due to its 1-dimensionally connected cage-like micropores. The unique micropore structure of the KL zeolite, however, can also inhibit the diffusion of bulky aromatic products out of the catalyst, which can cause unwanted side reactions that can decrease the aromatic yields. In this work, we investigated the effect of secondary mesoporosity, which can substantially facilitate molecular diffusion in Pt/KL during C6–C8 alkane aromatizations. The results showed that the hierarchically micro-/mesoporous Pt/KL synthesized with a subsequent dealumination/desilication method exhibited enhanced aromatic yields, compared with a conventional Pt/KL. In particular, in C7 and C8 aromatizations, the hierarchical Pt/KL showed substantially less formation of dealkylated aromatic products than the Pt/KL due to suppressed secondary hydrogenolysis. Pt supported on a solely mesoporous γ-Al2O3 also showed significantly suppressed secondary hydrogenolysis (dealkylation), which indicates that fast diffusion of alkylated aromatics out of the catalyst structure is important for suppressing the secondary hydrogenolysis. However, the solely mesoporous Pt/γ-Al2O3 showed markedly lower aromatization activities than the KL-supported catalysts, which indicates that Pt located inside the zeolite micropores is crucial for obtaining high catalytic activity due to the preorganization of n-alkanes in the micropores. The present results showed that the hierarchical Pt/KL provides the synergistically combined benefits of the Pt/KL and the mesoporous Pt catalyst, namely high aromatization activity and suppressed secondary hydrogenolysis, respectively.

Comparative study of bioethanol transformation catalyzed by Ru or Pt nanoparticles supported on KL zeolite

The catalytic properties of two noble metals, Pt and Ru, supported on KL zeolite were investigated in the selective transformation of bioethanol.

Magnetic Properties and the Superatom Character of 13-Atom Platinum Nanoclusters

13-atom platinum nanoclusters have been synthesized quantitatively in the pores of the zeolites NaY and KL. They reveal highly interesting magnetic properties like high-spin states, a blocking temperature, and super-diamagnetism, depending heavily on the loading of chemisorbed hydrogen. Additionally, EPR active states are observed. All of these magnetic properties are understood best if one considers the near-spherical clusters as analogs of transition metal atoms with low-spin and high-spin states, and with delocalized molecular orbitals which have a structure similar to that of atomic orbitals. These clusters are, therefore, called superatoms, and it is their analogy with normal atoms which is in the focus of the present work, but further phenomena, like the observation of a magnetic blocking temperature and the possibility of superconductivity, are discussed.

Comparative study of the hydrogenolysis of glycerol over Ru-based catalysts supported on activated carbon, graphite, carbon nanotubes and KL-zeolite

Supported ruthenium catalysts were prepared by incipient wetness impregnation of three different carbon materials: activated carbon (AC), high surface area graphite (HSAG) and multiwalled carbon nanotubes (CNT). Another catalyst was prepared by treating KL zeolite with RuCl3·xH2O in aqueous solution. All these samples were characterized by temperature programmed reduction (TPR), CO chemisorption coupled with microcalorimetry and transmission electron microscopy (TEM). The reduced catalysts were tested in the hydrogenolysis of glycerol in the liquid phase, under a reaction pressure of 8 MPa and isothermally at the reaction temperature of 453 K. The CO microcalorimetry measurements evidenced that electron donor properties of graphite and carbon nanotubes promote formation of electron-rich metal species (Ruδ−) in Ru/HSAG and Ru/CNT catalysts, which not only favors formation of 1,2-propanediol from glycerol but also enhances the successive CC cleavage, with formation of undesired products, mainly methane. For Ru/KL the occurrence of Brönsted acid sites, resulting of the reduction of the chlorinated ruthenium species bonded to the zeolite framework, was verified by temperature programmed desorption (TPD) of NH3. Furthermore, observations by TEM of the Ru/KL catalyst showed an important population of metal nanoparticles lower than 1 nm, part of which exhibits electron deficient character as indicated by the CO microcalorimetry. As a consequence, the transformation of glycerol into 1,2-PDO over Ru/KL seems to be promoted through formation of the intermediate acetol on acid sites, while for the scarcely acid Ru-carbon catalyst conversion of glycerol occurs mainly on metal sites, ethylene glycol being the preferred hydrogenolysis product.

Support Effects on Hydrogen Desorption, Isotope Exchange, Chemical Reactivity, and Magnetism of Platinum Nanoclusters in KL Zeolite

Platinum clusters were prepared by ion exchange of a KL zeolite, followed by oxygen calcination and hydrogen reduction, and characterized by electron paramagnetic resonance (EPR) and hyperfine sublevel correlation experiment (HYSCORE) spectroscopy. Simulations indicate that the cluster contains 12 equivalent platinum atoms. Therefore, the most likely structure is an icosahedral or cuboctahedral magic number Pt13 cluster with 12 platinum atoms at the surface. One atom in the center possesses only a small spin density. H/D desorption and readsorption experiments monitored via EPR and HYSCORE measurements provide information about the structure of the clusters and about the reversibility of the adsorption/desorption and isotope exchange process. Deuterium desorption experiments result in a value of 2.1 ± 0.2 eV for the D2 desorption energy of the Pt13 clusters dispersed in KL zeolite. This is more than double of the value for a (111) Pt single-crystal surface, revealing a finite size and/or support effect. O...

ChemInform Abstract: Maximum Hydrogen Chemisorption on KL Zeolite Supported Pt Clusters.

AbstractPt clusters supported on KL zeolites are prepared by aqueous ion exchange with [Pt(NH3)4]Cl and characterized by EPR spectroscopy, HRTEM, and EXAFS.

Maximum hydrogen chemisorption on KL zeolite supported Pt clusters

Platinum clusters supported on KL zeolites were characterized by EPR, HRTEM, and EXAFS. Two kinds of hydrogen chemisorption experiments both result in a saturation value of 2.9 hydrogen atoms per platinum atom, significantly more than that reported so far. A hydrogen coverage-dependent cluster restructuring is suggested.

Transesterification of soybean oil to biodiesel using zeolite supported CaO as strong base catalysts

A series of supported CaO catalysts with NaY, KL and NaZSM-5 zeolites as supports were prepared by microwave irradiation and used to catalyze the transesterification of methanol with soybean oil. The activities of synthesized catalysts were investigated compared to pure CaO. It was found that the activity of CaO catalyst increased after supported on zeolites, and the CaO/NaY exhibited the best performance. The improved catalytic activity was attributed to the increase of BET values and the concomitant increase of basic strength and basicity after dispersed on NaY zeolite through microwave irradiation. The prepared catalysts were characterized by several techniques such as X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, scanning electron microscopy (SEM) and the Hammett indicator method. In addition, the factors affecting biodiesel yield were investigated to optimize the reaction conditions. A biodiesel yield of 95% was achieved (using the CaO/NaY catalyst) with 30wt.% of CaO loading on NaY zeolite, methanol/soybean oil molar ratio 9, reaction temperature 65°C, reaction time 3.0h and catalyst/oil mass ratio 3%. The results indicated that the supported catalyst had the capacity of water resistance and acid resistance. A high yield (more than 80%) could be obtained when the acid value and the water content of oil were less than 4mg KOH/g and 1.5%, respectively.

Influence of the nature of support on Ru-supported catalysts for selective hydrogenation of citral

Ruthenium catalysts supported on KL zeolite, ZrO2 and graphite, prepared by incipient wetness impregnation, were characterized by N2 adsorption, H2 chemisorption, TEM and CO adsorption microcalorimetry and tested in the selective hydrogenation of citral in the liquid phase, at 5MPa and 323K. Characterization studies reveal that graphite promotes formation of electron-rich metal species (Run−) that difficult hydrogenation of the conjugated CC double bond of citral and indirectly favor the production of geraniol and nerol. For Ru/ZrO2 catalyst, the Ru↔ZrO2 interaction at the interface, with formation of Ru0Zrn+ species as evidenced by the TEM and CO adsorption microcalorimetry measurements, activates the carbonyl group of citral and enhances the selectivity towards unsaturated alcohols. Characterization measurements of Ru/KL evidence that metal nanoparticles placed inside of the zeolite channels block part of ruthenium loading and hinder the free transit of citral through the channels, thus reducing the overall hydrogenation activity of the catalyst. Furthermore, this steric hindrance impedes adsorption the conjugated CC double bond of citral and forces hydrogenation of the carbonyl group in terminal position. On the other hand, the fact that active sites on occluded particles are more resistant to poisoning than those easily accessible to citral enhances the selectivity towards geraniol and nerol, as the reaction time increases. The comparative analysis of the results indicates that, in order to increase the selectivity towards unsaturated alcohols, geometrical effects derived from the size, shape and location of ruthenium particles in the KL structure are more effective than the electronic modifications induced by graphite or zirconia on the ruthenium particles.