Sites In ZSM-5 Research Articles

Response of Hydrogen-Bonded Brønsted Acid Sites in ZSM-5 to Adsorption and H/D Exchange of Hydrocarbons

Response of Hydrogen-Bonded Brønsted Acid Sites in ZSM-5 to Adsorption and H/D Exchange of Hydrocarbons

Aromatics production from catalytic pyrolysis of waste cassava residue using La and P modified ZSM-5: Experimental and kinetic study

In the present study, selective transformation of waste cassava residues into aromatics via catalytic pyrolysis over as-prepared La and P modified ZSM-5 was systematically investigated, especially in the aspects of product regulation and kinetic analysis. Comprehensive physicochemical characterizations were undertaken for unveiling the essential structural properties of La and P modified ZSM-5. The introduction of La and P species significantly enriched the Lewis and Brønsted acid sites in ZSM-5, thus facilitating the directional pyrolysis of waste cassava residues into aromatics. Higher apparent activation energy for aromatics formation from waste cassava residues pyrolysis in the presence of La and P modified ZSM-5 might be due to the fact that the catalytic pyrolysis belonged to diffusion-controlled gas-solid reaction. In comparison with that of the parent ZSM-5, LaP2/ZSM exhibited better performance towards aromatics formation in waste cassava residues pyrolysis, wherein the relative content of aromatics reached 83.3% at a temperature of 650 °C. Gratifyingly, the relative content of aromatics was increased to 90.5% for the catalytic pyrolysis of acid pretreated cassava residue, in the case of LaP1/ZSM catalyst.

Boosting diethylamine selective oxidation over CuO/ZSM-5 catalyst by CeO2 modification

Selective catalytic oxidation of nitrogen containing volatile organic compounds (NVOCs) is very meaningful but still remains big challenge because it is very difficult to balance the oxidation activity and N2 selectivity of available catalysts. In order to develop excellent catalysts with high N2 selectivity in wide temperature window (△T) during NVOCs oxidation, CeO2 modified CuO/ZSM-5 catalysts with different CeO2 addition order were prepared for diethylamine (DEA) oxidation. Detailed characterizations of catalysts such as XRD, XPS, H2-TPR and NH3-TPD were conducted, it is found that addition of CeO2 obviously changes the geometrical and electronic states of CuO active sites in the catalyst, and CeO2 addition order affects the Ce3+ ratio on the catalyst surface. Promotion of CeO2 species enables the generation of more highly active Cu species (Cu+ species and finely dispersed CuO clusters) for DEA oxidation; and synergistic catalysis of the Ce3+ species, active Cu species and acid sites in ZSM-5 leads to the high N2 selectivity in wide temperature window (△T). Compared to the CeO2/CuO/ZSM-5 and CuO-CeO2/ZSM-5 catalysts, the CuO/CeO2/ZSM-5 catalyst exhibits moderate high oxidation activity with T99 of 230 °C and the widest △T of 150 °C for high N2 selectivity. Our primary results in this work provide the efficient strategy for improving catalyst employed for NVOCs selective oxidation.

Sustainable Aromatic Production from Catalytic Fast Pyrolysis of 2-Methylfuran over Metal-Modified ZSM-5

The catalytic fast pyrolysis (CFP) of bio-derived furans offers a promising approach for sustainable aromatic production. ZSM-5 modified by different metal species (Zn, Mo, Fe, and Ga) was employed in the CFP of bio-derived furans for enhancing aromatic production. The effects of metal species, metal loadings, and the weight hourly space velocity (WHSV) on the product distributions from the CFP of 2-methylfuran (MF) were systemically investigated. It is found that the introduction of Zn, Mo, Fe, and Ga on ZSM-5 significantly increases the MF conversion and aromatic yields. The maximum MF conversions of 75.49 and 69.03% are obtained, respectively, by Fe-ZSM-5 and Ga-ZSM-5, which boost the aromatic yield by 34.5 and 42.7% compared to ZSM-5. The optimal loading of Fe on ZSM-5 is 2%. Additionally, the highest aromatic yield of 40.03% is achieved by 2%Fe-ZSM-5 at a WHSV of 2 h−1. The catalyst characterization demonstrates that the synergistic effect of Brønsted and Lewis acid sites in Fe-ZSM-5 is responsible for achieving the efficient aromatization of MF. The key to designing improved zeolite catalysts for MF aromatization is the introduction of large numbers of new Lewis acid sites without a significant loss of Brønsted acid sites in ZSM-5. These findings can provide guidelines for the rational design of better zeolite catalysts used in the CFP of biomass and its derived furans.

Framework coordination of single-ion Cu2+ sites in hydrated 17O-ZSM-5 zeolite

The interfacial coordination chemistry of water solvated single Cu2+ sites in ZSM-5 is assessed through pulsed EPR spectroscopy and selective 17O isotopic labelling.

Catalytic fast pyrolysis with metal-modified ZSM-5 catalysts in inert and hydrogen atmospheres

The objective of this work was to determine the impact of hydrogen-activating metals on catalytic fast pyrolysis over ZSM-5 with and without hydrogen addition. Ga-, Ni-, Cu-, Co-, and Pt-modified ZSM-5 catalysts with metal-to-aluminum molar ratios of 2 were prepared by incipient wetness method. The catalysts were evaluated for hydrocarbon and oxygenate yields and for deactivation by analytical pyrolysis (Py-GC–MS), and post-reaction catalysts for characterization were prepared in a larger fixed-bed reactor. In inert atmosphere, there was a linear correlation between both the hydrocarbon yield and coke formation and density of strong acid sites, and the main impact of the metals was occlusion of active sites in ZSM-5, leading to proportional reductions in hydrocarbon and coke yields. With the addition of hydrogen, several metals (Pt, Ni, Cu, Co) activated hydrogen and further reduced coke formation, leading to improvements in the ratio of hydrocarbons to coke. No metal-modified catalyst out-performed fresh unmodified ZSM-5 in terms of hydrocarbon yield; however, Ni/ZSM-5 and Cu/ZSM-5 in hydrogen produced comparable hydrocarbon yields to ZSM-5 while reducing coke formation.

Evaluation of the molecular poisoning phenomenon of W sites in ZSM-5 via synchrotron X-ray powder diffraction.

The traditional investigation of complex catalyst poisoning phenomena is in the operation level: poisonings commonly attributed to macroscopic coke deposition and particle size change, etc. Here, we demonstrate that high-resolution SXRD can reveal the structure of the organic molecule-active site complex in a 3-D environment, leading to an understanding of the poisoning mechanism at the molecular level.

Zeolite Supported Bimetallic Catalyst System: The Effect of Metal Loading for Catalytic Pyrolysis of Jatropha Residue

Two transition metals were loaded on H-ZSM-5 zeolite to produce bimetallic zeolite supported catalysts for catalytic pyrolysis reaction. Ni and Co metal were loaded on H-ZSM-5 via wet impregnation method. The loading sequence was applied using one-step and two-step loading method. The different loading sequence affect surface properties of catalyst and catalytic activity in pyrolysis reaction. The bimetallic catalysts were prepared at Ni+Co metal loading content of 10+10 wt% (Ni:Co=1:1) to 10+20 wt% (Ni:Co=1:2 or 2:1). All bimetallic catalysts supported on H-ZSM-5 were calcined and characterized by X-ray Diffraction (XRD), Surface area analysis (BET) and Temperature Programmed Desorption of ammonia (NH3-TPD). The XRD patterns of bimetallic zeolite supported catalysts revealed that loading of two metals at high content affect crystalline structural of ZSM-5 support. All XRD patterns illustrated peaks characteristic of ZSM-5, cobalt oxide and nickel oxide. The NH3-TPD results showed number of acid sites of the catalyst which revealed that the acid sites of ZSM-5 support was weakened with transition metal added. The two-step loading of 10+20 wt% metals on ZSM-5 reduced the peak intensities of NH3 desorption due to the metal particles aggregate on acid sites of ZSM-5. The two-step 10+20 wt% bimetallic catalysts has the lowest surface acidity, followed by the one-step 10+20 wt%, the two-step 10+10 wt% and the one-step 10+10 wt% bimetallic catalysts, respectively. Jatropha residue was used for catalytic pyrolysis study. Jatropha residue and bimetallic catalyst was pyrolyzed at 500 °C in a pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). The product vapor was analyzed by GC/MS for the different groups of organic products such as fatty acid, aldehydes, ketones, aliphatic hydrocarbons, aromatic hydrocarbons and nitrogen compounds. The product from catalytic pyrolysis of jatropha residue with bimetallic zeolite supported catalysts enhance deoxygenation reaction that resulted in high aliphatic and aromatic hydrocarbons product. The one-step loading at ratio Ni:Co = 1:1 (10+10 wt%) gave the highest hydrocarbons product yield at 57.81%.

Effect of Location and Distribution of Al Sites in ZSM-5 on the Formation of Cu-Oxo Clusters Active for Direct Conversion of Methane to Methanol

A series of Cu/ZSM-5 materials were synthesized and tested for the selective oxidation of methane to methanol reaction in a three stage reaction. The efficiency of the catalysts is related to the ability of the zeolite framework to stabilize multinuclear Cu-oxo species, namely dicopper and tricopper oxo clusters. Spectroscopy characterization by EXAFS showed that the exchange with moderate Cu loadings led to preferential formation of trinuclear Cu complexes [Cu3(μ–O)3]2+ in HZSM-5. The concentration of Al pairs in ZSM-5 is found to limit the maximum concentration of multinuclear Cu-oxo species that can be formed. Above such maximum, inactive Cu species including Cu oxide nanoparticles are formed. Conversely, it was found that at low loadings the Cu speciation in Cu/ZSM-5 occurs as a mixture of Cu monomers and dimers. Furthermore, it was found that not only the structure of Cu-oxo clusters is relevant for the activation of methane, but also the local environment in which the cluster is embedded. Comparison of methane to Cu stoichiometries achieved for Cu/ZSM-5 and Cu/MOR systems containing the same type of active [Cu3(μ–O)3]2+ cluster shows that approximately 50 % of these clusters are inactive on ZSM-5. While MOR stabilizes the trinuclear clusters in highly constrained 8-MR side pockets, the possibility of ZSM-5 to stabilize part of these clusters in less constrained local environments might be the reason for a lower activity in methane oxidation.

Modification of acid sites in ZSM-5 by ion-exchange: An in-situ FTIR study

A study of the acid sites in Mn+-ZSM-5 zeolites (Mn+=H+, Al3+, Ca2+, and Ba2+), synthesized by ion-exchange from the NH4+ form, has been carried out using X-ray diffraction, 27Al MAS NMR, and in-situ FTIR spectroscopy. X-ray diffraction data indicate that the crystalline structure of ZSM-5 is maintained on ion-exchange. 27Al MAS NMR spectroscopy confirms that Lewis acid sites in all of the Mn+-ZSM-5 zeolites are mainly located on the tetrahedral aluminum atoms in the zeolite framework. However, octahedral extra-framework aluminum is another source of Lewis acid sites in H+-ZSM-5 and Al3+-ZSM-5. Brønsted acid sites are identified as originating from the hydroxyl groups that bridge Al and Si atoms. The acid sites in Mn+-ZSM-5 have been characterized by in-situ FTIR spectroscopy with pyridine as the probe molecule. FTIR spectroscopy demonstrated that the number of accessible acid sites and ratio of Lewis to Brønsted acid sites in Mn+-ZSM-5 can be effectively modified by ion-exchange. The number of acid sites in Mn+-ZSM-5 increases in the same order as the acidity of cations with Ba2+<Ca2+<NH4+<Al3+<H+. Though the strength of both Lewis and Brønsted acid sites is virtually identical for all Mn+-ZSM-5 zeolites, the ratio of Brønsted to Lewis acid sites varies as a result of the loaded cation. In addition, modification of acid sites in Mn+-ZMS-5 by loading different cations is discussed in terms of the accessibility of the surface of the zeolite channels and the reactivity of the cations with Brønsted acid hydroxyls. We conclude that larger cations can effectively constrict the zeolite channels and impede the ability of pyridine to access acid sites on the surface of the channels.

Impact of the local environment of Brønsted acid sites in ZSM-5 on the catalytic activity in n-pentane cracking

The impact of the zeolite Brønsted and Lewis acid site concentration on the catalytic cracking of alkanes was explored using n-pentane and H-ZSM-5 as examples. Rates normalized to strong Brønsted acid sites (i.e., the turnover frequencies, TOF) showed that the two samples with the highest Al content had much higher TOF than all other samples. This difference has been unequivocally linked to the presence of extra-lattice alumina. Post-treatment of the zeolites with ammonium hexafluorosilicate and static calcination was used to vary the concentration of extra-lattice alumina. After extraction of extra-lattice alumina from the samples with high TOF, all TOFs were identical. IR spectra of adsorbed pyridine and NH3, coupled with 27Al MAS NMR, showed that the overall enhanced activity is associated with tetrahedrally coordinated extra-lattice alumina in close proximity to strong Brønsted acid sites. The TOF of these sites is approximately 40-times higher than the TOF on normal Brønsted acid sites.

Comparative study of bimetallic Pt-Sn catalysts supported on different supports for propane dehydrogenation

Bimetallic PtSn catalysts supported on different supports (ZSM-5, γ-Al2O3, mesoporous alumina, SBA-15) were prepared and several techniques were employed to study the influence of different supports on the catalytic structure and reaction performances. According to the results of XRD, textual properties and acid measurements for the as-prepared catalysts, the selected support is the main factor to determine the porous character and the acidic properties of the corresponding catalyst. In our experiments, when the used support is mesoporous alumina, most uniform and narrow distribution of metallic particles can be obtained. Furthermore, the presence of this support promotes the occurrence of spillover hydrogen and strengthens the interaction between Sn species and the support. As a result, the oxidative state of Sn can be stabilized and the transformation of active sites is promoted, thus improving the catalytic stability and reaction selectivity. Compared with this, the existence of the strong acid sites in ZSM-5 induces more cracking reaction and the porous character of γ-Al2O3 increases the accumulation of the coke. Meanwhile, the use of SBA-15 support leads to the agglomeration of metallic particles and the easy reduction of tin species, which is disadvantageous to the reaction.

Electronic propensity of Cu(II) versus Cu(I) sites in zeolites to activate NO — Spin- and orbital-resolved Cu–NO electron transfer

Electronic factors responsible for the notable decline of NO activation by Cu(II) with respect to Cu(I) sites in zeolites are investigated within spin-resolved analysis of electron transfer channels between the copper center and the substrate. The results of natural orbitals for chemical valence (NOCV) charge transfer analysis for a minimal model of Cu(II) sites in zeolite ZSM-5 ({T1Cu}+ NO) are compared with those for Cu(I)–NO and referenced to an interaction of NO with bare Cu+ cations. The bonding of NO, which is an open-shell and non-innocent ligand, gives rise to a noticeable nondynamical correlation in the adduct with Cu(II) (reflected in a broken-symmetry solution obtained at the density functional theory (DFT) level). Four distinct components of electron transfer between the copper and NO are identified: (i) donation of an unpaired electron from the NO π∥* antibonding orbital to the Cu species, (ii) backdonation from copper d⊥ to the NO antibonding orbital, (iii) “covalent” donation from NO π∥ and Cu d∥ orbitals to the bonding region, and (iv) donation from the nitrogen lone pair to Cus,d. Large variations in channel identity and significance may be noted among studied systems and between spin manifolds: channel i is effective only in the bonding of NO with either a naked Cu+ cation or Cu(II) site. Channel ii comes into prominence only for the model of the Cu(I) site: it strongly activates the NO bond by populating antibonding π*, which weakens the N–O bond, in contrast to channel i depopulating the antibonding orbital and strengthening the N–O bond. Channels iii and iv, however, may contribute to the strength of the bonding between NO and copper, and are of minor importance for the activation of the NO bond. This picture perfectly matches the IR experiment: interaction with either Cu(II) sites or a naked Cu+ cation imposes a comparable blue-shift of NO stretching frequency, while the frequency becomes strongly red-shifted for a Cu(I) site in ZSM-5 due to enhanced π* backdonation.

C═C, C≡C, and C═O Bond Activation by Coinage Metal Cations in ZSM-5 Zeolites: Quantitative Charge Transfer Resolution

Adsorption and activation of ethene, ethyne, and formaldehyde by copper and silver sites in ZSM-5 are reinspected by quantitative assessment of the donation and backdonation processes between the three molecules and either models comprising bare cations (M+) or the cations embedded in zeolite framework (M(I)). ETS-NOCV analysis (decoupling the deformation density upon substrate bonding into independent components) reveals two predominant channels for electron transfer between adsorbed molecule and a cation or a cationic site, namely π*-backdonation from metal d orbitals to π* antibonding orbital of a substrate and σ-donation from π bonding or lone pair orbitals of the substrate to the cation. Alternative fragmentation of a complex modeling the cation embedded in zeolitic framework, treated as a three-part entity, allows for extracting predominant electron transfer channels from framework oxygens to the cation with bound substrate, namely, opposing σ-donation and supporting π*-backdonation. Critical analysis of charge flows between various parts of a complex system shows that effective activity of the cationic site comes from two contradictory processes and must be viewed as resulting from the framework effect on the sensitive balance between opposing electron transfer channels. Juxtaposition of density transfer channels between the substrate with CC and CO multiple bonds and either a bare metal cation or the cation embedded in a zeolite framework shows that σ-donation prevails over π*-backdonation for free cations, while for zeolitic sites their order is reversed. Quantitative analysis reveals that for title systems, activation decrease due to the reduction of σ-donation becomes outweighed by the increase in π*-backdonation for Cu(I) but not for Ag(I). Thus zeolitic framework, regarded as an electron reservoir, may either support (Cu+) or impair (Ag+) electronic processes underlying catalytic activation of multiple bonds.

A DFT study on the [VO]1+–ZSM-5 cluster: direct methanol oxidation to formaldehyde by N2O

The mechanism of direct oxidation of methanol to formaldehyde by N2O has been theoretically investigated by means of density functional theory over an extra framework species in ZSM-5 zeolite represented by a [(SiH3)4AlO4](1-)[V-O](1+) cluster model. The catalytic reactivity of these species is compared with that of mononuclear (Fe-O)(1+) sites in ZSM-5 investigated in our earlier work at the same level of theory (J. Catal. 2011, 282, 191). The [V-O](1+) site in ZSM-5 zeolite shows an enhanced catalytic activity for the reaction. The calculated vibrational frequencies for grafted species on vanadium sites on the surface are in good agreement with the experimental values. According to the theoretical results obtained in this study the [V-O](1+) site in the ZSM-5 catalyst has an important role in the direct catalytic oxidation of methanol to formaldehyde by N2O.

Theoretical studies of HNCO adsorption at stabilized iron complexes in the ZSM-5 framework

The stabilization of monomeric and dimeric iron complexes, such as FeOH, HOFeOFeOH and FeOFe, which are representative of the iron-exchange in the ZSM-5 structure, has been investigated using a DFT approach with a cluster model. A Si20O53H28 cluster was chosen to represent part of a single channel in the ZSM-5 structure. In this basic cluster, two Si atoms were replaced by two aluminum atoms to simulate the exchange sites in ZSM-5 and to determine energetically favorable locations for AlAl couples. The distance of the aluminum atoms varied from 4.8 to 7.5Å. Inside the clusters of the ZSM-5 structure, iron oxohydroxide and oxide species formed stable monomer and dimer configurations, which were bound to the oxygen atoms of AlO4- due the more negative charge and the stronger ion attraction compared to SiO4. The stabilization energy per iron site was about 3.5eV for the monomeric iron complex and 1.0–2.5eV for the different dimeric iron sites. These similar energy levels, in combination with a detailed study of the geometric structure of the iron and oxygen centers in Fe-ZSM-5, support our hypotheses that the distribution of iron sites follows the distribution of the Al atoms and that no preferred configurations exist for these interactions. However, when aluminum centers are present with a distance compatible to the FeFe distance the formation of a dimer is slightly favored. In addition, the molecular and dissociative adsorption of isocyanic acid on Fe-ZSM-5 has been investigated due to its importance for the selective catalytic reduction of NOx with urea.

DFT Study of Direct Methanol Oxidation to Formaldehyde by N2O on the [Fe]2+–ZSM-5 Zeolite Cluster

The mechanistic pathways of direct oxidation of methanol to formaldehyde by N2O were theoretically investigated by means of density functional theory (DFT) over an extra framework species in ZSM-5 zeolite represented by a [Si6Al2O9H14]2–[Fe]2+ cluster model. The catalytic reactivity of these species is compared with that of mononuclear Fe1+ and (FeO)1+ sites in ZSM-5 investigated in our earlier work at the same level of theory (J. Catal.2011, 282, 191). The formation of the grafted species including methoxy on the [Fe]2+ site was calculated to be thermodynamically more stable than on the [FeO]1+ site and less stable than on the [Fe]1+ site. The order of activation barrier values of a critical step, proton transfer from grafted methoxy to form formaldehyde and water, on these sites is as follows: [Fe]1+ > [Fe]2+ ≫ [FeO]1+. The calculated vibrational frequencies for grafted species on the iron site on the surface are in good agreement with the experimental values.

A DFT Study of Direct Oxidation of Benzene to Phenol by N2O over [Fe(μ-O)Fe]2+ Complexes in ZSM-5 Zeolite

Density functional theory (DFT) calculations were carried out in a study of the mechanism of benzene oxidation by N2O to phenol over an extra framework dimeric [FeOFe]2+ species in ZSM-5 zeolite represented by a [Si6Al2O9H14(Fe(μ-O)Fe)] cluster model. The catalytic reactivity of such a binuclear species is compared with that of mononuclear Fe2+ and (FeO)+ sites in ZSM-5 investigated in our earlier works at the same level of theory (J. Phys. Chem. C2009, 113, 15307; 2010, 114, 12580). The activation energies for the elementary reaction step involved in the benzene hydroxylation over the binuclear and the mononuclear iron sites are comparable. The major difference in the catalytic behavior of the systems considered is related to the ability of Fe3+-containing sites to promote side reactions leading to the active site deactivation. Regeneration of the active site via the phenol desorption is much less favorable than its dissociation resulting in the formation of very stable grafted phenolate species on both the [Fe(μ-O)Fe]2+ and (FeO)+ sites. In the case of Fe2+ sites such an alternative reaction path does not exist resulting in their stable catalytic performance. Benzene hydroxylation and phenol formation over the binuclear (Fe(μ-O)Fe)2+ sites in ZSM-5 are promoted in the presence of water. These computational findings are consistent with the experimental observations and allow their rationalization at the molecular level.

Formaldehyde activation by Cu(I) and Ag(I) sites in ZSM-5: ETS-NOCV analysis of charge transfer processes

Adsorption and activation of formaldehyde molecule by copper and silver sites in ZSM-5 was studied by combined QM/MM methodology. ETS-NOCV analysis was applied to decompose the deformation density (the difference between electron densities of composed systems and non-interacting fragments) into independent density flow channels. The analysis disclosed that the bonding of formaldehyde by the metal site may be described in terms of lone pair donation from carbonyl oxygen to the metal and π*-backdonation from metal d orbitals to antibonding orbital of CO. The former one is responsible for the formation of coordinate bond (“dative covalent”) and the latter one for the activation of carbonyl bond. Cu(I) site is more efficient with respect to both processes than Ag(I) one. Zeolitic environment is shown to weaken the bond between formaldehyde and M(I) with respect to free cations for both metals whereas it enhances π*-backdonation in the case of copper and diminishes π*-backdonation in the case of silver. Calculated red-shifts of CO frequencies agree well with the IR-measured ones and correlate with the degree of π*-backdonation which rationalizes as well very good performance of Cu(I) site compared to Ag(I) as the role of zeolitic framework in modifying exchanged cation properties.

From Electron Density Flow Towards Activation: Benzene Interacting with Cu(I) and Ag(I) Sites in ZSM-5. DFT Modeling

Cu(I) and Ag(I) sites in ZSM-5 and their interaction with adsorbed benzene are studied by DFT cluster modeling aided with NOCV analysis of charge transfer processes. The interplay between donation and back donation from the cation to the ad-molecule, reinforced by the framework environment correlates with benzene activation shown also by the red shift in calculated and measured IR frequencies. Copper sites have better activation ability due to its stronger interaction with the framework, serving as electron reservoir, and better match between d orbitals and π orbitals of benzene.