Outer Surface Of Zeolite Crystals Research Articles

Maximizing noble metal utilization in solid catalysts by control of nanoparticle location

Maximizing the utilization of noble metals is crucial for applications such as catalysis. We found that the minimum loading of platinum for optimal performance in the hydroconversion of n-alkanes for industrially relevant bifunctional catalysts could be reduced by a factor of 10 or more through the rational arranging of functional sites at the nanoscale. Intentionally depositing traces of platinum nanoparticles on the alumina binder or the outer surface of zeolite crystals, instead of inside the zeolite crystals, enhanced isomer selectivity without compromising activity. Separation between platinum and zeolite acid sites preserved the metal and acid functions by limiting micropore blockage by metal clusters and enhancing access to metal sites. Reduced platinum nanoparticles were more active than platinum single atoms strongly bonded to the alumina binder.

Effect of metal dispersion and support structure of Ni/silicalite-1 catalysts on non-thermal plasma (NTP) activated CO2 hydrogenation

Non-thermal plasma (NTP) activated heterogeneous catalysis is a promising alternative to thermal catalysis for enabling many challenging reactions (e.g. catalytic CO2 hydrogenation) under mild conditions. However, the mechanistic insight into the interaction between highly energetic electrons and vibrationally-exited reactive species with metal catalyst is still lacking. Here, catalytically active Ni nanoparticles supported on silicalite-1 zeolites with different configurations regarding the location of Ni active sites and support pore structures were comparably investigated using catalytic CO2 hydrogenation under the thermal and NTP conditions. Experimental results revealed that the performance of the NTP-catalysis depends on the configuration of the catalysts significantly. Specifically, catalysts with Ni active sites sit on the outer surface of zeolite crystals (i.e. microporous Ni/S1 and Ni/M-S1@Shell with steam-assisted recrystallised micro-meso-porous structure) showed relatively good catalytic performance at a low applied voltage of 6.0 kV. Conversely, the encapsulated catalyst with hierarchical meso-micro-porous structure (i.e. Ni/D-S1) which has relatively small (i.e. average Ni particle sizes of 2.8±0.7 nm) and dispersed Ni nanoparticles (i.e. Ni dispersion of ca. 2.5 %) demonstrated comparatively the best catalytic performance (i.e. CO2 conversion of ca. 75 %) at 7.5 kV. Additionally, under the NTP conditions studied, Ni on carbon-templated mesoporous silicalite-1 (Ni/M-S1) showed the worst selectivity to CH4, which was attributed to the poor accessibility of Ni active sites encapsulated in the enclosed mesopores. This study demonstrated the crucial role of catalyst design in NTP activated catalysis.

Sustainable Production of Acetonitrile from Microalgae via Catalytic Fast Pyrolysis with Ammonia over Ga/HZSM-5 Catalysts

We report on catalytic fast pyrolysis microalgae under ammonia atmosphere into acetonitrile over Ga/HZSM-5, which provides a long-term sustainable option for acetonitrile production and microalgae valorization. The effect of various catalysts, reaction conditions and species of microalgae on acetonitrile production was explored systematically. Under the optimized conditions, the maximum carbon yield of acetonitrile from Chlorella vulgaris was 23.4%, and the selectivity of acetonitrile in bio-oil was up to 77.2%. The possible reaction pathways were proposed according to the comprehensive experimental investigation on the conversion of model compounds and intermediates (proteins, carbohydrates, lipids, amino acids, carboxylic acids, furans, and amines). High-resolution transmission electron microscopy (TEM) analysis showed that Ga was present mainly at the outer surface of zeolite crystals, and a small amount of Ga was doped into the zeolite crystals.

State of the copper-containing component and the catalytic properties of Cu/ZSM-5 in selective NO reduction with propane

Supported Cu/ZSM-5 catalysts have been synthesized by ion exchange and impregnation using aqueous ammonia solutions of copper nitrate containing orbital-ordered copper ions. The state of the copper-containing component in the pore space of the catalysts and copper sorption in the catalysts have been investigated by a complex of physicochemical methods. The catalytic properties of Cu/ZSM-5 in the selective catalytic reduction of NO with propane are reported. The catalytic properties depend on the copper precursor structure and deposition method, which determine the size of the copper oxide clusters on the outer surface of zeolite crystals.

Methods of increasing the stability of the catalytic effect of MFI type zeolites and the total service life of catalysts based on them

Increasing the operational stability of MFI-type zeolite catalysts is topical for the creation of highly efficient catalysts for the conversion of methanol or C2-C4 hydrocarbon gases into aromatic hydrocarbons, i.e., important industrial processes. This study proposes three new approaches to increasing the stability of zeolite catalysts: selective dealumination on the outer surface of MFI zeolite crystals, structurally selective ion exchange on the outer surface of crystals, and the application of an isothermal (multitube) reactor. The effect of selective dealumination and structurally selective ion exchange conditions on the SiO2/Al2O3 molar ratio in the zeolite on the interregeneration period duration in the conversion of methanol into hydrocarbons has been studied. It has been established that the selective dealumination of the outer surface increased the interregeneration period duration by 3–5 times. Structurally selective ion exchange on the outer surface of zeolite crystals allows us to increase the interregeneration period duration by 2–4 times and to decrease the regeneration temperature and duration due to the change of the properties of deposited coke. The application of a multitube reactor facilitates the procedure of regeneration in comparison with an adiabatic reactor due to the formation of less condensed coke depositions.

New approaches to the study of localization of carbonaceous deposits on acidic zeolites

The discrete-successive coke micro oxidation (DSCMO) method has been improved in the direction of its sensibility increase. Two acid catalysts on the base of NaX zeolite, modified by mint of SiCl4 for the purpose of deactivating the outer surface of zeolite crystals, have been synthesized. The samples were deactivated in the reaction of isobutane with isobutene alkylation. The coke formed has been investigated by the use of improved DSCMO method. It has been shown that coke of different H/C ratios burning down alternates with burning of pure carbon, and on these grounds a conclusion has been drawn about the successive burning of carbonaceous deposits on the outer surface of zeolite crystals, in the great cavities, sodalite cells, and finally in the hexagonal prisms.

The beneficial effect of silica on the activity and thermal stability of PtCoFerrierite-washcoated cordierite monoliths for the SCR of NOx with CH4

PtCoFerrierite washcoated on a ceramic monolith is an active and selective catalyst for the SCR of NOx with methane. For our system, we found that the addition of a binder to the slurry used for the washcoat is necessary to improve the adherence and to obtain a selective catalyst. In our case, we used 2wt.% of Cabot Silica. Ultrasound experiments used to comparatively evaluate the coating adherence showed that silica prevents the washcoat loss due to attrition. Similar monolithic catalysts, but prepared without a binder, resulted in catalysts with poor activity for NOx reduction, but still active for the deep oxidation of methane with oxygen. Monoliths washcoated without a binder showed both Pt segregation to the outer surface of zeolite crystals and formation of the non-selective Co3O4. Segregated Pt clusters and cobalt oxide promote the methane combustion with oxygen, thus leaving the NOx molecules unreacted. Moreover, due to Pt segregation, the necessary intimate contact between Pt and Co active species is not favored in this catalyst, which also contributes to the low selectivity to N2.

Structure of Al–Lewis Site in Beta Zeolite Active in the Meerwein–Ponndorf–Verley Reduction of Ketone to Alcohol

H-beta zeolites differing in removal of the template by calcination in an oxygen or in a stream of ammonia, and in their subsequent steam treatment at various temperatures, as well as beta zeolites exchanged with alkaline metal ions, were investigated in the MPV reduction of 4- tert-butyl cyclohexanone to 4- tert-butyl hexanol using isopropanol. FTIR of the OH groups, of the C≡N groups of adsorbed d 3-acetonitrile, and of the framework T–O–T antisymmetric mode was used to estimate the concentration of the individual Al–Lewis sites. XPS was employed to monitor the migration of the Al species into the surface layers of the zeolite crystals. The VHF band at 3780 cm −1 and skeletal T–O–T bands at 901, 890, and 882 cm −1 were found for steam-treated (ST) H-beta. Both alkali metal ion exchange and treatment of zeolite by gaseous ammonia annihilated the bands at 3780 and 882 cm −1, but the bands at 901 and 890 cm −1 (siloxane bridges) were retained. While the bands at 3780 and 882 cm −1 correspond to one “partly” framework Al species bearing an OH group, we suggest that the band at 901 cm −1 reflects perturbation of the framework due to the presence of highly acidic extraframework AlO x counter ions exhibiting Lewis properties inside the zeolite channels. The inability to replace these AlO x complex ions by alkali metal ion exchange indicates their complex structure. These extraframework AlO x species localized inside the zeolite void volume of beta zeolites represent highly active sites in the stereoselective MPV reduction of ketone to cis-alcohol, but their migration to the outer surface of zeolite crystals provides higher yields of trans-alcohol.

IR spectroscopic studies on the surface chemistry of mordenites modified by ceria

Surface chemistry of mordenite-type zeolites modified by various methods was investigated. We found that after cerium ion exchange acidic groups were generated in the samples. Treatment with cerium salt resulted in a sample of modified surface character. Upon such modification the concentration of terminating silanol groups sitting mainly on the outer surface of zeolite crystals decreased substantially; however, this treatment also influenced the nature of acidic OH groups.

Changes in catalytic activity of MFI-type zeolites caused by dealumination in a steam atmosphere

Changes in the acidic properties and catalytic activity of MFI-type zeolite catalysts caused by dealumination in a steam atmosphere were examined. The reaction rate constant of the cracking of n-hexane was used as an index of the catalytic activity. The zeolite catalysts were exposed to a steam atmosphere for 3–20 h under a steam partial pressure of 1.0–101 kPa, and a temperature of 773 K. The amounts of acid sites induced by each kind of aluminum atoms inside and on the outer surface of the steamed zeolite crystals were measured separately by TPD methods with ammonia and 2,6-diethylaniline. Dealumination in a steam atmosphere was found to proceed as follows: first, tetrahedral aluminum atoms are extracted out of the framework and transformed into partially distorted octahedral aluminum atoms, which also induce acid sites. These atoms move to the outer surface of the zeolite crystals. During this movement, a part of them is pushed out of the crystals and turn into perfectly octahedral ones, which never induce acid sites. The remaining partially distorted octahedral aluminum atoms arrive at the outer surface of the crystals, and then they are pushed out of the crystals and turn into perfectly octahedrally coordinated aluminum atoms. With the progress of the dealumination, the catalytic activity increased at the beginning of the process, reached a maximum value, and then decreased. This phenomenon was considered to be dependent on the change in the amount of partially distorted octahedral aluminum atoms on the outer surface of the zeolite crystals.

Structure and Shape-Selective Properties of MFI Type Ferrisilicates. A Comparison with Aluminosilicate Analogues

A comparison of H-forms of Fe- and Al-substituted silicates of MFI structure and of a comparable crystal size is given from the point of view of the acidity of SiOHMe groups, activity as well as para-selectivity in toluene disproportionation and toluene alkylation with ethylene. It has been found that SiOHFe groups exhibit weaker acidity than SiOHAl groups which is reflected in ammonia desorption at lower temperatures and lower toluene conversion for ferrisilicates. The ferrisilicates exhibit lower para-selectivity compared to aluminosilicates; moreover, higher coking is observed. While the aluminosilicates contain mostly aluminum in the framework sites forming SiOHAl groups, the ferrisilicates contain, beside Fe linked in SiOHFe groups, also tricoordinated Fe3+ cations or even extraframework Fe species. The latter two species can be responsible for a considerably higher coking of ferrisilicates compared to aluminosilicates. Simultaneously, such Fe species can contribute to observed lower para-selectivity of ferrisilicates in comparison with aluminosilicates, when they are located on the outer surface of zeolite crystals.

Introduction of Cr(V), Mo(V) and V(IV) ions in cationic positions of high-silica zeolites by a solid-state reaction

Calcinatión of H-mordenite or H-ZSM—5 with some compounds of Cr(III), Cr(VI), Mo(V) and V(V) is accompanied by the migration of metal ions into zeolite channels. The ions, which migrate from the outer surface of zeolite crystals, are coordinated in the cationic positions of the zeolites. Most of the migrated ions are located in H-ZSM—5 channels as isolated cations (Cr(V), Mo(V), V(IV)) accessible to gas phase molecules. The accessibility of the cations is confirmed by the strong influence of O 2 adsorption on hyperfine structure (h.f.s.) of e.s.r. spectra for Cr(V), Mo(V) and V(IV). The possibility of cation introduction in high-silica zeolites by a solid-state reaction is determined by the presence of acid sites which may be considered as powerful ‘traps’ for migrating ions. The solid-state interaction at temperatures up to 820°C does not permit the exchange of Na + cations for polyvalent cations.

Formation of a Special Intermediate during Phase Transformation of Zeolite NaA to Nepheline

AbstractIn the process of phase transformation of zeolite NaA to nepheline under hydrothermal conditions a special intermediate occurs. This is due to the high reaction rate of nepheline formation in the presence of water molecules. Consequently, the usually observed amorphous material with high condensed silica does not occur as separate phase. – The intermediate which is formed due to the destruction of the long‐range order of zeolite is significantly short‐range ordered. The phase transformation process starts at the outer surface of zeolite crystals, thus generating a diffusion barrier for sorption uptake processes.

Introduction of transition metal ions in cationic positions of high-silica zeolites by a solid state reaction. Interaction of copper compounds with H-mordenite or H-ZSM-5

The calcination of H-mordenite or H-ZSM-5 mixtures with CuO results in the migration of copper(II) ions into zeolite channels. The Cu 2+-ions which migrate from the outer surface of zeolite crystals are coordinated in the same positions as Cu 2+-cations introduced by ion-exchange. The majority of Cu(II) is located in H-ZSM-5 channels as isolated, coordinatively unsaturated, covalent-bonded Cu 2+-cations. The coordinative unsaturation of Cu 2+-ions is confirmed by the strong influence of O 2 adsorption on hyper-fine splitting (hfs) of e.s.r. spectra. The quantity of Cu 2+-cations which may be introduced in H-ZSM-5 by the solid state reaction is related linearly to the Al 3+ content in the zeolite lattice. The interaction of H-ZSM-5 with CuCl 2, CuF 2, Cu 3(PO 4) 2 or Cu 2S results in the migration of Cu(II) or Cu(I) ions in cationic positions of zeolite. So, the thermal stability of high-silica zeolite frameworks allows the introduction of some polyvalent ions in cationic positions by the solid state reaction.