The aim of the study was to test the hypothesis that an isolation of the vanadium active sites is pivotal for high activity in oxidative dehydrogenation (ODH) of propane. A series of materials in which isolated vanadium ions were introduced into faujasite was synthetized. The nature of the VO x species was confirmed by different physicochemical techniques showing that they constitute highly dispersed VO x species of tetrahedral/square pyramid coordination. The samples were tested in ODH in air in the temperature range 400–500 °C under atmospheric pressure. Their activity was compared with analogous systems in which nonisolated, polymeric vanadium species were introduced into the same zeolite by wet impregnation. The resulting catalysts with isolated vanadium ions exhibited higher propylene selectivity than those containing polymeric species in temperature range 425–500 °C (23.6%–31.7% vs. 3.6%–10.5%), confirming the hypothesis. Additionally, the reaction mechanism was proposed based on density functional theory calculations, indicating that the first C─H bond breaking in propane is the rate determining step (the energy barrier equals to 27.5 kcal/mol). The formed propyl radical diffuses then to reach the new vanadium site, where the second C─H bond breaking occurs (the energy barrier equals to 10.5 kcal/mol).

The effect of adsorption of ethanol and ammonia on the basicity of Ga2O3, MgO, and ZnO was examined via IR studies of CO2 adsorption. Ethanol reacts with OH groups on Ga2O3, and MgO, forming ethoxyl groups. The substitution of surface hydroxyls by ethoxyls increases the basicity of the neighbouring oxygen. The ethoxyl groups that also form on ZnO do not contain surface OH groups, but the mechanism of their formation is different. On ZnO, ethoxy groups are formed by the reaction of ethanol with surface oxygens. The presence of ethoxyls on ZnO decreases the basicity because some surface oxygens are already engaged in the bonding of ethoxyl groups. The effect of ammonia adsorption on basicity is different for each oxide. For Ga2O3, ammonia adsorption increases the basicity of neighbouring oxygen sites. Ammonia is not adsorbed on MgO; therefore, it does not change the basicity of this oxide. Ammonia adsorbed on ZnO forms coordination bonds with Zn sites; it does not change the number of basic sites but changes how carbonate species are bonded to surface sites.

As it is commonly known, CO 2 reacts simultaneously with basic O 2- and basic OH sites on oxides forming carbonates and bicarbonates, which can be followed by infrared spectroscopy (IR). However, here, we succeeded to elaborate experimental conditions under which CO 2 reacted solely with O 2- forming CO 3 2- for ZrO 2 and CeO 2 , and calculated the extinction coefficients of diagnostic bands of carbonate and bicarbonate species. For the first time, the developed IR method enabled the concentrations of O 2- and basic OH for ZrO 2 , CeO 2 , Al 2 O 3 and CuO to be measured separately. Moreover, in the case of all IR studied oxides, the sum of concentrations of O -2 and basic OH basic sites was comparable with the concentration determined by pulse adsorption of CO 2 . Thus, the presented extinction coefficients can be applied for IR basicity studies of various basic catalysts. We also followed the effect of thermal treatment on basicity of oxides.

The adsorption of ethanol and ammonia changes the basic properties of alumina, and new basic sites are created. Ethanol reacts with surface Al-OH groups, forming ethoxy group Al-O-C2H5. The substitution of Al-OH by Al-O-C2H5 increases the negative charge of neighbouring oxygen atoms, and they became sufficiently basic to react with adsorbed CO2 forming carbonate species CO32−. These carbonates were found to be monodentate and bidentate species. Preadsorption of ammonia also increases the basicity of alumina, but the mechanism is different than for ethanol adsorption. Adsorbed ammonia interacts with surface Lewis acid sites being three-coordinated aluminium atoms. This interaction is accompanied by an electron transfer from ammonia molecules to surface sites, and increases the basicity of the neighbouring oxygens, which can react with the absorbed CO2. The carbonate species formed are polydentate ones.

The hydrogenation and decarbonylation of furfuryl aldehyde in the gas phase at 120 - 200 degrees C were studied over the catalysts containing 5 wt% of the active copper or nickel active phase deposited on SiO2, CeO2, and ZrO2. Structural and morphological features of the catalysts were thoroughly investigated by several physicochemical characterization techniques (XRD, BET/BJH method, XRF, SEM/EDS, H2-TPR and NH3-TPD). The influence of the catalysts' parameters such as different active phase (Cu or Ni) as well as different carriers (SiO2, CeO2, ZrO2) or reaction temperature was examined with respect to furfural conversion and selectivity to the most desirable reaction products: furfuryl alcohol, 2-methylfuran, and furan. The results clearly showed that Cu-systems catalyse mainly the furfuryl aldehyde hydrogenation process, while Ni-catalysts catalyse mainly the process of its decarbonylation. The Cu5.0ZrO2 catalyst exhibited excellent activity which led to 85 % selectivity to furfuryl alcohol at 200 degrees C. Among Ni-catalysts, the highest selectivity towards the desired products was shown by Ni5.0CeO2 (SFOL = 52 % at T = 120 degrees C; Sfuran = 41 % at T = 160 degrees C) and Ni5.0SiO2 (S2-MF = 26 % at T = 160 degrees C). Catalytic activity is correlated with the reducibility of the materials.

In our previous study, we elaborated a method of determination of concentrations of the basic sites O2− and OH− in a quantitative IR study of CO2 adsorption. Previous adsorption studies or TPD experiments only provided the total basicity without distinguishing between O2− and OH−. In this study, we determined the concentration of O2− and OH− on ZnO, Ga2O3, and MgO surfaces. The basicity of ZnO and MgO was found to be significantly higher than that of Ga2O3. The surface of ZnO was rich in O2−, the contribution of OH− was very small, and the Ga2O3 surface contained mainly OH−. For MgO, the contribution of O2− and OH− was comparable. According to the IR results, only a small fraction of all surface hydroxyls were sufficiently basic to react with CO2. The partial dehydroxylation changed the proportion of the concentrations of O2− and OH− on the oxides. We also elaborated upon a new method to determine the total concentration of basic sites via CO2 desorption monitored using IR. For all the oxides, we studied the sum of the concentrations of O2− and OH−, as determined in our quantitative IR studies, to find whether they were comparable with the total basicity determined in the desorption experiments.

The reaction of ethanol with surface OH groups on ZrO2, CuO/ZrO2, CuO, Al2O3, Ga2O3, NiO, and SiO2 was studied by IR spectroscopy. The basicity of oxides was followed by CO2 adsorption, and their ability to oxidize was investigated by H2-TPR. It has been found that ethanol reacts with surface OH groups forming ethoxy groups and water. Some oxides: ZrO2, CuO/ZrO2, Al2O3, and Ga2O3 contain several kinds of OH groups (terminal, bidentate, and tridentate) and terminal hydroxyls react with ethanol in the first order. Two kinds of ethoxyls are formed on these oxides: monodental and bidental ones. On the other hand, only one kind of ethoxy group is formed on CuO and NiO. The amount of ethoxy groups correlates with the basicity of oxides. The biggest amount of ethoxyls is produced on the most basic: ZrO2, CuO/ZrO2, and Al2O3, whereas the smallest amount of ethoxyls is produced on CuO, NiO, and Ga2O3, i.e., on oxides of lower basicity. SiO2 does not form ethoxy groups. Above 370 K ethoxy groups on CuO/ZrO2, CuO, and NiO are oxidized to acetate ions. The ability of oxides to oxidize ethoxyl groups increases in the order NiO < CuO < CuO/ZrO2. The temperature of the peak in the H2-TPR diagram decreases in the same order.

Ultrastabilization of zeolites is performed by an intense treatment: high-temperature steaming with further acid leaching to obtain a stable and very catalytically active material. This work shows that ultra-stable zeolite Y (USY) possesses small particles of amorphous material located in the porous structure, as detected via 29Si MAS NMR and STEM. They are washed out by the chemical treatment with 0.2 M TBAOH without deterioration or modification of the crystal structure of zeolite. Contrarily, the treatment of this zeolite with NaOH-containing solutions, results in major changes in porosity and acidity of the samples. TBAOH-treated sample showed significantly different NMR spectra of adsorbed 129Xe, containing two separate signals and powder-like line shapes recorded at low temperatures. It was ascribed to the higher loading of xenon atoms in supercages. The cleaning of the sample from the debris located in micropores increases the accessible volume in supercages, therefore xenon atoms could occupy the cages in higher quantities. This treatment method could be used for fine-tuning of ultra-stable zeolite Y crystals.

The reaction of ethanol with a surface of CeO2 was studied using IR spectroscopy. In some experiments, CeO2 was pretreated in a vacuum at 820 K which caused a partial reduction. In other experiments, CeO2 was reduced with hydrogen at 770 K. We also used CeO2 oxidized by oxygen treatment at 670 K. At low coverages, ethoxy groups and new surface OH groups were formed and water was not produced. On the other hand, at higher loading surfaces, Ce-OH was consumed and ethoxy groups and water were formed. Three kinds of ethoxyls were found on CeO2: monodentate, bidentate, and tridentate ones. They were characterized by various frequencies of symmetrical, asymmetrical, and combinational bands of C-C-O units. The reduction of CeO2 increased the contribution of tridentate ethoxyls and the oxidation increased the contribution of monodentate ones. At higher temperatures, ethoxy groups were oxidized to acetate ions with the formation of new surface OH groups. Monodentate ethoxyls were the most reactive and tridentate ones were the least reactive during oxidation. The amounts of acetate species were the highest for the oxidized CeO2.

The formation, properties, decomposition and reactions of ethoxy groups on ZrO2, CuO, and CuO/ZrO2 were followed by IR spectroscopy. The reaction of ethanol with terminal Zr-OH groups leads to the formation of monodendate ethoxy groups (type I), whereas the reaction of ethanol with tribridged Zr-OH grups results in the formation of bidendate ethoxyls (type II). In both cases, water is produced. Ethoxy groups of type II were also formed on CuO. The type of the surface species detected after interaction of ethanol with CuO/ZrO2 was the same as detected for both oxides (i.e., ZrO2 and CuO) separately. This suggests that no new phase was formed in the mixed oxide system. At higher temperatures, ethoxy groups were oxidized forming acetate ions. Gaseous ethanol present in the cell was oxidized to acetaldehyde without the intermediacy of ethoxy groups.

The commercially available zeolite HY and its desilicated analogue were subjected to a classical wet impregnation procedure with NH4VO3 to produce catalysts differentiated in acidic and redox properties. Various spectroscopic techniques (in situ probe molecules adsorption and time-resolved propane transformation FT-IR studies, XAS, 51V MAS NMR, and 2D COS UV-vis) were employed to study speciation, local coordination, and reducibility of the vanadium species introduced into the hierarchical faujasite zeolite. The acid-based redox properties of V centres were linked to catalytic activity in the oxidative dehydrogenation of propane. The modification of zeolite via caustic treatment is an effective method of adjusting its basicity—a parameter that plays an important role in the ODH process. The developed mesopore surface ensured the attachment of vanadium species to silanol groups and formation of isolated (SiO)2(HO)V=O and (SiO)3V=O sites or polymeric, highly dispersed forms located in the zeolite micropores. The higher basicity of HYdeSi, due to the presence of the Al-rich shell, aided the activation of the C−H bond leading to a higher selectivity to propene. Its polymerisation and coke formation were inhibited by the lower acid strength of the protonic sites in desilicated zeolite. The Al-rich shell was also beneficial for anchoring V species and thus their reducibility. The operando UV-vis experiments revealed higher reactivity of the bridging oxygens V-O-V over the oxo-group V=O. The (SiO)3V=O species were found to be ineffective in propane oxidation when temperature does not exceed 400 °C.

Catalytic conversion of ethanol is a promising method of hydrogen production. This reaction is catalysed by metals and oxides. One of such catalysts are mixed oxides CuO/ZrO2 (Cu/Zr) with the addition of other oxides. We studied by IR spectroscopy the transformations of ethanol on Cu/Zr with addition of ZnO, NiO. Additionally, the temperature-programmed reaction of ethanol on these oxides was followed. We studied also the temperature-programmed reduction of these oxides with hydrogen. The reaction of ethanol on CuO produced acetaldehyde and water but not hydrogen, indicating, that ethanol was oxidized over this oxide. On the other hand, acetaldehyde was produced from ethanol both by oxidation and dehydrogenation over Cu/Zr (both water and hydrogen were produced). Acetaldehyde was subsequently oxidized to acetic acid, which was next transformed to acetone. The addition of ZnO facilitated the formation of ace- tone evidencing that ZnO is effective catalyst for the reaction acetic acid -> acetone. The addition of NiO caused the decomposition of acetaldehyde to methane and CO. (C) 2022 Elsevier B.V. All rights reserved.

The properties of both Cu2+ and Cu+ ions in zeolite CuY were followed with NO and CO as probe molecules. Cu2+ was found to be located in SII, SII*, and SIII sites, whereas Cu+ was found in SII and SII* sites. The fine analysis of the spectra of Cu2+-NO and Cu+-CO adducts suggests that both in SII and in SII* sites two kinds of Cu cations exist. They differ in the positive charge, which may be related to the varying numbers of AlO4− in close proximity. The experiments of NO and CO adsorption and desorption evidenced that both Cu2+ and Cu+ sites of highest positive charge bind probe molecules most strongly but activate them to a lesser extent than the Cu sites of lowest positive charge. The experiments of reduction with hydrogen evidenced that the Cu ions of higher positive charge are first reduced by hydrogen. On the other hand, Cu sites of the lowest positive charge are first oxidized by oxygen. The experiments with CuNaY zeolites of various Cu contents suggest that the first introduced Cu (at low Cu contents) created Cu+, which was the most neutralized by framework oxygens. Such Cu cations are the most stabilized by framework oxygens.

One of the methods of IR studies of the heterogeneity of Si–OH–Al groups in zeolites is the investigation of the frequency shift of the band of free OH bands restored upon the adsorption of ammonia and subsequent desorption at increasing temperatures. We extended this method by following the shift of the band of the OH group interacting by hydrogen bonding with nitrogen. The advantage of nitrogen, compared with CO, which has been commonly used as a probe molecule in studies on hydrogen bonding, is that for nitrogen the frequency shift is smaller than for CO and therefore there is no overlapping of shifted OH band with the bands of ammonium ions. For zeolites NaHY, HMFI, and HBEA, the frequency shift of IR bands of both free and hydrogen-bonded Si–OH–Al with the increase of ammonia desorption temperature evidences the heterogeneity of these hydroxyls. On the other hand, in zeolite HFAU of Si/Al = 31, Si–OH–Al were found to be homogeneous. Heterogeneity of OH groups may be explained both by the presence of Si–OH–Al of various number of Al near the bridge and of Si–OH–Al of various geometry.

The article reviews different strategies towards obtaining mesoporous zeolites Y: desilication; surfactant templating and assembly of zeolite crystals. The impact of those methods on physicochemical properties is covered, with a special focus on the acidity of the samples measured with infrared (IR) spectroscopy. The methods of characterization of acidity are presented. Quaternary ammonium cations used for desilication lead to obtaining crystalline; mesoporous and highly acidic zeolites. Si-OH-Al groups of extremely high acidity can be produced by calcination in a humid atmosphere. When the conditions are optimized, post-synthetic surfactant templating allows crystalline mesoporous zeolite to be obtained with no loss of material. All mesoporous zeolites Y proved to be active catalysts in liquid phase isomerization, catalytic cracking, and other reactions.

The desilication of zeolite Y (of Si/Al = 31) that was previously dealuminated by steaming and acid treatment was studied. Desilication of zeolites of high Si/Al module in alkali solutions extracts both Si and Al from zeolite crystals, but while Si remains in solution, Al is reinserted into the zeolite grain. The main goal of our study was to follow the status of Al reinserted into zeolite during the desilication procedure, and its role in the formation of acid sites of the Brønsted and Lewis types. The properties of Al were followed by 27Al MAS NMR spectroscopy (for parent samples and zeolites treated either with NaOH or NaOH/tetrabutylammonium hydroxide), whereas the acid sites generated in the final stages were studied by IR spectroscopy with NH3 and CO as probe molecules. In non-desilicated zeolite, most of the Al was in a typically zeolitic tetrahedral coordination, while both NMR and quantitative IR studies of NH3 sorption evidenced that Al that was extracted by desilication and was subsequently reinserted had a tetrahedral coordination similar to amorphous aluminosilicates and showed an ion exchange ability. After the exchange of Na+ to NH4+ and decomposition of NH4+ ions, reinserted Al forms generated protonic sites from which some condensed at higher temperatures producing Lewis acid sites (with stoichiometry typical for zeolites i.e., the condensation of two protonic sites produces one Lewis site) but some other kept their character.

The process of reduction (by hydrogen and ethanol) and oxidation (by oxygen and NO) of Cu sites in dealuminated faujasite-type zeolites (of Si/Al = 31) was studied by infrared (IR) spectroscopy with CO (for Cu+) and NO (for Cu2+) as probe molecules. Two zeolites were studied: one of them contained mostly Cu+exch., whereas another one contained mostly Cu2+ and Cu+ox. The susceptibility of various forms of Cu for reduction were investigated. IR experiments of CO sorption evidenced that Cu+ox. was more prone for the reduction than Cu+exch. According to NO sorption studies, Cu2+exch. was reduced in the first order before Cu2+ox. Ethanol reduced mostly Cu2+ and, also, some amounts of Cu+. The treatment with oxygen caused the oxidation of Cu+ (both Cu+exch. and Cu+ox.) to Cu2+. The adsorption of NO at 190K produced Cu+(NO)2 dinitrosyls, but heating to room temperature transformed dinitrosyls to mononitrosyls and increased the Cu2+ content.

Dealuminated mazzite shows very high acid strength of Si-OH-Al groups. The aim of our study was to produce hierarchical mazzite, containing mesopore system, but preserving strong acidity. This was done by desilication of dealuminated mazzite (Si/Al = 30), carried out using NaOH/TBAOH solutions. Desilication of zeolite as strongly acidic as mazzite was not done before. During desilication ca. 50% of silicon was extracted, the structural microporosity was indeed preserved, and large volume of mesopores was formed too. IR studies evidenced that very high acid strength of Si-OH-Al groups was preserved. According to NMR results five types of Al species were present in the desilicated material. Distribution of aluminium between two tetrahedral sites was non-random, with high preference toward occupying T-2 positions located in the six-membered rings. The desilicated sample showed very high Bronsted acidity and high catalytic activity in the liquid-phase isomerization of alpha-pinene. Retention of the sample crystallinity, increased amount of Bronsted acid sites, their high acid strength and formation of large volume of 3D mesopores improving transport of reactants, all contributed to the enhanced catalytic activity of the desilicated mazzite.

Optimization of the procedure of desilication of dealuminated zeolite Y in a NaOH/tetrabutylammonium hydroxide (NaOH/TBAOH) mixture was done in order to obtain zeolites of optimal crystallinity, porosity and acidity and therefore of promising catalytic properties in the isomerization of alpha-pinene. High-silica zeolite Y (Si/Al = 31) was treated with NaOH/TBAOH mixture of various TBAOH content (0-100 mol %). Desilication was carried out at temperatures from 293 to 373 K. It was shown, that the optimal crystallinity, acidity, porosity and catalytic properties was obtained if zeolite was treated with the NaOH/TBAOH mixtures containing 10-70% of TBAOH. However, taking into account the cost of a desilication route (TBAOH is an expensive agent), the most economic variant of desilication using 10 mol % of TBAOH only was chosen. The experiments performed at different temperatures revealed that the optimal catalytic properties of a resultant material were obtained after desilication carried out at 353 K. The conversion of a-pinene on this sample was nearly doubled in comparison with other zeolites desilicated at lower or higher temperatures. IR experiments showed also that desilication performed at higher temperatures (above 318 K) produced samples with a new kind of OH groups at 3600 cm(-1). Finally, the experiments of pyridine adsorption demonstrated that this maximum is composed of the two kinds of OH groups exhibiting various properties, namely the acidic groups vibrating at 3600 cm(-1) and the non-acidic ones at 3620 cm(-1).

The properties of Cu ions in dealuminated faujasite-type zeolites (Si/Al = 31) containing 1, 2, and 5 wt.% of Cu were investigated by IR spectroscopy with CO and NO as probe molecules. Cu was introduced by impregnation into zeolites in both protonic (HFAU) and sodium (NaFAU) forms of zeolite. Four kinds of Cu species were found: Cu+exch., Cu+oxide, Cu2+exch. (square, planar, and square pyramidal), and Cu2+oxide (CuO). The proportions between these four kinds of Cu depended on the amount of Cu and on the form of zeolite to which Cu was introduced (HFAU or NaFAU). Zeolites with 1 wt.% of Cu introduced to HFAU (denoted as Cu(1)HFAU) contained only Cu+exch., whereas other forms of Cu were present in zeolites of higher Cu contents. The concentration of Cu+exch. was determined by quantitative IR studies of CO adsorption. According to the IR results, some Cu ions were situated inside hexagonal prisms and/or cuboctahedra, and were inaccessible to adsorbed molecules. IR studies also evidenced that Cu ions in oxide forms—Cu+oxide and Cu2+oxide (CuO)—were better electron donors than Cu in exchange positions (Cu+exch. and Cu2+exch).