Kinga Góra-Marek

Acid properties of NaKH-ferrierites of various exchange degrees studied by IR spectroscopy

Abstract The H/Na,K exchange was studied as one of the methods of modification of the acid properties of ferrierite. The concentration of Bronsted acid sites was determined, their acid strength and heterogeneity were followed by quantitative IR studies of ammonia desorption. The information on the Si status was obtained in MAS NMR experiments. IR studies showed that, even at low exchange degrees, hydroxyl groups both in 10-ring channels and in ferrierite cages were formed. In all the NaKH-ferrierites studied the concentration of Bronsted sites determined in our IR studies was practically identical as the concentration calculated from the zeolite composition. The acid strength of OH groups was found to increase with the H/Na,K exchange degree. The IR experiments of ammonia desorption evidenced that SiOHAl groups in our H-ferrierite were heterogeneous and several kinds of OH groups of various acid strength were found. It is possible that an increase of the acid strength with the exchange degree, i.e. with the amount of protons introduced is realized by the formation of less acidic hydroxyl groups at lower exchange degree, and more acidic hydroxyl groups at higher exchange degrees.

Catalytic dehydration of ethanol over hierarchical ZSM-5 zeolites: studies of their acidity and porosity properties

The catalytic activity of novel micro/mesoporous ZSM-5 in the dehydration process of alcohols has been studied with respect to their acidity and porosity. Samples characterized by similar Si/Al ratios in the range between 23 and 36 were chosen to ensure the ability to properly monitor the changes in catalytic performance related to the concentration of acid sites, as well as their accessibility and strength. Special attention has been paid to porosity studies involving low-temperature nitrogen sorption and the quasi-equilibrated temperature programmed desorption of hydrocarbons. Thus, the wide range of materials from typically microporous ZSM-5 to micro/mesoporous analogues obtained by different methods, but presenting many similar properties, allowed us to investigate the influence of porosity character on catalytic performance. Both the acidity and porosity properties, which were significantly perturbed by various synthesis and modification strategies, were clearly reflected in the catalytic performance of hierarchical zeolites, described as volcano curve dependence.

Acidity and accessibility studies of desilicated ZSM-5 zeolites in terms of their effectiveness as catalysts in acid-catalyzed cracking processes

The structural, textural and acidic characteristics of hierarchical ZSM-5 (Si/Al = 18–32), obtained with two desilication approaches, and the effect of these treatments on the reactivity in various cracking reactions of variable feedstock size and severity have been investigated. Emphasis is given to understanding the accessibility of acid sites; this was investigated by textural analysis, FTIR probe molecules (pyridine, trimethylacetonitrile and 2,4,6-trimethylpyridine) and reactions involving n-decane, 1,3,5-triisopropylbenzene (TIPB), and low and high-density polyethylene, LDPE and HDPE, respectively. Higher surface areas and a narrower pore size distribution were obtained for NaOHT both parameters are linearly dependent on the pivalonitrile and collidine accessibility factors, for LDPE and HDPE. The T5% for HDPE is more influenced by the accessibility factors than it is for the LDPE. This is interpreted to be the result of the branching degree of HDPE and LDPE; linear HDPE is more sensitive to the enhanced number of pore mouths of ZSM-5 channels on the mesopores. At high conversion, the influence on the T50% of the accessibility factors for HDPE and LDPE is weaker, suggesting that the cracking at this stage involves intermediate molecules of smaller size with fewer diffusional limitations. With respect to our own prior work, the chosen zeolite and the cracking of polyolefins gave more pronounced differences for the hierarchical ZSM-5.

Water thermoporosimetry as a tool of characterization of the textural parameters of mesoporous materials

Water thermoporosimetry (TPM) is a powerful method for studying the properties of porous materials, devoted especially for investigation of the samples that can be destroyed in drying process. However, this method is not very popular and relatively rarely used because of problems with proper measurement procedure and choosing correct equation for the result interpretation. This report focused on refinement of general experimental protocols for water TPM. For the first time, the role of various parameters on the TPM outcomes was deeply investigated and simple and fast experimental mode was proposed. Additionally, based on the series of mesoporous silica SBA-15, the calibration procedure was employed and the reliable calibration equations were established.

Acid properties of MCM-22 and delaminated ITQ-2 studied by IR spectroscopy

We studied the concentration, acid strength, heterogeneity, accessibility and the distribution of Si-(OH)-Al groups in MCM-22 (of Si/Al=15 and 50) and in their delaminated derivatives obtained from the same precursor. The results of our IR studies suggest that Al atoms first introduced into MCM-22 structure (at high Si/Al) form the hydroxyls both in the supercages and 10-membered channels, whereas Al introduced in the next order (at low Si/Al) forms mostly hydroxyls inside supercages. Hydroxyls in the supercages show higher acid strength than those in 10-membered channels. In MCM-22 zeolites hydroxyls are much more resistant towards dehydroxylation than in others zeolites (ZSM-5). What is more, delamination practically did not change the concentration of Bronsted sites, indicating that the protonic sites were not destroyed during this process. IR studies of CO adsorption showed that delamination did not produce extrazeolitic material.

Ag-loaded zeolites Y and USY as catalysts for selective ammonia oxidation

IR spectroscopic studies of NH3 and CO adsorption were applied to establish the status of Ag0 and Ag+ in silver loaded zeolites Y and USY. The nature of the silver particles and ions and their dispersion were found to be influenced by the type of support. Application of zeolite USY as support allowed to operate such catalysts at low temperatures with high selectivity to nitrogen (95%). Zeolite USY as support guaranteed a high concentration of uniformly dispersed metallic silver species and Ag+ cations with strong electron acceptor properties. The silver species of defined nature together with the highly acidic centres in AgUSY protected NH4+ ions against oxidation thus high selectivity to nitrogen was observed.

Demonstrating the Benefits and Pitfalls of Various Acidity Characterization Techniques by a Case Study on Bimodal Aluminosilicates

A new combination of a volumetric with a dynamic method to investigate the acidity properties of aluminosilicates is introduced. In the first step, the total acidity is determined volumetrically by the measurement of two-cycle adsorption (TCA) isotherms with ammonia as a probe, directly followed by a dynamic temperature-programmed desorption (TPD) experiment to define the acid strength distribution. Furthermore, the results obtained by the new direct combination of TCA and TPD are validated by comparison with an in-situ FTIR (Fourier transform infrared) study with the same probe molecule on the same materials. Both acidity characterization techniques are compared, and we comment on their complementarity, benefits, and pitfalls. The material under investigation is a new type of bimodal microporous and mesoporous material with zeolitic characteristics, synthesized by a mesotemplate-free method. The acidic nature of the novel material is compared to two reference materials: a crystalline zeolite and a mesoporous aluminum incorporated mesocellular foam (Al-MCF) with amorphous characteristics.

Nature of active sites in the Fe-TON Zeolites: Mössbauer and IR studies

Abstract Results on the stability of framework Fe atoms obtained for the ZSM-22 zeolites of different iron contents — members of the TON family - are presented. They show how the formation of extraframework iron species can modify the parent acidity of the Fe-TON zeolites. Mossbauer spectroscopy revealed that iron atoms were partially extracted from the framework during both high-temperature template elimination and activation of the NH4 form prior to the reaction. The extent of the Fe removal depended on the iron content and was higher for samples of low Si/Fe ratios. Unexpectedly, strength of the Brensted sites was found to be higher also for iron rich samples. It was postulated that an interaction between the extraframework iron species of a cationic character and the neighbouring bridging Si-OH-Fe groups enhanced the acidity of the existing Brensted sites.

The dependence on ammonia pretreatment of N−O activation by Co(II) sites in zeolites: a DFT and ab initio molecular dynamics study

AbstractThis work is focused on the donor properties of cobalt-exchanged cationic sites in zeolites. It is based on cluster and periodic density functional theory modeling for relevant {[Co(II)(NH3)n]–NO} adducts, where Co(II) means a cobalt cation embedded either in a periodic model of chabasite (CHA) zeolite or in model clusters. NO stretching frequencies were derived from MD trajectories and compared to harmonic values from cluster calculations. By relating calculated NO frequencies to experimental FTIR spectra, it was shown that the forms of {Co(II)-NO} adducts comprising three or four ammonia co-ligands dominate the spectrum taken in ammonia-saturation conditions while forms with two NH3 ligands prevail under intermediate ammonia saturation. Finally, this work confirms the critical dependence of Co(II) activation ability towards NO upon the center donor properties, reinforced by ligation of strong donor ammonia ligands. However, strongly bound ligands appear also to compete with interaction of the center with the electron-rich framework, and a balance must be observed to maintain optimal activation ability. Graphical abstractA snapshot from MD trajectory showing a fragment of periodic framework with twoCo(II)–NO centers, bound to one framework oxygen and strongly coordinating three ammonia ligands with four others forming the second coordination sphere

Extinction coefficients of CC and CC bands in ethyne and ethene molecules interacting with Cu+ and Ag+ in zeolites--IR studies and quantumchemical DFT calculations.

The values of extinction coefficients of CC and CC IR bands of ethyne and ethene interacting with Cu+ and Ag+ in zeolites were determined in quantitative IR experiments and also by quantumchemical DFT calculations with QM/MM method. Both experimental and calculated values were in very good agreement validating the reliability of calculations. The values of extinction coefficients of ethyne and ethene interacting with bare cations and cations embedded in zeolite-like clusters were calculated. The interaction of organic molecules with Cu+ and Ag+ in zeolites ZSM-5 and especially charge transfers between molecule, cation and zeolite framework was also discussed in relation to the values of extinction coefficients.