Naděžda Žilková

Synthesis, Characterization and Catalytic Applications of Organized Mesoporous Aluminas

The synthesis of organized mesoporous aluminas has opened a very interesting area for application of this type of materials, particularly as catalysts or catalyst supports. This review focuses on the individual synthesis routes to produce organized mesoporous aluminas with large surface areas and narrow pore size distributions, and on the evaluation of their textural, chemical and thermal properties and outlines examples of catalytic applications of organized mesoporous alumina‐based catalysts. We tried to rationalize the synthetic approaches to prepare organized mesoporous aluminas, to relate their properties to synthetic procedures used as well as to their catalytic behavior in different reactions. Utilization of various structure‐directing agents for “cationic,” “neutral,” “anionic,” “nanocasting,” and special approaches leading to scaffolding and lathlike organized mesoporous aluminas is discussed in the first part of this review, as well as textural and structural characterization and thermal stability of mesoporous aluminas synthesized by different synthetic approaches. In the second part, catalytic applications of organized mesoporous aluminas described in the open literature are evaluated from the standpoint of the importance of these reactions for technological applications.

Activity and selectivity of zeolites MCM-22 and MCM-58 in the alkylation of toluene with propylene

The influence of the pore architecture of zeolites MCM-22 and MCM-58 on their catalytic activity and selectivity in the gas-phase alkylation of toluene with propylene has been investigated. The results obtained with MCM-22 as catalyst revealed that, despite the expectations due to the presence of 10-membered-ring pores in the structure of this zeolite, no enhanced selectivity to p-cymene is observed. Moreover, also n-propyltoluenes, which are usually formed over three-dimensional 10-membered-ring zeolites (e.g., ZSM-5 and ZSM-11) via bimolecular transalkylation/isomerization reactions of cymenes with toluene, are only formed to a negligible extent. This has been rationalized by the assumption that most of the reactions occurring in this system take place on acid sites at or close to the external surface. This is supported by the FTIR-spectroscopic observation that p-cymene can reach only about one half of the bridging OH groups, while the smaller d3-acetonitrile reaches/covers virtually all remaining Bronsted and Lewis acid sites. Over zeolite MCM-58, relatively high selectivities to n-propyltoluenes were observed, which has been ascribed to the peculiar undulating 12-membered-ring channel system of this zeolite. It is proposed that the 1,2-p-ditolylpropane-type transition state required for the formation of n-propyltoluenes is preferentially located in positions similar to those which are occupied by the N-benzyl-1,4-diazabicyclo[2.2.2]octane cations used as templates for the synthesis of zeolite MCM-58.

Recent Advances in Reactions of Alkylbenzenes Over Novel Zeolites: The Effects of Zeolite Structure and Morphology

Alkylbenzenes form an important segment of petrochemical industry for the manufacture of widely used commodities and specialty products. Since the last review on this topic (8), numerous new zeolite-based catalysts have been synthesized, characterized and evaluated in various transformations of aromatic hydrocarbons. This comprehensive review covers major reactions of mono-, di-, and tri-alkylbenzenes such as disproportionation, alkylation, transalkylation, isomerization, etc., over different zeolite-based acid catalysts. During the last decade, significant progress was made in the synthesis and structure determination of novel zeolites, mesoporous single crystals, hierarchic zeolites and two-dimensional zeolites. These developments have enhanced the understanding of the role of zeolites (effects of structural type, morphology, acid sites, accessibility of acid sites, shape selectivity factors) in transformations of aromatics. In this review, the emphasis is on the influence of the type of acid sites, zeolite topology, and reaction conditions on the activity, selectivity and pathways of these reactions. Thermodynamics and reaction kinetics of transformations of aromatic hydrocarbons are also discussed. This article covers mostly literature published during the period of 2002–2013.

Nitrogen adsorption study of organised mesoporous alumina

As the arrangement of pores generally achieved in organised mesoporous alumina is not completely regular, gas adsorption methods appear important for characterisation of this material. One of the promising approaches to adsorption data analysis is based on the comparison of the adsorption isotherm of nitrogen at −196 °C on the porous solid under study with the standard adsorption isotherm of the same adsorbate on a reference non-porous solid. To obtain reliable standard nitrogen adsorption data for characterisation of the alumina porous structure, nitrogen adsorption isotherms on Degussa Aluminiumoxid C and α-alumina were carefully measured at −196 °C with ASAP 2010 and modified Accusorb 2100E instruments. Standard nitrogen adsorption data (adsorbed amount divided by surface area) were listed for 41 points in the relative pressure range 0.001 to 0.95. Their applicability was tested on organised mesoporous aluminas of various structural parameters using the comparison plot method. Although some samples of organised mesoporous alumina were calcined at relatively high temperatures, only the standard data on Degussa Aluminiumoxid C proved suitable for the comparative analysis of adsorption isotherms. The standard adsorption data on Degussa Aluminiumoxid C transformed to the statistical film thickness curve were used for calculation of Barrett–Joyner–Halenda (BJH) pore size distributions. The determined material parameters of the organised mesoporous aluminas were also supported by X-ray diffraction data and were consistent with the conditions of synthesis and heat treatment of these materials.

High activity of highly loaded MoS2 hydrodesulfurization catalysts supported on organised mesoporous alumina

Abstract Organised mesoporous alumina-supported Mo sulfide catalysts were prepared by the conventional impregnation with solution of ammonium heptamolybdate and by the method of thermal spreading of MoO3. The mesoporous alumina (surface area 400–600 m 2 g −1 , pore diameter 3–4.5 nm) exhibited much higher capacity for dispersion of Mo species than the conventional alumina. The activity in thiophene hydrodesulfurization of highly loaded mesoporous alumina-supported catalysts with the MoO3 content of 30 wt.% was almost two times higher than the activity of the conventional industrial Mo/Al2O3 containing 15 wt.% of MoO3

Rhenium Oxide Supported on Mesoporous Organised Alumina as a Catalyst for Metathesis of 1-Alkenes

New type of heterogeneous catalyst for metathesis of linear 1-olefins based on rhenium (VII) oxide supported on organized mesoporous alumina was developed. This catalyst operates under very mild reaction conditions without any solvent and the highest activity was achieved at the reaction temperature of 60 °C with selectivity to symmetrical olefin over 90%. The optimum loading of rhenium (VII) oxide was found to be between 9 and 15 wt% of Re. The initial activity was dependent on pore size diameter—the larger pore diameter, the higher initial catalytic activity under the same reaction conditions.

Selective synthesis of cumene and p-cymene over Al and Fe silicates with large and medium pore structures

The alkylation of benzene and toluene with isopropanol was studied in relation to time-on-stream (TOS) at temperatures of 520 and 570 K with the large pore zeolites Y, beta, mordenite and ZSM-12 and medium pore MFI silicates with isomorphously substituted Al and Fe in the framework. It has been found that the conversion of toluene and benzene does not correlate with the number and strength of bridging OH groups, as it does for alkylation reactions leading to xylenes and ethyltoluenes. The TOS behaviour of the conversion as well as the product composition indicated that the reaction rate is controlled by the desorption/transport rate of the bulky propylbenzenes and propyltoluenes being affected by the acidity and channel geometry of the zeolite. Formation of the undesired n-propylbenzene and n-propyltoluenes via bimolecular transalkylation reactions is enhanced by the acidity (strength and number of OH groups) and narrow intersecting channels of the molecular sieves. Thus, on acid catalysts with a more open structures (Y, beta, AlCl3, H3PO4) the formation of branched isomers is preferred. However, as these zeolites do not exhibit a high para-selectivity needed for synthesis of p-cymene, ferrisilicates of MFI structure possessing less acidic SiOHFe groups are favoured.

The effect of MFI zeolite lamellar and related mesostructures on toluene disproportionation and alkylation

The effect of MFI zeolite nanosheet morphology and related mesostructures on the accessibility of acid sites evaluated from FTIR spectra of different adsorbed probe molecules and conversions and selectivities in toluene disproportionation and alkylation with isopropyl alcohol was investigated. Two-dimensional MFI lamellar zeolite (1.5 nm thickness of layers) and related mesoporous sponge morphologies, most probably with 10-ring channel system not yet structurally fully determined, were synthesized and characterized by XRD, SEM, TEM, and adsorption measurements. FTIR spectroscopy using pyridine, deuterated acetonitrile and 2,6-di-tert-butyl pyridine evidenced high concentrations of accessible Lewis acid sites at the expense of Bronsted acid sites. Ordered mesoporous zeolites exhibited slightly higher toluene conversion in its alkylation with isopropyl alcohol than lamellar MFI or conventional MFI due to easy access to active sites. The lamellar zeolite consisting of less than a unit cell layer was slightly more active than MFI but still exhibited high selectivity to p-cymene. It evidences that alkylation of toluene with isopropyl alcohol proceeds mainly in the 10-ring channel system of MFI zeolite, although the effect of large external surface cannot be neglected. Increased para-selectivity compared with thermodynamic value is achieved even in the case of extremely thin zeolite nanosheets.

Experimental study of the effect of Si/Al composition on the aluminum distribution in (Al)MCM-41

Abstract The effect of framework Al content on the number of framework Al atoms on the surface of (Al)MCM-41 channels and the distribution of these Al atoms among single Al atoms and various Al–Si–Al and Al–Si–Si–Al pairs was investigated. The number of framework Al atoms on the wall surface was characterized by the ion exchange capacity for Na+ ions, number of Al pairs on the surface by exchange capacity for Co2+ ions. Two different geometrical arrangements of Al pairs were characterized by VIS spectroscopy of bare Co2+ ions. At low framework Al content, Al atoms occupy only T sites on the surface of the channels. In aluminum rich (Al)MCM-41, Al atoms are incorporated also into the inner volume of the channel walls and at Si/Al∼11 only about 50% of Al atoms are on the channels surface. Al atoms on the wall surface preferentially form Al–Si1,2–Al pairs, enabling siting of divalent cations. Number of single Al atoms increases with framework Al content and overcome 50% of surface Al atoms for Si/Al∼11. The Al–Si1,2–Al pairs are present in two cationic sites and also the distribution of Al pairs into these sites depends on the framework Al content.

Alkylation and disproportionation of aromatic hydrocarbons over mesoporous molecular sieves

Abstract Toluene alkylation with propylene and trimethylbenzene (TMB) disproportionation were investigated over mesoporous molecular sieves of MCM-41 type of different pore dimensions, possessing various Si/Al ratios or modified with heteropoly acids (HPA) of Keggin type. It was shown that the acid strength of (Al)MCM-41 was sufficient to activate propylene in the alkylation of toluene providing cymene selectivity over 96% while the rate of consecutive reactions was negligible. In contrast, TMB disproportionation proceeded at a significantly lower extent than over zeolites. Siliceous MCM-41 modified by HPA (HPA-MCM-41) exhibited a lower rate of cymene isomerization compared to (Al)MCM-41. Moreover, m -cymene was mainly formed at the expense of o -cymene. In TMB disproportionation over HPA-MCM-41 (HPA=H 6 PV 3 Mo 9 O 12 ) significantly higher concentrations of xylenes were reached in comparison with (Al)MCM-41. m - and o -xylenes as primary products of TMBs disproportionation were preferentially formed.