Acidic Mordenite Research Articles

Immobilization of aluminum chloride on MCM-41 as a new catalyst system for liquid-phase isopropylation of naphthalene

A great deal of effort has been made at searching for alternative catalysts to replace conventional Lewis acid catalyst aluminum trichloride (AlCl 3). In this paper, immobilization of AlCl 3 on mesoporous MCM-41 silica with and without modification was carried out. The catalytic properties of the immobilized catalyst systems for liquid-phase isopropylation of naphthalene were studied and compared with those of H/MCM-41 and H/mordenite. The structures of the surface-immobilized aluminum chloride catalysts were studied and identified by using solid-state magic angle spinning nuclear magnetic resonance (MAS NMR), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption, and X-ray diffraction (XRD) techniques. The catalytic activity of the immobilized catalysts was found to be similar to that of acidic mordenite zeolite. A significant enhancement in the selectivity of 2,6-diisopropylnaphthalene (2,6-DIPN) was observed over the immobilized aluminum chloride catalysts. Immobilization of aluminum chloride on mesoporous silica coupled with surface silylation is a promising way of developing alternative catalyst system for liquid-phase Friedel–Crafts alkylation reactions.

A Periodic Density Functional Theory Study of Intermolecular Isomerization of Toluene and Benzene Catalyzed by Acidic Mordenite Zeolite: Effect of the Zeolite Steric Constraints

A periodic density functional theory study of the isomerization reactions of toluene and benzene catalyzed by acidic mordenite is reported. Reaction energy diagrams including transition-state energies of disproportionation and direct transalkylation reactions are presented and analyzed. Alternative reaction pathways have been considered. The use of periodic structure calculations allows analysis of steric constraints that occur within zeolite micropores. General rules concerning the influence of steric constraints in relation with reaction mechanism are described.

The effect of adamantane addition on the conversion of n-heptane over H- and Pt/H-zeolites

H- and Pt/H-forms of zeolites Y and mordenite were studied in atmospheric n-heptane ( n-C 7) hydroconversion at 423–443 K. Adamantane (Ad) additive was found to increase the isomerization and suppress the cracking activity of the H-zeolites. It had minor or no influence on the hydroconversion activity of the Pt/H-forms. Species obtained from Ad adsorbed on zeolites were examined by IR spectroscopy. The Ad-saturated samples (Ad/H-zeolites) were boiled with n-C 7 and the liquid phase was analyzed by GC–MS. Results suggested that Ad was activated over the H-zeolites for further reactions via protolysis of its CH bonds. The Ad and the formed adamantyl carbenium ions (Ad +) participated in hydride transfer processes. In these processes, the Ad remained unconverted and acted as co-catalyst increasing n-C 7 isomerization selectivity. Results suggested that under conditions of n-C 7 hydroconversion no or minor amount of Ad + was generated on the Pt/H-zeolite catalysts. Over strong acid mordenite bulky carbocations and Ad compounds were rapidly formed contributing to the decay of the activity in n-C 7 conversion.

Molecular Modelling of N2O/NO Separation in Acid Mordenite: Comparison with Gas-Chromatographic Experiments

Molecular modelling, utilising the Grand Canonical Monte Carlo method, was employed to study the interactions between nitrous oxide (N2O) and nitric oxide (NO) with a solid matrix, namely mordenite, before and after charge equilibration. The results indicate that the charge equilibration leads to an adsorption which is independent of the pressure. Comparison with gas-chromatographic experiments indicate that charge equilibration is not allowed in acid mordenite. Therefore, in the case of polar molecules, their adsorption depends on the charge in the zeolite micropores.

A Periodic DFT Study of the Isomerization of Thiophenic Derivatives Catalyzed by Acidic Mordenite

A periodic density functional theory study of the isomerization reactions of alkylated benzothiophene derivatives (i.e., dimethylated and monomethylated dibenzothiophenes and monomethylated benzotiophenes) catalyzed by acidic mordenite zeolite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways are discussed in detail. The use of periodic structure calculations takes into consideration the electrostatic contributions and steric constraints that occur within the zeolite micropore. The isomerization reactions have been regarded as being helpful in hydrodesulfurization reactions of alkylated benzothiophene derivatives catalyzed by solid acid–metallic sulfide bifunctional catalysts.

Isomerization of n-butane over ultrastable H-Y zeolites with different Si/Al atomic ratio

Ultrastable H-Y zeolites with different Si/Al atomic ratios (3<Si/Al<11) are active and selective catalysts for the n-butane isomerization. The initial activity of these catalysts is lower than that measured on tungsta supported on zirconia catalysts (WO x /ZrO2) and acidic mordenite catalysts; however, the Bronsted acid sites of the ultrastable H-Y zeolites are stable and selective towards isobutane. No deactivation of the catalysts was observed after 5 h of time on stream. In contrast, WO x /ZrO2 and acidic mordenite catalysts under the same experimental conditions are largely deactivated in less than 1 h of time on stream. The stability of the ultrastable H-Y zeolite in comparison to H-mordenite catalysts may be due to the three-dimensional structure of H-Y made of large supercages interconnected by apertures of 12 oxygen atoms. This structure may favour the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. Acidic molecular sieves with monodimensional structure may favour the formation of the precursors of the coke responsible of the catalyst deactivation.

A periodic DFT study of intramolecular isomerization reactions of toluene and xylenes catalyzed by acidic mordenite.

A periodic density functional theory (DFT) study of the isomerization reactions of toluene and xylene catalyzed by acidic mordenite is reported. Monomolecular isomerization reactions have been considered and analyzed. The different reaction pathways have been discussed in detail. The use of periodic structure calculations allows consideration and analysis of zeolite electrostatic contributions and steric constraints that occur within zeolite micropores. Major differences in the details of protonation reaction pathways are found when periodic structures are used rather than small cluster models of the Brønsted acidic site. Complex relationships are found between zeolite topology and reaction pathways.

The liquid-phase reaction of hexane on acid mordenite

The batch reaction of hexane (H) on acid Mordenite at 160°C does not occur by consecutive carbocationic isomerization (H→2MP,3MP→2,3DMB→2,2DMB). Instead, a mixture of all the isomers in constant ratios was obtained at all conversions between 2% and 13%, as expected for quenching or desorption of an equilibrium mixture. The ratios of the monobranched isomers to the dibranched isomers did not correspond, however, to their equilibrium at the reaction temperature, or to the equilibrium of the corresponding carbocations. The existence of mechanistic differences between the reaction under these conditions and the experiments conducted either with nonoxidizing superacids in solution (the source of the accepted mechanistic model) or with a Pt-containing catalyst in the presence of hydrogen, in the gas phase, is indicated by these data.

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by acidic mordenite.

A theoretical study of the alkylation reaction of toluene with methanol catalyzed by the acidic Mordenite (Si/Al = 23) is reported. Cluster DFT as well as periodical structure DFT calculations have been performed. Full reaction energy diagrams of the elementary reaction steps that lead to the formation of the three xylene isomers are given. The use of periodical structure calculations allows one to account for zeolite framework electrostatic contributions and steric constraints that take place in zeolitic catalysts. Especially the steric constraint energy contribution has a significant effect on the energies and bond formation paths. The activation energy barrier of p-xylene formation is found to be approximately 20 kJ/mol lower than the corresponding values for the formation of its isomers. Computed host-guest binding energies according to the DFT method need a correction due to the absence of the dispersive interaction with the zeolite wall. Apparent activation energies obtained with this correction are in good agreement with experimental data.

Ab initio investigation of the adsorption of benzene in mordenite

The adsorption of benzene in pure-silica and acid mordenite has been investigated using density functional theory, both in the local density approximation and including generalized-gradient corrections. Benzene acts as a test molecule for studying the interaction between an aromatic hydrocarbon and a zeolite. Different adsorption modes have been studied for several acid sites. All configurations have been optimized by minimizing the total energy with respect to all lattice parameters and to the atomic coordinates. The strength of the adsorption is directly correlated to the local structural distortion of the zeolitic framework, especially of the acid site. Only if strong adsorption occurs, the molecule itself is slightly deformed. Furthermore, an analysis of the differential charges has been performed, indicating substantial polarization effects for the acid site as well as for the molecule. Finally, the red-shift of the OH-stretching frequencies due to the adsorption has been calculated.

The role of zeolite type on the lean NO x reduction by methane over Pd loaded pentasil zeolites

The lean selective catalytic reduction of NO x by methane over protonic palladium loaded ZSM-5, FER and MOR, as well as, on bimetallic Pd–Pt-HMOR was examined. Special emphasis was paid on the combined effects of water and SO 2 in the feed stream. Under dry conditions and in the absence of SO 2, the degree of NO x conversion at 450°C decreases as follows: Pd-HZSM-5>Pd-HMOR>Pd-HFER. Sulfur dioxide alone has no apparent effect on the activity for NO x reduction, but the coexistence of water and SO 2 inhibits both NO x and methane conversions. The extent of inhibition by water and SO 2 on NO x reduction is Pd-HFER>Pd-HZSM-5>Pd-HMOR. Acid mordenite doped with low levels of Pt and Pd leads to an active catalyst that is more tolerant to the presence of either water or SO 2 than the corresponding monometallic Pt- and Pd-HMOR. Nevertheless, NO x reduction is also inhibited at temperatures below 450°C when SO 2 and water are both present. TPD experiments of water over calcined samples of protonic Pd supported pentasil zeolites, Pd/γ-Al 2O 3 and Pt–Pd-HMOR with and without pretreatment in SO 2+O 2 indicate that sulfation of the surface increases water chemisorption by the support. Therefore, the observed decrease of NO x reduction on Pd-loaded zeolite catalysts when SO 2 and H 2O coexist in the feed stream may be due to enhanced water inhibition and presumably active site poisoning.

Reduction of nitrogen dioxide by propene over acidic mordenites: influence of acid site concentration, formation of by-products and mechanism

The reduction of nitrogen dioxide to nitrogen by propene was studied over a variety of acidic mordenite zeolites differing in their Si : Al ratio and thus, in their concentration of acid sites. The formation of by-products was monitored applying an ion–molecule reaction (IMR) mass spectrometer. It was found that at fixed conditions the yield of nitrogen increases with increasing concentration of acid sites, confirming that acid sites are the active catalytic centres in the reaction. Apart from nitrogen and nitric oxide, acrylonitrile and ammonia are formed as nitrogen containing gas-phase products in the reaction. In separate experiments, it was shown that acrylonitrile is hydrolysed by water over the acidic zeolites to yield ammonia and acrylic acid. When acrylonitrile is used as reducing agent for nitrogen dioxide, formation of nitrogen is strongly enhanced in the presence of water. Water also has a promoting effect on the formation of nitrogen in the reaction between nitrogen dioxide and propene. Acrylonitrile and its product of hydrolysis, ammonia, are considered to be intermediates of nitrogen dioxide reduction to nitrogen by propene over acidic zeolites.

Application of the TAP-2 reactor and FTIR in elucidating the mechanism of NO 2 reduction by propene over an acidic mordenite

A vacuum pulse method (Temporal-Analysis-of-Products (TAP) reactor) and in situ FTIR were applied to study the mechanism of the reduction of NO 2 to N 2 by C 3H 6 over an acidic H-mordenite catalyst. An adsorbed nitrile is formed on the catalyst surface presumably via a nitro or nitroso compound of propene. Upon reaction with water the nitrile yields adsorbed ammonium ions. Ammonium ions on the surface form nitrogen upon reaction with NO 2 and may therefore be considered as the decisive intermediate of NO 2 reduction by propylene over acidic mordenite catalysts.

An FTIR Study on the Mechanism of the Reaction between Nitrogen Dioxide and Propene over Acidic Mordenites

Nitrosonium ions, NO+, are formed upon reaction between NO, NO2, and the Brønsted acid sites of H–MOR. It is shown that the NO+ ions are highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can be hydrolysed to yield adsorbed ammonium ions, which are known to be efficient in reducing nitrogen oxides to nitrogen. The dehydration of propenal oxime appears to be the rate determining step in nitrile formation.

Steric aspects in methylamine and dimethylether synthesis over acidic mordenites

Mechanistic and steric aspects of direct amination of methanol over Brønsted acidic mordenites were investigated using in situ infrared (IR) spectroscopy and kinetic studies. It was found that the rate of formation of the primary product monomethylamine is linearly correlated to the total concentration of available methyl groups from methylammonium ions rather than the acid site concentration. This indicates scavenging of methyl groups by ammonia and amines is likely to be the major pathway for amine formation. Differences in selectivity to the different amines, i.e., mono-, di- and trimethylamine, are related to differences in hindrance for the diffusion of the larger molecules. Higher concentration of alkylammonium ions in the pores and higher substitution of sorbed molecules suppress formation of trimethylamine. The selectivity to dimethyl ether seems also to be controlled by such steric factors. Lowering the effective available space for formation and transport retards dimethyl ether formation or leads to more dimethyl ether reacting to amines. A concerted bimolecular mechanism involving weakly sorbed methanol species is proposed to be the main pathway for ether formation.

Aniline alkylation with ethanol over zeolites and vanadium modified zeolites prepared by solid state exchange method

Vapour phase aniline alkylation was carried out over HX, HY, HZSM-5, H-mordenite and over zeolites solid state ion-exchanged with vanadium. Calcination of V 2O 5-zeolite mixture in nitrogen reduced V 2O 5 to VO 2+ ion and these species migrated into the cationic positions in the zeolite channels. Three-dimensional and mildly acidic zeolites (ZSM-5, Y and X) showed higher catalytic activities for aniline alkylation than did unidimensional and strongly acidic mordenite. Addition of vanadia enhanced the alkylation activity of all the four zeolites studied.

Surface species in the direct amination of methanol over Brønsted acidic mordenite catalysts

Quantitative in situ infrared spectroscopy in combination with kinetic analysis is utilized to derive mechanistic aspects for the reaction of methanol with ammonia on Bronsted acidic mordenite. Under non-reactive conditions, a coadsorption complex between methanol and ammonia is found, in which only ammonia is in direct interaction with the Bronsted acid sites of the zeolite. This complex is proposed to be the precursor for the formation of protonated methylamines in the zeolite pores which are formed in sequential order up to tetramethylammonium ions. These methylamines are unable to desorb under reaction conditions in the absence of ammonia. They leave the surface either by ammonia adsorption assisted desorption or by scavenging of methyl groups from protonated methylamines by ammonia. Both steps are concluded to be potentially rate determining.

Sorption and Surface Chemistry of Aminoethanol and Ethanediamine on H-Mordenites

The sorption and surface chemistry of 2-aminoethanol and 1,2-ethanediamine on acidic mordenites was studied in situ by means of IR spectroscopy. Upon sorption on the hydroxyl groups of the zeolite, the amino group of aminoethanol and one amino group of ethanediamine is protonated. The extent of interaction of the second functional group with the zeolite decreases with increasing Si/Al ratio. On mordenites with a high acid site density the OH group of aminoethanol was strongly hydrogen bonded to the zeolite, while this interaction was less pronounced in the samples with a low site density. For ethanediamine sorbed on mordenites with a high acid site density the fraction of double-protonated molecules was higher than for samples with a low aluminum content. When aminoethanol and ammonia were coadsorbed on Si-rich mordenite with a large-pore volume, aminoethanol was preferentially adsorbed. On Al-rich samples with a small pore volume, the formation of a coadsorption complex is indicated in which aminoethanol...

Selectivity Enhancement in Methylamine Synthesis via Postsynthesis Modification of Brønsted Acidic Mordenite: An Infrared Spectroscopic and Kinetic Study on the Reaction Mechanism

Methylamine synthesis from methanol and ammonia over parent and modified Brønsted acidic mordenites is studied byin situinfrared spectroscopy and kinetic analysis to elucidate the role of elementary steps for activity and selectivity.In situinfrared spectroscopy reveals that all methylammonium ions are formed in the micropores of these catalysts. The formation of the chemisorbed methylamines, however, is not rate determining. Transient response experiments indicate that the desorption of these methylamines aided by adsorbing ammonia and/or the scavenging of methyl groups with ammonia constitutes the rate determining step. For a given catalyst, the selectivity strongly depends on the methanol conversion and the ammonia to methanol ratio of the feed. Upon modification of mordenite with tetraethoxysilane, the selectivity to the lower methylated amines is strongly enhanced. The transport limitations of the bulkier products, formed in high concentration in the pores, are concluded to cause the enhanced selectivity toward mono- and dimethylamine over the modified catalyst. Since a decrease in activity compared to the parent sample was not observed, it seems that the methyl-scavenging mechanism plays an important role over these catalysts.

IR study of nitrotoluene adsorption on oxide surfaces

IR spectra of 4-nitrotoluene on silica, titania, zirconia, silica–alumina, mordenite and sulfated zirconia, are reported. Ligation to Lewis acidic surface sites was the dominant mode of adsorption for all oxides except silica for which hydrogen bonding between silanol groups and nitro groups occurred. Zirconia and sulfated zirconia catalysed the slow reductive dimerisation of adsorbates to the corresponding azoxybenzenes. There was no detectable IR evidence for adsorbate protonation, even for the highly acidic mordenite and sulfated zirconia surfaces. Colour changes accompanying adsorption may not necessarily imply that protonation has occurred.