10-membered Ring Channels Research Articles

Ga 3(HPO 3) 4F 4(H 3DETA) (DETA=diethylenetriamine): A new open-framework fluorinated gallium phosphite with pentameric building unit

Ga 3(HPO 3) 4F 4(H 3DETA) is a new open-framework fluorinated gallium phosphite obtained by mild hydrothermal synthesis using diethylenetriamine as templated agent and characterized by single crystal X-ray diffraction, the powder X-ray diffraction, IR spectroscopy, TGA, ICP and elemental analyses. It crystallizes in the monoclinic space group C2/ c, a=12.741(6) Å, b=12.068(6) Å, c=11.988(5) Å, β=94.902(8) o, V=1836.6(15) Å 3, Z=4. The construction of 3D open-framework structure in the title compound may be viewed as the assembly of pentameric building units and HPO 3 groups. The pentameric building unit is the first to be found, which lead to form the three types of channels along a-, b- and c-axes, respectively, in gallium phosphite. The triprotonated DETA cations are inserted within the 10-membered ring channels and interact with anions of the framework via hydrogen bonds.

The roles of acidity and structure of zeolite for catalyzing toluene alkylation with methanol to xylene

A variety of zeolites, including MOR, MCM-22, SAPO-34, SAPO-11, SAPO-5 and ZSM-5 with different SiO 2/Al 2O 3 ratio, were used for catalyzing toluene alkylation with methanol to produce xylene. The catalytic activity of zeolite for the toluene alkylation is approximately proportional to the number of its mid-strength acidic sites, except for SAPO-34 with small channels and MOR with strong acidity. Moreover, the acidic sites for catalyzing toluene alkylation are weaker in strength than those for catalyzing toluene disproportionation, but slightly stronger than those for catalyzing the reaction of methanol to hydrocarbons. On the other hand, zeolites with 12-membered ring channels may lead to the further alkylation of xylene and the rapid deactivation by coking. Zeolites with 8-membered ring channels restrict toluene alkylation, only favorable for forming non-aromatic hydrocarbons. As a result, zeolites with 10-membered ring channels and mid-strength acidity show high catalytic selectivity and activity for methylation of toluene.

Mechanism of n-butene skeletal isomerization over HFER zeolites: a new proposal

The effect of time on stream (from 1 min to 44 h) on the rate and selectivity of n-butene isomerization and on the concentration of protonic sites interacting with ammonia or deuteroacetonitrile was determined over a HFER zeolite with a Si/Al ratio of 10. Aging for 44 h was shown to increase the selectivity from 55 to 95% and the turnover frequency for isomerization from ∼250 to 4000 h −1. All of this can be explained by considering the HFER pore system as equivalent to a series of noninterconnected nanoreactors (the 10-membered ring channels) into which the reactant molecules have to diffuse without any possibility of desorption before their exit while undergoing various successive bimolecular reactions. In nanoreactors with few protonic sites, n-butene isomerizes selectively through an autocatalytic process; in those with many sites a thermodynamic equilibrium mixture of propene, butenes, pentenes, and deactivating carbonaceous species is formed.

Structure and acidity of Mo/H-MCM-22 catalysts studied by NMR spectroscopy

A comprehensive study on physical and chemical properties of Mo/MCM-22 bifunctional catalysts has been made by using combined analytic and spectroscopic techniques, such as adsorption, elemental analysis, and 129Xe and 31P NMR of adsorbed trialkylphosphine oxide probe molecules. Samples prepared by the impregnation method with Mo loadings ranging from 2–10 wt.% have been examined and the results are compared with that obtained from samples prepared by mechanical mixing using MoO 3 or Mo 2C as agents. Sample calcination treatment is essential in achieving a well-dispersed metal species in Mo/MCM-22. It was found that, upon initial incorporation, the Mo species tend to inactivate both Brønsted and Lewis sites locate predominantly in the supercages rather than the 10-membered ring channels of MCM-22. However, as the Mo loading exceeds 6 wt.%, the excessive Mo species tend to migrate toward extracrystalline surfaces of the catalyst. A consistent decrease in concentrations of acid sites with increasing Mo loading <6 wt.% was found, especially for those with higher acid strengths. Upon loading of Mo > 6 wt.%, further decreases in both Brønsted and Lewis acidities were observed. These results provide crucial supports for interpreting the peculiar behaviors previously observed during the conversion of methane to benzene over Mo/MCM-22 catalyst under non-oxidative conditions, in which an optimal performance was achieved with a Mo loading of 6 wt.%. The effects of Mo incorporation on porosity and acidity features of the catalyst are discussed.

Xylene isomerization and aromatic alkylation in zeolites NU-87, SSZ-33, β, and ZSM-5: molecular dynamics and catalytic studies

The unique pore topology of zeolite NU-87, with 10-membered ring (MR) channels intersected by perpendicular 12-MR cavities, can offer new opportunities for alkylaromatics isomerization, as well as for aromatics alkylation. The catalytic performance has been studied for meta-xylene isomerization and disproportionation, as well as toluene alkylation with methanol, ethanol, propanol, and isopropanol. Product distribution was interpreted on the basis of pore topology and compared to zeolites with 12-MR ( β), 10- and 12-MR (SSZ-33), and 10-MR (ZSM-5) zeolites. The presence of cavities allows more space for the location of bulky intermediates and/or products, and also traps the molecules, allowing consecutive reactions that lead to thermodynamic equilibrium. Channels, on the other hand, allow diffusion without trapping, if their free diameter is large enough. 10-MR channels in NU-87 are smaller than in ZSM-5 and this limits diffusivity. Furthermore, trimethylbenzenes formed will tend to equilibrate, but only the smaller isomer, i.e., 1,2,4-trimethylbenzene, fits in the NU-87 cavity and will have a small chance to diffuse. Catalytic experiments as well as atomistic molecular dynamics are used to interpret the behavior of NU-87 and to explain the results in comparison with other structures.

Catalytic performance of MCM-22 zeolite for alkylation of toluene with methanol

MCM-22 zeolite with mid-strong acidity and openings of 10-membered ring channels may obtain a high catalytic activity and selectivity for alkylation of toluene with methanol. The acidic sites, for catalyzing alkylation of toluene with methanol, are weaker than that for catalyzing toluene disproportionation. Compared with silicon as a modifier, modification of MCM-22 with La(NO 3) 3 is a promising way to improve the catalytic selectivity of para-xylene. In addition, the experimental results also clearly indicate the characteristics of MCM-22 structure consisting of large intracrystalline cages, some of which may locate on surface of MCM-22.

Stepwise dehydration and change of framework topology in Cd-exchanged heulandite

Fully Cd-exchanged heulandite of Cd 4.00Na 0.01K <0.01Ca 0.09[Al 8.70Si 27.30O 71.75] · 29H 2O composition was used for stepwise dehydration experiments. The crystal was heated for approximately 12 h in a stream of hot air from 50 to 250 °C in steps of 50 °C. For X-ray single-crystal data collection the heating device was switched off and the crystal was immediately cooled to −173 °C. The crystal structures of the 50–200 °C data sets were refined in space group Cm, the 250 °C data were refined in C2/ m. The original crystal had 29 H 2O molecules per formula unit (pfu). Heating at 50, 100, 150, and 200 °C reduced the H 2O content to 27(2), 25(1), 26(1), and 13(1) molecules pfu. At 250 °C heulandite-Cd became essentially anhydrous. With loss of H 2O the cell parameters changed: a decreased slightly, b decreased strongly, c remained more or less invariant, and β increased leading to a decrease of the cell volume with dehydration. Due to the loss of the surrounding H 2O molecules the Cd ions moved from the center of the eight- and ten-membered rings towards the framework walls causing distortion of the heulandite channels at 250 °C. Parts of the structure transformed to a heat collapsed B-phase. This new topology differs from the original A-phase by broken T–O–T bonds and the appearance of new T–O–T connections reducing the free aperture of the 10-membered ring channels. This new structure forms domains (≈28%) coherently intergrown with the heat collapsed (A type) heulandite framework. It is suggested that in absence of H 2O and within strongly Cd populated domains, the bonding requirements of Cd 2+ extra-framework cations give rise to the altered T–O–T connections.

Synthesis of a new organically templated zeolite-like zirconogermanate (C4N2H12)[ZrGe4O10F2] with cavansite topologyElectronic supplementary information (ESI) available: tables of crystal data, including atomic coordinates, selected bond lengths and angles, and thermal parameters, and also a SEM image of FDZG-1. See http://www.rsc.org/suppdata/jm/b2/b209801f/

A new three-dimensional (3D) zeolite-type zirconogermanate, (C4N2H12)[ZrGe4O10F2] (designated as FDZG-1), with a low framework density of 13.5 T/1000 A3, has been hydrothermally synthesized by using organic piperazine as template and the inorganic salt ZrO(NO3)2·2H2O as the zirconium source at 160 °C in a mixture of pyridine and water as solvent. Single-crystal X-ray analysis reveals that its framework structure is constructed from a spiro-5 second building unit (SBU) or a parallelepiped-like building block and has a similar topology to cavansite (Ca(VO)Si4O10·4H2O). Large elliptic 10-membered ring channels are clearly observed along the crystallographic c axis and blocked by terminal fluorine atoms. Diprotonated piperazinium cations are located in the cavity center of its parallelepiped building block and form strong hydrogen bonds with oxygen atoms and fluorine atoms in the framework. This compound is not stable towards thermal removal of the organic template at above 350 °C.

High selectivity of MCM-22 for cyclopentanol formation in liquid-phase cyclopentene hydration

Highly effective formation of cyclopentanol through the liquid-phase hydration of cyclopentene has been attempted on various zeolites catalysts. MCM-22 zeolite was the most selective catalyst, which actively converted cyclopentene to cyclopentanol with a selectivity up to 99%. The effects on the hydration of catalyst preparation method, reaction atmosphere and temperature have been investigated for the MCM-22 catalysts. On the basis of the effect of reaction atmosphere, the mechanism of liquid-phase cyclopentene hydration was proposed. The thermodynamic equilibrium between cyclopentene and cyclopentanol was suggested to control greatly the cyclopentene conversion. The cyclopentene conversion was increased to 10% by increasing the water/cyclopentene ratio. Poisoning using organic amines with different molecular sizes revealed that the hydration occurred mainly in the 10-membered ring channels of MWW structure, which had an elliptic aperture smaller than that of MFI structure, exhibiting a significant shape selectivity by suppressing the etherification cyclopentanol.

Synthesis, Structure, and Properties of a Three-Dimensional Open-Framework Niobium Fluorogermanate (NGH-5)

The low-temperature hydrothermal synthesis and crystal structure of a new open-framework niobium germanate phase is reported. (C6H18N2)·Ge2.2Nb0.8O6.8F1.2 (NGH-5) was synthesized from GeO2, Nb2O5, and HF (aq) by using 1,4-trans-diaminocyclohexane (1,4-DACH) as an organic structure directing agent. The structure of NGH-5 was determined ab initio by using powder X-ray diffraction techniques and consists of a negatively charged three-dimensional Nb/Ge/O/F framework containing three orthogonal channel systems. Two of the channels are delimited by 10-membered rings of NbO5F octahedra and GeO4 tetrahedra; the third is delimited by an 8-membered ring. Charge-balancing (C6H18N2)2+ cations are encapsulated within the framework and are located at the center of the 10-membered ring channels. The NGH-5 framework is characterized by a low framework density and is closely related to the recently reported zirconium germanate ASU-15.

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Ferrierite zeolite was synthesized in fluoride medium using dicationic 1,6-bis(N-methylpyrrolidinium)hexane (MPH) as a bulky structure directing agent (SDA) in combination with tetramethylammonium (TMA) as a small co-structure-directing agent (co-SDA). When trimethylamine, pyrrole or tert-butylamine were used as co-SDA, amorphous and beta zeolite (BEA framework type) were obtained, suggesting that these amines do not play a strong role as co-SDA. Only when TMA was used as co-SDA in synthesis gels with moderate pH, ferrierite is obtained; in fact, we have found a dramatic influence of the pH of the synthesis gel on the ferrierite crystallization, since even using TMA as co-SDA, ferrierite was only obtained under moderate pH values. Interestingly, ICP and CHN chemical analyses suggest that the Al content is driven by the amount of SDAs able to be hosted within the ferrierite framework, more specifically, by the nitrogen (positive charge) content available to charge-balance the negative charge associated to the incorporation of Al. A computational study showed that the most stable configuration of the ferrierite system corresponds to the occupation of the FER cages by TMA, while MPH accommodates in the 10-membered ring channels of the FER structure; the packing arrangement of MPH determines the density of positive charges, and hence the Al incorporation. Moreover, the unit cell parameters of the as-made ferrierite samples indicate that the structure is slightly expanded along the [100] direction in order to accommodate the bulky MPH dication.

The Origin of the C7-Hydroconversion Selectivities on Y, β, ZSM-22, ZSM-23, and EU-1 Zeolites

We carry out experimental hydroconversion tests of n-heptane paraffin on five different zeolites: Y, β, ZSM-22, ZSM-23, and EU-1. The experimental selectivities clearly depend on the catalyst pore structures. Through force field simulations, we investigate adsorption and diffusion properties as well as model cyclopropane intermediate stabilities inside each zeolite framework. In open structures such as Y and β, no shape transition state selectivity or diffusion limitation are taking place. In the more restricted 10-membered ring (MR) channels of ZSM-22, ZSM-23, and 10-MR windows of EU-1, we show that the selectivities in branched isomers and cracked products are explained by the combined effects of transition state restriction and product diffusion limitations. In particular, the selectivities in monomethyl-branched products are correlated to their relative diffusion barriers, in agreement with earlier results of E. B. Webb III et al. (1999, J. Phys. Chem. B103, 4949). The explanation for this result has its foundation in the degree of symmetry of the monomethyl-branched paraffin. In ZSM-22 and ZSM-23, the 2,2- and 3,3-dMC5 are not formed on account of transition-state shape restrictions, whereas a slight amount of 2,3- and 2,4-dMC5 is formed and can diffuse out of the pores. In the EU-1 sieve, the large side pockets behave as the open Y or β structures, where all isomerization reactions can take place without restriction, but only the faster diffusing 2,3-dMC5 and 2,4-dMC5 products are released. The slower diffusing multibranched 2,2-dMC5 and 3,3-dMC5 paraffins are transformed via methyl shift or cracked inside the pockets producing a higher amount of cracked products in EU-1 than in ZSM-22 or ZSM-23.

Deactivation of medium pore zeolite catalysts by butadiene during n-butene isomerization

The deactivation of the 10-membered ring zeolites MFI, FER and TON was studied during the skeletal isomerization of n-butene in the presence of butadiene. The rate of deactivation per butadiene molecule was found to increase with the concentration of butadiene and the contact time. It decreased in the presence of Pt and hydrogen. FER and TON deactivated much faster than MFI. This was attributed to two factors: (i) Deactivation of TON occurred by pore-blocking, whereas direct poisoning of the acid sites by coke deposits took place on MFI. In the case of FER, coke was only deposited in the 10-membered ring channels, whereas the access to the 8-membered ring channels was blocked. (ii) The build-up of carbonaceous deposits on MFI was slower than that on the other zeolites, because MFI had a high activity for cracking oligomeric coke-precursors into smaller fragments, which desorbed. Although this cracking activity is responsible for the moderate selectivity of MFI for butene isomerization, it is beneficial with respect to the stability against deactivation by butadiene.

Molecular modeling studies on zeolite catalysts for shape-selective electrophilic substitution: formation of xylenes

Energy minimization methodology has been used to study the interaction of isomers of xylene with ZSM-5, mordenite and MCM-22 zeolites. Adsorption sites and the mechanism of diffusion of the reactant molecules in the alkylation of toluene and xylene isomers inside these zeolites were studied. When molecules diffuse through 10-member and 12-member ring channels, they pass through favorable and unfavorable adsorption sites. ZSM-5 shows significant selectivity for p-xylene over the o- and m-xylenes. In large pore mordenite, the difference between favorable and unfavorable adsorption sites is not significant for xylene isomers. In the case of MCM-22, there are significant energy barriers, and the diffusivity is not high in spite of the presence of large cages. Results of this study emphasize the importance of the shape and the size of molecules as well as the pore dimensions and architecture of zeolites in effectively controlling molecular diffusion characteristics. The diffusion characteristics of the molecules inside a zeolite are sensitive to its pore architecture. The results offer the reasons based on molecular level interactions, for the experimentally observed shape selectivities in the above three zeolites.

Synthesis and crystal structure of the hydrated magnesium diphosphate Mg2P2O7·3.5H2O and its high temperature variant Mg2P2O7·H2O

The synthesis and crystal structure of a novel hydrated magnesium diphosphate and its high temperature variant are described. Both structures were solved from powder X-ray diffraction data. The room temperature variant with composition Mg2P2O7·3.5H2O crystallises in the monoclinic space group P21/c (No. 14) with a=10.9317(1), b=8.05578(9), c=9.2774(1) Å, β=90.201(1)°, V=816.99(2) Å3 and Z=4. The structure consists of sheets stacked along [100] which are linked through MgO2(H2O)4 pillars into a three-dimensional framework with cavities containing water molecules. Within the sheets there are infinite edge-sharing chains of Mg octahedra along [010] which are cross linked by P2O74− groups. A high temperature variant exists around 200°C. The crystal structure of this compound with composition Mg2P2O7·H2O was solved and refined in the monoclinic space group C2/c (No. 15) with a=18.6596(4), b=7.9769(1), c=8.9757(2) Å, β=107.378(1)°, V=1275.01(4) Å3, Z=8. The transformation to Mg2P2O7·H2O involves removal of the water molecules in the cavities and the water molecules of the Mg octahedral pillars in Mg2P2O7·3.5H2O. The sheets in Mg2P2O7·3.5H2O however remain unchanged during the transformation as the water molecule coordinating Mg here is retained. These sheets are linked through tetrahedral MgO4 pillars into a three-dimensional structure containing infinite 10-membered ring channels along [001]. Both compounds have been further characterised by 31P MAS NMR spectroscopy, thermogravimetric analysis and high temperature powder X-ray diffraction.

Molecular Modeling of Carbonaceous Compounds Formed Inside the Pores of FER Zeolite during Skeletal Isomerization of n-Butene

Molecular mechanics calculations of isobutene, its dimer (2,4,4-trimethyl-2-pentene), its trimer (2,4,4,6,6-pentamethyl-2-heptene), and seven families of coke molecules formed during n-butene isomerization were carried out within the ferrierite pore system. Monte Carlo docking showed that most of these molecules are well solvated by the zeolite framework and are preferentially sited within one or two intersections along the 10-membered ring channel. The locations of most of the larger coke molecules in two adjacent intersections account for the significant influence of dehydrogenative coupling in the formation of coke molecules in FER-structured zeolites. The stability of the butene dimer is close to that of molecules of carbonaceous compounds, which are known to be trapped inside the zeolite pores, whereas the stability of a trimer intermediate is much lower. This is consistent with the bimolecular isomerization mechanism and autocatalytic process proposed previously. Of the coke molecules investigated, only fluoranthene is unstable in the FER pores. This suggests that fluoranthene is not completely located in the zeolite pores, and it is proposed that this molecule is formed at the pore mouth. The results of simulated diffusion calculations confirmed the minimum energy locations of biphenyl and naphthalene found by docking and were consistent with experimental data regarding the mobility of these two species in ferrierite.

RUB-23: a new microporous lithosilicate containing spiro-5 building units

A new microporous lithosilicate, Cs 10(Li,H) 14[Li 8Si 40O 96]·12H 2O, was synthesised under hydrothermal conditions. The crystal structure has been solved and refined from single-crystal data in the space group P2 1 to R=14%. The three-dimensional (3D) silicate framework contains lithium as cations on T sites in the centre of spiro-5 units that are charge-balanced by non-framework caesium, lithium and hydrogen cations. The hydrate water in the pores can be removed, and the structure is stable up to ∼300°C. The experimental density of the as-synthesised crystal is 2.62 g cm −3; the framework density is calculated to be 17.7 T/1000 Å. The channel system is one-dimensional for 10-membered ring (10-MR) channels which are connected to 8-MRs running parallel to the 10-MRs. The connection is established by 8-MR pore openings. In principle, this allows the 3D diffusion of (at least) small molecules. Chemical analysis, 29Si, 7Li and 133Cs solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, infrared (IR) spectroscopy and X-ray structure analysis are all consistent and agree with crystal–chemical reasoning based on the flexibility of [TO 4] units in silicates which are required to build 3-MRs as observed in spiro-5 units.

Vibrational spectroscopy of H 2, N 2, CO and NO adsorbed on H, Li, Na, K-exchanged ferrierite

The vibrational spectroscopy of H 2, N 2, CO and NO adsorbed on the zeolite ferrierite is described. The spectral modifications induced by the adsorption process on the protonic form are compared with those observed on the alkaline-exchanged forms. Different environments of Brønsted sites and of alkaline counterions in ferrierite are evidenced, which differ in the local electric field associated with the cationic species. These different environments are associated with the cation locations in the channels and cavities. In particular, ions located in the 10-membered ring channels are more available for the interaction with CO, N 2 and H 2, while ions in the cages form weaker adducts. On passing from lithium-, to sodium- and to potassium-exchanged samples, the local fields probed by H 2, N 2 and CO are increasingly dependent upon the distribution of the ions in the framework. In K-ferrierite the interaction cannot be solely described in terms of 1:1 K +/B (B=N 2, CO) adducts. As far as H-ferrierite is concerned, it is inferred from the observed shift of the ν(OH) mode upon interaction with H 2, N 2 and CO that the bridged strong acid groups show an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta.

FTIR investigation of the interaction at 77 K of diatomic molecular probes on MCM-22 zeolite

Three different Si(OH)Al sites have been singled out in MCM-22, absorbing at 3628, 3618 and 3585 cm −1, tentatively assigned to hydroxyls located in supercages, 10-membered ring channels and six-membered rings, respectively. FTIR study of the adsorption of CO and N 2 at the nominal temperature of 77 K allowed the measurement of their acidity, which appears close to that measured for H-ZSM-5 by means of the same technique, and it is not modified by dealumination. Lewis sites are present in small amounts in vacuum-activated samples. After dealumination, no Lewis sites are present, being extraframework Al 3+ species coordinatively saturated by OH groups.

Characterization and Catalytic Properties of Hydrothermally Dealuminated MCM-22

MCM-22 has been synthesized and dealuminated by hydrothermal treatments. The resulting solids were characterized by different techniques in order to know how the acid properties of the dealuminated solids were modified. It appears that the number of acid sites was decreased by hydrothermal treatments but the acid strength was not modified. Nondealuminated and dealuminated solids were used in the n-octane hydroconversion. Results suggested that for both types of solids the hydroisomerization is mainly occurring in the 10-membered ring (MR) channels whereas the hydrocracking is mainly coming from acid sites located in 12-membered ring channels. According to the observed distribution of monobranched isomers, MCM-22 shows some “shape-selective” character typical of the pentasil family, whereas, according to the absolute yield of isopentane in the hydrocracked products at low hydrocracking conversions, “MCM-22 behaves more like a 12 MR zeolite” (6). Similar conclusions were proposed in (7).