10-membered Ring Channels Research Articles

A Zinc Phosphate Possessing Ladder-like Layers Made Up of Three- and Four-Membered Rings and Infinite Zn−O−Zn Chains

A new open-framework zinc phosphate, [NH3(CH2)2NH(CH2)2NH3]2+2[Zn2PO4(HPO4)]-, possessing a layered architecture and consisting of alternating inorganic and organic layers, stabilized by multipoint hydrogen bonding has been synthesized hydrothermally. It has the following crystal data: monoclinic space group C2/c, a = 25.075(5) Å, b = 5.127(1) Å, c = 17.726(4) Å, β = 125.4(1)°, V = 1857.6(6) Å3, Z = 8, M = 748.3(1), Dcalc = 2.06 g cm-3, MoKα, and RF = 0.04. The structure is made up of ZnO4 and PO4 tetrahedra linked though vertexes. The connectivity between the two units creates a layered structure with steps, the layers consisting of three- and four-membered rings only. This is the first instance of such a network in open-framework zinc phosphates. The exclusive presence of three-membered ring chains creates steps in the layers as well as an infinite number of Zn−O−Zn one-dimensional chains. Hydrogen bond interactions between the inorganic layers give rise to pseudo 10-membered ring channels, in which the diprotonated diethyelenetriamine molecules reside. The unusual T atom (T being Zn or P) connectivity in this material suggests the possibility of synthesizing many such materials with novel connectivities and networks.

Selective formation of p-xylene with disproportionation of toluene over MCM-22 catalysts

Disproportionation of toluene has been studied over dealuminated MCM-22 zeolites. The dealumination of MCM-22 was performed by acid reflux and the combination of calcination or steaming with the subsequent acid reflux. These treatments effectively eliminated Al atoms from the framework site but left most of them in the crystallites as non-acidic extra-framework species. The activity of MCM-22 for the toluene disproportionation measured at relatively low temperatures (473–573 K) was comparable to that of mordenite and much higher than that of ZSM-5. Toluene disproportionation was found to occur mainly within the supercages of MCM-22 but not in the 10-membered ring (10-MR) channels of MCM-22 at these temperatures. The fraction of p -xylene in xylene isomers ( para -selectivity) formed on MCM-22 was higher than its equilibrium value, and increased further by the dealumination. p -Xylene formed within the supercages as a primary product would be isomerized to o - and m -xylene by the acid sites not only on the external surface but also within the 10-MR channels. The improvement in para -selectivity by the dealumination resulted from the suppression of isomerization activity probably because the dealumination treatments eliminated framework Al atoms predominantly on the external surface and inside the 10-MR channels compared with those inside the supercages.

Cracking Behavior of Zeolites with Connected 12- and 10-Member Ring Channels: The Influence of Pore Structure on Product Distribution

n-Heptane has been cracked on a CIT-1 zeolite which has connected 12- and 10-member ring (MR) channels, and its behavior was compared with that of MCM-22 with nonconnected 12- and 10-MR channels, and SSZ-24 and BETA with unidirectional and tridirectional 12-MR channels, respectively. The kinetic rate constant is highest for CIT-1, and the decay constant is lowest. From the selectivity point of view, its behavior can be better represented by a system with large cavities (the intersections between the 12- and 10-MR) connected by windows. This gives a behavior typical of that of large pore zeolites. CIT-1 produces a remarkably high selectivity toi-C4, and specially to isobutane. This zeolite shows promise as an additive in FCC operations.

Synthesis, electron paramagnetic resonance and electron spin echo modulation studies on synthesized NiAPSO-41 molecular sieve and comparison with ion-exchanged NiH-SAPO-41 molecular sieve

A. M. Prakash, M. Hartmann and L. Kevan, J. Chem. Soc., Faraday Trans., 1997, 93, 1233 DOI: 10.1039/A606398E

Adsorption and catalytic properties of MCM-22: The influence of zeolite structure

A combination of infrared spectroscopy and adsorption and microcalorimetric studies of molecules with different kinetic diameters, such as toluene, meta- and orthoxylene, and 1,2,4-trimethylbenzene, together with consecutive adsorption of these molecules, confirms the presence in MCM-22 of 10-membered ring channels and cavities of larger dimensions, both of which can be reached from the external surface. From the kinetic diameters of the adsorbates the dimension of the two types of void species has been estimated. The implications of pore topology on catalysis have been discussed on the basis of selectivity results obtained during alkylation of toluene by methanol and ethanol. The presence of the two types of unconnected cavities determines the ratios of para- to ortho isomers as well as the ratio of mono- to dialkylates.

Molecular shape selectivity of EUO zeolites

The micropores of the EUO zeolite consist of 10-membered ring (10-MR) channels with deep side-pockets circumscribed by 12-MRs. In decane isomerization and hydrocracking EUO zeolites crystallized in the presence of hexamethonium (HM) and dibenzyldimethylammonium (DBDMA) exhibit different molecular shape selective properties. The catalytic sites of EUO specimens crystallized with DBDMA are located in more spacious cavities than those of EUO crystals synthesized with HM. This disparity in molecular shape selectivity is explained by a different location of the framework aluminium atoms in the two materials, viz. by the absence of aluminium in the 10-MR channels in EUO-DBDMA. Such aluminium siting is in agreement with the location of the organic template cations in the pores of the as-synthesized zeolites suggested in literature and revealed by a molecular graphics modeling.

Adsorptive separation of para-alkylaromatic hydrocarbons

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).

Coke Formation and Its Effects on Shape Selective Adsorptive and Catalytic Properties of Ferrierite

Channels or cavities of ferrierite are blocked by carbonaceous deposits (coke) which are formed during butene treatments. The pore blocking inside ferrierite/alumina catalysts affects the yield and selectivity to isobutylene in the catalytic reaction of butene isomerization. Pore size distribution experiments show that the blocking of 10-member ring channels (4.2 x 5.4 A) and 8-member ring channels (3.5 x 4.8 A) of ferrierite by coke reduces the channel size smaller than that of the nitrogen molecule (4.09 A). TPD data show that ammonia uptake for coked samples of different times on stream is decreased from about 62% to 35% of that for the fresh sample. Uptakes for more bulky molecules such as 1-butene (2.99 x 4.71 A) and isobutylene (3.28 x 4.14 A) are severely reduced by coke formation (< 9% of 1-butene and isobutylene uptakes for the fresh sample). About 76.9% of the coke formed after 18 h is deposited inside the micropores (< 10 A) of ferrierite/alumina. Modification of pore shapes through such coke deposition favors reactions involving small molecules, such as butene isomerization to isobutylene. 45 refs., 12 figs., 4 tabs.

Oxyfluorinated Microporous Compounds: V. Synthesis and X-Ray Structural Determination of ULM-3, a New Fluorinated Gallophosphate Ga3P3O12F2, H3N(CH2)3NH3, H2O

Ga3P3O12F2, H3N(CH2)3NH3, H2O was obtained by hydrothermal synthesis (453 K, 24 hr, autogeneous pressure) from a mixture of Ga2O3, P2O5, HF, 1,3-diaminopropane, and H2O in the ratio 1:1:2:1.3:80. It is orthorhombic (space group Pbca (No. 61) with a = 10.154(1)Å, b = 18.393(2)Å, c = 15.773(2) Å, V = 2945.8(9) Å, Z = 8. The three-dimensional network is built up from corner-linked [Ga3(PO4)3F2] units composed of three PO4 tetrahedra, two GaO4F trigonal bipyramids, and one GaO4F2 octahedron. Fluorine atoms are shared between the octahedron and the bipyramid. The framework delimits 10-membered ring channels along [100] and 8-membered ones along [101] and [10-1]. The amine and the water molecule are inserted in the 10-membered ring channels. The hydrogen bonding scheme is discussed and shows that the water is principally linked to the amine within the tunnel and that the template might be the amine monohydrate. The thermal behavior of the title compound is studied by TGA and X-ray thermodiffractometry.