Zdenek Moravec

A Structurally Diverse Series of Aluminum Chloride Alkoxides [ClxAl(μ-OR)y]n (R = nBu, cHex, Ph, 2,4-tBu2C6H3)

A diverse series of aluminum chloride alkoxides, [Cl(x)Al(mu-OR)(y)](n) (R = (n)Bu, (c)Hex, Ph, 2,4-(t)Bu(2)C(6)H(3)), was synthesized using the reactions of dichlorethylalane (EtAlCl(2)) with cyclohexanol ((c)HexOH), n-butanol ((n)BuOH), and phenols (PhOH and 2,4-(t)Bu(2)C(6)H(3)OH). Eight molecular products were isolated and structurally characterized. The dimeric [Cl(2)Al(mu-O(c)Hex)(2)AlCl(2)] (1) was the smallest oligomer isolated among the cyclohexanolate derivatives. The adduct of 1 with cyclohexanol is a dinuclear molecule [Cl(2)(HO(c)Hex)Al(mu-O(c)Hex)(2)AlCl(2)] (2) which represents a possible intermediate in the conversion reaction leading to the formation of a trinuclear bicyclic [ClAl{(mu-O(c)Hex)(2)AlCl(2)}(2)] (3). Two polymorphic forms of 3 were isolated. Further coordination of cyclohexanol to the Lewis acidic five-coordinate aluminum atom in 3 provided [Cl(HO(c)Hex)Al{(mu-O(c)Hex)(2)AlCl(2)}(2)] (4) with octahedrally coordinated central aluminum. Compound 4 could be regarded as a precursor to the well-known Mitsubishi (tridiamond) tetranuclear species. The reactions of EtAlCl(2) with less sterically demanding (n)BuOH yielded a cyclic trimer, [Cl(2)Al(mu-O(n)Bu)](3) (5), and a unique trinuclear ionic species, [Cl(2)Al{(mu-OH)(mu-O(n)Bu)AlCl(HO(n)Bu)(3)}(2)]Cl (6) with a linear Al(mu-O)(2)Al(mu-O)(2)Al core. In the reactions with phenols, the aromatic groups preferentially stabilized dimeric structures of [Cl(2)Al(mu-OR)(2)AlCl(2)] (R = Ph, 7; 2,4-(t)Bu(2)C(6)H(3), 8). Since these compounds could be considered as intermediates in the nonhydrolytic condensation reactions of metal halides with metal alkoxides, a mixture of EtAlCl(2) with (c)HexOH was used as a precursor for the nonaqueous synthesis of alumina by alkylhalide elimination.

Non-aqueous template-assisted synthesis of mesoporous nanocrystalline silicon orthophosphate

The first synthesis of mesoporous nanocrystalline silicon orthophosphate Si5P6O25 is presented. The synthetic procedure is based on the non-hydrolytic sol–gel reaction in the presence of Pluronic P123 template and subsequent calcination in air. The condensation of silicon acetate, Si(OAc)4, and tris(trimethylsilyl)phosphate, OP(OSiMe3)3 (TTP), in non-aqueous solvents driven by elimination of trimethylsilyl acetate provides a homogeneous network with a high content of Si–O–P bonds and SiO6 moieties. After burning out the template, mesoporous silicon orthophosphate was obtained with surface areas up to 128 m2 g−1 and pore sizes around 20 nm. The nanocrystalline Si5P6O25 phase forms relatively easily (500 °C, 4 h) in comparison with other synthetic routes. All samples were characterized by SEM, TEM, elemental analysis, TGA, nitrogen adsorption, SAXS, 1H, 13C, 29Si, and 31P solid-state NMR spectroscopy, and powder XRD. These xerogels showed superior catalytic activity and selectivity in methylstyrene dimerization.

Novel non-hydrolytic templated sol–gel synthesis of mesoporous aluminosilicates and their use as aminolysis catalysts

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous aluminosilicate xerogels is presented. The polycondensation between silicon acetate, Si(OAc)4, and tris(dimethylamido)alane, Al(NMe2)3, leads to homogeneous aluminosilicate xerogels containing Si–O–Al linkages through dimethylacetamide elimination. The addition of Pluronic P123 and F127 templates provides stiff gels that are, after calcination at 500 °C, converted to stable mesoporous xerogels with a high surface area (>600 m2 g−1) and wormhole-type pores (d = 5.9 nm). The xerogels exhibit high catalytic activity in aminolysis of styrene oxide (82% conversion) with the turnover frequency up to 100.

Control of micro/mesoporosity in non-hydrolytic hybrid silicophosphate xerogels

Non-hydrolytic sol–gel reactions of acetoxysilanes with trimethylsilyl esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicon and phosphoryl groups connected together by networks of Si–O–P(O) linkages. These polycondensation reactions proceed cleanly and under mild conditions for a wide variety of precursor silanes RnSi(OAc)4−n (R = alkyl, aryl; n = 1, 2) and phosphoryl compounds RP(O)(OSiMe3)2 (R = alkyl, aryl) to provide hybrid xerogels, the final properties of which are a sensitive function of the organic substituents and the Si : P ratio of the precursors. The reactions of bridged acetoxysilanes (AcO)3Si–X–Si(OAc)3 and phosphoryl reagents (Me3SiO)2P(O)–X–P(O)(OSiMe3)2 have also been investigated and found to produce gels that exhibit large surface areas (up to 700 m2 g−1). The presence of SiO6 structural units in bridged-phosphoryl xerogels is related to their microporosity while the absence of such moieties in bridged-acetoxysilane networks is congruent with significant mesoporosity. Several important parameters are identified which can be used to tailor the properties of these hybrid matrices such that gels with specific polarity, porosity and surface area can be targeted at the time of synthesis.

Construction of Larger Molecular Aluminophosphate Cages from the Cyclic Four-Ring Building Unit

New molecular aluminophosphates of different nuclearity are synthesized by a stepwise process and structurally characterized. The alkane elimination reaction of bis(trimethylsiloxy)phosphoric acid, OP(OH)(OSiMe3)2, with trialkylalanes, AlR3 (R = Me, Et, (i)Bu), provides the cyclic dimeric aluminophosphates, [(AlR2{μ2-O2P(OSiMe3)2})2] (R = Me (1), Et (2), (i)Bu (3)). Unsymmetrically substituted cyclic aluminophosphonate [(AlMe2{μ2-O2P(OSiMe3)((c)Hex)})2] (cis/trans-4) is prepared by dealkylsilylation reaction of (c)HexP(O)(OSiMe3)2 with AlMe3. Molecules 1-4 containing the [Al2(μ2-O2P)2] inorganic core are structural and spectroscopic models for the single four-ring (S4R) secondary building units (SBU) of zeolite frameworks. Compound 1 serves as a starting point in construction of larger molecular units by reactions with OP(OH)(OSiMe3)2 as a cage-extending reagent and with diketones, such as Hhfacac (1,1,1,5,5,5-hexafluoropentan-2,4-dione) and Hacac (pentan-2,4-dione), as capping reagents. Reaction of 1 with 4 equiv of Hhfacac leads to new cyclic aluminophosphate [(Al(hfacac)2{μ2-O2P(OSiMe3)2})2] (5), existing in two isomeric (D2 and C2h) forms. Reaction of 1 with 2 equiv of OP(OH)(OSiMe3)2 and 1 equiv of Hhfacac provides a molecular aluminophosphate [AlMe{Al(hfacac)}2{μ3-O3P(OSiMe3)}2{μ2-O2P(OSiMe3)2}2{OP(OSiMe3)3}] (6), while by adding first the Hhfacac and using 3 equiv of OP(OH)(OSiMe3)2 we isolate [Al{Al(hfacac)}2{μ3-O3P(OSiMe3)}2{μ2-O2P(OSiMe3)2}2H{OP(O)(OSiMe3)2}2] (7). These molecules contain units in their cores that imitate 4=1 SBU of zeolite frameworks. Reaction with the order of component mixing 1, Hhfacac, OP(OH)(OSiMe3)2 at a 1:2:2 molar ratio lead to formation of a larger cluster [(Al(AlMe){Al(hfacac)}{μ3-O3P(OSiMe3)}2{μ2-O2P(OSiMe3)2}3)2] (8) containing both S4R and 4=1 structural units. Similarly, Hacac (pentan-2,4-dione) provides an isostructural [(Al(AlMe){Al(acac)}{μ3-O3P(OSiMe3)}2{μ2-O2P(OSiMe3)2}3)2] (9). Both molecules display Al centers in three different coordination environments.

Mesoporous SnO2–SiO2 and Sn–silica–carbon nanocomposites by novel non-hydrolytic templated sol–gel synthesis

A novel non-hydrolytic sol–gel (NHSG) synthesis of mesoporous tin silicate xerogels is presented. The polycondensation between silicon tetraacetate, Si(OAc)4, and tetrakis(diethylamido)tin, Sn(NEt2)4, resulting in acetamide elimination leads to tin silicate xerogels containing Si–O–Sn linkages. The addition of Pluronic P123 or F127 templates provides homogeneous stiff gels that are, after template removal by calcination at 500 °C in air, converted to stable mesoporous silica xerogels with large surface areas (476 m2 g−1) and dispersed SnO2 nanoparticles (6–7 nm). Heat treatment of the as-prepared tin silicate gels in an inert N2 atmosphere leads to reduction and transformation to Sn nanoparticles (70–150 nm) embedded in a silica–carbon matrix. The composition and morphology of the xerogels, volatile reaction byproducts, and thermal transformations were followed by elemental analyses, IR spectroscopy, thermal analysis TG-DSC, nitrogen adsorption measurements, solid-state NMR spectroscopy, DRUV-vis spectroscopy, electron microscopy, and HT powder XRD. The SnO2–SiO2 xerogels were tested as potential catalysts for aminolysis of styrene oxide with aniline and for the Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone. The resulting reaction systems displayed good activity and selectivity.

Sonochemical precipitation of amorphous uranium phosphates from trialkyl phosphate solutions and their thermal conversion to UP2O7

Insoluble amorphous precipitates containing uranyl and phosphate ions are obtained by sonication of solutions of three uranyl precursors, UO2(X)2, X=NO3, CH3COO, CH3C(O)CHC(O)CH3 (acetylacetonate, acac), in triesters of phosphoric acid, OP(OR)3, R=Me (trimethyl phosphate, TMP), Et (triethyl phosphate, TEP). TMP and TEP are used as high-boiling solvents and they serve also as a source of phosphate anions. Sonolysis experiments were carried out under flow of Ar at 40°C on a Sonics and Materials VXC 500W system (f=20 kHz, Pac=0.49 W cm(-3)). Powder X-ray diffraction (PXRD) reveals amorphous character of all obtained precipitates. The presence of uranyl and phosphate is evidenced by IR spectroscopy and ICP-OES analysis reveals the content of both U (38.6-43.4 wt%) and P (11.0-13.6 wt%). The thermal behavior of the substances was studied by TG/DSC analysis, which shows weight losses in the range of 19.21-24.08%. On heating the amorphous precipitates to 1000°C, crystalline uranium diphosphate UP2O7 is obtained in all cases as the only crystalline phase. Uranyl(VI) is reduced during thermolysis to U(IV) as there is no characteristic vibration of UO2(2+) in the IR spectra of solid UP2O7 products. The ICP-OES analysis of U and P content in precipitates allowed us to calculate the efficiency of precipitation of uranium from mother liquor and to compare it with the efficiency calculated from the data received by the PXRD and TG/DSC analyses. The efficiency of the uranium removal attained by our sonoprecipitation procedure was typically 30-35%. These sonochemical precipitation reactions providing insoluble uranium phosphates may be potentially interesting models for the description of behavior of uranium-containing waste or reprocessing streams.

Surface reactivity of non-hydrolytic silicophosphate xerogels: a simple method to create Brønsted or Lewis acid sites on porous supports

Non-hydrolytic sol-gel reactions of silicon acetates with trimethylsilyl (TMS) esters of phosphoric and phosphonic acids produce cross-linked matrices containing homogeneous dispersions of silicate and phosphoryl groups connected together by networks of Si-O-P(QO) linkages. The condensation degrees reach 80 to 90%. Residual organic groups (10 to 20%) were reacted with a variety of compounds (H2O, Me3SiOSiMe3, POCl3, SiCl4, AlMe3, Al(NMe2)(3), and AlCl3) in order to enrich the surface of these porous matrices with Bronsted (RP-OH) and Lewis (tetracoordinated Al) acid functional groups. The differences in the reactivity of RSi-OAc and RP-OSiMe3 groups were utilized for the selective modification at the silicon and phosphorus atoms. The reaction procedures were optimized and significantly porous silicophosphate materials with a high content of either hydroxyl groups or four-coordinated aluminium species were obtained. The activity and selectivity of prepared samples as catalysts for the dimerization of a-methylstyrene were tested. Excellent activities and moderate to very high selectivities were achieved suggesting the potential use of silicophosphate xerogels in heterogeneous catalysis.

Polyhedral Metallophosphonates and silicates: Synthesis,Functionalization, and Transformations

We synthesized molecular cyclic and polyhedral precursors to aluminophosphate and silicate materials and studied their substitution and nonhydrolytic sol-gel reactions.