Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption.

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH4)2K2[V4(µ2-OH)4(ox)4(pz)4]·9H2O (1, ox = oxalate and pz = pyrazole) and (NH4)2Na2[V4(µ2-OH)4(ox)4(4-mpz)4]·7H2O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K2[V6(µ2-OH)6(ox)6(Hdatrz)6]Cl2·29.5H2O (3) and [V8(µ2-OH)8(SO3)8(Hdatrz)8]·38H2O (4, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V4(µ2-OH)4} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V6(µ2-OH)6} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V8(µ2-OH)8(SO3)8} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO2 and O2 more favorably than N2, CH4 and H2 gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO2 adsorption.

Gas Adsorption of Mixed-Valence Trinuclear Oxothiomolybdenum Glycolates.

Trinuclear oxothiomolybdenum(IV) glycolates (H2glyc, glycolic acid) with 2-methylimidazole (2-mim), 4-methylimidazole (4-mim), and sulfite, Na2[MoIV3(µ3-S)(µ2-O)3(glyc)3(2-mim)3]·1.5H2O (1), (4-Hmim)6[MoIV3(µ3-S)(µ2-O)3(glyc)3(4-mim)3]2[MoVIO2(glyc)2] (2), and Na3(4-Hmim)[MoIV3(µ3-S)(µ2-O)3(SO3)(glyc)3(4-mim)]·8H2O (3), have been isolated in reduced media, where 4-methylimidazole trinuclear oxothiomolybdenum(IV) glycolates in 2 coprecipitate with dioxomolybdenum(VI) glycolate, exhibiting unusual mixed valences of 4+ and 6+. Large downfield shifts of glycolates have been observed in solid-state and solution 13C (1H) NMR spectra with coordination to Mo, indicating obvious dissociation of soluble 1 and 3 in solution. Investigations of the coordination modes and conversions among the three complexes give insight into the reactivities of trinuclear oxothiomolybdenum(IV) complexes. Channels with 3.1 × 7.0 Å2 diameters exist in 2, showing reversible O2 absorption of 65.03 mg at 29.9 bar compared with little or no adsorption of N2, H2, CO2, and CH4 at room temperature, respectively. Moreover, trinuclear 2- or 4-methylimidazole oxothiomolybdenum(IV) glycolates 1 and 3 show only a few adsorptions for O2 under the same conditions.

Formation of N-oxido copper ethylenediaminetetraacetate and propanediaminetetraacetate and their selective degradation to iminodiacetate and propanediaminediacetate.

N-Oxido copper(ii) ethylenediaminetetraacetate Na4n[Cu2(edtaO2)2(H2O)4]n·13nH2O (2) (H4edta = ethylenediaminetetraacetic acid, C10H16O8N2) and N-oxido copper(ii) 1,3-propanediaminetetraacetate Na5nOn[Cu2(HpdtaO2)2Cl]n·12.5nH2O (4) (H4pdta = 1,3-propanediaminetetraacetic acid, C11H18O8N2) were obtained from the reactions of copper(ii) edta and pdta respectively with hydrogen peroxide. The copper ions in 2 and 4 are hexa-coordinated by edtaO2 or pdtaO2 ligands, forming 1D chain structures. Further reactions of 2 and 4 at lower pH values result in the isolation of copper(ii) iminodiacetate K[Cu(ida)(H2O)2Cl] (3) (H2ida = iminodiacetate acid, C4H7O4N) and copper(ii) propanediaminediacetate [Cu2(pdda)2]n·nH2O (5) (H2pdda = propanediaminediacetic acid, C7H10O4N2), respectively, which show the selective degradation of ethylenediaminetetraacetate and propanediaminetetraacetate.

Carbonate and phosphite encaged in frameworks constructed from square lanthanum aminopolycarboxylates and sodium chloride.

Novel additives of lanthanum aminopolycarboxylates with inorganic anions, Na12n[La(edta)L]4n·8nNaCl·4nH2O (1: L = HPO32-; 2: L = CO32-) and K12n[La(cdta)(CO3)]4n·35nH2O (3) (H4edta = ethylenediaminetetraacetic acid; H4cdta = cyclohexanediaminetetraacetic acid), were obtained in alkaline solution. Structural analyses reveal that 1 and 2 are isomorphous and contain interesting square structures. HPO32- (CO32-) was encaged in the constructed tetranuclear frameworks. Tetranuclear lanthanum ethylenediaminetetraacetate was further encaged in superstructures of sodium chloride. 3 has a similar square structure, in which edta is replaced by cdta. All complexes are fully characterized via elemental, FT-IR, NMR, thermogravimetric and structural analyses. Solution 13C NMR spectra show that 1 and 2 dissociate into mononuclear units in water. Interestingly, 2 possesses 3.7 Å diameter holes inside its crystals, which can adsorb a small amount of O2 or CO2 selectively. The amounts of O2 and CO2 adsorbed increase gradually from 0.32 and 0.38 mg g-1 at 0.4 bar to 15.90 and 10.54 mg g-1 at 29.9 bar, respectively.

Photoinduced Formation of Peroxide Ions on La2O3 and Nd2O3 under O2: Effects of Excitation Wavelength and Crystal Structure.

Photoinduced formation of peroxide ions on La2O3 and Nd2O3 under O2 was studied by in-situ microprobe Raman spectroscopy with attention focused on the effect of excitation wavelength and crystal structure on the O2(2-) formation. It was found that photoexcitations at 633, 532, 514, and 325 nm can induce O2(2-) formation over La2O3 at 450 °C. By contrast, photoexcitation at 785 nm does not cause formation of O2(2-) up to 500 °C. Photoexcitation at 325 nm can induce O2(2-) formation on cubic Nd2O3 at 25 °C, but cannot induce O2(2-) formation on hexagonal Nd2O3 up to 200 °C. The significant difference in the behavior of O2(2-) formation over the Nd2O3 samples of the two structures can be related to the difference in the capacity to adsorb O2. Since the number of oxygen vacancies in cubic Nd2O3 is larger than that in the hexagonal one, the former has a higher capacity than the latter to adsorb O2. As a result, cubic Nd2O3 is more favorable to the reaction of O2 with O(2-) to generate O2(2-). The structural similarity between cubic Nd2O3 and Nd2O2(O2) may be another factor in favor of peroxide formation.

Dimeric 1,3-propanediaminetetraacetato lanthanides as the precursors of catalysts for the oxidative coupling of methane.

From neutral solutions, dimeric 1,3-propanediaminetetraacetato lanthanides (NH4)2[Ln2(1,3-pdta)2(H2O)4]·8H2O [Ln = La, 1; Ce, 2] and K2[Ln2(1,3-pdta)2(H2O)4]·11H2O [Ln = La, 3; Ce, 4] (1,3-H4pdta = 1,3-propanediaminetetraacetic acid, C11H18N2O8) were isolated in high yields. The reaction of excess strontium nitrate with 1 resulted in the formation of a two dimensional coordination polymer [La2(1,3-pdta)2(H2O)4]n·[Sr2(H2O)6]n·[La2(1,3-pdta)2(H2O)2]n·18nH2O (5) at 70 °C. Complexes 1-4 show a similar central molecular structure. The lanthanide ions are coordinated by two nitrogen atoms, four carboxy oxygen atoms from one 1,3-pdta ligand, two from the neighboring 1,3-pdta ligand forming a four-membered ring and two water molecules. Complex 5 has two kinds of dimeric lanthanum unit and extends into a 2D coordination polymer through strontium ions and bridged oxygen atoms, and forms a fourteen membered ring linked by oxygen atoms from carboxy groups of pdta. Complexes 1-4 are soluble in water. The (13)C{(1)H} NMR experiments for complex 1 were tested in solution. Thermal products from 1 and 5 show good catalytic activities towards the oxidative coupling reaction of methane (OCM). The conversion of methane and selectivity to C2 reached 29.7% and 51.7% at 750 °C for the product of 5. From TGA, XRD and SEM analyses, the thermal products from 1 and 5 are rod- and poly-shaped, which are assigned as lanthanum oxocarbonate and a mixture of La2O3, SrCO3 and La2O2CO3 for 1 and 5, respectively. The precursor method is favorable for the formation of regular shaped mixed oxides.

Effect of calcination temperature and pretreatment with reaction gas on properties of Co/γ-Al2O3 catalysts for partial oxidation of methane.

The effects of calcination temperature and feedstock pretreatment on the catalytic performance of Co/γ-Al(2)O(3) catalysts were studied for partial oxidation of methane (POM) to synthesis gas, with emphasis on the role of feedstock pretreatment. The physicochemical properties of the catalysts were characterized by N(2) adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), H(2) temperature-programmed reduction (H(2) -TPR), and Raman spectroscopy. The results showed that the pretreatment of the catalyst by reaction gas significantly improved the catalytic activity and stability for the POM reaction. On the other hand, the effect of calcination temperature was less significant. Although the initial activity was increased by an increased calcination temperature, the catalyst without the feedstock pretreatment suffered a rapid deactivation. The reaction-atmosphere pretreatment was revealed as a process that mainly modified the surface structure of the catalyst. In that process, the formation of a CoAl(2)O(4) -like compound led to high Co metal dispersion after reduction, and the transformation of the carrier into α-Al(2)O(3) occurred over the catalyst surface. Both the high dispersion of cobalt and the presence of α-Al(2)O(3) surface phase were assumed as the important factors resulting in an excellent catalytic performance in terms of high activity and high stability.

Mechanistic aspects of photo-induced formation of peroxide ions on the surface of cubic Ln2O3 (Ln = Nd, Sm, Gd) under oxygen.

The photo-induced formation of peroxide ions on the surface of cubic Ln2O3 (Ln = Nd, Sm, Gd) was studied by in situ microprobe Raman spectroscopy using a 325 nm laser as excitation source. It was found that the Raman bands of peroxide ions at 833-843 cm(-1) began to grow at the expense of the Ln(3+)-O(2-) bands at 333-359 cm(-1) when the Ln2O3 samples under O2 were continuously irradiated with a focused 325 nm laser beam at temperatures between 25-150 °C. The intensity of the peroxide Raman band was found to increase with increasing O2 partial pressure, whereas no peroxide band was detected on the Ln2O3 under N2 as well as on the samples first irradiated with laser under Ar or N2 followed by exposure to O2 in the dark. The experiments using (18)O as a tracer further confirmed that the peroxide ions are generated by a photo-induced reaction between O2 and the lattice oxygen (O(2-)) species in Ln2O3. Under the excitation of 325 nm UV light, the transformation of O2 to peroxide ions on the surface of the above lanthanide sesquioxides can even take place at room temperature. Basicity of the lattice oxygen species on Ln2O3 also has an impact on the peroxide formation. Higher temperature or laser irradiation power is required to initiate the reaction between O2 and O(2-) species of weaker basicity.

In situ Raman and pulse reaction study on the partial oxidation of methane to synthesis gas over a Pt/Al2O3 catalyst.

Catalytic partial oxidation of methane (POM) to synthesis gas (syngas) over Pt/Al(2)O(3) was investigated by in situ microprobe Raman and pulse reaction methods with attention focused on the mechanism of syngas formation in the oxidation zone (i.e., the catalyst zone in which O(2) was still available in the reaction feed). It was found that the amount of platinum oxide in the catalyst under POM conditions was below the detection level of Raman spectroscopy. Raman bands of carbon species that originated from methane dissociation were detected at the entrance of the catalyst bed under working conditions. The results of the pulse reaction study on POM as well as steam and CO(2) reforming of methane at 700 °C with a contact time of less than 1 ms over the catalyst suggest that pyrolysis of methane on reduced platinum sites followed by coupling of two surface hydrogen atoms to H(2) and partial oxidation of surface carbon species to CO are the major reactions responsible for syngas formation in the oxidation zone. Under the experimental conditions, steam and CO(2) reforming of methane play only a minor role in syngas formation in the same reaction zone. The contribution of the last two reactions increases with increasing contact time.

FTIR studies of zeolite matrix effects on the properties of palladium clusters confined in the supercages of NaX and NaY.

Palladium clusters have been synthesized by the "ship-in-a-bottle" approach in the supercages of NaX and NaY faujasite zeolites. In comparison with CO adsorbed on a bulk Pd electrode, the same molecule adsorbed on the Pd clusters electrodes evoked an enhanced IR absorption (EIRA). The enhancement factors have been determined to be about 38 and 51 in NaX and NaY, respectively. IR band centers of linear-bonded CO, bridge-bonded CO, and multi-bonded CO in NaX are measured, respectively, 12, 14, and 11 cm(-1) lower than those of the corresponding adsorption modes in NaY. The adsorption of CO and the oxidation of adsorbed CO in NaX matrix are faster than that in NaY matrix. These results suggest that part of the Pd2+ ions in NaX are located in sites III and III' that are near the 12-ring window of the supercage of zeolite, which lead to the formation of small Pd clusters. The present study is of significant importance in exploring the dependence of catalyst properties on structures, as well as in understanding and predicting the locations and properties of metal clusters in zeolites.

Mechanistic study of partial oxidation of methane to syngas using in situ time-resolved FTIR and microprobe Raman spectroscopies.

In situ time-resolved Fourier transform infrared (FTIR) and microprobe Raman spectroscopies were used to characterize the reaction mechanisms of the partial oxidation of methane to syngas over SiO(2)- and gamma-Al(2)O(3)-supported rhodium and ruthenium catalysts. The interaction of both pure methane and a methane/oxygen mixture at a stoichiometric feed ratio with an oxygen-rich catalyst surface led to the formation of CO2 and H(2)O as the primary products. For the H(2)-pretreated samples, the reaction mechanisms with the catalysts differ. Only Rh/SiO(2) is capable of catalyzing the direct oxidation of methane to syngas, while syngas formation over Rh/gamma-Al(2)O(3), Ru/SiO(2), and Ru/gamma-Al(2)O(3) can be achieved mainly via a combustion-reforming scheme. The significant difference in the mechanisms for partial oxidation of methane to syngas over the catalysts can be correlated to the differences in the concentration of oxygen species (O(2-)) on the catalyst surface during the reaction, mainly due to the difference in the nature of the metals and supports.