Electron Spin-echo Modulation Spectroscopy Research Articles

ChemInform Abstract: Synthesis and Characterization of Mn-Containing Cubic Mesoporous MCM-48 and AlMCM-48 Molecular Sieves.

Employing electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy, five Mn(II) species are found in three kinds of Mn-containing MCM-48 materials: ion-exchanged Mn−MCM-48, synthesized MnMCM-48, and synthesized MnMCM-48(Ca) with subsequent CaCl2 exchange. Mn(II) species I with a well-resolved sextet centered at g = 2.00 has distorted octahedral symmetry and is observed in hydrated ion-exchanged Mn−MCM-48 and in synthesized calcined MnMCM-48. Mn(II) species II with a group of single peaks between g = 11.32 and 2.62 with possible distorted tetrahedral symmetry and a significant zero field contribution is observed in dehydrated Mn−MCM-48 and in MnMCM-48. Mn(II) species III with a symmetric line at g = 2.00 and a line width around 410 G is assigned to tetrahedral symmetry and is observed in MnMCM-48(Ca) and in MnAlMCM-48(Ca). Mn(II) species IV with a sextet centered from 4.11 to 4.34 also has distorted octahedral symmetry and is obtained in partially dehydrated Mn−MCM-48 and Mn−AlM...

Adsorbate interactions of Cu(II) in mesoporous Cu-GaMCM-41 studied by electron spin resonance and electron spin echo modulation

Gallium-containing mesoporous MCM-41 materials (GaMCM-41) were synthesized by adjusting the gel equilibrium between the source gallosilicate mixture and a GaMCM-41 phase using an organic acid. The location of Cu(II) exchanged into the mesoporous GaMCM-41 and its interactions with various adsorbate molecules were investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The results are compared with those of Cu-AlMCM-41. In a fresh, hydrated Cu-GaMCM-41, Cu(II) is octahedrally coordinated to six water molecules and is located in a cylindrical channel and rotates rapidly at room temperature. Evacuation removes some of these water molecules, leaving the Cu(II) coordinated to fewer water molecules and anchored to oxygens in the channel wall. Dehydration produces one Cu(II) species located on the internal wall of a channel which is accessible to oxygen as evidenced by broadening of its ESR lines by oxygen. Adsorption of water, methanol, ammonia, pyridine, aniline, acetonitrile, benzene and ethylene on dehydrated Cu-GaMCM-41 materials causes changes in the ESR spectra of Cu(II), indicating complex formation with these adsorbates. Cu(II) fully coordinates with a maximum coordination number of polar adsorbates in MCM-41 mesopores. Cu(II) forms a complex with six molecules of methanol as evidenced by an isotropic room temperature ESR signal and ESEM data. Cu(II) forms square planar complexes with four molecules of ammonia, pyridine and aniline based on resolved nitrogen superhyperfine interactions and other ESR parameters. However, Cu(II) forms a complex with six molecules of acetonitrile based on the ESR parameters and ESEM data. Interestingly, Cu(II) does not fully coordinate with nonpolar molecules. Cu(II) interacts directly with only one molecule of benzene, and only part of Cu(II) interacts indirectly with one molecule of ethylene based on ESR and ESEM analyses.

Electron Spin Resonance and Electron Spin Echo Modulation Studies of Adsorbate Interactions with Cupric Ion on the Aluminum Content in Cu−AlMCM-41 Materials

Employing electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy, adsorbate interactions are studied in two types of ion-exchanged Cu−AlMCM-41 materials. One is structurally ordered Cu−AlMCM-41(60)-I, Cu−AlMCM-41(40)-I, Cu−AlMCM-41(20)-I, and the other is less structurally ordered Cu−AlMCM-41(15)-II. Four Cu(II) species are observed by ESR. Species A with ESR parameters g∥A = 2.406, A∥A = 0.0145 cm-1, and g⊥A = 2.081 is assigned to octahedral Cu(II) and is observed in hydrated Cu−AlMCM-41. Species B with g∥B = 2.317, A∥B = 0.0176 cm-1, and g⊥B = 2.076 is assigned to distorted octahedral Cu(II) and is observed in dehydrated samples. Species C with g∥C = 2.430, A∥C = 0.0125 cm-1, and g⊥C = 2.091 is also assigned to octahedral Cu(II) and is observed after adsorption of CH3OD. Species D contains square planar D1 Cu(II) with g∥D1 = 2.280, A∥D1 = 0.0168 cm-1, and g⊥D = 2.058, and distorted octahedral D2 Cu(II) with g∥D2 = 2.249 and A∥D2 = 0.0190 cm-1 after adsorption of ammonia. The...

Synthesis and Characterization of Mn-Containing Cubic Mesoporous MCM-48 and AlMCM-48 Molecular Sieves

Employing electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy, five Mn(II) species are found in three kinds of Mn-containing MCM-48 materials: ion-exchanged Mn−MCM-48, synthesized MnMCM-48, and synthesized MnMCM-48(Ca) with subsequent CaCl2 exchange. Mn(II) species I with a well-resolved sextet centered at g = 2.00 has distorted octahedral symmetry and is observed in hydrated ion-exchanged Mn−MCM-48 and in synthesized calcined MnMCM-48. Mn(II) species II with a group of single peaks between g = 11.32 and 2.62 with possible distorted tetrahedral symmetry and a significant zero field contribution is observed in dehydrated Mn−MCM-48 and in MnMCM-48. Mn(II) species III with a symmetric line at g = 2.00 and a line width around 410 G is assigned to tetrahedral symmetry and is observed in MnMCM-48(Ca) and in MnAlMCM-48(Ca). Mn(II) species IV with a sextet centered from 4.11 to 4.34 also has distorted octahedral symmetry and is obtained in partially dehydrated Mn−MCM-48 and Mn−AlM...

Electron Spin Resonance and Electron Spin Echo Modulation Studies on the Formation and Adsorbate Interactions of Ni(Ι) in Synthetic Clinoptilolite

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies were used to study the formation of Ni(Ι) by various reduction methods and various adsorbate interactions of Ni(Ι) in synthetic clinoptilolite, in which Ni(ΙΙ) was ion-exchanged into extraframework sites of clinoptilolite. Thermal reduction by dehydration at 573 K shows a single species assigned to Ni(Ι) ions. Two isolated Ni(Ι) ions are observed after hydrogen reduction at 623 K. Adsorption of methanol on hydrogen-reduced NiNaK−Clino forms a Ni(Ι)−(CD3OH)n complex resulting from the interaction of methanol with one of the isolated Ni(Ι) ions. Adsorption of ammonia on hydrogen-reduced NiNaK−Clino produces a prominent Ni(Ι)−(ND3)n complex by interaction with both Ni(Ι) ions. The 13C hyperfine structure and ESEM analysis parameters after 13CO adsorption indicate that Ni(Ι) interacts with one CO molecule and forms a Ni(Ι)−(CO)1 complex. Two Ni(Ι)−(C2D4)n complexes are generated after adsorption of ethylene on dehydrated Ni(II)NaK−Clino and subsequent heating below 623 K. At higher temperature, these species disappear with the concomitant formation of two Ni(Ι)−(C4D8)n complexes, indicating ethylene dimerization. 2D ESEM confirms the coordination of Ni(Ι) with butene. Adsorption of NO on dehydrated Ni(II)NaK−Clino produces two Ni(Ι)−NO+ complexes formed by transfer of the odd electron from NO to Ni(ΙΙ) ions. Along with two Ni(Ι)−NO+ species, another species assigned to the NO radical is also observed in NiNaK−Clino after NO adsorption on Ni(II)NaK−Clino and NaK−Clino.

Location and Adsorbate Interactions of Vanadium in VAPO-5 Molecular Sieve Studied by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies

Large-pore vanadium-incorporated aluminophosphate molecular sieve VAPO-5 has been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies to determine the vanadium location and interaction with various adsorbate molecules. As-synthesized VAPO-5 contains two paramagnetic vanadyl species A and B with distorted octahedral coordination. After calcination in O2 and exposure to moisture, only species A is observed with reduced intensity. Species A is identified as a VO(H2O)32+ complex coordinated to two framework oxygens bonded to aluminum. Species B is observed only in as-synthesized VAPO-5. Species B is suggested to be similar to species A in which one of the water ligands is replaced by hydroxyl interacting with a protonated amine. When calcined, hydrated VAPO-5 is dehydrated at elevated temperature, species A loses its water ligands and transforms to VO2+ ions coordinated to two framework oxygens (species C). Species C reduces its intensity significantly after treatment with O2 at high temperature, thus suggesting oxidation of V4+ to V5+. High-temperature H2 treatment of a previously oxidized sample shows more than one vanadium species. Weak 31P ESEM is observed in both as-synthesized and dehydrated VAPO-5. Simulation of the 31P ESEM spectrum of dehydrated VAPO-5 shows one phosphorus nucleus at 4.4 Å from the vanadium ion. Strong 27Al ESE modulations are observed in both as-synthesized and dehydrated VAPO-5, which suggests that the vanadium ion is located at sites nearer to framework aluminum than to phosphorus. These observations lead to the conclusion that vanadium incorporated into AlPO4-5 occupies a framework phosphorus site probably as a localized defect. When dehydrated VAPO-5 makes contact with D2O at room temperature, the ESR signal of species A is observed. This species is identified as a VO(D2O)32+ complex from the 2D ESEM spectrum. Adsorption of ND3 on VAPO-5 generates a new vanadium species F identified as a VO(ND3)22+ complex. Adsorption of deuterated methanol on dehydrated VAPO-5 results in another new vanadium species G, which is identified as a VO(CD3OH)22+ complex. When deuterated ethylene is adsorbed on dehydrated VAPO-5, another new vanadium species H identified as VO(C2D4)12+ is observed. Possible coordination geometries of these various complexes are discussed.

EPR and electron spin echo modulation spectroscopy of CrIII and CrV in CrAPSO-5 molecular sieve: Evidence for framework substitution

CrAPSO-5 molecular sieve was synthesized to incorporate CrIII into the molecular sieve framework using tripropylamine as a structure-directing agent. CrAPSO-5 was characterized by XRD, thermogravimetric analysis and IR spectroscopy. Treatment of as-synthesized CrAPSO-5 with a solution of CH3OH–HCl is effective in removing the tripropylamine. EPR shows CrIII with g1=5.20, g2=2.00 and g3=0.98, which is assigned to CrIII in a distorted octahedral coordination. Spectral simulation indicates a zero-field splitting parameter of 0.50 cm-1. CrIII in ion-exchanged (L)Cr-SAPO-5 also shows octahedral coordination and is located in the main channel. After dehydration, CrIII shows sharper EPR lines with g⊥=4.00 and g‖=2.00 which are assigned to CrIII in tetrahedral coordination. After calcination and dehydration, the EPR of CrAPSO-5 shows CrV in tetrahedral coordination, characterized by g⊥=1.97 and g‖=1.89. UV–VIS spectra support these coordination assignments. Electron spin echo modulation (ESEM) of 31P indicates that CrV substitutes for phosphorus in a framework site in calcined CrAPSO-5 and that CrV is in a six-ring window in dehydrated, oxidized (L)Cr-SAPO-5. Deuterium ESEM indicates, that, in CrAPSO-5, CrV interacts with one molecule of ethylene via a weak σ-bond, while in dehydrated, oxidized (L)Cr-SAPO-5, CrV interacts with one molecule of ethylene via a π-bond. These results seem consistent with framework substitution in a phosphorus site in CrAPSO-5.

Location of cupric ion and its adsorbate interactions in Cu(II)-exchanged mesoporous AlMCM-41 determined by electron spin resonance and electron spin echo modulation

The location of Cu(II) exchanged into mesoporous AlMCM-41 material and its interaction with various adsorbate molecules were reinvestigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. In contrast to previous work, it seems that Cu(II) is fully cordinated to adsorbates and interacts only with oxygen in an AlMCM-41 channel only in the absence of adsorbates. Cu(II) is octahedrally coordinated to six water molecules, as evidenced by an isotropic room temperature ESR signal. This species is located in an AlMCM-41 channel and rotates rapidly at room temperature. Evacuation at room temperature removes some of these water molecules, leaving the Cu(II) coordinated to fewer water molecules and anchored to oxygens in an AlMCM-41 channel wall. Adsorbate molecules such as water, methanol, ammonia, pyridine, aniline, acetonitrile, benzene and ethylene on dehydrated Cu-AlMCM-41 cause changes in the ESR spectrum of Cu(II), indicating complex formation with these adsorbates. Cu(II) forms a complex with six molecules of methanol, as evidenced by anisotropic room temperature ESR signal and ESEM data. Cu(II) also forms a square planar complex with four molecules of ammonia, pyridine and aniline, based on resolved nitrogen superhyperfine interaction and their ESR parameters. However, Cu(II) forms a complex with six molecules of acetonitrile, based on the ESR parameters. Only one molecule of benzene is directly coordinated to Cu(II). Only part of the Cu(II) coordinates weakly to one molecule of ethylene.

Evaluation of relative catalytic activities of Cu(II)-exchanged L, offretite and omega gallosilicates based on Cu(II) location and adsorbate interactions determined by CW and pulsed electron spin resonance

Three structurally related gallosilicates with L, offretite and omega zeolite structures were synthesized and cupric ion was ion-exchanged into each of these gallosilicates. The location of Cu(II) and its interaction with adsorbates in these Cu(II)-exchanged gallosilicates have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. In fresh hydrated state, Cu(II) is located in a main channel for these materials but with different coordination numbers to water. Cu(II) is octahedrally coordinated to six water molecules in L, while Cu(II) is coordinated to three water molecules and to framework oxygens in the main channel in offretite and omega. During evacuation at increasing temperature, Cu(II) moves from the main channel towards recessed sites. Dehydration produces predominantly one Cu(II) species located in recessed sites such as hexagonal prism for L and offertite and six-ring window of a gmelinite cage for omega based on a lack of broadening of its ESR lines by added oxygen. In L and offretite gallosilicates, there is evidence for back migration of Cu(II) from a hexagonal prism into a main channel to coordinate with adsorbates. Cu(II) in L gallosilicate forms complexes with bigger molecules such as benzene, pyridine and aniline. However, no complex formation between Cu(II) and each of these bulky molecules was observed due to a distorted 12-ring entrance in offretite which makes diffusion of these bulky molecules into the main channel difficult. In omega, the back migration of Cu(II) from a gmelinite cage to a main channel was blocked and thus coordination was possible only with small molecules which can diffuse into the gmelinite cage. The L structure is likely to be the best and comparable to ZSM-5 zeolite, offretite the next and omega the least good for catalytic efficiency for exchanged transition metal ions.

Reducibility and Adsorbate Interactions of Ti in Titanosilicate Molecular Sieve TS-1

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopy are used to study reducibility and adsorbate interaction of Ti in titanosilicate TS-1 molecular sieve. Various reduction methods were used to reduce Ti(IV) in TS-1 to Ti(III) which was then monitored by ESR. When TS-1, after dehydration and oxygen treatment at high temperature followed by evacuation (activation), is γ-irradiated at 77 K; an ESR signal withg‖=1.970 andg⊥=1.906 is observed for isolated Ti(III) centers. Radiation induced defect centers known as V centers are also observed after γ-irradiation. When activated TS-1 is treated with CO or H2at 673 K, an axial ESR signal withg⊥=1.968 andg‖=1.933 is observed which is suggested to be Ti(III)(CO)nand Ti(III)(H2)ncomplexes. When activated TS-1 is exposed to D2O at room temperature and subsequently γ-irradiated at 77 K, a new ESR signal withg=1.924 is observed. This species is identified as Ti(III)-(OD)1from2D ESEM data. Adsorption of CH3OD on activated TS-1 produces a new Ti(III) species withg=1.931. This species is identified as Ti(III)-(CH3OD)1from2D ESEM data. Adsorption of C2D4on activated TS-1 produces a new Ti(III) species withg⊥=1.968 andg‖=1.910. This species is identified as Ti(III)-(C2D4)1from2D ESEM data. Possible coordination geometries of the various Ti(III)-adsorbate complexes are discussed.

Electron Spin Resonance and Electron Spin Echo Modulation Studies on Reducibility, Location, and Adsorbate Interactions of Ni(I) in Ni(II)-Exchanged SAPO-34

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies were used to study the reducibility, location, and adsorbate interactions of Ni(I) in NiH−SAPO-34, in which Ni(II) was introduced into extraframework sites of SAPO-34 by partial ion exchange of H(I) by Ni(II). After dehydration at temperatures above 573 K as well as after γ-irradiation at 77 K, one nickel species assigned as isolated Ni(I) by ESR is observed. Along with the isolated Ni(I) species, a second species assigned to Ni(I)−(H2)n is also observed after hydrogen reduction at 573 K. Adsorption of water into reduced NiH−SAPO-34 forms an axially symmetric Ni(I)−(O2)n complex indicating water decomposition by Ni(I). Adsorption of methanol into reduced NiH−SAPO-34 forms two Ni(I)-methanol complexes suggested to be located at two different sites in the chabazite structure. Similarly, two Ni(I)−(C2D4)n complexes, the predominant one exhibiting axial symmetry and the other one having rhombic symmetry, are formed after eth...

Synthesis and Characterization of CuAPO-5 Molecular Sieves: Evidence for the Framework Incorporation of Cu(II) Ions

The hydrothermal synthesis of CuAPO-5 molecular sieves has been investigated by using Cu(II) oxide in the presence of tetraethylammonium hydroxide as a template. The CuAPO-5 material was characterized by X-ray powder diffraction, FT-IR spectroscopy, scanning electron microscopy, and elemental analysis. Supporting evidence for the framework location of copper was obtained from electron spin resonance and electron spin-echo modulation spectroscopy experiments.

Photoinduced electron transfer from alkylpyrenes embedded into DHP, DPPC and DODAC vesicles studied with electron paramagnetic resonance and electron spin echo modulation spectroscopies

Photoinduced electron transfer from alkylpyrene derivatives solubilized in cationic dioctadecyldimethylammonium chloride (DODAC), neutral dipalmitoylphosphatidylcholine (DPPC) and anionic dihexadecylphosphate (DHP) vesicles to interface water (D2O) was monitored by changing the pendant alkyl chain length of the alkylpyrenes and the interface charge of the vesicles. The amount of photoproduced pyrene cation radical defined as the photoyield decreased with increasing alkyl chain length of the alkylpyrene and with the interface charge from cationic DODAC to neutral DPPC to anionic DHP vesicles. The interpretation is that the increased alkyl chain length of alkylpyrene increases the electron transfer distance between the pyrene moiety as an electron donor and the interface water (D2O) as an electron acceptor. This occurs by deeper penetration of the alkylpyrene into the hydrophobic region of the vesicles for longer alkyl chains due to greater hydrophobic interaction of the alkylpyrene with the surfactant alkyl chains. This interpretation is supported by a decreasing deuterium modulation depth trend. The decreasing photoyields of alkylpyrene from DODAC to DPPC to DHP vesicles are due to an increasing energy barrier for electron transfer through the vesicular interface.

Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopic Studies of Cupric Ion−Adsorbate Interactions in Synthetic Clinoptilolite

The site locations and adsorbate interactions of Cu2+ in Cu2+-exchanged synthetic clinoptilolite have been determined by electron spin resonance and electron spin echo modulation spectroscopies. Strong effects of the H+, Li+, Na+, and K+ cocations have been found on the coordination number and on the location of the cupric ion. Cu2+ coordinates three molecules of water in hydrated CuH−clinoptilolite (CuH−Clino) and in CuLi−Clino, but only two molecules of water in hydrated CuNa−Clino and CuK−Clino. Two cupric ion sites are observed in many cases and are attributed to sites in ten-ring and eight-ring main channels. Activation to 400 °C is sufficient to remove these water molecules and cause migration of Cu2+ to an eight-ring intersecting channel. Adsorption of polar molecules such as water, ammonia, alcohols, and acetonitrile causes the migration of Cu2+ into the main channels to coordinate with the adsorbates. Cu2+ forms complexes with three molecules of ethanol and four of ammonia in CuH−Clino, but only coordinates to two molecules of ethanol or methanol and three of ammonia in CuNa−Clino, and to one molecule of ethanol or methanol and three of ammonia in CuK−Clino. Cu2+ coordinates four molecules of acetonitrile at the center of a main channel in clinoptilolite, and this coordination number is unaffected by the alkali metal cocations.

Adsorbate Interactions of Paramagnetic Palladium(I) Species in Pd(II)-Exchanged Na−MCM-22 Zeolite

Adsorbate interactions of paramagnetic palladium(I) species in Pd(II)-exchanged Na−MCM-22 zeolite are studied by electron spin resonance (ESR) and electron spin-echo modulation (ESEM) spectroscopies. Oxidation and subsequent evacuation at 825 K of Na−MCM-22 zeolite exchanged with Pd(NH3)4Cl2 results in the formation of a Pd(I) species. Monovalent palladium can be produced in PdNa−MCM-22 material by room temperature hydrogen reduction after dehydration and O2 treatment at high temperature. Addition of hydrogen to PdNa−MCM-22 after dehydration, O2 treatment, and evacuation (activation) is found to increase the concentration of Pd(I) at room temperature and at higher temperature is found to reduce Pd(I) to Pd(0)n (palladium clusters). Adsorption of O2 and H2O to an activated sample containing Pd(I) generates a Pd(II)−O2- species with g⊥ > g∥. Adsorption of CO on an activated sample results in a Pd(I)−(CO)2 complex which partially transforms to Pd(I)−CO on evacuation. Adsorption of ethylene on an activated sa...

Copper(II) ionic species in CuII-exchanged K-offretite aluminosilicate and comparison with CuII-exchanged K-offretite gallosilicate determined by electron paramagnetic resonance and electron spin echo modulation spectroscopies

The location of Cu II and its interaction with deuteriated adsorbates in Cu II -exchanged K-offretite aluminosilicate zeolite have been investigated by electron paramagnetic resonance (EPR) and electron spin echo modulation (ESEM) spectroscopies and compared with those in Cu II -exchanged K-offretite gallosilicate. Basically similar Cu II locations to those in CuK-offretite gallosilicate are observed in CuK-offretite aluminosilicate, but there are some interesting differences. It is found that in the fresh hydrated sample, Cu II is, in the main channel, coordinated to three water molecules and three framework oxygens in a six-ring window of an e-cage to form a distorted octahedral complex. Upon evacuation at increasing temperature, Cu II ions move from the main channel through the e-cages to hexagonal prism sites. However, the water coordinated to Cu II is more tightly bound in the aluminosilicate than in the gallosilicate. Dehydration produces two different Cu II species in the aluminosilicate, both believed to be located in recessed sites owing to the lack of broadening of its EPR lines by oxygen, while only one Cu II species is located in a recessed site in the gallosilicate. Adsorption of polar molecules such as water, alcohols, dimethyl sulfoxide, acetonitrile and ammonia cause changes in the EPR spectrum of the Cu II indicating migration into cation positions in the main channels where adsorbate coordination can occur. However, non-polar ethene does not cause migration of Cu II . Cu II forms complexes with two molecules of methanol, ethanol and propanol, and one molecule of dimethyl sulfoxide based on ESEM data. Cu II forms a trigonal bipyramidal complex with two ammonias in axial positions and three framework oxygens in a six-ring window of an e-cage based on EPR parameters and ESEM data, which is the same for Cu II in CuK-offretite gallosilicate.

CuIIlocation and adsorbate interaction in CuII-exchangedsynthetic Na-omega gallosilicate EPR and electron spin echo modulation studies

The location of CuII and its interaction with adsorbates in CuII-exchanged synthetic Na-omega gallosilicate have been studied by EPR and electron spin echo modulation (ESEM) spectroscopies. These results are compared with those of CuII-exchanged Na-omega aluminosilicate and also those of L and offretite gallosilicates of similar channel type and size, and the differences are discussed. In general, similar results to those for CuNa-omega gallosilicate are obtained for CuNa-omega aluminosilicate. It is concluded that, in fresh hydrated omega material, CuII is in a main channel coordinating to three water molecules and to framework oxygens in the main channel wall. A minor CuII diaquo species not seen in the gallosilicate is observed in the aluminosilicate. Upon evacuation at increasing temperature, CuII moves from the main channel to a gmelinite cage. Dehydration at 410°C produces one CuII species located in a six-ring window of a gmelinite cage, based on a lack of broadening of its EPR lines by oxygen. In L and offretite gallosilicates, there is evidence for back migration of CuII from a hexagonal prism into a main channel to coordinate with adsorbates. However, in omega the back migration from a gmelinite cage to a main channel seems to be blocked, as shown by very slow changes in the EPR spectra and differing coordination numbers for methanol, ethanol and propanol to CuII when alcohols are adsorbed. CuII does not form a complex with propanol or larger adsorbates in omega gallosilicate. It is suggested that, in omega, small adsorbates must diffuse into a gmelinite cage where CuII is located, to form CuII–adsorbate complexes. The slow changes in the EPR spectra correspond to the time for adsorbate diffusion into a gmelinite cage. CuII interacts with one molecule each of ethylene and acetonitrile, based on EPR and ESEM analyses. CuII forms a square-planar complex containing four molecules of ammonia, based on resolved nitrogen superhyperfine coupling.

Four pulse electron spin echo modulation studies of Cu(II) complexes in MCM-41 silica tube materials

The coordination geometry of Cu(II) complexes with water and ammonia has been studied by four pulse electron spin echo modulation spectroscopy in siliceous (L)Cu-MCM-41 and in aluminum-containing (L)Cu-AlMCM-41 where (L) denotes Cu(II) incorporation by liquid phase ionexchange. An analysis of the proton sum peaks in the echo modulation pattern of the water and ammonia ligands reveals significant differences in the Cu(II) coordination between MCM-41 and AlMCM-41. In the aluminum-containing material (L)Cu-AlMCM-41, Cu(II) coordinates to two molecules of water or ammonia and three framework oxygens in a square-based pyramidal coordination geometry. The base of the pyramid is formed by two adsorbate molecules together with two framework oxygens. A third framework oxygen is located at the apex of the pyramid. The cupric ion site is slightly shifted from the plane of the pyramid base towards the apex resulting in an off-plane position. In the siliceous material (L)Cu-MCM-41, [Cu(H2O)6]2+ and [Cu(NH3)4]2+ complexes are observed. The results of four pulse electron spin echo modulation experiments support a distorted octahedral coordination geometry for the [Cu(H2O)6]2+ complex in (L)Cu-MCM-41.

Adsorbate interactions of Cu(II) in Cu(II)-exchanged K-L gallosilicate studied by CW and Pulsed electron spin resonance

The interactions of Cu(II) in Cu(II)-exchanged K-L gallosilicate with adsorbates containing coordinative nitrogens in ammonia, pyridine, aniline, acetonitrile and hydrazine, and with the adsorbates carbon monoxide, benzene, propanol and dimethyl sulfoxide are investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. These results are compared with those in Cu(II)-exchanged K-L aluminosilicate and the differences are discussed. Adsorption of ammonia produces a complex containing four molecules of ammonia based on resolved nitrogen superhyperfine interaction. Upon equilibrium with pyridine, Cu(II) forms a complex containing four molecules of pyridine in CuK-L gallosilicate and a complex containing only three molecules of pyridine in CuK-L aluminosilicate based on resolved nitrogen superhyperfine. Upon adsorption of aniline and acetonitrile, Cu(II) forms complexes containing two molecules of each in CuK-L gallosilicate based on resolved nitrogen superhyperfine. However, no resolved nitrogen superhyperfine between hydrazine and Cu(II) is seen. Adsorption of carbon monoxide, benzene, propanol and dimethyl sulfoxide causes changes in the ESR spectrum of Cu(II), indicating migration of Cu(II) into cation positions in the main channels where adsorbate coordination can occur. Cu(II) forms complexes with one molecule of benzene and two molecules of propanol based on ESEM data in both K-L gallosilicate and K-L aluminosilicate. However, Cu(II) interacts directly with one dimethyl sulfoxide in K-L aluminosilicate but only indirectly at a longer distance with one dimethyl sulfoxide in K-L gallosilicate based on ESEM data.

Electron spin resonance analysis of paramagnetic Pd(I) species in Pd(II)-exchanged H-rho zeolite

Zeolite rho was synthesized and Pd(II) exchanged into it. Pd(II) was reduced to paramagnetic Pd(1) by a thermal activation process. The interactions of Pd(I) in zeolite H-rho with oxygen, water, methanol, ammonia, carbon monoxide and ethylene have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. The ESR spectrum of an activated sample shows the formation of one Pd(I) species. Pd(I) interacts with water vapor or molecular oxygen to form Pd(II)−O2, indicating decomposition of water. Equilibration with methanol results in a broad isotropic ESR signal which is attributed to the formation of small palladium clusters. ESEM shows that the Pd clusters coordinate one molecule of methanol. Adsorption of ammonia produces a Pd(I) complex containing four molecules of ammonia based upon resolved nitrogen superhyperfine coupling. Adsorption of carbon monoxide results in a Pd(I) complex containing two molecules of carbon monoxide based upon resolved13C superhyperfine coupling. ESR and ESEM results indicate that exposure to ethylene leads to two new Pd(I) species each of which coordinates one molecule of ethylene.