New method for the temperature-programmed desorption (TPD) of ammonia experiment for characterization of zeolite acidity: a review.

In this review, a method for the temperature-programmed desorption (TPD) of ammonia experiment for the characterization of zeolite acidity and its improvement by simultaneous IR measurement and DFT calculation are described. First, various methods of ammonia TPD are explained, since the measurements have been conducted under the concepts of kinetics, equilibrium, or diffusion control. It is however emphasized that the ubiquitous TPD experiment is governed by the equilibrium between ammonia molecules in the gas phase and on the surface. Therefore, a method to measure quantitatively the strength of the acid site (∆H upon ammonia desorption) under equilibrium-controlled conditions is elucidated. Then, a quantitative relationship between ∆H and H0 function is proposed, based on which the acid strength ∆H can be converted into the H0 function. The identification of the desorption peaks and the quantitative measurement of the number of acid sites are then explained. In order to overcome a serious disadvantage of the method (i.e., no information is provided about the structure of acid sites), the simultaneous measurement of IR spectroscopy with ammonia TPD, named IRMS-TPD (infrared spectroscopy/mass spectrometry-temperature-programmed desorption), is proposed. Based on this improved measurement, Brønsted and Lewis acid sites were differentiated and the distribution of Brønsted OH was revealed. The acidity characterized by IRMS-TPD was further supported by the theoretical DFT calculation. Thus, the advanced study of zeolite acidity at the molecular level was made possible. Advantages and disadvantages of the ammonia TPD experiment are discussed, and understanding of the catalytic cracking activity based on the derived acidic profile is explained.

Ammonia IRMS-TPD measurements on Brønsted acidity of proton-formed SAPO-34.

By utilizing the advantages of a combined method of IRMS-TPD of ammonia and DFT calculations, the solid acidity of HSAPO-34 was studied. The number, strength and structure of the Brønsted OH were measured experimentally. The quantitative measurements and DFT calculations supported the identification of Brønsted OH to account for the generation model of the Brønsted OH primarily located in the edge of the Si domain (island). The acid strength of SAPO-34 was slightly weaker than that of chabazite, a zeolite with the same structure. Thus, some important insights were obtained to understand the acid site generation of SAPO-34.

Enhancement in the catalytic activity of Pd/USY in the heck reaction induced by H2 bubbling.

Pd was loaded on ultra stable Y (USY) zeolites prepared by steaming NH(4)-Y zeolite under different conditions. Heck reactions were carried out over the prepared Pd/USY. We found that H2 bubbling was effective in improving not only the catalytic activity of Pd/USY, but also that of other supported Pd catalysts and Pd(OAc)2. Moreover, the catalytic activity of Pd/USY could be optimized by choosing appropriate steaming conditions for the preparation of the USY zeolites; Pd loaded on USY prepared at 873 K with 100% H2O gave the highest activity (TOF = 61,000 h⁻¹), which was higher than that of Pd loaded on other kinds of supports. The prepared Pd/USY catalysts were applicable to the Heck reactions using various kinds of substrates including bromo- and chloro-substituted aromatic and heteroaromatic compounds. Characterization of the acid properties of the USY zeolites revealed that the strong acid site (OH(strong)) generated as a result of steaming had a profound effect on the catalytic activity of Pd.

Highly dispersed Pd species active in the Suzuki-Miyaura reaction.

Structures of Pd/zeolites immersed in solvents were measured by in situ X-ray absorption fine structure (XAFS). Systematic studies revealed that the selection of an appropriate support (USY-zeolite), thermal treatment temperature of USY, solvent (o-xylene), H(2) partial pressure (6%), and the use of a Pd amine complex affect the structure of Pd. As a result, we found that monomeric Pd can be obtained in the USY support with H(2) bubbling in o-xylene. The structural properties of Pd correlate well with its catalytic performance in the Suzuki-Miyaura coupling reactions; a very high TON of up to 11,000,000 was obtained over the monomeric Pd.

Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites.

Brønsted acidity of H-chabazite (CHA) zeolites (Si : Al(2) = 4.2) was investigated by means of ammonia infrared-mass spectrometry/temperature-programmed desorption (IRMS-TPD) methods and density functional calculations. Four IR bands were observed at 3644, 3616, 3575 and 3538 cm(-1), and they were ascribable to the acidic OH groups on four nonequivalent oxygen sites in the CHA structure. The absorption band at 3538 cm(-1) was attributed to the O(4)H in the 6-membered ring (MR), and ammonia adsorption energy (DeltaU) of this OH group was the lowest among the 4 kinds of OH groups. The other 3 bands were assigned to the acidic OH groups in 8MR. It was observed that the DeltaU in 8 and 6MR were 131 (+/-3) and 101 kJ mol(-1), respectively. On the other hand, the density functional theory (DFT) calculations within periodic boundary conditions yielded the adsorption energies on these OH groups in 8 and 6MR to be ca. 130 and 110 kJ mol(-1), respectively, in good agreement with the experimentally-observed values.

Identification and measurements of strong Brønsted acid site in ultrastable Y (USY) zeolite.

By using the IRMS-TPD method in which IR (infrared) and MS (mass spectroscopy) worked together, acid sites of USY (ultrastable Y) zeolite were studied. A new band of OH playing a role of Brønsted acid was clearly detected on Na2H2-ethylenediaminetetraacetic acid (EDTA)-treated USY at 3595 cm(-1) during an elevation in temperature after the adsorption of ammonia. MS-measured TPD (temperature-programmed desorption) of NH3 and IR-measured TPD of the NH4+ cation coincided well to show that this zeolite consisted of the Brønsted acid sites. The MS-TPD profile at higher temperatures corresponded to the IR-TPD of the 3595-cm(-1) band, and therefore, this OH was identified as a strong acid site. From comparison between IR-TPD of OH and MS-TPD, numbers of three kinds of Brønsted OH (i.e., those in super and sodalite cages of a Y zeolite structure) and created strong Brønsted acid site were quantified. On the other hand, strength of the Brønsted acid site DeltaH was determined individually by a simulation method, where the corrected IR-TPD of OH was simulated based on the proposed equation. Thus, a new strong Brønsted acid site was identified in the EDTA-treated USY, and the amount and strength was measured quantitatively.

Quick XAFS studies on the Y-type zeolite supported Au catalysts for CO-O2 reaction.

Au/zeolite catalysts prepared with a deposition-precipitation method were characterized with quick XAFS (QXAFS) in combination with IR. The data were correlated with the catalytic performance in the CO-O(2) reaction conducted at 273 K. On the basis of the XANES analysis of Au loaded on H-Y, the deposited Au(2)O(3) was observed at the initial stage. The transformation of Au(2)O(3) to form metal Au clusters was observed at 473 K in a H(2) atmosphere. The fact was supported by the IR measurement of adsorbed CO and the subsequent reaction with O(2). Detailed clustering process of Au supported catalysts could be directly followed by EXAFS analysis. The growth of metal Au proceeded via the formation of a Au(55) cluster at 473 K. Then it agglomerated to give metal Au with diameter of 2 nm at 723 K. The addition of H(2) was effective to retard the sintering of Au clusters. A similar phenomenon was observed over Au loaded on USY zeolite. In marked contrast to the H-Y and USY supports, significantly agglomerated Au particles generated on Na-Y zeolite, indicating the importance of the presence of acid sites in keeping the Au clusters with highly dispersed form. The performance of 5 wt % Au loaded on H-Y and USY in the CO-O(2) reaction was remarkably sensitive to the pretreatment temperature and the gas atmosphere. The catalyst pretreated with hydrogen showed a two-spike pattern with respect to the pretreatment temperature. Namely, the optimum activity was observed after the pretreatment at 373 and 723 K, where the temperatures corresponded to the generation of Au(2)O(3) and metal Au clusters with 2 nm diameter as evidenced by QXAFS analysis, respectively. The reason for enhancement of the activity of Au/H-Y by the addition of H(2) in the pretreatment step could be attributed to the formation of metal Au with appropriate size. In contrast to the H-Y and USY support, Au loaded on Na-Y prepared under the same condition was almost inactive in the reaction due to the formation of aggregated metal Au.

Ammonia IRMS-TPD study on the distribution of acid sites in mordenite.

Using an IRMS-TPD (temperature programmed desorption) of ammonia, we studied the nature, strength, crystallographic location, and distribution of acid sites of mordenite. In this method, infrared spectroscopy (IR) and mass spectroscopy (MS) work together to follow the thermal behavior of adsorbed and desorbed ammonia, respectively; therefore, adsorbed species were identified, and their thermal behavior was directly connected with the desorption of ammonia during an elevation of temperature. IR-measured TPD of the NH4(+) cation was similar to MS-measured TPD, thus showing the nature of Brønsted acidity. From the behavior of OH bands, it was found that the Brønsted acid sites consisted of two kinds of OH bands at high and low wavenumbers, ascribable to OH bands situated on 12- and 8-member rings (MR) of mordenite structure, respectively. The amount and strength of these Brønsted hydroxyls were measured quantitatively based on a theoretical equation using a curve fitting method. Up to ca. 30% of the exchange degree, NH4(+) was exchanged with Na+ on the 12-MR to arrive at saturation; therefore, in this region, the Brønsted acid site was situated on the large pore of 12-MR. The NH4(+) cation was then exchanged with Na+ on 8-MR, and finally exceeded the amount on 12-MR. In the 99% NH4-mordenite, Brønsted acid sites were located predominantly on the 8-MR more than on the 12-MR. Irrespective of the NH4(+) exchange degree, the strengths deltaH of Brønsted OH were 145 and 153 kJ mol(-1) on the 12- and 8-MR, respectively; that is, the strength of Brønsted acid site on the 8-MR was larger than that on the 12-MR. A density functional theory (DFT) calculation supported the difference in the strengths of the acid sites. Catalytic cracking activity of the Brønsted acid sites on the 8-MR declined rapidly, while that on the 12-MR was remarkably kept. The difference in strength and/or steric capacity may cause such a difference in the life of a catalyst.

Cellular uptake of arginine-rich peptides: roles for macropinocytosis and actin rearrangement.

The use of membrane-permeable peptides as carrier vectors for the intracellular delivery of various proteins and macromolecules for modifying cellular function is well documented. Arginine-rich peptides, including those derived from human immunodeficiency virus 1 Tat protein, are among the representative classes of these vectors. The internalization mechanism of these vector peptides and their protein conjugates was previously regarded as separate from endocytosis, but more recent reevaluations have concluded that endocytosis is involved in their internalization. In this report, we show that the uptake of octa-arginine (R8) peptide by HeLa cells was significantly suppressed by the macropinocytosis inhibitor ethylisopropylamiloride (EIPA) and the F-actin polymerization inhibitor cytochalasin D, suggesting a role for macropinocytosis in the uptake of the peptide. In agreement with this we observed that treatment of the cells with R8 peptide induced significant rearrangement of the actin cytoskeleton. The internalization efficiency and contribution of macropinocytosis were also observed to have a dependency on the chain length of the oligoarginine peptides. Uptake of penetratin, another representative peptide carrier, was less sensitive to EIPA and penetratin did not have such distinct effects on actin localization. The above observations suggest that penetratin and R8 peptides have distinct internalization mechanisms.

Arginine carrier peptide bearing Ni(II) chelator to promote cellular uptake of histidine-tagged proteins.

Arginine-rich peptide-mediated protein delivery into living cells is a novel technology for controlling cell functions with therapeutic potential. In this report, a novel approach for the intracellular delivery of histidine-tagged proteins was introduced where a Ni(II) chelate of octaarginine peptide bearing nitrilotriacetic acid [R8-NTA-Ni(II)] was used as a membrane-permeable carrier molecule. Significant internalization of histidine-tagged enhanced green fluorescent protein (EGFP) into HeLa cells was observed by confocal microscopic observation in the presence of R8-NTA-Ni(II). Nuclear condensation characteristic in apoptotic cell death was also induced in the cells treated with a histidine-tagged apoptosis-inducing peptide [pro-apoptotic domain peptide (PAD)], indicating that the cargo molecules really went through the membrane to reach the cytosol. The apoptosis-inducing activity of the peptide thus delivered was compared with that of the PAD peptide covalently connected with the octaarginine peptide.

Energy-Dispersive XAFS Studies on the Spontaneous Dispersion of PdO and the Formation of Stable Pd Clusters in Zeolites.

Spontaneous dispersion and clustering processes of Pd were measured by means of the energy-dispersive EXAFS method. The spontaneous dispersion of bulky metal Pd into highly dispersed PdO was directly observed on the H-type zeolite in the atmosphere of O2. In contrast to H-type zeolites, simple oxidation of the agglomerated Pd was observed on Na-ZSM-5. The structural change of Pd was followed in the atmosphere of hydrogen. The clustering processes of metal Pd depended on the kind of zeolite, and these were categorized into three groups. The first group, i.e., Na-ZSM-5 and H-beta, showed monotonic agglomeration of metal Pd by increasing the reduction temperature. The second group consisted of H-ZSM-5 and H-mordenite where the formation of Pd6 clusters was found. On these zeolites, the generation of Pd6 clusters was reversibly observed upon the repetition of reduction and oxidation treatments. The third group consisted of H-Y and USY zeolites where the formation of Pd13 clusters was observed. From these findings, it was concluded that the crystal structure and acid sites of zeolites had profound influences on the dynamic behavior and the genesis of Pd clusters with various structures.

Topological stability and self-association of a completely hydrophobic model transmembrane helix in lipid bilayers.

Investigation of interactions between hydrophobic model peptides and lipid bilayers is perhaps the only way to elucidate the principles of the folding and stability of membrane proteins (White, S. H., and Wimley, W. C. (1998) Biochim. Biophys. Acta 1367, 339-352). We designed the completely hydrophobic "inert" peptide modeling a transmembrane (TM) helix without any of the specific side-chain interactions expected, X-(LALAAAA)(3)-NH(2) [X = Ac (I), 7-nitro-2-1,3-benzoxadiazol-4-yl (II), or 5(6)-carboxytetramethylrhodamine (III)]. Fourier transform infrared-polarized attenuated total reflection measurements revealed that I as well as II assume a TM helix in hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers. Dithionite quenching experiments detected no topological change (flip-flop) in the helix II for at least 24 h. Thus, the TM helix itself is a highly stable structure, even in the absence of flanking hydrophilic or aromatic amino acids which are suggested to play important roles in stable TM positioning. Helix self-association in lipid bilayers was detected by fluorescence resonance energy transfer between II and III. The peptide was in a monomer-antiparallel dimer equilibrium with an association free energy of approximately -13 kJ/mol. Electron spin resonance spectra of 1-palmitoyl-2-stearoyl-(14-doxyl)-sn-glycero-3-phosphocholine demonstrated the presence of a motionally restricted component at lower temperatures.

Possible existence of common internalization mechanisms among arginine-rich peptides.

Basic peptides such as human immunodeficiency virus type 1 (HIV-1) Tat-(48-60) and Drosophila Antennapedia-(43-58) have been reported to have a membrane permeability and a carrier function for intracellular protein delivery. We have shown that not only Tat-(48-60) but many arginine-rich peptides, including HIV-1 Rev-(34-50) and octaarginine (Arg(8)), efficiently translocated through the cell membranes and worked as protein carriers (Futaki, S., Suzuki, T., Ohashi, W., Yagami, T., Tanaka, S., Ueda, K., and Sugiura, Y. (2001) J. Biol. Chem. 276, 5836-5840). Quantification and time course analyses of the cellular uptake of the above peptides by mouse macrophage RAW264.7, human cervical carcinoma HeLa, and simian kidney COS-7 cells revealed that Rev-(34-50) and Arg(8) had a comparable translocation efficiency to Tat-(48-60). Internalization of Tat-(48-60) and Rev-(34-50) was saturable and inhibited by the excess addition of the other peptide. Typical endocytosis and metabolic inhibitors had little effect on the internalization. The uptake of these peptides was significantly inhibited in the presence of heparan sulfate or chondroitin sulfates A, B, and C. Treatment of the cells with the anti-heparan sulfate antibody or heparinase III also lowered the translocation of these peptides. These results strongly suggest that the arginine-rich basic peptides share a certain part of the internalization pathway.