Atsushi Satsuma

Effects of Brønsted and Lewis acidities on activity and selectivity of heteropolyacid-based catalysts for hydrolysis of cellobiose and cellulose

Heteropolyacids (H3PW12O40, H4SiW12O40) and salts of metal cations (Mn+) and PW12O403− (M3/nPW12O40) act as effective homogeneous catalysts for selective hydrolysis of cellobiose and cellulose to glucose and total reducing sugars (TRS), respectively, in an aqueous phase. For Bronsted acid catalysts, including mineral acid and heteropolyacids, the activity for both reactions increases with a decrease in the deprotonation enthalpies (DPE), indicating that stronger Bronsted acidity is more favorable. For M3/nPW12O40 of 11 kinds of metal ions (Ag+, Ca2+, Co2+, Y3+, Sn4+, Sc3+, Ru3+, Fe3+, Hf4+, Ga3+ and Al3+), the rate of cellulose hydrolysis increases with Lewis acidity of the cation, while the TRS selectivity was highest for cations with moderate Lewis acidity, such as Sn4+ and Ru3+. For the hydrolysis of cellobiose, cellulose and lignocellulose, H3PW12O40 and Sn0.75PW12O40 showed higher TRS yield than H2SO4.

Promotion effect of H2 on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al2O3

Abstract The effect of H2 on the selective reduction of NO by light hydrocarbons over Ag/Al2O3 catalyst under lean-exhaust conditions was investigated. The NO reduction activity at the low temperature region was markedly increased by adding H2. The NO reduction activity at high GHSV condition and SO2 tolerance were also improved by adding H2. Since the NO was reduced little by H2 in the absence of C3H8, H2 should act as a promoter of NO reduction by hydrocarbons over Ag/Al2O3. The results of reaction studies and in situ IR experiments showed that the possible reasons for promoting the effect by H2 on the HC-SCR were mainly caused by the activation of hydrocarbons.

Catalytic performance of Ag–Al2O3 catalyst for the selective catalytic reduction of NO by higher hydrocarbons

Silver–aluminum mixed oxide catalyst (Ag–Al2O3) prepared by the sol–gel method was studied for the selective reduction of NO by various alkanes in the presence of water vapor. As the carbon number of alkanes increases, the de-NOx activity and water tolerance were markedly increased. In the case of n-octane as a reductant, the presence of water vapor markedly promoted NO reduction. The results of reaction studies and in situ IR experiment showed that the possible reasons for the promoting effect by water vapor are the inhibition of the n-octane oxidation by O2 and the suppression of the poisoning effect caused by carboxylate and carbonate species. Among various alumina-supported transition metal catalysts, Ag–Al2O3 showed the highest activity for SCR by n-octane. Ag–Al2O3 showed higher NO conversion to N2 and selectivity than alumina-supported Pt and Cu-ZSM-5 catalysts for the selective reduction of NO by n-octane and i-octane.

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

A gamma-alumina-supported silver cluster catalyst--Ag/Al(2)O(3)--has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373 K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1 nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1) reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2) C-H activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3) H(2) desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid-base bifunctional support (Al(2)O(3)) than on basic (MgO and CeO(2)) and acidic to neutral (SiO(2)) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d(10) element--silver--through optimization of metal particle size and the acid-base natures of inorganic supports.

Direct Dehydrogenative Amide Synthesis from Alcohols and Amines Catalyzed by γ‐Alumina Supported Silver Cluster

Aromatic and aliphatic amides are functional groups of great importance in polymers, natural products, and pharmaceuticals. Synthesis of amides is mostly based on activated acid derivatives (acid chlorides and anhydrides) or rearrangement reactions induced by an acid or base. Alternative procedures include the Staudinger ligation, aminocarbonylation of aryl halides, oxidative amidation of aldehydes, and amidation of alcohols with excess amount of hydrogen acceptor. However, all these methods require stoichiometric amounts of various reagents and lead to equimolar amounts of byproducts. A more environmentally friendly protocol, first reported by Milstein et al., is the direct amidation from alcohols and amines catalyzed by Ru complexes accompanied by H2 removal. They proposed that the reaction is based on alcohol dehydrogenation to give an aldehyde intermediate which reacts with the amine to give a hemiaminal that is subsequently dehydrogenated to the amide. In this highly chemoselective reaction, a hydrogen acceptor is not needed and the “dearomatized” aminomethyl phosphinomethyl pyridine as a pincer ligand plays an active role in the hydrogen abstraction and liberation process (cooperative ligand). To date, only two homogeneous catalysts using platinum-group metal (PGM), Ru, with molecularly designed cooperative ligands, have been reported. However, these expensive catalysts do not tolerate secondary amines and are problematic in terms of the catalyst/product separation and necessity of special handling of metal complexes. From the environmental and economic point of view, a reaction with inexpensive (PGM-free) heterogeneous catalyst is desired. Metal clusters have interesting chemical properties, which are unusual for bulk solids. For example, it is well known that gold as an inert d element shows similar or higher catalytic activity for various reactions than PGM-based catalysts when several nanometer-sized gold clusters are supported on a specific support. Silver as a less expensive Group IB metal is very popular in research field of cluster synthesis. However, compared with a research of gold catalysis, surprisingly less attempts have been focused on specific catalysis of silver cluster as well as a structure–activity relationship of silver cluster catalysis. Among a few examples, g-alumina-supported silver cluster (Ag/Al2O3) [9,10] reported by our group acts as heterogeneous catalyst for the oxidant-free dehydrogenation of alcohols and one-pot C C cross-coupling reaction from secondary and primary alcohols. The unprecedented activity of Ag/Al2O3 for reactions initiated by C H activation of alcohols led us to investigate the possibility of using Ag/Al2O3 as new environmentally benign catalyst for the title reaction. Herein, we demonstrate the first example of a heterogeneously catalyzed reaction of alcohols with amines to form amides and H2 using the easily prepared and inexpensive heterogeneous catalyst, Ag/Al2O3. Systematic studies on the structure–activity relationship are also shown to provide a new synthetic strategy of C H activation catalyst using non-PGM material with inorganic cooperative ligands, that is, OH/OH groups on alumina. The catalyst precursors were prepared by impregnating oxides with an aqueous solution of silver nitrate followed by evaporation to dryness at 80 8C and calcination at 600 8C. By controlling the H2 reduction temperature, silver clusters (5 wt %) with similar size were supported on various metal oxides: Ag/MOx-5 (MOx =CeO2, MgO, ZrO2, Al2O3, SiO2). Our previous results of Ag K-edge extended X-ray absorption fine structure (EXAFS) showed that silver species in all samples are in a range 0.84–3.0 nm. For Al2O3-supported catalysts, a series of catalysts with average particle diameter from 0.73 to 30 nm was also prepared by changing the Ag content (1, 3, 5, 10, 50 wt %); the particle diameter increased with Ag content (see Figure S1 in the Supporting Information). [a] Dr. K.-i. Shimizu, K. Ohshima, Prof. Dr. A. Satsuma Department of Molecular Design and Engineering Graduate School of Engineering, Nagoya University Nagoya 464-8603 (Japan) Fax: (+81) 52-789-3193 E-mail : kshimizu@apchem.nagoya-u.ac.jp Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901896.

Direct CC Cross-Coupling of Secondary and Primary Alcohols Catalyzed by a γ-Alumina-Supported Silver Subnanocluster

Lets drink to that! Two alcohols (one primary and one secondary) can be coupled in an atom-efficient process by a hydrogen-autotransfer catalytic system in the form of silver subnanoclusters supported on gamma-Al(2)O(3). The recyclable heterogeneous catalyst promoted the one-pot C-C cross-coupling in the presence of a catalytic amount of the weak base Cs(2)CO(3) (see reaction mechanism).

Selective catalytic reduction of NO over supported silver catalysts- : practical and mechanistic aspects

Selective catalytic reduction of NO by hydrocarbons (HC-SCR) is one of the promising technologies for removal of NO in exhausts containing excess oxygen, such as diesel and lean burn gasoline engines. Supported Ag catalysts, especially Ag/Al2O3, are thought to be the promising candidates for use in diesel exhausts, as confirmed by several reports on engine bench tests. The HC-SCR performance of supported Ag catalysts is very sensitive to the reaction conditions, especially the type of hydrocarbons and the addition of H2. The control of reaction conditions would be key for practical use. The current research of supported Ag catalysts is reviewed from the viewpoints of practical use and the reaction mechanism, i.e., the reaction scheme, the role of surface adsorbed species, and the structure of active Ag species.

γ-Alumina-Supported Silver Cluster for N-Benzylation of Anilines with Alcohols

An alumina‐supported silver cluster with a Lewis acidic cocatalyst (polyvalent metal salts, such as FeCl3⋅6 H2O) acts as a heterogeneous and recyclable catalyst for the direct N‐alkylation of anilines with benzyl alcohols, which is driven by the borrowing hydrogen mechanism. Systematic studies on the effects of metal (Pt, Pd, Au, and Ag), silver particle size, support oxides (CeO2, ZrO2, Al2O3, SiO2), and Lewis acidic additives show four important factors required to achieve selective N‐alkylation of anilines: 1) metal with a weak metal–hydrogen bond energy (i.e., Ag), 2) smaller size of the silver cluster, 3) the support having both acidic and basic sites (i.e., Al2O3), and 4) additives with high Lewis acidity (i.e., FeIII salt). Fundamental information will be useful for the rational design of platinum‐group metal‐free heterogeneous catalysts for environmentally benign CN bond forming reactions.

Transamidation of amides with amines under solvent-free conditions using a CeO2 catalyst

Among various metal oxides, cerium oxide (CeO2) shows the highest catalytic activity for transamidation of picolinamide with n-octylamine. CeO2 acts as a reusable and effective heterogeneous catalyst for transamidation under solvent-free conditions. Transamidation of a variety of amides and amines produced the corresponding N-alkyl amides in high yields. This method provides the first example of a heterogeneous catalyst for transamidation using aliphatic amines as substrates. Characterization of acid–base properties and kinetic studies suggest that the cooperation of the weak Lewis acid sites and adjacent strong base sites play important roles in the transamidation reaction.

Promotion effect of hydrogen on surface steps in SCR of NO by propane over alumina-based silver catalyst as examined by transient FT-IR

The mechanistic cause of enhancement of C3H8-SCR activity by addition of H2 over Ag/Al2O3 was investigated with in-situ FT-IR spectroscopy. Under a flow of NO + C3H8 + O2, nitrates were mainly formed on Ag/Al2O3. An addition of H2 into a C3H8-SCR atmosphere increased the concentration of surface acetate significantly, but decreased the concentration of surface nitrates. Formation and consumption rates of acetate and nitrates were estimated with transient in-situ IR measurement. By the addition of H2, both formation rates of acetate and nitrates were increased. Moreover, both consumption rates of nitrates in a flow of C3H8 + O2 and acetate in a flow of NO + O2 were also increased by the addition of H2. From a comparison between the evolutions of adsorbed species (nitrate and acetate) and gaseous species (NO and C3H8), it was clarified that the NO reduction activity is controlled by partial oxidation of C3H8 to mainly surface acetate. The addition of H2 results in remarkable promotion of partial oxidation of C3H8 to mainly surface acetate, which is the rate-determining step of C3H8-SCR in the absence of H2.