Effect of the surface area of platinum on the activity of a bifunctional aluminoplatinum catalyst in the synthesis of alkylimidazoles from diamines and carbonyl acids

An investigation has been conducted into the effect of the acid and dehydrogenating functions of an aluminoplatinum catalyst on the synthesis of 2-methylimidazole from ethylenediamine and acetic acid. It has been established that formation of the intermediate 2-methylimidazoline involves the acid Al₂O₃ centers and its rate of formation is greater than the rate of its subsequent dehydrogenation to 2-methylimidazole on the Pt centers. The symbatic nature of the variations in the 2-methylimidazole yield and the surface area of the platinum in the aluminoplatinum catalyst has been demonstrated.N. D. Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, 117913 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 9, pp. 1992–1996, September, 1992.     

The influence of plasma chemical treatment of a platinum catalyst on its activity in the dehydrogenation of cyclohexane

The influence of plasma chemical treatments on the catalytic activity of 0.64 wt % Pt/SiO₂ and 1.0 wt % Pt/SiO₂ platinum catalysts in the dehydrogenation of cyclohexane was studied. The state of the surface of the catalysts was examined using X-ray photoelectron spectroscopy. Temperature hysteresis caused by the formation of active carbon was observed in flow experiments. It was shown that the reaction on the initial catalysts occurred on neutral and positively charged Pt particles, and that the active centers contained carbon. After catalyst treatment with a high-frequency plasma in H₂, its activity increased by many times because of the formation of a large number of low-activity centers on positively charged platinum particles also containing carbon. Glow discharge plasma in Ar sharply decreased catalytic activity, and the reaction then predominantly occurred on centers localized on neutral Pt particles, whereas centers on positive Pt particles were blocked. The state of the substrate (silica gel) did not change under the action of plasmas of both kinds.     

Effect of properties of aluminum oxide on selectivity of Pt/Al2O3 in dehydrogenation of n-alkanes

Conclusions The selectivity of Al-Pt catalysts (0.25% Pt) in the dehydrogenation of n-decane and n-hexane to n-alkenes depends on the properties of the aluminum oxide and increases in the order: Pt/-Al₂O₃2O₃s > Pt/-Al₂O₃.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2615–2618, November, 1978.     

The mechanisms of the formation of by-products in the catalytic synthesis of C-alkylimidazoles from diamines and carboxylic acids

The formation of 1-ethyl-2-methylimidazole (1-Et-2-MI) and 1-ethyl-2-methylimidazoline (1-Et-2-MIN), intermediates in the synthesis of 2-methylimidazole (2-MI) from ethylenediamine (EDA) and AcOH in the presence of a platinum-on-alumina catalyst, was studied. Using CH₃ ¹⁴COOH the incorporation of two acetate units into 1-Et-2-MI and 1-Et-2-MIN molecules was demonstrated. The same products were also obtained on dehydrogenation of 2-methylimidazoline (2-MIN) under mild conditions (220–230 °C). A part of the latter is transformed to N-ethyl ethylendiamine (EEDA) and EDA. These facts indicate that both 1-Et-2-MI and 1-Et-2-MIN result from hydrogenolysis of the imidazoline ring with subsequent ethylation. A reverse reaction, the C₅-cyclization of EEDA, was also observed. The formation of 2-MI is favored by increasing temperature.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 937–940, May, 1993.     

Nonoxidative conversion of methane and <Emphasis Type="Italic">n</Emphasis>-pentane over a platinum/alumina catalyst

Methane adsorption on the Pt–H/Al₂O₃ and Pt/Al₂O₃ catalysts begins at Т = 475°C and is accompanied by the appearance of hydrogen in the reaction medium. At a higher temperature is raised to 550°C, the amount of adsorbed hydrogen increases to 1.1 and 0.8 mol/(mol Pt), respectively. According to the calculated degree of methane dehydrogenation on platinum sites at Т = 550°C, the Н/C ratio is 1.3 (at/at) for the Pt–H/Al₂O₃ catalyst and 1.5 (at/at) for the Pt/Al₂O₃ catalyst. The introduction of n-pentane into the reaction medium increases the yield of aromatic hydrocarbons (benzene and toluene) by a factor of 8.8 over the arene yield observed in individual n-pentane conversion. A mass spectrometric analysis of the arenes obtained with the Pt/Al₂O₃ catalyst has demonstrated that 37.5% of the adsorbed methane is involved in the methane–n-pentane coaromatization yielding benzene and toluene.     

ESR studies on heterogeneous processes in irradiated SiO2/B2O3+H2O systems

Effect of B³⁺ (%B₂O₃) percentage ratio in silica gel on the nature and the mechanism of defect formation and yields of paramagnetic radiation centers (PC) have been studied. The nature of paramagnetic radiation centers of electron hole type has been revealed and their behavior has been investigated as a function of temperature changes. It has been found that the interaction between paramagnetic centers formed at SiO₂/B₂O₃ surface with absorbed water molecules (=1) leads to the formation of H and OH radicals. In addition the experiment shows that increased B³⁺ (%B₂O₃) percentage ratio in silica gel results in higher yields of paramagnetic centers and H-radicals. Possible mechanism of formation, localization and annihilation of non-equilibrium charge carriers (NCC) and their energy transfer to the adsorbed phase has been suggested.     

Influence of the formulation of catalysts deposited on cordierite monoliths for acetic acid oxidation

Monolithic catalysts are prepared by washcoating cordierite monoliths with different sols (Pt/Al₂O₃, Pt/CeO₂, Pt/ZrO₂, Pt/Al₂O₃CeO₂, Pt/Al₂O₃ZrO₂, and Pt/CeO₂ZrO₂). These sols are prepared by a sol–gel process and characterized by specific surface area (S_BET), inductively coupled plasma, hydrogen chemisorption, high-resolution transmission electron microscopy, field emission scanning electron microscopy, oxygen storage capacity, X-ray diffraction, temperature-programmed reduction, CO₂ chemisorption, and the model reaction of 3,3-dimethylbutene isomerization. The catalytic performances of the monolithic catalysts are then evaluated for the acetic acid oxidation. The nature of catalyst coating has been found to influence the adherence with the cordierite monolith and the presence of cerium in the catalyst appears to increase the adherence of the latter. Pt/CeO₂, Pt/Al₂O₃CeO₂, and Pt/CeO₂ZrO₂ are found to be the most reducible catalysts (oxygen storage capacity and temperature-programmed reduction) and to have the lowest acidities (3,3-dimethylbutene isomerization). CO₂ chemisorption shows that these catalysts possess a good basicity. From the relation established between the catalytic activity and the redox and acid–base properties it has been concluded that the reducibility is the key factor for a good catalytic activity although the basicity has a significant influence on the catalytic performance.     

Crystal structure of di-μ-amido-bis[diammineplatinum(II)] nitrate

Di-μ-amido-bis[diammineplatinum(II)] nitrate (1) was synthesized as a byproduct during preparation of tetraammineplatinum(II) nitrate. One possible pathway to produce 1 is that [(H₃N)₂Pt(μ- OH)₂Pt(NH₃)₂](NO₃)₂, a well-known complex forming on treatment of cis-Pt(NH₃)₂I₂ with AgNO₃, reacts with aqueous ammonia. The other possible pathway involves deprotonation of [Pt(NH₃)₄](NO₃)₂ to form monomeric Pt(NH₃)₃(NH₂)NO₃ followed by elimination of NH3. Crystals of 1 (from water) are monoclinic (C2/c) with a = 16.834(2) Å, b = 10.573(1) Å, c = 7.415(1) Å, β = 114.846(1)°, and Z = 4. The cationic portion consists of two symmetrical square-planar Pt centers with the inversion center at the midpoint of the Pt(1)???Pt(1A) vector. The Pt(II) ion is coordinated by four N atoms from two ammonia molecules and two bridging amido groups affording a slightly distorted square. The molecules are stacked in such a way that the planes of coordination squares turn out to be parallel to each other with a distance of 3.501 Å. Intermolecular Pt–H interaction between the μ-NH₂ hydrogens and the platinum(II) centers of the adjacent molecule are observed.     

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state‐of‐the‐art DFT methods and by a combination of supersonic molecular beams with high‐resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH₃ and CH₂ are specially stabilized at sites located at particles edges and corners by an amount of 50–80 kJ mol^?1. This stabilization is caused by an enhanced activity of low‐coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH₃ and CH₂ formation steps are endothermic on Pt(111) but markedly exothermic on Pt₇₉. An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol^?1 for first C? H bond scission and 40 kJ mol^?1 for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO₂ film on Cu(111). It is shown that CH₃ generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single‐crystal surfaces in which CH₃ was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.     

Specific properties of dehydrogenation centers in supported platinum catalysts

Cyclohexane dehydrogenation was used as a model reaction to study the dehydrogenation function of Pt/Al₂O₃ catalysts. Fresh and coked catalysts were tested in two ranges of reaction conditions: model conditions (T=280–340°C, P=1 atm) and the conditions used in the naphtha reforming process (T=420–460°C, P=15 atm). Intrinsic properties of the dehydrogenation active centers were found to be identical throughout the experimental conditions. The coke deposits only block a part of the active centers without their qualitative modification.     

Effect of plasmochemical treatment of platinum catalyst on dispersity of metal phase and its catalytic activity

We studied how treatment of 0.64 wt % Pt/SiO₂ catalyst with a glow-discharge plasma in O₂ and a high-frequency plasma in H₂ (HF H₂) influences the cyclohexane dehydrogenation reaction. The effect of plasmochemical treatment on the average diameter of platinum particles was established by the method of X-ray diffraction analysis. It was found that oxygen plasma increases the surface area of the metal by ∼15% and diminishes activity by raising the activation energy and reducing the number of active centers per unit surface of the metal. The HF H₂ plasma increases the reaction rate constant many times over, due to the number of active centers per unit surface of the metal rising sharply while the size of the Pt particles remains unchanged.     

Selective skeletal isomerization of alkanes over partially reduced MoO<Subscript>3</Subscript>

The surface area and the pentane isomerization activity of Pt/MoO₃ were enlarged by H₂ reduction. The enlargements was observed only when the reduction proceeded through the formation of hydrogen molybdenum bronze, H_xMoO₃. The catalytic activities of H₂-reduced MoO₃ with different noble metals for pentane isomerization and 2-propanol dehydration depended on the ability of noble metal to produce the H_xMoO₃ phases. H₂-reduced Pt/MoO₃ was more active for pentane isomerization than Pt/H, and its activity was comparable to that of Pt/HZSM-5. In heptane isomerization, H₂-reduced Pt/MoO₃ exhibited a lower activity than Pt/H, although heptane was isomerized very selectively. Strong adsorption of heptane onto H₂-reduced Pt/MoO₃ is likely to be a reason for its lower heptane isomerization activity.     

The dehydrogenation of cyclohexane on an AP-64 industrial platinum catalyst subjected to plasma chemical treatments

The influence of glow discharge plasma in oxygen and argon and high-frequency discharge plasma in hydrogen on the activity of the AP-64 (Pt/γ-Al₂O₃) catalyst in the dehydrogenation of cyclohexane was studied. The catalytic experiments were performed in a flow unit and under static conditions in a vacuum. Under flow conditions, catalyst treatment with plasmas in O₂ and Ar decreased the yield of benzene by ～50% but strengthened temperature hysteresis because of the formation of active carbon on the surface of the catalyst. Under static conditions, argon plasma and high-frequency discharge H₂ plasma multiply increased the rate of the reaction because of an increase in the number of active centers, whereas an oxygen plasma decreased the rate of the reaction by two times because of an increase in activation energy. The determination of the order of the reaction led us to suggest that the stage scheme of the reaction did not change after plasma chemical catalyst treatments.     

Synthesis and Crystal Structure of the 2-D Polymer {Cu(bipy)Pt(CN)4}∞ and Its Relationship to the Tetranuclear Compound [{Cu(bipy)(H2 O)Pt(CN)4}2]·2H2O

The reaction of (NBu₄)₂[Pt(CN)₄] with Cu(NO₃)₂·2.5H₂O and bipy in stoichiometric ratio in methanol solution yields the 2-D polymer {Cu(bipy)Pt(CN)₄}. The influence of water molecules and hydrogen bonding on the formation of a 2-D polymeric structure versus a tetranuclear cluster from Cu(bipy)Pt(CN)₄ units is discussed.     

Investigation of electronic structure of impurity and defect centers on the surface of silica by the MNDO method

The electronic structure of cluster models of fragments of a. silica surface with defect centers [stressed disiloxane bridges, radical (Si, SiO) and ionic (Si⁺, SiO^–) groups] and impurity centers (boron and aluminum atoms) has been investigated by the MO-LCAO-SCF method in the MNDO approximation. It has been shown on the basis of an analysis of the energetics of the defect structures (with complete optimization of the geometry of the clusters) that the dehydroxylation of silica with the formation of radical and ionic centers is energetically significantly more favorable than is the formation of disiloxane bridges. The influence of impurity boron and aluminum atoms on the properties of the surface fragments of SiO₂ has been considered. It has been shown that the migration of impurity boron atoms, which isomorphously replace silicon atoms in the lattice, from the bulk to the surface of silica is energetically unfavorable.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 407–418, July–August, 1988.     

Dehydrogenation of piperazine into pyrazine on alumina-platinum catalysts

Dehydrogenation of piperazine on alumina-platinum catalysts at 360–400°C was investigated. It was found that the alumina-platinum catalyst has low selectivity in the formation of pyrazine (yield 43%), and dehydrogenation is accompanied by decomposition and coke formation and products of decomposition in the formation of alkylpyrazines. The alumina-platinum catalyst in In₂O₃ and Re₂O₇ additives permits obtaining pyrazine with a yield of up to 80%. Dehydrogenation of piperazine is accompanied by reactions of decomposition, dehydroisomerization, and alkylation to a small degree, resulting in the formation of pyrrole, methylimidazole, and alkylpyrazines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1483–1488, July, 1990.     

Crystal and Molecular Structure of Platinum(II) trans-Bis-(1,1,1-trifluoro-4-iminopentan-2-onate)

This paper describes a new method for the synthesis of the -iminoketonate complex Pt(ktf)₂ using 1,1,1-trifluoro-4-iminopentan-2-one as a bidentate ligand. An X-ray study has been performed on the polycrystals and single crystals of the compound. The single crystal X-ray diffraction analysis was carried out on a CAD-4 diffractometer (MoK radiation, /2 scan mode). Crystal data: a = 10.217(2), b = 10.231(2), c = 10.924(2) , = 83.12(3), = 63.38(3), = 70.77(3)°, V = 963.4(3) ³, space group P1, Z = 2, d _calc = 1.721 g/cm³. The Pt atom has a distorted planar square environment; the average lengths of Pt–N and Pt–O coordination bonds coincide (1.97 ). The mean value of the chelate angle is 94.2°. The environment of Pt is completed to bipyramid by the hydrogen atoms of the neighboring molecules. The structure of the compound is molecular. The differently oriented molecules of the complex are linked into chains down the x axis with Pt...Pt distances of 5.10 . A comparative crystal-chemical analysis of the structures of Pt(ktf)₂ and Pd(ktf)₂ and their O,O-chelate analogs — Pt(II) and Pd(II) trifluoroacetylacetonates — has been carried out. An X-ray study of Pt(tfa)₂ and Pd(tfa)₂ has been accomplished, and crystal data are given.