Identification of decomposition reactions for HMDSO organosilicon using quantum chemical calculations

Hexamethyldisiloxane [HMDSO, (CH₃)₃-SiOSi-(CH₃)₃] is an important precursor for SiO₂ formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH₃ radical by Si C bond dissociation.     

Palladium(0)/PAr3‐Catalyzed Intermolecular Amination of C(sp3)?H Bonds: Synthesis of β‐Amino Acids

An intermolecular C(sp³) H amination using a Pd⁰/PAr₃ catalyst was developed. The reaction begins with oxidative addition of R₂N OBz to a Pd⁰/PAr₃ catalyst and subsequent cleavage of a C(sp³) H bond by the generated Pd NR₂ intermediate. The catalytic cycle proceeds without the need for external oxidants in a similar manner to the extensively studied palladium(0)‐catalyzed C H arylation reactions. The electron‐deficient triarylphosphine ligand is crucial for this C(sp³) H amination reaction to occur.     

Oxygen‐Promoted C?H Bond Activation at Palladium

[Pd(P(Ar)(tBu)₂)₂] ( 1 , Ar=naphthyl) reacts with molecular oxygen to form Pd^II hydroxide dimers in which the naphthyl ring is cyclometalated and one equivalent of phosphine per palladium atom is released. This reaction involves the cleavage of both C H and O O bonds, two transformations central to catalytic aerobic oxidizations of hydrocarbons. Observations at low temperature suggest the initial formation of a superoxo complex, which then generates a peroxo complex prior to the C H activation step. A transition state for energetically viable C H activation across a Pd peroxo bond was located computationally.     

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single-atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single-atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single-atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂-supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.     

Thermally Stable and Regenerable Platinum–Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single‐atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single‐atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single‐atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂‐supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary C?H Bonds

Achieving selective C H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C H bonds.     

Studies of the roles of Sn or Fe on γ-Al2O3-supported Pt catalysts by CO adsorption microcalorimetry and dehydrogenation reaction of C4 alkanes

CO adsorption microcalorimetry was employed in the study of γ-Al₂O₃-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the Pt/γ-Al₂O₃ catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N₂O₃ catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and Fe to the Pt/γ-Al₂O₃ catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was changed by adding Sn or Fe to Pt/γ-N₂O₃. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al₂O₃ catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C₄ alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity for the dehydrogenation of alkanes. Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training Program Foundation for the Talents by The State Education Commission of China.     

Unparalleled,Enormous Metal–Carbon Bond Strength in PdCH2I+

Thermalized Pd⁺ cations activate methyl iodide by selective cleavage of a C? H bond under formation of PdCH₂I⁺ and an H-atom. This finding implies that the interaction energy between the metal cation and the CH₂I fragment and thus the metal–carbon bond strength exceeds 103 kcal/mol. Theory predicts that the energetically most favorable isomer of this ion exhibits the Pd⁺? CH₂? I structure, which is stabilized by an unprecedented bridging interaction between the two heavy atoms Pd and I.     

Frontispiz: Mild and Versatile Nitrate‐Promoted C?H Bond Fluorination

A novel and facile C H bond fluorination proceeds under remarkably mild conditions (close to room temperature in most cases). Both aromatic and olefinic C(sp²) H bonds with a wide range of electronic properties are selectively fluorinated in the presence of a catalytic amount of simple, cheap, and nontoxic nitrate as the promoter. A Pd^II/Pd^IV catalytic cycle that is initiated by an in situ generated cationic [Pd(NO₃)]⁺ species was proposed based on preliminary mechanistic studies.     

The Dehydrogenation of Propane on Platinum–Tin Glass-Fiber Woven Catalysts

The reaction of propane dehydrogenation on platinum–tin catalysts supported onto different woven carriers (an aluminoborosilicate and two silica materials) was studied. It was found that the catalyst was rapidly deactivated by carbon deposits formed, and the rate of this reaction increased with the specific surface area of the glass-fiber woven material and the Pt content. It was established that the Pt: Sn ratio in surface platinum particles was about 6, and it increased to 39 after the reaction; this fact is indicative of a Sn loss, which led to an increase in the conversion of feed into carbon deposits that deactivated the catalyst. A mixture of propane and 5–10 vol % H₂ should be used for the stabilization of the catalytic system; in this case, the negative effect of hydrogen on the yield of propylene was minimal. On the catalyst supported onto a silica carrier under optimum conditions (550°C; propane space velocity, 480 h^–1), which correspond to minimum selectivity for the formation of carbon deposits, the yield of propylene was ~18%. The test glass-fiber woven catalyst was inferior to granulated platinum–tin catalysts in terms of catalytic activity; therefore, its use in the reaction of propane dehydrogenation is inexpedient.     

Atomically Dispersed Cobalt/Copper Dual-Metal Catalysts for Synergistically Boosting Hydrogen Generation from Formic Acid

The development of practical materials for (de)hydrogenation reactions is a prerequisite for the launch of a sustainable hydrogen economy. Herein, we present the design and construction of an atomically dispersed dual-metal site Co/Cu−N−C catalyst allowing significantly improved dehydrogenation of formic acid, which is available from carbon dioxide and green hydrogen. The active catalyst centers consist of specific CoCuN₆ moieties with double-N-bridged adjacent metal-N₄ clusters decorated on a nitrogen-doped carbon support. At optimal conditions the dehydrogenation performance of the nanostructured material (mass activity 77.7 L ⋅ g_metal⁻¹ ⋅ h⁻¹) is up to 40 times higher compared to commercial 5 % Pd/C. In situ spectroscopic and kinetic isotope effect experiments indicate that Co/Cu−N−C promoted formic acid dehydrogenation follows the so-called formate pathway with the C−H dissociation of HCOO* as the rate-determining step. Theoretical calculations reveal that Cu in the CoCuN₆ moiety synergistically contributes to the adsorption of intermediate HCOO* and raises the d-band center of Co to favor HCOO* activation and thereby lower the reaction energy barrier.     

Density Functional Theory Analysis of Anthraquinone Derivative Hydrogenation over Palladium Catalyst

A density functional theory (DFT) analysis was conducted on the hydrogenation of 2‐alkyl‐anthraquinone (AQ), including 2‐ethyl‐9,10‐anthraquinone (eAQ) and 2‐ethyl‐5,6,7,8‐tetrahydro‐9,10‐anthraquinone (H₄eAQ), to the corresponding anthrahydroquinone (AQH₂) over a Pd₆H₂ cluster. Hydrogenation of H₄eAQ is suggested to be more favorable than that of eAQ owing to a higher adsorption energy of the reactant (H₄eAQ), lower barrier of activation energy, and smaller desorption energy of the target product (2‐ethyl‐5,6,7,8‐tetrahydro‐9,10‐anthrahydroquinone, H₄eAQH₂). For the most probable reaction routes, the energy barrier of the second hydrogenation step of AQ is circa 8 kcal mol^?1 higher than that of the first step. Electron transfer of these processes were systematically investigated. Facile electron transfer from Pd₆H₂ cluster to AQ/AQH intermediate favors the hydrogenation of C=O. The electron delocalization over the boundary aromatic ring of AQ/AQH intermediate and the electron‐withdrawing effect of C=O are responsible for the electron transfer. In addition, a pathway of the electron transfer is proposed for the adsorption and subsequent hydrogenation of AQ on the surface of Pd₆H₂ cluster. The electron transfers from the abstracted H atom (reactive H) to a neighbor Pd atom (Pd_H), and finally goes to the carbonyl group through the C₄ atom of AQ aromatic ring (C₄).     

DFT (density functional theory) studies on cycloisomerization of 15–membered triazatriacetylenic macrocycle

The mechanism as well the stereochemistry of cascade cycloisomerization of 15–membered triazatriacetylenic macrocycle was investigated theoretically by using M062X/6–31+G(d,p) and M062X/LANL2DZ calculations. The results showed that the mechanism and outcome of the reaction depended on the absence and presence of a transition metal catalyst. So that, in thermal-induced condition, the reaction had to experience several suprafacial concerted reactions including Ene-reaction (DG^#=35.38 kcal/mol), Diels–Alder cycloaddition (DG^# = 17.16 kcal/mol), and sigmatropic H-shift rearrangement (DG^# = 56.21 kcal/mol) to produce diastereoselective fused cis–tetracyclic aromatic bearing a pyrrole moiety by following kinetic considerations. Also, the [2+2+2] cycloaddition mechanism was neglected in thermal–induced conditions because of high activation free Gibbs energy (DG^# = 63.90 kcal/mol). In the presence of palladium catalyst, Pd(0) formed an adduct by coordinating to C = C bonds and decreased the DG^# of the process to 29.58 kcal/mol, and consequently provided a facilitated media for the reaction to follow the [2+2+2] to produce more stable fused tetracyclic benzenoid aromatic by passing through the lower energy barrier.     

Preparation of highly active and reusable heterogeneous Al2O3–Pd catalysts by the sol–gel method using bayberry tannin as stabilizer

A novel heterogeneous Al₂O₃–Pd catalyst has been prepared by the sol–gel method; bayberry tannin (BT) was used as stabilizer to prevent the migration and aggregation of Pd species during calcination. According to N₂ adsorption/desorption determination, Al₂O₃–Pd has a mesoporous structure and its specific area is as high as 336.5 m²/g. Transmission electron microscopy observation indicated that the size of the Pd particles was greatly reduced by the presence of BT. On the basis of X-ray photoelectron spectra analysis, it was found that the most of Pd nanoparticles were dispersed in the pores, implying that BT can prevent migration of Pd particles from the pores to the outer surface of Al₂O₃ during calcination. For comparison, Al₂O₃–Pd^* was prepared by the sol–gel method but without use of BT. In the hydrogenation of acrylic acid, Al₂O₃–Pd had high catalytic activity and excellent reusability compared with commercial and traditionally prepared heterogeneous Pd catalysts. The turnover number of Al₂O₃–Pd is as high as 11,328.0 mol/mol after recycling seven times, which is much higher than that of a commercial Pd–C catalyst (8048.0 mol/mol).     

Catalytic behavior of a silica-supported polystannazane-copper complex for the oxidation of methanol to formaldehyde at mild reaction conditions

A very cheap catalyst, a silica-supported polystannazane-copper complex (abbreviated as SiO₂ Sn N Cu) was prepared by the reaction of the silica-supported polystannazane ligand with CuCl₂ in ethanol. The results showed that SiO₂ Sn N Cu can catalyze the oxidation of methanol to formaldehyde in high yield at mild conditions, i.e. at 40°C and 1 atm of oxygen. The catalyst could be recovered and could catalyze the reaction repeatedly.     

Mechanistic insight into the rhodium(III)-catalyzed ortho-selective coupling of diverse arenes with 4-acyl-1-sulfonyltriazoles: A computational study

A mechanistic study of the Cp*Rh(III)-catalyzed annulative coupling of benzimidates with 4-acyl-1-sulfonyltriazoles by C H activation was performed using density functional M06 method. It was demonstrated that the active catalyst during the coupling process should be the cation [Cp*Rh(OAc)]⁺ rather than the neutral Cp*Rh(OAc)₂ and Zn(OAc)₂ as proposed previously by the experimenters. A novel energetically feasible reaction pathway has been revealed theoretically in details. The acetic acid-mediated cyclization process was confirmed to be the rate-limiting step with an overall barrier of 24.3 kcal/mol, excluding any importance of the C H cleavage mechanism as supported by the kinetic isotope effect experiments. The major factors responsible for the preferred regioselectivity of N-pyrimidinylinoles with 4-acetyl-1-sulfonyltriazoles were discussed.     

VO3/CeO2(111)催化剂上丙烷氧化脱氢反应活性和选择性的密度泛函理论研究

低碳烯烃一直以来都是化工行业非常重要的基础原料,一般采用烷烃直接热裂解制得,但该方法耗能很大,经济价值有限.近年来,人们开始尝试利用氧化脱氢反应(ODH)方法制备低碳烯烃,并取得了巨大的研究进展,其中稀土氧化物负载钒氧化物催化剂具有良好的低碳烷烃氧化脱氢性能.本文分析了前人对于钒氧化物负载在CeO2表面的计算研究结果,并选取了最具代表性的VO3/CeO2(111)作为烷烃ODH制烯烃的模型催化剂,详细研究了丙烷在该催化剂体系中发生ODH反应机理.通过使用密度泛函理论,对丙烷在VO3/CeO2(111)催化剂上断裂第一根和第二根碳氢键的反应过程进行了理论模拟,并对比了丙烷制丙烯中碳氢键断裂先后的活化能及VO3/CeO2(111)催化剂材料自身的电子性质.结果表明,该催化剂的电子结构在丙烷氧化脱氢反应中扮演关键角色.在丙烷分子断裂第一根碳氢键的反应过程中,会产生两个自由电子,对其电子结构分析发现,其中的一个自由电子会局域在由VO3/CeO2(111)催化剂中五个相关氧原子的2p轨道所形成的新发生局域空轨道(NELS)上,这个独特的新发生局域空轨道只能接受一个电子,另一个电子则会通过丙基在CeO2表面发生吸附将电子传递到CeO2表面的Ce原子上;当丙烷分子进一步发生第二根碳氢键断裂反应时,同样会产生两个新的局域电子,其中一个电子局域在Ce的4f轨道上,此时CeO2表面存在两个局域电子,相互排斥,导致该催化剂上丙烷断裂第二根碳氢键所需的活化能远高于第一根碳氢键.综上,本文对VO3/CeO2(111)催化剂上低碳烷烃ODH反应独特的催化活性和选择性给出了较为细致的分析和解释.     

Synthesis and properties of PdSn/Al2O3 and PdSn/SiO2 prepared by solvated metal atom dispersed method

A series of solvated metal atom dispersion (SMAD) catalysts: Pd/SiO₂, Pd/Al₂O₃, Sn/SiO₂, Sn/Al₂O₃, Pd_xSn_y/SiO₂ and Pd_xSn_y/Al₂O₃. It was prepared by simultaneous evaporation of Pd and Sn. The metals were co-deposited at 77 K using acetone, 2-propanol and THF to produce colloids “in situ” all the supported catalyst were characterized by chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and TPR. This series of catalyst were tested for crotonaldehyde hydrogenation in gas phase to obtain crotyl alcohol.     

High Performance Pd,PdNi, PdSn and PdSnNi Nanocatalysts Supported on Carbon Nanotubes for Electrooxidation of C2C4 Alcohols

Nanocatalysts Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ supported on multi‐walled carbon nanotubes (MWCNTs) were successively synthesized by the chemical reduction method in the glycol‐water mixture solvent. Transmission electron microscopy results show that the prepared Pd, Pd₈Ni₂, Pd₈Sn₂ and Pd₈Sn₁Ni₁ nanoparticles are uniformly dispersed on the surface of MWCNTs. The average particle sizes of the nanocatalysts are 3.5–3.8 nm. Electroactivity of the prepared catalysts towards oxidation of ethanol, 1‐propanol, 2‐propanol, n‐butanol, iso‐butanol and sec‐butanol (C2? C4 alcohols) in alkaline medium was studied by cyclic voltammetry and chronoamperometry. The current density obtained for the electrooxidation of C2? C4 alcohols depends on the catalysts and the various structures of the alcohols. Addition of Sn or/and Ni to Pd nanoparticles enhances the electroactivity of the Pd/MWCNT catalyst. Furthermore, the ternary Pd₈Sn₁Ni₁/MWCNT catalyst presents the highest electroactivity for the oxidation of C2? C4 alcohols among the prepared catalysts. Electrocatalytic activity order among propanol isomers and butanol isomers is as follows respectively: 1‐propanol > 2‐propanol, and n‐butanol > iso‐butanol > sec‐butanol > tert‐butanol. This is consistent with the Mulliken charge value of the carbon atom bonded with hydroxyl group in the corresponding alcohol molecule.     

Nanosized hydrogenation catalyst based on palladium bisacetylacetonate and phosphine: Formation,the origin of activity,and properties

The nature and catalytic properties of a hydrogenation catalyst based on Pd(acac)₂ and PH₃ are considered. As demonstrated by a variety of physicochemical methods (IR and UV spectroscopy, ³¹P and ¹H NMR, electron microscopy, and X-ray powder diffraction), nanoparticles consisting of various palladium phosphides (Pd₆P, Pd_4.8P, and Pd₅P₂) and Pd(0) clusters form under the action of dihydrogen during catalyst preparation. The promoting effect of phosphine at low PH₃: Pd(acac)₂ ratios is mainly due to the ability of phosphine to increase the extent of dispersion of the catalyst.