Formation of isocyanic acid during the reaction of mixtures of NO, CO and H2 over supported platinum catalysts

On-line infrared spectroscopy has been used to show that isocyanic acid, HNCO, is a substantial primary product of the reaction between NO, CO and H₂ over a Pt/SiO₂ catalyst. At 230°C it accounts for more than 40% of the CO converted. No isocyanic acid is seen when Al₂O₃ is placed downstream of Pt/SiO₂, or when using a Pt/Al₂O₃ catalyst under the same conditions, due to rapid hydrolysis of HNCO on the alumina. Isocyanic acid may exist as a trace intermediate in automobile catalytic converters during their warm-up phase but it is unlikely to emerge from the pore system of aluminabased catalysts.     

Proton abstraction ability of MgO: a DFT cluster model study of the role of surface geometry

Methanol and isocyanic acid adsorptions on a defective MgO surface have been studied. Equilibrium geometries, adsorption energies, atomic and molecular charges and electronic densities were obtained using a density functional theory method. Oxide surface atoms with different coordination numbers show very different reactivity giving both molecular and dissociated adsorbed species. The methanol molecule requires lower coordination numbers of the active site than the isocyanic acid molecule for dissociative adsorption. The role of the acidic and basic sites has been considered and analyzed in terms of natural bond orbital charges. The main vibration frequencies have been compared with available infrared spectroscopic data.     

Insight into the chemical reaction of CH2OH with NO: Formation of N-hydroxy formamide as an isocyanic acid precursor

This study has explored the potential energy surface on the chemical reaction of CH₂OH with NO by using ab initio calculation. We have found the new reaction pathway producing N-hydroxy formamide, which can further decompose to generate isocyanic acid as a reducing agent of hydrocarbons selective catalytic reduction.     

Formation and reactions of isocyanic acid during the catalytic reduction of nitrogen oxides

Reactions which can produce and consume isocyanic acid (HNCO) over two types of catalysts active for the reduction of nitrogen oxides have been investigated. More than 1000 ppm HNCO can be produced by the reduction of 3000 ppm NO with H₂/CO mixtures over a Pt/SiO₂ catalyst. Complete hydrolysis of HNCO to ammonia and carbon dioxide occurs if even weakly catalytic materials, such as CeO₂/SiO₂ and BaO/SiO₂, are placed downstream. Isocyanic acid is also involved as an intermediate in the reaction of nitromethane over CoZSM5 and CuZSM5 under the conditions of hydrocarbon SCR. In the initial stages of reaction there is complete conversion through to N₂ with CuZSM5 but the process stops at ammonia with CoZSM5 at temperatures below 350°C. In both cases, but especially with CoZSM5, isocyanic acid becomes observable as the catalyst deactivates during continuous exposure at temperatures below about 290°C. In situ FTIR measurements indicate that deactivation is due to a reaction between isocyanic acid and ammonia which generates cyclic striazine compounds.     

Surface chemistry and kinetics of the hydrolysis of isocyanic acid on anatase

In order to meet the stricter NO_x and particulate emission limits for commercial vehicles, the selective catalytic reduction (SCR) with urea is currently seen having the highest potential. The conversion of urea into ammonia and carbon dioxide consists of two consecutive reactions, in which isocyanic acid is an intermediate that is hydrolyzed over TiO₂. The intrinsic kinetics and the surface chemistry for this reaction are explored. Up to a temperature of 132 °C the reaction was in the intrinsic kinetic regime (E_A = 73 kJ/mol), while at higher temperatures the reaction was limited by pore and external diffusion constraints, respectively. In the presence of NO, NH₃ and NO₂, the catalytic activity was negatively influenced, increasing in severity in the sequence mentioned indicating that nitrates formed from NO₂ were most effective in blocking cations and anions of TiO₂. IR spectroscopy indicates that dissociative adsorption of HNCO on TiO₂ forms Ti–NCO and hydrogen bonded OH species. In the presence of water, isocyanic acid was so rapidly hydrolyzed that only adsorbed ammonia was observed on the catalyst surface. The presence of NO, NH₃ and NO₂ retards hydrolysis leading to the appearance of isocyanate species on the surface.     

Amides and polyamides of m-phenylenedioxydiacetic acid

The reaction between m-phenylenedioxydiacetic acid (m-PhDDA) and phenoxyacetic acid (PhA) with isocyanic esters was studied in order to establish the working conditions for the synthesis of some polyamides prepared from m-PhDDA and diisocyanic esters. The polymers were characterized with respect to some physical properties as melting point, viscosity and IR-spectra. In the latter the peak at 1668–1700 cm^?1 was assigned to the C?O amide function. The thermogravimetrical curves show better thermal properties for the polymers obtained in high boiling solvents as trichlorobenzene (TCB) and tetraline (THN).     

FTIR Studies of the Origin of Deactivation during the Decomposition of Nitromethane on Co-ZSM5

In situFourier transform infrared spectroscopy has been used to observe the species deposited on Co-ZSM5 during the decomposition of nitromethane. At 553 K deactivation commences after 60 min. This is accompanied by the formation of bands due to NCO species between 2150 and 2300 cm⁻¹and the sudden development of a very strong band at 1662 cm⁻¹. The latter band can be assigned to the s-triazine compound, melamine, and/or derivatives of it, which cause deactivation by blocking zeolite channels. The most likely chemistry is an initial dehydration of nitromethane to isocyanic acid (HNCO) which forms melamine by reaction with ammonia derived by hydrolysis of HNCO. Cyanuric acid, the cyclic trimer of HNCO, may be an intermediate in the process.     

Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines

Urea-SCR, the selective catalytic reduction using urea as reducing agent, has been investigated for about 10 years in detail and today is a well established technique for DeNO_x of stationary diesel engines. It is presently also considered as the most promising way to diminish NO_x emissions originating from heavy duty vehicles, especially trucks.

The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the reduction of the required catalyst volume while still maintaining a high selectivity for the SCR reaction over a wide temperature range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Additional problems include the control strategy for urea dosing, the high freezing point of urea, and the long term stability of the catalyst.     

尿素醇解法合成碳酸乙烯酯

在干燥碱式碳酸锌催化剂的作用下,以尿素和乙二醇（EG）为原料合成碳酸乙烯酯（EC）。重点考察了催化剂用量、原料配比、反应温度、反应时间对反应结果的影响。并通过GC-MS、FTIR和XRD分析,初步探索了反应机理以及碱式碳酸锌的作用机理。结果表明：当催化剂用量为尿素质量的6%,原料配比n（EG）∶n（尿素）=1∶1,反应温度160℃,反应时间3h,反应压力为0.01MPa时,碳酸乙烯酯收率达到91.64%。尿素和乙二醇合成碳酸乙烯酯的反应中,尿素首先分解生成异氰酸,异氰酸再与乙二醇反应生成中间产物2-羟乙基氨基甲酸酯（2-HEC）,最后2-羟乙基氨基甲酸酯脱氨环化生成碳酸乙烯酯。所用碱式碳酸锌催化剂中存在Zn₄CO₃（OH）₆·H₂O和ZnO两种晶相,且以ZnO为主活性组分,两种晶相的协同作用提高了反应收率。     

Decomposition of Urea in the SCR Process: Combination of DFT Calculations and Experimental Results on the Catalytic Hydrolysis of Isocyanic Acid on TiO2 and Al2O3

In selective catalytic reduction (SCR) systems for diesel vehicles the injected urea solution decomposes to ammonia and isocyanic acid (HNCO), which reacts with water to another ammonia molecule and carbon dioxide over the SCR catalyst or a special urea decomposition catalyst. The second reaction step, i.e. the catalytic hydrolysis of HNCO was studied on the anatase TiO₂(101) surface and Al₂O₃(100) surface with ab initio density functional theory (DFT) calculations using a cluster model as well as with in situ diffuse reflectance infrared Fourier transform spectra (DRIFTS) investigations and kinetic experiments. The following mechanistic pathway has been identified to be most feasible: HNCO dissociatively adsorbs on the metal oxide surface as isocyanates, which are attacked by water, thereby forming carbamic acid at the surface. In a further step this intermediate is transformed to a carbamate complex, which leads to CO₂ desorption and consequently NH₃ formation. The comparison between the sum of the theoretical vibrational spectra of the reaction intermediates with the in situ DRIFT spectra also strongly supports the accuracy of the second reaction pathway. This mechanism holds also for the HNCO hydrolysis over γ-Al₂O₃ and the reactivity compared to TiO₂ was found to be consistent with the heights of the barriers in the energy diagrams. Based on these promising preliminary results a computational screening has been started in order to predict the most active metal oxides and surfaces for this reaction.     

d-生物素中间体合成工艺改进

通过Zn/TiCl4、H4AlLi/TiCl4、Zn Cu/TiCl3低价钛催化剂比较,选择了稳定的Zn Cu/TiCl3为催化剂,反应制得生物素中间体(6aR) 1,3 二苄基 4 (4 甲氧羰基丁基) 二氢 噻吩并[3,4 d]咪唑 2 酮(Ⅵ)。改用异氰酸苄酯为关环试剂环化合成了(7aR) 3 苯基 6 苄基 1H,3H 咪唑并[1,5 c]噻唑 5,7 二酮(Ⅲ),选用CH3COOH/(CH3CO)2O代替CH3COOH作溶剂,反应时间由17h缩短为3h,产率从75%提高到93%。以L 半胱氨酸盐酸盐为原料,依次以苯甲醛、异氰酸苄酯为关环试剂环化合成Ⅲ,以锌为还原剂于90℃下开环合成了N,N′ 二苄基 L 巯基海因(Ⅳ),经与己二酸单甲酯酰氯于0℃下的酯化反应引入侧链,Zn Cu/TiCl3为催化剂,反应得到Ⅵ,5步反应建立了生物素所需骨架结构,总产率达56%。     

A lumped model for thermal decomposition of urea. Uncertainties analysis and selective non-catalytic reduction of NO

A two-step mechanism for non-catalytic thermal decomposition of urea was investigated. It involves a first order reaction for the thermolysis of urea and a pseudo-first order reaction for hydrolysis of isocyanic acid. Rate constants of these reactions were tuned on experimental measurements of NH₃ and HNCO concentrations at 423-723 K and 1083-1383 K in isothermal flow reactors made of aluminum and ceramic, respectively. A regression analysis by taking the uncertainties in the kinetic parameters into account shown that the resultant pairs of Arrhenius expressions for thermo-hydrolyzing urea were different with a 90% confidence when obtained from the different sets of experimental data. Additional results of ammonia concentration from experiments of urea solution decomposition reported in the literature revealed the reliability of the Arrhenius expressions tuned on the reaction rates obtained in the highest range of temperature in the ceramic reactor. The consistency of these equations was also verified by a comparison between calculated and experimental rates of selective non-catalytic reduction of nitric oxide with urea as reducing agent. A statistical analysis based on the identification of possible tendencies in these residuals again evidences the weakness of the Arrhenius expressions based on the experiments performed at lower temperatures, which was attributed to a non-expected catalytic effect of aluminum reactor walls on reactions rates of urea thermal decomposition.     

PU固化酚醛环氧树脂在耐酸、耐热涂料中的应用

环氧树脂以其优良的性能在工业防腐涂料中发挥着重要作用,但其耐酸性差影响应用范畴。通过热塑性线型酚醛树脂（诺伏勒克）与环氧氯丙烷反应生成诺伏勒克环氧树脂,FZTDI—TMP加成物为固化剂,添加含有某种金属离子的颜填料作为催化剂,在常温下打开环氧基,使其在常温下与异氰酸根发生反应,生成含曙唑烷酮结构的聚合物,其涂膜具有良好的耐酸性和耐热性,对硅胶生产设施、工业烟囱等基材的防腐有明显效果。     

The reaction of sodium cyanate with wool and nylon and its effect on subsequent dyeing

This paper describes the reaction of the simplest isocyanate, isocyanic acid, conveniently generated in situ from sodium cyanate, with nucleophilic carboxylate and amino residues in polyamide fibres. Changing the pH conditions greatly affects the outcome of these reactions: under acidic conditions the carboxylate residues are selectively carbamoylated, whereas under neutral conditions both residues are carbamoylated. Such modifications greatly change the dyeing behaviour of the treated substrates. The carboxylate carbamoylation reaction leads to the formation of amides rather than the carbamoylcarboxylate, which is a reactive intermediate.     

The positive effect of hydrogen on the reaction of nitric oxide with carbon monoxide over platinum and rhodium catalysts

The effect of adding 330–4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200–250°C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO₂ rather than react with hydrogen to form water. This reaction is described by CO + NO +3/2H₂ CO₂ + NH₃ and accounts for 50–100% of the CO₂ formed with Pt/Al₂O₃ and 20–50% with Rh/Al₂O₃. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H₂ present.     

Kinetics of the esterification of phthalic anhydride with 2-ethylhexanol. I. Sulfuric acid as a catalyst

The kinetics of the esterification of mono-2-ethylhexyl phthalate in the presence of sulfuric acid as a catalyst has been investigated in an isothermal semibatch reactor. The reaction appears to be first order with respect to mono-2-ethylhexyl phthalate alone and the reaction rate does not depend on the concentration of 2-ethylhexanol. The kinetic parameters have been determined.     

Kinetics and mechanism of the hydrolysis and alcoholysis of alkoxysilanes

As part of our work with the use of silane coupling agents, we have been investigating the transformations of alkoxysilancs into siloxanes. The influence of a para-substituted phenyl group attached to the silicon was investigated and the rates of acid catalyzed hydrolysis and alcoholysis of these para-substituted phenylalkoxysilanes have been determined. The kinetics and mechanism of the reactions are presented. These reactions are of interest because of their role in the use of silane coupling agents as adhesion promoters, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. The hydrolysis reaction was found to be first order in acid, zero order in water and to have a Hammett p of -1.42 when catalyzed by sulfuric acid. These results are consistent with current opinion that the reaction mechanism is SN₁, and involves a positive intermediate, possibly a siliconium intermediate. The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloroacetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species.     

Esterification of fatty acid over Tunisian acid activated clay: kinetic study

Tunisian acid activated clay was investigated as catalyst in the esterification of stearic acid with ethyl alcohol, carried out in a semi-continuous reactor. Kinetic study shows that the reaction is first order with respect to acid concentration. The activation energy found was 21 kJ/mol. This result suggests that the reaction process was controlled by a diffusional step. A kinetic model, giving the apparent constant rate as a function of temperature and catalyst concentration, has been established. This equation has been then successfully applied for a complex mixture of fatty acid.     

Potassium persulphate-malonic acid redox pair initiated polymerization of methacrylamide

Summary The kinetics of aqueous polymerization of methacrylamide initiated by potassium persulphate-malonic acid redox pair has been investigated at 50±1°C under nitrogen atmosphere. The initial rate of polymerization has been found to be square root dependence to the initial concentration of potassium persulphate and malonic acid. The order of reaction with respect to monomer has been found to be unity. The overall energy of activation has been found to be 66±1 KJ/mol. The effect of addition of manganous sulphate and sulphuric acid has been investigated. On the basis of experimental results a suitable mechanism has been proposed.     

In situ preparation of lithographic resists containing glutarimide groups

Poly(acrylic acid) and poly(methacrylic acid) homopolymers, copolymers, and 1-μ-thick films have been converted in high yield to their corresponding cyclic imide derivatives by an in situ gas–solid phase reaction at 200°C with ammonia, methylamine, and ethylamine, respectively. Under similar reaction conditions with H₂S or CH₃SH as the reactive gas, sulfur was incorporated into the polymers in low yield. Under UV and electron beam irradiation, these cyclic derivatives degraded with main chain scission to eliminate isocyanic acid or alkyl isocyanate and to form olefins and ketenes. Poly(diacrylimide) was the exception and crosslinked. The poor solubility in organic solvents and the very high solubility in aqueous solutions of base limited the use of these films as positive resists. However, poly(methyl methacrylate) copolymers, containing 20%–25% cyclic groups, exhibited adequate solubility and sensitivity to be utilized as lithographic resists.