Mechanistic Insight Into the Application of Alumina-Supported Pd Catalysts for the Hydrogenation of Nitrobenzene to Aniline

Two Pd/γ-Al2O3 catalysts are examined for the vapor phase hydrogenation of nitrobenzene over the temperature range of 60–200 °C. A 1 wt % catalyst is selected as a reference material that is diluted with γ-alumina to produce a 0.3 wt % sample, which is representative of a metal loading linked to a candidate industrial specification aniline synthesis catalyst. Cyclohexanone oxime is identified as a by-product that is associated with reagent transformation. Temperature-programed infrared spectroscopy and temperature-programed desorption measurements of chemisorbed CO provide information on the morphology of the crystallites of the higher Pd loading catalyst. The lower Pd loading sample exhibits a higher aniline selectivity by virtue of minimization of product overhydrogenation. Reaction testing measurements that were undertaken employing elevated hydrogen flow rates lead to the proposition of separate reagent and product-derived by-product formation pathways, each of which occurs in a consecutive manner. A global reaction scheme is proposed that defines the by-product distribution accessible by the grades of catalyst examined. This information is helpful in defining product purification procedures that would be required in certain heat recovery scenarios connected with large-scale aniline production

The Solvation and Dissociation of 4‑Benzylaniline Hydrochloride in Chlorobenzene

A reaction scheme is proposed to account for the liberation of 4-benzylaniline from 4-benzylaniline hydrochloride, using chlorobenzene as a solvent at a temperature of 373 K. Two operational regimes are explored: “closed” reaction conditions correspond to the retention of evolved hydrogen chloride gas within the reaction medium, whereas an “open” system permits gaseous hydrogen chloride to be released from the reaction medium. The solution phase chemistry is analyzed by ¹H NMR spectroscopy. Complete liberation of solvated 4-benzylaniline from solid 4-benzylaniline hydrochloride is possible under “open” conditions, with the entropically favored conversion of solvated hydrogen chloride to the gaseous phase thought to be the thermodynamic driver that effectively controls a series of interconnecting equilibria. A kinetic model is proposed to account for the observations of the open system

The Solvation and Dissociation of 4‑Benzylaniline Hydrochloride in Chlorobenzene

A reaction scheme is proposed to account for the liberation of 4-benzylaniline from 4-benzylaniline hydrochloride, using chlorobenzene as a solvent at a temperature of 373 K. Two operational regimes are explored: “closed” reaction conditions correspond to the retention of evolved hydrogen chloride gas within the reaction medium, whereas an “open” system permits gaseous hydrogen chloride to be released from the reaction medium. The solution phase chemistry is analyzed by ¹H NMR spectroscopy. Complete liberation of solvated 4-benzylaniline from solid 4-benzylaniline hydrochloride is possible under “open” conditions, with the entropically favored conversion of solvated hydrogen chloride to the gaseous phase thought to be the thermodynamic driver that effectively controls a series of interconnecting equilibria. A kinetic model is proposed to account for the observations of the open system

Toward High Selectivity Aniline Synthesis Catalysis at Elevated Temperatures

In connection with an initiative to enhance heat recovery from the large-scale operation of a heterogeneously catalyzed nitrobenzene hydrogenation process to produce aniline, it is necessary to operate the process at elevated temperatures (>100 °C), a condition that can compromise aniline selectivity. Alumina-supported palladium catalysts are selected as candidate materials that can provide sustained aniline yields at elevated temperatures. Two Pd/Al2O3 catalysts are examined that possess comparable mean Pd particle sizes (∼5 nm) for different Pd loading: 5 wt % Pd/Al2O3 and 0.3 wt % Pd/Al2O3. The higher Pd loading sample represents a reference catalyst for which the Pd crystallite morphology has previously been established. The lower Pd loading technical catalyst more closely corresponds to industrial specifications. The morphology of the Pd crystallites of the 0.3 wt % Pd/Al2O3 sample is explored by means of temperature-programmed infrared spectroscopy of chemisorbed CO. Reaction testing over the range of 60–180 °C shows effectively complete nitrobenzene conversion for both catalysts but with distinction in their selectivity profiles. The low loading catalyst is favored as it maximizes aniline selectivity and avoids the formation of overhydrogenated products. A plot of aniline yield as a function of WHSV for the 0.3 wt % Pd/Al2O3 catalyst at 100 °C yields a “volcano” like curve, indicating aniline selectivity to be sensitive to residence time. These observations are brought together to provide an indication of an aniline synthesis catalyst specification suited to a unit operation equipped for enhanced heat transfer

Origin of Impurities Formed in a Polyurethane Production Chain. Part 2: A Route to the Formation of Colored Impurities

The quality of methylene diphenyl diisocyanate (MDI) products, which are valuable feedstocks in the industrial manufacture of polyurethanes, can be compromised by the presence of color, presumed to arise from trace impurities. One undesired branch in the synthesis chain originates with phosgenation of diaryl ureas, formed from reactions between aryl isocyanates and polyamine precursors. Subsequent key steps include, (i) breakdown of the primary compounds, substituted chloroformamidine-<i>N</i>-carbonyl chlorides (CCC), to give aryl isocyanide dichlorides, ArNCCl₂, (ii) an apparent equilibrium connecting CCC with aryl carbodiimides, and (iii) the thermolysis of ArNCCl₂ in the presence of MDI. Color formation is associated directly with the last process; it involves several events, including HCl elimination from reaction of ArNCCl₂ and MDI, formation of carbon-centered radicals, and a contribution from oxidation at the methylene bridge

Origin of Impurities Formed in the Polyurethane Production Chain. 1. Conditions for Chlorine Transfer from an Aryl Isocyanide Dichloride Byproduct

Phenyl and 4-methylphenyl isocyanide dichlorides are models for byproduct that may be formed in the later stages of certain polyurethane production chains. Photochemical electron paramagnetic resonance (EPR) studies (λ > 310 nm), using the spin trap, N-tert-butyl-α-phenylnitrone, confirm a previously made suggestion that ArNCCl2 can behave as a chlorine radical source. EPR spectra recorded during and after irradiation and supported by simulations evolve over time and indicate formation of the short-lived spin trap–Cl• adduct and a longer lived benzoyl-N-tert-butylnitroxide radical. Photolysis of C6H5NCCl2, either alone or mixed with methylene diaryl isocyanate species, in o-C6H4Cl2, a polyurethane process solvent, led to the formation of mixtures containing dichloro- and trichlorobiphenyl isomers

Origin of Impurities Formed in the Polyurethane Production Chain. 1. Conditions for Chlorine Transfer from an Aryl Isocyanide Dichloride Byproduct

Phenyl and 4-methylphenyl isocyanide dichlorides are models for byproduct that may be formed in the later stages of certain polyurethane production chains. Photochemical electron paramagnetic resonance (EPR) studies (λ > 310 nm), using the spin trap, <i>N</i>-<i>tert</i>-butyl-α-phenylnitrone, confirm a previously made suggestion that ArNCCl₂ can behave as a chlorine radical source. EPR spectra recorded during and after irradiation and supported by simulations evolve over time and indicate formation of the short-lived spin trap–Cl^• adduct and a longer lived benzoyl-<i>N</i>-<i>tert</i>-butylnitroxide radical. Photolysis of C₆H₅NCCl₂, either alone or mixed with methylene diaryl isocyanate species, in <i>o</i>-C₆H₄Cl₂, a polyurethane process solvent, led to the formation of mixtures containing dichloro- and trichlorobiphenyl isomers