Toward High Selectivity Aniline Synthesis Catalysis at Elevated Temperatures

Abstract

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/Al 2 O 3 catalysts are examined that possess comparable mean Pd particle sizes (∼5 nm) for different Pd loading: 5 wt % Pd/Al 2 O 3 and 0.3 wt % Pd/Al 2 O 3 . 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/Al 2 O 3 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/Al 2 O 3 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.

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

“…From here on in, the 1 wt % Pd/γ-Al 2 O 3 and 0.3 wt % samples will be referred to as GU-3 and GU-4, respectively. Table S1 summarizes the nomenclature of terms used to explain the five distinct Pd/Al 2 O 3 catalysts examined in this study and in two preceding articles. , …”

“…The surface morphology of GU-4 can be inferred from CO TP-IR measurements performed on GU-3. Both samples contain identical Pd crystallites due to the sample dilution process, which provides the opportunity to evaluate the morphology of low-loading Pd/Al 2 O 3 samples …”

“…The reaction is highly exothermic, exhibiting standard reaction enthalpies of −554.1 and −468.2 kJ mol –1 for the liquid and vapor phases, respectively . Given that ANL synthesis unit operations are typically linked to isocyanate manufacturing facilities that constitute integrated chemical complexes, the NB hydrogenation reaction possesses excellent credentials for the application of heat recovery strategies to produce superheated steam for use throughout a chemical complex . Indeed, Päβler and Freund have described a model-based design of energy-efficient reactors for ANL synthesis that is intended to exploit the heat recovery possibilities this reaction affords…”

“…This scenario can lead to complications in the catalytic conversion process, with elevated temperatures potentially compromising product selectivity via inadvertently providing access to chemical pathways that lead to by-product formation. As part of an initiative to improve the operational efficiency of certain ANL synthesis facilities associated with large-scale isocyanate production, the authors have recently considered the suitability of alumina-supported Pd catalysts for ANL synthesis operation at elevated temperatures . The matter of associating by-product formation with a specified catalyst formulation is a critical factor when considering the selection of a suitable postreaction purification stage (i.e., distillation unit) for any intended plant revisions at the industrial complex.…”

“…Previous work from the authors examined two catalysts: a commercially available 5 wt % Pd/γ-Al 2 O 3 catalyst (GU-1) and a 0.3 wt % Pd/γ-Al 2 O 3 catalyst (GU-2). The work established that the low loading Pd/Al 2 O 3 catalyst (0.3 wt % Pd) minimized the possibility of ANL overhydrogenation productsa favorable outcome for the industrial scenario, and one that guides specifications for a technical grade ANL synthesis catalyst . However, due to the low concentration of Pd surface atoms (∼4 μmol Pd (s) g (cat) –1 ), these materials are difficult to analyze directly via the IR technique.…”