Ammonia coverage on nickel catalysts in industrial amination is determined by the competitive chemisorption of NH₃, dissociated hydrogen, and amine intermediates on reduced nickel crystallites. On Ni(111) single crystals, temperature-programmed desorption measurements in ultrahigh vacuum place non-dissociative ammonia binding energies in the 0.6–0.8 eV range, while stepped surfaces such as Ni(210) and Ni(211) exhibit stronger binding and measurable dissociation at temperatures above 250 K. The parameter used to evaluate coverage in fixed-bed amination reactors is the effective surface coverage θ_NH3, calculated from the competitive Langmuir-Hinshelwood expression:

θ_NH3 = (K_NH3 p_NH3) / (1 + K_NH3 p_NH3 + K_H2 p_H2^0.5 + K_Am p_Am)

In production units using pre-reduced Ni/Al₂O₃ trilobe extrudates with a nominal diameter of 2.5–3.5 mm and a BET surface area of 150–250 m²/g according to ASTM D3663-20, direct surface coverage measurements are not available. Instead, the ammonia-to-substrate feed ratio, hydrogen uptake, and product amine distribution measured by ASTM D2074-07 or ASTM D4629-20 are used as indirect indicators. At fixed-bed reactor inlet temperatures above 120°C, maintaining primary amine selectivity above 95% in nitrile and carbonyl amination typically requires an ammonia partial pressure of 0.4–2.5 MPa. The actual requirement depends on whether the catalyst is a low-dispersion kieselguhr-supported powder, a high-dispersion alumina-supported extrudate, or an activated Raney nickel slurry. The central process conflict is that excessive ammonia partial pressure suppresses hydrogen coverage and can lead to nitrogen insertion into the nickel lattice in low H₂ zones, whereas insufficient ammonia coverage promotes condensation of surface-bound imines with primary amine products, shifting selectivity toward secondary and tertiary amines.

What Limits Ammonia Chemisorption at Low Hydrogen Partial Pressure in Trickle-Bed Amination?

In trickle-bed units processing fatty nitriles to primary fatty amines over Ni/Al₂O₃, the inlet H₂:NH₃ molar ratio is generally maintained between 1.0:1 and 4.0:1, with total pressures of 5–25 MPa. Low hydrogen partial pressure relative to ammonia is required to maintain a high ratio of NH₃ to imine intermediates on the nickel surface, but this same condition introduces a hydrogen dissociation limitation. On nickel surfaces, NH₃ and H atoms compete for the same threefold hollow and bridge sites; when p_H2 falls below the value needed to replenish surface hydrogen, the imine hydrogenation step becomes limited by the availability of adsorbed H. Published kinetic studies on Ni/Al₂O₃ indicate that ammonia reaction orders for primary amine formation can shift from positive to negative at hydrogen-to-ammonia ratios below 0.5:1. At 130–170°C and 2.0–3.0 MPa, operation near the low-H₂ boundary can also convert a fraction of the nickel crystallites to metastable Ni₄N. The nitride phase is accompanied by a crystallite volume expansion of approximately 5–15% and can increase pressure drop across fixed beds of 4–8 m length. In trickle-bed reactors with liquid hourly space velocities of 0.5–2.0 h^-1, the local H₂ concentration in the liquid film is further reduced by gas-liquid mass transfer resistance; therefore, an apparent optimum ammonia-to-substrate ratio established in a stirred batch autoclave cannot be transferred to a trickle-bed without correcting for wetting efficiency and axial temperature gradients.

Within these ranges, the most common field failure is not loss of catalytic metal but the development of a high-pressure-drop zone near the reactor outlet where ammonia concentration is lowest. In adiabatic reactors, the outlet temperature may be 15–35°C higher than the inlet temperature depending on reaction enthalpy; this axial temperature increase reduces ammonia coverage at the exact location where the ammonia-to-substrate ratio has been depleted by conversion. Industrial fixed-bed designs therefore use quench hydrogen or interstage ammonia injection to flatten the axial temperature and ammonia partial pressure profiles. In a radial-flow reactor processing cyclohexanone to cyclohexylamine, the ammonia-to-ketone molar feed ratio is commonly kept above 10:1 to compensate for the coverage loss across the catalyst bed, and the temperature rise is limited to ≤10°C per bed pass. Failure to maintain this ratio produces a product shift that is detectable as an increase in the secondary amine fraction determined by ASTM D2074-07, often before any significant loss in conversion is observed.

Slurry-phase aminations with Raney nickel in batch or continuous stirred-tank reactors use a different method of ammonia coverage control because ammonia is present as a condensed liquid rather than solely as a gas-phase component. In a 500 L pilot autoclave processing cyclohexanone to cyclohexylamine over activated Raney nickel at 120°C and 6 MPa total pressure, primary amine selectivity above 98% is maintained only when the ammonia-to-ketone ratio exceeds 10:1 and the hydrogen uptake rate remains above 0.4 mol H₂ per mol ketone per minute. The coverage of ammonia on the catalyst surface in this configuration is controlled by the ammonia concentration in the liquid phase, the partial pressure of hydrogen in the reactor headspace, and the mass transfer rate of hydrogen through the gas-liquid interface. In batch reactors with a liquid fill ratio of 60–70% and a turbine tip speed of 3.5–6.0 m/s, the steady-state θ_NH3 can be increased by raising ammonia partial pressure from 0.3 MPa to 1.0 MPa while maintaining total pressure through a reduction in nitrogen or methane diluent. The practical limit is the increasing ammonia solubility in the organic phase, which changes the adsorption equilibrium of imine intermediates and can cause the carbonyl substrate to partition away from the catalyst surface. In continuous slurry bubble columns with internal diameters of 100–500 mm and height-to-diameter ratios of 8:1–15:1, catalyst loadings above 10 wt% can reduce the volumetric mass transfer coefficient sufficiently to create ammonia depletion inside the bottom region of the reactor even when the inlet gas is stoichiometrically ammonia-rich. Published data for full-scale slurry bubble column coverage gradients are limited, but the onset of secondary amine products at constant conversion is used industrially as a diagnostic.

Thermal Desorption Characteristics of NHx Species on Step-Defect-Rich Raney Nickel

Temperature-programmed desorption and diffuse reflectance infrared spectroscopy on Raney nickel show that ammonia coverage is not a single adsorbed state but a distribution of molecular NH₃, NH₂, and NH fragments with different desorption activation energies. After saturation at 50°C and flushing with helium, high-coverage NH₃ desorbs in a broad peak centred near 320–400°C; recombination of NH and NH₂ fragments appears at 400–550°C as hydrogen is co-produced. The lower-temperature desorption peak corresponds to weak ammonia bound to nickel atoms adjacent to alumina or silica promoters, while the high-temperature feature arises from strongly bound NHx species on step and kink sites. In industrial reduction procedures, passivated nickel catalysts are pre-dried at 280–320°C under flowing nitrogen for 4–8 h before hydrogen reduction at 400–480°C. If moisture is not removed before reduction, residual water competes with ammonia for Lewis acid sites on the support and reduces the effective θ_NH3 at the start of the campaign. The practical consequence is that the same reactor feed produces different amine ratios depending on catalyst pre-treatment history, a batch-to-batch variance observed in manufacturing lines when catalyst drums are exposed to ambient air above 60% relative humidity. For this reason, catalyst loading procedures in multi-tube reactors require dry nitrogen blanketing and a maximum open-to-atmosphere time of 30 min.

The safe processing window for primary amine selectivity over high-nickel catalysts is often less than ±5°C when operating at fixed H₂:NH₃ ratio. A temperature increase from 145°C to 155°C at 0.8 MPa ammonia partial pressure can reduce θ_NH3 from approximately 0.55 to 0.42 on a Ni/SiO₂ catalyst with 18 wt% nickel, producing an increase in secondary amine content from 1.2 wt% to 4.6 wt% in the crude product as measured by ASTM D2074-07. The sensitivity is amplified in multi-tube fixed-bed reactors with tube inner diameters of 38–50 mm because the radial temperature gradient can exceed 8°C at heat-flux densities above 1.2 W/cm². In such tubes, the centreline catalyst operates with a lower ammonia coverage than the near-wall catalyst, and the resulting radial distribution of imine coupling products can be detected as a higher secondary amine content in product samples taken from the centre region during laboratory-scale single-tube simulator studies. Published data for full-scale radial coverage distributions are limited, but the practical response is to reduce the tube diameter, dilute the bed with inert spacers, or increase ammonia partial pressure rather than raise the coolant temperature.

When the Ammonia-to-Ketone Feed Ratio Drops Below the Imine-Coupling Threshold in Fixed-Bed Cyclohexylamine Production

In fixed-bed cyclohexylamine production from cyclohexanone, ammonia, and hydrogen over nickel-containing catalysts, the selectivity to the primary amine is governed by the relative surface coverage of ammonia and the hydrogenation rate of the imine intermediate. When the ammonia-to-ketone molar feed ratio drops below approximately 5:1, the surface concentration of imine becomes high enough to undergo nucleophilic attack by adsorbed cyclohexylamine, producing N-cyclohexylidenecyclohexanamine and subsequently dicyclohexylamine. The threshold is shifted by temperature and hydrogen partial pressure; at 150°C and 0.6 MPa ammonia partial pressure, the ratio may need to exceed 8:1 to maintain dicyclohexylamine below 2.0 wt% in the crude product. Fixed-bed reactors equipped with interstage ammonia injection use a ratio profile that starts at 10:1 at the first bed inlet and falls to 3:1 at the final bed outlet, while hydrogen is added per bed to maintain the H₂:NH₃ ratio above 1.5:1. The coverage requirement is therefore not a single setpoint but a trajectory that must be matched to the axial conversion and temperature profile. Operation below the trajectory promotes imine coupling and oligomerization, which increases the high-boiling fraction and eventually deposits carbonaceous residues on the catalyst surface.

Catalyst Pellet Geometry, Pressure Drop, and Axial Coverage Decay in Ammonia Recycle Loops

Ammonia recovery loops change the coverage requirement because the recycle stream contains residual hydrogen, inert gases, and small quantities of water that affect both the ammonia dew point and the partial pressure of ammonia at the reactor inlet. In continuous isopropylamine production from acetone and ammonia, an inlet ammonia partial pressure of 1.2 MPa can decline to 0.4 MPa at 80% acetone conversion, creating a terminal bed zone where the reaction is no longer coverage-protected. To avoid this, the overall ammonia-to-acetone molar ratio is maintained at 5:1–15:1 or an interstage knockout drum is used to recover and reintroduce ammonia before the final bed. The required ammonia partial pressure also depends on pellet geometry. Trilobe extrudates with a diameter of 2.5 mm provide lower pressure drop than 1.5 mm cylinders but reduce the external surface-to-volume ratio, which can make the coverage at the pellet centre more dependent on liquid-filled pores. In gas-phase amination with a high ammonia partial pressure, condensation can occur when the reactor wall temperature is below the ammonia dew point; this condition produces liquid ammonia films that alter reactant diffusion and may leach nickel from the support. Process piping and separator vessels in such loops are designed to ASME B31.3 with a design pressure of at least 1.1 times the maximum operating pressure, and pressure-relief valves are set to 110% of design pressure where local code requirements permit. The catalyst bed pressure drop is typically limited to 0.05–0.15 MPa for radial-flow reactors and 0.2–0.5 MPa for axial-flow reactors, depending on pellet crush strength measured by ASTM D4179-22.

During routine production, ammonia coverage is adjusted by monitoring the product amine profile against the analytical ranges referenced in Table 2 and by maintaining the catalyst pressure drop within the specified mechanical limit. A shift from primary to secondary amines at constant conversion indicates a loss of effective ammonia coverage, while an increase in bed pressure drop without a corresponding increase in throughput indicates nitride-related pellet expansion or mechanical breakage. The two failure modes require opposite corrective actions; therefore, any change in ammonia partial pressure must be made only after confirming hydrogen partial pressure and catalyst mechanical integrity.