# Example: Catalyst Selection for Aromatic Hydrogenation (Chemical Engineering)

**Status:** Draft
**Phase:** The Bedrock Phase

## What This Example Demonstrates

Context record structure (OE-0003) applied to a chemical process selection where the governing constraint is not performance but product purity, and where the physical form of the catalyst (heterogeneous vs. homogeneous) determines the feasibility of the entire downstream purification strategy (OE-0007).

## The Observation

Pilot-scale runs using a dissolved ruthenium catalyst system consistently produced product contaminated with residual ruthenium at concentrations of 12–18 ppm. The product specification for this pharmaceutical intermediate permits no more than 5 ppm of any residual metal. Separate bench-scale experiments using a solid palladium-on-carbon catalyst produced product with metal residue below the analytical detection limit of 0.1 ppm, and the reaction reached completion in less time. The dissolved catalyst system also required an additional liquid-liquid extraction step to attempt metal recovery, which added process complexity and generated additional waste streams.

## Engineering Translation

Catalyst selection in chemical process engineering is not solely a question of reaction rate or conversion. The physical nature of the catalyst — whether it dissolves in the reaction medium or remains as a distinct solid phase — creates cascading consequences for product purification, catalyst recovery, waste generation, and ultimately the economic viability of the process at production scale. The fundamental distinction here is between a catalyst that shares the same phase as the product (creating a separation problem) and one that does not (eliminating that problem entirely). This is an enduring concept in process design: the phase behavior of every component in the system determines the separation strategy, and the separation strategy often dominates the process economics.

## Context Record

| Field | Content |
|---|---|
| **Decision** | Select a heterogeneous palladium-on-carbon (Pd/C) catalyst over homogeneous ruthenium and nickel alternatives for a commercial-scale hydrogenation of an aromatic nitro compound to the corresponding aniline. |
| **Observation** | (1) Pilot-scale runs (50L reactor, 200 hours cumulative) with homogeneous Ru/TPPTS catalyst showed product contamination from ruthenium leaching at 12–18 ppm, exceeding the 5 ppm specification for pharmaceutical intermediate. (2) Pd/C bench tests (1L, 40 hours) showed no metal leaching above detection limit (0.1 ppm) and faster reaction kinetics. (3) The homogeneous catalyst required aqueous extraction for attempted metal recovery, adding a process unit operation that did not exist in the heterogeneous pathway. |
| **Alternatives** | (A) Ru/TPPTS homogeneous — rejected due to metal leaching exceeding product specification and the introduction of an aqueous extraction step for catalyst recovery that increased process complexity and waste generation. (B) Raney nickel — rejected due to pyrophoric handling risk at production scale, which creates a safety engineering burden that is disproportionate to the cost savings, and lower selectivity producing 3.2% over-hydrogenation byproducts compared to 0.4% for Pd/C. (C) Pd/C — selected as the only option satisfying both the purity constraint and the selectivity constraint simultaneously. |
| **Constraints** | Product must meet pharmaceutical-grade purity with metal residue below 5 ppm (regulatory requirement). Hydrogenation must achieve 99.5% or greater selectivity to avoid costly downstream purification of over-hydrogenation byproducts. Catalyst must be recoverable and recyclable for a minimum of 10 cycles to meet the process economics model. Process temperature must remain below 80°C due to thermal sensitivity of downstream intermediates in the synthesis sequence. Production scale is a 2000L reactor, which changes the heat removal and filtration considerations relative to bench and pilot scale. |
| **Reasoning** | The core reasoning rests on the phase behavior distinction between heterogeneous and homogeneous catalysis (OE-0007). Pd/C provides heterogeneous catalysis in which the active metal remains physically bound to the carbon support throughout the reaction, eliminating the fundamental mechanism by which metal enters the product stream. This is not an incremental improvement in separation efficiency — it removes the separation problem entirely by ensuring the catalyst and product never share the same phase. The heterogeneous nature also simplifies catalyst recovery: filtration of a solid from a liquid is a mature, scalable unit operation, whereas recovering a dissolved metal from a complex organic mixture requires multi-step extraction with associated yield losses and waste streams. The observed selectivity advantage (0.4% vs. 3.2% byproducts) further reduces downstream purification burden, compounding the economic benefit. The trade-off is higher initial catalyst cost, which the recycle requirement (10+ cycles) amortizes to an acceptable per-batch cost. |
| **Verification** | (1) Scale-up run at 200L: 50-hour continuous operation produced product at 99.7% purity with metal residue below the ICP-MS detection limit of 0.1 ppm, confirming that the leaching advantage observed at bench scale persists at intermediate scale. (2) Catalyst recycling study: a 12-cycle test demonstrated activity retention above 95% of the initial conversion rate through cycle 10, declining to 91% at cycle 12, which exceeds the 10-cycle minimum requirement and suggests the economic model is valid. (3) Thermal profile of the 200L run confirmed a maximum exotherm of 62°C, remaining within the 80°C constraint with margin, indicating that heat removal at production scale is tractable. |
| **Lineage** | Builds on catalyst screening study CR-CHEM-2023-007, which evaluated eight catalyst candidates at bench scale and identified Pd/C, Ru/TPPTS, and Raney nickel as the three viable finalists. The reaction kinetics model developed in that study provided the basis for the 200L scale-up design and informed the thermal analysis. This context record extends that screening work to a production-scale selection decision. |
| **Assumptions** | Palladium supply chain remains stable at the projected production volumes of approximately 50 batches per year. The carbon support does not degrade under the combined chemical and mechanical environment of the reactor beyond the tested 12-cycle window, meaning that long-term catalyst performance at cycle 20 or beyond is extrapolated rather than demonstrated. Filtration at 2000L scale achieves separation efficiency equivalent to bench-scale filtration — this assumption carries risk because the filter cake properties and flow dynamics change significantly with scale. No catalyst poisoning occurs from trace impurities in the feedstock that were not present in the pilot-scale material, since the pilot feedstock came from a different supplier lot. |
| **Open Questions** | At what reactor scale does heat removal become the binding constraint rather than selectivity, and does the answer change the catalyst form factor preference? Can the Pd/C catalyst be regenerated in-situ through a reductive treatment step, or does it require external reactivation that would interrupt production and add logistics complexity? |

## Self-Fading Assessment

This example builds a bridge from the abstract concept of heterogeneous versus homogeneous catalysis to the concrete consequence that phase behavior determines separation strategy, and separation strategy often dominates process economics. The reader has crossed this bridge when they can look at any mixed-phase reaction system and immediately identify which component is in which phase, what separation that implies, and whether that separation is a routine unit operation or an unsolved problem. Once this pattern is recognized, the specific details of palladium, ruthenium, and the nitro-to-aniline chemistry become incidental — the underlying principle of phase-determined separability applies equally to leaching, extraction, crystallization, and distillation decisions across all of chemical engineering.