The Quest for Sustainable Chemistry
In the heart of Sapporo, scientists forged bonds stronger than the chemical reactions they study.
Catalysis is the invisible engine of our modern world, the secret behind everything from the fuel in our cars to the medicines in our cabinets. For decades, the scientific community working on catalysts has been divided into distinct tribes: those who study homogeneous catalysis (where catalyst and reactants exist in the same phase, often a liquid), those who focus on heterogeneous catalysis (where the catalyst is in a different phase, typically a solid interacting with liquid or gas reactants), and those exploring enzymatic catalysis. In 2013, over 500 scientists from 30 nations gathered at Hokkaido University to bridge these worlds. Their goal was not just to share discoveries, but to forge a unified understanding of catalysis on the molecular level, a crucial step toward designing the sustainable technologies of the future 1 3 7 .
To appreciate the significance of this symposium, one must first understand the classic split in the world of catalysis.
In a heterogeneous reaction, the catalyst is in a different phase from the reactants. A simple example is a solid catalyst that speeds up reactions between gaseous reactants. Imagine tiny nickel particles facilitating the addition of hydrogen to vegetable oils to produce margarine. The reactants (hydrogen gas and liquid oil) interact on the solid surface of the nickel catalyst 5 .
Reactant molecules stick to active sites on the catalyst's surface.
The adsorbed molecules interact, often with their bonds weakened, forming new products.
The product molecules break away from the surface, freeing the active site for a new cycle 5 .
In contrast, a homogeneous reaction has the catalyst in the same phase as the reactants, usually a liquid. A classic example is the use of iron(II) or iron(III) ions to catalyze the slow reaction between persulfate ions and iodide ions in water. The catalyst provides an alternative reaction pathway that avoids the difficult collision between two negatively charged ions, dramatically speeding up the reaction 5 .
Each approach has its strengths; heterogeneous catalysts are often easier to separate and reuse, while homogeneous catalysts can be more selective and efficient. The mission of the ISHHC-16 was to break down the barriers between these fields, fostering a collaborative spirit to tackle global challenges 1 3 .
The 16th International Symposium on Relations Between Homogeneous and Heterogeneous Catalysis (ISHHC-16) was more than just a conference; it was a symbolic gathering. Held from August 4-9, 2013, its logo featured the Japanese word "Kizuna," meaning "bond," representing both the human connections within the scientific community and the chemical bonds they seek to understand 1 .
The symposium was structured to force cross-pollination between disciplines. The oral sessions were deliberately arranged to mix lectures on homogeneous and heterogeneous catalysis, compelling attendees to look at problems from different angles 7 . The scientific program covered a wide range of pressing topics, from energy and environmental catalysts to bio-inspired catalysts and catalysis for green and sustainable chemistry 1 .
The gathering featured seven plenary lectures from top-tier scientists like Prof. Gabor A. Somorjai (heterogeneous catalysis) and Prof. David Milstein (homogeneous catalysis), who stressed the importance of stronger collaborations with biological systems 7 . With 96 oral presentations and 230 posters, the event was a hotbed for new ideas, further energized by evening sessions on industrial challenges and international collaboration 1 7 .
A powerful example of the kind of hybrid catalysis the symposium championed is the development of a polymer-supported iron catalyst for the selective reduction of nitro compounds to amines. This research, representative of the field's direction, combines the selectivity of a homogeneous-style metal complex with the reusability of a heterogeneous support .
Aromatic amines are vital intermediates in producing dyes, pharmaceuticals, and agricultural chemicals. Traditionally, converting nitro groups to amines can involve hazardous hydrogen gas or inefficient methods. This experiment demonstrated a safer, "greener" alternative using an iron-based catalyst—an abundant and cheap metal—anchored onto a polymer solid support .
The experiment was a notable success. The catalyst efficiently converted a range of nitroarenes into their corresponding amines with high selectivity. The use of water-ethyl acetate as a solvent and acetic acid as a hydrogen donor made the process more environmentally friendly than methods relying on toxic solvents and hazardous molecular hydrogen .
Crucially, the heterogeneous nature of the catalysis was confirmed. The catalyst could be reused for five consecutive cycles without a significant loss in activity, demonstrating its robustness and practical potential for industrial applications. This aligns perfectly with the principles of green and sustainable chemistry emphasized at the ISHHC-16 .
| Nitroarene Substrate | Product Amine | Primary Application of Product |
|---|---|---|
| Nitrobenzene | Aniline | Production of polyurethanes, dyes, and pharmaceuticals |
| 4-Nitroanisole | 4-Anisidine | Intermediate for dyes and pharmaceuticals |
| 4-Chloronitrobenzene | 4-Chloroaniline | Synthesis of pesticides and herbicides |
| Feature | Advantage |
|---|---|
| Iron-based Catalyst | Abundant, inexpensive, and low toxicity compared to precious metals. |
| Polymer Support | Easy separation by filtration, enabling reuse and simplifying product isolation. |
| Water-Ethyl Acetate Solvent | More environmentally benign solvent system. |
| Acetic Acid Hydrogen Donor | Safer than using high-pressure hydrogen gas. |
| Chemoselectivity | Selectively reduces nitro groups even in the presence of other sensitive functional groups. |
The following table outlines some essential materials and methods frequently employed in the development and study of advanced catalysts, as seen in the featured experiment and throughout the field.
| Tool/Reagent | Function in Catalysis Research |
|---|---|
| Polymer Supports (e.g., PS-DVB) | Provides a solid, often porous, inert matrix to anchor active catalytic species, facilitating easy separation and reuse. |
| Transition Metal Salts (e.g., FeCl₃) | The source of metal ions (like iron) that form the active center of the catalyst, often capable of changing oxidation state. |
| Organic Ligands (e.g., 2-Mercaptobenzimidazole) | Molecules that bind to the metal center to modify its electronic and steric properties, tuning its activity and selectivity. |
| Hydrogen Donors (e.g., HCOOH, Acetic Acid) | Provides a source of hydrogen atoms for reduction reactions in a safer, more controlled manner than gaseous hydrogen. |
| Density Functional Theory (DFT) Calculations | A computational method used to model and understand reaction mechanisms, active sites, and predict catalytic activity at the atomic level 4 . |
DFT calculations provide atomic-level insights into catalytic mechanisms and active sites 4 .
Automated systems rapidly test thousands of catalyst combinations to identify promising candidates 6 .
Techniques like XRD, XPS, and TEM reveal catalyst structure and composition at the nanoscale.
The ISHHC-16 symposium was a testament to a evolving paradigm in catalysis. By fostering Kizuna—the bonds between researchers across traditional disciplinary lines—the event accelerated progress toward a more sustainable society. The insights gained from understanding catalysis at the molecular level are directly feeding into the design of new catalysts that are more efficient, selective, and based on earth-abundant elements 1 4 .
The work continues globally, from the theoretical models of DFT 4 to the high-throughput discovery of new materials 6 . As the field moves forward, the integration of homogeneous, heterogeneous, and enzymatic principles, championed by gatherings like the ISHHC, promises to be the catalyst for the next generation of chemical technologies, powering our world while preserving it for the future.
The proceedings of the ISHHC-16 were published as a special issue of the journal Topics in Catalysis in 2014. The next symposium in the series, ISHHC-17, was held in Utrecht, The Netherlands, in 2015 1 .