Transitioning from Traditional to Green Catalysts: A Systematic Review of Sustainable Catalysis in Industrial Chemistry

Table 5. Representative performance metrics heterogeneous nanocatalysts (primary studies only).

Note: †N/R = not reported using the conventional TOF metric; performance assessed by yield and selectivity. Direct TOF comparisons across reactions with different substrates and conditions should be interpreted with caution (Section 2.3). All studies in Table 5 are high-quality (CASP).

Across all Suzuki coupling studies, Pd nanocatalysts achieved TOF values 2.4–6. 1-fold higher than homogeneous Pd complex analogues, a consistent finding across the included studies [14-16]. SACs represent the most significant emerging trend, with each isolated metal atom constituting an accessible active site and maximising atom utilisation efficiency. The most rigorous LCA comparison to date, by [17] demonstrated that Ni-based SACs for C–O ester coupling already outperform Pd- and Ir-based homogeneous systems in terms of Global Warming Potential (GWP), material criticality, and cost per kilogram of product under 2024 conditions, with the advantage projected to widen under 2035 renewable-energy pricing ([19], contextual).

The magnetic Fe₃O₄@SiO₂ systems exhibited excellent practical recyclability via simple magnetic separation, retaining 90–98% of their activity after 5–12 cycles. Eight nanocatalyst studies reported environmental LCA, with cumulative energy demand savings of 40-73% (quality-weighted mean: 56%) compared to conventional comparators, and GWP savings of 35-62% (mean: 47%) compared to conventional comparators (reaction-step values; only three studies reported full value-chain LCA).

Other issues that were not captured by the performance metrics were the ecotoxicological profile of the released nanoparticles, Pd leaching under severe conditions (confirmed by X-ray absorption spectroscopy by [20]), and the geopolitical concentration of supply: Pd is sourced from approximately 78% from Russia and South Africa, and Ir supply is similarly concentrated in South Africa supply chain risks that industrial investors must consider separately from catalytic efficiency.

3.4. Metal-Organic Framework (MOF)-Based Catalysts

Eleven main studies (4 high-quality, 6 moderate, and 1 low) were conducted on MOF-based catalytic systems. The studied frameworks included mainly zirconium-based UiO-66 variants and iron-based MIL series, which were used for Knoevenagel condensation, Meerwein–Ponndorf–Verley (MPV) reduction, Diels–Alder reactions, asymmetric hydrogenation, CO₂ cycloaddition, and selective oxidation.

MOFs offer qualitatively distinct structural properties, including surface areas of 500–6,500 m² g⁻¹, programmable pore geometry, and capacity for active-site microenvironment design through Post-Synthetic Modification (PSM).

Confinement-mediated selectivity is the defining characteristic of MOF catalysts. Mechanistic evidence reported by [21] demonstrated that UiO-66FC achieves near-quantitative yields in MPV reduction through an allosteric conformational change at Zr nodes inaccessible to homogeneous Zr complexes or traditional heterogeneous supports. [22] showed that rational active-site microenvironment design in MOF pores opens reactivity patterns that are genuinely inaccessible to molecular catalysts. Across the 11 included studies, selectivities of 93–99% were observed in reactions with multiple competing functional groups, substantially exceeding the zeolite comparators (72–85%). Flow Diels–Alder and Friedel–Crafts reactions on ZrOTf-BTC gave TONs of 1,700 and 2,700, respectively, with a single MOF framework catalysing mechanistically distinct reactions, a platform generality separating MOFs from reaction-specific molecular catalysts [23].

Industrial implementation barriers have been well documented. Seven of the 11 MOF studies reported catalyst degradation or surface-area loss exceeding 25% in aqueous conditions at pH below 4 or temperatures above 150 °C. Three studies identified batch-to-batch synthesis reproducibility at the kilogram scale as a challenge for GMP compliance. Specialty organic linkers remain costly, although mechanochemical MOF synthesis and bio-derived linker analogues are beginning to address this barrier.

3.5. Photocatalytic Systems

Seven main studies were reviewed, two of which were high-quality, three were moderate, and two were low. These studies included semiconductor photocatalysis (TiO₂, g-C₃N₄, and ZnO), plasmonic metal–semiconductor composites (Pd/Au/CeO₂ and Ag/ZnO), and Z-scheme heterojunctions. The basic appeal of photocatalysis is that it replaces thermally activated reactions with light-activated reactions under ambient conditions, in principle, without the energy cost of thermal activation.

Graphitic carbon nitride (g-C₃N₄) is the most studied photocatalyst, valued for its metal-free composition, visible-light activity (bandgap 2.7 eV), ease of synthesis from low-cost precursors, and thermal stability at 600 °C [24-26]. Despite these attributes, the photocatalytic evidence base shows uniform and severe performance constraints. The quantum yield (Φ) varied between 4.2% and 31.7% across the included studies, with the highest values attained only by optimised Z-scheme heterojunction systems under LED irradiation. Bulk g-C₃N₄ produced by calcination has a surface area of approximately 10-m² g⁻¹, severely limiting substrate accessibility, the and rapid recombination of photogenerated electron–hole pairs further diminish the effective quantum efficiency. Morphological control, elemental doping, and heterojunction engineering improve performance but increase the synthesis complexity and cost.

Most importantly, all seven photocatalytic studies were performed at the laboratory scale (reaction volumes ≤500 mL), and none were validated at the pilot scale. The engineering challenges of photon flux distribution, mass transfer, and thermal management at reactor scales larger than a few litres are completely unaddressed in the current evidence base, which is the largest unaddressed engineering gap in all four catalyst families.

4. DISCUSSION

4.1. Comparative Performance Maturity and TRL Positioning

The synthesis of the evidence from all four families of catalysts provides a clear picture: there is no single universal best green catalyst, and the choice is context-dependent, depending on the type of reaction, the value of the product, the requirements for enantioselectivity, and the industrial infrastructure. This is consistent with the fit-for-purpose framework provided by [1] as contextual framing.

In the pharmaceutical sector, where enantioselectivity requirements are most demanding, biocatalysts lead to industrial readiness (TRL 7–9 for established enzymatic processes) and environmental performance. The quality-weighted 67% E-factor reduction (H-only: 55–79%) directly addresses the most persistent sustainability critique of pharmaceutical production. Heterogeneous nanocatalysts span the widest reaction scope and have achieved the broadest industrial penetration, with SACs representing the frontier where atom utilisation efficiency approaches its theoretical maximum. MOF-based systems occupy a distinct niche of confinement-mediated selectivity approaching enzymatic precision without enzymatic fragility, but their TRL of 3–5 reflects real unresolved challenges in hydrothermal stability, synthesis reproducibility, and linker cost. The disconnect between MOF laboratory capabilities and industrial implementability is primarily an engineering and materials science issue rather than a catalytic chemistry issue. Photocatalytic systems at TRL 2–4 is the most scale-up-challenged of the four families: the physics of photon flux distribution, mass transfer, and thermal management at reactor scales beyond a few litres have not been confronted in any included study.

4.2. Why Better Performance Has Not Produced Faster Adoption

Given the consistent and often dramatic performance advantages, why has green catalyst adoption remained slow and uneven? At least four interacting mechanisms can explain this gap.

First, reaction-level green chemistry metrics do not capture the full cost of technological transition. The E-factor and atom economy quantify what occurs within the reactor but do not consider catalyst acquisition and regeneration costs, capital expenditure for new reactor designs, and the regulatory cost of re-qualifying approved manufacturing processes. In the pharmaceutical industry, a single-step change in an approved process can trigger revalidation requirements costing millions of dollars and months of calendar time, even when the replacement catalyst demonstrably performs better [2]. Greener does not automatically mean cheaper when the full transition economics are accounted for.

Second, supply chain risks pose genuine uncertainty to industrial investors. The noble metals at the core of heterogeneous nanocatalysts Pd sourced approximately 78% from Russia and South Africa, and Ir, primarily from South Africa, expose manufacturers to price volatility and geopolitical supply disruption. The requirement for decades of reliability for industrial deployment is challenging, even if the materials are highly catalytic, because it is difficult to achieve with geographically concentrated materials.

Third, there is a lack of regulatory frameworks that follow catalyst innovation. There are no harmonised international standards for green catalyst characterisation, performance benchmarking, and life cycle impact assessment, which leads to uncertainty for both manufacturers and regulators. In the absence of agreed frameworks, each company will have to develop its own assessment methodology, which will waste resources and create inconsistencies that will hinder the learning process at the industry level.

Fourth, and most fundamentally, there is a structural gap in the evidence base. Only 19.1% (9/47) of the included studies reported integrated TEA. Considering the scarcity of integrated TEA and LCA evidence, it appears that there is not a reporting gap but a significant lack of knowledge. All studies in the current body of literature reported reaction-level performance measurements, which included yield, selectivity, Turnover Frequency (TOF), waste reduction, and recyclability. However, few studies (nine of 47, 19.1%) considered both economic and lifecycle impacts on industrial choices. A highly active laboratory catalyst with good environmental metrics may not be economically viable for commercial use if the catalyst requires expensive raw materials, recalcitrant regeneration, or if raw materials are scarce. Hence, the lack of integrated TEA/LCA evidence reduces the possibility of enabling EFIs to be made across a green catalyst system (as a whole) and existing conventional processes based on complete economic and environmental metrics. If this evidence is not available, industrial decision-makers cannot compare the full value chain economics of a green catalyst investment to a conventional process baseline.

4.3. Qualification of Environmental and Economic Evidence

There was no comparative study in the evidence base that showed conventional processes to be better than green catalysts in terms of the E-factor, atom economy, or LCA. This uniformity is likely due to publication bias, as studies with null or negative results for green catalysts are less likely to be submitted, accepted or indexed. This also mirrors the study design of most of the papers included, which specifically designed reaction-step comparisons under conditions favourable to green catalysts. Therefore, the quantitative data in Section 3 should be interpreted as direction indicators of benefits in similar reaction classes rather than absolute performance scores. Individual study data are more useful for technology selection than data ranges. This also reflects the study design of most of the included papers, which specifically constructed reaction step comparisons under conditions favourable to green catalysts. Therefore, the quantitative data in Section 3 should be read as directional indicators of benefits in comparable reaction classes, not as absolute performance scores. For technology selection, individual study data are more informative than aggregate data.

Where TEA data were available (nine studies), green catalysts generally showed higher initial catalyst and, in some cases, higher reactor capital costs, offset by savings in raw material consumption (higher atom economy), waste treatment (lower E-factor), and per-cycle cost (longer catalyst service life).

The most advanced TEA [17] for Ni SACs showed that it is cost-competitive with Pd-based homogeneous routes at 2024 energy prices, and the benefits are expected to grow significantly when renewable energy prices are applied in 2035.

4.4. Barriers to Industrial Adoption

Technical barriers are the most extensively documented barriers. All four catalyst families are vulnerable to deactivation under industrial process conditions through sintering, leaching, poisoning, or hydrolysis. The mechanisms vary by family: conformational changes in enzyme structure due to pH, temperature, or co-solvent exposure in biocatalysts; hydrothermal framework degradation in MOFs; charge recombination and photocorrosion in photocatalysis; and metal leaching in nanocatalysts (confirmed spectroscopically for Pd by [20]). A consistent finding is that deactivation mechanisms are more easily characterised at the laboratory scale than under the more demanding and variable conditions of industrial production.

Regulatory barriers, less prominent in primary chemistry literature but present throughout contextual sources, centre on the absence of harmonised standards. There are no agreed characterisation protocols or life cycle assessment frameworks, which means that each manufacturer must create its own assessment procedures, waste effort and preventing the industry from building knowledge. Barriers in the supply chain are most pronounced for noble-metal nanocatalysts and MOF systems with specialty organic linkers. Earth-abundant metal SACs must be developed in a geopolitically concentrated manner, not only because of environmental preferences but also because of supply security necessities. The competitiveness of Ni-, Fe-, and Cu-based SACs has already been proven in some reaction classes compared to Pd and Ir systems [17]. Although several green catalyst systems have been found to perform well in laboratory settings, problems in moving to an industrial scale affect many of them. While MOFs often work very well at low temperatures, pressures, and/or solvent/moisture concentrations, some of their structural stabilities may be difficult to achieve under industrially relevant temperatures, pressures, solvents, or moisture conditions. The same has been found to be true for photocatalytic systems, which have been shown to catalyse reactions efficiently at a small scale in the laboratory, but are less studied at an industrial scale because photon penetration, light distribution, and reactor engineering become increasingly complex at larger scales. Transferring biocatalytic systems from optimised laboratory conditions to continuous manufacturing processes also presents a number of obstacles, including enzyme stability, substrate compatibility, and productivity. These examples demonstrate that the performance of a reaction is not the only criterion for industrial use and underscore the need to consider the stability and scalability, economics, and life-cycle impacts of a reaction, as well as its performance [27].

4.5. Towards an Integrated Analytical Framework

This review proposes a three-level analytical bridge connecting the evidence in Section 3 to system-level sustainability claims that appear throughout the literature. The principles of green chemistry [3] outline the molecular goals of a catalytic reaction: atom economy, waste prevention, and catalytic rather than stoichiometric reagents consumption. Industrial ecology extends the scope to the facility and sector levels, examining how the efficiency of the reaction translates to material flows, energy use, and waste production within production networks. Circular economy logic can be applied to the entire life cycle of a product and catalyst. Is it possible to recover and regenerate the catalysts? Is it possible to reuse process waste as a feedstock for further reactions? Is it possible to design end-of-life into the synthesis process from the beginning? A green catalyst with a good E-factor which needs to be synthesised from virgin rare-earth metals each time is green in the narrow sense but not necessarily sustainable in the broad sense. A catalyst with a lower E-factor, which allows for closed-loop solvent recovery, is based on earth-abundant metals, and is designed for end-of-life recycling, may be more sustainably beneficial overall. This framework offers a common analytical lens for assessing the evidence in Section 3 and prioritising the research agenda below.

4.6. Research Priorities

Four priorities emerged from the synthesis as interventions most likely to accelerate industrial green catalyst adoption.

Full-value-chain LCA and integrated TEA should not be optional additions to publications that claim industrial relevance. Only 19.1% of the included studies reported this evidence. Major journals should develop and mandate consensus-based TEA and LCA reporting frameworks as standardised as the E-factor metric proposed by [2].

Pilot and industrial-scale validation is required, particularly for photocatalytic systems (where scale-up is most critical) and MOF-based catalysts (where there is a lack of stability under industrial conditions). Research funding should focus on research that challenges laboratory-proven concepts with engineering reality.

The extension of earth-abundant metal SACs and engineered biocatalysts to reaction scopes currently dominated by noble metals is the most strategically important frontier. Machine learning-guided catalyst design, high-throughput computational screening, and directed enzyme evolution provide enabling tools, but only when investment is deliberately targeted to this frontier rather than the further optimisation of Pd- and Rh-dependent systems.

International research programs that extend the evidence base beyond the current geographic concentration (China, Germany, USA, UK: 70.2% of the included studies) are needed. The green catalytic transition should be designed for global applicability, not just developed in high-income economies and subsequently adopted elsewhere.