Green synthesis of hydrogen peroxide using anthraquinone process with palladium catalyst

Hydrogen peroxide is one of the most important industrial oxidants, used in bleaching, disinfection, and chemical synthesis, yet its conventional anthraquinone Auto-Oxidation (AO) process generates substantial organic solvent waste and requires energy-intensive hydrogenation under high pressure. Heterogeneous palladium-catalysed hydrogenation of 2-ethylanthraquinone in an environmentally improved solvent system offers a greener pathway with lower pressure requirements and recyclable catalysts. This research evaluated four Pd-supported catalysts (Pd/Al2O3, Pd/SiO2, Pd/TiO2, Pd/C) at 2 wt% Pd loading for the hydrogenation step of the AO process, and optimised Pd loading (0.5-3 wt%) on the best-performing Al2O3 support. Reaction conditions (temperature 45 degrees C, H2 pressure 1.5 bar, 2-ethylanthraquinone 0.15 M in 2-pentanol/trimethylbenzene 1:1) were held constant. Pd/Al2O3 at 2 wt% Pd gave the highest H2O2 yield (387 mmol/L) and selectivity (91.3%) with acceptable Pd leaching (3.4 ppb). Pd/TiO2 showed superior stability over 10 reaction cycles (95.7% activity retention vs. 88.9% for Al2O3 and 79.2% for SiO2). Turnover Frequency (TOF) decreased with increasing Pd loading, indicating that higher dispersion at lower loading is more catalytically efficient per Pd atom. These results identify Pd/TiO2 as the preferred catalyst for a stable, green AO process and demonstrate the feasibility of operating the hydrogenation step at near-ambient pressure in an improved solvent system.

Illustrations explain the paper; charts are replotted from the paper's own table values.

AI-generated illustration

↗ Enlarge

Catalyst Comparison MatrixClose ×

AI-generated conceptual illustration (Palladium catalysts on alumina, silica, titania, and carbon supports in a chemical reactor). It explains the idea and does not show study data.

Open full-size image ↗

Replotted from source data

↗ Enlarge

H2O2 yield (mmol/L) and Selectivity (%): Al2O3 vs SiO2 vs TiO2 vs Carbon (AC)Close ×

Replotted from Table 1 (page 2) of the original paper. Every bar is the paper's own table value.

Open full-size image ↗

Replotted from source data

↗ Enlarge

Cycle 1 and Cycle 3 and more: Pd/Al2O3 2wt% vs Pd/SiO2 2wt% vs Pd/TiO2 2wt%Close ×

Replotted from Table 2 (page 3) of the original paper. Every bar is the paper's own table value.

Open full-size image ↗

Introduction

Global hydrogen peroxide production exceeds 4 million tonnes per year, with the anthraquinone Auto-Oxidation (AO) process accounting for over 95% of this output. The AO cycle involves catalytic hydrogenation of a 2-alkylanthraquinone dissolved in an organic solvent mixture (the working solution), followed by air oxidation of the resulting anthrahydroquinone to regenerate the anthraquinone and release H 2 O 2 , which is extracted into water. The palladium-catalysed hydrogenation step governs the overall productivity and selectivity of the cycle: competing over-hydrogenation produces anthrahydroquinone tetrahydro derivatives that are inactive in the oxidation step, representing both yield loss and working solution degradation [2] . Green chemistry improvements to the AO process focus on reducing the operating hydrogen pressure (currently 2-5 bar in commercial plants), replacing conventional aromatic solvent mixtures with lower-toxicity alternatives, and improving catalyst lifetime to reduce Pd waste. Supported Pd catalysts on oxide supports offer better thermal stability and lower leaching compared to Pd/C, but the choice of support affects both activity and selectivity through electronic metal-support interactions and pore diffusion effects [4] . This research at the Drakensberg National University, Johannesburg, evaluates four support materials under a reduced-pressure, improved-solvent protocol with a focus on both initial activity and 10cycle stability. Africa, February-September 2023 [7] .

Material and Methods

Material and Methods

Material

Material

Methods

Hydrogenation reactions were carried out in a 100 mL Parr autoclave with temperature control (±1 degree C) and H2 pressure supply (mass flow controlled). Reaction conditions: 45 degrees C, 1.5 bar H2, 50 mL working solution (0. [9] .

Results

Table 1). Pd/TiO 2 performed similarly in yield (371 mmol/L) but with lower Pd leaching (2.8 ppb) and, as shown in Table 2, superior stability. Pd/C showed the lowest selectivity (78.4%), consistent with the high surface area and surface heterogeneity of activated carbon promoting over-hydrogenation side reactions. The TOF analysis across Pd loadings revealed an inverse relationship: at 0.5 wt% Pd, TOF was 1847 h-1 versus 795 h-1 at 3 wt%, confirming that smaller, better-dispersed Pd nanoparticles (higher dispersion at lower loading, 58.7% vs. 33.2%) are more intrinsically active per Pd atom. However, absolute H 2 O 2 yield was higher at 2 wt% due to the greater total Pd surface area [14] . Pd/TiO 2 demonstrated outstanding stability with 95.7% activity retention after 10 cycles, compared to 88.9% for Pd/Al 2 O 3 and 79.2% for Pd/SiO 2 (Table 2). The strong metal-support interaction (SMSI) between Pd and TiO 2 , well established in the heterogeneous catalysis literature, stabilises Pd nanoparticles against sintering and leaching under the mild aqueous-organic reaction conditions. ICP-MS confirmed the lowest cumulative Pd leaching for Pd/TiO 2 (2.8 ppb per cycle), consistent with the reduced nanoparticle mobility on the TiO 2 surface [15] .

Catalyst	Cycle 1	Cycle 3	Cycle 5	Cycle 7	Cycle 10	Retention (%)
Pd/Al2O3 2wt%	387	381	373	359	344	88.9
Pd/SiO2 2wt%	342	329	313	293	271	79.2
Pd/TiO2 2wt%	371	368	364	360	355	95.7

Table text extracted from the original PDF; superscript letters are shown inline. The original table image is below.

View original table image

↗ Enlarge

Table 2Close ×

Catalyst stability: H2O2 production (mmol/L) over 10 consecutive reaction cycles for three best-performing catalysts

Open full-size image ↗

Original paper tableSource PDF · page 3 ↗

Support	Pd loading (wt%)	BET SA (m2/g)	Pd dispersion (%)	H2O2 yield (mmol/L)	Selectivity (%)	TOF (h-1)	Pd leach (ppb)
Al2O3	2.0	187.4	42.3	387±18	91.3	1247	3.4
SiO2	2.0	312.7	38.7	342±22	84.2	1103	4.1
TiO2	2.0	143.2	51.4	371±19	88.7	1197	2.8
Carbon (AC)	2.0	892.3	34.1	318±28	78.4	1026	6.7
Al2O3	0.5	189.1	58.7	287±21	89.4	1847	1.2
Al2O3	3.0	184.8	33.2	371±24	87.3	795	5.1

Table text extracted from the original PDF; superscript letters are shown inline. The original table image is below.

View original table image

↗ Enlarge

Table 1Close ×

Catalyst characterisation and H2O2 synthesis performance for four Pd-supported catalysts at 2 wt% Pd loading, and Al2O3-

Open full-size image ↗

Original paper tableSource PDF · page 2 ↗

Discussion

The superior stability of Pd/TiO 2 is consistent with TiO 2 's known role as a strong-interaction support that anchors Pd via oxygen vacancy-mediated bonding, limiting nanoparticle migration during repeated hydrogenation-oxidation cycles.

The slight activity advantage of Pd/Al 2 O 3 in initial yield (387 vs 371 mmol/L) likely reflects higher overall Pd surface area on the higher-dispersion Al 2 O 3 surface, but this advantage erodes over cycles as sintering occurs. For a commercial process where catalyst lifetime is the dominant operating cost driver after raw material costs, Pd/TiO 2 is the preferred candidate [14] . The reduced H2 operating pressure of 1.5 bar used in this research represents a meaningful safety and energy improvement over conventional AO plants operating at 2-5 bar. The 2-pentanol/trimethylbenzene solvent system is less acutely toxic than the alkylnaphthalene/phosphate ester mixtures historically used, reducing occupational exposure risk. Together with the low-leaching Pd/TiO 2 catalyst, these modifications advance the AO process toward greener operating conditions without compromising productivity supporting the relevance of this work to industrial H 2 O 2 manufacturers seeking process improvement [15] .

Conclusion

Four Pd-supported catalysts were evaluated for the anthraquinone process hydrogenation step under reducedpressure (1.

Figures and tables from the paper

↗ Enlarge

Figure 1Close ×

Left: H2O2 production over 120 min reaction time for different Pd loading levels on Al2O3. Right: radar chart comparing six

Open full-size image ↗

Original paper figureSource PDF · page 2 ↗

↗ Enlarge

Figure 2Close ×

H2O2 production over 10 consecutive reaction cycles for Pd/Al2O3, Pd/SiO2, and Pd/TiO2 catalysts (2 wt% Pd, 45°C, 1.5 bar H2)

Open full-size image ↗

Original paper figureSource PDF · page 3 ↗

1. Centi G, Perathoner S, Abate S. Direct synthesis of hydrogen peroxide: recent advances. Top Catal. 2008;48(1-4):41-51.
2. Samanta C. Direct synthesis of hydrogen peroxide from hydrogen and oxygen: an overview of recent developments in the process. Appl Catal A Gen. 2008;350(2):133-149.
3. Goor G, Glenneberg J, Jacobi S. Hydrogen peroxide. In: Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH; 2007.
4. Edwards JK, Solsona B, N EN, Carley AF, Herzing AA, Kiely CJ, Hutchings GJ. Switching off hydrogen peroxide hydrogenation in the direct synthesis process. Science. 2009;323(5917):1037-1041.
5. Campos-Martin JM, Blanco-Brieva G, Fierro JL. Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process. Angew Chem Int Ed. 2006;45(42):6962-6984.
6. Liu Q, Boereboom JM, Berends E. Supported palladium catalysts for the direct synthesis of H 2 O 2 . J Catal. 2006;237(2):419-430.
7. Blanco-Brieva G, Capel-Sanchez MC, de Frutos MP, Padilla JM, Campos-Martin JM, Fierro JL. New twophase solvent system for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Ind Eng Chem Res. 2008;47(21):8011-8015.
8. Lewis RJ, Hutchings GJ. Recent advances in the direct synthesis of H 2 O 2 . ChemCatChem. 2019;11(1):298308.
9. Krishnan S, Bhatt NK, Babu KS. Green oxidation chemistry: palladium-catalysed peroxide synthesis. Catal Today. 2020;356:258-267.
10. Taramasso M, Perego G, Notari B. Preparation of porous crystalline synthetic material comprising silicon and titanium oxides. US Patent 4,410,501; 1983.
11. Lunsford JH. The direct formation of H 2 O 2 from H2 and O2 over palladium catalysts. J Catal. 2003;216(12):455-460.
12. Freakley SJ, He Q, Harrhy JH, Lu L, Crole DA, Morgan DJ, Hutchings GJ. Palladium-tin catalysts for the direct synthesis of H 2 O 2 with high selectivity. Science. 2016;351(6276):965-968.
13. Han YF, Zhong Z, Ramesh K, Chen F, Chen L, White T, Tay Q, Yaakob SN, Wang C. Au promotional effects on the synthesis of H 2 O 2 directly from H2 and O2 on supported Pd-Au alloy catalysts. J Phys Chem C. 2007;111(24):8410-8413.
14. Piqueras CM, Balbueena P, Luyanda K. Anthraquinone process optimization using heterogeneous Pd catalysts. Catal Commun. 2021;149:106203.
15. Biasi P, Mikkola JP, Sterchele N, Salmi T, Russo N, Fino D, Centomo P, Zecca M. Applicability of the Langmuir-Hinshelwood model for the direct synthesis of H 2 O 2 . AIChE J. 2017;63(1):32-42.
16. Wang S, Zhao L, Wang W, Zhao Y, Zhang G, Ma X, Gong J. Morphology control of ceria nanocrystals for catalytic conversion of CO 2 with methanol. Nanoscale. 2013;5(12):5582-5588.
17. Dissanayake DP, Lunsford JH. Evidence for the role of colloidal palladium in the catalytic formation of H 2 O 2 from H2 and O2. J Catal. 2002;206(1):173-176.
18. Ntainjua EN, Edwards JK, Carley AF, Lopez-Sanchez JA, Moulijn JA, Herzing AA, Kiely CJ, Hutchings GJ. The role of the support in achieving high selectivity in the direct formation of hydrogen peroxide. Green Chem. 2008;10(11):1162-1169.
19. Choudhary VR, Jana P. Factors affecting the formation of by-products in the direct synthesis of H 2 O 2 from H2 and O2 over supported Pd catalyst. J Catal. 2007;246(2):434-439.
20. Mthembu N, Ndlovu A, Sithole N, Khumalo L. Towards greener H 2 O 2 production at lower pressure: catalyst comparison. S Afr J Chem. 2024;77:112-121.