TiO2 Nanotubes for Hydroformylation of Vinyl Acetate from Syngas

Hongyuan Chuai^1*

Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong, 999077, China

^*Corresponding author:  Hongyuan Chuai, Department of Applied Physics, Hong Kong Polytechnic University, Hong Kong, 999077, China.

Received: 18 June 2025; Accepted: 26 June 2025; Published: 30 June 2025

Introduction

Hydroformylation, the addition of syngas (CO/H₂) to olefins to form aldehydes, is a cornerstone of industrial chemistry, with annual production exceeding 22 million tons [1]. Rhodium-based catalysts dominate this field due to their high activity and selectivity under mild conditions [2]. However, challenges persist, including the high cost of Rh, its deactivation by functionalized olefins (e.g., vinyl acetate), and the requirement for efficient heterogeneous catalytic systems. TiO₂ nanotubes (TNTs), synthesized via hydrothermal methods, offer a compelling solution as catalyst supports. Their high surface area (up to 300 m²/g), nanotubular morphology, and tunable surface acidity make them ideal for stabilizing metal nanoparticles and facilitating reactant adsorption [3]. Chuai et al. explored the use of TNTs in hydroformylation, demonstrating their efficacy in enhancing Rh catalyst performance through strategic modifications.

This review synthesizes Chuai et al.’s contributions, covering:

1. Rh/TNTs and bimetallic systems(Rh-Ru) for vinyl acetate
2. Alkali/alkaline earth cation-modified TNTsto improve CO adsorption and Lewis acid-base interactions.
3. Rare-earth metal promotion(La³⁺) for creating LaO_x–Rh active sites.
4. Prospects for TNTsin other catalytic applications.

Rh/TNTs and Bimetallic Systems for Vinyl Acetate Hydroformylation

Rh/TNTs: Baseline Performance

Chuai et al. prepared Rh/TNTs via impregnation-photoreduction, achieving Rh nanoparticles uniformly dispersed on TNTs [4]. In vinyl acetate hydroformylation, Rh_0.25/TNTs showed 60% conversion and 60% aldehyde selectivity (TOF = 3168 h^-1). The regioselectivity for 2-acetoxypropanal (branched aldehyde) was 100%, attributed to the stability of a five-membered ring intermediate [5].

Key limitations:

• • Functional groups (e.g., acetate in vinyl acetate) coordinate with Rh, reducing activity.
• •Low H₂ adsorption capacity limits aldehyde yield.

Rh-Ru/TNTs: Synergistic Effects

To mitigate chelation effects, Chuai et al. developed Rh-Ru/TNTs, where Ru acts as a Lewis acid to attract vinyl acetate’s carboxyl group, freeing Rh for hydroformylation [4].

Key results:

• • Vinyl acetate: Conversion increased from 39% (Rh/TNTs, 1 h) to 45% (Rh-Ru/TNTs, 1 h), but aldehyde selectivity dropped from 82% to 57% due to Ru’s hydrogenation activity.
• • Cyclohexene: Ru had no promotional effect, confirming its role is specific to functionalized olefins.

Mechanism: Ru enhances CO adsorption (FT-IR peaks at 2066 cm^{-1 and 1997 cm-1) and weakens substrate-catalyst chelation [4].}

Alkali/Alkaline Earth Cation-Modified TNTs

Design and Characterization

Chuai et al. decorated TNTs with Li^{+, Na+, K+, Mg2+, Ca2+, and Sr2+ via impregnation [6]. These cations:}

• • Increased CO adsorption (FT-IR peak intensity at 2068 cm^-1).
• • Acted as Lewis acids to attract vinyl acetate (Lewis base).

Notable catalysts:

• • Rh₂₅/Li-TNTs-P5: 100% conversion, 67% selectivity (TOF = 3581 h^-1).
• • Rh₂₅/Mg-TNTs-P5: 99% conversion, 76% selectivity (TOF = 2826 h^-1) [6].

Promotion Mechanism

1. Electronic effects: Cations (e.g., Mg²⁺) donate electrons to Rh, facilitating CO insertion (XPS shifts in Rh 3d) [6].
2. Concentration effect: Cations pre-concentrate substrates near active sites.

Trend: Smaller cations (Li^{+ > Na+ > K+) showed higher polarizing power and better performance [6].}

Rare-Earth Metal Promotion: La-Decorated Rh/TNTs

La’s Unique Role

Chuai et al. introduced La³⁺ to Rh/TNTs, achieving:

• • Rh₂₅-La/TNTs: 89% conversion, 66% selectivity, TOF = 5796 h^-1 [7].
• • H₂-TPD: Enhanced H₂ adsorption (peak at 307°C) due to LaO_x–Rh sites [7].

Mechanism

La forms highly dispersed La₂O₃ (XPS BE = 835.3 eV) that interacts with Rh²⁺, creating LaO_x–Rh interfaces to boost H₂ activation and CO insertion [7].

Prospects for TNTs in Other Catalytic Applications

Fischer-Tropsch Synthesis (FTS)

TNTs’ high surface area and metal-support interactions could stabilize Co or Fe nanoparticles. La-promoted TNTs may mimic Co-La/SiO₂ systems, enhancing C₅₊ selectivity [8].

Hydrogenation Reactions

Ru/TNTs’ hydrogenation activity (observed in hydroformylation) could be leveraged for nitroarene reduction or bio-oil upgrading.

Environmental Catalysis

TNTs’ photocatalytic properties (e.g., for CO oxidation [9]) may be combined with metal sites for VOC degradation.

Conclusion

Chuai et al.’s work establishes TNTs as versatile supports for hydroformylation catalysts. Key advances include:

• • Bimetallic systems: Rh-Ru for enhanced activity in the hydroformylation of vinyl acetate.
• • Cation promotion: Alkali/alkaline earth metals optimize CO adsorption and Lewis acid-base interactions.
• • La decoration: Creates LaO_x–Rh sites for higher TOFs.

Future research should explore:

1. TNTs in tandem reactions(e.g., hydroformylation-hydrogenation).
2. Multifunctional TNTsfor coupled redox processes.
3. Scalable synthesisfor industrial adoption.

TNTs’ adaptability positions them as a platform for next-generation catalysts beyond hydroformylation.

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