Synergistic nanoalloy PdCu/TiO₂ catalyst for in situ hydrogenation of biomass-derived furfural at room temperature

Abstract

This study reports a highly selective and efficiently recyclable PdCu/TiO₂ nanocatalyst for the in situ hydrogenation of biomass-derived furfural to furfuryl alcohol at room temperature using the readily available triethylsilane and methanol as the hydrogen donor. The finite dispersion of PdCu nanoalloys on the surface of shape-controlled TiO₂ nanorods facilitated synergistic acid-redox and more surface-exposed active metal sites, as elucidated by in situ pyridine FT-IR, H₂-TPR, and CO pulse chemisorption studies, respectively. Controlled reactions revealed that Pd is the key to in situ hydrogen generation from the mixture of triethylsilane and methanol. The PdCu/TiO₂ nanocatalyst (2 wt% Pd and 3 wt% Cu) exhibited a 97% yield of furfuryl alcohol at room temperature. In contrast, only 27 and 7% yields of furfuryl alcohol were obtained over Pd/TiO₂ and Cu/TiO₂ catalysts, respectively, confirming the synergistic role of the PdCu nanoalloy in the catalytic transfer hydrogenation (CTH) of furfural. The PdCu/TiO₂ nanocatalyst showed versatile efficiency in the CTH of various furfurals to achieve optimum yields of furfuryl alcohols at room temperature. The efficient gram-scale synthesis of furfuryl alcohol, the high stability of the PdCu/TiO₂ nanocatalyst as confirmed by the hot-filtration study, and the excellent catalyst reusability with 97 and 94% yields of furfuryl alcohol in the 1^st and 5^th cycles, respectively, emphasize the practical application of the developed catalytic process for the CTH of biomass-derived platform molecules at room temperature.

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Green foundation

1. This study reports the development of a nanoalloy PdCu catalyst dispersed on TiO₂ for efficient catalytic transfer hydrogenation of biomass-derived furfural using the readily available hydrogen donors at room temperature without the need for high-pressure conditions, offering new insights for promoting related reactions towards sustainable biorefinery.

2. The high stability of the PdCu/TiO₂ nanocatalyst, excellent catalyst reusability, and efficient gram-scale synthesis emphasize the practical application of the developed catalytic process for biomass valorization.

3. Using biomass-derived hydrogen donors (ethanol or formic acid) and non-noble bimetal catalysts can provide a greener and more cost-effective process for the catalytic transfer hydrogenation reactions. However, this approach requires harsh reaction conditions as the non-noble metal catalysts alone are ineffective in releasing hydrogen from ethanol or formic acid. Thus, future studies should focus on the rational design of new catalytic materials, like high-entropy nanoalloys, which can activate hydrogen donors effectively and store more hydrogen because of their unique lattice strain and charge disparity properties.

1. Introduction

Biomass is a potential renewable hydrocarbon source for producing platform molecules, value-added chemicals, and biofuels towards carbon neutrality.^1–4 In particular, lignocellulosic biomass, consisting of three components (cellulose, hemicellulose, and lignin), is an excellent alternative to fossil fuels due to its inedible nature, abundance as agricultural waste and forest residues, and diverse functional groups. Heterogeneous catalysis plays a vital role in valorizing lignocellulosic biomass and its platform molecules by providing sustainable and practical routes for the biorefinery industry.^5–7 Furfural is one of the key platform molecules produced from hemicellulose via a two-step hydrolysis-dehydration approach.^8–12 The sustainable processing of furfural into chemicals and fuel-grade molecules relies on developing efficient heterogeneous catalysts with strong structural stability and high reusability performance.^13,14

Furfural can undergo various chemical transformations such as hydrogenation,^15–17 hydrodeoxygenation,^18–21 Diels–Alder reaction,^22–25 and hydroxymethylation.²⁶ Selective hydrogenation of furfural to furfuryl alcohol is one of the key reactions that can be carried out using either high-pressure H₂ gas^27–29 or liquid hydrogen carriers.^30–32 Furfuryl alcohol holds many industrial applications, especially in the resins industry (phenolic and epoxy resins), and in producing polyurethane foams and polyesters. It is a key reagent for the pharma, lubricant, and fragrance industries. It also finds vital applications in synthesizing lysine, vitamin C, ranitidine, and levulinic acid/esters.^8,32 The use of H₂ gas for the catalytic hydrogenation of furfural requires harsh reaction conditions (high pressures/temperatures) to activate molecular hydrogen and the subsequent adsorption of hydrogen species on the catalyst surface. For example, Yuan et al. reported that the Pd/Co_xO_y-based catalyst supported on SiAlO mixed oxide is effective at 20 bar H₂ pressure and 150 °C for the selective formation of furfuryl alcohol (99% selectivity) with 90% furfural conversion (Scheme 1, Fig. 1).²⁷ The PtFe-based catalyst (Pt₃Fe/CeO₂) developed by the Gao research group gave 76% conversion of furfural with 79% selectivity to furfuryl alcohol at 20 bar H₂ pressure and 100 °C (Scheme 2, Fig. 1).²⁸ A similar Pt-based catalyst (Pt/Fe-TiO₂) gave 99% conversion of furfural with 83% selectivity to furfuryl alcohol at 20 bar H₂ pressure and 40 °C (Scheme 3, Fig. 1).²⁹ Despite the conversion rates and product selectivities reported in the above articles, the use of high-pressure H₂ gas poses challenges to process sustainability and safety concerns for the biorefinery.

Fig. 1 Literature comparison for the hydrogenation of furfural into furfuryl alcohol using various metal-based heterogeneous catalysts. ^a Conversion of furfural. ^b Selectivity of furfuryl alcohol.

Liquid hydrogen carriers can provide greener and more sustainable catalytic routes by releasing hydrogen in situ for the selective conversion of biomass-derived platform molecules, including furfural.^33,34 The catalyst plays a dual role in releasing hydrogen from the liquid hydrogen carrier and subsequently activating hydrogen species under mild conditions. For example, Puthiaraj's group developed a Pd-based catalyst for the catalytic transfer hydrogenation (CTH) of furfural using 2-butanol as the hydrogen donor (Scheme 4, Fig. 1).³⁰ However, it requires a higher reaction temperature (120 °C) and a longer time (10 h) to achieve 98% conversion of furfural with 92% selectivity to furfuryl alcohol. Insignificant conversion and selectivity were obtained in the case of a Pd/Fe₂O₃ catalyst at 180 °C for 7.5 h (Scheme 5, Fig. 1).³¹ Recently, the group of Ruiz found that the Pd/Al₂O₃ catalyst is effective for the CTH of furfural, giving 80% yield of furfuryl alcohol in the presence of polymethylhydroxysiloxane (reducing agent) and ethanol (hydrogen donor) at room temperature (Scheme 6, Fig. 1).³² Although the above reports have provided alternative routes to high-pressure hydroprocessing of furfural, achieving optimum yields of furfuryl alcohol at room temperature without applying pressure remains a challenge.

Nanostructured metal-based catalysts containing two different metals have attracted considerable interest in several energy applications, including biomass valorization, due to their synergistic metal–metal-induced catalytic properties.^2,35,36 In particular, supported bimetallic nanoalloys have significant importance in heterogeneous catalysis due to the versatile advantages of tunable metal composition, superior lattice strain and entropy, high surface area, and enhanced redox properties to achieve higher catalytic activity and product selectivity in biomass valorization.^37–39 In addition, supported nanoalloys with controlled morphology, particle size, and alloy dispersion can have more surface-enriched active sites and a significant amount of coordinatively unsaturated metal sites with selective catalytic performance than conventional monometallic catalysts. Alloying a noble metal (e.g., Pd) with a base metal (e.g., Cu) can not only reduce noble metal consumption and overall catalyst cost but also provide selective hydrogenation sites by tuning the electronic structure of nanoalloys through optimal composition and uniform mixing of atoms.⁴⁰

The dispersion of nanoalloy particles on a shape-controlled metal oxide is considered an important class of heterogeneous catalysts because of the shape-tuned alloy-support interaction and selective surface/interface active sites.^41–43 The metal oxides with uniformly shaped particles can exhibit specific crystal facets, truncated surfaces (edges, corners, steps, and kinks), and abundant defect sites.^44–46 In particular, shape-controlled metal oxides with a redox nature are preferred as they provide strong anchoring sites for efficient dispersion of alloy species on the oxide surface. In this context, the nanorod-shaped TiO₂ material can be an excellent support for nanoalloys due to its distinct morphological, acidic, and redox properties. Unsaturated surface Ti^x+ sites and oxygen vacancies in TiO₂ nanorods can facilitate strong dispersion of nanoalloy species, such as PdCu with enhanced catalytic properties.⁴⁷

We have developed the room temperature CTH of furfural to furfuryl alcohol over a PdCu nanoalloy catalyst supported on the TiO₂ nanorods. The mixture of triethylsilane and methanol is used as a hydrogen donor. The synergistic role of the PdCu nanoalloy in the CTH of furfural was confirmed by several controlled reactions. The PdCu/TiO₂ nanoalloy catalyst is highly reusable without any leaching of active sites (confirmed by the hot filtration step) and has the potential for the scalability of the CTH of furfural at room temperature using a hydrogen donor. Several characterization techniques, such as powder XRD, XPS, TEM, STEM-EDAX, N₂ adsorption–desorption isotherm, H₂-TPR, CO-pulse chemisorption, and pyridine-adsorbed FT-IR were employed to elucidate the structure–activity properties of the developed PdCu/TiO₂ nanoalloy catalyst in the CTH of furfural at room temperature.

2. Experimental section

The synthesis procedure for the TiO₂ nanorods and the dispersion of Cu, Pd, and PdCu nanoparticles on TiO₂ nanorods (Fig. 2), materials characterization, and their catalytic activity studies were provided in the ESI.†

Fig. 2 Schematic representation of synthesis procedure of TiO₂ nanorods and PdCu/TiO₂ nanocatalyst.

3. Results and discussion

3.1. Catalyst characterization studies

The morphology, particle size, and elemental distribution of the catalysts were elucidated by TEM analysis. The TiO₂ material contains a significant number of rod-like morphology along with a few nanotubes (Fig. 3a). The average width and length of the nanorods were found to be 10 ± 1 nm and 121 ± 5 nm, respectively (Fig. 3a). The lattice fringes of the TiO₂ are evident in the HR-TEM image (Fig. S1, ESI†), with the d-spacings of ∼0.48 and 0.35 nm, corresponding to the (001) and (101) planes of anatase TiO₂, respectively.^48–50 Commercial TiO₂ has large aggregates of irregularly shaped particles with a broad size distribution of 10–600 nm (Fig. S2, ESI†). The spherical PdCu particles are clearly visible on the surface of TiO₂ in the PdCu/TiO₂ catalyst (Fig. 3b and c). The average size of the PdCu particles was found to be ∼4.2 nm. The STEM-EDAX elemental mapping images show high dispersion of O, Ti, Pd, and Cu in the PdCu/TiO₂ catalyst (Fig. 3d–h). It also confirms the formation of PdCu nanoalloy particles rather than the bimetallic particles, as there is no segregation of individual Pd and Cu species (Fig. 3g and h).⁵¹

Fig. 3 (a) TEM image of TiO₂, (b and c) TEM images of PdCu/TiO₂ catalyst, and (d–h) STEM-EDAX elemental mapping images of PdCu/TiO₂ catalyst.

As shown in Fig. 4a, the XRD peaks at 2θ = 25.4, 37.9, 48.1, 54.0, 55.1, 62.8, 68.8, 70.4, 75.1, and 82.8° belong to the anatase TiO₂ (JCPDS 21-1272).^52–54 In addition, the Cu/TiO₂ material showed a diffraction peak at 2θ = 35.6°, which is attributed to the (110) plane of metallic Cu crystallized in the FCC structure (JCPDS 04-0836).⁵⁵ Similarly, the Pd/TiO₂ catalyst showed two diffraction peaks at 2θ = 33.9 and 40.1°, corresponding to the (101) and (111) crystal planes of metallic Pd in the FCC structure.^56–58 The characteristic diffraction patterns associated with the oxide forms of Cu and Pd were not observed in the respective catalysts. The PdCu/TiO₂ catalyst does not contain the individual diffraction peaks of Pd and Cu species, indicating a high dispersion of the alloy particles, in agreement with TEM studies (Fig. 3e–h).⁵⁹ The presence of type IV isotherms with H₃ hysteresis loop in all the materials indicates the slit-shaped pores (Fig. 4b). The BET surface areas of TiO₂, Pd/TiO₂, Cu/TiO₂, and PdCu/TiO₂ catalysts were found to be 78, 96, 92, and 83 m² g⁻¹, respectively (Table S1, ESI†). An increase in the BET surface area of TiO₂ after adding Cu and Pd indicates the high dispersion of these species on the surface of TiO₂ nanorods. The pore volume and pore size of TiO₂, Pd/TiO₂, Cu/TiO₂, and PdCu/TiO₂ catalysts were found to be in the range of 0.038–0.050 cc g⁻¹ and 15.278–15.334 nm, respectively (Table S1, ESI†).

Fig. 4 (a) Powder XRD analysis and (b) N₂ adsorption–desorption analysis of the catalysts.

The XPS analysis of TiO₂, Cu/TiO₂, Pd/TiO₂, and PdCu/TiO₂ nanocatalysts elucidated the oxidation states of each metal as well as the metal–metal and metal–support interactions as reflected by the changes in the binding energies (BEs) of Ti, O, Cu, and Pd species. Two types of oxygen species are found in all catalysts (Fig. S3a, ESI†). The XPS peaks at 533.2 − 528.4 eV and 531.1 − 527.6 eV correspond to the surface-adsorbed hydroxyl species (oxygen vacancies, O_v) and the lattice oxygen (O_L), respectively.^53,60 The Ti 2p XP spectra show two peaks at 466.1 − 461.0 eV (Ti 2p_1/2) and 459.3 − 456.0 eV (Ti 2p_3/2), which belong to the Ti⁴⁺ peaks (Fig. S3b, ESI†).⁶¹ The change in the BEs of the O and Ti species after the addition of Cu and Pd indicates the metal–support interaction. The Cu 2p XP spectra of Cu/TiO₂ and PdCu/TiO₂ nanocatalysts show the different types of Cu species with BEs of 958.72 − 949.8 eV and 938.29 − 930.7 eV for Cu 2p_1/2 and Cu 2p_3/2, respectively (Fig. 5a).^62,63 The deconvolution of the Cu 2p XPS peaks shows that all catalysts contain both Cu⁰ and Cu²⁺ species. The presence of two satellite peaks (961.7 and 943.0 eV) in the Cu 2p XP spectra confirms the Cu²⁺ species.^64,65 The Pd 3d XP spectra show that all the catalysts contain Pd⁰ species with the BE difference of around 5.3 eV between Pd 3d_3/2 and Pd 3d_5/2 in all cases (Fig. 5b).^66,67 Higher BEs of metallic Pd observed in the PdCu/TiO₂ catalyst than in the monometallic Pd/TiO₂ catalyst indicate the strong Pd–Cu interaction.^59,68,69

Fig. 5 (a) Cu 2p XPS and (b) Pd 3d XPS spectra of the catalysts.

The H₂-TPR, in situ pyridine FT-IR, and CO-pulse chemisorption studies are carried out to investigate the redox ability, the nature of the acid sites, and the concentration of the active sites, respectively (Fig. 6, Table 1, and Table S2, ESI†). As shown in Fig. 6a, TiO₂ nanorods exhibit reduction peaks in the temperature range of 500–750 °C, indicating the need for higher temperatures to reduce the titania (entry 1, Table S2, ESI†). The peaks at 531.5 and 690.3 °C correspond to the reduction of surface and bulk reduction of Ti⁴⁺ to Ti³⁺.^52,70,71 The addition of Cu and Pd to TiO₂ not only decreased the reduction temperature (668.7 °C) of bulk TiO₂ but also increased the hydrogen uptake compared to that of pure TiO₂ (Table S2, ESI†). This provides strong evidence for the strong metal–support interaction and the H₂ spillover effect.⁷² In the case of Pd/TiO₂, the negative peak at 79.6 °C indicates the decomposition of the Pd–H phase (β-PdH) formed by the adsorption of hydrogen species on the metallic Pd at room temperature (entry 2, Table S2, ESI†).⁷² This hydrogen consumption is attributed to the increased H₂/Pd ratio resulting in the formation of the β-PdH phase (1.20 μmol g⁻¹, entry 2, Table S2, ESI†).^73,74 On the other hand, the Cu/TiO₂ catalyst showed a reduction peak at 170.6–386.9 °C with three overlapping peaks attributed to the reduction of Cu²⁺ to Cu⁺, Cu⁺ to Cu, and CuO_x with relatively strong interaction with the TiO₂ support (entry 3, Table S2, ESI†).^75–78 The alloying of Pd and Cu in the PdCu/TiO₂ catalyst promoted the reduction of PdCu at very low temperatures (54.2–138.4 °C) with the overlap of two peaks, confirming the accelerated redox nature of the catalyst (entry 4, Table S2, ESI†). The stronger the interaction between Pd and Cu, the easier the reduction of two metal sites. Thus, the high-intensity peak at a relatively low temperature corresponds to the reduction of surface Pd–CuO_x species.⁷⁹ In contrast, the lower intensity shoulder peak corresponds to the reduction of isolated CuO species. The significant decrease in the reduction temperature of CuO species in the PdCu/TiO₂ catalyst compared to the Cu/TiO₂ catalyst confirms the strong interaction between Pd and Cu. The absence of the β-PdH reduction peak in the PdCu/TiO₂ catalyst confirms the alloying of Pd with Cu.^80,81 The amount of H₂ uptake by PdCu species in the nanoalloyed catalyst (12.42 μmol g⁻¹, entry 4, Table S2, ESI†) is intermediate between the Pd/TiO₂ and Cu/TiO₂ catalysts (10.40 and 22.54 μmol g⁻¹, respectively, entries 2 and 3, Table S2, ESI†), which is attributed to the presence of fewer PdO and CuO species due to the formation of Pd–Cu alloy.^82,83

Fig. 6 (a) H₂-TPR and (b) in situ pyridine FT-IR analyses of the catalysts.

Table 1 Quantitative measurements of CO-pulse chemisorption analysis of the catalysts

S. no.	Catalyst	Metal dispersion (%)	Average particle size (nm)	Active surface area (m^2 g^−1)	Amount of active sites (CO uptake, μmol g^−1)
1	Cu/TiO2	3.39	25.6	21.74	26.70
2	Pd/TiO2	17.97	5.6	42.24	18.42
3	PdCu/TiO2	22.90	6.6	70.70	38.74

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In addition to the redox sites, the acid sites of the catalysts play a crucial role in the CTH of furfural.^84,85 Therefore, in situ pyridine FT-IR analysis was carried out to elucidate the nature of the acid sites (Lewis and Brønsted) present in the catalysts. The FT-IR spectra shown in Fig. 6b reveal three distinct peaks in all catalysts. The peaks at 1446 cm⁻¹, 1490 cm⁻¹, and 1591 cm⁻¹ represent the Lewis acid sites (LA), a combination of Lewis and Brønsted acid sites (LA + BA), and Lewis acid sites (LA), respectively.⁶³ The absence of a distinct peak corresponding to the Brønsted acid sites (BA) indicates the absence of bridged metal-OH-metal species. Thus, all catalysts exhibited the LA sites, which can increase the electropositivity of the carbonyl carbon by activating it through the carbonyl oxygen of the furfural group.^84–86

The concentration of active sites (CO uptake), metal dispersion, average particle size, and active surface area were estimated by CO-pulse chemisorption analysis (Table 1). The Cu/TiO₂ catalyst showed a CO uptake of 26.7 μmol g⁻¹ with 3.39% metal dispersion (entry 1, Table 1). On the other hand, the Pd/TiO₂ catalyst showed a higher metal dispersion (17.97%, entry 2, Table 1), although a lower CO uptake is observed (18.42 μmol g⁻¹, entry 2, Table 1) compared to the Cu/TiO₂ catalyst (entry 1, Table 1). The high dispersion of Pd is attributed to the smaller particle size (5.6 nm, entry 2, Table 1), while the lower CO uptake compared to the Cu/TiO₂ catalyst (5 wt% Cu) is due to the lower metal content (2 wt% Pd) in the Pd/TiO₂ catalyst. The nanoalloyed PdCu/TiO₂ catalyst showed a higher metal dispersion (22.90%, entry 3, Table 1) and a higher CO uptake (38.74 μmol g⁻¹, Table 1), which are due to the alloy formation providing more amount of active sites to the optimum extent compared to the supported monometallic catalysts.^87,88 This increasing trend is reflected in the same pattern for the active surface area from Cu/TiO₂, Pd/TiO₂ to PdCu/TiO₂ (Table 1).

3.2. Catalytic activity studies

3.2.1. Catalysts’ screening and reaction conditions optimization for the CTH of furfural. The CTH of furfural using the mixture of triethylsilane and methanol as hydrogen donor was carried out at room temperature for 4 h using 1 mmol furfural and 15 wt% catalyst. The isolated furfuryl alcohol was confirmed by ¹H, ¹³C, and DEPT NMR analyses (Fig. S4–S6, ESI†). Triethyl(methoxy)silane is a major by-product in the production of hydrogen from triethylsilane and methanol, which is confirmed by HR-MS analysis (Fig. S7, ESI†). There is no conversion of furfural under the blank (entry 1, Table 2), commercial TiO₂ (entry 2, Table 2), and nanorod-shaped TiO₂ (entry 3, Table 2). The Cu/TiO₂ catalyst showed negligible conversion of furfural (7%) as it plays a trivial role in the generation of hydrogen from triethylsilane and methanol (entry 4, Table 2). The Pd/TiO₂ catalyst showed 27% furfural conversion and 99% selectivity to furfuryl alcohol (entry 5, Table 2). On the other hand, the PdCu/TiO₂ nanoalloy catalyst showed 97% furfural conversion and 99% selectivity to furfuryl alcohol (entry 6, Table 2), indicating the synergistic role of PdCu nanoalloys for CTH of furfural at room temperature. In the case of molecular hydrogen, no conversion was obtained over the PdCu/TiO₂ nanocatalyst (entry 7, Table 2). This is due to the insufficient pressure available at room temperature to dissolve the dihydrogen gas in the reaction mixture to facilitate the hydrogenation of furfural. This highlights the importance of the hydrogen donor (triethylsilane and methanol) to facilitate the supply of hydrogen species to the reaction at room temperature. The use of commercial TiO₂ (anatase), which has irregular-shaped particles (Fig. S2, ESI†), as a support for the PdCu catalyst gave a moderate activity of 84% conversion and 72% selectivity (entry 8, Table 2), confirming the important role of TiO₂ morphology in enhancing the activity of the PdCu catalyst (entry 6, Table 2). In the case of ethanol (entry 9, Table 2) and isopropanol (entry 10, Table 2) as hydrogen donors, only 76% and 58% furfural conversions were achieved, respectively, which are much lower than the conversion (97%) obtained in the case of methanol (entry 6, Table 2). It indicates that methanol releases hydrogen effectively in the presence of the PdCu/TiO₂ catalyst compared to ethanol and isopropanol.

Table 2 Catalyst screening for the CTH of furfural at room temperature^a

S. no.	Catalyst	Conversion (%)	Selectivity (%)
			Furfuryl alcohol	Tetrahydrofuran-2-methanol
a Reaction conditions: furfural = 1 mmol, TES = 1.5 mmol, MeOH = 4 mL, catalyst = 15 mg, room temperature, and 4 h. b Anatase TiO2. c H2 gas. d Ethanol. e Isopropanol.
1	Blank	—	—	—
2^b	Commercial TiO2	—	—	—
3	TiO2 NR	—	—	—
4	Cu/TiO2	7	99	1
5	Pd/TiO2	27	99	1
6	PdCu/TiO2	97	99	1
7^c	PdCu/TiO2	—	—	—
8^b	PdCu/TiO2	84	72	—
9^d	PdCu/TiO2	76	99	1
10^e	PdCu/TiO2	58	99	1

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In order to study and understand the kinetics of the CTH of furfural over the PdCu/TiO₂ catalyst, time-on-stream studies were carried out under optimized conditions. The catalyst showed 48%, 72%, 88%, and 97% conversions of furfural in 1, 2, 3, and 4 h, respectively (Fig. 7a). This indicates a linear increase in the reaction rate up to 2 h, followed by a gradual increase as the reaction approaches equilibrium. On the other hand, the conversions of furfural were found to be 27%, 63%, 97%, and 97% with the increase of catalyst amount from 5, 10, 15, and 20 mg with 99% selectivity of furfuryl alcohol in all cases (Fig. 7b). This indicates that the reaction achieved optimum conversion and selectivity using 15 mg of the PdCu/TiO₂ catalyst. The effect of the amount of triethylsilane with respect to methanol was also investigated (Fig. 7c). The PdCu/TiO₂ catalyst showed 59%, 75%, and 97% conversions of furfural with 99% product selectivity over 1, 1.25, and 1.5 mmol of triethylsilane. There is no change in the conversion/selectivity with further increases in the amount of triethylsilane. The stability/heterogeneity of the PdCu/TiO₂ catalyst was determined by the hot filtration test under optimized reaction conditions (Fig. 7d). The catalyst was separated from the reaction mixture by centrifugation after 1 h and the reaction was then continued with the filtrate for the remaining 3 h. The conversion was found to be 48% at 1 h, but there was little change in the conversion (48.7%) after 4 h. This indicates that there is no leaching of active sites during the reaction. It also confirms that the reaction is completely heterogeneous and highly stable under the reaction conditions.

Fig. 7 (a) Effect of reaction time, (b) effect of catalyst amount, (c) effect of TES amount on the CTH of furfural, and (d) hot filtration study (reaction conditions: furfural = 1 mmol, PdCu/TiO₂ = 15 wt%, TES = 1.5 mmol, MeOH = 4 mL, RT, and 4 h).

3.2.2. Scalability, reusability, and kinetic studies. The gram-scale synthesis (1 g scale) and reusability studies were carried out under the optimized conditions to investigate the practical applicability of the CTH of furfural over the PdCu/TiO₂ catalyst. The gram-scale synthesis was carried out by increasing the amount of all the reagents and the catalyst by 10 times with respect to the optimized one under the same conditions (Fig. 8a). Specifically, furfural (10 mmol), PdCu/TiO₂ (150 mg), TES (15 mmol), and methanol (40 mL) were used for the gram-scale synthesis. The PdCu/TiO₂ catalyst gave 96% conversion of furfural and 99% selectivity towards furfuryl alcohol at room temperature for 5 h (Fig. 8a). For the reusability studies of the catalyst, the details of the purification and the regeneration of the catalyst have been given in the experimental section (ESI†). The PdCu/TiO₂ catalyst showed remarkable catalytic activity even after 5 cycles. The catalyst showed 97%, 96%, 94%, 93%, and 94% conversions of furfural in consecutive cycles with 99% selectivity of furfuryl alcohol in all cases (Fig. 8b).

Fig. 8 (a) Gram scale synthesis (reaction conditions: furfural = 10 mmol, PdCu/TiO₂ = 150 mg, TES = 15 mmol, methanol = 40 mL, RT, and 5 h). (b) Catalytic reusability study (reaction conditions: furfural = 1 mmol, PdCu/TiO₂ = 15 mg, TES = 1.5 mmol, RT, and methanol = 4 mL).

Since furfural is the limiting reagent, the CTH of furfural follows the first-order reaction i.e., −dC_F/dt = kC_F (C_F is the concentration of furfural and k is the rate constant). The rate constants were estimated by plotting −ln(1 − p) with reaction time at different temperatures (Fig. S17a, ESI†). The conversion p is defined as [1 − (C_F/C_F0)], where C_F and C_F0 are the final and initial concentrations of furfural, respectively. The relation between −ln k with 1/T (ln k = ln A − E_a/RT) gave the activation energy (E_a) of 120.85 kJ mol⁻¹ (Fig. S17b, ESI†).

3.2.3. Substrate scope studies. The broad applicability of the PdCu/TiO₂ catalyst for in situ hydrogenation was investigated at room temperature using different furfural derivatives under the optimized reaction conditions. Regarding the reactivity of the different substrates, electronic and steric effects are the most important parameters controlling the reactivity. In general, the electron-donating groups reduce the electrophilicity of the carbonyl group in furfural, thus reducing the susceptibility to nucleophilic attack at the electrophilic carbon. However, this effect is outweighed by the steric hindrance of the substituents during hydrogenation. The substituents at the 5-position of the furfural ring control the reactivity more effectively than those at other positions since they directly influence the distribution of the ring electrons and, thus, the reactivity of the carbonyl group. The presence of the methyl group at the 5^th position gave 83% conversion and 99% selectivity (entry 2, Table 3). This is because the methyl group is not significantly involved in electronic effects but shown a moderate steric hindrance and therefore reduces the reactivity compared to furfural (entry 1, Table 3). The hydroxymethyl group at position 5 gave 98% conversion and 99% selectivity (entry 3, Table 3). This is attributed to the electron-withdrawing effect (–I effect) of the hydroxyl group, which attracts the electron density of the ring to a certain extent. This, in turn, increases the electrophilicity of the carbonyl, and thus, the increased reactivity was obtained. The presence of halogens at the 5-position significantly influences the reactivity. Although the halogens exert both mesomeric and electron-withdrawing effects, the one that dominates during the reaction influences the reactivity of the substrates. The presence of –Cl, –Br, and –I gave 97%, 95%, and 91% conversion, respectively, with 99% selectivity in all cases (entries 4–6, Table 3). The slightly gradual decrease in conversion is attributed to the steric hindrance of the substituents as the size of the group increases. However, the mesomeric and –I effects of the halogens did not play a significant role in the reactivity as both effects cancel each other out. On the other hand, the –Br group in the 3^rd and 4^th position gave 62% and 86% conversions, respectively with 99% selectivity (entries 7 and 8, Table 3). This is attributed to the mesomeric effect of the –Br at the 3^rd position, which reduces the electrophilicity of the carbonyl and thus the reactivity. However, the decrease in reactivity is relatively less in the case of –Br at the 4-position, as it only represents a steric hindrance for the dihydrogen to approach the electrophilic carbonyl group. Similarly, the presence of the phenyl group in the 5^th position gave only 66% conversion with 99% selectivity (entry 9, Table 3). This decrease in conversion is due to the importance of the electron-donating effect of the phenyl group in combination with the steric hindrance. Similarly, the presence of 4-chlorophenyl and 4-bromophenyl substituents at the 5^th position further decreased the activity, i.e., 52% and 50% conversions were obtained, respectively, with 99% selectivity (entries 10 and 11, Table 3). This is due to their steric and mesomeric effects on the furan ring, which reduced the electrophilicity and, thus, the product yields. It is evident that the mesomeric effect of the bromophenyl group is greater than that of the chlorophenyl group. Furthermore, the presence of the –NO₂ group in the 5^th position resulted in 88% conversion with 99% selectivity (entry 12, Table 3). This is attributed to the –I effect of the –NO₂ group, which withdraws the electron density from the ring, and thus, the increased electrophilicity increases the product yields. Besides the hydrogenation of the aldehyde group of furfural, the hydrogenation of the –NO₂ group was also noticed (entry 12, Table 3).

Table 3 Substrate scope studies for the CTH of various furfural derivatives over PdCu/TiO₂ nanoalloy catalyst

Reaction conditions: furfural = 1 mmol, PdCu/TiO₂ = 15 mg, TES = 1.5 mmol, methanol = 4 mL, and room temperature.a Furfural conversion.b Furfuryl alcohol selectivity.

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4. Structure–activity relationship for the CTH of furfural

The PdCu/TiO₂ nanoalloy catalyst showed excellent activity in the CTH of furfural at room temperature (entry 6, Table 2). In order to understand the structure–activity relationship of the catalyst for the observed activity, some controlled reactions were carried out (Table S3, ESI†). When the CTH of furfural was carried out with triethylsilane or methanol alone, no activity was obtained. This confirms the importance of both triethylsilane and methanol as a combination for the hydrogen source. In addition, it is important to understand the properties and role of the catalyst in two different ways, namely, the role of the catalyst in hydrogen release and in the hydrogenation of furfural. The hydrogen-releasing activity was assessed by the extent of formation of the by-product (triethyl(methoxy)silane). The activity was attributed proportionally to the extent of the hydrogen release. Thus, the Cu/TiO₂ catalyst showed very low activity (furfural conversion = 7%, entry 4, Table 2), whereas the Pd/TiO₂ catalyst gave relatively higher activity (furfural conversion = 27%, entry 5, Table 2). This confirms the more effective hydrogen-releasing ability of Pd than that of Cu. This is attributed to the easy reducibility and higher reactivity of Pd than that of Cu. In other words, the primary requirement for hydrogen release and adsorption is the presence of M⁰ species (Pd⁰ and Cu⁰). Therefore, the large amount of Pd⁰ is mainly responsible for the hydrogen release, and the presence of Pd⁰ species was confirmed by the XPS analysis (Fig. 5). The PdCu/TiO₂ nanoalloy catalyst gave optimum catalytic activity of 97% furfural conversion with 99% furfuryl alcohol selectivity (entry 6, Table 2). This is attributed to the alloying and synergistic effect of the Pd and Cu, together with the proper dispersion of the PdCu alloy on the surface of the TiO₂ nanorods, which in turn established the hydrogen spillover phenomena. The formation of PdCu nanoalloys and their uniform dispersion were confirmed by powder XRD (Fig. 4a), XPS (Fig. 5), H₂-TPR (Fig. 6a), and the STEM-EDAX analyses (Fig. 3d–h). Among all the catalysts, the PdCu/TiO₂ nanoalloy catalyst showed a higher number of active sites (entry 3, Table 1), more Lewis acid sites (Fig. 6b), and a superior reduction capacity (Fig. 6a); thus, this catalyst showed optimal catalytic activity in the CTH of furfural at room temperature. The plausible mechanism involves the release of hydrogen from the triethylsilane and methanol on the metallic Pd sites, and the released hydrogen was activated on the PdCu nanoalloy to react with furfural (Fig. 9). The Lewis acid sites present in the PdCu/TiO₂ nanoalloy catalyst can activate furfural to react with hydrogen species to form furfuryl alcohol.^84,89 Moreover, the hydrogenation of furfural was selectively achieved at the carbonyl group rather than the ring hydrogenation over the PdCu/TiO₂ nanoalloy catalyst, indicating the synergistic alloying effect of PdCu. The excellent activity of the PdCu/TiO₂ nanoalloy catalyst in the CTH of furfural at room temperature, together with its remarkable stability (hot filtration test, Fig. 7d), scalability (Fig. 8a), and efficient reusability (Fig. 8b) elucidate the practical application of this catalytic approach for the sustainable hydrogenation of biomass-derived platform molecules, which will not only reduce dependence on fossil fuels but also provide solutions to achieve carbon neutrality.

Fig. 9 Possible mechanism for the CTH of furfural over the nanoalloy PdCu/TiO₂ catalyst using the mixture of triethylsilane and methanol as hydrogen donor at room temperature.

5. Conclusions

Here, the combination of Pd and Cu on TiO₂ nanorods effectively catalyzed the CTH of furfural (97% conversion) to furfuryl alcohol (99% selectivity) at room temperature in 4 h. The Pd⁰ sites facilitated in situ hydrogen release from the mixture of triethylsilane and methanol at room temperature. The stability and heterogeneous nature of the PdCu/TiO₂ catalyst during the reaction were confirmed by the hot filtration test. Moreover, the catalyst was found to be highly active even after the 5 cycles of reusability and the process is scalable. The activity of the PdCu/TiO₂ catalyst is mainly due to the presence of more active sites and the Pd–Cu synergy, as well as their high dispersion on the surface of the TiO₂ nanorods.

Data availability

The data supporting this article has been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

P.S. is thankful to the UGC for providing a fellowship. P.S. acknowledges the funding support from the Indian Institute of Technology Hyderabad (grant number SG-130) and SERB-CRG (CRG/2022/005932). The authors thank the DST-FIST grant (Grant No. SR/FIST/ET-I/2021/918(C)) for the use of in situ FT-IR, H₂-TPR, and CO-pulse chemisorption facilities.

References

1. W. Schutyser, S. Van den Bosch, T. Renders, T. De Boe, S.-F. Koelewijn, A. Dewaele, T. Ennaert, O. Verkinderen, B. Goderis, C. M. Courtin and B. F. Sels, Green Chem., 2015, 17, 5035–5045.
2. P. Sudarsanam, E. Peeters, E. V. Makshina, V. I. Parvulescu and B. F. Sels, Chem. Soc. Rev., 2019, 48, 2366–2421.
3. S. De, A. S. Burange and R. Luque, Green Chem., 2022, 24, 2267–2286.
4. X. Zhang, K. Wilson and A. F. Lee, Chem. Rev., 2016, 116, 12328–12368.
5. P. Sudarsanam, R. Zhong, S. Van den Bosch, S. M. Coman, V. I. Parvulescu and B. F. Sels, Chem. Soc. Rev., 2018, 47, 8349–8402.
6. L. Lin, X. Han, B. Han and S. Yang, Chem. Soc. Rev., 2021, 50, 11270–11292.
7. Z. Sun and K. Barta, Chem. Commun., 2018, 54, 7725–7745.
8. R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sádaba and M. López Granados, Energy Environ. Sci., 2016, 9, 1144–1189.
9. V. Choudhary, S. I. Sandler and D. G. Vlachos, ACS Catal., 2012, 2, 2022–2028.
10. Y. Luo, Z. Li, X. Li, X. Liu, J. Fan, J. H. Clark and C. Hu, Catal. Today, 2019, 319, 14–24.
11. L. Ye, Y. Han, X. Wang, X. Lu, X. Qi and H. Yu, Mol. Catal., 2021, 515, 111899.
12. A. Mittal, H. M. Pilath and D. K. Johnson, Energy Fuels, 2020, 34, 3284–3293.
13. X. Li, P. Jia and T. Wang, ACS Catal., 2016, 6, 7621–7640.
14. X. Zhang, S. Xu, Q. Li, G. Zhou and H. Xia, RSC Adv., 2021, 11, 27042–27058.
15. W. Fang and A. Riisager, Green Chem., 2021, 23, 670–688.
16. X. Chen, L. Zhang, B. Zhang, X. Guo and X. Mu, Sci. Rep., 2016, 6, 28558.
17. M. J. Taylor, L. J. Durndell, M. A. Isaacs, C. M. A. Parlett, K. Wilson, A. F. Lee and G. Kyriakou, Appl. Catal., B, 2016, 180, 580–585.
18. W. Ren, J. Tian, Z. Wang and M. Zhang, Appl. Catal., A, 2024, 685, 119894.
19. L. F. Sosa, P. M. de Souza, R. A. Rafael, R. Wojcieszak, E. Marceau, S. Paul, V. Briois, F. B. Noronha and F. S. Toniolo, ChemCatChem, 2024, 16(2), e202300890.
20. Y. An, Q. Wu, L. Niu, C. Zhang, Q. Liu, G. Bian and G. Bai, J. Catal., 2024, 429, 115271.
21. F. Yang, T. Zhang, J. Zhao, W. Zhou, N. J. Libretto and J. T. Miller, Appl. Catal., B, 2024, 340, 123176.
22. R. C. Cioc, E. Harsevoort, M. Lutz and P. C. A. Bruijnincx, Green Chem., 2023, 25, 9689–9694.
23. J. Gancedo, L. Faba and S. Ordoñez, Appl. Catal., A, 2024, 685, 119907.
24. F. Xu, Z. Li, L.-L. Zhang, S. Liu, H. Li, Y. Liao and S. Yang, Green Chem., 2023, 25, 3297–3305.
25. R. C. Cioc, M. Lutz, E. A. Pidko, M. Crockatt, J. C. van der Waal and P. C. A. Bruijnincx, Green Chem., 2021, 23, 367–373.
26. S. Nishimura, A. Shibata and K. Ebitani, ACS Omega, 2018, 3, 5988–5993.
27. E. Yuan, Y. Deng, C. Wu, G. Shi, P. Jian and X. Hou, Appl. Catal., A, 2024, 677, 119679.
28. X. Gao, S. Tian, Y. Jin, X. Wan, C. Zhou, R. Chen, Y. Dai and Y. Yang, ACS Sustainable Chem. Eng., 2020, 8, 12722–12730.
29. Z. Zhao, X. Li, X. Liu, H. Gao, A. Jia, S. Xie, X. Song, X. Liu, F. Yang and Q. Yang, ACS Catal., 2024, 14, 4478–4488.
30. P. Puthiaraj, K. Kim and W.-S. Ahn, Catal. Today, 2019, 324, 49–58.
31. D. Scholz, C. Aellig and I. Hermans, ChemSusChem, 2014, 7, 268–275.
32. M. E. Medina Ruiz, R. Maderuelo-Solera, C. P. Jiménez-Gómez, R. Moreno-Tost, I. Malpartida, C. García-Sancho, J. A. Cecilia, C. Len, J. M. Mérida-Robles and P. Maireles-Torres, ACS Sustainable Chem. Eng., 2024, 12, 14910–14920.
33. Z. An and J. Li, Green Chem., 2022, 24, 1780–1808.
34. P. Subha, K. Krishan and P. Sudarsanam, Sustainable Energy Fuels, 2024, 8, 3775–3800.
35. C. Su, S. Zou, J. Li, L. Wang and J. Huang, ChemSusChem, 2024, 17(20), e202400602.
36. Y. Wang, S. De and N. Yan, Chem. Commun., 2016, 52, 6210–6224.
37. R. Ferrando, J. Jellinek and R. L. Johnston, Chem. Rev., 2008, 108, 845–910.
38. L. Liu and A. Corma, Chem. Rev., 2023, 123, 4855–4933.
39. S. Feng, Y. Geng, H. Liu and H. Li, ACS Catal., 2022, 12, 14999–15020.
40. L. Yang, S. Shan, R. Loukrakpam, V. Petkov, Y. Ren, B. N. Wanjala, M. H. Engelhard, J. Luo, J. Yin, Y. Chen and C.-J. Zhong, J. Am. Chem. Soc., 2012, 134, 15048–15060.
41. Y. Zhou, Y. Li and W. Shen, Chem. – Asian J., 2016, 11, 1470–1488.
42. F. Zaera, ChemSusChem, 2013, 6, 1797–1820.
43. J. Wang, X. Chen, C. Li, Y. Zhu, J. Li, S. Shan, A. Hunt, I. Waluyo, J. A. Boscoboinik, C.-J. Zhong and G. Zhou, ACS Catal., 2024, 14, 5662–5674.
44. B. Ni and X. Wang, Adv. Sci., 2015, 2, 1500085.
45. Y. Li and W. Shen, Chem. Soc. Rev., 2014, 43, 1543–1574.
46. J. Pal and T. Pal, Nanoscale, 2015, 7, 14159–14190.
47. P. Sudarsanam, H. Li and T. V. Sagar, ACS Catal., 2020, 10, 9555–9584.
48. J. Li, K. Cao, Q. Li and D. Xu, CrystEngComm, 2012, 14, 83–85.
49. W. Zhang, Y. Xie, D. Xiong, X. Zeng, Z. Li, M. Wang, Y.-B. Cheng, W. Chen, K. Yan and S. Yang, ACS Appl. Mater. Interfaces, 2014, 6, 9698–9704.
50. J. Shi, C. Sun, M. B. Starr and X. Wang, Nano Lett., 2011, 11, 624–631.
51. D. Liu, Y. Li, M. Kottwitz, B. Yan, S. Yao, A. Gamalski, D. Grolimund, O. V. Safonova, M. Nachtegaal, J. G. Chen, E. A. Stach, R. G. Nuzzo and A. I. Frenkel, ACS Catal., 2018, 8, 4120–4131.
52. P. Sudarsanam, A. Köckritz, H. Atia, M. H. Amin and A. Brückner, ChemCatChem, 2021, 13, 1990–1997.
53. W. P. Utomo, H. Wu, R. Liu and Y. H. Ng, Green Chem., 2024, 26, 1443–1453.
54. Y. Cao, D. Chen, Y. Meng, S. Saravanamurugan and H. Li, Green Chem., 2021, 23, 10039–10049.
55. F. Rasera, A. S. Thill, L. P. Matte, G. Z. Girotto, H. V. Casara, G. B. Della Mea, N. M. Balzaretti, F. Poletto, C. Brito and F. Bernardi, ACS Appl. Nano Mater., 2023, 6, 6435–6443.
56. H. Tan, Y.-P. Xu, S. Rong, R. Zhao, H. Cui, Z.-N. Chen, Z.-N. Xu, N.-N. Zhang and G.-C. Guo, Nanoscale, 2021, 13, 18773–18779.
57. S. Navaladian, B. Viswanathan, T. Varadarajan and R. Viswanath, Nanoscale Res. Lett., 2009, 4, 181.
58. F. U. Khan, S. Mehmood, S. Liu, W. Xu, M. N. Shah, X. Zhao, J. Ma, Y. Yang and X. Pan, Front. Chem., 2021, 9.
59. X. Li, C. Wang, J. Yang, Y. Xu, Y. Yang, J. Yu, J. J. Delgado, N. Martsinovich, X. Sun, X.-S. Zheng, W. Huang and J. Tang, Nat. Commun., 2023, 14, 6343.
60. T. Wei, P. Ding, T. Wang, L.-M. Liu, X. An and X. Yu, ACS Catal., 2021, 11, 14669–14676.
61. B. Lin, J. Tang, J. Yang, M. Fu, D. Ye and Y. Hu, Appl. Catal., B, 2025, 361, 124619.
62. Z.-H. He, Z.-H. Li, Z.-Y. Wang, K. Wang, Y.-C. Sun, S.-W. Wang, W.-T. Wang, Y. Yang and Z.-T. Liu, Green Chem., 2021, 23, 5775–5785.
63. W. Xu, R. Tian, C. Gao, C. Wang, Y. Chen, R. Wang, J. Peng, S. An and P. Li, Sci. Rep., 2024, 14, 23604.
64. B. S. Solanki and C. V. Rode, Green Chem., 2019, 21, 6390–6406.
65. W. Gao, Y. Zhao, H. Chen, H. Chen, Y. Li, S. He, Y. Zhang, M. Wei, D. G. Evans and X. Duan, Green Chem., 2015, 17, 1525–1534.
66. E.-M. Felix, M. Antoni, I. Pause, S. Schaefer, U. Kunz, N. Weidler, F. Muench and W. Ensinger, Green Chem., 2016, 18, 558–564.
67. J. Yang, D. Wang, W. Liu, X. Zhang, F. Bian and W. Yu, Green Chem., 2013, 15, 3429.
68. X. Wang, Y. Yang, H. Zhong, T. Wang, J. Cheng and F. Jin, Green Chem., 2021, 23, 430–439.
69. M. Rahaman, K. Kiran, I. Z. Montiel, V. Grozovski, A. Dutta and P. Broekmann, Green Chem., 2020, 22, 6497–6509.
70. J. Guo, F. Dong, S. Zhong, B. Zhu, W. Huang and S. Zhang, Catal. Lett., 2018, 148, 359–373.
71. W. Wu, S. Bu, L. Bai, Y. Su, Y. Song, H. Sun, G. Zhen, K. Dong, L. Deng, Q. Yuan, C. Jing and Z. Sun, Nanoscale, 2023, 15, 5909–5918.
72. M. Yarar, A. Bouziani and D. Uner, Catal. Commun., 2023, 174, 106580.
73. J. Batista, A. Pintar, D. Mandrino, M. Jenko and V. Martin, Appl. Catal., A, 2001, 206, 113–124.
74. Z. Guo, Q. Huang, S. Luo and W. Chu, Top. Catal., 2017, 60, 1009–1015.
75. X. Meng, L. Meng, Y. Gong, Z. Li, G. Mo and J. Zhang, RSC Adv., 2021, 11, 37528–37539.
76. Z. Wang, Z. Niu, Q. Hao, L. Ban, H. Li, Y. Zhao and Z. Jiang, Catalysts, 2019, 9, 35.
77. Y. Maimaiti, M. Nolan and S. D. Elliott, Phys. Chem. Chem. Phys., 2014, 16, 3036.
78. F. Platero, A. López-Martín, A. Caballero, T. C. Rojas, M. Nolan and G. Colón, ACS Appl. Nano Mater., 2021, 4, 3204–3219.
79. F. Cai, L. Yang, S. Shan, D. Mott, B. Chen, J. Luo and C.-J. Zhong, Catalysts, 2016, 6, 96.
80. M. Cordoba, L. Garcia, L. Martinez Bovier, J. Badano, C. Betti, F. Coloma Pascual, M. Quiroga and C. Lederhos, Top. Catal., 2022, 65, 1347–1360.
81. S. R. Akuri, C. Dhoke, K. Rakesh, S. Hegde, S. A. Nair, R. Deshpande and P. Manikandan, Catal. Lett., 2017, 147, 1285–1293.
82. H. Pan, B. Ma, L. Zhou, Y. Hu, M. Shakouri, Y. Guo, X. Liu and Y. Wang, ACS Sustainable Chem. Eng., 2023, 11, 7489–7499.
83. W. Zhang, Y. Wang, B. Gu, Q. Tang, Q.-E. Cao and W. Fang, ACS Sustainable Chem. Eng., 2023, 11, 12798–12808.
84. Z. Zhu, L. Yang, C. Ke, G. Fan, L. Yang and F. Li, Dalton Trans., 2021, 50, 2616–2626.
85. D. S. S. Jorqueira, L. F. de Lima, S. F. Moya, L. Vilcocq, D. Richard, M. A. Fraga and R. S. Suppino, Appl. Catal., A, 2023, 665, 119360.
86. M. J. Gilkey, P. Panagiotopoulou, A. V. Mironenko, G. R. Jenness, D. G. Vlachos and B. Xu, ACS Catal., 2015, 5, 3988–3994.
87. J. Yang, Y. Fan, Z.-L. Li, Z. Peng, J.-H. Yang, B. Liu and Z. Liu, Mol. Catal., 2020, 492, 110992.
88. W. Tolek, N. Nanthasanti, B. Pongthawornsakun, P. Praserthdam and J. Panpranot, Sci. Rep., 2021, 11, 9786.
89. H. Hao, Y. Abe, H. Guo, X. Zhang and R. Lee Smith Jr., ACS Sustainable Chem. Eng., 2022, 10, 16261–16270.

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Footnotes

† Electronic supplementary information (ESI) available: BET surface area, pore volume, and pore size of the catalysts, HR-TEM images of TiO₂ nanorods, TEM images of commercial TiO₂ (anatase), quantitative measurements of H₂-TPR profiles, controlled reactions, NMR spectra of furfuryl alcohol, HR-MS spectrum of triethyl(methoxy)silane, HR-MS data of the products of the substrate scope studies, O 1s XPS and Ti 2p XPS spectra of the catalysts, HR-MS data, and kinetic studies of CTH of furfural. See DOI: https://doi.org/10.1039/d5gc00006h

‡ These authors contributed equally.

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