1 Introduction

At present, with the depletion of fossil resources and the deterioration of the environment, the production of chemicals from biomass has become a hot field [1]. Furfural (FUR), as an important biomass-derived platform, is easily obtained from hemicellulose, and the yearly production of FUR was about 200,000 Tm in the last decade [2, 3]. FUR can convert to many high value chemicals though different reactions, such as hydrogenation [4, 5], oxidation [6,7,8], oxidation-condensation [9,10,11], ring opening [12], esterification [13, 14], etc. Among them, around 62% of FUR is estimated to be converted into furfuryl alcohol (FAL) due to its industrial relevance for the manufacture of foundry resins [15]. Commercially, the FAL is produced by the direct hydrogenation over copper-chromite catalysts [16]; however, these catalysts have high toxicity and always produce more environmental problems, some other precious metals (e.g. Pt, Pd, Ir, Ru) [17, 18] was thus used in the direct hydrogenation reaction of FUR. While, the high pressure of H2 tends to hydrogenate the C = C bonds, resulting in a low selectivity of FAL. On the other hand, the high pressure of H2 is difficult to store and transport. Therefore, it is necessary to find an economical and efficient process to produce FAL from FUR.

Catalytic transfer hydrogenation (CTH) of α,β-unsaturated aldehydes to the corresponding alcohols using organic acid or alcohol instead of H2 as hydrogen donor is a green and sustainable pathway [19]. Because the CTH reaction has the characteristics of non-toxic, high atomic economy, there are many publications dealing with the performance of various catalysts for the CTH reaction of FUR to FAL in recent years. However, this process was always accompanied by several competing reactions and formed 2-Methy Furan (2-MF), 2-(diisopropoxymethyl)furan (DIPMF), 4-(furan-2-yl)-4-hydroxybutan-2-one (FHB), 4-(furan-2-yl)but-3-en-2-one (FB), 2-(isopropoxymethyl)furan (IPF), and so on, as shown in Scheme 1. FUR is usually produced from the acid catalytic hydrolysis of lignocellulose, which is accompanied by the process with an equal amount of formic acid, which can be used as a hydrogen source for the reduction of FUR [15]. Neeli et al. reported a Rh/ED-KIT-6 as the catalyst and formic acid as the H2-donor for the CTH reaction of FUR to FAL that gave a 99% selectivity to FAL with 98% conversion of FUR after 5 h at 100 °C [20]. However, the decomposition of formic acid will also produce CO2, which has a complex effect on the reaction. Not to mention that the acidity of formic acid will corrode the reactor. Alcohol, as a near neutral chemical, overcomes the disadvantages of formic acid as the hydrogen source and becomes an ideal hydrogen donor for CTH reaction. Acid or basic catalysts (e.g., composite metal oxide, zeolites, MOF) are considered to be effective catalysts for the CTH reaction. Very recently, Sancho et al. developed a ZrO2/Al2O3 catalyst for the CTH of FUR to FAL reaction using 2-propanol as the H-donor and solvent, giving a 95% FUR conversion with a 90% yield of FAL after 5 h at 130 °C [21]. The great number of acid sites and high specific surface area are beneficial to the formation of FAL. Some other complex catalysts were also prepared and used in the CTH of FUR to FAL [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36], as depicted in Table 1. Strangely, single metal oxide has less been studied in this reaction, although they are widely used in other CTH reactions of other reactants, such as cinnamaldehyde [37, 38], benzenepropanal [39], Levulinic acid [40, 41], cyclohexanone [42,43,44], crotonaldehyde [45] and so on.

Scheme 1
scheme 1

The CTH reaction of FUR and its competing reactions

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Table 1 Comparison of the catalytic performance of the as-prepared Al2O3 and other reported catalysts to the CTH of FUR to FAL using 2-propanol as H-donor
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Although composite metal oxides have been widely investigated in the CTH of FUR reaction, reports on FAL production from FUR over single metal oxide are surprisingly less [26, 28, 32, 34]. In this paper, a series of single metal oxides, includeing acidic (Al2O3), basic (MgO), acid–base bifunctional (ZrO2) and neutral (Fe2O3) catalysts were prepared and used to catalyze the CTH of FUR reaction. Several key experimental variables, such as the reaction temperature, the reaction time, the kind of alcohol and the catalyst dosage were optimized to attain the highest FUR conversion and FAL yield. The as-prepared Al2O3 catalyst is identified as the most effective for FAL production, giving a FUR conversion as high as 96.5% and FAL selectivity of 99% after 1 h at 150 °C using 2-propanol as the H-donor and solvent, outperforming most catalysts published in the literature. In addition, the FUR to catalyst mass ratio is significantly higher than those employed in the publications (Table 1).

2 Experimental

2.1 Materials

The precursor salts such as, Al(NO3)3∙9H2O, ZrOCl2∙8H2O, Mg(NO3)2, Fe(NO3)2 were supplied by Sinopharm Chemical Reagent Co., Ltd; the precipitant NH3∙H2O (25 ~ 28%) was obtained by Changzhou Yongfeng Chemical Co. Ltd; furfural (99%), furfuryl alcohol (99%) and solvent (e.g., methanol, ethanol, 1-propanol, 1-butanol, 2-propanol, 2-butanol) were purged from Aladdin. All chemicals were directly used in the experiments without further purification.

2.2 Catalyst Preparation

All the metal oxides were prepared by precipitation mothed. For Al2O3 (Scheme 2), 105 mL of an aqueous solution of 2.5 wt% NH3∙H2O were added in the three-neck flask. 350 mL of an aqueous solution of 0.078 M Al2(NO3)3 and 245 mL 2.5 wt% NH3∙H2O were added dropwise into the three-neck flask under strong stirring. Al2(NO3)3 and NH3∙H2O were completed simultaneously by adjusting the flow rate. After the process finished, the mixture solution was continued stirring for 0.5 h, then aged for 12 h at room temperature before the product precipitates were collected by filtration. The precipitate was then washed with deionized water until neutral. Finally, the solid powers were dried overnight at 110 °C and calcined at 500 °C for 5 h in a muffle furnace.

Scheme 2
scheme 2

Preparation route of Al2O3 catalyst

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ZrO2, MgO and Fe2O3 catalysts were synthesized by the same process as Al2O3, and their precursors were ZrOCl2·8H2O, Mg (NO3)2 and Fe (NO3)3, respectively.

2.3 Catalyst Characterization

X-ray diffraction (XRD) patterns of the catalysts were measured on a D/max 2500 PC X-ray diffractometer (Rigaku) with a graphite monochromator using Ni-filtered Cu-Kα (λ = 0.15406 nm) radiation at 40 kV and 40 mA. The patterns were recorded over 2θ = 10–70° at a rate of 10°/min. The surface area, pore volume and pore diameter data were obtained by N2 adsorption–desorption (ASAP 2020 instrument) at 77 K. The specific surface areas were calculated based on the Brunauer–Emmet–Teller method, and pore size distributions were determined by the Barret–Joyner–Halenda method. The acid properties of the samples were evaluated by NH3-TPD using ChemBET-3000. The samples (0.2 g) were first pretreated under He atmosphere (80 mL min−1) at 500 °C for 1 h and then cooled to 100 °C for adsorption of NH3/He (30 min). After purging with Ar to remove the reversibly adsorbed NH3, the sample was finally heated from 100 °C to 500 °C at a rate of 10 °C min−1 in flowing Ar.

2.4 Catalytic Reaction

The catalytic transfer hydrogenation reaction of FUR with alcohol was conducted in a stainless steel autoclave (25 mL). A typical procedure for the reaction is described as follows: 50 mg of catalyst, 2 mmol of FUR, 10 mL of alcohol were added into the autoclave. The autoclave is sealed and purged with N2 (0.8 MPa) 3 times to replace the air, finally, 0.8 MPa N2 was introduced to the autoclave. The reactor was heated to reaction temperature under stirring. After the reaction, the autoclave was cooled immediately using ice water, and the reaction products were analyzed by GC (SP-7860) with a FFAP capillary column and a flame ionization detector (FID). Conversion of FUR and selectivity of FAL were calculated according to the following calculations:

$$\mathrm{FUR \,\, conversion }\left(\mathrm{\%}\right)=\frac{\mathrm{moles \,\, of \,\, FUR \,\, reacted \,\,}}{\mathrm{moles \,\, of \,\, FUR \,\, in \,\, the \,\, feed \,\, }}\times 100\%$$
$$\mathrm{FAL \,\, selectivity }\left(\mathrm{\%}\right)=\frac{\mathrm{moles \,\, of \,\, FAL \,\, in \,\, the \,\, product \,\,}}{\mathrm{moles \,\, of \,\, FUR \,\, reacted\,\,}}\times 100\%$$

The catalytic activity was expressed as mass-specific rates according to the consumption of FUR and formation rate of FAL, which were obtained using the following equation:

$$\mathrm{FUR \,\, consumption \,\, rate}=\frac{\mathrm{moles \,\, of \,\, FUR \,\, consumed \,\, per \,\, hour \,\,}\left(\mathrm{mol}/\mathrm{h}\right)}{\mathrm{weight \,\, of \,\, catalyst \,\,}\left(\mathrm{g}\right)}$$
$$\mathrm{FAL \,\, formation \,\, rate}=\frac{\mathrm{moles \,\, of \,\, FAL \,\, formed \,\, per \,\, hour \,\,}\left(\mathrm{mol}/\mathrm{h}\right)}{\mathrm{weight \,\, of \,\, catalyst \,\,}\left(\mathrm{g}\right)}$$

3 Results and Discussion

3.1 Catalyst Characterization

Figure 1 shows the XRD patterns of prepared metal oxide catalyst. All samples showed their corresponding diffraction peaks. No impurity peaks appeared in those XRD patterns, indicating that the purity of all those catalysts is very high.

Fig. 1
figure 1

XRD spectra of metal oxide catalyst

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The texture properties of catalysts were analyzed by N2 physical adsorption–desorption isotherms at 77 K. The curves of adsorption–desorption isotherms and pore size distributions are shown in Fig. 2. All samples showed the Type IV isotherms, indicating the existence of mesoporous structure in all catalysts (Fig. 2A). Both prepared and commercial Al2O3 samples showed a long and narrow H3 type hysteresis loop at a wide relative pressure (P/P0 = 0.42 ~ 0.95), indicating that Al2O3 has a typical mesoporous feature, which agrees well with the literature [38]. ZrO2 presented a H2 type hysteresis loop at P/P0 = 0.65 ~ 0.88, MgO and Fe2O3 exhibited an H1 type hysteresis loop at a relatively narrow P/P0 = 0.90 ~ 0.98. In addition, the pore size distribution of Al2O3 and ZrO2 is relatively narrow compared to MgO and Fe2O3 samples (Fig. 2B). The detailed textural properties (surface area, pore volume and pore diameter) of samples were summarized in Table 2. The prepared Al2O3 had the largest BET surface area of 295 m2 g−1, as well as the highest total volume (0.36 cm3 g−1) and lowest pore diameter (3.7 nm), which is also in line with the TEM result (Fig. S1). The commercial Al2O3 lad a lower surface area (131 m2 g−1) and pore volume (0.22 cm3 g−1) than the as prepared Al2O3. Fe2O3 showed the lowest surface area (29 m2 g−1) and pore volume (0.05 cm3 g−1).

Fig. 2
figure 2

N2 adsorption–desorption isotherms (A) and the pore size distributions (B) of the prepared catalyst

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Table 2 The textural and acidity-basicity properties of the catalysts
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NH3-TPD and CO2-TPD were measured to study the acid–base properties of the prepared catalysts. Because the calcination temperature of those samples was 500 °C, the highest temperature of NH3 or CO2 desorption was thus set at 500 °C. As seen in Fig. 3A, the Al2O3 and ZrO2 samples had significantly broad peaks of NH3 desorption from about 180 °C to 500 °C. The peak temperature of Al2O3 (250 °C) is lower than that of ZrO2 (300 °C), suggesting that both Al2O3 and ZrO2 have weak acid sites and the acid strength of Al2O3 is slightly lower than that of ZrO2. In addition, the prepared Al2O3 has higher acidity than commercial Al2O3. Maybe this is why the prepared Al2O3 has better catalytic performance than commercial Al2O3. While, Fe2O3 has almost no obvious peak of NH3 desorption, indicating that there are a few acid sites in Fe2O3 sample. In addition, small NH3 desorption peaks also appeared in the MgO sample, which is consistent with literature reports [26]. Landau et al. also determined the surface acidity of MgO by the n-butylamine titration method [46]. The base properties of catalysts were shown in Fig. 3B, the CO2 desorption temperature of Al2O3 and ZrO2 were similar and relative low (110 °C), indicating the week basic sites existed on the surface of catalysts. As was expected that the MgO sample showed a strong base strength (peak temperature at 250 °C) and medium basic sites. The density of the surface acidic and basic sites was also calculated and listed in Table 2. The prepared Al2O3 showed the highest acidity (153.9 µmol g−1), flowed by ZrO2 (83.6 µmol g−1) and commercial Al2O3 (77.6 µmol g−1). Fe2O3 and MgO contained few acid sites, which were 3.1 and 9.9 µmol g−1 respectively. Expectedly, MgO as an alkaline oxide exhibited large numbers of basic sites (201 µmol g−1). Al2O3 and ZrO2 has few basic sites, and there are almost no basic sites on Fe2O3 surface. Those results are consistent with the data in the literature [26, 27, 39].

Fig. 3
figure 3

A NH3-TPD and B CO2-TPD of metal oxide catalyst

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3.2 Catalytic Performance

3.2.1 Catalyst Screening

The CTH reaction of FUR with 2-propanol was carried out at 150 °C for 1 h over different metal oxide catalysts and the results were shown in Table 3. It is seen that, except Fe2O3 catalyst, the main product was FAL (selectivity: 91.7 ~ 99.0%). A number of very minor byproducts were also detected and confirmed by GC–MS analysis (as shown in Scheme 1), including DIPMF (acetalization reaction between FUR and 2-propanol), 2-MF (excessive hydrogenation of FUR), FHB (reaction of FUR with acetone), FB (dehydration of FHB), IPF (etherification of FAL with 2-propanol) [27, 33, 47, 48]. The selectivity for those minor products was less than 2%, thus we did not list them in Table 3. As expected, no FAL was detected in the absence of catalyst, indicating that the noncatalyzed process could not occur in the CTH reaction of FUR and 2-propanol (Entry 1). Using Fe2O3 as the catalyst, only 17.6% conversion of FUR with 24.7% selectivity of FAL were obtained (Entry 2), meanwhile, about 60% selectivity of acetalization product (DIPMF) was formed by the acid site of the catalyst surface. ZrO2 as an acid–base catalyst offered a 45.8% FUR conversion and 96.3% FAL selectivity (Entry 3), Zr-based catalysts were also effective for the CTH reaction with other reactants in the reference [21, 27, 33, 49]. In addition, a 69.8% conversion and 93.4% selectivity were obtained over the base MgO catalyst (Entry 4), MgO was also found to be active for other CTH reactions [39, 50]. Very interesting, the highest FUR conversion (96.5%) and FAL selectivity (99.9%) were achieved with the acid Al2O3 catalyst (Entry 5), and showed the highest FAL yield (96.4%) and FAL formation rate (38.6 mmol gcat−1 h−1). The GC image was shown in Fig. S2. For comparison, the commercial Al2O3 was used in this reaction, only 44.1% FUR conversion was obtained (Entry 6), which is agreed will with the same reaction in the reference [51]. It indicated that the as prepared Al2O3 can effectively catalyze FUR to FAL compared to other metal oxide catalysts. It has been known that acidity sites play an important role in the CTH reaction when using alcohol as H-donor [21, 26, 37, 48, 52]. An attempt is made to correlate the FAL yield with the surface acidity and surface area of the catalysts (except for MgO) and the results were shown in Fig. 4. It is clear that the FAL yield increased with increasing the surface acidity and surface area. The Fe2O3 with near neutral and lowest surface area presented the lowest FAL yield (4.3%), and Al2O3 with the largest surface area and the highest density of surface acidic showed the highest FAL yield (95.5%). It should be mentioned that the basic MgO also gave a relatively high FAL yield (65.2%), which agrees with many earlier reports [39, 50].

Table 3 Catalyst screening for the CTH of FURa
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Fig. 4
figure 4

Dependence of the FAL yield to total acidity (Black filled triangle) and specific surface area (Blue filled circle)

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3.2.2 Effect of H-Donor

To reveal the influence of H-donor on the performance of Al2O3 catalyst, several alcohols include C1 to C4 alcohols, primary and secondary alcohols were chosen and used in the CTH of FUR to FAL reaction. It shows in Table 4 that all of the alcohols could offer the hydrogen atom for FUR. When primary alcohols (methanol, ethanol, 1-propanol and 1-butanol) were used as H-donor (Entry 1–4), the FUR conversion (28.5 ~ 48.5%) and FAL yield (1.3 ~ 39.4%) were much lower than that of secondary alcohol (2-propanol and 2-butanol). This is because secondary alcohols have a lower reduction potential than primary alcohols [38, 52]. The acetalization was the main reaction in methanol and thus showed the lowest FAL selectivity (2.6%) [32, 53]. 1-propanol and 2-propanol showed the higher FUR conversion (40.8% and 96.5%) than that of 1-butanol and 2-butanol (28.5% and 75%), respectively, which is due to the steric effect caused by the longer carbon chain of 2-butanol [53]. Combined with the FUR conversion and FAL selectivity, 2-propanol was the most effective H-donor and reaction solvent for the CTH reaction of FUR to FAL. Those results agree well with earlier literature over other catalysts [26, 30, 32, 53].

Table 4 The influence of alcohols in the CTH of FUR over Al2O3 catalysta
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3.2.3 Effect of Temperature and Time

The effect of the reaction temperature in the range of 110 ~ 150 °C and reaction time in the range of 5 ~ 60 min on the catalytic performance of Al2O3 catalyst were investigated using 2-propanol as the H-donor, the results are shown in Fig. 5. The reaction temperature and time had little effect on the selectivity of FAL but had a big influence on FUR conversion. The effect of temperature on FUR conversion and FAL selectivity at 60 min are shown in Fig. 5A. The FAL selectivity was higher than 95% regardless of temperature, and the FUR conversion increased from 42% to 96% when temperature increased from 110 °C to 150 °C. By measuring the effect of temperature on FUR consumption rates could obtain the apparent activation energy (Ea) over Al2O3 catalyst. The Arrhenius plots were shown in Fig. S3, and the value of Ea is 15.2 kJ/mol. Which is much lower than the results of other catalysts [26, 27, 31, 36, 47, 49, 54, 55]. The effect of reaction time on FUR conversion and FAL selectivity at 150 °C are shown in Fig. 5B. To our surprise, the FUR conversion reached almost 40% in just 5 min of the reaction. The consumption rate was as high as 186 mmol gcat−1 h−1. The FUR conversion increased to 96% with increasing the reaction time to 60 min. The FAL selectivity was always higher than 95%. Hence, the FAL yield could reach as higher as 96% for only one hour at 150 °C, and the catalytic performance of this catalyst was much higher than that of the most catalysts published in literatures, as listed in Table 1.

Fig. 5
figure 5

Effect of reaction temperature (A) and reaction time (B) on the catalytic performance of Al2O3 for the CTH reaction of FUR to FAL. Reaction conditions: 2 mmol FUR, 10 mL 2-propanol, 50 mg Al2O3, 0.8 MPa

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3.2.4 Effect of Catalyst Dosage

The effect of catalyst dosage on the catalytic performance of FUR to FAL was also studied at 150 °C and 1 h using 2-propanol as the H-donor. It can be seen from Fig. 6 that the selectivity of FAL was always higher than 90% regardless of the catalyst amount. The FUR conversion increased more than two times (from 26% to 56%) with increasing the catalyst dosage from 10 mg to 20 mg. Further increased the amount of Al2O3 to 50 mg, the conversion of FUR increased to 96% slowly. Therefore, the preferred Al2O3 amount was 50 mg in the CTH reaction of FUR to FAL.

Fig. 6
figure 6

The effect of the amount of Al2O3 on the CTH reaction of FUR to FAL. Reaction conditions: 2 mmol FUR, 10 mL 2-propanol, 0.8 MPa N2, T = 150 °C, t = 1 h

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3.2.5 CTH Reaction of Various Aldehydes

The CTH reaction of other aldehydes to the corresponding alcohols over as prepared Al2O3 catalyst using 2-propanol as the H-donor and solvent at 150 °C was also performed. As shown in Table 5, the prepared Al2O3 showed high activity of aldehydes and selectivity to the corresponding alcohols, especially to benzaldehyde (Entry 1), 66% conversion and 99% selectivity for benzyl alcohol after 1 h. For the derivatives of benzaldehyde (e.g., 4-nitrobenzaldehyde, p-methyl benzaldehyde, p-fluorobenzaldehyde), due to the steric hindrance, a longer reaction time or higher reaction temperature was needed to obtain the higher activity of Al2O3 catalyst (Entry 2–4). In addition, a 58% conversion of salicylaldehyde and 72% selectivity of 1,2-phenylenedimethanol were obtained (Entry 5). Moreover, Al2O3 can also catalyze the aliphatic aldehydes to the corresponding alcohols (Entry 6). Therefore, the prepared Al2O3 also showed a good catalytic performance for the other aldehydes. In combination with the simple preparation and low cost, the Al2O3 catalyst has a good industrial application.

Table 5 Catalytic performance of Al2O3 catalyst for other aldehydesa
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3.3 Possible Reaction Mechanism

As is well known, homogeneous Lewis acids can effectively promote the CTH process, whereas the Brønsted acids cannot catalyze this reaction. The mechanism for homogeneous CTH process is commonly accepted to involve a cyclic six-membered transition state in which both the reducing alcohol and the carbonyl compound are coordinated to the same Lewis acidic center [37, 56]. In a heterogeneous system, the surface Lewis acidic sites [37, 48], Brønsted acidic sites [52, 57, 58] and Basic sites [39, 50] were all found to be active for CTH reaction. Base on the nature of Al2O3 (abundant Lewis acidic sites) and previous studies on CTH reaction [26, 30, 37, 49, 51], we proposed a possible reaction mechanism. As shown in Scheme 3, 2-propanol was first activated and adsorbed on the surface Lewis acidic sites (Al3+) of Al2O3 to form the corresponding alkoxide (step I), followed by activation and adsorption of the C = O bound of FUR on the same Lewis acidic sites (step II). Subsequently, a six-membered ring transition state was formed between FUR and 2-propanol by the hydrogen transfer process (step III). Finally, ring-opening occurred (step IV) and the FAL was desorbed from Al2O3 along with acetone (step V) to complete the catalytic cycle.

Scheme 3
scheme 3

Possible reaction mechanism for the CTH of FUR to FAL over Al2O3 catalyst

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4 Conclusions

In summary, a series of single metal oxides with different acid–base nature include Al2O3, MgO, ZrO2 and Fe2O3 catalysts were prepared by precipitation mothed and then used to catalyze the CTH of FUR to FAL reaction. It is found that the as-prepared Al2O3 catalyst is identified as the most effective for FAL production, giving a FUR conversion as high as 96.5% and FAL selectivity of 99% after 1 h at 150 °C using 2-propanol as the H-donor and solvent. The Al2O3 has a large specific surface area and high density of surface acidic, which were beneficial to FAL production from FUR. The catalytic data also showed that reaction time, reaction temperature and catalyst dosage have little effect on FAL selectivity (> 95%) but had a big influence on catalyst activity. Compared to other alcohols, 2-propanol was the most effective H-donor for the CTH reaction of FUR to FAL. The Arrhenius plots showed an Ea value of 15.2 kJ/mol, which is much lower than other catalysts. The prepared Al2O3 also showed a good catalytic performance for the others aldehydes and has a good application in industry.