Highly active and durable WO₃/Al₂O₃ catalysts for gas-phase dehydration of polyols

Abstract

Gas-phase glycerol dehydration over WO₃/Al₂O₃ catalysts was investigated. WO₃ loading on γ-Al₂O₃ significantly affected the yield of acrolein and the catalyst with 20 wt% WO₃ loading showed the highest activity. The WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed higher activity and durability than the other supported WO₃ catalysts and zeolites. The number of Brønsted acid sites and mesopores of the WO₃/Al₂O₃ catalyst did not decrease after the reaction, suggesting that glycerol has continuous access to Brønsted acid sites inside the mesopores of WO₃/Al₂O₃, thereby sustaining a high rate of formation of acrolein. Dehydration under O₂ flow further increased the durability of the WO₃/Al₂O₃ catalyst, enabling the sustainable formation of acrolein. In addition, the WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed high activity for the dehydration of various polyols to afford the corresponding products in high yield.

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Introduction

The conversion of biomass-derived compounds instead of oil-derived resources to valuable chemicals is essential for the sustainable development of society.^1–5 Glycerol is one of the most important biomass-derived compounds. It is produced by the hydrolysis of glycerides such as those in vegetable oils and animal fats.^6–8 A wide range of commodity chemicals can be produced from glycerol through various reactions such as dehydration,^9–25 hydrogenolysis^26,27 and oxidation^28–30 over metal oxides, zeolites, metal phosphides and metal–organic frameworks. The dehydration of glycerol is a valuable reaction for the formation of acrolein, which is an important intermediate for the production of valuable compounds in the chemical and agricultural industries, such as acrylic acid and DL-methionine.^31–34 Since acrolein is produced by the oxidation of propylene over a Bi/Mo-mixed oxide catalyst,^35–40 a method for the production of acrolein from glycerol instead of oil-derived resources should be quite valuable.

Furthermore, solid acid-catalyzed dehydration of polyols has been reported to be a useful method for obtaining value-added chemicals (e.g. diols, aldehydes, ketones and alkenes).^41–48 Many of these reactions, however, usually require a low substrate feed rate and high reaction temperatures to achieve a high conversion rate. Hence, the development of a catalyst that shows high activity and selectivity to give the target products under mild conditions is still a challenge.

On the other hand, the deposition of coke on the catalyst surface contributes to deactivation of the catalyst in dehydration. The mechanism of coke formation and the relationship between coke formation and acid strength and/or porosity of the catalyst have been widely investigated.^49–54 Two deactivation mechanisms due to coke formation have been proposed so far; the direct deposition of coke at catalytically active acid sites and coke formation at the entrance of micropores, which hinders the substrate from accessing the active site inside the pore. A better understanding of the deactivation mechanism could be useful for the design of novel dehydration catalysts with high durability.

In the course of our investigation of the relationship between the structure of a two-dimensional tungsten oxide monolayer supported on metal oxides and their performance as acid catalysts,^55,56 we reported that supported WO₃ catalysts showed high activity for selective biomass conversion, such as the hydrogenolysis of glycerol and tetrahydrofurfuryl alcohol.^57–60 These results encouraged us to further explore the application of selective conversion of biomass-derived chemicals over supported WO₃ catalysts, in particular polyols to give value-added chemicals.

In this study, gas-phase glycerol dehydration over WO₃/Al₂O₃ catalysts was investigated. The optimization of WO₃ loading on Al₂O₃ enabled us to devise highly active and durable catalysts for glycerol dehydration. To reveal the effect of coke deposition at the catalyst surface on catalytic activity, the reaction under O₂ flow and a detailed characterization were carried out. Furthermore, the application of WO₃/Al₂O₃ catalyst to the dehydration of various polyols was investigated.

Experimental

Materials

Glycerol was purchased from Nacalai Tesque, Japan. Polyols, including ethylene glycol, 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol and 1,2-pentanediol, were purchased from Wako Pure Chemical Industries and Tokyo Chemical Industry, Japan. γ-Al₂O₃ (JRC-ALO-8), ZrO₂(JRC-ZRO-3, monoclinic), TiO₂(JRC-TIO-4, rutile) and Nb₂O₅ (JRC-NBO-1, orthorhombic) were supplied by Japan Reference Catalyst. (NH₄)₁₀W₁₂O₄₁·5H₂O and WO₃ were purchased from Wako Pure Chemical Industries, Japan. All zeolites such as H-ZSM-5(90), H-β(25), H-Y(5.5) and H-MOR(20), namely, JRC-Z5-90H, JRC-Z-HB25, JRC-Z-HY5.5 and JRC-Z-HM20, where the number in parentheses is the SiO₂/Al₂O₃ ratio, were supplied by Japan Reference Catalyst.

Catalyst preparation

Metal oxides and zeolites were calcined at 773 K for 3 h in flowing air. A series of WO₃-loaded catalysts were prepared by impregnation of supports with an aqueous solution of (NH₄)₁₀W₁₂O₄₂, dried at 353 K for 6 h, and then calcined at 1123 K for 3 h in flowing air.^55,56 The surface coverages of Al₂O₃, ZrO₂ and TiO₂ with a WO₃ monolayer were estimated by using the cross-sectional area of a WO₆ octahedral unit (0.22 nm⁻²)⁶¹ and were almost 100% at 20, 10 and 2.5 wt% WO₃ loading, respectively.⁶⁰

Catalytic dehydration of polyols

Catalytic polyol dehydration was performed in a fixed-bed down-flow glass reactor with an inner diameter of 6 mm at an ambient pressure of N₂ and O₂. Prior to the reaction, 100 mg of a catalyst was placed in the catalyst bed, and the catalyst was treated at 588 K with N₂ for 1.5 h. A 10 mol% aqueous solution of polyols was fed through the top of the reactor at a prescribed liquid feed rate together with a carrier gas flow of 20 mL min⁻¹. The total substrate feed rate was 5.1 mmol h⁻¹ and the composition of the mixture gases was substrate/H₂O/carrier gas = 1/9/10 (molar ratio). The liquid products were collected in an ice trap (273 K) and a dry ice–methanol trap (195 K) every hour, and analysed by a FID-GC (GC2014, Shimadzu) with a 30 m capillary column of Stabilwax (GL Science, Japan). Gaseous products were analysed by on-line TCD-GC (GC-8A, Shimadzu, Japan) with a 3 m packed column (Porapak-Q, GL Science, Japan). For all the catalysts tested, the carbon balance was >90%, with a few exceptions.

Characterization

X-ray diffraction (XRD) patterns of the catalyst were recorded by Rigaku SmartLab with Cu Kα radiation. The samples were scanned from 2θ = 10–70° at 10° min⁻¹ and a resolution of 0.01°. X-ray photoelectron spectroscopy (XPS) analysis of the catalysts was performed using a JEOL JPS-9010 MX instrument. The spectra were measured using Mg Kα radiation. All spectra were calibrated using C 1s (284.5 eV) as a reference. The surface properties of samples were obtained from N₂ isotherms obtained using a BELSORP-mini II (MicrotracBEL, Japan) at 77 K. The analysed samples were evacuated at 573 K for 3 h prior to the measurement. The surface area was estimated by the Brunauer–Emmett–Teller (BET) method. Micropore size distributions were determined by t-plot and micropore (MP) analysis. The Barrett–Joyner–Halenda (BJH) method was used to determine mesopore size distributions. FT-IR spectra were recorded by FT/IR-4200 typeA (JASCO, Japan) with a resolution of 4 cm⁻¹. A total of 64 scans were averaged for each spectrum. Each sample (30 mg) was pressed into a self-supporting wafer with a diameter of 20 mm. Catalysts were pretreated under 20 kPa of flowing O₂ at 773 K for 1.5 h and then evacuated. Catalysts were exposed to pyridine (0.5 kPa) at 303 K for 30 min, and then evacuated at 423 K for 30 min. NH₃ temperature programmed desorption (NH₃-TPD) was measured using a BELCAT II (MicrotracBEL, Japan). Fifty mg of the sample was loaded in a quartz reactor and pretreated under a He flow at 773 K for 1 h. NH₃ adsorption was carried out over 30 min with 5% NH₃/He at 373 K followed by purging with He for 15 min. The temperature was linearly increased from 373–873 K (10 K min⁻¹). The outlet flow was analysed by means of a TCD and Q-Mass (BELMASS, MicrotracBEL, Japan). Thermogravimetric analyses (TG) were performed using a DTG-60H (Shimadzu, Japan) The temperature range was 303–1273 K (10 K min⁻¹).

Results and discussion

Fig. 1 shows the effect of WO₃ loading over γ-Al₂O₃ on the gas-phase dehydration of glycerol at 588 K with WHSV by glycerol of 4.7 h⁻¹. The catalysts with 0 and 100 wt% WO₃ loading were γ-Al₂O₃ and bulk WO₃, respectively. Dehydration over WO₃/Al₂O₃ provided acrolein as a main product together with the formation of hydroxyacetone as a by-product. The selectivity for acrolein was around 80% regardless of the WO₃ loading on γ-Al₂O₃. In contrast, the WO₃ loading significantly affected the yield of acrolein. The yield of acrolein increased with an increase in WO₃ loading; WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed the highest acrolein yield. On the other hand, excess loading of WO₃ led to a decrease in the acrolein yield. Notably, γ-Al₂O₃ and WO₃ showed no activity for this reaction, suggesting that WO₃ loaded on γ-Al₂O₃ was the main active site for the dehydration of glycerol to form acrolein.

Fig. 1 Dehydration of glycerol to acrolein over WO₃/Al₂O₃ catalysts with various WO₃ loadings. ●: yield, □: selectivity. Conditions: catalyst (100 mg), WHSV by glycerol (4.7 h⁻¹), T = 588 K.

Since previous studies on the relationship between Brønsted acidity and the activity for glycerol dehydration to form acrolein have revealed that Brønsted acid sites were a main active site for acrolein formation,^10,62 the number of Brønsted acid sites (Brønsted acidity) on WO₃/Al₂O₃ catalysts with various WO₃ loadings was estimated by pyridine adsorbed FT-IR (Fig. 2 and S2†). Brønsted acidity increased with an increase in WO₃ loading, and WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading exhibited the highest Brønsted acidity. In contrast, the Brønsted acidity decreased gradually for catalysts with loadings above 20 wt%. Acrolein was formed in the highest yield with the catalyst with 20 wt% WO₃ loading which showed the highest Brønsted acidity. We reported that a two-dimensional WO₃ monolayer was formed on γ-Al₂O₃ when the WO₃ loading was below 20 wt% and Brønsted acid sites were generated at the boundaries between the WO₃ domains.^48,49 The close correlation between the catalytic activity and Brønsted acidity was observed. Some researchers reported that dehydration mechanism of glycerol and the formation of acrolein proceeds over Brønsted acid sites preferentially to over Lewis acid sites.^{9,10,16,18,62} Similarly, the active sites of this reaction should be the Brønsted acid sites at the boundaries between WO₃ domains on γ-Al₂O₃. After protonation of hydroxyl group at the secondary carbon of glycerol by the Brønsted acid sites at the boundaries between WO₃ domains, the dehydration and keto–enol tautomerism takes place to give 3-hydroxypropionaldehyde. The further dehydration of 3-hydroxypropionaldehyde can easily proceed because of the low stability to produce acrolein.

Fig. 2 Brønsted acidity of WO₃/Al₂O₃ catalysts with various WO₃ loadings.

Fig. 3 shows the durability of various solid acid catalysts in the dehydration of glycerol to acrolein. All the tested catalysts provided acrolein as a main product (Fig. S3†). Although WO₃/ZrO₂ with 10 wt% WO₃,⁶⁰ WO₃/TiO₂ with 2.5 wt% WO₃, H-ZSM-5(90) and H-β(25) showed high acrolein yield at the initial stage of the reaction, the rapid deactivation of these catalysts was observed. H-Y(5.5), H-MOR(20) and Nb₂O₅ showed low initial activity. On the other hand, WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed high catalytic performance regarding both activity and durability.

Fig. 3 Dehydration of glycerol to acrolein over various acid catalysts. ●: WO₃/Al₂O₃ with 20 wt% WO₃ loading, ▲: WO₃/ZrO₂ with 10 wt% WO₃ loading, ■: WO₃/TiO₂ with 2.5 wt% WO₃ loading, ◆: Nb₂O₅, ○: H-ZSM-5(90), △: H-β(25), □: H-Y(5.5), ◇: H-MOR(20). Conditions: catalyst (100 mg), WHSV by glycerol (4.7 h⁻¹), T = 588 K.

Coke deposition during dehydration is a well-known cause of catalyst deactivation.^49–54 Thus, to identify factors that contribute to deactivation, we evaluated the BET surface area of various solid acid catalysts before and after the reaction for 5 h as well as the amount of deposited coke (Table 1). Although coke was deposited on all of the catalyst surfaces, no remarkable relationship was observed between the amount of deposited coke and deactivation rates of catalysts. The BET surface areas of zeolite catalysts were drastically decreased after reaction. On the other hand, the decreases in the surface area of supported WO₃ catalysts were much smaller than those of zeolite catalysts. Particularly, WO₃/Al₂O₃ catalyst maintained a larger surface area than the other supported WO₃ catalysts. These results suggest that WO₃/Al₂O₃ catalyst maintained a large surface area during the reaction showed the highest acrolein yield and stability in the dehydration of glycerol.

Table 1 Physical properties of various catalysts before and after the reaction for 5 h

Catalyst	Amount of deposited coke^a/mg gcat^−1	BET surface area/m^2 gcat^−1
		Before reaction	After reaction^b
a Estimated by TG analysis.b Reaction under flowing N2 after 5 h.
WO3/Al2O3 (WO3: 20 wt%)	131	109	104
WO3/ZrO2 (WO3: 10 wt%)	82	48	69
WO3/TiO2 (WO3: 2.5 wt%)	35	14	18
H-ZSM-5 (90)	138	460	116
H-β (25)	198	660	103
H-Y (5.5)	225	749	25
H-MOR (20)	151	502	14

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Oxidative treatment is useful for removing the coke deposited at active Brønsted acid sites.^63–65 Hence, we examined the effect of the oxidative treatment of WO₃/Al₂O₃ and H-ZSM-5(90) after the catalytic run on the regeneration of catalytic performance. Treatment with O₂ flow (at 823 and 923 K for WO₃/Al₂O₃ and H-ZSM-5(90), respectively) successfully recovered the initial catalytic activity for both WO₃/Al₂O₃ with 20 wt% WO₃ loading and H-ZSM-5(90) (Fig. 4). This suggests that the coke deposited at active Brønsted acid sites can be removed by oxidative treatment, and WO₃/Al₂O₃ catalyst can be repeatedly used for the dehydration reaction.

Fig. 4 Effect of O₂ treatment on acrolein yield over (●) WO₃/Al₂O₃ with 20 wt% WO₃ loading and (○) H-ZSM-5(90) catalysts. Conditions: catalyst (100 mg), WHSV by glycerol (4.7 h⁻¹), T = 588 K. O₂ treatment was carried out after 5 h at the temperature at which deposited coke completely decomposed; 823 K (WO₃/Al₂O₃) and 923 K (H-ZSM-5), respectively.

Additionally, the dehydration of glycerol over WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading (Fig. 5) and H-ZSM-5 (Fig. S4†) under flowing N₂ and O₂ gas was investigated. The reaction under O₂ flow with both WO₃/Al₂O₃ and H-ZSM-5 catalysts showed constant acrolein yields of around 95% for 5 h, indicating that the reaction under an O₂ atmosphere was effective for protecting the catalysts from deactivation. Note that no excess oxidised products (such as acrylic acid) were obtained in this reaction condition.

Fig. 5 Dehydration of glycerol to acrolein over WO₃/Al₂O₃ catalysts with 20 wt% WO₃ loading and under flowing N₂ (circle) and O₂ (square). Conditions: catalyst (100 mg), WHSV by glycerol (4.7 h⁻¹), T = 588 K.

Table 2 shows the effect of the feed gas on the physical properties of WO₃/Al₂O₃ and H-ZSM-5 catalysts after the catalytic run. The amount of deposited coke in the reaction under O₂ was greater than that under N₂ over both catalysts. The acidity of catalysts after the reaction with flowing N₂ and O₂ was also estimated by NH₃-TPD (Fig. S6†). Note that pyridine adsorbed FT-IR measurement could not be used to estimate the acidity because coke deposition on the catalyst caused quite low IR transmittance. No difference in the acidity of the WO₃/Al₂O₃ catalyst between before and after the reaction was observed regardless of the reaction atmosphere. In contrast, the area of NH₃ desorption peak at 673 K was drastically decreased, indicating that the number of strong acid sites of H-ZSM-5 was decreased by the reaction. (Fig. S6(B)).† The micropore and mesopore distributions of WO₃/Al₂O₃ and H-ZSM-5 catalysts are shown in Fig. 6. No micropores were observed in the WO₃/Al₂O₃ catalyst. Furthermore, no significant change in pore size distribution after the reaction under both N₂ and O₂ was observed. On the other hand, H-ZSM-5 showed large number of micropores and mesopores. The catalytic runs drastically decreased the number of micropores in H-ZSM-5. These results suggest that coke deposition at the entrance of the H-ZSM-5 pore reduced the ability of glycerol to access the acid sites inside the micropores. Based on these results, we can conclude that the high activity and durability of WO₃/Al₂O₃ are responsible for the fact that the mesoporous nature of WO₃/Al₂O₃ was not prevented by coke formation at the entrance of pores and Brønsted acid sites.

Table 2 Physical properties of WO₃/Al₂O₃ with 20 wt% WO₃ loading and H-ZSM-5(90) catalysts after reaction under flowing N₂ and O₂

Catalyst	Carrier gas	Amount of deposited coke^a/mg gcat^−1	BET surface area/m^2 gcat^−1		Acidity^b/μmol gcat^−1
			Before reaction	After reaction^c	Before reaction	After reaction^c
a Estimated by TG analysis.b Estimated by NH3-TPD.c Reaction after 5 h.
WO3/Al2O3 (WO3: 20 wt%)	N2	131	109	104	296	104
	O2	221		69		297
H-ZSM-5 (90)	N2	138	460	116	158	48
	O2	170		131		30

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Fig. 6 Pore size distribution of the (A) micropore and (B) mesopore regions in WO₃/Al₂O₃ with 20 wt% WO₃ loading and (C) micropore and (D) mesopore regions in H-ZSM-5(90). Fresh catalyst (circle), after the reaction with N₂ (triangle) and O₂ (square) for 5 h.

The dehydration of polyols other than glycerol can also provide useful chemicals. Therefore, the dehydration of various polyols over WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading was investigated (Fig. 7). The reactions of polyols gave the corresponding aldehydes, dienes, ketones and ethers. In particular, the dehydration of polyols with hydroxyl groups at the 1,2-position such as glycerol, ethylene glycol, 1,2-propanediol, 1,2-butanediol and 1,2-pentanediol afforded the aldehydes (e.g. acrolein, acetaldehyde, propionaldehyde, butyraldehyde and valeraldehyde) as main products. On the other hand, the reaction of 1,3-butanediol mainly produced 1,3-butadiene through double dehydration. In contrast, the dehydration of 1,4-butanediol gave tetrahydrofuran as a sole product. These results indicate that WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading shows high activity and durability for the dehydration of various polyols.

Fig. 7 Dehydration of various polyols over WO₃/Al₂O₃ catalysts with 20 wt% WO₃ loading. Conditions: catalyst (100 mg), WHSV by substrate (4.7 h⁻¹), T = 588 K. (A) Ethylene glycol, (B) 1,2-propanediol, (C) 1,2-butanediol, (D) 1,2-pentanediol, (E) 1,3-butanediol and (F) 1,4-butanediol.

Conclusions

Gas-phase dehydration of polyols over WO₃/Al₂O₃ catalysts was investigated. WO₃ loading on γ-Al₂O₃ strongly affected the activity for the dehydration of glycerol to acrolein, and WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed the highest acrolein yield. Brønsted acid site on the WO₃/Al₂O₃ surface catalysed the formation of acrolein with high selectivity. WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed a higher product yield than the other supported WO₃ catalysts and zeolites. H-ZSM-5 has both micro- and mesopores and coke deposits at the entrances of those pores. On the other hand, WO₃/Al₂O₃ catalysts have no micropores, suggesting that glycerol can access surface Brønsted acid sites at the inside of the mesopores of WO₃/Al₂O₃ and acrolein is continuously formed with high durability during the reaction. No decrease in acrolein yield was observed under O₂ flow during dehydration over WO₃/Al₂O₃ catalyst. On the other hand, the amount of deposited coke under O₂ flow was greater than that under N₂, implying that coke deposited at the catalyst surface was removed by O₂. Furthermore, WO₃/Al₂O₃ catalyst with 20 wt% WO₃ loading showed high activity and durability for the dehydration of various polyols.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was partially supported by the Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB) (Grant JPMXP0112101003). This work was also supported in part by Grants-in-Aid for Scientific Research (B) (Grant 17H03459) and Scientific Research on Innovative Areas (Grant 17H06443) commissioned by MEXT of Japan. The XAFS experiments at SPring-8 were conducted with the approval (No. 2019A1717) of the Japan Synchrotron Radiation Research Institute (JASRI).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra08340b

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