Mesoporous nano-WO_x/ZrO₂: facile synthesis and improved catalysis

Abstract

A facile synthetic method was developed to prepare mesoporous nano-WO_x/ZrO₂ (MN-WZ). TEM, XRD, BET, UV-vis, Raman and catalytic tests were performed to investigate its mesoporous structure and acidity. Due to the abundant pores and corresponding permeability, the MN-WZ showed much improved catalytic performance on the conversion of n-pentane isomerisation.

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Solid acid catalysts played an important role in a great number of petroleum and chemical processes,^1,2 such as catalytic cracking,³ isomerization,^4,5 alkylation,⁶ hydration and dehydration,⁷ because of their high activity, convenient separation, and reduced corrosion of reactors. Tungsten oxide-based materials such as WO₃/Al₂O₃,^8,9 WO₃/TiO₂,^10,11 WO₃/SiO₂,^12,13 and WO₃/ZrO₂,^14–18 are considered promising solid acid catalysts, owing to their strong acidity and resulting notable activity for various catalytic reactions, especially the strongest one, WO₃/ZrO₂. In our previous work,^19,20 we made a series of nanosized WO_x/ZrO₂ catalysts via a two-phase interface hydrolysis (TPIH) method, whose aim was to improve its dispersion threshold and to make more effective acid sites. Results showed that nano-WO_x/ZrO₂ had higher dispersion threshold (ca. 0.372 g WO_x per g ZrO₂ and 6.2W atoms per nm²) and increased one time conversion rate than conventional ones on the catalytic reaction of n-pentane isomerisation. However, a serious problem, fast inactivation by coke, then followed. Actually, porous materials with macropore, mesopore or micropore can provide a high surface area and better permeability.^21–26 The better permeability would offer better coke-tolerance. This is to find one suitable method of preparing some porous nano-WO_x/ZrO₂ catalysts and to solve the problem of fast inactivation on n-pentane isomerisation.

This paper is aimed to develop some facile preparative method to fabricate porous nano-WO_x/ZrO₂ and extend to general preparation of nanocomposite. In this work, we developed a TPIH and polymer-template method to prepare mesoporous nano-WO_x/ZrO₂ (NM-WZ) and investigated its performance on the catalytic reaction of n-pentane isomerisation. Scheme 1 presents the process of preparation on NM-WZ by combining TPIH and polymer-template method. The water phase and the organic phase (e.g. toluene) were placed separately in a two-chambered Teflon-lined autoclave, and metal alkyl oxide precursors (M: W(OC₃H₇)₆ and Zr(OC₃H₇)₄) and polymer solution were added into the organic phase. Under the reacted temperatures over 100 °C, the two phases evaporated and diffused within the autoclave and hydrolysis reaction occurred at their interface area. At the same time, metal oxides crystallized within the confined spaces of swelled polymer matrix and the nano-WO_x/ZrO₂/polymer formed. The gel-like monolith of obtained composite can be easily separated from the parent organic solvent due to the characteristic segregation of the polymer below cloud point²⁷ (as shown in A, B and C parts). The organic phase could be recycled for the next round reaction and MN-WZ was obtained after direct calcination under 650 °C.

Scheme 1 Schematic diagram for the preparation process of mesoporous nano-WO_x/ZrO₂. M means metal alkyl oxide precursor.

Fig. 1a is the typical transmission electron microscope (TEM) image for MN-WZ prepared at thermal temperature of 150 °C and calcined at 650 °C for 12 h, which shows the product has abundant porous structures. The corresponding high-resolution TEM image (inset, Fig. 1a) shows that the pores, formed by the stack of nanoparticles. It appears to be interconnected and have wormlike shape. The determinate components of W–O–Zr are detected by energy dispersive (EDX) method (see ESI Fig. S1†). Fig. 1b shows the amplified mesoporous structure and observed crystal lattices. The inset shows one interesting observation of a heterojunction between WO₃ and ZrO₂. Powder X-ray diffraction (XRD) measurement and N₂ adsorption/desorption were carried out to investigate the characterizations of as-prepared MN-WZ. The characteristic peaks of t-ZrO₂ can be found completely in the XRD patterns as seen in Fig. 1c. The broad peaks imply the ZrO₂ are very small nanoparticles, which is in agreement with the observation from HRTEM. At the same time, the typical diffraction of m-ZrO₂ and m-WO₃ couldn't be detected, which implies the WO_x are highly dispersed on the surface of t-ZrO₂. The broad peak at the small-angle range from 0.6–1.5° shows the mesopores have a wide distribution of porous diameter. Fig. 1d is the curve of distribution of porous diameter based on nitrogen adsorption/desorption, which shows the products possess around 6 nm of diameter and rich mesopores. This result is in agreement with the observation of HRTEM, as well. The BET surface is calculated to 160 m² g⁻¹, which is 6 times higher than WO_x/ZrO₂ obtained by impregnated method.²⁸

Fig. 1 (a) Low resolution and (b) corresponding high resolution TEM images of MN-WZ, the arrow indicates the pore with diameter of around 5 nm (inset: further magnified image show the detailed observation of heterojunction between tetragnol-ZrO₂ and WO₃). (c) XRD pattens, (inset: diffraction at small angle) and (d) curve of pore distribution (inset: curve of N₂ adsorption/desorption).

The MN-WZ was further evaluated by UV-vis, Raman spectra and a fast catalytic test toward to acidity. As seen in Fig. 2a, the nano-WO_x-ZrO₂ without calcinations has two absorption peaks, 216.2 nm and 236.4 nm, which could be affiliated to the inherent absorption of nano ZrO₂ and WO_x, respectively. After thermal treatment at 650 °C for 8 h, the absorption peak locating at 216.2 nm changed to 219.6 nm, which corresponded to the slight growth of nano-ZrO₂; and that locating at 236.4 changed to a broaden absorption from 236.4 nm to 290.8 nm, which corresponded to the electron transition through the heterojunction between WO_x and ZrO₂ and a variety of WO_x states. Fig. 2b shows the Raman spectrum with the range from 720 cm⁻¹ to 1080 cm⁻¹. It can be seen that the sample before calcination just has one broad absorption peak, indicating the stretching modes of W–O or W–O–W (∼951.5 cm⁻¹) from a variety of tungstate species.²⁹ Then four peaks appeared after calcinations, which could be attributed to stretching modes of W–O–W (∼828.9 cm⁻¹, from WO₃), W–O–W or W–O (922.1 cm⁻¹ and 952.8 cm⁻¹, from two tungstate species), W=O oxo group (∼991.5 cm⁻¹ and 1020 cm⁻¹, from small oligomeric clusters). Fig. 2c shows the compared catalytic result on the reaction from cymene hydroperoxide (CHP) to phenol and acetone.^30,31 The referenced catalyst is zeolite MCM-22, which is one traditional industrial catalyst. It can be seen that, for MN-WZ, the conversion rate after 20 minutes was 53.6%, then rapidly increased to 86.1% after 120 minutes and kept almost unchanged. The referenced zeolite showed the conversion rate was always around 75%. The compared result revealed that our MN-WZ catalyst have a higher catalytic efficiency than referenced zeolite. These three results all implied that the MN-WZ has strong acidicity, which was from the interaction between W–O–Zr. That's to say, the polymer induced is simply one pore-template and there is no negative affection on the WO_x/ZrO₂ materials.

Fig. 2 (a) UV-vis absorption and (b) Raman spectrum of as-prepared MN-WZ. (c) The conversion of cymene hydroperoxide. (●) MN-WZ with W–O–Zr strong interaction, (calcined under 650 °C) and (▲) referenced zeolite MCM-22 catalyst.

To solve previous problem, as-prepared MN-WZ catalysts were tested preliminarily on conversion of n-pentane. Fig. 3 shows the compared catalytic results based on MN-WZ and TPIH-WZ. The TPIH-WZ lost its activity as quickly as reported previously. But the MN-WZ kept its activity aroud 22% and stabilized. This MN-WZ catalysts offered more improved catalysis on n-pentane isomerisation.

Fig. 3 Compared conversion of n-pentane catalyzed by MN-WZ and TPIH-WZ (ref.) at 523 K with WHSV of 2.25 h⁻¹.

As reported, working on the investigation of WO_x state on ZrO₂ is always an important work,^32,33 but the nature of the interaction between WO_x and ZrO₂ was still uncertain. Inspired by the observation of the heterojunction in HRTEM (Fig. 2b), we tried to build a relationship between W and Zr, which may help to understand the nature of WO_x on ZrO₂. The patterns of (001) crystal face tailored from simulated t-ZrO₂ (Fig. 4a) and m-WO₃ (Fig. 4c) are matched to the structures viewed on TEM images. Then the connected faces were located to the faces of t-ZrO₂ (110) (Fig. 4b) and m-WO₃ (010) (Fig. 4d). It could be seen that one corner Zr atom connected two four-coordinated oxygen atoms, and total eight four-coordinated oxygen atoms connected one central Zr atom. Eight bonds from four four-coordinated oxygen atoms were used to connect W atoms. Similarly, six-coordinated W atoms interlacedly connect each other via two-coordinated oxygen. Eight bonds from eight two-coordinated oxygen atoms were used to connect Zr atoms. As measured, a certain distance of W–W is 0.499 nm, which is much close to the distance of opposite Zr–Zr in one plane (0.515 nm). The distance might be the reason why m-WO₃ connected t-ZrO₂ with inclined edge. Herein, the refined structure, (four 0.25Zr + one 1.0Zr atoms and four 0.25W atoms connect oxygen to compose the WO_x/ZrO₂, the final molar ratio of Zr to W is 2 to 1) could be drawn as a speculation, which is well accordant to the structure of WO_x/ZrO₂ as ensured.^34,35

Fig. 4 (a) View of ZrO₂ (001) face (1, 3, 6, 8 atoms at front layer and 2, 4, 7, 9 atoms at back layer), (b) ZrO₂ (110) face after turning 90° from (001) face; (c) view of WO₃ (001) face (1, 3, 5, 7 atoms at front layer and 2, 4, 6, 8 at back layer), (d) WO₃ (010) face after turning 90° from (001).

Conclusions

In conclusion, a facile synthetic method is described to prepare mesoporous nano WO_x/ZrO₂ via a polymer confined two-phase hydrolysis reaction under thermal condition. The essence of our method is the combination of the interface hydrolysis and nano-confined nucleation process in the polymer matrix, through which the wall thickness (particle size) and pore size (removing of the polymer) could be adjusted easily. With the abundant pore structure and permeability, on the preliminary test of the n-pentane conversion, the prepared mesoporous nano-WO_x/ZrO₂ showed much improved catalytic performance. We think that, the method is general to easily prepare porous nanocomposites and the recycling of solvent make environment friendly and a little low-cost.

Experimentals

(1) In a typical synthesis, about 9 mL water was added into a Teflon lined stainless steel autoclave. Subsequently, 0.3 g zirconium isopropanol, 0.1 g tungsten isopropanol, and 0.3 g PEG were dissolved in 9 mL of toluene to serve as an organic phase. Then this solution was transferred into the inner container which was placed in a Teflon line above the surface of water. The autoclave was then sealed and placed in a preheated oven and heated statically at 150 °C for 24 h. After cooling down, the solid product was separated by picking up the gel from solvent and calcined under 600 °C for 5 h.

(2) The experiment of developing phenol and acetone from cymene hydroperoxide was carried out in a glass-flask with condensator. The as-prepared mesoporous nano-WO_x/ZrO₂ powders (0.2 g) were dispersed in a mixed solution with 10 mL acetone, 8 mL CHP/cymene (35% CHP in cymene) and kept stirring. The reaction temperature was controlled at 80 °C. The conversion was measured about every 30 minutes. The amounts of acetone, cymene and cymene hydroperoxide evolved were analysized by gas chromatography (Agilent GC-6820 TCD, Ar carrier).

(3) Preliminary catalytic performance isomerization of n-pentane was evaluated in a down-flow fixed-bed reactor. 0.1 g of mesoporous nano-WO_x/ZrO₂ was placed in the central region of the reactor, with each end filled with quartz sand. Before the reaction, the catalyst was pretreated at 673 K under flowing dry air for 2 h to remove the adsorbed moisture. Then, the reactor was cooled to the reaction temperature under flowing high-purity nitrogen. The reactant n-pentane was pretreated by out-gassing and filtration and then introduced via a SSI Series II digital pump. The flow rate was 0.006 mL min⁻¹, corresponding to a WHSV of 2.25 h⁻¹. Then, the liquid n-pentane was gasified in an evaporation chamber at 473 K. After that, the gaseous n-pentane underwent an isomerization reaction in the fixed-bed reactor. The products were analyzed by online gas chromatography with a HP-alumina capillary column and a flame ionization detector (FID).

(4) X-ray powder diffraction analysis of the products were carried out on a Bruker D8 X-ray diffractometer with CuKα radiation (λ = 1.5418 Å; 40 kV, 200 mA). The TEM and high resolution TEM analysis of the products were performed on FEI Tecnai 20 STWN. N₂ adsorption was measured on a Micromeritics Tristar 3000 instrument. The Raman property was investigated on a Dilor Labram-1B spectrometer and UV-vis property was investigated on a Varian cary-5000 spectrometer.

Acknowledgements

The authors thank the financial support from the SINOPEC and National Natural Science Foundation of China (NSFC 21373272). The authors also thank Prof. Yahong Zhang for polishing our language.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: EDX patterns of as-prepared mesoporous WO_x/ZrO₂ nanocomposite. See DOI: 10.1039/c6ra14951k

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