Al-doped ZnO nanocomposite as bifunctional catalyst for photocatalysis and synthesis of dimethyl carbonate

Accepted Manuscript K/Al-doped ZnO nanocomposite as bifunctional catalyst for photocatalysis and synthesis of dimethyl carbonate

Wenwen Jia, Lige Gong, Yongchen Shang PII: DOI: Reference:

S1387-7003(18)30965-1 https://doi.org/10.1016/j.inoche.2018.12.015 INOCHE 7203

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

1 November 2018 1 December 2018 17 December 2018

Please cite this article as: Wenwen Jia, Lige Gong, Yongchen Shang , K/Al-doped ZnO nanocomposite as bifunctional catalyst for photocatalysis and synthesis of dimethyl carbonate. Inoche (2018), https://doi.org/10.1016/j.inoche.2018.12.015

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ACCEPTED MANUSCRIPT K/Al-doped ZnO nanocomposite as bifunctional catalyst for photocatalysis and synthesis of dimethyl carbonate Wenwen Jiaa, Lige Gongb*, Yongchen Shanga* a

College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, 150025, PR

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China b

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College of Life Science and Technology, Harbin Normal University, Harbin, 150025, PR China

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Abstract In this paper, K/Al-doped ZnO nanocomposite was prepared by coprecipitation and impregnation method using sodium hydroxide as precipitation

microscopy

(SEM),

ultraviolet-visible

spectroscopy

(UV-Vis),

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electron

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agent. The title material was characterized by X-ray diffraction (XRD), scanning

Brunauer-Emmett-Teller (BET) and temperature-programmed desorption of carbon

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dioxide (CO2-TPD). K/Al-doped ZnO as photocatalyst exhibits highly efficient

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photocatalytic performance for degradation of four typical dyes (Methylene blue (MB), Congo red (CR), Rhodamine B (RhB) and Methyl orange (MO)) under

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UV-light irradiation. The photocatalytic decomposition rate in the order of CR (91.40%) > MB (82.47%) > RhB (54.96%) > MO (51.70%). The comparative

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experiments show that K/Al-doped ZnO composite possesses higher activity relative to ZnO, Al2O3 and Al-doped ZnO. Furthermore, the composite also has unique catalytic behavior for the one-pot synthesis of dimethyl carbonate (DMC) from methanol (MeOH), epoxy propane (PO) and CO2, which may be an effective and simple material to one-pot synthesize DMC in industry. Keywords:

K/Al-doped

ZnO

nanocomposite;

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photocatalytic

degradation;

ACCEPTED MANUSCRIPT coprecipitation; one-pot synthesis With the rapid development of the global economy, wastewater containing toxic contaminants and organic dyes were discharged to become one of urgent problem and the greatest threat for people’s health. To obtain clean, the effective measures and

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ways for the wastewater are very necessary. Thereinto, the photocatalytic plays an

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important role in degradation of organic toxicants under UV or visible light irradiation

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[1-7]. During the development of photocatalytic degradation, oxide composites as catalysts have the advantages of being widely available, environmentally-friendly,

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stable and cheaper, which have attracted wide attention. Thus far, various oxide

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composites as the photocatalysts in the literatures on photodegradation of dyes such as Ag/ZnO-TiO2 [8], Zn1-xMnxO [9], Ag3PO4/MnFe2O4 [10], CeO2/CuO [11],

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β-Ag0.333V2O5 [12]. Among the oxide composites, ZnO has been wide direct band-gap

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energy (3.37 eV) and a relatively large exciton binding energy (60 me V) used as a photocatalyst [13-15] to decompose the organic pollutants, which endow them with

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similar band-gap, excellent oxidation capacity, high chemical stability and lower cost

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to suitable substitute for TiO2 in waste treatment [16,17]. Therefore, many studies have been reported that ZnO is used as photocatalytic degradation of water pollutants [18]. However, due to the large band gap and the fast recombination rate of single phase photogenerated electron-hole pairs, ZnO can only be activated under the irradiation of high energy ultraviolet light. Hence, in order to improve the photocatalytic efficiency of ZnO, many scientists have proposed that ZnO dopped other metal oxide can improved the separation of electron-hole pairs and enhanced

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ACCEPTED MANUSCRIPT light absorption [19]. For example, Fe-doped ZnO [20], Ag-doped ZnO [21], Cd-doped ZnO [22], Er3+:YAlO3/Co- and Fe-doped ZnO [23], Er:YAlO/Fe-doped TiO2-ZnO [24] had been successfully synthesized, which used enhanced photocatalytic activity than pure ZnO. Therefore, metal doping has become the hot

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research topic to solve the material defects of ZnO.

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The different dopants into composite material have the following advantages: (1) the

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distribution of electrons can be changed; (2) the growth of grains is inhibited; (3) the particle size is reduced; (4) the specific surface area is increased; (5) the

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photocatalytic performance is improved.[25,26] To our knowledge, Al-doping has

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good application in various fields of fire retard, catalyst, surface protective coating and composite materials [27,28]. In 2016，M. Ahmad et al. demonstrated the crystal

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structure and photocatalytic activity of Al-doped TiO2 nanofiber [29]. In 2017,

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Casillas et al. reported an Al-Nd-Zn-x composite material to promote the photocatalytic activity of ZnO [30]. In particular, Al-doped ZnO nanocomposite can

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improve photocatalytic degradation of dye pollution due to their low cost,

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non-toxicity and high efficiency [31]. Al enter the ZnO lattice substitutionally as deep acceptors in combination with a neighboring O vacancy leads to the narrow of band gap energies and reduce the size of the grain [32]. Through this approach, Al-doped ZnO has the more effective and stable catalyst system than a single Zinc Oxide. However, the synthesis of Al-doped ZnO is very complex, which limits the development and application of these composites. Therefore, it is a challenging task to find a simple method to synthesize Al-doped ZnO nanocomposites.

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ACCEPTED MANUSCRIPT Noteworthy, K2CO3-doped can be synthesized in a simple way and widely used as catalysts. Liu's group studied an efficient catalyst CuO/K2CO3/MgAl2O4 [33], which can lean NOx storage and reduction at high temperatures. Zhang and his coworkers prepared an K2CO3/ZrO2 material to promote the transesterification of dimethyl

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carbonate with tetrahydrofurfuryl alcohol [34]. To the best of our knowledge, there

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are no reports on the composite material of K2CO3 as photocatalytic catalyst.

for improving the photocatalytic efficiency.

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Therefore, the research of K2CO3 mixed in Al-doped ZnO has potential significance

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In this work, K/Al-doped ZnO nanocomposite was prepared by simple mixing

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coprecipitation and impregnation method. K/Al-doped ZnO was used to degrade MB, CR, RhB and MO dyes under UV irradiation, which showed good catalytic activities.

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Moreover, the catalytic activity of K/Al-doped ZnO for propylene oxide, carbon

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dioxide and methanol to synthesis dimethyl carbonate was also investigated. The synthesis of K/Al-doped ZnO nanocomposite was done by facile impregnation

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process. For the synthesis, all the chemicals were analytical grade which were used as

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received without further purification. Potassium carbonate (17.5 wt%) was mixed well in 30 mL of deionized water under stirring at room-temperature. The resultant liquid mixture was added to the above prepared Al-doped ZnO nanocomposite, and the mixture was stirred for 24 h. Finally, the mixture was dried at 80 ℃ for 12 h and calcined at 600 ℃ for 5 h to obtain K/Al-doped ZnO. Fig. S1 depicts the schematic view of the synthesis steps. The morphology of K/Al-doped ZnO is shown in Fig. 1(a). As can be seen from Fig.

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ACCEPTED MANUSCRIPT 1(a) that K/Al-doped ZnO nanocomposite possesses rod-shaped morphologies, which is much different from the morphology of pure ZnO. The morphology of pure ZnO was observed as irregular nanoparticles and nanorods in sizes and diameter. (Fig. S2) From further observation, it is very interesting to see that the K/Al-doped ZnO

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nanocomposites were grown in good uniform shape and size, and without reunion

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phenomenon (Fig. 1(b), (c)). The XRD pattern of K/Al-doped ZnO nanocomposite

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sample is presented in Fig. 1(d). The results revealed that the main diffraction peaks found at 2θ = 31.86, 34.52, 36.36, 47.66, 56.72, 62.98, 66.50, 68.08 and 69.24, which

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can be attributed to the hexagonal wurtzite structure of ZnO (JCPDS Card

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No.36-1451). In addition, no other characteristic peaks can be observed, indicating that Al and K ions are highly dispersed or in amorphous form in the K/Al-doped ZnO

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nanocomposite. However, the existence of Al and K can be confirmed by the EDS

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spectra of samples in Fig. S3(a)-(f). It was found that Zn and O can be clearly identified in the backbone region of sample whereas Al and K can be found

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throughout the entire surfaces, indicating that the Al, K ions are uniformly doped in

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the ZnO matrix, which is combination with XRD results, proved that K/Al-doped ZnO was successfully prepared. The basicity of the catalyst were examined by CO2-TPD (Fig. 2(a)). Different desorption peaks represent different alkaline active centers, and the higher temperature of desorption peaks represents the intensity increased. At the same time, the higher area of desorption peaks represents the larger amount of alkaline sites. The

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ACCEPTED MANUSCRIPT three desorption peaks at 323-473 K, 473-673 K, 673-1073 K are suggested the presence of weak, moderate and strong basic sites, respectively. It is noted that a great deal of alkaline sites can be assigned to K/Al-doped ZnO catalysts. Fig. 2(b) shows the nitrogen adsorption/desorption isotherms for ZnO powder, Al-doped ZnO and

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K/Al-doped ZnO. The BET specific surface area for ZnO powder, Al-doped ZnO,

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K/Al-doped ZnO are 6.400 m2/g, 22.809 m2/g and 31.870 m2/g, respectively,

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indicating obvious increase for K/Al-doped ZnO compared with that of ZnO powder and Al-doped ZnO.

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UV-vis diffuse reflectance spectroscopy is an important technique to investigate the optical properties of the nanocomposite. The UV-vis absorption spectra of ZnO,

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Al-doped ZnO and K/Al-doped ZnO in the range of 250-800 nm are shown in Fig.

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S4(a). The absorption band of K/Al-doped ZnO is much stronger than the pure ZnO and Al-ZnO [35]. Besides, K/Al-doped ZnO also occurs a little red shift in the

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visible-light region. The extended absorption is attributed to high electronic

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interaction with the K/Al-doped on the surface of ZnO [36], and the shifting of the absorption bands was due to the interaction between the polymer matrix and nanoparticles [37]. The addition of K and Al doping also affects the energy gap and electron transition. The following equation was used to calculate the band gap energy (Eg): αhν = Α(hν − Eg)1/2. Where α, ν, Eg and Α are absorption coefficient, light frequency, band gap energy and a constant, respectively. The value of energy band gaps for pure ZnO, Al-doped ZnO and K/Al-doped ZnO are shown in Fig. S4(b). It

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ACCEPTED MANUSCRIPT was found that the energy gap of K/Al-doped ZnO was decreased greatly at 2.72 eV, compared to that of ZnO (3.02 eV) and Al-doped ZnO (2.99 eV), which will be improved the photocatalytic activities under visible light irradiation. The photocatalytic mechanism of K/Al-doped ZnO shows in Fig. S5. K/Al-doped

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ZnO is stimulated to produce h+ and e− under Hg lamp irradiation. •OH is generated

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from the oxidization of H2O by h+ and the reduction of O2 by e−. As a powerful

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oxidant, •O2- and •OH can decompose effectively the organic dyes.

To evaluate the photocatalytic performance of K/Al-doped ZnO, four common toxic

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organic dyes (RhB, MB, MO and CR) are examined under visible light irradiation. As

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shown Fig. 3, with the increased irradiation time, the main peak at ultraviolet regions are quickly diminishing and MB, CR, RhB, MO molecules are decomposed. It is

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observed when the illumination time are 90, 45, 80 and 35 min under UV-light

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irradiation, the photocatalytic decomposition rates defined as 1–C/C0 are 95.97% for RhB, 96.74% for MB, 89.47% for MO, 93.80% for CR, respectively. In order to

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better compare the photocatalytic degradation efficiencies of four dyes under the same

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conditions, a fixed degradation reaction time of 30 min was chosen. The order of degradation rate is CR (91.40%) > MB (82.47%) > RhB (54.96%) > MO (51.70%), which indicate that K/Al-doped ZnO structures possess excellent photocatalytic performance toward CR dye in 30 min. The different photocatalytic activities of K/Al-doped ZnO nanocomposite owe to the different structure and properties of the dyes.

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ACCEPTED MANUSCRIPT To demonstrate and compare the photocatalytic activities of four different samples, photocatalytic degradation experiments of CR are conducted at the same conditions. In Fig. S6, the degradation of CR dye in 35 min as follow: K/Al-doped ZnO (93.80%) > Al-doped ZnO (90.30%) > commercial ZnO powder (77.00%)>

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commercial Al2O3 powder (73.50%). Thus, the experimental results illustrate that the

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photodegradation of dye in the presence of K/Al-doped ZnO nanocomposite has

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strong photodegradation ability and unique universal advantage. In addition, K/Al-doped ZnO nanocomposites photocatalytic efficiency is highst than other similar

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photocatalysts (Table S1). In a practical industrial application, the photocatalysts are

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expected to maintain long-term stable and the reuse of the composite during reaction. The nanocomposite K/Al-doped ZnO was separated by centrifugation and washed

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with H2O for several times, then dried at 80 °C for 12 h and calcined. From the

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observed re-cyclic results in Fig. S7, no much significant change is observed after fifth run, and confirms that the K/Al-doped ZnO has certain stability under visible

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light.

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The catalytic activity of K/Al-doped ZnO for the one-pot synthesis of dimethyl carbonate (DMC) using epoxy propane (PO), CO2 and methanol (MeOH) was assessed. The change of different parameters (reaction temperature (T), reaction time (h) and molar ratio of MeOH to PO) were summarized to achieve optimal performance. The influence of temperature on the reaction was studied in Fig. 4(a). As the reaction temperature was varied from 120 to 200 °C, the catalytic performance gives a crest value at 160 °C. It is obvious that the conversion of PO and the

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ACCEPTED MANUSCRIPT selectivity of DMC came up to the maxima of 100% and 44.59%, respectively. However, when the reaction temperature was higher than 160 ℃, the conversion of PO and the selectivity of DMC dropped. This can be partially ascribed to side reactions increased. To explore the effect of reaction time, further DMC synthesis reactions

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were carried out showing in Fig. 4(b) (P=2 MPa; T=160 °C; mcatalyst=1 g; the molar

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ratio of PO:MeOH=2:3). All the results show that the conversion of PO is the best to 5

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h for the reaction time. The selectivity of DMC also showed pretty good promotion, but the change was not significant after 5 h. The change trend indicates that the

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reaction time played an important role in DMC synthesis. Fig. 4(c) presents plots of

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the feed molar ratio of MeOH to PO. The reaction was carried out using 1 g catalyst at 160 °C. In addition, the reaction time was 5 h and the initial pressure of CO2 was 2

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MPa. Increasing the molar ratio of MeOH to PO can improve the selectivity of DMC,

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but the selectivity of DMC decreased at the large ratios. The appropriate feed molar ratio of MeOH to PO was about 3:2, where the content of DMC was the largest after

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reaction. The results of recycling experiments using K/Al-doped ZnO catalyst are

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depicted in Fig. 4(d). The catalyst was separated by centrifugation and washed with H2O after the test, and then reused in the next cycle under the same conditions. PO still could be converted completely and the changes of DMC selectivity was not much after the catalyst had been recycled five times. In conclusion, K/Al-doped ZnO nanocomposite was successfully synthesized by simple

coprecipitation

and

impregnation

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method.

The

K/Al-doped

ZnO

ACCEPTED MANUSCRIPT nanocomposite exhibits the excellent photocatalytic ability for photo-degradation of four typical dyes under UV-light irradiation. The photocatalyst of K/Al-doped ZnO nanocomposite were larger photodegradation efficiencies than that of ZnO, Al2O3 and Al-doped ZnO. In addition, K/Al-doped ZnO was found to be a superior catalyst for

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the one-pot synthesis of DMC from methanol, epoxy propane and CO2, it may

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become an effective and simple catalyst to synthesize DMC in industry.

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Acknowledgments

This work was supported by the Natural Science Foundation of Heilongjiang Province

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(grant number B2015006), the Science and Technology Research Project Foundation

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of Education Department of Heilongjiang Province, China (grant number 12511472). References

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Figure caption

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Fig. 1. Typical (a)-(c) FESEM images and (d) X-ray diffraction patterns of

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K/Al-doped ZnO.

isotherm of ZnO, Al-ZnO and K/Al-doped ZnO.

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Fig. 2. (a) CO2-TPD profile of K/Al-doped ZnO; (b) N2 adsorption-desorption

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Fig. 3. Variations in absorption spectra of the organic dye solutions in the presence of

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K/Al-doped ZnO photocatalyst under UV-light irradiation for different time intervals: (a) RhB, (b) MB, (c) MO, (d) CR dye and (e) Photocatalytic degradation rates of RhB,

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MB, MO and CR dyes.

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Fig. 4. Effect of reaction temperature (a), reaction time (b) and molar ratio of MeOH to PO (c) on the conversion of PO and the selectivity of DMC; (d) The reusability of

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the catalyst.

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ACCEPTED MANUSCRIPT

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Fig. 1.

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Fig. 2.

Fig. 3. 18

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Fig. 4.

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ACCEPTED MANUSCRIPT K/Al-doped ZnO nanocomposite as bifunctional catalyst for photocatalysis and synthesis of dimethyl carbonate Wenwen Jiaa, Lige Gongb*, Yongchen Shanga* a

College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin, 150025, PR

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China b

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College of Life Science and Technology, Harbin Normal University, Harbin, 150025, PR China

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Graphical abstract

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The K/Al-doped ZnO nanocomposite exhibits high-efficient for photocatalytic

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degradation of azo dyes and a simple material to one-pot synthesize DMC .

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ACCEPTED MANUSCRIPT Highlights 

K/Al-doped ZnO nanocomposite was synthesized by a facile method.

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K/Al-doped ZnO exhibits high-efficient photocatalytic degradation ability for

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typical dyes. Photocatalytic activities of K/Al-doped ZnO is higher than that of ZnO, Al2O3

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K/Al-doped ZnO may be an effective and simple material to one-pot synthesize

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DMC in industry.

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and Al-ZnO.

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Figure 1

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