Effect of Morphological Structure of PtSnNa/ZSM-5 on Its Catalytic Performance i

Wang Yongjuan; Zhou Yuming; Zhou Shijian; He Qiang; Zhong Yangyang

(School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189)

Abstract: The ZSM-5 zeolite with flower-shaped crystallites (marked as ZFS) was synthesized through the hydrothermal preparation method. The properties of ZFS as a supporting material have been investigated in comparison with the traditional ZSM-5 zeolite. The prepared samples were characterized by XRD, N2 adsorption-desorption, TEM, NH3-TPD,XRF, and XPS techniques. The results showed that the formation of the flower-shaped crystallites directly influenced the physicochemical properties of ZFS material. In ZFS, the pore structure was changed, the acid content was decreased,and the surface properties were promoted. Besides, in the PtSnNa/ZFS catalyst, the amount of oxidized Sn species was remarkably increased and the dispersion of Pt was relatively improved. Meanwhile, due to these modifications, the capacity of the catalyst supported on ZFS to accommodate coke was greatly improved and the coke deposits on PtSnNa/ZFS were migrated from active metals to the support. Furthermore, as expected, the catalytic performance of the PtSnNa/ZFS catalyst for propane dehydrogenation to propylene and the stability of catalyst were significantly improved.

Key words: ZSM-5 zeolite; acid sites; propane dehydrogenation; coke

1 Introduction

In recent years, a variety of strategies have been employed to facilitate the diffusion of reactants in bulky moleculeinvolved or diffusion-controlled catalytic reactions in zeolite-catalyzed reactions, such as reducing the crystal size to shorten the diffusion paths[1-4], or raising the zeolite pore size to better facilitate the diffusion of bulky molecules[5-6]. Besides these measures for introducing mesoporosity within the microporous crystals, which can lead to the hierarchization of zeolite pore structure, are suggested as a possible solution[7-9].

For the propane dehydrogenation reaction, the properties of acidity, pore structure and crystallite size of catalysts can directly determine the extent of reaction, the reaction path, and the residence time[10]. ZSM-5 zeolite, one of the crystalline microporous materials, is widely used in various hydrocarbons involved reactions owing to its strong catalytic performance, excellent thermal,hydrothermal, and chemical stability[11-12]. But for the dehydrogenation reaction, due to the large amount of acid content and strong acid sites, the ZSM-5 zeolite is beneficial to the undesirable side reactions and is harmful to the dehydrogenation process. In our previous works,many efforts have been made to modify the properties of PtSn/ZSM-5 catalyst[13-16]. It is confirmed that the introduction of additive metals such as Na, K, Mg, etc.could be an effective method. The presence of promoters can neutralize the strong acid sites on the surface of catalysts and improve the relationship between metals and support. Moreover, several strategies have also been proposed to enhance the accessibility to active sites in ZSM-5, such as the use of nanocrystals[17], the composite materials[18-19], a low level of acid concentration provided by high Si/Al molar ratio, and incorporation of intracrystalline mesoporosity[20]. Besides, Kumar's group[21]observed that the catalyst supported on the ZSM-5 zeolite with smaller pore size exhibited higher catalytic conversion and stability but higher coke deposition than SBA-15 with larger pore size. Zhang, et al.[22]compared different kinds of catalytic carriers in propane dehydrogenation. They found that the carrier with mesoporous structure is favorable to the dispersion of metallic particles and the interaction between the metals and the support. Also, this mesoporous structure with a certain pore volume is beneficial to the catalytic capacity for accommodating the coke formed during the reaction.Thus, we propose that the ZSM-5 supported catalyst with smaller crystallite size could be beneficial to the catalytic dehydrogenation reaction, and the coke problem in propane dehydrogenation reaction would be partially solved by the presence of mesoporous structure.

Based on these reports, in order to modify the acidity and pore structure of ZSM-5 zeolite, a flower-shaped zeolite has been synthesized with smaller crystallite size and lower acidity. To obtain the information about the modification of active sites and catalytic structure,the prepared samples were characterized by several techniques, such as XRD, N2adsorption-desorption,TEM, NH3-TPD, XPS and TPO. Besides, the propane dehydrogenation tests of different catalysts were analyzed, and the stability of different catalysts was also studied.

2 Experimental

2.1 Material preparation

The ZSM-5 zeolite was hydrothermally synthesized and the reagents used in the synthesis included silica sol,sodium meta-aluminate, NaOH, hexanediamine, and distilled water. The molar composition of the precursor gel was 60SiO2: 0.15Al2O3: 3.5NaOH: 1400H2O:15.2NH2-(CH2)6NH2. Firstly, the mixture of these reagents was stirred for 3 h. After that, the mixture was transferred into a Teflon-lined autoclave and heated at 160 °C for 1 day. After being subject to natural cooling to room temperature, the resulting sample was collected and dried at 80 °C for one night, and then the dried powder was calcined at 550 °C for 8 h. After being NH4+-exchanged three times with an 1 mol/L NH4Cl solution, the NH4-ZSM-5 was obtained, and finally the proton form of ZSM-5 (H-ZSM-5) was obtained by calcining at 550 °C for 8 h.

ZFS was synthesized by adding polydimethyldiallyl ammonium chloride acrylamide (PDD-AM) in the precursor gel of ZSM-5, the molar composition of which was 60SiO2: 0.15Al2O3: 3.5NaOH: 1400H2O: 15.2NH2-(CH2)6NH2: 10PDD-AM. After being subject to stirring intensively for 3 h, the mixture was also transferred into a Te flon-lined stainless steel autoclave and was heated at 160 °C for 3 days. After crystallization, the product was filtered, washed with deionized water, dried at 80 °C, and calcined at 550 °C.

2.2 Catalyst preparation

The PtSnNa/ZSM-5 and PtSnNa/ZFS samples were prepared by the same sequential impregnation method.The sample powder was impregnated in an aqueous solution of 0.427 mol/L NaCl at 80 °C for 4 h, and then in a mixed solution composed of 0.033 mol/L H2PtCl6and 0.153 mol/L SnCl4. After that, the prepared catalyst was dried at 80 °C for 3 h. The nominal composition of the catalyst samples covered 0.5% of Pt, 1.0% of Sn, and 1.0%of Na.

To obtain larger and more stable particles, all of the prepared samples were fully agglomerated and shaped with binders[23]. The spaghetti-shaped extrudates had a diameter of 1.5 mm and a length of 3―8 mm. Finally,after having been completely dried, the samples were dechlorinated at 500 °C for 4 h in air and then were reduced in H2at 500 °C for 8 h.

2.3 Catalysts characterization

The X-ray diffraction (XRD) patterns of different samples were obtained on a D8 X-ray powder diffractometer coupled with a copper anode tube. The Kα radiation was selected with a diffracted beam monochromator. A scanning angle (2θ) ranging from 5° to 60° was recorded using step scanning and long counting time to determine the position of the peaks.

The elemental contents of the as-prepared samples were analyzed by X-ray fluorescence (XRF) measurements on a SWITZERLAND ARL 9800 XRF.

The nitrogen adsorption-desorption isotherms were measured at -196 °C on a Micromeritics ASAP2020M volumetric adsorption analyzer. Before measurements,the samples were degassed at 300 °C and 1×10-3torr. The Specific surface area was obtained using the BET method.The microporous volume was calculated based on the t-plot method.

The ammonia temperature programmed desorption (NH3-TPD) experiments of different samples were measured in a fixed-bed reactor of BEL Catal II under a flow of NH3/Ar and at a heating rate of 10 °C/min from 50 °C to 800 °C. The pretreatment was carried out at a flow rate of 30 mL/min for 1 h at 300 °C in Ar, with the NH3uptake amount recorded.

The transmission electron microscopy (TEM) studies were analyzed using a JEOL-2010 microscope operated at 200 kV. The samples were ultrasonically dispersed in the ethanol solution and the resulting suspension was dried on carbon films supported on copper grids.

The X-ray photoelectron spectra (XPS) were performed on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al Kα radiation (1 486.6 eV).The C1s peak at 284.6 eV was used as the reference for binding energies.

The thermogravimetric (TG) test was measured in a flow of air (30 mL/min) with a LCT thermogravimetric analyzer at temperatures ranging from room temperature to 700 °C at a temperature increase rate of 20 °C/min.,with 0.02 g of catalyst put in the analyzer.

The temperature programmed oxidation (TPO) analysis was measured with the same apparatus as that used for NH3-TPD analysis. About 0.05 g of sample were placed in a quartz reactor at room temperature, then the sample was heated up to 700 °C at a heating rate of 10 °C/min in a 5% O2/He gas mixture (at a flow rate of 30 ml/min).

2.4 Catalyst evaluation

Propane dehydrogenation reaction was carried out in a conventional quartz tubular micro-reactor. The catalyst(1.5 g) was placed into the center of the reactor. The dehydrogenation reaction was conducted under conditions covering a reaction temperature of 590 °C, a reaction pressure of 0.1 MPa, a H2/C3H8molar ratio of 0.25, and a propane weight hourly space velocity (WHSV) of 3.0 h-1.The reaction products were analyzed with an online GC-14C gas chromatograph equipped with an activated alumina packed column and a flame ionization detector(FID).

3 Results and Discussion

3.1 Characterization of zeolites

Figure 1 shows the XRD patterns of different samples. In particular, the pattern of ZSM-5 showed the representative peaks at 2θ of 7°―9° and 22°―24°, which were Specific for the MFI structure of pure ZSM-5[24]. After adding PDD-AM, these specific peaks still remained in the sample of ZFS. It implied that the structure of ZSM-5 was well preserved during the modification process. Besides,the intensity of these peaks in ZFS was strengthened as compared to the sample of ZSM-5, demonstrating that the crystallinity of ZFS was promoted relatively, especially for the peaks at 8.8° and 23.1°. Generally, the peak at a low angel of 8.8° was particularly sensitive to the pore channel structure[25]. Hence these variations in XRD patterns might be relevant with the modification of the pore channel structure in ZFS sample.

Figure 1 XRD patterns of different samples

TEM images are presented to directly observe the morphology of different samples in Figure 2. As shown in Figure 2(1), as evidenced by the regular synthetic procedure, the ZSM-5 sample exhibited a typical MFI structure with “boat-like” hexagonal shape with a size of 6-8 μm in length. Besides, the insert showing electron diffraction (SAED) pattern of selected area in Figure 2(1) confirms the formation of ZSM-5 crystallites.In comparison, after adding the PDD-AM during the synthetic procedure, the morphology of the target sample was totally changed. As shown in Figure 2(2), the zeolite crystallites in the well-de fined flower shape were obtained in ZFS. The SAED pattern in Figure 2(2) is similar to ZSM-5, suggesting that the zeolite crystallites are preserved in ZFS, which is in agreement with the XRD results. Moreover, compared with ZSM-5, the size of these crystallites becomes smaller in their radius range of 200―400 nm.

To further discuss the pore structure in different samples,the nitrogen adsorption-desorption isotherms of different samples are shown in Figure 3(a). It can be seen from Figure 3(a) that the sample of ZSM-5 represents the type-I isotherm with a small N2uptake at a higher relative pressure, which is typical for microporous materials without any mesoporous structure[26]. In contrast, the ZFS sample exhibits the typical type-IV isotherm with a H1 hysteresis loop as defined by IUPAC, which displays an uptake of nitrogen and hysteresis loop at a higher relative pressure of p/p0＞ 0.45, representing the presence of mesoporous structure[27]. As reflected in Figure 3(b), different samples, which are determined by the BJH model, are applied to the adsorption branch of the isotherms and the pore size distribution is observed[28].The pore size distribution of the ZFS sample has two obvious peaks which are specific for the mesoporous distribution. Based on the textural properties presented in Table 1, which are calculated by means of the nitrogen desorption isotherm, the average pore diameter is increased up to 3.6 nm in ZFS material. These findings indicate that intercrystallite mesopores are formed in the ZFS sample by adding PDD-AM. Moreover, the surface area of ZFS is increased to 355 m2/g compared with that of ZSM-5 (331 m2/g), especially for the external surface area (Sext= 241 m2/g). Besides, the total pore volumn(Vtotal) of ZFS (0.19 cm3/g) is also increased drastically.These findings should be attributed to the appearance of mesopores in the ZFS sample. According to the results of Gao's group[29], the support of catalyst in larger pore size distribution is more effective for resolving the diffusion and mass transfer limitations in catalytic reaction.Therefore, the catalyst supported on ZFS would have a better ability to accommodate coke during the reaction.Further discussions will be presented in the following section.

Figure 2 Transmission electron micrographs and the corresponding electron diffraction patterns (insert graphs)of selected area in different samples

Figure 3 N2 adsorption-desorption isotherms and the corresponding pore size distribution as obtained from the desorption branch of the different samples

Table 1 Characterization data of different samples

The NH3-TPD patterns of different samples are shown in Figure 4, with the NH3uptake of different samples summarized in Table 2. Apparently, in the sample of ZSM-5, there are two desorption peaks observed at low temperature (237 °C, peak I) and at high temperature(416 °C, peak II), corresponding to the weak and strong acid sites, respectively[30]. Figure 4 can con firm that ZFS material exhibits the same desorption peaks, but the intensity of each peak is different from that of ZSM-5 zeolite. This phenomenon could be directly observed in Table 2, showing that in comparison with the sample of ZSM-5, the total NH3uptake of ZFS is markedly reduced,mostly in terms of the uptake at high temperature. This fact indicates that the strong acid sites in ZFS sample are significantly slashed, while the weak acid sites are almost unaffected. To explain this, Barakov's group[31]found that larger-sized crystallites contribute to the increase of strong acid sites in zeolite, and also prolonging the crystallization time would lead to the decrease of concentration of acid sites. Thus smaller crystallites and longer crystallization time would lead to the reduction of strong acid sites.Besides, compared with ZSM-5 sample, the maximum values of both low and high temperature peaks in ZFS could shift toward lower temperature, indicating that the acid sites would become weaker.

Figure 4 NH3-TPD patterns of different samples

3.2 Physicochemical properties of catalysts

In order to find out the metallic information on the surface of each support, the chemical state of each component was tested by the X-ray photoelectron spectroscopy. The binding energy of Al2p, Si2p, and Sn3d5/2are summarized in Table 3. Figure 5 shows the XPS spectra corresponding to the Sn 3d5/2level of different catalysts after reduction at 500 °C. Judging from the deconvolution of the spectrum of PtSnNa/ZSM-5, three peaks at 485.3 eV, 486.6 eV,and 487.6 eV are obtained and assigned to different tin species. The component at a lower binding energy of 485.3 eV is attributed to metallic state of Sn (Sn0), while the other values are ascribed to the different types of tin oxides (SnO and/or SnO2)[32]. As regards tin oxides, the values of binding energy are so close that it is difficult to distinguish between SnⅡand SnIVby XPS analysis[33].Therefore, based on the data in Figure 5, the observed conclusion is that most of tin species in each catalyst are oxides. Furthermore, in order to analyze these results with great certainty, the XPS binding energy for different catalysts is presented in Table 3. It can be seen from Table 3 that the percentage of Sn0for PtSnNa/ZFS is 23.6%, which is much lower than that of PtSnNa/ZSM-5 (29.1%). However,the Sn species in zero state are easier to alloy with platinum to form a Pt-Sn alloy, leading to sintering and poisoning of Pt metal to cause deactivation of the catalysts[34]. Based on the above results, the amount of nonmetallic state of tin species (SnIIor SnIV) is more abundant in PtSnNa/ZFS as shown in Table 3. This significant amount of Sn species in oxidized state would facilitate the Pt-Sn interaction to establish a strong interaction between the tin species and the support. Therefore, during the high temperature reaction,it would be more difficult for Pt particles to agglomerate on the catalyst of PtSnNa/ZFS.

Figure 5 XPS pro files of different catalysts

Table 2 NH3-TPD results of different samples

Table 3 Binding energy of core electrons for the different catalysts

According to the results of XPS analysis, in order to directly observe the effect between the active sites of Pt and the zeolite support, the model of metallic properties on the surface of catalysts is proposed in Figure 6.Generally speaking, in the PtSn-based catalyst, there are two types of active Pt sites, viz.: Pt1and Pt2[35]that are dispersed on the surface of the support, and these two active sites are responsible for the different effects during the reaction. Platinum species can directly anchor to the support surface to form multiple Pt centers de fined as the Pt1sites, which are effective for the side reactions such as hydrogenolysis and coking. Meanwhile, the platinum species anchoring to the Sn oxide surface to form active sites with new “sandwich structure” are de fined as the Pt2sites. These Pt2active sites make a major contribution to the dehydrogenation of propane. As mentioned in XPS results, more tin oxides are detected in ZFS sample, thus the amount of Pt2sites is increased. This large amount of Pt2sites in PtSnNa/ZFS catalyst would greatly help to conduct the catalytic propane dehydrogenation. More discussions will be presented in the following section.

To directly observe the morphology and particle size distribution of metallic particles in each sample, the TEM analysis of different catalysts was performed. The TEM images and corresponding particle size distribution are presented in Figure 7. Judging from the TEM images of PtSnNa/ZFS, it can be easily found that the distribution of metallic particles is more uniform and concentrated,as compared with the sample of PtSnNa/ZSM-5.Reasonably, this difference may arise from better textual properties and stronger interaction between Pt and Sn species in PtSnNa/ZFS catalyst, which will further affect the agglomeration degree of Pt during the calcination process[22]. As it has been reported, the strong interaction between metallic particles would lead to the long range order of contiguous Pt atoms, which are interrupted by Sn atoms[36]. Therefore, upon combining the XPS results, more dispersive Pt particles are formed on the surface of PtSnNa/ZFS catalyst. For better understanding the platinum dispersion on the surface of catalysts, the corresponding particle size distribution is also obtained.It can be seen from Figure 7 that the average particle diameter of PtSnNa/ZFS (7.4 nm) is much smaller than that of PtSnNa/ZSM-5 (12.8 nm), and the particle size distribution of PtSnNa/ZFS is concentrated in the range of 5―10 nm. These results indicate that the dispersion of Pt particles of PtSnNa/ZFS is much better than that of PtSnNa/ZSM-5.

Figure 6 Model of the metallic properties on the surface of catalysts

3.3 Catalytic performance tests

The catalytic behaviors of different catalysts are depicted in Figure 8, with the data listed in Table 4. It can be directly seen from Figure 8(a) that the PtSnNa/ZSM-5 catalyst shows poor activity, and its catalytic deactivation is relatively fast. In contrast to the above data, the catalytic activity is clearly enhanced in the PtSnNa/ZFS catalyst. In combination with the data in Table 4,the initial conversion of PtSnNa/ZFS catalyst (41.9%)is much higher than that of PtSnNa/ZSM-5 (34.1%).According to the above findings, these results could be attributed to the increased metal dispersion and/or strong interaction between Pt, Sn, and the support. Figure 8(b)displays the propylene selectivity versus the reaction time for different catalysts. It can be observed that the propylene selectivity is also improved on the PtSnNa/ZFS catalyst. These phenomena can be interpreted as follows:The dehydrogenation catalysts provide important sites for cracking and hydrogenolysis on the metal surface, which would result in the rupture of C-C bonds of propane.These sites are quickly poisoned by coke, leading to the weakening of side reaction and the improvement of dehydrogenation process[21]. In addition, at high reaction temperature on PtSnNa/ZSM-5, some Pt particles would be agglomerated due to the weak interaction between Pt and the support. Thus, the undesired hydrogenolysis reaction could be carried out easily on the catalyst of PtSnNa/ZSM-5[37].

Figure 7 TEM images and (insert) corresponding particle size distribution of different catalysts

Figure 8 Propane conversion and propylene selectivity of different catalysts for propane dehydrogenation

In Table 4, the deactivation parameter (13.1%) is much lower in PtSnNa/ZFS catalyst than that of PtSnNa/ZSM-5 catalyst, denoting that the catalytic stability of the former is greatly improved. To explain this phenomenon, it should be noted that the Pt-based catalyst is bifunctional and two active centers (metal and acid active sites) might work collaboratively[38]. Therefore, a fine equilibrium between the metal sites and the acid sites on the support is required in the propane dehydrogenation process. In other words, a good matching between the metal function and the acid function is found in the PtSnNa/ZFS catalyst.And in this case, better propane conversion and catalytic stability are realized in the PtSnNa/ZFS catalyst. In order to further con firm the stability of different catalysts, the stability tests at the temperature of 590 °C were carried out. The propane conversion and propylene selectivity as a function of reaction time are shown in Figure 9.In comparison, after reaction for 100 h, the propylene selectivity of PtSnNa/ZFS catalyst is more than 97%while the propane conversion is above 33%, which are much higher than those achieved by the PtSnNa/ZSM-5 catalyst. Also, compared with our precious work[39], the catalytic activity and stability of PtSnNa/ZFS catalyst are sign ficantly improved, which should be mainly attributed to better dispersion of the active metal Pt, leading to better capacity of the catalyst to accommodate coke. The analysis of coke deposition is presented in the following section.

Table 4 Catalytic properties of different catalysts after conducting reaction for 8 h

Figure 9 Stability test of PtSnNa/ZFS and PtSnNa/ZSM-5 in propane dehydrogenation at 590 °C for 100 h

3.4 Coke deposits

In order to analyze the coke deposits on different catalysts, the TPO profiles of different catalysts are shown in Figure 10. As mentioned above, the platinum particles are mainly dispersed on the external surface of the catalyst, and then the carbon formed on the active metal during the reaction would be the main reason of the catalytic deactivation. It can be seen from Figure 10 that for both two different catalysts, two successive peaks in TPO pro files are observed. Particularly, the first peak at low temperature corresponds to the carbon deposits on active metal, while the second peak at high temperature represents the carbon deposits located on the external surface of support[40].

Figure 10 TPO pro files of different catalysts after running for 8 h

It can be seen from Figure 10 that for the PtSnNa/ZSM-5 catalyst, there are large amounts of carbon deposits both on the active metal and the external surface of support.In contrast, the intensity of each peak is much lower in the PtSnNa/ZFS catalyst. In Table 5, the quantitative analysis of coke clearly shows the total amount of coke on the PtSnNa/ZSM-5 catalyst (6.2%) is much higher than that on the PtSnNa/ZFS catalyst (2.3%). In general,coke formation on the catalyst involves several processes,viz.: (1) successive dehydrogenation/cyclization of alkyl chains; (2) n-alkane oligomerization; and (3) the Diels-Alder type reactions[41]. Since the olefins are the primary precursors of coke, the intrinsic acidity of the catalyst would promote the undesirable reactions such as cracking/isomerization, leading to the increase of carbon deposits. It is therefore proposed that strong interaction between Pt-Sn metals and the change of acidity in the catalyst can strongly influence the coke formation.Additionally, it should be noted that the coke deposits on the active metal are highly suppressed in PtSnNa/ZFS catalyst. The presence of Sn in the Pt-based catalyst could weaken the binding between hydrocarbon and Pt metals,and promote the migration of coke precursor from metals to the support[42]. Meanwhile, according to the result of NH3-TPD analysis, the weakened acidity in ZFS material would lead to a reduced coke formation rate in the PtSnNa/ZFS catalyst during the reaction[43], and make the coke precursor formed on the metal migrate to the support and acid sites[44].

Table 5 Characterization data for different catalysts after running for 8 h

From another point of view, according to the structural results, larger surface area and the existence of mesopores in ZFS may also affect the coking behaviors. The coke accommodation in PtSnNa/ZFS must be better than that of PtSnNa/ZSM-5. Based on the above results, the PtSnNa/ZFS catalyst would be a better catalyst with high propane conversion and catalytic stability.

4 Conclusions

In this work, the morphological structure, Pt particle dispersion and acid properties of different zeolite samples and their catalytic performance in propane dehydrogenation were studied. The formation of flower-shaped crystallites in ZFS not only modified the structural parameters but also influenced the acid properties. Moreover, these effects in combination with decreased crystallite size in ZFS would be beneficial to the dispersion of Pt particles. The weakened acid sites and large amount of tin oxides in ZFS suppressed the coke precursor from forming on the metals and could even facilitate the migration of coke precursor from active metals to the support. Also, the appearance of mesopores in ZFS improved its capacity of coke accommodation. Based on these findings, it is confirmed that this new flower-shaped PtSnNa/ZFS catalyst could be an effective catalyst for propane dehydrogenation with good catalytic activity and stability.

Acknowledgements: This work was funded by the National Nature Science Foundation of China (51673040), the Natural Science Foundation of Jiangsu Province (BK20180366,BK20171357), the Fundamental Research Funds for the Central Universities (No. 3207048418), the Fundamental Research Funds for Central Universities (2242018k30008), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (1107047002), the Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu (BA2018045), the Prospective Joint Research Project of Jiangsu Province (BY2016076-01), the Opening Project of Guangxi Key Laboratory of Clean Pulp& Papermaking and Pollution Control (KF201605), and the Scientific Innovation Research Foundation of College Graduates in Jiangsu Province (KYLX16_0266).