Boehmite Preparation via Alditols-Interacting Transformation of

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Boehmite Preparation via Alditols-interacting Transformation of Metastable Intermediates in Al-H2O Reaction Crystallization Hongqi Wang, Zhi Wang, Jianwei Guo, Zhihao Shi, Xuzhong Gong, and Jianwei Cao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01408 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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Crystal Growth & Design

Boehmite Preparation via Alditols-interacting Transformation of Metastable Intermediates in Al-H2O Reaction Crystallization Hongqi Wanga, Zhi Wanga, *, Jianwei Guoa, Zhihao Shia, Xuzhong Gonga, Jianwei Caoa a National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China

Abstract Boehmite was prepared efficiently by adding alditols to Al-H2O reaction crystallization in moderate condition. We demonstrate that it is the interaction between alditols and metastable intermediates that inhibits the decomposition of aluminate ions into gibbsite and favors the formation of boehmite, rather than the blocking of gibbsite growth sites as reported in the previous investigations. The analysis of the crystal formation process shows that the alditols play a role before the nucleation. The interaction simulated by DFT and the local HSAB theory suggests that the metastable intermediates act as soft bases while the alditols play the role of acids in the interaction and the higher the alditols’ softness, the stronger the inhibiting effect. The configuration of the interacting metastable intermediates and alditols was also verified by the comparison of both experimental and computational FTIR spectrum.

1. Introduction Boehmite, as one of the important aluminum oxyhydroxide polymorphs, is widely used in the catalyst 1, adsorbent 2, thin porous membrane 3. Besides, boehmite can also be used to fabricate high-efficiency white light-emitting diodes 4 and other photoluminescence materials 5. There are several boehmite preparation methods, that is decomposition from gibbsite 6, precipitation from sodium aluminate solution induced by boehmite seed and additive 7, sol-gel route of aluminum alcoholates 8 and hydrothermal method 9. Generally, the boehmite is always produced under high temperature or with the presence of a large amount of boehmite seeds 10. For instance, 230 g/L initial boehmite seed is needed to prepare boehmite from Bayer process liquors 11. The application of alditols for boehmite preparation has previously been investigated. The alditols, with the chemical formula being CH2OH-[CHOH]n-CH2OH, are especially characteristic of straight-chain alkanes with a hydroxyl group on each carbon atom. Barsha et al. 12 investigated the effect of several additives on boehmite precipitation from sodium aluminate solution and found that tartaric acid, xylose and glucose are favorable for the formation of boehmite. David et al. 13 synthesized the boehmite nanoparticles through the sol-gel route in the presence of alditols *

Corresponding author. Tel./fax: +86-010-82544818.

E-mail address: [email protected] (Zhi Wang)

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and attributed the control effect of alditols on the particle size and shape to adsorption of alditols on the boehmite surface. However, the previous researches generally attribute the effect of alditols to the blocking of gibbsite growth sites without giving a quantitative description about the inhibition ability and mechanism analysis at the molecular level. Watling 14 attributed the inhibiting effect to the adsorption on specific sites of the gibbsite seed crystals. But the alditols also inhibit the gibbsite crystallization in seed-free crystallization in the Al-H2O reaction crystallization and change the crystallization direction. Metastable intermediates are the pre-nucleation precursors in the crystallization which are crucial for the properties of the final crystal products. The understanding of nucleation is still elusive due to the micro/nano scale and stochastic nature. The most popular theory about the nucleation is the Classical Nucleation Theory (CNT). However, recent studies show that the nucleation does not follow the CNT, but the two-step nucleation mechanism which states that the nucleation is composed of the formation of a dense liquid phase and growth of the critical nucleus 15, 16 . Gebauer et al. 17 and Nielsen et al. 18 have demonstrated the existence of amorphous calcium carbonate clusters or precursor before nucleation through experiments. Wallace et al. 19 have probed the structure, dynamics and other properties of the amorphous clusters. Mrinal et al. 20 have verified that the keggin heteropolyanions crystallize from the aqueous solution via a two-step process, including the formation of the dense liquid-like phase and the metastable polymorphic crystals in equilibrium with the dense liquid-like phases. This research concentrates on the preparation of boehmite by controlling the transformation direction of metastable intermediates via alditols and the analysis of alditols-interacting mechanism. Both the experimental and Density Functional Theory (DFT) simulation were conducted to investigate the effect of alditols on boehmite formation in Al-H2O reaction crystallization and analyze the interaction mechanism between alditols and metastable intermediates thoroughly with the global and local Hard and Soft, Acids and Bases (HSAB) theory.

2. Experimental and Calculation 2.1 Chemicals and Experimental setup High-purity aluminum (purity >99.999%) were supplied by Xinjiang Joinworld Company and processed into the flake shape. The organic alkali ROH (choline hydroxide) was provided by Jinan Jinhui Chemical Co. D-sorbitol (BR), xylitol (BR) and D-mannitol (AR) were brought from Sinopharm Chemical Reagent Co. Ltd. High-purity water of 18 MΩ was used. All reagents were used as received. The reaction crystallization equipment used in the present work is as previously reported in 21. The experiment condition in this article is that 20 g/L aluminum reacts with 0.2798 mol/L organic alkali solution (500 mL) under 90 oC with different alditols added. The phase composition of the product crystals was determined on a Smartlab XRD. The morphology of the crystals was characterized by the SEM (JSM-6700F). The FT-IR spectrum was measured by Spectrum GX. The crystal was separated by the high speed centrifuge (HC-3515) of


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Zonkia with the speed being 10000 rpm for 4 min.

2.2 DFT Computational Details The density functional B3LYP method with 6-311+ G (d, p) basis set was used to calculate the geometry optimizations and vibration spectra. All calculations were performed with the Gaussian 09 program.

3. Results and Discussion 3.1 Effects of Alditols on the Process and Products of Al-H2O Reaction Crystallization Different alditols were used to prepare boehmite from Al-H2O reaction crystallization, including D-sorbitol, xylitol and D-mannitol. The Al-H2O reaction crystallization was chosen here due to the original generation of aluminate ions and strong driving force supplied by chemical reaction. In order to penetrate into the mechanism, the effect of alditols on the reaction and crystallization process was investigated separately. First, the effect of sorbitol addition mass on the Al-H2O chemical reaction rate was investigated as shown in Fig. 1. From the change of hydrogen generation rate which is in stoichiometric relation with aluminate ions, it can be seen that the sorbitol has little effect on the Al-H2O reaction in the former stage (before 1500 s). However, for the latter stage of the reaction, the reaction rate decreases with the increase of the sorbitol mass. The reaction in the appearance of 11 mmol/L sorbitol nearly stopped with the hydrogen mass flow rate fluctuating around 0.

Fig. 1 The effect of D-sorbitol mass on the Al-H2O reaction rate The effect of sorbitol addition mass on the crystallization process was studied by comparing the evolvement of the total crystal number which is derived from the chord length distribution



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measured by the Focused Beam Reflectance Method (FBRM). As illustrated in Fig. 2, the total crystal numbers all increase with the reaction process. With the increase of sorbitol mass, the total crystal numbers decrease significantly. When the addition mass of the sorbitol is larger than 1.1 mmol/L, the variation range of the crystal number is less significant. It can be inferred that the alditols inhibit the crystallization process and thus decrease the chemical reaction and crystallization rate. This is because the crystallization in Al-H2O reaction crystallization can be taken as the decomposition of aluminate ions which can generate the hydroxyl ions and the generated hydroxyl can accelerate the Al-H2O reaction in return. The presence of alditols impedes the aluminate decomposition and thus the regeneration of hydroxyl ions.

Fig. 2 The total crystal number change with reaction time under different sorbitol mass The addition of sorbitol not only decreases the reaction and crystallization rate in the later stage, but also changes the polymorph of the final products. As shown in Fig. 3, the XRD patterns of the final products show that the sorbitol can influence the product polymorphs significantly. In the alditols-free crystallization, the product is composed of bayerite and gibbsite and these polymorphs are all alumina trihydrate with the molecular formula being Al(OH)3. When 0.5 mmol/L sorbitol is added to the reaction crystallization system, boehmite appears in the final products. The composition proportion of boehmite gets larger with the increase of the sorbitol addition mass. The critical addition quantity for acquiring pure boehmite is as low as 1.4 mmol/L, which can also be confirmed by the SEM of the products as demonstrated in Fig. 4. The alumina trihydrate exhibits a habit of prism (gibbsite) or pyramid (bayerite) in the blank experiment as shown in Fig. 4 (a) and the boehmite appears in the final product with the aggregation form of flakes in Fig. 4 (c, d). What is more, the crystal size of the boehmite is decreasing with the increase of the addition of alditols based on the comparison of Fig. 4 (c) and (d) and it can be inferred that the driving force for the nucleation of boehmite is enhanced by the appearance of alditols due to the fast nucleation.


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Fig. 3 XRD patterns of products under different sorbitol mass

(a)

(b)



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(c)

(d) Fig. 4 SEM of the final products under different additive mass: (a) blank, (b) 0.5 mmol/L, (c) 1.4 mmol/L and (d) 5.5 mmol/L Other alditols, such as the xylitol and mannitol, were also used to test the effects of alditols on the Al-H2O reaction crystallization. The sorbitol and mannitol are isomers with the molecular formula being C6H14O6. The effect of xylitol, mannitol and sorbitol of 5.5 mmol/L on the crystallization rate is shown in Fig. 5 and it can be seen that the inhibition ability ranking from strong to weak is: xylitol > mannitol > sorbitol because the strong inhibitor can cause less crystals. What is more, the final crystals of the three alditols are all composed of boehmite.


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Fig. 5 The effect of alditol configuration on the crystallization rate The effect of alditols on the reaction and crystallization can be summarized as following. The Al-H2O reaction crystallization can be divided into two processes: the chemical reaction and crystallization. Reaction:

2Al+6H 2 O+2ROH → 2R + Al(OH) 4− +3H 2 ↑

(1)

n {R + [Al(OH)-4 ]} → MI → Al(OH)3 +R +OH-

(2)

Alditol n {R + [Al(OH)-4 ]}  → MI' → AlO(OH)+R +OH- + H 2O

(3)

Crystallization:

, where the MI in reaction (2) and (3) is referred to metastable intermediates. The reaction (1) is not affected by the alditols in the former stage. However, the appearance of the alditols can inhibit the crystallization reactions (2) and (3). The reaction (3) is favored compared with the reaction (2) based on the experiment even though the overall crystallization rate is lowered. From the perspective of metastable intermediates and the two-step nucleation theory, the MI in reaction (2) and the MI’ in reaction (3) are featured with different structure and stability. The above experiments also support the reaction pathway of metastable intermediates. If the growth sites inhibition dominates the crystallization process, the final products are supposed to be composed of small-size gibbsite crystals because the unattached growth units will nucleate in the form of gibbsite under high supersaturation. It can be inferred that the reaction crystallization system undergoes a different reaction pathway as shown in reaction (3).

3.2 Interaction between Alditols and Metastable Intermediates: Stage and Strength Nearly all the previous studies attributed the inhibiting effect of alditols on gibbsite crystallization to the adsorption on the alumina hydrate. Bronswijk et al. 22 examined the adsorption of ten selected C3, C4, C5 and C6 acyclic polyols on hydrated alumina and found that



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threo-threo configuration can promote adsorption of polyol than the erythro–erythro sequences. Helen 14 also investigated the inhibition effect of eight alditols on the gibbsite crystallization and thought that alditols adsorbed on the specific sites and reduce the available sites. David et al. 13 also observed the adsorption of polyols on the boehmite surface. However, their explanations are all based on the assumption of the presence of gibbsite crystal surface before the nucleation. But the adsorption theory may not explain the inhibition and direction-changing effect. Coyne et al. 23 reported the adsorbed gluconate ions can only cover 3.5% of seeds surface but can lead to 90% crystallization inhibition. What is more, the boehmite can be also acquired in the appearance of alditols without the induction of boehmite seeds according to Section 3.1. So it can be inferred that the alditols affects the Al-H2O reaction crystallization mainly by acting on the species in the aqueous solution rather than the crystal phase. The crystal formed right upon nucleation was also analyzed to ascertain the influencing stage of the alditols. The experiment condition is that 20 g/L aluminum reacts with 0.2798 mol/L organic alkali solution under 90 oC with and without 5.5 mmol/L sorbitol added. 40 mL suspension was taken out the crystallizer once nucleation took place and centrifuged to separate the solid from the aqueous solution. The TEM characterization of crystals derived from additive-free crystallization is shown in Fig.6. It can be seen that the acquired crystal right at nucleation is amorphous based on the Selected Area Electron Diffraction (SAED) in Fig. 6 (a). Several strips of solid were formed during this stage as shown by the black zone. It can be inferred that the amorphous solid are mainly composed of fast-solidified metastable intermediates. The Fig 6 (b), (c) and (d) are the characterization of the crystals of three morphologies that were separated at 10 min after the nucleation and (e) is an overview of the three morphologies by low magnification. The solid in Fig 6 (b) is still amorphous and the size gets larger than that in Fig. 6 (a). The crystalline bayerite and gibbsite are also formed during this period as shown in Fig.6 (c) and (d) based on the SAED and morphology. The bayerite exhibit a shape of hourglass while the gibbsite is featured with prism. It is known that all polymorphs of Al(OH)3 are layered compounds with different patterns. Gibbsite is characterized with ABBA, bayerite with ABAB and nordstrandite with a combination of alternate gibbsite and bayerite layering patterns, where A is referred to aluminum octahedral and B is hydroxyl groups 24, 25. It can be inferred that the metastable intermediates may be composed of loosely bonded unit with AB mode.

(a)


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(b)

(c)

(d)

(e) Fig.6 TEM of nuclei formed in the nucleation (a), 10 min (b, c, d, e) The crystals acquired with 5.5 mmol/L sorbitol added were characterized with XRD and



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SEM with the results shown in Fig. 7 and it can be seen that the single phase of boehmite was formed since the nucleation. It can be verified that the alditols are supposed to influence the metastable intermediates that are formed before nucleation, rather than inhibit the growth of gibbsite by adsorbing on the crystal surface.

(a)

(b) Fig.7 XRD and SEM characterization of nucleated crystal The metastable intermediates are derived from the aluminate ions due to the local densification in the supersaturated solution and they might be the instable clusters or intermediates that appear before nucleation and can be converted into stable polymorphs 17. The metastable intermediates are similar to the dense liquid phase in the two-step nucleation mechanism 20, 26, 27. Isabelle et al. also hypothesized the existence of such intermediates or clusters 28. The metastable intermediates are hard to detect and characterized due to the metastable nature and the detection limitation of the online equipment. It can be inferred that the metastable intermediates have similar properties to the aluminate ions and act as the electron donors. In the interaction between the alditols and metastable intermediates, the alditols act as the acid (electrophile) and the metastable intermediates act as the base (nucleophile). The Pearson’s HSAB theory provides a convenient tool to describe the interaction between two molecules, which indicates “hard acids prefer to coordinate to hard bases and soft acids prefer to coordinate to soft bases” 29, 30. The hardness (η) and softness (σ) can be derived from the DFT calculation based on the


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Frontier Molecular Orbital (FMO) energies 31. The energies of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are calculated by DFT B3LYP-6-311G (d, p) using the Gaussian 09 program. The corresponding hardness (η) and softness (σ) can be computed using the following equations (4) and (5):

η=

[ ELUMO - EHOMO ] 2

σ=

1

η

(4)

(5)

Different alditols in Fig. 8 were examined in the simulation to predict the interaction and the corresponding intensity. The alditols exhibited here range from C2H6O2 to C6H14O6 with different carbon atom numbers and structures.

Fig.8 Configurations of different alditols The hardness and softness of different alditols were calculated with the results shown in Table. 1. Different alditols are featured with different hardness and softness. As it can be seen, the softness of sorbitol is 0.29 eV-1, higher than that of erythritol being 0.271 eV-1. All the alditols in Table.1 can be arranged according to their softness values from high to low: xylitol > mannitol > arabinitol > sorbitol > ribitol > threitol > glycerol > erythritol > ethylene glycol > inositol. This sequence is in good agreement with the inhibiting rank reported by Paulaime et al. 32 through experiments and the results in Section 3.1. It is indicated that the alditols with strong inhibiting effect are featured with low hardness or high softness and the softness of the metastable intermediates is around 0.323 eV-1 due to the strong interaction with xylitol. The inostiol, as a cyclic polyol with high hardness, has little effect on the crystallization, which is in agreement with



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that reported by Dhiren 33. Table. 1 The hardness and softness of different alditols calculated by DFT

alditols HOMO(eV) LOMO(eV) Hardness(eV) Softness(eV-1) ethylene glycol -7.785 -0.160 3.812 0.262 glycerol -7.606 -0.397 3.605 0.277 erythritol -7.739 -0.347 3.696 0.271 threitol -7.592 -0.506 3.543 0.282 ribitol -7.514 -0.509 3.502 0.286 arabinitol -7.414 -0.564 3.425 0.292 xylitol -6.869 -0.683 3.093 0.323 mannitol -7.422 -0.668 3.377 0.296 sorbitol -7.434 -0.526 3.454 0.290 inositol -8.004 -0.200 3.902 0.256

3.3 Determination of the Interaction Configuration The sorbitol and aluminate ion are selected here to investigate the interaction configuration. Gerson et al. 34 confirmed that the Al(OH)4- monomer is the predominant specie in the aluminate solution. As discussed in Section 3.2, the sorbitol acts as the electrophile and the aluminate ion as the nucleophile. The carbon atoms labeling for sorbitol molecule is depicted in Fig. 9.

Fig .9 The configuration of sorbitol and the numbering of carbon atoms The possible sites for interaction between sorbitol and aluminate are verified by the DFT simulation. The six possible reaction sites are the oxygen atoms that corresponds to the six carbon atom, as depicted in Fig. 9. The six possible interaction sites were all optimized from the geometric and energetic aspects to determine the conformations. From the calculations, the aluminate ion forms relatively weak chemical bond with the O-4, O-5 and O-6 and the results are shown in Fig. 10. The three calculated bond lengths are all no less than 2.010 Å, longer than the regular Al-O bond length of 1.793 Å in aluminate ions.


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Fig.10 Optimized configurations of the interacting sorbitol and aluminate In order to ascertain the mechanism, the infrared spectra of organic aluminate solution added with sorbitol was measured and compared to the simulated IR spectra of O-4, O-5 and O-6 configuration in Fig. 11. From the comparison result shown in Fig.10, it can be seen that the bands at 875 cm-1 and 824 cm-1 in the experiment data are close to the overlap of those bands in O-4, O-5 and O-6 configuration simulated by the DFT and can be identified as the vibration of the interacting metastable intermediates and aluminate ion.

Fig.11 Experimental FTIR spectrum of organic aluminate solution added with sorbitol and theoretical IR spectrum of O-4, O-5 and O-6 configuration The local HSAB theory can be used to predict the reaction site-selectivity in the chemical reaction 35. The Fukui function f ( r ) and local softness σ ( r ) are mainly used to describe the local interactions and are obtained as 36:

f k+ = qk ( N + 1) − qk ( N )

for atom k as an electrophile

(6)

f k− = qk ( N ) − qk ( N − 1)

for atom k as a nucleophile

(7)

σ ( r ) = f ( r )σ

(8)

The results of Fukui function and local softness of oxygen atoms connected with the six carbon atoms of sorbitol and the aluminum atom in aluminate ion are shown as in Table. 2. From Table. 2, it can be seen that the O-4, O-5 and O-6 have the similar local softness to the aluminum atom of the aluminate ion. According to the local HSAB theory which states that the atoms with



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closer local softness can be considered as the most probable sites of interaction, the O-4, O-5 and O-6 are the favored interaction sites 37.

Table. 2 Fukui functions and local softness of aluminate and sorbitol

sorbitol

Atom

f(r)

σ(r) (eV-1)

∆σ(r)

O-1*

0.004

0.001

0.065

O-2

-0.001

0

0.064

O-3

-0.023

-0.007

0.057

O-4

-0.074

-0.021

0.043

O-5

-0.082

-0.024

0.040

O-6

-0.350

-0.101

-0.037

-0.181

-0.064

aluminate Al

* The O-Number refers to the oxygen atom connected to carbon number of sorbitol Based on the above results, it can be concluded that the application of alditols can prepare boehmite from Al-H2O reaction by affecting the pre-nucleation metastable intermediates in the crystallization, rather than by blocking the growth sites of gibbsite. The formation of boehmite may be attributed to the combination of alditols with metastable intermediates and the uniform bondings of aluminum atoms and oxygen atoms of hydroxyl ions are inhibited. This is because that the Al atom in gibbsite is connected by six hydroxyl ions uniformly (1/2 O ×6), but in boehmite the Al atom is connected unevenly by two O atoms that are shared by two Al atoms and four O atoms shared by four Al atoms (1/4O ×4 +1/2O ×2). Therefore, the crystallization of gibbsite is inhibited and the transformation from modified intermediates MI’ to boehmite is favorable when the alditols interact with the intermediates. The interaction form and strength is indicated well with the softness from the perspective of HSAB theory and this method is promising for the selection of additive for directional crystallization.

Conclusion In this study, the alditols were used to prepare single-phase boehmite from Al-H2O reaction crystallization in moderate condition. The critical sorbitol addition quantity for obtaining pure boehmite is as low as 1.4 mmol/L for the condition under which 20 g/L aluminum reacts with 0.2798 mol/L organic alkali solution at 90 oC. By analyzing the crystals that were formed right upon nucleation, it is concluded that the alditols can inhibit the decomposition of aluminate ions into gibbsite and favor the formation of boehmite by affecting the pre-nucleation metastable intermediates rather than by blocking the active sites on the gibbsite crystal surface. In the interaction, the metastable intermediates that can be converted to gibbsite or boehmite crystals are taken as soft bases while the alditols play the role of acid and it is found the higher the alditols’ softness, the stronger the inhibiting effect. The softness of metastable intermediates is estimated to


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be around 0.323 eV-1. The most probable sites of interaction were also determined by DFT simulation and local HSAB theory and verified by FTIR spectrum. The adjustment of metastable intermediates by additives is promising and beneficial for the nucleation mechanism analysis and the control of directional crystallization.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (51422405, 51374192).

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TOC-Synopsis Table of Contents Graphic:

Synopsis: The interaction between alditols and metastable intermediates inhibits the decomposition of aluminate ions into gibbsite and favors the formation of boehmite in Al-H2O reaction crystallization. In the interaction, the metastable intermediates act as soft bases while alditols play the role of acids and the higher the alditols’ softness, the stronger the inhibiting effect. For Table of Contents Use Only

Title: Boehmite Preparation via Alditols-interacting Transformation of Metastable Intermediates in Al-H2O Reaction Crystallization Authors: Hongqi Wang, Zhi Wang, Jianwei Guo, Zhihao Shi, Xuzhong Gong, Jianwei Cao


Boehmite Preparation via Alditols-Interacting Transformation of

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