Development of a Strategy To Reuse Spent Cr Adsorbents as Efficient

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The Development of a Strategy to Reuse Spent Cr-Adsorbents as Efficient Catalyst: From the Perspective of Practical Application Dedong He, Yaliu Zhang, Shuang Yang, Liming Zhang, Jichang Lu, Yutong Zhao, Yi Mei, Caiyun Han, and Yongming Luo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05206 • Publication Date (Web): 13 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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ACS Sustainable Chemistry & Engineering

The Development of a Strategy to Reuse Spent Cr-Adsorbents as Efficient Catalyst: From the Perspective of Practical Application

Dedong He,† Yaliu Zhang,† Shuang Yang,‡ Liming Zhang,‡ Jichang Lu,‡ Yutong Zhao,‡ Yi Mei,† Caiyun Han,‡ and Yongming Luo,*,‡

†

Faculty of Chemical Engineering, Kunming University of Science and Technology,

Kunming 650500, P. R. China. ‡

Faculty of Environmental Science and Engineering, Kunming University of Science and

Technology, Kunming 650500, P. R. China.

* Corresponding author Tel: +86-871-65103845 Fax: +86-871-65103845 E-mail: [email protected] (Y. Luo)

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Abstract Recycling waste Cr adsorbents as efficient catalysts reflects a new avenue of green chemistry and has important implications for environmental protection. In particular, the investigation of the effect of additional impurity ions is considered to be applicable for real Cr adsorbents. In the present work, the reused Cr adsorbents synthesis from streams containing competing ions were developed for CH3SH abatment and propane dehydrogenation. The good dispersion of Cr species was obtained through this special procedure of preparation, and its difference from the traditional impregnation method was also compared. Furthermore, these reused Cr adsorbents with the exposure of additional impurity ions provided different physicochemical properties, and different catalytic activities were shown accordingly. The results indicated that the impurity cations had little effect on the catalytic activity of the reused Cr adsorbents, while the impurity anions caused the destruction of siliceous framework from MCM-41 support, and lead to the formation of some inactive Cr(VI) species, thus exhibiting decreased reactivity. Consequently, avoiding the addition of some anions was considered to be the premise for reusing the waste Cr adsorbents. Keywords: Waste Cr Adsorbents; Reused as Catalyst; Additional Impurity Ions; Propane Dehydrogenation.


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INTRODUCTION The presence of hexavalent chromium (Cr(VI)) in aquatic environments, generally originated from different industrial activities, poses serious threats to environment safety and human health.1,2 Treatment of Cr(VI) pollution is therefore of great importance and gains ever-lasting attention recently. As an efficient and economical technique, adsorption is one of the most promising treatment processes for Cr removal from wastewater.3 Various adsorbents, including organic-inorganic hybrids silicon material, have been devoted for the removal of Cr(VI).4,5 Nevertheless, disposal aspects of the used adsorbents containing adsorbed Cr metals requires further attention, since the inappropriate disposal of these spent adsorbents may cause new pollution problem. Although acid and base treatments are claimed to desorb these adsorbents effectively,6,7 the requirement for treating the spent acid and base solutions containing high concentration of Cr(VI) is still a major disadvantage. Besides, another strategy called as landfill is considered as the preferred option,8 but the reserve of Cr-loaded solid waste in the environment also shows toxic effects on human health.9 Therefore, finding an alternative and environmental technology to tackle the problem of the spent Cr adsorbents is necessary. One attractive option for dealing with these undesired Cr adsorbents is to recycle them as highly efficient catalysts.10 In fact, the outstanding characteristic of the present Cr adsorbent, regarded as organic-inorganic hybrids silicon material, is its calcined product, which can eventually become MCM-41, and is known to be catalyst support for many heterogeneous catalysts. As a matter of fact, waste Cr adsorbents were recycled by calcination treatment and reused as Cr/MCM-41 catalyst to eliminate methyl mercaptan



(CH3SH), a typical compound of sulfur-containing VOCs,11,12 as shown in our recent study,10 which indicates a new avenue of green chemistry and has important implications for environmental protection. However, it was proposed to use chemical reagent of K2Cr2O7 to prepare Cr(VI) containing solutions and did not consider the influence of any additional impurity ions, so the results might not be applicable for real spent Cr-adsorbents, since many other chemical species in a given wastewater would be co-adsorbed or/and compete for adsorption with Cr on the adsorbent during wastewater treatment. Thus, it would be very important to study the effect of these other additional impurity ions on the physicochemical properties and catalytic reactivity of the reused Cr-adsorbent. In fact, investigation of the effects of other chemical species such as additional impurity anions and cations in the wastewaters on the catalytic performance of the reused Cr-adsorbents has already been carried out in our previous work,10 and it was shown that these impurity ions caused difference in the physicochemical properties of the reused Cr-adsorbent, thus leading to the difference in the catalytic reactivity for CH3SH abatment. However, detail analysis and explanations on these results are not given. Furthermore, the application of the reused Cr-adsorbent to other reactive system also fail to discuss. On the basis of these considerations, the reused Cr adsorbent of the present work was also developed to the reactive system of propane dehydrogenation (PDH). It is known that growing demand for propene as well as the vast amounts of shale gas worldwide has spurred an increased interest in the reaction of PDH,13,14 since propene is an important feedstock in the production of many chemicals, such as acrylonitrile, polypropylene and propylene oxide.15 Besides, the explanation for the different effects on the physicochemical


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properties as well as the catalytic activities of the reused Cr-adsorbents synthesis from streams containing additional impurity ions was illustrated. MATERIALS AND METHODS Synthesis of adsorbents and experiment of Cr(VI) column adsorption were provided in the literature reported by our team.10 In the typical column adsorption process, desired amount of adsorbents were packed into a glass column fixed-bed (with length of 200 mm and internal diameter of 11 mm) to ensure a bed height of 70 mm. Then, solutions (pH=2.0) containing 20 mg/L of Cr(VI) were led to the fixed-bed through peristaltic pump, with a flow rate of 2.2 mL/min. Meanwhile, some competing cations, including Pb2+ (Pb(NO3)2), Zn2+ (Zn(NO3)2), Cu2+ (Cu(NO3)2), Ni2+ (Ni(NO3)2), Ca2+ (Ca(NO3)2) and Mg2+ (Mg(NO3)2), as well as some anions, including F- (NaF), Cl- (NaCl), SO42- (Na2SO4) and SiO32- (Na2SiO4), at a concentration of 20 mg/L were added into the Cr(VI) solution. Notably, the whole column adsorption process was conducted at room temperature for 96 h. Subsequently, the spent Cr adsorbents were reused by calcination treatment in air at 550 oC for 6 h. The structure and physicochemical properties of the obtained samples were characterized by XPS, XRD, UV-Vis, FT-IR, H2-TPR, N2 adsorption-desorption,

29Si

MAS NMR and

SEM/Mapping. Details on characterization methods are recorded in the Supporting Information (SI). Catalytic evaluation for CH3SH abatement was recorded in detail in our previous studies.10,11,12 Meanwhile, the PDH reaction was carried out in the same reactor packed with 0.4 g of catalyst. The feed and reaction products of PDH were analyzed by an online



gas chromatograph equipped with TCD and FID detectors. More details are described in the SI. RESULTS AND DISCUSSION From Figure S1 in SI, it is seen that these additional impurity ions show little effect on Cr(VI) column adsorption process, which indicates that our adsorbent (organic-inorganic hybrid mesoporous silica materials) exhibits excellent selectivity for Cr(VI) adsorption. Meanwhile, the SEM/Mapping images of the reused Cr-adsorbent are shown in Figure 1. To our surprise, the sample of the reused Cr-adsorbent displays a good dispersion of Cr species, while poor Cr dispersion over the Cr/MCM-41 synthesized by traditional impregnation method are observed, as presented in Figure S2 in SI. The difference between the reused Cr adsorbent and the Cr/MCM-41 sample synthesized from impregnation method is compared in Figure S3 in SI. From Figure S3, different anchoring sites for the stabilization of Cr species are exhibited, on which anionic chrome oxo species (Cr2O72-) is anchored to the support by the positively charged ammonium groups (from template agent of CTAB) through electrostatic interaction,10 while the anchoring sites of Cr/MCM-41 from impregnation method are surface hydroxyl groups. From this point of view, the normal impregnation onto MCM-41 forms poor agglomerated catalysts, and this seems to be expected based on the anionic nature of the chromium oxide species (CrO42-) and the negative charge on the surface hydroxyls. Subsequently, cationic Cr complexes (e.g. Cr(NO3)3) are used to prepare the Cr/MCM-41, but the introduced Cr species are not highly dispersed on the support as expect (as shown in Figure S5 in SI), which further confirms the advantage of the present column adsorption method. In general, good dispersion of the


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active sites on the catalyst is expected to obtain good catalytic performance in heterogeneous catalysis.16 Thus, the above advantages over the reused Cr-adsorbent highlight its further application. Figure 2 illustrates catalytic activity for CH3SH abatement and PDH over the reused Cr-adsorbents. As to CH3SH abatement (Figure 2A), it is found that the additional impurity cations shows little effect on the catalytic activity of the reused Cr-adsorbents, but the samples synthesized from the impurity anions presents decreased reactivity. A similar result is also seen towards PDH reaction (Figure 2B and 2C), where sample with the exposure of additional impurity anions exhibits the decreased catalytic performance with regard to propane conversion and propene selectivity. Hence, further characterization studies are carried out to explain the above results. An investigation of Cr2p XP spectra in Figure 3 and Figure S6B indicates the existence of both Cr(VI) and Cr(III) states in the reused Cr-adsorbent samples. Although the introduced Cr species from the adsorption process is Cr(VI), the reduction of Cr(VI) to Cr(III) under calcination treatment has been clarified previously.10 Surprisingly, peak fitting of the Cr2p binding energy (BE) region for the tested samples (Figure 3) reveals that a higher relative percent of Cr(VI), about 52%, are presented on the reused Cr-adsorbent sample with the exposure of additional impurity anions, while lower relative percents of Cr(VI), at about 40% and 30%, are respectively calculated from these none and cations exposed samples. Si2p XP spectra of the adsorbent samples is shown in Figure S6A in SI. Clearly, the shift of the BE to the lower BE location over the anions exposed sample illustrates that the local Si environment on this sample is changed. Meanwhile, the XRD patterns of the reused



Cr-adsorbents are illustrated in Figure 4A. From Figure 4A, diffraction lines of Cr2O3 are seen in the XRD patterns of the none and cations exposed samples,17 but the addition of impurity anions cause the disappearance of crystal Cr2O3 phase. It is accepted that the generated HF under acidic condition may cause the destructive effects on the crystal structure of Cr2O3. Moreover, the UV-vis spectra of the tested adsorbents are compared in Figure 4B. It is observed that the intense peak centered at about 360 nm, expresses O-Cr(VI) charge transfer transitions of Cr(VI) species,18 is more pronounced on the reused Cr-adsorbent sample synthesis from streams containing competing anions, while other two bands at about 460 and 600 nm, which are characteristic for octahedral Cr(III) in CrOx clusters and the presence of crystalline Cr2O3,18,19 are found for the samples exposed with none and cations. Thus, the UV-vis analysis is in line with the XPS data, suggesting the presence of Cr(VI) species on the reused Cr-adsorbent sample synthesis from streams containing competing anions. Furthermore, Figure 4C shows the FT-IR result of the tested samples. It is seen that vibration frequency peaks at 560 and 640 cm-1, attributed to the contribution of Cr2O3 phase,20 are observed in these none and cations exposed samples, while they are absent in the sample with the exposure of competing anions. Besides, a weak IR band at about 900 cm-1, attributed to the vibration of Cr=O or Cr-O from Cr(VI) species,21 is seen in the anions exposed sample. As a result, FT-IR result is in accordance with the XRD, UV-vis and XPS analysis. Notably, a FT-IR band at 960 cm-1 is correlated with the vibration of Si-O bond,22 but the shift of this band to 952 cm-1 over the anions exposed sample, as seen in Figure 4C, again indicates the change of local Si environment.21 In addition, the H2-TPR profiles in Figure 5 shows that intense reduction peaks appear on


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the anions exposed sample, which gives account for the reduction of Cr(VI) species,17,18 hence, it again suggests the presence of more amount of Cr(VI) species. Notably, two peaks from the H2-TPR profiles can be attributed to the reduction of di/polychromates and monochromates Cr(VI) species, respectively.10,17,18 Obviously, the sample synthesis from streams containing additional impurity anions records higher relative content of di/polychromates Cr(VI) species. In our previous publications,23 it is demonstrated that active Cr(VI) species are proved to be beneficial for CH3SH conversion. Meanwhile, some reported literatures also indicate that the presence of high-valence state of Cr species (e.g. Cr(VI)) can be generally regarded as reactive sites during dehydrogenation reaction.24,25 In fact, catalytic tests toward CH3SH abatement and PDH reaction are carried out by using mechanical mixtures of CrO3 (Cr(VI)) and Cr2O3 (Cr(III)) with solid MCM-41 samples, since pure MCM-41 solid exhibits almost no activity for these reactions. As to both CH3SH abatement and PDH reaction, it is observed from Figure S7 in SI that significant activity is found by using a mechanical mixture of CrO3/MCM-41, while mechanical mixture of Cr2O3/MCM-41 shows low catalytic activity. From this point of view, Cr(VI) species are thought to be active centers and associated with an improvement in reactivity. Nevertheless, the reused Cr adsorbent with more amount of Cr(VI) species synthesis from streams containing competing anions does not show the expected high activity. In other words, the existed Cr(VI) species on this sample are not active. Consequently, in-depth explanation should be given. The N2 adsorption-desorption isotherms of the tested adsorbent samples are shown in Figure 6A. The none and cations exposed samples display type IV isotherms with an



inflection at P/P0 range between 0.2 and 0.4, which is characteristic for mesoporous MCM-41 structure.26,27 From Figure 6A, it is seen that the cations exposed sample shows a less sharp inflection, which illustrates a slight decrease in textural characteristic,27 since the sharpness of the inflection generally relates to the uniformity of mesopores.26 However, the anions exposed sample exhibits an obviously different isotherms, where its inflection position has shifted to the higher P/P0 location, suggesting the increase in the diameter of the mesopores.26 Meanwhile, the appearance of hysteresis loop also indicates the formation of mesopores and even some stacking pores on this sample, possibly originated from the dissolution of siliceous framework in MCM-41 support. Besides, pore size distributions from the physisorption data are also displayed in Figure 6A, and the results clearly confirm the generation of more amount of mesopores on the anions exposed sample. In fact, the XPS and FT-IR results have revealed the change of local Si environment on this sample, and considering that the added F- under acidic condition (pH = 2) may generate HF, thus leading to the destruction and dissolution of siliceous framework. To further illustrate this issue, SEM maps and ICP measurements of the post-treatment materials were carried out. It is seen from Figure S8 in SI that the cations and anions exposed materials display different support morphology expressions, and the Si content decreases from 38.4% (none impurity ions exposed sample) to 20.9% (anions exposed material). Besides, 29Si MAS NMR spectra of the samples were conducted, and the results are recorded in Figure 6B. Three distinct peaks, representing for Q2, Q3 and Q4,28 are observed in the none impurity ions exposed sample. In general, these peaks, Q2, Q3 and Q4, can be respectively assigned to (-O-)2Si(OH)2, (-O-)3Si(OH) and (-O-)4Si, where two, one and no OH groups are attached


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to the silicon atom.29 However, an decrease in the intensity and the change in the distribution of Q2, Q3 and Q4 on the sample with the exposure of anions clearly support the conclusion that the local Si environment of the sample has been changed. Furthermore, from Figure 6C and 6D, the Q2 peak is not detected in spectrum of the anions exposed sample, and the value of Q4/(Q3+Q2) in this sample (2.50) is higher than that in the none ions exposed sample (1.53). Thus, it is obtained that less OH groups are presented on the anions exposed sample. In addition, the high-frequency region from 3200 to 3700 cm-1 can be assigned to the stretching vibration of SiO-H bond.30 As a result, from Figure 6E, the decrease in the intensity of this band in the case of the anions exposed sample further suggests the presence of less OH groups. From above results, the generation of the inactive Cr(VI) species on the anions exposed sample can be illustrated as follows (Figure 7): the exposure of anions (e.g. F-) lead to the destruction of siliceous framework from MCM-41 support. At this time, the SBET of this sample has decreased to 350 m2/g (about 1000 m2/g of SBET are obtained on the none and anions exposed samples), and less OH groups are presented. Consequently, more amount of di/polychromates Cr(VI) species are formed, as evident from H2-TPR analysis (In fact, di/polychromates Cr(VI) species are generally proved to be inactive during many catalytic reactions17). These di/polychromates Cr(VI) species are difficult to be reduced by the ammonium groups (the reduction processes are stated in our previous work10). As a result, this effect causes the formation of some inactive Cr(VI) species. Besides, the collapse of the structure of the silica support could encapsulate a fraction of the Cr, which would also be used to explain the lack of Cr2O3 and would provide a simpler explanation for the



presence of higher amounts of unreducible Cr6+. To our surprise, the reused Cr adsorbent obtained from the Cr solution with no fluorine addition shows improved catalytic activity (results are not shown), which suggests that avoiding the presence of some anions (e.g. F-) is the premise for reusing the waste Cr adsorbents. CONCLUSIONS To summarize, the exploration of recycling the spent Cr adsorbents as a highly efficient catalyst has important environmental significance, and the investigation of the effect of other additional impurity ions on the physicochemical properties and catalytic reactivity is applicable for real spent Cr-adsorbents. It was shown that the reused Cr-adsorbent sample displayed a good dispersion of Cr species. Moreover, the impurity cations exhibited little effect on the catalytic activity of the reused Cr adsorbents, while the exposure of anions caused the destruction and dissolution of siliceous framework from MCM-41 support, thus causing the formation of some inactive Cr(VI) species. As a result, the sample synthesis from streams containing competing ions showed decreased reactivity for CH3SH abatment and PDH reaction. Acknowledgements The National Natural Science Foundation of China (U1402233, 21667016 and 21367015) is gratefully acknowledged for financial support. Supporting Information Available Details on Catalyst Characterization and Test Evaluation are available as Supporting Information. Reference


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Figure 1. SEM/Mapping images of the reused Cr adsorbent.


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Figure 2. (A) The effect of additional impurity ions on CH3SH abatement; The effect of additional impurity ions on (B) C3H8 conversion and (C) C3H6 selectivity during Propane dehydrogenation process.


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Figure 3. Cr 2p XP spectra of the reused Cr adsorbents.


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Figure 4. (A) XRD patterns; (B) UV-vis spectra and (C) FT-IR spectra of the reused Cr adsorbents.


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Figure 5. H2-TPR profiles of the reused Cr adsorbents.



Figure 6. (A) N2 adsorption-desorption isotherms; (B), (C) and (D) 29Si MAS NMR spectra; (E) FT-IR spectra of the reused Cr adsorbents.


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Figure 7. The diagram for the preparation of the reused Cr adsorbents.



For Table of Contents Use Only

Recycling waste Cr adsorbents as efficient catalysts reflects a new avenue of green chemistry and has important implications for environmental protection.


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Development of a Strategy To Reuse Spent Cr Adsorbents as Efficient

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