Cellulose (2016) 23:2539–2548 DOI 10.1007/s10570-016-0955-5 ORIGINAL PAPER The oxidation of viscose fiber optimized by response surface methodology and its further amination with PEI for CO2 adsorption Hui He . Xunan Hou . Beibei Ma . Linzhou Zhuang . Chuanfa Li . Shihong He . Shuixia Chen Received: 15 September 2015 / Accepted: 6 May 2016 / Published online: 17 May 2016 Ó Springer Science+Business Media Dordrecht 2016 Abstract Viscose fiber was oxidized with sodium periodate to prepare a reactive dialdehyde viscose fiber (DAVF) containing abundant aldehyde groups. A solid amine adsorbent (DAVF-PEI) with high amino density for CO2 capture was then prepared by modifying DAVF with polyethylenimine (PEI). Response surface methodology (RSM) based on a three-level, three-factorial design was used to optimize the synthesis conditions of DAVF, in which multiple linear regression equations of aldehyde content and fiber mass loss degree were constructed. The well-designed DAVF was then employed as a support to graft with PEI via Schiff base reaction to prepare a solid amine fiber (DAVF-PEI) for CO2 adsorption. The experimental results verified that DAVF-PEI possessed good thermo-stability and high CO2 adsorption capacity (4.11 mmol/g). DAVF-PEI also showed promising regeneration performance, Electronic supplementary material The online version of this article (doi:10.1007/s10570-016-0955-5) contains supplementary material, which is available to authorized users. H. He X. Hou B. Ma L. Zhuang C. Li S. He S. Chen (&) PCFM Lab, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China e-mail: cescsx@mail.sysu.edu.cn S. Chen Materials Science Institute, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China which could maintain almost the same adsorption capacity for CO2 after ten adsorption and desorption recycles. Keywords Dialdehyde viscose fiber Response surface methodology PEI CO2 adsorption Introduction Global warming is mainly attributed to an increasing atmospheric levels of CO2 arising from the unrestrained burning of fossil fuels (Deanna et al. 2010; Li et al. 2010; Siriwardane et al. 2001; Zhao et al. 2013). In order to make the separation process energyefficient, cost-effective, and available over a relatively wider range of temperatures and pressures, many solid adsorbents have been developed (Gray et al. 2005; Harun et al. 2012; Sayari et al. 2011; Zukal et al. 2009; Schrier 2012). Particularly, the amine-modified adsorbents using porous materials as substrates have been considered as the most promising adsorbents, as their large specific surface areas and pore volumes are favorable to enhancing the CO2 adsorption capacity (Drese et al. 2009; Pevida et al. 2008; Hicks et al. 2008; Wang et al. 2012; Serna-Guerrero and Sayari 2010). The CO2 adsorption capacity of the adsorbents mainly depends on the amine loading amount and the pore volume of the porous substrates. However, the increase of amine loading will block the pores of the 123 2540 adsorbents, thus reducing the CO2 adsorption capacity. So it is significant to develop alternative substrates to overcome the above defect. Many works advocating the usage of fibrous adsorbents for CO2 adsorption have been published by our group. Unlike porous adsorbents, fiber-based adsorption materials possess the superior properties of a large external surface area, short transit distance, low pressure drops, and flexibility, which make the fiber a promising substrate for CO2 adsorbents (Yang et al. 2010; Zhang et al. 2008; Zhuang et al. 2013; Wu et al. 2014). Besides the substrate, the amine content is also an important factor affecting the CO2 adsorption performance of the material. Because of its high amine content, PEI has been widely reported to modify materials used for gas separation. Wu et al. (2014) grafted acrylamide onto the surface of a polypropylene fiber and subsequently modified it with PEI to make a novel kind of solid amine fibrous adsorbent, which showed an adsorption capacity of 5.91 mmol/g. Although these solid amine fibers show great performance on CO2 adsorption, their harsh synthesis conditions still set restrictions on their large-scale applications. The preparation of solid amine fibers generally requires two steps including grafting and amination, and the amination step needs to be carried out under high temperature, which significantly increases the energy cost. Thus, the development of a simpler and lower energy cost preparation route will be beneficial to improve the synthesis of solid amine adsorbents. Dialdehyde cellulose, obtained by cellulose oxidation using sodium periodate (Calvini et al. 2006; Sirvio et al. 2011; Wu and Kuga 2006; Li et al. 2011), is able to directly react with amino reagents under low temperature for high reactivity of the aldehyde group, which not only simplifies the preparation process, but also reduces the production energy consumption. Thus, dialdehyde cellulose is a kind of potential fibrous substrate (Potthast et al. 2007). However, the high temperature and sodium periodate concentration can make the cellulose or dialdehyde cellulose degrade. In the preparation process, the crystallinity of the oxidized cellulose and the yield will both decrease with increasing oxidation level (Kim et al. 2004; Liu et al. 2012a, b). However, one major problem now is the absence of the available information concerning the optimization of the preparation conditions and the kinetics of viscose fiber (VF) 123 Cellulose (2016) 23:2539–2548 oxidation by sodium periodate. The detailed study on the optimization of the VF oxidation thus becomes the priority. The conventional reaction condition optimization technique is called one-variable-at-a-time. Its major disadvantage is that it does not include the interactive effects among the variables studied. Therefore, it does not depict the complete effects of the parameters on the response (Bezerra et al. 2008). In order to overcome this weakness, the response surface methodology (RSM) is adopted, as it could be well applied even though responses or sets of responses are simultaneously influenced by several variables (Liu et al. 2012a, b). RSM could not only describe the behavior of the objective well, but also make the statistical previsions to attain the best system performance. In this work, the factors affecting viscose fiber oxidation were investigated with RSM. Optimized dialdehyde viscose fiber (DAVF) was employed as a support to load PEI via Schiff base reaction to prepare a novel solid amine fiber (DAVF-PEI) for CO2 adsorption. The effect of the grafting monomer, adsorption performance, and regeneration ability of DAVF-PEI were also studied. Experimental section Materials and reagents Viscose fiber (VF) was provided by Jihua Group Ltd., China. Branched polyethylenimine (PEI, Mw = 600) was purchased from Aladdin. The others, including sodium periodate, oxammonium hydrochloride, sodium acetate, and ethanol with analytic reagent grade were purchased from Guangzhou Reagent Co. Deionized water was used to prepare all solutions in the study. Experimental design The preparation of DAVF was rationally designed using RSM of three factors, including reaction time, reaction temperature, and sodium periodate concentration, and for the purpose of statistical computations, the three independent variables were denoted as x1, x2, and x3, respectively. The design was composed of three levels (low, medium, and high, being coded as Cellulose (2016) 23:2539–2548 2541 -1, 0, and ?1), and a total of 20 runs were carried out in duplicate to optimize the level of the chosen variables. According to the preliminary experiments, the range and levels used in the experiments are selected and listed in Table 1. Preparation of DAVF In the general preparation procedures, 5.00 g VF was added to the sodium periodate solution with a given concentration (0.18–0.52 mol/l). After adjusting the pH to 4, the solution was sonicated at room temperature for 20 min, followed by being placed in a shaking bath at a certain temperature (30–72 °C) and shaken for a desired time (1–6 h). Then, the obtained material was washed with water several times until the produced iodine was completely removed (sodium thiosulfate was used to detect the presence of iodine through the color reaction between them). Then, it was filtered and dried at 60 °C, and the obtained oxidized VF containing aldehyde groups was called dialdehyde viscose fiber (DAVF). Grafting of PEI on DAVF DAVF (5.00 g) was added to 100 ml 10 wt% PEI solution. The mixture was continuously stirred at 45 °C for 24 h. After the reaction, the obtained material was stirred in 60 % aqueous ethanol for 2 h and washed with water until neutral. It was then extracted with ethanol for 12 h and dried at 60 °C, as illustrated in Scheme 1, DAVF loading PEI adsorbent; DAVF-PEI was thus obtained. Determination of the aldehyde content DAVF (0.10 g) was placed in a 250-ml beaker containing 1.39 g NH2OHHCl and 100 ml of 0.1 M sodium acetate buffer (pH 4.5). The beaker was covered with a thin rubber foil. The mixture was stirred for 48 h at room temperature with a magnetic Table 1 Independence factors and corresponding levels used for optimization stirrer. The reaction product, an oxime derivate of DAVF, was filtrated, washed with deionized water, and then dried at 60 °C; the nitrogen content of the oxime derivate of DAVF was determined through elemental analysis (EA). As illustrated in Scheme 2, the aldehyde contents of DAVF were determined based on the oxime reaction between the aldehyde group and NH2OHHCl, in which aldehyde groups were stoichiometrically converted into oxime groups (Sirvio et al. 2011). Determination of the mass loss degree of fiber The fiber mass loss degree (L, %) was obtained by calculating the difference of the weights of viscose fibers before (m0, g) and after (m, g) the oxidation reaction, as presented in Eq. (1). L ¼ ðm0 mÞ=m0 100 % ð1Þ Physical and chemical characterization Infrared (IR) spectra (Tensor-27 spectrometer), elemental analysis (Elementar, Vario EL), 400-MHz solid state 13C NMR analysis (AVANCE AV, Bruker), and X-ray photoelectron spectroscopy (ESCALAB 250, Thermo-VG Scientific) were used to confirm the structure of the polymers. CO2 adsorption experiment Breakthrough curves were used to characterize the CO2 adsorption performances of all samples; 1.00 g DAVF-PEI sample was tightly placed in an adsorption column (U = 1.3 cm), into which a dry nitrogen flow was introduced at a flow rate of 30 ml/min for 0.5 h to remove air and excess water in the tube. Then, the dry CO2/N2 mixed gas was introduced through the tube at a flow rate of 30 ml/min. The inlet/outlet concentrations of CO2 were analyzed every 2 min using a Techcomp 7900 gas chromatograph equipped with a Variables Real values of coded levels -1 Reaction time, x1(h) Reaction temperature, x2(°C) Sodium periodate concentration, x3(mmol/g) 0 ?1 3 4 5 30 45 60 0.25 0.35 0.45 123 2542 Cellulose (2016) 23:2539–2548 Scheme 1 Reaction between DAVF and PEI Scheme 2 Reaction between DAVF and NH2OHHCl thermal-conductivity detector (TCD). After adsorption, pure nitrogen gas at a flow rate of 30 ml/min was introduced through the tube at 90 °C to regenerate the used DAVF-PEI. The adsorption capacity was calculated as follows: Z t Q¼ ðCin Ceff ÞVdt=22:4W ð2Þ 0 where Q represents the adsorption capacity (mmol CO2/g), t is the adsorption time (min), and Cin and Ceff were the influent and effluent concentrations of CO2 (vol %), respectively. V is the total flow rate, 30 ml/ min; W and 22.4 are the weight of the sample (g) and molar volume of gas (ml/mmol), respectively. 123 Results and discussion RSM analysis of the preparation conditions of DAVF The optimum values of the variables are the main aim using the response surface method. According to the RSM analysis experimental data (the RSM experimental design matrix and corresponding experimental responses are listed in Table S1 of the supporting information), the optimal sodium periodate concentration, reaction temperature, and reaction time were 0.35 mol/l, 45 °C, and 4 h, respectively. The aldehyde content of DAVF in these conditions reached 8.07 mmol/g, while the fiber mass loss degree could Cellulose (2016) 23:2539–2548 2543 be controlled at 10.76 %. The three-dimensional response surface plot obtained from the method depicted an infinitive interaction between two test parameters on the aldehyde content of DAVF and fiber mass loss degree (Fig. 1), and the detailed analysis of the two-factor interaction is shown in the supporting information. We used the quadratic polynomial equation to express the response of the aldehyde content and fiber mass loss degree, which are shown in Eqs. (3) and (4). The results of the coefficient term for the quadratic equation are tabulated in Table S2 and S3 of the supporting information. Yaldehyde content =ðmmol/gÞ ¼ 26:46514 þ 3:23349 x1 þ 0:51422 x2 Fig. 1 Interactions between preparation conditions. a, b Twofactor interaction between the sodium periodate concentration and reaction temperature. c, d Two-factor interaction between the sodium periodate concentration and reaction time. e, f Twofactor interaction between the reaction time and reaction temperature þ 89:21896 x3 0:17667 x2 x3 0:36982 x21 5:19490 103 x22 110:52123 x23 ð3Þ 123 2544 Cellulose (2016) 23:2539–2548 Yfiber mass loss degree =% ¼ 4:70929 þ 2:10884 x1 þ 0:1005 x2 þ 0:19308 x3 0:046333 x1 x2 þ 1:52500 x1 x3 þ 0:52167 x2 x3 ð4Þ where x1 is the reaction time, x2 is the reaction temperature, and x3 is the sodium periodate concentration. For the multiple correlation coefficient of the regression Eqs. (3) and (4), their R-squared value and Adj R-squared value were both[90 %, indicating the high fitting degree of the model and the low error of the experiment. Meanwhile, the lack of fit item had no p value, which confirmed the validity of the two regression equations. Besides, the larger the F value was, the greater the significance for the Yaldehyde content and Yfiber mass loss degree could be; therefore, according to the F values in Table S2 and S3 provided in the supporting information, it could be known that the concentration of sodium periodate had the greatest influence on the Yaldehyde content, while the influence of the reaction temperature was minimal. The importance order of these influence factors on the influence of Yfiber mass loss degree should be the concentration of sodium periodate [ reaction temperature [ reaction time. Moreover, the validity of the multiple linear regression equations can be further verified by comparing the relative errors of the actual values and predicted values of the aldehyde content and fiber mass loss degree in Table S4 provided in the supporting information, which were -4.61 to 5.92 and -5.93 to 5.89 %, respectively. Structural characterization of DAVF and DAVF-PEI FT-IR spectra of VF, DAVF, and DAVF-PEI Chemical changes of the fibers in each step were characterized by FTIR (Fig. 2). It could be seen that the FTIR spectrum of VF showed peaks at 3446 cm-1 (-O–H stretching), 2923 cm-1 (-C-H stretching), 1060 cm-1 (-C–O–C pyranose ring skeletal vibration), and 897 cm-1 (b-glycosidic linkages). Compared with the spectrum of the original VF fiber, the spectrum of DAVF showed new peaks at 1160 cm-1 (-C-O stretching of the -C–O–C group), 1735 cm-1 and 1647 cm-1 (-C=O stretching), and 2891 cm-1 (-C-H stretching in aldehyde) (Kumari 123 Fig. 2 FT-IR spectra of VF, DAVF, and DAVF-PEI and Chauhan 2014), suggesting the successful oxidation of VF. After amination with PEI, a new peak appeared at 1630.78 cm-1, which could be ascribed to –C=N (characteristic of imine). Moreover, a series of characteristic adsorption peaks of DAVF-PEI appeared at 3339.60 cm-1 (N–H stretching vibration), 2899.62 cm-1 (C–H stretching vibration), 1630.78 cm-1 and 1476.41 cm-1 (amine N–H deformation vibration), 1630.78 cm-1 (secondary amine), 1476.41 cm-1 (non-conjugated C–N bonds), 1422.69 cm-1 (C–N stretching vibration), and 1369.98 cm-1 (C–N stretching vibration in amines), which also helped to verify the successful preparation of DAVF-PEI. 13 C NMR spectra of VF, DAVF, and DAVF-PEI The solid-state 13C NMR (Fig. 3) studies further confirmed the chemical structures of DAVF and DAVF-PEI. The 13C NMR spectrum of the VF had strong peaks at 60.46 and 62.42 ppm (C-6), 72.01, 74.79, and 79.44 ppm (C-2, C-3, and C-5), 83.66 and 88.74 ppm (C-4), and 104.89 ppm (C-1), respectively (Kumari and Chauhan 2014). For DAVF, due to the influence of C=O, the chemical shift of the C-1 peak had an offset and became closer to that of C-4. Meanwhile, the absorption peaks at 65–80 ppm became much broader, which could be attributed to C=O (70.64 ppm) and C-2, C-3, and C-5 (72.01, 74.79, and 79.44 ppm). With respect to DAVF-PEI, two characteristic absorption peaks appeared at Cellulose (2016) 23:2539–2548 2545 Fig. 3 13C NMR spectra of VF, DAVF, and DAVF-PEI 105.28 ppm (C=N) and 45–50 ppm (C–N of the primary amine, secondary amine, and tertiary amines, respectively), indicating the successful amination of DAVF. Chemical composition of VF, DAVF, and DAVF-PEI Chemical compositions of the fibers were verified by EA and XPS. From Table 2 it could be found that DAVF contained no nitrogen, and after amination the nitrogen content of DAVF-PEI increased to 19.59 %. In the C1s spectrum of DAVF (Fig. 4a), there were three peaks at 287.99, 286.55, and 284.92 eV, which were attributed to the C=O, C–O–C/C–O–H, and C–C/ C–H groups, respectively. The C=O group content on the DAVF surface was calculated to be 42.08 %, demonstrating that a large number of aldehyde groups Table 2 Elemental analysis of VF, DAVF, and DAVF-PEI Materials C/% H/% N/% VF 38.80 6.59 0 DAVF 38.51 6.31 DAVF-PEI 45.53 8.51 0 19.59 were generated during the oxidation process. Moreover, the content of C–O groups, including C–O–C and C–O–H, made up 37.50 % of the C species on the DAVF surface. The abundant hydroxyl and ester groups proved that VF could be selectively oxidized by sodium periodate. The N1s spectra (Fig. 4b) could be resolved into four peaks at 400.81 eV (7.40 %), 398.73 eV (37.28 %), 398.09 eV (45.01 %), and 399.73 eV (10.31 %); these peaks were the characteristics of imine, primary amine, secondary amine, and tertiary amines, respectively. (Xu et al. 2015) These data provided assertive evidence of the covalent bonding between PEI chains and the DAVF surface. Adsorption behavior of DAVF-PEI for CO2 Effect of aldehyde content on the adsorption properties of DAVF-PEI To investigate the influence of the aldehyde group content on the adsorption performance of DAVF-PEI, a series of DAVF samples with different aldehyde contents (2.14–8.07 mmol/g) were prepared and grafted with PEI and then were applied to CO2 adsorption. Figure 5a shows that with the increase of 123 2546 Cellulose (2016) 23:2539–2548 Fig. 4 XPS spectra of DAVF and DAVF-PEI. a C1s spectrum of DAVF. b N1s spectrum of DAVF-PEI Fig. 5 Effect of aldehyde group content on nitrogen contents (a) and adsorption capacities (b) of DAVF-PEI Fig. 6 Breakthrough curves of CO2 adsorption on DAVF-PEI with different nitrogen contents aldehyde group content from 2.14 to 8.07 mmol/g, the nitrogen content increased from 6.37 to 19.59 %, indicating that the content of the aldehyde group would determine the loading amount of PEI. As a result, the CO2 adsorption capacity accordingly increased from 1.07 to 4.11 mmol/g. The breakthrough curves presented in Fig. 6 showed that DAVF-PEI was able to thoroughly adsorb CO2 at the initial phase, and the effluent CO2 concentration remained zero for a certain time before a breakthrough occurred. Afterwards, it was observed that the higher the aldehyde group content was, the longer time it would take to break through. Different from the CO2 adsorption capacity, the amine utilization efficiency increased first and reached the maximum value at an aldehyde content of 5.13 mmol/g, and then it 123 decreased with the further increase of the aldehyde group. This was because the reaction between PEI and DAVF occurred mainly through the substitution of oxygen by nitrogen and turned C=O groups into C=N groups. With low aldehyde content, the aldehyde groups could be far apart from one another in space so that one PEI chain would tend to react with a small number of aldehyde groups, leading to a low C=N content. But as the aldehyde content exceeded 5.13 mmol/g, the surface aldehyde density became larger, making each PEI chain react with more aldehyde groups and resulting in a much larger C=N content. Since the C=N groups could not react with CO2, the effective amine sites declined, as did the amine utilization efficiency. Cellulose (2016) 23:2539–2548 2547 The well-designed DAVF was grafted with PEI via Schiff base reaction to prepare a solid amine fiber (DAVF-PEI) for CO2 adsorption. The effect of the aldehyde content was investigated. The experimental results indicated that DAVF-PEI possessed remarkable CO2 adsorption capacity (4.11 mmol/g) and good regeneration properties. Our work can provide an effective and low-energy-consumption method for the preparation of an excellent CO2 solid amine adorbent. Acknowledgments The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant No. 51473187), Natural Science Foundation of Guangdong Province (2014A030313192). Fig. 7 Breakthrough curves of CO2 adsorption on fresh and regenerated DAVF-PEI adsorbent (a) and the corresponding CO2 adsorption capacities (b) Regeneration performance To satisfy the demands of practical use, the adsorbent was processed for ten adsorption-desorption cycles, and the results are shown in Fig. 7. After ten cycles, the adsorption capacity of the adsorbent only slightly decreased by 2.92 % (Fig. 7b). Furthermore, the breakthrough curves of fresh and regenerated fibers displayed in Fig. 6a clearly showed that the regenerated fibers exhibited nearly the same adsorption behavior as the fresh ones. All these results confirmed that the solid amine adsorbent DAVF-PEI could stay stable after multiple regeneration cycles and maintain its adsorption capacity for CO2. Conclusions RSM was employed to optimize the synthesis parameters of dialdehyde viscose fiber (DAVF). The constructed multiple linear regression equations of the aldehyde content and fiber mass loss rate can successfully predict the VF oxidation results when given the conditions. The optimum synthesis conditions were found to be a sodium periodate concentration of 0.35 mol/l, reaction temperature of 45 °C, and reaction time of 4 h. Under these conditions, the aldehyde content of DAVF can be as high as 8.07 mmol/g, while the VF fiber mass loss rate was controlled at 10.76 %. References Amini M, Younesi H, Bahramifar N, Lorestani AAZ, Ghorbani F, Daneshi A, Sharifzadeh M (2008) Application of response surface methodology for optimization of lead biosorption in an aqueous solution by Aspergillus niger. 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