Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Organic−Inorganic Composites Based on Gel-Type Sulfonic Resin KU-2‑8 and Zirconia: Acid and Catalytic Properties in the Etherification Reaction of iso-Butylene with Ethanol Nina V. Vlasenko,* Yuri N. Kochkin, Tamila G. Serebrii, and Peter E. Strizhak
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LV Pisarzhevskii Institute of Physical Chemistry, National Academy of Sciences of Ukraine, Prosp. Nauky 31, Kyiv, 03028, Ukraine ABSTRACT: Although acid and catalytic properties of the sulfonic resins and zirconia are well-characterized, a little is known about their composites, particularly about the KU-2-8-ZrO2 composites, in which KU-2-8 is a commercial gel-type sulfonic resin. In this article, the acidity of the KU-2-8-ZrO2 composites has been studied by the ammonia quasi-equilibrium thermo-desorption technique and their catalytic characteristics in etherification of isobutylene with ethanol were defined. The KU-2-8-ZrO2 productivity in this reaction was found to be an order of magnitude higher compared to KU-2-8 and several times higher compared to the commercial macroporous sulfonic resin Amberlyst 15. A high productivity of the KU2-8-ZrO2 composites is achieved by the improved accessibility of the sulfonic resin acid sites for reagents. The composites do not swell in the polar medium and show high resistance to osmotic shock. The KU-2-8-ZrO2 composites with 70−80 wt % of KU-2-8 were found promising for industrial applications.
1. INTRODUCTION Among other applications, sulfonic resins are used as catalysts for the industrial production of alkyl tert-butyl ethers.1−5 Macroporous strongly acidic ion-exchange resin Amberlyst-15 has found industrial application for these purposes.1−6 The geltype sulfonic resins which are available on the market, particularly, Lewatit MonoPlus S 100 (LANXESS, Germany) and KU-2-8 (PJSC Azot, Cherkasy, Ukraine), are less used due to their worse catalytic performance. As compared to the macroporous sulfonic resins, the gel-type sulfonic resins have lower mass transfer characteristics limiting their use in catalysis, particularly under conditions of the flow reactor. The catalytic activity of sulfonic resins is limited mainly by the accessibility of their acid sites for reacting molecules.7−9 Moreover, the accessibility of the acid sites depends on the swelling of the polymer matrix in the reaction medium.10 The effect of swelling is less pronounced for Amberlyst 15 due to the macroporous structure of the sulfonic resin which is formed by the agglomerates of gel particles. As a result, most of the sulfonic groups are available for reagents even in the nonswollen state of the polymer.11−13 The gel-type sulfonic resin KU-2-8 does not have a porous structure.14 Most of the sulfonic groups are located within the granules.15 They are accessible to reagents only in a polar medium which provides swelling of the polymer matrix. In a nonpolar reaction medium, the accessibility of the acid sites in the gel-type sulfonic resin may be enhanced by decreasing the size of the catalyst granules. The main disadvantage in the practical use of the sulfonic resins as catalysts concerns their ability to swell in a polar medium. The catalyst swelling results in two undesirable consequences. The catalyst volume increases decreasing the © XXXX American Chemical Society
reactor free space resulting in increasing the hydraulic resistance of the system. Swelling also may decrease the lifetime of a catalyst caused by the catalyst destruction by a reaction media. Both these disadvantages can be overcome by using the composite catalysts which have the finely dispersed phase of a sulfonic resin distributed in the pores of the mineral component.16−18 These organic−inorganic composites possess simultaneously both the properties of the inorganic matrix and the organic component. In this connection, the fields of application of the organic−inorganic materials in sorption and catalytic processes are expanding. Moreover, organic− inorganic materials, due to their specific properties, are widely used in absolutely different fields of technology, for example, in the manufacture of membranes,19,20 sensors,21,22 electronic applications,23−25 nanocomposites,18 photovoltaic applications,26,27 biomaterials,28,29 fuel cells,30 solar cells,31−34 surface modification,35 flame retardants,36 and catalysis.37−55 The organic−inorganic composites may exhibit a high catalytic performance.50−55 Particularly, the use of the sol− gel technology resulting in the synthesis of the zeolite-based organic−inorganic acid composites having catalytic characteristics at the level of the industrial catalysts.56−60 Another example is given for the sulfonic acid resin Nafion deposited on mineral component.43,47,48 For example, Nafion deposited on Al2O3 is characterized by a high selectivity for the diisopropyl ether synthesis from isopropyl alcohol.61 The main attractive feature of the organic−inorganic acid catalysts is the possibility Received: Revised: Accepted: Published: A
April 16, 2018 June 18, 2018 July 23, 2018 July 23, 2018 DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
step size of 20 °C) and pumping out with holding at constant temperature until constant weight was achieved (typically 10− 15 min). Thus, experimental conditions corresponded to desorption at constant pressure, that is, desorption isobar. The total concentration of acid sites (C∑) was determined by the amount of ammonia sorption on a sample surface at 50 °C. The concentration of weak acid sites evaluated determined by the amount of ammonia desorbed in the temperature range 50−150 °C. The concentration of strong acid sites was determined by the residual amount of ammonia adsorbed at 150 °C. 2.4. Catalytic Experiments. The catalytic properties of organic−inorganic composites were studied in the ETBE synthesis. Experiments were carried out in a flow reactor with a fixed bed under 1 MPa in the temperature range 60−140 °C. Catalyst loading was 1.5 cm3, grain size of 1−2 mm, mass of catalyst was ∼0.5 g. The molar ratio ethanol/isobutylene was 1.5; LHSV of 1 h−1, carrier gas, helium (30 cm3/min). Ethanol and isobutylene were supplied out from vessels standing under 1.2 MPa pressure with the help of a dropwise sampler and “Whitey” needles. The space velocity of carrier gas (helium) was regulated by a needle and was registered by a differential manometer. The temperature in the reactor was established and controlled by the controller/programmer type 812 “EUROTHERM” (Great Britain). The pressure in the reactor and in the vessels with ethanol and isobutylene was registered by manometers. Reaction products were analyzed by gas chromatograph Agat (“Mashpriborkomplekt”, Russia) equipped with Chromaton N-AW column with 10% Carbowax 600 (3 mm i.d., 2 m length) and a thermal conductivity detector.
of forming the nonswelling granules of the required dimensions.62−64 A mineral component of the catalyst may not only prevent swelling but also enhance the resistance to osmotic shock of the catalyst that results in increasing the lifetime of the catalyst. A mineral component also increases the thermal stability of a composite catalyst which is of particular interest for acid catalysis. This work is aimed toward a development of the osmotically strong, nonswollen organic−inorganic composite catalysts for the ethyl tert-butyl ether synthesis. The organic part of the composite is based on the gel-type sulfonic resin KU-2-8. The organic−inorganic composite catalysts are prepared by a precipitation of the mineral component in the presence of the well-dispersed sulfonic resin followed by the calcination. The choice of zirconia as a mineral component of the composite catalyst is determined by a possibility of the zirconia solid phase formation from a precursor at a moderate temperature below the thermal stability of KU-2-8.65
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. We have investigated a series of organic−inorganic composites based on zirconia. As an active organic component, the industrial gel-type sulfonic resin KU-2-8 (H-form) was used as the polydisperse fraction. This polydisperse fraction was obtained by grinding the initial granules of 1−2 mm in a Fritsch Pulverisette 6 planetary ball mill. Mechanochemical treatment was carried out with silicon nitride balls 20 mm in diameter at 300 rpm for 1 h. The size of the obtained particles KU-2-8 determined by transmission electron microscopy was 10−50 nm. The ratio of KU-2-8 to metal balls in a ball mill was 1/6. The resulting fraction of the sulfonic resin in an amount corresponding to its required content in the composite was mixed with 50 mL of the Zr(OH)4 water suspension for 2 h. The mixture was kept for 10−12 h. The obtained solid was filtered and dried at room temperature. Then obtained solid was dried at 95 °C for 4 h and calcined at 250 °C for 30 min (temperature rising rate is 5 °C/min). The fraction with the size of 0.25−0.50 mm was selected for catalytic tests. At a varying initial concentration of Zr(OH)4 we obtained solids with the KU-2-8 content of 10, 20, 30, 40, 50, 60, 70, 80, and 90 wt %. 2.2. Resistance to Osmotic Shock. The resistance to osmotic shock for the prepared organic−inorganic composites was estimated qualitatively in a procedure similar to that described in the literature.66 The composite was kept in water for 1 h, then filtered and dried at 150 °C for 1 h. The procedure was repeated 10 times. If the composite was not destroyed, it was characterized by the high resistance to osmotic shock. Our studies have shown that all samples are characterized by the high resistance to osmotic shock, except the composite with the 90% KU-2-8 content which was totally destroyed during the treatment. Acidity measurements and catalytic studies were not performed for this sample. 2.3. Acidity Measurement. The acid properties of the samples were investigated by quasi-equilibrium thermal desorption (QE-TD) of ammonia, as described in ref 67. The samples were outgassed in the vacuum (0.133 Pa) at 150 °C until constant weight was attained. Ammonia sorption was carried out at room temperature until no change of weight was observed, and the surplus of ammonia was removed by pumping out. Then thermodesorption of ammonia was carried out in a quasi-equilibrium regime: the weight was recorded with the stepwise temperature rising (5 °C/min between steps,
3. RESULTS AND DISCUSSION 3.1. Acid Properties of Organic−Inorganic Composites. Figure 1 gives the dependences of total acid capacity, the
Figure 1. Dependence of the acid capacity of the organic−inorganic composites on the KU-2-8 content.
concentration of strong internal acid sites and the concentration of the weak external acid sites on the KU-2-8 content. The data at the two ends of the graph represent the behaviors of the pure constituents of the composites, to the left, pure ZrO2 phase and to the right, pure sulfonic resin KU-2-8. The ammonia sorption on the sulfonic resins involves two processes.68 The first, reversible process corresponds to the weak ammonia adsorption on the freely accessible acid sites which are located on the external surface of the sulfonic resin. B
DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 1. ETBE Synthesis over KU-2-8-ZrO2 Composites
Perhaps, the grinding of the initial bulk KU-2-8 results in the localization of some fraction of the sulfonic groups on the external surface of polymer micropellets. The inorganic component stabilizes fine particles of the sulfonic resin, preventing their sintering. In the composites with the high content of KU-2-8 above 70%, the internal acid sites are characterized by the highest concentration. Decreasing the content of the sulfonic resin below 70% results in increasing the concentration of the weak external acid sites and their concentration is always higher compared to the concentration of the strong internal acid sites. 3.2. Catalytic Performance of the Organic−Inorganic Composites. Catalytic studies show that pure ZrO2 does not exhibit appreciable catalytic activity in the ethanol etherification with isobutylene. The rest of the solids are fairly active in the temperature range of 60−140 °C. Therefore, as a matter of fact, zirconia plays a role of diluent in the composition of KU-2-8-ZrO2 catalysts. Besides the main product, ethyl-tertbutyl ether (ETBE), the only one byproduct, tert-butyl alcohol, was detected. Table 1 gives the data on the isobutylene conversion and the ETBE selectivity over the organic−inorganic composites. The pure KU-2-8 is characterized by the highest conversion of isobutylene. However, it has a low selectivity. Among the organic−inorganic composites, the highest conversion is achieved for a sample with 80% of KU-2-8. Conversion decreases with increasing the content of the inorganic component. The sample with 30% content of KU-2-8 shows anomalously low conversion, whereas acidic properties of this solid allow one to expect significantly higher activity. All solids show a nonmonotonic dependence of the conversion on the temperature. The temperature dependence of the conversion has a maximum at the optimal temperature of the process, Topt. The optimal temperature of the process shows a trend to decrease with increasing the content of the sulfonic resin in the composite. Moreover, this trend follows the dependence of the acidic properties of the organic− inorganic composites on the KU-8-2 content which is highlighted by gray in the Table 1. The optimal temperature of the process is 140 °C for the solids belonging to the first
This process is characterized by a low heat of the ammonia adsorption. The second process is characterized by the high heat of the ammonia adsorption which is close to the heat of the chemical reaction between ammonia and the sulfonic group. According to the second process, ammonia slowly diffuses through the polymer matrix to the internal acid sites and strongly interacts with them. In our study, we assume that the amount of ammonia desorbed in the temperature range 50−150 °C corresponds to the adsorption on the weak external acid sites of the sulfonic resin, whereas the amount of ammonia which remains at 150 °C reflects adsorption on the strong internal acid sites. Figure 1 shows that the concentration of acid sites varies nonmonotonically with an increase in the content of the sulfonic resin. The nonmonotonic tendency may be a result of the heterogeneity of the composition of two solids if each component has its own acidity. Increasing the content of the mineral component leads to a decrease of the composites total acidity. The dependence of the total acid capacity on the KU2-8 content has three local maxima: at 20, 50, and 80 wt % of KU-2-8. Accordingly, the dependence is characterized by two inflection points, at 40 and 60 wt % of KU-2-8. Therefore, reflecting a difference in the acidic properties, all samples may be divided into three groups. The first group is characterized by the KU-2-8 content below 40 wt % The second group has the KU-2-8 content in the range of 40−60 wt %. For the third group, the KU-2-8 content is above 60 wt %. These three groups are separated by vertical dashed lines in Figure 1. Increasing the KU-2-8 content follows the general trend in increasing the acidity of solids. However, inside each group, increasing the KU-2-8 content results in an appearance of a local maximum on the dependence of the total acidity, as well as internal and external acidity, on the KU-2-8 content. The inorganic component, ZrO2 is characterized by a relatively low acid capacity, which includes mainly external acid sites with a small content of internal acid sites. KU-2-8 has significantly higher acid capacity as compared to ZrO2. The acidity of KU-2-8 is formed exclusively by the internal acid sites. The addition of zirconia to the sulfonic resin results in the appearance of the external acid sites in the composites. C
DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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comparing to the pure Amberlyst-15 which exhibits the ETBE productivity of 5.8 mol/(s·gcat) at the same temperature.69 Our results indicate an unexpectable effect of the diluting the sulfonic resin by ZrO2 on the catalytic performance of the organic−inorganic composites KU-2-8-ZrO2. The effect is well pronounced for the value of the productivity which is related not to the total catalyst weight but to the weight of the sulfonic resin in the catalyst. Figure 2 shows the dependence of the
group with the KU-2-8 content less than 40 wt %. The second group of solids with the KU-2-8 content between 40 and 60 wt % is characterized by the optimal temperature of 120 °C. Finally, the lower value of the optimal temperature of the process, 100 °C, is observed for the third group of solids with the KU-2-8 above 60 wt %. All KU-2-8-ZrO2 composite catalysts are characterized by the higher selectivity compared to the pure sulfonic resin KU2-8. The highest selectivity of 90% is reached for the composite with the highest content of KU-2-8 (80%). Most of solids show the highest selectivity at a temperature of 140 °C. For industrial application, the ETBE productivity, calculated as the amount of product (moles of ETBE) formed on the unit of catalyst’s weight per second, is the relevant value for estimating the catalyst performance. Table 2 gives the ETBE productivity for various organic−inorganic composite solids KU-2-8-ZrO2 at various temperatures. Table 2. ETBE Productivity (r × 106 mol/(s × gcat)) of the Pure KU-2-8 and Various Composites KU-2-8-ZrO2 KU-2-8 content, wt %
60 °C
80 °C
100 °C
120 °C
140 °C
100 80 70 60 50 40 30 20 10
0.6 1.2 1.4 0.3 0.6 0.5 0 0.2 0.5
1.0 4.2 3.4 1.0 0.8 1.1 0 0.2 0.1
1.0 7.1 10.7 2.6 2.6 2.9 0.1 0.7 0.3
1.2 6.3 8.0 3.7 3.9 4.0 0.3 1.8 0.9
1.0 4.1 4.7 2.4 2.9 2.6 0.7 2.1 1.2
Figure 2. Dependence of the ETBE productivity (per gram of sulfonic resin) on the KU-2-8 content in the organic−inorganic catalysts.
ETBE productivity calculated in this way on the content of the sulfonic resin at various temperatures. The results presented in Figure 2 clearly show that the dilution of the KU-2-8 by ZrO2 significantly enhances the activity of sulfonic resin in the composite solid. Particularly, the catalytic activity of the sulfonic groups for the most active composite with 70% of KU2-8 is almost 16-times higher as compared to that for the pure sulfonic resin. The catalytic studies of the organic−inorganic acid composites presented in the literature show that the porous structure of the mineral carrier and the ordered arrangement of grafted organic groups on its surface increase the acid sites accessibility for reagent molecules, which increases both the activity of the acid catalysts and their productivity to the target product. As a result, the organic−inorganic acid materials may show higher catalytic performance as compared to the traditional acid catalysts.50−54 Particularly, the sulfonic acidfunctionalized MCM-41 is characterized by higher activity in the bisphenol synthesis comparing to zeolites H-ZSM-5, H−Y, and H-beta.50−52 Moreover, the phenol conversion and the selectivity to bisphenol is comparable to that of the sulfonic acid resin Amberlite-120, whereas the concentration of the acid sites of the sulfonic acid-functionalized MCM-41 is four times lower compared to that of the Amberlite-120. The sulfonic acid-functionalized MCM-41 gives a higher yield in the monoglycerides synthesis by esterification of glycerol with fatty acids compared to H-USY and Amberlyst-15.53 The high yield of dibutyl ether by the dehydration of butanol was found for the sulfonic acid functionalized SBA-15, which was attributed to the silica surface hydrophobicity that reduces the acid site deactivation by the water adsorption.54 The high catalytic activity of propyl sulfonic acid functionalized mesoporous silica was shown in the esterification of fatty acids.55 The propyl-SO3H-functionalized SBA-15 was reported as the best catalyst for the methyl oleate synthesis which performance exceeds even the commercial Amberlyst-15.55 All
ETBE productivity for gel-type sulfonic resin KU-2-8 is relatively low; the optimum temperature is 120 °C. However, already a small dilution of sulfonic resin by inorganic components (up to 70−80 wt % KU-2-8) contributes to a significant (7−8 fold) increase in the catalyst’s productivity, while reducing in the optimum temperature of process to 100 °C. A further increase of ZrO2 content in composite leads to a decrease in productivity and an increase in the optimum process temperature. The data presented in Table 2 clearly show that the organic−inorganic composites KU-2-8-ZrO2 containing 70% and 80% of the sulfonic resin exhibit the highest productivity. This may be associated either with the highest total acidity or with the highest content of the internal acid sites for these solids as it follows from the data presented in Figure 1. A comparison of the data presented in Table 2 and Figure 1 does not show a correspondence between the productivity and the total acidity of the solids. This is associated with the presence of the ZrO2 acid sites which are catalytically inactive. Recently we have shown that the strong acid sites of sulfonic resins localized within the polymer network are more active in the ETBE synthesis than the weak acid sites localized on the external surface.69 Therefore, the highest catalytic performance of the solids containing 70% and 80% of the sulfonic resin is mainly caused by the highest content of the internal acid sites. Among all studied solids, the organic−inorganic composite KU-2-8-ZrO2 containing 70% of the sulfonic resin has the highest productivity, 10.7 mol/(s·gcat), at the temperature of 100 °C. Unpredictably, the ETBE productivity of this solid is 10 times higher compared to the pure KU-2-8. Moreover, the ETBE productivity of this solid is almost two times higher D
DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
directions. As a result, the organic−inorganic composites KU2-8-ZrO2 with the KU-2-8 content of 70% shows a superior catalytic performance exhibiting an order of magnitude higher catalytic activity comparing to pure KU-2-8. These results highlight a way to enhance the catalytic performance of the sulfonic resins not only for the ETBE synthesis but also for other acid−base catalytic processes.
these examples show that combining both advantages of organic and inorganic components is a good strategy for designing new effective acid catalysts. Moreover, our studies show that even relatively small amounts of the inorganic component may significantly enhance the catalytic performance of the organic−inorganic composite. Comparison of the data presented in Figure 1 and Figure 2 indicates similar trends for both dependencies. This is a consequence of the relationship between the acidic and catalytic characteristics of the organic−inorganic composites. The dependence of the ETBE productivity on the KU-2-8 content in the organic−inorganic catalysts shows three local maxima which follow the local maxima for the dependence of the acid properties of these solids on the KU-2-8 content. For the first group, which is characterized by the KU-2-8 content below 40 wt %, the productivity maximum reaches at 10% of KU-2-8. For the second group with the KU-2-8 content in the range of 40−60 wt %, the productivity maximum reaches 40% of KU-2-8. For the third group with the highest KU-2-8 content, the maximum reaches at 70% of KU-2-8. The catalytic properties of the acid catalysts are directly correlated with their surface acidic characteristics since the chemical process mainly occurs on the surface of a catalyst. Particularly, it is well-established that the activity of the acid catalysts is strongly related to the density of acid sites responsible for the activation of reagents. The sulfonic acid containing materials are commonly used as solid acid catalysts in many reactions which are catalyzed at Bronsted acid sites. Sulfonic resins are used in processes proceeding under mild conditions, for example, etherification11−14,69 and dehydration.15 The catalytic activity of the sulfonic-acid based organic−inorganic composite depends on the density of acid sites, for example, sulfonic groups, and their accessibility. Our studies show that organic−inorganic composites may combine both the high density of acid sites and their enhanced accessibility. Our results also show that the impact of an inorganic component on the catalytic performance of the organic− inorganic composites causes a certain decrease in activity, which is manifested in a decrease in the conversion values. However, the selectivity of the composite catalysts is higher compared to the pure sulfonic resin. As a consequence of these opposing trends, the optimum composition of the composite catalyst contains a relatively small amount of inorganic component resulting in a solid containing 70 wt % KU-2-8 and 30 wt % ZrO2. The productivity of this composite catalyst in the ETBE synthesis is an order of magnitude higher compared to that of the pure KU-2-8, whereas the optimal temperature for the ETBE synthesis is lower.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Nina V. Vlasenko: 0000-0003-2162-5969 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Dr. V. L. Struzhko for preparing the samples of the organic−inorganic composites.
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REFERENCES
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4. CONCLUSION The organic−inorganic composite KU-2-8-ZrO2 with the KU2-8 content of 70%, which is prepared by mixing of the finedispersed fraction of sulfonic resin with the Zr(OH)4 water suspension followed by calcination, is developed as a highperformance catalyst for the ETBE synthesis by the etherification reaction of isobutylene with ethanol. The catalyst shows high resistance to osmotic shock. Only the sulfonic resin KU-2-8 plays its function for the etherification reaction, whereas acid properties of the organic−inorganic composites are defined by both components, KU-2-8 and ZrO2. Varying a content of the inorganic component in the organic−inorganic composite changes the activity and selectivity in opposite E
DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
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DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.iecr.8b01587 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
