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Letter pubs.acs.org/journal/ascecg
Photocatalytic Oxidation of Glucose into Formate on Nano TiO2 Catalyst Binbin Jin, Guodong Yao, Xiaoguang Wang, Kefan Ding, and Fangming Jin* School of Environmental Science and Engineering, State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P.R. China S Supporting Information *
ABSTRACT: Formic acid, as an excellent hydrogen-storage material, has recently become increasingly important. Previous conversion of biomass into formic acid suffers from problems of strict operation conditions such as the use of a high concentration of H2O2 oxidant and alkali at higher temperatures or the precious metal catalysts. Here, a straightforward photocatalytic method to selectively convert glucose into formate is first reported. Using a nano TiO2 catalyst, we show that glucose can be efficiently converted into formic acid with a high yield of 35% at ambient condition with 0.03 M NaOH concentration, and the proposed method also worked on xylose for formate production. The conversion of glucose into formic acid is mainly attributed to the accelerated formation of active oxidative radicals (O2•−, •OH) in the presence of hydroxyl ions, and also, hydroxyl ions adjust the charge of the TiO2 surface and control the adsorption of glucose and the desorption of formic acid. These results open up a new approach toward economical utilization of sustainable biomass and clean solar energy for formic acid production. KEYWORDS: Biomass conversion, Photocatalytic, Sustainable, Hydrogen storage
W
oxidant of H2O2 or O2 with a high concentration of alkali or the use of precious metal catalysts, which suffered from the challenge of intensive energy consumption and high alkali pollution, as well as a complex catalytic system. Photocatalytic conversion is considered as a promising technology for producing chemicals and energy from CO2, H2O, and biomass due to its excellent properties such as cleanness and energy savings.15−17 Developing artificial systems that can directly harvest and convert solar energy into usable or storable energy resources has been the dream of scientists for many years, and remarkable achievements have been obtained in controllable organic synthesis, CO2 reduction, and hydrogen production. However, compared to photocatalytic conversion of CO2 and hydrogen production, photocatalytic conversion of biomass is relatively minor, particularly for photocatalytic oxidation of biomass into formic acid/formate, which has not been reported previously. Generally, various radicals and photogenerated holes with strong oxidative activity would form in the photocatalytic reactions, particularly in the presence of alkali, as presented in the following reaction sequence:18,19
ith the increasing concerns over fossil fuel depletion and greenhouse gas emission, the use of biomass for the production of value-added chemicals or other energy products is becoming increasingly important since biomass is renewable, inexpensive, less polluting, and abundant. In the utilization of biomass for the sustainable production of high-value chemicals, biofuel, or hydrogen, various processes such as hydrothermal liquefaction,1−4 super/subcritical conversion,5 and fermentation6,7 have been proposed. Among the conversion of biomass into chemicals, formic acid/formate has attracted much attention recently because as an important chemical formate can serve as a leather tanning agent, as reducing agents in the dyeing industry, and as the raw material for environmentally friendly road deicer.8 More importantly, recent research has demonstrated that formic acid/formate has the potential to serve as an excellent hydrogen carrier in the context of a hydrogen energy economic picture because formic acid dehydrogenation can easily proceed under mild conditions by using catalysts.9−11 Thus, it is meaningful to produce formic acid/formate from biomass to solve the challenge of hydrogen production and storage simultaneously. We have investigated the conversion of glucose into formate under hydrothermal conditions with H2O2 as the oxidant, achieving 75% selectivity in formate production and 100% conversion in glucose.3 Recently, some interesting researches on catalytic conversion of biomass into formic acid have also been reported.12−14 However, the previous processes of biomass conversion into formic acid/formate generally involved strict operation conditions such as a high concentration of © 2017 American Chemical Society
TiO2 + hv → TiO2 (e− + h+)
(1)
h+ + OH− → •OH
(2)
Received: February 6, 2017 Revised: June 5, 2017 Published: June 21, 2017 6377
DOI: 10.1021/acssuschemeng.7b00364 ACS Sustainable Chem. Eng. 2017, 5, 6377−6381
Letter
ACS Sustainable Chemistry & Engineering O2 + e− → O2•−
(3)
O2•− + H 2O → •OH + OH−
(4)
possible that TiO2 was negatively charged in the alkaline medium.20,21 To test this assumption, the ζ-potential of the TiO2 catalyst was measured, and results showed that the TiO2 nanocrystals in the solution with 0.06 M NaOH had negative charges of −24.08 ± 0.15 mV (Figure 1), suggesting that
Currently, most efforts are focused on quenching these species by sacrificial agent addition in photocatalytic transformation. On the other hand, if these active oxidative species can be used to replace traditional oxidants (H2O2, O2) in the thermal-chemical conversion of biomass into chemicals, a new and green photo conversion of biomass without the addition of H2O2 or O2 oxidant can be achieved. Such a consideration inspired us to study photocatalytic conversion of biomass into formic acid/formate without the addition of H2O2 or O2 oxidant, and TiO2 was selected as the photocatalyst due to its high stability. In this study, alkali was used as the reaction accelerator, aiming to adjust the hydroxyl ions concentration because (1) according to photocatalytic reactions in eqs 1−4, the presence of alkali is favorable for the formation of radicals with strong oxidative activity, and (2) on the basis of electric double layer theory, the surface of TiO2 would be charged positive or negative in the solution under different pH values, affecting the adsorption of glucose and the desorption−adsorption of products. Results showed that the production of formate from glucose with 35% selectivity was achieved by using a general TiO2 catalyst under irradiation, and the control of alkali concentration was vital for the conversion of glucose into formate. To the best of our knowledge, it is the first observation of selective oxidative conversion of glucose into formate by using the combination of light and alkali to replace the traditional oxidant such as H2O2 or O2, and it also is the first time to directly improve the selectivity in target product formation through catalyst surface charge adjustment. First, glucose solution was irradiated in the presence and absence of NaOH at room temperature. As expected, in contrast to 9.7% glucose conversion in the absence of NaOH (Table 1, entry 2), the conversion of glucose was improved
Figure 1. Zeta potential of TiO2 in the presence and absence of alkali.
repulsive forces form between the TiO2 surface and formate ion (HCO2−) and then avoid the complete mineralization of formate ion. Namely, a “shield effect” is triggered in the presence of NaOH. Noteworthy, the reaction system temperature increased to 50 °C after light irradiation. To exclude the possibility that the conversion of glucose into formate was due to the thermal chemical effect, further experiments were conducted without irradiation. As shown in Table 1, trace formate formation (entry 4) was observed when no irradiation was applied. Clearly, the presence of irradiation was essential to drive the oxygenation process, and thus, the conversion of glucose into formate certainly was a photo driven process rather than a thermal driven process. However, a relatively high lactic acid yield was observed at the present conditions (50 °C). Generally, glucose is easily converted into lactic acid in the presence of alkali,22 and the enthalpy change for the conversion of glucose into lactic acid is approximately 74.28 kJ/mol (C6H12O6 → 2 C3H6O3 ΔH°298 = 74.28 kJ/mol), indicating that the lactic acid formation from glucose is endothermic. On the contrary, the oxidation of glucose into formic acid is typically exothermic. Thus, it is thermodynamically more favorable to produce lactic acid at higher temperatures, and a lower temperature is favorable for formate production. Then, we investigated the effect of temperature on formic acid and lactic acid production to enhance the yield of formate. As displayed in Figure 2, a decrease in the reaction temperature led to an increase in formate production. In contrast, the lactic acid production displayed a significant increase as the reaction temperature increased. As an important chemical, lactic acid has been widely applied in the chemical, food, and pharmaceutical industries, especially in the synthesis of lactic acid polymers due to its effective biodegradability.23,24 It is known that lactic acid is readily produced by the basecatalyzed conversion of carbohydrates. To investigate whether the irradiation can improve the formation of lactic acid from glucose in the present study, we conducted the experiments with and without irradiation. The results showed that no significant change in lactic acid yield occurred in the experiments with and without irradiation, and also a slight decrease in lactic acid yield was observed with irradiation (Figure S1). From these results, it is suggested that lactic acid is
Table 1. Effect of Reaction Conditions on Glucose Conversion and Selectivity in Formate Productiona entry
additive
glucose conversion (%)
1 2 3 4b
− TiO2 (10 mg) NaOH (0.06 M) + TiO2 (10 mg) NaOH (0.06 M) + TiO2 (10 mg)
trace 9.7 79.6 11.8
a
formate selectivity (%) 0 10.8 14.2 0
Irradiation, 3 h; 50 °C. bWithout irradiation.
about 8 times in the presence of NaOH (Table 1, entry 3), giving 79.6% glucose conversion. Meanwhile, we observed that a higher formate selectivity of 14.2% was also produced. These results demonstrated the photocatalytic conversion of glucose into formate, and the presence of alkali was crucial, which was mainly attributed to the role of oxidative radicals such as O2•− and •OH in the presence of NaOH. In addition to the oxidative role of radicals (O2•−, •OH), alkali (NaOH) might change the affinity strength of formate ions (HCO3−) with the TiO2 surface to avoid the complete mineralization of formate ion and then attain a high formate selectivity because formic acid is easily decomposed on a TiO2 catalyst. In the present reaction system, the pzc (pzc describes the condition when the electrical charge density on a surface is zero) pH was 6.8 for TiO2 particles (Degussa P25), and it was 6378
DOI: 10.1021/acssuschemeng.7b00364 ACS Sustainable Chem. Eng. 2017, 5, 6377−6381
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ACS Sustainable Chemistry & Engineering
less glucose was distributed into the solution, namely, the amount of glucose absorbed on the TiO2 catalyst increased. In the FT-IR analysis results of the collected catalysts (Figure 3c), corresponding enhanced absorption bands between 1500 and 1300 cm−1 were clearly observed, and the absorption strength increased with the increase in NaOH concentration. The bands between 1500 and 1300 cm−1 could be assigned to δCH2, δOCH, δCOH, and δCCH vibrations groups (Table S1).26 The appearance of the absorption bands would be attributed to the infrared radiation absorption of the glucose on the TiO2 surface. Similar observations about the FT-IR characterization for the adsorption of glucose on TiO2 were reported by Kim et al.27 Additionally, in the element analysis of the collected catalysts (Figure S2), a certain amount of carbon was also detected in the collected catalysts, and the residue carbon increased with the increase in NaOH concentration. With the consideration that glucose is a unique carbon resource in the present reaction system, the results suggested that the residue carbon is mainly from the adsorption of glucose on the TiO2 catalyst, which is consistent with FT-IR characterization results. Thus, it was reasonable to speculate that a higher glucose conversion rate with higher alkali concentration (0.06, 0.09, and 0.12 M NaOH) was because alkali improved not only the formation of oxidative substances but also the adsorption of glucose on TiO2 catalyst. However, from Figure 3a, a higher glucose conversion with a higher alkali concentration (0.06, 0.09, and 0.12 M NaOH) did not lead to a good selectivity of formate, while a good selectivity of formate occurred at a lower NaOH concentration of 0.03 M NaOH, which was probably because a higher alkali led to the mineralization of the formed formic acid. These results further show that control of NaOH concentration is key in the conversion of glucose into formate because the alkali can not only promote the oxidative species (e.g., •OH) formation for accelerating the oxidation of glucose into formic acid but can also stabilize the products (formic acid) by suppressing further mineralization. Figure 4a shows the effect of irradiation time on the formate yield. The formate yield increased from 0 to about 20 mM with
Figure 2. Effect of reaction temperature on the products distribution (TiO2, 10 mg; NaOH, 0.06 M; glucose, 0.01 M; 3 h).
formed by a base catalytic effect of OH−.25 When temperature decreased to 25 °C, 1.42 mM formate (23.4% selectivity) was produced, while the lactic acid formation was efficiently inhibited to 1.74 mM. Accordingly, a relatively lower temperature 25 °C was favorable for the conversion of glucose into formic acid. The role of alkali was further investigated by examining the products distribution and glucose conversion with the variation of NaOH concentration. As shown in Figure 3a, with the
Figure 3. (a) Effect of NaOH concentration on the products distribution (TiO2, 10 mg; glucose, 0.01 M; 25 °C; 3 h). (b) Effect of NaOH concentration on substrates distribution (TiO2, 10 mg; glucose, 0.01 M; 0 h). (c) FT-IR spectra of glucose, P25, and collected catalysts.
Figure 4. (a) Effect of reaction time on the products distribution (TiO2, 10 mg; NaOH, 0.03 M; glucose, 0.01 M; 25 °C). (b) IC variation in the solution (TiO2, 10 mg; NaOH, 0.03 M; glucose, 0.01 M; 25 °C).
increase in NaOH concentration, the conversion ratio of glucose displayed an increasing tendency. It is probably because higher hydroxyl ions concentration accelerates the formation of oxidative substances (e.g., •OH), leading to easy oxidation of glucose. Additionally, a higher NaOH concentration might increase the adsorption of glucose on TiO2, enhancing the oxidation of glucose. Then, to test the effect of alkali concentration on the adsorption of glucose on TiO2, glucose dispersion concentrations in the solution with different NaOH concentrations before irradiation were measured. Clearly, as shown in Figure 3b, with the increase in NaOH concentration,
the increase in irradiation time, and the selectivity in formate reached the highest value of 35% at 9 h. Moreover, accompanied by the increase in reaction time, complete conversion of glucose and 33.1% selectivity in formate production were obtained at 12 h. A further increase in the irradiation time gradually decreased the formate yield, which was most likely because of the further oxidation decomposition of the formic acid. It was supported by the increasing inorganic carbon (IC) level as shown in Figure 4b. Furthermore, it was 6379
DOI: 10.1021/acssuschemeng.7b00364 ACS Sustainable Chem. Eng. 2017, 5, 6377−6381
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ACS Sustainable Chemistry & Engineering Notes
noteworthy that the conversion rate of glucose decreased with the increasing reaction time, while a higher selectivity of formate production was observed. It was probably because OH− and O2, which supported the formation of reactive species (e.g., •OH, O2•−), were consumed with the reaction, slowing the reaction rate. We further investigated the conversion of xylose, the second most abundant monosaccharide in nature, into to formate. As shown in Table 2, the formate yield with xylose was slightly
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21277091 and 51472159), State Key Program of National Natural Science Foundation of China (No. 21436007), and Key Basic Research Projects of Science and Technology Commission of Shanghai (14JC1403100).
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Table 2. Effect of Different Catalysts on Glucose and Xylose Conversiona
a
substrate
catalyst
conversion (%)
formate (mmol/L)
glucose xylose glucose glucose
TiO2 TiO2 ZnO Fe2O3
51.10 65.04 34.97 10.89
8.50 10.21 4.19 0.65
Catalyst, 10 mg; Substrate, 0.01 M; NaOH, 0.03 M; 3 h; 25 °C.
higher than that with glucose, suggesting that the conversion method also worked on the conversion of xylose. Thus, the proposed methods are suitable for the conversion of both hexose and pentose. Finally, the photocatalytic activity of another two representative photocatalysts, ZnO (99.9%, Aladdin, 30 ± 10 nm) and α-Fe2O3 (99.5%, Aladdin, 30 nm), were also investigated. The results showed that the glucose conversion and the formate yield with both ZnO and α-Fe2O3 were much lower than those with TiO2, suggesting that the photocatalytic activities of ZnO and α-Fe2O3 were limited under the present conditions. Additionally, the stability of TiO2 was investigated. As displayed in Figure S3, no significant decrease in formate yield was observed in the recycle tests. It means that TiO2 could keep suitable stability in the present conditions for recycle use.
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CONCLUSION In summary, an efficient photocatalytic oxidization of glucose to formate has been developed under mild conditions by using a general TiO2 catalyst. A 35% yield of formate and high glucose conversion up to 100% were achieved at 25 °C with 0.03 M NaOH concentration. A significantly selective production of formate from glucose is mainly because hydroxyl ions can accelerate the utilization of photogenerated electrons and holes for the formation of active oxidative radicals (O2•−, •OH) and also trigger a unique “shield effect” for glucose adsorption and formic acid desorption.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00364. Experimental details, materials, methods, and characterizations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (F. Jin). ORCID
Fangming Jin: 0000-0001-9028-8818 6380
DOI: 10.1021/acssuschemeng.7b00364 ACS Sustainable Chem. Eng. 2017, 5, 6377−6381
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