TiO2 Catalyst and the Performance of Liquid

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Preparation of Au/TiO2 catalyst and the performance of liquid methanol catalytic oxidation to formic acid Da SHI, Jianfang Liu, and Shengfu Ji Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02506 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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Industrial & Engineering Chemistry Research

Preparation of Au/TiO2 catalyst and the performance of liquid methanol catalytic oxidation to formic acid Da Shi, Jianfang Liu, Shengfu Ji* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China * Corresponding author. Tel: +86-010-64419619; Fax: +86-010-64419619. E-mail address: [email protected]

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Abstract： Loading gold on TiO2 to prepare heterogeneous catalysts and use XRD, SEM-EDS, TEM and Raman to test the structure of the catalysts. The aerobic oxidation of aqueous methanol to produce formic acid was studied using gold catalysts. The results show that Au nanoparticles are highly dispersed on the TiO2 supporter with 4.4nm average particle sizes. By proper selection of the reaction conditions about temperature, pressure, amount of base and catalysts and time, yield of 51.4% of formic acid could be achieved at moderate temperatures and pressures. The preliminary mechanism of the oxidation pathway was also studied that the dehydrogenation of methanol to produce formaldehyde happened at first and following the continuous oxidation to formic acid. The CO2 as the major byproduct was produced from the intermediate. Key Words: Au/TiO2; O2; methanol; formic acid 1. Introduction Methanol, one of the simplest alcohols, is one of the most important chemical raw materials that can be used in energy as fuels or synthesis dye, perfume or explosive materials 1-3. In the transformation of Methanol catalysis, there are many high value-added chemicals include formaldehyde, DMM, MF, etc 4-6. However, the traditional preparation method has a long reaction process route with pollution and may cause reaction equipment corrosion seriously 7-9. For example, in the gas-phase conversion of methanol-to-methanol, methanol is oxidized to formaldehyde and then oxidized to formic acid. The feed concentration of methanol is low, and a large amount of gas is needed to circulate and the reaction selectivity is low 10,11. In the preparation of formic acid by liquid-phase methanol conversion, methanol is first converted to methyl formate and then to formic acid via hydrolysis. This process route is long, there is much wastewater discharge and the corrosion of the reaction pipe is serious. These preparation processes are time consuming with high cost 12,13. Once the high efficient catalyst is adopted, the pollution of the reaction can be reduced and the steps can be cut down, thereby strengthening the reaction process 14,15. Recent researchers have found that gold catalysts show high activity for alcohols selective oxidation 16. R. Wojcieszak 17 used Au/SiO2 as catalysts for methanol gas phase one-step oxidation to methyl formate. The methanol could achieve 88% of conversion for methanol and 72% of selectivity and 63% of yield for methyl formate under 80℃. Whiting 18 prepared Au-Pd/TiO2 catalysts for methanol gas phase oxidation to methyl formate and they could achieve 7% conversion of methanol and 100% selectivity and 7% yield of methyl formate under the following conditions as MeOH: O2: He with a molar ratio of either 5: 10: 85, or 5: 2.5: 92.5 and a total flow rate of 60 mL min-1 (GHSV = 12000 h-1). Also, the researchers found the reaction would happen on the surface between gold and TiO2 19. Also, many researchers tried to use gold nanoparticles to catalyze alcohols oxidation reactions such as aromatic alcohols, primary alcohols and secondary 2


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alcohols with oxygen or air 20,21. Following research, also find gold has obvious size effect that the smaller particle size always shows higher activity 22-24. Takashi 25 used various supporters loading Au for ethanol oxidation reaction and found TiO2 showed high conversion and selectivity for the reaction. As far as we know, there are no related literature reports about methanol liquid phase oxidation preparation of formic acid. According to the excellent performance of Au and TiO2 for the selective oxidation of alcohols, in this paper, Au/TiO2 catalyst was prepared and characterized by XRD, SEM-EDS, TEM and Raman. The structure of the catalyst, especially the nano-Au active component, was characterized. Then the performance for the production of formic acid by liquid phase transformation of methanol was studied through batch reactor. The relationship between different reaction conditions and catalyst performance were also investigated. 2. Experimental Section 2.1 Preparation of Catalysts This paper chose P25 as supporter and used impregnation method as reference 26. The typical preparation process is as following: weigh certain amount of HAuCl4·3H2O and dissolve it into deionized water to prepare solutions of the desired gold concentration (1.26 × 10−4 M). Then the supporter was added into the solution to make sure the loading weight percent from 0.2wt% to 1wt% for 12h. After that, fresh solutions of 0.1M NaBH4 (NaBH4/Au (mol/ mol) = 5) were added dropwise to each with stirring 3h. The slurry was filtered, and the resulting solid was washed thoroughly with distilled water. The catalysts dried under room temperature overnight to produce Au/TiO2. 2.2 Characterizations The X-ray diffraction (XRD) pattern was collected on a D/Max 2500 VB 2+/PC diffractometer (Rigaku, Japan) with Cu –Kα irradiation (λ = 1.5418 Å, 200 kV, 50 mA) in the range of 2θ value between 10° and 80°. Transmission electron microscopy (TEM) was performed with a ZEISS SUPRA55 transmission electron microscope operated at 200 kV accelerating voltage. Scan electron microscopy was operated by ZEISS scan electron microscope. EDS was operated by Oxford instrument. Raman spectrum was tested by LABRAM ARAMIS Raman spectroscopy (Horiba Jobin Yvon, France) with Nd:Yag laser source (λ = 532nm). 2.3 Catalytic Activity Test All catalytic activity test experiments were conducted in self-builded stirred batch reactor autoclaves with a total volume of 100 mL. First, 30 mL of 0.3mol/L methanol solution was transferred to the autoclave with the catalyst according to the molar ratio between methanol and Au from 500 to 2000. Following the certain amount of NaOH was added into the reaction system from 0 equivalent to 3 equivalent. Then the desired pressure from 10 to 35bar of technical air was applied in the autoclave. The autoclave was heated under stirring and kept at the reaction temperature for the desired reaction time, then cooled in an ice bath to room temperature. The gas phase was collected by collecting bag and analyzed on a Shimadzu GC-9A gas chromatograph equipped with a Nukol capillary column (15 m × 0.53 mm) by TCD detector. The liquid phase was separated by centrifuge and analyzed on Shimadzu 3



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Nexera-HPLC(Alltech OA-10308, 300 mm × 7.8 mm ). The results of GC and HPLC were calculated based on carbon balance. The conversion of methanol was calculated according to the molar before and after reaction. The selectivity of methyl formate was calculated based on carbon balance results. For the reusability test, after reaction the mixture was filtered and wash by water. Then dry the catalysts under room temperature overnight and weight it to calculate the desired volume of methanol solution. 3. Results and Discussion 3.1 XRD Figure 1 is the XRD spectrums of Au/TiO2 with Au loading from 0.2wt% to 1wt%. According to the XRD spectrums, the samples show obvious characteristic peaks of TiO2 at 25.3o, 37.8o and 48.1o 27. As to gold, the weak peaks only can be seen when the weight percent reaches to 1wt% at 44.4o 28. There are no peaks for gold when the weight percent from 0.2wt% to 0.6wt% until it reaches to 0.8wt%. According to the reference about silicon-gold nano-shell, the peak at 44.4o belongs to Au (200) crystal surface 29. When the loading percent is lower, it is hard to detect the nanoparticles due to smaller particle size and higher dispersity.

Figure 1 XRD spectrum of Au/TiO2 with different Au weight percent (A, TiO2, B, 0.2wt% Au/TiO2, C, 0.4wt% Au/TiO2, D, 0.6wt% Au/TiO2, E, 0.8wt% Au/TiO2, F, 1wt% Au/TiO2) 3.2 SEM-EDS and TEM According to the mapping results of SEM-EDS, the gold nanoparticles load on TiO2 with high dispersity corresponding with TEM results. The weight percent of gold is 0.91wt% after EDS mapping test (Figure 2), which is close to the theoretical one. In order to correspond with this result, we also did ICP test for 1wt%Au/TiO2 and the result is 0.90wt% and close to EDS one. From Figure 3, the HRTEM images show that the gold nanoparticles immobilize into the pore of TiO2 and on the supporter surface with the diameter of around 3-4nm as to the distribution graph. The 4


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average size is 4.4nm after calculating more than 200 nanoparticles. Compared with other traditional methods, the gold nanoparticles have smaller size 30. The high dispersity of gold nanoparticles and smaller size could provide high activity for the reaction 31. At the same time, as the increase in loading, the nanoparticle would increase. However, the change is very little with different weight percent. This is also the same with the reference 32 and the average nanoparticle size increase 0.2nm when the weight percent up to 10 times.

Figure 2 SEM-EDS images of 1wt%Au/TiO2

Figure 3 TEM images of 1wt%Au/TiO2 (Insert: Distribution of Au nanoparticles size in 1wt%Au/TiO2) 3.3 Raman Figure 4 is the Raman spectrums of Au/TiO2 catalysts with different loading. From Figure 4, TiO2 would have four peaks at 144, 399，516 and 639cm-1 33 that belongs to the vibration mode of Eg, A1g, B1g and an overtone of Eg respectively 34. With the weight percent increasing from 0.2wt% to 1wt%, the intensity of the peak 5



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decreases without any shift at 144cm-1. This indicates that there is no strong reciprocity between Au and TiO2. With the increasing weight percent, more gold nanoparticles are loading on the surface of supporter to reduce the intensity 35.

Figure 4 Raman Spectrums of samples 3.4 Effect of Temperature Figure 5 is the effect of temperature for the catalytic activity. It shows that the conversion of methanol has no obvious changes from 14.8% to 15.1%, when the temperature is under 100℃. However, the selectivity of formic acid is higher at this range as 96.9% and 96.4% respectively. When the reaction temperature increases to 100℃, the conversion grows up to 29.3% with the decrease of selectivity to 95.1%. During this period, the conversion of methanol shows great increase. With the growing temperature to 150℃, the conversion could increase to 37.6%, but the selectivity of formic acid falls down to 90.5% because of the production of CO2. Considering the each direction of reaction, this paper chose 100℃ as optical temperature. Noble-metal nanoparticles always have a low activity under high reaction temperature 36, but Au/TiO2 showed high activity even after 300 degrees during our following research with 55.7% conversion without losing activity after 2h as some of the Au nanoparticles loading into the pore of the TiO2. At the same time, in the following test, this paper researched the effect of reaction time and found the conversion kept stable after 2h, so the reaction time is 2h in the following research.

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Figure 5 Relationship between reaction temperature and conversion of methanol (reaction conditions: Methanol/Au = 2000 mol/mol, NaOH/methanol = 2:1, 20bar, 2h) 3.5 Effect of Weight Percent Figure 6 shows the evaluation of catalysts activity with different Au weight percent. The conversion of methanol and the selectivity and yield of formic acid are 96.9% and 14.6% respectively with 0.2wt% Au on TiO2. With the increasing weight percent of Au, the conversion of methanol grows up to 29.3% and the selectivity keeps stable as 95.1% with the yield growing to 27.9%. Because the Au active sites would increase with the higher loading, in order to improve the reaction efficiency 37. However, the loadings have a slight impact for the selectivity. It has the relationship with the reaction pathway as the methanol was oxide to formaldehyde as first before following oxidation to formic acid. Also, corresponding with reaction kinetics, according to reference of ethanol liquid phase oxidation by Au/TiO2 38, the first step to aldehyde is reaction rate control step and the second step to acid is fast. Hence, when we use lower loading catalyst, although the conversion is lower, there is no significant effect on selectivity.

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Figure 6 Relationship between Au weight percent and conversion of methanol ( Reaction conditions: NaOH/methanol = 2:1 mol/mol , 100℃, 2h,20bar) 3.6 Effect of usage of catalysts The effect of usage of catalysts is shown in Figure 7. The conversion of methanol increases from 28.1% to 34.7% with the growing usage of catalysts. Although the yield of formic acid increases, the changes are not obvious compared with the increasing catalysts usage from TOF results. Instead, when the usage of catalysts is lower, the TOF of reaction could be 7.8s-1, which is quite higher than the highest usage of catalysts. Hence, the optical usage of catalysts is when the molar ratio between methanol and Au is 2000:1.

Figure 7 Relationship between catalyst usage and conversion of methanol a-d: molar ratio between methanol and Au are 500,1000,1500,2000 (reaction conditions: NaOH/methanol = 2:1 mol/mol ,100℃, 20bar, 2h) 3.7 Effect of Pressure 8


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Figure 8 Relationship between reaction pressure and conversion of methanol (reaction conditions: Methanol/Au = 2000 mol/mol, NaOH/methanol = 2:1 mol/mol , 100℃, 2h) This paper also researched the effect of pressure from 10bar to 35bar as shown in Figure 8. It is clear that the conversion of methanol increases a lot from 20.8% to 29.3%, when the reaction pressure is lower than 20bar. At the same time, the selectivity almost keeps stable with a little decrease from 99.0% to 95.9%. If the pressure is keeping increased to 35bar, there is a little increase of the conversion to 31.2% and the selectivity fall down to 92.6%. When the pressure is lower than 20bar, the absorption of oxygen on Au surface does not reach at saturation, so the catalysts could provide more active oxygen atoms 39. If the pressure keeps increasing to higher than 20bar, there is a dynamic equilibrium between the absorption and decomposition of oxygen, so the conversion keeps stable. 3.8 Effect of Base Table 1 shows the effect of base for the catalytic activity. From the table, it has obvious effect about whether the reaction system contains base or not. When there is no base in the reaction, the dehydrogenation of methanol absorbed on the Au surface could not happen, so the following reaction stopped and the catalysts did not show activity for the reaction. According to the mechanism for alcohols oxidation by Au, Au nanoparticles would absorb methanol and OH in the reaction system and produce methoxy active intermediate through dehydrogenation. At the same time, Au would absorb oxygen and decompose it to active atoms to produce water with dehydrogenation 40. The catalysts would be died, if the absorption of methanol on Au could not participate in the following reaction. Hence, the conversion of methanol grows up to 19.3% when an equivalent NaOH is added into the reaction system. With keeping increase of NaOH to two equivalents, the conversion increases to 29.3% and the selectivity also grows from 89.6% to 95.9% at the same time. If keep increase the equivalents of base in the following texts, the conversion keeps stable. 9



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Table 1 Reaction activity with different base equivalent Molar Ratio Conversion/% Selectivity/% Yield/% (NaOH:MeOH) 1 19.3 89.6 17.3 2 29.3 95.9 28.1 3 30.2 93.7 28.3 Reaction conditions: Methanol/Au = 2000 mol/mol, 100℃, 20bar, 2h According to the above results, this paper postulated the following reaction mechanism as shown in Figure 9 compared with the research of liquid ethanol oxidation. At first, the methanol would be absorbed on Au surface to form methanol in absorption state. The OH- in the reaction system would attack the absorbed methanol to produce intermediate A on Au surface. Base on the results in section 3.8, it shows that the reaction could not be happened without base. Also according to the results from reference 41, the reaction could not happen for ethanol oxidation by Au without base, so base is necessary for the reaction. Following, the active intermediate A produces B after dehydrogenation 42. On the other pathway, the oxygen absorbed on Au decomposed to active atoms [O*] 43. It is easy for active oxygen atoms to reactive with absorbed B for dehydrogenation to produce intermediate D. In section 3.7, it shows that if the oxygen is exchanged to nitrogen before the reaction, there are also no products detected after reaction. During the subsequent reaction, the intermediate D would absorb OH group to produce E 44. The intermediate C would react with E through dehydrogenation with losing water to form absorbed F. After this, F would desorb from Au surface to produce formic acid. As the comparison experiments, this paper used kinds of acid supporter and base supporter and the resists show that the acid supporter have high activity as to base ones. Acid catalysts could improve the oxidation process in the reaction, which is the same as methanol liquid phase oxidation reaction 45.

Figure 9 Reaction mechanism of methanol oxidation 3.9 Reusability Figure 10 is the reusability of catalysts and it shows that the catalysts show great stability for the reaction. The conversion is 26.6% after 5 cycles compared with initial one with 29.3%. At the same time, the selectivity of catalysts keeps around 95.0% 10


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during the recycle test. As the great stability of catalysts for the reaction, it is important to determine the real active components. Through the following test, in order to investigate if the leaching Au is the real active components, this paper added the certain amount of cycled reaction solution into the fresh reaction system and reacted under the same conditions. The results prove that there is no activity for the leaching part. This also explains the great stability for the catalysts.

Figure 10 Reusability of catalyst (reaction conditions: Methanol/Au = 2000 mol/mol, NaOH/methanol = 2:1 mol/mol , 20bar,100℃, 2h) 4. Conclusion Au/TiO2 catalysts have been synthesized with weight percent from 0.2wt% to 1wt%. The Au nanoparticles are highly dispersed on supporter with around 4nm. Aerobic oxidation of methanol over Au/TiO2 heterogeneous catalysts has been studied through modification of reaction conditions. The reaction can achieve high yield for formic acid under 20bar, 100℃ after 2h with the addition of base. Furthermore, the reaction mechanism has been postulated based on the experimental results. The conversion of methanol and hydroxyl to intermediate is the first step following by dehydrogenation to produce formaldehyde and formic acid at last. The research achieved the liquid oxidation of methanol to produce formic acid and solve the industrial problem for producing formic acid, providing the further application in industry. Acknowledgements Financial support from the National Natural Science Foundation of China (grant no. 21573015) is gratefully acknowledged. References (1) Olah G A. Towards oil independence through renewable methanol chemistry. Angew. Chem. Int. Ed. 2013, 52, 104. (2) Kamarudin S K, Achmad F, Daud W R W. Overview on the application of direct 11



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TiO2 Catalyst and the Performance of Liquid

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