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Catalytic effect of SUS316 reactor surface on hydrolysis of benzamide in sub- and supercritical water Makoto Akizuki, Nanae Fujioka, and Yoshito Oshima Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02640 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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Industrial & Engineering Chemistry Research

Catalytic effect of SUS316 reactor surface on hydrolysis of benzamide in sub- and supercritical water

Makoto Akizuki*, Nanae Fujioka, Yoshito Oshima

Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan. E-mail: [email protected]. Tel. & Fax: +81 4 7136 4694.

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ABSTRACT

The catalytic effect of an SUS316 reactor surface on the hydrolysis of benzamide in sub- and supercritical water was investigated at 350–450 °C and 25–65 MPa. Tube reactors having different surface area to volume ratios were used to distinguish the effect of the homogeneous reaction and the surface catalyzed reaction. The surface of an SUS316 reactor acted as a heterogeneous catalyst for the hydrolysis reaction. Kinetic analysis indicated that surface Fe2O3 mainly contributes to the catalytic effect of SUS316. Both homogeneous and surface catalyzed reactions occurred, and the contribution of each reaction depended on water density. The surface-catalyzed reaction was dominant at low water density, and the contribution from the homogeneous reaction became larger with increasing water density. Furthermore, at 400 °C and 25 MPa, hydrolysis products promoted the reaction.


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1 INTRODUCTION Sub- and supercritical water have attracted considerable attention as promising media for waste treatment, organic synthesis, and bio-refinery applications. Because the properties of water change significantly depending on its temperature and pressure,1-3 the solvent properties can be adjusted to suit the characteristics of reactions. In order to conduct reactions in sub- and supercritical water, reactors made of Fe- or Nibased alloy are generally used. Particularly, SUS316, an austenitic stainless steel, is frequently used as a reactor material because of its relatively high corrosion resistance and low cost. It is well known that metals or metal oxides on reactor surfaces can promote several kinds of reactions.4,5 Consequently, from a scientific viewpoint, elucidating the effects of reactor surfaces is important for investigating the rates of reactions in homogeneous sub- and supercritical water. From an engineering viewpoint, elucidating the effects of reactor surfaces is necessary for scaling up the reactors or designing the reactors without the need for added catalysts. There have been many studies on the effects of reactor surfaces on reaction rates in sub- and supercritical water. For example, Lee and Gloyna examined the oxidation of acetic acid at 400 °C in water with a density of 0.25 g/cm3 using an SUS316 batch reactor, and reported that the conversion increased with increasing the surface area to volume ratio (S/V ratio) of the reactor.6 They also reported that the conversion increased with the number of times the reactor was used, and became constant after several runs. Segond et al. investigated the oxidation of ammonia at 530–630 °C and 14–28 MPa using an SUS316 flow reactor, and also reported that the rate constant increased with increasing S/V ratio.7


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In addition to oxidation reactions, several research groups have reported that the reactor surface affects the decomposition of formic acid. For example, Maiella and Brill decomposed formic acid at 310 °C and 27.5 MPa, and reported that the reaction rate was higher in reactors made of SUS316 and 90/10 Pt/Ir than that in a reactor made of Ti.8 The reactor-surface effect on the decomposition of formic acid has been extensively reviewed by Watanabe et al.4 The promotion of hydrogenation and dehydrogenation reactions by SUS316 reactor surfaces has also been reported. Fábos et al. investigated hydrogenation of cyclohexanone with formic acid at 200–250 °C using both an SUS316 reactor and a sapphire reactor, and found that SUS316 promoted the reaction.9 Furthermore, Comisar and Savage reported that the dehydrogenation of benzhydrol at 300 °C was catalyzed by a SUS316 reactor surface.10 Although there have been many studies concerning reactor surface effects, the effects of reactor surfaces on acid and base reactions, one of the most investigated reactions in sub- and supercritical water, have not been fully investigated. Torry et al. reported that the hydrolysis of dibenzyl ether at 380 °C in water with a density of 0.16 g/cm3 conducted in both an SUS316 reactor and a titanium reactor proceeded at the same rate.11 Duan and Savage performed the hydrolysis of N-methylacetamide at 200 °C using both a stainless steel reactor and a quartz reactor, and also concluded that the stainless steel had no catalytic effect because the rate of the reaction in both reactors was the same.12 Akiya and Savage reported that the dehydration of cyclohexanol at 380 °C in water with a density of 0.34 g/cm3 was promoted in a new stainless steel reactor, but was not promoted in a seasoned stainless steel reactor.13 Comisar and Savage investigated benzyl rearrangement at 300 °C, and reported the faster disappearance of benzil in a quartz reactor compared to that in a stainless steel reactor.10


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To the best of our knowledge, there are very few reports on the effect of stainless steel reactor surfaces on the reaction rate of acid and base reactions such as hydrolysis. However, because water properties change significantly with temperature and pressure, surface properties may vary according to the water properties, and thus the catalytic effect should be considered under different reaction conditions. In this study, we have investigated the hydrolysis of benzamide as a model reaction for acid and base reactions. Hydrolysis of amides in sub- and supercritical water is an important reaction not only as a fundamental acid/base reaction but also as a model reaction for the degradation of proteins and pharmaceuticals. Several studies on this reaction in homogeneous12,14-18 and heterogeneous catalysis systems19 have been reported. In general, the hydrolysis of amides at low temperatures proceeds in the presence of homogeneous acid or base catalysts.20 Conversely, the hydrolysis of amides has been reported to proceed even in the absence of catalyst in sub- and supercritical water.12,14-18 Although the reaction mechanism in sub- and supercritical water has not been elucidated sufficiently, some researchers have investigated the reaction mechanism through kinetic experiments. For example, Lee and Gloyna investigated the hydrolysis of acetamide, and reported that the reaction was first order with respect to acetamide.14 Duan et al. examined the hydrolysis of N-methylacetamide, and reported that the reaction was also first order with respect to the starting material.12 From the dependence of the reaction rate constant on pH, they concluded that the SN2 attack of a water molecule on the reactant was the rate determining step, and proposed to two possible mechanisms; one in which a water molecule acts as a general base or acid catalyst, and one in which two water molecules take part in the transition state to assist hydrolysis.


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In the case of hydrolysis with high initial concentrations, the existence of an autocatalytic effect has been reported.15,18 Okazaki et al. examined the hydrolysis of acetamide, and reported that the reaction rates could be represented by second-order reaction kinetics.15 They proposed that the reaction was catalyzed by OH− produced from ammonia, because ammonia is more dissociated than acetic acid under hydrothermal conditions and thus the solution became more basic as ammonia and acetic acid were produced by the hydrolysis. In this study, the catalytic effect of an SUS316 reactor surface on the hydrolysis of benzamide in sub- and supercritical water were investigated kinetically. By using reactor tubes of different inner diameters, the contributions of the homogeneous phase and the reactor surface on the apparent reaction rate constants were elucidated. The experiments were conducted at various temperatures and pressures so as to examine the effects of water properties on the catalytic effect of the reactor surface. 2 EXPERIMENTAL METHODS Reagents. Benzamide was purchased from Tokyo Chemical Industries Co., Ltd., Japan, and ammonium benzoate was purchased from Wako Pure Chemical Industries, Ltd., Japan. Both were used as received. Distilled water was prepared with distillation equipment (RFD240HA; Advantec Toyo Kaisha, Ltd., Japan). The aqueous solution of benzamide and the distilled water were degassed by bubbling with N2 gas prior to use. Pretreatment of reactor. As described later in section 3.3, the reaction conditions are mildly oxidative rather than reductive. In order to avoid changes in the reactor surface conditions during the reaction, the reactor was pretreated in supercritical water with 0.3 mmol/dm3 O2 at 500 °C and 25 MPa for more than 8 h.


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Experimental procedure. The experiments were conducted using a tubular plug-flow reactor, as shown in Figure 1. The aqueous solution of benzamide (4.13×10−2 mol/dm3) and the distilled water were pumped separately using high-pressure pumps (PU-2080 or PU-4180; JASCO Co., Japan), and distilled water was preheated so that the temperature at the reactor entrance was 400 °C. The volume ratio of the aqueous solution of benzamide to the distilled water was 1:9. The two streams were mixed and fed into the SUS316 reactor having four different inner diameters (0.25, 0.50, 1.00, and 2.17 mm) set in an oven. The S/V ratios [mm−1] of the reactors were 16, 8.0, 4.0, and 1.8, respectively. The stream emitted from the reactor was cooled immediately in a water-cooled heat exchanger, and depressurized by a back pressure regulator (26-1761; TESCOM Co., USA or SCF-Bpg; JASCO Co., Japan). The residence time, defined as the volume of the reactor divided by the volumetric flow rate, was controlled both by changing the reactor length and by changing the flow rate. The details of the experimental procedure are shown in the Supporting Information. Reactions with metal oxide catalysts. In order to investigate the catalytic effect of metal oxides on the SUS316 reactor surface, reactions with metal oxide catalysts (Fe2O3, Fe3O4, Cr2O3, and FeCr2O4) were conducted. Fe2O3, Cr2O3, and FeCr2O4 powders were prepared by (co)precipitation methods. Aqueous ammonia solution (Wako Pure Chemical Industries, Ltd., Japan) was added slowly to solutions of iron(III) nitrate ennahydrate, chromium(III) nitrate enneahydrate, and iron(II) chloride tetrahydrate (Kanto Chemical Co., Inc., Japan). The obtained precipitates were washed with distilled water and dried in a drying oven. Fe3O4 powder was purchased from Wako Pure Chemical Industries, Ltd., Japan, and used as received. All powders were pressed into pellets, calcined at 500 °C for 2 h in air (Fe2O3 and Cr2O3) or in 3% H2/Ar (Fe3O4, FeCr2O4), and then ground into 0.3–0.5 mm-diameter granules. The reactor was loaded


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with 0.079-0.15 g of the metal oxide catalysts, and both ends of the reactor were equipped with filters made of SUS316 to retain the catalysts. Analysis. Organics in the effluents were analyzed using high-performance liquid chromatography-ultraviolet (HPLC-UV) analysis (LC-980; JASCO Co., Japan) with a packed column (Finepak SIL C18S; JASCO Co., Japan). Ammonia in the effluents was separated using the same HPLC system with a packed column (Y-521; Showa Denko K. K., Japan), and analyzed by an ion conductivity detector (CDD-6A; Shimadzu Corp., Japan). The details of the HPLC and IC analyses are shown in the Supporting Information. Metal ions in the effluents were quantified using inductively coupled plasma mass spectrometry (ICP-MS) (ICPM-8500; Shimadzu Co., Japan). X-ray diffraction (XRD) (SmartLab; Rigaku Co., Japan) and N2 adsorption (NOVA 2200e; Quantachrome Co., USA) were used to characterize the metal oxide catalysts. Analysis of the SUS316 plate surface was conducted using X-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe; ULVAC-PHI Inc., Japan). 3 RESULTS AND DISCUSSION 3.1 Effect of S/V ratio on benzamide conversion at various reaction pressures. Figure 2 shows the S/V ratio dependence of benzamide conversion (X), defined as moles of consumed benzamide divided by moles of initial benzamide, at 400 °C and (a) 25 MPa, (b) 35 MPa, (c) 45 MPa, and (d) 65 MPa. The main reaction products are benzoic acid and ammonia under all reaction conditions, which indicates that hydrolysis is the main reaction under the reaction conditions investigated. For all reaction pressures, the reaction rates increase with increasing S/V ratio, and this result indicates that the reactor surface promotes the reaction. Furthermore, the reaction rates increase more with S/V ratio at 25 MPa than at the other reaction pressures. In


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addition, the conversion at 25 MPa in the initial stage of the reaction increases more slowly than that in the middle stage of the reaction, and this behavior indicates that the reaction products have some promotion effect on the reaction rate at 25 MPa. This phenomenon will be discussed in section 3.6. The positive dependence of reaction rate on S/V ratio suggests that metals or metal oxides on the reactor surface act as heterogeneous catalysts. However, another possibility is that metal ions leached from the reactor surface act as homogeneous catalysts. In order to investigate the effects of metal ions on the reaction rate, the leaching behavior of the main components of SUS316 were analyzed. Table 1 shows that the concentration of metal ions is low in the effluents. In addition, although the effect of reactor surface is larger at 25 MPa, the concentrations of metals in the effluents at 25 MPa are not larger than those under other reaction pressures. Therefore, it can be concluded that the reaction is not promoted by the homogeneous catalytic effect of leached metal ions. 3.2 Kinetic analysis of benzamide hydrolysis at 400 °C under different reaction pressures. In order to discuss the effect of S/V ratio quantitatively, kinetic analysis was conducted. Prior to the kinetic analysis, we investigated whether flow conditions affect the reaction rates. Experiments using two different reactor lengths (6.0 and 10 m) showed the same reaction rates (Figure S2 in the Supporting Information), indicating that the effect of flow conditions on the reaction rate under the reaction conditions examined can be ignored. Figure 3 shows the pseudo-first-order plot for benzamide conversion (X) at 400 °C. Because the data at 35–65 MPa follow straight lines, the reaction is considered as first order with respect to benzamide concentration. For the reaction at 25 MPa, the data in the region where residence


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time and ln(1-X) present a linear relationship are used for determining the pseudo-first order reaction rate constant. Figure 4 shows the dependence of the pseudo-first-order reaction rate constant (k) on S/V ratio calculated from the slopes in Figure 3. Error bars show the 95% confidence interval for each slope. k increases with S/V ratio regardless of the reaction pressure. Assuming the S/V ratio and k have a linear relationship, k can be described as the following equation: = + × ⁄ ratio

(1)

where khomogeneous is the intercept of the line, which corresponds to the reaction rate constant of the homogeneous reaction, and ksurface is the slope of the line, which is the rate constant of the reaction catalyzed by the reactor surface. The water properties and reaction data determined for each pressure are summarized in Table 2. These data suggest that the catalytic reaction on the reactor surface is dominant at 25 MPa and that both the homogeneous reaction and the catalytic reaction contribute at pressures of 35–65 MPa. The details of the dependence of khomogeneous and ksurface on the reaction pressure will be discussed in section 3.4. 3.3 Catalytic effect of metal oxides on the SUS316 reactor surface. The chemical species present on the SUS316 reactor surface were determined, and reactions with these chemical species were investigated. For determination of the chemical species, an SUS316 plate (Nilaco Corp., Japan) was treated in supercritical water under the same conditions used for reactor pretreatment, and XPS analysis of the surface was conducted. It was reported that multi-layer structure was observed on SUS316 surface exposed to oxidative supercritical water using transmission electron microscopy


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analysis, and the structure was identified as (Fe,Cr)2O3/(Fe,Cr)3O4/Cr2O3/Ni-rich steel/stainless steel from the outer to the inner layer21. Therefore, we focused on Fe and Cr, which is reported to compose outer layers of the oxide film. Figure 5 shows XPS spectra for (a) Fe 2p and (b) Cr 2p, and the data obtained before supercritical water treatment are also shown as a reference. According to XPS analysis in previous reports,22-24 the broad peak around 708–713 eV for the supercritical-water-treated SUS316 surface is a combination of peaks for Fe2O3, Fe3O4 (Fe3+), Fe3O4 (Fe2+). As same as the previous report, 22, 24 the peak can be divided into three peaks (Figure S3 in the Supporting Information), which indicates the existence of both Fe2O3 and Fe3O4. In addition, the peak for FeCr2O4 may be included in the broad peak. 23 No metallic Fe peak (706–708 eV) is observed in the spectrum after supercritical water treatment, but is observed before supercritical water treatment. The peak around 574–577 eV can be assigned to Cr2O3 and/or FeCr2O4.23, 24 These results indicate that Fe2O3, Fe3O4, Cr2O3, and FeCr2O4 are present on the SUS316 reactor surface. Subsequently, reactions with Fe2O3, Fe3O4, Cr2O3, and FeCr2O4 were examined at 400 °C and 25 MPa to investigate the catalytic effect of these oxides. The BET surface areas of the metal oxide catalysts are shown in Table 3. The surface areas of these metal oxide catalysts did not change during the use in supercritical water for 2.0-2.5 h. From the XRD analysis (Figure S4 in the Supporting Information), all the metal oxides are single-phase before the reaction. After the reaction, the crystalline phases of Fe2O3 and Cr2O3 are not changed. Fe3O4 is almost unchanged, but a small peak for Fe2O3 is observed. In the reaction with of FeCr2O4, some of the catalyst is converted to Fe2O3 and Cr2O3. This partial oxidation of Fe3O4 and FeCr2O4 indicates that the reaction conditions are mildly oxidative rather than reductive, even though the O2 dissolved in the distilled water was removed by degassing with bubbling N2 prior to use.


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Figure 6 shows the effect of the metal oxide catalysts on benzamide conversion at 400 °C and 25 MPa. The result of the conversion using no catalyst is also shown as reference. The residence time for the reactor packed with catalysts is defined as the vacant volume of the reactor divided by the volumetric flow rate. The reaction is promoted by all the metal oxide catalysts, as revealed by reference to the reaction in the absence of catalyst. For the comparison of the pseudo-first-order reaction rate constants, W/F, defined as the catalyst weight divided by the volumetric flow rate, was used as an indicator of reaction time. Because the reaction rates per surface areas of the catalysts are quite important to compare the reactivity of metal oxide, the reaction rate for each metal oxide surface was evaluated by dividing the W/F based pseudo-firstorder reaction rate constants by the BET surface area. The values for the metal oxide catalysts are in the order Fe2O3 (5.4 × 10−7 [(kg s/m3)/(m2/kg)]) > Fe3O4 (3.1 × 10−7) > Cr2O3 (2.0 × 10−7) > FeCr2O4 (5.6 × 10−8). Because Fe is the main component of SUS316, and Fe2O3 is stable in supercritical water, Fe2O3 is considered to be the main promoter of the reaction. The rates of the reactions catalyzed by Fe2O3 and the SUS316 reactor surface were compared. The S/V ratio of the reactor filled with Fe2O3 catalyst was calculated from the surface area of the Fe2O3 catalyst divided by the vacant volume of the reactor as 1.6 × 104 mm-1. Because some reactions in supercritical water have been reported to be diffusion limited, the contribution of diffusion processes on benzamide hydrolysis with an Fe2O3 catalyst was evaluated by the same procedure suggested by previous research.25 The estimated value of the Thiele modulus (Φ) is 17. Because the Thiele modulus is larger than 4, the reaction rate should be strongly affected by diffusion processes in the catalyst pores. 26 In the case of first-order-reaction, the effectiveness factor (η) calculated from the Thiele modulus is 5.8 × 10−2. In order to compare the reaction rate catalyzed by Fe2O3 and by the SUS316 reactor surface, the residence time based reaction rate


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constants were calculated. Dividing the apparent pseudo-first-order reaction rate constant by η, the reaction rate constant without the effect of diffusion processes is estimated to be 1.1 × 102 s-1. As the surface reaction is considered to be dominant in this reaction, equation (1) can be approximated as ≅ × / ratio, and therefore, ksurface for the Fe2O3 catalyzed reaction at 400 °C and 25 MPa is calculated as 7.0 × 10−3 mm s-1. Because the estimated values of ksurface for the Fe2O3 catalyst and the SUS316 surface (Table 2) are of the same order, it is consistent to suggest that Fe2O3 is the main contributor to the catalytic effect of the SUS316 reactor wall. 3.4 Pressure dependence of homogeneous and surface-catalyzed reaction rates at 400 °C. In this section, the effect of reaction pressure on khomogeneous and ksurface (Table 2 in section 3.2) is discussed. The data in Table 2 show that khomogeneous tends to increase with increasing reaction pressure. Duan et al. previously suggested that the attack of a water molecule on the reactant is the rate determining step in the hydrolysis of amides in the homogeneous phase of hot compressed water.12 In this mechanism, the reaction rate is thought to increase with water density because the concentration of the water increases. As the water density increases with increasing reaction pressure, the dependence of khomogeneous on the reaction pressure is consistent with the homogeneous reaction mechanism suggested in the previous report. ksurface is notably large at 25 MPa, and decreases with increasing reaction pressure to 35 MPa. Conversely, ksurface increases with increasing reaction pressure from 35 to 65 MPa. For heterogeneous catalysis in supercritical water, it has been reported that reactions are suppressed by increasing water density because water and the reactants adsorb competitively onto the active sites.27,28 Because water density increases significantly with pressure from 25 MPa to 35 MPa, as


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shown in Table 2, the notable decrease of ksurface can be attributed to the decrease in effective active sites caused by the competitive adsorption of water. In addition to the effect of water density, our previous study reported that the number of Brönsted acid sites increased with an increase in the ionic products of water (KW) with the use of a TiO2 catalyst, which has a moderate acid strength.27 Although the mechanism for the hydrolysis catalyzed by the SUS316 reactor surface is unclear, the acidity or basicity of the metal oxides on the reactor surface is believed to promote the reaction. Because both the acid strength and the base strength of metal oxides on the SUS316 reactor surface are not strong, the number of acid sites and base sites increases by dissociation of water molecules on the surface, and this may cause the increase in ksurface between 35 to 65 MPa. 3.5 Temperature dependence of homogeneous and surface-catalyzed reaction rates. Figure 7 shows the temperature dependence of benzamide conversion at (a) 25 MPa and (b) 35 MPa. Regardless of the reaction pressure and S/V ratio, the reaction rate increases with the reaction temperature. However, the dependence on the reaction temperature is quite different according to the reaction pressure. Applying the same procedure as that used in the previous section, khomogeneous and ksurface were calculated, and the results are summarized in Table 4. As shown in Table 2 and Table 4, the values of khomogeneous at 400 °C are larger than that those at 350 °C and 450 °C. The values of ksurface increase with increasing reaction temperature at both reaction pressures, and the dependence on the reaction temperature is larger at 25 MPa than at 35 MPa. These results can be explained by the effects of water density on khomogeneous and ksurface proposed in the previous section. In general, kinetic rate constants increase with increasing reaction temperature in accordance with the Arrhenius equation. The results in the previous section indicate that


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khomogeneous also increases with increasing water density. Because water density decreases with increasing reaction temperature, the reaction temperature has both a positive and negative effect on khomogeneous, and results in a maximum value at 400 °C. Increasing water density contributes to decreasing ksurface. Although the lower KW in lower density water has a negative effect on ksurface, both the increase in temperature and a decrease in water density caused by temperature increase promote the surface reaction, thus ksurface increases with increasing temperature. Furthermore, because the water density decreases with increasing temperature more rapidly at 25 MPa than at 35 MPa, ksurface increases more significantly at 25 MPa. These results also indicate that the reaction pressure itself has little effect on the reaction rate, and the change in water density is the actual reason for the change in the reaction rate. As discussed in the above sections, the catalytic effect of the SUS316 reactor surface strongly contributes to the hydrolysis of amides when the S/V ratio of the reactor is large and the water density is low (owing to either high temperature or low pressure, or both). Because a tubular flow reactor, which generally has larger S/V ratio than a batch reactor, was used in our experiments, and the range of the reaction pressures examined was large, the catalytic effect of the SUS316 reactor surface, which was not reported in the previous research by Torry et al.11 and Duan et al.,5 is considered for the first time in our research. 3.6 Effect of hydrolysis products on reaction rate. In order to investigate the effect of the reaction products (benzoic acid and ammonia) on the reaction rate, the hydrolysis of benzamide with ammonium benzoate were examined. The molar concentration of the ammonium benzoate added was the same as the initial molar concentration of benzamide. Figure 8 shows the benzamide conversion with and without ammonium benzoate. By addition of ammonium benzoate, the reaction at 25 MPa is promoted, but the reactions at 35 MPa and 45 MPa are not


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promoted. These results indicate that the benzoic acid and ammonia produced by hydrolysis of benzamide promotes the reaction at 25 MPa, as shown in Figure 2 (a). According to the kinetic analysis in section 3.2, the contribution of the homogeneous reaction is larger at 35 MPa and 45 MPa than at 25 MPa. Because the reactions at 35 MPa and 45 MPa are not promoted by the addition of ammonium benzoate, the hydrolysis products are not likely to act as homogeneous catalysts. A plausible explanation is that the benzoic acid and ammonia produced by the hydrolysis of benzamide cause an increase in solvent polarity, and this change in the solvent properties affects the reaction rate. In general, the reaction rate is higher in polar solvents if the transition state of the reaction is more polar than the reactants. Although the exact reaction mechanism for the hydrolysis catalyzed by the SUS316 reactor surface has not been fully elucidated in this research, the transition state is believed to be more polar than the reactant because acid or base catalyzed hydrolysis generally involves a polar transition state. Because the dielectric constant of water, which is an index of polarity, at 25 MPa is lower than at 35–65 MPa (Table 2), a small amount of the hydrolysis products cause considerable change in solvent polarity at 25 MPa, and thus the reaction is promoted by the hydrolysis products only at 25 MPa. In the hydrolysis reaction of dibenzyl ether in near critical water at 380 °C, Torry et al. observed an increase in the reaction rate upon the addition of NaCl, which they attributed to an increase in the polarity of the solvent.11 Experiments adding salts like NaCl to our reaction system will provide important information about the effect of hydrolysis products, which will be a topic for further investigation. 4 CONCLUSIONS


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The catalytic effect of an SUS316 reactor surface on the hydrolysis of benzamide in sub- and supercritical water was investigated. The following conclusions were drawn from the results: (1) The surface of an SUS316 reactor acts as a heterogeneous catalyst for the hydrolysis of benzamide. Fe2O3, Fe3O4, Cr2O3, and FeCr2O4 are thought to be present on the SUS316 reactor surface, and these metal oxides exhibit catalytic activity in the hydrolysis of benzamide. Among these metal oxides, Fe2O3 exhibits the highest activity per surface area, and is most likely the major contributor to the catalytic effect of the SUS316 reactor wall. (2) The results of kinetic analysis show that both the homogeneous reaction and the heterogeneous catalytic reaction occur in sub- and supercritical water, and the contributions of these two types of reaction differ depending on the water density. The homogeneous reaction rate at 400 °C tends to increase with increasing reaction pressure, and this tendency is consistent with the reaction mechanism for amide hydrolysis in sub- and supercritical water proposed in previous research. The reaction rate with heterogeneous catalysis at 400 °C significantly decreases with increasing reaction pressure from 25 to 35 MPa, and increases with increasing reaction pressure from 35 to 65 MPa. This can be explained by considering the two opposite effects of the reaction pressure. The increase in water density with increasing reaction pressure is considered to cause a decrease in the number of effective active sites due to the competitive adsorption of water on the reactor surface. Conversely, the increase of KW with increasing reaction pressure is reported to increase the number of acid and base sites by the dissociation of water molecules on the surface, and this effect is considered to promote the reaction. The temperature dependence of the reaction rate can be explained by considering the interdependence of the water density and the temperature.


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(3) The reaction at 400 °C and 25 MPa is promoted by reaction products, and this promotion effect is not observed at the other reaction pressures. One plausible explanation for this is that benzoic acid and ammonia produced by the hydrolysis of benzamide cause an particular increase in solvent polarity at 25 MPa due to the low dielectric constant of water at 400 °C and 25 MPa, and this change affects the reaction rate.


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FIGURES

Figure 1. Schematic diagram of the flow reactor used in this study.


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Figure 2. Benzamide conversion for each S/V ratio at 400 °C. (a) 25 MPa, (b) 35 MPa, (c) 45 MPa, (d) 65 MPa. S/V ratio: (□) 1.8, (●) 4.0, (◊) 8.0, (▲) 16.


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Figure 3. Pseudo-first-order plot for benzamide conversion at 400 °C. (a) 25 MPa, (b) 35 MPa, (c) 45 MPa, (d) 65 MPa. S/V ratio: (□) 1.8, (●) 4.0, (◊) 8.0, (▲) 16.


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Figure 4. Effect of S/V ratio on the pseudo-first-order reaction rate constant at 400 °C. Experimental data: (□) 25 MPa, (●) 35 MPa, (◊) 45 MPa, (▲) 65 MPa. Fitted line: (——) 25 MPa, (— –) 35 MPa, (– – –) 45 MPa, (·····) 65 MPa

Figure 5. XPS spectra of an SUS316 plate. (a) Fe 2p, (b) Cr 2p. (●) after supercritical water treatment, (◊) before supercritical water treatment.


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Figure 6. Effect of metal oxide catalysts on benzamide conversion at 400 °C and 25 MPa. (□) Fe2O3, (●) Cr2O3, (◊) Fe3O4, (▲) FeCr2O4, (○) no catalyst.

Figure 7. Temperature dependence of benzamide conversion at (a) 25 MPa and (b) 35 MPa. S/V = 1.8 : (●) 350 °C, (♦) 400 °C, (■) 450 °C, S/V = 8.0 : (○) 350 °C, (◊) 400 °C, (□) 450 °C.


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Figure 8. Effect of ammonium benzoate addition on benzamide conversion at 400 °C. (a) 25 MPa, S/V = 1.8, (b) 25 MPa, S/V = 8.0, (c) 35 MPa, S/V = 1.8, (d) 45 MPa, S/V = 8.0. (●) with ammonium benzoate, (◊) without ammonium benzoate.


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TABLES Table 1. Concentration of metal ions in effluents from the benzamide reaction at 400 °C.

Sample no.

Pressure

S/V ratio [mm-1]

Residence time Fe [s] [ppm]

Cr [ppm]

Ni [ppm]

Mo [ppm]

Mn [ppm]

1.84

74.3

0.007

0.006

0.019

0.005

0.020

4.00

10.5

0.042

0.007

0.020

0.010

0.020

3

8.00

6.5

0.000

0.005

0.019

0.005

0.020

4

1.84

105.9

0.000

0.005

0.021

0.007

0.020

1.84

211.8

0.035

0.006

0.020

0.010

0.020

6

4.00

29.8

0.031

0.007

0.020

0.014

0.020

7

8.00

18.7

0.106

0.011

0.039

0.028

0.040

[MPa]

1 2

25

5 35

8

45

8.00

21.8

0.000

0.005

0.020

0.026

0.021

9

65

8.00

24.6

0.001

0.005

0.019

0.022

0.022

Table 2. Rate constants and water properties for each reaction pressure at 400 °C. Water properties1-3

Rate constants Pressure

khomogeneous

ksurface

[10-3 s-1] 25 MPa

[10-3 mm s-1]

Density [102 kg/m3]

Log (KW 2 [mol /kg2])

Dielectric constant [-]

2.6 (±18)

2.2 (±2.0)

1.7

−16.6

2.5

35 MPa

2.9 (±3.6)

0.31 (±0.39)

4.7

−12.4

8.9

45 MPa

2.8 (±0.4)

0.43 (±0.03)

5.5

−11.7

11.2

65 MPa

3.9 (±2.9)

0.50 (±0.28)

6.3

−11.2

13.5


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Table 3. BET surface areas of metal oxide catalysts. BET surface area [m2/g] Catalyst before reaction

after reaction

Fe2O3

8.76

8.80

Fe3O4

4.29

4.30

Cr2O3

23.8

23.3

FeCr2O4

115

114


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Table 4. Rate constants and water properties at 350 °C and 450 °C.

Temper ature [°C]

Pressure [MPa]

S/V ratio -1

k [10-3 s-1]

[mm ]

1.8

0.74 (±0.08)

8.0

1.46 (±0.49)

1.8

1.07 (±0.14)

25

ksurface

Water density1

[10 s ]

[10-3 mm s1 ]

0.53

0.12

6.3

0.84

0.13

6.6

0*

27*

1.1

0.47

7.1

2.0

khomogeneous -3 -1

[102 kg/m3]

350 35 8.0

1.86 (±0.31)

1.8

19.0 (±4.1)

8.0

220 (±33)

1.8

13.3 (±1.0)

25 450 35 8.0

57.4 (±10.5)

*Although the intercept of S/V ratio vs. k gives a negative value, khomogeneous should not be negative. Therefore, khomogeneous is assumed to be zero, and ksurface is estimated by the slope of the fitting line which passes the zero point.


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SUPPORTING INFORMATION Details of experimental procedure, details of HPLC and IC analysis, and Figures S1 to S4 are provided as the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This work was supported in part by JSPS KAKENHI Grant Number 15K18262. The XRD analysis was performed using facilities at the Institute for Solid State Physics, The University of Tokyo. ICP-MS measurement was performed at the Kashiwa Branch, the Environmental Science Center, The University of Tokyo. The XPS analysis was performed using facilities at the Institute of Engineering Innovation, The University of Tokyo. Mr. Akira Yoko provided technical support for the XPS analyses, which is greatly appreciated.

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Abstract Graphics


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