Review pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Solid Acid/Base Catalysis in Sub- and Supercritical Water Makoto Akizuki* and Yoshito Oshima Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan S Supporting Information *
ABSTRACT: Solid acid/base catalysis in sub- and supercritical water (Sub&SCW) is a promising method to control organic reactions such as biomass conversions to chemicals and fuels, organic synthetic reactions, and degradation reactions. Synergistically using the unique characteristics of Sub&SCW and the catalytic properties of solid acid/base compounds, high reaction rates and/or high product selectivity can be achieved. This Review provides an overview of the use of solid acid/base catalysts for Sub&SCW reactions. Recent progress in the elucidation of the acid/base catalytic properties of solid catalysts in Sub&SCW and the effect of water properties on the catalysis is introduced. In addition, the application of solid acid/base catalysts in several research fields and the stability of these catalysts during reactions is discussed.
1. INTRODUCTION Sub- and supercritical water (Sub&SCW), defined as water near and above its critical point (374 °C, 22.1 MPa), is a potential medium for organic reactions such as organic syntheses1−3 and biomass conversions.4−6 Water in subcritical and supercritical conditions exhibits characteristics very different from those at ambient conditions. Many organic compounds can be dissolved in Sub&SCW due to its small dielectric constant (Figure 1a)7 and relatively large density (Figure 1b).8 The large ion product of water (KW = [H+][OH−]) in subcritical water and supercritical water at higher pressures (Figure 1c)9 is suitable for acid/base reactions, whereas KW is drastically small in supercritical water at lower pressures. These characteristics of water and the ability to adjust these properties by controlling temperature and pressure make Sub&SCW attractive solvents. Organic reactions in Sub&SCW have been conducted without catalysts, with homogeneous catalysts, and with solid catalysts. Particularly, reaction control using solid catalysts is attracting attention due to their ease of separation from solvents and their reusability. Because diffusion constant of water in subcritical and supercritical conditions is quite large (Figure 1d)10−12 compared to that at ambient conditions, reactions are less affected by mass transfer processes, which generally limits the reaction rates of solid catalysis. In addition, the large density of water miscible with organic compounds will suppress the deactivation of catalysts by organic catalyst poisons. Furthermore, the change in water properties such as ε and KW affects the surface properties of solid catalysts and the reactions occurring at the surfaces, and therefore it is thought that reactions using solid catalysts can be controlled by controlling the properties of water. © XXXX American Chemical Society
The major types of solid-catalyzed reactions carried out using Sub&SCW are complete oxidation, partial oxidation, hydrogenation, gasification, and acid/base reactions. Several researchers, including Savage13,14 and Kruse and Vogel,15 reviewed the work done in this field. Detailed reviews on oxidation reactions were published by Ding et al.16 and Savage et al.17 and Azadi and Farnood18 reviewed the available literature on gasification. In this Review, we focus on solid acid/base catalysis in Sub&SCW. Acid/base reactions are necessary for organic syntheses and biomass conversions, and various reactions have been investigated mainly without catalyst in Sub&SCW because Sub&SCW itself can act as acid/base catalyst.1,2,5 Moreover, reactions using solid acid/base catalysts are attracting attention as a method to achieve high reaction rates and/or high product selectivity, which could not be achieved by using the acid/base catalytic activity of Sub&SCW alone. Reviews including the use of acidic/basic catalysts are available on some types of reactions, but they are restricted to the type of reaction.6,19,20 Thus, a comprehensive review on the acid/base catalytic properties of solid catalysts and their application will provide us deep insight into solid acid/base catalysis as a reaction control method. We intentionally limit the temperature range of our review from 200 to 400 °C, in which range water shows unique solvent properties and adjustability. Regarding the solid acidcatalyzed reactions of biomass in water below 200 °C, many review articles have been published.21,22 Acidic/basic solids are Received: Revised: Accepted: Published: A
February 14, 2018 March 31, 2018 April 4, 2018 April 4, 2018 DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 1. Properties of water as a function of temperature. (a) dielectric constant,7 (b) density,8 (c) ion product,9 (d) diffusion coefficient.10−12 () 25 MPa, (··· ) 50 MPa, (○) saturated pressures of water.
also used as supports for Ni and precious metals, which are used for the catalytic gasification in water at high temperatures (ca. 350−650 °C);18 reactions using these supported metal catalysts are excluded in this review.
positive effect in overcoming the mass transfer limitation; however, the reaction rates and the porosity and size of the catalysts also affect the mass transfer process. Therefore, external mass transfer on shaped catalysts is thought to be negligible, but internal mass transfer in catalyst pores cannot be ignored. Because the mass transfer process affects the reaction rate and product selectivity in consecutive reactions, considering it is necessary in evaluating the apparent acid and base catalytic properties. 2.1. Investigation of Acidity and Basicity Using Model Reactions. One method to investigate the acidity and basicity of catalysts is by using test reactions. Watanabe et al.25 investigated effective OH− concentrations (the number of base sites) on several metal oxide surfaces using formaldehyde decomposition as the test reaction. They reported that the order of effective OH− concentrations was CeO2 > ZrO2 > MoO3 > rutile-TiO2 > anatase-TiO2 at 400 °C and 30 MPa, and that the order is related to the electronegativity of the metal ion. Because the electronegativity of the metal ion is generally used as an indicator of acid−base properties of metal oxide catalysts,26,27 their observation confirms that acidity/basicity is common to some extent in both gas phase and Sub&SCW. They also examined the acidity and basicity of these metal oxides using various test reactions (reactions of formaldehyde, acetic acid, 2-propanol, and glucose)28 and reported that MoO3 and anatase-TiO2 show acidity, whereas rutile-TiO2 and ZrO2 exhibit both acidity and basicity and CeO2 exhibits basicity in supercritical water at 400 °C and 25−35 MPa. Regarding the number of acid sites, Tomita et al.29,30 reported that the H+ concentrations (the number of Brønsted acid sites) on MoO3/Al2O3 and TiO2 surfaces increased with increasing pressure; they used propylene hydration as the test reaction. They mentioned that water dissociation on the surface
2. ACID AND BASE CATALYTIC PROPERTIES OF SOLID CATALYSTS IN SUB&SCW The acidity and basicity of catalysts, which can be defined by the types of acid/base sites (Brønsted/Lewis), strength of acid/ base sites, and number of acid/base sites, are strongly related to reaction rate and selectivity. Brønsted acid/base sites are defined as active sites donating/accepting protons, whereas Lewis acid/base sites are defined as active sites accepting/ donating electrons. Typically, protonic acids immobilized on solid and acidic hydroxyl groups on solid surfaces act as Brønsted acids, whereas metal cations exposed on solid surface acts as Lewis acids.23 In the case of basic sites, hydroxyl groups or oxygen atoms on the surface can act as Brønsted bases as well as Lewis bases.24 Because reaction mechanisms on these two types of acid/base sites are different, the reaction rate and selectivity strongly depend on the type of acid/base sites (Brønsted/Lewis). In addition, the acid/base strength is important in controlling solid acid/base reactions as it determines whether the acid/base sites can activate organics or not. Further, the number of acid/base sites is important because it determines the rate of reaction. In Sub&SCW, though the acidity and basicity of catalysts are thought to be common to some extent, they are different from those in vapor phase due to the existence of dense water. Therefore, elucidating the acidity and basicity in Sub&SCW is necessary to control the reaction rate and selectivity. In addition, elucidating the effect of mass transfer processes is also important. The large diffusivity of Sub&SCW exerts a B
DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research is promoted by a large KW at high pressures, which causes an increase in the H+ concentration at the surface. Using pressure dependence experiments, they determined that the H + concentrations were proportional to the 0.40th (MoO3/ Al2O3) or 0.43th (TiO2) power of KW. Using theoretical considerations on the dissociation of water on metal oxide surfaces, it was suggested that the H+ concentration is proportional to the 0.45th power of KW. Because water is often considered as a catalyst poison in typical solid acid catalysis, the increase in H+ concentration at high pressures is a unique feature of solid acid catalysis in Sub&SCW. An increase in the reaction rate was also observed at high pressures in cyclohexene reactions using WO 3 /ZrO 2 in subcritical water31and in 1-octene reactions using TiO2, Nb2O5, and Nb2O5/TiO2 in supercritical water.32 However, a constant decrease in the reaction rates was also observed with increasing pressure in some reactions such as glycerol dehydration using TiO2, Nb2O5, Nb2O5/TiO2, and WO3/TiO2 in supercritical water.32,33 The effect of the properties of water and reaction substrates on the rates of acid-catalyzed reactions were explained on the basis of the Langmuir−Hinshelwood model considering both the competitive adsorption of water and reaction substrate as well as the increase in H+ concentration with an increase in the water density. Concerning the types of acid sites, Akizuki et al.34,35 considered 1-octene and 2-octanol test reactions and reported that the types of acid sites (Brønsted/Lewis) of TiO2 changed depending on the density of water. They reported that TiO2 exhibits Lewis acidity in supercritical water at a low density of water, but its Brønsted acidity is increased in Sub&SCW under high water density conditions due to a large KW, and further, they proposed a quantitative acidity model on the basis of kinetic analysis (Figure 2). The product selectivity of cis-/trans-
water promotes the formation of Brønsted acid sites even under low water density conditions. 2.2. Effect of Mass Transfer Process on Catalysis. Despite the importance for reaction control, investigation on the mass transfer process in solid acid/base catalysis in Sub&SCW has been limited. Examining the mass transfer effect in reactions conducted in a batch reactor is quite difficult. On the basis of the value of the activation energy, Mo and Savage suggested that the reaction of palmitic acid using ZSM-5 zeolite at 400 °C and 25 MPa is diffusion limited.36 When the reaction is conducted in a flow reactor, the mass transfer effect can be evaluated by standard methods in reaction engineering. Akizuki et al. evaluated the mass transfer effect by comparing the apparent reaction rate and estimated diffusion rate in catalyst pores. During the hydrolysis of N-substituted amides using ZrO2 at 300 °C and 10 MPa, the reactions were almost limited by the surface reactions, but the mass transfer effect could not be ignored for several reactants such as benzamide and N-phenylbenzamide, because of their higher reaction rates.37 During the aldol condensation of acetone and benzaldehyde using Mg(OH)2 at 250−450 °C and 23−31 MPa, the mass transfer effect did not limit the reaction.38 To estimate the diffusion rate, estimating the binary diffusion coefficients of organics in Sub&SCW is necessary. Recently, Kraft and Vogel reviewed the binary diffusion coefficients determined experimentally in Sub&SCW and suggested that the diffusion coefficient near the critical point can be estimated using the Wilke-Chang equation, whereas that above the critical point can be estimated using the Stokes−Einstein equation.39 In addition, the mass transfer effect can be evaluated in a more straightforward manner by comparing the reaction rates using catalysts of different sizes, and this method is more accurate because the estimation of the diffusion coefficient is unnecessary. Akizuki and Oshima33 evaluated the effect of mass transfer on the dehydration of glycerol using TiO2 and WO3/TiO2 at 400 °C and 25−33 MPa using a series of granular catalysts of different sizes. The reaction using TiO2 was completely limited by surface reactions, whereas the reaction using WO3/TiO2 was limited by both surface reactions and mass transfer due to the higher surface reaction rate. Comparing the reactions at 25 and 33 MPa, although the diffusion rate was thought to be smaller at 33 MPa, the surface reaction rate was much smaller at this pressure because water inhibited the reaction at large water density conditions and hence the mass transfer effect was smaller at 33 MPa. 2.3. Interactions between Sub&SCW and Solid Surface. Elucidating the interactions between Sub&SCW and solid surfaces is another method to investigate the acid and base catalytic properties of solid catalysts in Sub&SCW. Particularly, the interactions between Sub&SCW and metal oxides have been investigated well. Although such studies are not focused on acid/base catalysis, the findings help us to understand the fundamental characteristics of the reaction field. In aqueous solutions, solid oxides are electrically charged and the pH resulting in a zero net surface charge is called the isoelectric point or zero point of charge (ZPC).40 Because the surface charge is related to the acidity or basicity of the surface hydroxyl groups, ZPC is an important index in solid acid/base catalysis.26 The ZPC at subcritical conditions was experimentally measured for several metal oxides, such as TiO2,41 Fe3O4,42−44 Fe2O3,43 CoFe2O4,44 NiFe2O4,44 ZrO2,45 and SnO2.46 Machesky et al.41 reported that the difference between
Figure 2. Schematic image of the acidity model of TiO2 in Sub&SCW.35
2-octene, which reflects the types of acid sites, can also be explained by the model. The changeability in the type of acid sites, which is dependent not on the catalyst species but on the reaction conditions, is a unique characteristic of solid acid catalysis in Sub&SCW. Akizuki at al. also investigated the types and number of acid sites of various metal oxide catalysts (TiO2, Nb2O5, Nb2O5/TiO2, WO3/TiO2) in supercritical water at 400 °C and 25−41 or 65 MPa using the 1-octene and glycerol reactions.32,33 In supercritical water, Nb2O5 and WO3/TiO2 mainly exhibited Brønsted acidity. Nb2O5/TiO2 shows both Brønsted and Lewis acidity, whereas TiO2 tends to show Lewis acidity under low water density condition, and further, their Brønsted acidity increases with increase in the density of water. In addition, the fact that Nb2O5 and Nb2O5/TiO2, which typically show only Lewis acidity in the gas phase, exhibit Brønsted acidity in supercritical water suggests that supercritical C
DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Scheme 1. Reaction Pathway of Cellulose
both iron and sulfur sites are available for chemical reactions. Moreover, Schreiner et al.52 investigated the roles of subcritical water (227 °C, 20 MPa) and pyrite surface on the formation and hydrolysis of peptides in detail by ab initio molecular dynamics simulation.
the ZPC values of rutile-TiO2 and 1/2pKW values is rather constant in the range of 25−250 °C and suggested that it may reflect similarities between the hydration behavior of surface hydroxyl groups and water. A similar trend between ZPC and KW has been reported for other metal oxides, such as Fe3O4 at 25−290 °C42 and ZrO2 at 25−200 °C.45 On the other hand, Barale et al.44 reported that although the temperature dependence (5−320 °C) curves of ZPC of CoFe2O4 and Fe3O4 and that of 1/2pKW are similar, the minimum values are not located at the same temperature. Hence, the ZPC is below the pH of neutral water at temperatures below 150 °C (CoFe2O4) or 250 °C (Fe3O4), but it is above the pH of neutral water at higher temperatures. The behavior of water molecules near solid surfaces is also important while considering surface reactivity and diffusion. Předota et al.47,48 investigated the interactions of water molecules and the (110) surface of rutile-TiO2 in liquid water up to 250 °C using molecular dynamics simulations. They reported that three regions of different viscosities and diffusivities exist from the surface. (1) two adsorbed layers of water molecules with apparently infinite viscosity and zero diffusivity, (2) an interfacial inhomogeneous region consisting of 2−3 layers, and (3) bulk-liquid behavior recovered as close as 15 Å from the surface. The second layer of water molecules becomes more loosely bound to the surface with increasing temperature. Abe et al.49 investigated the water structure in mesoporous silica up to 400 °C and 20 MPa by analyzing infrared spectra and reported that the structure of water near the silica surface becomes ice-like and it is at least 10 nm thick. Ishikawa et al.50 investigated the behavior of water molecules confined between the (1010) surface of SiO2 (quartz) at 25− 300 °C using molecular dynamics simulations. They reported that the self-diffusion coefficient of water near the surface is reduced even at high temperatures. It was also mentioned that because the activation energy of the diffusion process in confined geometries is close to that of bulk water, the diffusion mechanism is similar and the activation energy may be interpreted by the dissociation of the hydrogen bond. In addition to the research concerning metal oxides, the interaction between Sub&SCW and pyrite (FeS2) has been investigated from the interest of prebiotic chemical reactions occurred in the ancient earth. Stirling et al.51 studied the behavior of supercritical water on pyrite at 527 °C and 100 MPa (ρ = 0.5 g/cm3) by ab initio molecular dynamics simulation. They reported that, in contrast to the ambient conditions where the surface in fully covered with adsorbed water molecules, supercritical conditions effectively eliminate the water adsorption layer from the surface, suggesting that
3. REACTIONS CATALYZED BY SOLID ACID/BASE CATALYSTS IN SUB&SCW The reactions catalyzed by solid acid/base catalysts in Sub&SCW are summarized in Table S1 in the Supporting Information. Typical solid acid catalysts used in Sub&SCW are metal oxides of Ti, Zr, Nb, Mo, W, and Al, mixed metal oxides, metal phosphate of Zr, Ca, and Sr, zeolites, clays, such as montmorillonite, and carbon materials, such as graphene oxide. Sulfonated catalysts and other acid-treated catalysts are also used, particularly under lower temperature conditions. Solid base catalysts frequently used in Sub&SCW are metal oxides and hydroxides of Zr, Ce, Mg, and Ca, clays, such as hydrotalcite and bentonite, and ion-exchanged montmorillonite or zeolite. Note that the acidity and basicity of these catalysts play important roles in the reactions summarized in Table S1; however, other properties of these catalysts are also important for some reactions. For example, CeO2, which is known to have basic catalytic properties, is reported to have redox catalytic properties in Sub&SCW at 350 or 400 °C, and 25 MPa.53 3.1. Biomass to Chemicals (monosaccharide, 5hydroxymethylfural, furfural, and levulinic acid). The conversion of cellulose and hemicellulose contained in biomass into valuable chemicals in Sub&SCW is a major research area, and various studies on the reactions of C6-monosaccharides,54−61 C5-monosaccharides,58,62 cellulose,58−60,63−68 and biomass products58,69,70 have been conducted using solid acid/base catalysts. In these reactions, catalysts with acidity or both acidity and basicity are typically used. The simplified reaction pathways of cellulose and hemicellulose are shown in Schemes 1 and 2. Glucose, the monomeic component of cellulose, isomerizes into fructose and dehydrates into 5-hydroxymethylfurfural (5HMF). Watanabe et al.54,55 reported that in the reaction of glycerol at 200 °C, ZrO2 promoted the isomerization of glucose Scheme 2. Reaction Pathway of Hemicellulose
D
DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Scheme 3. Reaction Pathway of Biomass to Lactic Acid73,76
Scheme 4. Reaction Pathway of α-Pinene to Monocyclic Monoterpenes81
Xue et al.71 conducted reactions on glucose using basic clays at 260 °C and obtained a lactic acid yield of 12.37% and 10.84% using albite and potassium feldspar, respectively, as the catalysts. Cao et al.74 reported that a lactic acid yield of 39% can be obtained with 100% glucose conversion using Nb2O5 nanorods as the catalyst at 250 °C for 4 h, and suggested that high Lewis acidity of the catalyst contributed to the reaction. Wang et al.75 reported that a lactic acid yield of 67.6% with 100% cellulose conversion using Er-montmorillonite as the catalyst at 240 °C for 30 min. Yang et al.76 reported that xylose and xylan were converted to lactic acid with yields as high as 42% and 30%, respectively, when ZrO2 was used as the catalyst. After comparing reactions using ZrO2 and MgO catalysts, they reported that bifunctional ZrO2 is more efficient in lactic acid synthesis compared to the monofunctional base catalyst, MgO. The conversion of alginate derived from macroalgae to lactic acid using solid base catalysts was investigated by Jeon et al.77 They reported that CaO catalyzed the production of lactic acid with a maximum yield of 14.66% at 200 °C for 6 min. 3.3. Biomass to Chemicals (others). Apart from reactions on carbohydrate biomass, reactions on other biomass products and biomass-related materials have also been studied. The dehydration of glycerol, which is a byproduct of biodiesel synthesis, using solid acid catalysts was investigated by Akizuki et al.32,33,78,79 They reported that the acrolein yield increased with an increase in the content of tungsten in WO3/ TiO2 catalysts owing to their larger surface area and stronger acidity. An acrolein yield of 53% at a glycerol conversion of 92% was obtained using 10 wt %WO3/TiO2 as the catalyst at 400 °C and 33 MPa when W/F (catalyst weight divided by volumetric flow rate) = 1.6 × 103 kg-cat s/m3 (∼2 s).78 It was also reported that a Nb2O5/TiO2 catalyst also promoted glycerol dehydration to acrolein, although the selectivity of acrolein was less than that when WO3/TiO2 was used as the catalyst.79 Eom et al.80 investigated the conversion of phenethyl phenyl ether as a model compound into lignin using ZrO2 and NaZrO2 as the catalysts at 400 °C in hydrogen atmosphere. They reported that Na-ZrO2 exhibited superior activity due to basicity of the catalyst and the yields of phenol and styrene were 39% and 16%, respectively, at a phenethyl phenyl ether conversion of 97%. Akizuki et al.81 investigated the acid-catalyzed isomerization of α-pinene, a major component of turpentine oil, using TiO2 and WO3/TiO2 as the catalysts at 250 °C and 7 MPa (Scheme 4). The major products were limonene and terpinolene when TiO2 was used as the catalyst, and α-terpinene, γ-terpinene, and
into fructose, whereas anatase-TiO2 promoted isomerization and dehydration into 5-HMF. Because isomerization is a basecatalyzed reaction and dehydration is an acid-catalyzed reaction, the dual characters of anatase-TiO2 (acidity and basicity) led to a high 5-HMF yield. Daorattanachai et al.60 reported that a 5HMF yield of 21% was obtained at a glucose conversion of 60%; this conversion was achieved using α-Sr(PO3)2 as the catalyst at 220 °C for 5 min. Regarding the fructose reaction, Asghari and Yoshida56 reported that a 5-HMF yield of 48% at a fructose conversion of 80% could be obtained using Zr phosphate as the catalyst at 240 °C for 2 min. In addition to glucose and fructose, the dehydration of sorbitol, which is produced by the hydrogenation of glucose, to isosorbide was investigated. Otomo et al.61 reported that Beta-zeolite with a Si/Al ratio of 75 resulted in an isosorbide yield as high as 80% at 200 °C in ∼18 h. Furfural is produced by the dehydration of xylose, which is a major monomeric component of hemicellulose. Lam et al.62 reported a furfural yield of 62% at a xylose conversion of 83% using sulfonated graphene oxide as the catalyst at 200 °C for 35 min. Regarding cellulose conversion, the direct conversion of cellulose to 5-HMF by the depolymerization of cellulose into glucose and its consecutive reactions has been examined.58−60 Furthermore, because 5-HMF can be converted into levulinic acid, the direct conversion of cellulose into levulinic acid has also been investigated.63−65 Joshi et al.65 conducted cellulose reactions with as the ZrO2 catalyst at 120−200 °C and reported that a levulinic acid yield of 53.9% at 100% cellulose conversion at 180 °C for 3 h. Selective conversion of cellulose to glucose is another challenge that has been investigated. Foo et al.66 reported that sulfonated activated carbon treated in hot liquid water led to a moderate cellobiose conversion but a high glucose selectivity due to a larger fraction of weak acid sites. Misson et al.67 examined cellulose reaction using graphene oxide as the catalyst accompanied by microwave heating at 160−200 °C and obtained a glucose yield as high as 61% at 180 °C for 60 min. The conversions of biomass products, such as sugar cane bagasse,58,69 rice husk,69 corncob,69 and rice straw,70 into monosaccharide, 5-HMF, and furfral have also been investigated using solid catalysts. 3.2. Biomass to Chemicals (lactic acid). The conversion of carbohydrate biomass into lactic acid is also a topic of interest and reactions on glucose,71−74 cellulose,75 xylose,76 xylan,76 and alginate77 have been investigated. These reactions proceed via glyceraldehyde (Scheme 3),73,76 and promoted by base catalysts and/or Lewis acid catalysts. E
DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Scheme 5. Reaction Pathways of Citric Acid, Itaconic Acid, and 2-Hydroxyisobutyric Acid to Methacrylic Acid82
materials, such as high-diversity grass land perennials,86 rice husk,87 dried distiller’s grains with solubles,88 Russian olive seed,89 bagasse,90 sweet sorghum bagasse,91 and empty fruit bunch derived from oil palm residues92 were examined using solid acid/base catalysts. Shi et al.87 reported that a biocrude yield of 32.5% from rice husk using La2O3 as the catalyst at 300 °C for 10 min; further, the heating values of biocrude using metal oxide catalysts increased due to a reduction in the oxygen content. Long et al.90 investigated bagasse liquefaction using a series of MgMOX (M = Mn, Ni, Fe, Cr, Zn, and Al) catalysts. They reported 93.7% liquefaction with a water-soluble fraction of 59.5% using MgMnO2 as the catalyst at 250 °C for 15 min; lignin, the most recalcitrant fragment in biomass hydrothermal liquefaction, was efficiently depolymerized using catalysts. Regarding the liquefaction of empty fruit bunch derived from oil palm residues, Yim et al.92 screened metal oxide catalysts on the basis of their activity and low electronegativity and reported a maximum relative yield 1.44 times higher than that obtained without catalyst was obtained; in this case CeO2 was used as the catalyst at 390 °C and 25 MPa for 15 min. The production of biofuel from sludge was also investigated.93,94 Hammerschmidt et al.93 examined the continuous liquefaction of sludge using both ZrO2 and K2CO3 as the catalysts at 330−350 °C. They investigated the effect of several process parameters (temperature, residence time, feed/ recirculation ratio, etc.) on the products and estimated the energy gain of the process using the experimental data and theoretical calculations. Solid acid/base catalysis was also used for preprocessing or post processing of hydrothermal processes. Duan et al.95 reported the upgrading of algal bio-oil (produced by hydrothermal liquefaction) using zeolite catalysts with H2 at 400 °C. Onwudili and Williams96 investigated the reaction of pine wood sawdust using Nb2O5 catalyst at 280 °C and 8 MPa as a preprocessing method for supercritical water gasification. Note that for the conversion of biomass into fuels, solid catalysts containing precious metals or Ni, which are frequently used in gasification, are also used and show good activity.85,91 3.5. Olefin Hydration and Related Reactions. Olefin hydration to alcohol in Sub&SCW is expected to proceed fast due to the high affinity of olefins to Sub&SCW. Reactions of propylene,29,30 cyclohexene,31 and 1-octene32,34,35 have been examined using solid acid catalysts. Further, the dehydration of alcohol to olefins, which is thermodynamically dominant at higher temperatures, was studied using 2-propanol,28 2butanol,97 and 2-octanol.35 Tomita et al.29,30 investigated propylene hydration using MoO3/Al2O3 and TiO2 catalysts at 100−470 °C and 21−31 MPa. They reported that the conversion of propylene was enhanced at large KW conditions. In particular, 2-propanol was
isoterpinolene were obtained when WO3/TiO2 was used as the catalyst. In subcritical water using solid acid catalysts, α-pinene isomerized selectively into monocyclic monoterpenes owing to both the dominant Brønsted acid character due to large KW and the high reaction rates of solid acid catalysis. Pirmoradi and Kastner82 reported the base-catalyzed synthesis of methacrylic acid from biobased substrates related to the tricarboxylic acid cycle using hydrotalcite as the catalyst (Scheme 5). The catalyst was prepared by the calcination of raw hydrotalcite (Mg6Al2(CO3)(OH)15·4H2O). Using citric acid and itaconoic acid, they obtained methacrylic acid yields of 21% and 23%, respectively, with ∼100% conversion at 250 °C for 15 min. During the 2-hydroxyisobutylic acid reaction, a methacrylic acid yield of 71.5% at a conversion of 83% was achieved at 275 °C for 1 min. 3.4. Biomass to Fuels. The conversion of biomass into fuels is another major research subject in Sub&SCW. Both solid acid and solid base catalysts are used, and the reactions are conducted at relatively higher temperatures compared to the conversion of biomass to chemicals. Fatty acids are promising raw materials for biofuel production. Watanabe et al.83 studied the reaction of stearic acid using metal oxide catalysts (CeO2, Y2O3, and ZrO2) at 400 °C and 25 MPa for 30 min and reported that these catalysts promoted bimolecular decarboxylation to form CO2 and C16 alkenes. Mo et al.36,84 investigated the reactions of palmitic acid, stearic acid, oleic acid, and linoleic acid to aromatics using zeolite catalysts (Y, Beta, and ZSM-5) at 400 °C and 25 MPa for 3 h (Scheme 6). They obtained a total liquid and gas yield Scheme 6. Conversion of Palmitic Acid into Aromatics36
of 97 wt % (86% of the liquid product yield) from palmitic acid using ZSM-5 with a Si/Al ratio of 23 as the catalyst. In addition, Duan and Savage85 reported the liquefaction of the macroalgae, Nannochloropsis sp., which is known to contain substantial amounts of fatty acids, using solid catalysts, such as zeolite, at 350 °C. The liquefaction of carbohydrate biomass is another promising reaction and reactions of a number of biomass F
DOI: 10.1021/acs.iecr.8b00753 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
were produced by the bimolecular decarboxylation of acetic acid. The decomposition of formaldehyde using metal oxide catalysts at 400 °C and 30 MPa was also investigated by Watanabe et al.25 It was suggested that formaldehyde primarily decomposed according to the Cannizzaro reaction and selfdecomposition; the contribution of each reaction depended on the acidity/basicity of the metal oxide catalysts. The hydrolysis of amides (Scheme 9) is used for the wastewater treatment and monomer recycle from polyamides.
formed exclusively when MoO3/Al2O3 was used as the catalyst, whereas a small amount of acetone was also produced by dehydrogenation of 2-propanol using TiO2 as the catalyst. The production of acetone (as a byproduct) from 2-propanol was reported by Watanabe et al.28 using rutile-TiO2 and ZrO2 as the catalysts at 400 °C and 30 MPa owing to the basicity of these catalysts. Yuan et al.31 conducted cyclohexene hydration using WO3/ ZrO2 as the catalyst. They reported that the major product was cyclohexanol at 225−275 °C because the water/cyclohexene system was converted into a single phase. Increasing the temperature to 300 °C or higher resulted in the formation of condensed ring hydrocarbons. In reaction involving linear olefins with more than three carbons, double bond isomerization takes place in addition to hydration (Scheme 7). Siskin and Brons98 reported that the
Scheme 9. Hydrolysis of Amides
Scheme 7. Reaction Pathways of Liner Olefin The hydrolysis of N-substituted amides using ZrO2 as a solid base catalyst at 240−360 °C and 10−41 MPa was examined by Akizuki and Oshima.37 N-substitution largely affected the rate of the hydrolysis reaction.The N-substitution effect when ZrO2 was used as the catalyst was different from the cases when no catalyst was used and NaOH was used as a homogeneous base catalyst. The difference was discussed by kinetic analyses using a modified Taft equation.100 In addition, they reported that the surface of the SUS316 reactor promoted the hydrolysis of benzamide at 350−450 °C and 25−65 MPa due to the activity of Fe2O3 at the surface.101 The hydrolysis of polyamide using Al2O3-supported solid acids and zeolite catalysts was investigated by Wang et al.102,103 The production of ε-caprolactam and oligomers from monomer casting nylon was observed. They reported that Beta-zeolite with a Si/Al ratio of 25 exhibited the highest activity among zeolites and it accelerated the generation and consumption of linear oligomers as a result of microporous structure. In addition to these studies, reactions of cholesterol,104 n-C10 aliphatic compounds,98 and nitrogen-containing heterocycles105 were investigated using clay catalysts over long reaction times (1−13.5 days).
isomerization of 1-decene to 2-decene was promoted using Camontmorillonite catalyst at 250 °C for 5.5 days. This was the first report on olefin reactions using solid acid/base catalysts in Sub&SCW. Akizuki et al.34 conducted 1-octene reactions using TiO2 as the catalyst at 250−450 °C and 11−33 MPa and reported that double bond isomerization and hydration proceeded in parallel. The major products were 2-octene (cis/trans) and 2-octanol; inner octenes and their corresponding alcohols were formed as the byproducts. They also reported that the selectivity of hydration toward isomerization was maximum at ∼330 °C.35 3.6. Synthetic Reactions. Although a number of studies on organic synthesis in Sub&SCW have been conducted,1−3 synthetic reactions using solid acid/base catalysts have been scarcely reported. Aldol condensation of acetone and benzaldehyde (Scheme 8) using Mg(OH)2 as the catalyst at
4. STABILITY OF CATALYSTS IN SUB&SCW In solid catalysis, not only the initial activity of the catalysts but also the stability of the catalysts is important. When using solid acid/base catalysts in Sub&SCW, high temperature and the existence of dense water may induce changes in the surface properties, structural collapse, and elution of active components. On the other hand, the existence of dense water, which is miscible with organic compounds, can suppress the deactivation of catalysts caused by organic catalyst poisons. Because of both the positive and negative effects of Sub&SCW, analyzing the stability of catalysts in Sub&SCW is particularly important. In this section, the stability of typical solid/base catalysts during reactions is briefly reviewed (based on the literature described in the previous section). The stability of catalysts for biomass conversion, including solid acid/base catalysts in hydrothermal conditions (superheated steam and Sub&SCW), was reviewed by Xiong et al.,106 with emphasis on the physical structure. 4.1. Ti and Zr Oxides. TiO2 and ZrO2, catalysts with moderate acidity and basicity, are chemically stable and generally remain active during reactions in Sub&SCW, although changes in crystalline structure sometimes occur.
Scheme 8. Aldol Condensation of Acetone and Benzaldehyde
250−450 °C and 23−31 MPa was investigated by Akizuki et al.38 The major product was benzylidene acetone, irrespective of the reaction conditions. They reported that the temperature dependence of the reaction rate can be explained by the Eley− Rideal mechanism, whereas the pressure dependence at 400 °C showed some deviation. 3.7. Decomposition. The decomposition of some functional groups or chemicals has been investigated. The decarboxylation of acetic acid using ZrO2 as the catalyst at 400 °C and 25 MPa was investigated by Watanabe et al.99 They reported that the major products were CO2 and acetone, which G
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MgO was converted into Mg(OH)2 at 200 °C for 1 h77 or at 250 or 400 °C and 25 MPa for 5 min.38 The leached content of Mg was lesser (0.07%) compared to that of Ca(OH)2 during treatment in water at 200 °C for 1 h but more (9.45%) than that of Ca(OH)2 during alginate conversion to lactic acid at 200 °C for 1 h.77 Regarding MgMnO2, it retained its activity as a solid base catalyst during baggase reaction at 250 °C for 15 min over 5 batch runs when reused with calcination.90 4.4. Zr Phosphate. Zr phosphate used for converting fructose to 5-HMF at 240 °C was deactivated, but could be regenerated by treatment in phosphoric acid. The regenerated catalyst maintained its activity for 6−7 batch runs.56 4.5. Zeolites. Beta-zeolite retains its acid catalytic properties during reactions at low temperature. ZSM-5 can be used at higher temperatures, but is deactivated irreversibly when used repeatedly. Beta-zeolite with a Si/Al ratio of 25 retained its activity during monomer casting nylon reaction at 345 °C for 30 min over 4 batch runs, although a decrease in the surface area and micropore volume were observed.102 Beta-zeolite with a Si/Al ratio of 32 was deactivated during cellulose reactions with microwave heating at 210 °C for 30 min due to coking, but it could be regenerated by calcination.64 Further, Beta-zeolite with a Si/Al ratio of 75, when used for sorbitol conversion into isosorbide, gradually lost its activity at 200 °C for 18 h due to the deposition of carbonaceous materials. Upon calcination, the catalyst retained its activity for 4 batch runs.61 ZSM-5 with a Si/Al ratio of 30 lost almost all its activity during palmitic acid reaction at 400 °C and 24 MPa for 3 h due to severe coking.36 The deactivation could be suppressed by calcination after each batch run (for ZSM-5 with Si/Al ratios of 2384 and 3036); however, reduction in conversion, xylene yield, and toluene yield were observed. 4.6. Carbon Catalysts. Graphene oxide was deactivated during cellulose reaction with microwave heating at 180 °C for 60 min.67 On the other hand, sulfonated graphene oxide maintained its activity during xylose conversion to furfural at 200 °C for 35 min over 12 batch runs, although carbonaceous materials were accumulated on the surfaces.62 Sulfonated activated carbon and that treated in subcritical water at 200 °C showed stable activity during cellobiose reaction at 200 °C and 2.5 MPa over a flow period of 10.5 h.66 4.7. Clay Catalysts. Albite retained its reactivity when reused for 5 runs during glucose conversion to lactic acid at 260 or 280 °C for 140 s.71 The hydrotalcite catalyst, prepared by calcination of Mg6Al2(CO3)(OH)16·4H2O, when used in citric acid reaction or itaconic acid reaction at 250 °C for 15 min retained its activity up to the second batch run. Moreover, the yield of methacrylic acid increased in the second run due to the structural changes occurring in the hydrotalcite catalyst.82 Meanwhile, bentonite was deactivated during glucose conversion to acids at 275 °C for 90 s because of the reduction in its crystallinity.73 Er montmorillonite was deactivated during cellulose conversion to lactic acid at 240 °C for 30 min due to a combination of Er leaching, deposition of carbon species, and structural changes.75
The crystalline structure of anatase-TiO2 did not change after usage for 19 h at 250 °C and 7 MPa in a flow reactor81 and after 30 min at 400 °C and 30 MPa in a batch reactor.25 However, anatase-TiO2 was converted into rutile-TiO2 at 450 °C and 25 MPa in a flow reactor, which caused a reduction in the surface area.78 The transformation of anatase to rutile can be inhibited by the addition of more than 5 wt % WO333 or 10 wt % Nb2O532 to the catalyst. Regarding the crystalline structure of ZrO2, amorphous-ZrO2·XH2O and tetragonalZrO2 were changed into monoclinic ZrO2 at 400 °C and 30 MPa in a batch reactor.99 TiO2 retained its activity during sugar cane bagasse reaction at 250 °C for 5 min over 5 batch runs58 and during α-pinene reaction at 250 °C and 7 MPa for 6 h flow.81 ZrO2 also maintained its activity during sugar cane bagasse reaction at 250 °C for 5 min over 5 batch runs,58 but its activity during xylose conversion to lactic acid decreased at 200 °C for 40 min due to the formation of carboxylic acid anhydride groups on the surface.76 TiO2−ZrO2 retained its activity in corncob reaction at 250 °C for 5 min over 5 batch runs.69 Meanwhile, sulfonated ZrO2 was gradually deactivated during sugar cane bagasse reaction at 250 °C for 5 min, probably due to the leaching of sulfonic acid groups.58 Sulfonated ZrO2− Al2O3 was also deactivated during catalyst reuse in the hydrolysis of monomer casting nylon at 345 °C for 30 min.102 4.2. Nb and W Oxides. Nb2O5 is chemically stable and typically maintains its acid catalytic activity in Sub&SCW unless coking occurs. WO3 acts stably as a solid acid at low temperatures, but is deactivated at high temperatures and pressures due to leaching. Amorphous-Nb2O5·XH2O was deactivated during 2-butanol reaction at 240 °C and 5.1 MPa due to the formation of large faceted crystallites. On the other hand, although their initial activity was smaller than Nb2O5·XH2O, 4.1 and 8.9 wt % Nb2O5/carbon retained their activity for more than 25 h as embedding Nb 2 O5 in carbon prevents the growth of crystallites.97 The activity of Nb2O5 nanorods decreased during glucose conversion to lactic acid at 250 °C for 4 h due to coking, but when reused with calcination, their activity was retained over 4 batch runs.74 Nb2O5/TiO2 was slightly deactivated during glycerol reaction at 400 °C and 25 or 33 MPa for 6 h flow due to coking. The amount of formed coke was less at 33 MPa than at 25 MPa, because coke precursors were transferred from catalyst pores more effectively at 33 MPa. However, Nb2O5 leaching did not occur at both pressures.79 WO3/TiO2 maintained its activity during α-pinene reaction at 250 °C and 7 MPa over a 6 h flow period.81 However, it was deactivated during glycerol reaction at 400 °C and 25 or 33 MPa. The major reasons behind the deactivation were coking at 25 MPa or WO3 leaching at 33 MPa due to a large KW. Leaching of WO3 was significant during the heating-up period.79 4.3. Ca and Mg Hydroxides. CaO and MgO are converted into Ca(OH)2 and Mg(OH)2 in Sub&SCW, and these hydroxides remain active as solid bases. CaO is converted into Ca(OH)2 at 200 °C for 1 h. Leached Ca was 5.0% during treatment in water at 200 °C for 1 h and 8.6% during alginate conversion to lactic acid at 200 °C for 1 h; leaching seemed to be promoted by the produced organic acids. During the reaction, Ca(OH)2 retained its activity over 2 batch runs, but was deactivated in the third run due to coking. The deactivated catalyst could be regenerated by calcination.77
5. CONCLUDING REMARKS Solid acid/base catalysis in Sub&SCW has been extensively studied in this Review. The acid and base catalytic properties of solid catalysts in Sub&SCW and the effects of the properties of water on the said properties have been elucidated to a considerable extent mainly by using model reactions. Further H
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research using the other experimental methods, such as the direct observation of catalyst surfaces by spectrometry and understanding at the molecular level by quantum chemical calculations and molecular dynamics simulations, will enable us to predict and control catalytic properties in Sub&SCW. Many applications have been investigated especially biomass conversion to chemicals and fuels, and these reactions play important roles in biorefinery processes. Synthetic reactions have been investigated to a lesser extent thus far, but the use in the synthetic processes of high value-added chemicals will gain traction in the near future. Moreover, developing catalysts suitable for use in Sub&SCW by considering the effects of water on the surface properties and durability of catalysts is also necessary. Some well-known solid acid/base catalysts such as zeolites and clay catalysts are not highly stable in Sub&SCW, and their uses have typically been limited to reactions in low temperature. On the other hand, approaches to improve hydrothermal stability of catalysts by doping or substituting of metal cations107 and surface modification108,109 have been reported, and improving the stability of zeolites and clay catalysts by these methods will lead to a highly selective catalysis in Sub&SCW owing to their porous or layered structures. Both the reactions in Sub&SCW and the reactions catalyzed by solid acid/base catalysts are attracting attentions as promising reaction control methods with low environmental load. By combining these two methods, unique reaction controls including the adjustment for acid/base catalytic properties of solid catalyst using the characteristics of Sub&SCW can be obtained, and it can lead to the development of a novel environmentally friendly technology to control various organic reactions.
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Makoto Akizuki received his B.S. degree in Chemical Engineering from The University of Tokyo in 2008. He received his M.S. degree in 2010 and Ph.D. degree in 2013 from The University of Tokyo. These degrees are in Environmental Studies. From 2013, he is an assistant professor at Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo. His research interest is in environmentally friendly chemical processes using sub- and supercritical fluids. Particularly, he is focusing on the control of chemical reactions and catalysis using characteristics of sub- and supercritical solvents.
Yoshito Oshima received his Ph.D. degree in Engineering in 1993 from The University of Tokyo. He became a professor at Environmental Science Center, The University of Tokyo in 2003. From 2005, he is a professor at Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo. His research fields are application of supercritical fluid technology to treatment of hazardous waste, material recycle, organic synthesis, and nanoparticle synthesis. He is also interested in environmental safety in university’s laboratory focusing on behaviors of experimenters and air flows in experimental rooms.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b00753.
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Reactions catalyzed by solid acid/base catalysts in Sub&SCW (PDF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*M. Akizuki. E-mail:
[email protected]. ORCID
Makoto Akizuki: 0000-0001-9350-0036 Yoshito Oshima: 0000-0002-6222-3774 Notes
The authors declare no competing financial interest. I
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