1 Introduction
All kinds of chemical reactions involving water are going on in nature all the time, and the existence of water is also a prerequisite for the beginning and continuation of life. The study of the structure and properties of water and hydrothermal chemical reactions has important guiding significance for green chemical synthesis and understanding the origin of life.
In this paper, the chemical reactions in hydrothermal systems are reviewed from the perspective of condensed matter chemistry, starting from water molecules and their clusters. As the most common reaction medium, condensed liquid water is environmentally friendly and has a wide range of sources. In the hydrothermal system, water in the subcritical or critical state plays a unique role in the synthesis of inorganic functional materials and bio-hydrothermal synthesis. Hydrothermal method has irreplaceable advantages in the preparation of inorganic large single crystal, micro-nano materials, molecular sieves, porous materials and so on, and many breakthroughs have been made in the preparation of inorganic functional materials by hydrothermal synthesis. In recent years, with the improvement of research technology, people's understanding of water structure and water clusters has become more profound. Some scientific questions that have puzzled people for a long time, such as the structure of solid ice and the existence form of hydrogen bonds in supercritical water system, have gradually found answers in the research of researchers.
In this paper, we start with the properties of water under different temperature and pressure conditions, transition to the condensed state involved in hydrothermal systems, and finally focus on the synthesis of inorganic materials in hydrothermal systems, organic chemistry under hydrothermal conditions, and the hydrothermal synthesis of biomolecules, and summarize the condensed state problems and hydrothermal chemical reactions in hydrothermal systems. The physical and chemical properties of condensed water in hydrothermal chemistry and the control of reaction process by water as a reaction component were explored.It plays a vital role in the development of hydrothermal synthesis to prepare more functional advanced materials and help us learn more deeply about the chemical reactions in nature. It is hoped that this review can provide some reference for future hydrothermal chemistry research.
2 Structural characteristics of water molecules and molecular groups
2.1 Structure of water molecule
The water molecule (H
2O) is a V-shaped structure molecule with a bond angle of 104.45 ° formed by one O atom and two H atoms, and both O — H bond lengths are 95.84 pm (Fig. 1). In a water molecule, the electron cloud is biased toward the more electronegative O atom, which is partially negatively charged, while the two hydrogen atoms are partially positively charged. The positive and negative charge centers in the water molecule do not coincide, so the water molecule is a typical polar molecule. As a molecule with a large value of dipole moment μ (μ=6.23×10
-30C m), water is a good polar solvent
[1]. Many kinds of chemical reactions can be carried out in aqueous media, such as hydrolysis, metathesis, precipitation, acid-base, oxidation-reduction, etc. The study of the structure and properties of water is of great significance for further understanding the mechanism and law of chemical reactions.
2.2 Structure of water molecular group
Water in nature exists in the form of water clusters (H
2O)
N formed by N water molecules through hydrogen bond association. Due to the saturation and directionality of the weak force of hydrogen bonds, water clusters have different steric configurations. At present, the spatial configuration formed by the (H
2O)
5 of water clusters with N ≤ 5 has been deeply studied and understood. However, the spatial configuration of water clusters with N ≥ 6 is still controversial
[2]. The study of the cluster configuration of water molecules is mainly carried out by means of theoretical calculation prediction and experimental observation. The spatial configurations of water clusters composed of less than 20 water molecules can be searched in "The Cambridge Cluster Database", where The configurations and interaction energies of water clusters with different numbers of water molecules can be obtained based on the RHF/6-31 G (d, p) level
[3]. Hodges et al. systematically calculated the lowest possible energy configurations of N ≤ 21 clusters by using the TIP4P potential (Fig. 2)
[4]. The (H
2O)
20 containing 20 water molecules obtained by this empirical method has five configurations: edge-sharing pentagonal prism configuration, connected cubic configuration, coplanar pentagonal prism configuration, dodecahedron configuration and box-kite configuration, among which the box-kite configuration has the lowest energy.
2.3 Water molecular cage with guest molecules
The cage-like structure formed by water clusters provides space for the storage of size-matched guest molecules. Common gaseous alkane molecules with low carbon number can exist in the form of single molecules or mixed combinations of multiple molecules in the cage of water molecules.
Natural gas hydrate is a solid compound formed by non-stoichiometric natural gas and water at low temperature and high pressure
[5]. Unlike traditional mobile phase hydrocarbons, gas molecules are highly compressed in this class of solid crystalline compounds. Therefore, the research on the simulation and physicochemical properties of this kind of water molecular cage with guest molecules has guiding significance in energy gas storage and transportation, energy and coping with seasonal changes. In general, cage hydrates are formed when water contacts guest molecules with molecular sizes less than 0.9 nm at ambient temperatures below 27 ℃ and pressures greater than 0.6 MPa
[6]. Hydrate cages are made up of water molecules bonded together by hydrogen bonds to form a cavity in which individual molecules are "locked". According to the size of storable guest molecules, the structures of hydrate molecular cages can be divided into three categories (Figure 3): cubic structure I, which exists in nature in large quantities and can accommodate small guest molecules (molecular size is about 0.4 ~ 0.55 nm, such as CH
4, C
2H
6, CO
2, etc.)
[6]; Cubic structure II, which mainly exists in artificial environments and can be used to store guest molecules with slightly larger size (molecular size is about 0.6 ~ 0.7 nm, such as C
3H
8, C
4H
10, etc.); Hexagonal structure H, which can store small molecules and larger molecular mixtures (macromolecular size is about 0.8 ~ 0.9 nm, such as CH
4 and new hexane, CH
4 and cyclohexane, etc.). The number of water molecules in each unit in the cavity of structure I, II and H is 46, 136 and 34, respectively
[7]. In the methane hydrate cage with type I structure, each cage cavity can accommodate up to 8 methane molecules; In the methane hydrate cage with a type II structure, each cage cavity can accommodate 24 methane molecules at most; In the methane hydrate cage with H-type structure, each cage cavity can accommodate up to 10 methane molecules
[8]. The hydration reaction between guest molecules and water often occurs at the interface between the bulk guest molecules and the liquid aqueous solution. When the cavity of the hydration cage is occupied by guest molecules, the concentration ratio of water to guest molecules in the water molecule cages of the three structures is 17 ∶ 3
[6].
3 Existing form of water in hydrothermal and supercritical water system
As a clean, safe and inexpensive reaction solvent, condensed liquid water shows lower surface tension, viscosity and density at high temperature and high pressure. Many inorganic or organic substances that are not soluble at normal temperature and pressure will become soluble in the hydrothermal system, so water can promote chemical reactions in the hydrothermal system. In the hydrothermal reaction system, water in the subcritical or supercritical state promotes the chemical reaction process mainly from three aspects: accelerating the reaction rate between complex ions, strengthening the role of hydrolysis reaction, and significantly changing the redox potential of reactants. Water is heated to a temperature above its boiling point in a closed autoclave, and according to the Arrhenius equation dlnk/dT=E/RT2, an increase in temperature T will lead to an exponential increase in the reaction rate constant K.
3.1 Phase diagram of water
Fig. 4 is the phase diagram of water. It is composed of the vapor-liquid equilibrium line and the liquid-solid equilibrium line of water. From the phase diagram of water, the temperature-pressure relationship of gas, liquid and solid phases can be obtained.
When the triple point (TP) of water is reached, that is, the temperature and pressure are 0. 01 ℃ and 0. 612 kPa, respectively, the gas, liquid and solid phases of water coexist in equilibrium. When the temperature is 0 ℃, with the increase of pressure, the condensed water in the high pressure region has a homogeneous polycrystalline phenomenon, and there are many different crystalline forms of ice (such as ice I, ice II, ice III, ice IV, ice V, ice VI, etc.). Among them, ice I, which is common in nature, appears at 0 ℃; Ice Ⅲ appears at -22 ℃ and 220 MPa; Ice Ⅳ, Ⅴ and Ⅵ appeared respectively with the increase of pressure and temperature. The condition for the appearance of ice Ⅶ is reached at 81 ℃ and 2.17 GPa. At present, the study of ice species is still under exploration. In 2019, Millot et al. Compressed and heated liquid water to extreme conditions of 100 ~ 400 GPa and 2000 ~ 3000 K by laser shock wave in the laboratory
[9]. At this time, water is transformed into "super-ion ice" (ice ⅩⅧ) with the properties of both solid and liquid water. In ice ⅩⅧ, protons diffuse through lattice oxygen vacancies, and the ionic conductivity is as high as 100 S·cm
-1, comparable to that of metals
[10]. In 2021, German scientist Loerting discovered the 19th crystalline form of ice and named it Ice XXI
[11].
When liquid water at normal temperature is heated from 25 ℃, its density gradually decreases, the component of water in the gas phase gradually increases, and the difference between the liquid phase and the gas phase becomes smaller and smaller, until it reaches the critical point (CP, critical temperature T
c=374℃, critical pressure P
c=22.1 MPa), when the two phases of gas and liquid disappear, water at this time is called "supercritical water"
[12].
3.2 Properties of water in supercritical water system
At room temperature and normal pressure (25 ℃, 101.3 kPa), the density of liquid water is 1 g·cm
-3. With the increase of temperature, the density of liquid water decreases gradually, while the density of gas phase water vapor increases gradually. When the temperature reaches the critical condition, the gas-liquid phase disappears.At this time, water not only has the compressibility and expansibility of gas, but also has the good fluidity and thermal conductivity of liquid water. It is an excellent solvent and is widely used in environmental management, biomass conversion, separation and extraction, material preparation, organic chemical synthesis and so on
[14][15][16][17][18][19,20].
At or near the critical point, the hydrogen bond of water is broken, the density is lower, the diffusion rate is higher, the viscosity is lower, and the compressibility is higher. Above the critical point, water exists in a fluid state between gaseous water and liquid water. In this condition region, the physical properties of water, such as compressibility, viscosity and diffusivity, are very sensitive to changes in temperature and pressure.
The existence of hydrogen bonds is responsible for the three-dimensional topology and many anomalies of condensed water. In condensed liquid water, the hydrogen bonding force is stronger and the arrangement of water molecules is denser. The space utilization of water molecules in solid ice is not high, and the gap between molecules is relatively large. Therefore, when water freezes, its volume expands and its density decreases. Hydrogen bonding in supercritical water has been studied by neutron or X-ray diffraction, quasi-elastic neutron diffraction, deep inelastic neutron diffraction, nuclear magnetic resonance, mid-infrared spectroscopy, and Raman spectroscopy for decades
[21,22][23][24][25,26][27][28]. Franck et al. Believe that hydrogen bonds must exist in high-density supercritical water, but the hydrogen bond network that causes the abnormal properties of liquid water no longer exists in supercritical water
[29]. Schienbein et al. Made a detailed theoretical calculation of the evolution of the hydrogen bond network of water molecules from room temperature to supercritical conditions by using ab initio molecular dynamics simulation
[30]. According to their results, hydrogen bonds require vibrational periods that far exceed the lifetime of hydrogen bonds in supercritical water. In supercritical water, the low-frequency intermolecular vibration of water molecules is consistent with the isotropic van der Waals interaction force, so water in supercritical state is not a hydrogen-bonded fluid.
At 25 ℃, the dielectric constant of water is 78.46 F·m
-1, which is affected by temperature, pressure and density. The dielectric constant increases with the increase of molecular dipole moment and polarizability, and the study of dielectric constant can be used to judge the solubility of water. According to the formula, Frank et al. Related the dielectric constant of water with temperature and density, and concluded that the dielectric constant of water decreases with the increase of temperature and increases with the increase of density
[31].
Unlike liquid water at room temperature and normal pressure, which can dissolve most polar inorganic substances, some organic substances and gases become soluble in supercritical water, while inorganic substances that are soluble at room temperature become slightly soluble or insoluble in supercritical water
[32]. Using this property, many organic reactions can be carried out in supercritical water system, making supercritical water a cleaner "organic solvent" choice.
The diffusion coefficient is related to the viscosity of the medium. In the supercritical state, water has a gas-like fluidity and a diffusion coefficient much higher than that of normal water. Water with higher diffusion capacity will accelerate the mass transfer process in chemical reactions and chemical reactions controlled by diffusion. Butenhoff et al. Found that the binary diffusion constant near the critical point was much higher in the aqueous solution of NaNO
3 with high concentration than that at normal temperature and pressure
[33].
The ion product of water is K
w=a(H
+)a(OH
-), which is mainly affected by density and temperature (Fig. 5), and it reflects the degree of ionization of water. The ion product of water at normal temperature and pressure is K
w=1×10
-14, in which the proton concentration is 1×10
-7mol·L
-1. Near the supercritical point, the increase of temperature leads to a significant decrease in the density of water, and the ion product of water also decreases significantly. The change of ion product means the change of H
+ concentration and OH
- concentration in water, which has a significant effect on the hydrolysis reaction and acid-base equilibrium involving proton acid and base concentration. The Cl
-+H
2O=HCl+OH
- equilibrium constant of chloride ion hydrolysis at 500 ℃ and 200 MPa is 9 times higher than that at normal temperature and pressure
[30].
4 Condensed state in hydrothermal system
Condensed matter in hydrothermal systems refers to the study of chemical reaction processes in liquid water systems. Hydrothermal reactions are chemical reactions that occur above the boiling point of water and are often carried out in specific closed autoclaves. Water, as a reaction solvent or reactant, is in a subcritical or supercritical state, thus exhibiting a state different from that of water at normal temperature and pressure to participate in chemical reactions.
4.1 Solvation
As a polar solvent, condensed liquid water will produce solvation with other anions and cations through electrostatic attraction. Therefore, charged ions often exist in aqueous solution in the form of hydrated ion structure. For example, the ionic crystal NaCl dissolves in water (Fig. 6). In NaCl crystals, Na+ is positively charged and Cl- is negatively charged. In H2O, H is partially positively charged and O is partially negatively charged, as previously described. When the NaCl crystal meets the H2O molecule, the sodium ion will interact with the negatively charged O end of the H2O molecule through electrostatic attraction to form a hydrated cation with the sodium ion as the center and the H2O molecule as the shell. Similarly, there are hydrated anions in solution with chloride ions as the center and H2O molecules as the shell, and the direct interaction with chloride ions is partially positively charged H atoms, with O atoms facing outward. Overall, the NaCl aqueous solution appears electrically neutral.
In 2018, Peng et al. Used scanning tunneling microscopy combined with non-contact atomic force microscopy to observe the atomic level image of sodium ion hydrate for the first time in experiments
[35]. By constructing a hydrated sodium ion model on the (001) crystal face of NaCl and gradually increasing the number of water molecules solvating with Na
+ from 1 to 5, they found that when the number of water molecules forming hydrated cations with Na
+ is 3, the migration rate of hydrated ions is the fastest.
4.2 Crystallization of zeolite
According to different crystallization conditions, the hydrothermal crystallization mechanism of zeolite can be divided into solid-phase gel conversion mechanism, liquid-phase gel conversion mechanism and two-phase conversion mechanism. In the gelling stage, TEOS, water glass (Na2O·nSiO2), aluminum sulfate (Al2(SO4)3), etc., as silicon-containing and aluminum-containing precursors, are hydrolyzed to form hydrated anions such as silicate ([SiO2(OH)2]2-)) and aluminate ([Al(OH)4]-)), respectively. Amorphous aluminosilicate solid gel is formed by polycondensation between hydrated anions, and the gel grows into zeolite crystal after crystallization.
The solid phase transformation mechanism holds that the solution phase does not participate in the crystallization process of zeolite. Initially, the metal cations in the gel system are surrounded by water molecules under solvation in the form of hydrated cations, and silicate ions and aluminate ions exist in the form of gel. In alkaline medium, the amorphous aluminosilicate gel undergoes a catalytic depolymerization process to form the secondary building units required for the formation of zeolites. By rearranging around hydrated cations, aluminosilicates form small polyhedral nuclei with a specific geometric space structure. Finally, the zeolite crystals continue to grow and become larger.
Different from the solid phase gel inversion mechanism, in the liquid phase gel inversion mechanism, the amorphous gel is partially dissolved in the liquid phase solution composed of Na+, OH- and some silicate ions. As the gel framework is continuously dissolved in the liquid phase, the concentration of silicate and aluminate ions in the solution is continuously increased, and these ions are further combined to form secondary structural units, and finally crystal nuclei are formed, and the growth of zeolite crystals is promoted with the continuous increase of the number and size of the crystal nuclei.
4.3 Hydrothermal molten salt system
Due to the decrease of water polarity and density in supercritical water system, the solubility of inorganic substances soluble in water at normal temperature and pressure becomes poor, which hinders the chemical reaction of inorganic substances in supercritical water system. Voisin et al. Proposed a solvent system of hydrothermal molten salt, and the solubility of inorganic substances changed in the supercritical water system at high pressure
[36]. They selected the dissolution of Na
2SO
4 in NaOH hydrothermal molten salt as an example to confirm the utility of this novel solvent system (Fig. 7). The formation mechanism of hydrothermal molten salt is that the homogeneous aqueous electrolyte solution or salt is first heated under high pressure, and when the temperature of salt precipitation is reached, the salt precipitates in solid form. Because the temperature of the reaction system at which the precipitate is formed is higher than the melting temperature of the precipitate, the precipitate is immediately melted to form a hydrothermal molten salt system. When the system is cooled, the initial homogeneous electrolyte solution or salt will return to the initial state, and the whole process is completely reversible. The hydrothermal molten salt system generates a high-density solvent through a simple and cheap H
2O-NaOH system in supercritical water, dissolves a large amount of solid salt such as Na
2SO
4, and provides a possibility for the hydrothermal reaction of a high-concentration salt system.
5 Hydrothermal synthesis of inorganic materials
The earliest application of hydrothermal reaction can be traced back to 100 years ago, when people synthesized mineral materials and extracted mineral elements from ores by hydrothermal method. Subsequently, the chemical reactions that can be carried out in hydrothermal systems have been extended to the preparation of inorganic materials, the synthesis of porous materials, the imitation of natural geothermal processes, and biological hydrothermal reactions. The large-scale preparation of molecular sieve materials and quartz materials has made more and more scientists realize the importance of hydrothermal chemistry. Hydrothermal reaction systems have replaced other reaction systems to achieve the preparation of a variety of functional materials, which has greatly promoted the vigorous development of hydrothermal chemistry. For example, hydrothermal synthesis can accomplish many reactions at temperatures lower than those required for solid phase preparation, allowing the reaction to be carried out at lower temperatures. Hydrothermal synthesis can prepare inorganic materials with special chemical composition, structure and morphology, such as inorganic large single crystal, micro-nano materials, ultrafine powder materials, inorganic membrane materials and so on.
Hydrothermal synthesis of inorganic materials refers to the synthesis of required inorganic materials under high temperature and high pressure with aqueous solution as the reaction medium. Under this condition, the synthesized material has high purity and controllable size. The crystal can be fully grown under unconfined conditions, which is more conducive to the growth of large single crystals with high purity and few defects
[37]. In this section, the principle of hydrothermal synthesis of inorganic crystals is introduced, and the progress of hydrothermal synthesis of inorganic materials is discussed by taking the preparation of large-size single crystals and micro-nano materials as examples.
5.1 Hydrothermal synthesis mechanism of inorganic crystal
At present, the hydrothermal synthesis of inorganic crystals mainly adopts the temperature difference seed crystal hydrothermal method, and the synthesis process is carried out in a highly airtight reaction kettle, as shown in Figure 8. The raw material for cultivating the crystal is placed at the bottom of the reaction kettle, the seed crystal is hung at the upper part, and a certain amount of mineralizer is filled in the reaction kettle as a solution medium to improve the solubility and growth rate of the raw material. The raw materials are put into a reaction kettle and sealed, and then put into an oven for heating. With the gradual increase of the temperature in the kettle, the raw materials are continuously dissolved in the mineralizer to form a saturated solution. Because the temperature at the bottom of the reactor is higher than that at the top, strong convection will occur. When the high-temperature saturated solution at the bottom is transported to the low-temperature growth zone with seed crystal at the top, supersaturated solution will be formed and precipitated on the seed crystal. The remaining solute will flow back to the bottom and continue to be heated to participate in the crystal growth cycle, and eventually the crystal will grow larger
[38,39].
In the process of crystal growth by hydrothermal method, there are many factors affecting the morphology, growth rate and quality of crystals, such as the quality of culture material, the type and concentration of mineralizer, crystallization temperature and temperature difference, the filling degree of solution, the placement and orientation of seed crystals, etc. Culture material is the raw material for crystal growth, and the selection of culture material with high purity and small particle size can improve the growth rate and quality of crystals
[40]. In the synthesis process, the solubility of the compost can be greatly improved by selecting the appropriate mineralizer solution and the appropriate concentration
[41]. Because the mineralizer solution will corrode the inner wall of the autoclave under high temperature and high pressure, inert materials such as Au, Ag, Pt and other precious metal materials are usually used to make protective bushings to separate the mineralizer solution from the inner wall of the autoclave, so as to prolong the service life of the autoclaves
[42]. It is of great significance to select the appropriate crystallization temperature and control the appropriate temperature difference for the rapid growth of high-quality crystals
[43]. Generally, the growth rate of crystal will increase with the increase of temperature and temperature difference, but when the growth rate of crystal is too fast, part of the culture material will enter the crystal in the form of inclusions, thus affecting the preparation of high-quality crystals. Usually, a buffer (such as a baffle) is added into the kettle to adjust the temperature difference, so that the purpose of growing large-size crystals can be achieved
[44]. In addition, the filling degree of the solution, the placement and orientation of the seed crystal can also affect the growth of the crystal
[45]. Therefore, in the process of crystal preparation, various factors must be considered comprehensively to synthesize the required crystal materials.
5.2 Large single crystal
Hydrothermal method is a very important crystal synthesis method, especially in the synthesis of some bulk single crystals, which has great advantages. The largest single crystal formed in nature (> 1 kg of beryl crystal) and the human preparation of quartz crystals with a mass of several kilograms in a single experiment were achieved under hydrothermal conditions
[46]. Hydrothermal method can make the crystal grow fully under unrestricted conditions, and can be carried out at a temperature far below the melting point of the substance, so it is often used to prepare some bulk crystals with high melting point. In this section, the hydrothermal synthesis of quartz, zinc oxide, YAG (yttrium aluminum garnet), emerald and other four functional materials and the hydrothermal synthesis of inorganic crystals are introduced.
Quartz (SiO
2), commonly known as crystal, is a decorative material and a very important piezoelectric crystal (Fig. 9 a). Hydrothermal method is the earliest method used to prepare quartz, and it is also the only method to prepare quartz crystal
[47][37]. During the hydrothermal synthesis of quartz crystal, the raw materials for crystal cultivation are placed at the hot bottom of the autoclave, and the seed crystal is suspended at the upper part with lower temperature. The kettle is filled with 1.0~1.2 mol L
-1 NaOH as a mineralizer and additives LiF, LiNO
3 or Li
2CO
3. The temperature of the dissolution zone in the kettle is 360-380 deg C, and the temperature of the crystallization zone is 330-350 deg C. The hydrothermal synthesis of quartz crystal includes two processes: the dissolution of quartz in the medium and its growth on the seed crystal. First, quartz dissolves in the NaOH mineralizer :SiO
2+(2x-4)NaOH→N
SiO
x+(x-2)H
2O,(x>2),The main products of the reaction are Na
2Si
2O
5,Na
2Si
3O
7 and their dissociation and hydrolysis products NaSi
2 and NaSi
3 . The solute ions are then activated,The reaction process can be expressed as :NaSi
3 .+H
2O→Si
3O
6+Na
++2OH
- or NaSi
2 +H
2O→Si
2O
4+Na
++2OH
-. Finally, the activated ions precipitate on the seed surface and grow :Si-OH+(Si-O)
-→Si-O-Si+OH
-.
Pure crystal is colorless and transparent, and different colors of crystal can be prepared by doping different transition metal ions in the preparation process. For example, Balisky prepared purple quartz by doping a small amount of Fe
3+ with NH
4F solution as the growth solution
[48]. At present, the technology of hydrothermal synthesis of quartz is very mature, and the quartz crystal synthesized under hydrothermal conditions has almost no serious fault or dendritic structure
[49]. Most of the artificial quartz crystals on the market are synthesized by hydrothermal method, which has realized the industrialization of quartz materials.
Zinc oxide (ZnO) is a typical wide band gap semiconductor material, which has a melting point of 1975 ℃ and decomposes and sublimates at temperatures above 1300 ℃. It is difficult to synthesize ZnO by traditional melt method, while hydrothermal method can prepare high melting point materials at lower synthesis temperature
[50]. In 2004, Ohshima et al. Used a hydrothermal method to prepare 50 mm × 50 mm × 15 mm high-purity transparent ZnO single crystals at 300 ~ 400 ℃ by using a mixture of 3 mol·L
-1KOH and 1 mol·L
-1LiOH as mineralizers in a platinum-containing lining tube
[51]. Maeda et al. Also prepared large ZnO single crystals with high purity, high crystallinity and a diameter of 2 inch at 300 ~ 400 ℃ (Fig. 9b)
[52]. In 2006, Ehrentraut et al. Reported that 36 kg of ZnO could be produced from 100 seed crystals after 100 days of growth, which fully demonstrated the advantages of hydrothermal method in the throughput of crystal growth
[53]. At present, the hydrothermal method is the most suitable method for growing zinc oxide crystals, especially the success of 2 inch single crystal growth, which makes people full of hope for growing large size ZnO single crystals.
Garnet crystal is a natural mineral, which is widely used as a laser medium with high power and high repetition rate. The garnet crystal belongs to a cubic crystal system and is formed by connecting three polyhedrons, and the general formula is A
3B
2C
3O
12, wherein A is an atom of Y, Gd, Lu, La and the like, and occupies a dodecahedral site; B is Sc, Al, Ga, Fe and other atoms, occupying octahedral sites; C is an atom of Al, Ga, Fe, etc., occupying a tetrahedral site. They have the characteristics of high melting point and high thermal conductivity, and the hydrothermal method can realize the synthesis at a lower temperature. In the early 1960s, Laudise et al. Prepared YIG(Y
3Fe
5O
12) and YGG(Y
3Ga
5O
12) by hydrothermal method at 350 – 425 ° C
[54][55]. Subsequently, McMillen et al. Grew various metal-doped yttrium aluminum garnet crystals, such as Nd: YAG and Cr: YAG (Fig. 9c), in a hydrothermal environment of 600 ~ 640 ℃, and greatly improved their laser performance
[56,57].
Beryl (Be
3Al
2Si
6O
18) is a very valuable gemstone crystal. Hydrothermal method is one of the earliest methods used to produce beryl, which can simulate the growth environment of natural beryl
[58]. In 1982, Kodaira et al. Prepared beryl crystals at 1000 MPa and 500 ~ 800 ℃ using pure water and NaOH solution, and found that the infrared spectrum of beryl crystals prepared under high pressure hydrothermal conditions was the same as that of natural beryl crystals
[59]. Pure beryl crystals are white, but their color is changed by doping during preparation
[60]. Among them, the most common one is to replace Al
3+ with a small amount of Cr
3+ to prepare emerald crystals with emerald green color (Fig. 9D)
[61]. At present, acid solution or fluorine-containing solution is usually used as solvent to synthesize emerald crystals under hydrothermal conditions of 500 ~ 600 ℃ and 100 MPa
[62].
Compared with the traditional crystal synthesis method, the hydrothermal method can improve the crystallinity of the crystal; By optimizing the synthesis condition parameters, the crystal growth rate can be controlled and the defects can be reduced; Because the chemical reaction is carried out in a water system, the method is green, environment-friendly and less in pollution
[63]. At present, there are two main challenges in the field of crystal growth: the first is how to obtain crystal materials with perfect structure and larger size; The second is how to grow single crystals of materials with extreme thermodynamic characteristics, especially some crystalline materials with high melting point, high vapor pressure and easy decomposition
[64]. Hydrothermal method has great advantages in solving these two problems. Nowadays, many important inorganic crystals can be prepared by hydrothermal method
[65]. In addition to quartz, zinc oxide, YAG laser crystals, and emerald single crystals, hydrothermal or solvothermal techniques have also been used to grow some optical crystal materials (fig. 9e, f).
5.3 Inorganic micro-nano material
In addition to the synthesis of large single crystals, hydrothermal methods are often used to prepare micro-and nano-materials. Under hydrothermal conditions, water can participate in the reaction as a chemical component, as a solvent or mineralizer, and as a pressure transfer medium. Compared with other preparation methods, the hydrothermal synthesis of nanomaterials has the characteristics of uniform phase, high purity, good dispersion, no agglomeration, controllable shape and size
[67].
In recent years, hydrothermal methods have been widely used in the synthesis of nanomaterials, in which morphology and dispersion control are the key points. Nanomaterials with different morphologies can be synthesized by hydrothermal method. In 2001, Li et al. Synthesized metal bismuth nanotubes with quasi-layered structure by low-temperature hydrothermal method, which is the first single crystal nanotube formed by metal in the world
[68]. Subsequently, they used MnSO
4 and (NH
4)
2S
2O
8 as raw materials to synthesize MnO
2 nanowires under hydrothermal environment at 120 ° C for 10 H
[69]. Huang et al. Synthesized ZnO nanospheres by hydrothermal reaction of Zn(NO
3)
2·6H
2O at 95 ℃ for 1 H, and found that with the extension of hydrothermal reaction time, the microspheres gradually disappeared and transformed into short hexagonal zinc oxide microrods
[70]. Zeng et al. Synthesized W
18O
49 nanotubes from HCl and Na
2WO
4 in an aqueous solution of Na
2SO
4 under hydrothermal conditions at 160 – 200 ° C
[71]. Sun et al. Formed Pt nanoflowers composed of nanowires by mixing H
2PtCl
6 with an aqueous solution of HCOOH at room temperature for 16 H
[72].
In addition, the hydrothermal method is an effective synthesis method for the preparation of functional micro-nanomaterials such as nanoceramics, fast ionic conductors, and composite fluorides. The introduction of nano-ceramic particles, whiskers and fibers into the ceramic matrix can improve the mechanical properties and high temperature resistance of ceramics. Piticescu et al. Synthesized PZT piezoelectric nanoceramic materials using ZrCl
4, TiCl
4, Pb(NO
3)
2 and H
2O
2 as raw materials and KOH as mineralizer under hydrothermal conditions of 200 ° C and 0.45 MPa
[73]. Feng et al. Used LiF, LiOH, Ho
2O
3, Er
2O
3, NH
4HF
2, NH
4F and HF as starting materials.Microcrystals of complex fluoride LiHoF
4(3μm) and LiErF
4(3.7μm) were synthesized at 240 ° C
[74].
Hydrothermal method plays an important role in the synthesis of inorganic nanomaterials. Hydrothermal method has unique advantages in the synthesis of nanomaterials, but it is still limited by the long reaction period and the lack of direct observation of crystal morphology
[75]. In order to improve the application scope of hydrothermal method, the hydrothermal method can be combined with other synthesis methods such as solvothermal method, microwave hydrothermal method and sol-gel hydrothermal method to prepare inorganic functional materials, which can perfect the hydrothermal synthesis system.
6 Hydrothermal Synthesis of Porous Materials
Porous materials have been widely used in adsorption, separation, catalysis, microelectronics, molecular device manufacturing, biomedicine and other fields because of their advantages of ordered pores and large specific surface area. The development of porous materials with controlled pore size has become a rapidly growing interdisciplinary field. The porous material prepared by the hydrothermal synthesis method has the advantages of high purity, good crystal form and the like. In this section, we will discuss the research progress of hydrothermal methods in the synthesis of porous materials by introducing the synthesis of microporous materials, mesoporous materials and metal-organic frameworks in hydrothermal systems.
6.1 Microporous material
Zeolite molecular sieve is a typical representative of microporous materials, and its hydrothermal synthesis goes through the formation stage of hydrated aluminum silicate gel and its crystallization stage. The crystallization process mainly includes four steps: (1) condensation of polysilicate gel with aluminate anions; (2) zeolite nucleation; (3) growth of nucleus; (4) Zeolite crystals grow, sometimes with secondary nucleation
[76].
Since the discovery of natural molecular sieve in 1756, people began to study the preparation of zeolite molecular sieve materials in simulated natural environment. Until the 19th century, its microporous properties and characteristics in adsorption and ion exchange were gradually concerned by people. In 1956, Milton and Breck, chemists of Union Carbide Corporation in the United States, successfully synthesized Linde Type Y, A and X zeolites by hydrothermal synthesis under relatively mild conditions
[77]. In 1961, Barrer and Denny prepared zeolite materials with high Si/Al ratio and high purity by introducing quaternary ammonium salt cations into the hydrothermal system, which became a major breakthrough in this field
[78]. Since then, more and more new zeolites have come out. In 1982, Thompson and Huber improved the reaction engineering model for the synthesis of Linde A zeolite by introducing a nucleation step. For the first time, Linde Type A (LTA) microporous zeolite with a silica to alumina ratio of 1 ∶ 1 was obtained by hydrothermal synthesis using a population balance analysis method with a mixture of silicate solution and aluminate solution
[79]. Later, K Kühl et al. Prepared Linde Type X (LTX) zeolite with low silicon content and Linde Type Y (LTY) zeolite with high silicon content
[80]. Mordenite (MOR) zeolites with different structures and Si/Al ratios were synthesized by Kim and Ahn
[81]. In 1992, Huo et al. Prepared a microporous JDF-20 material (JDF stands for Jinlin David Faraday) in a non-aqueous solution in the presence of trace water
[82]. JDF-20 is a macrocyclic aluminophosphate molecular sieve composed of 20 tetrahedra (fig. 10), and the entry of heteroatoms into its lattice will form substituted M-JDF-20 (M is Li, Mg, Mn, Ga, B, Si, Sn, etc.). Under hydrothermal conditions, JDF-20 will crystallize into microporous AlPO-5 again
[83].
Hydrothermal method instead of traditional synthesis method to prepare microporous materials has the characteristics of green environmental protection and superior material properties, so it is widely used by researchers. Wang et al. First prepared hydrated sodium aluminum silicate from natural kaolin, and then prepared high-purity LTJ zeolite microporous material by hydrothermal method
[84]. In this study, the hydrothermal method was used to replace the calcination method with high energy consumption, which successfully solved the problem that the calcination method could not remove impurities such as quartz with higher thermal stability, and synthesized microporous materials with higher purity. Sevilla et al. Used nitrogen-rich microalgae as raw materials for hydrothermal carbonization in glucose solution, and the amino groups in amino acids reacted with the carbonyl groups in carbohydrates to increase the carbon and nitrogen contents in the products
[85]. During the KOH activation process, the carbon is oxidized to carbonate ions (6KOH+2C→2K+3H
2+2K
2CO
3) which are removed in the subsequent washing process, thus leaving a porous structure in the material. In this study, microalgae was successfully converted into porous carbon by hydrothermal carbonization, and the surface of the material was covered with a layer of carbon shell, which solved the problems of uneven structure, low carbon content and KOH burning in the traditional synthesis method, and improved the electrochemical performance of the material. In 2020, Tan et al. Carried out hydrothermal reaction under external high pressure and synthesized a series of zeolites with different degrees of crystallization, such as LTA and FAU (Faujasite), which confirmed for the first time that external pressure plays an important role in the crystallization process of zeolite materials synthesized by hydrothermal method
[86].
6.2 Mesoporous material
Mesoporous materials are porous materials with pore size between 2 and 50 nm. Its pore channels can accommodate molecules, so it has a wide range of applications in adsorption, separation, catalysis and other fields. Hydrothermal synthesis of mesoporous materials is often accompanied by the use of templates. For example, Huang et al. Successfully synthesized carbon monomers with two-dimensional hexagonal ordered mesostructure by one-step hydrothermal method using phenolic resin as precursor and mixed triblock polymer as template
[87]. Feng et al. Used a simple hydrothermal method to copolymerize triblock polymer and β-cyclodextrin and carbonize them to form ordered mesoporous carbides, which showed high specific capacitance and strong electrochemical stability when applied to supercapacitors
[88]. Fang et al. synthesized ordered mesoporous carbon microspheres with a sphere diameter of 20 ~ 140 nm by hydrothermal method using triblock copolymer as template and phenolic resin as carbon source (Fig. 11). The product has low cytotoxicity, high cell permeability and drug loading, and has application prospects in cell transmission and cell imaging
[89]. Jun et al. Used ordered mesoporous silica SBA-15 as a template to prepare CMK-3 mesoporous materials with uniform and ordered pore size by hydrothermal method for the first time without structural change in the process of template removal
[90]. In this study, the carbonization was completed at a low temperature, and the pores of the material were uniform and orderly. Liu et al. Used the soft template method to self-assemble the mesoporous polymeric nanospheres by hydrothermal treatment of Plankney F127, 1,3,5-trimethylbenzene (TMB), fluorocarbon surfactant FC4, etc., and then formed mesoporous carbon nanospheres after carbonization
[91].
Through the above studies, it can be found that the hydrothermal template method is widely used in the synthesis of mesoporous materials. Mesoporous molecular sieve materials with different pore sizes can be successfully synthesized by loading raw materials on the template and then removing the remaining pores of the template by hydrothermal method. Compared with microporous materials, the pore channels of mesoporous materials are beneficial to the diffusion of substances and have larger specific surface area, which provides advantages for their applications in catalysis, life science, energy, environment, functional materials and so on.
6.3 Metal-organic frameworks (MOFs)
As porous materials containing organic components, metal-organic frameworks have the advantages of adjustable pore size, flexible network, diverse components, easy functionalization and unique physical properties, but their thermal and chemical stability need to be improved
[92]. At present, MOFs have become the key materials of various functional systems, and have been widely used in many fields, such as electrocatalysis, batteries, structural design and electromagnetic wave absorption
[93⇓~95][96][97⇓~99][100,101]. Common methods for the synthesis of MOFs include ultrasonic method, microwave heating method, electrochemical method, mechanochemical synthesis method, hydrothermal method or solvothermal method, among which hydrothermal method is the earliest method for the preparation of MOFs materials.
Hydrothermal synthesis of metal-organic frameworks is usually carried out by mixing metal ions and organic ligands in solution at high pressure above 100 ℃. The structural characteristics of the material can be effectively regulated by adjusting the synthesis conditions, such as the functional group of the ligand, the concentration, the pH value of the solution, and other factors. In 1999, Yaghi et al. Synthesized a new type of MOF material, named MOF-5, and then used different terephthalic acid (BDC) derivatives as organic ligands to prepare a series of MOFs materials with different pore sizes based on the MOF-5 framework through hydrothermal method, and they found that the pore size of the material is closely related to the type of substituent on the BDC
[102][103]. Liu et al. Prepared six kinds of MOFs materials with different structures by using 5-thioisophthalic acid and 1,2,4-triazole as organic ligands and Zn
2+ and Cd
2+ as metal linkers under hydrothermal environment, and the structures of the final products were changed by changing the metal types and synthesis conditions, such as the presence or absence of NaOH
[104]. Zhang et al. Mixed NaVO
3, H
3BO
3, Zn(CH
3COO)
2, and ethylenediamine, and hydrothermally synthesized Zn
6V
12B
18, or Zn
6V
12B
18 for short
[105]. The pH value has a vital influence on the hydrothermal synthesis of the material: when the pH value is about 8, the purest product can be prepared; When the pH value is greater than or less than 8, no product and no crystal can be obtained. Later, they used a similar method to prepare Cd[C
3N
2H
11]
2[V
8O
20] materials, and confirmed the conclusion that the initial product, concentration, pH value, crystallization temperature, pressure and other factors will affect the state of the product
[106].
Thanks to their unique pore structure, MOFs materials have shown potential applications in drug loading. Pham et al. Hydrothermally synthesized MOFs materials of Fe-MIL-88B-NH
2 from ferric salts, 2-aminoterephthalic acid, Planik F127 and acetic acid
[107]. It was found that the change of acetic acid concentration led to the change of nanocrystal size, and MOFs could be loaded with different kinds of drugs by adjusting the pore capacity. Rezaei et al. Prepared metal-organic framework MIL-100 (MIL = Material from Institut Lavoisier) by hydrothermal method and used it for drug delivery
[108]. During the hydrothermal process for the synthesis of MIL-100 (Fe), the trivalent iron with octahedral structure was polymerized into trimers, which were further polymerized into hybrid super-tetrahedra, and finally three-dimensional crystallites were formed (Fig. 12). A product with optimal docetaxel (DTX) drug loading can be prepared by adjusting the solvent type and the drug ratio. After processing the three-dimensional microcrystalline structure by ultrasonic technology, they obtained nanoscale MIL-100 and tested its controlled release under different solvent systems and drug ratios. This study shows the potential of MOFs materials in the research and development of anticancer drugs.
7 Organic hydrothermal chemistry
As a clean and pollution-free liquid, water is used as a solvent for organic reactions to meet the requirements of green chemistry. Compared with water at normal temperature and pressure, the physical and chemical properties of water change significantly under hydrothermal conditions, and the solubility of many organic compounds in water will change, so that some organic reactions can be carried out in hydrothermal systems. In this section, organic hydrothermal chemistry will be introduced from four aspects: biomass conversion, organic conversion, C1 conversion and organic ligand conversion under hydrothermal conditions.
7.1 Biomass conversion in hydrothermal system
As one of the main components of biomass, lignocellulose is abundant and cheap, and it is one of the main raw materials for the preparation of bio-oil. The structure of lignocellulose is composed of cellulose, hemicellulose and lignin, which have different reaction mechanisms as starting materials for the preparation of bio-oil in hydrothermal system
[109]. Sasaki et al. Studied the hydrolysis reaction of cellulose in subcritical and supercritical water, and found that the disappearance rate of cellulose increased significantly when the temperature was 350 ℃
[110]. At 400 ℃, the hydrolysis rate of cellulose becomes faster, and the decomposition products are glucose and cellobiose. They believe that the breaking of hydrogen bonds in cellulose crystals makes cellulose dissolve in high temperature water, resulting in the hydrolysis yield of cellulose at 400 ℃ much higher than that at 320 and 350 ℃. Based on the research results of Oefner et al., Antal et al. And Luijkx, Aida et al. constructed the reaction pathway of D-xylose, a xylan monomer in hemicellulose, in subcritical and supercritical water (Fig. 13)
[111][112][113]. D-xylose is transformed into D-xylulose by Lobry de Bruyn-Alberda van Ekenstein transformation (LBET transformation); Furfural can be produced by the dehydration of D-xylulose, and glycolaldehyde and glyceraldehyde can be produced by the Retro Aldol reaction of D-xylose and D-xylulose, and glycolaldehyde and formaldehyde can be formed by the Retro Aldol reaction of glyceraldehyde. At the same time, glyceraldehyde can also be transformed into dihydroxyacetone by LBET. Glyceraldehyde and dihydroxyacetone can be converted into methylglyoxal by dehydration reaction, and methylglyoxal can be finally converted into lactic acid by benzilic acid rearrangement reaction. In the course of the study, Aida et al. Found that the kinetics of LBET transition, retro-aldol condensation and dehydration in D-xylose liquefaction reaction could be controlled by changing the water density of the reaction system
[114]. Barbier et al. Also studied the hydrothermal conversion of alkali lignin and phenolic model compounds at 370 and 390 ℃, under which lignin was converted into oligomeric aromatics, anisole compounds and oxygenated hydrocarbons with C
2~C
6 by hydrolysis
[115].
As the other two main components of biomass, the reaction mechanism of protein and oil in hydrothermal system has also attracted the attention of researchers. Using alanine and its derivatives as model amino acids, Sato et al. Studied two main pathways of their reactions under hydrothermal conditions: one is decarboxylation to carbonic acid and organic amine, and the other is deamination to ammonia and organic acid, and pointed out that the ratio of decarboxylation to deamination is related to the type of amino acid
[116]. Subsequently, Klingler et al. Studied the reactions of alanine and glycine as representatives of amino acids in subcritical and supercritical water, which also mainly undergo decarboxylation and deamination
[117]. In addition, Hu et al. Pointed out that the rate of oil hydrolysis is related to temperature, and high temperature increases the solubility of water in oil, so that oil hydrolysis can proceed faster
[118].
7.2 Organic reaction in hydrothermal system
Because the ion product of water changes significantly with temperature, some reactions that originally require additional acid-base catalysts can be carried out in hydrothermal systems. Taylor et al. Explained the acid catalytic mechanism of ether by studying the decomposition of MTBE in subcritical and supercritical water (Fig. 14)
[119]. In this study, it is assumed that all H
+ in water are generated by water dissociation, and MTBE first generates methanol and tertiary carbocation through protonation reaction and S
N1 dissociation reaction (as shown in 1) and 2) in Fig. 14); Subsequently, the tertiary carbocation reacts with water to produce tert-butyl alcohol and protonic hydrogen (as shown in 2) and 3) in Fig. 14), and at the same time, the tertiary carbocation can also produce isobutylene and protonic hydrogen through self-elimination reaction (as shown in 2) and 4) in Fig. 14), in which the proton from water participates in the reaction as a catalyst. Similarly, Xu et al. Found that when tert-butyl alcohol is dehydrated to isobutylene, the hydronium ion produced by water dissociation is sufficient to catalyze the reaction
[120].
The traditional synthesis of compounds containing isochromanone structure is complicated in reaction conditions and steps. Zhang et al. Used high temperature water as solvent to carry out Claisen rearrangement-nucleophilic addition tandem reaction.3-Methyl-5-hydroxy-7-carboxyisochroman-1-one was synthesized from dimethyl 5-allyloxyisophthalate at 160 ℃ for 72 H. This method used water as solvent, without additional catalyst, and the reaction conditions were mild
[121]. Duan et al. Synthesized 2-phenyl-2,3-dihydro-4 (1H) -quinazolinone from 2-aminobenzamide and benzaldehyde using high-temperature water as the reaction medium, avoiding the use of organic solvents and catalysts
[122].
7.3 C1 conversion in hydrothermal system
The synthesis of organic compounds, such as short chain alkanes, carboxylic acids, and alcohols, from CO2 and H2 occurs in the geological hydrothermal fluids. It is important to study the transformation of C1 compounds into other organic compounds in the laboratory by simulating the geological hydrothermal environment, which is of great significance for distinguishing biological and non-biological synthetic organic compounds.
Fischer-Tropsch synthesis (F-T synthesis) is a process developed by Fischer and Tropsch in 1925 to produce hydrocarbons or other hydrocarbons from CO and H
2 under transition metal catalysis. Mißbach et al. Used oxalic acid as a substitute for CO
2, CO and H
2, and mixed it with montmorillonite to carry out hydrothermal F-T synthesis at 175 ℃, and the products mainly included alcohols, non-cycloalkanes, alkanoic acids and other organic substances
[123]. Fu et al. Used carbon isotopes to reveal the F-T synthesis reaction pathway under hydrothermal conditions (Fig. 15): first, H
2 was used as a reducing agent to activate the catalyst surface,The CO
2 and the H
2 are adsorbed on the surface of the catalyst and then dehydrated and converted into hydroxymethylene (— CHOH), and the adjacent hydroxymethylene is dehydrated and condensed again to form a bond between two carbon atoms (the carbon chain can be extended by adding additional hydroxymethylene)
[124][125]; The dehydration product of the two hydroxymethylene groups can be converted into a saturated primary alcohol containing the same carbon number by hydrogenation, and the primary alcohol further reacts with water to form a carboxylic acid; Finally, H
2 dissolved in water can terminate the reaction by breaking the C — C bond of the alcohol or acid. Chen et al. Carried out the reaction of CO
2 with Fe
3O
4 in a hydrothermal system, and the main products obtained under most reducing conditions were acetaldehyde, ethanol, and acetic acid
[126]. By controlling the water content of the reaction system, they found that a small amount of H
2O was more favorable for the reduction of CO
2.
In addition, Tian et al. Hydrothermally synthesized phenol at 200 ℃ in the presence of iron powder using sodium bicarbonate as a CO
2 source. They believed that the possible reaction mechanism was that :NaHCO
3 was first decomposed into CO
2;CO
2 and (or) C
in the hydrothermal solution and adsorbed on the surface of metallic iron
[127]. At the same time, iron reacts with water to produce H
2 and Fe
2+ (rate-determining step); Formaldehyde was generated on the surface of iron powder by CO
2 and H
2, and phenol was finally produced by oxidative coupling and rearrangement reaction of formaldehyde as raw material. The pH value of the reaction system can affect the yield of phenol, and the yield of phenol can be significantly increased by experimental verification under the conditions of acidic solution, CO
2 and sufficient H
2.
He et al. Simulated the hydrothermal environment of the earth and prepared long-chain hydrocarbons containing up to 24 carbons at 300 ℃ and 30 MPa using NaHCO
3 as raw material, Fe as reducing agent and Co as catalyst
[128]. They proposed a mechanism for the reduction of NaHCO
3 to hydrocarbons by Fe and Co: HC
is first adsorbed on the Fe surface; Then it is reduced to COOH and transferred to the surface of Co, while Fe-OH is formed on the surface of Fe. Then, the bonded COOH is reduced to CO on the surface of Co, which is reduced and polymerized to long-chain alkanes several times. The synergistic effect of Co and Fe-OH in this reaction process enhances the adsorption of Co on the surface of Co, thus promoting the formation of long-chain hydrocarbons.
7.4 Transformation of organic ligand in hydrothermal system
Metal-organic framework materials are formed by coordination between organic ligands and metals, and the phenomenon that organic ligands are converted into new ligands in situ and participate in coordination may occur when such reactions are carried out in hydrothermal systems. In the hydrothermal synthesis of coordination polymer by Juibari et al. Using Co(NO
3)
2·6H
2O, 1,10-phenanthroline-5,6-dione, 2,2 '-bipyridine and sodium hydroxide as raw materials, 1,10-phenanthroline-5,6-dione was transformed into 2,2' -bipyridine-3,3 '-dicarboxylate ligand in situ
[129]. In situ decarboxylation has been reported in many studies on the synthesis of organic frameworks by Ay et al
[130⇓~132]. During the hydrothermal synthesis of lanthanide organic framework compounds at 160 ℃, some pyridine-2,5-dicarboxylic acids were decarboxylated, and these carboxyl groups were linked to piperazine to form piperazine-2,5-dicarboxylic acid ligands
[130]. As shown in Fig. 16, during the formation of three-dimensional lanthanide organic framework by pyridine-2,5-dicarboxylic acid (H
2pydc) and Nd salt, Dy salt, the H
2pydc ligand was in situ decarboxylated into 2-picolinic acid (Hpyc) ligand under hydrothermal conditions and coordinated with lanthanum salt in different coordination modes, while pyrazine, pyridine, 3-picolinic acid, 1-nitrosopiperazine and 1,4-dinitrosopiperazine were generated in solution
[131]. They found that pyridine-2,6-dicarboxylic acid decarboxylated to 2-picolinic acid ligand when they used "one-pot" hydrothermal synthesis of one-dimensional copper coordination polymer in the presence of pyridine-2,6-dicarboxylic acid, 1,10-phenanthroline, ZnSO
4·H
2O, CuSO
4·5H
2O and H
3BO
3[132]. In addition, Xu et al. Also found the decarboxylation reaction
[133].
Although some progress has been made in organic reactions in hydrothermal systems, there are still many problems that need to be solved urgently. For example, the bio-oil prepared in the hydrothermal liquefaction system has lower calorific value and higher acidity
[130]; Some organic reactions have low selectivity and yield under hydrothermal conditions. And whether more ligands can be converted into new ligands and participate in coordination under hydrothermal conditions.
8 Hydrothermal synthesis of biomolecules
Many rich chemical reactions occur in warm oceans and in microorganisms, and there is growing evidence supporting a hydrothermal origin of life. The synthesis of organic or biological species from inorganic materials in enzyme-free hydrothermal synthesis systems is important for understanding microbiology, ocean exploration, origin of life, and industrial applications. It has become an environmentally friendly and green synthesis method to prepare bio-organic molecules by simulating hydrothermal environment and using small molecules as starting materials for chemical reactions.
In 2004, Ricardo et al. Used formaldehyde, glyceraldehyde and borate to prepare four kinds of pentose including ribose under hydrothermal conditions, and found that the addition of borate was beneficial to the stabilization of pentose
[134]. Saladino et al. Introduced formamide into the hydrothermal system, and used CaCO
3 and different types of montmorillonite as a composite catalyst to produce purine and pyrimidine. They found that different types of montmorillonite catalyzed the reaction to produce different bases
[135]. The general strategy of researchers to synthesize nucleotides is to synthesize ribose and base separately, and then add phosphate to combine to obtain nucleotides. In 2009, Powner et al. Changed the traditional synthesis method and directly added phosphoric acid in the process of synthesizing ribose and base to synthesize pyrimidine nucleotide in one step
[136].
Fox et al. Hydrolyzed the product of the heating reaction of formaldehyde and ammonia to prepare a total of seven amino acids: aspartic acid, glutamic acid, serine, proline, valine, glycine and alanine, of which glycine is the main amino acid
[137]. Sulfides are believed to be formed near craters under early alkaline hydrothermal conditions. Kitadai et al. Prepared a series of reduced metal sulfides (Ag
2S, CdS, CoS, CuS, MnS, MoS
2, NiS, PbS, WS
2, ZnS) for the electroreduction of CO
2 to produce CO, and used this reaction to verify that α-amino acids were converted from CO as a raw material
[138]. This is consistent with their previous suggestion that the production of CO is a driving force of abiotic organic synthesis
[139].
Amino acids will form peptides by dehydration condensation, and the direct condensation of amino acids in solution to form polypeptides is not thermodynamically favorable. Marshall-Bowman et al. Studied the reaction equilibrium of amino acid condensation to dipeptide and its depolymerization by heating the aqueous solution of amino acid to 25, 70 and 250 ℃ respectively. They found that higher temperature was beneficial to the condensation of amino acid monomer to peptide, but the decomposition rate of peptide would also be accelerated at high temperature
[140]. Fig. 17 is a continuous reactor for oligomerization design by Imai et al. By simulating a submarine hydrothermal system, when a fluid contain glycine is continuously circulated through that hot and cold regions of the reactor, the glycine will form oligopeptides
[141]; If divalent metal ions are added to the reaction under acidic conditions, the oligoglycine will be elongated to hexameric glycine.
The study of hydrothermal synthesis of biomolecules not only has important academic value in exploring the origin of life, but also has guiding significance for human beings to learn from efficient enzyme catalysis for industrial catalysis and medical research. However, the preparation of high-yield biomolecules by hydrothermal methods is limited by their thermodynamic and kinetic limitations, and the future development of biohydrothermal chemistry is closely linked to the development of new catalysts.
9 Conclusion and prospect
Condensed matter and chemical reactions in hydrothermal systems are reviewed from the point of view of condensed matter chemistry. It begins with the introduction of water molecular structure and water molecular clusters formed by hydrogen bonding, and then transits to the changes of water structure and properties in hydrothermal systems.Finally, the importance of hydrothermal chemistry and condensed matter chemistry is discussed by introducing the typical chemical reactions under hydrothermal conditions, such as the preparation of inorganic functional single crystal materials, the synthesis of porous materials, organic hydrothermal and biological hydrothermal reactions.
Hydrothermal system is the only medium suitable for the preparation of bulk quartz, molecular sieves and other silicate materials. Compared with the traditional material synthesis methods, the large single crystal prepared by the hydrothermal system has the advantages of high crystallinity, low defect, easy industrialization and the like. In addition, hydrothermal method has also been widely used to prepare micro-and nano-materials with special morphologies, which can realize the controllable synthesis of material morphologies under mild conditions.
Porous materials have broad application prospects in catalysis, life science, energy, environment, functional materials and many other fields due to their good periodicity, controllable pores and high specific surface area. Under hydrothermal conditions, the solubility of reactants can be improved, the reactivity of substrates can be increased, the dissolution and rearrangement efficiency of gels can be greatly improved, and the synthesized porous materials have the advantages of high purity, good crystal form, large specific surface area and the like.
The change of water properties such as solubility and ion product under hydrothermal conditions has expanded its applications in organic reactions, such as hydrothermal biomass conversion, organic reactions with water as a catalyst, C1 conversion and ligand conversion. Water is a clean and pollution-free liquid, and the environment-friendly organic reactions using water as solvent or acid-base catalyst meet the requirements of green chemistry.
The hydrothermal synthesis of some biomolecules, such as nucleotides, amino acids, and peptides, has also been widely studied, which is of great significance for the study of the origin of life and chemical reactions in biological systems.
The vigorous development of chemical reactions in hydrothermal systems in material synthesis and organic transformation has promoted the in-depth study of condensed liquid chemistry. Hydrothermal chemistry has unique advantages in the synthesis of functionalized advanced materials by accelerating the kinetic process in the synthesis. The future research trend of hydrothermal chemistry is to develop the combination of hydrothermal method with other synthesis techniques such as microwave method, electrochemical method and sonic method. With the participation of other forms of energy, the thermodynamic problems of chemical reaction systems will be more complex, and materials will be more functional. However, the lack of in situ detection of water structure and the lack of in-depth exploration of the chemical reaction mechanism of water in hydrothermal systems have inhibited the research progress of hydrothermal chemistry. With the development of information technology, the study of the structure and properties of condensed water requires more comprehensive theoretical calculations to guide experimental breakthroughs. In the field of hydrothermal chemical synthesis, researchers can design better and greener new synthesis paths by using computational simulation methods.The relationship between the crystallization mechanism, structure and function of various crystal materials is analyzed more profoundly to seek the application potential of materials in other fields and expand the practical application of inorganic functional materials. Through the combination of theory and experiment, we can continue to improve the research of condensed water and hydrothermal synthesis system, understand the role of hydrothermal chemistry in nature, and ultimately further promote the industrialization of hydrothermal synthesis.
National Natural Science Foundation of China(21931005)
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