Quantum-confined superfluid reactions

A helium atom superfluid was originally discovered by Kapitsa and Allen. Biological channels in such a fluid allow ultrafast molecule and ion transport, defined as a quantum-confined superfluid (QSF). In the process of enzymatic biosynthesis, unique performances can be achieved with high flux, 100% selectivity and low reaction activation energy at room temperature, under atmospheric pressure in an aqueous environment. Such reactions are considered as QSF reactions. In this perspective, we introduce the concept of QSF reactions in artificial systems. Through designing the channel size at the van der Waals equilibrium distance (r0) for molecules or the Debye length (λD) for ions, and arranging the reactants orderly in the channel to satisfy symmetry-matching principles, QSF reactions in artificial systems can be realized with high flux, 100% selectivity and low reaction activation energy. Several types of QSF-like molecular reactions are summarized, including quantum-confined polymerizations, quasi-superfluid-based reactions and superfluid-based molecular reactions, followed by the discussion of QSF ion redox reactions. We envision in the future that chemical engineering, based on multi-step QSF reactions, and a tubular reactor with continuous nanochannel membranes taking advantage of high flux, high selectivity and low energy consumption, will replace the traditional tower reactor, and bring revolutionary technology to both chemistry and chemical engineering.


Introduction
The concept of an atomic superuid can be traced back to the 1930s, when Kapitsa and Allen et al. observed a 4 He uid below 2.17 K. 1,2 The nearly zero viscosity of the 4 He superuid means that there is no loss of kinetic energy, and its velocity through capillaries with varying diameters increases rapidly as its channel diameter decreases. 3 When the intrinsic diameter of a capillary is below 100 nm, the velocity of 4 He only depends on the temperature rather than pressure and channel length (Fig. 1a). 4,5 In biological ion or molecule channels, unique characteristics of ultrafast transport, high ux and low energy loss can be realized, with such phenomena being dened as a quantum-conned superuid (QSF). 6,7 In the process of enzymatic biosynthesis, the reactant molecules arrange in the channels orderly, which greatly reduces the reaction activation energy, realizing 100% selectivity and ultrahigh ux in the bodily environment. Such reactions are considered as QSF reactions, 8,9 such as DNA, ATP, natural rubber and fatty acid syntheses. For example, four kinds of deoxynucleotides are individually inserted into complementary strands in the DNA replication process with accurate positions and conformation due to base-pairing rules (Fig. 1b). 10 Thus, the desired DNA double helix can be polymerized with remarkably precise sequence and structure. During the preparation of ATP from ADP and phosphate, the ATP synthase catalyses the reaction with low energy consumption (Fig. 1c). 11,12 In the biosynthesis of natural rubber, the monomer of isopentenyl pyrophosphate is catalysed in vivo by rubber transferase, and the rubber molecules grow via "living carbocationic polymerization" with the continuous addition of monomers (Fig. 1d). 13 It is noteworthy that specic rubber transferases can regulate cis-trans stereoregulation, demonstrating symmetry-matching principles during the reaction process. The fatty acids are synthesized via a series of decarboxylative Claisen condensation reactions by fatty acid synthase, and the growing fatty acid chain is carried between these active sites of synthase (Fig. 1e). 14 These enzymatic reactions involve the highly-ordered arrangement of reactants in a conned nanospace, realizing QSF reactions with high ux, 100% selectivity and low reaction activation energy at room temperature, under atmospheric pressure in an aqueous environment.
In this perspective, we introduce the concept of QSF reactions in articial systems. Quantum-conned molecule super-uids and QSF-like molecule reactions, including quantum-conned polymerizations, quasi-superuid-based reactions and superuid-based molecule reactions are summarized. Then quantum-conned ion superuids and QSF ion redox reactions are discussed. We further look forward to the future chemical engineering based on QSF reactions, which would achieve excellent performances in terms of high ux, high selectivity and low energy consumption.

Quantum-confined molecule superfluids and QSF-like molecule reactions
Quantum-conned molecule superuids can be found in both biological and articial systems when the channel size is reduced to the distance of the van der Waals equilibrium distance (r 0 ) of molecules. Biological water channels with ultrahigh water ux comprise ordered water strands, indicating quantum method of transport (Fig. 2a). During the past decade, researchers found that the ultrafast water ow through an aligned carbon nanotube (CNT) membrane is 4-5 orders of magnitude higher than that predicted from conventional uid-ow theory. 15,16 Both experimental results and simulations have demonstrated that the conned water ux can increase up to seven orders of magnitude in the hydrophobic nanochannels compared to that of bulk water. 17 In the case of molecular dynamics (MD) simulations, water orientations and motions conned in nanochannels suggest that water molecules can arrange spontaneously in an ordered way, when the diameter of the CNT channel is below 8.6 A. But in the wider CNT channel, water molecules are disordered just as in bulk water. 18 Another MD simulation for water transport in CNTs with different diameters (1.66-4.99 nm) demonstrated that the enhancement of water ow velocity increased from 47 to 433, when the diameter of the CNT channel decreased (Fig. 2b). 19 Hummer and co-workers reported spontaneous and continuous lling of a one-dimensionally ordered chain of water molecules (about ve water molecules) in a hydrophobic CNT, which transited pulse-like through the nanotube (Fig. 2c). 20 It is noteworthy that they ignored the fact that the inlet of a CNT is hydrophilic, 15 although no inuence on their results was observed. Our previous simulation showed that water can only penetrate into a nanotube with a hydrophilic inlet. 21 Fig. 2d shows the formation process of a quantum-conned molecule superuid by decreasing the channel size to the distance of the van der Waals equilibrium distance (r 0 ) of the molecules. Therefore, a quantum-conned molecule superuid, either in biological or articial channels, shows unique performances, with high ux and low energy consumption.
In order to realize QSF molecule reactions in an articial system, the reactants should be arranged orderly in the channel to satisfy symmetry-matching principles, and the channel size should be designed at a distance of the van der Waals equilibrium distance (r 0 ) of the molecules, then QSF reactions can be realized with high performance, including high ux, 100% selectivity and low reaction activation energy. Arrangement of the reactant molecules orderly in the channel to satisfy symmetry-matching principles can realize a low reaction activation energy of the reactions, and such reactions are considered to be quantum-conned reactions. Specic Au(110) surfaces can serve as a conned space and efficient catalysts to lower the energy barriers, resulting in high-activity and selective reaction processes. Linear 18,19-dimethylidenehexatriacontane (DMH) alkanes adsorbed in Au (1 Â 3)-(110) have been shown to preassemble and pack closely in the 1D atomic channels along the (110) lattice direction, and then undergo polymerization with a low activation energy and reaction temperature (Fig. 3a). 22 Moreover, controlled synthesis of few-layer 2D polyimide crystals on the surface of water (air-water interface) was proved through the reaction between amine and anhydride monomers, assisted by surfactant monolayers, which resulted in polymers with high crystallinity and a thickness of $2 nm (Fig. 3b). 23 The formation of crystalline polymers was attributed to the pre-organization of monomers at the water-surfactant interface. Similarly, 2D conductive hybrid lamella with high crystallinity and electrical conductivity for intercalative charge storage have been fabricated in a 2D conned space (Fig. 3c). 24 Another example is supramolecular catalyzed polymerization, where cucurbit [8]uril (CB [8]) can ip along and elongate the polymer chain as a supramolecular connement agent and catalyst for the fabrication of covalent polymers under light irradiation, while the molecular weights of the obtained polymer could be increased by controlling the irradiation time or the monomer (Fig. 3d). 25 Similar supramolecular polymerization was reported to construct a NIR-II chromophore via the tailor-made assembly of organic radicals for photothermal conversion and therapy. 26 In addition, porous coordination polymers with nanochannels and basic interaction sites allowed the highly accelerated, stereocontrolled, and monomerselective polymerization of substituted acetylenes. 27 Therefore, quantum-conned polymerizations can achieve low reaction activation energy of the reactions.
Some previous studies have successfully realized quasi-superuid-based reactions with improved performance. 8 For example, Rh-based particles were conned inside CNTs to catalyze Fischer-Tropsch synthesis, and enhanced catalytic activity was achieved for the conversion of syngas to ethanol (Fig. 4a). The overall rate of ethanol yield inside the CNTs was higher by one order of magnitude than that outside the nanotubes even though the outer surface was more accessible. Chirally modied Pt catalysts were designed inside the CNTs, which could realize relatively higher enantioselectivity and activity than that loaded outside the CNTs for the asymmetric hydrogenation of a-ketoesters. 28 The enhancement was attributed to the nanoconnement and enrichment of the Pt nanocatalyst with the chiral modier cinchonidine inside CNT channels. Besides this, tandem hydrogenation reactions, such as nitrobenzene hydrogenation using hydrazine exhibited high catalytic efficiency and 99% selectivity of aniline, resulting from      (Fig. 4b). 29 When encapsulated in the nanochannels of SBA-15, SBA-16, MCM-41, the Co(III) complex catalysts reached 98% enantioselectivity and outstanding reusability in the hydrolysis of propylene oxide (Fig. 4c). 30 In a conned photosensitized oxidation system, alkenes were conned in the channels of Na-ZSM-5 zeolites, and the products were obtained selectively from singlet oxygen oxidation. 31 More recently, a heterogeneous catalyst system, consisting of AuPd alloy nanoparticles xed within aluminosilicate zeolite crystals, was reported for enhanced methanol productivity in methane oxidation by in situ generated hydrogen peroxide at mild temperature (70 C). 32 It can be concluded that these quasi-superuid-based reactions performed in nanoconned channels exhibited enhanced performances in terms of high selectivity and low reaction activation energy. Further reducing the channel size would allow superuidbased molecule reactions to proceed, realizing high ux and high selectivity. For example, electrocatalytic CO 2 reduction in solid-electrolyte devices containing ion-conducting solid polymers has been reported, resulting in continuous production of pure liquid fuel solutions (Fig. 5a). 33 By using a HCOOHselective and easily scaled Bi catalyst at the cathode, production of pure HCOOH solutions with concentrations of up to 12 M was demonstrated, also showing 100 h of continuous and stable generation of 0.1 M HCOOH with high selectivity and activity. In addition, a porous solid electrolyte was also demonstrated in use in the direct electrosynthesis of pure aqueous H 2 O 2 solutions up to 20% by weight and >90% selectivity. 34 Recently, the metal-organic framework (MOF) MFM-520 has been utilized to efficiently conne a NO 2 dimer with a high adsorption capacity of 4.2 mmol g À1 (Fig. 5b). 35 The N 2 O 4 conned inside MFM-520 nanopores was established at the molecular level, which was quantitatively converted into HNO 3 . More importantly, the MFM-520 was fully recovered with no loss of subsequent uptake capacity of NO 2 , indicating continuous clean-up and molecular reaction. Overall, superuid-based molecule reactions can realize excellent performance in terms of high ux and high selectivity.

Quantum-confined ion superfluids and QSF ion redox reactions
In addition to quantum-conned molecule superuids, quantum-conned ion superuids can also be found in both biological and articial channels, when the channel size is reduced to the distance of the Debye length (l D ) for ions. Ultrafast ion and molecule transport are observed in biological ion channels, and approximately 10 7 ions are allowed to transit in a single channel in 1 s at body temperature. 36 Electrocytes in electric eels can generate a high potential of $600 V and high current density of 500 A m À2 within 20 ms (Fig. 6a), 37,38 indicating the fast transmission of ions and molecules accurately in the form of a superuid through Na + and K + channels due to the coherence effect. 39 However, why does an electric eel not kill itself? This means that the resistance of electrocytes in the electric eel is minimal, and they do not generate a lot of heat to kill the animal. Other examples include a NaK nonselective channel, which enables only one fully hydrated Na + ion to be transported through a selective lter. 40 Similarly, a potassium lter from Streptomyces lividans is able to hold an ordered strand containing two K + ions of around 7.5 A apart and a single water molecule in between (Fig. 6b). 41,42 Besides this, each calcium channel in calmodulin can also simultaneously bind two Ca 2+ ions. 43 In an articial system, ultrafast ion transport has been reported in MOF channels. The porous ZIF-8 membrane that has an average pore size of $0.34 nm demonstrates a high LiCl/RbCl ion selectivity of $4.6, and an ultrafast ion transport rate (10 6 -10 8 ions per s) (Fig. 6c). 44 UiO-66 MOF channels with pores that are $0.6 nm in size show an ultrahigh F À transport rate (10 8 -10 10 ions per s) and ultrahigh F À /Cl À selectivity. 45 According to the theory of ionics, in liquid phase, counter-ions and co-ions are transported across microchannels based on disordered entropy-driven ion diffusion. 46 When the channel size decreases to the nanoscale level, specically close to a distance of two-fold the Debye length (l D ), the nanochannel is lled with relatively ordered counter-ions in the solution Chemical Science without any co-ions. 47 Further reducing the channel size to a distance of l D connes the counter-ions in the way of ordered strands, and endows them with ultrafast ion transport in an enthalpy-driven way without energy loss (Fig. 6d). Therefore, quantum-conned ion superuids either in biological or arti-cial channels show unique characteristics of high ux and low energy consumption. The success of the Li battery demonstrates QSF ion redox reactions in an articial system. In the charge-discharge process of a Li battery, Li redox reactions in the 2D conned layered structure have two characteristics, superdense ordering and superuidity, to realize high energy density and fast charge-discharge. In the Li battery, reversible superdense ordering of Li between two graphene sheets has been evidenced by in situ transmission electron microscopy (TEM) measurements and density functional theory (DFT) calculations (Fig. 7a). 48 Li atoms adopt close-packed ordering between the two carbon sheets, resulting in ultrahigh Li storage capacity, which far exceeds that expected from the formation of LiC 6 , the densest conguration known under normal conditions for Li intercalation within bulk graphitic carbon. On the other hand, when charged and discharged, ultrafast transport of Li ions occurs in a superuid way in the conned 2D channel at a distance of the Debye length (l D ) for Li ions, resulting in a fast charge-discharge rate (Fig. 7b). Among them, some anodes with excellent performance due to the unique 2D channel structure have been developed (Fig. 7c). For instance, a composite lithium metal anode fabricated by molten Li infusion into a layered rGO lm with nanoscale gaps was designed. The anode retains up to $3390 mA h g À1 of capacity, exhibits low overpotential ($80 mV at 3 mA cm À2 ) and a at voltage prole in a carbonate electrolyte. 49 As an attractive pseudocapacitive electrode material, MoS 2 nanoparticles with an expanded atomic lamellar structure have been incorporated in a lithium battery, and achieved a maximum power density of 5.3 kW kg À1 (with 6 W h kg À1 energy density) and a maximum energy density of 37 W h kg À1 (with 74 W kg À1 power density). 50 TiO 2 -B nanowires with a length of several hundred nanometers and a width of approximately 10 nm show excellent electron/ion transport properties and reaction kinetics in lithium intercalation, and exhibit an extraordinary rate performance as an anode material for lithium-ion batteries. 51 On the other hand, cathode materials for high rate performance have also been focused on (Fig. 7d). A LiMn 2 O 4 nanochain with beads of 100 nm has shown great promise for practical application as a high rate cathode material for Li ion batteries due to its unique subnanochannel structure. 52 A Li battery with LiFePO 4 nanoparticles wrapped with a N, S-co-doped graphene composite was shown to realize an ultrahigh rate and long-life for Li ion batteries, owing to the increased Li ion transport rate in its subnanochannels. 53 Besides this, nanocrystalline LiCoO 2 with nanosized cell parameters has been adopted as a cathode to achieve high-rate Li-ion intercalation. 54 It is noteworthy that although John B. Goodenough was awarded the Nobel Prize in Chemistry alongside M. Stanley Whittingham and Akira Yoshino in 2019 for the development of Li-ion batteries, they did not realize that besides high energy density and storage, QSF ion transport and redox reactions in the 2D conned channels of the anode and cathode are the key to a fast charge-discharge process, which is superior to a charge-discharge process dominated by ion diffusion.
To achieve superdense ordering and superuidity in QSF ion redox reactions, regulation of the intersheet spacing of a 2D layered structure is of signicant importance. As for capacitive energy storage, graphene sheets with subnanometer scale intersheet spacing have been used to form highly compact In order to realize QSF reactions in artificial systems, the designs of channels and reactions should be considered. As for the design of channels, the channel size should be designed with a van der Waals equilibrium distance (r 0 ) for the reactant molecules, to achieve high flux reactions. As for the design of the reactions, through regulating the chemical structure of the channels and the chemical properties of the reactant molecules (polarity, chirality, hydrogen bonding, etc.), to arrange the reactants orderly in the channel to satisfy symmetry-matching principles, high performance in terms of 100% selectivity and low reaction activation energy of the reactions can be realized. (c) One-step QSF reactions (A + B ¼ C) can be developed into multi-step QSF reactions (A + B ¼ C, C + D ¼ E, E + F ¼ G, G + H ¼ I, etc.), which can be used in future tubular reactors with continuous nanochannel membranes, to achieve high flux, high selectivity and low energy consumption reactions. It is expected that such a tubular reactor will replace the traditional huge tower reactor in future chemical engineering, and bring revolutionary technology to both chemistry and chemical engineering. carbon electrodes with a continuous superuid-based ion transport network, which achieve ultrahigh volumetric energy densities. The intercalation of Li ions into nanocrystalline layers results in capacitor behaviour, which can be increased as the crystallite size decreases. It has been found that uniquely structured anode and cathode materials can increase capacitance, even at a high rate, especially in a nanoconned space. 55,56 Chemically converted graphene hydrogel lms with an adaptive pore structure have been fabricated by capillary pressure to adjust intersheet spacing (Fig. 8a). 57 It is a convenient fabrication route that can be used to achieve a series of pore size lms in contrast to traditional techniques (Fig. 8b). The intersheet spacing of graphite is 0.335 nm, while it can vary from 5.360 to 0.670 nm via increasing the packing density of graphene (Fig. 8c). Consequently, a higher packing density or thinner intersheet spacing can improve conductivity and decrease sheet resistance (Fig. 8d). Therefore, reducing the intersheet spacing of the 2D layered structure to a distance of the Debye length (l D ) results in high ion-ux and superuidity, which are benecial for the realization of QSF ion transport and redox reactions.

QSF molecule reactions
In order to realize QSF molecule reactions in an articial system, on the one hand, reactant molecules should be arranged in order and be able to transform their molecular conguration to satisfy the symmetry-matching principles of the frontier molecular orbital theory, resulting in a lower reaction activation energy of the reactions and achieving 100% selectivity. On the other hand, the channel size should be designed at the van der Waals equilibrium distance (r 0 ) for molecules, then the reactant uid can achieve QSF-like ultrafast ow within the channels, resulting in high ux in the reactions. QSF molecule reactions can be further subdivided into QSF organic reactions (Fig. 9a) and QSF polymerization (Fig. 9b), both types exhibiting the characteristics of high ux, low reaction activation energy and 100% selectivity. It is noteworthy that the high ux feature enables the fast adsorption-desorption of reactant molecules on catalysts and reduces the possibility of catalyst deactivation or poisoning to extend the catalyst lifetime.
Based on the above research, one-step QSF reactions (A + B ¼ C) can be developed into multi-step QSF reactions (A + B ¼ C, C + D ¼ E, E + F ¼ G, G + H ¼ I, etc.), which can be carried out in a future tubular reactor with continuous nanochannel membranes, to achieve high ux, high selectivity and low energy consumption of the reactions (Fig. 9c). Such tubular reactor is expected to replace the traditional huge tower reactor in future chemical engineering, and would bring revolutionary technology to both chemistry and chemical engineering. It should be noted that heat transfer problems are an important issue in QSF reactions. When there are multi-step QSF reactions, we can design two adjacent reactions where one is exothermic and the other is endothermic, so their heat can offset each other. Heat transfer components can also be designed outside the tubular reactor when heating or cooling is needed. Moreover, there must be a free energy difference to make the QSF reaction devices work. Actually, ions or molecules in a biological system use concentration difference to obtain an energy difference. In an articial system, we can use concentration difference or pressure difference to obtain an energy difference.

Summary and outlook
In summary, inspired by enzymatic biosynthesis, we have introduced QSF reactions into articial systems to achieve high ux, 100% selectivity and low reaction activation energy at room temperature, under atmospheric pressure in an aqueous environment. In order to realize QSF reactions in articial systems, the design of channels and reactions should be seriously considered. As for the design of channels, according to the molecule reaction or ion redox reaction, we can design the channel size to be the van der Waals equilibrium distance (r 0 ) for molecules or the Debye length (l D ) for ions to achieve high ux in the reactions. As for the design of reactions, through regulating the chemical structure of the channels and the chemical properties of the reactant molecules (polarity, chirality, hydrogen bonding, ion interactions, etc.), to arrange the reactants orderly in the channel to satisfy symmetrymatching principles, a high performance of 100% selectivity and low reaction activation energy can be realized in the reactions. Additionally, the dynamic wettability of reactant molecules or ions in the channels should be considered in QSF reactions. 58 In future chemical engineering, based on multi-step QSF reactions, a tubular reactor with continuous nanochannel membranes can be used to achieve high ux, high selectivity and low energy consumption reactions, and is expected to replace the traditional huge tower reactor.
Still, there are many remaining challenges for QSF reactions. For instance, new technology in terms of characterization should be developed to understand the mechanism of QSF reactions. The problems of reaction ow, blockage, and membrane stability should be solved in actual chemical production. A new understanding of the QSF reaction by means of theoretical simulation should be explored. To solve these problems, we can continue to learn from nature and simulate natural systems to achieve better performance.

Conflicts of interest
There are no conicts to declare.