A countercurrent microflow strategy for simultaneous high selectivity and conversion in aromatic nitration

Domain: chemistry
Published: 2026-02-20
Editorial outcome: major_revision

Abstract
Aromatic nitration, a hazardously complex process, poses serious risks. A major challenge for the reaction is the trade-off effect between spatiotemporal conversion rate and selectivity, particularly the over-nitration side reactions that have plagued the field for nearly 200 years. We propose a countercurrent microflow mode between two microreactors, which boosts spatiotemporal conversion rate by over five times compared to the normal single-stage co-current microflow mode, and two orders of magnitude compared to traditional batch reactors. Meanwhile, we identify an inhibition mechanism of over-nitration. The generated H2O in the main reaction can in situ reduce the dissolution of nitroaromatics in the aqueous phase and effectively prevent over-nitration. Through synergistic control of both kinetics and thermodynamics in the microreaction process, high spatiotemporal conversion and selectivity are achieved simultaneously, overcoming the trade-off effect. Furthermore, we demonstrate the broad applicability of the microflow strategy across various aromatic nitration processes.

Introduction
Aromatic nitration is one of the top well-known chemical reactions, with nearly 200 years of history. It is widely used in the production of pharmaceuticals, dyes, pesticides, and energetic materials. Due to its low cost and high nitrating activity, the mixed acid composed of concentrated H2SO4 and HNO3 is the most widely used nitrating agent, and is almost the only choice of nitrating agent in the aromatic nitration industry1. The aromatic nitration with mixed acid is a typical fast and highly exothermic reaction, with the reaction medium being highly corrosive and posing significant risk of explosion2. As such, it is regarded as a classic hazardous chemical process. From the 1830s to the 1990s, aromatic nitration was predominantly carried out in stirred-tank reactors. Due to the limited transport rates of stirred tanks, the process was typically operated under low-temperature and low-reactant-concentration conditions to ensure safety. Since the 1990s, there has been a gradual shift toward the use of microreactors and flow chemistry for aromatic nitration. Microreactors offer superior heat and mass transport performance compared to stirred tanks and have a small liquid holdup, providing a higher level of safety3–5. However, the hazards of aromatic nitration not only exist in reaction process, but also arise during the storage, transportation, and separation of reaction products. This is primarily due to the side reactions in aromatic nitration that produce high-energy and thermally unstable byproducts. The two most prominent side reactions are over-nitration and oxidation. It has been shown that oxidation side reactions can be effectively suppressed in microreactors6. Nonetheless, over-nitration side reaction remains inevitable due to the complex parallel-consecutive reactions (Fig. 1a)7.

In stirred tanks, a relatively high mononitration selectivity could be achieved at the cost of significantly reduced spatiotemporal conversion rate8–10. In microreactors, the spatiotemporal conversion rate is greatly improved6,11–13. However, all existing microreaction technologies remain insufficient to control the selectivity of aromatic nitration. The intense heat release of the reaction causes unavoidable rises in interfacial temperatures, which significantly exacerbate over-nitration, resulting in notably lower mononitration selectivity in microreactors compared to stirred tanks. Taking the toluene nitration as an example, the selectivity of mononitrotoluene (MNT) in stirred-tank reactors can reach 95%, while in current microreactors, S MNT ranges between 64% and 95%14–16. The characteristics result in a pronounced trade-off effect between spatiotemporal conversion rate and selectivity even in the microreaction systems. Furthermore, based on the reaction mechanism of electrophilic substitution, the more active the nitration reaction is, the more significant the trade-off effect is (Fig. 1b)17–19. Unfortunately, no effective solution to this issue has been identified over the past two centuries. In addition, due to the kinetic characteristics of a second-order reaction, the low reactant concentrations in the later reaction stages lead to significantly reduced intrinsic reaction rates3,20,21. Current methods for achieving high aromatic conversion involve either extending the residence time or increasing the HNO3 feed. However, due to the characteristics of consecutive reactions, extended residence times exacerbate over-nitration and lead to high-pressure drops in flow chemistry systems. Similarly, increasing HNO3 feed intensifies competitive side reactions, further intensifying the over-nitration issue (Fig. 1a). To overcome these bottlenecks in aromatic nitration, it is imperative to develop new strategies that enhance spatiotemporal conversion rate without compromising selectivity.

Here, based on the principles of flow chemistry, we proposed to introduce the concept of countercurrent flow into flow chemistry to address the significant disparity in reaction rate and interfacial temperature distribution during the early and late stages under co-current conditions. A two-stage countercurrent kinetic control strategy featuring co-current flow intra-stage, countercurrent flow inter-stage is proposed. Based on the improved controllability of the interfacial temperature, the thermodynamic properties of the nitration system were further characterized, and the regulatory mechanism of interfacial thermodynamic property changes on reaction selectivity during the reaction was explored. Finally, by synergistically integrating kinetic and thermodynamic control, we achieve both a high spatiotemporal conversion rate and high selectivity in aromatic nitration simultaneously. The method is also extended to the nitration of other typical aromatics, such as benzene and chlorobenzene, to demonstrate the broad applicability of the proposed strategy.

Results
Comparison of reaction kinetics between co-current and countercurrent modes
According to the second-order kinetics, the relationship between aromatic conversion rate and residence time in a conventional single-stage co-current flow mode can be described as Eq. (1) (derivation in Supplementary Note 1)22.

x = 1 - (c0N - c0ar) / (c0N exp(k1(c0N - c0ar)t) - c0ar) (1)

where x is the aromatic conversion rate, c is the concentration, k1 is the observed reaction rate constant for mononitration and t is the residence time. Superscript 0 denotes the initial condition, and subscripts ar and N refer to the aromatic and HNO3, respectively. We assume isothermal conditions. The generated H2O has no effect on the reaction rate, as well as ignoring the over-nitration. Thus, k1 is constant.

Based on Eq. (1), Fig. 2a shows that over 90% conversion occurs within the first 10% of residence time due to high reactant concentrations. As the reaction progresses, the decreasing concentrations substantially slow down the reaction rate. In practice, H2SO4 dilution by the generated H2O reduces k1 overtime, making actual residence times longer than predicted. This is an inevitable result of the second-order reactions under co-current flow. However, the over-nitration, as the second step in the consecutive reactions, will be exacerbated when longer residence times are applied to achieve higher aromatic conversion, resulting in a reduced selectivity for mononitration, as shown in Fig. 2b.

To address this, we propose a countercurrent microflow mode of co-current flow intra-stage, countercurrent flow inter-stage by combining the idea of countercurrent flow and flow chemistry (Fig. 2c). This mode transforms the conventional single-stage co-current system into two co-current stages. Aromatic feed enters the first-stage and reacts with the mixed acid derived from the second-stage. Due to the excess of aromatic, HNO3 in the aqueous phase could be completely consumed, yielding an organic phase consisting unreacted aromatic and nitroaromatic products. This stream, combined with fresh mixed acid, enters the second-stage, where the remaining aromatic is fully nitrated. The mixed acid stream is recycled back to the first-stage, completing the countercurrent flow. In this configuration, aromatic conversion in each stage affects the reactant concentrations in the other, thereby influencing overall reaction efficiency (Fig. 2d). As the aromatic conversion in the first-stage (x ar,1) increases, the initial HNO3 concentration in the first-stage rises, enhancing its reaction rate. According to the mass conservation, this reduces the initial aromatic concentration in the second-stage, lowering its reaction rate. Consequently, the total reaction efficiency, as the sum of both stages, shows a nonlinear relationship with x ar,1, as further discussed in the next section. The residence times required in both modes are compared under identical conditions (Fig. 2d). For instance, to achieve 99.9% conversion of aromatic, the two-stage countercurrent demonstrates an over 5 times improvement in reaction efficiency (Fig. 2e). This efficiency gap widens with increasing target conversion. Furthermore, lower reactant concentrations in each stage of the two-stage countercurrent mode greatly reduce the heat release rate (HRR) compared to the single-stage co-current mode (Fig. 2f). The maximum of HRR in the countercurrent mode is only half that of the single-stage co-current mode, significantly reducing the difficulty of controlling the system interface temperature. A comparison of interfacial temperature evolution during the early stage of the reaction is shown in Fig. 2g. Under the co-current mode, the interfacial temperature rises by 51 °C within 1 s, whereas in the two-stage countercurrent mode, the temperature increases by only 15 °C, representing a 69.8% reduction.

In summary, the two-stage countercurrent mode significantly enhances reaction efficiency and facilitates the achievement of a high conversion rate, while simultaneously reducing the heat release rate of the system and improving the controllability of the aromatic nitration process. It should be noted that the two-stage countercurrent mode does not change the intrinsic reaction rate constant and activation energy, but rather redistributes the concentration gradients of the two reactants along the reactor, enabling each stage to operate under more favorable local conditions and thus enhancing overall spatiotemporal conversion rate and thermal uniformity.

Optimization of reaction efficiency of two-stage countercurrent mode
Each stage in the two-stage countercurrent mode operates under co-current microflow. Therefore, the total residence time required to achieve a target overall conversion is the sum of the residence times needed for each stage to achieve its respective conversion. Based on the second-order reaction kinetics, this relationship is expressed in Eq. (2) (derivation in Supplementary Note 2).

t = (1 / (k(c0N - c0ar))) * ln((c0N(1 - x ar)) / (c0N - c0ar * x ar)) + (1 / (k(c0N - c0ar(1 - x ar,1)))) * ln((c0N - c0ar(1 - x ar,1)) / (c0N - c0ar * x ar)) (2)

where t is the total residence time. x ar is the total target aromatic conversion. x ar,1 is the aromatic conversion achieved in the first-stage.

The variation of t with x ar,1 is illustrated in Fig. 3a. As x ar,1 increases, the residence time in the first-stage (t1) increases, while that in the second-stage (t2) decreases. t initially decreases and then increases with the increase of x ar,1. This indicates the existence of an optimal first-stage conversion ((x ar,1) opt), at which t is minimized. The effects of the reaction rate constant (k), the initial molar ratio of HNO3 to aromatic (M), and the overall target conversion (x ar) on (x ar,1) opt are further investigated (Fig. 3b–d). The results show that (x ar,1) opt decreases with the increase in M. However, when M is fixed, k and x ar only affect the values of t but do not change (x ar,1) opt. These findings suggest that optimizing x ar,1 is an effective method to minimize t, when M changes. (x ar,1) opt can be determined by Eq. (3).

(x ar,1) opt = 0.5 * (M - 1) / M (3)

This provides an effective method for optimizing the efficiency of the two-stage countercurrent mode for second-order reactions. Given a specific M, the respective aromatic conversion rate allocated to the two stages can be determined.

The kinetic regulation method described above is based on the principles of flow chemistry and established from the principles of second-order reaction kinetics. Therefore, this method is broadly applicable beyond aromatic nitration and can be extended to other second-order reaction systems.

Application of two-stage countercurrent kinetic regulation
The kinetic regulation strategy was applied to aromatic nitration, using toluene nitration as a representative example (Supplementary Note 3). As mentioned above, the enhancement of the spatiotemporal conversion rate originates from the optimized reactant concentration profiles governed by the law of mass action. The molar ratio of HNO3 to aromatic (M) serves as the key parameter that regulates the concentration distribution of the two reactants along the reactor. It also determines the allocation of conversion between the two stages in the design of the two-stage countercurrent mode, as described by Eq. (3). Unlike the single-stage co-current mode, which requires a large excess of HNO3 to achieve high toluene conversion, the two-stage countercurrent mode enables high toluene conversion with stoichiometric feeding, assuming over-nitration is negligible. However, in practice, a slight excess of HNO3 is necessary to account for its consumption via over-nitration side reactions and to help suppress equipment corrosion. Therefore, this study adopts M=1.01, representing a 1% excess of HNO3. According to Eq. (3), (x ar,1) opt is equal to 0.49. This indicates that the overall reaction efficiency is maximized when the first-stage conversion reaches 49%. Given the target overall conversion of 99.9%, the corresponding molar equivalent design of the microreaction system is illustrated in Fig. 4a.

Suppress over-nitration side reaction via thermodynamic regulation
Aromatic nitration suffers from the trade-off effect between spatiotemporal conversion rate and selectivity. The two-stage countercurrent mode significantly improves the spatiotemporal conversion rate, making suppression of over-nitration reaction essential to overcoming trade-off effect. Temperature (T), H2SO4 concentration (w H2SO4), and M are critical parameters influencing over-nitration. As shown in Fig. 4b–e, increasing T, w H2SO4, or M accelerates the reaction but also exacerbates the over-nitration due to the increased interfacial temperatures, reflecting the trade-off effect. Notably, the selectivity of over-nitration (S DNT) is highly sensitive to w H2SO4, and decreases greatly when w H2SO4 falls below 80% (Fig. 4c). Under the operating conditions explored in this work, S DNT is much lower than values reported in the literature (5–36%), where higher H2SO4 dosages are commonly employed.

H2SO4 serves as both catalyst and solvent, affecting both reaction kinetics and product distribution through physical solubility changes. Conventional aromatic nitration processes rely on high H2SO4 dosage to moderate temperature rise and ensure safety. In contrast, the superior transport property of microreactor allows for greatly reduced H2SO4 dosage while maintaining safety, enabling exploration of scientific blind spots in aromatic nitration. Based on the principles of flow chemistry, the H2SO4 dosage determines the volumetric flow rate ratio between the aqueous (Qa) and organic phases (Qo). The effect of Qa/Qo on the reaction performance in the second-stage microreactor was examined (Fig. 5a, b and Supplementary Fig. S2).

Decreasing H2SO4 dosage lowers Qa/Qo while increasing the initial HNO3 concentration, resulting in a higher initial reaction rate when Qa/Qo > 1 (Fig. 5a). Meanwhile, S DNT decreases with decreasing Qa/Qo (Fig. 5b). When Qa/Qo < 1, S DNT approaches zero, but the reaction rate also decreases due to insufficient H2SO4 concentration to sustain NO2+ generation. Figure 5c shows that lower H2SO4 dosage enhances the dilution effect of generated H2O, further reducing H2SO4 concentration and suppressing over-nitration. At Qa/Qo = 1, both high conversion and high selectivity are achieved. This operating condition corresponds to an optimal dilution window (Supplementary Note 12), in which the molar ratio n H2SO4/n HNO3 is maintained between 0.7 and 1.5.

To clarify the role of H2O, the effect of residence time on reaction performance and H2SO4 concentration was explored (Fig. 5d–f and Supplementary Fig. S3). Increasing residence times decreased both reaction rate and S DNT, indicating that over-nitration is more sensitive to H2O dilution than mononitration. The solubility of toluene and MNT in different H2SO4 concentrations was investigated (Fig. 5g). MNT is always more soluble than toluene in H2SO4, but as H2SO4 concentration decreases, the solubility of MNT decreases more significantly. These results imply that in conventional aromatic nitration processes (Fig. 5h), where H2SO4 dosage is large, the amount of generated H2O by the main reaction is low relative to the total H2SO4 dosage, resulting in minimal dilution effect. The interfacial temperature increases due to the heat released by the reaction, resulting in a higher solubility of MNT in the mixed acid, where over-nitration occurs. In contrast, under low H2SO4 dosage, the generated H2O effectively in situ dilutes H2SO4, greatly reducing MNT solubility and preventing its transfer into the acid phase. Meanwhile, the higher initial HNO3 concentration maintains the main reaction rate.

This mechanism enhances the reaction rate while simultaneously suppressing over-nitration side reactions in aromatic nitration. Its essence lies in the fact that the generated H2O in the main reaction in situ reduces the solubility of the product in H2SO4, thereby keeping the over-nitration side reaction under thermodynamically unfavorable conditions and effectively suppressing it. The concentration and dosage of H2SO4 are the key parameters for achieving this thermodynamic control, which we term the product inhibition mechanism (Fig. 5h).

Molecular simulations provide microscopic support for this mechanism (Supplementary Note 7). Nitroaromatic solvation in H2SO4 is dominated by directional hydrogen bond between the nitro group (-NO2) and the -OH groups of H2SO4, whereas solvation in H2O is significantly weaker due to longer hydrogen bond distances. As H2O accumulates, the hydrogen bond network of the acid phase transitions from an H2SO4-dominated to an H2O-dominated structure, markedly decreasing the solvating power of the acid phase. Furthermore, thermodynamic calculations reveal that increasing H2O content lowers the equilibrium concentration of nitronium ions (NO2+), thereby kinetically suppressing over-nitration. Consequently, the droplet interface becomes a region where over-nitration is simultaneously thermodynamically and kinetically disfavored, representing a general principle applicable to aromatic nitration systems.

Overcome trade-off effect in aromatic nitration via kinetics and thermodynamics regulation
The two-stage countercurrent mode serves as a reaction efficiency enhancement strategy based on kinetics, and the product inhibition mechanism offers a thermodynamic approach for selectivity control. Building on these principles, we developed a two-stage countercurrent microreaction system with Qa/Qo = 1 to emphasize the product inhibition effect, thereby enabling synergistic regulation through combined thermodynamic and kinetic control. Toluene nitration was first used as a model reaction to compare the performance of the two-stage countercurrent and single-stage co-current microreaction processes under identical operating conditions (Fig. 6a, b). With 1% excess HNO3, the two-stage countercurrent achieved a 99.9% toluene conversion, while the single-stage co-current mode reached only 69%. The two-stage process also demonstrated lower over-nitration side reaction selectivity, which was only 0.2%, 1–2 orders of magnitude lower than the 5–36% reported in the literatures (Supplementary Notes 8, 10 and 11). The Hatta number (Ha) under the operating conditions was evaluated (Supplementary Note 9). The calculated Ha values in both stages of the microreactor are consistently below 0.3 and decrease with increasing residence time, indicating that the reaction rate is governed by kinetics.

Through the synergistic control of kinetics and thermodynamics, we achieve high toluene conversion and MNT selectivity simultaneously (Fig. 6c). More importantly, the results reported in the literatures demonstrate a clear trade-off effect between spatiotemporal conversion rate and selectivity (Fig. 6d). Although relatively high conversion and selectivity were also achieved in batch reactors, this work improves the spatiotemporal conversion rate by two orders of magnitude (Supplementary Note 11), effectively overcoming the trade-off effect in nitration (Fig. 6d). Based on the product inhibition mechanism, the single-stage co-current also achieves high MNT selectivity, but it is difficult to reach a high conversion rate. As a result, the yield of MNT is significantly lower than that of the two-stage countercurrent (Fig. 6e). This limitation arises from the intrinsic coupling between reaction intensity, interfacial temperature rise, and residence time in the single-stage reaction mode.

We established a microflow strategy to overcome the trade-off effect based on the common laws of aromatic nitration kinetics and thermodynamics. To validate its universality, we extended it to the nitration reactions of benzene and chlorobenzene, with the reaction performance and solubility characteristics provided in Supplementary Notes 5 and 6. Due to the electron-donating effect of the methyl group, toluene exhibits stronger nitration reactivity, making the trade-off effect most prominent in toluene nitration. In contrast, the suppression of over-nitration side reaction in benzene and chlorobenzene is easier. Thus, applying the proposed method to the nitration of benzene and chlorobenzene yielded better results.

Methods
Materials
H2SO4 (98%) and HNO3 (98%) were provided by Beijing TongGuang Fine Chemicals Company. Other reagents were purchased from Aladdin Chemistry Co., Ltd. H2SO4 (<98%) was prepared by diluting H2SO4 (98%) with the deionized water. Other chemicals were used as received without any further purification. The only metallic component in contact with the mixed acid was the 316L stainless-steel pump head of the metering pump, which was operated at room temperature. The other components in contact with the mixed acid were fabricated from acid-resistant materials.

Reaction procedures
First, H2SO4 is diluted to the specified concentration. Next, according to the molar ratio of HNO3 to aromatics and the flow rate ratio of the two phases, a specified amount of HNO3 is added to the H2SO4 to obtain a mixed acid solution. The aqueous and organic phases are separately delivered using metering pumps (EMO AP-F, China). The two phases are dispersed in a microdisperser (CTFE) and reacted in a capillary microreactor (PTFE). The structure of the microdisperser is T-junction, which is consistent with our previous report22. The preheating coil, microdisperser, and capillary microreactor are all placed in a thermostat (CORIO CD-200F, Julabo, Germany) to maintain a constant temperature during the reaction process. Each reactor outlet is connected to a phase separator. At the outlet of the first-stage, the concentration of HNO3 is nearly zero, so it can be assumed that no further reaction occurs in the phase separator. The obtained organic phase is then delivered to the second-stage for further reaction. At the outlet of the second-stage, the phase separator contains H2SO4 at a lower concentration and at a temperature significantly lower than the reaction temperature, resulting in a very slow reaction rate. Therefore, it can also be assumed that no further reaction occurs. The organic phase obtained from the second phase separator is the final product.

For the long-term stability evaluation, the reactor was operated continuously for up to 10 h, and samples were periodically collected from the reactor outlet for compositional analysis (Section 10 in Supplementary Information).

Analysis
All samples were taken directly from the reactor outlet. The product was added to ice water to further dilute the H2SO4. After thorough mixing, methanol was added to obtain a homogeneous solution. The composition of the samples was analyzed using UPLC (Waters, ACQUITY UPLC I-CLASS System, column: PFP Column, 1.8μm, 3mm × 50mm, America, mobile phase: QH2O/QMeOH = 0.65/0.35, flow rate: 0.2mL/min, injection volume: 1μL). The content of each component was characterized using the internal standard method. GC–MS and UPLC–MS were employed for identification of by-products.

Determination of solubility of aromatics in H2SO4
First, H2SO4 was diluted to the specified concentration. An excess of aromatic was added to the H2SO4, and the mixture was stirred at 800 rpm for 6 h. After centrifugation to separate the phases, the upper organic phase was decanted, and a sample was taken from the lower aqueous phase. The aromatic hydrocarbon content was measured using the UPLC.

Data availability
The data that support the findings of this study are provided in the Source Data file, which constitutes the minimum dataset necessary to interpret and verify the results. Additional data are available in the manuscript and Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents a micro-reaction strategy utilizing a two-stage countercurrent flow architecture to address the long-standing trade-off between spatiotemporal conversion rate and selectivity in aromatic nitration with mixed acid. The experimental approach is systematic, and the results appear promising. However, several key aspects require further clarification and validation to firmly establish the claims, particularly regarding the proposed mechanisms and the comparative advantages of the countercurrent system. Prior to publication in Nature Communications, it is necessary to address the following comments.\nMajor comments:\n1. The actual effect of the countercurrent mode. Whereas the conventional co-current mode is limited in its later phase by low aromatic concentration, the countercurrent mode partitions these limitations: the first stage is constrained by low HNO3 concentration, and the second by low aromatic concentration. Therefore, the reported improvements in conversion efficiency and thermal management stem from operating each stage under more favorable conditions for one reactant, thereby raising the time-averaged reaction rate and reducing localized heat release. Although conversion improves significantly, the underlying reaction kinetics remain unchanged. The authors should clarify this distinction explicitly.\n2. Kinetics versus thermodynamics. In the manuscript, the authors claim to have kinetically and thermodynamically controlled the reaction process. However, I think they haven't clearly explained certain specific concepts, which may mislead readers:\na. In eqs. (1) and (2) and in Section 3 (“Application of two-stage countercurrent kinetic regulation”), the authors treat the reaction as second-order and suggest that kinetics can be tuned by changing M (the HNO3-to-aromatic molar ratio). However, concentration changes affect the reaction rate through mass action, not the intrinsic rate constant k, which is governed by the Arrhenius equation. The authors should clarify that the observed rate enhancement is due to optimized concentration profiles, not a change in fundamental kinetics.\nb. In Section 4 (“Suppress over-nitration side reaction via thermodynamic regulation”), H2SO4 concentration and temperature are presented as thermodynamic controls. While these factors influence both kinetics (via activation energy) and thermodynamics (e.g., solubility, phase behavior), the authors should explicitly distinguish their effects. For instance, H2SO4 acts as a catalyst and solvent, affecting both reaction rate and product distribution through physical solubility changes.\n3. Unclear mechanism for selectivity. The manuscript proposes that the generated H2O is a key intermediate during MNT formation, reducing MNT solubility and dissolution and thereby suppressing over-nitration in the aqueous phase. Since both countercurrent and normal modes generate H2O, what specific selectivity advantage arises from the countercurrent mode? Moreover, the proposed mechanism appears speculative and lacks direct evidence.\n4. Fundamental constraints are not resolved. Although the authors employ a countercurrent microflow mode, the reaction at each stage still seems to face the same constraints as the normal mode (Fig. 2a and 2b), suggesting the core trade-off remains unresolved.\n5. Uniqueness of the mode remains unclear. Many variables (t, Xar, M, T, WH2SO4 and Q) are explored under the countercurrent microflow mode, yet it remains unclear which factors are most critical in governing selectivity and conversion in their system.\nMinor comments:\n6. In Fig. 2h, different HNO3 dosages are used in stages 1 and 2. Is the total nitric acid consumption the same as in the normal mode?\n7. In Fig. 5e and 6b, why does the SDNT fraction decrease with increasing residence time?\n8. Normalize the literature data where applicable (see Fig. 6c-6e) to indicate whether the SDNT values from the literature are comparable in terms of temperature, WH₂SO₄, and M. If they are not, provide a reasonable performance envelope.\n9. The two-stage countercurrent system requires inter-stage phase separation and recirculation of the acid phase. What are the practical challenges and potential efficiency losses (e.g., product carry-over, acid phase degradation, flow stability) associated with this separation and recirculation in a continuous system, especially at larger scales?", "reviewer_2": "This manuscript presents an innovative \"two-stage countercurrent microreaction\" strategy that successfully overcomes the classic trade-off between spatiotemporal conversion rate and selectivity in aromatic nitration through synergistic kinetic and thermodynamic control. The study is well-designed and supported by substantial experimental data, offering a groundbreaking solution to a challenge in aromatic nitration. I recommend acceptance after minor revisions. Below are detailed comments:\n1. In Figure 2f, there are two orange lines. Are the HRR values of those lines measured from the first stage? This point should be expressed more clearly.\n2. Although low H2SO4 usage is emphasized, the effect of H₂SO₄/HNO3 ratio variation on selectivity is not systematically explored. Additional experiments with different HNO3 concentrations are recommended to define the \"optimal dilution window.\"\n3. After reducing the amount of sulfuric acid, the temperature was also raised to 90℃, which might accelerate the corrosion of the microreactor. (especially in hot zones). A long-term stability test should be evaluated.\n4. Does reduced over-nitration selectivity (SDNT) coincide with increased oxidative byproducts? Comprehensive byproduct analysis (e.g., HPLC-MS) is necessary to validate selectivity improvements fully.\n5. By optimizing reactant concentration distribution through a two-stage countercurrent operation, this work overcomes the efficiency bottleneck imposed by intrinsic second-order reaction kinetics. If analysis of the Damköhler number (or Hatta number) under actual operating conditions could be added, it would more rigorously validate the premise that mass transfer is not rate-limiting, thereby making the argument for its kinetic optimization strategy more comprehensive and compelling.\n6. The axis label should be \"t (min)\" instead of a slash, the current notation is confusing. Additionally, all abbreviations should be written in full the first time they appear. Please carefully review the entire text.", "reviewer_3": "This manuscript proposes a potentially transformative strategy for aromatic nitration by synergistically combining a novel two-stage countercurrent microflow system for kinetic regulation with a \"product inhibition mechanism\" for thermodynamic control. The work successfully addresses the persistent trade-off between spatiotemporal conversion rate and selectivity, demonstrating outstanding performance in toluene, benzene, and chlorobenzene nitration. However, before acceptance, the author must address the following major issues to strongly support their statement and fully clarify the reaction mechanism and practical application potential The essential revisions primarily concern:"}, "author_response": "Dear Editor:\nThanks so much for your attention and the reviewers’ suggestions on our manuscript\n“A Precise Microreaction Strategy to Overcome Trade-off Effect in Aromatic\nNitration via Kinetic and Thermodynamic Regulation”. All comments are very helpful\nfor improving the quality of our manuscript. We have revised the manuscript according\nto your kind advices and reviewers’ suggestions. Enclosed please find the responses to\nreviewers’ comments. In addition to the revised manuscript, we also uploaded one\nversion (named Marked Revision) with highlighted changes in blue. We sincerely hope\nthis manuscript will be finally acceptable and published on Nature Communications.\nThank you so much for all your help and we are looking forward to hearing positive\nresponse from you soon.\nReviewer #1\n1. The actual effect of the countercurrent mode. Whereas the conventional co-current\nmode is limited in its later phase by low aromatic concentration, the countercurrent\nmode partitions these limitations: the first stage is constrained by low HNO\n3\nconcentration, and the second by low aromatic concentration. Therefore, the reported\nimprovements in conversion efficiency and thermal management stem from operating\neach stage under more favorable conditions for one reactant, thereby raising the time-\naveraged reaction rate and reducing localized heat release. Although conversion\nimproves significantly, the underlying reaction kinetics remain unchanged. The authors\nshould clarify this distinction explicitly.\nThank you very much for your suggestion. The two-stage countercurrent mode\ndoes not affect the intrinsic reaction rate constant or activation energy. Instead, it\nredistributes the concentration gradients of the two reactants along the reactor, enabling\neach stage to operate under more favorable local conditions and thus enhancing overall\nspatiotemporal conversion rate and thermal uniformity. We have clarified this\ndistinction explicitly in the revised manuscript in the “Comparison of reaction kinetics\nbetween co-current and countercurrent modes” section as follows:\n“It should be noted that the two-stage countercurrent mode does not change the intrinsic\nreaction rate constant and activation energy, but rather redistributes the concentration\ngradients of the two reactants along the reactor, enabling each stage to operate under\nmore favorable local conditions and thus enhancing overall spatiotemporal conversion\nrate and thermal uniformity.”\n2. Kinetics versus thermodynamics. In the manuscript, the authors claim to have\nkinetically and thermodynamically controlled the reaction process. However, I think\nthey haven't clearly explained certain specific concepts, which may mislead readers:\na. In eqs. (1) and (2) and in Section 3 (“Application of two-stage countercurrent kinetic\nregulation”), the authors treat the reaction as second-order and suggest that kinetics can\nbe tuned by changing M (the HNO3-to-aromatic molar ratio). However, concentration\nchanges affect the reaction rate through mass action, not the intrinsic rate constant k,\nwhich is governed by the Arrhenius equation. The authors should clarify that the\nobserved rate enhancement is due to optimized concentration profiles, not a change in\nfundamental kinetics.\nb. In Section 4 (“Suppress over-nitration side reaction via thermodynamic regulation”),\nH2SO4 concentration and temperature are presented as thermodynamic controls. While\nthese factors influence both kinetics (via activation energy) and thermodynamics (e.g.,\nsolubility, phase behavior), the authors should explicitly distinguish their effects.\nFor instance, H2SO4 acts as a catalyst and solvent, affecting both reaction rate and\nproduct distribution through physical solubility changes.\nThank you very much for your important suggestion. We have revised the relevant\nsections accordingly in the revised manuscript.\na. This clarification has been added to section 3, “Application of two-stage\ncountercurrent kinetic regulation”:\n“As mentioned above, the enhancement of the spatiotemporal conversion rate\noriginates from the optimized reactant concentration profiles governed by the law of\nmass action. The molar ratio of HNO to aromatic (M) serves as the key parameter that\n3\nregulates the concentration distribution of the two reactants along the reactor. It also\ndetermines the allocation of conversion between the two stages in the design of the two-\nstage countercurrent mode, as described by Eq. (3).”\nb. This clarification has been added to section 4, “Suppress over-nitration side\nreaction via thermodynamic regulation”:\n“H SO serves as both catalyst and solvent, affecting both reaction kinetics and\n2 4\nproduct distribution through physical solubility changes.”\n“Its essence lies in the fact that the generated H O in the main reaction in situ\n2\nreduces the solubility of the product in H SO , thereby keeping the over-nitration side\n2 4\nreaction under thermodynamically unfavorable conditions and effectively suppressing\nit. The concentration and dosage of H SO are the key parameters for achieving this\n2 4\nthermodynamic control, which we term the “product inhibition mechanism” (Fig. 5h).”\n3. Unclear mechanism for selectivity. The manuscript proposes that the generated H2O\nis a key intermediate during MNT formation, reducing MNT solubility and dissolution\nand thereby suppressing over-nitration in the aqueous phase. Since both countercurrent\nand normal modes generate H2O, what specific selectivity advantage arises from the\ncountercurrent mode? Moreover, the proposed mechanism appears speculative and\nlacks direct evidence.\nThank you very much for your important comment. The product inhibition\nmechanism, as a thermodynamic regulation strategy, applies to both co-current and\ncountercurrent systems. However, it is more effectively manifested under the\ncountercurrent mode. As shown in Fig. 6b, even under co-current mode, the selectivity\nof the over-nitration side reaction is significantly lower than those reported in the\nliterature, confirming that the product inhibition mechanism is generally valid.\nIn the co-current mode, HNO and aromatics are simultaneously present at high\n3\nconcentrations at the reactor inlet, leading to a rapid reaction rate and intense heat\nrelease. The resulting interfacial temperature rise increases the solubility of MNT in\nH SO (Figs. 2g and 5g). In addition, since the activation energy of the over-nitration\n2 4\nreaction is higher than that of mononitration, elevated temperatures further promote its\noccurrence. Moreover, as shown in Fig. 6a, a longer residence time is required to reach\nhigh conversion in the co-current system, which enhances the probability of over-\nnitration as the second step of the consecutive reactions.\nIn contrast, the two-stage countercurrent mode significantly reduces the interfacial\ntemperature rise (Fig. 2g) and overall residence time (Fig. 2e). These effects suppress\nthe conditions favorable for over-nitration and further strengthen the inhibition effect\ninduced by the generated H O, resulting in higher selectivity.\nas a function of H SO concentration (Fig. 5g), and determined the dynamic variation\n2 4\nof H SO concentration with residence time (Fig. 5c,f). Molecular simulations further\n2 4\nreveal that the solvation of nitroaromatics in H SO originates from directional\n2 4\nhydrogen bonding between the nitro group (-NO ) and the -OH groups of H SO . This\n2 2 4\nprovides a molecular-level foundation for the product inhibition mechanism and is fully\nconsistent with the experimentally observed sharp decrease in nitroaromatic solubility\nas the H SO concentration decreases. These results demonstrate that the in-situ\n2 4\ngenerated H O weakens the solvating ability of the acid phase toward nitroaromatics,\n2\nthereby suppressing their diffusion into the acid phase. Moreover, we calculated the\nequilibrium concentration of nitronium ion (NO +) under different temperatures and\n2 4 2\nthe NO + activity. This indicates that the droplet interface becomes a region where the\n2\nover-nitration is simultaneously thermodynamically and kinetically disfavored.\nTherefore, by lowering the H SO fraction in the mixed acid, we intensify the\n2 4\ndilution effect induced by H O and effectively suppress the over-nitration. The\n2\nexperimental observations and molecular simulations provide direct evidence\nsupporting the proposed product inhibition mechanism.\n4. Fundamental constraints are not resolved. Although the authors employ a\ncountercurrent microflow mode, the reaction at each stage still seems to face the same\nconstraints as the normal mode (Fig. 2a and 2b), suggesting the core trade-off remains\nunresolved.\nThank you very much for your important comment. The trade-off effect in\naromatic nitration refers to the intrinsic difficulty of simultaneously achieving high\nspatiotemporal conversion rate and high selectivity. Its essence is that an increase in the\nreaction rate is inevitably accompanied by an increase in the heat release rate, leading\nto a high interfacial temperature rise (ΔT). Since the activation energy of the over-\nnitration side reaction is higher than that of mononitration, the elevated temperatures\naccelerate over-nitration and decreases the selectivity of the mononitration. The\nconstrain still exists in single-stage co-current reactors. Although high selectivity can\nbe achieved through the product inhibition mechanism in single-stage co-current\nreactors, it is obtained at the cost of low spatiotemporal conversion rate.\nIn contrast, the two-stage countercurrent mode moderates the reaction intensity of\neach stage while simultaneously enhancing the overall spatiotemporal conversion rate.\nBy combining “countercurrent inter-stage” with product inhibition mechanism, this\ndesign effectively overcomes the trade-off limitation of single-stage reactor.\n5. Uniqueness of the mode remains unclear. Many variables (t, Xar, M, T, WH2SO4\nand Q) are explored under the countercurrent microflow mode, yet it remains unclear\nwhich factors are most critical in governing selectivity and conversion in their system.\nThank you very much for your kind suggestion. The uniqueness of the two-stage\ncountercurrent mode lies in dividing a single-stage co-current reactor into a\ncountercurrent flow between two co-current reactors, leading to the improvement in\nspatiotemporal conversion rate. Therefore, a rational segmentation design is the core of\nthe two-stage countercurrent strategy. As shown in Eq. (3), the moral ratio of HNO to\n3\naromatic (M) is the key parameter for designing the conversion distribution in the two-\nstage countercurrent mode. It defines the spatial concentration gradients of the two\nreactants within each stage, thereby influencing both the reaction rate and the heat\nrelease rate. The mass fraction of H SO (w ) determines the solubility of aromatic\n2 4 H2SO4\nand nitroaromatics in H SO , while the H SO dosage determines whether the product\n2 4 2 4\ninhibition mechanism can function. When M is fixed, the H SO dosage is governed by\n2 4\nthe molar ratio of H SO to HNO (n /n ). Therefore, both w and\n2 4 3 H2SO4 HNO3 H2SO4\nn /n together determine the selectivity. Overall, M, w , and n /n\nH2SO4 HNO3 H2SO4 H2SO4 HNO3\nare the most critical parameters in governing selectivity and conversion in the two-stage\ncountercurrent mode.\nMinor comments:\n6. In Fig. 2h, different HNO3 dosages are used in stages 1 and 2. Is the total nitric acid\nconsumption the same as in the normal mode?\nThank you very much for your question. As shown in Fig. 4a, the total input of\nHNO is 1.01 molar equivalents (M = 1). In the two-stage countercurrent configuration,\n3\nconsumption, the remaining HNO is recycled back to the first-stage reactor. No\n3\nadditional HNO is added. Therefore, the overall HNO input and consumption are the\n3 3\nsame as that in the normal mode.\n7. In Fig. 5e and 6b, why does the SDNT fraction decrease with increasing residence\ntime?\nThank you very much for your question. The observed decrease in S with\nDNT\nincreasing residence time arises from the dilution effect of H SO by the generated H O,\n2 4 2\nwhich strengthens the product inhibition mechanism. In the early stage of the reaction,\nthe reaction rate and heat release rate are high. Although the generated H O lowers the\n2 4\nsolubility of mononitrotoluene (MNT) in H SO , leading to the formation of a small\n2 4\namount of DNT. As the reaction proceeds, the reaction rate decreases, leading to a lower\ninterfacial temperature rise and a lower H SO concentration. Under these conditions,\n2 4\nthe over-nitration side reaction becomes thermodynamically unfavorable, and its rate is\nmuch lower than that of the main nitration reaction. Consequently, the fraction of DNT\ncontinuously decreases with increasing residence time.\n8. Normalize the literature data where applicable (see Fig. 6c-6e) to indicate whether\nthe SDNT values from the literature are comparable in terms of temperature, WH₂SO₄,\nand M. If they are not, provide a reasonable performance envelope.\nThank you very much for your kind suggestion. We have added the detailed\nreaction conditions corresponding to the literature data (Figs. 6c-e) to the revised\nmanuscript (Table S1) to clarify their comparability. The literature results are all\nobtained in single-stage co-current microreactors. For comparison, we additionally\nconducted single-stage co-current experiments under the same operating conditions as\nthe two-stage countercurrent process.\nAs shown in Table S1, with only a 1% excess of HNO₃, the single-stage co-current\nmode achieved 69% toluene conversion at the same residence time, whereas the two-\nstage countercurrent mode reached 99.9%. This clearly demonstrates the advantage of\nthe countercurrent configuration in enhancing the spatiotemporal conversion rate.\nMoreover, the mononitration selectivity achieved in both systems was significantly\nhigher than the values reported in the literature, confirming that the product inhibition\nmechanism effectively improves selectivity.\nTable S1. Comparison of reaction performance of toluene nitration.\nReaction w x S STCR\nH2SO4,0 T MNT\nAuthors (Year) V (mL) M T (℃) t (min)\nmode (%) (%) (%) (mol/L·min)\nPanke et al. (2003)1 Batch 25 62 1.5 65 140 100 94.6 1.3×10-5\nHNO\nexcessive\nPanke et al. (2003)1 1.2 62 1.5 65 15.0 98.8 89.5 3.1×10-4\nFu et al. (2022)3 132 — 1.2 45 2.2 86 95 1.7×10-4\nYang et al. (2022)4 10 58 1.3 49 2.8 85 91.3 1.7×10-3\nHan et al. (2024)5 6.74 80 1.4 70 1.2 100 91.5 6.3×10-3\nContinuous\nHan et al. (2024)5 6.65 79 1.3 80 1.0 100 89.9 7.5×10-3\nHan et al. (2024)5 6.82 78 1.4 70 1.2 99.1 90.6 6.2×10-3\nSingle-stage co-current\n4 71.5 1.01 90 3.9 69 99.1 0.8×10-3\n(This work)\nTwo-stage countercurrent\n4 71.5 1.01 90 3.9 99.9 99.8 1.2×10-3\n(This work)\nRT* means room temperature.\n9. The two-stage countercurrent system requires inter-stage phase separation and\nrecirculation of the acid phase. What are the practical challenges and potential\nefficiency losses (e.g., product carry-over, acid phase degradation, flow stability)\nassociated with this separation and recirculation in a continuous system, especially at\nlarger scales?\nThank you very much for your important comment. Phase separation and inter-\nstage countercurrent flow are indeed the core features that distinguish the proposed two-\nstage countercurrent mode. In our microreaction nitration strategy, the initial H SO\n2 4\nconcentration is selected based on the low solubilities of both nitroaromatics and\naromatics in H SO . As nitration proceeds, the in-situ generated water further dilutes\n2 4\nH SO , and reduces the solubility of the organic phase in the aqueous phase. As a result,\n2 4\nthe amount of organic phase entrained in the aqueous phase after reaction is extremely\nlow. Meanwhile, the two-stage countercurrent mode achieves high aromatic conversion\nwithout requiring large excess of HNO . Therefore, the residual HNO in the aqueous\n3 3\nphase is also low. Furthermore, the content of by-products is extremely low, so the acid\nphase does not undergo oxidative degradation or coking due to impurity accumulation.\nThese characteristics greatly facilitate the H SO reconcentration and recycling.\n2 4\nRegarding phase separation, the mixed acid acts as the dispersed phase to enable\nthe product inhibition mechanism in this work. The interfacial tension between mixed\nacid and aromatics is relatively high (> 15 mN/m). It will increase as the water content\nincreases (for reference, the interfacial tension between water and toluene is 35 mN/m).\nThis favors rapid coalescence of mixed acid droplets. In addition, a large density\ndifference exists between the two phases (1.5 - 1.7 g/mL for mixed acid and 0.9 - 1.1\ng/mL for aromatics), enabling efficient gravity-based separation. This property also\nensures stable inter-stage countercurrent flow. In the microreactor, uniform channel\ndimensions ensure stable and robust flow behavior.\nWe believe that heat management is the most critical challenge in scaling-up of\nthe two-stage countercurrent system. As shown in Fig. 2g, although the countercurrent\nmode significantly reduces interfacial temperature rise, the initial reaction stage still\nexhibits relatively high heat release rates. In this work, PTFE microreactors were\nselected to maintain the organic phase as the continuous phase. Although PTFE material\nhas moderate thermal conductivity, the small liquid holdup and high specific surface\narea of microreactors provide effective heat transfer ability. However, at large industrial\nthroughputs, heat-management requirements will become more stringent, and reactor\ndesign must account for enhanced heat transfer capability.\nA summary of these considerations has been added to the “Discussion” section of\nthe revised manuscript.\nReviewer #2\n1. In Figure 2f, there are two orange lines. Are the HRR values of those lines measured\nfrom the first stage? This point should be expressed more clearly.\nThank you very much for your kind suggestion. The two orange lines in Fig. 2f\ncorrespond to the heat release rates (HRR) of the first-stage and second-stage\nmicroreactors in the two-stage countercurrent system, respectively. Since the two\nmicroreactors are controlled independently, the two lines represent the HRR profiles\nalong the flow direction (from inlet to outlet) for each stage. We have revised Fig. 2f in\nthe revised manuscript.\n2. Although low H SO usage is emphasized, the effect of H SO /HNO ratio variation\n2 4 2 4 3\non selectivity is not systematically explored. Additional experiments with different\nHNO concentrations are recommended to define the \"optimal dilution window.\"\nanalysis of the effect of the H SO /HNO molar ratio on the aromatic nitration\n2 4 3\nperformance, as shown in Fig. S1. The tested range spans from fuming HNO\n3\n(n /n = 0) to the conventional mixed acid compositions used in conventional\nH2SO4 HNO3\naromatic nitration technologies (n /n > 5). The results show that when fuming\nH2SO4 HNO3\nHNO is used as the nitrating agent (n /n = 0), the reaction rate is significantly\nlower than that obtained under H SO catalyzed conditions, while the extent of over-\n2 4\nnitration increases markedly. This is because with fuming HNO as the nitrating agent,\n3\nthe high HNO concentration leads to an intense reaction at the initial stage of reaction\n3\nand a substantial interfacial temperature rise. Although both toluene and MNT are\ninsoluble in fuming HNO , the elevated interfacial temperature promotes the over-\n3\nnitration reaction kinetically. As the reaction proceeds, H O generated by mononitration\n2\nrapidly dilutes HNO , suppressing NO + formation and resulting in a low overall\n3 2\nconversion. When H SO is added, it is used as both a catalyst and a dehydrating agent,\n2 4\neffectively absorbing the generated water and maintaining a higher reaction rate\ncompared to using fuming HNO alone. However, when the H SO dosage is low(0 <\n3 2 4\nn /n < 0.7), H O generated still dilutes H SO to a level insufficient for NO₂⁺\nH2SO4 HNO3 2 2 4\ngeneration. Conversely, when the H SO dosage is high (n /n > 1.5), the\n2 4 H2SO4 HNO3\nproduct inhibition mechanism cannot be realized. Considering both reaction rate and\nselectivity, our results indicate that maintaining n /n within 0.7 – 1.5 provides\nH2SO4 HNO3\nan optimal balance, enabling both high conversion and high mononitration selectivity.\nThis corresponds to an optimal HNO dilution window of 40–59 mol%.\nInformation.\nFig. S1. Effect of molar ratio of H SO to HNO (n /n )on reaction performance. a. Effect of\n2 4 3 H2SO4 HNO3\nn /n on toluene nitration. b. Effect of n /n on DNT selectivity.\nH2SO4 HNO3 H2SO4 HNO3\n3. After reducing the amount of sulfuric acid, the temperature was also raised to 90℃,\nwhich might accelerate the corrosion of the microreactor. (Especially in hot zones). A\nlong-term stability test should be evaluated.\nThank you very much for your important comment. In this work, to fully utilize\nthe product inhibition mechanism, the mixed acid phase was dispersed as droplets, with\nthe organic phase serving as the continuous phase. Therefore, both the micro-disperser\nand the microreactor were fabricated from oleophilic and acid-resistant fluoropolymers.\nThe micro-disperser was made of CTFE, and the microreactor was made of PTFE. To\nevaluate the material stability under operating conditions, we analyzed the metal ion\ncontent in the aqueous phase using ICP-OES, as shown in Table S2. The results indicate\nthat the reaction system remained chemically stable despite the strongly oxidizing and\ncorrosive characteristics of aromatic nitration system. The metering pump used to\ndeliver the mixed acid always operated at room temperature, and the presence of HNO\n3\nis known to reduce the corrosiveness of the mixture toward stainless steel. We also\nconducted a 10-hour continuous operation test, during which samples were collected\nevery hour for analysis (Fig. S2). Throughout the operation, both the toluene conversion\nand the over-nitration selectivity remained stable within the experimental error range,\ndemonstrating the robustness of the microreaction system.\nThese results confirm that the microreactor system maintained stable under the\ntested conditions. The corresponding results and discussion have been incorporated into\nthe revised manuscript and Supplementary Information.\nFig. S2. Long-term stability test. a. Toluene conversion as a function of time-on-stream. b. DNT\nselectivity as a function of time-on-stream.\nTable S2. Concentration of Metal Elements in Different Samples\nSample Fe (mg/L) Cr (mg/L) Ni (mg/L) Cu (mg/L) Al (mg/L)\n1* 0.04 ± 0.01 0 0 0 0\n2* 0.43 ± 0.02 0.05 ± 0.01 0 2.15 ± 0.04 3.29 ± 0.05\n3* 0.50 ± 0.04 0.06 ± 0.02 0 1.82 ± 0.11 3.37 ± 0.03\n* Sample 1 is a mixed acid raw material. Sample 2 is a mixed acid raw material pumped to the reactor\ninlet. Sample 3 is the aqueous solution after the reaction.\n4. Does reduced over-nitration selectivity (SDNT) coincide with increased oxidative\nbyproducts? Comprehensive byproduct analysis (e.g., HPLC-MS) is necessary to\nvalidate selectivity improvements fully.\nThank you very much for your kind suggestion. To verify whether reduced over-\nnitration is accompanied by increased oxidative byproducts, we performed a\ncomprehensive byproduct analysis using UPLC-MS and GC-MS, as shown in Figs. S3\nand S4.\nPossible byproducts in the toluene nitration system include:\nOver-nitration byproducts: 2,6-dinitrotoluene(2,6-DNT), 2,4-dinitrotoluene(2,4-\nDNT), 2,5-dinitrotoluene(2,5-DNT), 3,4-dinitrotoluene(3,4-DNT).\nOxidation and polymerization by-products: benzaldehyde, benzyl alcohol, p-\nnitrobenzaldehyde, 2,4-dinitro-o-cresol, 2-methylbenzophenone, 3-\nmethylbenzophenone and 4-methylbenzophenone.\nThe UPLC-MS and GC-MS results were consistent, confirming the reliability of\nproduct analyzation. GC–MS was used to quantify the product distribution. At the first-\nstage outlet (Fig. S4a), four main peaks were observed, corresponding to toluene, o-\nMNT, m-MNT, and p-MNT. The total content of over-nitration products was <0.05%,\nwhile oxidation/polymerization products were less than 0.01%. At the second-stage\noutlet (Fig. S4b), only o-MNT, m-MNT, and p-MNT were detected. The residual\ntoluene was < 0.1%, and the level of over-nitration products remained very low (<0.2%).\nConsistently, oxidation and polymerization byproducts were less than 0.01%.\nThese results clearly demonstrate that our proposed mciroreaction strategy\nsuppresses over-nitration side reactions without increasing the occurrence of oxidation\nor polymerization reactions.\nThe full chromatographic spectra, mass-fragmentation assignments, and detailed\ndiscussion have been added to the Supplementary Information and incorporated into\nthe revised manuscript.\nFig. S3. UPLC-MS spectra of products from the toluene nitration system under two-stage countercurrent\nmode. a, product from the organic phase of the first-stage microreactor outlet. b, product from the organic\nphase of the second-stage microreactor outlet.\nFig. S4. GC-MS spectra of products from the toluene nitration system under two-stage countercurrent\nmode. a, product from the organic phase of the first-stage microreactor outlet. b, product from the organic\nphase of the second-stage microreactor outlet.\n5. By optimizing reactant concentration distribution through a two-stage countercurrent\noperation, this work overcomes the efficiency bottleneck imposed by intrinsic second-\norder reaction kinetics. If analysis of the Damköhler number (or Hatta number) under\nactual operating conditions could be added, it would more rigorously validate the\npremise that mass transfer is not rate-limiting, thereby making the argument for its\nkinetic optimization strategy more comprehensive and compelling.\nThank you very much for your kind suggestion. We evaluated the relative\ncontributions of reaction kinetics and mass transfer under the actual operating\nconditions by calculating the Hatta number (Ha), as shown in Fig. S5. The calculation\nmethod follows our previously reported approach. The results show that, in both stages\nof the microreactor, the Hatta number remains below 0.3 and gradually decreases with\nincreasing residence time. This confirms that throughout the entire reaction process,\nmass transfer is significantly faster than the intrinsic reaction rate. Therefore, the overall\nreaction rate is kinetically controlled rather than mass-transfer limited.\nAs the reaction proceeds, the concentrations of both reactants and the catalytic\nacid decrease, further lowering the intrinsic reaction rate and thus reducing the Hatta\nnumber. These results establish that the enhancement in spatiotemporal conversion rate\nachieved by the two-stage countercurrent operation arises from the redistribution of\nreactant concentration gradients, rather than from any improvement in mass transfer.\nThis proves the effectiveness of the mechanism of the kinetic control strategy proposed\nin this work.\nFig. S5. Variation of Hatta number (Ha) with residence time (t) in two countercurrent reaction process.\na. Changes in Ha with t in the first-stage microreactor. b. Changes in Ha with t in the second-stage\nmicroreactor.\n6. The axis label should be \"t (min)\" instead of a slash, the current notation is confusing.\nAdditionally, all abbreviations should be written in full the first time they appear. Please\ncarefully review the entire text.\nThank you very much for your kind reminder. We have revised the axis labels in\nall figures by replacing the previous slash with the standard format. In addition, we\ncarefully checked the entire manuscript, figure captions, and Supplementary\nInformation, and ensured that all abbreviations are written in full at their first\nappearance and remain consistent throughout the text.\nReviewer #3\nthroughout the manuscript, avoiding overly absolute or potentially contentious terms.\nSuch as \" firstly proposed \"(line 82), \"the highest” (line 22, 290,338), etc\nThank you very much for your reminder. We have carefully re-examined the full\nmanuscript and revised the statements that may appear overly absolute or potentially\ncontentious.\ncountercurrent microreaction technology improves the spatiotemporal conversion rate\nof aromatic nitration by \"two orders of magnitude\" compared to traditional batch\nreactors. This claim is crucial for demonstrating the extent of technological\nadvancement in this work. However, the evidence provided in the current manuscript\nis insufficient to directly support this strong conclusion. Specifically, while the\nconclusion is stated, the manuscript lacks specific numerical values, experimental\nconditions, or cited sources for the spatiotemporal conversion rate of the traditional\nbatch reactors used for comparison.\nThank you very much for your reminder. We have added the reactor reaction\nconditions, space-time conversion rate, and references to the revised manuscript, as\nshown in Table S1. The relevant calculation details have been added to the\nSupplementary Information.\nTable S1. Comparison of reaction performance of toluene nitration.\nReaction w x S STCR\nH2SO4,0 T MNT\nAuthors (Year) V (mL) M T (℃) t (min)\nmode (%) (%) (%) (mol/L·min)\nPanke et al. (2003)1 Batch 25 62 1.5 65 140 100 94.6 1.3×10-5\nHNO\nexcessive\nContinuous\nPanke et al. (2003)1 1.2 62 1.5 65 15.0 98.8 89.5 3.1×10-4\nFu et al. (2022)3 132 — 1.2 45 2.2 86 95 1.7×10-4\nYang et al. (2022)4 10 58 1.3 49 2.8 85 91.3 1.7×10-3\nHan et al. (2024)5 6.74 80 1.4 70 1.2 100 91.5 6.3×10-3\nHan et al. (2024)5 6.65 79 1.3 80 1.0 100 89.9 7.5×10-3\nHan et al. (2024)5 6.82 78 1.4 70 1.2 99.1 90.6 6.2×10-3\nSingle-stage co-current\n4 71.5 1.01 90 3.9 69 99.1 0.8×10-3\n(This work)\nTwo-stage countercurrent\n4 71.5 1.01 90 3.9 99.9 99.8 1.2×10-3\n(This work)\nRT* means room temperature.\n3. In Figures 6c, d, e, the current presentation effectively highlights the advantages in\nthe performance outcomes but lacks a side-by-side comparison of the key operating\nconditions required to achieve them. To more convincingly demonstrate that the\nsuperiority of the proposed strategy stems from its intrinsic innovation rather than from\nsacrificing other parameters, it is recommended to provide a systematic comparison\ntable. This table should compare, at similar conversion levels, the spatiotemporal\nconversion rate, selectivity, and key experimental conditions.\nThank you very much for your kind reminder. We have added a detail comparison\ntable to the Supplementary Information (Table S1). This table compared the reaction\nparameters of the two-stage countercurrent process with that of both the single-stage\nco-current process used in this work and representative literature reports.\nAll nitration processes reported in the literature were carried out in single-stage\nco-current microreactors, differing only in reactor structure or reaction conditions.\nCompared with these studies, our work employs lower H SO concentration, smaller\n2 4\nexcess of HNO , and higher operating temperature. Since decreasing H SO\n3 2 4\nconcentration significantly reduces the reaction rate, a direct comparison under\nidentical conditions is necessary to evaluate reaction performance. Therefore, we\nconducted nitration experiments using a conventional single-stage co-current\nmicroreactor under the same reaction conditions as those used in the two-stage\ncountercurrent mode. The results show that, within the same residence time, the single-\nstage co-current system achieves only 69% conversion of toluene. At this point,\nachieving higher conversion would require several-fold longer residence times. In\ncontrast, the two-stage countercurrent mode reaches 99.9% conversion under the same\ntotal residence time, demonstrating its intrinsic advantage in enhancing spatiotemporal\nconversion efficiency. Furthermore, the H₂SO₄ dosage used in both the single-stage co-\ncurrent and two-stage countercurrent modes in this work is substantially lower than\nthose reported in the literature. Owing to the product inhibition mechanism, over-\nnitration remains at a very low level regardless of the flow mode. However, the two-\nstage countercurrent mode provides additional benefits by reducing the heat release rate\nand interfacial temperature rise in each stage, thereby achieving even lower over-\nnitration selectivity.\nIn summary, these results clearly demonstrate that the superiority of the proposed\ntwo-stage countercurrent strategy arises from its intrinsic innovation, rather than from\nsacrificing other reaction parameters.\n4. The manuscript only focuses on the by-product of DNT. Please provide a\ncomprehensive product analysis spectrum (such as GC-MS) to prove that at such a high\nconversion rate, other competitive side reactions such as oxidation and polymerization\nhave also been effectively controlled.\nThank you very much for your kind suggestion. To verify the suppression of side\nreactions in aromatic nitration, a comprehensive product analysis was performed using\nthe toluene nitration system as a representative sample. First, we identified all potential\nproducts and byproducts in toluene nitration system under batch conditions. As shown\nin Fig. S6, GC–MS confirmed the presence of the mononitration products (o-, m-, and\np-nitrotoluene) as well as possible side products, including:\nOver-nitration byproducts: 2,6-dinitrotoluene(2,6-DNT), 2,4-dinitrotoluene(2,4-\nDNT), 2,5-dinitrotoluene(2,5-DNT), 3,4-dinitrotoluene(3,4-DNT)\nOxidation and polymerization by-products: benzaldehyde, benzyl alcohol, p-\nnitrobenzaldehyde, 2,4-dinitro-o-cresol, 2-methylbenzophenone, 3-\nmethylbenzophenone and 4-methylbenzophenone\nBased on this profiling, we further conducted full product analysis under the two-\nstage countercurrent reaction conditions via GC–MS (Fig. S7). At the first-stage outlet\n(Fig. S7a), four main peaks corresponding only to toluene, o-MNT, m-MNT, and p-\nMNT were detected. The content of over-nitration byproducts is less than 0.05%, and\nno oxidation or polymerization byproducts were detected above the instrumental\ndetection limit (< 0.01%). At the second-stage outlet (Fig. S7b), only three main peaks\ncorresponding to o-MNT, m-MNT, and p-MNT were detected. The residual toluene\ncontent was less than 0.1%, the content of over-nitration products remained very low\n(< 0.2%), and similarly no oxidation or polymerization products were detected above\nthe detection limit (<0.01%).\nThese results clearly confirm that even at an overall conversion of 99.9%, over-\nnitration, oxidation and polymerization side reactions are effectively suppressed in the\ntwo-stage countercurrent microreaction system. The corresponding GC–MS spectra\nand discussion have been added to the Supplementary Information, and a summary\nstatement has been added into the revised manuscript.\nFig. S6. GC-MS spectra of products from the toluene nitration system under batch conditions.\nFig. S7. GC-MS spectra of products from the toluene nitration system under two-stage countercurrent\nmode. a, product from the organic phase of the first-stage microreactor outlet. b, product from the organic\nphase of the second-stage microreactor outlet.\n5. At present, the \"product inhibition mechanism\" is mainly explained from the\nperspective of macroscopic solubility differences. If molecular simulation or in-situ\nmonitoring of the acid phase composition during the reaction process can be combined\nto explore the influence of water molecule addition on the hydrogen sulfate bond\nnetwork, the active form of nitrifying agents, and the solvation structure of organic\nsubstances, the mechanism explanation will be more profound.\nThank you very much for your kind suggestion. We further performed molecular\nsimulations to investigate the solvation structure and hydrogen bonding interactions of\nnitroaromatic molecules in H SO and H O. The results are summarized below.\n2 4 2\nFig. S8. Molecular simulations of solvation and hydrogen bonding interactions of p-nitrotoluene in (a)\nH SO and (b) H O.\n2 4 2\np-nitrotoluene was used as a representative example. As shown in Fig. S8a, when\np-nitrotoluene is solvated in H SO , its nitro group (-NO ) forms strong and directional\n2 4 2\nhydrogen bonds with the -OH groups of H SO . Other sites on the aromatic ring show\n2 4\nnegligible interactions. This indicates that solvation of nitroaromatics in H SO is\n2 4\ndominated by nitro-H SO hydrogen bond, consistent with their high solubility in\n2 4\nH SO . When p-nitrotoluene is solvated in H O, -NO still forms hydrogen bonds with\n2 4 2 2\nH O molecules (Fig. S8b). However, the hydrogen bond distances are significantly\n2\nlonger and the interactions are weaker than those in H SO . Therefore, nitroaromatics\n2 4\nexhibit much lower solubility in H O than in H SO .\n2 2 4\nTable S3 summarizes the evolution of hydrogen bond networks at different water\ncontents. In anhydrous H SO , the hydrogen bond network is dominated by H SO -\n2 4 2 4\nH SO interactions, with 200 O-H bonds and 53 O-H···O hydrogen bonds. As water is\n2 4\nintroduced, the number of O-H groups originating from H SO decreases (from 200 to\n2 4\n37 at 75% water), while H O-derived O-H groups increase correspondingly (from 0 to\n2\n163). Thus, the hydrogen bond network gradually transitions from a H SO -dominated\n2 4\nstructure to a H O-dominated structure.\n2\nrises from 0% to 75%, indicating that the network becomes more extensively hydrogen\nbridged rather than being disrupted. However, this network is chemically less favorable\nfor nitroaromatic solvation. Because solvation of nitroaromatics depends specifically\non nitro-H SO hydrogen bonds (as shown in Fig. S8). As water increases, H SO\n2 4 2 4\nmolecular participates less in the hydrogen bond network, so the energy of breaking the\nexisting hydrogen bond network and solvating nitroaromatics becomes higher.\nTherefore, nitroaromatic molecules are thermodynamically disfavored from dissolving\ninto the aqueous phase. This microscopic insight is fully consistent with the\nmacroscopic solubility trends we observed experimentally. This is also a common rule\nin aromatic nitration systems.\nTable S3. Evolution of hydrogen bond network in H SO -H O mixtures at different H O molar fractions.\n2 4 2 2\nH O molar Number of O-H bonds Number of O-H bonds Total O-H···O Hydrogen\n2\nfraction (%) in H SO in H O Bonds\n2 4 2\n0 200 0 53\n25 143 57 58\n50 82 118 73\n75 37 163 76\n100 0 200 74\nWe further calculated the concentration of nitronium ion (NO +) at different\n2\ntemperature and H SO concentration (Fig. S9). Since NO + can only exist in the\n2 4 2\naqueous phase, aromatic nitration must occur either within the acid phase or at the phase\ninterface. The calculations show that the generation of water significantly decreases the\nequilibrium concentration of NO +. In other words, in-situ water locally dilutes H SO\n2 2 4\nat the interface, reducing the activity of NO +.\nacid phase while also kinetically suppressing the rate of over-nitration. As a result, the\ninterfacial region becomes a zone of simultaneous thermodynamic and kinetic\nsuppression of over-nitration.\nIn summary, these simulations demonstrate that the generated H O during the main\n2\nreaction simultaneously weakens interfacial solvation of nitroaromatics, preventing\ntheir diffusion into the acid bulk, and suppresses the activity of NO +, decreasing the\n2\ndriving force for over-nitration. These provide a direct molecular support for the\nproposed product inhibition mechanism. The discussion has been added into the\nSupplementary Information and revised manuscript.\nFig. S9. Effect of temperature and H SO concentration on NO + concentration.\n2 4 2\n6. The reaction medium has strong oxidizing and corrosive properties, which poses a\nsevere test to the material of the equipment. Please clearly identify the material and\nstructure of the reactor, and use ICP-MS or other detection methods to check if any\nsubstances from the reactor material have dissolved.\nThank you very much for your important comment. In this work, to fully utilize\nthe product inhibition mechanism, the mixed acid was dispersed as droplets while the\norganic phase served as the continuous phase. Therefore, both the microdisperser and\nthe microreactor were fabricated from oleophilic and acid-resistant fluoropolymers.\nThe micro-disperser was made of chlorotrifluoroethylene (CTFE), the microreactor of\npolytetrafluoroethylene (PTFE), and the phase separator of glass. All of the materials\nexhibit excellent resistance to the mixed acid of HNO and H SO . In our experimental\n3 2 4\nsetup, the only metallic component in direct contact with the mixed acid is the 316L\nstainless-steel pump head of the metering pump. All other parts are metal-free.\nTo assess whether any corrosion or dissolution occurred under operating\nconditions, we analyzed the aqueous phase composition at different process locations\nusing ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy). We\nmonitored characteristic elements associated with stainless-steel corrosion, including\nFe, Cr, Ni, Cu, and Al. As shown in Table S3, the analysis indicates trace levels of metal\nions derived from the minor leaching from the 316L stainless-steel pump head. More\nimportantly, no signals indicating corrosion and degradation of the micro-disperser,\nmicroreactor or phase separator were observed.\nThese results demonstrate that the reaction system remain stable under the strongly\noxidizing and corrosive conditions of aromatic nitration. The detailed material\nspecifications and ICP–OES analysis have been added to the Methods and\nSupplementary Information sections in the revised manuscript.\nTable S2. Concentration of Metal Elements in Different Samples\nSample Fe (mg/L) Cr (mg/L) Ni (mg/L) Cu (mg/L) Al (mg/L)\n1* 0.04 ± 0.01 0 0 0 0\n2* 0.43 ± 0.02 0.05 ± 0.01 0 2.15 ± 0.04 3.29 ± 0.05\n3* 0.50 ± 0.04 0.06 ± 0.02 0 1.82 ± 0.11 3.37 ± 0.03\n* Sample 1 is a mixed acid raw material. Sample 2 is a mixed acid raw material pumped to the reactor\ninlet. Sample 3 is the aqueous solution after the reaction.\n7. It is suggested to supplement a brief discussion on the amplification strategy of this\nmicro-reaction system, such as the issues of distribution uniformity and temperature\ncontrol faced by scale-up via numbering-up, in order to assess its industrial\ntransformation potential.\nThank you very much for your kind suggestion. The two-stage countercurrent\nmicroflow mode is characterized by “co-current intra-stage, and countercurrent inter-\nstage”. Therefore, each co-current microreactor can be independently scaled up via\nnumbering-up, following common microreactor scale-up strategies. In microreactor\nsystems, micro-dispersion, flow, and mass/heat transfer are strongly governed by\nchannel geometry and characteristic dimensions. To date, the micro-disperser,\nmicroreactor, phase separator, and material distributor can be easily developed into\nmodular units. By increasing the number of modules and operating them in parallel,\nuniform flow distribution and reproducible reaction performance can be reliably\nmaintained. In the aspect of thermal management, the high specific surface area of\nmicroreactors provides excellent heat transfer capability. More importantly, the two-\nstage countercurrent mode reduces the heat release rate within each stage microreactor,\nthereby lowering the thermal load and significantly simplifying temperature control\nduring scale-up. Therefore, even under parallel numbering-up, efficient heat removal\ncan be sustained. These features minimize the risk of localized hot spots that arise in\nconventional aromatic nitration processes. In summary, this modular design makes the\ntwo-stage countercurrent reaction mode highly scalable to industrial scale.\nA summary of these scale-up considerations has been added to the “Discussion”\nsection of the revised manuscript.\nReference\n1. Panke G, Schwalbe T, Stirner W, Taghavi-Moghadam S, Wille G. A Practical Approach of\nContinuous Processing to High Energetic Nitration Reactions in Microreactors. Synthesis 2003,\n2827-2830 (2003).\n2. Burns JR, Ramshaw C. A Microreactor for the Nitration of Benzene and Toluene. Chemical\nEngineering Communications 189, 1611-1628 (2002).\n3. Fu G, Ni L, Wei D, Jiang J, Chen Z, Pan Y. Scale-up and safety of toluene nitration in a meso-\nscale flow reactor. Process Safety and Environmental Protection 160, 385-396 (2022).\n4. Yang A, et al. Experimental investigation of mononitrotoluene preparation in a continuous-flow\nmicroreactor. Research on Chemical Intermediates 48, 4373-4390 (2022).\n5. Han B, Chen Y, Zou H, Yu G, Sheng C, Wang G. Study on characteristics of\ntoluene/chlorobenzene nitrification in different microreactors. Chemical Engineering Research\nand Design 205, 343-353 (2024)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have carefully considered and responded to each comment from the review and have vastly improved the quality of the manuscript. So, it can be accepted as it is.", "reviewer_3": "I have reviewed the revised manuscript titled “A Precise Microreaction Strategy to Overcome Trade-off Effect in Aromatic Nitration via Kinetic and Thermodynamic Regulation” and the authors’ detailed point-by-point response to the reviewers’ comments. The authors have made substantial and thoughtful revisions that have significantly strengthened the manuscript. All major and minor concerns raised during the initial review have been adequately addressed. Therefore, I am satisfied that the manuscript in its current form meets Nature Communications."}, "author_response": ""}]

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