Ultrafine Pd Nanoparticles Encapsulated in Mesoporous TiO2 Region Selectively Confined in Bamboo Microchannels: An Ultrastable Continuous‐Flow Catalytic Hydrogenation Microreactor

Plant‐based flow microreactors with natural channel structures, renewable properties, and environmental friendliness have increasingly gained popularity in heterogeneous catalysis. However, firmly immobilizing the catalysts simultaneously with ease and adaptability, maintaining great effectivity and long‐term stability, is still a fundamental challenge. Herein, a highly efficient and ultrastable bamboo‐based catalytic microreactor (CMR) containing mesoporous TiO2 (M‐TiO2)‐encapsulating ultrafine Pd nanoparticles (NPs) is constructed for the continuous‐flow hydrogenation of nitroaromatics. The fabrication of the Pd‐TiO2 catalysts in required bamboo microchannels (Pd‐TiO2/B CMR) mainly involves a two‐step region‐selective synthetic strategy with ultra‐low chemical usage, fast preparation, and low catalyst loading (0.007 wt%). The M‐TiO2 films: 1) provide abundant oxygen vacancies and enough open cavities to facilitate the growth of Pd NPs; 2) improve Pd dispersion and reduce particle size; 3) allow diffusion of reactants, and 4) induce strong metal‐support interactions for enhanced catalytic activity and stability. The optimized Pd‐TiO2/B CMR demonstrates high efficiency (>97%) and excellent stability (1,000 h) for the continuous‐flow hydrogenation of nitroaniline, even under intermittent operation (12 h on/12 h off for five cycles) or in a real aqueous matrix (>200 h), making it a promising candidate for Pd‐catalyzed hydrogenation.


Introduction
Microreactor technology combined with ongoing flow amalgamation is an appealing substitute to conventional group systems for heterogeneous catalysis owing to several unique benefits.3] One of the major challenges in developing more efficient chemical manufacturing processes is the design of porous microchannels for flow-catalysis reactors.Numerous studies have been conducted on developing a variety of flow reactors with microscale or nanoscale pores based on alumina membranes, [4] ceramic monoliths, [5] cellulose sponges, [6] silica monoliths, [7] carbon monoliths, [8,9] quartz capillaries, [10] and synthetic polymers. [11]he design of several plant-based catalytic microreactors has recently been reported, including Pd/Au/Ag/ Pd-Ag-MOF@wood, [12][13][14][15] Ag@rattan, [16] Au/Ag/MOFs@ sugarcane, [17][18][19] Ag/Cu@bamboo, [20,21] and enzyme@wood/bamboo. [22,23]Certain bamboo species are excellent sustainable resources for socioeconomic development because they grow quickly, mature sooner within 4-7 years compared to other trees and possess a woody structure. [24]Bamboo material also has excellent mechanical properties due to its fiber-reinforced cellular structure. [25,26]Furthermore, the microchannels inside the vascular bundles of bamboo have a very large specific interfacial area per unit volume, which is suitable for microreactors. [21]Seven of the United Nations' 17 goals for sustainable development have a direct connection to rattan and bamboo. [24]etal NPs' exceptional catalytic reactivity results from their extraordinary surface energy and enormous surface area, [27][28][29] making them very popular in fabricating plant-based microreactors for continuous-flow heterogeneous catalysis.5]21,30] This results in the metal catalysts being loaded throughout the sample rather than in the desired channel for liquid flow and forming a metal catalyst loading gradient from the surface to the interior of the monolithic support.The "dead catalysts" in the support reduce the catalytic efficiency of the microreactor.Moreover, metal NPs are unstable and are easily deactivated by migration-coalescence during long-term catalysis because of their high surface energy, thus reducing their catalytic performance. [31][33] Mesostructured metal oxides (TiO 2 , CeO 2 , SiO 2 , etc.) are among the most important catalyst supports.They have attracted great interest because of their large specific surface areas, host-guest interactions, and uniform channel interconnectivity for gas diffusion. [31]Among mesoporous metal oxides, M-TiO 2 has the broadest applicability; it can provide a high surface area and ordered pore structures that allow regular arrays of the metal NPs for enhanced catalytic stability. [34,35]Moreover, metal NPs are in close contact with M-TiO 2 , which may induce strong metal-support interactions for enhanced catalytic activity and stability. [34]We recently demonstrated that the F-modified mesoporous TiO 2 films can be firmly deposited on the bamboo surface by a low-temperature hydrothermal method. [36,37][40] Until now, there have been no reports that plant-based catalytic reactors can achieve ultrastable catalytic activity under continuous flow.
Here, a novel region-selective synthetic strategy was formulated to create a highly effective and ultrastable bamboo-based catalytic microreactor based on M-TiO 2 encapsulated ultrafine Pd NPs for the continuous-flow hydrogenation of nitroaromatics.The approach is a two-step process that includes forming a continuous M-TiO 2 coating on the internal surface, as shown in Figure 1a of the microchannel inside the vascular bundles of bamboo, followed by immobilization of active Pd NPs on the M-TiO 2 film.The outcome of Pd-TiO 2 /B CMR was completely categorized through a variety of methods.This supported the consistent placement of the M-TiO 2 film on the desired microchannel surface and the presence in the film of Pd NPs of small and narrowly distributed size.The Pd-TiO 2 /B CMR was tested for the continuous-flow catalytic hydrogenation of nitroaromatics, given their wide use in the chemical industry and frequent discharge in wastewater, resulting in severe environmental risks and hazards to human health. [41]The catalytic activity was evaluated during continuous enduring operation for about 1,000 h.The only process variables, Pd content, flow rate, initial concentration of nitroaromatics, and operation timing, were meticulously assessed.Finally, genuine aqueous matrices (lake and river water) were used to verify the system's adaptability.

Catalyst Characterization
As shown in Figure 1a, the fabrication of Pd-TiO 2 catalysts in the long microchannels inside the vascular bundles of bamboo mainly involves a two-step synthetic strategy, including the region-selective deposition of M-TiO 2 films on the inner walls of the bamboo microchannels, and followed by in situ encapsulation and reduction of Pd NPs into M-TiO 2 particles.Conventional strategies for synthesizing plant-based monolithic microreactors typically involve long-term hydrothermal modification, extensive chemical usage, or energy-consuming equipment.Here, the precursor solution flowed directly into the desired microchannels by the region-selective deposition and formed a catalytic Pd-TiO 2 heterostructure through a two-step hydrothermal reaction with low chemical usage and a fast preparation process.At the same time, the loading of the catalyst could be effectively reduced (Table S1, Supporting Information).Meanwhile, Pd NPs are fixed strongly within the TiO 2 mesostructures.The individual Pd NP is isolated and separated by the TiO 2 support, which can efficiently stop the metal transformation process during the long-term catalytic process, achieving an ultrastable continuous-flow catalytic hydrogenation microreactor.
Images of the Pd-TiO 2 /B CMR following various catalyst production stages are shown in Figure 1b.As seen, the end surface of the CMR's color changed each preparation stage subsequently, going from a light yellow to a straw hue and then to a dark brown.The latter color indicates a Pd NP-coated surface that has decreased Pd(0) species. [4]The optical microscopy image (Figure S1, Supporting Information) and Pd-TiO 2 /B CMR tangential-section image (Figure 1b2) showed that Pd NPs were primarily deposited on the internal walls of the lengthy microchannels found inside the bamboo vascular bundles.Numerous arterial bundles were found in the transaction, as depicted in Figure S2a, Supporting Information, and they were embedded within a matrix of ground parenchyma cells.Two metaxylem vessels, fibers, and metaphloem of sieve tubes with companion cells made up the vascular bundle system (Figure 1c).After Pd-TiO 2 catalyst modification, catalyst coatings were observed on the inner wall of the bamboo microchannels (Figure 1d). Figure S3, Supporting Information, shows the relative intensity of each element measured along the cross-section (yellow line) of the microchannel.The signals of titanium (sky blue) and palladium (pink) implied that the Ti and Pd elements were distributed across the wall layer of the microchannel, which confirmed that the Pd-TiO 2 catalyst was mainly confined to the inner surface of the bamboo microchannels.
Periodically, perforation plates and the lengthy protoxylem vascular channel's inner surface were covered in various pits.(Figure 1e,f and S2b, Supporting Information).The original surface of the channel was smooth and clean.M-TiO 2 films self-gathered with nano-sized particles of TiO 2 were evenly placed on the inner walls of the bamboo microchannel at 90 °C for 1 h of hydrothermal reaction (Figure 1g,h).Because very little liquid was injected into the microchannel and the reaction time was short, TiO 2 NPs were primarily intense vascular bundle walls.When an H 2 PdCl 4 solution was impregnated into mesoporous TiO 2 , an anionic metal complex, along with its high electrostatic interaction and TiO 2 substrate bearing positive charge, was the catalyst that caused metal ions to adhere to mesoporous backings. [42]After hydrothermal reduction, the Pd NPs were encapsulated in M-TiO 2 .The smooth M-TiO 2 films were etched into a rough surface of M-TiO 2 particles (Figure 1i,j, and S3c, Supporting Information).The scenario has increased the likelihood of the reactant contacting the Pd-TiO 2 catalyst, thereby improving the catalytic efficiency of the Pd-TiO 2 /B CMR under continuous flow. [43]he size of the Pd NPs, along with the morphology of the TiO 2 backing, was further examined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and element-mapping measurements.As shown in Figure 2a-c, it could be found that the Pd NPs with good crystallinity were monodispersed inside the amorphous TiO 2 matrix, and the mean size was 1.48 nm.The inset HRTEM image clearly shows distinct lattice fringes of the Pd NPs, and it has an interplanar gap of 0.22 nm, which is consistent with the (111) plane. [44]The bright regions in the HAADF image are sequestered Pd atoms, as shown by the accompanying STEM data (Figure 2d).Considering the significant mesoporous structures, as well as the incipient wetness impregnation employed in this study for introducing metal precursors by electrostatic interactions, we believe that most of the Pd NPs were encapsulated within the mesoporous TiO 2 framework.A small number of Pd NPs (approximately 10 nm in diameter) were present on the surface of the TiO 2 framework.This phenomenon is consistent with a previous report that states simply loading metal NPs on the support surface allows secondary growth because of the lack of confinement and strong Brownian motion. [45]Similarly, the Pd NPs' average size was 36.83 nm without the confining TiO 2 framework (Figure S4, Supporting Information), even though on the inside walls, the Pd NPs were uniformly distributed in the bamboo microchannels.Furthermore, the EDX elemental mapping images shown in Figure 2e-h confirm the even dissemination of Pd NPs within the M-TiO 2 framework without agglomeration.The Pd NPs were in adjacent interaction with the mesoporous TiO 2 , which could induce strong metal-support interactions for enhanced catalytic activity and stability.
Figure 3a depicts XRD patterns of OB CMR, TiO 2 /B CMR, and Pd-TiO 2 /B CMR.All CMR samples exhibited similar diffraction peaks at approximately 16°, 22°, and 35°, which can be attributed to the crystalline cellulose in the bamboo.The TiO 2 /B CMR and Pd-TiO 2 /B CMR exhibited a typical anatase TiO 2 phase pattern (JCPDS No. 71-1167).The peaks were extremely weak, indicating that the TiO 2 existed mainly in an amorphous form, supported by the HRTEM analysis.After Pd deposition, TiO 2 maintains its original microstructure and amorphous nature, with only slightly enhancing the diffraction peaks.Due to the low amount of Pd clusters 0.007 wt%, as determined by ICP-MS, no peaks related to Pd species were seen in the XRD pattern of Pd-TiO 2 /B CMR.Past research has found that the amorphous TiO 2 matrices generally have large surface areas, large pore structures, and abundant intrinsic oxygen vacancies (Vos), which can act as the anchoring sites for metal NPs. [46,47]Figure 3b shows a type-IV isotherm with an H 3 hysteresis loop, pointing to the TiO 2 film's typical mesoporous structure with a BET surface area of 178.1 m 2 g À1 .The many mesopores, which had pores with an average size of 3.4 nm, additionally provided space for Pd NP growth.However, the confinement of the mesopores can limit their overgrowth.For efficient reactant flow into the catalyst support mesopores, the metal NPs must not block the pore openings. [45]In this research, Pd NPs with an average size of 1.48 nm were readily incorporated into a mesoporous TiO 2 framework.The remaining space in the pores facilitated the fast mass transfer, rendering the metal NPs highly accessible to the reactants.
The catalytic performance is notably dependent on the catalyst's chemical state.To learn more about the surface chemical states of the Pd, Ti, O, and F species of the produced catalysts, XPS analysis (Figure S5, Supporting Information) was carried out.Compared with the OB CMR, characteristic Ti 2p and F 1s peaks appeared in the TiO 2 /B CMR, which demonstrated successful coating of the M-TiO 2 film. [36,37]Furthermore, a strong Pd 3d peak appeared for the Pd-TiO 2 /B CMR, indicating that the Pd NPs were immobilized in the films by TiO 2 .Figure 3c shows the O1s XPS spectral measurements of the CMRs.compared to that on the surface of the OB CMR, indicating that the Pd-TiO 2 films were densely deposited on the bamboo substrate.Figure 3d shows the XPS spectral analysis of TiO 2 /B CMR and Pd-TiO 2 /B CMR in the Ti 2p area.For TiO 2 /B CMR, Ti 2p 3/2 and 2p 1/2 can be attributed to the peculiar two peaks at 458.9 and 464.8 eV.After the introduction of Pd into TiO 2 , the peak at 464.6 eV shifted negatively, this may be explained by an electron transfer from Pd to Ti and the formation of a strong metal-support interaction, stabilizing the isolated Pd sites. [48]It was determined that the isolated surface terminal Ti-F groups on the TiO 2 crystal surface were responsible for the peak at 684.4 eV in the high-resolution F 1s XPS spectra of TiO 2 /B CMR and Pd-TiO 2 /B CMR, as shown in Figure 3e. [39]F À on the surface of TiO 2 was beneficial to the formation of surface Vos.According to Figure 3f, the Pd species existed in the metallic state since the Pd 3d 5/2, and Pd 3d 3/2 peaks of Pd-TiO 2 occurred at 335 and 340.2 eV, respectively.Two peaks from Pd 2p were also visible in the Pd 3d XPS spectra, which could have resulted from the surface oxidation of Pd metal or oxygen sharing between Pd 2p and Ti 2p. [49,50]The findings of the EPR used to identify Vo in the produced CMRs are displayed in Figure 3g.The g-value of paramagnetic Ti 3þ is reportedly between 1.94 and 1.99, and Vo at g = 2.004. [51]TiO 2 /B CMR and Pd-TiO 2 /B CMR exhibited an intense signal at g = 2.004, arising from unpaired electrons trapped by Vo.Notably, a downward EPR peak was observed for the Pd-TiO 2 /B CMR compared to TiO 2 /B CMR, which can be attributed to an alteration in the coordination field of the Pd NPs, demonstrating the presence of interactions among the Pd NPs and the TiO 2 support. [52]In addition, oxygen defects generally increase the electrical conductivity of TiO 2 , which may facilitate electron transfer from the Pd NPs to the support. [53]2.Catalytic Performance Evaluation

Operating Conditions Study
The hydrogenation of 4-NA is an important reaction in the chemical industry, as the product, 4-phenylenediamine (4-PD), can be reused to synthesize drugs and dyes. [54]The reduction of 4-NA to 4-PD about NaBH 4 was selected to assess the catalytic effectiveness of the Pd-TiO 2 /B CMR.The reaction can be seen directly by the hue of the solution and can be easily analyzed using UV-visible spectroscopy.In this experiment, 0.5 mM 4-NA and freshly made 0.25 M NaBH 4 were combined and passed via means of preparing at microreactor 0.15 mL min À1 .For comparison, TiO 2 /B CMR and Pd/B CMR were investigated in parallel.According to Figure S6, Supporting Information, the peak of absorption of 4-NA decreased marginally, different back to the initial peak of the preliminary 4-NA at 380 nm.No product absorption peak 4-PD was observed at 300 nm, showing that the mesoporous TiO 2 film confined in the bamboo microchannel acted as a catalyst support.The modest reduction in the absorption peak of 4-NA is due to the improved mass transfer in the bamboo with a microchannel of the M-TiO 2 coating that can be seen as typical conduct. [10]As expected, for the Pd-TiO 2 /B CMR encompassing 0.007 wt% Pd in identical operating situations, only at 300 nm could the 4-AP absorption peak be seen (Figure 4a).It was determined that 4-NA has a conversion effectiveness of up to 97.36%.The current situation demonstrated the role of Pd NPs as catalysts in the M-TiO 2 film and their extraordinarily high catalytic action in this reduction reaction.The catalytic efficiency remained as high as 97.75% even after 30 h of continuous running (Figure 4a).Under identical working circumstances, the 4-NA absorption maxima (380 nm) vanished at the start of the experiment for the Pd/B CMR comprising 0.005 wt% Pd. (Figure S7, Supporting Information).However, this absorption peak gradually reappeared during the experiment.The conversion efficiency of 4-NA by Pd/B CMR was 90.50% at 32 h.The concentration of PdCl 2 for the preparation of Pd/B CMR was the same as that of PdCl 2 for the preparation of Pd-TiO 2 /B CMR.However, the Pd content in the Pd-TiO 2 /B CMR was higher than that in the Pd/B CMR, indicating that the M-TiO 2 film, owing to its rich porous structure and higher specific surface area, was helpful in loading of Pd NPs.Moreover, when the concentration of PdCl 2 was reduced by half, the Pd content in the 1.5Pd-TiO 2 /B CMR decreased to 0.005 wt%.As shown in Figure 4b, the peak of absorption gradually reappeared at 380 nm during 30 h of continuous operation, and the catalytic efficiency was only 78.86% at 30 h.In conclusion, when enough Pd catalyst was included in the bamboo microchannels, the suggested CMR demonstrated high catalytic activity in the reduction reaction.
The prepared CMR was initially operated at steady-state under continuous flow.Subsequently, the initial 4-NA concentration and the flow rate, which have an important factor in the conversion efficiency, were varied to evaluate the performance of the CMR.Increasing the reactant concentration and the flow rate can yield more product.Therefore, we doubled the initial concentration of 4-NA and the flow rate of the reaction solution through the microreactor.Unfortunately, under both conditions, the adsorption peak reappeared at 380 nm and increased with the reaction time (Figure 4c,d).At the higher concentration of 4-NA, the catalytic efficiency was only 78.66% at 36 h and 90.60% at 30 h with the higher flow rate.Because the flow rate was indirectly proportional to the residence duration, it is not surprising that the efficiency was higher at a lower flow rate.

Long-Term Stability of Pd-TiO 2 /B CMR
Continuous-Flow Conditions: Stability over time is crucial for the industrial application of CMR.Durability tests of nitroaniline (1,000 h for 5-NA and 500 h for 2-NA) hydrogenation were performed at a rate of flow of 0.15 mL per minute and inlet nitroaniline concentration of 0.5 mM.Samples were collected at the outlet every 24 h.Surprisingly, the conversion efficiencies for both 4-NA and 2-NA remained above 96% during the entire test period, as shown in Figure 5a and S8a,b, Supporting Information, demonstrate that the Pd-TiO 2 /B CMR is stable for while being very effective hydrogenation of nitroaniline.It can also be seen from Figure 5a (inset) that the color of the reaction solutions of both 4-NA and 2-NA changed from yellow to colorless.Our method outperformed the stability among several common celluloses-, [6] metal-, [55] carbon-, [9] and polymer-based [56] catalysts, as well as strong glass [1] and ceramic reactors. [4]he excellent long-term catalytic stability of this flow reaction system may be attributed to: 1) bamboo having unique hierarchically permeable channel assembly (Figure S9, Supporting Information), which enables it to quickly diffuse into the small channel space and come into contact with the catalyst [21,57] ; 2) the Pd-TiO 2 catalysts forming on the bamboo microchannel surfaces; 3) the TiO 2 mesopores providing space for the distributed and limited growth of Pd NPs; 4) the high porosity facilitating the fast mass transfer, rendering the Pd NPs highly accessible to the reactants [31] ; 5) the strong metal-support interactions enhancing catalytic activity and stability, according to the very low Pd leaching observed in the CMR effluents (Figure 5b);  We recovered the specimens and found that the Pd-TiO 2 /B CMR was not mildewed, even after 1,000 h of continuous operation.This result is satisfactory for practical applications because bamboo is easily susceptible to fungi.However, after drying, the recovered Pd-TiO 2 /B CMR specimens exhibited dry shrinkage, up to a 19.32% reduction in diameter after 1,000 h (Figure S11, Supporting Information).This result indicates that during the long period of flow catalysis, some components of the bamboo matrix, such as lignin and hemicellulose, may be lost to the reaction liquid.It should be noted that the product 4-PD can be isolated by extraction in ethyl acetate, then drying over anhydrous sodium sulfate, and evaporation of the solvent under vacuum followed by column chromatography using n-hexane/ethyl acetate as an eluent.No such loss has been previously reported; however, no other plant-based catalytic reactors have achieved high-efficiency catalytic activity under long-term continuousflow conditions (Table S1, Supporting Information).60][61][62] Intermittent Operating Conditions: Intermittent operation is also important for catalyst lifetime and stability. [63]Catalytic hydrogenation was studied under intermittent operation conditions; 12 h on, followed by four cycles of 12 h off and 12 h on.A recently mixed mixture of 0.5 mM 4-NA and 0.25 M NaBH 4 passed through the Pd-TiO 2 /B CMR when the rate of flow was 0.15 mL min À1 .As shown in Figure S8c, Supporting Information, the adsorption band at 380 nm (due to 4-NA) reduced significantly after flowing the 4-NA/NaBH 4 water sample through the Pd-TiO 2 /B CMR, while a novel adsorption peak appeared at 300 nm (due to 4-PD).The conversion efficiency was up to 98.22% (Figure 5c), remained high (>97.28%)during the first 12 h of continuous operation, and was always above 95.79%throughout the remaining cycles.Note that the microreactor was not emptied during the off periods.In other words, the Pd-TiO 2 / B CMR stably catalyzes the hydrogenation of 4-NA under intermittent operating conditions.Such tests are rarely used for studying the stability of plant-based catalytic reactors.

Catalytic Process on Other Nitroaromatics
The catalytic conversions of other nitroaromatics with various substituents (2-NP and 4-NP) were carried out at the flow rate of 0.15 mL min À1 to explore the range and generality of the developed Pd-TiO 2 /B CMR (Figure 6).The Pd-TiO 2 /B CMR was flowing with a freshly made solution containing 0.5 mM nitroaromatic and 0.25 M NaBH 4 .The conversion efficiency of 2-NP in the initial step was 97.93% (Figure 6a,b).The catalytic efficiency remained high (>91.97%)during the first 12 h of continuous operation but dropped to only 41.19% after 24 h.The initial conversion efficiency of 4-NP was as high as 99.35% (Figure 6c,d) but dropped to 80.82% after 6 h and was only 33.25% after 24 h.During 24 h of continuous hydrogenation, the Pd-TiO 2 /B system suffered 56.7% deactivation to facilitate catalytic reduction of 2-NP and 66.1% deactivation to reduce 4-NP via catalysis.

Deactivation Mechanism
Previous research has suggested that charge-charge repulsion is one of the primary factors contributing to the great resistance to poisoning.Variables include the Pd NPs' morphological alterations and the catalytic reaction's surface passivation by other chemical species that might be involved in the deactivation of Pd-TiO 2 /B catalysts. [64]Further studies were carried out on the catalytic reduction of 4-NA with a Pd-TiO 2 /B CMR, previously consumed for the decrease of 4-NP and after undergoing a simple wash-dry process. [21]After 24 h of continuous operation, the catalytic efficiency was as high as 92.32%, undergoing only 6.9% deactivation for reducing catalytically 4-NA, indicating that the prior deactivation was reversible.It can also be directly proven that the morphological changes and surface passivation are not the main causes of catalyst poisoning, demonstrating a higher resistance for the Pd NPs encapsulated in M-TiO 2 against poisoning by the adsorption of 4-PD.This can be attributed to the fact that the Pd-TiO 2 nanocomposite possessed many negative charges on its surface (such as F À , supported by XPS), enabling it to strongly adsorb the positively charged 4-AP (as a result of its amino group being protonated).This accumulation fouls the active sites on the catalyst surface (Figure 6e). [65]4-PD is not protonated in the basic solution (pK a s of the conjugate acids are 3.3 and 6.1). [66]Due to the 4-PD amino groups' significantly lower absorbability than the 4-aminophenol amino groups, the asobtained 4-PD could desorb readily from the Pd-TiO 2 nanocomposite surfaces, and the reaction proceeded.Moreover, the deactivation of the Pd-TiO 2 /B CMR for the catalytic reduction of 4-NA was more severe than that of the Pd-TiO 2 /B CMR for the catalytic reduction of 2-NA, which may be attributed to the pK a values of 4-aminophenol (10.30) being higher than that of 2-aminophenol (9.71). [67]

Operation in an Environmental Water Matrix
The Pd-TiO 2 /B CMR is because the long-term catalytic process is an efficient and stable conversion of nitroaniline for more than 500 h in deionized water.The catalytic process and stability of the Pd-TiO 2 /B CMR were tested in Fengjia River and Qiandao Lake waters.The Fengjia River is incredibly murky (Figure 7a and S12, Supporting Information), with the total organic and inorganic carbon content at 6.2 and 13.5 mg L À1 , respectively.The Fengjia River water sample's UV-vis spectrum shows that 4-NA is not present.The highest absorption of 4-NA in river water happened at 380 nm after combining the water sample with 4-NA and NaBH 4 , indicating the stability of 4-NA in the murky river water sample.After passing the water sample through the Pd-TiO 2 /B CMR, a new absorption maximum was noticed at 300 nm, demonstrating the production of 4-PD.Even after 6 hours of constant operation, the conversion efficiency was up to 99.26% and maintained high (>90.04%)(Figure 7c).However, the effectiveness fell to 44.97% after 24 h.In contrast, as mentioned above, CMR used pure water from Qiandao Lake to produce the catalytic efficiency for the Pd-TiO 2 /B stable conversion.97.6% was achieved during 216 h of continuous operation (Figure 7b,c), which is nearly identical to the result obtained with deionized water.
The current study has far exceeded the catalytic performance of our previously reported Ag/B microreactor in clear Qiandao Lake water. [21]The presence of multiple cations in the water (Table S2, Supporting Information) did not play a significant part in the catalytic performance of the microreactor.Silt and organic matter adsorption led to the Pd-TiO 2 /B CMR's partial deactivation by the river water, and sediments were found at the bottom of the container (Figure S12, Supporting Information).This discovery is particularly significant since silt and organic materials in the real water matrix may cause catalyst fouling.Similar to how there was no noticeable decline in activity brought on by competing effects when inorganic salts were present.Before catalytic hydrogenation, microorganisms, organic debris, and suspended particulates would be mostly eliminated, but the actual water matrix was far from deionized water.In brief, the Pd-TiO 2 /B CMR can be employed as a promising catalyst in real-world applications due to its high efficiency and stability.
The bamboo-based CMR in our study was a miniature conventional flow reactor.The simplicity with which reaction conditions can be transferred between reactors and production sites without necessitating reoptimization makes the microreactor particularly appealing as a continuous-flow system. [68]Through the use of several reactors operating in parallel, this characteristic enables reactions that have been perfected in a lab to be scaled up.Through the use of several reactors operating in parallel, this characteristic enables reactions that have been perfected in a lab to be scaled up.As a proof-of-concept, as shown in Figure 7d, numerous bamboo tubes can fabricate large-scale catalytic reactors, which can be easily expanded.Thus, bamboo-based CMRs with nano-sized catalysts could speed the commercialization of catalyzed hydrogenation of organic compounds with high scalability, sustainability, real-time detection, and quick and continuous flow processing.

Conclusions
A highly efficient and ultrastable bamboo-based CMR containing M-TiO 2 encapsulated ultrafine Pd NPs was carefully crafted for continual flow hydrogenation of nitroaromatics.This method involves two steps, including the development of an ongoing continuous M-TiO 2 internal surface film of the desired bamboo microchannels, followed by a halt of active Pd NPs within the film, which only requires a small amount of precursor solution, a relatively short preparation time, and low Pd loading (0.007 wt%).The abundant oxygen vacancies and open cavities (%3.4 nm) in the M-TiO 2 films not only facilitated the growth of Pd NPs, but also ensured better dispersibility of the NPs.Reduced size (%1.48 nm) but also allowed for the diffusion of reactants, enabling contact with Pd, and inducing strong metal-support interactions with Pd for enhanced catalytic activity and stability.Even after 1,000 h, the resulting Pd-TiO 2 /B CMR maintained a conversion of around 97% for the decline of 4-NA during the continuous-flow reaction, exhibiting excellent catalytic ability and steadiness.In addition, the microreactor was tested in real aqueous matrices and under intermittent operation.It demonstrated strong catalytic activity, making it a promising candidate for Pd-catalyzed hydrogenation reactions under continuous flow.
Preparation of Pd-TiO 2 /B CMR: The preparation of Pd-TiO 2 catalysts in long bamboo vascular bundles that have tiny channels inside of them involved a two-step synthetic strategy.The bamboo rods were encased in 3 M tape to stop the evaporation of the precursor solution based on our previous method. [21]The silicone tube was tightly joined to both ends of the bamboo stick to create a flow-assisted preparation mechanism.First, a fresh TiO 2 precursor remedy was created by completely mixing (NH 4 ) 2 TiF 6 (0.1 M) and H 3 BO 3 (0.3 M) in deionized water.With a peristaltic pump operating at a flow rate of 3.3 mL min À1 , 1.0 mL of TiO 2 precursor solution was pumped through a bamboo stick and then sealed at both ends of the silicone tube.The sample was heated at 90 °C for one hour before being rinsed in flow mode with deionized water till the pH of the washing solution touched 7. To obtain TiO 2 /B CMR, the cleaned bamboo was dried for 12 h at 45 °C.The second H 2 PdCl 4 precursor solution (3.0 g L À1 ) was prepared by PdCl 2 and HCl solution (20 mM).They were combined, and the mixture was heated at 60 °C for one hour to dissolve the PdCl 2 powder thoroughly.A peristaltic pump was used at a 3.3 mL min À1 rate of flow, and 1.0 mL of H 2 PdCl 4 precursor solution was pumped through a TiO 2 /B CMR and then sealed at both ends of the silicone tube.The sample was heated at 80 °C for 0.5 h before being flow-washed with deionized water until the washing solution was colorless.For obtaining the Pd-TiO 2 /B CMR, the washed bamboo was dried up for 12 h at 45 °C.
For comparison, a 1.5 g L À1 H 2 PdCl 4 precursor solution was used to prepare the microreactor, denoted as 1.5Pd-TiO 2 /B CMR.The other reaction conditions were identical to those described above.A Pd/B CMR was prepared using the same method as Pd-TiO 2 /B CMR but without the deposition of TiO 2 films.The Pd contents of Pd/B CMR, 1.5Pd-TiO 2 /B CMR, and Pd-TiO 2 /B CMR were 0.005, 0.005, and 0.007 wt%, respectively.
Characterization: A field emission scanning electron microscope (SEM, Hitachi SU8010, Tokyo, Japan) and an Olympus SZ61 stereomicroscope were used to analyze the surface morphology (Olympus, Tokyo, Japan).Energy dispersive X-ray spectroscopy was utilized to analyze the elemental distribution (EDS, Bruker Xflash 6130).The HRTEM, TEM, element mapping, and HAADF-STEM analysis were carried out on a field-emission transmission electron microscope (FE-TEM, FEI Talos F200S, USA).The samples were taken from the vascular bundle's Pd-TiO 2 catalystloaded microchannels, and then they were sonicated in ethanol for 0.5 h to get the dispersed Pd-TiO 2 catalysts.A Rigaku Ultima IV X-ray diffractometer with Cu Kα irradiation was used to acquire the block samples' X-ray diffraction (XRD) patterns.The Micromeritics ASAP 2460 surface analyzer (Micromeritics, Norcross, GA, USA) was used to acquire the N 2 adsorption-desorption isotherms, and the BET surface areas were estimated from the desorption branch of the isotherm.The BET surface areas were then measured from the N 2 adsorption-desorption isotherms.The XPS data were collected using a Thermo ESCALAB 250Xi spectrometer with an Al K X-ray source (Thermo Scientific, Waltham, MA, USA).The pore-size distribution curves were derived from the desorption branch of the isotherm in Norcross, GA, USA.A Thermo ESCALAB 250Xi spectrometer (Thermo Scientific, Waltham, MA, USA) with an Al K X-ray source was used to gather the XPS results.The Bruker EMX PLUS spectrometer (Germany) was used to record electron paramagnetic resonance (EPR) spectra at a resonance frequency of 9.30 GHz and a power of 6.325 mW.The central portion of the specimen's absolute dried CMRs was removed to fully dissolve them in aqua regia for inductively coupled plasma mass spectrometry analysis to determine the Pd loading in the bamboo (ICP-MS, Agilent 7800).ICP-MS also looked into the leaching of Pd NPs in the Pd-TiO 2 /B CMR.Mercury intrusion porosimetry was used to measure the pore size distribution by a MicroActive AutoPore V 9600 (Micromeritics, America).The compression strength was tested using a universal material testing machine (CMT 6103, Sans Testing Machine Inc., Shenzhen, China).
Catalytic Performance of Pd-TiO 2 /B CMR: The nitroaromatics are reduced (4-NP, 2-NP, 4-NA, and 2-NA) with NaBH 4 was utilized to assess the efficiency of the catalytic Pd-TiO 2 /B CMR.This reaction can be conducted in an environment with no byproducts.It can be monitored using UV-visible spectroscopy. [69]Furthermore, the resulting products are essential components of both bulk and fine chemicals, including agrochemicals, dyes, pigments, and medicines, e.g., polymers. [54]A recently ready mixture of 0.5 mM nitroaromatic and 0.25 M NaBH 4 was passed through the CMR at 0.15 mL min À1 , corresponding to a residence time of 0.28 min.The items were gathered from the CMR channel and examined using UV-vis spectroscopy (Shimadzu UV-2550, Kyoto, Japan) at 380 nm, 400 nm, 412 nm, 415 nm, and for 4-NA, 4-NP, 2-NA, and 2-NP, respectively.The effectiveness of conversion (%) of the nitroaromatics was determined using the following equation: Conversion efficiency (%) = [(A 0 À A)/A 0 ] Â 100 Where A and A 0 are the absorbances of the nitroaromatics, the effluent, and influent solutions, correspondingly.

Figure 1 .
Figure 1.a) Schematic illustration of the rational stepwise design of a Pd-TiO 2 /B CMR for continuous-flow catalytic hydrogenation.b) Images of Pd-TiO 2 /B CMR at different stages of preparation.Tangential sections of (b1) OB CMR and (b2) Pd-TiO 2 /B CMR, respectively.SEM images of the vascular bundles of c) OB CMR and d) Pd-TiO 2 /B CMR embedded within a matrix of ground parenchyma cells.The mean diameters of (1) metaxylem, (2) phloem, and (3) protoxylem are 144, 151, and 36 μm, respectively.SEM images of bamboo microchannels and the corresponding images at higher magnification: e,f ) OB CMR, g,h) TiO 2 /B CMR, and i,j) Pd-TiO 2 /B CMR.

Figure 2 .
Figure 2. a) TEM and b) HRTEM images of the Pd-TiO 2 /B CMR.(b) Shows the enlarged image for Pd NP. c) The particle size distribution of the Pd NPs within Pd-TiO 2 /B CMR.d-h) HAADF-STEM images and the corresponding EDS mapping of the Pd-TiO 2 catalyst.
3 peaks at 531.1, 532.9, and 535.1 eV, equivalent to C─O, C═O, and O─H, respectively, are present.The O 1s XPS spectra of TiO 2 /B CMR may be divided into three peaks, Ti-O-corresponding peaks, at 530.1, 531.2, and 532.5 eV, Vo, and C─O, respectively.After Pd introduction, four peaks at 528.8, 530.1, 531.2, and 532.2 eV were evident for Pd-TiO 2 /B CMR, corresponding to Pd─O and Ti, Vo, and C─O.The C─O content on the surfaces of TiO 2 /B CMR and Pd-TiO 2 /B CMR gradually decreased

Figure 5 .
Figure 5. a) Long-term stability of Pd-TiO 2 /B CMR for the catalytic hydrogenation of nitroaniline in continuous-flow conditions.The insets in (c) are the corresponding experimental photographs.b) Leached Pd as a function of reaction time during the catalytic hydrogenation of 4-NA in continuous flow concentration of Pd in the outlet solution was collected every 50 h.c) Long-term stability of Pd-TiO 2 /B CMR for the catalytic hydrogenation of 4-NA under intermittent operation conditions.

and 6 )
the superior mechanical qualities of the bamboo substrate enabling its applicability in long-term flow catalytic processes (FigureS10, Supporting Information).

Figure 6 .
Figure 6.a) UV-vis spectra and b) corresponding conversion efficiency of the 2-NP/NaBH 4 solution after treatment with the Pd-TiO 2 /B CMR.c) UV-vis spectra and d) corresponding conversion efficiency of the 4-NP/NaBH 4 solution after treatment with the Pd-TiO 2 /B CMR and the 4-NA//NaBH 4 solution after treatment with the Pd-TiO 2 /B CMR that have used for the reduction of the 4-NP/NaBH 4 solution.e) Possible mechanism of Pd-TiO 2 /B catalyst poisoning.

Figure 7 .
Figure 7. a,b) UV-vis spectra and c) corresponding conversion efficiency of the 4-NA/NaBH 4 solution after treatment with the Pd-TiO 2 /B CMR using Fengjia River and Qiandao Lake as sample matrices.d) Multiple bamboo-based CMRs operated in parallel to form a large catalytic reactor.