Porous PdO-Flower Induced by Nanomicrostructure on Monolith with

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Kinetics, Catalysis, and Reaction Engineering

Porous PdO-flower Induced by Nano-microstructure on Monolith with Traditional Immersion-pyrolysis Technique for Hydrogenation Fan Yang, Di-Song Wang, Ya-Zhao Liu, Guang-Wen Chu, Yong Luo, and Jian-Feng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01876 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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Industrial & Engineering Chemistry Research

Porous PdO-flower Induced by Nano-microstructure on Monolith with Traditional Immersion-pyrolysis Technique for Hydrogenation

Fan Yang a,b,c, Di-Song Wang b,c, Ya-Zhao Liu b,c, Guang-Wen Chu a,b,c, Yong Luo b,c*, Jian-Feng Chen a,b,c*

aBeijing

Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, P.R. China bState

Key Laboratory of Organic-Inorganic Composites and Technology, and

cResearch

Center of the Ministry of Education for High Gravity Engineering

Technology, Beijing University of Chemical Technology, Beijing 100029, P.R. China

* Corresponding author Tel: +86 10 64446466; Fax: +86 10 64434784 E-mail address: [email protected]; [email protected]

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Abstract: Monolith catalysts fabricated with immersion-pyrolysis technique suffer the uncontrollable morphology of active component, leading to low conversion and reaction rate. Herein, we propose a strategy by introducing Co3O4 nano-microstructure on Ni foam to induce the construction of porous flower-like PdO morphology. Investigations revealed the flower structure has originally formed during the solvent evaporation after immersing Co3O4/Ni in PdCl2 solution. The nano-microstructure also restrained the spontaneous replacement of palladium ions to nickel atoms. Benefitting from these advantages, 0.51% PdO/Co3O4/Ni declared high catalytic reaction rate up to 9.44 mmol gPd–1 s–1 for α-methylstyrene (AMS) hydrogenation in a stirring tank reactor (STR) at 40 oC and 0.5 MPa of H2, 29.5 times superior to that of 5.99% PdOy/Ni. In a mass-transfer-enhanced rotating packed bed reactor, the modified monolith catalyst also exhibited slightly boosted AMS conversion and reaction rate than that in STR, demonstrating the flexible applicability prospects for various reactors.

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1. Introduction Monolith catalysts receive intense interest due to the reduced mass transfer limitation, improved heat transfer efficiency, and simplified separation from reaction products1-3. The wash-coating methods with nano/micro-particles as a secondary support on monolith substrate are widely adopted to enlarge the surface area of monolith substrates for improving the dispersion and adhesion of the active component4-10. But several issues exist in monolithic catalyst in aforementioned method11. On the one hand, the noteworthy amount of material usage with nano/microparticles coating enforces great impact on the noble metal supply, increasing the catalytic converters cost. On the other hand, the wash-coating process sometimes does not ensure uniform catalyst deposition, compromising the material utilization efficiency. Thus, coating with nano-array on monolith is developed to solve the aforementioned issues12-19. One typical example is ZnO nanorod array in situ grown on cordierite monolith in which a large amount of uniform and small Pt-nanoparticles (NPs) were adsorbed by immersing in Pt colloid, showing outstanding CO oxidation performance12. Although immersion-pyrolysis technique is one of the simplest and most energy-saving process for monolith catalyst fabrication, this technique was rarely employed for the fabrication of nano-microstructure modified monolith catalyst. To the best of our knowledge, the tuned active component morphology with immersionpyrolysis is not explored on the modified monolith substrate, leading to the unclearness of the relationships among nano-microstructure, active component morphology, and catalytic performance. 3



Besides coating, for metal monolith the spontaneous replacement reaction which is governed by the inherent reactivity series of metals is another approach to load active component via immersing the monolith in corresponding solution containing the precursor of active metal20-25. For example, Ru-NPs and Pt-NPs deposited on Ni foam have been prepared based on the replacement reaction between RhCl63+ or PtCl62+ ions and Ni atoms of the Ni foam substrate24; Ag/Al foil were synthesized through galvanic replacement reaction25. In theory, many materials including nickel, CrFe Alloy, stainless steel, aluminum foam, and copper foam can be taken advantage of the replacement property to construct monolithic catalysts. However, the surface area of metal substrate is usually too low to well disperse the active component. Moreover, it is reasonable to deduce that the excessive replacement is easy to take place during the immersion operation. In this situation many active metal atoms would be imbedded into the metal substrate which have no chance to contact with the reactant, causing the waste of active metal and decrease of catalyst activity. After immersion operation and before the solvent completely evaporated, the noble metal ions in the residual solution of precursor on the metal substrate surface would still react with the atoms of the substrate, making the replacement reaction inevitable to occur. Therefore, to well disperse the active component and control the replacement reaction, modification on the metal substrate is needed before immersion-pyrolysis. Herein, we report x% PdO/Co3O4/Ni foam (x represents the weight percent of Pdelement) monolith catalysts prepared with a strategy by introducing nanomicrostructured Co3O4 array on Ni foam substrate to induce the PdO morphology after 4


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immersion-pyrolysis and simultaneously suppress the replacement between palladium ions and nickel atoms on Ni foam. Various characterizations such as scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and transmission electron microscopy (TEM) were employed to identify the evolution of monolith catalyst morphology and composition during immersion-pyrolysis process. The α-methylstyrene (AMS) hydrogenation is a model reaction which is usually used to evaluate the performance of a monolithic catalyst in a reactor26-29. Therefore, the AMS hydrogenation performances of these PdO/Co3O4/Ni foam were systematically investigated in a stirring tank reactor (STR) in the temperature range of 40 ~ 60 oC and H2 pressure range of 0.1 ~ 0.5 MPa. The flexible applicability of the monolithic catalyst was also inspected in a mass-transfer-enhanced rotating packed bed (RPB) reactor.

2. Experimental Section All chemicals were purchased form Sigma-Aldrich and used without purification. 2.1 Synthesis of Co3O4/Ni foam The Co3O4/Ni foam preparation was performed with monolithic Ni foam (45 mm × 20 mm × 1.3 mm, 110 PPI)30,31. Ni foam was tilted against the wall of the autoclave at a certain angle to ensure the uniform growth of Co3O4 on the substrate surface32. 2.2 Synthesis of the monolithic catalysts The immersion-pyrolysis process was employed to prepare monolith catalyst in this work. Typically, a clear PdCl2 solution (20 mL) with a concentration of 25 mmol L–1 was prepared with PdCl2 solute and dilute HNO3 (4 mol L–1). One Co3O4/Ni foam 5



was totally immersed in the aforementioned solution at room temperature for five hours. Subsequently, the PdCl2/Co3O4/Ni foam was obtained by drying Co3O4/Ni foam with soaked PdCl2 solution at 80 oC for another five hours. Finally, 0.51% PdO/Co3O4/Ni foam was obtained after calcining PdCl2/Co3O4/Ni foam in a tube furnace at air atmosphere and 450 oC for two hours with heating rate of 1 oC min–1. Other PdO/Co3O4/Ni foam catalysts were prepared by varying the concentration of PdCl2 solution (Table 1) and the rest of the experimental conditions were identical with the preparation of 0.51% PdO/Co3O4/Ni foam. The 5.99% PdOy/Ni foam was prepared by using bare Ni foam and the same immersion-pyrolysis process as 0.51% PdO/Co3O4/Ni foam. 2.3 Materials Characterization Powder XRD analyses were tested by using a Rigaku X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å). FEI Nova NanoSEM 450 SEM were used to observe the surface morphologies of all samples at an accelerating voltage of 2 kV. XPS using a PHI model 590 spectrometer with multi-probes was conducted to analyze palladium element of the catalysts and the binding energies (BEs) were calibrated using the signal from adventitious carbon (284.6 eV). A transmission electron microscopy TEM (JEM2010F, JEOL, Japan) was used to observe the size and morphology of catalyst. A Micrometrics ASAP 2020 Surface Characterization Analyzer was used to analyze the Brunauer-Emmett-Teller (BET) surface area. Inductively coupled plasma-optical emission spectrometer (ICP-OES) data were collected on a Thermo iCAP-6300. 2.4 PdCl2 solution uptake of Co3O4/Ni foam and Ni foam 6


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To assess the solution uptake of Co3O4/Ni foam and bare Ni foam the weights were recorded before and after immersing them in PdCl2 solution, the weights were recorded before and after immersing them in PdCl2 solution for 10 seconds separately. The weight increase (η%) is calculated by η% = (M2 – M1)/M1

(1)

where M2 is the weight after soaking, M1 is the weight before soaking. 2.5 Catalytic hydrogenation AMS hydrogenation tests were carried out in the STR and RPB reactor, respectively. In the STR (Figure 1a), one monolith catalyst was fixed on the stirring paddle before hydrogenation (Figure 1b). A mixture of 50mol% AMS/cumene (120 mL) was put into the reactor and sealed. After flushed three times to remove the air using pure H2 of 0.5 MPa, the STR was charged to the desired pressure with H2. The hydrogenation process was then conducted at desired temperature with a stirring speed of 150 r/min. A gas chromatography (Shimadzu 2014C) with a flame ionization detector and HPINNOWAX column was used to identify the product. In the RPB reactor (Figure 1c), the packing zone was filled with stainless steel wire mesh first. Then one monolith catalyst was fixed on the surface of the packing zone (Figure 1d). When the reactor started, AMS/cumene (50mol%, 240 mL) liquid was elevated by the rotating riser with a rotating speed of 1000 r/min to the packed zone and dispersed into discrete liquids state by the wire mesh packing. These discrete liquids together with H2 were then contacted with the monolith catalyst and the AMS hydrogenation took place. After a short contact time the liquid was thrown onto the wall 7



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of the reactor under the centrifugal force and flowed down to bottom. This circulation of the liquid continued until stopping the motor of the RPB reactor. The reaction is described as following33: (2) The reaction rate is calculated by the following equation33: r = (dnAMS/dt)/mPd

(3)

where mPd is the mass of the Pd-element; dnAMS is the AMS consumption during a time interval (dt); r is described with the unit of mmol gPd–1 s–1. 3. Results and discussions 3.1. Physical characterization of catalysts The nano-microstructure Co3O4/Ni foam was successfully obtained as confirmed by XRD pattern (Figure 2). The nano-needle or nano-flake Co3O4 uniformly arranging on Ni foam substrate with an average length of ca. 2 μm (Figure 3b and c), matching with the reported Co3O4 in literature30-32. Based on BET analysis, the surface area of Co3O4 on Ni foam was up to 133.8 m2/g, much larger than that of bare Ni foam (2.90 m2/g)31,32. After Co3O4/Ni foam soaking PdCl2 solution and followed by drying, the PdCl2 (020) crystal plane (JCPDS no. 010228) was detected, implying the existence of PdCl2 on Co3O4/Ni foam (Figure S2). The XPS result also confirmed the existed PdCl2 with strong peaks of Pd3d5/2 and Pd3d3/2 centered at 336.8 and 343.1 eV (Figure S3), respectively, which matched well with the reported data for Pd2+ in PdCl2 composition34. After pyrolysis, the PdO (200) crystal plane (JCPDS no. 461043) was identified by the peak position at ca. 31.7o (Figure 2, insertion), verifying the existence of PdO on this 8


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monolith catalyst. Meanwhile, the XPS spectrogram also confirmed the PdO (Figure S4) which was produced by the decomposition of PdCl2 in air at 450 oC. As revealed by ICP-OES (Table 1), the loading amount of Pd-element was 0.51wt%. This monolith was named by 0.51% PdO/Co3O4/Ni foam for easy understanding and comparison. For bare Ni foam substrate with immersion-pyrolysis process, the name of 5.99% PdOy/Ni foam was used (Table 1). This is because both Pd2+ and Pd0 were found in relative XPS spectra (Figure S5) which confirmed the existence of PdOy. As proved by the XRD pattern in Figure S1, the metallic Pd has already generated by the replacement reaction between the Pd2+ ions and Ni atoms during the immersing process. Thus, these metallic Pd atoms might be incompletely oxidized during calcining PdCl2 (Figure 2), leading to the formation of 5.99% PdOy/Ni foam. Surprisingly, SEM-EDS (Figure 3e, 4, S7) and TEM images (Figure 5a) revealed there existed an interesting flower-like morphology of PdO on the surface of Co3O4. The PdO-flowers displayed many pores on their large petals. The pore feature could be also proved by the obvious hysteresis loop in N2 adsorption-desorption isotherm (Figure S8). By pore size distribution analysis, the pores were found to be ca. 5 nm or 11 nm in average diameter, while there were no obvious pores on Co3O4/Ni foam and Ni foam (Figure S9). To the best our knowledge, this unique flower-like PdO morphology was obviously different from that prepared via immersion-pyrolysis process30. Generally, immersing-pyrolysis with high concentration of precursor usually results in seriously aggregation of active component without ordered or uniform morphology, just as the granulated Pd-NPs in 5.99% PdOy/Ni foam prepared in this 9



work with ca. 30 nm in diameter (Figures 5b, and S10b~c). Thus, it is rationally to deduce that the Co3O4 nano-microstructure was responsible for the highly ordered porous PdO-flower morphology. To investigate this interesting effect of nano-microstructure, a series of PdO/Co3O4/Ni foam and their evolution at each stage of the preparation process were tracked. Interestingly, a more smooth and intact flower-like morphology of PdCl2 precursor has already formed after drying the Co3O4/Ni foam with soaked PdCl2 solution (Figure 3d). Thus, subsequent pyrolysis retained the morphology and generated porous PdO-flowers uniformly arranging on Co3O4/Ni foam (Figures 3e and S7). At the same time, at low concentration of 5 mmol L–1, PdCl2 solute was not enough to form a regular structure, but still uniformly adsorbed on the surface of the Co3O4, leading to produce highly dispersed PdO-NPs (Figure S12a). As the concentration elevated to 15, 35, and 45 mmol L–1, more and more flower-like PdCl2 solute liberated, finally presenting more PdO-flowers (Figures S12b and S13). Thus, as shown in Figure 3a, the Co3O4 nano-microstructure was supposed to induce the crystallization and growth of PdCl2 solute to foam flower-like morphology after the immersing-drying process36. On the contrary, the surface area of bare Ni foam had no inducing effect and was too small to disperse the PdCl2 solute, resulting in the irregular morphology of PdCl2 (Figure 20a) which finally caused the serious aggregation of PdOy-NPs on Ni foam after pyrolysis (Figures 5b, S10b, and S10c). Moreover, the 5.99% PdOy/Ni foam showed a large amount of PdOy-powder dropped from the monolith after pyrolysis, while no such phenomenon was observed in 10


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0.51% PdO/Co3O4/Ni foam (Figure S14). This fact demonstrates the nanomicrostructure on Ni foam could enhance the stability of monolith catalyst by well dispersing and immobilizing the PdO flowers. And the PdOy-NPs of 5.99% PdOy/Ni foam may lack of strong interaction with the substrate, and thus a portion of them fell off during pyrolysis, leading to the waste of the noble metal element. Meanwhile, the strong peaks of metallic Pd on bare Ni foam after immersing in PdCl2 solution (Figure S1) implied a serious replacement reaction between Pd2+ and Ni atoms on nickel foam. Although the immersing-pyrolysis experimental conditions of bare Ni foam were the same as Co3O4/Ni foam, the Pd-element loading amount of 5.99% PdOy/Ni foam fiercely raised up to 5.99wt%, 11.74 times higher than that of 0.51% PdO/Co3O4/Ni foam (Table 1). Notably, the surface area of bare Ni foam was too small to well disperse PdOy-NPs with such high loading amount. The Pd-atoms might deposit randomly and many of them were supposed to be embedded in the Ni foam framework during the replacement reaction, also leading to waste of noble metal as implied by AMS conversion and reaction rate below. Thus, the replacement reaction was not expected and had better to be avoided. No peaks of metallic Pd in the XRD pattern (Figure S2) and XPS spectra (Figure S3) of PdCl2/Co3O4/Ni foam, however, were observed, demonstrating that the uniformed Co3O4 array restrained the replacement reaction and saved the noble metal element. Besides, the larger surface area of Co3O4 may be in favor of soaking more PdCl2 solution. The soaked PdCl2 solution on Co3O4/Ni foam was discovered to be higher than that on Ni foam (Figure 6). For example, the increase of Co3O4/Ni foam reached 11



up to 88.04%, while the increase of bare Ni foam was only 79.79% after immersing in PdCl2 solution with a concentration of 25 mmol L–1. The hydrophilia, capillarity, and large surface area of Co3O4 nano-microstructure may account for the higher loading of PdCl2 solution. Thus, this test implies that nano-microstructure could be used to improve Pd-element loading on the surface of monolith. In brief, the nano-microstructure Co3O4 had three effects in the preparation of PdO/Co3O4/Ni foam. First, Co3O4 was responsible for the formation of porous Pdflower after pyrolysis by originally inducing PdCl2 precursor to form flower-like morphology. Second, the nano-microstructure avoided the waste of Pd-element by well anchoring and dispersing PdO-flower and simultaneously suppressing the replacement between Pd2+ ions and Ni atoms. Third, Co3O4 also slightly improved the Pd-element loading by soaking more PdCl2 solution. 3.2. Catalytic activity To explore the catalytic performance of PdO/Co3O4/Ni foam, a typical model multiphase reaction of AMS hydrogenation was chosen. Considering the Pd-element loading amount of most reported catalysts is 0.5wt% (Table 2), we firstly focused on the hydrogenation performance of 0.51% PdO/Co3O4/Ni foam in the STR (Figure 1a). As shown in Figure S15, Co3O4/Ni foam showed inefficient AMS conversion, demonstrating its poor activity. The 0.51% PdO/Co3O4/Ni foam exhibited a boosted AMS conversion with an increase up to 51.20% than that of Co3O4/Ni foam. The 5.99% PdOy/Ni foam which was prepared with the same immersing-pyrolysis procedure of 0.51% PdO/Co3O4/Ni foam only displayed a slight conversion increase of only 13.4% 12


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than Ni foam (Figure S13) and a longer time of 7 h to completely convert AMS than that of 0.51% PdO/Co3O4/Ni foam (5.6 h). So 0.51% PdO/Co3O4/Ni foam is better for AMS hydrogenation than the other three monolith catalysts. Meantime, after AMS hydrogenation, this used catalyst was analyzed by XPS spectra which presented metallic Pd (Pd0) and Pd2+ (Figure S6), probably being explained by the reduction of PdO under the H2 environment. The metallic Pd may act as the active component during AMS hydrogenation, such like the reported Pd-based catalyst in Table 2. Beyond the conversion, the rate of catalytic reaction is more important parameter for catalyst, especially for the noble metal catalyst. After deducting the amount of AMS converted by Co3O4/Ni foam (Figure S15), the hydrogenation reaction rate based on the mass of Pd-element in 0.51% PdO/Co3O4/Ni foam was revealed up to 9.44 mmol gPd–1 s–1, 29.5 times higher than that of 5.99% PdOy/Ni foam (0.32 mmol gPd–1 s–1) (Figure 7). This surprising rate increase of 0.51% PdO/Co3O4/Ni foam could be attributed to the porous PdO-flower on Co3O4 nano-microstructure. This is because during AMS hydrogenation partial PdO were reduced to metallic Pd with the retained morphology as reveal by the SEM image and XPS spectra which may provide more active sites for catalytic hydrogenation (Figure 3e). On the contrary, Pd5.99/Ni foam displayed the sharply decrease of reaction rate owing to the seriously aggregated large Pd-NPs on the surface and embedded Pd-atoms in Ni foam framework as described above. It can be concluded that the modification of nano-microstructured Co3O4 on monolithic catalyst could enhance the conversion and reaction rate, and decrease the usage of noble metal at the same time. 13



Varying the Pd-element loading amount on Co3O4 modified monoliths could tune the complete AMS conversion time (Figure 8). The catalyst with higher loading amount of active metal had fast conversion. For example, 0.89% PdO/Co3O4/Ni foam completely converted AMS to cumene in 4.2 h, while the 0.30% PdO/Co3O4/Ni foam needed a prolonged time of 7 h. The reaction rate is following the order of 0.10% PdO/Co3O4/Ni foam > 0.30% PdO/Co3O4/Ni foam >0.51% PdO/Co3O4/Ni foam > 0.72% PdO/Co3O4/Ni foam > 0.89% PdO/Co3O4/Ni foam (7.71 mmol gPd–1 s–1). All the reaction rates are higher than that of 5.99% PdOy/Ni foam and most of the reported (Table 2), even including the powder catalysts with well dispersed Pd-NPs33,36-41. As temperature increased, the reaction rate improved slightly and the conversion time was shortened (Figure S17). Similarly, the pressure of H2 also affected the conversion and reaction rate (Figure S18). These data showed the monolith catalyst would have a higher activity at a higher temperature and pressure of H2, well matching with that in literature38-42. The durability of monolith catalyst is vital for practical application. Five runs of AMS hydrogenation of 0.51% PdO/Co3O4/Ni foam were performed. Significantly, the total conversion time of each run kept steady at 5.5 ~ 5.6 h and the hydrogenation reaction rate stabilized with a range of 9.40 ~ 9.45 mmol gPd–1 s–1 (Figure 9). The SEM image showed the morphology of the catalyst after five runs of AMS hydrogenation were still porous and flower-like (Figure S19), having no visible change compared with the fresh one. And the ICP-OES result also revealed the Pd-element content of the catalyst was 0.50wt%, being comparable with that of the fresh one. Therefore, both the 14


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SEM-morphology and ICP-OES results of the used catalyst proved the high stability of 0.51% PdO/Co3O4/Ni monolithic catalyst which well explained the steady catalytic performance. This stable performance demonstrates the integrity of the monolithic catalyst due to the strong adhesion among PdO-flowers, Co3O4, and Ni foam30,31, which indicates the great potential of the nano-microstructure modified monolith catalyst applying for chemical hydrogenation. Normally, one kind of monolithic catalyst usually corresponds to one type of reactor1. The universality of this monolithic catalyst for different reactors is also worthy paying attention besides the high catalytic activity and excellent durability. RPB reactor can generate centrifugal acceleration up to several hundred times of gravitational acceleration, being regarded as a promising process intensification technology for chemical industry43,44. Under the large centrifugal field, liquid is broken into liquid droplets, films, and ligaments, resulting in an increase of the gas-liquid interfacial area in gas-liquid system40. Thus, RPB could enhance the mass transfer coefficient up to 1 ~ 3 order of magnitudes compared with traditional packed bed, declaring great potential to improve the catalyst efficiency. But up to now, only several monolith catalysts was reported relevant to RPB37,46. We further explored the application of 0.51% PdO/Co3O4/Ni foam in the RPB reactor (Figure 1b). As displayed in Figure 10, the AMS conversion in the RPB reactor reached up to 6.9% in 30 minutes, higher than that in STR (4.1%). The hydrogenation reaction rate in the RPB reactor was effectively improved up to 17.23 mmol gPd–1 s–1, 1.5 times higher than that in the STR with a value of 11.54 mmol gPd–1 s–1 at 60 oC and 0.5 MPa of H2. The enhanced AMS conversion 15



and reaction rate can be explained by the intensified mass transfer between H2 gas, AMS liquid, and solid catalyst in the RPB reactor46. Different from the environment in the STR with the H2 gas diffusing into the successive AMS solution, the AMS/cumene was cut into discrete tiny liquid droplets, thin liquid films, and fine liquid ligaments in H2 atmosphere by the inner stainless steel wire mesh in the RPB reactor, resulting in a larger gas-liquid contacting area and the renewal rate of the gas-liquid interface47,48, leading to a high hydrogenation conversion and reaction rate. Consequently, the Co3O4 modified monolith catalyst is proved to be not only suitable for STR, but also appropriated for RPB reactor, implying a promising application prospect for various types of reactors.

4. Conclusions In this work, a nano-microstructure modification strategy for monolithic catalyst fabrication is developed to control the morphology of active component using the simplest immersion-pyrolysis technology. The nano-microstructure on monolith directionally induced the formation of flower-like PdCl2 solute during the immersing drying process with PdCl2 solution, thus eventually favored the construction of porous PdO-flowers on Co3O4/Ni foam monolith. The PdO-flowers was reduced to PdOy which efficiently enhanced AMS hydrogenation conversion and reaction rate at a wide range of temperature and H2 pressure in STR. Particularly, the reaction rate of 0.51% PdO/Co3O4/Ni foam reached up to 9.44 mmol gPd–1 s–1, 29.5 times higher than that of 5.99% PdOy/Ni foam and higher than most of the reported in literature. Benefiting from 16


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the strong adhesion among PdO-flower, Co3O4, and Ni foam, the modified monolith showed excellent durability with steady AMS conversion and reaction rate after five runs of hydrogenation reaction. The monolith catalyst was also suitably to be applied in mass-transfer-enhanced RPB reactor with slightly improved catalytic performance. By allowing to utilize the simplest immersion-pyrolysis technique and simultaneously controlling the active component morphology this strategy opens a new avenue to fabricate monolithic catalysts for various reactors.

Acknowledgment This work was supported by the National Key R&D Program of China (No. 2017YFA0206801), the National Natural Science Foundation of China (Nos.21725601, 21676009, and 21436001), and China Postdoctoral Science Foundation (No. 2018M631312).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. XRD pattern, XPS spectra, SEM images, photographs of the monolith catalysts, and catalyst performance.

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jet impaction on the single-layer stainless steel wire mesh in a rotating packed bed reactor. AIChE J. 2019, 65, DOI: 10.1002/aic.16597. (48)Sang, L; Luo, Y.; Chu, G.W.; Sun, B.C.; Zhang, L.L.; Chen, J.F. A three-zone mass transfer model for a rotating packed bed. AIChE J. 2019, 65, DOI: 10.1002/aic.16595.

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Figure 1. Schematic of STR (a) with the three-dimensional (3D) diagram stirring paddle (b) and RPB reactor (c) with the 3D diagram of RPB and the riser (d). (1), stirring paddle anchoring with monolithic catalyst in STR; (2), rotating packed bed anchoring with monolithic catalyst in RPB reactor; (3), riser of RPB; (4), controller of temperature and motor; (5), motor; (6), inlet of H2; (7), outlet of H2; (8), bolt; (9), AMS/cumene. (The arrow with dotted line represents the direction of liquid movement in RPB reactor).

25



Figure 2. XRD patterns of Ni foam, 5.99% PdOy/Ni foam, Co3O4/Ni foam, and 0.51% PdO/Co3O4/Ni foam.

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Figure 3. Schematic illustration of the synthesis process of PdO/Co3O4/Ni foam (a) and SEM images of Ni foam (b), Co3O4/Ni foam (c), flower-like PdCl2 on Co3O4/Ni foam prepared with 25 mmol L–1 PdCl2 solution (d), porous PdO-flower on Co3O4/Ni foam (e).

27



Figure 4. SEM and EDS images of 0.51% PdO/Co3O4/Ni foam. (a), SEM image; (b), Pd; (c), O; (d), Co; (e), Ni. Scale bar: 1 μm.

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Figure 5. TEM images of porous PdO-flower in 0.51% PdO/Co3O4/Ni foam (a) and seriously aggregated PdOy-NPs in 5.99% PdOy/Ni foam (b).

29



160 Ni foam Co3O4/Ni foam

120

/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80 40 0

5

15

25

35

-1 45 / mmol L

Figure 6. Different weight increase of Ni foam and Co3O4/Ni foam after soaking in the solution with varied PdCl2 concertation.

30


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rate / mmol gpd-1 s-1

Page 31 of 37

9

0.51% PdO/Co3O4/Ni foam

6 3 0

5.99% PdOy/Ni foam

Figure 7. Hydrogenation reaction rate of 5.99% PdOy/Ni foam and 0.51% PdO/Co3O4/Ni foam. Conditions: 40 oC and 0.5 MPa of H2.

31



(a)

(b) 100 80 60

0.10% PdO/Co3O4/Ni 0.21% PdO/Co3O4/Ni

40


20 0


0

2

4

6

8

10


AMS Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37


20

0.30% PdO/Co3O4/Ni 0.51% PdO/Co3O4/Ni 0.72% PdO/Co3O4/Ni 0.89% PdO/Co3O4/Ni

10

0

t/h

Figure 8. AMS conversion (a) and hydrogenation reaction rate of x% PdO/Co3O4/Ni foam monoliths (b). Conditions: 40 oC and 0.5 MPa of H2.

32


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100


(b)

(a) AMS Conversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 first run

50

secend run third run fourth run

25

fifth run

1

2

3

4

5

6

12

first secend run run

third run

fourth fifth run run

8 4 0

t/h Figure 9. Durability of 0.51% PdO/Co3O4/Ni foam at 40 oC and 0.5 MPa of H2. (a), AMS conversion; (b), the hydrogenation reaction rate.

33



(b)

8 6


(a)

AMS Coversion / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

RPB STR

4 2 0

Page 34 of 37

0

10

20 t / min

30

18 12

RPB STR

6 0

Figure 10. Comparison of AMS conversion (a) and the hydrogenation reaction rate of 0.51% PdO/Co3O4/Ni foam in STR and RPB reactor. Conditions: 60 oC and 0.5 MPa of H2.

34


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Table 1. Pd-element loading amount (wt%) of the catalysts evaluated by ICP-OES analysis and the PdCl2 concentration used in the preparation process. No.

Monolith catalyst

Concentration of PdCl2 solution / mmol L–1

Pd loading amount / wt%

1 2 3 4 5 6 7

5.99% PdOy/Ni foam 0.10% PdO/Co3O4/Ni foam 0.21% PdO /Co3O4/Ni foam 0.30% PdO/Co3O4/Ni foam 0.51% PdO/Co3O4/Ni foam 0.72% PdO/Co3O4/Ni foam 0.89% PdO/Co3O4/Ni foam

25 5 10 15 25 35 45

5.99 0.10 0.21 0.30 0.51 0.72 0.89

35



Page 36 of 37

Table 2. Comparison of AMS hydrogenation reaction rate of different catalysts in various reactors.

No.

Catalysts

Reactors

1

0.51% PdO/Co3O4/Ni foamThis work

STR

2 3 4 5 6 7 8 9 10 a

foamThis work

5.99% PdOy/Ni 6.57% Pd/Ni foam 37 0.5% Pd/Al2O3 38 0.5% Pd/Al2O3 39 1.0% Pd/Al2O3 40 2.0% Pd/Al2O3 41 0.5% Pd/Al2O3 33 0.5% Pd/Al2O3 33 1.0% Pd/Al2O3 42

Rate / mmol

gPd–1 s–1

RPB reactor STR FBRa STBRb PTBRc RTBRd TSCMe SRf FTMRg MMRh

9.44 11.54 17.23 0.32 0.8 0.16 0.86 0.34 1 8.3 9 8.6

Condition: H2 pressure and temperature 0.3 MPa, 40 oC 0.5 MPa, 60 oC 0.5 MPa, 60 oC 0.3 MPa, 40 oC 0.1 MPa, 25 oC 0.3 MPa, 50 oC 0.1 MPa, 41 oC 0.1 MPa, 40 oC 0.1 MPa, 46 oC 1 MPa, 53 oC 0.1 MPa, 40 oC 0.28 MPa, 40 oC

fixed bed reactor; b structured trickle-bed reactor; c pulsed trickle-bed reactor; d rotating tricklebed reactor; e tubular-supported ceramic membrane; f slurry reactor; g flow-through membrane

reactor; h mesh microreactor. All subscripts of Pd represent of the weight percent according to the relative literature.

36


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Table of Contents (TOC)

37


Porous PdO-Flower Induced by Nanomicrostructure on Monolith with

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