Continuous and complete conversion of high concentration p-nitrophenol in a flow-through membrane reactor

TL;DR: In this article, a green and effective method for the continuous and complete conversion of high concentrations of p-nitrophenol (PNP) using a flow-through membrane reactor and less NaBH4 was reported.

Abstract: Here, we report on a green and effective method for the continuous and complete conversion of high concentrations of p-nitrophenol (PNP) using a flow-through membrane reactor and less NaBH4 The catalytic membrane was successfully fabricated by loading Pd nanoparticles onto the surface of a branched TiO2 nanorod-functionalized ceramic membrane The modification with branched TiO2 nanorods can significantly improve the loading amount of Pd nanoparticles onto ceramic membranes, resulting in enhanced catalytic performance With 6 mg of Pd, 93 L m−2 hr−1 of flux density and 804 cm2 of membrane surface area in the flow-through membrane reactor, PNP at a concentration of 4,000 ppm can be converted to high-value p-aminophenol using less NaBH4 (using a molar ratio of NaBH4:PNP of 96) within 24 s at 30°C More importantly, the conversion can be continuously and stably performed for 240 min

Summary

Introduction

• P-Nitrophenol (PNP) is a common environmental pollutant in water and is therefore a great public concern.
• It is usually used as a precursor or a synthetic intermediate in the industrial manufacturing of analgesics, pharmaceuticals, insecticides, dyes and other chemicals.
• Therefore, the complete elimination of this compound from industrial effluents is a matter of concern for environmental protection.
• The conventional physical methods (sedimentation, filtration, adsorption, etc.) transfer the contaminants into other forms and cannot solve the waste disposal problem.
• 15,16 Conventional metal-acid reduction methods employ reagents such as iron-acids or tin-acids.

Preparation of catalytic membranes

• Synthesis of TiO2 nanorods and branched TiO2 nanorods on ceramic membranes TiO2 nanorod arrays on sheet Al2O3 ceramic membranes (circular, 3.2 cm in diameter, 1.6 mm in thickness and 30% in porosity) were obtained via the following two-step hydrothermal method.
• 26 Twenty mL of concentrated hydrochloric acid (Analytical Reagent, This article is protected by copyright.
• Hydrothermal synthesis was initiated when the autoclave was placed in an electric oven and maintained at 150°C for 5 hours.
• Chloride ions can accelerate the growth of TiO2 in the direction of the top (001) crystal facet.29.
• The branched TiO2 nanorod-modified ceramic membranes were further modified with N-(β-aminoethyl)-γ-aminopropyl trimethoxy silane by immersing them in 50 mL of This article is protected by copyright.

Characterization of Catalytic Membranes

• The crystal structure of the catalytic membranes was determined by X-ray diffraction (XRD, Miniflex-600) analysis at a scan rate of 10° per min in the range of 5-70°.
• The surface morphology of the catalytic membranes was characterized using a field emission scanning electron microscope (FESEM, Hitachi S-4800II).
• Area of the catalytic membranes was measured by N2 physisorption using a physical adsorption apparatus (Micromeritics, ASAP 2020).
• The pore size of the catalytic membranes was determined using a mercury intrusion porosimeter (Poremaster GT-60).

Pd nanoparticles in catalytic membranes

• EDS mapping was performed on the three catalytic membranes to investigate the distribution of elemental Pd. In Figure 4, the red dots represent the Pd signal.
• The distribution of Pd on the surface of Pd/BTiO2-CM is particularly dense and uniform, confirming that modification with branched TiO2 nanorods can provide additional available surface for the loading of Pd nanoparticles.
• As expected, the total quantity of Pd in the reactor consisting of the Pd/BTiO2-CM is the highest (3 mg).
• Taking into account of the errors of measurement, the average diameters of Pd nanoparticles in the three catalytic membranes are almost the same (about 4.2 nm).
• The decline in the PNP conversion using Pd/CM and Pd/BTiO2-CM were 15.6% and 8.4%, which are less than the leaching percentage of Pd.

Evaluation of Catalytic Performance

• Reduction of PNP was performed in a flow-through membrane reactor system in two modes (cycle mode and continuous mode).
• Samples of the reaction mixture were taken from the sampling port in intervals of 5 minutes and were analyzed by high-performance liquid chromatography (HPLC, Agilent 1200 series).
• The flow rate of the reaction mixture through the reactor system was controlled by the peristaltic pump.
• Furthermore, in order to improve the catalytic efficiency, two membrane modules were connected in series.

Characterization of catalytic membranes

• As illustrated in Figure 2, the diffraction lines of Pd/TiO2-CM and Pd/BTiO2-CM show similar patterns.
• Compared to Pd/TiO2-CM, the diffraction peaks of TiO2 relative to the Al2O3 peaks are more intense with the Pd/BTiO2-CM due to the higher coverage density of TiO2 on the ceramic membrane after synthesis of branches.
• Pd in the XRD patterns, which may be attributed to the low content and high dispersion of Pd nanoparticles.
• The morphology of the branched TiO2 nanorods obtained after 16 hours of treatment is shown in the image of Pd/BTiO2-CM.

Conclusions

• In summary, a novel flow-through catalytic membrane reactor system was designed and constructed for the continuous complete conversion of high concentration p-nitrophenol.
• The This article is protected by copyright.
• Modification of ceramic membranes with branched TiO2 nanorods can enhance the Pd content but renders parts of the Pd nanoparticles unusable.
• High concentrations of p-nitrophenol can be completely converted with less consumption of NaBH4 in a short time in the flow-through catalytic membrane reactor, which can be attributed to the small Pd nanoparticles, a higher Pd content and rapid mass transfer.
• The conversion efficiency remained at 100% for 240 minutes, and no byproducts were detected.

Full text

REACTION ENGINEERING, KINETICS AND
CATALYSIS
AIChE Journal
DOI10.1002/aic.16692
Continuous and complete conversion of high concentration p-nitrophenol
in a flow-through membrane reactor
Jianfeng Miao, Jia Lu, Hong Jiang, Yefei Liu and Weihong Xing
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University,
Nanjing 210009, P.R. China
Xuebin Ke*
School of Engineering and Computer Science, University of Hull, HU6 7RX, United
Kingdom
Rizhi Chen*
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University,
Nanjing 210009, P.R. China
*Corresponding author. X.Ke@hull.ac.uk; rizhichen@njtech.edu.cn
This article has been accepted for publication and undergone full peer review but has not been through
the copyediting, typesetting, pagination and proofreading process which may lead to differences between
this version and the Version of Record. Please cite this article as doi: 10.1002/aic.16692
© 2019 American Institute of Chemical Engineers
Received: Jan 14, 2019;Revised: May
23, 2019;Accepted: Jun 04, 2019
This article is protected by copyright. All rights reserved.
This is the peer reviewed version of the following article: Miao, J, Lu, J, Jiang, H, et al. Continuous and complete
conversion of high concentration p-nitrophenol in a flow-through membrane reactor. AIChE J. 2019;e16692, which has
been published in final form at https://doi.org/10.1002/aic.16692. This article may be used for non-commercial
purposes in accordance With Wiley Terms and Conditions for self-archiving.

Abstract
Here, we report on a green and effective method for the continuous and complete
conversion of high concentrations of p-nitrophenol (PNP) using a flow-through membrane
reactor and less NaBH
4
. The catalytic membrane was successfully fabricated by loading Pd
nanoparticles onto the surface of a branched TiO
2
nanorod-functionalized ceramic membrane.
The modification with branched TiO
2
nanorods can significantly improve the loading amount
of Pd nanoparticles onto ceramic membranes, resulting in enhanced catalytic performance.
With 6 mg of Pd, 93 L·m
-2
·h
-1
of flux density and 8.04 cm
2
of membrane surface area in the
flow-through membrane reactor, PNP at a concentration of 4000 ppm can be converted to
high-value p-aminophenol using less NaBH
4
(using a molar ratio of NaBH
4
:PNP of 9.6)
within 24 seconds at 30
o
C. More importantly, the conversion can be continuously and stably
performed for 240 minutes.
Keywords: p-Nitrophenol, continuous and complete conversion, Pd nanoparticles, branched
TiO
2
nanorods, flow-through membrane reactor
This article is protected by copyright. All rights reserved.

Introduction
p-Nitrophenol (PNP) is a common environmental pollutant in water and is therefore a
great public concern.
1
It is usually used as a precursor or a synthetic intermediate in the
industrial manufacturing of analgesics, pharmaceuticals, insecticides, dyes and other
chemicals.
2-4
PNP may bring about significant health hazards because of its carcinogenic
toxicity. Short-term inhalation or ingestion in humans can cause headaches, drowsiness,
nausea and cyanosis, even at low concentrations.
5,6
The concentration of PNP in industrial
wastewater is usually much higher than 500 mg/L.
7
Therefore, the complete elimination of
this compound from industrial effluents is a matter of concern for environmental protection.
Various methods have been proposed for the treatment of PNP-contaminated wastewater.
8
The conventional physical methods (sedimentation, filtration, adsorption, etc.) transfer the
contaminants into other forms and cannot solve the waste disposal problem.
9
Biological
methods may require long treatment time, and complete degradation may be impossible,
This article is protected by copyright. All rights reserved.

especially for effluents containing PNP at a high concentration.
10-12
Thus, the development of
effective chemical methods to convert PNP to high-value products or to achieve complete
degradation is urgently needed.
Catalytic reduction of PNP to p-aminophenol, an important chemical intermediate, is a
feasible method for turning waste into a renewable resource.
13,14
In addition, aromatic amines
are less toxic and considerably easier to mineralize than their corresponding
nitroaromatics.
15,16
Conventional metal-acid reduction methods employ reagents such as
iron-acids or tin-acids.
The major disadvantage of such reduction processes is the generation
of large amounts of metal oxide sludge that is associated with severe pollution problems.
17
One-step hydrogenation of PNP in the presence of metal catalysts or supported metal
catalysts is considered to be the most promising process because of its high efficiency and
environmentally friendly properties.
18
As a strong reducing agent, NaBH
4
can effectively
reduce PNP to p-aminophenol under mild operation conditions (room temperature and
atmospheric pressure).
19
Thus, the direct reduction of PNP with NaBH
4
as a reducing agent is
considered to be an efficient and greener catalytic route for the conversion of PNP.
In the practical treatment of industrial wastewater, fixed-bed reactors and slurry reactors
inevitably develop problems. For a fixed-bed reactor, fine catalysts cannot be directly used,
and their inner surface cannot be fully utilized. In addition, the regeneration and replacement
of catalysts are inconvenient. In a slurry reactor, metal nanoparticles that are present on the
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powdered catalysts easily aggregate or leak, and it is difficult to separate them from the
reaction system.
20
Catalytic membranes exhibit good particle distribution, and no additional
separation is required.
21,22
Moreover, the porous membrane structure and flow-through mode
can enhance the catalytic efficiency by increasing mass transfer.
23
These advantages allow
flow-through catalytic membrane reactors to efficiently treat industrial wastewaters that
contain high concentrations of PNP. Wang et al.
24
developed a novel poly (vinylidene fluoride)
membrane with Pd/poly (methacrylic acid) microspheres immobilized inside the membrane
pores. Its use in the catalytic reduction of PNP indicated that a conversion of 99.8% could be
achieved in a cross-flow model. Domènech et al.
25
reported on the synthesis of Pd
nanoparticles in sulfonated polyethersulfone membranes. The catalytic performance was
evaluated by following the reduction of PNP in the presence of NaBH
4
. Greater than 90% of
the PNP was reduced within 4 hours using a single reaction step, and deactivation was
observed after consecutive catalytic cycles. Our group
26
successfully demonstrated that the
use of TiO
2
nanorod-functionalized ceramic membranes is an effective approach for
enhancing the loading amount of Pd and the corresponding catalytic activity. However, the
catalytic membranes obtained when used directly in a batch reactor could not achieve
continuous conversion of PNP. Furthermore, although complete conversion of PNP can be
achieved, a high molar ratio of NaBH
4
to PNP like 100 is required.
27,28
An excess of the
reductant NaBH
4
will increase the treatment cost significantly. Therefore, the efficient
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Frequently asked questions

Q1. What is the effect of the flow-through catalytic membrane reactor?

High concentrations of p-nitrophenol can be completely converted with less consumption of NaBH4 in a short time in the flow-through catalytic membrane reactor, which can be attributed to the small Pd nanoparticles, a higher Pd content and rapid mass transfer. 

Q2. What have the authors contributed in "Continuous and complete conversion of high concentration p-nitrophenol in a flow-through membrane reactor" ?

Here, the authors report on a green and effective method for the continuous and complete conversion of high concentrations of p-nitrophenol ( PNP ) using a flow-through membrane reactor and less NaBH4. The catalytic membrane was successfully fabricated by loading Pd nanoparticles onto the surface of a branched TiO2 nanorod-functionalized ceramic membrane. The modification with branched TiO2 nanorods can significantly improve the loading amount of Pd nanoparticles onto ceramic membranes, resulting in enhanced catalytic performance. 

Q3. What is the important chemical intermediate for the conversion of PNP?

Catalytic reduction of PNP to p-aminophenol, an important chemical intermediate, is afeasible method for turning waste into a renewable resource. 

Q4. What is the promising process for the conversion of PNP?

17 One-step hydrogenation of PNP in the presence of metal catalysts or supported metal catalysts is considered to be the most promising process because of its high efficiency and environmentally friendly properties. 

Q5. What is the potential of the catalytic membrane reactor?

The catalytic membrane reactor developed here provides a promising prospect for practical applications of industrial wastewater treatment. 

Q6. How long did the membrane remain in ethanol?

Once the reaction process was completed (60 minutes for each run), the catalytic membrane was removed and immersed in ethanol for 1 hour, then applied to the next reaction. 

Q7. What is the effect of the flow-through mode on the catalytic efficiency of the membrane?

In future work, the authors will attempt to further reduce the amount of NaBH4, enhance the catalytic efficiency of Pd and the stability of the catalytic membrane, and improve the economic efficiency of the catalytic membrane reactor. 

Q8. How was the surface area of the catalytic membrane in contact with the reactant?

The surface area of the catalytic membrane in contact with the reactant solution was 8.04 cm2 and the thickness of the catalytic membrane was 0.16 cm. 

Q9. What is the total residence time of the reaction solution in the two successive catalytic membranes?

The total residence time of the reaction solution in the two successive catalytic membranes is defined as:residence time (s) = pore volume of membranes (cm3)volumetric flow rate of reaction solution (cm3/s)This article is protected by copyright. 

Q10. What is the reason for the improvement of catalytic activity?

the authors can deduce that modification with branched TiO2 nanorods can effectively increase the surface area, resulting in increased loading amounts of Pd, which is one of the reasons for the improvement of catalytic activity. 

Q11. What are the features of the flow-through catalytic membrane reactor?

These features indicate that the reaction solution can more easily pass through the membrane and contact the active sites in the pores in their research, resulting in enhanced catalytic efficiency23. 

Q12. What is the effect of the synthesis of TiO2 nanorods on the ceramic membrane?

In this work, the synthesis of TiO2 nanorods and branched TiO2 nanorods on the ceramic membranes makes the membrane pore size decrease (Table 1), leading to the reduced water flux. 

Q13. How many particles were tested in a triplicate?

To obtain reproducible results, the particle size of the Pd was evaluated by counting more than 100 particles and samples were tested in triplicate. 

Q14. What are the advantages of flow-through catalytic membrane reactors?

These advantages allow flow-through catalytic membrane reactors to efficiently treat industrial wastewaters that contain high concentrations of PNP. 

Q15. What is the way to reduce PNP?

the direct reduction of PNP with NaBH4 as a reducing agent is considered to be an efficient and greener catalytic route for the conversion of PNP. 

Q16. How much is the conversion efficiency of PNP in the first run?

For Pd/CM, after 5 cycles, the conversion efficiency of PNP was only 74.0%, which is significantly lower than that in the first run (87.7%). 

Q17. How long can PNP be reduced in a flow-through membrane reactor?

it can be concluded that the contaminant PNP can be continuously and thoroughly reduced to p-aminophenol within 240 min in a flow-through membrane reactor.