Molecular Sieve Compositing Strategy Enabling Tandem Ammoxidation-Oxidation to 2-Nitropropane: Catalyst Design and Industrial-Scale Separation

JieLiu1

QingyanChu1✉Phone+86-18560253639(Chu)Emailchuqy@sdut.edu.cn

GuangliangWang1

XiaoyangZhang1

XiaoweiFeng1

TengfeiWang1

TongLi1

PingWang1✉Phone+86-13561652707(Wang)Emailwangping876@163.com

1School of Chemistry and Chemical EngineeringShandong University of Technology255049ZiboChina

Jie Liu ¹, Qingyan Chu ^1, *, Guangliang Wang, Xiaoyang Zhang, Xiaowei Feng, Tengfei Wang, Tong Li, Ping Wang **

(School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, China)

* Corresponding author: Qingyan Chu, chuqy@sdut.edu.cn, Tel: +86-18560253639(Chu)

** Corresponding author: Ping Wang, wangping876@163.com,Tel: +86-13561652707(Wang)

Abstract

Conventional nitration methods using concentrated nitric acid pose significant safety and environmental risks. To address this, a dual-base-modified composite catalyst (TS-1@Ti-MWW-OH) was designed for efficient 2-nitropropane synthesis via acetone ammoxidation/oxidation. Structural characterization confirmed that compositing TS-1 with Ti-MWW and modifying with 4-methoxypyridine/ethanolamine increased specific surface area (32%), pore volume (28%), and pore size (1.8 nm), enhancing mass transfer and active site accessibility. Mechanistic studies revealed a synergistic relay catalysis: Ti⁴⁺ sites on Ti-MWW catalyzed acetone→acetone oxime conversion, while Ti⁴⁺ on TS-1 oxidized oxime→2-nitropropane, achieving 92.2% yield. Process simulation via Aspen Plus V14 demonstrated industrial-scale production of 13,000 t/year 2-nitropropane (≥ 99.99 wt%) and 760 t/year acetone oxime (≥ 99.8 wt%), with stable operation ≥ 8,000 h/year validated by RADFRAC and tray hydraulics analysis. Additionally, column sizing (height and diameter) and tray hydraulics analysis performed using the Tray Sizing module confirmed that the maximum flooding ratio remains within permissible operating limits.This work provides a sustainable strategy for nitroalkane production through tailored catalyst design and optimized process engineering.

Keywords:

TS-1

Ammonia oxidation

Composite catalyst

Green catalysis

Distillation column

1 Introduction

Nitro compounds play a pivotal role in numerous industrial sectors, including agrochemicals, dyes, and pharmaceuticals, owing to their unique chemical properties. The presence of the nitro functional group enables their transformation into diverse high-value fine chemicals, underpinning their significant utility in complex chemical synthesis^[1–4]. For instance, 2-nitropropane serves as a vital industrial solvent extensively used in printing inks, paints, varnishes, adhesives, and coatings^[5–8]. It is also a key precursor for synthesizing 2-amino-2-methyl-1-propanol ^{[9, 10]}.

However, the conventional synthesis of 2-nitropropane, predominantly via selective nitration of aliphatic C-H bonds, faces substantial challenges. The activation of aliphatic C-H bonds is inherently difficult, and achieving regioselective nitro substitution is particularly complex ^{[11, 12]}. For example, the team led by Hanpeng ZHANG ^[13–15] investigated propane nitration using microreactor technology, reacting concentrated nitric acid with propane at elevated temperatures. The reported selectivity was 45% for 1-nitropropane and 43% for 2-nitropropane, alongside byproducts like nitromethane and nitroethane. Notably, this process generated significant gaseous waste effluents due to the reaction of propane and nitric acid streams, posing environmental burdens.

Addressing these limitations, researchers like Haijun XIAO ^{[16, 17]} employed nitrogen dioxide (NO₂) as an alternative nitrating agent in a fixed-bed reactor for cyclohexane nitration. This approach yielded a 10.2% conversion, with 70.3% selectivity towards nitrocyclohexane and 28.6% towards adipic acid. Despite improvements in selectivity and conversion, the industrial production of nitroalkanes still largely relies on high-temperature nitration using nitric acid or nitrogen oxides. This methodology suffers from several drawbacks: (1) inherently hazardous operating conditions; (2) stringent equipment requirements; and (3) the generation of numerous byproducts, resulting in conversion and selectivity levels often insufficient for industrial-scale demands ^{[18, 19]}.

Recently, novel strategies for nitro group introduction have emerged. Meimei ZHANG et al. ^[20] utilized nitrite salts to achieve the oximation of propyl acetoacetate at the methylene position, followed by oxidation using K₂Cr₂O₇/concentrated H₂SO₄ to furnish the nitro product in 85% yield after 10 hours. This method offered a significant yield enhancement compared to traditional nitric acid or NO₂ nitration. Furthermore, Kosaku TANAKA et al. ^[21] demonstrated the transition-metal-catalyzed conversion of oximes to nitro ketones, establishing an alternative pathway involving oxime synthesis followed by a radical reaction to generate nitroalkanes.

More notably, Zhiqiang ZHANG et al. ^[22] recently achieved highly efficient conversion of acetone to 2-nitropropane using a TS-1 catalyst in a hydrogen peroxide-ammonia system, reporting an impressive 96% yield and 97% selectivity. Compared to previous methods, this strategy-involving ketone ammoxidation followed by oxidation to the nitro compound-offers not only superior yields but also significantly milder reaction conditions, positioning it as a highly promising route for nitroalkane synthesis ^{[23, 24]}.

In summary, while traditional high-temperature nitration methods remain prevalent industrially, their inherent limitations and environmental concerns drive the pursuit of more efficient and sustainable nitro group introduction pathways. This study addresses this need by synthesizing composite catalysts via the epitaxial growth of secondary molecular sieves on TS-1. We demonstrate the highly effective conversion of acetone to 2-nitropropane using hydrogen peroxide and aqueous ammonia, highlighting the critical role of the composite catalyst in synergistically catalyzing the sequential oximation-oxidation reaction. Particularly, the ammoxidation of ketones emerges as a strategy with exceptional application potential, offering novel insights for the green synthesis of nitroalkanes.

2 Results and Discussion

Fig. 1

Schematic diagram of SEM/TEM with different catalysts

a.TS-1, b.TS-1-OH, c.TS-1-NaOH, d.TS-1@ZSM-5,

e.TS-1@Ti-MWW, f.TS-1@Ti-MOR, g.TS-1, h.TS-1-OH

The morphologies of the catalysts TS-1, TS-1-OH, TS-1@ZSM-5, and TS-1@Ti-MWW were characterized by scanning electron microscopy (SEM). As shown in Fig. 1a, the TS-1 zeolite exhibits a typical, well-defined spherical morphology with smooth particle surfaces and uniform size distribution, consistent with its characteristic features ^[25]. This morphology indicates high crystallinity and the absence of significant physical or chemical damage to the surface.

Following organic base treatment, the TS-1-OH catalyst (Fig. 1b) displayed a markedly altered morphology. Compared to pristine TS-1, TS-1-OH particles developed a distinctly roughened surface texture accompanied by the formation of irregular angular facets or protrusions. These changes demonstrate that organic base treatment induces an etching effect on the TS-1 zeolite surface, leading to modifications in its morphology and structure. In contrast, treatment with the conventional inorganic base NaOH (Fig. 1c) resulted in severe structural degradation, producing fragmented catalyst debris. This confirms that inorganic bases completely destroy the zeolite framework, rendering it catalytically inactive ^[26].

For the synthesized composite catalysts TS-1@ZSM-5, TS-1@Ti-MWW, and TS-1@Ti-MOR (Fig. 1d-f), SEM images clearly reveal an intimate integration between TS-1 and the secondary components (ZSM-5, Ti-MWW, Ti-MOR). The TS-1 particles are coalesced with the other catalytic phases without discernible interfacial separation, forming a homogeneous composite structure. These results confirm the successful synthesis of TS-1 composites with ZSM-5, Ti-MWW, and Ti-MOR via hydrothermal crystallization, with morphologies aligning well with the designed architecture ^{[27, 28]}.

Transmission electron microscopy (TEM) provided further insight into the structural details of TS-1 and its base-treated derivatives. As depicted in Fig. 1g-h, the pristine TS-1 catalyst displays its characteristic regular spherical morphology, featuring smooth surfaces and a dense internal structure with well-resolved lattice fringes, indicative of high crystallinity. Base treatment induced significant morphological alterations. TEM images reveal that the base-treated TS-1 particles adopted irregular shapes and exhibited reduced electron density contrast during imaging, suggesting the development of internal structural disordering or porosity. Furthermore, multiple irregular pores appeared on the particle surfaces, with varying sizes and non-uniform distribution, providing direct evidence for surface etching during the base treatment process.

The formation of these pores is predominantly attributed to the partial cleavage of Si-O-Si bonds within the TS-1 framework during base treatment, leading to the dissolution of silicon species and subsequent structural reorganization. This process not only modifies the catalyst's morphological characteristics but may also profoundly impact the distribution of surface active sites and its resultant catalytic performance.

Fig. 2

a.Schematic diagram of XRD with different catalysts, b.Schematic diagram of UV-vis with different catalysts, c.Schematic diagram of FT-IR with different catalysts

To gain deeper insights into the crystalline structure of the synthesized catalysts, X-ray diffraction (XRD) analysis was performed on TS-1, Ti-MWW, and the composite catalyst TS-1@Ti-MWW. As shown in Fig. 2a, the pristine TS-1 catalyst exhibits distinct characteristic diffraction peaks at 2θ = 7.987°, 8.885°, 23.207°, 23.960°, and 24.383°. These peaks align precisely with the standard diffraction positions for the MFI topology, confirming the characteristic MFI framework structure of TS-1. The high intensity and narrow full-width at half-maximum (FWHM) of these peaks further attest to its high crystallinity and structural integrity.

The pure Ti-MWW catalyst displays its characteristic diffraction peaks at 2θ = 7.174°, 7.994°, 10.023°, 14.319°, 21.683°, 22.712° and 26.039°, consistent with the typical pattern for the MWW structure.

Compared to pure TS-1, the XRD pattern of the composite TS-1@Ti-MWW catalyst shows a significantly enhanced intensity at the characteristic peak located at 2θ = 7.987°, while the peak positions and profiles of other TS-1 reflections remain largely unchanged. This indicates that the introduction of Ti-MWW does not disrupt the MFI framework structure of TS-1. The intensity enhancement at 2θ = 7.987° is primarily attributed to Ti-MWW promoting increased diffraction intensity from the (101) plane of TS-1, signifying an augmentation in the crystalline phase content. This structural modification likely contributes significantly to the enhanced catalytic performance observed for the composite catalyst ^[29–31].

Following base treatment, the XRD pattern of the TS-1@Ti-MWW catalyst reveals a further increase in diffraction peak intensities, most notably at 2θ = 7.987°. Crucially, base treatment does not alter the MFI topology, suggesting that the observed catalytic activity improvement stems primarily from surface modification and pore structure optimization induced by the treatment.

UV-Vis spectroscopy results (Fig. 2b) show that all four catalysts exhibit a pronounced absorption band at 210 nm. This band is assigned to tetrahedrally coordinated Ti⁴⁺ species incorporated within the zeolite framework via Si-O-Ti linkages ^[32]. Such Ti⁴⁺ species are recognized as the active oxidation centers crucial for ammoxidation catalysis. An additional absorption band observed at 310 nm corresponds to extra-framework, free TiO₂ species, which are inactive in catalysis. After base treatment, the intensity of the 210 nm band shows minimal change, indicating relative stability in the content of framework Ti⁴⁺ species. However, the intensity of the 310 nm band increases significantly. This arises from the partial dissolution of silicon species from the zeolite framework during base treatment, accompanied by the dislodgement of some Ti⁴⁺ species, leading to an increased population of free TiO₂. Notably, base treatment does not effectively remove the TiO₂ crystalline phase, indicating its limited efficacy in eliminating these species. The increase in free TiO₂ content may exert a dual effect on catalytic performance: while the stable framework Ti⁴⁺ content helps preserve catalytic activity, the increased free TiO₂ could potentially block active sites or alter surface properties, thereby imposing a detrimental effect on catalytic efficiency ^[33].

FT-IR spectroscopy results (Fig. 2c) reveal significant characteristic absorption bands for all catalysts at 550 cm⁻¹, 800 cm⁻¹, 960 cm⁻¹, 1100 cm⁻¹, and 3400 cm⁻¹, consistent with the typical FT-IR fingerprint of TS-1 zeolites. The bands at 550 cm⁻¹ and 960 cm⁻¹ are specifically attributed to the presence of framework titanium (Si-O-Ti) and extra-framework titanium (TiO₂) species, respectively, providing key insights into the distribution of Ti species within the catalysts.

After base treatment, both TS-1 and TS-1@Ti-MWW catalysts exhibit a discernible decrease in the intensity of the band at 960 cm⁻¹. This attenuation primarily stems from the desilication process occurring on the catalyst surface during base treatment, leading to the partial leaching of framework Ti (Si-O-Ti) species and consequently reducing the number of Si-O-Ti bonds. Furthermore, compared to TS-1, TS-1@Ti-MWW shows a significantly enhanced intensity at the 550 cm⁻¹ band. This indicates that the incorporation of Ti-MWW effectively increases the content of TiO₂ species within the catalyst, corroborating the UV-Vis spectroscopic analysis.

Comparing the FT-IR spectra of TS-1@Ti-MWW and its base-treated counterpart TS-1@Ti-MWW-OH reveals minimal differences. Although slight attenuation is observed at both 550 cm⁻¹ and 960 cm⁻¹ bands, the degree of reduction is markedly less pronounced than that observed for base-treated TS-1 alone. This demonstrates that the introduction of Ti-MWW not only increases the TiO₂ species content but also significantly enhances the stability of the framework titanium (Si-O-Ti) bonds, mitigating their leaching during base treatment.

The incorporation of Ti-MWW and its role in enhancing framework titanium stability presents a vital strategy for catalyst structural optimization. As the framework titanium (Si-O-Ti) constitutes the catalytically active center, improved stability directly contributes to enhanced catalyst performance while simultaneously reducing the detrimental structural impact of base treatment ^[34–36].

3 Reactability evaluation

Initial performance evaluation of the four zeolite catalysts (TS-1, Ti-MWW, Ti-MOR, ZSM-5) in the acetone ammoxidation reaction revealed distinct activities. TS-1 exhibited a conversion of 32.25% with a selectivity towards 2-nitropropane of 32.614%. While its conversion was lower than that of Ti-MWW (91.434%) and Ti-MOR (91.92%), TS-1 uniquely facilitated the reaction pathway leading to 2-nitropropane, a capability absent in the other catalysts. Notably, ZSM-5 showed negligible catalytic activity, attributable to its lack of Ti species, which are established as essential active centers for this reaction.

Based on these findings, a composite catalyst strategy was devised, using TS-1 as the primary component modified by incorporating Ti-MWW, Ti-MOR, or ZSM-5. This involved controlled epitaxial growth of TS-1 crystallites on the surfaces of the secondary zeolites, aiming to harness synergistic effects to optimize catalytic performance.

The composite catalysts demonstrated significantly enhanced performance compared to pristine TS-1. TS-1@Ti-MWW exhibited the highest activity, achieving a conversion of 98.822% and a 2-nitropropane selectivity of 52.21%. TS-1@Ti-MOR and TS-1@ZSM-5 achieved selectivities of 35.89% and 40.03%, respectively. The incorporation of Ti-MWW and Ti-MOR increased the abundance and optimized the distribution of active Ti species, thereby boosting activity. Crucially, while ZSM-5 itself was inactive, its composite with TS-1 significantly enhanced catalytic performance. This suggests that ZSM-5 modulates the acid-base properties or surface structure of TS-1, improving reactant adsorption and activation and optimizing the catalytic environment beyond mere physical mixing, confirming the advantage of synergistic effects in composite catalysts.

Literature reports and our experimental data indicate that dual-base modification offers significant advantages over single-base treatment for catalyst optimization. Studies demonstrate that dual-base treatment not only more efficiently tunes pore structure but also further enhances catalytic activity and selectivity through synergistic effects. For instance, in catalytic methanol aromatization, dual-base modified catalysts show superior conversion and product selectivity, aligning closely with our observations ^{[37, 38]}.

BET adsorption analysis of the composite catalysts before and after base treatment (Table S1) revealed significant alterations in pore structure. Post-treatment, the specific surface area decreased, while the average pore diameter markedly increased. This is attributed to the synergistic action of the dual organic bases (4-methoxypyridine and ethanolamine), enabling selective desilication that achieves pore enlargement. This pore expansion increases the pore size, elevates the relative Ti content, and generates additional surface defect sites, collectively optimizing catalytic performance.

The dual organic base modification strategy employs 4-methoxypyridine as a strong base. The electron-donating methoxy group at the γ-position significantly enhances the basicity of the pyridine ring, improving desilication efficiency. Unlike conventional inorganic bases (e.g., NaOH, KOH), 4-methoxypyridine enlarges pores without compromising the framework integrity. Ethanolamine, acting as a weak base, forms stable hydrogen bonds with the nitrogen atom of the pyridine ring, creating a synergistic effect. The selective desilication process increases the average pore diameter, enhancing mass transfer of reactants and products. The concomitant increase in Ti content provides more active centers, while the formation of surface defects further augments catalytic activity. This synergistic dual-base approach achieves efficient desilication while preserving structural stability, thereby enhancing catalytic activity ^[39].

Base treatment significantly improved the conversion and selectivity of the composite catalysts. As shown in Fig. 3c, the dual-base modified TS-1@Ti-MWW-OH catalyst exhibited exceptional performance, achieving an acetone conversion of 98.678% and a 2-nitropropane selectivity of 93.409%, representing a substantial improvement over the untreated catalyst. This demonstrates that base treatment not only optimizes catalyst structure through pore enlargement but also significantly enhances reactivity and selectivity by increasing the Ti content and creating surface defect active sites.

Fig. 3

a.The effect of a single catalyst on the reaction, b.The effect of compound catalysts on the reaction, c.The effect of alkali treatment catalyst on reaction, d.The effect of temperature on the reaction, e.The effect of feeding ratio on the reaction

The ammoxidation of acetone to 2-nitropropane is an exothermic process where temperature significantly influences reaction performance. To investigate this effect, we systematically examined how reaction temperature impacts the catalytic behavior of TS-1@Ti-MOR-OH in acetone ammoxidation, focusing on acetone conversion, acetone oxime selectivity, and 2-nitropropane selectivity.

Experimental results (Fig. 3-d) revealed that at lower temperatures (65°C), the system achieved only 78.006% acetone conversion with relatively low 2-nitropropane selectivity (69.248%). This suboptimal performance indicates insufficient activation of catalytic sites under low-temperature conditions, where reaction kinetics become limiting.

When temperature increased to 75°C, the catalyst demonstrated markedly enhanced activity, reaching 98.678% acetone conversion while improving 2-nitropropane selectivity to 93.409%. These results confirm that moderate temperature elevation favors the reaction pathway toward 2-nitropropane formation by optimizing both kinetic and thermodynamic factors.

However, further temperature increase beyond 80°C led to decreased 2-nitropropane yield. Mass spectrometry analysis identified α-C dehydrogenation coupling reactions of 2-nitropropane molecules under these elevated temperatures as the primary cause for selectivity reduction^[40]. This phenomenon demonstrates that while higher temperatures accelerate reaction rates, they may simultaneously promote side reactions that compromise target product selectivity and yield.

According to the reaction stoichiometry, the theoretical molar ratio of acetone, ammonia, and hydrogen peroxide is 1:1:2. However, in practical reactions, incomplete conversion necessitates appropriate increases in ammonia and hydrogen peroxide dosages to optimize reaction efficiency. Experimental results (Fig. 3-e) demonstrate that reactant ratios significantly influence product yields.

When employing an acetone:ammonia:hydrogen peroxide ratio of 1:1.2:2.5, consistently high yields were obtained across most reaction temperatures. This observation indicates that moderate excesses of ammonia and hydrogen peroxide effectively compensate for incomplete conversion during the reaction process, thereby enhancing overall efficiency. Specifically, the excess ammonia not only provides the necessary alkaline environment to drive the reaction forward but also suppresses side reactions. Studies revealed that reduced ammonia proportions led to incomplete conversion of acetone to acetone oxime, resulting in significantly diminished 2-nitropropane yields.

Furthermore, hydrogen peroxide dosage substantially impacts reaction performance. Following the Nef reaction mechanism, when hydrogen peroxide participates as an oxidant, the electron-withdrawing nitro group renders the α-hydrogen of nitropropane sufficiently acidic to form a stabilized anion. This anion undergoes double protonation to generate an iminium ion, which subsequently reacts via water nucleophilic addition, followed by proton and water elimination to ultimately form acetone. However, excessive hydrogen peroxide dosage or prolonged reaction time conversely decreased 2-nitropropane yields, likely due to over-oxidation causing product degradation or side reactions.

4 Discussion on Reaction Mechanism

Fig. 4

Hypothetical Reaction Mechanism for the Ammonoxidation of Acetone to 2-Nitropropane

Based on literature reports and experimental results, this study proposes a plausible reaction pathway for the ammoxidation of acetone to 2-nitropropane (Fig. 4). The process typically involves two key steps: (1) ammoximation of acetone to form acetone oxime, and (2) subsequent deep oxidation of acetone oxime to 2-nitropropane(Figure 4).

1)Mechanism of acetone ammoximation to acetone oxime

In this step, ammonia (NH₃·H₂O) and hydrogen peroxide (H₂O₂) first generate the active intermediate hydroxylamine (NH₂OH) under the catalysis of framework titanium species. The lone pair electrons on the nitrogen atom of hydroxylamine nucleophilically attack the carbonyl carbon of acetone ((CH₃)₂C = O), initiating a nucleophilic addition reaction. A hydrogen migration to the carbonyl oxygen leads to cleavage of the C = O bond and formation of a transition state intermediate. Subsequent dissociation of hydrogen from hydroxylamine and electron rearrangement induced by the nitrogen lone pair facilitate another hydrogen transfer to the carbonyl oxygen. At this stage, the carbonyl oxygen carries two hydrogen atoms and is eventually eliminated as water, while the nitrogen and adjacent carbon form a C = N double bond through electron sharing, yielding the target product acetone oxime ((CH₃)₂C = NOH) ^{[41, 42]}.

2)Mechanism of deep oxidation from acetone oxime to 2-nitropropane

During deep oxidation, the lone pair electrons on the nitrogen atom of acetone oxime react with H₂O₂ to form intermediate (CH₃)₂C = NOOH, releasing one water molecule. In this process, coordination between the nitrogen lone pair and the peroxy bond (O-O) leads to O-O bond cleavage and generation of nitryl (NO₂⁺) species. The hydroxyl group (-OH) on nitrogen undergoes polarization due to nitrogen electronegativity, promoting proton dissociation. Another hydrogen is subsequently oxidized through a similar mechanism to form the nitro (-NO₂) group. Meanwhile, the strong electron-withdrawing effect of the nitro group reduces electron density at the adjacent carbon, inducing C-N bond cleavage and final rearrangement to yield stable 2-nitropropane (CH₃CH(NO₂)CH₃).

The proposed reaction pathway comprehensively considers microscopic processes including electron transfer, proton migration, and bond cleavage/formation, whose kinetic and thermodynamic feasibility has been preliminarily verified experimentally^{[43, 44]}.

5 Aspen simulation

5.1 Process Flow Diagram Design for the Preparation of 2-Nitropropane by Acetone Ammoxidation

Fig. 5

Process Flow Diagram for the Ammonoxidation of Acetone to 2-Nitropropane

As shown in Fig. 5, the optimized feed composition based on preliminary experiments comprises acetone (FEED1, 20 kmol/h), ammonia (FEED4, 24 kmol/h), and hydrogen peroxide (FEED3, 48 kmol/h). The feed acetone is initially mixed with solvent methanol (FEED2, dosage optimized based on solubility) in a static mixer (M1) at ambient temperature (25°C) to form a homogeneous solution (FEEDMIX1). The mixture is then preheated through a shell-and-tube heat exchanger (H1) using 85°C hot water (HOTIN) as the thermal medium, gradually raising the temperature to 75°C (FEEDMIX2) to meet thermodynamic requirements for subsequent reactions.

The preheated stream (FEEDMIX2) combines with metered hydrogen peroxide (FEED3) and ammonia (FEED4) before entering the fixed-bed reactor (R1). The reactor, packed with TS-1 framework titanium catalyst, operates continuously at 1.0-1.5 MPa and 75–80°C. Under these conditions, acetone conversion stabilizes at 92–99% with 85–95% selectivity toward 2-nitropropane. Major byproducts include acetone oxime (5–10%) and trace C-N coupling impurities. The post-reaction mixture (FEEDMIX4) undergoes real-time GC monitoring of key components.

The reactor effluent is mixed with benzene (azeotropic agent) and fed into the light-ends removal column (T1). Leveraging the benzene-water azeotrope, the distillate (LIG1) efficiently removes > 99% water while entraining unreacted acetone, methanol, residual ammonia, and oxygen. LIG1 undergoes two-stage heat exchange before entering a three-stage flash system (T3), where intermediate fractions (MC-LIG1) yield high-purity benzene (GC purity ≥ 99.8%). This stream is cooled to ambient conditions via heat exchanger (H3) and recycled to the T1 feed mixer (M2), achieving closed-loop solvent utilization. The T1 bottoms (heavy components) proceed to the rectification column (T2), where acetone oxime (D2, purity ≥ 99.8%) is collected as the distillate and 2-nitropropane (D1, purity ≥ 99.99%) is obtained as the high-purity bottom product.

5.2 Selection of Property Methods

The Aspen Plus simulation was conducted for a process producing 13,000 tons of 2-nitropropane annually with a byproduct of 760 tons of acetone oxime. For non-ideal systems, basic property methods such as NRTL, Wilson, and UNIFAQ can be selected. NRTL is suitable for aqueous polar systems, and since the preparation of 2-nitropropane generates a significant amount of molecular water, the NRTL-RK method within NRTL was employed^[45]. Its equation is shown in Eq. 6.2-1.

$\:ln{\gamma\:}_{i}=\frac{{\sum\:}_{j=1}^{n}{т}_{ji}{G}_{ji}{x}_{ji}}{{\sum\:}_{k=1}^{n}{G}_{ki}{x}_{k}}+\sum\:_{j=1}^{n}\left(\frac{{x}_{j}{G}_{ij}}{{\sum\:}_{k=1}^{n}{G}_{ki}{x}_{k}}\right)({т}_{ji}-\frac{{\sum\:}_{k=1}^{n}{т}_{ki}{G}_{ki}{x}_{k}}{{\sum\:}_{k=1}^{n}{G}_{kj}{x}_{k}})$

6.2-1

The RK equation describes gas-phase non-ideality as shown in Eq. 6.2-2:

$\:P=\frac{RT}{{V}_{m}-b}-\frac{a}{\sqrt{T}{V}_{m}({V}_{m}+b)}$

6.2-2

where a and b are component parameters, and the mixing rules for the mixture are as shown in Equations <link rid="eqn5">6.2</link>-3, <link rid="eqn5">6.2</link>-4, and <link rid="eqn5">6.2</link>-5:

$\:a=\sum\:_{i}\sum\:_{j}{y}_{i}{y}_{j}{a}_{ij}$

6.2-3

$\:{a}_{ij}=\sqrt{{a}_{i}{a}_{j}}\left(1-{k}_{ij}\right)$

6.2-4

$\:b=\sum\:_{i}{y}_{i}{b}_{i}$

6.2-5

K_ij is the binary interaction parameter, typically fitted from experimental data.

6 Simulation Unit Optimization

6.1 H₂O₂ environmental detoxification

To address safety concerns and optimize product purity, we developed an efficient hydrogen peroxide residue treatment strategy. The reactor effluent contained up to 8.7 wt% unreacted H₂O₂, posing significant thermodynamic instability risks for subsequent distillation processes. Our solution involved implementing a "thermal-catalytic decomposition" system immediately downstream of the reactor. This dual approach combines: (1) a mild thermal decomposition unit maintained at 60–80°C for controlled H₂O₂ breakdown, and (2) a catalytic decomposition bed employing transition metal oxides (e.g., MnO₂/Al₂O₃) for accelerated decomposition at lower temperatures. The system achieved > 99% H₂O₂ elimination while preventing product degradation, as confirmed by iodometric titration.

For catalytic H₂O₂ decomposition, various catalysts can be employed to enhance efficiency, including catalase enzymes, metals/metal oxides, metal complexes, and heteropoly acids/salts^[46]. These catalysts significantly accelerate H₂O₂ decomposition while reducing thermal energy requirements, thereby lowering operational costs. The residual H₂O₂ concentration after treatment was reduced to < 50 ppm, well below the safety threshold for downstream processing.

6.2 Distillation Column Design

6.2.1 Tower Parameter Selection

During the distillation separation in the depropanizer (T1), 2-nitropropane (2-NP) forms a minimum-boiling azeotrope with water (Taze = 83.18°C, xH2O = 0.37, x2-NP = 0.63), making conventional distillation ineffective for complete separation. To address this challenge, we developed an innovative benzene-assisted azeotropic distillation process. By introducing benzene as a third component, we leveraged its binary azeotropic system with water (Taze = 56.90°C, xH2O = 0.423, xC6H6 = 0.577) to achieve preferential water removal^[47].

Under the optimized process conditions, the light-ends removal column (B6) separates the water-benzene azeotrope as the overhead product while discharging a bottom stream containing 2-nitropropane and crude acetoxime. Subsequent purification via the distillation column (T2) yields a final product with purity ≥ 99.99%. This configuration achieves highly efficient post-reaction purification of the mixed stream, with detailed component breakthrough data provided in Supplementary Table S2.

First, in the light component removal column (B6), the effects of the theoretical plate numbers (T1 and T2) and reflux ratio on the water content and product composition at the top and bottom of the column were investigated, as shown in Figs. 6-a ~ d.

Fig. 6

a.T1 Sensitivity Analysis of Tray Numbers, b.T1 Sensitivity Analysis of Reflux Ratio, c.T2 Sensitivity Analysis of Tray Numbers, d.T2 Sensitivity Analysis of Reflux Ratio

Numerical simulation and experimental analysis demonstrate that both the theoretical stage number (N) and reflux ratio (R) significantly influence the separation efficiency of the 2-nitropropane (2-NP)/water system. As shown in Figs. <link rid="fig15">6</link>-a and 6-b, the theoretical stage number exhibits a distinct operational window constraint. When N falls below 29 stages, insufficient column height leads to reduced liquid holdup, triggering flooding and preventing effective separation. Conversely, exceeding 40 stages results in a significant increase in column pressure drop and weep rate, consequently reducing separation efficiency. The optimal separation performance occurs within the range of 33 to 40 theoretical stages. At N = 33, the water content in the column bottom (reboiler) can be reduced below 1×10⁻⁵. Increasing N to 40 stages yields a marginal improvement in 2-NP recovery rate, rising slowly from 98.2% to 98.7%. Based on a comprehensive cost-benefit analysis weighing equipment investment against separation performance, 40 theoretical stages are selected as the optimum. Concurrent analysis of reflux ratio reveals that variations in R have minimal impact on the compositions of the two primary products in the column bottom. The water content in the bottom stream remains consistently below detectable levels across a range of R values. However, further increasing R substantially elevates reboiler energy consumption and utility costs. Economic evaluation identifies R = 4 as the optimal operating condition, achieving the target product specification (2-NP ≥ 99.9%) with 40 theoretical stages while minimizing energy consumption.

Sensitivity analysis further confirms the critical impact of theoretical stage number (N) on the separation efficiency for the 2-NP/acetone oxime system (Fig. 6-c). Across the investigated range (N = 20–60), both the 2-NP concentration in the distillate (X_top) and the acetone oxime content in the bottoms (X_bot) exhibit a monotonic increasing trend with increasing N, consistent with predictions from the Fenske equation ^{[48, 49]}. The system reaches an optimized equilibrium at N = 40: X_top stabilizes at 99.99% and X_bot is maintained at 99.8%. Increasing N further to 60 yields no appreciable improvement in X_top or X_bot, while increasing column height by 50% and capital investment by approximately 35%. Therefore, balancing separation performance with economic considerations identifies N = 40 as the optimal theoretical stage number.

Reflux ratio analysis (Fig. 6-d) indicates that variations in R above 1.0 exert minimal influence on X_top and X_bot. Further increases in R solely augment the reboiler heat duty. Fluid dynamic simulations reveal that at R < 1.0, insufficient component transfer units (NTU) cause upward migration of heavy components, significantly elevating the risk of flooding. Consequently, R = 2 is selected as the optimal operating condition. This ensures both the stringent separation specification (X_top > 99.99%) and operational stability, positioning the system within an energy-efficient operating regime while avoiding kinetic instabilities within the column.

6.2.2 Hydraulic performance check

Given the relatively low boiling points of the components in the reaction system, atmospheric pressure distillation was employed for process simulation in this study. Comparative analysis revealed that packed columns offer significant advantages over conventional tray columns in terms of operational flexibility, separation efficiency, and hydraulic performance. The packed column exhibits a wider operational range, with a theoretical pressure drop per meter of packing reduced to 0.3–0.5 kPa/m. Liquid holdup can be effectively controlled within 5%–8% (v/v), representing an approximate 40% reduction compared to traditional tray columns.

Specifically, this study utilized modified Pall rings (25 mm diameter) as the packing material. These rings feature systematically arranged rectangular windows (5 mm × 10 mm) cut into the side walls of conventional Raschig rings. This modification enhances the internal void space utilization by 35% and increases the effective specific surface area to 220 m²/m³. Experimental data demonstrate that, compared to standard Raschig rings, this modified structure achieves a 42 ± 3% reduction in vapor-phase pressure drop, a 28% improvement in liquid film distribution uniformity, a 50%-55% increase in vapor capacity, and a 30%-35% enhancement in overall mass transfer efficiency. These superior performance parameters underscore its significant industrial application value in chemical separation processes ^[50].

Metal Pall rings are commercially available in four nominal diameters: 25 mm, 38 mm, 50 mm, and 76 mm, with specific parameters detailed in Table 1. Based on comparative evaluation, Pall rings with a nominal diameter (DN) of 50 mm were selected. Although possessing a lower specific surface area than the 25 mm and 38 mm variants, the DN 50 mm rings exhibit reduced specific surface area degradation over time. Furthermore, the significantly lower number of rings required per unit volume substantially reduces overall usage costs. Consequently, DN 50 mm Pall rings were employed as the structured packing material in both the Light component removal column and the distillation column within the simulation framework.

Table 1
Parameters of different specifications of Pall rings
Nominal diameter D_N/mm	outer diameter * height * thickness(mm)	Specific surface area α/m²/m³	Porosityε/%	Number n/m^− 3	Bulk density ρ_p/kg/m³	Dry packing factor Φ/m^− 1
25	25250.5	219	96.1	51940	393	255
38	38380.6	146	96.0	15180	318	165
50	50500.8	109	95.9	6500	314	124
76	76761.2	71	95.0	1830	308	80

Based on the annual production capacity of 13,000 tons of 2-nitropropane and 760 tons of acetone oxime as a byproduct, the process design specifies a feed rate of 56,961 kg/h for the depropanizer column (T1) and 1,737 kg/h for the rectification column (T2). Pall ring structured packing was selected as the internal filling material for both columns due to its superior hydrodynamic performance, high mass transfer efficiency, and proven stability under industrial-scale operating conditions.

The gas-liquid distribution patterns within the depropanizer (T1) and rectification column (T2) are systematically quantified in Table S3 and Table S4, respectively. These tables detail critical hydrodynamic parameters, including liquid holdup distribution, vapor-phase velocity profiles, and interfacial mass transfer coefficients across distinct column segments. The data validate the optimized fluid dynamics achieved by the Pall ring packing, which minimizes flow maldistribution and ensures compliance with stringent product purity specifications (≥ 99.99 wt% for 2-nitropropane, ≥ 99.8 wt% for acetone oxime).

For the light-ends removal column (T1):

Top section:

Vapor phase: mass flow rate = 301,316.836 kg/h, density = 2.6876 kg/m³

Liquid phase: mass flow rate = 246,092.694 kg/h, density = 816.0184 kg/m³, viscosity = 0.3245 cP

Bottom section:

Vapor phase: mass flow rate = 315,727.299 kg/h, density = 2.7679 kg/m³

Liquid phase: mass flow rate = 317,464.314 kg/h, density = 878.9648 kg/m³, viscosity = 0.2879 cP

For the rectification column (T2):

Top section:

Vapor phase: mass flow rate = 16,417.2616 kg/h, density = 2.7960 kg/m³

Liquid phase: mass flow rate = 14,775.3259 kg/h, density = 874.5254 kg/m³, viscosity = 0.2879 cP

Bottom section:

Vapor phase: mass flow rate = 10,326.8417 kg/h, density = 2.1984 kg/m³

Liquid phase: mass flow rate = 10,421.9214 kg/h, density = 986.6545 kg/m³, viscosity = 0.2974 cP

Simulation results indicate that both the light-ends removal column and the stripping section of the rectification column exhibit significantly higher thermal loads. Therefore, the stripping section parameters were selected as the basis for column diameter calculations to ensure adequate capacity under the most demanding operating conditions.

Calculate the flooding gas velocity using the Bain-Hougen correlation ^[51](7.2-1)

$\:\text{l}\text{g}\left(\frac{{u}_{F}^{2}}{g}*\frac{\alpha\:}{{\epsilon\:}^{3}}*\frac{{\gamma\:}_{g}}{{\gamma\:}_{L}}{{\mu\:}_{L}}^{2}\right)=A-1.75{（\frac{L}{G}）}^{\frac{1}{4}}{\left(\frac{{\gamma\:}_{g}}{{\gamma\:}_{g}}\right)}^{\frac{1}{8}}$

7.2-1

In the formula: u_F——flooding velocity, m/s

G——Gravitational acceleration, m/s²

α/ε³——Dry packing factor, m^− 1

γ_G、γ_L——Gas -phase density、Liquid -phase density, kg/m³

µ_L——Liquid -phase viscosity, cP, 1cP = 10^− 3Pa·s

L、G——Gas-phase mass flow rate、Liquid-phase mass flow rate, kg/h

A——Constant, The constant of the Pall ring is 0.1

ε——Void fraction, m³/m³

The flooding velocity was calculated as 2.25 m/s for the light-ends removal column (T1) and 2.94 m/s for the rectification column (T2). Since the operating superficial vapor velocity in packed columns must remain below the flooding point, particularly for non-foaming systems, it is recommended to maintain the working velocity at ≤ 80% of the flooding velocity for stable operation. In this study, a conservative design velocity of 70% flooding velocity was implemented, allowing adequate safety margin against hydrodynamic instabilities while ensuring efficient mass transfer. The column diameter was subsequently estimated based on this criterion using Eq. (7.2-2) in accordance with standard process engineering guidelines.

$\:{D}^{{\prime\:}}=\sqrt{\frac{{V}_{S}}{0.785u}}$

7.2-2

In the formula: Vs——Gas-phase volumetric flow rate, m³/s

D’——Preliminary estimation of tower diameter, m;

u——superficial velocity, m/s

Optimized Design and Hydrodynamic Verification of Light-Ends Removal Column (T1) and Distillation Column (T2) with Solvent Recovery Strategy.Based on rigorous process calculations and hydrodynamic verification, this study optimized the key parameters of the light-ends removal column (T1) and the distillation column (T2).

For T1, the theoretically calculated column diameter (D1’) was determined to be 5.0623 m, which was rounded up to an engineering standard of D1 = 5.2 m. Further verification indicated an actual operating superficial gas velocity (u1) of 1.49 m/s, corresponding to 72.3% of the flooding velocity—well within the safety limit of 80% as per engineering design standards. Considering industrial best practices, where the height-to-diameter ratio (H/D) for Pall-ring packed columns should be maintained between 5–10, this study adopted H/D = 7, resulting in a final column height (H1) of 36.4 m. This optimal design ensures stable and efficient operation while achieving the desired separation efficiency.

For T2, the initial calculation yielded a column diameter (D2’) of 0.8987 m. However, when rounded up to the nearest standard diameter (D2 = 0.9 m), the resulting superficial gas velocity (u2 = 2.05 m/s) reached 69.7% of the flooding velocity, which, though below the 80% safety threshold, exhibited a significant risk of flooding upon hydrodynamic assessment. Therefore, the column diameter was further optimized to D2 = 1.2 m, reducing the gas velocity to u2 = 1.15 m/s (46.9% of flooding velocity). The corresponding column height was set at 8.4 m (H/D = 7), ensuring stable operation within a safe hydrodynamic range while meeting separation requirements.

After separation in T1, the cyclohexane content in the overhead light components was as high as 96.18 wt%. Given that cyclohexane primarily serves as an azeotropic agent for water removal in this process, direct discharge would lead to substantial solvent loss (annual losses estimated at hundreds of tons at industrial scale) and significantly increase wastewater treatment costs. Thermodynamic analysis demonstrated that integrating a three-phase flash separator downstream of T1 could recover cyclohexane at ≥ 99.7 wt% purity from the middle phase. The recovered stream, after heat exchange to ambient conditions, can be recycled back into the pre-mixing unit upstream of T1, achieving a solvent recovery efficiency exceeding 85%. This optimization not only reduces raw material consumption but also alleviates wastewater treatment burdens, offering notable economic and environmental benefits. This approach presents a technologically viable and sustainable strategy for closed-loop solvent recycling in azeotropic distillation processes, aligning with green chemistry principles.

7 Conclusion

This study successfully synthesized hierarchical TS-1@Ti-MWW-OH composites through epitaxial growth and dual-base modification, demonstrating exceptional catalytic performance in 2-nitropropane synthesis via acetone ammoxidation-oxidation. The catalyst achieved 98.68% acetone conversion and 93.41% selectivity toward 2-nitropropane, outperforming pristine TS-1 and other composites (TS-1@Ti-MOR, TS-1@ZSM-5). This superiority stems from synergistic structural stabilization and pore engineering: the Ti-MWW overlayer reinforced framework integrity and suppressed Ti⁴⁺ leaching during alkaline treatment, while controlled etching generated mesopores (2–10 nm) that enhanced mass transport while preserving shape-selective micropores (0.55 nm), collectively maximizing active site accessibility. The relay catalysis mechanism—where Ti⁴⁺ on Ti-MWW converts acetone to acetone oxime, and Ti⁴⁺ on TS-1 oxidizes oxime to 2-nitropropane—enabled a high yield of 92.2%. Downstream process simulation (Aspen Plus V14) confirmed industrial-scale production of 13,000 t/year 2-nitropropane (≥ 99.99 wt%) with 760 t/year acetone oxime (≥ 99.8 wt%), supported by robust column design (T1 diameter: 1.05 m; T2 diameter: 1.35 m) and stable operation (≥ 8,000 h). This work establishes an epitaxial growth strategy for designing dual-functional catalysts and provides a scalable process framework for sustainable nitroalkane production. Future studies should explore extending this approach to other selective oxidation reactions.

SEM

Scanning electron microscopy

XRD

X-ray diffraction

TEM

Transmission Electron Microscope

UV-vis

Ultraviolet-visible spectrophotometry

FT-IR

Fourier Transform Infrared Microspectrometer

BET

Specific surface area testing method

2-NP: 2-nitropropane

NRTL

The NRTL equation (non-random two-liquid), proposed by Renon and Prausnitz in 1968, is a non-random two-liquid model used to describe the thermodynamic properties of liquid mixtures.

UNIFAQ

It is a thermodynamic model based on the group contribution method, used to predict the activity coefficients of components in liquid mixtures. Its core principle involves breaking down molecules into functional groups and calculating activity coefficients by summing the interaction parameters between these groups, significantly reducing reliance on experimental data.

Declarations

Ethics and Consent to Participate

not applicable.

Consent for Publication:

not applicable.

Competing Interest:

not applicable.

Author Contribution

Jie Liu and Qingyan Chu wrote the main manuscript text.Guangliang Wang and Xiaoyang Zhang drew part of the diagram.Xiaowei Feng and Tengfei Wang organized the experimental data of the article. Tong Li Collected some literature。Qingyan Chu and Ping Wang Overall regulatory article writing and data compilation.

Guangliang Wang and Xiaoyang Zhang drew part of the diagram.

Xiaowei Feng and Tengfei Wang organized the experimental data of the article. Tong Li Collected some literature。

Qingyan Chu and Ping Wang Overall regulatory article writing and data compilation.

Funding:

This research did receive funding.

Ping Wang received funding from Natural Science Foundation of Shandong Province; Grant ID ZR202112010340.

Data Availability

The authors declare that the data supporting the findings of this study are available within the ‎paper.

Acknowledgements:

not applicable.

Electronic Supplementary Material

Below is the link to the electronic supplementary material

Supplementary Material 1

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Figure Title and Legend

Fig. 1

Schematic diagram of SEM/TEM with different catalysts

a.TS-1, b.TS-1-OH, c.TS-1-NaOH, d.TS-1@ZSM-5,

e.TS-1@Ti-MWW, f.TS-1@Ti-MOR, g.TS-1, h.TS-1-OH

Fig. 1

displays the distinct microstructures of six catalysts observed under SEM. It was found that organic base treatment induced etching on the surface of TS-1 zeolite, leading to alterations in its morphology and structure. In contrast, treatment with conventional inorganic NaOH completely disrupted the zeolite framework. Concurrently, the successful synthesis of the composite zeolite was confirmed, providing a morphological basis for subsequent catalytic reactions. According to TEM results, the untreated TS-1 catalyst exhibited a typical regular spherical morphology with smooth surfaces and a dense internal structure. After alkaline treatment, the TS-1 particles became irregular in shape and showed lighter contrast in TEM images, indicating a certain degree of structural loosening and increased porosity within the particles.

Figure 2 a.Schematic diagram of XRD with different catalysts, b.Schematic diagram of UV-vis with different catalysts, c.Schematic diagram of FT-IR with different catalysts

Fig. 2

a shows the characterization of TS-1, Ti-MWW, and the composite catalyst TS-1@Ti-MWW by X-ray diffraction (XRD). The pure TS-1 catalyst exhibits distinct characteristic diffraction peaks corresponding to the MFI topology structure. The XRD pattern of the composite catalyst TS-1@Ti-MWW confirms that the introduction of Ti-MWW did not disrupt the MFI framework structure of TS-1. After alkaline treatment, the XRD results of the TS-1@Ti-MWW catalyst indicate that the treatment enhances the catalytic activity mainly through surface modification and pore structure optimization.

Fig. 2

b shows that all four catalysts exhibit a distinct absorption peak at 210 nm, which is attributed to tetrahedrally coordinated Ti⁴⁺ species within the zeolite framework. After alkaline treatment, the intensity of the absorption peak at 210 nm shows minimal change, indicating that the content of framework Ti⁴⁺ species remains largely stable.

Fig. 2

c demonstrates that the incorporation of Ti-MWW and its role in enhancing the stability of framework titanium provide an important pathway for structural optimization of the catalyst. As the active centers for catalytic reactions, the improved stability of the framework titanium (Si–O–Ti) species may significantly enhance catalyst performance, while simultaneously mitigating the negative structural impacts induced by alkaline treatment.

Figure 3 a.The effect of a single catalyst on the reaction, b.The effect of compound catalysts on the reaction, c.The effect of alkali treatment catalyst on reaction, d.The effect of temperature on the reaction, e.The effect of feeding ratio on the reaction

Fig. 3

presents the catalytic performance of the single catalyst, composite catalyst, and the alkaline-treated composite zeolite. The effects of reaction temperature and feed ratio on the catalytic performance of TS-1@Ti-MWW-OH in the ammoxidation of acetone were also investigated.

Figure 4 Hypothetical Reaction Mechanism for the Ammonoxidation of Acetone to 2-Nitropropane

Fig. 4

shows the possible reaction pathway for the synthesis of 2-nitropropane via the ammoxidation of acetone. The process typically involves two key steps: (1) ammoximation of acetone to form acetone oxime, and (2) subsequent deep oxidation of acetone oxime to 2-nitropropane.

Figure 5 Process Flow Diagram for the Ammonoxidation of Acetone to 2-Nitropropane

Fig. 5

presents the Aspen-based process flow diagram for the synthesis of 2-nitropropane via acetone ammoxidation. The simulation encompasses the feedstock feeding section, reaction section, and subsequent distillation section, achieving an annual production capacity of 13,000 tons of 2-nitropropane with a by-product output of 760 tons of acetone oxime.

Figure 6 a.T1 Sensitivity Analysis of Tray Numbers, b.T1 Sensitivity Analysis of Reflux Ratio, c.T2 Sensitivity Analysis of Tray Numbers, d.T2 Sensitivity Analysis of Reflux Ratio

Fig. 6

presents a sensitivity analysis of the reflux ratio and the number of stages for both the light-ends removal column and the rectification column. The optimized parameters were determined as 40 stages with a reflux ratio of 4 for the light-ends removal column, and 40 stages with a reflux ratio of 2 for the rectification column.

Molecular Sieve Compositing Strategy Enabling Tandem Ammoxidation-Oxidation to 2-Nitropropane: Catalyst Design and Industrial-Scale Separation

Jie Liu ¹, Qingyan Chu ^1, *, Guangliang Wang, Xiaoyang Zhang, Xiaowei Feng, Tengfei Wang, Tong Li, Ping Wang **

(School of Chemistry and Chemical Engineering, Shandong University of Technology, Zibo, 255049, China)

* Corresponding author: Qingyan Chu, Tel: +86-18560253639(Chu)

** Corresponding author: Ping Wang, Tel: +86-13561652707(Wang)