Manganese and Cobalt Doped Hierarchical Mesoporous Halloysite-Based Catalysts for Selective Oxidation of p- Xylene to Terephthalic Acid

: Bimetallic MnCo catalyst, supported on the mesoporous hierarchical MCM-41/halloysite nanotube composite, was synthesized for the first time and proved its efficacy in the selective oxidation of p- xylene to terephthalic acid under conditions of the AMOCO process. Quantitative yields of terephthalic acid were achieved within 3 h at 200 – 250 °C, 20 atm. of O 2 and at a substrate to the Mn + Co ratio of 4 – 4.5 times higher than for traditional homogeneous system. The influence of temperature, oxygen, pressure and KBr addition on the catalyst activity was investigated, and the mechanism for the oxidation of p -toluic acid to terephthalic acid, excluding undesirable 4-carboxybenzaldehyde, was proposed. influence of the oxygen pressure and KBr presence on the effectiveness of the p- xylene oxidation 1 . E.I.; validation, E.K. and A.M.; formal analysis, E.I.; investigation, A.Z. and A.G.; resources, E.I.; data curation, A.Z.; writing — original draft preparation, A.Z. and A.G.; writing — review and editing, A.M. and E.K.; visualization, A.G. and A.Z.; supervision, V.V.; project administration, V.V. and A.M.; funding acquisition, E.I.

In spite of the obvious advantage of the AMOCO process, such as quantitative yield of high purity terephthalic acid, acetic acid medium with bromides cause corrosion that makes it necessary to use expensive titanium reactors [11][12][13]. It challenges the development of new environmentally friendly, safer and less corrosive reaction media, promoters and additives [7]. Catalyst heterogenization is considered as one of the possible ways for improving the AMOCO process Thus, it was earlier demonstrated, that the use of CoO and Co3O4 as catalysts together in the presence of MnO, NiO or CeO co-catalysts and p-toluic acid as a promotor allowed for oxidation without KBr [14]. Nonetheless, this system appeared to be much less effective in comparison with the conventional homogeneous system, including Mn(OAc)2, Co(OAc)2 and KBr, and the yield of terephthalic acid did not exceed 65%-70% within 8 h [14].
High yields of terephthalic acid (99% within 2 h) were obtained in the presence of polynuclear μ3-oxo-bound complexes of Co and Mn, encapsulated in the cavities of Y zeolite, under the conditions similar to AMOCO process (KBr, 200 °C, 610 atm of the air) [16]. The said catalyst appeared as highly resistant to the metal leaching due to close sizes of polyoxo metal clusters and zeolite cavities [8,16].
In this connection, halloysite-based materials appeared to be promising as heterogeneous catalysts for p-xylene oxidation under the typical conditions. Halloysite is a natural clay with the rolled tubular structure, appearing as a multiwall nanotube (halloysite nanotube, HNT) with a length of 0.5-1.5 mm, an outer diameter of 50-60 nm and an inner cavity diameter of 10-15 nm ( Figure 1) [17,18]. Halloysite clays were successfully applied as carriers for the tubular Ru nanocatalysts, revealing high activity in the hydrogenations of aromatic compounds and phenols [19][20][21][22][23]. Grafting of the ordered mesoporous materials, such as MCM-41 or SBA-15, onto halloysite template allows us to obtain new hierarchical systems with stronger mechanical properties and surface area up to 650 m 2 /g ( Figure 2) [21,24,25]. La-doped MCM-41/HNT composite revealed high efficacy as a sulfur-reducing additive for FCC (fluid catalytic cracking) catalyst, resulting in decrease of sulfur content by 25% and in the yield of gasoline fraction of about 45% [24,26,27]. Modified with CaO and MgO, MCM-41/HNT and SBA-15/HNT composites demonstrated high activity in the cracking of sulfones, formed after the oxidative desulfurization of diesel fraction, decreasing sulfur content from 450 up to 100 ppm [28]. The catalysts said were recycled several times without significant loss of activity, and with high resistance to metal leaching and structure maintenance under the reaction conditions [28]. In this work, we present for the first-time synthesis of new heterogeneous bimetallic MnCo catalyst, based on mesoporous hierarchical MCM-41/HNT composite that can be successfully applied for quantitative oxidation of p-xylene to terephthalic acid in the AMOCO process, proving its efficiency, that was 4-4.5 times higher, as compared with a traditional homogeneous system.

The Synthesis and Characterization of Hierarchical Mesoporous MCM-41/HNT Composite, Doped with Mn and Co
Bimetallic MnCo catalyst, based on MCM-41/HNT composite was synthesized by the wetness impregnation method. Herein Mn 2+ and Co 2+ were deposited from the water solution of Mn(OAc)2 and Co(OAc)2 in molar ratio of 1:10 (Scheme 2) [16]. Mn(OAc)2 and Co(OAc)2 tetrahydrates were chosen as sources for Mn 2+ and Co 2+ respectively to avoid the influence of other counter-anions on the adsorption and oxidation processes, and because of acetic acid, used as a solvent in the conventional oxidation process [2,8]. The material obtained was characterized by atomic emission spectroscopy with inductively coupled plasma (ICP-AES), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The physical and chemical properties are listed in Table 1. As seen from Table 1, weight content of Mn and Co in the sample reached 0.15% and 1.29% respectively, which was approximately three times less than corresponding theoretical values. Herein Co/Mn ratio appeared as 8.6:1, which was in accordance with the literature data [29].

Oxidation of p-xylene in the Presence of Hierarchical Mesoporous MCM-41/HNT Composite Doped with Mn and Co
The catalyst synthesized was tested in the liquid-phase oxidation of p-xylene and compared with traditional homogeneous system Mn(OAc)2/Co(OAc)2/KBr. It should be noted, that p-xylene oxidation is a multi-stage radical process (Scheme 3) [2,7,8,46,47]. Its kinetics is strongly affected by the reaction conditions, such as temperature, oxygen pressure, substrate concentration, substrate: Co:Mn ratios, KBr loading, etc. [2,7,8]. Herein KBr is required as a free radical source, and Mn(OAc)2 acts as promotor at the first stage of reaction-activation of the methyl group in the p-xylene molecule [2,10]. As a rule, p-xylene conversion to p-toluic acid proceeds fast, whereas oxidation thereof to terephthalic acid is a slow highly activated process [2,8]. The use of acetic acid as a solvent promotes dissolution of both KBr and p-xylene, the ion-radical oxidation process, and deposition of terephthalic acid due to its poor solubility. The TPA product is easily isolated from the reaction medium as white needle-like glittering crystals [2].
The best conditions for p-xylene oxidation, found out in academic studies and widely used in industry (AMOCO process) are as follows: temperature of 180-225 °C, oxygen pressure of 15-30 atm, p-xylene to acetic acid molar ratio of 0.08-0.16, and molar ratios Mn/Co and Co:Br being 1:8-10 and 3-4 respectively [2,7,8,[48][49][50]. We have used a temperature of 200 °C, oxygen pressure of 20 atm and molar ratios p-xylene/acetic acid and Mn/Co of 0.08 and 1:10 respectively, both in homogeneous and heterogeneous processes. Herein the reaction turnover frequency (TOF) was calculated as the amount of substrate reacted (νsubstr) per mole of metal (ν(Mn + Co)) per unit of time with account of each product yield and the number of oxygen atoms added, according to formula: where ω is the yield of the certain product, expressed in the unit fractions, N/I is a not identified product (presumably 4-hydroxymethylbenzoic acid) and t is the minimal reaction time, for which the reaction progress is measured. The results obtained are listed in Table 2. As one can see, both the homogeneous Mn(OAc)2/Co(OAc)2 system and heterogeneous Mn II 1Co II 10@MCM-41/Hall catalyst give a quantitative conversion and similar product distribution with a TPA yield of 94%-95% within 3 h ( Table 2). Similar results were obtained earlier for μ-oxo-bridged complexes of Mn and Co, encapsulated into the zeolite Y pores [16]. It should be noted, that near to the quantitative yield of terephthalic acid in the presence of heterogeneous hierarchical catalyst Mn II 1Co II 10@MCM-41/HNT was achieved at much higher substrate to the Mn + Co ratio, giving rise to the higher TOF value ( Table 2). It proves the efficacy of the new heterogeneous catalyst developed, comprising of the MCM-41/HNT composite, for the AMOCO process. On the other hand, due to the same loading of KBr for both homogeneous and heterogeneous systems, Co:Br in the latter appeared as high as 1:1, resulting in the increased concentration of Br· radicals and, as a consequence, the easier activation of methyl groups in p-xylene and p-toluic acid molecules.
Temperature, oxygen, pressure and the presence of KBr were found to be crucial factors for the effective p-xylene oxidation process, which was in accordance with the literature data [29]. As seen from Table 3, decrease in oxygen pressure down to 5 atm resulted in corresponding decrease in conversion to 88%-89%, and p-toluic acid appeared as the major reaction product with the yield of 50%-63% for both homogeneous and heterogeneous catalysts. Moreover, lower metal loading in heterogeneous system gave rise to a lower rate of further oxidation of p-toluic acid, characterized by the higher activation energy [8], resulting in the extremely low yield of terephthalic acid, about 0.5% (Table 3, Entry 2).
The removal of KBr from the reaction medium led to abrupt downfall in conversion for heterogeneous Mn II 1Co II 10@MCM-41/HNT catalyst even at high oxygen pressure, when all other conditions are equal (Table 3, Entry 3). p-Toluic aldehyde was found to be the major reaction product with the yield of 1% only. The results obtained seemed to be connected not only with the absence of the source of Br· radicals, initiating the oxidation stepwise process, but also with the extremely low concentration of Mn 3+ ions in the system, being also responsible for methyl group activation in the substrate. For the successful oxidation of p-xylene, Pd-containing systems were suggested [51][52][53] or compounds, such as N-hydroxyphthalimide, being able to generate stable free radicals [8,54,55]. In these cases, however, a very long time (12-48 h) is needed to obtain the yield of terephthalic acid, exceeding 70%. The influence of temperature on the rate of the p-xylene oxidation and product distribution in the presence of Mn II 1Co II 10@MCM-41/HNT heterogeneous catalyst is presented in Table 4. As one can see, the quantitative yield of terephthalic acid may be obtained in 5 h at 200 °C and in 3 h at 250 °C, and the TOF values being 401.7 and 424.8 h −1 respectively (Table 4, Entries 6 and 8-9). Hence, an increase in temperature from 200 to 250 °C results in a slight rise of the catalyst activity and, therefore, the oxidation rate only.
Vice versa, decrease in temperature down to 150 °C led to an abrupt downfall in activity, in accordance with literature data [29,50]. Herein the p-xylene conversion did not exceed 40% even after 5 h, with p-toluic aldehyde being obtained as the major reaction product (Table 4, Entry 3). As seen from Tables 2-4, the yield of 4-carboxybenzaldehyde in the presence of heterogeneous Mn II 1Co II 10@MCM-41/Hall catalyst (under 20 atm of O2) did not exceed 0.5%, and 4-hydroxymethylbenzoic acid being formed as the main by-product with the yield up to 5%. At low oxygen pressures the yield of 4-carboxybenzaldehyde increased and reached 12% for homogeneous Mn(OAc)2/Co(OAc)2 system and 4% for heterogeneous Mn II 1Co II 10@MCM-41/HNT catalyst (Table 3, Entries 1-2). It should be noted, that 4-carboxybenzaldehyde is the undesirable product of p-xylene oxidation: it is slowly oxidized to terephthalic acid and cocrystallizes with it at separation due to structural similarity [2,7,8].
One may assume, that, when Mn II 1Co II 10@MCM-41/Hall is used as the catalyst, the oxidation passed through the intermediate alcohol formation (Scheme 3, right). Due to the low metal content and/or specific microenvironment of active sites in the Mn II 1Co II 10@MCM-41/HNT catalyst, caused by the interaction of Mn (II) and Co (II) with halloysite and/or MCM-41 matrix, ArCH2OO· radical apparently interacted not with Co 2+ , resulting in aldehyde formation, but with each possible reactant molecule, resulting in the alcohol formation. Noticeably, the oxidation of p-xylene to p-toluic acid mainly proceeded through the formation of p-toluic aldehyde, and only traces of p-toluic alcohol were observed at low conversions of p-xylene. Hence, the "alcohol pathway" was presumably the characteristic of the oxidation p-toluic acid. Thus obtained 4-hydroxymethylbenzoic acid underwent further Co (II) catalyzed oxidation to terephthalic acid, initiated by O2 or Br· as radicals.
We think the explanation above is consistent with the influence of oxygen pressure and KBr observed, as well as with a high reaction rate even at very low metal loading, for the selective oxidation of p-xylene to terephthalic acid in the presence of heterogeneous Mn II 1Co II 10@MCM-41/HNT catalyst. Therefore, MCM-41/HNT composite, doped with Mn and Co, may be considered as a prospective catalyst for the AMOCO process, far superior to the original homogeneous Mn(OAc)2/Co(OAc)2 system. Unfortunately, metal leaching took place during the reaction. The quantity of leached metals depended on the reaction temperature and time: a higher temperature and longer reaction time resulted in less metal maintained in the catalyst (Table 5). Herein Co leached the first followed by Mn. When carrying out the reaction for the longest time (5 h), the concentrations of both Mn and Co dropped approximately by 13-15 times (Table 5, Entry 5). XPS analysis revealed no Mn or Co on the surface of the samples recycled. Nonetheless it should also be noted, that catalyst, based on MCM-41/HNT composite possesses very low packed density and, simultaneously, high apparent adsorption capacity, and, therefore, occupy all of the reaction volume (5 mL of AcOH vs. 150 mg of catalyst). To extract reaction products for HPLC analysis, up to 40 mL of DMSO (the best solvent for terephthalic and p-toluic acids) was required, that strictly reduced the possibility for the hot filtration and recycling tests. Mn and Co content in the catalyst recycled was measured just after filtration from DMSO solution. Therefore, the metal leaching could take place not only during the reaction, but also at washing by DMSO.
Hence one may conclude, that pores and cavities of MCM-41 and halloysite moieties act as microreactors for the oxidation of p-xylene under the reaction conditions. However these pores appear too wide and not able to effectively retain Mn and Co species inside the carrier at filtration and washing as compared with polynuclear μ3-oxo-bound complexes of Co and Mn, encapsulated in the cavities of Y zeolite [16]. This challenges for the further development and modification of MCM-41/HNT composites to provide more effective metal retention and leaching resistance under the severe reaction conditions, to make possible the repeated use of Mn II 1Co II 10@MCM-41/HNT catalyst in p-xylene oxidation.

Analyses and Instrumentations
Analysis by transmission electron microscopy (TEM) was carried out using LEO912 AB OMEGA (Carl Zeiss, Jena, Germany) and JEM-2100 JEOL microscopes (Jeol Ltd., Tokyo, Japan) with an electron tube voltage of 100 kV.
The isotherms of nitrogen adsorption/desorption were measured at 77 K on Micromeritics Gemini VII 2390 t instrument (Micromeritics Instrument Corp., Norcross, GA, USA). Before the measurements, the samples were degassed at 350 °C for 4 h. The specific surface area was calculated with the Brunauer-Emmett-Teller (BET) and Langmuir methods applied to the range of relative pressures P/P0 = 0.05-0.30. The pore volume and pore size distributions were determined from the adsorption branches of the isotherms based on the Barrett-Joyner-Halenda (BJH) model.
Weight content of manganese and cobalt in the catalyst was determined by means of atomic emission spectroscopy with inductively coupled plasma (ICP-AES) and X-ray fluorescent spectrometry (XFS). ICP-AES analysis was performed on the IRIS Intrepid II XPL instrument (Thermo Electron Corp., Waltham, MA, USA) in the radial observation modes at wavelengths of 257.6 and 259.4 nm for Mn and 228.6 and 237.9 nm for Co. XFS analysis was conducted on the ARL PERFORM'X spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with Rh anode and tube diameter of 3 mm in the etalon-free mode with the relative error of 5%.
X-ray photoelectron studies (XPS) were carried out on a Versa Probe II instrument (ULVAC-PHI Inc., Hagisono, Chigasaki, Kanagawa, Japan), equipped with a photo-electronic hemispherical analyzer with retarding potential OPX-150. X-ray radiation of the aluminum anode (Al Kα = 1486.6 eV) with a tube voltage of 12 kV, emission current of 20 mA and power of 50 W was used to excite photoelectrons. Photoelectron peaks were calibrated with respect to the carbon C 1s line with a binding energy of 284.8 eV.
Qualitative and quantitative analysis for the products of the p-xylene oxidation was carried out by means of high-performance liquid chromatography (HPLC) similar to the procedure, earlier described in literature [56] on the Agilent 1100 Series instrument (Agilent Technologies, Santa Clara, CA, USA) with a Zorbax SB-C18 column (5 μm, 2.1 mm × 150 mm) and UV detector. The eluent consisted of methanol (25%), acetonitrile (25%) and water (50%). For better peak resolution, phosphoric acid was added with concentration of 1 mL per 1 L of the eluent. The flow rate was 0.1 mL/min and the injection volume was 0.1 μL. Chromatograms were recorded at 210, 230, 254 and 280 nm simultaneously and analyzed on a computer using Agilent 1100 Software (Agilent Technologies, 2008, Santa Clara, CA, USA). Conversion of p-xylene and the yield of each oxidation product were calculated using calibrating curves with exponential approximation.

The Synthesis of Mn/Co-Containing Oxidation Catalyst Based on the MCM-41/HNT Composite
Deposition of Mn and Co on the MCM-41/HNT composite was performed according to the following procedure [16]. Of MCM-41/HNT composite 1000 mg and 20 mL of distilled water were placed into a 100 mL one-neck round-bottom flask, equipped with a reflux condenser and a magnetic stirrer. Then 20 mg of Mn(OAc)2 × 4H2O (0.08 mmol) and 200 mg of Co(OAc)2 × 4H2O (0.8 mmol) were placed into a chemical beaker and dissolved in 10 mL of distilled water at room temperature while stirring. Then the resulting solution of the pink-purple color was added dropwise to the suspension of MCM-41/HNT composite in water at room temperature while stirring, and additional 10 mL of distilled water was passed through the dropping funnel to wash the residues of Mn(OAc)2 and Co(OAc)2 solution. The reaction was carried out at 60 °C for 12 h. After that the material obtained was centrifuged, washed twice with ethanol for the better removal of water and then dried in the air. The catalyst obtained was isolated as a pale-pink powder, weighing 1065 mg (92%).

Protocol for the Catalytic Experiments
Oxidation of p-xylene in the presence of Mn/Co MCM-41/HNT catalyst was carried out according to the literature procedures [16,29]. Here 150 mg of the catalyst, 4.5 mg of KBr, 0.5 mL of p-xylene and 5 mL of acetic acid were placed in a titanium autoclave, equipped with a magnetic stirrer. The autoclave was sealed, filled with oxygen up to a pressure of 2 MPa and placed in an oven with a thermostat control. Then the oxidation reaction was conducted at 150, 200 or 250 °C for 1, 3 or 5 h. After reaction, the autoclave was cooled down to room temperature and depressurized. The reaction products were additionally diluted by DMSO as the best solvent for terephthalic acid and, after the catalyst sedimentation and filtration, analyzed by the HPLC method.
The catalyst activity (TOF = turnover frequency) was calculated as the amount of reacted substrate (νsubstr) per mole of metal (ν(Mn + Co)) per unit of time with account of the yield of each product and number the oxygen atoms added, according to the formula: where ω is the yield of the certain product, expressed in the unit fractions, N/I is a not identified product (presumably 4-hydroxymethylbenzoic acid) and t is the minimal reaction time, for which the reaction progress is measured.
Each experiment at the same conditions was carried out two or three times, with the results differing by no more than 5% from the corresponding average value. These average values were presented in Tables 2-4. The measurement error did not exceed 5%.

Conclusions
Heterogeneous bimetallic Mn/Co-containing catalyst, based on MCM-41/halloysite composite, have been synthesized for the first time and tested in p-xylene oxidation under the conditions of the AMOCO process. It was demonstrated, that in the presence of bimetallic heterogeneous catalyst Mn II 1Co II 10@MCM-41/HNT and KBr as a free radical source, the quantitative yield of terephthalic acid can be obtained in 3 h at temperature of above 200 °C, and oxygen pressure of 20 atm. The substrate to the Mn + Co ratio was 3.5-4 times higher than that for the traditional homogeneous Mn(OAc)2/Co(OAc)2 system, hence proving a high efficacy and superiority of the heterogeneous catalyst, based on the hierarchical material MCM-41/HNT. The influence of the oxygen pressure, temperature and KBr presence on the catalyst activity and product distribution was investigated. It was established that a decrease in oxygen pressure to 5 atm. resulted in the corresponding decrease of p-xylene conversion to 88%-89%, with p-toluic acid obtained as the major reaction product with the yield up to 63% within 3 h. Decrease in temperature from 200 to 150 °C led to the abrupt downfall in the reaction rate. Herein the conversion did not exceed 40% after 5 h and p-toluic aldehyde was the major reaction product. Vice versa, rise in temperature from 200 to 250 °C did not result in the significant increase of the reaction turnover frequency. The presence of KBr was found to be crucial for the effective process of the oxidation of p-xylene, whose conversion did not exceed 2%, when KBr was removed from the reaction medium.
It was found that in the presence of Mn II 1Co II 10@MCM-41/HNT the further oxidation of p-toluic acid to terephthalic acid mostly proceeded through the formation of 4-hydroxymethylbenzoic acid, thus eliminating the stage of undesirable 4-carboxybenzaldehyde. This pathway supposes the diminished role of the Co (II) in the oxidation of p-toluic acid and, therefore, allows the elevated substrate to catalyst ratios, but requires high oxygen pressures and Br· radicals as initiators.
Halloysite clay is a cheap and available in thousands tons aluminosilicate, which makes it a prospective nanomaterial for catalysts support. Despite the low resistant to metal leaching under the reaction and separation conditions the heterogeneous catalyst showed a phenomenally high activity in the oxidation of p-xylene to terephthalic acid under the conditions of AMOCO industrial process. This Mn II 1Co II 10@MCM-41/HNT catalyst based on new hierarchical support with halloysite aluminosilicate nanotubes could be easily scaled up after stability improvement.