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

Abstract

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 O₂ 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.

1. Introduction

Terephthalic acid (TPA) and its dimethyl ester are important monomers in thermoresistant and mechanically stable polymer production, such as polyethylene terephthalate (PET) and poly-paraphenylene terephthalamide (Kevlar) [1,2]. The world production of terephthalic acid has exceeded 5 Mt/yr. is still growing [1,2]. Currently up to 70% of terephthalic acid is produced by direct oxidation of p-xylene through the AMOCO/Mid-Century process being developed in the 1960s [1,2,3,4,5,6].

p-xylene in the AMOCO process is subjected to oxidation by molecular oxygen or air in the presence of homogeneous Mn and Co catalysts with KBr as a promotor and free-radical source in acetic acid medium under the severe conditions (170–220 °C, 15–30 atm of O₂ or 50–70 atm of air), giving terephthalic acid yield of 90–97% within 4–12 h (Scheme 1) [7,8,9,10].

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].

In the presence of MCM-41 doped with Fe and Cu, terephthalic acid underwent further oxidation to 2,5-dihydroxy-1,4-terephthalic and p-benzoquinone-2,5-dicarboxylic acids even under much milder conditions, than in the AMOCO process (H₂O₂ as an oxidant, 80 °C, AcOH, CH₃CN, 5 h) [15]. The highest selectivity to terephthalic acid did not exceed 45% at a p-xylene conversion of 10% in the neat acetonitrile. It was found out, that iron additive increased the selectivity to terephthalic acid, whereas copper addition, vice versa, favored the further oxidation of TPA to 2,5-dihydroxy-1,4-terephthalic and p-benzoquinone-2,5-dicarboxylic acids [15].

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 m²/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.

2. Results and Discussion

2.1. 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 Mn²⁺ and Co²⁺ 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 Mn²⁺ and Co²⁺ 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. Physical and chemical properties of the synthesized Mn^II₁Co^II₁₀@MCM-41/HNT catalyst.

Mn (wt. %)	Co (wt. %)	Surface Concentration by XPS, at. %							Mn Valency States at Mn 2p3/2 Line, at. %		Co Valency States at Co 2p3/2 Line, at. %
		Mn	Co	Si	Al	C	N	O	MnO (eV)	[MnO4] Bound (eV)	CoO (eV)	[CoO6] Bound (eV)
0.15	1.29	<0.1	0.7	25.9	2.5	5.8	0.6	64.2	79.8 (642.5)	20.2 (646.9)	15.3 (780.0)	84.7 (782.5)

.

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].

According to XPS data (Table 1, Figure 3), both Mn and Co were presented in the forms of simple oxides MnO [30,31,32] and CoO [33,34,35] and bound complexes [MnO₄] [36,37] and [CoO₆] [32,33,34,38,39,40,41], arising from initial Mn(OAc)₂ and Co(OAc)₂ tetrahydrates as well as from the aluminosilicate. Herein MnO form strongly predominated over [MnO₄] bound form for Mn, whereas for Co [CoO₆] form, vice versa, predominated over CoO, that might be due to the much stronger oxygen affinity for Co in comparison with Mn [42,43]. Taking in account the presence of nitrogen in the XPS spectra (Table 1), carbon in the sample, found out in –CH₂CH₂–, –CH₂CH₂N– and H₃CC(=O)– forms [44,45], may be related not only to adsorbed acetate anions, but also to cetyl trimethyl ammonium bromide template, partly remained in the MCM-41/HNT composite after calcination.

As TEM analysis showed (Figure 4), deposition of manganese and cobalt acetates did not destruct the MCM-41/HNT matrix. The MCM-41 framework observed consisted of regular hexagonal pores of 2.9 nm in diameter (Figure 4B) and inner located halloysite nanotubes with the inner diameter of 12–15 nm and outer diameter of 31–35 nm, and interplanar space of 3.2 nm (Figure 4A).

2.2. 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 (ν_substr) 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:

$${\mathrm{TOF}(\mathrm{O}}_2)=\frac{{\nu }_{substr}*({\mathsf{\omega }}_{pTAld}*1 +{ \mathsf{\omega }}_{\mathrm{pTAc}}*2+{\mathsf{\omega }}_{4-\mathrm{CBA}}*3 +{ \mathsf{\omega }}_{\mathrm{TPA}}*4+{ \mathsf{\omega }}_{\mathrm{N}/\mathrm{I}}*3)}{{\nu }_{(Mn+Co)}*t},$$

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. Comparison of homogeneous system Mn(OAc)₂/Co(OAc)₂ and heterogeneous Mn^II₁Co^II₁₀@MCM-41/HNT catalyst in the liquid-phase oxidation of p-xylene ¹.

Entry	Catalyst	Conv., %	Product Yields, % mol.					TOF, h−1
			pTald	pTac	4-HMBA	4-CBA	TPA
1	Mn(OAc)2/Co(OAc)2	98.0	0.15	0.95	1.2	0.5	95.2	37.5
2	MnII1CoII10@MCM-41/HNT	99.0	0.2	1.0	4.0	-	93.8	142.5

¹ Reaction conditions are: 0.5 mL (4.055 mmol) of p-xylene, 5 mL of AcOH, 4.5 mg (0.038 mmol) of KBr, 200 °C, 20 atm of O₂, 3 h–for both homogeneous and heterogeneous processes; 32 mg (0.127 mmol) of Co(OAc)₂ and 3.1 mg (0.13 mmol) of Mn(OAc)₂ for homogeneous system; 150 mg of bimetallic catalyst for the heterogeneous system.

.

As one can see, both the homogeneous Mn(OAc)₂/Co(OAc)₂ system and heterogeneous Mn^II₁Co^II₁₀@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^II₁Co^II₁₀@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. The influence of the oxygen pressure and KBr presence on the effectiveness of the p-xylene oxidation ¹.

Entry	Catalyst	P(O2), atm	KBr	Conv., %	Product Yields, % mol.						TOF, h−1
					pTald	pTac	TPAld 2	4-HMBA	4-CBA	TPA
1	Mn(OAc)2/Co(OAc)2	5	Yes	89.1	11.7	49.8	x	12.6	11.8	3.2	19.0
2	MnII1CoII10@MCM-41/HNT	5	Yes	87.9	14.1	62.8	0.25	6.2	4.1	0.4	63.2
3	MnII1CoII10@MCM-41/HNT	20	No	2.2	1.1	0.4	-	0.1	0.1	0.5	1.6

¹ Reaction conditions are: 0.5 mL (4.055 mmol) of p-xylene, 5 mL of AcOH, 4.5 mg (0.038 mmol) of KBr, 200 °C, 3 h—for both homogeneous and heterogeneous processes; 150 mg of bimetallic catalyst for heterogeneous system; 32 mg (0.127 mmol) of Co(OAc)₂ and 3.1 mg (0.13 mmol) of Mn(OAc)₂ for homogeneous system. ² TPAld is terephthalic aldehyde.

, 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^II₁Co^II₁₀@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 Mn³⁺ 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^II₁Co^II₁₀@MCM-41/HNT heterogeneous catalyst is presented in

Table 4. The influence of the temperature on the effectiveness of the p-xylene oxidation in the presence of Mn^II₁Co^II₁₀@MCM-41/HNT catalyst ¹.

Entry.	T, °C	t, h	Conv., %	Product Yields, % mol.					TOF, h−1
				pTald	pTac	4-HMBA	4-CBA	TPA
1	150	5	36.8	28.8	5.2	1.9	0.1	0.8	15.7
2	150	3	25.9	21.1	2.5	1.8	-	0.5
3	150	1	10.2	8.0	0.5	1.5	-	0.2
4	200	5	99.8	0.1	0.2	2.3	-	97.2	401.7
5	200	3	99.0	0.2	1.0	4.0	-	93.8
6	200	1	98.8	1.9	11.7	0.2	-	85.0
7	250	5	100	-	-	0.5	-	99.5	424.8
8	250	3	99.9	-	0.5	1.8	-	97.6
9	250	1	99.5	-	4.5	2.1	-	92.9

¹ Reaction conditions are: 150 mg of bimetallic MnCo heterogeneous MCM-41/HNT-based catalyst, 0.5 mL (4.055 mmol) of p-xylene, 5 mL of AcOH, 4.5 mg (0.038 mmol) of KBr and 20 atm. of O₂.

. 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 Table 2, Table 3 and Table 4, the yield of 4-carboxybenzaldehyde in the presence of heterogeneous Mn^II₁Co^II₁₀@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^II₁Co^II₁₀@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^II₁Co^II₁₀@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^II₁Co^II₁₀@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 Co²⁺, 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^II₁Co^II₁₀@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. Comparative Mn and Co weight content in the recycled samples of Mn^II₁Co^II₁₀@MCM-41/HNT catalyst depending on reaction conditions.

Entry	Mn (wt. %)	Co (wt. %)	Reaction Conditions
1	0.15	1.29	Initial catalyst
2	0.15	0.22	150 °C, 20 atm. of O2, 3 h
3	0.03	0.13	200 °C, 20 atm. of O2, 3 h
4	0.04	0.09	250 °C, 20 atm. of O2, 3 h
5	0.01	0.10	200 °C, 20 atm. of O2, 5 h

). 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.

We suppose, the metal leaching observed occurs due to dissolution of the adsorbed Mn and Co species by acetic and hydrobromic acids. The last one is resulted from the interaction of Br· radicals with substrate or intermediate compounds, bearing H-atoms at side carbons (Scheme 3). 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^II₁Co^II₁₀@MCM-41/HNT catalyst in p-xylene oxidation.

3. Experimental

3.1. Chemicals

The following substances were used as substrates and reference compounds: p-xylene 4-H₃CC₆H₄CH₃ (pX; Reachim, Purum, Moscow, Russia); p-toluic aldehyde 4-H₃CC₆H₄C(=O)H (pTAld) (Aldrich, 97%; Steinheim, Germany); p-toluic acid 4-H₃CC₆H₄C(=O)OH (pTAc; Aldrich, 98%; Steinheim, Germany); 4-carboxybenzaldehyde 4-HO(O=)CC₆H₄C(=O)H (4-CBA; Aldrich, 97%; Steinheim, Germany) and terephthalic acid p-HO(O=)CC₆H₄C(=O)OH (TPA; Acros Organics, 99+%; Geel, Belgium).

For the synthesis of Mn/Co oxidation catalyst based on MCM-41/HNT composite the following reagents were used: manganese (II) acetate tetrahydrate (CH₃COO)₂Mn × 4H₂O (Aldrich, ≥99%; Steinheim, Germany), cobaltous (II) acetate tetrahydrate CH₃COO)₂Co × 4H₂O (Aldrich, ≥98%; Steinheim, Germany) and MCM-41/HNT composite, earlier prepared according to literature procedure [24].

Acetic acid CH₃COOH (Chimmed, Chemical Grade; Moscow, Russia), dimethyl sulfoxide (DMSO) (H₃C)₂SO (Ruschim, Imp; Moscow, Russia) and potassium bromide KBr (Chimmed, Chemical Grade; Moscow, Russia) were used as solvent and additive respectively in the procedure of catalytic experiments.

Double-distilled water, methanol CH₃OH (J.T. Baker, HPLC grade; Gliwice, Poland) and acetonitrile CH₃CN (J.T. Baker, HPLC grade; Gliwice, Poland) were used as solvents and phosphoric acid H₃PO₄ (Component-Reactive, Chemical Grade; Moscow, Russia) was used as a pH regulating additive while conducting HPLC analysis.

3.2. 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.

3.3. 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%).

• ω_Mn (XFS): 0.15%.
• ω_Co (XFS): 1.29%.

XPS (eV): 74.5 (Al₂O₃, Al 2p, 2.5%); 103.5 (SiO₂, Si 2p, 25.9%); 119.5 (Al₂O₃, Al 2s); 154.5 (SiO₂, Si 2s); 284.9 (–CH₂CH₂–, C 1s, 1.27%), 285.9 (–CH₂CH₂N, H₃CC(=O)–, C 1s, 3.15%), (H₃CC(=O)–, –CH₂N⁺, C 1s, 0.82%), 290.1 (H₃CC(=O)–, C 1s sat, 0.55%); 400.5 (–CH₂CH₂N, [NR₄]⁺, N 1s, 0.6%); 530.6 (MnO_x, CoO_x, O 1s, 1.7%), 532.1 (CH₃C(=O)O–, (Al₂O₃)_x*(SiO₂)_y, O 1s, 14.8%), 533.1 (SiO₂, O 1s, 43.0%), 534.6 (O…H₂O, O 1s, 4.6%); 642.5 (MnO, Mn 2p_3/2, 0.08%), 646.9 ([MnO₄] bound, Mn 2p_3/2, 0.02%), 654.0 (MnO, Mn 2p_1/2), ([MnO₄] bound, Mn 2p_1/2); 686.5 (O–Si–F, F 1s, 0.2%); 780.0 (CoO, Co 2p_3/2, 0.05%), 782.5 ([CoO₆] bound, Co 2p_3/2, 0.28%), 785.2 (CoO, [CoO₆] bound, Co 2p_3/2 sat, 0.14%), 788.2 (CoO, [CoO₆] bound, Co 2p_3/2 sat, 0.17%), 791.6 (Co 2p_3/2 sat, 0.06%), 794.6 (CoO, Co 2p_1/2), 798.4 ([CoO₆] bound, Co 2p_1/2), 802.4 (CoO, [CoO₆] bound, Co 2p_1/2 sat), 805.1 (CoO, [CoO₆] bound, Co 2p_1/2 sat), 808.9 (Co 2p_1/2 sat, 0.06%).

3.4. 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 (ν_substr) 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:

$${\mathrm{TOF}(\mathrm{O}}_2)=\frac{{\nu }_{substr}*({\mathsf{\omega }}_{pTAld}*1 +{ \mathsf{\omega }}_{\mathrm{pTAc}}*2+{\mathsf{\omega }}_{4-\mathrm{CBA}}*3 +{ \mathsf{\omega }}_{\mathrm{TPA}}*4+{ \mathsf{\omega }}_{\mathrm{N}/\mathrm{I}}*3)}{{\nu }_{(Mn+Co)}*t},$$

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 Table 2, Table 3 and Table 4. The measurement error did not exceed 5%.

4. 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^II₁Co^II₁₀@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^II₁Co^II₁₀@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^II₁Co^II₁₀@MCM-41/HNT catalyst based on new hierarchical support with halloysite aluminosilicate nanotubes could be easily scaled up after stability improvement.