Catalytic roles of Mo-based sites on MoS2 for ethanolysis of enzymatic hydrolysis lignin into aromatic monomers

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Mo-based catalysts have been successfully employed in Kraft lignin (KL) depolymerization.For instance, Ma et al. [12] achieved the complete depolymerization of KL over a MoC 1− x /activated carbon catalyst in ethanol at 280 • C for 6 h, forming C8-C10 esters, C6 alcohols, arenes, monophenols and benzyl alcohols as main products [12].Afterwards, similar products were also obtained from the depolymerization of KL with Mo/Al 2 O 3 , Mo 2 N/Al 2 O 3 and MoO 3 as catalysts, and Mo(OC 2 H 5 ) 5 was proposed as the active species for KL solvolysis reaction over varied Mo-based catalysts [13].High yield of aromatic monomers, up to 575 mg/g KL, was obtained when using MoC 1− x /CuMgAlO x as a catalyst at 330 • C [14].The depolymerization of EHL was focused recently.Mai et al. [15] and Bai et al. [9] depolymerized EHL into alkylphenols, such as isopropylphenols, butylphenols and pentylphenols, which are the precursors for synthesis of surfactant and pharmaceutical chemicals.[16,17] However, both the WO 3 / γ-Al 2 O 3 catalyst used by Mai et al. [15] and NiMo/γ-Al 2 O 3 catalyst used by Bai et al. [9] were significantly deactivated after 1 time use.Sang et al. [10,11] depolymerized EHL with unsupported Ni catalysts prepared via nickel formate decomposition, achieving complete EHL liquefaction with 20-30 wt% monomer yields, where H 2 was regarded as essential for formation of high yield of monomers and elimination of char.
Transition metal sulfides such as MoS 2 [18][19][20][21][22], VS 2 [23,24] and mixed sulfide catalysts (e.g., sulfided CoMo and NiMo catalysts) [25][26][27][28] have already been reported as good candidates in lignin depolymerization reaction or hydrodeoxygenation of lignin model compounds.Typically, Kumar et al. [28] reported a highly efficient, solvent free approach of depolymerization of KL to aromatic monomers over a series sulfide NiMo and CoMo catalysts on various acidic and basic supports (i.e., Al 2 O 3 , ZSM-5 and MgO-La 2 O 3 ).The higher total monomer yield of 26.4 wt% with 15.7 wt% alkylphenols was obtained over sulfide NiMo/MgO-La 2 O 3 at 350 • C for 4 h and initial 10 MPa H 2 .However, the preparation methods of those reported sulfide catalysts were cumbersome, which were generally composed of an initial wet impregnation and a subsequent high temperature vulcanization under H 2 S or in-situ addition of sulfur source (i.e., dimethyldisulphide).MoS 2 can be conveniently synthesized with a hydrothermal method in a single step [22,29,30], which is not examined for lignin solvolysis reaction.
In our recent work, we reported that a one-step hydrothermally synthesized MoS 2 showed high activity for the conversion of guaiacol to alkylphenols in supercritical methanol without any gaseous hydrogen input [30].Herein, we extended our work with hydrothermally prepared MoS 2 catalysts with different Mo and S precursors for the depolymerization of EHL in ethanol without hydrogen.Based on the characterization of catalysts and products, the reaction pathway and roles of different Mo-based active sites were discussed.The stability of MoS 2 catalyst was also examined.

Synthesis of the catalysts
The molybdenum sulfide catalyst samples were prepared with a hydrothermal method, as described previously [30], using different precursors of Mo and S, i.e., ammonium molybdate (NH 4 ) 2 MoO 4 ), sodium molybdate (Na 2 MoO 4 )), thiourea (NH 2 CSNH 2 ) and thioacetamide (CH 3 CSNH 2 )).The STA-MoS 2 sample was prepared with dissolving Na 2 MoO 4 (~6 g) and CH 3 CSNH 2 (~9.4 g) in a mixture of equal volume (15 mL) of deionized water and ethanol.After ultrasonication for 15 min, the solution was treated with few drops of concentrated hydrochloric acid (~3 mL) to form slurry, which was then transferred into a 50 mL stainless-steel autoclave and underwent a hydrothermal reaction at 200 • C for 24 h.To prepare ATA-MoS 2 sample, both (NH 4 ) 2 MoO 4 (~0.80 g) and CH 3 CSNH 2 (~1.50 g) were dissolved in 30 mL deionized water with ammonium hydroxide (6 mL).Then, the mixture was sonicated at room temperature for 30 min and transferred into the autoclave, which was kept at 200 • C for 20 h.AT-MoS 2 was obtained after dissolving (NH 4 ) 2 MoO 4 (0.18 g) and NH 2 CSNH 2 (0.65 g) in 28 mL deionized water, subsequently, hydrochloric acid was added into mixture until the pH was ~1, and the whole mixture was transferred to the 50 mL autoclave.The autoclave was heated to 200 • C for 12 h.AS-MoS 2 was synthesized with (NH 4 ) 2 MoO 4 (~2.3 g), sublimed sulfur (~0.75 g) and hydrazine hydrate (~12 mL), which were initially dispersed in 60 mL deionized water followed with 20 min of ultrasonication treatment.The obtained suspension was kept at 200 • C for 24 h in 100 mL autoclave.After each hydrothermal reaction, the autoclave was cooled down to the room temperature and the precipitate was obtained via vacuum filtration.Subsequently the precipitate was washed several times with ethanol until the filtrate became neutral.Finally, the collected solid was dried in vacuum at 60 • C overnight prior to the characterization and further use.

Catalyst characterization
All catalyst characterization methods and their instruments were used as reported in our previous work [30].Briefly, specific surface area (S BET ), pore size distribution and pore volume of the all fresh hydrothermal MoS 2 , C-MoS 2 and used STA-MoS 2 samples were analyzed using a Quantachrome Autosorb-1 measured at 196 •C.X-ray diffraction (XRD) patterns of the freshly synthesized and used samples were tested at a scanning rate of 10 • /min in the diffraction angle range (2θ) from 10 • to 90 • at room temperature on Rigaku D/max 2500 v/pc.Raman spectra for the used samples were collected using a LabRAM HR Evolution microscopic confocal Raman spectrometer (HORIBA Hobin Yvon S.A.S., France).All measurements were done under the 532 nm He-Ne laser-excitation.
The chemical states of the fresh and used samples were revealed by X-ray photoelectron spectroscopy (XPS).XPS spectra of those samples were recorded with a PHI 1600 ESCA system spectrometer equipped with a Mg Kα X-ray source (hv = 1253.6eV).The binding energy (BE) of C 1 s peak at 284.6 eV (C-C/C-H) was used as an internal standard.

Catalytic reaction
The ethanolysis of EHL was carried out in a 300 mL stainless steel autoclave reactor (Kemi Co. Ltd, Hastelloy).Typically, 1.0 g of EHL, 80 mL of ethanol and 0.5 g of catalyst were introduced into the reactor.After purging the reactor with nitrogen gas for 5 times, it was heated to the desired temperature (260-330 • C) with a stirring speed of 600 rpm.After completing the reaction, the reactor was cooled down rapidly to room temperature, and the liquid products were collected by suction filtration.
In the recycle test, the used MoS 2 catalyst was collected after washing with 10 mL of ethanol.Prior the next use, the solid was dried at 60 • C for 2 h in vacuum.

Product analysis
With anisole as an internal standard, the liquid products were analyzed and quantified using gas chromatograph equipped with a flame ionization detector (GC-FID, Agilent Technologies, model 6890) and a HP-5 MS capillary column (Agilent, 30 m × 0.25 mm × 0.25 µm).The same GC and quantitative analysis methods were followed as given in our previous work [30].The yield of overall aromatic monomers or individual grouped products (i.e., alkyl-substituted phenols (A-Ps) and other aromatic products (OPs)) was calculated according to Eq. ( 1).The selectivity (S) of complex A-Ps (isopropyl-and butyl-substituted phenols) in overall A-Ps was symbolized as S and was calculated with Eq. ( 2).The yield (Y/%) of products from the conversion of model compounds were calculated with Eq. ( 3).All the results were confirmed with triplicate experiments and the average values were taken for calculation.In the following equations, M and n represents the mass and mole of corresponding chemicals, respectively.
The FTIR spectra of the bare EHL and EHL ethanolysis products were recorded with a IRAffinity-1S FT-IR spectrophotometer of Shimadzu.Matrix assisted laser desorption/ionization time-of-light mass spectroscopy (MALDI-TOF-MS) of the liquid products were measured to identify the molecular weight distribution with an Autoflex tof/toflll of Bruker Dalton Corporation.A 15 g/L solution of 2,5-dihydroxyl benzoic acid (DHB) (Sigma) in ethanol was used as matrix.The two-dimensional heteronuclear single quantum coherence-nuclear magnetic resonance (2D-HSQC NMR) spectra were recorded on a Bruker AVAVCE III HD 400 MHz.The deuterium generation reagent was DMSO-d 6 .

Catalyst characterization
Although the partial characterization of C-MoS 2 and STA-MoS 2

Activity
The total-ion chromatogram (TIC) of the liquid products obtained from STA-MoS 2 catalyzed EHL ethanolysis at 320 • C and the corresponding structure are depicted in Fig. 3(a).Twenty-seven aromatic monomers were quantified, and, among these monomers, alkylphenols (A-Ps) are the main products.The extend GC-TIC with retention time of 8-12 min and with other MoS 2 as catalysts were also plotted along with their molecules as listed in Fig. S2 and Fig. S3, respectively.Fig. 3(b) show the yields of overall aromatic monomers and A-Ps obtained from non-catalytic reaction and catalytic reactions over different MoS 2 catalysts.Without a catalyst, EHL was converted with ethanol into 50.1 mg/ g EHL aromatic monomers, mainly including ortho-ethylphenol, orthomethylguaiacol, ortho-ethylguaiacol, ethyl ferulic acid and methyl pcoumarate (Fig. S1), with formation of a certain amount of char.When a catalyst was added, EHL was completely liquified without formation of char.ATA-MoS 2 gave the highest overall aromatic monomer yield (202.8 mg/g EHL), followed by STA-, AS-, AT-and C-MoS 2 with the corresponding yield of 175.6, 142.3, 133.7 and 102.8 mg/g EHL, respectively.Furthermore, the yield of A-Ps among overall aromatic monomers can be considered as a standard to measure the activity of catalyst for EHL ethanolysis into highly value-added chemicals.The decreasing order of the A-Ps yield for different catalyst is as follows: STA-MoS 2 (111.9 mg/g EHL) > ATA-MoS 2 (103.9 mg/g EHL) > AS-MoS 2 (88.9 mg/g EHL) > AT-MoS 2 (84.2 mg/g EHL) > C-MoS 2 (41.7 mg/g EHL).Due to its relatively higher activity towards A-Ps formation, we further used STA-MoS 2 for examining the effect of the reaction conditions and recyclability test.

FTIR analysis
The FTIR spectra of EHL, and the bio-oils obtained with and without catalysts are plotted in Fig. 5, and the assignments of all the bands are listed in Table S1 [21,35,36].The bands for hydroxyl groups at 3402 cm − 1 , aryl groups at 1595 cm − 1 and carbonyl groups at 1110 cm − 1 were found to be existed on both EHL and reaction products.Other typical functional group bands, i.e., methyl, methylene and methoxy, were observed in the liquid products, which reveals the existence of oxygen-containing aromatic compounds predominately.Specifically, the intensity order of alkyl groups characterized by the band at 2868-2966 cm − 1 is as follows: STA-MoS 2 > C-MoS 2 ~ without catalyst > EHL (Fig. 5(a-d)).Alkyl groups were dominantly released or formed during the EHL ethanolysis process, which resulted in the intensity improvement in the bio-oils obtained with or without a catalyst [37,38].The higher yield of A-Ps obtained over STA-MoS 2 than that over C-MoS 2 or without catalyst (Fig. 3(b)) clearly indicates the higher alkylation activity, and the formed substituted aliphatic side chain in A-Ps explained the relatively stronger intensity of alkyl groups over STA--MoS 2 (Fig. 5(d)).The hydrothermally prepared MoS 2 (STA-MoS 2 ) has already been proved to exhibit a higher alkylation activity than C-MoS 2 in previous work [30].The peak contributed to the aromatic C-H out-of-plane stretching (821 cm − 1 ) from EHL was slowly reduced (Fig. 5  (b-d)) over MoS 2 .In more details, a very weak band at 821 cm − 1 clearly revealed the release of guaiacyl units from EHL over STA-MoS 2 was poorer than that over the other catalysts studied [30,37].

MALDI-TOF-MS analysis
The MALDI-TOF-MS technique is employed to monitor the change of molecular weight of the lignin fragments after reaction.Fig. 6(a) shows the molecular weight distribution of the bio-oils with the m/z range of 331-1311 for the dissolution of EHL without catalyst at 320 • C for 6 h.In the m/z range of 331-400, the m/z difference of 14 for each adjacent peak may be due to the insertion or removal of -CH 2 -functional group.Furthermore, all the obtained fragments from the conversion of EHL is soluble in ethanol [12].As shown in Fig. 6(b), the m/z fragment was significantly reduced to 259-996 range on STA-MoS 2 catalyst, which confirms the ability to depolymerize EHL linkage in ethanol.While the higher overall aromatic yield was achieved at 12 h, the m/z peak distribution of liquid products mainly center on 367 with a broader m/z range of 259-1088 than that for 6 h (Fig. 6(c)), which may result from the dominant depolymerization reaction accompanied by the repolymerization of small molecules as the reaction time increase [13].

Table 2
The selectivity of complex A-Ps in overall A-Ps during the EHL ethanolysis reaction with different reaction time, and the Mo 5+ and S 2 2-content on the spent catalyst according to the XPS data. in the range of 0.46-0.65 and 0.47-0.62,respectively.Based on the XPS results, three different species were found to be existing on the surface of the hydrothermally synthesized MoS 2 catalysts, namely Mo sulfide (MoS 2 having Mo 4+ with S 2-species), Mo oxysulfide (MoO x S y having Mo 5+ with S 2 2-species) and Mo oxide (MoO 3 with Mo 6+ ) [44][45][46][47].Nevertheless, MoO x S y and MoO 3 phases were not identified in the XRD patterns of the fresh hydrothermal MoS 2 samples which may be due to their amorphous structure.No MoO x S y phase exists on C-MoS 2 as there was complete absence of Mo 5+ peak in XPS (Fig. 2 (a)).The complex A-Ps (iPr-Ps and Bu-Ps) from EHL depolymerization are expected in higher proportion due to their widely industrial application value, which are generally acted as important raw material for the production of oil-soluble phenolic resin, surfactant, antioxygen and flame retardant [48,49].The relationship between the selectivity of complex A-Ps in total A-Ps yield and the content of Mo 5+ and S 2 2-on MoS 2 surface is shown in Fig. 9(c).The highest selectivity of complex A-Ps was achieved with AT-MoS 2 as a catalyst (78.6%), which was followed with ATA-, AS-, STA-and C-MoS 2 with the complex A-Ps selectivity of 74.6%, 72.6%, 66.6% and 5.5%, respectively.The selectivity of complex A-Ps increased with the increase of the content of Mo 5+ and S 2 2on MoS 2 surface, indicating that MoO x S y phase is the active sites for DMO and alkylation process to produce the main products of complex A-Ps.Therefore, AT-MoS 2 showed the highest DMO activity, which was also proved by the 2D-HSQC analysis as shown in Fig. 7.Moreover, the signals of S units disappeared when hydrothermal MoS 2 was added (Fig. 7(a-e)), while the signals of G and H units appeared.Therefore, the yield of G units and their derivatives, which were obtained through the DMO of S units, could also show the DMO activity of hydrothermal MoS 2 .As shown in Fig. 9(d), the yield of G units and their derivatives was decreased as follows: AT-MoS 2 (18.8 mg/g EHL) > ATA-MoS 2 (16.5 mg/g EHL) > AS-MoS 2 (15.4 mg/g EHL) > STA-MoS 2 (13.5 mg/g EHL), which was perfectly consistent with the decrease order of Mo 5+ and S 2 2-contents.
In addition, the properties of STA-MoS 2 after reaction for different time (1, 2, 4 and 6 h) was further measured to observe the change of catalyst during EHL ethanolysis.As shown in Table 2, based on the XPS data, the content of Mo 5+ and S 2 2-were increased to 31.8% and 22.4% respectively after 1 hour, and further decreased with the extension of reaction time.After 6 h (used STA-MoS 2 after 1 run), the Mo 5+ and S 2 2content were deceased to 24.3% and 12.9%, which were higher and lower respectively than that of the fresh sample.The results indicated Mo 5+ and S 2 2-on STA-MoS 2 was generated at the beginning of reaction in ethanol, and then gradually consumed during the reaction.Moreover,  the selectivity of complex A-Ps in the overall A-Ps was gradually decreased from 81.9% (1 h) to 66.6% (6 h).The results further verified that Mo 5+ and S 2 2-might be the actives sites on MoS 2 surface for its DMO activity in the EHL ethanolysis process, which played a significant role in the achievement of abundant complex A-Ps.

Possible mechanism of complex A-Ps formation
Ma et al. [13] proposed a radical mechanism for Kraft lignin conversion in supercritical ethanol.Hydrogen and ethyl radicals were generated from ethanol (Fig. 10(a)).Therefore, we further proposed the pathway of A-Ps formation based on the 2D-HSQC NMR (Fig. 7) and intermediates conversion results (Fig. 8).Meanwhile, referred to the published work [36], the cycles of Mo-based species in EHL ethanolysis for the reduction reaction via acceptance of electrons from the conversion of radicals (hydrogen and ethyl radicals) into cations, and the oxidation reaction via discharge of electrons was proposed in Fig. 10(b).The discharged electrons would be joined in the A-Ps formation.As shown in Fig. 10(c), β-O-4 linkage in EHL structure was broken during the non-catalytic ethanolysis of EHL to release methoxy-containing S units, which were further demethoxylated into G units over MoS 2 and then into complex A-Ps through DMO/alkylation reaction with the effect of the discharged electrons and cations (Pathway a ′ ).On the other hand, several phenolic monomers obtained from non-catalytic ethanolysis further undergo DMO and alkylation reaction over MoS 2 to form complex A-Ps (Pathway b′).In more detail, the -C-C-COO-group of HFA and CA units were removed to form acetic acid and formic esters, which are the by-products of EHL ethanolysis (Fig. S2).

Stability and deactivation of STA-MoS 2
The reusability of the STA-MoS 2 catalyst is shown in Fig. 11, the yield for overall aromatic monomers and A-Ps did not obviously change in initial 2 runs, but generally decreased to 125.4 and 80.7 mg/g EHL respectively in the 4th run.Similar yields of overall aromatic monomers (133.3 mg/g EHL) and A-Ps (87 mg/g EHL) were further obtained in the 5th run.These results imply that STA-MoS 2 is recyclable at least for three catalytic runs.
In the XRD patterns of used STA-MoS 2 after 5 runs, the intensity of diffraction peak at 34 • assigned to (101) plane decreased, and new crystalline phases of monoclinic MoO 3 (PDF#47-1081) and hexagonal MoO 2 (PDF#50-0739) appeared (Fig. 12(a)), which indicate the occurrence of oxidization for MoS 2 .Due to the oxidization, Mo 4+ was decreased to 40.7%, and the Mo 6+ and SO x contents of used STA-MoS 2 after 5 runs were significantly increased to 32.3% and 13.9% respectively (Fig. 12(b)).Hence, the Mo 6+ /Mo 5+ and (Mo 6+ +Mo 5+ )/Mo 4+ of the used STA-MoS 2 after 5 runs were increased to 1.51 and 2.10, respectively, exceeding the above-mentioned proper ratio range.The loss of atomic sulfur content was also observed on the used catalyst.As shown in Fig. 12(d), the sulfur content on STA-MoS 2 decreased from 74.8% to 56.6% after 1 run, and further decreased to 43.2% after 5 runs.However, the oxygen content on STA-MoS 2 gradually increased from 1.4% to 7.6% after 1 run, and further increased to 28.3% after 5 runs.These results clearly revealed the sulfur-oxygen exchange occurred gradually in EHL ethanolysis reaction.Badawi et al. [50] also reported that the produced water during hydrodeoxygenation process of oxygenated compounds over MoS 2 led to the formation of vacancies by removing sulfur atoms from the metallic edge with oxygen atoms, and the sulfur-oxygen exchanges on the surface of MoS 2 was resulted in the continuous deactivation of catalyst.In addition, Raman spectrum revealed that both amorphous and graphite carbon coexisted on the surface of used STA-MoS 2 after 5 runs as shown in Fig. 12(c).Therefore, the carbon deposition and the sulfur-oxygen exchanges which is resulted from the oxidization on MoS 2 are responsible for the deactivation of the catalyst.
the textural data of the fresh C-MoS 2 and synthesized-MoS 2 samples.STA-MoS 2 (108 m 2 /g) and AT-MoS 2 (112 m 2 /g) have large specific surface areas.ATA-MoS 2 shows a moderate surface area (57 m 2 /g).Nevertheless, the surface areas of C-MoS 2 and AS-MoS 2 are only 4 and 6 m 2 /g, respectively.Correspondingly, the pore volumes of STA-MoS 2 (0.47 cm 3 /g) and AT-MoS 2 (0.41 cm 3 /g) are much larger than those of ATA-MoS 2 (0.19 cm 3 /g), C-MoS 2 (0.02 cm 3 /g) and AS-MoS 2 (0.04 cm 3 /g).Nevertheless, different catalysts have similar pore diameters which are in the ranges of ~3.3-4.3 nm.The high surface area and pore volume of STA-MoS 2 were decreased to 96 m 2 /g and 0.40 cm 3 /g after 1 run, respectively, and further drastically decreased into 15 m 2 /g and 0.04 cm 3 /g after 5 runs, respectively.The XRD patterns of the fresh and used samples are given in Fig.1.Fresh C-MoS 2 exhibits all the specific diffraction peaks of the hexagonal MoS 2 phase (PDF#17-0744).For catalyst samples prepared via hydrothermal route, only two broad peaks at 33 • and 58.8 • were recognized, which are attributed to (101) and (110) planes of MoS 2 with the corresponding d-spacing of 0.271 and 0.158 nm, respectively.After 1 run, the diffraction angle of (101) and (110) lattice plane of STA-MoS 2 increased by 1.0 • and 1.7 • , respectively, and the d-spacing assigned to these two planes decreased by 0.008 and 0.004 nm, respectively.The XPS spectra of different MoS 2 samples are illustrated in Fig. 2. Fig. 2(a) presents the XPS of Mo 3d (~228-237 eV) signals of different MoS 2 samples Fig. 2(b) presents the XPS of S 2p (~160-170 eV) signals of the MoS 2 samples.Only the peaks of S 2-(2p 1/2 and 2p 3/2 ) in C-MoS 2 were detected, while the peaks of S 2-, S 2 2-and SO x can be distinguished from the signals of hydrothermal MoS 2 samples.The contents of SO x , S 2 2-and S 2-on STA-and AS-MoS 2 are in similar

(
118.6 mg/g EHL).The effect of the reaction time on yields of overall aromatic monomers and A-Ps with STA-MoS 2 as the catalyst at 320 • C was also examined, and the results are plotted in Fig. 4(b).The overall aromatic monomer and A-Ps yields were increased monotonically with increasing the reaction time and reached the highest 226.4 and 140.7 mg/g EHL respectively for 12 h.
, the signals of -CH 3 , methoxyl groups, β-O-4 alkyl ether (A''), resinol structure (B'') containing β-β and α-O-γ linkages, phenylcoumarane structure (C'') containing β-5 and α-O-4 linkages, structural units of lignin (p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S)), p-coumaric acid (CA) and ferulic acid derivatives (FA) were detected in EHL [39,40].As shown in Fig. 7(a), the intensity of the -CH 3 signal in the H-C-C-region after EHL ethanolysis without catalyst becomes more stronger than that in EHL.The structure of β-O-4 alkyl ether disappeared after noncatalytic ethanolysis.Moreover, the C-H cross signals of alkyl side chains and ester side chains (ES), such as the C α -H α signals (para-methyl (PM α ), para-ethyl (PE α ) and ES α ), and the C β -H β signals (para-propyl (PP β )), are recognized.In the H-C-O-regions, the signals of methoxy group and C γ -H γ (ES γ ) were detected.In the C(Ar)-H correlation region, the signals of S unit, CA and hydrogenated FA (HFA) were also detected.When the hydrothermally synthesized MoS 2 was added, the intensity of -CH 3 signals were significantly increased and contributed to the highest for AT-MoS 2 .Nevertheless, the signals of methoxy groups disappeared.Meanwhile, the signals of para-propanol side chains (PPol α and PPol β ) in the H-C-C-region, the G and H units in C(Ar)-H region appeared, which revealed the hydrothermal MoS 2 showed fine demethoxylation (DMO) activity to realize the conversion of S units into G and H units via removing methoxy groups.The highest intensity of G and H units were achieved with AT-MoS 2 , which indicates that AT-MoS 2 showed highest DMO activity.In more detail, the signals of dimers containing α-O-4 (B ′ ) and β-β linkages (C ′ β and C ′ γ ) in H-C-O-region were only identified when AT-MoS 2 was used as a catalyst.The results indicated AT-MoS 2 exhibited poorest activity of EHL depolymerization among all the hydrothermal MoS 2 , which was consistent with the yield of aromatic monomers as shown in Fig. 3(b).

Fig. 9 .
Fig. 9.The effect of (a) Mo 6+ /Mo 5+ and (b) (Mo 6+ +Mo 5+ )/Mo 4+ calculated by the XPS data of different types of MoS 2 on the yield of overall aromatic monomers, (c) The effect of contents of Mo 5+ and S 2 2-species on the selectivity of complex A-Ps (iPr-Ps and Bu-Ps) in the overall A-Ps obtained from EHL ethanolysis over different types of MoS 2 , (d) Yield of G unit and its derivatives in the liquid products obtained from EHL ethanolysis over different hydrothermal MoS 2 .

Fig. 10 .
Fig. 10.(a) Conversion of hydrogen and ethyl radicals from ethanol to discharge electrons, (b) catalytic cycle of different Mo-based species, and (c) proposed reaction pathways for complex A-Ps formation from EHL ethanolysis over MoS 2 .(iPr-Ps: isopropyl substituted phenols, Bu-Ps: butyl substituted phenols.Complex A-Ps were composed of iPr-Ps and Bu-Ps).

Fig. 12 .
Fig. 12.(a) XRD patterns, (b) XPS spectra of Mo 3d and S 2p, and (c) Raman analysis of used STA-MoS 2 after 5 runs.(d) Relative atomic contents of Mo, S and O on the surface of fresh and used (after 1 and 5 runs) STA-MoS 2 catalyst by XPS (Note: Only Mo, S and O are semiquantitative and the total content of Mo, S and O for each sample was round to 100%).

Table 1
Texture data of the commercial-and hydrothermally synthesized-MoS 2 samples.