Modification Over MCM-41 for Methyl Mercaptan Catalytic Decomposition

processes

Article Promotional Effects of Rare-Earth Praseodymium (Pr) Modiﬁcation over MCM-41 for Methyl Mercaptan Catalytic Decomposition

Xiaohua Cao 1,2,3, Jichang Lu 1,2,3,*, Yutong Zhao 1,2,3, Rui Tian 1,2,3, Wenjun Zhang 1,2,3, Dedong He 2,3,4 and Yongming Luo 1,2,3,*

1 Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China; [email protected] (X.C.); [email protected] (Y.Z.); [email protected] (R.T.); [email protected] (W.Z.) 2 The Innovation Team for Volatile Organic Compounds Pollutants Control and Resource Utilization of Yunnan Province, Kunming 650500, China; [email protected] 3 The Higher Educational Key Laboratory for Odorous Volatile Organic Compounds Pollutants Control of Yunnan Province, Kunming 650500, China 4 Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China * Correspondence: [email protected] (J.L.); [email protected] (Y.L.)

Abstract: Praseodymium (Pr)-promoted MCM-41 catalyst was investigated for the catalytic decom-

position of methyl mercaptan (CH3SH). Various characterization techniques, such as X-ray diffraction (XRD), N2 adsorption–desorption, temperature-programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO -TPD), hydrogen temperature-programmed reduction (H -TPR), and X-ray 2 2 photoelectron spectrometer (XPS), were carried out to analyze the physicochemical properties of material. XPS characterization results showed that praseodymium was presented on the modified Citation: Cao, X.; Lu, J.; Zhao, Y.; catalyst in the form of praseodymium oxide species, which can react with coke deposit to prolong Tian, R.; Zhang, W.; He, D.; Luo, Y. Promotional Effects of Rare-Earth the catalytic stability until 120 h. Meanwhile, the strong acid sites were proved to be the main Praseodymium (Pr) Modification active center over the 10% Pr/MCM-41 catalyst by NH3-TPD results during the catalytic elimination over MCM-41 for Methyl Mercaptan of methyl mercaptan. The possible reaction mechanism was proposed by analyzing the product Catalytic Decomposition. Processes distribution results. The final products were mainly small-molecule products, such as methane (CH4) 2021, 9, 400. https://doi.org/ and hydrogen sulfide (H2S). Dimethyl sulfide (CH3SCH3) was a reaction intermediate during the 10.3390/pr9020400 reaction. Therefore, this work contributes to the understanding of the reaction process of catalytic decomposition methyl mercaptan and the design of anti-carbon deposition catalysts. Academic Editor: Francesco Parrino

Keywords: rare earth; MCM-41; CH3SH decomposition; stability; reaction mechanism Received: 31 December 2020 Accepted: 3 February 2021 Published: 23 February 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Methyl mercaptan (CH3SH) is one of the typical unconventional volatile organic com- published maps and institutional affil- pound (VOC) species, which exhibits high toxicity, volatility, and causticity [1,2]. People iations. exposed to high concentrations of CH3SH for a long time contributes to the presentation of mental illnesses [2,3]. In addition, methyl mercaptan not only participates in a series of complex reactions in air pollution events, such as PM2.5 (Fine Particulate Matters, ϕ < 2.5µm) [4], ozone layer depletion, and acid rain [4,5], but also corrodes equipment and poisons the catalyst during various processes. More importantly, there are wide industrial Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. sources for the CH3SH emission, including wood-pulping industries, waste water treat- This article is an open access article ment, sanitary landfills, petroleum refining, the natural gas industry and energy-related distributed under the terms and activities [6–9]. Therefore, on account of increasingly rigorous catalytic technical standards conditions of the Creative Commons and stringent environmental regulations, improved removal of CH3SH is imperative. Attribution (CC BY) license (https:// The reported methods currently used for the removal of CH3SH include alkaline creativecommons.org/licenses/by/ treatment [10], biological degradation [11], adsorption [12–14] incorrect ref order, 12 de- 4.0/). tected after 9. You jumped the numbers in between., hydrodesulfurization, catalytic

Processes 2021, 9, 400. https://doi.org/10.3390/pr9020400 https://www.mdpi.com/journal/processes Processes 2021, 9, 400 2 of 14

oxidation [15,16], and catalytic decomposition [17–21]. Among the above, the method of catalytic decomposition has been considered an effective option because of its low waste, lack of need of other reactants, and its ease of implementation. At present, nano-CeO2 and HZSM-5-based catalysts have attracted wide attention for the process of the catalytic decomposition of CH3SH owing to their high concentration of oxygen vacancies [18] and tunable acid–base properties [19], respectively. However, these catalysts have drawbacks in the form of poor stability and the high temperatures required for complete conversion. Huguet et al. [19] showed that poisonous CH3SH can be converted into CH4 and ◦ H2S small-molecule products at high temperature (600 C) using pure HZSM-5 zeolite. However, the stability for CH3SH abatement over HZSM-5 catalyst was no more than 10 h. Furthermore, Lu et al. [22] reported that the rare earth (RE) elements La-modified HZSM-5 catalyst could increase basic sites (lanthanum oxy-carbonates composites) and decrease strong acid sites to improve the stability (nearly 80 h time-on-stream (TOS) test). However, the deactivation of above HZSM-5-based catalysts were due to the blockage of microporous channels by deposited coke. Compared with HZSM-5-based catalysts, the ◦ conversion of CH3SH can reach 100% at low temperature (450 C) over nano-CeO2 [20,23] and modified Ce0.75RE0.25O2−δ (RE = Sm, Gd, Nd, and La) [17,24], but the ultrashort stability was still presented for these catalysts (about 8 h TOS test) without obvious improvement. These results suggest that the problem of deposited coke over these two types of catalysts for CH3SH decomposition has not been resolved yet, due to the fact that the reaction mechanism of the catalytic degradation of methyl mercaptan is ambiguous. As reported in the literature, praseodymium plays a vital role in removing coke during the methane reforming reaction [25,26]. Praseodymium, as an adjacent element with cerium, can exhibit good stability due to the presence of various stoichiometrically defined oxides (PrnO2n−1), like cerium oxide, which can accelerate the elimination of coke deposit [25–28]. Sierra et al. [25] added Pr into La1−xAxNiO3d perovskites as catalyst, which shows the highest catalytic activity and stability in the dry reforming of methane via the reaction between PrO2 and carbon residues gasifying the carbon deposits [26]. Abello et al. [29] indicated that a series of Pr (0–7 wt.%)-modified MgAl2O4 spinel oxide-supported Ni catalysts decreased the deactivation rate by influencing the type and amount of coke deposit. Therefore, praseodymium has been deemed a good promoter to eliminate carbon deposits for methyl mercaptan abatement. Moreover, it has been evidenced that the catalytic performance is significantly affected by different supports. Coke deposition ultimately leads to catalyst rapid deactivation by blocking the microporous channel of HZSM-5 support [19,22,30]. Al2O3 carrier has a high content of strong acidic sites, which results in easily deposited coke on the surface of catalyst compared with HZSM-5 [31–33]. Among numerous support materials, mesoporous MCM-41 support has been regarded as a potential support candidate because of its large specific area, small diffusion hindrance, and uniform-sized pores [34–40]. As far as we know, no investigation on Pr-supported MCM-41 catalyst for catalytic decomposition of CH3SH has been conducted so far. On the basis of this background, a certain amount of Pr was impregnated to the MCM-41 support with large specific area and small diffusion hindrance, and the obtained Pr-modified MCM-41 sample was used for methyl mercaptan elimination. The synthesized samples were carried out by N2 adsorption–desorption, X-ray diffraction (XRD), NH3- TPD, CO2-TPD, H2-TPR, and X-ray photoelectron spectrometer (XPS) characterizations to investigate the effect of the modification of rare metal Pr on the physicochemical properties of the MCM-41 zeolite catalyst. In addition, the reaction mechanism of eliminating methyl mercaptan was investigated in detail by product distribution results, which were detected by flame ionization detector (FID) and flame photometric detector (FPD). Processes 2021, 9, 400 3 of 14

2. Experimental 2.1. Chemicals and Materials Praseodymium nitrate hexahydrate, cetyltrimethylammonium bromide (CTAB), tetraethylorthosilicate (TEOS), ammonia, and other chemical reagents were obtained from Aladdin Biochemical Technology Co., Ltd, Shanghai, China. All of the above chemicals were at least analytical grade. Methyl mercaptan (5000 ppm CH3SH, N2 as balance gas) was purchased from Dalian Special Gases Co. Ltd., Dalian, China.

2.2. Catalyst Synthesis 2.2.1. The Preparation of the MCM-41 Support Organic–inorganic hybrid mesoporous silica materials were prepared by the sol-gel method. Cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) were used as template and silicon source, respectively. An ammonium solution (NH3·H2O) was applied as pH control agent. Firstly, 14 g of CTAB and 54 mL of NH3·H2O were dissolved at room temperature into 635 mL of deionized water. Subsequently, 57.6 mL of TEOS was slowly added into the mixture under continuous stirring. The obtained gel was ﬁltered and washed with deionized water, dried at 105 ◦C for 24 h, and then calcined at 550 ◦C for 5 h to remove the template.

2.2.2. The Synthesis of Pr/MCM-41 Catalyst The Pr/MCM-41 sample was successfully synthesized with an incipient wetness impregnation method using praseodymium nitrate hexahydrate (Pr(NO3)3·6H2O, AR, 99.95%) as the precursor and mesoporous MCM-41 molecular sieve as support. Firstly, the calculated amount of Pr(NO3)3·6H2O was dissolved in 8 mL of deionized water to obtain 10 wt% Pr. Then 2 g of the MCM-41 support was mixed with the above precursor solution with continuous stirring. Subsequently, the resulting mixture was placed in an oven and dried at 110 ◦C for 12 h. After that, the dried sample was calcined at 550 ◦C for 5 h. The obtained sample was denoted as 10% Pr/MCM-41 catalyst.

2.3. Characterization Nitrogen adsorption–desorption isotherms were conducted on a NOVA 4200e Sur- face Area and Pore Size Analyzer with the assistance of the NovaWin-CFR software. The samples were degassed under high vacuum at 300 ◦C for 3 h before analysis and then measured at −196 ◦C. The Brunauer, Emmett & Teller Method (BET) was used to calculate the specific surface areas of catalyst. The pore size distribution and pore volume were obtained from the Barrett, Joyner & Halenda Method (BJH). X-ray diffraction (XRD) patterns were obtained with a Rigaku D/max-1200 diffractometer equipped with Cu Kα radiation in the range of 10◦ to 90◦ at a scanning rate of 10◦/min. The surface acidity and basicity properties of catalysts were respectively characterized by the temperature-programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD), using gas chromatography (GC) with a thermal conductivity detector (TCD). In the NH3-TPD experiments, all catalysts (100 mg) were first treated under helium (He, 30 mL/min) at 450 ◦C for 30 min and cooled ◦ down to 100 C. Then, 30 mL/min of NH3 (10 vol.% NH3 in He) flow was introduced into the reactor for 1 h to saturate the acid sites of the measured samples. In order to remove the physiosorbed ammonia, the prepared sample was swept with He for 1 h. Finally, a NH3-TPD analysis was carried out at a gas flow rate of 30 mL/min in He, ranging from ◦ ◦ 100 to 800 C (10 C/min ramp rate). Meanwhile, CO2-TPD was analyzed with similar ◦ conditions to the NH3-TPD procedure. Additionally, the adsorption CO2 at 30 C and the desorption was registered from 30 ◦C to 900 ◦C. The redox properties of the sample were generally performed by hydrogen temperature-programmed reduction (H2-TPR) in a fixed-bed with a quartz tube reactor. Before H2-TPR measurements, 100 mg of the catalyst ◦ was pretreated in the flow of mixture gas (10 vol.% H2/Ar, 30 mL/min) at 100 C for 1 h to ◦ ◦ remove adsorbed water. The heating rate of H2-TPR results, ranging from 100 C to 800 C, ◦ was 10 C/min in the 10 vol.% H2/Ar. A TCD detector recorded the H2 consumption Processes 2021, 9, 400 4 of 14

during the measurement. X-ray photoelectron spectroscopy (XPS) was measured on an Escalab 250Xi spectrometer by using Al Kα anode as the excitation source. The binding energies (BE) of Pr 3d, O 1s, and Si 2p were corrected by the C 1s peak at 284.8 eV.

2.4. Catalytic Activity and Stability Measurement

The catalytic decomposition CH3SH experiment was carried out in a ﬁxed bed mi- croreactor under atmospheric pressure. First, 0.2 g of catalyst was sieved to sizes in the range of 40 to 60 mesh and placed into a quartz tube reactor. CH3SH as the reactive gas, with the concentration of 5000 ppm (30 mL/min), was introduced into the system. Then, 0.2 g of the 10% Pr/MCM-41 catalyst was used to evaluate the catalytic stability at 600 ◦C. The reactants were detected by the online gas chromatography (GC) equipped with FID and FPD detectors. The catalytic conversion of CH3SH (XCH3SH) was deﬁned as follows:

C[CH SH] − C[CH SH] X (%) = 3 in 3 out × 100% CH3SH C [CH3SH]in

Here, C[CH3SH]in and C[CH3SH]out represent the initial concentration of methyl mercaptan before the reaction and the remaining concentration after the reaction, respectively. The reaction rate of CH3SH was calculated as µmol of CH3SH converted per second per SBET as follows: × V × ( ) 0.5% V XCH3SH % Reaction rate = m SBET × m(catalyst)

where 0.5% is the concentration of CH3SH, V is the gas ﬂow rate of CH3SH, and Vm is the molar volume at one standard atmosphere. SBET is determined based on the BET method. m(catalyst) represents the quality of catalyst during the activity test.

3. Results and Discussion 3.1. Structure of Support and Modified Pr/MCM-41 Catalyst Nitrogen adsorption–desorption isotherms and pore size distribution of fresh MCM- 41 and 10% Pr/MCM-41 catalysts are displayed in Figure1A,B, and the related data are shown in Table1. From Figure1A, the isotherms of MCM-41 and 10% Pr/MCM-41 catalysts are related to a IV-type with a H1 type hysteresis loop [41–43], which reflects the typical mesoporous structure. The mesoporous structure of the modified Pr/MCM-41 catalyst did not notably change. From Figure1B, after the praseodymium species was introduced, the intensity of the pore diameter centered at about 3.7 nm significantly decreased and some new pore appeared at 5 nm, indicating some praseodymium species was deposited on the external surface, and the others were introduced in the porosity, filling the pores of the MCM-41 the support. Table1 lists the specific surface areas and pore volume of these two catalysts. The SBET and pore volume of the unmodified MCM-41 support were measured to be 1289 m2/g and 0.379 cc/cm3, respectively. After the addition of 10 wt% Pr loading, the SBET and pore volume of the 10% Pr/MCM-41 catalyst significantly decreased to 920 m2/g and 0.111 cc/cm3, respectively, which was a normal phenomenon when some additional species was introduced into the supports.

Table 1. Textural properties of fresh MCM-41 and 10% Pr/MCM-41 catalysts.

S Pore Volume Pore Diameter Sample BET (m2/g) (cc/cm3) (nm) MCM-41 1289 0.379 3.742 10% Pr/MCM-41 920 0.111 3.382 ProcessesProcesses 20212021,, 99,, x 400 FOR PEER REVIEW 55 of of 13 14

FigureFigure 1. 1. TheThe N N22 adsorption–desorptionadsorption–desorption isotherms isotherms ( A) and pore sizesize distributiondistribution plot plot ( B(B)) of of the the fresh freshMCM-41 MCM and‐41 10% and Pr/MCM-4110% Pr/MCM catalysts.‐41 catalysts.

TheThe structural structural property property of of catalysts catalysts was was characterized characterized by by X‐ray X-ray diffraction diffraction (XRD), (XRD), as shownas shown in Figure in Figure 2. The2. The pattern pattern consisted consisted of a very of a verybroad broad peak peakfor 10% for Pr/MCM 10% Pr/MCM-41.‐41. Com‐ paredCompared with the with MCM the MCM-41‐41 support, support, the peak the intensities peak intensities weakened weakened obviously, obviously, indicating indicating that thethat hexagonal the hexagonal mesoporous mesoporous structure structure of MCM of MCM-41‐41 was was partly partly damaged damaged after after the the addition addition ofof Pr. Pr. This This may may have have been been due due to to some some Pr Pr species species entering entering and and blocking blocking the the pores pores of of the the MCMMCM-41‐41 support support [43]. [43]. According According to to the the standard powder diffraction ﬁlesfiles of PrPr66O11,, no no correspondingcorresponding pattern pattern of of any any bulk bulk praseodymium praseodymium species species phase phase was was observed observed for for the the supportedsupported catalysts, catalysts, indicating indicating that that praseodymium praseodymium species species were were well well dispersed dispersed on on the the MCMMCM-41‐41 support support [44]. [44].

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Figure 2. The detailed X‐ray diffraction (XRD) profile of the fresh MCM‐41 and 10% Pr/MCM‐41 Figure 2. The detailed X-ray diffraction (XRD) profile of the fresh MCM-41 and 10% Pr/MCM-41 catalystsFigure 2. comparedThe detailed with X‐ray the diffraction standard powder(XRD) profile diffraction of the files fresh of MCM Pr6O‐1141. and 10% Pr/MCM‐41 catalysts compared with with the the standard standard powder powder diffraction diffraction files files of of Pr Pr6O611O. 11. 3.2. The Surface Acid–Base and Reducibility of Support and Pr/MCM‐41 Catalysts 3.2. The Surface Surface Acid–Base Acid–Base and and Reducibility Reducibility of of Support Support and and Pr/MCM Pr/MCM-41‐41 Catalysts Catalysts The surface acidity and basicity were performed via the NH3‐TPD and CO2‐TPD The surface acidity and basicity were performed via the NH3-TPD and CO2-TPD measurementsThe surface to acidity evaluate and the basicity 10% Pr/MCM were performed‐41 catalyst via (Figure the NH 3).3‐TPD As shownand CO in2‐ TPDFigure measurements to to evaluate evaluate the the 10% 10% Pr/MCM Pr/MCM-41‐41 catalyst catalyst (Figure (Figure 3).3). As As shown shown in in Figure Figure 3A, a small CO2 desorption peak, observed at 100 ◦°C to 300 ◦°C in the low‐temperature 33A,A, a small CO 22 desorptiondesorption peak, peak, observed observed at at 100 100 °CC to to 300 300 °C Cin in the the low low-temperature‐temperature region, is assigned to the adsorption of surface OH groups, which can be associated with region, is assigned to to the the adsorption adsorption of of surface surface OH OH groups, groups, which which can can be be associated associated with with the weak basic sites. Meanwhile, the high‐temperature desorption peak at 600 °C to◦ 850 °C, the weak basic basic sites. sites. Meanwhile, Meanwhile, the the high high-temperature‐temperature desorption desorption peak peak at 600 at °C 600 to 850C to °C, 850 connected◦ with coordinatively unsaturated O2 anions, is ascribed to the strong basic sites connectedC, connected with with coordinatively coordinatively unsaturated unsaturated O2 anions, O2 anions, is ascribed is ascribed to the strong to the strongbasic sites basic [45–47]. These basic sites are conducive to the adsorption of acidic methyl mercaptan mole‐ sites[45–47]. [45 These–47]. These basic sites basic are sites conducive are conducive to the adsorption to the adsorption of acidic of methyl acidic mercaptan methyl mercaptan mole‐ cules. Figure 3B shows the NH3‐TPD profiles of the 10% Pr/MCM‐41. The high‐tempera‐ molecules.cules. Figure Figure 3B shows3B shows the NH the3‐TPD NH profiles3-TPD of profiles the 10% of Pr/MCM the 10%‐ Pr/MCM-41.41. The high‐tempera The high-‐ ◦ temperatureture desorption desorption peakpeak atat peak~700~700 °C at°C ~700is is ascribed ascribedC is ascribed to to the the strong tostrong the strongacid acid sites sites acid [48–50], sites[48–50], [48 which– which50], whichcan can cleave the C‐S bond to remove CH3SH [23,31]. As is well known, the pure MCM‐41 sup‐ cancleave cleave the C the‐S bond C-S bond to remove to remove CH3SH CH [23,31].3SH [23 As,31 is]. well As is known, well known, the pure the MCM pure‐41 MCM-41 sup‐ portsupport did not did havehave not have obviousobvious obvious basic basic and basic and acid acid and sites sites acid [43,51–53]. [43,51–53]. sites [43, 51After After–53 ].the the After modification modification the modification of ofPr, Pr, the the of strengthPr, the strength of thethe acidacid of the‐‐basebase acid-base sites sites is is significantly significantly sites is significantly enhanced, enhanced, enhanced, which which is isbeneficial which beneficial is beneficialto tothe the adsorp adsorp to the‐ ‐ 3 2 adsorptiontion and activation and activation ofof acidacid ofCH CH acid3SHSH CH molecules. molecules.3SH molecules. Figure Figure 4 Figure 4shows shows4 Hshows H2‐TPR‐TPR H profiles 2profiles-TPR profilesof ofthe the fresh of fresh the MCMfresh MCM-41‐41 and 10%Pr/MCM10%Pr/MCM and 10%Pr/MCM-41‐‐4141 samples, samples, samples, which which were were which used used were to to investigate usedinvestigate to investigate the the redox redox proper the proper redox‐ ‐ propertiesties of catalysts. of catalysts. However,However, However, nono obvious obvious no obvious peaks peaks were peaks were detected weredetected detected over over the overthe catalyst catalyst the catalyst due due to due tothe the to highlythe highly dispersed dispersed PrPr66OO Pr1111 6species speciesO11 species over over the over the mesoporous mesoporous the mesoporous MCM MCM‐ MCM-4141‐41 support support support [28]. [28]. [ 28].

Figure 3. Cont.

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Figure 3. CO2‐TPD (A) and NH3‐TPD (B) patterns of the 10% Pr/MCM‐41 catalyst. Figure 3. CO22‐-TPDTPD ( (AA)) and and NH NH33‐TPD-TPD ( (BB) )patterns patterns of of the the 10% 10% Pr/MCM Pr/MCM-41‐41 catalyst. catalyst.

Figure 4. H2‐TPR profiles of MCM‐41 and 10% Pr/MCM‐41 catalysts. Figure 4. H22‐-TPRTPR profiles profiles of of MCM MCM-41‐41 and and 10% 10% Pr/MCM Pr/MCM-41‐41 catalysts. catalysts. 3.3. The Surface Composition of the Support and Pr/MCM‐41 Catalysts 3.3. The XPS Surface spectra Composition were conducted of the Support to analyze and thePr/MCM Pr/MCM-41 element‐41 oxidationCatalysts states and chemical componentXPS spectra of the were surface conducted species. The to analyze XPS spectra the element results of oxidation Pr 3d, O states 1s, and and Si 2pchemical are displayedcomponent in ofFigure the surface 5A–C, respectively.species. The The The XPS XPS binding spectra spectra energy results results position, of of Pr Pr 3d, 3d, content, O 1s, and and Si ration 2p are fordisplayed each assigned in Figure peak 55A–C,A–C, are quantified respectively.respectively. and The Thepresented binding binding in energy Tableenergy 2. position, position,From Figure content, content, 5A, the and and peaks ration ration atfor 929.5 each eV assigned and 950.3 peak eV are quantified quantifiedattributed to and the presented Pr 3d signal in ofTable Pr3+2 2.species.. From From Figure FigureThe peaks5 A,5A, the at the 933.9 peaks peaks eVat 929.5 929.5and 954.8 eV eV and and eV 950.3 are 950.3 attributed eV eV are are attributed attributedto Pr4+ species, to tothe the Prindicating Pr3d 3dsignal signal the of coexistence Pr of3+ Pr species.3+ species. of The Pr4+ peaks Theand Pr peaks at3+ 933.9in at theeV933.9 andPr6 eVO 954.811 andspecies eV 954.8 are over eVattributed the are fresh attributed to 10% Pr 4+Pr/MCM species, to Pr4+‐ 41indicatingspecies, catalyst indicating [50,54,55].the coexistence the The coexistence ratio of Pr of4+ Prand4+/Pr of Pr Pr3+3+ in4+ of the 10%3+ Pr/MCM‐41 catalyst is 2.36, evidencing that the content of Pr4+ is higher than4+ 3+ theand Pr Pr6O11in species the Pr over6O11 thespecies fresh over 10% thePr/MCM fresh‐ 10%41 catalyst Pr/MCM-41 [50,54,55]. catalyst The ratio [50,54 of,55 Pr]./Pr The thatofratio the of of 10%Pr Pr3+4+ inPr/MCM/Pr the3+ freshof‐41 the catalyst. catalyst 10% Pr/MCM-41 Theis 2.36, O 1s evidencing spectra catalyst of the isthat 2.36, unmodified the evidencing content MCM of thatPr‐4+41 is the were higher content only than of deconvolutedthatPr4+ ofis higherPr3+ in intothe than fresha thathigh catalyst. of intensity Pr3+ in The thepeak O fresh 1sat spectra533.0 catalyst. eV, of whichthe The unmodified O was 1s spectra ascribed MCM of to the ‐the41 unmodified wereoxygen only speciesdeconvolutedMCM-41 in werethe MCM into only ‐a deconvoluted41 high support intensity (Figure into peak a5B). high at As 533.0 intensity displayed eV, peak which in Figure at 533.0was 5C, ascribed eV, an which intense to was the peak ascribed oxygen of MCMspeciesto the‐41 oxygen in at the 103.8 MCM species eV,‐41 presented in support the MCM-41 in(Figure the supportSi 5B). 2p XPSAs (Figuredisplayed spectra,5B). can in As Figurebe displayed assigned 5C, an in to intense Figure the Si ‐5peakOC,‐Si an of

fromMCMintense the‐41 peak SiO at 103.82 ofphase MCM-41 eV, [31]. presentedFurthermore, at 103.8 in eV, the presentedthe Si O2p 1s XPS and in spectra, theSi 2p Si spectra 2p can XPS be of spectra, assignedthe 10% can Pr/MCM to be the assigned Si‐‐41O ‐Si were also tested for comparison. The positions of the O and Si species both shift to the fromto the the Si-O-Si SiO2 phase from the[31]. SiO Furthermore,2 phase [31 ].the Furthermore, O 1s and Si 2p the spectra O 1s and of the Si 2p10% spectra Pr/MCM of the‐41 were10% Pr/MCM-41 also tested for were comparison. also tested The for comparison.positions of the The O positions and Si species of the Oboth and shift Si species to the

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bothlower shift binding to the energy lower after binding the energyaddition after of Pr, the indicating addition of that Pr, indicatingthere is an thatinteraction there is be an‐ interactiontween praseodymium between praseodymium and silica. and silica.

Figure 5. Pr 3d ((AA),), OO 1s 1s ( B(B),), and and Si Si 2p 2p (C (C) X-ray) X‐ray photoelectron photoelectron spectrometer spectrometer (XPS) (XPS) spectra spectra of MCM-41 of andMCM 10%‐41 Pr/MCM-41and 10% Pr/MCM catalysts.‐41 catalysts.

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Table 2. Physical parameters and chemical and surface compositions of Pr, O, and Si in fresh MCM-41 and 10% Pr/MCM- 41 catalysts.Table 2. Physical parameters and chemical and surface compositions of Pr, O, and Si in fresh MCM‐41 and 10% Pr/MCM‐41 catalysts. 4+ 3+ Pr Pr Si OSi-O-Si 4+ 3+ Sample Content BindingPr Energy Content BindingPr Energy Pr4+/Pr3+ BindingSi EnergyOSi‐O‐SiBinding Energy (%) (eV) (%) (eV) Binding(eV) (eV) Sample Content Binding Energy Content Binding Energy Pr4+/Pr3+ Binding Energy Energy MCM-41 —(%) —(eV) —(%) —(eV) — 103.85(eV) 533.03 10%Pr/MCM-41 70.24 954.83 29.76 933.95 2.36(eV) 103.47 532.91 MCM‐41 — — — — — 103.85 533.03 10%Pr/MCM‐41 70.24 954.83 29.76 933.95 2.36 103.47 532.91 3.4. CH3SH Reaction Rate and Stability of Catalyst

3.4. CHThe3SH evaluation Reaction Rate of and the Stability reaction of Catalyst rate and stability for CH3SH decomposition is sum- marized in Figure6A,B, respectively. The reaction rates of CH SH over MCM-41 and The evaluation of the reaction rate and stability for CH3SH decomposition3 is summa‐ 10%rized Pr/MCM-41 in Figure 6A,B, catalysts respectively. are displayed The reaction in rates Figure of 6CHA.3 ItSH can over be MCM found‐41 that and the10% reaction ratePr/MCM increases‐41 catalysts with anare increase displayed in in the Figure temperature 6A. It can over be found these that two the catalysts. reaction Therate reaction − − ◦ ◦ rateincreases of MCM-41 with an increase is 0.0001 in and the temperature 0.0009 (µmol over·s these1·m 2two) at catalysts. 400 C and The 450reactionC, respectively.rate However,of MCM‐41 the is 0.0001 reaction and rate0.0009 of (μ themol 10%∙s−1∙m Pr/MCM-41−2) at 400 °C and is 0.0054450 °C, andrespectively. 0.0199 ( µHowever,mol·s−1 ·m−2) at 400the reaction◦C and rate 450 of◦C, the respectively. 10% Pr/MCM Thus,‐41 is 0.0054 the reaction and 0.0199 rate ( ofμmol the∙s− 10%1∙m−2) Pr/MCM-41 at 400 °C and is nearly 54450 and °C, respectively. 22 times that Thus, of MCM-41 the reaction catalyst rate of at the 400 10%◦C Pr/MCM and 450‐41◦C, is indicatingnearly 54 and that 22 MCM-41 times that of MCM‐41 catalyst at 400 °C and 450 °C, indicating that◦ MCM‐41 has almost has almost no activity in this temperature range. At 600 C, almost a complete CH3SH conversionno activity in was this achievedtemperature over range. the At 10% 600 Pr/MCM-41 °C, almost a complete catalyst. CH From3SH Figure conversion6B, the was variation achieved over the 10% Pr/MCM‐41 catalyst. From Figure 6B, the variation of CH3SH con‐ of CH SH conversion as the reaction time progressed on the 10% Pr/MCM-41 catalyst at version3 as the reaction time progressed on the 10% Pr/MCM‐41 catalyst at 600 °C was 600 ◦C was investigated. Excellent stability can be observed in that the 10% Pr/MCM-41 investigated. Excellent stability can be observed in that the 10% Pr/MCM‐41 maintains maintains 100% of CH SH conversion even after 120 h time-on-stream test. The reason 100% of CH3SH conversion3 even after 120 h time‐on‐stream test. The reason why the cat‐ whyalyst has the an catalyst ultralong has stability an ultralong is because stability the praseodymium is because theoxide praseodymium species can react oxide with species cancarbon react residues with to carbon gasify residuesthe carbon to deposits gasify on the the carbon surface deposits of the 10% on Pr/MCM the surface‐41 cata of‐ the 10% Pr/MCM-41lyst. catalyst.

Figure 6. (A) Reaction rate of CH3SH at 400 °C and ◦450 °C over MCM◦ ‐41 and 10% Pr/MCM‐41 cata‐ Figure 6. (A) Reaction rate of CH3SH at 400 C and 450 C over MCM-41 and 10% Pr/MCM-41 lysts. (B) Conversion of CH3SH as a function of time at 600 °C on spent◦ 10% Pr/MCM‐41 catalyst. catalysts. (B) Conversion of CH3SH as a function of time at 600 C on spent 10% Pr/MCM-41 catalyst. 3.5. Product Distribution 3.5. Product Distribution Reaction products distribution results of the 10% Pr/MCM‐41 catalyst are shown in FigureReaction 7. Apparently, products the distributioncatalytic performance results of for the eliminating 10% Pr/MCM-41 CH3SH is catalystinfluenced are by shown in Figurethe reaction7. Apparently, temperature the from catalytic 300 °C performance to 600 °C. Below for 475 eliminating °C, CH3SH CH firstly3SH decomposed is influenced by the ◦ ◦ ◦ reactionand formed temperature a small amount from of 300 dimethylC to 600 sulfideC. (CH Below3SCH 4753, DMS)C, CH and3SH hydrogen firstly decomposed(H2), and and formedthe reaction a small equation amount was 2CH of dimethyl3SH → CH sulfide3SCH3 + (CH H2.3 Then,SCH 3the, DMS) concentration and hydrogen of CH3SCH (H2),3 and the reactiongradually equation decreased was with 2CH the rising3SH → temperature,CH3SCH3 indicating+ H2. Then, that the there concentration was an intermedi of CH‐ 3SCH3 graduallyate species decreasedduring the reaction. with the From rising 475 temperature, °C to 600 °C, indicatingCH3SCH3 was that further there decomposed was an intermediate into small molecular products such as methane◦ (CH4) and◦ hydrogen sulfide (H2S), and the species during the reaction. From 475 C to 600 C, CH3SCH3 was further decomposed reaction equation is CH3SCH3 → CH4 + H2S. At the high temperature of 600 °C, a certain into small molecular products such as methane (CH4) and hydrogen sulfide (H2S), and the ◦ reaction equation is CH3SCH3 → CH4 + H2S. At the high temperature of 600 C, a certain concentration (~1000 ppm) of COS was detected by FPD. Meanwhile, a small amount of ProcessesProcesses2021 2021, 9,, 400 9, x FOR PEER REVIEW 10 of 13 10 of 14

concentration (~1000 ppm) of COS was detected by FPD. Meanwhile, a small amount of other sulfur and carbon species, such as CS2,C2H4 and C2H6, were detected in the reaction other sulfur and carbon species, such as CS2, C2H4 and C2H6, were detected in the reaction (detailed results are not shown in Figure7). (detailed results are not shown in Figure 7).

FigureFigure 7. 7. TheThe product product distribution distribution in the in the10% 10% Pr/MCM Pr/MCM-41‐41 catalyst catalyst for methyl for methyl mercaptan mercaptan abate‐ abatement. ment.

3.6. Possible Reaction Mechanism and Active Sites for the Conversion of CH3SH over the 10% Pr/MCM-413.6. Possible Reaction Catalyst Mechanism and Active Sites for the Conversion of CH3SH over the 10% Pr/MCM‐41 Catalyst It has been reported that redox and strong acid sites play an important role for It has been reported that redox and strong acid sites play an important role for elim‐ eliminating CH3SH. He et al. [20,23] observed that microwave-assisted rapid synthesized inating CH3SH. He et al. [20,23] observed that microwave‐assisted rapid synthesized◦ nano-CeOnano‐CeO2 2cancan completely completely decompose decompose CH3SH CH at3 lowSH temperature at low temperature (450 °C) due (450 to theC) rel due‐ to the relativelyatively higher higher redox redox from from the abundant the abundant active active oxygen oxygen species. species. This indicates This indicates that strong that strong redoxredox properties properties can can improve improve the the catalytic catalytic activity activity at low at temperature. low temperature. The 10% The Pr/MCM 10% Pr/MCM-‐ ◦ 4141 catalyst completely completely decomposes decomposes CH3 CHSH 3atSH high at hightemperature temperature (600 °C) (600 due toC) the due poor to the poor redox.redox. Thus, Thus, the the main main active active sites sites of the of the10% 10% Pr/MCM Pr/MCM-41‐41 catalyst catalyst can only can be onlystrong be acid strong acid sites,sites, which which can can cleave cleave the the C‐S C-S bond bond to improve to improve the catalytic the catalytic activity. activity. The possible reaction mechanism for eliminating CH3SH is proposed as follows, as The possible reaction mechanism for eliminating CH3SH is proposed as follows, as shown in Figure 8. At first, the acid CH3SH molecule is absorbed by the basic sites of the shown in Figure8. At ﬁrst, the acid CH 3SH molecule is absorbed by the basic sites of the surfacesurface of of the the 10% 10% Pr/MCM Pr/MCM-41‐41 catalyst. catalyst. Then Then the absorbed the absorbed reactants reactants migrate migrate to the active to the active centers, such as strong acid sites. After that, the S‐H bond of CH3SH firstly dissociates to centers, such as strong acid sites. After that, the S-H bond of CH3SH ﬁrstly dissociates to CH3S and H with the 10%Pr/MCM‐41 catalyst (CH3SCH3, H2). Subsequently, CH3S can CH3S and H with the 10%Pr/MCM-41 catalyst (CH3SCH3,H2). Subsequently, CH3S can decompose to CH3 and S via the breakage of the C‐S bound by the strong acid sites. The decompose to CH and S via the breakage of the C-S bound by the strong acid sites. The formation of CH3 and3 S can be hydrogenated to CH4 and H2S. Finally, the gas products formationdesorb easily of from CH3 theand surface S can of be the hydrogenated catalysts. to CH4 and H2S. Finally, the gas products desorb easily from the surface of the catalysts.

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FigureFigure 8. 8. SchematicSchematic reaction reaction mechanism mechanism for catalytically for catalytically eliminating eliminating methyl mercaptan methyl mercaptanover 10% over 10% Pr/MCMPr/MCM-41‐41 catalyst. catalyst.

4.4. Conclusions Conclusions InIn this this work, work, a Pr a Prmodified modified MCM MCM-41‐41 sample sample presented presented longer‐term longer-term stability stabilityand and maintainedmaintained 100% 100% of ofthe the CH CH3SH3 SHconversion conversion for 120 for h. 120 The h. effect The of effect the 10% of the Pr/MCM 10% Pr/MCM-41‐41 catalyst on CH3SH catalytic decomposition was studied by a series of characterization catalyst on CH3SH catalytic decomposition was studied by a series of characterization techniques. As proved by XRD and N2 adsorption–desorption results, the 10% Pr/MCM‐ techniques. As proved by XRD and N2 adsorption–desorption results, the 10% Pr/MCM-41 41sample sample was was successfully successfully synthesized and and praseodymium praseodymium oxide oxide species species were were highly highly dis- dispersed over the MCM‐41 support. H2‐TPR profiles hardly showed any obvious reduc‐ persed over the MCM-41 support. H -TPR profiles hardly showed any obvious reduction tion peak, indicating that the 10% Pr/MCM2 ‐41 catalyst possesses poor redox performance. peak, indicating that the 10% Pr/MCM-41 catalyst possesses poor redox performance. Meanwhile, CO2‐TPD and NH3‐TPD results suggested that 10%Pr/MCM‐41 catalyst with strongMeanwhile, basic/acid CO sites2-TPD promoted and NH the3-TPD adsorption results and suggested the break thatof C‐ 10%Pr/MCM-41S bonds of acid methyl catalyst with mercaptanstrong basic/acid molecules. sites Thus, promoted the main the active adsorption sites of and the the 10% break Pr/MCM of C-S‐41 bonds catalyst of for acid methyl CHmercaptan3SH catalytic molecules. decomposition Thus, were the the main strong active acid sites sites. of In theaddition, 10% Pr/MCM-41the products dis catalyst‐ for tributionCH3SH was catalytic employed decomposition to clarify the were reaction the strongmechanism. acid sites.In consideration In addition, of the poor products dis- catalytictribution activity was employedof the 10% Pr/MCM to clarify‐41 the catalyst reaction at low mechanism. temperature, In the consideration regulation of the of the poor redoxcatalytic performance activity ofof thethe 10%catalyst Pr/MCM-41 in the elimination catalyst of at sulfur low‐ temperature,containing volatile the regulationorganic of the compoundsredox performance (VOCs) is ofthe the focus catalyst in the future in the research. elimination of sulfur-containing volatile organic compounds (VOCs) is the focus in the future research. Author Contributions: Conceptualization: J.L. and Y.L.; Data curation: X.C., J.L. and Y.L.; Formal analysis: X.C., W.Z. and D.H.; Investigation: X.C. and Y.Z.; Methodology: Y.Z. and R.T.; Software: X.C.Author and R.T.; Contributions: Supervision: Conceptualization:J.L., W.Z., D.H. and Y.L.; J.L. Validation: and Y.L.; DataR.T. and curation: W.Z.; Visualization: X.C., J.L. and W.Z. Y.L.; Formal andanalysis: D.H.; Writing–original X.C., W.Z. and draft: D.H.; X.C.; Investigation: Writing—review X.C. and editing: Y.Z.; Methodology: J.L. and Y.L. All Y.Z. authors and have R.T.; Software: readX.C. and and agreed R.T.; Supervision:to the published J.L., version W.Z., of D.H. the manuscript. and Y.L.; Validation: R.T. and W.Z.; Visualization: W.Z. and D.H.; Writing–original draft: X.C.; Writing—review and editing: J.L. and Y.L. All authors have Funding: This research work was supported by the National Natural Science Foundation of China (Grantread and Nos. agreed 21966018, to the 42030712, published 21667016, version and of 21666013), the manuscript. and Science Research Fund Project of YunnanFunding: ProvincialThis research Department work of was Education supported (Grant by No. the 2020J0060). National Natural Science Foundation of China Institutional(Grant Nos. Review 21966018, Board 42030712, Statement: 21667016, Not applicable. and 21666013), and Science Research Fund Project of Yunnan Provincial Department of Education (Grant No. 2020J0060). Informed Consent Statement: Not applicable. DataInstitutional Availability Review Statement: Board All Statement: data used toNot support applicable. the findings of this study are included within the article. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Data Availability Statement: All data used to support the findings of this study are included within References the article. 1. Deshmukh, S.; Jana, A.; Bhattacharyya,Conflicts of N.; Interest: Bandyopadhyay,The authors R.; Pandey, declare R.A. no conflict Quantitative of interest. determination of pulp and paper industry emissions and associated odor intensity in methyl mercaptan equivalent using electronic nose. Atmos. Environ. 2014, References 1. Deshmukh, S.; Jana, A.; Bhattacharyya, N.; Bandyopadhyay, R.; Pandey, R.A. Quantitative determination of pulp and paper industry emissions and associated odor intensity in methyl mercaptan equivalent using electronic nose. Atmos. Environ. 2014, 82, 401–409. [CrossRef] Processes 2021, 9, 400 12 of 14

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