The Pore Confinement Effect of FDU-12 Mesochannels on MoS 2 Active Phases and Their Hydrodesulfurization Performance

FDU-12 silica with highly ordered face-centered cubic mesoporous structure is developed as support to prepare Mo/FDU-12 catalysts for hydrodesulfurization (HDS) of dibenzothiophene (DBT). A series of Mo/FDU-12 catalysts are synthesized by using incipient wetness impregnation method with different MoO 3 loadings (6, 8, 10, 12, and 15wt.%). The objective of this work is to explore the pore confinement effect of FDU-12 mesochannels on the MoS 2 morphology with various metal loadings. It is found that, as increasing MoO 3 loadings from 6 to 15 wt.%, the MoS 2 nanocrystallites transform from monolayer to multilayer and the morphology changes from straight layered to curved and then to ring-like and finally to spherical-like morphology due to the restriction of cage-like pore channels of FDU-12 support. The HDS results show that the catalytic activity increases first and then decreases with the best HDS performance at theMoO 3 loading of 10wt.%. In addition, we compared theHDS activity ofMo catalyst supported on FDU-12 with that on the commercial γ-Al 2 O 3 and SBA-15; the result exhibits that FDU-12 is superior to the other two supports due to its large pore size and ordered three-dimensional open pore channels.


Introduction
Ordered mesoporous materials with high surface area, large pore volume, ordered pore structure, and good thermal and mechanical stabilities have a broad range of applications in catalysis, adsorption, energy storage, and nanodevices due to their unique physical and chemical properties [1][2][3][4].In general, the ordered mesostructures can be classified as twodimensional (2D) and three-dimensional (3D) architecture according to the pore symmetry.The typical 2D mesoporous materials are SBA-15, MCM-41, and so forth [5,6] and the 3D mesoporous materials usually include MCM-48, SBA-16, KIT-6, and FDU-12 [7,8], whose property is considered to be superior to the former because the 3D channel is favorable for the mass transfer and diffusion of guest molecules.The mesoporous materials usually act as host to support the guest species or behave as a nanoreactor to provide a space for reaction, which are widely used in catalysis.When the active phase precursor is incorporated into the pores of the support, it can be inevitably restricted by the pore size and pore structure and, therefore, it is interesting to explore the pore confinement effect on the active phases.
One of the main applications of the mesoporous materials in catalysis field is as catalyst supports for hydrodesulfurization (HDS).HDS is considered to be the most effective method to bring down the sulfur content in fuels and produce ultra-low-sulfur clean fuels to fit the more and more stringent environmental policy [9,10].Among different materials recently as supports for HDS catalysts, the mesoporous silica materials with ordered pore structures such as SBA-15, MCM-41, and KIT-6 have attracted widespread attention [11][12][13][14][15]. FDU-12 is a kind of 3D material with a face-centered cubic (Fm-3m) symmetry and its particular property of highly open channel networks makes it possible for faster diffusion of reactants and products during the reaction without pore blockage as happened in MCM-41 and SBA-15 with linear pore channel structure [13].But unfortunately, to the best of our knowledge, there is no report to investigate the pore confinement effect of the cage-like pore structure of FDU-12 on the active phases with the increase of metal loadings and further on the variation of HDS activity.
Herein, we apply the 3D cubic cage-like mesostructured FDU-12 as the support to prepare a series of Mo/FDU-12 catalysts with different MoO 3 loadings (6-15 wt.%) in order to systematically explore the pore confinement effect on the MoS 2 active phases and the HDS performance.The results show that the morphology of MoS 2 changes from straight layered to curved and then to ring-like and spherical-like morphology as increasing MoO 3 contents (Scheme 1) owing to the restriction of cage-like pore channels of FDU-12 support and the best HDS activity is found at MoO 3 loading of 10 wt.% with curved active phases.Moreover, the comparison of HDS activity is done with commercial -Al 2 O 3 and SBA-15 supported catalysts, demonstrating the importance of the pore size and pore structure of catalyst support and highlighting the efficacy of FDU-12.

The Preparation of FDU-12 Support and Mo(𝑥)/FDU-12
Catalysts.Mesoporous structured pure FDU-12 silica was synthesized according to the literature [8] using nonionic block copolymer F127 as template and TMB together with KCl as additives.In a typical synthesis, 2.0 g of F127 and 10.0 g of KCl were completely dissolved in 2 M HCl (120 mL) at 15 ∘ C, then 2.4 g of TMB was added, and the mixture was stirred at 15 ∘ C for 6 h, and next 8.32 g of TEOS was added to the above mixture.After stirring for 24 h at 15 ∘ C, the mixtures were directly transferred into an autoclave for hydrothermal treatment at 150 ∘ C for 24 h.The solid product was collected by filtration, washed with water, dried, and then calcined at 550 ∘ C for 5 h to remove the templates.
Mo catalysts supported on FDU-12 were prepared by incipient wetness impregnation method reported elsewhere by using appropriate concentration of ammonium molybdate tetrahydrate as Mo source.The MoO 3 loadings of the catalysts were 6, 8, 10, 12, and 15 wt.%, respectively.After impregnation, all the catalysts were dried at 80 ∘ C for 12 h and calcined at 500 ∘ C for 4 h in static air atmosphere denoted by Mo()/FDU-12, in which  represents the MoO 3 content.For comparison, -Al 2 O 3 and mesoporous SBA-15 supported catalysts with MoO 3 content of 10 wt.% were prepared according to the same loading method.

Catalyst Characterization.
Small angle X-ray scattering (SAXS) profiles of the support were obtained from Bruker NanoSTAR with a 2D detector and X-ray beam pinhole collimated (40 kV, 30 mA).N 2 adsorption-desorption measurements of the support and corresponding catalysts were performed on a Micromeritics ASAP 2002 instrument (USA).The specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) method and the pore size distributions were obtained using the Barrett-Joyner-Halenda (BJH) method [16].The samples to be measured were firstly degassed in the preparation station at 180 ∘ C and a vacuum of 10 −5 Torr for 15 h and then switched to the analysis station for adsorption-desorption experiment at −196 ∘ C. Powder Xray diffraction (XRD) experiments were performed using a Philips X'Pert diffractometer equipped with Cu K radiation (wavelength 1.5406 Å).XRD patterns were collected with the 2 range between 8 ∘ and 70 ∘ , a step size of 2 ∘ , and a counting time of 60 s per step.UV-vis diffuse reflection spectroscopy (UV-vis DRS) experiments were performed on a Hitachi U-4100 UV-vis spectrophotometer with the integration sphere diffuse reflectance attachment.The powder samples were loaded into a transparent quartz cell and were measured in the region of 200-600 nm under ambient conditions.The standard support reflectance was used as the baseline for the corresponding catalyst measurement.
The morphological features of the catalysts were obtained from high resolution transmission electron microscopy (HRTEM) with a Tecnai F20 at 200 kV.The powder samples were grounded smoothly in an agate mortar and dispersed in ethanol in an ultrasonic bath for several minutes.Xray photoelectron spectroscopy (XPS) measurements of the sulfided catalysts were performed on a VG ESCA Lab 250 spectrometer using AlK radiation.To quantify the contents of Mo 4+ , Mo 5+ , and Mo 6+ species, the XPS spectra obtained were fitted using XPSPEAK software (version 4.1).A Shirley background was applied and the Mo 3d spectra were deconvoluted by fitting the experimental spectra to a mixed Gaussian-Lorentzian function.

Catalytic Activity.
The HDS activity tests were performed in a continuous fixed bed reactor with 0.5 g catalyst.Before the catalytic activity testing, the catalysts were sulfided with a mixture of 3 wt.%CS 2 and cyclohexane at 360 ∘ C for 4 h, under 4.0 MPa H 2 pressure.After sulfidation, 1 wt.%DBT in heptanes used as a model compound was fed to the reactor under the conditions of reaction temperature of 360 ∘ C, H 2 /oil of 300, and weight hourly space velocity (WHSV) of 13 h −1 .After evaluation, the reaction products were collected and the sulfur content of products was analyzed by a WK-2C type microcoulomb meter.
Assuming a pseudo-first-order reaction for HDS of DBT, the activity of the catalysts can be expressed by [17] where  is the feeding rate of the reactant in mol s −1 ,  is the catalyst mass in grams,  is the total conversion of DBT, and  HDS is the rate constant of HDS in mol g −1 s −1 .

Characterization of FDU-12 Support and Mo(𝑥)/FDU-12
Catalysts in Oxidic State.The SAXS pattern of the pure silica FDU-12 hydrothermally treated at 150 ∘ C is shown in Figure 1.
It exhibits six characteristic peaks, which should be assigned to the (111), ( 220), (311), (420), (333), and (531) reflections of a face-centered cubic structure [8], indicating the successful synthesis of highly ordered FDU-12 mesoporous silica.Figure 2(a) presents the N 2 adsorption-desorption isotherms of FDU-12 support and Mo()/FDU-12 catalysts and they all exhibit type IV isotherm with H2 hysteresis loop demonstrating the cage-like mesostructure.The shape of the isotherm and the hysteresis loop does not suffer evident changes after the incorporation of Mo species.The pore size distribution curves calculated from the adsorption branch  Note: [a] specific surface area calculated by BET method, [b] pore volume, [c] pore diameters calculated from the adsorption branch, and [d] window sizes obtained from the desorption branch by using the BJH method.
(Figure 2(b)) and desorption branch (Figure 2(c)) show a monomodal distribution with the pore diameter (cage size) centered at ca. 17 nm and window size centered at ca. 8.5 nm, respectively, and it is clear that, after the incorporation of Mo species, both of the cage and window size remain essentially unchanged.
The textural and structural characteristics (the specific surface area  BET , total pore volume   , and pore diameter   ) of the FDU-12 support and corresponding catalysts obtained by N 2 sorption results are summarized in Table 1.FDU-12 support has a BET surface area of 346.4 m 2 /g and total pore volume of 0.90 cm 3 /g and Mo(6)/FDU-12 catalyst only has a slight drop in surface area and pore volume since an increase in the sample's density after the incorporation of Mo species.However, this drop gradually becomes more apparent with the increase of MoO 3 contents, especially for Mo(15)/FDU-12 catalyst because the excess loadings may lead to the agglomeration of Mo species, partially blocking the pores of FDU-12.
More information about the aggregation and coordination state of the oxidic Mo species in Mo()/FDU-12 catalysts was provided by XRD and UV-vis DRS.The powder XRD patterns (Figure 3 2 = 24 ∘ , typical of amorphous silica.At lower MoO 3 loadings (6, 8, and 10 wt.%), there is no signal of any crystalline phase indicating the well-dispersed Mo species over the support.With the increase of MoO 3 loadings, the signals of MoO 3 crystallite phase are detected on Mo(12)/FDU-12 catalyst, but the signal strength is very weak.Nevertheless, with respect to Mo(15)/FDU-12 catalyst, there are four characteristic peaks that can be clearly observed at 2 = 12.7, 23.4,25.7, 27.4 ∘ , which are attributed to the bulk MoO 3 crystallite phase (PDF 05-0508).The excess loadings may result in the aggregation of metal oxides on the pure silica support due to the weak support-metal interaction and it will form large particles after the high-temperature calcination, which are hard to be sulfided and thereby unfavorable for the catalytic activity.
Figure 3(b) shows DRS spectra of the Mo()/FDU-12 catalysts and the absorption band in the spectra corresponds to the ligand-to-metal charge transfer (LMCT) O 2− → Mo 6+ .It is common knowledge that the position of this LMCT absorption band depends intensely on the local symmetry around the Mo 6+ species and their aggregation state [18].As reported elsewhere [19], the isolated molybdate species in tetrahedral coordination (Td) show a typical absorption band at about 250 nm; nevertheless the signal of polymolybdate species in octahedral coordination (Oh) is examined at the 280-330 nm range and its position is influenced by the aggregate size.Besides, both types of Mo 6+ species exhibit the second strong absorption band at about 220 nm.As can be seen from the spectra, a mixture of Mo species in tetrahedral and octahedral coordination is present in all Mo()/FDU-12 catalysts, but the Mo absorption edges produce a slight red shift with increasing the MoO 3 loadings, indicating the occurrence of the large MoO 3 clusters, which is consistent with a decrease in the dispersion of Mo species detected by XRD (Figure 3(a)).

Characterizations of Mo(𝑥)/FDU-12 Catalysts in Sulfided
State. Figure 4 displays the HRTEM images of the five Mo/FDU-12 catalysts with different MoO 3 loadings to obtain more information about the morphology and dispersion of  ).It can be concluded that, with the increase of MoO 3 loadings from 6 to 15 wt.%, the morphology of MoS 2 active phases changes from straight layered to curved and then to ring-like and sphericallike morphology, revealing that MoS 2 morphology is strongly affected by the pore structure of support.The Mo 3d XPS spectra of the sulfided Mo catalysts supported on FDU-12 were measured and employed to determine the state of Mo species after sulfidation (Figure 5).The binding energies of the Mo 3d 5/2 and Mo 3d 3/2 levels for Mo 4+ (MoS 2 ) are about 229.1 and 232.0 eV, respectively, those for Mo 5+ (MoO x S y ) are about 230.5 and 233.8 eV, and those for Mo 6+ (MoO 3 ) are about 232.8 and 236.0 eV [21,22], and the binding energy at about 226.5 eV is ascribed to the S 2s level.The sulfidation degree of Mo species, Mo sulfidation , is expressed as Mo 4+ /(Mo 4+ + Mo 5+ + Mo 6+ ) ratio and the corresponding results are calculated in Table 2.It is shown that the sulfidation degree of the Mo catalysts increases with the increase of metal loadings and reaches the highest value (69.0%) for Mo(10)/FDU-12 catalyst, due to the formation of well-dispersed MoS 2 nanoparticles with suitable stacking layer numbers.However, with the further increase of MoO 3 loadings, the Mo sulfidation decreases and downgrades to the lowest value (37.5%) for Mo(15)/FDU-12 catalyst because of the existence of bulk MoO 3 crystallites which are hard to be sulfided.The surface S/Mo atomic ratios of Mo()/FDU-12 catalysts were also determined by XPS and the results are given in Table 2.The S/Mo mole atomic ratio presents a similar trend which is in good accordance with the results of sulfidation degree.   in a similar law for the five catalysts are summarized in Table 2.It is shown that the HDS activity of Mo()/FDU-12 catalysts increases from 70.6% to 91.7% and then decreases to 60.4% with the best HDS performance at MoO 3 loading of 10 wt.%.Mo(10)/FDU-12 catalyst possesses the highest  HDS (0.42), which is about twice higher than Mo(6)/FDU-12 catalyst ( HDS of 0.22) and almost three times better than Mo(15)/FDU-15 catalyst ( HDS of 0.16).The low HDS ratio at low metal loadings is due to the limited active sites over the support and the monolayer morphology with low activity.Further increasing the MoO 3 loadings, the MoS 2 active phases become curved morphology with suitable stacking layers which has been reported to have more active sites than straight ones [23]; thus the catalyst exhibits the best HDS activity.However, when more Mo precursors enter into the cage-like pores of FDU-12, it happens to aggregate and forms spherical-like morphology because of the pore confinement effect, and such morphology leads to the dramatic decrease of the amounts of active sites and thus results in a poor HDS performance.
In addition, in order to reflect the superiority of threedimensional mesostructure and explore the support effect, it is interesting to compare the catalytic activities of Mo/FDU-12 catalyst with using SBA-15 and -Al 2 O 3 as supports at the same MoO 3 loading of 10 wt.%.SBA-15 has a twodimensional mesostructure and -Al 2 O 3 usually has irregular pore channels and nonuniform pore sizes and the textural properties of Mo(10)/SBA-15 and Mo(10)/-Al 2 O 3 are given in Table 1.The results indicate that FDU-12 supported catalyst is more active (HDS ratio of 91.7%) than the other two catalysts with HDS ratio of 86.0% and 80.2%.It is well known that the restrictive diffusion factor, (), is usually used to describe the diffusion limitations [24].() value is inversely proportional to  ( represents the ratio of the molecular diameter to the pore diameter) and thus, for a fixed molecular diameter, the larger the pore diameter, the smaller the restrictive diffusion effect.The window size (which is the main factor to affect the molecular diffusion) of Mo(10)/FDU-12 and the pore size of Mo(10)/SBA-15 and Mo(10)/-Al 2 O 3 are 8.5, 7.1, and 5.9 nm, respectively; thus the difference of () value is quite small, especially for Mo(10)/FDU-12 and Mo(10)/SBA-15.Therefore, the better HDS performance for Mo(10)/FDU-12 should be attributed to its three-dimensional open structure which is more favorable for the molecules diffusion in the pore channels to increase the accessibility to the active sites as demonstrated in Scheme 2.

Conclusion
In this study, cage-like mesostructured FDU-12 material was applied as support to synthesize a series of Mo/FDU-12 catalysts with different MoO 3 loadings in order to explore the pore confinement effect on active phases and the HDS activity.The dispersion and the morphology of MoS 2 vary with the increase of MoO 3 loadings.It is intriguing to note that due to the restriction of the cage-like pores of FDU-12 support, the layered MoS 2 crystallites transform from straight to slightly curved then to ring-like and finally to sphericallike morphology in the pores.For the HDS evaluation, Mo(10)/FDU-12 with the proper metal loading exhibits the highest HDS activity since the well-dispersed and curved MoS 2 nanoparticles with suitable stacking layer numbers expose more active sites.Furthermore, it is found that FDU-12 with 3D open pore structure and large pore size is more favorable for reactants diffusion and thus it shows to be superior as catalyst support compared with -Al 2 O 3 and SBA-15.

2 Scheme 1 :
Scheme 1: The schematic diagram to illustrate the change of the morphology of MoS 2 active phases with different MoO 3 loadings supported on FDU-12.

3. 3 .
HDS Catalytic Activity.HDS catalytic behaviors of Mo()/FDU-12 catalysts were evaluated in a continuously flowing tubular fixed bed microreactor by using DBT as a model compound which is a representative sulfur-containing compound for diesel fuel.By comparison, the values of the pseudo-first-order rate constant  HDS which is present