Fig. 1. BT formation rates of Mo/M and Mo/M_x (x = 200, 250, 300, 350, 400, 450, 500, 600, and 700) as a function of TOS. Solid black squares corresponding to Mo/M are enlarged for visibility. Gray arrows, which denote the changes in performance compared to Mo/M, are added for eye-guidance. All MDA reactions were conducted at 700 °C and atmospheric pressure.

Fig. 2. Characterization of Mo/M and Mo/M_x (x = 400, 500, and 600). (a) XRD patterns. (b) Ar adsorption-desorption isotherms at 87 K (adsorption: solid and desorption: open). (c) H-K pore size distributions from 0.4 to 0.8 nm. (d) BJH pore size distributions from 2 to 10 nm. In (a), for a fair comparison, the simulated XRD patterns of MoO3 and all-silica MWW type zeolite are included at top and bottom, respectively, and inverted red triangles indicate the peaks arising from ɑ-alumina, which was used as an internal standard. MoO3 was obtained by calcining the Mo precursor (ammonium heptamolybdate tetrahydrate) under the same conditions used for the four Mo-impregnated catalysts. In (b) and (c), the solid or open black squares corresponding to Mo/M are enlarged for visibility. In (b), the inset shows a narrower axis for the adsorbed amount. In (c), for clarity, schematics of MCM-22 and its pore network system are provided. The pore size distributions in (c) and (d) were calculated from Ar adsorption branches.

Fig. 3. Acid characterization of the H-MCM-22 support series (M and M_x; x = 400, 500, and 600) and their Mo-impregnated catalysts (Mo/M and Mo/M_x; x = 400, 500, and 600). NH3-TPD profiles (a,b) and Py-adsorption FT-IR spectra (c,d). In (a), the dashed line emphasizes the decrease in acid strength. In (c) and (d), BAS and LAS stand for the Br?nsted acid sites and Lewis acid sites, respectively. Additionally, offset values for absorbance are indicated.

Fig. 4. 27Al MAS NMR spectra of the H-MCM-22 support series (M and M_x; x = 400, 500, and 600) (a) and their Mo-impregnated catalysts (Mo/M and Mo/M_x; x = 400, 500, and 600) (b).

Fig. 5. BT formation rates (as in Fig. 1) (a) and CH4 conversions (b) of Mo/M and Mo/M_x (x = 400, 500, and 600). In (a), the naphthalene (naph) formation rates for the corresponding catalysts are shown in the inset. In (b), the inset shows a narrower axis for CH4 conversion. All MDA reactions were conducted at 700 °C and atmospheric pressure for ca. 32 h.

Fig. 6. BT (same as Fig. 5(a) with the initial TOS omitted) and C2 formation rates of Mo/M (a), Mo/M_400 (b), Mo/M_500 (c), and Mo/M_600 (d) as a function of TOS. Shaded orange regions indicate the specific TOS wherein the C2 formation rate decreased. Linear fitting equations of C2 formation rates before the point of decline were calculated for quantitative comparison.

Fig. 7. Mo 3d XPS spectra (a) and H2-TPR profiles (b) of Mo/M and Mo/M_x (x = 400, 500, and 600). In (a), the deconvoluted spectra show typical peaks of the Mo6+ species in the calcined catalysts. Additionally, offset values for intensity are indicated. In (b), the H2-TPR profile of Mo/ITQ-1(C) is also included for comparison.

Fig. 8. Coke and deactivation analyses of spent Mo/M and Mo/M_x (x = 400, 500, and 600). Accumulated amounts of coke determined from TGA (a) and combined TG and Ar physisorption analyses (b) as a function of reaction time. The amounts of soft and hard coke in (a) were obtained by decoupling the two representative peaks at the low and high temperatures in the TGA profiles shown in Fig. S13. In (a), numerical values (i.e., slopes from linear fittings) are included to signify the estimated accumulation rates (mgcoke·g?1·h?1) of soft coke. (c) Ratios of accumulated amounts of coke in 12-MR pores to that in 10-MR pores as a function of reaction time. Accumulated amounts of coke in the 10-MR or 12-MR pores as a function of reaction time (d1) along with BT and C2 formation rates as shown in Fig. 6 (d2). The accumulated amounts of coke in each pore were determined by multiplying the difference between the fresh and spent catalyst pore volumes to an assumed coke density (i.e., (V10-MR/12-MR,fresh - V10-MR/12-MR,spent) × d1 or 2). Shaded gray regions in (b), (c), and (d1) indicate reaction times during which a change in coke density was assumed to occur. Dashed lines in (d1)-(d2) are included for eye-guidance in which a change in color denotes possible trend changes (orange-to-red and gray-to-black).

Scheme 1. Schematic of proposed MDA reaction, showing activation, induction, and deactivation stages.

Fig. 9. (a) BT yields of Mo/M and Mo/M_x (x = 400, 500, and 600) as a function of TOS fitted by Eq. (1). (b) Deactivation coefficients of Mo/M and Mo/M_x (x = 400, 500, and 600) plotted against the corresponding total amounts of BT produced during the 32-h reaction. The dashed curve is added for eye-guidance.

Scheme 2. Generalized scheme for the four representative Mo-impregnated catalysts (Mo/M and Mo/M_x; x = 400, 500, and 600). Starting with either pristine or hydrothermally treated H-MCM-22 supports, various effects on the C2 (before decrease) and aromatic (BT and naphthalene) yields and coke formation behaviors (especially in relation to the pore systems, as in Fig. 8(c)) depended on the amount of BAS in each H-MCM-22 support and, simultaneously, the distribution of Mo in the resulting Mo-impregnated catalyst.

Fig. 10. MDA performance of Mo/M and Mo/M_400 (red dashed ellipse) along with those of Mo-impregnated MCM-22 (solid black squares) catalysts prepared in this study and Mo-impregnated MCM-22 (open squares) and ZSM-5 (open circles) catalysts reported in the literature [51,74,76,77,85,107,118,122,128,160-170]. Detailed information about the catalysts and their MDA performance is given in Table S3. For fair comparison, we considered the results obtained after a TOS of ca. 6-7 h. The x- and y-axes correspond to CH4 conversion and BT/benzene selectivity, respectively, and the curves indicate the contour of the BT or benzene yield. Depending on the availability of data, the selectivities can either refer to BT selectivities or benzene selectivity alone. Unless otherwise indicated, data were gathered from MDA reactions having similar conditions employed in this study (ca. 700 °C, 1 atm, and 1500-1800 mL·g?1·h?1). Numbers in parentheses (i.e., (n1, n2)) denote the nominal weight percent loading of Mo (n1) and the Si/Al ratio (n2) of the support. Gray arrows are added to point the change from pristine catalysts to treated ones. Error bars for Mo/M and Mo/M_400 correspond to standard deviations calculated from five runs of MDA.