III. CATALYTIC CRACKING OF N-HEXANE FOR PRODUCING LIGHT
3.2. CATALYST TEST
The catalytic performance of the catalysts in both powder and monolith forms were investigated in the conversion of n-hexane to light olefins at various reaction temperatures. The n-hexane conversion rates (Xn-hexane) as a function of time on stream are exhibited in
Figure 7. All catalysts showed enhanced activity in n-hexane conversion under higher reaction temperature. Moreover, although ZP showed slightly higher n-hexane conversion than ZM and SZM under both reaction temperatures at initial stages of the reaction, it experienced an obvious decline within 24 h on stream. The monolithic catalysts, ZM and SZM, displayed a stable n-hexane conversion during the investigated time on stream. It is generally believed that coke formation in zeolite catalysts is the cause of its deactivation [63]. Factors such as external surface area [32], strong acid sites amount [64] and acid site density [65] influence the amounts of coked deposition. As discussed in the previous section, the strong acid sites amount of the monolith catalysts ZM and SZM are lower than that of ZP, leading to less deactivation by coke formation. It is apparent that the fabrication of monoliths, which possess diluting binder and monolith channels, decreased the acid sites density hence retarded the coking rate and extended the catalyst lifetime. The fact that SZM
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showed slightly higher n-hexane conversion than bare ZM is in agreement with NH3-TPD data in which SAPO-34 growth enhanced the total acid sites amount of the monolith. Furthermore, the hierarchical zeolite monoliths with macro-meso-microporosity favored the mass transfer of the intermediates and products hence suppressed secondary reactions such as aromatics polymerization and reduced coke formation [24].
0 5 10 15 20 25 60 65 70 75 80 85 90 95 100 ZP ZM SZM Xn -h exan e ( %) Time on stream (h) (a) Reaction Temperature: 600oC 0 5 10 15 20 25 60 65 70 75 80 85 90 95 100 Reaction Temperature: 650oC ZP ZM SZM Xn -h exan e ( %) Time on stream (h) (b) 0 5 10 15 20 25 60 65 70 75 80 85 90 95 100 Reaction Temperature: 600oC YP YM SYM Xn- h ex a n e (%) Time on stream (h) (c) 0 5 10 15 20 25 60 65 70 75 80 85 90 95 100 Reaction Temperature:650 oC YP YM SYM Xn -h ex a n e (%) Time on stream (h) (d)
Figure 7. Conversion of n-hexane as the function of time on stream on the investigated HZSM-5 and HY zeolites at (a) (c) 600 °C and (b) (d) 650 °C. Reactant, n-hexane;
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In the case of HY zeolite, powder (YP) and bare monolith (YM) showed similar activity in n-hexane conversion at 600 °C and 650 °C, as can be observed in Figure 7c and 7d respectively. The n-hexane conversion over HY zeolite monolith coated with SAPO-34 (SYM) was higher than the other two HY zeolite catalysts. This outstanding conversion rate could be attributed to the superior total acid sites amount by SAPO-34 growth. Unlike HZSM-5 zeolite catalyst, all HY catalysts, regardless of catalyst structure, retained their activity in n-hexane conversion within 24 h on stream. The comparison of the external surface area between ZP and YP could explain the remarkable stability of HY zeolite catalyst. It is typically considered that the coke formation mainly occurs on the external surface of the zeolite crystal [33]. YP with an external surface area of 303 m2g-1, much higher than ZP with 168 m2g-1, suffered much less deactivation caused from coke deposition. In addition, HY zeolite framework bears cages and channels with much larger dimension than that of HZSM-5 zeolite, as discussed in previous sections. Larger space lessened the coke deposition which was confined in cages of zeolite crystals.
The products obtained from the n-hexane conversion over the investigated catalysts were found to be paraffin (C1–C5), olefins (C2=–C5=) and BTX (benzene, toluene, and xylene). Figure 8 a-d summarizes the selectivity to various hydrocarbons in three separate periods of the reaction: 1 h on stream, the initial period of the reaction; 10 h on stream, the medial period of the reaction; 24 h on stream, the final period of the reaction. To be conclusive, ethylene (C2=), propylene C3= and butylene (C4=), known as light olefins, are stacked in an individual column and plotted along with BTX, paraffin and other hydrocarbons represented by assorted color. In each period of the reaction, catalysts are
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displayed and compared in the following order: powder zeolite, bare monolith and monolith with coated SAPO-34.
0 10 20 30 40 50 60 70 C=2 C=3 C=4 BTX paraffin Others 24h 10h SZM ZM Sel ecti vi ty (%) ZP 1h Time on Stream Reaction Temperature: 600oC SZM ZM ZP ZP ZMSZM (a) 0 10 20 30 40 50 60 70 C=2 C=3 C=4 BTX paraffin Others 24h 10h SZM ZM Sel ecti vi ty (%) ZP 1h Time on Stream Reaction Temperature: 650oC SZM ZM ZP ZP ZMSZM (b) 0 10 20 30 40 50 60 70 C=2 C=3 C=4 BTX paraffin Others 24h 10h SYM YM Sel ecti vi ty (%) YP 1h Time on Stream Reaction Temperature: 600oC SYM YM YP YP YMSYM (c) 0 10 20 30 40 50 60 70 C=2 C=3 C=4 BTX paraffin Others 24h 10h SYM YM Sel ecti vi ty (%) YP 1h Time on Stream Reaction Temperature: 650oC SYM YM YP YP YMSYM (d)
Figure 8. Product distribution on the investigated HZSM-5 and HY zeolites at (a) (c) 600 °C and (b) (d) 650 °C. Reactant, n-hexane; WHSV, 5 h-1; time on stream, 24 h,
pressure, 1.01 bar.
Figure 8a and 8b illustrate the product distribution over different HZSM-5 zeolite samples at 600 °C and 650 °C respectively. In all stages, bare monolithic HZSM-5 showed higher selectivity to light olefins and lower BTX selectivity than its powder counterpart (ZP), at both temperatures. It is well accepted that n-hexane cracking is initiated with the
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formation of carbenium ion and then undergoes β-scission to give carbenium ion of lower carbon number and light olefins. The successive reactions of these light olefins produce BTX [66]. Javaid et al. reported that smaller acid concentration (total acid sites amount per unit mass) suppressed the formation of BTX [67, 68]. These findings match with the NH3- TPD results: the acid sites amount decreased for monolith compared to powder sample. The same trend was also observed for HY powder (YP) and monolith catalysts. The highest selectivity to light olefins on HZSM-5 zeolite (53.0%) was found over ZM at 650 °C in 24 h on stream while the highest on HY zeolite (57.9%) was obtained at 600 °C in 24 h on stream. It is noteworthy that the BTX selectivity on YP and YM were significantly small. This could be related to the large channels of the FAU framework which provide plenty of room for the reaction and adequate diffusion for the light olefins so that aromatization was suppressed.
The effect of SAPO-34 growth on the products distribution varied with the reaction temperature, as well as the zeolite type. The presence of SAPO-34 on the monolith surface increased the light olefin selectivity and decreased BTX selectivity at 600 °C on HZSM-5 zeolite whereas a reverse trend was observed at 650 °C. This could be explained by the fact that the aromatization of olefins on HZSM-5 is influenced by reaction temperature [38]. High reaction temperature favors the formation of BTX from hydrocarbons of lower carbon number. At 650 °C, boosted aromatization resulted in the reduction in light olefins and enhancement in BTX production. However, at 600 °C the acidity enhancement by SAPO- 34 growth on the monolith might promote the production of light olefins, but further aromatization to BTX was thermodynamically limited. On HY zeolite monoliths, SAPO- 34 growth dramatically increased the BTX selectivity compared to the bare sample. The
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significant improvement is the results of both surface property modification and the acidity enhancement. The highest BTX selectivity reached up to 27.5% over SYM at 600 °C.
To verify the cause of catalyst deactivation and benefits of monolithic catalysts, TGA of the spent catalysts after 24 h of n-hexane cracking at 650 °C was carried out in the temperature range of 30 – 900 °C in a 60 mL min-1 air flow. Both TGA and corresponding DTA profiles were plotted and displayed in Figure 9. All catalysts exhibited a significant
100 200 300 400 500 600 700 800 900 0.0 0.2 0.4 0.6 0.8 1.0 YP YM SYM ZP ZM SZM D eriv a tiv e W eig h t (%/ o C ) Temperature (oC) 40 50 60 70 80 90 100 Weight Loss (%)
Figure 9. TGA (lower) and DTA (upper) profiles of the spent catalysts after n-hexane cracking at 650 °C.
weight loss above 600 °C which represents the formation of ‘hard coke’ due to the activity on acid sites [69]. A peak in DTA profile for HZSM-5 powder (ZP) was observed at lower temperature between 450 and 500 °C. It is attributed to the ‘soft coke’ associated with the
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condensation and subsequent growth of coke precursor in the catalyst pore system. Owing to the relatively high acid sites amount as well as the restricted diffusion of intermediates and products in the confined cages and channels, the most severe coke formation was found in HZSM-5 powder. It was in agreement with the rapid deactivation of ZP therefore related the good performance of monolithic catalysts with their hierarchical porous structures.
4. CONCLUSION
In summary, it is feasible and advantageous to fabricate monolithic catalysts with 3D printing technology since it provides a rapid, cost-efficient and facile way of manufacture. The framework of the crystal and activity of the zeolite retained after fabrication into the monolith. The surface area and porosity were modified by the monolith fabrication but comparable to the power zeolite. Our catalytic results showed that zeolite acidity was also influenced by formulation into 3D-printed monoliths. The overall changes in the catalyst properties promoted the stability of HZSM-5 catalyst. The 3D-printed HZSM-5 monolith enhanced the selectivity to light olefins as a result of hierarchical pores and moderated acidity. SAPO-34 growth significantly tailored the characterizations of the zeolite monoliths. The most noteworthy effect was the increase of selectivity to BTX over 3D-printed HY monolith. As a pioneering research in the 3D-printed active catalyst, the resulst presented in this work demonstrated this technique to be a promising alternative fabrication approach to synthesize structured catalysts in desired configurations with comparable or even better catalytic performance than their powders analogues.
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ACKNOWLEDGEMENT
We thank the University of Missouri Research Board (UMRB) for supporting this work and Materials Research Center (MRC) of Missouri S&T for SEM and XRD.
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IV. 3D-PRINTED ZEOLITE MONOLITHS WITH HIERARCHICAL POROSITY FOR SELECTIVE METHANOL TO LIGHT OLEFINS REACTION
Xin Lia, Fateme Rezaeia, Ali Rownaghia, *
a Department of Chemical and Biochemical Engineering, Missouri University of Science
and Technology, Rolla, MO 65409, USA * E-mail: [email protected]
ABSTRACT
Herein, we report the synthesis of zeolite monoliths with various compositions and hierarchical porosity using 3D printing technique. In particular, several 3D-printed monoliths were synthesized from HZSM-5 and HZSM-5/silica followed by the growth of SAPO-34 crystals on as-synthesized monoliths via secondary growth method. The 3D- printed zeolite monoliths exhibited hierarchical porosity with pore sizes ranging from 1.5 nm to 1 µm. Characterization results revealed enhancement in mesopore volume and moderation of catalyst acidity as a result of formulation into the monolith structure. It was also found that incorporation of amorphous silica into the HZSM-5 monoliths further reduced the acid sites density. The obtained monoliths were evaluated in methanol-to- olefins (MTO) reaction and found to exhibit higher stability than their powder counterparts.