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Kinetics, Catalysis, and Reaction Engineering

Flower-like hierarchical Y with dramatically increased external surface: a potential catalyst contributing to improving pre-cracking for bulky reactant molecules Yanze Du, Qinglan Kong, Zhihong Gao, Zhijian Wang, Jiajun Zheng, Bo Qin, Meng Pan, Wenlin Li, and Ruifeng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00751 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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Industrial & Engineering Chemistry Research

Flower-like hierarchical Y with dramatically increased external surface: a potential catalyst contributing to improving pre-cracking for bulky reactant molecules

Yanze Du 1, 3, Qinglan Kong 1, Zhihong Gao1, Zhijian Wang2, Jiajun Zheng*, 1, Bo Qin,3 Meng Pan 1

, Wenlin Li 1, Ruifeng Li*, 1

(1 Research Centre of Energy Chemical & Catalytic Technology, Taiyuan University of Technology, Taiyuan 030024, China; 2

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China;

3

Fushun Research Institute of Petroleum and Petrochemicals, SINOPEC Fushun 113001, China)

Abstract: Crude oil becomes more and more difficult to be refined because of the increasing large heavy oil molecules. Increased external surface of catalysts contributes to promoting pre-cracking of heavy or extra-heavy oils molecules. Flower-like hierarchical Y zeolite with considerably increased external surface was synthesized without using any organic templates by a hydrothermal procedure. Because the isolated nanoparticles are unstable, the primary nanocrystals gather via self-assembly into loose aggregates. The inner crystals, which act as the “pistils”, are difficult to grow up because of the confined spaces, while the outer crystals in the aggregates can further grow up and form the oriented sheet “petals”. The increased exterior surface offers the catalysts dramatically elevated conversion when tested in the catalytic cracking of tri-isopropylbenzene. The result also suggests that the activity of the catalysts in large reactants involved reactions may generally depend on its external surface properties. Keywords: nanocrystals; hierarchical; synthesis; flower-like; Y zeolite

Corresponding Author: Email: [email protected]; [email protected] (Ruifeng Li) Fax: +86 351 6018384 Post Address: Institute of Special Chemicals, Taiyuan University of Technology 79# West Yingze Street, Taiyuan 030024, China

1

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1. Introduction Crude oil has been becoming more and more difficult to be refined because of the larger and larger heavy oil molecules. Elevated external surface of catalysts used in FCC contributes to promoting preliminary cracking of heavy or extra-heavy oils molecules. In the past decades, it has been taking researchers plenty of time to prepare or synthesize hierarchical zeolite

1-3

with significantly shortened

micropore channels as well as dramatically increased external surface area and boomed pore windows by introducing a complex meso- or/and macropore structures via non-templating 11-18

4-10

, and templating

approaches, etc. so as to solve the challenges involving diffusion limitation 5, 19 or macromolecules

reaction

4, 14, 20

in zeolite channel systems, particularly in oil refining industry and petrochemical

industry. No-template means, such as dealumination 5-6

9-10

by steaming or/and by acid leaching, or desilication

by alkaline etching, or desilication followed by dealumination, could create meso- or/and

macropores in zeolite crystals, which are usually called post-treated strategy. On the other hand, in templating method, various template agents such as carbon black or carbon nanotubes (CTNs)16, surfactants 14, polymers

13

and inorganic nanoparticles

18

are reported to be embedded into the zeolite

crystals during the crystallization. After removing the templates, meso- or/and macropores are therefore generated in zeolite crystal where the templates used to be located methods, hard template agents

13, 21-23

14, 12-18

. In the template

were firstly reported to be employed to generate mesopores

inside zeolite crystals. However, multisteps followed by complex processes are often needed to be adopted so as to overcome the challenge from the incompatibility 24 between hard template agent and sliicon-alumininm precursor species. For the past few years, soft templating methods 11-12 using cationic resin

14

or organosilane surfactant

25

as mesopore-creating agent, have been attracting a lot of

interests and attentions of researchers because it is highly-efficient and very easy for building mesopore system and controlling mesopore sizes. However, such a synthesis strategy either the hard or the soft templating approach is still facing severe challenge during the preparation of hierarchical zeolites. In facts, whether in hard templating or soft templating approach, the interactions and the combination forces between the templates and the precursors yielding the hierarchical zeolite play a key important role in creating mesopores in zeolite crystals. The failure in some direct approaches of synthesizing hierarchical zeolite with traditional hard or soft templating method can be ascribed to the brittle binding 2


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forces between mesopore structure-directing agent and zeolite precursors 6, 12, 26-27. As a consequence, a pseudo meso-zeolite mingled with purely microporous zeolite phase and an amorphous form mesopore phase was formed

24

by phase separation during the zeolitization of the precursors because of

the incompatibility between templates and zeolite framework. As a result, design and synthesis of hierarchical zeolite in order to overcome the challenges in macromolecules involved reaction 4, 14, 20, 28-36 by the templating approaches are still confusing the researchers in this field. In the present work, flower-like hierarchical Y zeolite with dramatically increased external surface area was synthesized without any templates by a classic hydrothermal synthesis procedure. As compared to the reference Y zeolite catalyst, the as-synthesized flower-like hierarchical Y zeolite catalyst exhibits a considerably increased external surface, which not only contributes to improving accessible surface acid sites so as to be more efficiently utilized by the large reactant molecules, but also guarantee the large molecules to be carried out hierarchical cracking, in which, the reactant can be pre-cracked on the accessible external and the pre-cracked product can be further cracked into the aimed products by the strong acid sites in the confined microporous channel. Catalytic performances are also drawn a correlation with the external surfaces of the catalyst, and the result suggests that the activity of the catalysts in the reaction involving large molecule reactants may generally depend on its external surface properties given that the catalysts have the similar acidity centers. 2. Experiment Section 2.1 Synthesis Seeds of faujasite were obtained using a synthesis recipe of our previous work 3. A clear solution was prepared by sequentially adding 2.34 g of sodium aluminate (Sinopharm Chemical Reagent Co., Ltd, 41 wt.% Al2O3, 35 wt.% Na2O), 5.86 g of sodium hydroxide (Tianjin Guangfu Technology Development Co., Ltd, 96 wt.% NaOH) into 31.5 mL of water. Sodium silicate with a volume of 22.5 mL (Qingdao Haiyang Chemical Co., Ltd; 6.28 mol/L SiO2; 3.16 mol/L OH-) was then added gradually and the obtained gel was vigorously stirred for about 2 h and aged at 35℃ for 12~18 h under static conditions. Flower-like hierarchical Y zeolite samples were obtained using the synthesis recipe of (10.7~18.3) Na2O: (10.4~22.6) SiO2: 1Al2O3: 815H2O: (0~57.2) NaCl. Typically, 1.22 g of sodium aluminate along with 4.0~7.0 g of sodium hydroxide and 0~16g of sodium chloride (Tianjin Kemiou Chemical Reagent Co., Ltd; 99 wt.% NaCl) was consecutively dissolved in 66 mL of water with stirring. After the mixture 3



cleared, 6~12 mL of colloidal silica (Qingdao Haiyang Chemical Co., Ltd, 30 wt.% SiO2) was dispersed gradually into the abovementioned clear solution with a vigorously stirring. After 2 h, the final gel mixture was placed in a 100 mL autoclave and stayed at 70 ℃ for 1 day under static conditions and autogenous pressure. After crystallization, the final solid product was obtained by filtration, washed, and dried for 1 day at 110 ℃, and denoted as YOH-a, YNA-b or YSA-c. Here, “a”, “b” and “c” stand for the OH-/SiO2, Na+/H2O, and SiO2/Al2O3 ratios in the gel precursors, respectively. A reference sample was obtained according the recipe of 2.4 Na2O: 5 SiO2: Al2O3: 250 H2O. Typically, 3.98 g of sodium aluminate was dispersed gradually into 49 mL of water, and 13 mL of sodium silicate was then slowly added into the aforementioned solution under vigorously stirring. After about 2 h, 4 mL of the as-made faujasite seeds was added and uniformly dispersed, and then 0.5 mL of sulfuric acid (Tianjin Kemiou Chemical Reagent Co., Ltd, 98 wt.% H2SO4) was added gradually with stirring. The obtained gel mixture was kept at 90 ℃ for 1 day under static condition. The solid sample was collected after filtered, washed, dried, and named as Yr. The as-prepared powder products were carried out an ion-exchanged procedure in an NH4Cl solution with a concentration of 1.0 M at room temperature. The ratio of the zeolite powder (g) to the solution (mL) is 1:10, twice, 2 h each time. The NH4+-form samples were then transferred into H+-form Y zeolite catalysts by calcination at 550℃ for 5 h. 2.2 Characterization Crystal structure of Na+-form samples were recorded from 2θ=5° to 2θ=35° on a Rigaku Dmax/2500 X-ray diffractometer (XRD) using CuKα radiation operated at 40 kV and 40 mA. SEM images were collected on a JEOL/JSM-6700F microscope (SEM) equipped with an energy-dispersive x-ray spectroscopy (EDS); TEM images of Na+-form samples were obtained on a JEM-200CX transmission electron microscopy (TEM) equipped with a selected area electron diffraction (SAED). Nitrogen adsorption/desorption data were tested at 77 K on an ASAP2400 automatic adsorption apparatus. Microporous structure and mesoporous structure, for example, the size distribution of mesopore and mesoporous volume, were determined by applying the t-plot analysis and the BJH model as using the adsorption branch of the isotherm, respectively. Infrared spectra of the Na+-form samples, which were dispersed in KBr pellets in advance, were investigated on a SHIMADZU FTIR-8400 spectrometer. Solid-state NMR spectra of the Na+-form samples were recorded on a Bruker Avance III spectrometer.

4


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Ammonia adsorption experiment of the H+-form catalysts were handled on an AutochemΠ 2920 apparatus coupled with a thermal conductivity detector (TCD). 2.3 Catalytic test Triisopropylbenzene (TIPB) was used as a probe molecule, and its catalytic cracking over the H+-form catalysts was performed under ordinary pressure in a fixed-bed MAT (micro-reactor test) connected with a reaction pipe with an internal diameter of 6 mm. Before catalysis tests, the H+-form catalyst powder was made into 20~40 meshes particulate in advance and activated at 550 ℃ for 2 h under a nitrogen flow with a flowing velocity of 50 mL/min. The activated catalyst was then kept in situ under the presupposed reaction temperature. TIPB was injected by a micro-injection pump, and TIPB cracking were handled at 450 ℃ over 50 mg of catalyst, catalyst: reactant=0.5:1(g/g), reaction times was controlled between 5 and 8 seconds, nitrogen flow for purging was controlled at a flowing rate of 20 mL/min. Cracked products were determined on an Agilent 7890B GC equipped with a flame ionization detector (FID) and a simulated distillation D2887 column produced by China Petrochemical Research Institute. 3. Results and Discussion 3.1 Structural Analysis of Catalysts It can be inferred from the XRD patterns (Figure 1), all the samples display characterization diffraction peaks at 2θ=6.16°、10.08°、15.72°、20.43°, and 23.67° corresponding to FAU topological structure (JCPDS No.39-1380), and no any other impure zeolite phase can be detected, indicating that the as-made samples are pure-phase FAU-type zeolite. Figure 1 also exhibits that enhancing the OH-/H2O ratio in the gel precursors results in a widened half-peak width corresponding to the peaks at 6.16°, 10.08°, 15.72°, and 23.67°. Relative crystallinity (Table S1), which is determined by comparing the peak areas of the XRD pattern of the samples to those of Yr, decreases with the increased alkalinity in the gel precursors. Size of crystal particle determined by using the Scherrer equation according their XRD patterns was shown in Table S1. It can be inferred that the increased alkalinity in the gel precursors gives the as-synthesized sample a decreased crystal size. As compared with YOH-0.032, the additionally added NaCl in the precursors also causes the varied half-peak width for YNA-b samples (Figure S1). The gel precursor with an added amount of Na+ in the gel precursor, for instance, the precursor yielding YNA-0.04, may attribute to promoting crystal nucleation and growth, which causes the more poignant characteristic peaks as compared with YOH-0.032 sample. However, further 5



increased Na+ content along with the presence of anion, i.e. the Cl-, causes the outstandingly increased half-peak width, which may be caused by the small crystal particles

37, 38

. Figure S2 shows that lower

Si/Al ratios, for example, Si/Al ratios of 11~15 in the gel precursors, also give the as-synthesized products obviously broadened half-peak width. While further increasing the Si/Al ratios to 17~23 in the precursors results in the more poignant characteristic peaks. As shown in Figure 2, Figure S3, and Figure S4, zeolite particles in the as-synthesized samples exhibit loose aggregates with a flower-like morphology. Zeolite particles in the reference Yr sample (Figure 2K-L) exhibit typical octahedral morphology with sizes of about 500~800nm, and look rather dense and smooth. The particles in Figure 2, Figure S3, and Figure S4 are very loose with a coarse external surface. Lower OH-/H2O ratio of 0.032~0.039 in the gel precursor gives the particles in the as-synthesized YOH-0.032 and YOH-0.039 samples a flower-like morphology, and those “flowers” are further composed of 50~100 nm primary nanoparticles. As shown in Figure 3, the “flowers” with the “petals” and the “pistils” are obviously observed. HRTEM image and the corresponding SAED image display that the “petals” in the flower-like aggregations are the Y zeolite monocrystalline sheet (Figure 3B, 3D) with a size of about 50~100 nm, simultaneously, the HRTEM image also reveal that the “petals” in the “flower” have extensive and orderly arraying microporous channels, suggesting a high crystallinity (Figure 3C)

39

. Figure 3B-C also reveals a non-uniform orientation of the micropores in

the “pistils” and “petals”. Figure 2E-J also shows that the increased OH-/H2O ratio in the precursor gives smaller “flowers” consisting of the more small primary nanoparticles in the final products. Additionally added NaCl in the precursors yielding YOH-0.032 also gives the final products the flower-like zeolite crystals (Figure S3). It can be seen from Figure S3D and the corresponding TEM image as shown in Figure S5 that the “pistils” and “petals” in the flower-like Y zeolite are more obviously observed, and the “petals” in YNA-0.06 are about 50 nm, which is smaller than the 80~100 nm of the one in YOH-0.039 (Figure 3B). Figure S3 also displays that the additionally added NaCl in the precursor offers the flower-like particles in YNA-b a more loosely aggregated morphology. It was reported that Si/Al ratios of precursor gel can be used to adjust and control the morphology of zeolite crystals. Very recently, ZSM-12 zeolite with different morphologies were reported to be fabricated by changing the H2O/SiO2 ratio, and the morphology of the as-synthesized zeolite was changed from compact structures to loosely polycrystalline aggregates composed of primary nanocrystallite.4 The relatively lower Si/Al ratios of 11~15 in the precursors give the as-synthesized 6


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samples, for example, YAS-15 a similar flower-like morphology (Figure S4I-J). The flower-like crystals in the YAS-15 are about 300 nm, slightly smaller than the 500 nm of the one in YOH-0.032 (Figure 2A-B), and are further composed of 50~100 nm primary sheet crystals, namely, the “petals” (Figure S6). The extensive and well-ordered arraying for the microporous channel also suggests the “petal” a high crystallinity (Figure S6C). Direct evidence, which proves the existence of mesopores structure resulted from interspaces of the aggregated “pistils”, is supported by the HRTEM image (Figure S6A inset). Further increased Si/Al ratios in the gel precursors yielded sphere-like and relatively dense polycrystalline aggregates (Figure S4K-P). Despite

assembly

of

nanocrystallite

into

hierarchical

materials

has

received

more and more attentions because of the significances ether in academic or in industrial application, most of such work reported in the opened literatures usually need a secondary template

40

. The

abovementioned results display that “flower-like” polycrystalline Y zeolite crystals can be obtained by changing the chemical components of the precursors. Recently, opened literatures reported that anions 25, 38

and Si/Al ratio in gel precursors 4 affected the crystals growth process, and were therefore used to

design and fabricate hierarchical materials by morphological control

25

or adjusting grain size

41-42

. In

present work, the synthesis system of flower-like hierarchical Y zeolite was regulated to be prone to forming nanocrystals. Relatively lower hydrothermal synthesis temperature and high alkali metal ion content in the synthesis solution not only attribute to nucleation but also conduce to confining the crystal size by inhibiting crystal growth 38, 43. Na2O/SiO2 ratio in the precursor yielding YOH-a is about 1, much higher than the 0.5 of the one in the precursor yielding Yr, and the former was hydrothermally treated at 70 ℃ while the later was handled at a relatively high crystallization temperature of 90 ℃. Although both of crystal growth and zeolite nucleation mechanisms are extremely complicated, while, it is obvious that precursor grain size was decided by base metal ions

44, 45

. Particularly, during the

initial condensation stage of sol-gel precursor, a high Na+ ion concentration in the synthesis system attributes to forming smaller zeolite particles by restraining the polymerization extent.

43

Primary

nano-sized zeolite crystals are therefore formed in the initial stage (Scheme 1.(a)-(c)). Concurrently, due to the instability in the hydrothermal synthesis system 40, the dissociative nanocrystals attract each other and form loose aggregates by self-assembly (Scheme 1.(c)-(d)). The inner nanocrystals, which act as the “pistils”, are difficult to grow up because of the confined spaces, while the nanocrystals outer the aforementioned aggregates can further grow epitaxially and form the oriented “petals” (Scheme 7



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1.(d)-(e)). From a thermodynamic point of view, the non-uniform orientation of the nanocrystals in the aggregates attributes to keeping the flower-like crystals with a stable polycrystalline aggregation state 38

. The results handled by N2 adsorption-desorption experiment display that all the YOH-a zeolite

samples have composite isotherms composed of type-I and type-IV. All the curves present significantly increased N2 adsorption after p/p0≈0.8, which is caused by the capillary agglomeration

38

in meso-

or/and macropores resulted from interparticles (the “pistils” or/and the “petals”) spaces. The pore diameter distribution (Figure 4 inset), which was gotten by adopting BJH model using the adsorption branch according the isotherm, states that the meso- and macropore structure created in the as-prepared samples possess a broad pore diameter distribution from 10 nm to over 100 nm, which can be caused by the interparticles spaces

38

as the aforementioned results observed by TEM images (Figure S6 and

Figure S7). As descripted in Table 1, the flower-like hierarchical Y have a large Sext (external surface area) ranging from 98 to186 m2/g, which is about 3~6 times as much as the 30 m2/g of the reference Yr catalyst. The remarkably elevated Sext, which may be caused by the results of the nanocrystallization of primary crystallites in the flower-like Y loosely aggregating materials, means boomed pore windows, which can be accessible and be utilized by bulky reactant. Table 1 also explains that the Smic (micropore area) and Vmic (micropore volume) of the hierarchical “flower-like” Y decreases with different degrees as compared to the 816 m2/g and the 0.36 cm3/g of the reference Yr, respectively. The weakened micropore structure in the “flower-like” Y zeolite may be caused by the degraded order in the 3D pore network because of the nanocrystallization of primary crystallites

22, 38

. It can be inferred

from Table 1 that a high alkalinity in the gel precursor gives the final YOH-a product with an elevated external surface area, which can be attributed to the continuously decreased size of the primary nanocrystals in the “flower-like” polycryatslline aggregations as detected by the aforementioned SEM characterization (Figure 2). Combing with the results as collected by SEM and TEM, here, the result indicates that the as-made product is hierarchal Y zeolite, possessing the inherent micropore channel of faujasite as well as artificial meso- and macropore structures resulted from the intercrystalline void due to the primary nanocrystals accumulation. The co-existence of the inherent micropores as well as the fabricated meso- and macropores (10~100 nm) provide the flower-like hierarchical zeolite catalysts with combined properties, for example, the shape selectivity and acidity accessibility, that may give the as-made zeolite catalysts a potential application in promoting a hierarchical cracking of large reactant. 8


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FT-IR of Na+-form YOH-a, YNA-b and YSA-c samples are displayed in Figure 5, Figure S7, and Figure S8, respectively. All of the sample exhibit the similar characteristic absorption peaks attributed to faujasite

46-48

despite of incompletely identical bands position and vibration intensity. As shown in

Figure 5, YOH-a catalysts display the absorption bands associated with the double-rings vibration at 570 cm-1, as well as the symmetrical stretching vibrations at both of 687 cm-1 at 764 cm-1, and asymmetrical stretching vibration of tetrahedral TO4 units at about 1001~1012 cm-1. 2, 47, 48 Shift of the absorption peaks located at 1000~1100 cm-1 usually means a varied framework Si/Al ratio

46

. The

asymmetric stretching vibration absorption corresponding to the YOH-0.045, YOH-0.042, YOH-0.039, and YOH-0.032 are respectively located at 1001 cm-1, 1007 cm-1, 1005 cm-1, and 1012 cm-1, implying the as-synthesized samples variable framework Si/Al ratios 46. Figure S7 displays that the added NaCl in the precursors causes the asymmetric stretching vibrations band shifts from the 1012 cm-1 of YOH-0.032 to ~1000 cm-1. The increased Si/Al ratios in the gel precursors also results in the asymmetric stretching vibrations band in samples YAS-c shifts from the 1005 cm-1 of YAS-11 towards the 1015 cm-1 of YAS-23, also indicating a somewhat varied framework Si/Al ratio. Figure 6 is solid-state NMR spectrum of the reference catalyst Yr and YOH-0.039 samples. The resonances around -108 ~ -105.0 ppm is associated with Si(4Si, 0Al) sites, and the resonance situated on -105~ -95 ppm is associated with Si(3Si, 1Al) sites; moreover, the signals located at -95 ~ -88ppm, at -92 ~ -86 ppm, and at -86 ~ -80 ppm, are respectively assigned to Si(2Si, 2Al), to Si(1Si, 3Al), and to Si(0Si, 4Al) 49, 50 (Figure 6A-B). The framework Si/Al ratios were calculated according to the Si solid-state NMR spectrum and the results explain that the Si/Al ratios in the reference Yr zeolite and YOH-0.039 are 1.63 and 2.23, respectively. As shown in Figure S9, the detected even surface Si/Al ratio of YOH-0.039 is about 2.36, slightly higher than the 2.23 calculated by Si solid-state NMR spectrum. Both of Al solid-state NMR spectra (Figure 6C-D) of Yr and YOH-0.039 exhibit an intense peaks centering at 55-62 ppm, which is associated with the tetra-coordinated framework Aluminum 51. No any signals associated with octahedral-coordinated Aluminum can be detected at around 0 ppm. Moreover, signal peak centering at 55-62 ppm in sample YOH-0.039 is obviously wider than that in Yr zeolite, which may be a reflection of deteriorated homogeneity of the framework aluminum atoms 52 in the flower-like zeolite caused by the more defects because of the nanocrystallization of the primary crystals as observed in Figure 2 and Figure 3.

9



As shown in Figure 7, all the TPD profiles of the four flower-like hierarchical Y zeolites have two NH3 desorption peaks, one centers at about 233 ℃, attributing to the NH3 desorbed from weak acid sites, and the other is around at 327 ℃, corresponding to the desorbed NH3 from the medium-strong acid sites. However, YOH-0.045 displays outstanding weakened medium-strong acid sites which can be ascribed to the dramatically decreased micropore properties because of the nanocrystallization of the crystals in the as-synthesized sample. As shown in Table S1, the total acid amounts show a considerable difference in the four catalysts. It can be seen that the acidity amount of the four catalysts has an order as following: YOH-0.039>YOH-0.032>>YOH-0.042>YOH-0.045. The considerable differences in the acid sites can be caused by the altered micropore properties because most acid centers exist in the micropore channels. Table 1 has displayed that the sample YOH-0.042 and YOH-0.045 possess the rather low micropore areas and volume as compared with YOH-0.032 and YOH-0.039. As shown in Figure S10 and Table S1, YSA-c catalysts have approximate equivalence in the acid amounts, which can be attributed to the similar micropore areas as shown in Table 1. The result also implies that the different ratio of Si to Al in the precursors mainly changes the morphology of the crystals in the final products, and does not considerably affect the acid amounts. 3.2 Catalysis Tests Combing with the results obtained by the SEM and TEM images, the nitrogen adsorption-desorption data in Table 1 suggests that nanocrystallization of the Y zeolite crystal offers the flower-like hierarchical Y zeolite catalyst an outstandingly elevated external area. It is obvious that the increased external surface area contributes to improving accessible acid sites for large reactant molecules, and therefore can be effectively utilized by the reactant. In order to verify the increased external surface areas playing a role in elevating the catalytic performances involving bulk molecules, catalytic cracking of tri-isopropylbenzene is selected as the probe reaction. The results tested on MAT were shown in Table 1, suggesting that the conversion of tri-isopropylbenzene generally depends on the Sext of the catalysts. The reference Yr, which has the least Sext, displays the lowest catalytic activity in catalytic cracking of tri-isopropylbenzene. Obviously, with an increased Sext, an elevated catalytic activity is obtained. It can be seen from Table1 and Figure 7 that YOH-0.039 has the largest acid amount while does not have the highest activity, indicating that not all the acid centers detected by the NH3 molecule are really accessible to the reactant. Tri-isopropylbenzene has a kinetic diameter of ~0.94 nm 53, distinctly larger than the aperture diameter (~0.74 nm) of the inherent micropore channel 10


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in faujasite. No doubt, it is rather difficult for the large tri-isopropylbenzene molecule to enter into or pass through the micropore channels of the catalysts because of the extremely diffusional resistance. As a result, it is anticipated that tri-isopropylbenzene molecules should be mainly cracked by the accessible acid sites located at the external surfaces and near the micropore windows in the catalysts 38. For large reactants, the larger Sext equals to the more efficient acidy sites can be really accessible and efficiently utilized. Plot regarding the conversion of tri-isopropylbenzene and Vmeso/Vtotal is shown in Figure 8A. It can be seen that the lager Vmeso/Vtotal ratio approximately gives the catalyst a higher conversion of tri-isopropylbenzene. Yr catalyst has the lowest activity in cracking tri-isopropylbenzene, which can be caused by the smallest Sext because of the largest crystal size as shown in Figure S3K-L. However, a discrete relationship between the Vmeso/Vtotal ratios and the conversion of tri-isopropylbenzene over the different catalyst is found. Besides the external surface areas, the acidity in the catalysts also affects the conversion of tri-isopropylbenzene. The catalysts obtained by different procedure, such as, obtained by hydrothermal crystallization of the precursors with different OH-/H2O, different Na/H2O, or different Si/Al ratios, may have different acidity. That also brings considerable affect for cracking tri-isopropylbenzene. Figure 8B exhibits that the conversion of tri-isopropylbenzene over YOH-a catalysts also displays a discrete relation with the Vmeso/Vtotal ratios because of the considerable difference in acidity of YOH-a as shown in Figure 7 and Table S1. Despite of possessing a lower Sext (Table 1), YOH-0.039 has relatively higher acid amount as shown in Figure 7 and Table S1, which still gives the catalyst the higher conversion of tri-isopropylbenzene, indicating that catalytic cracking of large reactant molecules depends on not only the external surface areas, but also the surface acidity. In order to avoid the interference of acidity with the dependency relationship between external surface areas and the conversion, the dependency relationship of YAS-c catalysts, which have the similar acidity (Figure S10 and Table S1), were extracted from Figure 8A and shown as that in Figure 8C-D. The result suggests that the activity of the catalyst used in cracking tri-isopropylbenzene is generally decided by its external surface properties. However, the increased external surface areas often sacrifices the micropore properties of the catalyst 54, which may partially lose the surface acidity. In a general way, two things should be considered as designing a highly-efficient FCC-catalyst. One is to promote the pre-cracking of bulky reactant which cannot enter into the micropore channels of the main active component. The enhance of 11



the Sext of zeolite catalyst conduces to facilitating the pre-cracking of the bulk reactant because of the increased accessible acidy sites which can be utilized by the bulky reactant. The other one is the secondary cracking of the pre-cracked molecules in the restricted spaces (micropore channels), where the strong acid centers realize the transformation of the pre-cracked products into smaller desired molecules 38. The abovementioned designed strategy fabricates a hierarchical cracking process so as to tackle the challenges of the heavy crude oil on the FCC-catalysts. Excessively weakened micropore properties may depress the hierarchical cracking of the pre-cracked products, and do not bring an ideal catalyst. Therefore, how to make the macromolecules to be efficiently cracked into the aimed products not only involves the pre-cracking of the macromolecules, but also relates to the secondary cracking of the pre-cracked products into the targeted products, such as the gasoline, diesel oils, or the light olefin. To optimize or to control the ratio of external surfaces relatively to the micropore surfaces should be vital important to design and build a highly-efficient catalyst used in FCC. 4. Conclusions Flower-like hierarchical Y zeolite was synthesized without any templates by a traditional hydrothermal synthesis procedure. Nanno-scale primary crystals take part in constructing the flower-like Y zeolite, and offer the hierarchical zeolite considerably increased external surface areas, which contributes to improving the acid accessibility, and therefore can be effectively utilized by the reactant. Catalytic performances of the synthesized hierarchical Y used in the catalytic cracking of tri-isopropylbenzene were drawn a correlation with the external surfaces of the catalyst, and the results suggested that the activity of the catalyst used in cracking large reactant molecules strongly depends on its external surfaces. Thus, the method presented in the present work may provide a highly-efficient active component used in the FCC-catalysts in order to promoting the pre-cracking of bulky reactants. The present work also confirms hierarchical zeolites can be fabricated at a low-cost way without secondary templates by controlling the synthesis condition and adjusting the chemical compositions of the precursors. ■ AUTHOR INFORMATION Corresponding Authors *Email: [email protected]. TEL: +86 6018384 *Email: [email protected]. TEL: +86 6018384 ORCID Jiajun Zheng: 0000-0001-7292-2678 Zhijian Wang: 0000-0002-9659-5242 Wenlin Li: 0000-0002-3761-3033 12


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Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS We acknowledge the NSFC (U1463209; 21371129; 21376157) and Sinopec (116050) for financial support. ■Supporting Information Relative crystallinity, Size of crystal particle, and Acidity (Table S1), XRD patterns (Figure S1, Figure S2), SEM images (Figure S3, Figure S4), High-magnification TEM images (Figure S5, Figure S6), FT-IR spectra (Figure S7, Figure S8), EDS analyzed results (Figure S9), NH3-TPD (Figure S10).(PDF) ■ REFERENCES (1) Sharon, M.; Boltz, M.; Liu, J.X.; Pérez-Ramírez, J. Engineering of ZSM-5 zeolite crystals for enhanced lifetime in the production of light olefins via 2-methyl-2-butene cracking. Catal. Sci. Technol 2017, 7, 64. (2) Zheng, J.J.; Zeng, Q.H.; Yi, Y.M.; Wang, Y.; Ma, J.H.; Qin, B.; Zhang, X.W.; Sun, W.F.; Li, R.F. The hierarchical effects of zeolite composites in catalysis. Catal. Today 2011, 168, 124. (3) Zheng, J.J.; Zeng, Q.H.; Zhang, Y.Y.; Wang, Y.; Ma, J.H.; Zhang, X.W.; Sun, W.F.; Li, R.F. Hierarchical porous zeolite composite with a core-shell structure fabricated using β-zeolite crystals as nutrients as well as cores. Chem. Mater. 2010, 22, 6065. (4) Zhao, Y.; Zhang, H.B.; Wang, P.C.; Xue, F.Q.; Ye, Z.Q.; Zhang, Y.H.; Tang, Y. Tailoring the morphology of MTW zeolite mesocrystals: Intertwined classical/nonclassical crystallization. Chem. Mater. 2017, 19(8), 3387. (5) Groen, J.C.; Bach, T.; Ziese, U.; Donk, A.; de Jong, K.P.; Moulijn, J.A.J.; Pérez-Ramírez, J. Creation of hollow zeolite architectures by controlled desilication of Al-zoned ZSM-5 crystals. J. Am. Chem. Soc. 2005, 127, 10792. (6) Pérez-Ramírez, J.; Christensen, C.H.; Egeblad, K.; Groen, J.C. Hierarchical zeolites: enhanced utilisation of microporous crystals in catalysis by advances in materials design. Chem. Soc. Rev. 2008, 37, 2530. (7) Valtchev, V.; Balanzat, E.; Mavrodinova, V.; Diaz, I.; El Fallah, J.; Goupil, J.-M. High energy ion irradiation-induced ordered macropores in zeolite crystals. J. Am. Chem. Soc. 2011, 133, 18950. (8) Zhang, B.; Zhang, Y.; Hu, Y.; Shi, Z.; Azhati, A.; Xie, S.; He, H.; Tang, Y. Microexplosion under microwave irradiation: A facile approach to create mesopores in zeolites. Chem. Mater. 2016, 28,2757.

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TABLES:

Table 1. Nitrogen adsorption-desorption data and MAT results Samples SBET Smic Sext Vmic Vmeso Vtotal Sext/ SBET Vmeso/ Vtotal Conversion(%)* 2 2 2 3 3 3 (m /g) (m /g) (m /g) (cm /g) (cm /g) (cm /g) YOH-0.032 746 640 106 0.26 0.25 0.51 0.14 0.49 46 YOH-0.039 758 660 98 0.27 0.25 0.52 0.12 0.48 59 YOH-0.042 729 569 160 0.23 0.26 0.49 0.22 0.53 57 YOH-0.045 729 543 186 0.21 0.58 0.79 0.26 0.72 62 YSA-11 836 709 127 0.31 0.28 0.59 0.15 0.47 74 YSA-12 851 740 111 0.33 0.20 0.53 0.13 0.38 67 YSA-13 790 614 176 0.25 0.31 0.56 0.22 0.55 83 YSA-14 866 708 158 0.30 0.30 0.60 0.18 0.50 77 YSA-15 853 731 122 0.32 0.27 0.59 0.14 0.46 69 Yr 846 816 30 0.36 0.02 0.38 0.03 0.05 25 * The conversion of tri-isopropylbenzene. Reaction temperature: 450℃; Filled catalyst: 50 mg; Catalyst/reactant=0.5 g/g.

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0

2

4

6

8

10 10

(1) 8

(2)

6

(3) 4

(4) (5) 2

(6) 0

5

10

15

20

25

30

35

2Theta/degree

Figure 1. XRD patterns of flower-like hierarchical YOH-a zeolite. (1): Yr; (2): YOH-0.032; (3): YOH-0.039; (4): YOH-0.040; (5): YOH-0.042; (6): YOH-0.045

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Figure 2. SEM images of reference Yr and flower-like hierarchical YOH-a zeolite. A, B: YOH-0.032; C, D: YOH-0.039; E, F: YOH-0.040; G, H: YOH-0.042; I, J: YOH-0.045; K, L: Yr

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Figure 3. TEM images of the flower-like hierarchical YOH-0.039. A: Low-magnification TEM image; B and C: High-magnification TEM images; D: the corresponding SAED pattern.

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700

0.8

dV/dlogD(mL/g)

600

Adrobed volume (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


500

(1) (2) (3) (4) (5)

0.6

0.4

0.2

0.0 10 100 Pore diameter(nm)

400

300

200

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (p/p0)

Figure 4. N2 adsorption-desorption isotherms and the corresponding BJH pore size distributions (inset) of hierarchical YOH-a zeolite. (1): YOH-0.045; (2): YOH-0.042; (3): YOH-0.040; (4): YOH-0.039; (5): YOH-0.032

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(a) (b) 1001

(c)

(d) 570

687

764

1012 400

600

800

1000

1200

1400

-1

Wavenumber/ cm

Figure 5. FT-IR spectra of hierarchical YOH-a zeolite. (a): YOH-0.045; (b): YOH-0.042; (c): YOH-0.039; (d): YOH-0.032

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(A) YOH-0.039

(B) Yr 4

[Si/Al]=2.23

3

[Si/Al]=1.63

5

4

3

2

5

2 1

1

-110 -105 -100

-95

-90

-85

-80

-110 -105 -100

-95

-90

-85

-80

70

75

Chemical shift(ppm)

Chemical shift (ppm)

(C) 58.3 YOH-0.039

(D) Yr

58.3

63.8

45

50

55

60

65

70

75


45

50

55

60

65


Figure 6. Solid-state NMR spectra. (A): 29Si NMR spectra of YOH-0.039; (B): 29Si NMR spectra of Yr; (C): 27Al NMR spectra of YOH-0.039; (D): 27Al NMR spectra of Yr.

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233

YOH-0.032 YOH-0.039 YOH-0.042 YOH-0.045 327

TCD signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

200

300

400

500

600

o

Temperature/ C

Figure7. NH3-TPD of hierarchical YOH-a zeolite samples.

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65

(A)

Conversion of triisopropylbenzene (%)


90 80 70 60 50 40 30

20 0.0

0.1

0.2

0.3

0.4

0.5

0.6

(B)

60

55

50

45

40 0.35

0.7

0.40

0.45

Vmeso/VTotal

85

(C)

80

75

70

65 0.12

0.14

0.16

0.18

0.50

0.55

0.60

0.65

0.70

0.75

Vmeso/VTotal


85 Conversion of triisopropylbenzene (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.20

0.22

(D)

80

75

70

65

60 0.35

0.24

0.40

Sext/SBET

0.45

0.50

0.55

Vmeso/VTotal

Figure 8. Plots regarding the conversions of tri-isopropylbenzene and the surface properties of the catalysts. (A): The relation between conversions and Vmeso/Vtotal; (B): The relation between the conversions and the calculated Vmeso/Vtotal of YOH-a with different acidities; (C): The relation between the conversions and the calculated Sext/SBET of YSA-c with similar acidities; (D): The relation between the conversions and the calculated Vmeso/Vtotal of YSA-c with similar acidities.

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Scheme 1. The proposed forming process of the flower-like Y zeolite

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Primary nanocrystals are created during the initial reaction period and self-assemble into polycrystalline aggregates. The inner nanocrystals, which act as the “pistils”, are difficult to grow up because of the confined spaces, while the nanocrystals outer the aggregates grow epitaxially and form the oriented “petals”. 271x195mm (150 x 150 DPI)


Flowerlike Hierarchical Y with Dramatically ... - ACS Publications

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