Insight into the Contribution of Isolated Mesopore on Diffusion in

Subscriber access provided by Kaohsiung Medical University

Thermodynamics, Transport, and Fluid Mechanics

Insight into the contribution of isolated mesopore on diffusion in hierarchical zeolites: the effect of temperature Huimin Zheng, Dong Zhai, Liang Zhao, Chenggen Zhang, Suyuan Yu, Jinsen Gao, and Chunming Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00515 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 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

Industrial & Engineering Chemistry Research

Insight into the contribution of isolated mesopore on diffusion in hierarchical zeolites: the effect of temperature Huimin Zheng1,2, Dong Zhai1,3, Liang Zhao1,*, Chenggen Zhang2, Suyuan Yu2, Jinsen Gao1, and Chunming Xu1

1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing),

18 Fuxue Road, Beijing 102249, P. R. China 2

Faculty of Chemistry and Material Science, Langfang Teachers College,

100 Aiminxi Road, Langfang 065000, Hebei Province, P. R. China 3

Department of Physics and International Centre for Quantum and Molecular Structures,

Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China

Email:

Liang Zhao([email protected]);

Tel:

86-10-89739078

Fax:

86-10-69724721

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

Page 2 of 34

Abstract In recent years, hierarchical zeolites are becoming more and more attractive as a solution to the diffusion restrain of classical zeolites. There have been many theoretical as well as experimental developments that deepened our understanding of mass transfer phenomena in hierarchical materials. However, there appears to be no consensus on the role of disconnected mesopores played in molecular transport. In this paper, molecular simulation is conducted to study the diffusion of benzene in purely microporous FAU zeolite and hierarchical FAU models which consist of both micro-porosities of FAU and a cylindrical mesopore with various radiuses (0.74-17.5 nm) at 300-800 K. The usage of the disconnected mesopores at moderate adsorbate loading is testified on the molecule level. In fact, under the reaction temperature of catalytic reactions, the disconnected mesoporous are likely to be fully utilized and beneficial to the reaction process. The temperature dependence illustrated shows a remarkably good agreement between the theoretical predictions and the experimental results. It therefore confirms that the disconnected mesopore may weaken and subdue the resistance of diffusion at high temperatures at moderate loading. Based on a thorough quantitative analysis of the adsorption interactions, the mechanisms leading to these phenomena are further explained.

Keywords: mesopore connectivity; hierarchical zeolite; molecular simulation; mass transport


Page 3 of 34 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


1. Introduction Zeolites have been widely used in industry as heterogeneous catalysts due to its unique pore structure, suitable acidity and hydrothermal stability1,2. Particularly, faujasite type (FAU) zeolite plays an irreplaceable role in petrochemical industry, especially in the fluidized-bed catalytic cracking (FCC) process3,4. However, the micropores structure of FAU zeolite (0.74 nm) often brings about the limitations of mass transport, which are victims of its own coking and deactivation5,6. In recent years, hierarchical zeolites are garnering attention as a solution to the diffusion restrain of classical zeolites2,6-16. For example, García-Martínez et al.13 found that hierarchical Y zeolite can prevent coking significantly and improves the selectivity in product compared with traditional counters, which were automatically attributed to the facilitated diffusion, just as a lot of other studies did2,14-17. In this decade, more and more investigations have directly assessed the improved transport in hierarchical zeolite18,19. For example, Groen et al.19 achieved 2 orders of magnitude faster diffusion of neopentane in hierarchically ZSM-5 zeolites based on tapered element oscillating microbalance technique.

Many researchers believe that the connectivity of the mesopore is crucial in the improvement of diffusion performance of hierarchical zeolite as compared to parent microporous material. Direct diffusion measurements-PFG NMR performed at 173-213 K by Kortunov et al.20 found that the intra-crystalline diffusion of guest molecules was essentially unaffected by disconnected mesopores in USY zeolite. However, the approaches widely used in the industry are mostly not capable to achieve mesopores that are perfectly connected with each other21-26. Practically, most of the hierarchical zeolite contains mesopores with only limited connectivity. The PFG NMR study has also found that hierarchical NaCaA zeolites with poor mesopore connectivity could benefit mass transfer of ethane at high temperatures27, in accordance with the prediction of previous theoretical studies28,29. Although many researchers have studied the contribution of disconnected mesopores on diffusion30, when taking the diversity of the 3 ACS Paragon Plus Environment


Page 4 of 34

operation condition in catalytic and separation systems into account, the role of disconnected mesopores on diffusion at different temperatures is still far from clear. Further research on molecular level is important for establishing correlations between these confusing results delivered by different methods and various experimental conditions. More importantly, this research is helpful for the establishment of structure-dynamics correlations, which is a knowledge-based guide not only for material selection and structure design, but also for further performance enhancement in heterogeneous catalysis and mass separation. Here we report the results of a molecular simulation which studies the molecular diffusion in hierarchical FAU zeolites in comparison with their purely microporous equivalent with suitably chosen guest molecule----benzene, which is highly catalytically-interesting, to characterize the role of disconnected mesopore on diffusion properties at various temperatures. The adsorption behaviors of the systems are also investigated as the background and reference of the diffusion research.

2. Models and Methods 2.1 Models of zeolite The framework of FAU zeolite was shown by a neutron diffraction study reported by Fitch et al.31, which has a cubic unit cell with cell length a =24.8536 Å. The accessible space for aromatic molecules was the supercage (SC) with a diameter of ∼13 Å formed by sodalite cages (SODs), as shown in Figure 1a. Each two SCs were connected by 12-membered oxygen window (12-T ring) with a diameter of ~7.4 Å. In order to study the effect of disconnected mesopore on the diffusion properties, models of adsorbents were constructed: the all-Si FAU zeoite model with perfect crystal structure (named S-0), and hierarchical all silica FAU models (H-Models) with the introduction of a single mesopore named by the radius of the cylindrical hole in Å ( S-7.4, S-10, S-12.5, S-15, and S-17.5). Both models consist of 8 (2 × 2 × 2) unit cells of FAU and no size effect have been noticed for the systems in this study32. It was possible that the 4 ACS Paragon Plus Environment

Page 5 of 34 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


diffusivities (D) in the presence of cations could change noticeably. However, Ghorai et al.33 found that in the case of FAU systems with (Si/Al=3.0) and without cations (Si/Al=∞), the same trend of D was demonstrated with respect to adsorbates size. In addition, we have observed that the adsorption and diffusion mechanisms were basically the same for benzene in microporous HY zeolite and is irrelevant with Si:Al ratio34,35. Further, when the Si:Al ratio is large, the presence of cations would not block the motion of guest species, and a strong interaction between the cations and the π-electron clouds of the adsorbate would be absence. These standpoints justified our use of a cation free zeolite for this simulation. The model of S-0 could be easily constructed in accordance with the crystal structure of IZA Database. The fabrication of H-models was under the following procedures, using S-15 as an example. First of all, a cylinder mesopore of radius r = 15 Å was caved out of S-0 model, so that the shortest distance between oxygens on the opposite sides of the inner wall of the mesopore is around 3 nm. Secondly, the inner face of the mesopore was fully hydroxylated by saturating all oxygen dangling bonds with hydrogen atoms to ensure the electroneutrality of the simulation box. Last, the patterns of -Si(OH)3 and -Si-H were deleted and retured back to the second step for compensation, resulting in a 2 × 2 × 2 supercell with 3600 atoms. Thus the diffusion path was constituted of micropore segments interconnected by disconnected mesopore in the obtained hierarchical pore architectures of S-15, disconnected with the external reservoir. The SODs of HY zeolite framework, which are inaccessible for benzene molecules, were blocked artificially in our simulations36-40. Figure 1b displays the atomistic models of S-0 and S-15 respectively. The pores of S-15 were divided into three main regions, i.e., mesopore with fully hydroxylated inner surface (Meso), perfect SCs (P-SC) and SCs defected by mesopore fabrication incorporating with hydroxy groups (D-SC).



a)

b)

Page 6 of 34

P-SC SOD Meso 15 Å

SC

D-SC

Figure 1. a) One SC and SODs of FAU. b) Snapshots of optimized atomistic models and pore structure of: (left) S-0 and (right) S-15. The silicon atoms, oxygen atoms, and hydrogen atoms (saturated the OII dangling bonds at the mesopore surface) are colored yellow, red and white respectively. 2.2 Simulation The commercial software Materials Studio from Accelrys, Inc. was used to calculate adsorption and diffusion properties of S-0 and H-models. The COMPASS force field41 which has been successfully applied for benzene/FAU system42-44 was chosen to describe the interactions between atoms in the simulations41,42. Two different partial charge values were assigned to oxygen (O) centers, depending on their position in the framework, OI and OII for Si-O-Si and Si-O-H bridges, respectively. We took the charges q(Si) =1.6 |e| and q(OI) = -0.8 |e| from Ref 45, in which placed themselves on the reasonable middle ground between simplicity of the charge description and validity of the local environment. The charges of H atoms used for saturating OII dangling bonds at the external surface were taken as 0.27 |e|46. The charges of OII (q(OII) = -0.67 |e|) were restricted by the relation: 2(q(OII) + q(H)) = q(OI) to preserve the neutrality of the zeolite lattice. The charges of benzene were chosen as the same with most common charges: hydrogen atoms carry a charge of +0.153 |e| and carbon atoms a charge of -0.153 |e|47-50. The summation methods of van der Waals interactions and electrostatic interactions were applied as atombased and Ewald respectively50-54. The cutoff value of van der Waals interaction was set as large as 2.4


Page 7 of 34 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


nm55, and the calculation accuracy of electrostatic potential energy was 0.004 kJ/mol. All the simulations are applied with the periodic boundary conditions55,56. The employed simulation scheme consists of energy minimization (EM), Monte Carlo (MC), and molecular dynamic (MD) simulations. There are three major steps. Firstly, Smart Minimizer method32, 57 were adopted in EM simualtions to optimize the internal coordinates of the initial structure of S-0 and S15 models as well as the adsorbate. Secondly, grand canonical Monte Carlo (GCMC) simulations were carried out to predict the benzene absorption isothermals, and NVT-MC simulations at benzene loading of 20 molecules/UC using the Metropolis technique were applied to obtain the minimum energy configurations and interaction energy for the guest-host system32. Both equilibration and production steps were set to 5 × 107 for all MC simulations58. The convergence of our simulations was affirmed by recomputing several loading points with more MC steps. Finally, 10 ns MD simulations in the NVT ensemble were performed at benzene loading of 20 molecule/UC after 2 ns MD utilised for system equilibration, in order to evaluate the diffusion coefficient. The 1 × 105 conﬁgurations frames were saved for each NVT-MC and MD simulation so that accurate distribution properties of adsorbates could be obtained. The diffusion coefficients (D) were evaluated from the mean square displacement (MSD), according to the Einstein relation: 2 1 d N (1) lim ∑  ri ( t ) − ri ( 0 )  , 6 N t →∞ dt i =1 where N is the number of diffusive atoms in the system. Furthermore, the diffusion coefficient

D=

components Dx, Dy, and Dz in x-, y-, and z-direction were calculated by replacing r with rx, ry, and rz in equation 1, respectively. The above differential is approximated here by the ratio of the MSD and the time difference, i.e., the slope of the graph, a. Since the value of the MSD is already averaged over the number of atoms N, the equation simplifies to:

d = a / 6. The diffusion activation energy is expressed by Arrhenius equation:

(2) 7

ACS Paragon Plus Environment


−

Page 8 of 34

E

D = Ae RT , (3) where D is the diffusion coefficient, A is the pre-exponential factor and E is the diffusion activation energy. The equation is changed to: E ln D = ln A − . (4) RT Make a line of lnD and 1/T, and we can get E from the slope of the line.

3. Results and discussion 3.1 Adsorption isothermals and interactions Although our focus is on the diffusion, the adsorption properties as isothermals and interaction energies are very important not only for evaluating the validity of models and force field, but also for understanding diffusion properties of adsorbates especially in micropores. Adsorption isothermals and interactions of benzene in S-0 and H-models (S-7.4, S-10, S-12.5, S-15 and S-17.5) were calculated at 20 molecules/UC at 300, 400, 500, and 800 K, respectively (as shown in Figure SI1 in Supporting Information). The S-15 was chosen as an example for the comparation with S-0 model since H-models have similar adsorption properties. Adorption isothermals of S-0, and S-15 models at 20 molecules/UC at 300, 500, and 800 K are shown in Figures 2. The isothermal of S-0 is consistent with the simulation and experimental results of Zeng et al.50, Takahashi et al.59, and our previous research60, which proves that the used models and parameters are reliable. For all temperatures, the isotherm curves obtained on S-0 are of type I of IUPAC classification. However, the isothermals of S-15 at 300 appear as S-shape and tend to vanish gradually at higher temperatures. This phenomena exiting in all H-models (as shown in Figure SI1 in Supporting Information), indicating a totally different adsorption behavior of hierarchal zeolites at different temperatures. As expected, the onset pressures of adsorption isothermals increase with increasing temperature for both models which indicates the decline of adsorptivity at higher temperture. The influence of temperature on the diffusion properties of the concerned systems will be further discussed below.


Page 9 of 34

The saturate loadings for S-0 and S-15 at 300 K are ~40 and 55 molecules/UC, respectively. The outcome shows that the calculational results are in good agreement with the experimental result of 39.5 molecules/UC61. Interestingly, the relative adsorption amount of H-modlels and S-0 were reversed with the increase of pressure at all temperatures (as shown in Figure SI1 in Supporting Information). The lower adsorption capacity of S-15 at lower pressure range is ascribed to the destruction of micropores in hierarchical zeolite. This result was also found in experiment isothermal of HY zeolite62. The adsorbate loading of S-15 overruns the loading of S-0 model with an intersection at p = 0.01 (300 K), 500 (500 K), and 10000 kpa (800 K), corresponding with the loading of 38 (300 K), 33 (500 K), and 25 (800 K) molecules/UC respectively. It seems that the fabrication of disconnected mesopore can only benefit the adsorption behavior at pressures (loadings) above the intersection. In the next section, the diffusion properties are studied at a moderate loading of benzene (20 molecules/UC) at 300-800 K which makes sure that the results are applicable to a broad loading range to illustrate the usage of the isolated mesopore.

100 S-0 300 K S-15 300 K S-0 500 K S-15 500 K S-0 800 K S-15 800 K

90 80

Loading (molecule/UC)

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


70 60 50 40

38 molecules/UC

Mesopore Contribution

33 molecules/UC

30

25 molecules/UC

20 10 0 1E-5

0.01

10

10000

1E7

Pressure (Kpa)

Figure 2. Isothermals of benzene in S-0 and S-15 models at 300, 500, and 800 K. The total adsorption energy of the model system (Uad = Uads-zeo + Uads-ads) and the interplay between Uadszeo

and Uads-ads interactions are essential for dynamic property analysis in zeolites. Figure 3 shows Uad and

the contribution of Uads-ads and Uads-zeo for benzene (loading: 20 molecules/UC) in S-0 and S-15 at 20 molecules/UC at 300-800 K. As can be seen, the change of Uad of S-0 with temperature is not evident, 9 ACS Paragon Plus Environment


which is due to the mutual effect of gradually increase in Uads-ads caused by larger kinetic energy and the decrease in Uads-zeo arising from delocalized adsorption on less energetic favorable regions with increasing temperature63. The Uad of S-15 exhibits a decrease trend because of more complex changes in both Uads-zeo and Uads-ads . At 300-500 K, the change of Uads-zeo and Uads-ads of S-15 are similar with that of S-0 which is attributed to the same reasons in consistent with results illustrated above. On the contrary, at 500-800 K a slight increase is observed in Uads-zeo which indicates an increased binding strength of benzene, and Uadsads

is greatly reduced indicating that decline of adsorbate density. The energetic of adsorption in

hierarchical zeolite suggests that the adsorption mechanisms at the two temperature ranges with a dividing point (D-P) of 500 K are different. The adsorption is largely dominated by micropore adsorption at 300-500 K. However, the mesopore takes more part in the adsorption at 500-800 K. 55 50 45

S-0 Uad 40

U (kJ/mol)

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

Page 10 of 34

35

S-15 Uad S-0 Uads-zeo S-15 Uads-zeo S-0 Uads-ads

10

S-15 Uads-ads

5

0 100

200

300

400

500

600

700

800

900

T (K)

Figure 3. Uad of benzene at 20 molecules/UC in S-0 and S-15 models comprising the Uads-ads and Uads-zeo contributions at 300-800 K. The values reported are accurate to 0.1 kJ/mol. Comparing Uad of the two models at the same temperature, the value of S-15 is generally lower than that of S-0. This can be explicitly observed when the temperature is within 500-800 K, which indicates that the introduction of mesoporosity decreases the strength of adsorption at all temperatures to a different extent. For the decline of Uads-zeo in S-15 compared with S-0, there are two fundamentally different explanations: more concentrated adsorption of benzene in microporous structure, which forces benzene to 10 ACS Paragon Plus Environment

Page 11 of 34 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


adsorb on unstable sites inside the supercage due to unprosperous adsorption into mesopore (labeled “SC loading factor”); or unfavorable adsorption strength of benzene in mesopore if its accessible (labeled “Mesopore contribution factor”). In the former case, a corresponding higher Uads-ads for S-15 could be expected, while in the latter case Uads-ads tends to decrease for S-15 model compared with S-0 ascribing to a less confined pore space. Actually, the Uads-ads values of S-15 present to be 0.7 and 1 kJ/mol larger at 300 and 500K, and 0.6 kJ/mol smaller at 800 K than these of S-0 model. This illustrates that the energy difference between two models is dominated by “SC loading factor” at 300-500 K, while “Mesopore contribution factor” becomes the main reason at 500-800 K. More importantly, this finding suggests that although the predominant interaction is Uads-zeo which is one order of magnitude larger than that of Uads-ads, a sole focus on Uads-zeo might omit important information of the adsorption process. In order to confirm the statement above, the adsorbates distribution in S-15 model needs to be investigated. For this purpose, the pores of S-15 were divided into three main regions, i.e., Meso, P-SC and D-SC. The position of the molecules is represented by the center of mass (COM). Figure 4 shows the distribution of benzene in terms of occupation probability (i.e., the ratio of loading in a specified region to the loading of the whole system) of three regions of S-15 at 20 molecules/UC at 300, 500, and 800 K. As shown, benzene adsorbs preferentially in SCs with the occupancy almost constantly within P-SC (38%) and D-SC (62%) at 300 and 500 K. Clearly, at both 300 and 500 K the benzene molecules are barely detected in Meso, suggesting that the “Mesopore contribution factor” can be discarded. These results in a higher concentration of benzene in the SCs of S-15 as compared to S-0, confirming that the “SC loading factor” is responsible for the energy difference of adsorption noticed above at low temperatures (higher Uads-ads and lower Uads-zeo for S-15 compared with S-0). At 800 K the occupy ratio slightly decreases to 36% for P-SC, while significantly reduces from 62% to 42% for D-SC, and it is worth to mention that benzene can freely access Meso at this temperature (~ 0 for 500 K and 22% for 800 K), supporting the claim that the “Mesopore contribution factor” in all likelihood can



become dominant corresponding with the striking decrease of Uads-ads in S-15 at 800 K. This result is consistent with the prospection of experimental study of ethane in the mesoporous NaCaA zeolites with increasing temperature27.

100

80 Occupation probability (%)

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

Page 12 of 34

P-SC D-SC Meso

60

40

20

0 300

500

800

T (K)

Figure 4. The occupation probability of benzene in P-SC, D-SC, and Meso of S-15 at 20 molecules/UC at 300, 500, and 800 K. Till now, it is still unclear whether “SC loading factor” plays a role in the energy difference between S-0 and S-15 at 800 K. As shown by the density contours of benzene at 20 molecules/UC in Figure 5, it is obvious that a higher density is observed for microporous space of S-15 at 300 and 500 K as compared to S-0. However, the density of benzene in microporous space of S-15 is obviously lower than that of S-0 at 800 K, which demonstrates the absence of “SC loading factor”. The results above illustrate that the contribution of mesopore to the adsorption properties of hierarchical materials is significantly affected by temperature and it is likely that the diffusion can only benefit from introduction of mesopore at temperatures that are high enough to allow the occupation of benzenes in it. With the aim to clearly elucidate the role of temperature in the diffusion process for hierarchical zeolite, the distribution and diffusivity of benzene are investigated.


Page 13 of 34 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


Figure 5. Density distributions of benzene in the frameworks of S-0 and S-15 at 20 molecules/UC at 300, 500, and 800 K. 3.2 Diffusivity The D of benzene in S-0 and H-models calculated from MSDs for the COMs of benzene at 20 molecules/UC at different temperatures is shown in Figure SI2, (Supporting Information). The diffusivities of S-0 model have been compared with the reported data from microscopic methods35, and the diffusivities obtained for S-0 is on the same order of magnitude as both MD and 2H NMR studies at the same temperature48,64,65. Two observations can be made based on Figure 6, which compares D of S-0 and the representative H-model (S-15) at 20 molecules/UC at different temperatures. First, the enhanced diffusion of benzene in both models with increasing temperature can be ascribed to the enhancement of molecules kinetic energy at a higher temperature which is in line with general expectation. The Arrhenius plot of diffusion coefficients of benzene in S-0 and H-models at 20 molecules/UC can be found in Figure SI3 of Supporting Information. The apparent activation energy of diffusion of S-0 and S-15 at benzene loading of at 20 molecules/UC is shown in Figure 7 to illustrate the temperature dependence of D. As shown in Figure 7, the application of Arrhenius equation to the measured D data yields a single value of ES-0 (5.7 kJ/mol) within the temperature range 300-800 K, which is in reasonable agreement with the value obtained by 2H NMR for zero defect dealuminated-Y (~10 kJ/mol)48. While two temperature ranges with an D-P at 500 K are characterized by slightly different ES-15 (form 11.8 kJ/mol at 300-500 K to 11.5 13 ACS Paragon Plus Environment


kJ/mol at 500-800 K), which could be due to different diffusion mechanisms at different temperatures that might result in favorable diffusion barrier at 500-800 K66. The value of ES-15 is larger than the value of ES0

at all temperatures, indicating that the favorable diffusion barrier is not an excuse for better transport in

hierarchical zeolite compared with the mesopore-free one.

22 S-0 S-15

20 18 16 14 2

12

9

D*10 (m /s)

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

Page 14 of 34

10 8 6 4 2 0

300

400

500

600

700

800

T (K)

Figure 6. Self-diffusion coefficients of benzene in S-0 and S-15 models at 20 molecules/UC at 300-800K in unit of 10-9 m2/s. The diffusivity of S-15 at 300 and 500 K is 1.4 × 10-9 m2/s-1 and 1.8 × 10-9 m2/s-1 lower than that of S-0. However, the mass transfer of S-15 at 800 K notably exceed that of S-0, and the diffusivity differences between S-0 and S-15 is 1.2 × 10-8 m2/s-1. Obviously, at lower temperatures of 300-500K, the built of mesopore has no advantage in diffusivity over intact FAU zeolite, which is in line with the PFG NMR study of Kortunov et al.20 which showed that the diffusion of guest molecules was essentially unaffected by mesopores in USY at 173-213 K. In addition, the diffusivity in H-models with larger mesopore is found to be smaller than those with the smaller mesopores (see Figure SI2, Supporting Information). This result is not unexpected because we have noticed that the benzene molecules are not able to access the mesopore at low temperatures at moderate loading, making the ‘‘tortuosity’’ of the framework increase with increasing volume fractions of the closed mesopore27. However, the mesopore fabrication leads to an order-of-magnitude increase in the diffusivities of benzene at high temperatures (500-800 K) in 14 ACS Paragon Plus Environment

Page 15 of 34

comparison to the mesopore-free zeolite, confirming that the development of diffusion behavior of benzene in hierarchical zeolite is heavily influenced by temperature. These findings further explained why the mesopore construcution of FAU zeolites can enhance the performance of catalyst at reaction temperatures which are generally higher than 500 K.

-17

S-0 S-15

ES-15, 300-500 K=11.8 kJ/mol -18

-19

ES-0=5.7 kJ/mol

lnD

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


-20

-21

ES-15, 500--800 K=11.5 kJ/mol -22 0.001

0.002

0.003

0.004

1/T

Figure 7. Arrhenius plot of diffusion coefficients of benzene at 20 molecules/UC in S-0 and S-15 models. Error bars of the diffusion coefficients have similar dimensions of the symbols and are not shown. The line shows the result of the fitting of the diffusivities by the Arrhenius law. ES-0 represents active energy of S-0 at 300-800 K. ES-15, 300-500 K and ES-15, 500-800 K represent active energy of S-15 at temperatures below and above 500 K. In order to further understand this important feature of diffusivities, the results from adsorption simulations above should be taken into account. Notice that the behavior of D is somewhat analogous with Uads-ads in Figure 3, illustrating the tenacious bond between them. As demonstrated above, at low temperatures (300-500 K) “SC loading factor” results in lower adsorption energy of S-15 while at high temperatures (500-800 K) “Mesopore contribution factor” is responsible. In the case of diffusion process, the “SC loading factor” would cause diffusion restrains in S-15 at 300-500 K due to a higher loading of adsorbate molecules inside SCs. As for temperatures higher than 500 K, the “Mesopore contribution



factor” would benefit the transport of adsorbates within the mesopore of S-15, and it is also possible that the diffusion inside the micropores of S-15 can be affected as well (as elaborated in the next section).

30 S-0 S-7.4 S-10 S-12.5 S-15 S-17.5

25 20

9

2

D*10 (m /s)

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

Page 16 of 34

15 10 5 0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3

-1

10 /T (K )

Figure 8. Diffusivities in S-0 and H-models at benzene loading of 20 molecules/UC at different temperatures. Four lines are shown for guidance. The dash black line, solid pink line, and solid red line represent the best linear fit to the diffusivities in S-0 (at 300-800 K), S-15 (at 300-500 K when mesopore are forbidden for benzene diffusion), and S-17.5 (500-800 K when mesopore are accessible for benzene diffusion), respectively. The solid black line is shown for guidance with the slop corresponding with isosteric heat of adsorption Eads (Eads = 54.4 kJ/mol, taken from Ref. [35]) for benzene adsorption in FAU zeolite at 20 molecules/UC. Fig. 8 shows the diffusivities of benzene in S-0 and H-models at 20 molecules/UC at 300-800 K. The slope of the best linear fit to the diffusivities in S-0 (dash black line) at 300-800 K is very similar to the best linear fit to the diffusivities in S-15 (solid pink line) at low temperatures of 300-500 K when mesopore are forbidden for benzene diffusion. This result resembles the experimental research using PFG NMR27, which found resembling slopes of the linear fit line of ethane diffusivities at different temperatures in microporous NaCaA zeolite and hierarchical NaCaA zeolites. The presence of mesopores causes no benefit on the mass transfer, because the mesopores are filled with other adsorbates (cyclohexane) and essentially closed for the diffusing of ethane molecules. With increasing temperature, 16 ACS Paragon Plus Environment

Page 17 of 34 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


they also found that the diffusivities of ethane in the hierarchical samples increase faster than those in microporous samples. This leads to the exceedance of the diffusivities in hierarchical zeolites over the diffusivities in parent microporous material. Despite the normal Arrhenius dependence, there are other reasons that should not be ignored to explain the enhancement of diffusivity of benzene in H-models with increasing temperature. On the one hand, the reason of the temperature dependence of the diffusivity in the mesoporous zeolites is attributed to different contribution of mesopore27. The benzene molecules are not able to get into the mesopore at low temperatures (as illustrated in Figure 4 and Figure 5) at 20 molecules/UC. This results in the compensating displacements back into the micropores, which in turn reduced the diffusivity in the microporous space. With increasing temperature, the diffusivity in microporous space would be increased because of the increasing contribution of mesopore which results in reduced anti-correlations in molecular displacement27. On the other hand, faster diffusion inside the mesopore compared with the micropores would also contribute to the overall diffusivity at temperatures high enough for the accessible of the mesopore (such as 500 K for S-15). As expected, the slope of the best fit line (that is, the slope in the Arrhenius plot of the diffusivity) of the benzene diffusivities in H-models at high temperatures increases with increasing volume fraction of the mesopore. The value increased is between isosteric heat of adsorption Eads (Eads = 54.4 kJ/mol for benzene adsorption in FAU zeolite at 20 molecules/UC taken from Ref. [35], slop of the solid black line) and the slope of the best fit line of S-0 (dash black line). The temperature dependence illustrated above shows a remarkably good agreement on both the experimental results27 and the theoretical predictions28, verified that the diffusion processes at low and high temperatures are correspondence with the different effects. This indicates that the disconnected mesopore may subdue the resistance of diffusion at high temperatures at moderate loading. Therefore, the PFG NMR study performed by Kortunov et al.20 at 173~213 K found no benefit of disconnect mesopores on the diffusion of n-octane in hierarchical zeolite. This is likely caused by the “SC loading factor”.



Page 18 of 34

So far, the discussions on the distribution of benzene molecule is based on the adsorption results, and an accurate insight into the temperature dependence of diffusion behavior in hierarchical zeolite models can be gleaned from investigations of adsorbate distribution evolved during the diffusion process. 3.3 Distribution of adsorbates in diffusion process The distribution law of benzene in the H-models and S-0 is studied by statistical analysis. Figure 9 shows the Radial distribution functions (RDFs) of COM of benzene and the central axis of the mesopore (mesoc) of S-15 (COM-mesoc) in the diffusion process at 20 molecules/UC at different temperatures. The first peak (peak A) locate around 1.2 nm in g(r) are clearly attributed to the presence of benzene which is located in the mesopore near the inner wall of it. However, the peak A is not shown for 300 K of S-15, which means that the benzene molecule cannot go inside the mesopore at room temperature. The peak A appears and gradually increases while the temperature increases. This is consistent with the result of the adsorption studies in which benzene sitting inside the inner wall of mesopore at 500-800 K is shown. Different from 300-500 K with zero distribution near mesoc, the diffusion inside all the regions of mesopore tends to be quite frequent at 800K. This observation applies to all H-models (see Figure SI4, Supporting Information).


Page 19 of 34

1.5

1.0 g(r)

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


300 K 350 K 400 K 450 K 500 K 550 K 600 K 700 K 800 K

c

S-15 COM-Meso

Peak A 800 K

0.5 500 K

0.0 0.0

0.3

0.6

0.9

1.2

1.5

r(nm)

Figure 9. The RDFs of COM of benzene and the central axis of the mesopore (mesoc) of S-15 (COM-mesoc) in the diffusion process at 20 molecules/UC at different temperatures. The relative concentration profile of benzene perpendicular to (1 0 0) plane of S-0 and S-15 at 20 molecules/UC are shown in Figures 10. As expected, the adsorbates reveal identical balanced distribution curves at all temperatures for S-0 model, signifying an even distribution of benzene in all SCs. In the case of benzene density in S-15 model, a marked increasing of SCs and decreasing of Meso compared with S0 are shown at low and moderate temperatures (300 and 500 K), representing more crowd distribution and leading to the increase in Uads-ads interactions in micropores which hinder the diffusion of benzene. This result further confirms the contribution of “SC loading factor” in diffusion at low temperatures. However, at 800K the relative concentration of benzene in the SCs of S-15 becomes similar with S-0, clearly stating the absence of “SC loading factor”. Meanwhile, a relative lower concentration of benzene is found for Meso in comparation with S-0, suggesting a relative loose interaction between adsorbates in mesopore compared with benzene inside the SC, which is consistent with the subdued Uads-ads values of S-15 than that of S-0 model in Figure 3. The distribution of benzene illustrated by Figure 10 could be furthermore confirmed by the RDFs of benzene COMs in S-0 and S-15 models at 20 molecules/UC (Figure 11). As anticipated, the first peak (peak B) of g(r) centered at 0.54 nm are more pronounced for S-15 model than that of S-0 at 300 and 500K, while the reverse is true for 800 K, which demonstrates that the distribution 19 ACS Paragon Plus Environment


of benzene is harmed and optimized by the introduction of mesoporosity for low and high temperatures respectively. These results further explain the lower diffusivity of benzene in S-15 than S-0 at 300-500 K, but the reverse at 500-800 K. Similar temperature dependence of g(r) can be found with other H-models, and the effect of temperature increases with the degree of mesopore construction (Figure SI5, Supporting Information).

2.1

S-0 300 K S-0 500 K S-0 800 K S-15 300 K S-15 500 K S-15 800 K

1.8 Relative concentration

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

Page 20 of 34

1.5 1.2 0.9 0.6 0.3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

r (nm)

Figure 10. Relative concentration profile of benzene generated for the 3D lattice of S-0 and S-15 perpendicular to the (1 0 0) plane at 20 molecules/UC at 300, 500, and 800K. Till now, we can conclude that the increase of diffusivity in H-models compared to S-0 is caused by different decisive effects. That is, “SC loading factor” at low temperatures, while “Mesopore contribution factor” at higher temperatures. Based on the diffusivities, the temperature dependence of the connectivity of mesopore and micropores, which can be illustrated by the time scale required for the benzene molecules to traverse the micro- and mesoporous pore spaces, is still unclear. Generally speaking, if the diffusivity in micropores is slower than that in the mesopore, the particles might be confined in a specified domain, leading to a “slow exchange” diffusion pattern. The diffusion pattern would be characterized as “fast exchange” if the opposite is true30. In the next section, this will be further evaluated by tracing benzene molecules at different initial locations in MD simulations. 20 ACS Paragon Plus Environment

Page 21 of 34

10 9

S-0 S-15

Peak B

8

800 K

7 6

g(r)

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 K

5 4 3

300 K

2 1 0 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

r (nm)

Figure 11. RDFs of benzene COMs in S-0 and S-15 models at 20 molecules/UC at 300, 500, and 800 K. 3.4 Connectivity of mesopore and micropores Three MSDs can be calculated separately according to benzene initially located in P-SC, Meso, and D-SC of H-models in MD simulation respectively. It is very interesting to find out that the MSDs in the three regions of H-models are indistinguishable with each other regardless of temperature. This results in almost identical diffusion coefficients (Figure SI6 and SI7, Supporting Information). This result shows that for benzene molecule with a kinetic diameter of 0.58 nm67, the connectivity of all three main regions of S-15 could be considered excellent. That is, the diffusion property can benefit from the mesopore fabrication as long as the diffusion into mesopore is viable by temperature. This can be further illustrated by the 2D tracing maps of benzene molecules initially located at different regions before MD simulation. As shown in Figure 12, the benzene molecules are able to migrate freely among different regions, resembling “fast exchange” diffusion pattern despite of the initial locations as long as the Meso region is accessible at a specified temperature.



Page 22 of 34

Figure 12. 2D tracing maps in xy plane of S-15 model for benzene molecules initially located in P-SC (black), D-SC (red), and Meso (green) at 300, 500, and 800 K. The positions of the molecules are taken from the first 1ns of total 10 ns MD simulations.


Page 23 of 34

40000 all x y z

35000 30000 25000

2

MSD (Å )

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


20000 15000 10000 5000 0 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 t (ps)

Figure 13. Representive MSDs of benzene in S-0 at 20 molecules/UC in three dimensions at 500 K. Although the diffusion in S-0 is isotropic in the three dimensions and is free from temperature influence as confirmed by MSDs of benzene in S-0 at 500 K in Figure 13, the effect of mesopore fabrication on the diffusion coefficient of H-models is expected to be anisotropic especially in z direction, considering the cylinder mesopore structure introduced. Figure 14 shows the isotropic average MSDs of benzene molecules in S-0 and the representative H-model S-15, as well as the Cartesian anisotropic components x, y, and z for S-15 with the comparison of one representative Cartesian isotropic component x for S-0. As we can see, the anisotropic of diffusivities for the three Cartesian components is not obvious at 300 and 500 K, and the MSDs in z direction slightly outperform that in x and y directions which are almost identical with each other. The MSDs of S-15 in three directions are generally smaller compared with S-0 at 300 and 500 K. However, at 800 K the MSD components of S-15 are apparently anisotropic with MSDs in z direction, that is, along the axis of mesopore, the values in z direction notably exceed that in x and y directions. Meanwhile, The MSDs of S-15 in three directions are generally larger compared with S0 at 800 K, manifesting that cylindrical mesopore facile diffusion in S-15 in all directions, especially in axis direction. Therefore, the diffusion results, together with the energies from adsorption and corroborative distribution analysis, present a consistent body of evidence that supports the predominance



of disconnected mesopore of hierarchical FAU zeolite on the diffusion of benzene when the temperature is high enough.

10000 S-0 S-15 S-0, x S-15, x S-15, y S-15, z

8000

MSD (Å)

6000

300 K

4000

2000

0 0

1000

2000

3000

4000 t (ps)

5000

6000

7000

8000

35000 S-0 S-15 S-0, x S-15, x S-15, y S-15, z

30000 25000

MSD (Å)

20000

500 K

15000 10000 5000 0 0 120000

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 t (ps) S-0 S-15 S-0, x S-15, x S-15, y S-15, z

100000 80000 MSD (Å)

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

Page 24 of 34

800 K

60000 40000 20000 0 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 t (ps)

Figure 14. MSDs of benzene at 20 molecules/UC in S-15 separated on three dimensions with the comparison of S-0 at 300, 500 and 800 K.

4. Conclusions In this paper, the adsorption and diffusion of benzene within microporous FAU zeolite model S-0 and Hmodels with both microporosities of FAU and disconnected mesopore carved out are studied by MC and MD simulations at 300-800 K. We found that disconnected mesopore has no advantage on diffusivity over conventional FAU zeolite at 300-500K. However, the diffusivity of benzene is significantly 24 ACS Paragon Plus Environment

Page 25 of 34 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


enhances by mesoporosity at higher temperature between 500-800 K. Together with energy and distribution analysis in the molecular level, we conclude that this is caused by different controlling effects: “SC loading factor” at 300-500 K and “Mesopore contribution factor” at 500-800 K respectively. Our result is a good proof of classical transport theory which indicates that isolated mesopore could only improve diffusion performance under limited conditions. Our results suggested that the enhancement of diffusion caused by disconnected mesopore in hierarchical zeolite might have been underestimated, since almost all of the fundamental understanding of diffusion is obtained at temperatures deviate from those of the usual technological use of nanoporous materials. For example, the cracking reactions are commonly performed at elevated temperatures. It seems reasonable to ascribe the enhanced catalytic performance of hierarchical zeolite over traditional counter zeolite, at least partly, to improve mass transport even if most mesopores do not constitute an interconnected accessible network. Furthermore, the interconnectivity of all three main regions of H-model is excellent since the diffusivities are irrelevant with initial position of benzene molecule at all considered temperatures. That is to say, the mesopore fabrication can benefit the diffusion property of benzene in the overall skeleton structure of Hmodels, as long as it can approach mesopore at a sufficiently high temperature.

Acknowledgments The authors acknowledge the financial support of TOTALS.A., Scientific Division, Paris. This study was also sponsored by the National Natural Science Foundation of China (Grant No 21476260, 21336011, and 21236009), Science Foundation of China University of Petroleum, Beijing (2462015YQ0311), the Natural Science Foundation of Heibei province (Grant No E2018408043), the Youth Talent Program of Colleges in Hebei Province (Grant No BJ2017043), and Science and Technology Program of Langfang, China (Grant No 2017013010). 25 ACS Paragon Plus Environment


Page 26 of 34

Supporting Information Supplementary data associated with this article, including the detailed information of isothermals, MSDs and self-diffusion coefficients of benzene in both S-0 and H-models, as well as RDFs of benzene COMs and COM-mesoc, can be found in the online version. This material is available free of charge via the Internet at http://pubs.acs.org.

Abbreviations 12-T ring = 12-membered oxygen window COM = center of mass COM-mesoc = radial distribution function of the COM of benzene and the central axis of the mesopore D = diffusion coefficient D-SC = supercage defected by mesopore fabrication incorporating with hydroxy groups EM = energy minimization E = apparent activation energy of diffusion FAU = faujasite FCC = fluidized-bed catalytic cracking GCMC = grand canonical Monte Carlo H-Model = hierarchical all silica FAU zeolite model D-P = dividing point MC = Monte Carlo MD = molecular dynamics molecules/UC = molecules per unit cell MSD = mean square displacement Meso = mesopore with fully hydroxylated inner surface mesoc = central axis of the mesopore O = oxygen OI = oxygen of Si-O-Si bridge in the framework OII = oxygen of Si-O-H bridge in the framework P-SC = perfect supercage 26 ACS Paragon Plus Environment

Page 27 of 34 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


SC = supercage SOD = sodalite cage S-0 = all silica FAU zeoite model with perfect crystal structure S-7.4, S-10, S-12.5, S-15, and S-17.5 = hierarchical FAU zeolite models contain a cylindrical mesopore with a radius of 7.4, 10, 12.5, 15, and 17.5 Å, respectively Uad = averaged adsorption energy of the model system Uads-zeo = averaged interactions of adsorbate and the zeolite framework Uads-ads = averaged interactions between pairs of adsorbates

References 1. Hattori, H. Heterogeneous basic catalysis. Chem. Rev. 1995, 95, 537. 2. Sazama, P.; Sobalik, Z.; Dedecek, J.; Jakubec, I.; Parvulescu, V.; Bastl, Z.; Rathousky, J.; Jirglova, H. Enhancement of Activity and Selectivity in Acid-Catalyzed Reactions by Dealuminated Hierarchical Zeolites. Angew. Chem. Int. Ed. 2013, 52, 2038. 3. Ong, Y. K.; Bhatia, S. Hydrodynamic study of zeolite-based composite cracking catalyst in a transport riser reactor. Chem. Eng. Res. Des. 2009, 87, 771. 4. Harding, R.; Peters, A.; Nee, J. New developments in FCC catalyst technology. Appl. Catal. A. 2001, 221, 389. 5. Perez-Ramirez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Hierarchical zeolites, enhanced utilisation of microporous crystals in catalysis by advances in materials design. Chem. Soc. Rev. 2008, 37, 2530. 6. Keller, T. C.; Isabettini, S.; Verboekend, D.; Rodrigues, E. G.; Perez-Ramirez, J. Hierarchical highsilica zeolites as superior base catalysts. Chem. Sci. 2014, 5, 677. 7. Li, B.; Hu, Z.; Kong, B.; Wang, J.; Li, W.; Sun, Z.; Qian, X.; Yang, Y.; Shen, W.; Xu, H.; Zhao, D. Hierarchically tetramodal-porous zeolite ZSM-5 monoliths with template-free-derived intracrystalline mesopores. Chem. Sci. 2014, 5, 1565. 8. Na, K.; Choi, M.; Ryoo, R. Recent advances in the synthesis of hierarchically nanoporous zeolites. Micropor. Mesopor. Mat. 2013, 166, 3. 9. Tao, Y.; Kanoh, H.; Abrams, L.; Kaneko, K. Mesopore-Modified Zeolites, Preparation, Characterization, and Applications. Chem. Rev. 2006, 106, 896. 10. Thommes, M.; Mitchell, S.; Pérez-Ramírez, J. Hierarchical MFI Zeolites by Advanced Water and Argon Sorption Studies. J Phys. Chem. C. 2012, 116, 18816. 27 ACS Paragon Plus Environment


Page 28 of 34

11. Zhu, J.; Meng, X.; Xiao, F. Mesoporous zeolites as efficient catalysts for oil refining and natural gas conversion. Front. Chem. Sci. Eng. 2013, 7, 233. 12. Christensen, C. H.; Johannsen, K.; Schmidt, I.; Christensen, C. H. Catalytic Benzene Alkylation over Mesoporous Zeolite Single Crystals: Improving Activity and Selectivity with a New Family of Porous Materials. J Am. Chem. Soc. 2003, 125, 13370. 13. Garcia-Martinez, J.; Li, K.; Krishnaiah, G. A mesostructured Y zeolite as a superior FCC catalyst from lab to refinery. Chem. Commun. 2012, 48, 11841. 14. Yang, X. Y.; Leonard, A.; Lemaire, A.; Tian, G.; Su, B. L. Self-formation phenomenon to hierarchically structured porous materials, design, synthesis, formation mechanism and applications. Chem. Commun. 2011, 47, 2763. 15. Houžvička, J.; Jacobsen, C. J. H.; Schmidt, I. 26-O-02-Design of zeolite catalyst for paraffin isomerisation. Stud.. Surf. Sci. Catal. 2001, 135, 58. 16. Schmidt, I.; Krogh, A.; Wienberg, K.; Carlsson, A.; Brorson, M.; Jacobsen, C. J. H. Catalytic epoxidation of alkenes with hydrogen peroxide over first mesoporous titanium-containing zeolite. Chem. Commun. 2000, 21, 2157. 17. Xiao, F. S.; Wang, L.; Yin, C.; Lin, K.; Di, Y.; Li, J.; Xu, R.; Su, D. S.; Schlögl, R.; Yokoi, T.; Tatsumi, T. Catalytic Properties of Hierarchical Mesoporous Zeolites Templated with a Mixture of Small Organic Ammonium Salts and Mesoscale Cationic Polymers. Angew. Chem. Int. Ed. 2006, 45, 3090. 18. Zhao, L.; Shen, B.; Gao, J.; Xu, C. Investigation on the mechanism of diffusion in mesopore structured ZSM-5 and improved heavy oil conversion. J Catal. 2008, 258, 228. 19. Groen, J. C.; Zhu, W.; Brouwer, S.; Huynink, S.J.; Kapteijn, F.; Moulijn, J. A.; Pérez-Ramírez, J. Direct Demonstration of Enhanced Diffusion in Mesoporous ZSM-5 Zeolite Obtained via Controlled Desilication. J Am. Chem. Soc. 2006, 29, 355. 20. Kortunov, P.; Vasenkov, S.; Kärger, J.; Valiullin, R.; Gottschalk, P.; Fé, Elía, M.; Perez, M.; Stöcker, M.; Drescher, B.; McElhiney, G. The role of mesopores in intracrystalline transport in USY zeolite, PFG NMR diffusion study on various length scales. J Am. Chem. Soc. 2005, 127, 13055. 21. Kirchner, T.; Shakhov, A.; Zeigermann, P.; Valiullin, R.; Kärger, J. Probing mesopore connectivity in hierarchical nanoporous materials. Carbon 2012, 50, 4804. 22. Tao, Y.; Kanoh, H, Kaneko, K. ZSM-5 Monolith of Uniform Mesoporous Channels. J Am. Chem. Soc. 2003, 125, 6044. 23. Jacobsen, C. J. H.; Madsen, C.; Houzvicka, J.; Schmidt, I.; Carlsson, A. Mesoporous Zeolite Single Crystals. J Am. Chem. Soc. 2000, 122, 7116. 28 ACS Paragon Plus Environment

Page 29 of 34 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


24. Cepak, V. M.; Hulteen, J.C.; Che, G.; Jirage, K. B.; Lakshmi, B. B.; Fisher, E. R.; Martin, C. R.; Yoneyama, H. Chemical Strategies for Template Syntheses of Composite Micro- and Nanostructures. Chem. Mater. 1997, 9, 1065. 25. Kim, S. S.; Shah, J.; Pinnavaia, T. J. Colloid-Imprinted Carbons as Templates for the Nanocasting Synthesis of Mesoporous ZSM-5 Zeolite. Chem. Mater. 2003, 15, 1664. 26. Tao, Y.; Kanoh, H.; Kaneko, K. Uniform Mesopore-Donated Zeolite Y Using Carbon Aerogel Templating. J Phys. Chem. 2003, 107, 10974. 27. Mehlhorn, D.; Valiullin, R.; Kärger, J.; Cho, K.; Ryoo, R. Exploring the hierarchy of transport phenomena in hierarchical pore systems by NMR diffusion measurement. Micropor. Mesopor. Mat. 2012, 164, 273. 28. Zeigermann, P.; Kärger, J.; Valiullin, R. Diffusion in microporous materials with embedded mesoporosities. Micropor. Mesopor. Mat. 2013, 178, 84. 29. Zeigermann, P.; Naumov, S.; Mascotto, S.; Kärger, J.; Smarsly, B. M.; Valiullin, R. Diffusion in hierarchical mesoporous materials: applicability and generalization of the fast-exchange diffusion model. Langmuir 2012, 28, 3621. 30. Schneider, D.; Mehlhorn, D.; Zeigermann, P.; Kärger, J.; Valiullin, R. Transport properties of hierarchical micro-mesoporous materials. Chem. Soc. Rev. 2016, 45, 3439. 31. Fitch, A. N.; Jobic, H.; Renouprez, A. A Localization of Benzene in Sodium-Y Zeolite by Neutron Diffraction. J Phys .Chem. 1986, 90, 1311. 32. Zheng, H. M.; Zhao, L.; Yang, Q.; Gao, J. S.; Shen, B.J.; Xu, C.M. Influence of Framework Protons on the Adsorption Sites of the Benzene/HY System. Ind. Eng. Chem. Res. 2014, 53, 13610. 33. Ghorai, P. K.; Yashonath, S.; Demontis, P.; Suffritti, G. B. Diffusion anomaly as a function of molecular length of linear molecules, levitation effect. J Am. Chem. Soc. 2003, 125, 7116. 34. Zheng, H. M.; Zhao, L.; Yang, Q.; Ji, J. J.; Gao, J. S.; Xu, C. M.; Luck, F. Unraveling the Adsorption Mechanism of Mono- and Diaromatics in Faujasite Zeolite. ACS Appl. Mat. Interfaces. 2015, 7, 10190. 35. Zheng, H. M.; Zhao, L.; Ji, J. J.; Yang, Q.; Gao, J. S.; Molecular origin of decreased diffusivity with loading for benzene/HY. AIChE J 2016, 8(62), 2778. 36. Bates, S. P.; Well, W. J. M. V.; Santen, R. A. V.; Smit, B. Energetics of n-alkanes in zeolites, a configurational-bias Monte Carlo investigation into pore size dependence. J Am. Chem. Soc. 1996, 118, 6753. 37. Beerdsen, E.; Dubbeldam, D.; Smit, B.; Vlugt, T. J. H.; Calero S. Simulating the effect of nonframework cations on the adsorption of alkanes in MFI zeolites. J Phys. Chem. B. 2003, 107, 12138. 29 ACS Paragon Plus Environment


Page 30 of 34

38. Granato, M. A.; Vlugt, T. J. H.; Rodrigues, A. E. Molecular simulation of propane-propylene binary adsorption equilibrium in Zeolite 4A. Ind. Eng. Chem. Res. 2006, 46, 321. 39. Krishna, R. Describing the Diffusion of Guest Molecules Inside Porous Structures. J Phys. Chem. C. 2009, 113, 19756. 40. Bae, Y. S.; Yazaydın, A. Ö.; Snurr, R. Q. Evaluation of the BET Method for Determining Surface Areas of MOFs and Zeolites that Contain Ultra-Micropores. Langmuir 2010, 26, 5475. 41. Sun, H. COMPASS, An ab initio force-field optimized for condensed-phase applications overview with details on alkane and benzene compounds. J Phys Chem B. 1998, 102, 7338. 42. Jirapongphan, S. S.; Warzywoda, J.; Budil, D. E.; Sacco, A. Enantioseparation of phenylglycinol in chiral-modified zeohte HY, A molecular simulation study. Micropor. Mesopor. Mat. 2007, 103, 280. 43. Arı, M. U.; Ahunbay, M. G.; Yurtsever, M.; Erdem-Şenatalar, A. Molecular Dynamics Simulation of Water Diffusion in MFI-Type Zeolites. J Phys. Chem. B. 2009, 113, 8073. 44. Fermeglia, M.; Pricl, S. Prediction of phase equilibria for binary mixtures by molecular modeling. AIChE J 2001, 47, 2371. 45. Jousse, F.; Auerbach, S. M.; Vercauteren, D. P. Correlation effects in molecular diffusion in zeolites at infinite dilution. J Phys. Chem. B. 2000, 104, 2360. 46. Ekhteiari, Salmas. R, Demir, B.; Yıldırım, E.; Sirkecioğlu, A.; Yurtsever, M.; Ahunbay, M. G. Silver– Sodium Ion Exchange Dynamics in LTA Zeolite Membranes. J Phys. Chem. C. 2013, 117, 1663. 47. Demontis, P.; Yashonath, S.; Klein, M. L. Localization and mobility of benzene in sodium-Y zeolite by molecular dynamics calculations. J Phys. Chem. 1989, 93, 5016. 48. Bull, L. M.; Henson, N. J.; Cheetham, A. K.; Newsam, J. M.; Heyes, S. J. Behavior of benzene in siliceous faujasite, a comparative study of deuteron NMR and molecular dynamics. J Phys. Chem. 1993, 97, 11776. 49. Snurr, R. Q.; Bell, A. T.; Theodorou, D. N. Prediction of adsorption of aromatic hydrocarbons in silicalite from grand canonical Monte Carlo simulations with biased insertions. J Phys. Chem. 1993, 97, 13742. 50. Zeng, Y.; Ju, S.; Xing, W.; Chen, C.. Computer Simulation of Benzene Adsorbed in All-Silica Y and NaY Zeolites. Ind. Eng. Chem. Res. 2007, 46, 242. 51. Song, L.; Sun, Z.; Duan, L.; Gui, J.; McDougall, G. S. One stage radical debridanent interbody fusion and instrumentation in the treainent of spinal tuberculosis. Micropor. Mesopor. Mat. 2007, 104, 115. 52. Ewald, P. P. Die Berechnung optischer und elektrostatischer Gitterpotentiale. Ann. Phys. 1921, 64, 253. 30 ACS Paragon Plus Environment

Page 31 of 34 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


53. Tao, Y.; Xue, Q.; Liu, Z.; Shan, M.; Ling, C.; Wu, T.; Li, X. Tunable hydrogen separation in porous graphene membrane, first-principle and molecular dynamic simulation. ACS Appl. Mat. Interfaces. 2014, 6, 8048. 54. Si, L.; Guo, D.; Xie, G.; Luo, J. Mechanical properties and interface characteristics of nanoporous low-k materials. ACS Appl. Mat. Interfaces. 2014, 6, 13850. 55. Bai, P.; Siepmann, J. I.; Deem, M. W. Adsorption of glucose into zeolite beta from aqueous solution. AIChE J 2013, 59, 3523. 56. Brémard, C.; Ginestet, A. G.; Maire, M. L. Sorption Sites, Energetics, and Reactions of Molybdenum Hexacarbonyl and Benzene Cosorbed in Faujasitic Zeolites. J Am. Chem. Soc. 1996, 118, 12724. 57. Beck, L. W.; Xu, T.; Nicholas, J. B.; Haw, J. F. Kinetic NMR and Density Functional Study of Benzene H/D Exchange in Zeolites, the Most Simple Aromatic Substitution. J Am. Chem. Soc. 1995, 117, 11594. 58. Wu, J. Y.; Liu, Q. L.; Xiong, Y.; Zhu, A. M.; Chen, Y. Molecular simulation of water/alcohol mixtures' adsorption and diffusion in zeolite 4A membranes. J Phys. Chem. B. 2009, 113, 4267. 59. Takahashi, A.; Yang, F. H. Y.; Yang, R. T. Aromatics/Aliphatics Separation by Adsorption, New Sorbents for Selective Aromatics Adsorption by π-Complexation. Ind. Eng. Chem. Res. 2002, 41, 2487. 60. Zheng, H. M.; Zhao, L.; Yang, Q.; Dang, S. Q.; Wang, Y. X.; Gao, J. S.; Xu, C. M. Insight into the adsorption mechanism of benzene in HY zeolites, the effect of loading. RSC Advances 2016, 6, 34175. 61. Mentzen, B. F. Structural investigations on zeolitic faujasite/benzene host/guest systems (Si/Al = 2.43 and ≈ 200) by using combined powder diffraction, adsorption calorimetry and molecular mechanics simulations. CR Chim. 2005, 8, 353. 62. Mańko, M.; Chal, R.; Trens, P.; Minoux, D.; Gérardin, C.; Makowski, W. Porosity of micromesoporous zeolites prepared via pseudomorphic transformation of zeolite Y crystals, A combined isothermal sorption and thermodesorption investigation. Micropor. Mesopor. Mat. 2013, 170, 243. 63. Yashonath, S.; Demontis, P.; Klein, M. L. Temperature and concentration dependence of adsorption properties of methane in NaY, a molecular dynamics study. J Phys. Chem. 1991, 95, 5881. 64. Rungsirisakun, R.; Nanok, T.; Probst, M.; Limtrakul J. Adsorption and diffusion of benzene in the nanoporous catalysts FAU, ZSM-5 and MCM-22, a molecular dynamics study. J Mol. Graph. Model. 2006, 24, 373. 65. Auerbach, S. M.; Bull, L. M.; Henson, N. J.; Metiu, H. I.; Cheetham, A. K. Behavior of benzene in Na-X and Na-Y zeolites, Comparative study by {sup 2}H NMR and molecular mechanics. J Phys. Chem. 1996, 100, 5923. 31 ACS Paragon Plus Environment


Page 32 of 34

66. Christensen, C. H.; Johannsen, K.; Schmidt, I.; Christensen, C. H. Catalytic Benzene Alkylation over Mesoporous Zeolite Single Crystals, Improving Activity and Selectivity with a New Family of Porous Materials. J Am. Chem. Soc. 2003, 125, 13370. 67. Van, D. P. S.; Van, A.T.; Bozbiyik, B.; Lannoeye, J.; Vos, D. E. D.; Baron, G. V.; Denayer, J. F. M. Adsorptive Characterization of the ZIF-68 Metal-Organic Framework, A Complex Structure with Amphiphilic Properties. Langmuir. 2014, 30, 8416.


Page 33 of 34 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


TOC graphic



TOC 60x38mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 34 of 34

Insight into the Contribution of Isolated Mesopore on Diffusion in

Recommend Documents