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Reports: DNI552089-DNI5: Metal Clusters Sequestered in Micropores of Hierarchical Lamellar Zeolites: Protected Active Sites and Enhanced Mass Transport for Heteroatom-Tolerant Catalysts

Dongxia Liu, University of Maryland

Unit-cell thick zeolites, or in a more general case, two-dimensional (2D) lamellar zeolites, are an innovative type of meso-/microporous materials inside the hierarchical zeolite family. As the characteristic length of the micropore domains shrinks in lamellar zeolites and, as a consequence, the fraction of external/mesoporous zeolite surface area becomes comparable to micropore surface area, the distribution of the Brønsted acid sites and metal or metal oxide sites, if the zeolite is used as catalyst support, become distinct from that of microporous zeolites. The active sites on the external surface of hierarchical lamellar zeolites result in higher reaction rates for diffusion-limited reactions and are, however, accompanied by the absence of the shape-selective environment of active sites in micropores. Our work aims to report an understanding of the interplay among the textural properties, active site distributions, and catalytic performances of lamellar MFI zeolite catalysts. We have created a dual template synthesis strategy to tune the meso-/microporosity of lamellar MFI zeolites (Figure 1).

                          (A)                                           (B)                                         (C)

                          (D)                                           (E)                                         (F)

Figure 1. SEM images showing morphologies of Mo loaded lamellar MFI catalysts: (A) Mo-MFI-10/0, (B) Mo-MFI-10/2, (C) Mo-MFI-10/5, (D) Mo-MFI-10/8, (E) Mo-MFI-10/12, and (F) Mo-MFI-10/36, respectively.

  By adding a molecular template (tetrapropylammonium hydroxide, TPAOH) to lamellar MFI synthesis recipe, a series of zeolites with systematical modulatable meso-/microporosity of the lamellar MFI zeolite catalysts were created. Molybdenum loaded lamellar zeolites (Mo/lamellar MFI) were employed as the catalyst systems to explore the spatial distribution and catalytic performances of metal and acid active sites in lamellar zeolite catalysts in direct methane aromatization (DMA) reactions. The textural and acidity properties of the Mo-zeolite catalysts were measured for understanding the distribution and the number of the active sites in the catalysts in the catalytic reactions. The loading of the metal into the catalyst was 1.3wt% and the sample was treated at 710ºC for 5 hours before the property characterizations. Measured surface area and pore porosity from Ar isotherm data show that lamellar zeolites have tunable surface area and pore volume. The loading of Mo into the catalyst lowers the pore volume and surface area of the zeolites. The measured number of Brønsted acid sites in the zeolites and Mo-loaded zeolites, respectively, by the dimethyl ether titration and elemental analyses show that (i) all of the MFI zeolites have comparable number of acid sites (ii) the number of free acid sites on the catalyst surface were tunable with the meso/microporosity of the zeolite catalyst. Comparing the results in Table 1, we can conclude that the mesoporous structure in lamellar zeolites facilities the dispersion of the metal species that forming a higher number of the interacting Mo-H+ species, which are responsible for the tunable spatial distribution of the metal and acid sites.  

Table 1. Composition and acidity of the lamellar MFI catalysts before and after Mo loading.

Catalyst

 

Before Mo loading

 

After Mo loading

Si/Ala

Si/Alb

Si/Alc H+(%) c

Si/Ald

fext, H+(%)e

 

Si/Alb

Si/Moa

Si/Moc

fext, H+(%)f

MFI-10/0

32

33

27

31

   12

 

140

126

131

10

MFI-10/2

32

33

33

33

14

 

105

124

191

12

MFI-10/5

31

29

46

35

10

 

76

96

77

12

MFI-10/8

33

28

34

32

9

 

66

103

64

7

MFI-10/12

32

32

56

40

8

 

55

101

61

10

MFI-10/36

26

21

38

28

5

 

42

98

23

1

a Determined by the ICP-OES; b Determined by DME titration ; c Measured by XPS; d Average Si/Al ratio, calculated from (Si/Ala + Si/Alb + Si/Alc )/3; e Determined by DTBP titration during methanol dehydration reaction; f Measured by DME titration in the presence of DTBP.  

    Figure 2. Production rates of benzene (A), toluene (B) and naphthalene (C), respectively, and selectivity of aromatics at TOS of 5 h (D) in DMA reactions.

Concurrently, our focus has evolved to study the catalytic activity, selectivity, and coke formation on the catalyst in the DMA reactions (Figure 2). For all the investigated catalyst systems, the aromatic product formation sharply increased in the initial period of the reaction (i.e., induction period) and reached a maximum after about 1 h. Afterwards, the production formation rate decreased with time-on-stream (TOS). The increase in induction time with increasing zeolite mesoporosity and external surface areas is ascribed to the higher degree dispersion of Mo species in lamellar MFI and lamellar MWW zeolites, which is consistent with that indirectly inferred from the Ar isotherm measurement and DME titrations of Brønsted acidity discussed above. After the reaction induction period, the methane conversion over Mo/MFI catalysts followed a volcano-shape plot, consistent with their distribution of the active sites in these catalysts (Figure 3).

Figure 3. Interdependence between active site distribution, catalytic performance, and textural properties (described as the product (Vmicro/Vtotal) x (Sext/SBET)) of the Mo/lamellar MFI catalysts. (A) Active site distribution, described as the product (fext, Mo) x (fext, H+), versus hierarchical factor; (B) Aromatic product formation rate at TOS 10 h versus hierarchical factor; (C) Selectivity to naphthalene at TOS of 10 h versus hierarchical factor; and (D) Coke distribution (fext, coke and fint, coke) versus relative external surface area of the catalysts, respectively. In summary, our work has shown the consequences of fine tuning the meso-/microporous zeolite structures on distribution and activity of active sites in Mo-lamellar MFI zeolites. The current research directs the PI to pursue the shape selective characteristics of meso-/microporous zeolite catalytic systems ubiquitous in petrochemical field for methane aromatization, hydrocracking, and hydroisomerization of heavy hydrocarbons where the mass transport and catalyst deactivation limit the catalytic reactions. More generally, the PI aims to develop diverse strategies for isolating metal and metal oxide clusters in restrictive environments of hierarchical zeolites to ultimately lead to a new class of catalytic materials that are stable under aggressive reaction environments. Accompanying the development on the PI’s career in the catalyst development for efficient catalytic processes, the graduate student working on this project is receiving a systematic training in the field of materials and catalysis. This will lead to the successful education of our next generation of scientists and engineers.