442978 Synthesis of Zeolitic Enwrapped Catalysts By Chemical Vapor Deposition

Monday, November 9, 2015
Exhibit Hall 1 (Salt Palace Convention Center)
Yijia Sun, Shoucheng Du, Chunxiang Zhu and George M. Bollas, Chemical and Biomolecular Engineering, University of Connecticut, Storrs, CT

Synthesis of Zeolitic Enwrapped Catalysts by Chemical Vapor Deposition

Yijia Sun*, Shoucheng Du, Chunxiang Zhu, George M. Bollas

* Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, 191 Auditorium Road, Unit 3222, Storrs, CT, 06269-3222, USA. Email:yijia.sun@uconn.edu


Zeolites have been widely used as molecular separation media [1] and in membrane reactors [2] due to their unique pore channel structure and advantages in spatial selectivity. They are also used in catalytic conversions in petro-chemical industry [3]. Recently, novel zeolite enwrapped metal catalysts attracted significant attention [4,5]. Zeolite enwrapped catalysts have a core/shell structure, where the zeolite membrane (shell) and enwrapped catalyst (core) provide independent catalytic functionalities. In these catalytic structures, the reactants need to pass through the zeolite membrane first and react on the core catalyst to form products.  In order to exit the catalyst, the intermediate products must diffuse out through the pore channels, where they are catalyzed and produce final products. Except for catalytic activity, the core usually has high adsorption capacity and the shell possesses separation properties. In the preparation of zeolite enwrapped catalysts, the zeolite membrane is coated directly onto the core catalyst surface. The common method of preparing these zeolite membranes is the so-called secondary growth method, where a seeding layer is deposited on the support and then undergoes crystallization in the presence of a structure directing agent (SDA) and silica source. Hydrothermal synthesis is usually used in membrane preparation. The schematic illustration is shown in Figure 1.

Figure 1. Schematic illustration of zeolite membrane preparation by hydrothermal synthesis.

In liquid-phase hydrothermal synthesis, zeolite composite membranes are prepared by secondary growth in a Teflon-lined autoclave in the presence of template solution [6]. However, hydrothermal synthesis only consumes a small amount of template substrates in the synthesis solution, thus generating significant amount of waste. The high alkalinity of the substrates causes corrosion on the support and destroys the active sites. Moreover, it is hard to control the reaction rate and crystalline thickness as hydrothermal synthesis is operated in a closed system. In this work, we explore a novel method to prepare zeolite capsuled catalysts: chemical vapor deposition. Chemical vapor deposition has the capacity of producing dense and pure materials. Besides, it has the ability to control the crystal structure and deposition rate. Moreover, the whole process presents merits in preventing corrosion and reducing the seed loss. To implement this strategy, briefly, two seeding techniques, hydrothermal synthesis (HS) and sol-gel formation (GF), were applied to prepare different types of zeolite seeds. Subsequently, γ-Al2O3 pellets coated with zeolite precursors from HS were subjected to chemical vapor deposition of gaseous phase SDA, with which the zeolite seeds could react and be transformed into zeolite membranes. γ-Al2O3 pellets were also coated with sol-gel, after which the coated pellets were exposed to a vapor phase silica source: tetraethyl orthosilicate (TEOS). This strategy incorporates the benefits of chemical vapor deposition as well as avoids the disadvantages of hydrothermal synthesis. Schematic illustrations of the two methods are presented in Table 1. Detailed preparation procedures will be presented but are not discussed here.

Table 1. Schematic illustrations of the synthesis methods (HS-CVD and GF-CVD) used in this study.


Secondary Growth


Hydrothermal synthesis



Sol-gel formation



Figure 2 shows the XRD patterns of the parent alumina and zeolite coated materials synthesized by HS-CVD and GF-CVD methods. Due to the thin and small quantity of the zeolitic membranes formed, the peaks that represent alumina are still dominant in the XRD patterns of the coated materials. Other than the peaks from alumina, the peaks of the zeolite from the HS-CVD method are clear in the ranges 2θ= 8-9 and 20-25°. The characteristic peaks of the zeolite in the GF-CVD synthesized material have very low intensities. The formation of zeolite crystals on the alumina core is further verified using Scanning Electron Microscopy (SEM). In Figure 3, unevenly distributed zeolite seeds are observed after the seeding step in the GF-CVD method. After CVD of TEOS on seeded supports, clear crystal coverage is observed. The zeolite crystals are growing individually with random orientation and few occurrences of intergrowth. In order for the membrane to possess high separation performance, it is preferred that the crystals are oriented. Membrane separation performance could be improved by varying the alkalinity and SDA concentration in the precursor [7].

Figure 2.  XRD patterns of (a) parent alumina, (b) material after HS-CVD, (c) material after GF-CVD. Peak identification: (+) alumina, (*) zeolite crystals after HS-CVD, (o) zeolite crystals after GF-CVD.

Figure 3. SEM images of materials after seeding (left) and secondary growth (right) using the GF-CVD method.

The pore size distributions of the parent alumina and the zeolite coated materials are shown in Figure 4a. A decrease in pore diameters is observed in the coated materials synthesized by both methods. In the HS-CVD material, a significant increase in pore volume is observed, due to the increase in structured channels in the zeolite crystals. In the GF-CVD material, a decrease in the mesopore volume in the pellets is observed, as shown in Figure 4a and 4b. According to the SEM images of Figure 3, alumina pellets are covered by a layer of zeolite crystals, possibly blocking the original pore channels of the parent alumina. This can be potentially addressed by using a different seeding technique, such as wetting the alumina support with a liquid agent before rubbing the zeolite dry crystals [8].

Figure 4. (a) Pore size distributions and (b) nitrogen adsorption/desorption isotherm curves of parent alumina, material after HS-CVD and material after GF-CVD.

In summary, two methods (HS-CVD and GF-CVD) were investigated for synthesizing zeolite enwrapped core/shell catalysts. The characterization results showed the success and feasibility of using CVD in zeolite membrane synthesis. Future work includes optimization of the microstructure to reduce randomness in the crystal orientation and tuning of CVD parameters to control the membrane thickness. Separation and pervaporation tests are required to test membrane performances.


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