(387i) Investigation on the Effective Pore Width of Multipore Zeolites with Different Pore Sizes

Zeolites are widely used as acidic catalysts in the heterogeneous catalytic reactions. The most important characteristic of these materials is the shape selectivity which is strongly determined by the crystal structure of zeolites. This has triggered the synthesis of new zeolite framework structures which showed the improved catalytic performance in methanol-to-olefins [1], dimethyl ether-to-olefins reaction [2], cracking of hydrocarbons [3], and alkylation reaction [4], etc. The unique catalytic performance of these zeolites comes from the multiple channel dimensionality composed by different pore sizes. However, in multipore zeolites, a careful inspection of the micropore structure is required to understand the effective pore width of these materials.

The constraint index (CI) test originally developed in the Mobil laboratories for the comparison of the relative pore sizes and the shape selectivities of zeolite catalysts [5]. We investigated the CI value (Eq. 1) of the recently reported zeolites, namely TUN- (10-membered ring (ring)×10-ring) and MSE-type (12×10×10-ring) zeolites in comparison with MFI- (10×10-ring) and *BEA-type (12×12-ring) zeolites in order to understand the effective pore width of these multipore zeolites. Furthermore, we investigated the effect of acidic properties (distribution and number of acid sites) on the CI value in attempt to rationalize their unexpected behavior in the CI value.

CI = log(1-X_n-hexane)/log(1-X_{3methylpentane)} (1)

2. Experimental

The TUN- [6] and MSE-type zeolites [2,3] were synthesized by following the previous reports. The MFI zeolite (Si/Al = 15) was synthesized with the gel composition of 1 Si: 0.066 Al: 0.37 Na: 25 H₂O. MFI samples with the different Al contents were synthesized with the synthesis gels of 1 Si: x Al: 0.1 Na: 0.25 tetrapropylammonium hydroxide (TPA): 40 H₂O (x = 0.02 or 0.04). MFI zeolites with the different Al distributions were synthesized by using the TPA without Na cations or the use of pore-filling agent (2,2-dimethyl-1,3-propanediol) instead of TPA [7,8]. Zeolite Beta (CP814E) was obtained from Zeolyst International. The prepared samples were characterized by XRD, ICP-AES, SEM, TG-DTA, ²⁷Al MAS NMR, NH₃-TPD and FTIR.

Catalytic reactions were performed by using a fixed-bed reactor. After pretreatment at 600 ºC for 1 h under the air flow, the temperature was decreased to 350 ºC, and then cracking reactions was started when n-hexane (nC6) or 3-methylpentane (3MP) were introduced into the reactor.

3. Results and discussion

Figure 1 shows the first-order plots of n-hexane (nC6) and 3-methylpentane (3MP) over MFI-, TUN-, MSE-, and *BEA-type zeolites, thus the slope of points represents the rate constants of nC6 and 3MP over these zeolites. The CI value of MFI was the largest among the studied samples. This could be due to the largest confinement effect caused by the smallest 10-ring channels of the MFI structure on the 3MP and its reaction intermediates. *BEA showed the lowest CI value indicating that the effective pore width of this material is the largest among the studied samples even compared to the MSE which has the largest pore diameter in its 12-ring channel. TUN zeolite consisting of two 10-rings presented an intermediate behavior as compared to MFI and *BEA, probably due to its larger pore sizes than those of 10-rings in the MFI structure. The CI value increased with decreasing Al contents in the MFI structure. Furthermore, the higher CI value of MFI prepared by the pore-filling agent in comparison with the MFI synthesized exclusively using the TPA cations indicates that the difference in the local environment of Al atoms could affect the effective pore width of zeolites with the same zeolite topology [7,8] and it should be carefully considered for the comparison of the CI values. The data here shows that the comparison of the CI value is a good method to compare the effective pore width of multipore zeolites with different pore sizes. Furthermore, the acidic properties such as the number of acid sites and their distribution in the zeolite framework should be carefully considered for the meaningful comparison of the CI value.

Reference

[1] M. Yoshioka et al, ACS Catal., 5 (2015) 4268.

[2] S. Park et al., J. Catal., 319 (2014) 265.

[3] S. Inagaki et al., 46 (2010) 2662.

[4] J.M. Serra et al., J. Catal., 227 (2004) 459.

[5] W.O. Haagr et al., Faraday Discuss. Chem. Soc., 72 (1981) 317.

[6] S. Hong et al., J. Am. Chem. Soc. 129 (2007) 10870.

[7] T. Biligetu et al., J. Catal. 353 (2017) 1.

[8] S. Park et al., Catal. Today 303 (2018) 64.