High-surface-area Magnesium Fluoride: Preparation by Template Method and Catalytic Activity for the Dehydrofluorination of HFC-152a

doi:10.15541/jim20180074

Abstract

Abstract:

Magnesium-based solid acid catalysts exhibit satisfactory catalytic performance for the preparation of fluorocarbons. In the present study, magnesium fluoride with high surface area was obtained by template method. The influences of SiO₂ template doping amount on phase structure, porosity, and surface acidity of MgF₂ were investigated. Physical and chemical properties of the as-prepared MgF₂ were characterized by N₂ physical adsorption-desorption iso-therms, X-ray powder diffraction (XRD), NH₃-temperature programmed desorption (NH₃-TPD), transmission electron microscope (TEM), and X-ray photoelectron spectroscope (XPS). The catalytic performance was evaluated for the dehydrofluorination of 1,1-difluoroethane (HFC-152a, CH₃CHF₂) to vinyl fluoride (VF, CH₂=CHF). Results show that the doping amount of SiO₂template plays a major role in the specific surface area, crystal size and acidity of MgF₂. With the presence of 14 mol% SiO₂ template in the Mg(CH₃COO)₂ and NH₄F solution, the derived MgF₂ possesses a specific surface area as high as 304 m²/g, which is almost 2.5 times higher than that of MgF₂ prepared without SiO₂ template. In addition, with suitable doping amounts of SiO₂ template, smaller crystal sizes and under-coordinated sites (five-fold and four-fold coordination Mg species) are derived. Consequently, with enhanced amounts of under-coordinated sites, acidic sites (Lewis acid) of MgF₂ are improved significantly. As the dehydrofluorination of 1,1-difluoroethane catalyzed by Lewis acid, as a result, the conversion of 1,1-difluoroethane is increased dramatically. In summary, it is confirmed that template method with SiO₂ as the template is an effective route for the fabrication of MgF₂ catalyst.

Key words: template method, magnesium fluoride, solid acid catalyst, dehydrofluorination of HFC-152a

CLC Number:

O643.36

DING Shan-Shan, CHEN Xin-Xin, LI Yu-Zhen, HAN Wen-Feng, LV De-Yi, LI Ying, TANG Hao-Dong. High-surface-area Magnesium Fluoride: Preparation by Template Method and Catalytic Activity for the Dehydrofluorination of HFC-152a[J]. Journal of Inorganic Materials, 2018, 33(11): 1186-1192.

Figures/Tables 12

Table 1 Crystal structures and pore structures of x%SP-MgF2

Samples	Surface area /(m²•g^-1)	Pore volume /(cm³•g^-1)	Pore size/nm	Crystal size/nm
SP-15	111	0.31	34	-
MgF₂	120	0.16	10.0	7.6
9%SP-MgF₂	261	0.17	3.4	6.4
14%SP-MgF₂	304	0.22	3.6	5.8
20%SP-MgF₂	295	0.22	3.6	6.2
50%SP-MgF₂	296	0.23	3.0	7.6

Fig. 1 N2 physical sorption isotherms (a) and the pore-size distribution (b) of x%SP-MgF2

Fig. 2 XRD patterns of x%SP-MgF2

Fig. 3 TEM images of MgF2 and x%SP-MgF2 ^Dotted spheres emphasize the formation of pores following the removal of SP-15 sphere

Fig. 4 NH3-TPD curves of x%SP-MgF2

Table 2 Acid intensity and acid amount of x%SP-MgF2

Samples	Total acid	Medium acid	Strong acid
MgF₂	1.00	0	1.00
9%SP-MgF₂	5.53	3.23	2.30
14%SP-MgF₂	8.67	4.38	4.29
20%SP-MgF₂	5.58	3.32	2.26
50%SP-MgF₂	3.41	2.30	1.11

Table 3 Content of saturated-coordination Mg (SCMg), F(SCF) and under-coordinated Mg (UCMg) and F(UCF) of x%SP-MgF2

Samples	SCMg/%	UCMg/%	SCF/%	UCF/%
MgF₂	81.5	18.5	79.3	20.7
9%SP-MgF₂	53.1	46.9	51.1	48.9
14%SP-MgF₂	30.6	69.4	31.9	68.1
20%SP-MgF₂	55.5	44.5	56.5	43.5
50%SP-MgF₂	66.6	33.4	69.6	30.4

Fig. 5 Mg1s and F1s XPS spectra of x%SP-MgF2

Fig. 6 Activity of x%SP-MgF2 in the dehydrofluorination of HFC-152a at 300℃

Fig. 7 The correlation between catalytic performance, acid amount of MgF2 and x% SP-15

Fig. 8 TG curves and macro appearance of the fresh and used 14%SP-MgF2

Fig. 9 Illustration of the synthetic process of x%SP-MgF2 using SP-15 as template

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