Article pubs.acs.org/Organometallics
Selective Ethylene Oligomerization with Chromium-Based Metal− Organic Framework MIL-100 Evacuated under Different Temperatures Suyan Liu,†,‡ Ying Zhang,*,†,§ Yang Han,§ Guangliang Feng,§ Fei Gao,§ Hui Wang,§ and Ping Qiu§ †
The State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, Hebei, P. R. China § Department of Materials Science and Engineering, China University of Petroleum, Beijing 102249, P. R. China ‡
S Supporting Information *
ABSTRACT: MIL-100(Cr) was synthesized and evacuated under different temperatures to generate a series of heterogeneous catalysts for ethylene oligomerization. These catalysts showed moderate catalytic activities for ethylene oligomerization but high selectivities to low carbon olefins C6, C8, and C10. Moreover, the oligomer distribution was different depending on the evacuation temperature. The XPS results showed the reduction of some CrIII active sites in the MIL-100(Cr) structure to CrII active sites, which made the catalysts show polymerization activities. The MIL-100(Cr)250 catalyst evacuated at 250 °C exhibited the highest oligomerization and polymerization activities up to 9.27 × 105 g/(molCr·h) and 0.99 × 105 g/(molCr·h) respectively. The oligomerization selectivity to low carbon olefins C6, C8, and C10 was about 99%. The byproduct polymer from MIL-100(Cr)-250 belonged to linear polyethylene with ultrahigh molecular weight and broad molecular weight distributions. This work demonstrated that MOFs containing coordinatively unsaturated metal sites might be a promising selective catalyst for ethylene slurry oligomerization.
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agent such as AlEt3.6 Recently, the famous BP trimerization system was based upon PNP ligands in combination with a soluble chromium source and MAO. Modification of this ligand system by researchers from Sasol led to the discovery of selective tetramerization of ethylene to α-octene.3 More recently, Duchateau et al. reported another selective ethylene tetramerization chromium-based catalyst employing ligands with a PN(CH2)3NP backbone structure.7 Compared with selective ethylene trimerization, selective ethylene tetramerization was rare. Moreover, most of the selective ethylene oligomerization catalysts are homogeneous. Heterogeneous catalysis is an environmentally friendly alternative to the traditional homogeneous ones due to easy separation of catalysts from products. The universal approach to the preparation of heterogeneous catalysts is to immobilize molecular transition metal catalysts on conventional porous solids, such as MCM-418 and Y.9 Recently, there were a few publications reporting Metal−Organic Frameworks (MOFs), a new member of porous crystalline materials family, as support for the preparation of heterogeneous catalysts for ethylene oligomerization and polymerization. For example, Canivet and co-workers10 reported a one-pot postfunctionalization method
INTRODUCTION Ethylene oligomerization has been dominated for ages by nonselective ones which yield a wide spectrum of linear αolefins often following a Schulz−Flory distribution or a Poisson distribution.1 The light fraction of linear α-olefins (LAOs), particularly C4, C6, and C8, are employed as comonomers for the production of linear low-density polyethylene (LLDPE) copolymers.2 Continually increasing demand for these shortchain oligomers has led to great interest in selective ethylene oligomerization technologies.3 Selective ethylene oligomerization systems have been reported for chromium, titanium, zirconium, tantalum, and hafnium homogeneous catalysts.4 A modified Ziegler catalyst system Ti(OBu)4/AlEt3 catalyzed the selective dimerization of ethylene to α-butene with high efficiency and was exploited to develop the α-butol process. Among all the selective ethylene trimerization and tetramerization catalysts, the chromium-based catalysts are considered as being the most numerous, active, and selective.5 The early chromium catalytic systems relied on ligands such as pyrrolides and carboxylates, while the more recent ones were based on diverse multidentate ligands displaying a donor combination such as PP, PNP, NPN, SNS, NNN, NNS, and NNO. For example, the Phillips trimerization catalyst as the first commercialized ethylene trimerization system was composed of a chromium source, 2,5-dimethylpyrrole, and an alkylating © XXXX American Chemical Society
Received: November 2, 2016
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DOI: 10.1021/acs.organomet.6b00834 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics for anchoring a molecular nickel complex into a mesoporous metal−organic framework (Ni@(Fe)MIL-101) to generate a very active catalyst for the liquid-phase ethylene dimerization to selectively form α-butene. Liu and co-workers11 generated a Crbased heterogeneous catalyst through postsynthetically modified IRMOF-3 for ethylene polymerization. It is notable that the preparations of these MOF-supported catalysts involved postsynthetic modification. As is well-known, some MOFs possess coordinatively unsaturated metal sites (CUS), which are definitely and firmly distributed due to the crystalline nature of MOFs and can conveniently act as active catalytic sites. However, there are rare reports on MOFs with CUS used for catalysts in ethylene oligomerization. As far as we know, Mlinar and co-workers12 reported two Ni2+-containing metal−organic frameworks, which were active for the oligomerization of propene in the gas phase and exhibited activities comparable to Ni2+-exchanged aluminosilicates but maintained high selectivities for linear oligomers. Taking into account the dominant role of Cr-based catalysts in the selective ethylene oligomerization and polymerization,13−15 we are interested in the ethylene conversion with Cr-MOF with CUS. Férey et al. reported a Cr-MOF named MIL-100(Cr) with a zeolite-type architecture built up from benzene-1,3,5-tricarboxylate and trimeric chromium octahedral cluster.16 MIL-100(Cr) has excellent thermal stability in air up to 275 °C and possesses two types of mesoporous cages (25 and 29 Å), accessible through microporous windows of 6.5 Å. There are two water molecules bound to the chromium sites per trimer of chromium octahedral, which can be evacuated upon heating in vacuum generating coordinatively unsaturated sites (CUS). Herein MIL-100(Cr) was synthesized according to our previous work17 and evacuated under different temperatures to generate a series Cr-based heterogeneous catalysts MIL100(Cr)-t, wherein t denoted evacuation Celsius temperature, t = 150, 200, 275, 300, and 350. Their ethylene oligomerization catalytic performances were investigated, and the polymer byproducts were also characterized. Depending on the evacuation temperatures, they showed different oligomerization and polymerization performances. Their catalytic activities for ethylene oligomerization were moderate, but selectivities to low carbon olefins C6, C8, and C10 were high. This work demonstrated that MOFs containing coordinatively unsaturated metal sites might be a promising selective catalyst for ethylene slurry oligomerization.
Figure 1. PXRD patterns of (a) MIL-100(Cr), (b) MIL-100(Cr)-150, (c) MIL-100(Cr)-200, (d) MIL-100(Cr)-250, (e) MIL-100(Cr)-300, and (f) MIL-100(Cr)-350.
Figure 2. Nitrogen adsorption−desorption isotherms of (a) MIL150(Cr), (b) MIL-100(Cr)-200, (c) MIL-100(Cr)-250, (d) MIL100(Cr)-300, and (e) MIL-100(Cr)-350.
The pore structure parameters of the MIL-100(Cr)-t catalysts calculated from nitrogen adsorption isotherms were listed in Table 1. With a gradually increasing evacuation temperature to 250 °C, the specific surface area and pore volume increased progressively. The catalyst MIL-100(Cr)-250 displayed the largest Langmuir surface area of 3323 m2·g−1,
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RESULTS AND DISCUSSION Characterization of the MIL-100(Cr)-t Catalysts. The XRD patterns of the as-synthesized MIL-100(Cr) and MIL100(Cr)-t catalysts (Figure 1) were coincident with those reported in the literature.16 With the evacuation temperature increasing from 150 to 250 °C, the diffraction peaks gradually became strong. Further increasing the evacuation temperature to 300 and 350 °C led to the decrease of the diffraction peaks and even disappearance of the diffraction peak at 2θ = 2.05°, indicating an obvious decrease in crystal order. The N2 adsorption−desorption isotherms on the MIL100(Cr)-t catalysts shown in Figure 2 were between type I and IV with a slight secondary uptake, which indicated the presence of both micro- and mesopores. No hysteresis loops were observed. These isotherms were consistent with that on the fully evacuated MIL-100(Cr) sample reported in the literature.16
Table 1. Specific Surface Area and Pore Volume of the MIL100(Cr)-t Catalyst
B
catalyst
SBET (m2·g−1)
SLangmuir (m2·g−1)
pore volume (cm3·g−1)
MIL-100(Cr)150 MIL-100(Cr)200 MIL-100(Cr)250 MIL-100(Cr)300 MIL-100(Cr)350
1905
2686
1.04
1954
2754
1.08
2348
3323
1.29
1930
2725
1.07
1917
2721
1.10
DOI: 10.1021/acs.organomet.6b00834 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Figure 3. XPS results of (a) MIL-100(Cr)-150, (b) MIL-100(Cr)-250, and (c) MIL-100(Cr)-350.
BET surface area of 2348 m2·g−1, and pore volume of 1.29 cm3· g−1. These values are much higher than the BET surface area of 1900 m2·g−1 and pore volume of 1.10 cm3·g−1 reported for the MIL-100(Cr) material,18 suggesting the successful synthesis and complete evacuation of pore channels of MIL-100(Cr) in this work. When the evacuation temperature was higher than 250 °C, the specific surface area and pore volume began to decrease. However, the MIL-100(Cr)-350 catalyst still exhibited the Langmuir surface area of 2721 m2·g−1, BET surface area of 1917 m2·g−1, and pore volume of 1.10 cm3·g−1, indicating the pore structure was still retained. The change trends of the specific surface area and pore volume depending on evacuating temperatures were the same as those of the XRD intensities. For the as-synthesized MIL-100(Cr), there are free water molecules condensed in cages, bound water molecules, and F−/ OH− counteranions coordinated to the chromium atoms, which can be removed through heating in vacuum. According to the literature,19 the free water molecules can be evacuated quite easily at room temperature under vacuum and the total removal of the bound water molecules requires both vacuum and heating (
