silica catalyst

Accepted Manuscript Title: High-resolution 29 Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst Authors: Ruihua Cheng, Xuee Liu, Yuwei Fang, Minoru Terano, Boping Liu PII: DOI: Reference:

S0926-860X(17)30201-6 http://dx.doi.org/doi:10.1016/j.apcata.2017.05.011 APCATA 16233

To appear in:

Applied Catalysis A: General

Received date: Revised date: Accepted date:

15-1-2017 8-5-2017 12-5-2017

Please cite this article as: Ruihua Cheng, Xuee Liu, Yuwei Fang, Minoru Terano, Boping Liu, High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst, Applied Catalysis A, Generalhttp://dx.doi.org/10.1016/j.apcata.2017.05.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Article submitted to Applied Catalysis A: General

High-resolution

29

Si CP/MAS solid state NMR

spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst Ruihua Cheng, a Xuee Liu,a Yuwei Fang,b Minoru Terano, b Boping Liu*,a a

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Meilong Road 130, Shanghai 200237, China b

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-

1 Asahidai, Nomi, Ishikawa 923-1292, Japan

Graphical Abstract

1

Highlights 

The type and amount of silanols on Phillips Cr/silica catalyst were identified by the highresolution solid state NMR.



The amount of residual silanol groups of the catalysts treated at different temperatures was compared with the silica gel counterparts.



The presence of the grafted chromate species obstructed further removal of the residual single silanols.



The role of silanols on the formation of surface chromate species on the well-defined models was theoretically studied.

ABSTRACT Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and

29

Si CP/MAS NMR allowed the

identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800 °C. For the catalysts treated at temperatures lower than 300 °C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 °C. The geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. Further increasing the calcination temperatures from 300 to 800 °C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation

of

surface

chromate

species

on

the

well-defined

polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most 2

thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time. Keywords: Phillips CrOx/SiO2 catalyst; Geminal silanols; High-resolution 1H MAS NMR;

High-resolution

29

Si

CP/MAS

NMR;

Theoretical

calculations;

Polyoligomericsilsesquioxane 1. Introduction Phillips Cr/silica catalyst is one of the most commercial catalysts for ethylene polymerization and accounts for about half of current production of high-density polyethylene (HDPE) due to its unique properties. [1-5] The molecular weight distribution (MWD) of the product is typical polydispersity index larger than 10, while that of Ziegler-Natta Ti-based catalyst is only 4. The broad MWD accompanied with the small amount of long chain branches in the main chain makes the PE by Phillips catalyst with excellent performances for extrusion blowing to produce the pipe and hollow containers et al.. The broad MWD character mainly derivates from the heterogeneity of Cr active site on the amorphous silica gel supported Phillips catalyst. Generally, the catalyst is prepared by the impregnation of an aqueous solution of chromium (VI) species [e.g. CrO3], or various Cr(III) salts usually the chromate acetate, on the silica gel followed by drying and calcination (300-900 ºC) in dried air resulting in the formation of complex surface chromate species in the forms of hexavalent state, undesirable aggregated Cr2O3, and some degradative reaction of chromate at the surface with lower valence states. [2, 6-9] Further, the Cr(VI) species might be reduced to the active Cr(II) species to initiate the ethylene polymerization.[5] Up to now, the molecular structure of the anchored Cr species on Phillips catalyst has been a strong point of discussion with various indirect and direct approaches have being carried out in this regard. Generally, the molecular structures of the chromate 3

species are reported in the forms of monochromate, dichromate, or even polychromate. Rebenstorf and Larsson [10] suggested that on the IR spectra of CO adsorption on Phillips catalyst reduced by CO, the decrease of one mol CO per Cr(II) at low temperatures can be interpreted as the replace of four single bonded CO by two bridging CO pro dinuclear chromium (II) surface complex. Weckhuysen et al. [11] found the monochromate was dominant on industrial pyrogenic silica supported Cr/silica catalyst by Raman and diffuse reflectance spectroscopy (DRS), while amounts of polychromate also presented at higher Cr loadings. Combined the FTIR, Raman, and UV-vis spectroscopic results, monochromate species were identified anchored on the silica aerogel monolite supported Cr/silica (0.5 wt% Cr) catalyst with CrO3 as raw material. For Phillips model catalysts, volatile CrO2Cl2 was reported to react with various oxide surfaces to produce highly dispersed monochromate sites. [1, 2, 12, 13] Thüne and his coworkers [14] found the silica-bound monochromates formed on a flat CrOx/silica/Si(100) model Phillips catalyst by extended X-ray absorption fine structure (EXAFS). Hogan [15] suggested that the monochromate favorably formed on the surface of Cr/silica catalyst after high-temperature activation in O2. On the contrary, Zecchina et al. [16] observed that the CrO3 was bound to the silica surface mainly as dichromate in the whole 0-5 wt% range of Cr loading, while monochromate represented only a minor component after calcination at 600-750 C. By the measurement of molar ratios of Δ[OH]/[Cr], McDaniel [7] rationalized the above conflicting evidences and suggested that the initially bonding was monochromate at 200 C (Δ[OH]/[Cr]=2) and the dichromate became dominant at 500 C (Δ[OH]/[Cr]=1), while polychromates might be formed above 800 C (Δ[OH]/[Cr]<1). Based on diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and DRS results, Paukshtiset al. [17] proved that the reactions of CrO3 vapor with the silica pre-dehydroxylated at 250, 400, and 800 C, yielding monochromates, mono- and di-chromates, poly-chromates, respectively, which agreed well with those reported by McDaniel. [7] Controlling the mono- and dichromate structures on silica surface by raw materials of (Cr(η3-allyl)3 and Cr2(η3allyl)4, Terano and coworkers [18] found that the dichromate model catalyst produced 4

more short branches as the industrial Phillips catalyst. In fact, it is difficult to differentiate the three types of chromates, because it is almost impossible to get the precise contents for the industrial Phillips catalyst calcined between 600 - 800 C. That is to say, the conflicting experimental evidence and interpretations about the chromate species are obtained owing to the various Cr loadings, treatment methods, and preparation conditions, thus no unifying picture could be given. [1, 2, 11] The difficulty in a clear understanding of the Phillips catalyst mainly comes from the complexity of the amorphous silica gel support, owing to the high surface area and considerable micro- and meso-pores facilitating the anchoring the Cr species through the esterification reaction with surface silanol groups, as well as the quite low fraction among all sites. [1, 19-21] The surface behavior is dominated by the chemistry of the terminal silanol groups. Various groups exist on the silica gel surface, including siloxane bridges (≡SiOSi≡), geminal silanols (two silanol groups attached to the same silicon atom), and single silanol groups (≡SiOH), which are either isolated (no hydrogen bonds possible) or vicinal silanols (silanols hydrogen bonded each other). Their concentrations and types greatly depend on the temperature and history of the thermal pretreatments of the catalyst during preparation. [1] In turn, the silica support is not independent and can function as both a ligand and an activator. The silanol (single or geminal) may exhibit various reactivities towards chromium species, and the second metal active sites to form different active site precursors, yielding polymer chains with different microstructures. [22, 23] Meanwhile, the residual hydroxyl group on the catalyst surface might be a necessary source forming active site by providing the first proton during the initiation stage in polymerization, [24, 25] as well as the strong influence on polymerization activity.[2, 26] Unfortunately, the roles of various silanols in the process of chromium oxide grafting on silica support and the catalyst–support interaction are poorly understood. The amount of silanol concentration obtained in McDaniel’s work [7] only represented the total reacted surface silanol groups on the Phillips catalyst without specification of the silanol types limited by the CH3MgI method. The role of various silanol groups only 5

considered on the model Phillips catalyst by the direct reaction between CrO2Cl2 and hydroxyl groups of the silica group to decrease the complexity of chromate species on silica. By IR spectroscopy, Nishimura and Thomas [27] found the vapor of CrO2Cl2 selectively reacted with the vicinal OH groups on silica surface. Combined IR, XANES, EXAFS spectroscopy methods, and DFT calculation on the interaction of CrO2Cl2 with silica model, Scott and coworkers [12] suggested that the CrO2 fragment grafted to nonvicinal hydroxyls at lower temperatures (200 and 450 ºC), and the chromasiloxane rings created from vicinal hydroxyls located on adjacent silicon atoms for the silica dehydrated at higher temperature (800 ºC). But this model was far from the real Phillips catalyst due to the formation of CrO2 through the vapor of CrO2Cl2 on the partially dehydrated silica gel. Indeed, the silanols on Phillips catalyst requires direct information from other techniques. High-resolution solid state 29Si NMR spectroscopy, using magic angle spinning (MAS), cross polarization (CP), and high power 1H decoupling, is a very powerful way to distinguish the geminal silanols and single silanols on heterogeneous silica gel supported catalysts. It could provide a promising and valuable approach for the characterization of the silica surface (e.g., silanol types, concentrations of geminal and single silanol sites) and the change during thermal treatment. [28, 29] As early as in 1983, Sindorf and Maciel [28] found that the relative apparent population of geminal sites decreased till 300 ºC, followed by a sharp increase to maximum at 650 ºC, and then decreased again at higher dehydration temperatures by 39.75 MHz 29Si CP/MAS NMR. However, the results were doubted owing to the relatively poor resolution of the solid state NMR technology at that time. Schnellbach et al. [30] utilized the solid state NMR to probe Union Carbide Cp2Cr/silica catalyst in 1H NMR spectra, and inferred the presence of one mononuclear and two dinuclear surface-attached Cr complexes. Focused on the close relationship between 29Si CP/MAS NMR longitudinal relaxation times and the adjacent chromium nuclei, Chudek et al. [31, 32] studied the oxidation states of the paramagnetic chromium species on Phillips catalyst without considering the types of silanol sites. The precise characterization of the geminal silanols and single 6

silanols depended greatly on the resolution of the solid state NMR instruments. A significant error would be caused due to the relatively low surface concentration for geminal silanols. Fortunately, solid state NMR technology with high resolution has been advanced so much in the last decade making it possible to elucidate this important point. Recently, some theoretical approaches based on molecular modeling for chromium species supported on various well-defined systems mimicing the complex silica surface were reported efficiently in achieving a deeper mechanistic understanding of the Phillips-type catalyst on a molecular and atomic level. Generally, some small and simple clusters containing Si, O, and Cr were involved in these model systems. Espelid and Børve [33, 34] theoretically evaluated the monomeric and dimeric chromium species on silica cluster models as viable starting structures for ethylene polymerization. Guesmi and Tielens [35] reported an amorphous silica surface slab containing 120 atoms (Si27O54·13H2O) represented the amorphous character of the hydrated and dehydrated silica surface involving different silanol type, and a higher stability of mono- and di-oxo chromium species was confirmed in comparison with chromium-hydroxyl species. The strategies emerge the combination of the theoretical and experimental studies. We [36] developed a silica surface model containing 37 Si atoms through supporting of a six-membered chromasiloxane ring onto a silica surface cutting from the β-cristobalite crystal structure to resemble the amorphous silica. The results were in good agreement with the experimental IR and EXAFS spectra. The combination of the solid state NMR and DFT studies were popular in the structural characterization of the catalytically significant sites on solid catalyst surfaces. [37] In this work, high-resolution solid state NMR were carried out to discriminate the geminal and single (isolated and vicinal) silanols reactivity in the esterification of Cr(IV) active sites on Phillips catalysts precursors, giving rise to relations between geminal silanols and Cr species. Moreover, based on the proposed structures, a fully hydroxylated polyoligomericsilsesquioxane (POSS) containing both geminal and single silanols were built to facilitate the CrO3 grafting on an amorphous silica surface. 7

Furthermore, the theoretical results concerning the role of single and geminal silanols based on these model sites provided much deeper understanding for the real catalyst system. 2. Experimental and computational 2.1. Chemicals Detailed specifications of each raw material can be found in our previous report. [38] Nitrogen and pure air were purchased from Uno Sanso Co.. Molecular sieves 4A and molecular sieves 13X purchased from Wako Pure Chemical Industries, Ltd. were used as moisture scavenger for gas purification. Chromium (III) acetate was also purchased from Wako Pure Chemical Industries, Ltd. Q-5 reactant catalyst (13 wt% of copper (II) oxide on alumina, Aldrich) was used as oxygen scavenger for gas purification. The silica gel ES70X (Matthew Frosdyck) donated from Asahi Kasei Co., was used as the catalyst support. The as-prepared Phillips catalyst ES370X with 1.0 wt% Cr loading(defined as PCP120), was also obtained from Asahi Kasei Co. by impregnation of aqueous solution of chromium (III) acetate onto the silica gel ES70X, followed by drying in air around 120 ºC for a certain period of time. More specific information concerning the preparation conditions of the commercial Phillips catalyst precursors PCP120 could not be given due to confidential requirements. 2.2. Catalyst preparation The silica gel support and PCP120 catalyst calcined at various temperatures were performed using a spouted fluidized-bed quartz reactor under a procedure similar to our previous report. [39] The quartz reactor was set vertically into an electronic heater equipped with a temperature-program-controller. About 15 g as-prepared catalyst PCP120 was added into the reactor, and the temperature was ramped from room temperature to 200, 300, 400, 600, 800 ºC and hold for 6 h in air with a flow rate of 200 mL min-1, respectively. Then the samples were cooled in N2 flow. The obtained five samples were named as PC200, PC300, PC400, PC600, and PC800, respectively. All the gases were purified further before used. For comparison, the ES70X was treated 8

similarly at 200, 300, 400, 600, and 800 ºC for 6 h to obtain SG200, SG300, SG400, SG600, and SG800 samples, respectively, which were detailed listed in Table 1. After preparation, the catalyst sample was sealed into several large glass tubes within N2 for storage. Finally, each catalyst sample was further distributed and sealed in small glass ampoule bottles under N2 atmosphere before use. The amount of catalyst in each small glass ampoule bottle could be precisely weighed about 100 mg. 2.3. Solid state NMR Silica gel and catalyst samples were characterized by a Varian UNITY-400 spectrometer at room temperature with a Varian Room Temperature/Cross Polarization Magic Angle Spin (RT/CP MAS) probe. Each sample (ca. 80 mg) was put and pressed tightly into a 7-mm zirconia rotor under N2 atmosphere in a glove box, which had been purged by N2 overnight before sample setting. The 1H MAS and 29Si CP/MAS NMR spectra were obtained operating at 400.47 and 79.56 MHz, and recorded for samples rotating at ca. 3 kHz. 1H MAS NMR spectrum was obtained with a 30° pulse of 3 μs, a relaxation delay of 1 s and total acquisition time of 10 min.

29

Si CP/MAS NMR

spectrum was obtained with a contact time of 500 μs, a 90° pulse of 3 μs, a relaxation delay of 2.5 s and a total acquisition time of 10 h. 1H and

29

Si NMR spectra of the

samples were referenced relative to external tetramethylsilane (TMS). 2.4. Computational detail Amorphous silica gel was simulated by a POSS cluster model. The geometries of various models were optimized by DFT method (RB3LYP, basis set 6-31G**, multiplicity: singlet for Cr(VI) sites and triplet for Cr(II) sites) using SPARTAN’04 Windows developed by Wavefunction, Inc. [40, 41] Each Cr atom in the equilibrium structures of molecular models of monochromate Cr(VI) ester (named as MCr-X) and dichromate Cr(VI) ester (named as DCr-X) was directly linked to the silsesquioxane in the optimized ground state. Scheme 2 presents the monochromate (M) or dichromate (D) fragments attached on the POSS spots with geminal and single (vicinal or isolated)

9

silanols. The calculated enthalpy for the formation of Cr(VI) ester on the POSS model is defined as: ΔH=ΔHCal, Cr-ester+2 ΔHCal, H2O -ΔHCal, POSS – n ΔHCal, CrO3 Where ΔHCal, Cr-ester, ΔHCal, POSS, ΔHCal, CrO3, ΔHCal, H2O, are the total enthalpies of Cr(VI) ester model, POSS model, CrO3, and H2O, respectively. n, which is denoted the number of the chromium number involved in the reaction, is dependent on the formation of monochromate (n=1) or dichromate (n=2) species. 3. Results and discussion 3.1. 29Si CP/MAS NMR High-resolution solid state

29

Si NMR, which was obtained by magic angle

spinning (MAS), cross polarization (CP), and high power 1H decoupling offers an effective method for the characterization of silica gel surfaces. In

29

Si MAS NMR

without cross-polarization (CP) (not shown in this paper), all spectra showed only one peak assigned to the siloxane bridge near -111 ppm with similar intensity. The information was very limited due to the low content of single (isolated or vicinal) and geminal silanols compared with that of the siloxane bridges. Therefore, crosspolarization technique was used to enhance the signal of these silanol groups.

29

Si

CP/MAS NMR spectra can distinguish single (either isolated or vicinal) silanols and geminal silanols from siloxane bridges. Fig. 1 presents 29Si CP/MAS NMR signal of the silica gels and Cr/silica catalysts calcined at various temperatures. The spectra of silica gels calcined below 300 ºC were broad and dominated by intense line at -100 ppm along with two shoulders at -90 and -111 ppm. And the resonance at -90, -100, and 111 ppm were assigned to the geminal silanols, single (either isolated or vicinal) silanols, and siloxane bridge, respectively, which were analogous to the literature. [28, 29] The relative peak intensities cannot intrinsically be considered a quantitative measure of their relative populations, as they were detected by the particulars of the CP processes of each silicon resonance, which depended upon the proximity of protons and the contact time used in experiment. Considering the same measurement conditions for 10

all 29Si CP/MAS NMR spectra, the effect of contact time could be ignored in this work. Thus, the peak intensity only depended on the amount of the proximal protons in samples. Accordingly, the intensity of the peak assigned to siloxane decreased with the decrease of the amount of protons under different calcination temperatures. As mentioned previously, the thermal activation of Phillips catalyst was the dispersion of chromate species on silica surface and reaction with silanol groups. The esterification reaction of the silanol group and the chromate presented in equilibrium during this reaction. At the high purified N2 flow, the equilibrium can be shifted towards the forwards direction of the formation of chromate by removing the water as soon as it was formed. Thus, the intensity change of siloxane bridge groups may be ignored, and only geminal and single silanols were considered in this paper. In Fig. 1A, the intensity of the main peak for silica gel assigned to single silanol at -100 ppm decreased gradually with the increase of calcination temperatures due to the dehydration of adsorbed water and silanol group, and only a slight peak remained after calcined at 800 C. The peak assigned to geminal silanols showed similar decreasing tendency, and cannot be detected above 300 ºC in this experimental condition. This was consistent with the FTIR results that a very low concentration of geminal hydroxyls existed. [42] In this work, all 79.56 MHz. In

29

29

Si NMR spectra were obtained at

Si CP/MAS NMR spectra, a weaker peak intensity, broader peak

width, as well as the stronger noise with dehydration of silanols were obviously at lowresolution NMR spectra. For the PCP120 (Fig. 1B (b)), an obvious decline of the peak intensities for -90 and -100 ppm were observed compared with the silica counterparts (Fig. 1A(b)), reflecting attenuation of the silanols through the incorporation of Cr species on silica surface. And a large amount of geminal and single silanols were removed by the dehydration above 300 ºC. Relative to the intensity of the siloxane bridge at -111 pm, it seemed that the isolated silanol peaks at -100 ppm for the PC 600 and PC 800 catalysts were slightly stronger than those for the silica SG600 and SG800 samples after high temperatures treatment.

11

To conveniently study the change of silanols on the silica after Cr species incorporation, the amount of geminal and single silanols were compared. Figs 2 and 3 show the plots of geminal and single silanols in all silica gel and catalysts samples, respectively. Here, the raw silica was regarded as a basic point to compare with the other calcined samples. In comparison of geminal silanols for silica gel and calcined catalysts samples in Fig. 2, the peak intensities in silica gel (SG120) and catalysts (PCP120) were apparently lower than that of raw silica. However, this result contracts with the statement in literature [43] that when the treated temperatures lower than 175 °C, only the physisorbed water on silica was desorbed, whereas the silanol groups almost had no obvious change. Contrarily, Bermudez [44] showed by NMR technique that the treatment of drying at 110 °C not only removed physisorbed water on silica, but also dehydroxylated some silanol groups and eliminated some internal water. It was also found that the geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. For the samples treated at temperatures lower than 300 °C, the amount of residual geminal silanols on catalysts were much lower than that of the silica gel counterparts. Two probabilities may be proposed: i) the existence and dispersion of chromium species promoted the removal of geminal silanols; ii) the chromium species directly reacted with geminal silanols even at 120 °C. Certainly, this direct reaction also includes two possible ways: one occurred within one geminal pair, and the other was reacted between one geminal and another adjacent silanol as either geminal or single silanols. Thus, the structures were shown in Scheme 3 probably exist. Two cases were all considered in a following theoretical investigation. Fig. 3 shows the comparison of single silanols between silica gels and catalysts in dependence of calcination temperatures. Similar with the cases of geminal silanols, the amount of single silanols on silica gel was still much smaller than that on catalysts calcined at temperatures below 300 °C. It was likely that the dehydration of single silanols or the reaction between chromium species and single silanols occurred even at 120 °C owing to the presence of chromium species. However, given the samples calcined at temperatures higher than 300 °C, the amount of residual single silanols on 12

catalysts were more than those on the silica gel counterparts. This interesting result indicated that the existence of chromate species probably hindered the removal of residue single silanols at high temperatures. As a proposed structure in Scheme 4, the dehydroxylation of a pair of nearest silanols becomes much more difficult due to the spatial hindrance of chromate species between these two remaining Si–OH bond. 3.2. 1H MAS NMR The solid state 1H MAS NMR spectra in Fig. 4 revealed the samples of the silica gel and catalyst calcined at different temperatures, respectively. There were three peaks in each spectrum: one main peak centered at 1.7~1.8 ppm was assigned to the proton species, and the other two near 12 and -10 ppm were the satellite peaks of the main peak whose intensity decreased with the decrease of main peak. These proton peaks are analogous to with Haukka’s work in literature, [45] in which the main peak can be divided into two peaks; the broad peak was attributed to hydrogen-bonded silanol and the sharp one to the isolated one. With the increase of calcination temperatures, the intensity of the broad peak decreased accompanied with a narrowering of the main sharp peak. 1H MAS NMR spectra of PC600 and SG600 showed that the hydrogenbonded silanols are almost completely removed and all the silanols are essentially isolated. From Fig. 4, it was found that the chemical shift of proton in silanol of calcined catalysts of PC200, PC300, PC400, and PC600, were about 0.06 ppm higher than that in the counterparts of the calcined silica gel. This slightly difference may be due to the electron withdrawing effect of the adjacent surface chromium species. Similar spectroscopic evidence for the interaction of silanol groups and low-valence chromium ions was also obtained using a model catalyst prepared from CrO2C12. [27] For raw silica gel, the main peak at 3.90 ppm was due to the strongly hydrogen-bonded water molecules physisorbed in the micropores on silica surface. Similarly, the very broad main peak of PCP120 at 3.29 ppm was also due to the residual physisorbed water in the micropores, which might exist even at higher temperature. Fig. 5 shows the change of chemical shift of proton for the main peak in silica gel and catalyst and their dependence on calcination temperatures. With the increased calcined temperatures, the chemical 13

shift of the proton on silica gel maintained at 1.75 ppm, which was slightly lower than the catalysts at 1.80 ppm suggesting the presence of the chromate species might obstruct the removal of certain silanol groups during calcinations. And the result was consistent with the above 29Si CP/MAS NMR conclusion. In summary, the high-resolution solid state 29Si CP/MAS NMR and 1H MAS NMR showed the amount of the residual geminal and single silanols on the Phillips catalysts were less than on the silica gel counterparts treated at the temperatures below 300 °C. Conversely, given the samples calcined at temperatures higher than 300 °C, the amount of the residual geminal silanols of the silica gel were similar to those of the catalysts samples, while that of the residual single silanols in silica gel were lower than those in the catalysts counterparts. The study focused on the role of the geminal and single silanols in the formation of Cr(VI) ester on the surface of silica gel support seemed worthwhile. 3.3. Computational modeling For Phillips catalyst, silanol groups are directly involved in the location of chromate on silica surface during impregnation, the activation of the hexa-valent Cr into the lower valence state during calcination, and even the performance during ethylene polymerization. [1] During the preparation process, increasing the calcination temperatures for Cr/silica catalysts is accompanied by the dehydroxylation of silica gel and the anchorage of Cr species on the surface of silica gel by consumption of silanol pairs. The residual silanols after calcination was suggested to play a role in the polymerization process, either as a hypothetical source of the active site by providing the first proton during the initiation stage of polymerization, [24, 25] as well as the strong influence on polymerization activity. [2, 26] Hence, the influence of silanol groups to the nearby Cr active site was crucial. However, the experimental measurements are not available to this effect. The possible location of the Cr(VI) ester involved adjacent silanols in the catalyst preparation process may be explored using the DFT calculation based on simplified POSS models. 14

The POSS with well-defined cubic core structures presented in Scheme 1 is considered to bridge the gap between the realistic silica gel and homogeneous support among many attempts. More importantly, the POSS-based systems can be considered as a close molecular analogy of active heterogeneous catalysts. [46-48] Furthermore, Feher et al. [49, 50] reported that the POSS supported Cr-based catalyst have been successively synthesized from CrO3 and exhibited real activity in polymerizing ethylene. As part of an application of silsesquioxane-based systems as Phillips-type catalyst models, we [41] have theoretical studied the electronic properties and thermodynamic effects of model silsesquioxane-supported catalysts with ligands as silanol and fluorine. Here, a polyoligomericsilsesquioxane (POSS) bearing both single and geminal silanols was modeled to probe the role of geminal silanol in the chromate grafting on silica support. Scheme 1 shows the Cr (VI)-ester on the POSS model with various kinds of geminal and vicinal silanols. The mechanism of Cr species attachment on silica support was simplified as esterification to pairs of surface silanol group to form surface chromate or dichromate species. Scheme 2 illustrates the attachment of monochromate (M) or dichromate (D) fragments on the POSS sites with the interaction of geminal and single (vicinal or isolated) silanols. Focused on the great change of geminal silanols in the early stage of calcination, the structures were mainly considered with chromate species reacted with two adjacent hydroxyls either within a pair of geminal silanols or a geminal with single silanol member, as deriving from Schemes 3 and 4. Thus, given the different possibilities for the arrangement of chromate on the silica surface, six formed Cr-O-Si structures (in Scheme 5) were chosen. The corresponding enthalpy for the formation of Cr (VI) ester on six kinds of silanol pairs are presented in Table 2. The DFT calculation suggested that the thermodynamic priority of Cr anchoring on the silica support strongly depended on the Cr center number and the location. MCr-2, giving rise to the monochromate sites directly grafting to the geminal and adjacent single member silanols pairs to form an 8membered chromasiloxane ring, was the exothermic way with enthalpy of -27.56 kJ 15

mol-1. And this was the most accessible way among the six studied models in this work. The similar model of MCr-1 in a 6-membered chromasiloxane ring presented slightly endothermic energy of 4.67 kJ mol-1. As shown in Scheme 5, the strain in the chromasiloxane ring was decreased, which was beneficial for the stability of chromasiloxane upon Cr location. DCr-2 presented the value of enthalpy of 17.15 kJ mol-1, which was about 45.7 kJ mol-1 higher than the MCr-2 counterparts. And the dichromate DCr-1 showed the enthalpy of 25.90 kJ mol-1. The enthalpy for MCr-3 and DCr-3 were as high as 79.14 and 52.33 kJ mol-1, respectively, suggesting the instability for either the monochromate or dichromate anchoring within a geminal pairs owing to the unfavorable geometrical arrangement. They probably served as the poor sites for the initial anchoring due to the highly strained Si-O-Si bonds. It was found that one geminal silanol together with the adjacent single silanol group was more accessible for supporting Cr species, resulting that the MCr-2, MCr-1, and DCr-2 with superior potential formed. This finding supported the proposal in the previous NMR analyses that the reaction of Cr with geminal silanol group improved the consumption of the geminal silanol at temperatures below 300 C. In addition, the geminal silanol also played an important role in the condensation of the silanol in the silica gel, that one silanol from a geminal pair was facile for the condensation. [51] Based on the calculation, it was also suggested that the anchoring chromium species accompanied the presence of an adjacent silanol on POSS model. Consequently, an additional hindrance to those residual single silanol sites created during the esterification may account for their difficult removal. This is supported by the NMR experimental observation that certain silanol groups were unfavorable to displace even at the temperatures higher than 800 C presumably due to the presence of Cr species nearby. 3.4. Speculation on formation mechanism for real system The use of the aforementioned simple models facilitated relatively expedient initial calculations, which were used in turn for expansion to more realistic models. The Phillips catalyst was previously prepared through the impregnation of silica gel in the aqueous CrO3 solution, which was substituted by lower toxic chromium(III) acetate in 16

recent years. For this case, a plausible mechanism concerning the interaction of the surface silanol groups, especially the geminal and vicinal silanols, with CrO3 during the preparation of Phillips catalyst was proposed and illustrated in Scheme 6. It was drawn on the basis of experimental NMR results on the real Phillips Cr/Silica catalyst system and the theoretical DFT investigation of Cr (VI)-ester grafting onto the POSS model. The interaction of the adjacent silanols pairs and CrO3 led to the formation of chromate species. Generally, the formation of a surface monochromate ester was more favorable than that of the dichromate ester counterpart in the esterification. As far as the theoretical enthalpy was concerned, the priority of the anchorage of chromium species on silica gel may be in the following order. The monochromate site was most favored by directly grafted to the geminal and adjacent single member silanols pairs to form a chromasiloxane ring. The priority of the formation of dichromate occupied the same sites declined. Dichromate occurred on a geminal pairs site became much less likely. Also, the case of mono Cr ester formed on a geminal pairs site seemed to be ruled out. 4. Conclusions In this work, the role of geminal silanols over amorphous silica support in the localization of chromium species for Phillips catalyst has been achieved. The results were based on the identified NMR silanol signals of silica gel reference and Phillips Cr/silica catalysts at various thermal treatment temperatures, as well as the DFT calculation of the formation of monochromate or dichromate species on a well-defined POSS model containing geminal and single silanols. In addition, the theoretical results were expanded into the real silica gel system to explore the chromate location. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300 °C. The geminal silanols almost disappeared on the catalysts at 120 °C, while that for the silica gel occurred at 300 °C. With further increased the calcinations temperatures from 300 to 800 °C, the amount of the residual single silanol groups (including isolated and vicinal silanols) of catalysts was higher than that of silica, suggesting the existence of chromate species obstructed the removal of the single silanol groups. According to the calculations about the 17

structures of chromium anchoring on POSS models with various Cr–O–Si bonds, it further suggested that the cases for monochromate were most favorable located on pairs of a single and an adjacent geminal silanol groups accompanied with a geminal silanol left. As a result, the replacement of this silanol may be unfavorable even at higher temperatures. And the formation of monochromate presents somewhat priority. Due to the analogy of silsesquioxane with silica, the theoretical results was expanded into the real silica gel system, and a monochronium anchored to adjacent single and geminal silanols pairs to form a chromasiloxane ring seemed the most facile location on the silica surface. Acknowledgement We gratefully thank the support by the Fundamental Research Funds for the Central Universities, research program of Introducing Talents of Discipline of university (B08021), and research program of the State Key Laboratory of Chemical Engineering.

18

REFERENCES 1. Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. Chem. Rev. 105 (2005) 115-183. 2.

McDaniel, M. P. Adv. Catal. 33 (1985) 47-98.

3. McDaniel, M. P. In Handbook of Heterogeneous Catalysis, 2nd ed.; Ertl, G., Knözinger, H., Schüth, F., Weitkamp J. Eds.; Wiley-VCH: Weinheim, 2008; p 3733. 4.

McDaniel, M. P. Adv. Catal. 53 (2010) 123-606.

5. Cheng, R.; Liu, Z.; Zhong, L.; He, X.; Qiu, P.; Terano, M.; Eisen, M. S.; Scott, S. L.; Liu, B. Adv. Poly. Sci. 257 (2013) 135–202. 6.

McDaniel, M. P. J. Catal. 76 (1982) 37-47.

7.

McDaniel, M. P. J. Catal. 67 (1981) 71-76.

8. M. McDaniel, Appl. Catal. A: Gen. (2016), http://dx.doi.org/10.1016/j.apcata. 2016.12.001 9.

Liu, B.; Terano, M. J. Mol. Catal. A: Chem. 172 (2001) 227-240.

10.

Rebenstorf, B.; Larsson, R. J. Mol. Catal. 11 (1981) 247-256.

11. Weckhuysen, B. M.; Wachs, I. E.; Schoonheydt, R. A. Chem. Rev. 96 (1996) 3327-3349. 12. Demmelmaier, C. A.; White, R. E.; van Bokhoven, J. A.; Scott, S. L. J. Catal. 262 (2009) 44-56. 13.

McDaniel, M. P. J. Catal. 76 (1982) 17-28.

14. van Kimmenade, E. M. E.; Kuiper, A. E. T.; Tamminga, K.; Thüne, P. C.; Niemantsverdriet, J. W. J. Catal. 223 (2004) 134-141. 15.

Hogan, J. P. J. Polym. Sci., Part A: Polym. Chem. 8 (1970) 2637-2652.

16. Zecchina, A.; Garrone, E.; Ghiotti, G.; Morterra, C.; Borello, E. J. Phys. Chem. 79 (1975) 966-972. 17. Panchenko, V. N.; Zakharov, V. A.; Paukshtis, E. A. Appl. Catal. A 313 (2006) 130-136. 18.

Tonosaki, K.; Taniike, T.; Terano, M. Macromol. React. Eng. 5 (2011) 332-339.

19. Zhuravlev, L. T.,Colloids and Surfaces A:Physicochem. Eng. Aspects 173 (2000) 1-38. 19

20. Vansant, E. F.; Voort, P. V. D.; Vrancken, K. C. Stud. Surf. Sci. Catal. 93 (1995) 59-77. 21. Pullukat, T. J.; Patterson, R. E. In Handbook of Transition Metal Polymerization Catalysts. Hoff, R., Mathers, R. T., Eds.; John Wiley & Sons: Hoboken, 2010; p 29. 22. Cheng, R.; Xue, X.; Liu, W.; Zhao, N.; He, X.; Liu, Z.; Liu, B. Macromol. React. Eng. 9 (2015) 462-472. 23. Zhang, S.; Dong, Q.; Cheng, R.; He, X.; Wang, Q.; Tang, Y.; Yu, Y.; Xie, K.; Da, J.; Liu, B. J. Mol. Catal. A: Chem. 358 (2012) 10– 22. 24. Groeneveld, C.; Wittgen, P. P. M. M.; Van Kersbergen, A. M.; Mestrom, P. L. M.; Nuijten, C. E.; Schuit, G. C. A. J. Catal. 59 (1979) 153-167. 25. Wittgen, P. P. M. M.; Groeneveld, C.; Zwaans, P. J. C. J. M.; Morgenstern, H. J. B.; van Heughten, A. H.; van Heumen, C. J. M.; Schuit, G. C. A. J. Catal. 77 (1982) 360-381. 26. Li, X.; Cheng, R.; Luo, J.; Dong, Q.; He, X.; Li, L.; Yu, Y.; Da, J.; Liu, B. J. Mol. Catal. A: Chem. 330 (2010) 56-65. 27.

Nishimura, M.; Thomas, J. M. Catal. Lett. 21 (1993) 149-155.

28.

Sindorf, D. W.; Maciel, G. E. J. Am. Chem. Soc. 105 (1983) 1487-1493.

29.

Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 87 (1983) 5516-5521.

30. Schnellbach, M.; Koehler, F. H.; Bluemel, J. J. Organomet. Chem. 520 (1996) 227-230. 31. Chudek, J. A.; Hunter, G.; McQuire, G. W.; Rochester, C. H.; Smith, T. F. S. J. Chem. Soc., Faraday Trans. 92 (1996) 453-460. 32. Chudek, J. A.; Hunter, G.; Rochester, C. H.; Smith, T. F. S. J. Catal. 136 (1992) 246-251. 33.

Espelid, O.; Børve, K. J. J. Catal. 205 (2002) 366-374.

34.

Espelid, O.; Børve, K. J. J. Catal. 206 (2002) 331-338.

35.

Guesmi, H.; Tielens, F. J. Phys. Chem. C 116 (2012) 994-1001.

36. Zhong, L.; Lee, M. Y.; Liu, Z.; Wanglee, Y. J.; Liu, B. P.; Scott, S. L. J. Catal. 293 (2012) 1-12. 37. Williams, L. A.; Guo, N.; Motta, A.; Delferro, M.; Fragalà, I. L.; Miller, J. T.; Marks, T. J. PNAS 110 (2013) 413-418. 38.

Liu, B.; Fang, Y.; Terano, M. J. Mol. Catal. A: Chem. 219 (2004) 165-173. 20

39.

Fang, Y.; Liu, B.; Terano, M. Appl. Catal., A 279 (2005) 131-138.

40.

Fujimoto, H.Acc. Chem. Res. 20 (1987) 448-453.

41.

Liu, B.; Fang, Y.; Xia, W.; Terano, M. Kinet. Catal. 47 (2006) 234-240.

42.

Hoffmann, P.; Konzinger, E. Surf. Sci. Rep. 188 (1987) 181.

43. Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry. Wiley-Interscience: New York, 1979; p 652. 44.

Bermudez, V. M.J. Phys. Chem. 74 (1970) 4160.

45.

Haukka, S.; Lakomaa, E. L.; Root, A.J. Phys. Chem. 97 (1993) 5085-5094.

46.

Duchateau, R. Chem. Rev. 102 (2002) 3525-3542.

47.

Quadrelli, E. A.; Basset, J. M. Coord. Chem. Rev. 254 (2010) 707-728.

48. Dijkstra, T. W.; Duchateau, R.; van Santen, R. A.; Meetsma, A.; Yap, G. P. A. J. Am. Chem. Soc. 124 (2002) 9856-9864. 49.

Feher, F. J.; Blanski, R. L. Chem. Commun. (1990) 1614-1616.

50. 107.

Feher, F. J.; Blanski, R. L. Makromol. Chem., Macromol. Symp. 66 (1993) 95-

51. Bunker, B. C.; Haaland, D. M.; Michalske, T. A.; Smith, W. L. Surf. Sci. 222 (1989) 95.

21

Figure Captions Fig. 1.

29

Si CP/MAS NMR spectra forsamples calcined at various temperatures: (A)

silica gel: (a) Raw SiO2, (b) SG120, (c) SG200, (d) SG300, (e) SG400, (f) SG600, and (g) SG800;(B) ES70X, silica gel (ES70X), and Phillips catalysts: (a) Raw SiO2, (b) PCP120, (c) PC200, (d) PC300, (e) PC400, (f) PC600, and (g) PC800. (I) geminal silanol, (II) single silanol (including isolated and vicinal silanols) and (III) siloxane bridge.

Fig. 2. Dependence of relative intensity of geminal silanol on calcination temperatures (Corresponding temperature for raw SiO2 was regarded as room temperature 25 ºC).

Fig. 3. Dependence of relative intensity of single silanol (including isolated and vicinal silanols) on calcination temperatures (Corresponding temperature for raw SiO2 was regarded as 25 ºC).

Fig. 4. 1H MAS NMR spectra of samples calcined at various temperatures: (A) silica gel, (a) Raw SiO2, (b) SG120, (c) SG200, (d) SG300, (e) SG400, (f) SG600, and (g)SG800; (B)Phillips catalysts, (a) Raw SiO2, (b) PCP120, (c) PC200, (d) PC300, (e) PC400, (f) PC600, and (g) PC800.

Fig. 5. Dependence of relative chemical shift of 1H MAS NMR spectra of silica gel and Phillips catalysts oncalcination temperatures (Corresponding temperature for raw SiO2 was regarded as room temperature 25 ºC).

22

Scheme Captions

Scheme 1. Schematic presentation of polyoligomericsilsesquioxane (POSS) model.

Scheme 2. Computational models of esterification reactions of monochromte and dichromate species grafted on POSS model.

Scheme 3. Plausible structure of chromate species formed by reaction between CrO3 and geminal silanols on silica gel.

Scheme 4. Plausible existence of single silanol group isolated by adjacent chromate species on catalyst surface accounted for hindrance effect of dehydroxylation process on catalyst surface at high temperatures (>300 ºC).

Scheme 5. Computational models for Cr(VI) ester on six kinds of silanol pairs on POSS.

Scheme 6. Plausible grafting priority for CrO3 on different silanol sites on silica surface.

23

II

II

Relative intensity (a.u.)

Relative intensity (a.u.)

III

I a b c d e

III I a b c d e f g

f g

–20 –40 –60 –80 –100 –120 –140 –160 ppm

–20 –40 –60 –80 –100 –120 –140 –160 ppm

(A)

(B)

Fig. 1. 29Si CP/MAS NMR spectra for samples calcined at various temperatures: (A) silica gel: (a) Raw SiO2, (b) SG120, (c) SG200, (d) SG300, (e) SG400, (f) SG600, and (g) SG800;(B) ES70X, silica gel (ES70X), and Phillips catalysts: (a) Raw SiO2, (b) PCP120, (c) PC200, (d) PC300, (e) PC400, (f) PC600, and (g) PC800. (I) geminal silanol, (II) single silanol (including isolated and vicinal silanols) and (III) siloxane bridge.

24

15 Silica Catalyst

3

Intensity of peak( 10 , a.u. )

12 9

×

6 3 0 0

100

200 300 400 500 600 700 o Calcination temperature ( C)

800

Fig. 2. Dependence of relative intensity of geminal silanol on calcination temperatures (Corresponding temperature for raw SiO2 was regarded as room temperature 25 ºC).

25

Fig. 3. Dependence of relative intensity of single silanol (including isolated and vicinal silanols) on calcination temperatures (Corresponding temperature for raw SiO2 was regarded as 25 ºC).

26

b c d e f g 30

20

10

0 -10 -20 ppm

(A)

-30

Relative intensity (a.u.)

Relative intensity (a.u.)

a

a b c d e f g 30

20

10

0 -10 ppm

-20

-30

(B)

Fig. 4. 1H MAS NMR spectra of samples calcined at various temperatures: (A) silica gel, (a) Raw SiO2, (b) SG120, (c) SG200, (d) SG300, (e) SG400, (f) SG600, and (g)SG800; (B)Phillips catalysts, (a) Raw SiO2, (b) PCP120, (c) PC200, (d) PC300, (e) PC400, (f) PC600, and (g) PC800.

27

4.0 Silica Catalyst

Chemical shift (ppm)

3.5

3.0

2.5

2.0

1.5 0

100

200

300

400

500

600

700

800

o

Calcination temperature ( C)

Fig. 5. Dependence of relative chemical shift of 1H MAS NMR spectra of silica gel and Phillips catalysts on calcination temperatures (Corresponding temperature for raw SiO2 was regarded as room temperature 25 ºC).

28

Scheme 1. Schematic presentation of polyoligomericsilsesquioxane (POSS) model.

29

Si

H

O

Si

O

H O O O O Si O H O H O Si O Si H O O O O O H Si O Si O O H H O

Si

Cr

O

O

O

O

H

H

O

O

H

H

MCr-X

+ 2

O H

H

M

+

O O

O

O Cr

H

O

O Cr

O H

DCr-X

+ 2 H

O H

D

Scheme 2. Computational models of esterification reactions of monochromte and dichromate species grafted on POSS model.

30

O

O O Cr

H

O

O

O

O Cr

O Si

Si

O Si

SiO2

SiO2

Scheme 3. Plausible structures of chromate species formed by reaction between CrO3 and geminal silanols on silica gel.

31

O

O Cr

O

O

H O

Si O Si O

H O

O

Si O Si O Si

O Cr O O

H

O O Si O Si

Scheme 4. Plausible existence of single silanol group isolated by adjacent chromate species on catalyst surface accounted for hindrance effect of dehydroxylation process at high temperatures (>300 ºC).

32

HH HH OO OO SiSi OO SiSi HH OO OO OOOO OO OH Si Si OO SiSi O Cr O H O Si OO HSi Si O H O OO H O Si OO O O OO OO OO H Cr Si O Si H O Si O Si O O OO H HH

MCr-1 H O

MCr-2 O O Cr Si O O

H O

Si O O O O O Si O H Cr H O Si O Si O O O O O O O H Si O Si O Cr O O O H DCr-1

Si O O OO O Si O Si O O H O Si O H Si H O O O O O H Si O Si O O H H M MCr-3 H

H

O O Si

O O Cr H O O O Si O Si H O Cr O O OO O O Si O Si O H O H O Si O Si H O O O O O H Si O Si O O H H

H H O O O Si O Si O Cr H O O O OO O Si O Si O Cr O O H O Si O Si H O O O O O O Si O Si H O O H H

DCr-2

H O Si O H O

DCr-3

Scheme 5. Computational models for Cr(VI) species on six kinds of silanol pairs in POSS.

33

H

H H O

H

H

O

O

H

O

Si

O

Si

Si

H H

O

O

Si Si

O Si

O

H O

Cr

O

H

Si

H

Si

SiO2

High

O Si

O

O

O

O

O

Cr

O

H

O Si

Si

O

O

SiO2

O

O

O Cr

Cr

O

O

H

Si Si

O Si

Si

O

O

O

Cr

+ CrO3

O

SiO2

Si

O

H

Si

O

O Si

Si

Si

Si

H

Cr O

O

O

H

Si Si

O

O

H

O

H

SiO2

Priority of formation

O

O Si

Si

H O Si Si

O Si

Si

SiO2

O Si

Low

Scheme 6. Plausible grafting priority for CrO3 on different silanol sites on silica surface.

34

Table 1. Preparation conditions of catalysts calcined at different temperatures (PCP: Phillips catalyst precursor; PC: Phillips catalyst; SG: Silica gel) Sample

Conditions

Raw SiO2

Crosfield ES70X

PCP120

catalyst precursor dried at 120 C after impregnation

PC200

calcined at 200C for 6 h in dry air

PC300

calcined at 300C for 6 h in dry air

PC400

calcined at 400C for 6 h in dry air

PC600

calcined at 600C for 6 h in dry air

SG120

calcined at 120C for 6 h in dry air

SG200

calcined at 200C for 6 h in dry air

SG300

calcined at 300C for 6 h in dry air

SG400

calcined at 400C for 6 h in dry air

SG600

calcined at 600C for 6 h in dry air

35

Table 2. Calculated enthalpies for Cr(VI) esters.

Ea/kJ mol-1

MCr-1

MCr-2

MCr-3

DCr-1

DCr-2

DCr-3

4.76

-27.56

79.14

25.90

17.15

52.33

36