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Title

High‑resolution spectroscopy (XPS, 1^H MAS solid‑
state NMR) and DFT investigations into Ti‑
modified Phillips CrO̲x/SiO̲2 catalysts

Author(s)Cheng, Ruihua; Xu, Chen; Liu, Zhen; Dong, Qi; He,
Xuelian; Fang, Yuwei; Terano, Minoru; Hu, Yatao;
Pullukat, Thomas J.; Liu, Boping

Citation

Journal of Catalysis, 273(2): 103‑115

Issue Date

2010‑06‑16

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/10719

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Ruihua
Cheng, Chen Xu, Zhen Liu, Qi Dong, Xuelian He,
Yuwei Fang, Minoru Terano, Yatao Hu, Thomas J.
Pullukat, Boping Liu, Journal of Catalysis,273(2), 2010, 103‑115,
http://dx.doi.org/10.1016/j.jcat.2010.05.002

Description

Japan Advanced Institute of Science and Technology

Full paper submitted to Journal of Catalysis

High resolution spectroscopy (XPS, 1H MAS solid state NMR) and DFT
investigations on Ti-modified Phillips CrOx/SiO2 catalysts

Ruihua Chenga, Chen Xua, Zhen Liua, Qi Donga, Xuelian Hea, Yuwei Fangb, Minoru
Teranob, Yatao Huc, Thomas J. Pullukatc, Boping Liua*aState Key Laboratory of Chemical Engineering, East China University of Scienceand Technology, Meilong Road 130, Shanghai 200237, P.R.Chinab

School of Materials Science, Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japanc

Research and Development Center, The PQ Corporation, 280 Cedar Grove Road,
Conshohocken, PA, 19428-2240 USA*Corresponding Author:
Boping Liu: boping@ecust.edu.cn
Tel/Fax: 0086-21-64253627

1

Abstract
Ti-modified Phillips catalyst is a most important industrial catalyst widely usedin ethylene polymerization, but the mechanism still remains mysterious. In this work,
Ti-modified Phillips catalysts were characterized by high resolution X-ray
photoelectron spectroscopy (XPS) and 1H magic-angle-spin solid state nuclear
magnetic resonance (1H MAS solid state NMR) combined with density functional
investigations on the effects of Ti-modification on promotion of polymerization
activity, and regulation of microstructures of the polymer chains. XPS data revealed
Ti-modification caused increase of electron-deficiency and photo-stability of thesurface chromate species. 1H NMR provided the first direct evidence of surface
residual Ti-OH groups. Modeling results rationalized well the effects of
Ti-modification on promotion of polymerization activity, extension of molecularweight distribution (MWD) to lower MW region, improvement of the distribution of
inserted co-monomer and enhancement of 2,1-insertion in regioselectivity of Phillips
catalysts. It was the first time the Ti-modification on Phillips catalysts weretheoretically elucidated.
Keywords: Phillips CrOx/SiO2 catalyst; Ti-modification; X-ray photoelectron
spectroscopy; 1H magic-angle-spin solid state nuclear magnetic resonance; Density
functional theory2

1. Introduction
Phillips catalyst, with highly dispersed chromium oxide covalently anchored on
silica gel surface, is one of the most important industrial olefin polymerization
catalysts and nowadays is roughly producing ten million tons of high density
polyethylene (HDPE) about 50% of the world HDPE market [1-3]. Its HDPE products
are featuring with ultra-broad molecular weight distribution (MWD), long chain
branches, and specially applicable for blow molding products like hollow containers
and pipes. In the last decades, the applications from exclusive Phillips HDPE products
for the gasoline tanks of the automobile industry and ultra large size plastic pipes andcontainers make a successively increasing market demanding.
Due to its great commercial importance, the Phillips catalyst has been intensively
studied since its discovery in the early 1950s. Despite the great research efforts, theacademic progresses about this catalyst are lagging far behind its successfulcommercial applications. Most importantly, the nature of the active sites and the exact
initiation/polymerization

mechanisms

on

the

Phillips

catalyst

still

remaincontroversial [1-3]. The difficulties for basic studies on this catalyst system mainly
derived from the low percentage of active Cr species, the complexity ofheterogeneous catalyst system, the multiple valence states of Cr, the fast
encapsulation of active sites by produced polymer, and the very fast polymerization
rate.
Phillips catalyst is usually prepared by a simple impregnation of porous silica gel
support with aqueous solution of Cr(III) acetate and a subsequent drying followed by
an important calcination process in dry air at a certain temperature between 500C
and 900 C to obtain a calcined catalyst stored in N2 atmosphere [1, 3]. During thecalcination process, the Cr(III) acetate decomposed into CrO3 followed byesterification reaction with the surface hydroxyl groups on silica gel resulting in theformation of chromate species including monochromate, dichromate, or evenpolychromate. The most unique feature of Phillips catalyst is that the calcined catalyst
could be directly activated by ethylene monomer for ethylene polymerization without
using any organometal cocatalysts. The performance of the calcined Phillips catalyst3

is known to be greatly dependent on the history of calcination process in terms ofeither physical or chemical modifications. During the past several decades, a series ofimportant chemically modified Phillips catalysts have been successfully developed
including the modifications of the silica gel support by titanium, aluminum, fluorine,
etc [1-3]. Ti-modified Phillips catalyst, which was first developed by Phillips
Petroleum Co. in the late 1960s, has been becoming a most important industrial
catalyst widely used in the Phillips ethylene polymerization processes [2-10]. The
Ti-modification could not only efficiently improve the catalyst activity but also
provide an important method for regulation of the molecular weight (MW), MWD,
and inserted co-monomer distribution in polymer products, as the Phillips catalyst is
not sensitive to chain transfer with hydrogen as Ziegler-Natta and metallocene
catalysts.
In spite of the commercial importance, Ti-modified Phillips catalysts have notbeen fully investigated in the literature. In general, Ti-modification in terms of titania
incorporation to Phillips catalyst could be fabricated by either co-gel or surface
impregnation method. The co-gel method incorporated titania by adding titanium salts
like TiCl4 into the silicate solution for the sol-gel process and led to the dispersion oftitania in the bulk of the synthesized silica gel support, while the surface impregnation
method incorporated titania on the surface through simple impregnation of titanium
ester onto silica gel support followed by calcination at high temperatures [2-7].
Nowadays, Ti-modified Phillips catalysts by both methods are applied in commercial
polyethylene processes for production of HDPE with only slight difference inpolymer properties [2]. According to Weckhuysen and coworkers [9], the surface
titania species over silica gel enhanced the concentration of surface polychromate
species leading to comparable amount of surface monochromate and polychromate
species based on Raman characterization. McDaniel et al. [2] reported that
Ti-modification could induce decrease in thermal stability of Phillips catalysts
resulting in the occurrence of earlier catalyst sintering at lower calcination
temperatures. Thus, an optimal titania loading for commercial Ti-modified Phillips
catalyst was usually less than 5 wt% Ti. Generally, Ti-modification could play the4following four most important roles in the performances of Phillips catalysts [2, 3, 10].
i) Ti-modification could increase the catalytic polymerization activity in terms of the
Ti-promotion effect with a shorter induction period and higher maximum
polymerization rate. ii) Ti-modification could broaden the MWD of the polymer
product to low MW region and thus decrease the average MW. As observed by
McDaniel, with the gradual increasing of the Ti loading, the MWD graduallyextended to lower MW part without any variation at the high MW end. iii)
Ti-modification could obviously decrease the incorporation of co-monomer (1-hexeneor 1-butene) in the low MW region and increase the density of the polyethylene. Suchvariation of the distribution of inserted co-monomer in polyethylene chain
significantly improved the mechanical properties of the final products. As declared by
McDaniel [2], there existed two seemingly contradictory points as follows. One wasthat Ti-modified Cr active sites preferred to produce low MW polyethylene with low
co-monomer incorporation, while low MW polyethylene made by Cr active sites
without Ti-modification showed high co-monomer incorporation. The other was thatthe chain transfer response to co-monomer of Ti-modified Phillips catalyst was muchgreater than that of Ti-free Phillips catalyst, while the product of Ti-modified Phillips
catalyst incorporated much less co-monomer contents. iv) Ti-modification could
obviously increase the trans-internal unsaturation chain ends. Based on thisexperimental fact, McDaniel suggested that 2,1-insertion versus 1,2-insertion might
be enhanced during ethylene copolymerization with -olefin indicative of a variationin regioselectivity of the catalyst. In summary, the mechanistic understanding on thefour above-mentioned most important roles of Ti-modification for Phillips catalysts
has not been achieved yet. Accordingly, these unique effects should be derived fromthe change of active site distribution through possible formation of new active Cr
species related to Ti-modification. As shown in scheme 1, Pullukat and coworkers
firstly proposed a site model of monochromate species directly linked to the surface
titania through two Cr-O-Ti bridges [5, 10]. McDaniel et al. [2] proposed similar sitemodel based on experimental investigations on a Ti-modified Phillips catalyst
prepared from CrO2Cl2. Moreover, the linkage of chromate species onto surface
5

titania through Cr-O-Ti bridge was also indirectly confirmed by spectroscopic
characterizations with diffuse reflectance spectroscopy (DRS), electron paramagnetic
resonance (EPR), and temperature-programmed reduction (TPR) [7, 13]. Based on theprevious understanding in the literature, detailed mechanistic investigations on
Ti-modified Phillips catalysts are in high demand for further progress in thisimportant field.
Amongvarioussurfaceanalyticaltechniques,

highresolution

X-ray

photoelectron spectroscopy (XPS) with high surface sensitivity (typical sampling
depths of 1-2 nm) is one of the most powerful methods for both quantitative andqualitative characterizations of surface chromate species over Phillips catalyst withrelatively low Cr loading [10, 12, 14-17]. The first XPS study on Ti-modified Phillips
catalyst was reported by Pullukat and coworkers [12] in 1980. They found the binding
energy (BE) of Cr 2p3/2 for chromate species decreased from 579.6 eV to 578.0 eV
after Ti-modification suggesting the increase in electron density and decrease inoxidizing capacity for the chromate species. This means Ti-modified chromate species
became much more difficult to be reduced by ethylene monomer, which wascontradictory to the experimental evidence that Ti-modification facilitated the
reduction of chromate species featuring with shorter induction period and higher
polymerization rate. This discrepancy probably originated from the following reasons.
Firstly, improper reference peak C 1s=285.0 eV from carbon contamination was
selected for their correction of charging effect during XPS measurement. We [17]have shown that reference peak C 1s=285.0 eV from carbon contamination mightcause significant error for BE value due to the different carbon contamination fordifferent samples, and the Si 2p peak at 103.3 eV from silica gel support was ideal forthe charging effect corrections. Secondly, the resolution of the XPS instrument was
not high enough to get accurate data for catalysts with low Cr loading. The second
XPS study on Ti-modified Phillips catalyst was reported by McDaniel et al. [10] in
1983. They found that the Ti species tended to migrate from the bulk phase of silica
gel support to its surface during calcination for co-gel type Ti-modified Phillips
catalysts through measuring the intensity ratio of the Ti 2p and Si 2p peaks. The BE
6values of the chromate species with and without Ti-modification were not measured.
After more than 30 years of technical development, the XPS instrument has been
improved significantly with high resolution for not only qualitative but also precise
quantitative characterization of surface chromate species, the precursor of the active
sites, for industrial Phillips catalysts with relatively low Cr loading. Therefore, much
deeper mechanistic insight on the Ti-modified Phillips catalyst through XPS method
becomes possible and is urgently needed. Following our previous series ofinvestigations on Phillips catalysts by high resolution XPS method [16, 17], in thiswork the BE value and the full width at half maximum (FWHM) of Cr 2p3/2 peak forthe surface chromate species with or without Ti-modification were monitored in termsof catalyst preparation methods, calcination temperatures, and Ti contents. We [14,16-18] have previously found that the XPS data obtained within 10 min acquisition ofhigh resolution scanning could reflect the original physico-chemical nature of surface
chromate species with negligible photo-reduction. However, longer XPS acquisition
time (over 10 min) could induce photo-reduction of some labile hexa-valent chromate
species into tri-valent Cr species due to the soft X-ray irradiation [14, 16-18]. Our
more recent report [16] revealed an interesting relationship between thephoto-stability of the surface chromate species during XPS measurements and thepolymerization activity for Phillips catalysts calcined at different temperatures. It was
found the photo-stability of the chromate species on Phillips catalysts increased with
the increasing calcination temperatures (200 -800 C), which was corresponding tosimultaneous increase of oxidizing capacity of chromate species and polymerization
activity as well. Although the mechanism of the above-mentioned correlation between
the photo-stability and oxidizing capacity of chromate species and polymerization
activity was still unclear, the XPS characterization in terms of photo-stability could be
utilized in this work for the precise investigations on the effects of Ti-modification
over Phillips catalysts.
During the last decade, computational molecular simulation based on density
functional theory (DFT) is becoming more and more important and powerful to getmolecular-level insight into Cr active site and its polymerization mechanism of
7

Phillips catalyst. Espelid and Borve [19-21] have studied on three possible
polymerization mechanisms involving Cossee mechanism, carbene mechanism, andmetallacycle mechanism on Phillips catalyst by DFT method using mononuclear,dinuclear disiloxanochromium (II) catalyst models, and found that the metallacyclic
initiation presented the lowest energy barrier. Very recently, Scott and coworkers [22]confirmed the mononuclear disiloxanochromium(II) 6-ring model might be the mostplausible active site, which showed much higher polymerization activity than that of
the mononuclear trisiloxanochromium(II) 8-ring model through DFT method
combined with experiments. Damin et al. [23] studied on the molecular modeling of
the mononuclear disiloxanochromium(II) model adsorbed with probe molecules of
CO and N2 and found that more accurate simulated IR spectra comparing toexperimental IR data was obtained with increasing the percentage of Hartree-Fockexchange in the hybrid density functional. Ziegler and his coworkers [24] studied theethylenepolymerization

mechanisms

by

DFT

methodoveraneutraldisiloxanochromium (IV)-carbene model and found that a transformation from the
Cr-carbene into a cationic Cr-C site followed by the Cossee type chain propagation
might be most plausible. By the combination of DFT and paired interacting orbitals(PIO), we previously studied the intermolecular orbital interactions between ethylene
monomer and various monochromate models of Phillips catalyst in terms of theeffects of supporting of chromium compounds [25] and the fluorination of the silica
gel support surface [26]. More recently, our DFT and PIO studies on the effects ofoxidation state and surface hydroxyl on the polymerization activity of various
disiloxanomolybdenum-methyl model catalysts disclosed that divalent Mo2+ might be
the most plausible active site for silica gel supported-MoOx ethylene polymerization
catalysts [27]. However, up to now, no molecular modeling study focusing on theeffects of Ti-modification for Phillips catalyst has been reported.
In this work, the effects of Ti-modification on Phillips catalysts with respect toits promotion of polymerization activity, extension of MWD to lower MW region,
improvement of the distribution of the inserted co-monomer, and the enhancement of
2,1-insertion in regioselectivity were systematically investigated by DFT method
8

combined with high resolution XPS and 1H magic-angle-spin solid state nuclear
magnetic resonance (1H MAS solid state NMR) characterization. As for theexperiments, the oxidation state and photo-stability of the surface Cr species on
Ti-modified Phillips catalysts with dependence on the XPS acquisition time was alsocharacterized in order to correlate with its electronic properties and polymerization
behavior as well. High resolution 1H MAS solid state NMR method was applied toconfirm the existence of surface residual Ti-OH groups on Ti-modified Phillips
catalysts, which verified the reliability of the active site models constructed forcomputational molecular modeling. Much deeper mechanistic understanding on theeffects of Ti-modification over Phillips catalysts was demonstrated.
2. Experimental and computational
2.1 Catalysts
In this work, eight Phillips catalysts (named as Ph-1 to Ph-8) with or without
Ti-modification stored within small N2-filled and sealed stainless steel containers
were all donated from PQ Corp. USA. Among them, seven catalysts from Ph-2 toPh-8 including five Ti-modified catalysts (Ph-2, Ph-4, Ph-5, Ph-7, and Ph-8) madeby surface impregnation method and two catalysts (Ph-3 and Ph-6) without
Ti-modification were commercial ethylene polymerization catalysts produced by PQ
Corp. The chromium content for each catalyst sample was ca. 1 wt.% Cr. The Ph-1
containing 2.22 wt.% Ti was a co-gel type Ti-modified Phillips catalyst preparedaccording to Dietz’s patent [6]. The specific preparation procedure for Ph-1 is shownas follows. Firstly, after 43 g sulfuric acid (95%) was mixed with 4.25 g (2.5 ml) TiCl4,
water was added to reach a total volume about 270 mL. Then, 255 g sodium silicate
solution (Na2O3.2SiO2, 38 wt%, =1.39 g mL-1) was added and mixed at 16.2 C.
Gelation occurred at pH= 6 and aged for about 4 h at 90.0 C. The gel was washed,dried, and then impregnated with a chromium acetylacetonate in ethyl acetate
followed by drying at 150 C for 2 h under vacuum. Finally, 8 g dried sample wasactivated at 593 C for 5 h within a 1-inch-diameter fluidized quartz tube reactor. The
temperature of the whole calcination was programmed and linearly increased to 593
C at 2 C min-1. Fluidizing gas media with a flow rate of 300 mL min-1 was changed
9

from N2 into dry air when temperature reached 300 C. During the cooling stage, dry
air was switched back to N2 at 100 C for purging the catalyst for 1 h followed bytransfer into a storage vessel. As for the industrial catalysts (Ph-2 to Ph-8) donatedfrom PQ Corp. [28], the detailed catalyst preparation procedures would not be given
herein due to the confidential reason, and only a simplified introduction of thecommercial Phillips catalyst prepared by PQ Corp. was shown as follows. GradeMS-3050 silica gel with BET surface area of 480 -550 m2 g-1, pore volumes of 3.0 mL
g-1, average pore diameter of 24 nm, average particle size of 90 m made by PQ Corp.
was pre-dried at 170 C to remove the physically-adsorbed water and used as support.
For the Ti-modified catalysts made from surface impregnation method, titanium
isopropoxide was dissolved in dry n-heptane to impregnate onto the pre-dried silica
gel support followed by drying at 250 C for 2 h in N2. And then, the obtained
samples were further impregnated with a methanolic chromium acetate solution
followed by drying at 150 C for 2 h in N2. Hereafter, the following preparation
procedures were similar to that of the Ph-1 catalyst except the final calcination
temperatures. Here, the catalysts with different final calcination temperatures and Ti
contents were referred to as Ph-2 (593 C), Ph-4 (650 C), and Ph-7 (820 C) with
2.38 wt.% Ti loading, and Ph-5 (650 C), and Ph-8 (820 C) with 3.45 wt.% Ti
loading. Two Ti-free catalysts Ph-3 (650 C) and Ph-6 (820 C) were prepared similar
to the Ti-modified catalysts by surface impregnation method skipping theimpregnation of Ti compound. The BET surface areas and pore volumes of all thecatalysts were measured at -196 °C using N2 after evacuating the catalysts at 300 °C
for 1 h. The chemical compositions (Ti and Cr loading) and textural properties
including BET surface area, pore volume, and average pore diameter of the catalysts
are listed in Table 1.
2.2 XPS characterization
All the catalyst samples were characterized by high resolution XPS method.
Details concerning the XPS measurements could be found in our previous reports [16,17]. Only a simplified introduction was given here. XPS data were obtained on a
Physical Electronics Perkin-Elmer Model Phi-5600 ESCA spectrometer equipped10with a hemispherical electron analyzer working in the constant pass energy mode. A
monochromated Al K radiation (1486.6 eV) operated at 300 W was used as the
X-ray source. Each sample was embedded on a conductive copper tape to form asample disc in a diameter of 5mm and fixed on a sample holder. The sample holder
was then put into the vacuum transfer vessel (Phi Model 04-110, Perkin-Elmer Co.,
Ltd.), which can be connected to the sample introduction chamber on the XPS
instrument for sample transfer without atmospheric exposure. The above procedures
were carried out in a glove box, which had been purged overnight with nitrogen gas inadvance. The prepared sample was degassed in the introduction chamber to 10−7 Torrbefore entering the main chamber, where the pressure was kept below 3×10-9 torr
during XPS data acquisitions. Firstly, low resolution survey scan between 0 - 1000 eV
was carried out on each sample within 2 min for a preliminary survey of all surface
elements. Then, high resolution XPS scan measurements for Cr 2p, Si 2p regions ofeach sample were performed within 10 min, 30 min, and 2 h. A neutralizer was used
to reduce the charging effect to obtain a better signal to noise ratio. The XPS dataobtained within 10 min acquisition could reflect the original physico-chemical natureof surface chromate species with negligible photo-reduction. The dependence ofphoto-stability of the chromate species on the XPS acquisition time (from 10 min to 2h) was investigated to correlate with its electronic property and the catalyst
polymerization behavior [14, 16-18]. All binding energies (BE) were referenced to theSi 2p peak of silica gel at 103.3 eV for the charging effect correction. Multiplet
fittings of the Cr 2p XPS curves by the Gaussian–Lorentzian method were carried out
to determine the mixed states of valences in all the samples. The fitting for each curve
was repeated for several times to ensure a reproducible result.
2.3 Solid-state NMR characterization
The six catalyst samples (Ph-3 to Ph-8) were characterized by high resolution 1H
MAS solid sate NMR method at room temperature. A Varian UNITY-400
spectrometer with a Varian Room Temperature/Cross Polarization Magic Angle Spin
(RT/CP MAS) probe operating at 400.47 MHz was used. Each sample (ca. 80 mg)
was put and pressed tightly into a 7-mm zirconia rotor under N2 atmosphere in a glove
11

box, which had been purged by N2 overnight before sample setting. The 1H MAS
solid sate NMR spectra were obtained with a 30° pulse of 3 μs, a relaxation delay of 1s and total 10 min acquisition time for each sample at a rotating speed of ca. 3 kHz.
The spectrum of each sample was referenced to the 1H NMR spectrum of the externalreference tetramethylsilane (TMS) at 0 ppm.
2.4. Theoretical method
All the DFT calculations were performed with the Gaussian 03 software package[29]. The calculations were based on the nonlocal three-parameter UB3LYP hybrid
DFT method, which was named as Becke’s gradient-corrected exchange-correlation
density function [30], and combining with Lee, Yang, and Parr’s nonlocal correlation[31]. The LANL2DZ (Los Alamos National Laboratory second Double-Zeta) basis set[32] with relativistic effective core potential (RECP) of Hay and Wadt was used for Cr
and Ti, and the 6-31G(d, p) basis set was used for the rest atoms including C, H, O, Si.
The stationary structures were optimized without any symmetry constraints, andfrequency calculations at the same harmonic frequencies. All energies reported
referred to Gibbs free energy corrections to the total electronic energies at 298.15 K.
Each reactant and product pair with minimum Gibbs free energy was used to search
for the transition state, which was verified by Intrinsic Reaction Coordinate (IRC)
calculations ensuring that each transition state was directly connected to the involved
reactant and product geometries [33]. Frequency calculations were all carried out toensure that all the stationary points (reactant and product) possessed no imaginary
vibrations and each transition state possessed only one imaginary vibration. The
Mulliken charge of chromium atom was also calculated by Gaussian 03 program foranalysis of the charge distribution on each center.
For our theoretical DFT calculations, six mononuclear chromium cluster models
including three hexavalent chromate sites (a0, b0, c0) and three divalent Cr sites (a1,b1, c1) were utilized to mimic various Ti-modification environments on Phillips
catalyst surface (shown in scheme 2). Among these models, a0 and a1 weremononuclear

disiloxanochromium(VI)

model

and

mononuclear

disiloxanochromium(II) model, respectively, which were most popularly accepted in
12the literature [19, 22, 23]. Models b0 and b1 represented Ti-modified Cr(VI) and Cr(II)catalysts, respectively, with a Ti/Cr atomic ratio of 1:1. Models c0 and c1 represented
Ti-modified Cr(VI) and Cr(II) catalysts, respectively, with a Ti/Cr atomic ratio of 2:1.
All the Ti-modified models have been previously proposed by Marsden [7], Pullukat[12], and McDaniel [2, 10].

3. Results and discussion
3.1. Experimental study of effects of preparation methods
The catalytic properties of Phillips catalysts are strongly dependent on thephysico-chemical states of the chromate species [3]. This may be influenced by
preparation methods. Thus, the effects of the preparation method either co-gel orsurface impregnation for the Ti-modified Phillips catalysts were first investigated.
Sample Ph-1 with 2.22 wt.% Ti was prepared by co-gel method, and sample Ph-2
with similar Ti-loading (2.38 wt.% Ti) was prepared by surface impregnation method.
Both of them were similarly treated at 593 C.
According to our previous reports [14, 16-18], the XPS data obtained within 10
min acquisition of high resolution scanning could reflect the original physic-chemical
nature of surface chromate species (expressed as Cr(VI)Ox,surf) with negligible
photo-reduction, and longer XPS acquisition time (over 10 min) could induce
photo-reduction of some labile hexa-valent chromate species into tri-valent surface
stabilized Cr species (expressed as Cr(III)Ox,surf) due to the soft X-ray irradiation [14,16-18]. The photo-stability of the chromate species subjected to the soft X-ray
irradiation during XPS measurement was further utilized to evaluate the electronic
property and polymerization behavior.
The XPS spectra of the Cr 2p core level of the surface chromium species withdependence on XPS acquisition time (10 min, 30 min, and 2 h) for the two samples
were presented in Fig. 1. All spectra for Ph-1 and Ph-2 were comprised of two main
doublet features corresponding to Cr 2p3/2 and Cr 2p1/2 photoelectrons, respectively[11]. The individual doublet of each oxidation state was resulted from deconvolution
by the peak fitting procedure to estimate the atomic composition of surface chromium
13species existed in different oxidation states. The specific details relating to theassignments of oxidation states for all surface chromium species based on BE valuesof Cr 2p3/2 level of the samples were detailed in our previous reports [16, 17]. For
Ph-1 and Ph-2 catalysts, the Cr component with BE value of Cr 2p3/2 level around 580
eV was assigned to be Cr(VI)Ox,surf species, and the Cr component with BE value of
Cr 2p3/2 level around 577 eV was assigned to be Cr(III)Ox,surf species. Accordingly, thecorresponding BE, FWHM values of Cr 2p3/2 level, assignment and atomic percentage
of different oxidation states of surface Cr species were illustrated in Table 2. The 10
min XPS data showed the existence of only a single surface Cr(VI)Ox,surf species on
Ph-1 and Ph-2 catalysts with BE of 580.1 and 580.0 eV and FWHM of 5.4 and 5.6 eV,
respectively. Thus, the electronic difference reflected from 10 min XPS data between
the samples prepared by the two methods was small. After prolongation of the XPS
acquisition time, a new surface Cr(III)Ox,surf species appeared on both Ph-1 and Ph-2
catalysts due to photo-reduction of Cr(VI)Ox,surf species during XPS measurement[16]. It was very interesting to find that the atomic percentage of Cr(III)Ox,surf on Ph-1
of 56.9% (30 min XPS acquisition) and 74.3% (2 h XPS acquisition) were obviously
higher than that on Ph-2 of 30.6% (30 min XPS acquisition) and 61.3% (2 h XPS
acquisition) indicating much higher photo-stability of Ph-2 compared than that of
Ph-1. According to our previous report on series of Phillips catalysts calcined at
different temperatures (200-800 °C) [16], the catalyst calcined at higher temperatureswith higher oxidizing capacity of chromate species and polymerization activity
showed better photo-stability of the surface chromate species during XPS
measurements. Accordingly, the oxidizing capacity of chromate species andpolymerization activity for the Ph-2 catalyst with a better photo-stability should behigher than that of Ph-1 with similar Ti loading. It is reasonable considering a more
effective use of the titania by surface impregnation method than that of the co-gel one
with partial Ti species in the bulk of silica gel [34], and Ti-modified Phillips catalysts
made by surface impregnation method are more popularly used for commercial
application. Therefore, the following research was focused on the catalysts preparedby surface impregnation method.
14

3.2. Experimental study of effects of calcination temperature and Ti-loading
Calcination temperature and Ti-loading are crucial factors to determine thecatalytic performance of Ti-modified Phillips catalyst. The practical calcination
temperature is usually between 500 -900 C in industrial processes. Among thistemperature region, the thermal-induced reduction of chromate species and catalyst
sintering could be negligible. Two series of catalysts calcined at 650 and 820 C wereconsidered in this work. Three catalysts were calcined at 650 C: Ph-3 (Ti-free), Ph-4
(2.38 wt.% Ti) and Ph-5 (3.45 wt.% Ti), and another three catalysts were calcined at
820 C: Ph-6 (Ti-free), Ph-7 (2.38 wt.% Ti), and Ph-8 (3.45 wt.% Ti) with different
Ti loading were characterized by XPS method. Figs. 2 and 3 show the Cr 2p spectraof the six catalysts with dependence of XPS acquisition time from 10 min to 30 min
and 2 h. The corresponding BE, FWHM values of Cr 2p3/2 level, assignment andatomic percentage of different oxidation states of surface Cr species of each catalyst
were illustrated in Table 3. The XPS data obtained at 10 min acquisition time could
reflect the original nature of surface chromate species of the catalysts. It was foundthat only single component of surface chromate species was detected for all thecatalysts based on the 10 min XPS data. For Ph-3, Ph-4, and Ph-5 catalysts calcinedat 650C, the BE values of Cr 2p3/2 level were 580.6, 580.9 and 581.1eV, respectively.
For Ph-6, Ph-7, and Ph-8 catalysts calcined at 820C, the BE values of Cr 2p3/2 level
were 580.8, 581.1 and 581.2eV, respectively. The slightly increasing of BE values
indicated that the electron-deficiency of surface chromate species increased withincrease of Ti-loading. The slight increase of the FWHM values after Ti-modification
indicated the broadening of the distribution of surface chromate species. The effect of
calcination temperature could be observed from the comparison of the two series ofcatalysts calcined at 650C and 820C. When the calcination temperature increased
from 650C to 820 C, the BE values of Ph-3 and Ph-6 (without Ti-modification) was
slightly increased from 580.6 to 580.8 eV, and the corresponding FWHM values
broadened from 4.9 to 5.8 eV, respectively. Similar tendency was observed between
Ph-4 and Ph-7 (with 2.38 wt.% Ti, with BE from 580.9 to 581.1 eV, with FWHM
from 5.3 to 6.3 eV) and also between Ph-5 and Ph-8 (with 3.45 wt.% Ti, with BE
15

from 581.1 to 581.2 eV, with FWHM from 5.3 to 6.3 eV). This means the increase of
calcination temperature showed similar effect with increase of Ti-loading in terms ofincrease of the electron-deficiency of surface chromate species, which could berationalized by the removal of more electron-donating surface hydroxyl groups andthe increase of surface tension due to dehydroxylation at higher calcination
temperatures [35].
As can be seen from Figs. 2 and 3, and Table 3, when the XPS acquisition time
was increased from 10 min to 30 min and 2 h, surface Cr(III)Ox,surf species appearedon all catalysts due to photo-reduction of Cr(VI)Ox,surf species during XPS
measurement [16]. The lower amount of Cr(III)Ox,surf species reflected the higher
photo-stability of the catalyst. After 30 min XPS acquisition, the Ti-free Ph-3 catalyst
presented a considerable portion of 31.5% Cr(III)Ox,surf species converted from
Cr(VI)Ox,surf species then to a dominant portion of 65.3% after 2 h XPS measurement
accompanied by a large increase of FWHM from 4.9 up to 6.3 eV. Comparably, for
the Ti-modified samples, the corresponding portion of Cr(III)Ox,surf were 31.0% and28.2% for Ph-4 and Ph-5 after 30 min XPS acquisitions, and increased to 51.3% and49.9% after 2 h XPS acquisition, respectively. After 30 min XPS acquisition, theTi-free Ph-6 catalyst presented a considerable portion of 35.5% Cr(III)Ox,surf species
converted from Cr(VI)Ox,surf species then to a dominant portion of 70.5% after 2 h
XPS measurement accompanied by a large increase of FWHM from 5.8 up to 7.1 eV.
Comparably, for the Ti-modified samples, the corresponding portion of Cr(III)Ox,surfwere 34.5% and 33.9% for Ph-7 and Ph-8 after 30 min XPS acquisitions, andincreased to 60.5% and 57.6% after 2 h XPS acquisition, respectively. It was very
interesting to find that Ti-modification could obviously decrease the amount of the
Cr(III)Ox,surf species and thus increase the photo-stability of the Phillips catalysts
mainly based on the 2 h XPS data. This was also consistent with the experimental factof increasing polymerization activity after Ti-modification for Phillips catalysts
calcined at the same temperature. However, the mechanism about the increase ofphoto-stability of Phillips catalyst by Ti-modification was still unclear. It seems
contradictory that chromate species with increased electron-deficiency due to the16presence of more electronegative Ti(IV) in the neighborhood showed even higher
photo-stability during XPS acquisition. Further mechanistic elucidation of thephoto-induced reduction of surface chromate species by soft X-ray irradiation during
XPS measurement is needed in order to rationalize this point.
As demonstrated above, the gradual shifted towards higher BE and FWHM
values of Cr 2p3/2 level for the surface chromate species in the Ti-modified catalyst
with increase of Ti content suggested that the Ti-modification increased the oxidizing
capacity of the surface chromate species associated with Ti and thus could shorten the
induction period and increase the polymerization activity as well. It is reasonable tosuggest the presence of at least two kinds of Cr(VI)Ox,surf species in the Ti-modified
catalyst: one species with (Cr-O-Si) neighbors, and the other with at least one(Cr-O-Ti) neighbor as shown in scheme 1. Only those Cr(VI)Ox,surf species with atleast one (Cr-O-Ti) neighbor having higher electron-deficiency than that in the Ti-freecatalyst may be attributed to the high polymerization activity in experiment, which
was confirmed in our DFT calculations, and would be introduced in detail in thefollowing sections. This also coincided with the evidence that 0.5 wt.% Cr on puretitania support was prone to be reduced and formed a large fraction in oxidation stateof +5 [36]. And the H2-TPR signal also gave a direct indication of the more easily
reducible species in Ti-modified Phillips catalyst [12].
Another powerful tool to detect the effect of Ti-modification on the surface
physico-chemical nature is high resolution 1H MAS solid state NMR. Due to thespectral overlap of Ti-OH and Si-OH, this distinction is very difficult to be obtainedby infrared spectroscopy [37]. Fortunately, 1H MAS solid state NMR with highresolution could allow differentiating the Si-OH and the Ti-OH species, yielding
thereby useful information on the surface structures on silica gel support modified bytitania. Fig. 4 (A) and (B) present the 1H MAS solid state NMR signals of Ph-3, Ph-4,
and Ph-5 catalysts calcined at 650 C, and Ph-6, Ph-7, and Ph-8 samples calcined at
820 C, respectively. There were three peaks in Ph-3 spectrum: one main peak near
1.7~1.8 ppm was assigned to the Si-OH species, and the other two near 9 and -8 ppm
were the satellite peaks of the main peak, whose intensities decreased with the17decrease of main peak. Similar spectra presented for Ph-4 and Ph-5 except a weakband at 6 ppm, which could be attributed to Ti-OH on amorphous but anatase-liketitania [38]. Though this was the residual Ti-OH signal after the treatment of thecatalyst at high temperatures, it was the first NMR evidence to confirm the existenceof the residual Ti-OH groups on the Ti-modified Phillips catalyst. Thus the surface
titania sites might render the Cr site incorporation [2]. The result also indirectly
confirmed the reliability of our calculation cluster models with one or two Cr-O-Ti
bridges designed for Ti-modified Phillips catalysts. The direct observation of surface
residual Ti-OH groups on the Ph-7 and Ph-8 catalysts calcined at 820 C in Fig. 4(B)were similar to those of their counterparts calcined at 650 C.
In summary, the high resolution XPS studies on Ti-modified Phillips catalysts
clearly confirmed that Ti-modification led to the increase of electron-deficiency andthe photo-stability of the surface chromate species and thus rationalized well thehigher oxidizing capacity of chromate species, shorter induction period and higher
polymerization activity after Ti-modification. The plausible formation of active Cr
species supported on silica gel support through one or two Cr-O-Ti bridges on
Ti-modified Phillips catalysts was further proved by high resolution 1H MAS solidstate NMR method through the first direct evidence of the surface residual Ti-OH
groups. These experimental approaches proved the reliability of the catalyst models
and provided a solid basis for our theoretical and computational molecular modelingfor Ti-modified Phillips catalysts. The basic understanding of Ti-modification on
Phillips catalyst solely from experimental approaches is limited at its stronger
Brönsted acidity, which could be clearly observed from our 1H MAS solid state NMR
characterization. However, as a matter of fact, the unique features for the Ti-modified
Phillips catalyst were not entirely derived from the acidity of Brönsted sites in thepresence of titania. It is known that many other oxide additives which also enhance
Brönsted acidity do not present similar effects [2]. Surely, some otherphysico-chemical factors may also contribute to the unique performances of
Ti-modified Phillips catalyst, which could be possibly elucidated by the following
DFT calculations.
18

3.3. Theoretical study of Ti-modification on catalyst activity
Based on the above experimental investigations, six mononuclear chromium
cluster models including three hexavalent Cr(VI)Ox,surf site models (a0, b0, c0) andthree divalent Cr(II)Ox,surf site models (a1, b1, c1) with 0, 1 or 2 Cr-O-Ti bridges were
utilized to mimic all the possible Ti-modification environments on Phillips catalyst
surface (shown in scheme 2) in our molecular modeling. The effects of
Ti-modification on Phillips catalysts with respect to its promotion of polymerization
activity, extension of MWD to lower MW region, improvement of the distribution of
the inserted co-monomer, and the enhancement of 2,1-insertion in regioselectivity
were systematically investigated by DFT method. Firstly, the promotion effect of
Ti-modification in terms of polymerization activity enhancement was studied.
Activity of Phillips catalyst was mainly determined by several reaction steps in thewhole process of polymerization including reduction of Cr(VI)Ox,surf species, chain
initiation, and chain propagation, which were calculated with respect to the effects of
Ti-modification.
Cr(VI)Ox,surf species, which formed during calcination of catalyst, has to be
reduced to lower valence state to form Cr active site. Requiring no extra reducing
agent like Al-alkyl cocatalyst, Cr(VI)Ox,surf species is considered to be reduced byethylene monomer to divalent Cr(II)Ox,surf, which has been generally accepted as the
final precursor of active site [3, 39-41]. All the optimized geometries of ground state
for Cr(VI)Ox,surf and Cr(II)Ox,surf models were listed in Table 4. The calculated bonddistances d(Cr-O), 1.75 Å and 1.75 Å in a0 were in the same range of 1.706-1.782Å
as that in the synthesized CrO2(OSiPh3)2 and [CrO2{(OSiPh2)2O}]2 molecules [42, 43].
And the bond distance of d(Cr-O) was also favorably agreed with 1.77 Å in Espelid’s
work [19], and 1.75 Å in Scott’s work [22] based on the same model. Besides, the
Cr-O single bond lengths were somewhat lengthened in Ti-involved models, as 1.77 Å,
1.76 Å for b0, and 1.77 Å, 1.77 Å for c0. Meanwhile, the O-Cr-O bond angle alsoslight opened from 104.7° for a0, to 105.1° for b0, and 106.0° for c0. Compared tothe a0 (0 presented the Cr in oxidation of +6) model, Ti-modified models showed an
obviously increased Mulliken charges on Cr(VI): +1.11 for a0, +1.27, and +1.32 for19

Ti-modified sites b0 (Ti:Cr=1:1) and c0 (Ti:Cr=2:1), respectively. The increase of
Mulliken charges on the Cr(VI) was the direct indication of increase of electron
deficiency of the Cr(VI) site with Ti-modification, which facilitated the reduction ofchromate species, thereby resulting in a shorter induction period in polymerization
process. And this was consistent well with the increase of BE of the surface chromate
species after Ti-modification by XPS method as demonstrated in the previous
sections.
After the reduction of Cr(VI)Ox,surf species into Cr(II)Ox,surf species, the initiationof the first polymer chain should occur on this Cr(II) site triggering the ethylene
polymerization. However, the chain initiation step still remains mysterious [1, 44-48].
One proposed mechanism of chain initiation involves the direct formation of
Cr(III)-ethyl active site from Cr(II) site and coordinated ethylene with an extrahydrogen atom. However, the origin of this extra hydrogen atom is postulated fromsilanol [49] or other surface ligands [3] without reasonable interpretation. In recent
years, it is becoming widely accepted that the first polymer chain might be initiatedthrough a metallacyclic mechanism [3, 47, 50-53]. As shown in scheme 3, theinitiation occurred through the formation of chromacyclopentane five-membered ringfrom a oxidative coupling reaction between Cr(II)Ox,surf species and two coordinatedethylene molecules, then the first polymer chain grows in loop style by continuous
ethylene insertion into the Cr(IV)-C bonds. The results of the formation ofchromacyclopentane calculated by DFT were shown in Table 5. The energy barrier ofthe initiation reaction for al, bl, and c1 models was 6.1, 6.8, and 6.9 kcal mol-1,
respectively. The slight increase of the energy barriers suggested that Ti-modification
was slightly unfavorable for the initiation reaction. As shown in Table 4, Mulliken
charges on Cr(II) site were +0.61 for al, +0.90 for b1, and +0.77 for c1. It isnoticeable that the increase of Mulliken charge with the increase of the Ti contents
involved in the models was not monotonous. It is likely that the asymmetric structureof the b2 model led to its much higher Mulliken charge on the Cr(II) site. This could
be also clearly observed from its structural parameters shown in Table 2. The Cr-O
single bonds were 1.78/1.78, 1.85/1.82, 1.78/1.78 Å for al, b1, and c1 models,20

respectively, and the corresponding O-Cr-O bond angles were 118.7, 114.7, and122.2.
After initiation of the first polymer chain on Phillips catalyst, hundreds ofpolymer chains would be formed on the same active site during chain propagation
step, thus, the chain propagation reactions during this step make more important
contribution to the polymerization activity than the initiation reaction. Different with
the initiation reaction, the chain propagation from the second polymer chain on eachactive site is generally considered to follow the classical Cossee mechanism, in whicha growing polymer chain propagates in a linear Cr-(CH2)n-CH3 pattern with anincrease of two methylene units as each coordinated ethylene inserts into the Cr-alkyl
bond (as shown as reaction a in scheme 4) [2, 19, 44]. In this study, the propagating
polymer chain was simplified as a methyl group, thus the Cr-methyl bond was
designated as the active site model for continuous insertion of ethylene monomers. In
Table 5, the Mulliken charge on Cr active site of the Cr-methyl structures increased
from +0.74 for unmodified site model (Ti:Cr=0:1), to +0.83 for Ti-modified sitemodel (Ti:Cr=1:1), and +0.89 for Ti-modified site model (Ti:Cr=2:1), and thecorresponding energy barriers of the first ethylene insertion were 19.2, 18.4, and 17.7
kcal mol-1, respectively. It indicated that the Cr active site became more
electron-deficient and thus achieved lower propagation energy barriers due toTi-modification. The obvious decrease of energy barrier for ethylene insertion during
chain propagation indicated that Ti-modification might be one of the main factors
bringing the Ti-promotional effects to enhance the polymerization activity of Phillips
catalyst.
In summary, the promotional effect of Ti-modification in terms ofpolymerization activity enhancement was found to be mainly derived from theincrease of electron deficiency of Cr(VI)Ox,surf species resulting in the easier reduction
into Cr(II)Ox,surf species and chain propagation reaction. Ti-modification was found tobe slightly unfavorable for the initiation reaction. The increased Mulliken charge on
Cr sites involved in Ti-contained Cr(VI), Cr(II) and Cr-methyl active sites provided
solid evidences for the fact that electron density of Cr site decreased obviously
21because of the electron-withdrawing effect of Ti(IV). This was consistent with many
experimental reports [2, 3, 5, 10, 54].
3.4 Theoretical study of Ti modification on microstructures of polyethylene chains
According to experimental reports [2, 3], Ti-modified Phillips catalysts are
known to be able to regulate the microstructures of the polyethylene chains in termsof extension of MWD to lower MW region, improvement of the distribution of
inserted co-monomers through decrease the co-monomer incorporation in the low
MW region. The energetic information about chain transfer for ethylene
homo-polymerization, chain propagation, and chain transfer during ethylene
copolymerization with -olefin was further calculated by DFT method in order toelucidate the specific mechanisms behind the two seemingly contradictory points
declared by McDaniel [2]. One was that Ti-modified Cr active sites preferred to
produce low MW polyethylene with low co-monomer incorporation, while low MW
polyethylene made by Cr active sites without Ti-modification showed highco-monomer incorporation. The other was that the chain transfer response to
co-monomer of Ti-modified Phillips catalyst was much greater than that of Ti-freePhillips catalyst, while the product of the Ti-modified Phillips catalyst incorporated
much less co-monomer contents.
The MW and MWD of the polyethylene are known to be determined by the
relative rate between chain propagation and chain transfer. β-H elimination tomonomer is believed to be the predominating chain transfer mode for Phillips catalyst[2]. As for ethylene homopolymerization, the chain transfer mode was shown asreaction b in scheme 4. According to Table 5, energy barrier of chain transfer was 25.9
kcal mol-1 for Ti-free model, which was higher than 24.7 and 24.1 kcal mol-1 for
Ti-modified site models of Ti:Cr=1:1 and Ti:Cr=2:1, respectively. According to theenergy barrier for chain propagation discussed in the previous section, those for chain
propagation and chain transfer were simultaneous decreased by Ti-modification, while
the later decreased more. This means chain transfer rate was enhanced more thanchain propagation rate after Ti-modification. Consequently, Ti-modification would
result in an increased low MW fraction of polyethylene, and thus the MWD
22

broadened to the lower MW region. The calculation results provided deeper
theoretical understanding on the experimental facts regarding the effects of
Ti-modification on the MW and MWD of polyethylene over Phillips catalyst.
Ethylene copolymerization with -olefin such as 1-butene or 1-hexene overPhillips catalyst is the key method to control the density and microstructures of thepolyethylene products in industrial processes [9]. The dominant primary 1,
2-insertions of 1-butene and 1-hexene and subsequent chain transfer by β-hydride
elimination to ethylene monomer were investigated by DFT method on the basis of
the three type models with 0:1, 1:1, and 2:1 Ti/Cr atomic ratios. The energy data werelisted in Table 5 for direct comparison. For the Ti-free catalyst model, the energy
barrier of insertion was increased from 19.2 kcal mol-1 for ethylene, to 21.7 and 21.8
kcal mol-1 for 1-butene and 1-hexene, respectively, reflecting the general discipline oflower polymerization rate for -olefin compared with that for ethylene. For the
Ti-modified catalyst models, the energy barrier of 1-butene insertion was decreased
from 21.7 kcal mol-1 for Ti-free site model (Ti:Cr=0:1), to 20.2 and 19.6 kcal mol-1 for
two Ti-modified site models (Ti:Cr=1:1 and Ti:Cr=2:1). The energy barrier of
1-hexene insertion was also decreased from 21.8 to 21.2 and 20.3 kcal mol-1 for the
Ti-free site model (Ti:Cr=0:1), and two Ti-modified site models (Ti:Cr=1:1 andTi:Cr=2:1), respectively. It indicates that Ti-modification could promote the activity
not only for ethylene homo-polymerization but also for co-polymerization with
-olefins for Phillips catalyst. A comparison between 1-butene and 1-hexene
insertions showed the general tendency of lower copolymerization rate for -olefin
with larger carbon atoms. In order to obtain the distribution of the inserted
co-monomers in polyethylene chains [2], the difference of energy barrier between
co-monomer insertion and chain transfer, which proceeded with a tertiary β-hydride
elimination after the 1-butene or 1-hexene insertion, was also calculated. In Table 5,for the case of 1-butene as co-monomer, the energy barrier difference decreased due
to Ti-modification from 4.4 kcal mol-1 for Ti-free site model (Ti:Cr=0:1) to 3.3 kcal
mol-1 for Ti-modified site model (Ti:Cr=1:1) and 2.1 kcal mol-1 for Ti-modified sitemodel (Ti:Cr=2:1). Similar tendency was also observed for the case of 1-hexene as23

co-monomer. The energy barrier difference between 1-hexene 1,2-insertion andsubsequent chain transfer decreased due to Ti-modification from 2.5 kcal mol-1 for
Ti-free site model (Ti:Cr=0:1) to 2.1 kcal mol-1 for Ti-modified site model (Ti:Cr=1:1)
and 1.8 kcal mol-1 for Ti-modified site model (Ti:Cr=2:1). Thus, Ti-modification
created an obvious effect to promote chain transfer after co-monomer incorporation.
Moreover, insertion of 1-hexene created more dominant effect than that of 1-butene.
According to the calculation results in the previous section, Ti-modified catalyst
model tended to produce polyethylene with lower MW. Therefore, it was reasonable
to conclude that Ti-modified catalyst model tended to make low MW polyethylene
with much less co-monomer insertion because the inserted co-monomer mainly led tochain transfer reaction and left the inserted co-monomer at the chain end. As a result,the increased chain termination by co-monomer resulted in less short chain branches(SCBs) in low MW fraction and higher density of the polymer for Ti-modified
Phillips catalyst. This lowering of the co-monomer incorporation in the low MW
region was highly desirable for improvements of polymer mechanical properties [9].
These molecular modeling results provided wonderful theoretical explanation to theexperimental results reported by McDaniel [2, 3, 10] that larger vinylidene chain ends
presented in polymers made by Ti-modified Phillips catalysts and MI response to
1-hexene by Ti-modified Phillips catalyst was much greater than that by Ti-freePhillips catalysts. The results based on the simple theoretical models also rationalized
very well the two seemingly contradictory points declared by McDaniel [2].
3.5 Theoretical study of Ti modification on regioselectivity of catalyst
For traditional heterogeneous olefin polymerization catalysts such as
Ziegler-Natta and Phillips catalysts, the regioselectivity is usually very high in theform of 1,2-insertion (primary insertion) of -olefins. 2,1-insertion (secondary
insertion) is not preferential mainly due to steric hindrance. McDaniel [2] suggested
that Ti-modification might enhance the 2,1-insertion based on the experimental
evidence of increased trans-internal unsaturation chain ends in the polyethylene
chains during ethylene copolymerization with 1-hexene on Ti-modified Phillips
catalyst. The effect of Ti-modification on the reaction energies of 1,2-insertion and24

2,1-insertion of -olefins for co-polymerization of Phillips catalyst was calculated by
DFT method based on the three catalyst models with Ti:Cr atomic ratios of 0:1, 1:1,
and 2:1. The regiospecific insertion of 1-butene on the catalyst site model (Ti:Cr=0:1)
was shown in scheme 5. All the energy barriers of the two insertion modes for both
1-butene and 1-hexene on the three catalyst models were listed in Table 6.
The much lower energy barriers for the co-monomer with 1,2-insertion than thatfor 2,1-insertion on all the catalyst models suggested the dominant 1,2-insertion withlower steric hindrance was not changed by the Ti-modification. It should be noted thatthe energy barriers for both 1,2-insertion and 2,1-insertion of co-monomer all
decreased after Ti-modification, which is in consistence with the Ti-promotion effecton activity. Moreover, the energy barriers for 2,1-insertion of co-monomer decreased
more than that for 1,2-insertion, which could be observed from the lower difference ofenergy barriers between 1,2-insertion and 2,1-insertion as shown in Table 6. For
1-hexene, the difference of energy barriers between 1,2-insertion and 2,1-insertion
were 1.9, 1.2, and 1.8 kcal mol-1 for Ti-free site model (Ti:Cr=0:1) and the two
Ti-modified site models (Ti:Cr=1:1 and Ti:Cr=2:1), respectively. For 1-butene, thedifference of energy barriers between 1,2-insertion and 2,1-insertion were 2.2, 1.2,
and 1.8 kcal mol-1 for Ti-free site model (Ti:Cr=0:1) and the two Ti-modified sitemodels (Ti:Cr=1:1 and Ti:Cr=2:1), respectively. The decrease of both the energy
barriers of the 2,1-insertion and the difference of energy barriers between
1,2-insertion and 2,1-insertion of co-monomer was a clear indication of theenhancement of 2,1-insertion by Ti-modification on Phillips catalyst. Our theoreticalresults perfectly validated McDaniel’s suggestion of the plausible enhancement of the
2,1-insertion versus 1,2-insertion during ethylene copolymerization with -olefin on
Ti-modified Phillips catalyst.
4. Conclusions
In this work, the effects of Ti-modification with respect to its promotion ofpolymerization activity, extension of MWD to lower MW region, improvement of the
distribution of the inserted co-monomer, and enhancement of 2,1-insertion in theregioselectivity of Phillips catalysts were systematically investigated by DFT method25

in combination with experiments using high resolution XPS and 1H MAS solid state
NMR characterization. The high resolution XPS studies on Ti-modified Phillips
catalyst clearly demonstrated that Ti-modification led to the increase ofelectron-deficiency and the photo-stability of the surface chromate species and thus
rationalized well the higher oxidizing capacity of chromate species, shorter induction
period and higher polymerization activity after Ti-modification. The Ti-modified
catalyst by surface impregnation method showed a better photo-stability and possibly
better catalytic performance than that by co-gel method with similar Ti-loading. The
plausible formation of active Cr species supported on silica gel support through one or
two Cr-O-Ti bridges on Ti-modified Phillips catalysts and the validity of the catalyst
models constructed for molecular modeling was further proved by high resolution 1H
MAS solid state NMR method through the first direct evidence of the surface residual
Ti-OH groups.
As for the theoretical modeling results, firstly, the promotional effect of
Ti-modification in terms of polymerization activity enhancement was found to bemainly derived from the increase of electron deficiency of Cr(VI)Ox,surf species
resulting in its easier reduction into Cr(II)Ox,surf species, and lower energy barrier forthe chain propagation reactions, although Ti-modification was found to be slightly
unfavorable for the initiation reaction. Secondly, the energy barriers for chain
propagation and chain transfer were simultaneous decreased by Ti-modification, while
the later decreased more. This suggested that chain transfer rate was enhanced morethan chain propagation rate after Ti-modification. Consequently, Ti-modification
could result in an increased low MW fraction of polyethylene, and thus MWD would
be broadened to the lower MW region. The difference of energy barrier between
co-monomer insertion and subsequent chain transfer decreased due to Ti-modificationand created a significant effect to promote chain transfer after co-monomer
incorporation. Moreover, insertion of 1-hexene created more dominant effect than thatof 1-butene. It was also reasonable to conclude that Ti-modified catalyst model tended
to make low MW polyethylene with much less co-monomer insertion because theinserted co-monomer mainly led to chain transfer reaction and left the inserted
26

co-monomer at the chain end. As a result, the increased chain termination byco-monomer resulted in less short chain branches (SCBs) in low MW fraction andhigher density of the polymer for Ti-modified Phillips catalyst. Thirdly, the decreaseof both the energy barriers of the 2,1-insertion and the difference of energy barriers
between 1,2-insertion and 2,1-insertion of co-monomer was a clear indication of theenhancement of 2,1-insertion by Ti-modification on Phillips catalyst, which perfectly
validated McDaniel’s suggestion of the plausible enhancement of the 2,1-insertion
versus 1,2-insertion during ethylene copolymerization with -olefin on Ti-modified
Phillips catalyst.
The combined spectroscopic and theoretical modeling results, which rationalized
the experimental facts well, provided much deeper mechanistic understanding on the
Ti-modified Phillips catalysts and a solid basis for further development in thisimportant field.

Acknowledgements
The authors thank the financial support by Key Projects in the National Science &Technology Pillar Program during the 11th Five-Year Plan Period (2007BAE50B04)
and National Natural Science Foundation of China (20774025), the research programof the State Key Laboratory of Chemical Engineering, Shanghai Pujiang Talent Plan
Project (08PJ14032) and the Program of Introducing Talents of Discipline to
Universities (B08021), and also the kind donation of the Ti-modified Phillips catalysts
from PQ Corporation.

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30

Figure/scheme captions
Figure 1. Dependence of XPS spectra of Cr 2p level for catalysts Ph-1 by co-gel
method and Ph-2 by surface impregnation method on XPS acquisition time: (a) 10
min; (b) 30 min; (c) 2 h.
Figure 2. Dependence of XPS spectra of Cr 2p level for A) Ph-3, B) Ph-4 and C)
Ph-5 catalysts calcined at 650 C on XPS acquisition time: (a) 10 min; (b) 30 min; (c)2 h.
Figure 3. Dependence of XPS spectra of Cr 2p level for A) Ph-6, B) Ph-7, and C)
Ph-8 catalysts calcined at 820 C on XPS acquisition time: (a) 10 min; (b) 30 min; (c)2 h.
Figure 4. 1H MAS solid state NMR spectra for (A) Ph-3, Ph-4, and Ph-5 catalysts
calcined at 650 C, and (B) Ph-6, Ph-7, and Ph-8 catalysts calcined at 820 C. (*peak of surface Ti-OH groups)

Scheme 1. Proposed formation of Ti-O-Cr bridges on Ti-modified Phillips catalyst inreference 7.
Scheme 2. Models for Cr(VI) sites (a0 for Ti:Cr=0:1, b0 for Ti:Cr=1:1, c0 for
Ti:Cr=2:1) and Cr(II) sites ( a1 for Ti:Cr=0:1, b1 for Ti:Cr=1:1, c1 for Ti:Cr=2:1) for
Phillips catalyst.
Scheme 3. Chain initiation over a1 model through metallacycle mechanism.
Scheme 4. a) Chain propagation through classical Cossee mechanism, b) Chaintransfer by β-H elimination to monomer, for Phillips catalyst.
Scheme 5. Regiospecific insertion of 1-butene as co-monomer in 1,2-orientation and
2,1-orientation for Phillips catalyst.

31

Table 1. Physical properties of Phillips catalysts donated from PQ Corporation.

a)

Temp.

SBET

PVol

PDiam

Cr

Ti

(C)

(m2 g -1)

(mL g-1)

(nm)

(wt%)

(wt%)

Co-gel

593

502

2.52

20.0

0.9

2.22

Ph-2

S. I. a)

593

503

2.33

18.5

1.0

2.38

Ph-3

-

650

505

2.34

18.6

1.0

0.00

Ph-4

S. I. a)

650

494

2.22

18.0

1.1

2.38

Ph-5

S. I. a)

650

499

2.17

17.4

1.1

3.45

Ph-6

-

820

505

2.34

18.6

1.0

0.00

Ph-7

S. I. a)

820

494

2.22

18.0

1.1

2.38

Ph-8

S. I. a)

820

499

2.17

17.4

1.1

3.45

Cat.

Type

Ph-1

Titania was introduced by surface impregnation method.

32

Table 2. XPS data from multiplet fitting of Cr 2p spectra with XPS acquisition timewithin 10 min, 30 min, and 2 h for Ti-modified Phillips catalysts Ph-1 and Ph-2
prepared with co-gel and surface impregnation methods, respectively.

Cat.

Ph-1

Temp.

Ti

(ºC)

(wt%)

593

2.22

XPS
Measurement

BE

FWHM

AssignmentCont.

time (min)

(eV)

(eV)

of Cr species

(%)

10

580.1

5.4

Cr(VI)

100

30

580.1

5.7

Cr(VI)

43.1

577.1

5.7

Cr(III)

56.9

580.1

6.0

Cr(VI)

25.7

577.3

6.3

Cr(III)

74.3

10

580.0

5.6

Cr(VI)

100

30

580.0

5.6

Cr(VI)

69.4

577.2

5.2

Cr(III)

30.6

580.0

5.9

Cr(VI)

38.7

577.0

6.0

Cr(III)

61.3

120

Ph-2

593

2.36

120

33

Table 3. XPS data from multiplet fitting of Cr 2p spectra with XPS acquisition timewithin 10 min, 30 min, and 2 h for Ph-3 ~ Ph-8 Phillips catalysts calcined at different
temperatures.

Cat.

Ph-3

Temp.

Ti

(ºC)

(wt%)

650

-

XPS
Measurement

BE

FWHM

AssignmentCont.

time (min)

(eV)

(eV)

of Cr species

(%)

10

580.6

4.9

Cr(VI)

100

30

580.6

6.1

Cr(VI)

68.5

577.1

4.2

Cr(III)

31.5

580.6

6.3

Cr(VI)

34.7

577.0

5.3

Cr(III)

65.3

10

580.9

5.3

Cr(VI)

100

30

580.9

6.8

Cr(VI)

69.0

577.2

4.7

Cr(III)

31.0

580.9

6.8

Cr(VI)

48.7

577.1

4.8

Cr(III)

51.3

10

581.1

5.3

Cr(VI)

100

30

581.1

6.6

Cr(VI)

71.9

577.1

4.7

Cr(III)

28.1

581.0

6.3

Cr(VI)

50.1

576.9

4.5

Cr(III)

49.9

10

580.8

5.8

Cr(VI)

100

30

580.8

6.9

Cr(VI)

64.5

577.2

5.0

Cr(III)

35.5

580.8

7.1

Cr(VI)

29.5

576.9

6.3

Cr(III)

70.5

10

581.1

6.3

Cr(VI)

100

30

581.1

6.6

Cr(VI)

65.5

120

Ph-4

650

2.38

120

Ph-5

650

3.45

120

Ph-6

820

-

120

Ph-7

820

2.38

34

577.5

4.8

Cr(III)

34.5

581.1

6.8

Cr(VI)

39.5

577.2

5.6

Cr(III)

60.5

10

581.2

6.3

Cr(VI)

100

30

581.2

6.6

Cr(VI)

66.1

577.3

4.1

Cr(III)

33.9

581.2

6.5

Cr(VI)

42.4

577.0

5.8

Cr(III)

57.6

120

Ph-8

820

3.45

120

35

Table 4. Geometric parameters and Mulliken charges for Cr(VI) site models (a0 for
Ti:Cr=0:1, b0 for Ti:Cr=1:1, c0 for Ti:Cr=2:1) and Cr(II) site models (a1 for
Ti:Cr=0:1, b1 for Ti:Cr=1:1, c1 for Ti:Cr=2:1).a)
Models Cr-O1 Cr-O2 O1-Cr-O2 q(Cr)

a)

a0

1.75

1.75

104.7

+1.11

b0

1.77

1.76

105.1

+1.27

c0

1.77

1.77

106.0

+1.32

a1

1.78

1.78

118.7

+0.61

b1

1.85

1.82

114.6

+0.90

c1

1.78

1.78

122.2

+0.77

Bond length in Å, bond angles in degrees.

36

Table 5. Energy barriers for chain initiation, chain propagation, and chain transfer
reactions on various models of Phillips catalysts a).
Models

Initiation Chain propagation b)

Chain transfer c)

Δ d)

C2H4

C2H4 1-C4H8 1-C6H12 C2H4 1-C4H8 1-C6H12 C2H4 1-C4H8 1-C6H12

Ti:Cr=0:1

6.1

19.2

21.7

21.8

25.9 26.1

24.3

6.7

4.4

2.5

Ti:Cr=1:1

6.8

18.4

20.2

21.2

24.7 23.5

23.3

6.3

3.3

2.1

Ti:Cr=2:1

6.9

17.7

19.6

20.3

24.1 21.7

22.1

6.4

2.1

1.8

a)

Energy barrier in kcal mol-1b)

Based on primary 1,2-insertionc)Based on β-H elimination to monomer after 1,2-insertion of the corresponding
monomer or co-monomerd)Energy barrier difference between ethylene insertion of chain propagation and chaintransfer steps in kcal mol-1

37

Table 6. Energy barriers through different regiospecific insertion modes withco-monomers in terms of regioselectivity on various models of Phillips catalysts a)
Model

Insertion modes

1-Hexene

Ti:Cr=0:1

1,2-insertion

21.8

2,1- insertion

23.7

1,2- insertion

21.2

2,1- insertion

22.4

1,2- insertion

20.3

2,1- insertion

22.1

Ti:Cr=1:1

Ti:Cr=2:1

Δb)
1.9

1.2

1.8

1-Butene
21.7
23.9
20.2
21.4
19.6
21.4

Δb)
2.2

1.2

1.8

a)

Energy barrier in kcal mol-1b)Energy barrier difference between 2,1- insertion and 1,2- insertion in kcal mol-1 foreach active site model

38

Figure 1.

Cheng et al.

39

Figure 2.

Cheng et al.

40

Figure 3.

Cheng et al.

41

Figure 4.

Cheng et al.

42

Si

O

O

Ti

O

O

a (Ti:Cr=0:1)

Ti

O

Cr

Cr
Si

O

O

Si

O

b (Ti:Cr=1:1)

Scheme 1.

43

O
Cr

O

Ti

O

c (Ti:Cr=2:1)

O

O
Cr
Si

Ti
H

Black and white for Scheme 2

O
Cr
Si

Ti
H

Scheme 2.

44a1

Scheme 3.

45Scheme 4.

46

Scheme 5.

47