Vol.:(0123456789)1 3

Catalysis Letters (2018) 148:2683–2695 https://doi.org/10.1007/s10562-018-2461-x

Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

Hai Wang1,2 · Peng Zhang2 · Panpan Zhou1 · Renwei Xu2 · Yu Tang1

Received: 1 April 2018 / Accepted: 11 June 2018 / Published online: 20 June 2018 © Springer Science+Business Media, LLC, part of Springer Nature 2018

AbstractMetallocene compound is supported on methylaluminoxane (MAO) and the carrier to form the metallocene catalyst, where (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 are two typical representatives of the metallocene compounds. The difference in the performances of the two compounds (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 was calculated by DFT. The introduc-tion of methyl groups led to an increase in the number of Mulliken charge and a decrease in the dipole distance. The results fully confirmed that the methyl group showed an electron-donating effect. The catalyst formed by the metallocene compound (n-BuCp)2ZrCl2 in the olefin polymerization process followed the a-agostic mechanism of ground state and transition state, and dehydrogenation occurred during the polymerization process. In contrast, the catalyst formed by the metallocene com-pound (1,3-Me,n-BuCp)2ZrCl2 in the olefin polymerization process was based on the Green–Rooney mechanism (hydrogen anion migration), and metal–hydrogen bond and small molecule alkane were formed during the polymerization reaction. However, there were some differences in the mechanism of the homopolymerization and copolymerization when using metallocene catalysts for ethylene polymerization. Ethylene/1-alkene reacted to produce copolymers containing unsaturated bonds, and the results were confirmed by FT-IR analysis. The significant effect of the ligand structure on the metallocene catalyst is beyond imagination, which will lead us to find a suitable ligand structure to meet the high catalytic activity and low hydrogen evolution.

Graphical Abstract

Keywords Density functional theory · Metallocene compounds · Transition state · Dehydrogenation

1 Introduction

Metallocenes [1] and post-metallocenes [2] have opened the way for new polymer materials, especially copolymers of ethylene and 1-alkenes, which now have a significant market

* Panpan Zhou zhoupp@lzu.edu.cn

* Yu Tang tangyu@lzu.edu.cn

Extended author information available on the last page of the article

2684 H. Wang et al.

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share in both commodity and high value-added polyolefin [3–6]. The main purpose of this field study is to design met-allocene complexes with high catalytic activities that are capable of producing polymers with specific physical prop-erties. Metallocenes are composed of a metal atom locat-ing between two planar aromatic ligands. Zirconocenes, Cp*2ZrCl2 (Cp* is a substituted or unsubstituted cyclopen-tadienyl ligand, Cp = η5-C5H5) are industrially important catalysts for the polymerization of α-olefins. It has been established that there is a direct relationship between the structure of the metallocene catalyst and its behavior during the polymerization [7–9]. Theoretical calculations are pow-erful tools for chemists to participate in the interpretation of experimental results [10]. In the frame of the density func-tional theory (DFT) and based on the energy of the highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO), global chemistry descriptors such as hardness [11], electronegativity [12], softness [13], elec-trophilicity [14], and chemical potential have contributed greatly to the evaluation of quantum chemistry and have been very successful in predicting trends in global chemical reactivation of compounds. The Frontier molecular orbital theory suggests that the smaller the band gap formed by a molecule, the easier it is to be excited. Methylaluminox-ane (MAO) [2] as cocatalyst can react with the metallocene compound to form cationic active center, and the band gap produced by the cationic active center becomes smaller, thereby becoming more reactive, which is the role of MAO in metallocene catalysts. Metallocene catalysts sometimes undergo dehydrogenation reactions during the slurry polym-erization process and release different amounts of hydrogen. These reactions are pronounced, for example, in copolym-erization reactions of ethylene with 1-alkenes [15–17]. Sev-eral routes leading to the H2 generation have been proposed. One is the σ-bond metathesis of the vinylic C–H bond in a polymer chain [18]. There are differences in the hydrogen content released by different metallocene catalysts in the olefin polymerization process, which is mainly related to the ligand structure of metallocene compounds. Ligand struc-tures cause differences in steric hindrance and electronic effects, which in turn lead to changes in the olefin polym-erization mechanism of metallocene catalysts. Cooney et al. [19] indicates that alkyl groups can exhibit electron-donating or electron-withdrawing effect, the electronic effect exhib-ited by the alkyl group is related to its attached functional group. Some researchers put forward different views on the details of the chain growth mechanism and establish several specific chain growth mechanism models (Fig. 1) [20–22]. A widely accepted model is that the hydrogen on the a-C of the growth chain acts as an electron-donating on the transition metal cationic, which is called the a-agostic effect.

The a-agostic effect significantly reduces the energy of the transition state during the monomer insertion, allowing

the chain growth to proceed smoothly. This is one of the reasons why metallocene catalysts have very high catalytic activity. Ziegler et al. [23] used DFT molecular simulation method to calculate and analyze the chain growth process of Cp2ZrCl2/MAO-catalyzed ethylene polymerization. The conclusion is consistent with the chain growth model con-sidering the a-agostic effect. McGuinness et al. [24] used Phillips catalyst to investigate the chain-growth polym-erization mechanism. Through analysis of low molecular weight oligomers/polymers formed during ethylene/a-olefin copolymerization with labeled monomers, it was shown that the isotopomer distribution was inconsistent with a metallacycle mechanism. Further analysis of poly-mer formed by copolymerization of labeled ethylene was used to rule out the Green–Rooney mechanism. The results supported the notion of chain growth via the Cossee–Arl-man process.

In this paper, the HOMO and LUMO orbital of dif-ferent metallocene compounds were calculated by DFT, and the correlations between the band gap size, catalytic activity and hydrogen release were analyzed. Among the listed metallocene compounds, the two compounds (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 were mainly analyzed because of the large difference in performance between them. The calculation of the transition state and the reaction barrier was mainly focused on the reasons why the olefin polymerization of the two metallocene cata-lysts followed the difference in the reaction mechanism. The current work was to provide theoretical basis for the discovery of the metallocene compound with high activity, low hydrogen release.

Fig. 1 Chain grow mechanism model of olefin polymerization

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2 Experiment and Computation

2.1 Materials

Metallocene compounds were purchased from Alfa Aesar, a Johnson Matthey Company, purity is 97%. MAO was pur-chased from Albemarle Corporation and the molar concen-tration is 1.0 mol/L. N-hexane and toluene were purchased from Lanzhou Petrochemical Company, the water content in which were removed using metal sodium. Silica gel was purchased from Grace Company in the United States for use in catalyst loading experiments where different gradient activation temperatures were required.

2.2 Catalyst Loading and Polymerization Experiments

5.0 g silica gel carrier was added to the reactor, and 10% solution of MAO toluene was added to keep the aluminum content at about 10%, and the reaction was carried out at 0–20 °C for 2–4 h. The compound was added to the reactor and the zirconium content was maintained at 0.2–0.4%. The Al/Zr molar ratio in the reaction solution was 200:1. The temperature of the reaction solution was heated to 40–70 °C, and the reaction was carried out at a constant temperature for 3–6 h. After the reaction, the temperature was gradually lowered to room temperature, and then the treated toluene solution and the hexane solution were washed several times, and the catalyst was dried by vacuum filtration. The catalyst was then sealed with an inert gas for the next polymeriza-tion reaction.

The catalyst was subjected to slurry polymerization using the US Zytel process. The reaction temperature was controlled at about 80 °C, the pressure was controlled at 1.0 MPa, and no hydrogen was added during the reaction. The final hydrogen content of the gaseous components was tested using an on-line chromatograph GC2000e by Agilent Corporation.

2.3 Computational Details

The structure of metallocene compounds was optimized by using the DMol3 module in the material studio package [25, 26]. A generalized gradient approximation (GGA) functional consisting of Becke’s exchange and correlation expression proposed by Perdew et al. [27], localized double numeri-cal basis sets with polarization functions (DND).In the SCF iterative calculation of the energy of the structures, orbital (HOMO/LUMO), frequency and population analysis was selected as properties parameter. The transition states (TS) were found by using the generalized linear synchronous and

quadratic (LST/QST) proposed by Govind from the origi-nal methods of Halgren and Lipscomb [28–31].For the TS calculations, the LST was first maximized and then energy minimization was performed in the direction of conjugate with the relevant reaction path. The TS approximation found in this method was used to find the secondary synchroniza-tion transmission (QST) maximization and perform another conjugate gradient minimization. The cycle repeats until a minimum point was located. The convergence criterion for transition state calculations used atolerance for all rms (root mean square) force values per atom as 0.25 eV/Å. The LST/QST methods using the same convergence criteria were used in searching for TS [32–35]. Calculating the possible vibra-tion frequencies of the TS was identified, showing that each discovered TS had only one imaginary frequency. In order to correct the energy of barrier, Fine was selected for the integration accuracy, GGA and RPBE were selected for the exchange of correlation functions, DNP was selected for the basis set and basis file chose 3.5.

3 Results and Discussion

3.1 The Formation of Active Centers

The metallocene compound undergoes an alkylation reac-tion with MAO to form a cationic active center [36, 37]. MAO first replaces the two halogens in the metallocene compounds with alkyl groups, and then one of the alkyl groups escapes the transition metal with the formation of negative ions, making the metallocene compound become e an electron-deficient active center (Fig. 2).

There are actually two states of the active center: one is a tight ion pair, where the transition metal has a strong effect on the aluminum of the MAO, and the other is a near free state of the metallocene cationic. Both types of active sites

Fig. 2 Formation of metallocene active center

2686 H. Wang et al.

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can catalyze the polymerization of olefins, but free metal-locene cationic has a higher catalytic activity. Chain growth occurs mainly in the free state of the active center, and met-allocene catalyst polymerization is very close to the single active center system. Thus, the next simulation is based on a similar active center structure.

3.2 Calculations of Orbital (HOMO/LUMO)

The HOMO/LUMO energy levels are collectively referred to as the front-line orbit, which refer to the Highest Occupied Molecular Orbital and the Lowest Unoccupied Molecular Orbital, respectively. The difference in the HOMO’s energy level and the LUMO’s energy level is called the band gap. The band gap can sometimes serve as a measure of the excit-ability of the molecule: the smaller the energy, the more easily a molecule’s electrons will be excited. The band gap formed by different metallocene compounds was calculated by the DMol3 module in the DFT (Table 1).

The band gap produced by the Cp2ZrCl2 compound is the largest of the listed compounds. The band gap produced by the new compounds is reduced to varying degrees when other functional groups are introduced on the cyclopen-tadienyl group. The band gap of the (n-BuCp)2ZrCl2 and (i-BuCp)2ZrCl2 compounds is the smallest of the listed com-pounds. According to the theory of front-line orbit, these two compounds are easily reacted. There is a certain differ-ence between the band gap produced by the cationic active center and the metallocene compound itself (Table 2).

Comparing Tables 1 and 2, the band gap formed by the cationic active centers is obviously smaller than the band gap formed by the corresponding compounds, indi-cating that the active centers are more reactive, which is the effect of MAO on the metallocene compounds. The orbital formed by the different cationic active sites were calculated by DFT (Fig. 3). In the listed HOMO/LUMO orbital, only the phases formed by the three compounds Cp2ZrMe+, (MeCp)2ZrMe+, and (1,3-Me,n-BuCp)2ZrMe+ have symmetry, and the phases of other compounds do not have symmetry. Only the methyl groups of the active centers of (EtCp)2ZrMe+, (n-PrCp)2ZrMe+, (1,3-n-Pr,n-BuCp)2ZrMe+ and (1,3-i-Bu,n-BuCp)2ZrMe+ show nega-tive phase of the wave function in the HOMO orbital, and the methyl groups carried by other cationic active centers show the positive phase of the wave function. In the LUMO orbital, only the methyl groups of the cationic active cent-ers of (n-BuCp)2ZrMe+, (1,3-n-Pr,n-BuCp)2ZrMe+and (1,3-i-Bu,n-BuCp)2ZrMe+ show the positive and negative phase of the wave function at the same time. The differ-ences in the positive and negative phases exhibited by the HOMO and LUMO orbital exhibit differences in the nature of the cationic active centers themselves. These differences Ta

ble

1 T

he b

and

gap

form

ed b

y di

ffere

nt m

etal

loce

ne c

ompo

unds

Ener

gy/H

aC

p 2Zr

Cl 2

(MeC

p)2Z

rCl 2

(EtC

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(n-P

rCp)

2ZrC

l 2(n

-BuC

p)2Z

rCl 2

(i-B

uCp)

2ZrC

l 2(1

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e,n-

BuC

p)2Z

rCl 2

(1,3

-Et,n

-B

uCp)

2ZrC

l 2(1

,3-n

-Pr,n

-BuC

p)2

ZrC

l 2(1

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Bu,

n-B

uCp)

2Zr

Cl 2

△E

0.11

520.

1137

0.11

350.

1084

0.10

690.

1069

0.11

210.

1105

0.11

070.

1092

2687Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

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are mainly manifested in the polymerization activity of the formed catalyst, and the hydrogen content released during the polymerization.

3.3 Polymerization Evaluation Experiment

The catalysts were prepared for the metallocene compounds listed under the same conditions, and the slurry polymeri-zation was carried out at a temperature of 83 °C and an ethylene pressure of 1.2 MPa. The catalyst polymerization activity, polymer molecular weight, and released hydrogen content were calculated (Table 3).

The catalyst formed by the compound (n-BuCp)2ZrCl2 has the highest polymerization activity, followed by com-pounds (i-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2. The listed metallocene catalysts generate hydrogen during the polymerization of olefins, which is also confirmed by Karol et al. [38].The metallocene catalysts formed by the (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 compounds have a little difference in olefin polymerization activity, but there is a large difference in hydrogen release. The difference between the two compounds lies in a methyl group, and the introduction of methyl group can produce a certain steric hindrance effect and electron effect, resulting in a change in the mechanism of catalyzed olefin polymerization.

3.4 Population Analysis

The chemical reactivity of a molecule is related to the charge distribution. The population is the distribution of electrons in an atomic orbital. The Mulliken [39] population analysis (Fig. 4) for (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 compounds were calculated by DMol3.

The introduction of a methyl group results in an increase in the charge density of the central metal atom of the com-pound and the two halogen ligands, indicating that the methyl group exhibits an electron-donating effect. The electronic effect presented by methyl is related to the dipole distance. The dipole magnitudes of the two compounds and the dipole moment vector in initial coordinates were calcu-lated (Table 4).

The introduction of methyl group leads to a decrease in the dipole magnitude of the (1,3-Me,n-BuCp)2ZrCl2 com-pound, indicating that the methyl group exhibits an electron-donating effect, which also confirms the results of the Mul-liken population analysis. This electron-donating effect of the methyl group is also the a-driving electron effect [40].

3.5 Productions of Hydrogen and Transition State

The transition states (Fig.  5) produced by the ethylene polymerization of the two metallocene catalysts were Ta

ble

2 T

he b

and

gap

of th

e ca

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c ac

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cent

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(EtC

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rMe+

(n-P

rCp)

2ZrM

e+(n

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p)2Z

rMe+

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2ZrM

e+(1

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e,n-

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rMe+

(1,3

-Et,n

-B

uCp)

2ZrM

e+(1

,3-n

-Pr,n

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2ZrM

e+(1

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n-B

uCp)

2ZrM

e+

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470.

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0.09

920.

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0.08

210.

0913

2688 H. Wang et al.

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simulated by DMol3 and the frequency of the transition state was calculated by vibration.

Only one imaginary frequency appears in the calcula-tion result of the vibration (Table 5). According to the tran-sition state theory, the imaginary frequency corresponds to the transition state (Fig. 5). The catalyst formed by the compound (n-BuCp)2ZrCl2 undergoes dehydrogenation in the transition state produced during the polymerization of ethylene. On the contrary, the transition state produced dur-ing the polymerization of ethylene in the catalyst formed by the compound (1,3-Me,n-Bu-Cp)2ZrCl2 has the formation of metal-hydrogen bonds. The difference between the two transition state structures lies in the polymerization mecha-nism. Kissin et al. [41] obtained the homopolymerization mechanism of metallocene ethylene by experiments, the active sites involved in catalysis are Cp2Zr+-polymer (where Cp represents any cyclopentadienyl π-ligand) metal cationic polymer (Fig. 6). Kissin believes that the precondition for the generation of metal-hydrogen bonds is the presence of hydrogen gas (step 3), and dehydrogenation occurs only when metal–hydrogen bonds exist together with unsaturated bonds (step 5). However, it is confirmed that hydrogen gas

still exists in the polymerization gas without hydrogen addi-tion through the homopolymerization experiment of metal-locene catalyst ethylene slurry, indicating that the dehydro-genation reaction have taken place. The results also confirm that the dehydrogenation reaction can take place without the need for hydrogen by simulating the transition state of metallocene olefin polymerization (Fig. 7). The same is true for the generation of metal-hydrogen bonds (Fig. 8).

The polymer ization mechanism (Fig.   7) of (n-BuCp)2Zr(C2H4)Me+ is carried out according to the “a-agostic mechanism of the ground state and transi-tion state”, and the polymerization mechanism (Fig. 8) of (1,3-Me,n-BuCp)2Zr(C2H4)Me+ is based on the “Green-Rooney mechanism”, that is, the migration of hydrogen negative ion.

3.6 The Energy Barrier of Transition State

The energy of transition state and barrier generated during the ethylene polymerization of the two metallocene cata-lysts was calculated (Tables 6, 7). In order to modify the energy of barrier, the energy of the transition state is defined

Fig. 3 HOMO/LUMO formed by different cationic active centers, where blue represents the positive phase of the wave function (state) and yel-low represents the negative phase of the wave function (state)

2689Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

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as ELST, and the energy of the optimized transition state is defined as EOpt. The energy barrier is modified based on the difference between the two energies (EOpt − ELST). For the cationic (n-BuCp)2ZrMe+, the transition state of the first ethylene insertion results in an optimized energy difference of − 40.26 kcal/mol, on the other hand, the transition energy resulting from the second ethylene insertion produces an optimized energy of − 34.15 kcal/mol. Similarly, for the cationic (1,3-Me,n-BuCp)2ZrMe+, the transition state of the first ethylene insertion results in an optimized energy differ-ence of − 40.13 kcal/mol and the second ethylene insertion produces an optimized energy of − 18.91 kcal/mol. Accord-ing to the above optimized energy, the energy is corrected (Table 8).

Based on the above energy barrier values, the transition states produced when the two metallocene catalysts are polymerized with ethylene can be obtained (Figs. 9, 10).

The metallocene catalyst formed by the (n-BuCp)2ZrMe+ compound in the polymerization of olefins shows that the first insertion energy barrier is lower than the secondary insertion energy barrier. Of course, this is related to the formation of the metallocene catalyst in the polymerization process to release hydrogen. The micro-content of hydrogen contributes to the improvement of the catalytic activity of the metallocene catalyst, but the enrichment of hydrogen will cause the metallocene catalyst to lose its activity. On the other hand, transition state forms a Zr–cyclopropyl bond, and in order to allow the next monomer to be coordinated to the Zr, the Zr–cyclopropyl group must be rotated, and this rearrangement requires a certain amount of energy, the barrier is even higher than the barrier of the monomer insertion chain and becomes the main barrier in the whole chain growth process. Of course, the olefin polymerization process of the catalyst formed by (1,3-Me,n-BuCp)2ZrMe+ compound is similar, except that the decay of the catalytic activity is not very significant. Although the dehydrogena-tion reaction does not take place directly in the transition state, but the metal–hydrogen bonds have been formed, and the accumulation of metal–hydrogen bonds inevitably leads to the generation of hydrogen, which has been confirmed by polymerization experiments. The presence of a small amount of hydrogen in the process is the main reason why the catalytic activity can be maintained, and it also confirms that the metallocene catalyst is more sensitive to hydrogen.

3.7 Polymer Double Bond Content Test

There are some differences in the reaction mechanism of homopolymerization and copolymerization of metallocene ethylene. The mechanism of copolymerization of metallo-cene ethylene/1-olefins was discussed by Kissin (Fig. 11). Unsaturated bonds are produced during the metallocene ethylene/1-olefin copolymerization (step 8, 10). The Ta

ble

3 S

lurr

y po

lym

eriz

atio

n re

sults

from

diff

eren

t met

allo

cene

cat

alys

ts

Con

tent

Cp 2

ZrC

l 2(M

eCp)

2ZrC

l 2(E

tCp)

2ZrC

l 2(n

-PrC

p)2Z

rCl 2

(n-B

uCp)

2ZrC

l 2(i-

BuC

p)2Z

rCl 2

(1,3

-Me,

n-B

uCp)

2ZrC

l 2(1

,3-E

t,n-

BuC

p)2Z

rCl 2

(1,3

-n-P

r,n-

BuC

p)2Z

rCl 2

(1,3

-i-B

u,n-

BuC

p)2Z

rCl 2

Act

ivat

ion

[Kg·

(mol

 h)−

1 × 10

− 4 ]

1.20

2.64

3.12

2.08

5.91

5.03

4.13

1.61

1.18

2.24

Hyd

roge

n (p

pm)

100

122

115

5726

222

032

8410

279

Poly

mer

Mw

× 10

−4

31.2

24.6

21.7

42.0

18.2

20.7

51.5

39.4

30.1

40.8

2690 H. Wang et al.

1 3

unsaturated bond in the copolymer was tested by Fourier transform infrared spectroscopy (FT-IR) (Table 9).

The content of vinylidene in metallocene polyethyl-ene is significantly higher than that of vinyl. The content of unsaturated bond in ethylene polymer formed by the (n-BuCp)2ZrCl2 compound is obviously higher than that of the (1,3-Me,n-BuCp)2ZrCl2 compound. Of course, the unsaturated bond values in the two metallocene copolymers are still high by comparing the metallocene copolymer 1018CA produced by Exxon. The presence of unsaturated bonds will exacerbate the defect in the processing perfor-mance of metallocene polymers. Therefore, the search for

suitable ligand structure of metallocene compounds will be a key point for solving the technical problems of metallocene catalyst development.

Fig. 4 Mulliken population analysis of two compounds, where the green ball represents chlorine atom, the brown ball represents zirconium atom, the white ball represents hydrogen atom, and the black ball repre-sents carbon atom

Table 4 Dipole magnitudes and vector coordinates produced by the two compounds

Compounds X/au Y/au Z/au Dipole magni-tude/au

(n-BuCp)2ZrCl2 0.695 0.707 1.784 2.041(1,3-Me,n-BuCp)2ZrCl2 0.879 0.894 1.0608 1.642

Fig. 5 The structures of transition state

Table 5 The vibration of transition states

(n-Bu-Cp)2Zr(C2H4)Me+ (1,3-Me,n-BuCp)2Zr(C2H4)Me+

Frequency (1/cm) Intensity (km/mol)

Frequency (1/cm) Intensity (km/mol)

− 5075.87 51.40 − 1226.030 85.5852.42 1.79 37.24 2.4870.06 0.30 39.88 0.7189.38 0.09 63.09 5.01

100.07 2.61 77.13 4.42115.61 0.63 84.76 2.35128.55 1.32 95.82 6.20143.22 0.45 106.14 10.39146.55 1.93 118.01 0.29152.44 2.29 132.65 1.34

2691Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

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Fig. 6 Reaction mechanism of metallocene ethylene homopo-lymerization

Fig. 7 Transition state pathways formed by (n-BuCp)2ZrCl2 compound

2692 H. Wang et al.

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Fig. 8 Transition state path-ways formed by (1,3-Me,n-BuCp)2ZrCl2 compound

Table 6 Calculation of transition state energy generated by the first insertion

Energy (kcal/mol) (n-BuCp)2Zr(C2H4)Me+ (1,3-Me,n-BuCp)2Zr(C2H4)Me+

Energy of reactant − 2736694.34 − 2786062.42Energy of product − 2736726.54 − 2786081.26Energy of transition state − 2736609.75 − 2785993.72Energy of optimized transition state − 2736569.49 − 2786033.85Location of transition state 0.81 0.70Energy of reaction − 22.20 − 18.84Energy of barrier 55.34 68.70

Table 7 Calculation of transition state energy generated by the second insertion

Energy (kcal/mol) (n-BuCp)2Zr(C2H4)2Me+ (1,3-Me,n-BuCp)2Zr(C2H4)2Me+

Energy of reactant − 2787563.28 − 2835408.56Energy of product − 2787620.41 − 2835421.21Energy of transition state − 2787435.28 − 2835360.93Energy of optimized transition state − 2787469.43 − 2835379.84Location of transition state 0.64 0.52Energy of reaction − 12.31 − 24.86Energy of barrier 77.59 52.74

2693Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

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4 Conclusion

The band gap formed by different metallocene compounds and their cationic active centers were investigated by DFT. Metallocene catalysts formed by the two compounds (n-BuCp)2ZrCl2 and (1,3-Me,n-BuCp)2ZrCl2 exhibited large differences in ethylene polymerization. However, they had only one methyl group difference in structure. The results showed that the methyl group exhibits elec-tron-donating effect by analyzing the Mulliken population and dipole magnitude. The metallocene catalyst formed by

the compound (n-BuCp)2ZrCl2 with ethylene polymeri-zation proceed according to the a-agostic mechanism of the ground state and transition state. The dehydrogenation occurred in the transition state structure, which was the main reason why the energy barrier of the second insertion reaction was higher than that of the first insertion. How-ever, the metallocene catalyst formed by the compound (1,3-Me,n-BuCp)2ZrCl2 and ethylene polymerization proceed according to the Green–Rooney mechanism. The metal–hydrogen bond was generated in the transition state structure, which was one of the reasons for the presence

Table 8 The result of energy barrier correction

Energy (kcal/mol)

(n-BuCp)2Zr(C2H4)Me+ (1,3-Me,n-BuCp)2Zr(C2H4)2Me+

(n-BuCp)2Zr(C2H4)2Me+

(1,3-Me,n-BuCp)2Zr(C2H4)2Me+

Energy of barrier

15.08 28.57 43.44 33.83

Fig. 9 Transition state formed by the polymerization of cationic (n-BuCp)2ZrMe+ and ethylene

Fig. 10 Transition state formed by the polymerization of cati-onic (1,3-Me,n-BuCp)2ZrMe+ and ethylene

2694 H. Wang et al.

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of the unsaturated bond in the polymer chain. Therefore, this provided a theoretical basis for studying the olefin polymerization mechanism of metallocene catalysts. The next task is mainly to find a metallocene catalyst with high activity and low hydrogen emission.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Projects 21471071, 21431002).

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Fig. 11 Mechanism of metal-locene ethylene/1-alkene copo-lymerization

Table 9 Double bond content test result in ethylene copolymerization

0.2 g of the sample is weighed and heated at 155 °C for 3 min, film is gained under a pressure of 2.0 MPa. The film is detected by FT-IR and the vinyl content is calculated based on the vinyl peak (909 cm−1) [42]

Polymer RCH=CH2 rela-tive content

R1R2C=CH2 relative content

(n-BuCp)2ZrCl2/mPE 0.07–0.13 0.21–0.27(1,3-Me,n-BuCp)2ZrCl2/mPE 0.04–0.07 0.10–0.15Exxon1018CA 0.03–0.04 0.06–0.13

2695Factors Affecting Dehydrogenation and Catalytic Activity: Methyl Substituent

1 3

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Affiliations

Hai Wang1,2 · Peng Zhang2 · Panpan Zhou1 · Renwei Xu2 · Yu Tang1

1 Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China

2 Lanzhou Petrochemical Research Center, Petrochina, Lanzhou 730060, People’s Republic of China