Adsorption of Titanium Tetrachloride on Magnesium Dichloride Clusters

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article Cite This: ACS Omega 2018, 3, 9921−9928

http://pubs.acs.org/journal/acsodf

Adsorption of Titanium Tetrachloride on Magnesium Dichloride Clusters Peter Zorve and Mikko Linnolahti* Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

Downloaded via 91.200.81.28 on August 26, 2018 at 08:02:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Magnesium dichloride and titanium tetrachloride are key components in heterogeneous Ziegler−Natta olefin polymerization catalysis. We have determined the preferred binding modes of titanium tetrachloride on magnesium dichloride clusters at the M06-2X level of theory, thus accounting for dispersion. A systematic study was carried out to locate the lowest energy isomers of (MgCl2)n(TiCl4)m complexes. Altogether ca. 1000 complexes with n ranging from 1 to 19 and m from 1 to 13 were studied. In line with the previous literature, the results consistently show that adsorption of TiCl4 onto MgCl2 preferably leads to octahedral six-coordination for both Mg and Ti atoms. This can be achieved by mononuclear binding of TiCl4 and binuclear binding of Ti2Cl8 on (110) and (104) surfaces of MgCl2, respectively. The preferred octahedral six-coordination is also achieved by binding of TiCl4 on defect sites, of which the most striking example is trinuclear binding mode at corners of the clusters. Overall, the results highlight the relevance of multinuclear binding of TiCl4 on MgCl2.

■

INTRODUCTION

Little is known about the reactions at atomic-level detail; there is not even consensus on how TiCl4 adsorbs on the MgCl2 support in the absence of other catalyst components. The binding modes have been widely studied by quantum chemical calculations, though, with varying results and conclusions.10,11,30−36 Four commonly discussed binding modes are illustrated in Figure 1C. Adopting a previously defined naming convention, which specifies the coordination number of Ti followed by the MgCl2 surface, these are 4-110, 5-104, 6-110, and 6-104.7 In addition, 5-110 mode has been proposed but later questioned.11,37 Spectroscopic experiments indicate octahedral six-coordination of Ti atoms,21,38 which also rules out the 4-110 and 5-104 modes. Alongside with 6110 and 6-104, defects on the surfaces generate environments allowing for the octahedral six-coordination.36,39−43 Octahedral six-coordination of Ti has been widely considered to signal mononuclear 6-110 binding or mononuclear binding on a defect site of MgCl2.40 The mononuclear binding is generally concluded to be favored over binuclear octahedral 6-104 binding based on the weaker binding energy of the latter. However, as has been shown recently, this is a biased conclusion, omitting the higher reactivity of the (110) surface.7 In a systematic computational study reported herein, we determine the preferred binding modes of TiCl4 on selected (MgCl 2 ) n clusters up to n = 19 based on thermodynamic stabilities of the products rather than binding

Heterogeneous Ziegler−Natta (ZN) catalysis is currently the main technique for industrial production of polyolefins.1,2 The current state-of-the-art ZN catalysts are composed of MgCl2, TiCl4, and a Lewis base (internal donor).3 Contacting the ternary mixture with a trialkyl aluminum cocatalyst and another and Lewis base (external donor) yields the catalytically active species with reduced oxidation states of Ti.2,4 Each of these components play a significant role in the polymerization process, though their precise functions are not fully understood and the structure of the catalyst is ill-defined because of a high complexity of the overall system.1−3 The catalytic reactions take place on the surfaces of the MgCl2 support.5,6 The bulk MgCl2 is a crystalline layered material, which exists in three polymorphs: rhombohedral αMgCl2, hexagonally closed-packed β-MgCl2, and rotationally distorted δ-MgCl2. The layers are held together by dispersion and are composed of octahedral six-coordinated Mg atoms bound to three-coordinate Cl atoms (Figure 1A).7−10 Lateral cut of the MgCl2 sheets exposes catalytically relevant (104) and (110) surfaces with five- and four-coordinate Mg atoms, respectively (Figure 1B),11 which adsorb and coadsorb TiCl4, aluminum alkyls, and Lewis bases.12−20 The (110) surface is less stable than the (104) surface because of the lower coordination numbers of surface Mg atoms,18,19,21 but adsorption of other catalyst components may reverse the stability order in favor of (110).22,23 Reactions taking place on the surface eventually lead to the formation of active sites for olefin polymerization.7,19,24−29 © 2018 American Chemical Society

Received: August 3, 2018 Accepted: August 14, 2018 Published: August 24, 2018 9921

DOI: 10.1021/acsomega.8b01878 ACS Omega 2018, 3, 9921−9928

ACS Omega

Article

Figure 1. (A) Structure of the MgCl2 layer, (B) (110) and (104) surfaces, and (C) binding modes of TiCl4. Mg, Cl, and Ti atoms are shown in yellow, green, and white, respectively.

energies. As it turns out, multinuclear binding modes of TiCl4 are highly relevant on the (104) surface and possibly on defect sites of MgCl2 as well.

Figure 2. Optimized lowest energy structures of (MgCl2)n−(TiCl4)m, where n = 1−4 and m = 1−8. Mg, Cl, and Ti atoms are shown in yellow, green, and white, respectively.

RESULTS AND DISCUSSION Binding of TiCl4 on (MgCl2)n Clusters (n = 1−4). We begin by a systematic and thorough exploration of the binding of TiCl4 on (MgCl2)n, where n = 1−4, to form (MgCl2)n− (TiCl4)m complexes. The (MgCl2)n clusters were obtained from a previous work by Luhtanen, et al.8 Here and in all that follows, all theoretically possible structures were initially constructed for each cluster followed by structure optimization. This resulted in optimization of 31 (MgCl2)−(TiCl4)m complexes, where m = 1−5, 63 (MgCl2)2−(TiCl4)m complexes, where m = 1−6, 144 (MgCl2)3−(TiCl4)m complexes, where m = 1−8, and 303 (MgCl2)4−(TiCl4)m complexes, where m = 1−8. The located lowest energy structure of each combination of n and m is shown in Figure 2. We use the nomenclature (n,m) to specify the number of MgCl2 (n) and TiCl4 (m) units in the presentation and discussion of the results. The driving force for the reactions is the tendency of both Mg and Ti to attain octahedral six-coordination. In the case of monomeric MgCl2, this is achieved for Mg, as well as for each Ti, upon binding of five TiCl4 molecules (1,5), thus leaving no further reactive sites. The (MgCl2)2 dimer attains octahedral six-coordination for the Mg atoms at (2,6), the trimer at (3,8), and the tetramer at (4,6). Electronic energies and Gibbs energies for the reaction (MgCl2)n + (TiCl4)m → (MgCl2)n−(TiCl4)m calculated by the M06-2X/TZVP method44,45 are given in Table 1. Energies

improve along gradual saturation of the (MgCl2)n clusters by the TiCl4 molecules. However, Gibbs energies show that entropy plays a major role in the process, preventing full saturation of the small clusters. For the monomer, dimer, and trimer, the minimum in Gibbs energy was found at m = 2, resulting in chain structures with four-coordinate magnesiums, terminated by five-coordinate titaniums at each end. The tetramer shows a distinctly different behavior. It finds the minimum in Gibbs energy at m = 6, where each Mg atom and four out of six Ti atoms obtain octahedral six-coordination in resemblance to the basal MgCl2 surface. To demonstrate the importance of dispersion in arriving at these conclusions, we report the reaction Gibbs energies in Table 1 also by the popular PBE0 method46,47 lacking description for dispersive interactions. In line with previous studies,7,48 PBE0 predicts much weaker binding between magnesium dichloride and titanium tetrachloride, particularly at large values of m. Consequently, the PBE0 method ends up favoring the (4,2) structure for the tetramer with two fourcoordinate magnesiums, in resemblance to the (110) surface of MgCl2. Binding of TiCl4 on (MgCl2)7. On the basis of the above results, considering dispersion by the M06-2X method, one could also envisage that MgCl2 clusters larger than the tetramer favor full coverage by TiCl4 to obtain octahedral sixcoordination for Mg, which is also possible for the Ti atoms. We first show that this is the case for the (MgCl2)7 heptamer,

■

9922


ACS Omega

Article

Table 1. Energies and Gibbs Energies (kJ mol−1) for the Reaction (MgCl2)n + (TiCl4)m → (MgCl2)n−(TiCl4)m n

m

ΔrE/M06-2X

ΔrG/M06-2X

ΔrG/PBE0

1 1 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4

1 2 3 4 5 1 2 3 4 5 6 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

−73.6 −153.8 −186.5 −259.5 −315.7 −82.2 −163.6 −196.8 −273.1 −323.0 −363.9 −81.3 −162.4 −195.7 −268.0 −316.6 −361.6 −436.8 −492.2 −75.1 −147.0 −231.0 −311.9 −342.0 −450.2 −497.2 −542.1

−32.4 −64.8 −44.8 −30.4 −24.2 −32.3 −66.3 −50.1 −39.9 −27.6 −9.3 −33.4 −67.0 −46.9 −38.4 −32.2 −9.5 −6.7 −0.3 −24.3 −51.2 −58.9 −71.2 −78.3 −87.8 −79.1 −65.2

−18.0 −34.4 −7.9 94.8 144.4 −17.1 −33.3 −7.6 79.0 133.3 169.9 −16.4 −32.9 −7.0 83.1 108.1 189.0 257.1 309.3 −7.7 −16.1 30.0 73.1 102.0 138.8 177.5 221.5

which is the simplest hexagonal structure with one saturated octahedral magnesium and six unsaturated surface magnesiums. The latter are the reactive sites, two of them having fivecoordination and four four-coordination, similar to the (104) and (110) MgCl2 surfaces, respectively. Altogether 141 (7,m) complexes, where m = 1−9, were optimized. The located lowest energy structure of each combination of n and m is shown in Figure 3 with energies and Gibbs energies for the reaction (MgCl2)7 + (TiCl4)m → (MgCl2)7−(TiCl4)m given in Table 2. The calculated Gibbs energies show that the reactions are much more exergonic for the heptamer than for the monomer to the tetramer discussed above. The first TiCl4 molecule makes a bidentate binding to the more reactive four-coordinate surface magnesiums, and upon reaction of three TiCl4 molecules, all surface magnesiums reach five-coordination. The reaction continues further until each magnesium attains octahedral six-coordination, which is reached at m = 7. However, because of the preference of also titanium for attaining octahedral six-coordination, the reaction continues further until (7,8), where the six-coordination of each Ti is obtained by trinuclear binding of TiCl4. We are not aware of any previous reports of trinuclear TiCl4 on MgCl2, possibly because the calculations have typically dealt with idealized (104) and (110) MgCl2 surfaces. Formation at a corner suggests that the trinuclear binding mode may be relevant at defect sites of the MgCl2 surface. Extension to Larger (MgCl2)n Clusters (n = 14, 19). As a first step to evaluate how the above results on small MgCl2 clusters translate to MgCl2 surfaces, we turn the attention to two isomers of (MgCl2)14, which are adopted from a previous work, including their namings.23 The pipe and diamond

Figure 3. Optimized lowest energy structures of (MgCl2)7−(TiCl4)m, where m = 1−9. Mg, Cl, and Ti atoms are shown in yellow, green, and white, respectively.

Table 2. Energies and Gibbs Energies (kJ mol−1) for the Reaction (MgCl2)7 + (TiCl4)m → (MgCl2)7−(TiCl4)m m

ΔrE/M06-2X

ΔrG/M06-2X

1 2 3 4 5 6 7 8 9

−131.3 −264.0 −336.8 −409.4 −478.8 −566.9 −635.9 −723.8 −729.3

−65.7 −139.8 −161.2 −172.9 −185.2 −196.7 −203.6 −217.3 −182.8

isomers of (MgCl2)14 provide simplified models for the (110) and (104) surfaces of MgCl2, respectively (Figure 4 top). Both models contain 4 octahedral six-coordinated Mg atoms that do not participate in reaction with TiCl4 and 10 reactive surface Mg atoms, of which 6 are four-coordinate and 4 are fivecoordinate in the pipe model, whereas 6 are five-coordinate and 4 are four-coordinate in the diamond model. The pipe isomer is 23.0 kJ mol−1 lower in energy, indicating that the adjacent five-coordinate magnesiums introduce certain strain to the diamond isomer because of its small size. For comparison, we, therefore, also include a larger model for the representation of the (104) surfaces, hexagonal (MgCl2)19 9923


ACS Omega

Article

Figure 4. Optimized lowest energy structures of (MgCl2)14− (TiCl4)m, where m = 8−11, for pipe and diamond models. Mg, Cl, and Ti atoms are shown in yellow, green, and white, respectively.

cluster, which has 7 octahedral six-coordinated magnesiums and 12 reactive surface magnesiums, of which 8 are fivecoordinate (Figure 5 top). The (MgCl2)19 cluster is 12.2 kJ mol−1 per MgCl2 unit lower in energy than the diamond isomer of (MgCl2)14, indicating reduced strain, and 10.6 kJ mol−1 per MgCl2 unit lower in energy than the pipe isomer of (MgCl2)14 because of the higher average coordination numbers. Because of thousands of possible configurations for TiCl4 binding on these larger clusters, we limit the number of bound TiCl4 molecules to the domain of full saturation of surfaces, that is, m = 8−11 for (MgCl2)14 and m = 10−13 for (MgCl2)19. This narrows down the studied (14,m) complexes to 204 and (19,m) complexes to 147. The located lowest energy structures are illustrated in Figures 4 and 5 with energies and Gibbs energies for the reaction (MgCl2)n + (TiCl4)m → (MgCl2)n− (TiCl4)m given in Table 3. In each case, TiCl4 adsorption continues until all magnesium atoms have reached octahedral six-coordination, which takes place at m = 10 and m = 12 for n = 14 and n = 19, respectively. Comparison of pipe and diamond isomers of n = 14 shows that the latter binds TiCl4 much more strongly (−337.9 kJ mol−1 vs −234.7 kJ mol−1). One could argue that this is because the pipe isomer is more stable and hence less reactive than the diamond isomer, but the stability difference is only a minor contribution as is revealed by comparison of the (14,10)

Figure 5. Optimized lowest energy structures of (MgCl2)19− (TiCl4)m, where m = 10−13. Mg, Cl, and Ti atoms are shown in yellow, green, and white, respectively.

isomers (Figure 6), where the diamond isomer is favored by 84.4 kJ mol−1 in Gibbs energy. Five binding modes of TiCl4 can be identified for the two isomers of (14,10), as illustrated in Figure 6. Mononuclear binding on the (110) surface (A) and binuclear binding on the (104) surface (B) are the dominant binding modes resulting in the preferred octahedral six-coordination of the Ti atoms. The pipe isomer features a mononuclear binding mode C, which is analogous to the 5-104 binding mode (Figure 1C). Yet another binding mode (D) is found at a corner resembling defect site. With two bridging chlorides, D is analogous to the 4-110 9924


ACS Omega

Article

Table 3. Energies and Gibbs Energies (kJ mol−1) for the Reaction (MgCl2)n + (TiCl4)m → (MgCl2)n−(TiCl4)m, Where n = 14, 19 n 14 14 14 14 14 14 14 14 19 19 19 19

pipe pipe pipe pipe diamond diamond diamond diamond

m

ΔrE/M06-2X

ΔrG/M06-2X

ΔrG-c/M06-2Xa

8 9 10 11 8 9 10 11 10 11 12 13

−656.7 −772.4 −845.1 −845.6 −804.1 −873.3 −980.9 −965.2 −919.1 −983.8 −1049.4 −1026.1

−215.4 −232.7 −234.7 −185.5 −287.3 −305.4 −337.9 −285.9 −289.8 −299.9 −315.0 −244.0

−350.4 −401.8 −425.3 −389.3 −447.2 −481.0 −538.8 −495.2 −484.6 −511.3 −541.5 −486.2

saturate the diamond isomer of n = 14 (Table 3). As Figure 7 illustrates, (19,12) cannot attain the optimal octahedral six-

a Correction to condensed phase by multiplication of the TΔS term by 2/3.

Figure 7. Top: (MgCl2)19−(TiCl4)12 with identification of the binding modes of TiCl4. Bottom: Removal of the two terminal chlorines from (MgCl2)19 yields optimal binding modes of TiCl4 for each site but has a wrong overall stoichiometry.

coordination for each Ti because it would lead to a wrong overall stoichiometry. Consequently, two of the adsorbed TiCl4 molecules are forced to adopt monodentate binding of TiCl4, leading to unfavorable four-coordinate surface titaniums. Figure 8 summarizes the relevant binding modes of TiCl4 on MgCl2 based on this work. The primary binding modes on pristine (110) and (104) surfaces are mononuclear A and binuclear B, respectively, both binding modes providing Ti atoms with octahedral six-coordination closely resembling the structure of the basal MgCl2 layer. Octahedral six-coordination is also attained by trinuclear binding of TiCl4 at a corner site having four-coordinate Mg and by mononuclear binding on a (104) surface site having an extra terminal Cl. Other binding modes (C, D, and F) can only be present at environments not allowing for octahedral six-coordination because of the immediate surroundings. The calculated reaction Gibbs energy (−337.9 kJ mol−1, Table 3) to form the diamond isomer of (14,10), which features octahedral six-coordination of each Ti with domination of the binuclear binding mode (B), corresponds to average adsorption Gibbs energy of −33.8 kJ mol−1 per TiCl4. For (19,12), the average adsorption Gibbs energy decreases to −26.3 kJ mol−1 per TiCl4. The drop is contributed by two factors as follows: (1) the presence of unsaturated Ti in (19,12) and (2) higher strain and hence higher reactivity of (MgCl2)14. A logical assumption from the latter factor is that beyond n = 19, adsorption Gibbs energies per TiCl4 continue

Figure 6. Comparison of pipe (top) and diamond (bottom) isomers of (MgCl2)14−(TiCl4)10 with labeling of the (104) and (110) surfaces and identification of the binding modes of TiCl4 (A−E).

binding mode but is stabilized by the extra terminal Cl at the corner of the pristine (MgCl2)14 cluster, thus reaching fivecoordination. Similarly, in the diamond isomer, the terminal Cl increases the coordination number of the mononuclear TiCl4 at the defect site from five to six (E), ending up with the optimal octahedral six-coordination for each titanium. Note that E is analogous to 5-104 on the ideal (104) surface. Octahedral six-coordination is the primary reason for the diamond isomer of (14,10) being thermodynamically favored over the pipe isomer. Moving to n = 19, the minimum in Gibbs energy is located at (19,12) with octahedral six-coordination of each magnesium. Adsorption of 12 TiCl4 molecules to saturate n = 19 is less exergonic than the adsorption of 10 TiCl4 molecules to 9925


ACS Omega

Article

size, indicating that the adsorption of TiCl4 is not strong enough to affect the growth of MgCl2 crystallites. Table 4. Relative Stabilities (ΔG/n, kJ mol−1) of (MgCl2)n− (TiCl4)m Products Formed from n/m = 1:1 Stoichiometry of the Reactants n 1 2 3 4 7 14 pipe 14 diamond 19

reaction

ΔG/n

MgCl2 + TiCl4 → (MgCl2)−(TiCl4) (MgCl2)2 + (TiCl4)2 → (MgCl2)2−(TiCl4)2 (MgCl2)3 + (TiCl4)3 → (MgCl2)3−(TiCl4)2 + TiCl4 (MgCl2)4 + (TiCl4)4 → (MgCl2)4−(TiCl4)4 (MgCl2)7 + (TiCl4)7 → (MgCl2)7−(TiCl4)7 (MgCl2)14 + (TiCl4)14 → (MgCl2)14−(TiCl4)10 + 4TiCl4 (MgCl2)14 + (TiCl4)14 → (MgCl2)14−(TiCl4)10 + 4TiCl4 (MgCl2)19 + (TiCl4)19 → (MgCl2)19−(TiCl4)12 + 7TiCl4

137.6 66.5 53.7 45.1 24.2 8.7 2.7 0.0

The same conclusion can be drawn from infinitely large periodic models based on previous work, where both 6-104 and 6-110 surfaces lie above the fully saturated basal MgCl2 layer in Gibbs energy (but not in electronic energy),7 and thus, thermodynamics prefers MgCl2 to grow in any direction rather than adsorbed TiCl4 cutting the growth. The situation becomes markedly different in the presence of internal donors, which can bind MgCl2 much stronger and which can hence be used for controlling the shape and size of the (MgCl2)n− adsorbate complexes.54,55

Figure 8. Relevant binding modes of TiCl4 on MgCl2. A and B are mononuclear and binuclear binding on the pristine (110) and (104) surfaces, respectively. E is a bidentate TiCl4 binding resulting in the optimal six-coordination of Ti. C, D, and F are TiCl4 bindings at MgCl2 environments that do not allow octahedral six-coordination of Ti.

■

CONCLUSIONS Using the M06-2X/TZVP method and thus accounting for dispersion, we have carried out a systematic computational study on (MgCl2)n(TiCl4)m clusters (n = 1−19, m = 1−13) to locate the lowest energy structures of the clusters as a function of n and m and to determine the preferred binding modes of titanium tetrachloride on magnesium dichloride. With the exception of the smallest (MgCl2)n clusters (n = 1−3), (TiCl4)m adsorbs on (MgCl2)n in such a way that each Mg atom attains octahedral six-coordination, which is in line with previous experiments and computations. Optimally, the adsorption simultaneously results in octahedral six-coordination of each Ti atom so that the (MgCl2)n(TiCl4)m clusters resemble the basal MgCl2 surface. However, the optimal scenario is not always possible because of the immediate surroundings of the adsorption site and stoichiometric requirements, leading to coordination numbers of less than six under such circumstances. The results indicate that multinuclear binding modes of TiCl4 are more relevant than those that have been considered previously. This becomes evident from the overall structural characteristics of the lowest energy (MgCl2)n(TiCl4)m clusters, which are abundant in binuclear binding mode on sites resembling the (104) surface and even feature trinuclear binding mode at a corner resembling defect site. The preference for multinuclear binding is further supported by comparison of the relative stabilities of isomers featuring different sites.

decreasing as a function of n. The assumption can be confirmed by comparison to a previous study employing infinitely large periodic models, where the calculations have been carried out at the same level of theory (M06-2X/TZVP) and where, though, the TZVP basis set was slightly different because it was optimized for periodic calculations.7 Full coverage of the infinite pristine (104) surface by binuclear TiCl4 adsorption (6-104) gives adsorption Gibbs energy of −16.0 kJ mol−1 per TiCl4, which is roughly the value toward which the (n,m) clusters dominant in the (104) surface converge as a function of n. Further, to provide an estimate for the Gibbs energies of the reaction at condensed phase, we use a procedure, justified and employed previously, of multiplication of the TΔS term by 2/3 to correct for solvation entropy.49−53 The results are given in the right column of Table 3 (ΔrG-c/M06-2X). The correction does not affect the preferred (n,m) configuration, but it systematically increases the strength of TiCl4 adsorptionon average by 19 kJ mol−1 per TiCl4. A further question of interest is if TiCl4 binds strong enough to cut the growth of MgCl2 and thus affect the preferred size of the (n,m) clusters. In this regard, n = 7 versus n = 14 provides a useful comparison. The energy of the reaction (MgCl2)7(TiCl4)8 + (MgCl2)7(TiCl4)8 → (MgCl2)14(TiCl4)10 (diamond) + 6TiCl4 is 14.5 kJ mol−1, thus favoring (7,8) over (14,10). However, entropy reverses the reaction clearly in favor of (14,10) with ΔrG = −274.0 kJ mol−1 (or ΔrG-c = −187.3 kJ mol−1 with the above correction for solvation entropy). An alternative way to shed light on this is to consider n/m = 1:1 stoichiometric compositions as a function of size (Table 4). The stabilities continue improving as a function of

■

THEORETICAL METHODS All structures were fully optimized by the M06-2X metahybrid GGA functional of the Minnesota 06 series,44 in combination 9926


ACS Omega

Article

with TZVP basis set by Ahlrichs and coworkers.45 The M06 functionals have turned out as the method of choice for systems involving dispersive interactions arising from metals bridged by halide and alkyl groups,48,56−58 and we have previously used the method for describing MgCl2- and TiCl4containing systems analogous to the systems reported here.7,59,60 Vibrational frequencies were calculated by the harmonic approximation to verify the structures as true minima in the potential energy surface and to obtain Gibbs energies, which were calculated at T = 298 K and p = 1 atm. Condensed phase Gibbs energies were estimated from the reported gasphase calculations by multiplication of the TΔS term of G = H − TΔS by 2/3 as suggested and employed in the previous literature.49−53 All calculations were carried out by Gaussian 09.61

■

(8) Luhtanen, T. N. P.; Linnolahti, M.; Laine, A.; Pakkanen, T. A. Structural Characteristics of Small Magnesium Dichloride Clusters: A Systematic Theoretical Study. J. Phys. Chem. B 2004, 108, 3989− 3995. (9) Bassi, I. W.; Polato, F.; Calcaterra, M.; Bart, J. C. J. A New Layer Structure of MgCl2 with Hexagonal Close Packing of the Chlorine Atoms. Z. für Kristallogr. - Cryst. Mater. 1982, 159, 297−302. (10) Lin, J.; Catlow, C. R. A. Quantum Mechanical Study of Ti/ MgCl2-Supported Ziegler-Natta Catalysts. J. Catal. 1995, 157, 145− 152. (11) Monaco, G.; Toto, M.; Guerra, G.; Corradini, P.; Cavallo, L. Geometry and Stability of Titanium Chloride Species Adsorbed on the (100) and (110) Cuts of the MgCl2Support of the Heterogeneous Ziegler−Natta Catalysts. Macromolecules 2000, 33, 8953−8962. (12) Ferreira, M. L.; Castellani, N. J.; Damiani, D. E.; Juan, A. The Co-adsorption of tetramethylpiperidine and TiCl4 on β-MgCl2. A theoretical study of a Ziegler-Natta pre-catalyst. J. Mol. Catal. A: Chem. 1997, 122, 25−37. (13) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. Theoretical investigations on Ziegler-Natta catalysis: Alkylation of the TiCl4 catalyst. J. Mol. Catal. A: Chem. 1997, 120, 143−147. (14) Seth, M.; Ziegler, T. Polymerization Properties of a Heterogeneous Ziegler−Natta Catalyst Modified by a Base: A Theoretical Study. Macromolecules 2003, 36, 6613−6623. (15) Taniike, T.; Terano, M. Density Functional Calculations for Electronic and Steric Effects of Ethyl Benzoate on Various Ti Species in MgCl2-Supported Ziegler-Natta Catalysts. Macromolecular Symposia; Wiley Online Library, 2007; Vol. 260, pp 98−106. (16) Cavallo, L.; Del Piero, S.; Ducéré, J.-M.; Fedele, R.; Melchior, A.; Morini, G.; Piemontesi, F.; Tolazzi, M. Key Interactions in Heterogeneous Ziegler−Natta Catalytic Systems: Structure and Energetics of TiCl4−Lewis Base Complexes. J. Phys. Chem. C 2007, 111, 4412−4419. (17) Taniike, T.; Terano, M. A Density Functional Study on the Influence of the Molecular Flexibility of Donors on the Insertion Barrier and Stereoselectivity of Ziegler-Natta Propylene Polymerization. Macromol. Chem. Phys. 2009, 210, 2188−2193. (18) Taniike, T.; Terano, M. Coadsorption Model for First-Principle Description of Roles of Donors in Heterogeneous Ziegler-Natta Propylene Polymerization. J. Catal. 2012, 293, 39−50. (19) Bazhenov, A. S.; Denifl, P.; Leinonen, T.; Pakkanen, A.; Linnolahti, M.; Pakkanen, T. A. Modeling Coadsorption of Titanium Tetrachloride and Bidentate Electron Donors on Magnesium Dichloride Support Surfaces. J. Phys. Chem. C 2014, 118, 27878− 27883. (20) Kumawat, J.; Gupta, V. K.; Vanka, K. Effect of Donors on the Activation Mechanism in Ziegler-Natta Catalysis: A Computational Study. ChemCatChem 2016, 8, 1809−1818. (21) Brambilla, L.; Zerbi, G.; Piemontesi, F.; Nascetti, S.; Morini, G. Structure of MgCl2-TiCl4 complex in co-milled Ziegler-Natta catalyst precursors with different TiCl4 content: Experimental and theoretical vibrational spectra. J. Mol. Catal. A: Chem. 2007, 263, 103−111. (22) Credendino, R.; Pater, J. T. M.; Correa, A.; Morini, G.; Cavallo, L. Thermodynamics of Formation of Uncovered and Dimethyl EtherCovered MgCl2 Crystallites. Consequences in the Structure of Ziegler-Natta Heterogeneous Catalysts. J. Phys. Chem. C 2011, 115, 13322−13328. (23) Turunen, A.; Linnolahti, M.; Karttunen, V. A.; Pakkanen, T. A.; Denifl, P.; Leinonen, T. Microstructure Control of Magnesium Dichloride Crystallites by Electron Donors: The Effect of Methanol. J. Mol. Catal. A: Chem. 2011, 334, 103−107. (24) Stukalov, D. V.; Zakharov, V. A. Active Site Formation in MgCl2−Supported Ziegler−Natta Catalysts. A Density Functional Theory Study. J. Phys. Chem. C 2009, 113, 21376−21382. (25) Correa, A.; Piemontesi, F.; Morini, G.; Cavallo, L. Key Elements in the Structure and Function Relationship of the MgCl2/ TiCl4/Lewis Base Ziegler−Natta Catalytic System. Macromolecules 2007, 40, 9181−9189.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01878. Absolute values of electronic energies, enthalpies, entropies, and Gibbs energies together with Cartesian coordinates of the reported structures (PDF) (XYZ)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: mikko.linnolahti@uef.fi. Phone: +358505926855. ORCID

Mikko Linnolahti: 0000-0003-0056-2698 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The computations were made possible by use of the Finnish Grid Infrastructure and Finnish Grid and Cloud Infrastructure resources (urn:nbn:fi:research-infras-2016072533).

■

REFERENCES

(1) Kashiwa, N. The discovery and progress of MgCl2-supported TiCl4 catalysts. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1−8. (2) Kashiwa, N.; Yoshitake, J. The Influence of the Valence State of Titanium in MgCl2-Supported Titanium Catalysts on Olefin Polymerization. Makromol. Chem. 1984, 185, 1133−1138. (3) Sita, L. R. In Synthesis of Polymers: New Structures and Methods, First Edition; Schlüter, A. D.; Hawker, C. J.; Sakamoto, J., Eds.; John Wiley & Sons: Hoboken, NJ, 2012; pp 25−66. (4) Kissin, Y. V.; Liu, X.; Pollick, D. J.; Brungard, N. L.; Chang, M. Ziegler-Natta Catalysts for Propylene Polymerization: Chemistry of Reactions Leading to the Formation of Active Centers. J. Mol. Catal. A: Chem. 2008, 287, 45−52. (5) D’Amore, M.; Thushara, K. S.; Piovano, A.; Causà, M.; Bordiga, S.; Groppo, E. Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler−Natta Catalysts. ACS Catal. 2016, 6, 5786−5796. (6) Magni, E.; Somorjai, G. A. Preparation of a Model Ziegler-Natta Catalyst Surface Science Studies of Magnesium Chloride Thin Film Deposited on Gold and Its Interaction with Titanium Chloride. Appl. Surf. Sci. 1995, 89, 187−195. (7) Linnolahti, M.; Pakkanen, T. A.; Bazhenov, A. S.; Denifl, P.; Leinonen, T.; Pakkanen, A. Alkylation of Titanium Tetrachloride on Magnesium Dichloride in the Presence of Lewis Bases. J. Catal. 2017, 353, 89−98. 9927


ACS Omega

Article

(26) Mikenas, T. B.; Koshevoy, E. I.; Zakharov, V. A.; Nikolaeva, M. I. Formation of Isolated Titanium(III) Ions as Active Sites of Supported Titanium-Magnesium Catalysts for Polymerization of Olefins. Macromol. Chem. Phys. 2014, 215, 1707−1720. (27) Busico, V.; Causà, M.; Cipullo, R.; Credendino, R.; Cutillo, F.; Friederichs, N.; Lamanna, R.; Segre, A.; Van Axel Castelli, V. Periodic DFT and High-Resolution Magic-Angle-Spinning (HR-MAS)1H NMR Investigation of the Active Surfaces of MgCl2-Supported Ziegler−Natta Catalysts. The MgCl2Matrix. J. Phys. Chem. C 2008, 112, 1081−1089. (28) Giannini, U. Polymerization of Olefins with High Activity Catalysts. Makromol. Chem., Suppl. 1981, 5, 216−229. (29) Dwivedi, S.; Taniike, T.; Terano, M. Understanding the Chemical and Physical Transformations of a Ziegler-Natta Catalyst at the Initial Stage of Polymerization Kinetics: The Key Role of Alkylaluminum in the Catalyst Activation Process. Macromol. Chem. Phys. 2014, 215, 1698−1706. (30) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. Theoretical investigations on Ziegler-Natta catalysis: models for the interactions of the TiCl4 catalyst and the MgCl2 support. Surf. Sci. 1995, 334, 289−294. (31) Martinsky, C.; Minot, C.; Ricart, J. M. A theoretical investigation of the binding of TiCln to MgCl2. Surf. Sci. 2001, 490, 237−250. (32) Seth, M.; Margl, P. M.; Ziegler, T. A Density Functional Embedded Cluster Study of Proposed Active Sites in Heterogeneous Ziegler−Natta Catalysts. Macromolecules 2002, 35, 7815−7829. (33) Lee, J. W.; Jo, W. H. Theoretical investigation on the model active site for isotactic polypropylene in heterogeneous Ziegler-Natta catalyst: A density functional study. J. Organomet. Chem. 2007, 692, 4639−4646. (34) Taniike, T.; Terano, M. Reductive Formation of Isospecific Ti Dinuclear Species on a MgCl2(110) Surface in Heterogeneous Ziegler-Natta Catalysts. Macromol. Rapid Commun. 2008, 29, 1472− 1476. (35) Stukalov, D. V.; Zilberberg, I. L.; Zakharov, V. A. Surface Species of Titanium(IV) and Titanium(III) in MgCl2-Supported Ziegler−Natta Catalysts. A Periodic Density Functional Theory Study. Macromolecules 2009, 42, 8165−8171. (36) D’Amore, M.; Credendino, R.; Budzelaar, P. H. M.; Causá, M.; Busico, V. A Periodic Hybrid DFT Approach (Including Dispersion) to MgCl2-Supported Ziegler-Natta Catalysts - 1: TiCl4 adsorption on MgCl2 crystal Surfaces. J. Catal. 2012, 286, 103−110. (37) Boero, M.; Parrinello, M.; Terakura, K. First Principles Molecular Dynamics Study of Ziegler−Natta Heterogeneous Catalysis. J. Am. Chem. Soc. 1998, 120, 2746−2752. (38) Brambilla, L.; Zerbi, G.; Nascetti, S.; Piemontesi, F.; Morini, G. Experimental and calculated vibrational spectra and structure of Ziegler-Natta catalyst precursor: 50/1 comilled MgCl2-TiCl4. Macromol. Symp. 2004, 213, 287−302. (39) Cheng, R.-h.; Luo, J.; Liu, Z.; Sun, J.-w.; Huang, W.-h.; Zhang, M.-g.; Yi, J.-j.; Liu, B.-p. Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over ZieglerNatta catalyst: A DFT study. Chin. J. Polym. Sci. 2013, 31, 591−600. (40) Credendino, R.; Liguori, D.; Fan, Z.; Morini, G.; Cavallo, L. Toward a Unified Model Explaining Heterogeneous Ziegler-Natta Catalysis. ACS Catal. 2015, 5, 5431−5435. (41) Bazhenov, A.; Linnolahti, M.; Pakkanen, T. A.; Denifl, P.; Leinonen, T. Modeling the Stabilization of Surface Defects by Donors in Ziegler-Natta Catalyst Support. J. Phys. Chem. C 2014, 118, 4791− 4796. (42) Kuklin, M. S.; Bazhenov, A. S.; Denifl, P.; Leinonen, T.; Linnolahti, M.; Pakkanen, T. A. Stabilization of Magnesium Dichloride Surface Defects by Mono- And Bidentate Donors. Surf. Sci. 2015, 635, 5−10. (43) Costuas, K.; Parrinello, M. Structure and Chemical Activity of Point Defects on MgCl2(001) Surface. J. Phys. Chem. B 2002, 106, 4477−4481.

(44) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (45) Schäfer, A.; Huber, C.; Ahlrichs, R. Fully Optimized Contracted Gaussian Basis Sets of Triple Zeta Valence Quality for Atoms Li to Kr. J. Chem. Phys. 1994, 100, 5829−5835. (46) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (47) Adamo, C.; Barone, V. Toward Reliable Density Functional Methods without Adjustable Parameters: The PBE0 Model. J. Chem. Phys. 1999, 110, 6158−6170. (48) Credendino, R.; Minenkov, Y.; Liguori, D.; Piemontesi, F.; Melchior, A.; Morini, G.; Tolazzi, M.; Cavallo, L. Accurate experimental and theoretical enthalpies of association of TiCl4 with typical Lewis bases used in heterogeneous Ziegler-Natta catalysis. Phys. Chem. Chem. Phys. 2017, 19, 26996−27006. (49) Tobisch, S.; Ziegler, T. Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by the Cationic Group 4 [(η5Cp-(CMe2-bridge)-Ph)MII(ethylene)2]+(M = Ti, Zr, Hf) Active Catalysts: A Density Functional Investigation of the Influence of the Metal on the Catalytic Activity and Selectivity. J. Am. Chem. Soc. 2004, 126, 9059−9071. (50) Ehm, C.; Cipullo, R.; Budzelaar, P. H. M.; Busico, V. Role(s) of TMA in polymerization. Dalton Trans. 2016, 45, 6847−6855. (51) Zaccaria, F.; Ehm, C.; Budzelaar, P. H. M.; Busico, V. Accurate Prediction of Copolymerization Statistics in Molecular Olefin Polymerization Catalysis: The Role of Entropic, Electronic, and Steric Effects in Catalyst Comonomer Affinity. ACS Catal. 2017, 7, 1512−1519. (52) Zaccaria, F.; Cipullo, R.; Budzelaar, P. H. M.; Busico, V.; Ehm, C. Backbone Rearrangement during Olefin Capture as the Rate Limiting Step in Molecular Olefin Polymerization Catalysis and Its Effect on Comonomer Affinity. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2807−2814. (53) Linnolahti, M.; Collins, S. Formation, Structure, and Composition of Methylaluminoxane. ChemPhysChem 2017, 18, 3369−3374. (54) Cheruvathur, A. V.; Langner, E. H. G.; Niemantsverdriet, J. W.; Thüne, P. C. In Situ ATR-FTIR Studies on MgCl2-Diisobutyl Phthalate Interactions in Thin Film Ziegler-Natta Catalysts. Langmuir 2012, 28, 2643−2651. (55) Pirinen, S.; Koshevoy, I. O.; Denifl, P.; Pakkanen, T. T. A Single-Crystal Model for MgCl2-Electron Donor Support Materials: [Mg3Cl5(THF)4Bu]2 (Bu = n-Butyl). Organometallics 2013, 32, 4208−4213. (56) Correa, A.; Bahri-Laleh, N.; Cavallo, L. How Well Can DFT Reproduce Key Interactions in Ziegler-Natta Systems? Macromol. Chem. Phys. 2013, 214, 1980−1989. (57) Ehm, C.; Antinucci, G.; Budzelaar, P. H. M.; Busico, V. Catalyst Activation and the Dimerization Energy of Alkylaluminium Compounds. J. Organomet. Chem. 2014, 772−773, 161−171. (58) Ehm, C.; Budzelaar, P. H. M.; Busico, V. Calculating Accurate Barriers for Olefin Insertion and Related Reactions. J. Organomet. Chem. 2015, 775, 39−49. (59) Nissinen, V.; Linnolahti, M.; Pakkanen, T. T. Methoxymagnesium chloridestructure and interaction with electron donors: experimental and computational study. J. Phys. Chem. C 2016, 120, 21505−21511. (60) Nissinen, V. H.; Linnolahti, M.; Bazhenov, A. S.; Pakkanen, T. T.; Pakkanen, T. A.; Denifl, P.; Leinonen, T.; Jayaratne, K.; Pakkanen, A. Polyethylenimines: Multidentate Electron Donors for Ziegler-Natta Catalysts. J. Phys. Chem. C 2017, 121, 23413−23421. (61) Frisch, M. J.; et al. Gaussian 09, Revision C.01, 2009.

9928


Adsorption of Titanium Tetrachloride on Magnesium Dichloride Clusters

Recommend Documents