silica catalyst

Journal Pre-proofs Ethylene Polymerization by Hydrocarbon-Reduced Cr/Silica Catalyst Masud Monwar, Carlos Cruz, Jared Barr, Max McDaniel PII: DOI: Reference:

S0021-9517(20)30425-5 https://doi.org/10.1016/j.jcat.2020.10.019 YJCAT 13945

To appear in:

Journal of Catalysis

Received Date: Revised Date: Accepted Date:

23 July 2020 7 October 2020 13 October 2020

Please cite this article as: M. Monwar, C. Cruz, J. Barr, M. McDaniel, Ethylene Polymerization by HydrocarbonReduced Cr/Silica Catalyst, Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.10.019

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ETHYLENE POLYMERIZATION BY HYDROCARBON-REDUCED CR/SILICA CATALYST by Masud Monwar, Carlos Cruz, Jared Barr, Max McDaniel* Chevron Phillips Chemical Company LP Phillips 66 Research Center Bartlesville, OK 74003 USA

ABSTRACT The Phillips Cr(VI)/silica catalyst, used worldwide to manufacture polyethylene, must first be reduced and alkylated to display polymerization activity. This is accomplished commercially by exposure to ethylene, in a reaction that is still uncertain after 60 years of investigation. Here we report that other non-olefin hydrocarbons can also be used in this self-alkylation mechanism, and we track, through isotopic labeling and other analyses, the fate of such hydrocarbons when used to initiate (reduce and alkylate) Cr(VI). A part of the reductant hydrocarbon is converted into oxygenated ligands, which remain attached as a permanent component of the active catalyst, thus influencing its behavior. Another part is used to alkylate the Cr, and it becomes incorporated into the first polymer chain. Each hydrocarbon reductant produces a unique catalyst, with unique ligands, displaying its own activity and sometimes producing distinctive polymer. The implication of this situation, in contrast to more tradition findings from the field, is one of opportunity to create novel varieties of new Phillips catalysts. * [email protected] Keywords: Phillips catalyst ethylene polymerization polyethylene chromium catalyst deuterium NMR deuterium labeling of polyethylene

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Graphical Abstract Cr(VI)/silica catalyst was initiated by alkanes and aromatics, sometimes isotopically labeled, and their fate was monitored through D-NMR, GC-MS, etc.

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INTRODUCTION The Phillips polymerization catalyst, Cr/silica, is used to manufacture a large share of the world's polyethylene, which is the most widely used polymer. Discovered in 1951 at Phillips Petroleum [1], and thus named the "Phillips catalyst", it has been studied for over 60 years by both academic and industrial scientists [2,3,4]. However, one commercially important question remains unclear: namely, how the Cr(VI) site initially reacts with ethylene to self-alkylate, thus beginning its first PE chain [2]. Upon contact with ethylene under commercial polymerization conditions (70-150°C), the hexavalent chromium catalyst undergoes a sometimes-slow reduction and alkylation step. Then, the Cr active site comes alive and rapidly produces thousands of PE chains through chain transfer during its brief one-hour industrial lifetime. The process is illustrated in Scheme 1 below. Scheme 1 Site initiation followed by continuous chain transfer on the same active site. Once the Cr site has become reduced and alkylated through an unknown mechanism, possibly also producing an unknown ligand X, PE chains are continuously terminated and re-initiated.

To better understand this elusive initiation process, a very early study by a Phillips licensee [5] was published which became widely accepted for 40 years. It attempted to identify the redox products of the reduction of Cr(VI)/silica by ethylene. It concluded that surface chromate is reduced by ethylene to produce two formaldehyde molecules and a divalent active site. Some later work by others also seemed to confirm this conclusion [6,7]. The residual formaldehyde ligand was assumed to then be displaced by excess ethylene, yielding the "naked" and active divalent catalyst. This seems implausible not only from the known poisoning behavior of aldehydes, but it also did not address the self-alkylation issue. Recent years, however, have brought a re-evaluation of this important issue. In work from this laboratory [8,9,10], it was noticed that neither aldehydes nor other oxygenated byproducts could be detected in the recycle streams of mega-reactors, where they could have been concentrated, had they been liberated from the Cr site. And in laboratory studies, only by hydrolysis, and not through heat, could oxygenates be removed from an olefin-reduced catalyst [8,9]. Furthermore, aldehydes were deliberately added to the "naked" divalent catalyst but could not be desorbed by heat [8]. Even before these reports, the Turin group presented spectroscopic evidence for the formation of, not formaldehyde, but formate, when ethylene was oxidized by the hexavalent catalyst [11]. Later data from this laboratory also found evidence for the formation of carboxylate [8], and in a second study [9], the many redox products of the reduction step were catalogued in some detail. In addition to carbonyls, carboxylates were again identified as redox/hydrolysis products, mainly formate from ethylene but also some acetate. A "naked"

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divalent catalyst was even doped with 1 equivalent of acetic acid, and found to have activity similar to, and to make polymer like, the ethylene-reduced catalyst [9]. Still other publications speculated on potential alkylation pathways [4,12,13,14,15,16]. Together, this new data suggests that surface chromate is reduced by ethylene to produce both an oxygenated ligand that remains attached to the site, and also an alkyl group which initiates the first PE chain on each site. For example, the possible coexistence of a methyl group and an attached carboxylate on the same site has been previously confirmed for molybdenum and tungsten species [17]. In still other work from this laboratory [18,19] it was reported that alkanes could also be used to reduce Cr(VI)/silica and thus generate active polymerization catalysts. Although that reduction was found to be very slow at 25°C, recent work indicates that it can be greatly accelerated by light [18,20,21]. The resultant activity and polymer character were found to differ somewhat from that of ethylene-reduced catalyst. This implies that the remaining oxygenated ligand is different from that obtained from reduction by ethylene. In fact, it suggests that there is not a single path to reduction/alkylation of the catalyst, but that many avenues for site initiation could exist, each of which generates a unique set of ligands that influence polymerization behavior, perhaps producing distinctive polymers as well. Different initiation pathways might even exist on the same catalyst using the same reductant [2,7,11,22]. This would be consistent with the organic synthesis literature, which describes Cr(VI) as a strong but non-selective oxidant [23,24]. It is also consistent with other recent experience from this laboratory [21]. The current report describes our second study using isotopic labeling to trace the fate of initiating species [10]. In this study we investigate the reduction and alkylation process of Cr(VI)/silica by using deutero-labeled hydrocarbons as reductants, including alkanes and aromatics. Then the reduced catalyst was allowed to polymerize normal proteo-ethylene, and that polymer was finally analyzed by 2H-NMR.

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EXPERIMENTAL Catalyst Preparation The NMR experiments of this study used a Cr/silica-titania catalyst having 4.2 wt% titania and 1 wt% Cr. It had high porosity, 2.5 mL/g and 550 m2/g [25]. Cr/silica-titania catalysts tend to behave in similar ways to those of Cr/silica but they are usually more active, which was helpful in these experiments to quickly achieve the productivity target. They have been wellcharacterized in other studies by various forms of spectroscopy [26,27,28,29,30,31,32,33,34,35,36]. Calcination of the catalyst was accomplished in a 4.75 cm diameter vertical quartz tube. Dry air was passed up through a sintered quartz disk which supported the fluidized catalyst bed. The air had a linear velocity of 1.5 cm/s to fluidize the catalyst. The air and nitrogen used in this study were dried by passage through a 13X molecular sieve column. The catalyst bed was then heated, at the rate of 400°C/h to 870°C, at which temperature it was fluidized while still in dry air for another three hours. This method is known to oxidize and bind the Cr into surface chromate and dichromate almost quantitatively [2,37]. Finally, the catalyst was cooled to 25°C while fluidizing in dry air, and then the air was flushed out by nitrogen for 30 minutes, followed by discharge and capture. At no time after calcination was the catalyst exposed to the atmosphere. The experiments in which hydrolysis solutions were analyzed used a wide assortment of catalysts. These are listed in the Supplementary Information file. The support was varied, including silica, silica-titania, silica-alumina, and alumina. The Cr loading was also varied between 0.28 wt% to 10 wt% total Cr. And the calcination temperature was also varied between 400°C up to 870°C.

Catalyst Reduction In these experiments the hexavalent starting catalyst described above was then reduced by exposure to various hydrocarbons, often isotopically labeled, at 25°C. A 75 mL quartz flask was loaded under dry nitrogen with about two grams of the catalyst. Then 0.5 mL of the reductant (cyclohexane-d12, n-hexane-d14, or toluene-d8) was injected into the flask. The flask was continuously rotated under blue fluorescent light as previously described [18,20,21] for 3-5 hours. This was done to activate the Cr(VI) and to accelerate its reactivity with the hydrocarbon. Previous reduction experiments established that blue, white, or UV light would activate the Cr(VI), whereas red light was much less effective [21]. After the reduction treatment, the color change was noted, aliquots of the catalyst were taken for polymerization experiments, and sometimes the remainder of the catalyst was quenched by the addition of 20 mL ethanol to the flask. In a large number of other experiments, listed in the Supplementary Information file, a wide variety of Cr(VI) catalysts were treated with various types and amounts of proteohydrocarbons, and reduced in different types of light for various times. These catalysts were then quenched (hydrolyzed) by injecting water, methanol, or methanol-water solutions. The quench solution also contained a small but known amount of reference compound for calibration of experimental peak areas. Usually the reference compound was a C3 or C4 glycol, chosen because it was unlikely to overlap with other experimental GC peaks.

Analysis of Hydrolysis Products Hydrolysis products were determined through gas chromatography using an Agilent 7890B gas chromatograph, which was equipped with both flame ionizing and mass spectral analysis. An all-purpose capillary column (Agilent J&W VF-5ms, 30 m x 0.25 mm x 0.25 μm)

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was used with variable temperature. Approximately 0.5 μL sample aliquots were injected into a GC port held at 250 °C using a split ratio of 10:1. The carrier gas was ultra-high purity helium and was electronically controlled throughout the run to a constant flow rate of 1.2 mL/min. Initial column temperature was held at 50 °C for 5 min, ramped at 20 °C/min to 250 °C, and then held at 250 °C for 19 min. Spectral assignment was made via mass correlation using an Agilent 5977B mass spectrometer connected to the GC unit using electron ionization at 70 eV. The nominal mass range scanned was 14-400 m/z using a scan time of 0.5 sec. Nominal detector voltage used was 1200 V. For calibration purposes both the FID and MS detectors were sometimes used in sequence on the same or reference samples. The chromatograms obtained from these analyses are shown in the Supplementary Information. In many experiments, and especially those in which the catalyst was reduced by light hydrocarbons, such as methane, ethane or ethylene, another chromatography test was used in addition to the one described above. This test had been specifically devised by the Phillips 66 Analytical Dept. to detect and quantify trace amounts of light oxygenates in various refinery operations, that is, C1-C4 alcohols, aldehydes, and ketones. In this study, the hydrolysis solution was usually also submitted for this standardized procedure. It used an Agilent 6890 gas chromatograph having a flame-ionizing detector (FID). It used a Restek Stapilwax column (P/N 10658) designed and gated specifically to separate and detect light alcohols. This data is also included in the Supplementary Information file. Still another method was also used to analyze the hydrolysis products produced in this study, especially when the catalysts were reduced by ethane and methane. Hydrolysis solutions were submitted for a standardized ion chromatography procedure developed by the Phillips 66 Analytical Dept. to detect and quantify trace quantities of light (C1-C6) carboxylic acids in aqueous solutions. The sample was first neutralized with a solution of sodium hydroxide to put the acids into their anionic form. Then a small amount of the sample was injected through the ion column. The chromatograph used was a Dionex IC-3000 with two channels and an ICE-AS1 column and guard. Results were reported in mg of carboxylate per liter of solution. The results of this test are also listed in the Supplementary Information file.

Polymerization After the reduction treatment, the catalyst vessel was evacuated for about 30 minutes at 25°C to remove any excess reductant. Then, aliquots were taken and allowed to polymerize proteo-ethylene (C2H4) for a short time, usually about 15 minutes, to make about 1-2 g of PE per g of catalyst. Usually about 2.0 g of catalyst was used, and the polymer yield was deliberately kept low in order to maximize the NMR signal contribution from the very first PE chains produced. This polymerization reaction was conducted at 50°C at 170-200 kPa. Unless otherwise indicated, the polymerization was conducted from the gas phase, that is, no solvent was used. After completion of the polymerization, 15 mL of isopropanol was added to the reaction mixture to stop the polymerization and to hydrolyze live chains. After polymerization, the catalyst/polymer solid was allowed to settle in the reaction vessel. After a nitrogen purge at 25°C for 18 hours, the dry polymer/catalyst was recovered. In some experiments the reduced catalyst was tested at higher ethylene pressure in a 2.2 stainless steel autoclave. About 50 mg of the catalyst was charged to the autoclave under dry nitrogen, followed by 2 L of isobutane liquid. The temperature was increased to 105°C and ethylene was supplied on demand at 3.79 MPa for about 1 hour.

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Polymer Purification To prevent the paramagnetic catalyst from interfering with subsequent NMR analysis of the polymer, the catalyst was first removed from the polymer. To accomplish this, the reaction product from above was added to about 200 mL of 1,2,4-trichlorobenzene (TCB, Aldrich reagent grade) which had been heated to 150°C under a nitrogen blanket. The mixture was allowed to stir for 1 h at 150°C while still under N2, after which it was passed through a medium sintered glass filter funnel by force of vacuum, which usually took about one minute to remove the catalyst. The hot TCB/polymer solution passed through the filter and into a cooling bath of 2L of isopropanol at 25°C. Polymer immediately precipitated and after being stirred for another 30 min at 25°C, it was filtered out onto paper filter. The filtrate was then dried overnight in the air, followed by a final drying step under vacuum at 60°C for one hour. The polymer was then separated from the paper. White flakes of dry polymer were obtained, with recovery typically 70–90% of the polymer yield as calculated from the change in weight during polymerization,

GPC Analysis Molecular weights and molecular weight distributions were obtained using a PL 220 GPC/SEC high temperature chromatography unit (Polymer Laboratories, now an Agilent Company) with 1,2,4-trichlorobenzene (TCB) as the solvent, and with a flow rate of 1.0 mL/minute at a temperature of 145 C. BHT (2,6-di-tert-butyl-4-methylphenol) was used as a stabilizer in the TCB at a concentration of 0.5 g/L. An injection volume of 400 µL was used with a nominal polymer concentration of 0.5 mg/mL. Dissolution of the sample in stabilized TCB was carried out by heating at 150 C for about 4 hours with occasional, gentle agitation. Three Waters Styrogel HMW-6E columns were used and were calibrated with the integral method using a broad linear polyethylene standard (Chevron Phillips Chemical Company LP’s Marlex® RTM BHB 5003 polyethylene resin) for which the molecular weight distribution had been determined. An IR4 detector (Polymer Char, Spain) was used for the concentration detection.

NMR Analysis Sample Preparation for NMR Data Collection For NMR analysis, all the samples were prepared in 10 mm NMR tubes. About 0.3 g of deuterium-labeled polyethylene samples was dissolved in a mixture of 2.5 mL 1,2,4trichlorobenzene (TCB) and 1.20 g of 1,4-dichlorobenzene-d4 (DCB-d4) for 1H and 13C NMR data collection. For solution-state deuterium (2H) NMR data collection, about 0.3 g of the polyethylene samples and the model compound were dissolved in 2.5 ml of non-deuterated TCB solvent. The sample and the solvent (or solvent mixture) were heated in a heating block at 130 C for 4–5 hours. The mixture was occasionally stirred with a stainless-steel stirrer to ensure homogeneous mixing. The resulting solution was then left overnight (for 15–16 hours) in the heating block at 112 C to ensure complete disentanglement of the polymer chains. The final concentrations of the resulting solutions were about 5-7 wt%. Acquisition of 1H, 2H and 13C NMR Spectra The NMR data were collected in a 500 MHz NMR instrument comprised of a 500 MHz Oxford magnet and Bruker’s Avance III HD console. A 10 mm BBOI probe fitted with z-gradient was used for 1H, 2H and 13C NMR data collection. The deuterium lock channel of the instrument was used for 2H NMR data collection. All the NMR data were collected at 125C and the sample was equilibrated at 125C for 15 minutes before the start of data acquisition. The data were collected and processed with Bruker’s Topspin software (v.3.2).

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The 1H NMR data were collected with standard pulse sequence using the standard parameter set including: a 7.4 sec 90 pulse width, a 7.5 kHz spectral window, 5.0 sec relaxation delay and 5.0 sec acquisition time. 1024 transients were averaged to obtain enough signal-to-noise ratio (SNR) to detect the signals originated from terminal olefins. The data was zero filled with 131k data points and exponentially weighted with 0.30 Hz line-broadening before Fourier transformation. The spectrum was referenced with the residual proton peak of (7.16 ppm) DCB-d4 solvent. The 2H NMR (deuterium) data were collected with standard pulse sequence using the standard parameter set including: a 225 sec 90 pulse width, a 1.15 kHz spectral window, 2.0 sec relaxation delay and 0.99 sec acquisition time. 16k transients were collected and averaged to obtain enough SNR to detect the methyl signal. The data was zero filled with 8k data points and exponentially weighted with 2.0 Hz line-broadening before Fourier transformation. The spectrum was referenced with the natural abundance deuterium peak of non-deuterated TCB solvent (the chemical shift of the central peak of the triplet is δ ~7.2 ppm). The 13C NMR spectra of the polyethylene samples were collected with standard pulse program using the standard parameter set including: a 13.0 µsec 90° pulse width, a 21.7 kHz spectral window, 7.0 sec relaxation delay, 3.0 sec acquisition time. 8k transients were collected in an overnight experiment and full NOE was exploited during data collection to improve the SNR at a reasonable amount of time. The data was zero filled to 131k data points and exponentially weighted with a 1.0 Hz line-broadening before Fourier transformation. Quantification of the Signal To translate the 2H-NMR signal into physical quantities, the following protocol was used. In each experiment, the amount of reduced catalyst was weighed into a flask prior to polymerization. Then, following polymerization, and after evacuation of the solvent overnight, the polymer/catalyst powder was weighed again, to determine the amount of polymer made. For NMR analysis, a small sample of the purified polymer was again carefully weighed, and then a measured amount of NMR solvent was added. In Experiments 1 and 2, that solvent was 1,2,4-trichlorobenzene. In Experiment 3 it was 1,1,2,2-tetrachloroethane, to avoid overlapping resonance with the incorporated toluene-d8 reductant. As the NMR spectrum was obtained, both solvents produced a deuterium resonance due to natural abundance. This signal from the solvent was then compared to the experimental D-resonances from the polymer to quantify the physical amounts of these NMR species in the polymer.

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RESULTS AND DISCUSSION Hydrolysis of Reduced Cr(VI) Catalyst During commercial operations, the hexavalent Cr/silica catalyst is reduced in the reactor upon contact with ethylene to one or more lower-valent active species which are then responsible for the polymerization. However, non-olefinic hydrocarbons can also be used to reduce Cr(VI) catalysts [18]. Unlike CO-reduced catalysts, they do not chemiluminesce, with the exception of methane-reduced Cr/silica. Nevertheless, these hydrocarbon-reduced catalysts are usually found to be active for ethylene polymerization. Table 1 lists a few examples of such catalysts, and their polymerization activity as well as a few basic characteristics of the resultant polymer. Table 1 Comparison of catalysts reduced by various hydrocarbons. Activity* HLMI Mw Mw/ LCB 6 Reductant Catalyst Color Poly'm gPE/g/h dg/min kg/mol Mn /10 C Not reduced B Orange C 2577 220 98 7.9 11.2 Ethane B Blue C 2000 113 120 12.5 7.7 n-Pentane B Blue C 3188 154 100 6.8 8.8 n-Hexane B Blue C 2597 139 102 10.3 10.0 Cyclohexane B Blue C 938 46.7 162 10.3 4.2 Decalin B Blue C 2094 199 108 7.5 5.9 Toluene B Blue-black C 2614 203 107 9.6 7.8 Not reduced A Orange D 6632 62.0 Methane A Green D 9256 66.6 Not reduced A Orange C 2663 68.5 Methane A Green C 4536 89.9 Catalyst: (A) Cr/silica-titania 850°C, (B) Cr/silica-titania 870°C Polymerization: (C) 105°C homopolymer, (D) 100°C with 1-hexene *Activity includes induction time Figure 1 MW distribution curves of PE from Cr(VI) catalyst reduced by various hydrocarbons.

Notice in Table 1 that the activity and polymer character can vary with the reduction treatment. This is reasonable if, as we believe [8,9,11,38], the oxidized reductants are left on the catalyst as permanent ligands, which would make the active site a little different on each of the catalysts in Table 1. For example, reduction by ethane seemed to broaden the MW

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distribution, reduction by cyclohexane increased the molecular weight and lowered the amount of long-chain branching, and methane improved the activity. Figure 1 shows the molecular weight distributions from some of the polymers in Table 1. Note that, although they are not identical, they all have a similar medium-to-narrow MW distribution, which is expected as a result of the particular silica-titania used as the support, and the high calcination temperature chosen. Despite the polymer differences in Table 1, it is important to put this diversity into perspective, with respect to the entire universe of chromium polymerization catalysts. When considered from that wider viewpoint, the variations in Table 1 appear to be more subtle. For example, the degree of long-chain branching from the polymers in Table 1 is very typical of the original Cr(VI) catalyst, as is the molecular weight. And, in no case was the polymer found to be highly branched from the copolymerization of in-situ produced 1-hexene or light olefins, as is typically produced by supported organochromium catalysts [2]. Neither do the polymers of Table 1 resemble those produced when metal alkyl cocatalysts are added, which tend to have a broader MW distribution, a high-MW tail, and to again have in-situ generated branching [2,39,40,41,42,43]. On both types of catalysts, organo-Cr and metal alkyl treated, the oligomerization behavior is thought to be caused by mono-attached chromium, as illustrated in Scheme 2. Recently the Turin group has produced some strong spectroscopic evidence that also seems to support this idea [44]. Their study treated both Cr(VI) and Cr(II) catalysts with aluminum alkyl. The use of Cr(VI) on other supports, such as alumina or aluminophosphate [45,46,47], produces quite different polymer from Cr(VI)/silica-generated polymer, typically higher in molecular weight, and sometimes bimodal. In fact, variation of the calcination temperature produces much larger changes in polymer character than we see in Table 1. Consequently, although the ligands left from reduction do produce commercially significant differences in polymerization behavior, we conclude that the ligands contributed by the support itself can also have a very strong, perhaps stronger, influence on the active site, than the ligands left from reduction. The polymers in Table 1 appear to be variations within a common catalyst family, that is, the Cr has a similar attachment to a similar support. Scheme 2 Formation of the mono-attached Cr species upon deposition of an organoCr compound on silica. R is allyl, benzyl, neopentyl, benzene, cumene, 2,4-dimethylpentadienyl, etc.

Reduction of Cr(VI) by various hydrocarbons must lead to oxygenated by-products, as has been established for reduction by olefins [8,9,11,18]. However, once again we have found that such by-products remain on the catalyst as ligands, and cannot be removed by simple heating, even up to 300°C or by extraction with hydrocarbons. Instead, these oxygenated byproducts can only be liberated through hydrolysis, when the reduced catalysts are exposed to water or an alcohol. GC-MS analysis of the resultant hydrolysis solutions typically indicates the formation of the corresponding alcohol or carbonyl from the reductant hydrocarbon, regardless of whether the hydrolysis solution is acidic, basic, has a neutral pH, or is just an alcohol. As part of this study a large number of reduction experiments have been performed using widely varying catalyst types, reductants, and reducing and hydrolysis conditions. These experiments have been described in detail in the Supplementary Information file accompanying this report. However, Table 2 condenses this larger data base into a simplified list of some of

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the major oxygenated by-products that were usually observed for each reductant. The products obtained were rather consistent, although the relative amounts varied between experiments with reaction conditions. The amounts of each product in Table 2 was obtained by averaging the multiple runs described in the Supplementary Information file. Table 2 Products obtained from the reduction of Cr(VI)/silica-titania catalysts. Color Green

Highest Prod./Cr 0.72

Ethane

Blue

0.51

Isobutane

Blue

0.45

Cyclopentane

Blue

0.74

Cyclohexane

Blue

0.78

n-Hexane

Blue

0.72

Dark Blue

0.70

n-Pentane

Blue

1.05

i-Pentane

Blue-Green

0.85

Reductant Methane

Toluene

Calc. Avg Relative Valence Product Amount* 4.6 Methanol 97% Formic acid 3% 4.9 Ethanol 98% Acetic acid 2% 5.0 t-Butanol 52% i-Butanol 38% i-Butanal 7% 4.6 Cyclopentanol 75% Cyclopentanone 17% Cyclopentane oxide 6% 2-Cyclopenten-1-one 2% 3.9 Cyclohexanol 65% Cyclohexanone 27% 2-Cyclohexen-1-one 3% Cyclohexene oxide 3% 4.0 2-Hexanol 25% 2-Hexanone 23% 3-Hexanol 20% 3-Hexanone 17% 1-Hexanol 15% 3.2 Benzaldehyde 42% Benzophone-type dimers 29% Benzyl alcohol 17% 2&4 Me-Phenols 10% 3.1 2&3-Pentanols 46% 2-Pentanone 25% 1-Pentanol 16% 3-Pentanone 10% 2-Penten-1-one 2% 2-Pentenal 1% 4.0 2-Me-2-butanol 30% 3-Me-1-butanol 20% 3-Me-2-butanone 18% 2-Me-1-butanol 16% 3-Me-2-butanol 13% 3-Me-3-buten-2-one 3%

* average over multiple runs

Cr(VI) has been reported in the organic synthesis literature to be a strong but nonselective general oxidant for olefins [23,24], which was also found to be the case for Cr(VI)/silica catalysts in our earlier work with olefins [8,9]. And it also now seems to be true for the oxidation of alkanes [18,21]. The major products were usually alcohols and carbonyls, with sometimes smaller amounts of alkenol or alkenone species. The proportion of alkanol and alkanone hydrolysis products was quite variable between catalysts, reductants and experimental conditions. In some cases, no carbonyls were obtained at all. This raised the question as to whether the ketones and aldehydes produced from alkanes might originate from the secondary oxidation of liberated alkanols by unreduced Cr(VI) after quenching. As a test, virgin Cr(VI) catalyst was added in small amounts to a dilute aqueous

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solution of 1-pentanol. Immediate oxidation to small amounts of pentanal was observed, indicating that secondary oxidation is at least possible. In another test (R-29 in the Supplementary Information file), a catalyst reduced by npentane was quenched with a dilute aqueous solution of ascorbic acid, which is known to quickly reduce hexavalent Cr to the trivalent state [48]. Again 2- and 3-pentanols were the main products, and in high yield, followed by 1-pentanol. However, this time, no pentanones were produced. This might indicate that ketones are the result of secondary oxidation of alcohols by unreduced Cr(VI) sites. But it could also mean that the reduced chromium still had considerable oxidizing capacity when the hydrolysis solution was added, perhaps in the form of Cr(IV). Given the strong color change upon reduction by alkanes, and the sometimes large amount of product generated, it is difficult to believe that very much Cr(VI) remained in many of these reduction experiments. Usually about one or a little less oxygenated by-product was hydrolyzed per chromium, which possibly suggests Cr(IV) as a “reduced” species, but still having oxidizing capacity. In an effort to determine the amount of oxidizing power still left after reduction by hydrocarbons, some of these reduced catalysts were titrated in a standard test used to determine the amount of Cr(VI) present after calcination. A sample of the reduced catalyst was added to an aqueous dilute sulfuric acid solution. The solution had been de-oxygenated, and both the transfer and subsequent titration were conducted under nitrogen. Ferrous ammonium sulfate solution was used as the titrant to the ferroin endpoint. The test measures the oxidizing power down to the trivalent form. A sample reduced by isopentane was found to have an average valence of 3.3, that is, nearly full reduction. Another sample, reduced by ethane, was found to have an average valence of 4.1. Again, the source of this remaining oxidizing power is uncertain. It could be attributed to unreduced Cr(VI) sites, but this does not quite agree with the number of oxygenated hydrolysis products sometimes released. Rather, it may suggest that much of the reduced Cr is still in the form of Cr(IV), which was also titrated. Although both primary and secondary positions on a reductant alkane were attacked by the Cr(VI), it is the latter, methylene positions, that might be expected to be most reactive. In general, this was indeed found to be the case. For example, the oxidation of n-pentane or nhexane produced oxygenates mostly at the 2 and 3 positions, along with smaller amounts of the 1-alcohol. Thus, when n-hexane was used as the reductant, the main hydrolysis products were 2-hexanol, followed by 2-hexanone, 3-hexanol, 3-hexanone and 1-hexanol respectively. Likewise, when n-pentane was used as the reductant, hydrolysis produced a similar pattern, namely 2&3-pentanols > 2-pentanone > 1-pentanol > 3-pentanone [23]. Unfortunately, the 2and 3-pentanols could not be cleanly separated in the chromatographic analysis. A few other products were also often released in smaller quantities, including alkenones and alkenols. It was usually the 2-position that was most favored over the 3-position, perhaps from statistical or steric considerations. Tertiary carbons were still more reactive, as shown by the products from isobutane and isopentane, with alcohols as the only possible product on the tertiary position. Even in the presence of secondary and primary carbons, tertiary carbons were usually most favored [18]. However, the other positions also reacted with lower yields. When the alkane contained primary and tertiary carbons, but no secondary carbons, as in isobutane, the tertiary carbon was again most favored. Reduction of the catalyst by toluene produced benzaldehyde, benzyl alcohol, methylphenols and benzophenone-type dimers. Also listed in Table 2 is the remaining average valence state of the Cr after reduction, calculated from the product distribution, as shown in the Supplementary Information. For example, formation of alcohols were considered as 2-electron reductions, and of ketones as 4-

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electron transfers. This computed valence would be an upper bound on the actual average oxidation state, since it only uses the detected oxygenated products in the calculation. That is, it would not count other potential reduction pathways that generate products not detected, such as hydrogen, alkanes, or perhaps alkenes. Some of the minor products detected, such as alkeneones, could require a 6-electron transfer if the product is H2O, or a four-electron transfer if the product is H2. The former would imply that two Cr sites are involved, perhaps in a secondary reaction after quenching as discussed above. But such products were usually only present in small quantities. Earlier hydrolysis studies have already been reported in which Cr(VI)/silica was reduced by olefins [8,9]. Alkenes generated wider assortment of oxidation products than alkanes, including carboxylates, carbonyls, olefinic oxgenates, but much less of the simple alcohols observed here from reduction of alkanes. Oxidation of olefins also sometimes resulted in products that indicated the cleavage of vinyl bond, whereas carbon-carbon bond clevage was not observed in the present study using other hydrocarbons. Alkylation of Cr by Cyclohexane It is also of interest to analyze the polymer produced after reduction of Cr(VI) by hydrocarbons. In the experiments that follow, Cr(VI)/silica-titania catalyst, calcined at 870°C, was reduced by three perdeutero-hydrocarbons. After reduction, the excess reductant was then evacuated out of the system, and proteo-ethylene (C2H4) was added to produce a small amount of polymer. This polymer was then dissolved, filtered, precipitated, and finally analyzed by 2HNMR. This procedure was developed to track the reductant. In Experiment 1 a sample of Cr(VI) catalyst was reduced by cyclohexane-d12. The color changed from the original orange to blue, indicating reduction, and as shown in Table 1, the cyclohexane-reduced catalysts were found to be active for ethylene polymerization. In Experiment 1, low-pressure proteo-ethylene (C2H4) was added to allow polymerization for about 3 minutes. When the polymer was analyzed by 2H NMR, the spectrum shown in Figure 2 was obtained. It indicates the presence of deuterium in the polymer. Several signals were detected, including D-methyl, D-methylene, possibly D-methyne, D-allyl and even some D-vinylic species. Figure 2 2H-NMR spectra of PE made by two alkane-reduced catalysts.

13

Scheme 3 illustrates how cyclohexane could be incorporated into the polymer chain. Four pathways are shown, which are not exhaustive, but they do represent the main reactions that seem to be involved. In each pathway the Cr(VI) reacts with a C-D bond on the cyclohexane. In the upper pathway (A), the cyclohexane does not alkylate the Cr. In fact, these sites might even be alkylated by ethylene later. But deuterium would not be incorporated into the polymer. Instead, only oxygenated non-polymeric by-products would be formed in pathway (A). Scheme 3 Pathways in which cyclohexane-d12 could be incorporated into a PE chain.

Cyclohexanol is represented as a product in Scheme 3 because it was the main redox by-product that was observed in the hydrolysis experiments. That should be a 2-electron transfer to yield Cr(IV). However, some cyclohexanone was also observed a product. This may indicate a 4-electron transfer to yield Cr(II) and H2O, or a three-electron transfer to yield Cr(III) and 1/2 H2. Another possible origin for the ketone is through secondary reaction after hydrolysis in which unreduced Cr(VI) sites oxidize the free cyclohexanol to cyclohexanone. After hydrolysis, disproportionation between Cr(II) and Cr(IV) may also occur. In the pathways (B), (C) and (D) of Scheme 3, Cr(VI) is reduced by cyclohexane-d12 to yield three different deuterated alkyl products, each of which can then initiate the first polymer chain when ethylene is added. In pathway (B), C-D bond rupture occurs on the reductant to place a single deuteride group on the Cr. Similarly, in pathway (C) the Cr-alkyl group is the cyclohexyl group, which can potentially also begin the first chain upon ethylene polymerization. Finally, pathway (D) is similar to (C) except that the alkyl group is now a cyclohexenyl group. The pathways shown in Scheme 3 could potentially result in five different initiating species on the first polymer chain. These are shown in Scheme 4. The NMR resonance values expected from each deuterium are also shown, which were obtained through computer simulation [49]. Because there is minimal overlap between the three pathways, that is, little shared NMR resonance, it is not difficult to estimate the relative contributions of each pathway using the observed NMR signals. For example, pathway (B) is the only one that can produce a D-methyl group and pathway (D) is the only pathway that can contribute the D-vinyl and D-allyl resonance.

14

Scheme 4 Expected 2H NMR chemical shifts of five possible products of reduction by cyclohexane-d12.

In Table 3 the five species in Scheme 4 have been combined in a proportion that attempts to replicate the observed NMR spectrum. On the left of the table in columns 1 and 2, we tabulate the chemical shifts of the observed NMR peaks along with their assignments. On the right of Table 3 in column 8 we see the relative intensity, or D-area, of each peak observed, expressed as a percent of the total D area. Keep in mind that these values are expressed as percents of all D atoms, or D resonance, not as percent of chains or initiating species. In the middle (columns 3-7) we list how many D atoms would be contributed by each path or species. For example, initiation by a deuteride in path (B) would incorporate one D atom into the PE chain as methyl. Or, initiation by a cyclohexane molecule in path (C) would contribute 11 D atoms into the chain as methylenes, and so on. Then, at the lower part of Table 3 there a row entitled "Percent of D-chains". This row indicates in what proportion each of the pathways, or more precisely the initiating D-species in Scheme 4 (i.e. D-initiated chains, not D atoms), has been combined to replicate the observed spectrum. At first the numbers entered here are just a guess. But from this guess it is then possible to calculate, using the D-atom distributions in the middle part of the table (columns 3-7 again), the predicted D-atom distribution by NMR resonance. That calculation is what is listed in column 9 ("Best fit"). By comparing column 8 with column 9 one can judge how successful Scheme 4 was at reproducing the observed NMR spectrum. Of course, the first guess produced a terrible fit, but with the help of an iterative program, the "Best-fit" can indeed be obtained, and that is what is shown in column 9 of the finished Table 3. Finally, the lower row entitled "Percent of D by path" is just a re-calculation of the species distribution shown in the row below ("Percent of D-chains"), except that now the distribution is weighted by D-atoms rather than D-species. Thus, going down Table 3, the first resonance, which has chemical shift of 0.95 ppm, is assigned to D-methyl, and is the result of incorporation of deuterium into the initiating group as shown in pathway (B), most likely in the form of ―CH2D. Pathway (B) is the only one in Scheme 3 capable of producing a methyl group. The magnitude of this peak is significant, indicating that path (B) accounts for a respectable percentage of first chains. The D-methyl signal is statistically much too strong (compared to methylenes) to be explained by simple H-D exchange.

15

Table 3 2H-NMR analysis of the polymer produced by cyclohexane-d12 reduction of Cr(VI). Chem.

Shift D/Chain from Each Path ppm Assignment (B) (C) (D1) (D2) (D3) 0.95 Methyl, ‒CH2D 1 0 0 0 0 Methylene, –CD2– 1.37 0 11 0 0 0 1.67 Methylene, –CD2– 0 0 4 4 3 2.06 Allyl, –CD2–C= 0 0 4 3 4 5.26 Vinyl, –CD= 0 0 1 2 2 Percent of D by path 5% 40% 39% 16% 0% Percent of D-chains 33% 25% 30% 12% 0% GPC Mn = 8.29 kg/mol D-chains as % of all chains = Chains containing D per Cr = Chains initiated with cyclohexane = Chains initiated with D (path B) = Minimum active Cr =

D by Peak Actual Best Fit Obsv. Calc'd 6% 5% 39% 35% 30% 27% 20% 25% 6% 8%

7.9% 0.011 0.008 0.004 1.2%

The next D-signal, with a chemical shift of 1.37 ppm, is assigned to methylene or perhaps also methyne resonances, since the two are predicted to lie very close. This is probably from the incorporation of cyclohexane as the initiating species as shown in pathway (C) of Scheme 3. (The expected methyne resonance could also be included in the next resonance, at 1.67 ppm.) The last three D-resonances, at 1.67, 2.06 and 5.26 ppm, are assigned to methylene, allyl and vinylene species as indicated in pathway (D) of Scheme 3. Note that alkenols and alkenones were also frequently detected as minor hydrolysis products (see Table 2 and the Supplementary Information). Only one double bond resonance was observed at 5.261 ppm which is characteristic of vinylene, or ‒CD=CD‒. A terminal vinyl group, ‒CD=CD2, should have also shown a second resonance at 4.6-4.8 ppm, which is missing in the spectrum. This is consistent with the double bond being part of the cyclohexyl structure, and not on the end of a PE chain created from normal chain transfer (which even if present, would not include deuterium). Thus, these signals, together with the GC-MS of hydrolysis products, suggest a reaction similar to that in Scheme 3 path (D) in which a vinyl group can be formed upon reduction. Since the most abundant redox product found in the hydrolysis solution was cyclohexanol (Table 2), we looked for evidence of the alcohol in the polymer as well. An alcohol group should have produced a prominent resonance at 3.6-3.8 ppm. But nothing could be seen in the spectrum in this region. Similarly, smaller amounts of cyclohexanone were also produced by hydrolysis. A ketone group in the polymer should have produced resonance at about 2.5 ppm, but again nothing was observed in the NMR spectrum in this region. Therefore, it seems unlikely that oxygenated species, although produced as ligands, were incorporated into the polymer. Thus, Table 3 sums up the contributions of each pathway to each NMR resonance, in which the relative amount of each pathway was decided by computer iteration to obtain the best overall fit to the observed NMR spectrum. At the bottom of Table 3 one can see the relative contributions of each pathway. They are listed first as weighted by D content, and below in terms of PE chains. Thus, pathway (B) accounts for about 5% of the measured deuterium in the polymer, and about 33% of D-containing chains, being initiated with a single deuterium. 16

Similarly, about 67% of the D-containing chains incorporated an intact cyclohexane-d12 group, from pathways (C) and (D), which accounts for about 95% of the measured deuterium in the polymer. Also shown at the bottom of Table 3 are some further derivatives of the "best fit". Note that D-initiated PE chains accounted for about 7.9% of all the chains measured by GPC. To obtain this number, the average D content per D-chain was determined by multiplying the D/chain contribution from each pathway, as shown in the middle of Table 3, by the relative proportion of each initiating species, as shown in the lower row ("Percent of D-chains). The sum of these products yields the average D-content per D-chain, that is, all chains containing D. The total number of moles of D was determined by NMR, and when that value was divided by the average number of D atoms in each D-initiated chain, this provides the number of moles of all D-chains. The total number of all chains (including those containing no D) is determined from the GPC average molecular weight (Mn). Thus, D-chains divided by all chains yields about 7.9% in this example. This is confirmed by 13C-NMR measurements, which indicated that the remaining 92% non-deuterated chains were initiated by –CH3 groups, and many were terminated by nondeuterated t-vinyl groups. These non-deuterated other chains can originate from two sources: 1) on the same D-reduced Cr sites through chain transfer after liberation of the first D-chain (as shown in Scheme 1), and/or 2) on other non-deuterated Cr sites produced from later reduction and alkylation by proteo-ethylene. Other consequences or derivatives of the data in Table 3 include the following. Dlabeled chains accounted for about 0.011 chains produced per Cr, of which 0.008 chains/Cr were initiated by cyclohexane-d12 as in pathways (C) and (D), and 0.004 chains were initiated by a single deuterium as shown in pathway (B). Thus, summing the two, one could say that in this experiment, at least 1.2% of the Cr was active. This is not very high in comparison to the two following experiments, or to other methods. However, unlike commercial production, this is to be expected when working at low temperature, low ethylene concentration, and short residence time. Alkylation by n-Hexane In a similar test designated Experiment 2, another sample of the same Cr(VI) catalyst was reduced with n-hexane-d14. The color changed from the original orange to light blue, indicating reduction of Cr(VI). This treatment was followed by evacuation of excess reductant and then polymerization of proteo-ethylene (C2H4). The resultant polymer was then recovered and filtered, followed by 2H NMR analysis, as in Experiment 1. As noted in Table 2, reduction of Cr(VI) catalyst by n-hexane, followed by hydrolysis, yielded several oxygenate isomers, with the 2-position favored. 2-Hexanol was the most abundant product. Figure 2 shows the resulting 2H-NMR spectrum, overlaid with that from Experiment 1, so that it compares reduction by cyclohexane-d12 with reduction by n-hexane-d14. The polymer was again found to have incorporated deuterium. The two spectra are very similar with the same five resonances being evident. The one notable difference is the magnitude of the peak at 0.915 ppm, which is much larger in Experiment 2. This signal is assigned to initiating Dmethyl groups, and consequently the observed difference in signal strength is understandable, given that incorporation of cyclohexane into the chain contributed no methyl groups, whereas incorporation of n-hexane at the favored 2-position can contribute two D-methyl groups as ―CD3. Scheme 5 illustrates how n-hexane could be incorporated into the polymer chain. It is analogous to the pathways shown in Scheme 3, except that incorporation of n-hexane could

17

produce up to 18 possible isomers to consider. Four main pathways are again shown, which are not exhaustive, but they do represent the main reactions that could be involved. Once more, each pathway in Scheme 5 starts with an attack by Cr(VI) on various C-D bonds on the nhexane-d14, and the 2-position has been used for illustration, given its prominence in the hydrolysis products. In pathway (A) of Scheme 5, the n-hexane does not alkylate the Cr, and these sites, if active, may be activated later by ethylene without incorporation of deuterium. Therefore pathway (A) does not contribute to the 2H-NMR spectrum. It produces only oxygenated nonpolymeric redox products. 2-Hexanol is shown as the main product, as was observed in the hydrolysis solutions. However, once again the ketone and other isomers could also be formed. Pathway (B) in Scheme 5 produces a Cr-D species, to which ethylene then adds, producing a ―CH2D methyl end-group. In pathway (C) the Cr becomes alkylated by n-hexane at the favored 2-position. Again, since hydrolysis products favor the 2-position, we have assumed a similar preference for the alkylated species. Alkylation at the 2-position would produce two D-methyl chain end-groups, that is ―CD3. In pathway (D) the n-hexane reductant is also oxidized to a hexene species, which is consistent with the small amounts of alkenols or alkenones that are often observed among the hydrolysis products. For simplicity, Scheme 5 shows Cr(IV) as the final oxidation state, but Cr(II) and Cr(III) could also be possible, depending on the oxidation products. Unlike Experiment 1, however, both types of vinyl resonance were observed, ‒CD= at 5.2 ppm and =CD2 at 4.8 ppm. The latter species was not possible from cyclohexane, which only allowed internal vinyls. Scheme 5 Pathways in which n-hexane-d14 could be incorporated into a PE chain.

18

Once again, there was no NMR evidence for oxygenates in the polymer. The resonance at 3.4-3.6 ppm from an alcohol, or at 2.5 ppm from a ketone, was not apparent in the NMR spectrum in Figure 2. In Experiment 1, only five possible initiating species where shown in Scheme 4, which was fairly easy to work with. In Experiment 2, however, the reactions in Scheme 5 with nhexane could potentially produce 18 different alkyl species capable of initiating a PE chain. With so many isomers to choose from, it is not difficult to find many combinations that successfully reproduce the observed 2H-NMR spectrum in Figure 2. Thus. it is not possible to propose an exact identity for the observed initiating species. It is even possible that the observed polymer signals contain some combination of all 18 alkylating species. Instead, we have selected just four of these species to use as an illustration for pathways (C) and (D). They are shown in Scheme 6 along with their expected NMR chemical shifts from computer simulation [49]. These four species were chosen to be representative, and also to be consistent with the observed redox products. Consequently, the four species all originate with the Cr attacking the 2-position of n-hexane. The relative amounts of these five initiation pathways (pathway (A) now also included) were then varied by computer iteration to determine a "best fit" to the observed NMR spectrum. It should be emphasized, however, that there are many possible "best fits" and that this process is mainly for illustration. Scheme 6 Expected 2H NMR chemical shifts of five possible products of reduction by n-hexane-d14.

Table 4 is comparable to Table 3 in Experiment 1, and the calculations involved have already been explained. The table lists the chemical shifts, assignments and the magnitudes of the observed 2H-NMR resonances in Experiment 2. The presence of D-methyl, D-methylene, and possibly D-methyne signals indicates that n-hexane-d14 was incorporated intact into some of the chains, almost certainly as the starting group on those Cr sites. The presence of so much D-methyl suggests that n-hexane-d14 preferentially reacted with Cr at the 2- and 3-positions (the 1-position would yield only half as many methyls), which were also favored in the redox product analysis (Table 2). This would account for the large difference in D-methyl signal intensity versus Experiment 1. The contributions of each pathway (isomer) to each of the observed signals is summarized at the right of Table 4 to produce a comparison between the calculated or "best fit" spectrum, versus the observed 2H-NMR spectrum. The match is quite close, but with so many isomers being possible, many other combinations of isomers could also work just as well.

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Table 4 2H-NMR analysis of the polymer produced by n-hexane-d12 reduction of Cr(VI).

Chem Shift D Contribution from Each Path ppm Assignment (B) (C) (D1) (D2) (D3) 0 3 3 6 1 0.92 Methyl, ‒CH2D or ‒CD3 0 7 2 0 6 1.35 Methylene, –CD2– 0 5 2 0 0 1.65 Methylene, –CD2– 2.05 Allyl, –CD2–C= 0 0 2 2 4 0 0 2 0 0 4.80 Vinyl, =CD2 5.26 Vinyl, –CD= 0 0 0 1 1 33% 0.4% 36% 3% 28% Percent of all D Percent of all D-chains 5% 31% 3% 28% 34% GPC Mn = 11.2 kg/mol D-chains as % of all chains = 14.7% Chains containing D per Cr = 0.085 Chains initiated with n-hexane = 0.081 Chains initiated with D (path B) = 0.004 Minimum active Cr = 8.5%

D by Peak Actual Best Fit Obsv. Calc'd 25.3% 25.3% 37.9% 37.9% 15.2% 13.1% 16.0% 14.6% 0.5% 0.5% 5.0% 8.6%

Also shown at the bottom of Table 4 are the results of some derivative calculations based on the data in Table 4. The contributions of each pathway are listed first as weighted by D-content, and second in terms of initiating species or PE chains. Thus, for this particular "best fit", only 0.4% of all measured deuterium came from pathway (B), initiation by a single D atom, whereas 99.6% came from intact incorporation of n-hexane-d14. Or, one could say that 5% of D-initiated chains came from pathway (B) and 95% of D-chains originated by the incorporation of intact n-hexane-d14 in pathways (C) and (D). Keep in mind that selecting a different set of starting species, rather than those in Scheme 6, can produce a different "best fit", that is, a different combination of species, which can change these numbers a little. Note also that D-labeled PE chains accounted for about 14.7% of all chains detected by GPC. This amounted to 0.085 D-initiated chains per chromium atom. These numbers were calculated as described in Experiment 1. And as explained above, the non-D-labeled PE chains could have resulted from later chain transfer on D-initiated sites, as in Scheme 1, or alternatively from virgin Cr(VI) sites that were not initiated until the introduction of proteo-ethylene. Nevertheless, one could say (from this particular "best fit") that at least 8.5% of the Cr was active for polymerization in this short experiment. Finally, another interesting observation was made during these experiments. Reduction by n-hexane, followed by ethylene polymerization, produces even-numbered oligomers, as detected by GC-MS analysis out to about C26. Of course, even-numbered oligomers are common in normal polymerization, since all chains after the first one would be initiated with ethylene. However, when the experiment was repeated, but with n-pentane as the reductant instead of n-hexane, a series of odd-numbered oligomers were also detected in the polymerization solvent, including C9, C11, C13, C15, C17, C19, C21, C23, and C25, in addition to the expected even-numbered series. This is another suggestion of initiation of the first chain by Cr-alkyl on some sites, that is, alkyl being C6 in one case and C5 in the other. Alkylation by Toluene As shown in Table 1, aromatics can also be used to reduce Cr(VI) catalysts. The color changed from orange to blue-black, indicating reduction, and this also results in a catalyst active for ethylene polymerization. Upon quenching such toluene-reduced catalysts with water or methanol, benzylaldehyde was recovered as the main redox hydrolysis product, as summarized 20

in Table 2. Benzyl alcohol, methylphenols and benzophenone-type dimers were also obtained. This suggests that the benzyl protons on toluene were most reactive, but the ring protons also participated to some degree. In Experiment 3, another sample of the Cr(VI)/silica-titania catalyst was reduced with toluene-d8 using the same protocol described in Experiments 1 and 2. As expected, the color changed from orange to blue-black, indicating reduction of the Cr(VI). Then excess reductant was evacuated out of the system, and afterward the catalyst was allowed to polymerize proteoethylene at 80°C. Upon recovery and purification of the polymer, 2H-NMR analysis of the polymer again revealed the presence of deuterated species incorporated into the polymer. The spectrum obtained is shown in Figure 3. In Experiments 1 and 2, trichlorobenzene (TCB) was used as the NMR solvent to dissolve the polymer. To quantify the experimental NMR resonances, their area was compared to that of the naturally abundant deuterium in the TCB solvent. Being aromatic, these TCB signals served well as reference peaks, because their chemical shift was well away from the that of experimental peaks being studied. For this reason, the solvent in Experiment 3 had to be changed to trichloroethane, in order to avoid overlap with any aromatic signals coming from the experimental polymer. Thus, notice in Figure 3 that there is indeed aromatic resonance from the experimental polymer, which can only mean that toluene was incorporated intact into some the polymer. Notice also the new benzyl resonance at about 2.4 ppm. Figure 3 2H-NMR spectrum obtained from PE made using toluene-d8 reduced catalyst.

Scheme 7 shows how reduction of Cr(VI) catalyst by toluene-d8 could lead to incorporation of deuterium into the polymer. Analogous to Schemes 3 and 5 in Experiments 1 and 2, Scheme 7 shows four pathways in which the Cr(VI) catalyst could react with toluene-d8. A fifth method is also discussed further below. In pathway (A) there is no alkylation of the Cr, and therefore this path does not contribute to the 2H-NMR spectrum. If active at all, such sites may initiate later with ethylene. Pathway (A) shows benzaldehyde as the product, which is consistent with the hydrolysis products in Table 2.

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Pathway (B) in Scheme 7 shows how toluene-d8 could create deuterated methyl groups, or ‒CH2D. This would explain the presence of the 2H-NMR resonance signal at the chemical shift of 0.975 ppm, labeled "Methyl" on the spectrum shown in Figure 3. Pathway (C) in Scheme 7 shows Cr(VI) attacking the benzyl group on toluene-d8 to alkylate the Cr with a benzyl group. This would explain the presence of aromatic resonance in the NMR spectrum, at a chemical shift of about 7.3. Analysis of hydrolysis solutions from toluene-reduced catalyst also indicates the presence of methylphenols, which indicates that, although the benzyl protons are most reactive, Cr(VI) is also capable attacking the aromatic hydrogen on the benzene ring. Therefore, this type of alkylation is shown in pathway (D) of Scheme 7. This mechanism would also incorporate aromatic species into the polymer chain, which in addition to pathway (C) contribute to the observed NMR resonance at chemical shifts of about 7.3. Actually, in terms of NMR resonance, there is little difference between pathways (C) and (D). That is, both pathways result in species with about the same chemical shift. There is a difference, however, in the relative amounts of these species. Notice that pathway (C) should produce a phenyl to benzyl deuterium ratio of 5:2, whereas for pathway (D) it is 4:3. Thus, it is interesting to note that the observed phenyl to benzyl D ratio was almost exactly 5:2. This is illustrated in Scheme 8, which shows all of the possible isomers that could be produced from alkylating Cr in pathways (B), (C) and (D). Unlike Experiment 2, in this case there are a reasonably small number of initiating species to consider. Scheme 7 Pathways in which toluene-d8 could be incorporated into a PE chain.

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Scheme 8 Expected 2H NMR chemical shifts of the six initiating species by toluene-d8.

Finally, there is also a strong NMR signal in Figure 3 with a chemical shift of about 1.37 ppm. This is assigned to methylene. However, Scheme 7 cannot explain the presence of simple methylene. We believe that this signal originates by H-D exchange during the reduction and polymerization reactions, in the presence of a strong Lewis-acidic catalyst. This outcome is listed as pathway (E) in Scheme 8. H-D exchange could happen at several times during the reactions conducted in these experiments. Two examples are shown in Scheme 9. In the upper part of Scheme 9, the deuterated ligand is seen exchanging with H on the growing polymer chain. In the lower half of Scheme 9, free residual toluene-d8 is shown exchanging with the growing polymer chain. Actually, some effort was made during these experiments to evacuate excess residual toluened8 before ethylene was introduced. Nevertheless, it is always possible that small amounts remained. In addition, there are many other mechanisms by which H-D exchange could occur, both in Experiment 3, where it is easy to see, and also in Experiments 1 and 2, where the resultant methylene would be invisible in the presence of so much methylene from the incorporated reductant. Again, there was no apparent evidence of oxygenated species in the polymer. A phenol group would have displayed a resonance at about 6.7 ppm, or a benzyl alcohol group at 4.6 ppm, both of which were missing. The chemical shifts of the observed 2H-NMR peaks are listed in Table 5 along with the relative amounts of each. The deuterium contributions are listed both by NMR peak type as well as by reaction pathway. The presence of aromatic and benzyl resonance indicates that toluened8 was incorporated intact into some of the PE chains, most likely through initiation of the first chain produced from various Cr sites. The D-contributions of each pathway are also shown in Table 5. These values were determined by using an iterative program to obtain the best match to the observed D-content by resonance type. Since there is no way to distinguish between the three isomers from path (D), all of these isomers have been combined in the table under the title of (D1). Even then, however, the contribution from path (D) was small. This preference for reaction with the benzyl hydrogen, versus those on the ring, is consistent with the reduction studies discussed above and the known chemistry of toluene. The two right columns in Table 5 compare the calculated

23

"best fit" data to that actually observed in the 2H-NMR spectrum shown in Figure 3. In this example, the fit is perfect due to the lack of much NMR overlap between the proposed species. Scheme 9 (Pathway E) H-D exchange could be possible in many ways, such as between: 1) above, the PE chain and deuterated ligands, and 2) below, the PE chain and free toluene-d8 that was not entirely evacuated away.

Also shown at the bottom of Table 5 are some further derivatives of the best-fit data. Note that D-labeled PE chains accounted for about 36% of all PE chains, as detected by GPC. This number was calculated as described in Experiment 1. Again, the non-deuterated PE chains could have originated on: 1) non-deuterated Cr sites, those slow to react with toluened8; or 2) on Cr sites that were initiated by deuterated species, but as the 2nd or 3rd chain produced through chain transfer (Scheme 1) which would not contain deuterium. Table 5 2H-NMR analysis of the polymer produced by toluene-d8 reduction of Cr(VI).

Chem Shift Assignent ppm 0.98 Methyl 1.37 Methylene 1.8-2.9 Benzyl 7.3-7.4 Phenyl Percent of all D (best fit) Percent of all D chains GPC Mn = 11.7 kg/mol

D Contribution from each path (B) (C) (D1) (D2) (D3) (E) 1 0 0 0 0 0 0 0 0 0 0 1 0 2 3 3 3 0 0 5 4 4 4 0 13% 49% 1.0% 0% 0% 37% 22% 12% 0.3% 0% 0% 65% D-chains as % of all chains = Chains containing D per Cr = Chains initiated with toluene = Chains initiated with D (path B) = Minimum active Cr =

Actually Observed 12.8% 37.4% 14.4% 35.4%

Best fit Calc'd 12.8% 37.4% 14.4% 35.4%

36% 0.49 0.06 0.11 17.2%

About 0.49 D-chains were produced per Cr. This number counts all chains containing deuterium from all pathways. In contrast, about 0.06 of these chains were initiated by intact toluene-d8 through pathways (C) and (D), and about twice that number (0.11 chains/Cr) were initiated by a deuteride through pathway (B). This suggests that in this experiment at least 17% of the Cr was active.

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D-Incorporation During Polymerization In Experiment 4, another sample of the same Cr(VI) catalyst was reduced by proteo-nhexane (n-C6H14), using the same protocol described above. Then, after evacuation for half an hour, it was allowed to polymerize proteo-ethylene at 50°C for 25 minutes. Midway through the polymerization, n-hexane-d14 was added as a solvent, and then the polymerization was continued for another 65 min. After the desired amount of polymer had been made, the reaction was quenched with an injection of isopropanol, and the polymer was recovered and purified. 2H

NMR analysis of the polymer again confirmed the presence of deuterium in the polymer. However, only D-methylene was detected, no other species. This indicates that the only mechanism of deuterium incorporation was through H/D exchange during polymerization, as illustrated, for example, in the lower half of Scheme 9. Integration of the NMR signal indicated that about 0.22 D atoms had been transferred per chain, or one could say that about a quarter of chains contained 1 deuterium. This result indicates that the incorporation of reductant molecules into the PE chain occurs during the initiation of the first chain. After that, the only deuterium incorporated is through incidental H/D exchange to produce D-methylene groups.

CONCLUSIONS Cr(VI) catalysts can be reduced by olefins, alkanes, aromatics and other hydrocarbons to produce active catalysts. Oxygenated by-products are formed, and a clue about their identity comes from subsequent hydrolysis and analysis of the quench solution. When reduced by olefins, the products are complex and have been extensively cataloged in earlier reports [9,11]. The redox products from reduction by alkanes have now also now been studied, both in this report and previous ones [18,21]. Reduction by alkanes may be somewhat less complex. Table 2 summarized the results of many reduction experiments and analyses. These oxygenated ligands become a permanent part of the catalyst site, influencing its activity and the character of the polymer it produces. In this study, Schemes 3, 5 and 7 speculated on how a variety of both oxygenated and non-oxygenated products could be simultaneously produced upon reduction of Cr(VI), depending on the reaction mechanism, and the final oxidation state of the Cr. Using deuteriumlabeled hydrocarbons has shown that the reductant can also become incorporated into the polymer chain. After reduction and alkylation, subsequent polymerization of proteo-ethylene yielded many deuterated species in the polymer. Deuterated alkyl species, left attached to the Cr from the reductant, most likely become initiating species for the first PE chain produced on at least some of the active sites. The wider picture developed by this report, and by the preceding reports [8,10,9,18,21], as well as those from the Turin group [11,50], and also the Scott group [12,14,51], is that the process of Cr site activation (reduction and alkylation) can proceed not through just one, but many different pathways, depending on the catalyst, the reaction circumstances, and it may even vary between Cr sites on the same catalyst [11,18,21].

ACKNOWLEDGEMENTS The authors wish to thank Chevron-Phillips Chemical Co. for supporting this research. We are also indebted to Kathy S. Clear for help in the synthesis of catalysts.

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RESEARCH HIGHLIGHTS 

Active sites on Phillips Cr(VI)/silica polymerization catalysts are normally reduced and alkylated by ethylene, but non-olefinic hydrocarbons have now also been found to react.



Reduction of Cr(VI) by hydrocarbons results in oxygenated ligands, as well as Cr alkyls or hydride species, from which the first polymer chain grows.



Polymer character, and Cr activity, are controlled by ligands left after initiation.



Isotopic labeling of the reductant, during Cr/silica initiation, indicates that the same hydrocarbon often begins the first PE chain during ethylene polymerization.

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