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Stabilizing Single Sites on Solid Supports: Robust Grafted Ti(IV)Calixarene Olefin Epoxidation Catalysts via Surface Polymerization and Cross-Linking Yijun Guo,† Andrew Solovyov,† Nicolás A. Grosso-Giordano,† Son-Jong Hwang,‡ and Alexander Katz*,† †

Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720, United States ‡ Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *

ABSTRACT: This manuscript develops a surface polymerization and cross-linking approach for the stabilization of single-site catalysts on solid surfaces, which is demonstrated here for grafted Ti(IV)-calixarene Lewis acids on silica. Our approach relies on cationic polymerization that is initiated by an adsorbed B(C6F5)3 and uses styrene as the monomer and diisopropenylbenzene as the cross-linking agent. The mildness of this polymerization method is demonstrated by its lack of blocking micropores and only slight consumption of mesopore internal surface area on the basis of N2 physisorption data at 77 K, both of which are in contrast to previously reported surfacepolymerization approaches. Catalysis of samples before and after polymerization and cross-linking was investigated with a probe reaction consisting of the epoxidation of 1-octene with tert-butyl hydroperoxide as oxidant, which is known to be catalyzed by Lewis-acid sites, and a comparison of catalyst hydrolytic stability was performed. Added water in the latter was used as a a trigger to induce site aggregation, as a stress test to determine the effectiveness of site protection by our polymerization approach. Consistent with the N2 physisorption data, catalysis data demonstrate that surface polymerization does not block small-molecule reactant and product access to Lewis-acid sites on the surface, since the conversion remains essentially unchanged before and after surface polymerization and cross-linking. DR UV−vis, TGA, and catalysis data reveal that the grafted Ti(IV)-calixarene sites on silica maintain their catalytic activity even after being treated with corrosive protic stress-test solution. In sharp contrast, grafted sites without the polymer layer leach nearly all of their calixarene and Ti contents during similar stress testing, resulting in the near complete loss of catalytic activity. We hypothesize that the surface polymer acts as a nanoreactor gatekeeper, which prevents the large Ti(IV)-calixarene site from leaching and keeps surface complexes as single sites grafted on the silica surface, by blocking access for the migration of sites from the surface to bulk solution. KEYWORDS: Lewis acid catalyst, surface polymerization, olefin epoxidation reaction, leaching, nanoreactor gatekeeper

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INTRODUCTION Approximately 47% of commodity propylene oxide produced industrially worldwide is synthesized via propylene epoxidation using organic hydroperoxide as oxidant.1 Conventional isolated Ti(IV) sites grafted on the surface of silica are known to be highly selective and active catalysts for this reaction;2−5 however, catalyst deactivation continues to be a significant issue.6−8 A proposed deactivation mechanism involves aggregation of isolated Ti sites grafted on silica into more coordinatively saturated aggregates.9−15 Such aggregates are less catalytically active for at least two reasons. First, some metal is buried within the internal bulk of the aggregate and as such is less accessible or entirely inaccessible to incoming reactants. Second, the metal coordination environment within such an aggregate possesses diminished Lewis acidity and reactivity, due to the higher coordination number, as previously demonstrated in well-defined molecular systems where the © 2016 American Chemical Society

coordination number of grafted isolated Ti sites on silica was systematically varied using a permanently bound organic ligand.16,17 Previously, grafted Ti(IV)-calixarene complexes on silica have been demonstrated as active olefin epoxidation catalysts, which have a rate that is 2-fold higher than that of a conventional site consisting of an inorganic grafted Ti(IV) on silica, lacking the organic ligand, for the epoxidation of an internal alkene using organic hydroperoxide as oxidant.16−18 Within this class of grafted Ti(IV)-calixarene on silica catalysts, the calixarene helps preserve the single-site nature of the isolated Ti(IV) grafted on silica Lewis acid site; however, these catalysts have yet to be tested under flow conditions at Received: July 17, 2016 Revised: September 11, 2016 Published: September 14, 2016 7760

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ACS Catalysis temperatures above 80 °C, which is more pertinent to conditions where grafted Ti-on-silica catalysts are used in industry for epoxidation. This is a harsh condition from the perspective of challenging the catalyst under corrosive proticsolvent conditions at moderate temperatures and in the midst of ligand-transfer processes accompanying the catalytic cycle. As we demonstrate here, this can lead to significant calixarene and even grafted Ti leaching from the silica support. Historically, methods for improving the robustness and stability of grafted Ti(IV)-on-silica epoxidation catalysts have employed hydrophobic surface modification of the catalyst surface.19−23 In addition, previously, elegant in situ polymerization methods have been described, which increased the robustness of catalysts consisting of grafted Ti centers on mesoporous SBA-15 as silica support. This approach relied on synthesis of a Ti-silsesquioxane complex, which consisted of an isolated Ti site and an organic vinyl group. This complex was further immobilized onto the silica support surface by wet impregnation and copolymerization with styrene and divinylbenzene. The resulting catalyst showed much higher activity than its homogeneous counterpart in H2O2-cyclooctene twophase reaction media.24 A separate type of approach for endowing robustness to active sites on supports does not involve covalent linkages that act to stabilize the active site; instead, it involves mechanical encapsulation of a catalyst into the mesoporous cage of a solid support (SBA-16) and has been demonstrated for homogeneous Co or Ru chiral complexes. In this approach, a nanoreactor is formed by the mesopores, which effectively entraps catalyst therein. The encapsulated catalyst has been previously reported to exhibit performance that is comparable to that of the homogeneous catalyst in solution, in a manner that did not block sites in comparison to the surfacepolymerization approach above. Crucially, due to entrapment inside of the nanocages, the catalyst could be recycled for more than 10 times without loss of performance, which is consistent with a lack of active-site leaching.25−27 Here, in this paper, we develop a very mild surfacepolymerization approach for increasing the robustness of grafted Ti(IV)-calixarene sites on a silica surface, in the face of corrosive environments accompanying olefin-epoxidation catalysis under flow conditions. The resulting catalysts represent a new generation of grafted calixarenes with greatly improved stability, while still possessing similar accessibility of the original grafted Ti(IV)-calixarene sites on silica. Within the context of this surface-polymerization approach, we investigate whether or not the site itself requires a vinyl substituent for covalent attachment to the polymer, as in the silsesquioxane example above, or whether solely noncovalent interactions between polymer and sites are sufficient in order to endow more robust character, as in the entrapped example above. The former scenario involves a mechanism of covalent anchoring of the site for its subsequent stabilization, as a result of direct bonding to surface-polymer chains, whereas the latter instead involves a nanoreactor type of mechanism, wherein the leaching of sites is mechanically blocked by a gatekeeper. Such a blockage stabilizes grafted sites on the surface by preventing them from accessing bulk solution, where they are washed out. We investigate these two possibilities within the context of our surface polymerization strategy, developed here for sites based on grafted Ti(IV)-methyl-calixarene (Ia) and Ti(IV)vinyl-calixarene (IIa) complexes on silica. A central question reduces to whether there is an advantage to using our surface-

polymerization approach with a vinyl-containing site of IIa relative to one that lacks the vinyl substituent in Ia. The design of these two sites is guided by the fact that only IIa can form a covalent bond to a growing polymer chain, due to the presence of the vinyl substituent, which Ia lacks, as shown in Figure 1.

Figure 1. Ti-methyl-calixarene (Ia) and Ti-vinyl-calixarene (IIa) precursors anchored on a silica surface.

For both sites, polymerization was conducted either with or without cross-linking, using either styrene/diisopropenylbenzene (95/5 v/v) as monomers to affect cross-linking or styrene alone without cross-linking, as shown in Figure 2. The comparison of the stability of these two sites after polymerization informs on which mode of action the surface polymer layer uses in order to preserve the stability of grafted Ti(IV)calixarene Lewis acid sites on silica: a nanoreactor involving solely noncovalent connectivity between sites and polymer versus a mechanism that relies on covalent connectivity between the two. While the experiments described above are conducted within the context of a grafted-Ti catalyst system for olefin epoxidation involving organic hydroperoxide as oxidant, the broader question relating to the two mechanisms (mechanical stabilization via a gatekeeping mechanism versus covalent attachment of surface polymer chains to the site) of stabilizing a surface-anchored site are general. As such, the results of this study are expected to have broader ramifications into mechanisms of stabilizing other homogeneous and single sites on solid surfacesand inform as to possible mechanisms for such stabilization.

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EXPERIMENTAL SECTION Polymerization of Ti(IV)-Calixarene-SiO2. The polymerization reaction was carried out via cationic polymerization in CH2Cl2 initiated by B(C6F5)3 at ambient temperature. A B(C6F5)3 (12.5 mg, 0.024 mmol) solution in CH2Cl2 (1 mL) was added dropwise to a slurry of 0.5 g of Ti-calixarene-SiO2 in 2 mL of CH2Cl2 with stirring. After the mixture stood for 5 min, 1 mL of a styrene/diisopropenylbenzene mixture (0.95/ 0.05 v/v) (cross-linked polymer), or 1 mL of styrene (linear polymer) was introduced to the slurry, which was then stirred for 16 h at room temperature. The polymerized material was recovered by filtration and washed with CH2Cl2 (5 mL × 8). The resulting yellow/orange powder was dried under dynamic vacuum at room temperature for 3 h and then at 120 °C for 1 h. Batch Catalytic Epoxidation of 1-Octene with TBHP. In a typical olefin epoxidation experiment, 25 mg of Ticontaining catalyst was introduced to a 25 mL round-bottom Schlenk flask. The reactor was sealed with a PTFE-coated rubber septum, degassed under dynamic vacuum at 25 °C for 16 h, and then heated at 120 °C for 1 h to remove any residual 7761

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Figure 2. (a) Ti(IV)-calixarene sites grafted on SiO2 surface without polymerization. (b) Linear polymerized Ti(IV)-calixarene sites grafted on SiO2 surface via styrene, without a cross-linker. (c) Polymerized and cross-linked Ti(IV)-calixarene sites grafted on a SiO2 surface via a mixture consisting of styrene/diisopropenylbenzene. R indicates either an alkyl (derived from site in Ia) or vinyl (derived from site in IIa) substituent. The vinyl substituent may be covalently attached to the polymer network in (b) and (c) according to one of the hypotheses of this work.

powder was then weighed into a Schlenk tube and added to a freshly prepared reaction mixture (20 mL of octane and 0.66 mL of 1-octene). A 0.11 mL portion of TBHP in nonane was added in order to start the catalytic reaction.

moisture. The reactor was cooled to room temperature and back-filled with Ar. A 20 mL portion of octane (solvent) and 0.66 mL of 1-octene (4.20 mmol, 6.37 M) were added to the flask. The mixture was heated to 50 °C in an oil bath with vigorous magnetic stirring, and then 0.11 mL of tert-butyl hydroperoxide (TBHP, Aldrich, ∼5.5 M solution in nonane) was added to initiate the olefin epoxidation reaction. Aliquots (0.2 mL) were taken using a syringe, filtered to remove the catalyst, and analyzed using an Agilent 6890 GC system equipped with a flame ionization detector using an HP-1 methylsilicone capillary column. Nonane present in the TBHP solution was used as an internal standard. TON was calculated on the basis of moles of epoxide produced per mole of Ti sites. Conversion was calculated on the basis of moles of TBHP consumed over moles of TBHP initially in the reaction mixture. Batch and Flow Leach Stress Tests. Batch leach stress testing was conducted according to a published procedure with slight modification.28 A 50 mg portion of the catalyst was stirred on a stirring plate in the presence of 5 mL of isopropyl alcohol and 5 mL of DI water at room temperature for 4 h. The catalyst was then filtered on a fritted funnel and washed with 6 × 3 mL toluene. The resulting catalysts were dried under dynamic vacuum at room temperature for 3 h and 120 °C for 1 h before TGA. Flow leaching stress testing was conducted by washing 50 mg of material on a fritted funnel using a simulated epoxidation reaction solution (10 × 1 mL). The simulated solution contained 5.48 M 1-octene, 0.53 M tert-butyl alcohol, 0.36 M tert-butyl hydroperoxide, 0.17 M nonane, 0.02 M water, and 0.01 M acetone. The catalyst was then washed with 6 × 3 mL of toluene. The resulting catalysts were dried under dynamic vacuum at room temperature for 3 h and 120 °C for 1 h before TGA and catalytic recyclability tests. Catalyst Recyclability Tests. After flow leach stress tests, the resulting catalysts were placed under dynamic vacuum at room temperature for 3 h and 120 °C for 1 h to remove any leaching stock solution that was entrapped. 25 mg of the dried

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RESULTS AND DISCUSSION Synthesis of Polymerized and Cross-Linked Grafted Ti(IV)-Calixarene Catalysts. As shown in Figure 2, polymerization performed with only styrene (i.e., lacking a cross-linking monomer) results in a one-dimensional linear polystyrene on the silica surface, as shown by materials Ib (derived from Ia) and IIb (derived from IIadesigned to probe the effect of having a vinyl substituent on the calixarene directly for possible covalent bonding with polymer), while the expected corresponding catalysts synthesized with two-dimensional crosslinking are represented by Ic and IIc. In both cases, the Lewis acid catalyst B(C6F5)3 was used for initiating surface cationic polymerization of styrene and cross-linking monomer.29,30 This compound was previously shown to adsorb to silica and induce dehydration of surface silanol groups to yield chemisorbed  SiOB(C6F5)2 sites on the silica surface, which we hypothesize to facilitate polymerization close to the silica surface, when using our approach.31 Some SiOB(C6F5)2 sites could be observed to be remaining behind on the silica support surface after polymerization and catalyst synthesis, via 11B MAS NMR spectroscopy (Figure S5 in the Supporting Information). A control catalyst was therefore synthesized in the absence of grafted Ti(IV)calixarene. This catalyst consisted of B(C6F5)3-treated silica after polymerization using a mixture of styrene and diisopropenylbenzene (95/5 v/v) monomers. We assessed the catalytic significance of remaining boron in this control material by measuring its epoxidation catalytic activity. Because there was no activity in this control, we conclude that any remaining chemisorbed B(C6F5)3 had no catalytic consequen7762

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Table 1. Comparison of Hydrolytic Stability and TON for 1-Octene Epoxidation for Various Grafted Ti-Containing Catalysts and Their Polymerized Products

a Content before stress leaching test. bOrganic content after flow stress leaching test on a fritted funnel. Organic content (%) = 100 × (organic content after leaching test)/(organic content before leaching test). cTON after 5 h reaction with Ib,c. dTON after 5 h reaction with Ibl,cl. eA dash for an entry indicates not applicable.

ces (Figure S8 in the Supporting Information) for the data reported here. The calixarene and Ti concentrations in Ia and IIa were measured using TGA and a spectrophotometric assay, respectively. As summarized in Table 1, the Ti concentration is 124 mmol/g in Ia and 98 μmol/g in IIa, with the Ti:calixarene ratio being nearly unity for both, as expected for each grafted Ti being complexed to a cone-calixarene ligand. In order to assess possible pore blockage, the N 2 physisorption isotherm and calculated pore-size distribution are compared before and after polymerization and cross-linking in Figure 3 and Table 2. The BET surface area and pore volume decreases only slightly after surface polymerization and crosslinking, from an initial value of 281 m2 g−1 (Ia) to 245 m2 g−1 (Ic) and from 0.43 cm3 g−1 (Ia) to 0.39 cm3 g−1 (Ic), respectively. Furthermore, when isotherms for Ia and Ic are compared, the low-pressure region in Figure 3a nearly overlaps. This suggests a complete lack of micropore blockage by surface polymer in Ic. Indeed, within the mesopore region, the two hysteresis loops are similar in width, which indicates that the pore-size distributions do not change appreciably as a result of surface polymerization and cross-linking in Ic.32 The magnitude of the uptakes in the mesopore region of the isotherm show only slight partial blockage of mesopores, consistent with a thin layer of polymer and cross-linking agent residing there. Had surface polymerization occurred solely on the external surface of the silica particle, no mesopore blockage would have been observed. We therefore conclude that styrene polymerization

on a material consisting of grafted Ti(IV)-calixarene sites in Ia selectively occurs in internal mesopores rather than micropores and forms in thin layers that do not completely block pores. Our observed lack of micropore blockage is in stark contrast with data from previously reported surface polymerization strategies in porous catalyst materials. For example, in 2005, Ryoo et al. employed free radical polymerization to imbibe polymer in mesopores of silica via a wet-impregnation approach.33 The resulting material had a polymer loading varying from 10% up to 30% by weight, much higher than the amount of polymer incorporated in catalysts here in this paper (vide infra). Adopting a similar free-radical polymerization approach, Li et al. elegantly immobilized titanium silsesquioxane complexes on the surface of mesoporous SBA-15.24 Although both of these approaches synthesize a polymer coating on the silica surface, they form polymer in a manner that significantly blocks micropores as well as mesopores. The latter is manifested by a broadening of the hysteresis loop in the N2 adsorption isotherm following polymerization.24 Here, in contrast, we discovered a much gentler surface-initiated cationic polymerization of styrene, using chemisorbed B(C6F5)3 on silica, which results in such a thin layer of polymer coating that only partially occupies mesopores of silica, leaving micropores largely unperturbed and intact for facilitating facile internal mass transport. Catalytic Activity of Polymerized and Cross-Linked Grafted Ti(IV)-Calixarene Catalysts. In order to characterize the catalytic accessibility of sites in our grafted Ti(IV)7763

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these values were 27.1 and 29.7%, respectively. Vinylsubstituted catalysts IIa,c gave nearly identical TON and conversion values (Figure S7). This allows us to conclude that surface polymerization and cross-linking of sites in both Ia and IIa did not block sites, as well as the fact that these treatments did not decrease the number of Ti(IV)-calixarene complexes on the catalyst, since, as previously shown, a decrease in the number of Ti(IV)-calixarene complexes would otherwise lead to a decreased catalytic activity for these single-site materials which is not observed.16 This result further underscores the gentle nature of our surface polymerization and cross-linking synthetic approach, as discussed above on the basis of N2 physisorption data, and specifically the lack of observed micropore blockage after polymerization, which facilitates free transport of reactants and products. The observed lack of Ti(IV)-calixarene site leaching during polymerization and cross-linking is not surprising, given the aprotic solvent conditions for effecting polymerization and cross-linking. No differences in epoxide selectivity (moles of epoxide produced per mole of TBHP reacted) were observed between any of the samples, and these values are typical of what has been reported previously for the same reaction performed under our conditions.17 Solid-State NMR Spectroscopy of Polymerized and Cross-Linked Grafted Ti(IV)-Calixarene Sites. TGA data of Ia,c are shown in Figure S9 in the Supporting Information. The resultant polymerized and cross-linked catalyst Ic has 11.8 wt % of organic species, compared with 9.2 wt % of organics before polymerization in material Ia. The observed organic content increase is consistent with some polymer successfully incorporated into sample Ia. We estimated the number of monomer units per calixarene incorporated into the catalyst, on the basis of TGA results before and after polymerization and cross-linking. According to data in Figure S9, the 2.6 wt % increase in organic content for Ic relative to Ia corresponds to around 3 repeat monomer units per grafted calixarene (assuming the molecular weight of a monomer unit corresponds to 104). On calculation by the same method with data in Figure S9, Ib shows 2 repeat monomer units per calixarene. Similarly, IIc exhibits a non-calixarene organic loading corresponding to 5 monomer repeat units per calixarene. Thus, in all cases we investigated here, there are relatively low loadings of polymer relative to calixarene sites. Indeed, from a length perspective, this hardly represents enough polymer to wrap around a single calixarene site, let alone the length between sites, which has been previously shown to be a distance of 2 nm on average at the highest possible grafted calixarene density, corresponding to the jamming limit for random irreversible chemisorption.17 Given just how little polymer was incorporated, questions related to this are as follows. How does the polymer arrange itself on the silica surface? Does it have any covalent contact with the grafted Ti(IV)-calixarene sites on the surface in the case of the vinyl substituent on the calixarene? In particular, the vinyl substituent of the site in catalyst IIa may in principle actively participate in the surface polymerization by forming a covalent bond to the surface polymer. On the other hand, it

Figure 3. (a) Nitrogen physisorption isotherms of Ia (blue line) and Ic (red line) surface polymerization and cross-linking. (b) Pore-size distributions calculated from the NLDFT model using desorption isotherms.

Table 2. Properties of Catalysts Ia,c

a

material

SBET (m2 g−1)

Vmesoa (cm3 g−1)

polymerized SiO2@ 800 Ia Ic

223

0.36

281 245

0.43 0.39

Ti/calixarene ratio

1.0

Vmeso is the mesopore volume, calculated from the t plots.

calixarene epoxidation catalysts before and after polymerization, we chose to assess the activity of 1-octene epoxidation, as a model test reaction with organic hydroperoxide TBHP as oxidant at 50 °C (Scheme 1). The turnover number (TON) and conversions of Ia−c and IIa,c as a function of reaction time are presented in Figure 4, Figure S7 in the Supporting Information, and Table 1. Over a 5 h period of reaction, catalyst Ia achieved a TON of 28.8 and conversion of 31.3%, Ib achieved a TON of 27.4 and conversion of 31.5%, and for Ic, Scheme 1. Epoxidation of 1-Octene with TBHP

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Figure 4. (a, b) Activity of 1-octene epoxidation reaction representing comparison of Ia (hollow blue diamond) with Ib (solid red diamond) and Ibl (solid green triangle) for 1-octene epoxidation reaction. (c, d) Comparison of Ia (hollow blue diamond) with Ic (solid red diamond) and Icl (green triangle). TON = (mol of oxidation products)/(mol of Ti). Conversion (%) = (mol of TBHP consumed)/(mol of initial TBHP).

can be attributed to phenyl groups of the polymer backbone.37 More interestingly, the intensity of the vinyl resonance at ∼120 ppm remains almost the same for IIa−c. While in the case of the cross-linked polymer in IIc such residual vinyl groups may be due to unpolymerized double bonds on the diisopropenylbenzene cross-linker, the only interpretation of this resonance remaining in the spectrum of material IIb is a lack of covalent contact between the calixarene lower-rim substituent and linear polymer. This strongly suggests that this polymerization process does not covalently engage the Ticalixarene complex, thus leaving the vinyl calixarene substituent intact. As a result, we focus our attention here on demonstrating stability with sites derived from Ia, since the NMR spectroscopic data above suggest that covalent attachment between polymer and grafted site may not be occurring and/or necessary for site stabilization in our system. Hydrolytic Stability of Polymerized and Cross-Linked Grafted Ti(IV)-Calixarene Catalysts. The most crucial attribute of catalysts after polymerization is the stability to protic solvent environments, including hydrolytic stability. To investigate this, we conducted several different leaching stress tests in batch and flow mode, using Ia−c and also IIa,c, before and after polymerization and cross-linking. The batch leaching stress test was conducted by treating catalysts with a 1/1 v/v water/IPA mixture in a batch mode for 4 h at room temperature, whereas the flow leaching stress test was conducted by passing a corrosive stock solution consisting of protic solvent and hydroperoxide (composition details are described in the Experimental Section) through a bed comprising the catalysts for 20 min at room temperature. Both tests are relevant to catalyst performance because industrial epoxidation catalysis often involves some water and excess alcohol present in the reaction medium. In addition, the

may remain inert and act the same as the methyl substituent of Iaunable to form covalent bonds to the polymer. To gain insight into these two different possibilities, 13C CP/ MAS NMR spectroscopy was used to characterize the materials IIa (before polymerization), IIb (after polymerization), and IIc (after polymerization and cross-linking), as shown in Figure 5.

Figure 5. 13C CP/MAS NMR spectrum of IIa (a), IIb (b), and IIc (c).

The sharp resonance around 30−35 ppm comes from the tertbutyl substituents on the top of the cone calixarene ligand. The spectrum of IIa demonstrates low intensity but a distinct vinyl resonance at 115 ppm. Resonances at 128 ppm and 146 ppm can be assigned to calixarene phenyl groups.36 After polymerization, new resonances for IIb,c appear at 35−55 ppm, which can be assigned to alkyl carbons (CH2 and CH(phenyl) groups) of the polymer backbone. Resonances at 128 and 146 ppm become more predominant after polymerization. These 7765

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corrosiveness of protic solvents toward calixarene leaching, in a flow mode. In addition, Ti elemental analysis shows that less than 9% of the original titanium content remains on the catalyst following this leaching test. In comparison, under similar treatment conditions, Ib is able to retain 95% of its initial Ti contents and 92% of its initial organic content, as shown in Figure 7b. Under flow stress-test treatment, Ic retains 98% of

water content serves as an accelerated aging test of the catalyst, to promote site aggregationa hypothesized deactivation mechanism (vide supra). TGA data on catalysts before and after leaching tests were employed in order to quantify organic contents of the catalysts. To enable a comparison of organic content despite adventitious chemisorption of alkoxide species on the silica support from the solvents used during leaching (e.g., synthesis of silicon-surface alkoxides when using alcohol as solvent), all plots were normalized with respect to a silicasupport control sample during analysis of TGA data, which consisted of blank silica support that was treated under the same stress-test conditions as for the catalyst sample.28 TGA data for Ia,c before and after the batch stress test of hydrolytic stability are shown in Figure 6. Ia lost 80% of its

Figure 6. Thermogravimetric analysis of Ia and Ic before (solid blue line) and after (dashed red line) treatment with a mixture of 5 mL of isopropyl alcohol and 5 mL of DI water for 4 h.

original organic content, demonstrating the corrosiveness of the protic solvents to grafted calixarene species. In stark contrast, under the same treatment conditions, Ic showed no evidence of organic leaching. The slightly higher organic weight content (in normalized units in Figure 6b) of sample Ic after the stress test in comparison to that before (at 200 °C) represents the uncertainty in this measurement (10%) and could be due to slight variations in alkoxide formation versus the control silica surface as a result of the stress-test treatment. We conducted the same batch stress test with IIa,c (in normalized units in Figure S10 in the Supporting Information). IIa lost over 90% of its original organic content. However, in contrast, IIc is able to maintain over 95% of its organic content. Both Ic and IIc highlight the effectiveness of a surface polymerization and cross-linking approach, even in a site Ia that is unable to make covalent contact with the surface polymer chain. Next, we performed a similar stress test under flow conditions with Ia−c. Under this condition, Ia retains less than 5% of its original organic content, again demonstrating the

Figure 7. Thermogravimetric analysis of Ia (a), Ib (b) and Ic (c) before (solid blue line) and after (dashed red line) treated with simulated leaching stock solution under flow.

its initial Ti contents and 95% of its initial organic content, as shown in Figure 7c. Materials consisting of polymerized Ib and polymerized and cross-linked Ic both exhibit no detectable calixarene leaching and titanium loss after undergoing the flow stress-test treatment. Both stress tests, batch and flow, highlight the importance of polymerization and, specifically, polymerization with cross-linking for preventing leaching of the types of grafted Ti(IV) active sites as in Ia, when these compounds are subjected to harsh conditions. Ti(IV) Site Characterization after Leaching Stress Test. Although TGA and Ti elemental analysis show that more than 7766

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hand, for Ic, the catalyst after polymerization and cross-linking, both absorption bands stay intact before and after the same stress test. These data highlight the importance of having a polymer overlayer on the catalyst surface, in order to impart robustness and preservation of catalytic activity following stress testing. At this time, we cannot rule out that a portion or all of this polymer may be residing near the external surface of the catalyst particle, rather than within internal pores. However, we deem such a possibility as less likely given the lack of internalpore blocking observed as well as the fact that the monomer and polymerization are expected to occur uniformly throughout the particle, where there are Lewis-acid sites available.

90% of the Ti and calixarene ligand can be preserved on the surface following leaching stress tests by our surface polymerization and crosslinking approach, it is possible that decreased catalytic activity still results afterward, which could be due to growth of aggregated Ti-oxide domains, as opposed to a loss of Ti content from the support surface,6,13,15 since this has been a hypothesized mode of catalyst deactivation previously.6 Thus, we employed DR UV−vis to probe the Ti(IV) coordination environment and examined the recyclability of catalysts Ib,c recovered after the flow leaching stress test described above. We thus denote Ibl,cl to represent catalysts Ib,c that have been recovered after the flow leaching test, respectively. These catalysts were dried at 120 °C under dynamic vacuum prior to use after stress testing. A fresh mixture of octane, 1-octene, and TBHP was used to initiate the catalytic reaction with Ibl and Icl as catalysts, and these data are plotted in Figure 4. In both cases, the activity (as measured by TON versus time) of the catalysts after the stress test was identical with that of the corresponding catalysts before. Indeed, for both Ibl and Icl, after 5 h of reaction time, the TON reached around 30 and the TBHP conversion was around 40%. This excellent recyclability demonstrates successful preservation of grafted Ti(IV) active sites via our surface polymerization approaches, both with and without cross-linking, on application to site Ia, which is not anticipated to form a covalent connection to the surface polymer. As an obvious control, no catalytic activity was observed for Ia following similar flow stress testing (for Ial), consistent with its single-site nature and nearly complete loss of Ti and calixarene content following flow stress testing. We also investigated the chemical environment of grafted Ti(IV) sites before and after leaching with diffuse-reflectance (DR) UV−vis spectroscopy in Figure 8, leveraging on the

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CONCLUSIONS

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ASSOCIATED CONTENT

Our surface polymerization and cross-linking approach forms a two-dimensional polymer on the catalyst and endows greatly enhanced robustness and stability to grafted Ti(IV)-calixarene sites on silica. TGA data demonstrate around a 2−4% increase in organic content after polymerization, which represents two to five repeating styrene units per Ti(IV)-calixareneless than that required to bridge the distance between grafted calixarene sites. Nitrogen physisorption data at 77 K demonstrate that the as-synthesized polymer only occupies mesopores but not micropores on the silica support, highlighting the unique gentleness of this method in not blocking accessibility to sites. However, this polymer overlayer effectively protects grafted-Ti Lewis acid sites from leaching and aggregation during both batch and flow leaching stress tests, while still being porous enough to accommodate the transport of reactant and product. When they are treated with a harsh leaching solution, the Lewis acid catalysts after polymerization exhibit no detectable calixarene leaching, while those without polymer leached over 95% of their grafted Ti(IV)-calixarene complex during tests. Most importantly, the epoxidation catalysts after polymerization and after aggressive flow stress testing with a protic solution exhibit activities and conversions that are comparable and are almost identical with those before polymerization. This conclusion is further reinforced with DR UV−vis spectroscopy of catalysts after polymerization and flow stress testing, which also shows virtually no difference in comparison with catalysts prior to polymerization. This surface-polymerization method therefore enables synthesis of grafted Lewis-acid catalysts that are both highly active and more resistant to leaching in the presence of corrosive catalytic poisons. The mechanism of action of the surface polymer is one of a gatekeeper, which keeps grafted Ti-calixarene sites within a nanoreactor comprising a silica pore surface, as the mechanism of the observed stabilization. Such a mechanism explains why a vinyl substituent on the calixarene is not required to achieve comparable levels of stabilization in leaching stress tests. A corollary of this is the ability to achieve function with a relatively small amount of surface polymer, which is insufficient to even bridge the distance between grafted calixarene sites. We expect this surface polymerization and cross-linking method to be generalizable to other single-site solid catalyst systems, where prevention of aggregation and leaching of grafted sites can be crucial to performance.

Figure 8. Diffuse reflectance UV−vis of Ic (a, solid red line), Icl (b, dashed red line), and Ial (c, dashed black line) treated with simulated leaching stock solution.

previous report of the DR UV−vis of Ia.17 Samples Ic,cl both exhibit two distinct absorption bands. The broad absorption band at 330−480 nm represents calixarene-Ti(IV) (LMCT) bands and is similar to that in previous reports.17,34,35 With Ia, the species without polymer protection, both this band and absorption bands in the 230−280 nm window arising from oxygen to metal charge transfer disappear almost completely after flow stress testing, as shown in Figure 8c. This suggests nearly complete removal of the Ti(IV)-calixarene complex after the flow leaching stress test (consistent with removal of calixarene and Ti sites from Iavide supra). On the other

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01998. 7767

DOI: 10.1021/acscatal.6b01998 ACS Catal. 2016, 6, 7760−7768

Research Article

ACS Catalysis

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(17) Notestein, J. M.; Iglesia, E.; Katz, A. J. Am. Chem. Soc. 2004, 126, 16478−16486. (18) Notestein, J. M.; Andrini, L. R.; Requejo, F. G.; Katz, A.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 15585−15595. (19) Wulff, H. P. Epoxidation Process with Improved Heterogeneous Catalysis. US Patent 3923843, 1975. (20) Tatsumi, T.; Koyano, K. A.; Igarashi, N. Chem. Commun. 1998, 325−326. (21) Corma, A.; Domine, M.; Gaona, J. A.; Jordá, J. L.; Navarro, M. T.; Rey, F.; Pérez-Pariente, J.; Tsuji, J.; McCulloch, B.; Nemeth, L. T. Chem. Commun. 1998, 2211−2212. (22) Sever, R. R.; Alcala, R.; Dumesic, J. A.; Root, T. W. Microporous Mesoporous Mater. 2003, 66, 53−67. (23) Jarupatrakorn, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8380−8388. (24) Zhang, L.; Abbenhuis, H. C. L.; Gerritsen, G.; Bhriain, N. N.; Magusin, P. C. M. M.; Mezari, B.; Han, W.; van Santen, R. A.; Yang, Q.; Li, C. Chem. - Eur. J. 2007, 13, 1210−1221. (25) Yang, H.; Li, J.; Yang, J.; Liu, Z.; Yang, Q.; Li, C. Chem. Commun. 2007, 1086−1088. (26) Yang, H.; Zhang, L.; Zhong, L.; Yang, Q.; Li, C. Angew. Chem., Int. Ed. 2007, 46, 6861−6865. (27) Sensarma, S.; Bouh, A. O.; Scott, S. L.; Alper, H. J. Mol. Catal. A: Chem. 2003, 203, 145−152. (28) Winner, L.; Daniloff, G.; Nichiporuk, R. V.; Solovyov, A.; Katz, A. Top. Catal. 2015, 58, 441−450. (29) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345−354. (30) Erker, G. Dalton Trans. 2005, 1883−1890. (31) Wanglee, Y.-J.; Hu, J.; White, R. E.; Lee, M.-Y.; Stewart, S. M.; Perrotin, P.; Scott, S. L. J. Am. Chem. Soc. 2012, 134, 355−366. (32) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquérol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603−619. (33) Choi, M.; Kleitz, F.; Liu, D. N.; Lee, H. Y.; Ahn, W. S.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 1924−1932. (34) Marchese, L.; Maschmeyer, T.; Gianotti, E.; Coluccia, S.; Thomas, J. M. J. Phys. Chem. B 1997, 101, 8836−8838. (35) Oldroyd, R. D.; Sankar, G.; Thomas, J. M.; Ö zkaya, D. J. Phys. Chem. B 1998, 102, 1849−1855. (36) Dire, S.; Babonneau, F. J. Non-Cryst. Solids 1994, 167, 29−36. (37) Feil, F.; Harder, S. Macromolecules 2003, 36, 3446−3448.

Materials and general methods, synthesis and grafting of Ti(IV)-calixarene complex, 1H NMR, 13C NMR, 1H−1H COSY, and high-resolution mass spectrum of 5,11,17,23tetra-tert-butyl-25,26,27-trihydroxy-28-pentenoxy-Ti(IV)-calix[4]arene precursor, 11B MAS NMR, TGA, 1octene epoxidation reaction conversion vs time, nitrogen physisorption isotherms and their pore size distributions calculated from the NLDFT model, and TGA normalized unit calculation equation (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for A.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Lyondell Chemical Co. for financial support of this research and thank Dr. Sandor Nagy and Dr. Vu Dang for helpful discussions. The authors also acknowledge a grant from the Basic Energy Sciences division of the U.S. Department of Energy (DE-FG02-05ER15696) for partial funding of this work. The NMR facility at Caltech was supported by the National Science Foundation under Award No. DMR-520565.

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REFERENCES

(1) Ullmann, F. Propylene Oxide. In Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Baer, H., Bergamo, M., Forlin, A., Pottenger, L. H., Lindner, J., Eds.; Wiley-Interscience: Weinheim, Germany, 2012; pp 1−26. (2) Sheldon, R. A.; Van Doorn, J. A. J. Catal. 1973, 31, 427−437. (3) Sheldon, R. A.; Van Doorn, J. A.; Schram, C. W. A. J. Catal. 1973, 31, 438−443. (4) Sheldon, R. A. J. Mol. Catal. 1980, 7, 107−126. (5) Wattimena, F.; Wulff, H. P. A Process for Expoxidizing Olefins with Organic Hydroperoxides. Great Britain Patent GB 1249079, 1971. (6) Oyama, S. T. Homogeneous and Heterogeneous Epoxidation Catalysis, 1st ed.; Buijink, J. K. F., Lange, J.-P., Bos, A. N. R., Horton, A. D., Niele, F. G. M., Eds.; Elsevier: Amsterdam, 2008; Chapter 13, p 362. (7) Clerici, M. G.; Kholdeeva, O. A. Liquid Phase Oxidation via Heterogeneous Catalysis: Organic Synthesis and Industrial Applications, 1st ed.; Wiley-Interscience: Hoboken, NJ, 2013; Chapter 4, pp 127− 219. (8) Hutter, R.; Mallat, T.; Baiker, A. J. Catal. 1995, 153, 177−189. (9) Trukhan, N. N.; Romannikov, V. N.; Paukshtis, E. A.; Shmakov, A. N.; Kholdeeva, O. A. J. Catal. 2001, 202, 110−117. (10) Kholdeeva, O. A.; Trukhan, N. N.; Vanina, M. P.; Romannikov, V. N.; Parmon, V. N.; Mrowiec-Biaℏoń, J.; Jarzębski, A. B. Catal. Today 2002, 75, 203−209. (11) Fraile, J. M.; Garćia, J. I.; Mayoral, J. A.; Vispe, E. Appl. Catal., A 2003, 245, 363−376. (12) Wu, P.; Sugiyama, H.; Tatsumi, T. Stud. Surf. Sci. Catal. 2003, 146, 613−616. (13) Gianotti, E.; Bisio, C.; Marchese, L.; Guidotti, M.; Ravasio, N.; Psaro, R.; Coluccia, S. J. Phys. Chem. C 2007, 111, 5083−5089. (14) Yu, W.; Zhang, Z.; Wang, H.; Ge, Z.; Pinnavaia, T. Microporous Mesoporous Mater. 2007, 104, 151−158. (15) Guidotti, M.; Pirovano, C.; Ravasio, N.; Lázaro, B.; Fraile, J. M.; Mayoral, J. A.; Coqc, B.; Galarneau, A. Green Chem. 2009, 11, 1421− 1427. (16) Notestein, J. M.; Andrini, L. R.; Kalchenko, V. I.; Requejo, F. G.; Katz, A.; Iglesia, E. J. Am. Chem. Soc. 2007, 129, 1122−1131. 7768

DOI: 10.1021/acscatal.6b01998 ACS Catal. 2016, 6, 7760−7768