Continuous monitoring the oxyfunctionalization of hexane by aqueous H2O2 over TS-1 related catalysts

Applied Catalysis A: General 241 (2003) 167–184

Continuous monitoring the oxyfunctionalization of hexane by aqueous H2 O2 over TS-1 related catalysts Istvan Halasz∗ , Mukesh Agarwal, Eric Senderov, Bonnie Marcus R&D Center, PQ Corporation, 280 Cedar Grove Road, Conshohocken, PA 19428, USA Received 18 March 2002; received in revised form 7 August 2002; accepted 7 August 2002

Abstract The title reaction was studied over three TS-1 type catalysts at atmospheric pressure and at temperatures from 40 to 60 ◦ C. By monitoring the consumption of H2 O2 continuously we found that the selective oxidation of n-hexane to 2- and 3-hexanols and -hexanones proceeds usually faster than previous experiments suggest, but the reaction stops before reaching total hydrocarbon conversion despite ample H2 O2 supply. One of our catalysts permitted >99% H2 O2 utilization even in the absence of any homogenizing co-solvent. This material has an MFI structure but does not show significant UV absorption near 48,000 cm−1 , which is a benchmark of the isomorphously substituted TS-1. Our other TS-1 catalyst, with a strong UV band near 48,000 cm−1 , promoted the oxidation of n-hexane by aqueous H2 O2 best in the presence of methanol. The hydroxyl content of catalysts and their Ti4+ coordination were compared by dispersive Raman and Fourier transform (FT) mid-infrared (MIR), near infrared (NIR), and UV spectroscopy. Spectra of hydrated and dehydrated samples were scanned from 900 to 52,000 cm−1 in a single DRIFT cell before and after in situ dehydration at ∼10−3 Pa pressure at temperatures from 25 to 400 ◦ C. High-resolution FT-UV measurements indicated an array of charge transfer transitions for Ti4+ ions in every sample. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Selective oxidation; Oxyfunctionalization; n-Hexane; Hydrocarbon; TS-1; Catalyst; Microporous; Titanium silicate

1. Introduction TS-1, the well-known microporous titanium silicate proved first that a true heterogeneous catalyst can be active and selective for the liquid phase oxidation of organic compounds with dilute aqueous H2 O2 which is environmentally benign and safe to handle [1]. Amorphous silica supported titania catalysts had been used for liquid phase oxidation in non-aqueous media, but they lost activity in the presence of water unless transformed into water tolerant crystalline TS-1 by “wetness-impregnation” [2–6]. TS-1 has been ∗ Corresponding author. E-mail address: [email protected] (I. Halasz).

found to be an active and selective catalyst for a variety of reactions that has spawned vigorous research for other redox molecular sieve catalysts [7–11]. A challenging reaction is the oxyfunctionalization of paraffins ( parum affinis = lack of reactivity) in aqueous environment at near ambient conditions. Only bio-catalysts (enzymes) and superacids had been known to promote this process before 1990 when two research groups recognized independently the unique catalytic activity and selectivity of TS-1 [12,13]. During the past decade numerous laboratories have tested the effect of TS-1 structure and reaction parameters on the paraffin conversion, H2 O2 efficiency, and product distribution [4,7,8,11,14–39]. It has been clarified that H2 O2 can selectively attack the secondary and tertiary

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 6 4 - 7

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C–H bonds of saturated hydrocarbons with the help of TS-1 that results in the selective formation of corresponding mono alcohols and ketones. However, the mechanism of this process is still not well understood. Most researchers agree that a radical mechanism is involved in the TS-1 catalyzed paraffin oxidation [3,4,7–10,19,20] just like in nearly every homogeneous [40,41], bio-catalytic [41–43], and heterogeneous catalytic [44–46] paraffin oxidation process. Ionic, non-radical reaction pathways with heterolytic C–H bond cleavage have only been proposed for paraffin oxidation in superacids [47] and over certain acid-base type heterogeneous catalysts [48–50]. However, mostly ionic reaction mechanisms have been considered for oxidation of olefins, alcohols, and other non-paraffin reactants over TS-1 [4,10,19,20,35]. Numerous studies suggest that Ti–O–OH∗ type peroxo intermediates should initiate the oxidation process with aqueous H2 O2 over TS-1 and related microporous titanium silicates [2,4,8–11,19,20,29,35,36,51–54]. The underlying experimental evidence involves the visual observation of a white to yellow color change upon exposing the catalyst to H2 O2 and the IR, UV, and ESR spectroscopic observation of interactions between the H2 O2 molecules and Ti–O–Si bonds [4,10,35,53–57]. In contrast to this prevalent reactivity of the hydrophilic H2 O2 with the TS-1 lattice, the hydrophobicity of TS-1 is frequently emphasized as a marked difference between this catalyst and the hydrophilic amorphous TiO2 /SiO2 which attracts both H2 O and H2 O2 that in turn poison its active sites [4,7,10,11,13,15,20,27,29–31,35,54,58–62]. Since hydrophobic zeolites do not accommodate easily polar molecules [63–69], the polar H2 O2 can hardly compete with the non-polar hydrocarbon molecules for entering the hydrophobic TS-1 channels. To accelerate the oxidation of paraffins by aqueous H2 O2 , co-solvents, most frequently methanol or acetone, are added to the reactant mixture [7,11,13, 29,31,54]. However, the role of these co-solvents is elusive. The dielectric constants of methanol (ε ∼ 33) or acetone (ε ∼ 21) are lower than those for either H2 O (ε ∼ 80) or H2 O2 (ε ∼ 75), but still much higher than the typical ε < 2 values for paraffins [70]. Thus, for example, the low oxidation rate of methanol has been attributed to the preferred hydrocarbon adsorption in the hydrophobic TS-1 channels [10]. The more polar aqueous H2 O2 should be even less capable for

adsorption than methanol. Moreover, the amount of co-solvents is much lower than needed to homogenize the non-miscible aqueous and hydrocarbon phases. It is possible that the partition of reactants in these solvents affect the conditions of physisorption hence also affect the outcome of the oxidation process [71]. Very recently, Clerici [54] proposed that co-solvents should be added to the reactant mixture in order to help the H2 O2 molecules in hydrolyzing Ti–O–Si bonds and to stabilize the generated lattice defects as active sites for the catalytic process. It will be shown in this paper that, like olefins [58,72–75], saturated hydrocarbons can quickly react with aqueous H2 O2 also in the absence of any co-solvent when the catalyst structure is adequate. Isolated Ti(OSi)4 units are often cited as important features of TS-1 [10,20,35]. A defect-free, isomorphously substituted MFI structure must contain all Ti4+ ions in tetrahedral coordination likely occupying certain preferred lattice positions similarly to the non-randomly arranged Al3+ ions of ZSM-5 [76–82]. Formation of Ti–O–OH∗ or related intermediates assumes the breakage of Ti–O–Si bonds in such isomorphous TS-1 structures when exposed to H2 O2 . On the other hand, the MFI framework of some catalytically active TS-1 type materials was found to be distorted by not isomorphously substituted titanium ions and other structural defects that might activate H2 O2 without the need of hydrolyzing Ti–O–Si bonds [83–87]. Catalytic experiments are presented in this paper over both types of these microporous titanium silicates. H2 O2 is an expensive reactant that can decompose during a long reaction especially in the presence of TiO2 [4,19,56,59,61]. Therefore, we minimized its contact time with the catalyst by using a laboratory scale semi-batch reactor system that can keep the bulk of this oxidant outside the reaction mixture until needed. This arrangement also permits continuously monitoring of H2 O2 reaction. It will be shown that the oxidation of n-hexane is faster and concludes quicker than previous experiments suggest. 2. Experimental 2.1. Catalysts and materials n-Hexane (98% purity), methanol (99.98% purity), and 30% aqueous H2 O2 solution were purchased

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from Sigma–Aldrich. For accurate stoichiometric calculations, the concentration of H2 O2 was periodically tested by the classic permanganometric titration method. Hexanol and hexanone isomers for GC calibrations were >98% purity materials from Fluka and Aldrich. Anatase and rutile powders for spectroscopic comparison were >99.9% purity samples from Aldrich. The three TS-1 catalysts, that we call TS-PQ-A, TS-PQ-B, and TS-PQ-C for easier distinction, were synthesized according to a pending patent description [88] with slight modifications in the procedure. Powder XRD measurements (Phillips Xpert diffractometer) indicated MFI structure and high crystallinity for all TS-1 samples. The average particle size was 0.1–0.3 mm measured by a Hitachi 3500N SEM. Some other parameters of the three TS-1 samples are shown in Table 1. Spectroscopic measurements are detailed in the corresponding sections of this paper. 2.2. Catalytic experiments Catalytic measurements were made in a 100 ml three-necked glass flask equipped with a water-cooled reflux, stirrer, Type JJ9 Cole Palmer redox (ORP) electrode, and a type T thermocouple for temperature reading and control. The reactor was heated externally using either a Glass Col or an Electromantle ME type heater controlled from a Staco Series 500 temperature controller that reads the temperature directly from a type T thermocouple submerging into the reacting

Table 1 Data pertaining to the titanium contents of catalysts Sample

Si/Ti

SiOTi (%)

TiO2 (ppm)

TS-PQ-A TS-PQ-B TS-PQ-C

55 82 62

89 59 99

∼200 ∼4700 <100

The Si/Ti molar ratio was measured by XRF. SiOTi (%) is the percent of Ti4+ ions attached to the silicate lattice via Si–O–Ti bonds; it is estimated from the area ratio of α = 960 cm−1 /800 cm−1 IR bands (Fig. 4) assuming that 100% corresponds to αmax = 1.6, observed for a typical isomorphously substituted TS-1 [90] (see text for further details). TiO2 is the anatase content in the sample calculated from the area of calibrated Raman peaks at 144 cm−1 (Fig. 5).

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liquids. The temperature oscillated within <±2 ◦ C around the set value during reaction. A variable speed magnetic stirrer was used for experiments with <1000 rpm and a PolyScience Model X-120 homogenizer for stirring rates from 5000 to 30,000 rpm. The ORP electrode connected to a Chemcadet controller reads H2 O2 concentration in the reactant mixture and keeps it between about 0.09 and 0.11 mol/l by starting and stopping a Masterflex liquid pump that doses 30% aqueous H2 O2 from a Teflon container. Blank experiments at 50 ◦ C with various ratios of 20 to 40 ml n-hexane and 0 to 30 ml methanol indicated that the electrode signal changes more than 100 mV when a few drops of H2 O2 (∼0.1 mol/l) is added. The ORP controller reads signals with ±1 mV accuracy. Thus, this system is highly sensitive to the presence or absence of H2 O2 . The H2 O2 container was placed onto an Ohaus Precision Standard balance and the weight was continuously monitored during the catalytic reaction. Weights were registered every 20 s. End of the reaction was declared when the signal from the balance did not change for 10 min or more. For a typical measurement, 1 g TS-1 sample was placed into the reactor and saturated with about 0.6 ml aqueous 30% H2 O2 solution. Thereupon, 39.4 ml n-hexane was added, the stirrer started, and the heater was set to a desired value. The ORP controller was turned on when the temperature of reactant stabilized at the set value. Reaction temperatures were varied from 25 to 65 ◦ C, stirring rates from 500 to 30,000 rpm, the pH of H2 O2 solution from 4 to 9, molar ratios of n-hexane/Ti from 100 to 1300, volume ratios of methanol/n-hexane from 0 to 1, and dosing rate of H2 O2 from 0.2 to 3 ml/min. After reaction, the liquids were homogenized with acetone or methanol and the solid was filtered out from the liquid phase for GC-MS analysis. Blank experiments without catalyst indicated immeasurably low H2 O2 consumption during a period of 120 min that is substantially longer than the longest oxidation time at our reaction conditions (∼60 min). Doing blank experiments with only H2 O2 on the catalyst without n-hexane has no meaning at our experimental arrangement. The utilization of H2 O2 for hexane oxidation versus decomposition was calculated from the material balance of reacted n-hexane and H2 O2 molecules according to the following stoichiometry:

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2.3. Product analysis and catalyst characterization The organic phases were analyzed on a HP-6890 type GC-MS using a 30 m long SPB50 capillary column with temperature programming from 40 to 300 ◦ C. Catalysts were tested by mid-infrared (MIR), near infrared (NIR), UV, and laser Raman spectroscopy using Nicolet Magna 550, Kaiser Optical Systems HoloProbe, and Bruker IFS66/S spectrometers. Raman spectra were obtained under atmospheric conditions in a homemade sample holder using a 200 mW Nd: YAG diode pumped laser frequency doubled to 532 nm. The laser power on the sample varied from 30 to 90 mW and was measured using an Orion/PD type power meter. The scattered radiation from the sample was directed onto a charge coupled device (CCD) detector via a Mark II type probe. Fourier transform (FT) MIR spectra were measured at atmospheric conditions using a single bounce diamond attenuated total reflectance (ATR) accessory from ASI Applied Systems. To determine the hydroxyl content and compare spectra of hydrated and dehydrated samples, either a vacuum diffuse reflectance (DRIFT) cell from Harrick Scientific or a vacuum transmission (T) cell from CIC Photonics was used. Both sample holders have CaF2 windows and can be attached to oil-free high vacuum pumps working at ∼10−6 Pa. The DRIFT cell can be heated up to a nominal 600 ◦ C but this temperature is measured externally and a thermocouple placed directly into the sample showed a maximum of ∼400 ◦ C. Thus, we accept this internal measurement as actual temperature. Self-supporting sample pellets were fabricated using a CE 98 press from International Crystal Laboratory. The wafers were placed into a sample holder inside the water-cooled T cell. This sample can be heated to 700 ◦ C by controlling its temperature by a thermocouple mounted

directly into the sample holder inside the vacuum chamber. FT-NIR and FT-UV measurements were made using the same DRIFT cell that was used for the FT-MIR measurements by switching beam splitters and detectors for the desired range. Thus, the calcined and evacuated samples were characterized from 900 to 52,000 cm−1 without touching or moving them. CaF2 powder was used as a background material. All DRIFT results were converted to absorption spectra using the Kubelka–Munk function. 3. Results 3.1. Catalytic measurements Figs. 1–3 show the turnover number (TON) of H2 O2 as a function of time in the oxidation of n-hexane at various conditions. These data were calculated from the original H2 O2 weight changes as recorded by the computer without smoothing. Fig. 1 shows the TON on TS-PQ-A at 45 ◦ C. Cosolvents were not added to this reaction mixture. Runs #1, #2, and #3 represent independent experiments at identical reaction conditions using fresh catalyst and reactants each time. The overlaying lines demonstrate good experimental repeatability. At these conditions, the reaction typically stopped after about 2500 s reaching about 1700 [mol H2 O2 /mol Ti4+ ] TON (note that TON is calculated from the total Ti content of samples). The corresponding 60% n-hexane conversion (TON ∼ 180 mmol n-hexane/0.11 mmol Ti4+ ) approximates the total amount of H2 O2 reacted. Consequently, H2 O2 converted the paraffin into 2-hexanol, 3-hexanol, 2-hexanone, and 3-hexanone with excellent efficiency. In agreement with many reports [12,13,16–21,24,26,29,32,33] we could not detect other products. The ratio of various hexanol and hexanone molecules depends on the reaction conditions

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Fig. 1. Oxidation of n-hexane by H2 O2 over TS-PQ-A. Each run was carried out with fresh catalyst. Reaction conditions: 45 ◦ C; 0.5 g catalyst; 300 mmol n-hexane; stirring rate 5000 rpm; 0.5 ml/min pump speed for H2 O2 (30.8%) dosing.

Fig. 2. Oxidation of n-hexane by H2 O2 over various TS-PQ samples. Reaction conditions for TS-PQ-A and TS-PQ-B are the same as those in Fig. 1 and also for TS-PQ-C except that 2 g catalyst was used at 55 ◦ C reaction temperature.

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Fig. 3. Oxidation of n-hexane by H2 O2 over TS-PQ catalysts. Reaction conditions for TS-PQ-A: 55 ◦ C; 0.5 g catalyst; 300 mmol n-hexane; stirring rate 5000 rpm; 1 ml/min pump speed for H2 O2 (30.8%) dosing. Reaction conditions for TS-PQ-C: 45 ◦ C; 1 g catalyst; 300 mmol n-hexane; 10 ml methanol, stirring rate 5000 rpm; 1 ml/min pump speed for H2 O2 (30%) dosing.

and a correlation has not been found with any specific reaction parameter. Therefore, we focus on the overall conversion of n-hexane in this paper. The calculated H2 O2 efficiencies are shown in the penultimate column of Table 2 that also compares the best reaction rates reported for the liquid phase oxidation of n-hexane over TS-1. Data for Runs #1 and #2 are shown in the first row and Run #3 in the second row of Table 2. The turnover rates (TOR) were calculated at the turning points of curves in Fig. 1 where the oxidation process starts and stops. Regardless of the reaction conditions, we have always found a similar limiting conversion in our experiments. There is no use to add further H2 O2 to the mixture beyond this turning point because it cannot trigger further paraffin oxidation. Since the typical batch experiments with premixed H2 O2 /n-hexane combinations run for 1 h or more but the oxidation finishes faster, the additional agitation of unreacted H2 O2 might lead to undesired spontaneous decomposition rarely permitting greater than 90% H2 O2 efficiency (Table 2). Run #3 in Fig. 1 indicates that occasionally the oxidation can continue longer at a given reaction

condition than in most other runs (we made several experiments at these conditions but the extended reaction time appeared only twice). Since we attempted to repeat every experiment at identical reaction parameters, the reasons for these runaway conversions are not clear. They reduce the repeatability of TON to approximately ±17%. As the time dependence of TON in Fig. 1 demonstrates, these odd reactions (Run #3) proceed approximately as fast as the regular ones (Runs #1 and #2). Thus, it is unlikely that random kinetic or transport phenomena play a role. We have not observed temperature fluctuations and the stirring rate between 500 and 25,000 rpm had little effect on the outcome of these reactions (but it was found to be an important overall reaction variable that we will address in a separate report). Both the reaction rate and the efficiency of H2 O2 remained largely unchanged in the prolonged reaction (Table 2). The experimental result in row 3 of Table 2 shows that the oxidation of n-hexane over TS-PQ-A substantially accelerated at slightly elevated temperature without compromising the efficiency of H2 O2 . This kinetic effect of temperature is not surprising. It emphasizes

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Table 2 Comparison of the catalytic activities and selectivities of various TS-1 type materials in the oxidation of n-hexane (n-H) by H2 O2 (average 30% in H2 O) No.

Catalyst

Si/Ti

n-H/zeolite (mol/g)

Solvent/n-H

Temperature ( ◦ C)

Time (min)

TOR H2 O2

TOR n-H

H 2 O2 efficiency (%)

Reference

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

TS-PQ-A TS-PQ-A TS-PQ-A TS-PQ-A TS-PQ-A TS-PQ-A TS-PQ-B TS-PQ-C TS-PQ-C TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1 TS-1

55 55 55 55 55 55 82 62 62 58 55 63 30 60 79 60 35 42 35 35 53 29 80

0.6 0.6 0.6 0.3 0.023 0.15 0.6 0.15 0.3 0.79 0.22 0.24 0.15 1.54 0.38 1.54 0.24 0.31 0.041 0.064 n.a. 0.06 0.4

0 ml/ml 0 ml/ml 0 ml/ml 0 ml/ml 0.5 ml MetOH/ml 1 ml MetOH/ml 0 ml/ml 0 ml/ml 0.3 ml MetOH/ml 1.5 ml Ac/ml 3 ml Ac/ml 3 ml Ac/ml 3 ml Ac/ml 0.1 ml MetOH/ml 0 ml/ml 1 ml MetOH/ml 0.3 ml MetOH/ml 0.25 ml MetOH/ml 9.4 ml MetOH/ml 0 ml/ml 0.4 ml MetOH/ml 10.8 ml MetOH/ml 10 ml MetOH/ml

45 45 55 40 60 50 45 55 45 100 100 100 100 60 50 50 55 55 55 55 55 50 55

42 67 28 3 8 15 22 17 38 180 60 60 60 180 120 180 140 60 140 120 300 180 1440

41.00 38.00 61.00 10.10 6.20 18.30 18.30 5.00 14.20 7.50 n.a. n.a. 1.50 n.a. n.a. n.a. 0.40 n.a. 0.03 n.a. n.a. 0.24 1.50

41.00 38.00 61.00 10.10 5.80 18.30 5.20 3.10 14.20 2.90 7.00 11.70 2.00 0.20 0.08 0.20 2.70 3.30 0.30 0.20 0.04 0.09 0.53

99.2 99.5 99.7 99.5 94.2 99.9 28.0 63.1 84.8 70 >80 n.a. 63 n.a. n.a. n.a. 85 91 93 n.a. n.a. 37 35

– – – – – – – – – [2] [19] [18] [6] [13] [15,17] [15] [20] [23] [21] [21] [30] [32] [33]

Solvent is either methanol (MetOH) or acetone (AC); TOR: turnover rate (mol reactant/mol Ti/min); H2 O2 efficiency: efficiency of H2 O2 for the oxyfunctionalization of hydrocarbon vs. total H2 O2 usage assuming that 1 mol H2 O2 reacts with 1 mol n-hexane to yield 1 mol 2-hexanol or 1 mol 3-hexanol and 2 mol H2 O2 reacts with 1 mol n-hexane to yield 1 mol 2-hexanone, or 1 mol 3-hexanone.

however that solvent is not needed for this reaction and the time before the leveling-off of conversion can be much shorter than early batch experiments suggest. An extended reaction time can only reduce the efficiency of H2 O2 (compare, for instance, with rows 17 through 23 in Table 2). The experimental result in row 4 of Table 2 indicates that high reaction rate can be attained at as low as 40 ◦ C temperature when the aqueous H2 O2 solution was set to slightly acidic (pH ∼ 4) with HCl. The positive effect of acidity has been noted before [20] but paraffin oxidation over TS-1 has not been measured below 50 ◦ C. Similar to the previous experiments, H2 O2 selectively participates in the oxidation process at 40 ◦ C and its spontaneous decomposition is negligible. However, the reaction levels off within a very short time (3 min) reaching only about 2.3% n-hexane conversion conceivably because acid catalyzed byprod-

ucts quickly block the micropores. Experiments in rows 5 and 6 of Table 2 demonstrate that the presence of a co-solvent rather suppresses the turnover rate over this catalyst even at acidic conditions. Fig. 2 compares the catalytic activity of TS-PQ-A with that of TS-PQ-B and TS-PQ-C at similar conditions as those in Fig. 1 except that we quadrupled the amount of TS-PQ-C. Clearly, the reaction starts over every catalyst practically as soon as the reaction temperature is set but the limiting conversion is reached over TS-PQ-A much later than over the other two catalysts. The turnover number on TS-PQ-C remained more than one order of magnitude lower than over TS-PQ-A so we use a secondary Y-axis to compare data. Moreover, the oxidation proceeded smoother over TS-PQ-A than over the other two materials on which stepwise activity increase, “incubation”, activity increase, etc. occurs. We have no clear explanation

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for these “incubation” periods. They might reflect slow reactant or product diffusion. Rows 1, 7, and 8 in Table 2 give further details about experiments in Fig. 2. The low limiting turnover numbers of samples TS-PQ-B and TS-PQ-C are coupled with low reaction rates and low H2 O2 efficiency. We could not find a good reaction parameter combination for TS-PQ-B that contains most titanium in extra lattice position (see Table 1 and next section). Fig. 3 shows however, that the activity of TS-PQ-C substantially increased in the presence of methanol. Row 9 in Table 2 indicates that the reaction rate and the efficiency of H2 O2 also increased. Since we have not observed similar effect with methanol over TS-PQ-A there must be a pronounced difference between the active sites of these catalysts. Fig. 3 also shows a substantial initial delay in the catalytic activity of TS-PQ-C. For not clearly understood reasons, the reaction on TS-PQ-A also started with some delay at 55 ◦ C (Fig. 3; Table 2, row 3) compared to that at 45 ◦ C (Fig. 1; Table 2, rows 1 and 2).

3.2. Spectroscopic studies The high frequency regions of FT-MIR spectra of TS-PQ samples are compared in Fig. 4. The ∼810 cm−1 band has been assigned to Si–O stretching vibrations long time ago [89]. This band is obviously independent of the titanium content of MFI structure. The absorption band near 960 cm−1 is the well-known fingerprint signal of Ti–O–Si bridges whose intensity was found to be proportional with the lattice Ti4+ content of TS-1 [52,56,90]. To quantify the degree of titanium atoms connected to the crystal lattice via Ti–O–Si bonds, we calculated the α = (960 cm−1 /810 cm−1 ) intensity ratio for our samples (Table 1). According to Leofanti et al. [91], αmax ∼ 1.6 when all Ti atoms are built into a typical isomorphously substituted TS-1 lattice. We have not found an exception from this rule. Data in Table 1 suggest that nearly 100% of the Ti content in TS-PQ-C (α = 1.59) should be bound to the crystal lattice while about 10 and 40 % of all Ti atoms should lack

Fig. 4. Intensity of the 968 and 804 cm−1 bands in the FT-MIR ATR spectra of TS-PQ samples at ambient conditions. For numerical values, see Table 1.

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Fig. 5. Intensity of the 144 cm−1 Raman band of anatase in various samples. Table 1 shows quantitative data for the TS-PQ samples.

Ti–O–Si bonds in TS-PQ-A (α = 1.42) and TS-PQ-B (α = 0.94), respectively. Note that Leofanti et al. [91] obtained the αmax ∼ 1.6 value for a TS-1 sample with approximately Si/Ti ∼ 40 ratio. One could expect lower α max for samples with higher Si/Ti ratios, such as those in Table 2. Yet data indicates that α max is not sensitive to small differences in the total titanium content. Fig. 5 compares the Raman spectra of TS-PQ samples. In accordance with the FT-MIR calculations, a substantial amount of TiO2 (anatase) was found in TS-PQ-B. It is well known that the high reflectivity of TiO2 permits identification of small amounts from this oxide by Raman spectroscopy, which is more sensitive than powder XRD [10,28,35,56,90,92–96]. A host of studies indicate that only anatase forms from the three possible TiO2 crystal phases when titanium is not been built perfectly into the crystal lattice during synthesis. The characteristic Raman peaks for anatase appear at around 144, 395, 519, and 639 cm−1 . We used the 144 cm−1 vibration band to calibrate the quantity of this oxide in our TS-PQ

samples. The calculated values are shown in the last column of Table 1. For comparison, Fig. 5 also shows a silicalite and a calibration sample containing 2000 ppm anatase. According to our experience, calibration mixtures with less than 2000 ppm TiO2 cannot provide repeatable results because the solids cannot be homogenized. Thus, we use the electronic capability of spectrometer software to quantify smaller amounts of anatase down to an estimated 50 ppm level. To set up a calibration line, we made five different silicalite/anatase mixtures with 2000–6000 ppm anatase concentrations. Each data point could be recovered within ±5% accuracy in repeated measurements. UV spectroscopy is a widely used technique to characterize the coordination and electronic states of titanium atoms in TS-1. Silicalite does not give appreciable UV signal in the usually probed 25,000– 50,000 cm−1 range. Fig. 6 compares the FT-UV DRIFT spectra of TS-PQ samples at 128 cm−1 resolution that is about twice as high as the best resolution reported for titanosilicates thus far (further resolution increase up to 0.5 cm−1 is possible on

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Fig. 6. FT-UV DRIFT spectra of the TS-PQ catalysts at 25 ◦ C and 3 × 10−3 Pa.

our spectrometer). It is already commonplace in the pertinent literature that bulk TiO2 gives charge transfer bands approximately in the 26,000–32,000 cm−1 region, finely dispersed amorphous TiO2 in the 30,000–38,000 cm−1 region, and tetrahedral isolated Ti4+ ions in the 48,000–50,000 cm−1 region of the UV spectra of MFI type titanosilicalites [8,34,35,56, 59,61,92,97–102]. It is known that some other structures, including for example ETS-10 [103,104], Ti-MCM-41 [105], and Ti-MWW [106], contain octahedral Ti4+ ions bound to the crystal lattice and also absorb between 32,000 and 39,000 cm−1 . The assignment of peaks in the 40,000–48,000 cm−1 range is controversial partly because they seem to change positions depending on the hydration of samples. The 45,100 cm−1 maximum of TS-PQ-A resembles the spectra of SiO2 –TiO2 gels [96,97] or acid treated Ti-MWW [106]. This band reportedly does not interact with moisture [97]. The 47,800 cm−1 maximum of TS-PQ-C is typical for TS-1 structures that contain exclusively isomorphously substituted, isolated, tetrahedral Ti4+ ions especially after dehydration at 500 ◦ C [90]. However, spectra in Fig. 6 were mea-

sured on samples kept at ambient conditions, hence they were exposed to moisture. These high-resolution spectra indicate that both TS-PQ-A and TS-PQ-C contain a variety of Ti4+ ions. The peak around 49,200 cm−1 likely belongs to isolated tetrahedral Ti4+ ions of the MFI lattice. The other peaks between 45,000 and 48,000 cm−1 indicate that TS-PQ-C has a host of bonding and/or coordination environments for its tetrahedral Ti4+ ions although it looks like a perfect, isomorphously substituted TS-1 structure at the widely applied >250 cm−1 resolutions. Energetic differences between preferably occupied lattice positions [78–82,87] or lattice defects with various combinations of –OH and/or –O–Si ligands [83–87,90] can be considered as causes for the heterogeneity of tetrahedral Ti4+ ions. Fig. 6 also indicates that only a small fraction of titanium atoms give charge transfer signals near 45,000 cm−1 in TS-PQ-C. In contrast, TS-PQ-A shows the strongest absorption around this wavelength and only the small bands at 47,200, 48,000, and 49,200 cm−1 hint on the presence of isomorphously substituted TS-1 like structure. Consequently, the

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Fig. 7. FT-UV DRIFT spectra of the TS-PQ catalysts after 1 h in situ treatment at 400 ◦ C at 3 × 10−3 Pa.

bonds of most tetrahedral Ti4+ ions can be activated at lower energy in TS-PQ-A than in TS-PQ-C. This low activation energy might facilitate the activation of reactant molecules on TS-PQ-A compared to that on TS-PQ-C in the absence of co-solvents. The broad shoulder around 38,000 cm−1 in spectra of TS-PQ-A might represent variously positioned TiO2 molecules, small TiO2 particles confined into the cavities of the crystal, or other octahedral TiOx units attached to the MFI structure [96–98,103–106]. The observed difference in the FT-UV spectra of TS-PQ-A and TS-PQ-C coincides with their different catalytic activities, H2 O2 utilization, and tolerance to co-solvent in the oxidation of n-hexane (Table 2). Fig. 7 shows the FT-UV DRIFT spectra after 1 h in situ treatment at 3 × 10−3 Pa at 400 ◦ C. The spectrum of TS-PQ-B did not change much after dehydration except gaining a sharper resolution for the bulk TiO2 particles. The peaks near 45,000 cm−1 remained largely intact in the TS-PQ-A spectrum in good accordance with their reported moisture insensitivity [97]. However, the intensity of lower energy bands of TS-PQ-A increased significantly presumably due to the rela-

tively easy removal of the hydrate sphere from the vicinity of non-tetrahedral Ti4+ ions. The largest peaks of the hydrated TS-PQ-C diminished after heating. Boccuti et al. [107] emphasize that a band near 48,000 cm−1 should be typical for hydrated TS-1 but in their UV spectra this band shifts toward 42,000 cm−1 in the presence of H2 O vapor [10,107]. The spectrum of TS-PQ-C in Fig. 7 is quite similar to the spectrum of hydrated TS-PQ-A in Fig. 6. It appears therefore that the 46,000–48,000 cm−1 UV bands can mostly be associated with H2 O and/or –OH coordinated tetrahedral Ti4+ ions while the bands around 45,000 cm−1 probably belong to Ti4+ ions inaccessible to moisture. Considering the hydrophobic nature of these microporous materials it is possible that the former bands reflect the behavior of less hydrophobic external Ti4+ ions on the surface of micro-crystals and the 45,000 cm−1 bands represent internal Ti4+ sites in the microchannels. The coordination symmetry and bond structure of titanium is especially ill defined in the 40,000–45,000 cm−1 range [98]. The thermal behavior of hydrated TS-PQ-A and TS-PQ-C samples is the opposite of that reported for

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Fig. 8. FT-MIR DRIFT spectra of TS-PQ-A and TS-PQ-C after 1 h in situ treatment at 400 ◦ C at 3 × 10−3 Pa. Prolonged heating for 2 h did not cause measurable change in the spectra.

hydrated TiSiOx mixtures that typically have peak maxima in the 40,000–45,000 cm−1 range which changes to 45,000–48,000 cm−1 (blue shift) upon dehydration [90,97,99]. The dehydrated TS-PQ-B shows a slight change toward this direction. Fig. 8 shows the characteristic hydroxyl vibrations from the FT-MIR DRIFT spectra of TS-PQ-A and TS-PQ-C at the same conditions as those for Fig. 7. It is well known that evacuation at 500 ◦ C does not fully eliminate the hydroxyl content from any MFI structure including hydrophobic silicalites [19,35,68,92,102,108–110]. However, these hydroxyl peaks are small compared, for example, to the 1875 cm−1 Si–O combination vibrations [92] that do not change significantly with the degree of hydration. The hydroxyl peaks of non-calcined samples were only slightly bigger than those in Fig. 8. The 3730 cm−1 band in Fig. 8 corresponds to the fundamental stretching vibration of isolated internal Si–OH groups [109]. The amount of these silanols can be reduced at relatively mild steaming conditions in silicon rich zeolites due to the “healing” of defects they represent [111]. The broad, big peak between about 3200 and 3650 cm−1 is usually associated with

an array of hydrogen bonded internal and external silanol groups [109]. By analogy, titanol groups might also be present. It is known that TiO2 also gives vibrations near 3730, 3690, and 3300 cm−1 depending on its oxygen and heat treatment [112]. There are notions in the literature that these silanol and/or titanol groups might exhibit Brönsted acidity [92,113] but our pyridine adsorption experiments could not confirm this. However, the 1480 and 1610 cm−1 peaks of pyridine adsorbed at room temperature disappeared from our spectra only after treatment at 250 ◦ C and ∼2 × 10−5 Pa that is indicative of weak Lewis acidity in agreement with other relevant studies [10,114]. The small shoulders near 3700 cm−1 in Fig. 8 presumably refer to internal or external terminal silanol groups shifted down from the usual 3715–3725 cm−1 energy level of ZSM-5 [108–110]. According to Fig. 8 the dehydrated TS-PQ-A contains more isolated internal hydroxyl groups (3730 cm−1 ) than TS-PQ-C. Thus, TS-PQ-C is decorated with more hydroxyl related internal defects than TS-PQ-A. These hydroxyls are also shown in the FT-NIR DRIFT spectra in Figs. 9 and 10 that also give some details about the water content of these samples.

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Fig. 9. FT-NIR DRIFT spectra of TS-PQ-A in hydrated form (25 ◦ C) and after 1 h in situ treatment at 400 ◦ C at 3 × 10−3 Pa: (a) hydroxyl overtone range; (b) water combination vibrations.

Fig. 9a illustrates that the non-calcined “hydrated” TS-PQ-A sample contains a few water molecules that disappear at elevated temperatures. These characteristic combination vibrations of molecular water at 5300 cm−1 [96,115,116] are well separated

from other vibrations unlike the 3200, 3445, and 1650 cm−1 MIR bands of H2 O that overlap with the hydrogen bonded stretching vibrations of surface OH groups around 3500 cm−1 and the overtones of Si–O bond vibrations near 1640 cm−1 [92,109,112,117].

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Fig. 10. FT-NIR DRIFT spectra of TS-PQ-C in hydrated form (25 ◦ C) and after 1 h in situ treatment at 400 ◦ C at 3 × 10−3 Pa: (a) hydroxyl overtone range; (b) water combination vibrations.

The 7300 cm−1 peaks in Fig. 9b are the first overtones of the 3730 cm−1 fundamental hydroxyl vibrations (Fig. 8) [117]. There is not much difference between these isolated hydroxyls before and after heating. Consequently, changes in the FT-UV spectra of TS-PQ-A (Figs. 6 and 7) are either only related to the loss of physisorbed H2 O or reflect thermal

bond rearrangement without substantial hydroxyl change. Fig. 10a indicates that the non-calcined TS-PQ-C contains somewhat more liquid H2 O than TS-PQ-A. Interestingly, a little molecular water seems to remain on this sample even after 2 h in situ evacuation at 400 ◦ C and ∼3 × 10−3 Pa. It is possible that the

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shoulder at 7235 cm−1 in Fig. 10b also reflects the physisorbed water together with other hydrogen bound hydroxyls [118]. A very recent study suggests that even the 3740 cm−1 MIR peak (or its 7300 cm−1 NIR counterpart) might reflect isolated O–H vibrations from oriented, non-hydrogen bonded H2 O molecules attached to hydrophobic surfaces (such as TS-1) [119]. The slight increase of the 7300 cm−1 band in the FT-NIR spectrum of TS-PQ-C after heat treatment suggests the formation of isolated titanol or silanol groups. Thus, TS-PQ-C seems to be more sensitive to moisture than TS-PQ-A that might be associated with the observed differences in their catalytic activity and selectivity.

4. Discussion Forcing paraffins to react at low temperatures is a difficult task hence the oxyfunctionalization of n-hexane by H2 O2 can measure well the activity and selectivity of catalysts. Many researchers use n-hexane as a probe molecule for testing TS-1 and related catalysts because this paraffin is usually more reactive than most other linear alkanes [10,12,13,20]. To contrast our reaction rates and H2 O2 efficiencies with the best data published thus far, we placed the widely varied reaction parameters, including the titanium content of catalysts, onto a uniform platform in Table 2. It is clear from this table that TS-PQ-B performs below the level of the best published catalysts while TS-PQ-C is quite competitive with them and TS-PQ-A is superior with respect of both the reaction rate and the efficiency for utilizing H2 O2 . Spectroscopic data (Table 1, Figs. 4–7) indicate that a significant part of the titanium atoms of TS-PQ-B is in TiO2 form not connected chemically to the silicate framework. Thus, the low H2 O2 efficiency of this catalyst is presumably due to spontaneous H2 O2 decomposition over TiO2 [10,19,20]. Spectra of TS-PQ-C suggest that this catalyst is most similar to the well-known TS-1 hence its catalytic activity and H2 O2 efficiency are also similar to those of TS-1. Methanol as a co-solvent helps the oxidation over this catalyst that is also typical for TS-1 [7,11,13,29,31,54]. The high activity and selectivity of TS-PQ-A is somewhat surprising. It clearly contains extralattice

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TiO2 , barely contains isolated tetrahedral Ti4+ ions in lattice positions, and has plenty of defects with non-tetrahedral Ti4+ (Table 1, Figs. 4–7). These are in a sharp contrast with the desired phase purity and isolated tetrahedral Ti4+ of isomorphously exchanged TS-1 [10,20,35]. On the other hand, TS-PQ-A seems also to differ from the amorphous SiTiOx composite catalysts because it has a pronounced MFI crystal structure, its TiO2 content does not promote the decomposition of H2 O2 , and it is active in the aqueous media [3–5]. Note that the conversion limits in Figs. 1–3 hint on catalyst deactivation on all three TS-1 type samples. The reasons have not been elucidated unambiguously. Tatsumi et al. [13] assumed that confined oxidation products gradually block the access of active sites and this process ultimately leads to the observed leveling-off of TON. Based on the low catalytic activity of epoxy pretreated TS-1, Langhendries et al. [71] proposed that the fast catalyst deactivation might be due to both strong adsorption of products and pore blocking by secondary products (polymers). These internal processes are quite difficult to check, but we observed a pronounced catalyst color change from white to gray in nearly every experiment. If this color change indicates confined oxidation products, it is reasonable to assume that their random formation can occasionally permit the reactant molecules to reach the Ti4+ related active sites longer than normal leading to an increased turnover number (conversion) as illustrated in Fig. 1. TS-PQ-A does not require a co-solvent for attaining superior catalytic activity for the liquid phase oxidation of the saturated hydrocarbon. The bonds and coordination states of Ti4+ ions in the hydrated TS-PQ-A closely resemble those in the dehydrated TS-PQ-C that has a well-built TS-1 structure only in its hydrated form (Figs. 6 and 7). The surface hydroxyl structure of TS-PQ-A barely changes upon dehydration (Figs. 8–10). It appears that TS-PQ-A is an MFI structured silicalite that contains plenty of tetrahedral and non-tetrahedral titanium related defect sites. The low activation energy of these irregularities (Figs. 6 and 7) might serve as “prefabricated” catalytically active sites in contrast to a pure, isomorphously substituted TS-1 that requires breaking relatively high-energy bonds before its Ti4+ ions can coordinate the reactants. Since these active defect sites are part of the

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original structure in TS-PQ-A, they are permanently present for the reactant molecules providing thereby an extremely high turnover number. In TS-1, bonds break and form during the catalytic process that can reduce the average turnover number and the overall turnover rate. Data in Table 2 also indicate that the outcome of the reaction depends on a combined effect of various reaction parameters in addition to the quality of catalyst. To elucidate the effect of these parameters, we have launched a statistically designed series of catalytic experiments that will be described elsewhere.

Acknowledgements The authors thank Mike Bennett and Richard Hinchey for thoughtful discussions about the structure of catalysts. We also thank Anthony Marcus and Mark Kohl for the XRD, XRF, and GC-MS measurement.

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