Martina
Aigner
a,
Nicolás Andrés
Grosso-Giordano
a,
Christian
Schöttle
a,
Alexander
Okrut
*a,
Stacey
Zones
*b and
Alexander
Katz
*a
aDepartment of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, CA 94720, USA. E-mail: alexander.okrut@berkeley.edu; askatz@berkeley.edu
bChevron Energy Technology Company, Richmond, CA 94801, USA. E-mail: SIZO@chevron.com
First published on 12th October 2017
Building on our previous study of delaminated-zeolite catalysts for harsh tail-end conditions in an epoxidation flow reactor employing organic hydroperoxide as oxidant, this manuscript compares approaches for delamination in catalyst prepration. In one, a mild method of delamination is used for synthesis of Ti-UCB-4, in which fluoride in organic solvent is used as a mineralizing agent to affect delamination, while in another, catalyst Ti-SSZ-70-DEL-HIGHPH is synthesized by delamination under high-pH conditions and results in the highest external surface area, similar to that previously reported for ITQ-2. We also compare both materials to the calcined 3-D zeolite consisting of Ti-SSZ-70, a control which underwent no delamination treatment. Results of long-term flow reactor catalytic testing demonstrate a distinct 2.5-fold increase in reaction-rate constant k′ on a mass basis for Ti-UCB-4 relative to 3-D Ti-SSZ-70, and a lack of long-term deactivation for both catalysts. In contrast, for Ti-SSZ-70-DEL-HIGHPH, due to deactivation, no steady-state behavior is observed for either conversion or selectivity with increasing times on stream. A combination of data from powder X-ray diffraction (PXRD), nitrogen physisorption at 77 K, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and UV-visible (UV-vis) spectroscopy demonstrate Ti-SSZ-70-DEL-HIGHPH to be comprised of a combination of crystalline and amorphous phases. Control experiments demonstrate a negative synergy on catalysis when both phases are combined in a single catalyst, which leads to decreased conversion, at levels below values predicted based on the linear combination of the two phases present.
To address this, we have previously proposed the use of a crystalline zeolitic catalyst, based on delaminated zeolite Ti-UCB-4, for the tail-end section of the PO synthesis reactor, which is in contrast to either the soluble (Mo naphthenate) or amorphous (Ti Lewis-acid sites grafted on amorphous silica) nature of conventional PO synthesis catalysts.1,3–5 Delaminated zeolite UCB-4 was used as a support because of its high external surface area, which enables accessibility of catalytic sites to bulky reagents, such as organic hydroperoxides. Previously, we compared Ti-UCB-4 with the conventional solid catalyst based on grafted Ti sites on an amorphous silica support, in the epoxidation of 1-octene with EBHP, which was used as a probe reaction that is directly relevant to PO synthesis, under tail-end conditions in a continuous flow microreactor. Reviews of such microreactors are available elsewhere.6 Our relevant reaction is shown in Scheme 1.7 Tail-end conditions consisted of high temperature (110 °C) and high concentrations of 1,2-epoxyoctane and 1-phenylethanol (corresponding to greater than 80% organic hydroperoxide conversions, based on prior reported concentrations2,5). Our desire to use 1-octene as well as the choice of this temperature is related to it being a reliable surrogate for propylene in testing epoxidation catalysts,2 and our current flow system's limitation to work around atmospheric pressure (i.e. below the boiling point of 1-octene). Our results demonstrated a lack of deactivation,2 displaying true steady state behaviour with time on stream, which was not observed for the amorphous-silica catalyst, and 9% higher selectivity of organic oxidant being converted to epoxide for the crystalline Ti-UCB-4 catalyst. Our data also indicated a lack of dependence of selectivity on organic hydroperoxide conversion for this reaction under our conditions.7
Here, in this manuscript, we were compelled to investigate an alternative approach for delamination, which consists of high pH conditions and is known to result in delaminated materials with much higher external surface areas, compared with those that we have investigated previously. This high-pH approach is inspired by the elegant work of Corma et al., who reported the synthesis of ITQ-2,8,9 and, in particular, Ti-ITQ-2 as an active olefin epoxidation catalyst in batch-reactor mode.10,11 In particular, an appealing feature of the ITQ-2-based materials is their significantly higher external surface area, which is reported to be in excess of 700 m2 g−1, more than 8-fold higher compared to the 3-D zeolite without delamination. This higher external surface area is in contrast to the 1.5-fold enhancements that are typically observed when delaminating SSZ-70 using fluoride anion as mineralizing agent in organic solvent, for UCB-4-based materials, which typically have a surface area of 125 m2 g−1.12,13
Yet, while UCB-4-based materials lack amorphous silica as an impurity phase (no Q2 resonance in 29Si CP/MAS NMR spectrum of material),14 some amorphous silica phase has been previously reported to result as an impurity phase during ITQ-2 synthesis.9,15 Our comparison of high pH versus organic fluoride mineralizing agents for affecting SSZ-70 layered zeolite precursor delamination thus reduces to a question of the possible importance of crystalline-phase purity and high external surface area, when using grafted Ti catalyst sites for olefin epoxidation. In addition, to shed further light on the question of crystalline-phase purity, we investigate a catalyst consisting of grafted Ti sites on a mechanical equal-mass mixture of amorphous silica (506 m2 g−1)7 and crystalline UCB-4 as catalyst support materials. This latter mechanical mixture serves as a control, where the length scale separating amorphous and crystalline phases within the catalyst spans microns (particle size of silica is on the order of 32–63 μm), compared with possible nanoscale phase separation during the high pH hydroxide synthesis. Separately, we also analyze the catalytic performance of a catalyst consisting of the three-dimensional SSZ-70 (no delamination) as a support, in order to directly assess the benefits of delamination in olefin epoxidation catalysis. This comparison is shown in Fig. 1. In all cases, Ti grafting to zeolite surfaces can occur in organized “silanol nests” (shown in Fig. 1), which result from deboronation.
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Fig. 1 Schematic description of the synthesis of Ti-SSZ-70, Ti-UCB-4 and Ti-SSZ-70-DEL-HIGHPH. Based on the known structure of related MCM-22 zeolite (which like SSZ-70 also consists of a layered-zeolite precursor consisting of 12-membered-ring cups),13 the vast majority of grafted Ti atoms in Ti-UCB-4 and Ti-SSZ-70 are located in the framework as shown in Fig. 1, where they are coordinated to four framework oxygen atoms, as opposed to Ti grafted on isolated external-surface silanols, which are not within a silanol nest. Ti-SSZ-70-DEL-HIGHPH shows besides this framework mesoporous silica, which we assign to be formed due to the harsh conditions (high-pH) during synthesis. |
Previously, in Brønsted-acid catalyzed toluene alkylation with propylene, we demonstrated that the benefits of SSZ-70 delamination included increased surface area as well a shorter diffusions paths to internal active sites. Here, due to the steric bulk of the reagents, we do not expect to have catalytic contributions from internal active sites. From this perspective alone, we hypothesize the benefit of delamination to be limited to the moderate enhancement in external surface area as a result of delamination. The latter implicitly assumes the rates per Ti site to be the same in catalysts that have been delaminated compared with those that have not been, consisting of the same SSZ-70 framework topology. However, due to the complex relationship between Lewis-acid site structure and catalytic function in general,16 it may be that there are other benefits to delamination for epoxidation catalysis, which are above and beyond those predicted by external surface area only, and may involve a different nature in the types of active sites that are formed, as a result of the delamination procedure. This possibility also forms a motivating factor in the current study. Because heterogeneous catalysts comprise nearly 80% of all industrial catalysts,18 the learnings here can be relevant to other systems as well, in which crystalline catalyst frameworks replace those based on amorphous ones.
Powder X-ray diffraction (PXRD) patterns were measured using a Bruker GADDS D-8 diffractometer and Cu-Kα radiation. Data were collected in the 2θ range from 3° to 30° with a step size of 0.02° and a dwell time of 2 s. PXRD peaks at 2θ values of lower than 5° are not discussed due to instrument limitations.
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was conducted with a FEI Tecnai F20 operated at 200 keV. Samples suitable for HAADF-STEM were prepared on amorphous Lacey-carbon films deposited on copper grids (Ted Pella, Inc.).
Diffuse-reflectance Ultraviolet-visible (UV-vis) spectroscopy for solid materials was performed on a Cary 400 spectrophotometer (Varian) fitted with a Praying-Mantis diffuse reflectance attachment from Harrick Scientific Instruments. Samples were measured with an average time of 0.166 s and a wavelength data interval of 0.5 nm, resulting in a scan rate of 180.7 nm min−1. The results are corrected by measuring a baseline of polytetrafluoroethylene and reflectance data were converted into Kubelka–Munk pseudoabsorbance units (F(R)).17 The determination of the Ti contents of all catalysts was performed by Galbraith Laboratories Inc. (Knoxville, TN). Liquid samples from catalysis were analyzed using an Agilent gas chromatography (GC) system consisting of a GC 6890A plus with helium as the make-up gas, air as the utility gas, and a high-resolution capillary column with 50.0 m length, 320 μm diameter, and 1.05 μm in film thickness. The gas chromatograph is equipped with a flame ionization detector (FID). The temperature program runs from 80–180 °C.
A general observation is the broad scattering of the selectivity values of EBHP to 1,2 epoxyoctane. This is caused by the deviation when measuring the relatively small signal change during 1,2-epoxyoctane formation, which is divided by the relatively large signal change during EBHP consumption. Uncertainties were calculated based on the standard deviation. For all the runs, the flow rate was lowered after 10–12 hours to increase and set the conversion and to avoid situations in which the EBHP conversion approached 100% within uncertainty, at the beginning of catalysis.
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Fig. 3 N2 adsorption/desorption isotherms as a function of relative pressure of B-SSZ-70, UCB-4 and SSZ-70-DEL-HIGHPH. |
Material | Micropore volume [ml g−1] | External surface area [m2 g−1] | Total pore volume [ml g−1] |
---|---|---|---|
B-SSZ-70 | 0.17 | 69 | 0.31 |
UCB-4 | 0.11 | 101 | 0.40 |
Ti-SSZ-70-DEL-HIGHPH | 0.21 | 579 | 0.93 |
Microporosity in these materials is evident by their high uptakes at low relative pressures, and is quantified to be 0.17 mL g−1 for calcined B-SSZ-70, decreasing to 0.15 mL g−1 in delaminated zeolite UCB-4, consistent with previously reported results.7,12,21,22 We have previously described this decrease in micropore volume between the calcined three-dimensional versus delaminated material as being due to the removal of some microporosity in the delaminated material, which would otherwise be formed in between layers, during condensation of precursor layers when synthesizing the three-dimensional zeolite.7,12,21,22 In contrast, the SSZ-70-DEL-HIGHPH is measured to have a higher micropore volume of 0.21 mL g−1 when compared to calcined B-SSZ-70. This increase in micropore volume cannot be explained by delamination alone and could be a result of the formation of amorphous microporous silica, which could be synthesized under delamination conditions. Specifically, during the acidification step, in principle it is possible to precipitate microporous amorphous silicates from dissolved silicate ions, since acid-catalyzed condensations can yield microporous materials.23 If this occurs during synthesis of SSZ-70-DEL-HIGHPH, then the amorphous phase could also be partially responsible for the increased external surface area of this material.
The aforementioned changes to the exterior surface area and micropore volume motivate two crucial questions in this manuscript: (i) is the 1.5-fold difference in surface area obtained between B-SSZ-70 and UCB-4 significant, when using these materials as supports for catalysis? and (ii) is the much greater external surface area of SSZ-70-DEL-HIGHPH beneficial, when using this material as a support for epoxidation catalysis? We address these questions below, with the understanding that their answers may also be tied in with the type of grafted Ti site that is synthesized on each of the three materials, which are summarized in Fig. 1.
Material | Ti-SSZ-70 | Ti-UCB-4 | Ti-SSZ-70-DEL-HIGHPH | Ti–SiO2/Ti-UCB-4 (1![]() ![]() |
---|---|---|---|---|
Ti-content [wt%] | 0.22 | 0.41 | 0.77 | 0.97 |
Ti per external surface area [μmol m−2] | 0.67 | 0.85 | 0.28 | 0.67 |
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Fig. 5 Diffuse-reflectance UV-vis spectra for Ti-SSZ-70, Ti-UCB-4, Ti-SSZ-70-DEL-HIGHPH, and Ti– SiO2/Ti-UCB-4-MIX. |
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Fig. 6 Catalysis data for Ti-SSZ-70, Ti-UCB-4, Ti-SSZ-70-DEL-HIGHPH, and Ti-UCB-4/Ti–SiO2-MIX under tail-end conditions. Conversion of EBHP (•), selectivity of 1,2-epoxyoctane (△). |
Material | Crystalline | Conversion reaches steady state | Average selectivity | Selectivity reaches steady state | Ti-sites in framework | Visual color of spent catalyst | Reaction-rate constant k′ mass-based [mL h−1 g−1] | Reaction-rate constant k titanium content-based [mL h−1 g−1] |
---|---|---|---|---|---|---|---|---|
Ti-SSZ-70 | Yes | Yes | 74% (±5%) | Yes | Yes | Light yellow | 29 | 13 × 103 |
Ti-UCB-4 | Yes | Yes | 73% (±2%) | Yes | Yes | Light yellow | 72 | 17 × 103 |
Ti-SSZ-70-DEL-HIGHPH | Partially | No | 60% (±4%) | No | Partially | Yellow | 20 | 2 × 103 |
Ti–SiO2/Ti-UCB-4-MIX | Partially | No | 67% (±4%) | No | Partially | Yellow | 29 | 3 × 103 |
Ti–SiO2 | No | No | 63% (±4%) | No | No | Yellow | 97 | 6 × 103 |
In comparison, when using the delaminated zeolite Ti-UCB-4 as catalyst, the EBHP conversion drops slightly, from an 89% initial conversion to 84% after the first 72 hours of time on stream, as shown Fig. 6. When compared to Ti-SSZ-70, the steady state appears to be reached earlier and the initial drop in EBHP conversion is lower (5% vs. 12%) for Ti-UCB-4. Moreover, the reaction-rate constant on a mass basis k′ for Ti-SSZ-70 of 29 mL h−1 g−1 is lower than k′ for Ti-UCB-4 consisting of 72 mL h−1 g−1. This enhancement in k′ for delaminated Ti-UCB-4 relative to 3-D Ti-SSZ-70 can be partially elucidated on the basis of the 1.5-fold higher external surface area for the former, which we described above leads to the same 1.5-fold higher grafted Ti content for the former (i.e. directly connected to the trend in external surface area). Yet the observed 2.5-fold difference in k′ between these two materials suggests that factors beyond external surface area may also be at play. To further investigate this, we calculated k values on a per Ti basis. The value of k for 3-D Ti-SSZ-70 of 13 × 103 mL h−1 g−1 is significantly lower than the value observed for delaminated Ti-UCB-4 of 17 × 103 mL h−1 g−1. At this point, we do not know the origin of this per-site enhancement in the delaminated material, but acknowledge that it could be a combination of effects consisting of initial deactivation under the harsh tail-end conditions of epoxidation catalysis in the flow reactor here, as well as the possibility of any intrinsic differences within the grafted Ti site as a result of the delamination procedure. The latter we deem to be less likely based on the similarity of the diffuse-reflectance UV-Vis data in Fig. 5, which characterizes the grafted Ti site in both materials. Previously, the reaction rate constants were calculated by using an averaged EBHP conversion over the entire measured time-on-stream period.7 As a result of these different approaches for calculating the reaction rate constants, k and k′ now appear slightly lower for Ti–SiO2 and Ti-UCB-4 than in these former reports.7 The selectivity is defined as the fraction of moles 1,2-epoxyoctane formed per mol EBHP consumed.
Regarding the selectivityfor 1,2-epoxyoctane shown in Fig. 6, which is defined as the fraction of moles 1,2-epoxyoctane formed per mol EBHP consumed, it is stable for Ti-SSZ-70 and averages at 74% (±5%), which is similar to the observed selectivity for Ti-UCB-4, which averages at 73% (±2%) and is also stable. The similarity of the selectivity shown by both materials is consistent with delamination not appreciably changing the nature of the grafted Ti site within the SSZ-70-topology framework. In summary, we observe an increased activity for 2-D Ti-UCB-4 relative to 3-D Ti-SSZ-70 of 2.5-fold on a mass basis, and attribute this to a combination of higher surface area and slightly higher per-Ti-site activity, while selectivity for the two catalysts is comparable.
Catalysis data and reaction-rate constants of Ti-SSZ-70-DEL-HIGHPH are also shown in Fig. 6. The EBHP conversion is observed to decrease from 77% to 51% during the first 162 h of time on stream, without any evidence of this catalyst reaching a steady state in either the conversion or the selectivity. Due to our evidence of an amorphous-silica phase in Ti-SSZ-70-DEL-HIGHPH (vide supra), we chose to compare the catalytic performance of Ti-SSZ-70-DEL-HIGHPH with a catalyst that consists of an equal-mass mechanical mixture of amorphous Ti–SiO2 and crystalline Ti-UCB-4. This latter catalyst exhibits an EBHP conversion that initially drops from 64% to 49% in 183 h, and, like Ti-SSZ-70-DEL-HIGHPH, it continues deactivating without reaching a steady state. A comparison of the activities for Ti-SSZ-70-DEL-HIGHPH and the Ti–SiO2/Ti-UCB-4-MIX indicates that the former exhibits a lower reaction-rate constant per mass k′ of 20 mL h−1 g−1, compared to 29 mL h−1 g−1 for the latter. On a per Ti basis, the calculated k values for both catalysts were similar at 2 × 103 mL h−1 g−1 and 3 × 103 mL h−1 g−1, respectively. These data are consistent with a similar ensemble of grafted Ti sites, on both amorphous and crystalline phases, in both Ti-SSZ-70-DEL-HIGHPH and Ti-UCB-4/Ti–SiO2-MIX catalysts. It suggests this ensemble does not change whether the mixing between the amorphous and crystalline phases is on the nanoscale (as observed in Fig. 6) for Ti-SSZ-70-DEL-HIGHPH versus on the micron particle-size scale of the mechanical mixture in Ti-UCB-4/Ti–SiO2-MIX. This is quite profound given the nearly 1000-fold difference in length scale of phase mixing (relating to mixing of amorphous versus crystalline phases) between these two catalysts. The average selectivities for Ti-SSZ-70-DEL-HIGHPH and Ti-UCB-4/Ti–SiO2-MIX catalysts are 60% (±4%) and 67% (±4%), respectively, and decrease with time on stream. The lack of an observed steady state in the selectivity differentiates these partially amorphous catalysts from the pure crystalline catalysts consisting of Ti-UCB-4 and Ti-SSZ-70, which achieved a stable selectivity of around 74%.
We also compared these two partially amorphous catalysts to a pure amorphous catalyst, Ti–SiO2, over the long-term flow reactor conditions used here in this manuscript. The catalysis data and reaction-rate constants are shown in Fig. 6 and Table 3 for Ti–SiO2. This catalyst is observed to deactivate from an initial EBHP conversion of 88% to 68% after 187 hours of time on stream, without reaching a steady state. The lack of an observed steady state even after long times on stream for Ti–SiO2 is similar to what is observed for Ti-SSZ-70-DEL-HIGHPH and Ti-UCB-4/Ti–SiO2-MIX. The calculated k′ value of 97 mL h−1 g−1 for Ti–SiO2 is significantly higher than for Ti-SSZ-70-DEL-HIGHPH and Ti-UCB-4/Ti–SiO2-MIX, and the calculated k for Ti–SiO2 of 6 × 103 mL h−1 g−1 is 2-to-3-fold higher on a per Ti basis when compared to both partially amorphous catalysts. We surmise that the observed increase in catalyst activity for the fully amorphous Ti–SiO2 when compared with the partially amorphous Ti-SSZ-70-DEL-HIGHPH and Ti-UCB-4/Ti–SiO2-MIX catalysts is due to higher grafted Ti site densities, as described in Table 2, as well as higher per-Ti-site activity in Ti–SiO2. We do not yet understand the origin of the latter, but it points to a negative synergistic effect under tail-end reaction conditions, when a catalyst possesses both amorphous and crystalline phases, in that the catalyst activity is worse on a per-Ti basis than the linear combination of the two phases separately.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7re00138j |
This journal is © The Royal Society of Chemistry 2017 |