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Pyrolysis of Phenethyl Phenyl Ether Tethered in Mesoporous Silica. Effects of Confinement and Surface Spacer Molecules on Product Selectivity Michelle K. Kidder,† Alan L. Chaffee,‡ My-Huong T. Nguyen,‡ and A. C. Buchanan, III*,† † ‡
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6197, United States School of Chemistry, Monash University, Victoria 3800, Australia ABSTRACT: There has been expanding interest in exploring porous metal oxides as a confining environment for organic molecules resulting in altered chemical and physical properties including chemical transformations. In this paper, we examine the pyrolysis behavior of phenethyl phenyl ether (PPE) confined in mesoporous silica by covalent tethers to the pore walls as a function of tether density and the presence of cotethered surface spacer molecules of varying structure (biphenyl, naphthyl, octyl, and hexadecyl). The PPE pyrolysis product selectivity, which is determined by two competitive free-radical pathways cycling through the two aliphatic radical intermediates (PhCH 3 CH2OPh and PhCH2CH 3 OPh), is shown to be significantly different from that measured in the liquid phase as well as for PPE tethered to the exterior surface of nonporous silica nanoparticles. Tailoring the pore surface with spacer molecules further alters the selectivity such that the PPE reaction channel involving a molecular rearrangement (O C phenyl shift in PhCH2CH 3 OPh), which accounts for 25% of the products in the liquid phase, can be virtually eliminated under pore confinement conditions. The origin of this change in selectivity is discussed in the context of steric constraints on the rearrangement path inside the pores, surface and pore confinement effects, pore surface curvature, and hydrogen bonding of PPE with residual surface silanols supplemented by nitrogen physisorption data and molecular dynamics simulations.
’ INTRODUCTION There has been considerable recent interest in understanding the effects of confinement of organic molecules in nanoporous solids, including ordered mesoporous metal oxides, on their chemical and physical properties as well as their chemical reactivity.1 17 Envisioned new applications include the use of these solids as nanoreactors for organic synthesis including solid-phase synthesis,3,9 12 for drug and gene delivery13,14 including stimuliresponsive release,15 and for design of asymmetric catalysts for the synthesis of chiral organic molecules.5,15 17 Our research has been focused on the effects of pore confinement and surface properties on the pyrolysis rates and product selectivities for organic molecules confined in mesoporous silicas utilizing covalent tethers.18 22 Additional insights into tethered molecular dynamics under pore confinement conditions have come from fluorescence spectroscopy with fluorescent tags,23 quasi-elastic neutron scattering (QENS) experiments,24 and molecular modeling and dynamics simulations.18 Ordered mesoporous silicas continue to find widespread use as supports for diverse applications such as catalysis, separations, and sensors.6,16,17,25 27 They provide a versatile platform for these studies because of their high surface area and the ability to control pore size, topology, and surface properties over wide ranges. Furthermore, there is a diverse array of synthetic methodologies for tailoring the pore surface with organic, organometallic, r 2011 American Chemical Society
and inorganic functional groups.28 30 Typically, grafting of organic moieties to silica surfaces involves the reaction of the surface silanol groups (tSi OH) with silane coupling agents such as silyl chlorides (R3Si Cl), silyl alkoxides (R3Si OR), and disilazanes (R3Si NH SiR3), which results in siloxane tethers (tSi O SiR3) to the surface.28 30 However, we wish to have a temporary tether to the surface such that it can be cleaved after the pyrolysis reaction permitting recovery and analysis of surface-bound products. We have found that functionalizing mesoporous silicas with a phenol functional group provides such a “reversible” tether. This condensation reaction establishes a silyl aryl ether linkage (tSi O Caryl) to the silica surface that is thermally stable to ca. 500 °C, permitting the study of chemical reactions on the surface at high temperatures while being easily cleaved at room temperature with aqueous base for recovery of surface products.18 22 Aliphatic alcohols can also be tethered to the silica surfaces by a similar condensation reaction.23 This surface reaction technique also allows for precise control over molecular grafting densities, control over surface molecular orientation through the position of the active OH group in the organic precursor (e.g., para- vs meta-substituted phenol), and the ability to add a second grafted organic molecule of different Received: March 17, 2011 Published: June 22, 2011 6014
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Figure 1. Pyrolysis products for mesoporous silica-confined PPE and key chain propagation steps in the reaction mechanism. Jagged line represents silyloxy tether to the silica surface.
structure that could serve as an inert surface spacer altering the local surface nanostructure or as a reactant such as a hydrogen donor. Depending on the type of free-radical reaction occurring during pyrolysis, we have found that confinement can lead to modest increases18,19,21 or decreases20 in reaction rates and that rates can be influenced by the surface molecular orientation of a second, hydrogen-donating molecule and also by pore size.22 The organic molecules of interest in our studies are models for structural constituents present in lignin, which is being actively investigated as a potential source of renewable chemicals.31 In a preliminary communication, we briefly reported on the pyrolysis of mesoporous silica (MCM-41 and SBA-15) confined phenethyl phenyl ether (PhCH2CH2OPh, PPE),21 which is a simple model of the dominant β-O-4 aryl ether linkage present in lignin derived from woody biomass.31 Pyrolysis of PPE in mesoporous silica at 375 °C provides two sets of products shown in Figure 1 comparable to the products observed previously from pyrolysis of PPE in the solution or gas phase.32 The radical chain propagation steps leading to product formation are also shown in Figure 1. Product selectivity is determined by competitive hydrogen abstraction at the R- and β-carbons (eqs 2 and 5) by chain carrying phenoxyl (gas-phase) and benzyl (surface-tethered) radicals as long as subsequent β-scission and rearrangement steps (eqs 3 and 4) are fast. We found that the R/β-product selectivity was increased
(ca. 3-fold) in the mesoporous silicas compared with that on the exterior surface of a nonporous fumed silica (Cabosil) indicating pore confinement can play a role in altering product selectivity in free-radical reactions. In this paper, we explore this concept in more detail with support of new nitrogen physisorption measurements and molecular dynamics (MD) simulations, as well as present new findings that the product selectivity can be controlled and dramatically altered from homogeneous phase behavior through a combination of pore confinement and tailoring of the pore surface environment with a second grafted “spacer” molecule of variable structure. Systems investigated are shown in Figure 2.
’ RESULTS AND DISCUSSION Pore Confinement and PPE Graft Density Effects on Product Selectivity. Pyrolysis of PPE tethered to mesoporous
silica surfaces at 375 °C produces the same principal products and follows the same reaction pathway (Figure 1) as observed previously for PPE in the liquid or gas phase32 as well as when tethered to a nonporous, fumed silica surface, Cabosil.33 The free-radical chain reaction is initiated by homolysis of the weak C O bond whose BDE has recently been calculated by DFT (M06-2X) to be 69.5 kcal mol 1, considerably smaller than the 6015
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Figure 2. SBA-15 silica-confined two-component surfaces investigated.
Table 1. Pyrolysis Data for Surface-Confined Phenethyl Phenyl Ether at 375 °C surface composition a
on SBA-15
surface graft densityb (nm 2)
spacer/PPE ratio
no. of pyrolyses
PPE conversion range (%) 22.1 24.3
ratec (% h 1)
path selectivityd (R/β)
11.8 15.9
12.8 ( 2.4 25.7 ( 1.8
PPE PPE
1.16 0.27
n.a. n.a.
9 8
6.1 6.6
PPE/BP
0.22/0.64
2.9
5
4.3
9.1
11.8
43.6 ( 1.6
PPE/BP
0.16/0.93
5.8
5
6.3
19.0
10.8
93.6 ( 5.9
PPE/NAP
0.22/0.62
2.8
5
5.9
10.3
7.2
26.5 ( 0.8
PPE/OCT
0.30/0.79
2.6
5
6.5
14.4
11.7
41.8 ( 3.7
PPE/HEXD
0.23/0.60
2.6
6
5.1
21.0
18.6
76.7 ( 3.4
a
Mesoporous silica with grafted organic molecules shown in Figure 2. b Molecular density of grafted groups (molecules per nm2 surface area) corrected for the weight of the organic moiety; for two-component surfaces the PPE graft density is shown first and the spacer graft density second. c Initial pyrolysis rate measured from the slope (linear regression) of a plot of percentage PPE conversion versus reaction time; typical error is (5 10%. d R/βReaction path selectivity based on product yields (see Figure 1).
aliphatic C C bond at 77.1 kcal mol 1.34 The chain propagation steps (kinetic chain length >30) are shown in Figure 1 (eqs 2 5), while chain termination occurs primarily through benzyl radical coupling. Small amounts (ca. 3 5%) of products identified by GC MS as isomers of ≈PhCH2CH2PhOH (≈ denotes surface-attached) were also detected. These products arise from recombination of the incipient ≈PhCH2CH2• and PhO• radicals produced in the initial homolysis step, where phenoxyl radicals are known to react at the ortho- and para-ring carbons via the keto resonance forms followed by rearrangement to the final products.20 The R/β-product selectivity is defined as the yield of surfacetethered styrene plus gas-phase phenol relative to the surfacetethered toluene plus gas-phase benzaldehyde (Figure 1, eq 1). In the liquid or gas phase, steady-state kinetic analysis showed that this selectivity is determined by the relative rates of competitive hydrogen abstraction at the R- and β-carbons by the chain carrying phenoxyl and benzyl radicals as long as subsequent β-scission and rearrangement steps are comparatively fast, which was demonstrated to be the case.32 In the earlier communication, we reported that the R/βproduct selectivity for PPE confined in two MCM-41 samples (2.5 and 2.0 nm mean pore diameter) and one SBA-15 sample (5.8 nm pore diameter) at saturation PPE graft densities increased to a value of 14 17 compared with a selectivity of 5.0 ( 0.8 when confined to the external silica surface of Cabosil.21 We have now measured this selectivity in a larger pore size SBA-15
silica (6.8 nm) prepared for use in nitrogen physisorption measurements (vide infra) and obtained a selectivity value comparable to that of the other mesoporous silicas of 12.8 ( 2.4 (Table 1). Hence, there is a clear influence of pore confinement on increasing product selectivity (which will be discussed in more detail later) with only a small influence of pore size over the 2.0 6.8 nm range. Interestingly, we also find that the R/βproduct selectivity for PPE confined in mesoporous silica is sensitive to the density of grafted molecules. If the graft density is decreased from 1.16 (saturation) to 0.27 PPE molecules nm 2, the selectivity increases further to a value of 25.7 ( 1.8 (Table 1). To understand the origin of these effects, we begin by reanalyzing the behavior of PPE in solution or gas phases in light of recent computational and experimental studies.35 37 The early experimental studies found that the R/β-product selectivity at 375 °C was only 3.1 ( 0.3 over a wide range of concentration.32 This selectivity is quite small given that the R-pathway cycles through a secondary benzylic radical (Figure 1, eqs 2a and 5a) while the β-pathway cycles though a secondary aliphatic radical (Figure 1, eqs 2b and 5b). In a recent DFT computational investigation of the relative rates of hydrogen abstraction in PPE, we found that the R-benzylic radical is more stable by 6.8 kcal mol 1.35 This energy difference would predict an R/β-product selectivity of nearly 200 if the thermochemical stability of these radicals were the only factor involved. However, we find that the difference in energy in the R- and β-barrier heights for hydrogen 6016
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The Journal of Organic Chemistry abstraction by the benzyl and phenoxyl radicals are only ca. 2.5 and 0.3 kcal mol 1, respectively.35 The very small difference in the case of the electrophilic phenoxyl radical can be attributed to the highly polarized transition state for hydrogen abstraction at the β-methylene site, which is stabilized by the adjacent ether oxygen in PPE. This polar effect in the hydrogen abstraction by phenoxyl radical is the key contributor to the small values of the R/β-product selectivity observed experimentally. In fact, using rate constants calculated for the four key hydrogen transfer steps (Figure 1, eqs, 2a, 2b, 5a, and 5b) and steady-state kinetic analysis, we computed an overall R/β-product selectivity value of 2.4 at 375 °C, which is in excellent agreement with the experimental value of 3.1.35 The small increase in R/β-product selectivity (to 5.0) observed for PPE tethered to nonporous Cabosil surfaces can be attributed to a small substituent effect resulting from stabilization of the R-benzylic radical in PPE by the p-silyloxy linkage to the silica surface. This has been confirmed by recent experimental studies of the pyrolysis of hydroxy- and methoxy-substituted PPEs where R/β-product selectivities of 5.1 and 7.4 were measured forp-HOPhCH2CH2OPh and p-CH3OPhCH2CH2OPh,respectively,36 as well as by computational investigations that reproduce these substituent effect trends.37 Hence, the 3-fold increase in R/βproduct selectivity for PPE at saturation graft density in mesoporous silicas compared with Cabosil reflects a real impact of confinement in the nanoporous environment. It does not appear likely that this change in product selectivity could result from changes in the inherent selectivity for hydrogen abstraction by chain carrying radicals at the R- and β-sites in PPE. A possible explanation would be that the O C phenyl shift for the β-radical is impeded in the mesoporous silicas such that the rate of this key step is reduced. A decreased rate for the phenyl shift step would allow β-radicals to be quenched by hydrogen transfer to reform PPE molecules or react by hydrogen transfer with other PPE molecules to form the thermochemically favored R-radicals resulting in an increase in the R/β-product selectivity. A key clue supporting this hypothesis comes from a study of the thermolysis under vacuum of β-phenylisovaleryl peroxide adsorbed onto the surface of a silica gel.38 It was reported that silica surface interactions severely impeded the related 1,2-phenyl shift (C C) for the classic neophyl radical rearrangement, PhC(CH3)2CH2• f •C(CH3)2CH2Ph. In the case of mesoporous silica-confined PPE, hydrogen-bonding interactions between the ether oxygen and residual surface silanols (under vacuum and in the absence of solvent) could provide an additional driving force for such surface interactions beyond van der Waals interactions. Evidence supporting this hypothesis is revealed from nitrogen physisorption measurements and molecular modeling/simulations described below and also explains the further increase in R/β-product selectivity observed at lower PPE graft densities. Nitrogen Physisorption and Molecular Modeling/Dynamics Characterization. The nitrogen physisorption isotherms and typical BJH (Barret Joyner Halenda; based on the modified Kelvin equation) pore size distributions are shown in Figure 3 for SBA-15 and SBA-15 grafted with PPE molecules at saturation (1.10 nm 2) and 25% of saturation (0.27 nm 2) graft densities. The BET specific surface areas, mean pore diameters, and pore volumes are reported in Table 2. The SBA-15 silica exhibits the well-known type IV isotherm with the large adsorption desorption hysteresis loop that is likewise observed for the PPE-grafted samples. As seen in Figure 3 and Table 2, the surface
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Figure 3. Nitrogen physisorption analysis of SBA-15 with grafted PPE at different densities. Top: adsorption/desorption isotherms. Bottom: pore size distributions.
Table 2. Chemical and Nitrogen Physisorption Analysis for Chemically Modified SBA-15 Samples surface graft materiala SBA-15 PPE
pore
densityb
surface areac
pore diameterd
volume
(nm 2)
(m2 g 1)
(nm)
(cm3 g 1)
970 320
6.8 5.4
1.0 0.53
n.a. 1.10
PPE
0.27
563
6.2
0.85
SBA-15
n.a.
860
6.6
0.67
PPE/OCT
0.30/0.79
251
4.9
0.37
PPE/HEXD
0.23/0.60
147
4.9
0.26
PPE/BP
0.22/0.64
228
5.6
0.29
PPE/BP
0.16/0.93
224
4.9
0.30
SBA-15 PPE/NAP
n.a. 0.22/0.62
752 409
6.8 5.6
0.88 0.51
a
Mesoporous silica with grafted organic molecules shown in Figure 2. Molecular density of grafted groups (molecules per nm2 surface area) corrected for the weight of the organic moiety; for two-component surfaces the PPE graft density is shown first and the spacer graft density second. c BET specific surface area. d Mean pore diameter determined by the BJH method on the adsorption branch. b
area and mean pore diameter decrease with increasing PPE graft density as expected. It is interesting that, even at saturation graft density, the mean pore diameter decreases by only 1.4 nm. Since 6017
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Figure 4. Models of mesoporous silica with 2.9 nm pore diameter containing tethered PPE molecules at densities of 1.33 nm 2 (top) and 0.38 nm 2 (bottom). Yellow, silicon; red, oxygen; gray, carbon; white, hydrogen.
the length of a fully extended PPE molecule bound to the surface is ca. 1.2 nm, this suggests that not all of the PPE molecules are attaining a fully extended conformation normal to the surface of the cylindrical pore, which could reduce the mean pore diameter by up to 2.4 nm. This would be consistent with the concept that some fraction of the PPE molecules are lying down on the pore surface which, as described above, could reduce the rate of the O C phenyl shift rearrangement step. This conclusion is supported by molecular modeling and dynamics simulations (Materials Studio, Compass force field) using the recently reported model of MCM-41 hexagonal mesoporous silica whose unit cell is derived from R-quartz and has been used for analyzing the dynamics of hybrid materials containing tethered organic molecules.39 The native silica model contains 3.1 silanols (SiOH) nm 2 and a pore diameter of 2.9 nm. PPE graft positions were chosen on the basis of the sequential order for greatest calculated energy relief (determined from energy differences between the mean energies determined from MD runs of substrate and product structures) as previously described for aminopropyl-39 and 1,3-diphenylpropyl-tethered18 mesoporous silicas. The smaller pore diameter of 2.9 nm used in the model, compared with SBA-15 at 6.6 6.8 nm used in the pyrolysis study, should not have a significant impact on the models for PPE, which have plenty of room in the 2.9 nm pore. Figure 4 shows typical structures obtained from 1.01 ns MD simulations at 400 °C for mesoporous silica containing grafted
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PPE molecules at densities of 0.38 nm 2 (bottom) and 1.33 nm 2 (top). These PPE graft densities in the models are consistent with those for the samples studied experimentally. As seen in Figure 4 for PPE at saturation graft density, not all of the molecules are fully extended into the pore interior. In any given frame of the MD simulation, several PPE molecules are lying down on the silica pore surface as shown despite the high temperature consistent with favorable PPE surface interactions under vacuum. This model appears relevant to the current pyrolysis studies that are also conducted under vacuum at a slightly lower temperature (375 °C). The dynamics of PPE confined in MCM-41 have also been examined by quasi-elastic neutron scattering and compared with that of the hydrocarbon analogue 1,3-diphenylpropane (DPP; PhCH2CH2CH2Ph) to discern the role of the ether oxygen in PPE on the molecular dynamics at the ps to ns time scale.40 Interestingly, over the temperature studied (240 320 K), the relaxation times for PPE were found to be slower than for DPP. This indicates that the slower dynamics (and higher activation energies) observed for PPE relative to DPP are a consequence of the presence of the ether oxygen that produces stronger interactions with the pore surface, presumably through hydrogen bonding with surface silanols. In a recent report, Wang et al. examined the dynamics for covalently tethered amines in mesoporous silicas by the use of solid-state deuterium NMR techniques and showed that the dynamics on the microsecond time scale depend on the tethered amine structure.41 It was concluded that the dynamics of tethered secondary propyl amines was influenced by hydrogen bond formation with the residual silanol groups on the surface. It would also seem likely that at lower PPE graft densities, the less crowded environment depicted in Figure 4 (bottom) would provide a greater opportunity for PPE molecules to interact with the silica pore surface. Qualitatively, this appears evident from the MD simulations. If we hypothesize that hydrogen bonding of the PPE ether oxygen with residual surface silanols is a contributing driving force for this surface interaction, we can examine it in more detail through calculation of the radial distribution function (also known as the pair correlation function) for the PPE ether oxygens relative to all residual silanol protons. For the specific atoms of interest (i.e., the ether oxygens and all silanol protons), the radial distribution function g(r) for them represents the probability of finding these atoms at a particular distance, r, from each other over the course of the whole molecular dynamics simulation of the 3-D periodic cell. The 7-, 3-, and 2-PPE tether radial distribution plots (Figure 5) correspond to PPE graft densities of 1.33, 0.58, and 0.38 nm 2, respectively. Typical O H 3 3 3 O, N H 3 3 3 O, N H 3 3 3 N hydrogen bonds in molecular systems are considered to occur at H 3 3 3 X separations e 3.0 Å, with moderate to strong hydrogen bonds (g4 kcal mol 1) occurring at separations e 2.2 2.5 Å.42 45 As a visual aid in Figure 5, a vertical line is drawn at 2.5 Å. In both the 3- and 2-PPE tether models, it is apparent that there is considerably more hydrogen-bonding interaction between PPE molecules and the silica surface than can occur at the higher saturation PPE graft density (7-tethers). The observed 2-fold increase in the R/βproduct selectivity in the pyrolysis of PPE at low graft density (0.27 nm 2, more closely corresponding the 2-PPE tether model) is therefore consistent with the premise that the rearrangement path involving the O C phenyl shift in the β-radical is more sterically hindered by increased hydrogen-bonding interactions with the surface at the lower graft densities. 6018
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Figure 5. Radial distribution functions for ether oxygens in PPEs versus silanol protons on the MCM-41, 2.9 nm pore surface (1.01 ns MD simulation at 400 °C; vertical marker at 2.5 Å). The 7-, 3-, and 2-PPE tether models correspond to PPE graft densities of 1.33, 0.58, and 0.38 nm 2.
Influence of Surface Spacer Molecules. We have also investigated the impact on PPE pyrolysis of tailoring the pore surface with a second tethered molecule of varying structure that can serve the function of a surface spacer thereby altering the local surface nanostructure. The spacer molecules examined are shown in Figure 2 and include two aromatics, biphenyl (BP) and 2-naphthyl (NAP), as well as two aliphatics of differing lengths, n-octyl (OCT) and n-hexadecyl (HEXD). These spacer molecules are thermally stable at the temperature of this investigation. The materials were prepared at saturation surface coverage by cocondensation of the phenol/alcohol precursors with the mesoporous silica surface, and the spacer/PPE graft ratio was maintained at a comparable value of 2.6 2.9. In the BP spacer case, a sample at higher dilution (BP/PPE = 5.8) was also prepared for comparison. The PPE pyrolysis products for these two-component surfaces are unchanged (Figure1). Although interactions between PPE radical intermediates and the spacer molecules are feasible, no products were identified attributable to reaction of the spacer molecules. The pyrolysis data for these samples are also reported in Table 1. The PPE pyrolysis rate appears relatively uninfluenced by the presence of the spacer molecules. However, the presence of the spacer molecules leads to significant increases in the PPE R/β-product selectivity. The dependence on spacer structure is revealed from comparison of the samples with spacer/PPE graft ratios of 2.6 2.9. The PPE R/β-product selectivity increases in the order NAP < OCT ∼ BP < HEXD with the selectivity reaching a value of 77 for the HEXD spacer. In the case of the BP spacer, increasing the BP/PPE graft density from 2.9 to 5.8 also results in an increase in product selectivity from 44 to 94 showing that spacer:PPE graft ratio is another parameter that can play a role in altering the product selectivity. PPE R/β-product selectivities at 375 °C of 77 94 are quite remarkable when compared with the corresponding fluid-phase selectivity of only 3. As discussed above, our hypothesis is that the increased selectivity under pore confinement is primarily a result of a reduced rate for the β-radical pathway involving the O C phenyl shift that can be sterically hindered by interactions with the silica surface. This effect appears to be magnified in the presence of the spacer molecules the degree to which depends on the spacer structure. It is also possible that the spacer molecules alter the inherent selectivity for hydrogen abstraction by PhO• and surface-attached
Figure 6. Nitrogen physisorption analysis for selected samples of SBA15 with grafted PPE and spacer molecules. Abbreviations are defined in Figure 2 and the ratio of spacer to PPE graft densities are given in parentheses. Top: adsorption/desorption isotherms. Bottom: BJH pore size distributions.
PhCH2• at the R- and β-carbons in PPE (Figure 1, eqs 2 and 5) compared with the corresponding saturated graft density surface of PPE alone. However, the origin of such an effect (e.g., electronic, steric) and its dependence on spacer structure are not readily apparent. To probe possible causes for this spacer structure dependence, we have examined the nitrogen physisorption behavior of these samples. Selected isotherms are shown in Figure 6 while the surface area, pore size, and pore volume data for all samples are collected in Table 2. When we compare the four spacer molecules at the similar spacer:PPE graft ratios of 2.6 2.9, interesting changes can be noted in the BET surface areas as a function of spacer structure. When compared to the native underivatized silicas, the reduction in available surface area for the PPE:spacer samples (relative to the native silica employed, calculated from Table 2) increases in the order NAP < OCT ∼ BP < HEXD (343, 609, 632, 713 m2 g 1 respectively) with the hexadecyl spacer resulting in the largest decrease in BET surface area of 713 m2 g 1. We note that the increase in PPE R/β-product selectivities as a function of spacer structure described above follow the same trend as this decrease in BET surface area. Relative to a saturated coverage of PPE, the increase in selectivity is also NAP < OCT ∼ BP < HEXD (13.7, 29.0, 30.8, 63.9 respectively, calculated from Table 1). 6019
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Figure 8. Comparison of PPE pyrolysis path selectivity in the liquid phase,32 attached to non porous silica nanoparticles (Cabosil),33 and confined in SBA-15 mesoporous silica. Abbreviations are defined in Figure 2, and the ratio of diluent/spacer to PPE is given in parentheses. Figure 7. Model of mesoporous silica with 2.9 nm pore diameter containing two tethered PPE molecules in the presence of five tethered (top) octyl and (bottom) hexadecyl molecules. Yellow, silicon; red, oxygen; gray, carbon; white, hydrogen.
This correlation suggests the possibility that spacers such as HEXD are more effective at pinning the PPE molecules onto the silica surface resulting in increased PPE-silica surface interactions that hinder the O C phenyl shift. Molecular models of the PPE/OCT and PPE/HEXD systems in the MCM-41 silica (2.9 nm pore) are shown in Figure 7 for comparison. The two PPE molecules were held in the same location as in Figure 4 (bottom) with the aliphatic spacers occupying the other five sites previously occupied by PPE molecules. The OCT spacer is similar in length to the PPE molecule (1.1 1.2 nm) giving a similar looking structure to the saturation coverage of PPE (Figure 4, top). However with the longer HEXD chain (ca. 2.1 nm), the pore is clearly more congested, several of the HEXD molecules are folded, and the MD simulations indicate that the PPE spends more time interacting with the surface over the length of the simulation (1.01 ns). Comparison of PPE-silanol radial distribution functions, while probably acceptable for smaller BP, NAP, and OCT spacers, would likely not be well-represented for the HEXD spacer in the 2.9 nm pore diameter mesoporous silica model (rather than the ca. 6.7 nm pore diameter of the SBA-15) used in the current study. Hence, future modeling/simulation work will require development of a model for the SBA-15 at 6.7 nm pore diameter preferably with random attachments of PPE and spacer molecules over an extended silica structure with multiple unit cells.
Sorting out Surface Attachment, Pore Confinement, and Spacer Effects. In Figure 8, the PPE R/β-product selectivities at
375 °C are compared for the fluid phase, PPE attached to Cabosil, PPE attached to SBA-15, as well as for samples diluted with spacer molecules. The fluid phase reactions were previously performed in sealed Pyrex tubes in a temperature-controlled sand bath.32 The reactions were conducted for the neat liquid (3.8 M) as well as in solutions of varying concentration (dilutions to 0.5 M) employing biphenyl as an inert diluent. The product selectivity for the neat liquid is 3.1 indicating that the products originating from the thermochemically less stable β-radical intermediate are a significant fraction of the products, and the basis for the prominence of this pathway has been discussed in detail above. Relevant to the current discussion, we note that this product selectivity is unchanged when PPE is diluted in BP at a BP/PPE ratio of 8.2 indicating the lack of a solvent dilution effect on the fluid phase selectivity. As shown in Figure 8, this stands in stark contrast to the behavior of PPE confined to the external surface of Cabosil33 or to the interior pore surfaces of SBA-15 silica where the presence of coattached BP (and other surface spacer molecules) results in selectivity increases. In the case of Cabosil, the increase in the product selectivity to a value of 5.0 for PPE in the absence of a spacer molecule results primarily from a substituent effect of the p-silyloxy surface linkage as discussed above. The addition of BP or NAP spacer molecules on the surface at spacer/PPE ratios of 11.8 and 6.8, respectively, resulted in an additional 4-fold increase in selectivity to a value of ca. 20. 6020
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The Journal of Organic Chemistry This indicates that even on the external surface of the silica nanoparticles, the restrictions on diffusion and presumably enhanced surface interactions in the presence of spacer molecules can result in altered selectivity, although not as significantly as when tethered inside the pores of mesoporous silica. Polarz and Kuschel have recently discussed in some detail the effects of “confining reaction fields” and the differences that can occur on chemical transformations depending on whether the reaction occurs on a surface with a locally positive convex curvature (nanoparticle), zero curvature (flat surface), or negative concave curvature (pore).1a We have also observed recently that the differences in local surface curvature experienced for tethered molecules on Cabosil and MCM-41 silica can lead to different reaction rates for the pyrolysis of confined 1,3-diphenylpropane in the presence of a surface-tethered hydrogen donor (3-fluorene).22 The increase in rate for the MCM-41 case was attributed to improved geometries for rate-determining bimolecular hydrogen transfer steps on the pore surface.22 Confinement of PPE in SBA-15 without spacer molecules resulted in a ca. 3-fold increase in selectivity to a value of 14.2 compared with the corresponding Cabosil case. It would appear consistent with the premise that the rearrangement path is inhibited by PPE silica surface interactions that this interaction would be enhanced within the pore environment possessing locally negative concave curvature because of a more favorable local geometry for PPE to interact with the surface. Similar to the Cabosil system, the presence of surface spacers in the SBA-15 resulted in significant increases in product selectivity. However, the selectivities were dependent on spacer structure and spacer/ PPE ratio as illustrated in the BP spacer case. It is somewhat difficult to compare the effects of spacers on Cabosil and SBA-15 because of the different spacer/PPE ratios employed and the commonality of only two spacer structures. The most notable comparison is for the BP spacer where at a BP/PPE ratio of 6.8 a ca. 7-fold increase in product selectivity to a value of 93.6 was observed compared with the 4-fold increase observed on Cabosil at a BP/PPE ratio of 11.8. There are many factors that could contribute to the changes observed in PPE product selectivity, but it is clear that alterations in the local environment of the “confining reaction field”1a such as the nature of the surface curvature, graft density, and location of spacer molecules of variable structure and graft density play critical roles. For PPE pyrolysis where one pathway, the rearrangement path through the β-radical intermediate, is sensitive to the local surface environment, this can be exploited to make dramatic changes in product selectivity compared with fluid-phase behavior. The result in this current example is that the rearrangement path that accounts for 25% of the pyrolysis products in the liquid phase can be virtually eliminated under pore confinement conditions through tailoring the local environment.
’ CONCLUSIONS Pyrolysis of PPE confined in mesoporous silica occurs via a free-radical chain mechanism that produces two sets of products whose selectivity is significantly altered from that observed in the liquid or gas phases as well as when confined to the external surface of nonporous silica nanoparticles. It appears that the increase in product selectivity in the mesoporous silica is a consequence of steric inhibitions on the rearrangement pathway, which involves an O C phenyl shift in the PhCH2CH 3 OPh radical intermediate, resulting from interactions with the pore
ARTICLE
wall surface. Hydrogen bonding of the ether oxygen in PPE with residual surface silanols provides a driving force for this interaction, which is significant under vacuum even at pyrolysis temperatures (375 °C) as revealed by MD simulations of relevant models. The selectivity increases further at lower PPE graft densities as well as in the presence of surface spacer molecules where enhanced hydrogen bonding with the pore surface can occur. When compared with results on the Cabosil nonporous silica nanoparticles, it is apparent that the locally concave surface curvature of the mesoporous silica pores provides an environment where PPE molecules interact more frequently with the silica surface than on the locally convex silica surface of Cabosil. PPE represents the simplest, unsubstituted structural model of the dominant β-O-4 aryl ether linkage present in lignin.31 We and others have examined in the gas and liquid phases the pyrolysis of more complex models containing phenol and alcohol substituents that naturally occur for this linkage in lignin.31,36 Multifunctional mesoporous silica-based catalysts are currently being examined for upgrading products initially generated from thermochemical conversion of lignocellulosic biomass,46 The larger pore of these catalysts compared with zeolites affords higher yields of desirable phenolic and hydrocarbon products.46 However, little is known about the potential role of confinement effects within these mesoporous catalysts on chemical reactivity. Our research provides fundamental insights on a confined lignin model compound whose inherent reactivity is altered by interactions with the mesoporous silica surface and is dependent on the local nanostructure. In the pyrolysis of lignocellulosic biomass, initial products are often passed directly through the catalysts bed (MCM-41 based) to upgrade the bio-oil product.46 It is conceivable that thermally produced lignin monomers, dimers, trimers, etc., containing these phenolic/alcoholic substituents could experience some confinement within the mesoporous silica-based catalyst bed through reactions with the silica surface. In preliminary studies, we have found that passing HOPhCH2CH2OPh (HOPPE) through a bed of MCM-41 (2.9 nm pore diameter) under vacuum at 375 °C results in significant attachment of the organic to the silica surface and an altered product selectivity compared with HOPPE alone in the gas phase that is consistent with the results presented in this paper. Our future research will study the thermal and catalytic reactions for lignin model compounds in mesoporous silica based materials in more detail.
’ EXPERIMENTAL SECTION General Experimental Methods. Organic analysis by GC-FID was performed on a gas chromatograph equipped with an autosampler and a 30 m 0.25 mm DB-5 (5%-phenylmethylpolysiloxane, 0.25 μm thick film) capillary column. Detector response factors required for quantitative analysis were determined relative to 2, 5-dimethylphenol and 4-(2-phenylethyl)phenol as internal standards. Mass spectra were recorded at 70 eV on a GC/MS equipped with an identical column and temperature program conditions to the GC FID analyses. Product identification was made through comparison of GC retention times and MS fragmentation patterns of authentic commercial or synthesized samples as detailed below. BET specific surface areas of the organic-derivatized mesoporous silicas were obtained from nitrogen adsorption desorption isotherms measured at 77 K on samples outgassed at 100 °C for a minimum of 8 h prior to analysis. Native mesoporous silicas were dried at 200 °C for 4 h prior to outgassing. Pore size distributions were analyzed by the BJH method. 6021
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The Journal of Organic Chemistry Materials. SBA-15 hexagonal mesoporous silica was prepared similarly to a previously reported method,47 with a few modifications that reproducibly lowered the micropore volume to less than 10%. Briefly, ca. 8.0 g of poly(ethylene oxide)-block-poly(propylene oxide)block-poly(ethylene oxide), EO20-PO70-EO20 (Pluronic 123), was dissolved in 60 mL of DI water and 240 mL of 2 M HCl with stirring at 35 °C. To this was added 17.0 g of tetraethyl orthosilicate which was stirred for an additional 24 h. The mixture was cured at 90 °C overnight without stirring. The solid formed was recovered by filtration and washed with DI water and EtOH until neutral pH. The solid was refluxed in 600 mL of EtOH for 4 h, filtered, washed with EtOH and then water, followed by air drying. The solid was then calcined under an air flow while heating the oven in a stepwise fashion. This involved heating from room temperature to 150 °C where it was held for 1 h, followed by temperature increases of 100 °C every hour thereafter until 550 °C was reached where the sample was then held overnight. This produced SBA15 samples with mean pore diameters of 6.6 6.8 nm and specific surface areas of 750 970 m2/g. The organic substrate, p-HOPhCH2CH2OPh (p-HOPPE), was synthesized and purified as described previously.48 The precursor spacer molecules, n-hexadecyl alcohol, n-octanol, and 4-phenylphenol, were commercially available and used as received with measured GC purities of 99.7+%. 2-Naphthol was purified through recrystallization from ethanol, followed by sublimation in vacuo, to give a 99.9+% purity by GC. High purity dichloromethane, acetone, and HCl were commercially available and used as received, while benzene was distilled over sodium immediately prior to use. Preparation of Surface-Attached Substrates. The preparation of single-component surfaces of silica-immobilized PPE has been described previously.21 Two component surfaces were prepared in a similar fashion by coadsorbing the two precursors onto the surface of dried SBA-15 (200 °C, 4 h) at the desired molar ratio (i.e., 3:1 spacer/ PPE) from a benzene slurry. The benzene was removed via solvent evaporation on a rotary evaporator. Surface attachment was performed via condensation of the phenols on the mixed silica powder in an evacuated (5 10 5 Torr) sealed Pyrex tube at 215 °C for 1 h in a fluidized sandbath. Removal of any unattached reactants was accomplished by subsequent sublimation under dynamic vacuum (5 10 3 Torr) using a stepwise temperature ramp of 10 °C every 10 min from 205 to 265 °C. This was followed by extraction with dichloromethane, and drying in a vacuum oven at 45 °C and 20 mmHg overnight. The resulting white powders were stored in a vacuum desiccator prior to use. To analyze the organic surface coverage, ca. 25 mg of sample was dissolved in 30 mL of 1 N NaOH overnight. Internal standards, 4-(2phenylethyl)phenol, and 2, 5-dimethylphenol in 1 N NaOH were added, and the solution was acidified with concd HCl and extracted with dichloromethane, 3 10 mL. The combined organic layers were washed with brine, 1 10 mL, dried over MgSO4, and filtered and the solvent removed under reduced pressure. Silylation with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) in pyridine (2.5 M, 0.30 mL) produced the corresponding trimethylsilyl ether derivatives that were used for the GC quantitative analysis. Surface coverages and chemical purities (>99.5%) were then measured by GC FID. Pyrolysis Procedure. A weighed amount of sample (ca. 50 mg) was placed in one end of a T-shaped Pyrex tube (8 mm o.d.), plugged with a small piece of glass wool, evacuated, and flame-sealed at
