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Catalyst Deactivation in Pyridine-Assisted Selective Dehydration of Methyl Lactate on NaY Brian M Murphy, Michael P. Letterio, and Bingjun Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03166 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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Catalyst Deactivation in Pyridine-Assisted Selective Dehydration of Methyl Lactate on NaY

Brian M. Murphy, Michael P. Letterio and Bingjun Xu*

Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark DE, 19716 *[email protected]

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Abstract Acrylic acid is a major commodity chemical currently produced almost entirely from petroleum-derived sources. The catalytic dehydration of methyl lactate is a promising renewable route to producing this vital chemical feedstock, however, enhancing the selectivity toward the dehydration pathway and catalyst stability remain challenging. We demonstrate a selectivity for dehydration products of ~90% over a period of 30 hours on NaY by introducing a small amount of pyridine in the reactant feed (pyridine:methyl lactate = 1:10). This increase in selectivity is attributed to the inhibition of side pathways, i.e., decarbonylation and coking, both of which are catalyzed by surface Brønsted acid sites generated in situ, rather than the acceleration of the dehydration pathway. Catalyst deactivation has been shown to proceed through drastically different mechanism in the absence and presence of pyridine in the feed via a combination of activity tests, thermogravimetric analysis, N2 and transmission FTIR spectroscopic investigations. Coke formation is the primary cause of catalyst deactivation in the pyridine-free feed, whereas the accumulation of intact bulky and high boiling point acid/base complexes, e.g., pyridinium acrylate and pyridinium lactate, in the zeolite pores limits the access of reactant to the catalytic sites. Regeneration at 330-450 °C in inert atmosphere does not have any effect on the deactivated catalyst in the pyridine-free feed, but partially restores the catalytic activity of the spent catalyst in pyridine-spiked feed. Porous catalysts with more open structure that facilitate the desorption of the bulky acid/base complexes are expected to be more resistant to deactivation.

Keywords: methyl lactate, dehydration, decarbonylation, NaY, pyridine


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1. Introduction The development of efficient and economically competitive processes to produce valuable chemicals from biomass-based feedstocks is critical to reducing dependence on fossil carbon sources for a sustainable future.1-6 A key to developing effective biomass upgrading processes is the concept of platform chemicals, low-cost compounds that can function as flexible building blocks for a variety of valuable products.4,7 Lactic acid (LA) has been identified (with its alkyl esters) as a desirable platform chemical because it can be produced via the fermentation of biomass-derived cellulose at yields as high as 90%, with work on high-throughput catalytic routes from glucose ongoing.8-18 Valuable chemicals can be produced directly from LA, including acrylic acid (AA),19 1,2-propanediol,20 acetaldehyde (AD),21-25 2,3-pentanedione,26-27 and lactide (for poly-lactic acid).28 AA, which is currently produced from petroleum-derived propylene, is a high volume (~8 × 106 tons/year including alkyl esters), high growth (5%/year), and high value monomer for the polymer industry, and its polymer acrylate derivatives have a diverse range of commercial applications including paints, adhesives, coatings, and absorbent polymers.7,29 LA and lactates can be produced directly from cellulose through glucose with 100% theoretical atom efficiency, and therefore are promising renewable feedstocks for AA production. Despite the apparent structural similarity between LA and AA, selective dehydration of the former remains challenging.7 To this end, LA and lactates are excellent model compounds to investigate the design principles of solid catalysts which allow for selective activation of certain functional groups while preserving others in highly oxygenated multifunctional reactants. 3-hydroxypropanoic acid (3-HP) is another potential starting material, but it is much more expensive than LA.30 AA can also be synthesized from other sustainable feedstocks, particularly


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glycerol, which has received increasing attention recently due to its abundance as a byproduct of biodiesel production.31-34 There is a significant body of literature on the production of renewable AA and acrylates from LA and lactates via catalytic dehydration over a variety of solid acid catalysts. LA is often used as the reactant, but the acid’s tendency to self-oligomerize makes accurate catalytic measurements challenging; the use of use of lactate esters, i.e., methyl lactate (ML) can alleviate this concern.35 Dehydration activities of LA and ML have been reported to be similar.36 Inorganic salt catalysts have been studied extensively, reaching dehydration selectivities as high as 78%, but generally require high temperatures and low space velocity due to low intrinsic activities.29,36-43 Zeolitic materials, especially alkali-metal forms of FAU, MFI, and *BEA zeolites, have received significant attention, and usually show much higher intrinsic activity than salts. Selectivities as high as 75-80% over zeolite catalysts have been reported after tuning the ratio and/or strength of surface acidic and basic sites via alteration of surface balancing cations or salt impregnation.44-55 The major byproduct over these catalysts is acetaldehyde, which is formed efficiently (though not exclusively) by a Brønsted acid site catalyzed decarbonylation mechanism (Scheme 1, steps 5-6).55-57 Rapid catalyst deactivation is typical on zeolite catalysts, which is generally believed to be due to site blocking by coking. This issue was recently investigated in detail by Näfe, et al. (2015) and Lari, et al. (2016), who reported that switching to catalysts with an MFI topology or introducing mesopores via hierarchical zeolite structures could reduce catalyst coking and deactivation.53-54 A number of dehydration mechanisms have been proposed in the literature via reactivity, kinetic, and spectroscopic investigations. Tam, et al. proposed that sodium lactate formed via surface ion exchange, was a key intermediate in pathways for converting LA to both AA and 2,3-


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pentanedione over inorganic sodium salt catalysts.40 We have demonstrated a similar ion exchange process between organic molecules with acidic protons and Na+-form zeolites, creating surface Brønsted acid sites and adsorbed organic sodium salts;56 later, we showed that alkyl esters undergo an analogous reaction (Scheme 1, steps 1-2).58 The surface sodium lactate species generated by the ion exchange has been established as the active species required for selective

Scheme 1. Proposed mechanism for ML conversion by dehydration (to acrylates, left) and decarbonylation (to acetaldehyde, right) over NaY involving initial in situ generation of Brønsted acid sites by ion exchange with the assistance of water (center).56,58,59


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dehydration on NaY, similar to the conclusion reached by Tam, et al.40,56 Acrylic acid is the major dehydration product, and each Na+ site has been shown to catalyze multiple turnovers, leading to the conclusion that a reverse ion exchange mechanism precedes product desorption and frees the Na+ site for subsequent reaction (Scheme 1, steps 3-4). Sun, et al. proposed a Lewis acid-base cooperative dehydration mechanism that could help rationalize multiple literature reports where tuning the acid-base characteristics of the catalyst increased dehydration selectivity.47 Our previous study showed that using a small amount of pyridine (Py) as an additive in the reaction feed strongly suppresses the decarbonylation pathway by blocking the in situ generated Brønsted acid sites without a significant effect on the dehydration rate.59 This is consistent with a Lewis acid-base cooperative mechanism: the basic sites promote the ion exchange (by accepting a proton from water) while forming surface sodium lactate, while the Lewis acid sites catalyze dehydration. Brønsted acid sites, regardless of their origin (present prior to or generated during reaction), only promote the undesired decarbonylation and coking pathways, while Lewis acid sites are the active sites for selective dehydration (Scheme 1). Thus, by introducing pyridine into the reactant feed, active sites for the side reactions are selectively blocked, improving dehydration selectivity.59 Interestingly, although catalyst deactivation is generally believed to be caused by Brønsted acid site-catalyzed coking, the inclusion of pyridine in the reactant feed to titrate these sites does not significantly slow initial catalyst deactivation.60 In this work, we investigate the effect of pyridine, used as an additive in the reactant feed, on catalyst deactivation. We show that including pyridine in the feed increases the selectivity to dehydration substantially and changes the deactivation mechanism. Thermogravimetric analysis (TGA), transmission FTIR, and N2 adsorption tests performed on spent catalysts reveal that the causes of catalyst deactivation with and without pyridine in the feed are fundamentally different.


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With no pyridine in the feed, the accumulation of carbonaceous deposits (coke) is the main cause of catalyst deactivation. In contrast, when pyridine is continuously fed with the reactant, adsorbed bulky organic species such as pyridinium acrylate and pyridinium lactate accumulate in the cavities and channels of NaY, precluding reactant access to surface active sites and leading to catalyst deactivation.

2. Experimental 2.1 Materials Preparation NaY (nominal Si/Al = 2.5) was purchased from Zeolyst International (CBV-100, Lot 100031103472). The siliceous silicalite-1 sample was synthesized using the following procedure: 1.85 g Cabosil M-5 was mixed with 6.89 g TPAOH and 23.6 g H2O under stirring for 1 h, and then HF (0.65 g) was added to the mixture. The final gel composition of 50 H2O:0.44 TPAOH:1 SiO2:0.5 HF was placed in Teflon-lined autoclave at 175 °C for 5 days. All zeolite samples were calcined at 500 °C for 6 h with a 1 °C/min temperature ramp before use. Double deionized water was produced in-house by a Thermo Barnstead Mega Pure-1 Still deionizer. All chemicals (methyl lactate [98%], acetaldehyde [99.5%], acrylic acid [99%], methyl acrylate [99%], methanol [99.9%], propanoic acid [99.5%], 2,3-pentanedione [99%], and pyridine [99.8%],) were obtained from Sigma Aldrich and used without further purification.

2.2 Materials Characterization 2.2.1

X-Ray Diffraction


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Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Discover powder diffractometer with a Cu Kα source (λ = 1.5418 Å) over the range 2θ = 5°-50° with a step size of 0.025° and 2 s per step.

2.2.2

N2 Adsorption Textural characterization of the samples was determined using N2 physisorption at -196

°C. Isotherms were collected on a Micrometrics ASAP 2020 instrument, and micropore volumes were calculated using the t-plot method. Prior to the adsorption measurements, fresh and calcined spent catalyst samples were degassed at 250 °C for 6 h. Spent catalyst samples that did not undergo calcination were degassed at 225 °C for 6 h, so as to remove adsorbed water but not organic species; this temperature was chosen based on the TGA results in inert N2.

2.2.3

Thermogravimetric Analysis Thermogravimetric analysis (TGA) was performed on a Mettler-Toledo TGA 2

instrument using the following procedure: either air or N2 was flowed at 50 mL/min, and the sample was heated at a 2 °C/min temperature ramp to 500 °C followed by a 1 h hold at 500 °C to ensure that the relevant desorption/reaction processes were complete.

2.2.4

Solid-state Nuclear Magnetic Resonance Solid state

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Si (99.83 MHz, 10 kHz rotational speed, 2048 scans, and 30 s relaxation

delay) and 27Al (130.29 MHz, 10 kHz rotational speed, and 512 scans) MAS NMR experiments were performed on a Bruker AVIII500 NMR spectrometer with a 44 mm HX MAS probe for all measurements.

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Si and

27

Al MAS NMR measurements showed the Si/Al ratios of the NaY


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catalyst to be 2.2. We employed a standard method of calculating the Si/Al ratio from 29Si NMR data described by Engelhardt, et al., using the relative area of each Si peak (corresponding to various Si(nAl) coordinations).61

2.3 Transmission FTIR Spectroscopy Fourier transform infrared (FTIR) spectra were obtained on an Agilent Cary 660 FTIR Spectrometer equipped with a MCT detector (128 scans at a spectral resolution of 2 cm-1) with a custom transmission cell. A vacuum level of 90%) and enhance the deactivation effects. This is in contrast to our previous kinetic studies (ML conversion < 30%),

Figure 1. Reactivity of ML solution over NaY with (a) no pyridine additive over 6 h TOS and (b) Py/ML = 1/10 molar ratio over 10 h TOS at 330 °C. NaY loading = 100 mg; ML concentration in aqueous solution = 30 wt%, aqueous solution flow rate = 1 mL/h, N2 carrier gas flow rate = 50 mL/min.


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which focused on the formation rates and mechanisms of recoverable products and the effect of pyridine on these processes.56,59 With no pyridine in the feed, i.e., Py/ML = 0, ML is completely converted at 20 min time on stream (TOS), with nearly linear deactivation to 33% conversion at 6 h TOS (Figure 1a). Selectivity to dehydration products (SDH, AA and methyl acrylate) is initially 45% and climbs to ~70% after 2 h (Figure S2a). Side reactions are substantial: initial selectivity to acetaldehyde is almost 30%, and more than 20% of the fed carbon is unaccounted for (carbon balance (CB) < 80%). We attribute the low initial carbon balance to the rapid accumulation of coke and coke precursors, and TGA under air after 1 h TOS shows that surface carbon deposits already account for 22% of the dry catalyst mass (vide infra). After ~2 h TOS, the catalyst reaches a quasi-steady state, where the selectivity for dehydration products (SDH) and acetaldehyde (SAD) remain steady at ~70 – 75% and 20 – 25%, respectively, despite unclosed carbon balances (~95%) and continued deactivation. Textural characterization by N2 adsorption of the 6 h TOS spent catalyst (NaY-S0) showed a ~90% decrease in pore volume, 0.327 cm3/g to 0.031 cm3/g, consistent with deactivation by coking (Table 1). Yields of minor products such as propanoic acid (PA) and 2,3-pentanedione (PD) are less than 1%. Methanol is recovered with high initial yields (85-90%), because both the ML dehydration and decarbonylation pathways require hydrolysis of the ester bond (Table S1). The hydrolysis proceeds through the dissociative adsorption of methyl lactate on NaY to form sodium lactate and adsorbed methoxy group, which is subsequently removed by reacting with water.56 In contrast, Brønsted acid sites are only selective to decarbonylation, which we speculate proceeds via hydrolysis leading to methanol and a surface lactate-derived acylate species similar to the structure proposed by Gumidyala, et al. for reactions involving acetic acid over Brønsted acidic zeolites (Scheme S1).65-66


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Introducing pyridine into the reaction feed at a Py/ML molar ratio of 1/10 effectively suppresses the decarbonylation pathway to acetaldehyde and enhances SDH (Figure 1b). The initial ML conversion is slightly suppressed from >99% to 89% and decreases rapidly in the first 2 h TOS to 55%, a value not reached until 4 h TOS in the Py/ML = 0 case (Figure 1a). In contrast to the linear deactivation observed in the absence of pyridine, after 2 h TOS deactivation slows significantly: ML conversion decreases only 15% between 2 and 10 h TOS. Total deactivation over 10 h TOS with Py is 54%, versus 67% deactivation without, suggesting that the inclusion of Py slows catalyst deactivation. Pyridine suppresses the initial acetaldehyde yield (YAD) almost an order of magnitude, from 28% to 3%, and between 1-10 h TOS it stabilizes at 12%. This result matches well with the factor of 20 decrease in the turnover frequency for acetaldehyde formation observed in the kinetic regime, and both can be attributed directly to pyridine adsorption on Brønsted acid sites generated in situ via water-assisted ion exchange between ML and NaY as described previously (Scheme 1).59 Although both Brønsted and Lewis sites are present simultaneously, the Lewis acidic Na+ on NaY is unable to adsorb pyridine at the reaction temperature, and pretreating NaY with pyridine prior to reaction has no effect because Brønsted acid site generation requires interaction with the reactant solution. In contrast, transmission FTIR experiments show that pyridine remains adsorbed to both intrinsic (present prior to reaction) and in situ generated Brønsted acid sites at 330 °C.56,59 Blocking of the in situ generated Brønsted acid sites is likely a contributing factor in the reduced initial ML conversion (100% to 89%), as Brønsted acid site catalyzed decarbonylation is a significant side reaction in the absence of pyridine.21,23,56 Initial yield of dehydration products (YDH), i.e., AA and methyl acrylate, is similar with or without pyridine (40 – 45%), including an initial increase in the first 1 h TOS. However, YDH is much more stable at Py/ML = 1/10 than at lower pyridine


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concentrations (Figures S2b and S3), decreasing only 13% from a maximum of 48% (1 h TOS) to a minimum of 35% (10 h TOS). Without pyridine, YDH decreases 36% between 1 h TOS (57%) and 6 h TOS (21%). Moreover, in the presence of pyridine the YDH trace overlaps with the ML conversion trace between 4–10 h TOS, i.e., selectivity for dehydration products (SDH) is essentially 100% (Figures 1b and S2b). To understand the selectivity trend, it is helpful to note the pyridine uptake trend over time. Initially, most of the pyridine is missing in the effluent (95% uptake at 20 min TOS), but no pyridine decomposition products are detected, suggesting that pyridine is adsorbed on the catalyst rather than converted or decomposed. This is consistent with the strong adsorption of pyridine on in situ generated Brønsted acid sites.59 The pyridine uptake drops precipitously with TOS, and recovery is essentially ~100% at TOS ≥ 4 h (Figure 1b). By integrating the area under the pyridine uptake trace, the total pyridine uptake at Py/ML = 1/10 is estimated to be 1.6 × 10-5 mol/gcat, which corresponds to ~3 pyridine molecules per supercage. Literature estimates indicate

Figure 2. Product distribution of ML dehydration on NaY at various Py/ML molar ratios. (a) Changes in cumulative ML conversion and various product selectivities; (b) overall dehydration yield (product of ML conversion and SDH) (c) molar pyridine uptake; the dashed line represents the amount of accessible Al sites in the catalyst bed (in moles). Reaction conditions are identical to Figure 1. All data taken at 6 h TOS with the exception of 1/10* at 10 h TOS.


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that the maximum number of pyridine molecules per supercage in zeolite Y is ~4, limited by a combination of available balancing cations as adsorption sites and the volume of the FAU supercage.61–64 Textural characterization by N2 adsorption of the spent catalyst used in the Py/ML = 1/10 case shows very little available pore volume (0.008 cm3/g); the discrepancy between the lack of available pore volume and relatively high ML conversion will be discussed in detail later (Table 1). Therefore, access to the cations of NaY by ML appears to be limited at long TOS due to the accumulation of pyridine-related species. As the concentration of pyridine in the reactant feed ([Py]f) increases from Py/ML = 0 to Py/ML = 1/10, SDH increases from 67% to 87% while selectivity for acetaldehyde (SAD), ML conversion, and coke formation all decrease (Figure 2a). The effect of [Py]f on product distribution is depicted by plotting the cumulative ML conversion and product selectivities over 6 h TOS as a function of the Py/ML ratio in the feed in Figure 2a (time-resolved product distribution data of all runs can be found in Figure S3). [Py]f shows a strong positive correlation with SDH and negative correlations with SAD and total ML conversion (Figure 2a). When all 10 h TOS data are used for the Py/ML = 1/10 case (indicated by 1/10*), SDH increases further (80% to 87%) while ML conversion decreases (53% to 45%) and SAD remains constant at 3.5%. As [Py]f increases, selectivity for coke ("Lost Carbon") decreases slightly (10% to 6%) and selectivity for 2,3-pentanedione increases (1% to 3%). However, pyridine concentration has only a marginal effect on overall YDH, which remains constant at ~40% because increasing SDH is offset by declining overall ML conversion (Figure 2b). Thus, the enhanced SDH is attributed mainly to the suppression of side reactions, rather than the acceleration of the desired pathway, which is consistent with the reduced overall ML conversion at high pyridine concentrations as well as our previous work.59 Further, this supports our previous conclusion that Lewis acid sites are the


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active site for dehydration, as pyridine does not adsorb to these sites at 330 °C and therefore should not affect the dehydration activity.59 Total pyridine uptake increases with increasing feed concentration, which is likely due to competition between pyridine adsorption on Brønsted acid sites and Brønsted acid-catalyzed coke formation, with the former becoming more competitive at higher [Py]f (Figure 2c).59 For instance, at low [Py]f, i.e., Py/ML = 1/1000, total SDH and SAD over 6 h TOS are only slightly different than without pyridine, and there is only a marginal change in initial catalyst activity (Figure 2a and S3a). We also observe that pyridine uptake is 100% for the first 3 h TOS; these results indicate an excess of available Brønsted acid sites for adsorption and catalysis when the pyridine concentration is low (Figure S3a). More quantitatively, under Py/ML = 1/10 conditions the initial pyridine uptake is nearly 100%, and an excess of Brønsted acid sites in the high [Py]f case implies ~99% of the Brønsted sites remain exposed under Py/ML = 1/1000 conditions (Figure 1b). The total molar uptake of pyridine increases monotonically with [Py]f, and the catalyst deactivation trends in the intermediate ratios appear to be a mixture of the no pyridine (linear) and Py/ML = 1/10 (two-regime) trends, indicating direct competition between coking and pyridine adsorption (Figure S3). Therefore, the drop in pyridine uptake after 3 h TOS under Py/ML = 1/1000 is more likely due to elimination of adsorption sites by coking than pyridine saturation, as the catalyst is rapidly deactivating and the total amount of pyridine uptake over 6 h is more than one order of magnitude lower than in the Py/ML = 1/10 case (Figure 2c). While pyridine uptake is still non-zero at 6 h TOS at Py/ML = 1/1000, uptake becomes negligible at or before 6 h TOS when the Py concentration is equal to or higher than Py/ML = 1/500, and the time to saturation decreases with increasing [Py]f (Figure S3). More coking at lower [Py]f can be qualitatively confirmed: spent catalysts used in low [Py]f feeds have a much darker color (Figure S4). Even though the catalyst is adsorbing more pyridine


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at high [Py]f, NaY still deactivates rapidly and the carbon balance remains unclosed initially in all cases, suggesting both coking and adsorption processes contribute to deactivation. In the Py/ML = 1/10 case, substantially more pyridine is fed than in the other cases, and pyridine uptake is always less than 100%, indicating the feed is providing the alkaline additive in excess of what is required to fully quench all Brønsted acid sites present; these sites have been shown to rapidly catalyze coke formation, and therefore coke formation should decrease at high [Py]f.59-60 The contradiction between the apparent quenching of Brønsted sites, responsible for pyridine uptake and substantially reduced YAD, and the observed rapid deactivation even in the presence of pyridine is investigated in detail in the following sections.

3.3 Thermogravimetric analysis of spent catalysts Thermogravimetric analysis (TGA) experiments of all spent catalysts under flowing air suggest that changing [Py]f alters the loading and nature of the carbon-containing surface deposits present after reaction (Figure 3a). The post-reaction characterization is motivated by the phenomenological observation that the spent catalysts for the no pyridine and Py/ML = 1/10 experiments show different colorations after reaction (Figure S4) despite both deactivating rapidly (Figure 1). The mass loss during air TGA of the fresh, unused NaY sample below 200 °C corresponds to desorption of adsorbed water, which accounts for ~17% of the dry mass. Mass loss on all spent catalysts starts at room temperature, lower than on unused NaY, indicating a fraction of water is adsorbed on surface deposits rather than NaY, leading to more facile desorption. The Py/ML ratio is correlated with the amount of water adsorbed on a given sample, from 8% of dry mass for Py/ML = 0 to 15% for Py/ML = 1/10 with a distribution in between, reflecting a substantial difference in the hydrophilic/hydrophobic character of spent catalysts


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Figure 3. Thermogravimetric analysis of (a) unused NaY and all spent catalysts under an oxygencontaining atmosphere; (b) total mass loss at T > 300 °C for all catalysts shown in (a); (c) NaY-S0 and NaY-S10 under N2 feed; (d) Difference in mass loss at T > 250 °C between oxygen-containing and inert atmospheres for NaY-S0 and NaY-S10; (e) TGA of NaY sample used with no pyridine after 1 h TOS in air.

dependent on [Py]f. The spent samples of the two limiting cases, designated hereafter as NaY-S0 (used under Py/ML = 0 conditions) and NaY-S10 (used under Py/ML = 1/10 conditions) for brevity, will be investigated in more depth. For TGA experiments performed in flowing air, the mass loss beginning at 250-300 °C is typically attributed to combustion of carbon-containing deposits given the relatively high temperature and lack of an analogue in the unused NaY sample. The onset temperature of this downslope decreases with the increasing [Py]f, from 290


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°C for Py/ML = 0 to 250 °C for Py/ML = 1/10, which offers the first indication that the surface deposits on NaY-S10 may be of a fundamentally different nature than on NaY-S0. Carbon-rich coke deposits from LA dehydration reactions on NaY have been previously shown in other works to begin combusting at ~300 °C, matching the NaY-S0 curve well.54 The total mass of carbon-containing deposits decreases with increasing [Py]f, from ~25 dry wt% on NaY–S0 to ~15 dry wt% on NaY–S10, consistent with the higher total ML conversion and lower overall carbon balances observed at low [Py]f (Figure 3b). To further elucidate the effect of pyridine, TGA was also performed in N2, and the results also point to a change in the nature of the carbon-containing catalyst deposits due to pyridine (Figure 3c). There is very little mass loss at T < 400 °C on NaY-S0 under N2, suggesting a lack of volatile surface species after reaction without pyridine. Roughly 6% of the final catalyst mass is lost at T > 400 °C, which in the absence of O2 can be attributed to dehydrogenation, dehydration, and C-C bond forming reactions which occur in coke deposits at high temperatures under inert atmospheres in the presence of Brønsted acid sites, collectively referred to as catalyst aging.67-69 Textural analysis by N2 adsorption corroborates this attribution, after treatment under N2/H2O atmosphere at 400 °C for 4 h, the available pore volume in NaY-S0 increases from 0.031 cm3/g to 0.082 cm3/g (Table 1). Catalyst aging produces highly polymaromatic species which are denser than the starting materials, leading to the increase in available volume; however, little carbon is removed from the surface and so no active sites are recovered (vide infra). The difference in mass loss at T > 250 °C under air and N2 is much larger for NaY-S0 than for NaYS10: ~15% of final mass and 5% of final mass, respectively (Figure 3d), again indicative of fundamentally different surface deposits (Figure 3d). The similar behavior of the NaY-S10 sample under air and N2 suggests that there is a large contribution from desorption, which is


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more dependent on temperature than atmospheric conditions. NaY-S10 also shows significant low-temperature mass loss regardless of atmosphere, indicating water adsorption either due to the hydrophilicity of the surface deposits or the retention of available surface Na+. Textural characterization of NaY-S10 shows very little available pore volume, 0.008 cm3/g, indicating the former possibility is more likely (Table 1). TGA under air after 1 h TOS and [Py]f = 0 shows the low initial carbon balances are due to the rapid accumulation of carbonaceous deposits and coke precursors on the catalyst surface, and also that the nature of these deposits changes over time. The pyridine-free reaction was quenched after 1 h TOS by shutting off the reactant feed and rapidly cooling the catalyst bed to room temperature. At this time, carbon-based adsorbates or deposits are ~22% of the dry mass of the catalyst, very close to the ~25% observed after 6 h of reaction (Figure 3e). This is consistent with the observed carbon balances, which are initially low (< 80%) but after the first hour TOS rise to consistently above 90%. However, the onset temperature of the downslope after 1 h TOS sample is only 225 °C, much lower than 290 °C observed after 6 h TOS (Figure 3e). This indicates that the carbonaceous deposits formed at short TOS are less polyaromatic and more similar in nature to the ML reactant than those observed at long TOS. It has been proposed that coke formation may be caused by slow reactant desorption from the catalyst surface, allowing for Brønsted acid-catalyzed reactions to gradually form heavier non-volatile species and block H+ sites.70 We have shown that ML adsorbs strongly to NaY due to the formation of sodium lactate via ion exchange; this species is IR-visible under vacuum up to 400 °C .56 Therefore, the in situ generated Brønsted acid sites may react with the strongly adsorbed species, causing the accumulation of coke and coke precursors.70-71 The species that combust at relatively low temperatures in TGA are often referred to as "soft" coke, and are typically rich in H and O.67


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Prolonged exposure to high temperatures and/or reactants in the presence of surface H+ sites leads to subsequent reactions toward the eventual formation of highly polyaromatic species that combust at higher T than the early deposits or coke precursors; this is similar to the aging of coke described earlier.67-69 At long times, the total accumulation rate slows: after 22% deposited in the first hour, we observe only a 3% increase in the subsequent 5 h TOS. H2, H2O, and COx are all possible products of the aforementioned polyaromatization reactions, and our inability to detect these species via FID could account for discrepancy between unclosed carbon balance and lack of mass accumulation on the catalyst.69

3.4 Spectroscopic identification of adsorbed species on spent catalysts Transmission FTIR of pyridine and AA on a siliceous MFI zeolite sample (silicalite-1) indicate that the acid/base pair form an adsorbed acid-base complex that desorbs at higher temperature than either pyridine or AA alone. When pyridine is introduced to silicalite-1 at 75 °C, only weak bands at 1597 cm-1, 1490 cm-1, and 1445 cm-1 corresponding to molecularly adsorbed pyridine appear (Figure 4a[i]).59,72 Complete desorption of pyridine is observed at T < 200 °C, which is expected due to the weak interaction between pyridine and the non-acidic silicalite-1 (Figure 4a[vii–viii]). Upon introducing AA to silicalite-1 at 75 °C, characteristic bands of molecularly adsorbed AA appear: νC=O at 1722–1711 cm-1, νC=C at 1617 and 1635 cm-1, and δC-H at 1408 and 1430 cm-1 (Figure 4b[i]).73 As a control, temperature programmed desorption of adsorbed AA was conducted without pyridine, and desorption is complete at T < 150 °C (Figure 4b). The spectra of the combined acid and base species are substantially different: when pyridine is introduced to an AA-covered surface at 75 °C, a sharp band at 1580 cm-1 and a broader band at 1541 cm-1 appear in addition to the previously observed molecular pyridine


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bands (Figure 4c[ii]). 1541 cm-1 is characteristic of protonated pyridine, or pyridinium, formed in an acid/base reaction with adsorbed AA.74 Cationic pyridinium must be balanced by the acrylate


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anion, and we attribute the sharp 1580 cm-1 band (FWHM = 7 cm-1) to the νO-C-O mode of this species. We have previously attributed the same mode of sodium acrylate to a 1575 cm-1 band,


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and a small 5 cm-1 blueshift is possible due to the difference in the balancing cation.40,58-59,73,75 Slowly increasing the temperature shows that although the 1580 and 1541 cm-1 bands fade by

Figure 4. Transmission FTIR of various species on silicalite-1. (a) pyridine; (b) AA; (c) AA followed by pyridine; (d) zoomed-in spectra the 1525 – 1475 cm-1 region of (c).


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150 °C (Figure 4c[iii-iv]) the 1490 cm-1 band persists to 250 °C (Figure 4d[iii-vii]), much higher than when pyridine or AA was introduced alone. The 1490 cm-1 feature is known as the combination band because it appears when pyridine is adsorbed on either Brønsted or Lewis acid sites, but when adsorbed on the latter it is always accompanied by the 1445 cm-1 Lewis acid band.59,74 The absence of the 1445 cm-1 band when co-adsorbed with AA on silicalite-1 at 150 °C (Figure 4c(iv)) indicates that at higher temperature the 1490 cm-1 band can only correspond to pyridinium.59 These spectral signatures suggest the formation of an acid-base pyridinium acrylate species, which has a higher desorption temperature than either pyridine or AA alone, suggesting that it could be due to its intrinsic higher boiling point or trapped in the small pores of the MFI framework. It follows that AA formed in the dehydration of ML could react with pyridine to form pyridinium acrylate and get trapped in the pores of NaY, both by adsorption and by the steric limitation of the pore opening. This is corroborated by the significant pyridine uptake observed in the activity study, up to 3 pyridine molecules per NaY supercage. The gradual accumulation of pyridinium acrylate in the channels and supercages of NaY could limit access to the Na+ sites and cause catalyst deactivation. Transmission FTIR spectroscopy of NaY-S0 and NaY-S10 indicates that at high [Py]f catalyst deactivation is caused primarily by zeolite pore filling due to the accumulation of bulky organic species, while at low [Py]f deactivation is due to coking. The spent catalysts were ground and pressed into self-standing wafers for transmission FTIR experiments without adding diluting salt, e.g., KBr, to avoid any potential reaction in the process of heating. In the high wavenumber region (3800–2800 cm-1) at 50 °C, the spectrum of NaY-S0 shows a broad band between 3750– 3000 cm-1, which is attributed to a combination of adsorbed water and O-H modes in the surface deposits (Figure 5a[i]), corroborated by a band at 1645 cm-1 attributed to the H–O–H bending


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mode of water in the low wavenumber region (2000 – 1300 cm-1, Figure 5b[i]).76-77 The 1645 cm-1 band decreases significantly in intensity by 200 °C and disappears completely before 300

Figure 5. Transmission FTIR spectra of (a) and (b) NaY-S0, (c) and (d) NaY-S10 between 50 and 450 °C. Spectral windows 3800 cm-1 –3000 cm-1 and 2000 cm-1–1300 cm-1 28 ACS Paragon Plus Environment

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°C, which is consistent with the TGA results above (Figure 3 and Figure 5a[ii-vi]). At and above 250 °C, the spectroscopic signatures of the NaY-S0 sample point to the presence of carbonaceous surface deposits (Figure 5a and b[v-xi]). In particular, the strong and broad 1580 cm-1 band is characteristic of C=C bond stretching modes in the polycyclic aromatic compounds that make up carbonaceous, turbostratic deposits on catalysts; these deposits are commonly referred to as coke.78 The broad nature of this band (FWHM = 45 cm-1) has been attributed to the fact that these deposits are comprised of a wide variety of species, both polyaromatic and disorganized, containing similar bonds with a range of vibrational modes. As a composite of many different bond modes, the 1580 cm-1 band is broader than a band corresponding to one specific vibrational mode.60,79 The 2960 cm-1, 2928 cm-1 and 2870 cm-1 bands are replicated exactly in multiple previous studies and are known to be caused by –CH, –CH2, and –CH3 stretching modes in carbonaceous deposits, respectively; the weaker bands at 1456 cm-1 and 1380 cm-1 are attributed to the bending modes of these same species (Figure 5a and b). While no one band is definitive, the simultaneous presence of all of these band provides substantial evidence of carbonaceous deposits or coke.60,64,80-81 The 1580 cm-1 band has been previously identified as the strongest and most definitive band of coke formation, and the intensity of this "coke band" has been directly correlated to rate decreases of model Brønsted acid site-catalyzed reactions by other authors, showing that the formation of carbonaceous deposits (as measured by the broad 1580 cm-1 feature) leads to catalyst deactivation by blocking active sites.60 Therefore, tracking the intensity of this band, as long as the other modes identified above are present as well, is a simple and direct spectroscopic method of assessing coke formation on the surface of a zeolite catalyst. Uniquely, in the NaY-S0 sample the 1580 cm-1 band is accompanied by a strong band at 1680 cm-1 (Figure 5b). The retention of functional groups from the starting material into


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the carbonaceous deposits is common, especially for species with high adsorption strength such as sodium lactate and acrylate. Therefore, the 1680 cm-1 band is tentatively attributed to an α,β -unsaturated carbonyl stretching mode, which may correspond to residual carbonyl groups from the lactate and/or acrylate precursors that are retained in the carbonaceous deposits.81-82 The formation of polyacrylate species is also possible under the reaction conditions used in this study, especially considering the lack of polymerization inhibitor used, but these species would likely have similar deactivation effects as polyaromatic coke, and distinguishing between the two is outside the scope of this work. In contrast to NaY-S0, FTIR spectra of NaY-S10 provide evidence for the presence of adsorbed pyridinium on the catalyst surface. At T < 350 °C, the spectra have “flathead” bands, typically caused by signal saturation, at 3750 – 3000 cm-1 and 1750 cm-1 – 1550 cm-1. The saturation is attributed to a high concentration of IR-absorbing species in the catalyst wafer (Figure 5c and d[i-iii]), e.g., adsorbed ML, sodium acrylate and/or lactate, and pyridine (suggested given the properties of the feed and the location of the bands). Further reducing the thickness of the self-standing catalyst wafer in order to reduce the absorption in these regions proved challenging. Notably, there is a sharp band at 1488 cm-1, visible even at 50 °C, which is not present in the NaY-S0 spectra. This is indicative of adsorbed pyridinium on the catalyst surface, as detailed in previous works.59,72,81 Control experiments using pyridine on HY show that at high temperatures, the combination band near 1490 cm-1 shifts towards 1485 cm-1, and by 330 °C has moved to ~1488 cm-1 (Figure S5).59 When dosing ML, H2O, and pyridine at 200 °C the characteristic 1540 cm-1 pyridinium band is clearly present, but upon increasing the temperature this relatively weak band fades while combination band slowly shifts to ~1485 cm1 59

. In Figure 5d a strong 3085 cm-1 band corresponding to the C-H aromatic stretching mode of


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pyridine is also present; this band always accompanies the 1485 cm-1 combination band. Most C–H modes of non-nitrogenous single ring aromatics appear at wavenumbers below 3070 cm-1, confirming our assignment to pyridine rather than a hydrocarbon aromatic species.83 The 3115 cm-1 band present is attributed to a pyridinium acrylate-related species, as this band is only observed when pyridine is introduced to a NaY surface pre-treated with either lactate (which then dehydrates) or acrylate.59 The aforementioned characteristic bands for –CHx in coke (2960 cm-1, 2939 cm-1, 2870 cm-1) are also present in low intensities, suggesting the sample is not completely coke-free.60,80-81 Above 350 °C, the νC-H bands of pyridinium between 3200 cm-1 – 3000 cm-1 and the combination band at 1488 cm-1 disappear, indicating that pyridinium desorbs from the catalyst surface in this temperature range (Figure 5d[viii-ix]). Concurrently, the flathead band between 1750–1550 cm-1 sharpens into two bands centered around 1680 cm-1 and 1580 cm1

, along with the appearance of weak bands in the C-H bending region between 1460 cm-1 – 1360

cm-1. The spectrum of NaY-S10 at 450 °C closely resembles the coked NaY-S0 spectrum at the same temperature (Figure 5d[viii-ix]). The growth of these coke-related bands corresponds with the disappearance of the 1488 cm-1 pyridine band, suggesting that coking is initiated after pyridine desorption, which matches well with the results of the activity and TGA experiments. Although the FTIR results provide significant insight into the mechanistic causes behind the activity and TGA data, the high concentration of adsorbed species causing “flathead” bands on the spent catalysts limits the ability to specifically identify the composition and structure of adsorbates, especially at low temperatures.


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In order to identify key surface adsorbates, multiple sequential doses of the reactant solution (30 wt% aqueous ML with Py/ML = 1/10) were introduced to the NaY wafer at 330 °C in the transmission FTIR cell to simulate the flow reaction under controlled conditions. After annealing a fresh NaY wafer, the solution is dosed at ~100 mTorr for ~10 s via a vacuum manifold; spectra are taken both immediately following evacuation and after a 15 min hold under vacuum (cell pressure < 10-5 Torr). This process is repeated multiple times until the intensity of the band corresponding to the lactate species (1603 cm-1) plateaus. After the last dose, the

Figure 6. Multiple doses of Py/ML = 1/10 reactant solution on NaY at 330 °C, with 15 min hold under vacuum between each dose. Spectral window (a) 3800 – 2800 cm-1 and (b) 1700 – 1300 cm-1. In the Figure, 'm' represents minutes elapsed after the initial solution dose.


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catalyst is monitored for 7 h to probe any reaction and/or desorption of the adsorbed species under vacuum at 330 °C. The catalyst is initially devoid of adsorbates with only the 3740 cm-1 external silanol group band visible; for clarity, the NaY spectral background is subtracted in all subsequent traces (Figure 6[i]).58,74 Upon the first introduction of the ML/pyridine mixture, a broad band at 3600 – 3200 cm-1 and a series of narrower bands at 3150 – 2900 cm-1 appear, corresponding to adsorbed hydrogen-bonded water and νC-H modes in ML and pyridine, respectively (Figure 6a[ii]).56 The water band disappears within 15 min under vacuum at 330 °C, which is consistent with our previous work and literature (Figure 6a[iii]).58,72 The 1603 cm-1 band, previously attributed to the νO-C-O stretching mode of adsorbed sodium lactate, appears immediately after dose (Figure 6b[ii]). After 15 min hold under vacuum, the absorbance has gone down and a shoulder centered at 1575 cm-1, characteristic of the νO-C-O stretching mode of adsorbed sodium acrylate, emerges (Figure 6b[iii]).56,59 The concurrent diminishment and growth of the 1603 cm-1 and 1575 cm-1 bands, respectively, indicates that sodium lactate is actively being dehydrated to sodium acrylate (Figure 6b[iii]). This is strong evidence for Lewis acid sites, rather than Brønsted acid sites, catalyzing lactate dehydration, because the characteristic band corresponding to Brønsted acid sites on faujasite (at ~3645 cm-1) is not detected. All in situ generated Brønsted sites are quenched by adsorbed pyridine, evidenced by the 1485 cm-1 band which we have previously shown can only be due to pyridinium formation at this temperature.59 The absence of the 1541 cm-1 band corresponding to pyridinium is likely due to the broad nature of this feature under the given conditions and the presence of strong neighboring acrylate and lactate bands (Figure 4c[ii]). Six more doses are introduced at 330 °C interspersed by 15 min intervals of evacuation. Although the sodium lactate band is always much stronger than the sodium acrylate band immediately after each dose, during the 15 min vacuum holds the former


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always diminishes while the latter grows. Assuming roughly equal extinction coefficients for the νO-C-O mode in sodium lactate and acrylate (discussed in more detail below) the diminishment of the lactate band cannot be fully compensated by the increase in the acrylate band, indicating that a fraction of lactate undergoes desorption rather than dehydration, likely because the removal of gas-phase species by evacuation shifts the ion exchange equilibrium. During dose, the surface coverage of sodium lactate is in equilibrium with the gas phase concentration of ML in the cell, but the reverse reaction will occur when the gas phase ML concentration decreases due to pumping, following the Le Chatelier’s principle. No broad bands at 1680 cm-1 or 1580 cm-1 evolve over time, indicating the diminishment of the lactate band is not due to coking, again suggesting that pyridine is suppressing this pathway. In addition to the 1575 cm-1 band, the 1603 cm-1 band develops a shoulder centered around 1627 cm-1 over time (Figure 6b[iii]). Both acrylates and pyridinium have vibrational bands in this region, thus the intensity of this band cannot be correlated to a specific mode or species. Bands at 1425 cm-1 and 1364 cm-1 appear after each dose and disappear during each 15 min hold (Figure 6b[ii-xv]), and are consistent with bands observed of molecularly adsorbed LA on NaY, formed via ML hydrolysis, but not those originating from adsorbed ML, AA, lactate or acrylate (Figure S6).59,73,75 It is important to note that there are differences in the conditions of the multi-dose FTIR spectroscopic and reactivity experiments: under true reaction conditions, the catalyst is continuously exposed to the reactants at much higher pressures (760 Torr vs. ~0.1 Torr) for a much longer time (6-10 h TOS vs. ~10 s increments), meaning the current experiment likely captures on the initial stages of adsorption and accumulation. Therefore, the uptake of ML and pyridine in the presence of the reactant feed is likely substantially higher, leading to saturated IR signals as observed in the spectra of NaYS10 (Figure 5c and d).


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Peak deconvolution and quantification of the traces in Figure 6 show that the integrated area of the pyridinium band (1485 cm-1) correlates well with the sum of the area of adsorbed sodium lactate and acrylate bands, indicating that the pyridinium cation is balancing the charges of both lactate and acrylate anions. All spectra in Figure 6 are obtained under vacuum, so the observed bands are entirely attributed to adsorbed species. The spectral region of 1700 – 1450 cm-1 is deconvoluted into four bands for a semi-quantitative analysis of the relative concentrations of surface species: 1627 cm-1 (potential contributions from both acrylate and pyridinium), 1603 cm-1 (νO-C-O mode of sodium lactate), 1575 cm-1 (νO-C-O mode of sodium acrylate), and 1485 cm-1 (pyridinium); an example trace is shown in Figure 7a. Deconvolution was not performed for later traces (Figure 6b[xix]–[xxii]) as the bands are too weak for accurate quantification. The only Brønsted acid sites on the catalyst are generated by interactions between the surface and reactant molecules, and therefore by stoichiometry each pyridinium ion should correspond to one ion exchanged species, i.e., sodium lactate (via ML hydrolysis and ion

Figure 7. Analysis of the IR spectra of the multiple-dose experiment presented in Figure 6b. (a) Multipeak fitting analysis on trace [vii] of Figure 6b; (b) Area of 1485 cm-1 band (black) with the linear combination model (red) for each trace.


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exchange) or acrylate (via lactate dehydration). Given the variety of anionic (lactate, acrylate and zeolite framework) and cationic (Na+ and pyridinium) species coexisting in the zeolite pore, it is unlikely that all species exist in well-defined ion pairs, but overall each pyridinium should still balance one anionic species, and it follows that the integrated area of the pyridinium band can be expressed as a linear combination of integrated areas of the lactate and acrylate bands: 𝐴𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 𝑎𝐴𝑙𝑙𝑙𝑙𝑙𝑙𝑙 + 𝑏𝐴𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎

(1)

in which Apyridinium, Alactate and Aacrylate are the integrated areas of the 1485, 1603 and 1575 cm-1 bands, respectively. a and b represent extinction coefficients of the bands corresponding to sodium lactate and sodium acrylate, normalized by the extinction coefficient of the pyridinium band. This simple model fits the experimental data well with a = 0.152 and b = 0.139, confirming charge balance is maintained within the zeolite pore (Figure 7b). Moreover, the a and b coefficient values are similar, validating our earlier hypothesis that the extinction coefficients of lactate and acrylate are similar due to the bands arising from similar modes. After the last reactant solution dose at 90 min (Figure 6b[xiv]), the lactate, acrylate and pyridine species desorb very slowly over a time scale of hours, and the relationship between the three peaks holds even during this process, suggesting simultaneous desorption of acid/base pairs, i.e., pyridinium lactate or pyridinium acrylate (Figure 7b). The integrated area of the 1575 cm-1 band increases monotonically during the dosing period, indicating that dehydration of lactate occurs throughout the dosing period and the rate of dehydration is higher than the rate of pyridinium acrylate desorption (Figure S7a). In contrast, the integrated area of the 1603 cm-1 band oscillates with the dosing/evacuation cycles (Figure S7b). The lack of evidence for coke species (e.g., 1580 cm-1 or 1680 cm-1 bands) at any point shows that the disappearance of spectral bands is mainly


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attributable to desorption, and further corroborates the hypothesis that the presence of pyridine suppresses coking by titrating surface Brønsted acid sites. Based on the insights gained from the spectroscopic, reactivity, and TGA investigations, we propose that in the presence of pyridine, catalyst deactivation is due to pore clogging by multiple coexisting ionic species (Scheme 2). Initially, the zeolite pores contain only Na+ ions, having been dehydrated by the thermal pretreatment; when the reactant flow starts, ML, H2O, and pyridine diffuse into the pores (Scheme 2i). ML and H2O combine in an ion exchange process to form Brønsted acid sites and methanol; without pyridine, the reaction will rapidly proceed to coke and acetaldehyde as observed previously (Scheme 2ii).50 However, pyridine can quench the surface Brønsted sites, forming pyridinium; at this point multiple cationic (pyridinium and Na+) and anionic (zeolite framework, lactate, and acrylate) species coexist in the zeolite pores, with dynamic charge balancing rather than existing in fixed ion pairs (Scheme 2iii). Sodium lactate is undergoing Lewis acid-catalyzed dehydration to sodium acrylate. It follows that pyridinium lactate and pyridinium acrylate effectively, albeit transiently, exist in the zeolite pore, and they slowly desorb as LA/pyridine and AA/pyridine, respectively (Scheme 2v and xi). This explains the reduced pyridinium and lactate bands over 15 min under vacuum after each application in the multi-dose transmission FTIR experiment, and the reduction of all bands with time under vacuum (Figure 6b). In contrast, the pyridinium bands on HY show negligible change during 5 h hold under vacuum at 330 °C, suggesting a different process in the NaY/reactant solution case (Figure S5). However, desorption of the bulky acid-base pair species is slow, requiring at least 5 h hold under vacuum at 330 °C, and possibly rate limiting under reaction conditions with a constant feed (Scheme 2xii). This hypothesis is consistent with a variety of observations of the NaY-S10 catalyst, including: (1) saturation of the 1750-1550 cm-1


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Scheme 2. Proposed reaction and catalyst deactivation mechanisms for ML dehydration in the absence and presence of pyridine on NaY.

region (νO-C-O , νC=O and νC=C modes) in FTIR, likely a result of large quantities of organic species trapped in the zeolite pores (Figure 5d); (2) TGA results suggesting H- and O-rich surface deposits (Figure 3a); (3) pyridine uptake during activity tests showing ~3 pyridines per NaY supercage when the maximum is approximately 4 (Figure 2c); and (4) the negligible available pore space after 10 h TOS (Table 1). The latter suggests a continuous


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ACS Catalysis

desorption/replacement process, or perhaps direct replacement of product acrylate by reactant lactate. A direct prediction of the hypothesis is that deactivated catalysts can be regenerated, at least in part, by thermal treatment in an inert atmosphere to desorb the bulky species, rather than the typically oxidative environment to combust the accumulated coke.

3.5 Catalyst Regeneration When catalysts used under a Py/ML = 1/10 feed are regenerated by thermal treatment in an inert atmosphere, activity is partially restored and the extent of recovery is dependent on the treatment temperature; in contrast, the Py/ML = 0 catalyst cannot be regenerated with the same procedure. An N2/H2O atmosphere was employed to test the hypothesis that pyridine causes a change in the deactivation mechanism from coking (with the pyridine-free feed) to the accumulation of bulky organic species in the pores (in the presence of pyridine). Using air as a regeneration feed does in fact completely regenerate the catalysts (Figure S8), but it does so regardless of the mechanism of catalyst deactivation. The N2/H2O atmosphere has a minimal effect on carbonaceous surface species, i.e., coke deposits, but does not inhibit desorption of more volatile organic species; further, the extent of desorption should be a function of the regeneration treatment temperature (TR). NaY-S0 shows no change in activity after regeneration at either 330 °C or 400 °C, evidence for catalyst coking because the inert atmosphere cannot remove the surface coke which forms in the absence of pyridine (Figure 8a, Figure S9a). The slight increase in pore volume of NaY-S0 after treatment at 400 °C is attributed to aging reactions, e.g., dehydrogenation, without removal of the polyaromatic species covering the active sites (Table 1). In contrast, NaY-S10 shows substantial activity recovery after thermal treatment under N2/H2O, and the extent of regeneration is dependent on TR (Figure 8b and c, Figure S9).


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Taking the difference between the last data point prior to and the first data point after N2/H2O treatment, ML conversion increases 8% at TR = 330 °C, increasing to 25% at TR = 450 °C. While the untreated NaY-S10 sample has only 0.008 cm3/g of available pore volume for N2 phsyisorption, TR has a positive correlation with the pore volume after treatment: 0.075 cm3/g at 350 °C to 0.193 cm3/g at 450 °C (Table 1). This is consistent with increased desorption of the proposed bulky organic species, i.e. pyridinium acrylate and lactate, at higher temperatures, making active sites available for reaction upon the resumption of the ML feed (Table 1). Interestingly, acetaldehyde yield is insensitive to TR, although the side product is always present in low yields ( 350 °C after pyridine desorption from NaY-S10. H2O was included in the feed to simulate the reaction environment in the absence of reactant/pyridine; it is possible that some framework dealumination by steaming is occurring, which may contribute to the incomplete regeneration (O2 regeneration of the NaY-S10 catalyst showed ~15% loss in micropore volume). The negligible increase in acetaldehyde yield suggests the formation of extraframework alumina (which catalyzes decarbonylation) is minimal. Thus, there is a tradeoff in the choice of TR: elevated TR facilitates the desorption of trapped organic species but also induces coking of trapped species to permanently deactivate a fraction of sites, while lower TR is not as effective in desorbing trapped species but lowers the probability of coking. Textural characterizations of NaY-S0 and NaY-S10 clearly indicate differing deactivation mechanisms based on the inclusion of pyridine, and suggest the additive may somewhat complicate the catalytic cycle. The available pore volume on NaY-S0 is 0.031 cm3/g, a ~90% reduction from fresh NaY (0.327 cm3/g), while conversion decreases ~70% (from 100% to ~30% at 6 h TOS). Deactivation by coking means micropore volume should be correlated with activity: as polyaromatic species accumulate, they both fill volume and block active sites. Transport effects, not kinetics, is limiting at high reactant conversions, meaning each active site is not operating at full capacity. NaY-S10 has a slightly higher ML conversion (~35%) at 10 h TOS) than NaY-S0 at 6 h TOS, however, with a much lower pore volume (0.008 cm3/g),


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indicating that coking cannot be the main deactivation mechanism for NaY-S10. The almost complete lack of available pore volume and negligible pyridine uptake at long TOS suggests that under reaction conditions there may be a constant desorption/replacement cycle, either: 1) There is pyridine “turnover” on the catalyst surface, i.e., pyridine desorption makes surface Na+ available for reaction with ML and water, and the resulting Brønsted acid site is titrated by incoming feed pyridine; or 2) Lactate can directly replace acrylate in sodium or pyridinium acrylate species in the zeolite pore, without adsorbed pyridine leaving the catalyst surface. In order to further confirm that desorption of adsorbates contributes directly to regeneration, analysis of the effluent stream was continued during regeneration at TR = 400 °C. During the reaction (first 4 h TOS), pyridine is adsorbed into the catalyst bed as described above, and ~40% of adsorbed pyridine is detected in the effluent during the regeneration, mostly within the first 20 min after the reaction feed is shut off (Figure S10a). This value is obtained by correcting the amount of pyridine detected in the regeneration with that of a control experiment in which no catalyst is loaded in the reaction tube under otherwise identical conditions. After regeneration, pyridine uptake increases, suggesting re-adsorption on newly generated Brønsted acid sites, however, the uptake does not recover to the value observed at 20 min TOS over the fresh catalyst, supporting the hypothesis that permanent deactivation, i.e. coking, occurs during regeneration due to the high TR. Additionally, both reactants and products are still noticeable in the effluent stream in significantly decreased quantities after the feed is shut off (Figure S10b and c). No ML was detected 40 min after the feed is stopped (2nd data point) regardless of whether pyridine was included in the reactant solution; this could serve as a calibration regarding the time needed to purge all residual non-adsorbing reactants/products. The difference in the amount of ML detected 20 min after switching off the reactant feed in the Py = 0 and Py/ML =


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ACS Catalysis

1/10 cases can be accounted for by the difference in ML conversion immediately before the switch (4 h TOS). AA yield right before the switch (4 h TOS) in the pyridine-free feed (33%) is slightly higher than with the pyridine-spiked feed (31%), however, 20 min into the regeneration, the amount of AA detected with the pyridine-spiked feed is roughly double that of the pyridinefree feed (18.7 µmol vs. 9.6 µmol). The continued observation of AA in the Py/ML = 0 case may be attributed to slow reverse ion exchange and desorption of residual sodium or pyridinium acrylate in the catalyst bed. The increased amount of AA evolved during regeneration in the Py/ML = 1/10 feed compared to the pyridine-free feed is attributed to the AA from the desorption and dissociation of pyridinium acrylate accumulated in zeolite pores during reaction. Integrating under the AA and pyridine recovery curves for Py/ML = 1/10 shows that the AA/pyridine mole ratio during regeneration is close to 1 (~0.9), which lends support to the proposed formation of acid-base complexes within the zeolite pore (Scheme 2). Additionally, we performed an experiment where pyridine was introduced (Py/ML = 1/10) for 1 h TOS, then removed from the reactant solution while the reaction was continued for a further 6 h TOS (Figure S11). YAD increased over this period from ~5% to ~10%, while the catalyst slow deactivated from 60% ML conversion to 45%, indicating that pyridine is slowly desorbing from Brønsted acid sites under reaction conditions at 330 °C, and a constant pyridine feed is required to maintain the exceptionally high SDH observed above. We expect that analogous results would be obtained if pyridine were removed from the feed at a different TOS.


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Post-reaction characterizations show that catalyst deactivation by loss of Na+ sites through dealumination or permanent replacement by H+ is at most a minor contribution. The activities of both NaY-S0 and NaY-S10 are completely regenerated by calcination in air at 500 °C for 4 h (Figure S8). Previously, we have shown that alumina is active for ML decarbonylation to acetaldehyde, and the lack of change in YAD suggests no significant amount of extraframework alumina is formed.56 The activity results also suggest that Na+ is not permanently replaced by H+ as a resut of the surface ion exchange process; surface Brønsted sites are even more strongly correlated with acetaldehyde formation, and pyridine adsorption FTIR of calcined spent samples of NaY-S0 and NaY-S10 show no evidence of pyridinium bands at 1540 cm-1 (Figure S12). A 30 h experiment in a pyridine-spiked feed (Py/ML = 1/10) with the catalyst regenerated at 450 °C for 4 h every 10 h TOS was conducted was conducted to investigate the effectiveness of multiple regenerations in N2/H2O (Figure 9). ML conversion drops from 98% initially to 32%

Figure 9. 30 h TOS reactivity test of a Py/ML = 1/10 feed over NaY with two 450 ºC regeneration


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ACS Catalysis

at 10 h TOS before the first regeneration, consistent with results presented earlier (Figure 1b). The carbon balance is

Catalyst Deactivation in Pyridine-Assisted ... - ACS Publications

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