Catalytic Dehydration of Methyl Lactate: Reaction Mechanism and Selectivity Control ⇑ Brian M

Journal of Catalysis 339 (2016) 21–30

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Journal of Catalysis

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Catalytic dehydration of methyl lactate: Reaction mechanism and selectivity control ⇑ Brian M. Murphy, Michael P. Letterio, Bingjun Xu

Center for Catalytic Science and Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, United States article info abstract

Article history: Catalytic dehydration of lactic acid and its esters is a promising approach to renewably produce acrylic Received 12 September 2015 acid and its esters. Molecular level understanding of the dehydration reaction mechanism on NaY has Revised 6 March 2016 been achieved via a combination of reactivity and in-situ transmission Fourier transform infrared Accepted 22 March 2016 (FTIR) spectroscopic investigations. Brønsted acid sites generated in-situ with the assistance of water have been identiﬁed as the primary active sites for the dehydration pathway. The key branching point between the desired dehydration and undesired decarbonylation pathways is the dissociation of methyl Keywords: lactate on NaY to form adsorbed sodium lactate and methyl groups. Brønsted and Lewis acid sites mainly Dehydration catalyze the dehydration of adsorbed sodium lactate, whereas the decarbonylation pathway to acetalde- Methyl lactate Acrylic acid hyde dominates when methyl lactate is not dissociated. Similar mechanistic steps are likely followed in NaY the catalytic dehydration of lactic acid to acrylic acid. The mechanistic understanding gained will enable Mechanism rational design of catalysts for selective dehydration of methyl lactate. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction made major progress in developing 3-hydroxypropanoic acid (3-HP) dehydration processes as a renewable route to AA, but the As concern about climate change caused largely by anthro- high cost of 3-HP remains a concern [6]. pogenic greenhouse gas emissions continues to increase, more Lactic acid (LA) and alkyl-lactate esters are promising biomass- attention has been given to the efficient utilization of biomass, based renewable chemical feedstocks. They can be produced the only renewable carbon source for the production of fuels and selectively from cellulose via fermentation, and work on high- chemicals [1–5]. Biomass-based commodity chemicals are cur- throughput thermo-catalytic routes is ongoing [7]. LA has been rently more expensive than those derived from petroleum, but identified by the Department of Energy as a ‘‘platform molecule” the recent surge in natural gas production promises new opportu- which will play a critical role in a biomass-based future due to nities in renewable fine chemical synthesis. Production of key ole- its versatility; acrylic acid, acetaldehyde, 2,3-pentanedione, and fin feedstocks such as propylene and butadienes, which are lactide (a bio-polymer starting material) can all be produced from difficult to obtain from C1 and C2 starting materials, has decreased LA [8]. Catalytic dehydration of LA to AA has received significant because processes based on naphtha cracking that produce high attention in the last two decades, although achieving high selectiv- yields of these chemicals are being phased out. Biomass, which is ity has been challenging. Inorganic salts have been shown to be abundant and contains unique chemical building blocks, has the effective catalysts for the dehydration reaction, typically at high potential to fill this gap [4]. For instance, acrylic acid (AA) and its temperature (>375 °C) and long space time [9–12]. Zeolites, espe- esters are major commodity chemicals with a wide range of cially alkali-cation exchanged faujasites, show high activity in the industrial applications (detergents, absorbent polymers, coatings, conversion of LA and its esters, however, controlling selectivity is adhesives, paints, etc.) and an annual demand of roughly 8 million a major issue [13–19]. Although the dehydration activity of cata- tons. Nearly all acrylates are currently produced via the catalytic lysts is often correlated to acid/base properties, works attempting selective oxidation of petroleum-derived propylene; switching to to fine-tune these characteristics have not been able to signifi- a biomass-based feedstock would improve the sustainability of cantly improve selectivity, indicating that key factors for selectivity these widely used chemicals [5]. BASF and Dow Chemical have control have yet to be identified [10,13–19]. Additionally, the accuracy of the methods employed for zeolite acid/base characterization has been questioned [20]. Wadley et al. (1997) proposed ⇑ Corresponding author. sodium lactate as the active dehydration species after observation E-mail address: [email protected] (B. Xu). http://dx.doi.org/10.1016/j.jcat.2016.03.026 0021-9517/Ó 2016 Elsevier Inc. All rights reserved. 22 B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30 in ex-situ, post-reaction Fourier transform infrared (FTIR) spec- resulting material was then calcined using the same procedure as troscopy experiments [21], and Yan et al. (2014) proposed a reac- the other zeolite samples. The incorporation of Sn into the zeolite tion mechanism for dehydration on alkali-metal zeolites with framework was confirmed by performing the model reaction of sodium lactate as an intermediate, but direct experimental evi- glucose isomerization to fructose, as described by Moliner et al. dence supporting this mechanism is scarce [18]. The lack of solid [24]. This characterization test seeks to match the isomerization mechanistic understanding for lactic acid dehydration or any side rate to a literature standard, as it is known that framework and reactions (particularly the formation of acetaldehyde, the major extraframework tin perform the glucose to fructose conversion at side product) makes the rational design of more active and significantly different rates [25,26]. Additionally, we conducted selective catalysts challenging. in situ FTIR adsorption of pyridine on the sample, and only In this work, we present a mechanistic study of the dehydration observed Lewis acid sites as expected with framework Sn (Fig. S4). of methyl lactate (ML) to acrylic acid and methyl acrylate on sodium exchanged faujasite zeolites through a combination of 2.2. Materials characterization reactivity and in-situ FTIR spectroscopic investigations. The dissociation of ML that leads to the formation of surface bound 2.2.1. X-ray diffraction sodium lactate has been identified as a key step in the selective Powder X-ray diffraction (XRD) patterns were collected on a dehydration route. Both Brønsted and Lewis acid sites are capable Bruker D8 Discover powder diffractometer with a Cu Ka source of catalyzing the desired dehydration reaction, however, the for- (k = 1.5418 Å) over the range 2h = 5–50° with a step size of mer are substantially more active. Additionally, we show the 0.025° and 2 s per step. decarbonylation pathway to acetaldehyde dominates when

Brønsted acid sites are unaccompanied by sodium cations. Further, 2.2.2. N2 adsorption a comprehensive dehydration mechanism has been proposed for Textural characterization of the samples was determined using both LA and ML. N2 adsorption at 77 K. Isotherms were collected on a Micrometrics ASAP 2020 instrument, and micropore volumes were calculated using the t-plot method. Prior to the adsorption measurements, 2. Experimental all samples were degassed at 623 K for 6 h.

2.1. Materials preparation 2.2.3. 29Si-solid-state nuclear magnetic resonance 29Si MAS-NMR experiments were performed on a Bruker AVIII NaY (nominal Si/Al = 2.5) was purchased from Zeolyst Interna- 500 solid-state NMR spectrometer operating at a Larmor frequency tional (CBV-100). The following procedure was followed to synthe- of 500.138 MHz for 1H and 99.362 MHz for 29Si. A 44 mm HX MAS size partially exchanged NaHY zeolites: 1 g of NaY was introduced probe was used for all measurements. All spectra were collected at to a NH4NO3 solution with a predetermined molar ratio where a MAS frequency of 10 kHz ± 2 Hz using a Bruker MAS controller. + + 29 NH4/Na = X (X = 0.05, 0.1, 0.2, 0.3, and 0.5), and the corresponding Si single-pulse and cross-polarization experiments were per- sample was denoted as NaHY-X (where X = 0.05, 0.1, 0.2, 0.3, or formed on each sample: for single-pulse experiments, a 90° pulse 0.5). The suspension was stirred for 30 min at room temperature, with a duration of 4.3 ls and a recycle delay of 30 s was used. followed by vacuum filtration and drying at 80 °C for 3 h. 29Si solid-state NMR measurements showed that the Si/Al ratio of the 2.2.4. Brønsted acidity characterization exchanged catalysts was altered by less than 5% from the unex- Brønsted acidity of solid acid catalysts was measured via the changed NaY, whose Si/Al ratio was determined to be 2.2. Also pur- decomposition of isopropylamine into ammonia and propylene, chased from Zeolyst International: NH4Y (nominal Si/Al = 2.5, CBV- carried out using a microreactor system similar to that described 500), HY (nominal Si/Al = 30, CBV-760), and HZSM-5 (nominal Si/ by Kresnawahjuesa et al. [27]. A flow reactor with an on-line Agi- Al = 140, CBV-28014). 29Si solid-state NMR measurements of the lent 7890A gas chromatograph (GC) equipped with an HP-PLOT Q CBV-500 sample showed the framework Si/Al ratio to be 3.6, here- column was used. All gas lines leading from the reactor to the GC after denoted as HY(3.6). The CBV-760 and CBV-218014 samples were heat traced with temperature maintained at or above 75 °C. are denoted as HY(30) and HZSM-5(140), respectively. NMR was 10–100 mg of sample was charged into a quartz tube and loaded not performed on the high-silica samples, as the low alumina con- into the reactor. First, the sample was heated to 500 °Cat tent makes NMR quantification difficult. However, isopropylamine 30 °C/min in flowing He (50 mL/min) and held for 45 min. After measurements of the acid site density agree well with the nominal cooling to 100 °C, the sample was exposed for 30 min to flowing Si/Al values. All zeolite samples were calcined at 500 °C for 6 h He saturated with isopropylamine at room temperature via a with a 1 °C/min temperature ramp before use. bubbler. The sample was then heated to 200 °C and held for Double deionized water was produced in-house by a Thermo 90 min to desorb excess isopropylamine and ensure a 1:1 ratio of Barnstead Mega Pure-1 Still deionizer. All chemicals were obtained adsorbed isopropylamine to Brønsted acid sites. Finally, the from Sigma Aldrich and used without further purification. High temperature was ramped to 500 °C at a rate of 30 °C/min. The GC surface area c-alumina (220 m2/g) was purchased from Alfa Aesar. sampling loop was immersed in liquid nitrogen to collect the des- Zeolite beta with framework Sn (Sn-Beta) was prepared accord- orbed products, which were subsequently quantified via a thermal ing to the procedure described by Corma [22,23]. First, 7 g of tetra- conductivity detector (TCD) and flame ionization detector (FID). ethyl orthosilicate (TEOS, Sigma Aldrich, 98%) was hydrolyzed in 7.48 g of tetraethylammonium hydroxide (TEAOH, Sigma Aldrich, 2.3. In-situ transmission FTIR spectroscopy 40 wt%) while stirring at room temperature. To this solution,

0.096 g of SnCl4Á5H2O (Strem Chemicals, 98% reagent grade) and In-situ Fourier transform infrared (FTIR) spectra were obtained 0.62 g of DI water were added, and the solution was stirred at room on an Agilent Cary 660 FTIR Spectrometer equipped with a MCT temperature until the ethanol formed by hydrolysis was evapo- detector (128 scans at a spectral resolution of 2 cmÀ1) with a cus- rated. Next, 0.1 g of calcined, siliceous zeolite Si-Beta in 0.5 g of tom in-situ transmission cell. A vacuum level of <0.01 mTorr in the DI water was introduced as seed, followed by adding 0.77 g of transmission cell can be reached through a vacuum manifold, 48 wt% HF. The crystallization was carried out in rotating, which is connected to a mechanical pump and a diffusion pump. Teflon-lined, stainless-steel autoclaves at 413 K for 14 days. The A self-standing zeolite wafer was loaded to a custom-made sample B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30 23 holder, followed by annealing at 450 °C under vacuum to com- concentration due to the relatively high conversions used in this pletely remove adsorbed water. After the sample was cooled to study (up to 30%). The relatively high conversions were used to the desired temperature, 100 mTorr of the selected species was ensure the accurate quantification of product distribution due to introduced to the transmission cell via the vacuum manifold. the high boiling point of the reactant and catalyst deactivation. The linear relationship between the catalyst loading vs. ML consumption rate (Fig. 1b, with the fitted line extended to pass 2.4. Catalytic activity evaluations through the origin) suggests this effect is minimal.

2.4.1. Reaction setup and product analysis 2.4.2. Experimental conditions for sodium lactate impregnated on Catalytic activity was evaluated using an up-flow fixed bed silica reactor with an inner diameter of 4 mm at atmospheric pressure. Sodium lactate supported on silica (175–225 m2/g) was pre- In a typical experiment, 50 mg of catalyst (20–40 mesh) was pared with an incipient wetness impregnation method, followed charged into the reactor via a quartz tube with i.d. = 4 mm. A ther- by drying at 80 °C for 3 h without calcination. The total weight mocouple was placed directly above the catalyst bed inside the percent of sodium in the final product was the same as the weight reactor tube to ensure accurate temperature measurement. The percent of sodium in NaY (~11 wt%). To avoid the decomposition of thermocouple has no detectable catalytic activity at the reaction the impregnated sodium lactate in the catalyst pretreatment temperature. Catalysts were pretreated at 500 °C for 2 h in N at 2 protocol, the conditions were altered. After charging 50 mg of cat- a flow rate of 50 mL/min and then cooled to the reaction tempera- alyst into the quartz reactor tube, the temperature was ramped at ture (330 °C), followed by the introduction of reactant. A syringe 1 °C/min to the reaction temperature (330 °C). Beginning at 150 °C pump was used to inject an aqueous solution of 30 wt% ML at (roughly the boiling point of methyl lactate), the reactant solution 1 mL/h into the reactor. Weight hourly space velocity (WHSV) was introduced to the catalyst. Product sampling began 20 min was varied between 200 and 1670 g /g to ensure reactant solution cat after the temperature stabilized at 330 °C. In a prior test, reactant the intrinsic activity of each catalyst was accurately measured. conversion below 330 °C was <5%, therefore catalyst deactivation Reactivity studies of LA typically employ cold baths to condense during the temperature ramp up to 330 °C is minimal. products prior to analysis and quantification due to the propensity of the lactide dimer to clog the thin gas lines required for GC oper- ation. However, the accuracy of this method is debated in the liter- 3. Results and discussion ature [28,29] due to the possibility of dimer formation even at low temperatures. Therefore, reactants and products in the effluent 3.1. Catalyst characterization from the reactor are kept in the gas phase for on-line gas chromatograph/mass spectrometer (GC/MS, Agilent 7890B GC with a XRD spectra (Fig. S1) and pore volumes of all samples are con- DB-FFAP column and FID for quantification and Agilent 5973 MS) sistent with the literature [34,35] and the small changes in pore 29 analysis every 20 min. Conversion, product yields, total carbon volume and Si/Al ratio (as measured by N2 adsorption and Si balance, and turnover frequency (TOF) for each species were SS-MAS NMR, respectively) of the faujasite samples before and calculated as follows: after the ion exchange show that the room temperature ion

n Methyl lactate conversion ¼ 1 À ML Â 100% n ; ML feed ni Individual product yields ðYiÞ¼ Â 100% n ; ML feed 4 Â n þ 5 Â Y þ 4 Â Y þ 4 Â Y þ 3 Â Y þ 3 Â Y þ 3 Â Y þ Y Carbon balance ðCBÞ¼ ML PD MA MPyr AA PA AD M Â 100% 400 Y i Turnover frequency ðTOFiÞ¼ Â FML nAl

where nx is mole of species x, ML is methyl lactate, MA is methyl exchange process does not degrade the zeolite structure (Table 1). acrylate, AA is acrylic acid, PA is propanoic acid, AD is acetalde- Proton concentration in the NaHY-X samples (measured by hyde, MPyr is methyl pyruvate (a minor dehydrogenation product), isopropylamine decomposition) increases monotonically with + M is methanol, nAl is the mole number of aluminum atoms in increased concentration of NH4 in the exchange solution as the catalyst sample, and FML is the molar ﬂow rate of ML fed to expected. the reactor per hour. A 1:1 mole ratio between COx and acetaldehyde is assumed. TOF was calculated based on a per Al or Sn atom 3.2. Reactivity on NaY basis for zeolites; catalyst mass for alumina; and on a per Na+ basis for silica supported sodium lactate. TOF measurements were con- An aqueous solution of ML (30 wt%) reacted for two hours over ducted in the kinetically limited regime, which is conﬁrmed by NaY diluted with inert quartz at 330 °C shows a maximum conver- direct experimental observations (Fig. 1b) and several estimation sion of 33%, with acrylates, acetaldehyde, and coke as the major methods (Supplementary Information) [30–33]. Finally, the products (Fig. 1a). Because a lactate ester is used as the reactant, presented TOFs are average values over the entire catalyst bed, the dehydration pathway produces a mixture of AA (major) and as reaction rates could change slightly with changes in reactant methyl acrylate (minor), with a combined selectivity of 11%. 24 B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30

Fig. 1. (a) Conversion of ML (XML) and selectivity to major products on NaY at 330 °C over 2 h. (b) Initial ML consumption rate as a function of NaY loading at 330 °C.

Table 1 + + Textural and acidic properties of the zeolite samples. (In NaHY-X, X refers to the ratio of NH4/Na used to exchange ammonium ions into the zeolite framework).

NaY NaHY-0.05 NaHY-0.1 NaHY-0.2 NaHY-0.3 NaHY-0.5 HY(3.6) HY(30) HZSM-5 (140) Sn-⁄BEA Nominal Si/Al Ratio 2.5 2.5 2.5 2.5 2.5 2.5 2.5 30 140 125a Measured Si/Al Ratio 2.2 2.3 2.3 2.3 2.3 2.3 3.6 – – – + Brønsted acid site density (lmol H /gcat) 0 33.4 152 280 392 601 1112 110 3 – Pore volume (cm3/g) 0.33 0.32 0.33 0.32 0.32 0.31 0.28 0.29 0.20 0.23

a Si/Sn ratio.

Acetaldehyde is formed at 14.5% selectivity, mainly via a decar- deviate signiﬁcantly from the trend, implying transport limitations bonylation pathway, although decarboxylation is a minor pathway under these conditions. All further tests were conducted in the

(CO:CO2 >20:1). The low carbon balance (77% at 20 min TOS) and kinetically limited regime with ML conversions below 35%. negligible formation of minor products such as propanoates, pyru- vates, and acetates indicates that coke formation is significant, 3.3. Identifying active sites for acetaldehyde production leading to deactivation of >50% over 2 h reaction period tested. Furthermore, the activity of NaY as a catalyst is similar whether Brønsted acid sites present on the catalyst prior to the introduc- ML or LA is used as the reactant molecule. Using LA in water tion of reactant are very efficient at catalyzing ML decarbonylation (19–21 wt%) as the reactant solution, Näfe et al. (2015) showed to produce acetaldehyde, the major side product from reactions of that NaY (Si/Al = 2.4) has a selectivity of 40% to AA and 20% to either ML or LA on NaY. Acetaldehyde, CO and methanol are the acetaldehyde after 100 min TOS at 100% conversion. During the main products detected when reacting ML on HY(3.6) at 330 °C, reaction, dehydration selectivity initially improves up to 60% as with no detectable dehydration activity, similar to reports on other the catalyst deactivates before decreasing steadily with time on solid Brønsted acid catalysts (Fig. 2a, Table S1) [36]. A small stream (TOS) up to 5 h [19]. Comparable results have been amount of CO2 is also detected via mass spectrometry, less than reported recently by other groups [14–18]. Using ML in water 5% of the amount of CO, which suggests that decarbonylation of (30 wt%) as the reactant solution over NaY (Si/Al = 2.2) at 350 °C ML is the dominant pathway in the aldehyde formation, while with a similar reactant feed rate, we observe a selectivity of decarboxylation is minor [37]. In order to confirm the relationship 35% and 25% to dehydration and acetaldehyde, respectively between acetaldehyde selectivity and Brønsted acid site density on (Fig. S2). The dehydration selectivity trend is very similar to LA, the catalyst, an increasing fraction of Na+ cations are exchanged to with an initial increase to 55% followed by a decrease with TOS protons in NaY, and the turnover frequency (TOF) of ML conversion up to 6 h. The acetaldehyde selectivity decreases monotonically to acetaldehyde and dehydration products (referred to as TOFAD in both cases. The remarkable similarities in activity over NaY and TOFDH, respectively) are measured at 20 min TOS (Fig. 2a). between ML and LA indicate that a common reaction mechanism The density of protons in the NaHY-X samples is quantified by iso- is likely, and this hypothesis will be explored in more detail in propylamine decomposition and confirmed by adsorption of pyri- the following sections. Catalytic activities measured at high con- dine monitored by in-situ FTIR spectroscopy (Fig. 2b). versions are likely limited by heat or mass transport (Fig. 1b), In-situ transmission FTIR spectra of pyridine adsorption at and thus do not reflect intrinsic activity of the catalysts. The linear 200 °C on NaY, NaHY-X, and HY(3.6) catalysts show increasing relationship between catalyst loading and initial ML conversion intensity of vibrational bands corresponding to Brønsted acid sites rate indicates that the number of available active sites limits the and a migration of the band corresponding to Lewis acid sites on reaction rate, suggesting a kinetic regime [32]. Reactions at higher solid acids with increasing proton concentration (Fig. 2b). Pyridine loadings (15 mg NaY and above, WHSV 6 67 greactant solution/gcat) adsorbed on NaY shows a very strong Lewis acid band at B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30 25

Fig. 2. (a) Conversion and carbon balance (left axis), TOFAD and TOFDH (right axis) as a function of intrinsic Brønsted acid site density on NaY, NaHY, and HY(3.6). TOF values were calculated with data at 20 min TOS. The dotted line serves as the eye-guiding line for the trend. (b) In-situ FTIR of pyridine adsorbed on NaY, NaHY-X, and HY(3.6) at 200 °C.

Fig. 3. (a) TOFAD and (b) TOFDH of proton form zeolites HY(3.6), HY(30), and HZSM-5(140) as a function of Brønsted acid site density, with two NaHY-X catalysts of similar + ⁄ ⁄ [H ] for comparison. (c) TOFAD on NaHY-X samples as a function of Brønsted acid site density. TOFAD on NaY (not shown) is inﬁnity because no Brønsted acid sites were detected by the isopropylamine decomposition measurement.

À1 + + 1441 cm but no observable Brønsted acid band (Fig. 2b(i)), which shut off, with TOFAD rising steadily with increasing [H ]. At [H ] is consistent with the results obtained from isopropylamine < 200 lmol/gcat, TOFDH and TOFAD are comparable. Meanwhile, + + + decomposition measurements ([H ]=0lmol/gcat) as well as the lit- in the absence of Na , low [H ] leads exclusively to acetaldehyde À1 erature [38]. The 1543 cm band corresponding to the pyridinium formation, shown by the lack of dehydration products (TOFDH =0) ion formed during interactions with Brønsted acid sites is visible on HY(30) and HZSM-5(140), which have [H+] values close to those on NaHY-0.05, and grows in intensity as Brønsted acid site density of NaY and NaHY-0.1 (Fig. 3a). Additionally, the large increase in increases up to HY(3.6) (Fig. 2b(ii–vi)). Conversely, the Lewis inter- TOFAD shown by the low-alumina zeolites suggests that the action band decreases in intensity and shifts from 1441 cmÀ1 to intrinsic acidity of the catalyst has a strong effect on the catalytic 1453 cmÀ1 with increasing surface proton concentration. The shift activity for converting methyl lactate. These results indicate that in the Lewis band has been attributed to the interaction between in addition to the presence of Brønsted acid sites, a high [Na+]is pyridine and surface protons, without substrate protonation to needed to mediate the dehydration pathway. [H+]=200 pyridinium [39]. lmol/gcat in NaHY-X, the threshold below which dehydration reac- + + The signiﬁcant changes in both TOFAD and TOFDH with tion is appreciable, corresponds to a [Na ]/[H ] ratio of 8. TOF increasing Brønsted acid site density in NaHY-X catalysts (with a toward acetaldehyde formation calculated on a per H+, rather than corresponding decrease in [Na+]) suggest that a high [Na+]/[H+] Al atom, basis (as measured by isopropylamine decomposition and ⁄ + + ratio is required for promoting the dehydration pathway (Fig. 2a). referred to as TOFAD) becomes independent of [H ] when [H ] + At [H ]>200 lmol/gcat, the dehydration pathway is completely exceeds 200 lmol/gcat (Fig. 3c). In contrast, the activity of each 26 B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30

Brønsted acid site appears to increase at lower [H+]. This could be mode of adsorbed sodium acrylate salt, respectively (Fig. 4b(i–iv)) attributed to the fact that [H+] is quantiﬁed with an ex-situ tech- [21,42]. Brønsted acid sites are produced by an ion exchange pro- nique (isopropylamine decomposition), while Brønsted acid sites cess between the acidic hydrogen of the carboxylic acid group in are created in-situ via an ion exchange mechanism, which will be lactic acid and the surface Na+ cation (Step 1, Scheme 1). A similar discussed in detail in the following sections. process has been systematically shown for a series of organic acids The presence of the a-hydroxyl group appears to play a key role on several types of zeolites [42]. There is no evidence that these in the Brønsted acid mediated decarbonylation pathway, and we Brønsted acid sites are in any way different from those intrinsically speculate that it is facilitated by the cleavage of a relatively weak present in a proton-form zeolite such as HY, and therefore they OAH bond, rather than a CAH bond, to recover the Brønsted acid should exhibit the same properties and catalytic activity. The con- site. Neither AA nor propanoic acid show any appreciable conver- current appearance and reduction of the Brønsted acid band and sion over HY(3.6) at 330 °C, suggesting that the presence of the adsorbed dehydration intermediate (acrylate) band leads to our a-hydroxyl group facilitates the decarbonylation reaction. Proto- hypothesis that the Brønsted acid sites created in-situ via ion nation of the hydroxyl group is generally believed to lead to dehy- exchange are the active sites for lactate dehydration. The ion dration, and we speculate that decarbonylation is preceded by the exchange between the acidic proton of LA and the surface sodium protonation of the carboxylic acid or ester group by a Brønsted acid cation should produce adsorbed sodium lactate, which has a char- + + À1 site, forming [CH3CH(OH)CO2H2] or [CH3CH(OH)CO2CH3H] . This acteristic OACAO stretching band at 1600 cm [21]. The lack of is followed by the cleavage of C1AC2 bond and the formation of a vibrational band for lactate below 200 °C is attributed to the + CO and H2O or methanol. In contrast, the presence of Na and efﬁcient dehydration of adsorbed lactate by Brønsted acid sites to the formation of sodium lactate via ion exchange appear to sup- acrylate (Step 2, Scheme 1). Finally, LA leaves the surface via press activation of the carboxyl group by Brønsted acid sites, thus desorption after a reverse ion exchange process between sodium enhancing the selectivity to the dehydration pathway. Further, the acrylate and the proton (Step 3, Scheme 1). At temperatures above presence of the a-hydroxyl facilitates cleavage of the C1AC2 bond 250 °C, the Brønsted acid site and adsorbed acrylate bands are no by donating the relatively acidic hydroxyl hydrogen to recover longer visible, replaced with the expected band at 1604 cmÀ1 the surface H+, in comparison to a hydrogen bonded to a carbon corresponding to adsorbed lactate (Fig. 4(v–viii)) [21]. At higher atom, as in propanoic acid and AA. The elucidation of the decar- temperatures, it is likely that carbonaceous deposits from bonylation mechanism will be the subject of future studies. substrate decomposition have covered the Brønsted acid sites, leading to the disappearance of the 3646 cmÀ1 band [44]. 3.4. The role of Na+ in the selective catalytic dehydration of ML and LA 3.4.2. Catalytic dehydration of methyl lactate on NaY in the presence of 3.4.1. Identifying the active sites for catalytic LA dehydration on NaY water In-situ transmission FTIR experiments of LA on NaY suggest that In-situ transmission FTIR results show that water plays a key the active sites for dehydration on zeolites are Brønsted acid sites role in the dehydration of ML (which does not have an acidic created by ion exchange between adsorbed lactic acid and surface proton for ion exchange) by generating Brønsted acid sites that Na+ on the zeolite. Upon introduction of LA to NaY at 100 °C, vibra- are capable of catalyzing dehydration reaction following a mecha- tional bands corresponding to water (3695 cmÀ1 and 1645 cmÀ1) nism similar to that of LA. Since the reactant (LA or ML) is typically [39] and the carbonyl group of LA (1712 cmÀ1) [21,41] are dissolved in a solvent (water in the present study) as feed to the observed, indicating molecular adsorption of both species (Fig. 4 reaction, it is important to understand solvent’s role in the reac- (i)). Between 100 °C and 250 °C, bands at 3646 cmÀ1 and tion. Upon introduction of ML to NaY at 100 °C, vibrational bands 1576 cmÀ1 are also visible; these are attributed to Brønsted acid corresponding to the carbonyl group of ML (1730 cmÀ1) [41] and sites on the surface of zeolite Y [37] and the OACAO stretching water (3695 cmÀ1) are observed, indicating that ML is molecularly

Fig. 4. FTIR spectra of lactic acid on NaY from 200 °C to 450 °C: (a) 3800–3000 cmÀ1, and (b) 2000–1300 cmÀ1. B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30 27

Scheme 1. Proposed mechanism for catalytic dehydration of LA on NaY. (1) Ion exchange between the adsorbed lactic acid and the sodium balancing cation, creatinga surface Brønsted acid site and adsorbed sodium lactate; (2) Brønsted acid site catalyzed dehydration; and (3) Reverse ion exchange between adsorbed sodium acrylate and surface proton leading to product desorption.

Fig. 5. FTIR spectra of methyl lactate and water on NaY from 200 °C to 300 °C: (a) 3800–3000 cmÀ1, (b) 2000–1300 cmÀ1.

adsorbed on the surface (Fig. 5(i)). As the temperature is raised to peak at 1645 cmÀ1 attributed to the bending mode of adsorbed À1 225 °C, the mOACAO band (1603 cm ) corresponding to adsorbed water (Fig. 5(iv)) [40]. This is clear evidence that water promotes sodium lactate appears, indicating the dissociation of ML at the the formation of Brønsted acid sites on the ML preadsorbed sur- ester bond into adsorbed sodium lactate and a methyl group face. We propose that ML dissociates on NaY, forming adsorbed (Fig. 5(ii–iii)). No evidence of an OH stretching band corresponding sodium lactate and a methyl group on the bridging oxygen À1 to Brønsted acid sites on faujasite (3645 cm ), is observed, (Step 1, Scheme 2). Water then hydrolyzes the OACH3 bond, lead- which is expected because ML does not contain any protons acidic ing to the formation of methanol (which quickly desorbs) and a enough to exchange with the zeolite surface [43]. Before introduc- proton (Step 2, Scheme 2). The surface residence time of adsorbed ing water, the surface with dissociated ML is cooled slightly to methyl groups is likely very short in the presence of water vapor 200 °C. Upon introduction of water, the signature band of Brønsted [44]. An alternative pathway leading to the formation of adsorbed acid sites on faujasites (3647 cmÀ1) appears concurrently with a sodium lactate and methanol could involve the hydrolysis of 28 B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30

Scheme 2. Proposed mechanism for ML dehydration on NaY in the presence of water. (1) Ion exchange between adsorbed methyl lactate and the sodium balancing cation, creating adsorbed sodium lactate and a methyl group; (2) hydrolysis of the OACH3 bond by water, producing methanol and Brønsted acid site; and (3) Brønsted acid site catalyzed dehydration, followed by (4) reverse ion exchange and desorption of desired products. methyl lactate via the formation of a tetrahedral ortho ester Table 2 Catalytic properties of non-aluminosilicate zeolite catalysts. (CH3CH(OH)C(OH)2OCH3), which subsequently dissociates to lactic acid and methanol (Scheme S1). LA then ion-exchanges with NaY Catalyst ML Conversion Carbon Balance DH TOF AD TOF to form sodium lactate and a Brønsted acid site, as demonstrated (%) (%) (hÀ1) (hÀ1) in Fig. 4. Similar ion-exchange processes between Na exchanged Sn-Beta 17 88 0a 251a zeolites and organic molecules have also been reported [43]. The Alumina 22 86 0b 0.035b c c presence of excess water (mole ratio of water to ML 13.5:1) Na-Lac/SiO2 13 91 0.11 0.12

° a under reaction conditions (330 C) could facilitate this hydrolysis Normalized by number of framework Sn atoms. b mechanism, and the quick reaction between LA and NaY could also Normalized by gcat. move the equilibrium to the formation of the ortho ester. Both c Normalized by number of Na+ ions. mechanisms are consistent with the formation of adsorbed lactate species and the formation of Brønsted acid sites. Once surface Brønsted sites are present, a mechanism similar to that for LA is IPA and water are incapable of introducing protons to the surface most likely responsible for the formation of dehydration products via ion exchange (Fig. S3) [43], the IPA dehydration activity of (Scheme 2 and Scheme S1). NaY is entirely attributable to Lewis acid sites, implying that these Experimental evidence suggests a surface-mediated hydrolysis types of sites are able to catalyze dehydration in general, but do so is the major mechanism for methyl lactate de-esterification. In much less efficiently than Brønsted sites. As observed with the control experiments when the catalyst bed was replaced by inert Brønsted acidic catalysts performing decarbonylation, the acid quartz wool, little ML de-esterification (<0.5%) occurs at 330 °C. strength is strongly correlated with activity, and it stands to reason Additionally, no detectable increase in the de-esterification reac- that the same effect would be observed for ML dehydration. NaY tion was observed in 10 h, indicating that the uncatalyzed dissoci- also showed significant dehydration activity, meaning we cannot ation of ML in the aqueous solution to methanol and lactic acid is totally rule out the possibility of Lewis acid-catalyzed dehydration, not a major pathway. In addition, the large amount of methanol though Brønsted acid sites are likely the dominant dehydration formed during reaction suggests the dissociation of ML is a key, sites. if not indispensible, step in the dehydration pathway (Table S2). 3.5. The critical role of the adsorbed lactate in the selectivity control 3.4.3. Lewis acid mediated dehydration In addition to the in-situ generated Brønsted acid sites, Lewis Sn-Beta and alumina, purely Lewis acidic materials [43], are acid sites on NaY, i.e., Na+ [45], are also able to catalyze dehydra- employed to probe whether the formation of a lactate salt is nec- tion reactions, albeit significantly less efficiently than Brønsted essary to promote selective dehydration on Lewis acid sites ⁄ acid sites. Isopropanol (IPA, 30 wt% in H2O) dehydration is used (Table 2). Neither Sn- BEA nor alumina shows any selectivity to as a model reaction to investigate the relative dehydration activi- dehydration, with acetaldehyde as the main detected product. ties of secondary alcohols on Brønsted and Lewis acid sites at Unlike aluminosilicate zeolites, the neutral framework of Sn-⁄BEA

150 °C (Table S2). HY(3.6) shows significantly higher TOFDH (to (no balancing cations) owing to the tetravalent nature of the propylene) NaY, which is consistent with literature [46,47]. Since framework Sn atoms, precludes the formation of adsorbed lactate B.M. Murphy et al. / Journal of Catalysis 339 (2016) 21–30 29 species via ion exchange (Schemes 1 and 2). Similarly, alumina sites produced during the reaction by forming non-acidic pyri- possesses Lewis acid sites but no capacity for surface ion exchange. dinium; and (2) introducing base sites to the catalyst surface close In order to confirm that acrylates were not forming on the surface to metal cations, so that Brønsted acid sites can be quenched effec- and then rapidly polymerizing to non-volatile products, a 30 wt% tively upon formation. Further, water plays a unique role in gener- aqueous solution of AA was tested on NaY, Sn-Beta, and alumina ating catalytic sites, and thus its concentration in the feed could with identical catalyst loadings (Table S3). The results show that impact the product distribution. More generally, solvents in the although all three catalysts exhibited some AA conversion (ranging feed could be an important variable to optimize the selectivity between 4% and 8%), no species other than AA was observed in the for dehydration. Investigating the role of solvents, surface cations, product streams. We attribute the missing carbon to coking on the and reactant could provide substantial insights into control. catalyst surface. It is unlikely that AA formed on Sn-Beta and alumina via ML dehydration, if there is any, would be completely 4. Conclusions lost to coking, because a similar level of AA coking was observed on NaY and AA is a major product. Therefore, we conclude no In summary, Brønsted acid sites on NaY generated in-situ AA is produced on Lewis acidic catalysts in the absence of through ion exchange between reactants and Na+, have been iden- ion-exchange. The lack of dehydration products over Sn-Beta or tified as the primary active sites in the catalytic dehydration of alumina supports the hypothesis that the presence of a surface methyl lactate to acrylic acid and methyl acrylate by a combination cation to balance the lactate anion is key to activate the dehydra- of reactivity and in-situ spectroscopic investigations. A reaction tion pathway, regardless of the nature of the active sites. However, mechanism for the catalytic dehydration of lactic acid and methyl TOFAD on Sn-Beta is several orders of magnitude higher than alu- lactate on NaY has been proposed, which consists of the dissocia- mina, suggesting that the rate of decarbonylation is strongly tion of the reactant to form sodium lactate via ion exchange, affected by the acid strength. acid-mediated dehydration, and product formation via reverse Sodium lactate supported on inert silica is employed to probe ion exchange. The branching point between the desired dehydra- whether sodium lactate alone, in the absence of acid sites, is able tion and undesired decarbonylation pathways is identified as the to mediate the dehydration reaction (Table 2). The lack of dissociation of the reactant on NaY. Understanding of the branch- pyridine adsorption in the FTIR spectrum on sodium lactate/silica ing point could lead to modified reaction systems designed to pro- (Na-Lac/SiO2), even at room temperature indicates that no mote dissociation on the surface while inhibiting decarbonylation. Brønsted or Lewis acid sites are present on this catalyst (data not Decarbonylation dominates on both Brønsted and Lewis acid sites shown). The Na-Lac/SiO2 catalyst showed low activity (13% conver- for molecular methyl lactate; whereas dehydration becomes the sion at 20 min TOS) in converting ML to both acetaldehyde and main reaction channel when methyl lactate dissociates on the cat- dehydration products, in the absence of acid sites. This suggests alyst to form the adsorbed sodium lactate intermediate. The acid that while the presence of the lactate species is key to dehydration, strength of the catalyst also appears to have a substantial effect acid sites are necessary to accelerate the reaction. on decarbonylation activity, as we see variations over orders of The product distribution of ML reacted over the various cata- magnitude due to the use of more strongly acidic materials; this lysts tested clearly demonstrates that the formation of adsorbed may prove to be true for the dehydration reaction as well. The sodium lactate is key to promoting dehydration and suppressing mechanistic insights gained, especially the identification of the the competing decarbonylation pathway, which has been sug- mechanistic step for selectivity control, will guide the design of gested by Wadley et al. based on ex-situ FTIR experiments [21]. selective dehydration catalysts for lactic acid and its esters. Meanwhile, the low activity of Na-Lac/SiO2 indicates that acid sites are needed to catalyze the dehydration reaction. We have no evi- Acknowledgments dence to suggest that Brønsted acid sites generated in-situ via ion exchange or hydrolysis behave any different than those present We gratefully acknowledge the support of the National Science on the zeolite prior to reaction, as in HY, therefore should be able to Foundation, CBET under the grant number CBET-1437129. M.L. catalyze all typical Brønsted acid-assisted reactions. However, the acknowledges the support of NSF REU program under the same strength of these sites may have a large effect on reaction rates grant. We would like to thank Dr. Weihua Deng for assistance in + and product distributions. Rather, the presence of high [Na ] leads performing isopropylamine decomposition experiments to charac- to dissociation of LA and ML to adsorbed sodium lactate, which terize our zeolite catalysts, and Ms. Molly Koehle for her assistance directs activity toward dehydration. When the concentration of in preparing and characterizing the Sn-Beta sample. The authors + l Brønsted acid sites is large ([H ] > 200 mol/gcat), the dehydration would also like to acknowledge Dr. Guangjin Hou for his assistance pathway is overwhelmed by the high activity of Brønsted sites. The in solid-state NMR measurements. unique combination of LA/ML dissociation, forming adsorbed lactate, and the presence of Brønsted acid sites generated in-situ is Appendix A. Supplementary data key to the dehydration selectivity observed on NaY and catalysts with similar low intrinsic Brønsted acidity. The identification of Supplementary data associated with this article can be found, in the branching point (reactant dissociation/ion exchange to sodium the online version, at http://dx.doi.org/10.1016/j.jcat.2016.03.026. lactate) for the desired dehydration and undesired decarbonylation pathways in this work sets the stage for the rational design of catalytic systems for the dehydration of LA and ML. The presence of References exchangeable metal cations should be an indispensable feature of [1] D.M. Alonso, J.Q. Bond, J.A. Dumesic, Green Chem. 12 (2010) 1493–1513. selective LA and ML dehydration catalysts. In addition, the density [2] U.S. Department of Energy, U.S. Billion-Ton Update: Biomass Supply for a of Brønsted acid sites, either intrinsic or generated in-situ via ion Bioenergy and Bioproducts Industry, R.D. Perlack, B.J. Stokes (Leads), ORNL/ exchange must be minimized, because they catalyze the decar- TM-2011/224, Oak Ridge National Laboratory, Oak Ridge, TN, 2011, pp. 227. 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