Sulfated Zirconia Catalysts for D-Sorbitol Cascade Cyclodehydration

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Sulfated zirconia catalysts for D-sorbitol cascade cyclodehydration to isosorbide: impact of zirconia phase Xingguang Zhang, Abdallah Rabee, Mark Isaacs, Adam Fraser Lee, and Karen Wilson ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03268 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Sustainable Chemistry & Engineering

Sulfated zirconia catalysts for D-sorbitol cascade cyclodehydration to isosorbide: impact of zirconia phase Xingguang Zhang,1 Abdallah I.M. Rabee,2 Mark Isaacs,3 Adam F. Lee,4 and Karen Wilson4*

1College

of Chemical Engineering, Jiangsu Key Lab for the Chemistry & Utilization of Agricultural and Forest Biomass, Nanjing Forestry University, Nanjing, China

2

Chemistry Department, Faculty of Science, Minia University, El-Minia, 61519 Egypt 3Department

4Applied

of Chemistry, University College London, London, WC1H 0AJ, UK

Chemistry and Environmental Science, School of Science, RMIT University, 124 La Trobe Street, Melbourne, VIC 3001, Australia

*Corresponding author: [email protected]

ABSTRACT Isosorbide is a widely touted intermediate for the production of bio-renewable polymers and plastics, accessible through the aqueous phase cascade conversion of D-sorbitol to isosorbide via 1,4-sorbitan. However, existing routes to isosorbide typically employ mineral acids under forcing conditions, and hence alternative heterogeneously catalysed processes are highly desirable. Aqueous phase D-sorbitol conversion was therefore investigated over families of sulfated zirconia (SZ) solid acid catalysts, with the effect of employing monoclinic, tetragonal ZrO2, or Zr(OH)4 as the parent support compared. The cascade proceeds via a stepwise dehydration to 1,4-sorbitan and subsequently isosorbide, with the latter favored over stronger acid sites. Monoclinic SZ exhibits superior activity to tetragonal SZ, reflecting a higher acid site density and pyrosulfate formation at lower SO42- loadings

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than over the other supports. Isosorbide selectivity at iso-conversion was proportional to acid site density, but independent of zirconia phase. KEYWORDS: Sorbitol dehydration, Solid acid, Sulfated zirconia, biorefining, platform chemicals

INTRODUCTION: Societal concern over the impact of atmospheric emissions, environmental pollution, and global warming, associated with the unsustainable use of fossil based resources, is driving efforts to reduce the carbon footprint of consumer products and materials through the use of renewable or recycled feedstocks.1-2 Biorefining of waste biomass is one of the most promising options to sustainably deliver organic molecules for commodity and specialty chemicals or fuels synthesis. Routes to either ‘dropin’ replacements, that can directly substitute for existing petroleum-derived chemicals, or new high performance platform chemicals possessing unique properties, for consumer products are of especial importance.3-4 A plethora of (largely untapped) waste-derived carbohydrates are available to underpin a sustainable biorefinery,5, 6 including cellulose (the dominant component of lignocellulosic biomass, being 30-50 wt%) from agricultural residues, forestry and municipal solid waste, grass sugars from non-food sources (e.g. Ryegrass)7 and sugar beet pulp,8 and waste from the food and dairy industry. Carbohydrate valorization is the focus of significant industrial and academic interest, wherein processes to selectively deoxygenate highly functional C5-C6 sugar oligomers to their target product (contrasting with selective oxidation steps required for crude oil feedstocks)9 are urgently sought. Deoxygenation processes employing H2, such as hydrodeoxygenation or hydrogenolysis, are costly due to the requirement for a sustainable source of H2 and current dependence on precious metal catalysts;10 catalytic decarboxylation or dehydration using earth abundant (non-precious metal) catalysts are therefore particularly attractive. Isosorbide is a bicyclic molecule formed by the cyclodehydration of D-sorbitol, (Scheme 1) and has attracted interest for applications including isosorbide polycarbonate plastics,6 high-temperature


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polyethylene terephthalate (PET) production,11 isosorbide diesters plasticisers for flexible polyvinyl chloride production,12 and isosorbide diethers as biogenic fuel additives or high-boiling solvents.13, 14

HO HO

OH OH

HO

-H2O OH

HO

H+

H+

O HO

HO

-H2O

H HO

O

O

OH OH

D-Sorbitol

1,4-Sorbitan

Isosorbide

Scheme 1. Acid catalyzed cyclodehydration of D-sorbitol to 1,4-sorbitan and isosorbide.

D-Sorbitol, a precursor to isosorbide, is readily produced from carbohydrates,15, 16 with up to 90% yields possible via the one-pot hydrolytic hydrogenation of cellulose,17, 18, 19-21 and is consequently identified as one of the top-ten bio-derived platform chemicals by the US Department of Energy.2 However, current routes to isosorbide from D-sorbitol employ liquid sulfuric acid for the catalytic cyclodehydration,22, 23 which is undesirable due to the hazards in handling corrosive mineral acids, and the large volume of contaminated water waste produced in quenching the reaction to isolate isosorbide. Solid acid catalysts such as zeolites,24,

25, 22, 26

sulfated oxides,27 ZrO228 and TiO2,29 phosphated

oxides,25 sulfonic resins,30,31 sulfonic acid silicas,26, 32 silicotungstic acid,33 and Ru-Cu bimetals,34 have attracted attention as possible replacements for such homogeneous acid catalysts. Hot compressed water (itself acidic) has also been used in isosorbide synthesis, but requires temperatures >317 C and offers lower isosorbide yields (57 %)35 than H2SO4 (135 C and 77%).36 Sulfated zirconia (SZ) catalysts comprise earth abundant elements, are non-toxic, and the amphoteric character of the parent ZrO2 provides tunable acidity and in turn selectivity for dehydration,37,

38

isomerization,39

esterification,40 alkylation,41 and acetalization.42 The liquid phase cyclodehydration of D-sorbitol to isosorbide over SZ is reported,28 albeit under high temperature conditions wherein catalyst stability and reactant/product degradation to humins is problematic. Dehydration of molten D-sorbitol is also


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often studied under reduced pressure,28, 43, 29 impractical for any large-scale cascade process starting from cellulose which would afford D-sorbitol in the aqueous phase,25 hence catalysts for D-sorbitol dehydration must be tolerant to high water concentrations. Zirconia is a versatile amphoteric support whose acid-base properties may be readily tuned through e.g. impregnation with sulfate groups, whose density regulates corresponding acid strength, activity and selectivity.37, 39, 44 Surprisingly while there are many studies of SZ preparation, and the impact of sulfate on stabilizing the tetragonal phase of zirconia, there are few reports on catalysis by SZs prepared from pure phases.45 Early work attributed the inferior activity of monoclinic (m-) SZ for gas phase butane isomerization to a sub-optimal arrangement of sulfate species relative to its highly active tetragonal (t-) counterpart,46 leading to mSZ being largely discounted in catalysis. However, more recent studies show that despite its significantly lower surface area, the specific activity of (predominantly) m-SZ is ‘only’ four times less than that of t-SZ for n-butane isomerization, and hence the relative intrinsic activity of the two phases remains unclear.47-48 Significant differences exist between the surface terminations of m- and t-ZrO2, including the density of acid-base pairs and mode of hydroxyl coordination,49,50 that have yet to be exploited to tune the acidity of SZ for liquid phase reactions. Here we investigate the impact of acid site density and zirconia phase in the activity and selectivity of SZ for the cascade cyclodehydration of D-sorbitol to isosorbide. Sulfation of m- and t-ZrO2 creates a more uniform distribution of sulfate geometries than conventional (mixed phase) SZ materials prepared from amorphous Zr(OH)4 precursors. The latter exhibit a broad distribution of mono-, poly-, and pyrosulfate species, whereas m-ZrO2 favors C3v coordinated SO4 environments and t-ZrO2 favors C2v coordinated sulfate. The high surface density of C3v sulfate species on m-ZrO2 promotes formation of strongly acidic pyrosulfate S2O72- species at low SO4 loadings,44, 51 which exhibit high activity for D-sorbitol cyclodehydration to isosorbide.43


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EXPERIMENTAL Catalyst preparation A series of SO4/ZrO2 (SZ) catalysts with different SO42- loadings were prepared by impregnation of 4 g Zr(OH)4 (MEL Chemicals - XZO 880/01) with 40 ml H2SO4(aq) (Thermo Scientific Fisher (37 vol% in water) with molarities spanning 0.01-0.5M, as previously reported.37 The slurry was stirred for 5 h at ambient temperature (conditions previously determined as optimal37), then filtered and dried at 80 °C overnight, and finally calcined at 550 °C for 3 h (ramp rate 5 C.min-1). Catalysts were stored in air and used without further pretreatment. Catalysts are designated xSZ where x denotes the H2SO4 molarity used during impregnation. Two similar SZ series was prepared by sulfation of either pure m- or t-ZrO2, designated xm-SZ or xt-SZ respectively, where x denotes the H2SO4 molarity. m-ZrO2 was prepared by calcining Zr(OH)4 at 700 C in a muffle furnace for 3 h (ramp rate 5 C.min-1). t-ZrO2 was prepared via a citrate-mediated, sol-gel combustion method,52 in which 5.5 g zirconyl chloride hydrate (ZrOCl2-8H2O, 99% ResearchLab Fine Chem Industries/India) was mixed with an equimolar amount of citric acid (Sigma-Aldrich 99.5 %, 3.6 g) in 100 ml deionized water, and stirred at room temperature until a clear solution formed. The resulting solution was heated on a hotplate for 6 h at 85 C until a glassy gel formed, and then dried overnight in an oven at 200 C, ground to a fine brown powder in a mortar and pestle, and finally calcined in static air at 550 C for 5 h (5 C.min-1) to yield a white powder.

Catalyst characterisation Surface area and pore size analysis was performed by N 2 physisorption employing a Quantachrome Nova 2000e porosimeter system with NovaWin software version 11. Samples were outgassed at 120 °C for 2 h prior to analysis, with surface areas calculated using the Brunauer-Emmet-Teller (BET) method over the range P/P0 = 0.03-0.18 where a linear relationship was maintained. Pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) model applied to the desorption


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branch of the isotherm. Sample crystallinity was evaluated by powder Xray diffraction (XRD) using a Bruker D8 Advance diffractometer (Cu Kα radiation) between 2θ = 10.0º to 80.0º (step size of 0.02º). Xray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis HSi photoelectron spectrometer equipped with a charge neutralizer and monochromated Al Kα Xray source (1486. 6 eV). Spectra were recorded at normal emission using an analyzer pass energy of 40 eV and Xray power of 225 W, with quantification and spectral fitting conducted using CasaXPS version 2.3.14. Sulphur loadings were obtained using a Thermo Flash 2000 organic elemental analyzer, calibrated to a sulphanilimide standard, with the resulting chromatograms analyzed using Thermo Scientific's Eager Xperience software. Vanadium pentoxide was added to aid sample combustion. Temperatureprogrammed decomposition of propylamine to propene and NH 3 via the Hoffman elimination reaction was employed to quantify acid loading and strength. n-Propylamine (≥99 %, Sigma-Aldrich) was added by pipette to samples which were then dried for 2 h, with physisorbed propylamine removed by degassing at 30 °C overnight under vacuum. Samples were then heated in a Mettler Toledo TGA/DSC 2 STARe System equipped with a Pfeiffer Vacuum ThermoStarTM GSD 301 T3 mass spectrometer under flowing N2 (30 cm3.min-1) from 40-1000 °C at a ramp rate of 10 °C.min-1.

Catalytic tests Sorbitol cyclodehydration reactions were performed in a 100 ml Parr Autoclave 5500 system, fitted with overhead stirrer, sampling dip tube and 50 ml quartz liner. Optimization of reaction conditions was first performed using 0.15 g 0.1-SZ (having the highest acid site density of conventionally prepared SZ materials) at 140-220 C, using 0.4 g of D-sorbitol, 40 ml deionised water with 0.1 ml dimethyl sulfoxide (DMSO, Sigma Aldrich 99.5 %) as an internal standard. In a typical procedure, the reaction mixture was sampled cold without catalyst, and subsequently 0.15 g of catalyst was added, the reactor sealed and purged three times with N2 to remove air from the headspace, and heated under static conditions to the target reaction temperature. Once the desired reaction temperature was attained


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reactor stirring (900 rpm) was commenced, with reactions performed for 1-6 h. Samples were periodically withdrawn for analysis by high-performance liquid chromatography (HPLC) on an Agilent 1260 Infinity system equipped with a refractive index (RI) detector, and Hi-Plex Ca-Duo column held at 80 °C, and using deionized water as the mobile phase (0.6 ml.min-1). Response factors were determined from calibration curves of standards for D-sorbitol (≥98 %), D-mannitol (98 %), and 1,4-sorbitan (>99%) (all Sigma Aldrich), and isosorbide (98 %, ACROS Organics™). Individual data points were obtained from triplicate injections. D-sorbitol conversion, product selectivity, product yield, and carbon mass balance were calculated according to Equations (1-4):

Conversion (%) =

Moles D−sorbitol 𝑡=0 − Moles D−sorbitol 𝑡=i Moles D−sorbitol 𝑡=0

Relative Selectivity (%) =

Yield (%) =

Moles 1,4−sorbitan or isosorbide  moles of all products

Moles 1,4−sorbitan or isosorbide Moles D−sorbitol 𝑡=0

Mass balance (%): =

× 100

(Equation 1)

× 100

(Equation 2)

× 100

Moles of carbon in (unreacted D−sorbitol+ products) Moles of carbon in D−sorbitol 𝑡=0

(Equation 3)

× 100

(Equation 4)

Following optimization, all reactions were performed at 180 C using 0.4 g catalyst over 2 h to acquire accurate kinetic data. Activities and productivities were determined by normalizing the initial rates of D-sorbitol conversion or 1,4-sorbitan/isosorbide production to the catalyst mass. Initial rates were determined by fitting the linear portion of the reactant conversion or product formation profiles using data points recorded for D-sorbitol conversion 0.025 M, in accordance with literature reports.37 This phase transition was accompanied by an initial increase in surface area, with a maximum of 137 m2g-1 attained for [H2SO4] ~0.05 M, and subsequent decrease for [H2SO4] >0.25 M accompanying zirconium sulfate crystallisation.53

Table 1. Physicochemical properties of SZ as a function of H2SO4 concentration. Surface dAcid site Acid diam. S S loading site / nm loading loading / / mmol.g-1 density / wt% wt% / nm-2 0 Zr(OH)4 93 0 0.07 0.015SZ 119 3.4 0.73 1.2 1.5 0.10 0.51 0.02SZ 117 3.4 0.88 1.4 1.8 0.12 0.62 0.025SZ 126 3.4 1.11 1.7 2.5 0.17 0.81 Zr(OH)4 0.05SZ 137 3.4 2.18 3.0 3.8 0.35 1.54 0.075SZ 133 3.4 2.28 3.2 4.0 0.38 1.72 0.1SZ 134 3.4 3.10 4.4 4.6 0.48 2.16 0.25SZ 131 3.4 4.08 5.9 5.9 0.42 1.94 0.5SZ 47 3.9 6.02 24.3 6.1 0.25 3.22 m-ZrO2 31 0 0.025m-SZ 32 0.21 1.2 0.04 0.82 1.2 m-ZrO2 0.05m-SZ 31 0.36 1.7 0.05 0.96 2.2 0.1m-SZ 31 0.46 1.9 0.06 1.13 2.8 0.5m-SZ 30 0.57 2.0 0.06 1.19 3.5 t-ZrO2 63 0 5.1 0.025t-SZ 69 0.48 1.2 0.07 0.64 5.1 1.3 t-ZrO2 0.05t-SZ 68 0.57 1.3 0.09 0.84 5.8 1.6 0.1t-SZ 67 0.59 1.4 0.10 0.89 5.8 1.7 0.5t-SZ 67 0.90 1.9 0.12 1.11 5.9 2.5 a b c d From: BJH analysis on desorption branch; CHNS elemental analysis; XPS analysis; TGA-MS of propylamine decomposition. Support

Catalyst

BET S.A. / m2g-1

aPore

bBulk

SO4 density / nm-2

c


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Sulfation of pure phase m- and t-zirconia with increasing SO4 loading has not been previously reported; both parent zirconias possessed lower surface areas (relative to Zr(OH) 4) of 31 and 67 m2g-1 respectively, which however remained unchanged upon sulfation; the crystalline nature of both pure zirconias presumably renders them stable towards sulfate-induced restructuring, in contrast to amorphous Zr(OH)4,54 whose structure collapses under corrosion from 0.5M H2SO4.37 Sulfate contents of SZ, t-SZ and m-SZ track their surface areas, with bulk and surface sulfur loadings reaching a plateau >0.25 M [H2SO4] for SZ, and m-SZ and t-SZ saturating >0.05 M [H2SO4] (Figure S4). The presence of a high binding energy S 2p photoemission peak at 168.5-169.2 eV confirmed the presence of surface SO4 species in all samples (Figure S5). For SZ, a small increase in binding energy of the SO4 feature and concomitant peak broadening was noted with increasing sulfur coverage. The latter observations suggest the emergence of multiple co-existing SO4 species at high S loading for SZ, as previously reported for SZs and attributed to a transition from bidentate to monodentate sulfate, resulting in a reduction in charge withdrawal from the zirconia substrate.39, 44 In contrast, m-SZ and tSZ only exhibited a single coverage-invariant S 2p feature centered at 168.9 eV, and hence unique sulfate species for all S loadings. In all materials, the surface sulfur content was consistently higher than that of the bulk, consistent with surface sulfate functionalization. Acid site loadings (Table 1) increased with SO4 content for all three zirconia supports (with the exception of 0.25SZ and 0.5SZ which lose surface area due to bulk Zr(SO4)2 formation under these highly corrosive conditions). Acid sites were characterized by n-propylamine thermal decomposition. A linear correlation between the resulting acid site density and SO4 loading was observed for all SZ samples (Figure 1), with the acid site densities for comparable S loadings increasing in the order m-SZ > t-SZ > SZ, mirroring their respective surface areas. The nature of surface sulfates was further probed by DRIFTS (Figure 2) which reveal that Zr(OH)4 sulfation is accompanied by the emergence of strongly overlapping absorptions spanning 1350-960 cm-1 attributed to νS=O and νS-O vibrations. The literature consensus is that bands at 1240 and 1142 cm-


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1

arise from νS=O vibrations in chelating (bridging) bidentate sulfate coordinated to zirconium cations,

whereas those at 1076, 1040 and 960 cm-1 arise from corresponding νS-O vibrations.55-56 SO4 bound in a C3v-like (tri or unidentate - species I) or C2v-like (bidentate or mono or binuclear - species II) geometries (Scheme 2) gives rise to three and four IR active modes respectively.57

2.5

Acid site density / H+.nm-2

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⚫ m-SZ ⚫ t-SZ ⚫ SZ

2

1.5

1

0.5

0 0

0.2

0.4

0.6

SO4 content /

0.8

1

mmol.g-1

Figure 1. Correlation between SO4 loading and acid site density determined from TG-MS decomposition of n-propylamine to propene over SZ, t-SZ and m-SZ catalysts.

Scheme 2. Possible sulfate configurations for sulfated zirconia.


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m-SZ

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


III

I

t-SZ

I

I

II II

SZ

II

II

0.025M 0.1M

0.025M 0.1M

0.025M 0.1M 1600 1400 1200 1000

800

600

400

Wavenumber / cm-1 Figure 2. DRIFT spectra of SO4 bands (normalized to surface area) for SZ, t-SZ and m-SZ catalysts sulfated using 0.025 M and 0.1 M H2SO4. Dotted and dashed lines indicate positions of key vibrational modes for sulfate species in geometries I, II and III.

The poorly resolved spectra in Figure 2 for 0.025-SZ is thus attributed to co-existing C3v and C2v species, with the loss of resolution for 0.1-SZ reflecting the formation of multiple, co-existing sulfate geometries (including pyrosulfate structures - species III), as previously proposed. 37, 39, 44,51 Sulfation of t-ZrO2 resulted in a similar IR fingerprint, albeit with slightly improved resolution, possibly reflecting more homogeneous SO4 environments on the crystalline support, and a weaker contribution from bulk Zr(SO4)2. m-ZrO2 likewise shows significantly improved spectral resolution for all S loadings, with only three well-resolved peaks at 1250, 1140 and 1005 cm-1 observed, indicating a C3v SO4 is favored. The acid strengths of SZ, m-SZ and t-SZ were determined from the temperature at which reactivelyformed propene evolved from n-propylamine decomposition during heating (Figure S6). Figure 3 shows that SZ and t-SZ exhibit similar propene desorption profiles, which peak maxima at 430-425 C, and hence possess similar acid strength, consistent with the similar structural properties and surface


11


SO4 environments. In contrast, xm-SZ exhibited two propene desorptions at 422 and 330 C, the lower desorption peak indicating that ~35 % of the sulfate resides in unique strong acid sites. Figure S6 shows that the resolution of these features degrades as the SO 4 content and propene desorption increases. Fitting of the propene desorption from 0.1m-SZ (Figure S7a) confirms that 40% of the 41 amu signal from m-SZ can still be attributed to strong acid sites (320-370C), with the intensity of the 370C component increasing with sulfate loading (Figure S7b). In contrast for t-SZ the intensity of weak features at 320-370C do not evolve with increased SO4 coverage.

⚫ m-SZ ⚫ t-SZ ⚫ SZ

41 amu desorption signal

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200

300

0.025 M

400

500

600

Temperature / C Figure 3. Propene desorption (41 amu MS signal (normalized to surface area)) following temperature programmed reaction of n-propylamine over SZ, t-SZ and m-SZ catalysts sulfated using 0.025M H2SO4.

These observations are rationalized as follows. H 2SO4 adsorption over Zr(OH)4 results in initial surface bound HSO4 (bisulfate), which on calcination condenses with an adjacent Zr-OH group to form bidentate sulphate and a Zr n+ Lewis acid center. The different surface structures of t- and m-ZrO2 appear to influence the mode of bisulfate coordination and restructuring during calcination giving rise to: (i) a higher density of Zr4+-O2- acid-base pairs over m-ZrO2 than t-ZrO2;49,50 and (ii) hydroxyl groups predominantly bi-bridged over on t-ZrO2, whereas m-ZrO2 favors tri-bridged hydroxyls.50 We


12

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propose that SO4 preferentially binds to Zr4+ sites in zirconia via two oxygens, but (if the surface geometry permits) may also form an adduct via a dative interaction between one S=O bond and a Lewis acidic Zr4+ site,58 resulting in a C3v coordinated species, which appears to be favored over mZrO2 surfaces. High densities of C3v oriented species could favor pyrosulfate anion (S2O72-) genesis during bisulfate condensation: 2HSO4−→S2O72-+H2O,44, 58 and thus the strong acid sites on m-SZ arise result from pyrosulfate species (Scheme 3). The mixed SO4 geometries present on t-SZ and SZ do not favour such interactions until very high SO 4 densities. The Lewis:Brønsted character of all three catalyst families was similar at comparable sulfate density (Figure S8).

Scheme 3. Formation of C3v sulfate species at low coverage and formation of pyrosulfate species at higher coverage on ZrO2 (red = oxygen; blue = zirconia; yellow = sulfur).

D-Sorbitol cyclodehydration The impact of reaction temperature on D-sorbitol cyclodehydration was first investigated over 0.1SZ to identify conditions that favor isosorbide formation. 1,4 Sorbitan and isosorbide were the only detectable products, however as show in Figure 4a and S9 while the activities for D-sorbitol conversion and isosorbide productivity increase with reaction temperature, significant coloration of the reaction mixture occurs >180 C due to oligomeric by-products and humin formation, corroborated by carbon mass balances (Figure 4b) which decrease significantly >200 C (from >90 % to ~60 %).


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Activity (⚫) / mmol.h-1.g-1

a) 9

9

6

6

3

3

0 140

160

180

0 220

200

Isosorbide productivity () / mmol.h-1.g-1

Temperature / C

b) 100

70

95

60

Selectivity / %

90 50 40

85 80

◼ 1,4-Sorbitan  Isosorbide

75

30

70

20

65 60

10

55

Carbon mass balance (◆) / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

0 140

160

180

200

220

Temperature / C

Figure 4. Impact of reaction temperature on D-sorbitol cylcodehydration to isosorbide over 0.1-SZ: a) activity for D-sorbitol conversion and isosorbide for; and b) selectivity to 1,4-sorbitan and isosorbide, and carbon mass balance. All data after 1 h reaction. Conditions: 0.15 g catalyst; 0.4 g D-sorbitol; 40 ml water; and 0.1 ml DMSO internal standard.

Selectivity to isosorbide versus the 1,4-sorbitan intermediate increases with reaction temperature, with maximum isosorbide selectivity attained at 180-200 C (Figure 4b), indicating that the second dehydration step is thermodynamically challenging. Indeed, the activity of 0.1-SZ for 1,4-sorbitan


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dehydration was only 0.27 mmol.h-1g-1 (Figure S10) compared with 1.6 mmol.h-1g-1 for D-sorbitol conversion, and hence the second dehydration step appears rate-limiting for isosorbide production.

Activation energies of 48 kJ.mol-1 are reported for D-sorbitol dehydration catalyzed by homogeneous methane sulfonic acid43 and sulfuric acid59, slightly lower than that for 1,4-sorbitan dehydration to isosorbide catalyzed by sulfuric acid of 51 kJ.mol-1.59 The present activation barrier for D-sorbitol dehydration over 0.1-SZ was 87 kJ.mol-1 (Figure S9d), in good agreement with reports for other solid acid catalysts such as Hβ zeolites (89 kJ.mol -1)22 and Amberlyst-15 (72 kJ.mol-1),60 and much lower than that for silicotungstic acid (140 kJ.mol-1).33 In the present work, a reaction temperature of 180 C provided the optimal balance of isosorbide productivity and carbon mass balance, and hence was selected for subsequent comparison of SZ, m-SZ and t-SZ catalysts. Figure 5a shows the acid site loading dependence of D-sorbitol specific activity for SZ, m-SZ, and t-SZ catalysts (derived from reaction profiles in Figure S11). In all cases, the rate of D-sorbitol conversion linearly increased with acid site loading, however m-SZ exhibited the strongest dependence. Pure t-SZ was less active than SZ for similar acid site loadings; this observation that cannot be simply attributed to the lower surface area of the former, since that of m-SZ (~30 m2.g-1) is significantly lower than either t-SZ (~68 m2.g-1) or SZ (~50-140 m2.g-1) and yet outperforms both. The superior catalytic ability of m-SZ is also evidenced by a four-fold increase in isosorbide productivity relative to t-SZ and SZ at similar acid site loadings (Figure 5b). Corresponding TOF’s for isosorbide productivity (Figure S12) calculated based on total acid site loadings, show t-SZ and SZ samples exhibit comparable productivities of 1 h-1 consistent with a common acid site over the tetragonal phase. In contrast for m-SZ, isosorbide productivity increases continuously with sulfate content. These observations are consistent with both DRIFTS and propylamine TPD, which reveal the presence of strongly acidic C3v pyrosulfate species for m-SZ which evolve with sulfate loading indicated in Figure S7b, whereas t-SZ and SZ exhibit similar, weaker acid fingerprints characteristic


15


of C2v surface sulfate. Recycle tests reveal ~30-45% deactivation arising from S loss on the first cycle for different zirconias, presumably from weakly chemisorbed monodentate sulfate, after which steady state performance was attained (Table S2).

a) 1.4

Activity / mmol.h-1.g-1

1.2 1 0.8 0.6 0.4 0.2 0 0

0.1

0.2

Acid site loading /

0.3

0.4

mmol.g-1

b)

Isosorbide productivity / mmol.h-1.g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.25

0.2

0.15

0.1 ⚫ m-SZ ⚫ t-SZ ⚫ SZ

0.05

0 0

0.1

0.2

0.3

0.4

Acid site loading / mmol.g-1

Figure 5. D-sorbitol cyclodehydration to isosorbide over xM SZ, m-SZ and t-SZ catalysts: a) activity for D-sorbitol conversion and b) isosorbide productivity. Conditions: 0.4 g catalyst; 0.4 g D-sorbitol; 40 ml water; 0.1 ml DMSO internal standard; 180C; reactions run for 2 h.


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Figure 6 compares the selectivity to 1,4-sorbitan versus isosorbide for the different catalysts at 10 % iso-conversion (to eliminate mass transport limitations and minimize side reactions). Isosorbide selectivity was directly proportional to acid site density across all catalysts. Previous kinetic studies suggest that D-sorbitol cyclodehydration to 1,4-sorbitan is faster than the subsequent cyclodehydration of 1,4-sorbitan to isosorbide,61 consistent with the higher activation barrier for the latter. It is therefore expected that the second dehydration step (1,4-sorbitan→isosorbide) would be favored by stronger acids such as pyrosulfate that form at high surface sulfate densities (most easily achieved over the low

1,4-Sorbitan selectivity (⚫,⚫,⚫) / %

surface area m-SZ).

90

90

80

80

70

70

60

60

50

50

40

40

30

30 ⚫, m-SZ ⚫, t-SZ ⚫, SZ

20 10

20 10

0.2

0.7

1.2

1.7

Isosorbide selectivity (, , ) / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2

Acid site density / H+.nm-2

Figure 6. Relationship between 1,4 sorbitan or isosorbide selectivity at 10 % iso-conversion and catalyst acid site density during D-sorbitol cyclodehydration over SZ, m-SZ, and t-SZ. Conditions: 0.4 g catalyst; 0.4 g D-sorbitol; 40 ml water; 0.1 ml DMSO internal standard; 180C.


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The highest isosorbide selectivity of 62 % is in accordance with previous studies wherein selectivities of 61 % and 56 % were reported respectively for mixed-phase SZ28 and HPW62, albeit using molten D-sorbitol at near 100 % conversion and far higher temperatures (210 C and 250 C respectively), conditions under which very poor carbon mass balances are anticipated. D-Sorbitol conversion and product yields after 2 h reaction (Table S1) show that the isosorbide yield for the most active m-SZ catalyst is seven times that of a conventional SZ possessing a similar SO 4 content. The benefits of m-SZ for D-sorbitol cyclodehydration are readily apparent from a comparison of the Turnover Frequencies (TOFs) for D-sorbitol conversion and isosorbide production with those for homogeneous Brønsted acid catalysts at 160 C.63 Stensrud achieved a TOF(sorbitol) of 7.6 h-1 and TOF(isosorbide) of 0.4 h-1 employing H2SO4, whereas our 0.5-m-SZ catalyst delivered TOF(Sorbitol) of 18.9 h-1 and a TOF(isosorbide) of 3.25 h-1. In addition to the superior activity, m-SZ also obviates the hazards associated with handling corrosive liquid acids, and subsequent waste production through requisite quenching and neutralization steps. The relative waste production arising from isosorbide produced by H2SO4 catalysed cyclodehydration64 is estimated as follows: 0.65 mmol isosorbide (Mr = 146) was produced by 6.5 mol H2SO4, which if quenched using CaO would produce 6.5 mol CaSO4 (Mr = 137). Assuming the same mol% H2SO4 was employed in process scale-up, 10 g CaSO4 waste would be generated for every kg of isosorbide, in addition to a large volume of aqueous waste. m-SZ thus offers a more atom efficient and faster route to isosorbide from D-sorbitol, employing only earth abundant elements.

CONCLUSIONS The aqueous phase cascade conversion of D-sorbitol to isosorbide via 1,4-sorbitan, was explored over families of sulfated zirconia solid acid catalysts prepared from Zr(OH)4, and pure phase m-ZrO2 and t-ZrO2. Sulfation of Zr(OH)4 with >0.025M H2SO4 yields t-SZ that exhibits similar acid properties to those obtained by direction sulfation of preformed t-ZrO2, with isolated sulfate groups coordinated


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predominantly in a C2v geometry. In contrast, direct sulfation of m-ZrO2 favors C3v coordinated, strongly acidic pyrosulfate species due to the low zirconia surface area and concomitant high sulfate density (close proximity between neighboring SO4 units promoting polysulfates). D-sorbitol dehydration to isosorbide proceeds via a stepwise cyclodehydration to 1,4-sorbitan. m-SZ exhibits superior activity and isosorbide productivity compared to t-SZ and SZ possessing similar acid site loadings, attributed to preferential stabilization of pyrosulfate over the former. At low D-sorbitol conversion, isosorbide selectivity was proportional to acid site density, but independent of zirconia phase, attributed to competitive adsorption of 1,4-sorbitan and D-sorbitol. The excellent performance of m-ZrO2 indicates that the future synthesis of high area m-ZrO2 should prove fruitful for further enhancing isosorbide production. Continuous flow operation employing low D-sorbitol concentrations should also promote adsorption and consequent dehydration of the 1,4-sorbitan intermediate.

ASSOCIATED CONTENT Supporting Information. Catalyst synthesis procedure and characterization data, along with full details of catalytic reaction data is supplied as Supporting Information.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Funding Sources


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This research was supported by the EPSRC and UK Catalysis Hub under grants EP/K014749/1 and EP/K014706/1, and the British Council through the Global Innovation Initiative under the GB3-Net project.

ACKNOWLEDGMENT XZ, KW and AFL thanks the UK Catalysis Hub and EPSRC and the for the award for funding under (EP/K014706 and EP/K036548/2). KW acknowledges the Royal Society for the award of an Industry Fellowship, and AFL thanks the EPSRC for the award of a Leadership Fellowship (EP/G007594/2). MEL Chemicals are gratefully acknowledged for their supply of zirconia supports.

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Pyrosulfate species on monoclinic sulfated zirconia are highly active for D-sorbitol cyclodehydration to isosorbide 338x190mm (96 x 96 DPI)


Sulfated Zirconia Catalysts for D-Sorbitol Cascade Cyclodehydration

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