new catalytic materials for clean technology

14 New Catalytic Materials for Clean Technology Structure-Reactivity Relationships in Mesoporous Solid Acid Catalysts K. Wilson1∗, A. F. Lee2, M. A. Ecormier2, D. J. Macquarrie1 and J. H. Clark1 14.1. MESOPOROUS SOLID ACIDS AND CLEAN TECHNOLOGY The syntheses of many fine and speciality chemicals rely on homogeneous mineral acids, bases or metal salts, which are frequently used in stoichiometric amounts. Tightening legislation on the treatment and disposal of excess toxic waste, generated during the separation and neutralisation of products from these processes, is driving industry to consider cleaner technologies, including the use of heterogeneous catalysis. Of particular concern is the wide range of liquid phase industrial reactions, which rely on the use of inorganic or mineral acids. While some of these processes are catalytic, many require stoichiometric amounts of acid (e.g. acylation using AlCl3). Zeolites are widely used as solid acid catalysts in gas phase chemistry, however their use in liquid phase organic synthesis is limited by small pore sizes (90 %) thiol inclusion during the sol-gel preparation. Surface S levels shown in figure 14.1 (inset) determined by X-ray photoelectron spectroscopy (XPS) mirror this bulk trend indicating the modified silicas possess a uniform composition. The creation of surface sulphonic acid functionality via thiol oxidation was followed by XPS and Raman spectroscopy (Figure 14.2). XPS indicates a single oxidic sulphur environment associated with surface bound S(VI) forms following oxidation by H2O2 (a small subsurface reduced sulphur signal remains). Figure 14.2 (inset) shows the Raman spectra of unoxidised and oxidised samples which confirms the identity of the sulphoxy species as RSO3H41 from the appearance of two new bands at 1040 and 1100 cm-1, attributed to the symmetric and asymmetric vibrational modes of SO3- respectively. It should be noted that these Raman measurements represent the first direct identification of surface sulphonic acid groups on organically modified silica. In conventional IR measurements these S=O stretching vibrations are obscured by Si-O and Si-OH modes from the support. In addition, the retention of modes at 1250 and 1300 cm-1, assigned to CH2-S and CH2-Si wagging modes, confirm the integrity of the alkyl linker between the

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sulphonic acid and silicon centres. All resulting materials possessed pure Brønsted acid sites and were thermally stable to 250 °C across the composition range.

8

S 2p XP Intensity

12

6

0

Actual S Loading (%)

10

2 4 6 8 10 Bulk S loading (%)

4

2

0

0

2

4

6

8

10

12

Theoretical S Loading (%)

FIGURE 14.1. Bulk (main) and surface (inset) S content of sol gel sulphonic acid silicas

The corresponding reactivity of these materials was assessed in the esterification of butan-1-ol with acetic acid, (Figure 14.3), a reaction typically catalysed by strong Brønsted acids e.g. H2SO4 and susceptible to diffusion-limitations using small cage zeolites. Initial reaction rates determined by periodic off-line GC sampling reveal a linear increase with S loading and thus the number of sulphonic acid sites up to ~7 % beyond which the rate begins to plateau. Independent of the sulphonic acid loading the reaction was 100 % selective towards butyl acetate, with average single-site turnover frequencies calculated at 0.7 ± 0.1 molecules min-1. Conversion levels after 6 h reaction are shown in Table 14.1. The results for the high loading sulphonic acid catalysts compare favourably with reports of vapour phase butan-1-ol esterification using MCM-41 supported heteropoly acids.42 In the latter case the higher operating temperatures (~110 °C) required during plug flow reactor operation resulted in competing acid catalysed dehydration and etherification side reactions, which reduced the acetate selectivity to 80-85 %. The nanoporous nature of our templated sulphonic acid silicas was verified by measuring their adsorption isotherms across the MPTS range as shown in Figure 14.4, with well-defined pore-size distributions (PSD) obtained for all sulphonic acid loadings. The BET and porosity data extracted from these isotherms are shown in Figure 14.5 from which it can be seen that the surface area is approximately constant over the range 1 to 4 % S, with further increases in the MPTS content reducing the surface area.

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S 2p XP Signal

Oxidised

ρw (CH2 -Si) νs(SO3) νa(SO3)

Unoxidised

ρw (CH2 -S) Unoxidised ν Si-OH, Si-OR, Si-O-Si 950

1150 1350 Wavenumber (cm-1)

S

Oxidised

SOx

166

170 Binding Energy (eV)

FIGURE 14.2. S 2P XP (main) and Raman spectra (inset) showing oxidation of thiol functionalised silica to sulphonic acid

Initial rate / mmols min-1

0.05 0.05 0.04 0.04 0.03 0.03 0.02 0.02 0.01 0.01

0

0

0

2

4

6

8

10

12

Bulk S loading / %

FIGURE 14.3. Initial reaction rates for sulphonic acid silica in esterification of 1-butanol by acetic acid


301

TABLE 14.1: Conversion levels after 6 hours reaction in the esterification of 1-butanol by acetic acid over sulphonic acid silica catalysts.

Sulphonic Acid Loading wt %

Butan-1-ol Conversion %

Background 1.25 1.85 2.85 6.01 7.01 11.62

1 M H2SO4 the surface loading begins to level off, however the total loading continued to rise, approaching the final surface value of 5 wt% S for the 2.5 M treatment. Complete surface sulphation thus requires bulk sulphur loadings in excess of 1.7 wt %.

- 15 -15 3.0 3.0

H2SO4 Molarity / M

FIGURE 14.6. Surface () and bulk () Sulphur content and resulting Hammett acidity () of impregnated SO4/ZrO2 catalysts as a function of H2SO4 molarity

The acidity of these sulphated zirconias as estimated using Hammett indicators also shows a strong dependence on the molarity of the impregnating H2SO4 solution. It is interesting to note that the trend in H0 exhibits a direct correspondence with the surface SO4 coverage. The S XP spectra show no evidence for new chemically distinct SOx

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species at high loadings, hence superacidity does not arise from a change in the S oxidation state or significant restructuring of the sulphate local environment. Indeed the sulphur 2p XP spectra revealed that a single broad state at ~169.4 eV exists for all samples, consistent with a unique SO4 environment.43 Given that a full Zr(SO4)2 monolayer (ML) would contain ~22.6 wt% S, much greater than the limiting value of 5 wt%, this suggests that a maximum of only one fifth of the zirconia surface (corresponding to ~0.2 ML) undergoes sulphation. These values represent the first quantitative determination of surface sulphur within sulphated zirconias.44 Having demonstrated that a range of sulphated zirconias could be prepared exhibiting tuneable surface acidity, the rearrangement of α-pinene was selected to probe the effect of acid strength on catalytic performance.45 This is a particularly interesting reaction as the product selectivity is known to vary with catalyst acid strength; weak acids favouring camphene formation, while stronger acids result in monocyclic products46,47 as shown in scheme 14.4 below.

+

Solid acid/base

+

... Polycyclic

Camphene Solid acid

β -pinene

α-pinene

+ Limonene

+

... Monocyclic

SCHEME 14.4. Major products formed in solid acid catalysed isomerisation of α-pinene

The corresponding catalytic properties of the sulphated zirconias towards α-pinene isomerisation were subsequently explored at 60 °C using 63 mmols of α-pinene and 100 mg of air-exposed catalyst with 0.2 cm3 of tetradecane as an internal standard. Figure 14.7 shows the calcined zirconia support possesses poor activity with a corresponding limiting conversion of only 5 % after 9 h reaction. Sulphation enhances catalyst performance with initial rates showing a direct correlation with measured surface sulphate coverage (ranging from ~0.08 to 0.2 ML SO4) and thus acid site density. The intrinsic (area normalised) catalyst activities are almost independent of sulphate loading and thus acid site strength at ~0.35 ±0.05 mmolh-1gcat-1m-2. α-Pinene conversion mirrored the activity trends with a maximum of 66 % obtained for the 4.03 wt% catalyst. Sulphation also induces a significant switchover in selectivity from essentially pure polycyclic camphene over unsulphated zirconia towards the monocyclic limonene product, in striking agreement with the acid strength predictions from the Hammett indicators. Small amounts of α-terpinene, γ-terpinene and p-cymene were also observed as residual secondary by-products, most notably for the superacidic catalysts.

305

NEW CATALYTIC MATERIALS FOR CLEAN TECHNOLOGY 100 0.4

Camphene Limonene

60

0.3

0.2

40

0.1

20

0 0

0.44

1.74

2.49

4.03

Activity / mmolh-1 gcat -1 m-2

Selectivity / %

80

0

Total Sulphur Loading / wt%

FIGURE 14.7. Activity and selectivity of SO4/ZrO2 catalysts in the isomerisation of α-pinene as a function of sulphur loading (Reprinted from ref [44] with permission from Elsevier)

Corresponding XRD measurements revealed in that the onset of superacidity is also correlated with the degree of recrystallisation of the parent zirconia, which is in turn dependent on SO4 content. Figure 14.8 shows the powder X-ray diffraction for the different impregnated samples after calcination at 550 °C. The uncalcined zirconia precursor contained only a single weak, broad reflection centred around 2θ = 30°, indicating essentially amorphous character. In contrast the calcined zirconia support exhibited reflections arising from both monoclinic (2θ = 24.7, 28.4, 31.6°)48 and tetragonal (2θ = 30.3, 35.3, 50.7, 59.9, 60.6 and 63.5°)49 zirconia phases. All the sulphated materials were likewise highly ordered, however increasing the sulphur loading above 0.44 wt% caused a sharp increase in the intensity of the tetragonal phase and concomitant loss of the monoclinic phase. Although sulphation was previously reported to induce a similar structural transformation in the support from amorphous to tetragonal zirconia,50 our results show there is a threshold sulphur coverage for this evolution; loadings >0.44 wt% are necessary to effect a complete monoclinic → tetragonal transition. Peak shape analysis showed the tetragonal phase comprised small crystallites of ~145 Ă independent of sulphur loading; high temperature calcination (>650 °C) has been shown to eliminate differences in sulphated-zirconia crystallite sizes.51 The monoclinic phase present in the calcined and 0.44 wt% zirconia samples possessed a similar average crystallite size. Neither preparative route gave rise to crystalline, stoichiometric zirconium-sulphate. Prior to calcination the impregnated samples possessed high surface areas in excess of 250 m2g-1 and show a slight decrease with sulphur loading. Calcining at 550 °C reduced the surface areas of all samples (although they remain >50 m2g-1 in all cases). Pore size distribution measurements revealed that the resulting calcined materials were

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mesoporous, with well defined pore sizes ranging between 33 and 46 Å, with no systematic variation in pore diameter with sulphur content.

Intensity (Arb Units)

4.03 %

2.49 %

1.71 % 0.44 % Calcined ZrO2 ZrO2

20

40

60

80

2θ θ / degrees FIGURE 14.8. XRD spectra of calcined SO4/ZrO2 as a function of sulphur loading (Reprinted from ref [44] with permission from Elsevier)

Figure 14.9 shows the effect of calcination and sulphur loading on the thermal stability of impregnated zirconias as studied via temperature-programmed reduction (TPR) and thermogravimmetric analysis (TGA). The zirconia precursor showed no reduction at temperatures below 700 °C in accord with the literature.52,53 Morterra et al54 reported that SO2 is predominantly evolved during thermal-reduction of sulphated zirconias hence the peaks observed between 400 and 700 °C for treated samples can be associated with the reduction of sulphoxy species. The calcination step increased the thermal stability of both low and high sulphur loading materials. The TPR profile for the 1.74 wt% sample exhibits a single peak indicating a relatively homogeneous sulphoxy environment. Although higher sulphur loadings led to increased thermal stability they also reduced sample homogeneity resulting in the emergence of a second weaker reduction state ~50 °C below the main peak. These observations suggest that a single SO4 species predominates up to the saturation coverage determined by XPS, beyond which sulphur is incorporated within various subsurface environments. The corresponding thermogravimetric analysis confirmed this principal high temperature SOx decomposition process and also identified a small mass loss at 100 °C associated with adsorbed water desorption.

307

NEW CATALYTIC MATERIALS FOR CLEAN TECHNOLOGY 6 SOx

H2 O

d(weight)/d(temperature)

TCD Signal / µ V

5 4 4.03 wt % calcined

3 2

1.74 wt % calcined

1

ZrO2 0

0

200

400

600

Temperature / °C

FIGURE 14.9. Temperature Programmed Reduction of calcined 1.74 and 4.03 wt % SO4/ZrO2 catalysts

DRIFTS measurements shown in Figure 14.10 also reveal that the surface sulphoxy species present on SO4/ZrO2are dependent of S content. Following sulphation new features appear in the region 1400-900 cm-1 assigned to the sulphoxy vibrations.55 The principal SOx components lie at 1240-1265 (S=O stretch), 1134-1152 (νs O=S=O) and 1045 cm-1 (S-O stretch). It is interesting to note that the corresponding asymmetric O=S=O stretch, expected ~1300-1400 cm-1 was of very low intensity except for the lowest loading 0.44 wt% sample (indicative of bidentate sulphate).56,57 Higher sulphur loadings enhance the SOx vibrational intensities but degrade their resolution resulting in a broad spectral envelope for the 4.03 wt% sample. The observed correlation between acidity and surface sulphate loading can now be understood by considering the Clearfield model for acid site formation in sulphated zirconia. Exposure of ZrO2 or Zr(OH)2 surfaces to H2SO4 solutions is proposed to result in surface bisulphate groups (HSO4) in a bidentate coordination. During subsequent calcination bisulphate can undergo condensation with adjacent hydroxyl groups to form a neutral bidentate sulphate species together with a Lewis acid centre located on the zirconia support (scheme 14.5A). The latter can readily interconvert to yield weak Brønsted sites on coordination with water. Our DRIFTS measurements reveal that weak sulphuric acid solutions result in a low surface coverage of bidentate SO4 groups exhibiting strong covalent character. The corresponding support shows incomplete crystallisation (Figure 14.8, 0.44 wt% sample) and therefore partial hydration, consistent with a surface in which excess hydroxyl functionality was available for complete neutralisation of bisulphate. Thus as the H2SO4 concentration and initial bisulphate loading rises, the degree of reactive surface dehydroxylation is likewise expected to increase, as indeed evidenced by the appearance of the tetragonal phase of zirconia (usually formed via thermal. dehydroxylation) for loadings ≥1.71 wt%. Above a critical bisulphate coverage there may be insufficient hydroxyl groups for the condensation reaction to proceed to

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completion during calcination. This would result in the stabilisation of permanently bound, bisulphate Brønsted acid sites as postulated by Clearfield58 and Chen,59 and the generation of new strong acid sites with an Ho value similar to sulphuric acid (scheme 14.5B). Both our Hammett indicator measurements and the selectivity switchover from polycylic to monocylic products during α-pinene isomerisation support this model. The weaker covalent character of bisulphate versus sulphate would also account for loss of the asymmetric O=S=O stretch above 0.44 wt% S. Hence increasing the sulphate loading, and thus surface bisulphate:hydroxyl ratio, seems to induce a transition from ZrO2derived, weak Lewis/Brønsted acid sites to bisulphate-derived, strong Brønsted acid sites.

O

O Calcined ZrO2

O Zr

0.44 %

S O

O Zr

1.74 %

νs SOx

Transmittance

2.49 % 4.03 %

2000 2000

1800

1600 1600

1400

1200 1200

1000

800 800

Wavenumber / cm-1

FIGURE 14.10. DRIFTS of SO4/ZrO2 catalysts as a function of H2SO4 Molarity (Reprinted from ref [44] with permission from Elsevier)

In summary, we have demonstrated that direct wet impregnation routes permit the synthesis of mesoporous sulphated zirconias with tuneable structural and catalytic properties. The molarity of H2SO4 impregnating solution controls the surface sulphate coverage which attains a maximum of ~0.2 ML SO4 for acid concentrations >2.5 M. Sulphation induces a concomitant crystallisation of amorphous zirconia to the tetragonal phase while suppressing the monoclinic phase, and the formation of superacidic sites. The emergence of these superacid sites above a critical threshold SO4 coverage of 0.08

309


ML (0.44 wt% total S) appears associated with this bulk structural transformation and a corresponding change in the surface bisulphate concentration. These observations can also be correlated with catalyst performance in the isomerisation of α-pinene which has been successfully used to probe their surface acidity; low S loadings/weak acid sites favours camphene while high S loadings/strong acid sites promote limonene production. Thus it can be expected that when preparing dispersed SO4/ZrO2/MCM materials careful attention must be made to control both the ZrO2 loading and corresponding H2SO4 concentration during the sulphation step.

A H Zr

O O

Zr

H O O

O O Zr

H Zr

O O

OH S O

O Zr

O

O H O O

H Zr

O O

∆T Zr

O Zr

S O

Zr

O

B

O Zr

OH

OH S O O O Zr Zr Zr O O O S

O

H

O

O

∆T

O Zr

O S O

O

O

Zr

O O

Lewis Site

O

O Zr

+ H2O

H

O

Zr

Weak Brønsted site

Lewis Site

O

O Zr

Zr

O

+

H H H O O O Zr Zr Zr O O O

Strong Brønsted site

OH S

O Zr

O

S

O Zr

O

+ H2O

O Zr

O

+ O OH H H S O O O O Zr Zr Zr O O O

S

SCHEME 14.5. Generation of acid sites on SO4/ZrO2 (Reprinted from ref [44] with permission from Elsevier)

14.4. CONCLUSIONS AND OUTLOOK In the development of solid acid catalysts for use in liquid phase processes the ability to control both the active site density and distribution is paramount. To improve reaction efficiency and minimise waste, catalysts with high dispersion which afford control over acid site type and strength are required. Through careful control of preparation conditions, we have demonstrated that for a pure Brønsted acid catalyst it is possible to vary the site loading in a controllable fashion using sol gel methodologies. The ability to control reaction selectivity is also of great importance. A simple direct sulphation method can be used to generate SO4/ZrO2 catalysts with variable solid acid strength and concomitant tuneable selectivity in isomerisation reactions. The ability to tailor both the active site and gross catalyst morphology is the ultimate goal of heterogeneous catalysis research, and the availability of tuneable nanostructured materials has led to new and exciting developments in liquid phase solid acid catalysis. These materials permit greater control over active site homogeneity and associated control over reaction selectivity. Interdisciplinary collaboration between chemists, materials scientists and chemical engineers in both industry and academia is essential if viable heterogeneous alternatives to traditional homogeneously catalysed processes are to be developed.

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ACKNOWLEDGEMENTS Financial support by the UK Engineering and Physical Sciences Research Council under grant GR/M20877, the Royal Society through the provision of an equipment grant (to KW) and BP Chemicals is gratefully acknowledged. MAE thanks the University of Hull for financial assistance.

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