Preparation of solid catalysts with ionic liquid layer (SCILL catalysts:

Ionic liquid coating was achieved by weighing a predetermined amount of 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)amide, [EMIm][NTf2], into a 100 ml round

bottomed flask. Thereafter, the required amount of the catalyst was added followed by 5 ml of acetone. The mixture was gently swirled to allow proper distribution of ionic liquid onto the support. Acetone was then removed in a rotary evaporator yielding the SCILL catalysts. All the catalysts preparation steps are summarized in scheme 6.1.

Scheme 6.1: Illustration of the preparation of mesoporous metal oxides, immobilized nanoparticles and SCILL-type catalysts.

6.2.7. Catalytic studies

Prior to catalytic evaluation, catalysts were sieved with a 90 µm molecular sieve and only particles > 90 µm were used. The catalyst (200 mg) was layered on silica-100 (1 g) guard bed in a continous flow fixed bed reactor equipped with a stainless steel tube with a diameter of 10

mm, and temperature controllers. The reactor is also equipped with an online gas chromotograph. For details in the reactor setup and reactant ratios see supplementary information.

6.2.8. Catalyst characterization

The physico-chemical properties of the catalysts were determined using various caracterization methods. A 3 ml quartz cuvette was used for the UV-vis monitoring of the formation of Pt nanoparticles. A spectrum of aqueous solution of PAMAM dendrimer was obtained followed by spectrum of Pt2+-PAMAM complex. After the addition of sodium borohydride the spectrum of the reduced contents was obtained. All spectroscopic data were obtained between λ = 200 and 800 nm on a Shimadzu 1800 spectrophotometer using de-ionized water as a blank.

Prior to p-XRD analysis, all samples were pulverized to fine powder. Thereafter, the samples were analyzed using a Rigaku MiniFlex 600 difractometer equipped with Cu Kα1 radiation source (λ = 0.154 nm). Wide angle measurements were carried out between 10 and 90 degrees 2θ with a scan rate of 0.5°/min. Low angle measurements were carried out between 0.5 and 10 degrees 2θ with a scan rate of 0.1°/min. Matching of the crystallographic information was done using Match! 2 software [26].

Transmission electron microscope images were obtained using a Jeol-Jem 2100F electron microscope operating at 200 kV. Prior to analysis, the catalysts were dispersed in ethanol and sonicated for 40 minutes, deposited on carbon coated Cu-grids and dried at room temperature. Determination of particle size distribution and average diameter was done using ImageJ software [27].

Temperature programmed reduction results were obtained using a Micromeritics Autochem II chemisorption analyzer. Approximately 30 mg of catalyst was degassed at 90 °C for 12 hours and placed on top of deactivated glass wool. The temperature range for the analysis spans from 30 to 900 °C with a temperature ramp rate of 10°/min. The analysis was done with a gas mixture containing 10% hydrogen and 90% helium.

Nitrogen adsorption-desorption measurements were performed using a Micromeritics Tristar surface area and porosity analyzer. Between 20 to 30 mg of the catalysts were degassed at 90 °C for 12 hours prior to analysis and analyzed at -196 °C.

The percentage dispersion, and the specific surface area and crystallite size of the immobilized nanoparticles were determined by hydrogen chemisorption using a Micromeritics ASAP 2020 Surface Area and Porosity Analyzer. Thus, an appropriate amount of the catalyst was analyzed by hydrogen as adsorptive gas at temperature of 35 °C, with the equilibration interval of 20 seconds. H2 chemisorption analysis is perhaps one of the proper methods for determining the

surface areas and the crystallite sizes of the supported metal nanoparticles [28, 29].

Chemisorption method using organothiol (2-mercaptobenzimidazole, 2-MBI) as probe ligand was also used to determine the specific surface area and crystallite size of the platinum nanoparticles after immobilization [30]. The reaction was carried out at room temperature and atmospheric pressure by suspending an appropriate amount of the catalysts in 5 mL of various concentrations of organothiol solution. After stirring for 24 hours, the suspension was centrifuged and the photometric analysis was performed using UV-vis spectrophotometer. The amount of ligand adsorbed per gram of the nanoparticle was determined as the difference between the amount of ligand added and the amount of unadsorbed ligand. Langmuir–isotherm plot was used to determine the effective surface area per gram of the nanoparticle. Thus, the reciprocal of the slope of the Langmuir plot was multiplied by Avogadro’s number and the cross-section of the probe ligand (equation 1) [31].

𝐴_𝑚 = 1

𝑚× 𝑁𝐴 × 𝜎

(1)

where 𝐴𝑚 is the platinum active surface area per gram of particle, m is the slope of isotherm

plot, 𝑁_𝐴 is Avogadro’s number, and σ is the average surface area which each 2-MBI molecule occupies (3.56 nm-2). The average crystallite size, d, that is the diameter of the platinum nanoparticles can be calculated using equation 2 [31].

d = 𝐹𝑔 𝜌 × 𝐴_𝑚

(2)

6.3. Results

6.3.1. Catalyst characterization

During the synthesis of Pt40-DENs, a UV-vis spectrophotometer was used to monitor the

synthesis. The Pt2+-PAMAM dendrimer complex showed two ligand-metal charge transfer bands (LMCT) at λ = 201 and 246 nm as can be seen in Figure S6.1 in the supplementary information. Upon reduction with sodium borohydride there was a decrease in the intensities of the two LMCT bands signifying less coordination of the metal ions to the tertiary amine within the dendrimer scaffold. An increase in absorbance at higher wavelengths confirmed the formation of Pt nanoparticles encapsulated within the PAMAM dendrimer. This result is consistent to those reported in literature [32].

To confirm the crystilline phases and the order of crystallinity of the synthesized mesoporous metal oxides p-XRD analysis was performed. The low angle measurements revealed the highly crystalline nature of the synthesized mesoporous oxides and also confirmed that the materials were templated by a highly ordered KIT-6, Figure 6.1(a). Peaks between 0 and 2° 2θ range are indicative of a highly ordered materials with long range order. High angle measurements, Figure 6.1(b), suggest Co3O4, MnO2 and CeO2 forms with JCPDS, 96-900-3477 and 96-900-

Figure 6.1. Powder-XRD patterns for templating KIT-6 and the corresponding metal oxide replicas in (a) low angle pattern and (b) high angle pattern. (c) Type IV hysteresis loops indicating mesoporous nature of KIT-6 and the corresponding mesoporous metal oxide replicas. (d) Cumulative pore volume distribution showing the average pore size of KIT-6 and the corresponding mesoporous metal oxides

The presence of type IV hysteresis loops shown in Figure 6.1(c) suggest the mesoporous nature of these materials. The presence of randomly located disordered micropores in the 3-D structure of the templating KIT-6 results in the appearance of two different pore sizes from the pore distribution plots due to the difficulty of filling the different sized pores [15]. To obtain the actual average pore diameter cumulative analysis was used. The average pore size was calculated from the cumulative pore volume plots. This kind of analysis takes into account 50% of nitrogen mass absorbed and it is correlated to the average pore size. Upon immobilization of Pt nanoparticles the surface properties were slightly altered. The BET surface area decreased for all the metal oxides. The pore volumes decreased signifying occupation of the pores by Pt nanoparticles. Furthermore, the pore diameters of all the mesoporous metal oxides increased upon immobilization of Pt nanoparticles. Table 6.1 shows the surface properties of the

templating KIT-6 and the corresponding metal oxides and Figure 6.1(d) shows the cumulative pore size distribution curves. The nitrogen sorption measurements for immobilized Pt nanoparticle are also summarized in Table 6.1 and the corresponding plots can be found in the supplimentary information.

Table 6.1: Surface properties of the templating KIT-6 and corresponding mesoporous metal oxides.

Entry SBET (m2/g) Vpore (cm3/g) Dpore (nm)

KIT-6 440 0.14 4 - - - CeO2 130 0.41 10 Pt/CeO2 108 0.27 11 Co3O4 80 0.21 8 Pt/ Co3O4 22 0.09 17 MnO2 18 0.14 31 Pt/MnO2 10 0.09 34

The synthesized mesoporous metal oxides show structures that resemble the porous nature of the template, KIT-6. The interpenetrating bicontinuous structures of the mesoporous replicas is a feature confirming that these materials were templated inside the channels of KIT-6. TEM micrographs in Figure S6.2 in the supplementary information reveal these interpenetrating bicontinuous structures and the channels in-between them that represent pores.

The use of the dendrimer as a template and stabilizer led to the formation of well despersed, spherical nanoparticles with a narrow size distribution. The average size was found to be 3.5 ± 0.4 nm. After immobilization of well despersed and uniform Pt nanoparticles, mesoporous metal oxides retain their structures and nanoparticles can be seen both on the surface and inside the pores of the mesoporous metal oxides. Due to decreasing electron transparency of the support, nanoparticles suffer from low contrast, thus, observing and measuring nanoparticles after immobilization was difficult. The particle size was then determined using H2

chemisorption and organothiol adsorption techiques. Figure 6.2(a) shows Pt40-DENs with

oxide and Pt nanoparticles immobilized on the synthesized mesoporous cobalt in Figure 6.2(c- e), repectively.

Figure 6.2. TEM image of (a) Pt40-DENs and (b) particle size distribution. TEM images of (c) KIT-6, (d) cobalt oxide and (e) Pt nanoparticles immobilized on the coresponding cobalt oxides.

The mesoporous metal oxides were further characterized by XPS and the results obtained are in agreement with those obtained from powder XRD in the sense that the XPS analysis proved that indeed the Co3O4 is a spinel structure. The presence of both Co2+ and Co3+ cations were

seen from the peaks in the region with binding energies of 770 to 800 eV (Figure 6.3a). The XPS plots for other materials can be seen in Figure S6.6 in the supplementary information. Important to note is the presence of Pt on the surface of mesoporous metal oxides after immobilization. However, the XPS analysis showed that the Pt nanoparticles are oxidized to PtO2 nanoparticles during the immobilization process. The oxidation of Pt to PtO2

nanoparticles is most likely to take place during the removal of the dendrimer as high temperatures are required for calcination in air. Analysis by XPS showed the presence of Pt4+ cation as Pt 4f electrons can be observed. Figure 6.3b shows XPS peaks in the binding energy region between 68 and 79 eV corresponding to Pt4+ electrons.

Figure 6.3. XPS plots a) showing the Co 2p electrons and b) showing the Pt 4f electrons.

The active surface area and size of the as-prepared nanoparticles after immobilization on the supports was evaluated using H2 chemisorption and organothiol adsorption techniques. Figure

6.4(a) shows the isotherm for H2 chemisorption for PtO2/Co3O4 as revealed by H2

chemisorption analysis. The surface areas per gram of metal of the immobilized platinum nanoparticles on Co3O4 and MnO2 mesoporous supports were revealed to be 58.2 m2/g and

42.5 m2_{/g respectively; with the particle sizes of 4.8 nm for PtO}

2/Co3O4 catalyst and 6.6 nm

for PtO2/MnO2 catalyst. The catalyst with larger surface area was characterized by a relatively

small particle size. The platinum nanoparticles were rather homogeneously dispersed over the supports, with the metal dispersion of 23.5% and 17.2% for PtO2/Co3O4 and PtO2/MnO2

catalysts, respectively.

The Adsorption isotherm of the organothiol on Pt/Co3O4 and the Langmuir isotherm of the

adsorption of the ligand on PtO2/Co3O4 are shown in Figure 6.4(b). The adsorption isotherm

has a curved shape, whereas a straight line with a slope of 3299 was obtained for the Langmuir isotherm. The surface areas of the platinum nanoparticles were revealed to be 65.0 m2/g of metal, and the platinum particle sizes that are assumed nearly spherical and mono-dispersed were found to be 4.3 nm in diameter. From PtO2/Co3O4 catalyst, a good agreement was found

between platinum surface area and particle sizes obtained from H2 chemisorption and thiol

Figure 6.4. (a) Isotherm for H2 chemisorption on PtO2/Co3O4, and (b) Adsorption isotherm of

2-MBI on PtO2/Co3O4 and Langmuir isotherm of the adsorption of 2-MBI on PtO2/Co3O4.

The specific surface area is the essential information acquired from the chemisorption methods; nevertheless, it is worth comparing the obtained immobilized particle sizes with those obtained by TEM before immobilization. The sizes of the platinum nanoparticles before immobilization as determined from TEM (3.5 nm) do differ significantly with those determined by chemisorption methods after immobilization for PtO2/Co3O4 catalyst (average 4.6 nm). This

signifies instability of Pt nanoparticles, which undergo oxidation to PtO2 nanoparticles. The

same applies to MnO2-containing catalyst, the sizes of the platinum particles increase

moderately after immobilization (6.6 nm).

The hydrogen temperature-programmed reduction of the spinel cobalt oxide showed the reduction peak of the easily reduced Co3+ ions followed by the reduction of Co2+ ions. All the mesoporous metal oxides appeared to be reduced at temperatures above 300 °C. The results obtained are similar to those obtained by Chen et al [33]. Figure 6.5 shows reduction temperatures of pure and PtO2-loaded mesoporous cerium oxides with corresponding shifts in

temperatures upon immobilization and Figure S6.3 in the supplementary information shows H2-TPR plots for the remaining pure and PtO2 loaded mesoporous metal oxides.

Figure 6.5. Reduction temperatures of pure CeO2 (black line) and PtO2/CeO2 (red line) with

corresponding shifts in temperatures upon immobilization of PtO2 nanoparticles.

Table 6.2: Reduction temperatures of empty mesoporous metal oxides and PtO2-loaded

mesoporous metal oxides.

MMO PtO2/MMO

1st 2nd 1st 2nd

𝐶𝑒𝑂2 520 700 334 667

𝐶𝑜3𝑂4 375 620 328 577

𝑀𝑛𝑂₂ 360 390 312 393

The reduction temperatures shifted to lower temperatures upon immobilization of PtO2

nanoparticles as shown in Table 6.2. This suggests an altered electronic structure upon immobilization of PtO2 nanoparticles. In the presence of PtO2, H atoms are likely to be

provided easily and the reduction would occur at lower temperatures as compared to the samples without PtO2. Shifting to lower temperatures of the reduction peaks suggest an

increased oxidation capability as catalysts for these materials. The reaction equations for the reduction of the mesoporous metal oxides can be written as in equations 3-5:

𝐶𝑜3+→ 𝐶𝑜2+→ 𝐶𝑜0 (3)

𝑀𝑛4+→ 𝑀𝑛3+→ 𝑀𝑛2+ (4)

6.3.2. Catalytic evaluation

Evaluation of the catalytic activity of the synthesized mesoporous metal oxides was performed at different temperatures. The inert support, SiO2, did not show activity at the temperature range

studied. Amongst the mesoporous metal oxides, Co3O4 showed higher activity than CeO2 and

MnO2. For all three mesoporous metal oxides, the activities increased with increasing

temperature, Figure 6.6(a). Although the ethanol conversions were quite low, see Table S1 in the supplementary information, all these mesoporous metal oxides were 100% selective towards acetaldehyde. However, immobilization of PtO2 on the mesoporous metal oxides

affected negatively on the acetaldehyde selectivity as acetic acid was formed as one of the products. The activity of PtO2 nanoparticles obtained by immobilization on inert SiO2 is

approximately 2500 fold than those of pure mesoporous metal oxides, Figure 6.6(b). This indicates that mesoporous metal oxides are poor catalyst when used in their pure form.

Figure 6.6. Catalytic activity of (a) the synthesized mesoporous metal oxides at different temperatures and constant pressure, 3 bar and (b) Activity of PtO2 nanoparticles immobilized

on SiO2 (black bars) at constant temperature and pressure and the contribution of MMOs and

the synergy between MMO and PtO2 nanoparticles (red bars).

Temperature studies conducted with the use of PtO2 nanoparticles immobilized on mesoporous

metal oxides indicate increasing activity with increasing temperature. Although relatively low conversions were obtained at low temperatures, see Table S6.2 in supplementary information, reasonably high percentage of the product was the desired acetaldehyde with very small percentage of acetic acid. However, as the temperature increased both acetaldehyde and acetic acid amounts decreased and formation of other products, notably carbon dioxide (CO2) was

evident and the percentage conversion of ethanol increased with temperature. Figure 6.7 shows conversions and product selectivity at constant pressure and different temperatures.

Figure 6.7. Effect of temperature on conversion of ethanol and product selectivity at constant pressure catalyzed by (a) PtO2/CeO2 and (b) PtO2/MnO2 as catalysts.

In an attempt to curb the formation of carbon dioxide and to increase selectivity towards the desired acetaldehyde, an ionic liquid layer was introduced on the solid PtO2 nanoparticles

containing catalysts. Although the selectivity towards acetaldehyde increased after the introduction of the ionic liquid layer, the amount of ethanol converted decreased. However, the activity of the SCILL catalysts increased with increasing temperature, see Table S6.3 in the

supplementary information. The amount of 1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)amide, [EMIm][NTf2], in the SCILL catalyst plays a role in

determining the activity and selectivity of the catalyst. As the amount of [EMIm][NTf2]ionic

liquid was increased from 3% to 10% the catalysts achieved better selectivity towards acetaldehyde and conversions decreased significantly with increasing amount of [EMIm][NTf2]ionic liquid in the catalytic system. One interesting factor to note with the

SCILL catalysts is the increased conversions with increase in residence time of the gas feed at constant flow rate. At temperatures as high as 160 °C acetaldehyde selectivity was more than 90% for both 3% [EMIm][NTf2]/PtO2/MnO2 and 3% [EMIm][NTf2]/PtO2/Co3O4. Figure 6.8

shows the effect of variation in amount of ionic liquid in the SCILL catalyst and effect of variation in pressure.

Figure 6.8. Influence of the [EMIm][NTf2] ionic liquid on (a) PtO2/MnO2 and (b) PtO2/Co3O4

activity and acetaldehyde selectivity at 3 bar and 120 °C. Pressure effect on the activity of (c) PtO2/MnO2 and (d) PtO2/Co3O4 at constant temperature and [EMIm][NTf2] loading.

6.4. Discussion

The nature of nanocasting experiments is known to produce highly ordered structures with well-defined shapes. The bicontinuous interconnected channels of the mesoporous replicas with well-defined pore structures and relatively high surface areas compared to conventional mesoporous oxides prepared by sol-gel method indicate the success of the nanocasting technique. Furthermore, the thick wall structure of the KIT-6, used as a nanocasting template, enabled the synthesis of large pore-sized mesoporous oxides. This then enabled the immobilization of Pt nanoparticles into the pores. Generation 4 of PAMAM dendrimer having a diameter of 4.5 nm can be easily incorporated inside the pores of the as-synthesized mesoporous metal oxides because all their pore diameters are greater than the diameter of the encapsulating PAMAM dendrimer. Prior to the removal of the PAMAM dendrimer the nanoparticles remain in their original state, that is, they remain encapsulated within the dendrimer and with average diameter of 3.5 nm. However, upon removal of the dendrimer

through calcination, the nanoparticles are susceptible to agglomeration because of the mobility on the surface of the support and confined space within the pores of the mesoporous metal oxides. In addition, the immobilized nanoparticles are susceptible to oxidation because calcination in this work was performed in air at temperatures as high as 550 ºC. Oxidation and agglomeration of the nanoparticles of nanoaprticles in the process of removing the templating dendrimer result in increased particle size. It is important to note that it was very difficult to determine the size of immobilized nanoparticles due to high contrast of the supports. The use of hydrogen chemisorption and organothiol adsorption techniques enabled the determination of the nanoparticle sizes after immobilization. Both sets of results indicated an increase in the