Section CVapor phase hydrogenolysis of glycerol to propane diols by using promoted tungsten platinum supported TiP catalysts3.1. Introduction:The hydrogenolysis of glycerol has been studied by several researchers in the presence of various heterogeneous catalysts such as Cu based 1-2, Raney Ni 3 and noble metal (Pt, Ru, Rh, Iretc) catalysts.4-7 Among the noble metals, Pt based catalysts presented high catalytic ctivity and thermal stability towards glycerol hydrogenolysis 8-9.
Moreover, several researchers have employed Pt based catalysts for selective production of 1,3-PD but their yields are still not satisfying.10-11 The modification of Pt catalysts is indeed necessary to further improve the catalytic performance in the glycerol hydrogenolysis. Chary et al. 12 reported the selective hydrogenation of glycerol to 1,3-propane diol on Pt-10WO3/SBA-15 catalyst and attain utmost selectivity (42%) to 1,3-PDO at 2 wt% platinum loading.In this section of the chapter vapor phase hydrogenolysis of glycerol to 1,3-propane diols (1,3-PD) with various loadings of 2Pt-XWO3/TiP catalyst with WO3 content ranging from 5-20wt% and 2pt wt% of pt in gas phase under atmosphere pressure. The catalysts have prepared in sequential method or Co.
impregnation were steadily characterized by X-rd, SEM, Py-FTIR, NH3-TPD, CO-Chemisorptions and surface area measurements.Concomitantly, the effect of reaction parameters such as catalyst loading, feed flow rate, reaction temperature, H2 pressure and reaction time have studied for the examination of the optimized reaction conditions. More specifically, the significant synergistic effect of Pt and W on increasing the activity of the catalysts during glycerol hydrogenolysis has been discovered. The catalysts exhibited unprecedented activity for selective formation of 1,3-propanediol via hydrogenolysis of glycerol.
3.2. Experimental Section:3.2.1 Catalyst preparation:3.
2.1.1 Preparation of TiP:In a distinctive hydrothermal process 13, Titanium phosphate (TiP) was synthesized by using a mixture of n-butanol and titanium n-butoxide (TBT) to which 30 mL of 0.1 M phosphoric acid solution was added drop wise and kept under stirring for 2 h at room temperature. The mixture obtained was aged for 24 h at 80 °C in a teflon-lined autoclave. The product was then filtered, washed with water, dried at 60 °C for 12 h and finally calcined at 500 °C for 2 h.3.2.
1.2 Synthesis of Pt-WO3/TiP catalyst:The Pt precursor and W precursor of the catalysts were the chloroplatinic acid hexahydrate (H2PtCl6 .6H2O) (analytical grade, produced by Sigma Aldrich Co., Ltd) and ammonium metatungstate hydrate ((NH4)6 (H2W12O40) H2O), in that order. The catalysts were prepared by co-impregnation on the TiP. For the catalysts prepared by co-impregnation method, TiP was first impregnated with W precursor salt solution, followed by the introduction of Pt precursor salt solution.
For the catalysts prepared by co-impregnation method, a solution containing a mixture of Pt and W precursors salts was impregnated on TiP. The resulting solids were dried at 110 oC and calcined at 500 oC for 3 h in air after each impregnation step. The amounts of Pt (2 wt%) and WO3 (5-20 wt%) , respectively. The prepared catalysts were selected as XPt-YWO3/TiP where X for Pt loading and Y refers to WO3 loading.3.3 Results and Discussion:3.3.1 Characterization of catalyst:3.
3.1.1 X-rd Analysis: Figure. 3.1: XRD patterns of the Pure TiP, 10-WO3/TiP and 2Pt-XWO3/TiP catalystsa) Pure TiP b) WO3/TiP c) 2Pt-5W/TiP d) 2Pt-10W/TiP e) 2Pt-15W/TiP f) 2Pt-20W/TiPFigure 3.
1 exhibits the X-ray diffraction studies of pure TiP, WO3/TiP catalysts and of 2Pt-XWO3/TiP catalysts which are with various WO3 loadings ranging 5-20 wt %. The X-rd results suggest that the broad peak in the range 15-38o is due to their amorphous nature (14). The amorphous nature of the titanium phosphate is consistent with the results reported by Bhaumik et al. 15. In Figure 3.
1, the 2Pt-XWO3/TiP catalysts with WO3 loading below 10 wt%, show no peak indicating that the WO3 is present in a well dispersed amorphous state and the samples above 10 wt% loadings have shown crystalline peaks of WO3. The additional peaks observed for various WO3 loadings of 10 wt % and above, in the 2? region 23.08°, 23.58°, 24.
26°, 26.54°, 28.75° and 33.28° are characteristic of monoclinic WO3 16-17. XRD patterns have shown three sharp diffraction peaks at around 39.
7°,46.3° and 67.3°, which can be ascribed to the (111), (200) and (220) lattice planes of Pt (Ref.pattern.
00-001-1311). The intensity of the peaks in platinum loaded catalysts is not changed which is a clear indication that platinum content (2 wt%) remained constant in all the samples. 22.214.171.124 SEM AnalysisSEM analysis of 2Pt-XW/TiP catalysts with various loadings of WO3 have been carried out to study the morphology and surface topology of the catalysts and the results are shown in Figure.
3.2. The SEM analysis of 2Pt/TiP material showed (Figure 3.2) micron sized uneven shaped agglomerates of many smaller particles and 2Pt-XW/TiP catalyst shows that agglomerates of the small granular type platinum particles.
However, interestingly, the addition of WO3 over the TiP samples affected the morphology of the parent samples.Figure 4.2: SEM images of Pure TiP and Pt-WO3/TiP 3.3.1.
3 Temperature programmed desorption of ammonia (NH3-TPD)The NH3-TPD and/or Pyridine are the best methods in use to assess the acidity of solid acid catalyst as well as the strength of acidic sites. The TPD- NH3 desorption profiles of all samples are shown in Figure 4.3. Generally, the ammonia TPD peaks can be classified into three different strength of acidic regions, these are weak acidic sites at 150- 300 oC, moderate acidic sites at 300-450 oC and strong acidic sites at above 450 oC, respectively 18.As results are shown in figure 4.3 NH3-TPD profile only two peaks are found all the samples.
The TPD profile of Pure TiP catalyst shows a weak acidic peak at 70-160 oC whereas Pt-WO3/TiP samples exhibited two peaks i.e., a weak acidic peak at 70-160 oC and a broad peak at strong acidic region 450-550 oC. The total acidity values are increased with the incorporation of WO3 on TiP support for lower loadings (5 and 10 wt%) catalysts whereas decreased for higher loadings (15 and 20 wt%) , this might be due to condensation of W-OH groups and formation of 3-D WOx clusters which reduce acidity at higher loadings 19. The acidity of the WO3/TiP catalysts is ascribed to the presence of P–OH groups and also well dispersion of WO3 particles on the surface are responsible for the acidity.
Ammonia TPD analysis suggested that the amount of acidity & strength of acidic sites are increasing to addition of WO3 to the TiP support.3.1.
4 Figure: Results of NH3-TPD of various Pt-WO3/TiP Catalyst 126.96.36.199 CO-Chemisorption:In order to estimate the dispersion of platinum on 2Pt-XWO3/TiP catalysts with various WO3 loadings (5-20 wt%), metal surface area and average particle size of platinum, CO chemisorptions method was employed and results are presented in table 3.1. The dispersion of platinum on XWO3/TiP catalyst with different WO3 loadings (5 to 20 wt%) and it was demonstrated that platinum dispersion was decreases from 11.37 to 8.
74% on XWO3/TiP catalyst. the results are shown table 3.1, 2Pt-XWO3/TiP catalysts with various WO3 loadings (5 to 20 wt%), these catalysts the metal surface area was found to be in the range of 0.
43-0.54 m2/g of catalyst whereas the particle size different between 10.2 to 12.9 nm. Therefore CO chemisorption results exhibit that 2Pt-10WO3/TiP catalyst notably displayed high Pt dispersion and small Pt particle size compared to other catalysts. These results were found to be in agreement with the findings of XRD studies.Table 3.
1: Results of Dispersion, CO uptake, metal area, and average particle size of Pt-WO3/TiP catalystsCatalyst Dispersion (%) CO Uptake(µmol/g) Metal surface area (m2/g)cat Particle size (nm)2Pt-5W/TiP11.37 10.9 0.54 10.202Pt-10W/TiP10.07 10.9 0.49 11.
252Pt-15W/TiP9.22 6.7 0.
32 12.292Pt-20W/TiP8.74 3.6 0.21 12.
9188.8.131.52 BET surface area and pore size distribution studies:Table 3.2: Physico chemical properties of Pt-WO3/TiP catalystsCatalyst BET surface area (m2/g) Total pore volume (cc/g) pore diameter( ?) bAcidity(µmol/g)Pure TiP252.0 0.
31 84.1 15710W-TiP 204.5 0.27 86.5 1752Pt/5W-TiP 180.5 0.25 96.3 2002Pt/10W-TiP 165.
2 0.20 120.8 2102Pt/15W-TiP 152.6 0.18 150.4 1952Pt/20W-TiP 130.
7 0.10 175.2 186bAcidity from TPD AnalysisThe textural properties such as surface area, total pore volume and pore diameter obtained for all the catalysts such as pure TiP, 10WO3/TiP and XPt-XWO3/TiP (optimal loadings 2 wt% Pt and 10 wt% WO3) from physisorption measurements, are presented the table 3.2. The surface area and pore volume of pure TiP are 251 m2/g and 0.
31 cc/g respectively , where as the metal impregnated TiP samples have shown their surface area in the range of 204-130.7 m2/g and pore volume in the range of 0.27?0.10 cm3 g?1. The results are demonstrated in table 3.
2, where it is observed that the impregnation of WO3 and platinum over TiP support decreases both the surface area and pore volume of the catalysts as compared to the pure TiP and it may be due to blockages of the pores of TiP support. However, the pore diameter increases with the active phase loading on the support. 3.1.
6 Py. Adsorbed FTIR studies:Pyridine adsorbed FT-IR analysis is a useful tool to examine the nature and amount of acidic sites. Generally, pyridine FTIR analysis affirms that the IR bands appear at 1540-1548 cm-1 are due to Bronsted acidity and bands at 1445-1460 cm-1 are due to Lewis acidity and the additional IR bands correspond to the combination of both Lewis and Bronsted acidic sites are appearing at 1490-1500 cm-1 20.Figure 4.4: Pyridine adsorbed FT-IR profile of pure TiP and Pt-XWO3/TiP catalysts.
a) Pure TiP b) WO3-TiP c) Pt-5W/TiP d) Pt-10W/TiP e) Pt-15W/TiP f) Pt-20W/TiPFigure 4.4 exhibits the pyridine adsorbed FT-IR spectra of pure TiP and Pt-XWO3/TiP catalysts in the region 1400-1600 cm-1. All the samples containing bronsted acidic sites appear at 1545 cm-1, combination of both lewis and bronsted acid sites appears at 1490 cm-1 and lewis acidic sites appear at 1445 cm-1 shown in figure 4.4.
It is observed that with the increase of WO3 loadings on the support, the intensity of IR adsorption peak corresponding to the lewis acidity decreases marginally. which is combination of both lewis and bronsted acidic sites increased with WO3 loadings up to 10 wt% and decreased slightly with further increase in WO3 loadings that is above 10 wt%. The Brønsted acidity of WO3/TiP catalysts increases originally then there is no great effect until 10 wt% WO3/TiP, furthermore, in 15 and 20 wt% WO3/TiP a small decreases.3.2 Catalytic Activity: 3.2.1 Effect of WO3 loading:The impact of WO3 loadings (5-20wt %) on TiP support amid gas phase hydrogenolysis of glycerol over 2Pt-XWO3/TiP catalyst and the results are given in table 3.
2. The results were clearly recommend that introducing of WO3 into TiP support is more active then compare to pure TiP support. Both conversion of glycerol from 63% to 85% and selectivity of 1,3-PDO from 42% to 51% are found to increases because of addition of WO3 loadings (5 and 10 wt %) on TiP support. But more increasing the WO3 loadings (15 and 20 wt %), both the conversion and selectivity decreases. The incorporation of WO3 into TiP catalysts shows greater selectivity to 1,3-PDO due to enlarge bronsted acidity attain through the dispersed acidic sites of incorporated WO3 over TiP support.The catalyst 2Pt-10WO3/TiP has demonstrated conversion and selectivity increases then other catalysts. It was found that synergistic effect amid Pt and W is various at WO3 loadings of 10wt% 2Pt/TiP catalyst on increasing the activity of the catalysts while glycerol hydrogenolysis.
Table 3.3: Effect of tungsten loading on hydrogenation of glycerol to 1, 3 PDL S. No W-Loadings Conversion (%) Selectivity (%)1,3-PD 1,2-PD HA PropanolsOthers1 2Pt/5W-TiP 63 42 18 15 20 52 2Pt/10W-TiP 85 51 14 11 16 63 2Pt/15W-TiP 81 46 16 9 24 54 2Pt/20W-TiP 69 37 19 15 22 7Reaction conditions: 0.5 g catalyst, 2Pt-XWO3/TiP catalyst (X = 5 to 20 wt%); Reaction temperature: 210 0;, H2 flow rate: 90 mL/min; WHSV-1.021 h-1;1,3-propanediol (1,3-PDO); 1,2-propanediol (1,2-PDO); Propanol (1-PrOH, 2-PrOH); Hydroxyacetone (HA).3.
2.2 Effect of Pt loading:The consequence of Pt loadings on hydrogenolysis of glycerol was studied over a series of XPt-10WO3/TiP catalysts with different platinum loading (0.5-3 wt%) in order to get and find the best performed catalyst. The results exhibited in table 3.
4 convey that the consequence of increased Pt loadings on conversion of glycerol from 60 to 85% and selectivity of 1,3-PDO is positive from 35 to 51% obtain at 2 wt% loadings. However, the glycerol conversion and selectivity to 1,3-PDO slightly decreased on 3 wt% Pt-10WO3/TiP catalyst which is likely due to agglomeration of Pt on TiP, well evident from the characterization results. The other hydrogenation reaction product 1,2-PDO formed in minor amounts and other products 1-Propanol, 2-propanols are formed in excesses of hydrogenation reaction.
Therefore, among the series of catalysts (0.5-3 wt% Pt-WO3/TiP), an optimal amount of platinum (2 wt%) on 10WO3/TiP catalyst was found to be the best catalyst giving maximum conversion and selectivity to 1,3-PDO. S. No Metalloadings Conversion % Selectivity %1,3 PDL 1,2 PDL HA PropanolsOthers1 0.
5Pt/10W-TiP 60 35 13 18 26 82 1Pt/10W-TiP 72 46 15 13 21 53 2Pt/10W-TiP 85 51 14 11 16 64 3Pt/10W-TiP 79 48 11 13 19 9Reaction conditions: 0.5 g catalyst, XPt-10WO3/ TiP catalyst (X = 0.5 to 3.0 wt%); Reaction temperature: 210 0C, H2 flow rate: 80 mL/min, WHSV-1.021 h-1, 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), Propanol (1-PrOH,2-PrOH), Hydroxyacetone (HA).3.
2.4 Effect of reaction temperature:The impact of the reaction temperature (210-250) on the glycerol conversion and selectivity of the products, reactions were carried out at 210, 220, 230, 240 and 250 oC respectively under atmospheric pressure over Pt-WO3/TiP catalysts (optimal loadings 2 wt% Pt and 10 wt% WO3) results are demonstrated in Figure 4.5. The results are shown in Figure 4.5, with the temperature increasing from 180-250 oC (21), there was a monotonically increases in the glycerol conversion from 69 to 90%. The highest 1,3-PDO selectivity of 51 % was obtained at reaction temperature 210 oC, However, the selectivity of 1,3-PDO decreased with the further increase in temperature.
A similar trend is observed in the selectivity of 1,2-PDO whereas a gradual increase in the selectivity of HA and propanols (1-PrOH,2-PrOH). At higher temperature (>210) formation of other byproducts are formed such as ethanol, methanol, ethylene glycol and acetone due to excessive C-C cleavage reactions 22.Figure 4.5: Effect of the reaction temperature on hydrogenolysis of glycerol to propanediolsReaction conditions: 0.5 g catalyst, Reaction temperature: 170, 190, 210, 230 and 250 0C, H2 flow rate: 90 mL/min, WHSV-1.
021h-1, 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), Propanol (1-PrOH,2-PrOH), Hydroxyacetone (HA).3.2.
5 Hydrogen flow rate:The Effect of hydrogen flow rate on glycerol hydrogenation over the Pt-WO3/TiP catalyst results are shown in Figure 4.6. Figure 4.6 demonstrated glycerol conversion and selectivity to products as glycerol is one of the reactants in the glycerol hydrogenolysis reactions. It can be that the glycerol conversion enhanced from 69% to 90% was obtained with increased the hydrogen flow rate from 40 mL/min to 120 mL/min. The selectivity of 1,3-PDO was increasing from 36 to 51% with increases in the hydrogen flow rate from 40 mL/min to 80 mL/min, furthermore increases of hydrogen flow rate above 80 mL/min with simultaneously decreases in the selectivity of 1,3-PDO.
A similar trend of hydrogen flow rate on glycerol conversion and selectivity was reported in previous studies 23. Figure 4.6: Effect of hydrogen flow rate on hydrogenolysis of glycerol to propanediolsReaction conditions: 0.5 g catalyst, Reaction temperature: 210 0C, H2 flow rate: 40, 60, 80,100 and 120 mL/min, WHSV-1.021 h-1, 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), Propanol (1-PrOH,2-PrOH), Hydroxyacetone (HA).
3.2.6 Effect of glycerol concentration:The influence of glycerol concentration on glycerol hydrogenolysis reaction over Pt-WO3/TiP catalyst at 210 oC was studied by using varying concentration of glycerol (5, 10, 15 and 20 wt%) over the optimized catalyst 2Pt-10WO3/TiP and the results are exhibited in Figure 4.7. As probably, the glycerol conversion decreased with increase in the amount of glycerol, this observation is obvious because glycerol is a highly viscous liquid. The results are shown on figure 4.
7, the conversion of glycerol increased from 68% to 85% and selectivity of 1,3-PDO increased from 35% to 51% was obtained with increased glycerol concentration from 5-10 wt%, furthermore increases of glycerol concentration from 10-20 wt% both conversion and selectivity of 1,3-PDO decreased with the formation of undesired byproducts. The highest selectivity of 1,3-PDO was achieved with 10 wt% glycerol feed. Miyazawa et al.24 Figure 4.7: Effect of WHSV on hydrogenolysis of glycerol to propanediosReaction conditions: 0.
5 g catalyst, Reaction temperature: 210 0C, H2 flow rate: 80 mL/min, WHSV-1.021, 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), Propanol (1-PrOH,2-PrOH), Hydroxyacetone (HA).3.2.3 Effect of weight hour space velocity (WHSV):The influence of weight hour space velocity (WHSV) on glycerol hydrogenolysis reaction over Pt-WO3/TiP (optimal loadings 2 wt% Pt and 10 wt% WO3) catalyst at 210 oC with the varying 10 wt% glycerol feed flow from 0.
5-2.0 mL/h (corresponding WHSV =1.02-4.08 h-1) results are demonstrated in figure 4.8.
The results are shown in figure 4.8 the glycerol conversion decreased from 85 to 73 % and selectivity of 1,3-PDO decreased from 51 to 35% was obtained with increasing WHSV from 1.021 to 4.084 h-1. In contrast, an increase in the selectivities of 1,2-PDO, HA and Propanols was observed with the increase of WHSV. Therefore, the result imply that a WHSV 1.02 h-1 might allow secondary hydroxyl group of glycerol to be activated and make the hydrogenolysis reaction more selective to 1,3-PD.
Figure 4.8: Reaction conditions: 0.5 g catalyst, Reaction temperature: 210 0C, H2 flow rate: 80 mL/min, WHSV-1.021, 2.042, 3.
063 and 4.084 h-1, 1,3-propanediol (1,3-PDO), 1,2-propanediol (1,2-PDO), Propanol (1-PrOH,2-PrOH), Hydroxyacetone (HA).3.2.7 Time on stream: The catalytic results during gas phase hydrogenolysis of glycerol over 2Pt-10WO3/TiP catalyst at 210 oC reaction and the results are shown in figure 4.
9. The glycerol conversion and selectivity of 1,3-PDO are gradually increases with increasing reaction time and reaches 85% and 51% after 4 hr of time (Figure 4.9).
Thereafter, the maximum conversion and selectivity constant up to 5 h and finally it decreases to 73% and 39% after 10 hr. This behavior of decrease in glycerol conversion and selectivity of 1,3-propandiol is attributed to the catalyst deactivation by coke formation as demonstrated by CHNS analysis (Table 3.4). The time on stream results suggest the catalyst is quite stable for a long period of time resulting in good catalytic activity during glycerol hydrogenolysis.Figure 4.
9: Time on stream on hydrogenolysis of glycerol to 1,3-PDOReaction conditions: 0.5 g catalyst, Reaction temperature: 210 0C, H2 flow rate: 80 mL/min, WHSV-1.0213.4 Spent catalyst:Catalyst Conversion (%) Selectivity (%)of 1,3-PD BET surface area (m2/g) Acidity(µmol/g ) CHNSAnalysis2Pt-10W/TiP (Fresh) 85 51 165.
2 210 —2Pt-10W/TiP (Used) 81 48 162.3 203 3.86The spent catalyst of 2Pt-10WO3/TiP was characterized by X-rd, SEM, NH3-TPD, BET surface area and the results are compared with those of fresh catalyst the results are shown in figure 3.10.
the used catalyst was first treated in air at 300 oC for 3 h followed by reduction in H2 flow at 300 oC for 2 h in order to consider the reusability of catalyst. The reaction was repeated with regenerated catalyst under the same reaction conditions. The used catalyst was characterized to understand the changes that the catalyst has undergone during glycerol hydrogenolysis reaction and the results are shown in Table 3.4. When this reaction performed over the spent catalyst at same reaction conditions, the glycerol conversion and selectivity of 1,3-PDO both are decreased from 85 to 83% and 51% to 48.6%, respectively. The results are exhibited in Table 3.4.
The results are exhibited in table 3.4 the acidity and BET surface area of the spent catalyst was decreased to campared to the fresh catalyst due agglomerisation of metal particles on the support evident from CHNS analysis. The SEM studies of the fresh and used catalysts shown in figure 3.10 (A), it was definite that there is no such particular structure differences, which suggested that the structure of this catalyst was stable. The XRD patterns shows that the spent catalyst intensity of peak increases compared to fresh catalyst. Figure 3.10 :(A) SEM images and (B) XRD patterns of fresh and used 2Pt-10WO3/TiP catalystCatalyst Nitrogen (%) Carbon (%) Hydrogen (%) Sulphur (%)2Pt/10W-AL (Calcined) — 0.08 0.
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