Cryochemical approach to develop catalysts for intensification of

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Kinetics, Catalysis, and Reaction Engineering

Cryochemical approach to develop catalysts for intensification of hydrodesulphurization reaction Biswajit Saha, Debdeep Bhattacharjee, Pavitra Sandilya, and Sonali Sengupta Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Industrial & Engineering Chemistry Research

Cryochemical approach to develop catalysts for intensification of hydrodesulphurization reaction Biswajit Saha1, Debdeep Bhattacharjee1, Pavitra Sandilya2, Sonali Sengupta*1

1Chemical 2Cryogenic

Engineering Department, Indian Institute of Technology Kharagpur,

Engineering Centre, Indian Institute of Technology Kharagpur, India-721302

*Corresponding

author. Tel: +91-3222-283954 Fax : +91-3222-282250

E mail : [email protected] (Prof. Sonali Sengupta)

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Industrial & Engineering Chemistry Research 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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Abstract In the present work, Ni-Mo/Al2O3 and Co-Mo/Al2O3 catalysts were synthesized both by conventional and cryochemical co-precipitation methods and their performances for hydrodesulfurization of a model fuel have been explored. The catalysts were characterized by BET, XRD, FESEM, TEM, AFM, TPD, XPS and FTIR. Analyses showed that catalysts prepared by cryochemical technique impart higher dispersion, surface area, and chemical homogeneity compared to the conventionally prepared ones. Hydrodesulphurization of a model fuel, comprising of dodecane with a definite concentration of dibenzothiophene as a sulfur compound, was carried out in batch mode using in situ generated hydrogen obtained by ethanol reforming. Experimental results showed that Ni-Mo/Al2O3 catalyst is superior to Co-Mo/Al2O3 for the reaction, and cryochemical catalyst performed better than conventional catalyst at all temperatures. The absence of any mass transfer resistance, external and internal is established, and the reaction is intrinsically kinetically controlled. The endothermicity of the reaction was proved by thermodynamic analysis.

Keywords: cryochemical catalyst; hydrodesulphurization; dibenzothiophene; in situ generated hydrogen, kinetics


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1. Introduction Hydrodesulfurization (HDS) is the usual practice in a refinery to remove sulfur from fuel in the presence of a catalyst. The main reason for sulfur removal is to reduce sulfur oxides emission into the environment, as well as to reduce the chance of catalyst poisoning in secondary processing stages in the refinery. The environmental regulations concerning the sulfur content in the diesel fuels are becoming stringent day by day. The maximum sulfur concentration in engine fuels has been restricted to 10 ppm in the USA in 2016 and 12 ppm in EU by 2020. According to the protocol BS IV stage Indian government restricted the maximum sulfur content in diesel fuel as 50 ppm , whereas, for BS VI, this limit has become 10 ppm maximum, which will be implemented nationwide by 2020. These restrictions demand the development of new generation catalysts which will meet the strict environmental regulations. The conventional HDS is often carried out under high temperature and pressure using external hydrogen as a desulfurizing agent. High temperature and pressure conditions are closely associated with the low reactivity of S-compounds with hydrogen molecules1. Hence, if the hydrogen atoms required for the HDS can be provided in situ by the reforming reaction of alcohol, relatively milder reaction temperature and pressure can be expected. In effect, although in situ hydrogenation has been studied before, yet little work has been reported for the HDS of fuels using in situ hydrogen formation. Liu and Flora for the first time stated the HDS of dibenzothiophene (DBT) using in situ generated hydrogen by water gas shift reaction along with external gaseous hydrogen in the presence of dispersed Mo catalysts2. The reforming of alcohol provides the necessary hydrogen required for HDS. The reforming process is highly temperature dependent and is favoured by higher temperature. Reforming



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reaction occurs at a temperature range of 500°–1000°C. On the other hand, HDS is a slow process occurring at a temperature range of 320°–420°C 3-5. As the reforming and hydrogenation reactions are taking place together in the present study, to favour both the reactions, the temperature is kept in the range of 350–400°C. The maximum DBT conversion at 380°C within this temperature range can be explained by the fact that at this temperature both the reactions take place at their best conditions 6-8. Cryochemical technology is a combination of low and high-temperature processes which can avoid undesirable changes in the system: low temperature processes include cryo-crystallization, freeze-drying, cryo-extraction, cryo-precipitation, cryo-impregnation, and cryo-grinding9. Cryochemical technology can be successfully employed in the preparation of catalysts. In conventional catalyst synthesis, the process begins with the mixing of an aqueous solution of salts; the ratio of salt concentrations corresponds to the stoichiometry of the product. The first stage of the cryo-synthesis process starts by atomizing the conventionally prepared salt solution to form spheroids under cryo-cooling. The common cooling agent is liquid nitrogen which is inert with the salt solution. In the second stage, the spheroids are contacted with either acid or base to form a precipitate at the desired pH at a sub-zero temperature, which after filtration is dried and calcined10 . A few studies have been carried out in the field of material synthesis through the cryo chemical process. Landsberg and Campbell first proposed the freeze-drying synthesis of metal powders11 and further development was done by Johnson12 and Gallagher13. After freeze-drying, the dehydrated salts were heated at various rates to complete oxide formation. Lopato reported the preparation of Al2O3-ZrO2-MgO system by cryo-chemical route and examined their characteristics by changing the sintering temperatures14. These scientists inferred that the oxide


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materials produced at different sintering temperatures have different polymorphic forms with wide variations in the specific surface area. Trusova prepared highly dispersed NiO on titanium silicate following cryo-chemical approach

15.

The aqueous alcoholic solution of Ni acetate was

sprayed into liquid nitrogen to prepare cryo-granules and then thermally annealed with titanium silicate. This composite was used as a catalyst in desulfurization of thiophene in hexane in a flow reactor. Parida prepared metal sulfated-zirconia by both conventional oven drying and freezedrying, and observed significant differences in textural properties of the catalysts prepared by those two methods

16.

Preparation of bimetallic nanoparticles with a uniform distribution of

silver and lead on methyl acrylate has been reported for alloy preparation by cryo-chemical route17. An USA patent describes the production of mixed metal oxides by cryochemical route 18. Vit studied the synthesis of ultra-dispersed inorganic materials, such as NH4NO3, KNO3, NaNO3, NH4ClO4, etc., used in the fabrication of functional ceramics by the cryo-chemical process19. In the present work, the authors explored the HDS of model fuel containing DBT on two different types of catalysts, Co-Mo/Al2O3 and Ni-Mo/Al2O3prepared by conventional (conv) and cryochemical (cryo) methods. The reaction was done using in situ generated hydrogen by ethanol reforming reaction in the complete absence of external hydrogen. It was reported in the literature that, HDS of DBT over Mo-alumina might occur through either of the two mechanistic pathways: Direct desulfurization (DDS) and hydrogenation (HYD) routes20-22. DDS is a one-step process which involves direct abstraction of S atom from DBT and formation

of

biphenyl

(BP)

whereas

HYD

route

produces

an

intermediate,

hexahydrodibenzothiophene by direct hydrogenation of phenyl ring of DBT followed by the elimination of S atom to form cyclohexylbenzene (CHB). It has been reported that HDS of DBT over Co-Mo/Al2O3 proceeds mainly via DDS route23, 24. By adding a certain amount of Ni in



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place of Co and presence of excess amount of H2, HDS of DBT can shift its route to HYD22, 25. It was observed that Co or Ni could provide active centers for the cleavage of the C-S bond in the DBT through their electron-donating effect, and consequently increase the activity of DDS 26, 27. Mechanisms of both the processes are shown in Scheme.1 (supporting information)

21.The

reaction stoichiometry of HDS of DBT for two pathways are shown below

C12 H 8 S ( DBT )  2 H 2  C12 H10 (Biphenyl)  H 2 S C12 H 8 S ( DBT )  5 H 2  C12 H16 (Cyclohexylbenzene)  H 2 S 2. Experimental Method 2.1 Catalyst preparation 2.1.1 Materials Chemical reagents used in the current work were of analytical grade and used without further purification. Aluminium nitrate nonahydrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, ammonium heptamolybdate tetrahydrate, ammonia solution and N-methyl-2pyrrolidone (NMP) were purchased from Merck, India. DBT was procured from Spectrochem Private Ltd., India. Liquid nitrogen was supplied by LN2 laboratory, Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur, India. 2.1.2 Preparation by conventional co-precipitation method An aqueous solution of aluminium nitrate nonahydrate of concentration 2000 g/l was prepared by continuous stirring. To impregnate Co or Ni (3, 5 and 8 wt% for each) and Mo (19 wt%) on Al2O3, aqueous solutions of cobalt nitrate or nickel nitrate and ammonium heptamolybdate of desired concentrations were added to the aluminium nitrate solution to form a homogeneous mixture of all salts. The pH of the solution mixture was adjusted at the range of 8-9 by adding 25% aqueous ammonia and the mixture was left for 24 h for precipitation. The precipitate was 6 ACS Paragon Plus Environment

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filtered, dried at 120°C and then calcined at 850°C for 8 hrs for cobalt catalyst and 550⁰C for 6 hrs for nickel catalyst 22.

2.1.3 Preparation by cryochemical co-precipitation method The homogeneous mixture of salt solution prepared as before (Section 2.1.2) was dropped to liquid N2 using a syringe pump (20 ml syringe, 1.4 µl/hr to 2200 ml/h,

Holmarc Opto-

Mechatronics (P) Ltd. India) with volumetric flow rate of solution 1.5 l/h to form cryo-granules under constant stirring. The droplets formed from needle are estimated to have volume about520µl. The cryo-granules so formed are contacted with ammonia solution to co-precipitate the mixed cation hydroxides at 0°C. The precipitate was filtered, dried and calcined as described in section 2.1.2. Figure S1 in supporting information depicts cryo process of preparation of catalyst; the cryo-granules of cobalt aluminium nitrate and nickel aluminium nitrate mixed solutions prepared in our laboratory are shown in Figure S2(a) and (b) in supporting document respectively. 2.2 Catalyst Characterization Catalysts were characterized by Scanning Electron Microscope (SEM) (Zeol and Zeiss with Oxford EDX detector), Brunauer-Emmett-Teller (BET) surface area analyzer (AS1 MP/ChemiLP, USA), X-ray Diffraction (XRD) (Panalytical 3050/60) with beryllium filtered Cu Kα(1.541 A) and operating at 40 KV and 30 mA, Transmission Electron Microscope (TEM) (Zeol JEM 2100), Atomic Force Microscope (AFM) (5500 Agilent Technologies, USA),Temperature Programmed Desorption (TPD) (Chembet-3000, USA), XPS machine is equipped with hemispherical electron analyzer with Al-X ray source (1486.6 ev, 350 W and 30 mA) with



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Carbon 1S signal 284.5 ev was used as internal energy references and Fourier Transform Infrared Spectroscopy (FTIR) (Perkin Elmer Spectrum 100).

2.3 Hydrodesulphurization of model fuel HDS of model fuel was performed in a 250 ml stainless steel rocking type batch autoclave reactor connected with an automatic digital temperature controlling unit. A typical experimental run used 100 ml reaction mixture, consisting of 90 ml of model fuel comprising of 1000 ppm DBT in dodecane and 10 ml water-ethanol mixture (6:4 v/v) with 0.5 g of catalyst. The in situ ethanol reforming reaction with HDS was studied using Co and Ni-based catalysts which are reported elsewhere 6, 28-32. The ethanol reforming reaction is shown below:

The reaction pressure at constant 380°C temperature was found to vary in the range of 35-40 Kg/cm2. Liquid samples of 1 ml at each time were collected from the autoclave at definite time intervals and were analyzed by high-performance liquid chromatography (HPLC) (Perkin Elmer, Series 200) with reversed phase Agilent SB C-18 column and a Perkin Elmer Series 200UV/VIS detector set at 254 nm. The mobile phase used was 90%methanol in water. The experiments were carried out at different temperatures (300°C to 400°C) on both catalysts prepared by conv and cryo co-precipitation to determine the best operating condition. The weight percentages of Ni and Co in the catalysts were varied from 3 to 10 to find out the effects of the promoters in the catalyst on reaction, while the weight percentage of Mo was kept constant at 19% throughout the experiment. 8 ACS Paragon Plus Environment

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2.4 Performance study on reuse of the spent catalyst To study the reusability, the spent catalysts were collected from the reactor, and regenerated in two different ways, to remove the reactants and products present both on the surface, and inside the pores of the catalyst before reuse33. In the first method, the spent catalyst was washed with N-methyl-2-pyrrolidone (NMP) for 1 h under continuous stirring followed by drying. In the second method, the spent catalysts were washed by NMP and then regenerated for 3 hrs at 600°C. 3. Results & Discussions 3.1 Catalyst characterization 3.1.1 BET surface area and pore volume analysis The textural properties of alumina support and catalysts (conv and cryo) were measured using N2-physisorption method. The data for BET surface area and pore volume are presented in Table 1. It is observed from the table that with an increase in the wt% of metal up to 8%, the surface area and pore volume of the catalyst increase and then decrease at 10 wt%. The highest surface area and pore volume were observed for cryo 8 wt% Ni-Mo/alumina catalyst among all the catalysts, which are 286 m2/g and 0.536 cc/g respectively. The increase in surface area and pore volume with increase in metal loading may be explained by the logic that, oxides of Co and Ni have their own surface area and pores. These contribute to the catalyst surface area and pore volume without blocking pores of the support or blocking only the micropores. On the other hand, the increase in metal loading more than 8 wt% might block the macropores to such an extent that total surface area decreases. For a given metal (Co or Ni) and wt% loading, higher surface area and pore volume were observed for cryo catalysts compared to its conv counterpart. This is due to the creation of more irregularities and roughness on the surface of the catalyst



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prepared by cryo method compared to that by the conv method which is established by the AFM image study. The nickel catalyst provides a higher surface area than cobalt catalyst (for a given metal wt% and the same method of preparation). The result exactly obeys the trend of the nature of Co and Ni conventional catalysts as described in the literature8, 34,35. Hence, all the subsequent experimental runs and characterizations were performed on this catalyst. Both types of the catalyst are prepared by co-precipitation technique. The conventional route is, however, characterized by continuous variation in the physicochemical conditions with time, leading to the inhomogeneity of the precipitate. Such inhomogeneity becomes more pronounced in case of multicomponent catalysts because the rate and extent of precipitation of the individual components differ considerably. Thus, while obtaining Co and Ni conv catalysts on alumina by hydroxide precipitation, one can expect a great difference in precipitation rates of components, because the solubility products of Ni(OH)2 and Co(OH)2 (2×10-15 and 5.92×10-15 respectively at 25°C) differ considerably from that of Al(OH)3 (1.9×10-33 at 25°C) at the same condition. From this point of view, lower temperature favours simultaneous precipitation of all the components by decreasing their solubility products, causing thereby more chemically homogeneous and dispersed catalyst system. The cryochemical method significantly increases the homogeneity of the micro-component distribution on the substrate surface during cryo-impregnation. Cryoprecipitation converts salts directly to hydroxides before calcination. Thus possible rehydration of salts before conversion to oxides is avoided36,37. 3.1.2 SEM analysis The SEM images Figure S3 (a) and (b) in the supporting document display the morphology of 8 wt% Ni-19%Mo/Al2O3catalysts prepared by conv and cryo ways respectively. The SEM images show that both types of catalysts (conv and cryo) acquire near homogeneous structure which


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follows the earlier report

38.

The uniform dispersion of Ni and Mo in both the catalysts can be

ascribed to the co precipitation method which leads to perfect mixing of Ni and Mo and formation of bimetallic oxides on alumina. From these figures, it is observed that the surfaces of both the catalysts are rough and amorphous but the rough parts are sharper for the cryo catalyst. SEM images could not provide enough information about the difference in surface morphology of the two types of catalysts. Hence, the surface morphology is further explained by AFM image analysis. 3.1.3 AFM analysis The topographic and phase contrast images of both conv and cryo Ni-Mo/Al2O3 catalysts obtained from AFM analysis are shown in Figure 1(a) to (d). The topographic images (Figure 1(a) and (b)) show columns of metal oxide grains, although large variations are observed in both the catalysts. The topographic image essentially shows a group of grains, appear in a bright color which is located at different heights to those grains that appear dark, and situated at lower positions. The average height difference between these two types of grains are observed to be 20 nm for conv catalyst, whereas, for cryocatalyst, this height difference between the grains is found to be 40 nm. The phase contrast image can reveal a distinctly different image than the topography. These images shows clear and well-defined details of the grains and their boundaries, texture with the superior microstructural content of the solid phase of the catalyst, which are normally obscured in topographic images39. Figure 1(c) and (d) depict the phase contrast images of conv and cryo 8 wt% Ni-Mo/Al2O3 catalysts. From the images it is clear that the grains are finer and better distributed throughout the matrix in cryocatalyst compared to the conv one. There is large difference between the phase contrast signals of two types of catalysts, which are 22.5º and 100º for conv and cryo catalysts respectively, which highlights the higher



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extent of roughness in cryo-catalyst. The RMS roughness factors obtained from AFM results are 2.77 and 3.65 nm for conv and cryo catalysts respectively. More roughness in cryocatalyst leads to enhanced surface irregularities, finer grains with higher grain boundary areas and these lead to the higher surface area of the cryo catalyst. 3.1.4 EDX analysis The elemental compositions of 8 wt% Ni conv and cryo catalysts were investigated using energy dispersive X-ray spectroscopy (EDX). The EDX results are tabulated in the supporting document as Table S1 for both conv and cryo catalysts. It is clear from the data that the weight percentages of nickel and molybdenum are nearly 8 and 19 respectively in both the prepared catalysts, which are in good agreement to the ratio of the concentration of the precursor salts taken for synthesis the catalyst. 3.1.5 TEM analysis Transmission electron microscopy was conducted to observe the morphology and particle size analysis of 8 wt% Ni conv and cryo catalysts. Figure S4 (a) and (b) in the supporting document display the TEM images of both catalysts. The results reveal that the catalysts are in the shape of fine needle-like nanograins, the grains of cryo catalyst are smaller than that of the conventional one, the size of the grains are compared in the scales of 50 and 20 nm for conv and cryo catalysts respectively. The average particle size of 8 wt% Ni impregnated alumina catalyst (conv) is 46.7 nm and particle size distribution curve of this catalyst is also shown in Figure S5. Agglomeration of nanoparticles is observed in conv catalysts. Method of catalyst preparation may determine the extent of agglomeration. The conventional method leads to more agglomeration of the nanoparticles resulting in low surface area, while cryochemical method gets its merit to produce the catalyst of lesser agglomeration and in turn, acquires higher surface area.


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3.1.6 XRD analysis The amorphous nature of both the catalysts is clear from their X-ray diffractogram shown in Figure 2 and Table (2a & 2b) compiles the details of the XRD results with JCPDS software. While analyzing the peaks, it is observed that aluminium molybdenum oxide and nickelmolybdenum oxide peaks are present in both the catalysts, but a peak of nickel aluminium oxide is additionally present in the cryo catalyst. This explains higher dispersion and chemical homogeneity in cryocatalyst compared to the conventional one. 3.1.7 TPD analysis The temperature programmed desorption (TPD) curves of 8 wt% Ni conv and cryo catalysts are represented in Figure 3. The acidity of nickel catalysts was characterized by TPD using NH3 as a probe molecule. In this method, the total acidity and acid strength were determined from the total peak area of TPD at different temperatures. NH3-TPD can differentiate sites of sorption strength only and cannot distinguish between Bronsted and Lewis acid sites40. The NH3-TPD curve of both 8 wt% Ni conv and cryo catalysts show peaks of similar nature, but more intense peaks are observed for cryocatalyst. The peak at 175⁰C corresponds to desorption of NH3 from weak and medium acid sites, while desorption peaks over 700°C could be associated to NH3 desorption from strong acid sites for both the catalysts41. It is noted that most of the NH3 desorption occurred below 300°C. The considerable increase in peak area for cryocatalyst indicates the higher acid density or strength compared to the conv catalyst. NH3 gas consumptions for cryo and conv catalysts are 12.82 and 10.79 mmol NH3/g of catalyst respectively. Higher acidity of cryocatalyst enables stronger attraction to the electron cloud of DBT than the conv catalyst.



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3.1.8 TPR analysis H2-TPR profiles of the both conv and cryo of 8 wt% Ni catalysts are presented in Figure 4. The TPR profiles of the catalysts are different. The cryo catalyst shows no distinct peak but a hump at 550⁰C which may be attributed to the reduction step of Ni2O3 to NiO. The reduction peak of Mo6+ to the polymeric octahedral Mo species at that temperature also contributed to the hump42,43. The peaks at 8000C and higher temperature are merged with the previous lowtemperature peak and are not that prominent. This may be attributed to the deep reduction of Mo species during the decomposition of the loaded compound and difficult reduction of tetrahedral Mo-species. The reduction of conv catalyst is shown by distinct peaks at 5500C, 8000C and 9750C. In the first step, Ni2O3 was reduced to NiO at 550⁰C and the second step involved in the reduction of NiO to Ni at 800⁰C. The peak appearing at 975⁰C was attributed to the reduction of vary small metal particles and mixed metal-support oxides

44-46.

The addition of Ni to alumina

promoted the reduction of Mo species. 3.2 Effect of metal weight loading on conversion Reactions with bimetallic catalysts at 380°C using Ni (3, 5, 8 and 10 wt%) with 19 wt% Mo/alumina and Co (3, 5, and 8 wt%) with 19 wt% Mo/alumina prepared by both conventional and cryochemical ways, were done keeping all other parameters constant. Effects of variation of loading of Co and Ni in the catalyst, prepared by both the methods are presented in Figure 5 and 6 respectively. Figures show that conversion of DBT increased with increase in metal loading up to 8 wt% for both types of catalysts 7. For Ni catalyst, 10 wt% showed lower conversion (80%) than 8 wt% (86%) which may be due to its lower surface area and pore volume as shown in Table 1. Moreover, for same metal and fixed wt%, cryo catalyst 14 ACS Paragon Plus Environment

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showed higher conversion than conv catalyst. The conventionally prepared Co-Mo/Al2O3 with 8 wt% Co was found to give DBT conversion of 40% at 380°C and 240 min reaction time, while the same catalyst prepared by cryochemical method showed a conversion more than 50%. The same is true for Ni-Mo/Al2O3 catalysts and the highest conversion (86%) was achieved by 8 wt% Ni cryocatalyst among all the catalysts tested. 3.3 Test of catalytic activity of Mo/Al2O3 To explore the role of Mo on HDS, reactions with conv and cryo19 wt% Mo/Al2O3 were done at 380⁰C keeping all other parameters constant. Increased conversion for cryo catalyst was established by Figure S6 of the supporting document, producing conversion results of 18 and 28% for conv and cryo catalysts respectively. These conversions are much less compared to that by Co- or Ni-Mo on alumina catalysts. The effect of Ni or Co on the activity of the catalyst is thus well established. Co or Ni acts as catalyst promoter which enhances the catalytic activity. 3.4 Operating temperature variation The operating temperature of HDS was varied from 300°C to 400°C to find out its effect on the reaction. Conversions for Co and Ni catalysts versus temperature are shown in Figure 7 and 8 respectively. Conversion increased gradually as the temperature increases from 300⁰C to 380⁰C and then decreased above 380°C. The decrease in conversion beyond 380°C may be explained by deactivation due to pore blockage or sintering of catalyst. Hence, 380°C can be considered as the optimum temperature keeping other parameters constant. The interesting result observed is that the conversion obtained by conv 8wt% Ni is lesser than that obtained using cryo 3 wt% Ni catalyst. Almost similar result is found for Co catalyst too. The type of dependence on temperature is well supported by the endothermic nature of the HDS reaction as reported



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elsewhere47-50. In the studies reported in the literature, the experiments were carried out in a temperature range of 350°-360°C with the use of external hydrogen gas for HDS reaction.

3.5 Effect of variation of DBT concentration in the feed The effect of DBT concentration in the feed on conversion using 8 wt% Ni-Mo/alumina catalysts (both cryo and conv) is depicted in Figure 9. In the array of results, it is found that a conversion of 94 % is achieved by the cryocatalyst for the feed of 500 ppm DBT. It is the greatest extent of conversion among all the experiments in this study. The lowest conversion of 68% is obtained for the feed of 1500 ppm DBT concentration using the conventional catalyst. Moreover, the conversion of DBT (89%) with 500 ppm concentration using conv Ni catalyst is found to be almost the same as that for DBT (86%) of 1000 ppm with cryo Ni catalyst. It is clear that the conversion increases with the decrease in DBT concentration of the feed for the same type of catalyst keeping other parameters constant. These observations unambiguously established the superiority of the cryo catalyst over the conventional one. 3.6 Role of ethanol reforming on HDS reaction HDS reaction was carried out without ethanol reforming to find out the role of in situ hydrogen which is one of the reactants in HDS. The conversion results of HDS of DBT for 8 wt% Ni conv and cryo catalysts without ethanol reforming reaction is elucidated in the Figure S7 of supporting document. The conversions are found to be almost nil even after 4 h. This result reflects that ethanol reforming reaction works as the only source of hydrogen for HDS.


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3.7 Turnover Frequency (TOF) of Ni catalysts The turnover number (TON) and the turnover frequency (TOF) are two important parameters for a catalytic reaction.TON and TOF are calculated for both conv and cryo 8 wt% Ni catalysts51. TON of Ni conv and cryo 8 wt% catalysts are calculated to be 3.87×1020 and 4.48×1020 molecules/g respectively. The TOF of the same catalysts are found as 2.68×1016 and 3.11×1016 molecules/g/sec respectively. The TOF is the reflection of catalytic efficiency and here the activity of cryo Ni 8 wt% catalyst is found to be higher compared to that of conv Ni 8 wt% catalyst. Ni dispersion (%) can be calculated using the Eq (1)52 r

Dispersion (%) = n .TOF tot

(1)

Where r is the rate of hydrogenation (molecules converted per second), ntot is the g of Ni in the reactor. “r” value is obtained from TPR data. According to the equation, the Ni dispersion (%) for Ni 8 wt% conv and cryo catalysts is found to be 11.6 and 12.5 respectively. Hence, Ni is more dispersed on the matrix of cryo catalyst than conv. 3.8 Effect of mass transfer resistance on reaction rate The reaction was done in a rocking autoclave where the rocking speed could not be varied to find out the optimum speed at which the reaction can overcome the external mass transfer resistance. Moreover, it cannot be definitely said whether the internal diffusional resistance has any influence on the reaction kinetics. Hence, in this aim, the following calculations are made based on experimental data to find out the diffusional effect, if any, is there to affect the rate kinetics.



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3.8.1 Effect of external mass transfer resistance To study the effect of external mass transfer resistance on reaction rate, gas phase diffusivity coefficient value, DAB was calculated from equation 2 which is known as Fuller Schettler and Giddings correlation 53.

3

10  T DAB 

1.75

1 1 [  ] MA MB

1 2

1 3 2

1 3

P[( VA )  ( VB ) ]

...........(2)

Where, DAB is the diffusivity of DBT (A) in hydrogen (B), T is 6530K, MA is 184.25 g/mol, MB is 2.01 g/mol, P is 28.8 atm, VA is 230.84 cm3/g-mol and VB is 3.96 cm3/g-mol. Putting all the values in equation 2, DAB is calculated to be 0.034 cm2/sec. To determine the solid-gas phase mass transfer coefficient𝑘𝑠𝑔, equation 3 (Frossling correlation) is recalled. 1

1

Sh  2  0.6  Re 2  Sc 3 ........(3)

Considering stagnant reactant fluid, the values of Reynold’s number (Re) and Schmidt number (Sc) number are considered negligible 54,55. Hence, Sherwood number (Sh) is considered as 2. As a result, the equation 3 is reduced to equation 4,

Sh 

k sg  d p DAB

 2..........(4)

If 𝑑𝑝=114.48×10-4 cm and 𝐷𝐴𝐵=0.034 cm2/s, then 𝑘𝑠𝑔 was found as 5.93 cm/s. The value of 𝑎𝑝 was calculated using equation 5


Page 19 of 45 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


ap 

6 w ........(5) p dp

Putting the values of 𝑤=5.5×10-3 g/cm3and 𝜌𝑝=2.98 g/cm3, the calculated 𝑎𝑝is 0.96 cm2/cm3.

Now, 𝑘𝑠𝑔 × 𝑎𝑝 × 𝐶𝐴0= 1.779×10-4mol/cm3.s

The initial rate ( ― 𝑟𝐴)𝑖𝑛𝑡was calculated from experimental data as 5.20 ×10-9mol/cm3.s

It has been observed that 1 1 ≫ ( ― 𝑟𝐴)𝑖𝑛𝑡 𝑘𝑠𝑔 × 𝑎𝑝 × 𝐶𝐴0 as, 1.9×108>>5621.13

The above inequality proved the absence of solid gas external mass transfer resistance

55

for the

reaction. 3.8.2 Effect of internal mass transfer resistance Determination of Wiesz-Prater criterion (𝐶𝑊𝑃)55,56 to find out the effect of internal mass transfer resistance on reaction rate is important. Equation 6 represents the mathematical form of this criterion, where the effective diffusivity (De) can be determined by using equation 7.

CWP 

(rA )obs   p  R p 2 De  C A

.......(6)



De 

DAB     C



Page 20 of 45

........(7)

Now, considering only the bulk diffusion and neglecting Knudsen diffusion, and taking the values of porosity as 0.4, tortuosity as 3 and constriction factor 0.8 55,56, the effective diffusivity (De) is calculated as 3.62×10-3 cm2/sec.

Putting the value of (-rA)obs= 1.9×10-9mol/cm3.s, obtained from experimental data, in equation 6, the Wiesz-Prater criterion was found to be 1.49×10-5, which was much smaller compared to 1 (CWP

Cryochemical approach to develop catalysts for intensification of

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