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Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene
Journal of Saudi Chemical Society (2017) xxx, xxx–xxx
King Saud University
Journal of Saudi Chemical Society www.ksu.edu.sa www.sciencedirect.com
ORIGINAL ARTICLE
Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene Nabil Al-zaqri a, Ali Alsalme a, Syed F. Adil a, Ahmad Alsaleh a, Saad G. Alshammari a, Saud I. Alresayes a, Raja Alotaibi b, Mohammed Al-Kinany b, Mohammed Rafiq H. Siddiqui a,* a b
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia
Received 25 March 2017; revised 18 April 2017; accepted 11 May 2017
KEYWORDS Heteropoly acids; Nickel; Cobalt; Hydrodesulfurization (HDS); Thiophene; Brønsted/Lewis acidity
Abstract Phosphomolybdic acid supported on silica (PMoA), and phosphomolybdic acid supported on silica substituted with nickel (Ni-PMoA), cobalt (Co-PMoA), are prepared with different loadings. All catalysts have been characterized using thermogravimetric analysis (TGA), BET surface area, Solid-state 31P NMR spectra, FT-IR, Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Powder X-ray diffraction analysis (XRD). The surface areas of the catalysts are slightly lower than the surface area of the silica used in making these catalysts, while the XRD studies indicated the amorphous nature of the prepared catalysts. All catalysts are tested for their activity in the hydrodesulfurization of thiophene in the temperature range of 300–500 °C. Under similar conditions the silica supported catalyst i.e. PMoA, showed lower hydrodesulfurization activity, compared to the nickel and cobalt substituted heteropoly acids, i.e. Ni-PMoA and Co-PMoA, indicating enhancement of catalytic activity with substitution of nickel and cobalt. Moreover, the nickel-substituted catalyst, Ni-PMoA, showed slightly higher activity than the cobalt-substituted ones, Co-PMoA. At 500 °C, Ni-PMoA gave a 99.6% HDS conversion of thiophene whereas the Co-PMoA yielded a 98.3%. HDS conversion of thiophene. Brønsted/Lewis acidity of the catalyst precursor appear to be essential for the catalytic hydrodesulfurization. Ó 2017 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author. E-mail address: [email protected] (M.R.H. Siddiqui). Peer review under responsibility of King Saud University.
Production and hosting by Elsevier
1. Introduction Sulfur dioxide (SO2) is a very serious environmental pollutant which causes a series of environmental and health issues. For instance, when it combines with water and air, it forms sulfuric acid, which is the main component of acid rain that leads to
http://dx.doi.org/10.1016/j.jscs.2017.05.004 1319-6103 Ó 2017 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
2 acidified soils, lakes, streams, accelerated corrosion of buildings and monuments, and also reduce visibility. High concentrations of SO2 in air may results in serious breathing problems, respiratory illnesses and various other health related ailments [1,2]. Particularly, sulfur based compounds that are largely found as impurities in various petroleum fractions are the major cause of these problems. Notably, among various impurities, the major refractive sulfur species in diesel are dibenzothiophenes and 4, 6-substitued dibenzothiophenes (DBTs) [3]. The presence of sulfur compounds in crude oil and heavy fractions is an undesirable issue. It can lead to corrosion in oils and lubricants and poisonous emissions such as SO2 and H2S when the fuel is burned. Several processes have been proposed to deal with the problem of removing these compounds [4]. Recently, awareness about environmental protection has increased and consequently legislation to limit sulfur contents in fuels has been strictly applied throughout the world [5]. Nowadays, high quality of petroleum products is required to reduce environmental problems induced by the SOx emissions to the environment. For this purpose, more active and selective catalysts for the hydro-treatment processes have been proposed. In order to reach ultra-low sulfur levels (<15 ppm), the development of a new generation of catalysts with very high hydrodesulfurization (HDS) activity is highly desirable. At present, the hydrodesulfurization (HDS) process is an efficient method for sulfur removal from gasoline and diesel of high sulfur content [6]. In this process organic sulfur compounds in the liquid fuels are broken down to H2S using catalysts [7], followed by the removal of H2S. Using this process the sulfur content in fuels can be reduced to about 10 ppm [8]. For this process several mixed transition metals and metal oxides based catalysts have been reportedly employed. For example, various HDS reactions are reported to be catalyzed by using CoMo [9], NiW [10], or the trimetallic NiMoW catalysts supported mainly on c-Al2O3 [11] and many other
N. Al-zaqri et al. supports containing the active acidic or basic sites such as SiO2 [12–15], ZrO2 [16], TiO2 [17], MgO [18]. In a recent study, molybdenum catalysts supported on alumina were investigated for the effect of nickel and phosphorus on activity of hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine [19]. The presence of nickel into molybdenum-containing catalysts strongly promoted HDS activity. In another study, a weak promoting effect of phosphorus was observed, in the case of NiMoAl2O3 catalyst modified with phosphorus [20]. Recently, the popularity of the heteropoly acids as catalysts for various organic transformations has grown [21–24], especially, the Keggin type heteropoly acids (HPAs) have been proved to be excellent catalysts for various reactions [25–28] due to their simple synthesis, relatively high thermal stability [29] and very strong acidity and redox properties [30]. Although, several new industrial processes based on HPA catalysis have been developed and commercialized in the last two decades [31]. One of the most important properties of heteropoly anions is that they can be isolated as solids when coupled with an appropriate counter cation, e.g. H+, NH+ 4 . Heteropoly acids (HPA) are the hydrogen (acidic) forms and are utilized primarily as catalysts, and is reflected in the statistic that 80–85% of patent applications concerning the polyoxometalates are related to catalysis [32]. Nevertheless, there are few scientific reports available on the application of HPAs with respect to the hydrodesulfurization reaction. In our earlier studies we have demonstrated the excellent ability of heteropoly acid to act as catalyst precursors in the hydrodesulfurization reactions, where these catalysts have performed better than the commercial catalysts [33]. With our continued interest in heteropoly acids and hydrodesulfurization, this study was carried out to gain further insight into the mechanism of supported heteropoly acids in the presence of nickel and cobalt (cf. Scheme 1). Additionally, the aim of this study is also to get an insight into the role of Brønsted and Lewis acidity and its relation to HDS activity Scheme 2.
Scheme 1 Graphical representation of the synthesis of PMoA, Ni-PMoA, Co-PMoA catalysts and their evaluation as hydrodesulfurization catalyst.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
Scheme 2 thiophene.
Reaction
pathway
of
hydrodesulfurization
of
2. Experimental 2.1. Materials and methods Phosphomolybdic acid hydrate H3Mo12PO4020H2O, Ni (NO3)26H2O, Co(NO3)26H2O and thiophene (99%) were from Sigma–Aldrich and Aerosil 300 silica support (SBET = 300 m2 g1) from Degussa. Catalysts were characterized by several techniques including, thermogravimetric analysis (TGA). Shimadzu TGA-50 Thermogravimetric Analyzer is used to perform the thermogravimetric experiments using a heating rate of either 10 °C per minute to raise the temperature from room temperature to 800 °C, under a nitrogen gas flow. Surface area and porosity analysis is carried out on a Micromeritics ASAP2010 system using nitrogen sorption at 77 K. The surface areas were calculated using a multipoint Brunauer–Emmett–Teller (BET) model. Solid-state 31P-NMR spectra were recorded at 9.4 T using a JEOL resonance ECX500 II. The chemical shift of the 31P resonances is in ppm using 85% H3PO4 as internal standard. Fourier transform infrared spectroscopy (FT-IR) studies were carried out in transmission mode using a Perkin Elmer precisely spectrum 100 (FT-IR) spectrometer. Pyridine Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy is used to examine the state of HPA on SiO2 as well as to determine the nature of acid sites (Brønsted and Lewis) by adsorption of pyridine. The measurements were performed using a Nicolet Model Nexus FTIR-Raman spectrometer at room temperature under N2 atmosphere to prevent interference by the gaseous environment in the chamber. Powder X-ray diffraction studies were carried out on a Rigaku Ultima (IV) using Co anticathode, i.e. CoKa radiation with k = 1.7889 A˚, 40 kV and 40 mA. 2.2. Catalyst preparation All the catalysts are prepared by following our earlier method [34]. 1. H3PMo12O40 (HPMo) supported on silica pre-catalyst, HPMo/SiO2, PMoA, with 15 wt% HPMo loading based on anhydrous HPMo is prepared by wet impregnation. The impregnation of SiO2 with H3PMo12O40 is carried out in aqueous slurry at 40–50 °C with continuous stirring for 24 h. Solid residue is isolated through rotary evaporation at 45 °C, which is dried at 150 °C under vacuum conditions and calcined at 300 °C under nitrogen flow for 3 h
3
with a heating rate of 5 °C min1. The pre-catalyst is then grounded into a powder with a particle size of 45–180 lm. 2. Silica-supported Ni(II) and Co(II) substituted phosphomolybdate pre-catalysts are prepared employing wet impregnation of SiO2 with HPMo and the required amounts of Ni(NO3)2 or Co(NO3)2 with Ni(II) or Co(II)/ HPMo molar ratio of 2:1 (i.e. Ni/Mo atomic ratios of 1:6; Co/Mo atomic ratios of 1:6), followed by isolation, drying and calcination at 300 °C as above. Hereafter, these pre-catalysts are designated as Ni-PMoA, Co-PMoA, respectively. All of the catalysts are calcined at 300 °C for 3 h under N2. The temperature of furnace is raised from room temperature to 300 °C a heating rate of 5 °C min1, with a N2 flow of 20 mL min1 and this temperature is maintained for three hours. 2.3. Catalysts testing The prepared catalysts H3PMo12O406H2O/SiO2, (PMoA), (Ni-PMoA), (Co-PMoA) are tested for the hydrodesulfurization activity of thiophene at different temperatures varying from 300 °C to 500 °C. Hydrodesulfurization of thiophene is carried out at 300– 500 °C using a conventional fixed bed flow reactor. Thiophene is introduced into the reactor by passing hydrogen (10 mL min1) through a thiophene with octane 250 ppm at room temperature. Reaction conditions are as follows: Catalyst weight = 0.6 g, H2 = 10 mL min1, flow octane with thiophene 0.2 mL min1. GC analysis are conducted on the (Varian Chrompack CP3800 instrument with a 30 m 0.320 mm, 0.25 mm film thickness capillary column and a flame ionization detector). Gas-phase catalyst testing is performed in a continuous flow fixed bed reactor. A stainless steel reactor 28 cm3 in length and 0.9 cm internal diameter is used to carry out the gas phase reaction at reaction temperatures up to 300 °C. After the reaction temperature in the furnace is attained, the desired H2 flow is set and then redirected through to the saturator set, using the auto-injector; online GC injections are then made every 36 min. The selectivity is defined as the percentage of thiophene converted into a particular product taking into account the reaction stoichiometry. The reactants and potential products of the reactions were calculated using the internal standard method. 3. Results and discussion 3.1. Thermogravimetric analysis (TGA) The prepared catalysts PMoA, Ni-PMoA, and Co-PMoA are subjected TGA studies to measure the amount of physisorbed water removed at temperatures up to 150 °C. Thermogravimetric analysis (TGA) of the (H3PMo12O4.6H2O) catalyst shown in Fig. 1 was performed in N2 on a fresh sample prior to catalyst pretreatment, two weight losses are observed: one at 100–180 °C, corresponding to the loss of water, and the other at 460–480 °C, corresponding to the decomposition of the Keggin structure accompanied by the evolution of water.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
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3.2. Surface area and porosity studies The physisorption of N2 at 77 K provided information on the porosity of the catalysts and their precursors. To determine the surface area and characterize the porous texture of a catalyst, the first stage is to generate the N2 adsorption isotherm, which is found by plotting the volume of N2 adsorbed against its relative pressure. The prepared samples are characterized by BET analysis in order to find out the surface area, pore volume, and pore diameter, which are tabulated in Table 1 below. The cobalt promoted catalyst is found to possess the highest surface area with 256 (m2/g). However, all the samples are found to possess surface area within the range 230–256 (m2/g) and there is no significant differences in the pore volume and pore diameter. 3.3. Infrared spectroscopy (FT-IR) The experimental procedure for the FT-IR measurements is described in previous work using infrared spectroscopy [34]. The majority of the characteristic infrared active bands of the Keggin anion can be found in the fingerprint region of the spectrum which lies between 1200 cm1 and 500 cm1. From the FT-IR spectrum of Ni-PMoA, it is noticed that it possess signals at 958.18 cm1, 808.41 cm1 respectively, which could be due to the stretching vibrations of mas (Mo– Ot), mas (Mo–Oc–Mo). Similarly, from the FT-IR spectrum of H3PMo12O40xH2O/SiO2 promoted with Co signals at 959.84 cm1 and 811.73 cm1 is attributed to the stretching vibrations of mas (Mo–Ot), mas (Mo–O–Mo) respectively. However, it appears to be slightly different from signals obtained for H3PMo12O40xH2O/SiO2 at 961, 870 and 783 cm1 which is assigned to the stretching vibrations mas (Mo-Ot), mas (MoOb-Mo) and mas (Mo-Oc-Mo), respectively [35]. The Mo-OMo bridges belong to the MoO6 octahedra and the subscripts b, c and t indicate corner-sharing, edge-sharing and terminal oxygen, respectively. These variations could be due to the interaction of the Ni and Co ions with the atoms present in the Keggin structure. Similar variations are observed in vibra-
Figure 1
Table 1 Structural parameters of the catalysts H3Mo12PO40 xH2O/SiO2 (PMoA) promoted with Ni and Co. Catalyst
SBET (m2/g)
Pore volume (cm3/g)
Pore diameter (A˚)
SiO2 PMoA Ni-PMoA Co-PMoA
330 244 232 256
0.77 0.67 0.68 0.75
128.3 116.0 116.7 117.8
tion bands corresponding to mas (P-O) and d (O-P-O) for H3PMo12O40.6H2O, which appears at 1103 cm1 mas (P-O in central tetrahedron) in the Ni-PMoA and Co-PMoA. In all cases, an additional band at 1614 cm1 is observed, that is assigned to bending vibrations of mass (H2O) in the secondary structure of the Keggin species. The spectra obtained are given in Figs. 2 and 3. 3.4. FTIR with pyridine (DRIFT) The catalysts are also characterized using FT-IR spectroscopy with adsorbed pyridine. It is well established from the literature that the bands located at 1640 cm1 and 1540 cm1 are due to the adsorption of pyridinium ions from pyridine at the Bro¨nsted acid sites [36], while the bands in the region in the regions between 1600 to 1630 cm1 and between 1440 to 1445 cm1 are attributed to coordinately adsorbed pyridine on Lewis acid sites. The band located at 1490 cm1 is associated with both Bro¨nsted and Lewis acid sites [37]. The obtained spectra suggest that all the catalysts exhibited IR bands corresponding to Bro¨nsted acid sites (1545 cm1) and Lewis acid sites (1450 cm1). IR spectra at 1490 cm1 corresponding to both Bro¨nsted acid sites and Lewis acid sites are overlapped. Table 2 shows the acid type sites and Bro¨nsted/ Lewis ratio of different catalysts. Fig. 4 shows the Drift spectra of various catalysts, depicting peaks characteristic of Brønsted and Lewis acid sites. The used
TGA curves of the synthesized catalyst (A) Ni-PMoA and (B) Co-PMoA.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
5
3.5. Solid state
31
P-NMR
The dried and calcined catalyst, Ni-PMoA and Co-PMoA are subjected to 31P solid-state NMR studies. Fresh unsupported [PMo12O40]3 anions exhibit a characteristic peak at chemical shift at d = 8.5. The silica-supported H3PMo12O40 calcined at 300 °C, is expected to be stable like the corresponding bulk HPA. This is confirmed by the presence of a strong peak of the Keggin unit at 2.6 ppm, The Ni-PMoA and Co-PMoA shows peaks (around 1.7 ppm) indicating that the structure of polyoxometallate remains intact on substitution with Ni or Co. The spectrum for Ni-PMoA is given in Fig. 5. Figure 2
FT-IR spectrum of the synthesized catalyst Ni-PMoA.
3.6. Temperature programmed reduction (TPR) studies From the data obtained it can be understood that all the catalyst prepared including the non-substituted HPMo, exhibit two reduction peaks around 639–646 and 778–810 °C. The Keggin structure of HPMo would probably collapse to form MoO3 and P2O5 before the reduction takes place [38]. These peaks can be attributed to reduction of Mo(VI) to Mo(IV). As Mo(VI) is in large excess over Ni(II) and Co(II) in the Ni-PMoA and Co-PMoA catalysts respectively, reduction of Ni(II) and Co(II) is probably masked by the Mo peaks. Very similar H2-TPR profiles have been reported previously for non-promoted HPMo/TiO2 [15], MoO3/SiO2 and NiOMoO3/SiO2-Al2O3 [19]. The TPR profiles obtained are given in Fig. 6.
Figure 3
FT-IR spectrum of the synthesized catalyst Co-PMoA.
3.7. XRD Table 2 Brønsted (B) versus Lewis (L) acidity of catalysts from DRIFTS of adsorbed pyridine. Catalyst
Acid site type
B/L
PMoA Ni-PMoA Co-PMoA
B+L B+L B+L
1.4 0.39 1.2
The XRD analysis of samples is carried out in order to ascertain the crystal structure of the samples prepared, however the patterns acquired indicate the amorphous nature of samples prepared. 3.8. Catalytic studies The catalytic activity of the prepared catalysts are carried out for the hydrodesulfurization of thiophene at temperatures of 300, 400 and 500 °C for 120 min, to study the initial rate of the reaction and compare the effect of Ni and Co substitution. From the conversion product obtained (mol %) it is found that the catalytic activity decreases with time on stream and is
Figure 4 PMoA.
DRIFT spectra of (a) PMoA (b) Ni-PMoA (c) Co-
catalyst did not show any peaks in this region, indicating that the precursor loses all its acidic character during the activation or reaction process to form the catalyst for hydrodesulfurization.
Figure 5
NMR spectrum of Ni-PMoA.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
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Figure 6
TPR spectrum of the catalysts (A) Ni-PMoA and (B) Co-PMoA.
reduced to about 50% for PMoA. The nickel substituted heteropoly acid (Ni-PMoA) starts off with 70.6% conversion however the activity decreases significantly from 70.6 to 41.6% with time, which is consistent with our earlier reported studies wherein we found that the surface of the catalyst is covered with coke during the reaction [33]. The graphical representation of results is given in Fig. 7. From the conversion data obtained it is found that the catalytic activity decreases with time-on-line and the undoped catalyst (PMoA) displays lesser catalytic efficiency than the doped catalyst. The activity of the catalyst Ni-PMoA yields a 70.6% conversion product after 30 min of feed time, which reduces by 30% at the end of the experiment. However, it is found that there is a depreciation in the catalytic performance over timeon-line use. The graphical representation of results is given in Fig. 8. When the catalytic activity is carried out at 400 °C there appears to be a marked increase in the activity of all the catalysts when compared to the results obtained at 300 °C, however similar trend in the activity of the catalysts is observed. The Ni promoted catalyst, Ni-PMoA, show an initial activity of more than 98%, which is 28% more than the reaction carried out at 300 °C. More interestingly it is also observed that the decrease in activity with time-on-line for the catalyst i.e.
15%, is less than the decrease in activity with time-on-line for the reaction carried out at 300 °C i.e. 30%, which means that the catalyst preforms much better at this reaction temperature. However, the pattern in the decrease in catalytic performance with time-on-line is found to be similar, nevertheless the decrease in activity with time-on-line at this reaction temperature, i.e. 400 °C, is almost half of that observed when the reaction is carried out at 300 °C. However, with the Co promoted catalyst, Co-PMoA, at 400 °C reaction temperature, the initial conversion percentage found is 93% and decrease in conversion with is found to be 18% after 120 min. The hydrodesulfurization reactions carried out at 500 °C showed a much higher desulfurization of thiophene compared with reactions carried out at 300 °C and 400 °C. The initial activity of Ni promoted catalyst, Ni-PMoA, appears to be the highest, with 99.6% conversion with the least decrease in the time-on-line catalytic activity. However, when the comparison of the conversion results between Ni and Co promoted catalysts are carried out and it is observed that at 500 °C reaction temperature, the Co promoted catalyst, Co-PMoA, starts off with comparable conversion product as that of the Ni promoted catalyst, Ni-PMoA. Moreover, with passage of time the depreciation in catalytic performance of the Co promoted catalyst, Co-PMoA, is observed to be lower than that of the Ni
Figure 7 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at reaction temperature 300 °C.
Figure 8 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at 400 °C reaction temperature.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
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promoted catalyst Ni-PMoA. The Ni promoted catalyst, NiPMoA, displays 12% decrease in catalytic performance while an 8% decrease is observed with the Co promoted catalyst. The graphical representation of results is given in Fig. 9. 3.9. Reaction rate constants for simultaneous HDS of thiophene In homogeneous systems, it is attempted to fit the data to a simple first order kinetic equation. Consider the acidcatalyzed reaction Cat
A þ B ! P1 þ P2 In order to compare the reactivities of sulfur compound, hydrodesulfurization behavior of thiophene is analyzed according to the pseudo-first-order kinetic equation. The integral rate equation is shown below.
Figure 11 Rate constant of thiophene hydrodesulfurization at different temperatures over Ni-PMoA catalyst.
lnð1 XÞ ¼ kt where X is the conversion of thiophene and k is the rate constant (min1). The Pseudo first order plots are given below in Figs. 10–12. The rate constants obtained at different temperatures showed that the hydrodesulfurization of thiophene by Ni-PMoA is faster over all catalyst.
Figure 12 Rate constant of thiophene hydrodesulfurization at different temperatures over Co-PMoA catalyst.
3.10. Residual sulfur content
Figure 9 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at 500 °C reaction temperature.
The residual sulfur content in the effluent gases is a parameter in order to ascertain the effective performance of the desulfurization catalyst wherein the higher volumes of residual sulfur (ppm) obtained, the better the catalytic performance, hence residual sulfur content is calculated by analysis of effluent gasses and the values obtained are tabulated in the Table 3. From the results obtained it is observed it can be said that among the catalysts PMoA, Ni-PMoA and Co-PMoA, the nickel promoted phosphomolybdic acid supported on silica (Ni-PMoA) catalyst is the best catalyst for the hydrodesulphurization of thiophene. 4. Conclusions
Figure 10 Rate constant of thiophene hydrodesulfurization at different temperatures over PMoA catalyst.
It can be concluded that although several factors are responsible for the catalytic desulfurization, the B/L ratio also plays a significant role in the catalytic desulfurization of thiophene. Supported heteropoly acids and substituted heteropoly compounds are which can be easily prepared in the lab act as extremely efficient catalysts for the desulfurization. Although these compounds are just catalyst, precursors they lead to the formation of active catalyst during the reaction process, probably by a fine dispersion of metals and their reduction. Overall, we can conclude that the catalytic activity of nickel-promoted catalyst Ni-PMoA appear to be the best
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
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N. Al-zaqri et al. Table 3
Sulfur removed (ppm) from the reaction mixture.
Catalyst
Temp. °C
Total sulfur in feed (ppm)
Sulfur removed (ppm) after X mins 30 (mins)
60 (mins)
90 (mins)
120 (mins)
PMoA
300 °C 400 °C 500 °C
1000 1000 1000
547 945 983
494 882 961
325 853 820
256 816 779
Ni-PMoA
300 °C 400 °C 500 °C
1000 1000 1000
706 979 996
601 927 985
500 877 947
416 828 875
Co-PMoA
300 °C 400 °C 500 °C
1000 1000 1000
641 931 983
584 827 958
462 789 937
424 744 902
amongst the series studied. Furthermore, all the catalysts appear to show more stable time-on-line activity as the temperature increases from 300 °C to 500 °C. This may be due to slower desorption of thiophene at 300 °C. Although the catalyst loses the acidic properties during the reaction [34] moreover there appears to be a correlation between the initial activity and Brønsted/Lewis acidity of the catalyst precursor as the higher B/L appear to show higher initial activity. Ortis-Islas et al. have demonstrated that both the Brønsted and Lewis acidity gets lower with temperature and almost all the acidity of molybdophosphoric acid on titania is lost around 400 °C [39]. We believe that the presence of Brønsted and Lewis acidity in the catalyst precursors helps in creating active sites of the catalysts during reduction process, probably by finer distribution of the metals on the surface of the support and thus plays an important role in the resulting catalyst. Further studies are being carried out to understand thoroughly this behavior, which in turn can help in understanding the catalytic mechanism for the hydrodesulfurization reactions, which can aid in designing of catalysts with improved performance. Acknowledgements This research was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia, Award No. 11-NAN-1852-02. References [1] D. Ierodiakonou, A. Zanobetti, B.A. Coull, S. Melly, D.S. Postma, H.M. Boezen, J.M. Vonk, P.V. Williams, G.G. Shapiro, E.F. McKone, Ambient air pollution, lung function, and airway responsiveness in asthmatic children, J. Allergy. Clin. Immunol. 137 (2) (2016) 390–399. [2] M.G. Ghozikali, B. Heibati, K. Naddafi, I. Kloog, G.O. Conti, R. Polosa, M. Ferrante, Evaluation of Chronic Obstructive Pulmonary Disease (COPD) attributed to atmospheric O3, NO2, and SO2 using Air Q Model (2011–2012 year), Environ. Res. 144 (2016) 99–105. [3] Z. Cao, P. Du, A. Duan, R. Guo, Z. Zhao, H. lei Zhang, P. Zheng, C. Xu, Z. Chen, Synthesis of mesoporous materials SBA16 with different morphologies and their application in dibenzothiophene hydrodesulfurization, Chem. Eng. Sci. 155 (2016) 141–152.
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Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
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King Saud University
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ORIGINAL ARTICLE
Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene Nabil Al-zaqri a, Ali Alsalme a, Syed F. Adil a, Ahmad Alsaleh a, Saad G. Alshammari a, Saud I. Alresayes a, Raja Alotaibi b, Mohammed Al-Kinany b, Mohammed Rafiq H. Siddiqui a,* a b
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia
Received 25 March 2017; revised 18 April 2017; accepted 11 May 2017
KEYWORDS Heteropoly acids; Nickel; Cobalt; Hydrodesulfurization (HDS); Thiophene; Brønsted/Lewis acidity
Abstract Phosphomolybdic acid supported on silica (PMoA), and phosphomolybdic acid supported on silica substituted with nickel (Ni-PMoA), cobalt (Co-PMoA), are prepared with different loadings. All catalysts have been characterized using thermogravimetric analysis (TGA), BET surface area, Solid-state 31P NMR spectra, FT-IR, Diffuse Reflectance Infrared Fourier Transform (DRIFT) and Powder X-ray diffraction analysis (XRD). The surface areas of the catalysts are slightly lower than the surface area of the silica used in making these catalysts, while the XRD studies indicated the amorphous nature of the prepared catalysts. All catalysts are tested for their activity in the hydrodesulfurization of thiophene in the temperature range of 300–500 °C. Under similar conditions the silica supported catalyst i.e. PMoA, showed lower hydrodesulfurization activity, compared to the nickel and cobalt substituted heteropoly acids, i.e. Ni-PMoA and Co-PMoA, indicating enhancement of catalytic activity with substitution of nickel and cobalt. Moreover, the nickel-substituted catalyst, Ni-PMoA, showed slightly higher activity than the cobalt-substituted ones, Co-PMoA. At 500 °C, Ni-PMoA gave a 99.6% HDS conversion of thiophene whereas the Co-PMoA yielded a 98.3%. HDS conversion of thiophene. Brønsted/Lewis acidity of the catalyst precursor appear to be essential for the catalytic hydrodesulfurization. Ó 2017 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
* Corresponding author. E-mail address: [email protected] (M.R.H. Siddiqui). Peer review under responsibility of King Saud University.
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1. Introduction Sulfur dioxide (SO2) is a very serious environmental pollutant which causes a series of environmental and health issues. For instance, when it combines with water and air, it forms sulfuric acid, which is the main component of acid rain that leads to
http://dx.doi.org/10.1016/j.jscs.2017.05.004 1319-6103 Ó 2017 King Saud University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
2 acidified soils, lakes, streams, accelerated corrosion of buildings and monuments, and also reduce visibility. High concentrations of SO2 in air may results in serious breathing problems, respiratory illnesses and various other health related ailments [1,2]. Particularly, sulfur based compounds that are largely found as impurities in various petroleum fractions are the major cause of these problems. Notably, among various impurities, the major refractive sulfur species in diesel are dibenzothiophenes and 4, 6-substitued dibenzothiophenes (DBTs) [3]. The presence of sulfur compounds in crude oil and heavy fractions is an undesirable issue. It can lead to corrosion in oils and lubricants and poisonous emissions such as SO2 and H2S when the fuel is burned. Several processes have been proposed to deal with the problem of removing these compounds [4]. Recently, awareness about environmental protection has increased and consequently legislation to limit sulfur contents in fuels has been strictly applied throughout the world [5]. Nowadays, high quality of petroleum products is required to reduce environmental problems induced by the SOx emissions to the environment. For this purpose, more active and selective catalysts for the hydro-treatment processes have been proposed. In order to reach ultra-low sulfur levels (<15 ppm), the development of a new generation of catalysts with very high hydrodesulfurization (HDS) activity is highly desirable. At present, the hydrodesulfurization (HDS) process is an efficient method for sulfur removal from gasoline and diesel of high sulfur content [6]. In this process organic sulfur compounds in the liquid fuels are broken down to H2S using catalysts [7], followed by the removal of H2S. Using this process the sulfur content in fuels can be reduced to about 10 ppm [8]. For this process several mixed transition metals and metal oxides based catalysts have been reportedly employed. For example, various HDS reactions are reported to be catalyzed by using CoMo [9], NiW [10], or the trimetallic NiMoW catalysts supported mainly on c-Al2O3 [11] and many other
N. Al-zaqri et al. supports containing the active acidic or basic sites such as SiO2 [12–15], ZrO2 [16], TiO2 [17], MgO [18]. In a recent study, molybdenum catalysts supported on alumina were investigated for the effect of nickel and phosphorus on activity of hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine [19]. The presence of nickel into molybdenum-containing catalysts strongly promoted HDS activity. In another study, a weak promoting effect of phosphorus was observed, in the case of NiMoAl2O3 catalyst modified with phosphorus [20]. Recently, the popularity of the heteropoly acids as catalysts for various organic transformations has grown [21–24], especially, the Keggin type heteropoly acids (HPAs) have been proved to be excellent catalysts for various reactions [25–28] due to their simple synthesis, relatively high thermal stability [29] and very strong acidity and redox properties [30]. Although, several new industrial processes based on HPA catalysis have been developed and commercialized in the last two decades [31]. One of the most important properties of heteropoly anions is that they can be isolated as solids when coupled with an appropriate counter cation, e.g. H+, NH+ 4 . Heteropoly acids (HPA) are the hydrogen (acidic) forms and are utilized primarily as catalysts, and is reflected in the statistic that 80–85% of patent applications concerning the polyoxometalates are related to catalysis [32]. Nevertheless, there are few scientific reports available on the application of HPAs with respect to the hydrodesulfurization reaction. In our earlier studies we have demonstrated the excellent ability of heteropoly acid to act as catalyst precursors in the hydrodesulfurization reactions, where these catalysts have performed better than the commercial catalysts [33]. With our continued interest in heteropoly acids and hydrodesulfurization, this study was carried out to gain further insight into the mechanism of supported heteropoly acids in the presence of nickel and cobalt (cf. Scheme 1). Additionally, the aim of this study is also to get an insight into the role of Brønsted and Lewis acidity and its relation to HDS activity Scheme 2.
Scheme 1 Graphical representation of the synthesis of PMoA, Ni-PMoA, Co-PMoA catalysts and their evaluation as hydrodesulfurization catalyst.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
Scheme 2 thiophene.
Reaction
pathway
of
hydrodesulfurization
of
2. Experimental 2.1. Materials and methods Phosphomolybdic acid hydrate H3Mo12PO4020H2O, Ni (NO3)26H2O, Co(NO3)26H2O and thiophene (99%) were from Sigma–Aldrich and Aerosil 300 silica support (SBET = 300 m2 g1) from Degussa. Catalysts were characterized by several techniques including, thermogravimetric analysis (TGA). Shimadzu TGA-50 Thermogravimetric Analyzer is used to perform the thermogravimetric experiments using a heating rate of either 10 °C per minute to raise the temperature from room temperature to 800 °C, under a nitrogen gas flow. Surface area and porosity analysis is carried out on a Micromeritics ASAP2010 system using nitrogen sorption at 77 K. The surface areas were calculated using a multipoint Brunauer–Emmett–Teller (BET) model. Solid-state 31P-NMR spectra were recorded at 9.4 T using a JEOL resonance ECX500 II. The chemical shift of the 31P resonances is in ppm using 85% H3PO4 as internal standard. Fourier transform infrared spectroscopy (FT-IR) studies were carried out in transmission mode using a Perkin Elmer precisely spectrum 100 (FT-IR) spectrometer. Pyridine Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy is used to examine the state of HPA on SiO2 as well as to determine the nature of acid sites (Brønsted and Lewis) by adsorption of pyridine. The measurements were performed using a Nicolet Model Nexus FTIR-Raman spectrometer at room temperature under N2 atmosphere to prevent interference by the gaseous environment in the chamber. Powder X-ray diffraction studies were carried out on a Rigaku Ultima (IV) using Co anticathode, i.e. CoKa radiation with k = 1.7889 A˚, 40 kV and 40 mA. 2.2. Catalyst preparation All the catalysts are prepared by following our earlier method [34]. 1. H3PMo12O40 (HPMo) supported on silica pre-catalyst, HPMo/SiO2, PMoA, with 15 wt% HPMo loading based on anhydrous HPMo is prepared by wet impregnation. The impregnation of SiO2 with H3PMo12O40 is carried out in aqueous slurry at 40–50 °C with continuous stirring for 24 h. Solid residue is isolated through rotary evaporation at 45 °C, which is dried at 150 °C under vacuum conditions and calcined at 300 °C under nitrogen flow for 3 h
3
with a heating rate of 5 °C min1. The pre-catalyst is then grounded into a powder with a particle size of 45–180 lm. 2. Silica-supported Ni(II) and Co(II) substituted phosphomolybdate pre-catalysts are prepared employing wet impregnation of SiO2 with HPMo and the required amounts of Ni(NO3)2 or Co(NO3)2 with Ni(II) or Co(II)/ HPMo molar ratio of 2:1 (i.e. Ni/Mo atomic ratios of 1:6; Co/Mo atomic ratios of 1:6), followed by isolation, drying and calcination at 300 °C as above. Hereafter, these pre-catalysts are designated as Ni-PMoA, Co-PMoA, respectively. All of the catalysts are calcined at 300 °C for 3 h under N2. The temperature of furnace is raised from room temperature to 300 °C a heating rate of 5 °C min1, with a N2 flow of 20 mL min1 and this temperature is maintained for three hours. 2.3. Catalysts testing The prepared catalysts H3PMo12O406H2O/SiO2, (PMoA), (Ni-PMoA), (Co-PMoA) are tested for the hydrodesulfurization activity of thiophene at different temperatures varying from 300 °C to 500 °C. Hydrodesulfurization of thiophene is carried out at 300– 500 °C using a conventional fixed bed flow reactor. Thiophene is introduced into the reactor by passing hydrogen (10 mL min1) through a thiophene with octane 250 ppm at room temperature. Reaction conditions are as follows: Catalyst weight = 0.6 g, H2 = 10 mL min1, flow octane with thiophene 0.2 mL min1. GC analysis are conducted on the (Varian Chrompack CP3800 instrument with a 30 m 0.320 mm, 0.25 mm film thickness capillary column and a flame ionization detector). Gas-phase catalyst testing is performed in a continuous flow fixed bed reactor. A stainless steel reactor 28 cm3 in length and 0.9 cm internal diameter is used to carry out the gas phase reaction at reaction temperatures up to 300 °C. After the reaction temperature in the furnace is attained, the desired H2 flow is set and then redirected through to the saturator set, using the auto-injector; online GC injections are then made every 36 min. The selectivity is defined as the percentage of thiophene converted into a particular product taking into account the reaction stoichiometry. The reactants and potential products of the reactions were calculated using the internal standard method. 3. Results and discussion 3.1. Thermogravimetric analysis (TGA) The prepared catalysts PMoA, Ni-PMoA, and Co-PMoA are subjected TGA studies to measure the amount of physisorbed water removed at temperatures up to 150 °C. Thermogravimetric analysis (TGA) of the (H3PMo12O4.6H2O) catalyst shown in Fig. 1 was performed in N2 on a fresh sample prior to catalyst pretreatment, two weight losses are observed: one at 100–180 °C, corresponding to the loss of water, and the other at 460–480 °C, corresponding to the decomposition of the Keggin structure accompanied by the evolution of water.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
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N. Al-zaqri et al.
3.2. Surface area and porosity studies The physisorption of N2 at 77 K provided information on the porosity of the catalysts and their precursors. To determine the surface area and characterize the porous texture of a catalyst, the first stage is to generate the N2 adsorption isotherm, which is found by plotting the volume of N2 adsorbed against its relative pressure. The prepared samples are characterized by BET analysis in order to find out the surface area, pore volume, and pore diameter, which are tabulated in Table 1 below. The cobalt promoted catalyst is found to possess the highest surface area with 256 (m2/g). However, all the samples are found to possess surface area within the range 230–256 (m2/g) and there is no significant differences in the pore volume and pore diameter. 3.3. Infrared spectroscopy (FT-IR) The experimental procedure for the FT-IR measurements is described in previous work using infrared spectroscopy [34]. The majority of the characteristic infrared active bands of the Keggin anion can be found in the fingerprint region of the spectrum which lies between 1200 cm1 and 500 cm1. From the FT-IR spectrum of Ni-PMoA, it is noticed that it possess signals at 958.18 cm1, 808.41 cm1 respectively, which could be due to the stretching vibrations of mas (Mo– Ot), mas (Mo–Oc–Mo). Similarly, from the FT-IR spectrum of H3PMo12O40xH2O/SiO2 promoted with Co signals at 959.84 cm1 and 811.73 cm1 is attributed to the stretching vibrations of mas (Mo–Ot), mas (Mo–O–Mo) respectively. However, it appears to be slightly different from signals obtained for H3PMo12O40xH2O/SiO2 at 961, 870 and 783 cm1 which is assigned to the stretching vibrations mas (Mo-Ot), mas (MoOb-Mo) and mas (Mo-Oc-Mo), respectively [35]. The Mo-OMo bridges belong to the MoO6 octahedra and the subscripts b, c and t indicate corner-sharing, edge-sharing and terminal oxygen, respectively. These variations could be due to the interaction of the Ni and Co ions with the atoms present in the Keggin structure. Similar variations are observed in vibra-
Figure 1
Table 1 Structural parameters of the catalysts H3Mo12PO40 xH2O/SiO2 (PMoA) promoted with Ni and Co. Catalyst
SBET (m2/g)
Pore volume (cm3/g)
Pore diameter (A˚)
SiO2 PMoA Ni-PMoA Co-PMoA
330 244 232 256
0.77 0.67 0.68 0.75
128.3 116.0 116.7 117.8
tion bands corresponding to mas (P-O) and d (O-P-O) for H3PMo12O40.6H2O, which appears at 1103 cm1 mas (P-O in central tetrahedron) in the Ni-PMoA and Co-PMoA. In all cases, an additional band at 1614 cm1 is observed, that is assigned to bending vibrations of mass (H2O) in the secondary structure of the Keggin species. The spectra obtained are given in Figs. 2 and 3. 3.4. FTIR with pyridine (DRIFT) The catalysts are also characterized using FT-IR spectroscopy with adsorbed pyridine. It is well established from the literature that the bands located at 1640 cm1 and 1540 cm1 are due to the adsorption of pyridinium ions from pyridine at the Bro¨nsted acid sites [36], while the bands in the region in the regions between 1600 to 1630 cm1 and between 1440 to 1445 cm1 are attributed to coordinately adsorbed pyridine on Lewis acid sites. The band located at 1490 cm1 is associated with both Bro¨nsted and Lewis acid sites [37]. The obtained spectra suggest that all the catalysts exhibited IR bands corresponding to Bro¨nsted acid sites (1545 cm1) and Lewis acid sites (1450 cm1). IR spectra at 1490 cm1 corresponding to both Bro¨nsted acid sites and Lewis acid sites are overlapped. Table 2 shows the acid type sites and Bro¨nsted/ Lewis ratio of different catalysts. Fig. 4 shows the Drift spectra of various catalysts, depicting peaks characteristic of Brønsted and Lewis acid sites. The used
TGA curves of the synthesized catalyst (A) Ni-PMoA and (B) Co-PMoA.
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
5
3.5. Solid state
31
P-NMR
The dried and calcined catalyst, Ni-PMoA and Co-PMoA are subjected to 31P solid-state NMR studies. Fresh unsupported [PMo12O40]3 anions exhibit a characteristic peak at chemical shift at d = 8.5. The silica-supported H3PMo12O40 calcined at 300 °C, is expected to be stable like the corresponding bulk HPA. This is confirmed by the presence of a strong peak of the Keggin unit at 2.6 ppm, The Ni-PMoA and Co-PMoA shows peaks (around 1.7 ppm) indicating that the structure of polyoxometallate remains intact on substitution with Ni or Co. The spectrum for Ni-PMoA is given in Fig. 5. Figure 2
FT-IR spectrum of the synthesized catalyst Ni-PMoA.
3.6. Temperature programmed reduction (TPR) studies From the data obtained it can be understood that all the catalyst prepared including the non-substituted HPMo, exhibit two reduction peaks around 639–646 and 778–810 °C. The Keggin structure of HPMo would probably collapse to form MoO3 and P2O5 before the reduction takes place [38]. These peaks can be attributed to reduction of Mo(VI) to Mo(IV). As Mo(VI) is in large excess over Ni(II) and Co(II) in the Ni-PMoA and Co-PMoA catalysts respectively, reduction of Ni(II) and Co(II) is probably masked by the Mo peaks. Very similar H2-TPR profiles have been reported previously for non-promoted HPMo/TiO2 [15], MoO3/SiO2 and NiOMoO3/SiO2-Al2O3 [19]. The TPR profiles obtained are given in Fig. 6.
Figure 3
FT-IR spectrum of the synthesized catalyst Co-PMoA.
3.7. XRD Table 2 Brønsted (B) versus Lewis (L) acidity of catalysts from DRIFTS of adsorbed pyridine. Catalyst
Acid site type
B/L
PMoA Ni-PMoA Co-PMoA
B+L B+L B+L
1.4 0.39 1.2
The XRD analysis of samples is carried out in order to ascertain the crystal structure of the samples prepared, however the patterns acquired indicate the amorphous nature of samples prepared. 3.8. Catalytic studies The catalytic activity of the prepared catalysts are carried out for the hydrodesulfurization of thiophene at temperatures of 300, 400 and 500 °C for 120 min, to study the initial rate of the reaction and compare the effect of Ni and Co substitution. From the conversion product obtained (mol %) it is found that the catalytic activity decreases with time on stream and is
Figure 4 PMoA.
DRIFT spectra of (a) PMoA (b) Ni-PMoA (c) Co-
catalyst did not show any peaks in this region, indicating that the precursor loses all its acidic character during the activation or reaction process to form the catalyst for hydrodesulfurization.
Figure 5
NMR spectrum of Ni-PMoA.
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N. Al-zaqri et al.
Figure 6
TPR spectrum of the catalysts (A) Ni-PMoA and (B) Co-PMoA.
reduced to about 50% for PMoA. The nickel substituted heteropoly acid (Ni-PMoA) starts off with 70.6% conversion however the activity decreases significantly from 70.6 to 41.6% with time, which is consistent with our earlier reported studies wherein we found that the surface of the catalyst is covered with coke during the reaction [33]. The graphical representation of results is given in Fig. 7. From the conversion data obtained it is found that the catalytic activity decreases with time-on-line and the undoped catalyst (PMoA) displays lesser catalytic efficiency than the doped catalyst. The activity of the catalyst Ni-PMoA yields a 70.6% conversion product after 30 min of feed time, which reduces by 30% at the end of the experiment. However, it is found that there is a depreciation in the catalytic performance over timeon-line use. The graphical representation of results is given in Fig. 8. When the catalytic activity is carried out at 400 °C there appears to be a marked increase in the activity of all the catalysts when compared to the results obtained at 300 °C, however similar trend in the activity of the catalysts is observed. The Ni promoted catalyst, Ni-PMoA, show an initial activity of more than 98%, which is 28% more than the reaction carried out at 300 °C. More interestingly it is also observed that the decrease in activity with time-on-line for the catalyst i.e.
15%, is less than the decrease in activity with time-on-line for the reaction carried out at 300 °C i.e. 30%, which means that the catalyst preforms much better at this reaction temperature. However, the pattern in the decrease in catalytic performance with time-on-line is found to be similar, nevertheless the decrease in activity with time-on-line at this reaction temperature, i.e. 400 °C, is almost half of that observed when the reaction is carried out at 300 °C. However, with the Co promoted catalyst, Co-PMoA, at 400 °C reaction temperature, the initial conversion percentage found is 93% and decrease in conversion with is found to be 18% after 120 min. The hydrodesulfurization reactions carried out at 500 °C showed a much higher desulfurization of thiophene compared with reactions carried out at 300 °C and 400 °C. The initial activity of Ni promoted catalyst, Ni-PMoA, appears to be the highest, with 99.6% conversion with the least decrease in the time-on-line catalytic activity. However, when the comparison of the conversion results between Ni and Co promoted catalysts are carried out and it is observed that at 500 °C reaction temperature, the Co promoted catalyst, Co-PMoA, starts off with comparable conversion product as that of the Ni promoted catalyst, Ni-PMoA. Moreover, with passage of time the depreciation in catalytic performance of the Co promoted catalyst, Co-PMoA, is observed to be lower than that of the Ni
Figure 7 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at reaction temperature 300 °C.
Figure 8 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at 400 °C reaction temperature.
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Evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst
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promoted catalyst Ni-PMoA. The Ni promoted catalyst, NiPMoA, displays 12% decrease in catalytic performance while an 8% decrease is observed with the Co promoted catalyst. The graphical representation of results is given in Fig. 9. 3.9. Reaction rate constants for simultaneous HDS of thiophene In homogeneous systems, it is attempted to fit the data to a simple first order kinetic equation. Consider the acidcatalyzed reaction Cat
A þ B ! P1 þ P2 In order to compare the reactivities of sulfur compound, hydrodesulfurization behavior of thiophene is analyzed according to the pseudo-first-order kinetic equation. The integral rate equation is shown below.
Figure 11 Rate constant of thiophene hydrodesulfurization at different temperatures over Ni-PMoA catalyst.
lnð1 XÞ ¼ kt where X is the conversion of thiophene and k is the rate constant (min1). The Pseudo first order plots are given below in Figs. 10–12. The rate constants obtained at different temperatures showed that the hydrodesulfurization of thiophene by Ni-PMoA is faster over all catalyst.
Figure 12 Rate constant of thiophene hydrodesulfurization at different temperatures over Co-PMoA catalyst.
3.10. Residual sulfur content
Figure 9 Comparison of catalytic hydrodesulfurization of thiophene employing PMoA, Ni-PMoA and Co-PMoA catalysts at 500 °C reaction temperature.
The residual sulfur content in the effluent gases is a parameter in order to ascertain the effective performance of the desulfurization catalyst wherein the higher volumes of residual sulfur (ppm) obtained, the better the catalytic performance, hence residual sulfur content is calculated by analysis of effluent gasses and the values obtained are tabulated in the Table 3. From the results obtained it is observed it can be said that among the catalysts PMoA, Ni-PMoA and Co-PMoA, the nickel promoted phosphomolybdic acid supported on silica (Ni-PMoA) catalyst is the best catalyst for the hydrodesulphurization of thiophene. 4. Conclusions
Figure 10 Rate constant of thiophene hydrodesulfurization at different temperatures over PMoA catalyst.
It can be concluded that although several factors are responsible for the catalytic desulfurization, the B/L ratio also plays a significant role in the catalytic desulfurization of thiophene. Supported heteropoly acids and substituted heteropoly compounds are which can be easily prepared in the lab act as extremely efficient catalysts for the desulfurization. Although these compounds are just catalyst, precursors they lead to the formation of active catalyst during the reaction process, probably by a fine dispersion of metals and their reduction. Overall, we can conclude that the catalytic activity of nickel-promoted catalyst Ni-PMoA appear to be the best
Please cite this article in press as: N. Al-zaqri et al., Comparative catalytic evaluation of nickel and cobalt substituted phosphomolybdic acid catalyst supported on silica for hydrodesulfurization of thiophene, Journal of Saudi Chemical Society (2017), http://dx.doi.org/10.1016/j.jscs.2017.05.004
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N. Al-zaqri et al. Table 3
Sulfur removed (ppm) from the reaction mixture.
Catalyst
Temp. °C
Total sulfur in feed (ppm)
Sulfur removed (ppm) after X mins 30 (mins)
60 (mins)
90 (mins)
120 (mins)
PMoA
300 °C 400 °C 500 °C
1000 1000 1000
547 945 983
494 882 961
325 853 820
256 816 779
Ni-PMoA
300 °C 400 °C 500 °C
1000 1000 1000
706 979 996
601 927 985
500 877 947
416 828 875
Co-PMoA
300 °C 400 °C 500 °C
1000 1000 1000
641 931 983
584 827 958
462 789 937
424 744 902
amongst the series studied. Furthermore, all the catalysts appear to show more stable time-on-line activity as the temperature increases from 300 °C to 500 °C. This may be due to slower desorption of thiophene at 300 °C. Although the catalyst loses the acidic properties during the reaction [34] moreover there appears to be a correlation between the initial activity and Brønsted/Lewis acidity of the catalyst precursor as the higher B/L appear to show higher initial activity. Ortis-Islas et al. have demonstrated that both the Brønsted and Lewis acidity gets lower with temperature and almost all the acidity of molybdophosphoric acid on titania is lost around 400 °C [39]. We believe that the presence of Brønsted and Lewis acidity in the catalyst precursors helps in creating active sites of the catalysts during reduction process, probably by finer distribution of the metals on the surface of the support and thus plays an important role in the resulting catalyst. Further studies are being carried out to understand thoroughly this behavior, which in turn can help in understanding the catalytic mechanism for the hydrodesulfurization reactions, which can aid in designing of catalysts with improved performance. Acknowledgements This research was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Riyadh, Saudi Arabia, Award No. 11-NAN-1852-02. References [1] D. Ierodiakonou, A. Zanobetti, B.A. Coull, S. Melly, D.S. Postma, H.M. Boezen, J.M. Vonk, P.V. Williams, G.G. Shapiro, E.F. McKone, Ambient air pollution, lung function, and airway responsiveness in asthmatic children, J. Allergy. Clin. Immunol. 137 (2) (2016) 390–399. [2] M.G. Ghozikali, B. Heibati, K. Naddafi, I. Kloog, G.O. Conti, R. Polosa, M. Ferrante, Evaluation of Chronic Obstructive Pulmonary Disease (COPD) attributed to atmospheric O3, NO2, and SO2 using Air Q Model (2011–2012 year), Environ. Res. 144 (2016) 99–105. [3] Z. Cao, P. Du, A. Duan, R. Guo, Z. Zhao, H. lei Zhang, P. Zheng, C. Xu, Z. Chen, Synthesis of mesoporous materials SBA16 with different morphologies and their application in dibenzothiophene hydrodesulfurization, Chem. Eng. Sci. 155 (2016) 141–152.
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