Catalyst Deactivation and Reactor Fouling during Hydrogenation of

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Catalysis and Kinetics

Catalyst deactivation and reactor fouling during hydrogenation of conjugated cyclic olefins over commercial Ni-Mo-S/#-Al2O3 catalyst Ali Alzaid, Jason Wiens, John D. Adjaye, and Kevin J. Smith Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00791 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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Energy & Fuels

Catalyst deactivation and reactor fouling during hydrogenation of conjugated cyclic olefins over commercial Ni-Mo-S/γ-Al2O3 catalyst

By

Ali Alzaid1 Jason Wiens2 John Adjaye2 and Kevin J. Smith1*

1

Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC CanadaV6T 1Z3

2

Syncrude Canada Ltd., Edmonton Research Centre, 9421 - 17th Avenue, Edmonton, Alberta Canada T6N 1H4

* Corresponding author: email: [email protected] Tel/Fax: 604-822-3601/604-822-6003

1 ACS Paragon Plus Environment

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Abstract The dimerization of conjugated cyclic olefins during hydrogenation at low temperature (≤ 250°C), on a spent commercial Ni-Mo-S/γ-Al2O3 catalyst, is reported. Hydrogenation of 4methylstyrene versus α-methylstyrene showed that the methyl group attached to the vinyl group of α-methylstyrene decreased dimer yield due to steric hindrance, while the yield of hydrogenated products remained high. The addition of 20 wt% cyclohexene to 4-methylstyrene and reaction at lower temperature (200 versus 250°C) decreased the 4-methylstyrene hydrogenation rate. Increased concentration of 4-methylstyrene and lower reaction temperature increased dimer and gum yields. The data indicate that dimers are precursors to gum formation and that catalyst deactivation is linked to gum formation that results in increased carbon content and decreased BET surface area of the used catalyst. Furthermore, an increase in pressure drop across the fixed-bed reactor with time-on-stream (TOS), observed with 4-methylstyrene as reactant but not with α-methylstyrene, is consistent with cumulative gum deposition in the catalyst bed. The pressure drop is well described by the Ergun equation, assuming that gum deposition reduces bed voidage with TOS.

Keywords: catalyst, hydrogenation, hydrotreating, gum formation, catalyst deactivation, dimerization, methylstyrene, cyclohexene


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Introduction:

Canada’s oil sands reserves represent a strategic source of energy corresponding to 10.5% of the world's energy reserves.1 Surface mined Canadian oil sands is upgraded by carbon rejection or H2 addition processes, following initial extraction, washing and distillation steps. The resulting oils are hydrotreated to remove metal, sulphur and other contaminants.2

Fluid coking uses carbon rejection to increase the H/C molar ratio of the oil extracted from the Canadian oil sands. The naphtha product from a fluid coker contains high amounts of olefins (20 wt%, Br # = 57-65.8 g/100 g) and conjugated olefins (4.4 wt%, diene value = 8.3-9.7 g I2/100 g).3 Direct hydrotreating of the naphtha product from a fluid coker results in olefin/conjugated olefin oligomerization reactions2, 3 and the formation and deposition of gums that result in rapid catalyst deactivation and reactor fouling, causing early reactor shutdown. Consequently, prior to hydrotreating, the naphtha product is hydrogenated at low temperature (< 250°C) to decrease the naphtha olefin/conjugated olefin content and thereby limit gum formation in the hydrotreater. However, catalyst deactivation and reactor fouling may also occur during low temperature hydrogenation, especially as the catalyst ages and loses some of its hydrogenation activity.3, 4

The hydrogenation of unsaturated hydrocarbons occurs on noble metal (Ru, Pd, Pt, Ir and Cu) and metal sulphide (Ni, Mo, Co, W) catalysts.5-10 Although Co-Mo-S/Al2O3 and Ni-Mo-S/Al2O3 catalysts have been widely studied for the hydrogenation of olefins, in part because of their relatively low cost and strong resistance to poisoning by sulphur and other contaminants,11-16 the role of dimerization reactions in catalyst deactivation and reactor fouling during naphtha


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hydrotreating over commercial Ni-Mo-S/γ-Al2O3 at industrially relevant reaction conditions, has not been reported. Such studies are essential to understand and resolve the issue of catalyst deactivation and reactor fouling that leads to premature shutdown in commercial hydrotreaters.

In previous work, a series of conjugated olefins in decalin were hydrogenated at 250°C, 3.4 MPa H2 and a liquid hourly space velocity (LHSV) of 1-2 h-1, using a spent, commercial Ni-Mo-S/γAl2O3 catalyst.4 The dimerization of several conjugated olefins (linear and cyclic) was shown to occur during the hydrogenation at the chosen reaction conditions; whereas, non-conjugated diolefins had very low yields of dimerization products. For one series of experiments using isoprene as the model conjugated olefin reactant, the gum content of the product oil increased with increased yield of dimers formed during hydrogenation. In the present study, catalyst deactivation and reactor fouling is examined more thoroughly, with a view to a better understanding of the relationship between dimer formation, gum formation and catalyst deactivation. Hence, hydrogenation of model reactants was conducted in a fixed-bed reactor operated at 200 - 250°C, 3.4 MPa H2 and a liquid hourly space velocity (LHSV) of 1-2 h-1 for up to 30 days time-on-stream (TOS). The effects of steric hindrance, olefin addition, and operating conditions were investigated by hydrogenating α–methylstyrene or 4-methylstyrene over a spent commercial Ni-Mo-S/γ-Al2O3 catalyst. In some cases, cyclohexene was added to the feed. Experiments were conducted for different TOS periods so that the change in catalyst properties with TOS could be determined by characterization of the catalyst recovered after the extended reaction period.


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Experimental

A spent, commercial Ni-Mo-S/γ-Al2O3 catalyst, taken from a commercial hydrotreater, was used throughout the study. The catalyst had been soxhlet extracted after removal from the reactor and was used without further pretreatment. The Brunnauer-Emmett-Teller (BET) surface area, pore volume and pore size of the spent catalyst were determined from N2 adsorption/desorption isotherms measured at 77 K using a Micromeritics ASAP 2020 analyzer. The catalyst sample was first degassed at 1.33 kPa/s and heated at a ramp rate of 10°C/min until 0.04 kPa and 250°C, holding the temperature for 240 min. The sample was then cooled and analyzed using N2 adsorption at 77 K. The carbon, hydrogen, nitrogen and sulphur content of the catalysts was determined by elemental CHNS analysis using a Perkin-Elmer 2400 Series II CHNS/O analyzer. The Al, Mo, P and Ni content of the catalyst was determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES) in accordance with the US EPA 6010C method. This analysis was performed by Canadian Microanalytical Service Ltd. in Delta, BC.

The hydrogenation reactions were carried out in a micro-scale, trickle-bed reactor (length 50 cm; hot zone 30 cm) with an internal diameter of 1.18 cm, operated in gas and liquid downflow mode. The catalyst was loaded into the reactor with glass beads and SiC diluent in order to control heat and mass transfer effects. The packing at the bottom of the reactor consisted of 3 mm glass beads (height 15.3 cm) followed by three layers of SiC with mesh sizes #16, #46 and #80 and bed height 3.5, 1.4 and 1.4 cm, respectively. The 2 grams of catalyst were then loaded on-top of the SiC in two layers, each layer consisting of SiC (mesh size #80, height 2.3 cm) and


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1 gram of catalyst (approximately 1.1 cm height). Above this bed more SiC was added with mesh sizes # 80, #46 and #16 and bed heights each of 1.1 cm. Finally, the top of the bed was loaded with 3 mm glass beads (height 18.2 cm). This catalyst loading was developed for microscale fixed bed hydrotreaters with 10-12 mm internal diameter and 2-3 mm trilobe commercial catalyst to eliminate poor catalyst wetting, wall effects and backmixing.17-20 In the present study, the ratio of bed height to equivalent pellet diameter ( ⁄ ) and reactor diameter to equivalent pellet diameter ( ⁄ ) were larger than 30 and 4, respectively, satisfying the criteria for negligible axial mass dispersion21, 22 and wall effects.21-23 Furthermore, calculation showed that both the Mears criterion and the Weisz-Prater criterion were met under the reaction conditions of the present study, ensuring that mass transfer effects were minimal, as detailed elsewhere.24

Most of the hydrogenation tests were performed using 8.4 wt% 4-methylstyrene in decalin, equivalent to a diene value of 18 g I2 / 100 g, double the diene value in a typical naphtha feed,3 in order to emphasize the degree of dimerization. This aids the investigation of the impact of dimerization reactions on catalyst deactivation and reactor fouling, given the relatively short time-on-stream period of 30 days. One test was completed to assess the effect of concentration using 4.2 wt% 4-methylstyrene, which approximates the actual diene value in an industrial feed (diene value = 9 g I2 / 100 g)3 and another test used 8.4 wt% α-methylstyrene in order to investigate the steric hindrance effect of the methyl group attached to the vinyl group. In some tests, 20 wt% cyclohexene was added to the feed to represent the olefin content in a typical naphtha feed3 and to determine the impact of olefin content on catalyst deactivation and reactor fouling.


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The reactor was operated for 3, 12, 21, and 30 days of continuous operation, starting with a new catalyst bed for each experimental period so that the change in catalyst properties with TOS could be determined. The reactor was operated at commercially relevant conditions (200-250°C, 3.4 MPa H2, LHSV of 2 h-1 and H2/feed ratio of 392 mL(STP)/mL).3

The reactor liquid product was collected every 24 hours from a condenser held at room temperature. The gas flowed from the condenser to a scrubber containing NaOH (1 M) where it was scrubbed of H2S and then vented. Part of the gas was directed toward a gas chromatograph (GC) for periodic analysis. In order to identify and quantify the different chemical components, the liquid product was analyzed using a Shimadzu QP-2010S GC-MS equipped with a Shimadzu RXI-5MS column (ID 0.25 mm, length 30 m, film 0.25 µm) and AOC-20i auto-sampler (10 µL syringe). From the GC-MS results, the fractional conversion (X), selectivity (S), and yield (Y) were calculated as follows: AB+C

= 1 −

() + () () = () () + () + ()

=

() () + ()

=

() () + () + ()

where CW(j) is the weight fraction of component j in the liquid product, wt% and CW(j0) is the weight fraction of component j in the liquid feed, wt%.


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To determine the overall liquid mass balance, a scale placed under the feed flask was used to measure the mass of liquid fed to the reactor. The measured mass was compared to the mass of liquid product. For all the experimental tests reported, the mass balance was > 92%.

The liquid was also analyzed using other techniques including gum content by jet evaporation (ASTM D381-12), bromine number/olefin content by electrometric titration (ASTM D1159-07), diene value by maleic anhydride addition reaction (UOP 326-08) and

13

C nuclear magnetic

resonance (13C NMR). Details of these analytical procedures are provided in the Supporting Information.

Experimental data are reported based on the average of at least 3 analyses completed for each sample collected. In addition, repeat experiments at a standard set of operating conditions were used to estimate the overall experimental error. The estimated error for reported conversions and product yields was ± 10%.4, 24

Results and discussion

Catalyst stability during the hydrogenation of 8.4 wt% 4-methylstyrene or α-methylstyrene in decalin was investigated first. The chosen conjugated cyclic olefin content of the feed corresponds approximately to twice the conjugated olefin value of a typical naphtha feed.3

Figure 1 reports the product yields as a function of TOS for 4-methylstyrene and αmethylstyrene hydrogenation at the reaction conditions shown. The dimer yield from 4-


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methylstyrene increased from 11.2 ± 0.4 wt % after 3 days to 32.4 ± 1.2 wt% after 30 days TOS, with a corresponding increase in gum content from 2626 ± 35 to 3152 ± 42 mg/100 mL. Figure 1 shows that with α-methylstyrene, the dimer yield was < 2 wt% and the gum content was < 40 mg/100 mL after 30 days TOS. Compared to 4-methylstyrene, the vinyl group of αmethylstyrene is sterically hindered by the methyl group. Since the reaction conditions were identical, the differences in dimer yield support the proposal that the sterically hindered vinyl group of α-methylstyrene decreases dimerization compared to 4-methylstyrene, consistent with similar results obtained for linear and cyclic conjugated olefins, reported previously.4 At the chosen operating conditions, the conversions of both 4-methylstyrene and α-methylstyrene reached 100% and remained at that level after 30 days TOS (Table 1 and 2). After 3 days TOS, 4-methylstyrene was mainly hydrogenated to 1-ethyl-4-methylbenzene with a yield of 78.0 ± 2.8 wt%. The completely hydrogenated product 1-ethyl-4-methylcyclohexane was also formed with a yield of 10.7 ± 0.4 wt%. The main dimerization product from 4-methylstyrene was 1,3-di-(4'methylphenyl)butane produced with a yield of 10.0 ± 0.4 wt%. Another dimer, 1,4-di-(4'methylphenyl)butane, was also detected with a yield of 1.2 wt%. After 30 days TOS, the hydrogenated product yield decreased, with the yield of 1-ethyl-4-methylbenzene decreasing to 56.6 ± 2.0 wt%; whereas, the yield of dimers increased to 30.3 ± 1.1 wt% and 2.1 ± 0.1 wt% for 1,3-di-(4'-methylphenyl)butane and 1,4-di-(4'-methylphenyl)butane, respectively. Note that the yield of 1-ethyl-4-methylcyclohexane was relatively unchanged with TOS suggesting that this product may be equilibrium controlled.

Confirmation of the formation of dimers and other oligomers was also sought through 13C-NMR analysis of the liquid product collected after 30 days TOS. Analysis of the liquid product


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confirmed the presence of 1-ethyl-4-methylbenzene, as shown in Figure S1 and Table S1 (Supporting information). Other peaks of the 13C-NMR spectrum could be assigned to decalin or to the

13

C-NMR solvent, CD2Cl2 (53.30-54.38 ppm), as shown in Table S2. No dimers were

detected by 13C-NMR due to their very low concentration in the liquid product. Consequently, a diluted gum sample was analyzed as shown in Figure S2. Most of the peaks of the13C-NMR spectrum can be assigned to 1,3-di-(4'-methylphenyl)butane as shown in Table S3. The peaks at 53.30-54.38 ppm belong to CD2Cl2 (13C-NMR solvent). Other peaks relate to the other dimer, 1,4-di-(4'-methylphenyl)butane, but due to the low intensity they are difficult to assign.

Since gum content determination involves evaporating the sample under controlled temperature (232°C well temperature) and steam flow (1000 mL/s), elemental CHNS analysis was also completed on the same gum sample to estimate the gum O content that may result from reaction with steam or trace oxygen in the reactant feed. The C, H, N, S elemental composition of the gum was determined to be 87.5 ± 1.1 wt%, 10.6 ± 1.1 wt%, 0.94 ± 0.03 wt% and 1.01 ± 0.02 wt%, respectively, indicating that the O content of the gum was below the detection limit of the analyzer. The results confirm that the gum consists mainly of carbon and hydrogen and that the oxygen content is insignificant.

The dimer yield increase with TOS when hydrogenating 4-methylstyrene was also accompanied by an increase in gum content from 2626 ± 35 mg/100 mL after 3 days TOS to 3152 ± 42 mg/100 mL after 30 days TOS (Table 1). The initial high hydrogenation activity of the catalyst results in complete hydrogenation of the olefins and conjugated diolefins, as indicated by the zero value of both the Br # and the diene value of the liquid product after 3 days TOS (Table 1).


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After 30 days TOS, the Br # and the diene value increased to 0.7 g/100 g and 0.3 I2 /100 g, respectively, indicative of a small decrease in catalyst hydrogenation activity (Table 1).

For α-methylstyrene, the main hydrogenated products were (1-methylethyl)benzene with 81.6 ± 2.9 wt% yield and (1-methylethyl)cyclohexane with 17.1 ± 0.6 wt% yield, after 3 days TOS (Table 2). The total hydrogenated product yield remained almost the same after 30 days TOS, but with increasing selectivity toward (1-methylethyl)benzene (84.4 ± 3.0 wt% yield) versus (1methylethyl)cyclohexane (13.8 ± 0.5 wt% yield). Dimer formation remained low throughout with 2,4-diphenyl-4-methyl-2-pentene and 2,4-diphenyl-4-methylpentane dimers formed with < 2 wt% yield. The gum content reflected the low yield of dimer formation with < 40 mg/100 mL gum formed. Both bromine number and diene value decreased compared to the feed values and remained < 0.2 g/100 g and < 0.2 g I2 /100 g, respectively, reflecting the stablility of the catalyst hydrogenation activity.

Proposed reaction steps for the dimerization of 4-methylstyrene and α-methylstyrene, on Bronsted acid sites, are shown in Figures 2 and 3, respectively. Okuhara et al.9 noted that the isomerization of conjugated olefins proceeds via a carbocation mechanism on the S layer of MoS2. Sufficient proton activity is available on the S layer to form a tertiary carbocation intermediate from 2-methyl-1-butene, which is then converted to 2-methyl-2-butene. Yang and Satterfield25 further explained that dissociative adsorption of H2S on Ni-Mo-S/γ-Al2O3 can convert a S vacancy to a -SH group and a Bronsted acid site. On industrial catalysts, the added P2O5 promoter can also be a source of acid sites (P content of NiMoS/Al2O3 catalyst is 2.29 wt %, Table 3). Hence, for 4-methylstyrene, we assume that dimerization starts with the formation


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of an allylic carbocation by reaction with a Bronsted acid site. The reaction then proceeds with the addition of a 4-methylstyrene molecule followed by reaction with a Bronsted base to form 1,3- or 1,4-di-(4'-methylphenyl)butene. 1,3- and 1,4-di-(4'-methylphenyl)butene are then hydrogenated to yield the observed products 1,3- and 1,4-di-(4'-methylphenyl)butane, respectively. The dimerization of α-methylstyrene proceeds in a similar manner with the formation of an allylic carbocation by reaction with a Bronsted acid, followed by addition of αmethylstyrene and the reaction with a Bronsted base to form 2,4-diphenyl-4-methyl-2-pentene. Part of 2,4-diphenyl-4-methyl-2-pentene is then hydrogenated to form 2,4-diphenyl-4methylpentane. As seen in Figure 3, the methyl group attached to the vinyl group not only limits activation of the double bond, but also the addition of α-methylstyrene to the allylic carbocation to form a dimer (step 2), and this is the likely cause of low dimer yield from α-methylstyrene.

The hydrogenation of 4-methylstytrene in the presence of 20 wt% cyclohexene was also examined to mimic the olefin content of a fluid coker naphtha (~20 wt%). Figure 4 compares the 4-methylstyrene conversion and the liquid product gum content, dimer yield and dimer selectivity for the case of 4-methylstyrene/decalin feed and the case of 20 wt% cyclohexene added to the 4-methylstyrene/decalin feed. The detailed product yields, conversion, gum content, Br # and diene values are listed in Table S4 for 4-methylstyrene with cyclohexene addition. The cyclohexene was only converted to cyclohexane so that the yield of cyclohexane is equal to the cyclohexene conversion. As shown in Figure 4, the conversion of 4-methylstyrene in the presence of cyclohexene decreased significantly with TOS from 100% after 3 days TOS to 75.3 ± 2.7 w% after 30 days TOS. This indicates a severe loss in catalyst activity that caused a significant decrease in hydrogenation activity, as shown by the decrease in the yield of the


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hydrogenated product 1-ethyl-4-methylbenzene (Table S4). The loss in hydrogenation activity was also reflected in an increase in Br # from 0.9 g/100 g after 3 days TOS to 33.5 ± 1.1 g/100 g after 30 days TOS and an increase in diene value from 0 to 2.5 ± 0.3 g I2 / 100 g. Similarly, the dimer yield increased from 6.5 ± 0.2 wt% after 3 days TOS to 25.7 ± 0.9 wt% yield after 30 days TOS. These changes in dimer yield and hydrogenated product yields (Table 1 and S4) with TOS were similar to those observed without cyclohexene in the feed (Figure 4). However, in the latter case, 4-methylstyrene conversion remained at 100 %. Figure 4 also reports the dimer selectivity versus TOS. Although the dimer yield was lower in the 4-methylstyrene + cyclohexene test compared to the 4-methylstyrene test, the selectivity was almost the same after 21 days TOS and slightly higher after 30 days TOS.

From these results, we conclude that the main impact of the addition of 20 wt% cyclohexene to 4-methylstyrene was a reduction in 4-methylstyrene hydrogenation, due to competitive adsorption on hydrogenation sites between the two reactants. The apparent higher catalyst deactivation rate of the catalyst in the presence of cyclohexene is likely due to fewer sites available for 4-methylstyrene activation because of the competitive adsorption of cyclohexene. Hence, 4-methylstyrene conversion is more sensitive to gum deposition in the case of cyclohexene in the feed, resulting in a higher catalyst deactivation rate, even though the gum content of the product was lower than in the case of no cyclohexene in the feed.

To examine the effect of operating at a temperature lower than 250°C, the hydrogenation of 8.4 wt% 4-methylstyrene and 20 wt% cyclohexene in decalin was measured at 200°C. The results are compared to those obtained at 250°C in Figure 5 and the detailed product analyses are


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reported in Table S5. The data indicate a faster decrease in conversion at 200°C, from 100 to 70.4 ± 2.5 %, compared to 100 to 75.3 ± 2.7 % at 250°C, after 30 days TOS. This change in conversion was accompanied by a lower yield and selectivity to hydrogenated products, higher yield and selectivity to dimers and higher gum content at 200°C versus 250°C reaction temperature. This indicates that the availability of a higher concentration of 4-methylstyrene at the lower temperature enhanced dimerization and gum formation and therefore resulted in catalyst deactivation.

To confirm the effect of conjugated olefin concentration on catalyst deactivation and reactor fouling as inferred above, a feed consisting of 4.2 wt% 4-methylstyrene and 20 wt% cyclohexene in decalin was hydrogenated at 250°C, 3.4 MPa H2, LHSV=2 h-1 and H2/feed=392 mL(STP)/mL. Figure

6

compares

the

conversions,

gum

contents,

4-methylstyrene

hydrogenated

products/dimers yields and selectivities, with details provided in Table S6. Although the 4methylstyrene conversions were the same at 3 days TOS, at both concentrations, the overall 4methylstyrene conversion remained at 100% until 21 days TOS with 4.2 wt% 4-methylstyrene content and then decreased to 93.8 ± 3.3%. On the other hand, with 8.4 wt% 4-methylstyrene, the results show a faster decrease in conversion reaching 75.3 ± 2.7 % at 30 days TOS. This indicates lower loss of overall catalyst activity with lower conjugated olefin content in the feed. The lower catalyst deactivation resulted in higher yields and selectivities of hydrogenated products which remained above 80 wt% after 30 days TOS in the case of the 4.2 wt% 4methylstyrene compared to yields and selectivities below 60 wt% with 8.4 wt% 4methtylstyrene. With regard to dimers, higher yields and selectivities were obtained for the 8.4 wt% versus 4.2 wt% 4-methylstyrene following the same trend as the loss in catalyst activity


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indicated by the conversion results. The gum formation was the most sensitive to conjugated olefin content and increased more than 5-fold when the conjugated olefin content in the feed doubled.

As before, these results indicate that catalyst deactivation increased with increased 4methylstyrene concentration, resulting in higher dimerization yield. The loss in catalyst activity is also shown by the decrease in cyclohexene hydrogenation activity, as reflected in the cyclohexene conversion to cyclohexane. The activity loss was more severe when 8.4 wt% 4methylstyrene was used versus 4.2 wt%, as shown in Table S4 and S6, respectively. The loss of hydrogenation activity is also indicated by the increase in Br number which was more dramatic for the 8.4 wt% versus 4.2 wt% 4-methylstyrene feeds.

Relationship between dimer formation, catalyst deactivation and reactor fouling:

The relationship between dimer formation and gum formation is summarized in Figure 7 for all experiments completed as part of this study. The data show a clear trend of increasing gum content with increasing dimer yield in the liquid product, suggesting that the dimers are precursors to gum formation, with higher concentration of gum formed in the presence of higher dimer concentration.

To relate the dimer formation and gum formation to reactor fouling, the average pressure drop measured across the reactor as a function of TOS is reported in Figure 8, comparing 4- and αmethylstyrene as reactants. The reaction with 4-methylstyrene caused a pressure drop that


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increased with TOS, correlating with increased dimer yield and gum formation. On the other hand, no significant pressure drop was detected for α-methylstyrene, which produced a very low yield of dimers (< 2wt%) and gum (< 40 mg/100 mL). Hence, the pressure drop across the reactor is assumed to be caused by the deposition of gum on the reactor bed, decreasing the average reactor bed voidage. Consequently, with higher gum content, more gum deposits on the catalyst bed causing higher pressure drop, as observed with increasing TOS. Assuming that the pressure drop increase is caused by a decrease in average reactor bed voidage, the modified Ergun equation (See Supporting Information) was used to estimate the required change in average reactor bed voidage to achieve the measured pressure drop.26 Figure 8 shows the model fit versus measured pressure drop obtained by varying the average reactor bed voidage with TOS as a model parameter. Based on the model, the average reactor bed voidage decreased from 0.39 to 0.12. The large decrease in average reactor bed voidage was visually noticed after reactor shutdown as the bed was plugged with gum and unloading of the bed was not possible without using external force. Assuming the decrease in reactor bed voidage is caused by cumulative gum deposition, the total volume of gum deposited in the bed after 30 days TOS was estimated to be 8.8 mL. Assuming a gum density of 0.9 g/mL, the mass of deposited gum is 7.9 g. Hence a minimum oil gum content of ~221 mg/100 mL would be required if all the gum was deposited on the catalyst during the 30 days TOS. Given that the measured gum content in the liquid product reached 2469 ± 33 mg/100 mL at 30 days TOS for 4-methylstyrene + cyclohexene as reactant, the deposition of this amount of gum is certainly feasible. Hence, the deposition of < 10 wt% of the measured gum content in the liquid product results in a significant change in pressure drop.


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The “as received” properties of the catalyst used in this study are reported in Table 3; whereas, the carbon content and BET surface area of the catalyst recovered after 3, 12, and 21 days TOS are reported in Figure 9. The data show an overall increase in carbon content and decrease in BET surface area of the catalyst compared to the catalyst properties at the start of the experiments (TOS = 0 days). Note that it is expected that the deposition of gum on the catalyst will vary with bed height resulting in variability in the carbon content and BET area of the used catalyst. Since sampling the catalyst from different bed heights was not possible, the entire sample recovered from the reactor was randomly sampled for analysis. Gum formation was also detected on the glass beads and silicon carbide, but due to the very small quantity, it was not quantified.

Comparing the properties of the catalysts recovered after reaction with 4- and α-methylstyrene, shows that the decrease in BET surface and the increase in carbon content was larger for 4methylstyrene versus α-methylstyrene (Figure 9), in agreement with the higher dimer formation and gum content in the liquid product with 4-methylstyrene versus α-methylstyrene as reactant. Comparing 4-methylstyrene+cyclohexene, reacted at 200 and 250°C, the BET surface area was lower and the carbon content was higher at 200°C versus 250°C after 30 days TOS, corresponding to the higher dimer yields and gum content measured for the reaction at 200 °C versus 250°C, as shown in Figure 9. These results support the conclusion that dimerization of the conjugated olefin leads to gum formation that then deposits on the catalyst, resulting in reduced bed porosity and increased pressure drop.


Energy & Fuels 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

Page 18 of 35

Finally we note that dimerization of 4-methylstyrene reported herein occurs at significantly lower temperatures than that which occurs thermally.27, 28 According to Fan et al.,27 the fouling propensity of conjugated olefins present in coker gas oil (CGO) is very low at 200 °C; whereas, at temperatures between ∼250 and 325 °C, the fouling propensity increases because of polymerization of unsaturated olefins. Above 350 °C, fouling is observed to increase significantly because of thermal cracking reactions that produce olefins and conjugated olefins. The fouling deposits of CGO exhibit typical polyaromatic coke structures. In contrast, results from the present study suggest that in the case of catalytic reactions, higher temperatures that ensure fast hydrogenation of the conjugated olefins result in less gum formation (Figure 5), at least in the temperature range 200 to 250 °C.

Conclusions

Hydrogenation of conjugated olefins on a spent commercial Ni-Mo-S/γ-Al2O3 catalyst, yields dimers that lead to gum formation and catalyst deactivation. By comparing 4- and αmethylstyrene hydrogenation, it is shown that the sterically hindered vinyl group of αmethylstyrene results in decreased dimerization of the conjugated olefin. The addition of 20 wt% cyclohexene to 4-methylstyrene or reaction at lower temperature decreased hydrogenation of the 4-methylstyrene. Increased concentration of 4-methylstyrene in the feed and lower reaction temperature increased the yield of dimerization products and gums. The overall trend of increasing gum yield with increasing dimer yield suggests that the dimers are precursors to gum formation. Catalyst deactivation was linked to carbon deposition on the catalyst caused by dimer


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and gum formation; increased dimer and gum formation were accompanied by an increased carbon content and decreased BET surface area of the used catalysts. Furthermore, the observed increase in pressure drop with TOS, only observed with 4-methylstyrene as reactant and not αmethylstyrene, are evidence that the increase in pressure drop is caused by cumulative gum deposition in the catalyst bed. The pressure drop is well described by the Ergun equation, assuming that the gum deposition reduces bed voidage with TOS.

Acknowledgement Funding from Syncrude Canada Ltd. and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. Ali Alzaid is also sincerely grateful for the Research and Development Center of Saudi Aramco for the M.A.Sc. and Ph.D. scholarship.


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Page 20 of 35

Table 1: Hydrogenation of 4-methylstyrene. Reaction conditions: 8.4 wt% 4-methylstyrene in decalin at 250°C, 3.4 MPa H2, LHSV=2 h-1, H2/feed=392 mL(STP)/mL. Yield, wt% Product

Structure

TOS, days 0

3

12

21

30

-

78.0 ± 2.8

64.3 ± 2.3

62.1 ± 2.2

56.6 ± 2.0

-

10.7 ± 0.4

14. 2± 0.5

13.6 ± 0.5

11.0 ± 0.4

-

10.0 ± 0.4

19.7 ± 0.7

22.4 ± 0.8

30.3 ± 1.1

-

1.2 ± 0

1.8 ± 0.1

1.9 ± 0.1

2.1 ± 0.1

Total dimers, wt%

-

11.2 ± 0.4

21.5 ± 0.8

24.3 ± 0.9

32.4 ± 1.2

Conversion, %

-

100

100

100

100

Gum content mg/100 mL

204 ± 2

2626 ± 35

2746 ± 37

2797 ± 38

3152 ± 42

Bromine number, g / 100 g

11.4 ± 0.4

0

0.2 ± 0

0.3 ± 0

0.7 ± 0

Diene value, g I2 / 100 g

8.9 ± 1.1

0

0.2 ± 0

0

0.3 ± 0

1-ethyl-4methylbenzene

1-ethyl-4-methylcyclohexane

1,3-di-(4'methylphenyl)butane 1,4-di-(4'methylphenyl)butane


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Energy & Fuels

Table 2:

Hydrogenation of α-methylstyrene. Reaction conditions: 8.4 wt% α-methylstyrene in decalin at 250°C, 3.4 MPa H2, LHSV=2 h-1, H2/feed=392 mL(STP)/mL. Yield, wt%

Product

Structure

TOS, days 0

3

12

21

30

-

81.6 ± 2.9

83.0 ± 2.9

83.3 ± 2.9

84.4 ± 3.0

-

17.1 ± 0.6

15.9 ± 0.6

15.3 ± 0.5

13.8 ± 0.5

-

1.3 ± 0

1.1 ± 0

0.8 ± 0

1.0 ± 0

-

0

0

0.6 ± 0

0.7 ± 0

Total dimers, wt%

-

1.3 ± 0

1.1 ± 0

1.4 ± 0.1

1.7 ± 0.1

Conversion, %

-

100

100

100

100

Gum content mg/100 mL

46 ± 1

34 ± 0

15 ± 0

28 ± 0

11 ± 0

Bromine number, g / 100 g

11.9 ± 0.4

0.2 ± 0

0

0

0

Diene value, g I2 / 100 g

2.5 ± 0.3

0.2 ± 0

0.2 ± 0

0.2 ± 0

0.1 ± 0

(1-methylethyl)benzene

(1-methylethyl)cyclohexane 2,4-diphenyl4-methyl-2-pentene 2,4-diphenyl4-methylpentane


Energy & Fuels 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

Table 3:

Page 22 of 35

Properties of Ni-Mo-S/Al2O3 catalyst as used in the present study

Catalyst property

As received

Elemental composition, wt % Al

26.80

Ni

2.25

Mo

9.76

P

2.29

BET Surface area, m2/g

156 ± 6

Pore volume, cm3/g

0.28 ± 0.01

Pore size, nm

7.22 ± 0.02

CHNS analysis, wt % C

11.70 ± 0.18

H

0.82 ± 0.01

N

0.36 ± 0.02

S

5.14 ± 0.19


Page 23 of 35

Dimer yield, wt %

60

4-methylstyrene α-methylstyrene 40

20

0

Gum content, mg/100mL

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

Energy & Fuels

3000 2500 2000 1500 1000 500 0 0

5

10

15

20

25

30

Time-on-stream, days Figure 1:

Dimer yield and gum content of liquid product from 4-methylstyrene and αmethylstyrene hydrogenation. Reaction conditions: 8.4 wt% conjugated olefin in decalin, 250°C, 3.4 MPa H2, LHSV of 2 h-1 and H2/feed=392 mL(STP)/mL.


Energy & Fuels 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

Figure 2:

Proposed dimer formation reaction steps from 4-methylstyrene.

Figure 3:

Proposed dimer formation reaction steps from α-methylstyrene.

Page 24 of 35


Page 25 of 35

100 3000

2500 80

70

60

FEED: 4-methylstyrene 4-methylstyrene + cylohexene

50


4-Methylstyrene conversion, wt %

90

2000

1500

1000

40 500 30 0 0

5

10

15

20

25

30

50

50

45

45

40

40

35

35

Dimer selectivity, wt %

Dimer yield, wt %

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

Energy & Fuels

30 25 20 15

5

10

15

20

25

30

0

5

10

15

20

25

30

30 25 20 15

10

10

5

5

0

0

0 0

5

10

15

20

25

Time-on-stream, days

Figure 4:

30


Reactant conversion and product yield/selectivity during hydrogenation of 4methylstyrene or 4-methylstyrene plus 20 wt % cyclohexene. Reaction conditions: 8.4 wt% 4-methylstyrene in decalin, with or without cyclohexene reacted at 250°C, 3.4 MPa H2, LHSV of 2 h-1 and H2/feed=392 mL(STP)/mL.


Energy & Fuels

3000 Gum content, mg/100mL

4-Methylstyrene conv., wt %

100

90

o

80

250 C o 200 C

70

2500 2000 1500 1000 500 0

Hydrogenated prodcut selectivity, wt %

Hydrogenated product yield , wt%

100 90 80 70 60 50 40 30

100

90

80

70

60

50

60

40

DImer Selectivity, wt %

50 30

Dimer yield, wt %

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

Page 26 of 35

20

10

0

40 30 20 10 0

0

5

10

15

20

25

30

0


Figure 5:

5

10

15

20

25

30


Reactant conversion and product yield/selectivity during hydrogenation of 4methylstyrene plus 20 wt % cyclohexene. Reaction conditions: 8.4 wt% 4methylstyrene in decalin, with or without cyclohexene reacted at 200 or 250°C, 3.4 MPa H2, LHSV of 2 h-1 and H2/feed=392 mL(STP)/mL. 26 ACS Paragon Plus Environment

Page 27 of 35

3000 Gum content, mg/100mL

4-Methylstyrene conv., wt %

100

90

80

8.4 wt% 4-methylstyrene 4.2 wt % 4-methylstyrene

70

2500 2000 1500 1000 500 0

Hydrogenated product selectivity, wt %

Hydrogenated product yield, wt %

100 90 80 70 60 50 40 30

100

90

80

70

60

50

60

40

DImer Selectivity, wt %

50 30

Dimer yield, wt %

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

Energy & Fuels

20

10

0

40 30 20 10 0

0

5

10

15

20

25

30

0

5


Figure 6:

10

15

20

25

30


Reactant conversion and product yield/selectivity during hydrogenation of 4methylstyrene plus 20 wt % cyclohexene. Reaction conditions: 4.2 wt % or 8.4 wt% 4-methylstyrene in decalin, with or without cyclohexene reacted at 250°C, 3.4 MPa H2, LHSV of 2 h-1 and H2/feed=392 mL(STP)/mL. 27 ACS Paragon Plus Environment

Energy & Fuels

4000 8.4wt% 4-methylstyrene 8.4wt% α-methylstyrene 8.4wt% 4-methylstyrene + 20 wt% cylohexene 8.4wt% 4-methylstyrene + 20 wt% cylohexene 4.2wt% 4-methylstyrene + 20 wt% cylohexene

3000


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

Page 28 of 35

2000

1000

0 0

5

10

15

20

25

30

35

40

Dimer yield, wt % Figure 7

Gum content versus 4-methylstyrene dimer yield during hydrogenation at 250°C, 3.4 MPa H2, LHSV=2 h-1, H2/feed=392 mL(STP)/mL.


Page 29 of 35

α-methylstyrene

160 ____

140

Average daily pressure drop, kPa

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

Energy & Fuels

4-methylstyrene Ergun equation

120

100

80

60

40

20

0 0

5

10

15

20

25

30

Time-on-stream, days 0.6

0.7

0.8

0.9

(1-εBED) Figure 8:

Average pressure drop across reactor versus TOS during hydrogenation of 8.4 wt% α-methylstyrene in decalin [] or 8.4 wt% 4-methylstyrene + 20 wt% cyclohexene in decalin (o) at 250°C, 3.4 MPa H2, LHSV=2 h-1, H2/feed=392 mL(STP)/mL. Solid line represents Ergun Equation calculation of pressure drop as a function of 1-εB, where εB is the assumed bed voidage change due to gum deposition. 29 ACS Paragon Plus Environment

Energy & Fuels

30

C content, wt %

25

20

15

10

160 140

2

BET area, m /g

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

Page 30 of 35

120 100 80 60 0

5

10

15

20

25

30

TOS, days Figure 9

Carbon content and BET surface area of Ni-Mo-S/γ-Al2O3 catalyst after reaction at 200 or 250°C, 3.4 MPa H2, LHSV of 2 h-1, H2/feed=392 mL(STP)/mL, for various TOS periods. catalyst: spent Ni-Mo-S/γ-Al2O3). Feed: () – 8.4 wt% αmethylstyrene; (∆) – 8.4 wt % 4-methylstyrene; (O) – 8.4 wt% 4-methylstyrene + 20 wt% cyclohexene; (▄) – 4.2 wt% 4-methylstyrene + 20 wt% cyclohexene; (•) 8.4 wt% 4-methylstyrene + 20 wt% cyclohexene reacted at 200 °C. 30 ACS Paragon Plus Environment

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References:

1. Energy Information Administration Trend 1980 - 2010. Energy Information Administration. International Energy Statistics: Energy Intensity | Country: Oman | Indicator: Energy Intensity Total Primary Energy Consumption per Dollar of GDP (Btu per Year 2005 U.S. Dollar, 19802010. Data-Planet™ Statistical Ready Reference by Conquest Systems, Inc. Dataset-ID: 004014-0412015. 2. Gray, M. R. Upgrading Oilsands Bitumen and Heavy Oil; University of Alberta Press: 2015; . 3. Sok Yui Removing diolefins from coker naphtha necessary before hydrotreating Oil & Gas Journal 1999, 97, 64. 4. Alzaid, A.; Wiens, J.; Adjaye, J.; Smith, K. J. Impact of molecular structure on the hydrogenation and oligomerization of diolefins over a Ni-Mo-S/γ-Al2O3 catalyst Fuel 2018, 221, 206-215. 5. Dobrovolná, Z.; Kacer, P.; Cervený, L. Competitive hydrogenation in alkene–alkyne–diene systems with palladium and platinum catalysts Journal of Molecular Catalysis A: Chemical 1998, 130, 279-284. 6. Sales, E. A.; de Jesus Mendes, M.; Bozon-Verduraz, F. Liquid-phase selective hydrogenation of hexa-1,5-diene and hexa-1,3-diene on palladium catalysts. Effect of tin and silver addition Journal of Catalysis 2000, 195, 96-105.


Energy & Fuels 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

Page 32 of 35

7. Wells, P. B.; Bates, A. J. The hydrogenation of alkadienes, Part II. The hydrogenation of buta1,3-diene catalysed by rhodium, palladium, iridium, and platinum wires J. Chem. Soc. A 1968, 3064-3069. 8. Goetz, J.; Murzin, D. Y.; Touroude, R. A. Kinetic aspects of selectivity and stereoselectivity for the hydrogenation of buta-1,3-diene over a palladium catalyst Industrial & Engineering Chemistry Research 1996, 35, 703-711. 9. Okuhara, T.; Itoh, H.; Miyahara, K.; Tanaka, K. Hydrogenation of dienes and the selectivity for partial hydrogenation on a molybdenum disulfide catalyst Journal of Physical Chemistry 1978, 82, 678-682. 10. Wambeke, A.; Jalowiecki, L.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. The active site for isoprene hydrogenation on MoS2γ-Al2O3 catalystsJournal of Catalysis 1988, 109, 320-328. 11. Bartholomew, C. H.; Farrauto, R. J. Fundamentals of Industrial Catalytic Processes; Wiley: 2006; , pp 996. 12. Anderson, J. R.; Boudart, M. Catalysis Science and Technology; Springer Berlin Heidelberg: 1996; Vol. 11. 13. Badawi, M.; Vivier, L.; Duprez, D. Kinetic study of olefin hydrogenation on hydrotreating catalysts Journal of Molecular Catalysis A: Chemical 2010, 320, 34-39. 14. Brémaud, M.; Vivier, L.; Pérot, G.; Harlé, V.; Bouchy, C. Hydrogenation of olefins over hydrotreating catalysts: Promotion effect on the activity and on the involvement of H2S in the reaction Applied Catalysis A: General 2005, 289, 44-50. 32 ACS Paragon Plus Environment

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15. Badawi, M.; Vivier, L.; Pérot, G.; Duprez, D. Promoting effect of cobalt and nickel on the activity of hydrotreating catalysts in hydrogenation and isomerization of olefins Journal of Molecular Catalysis A: Chemical 2008, 293, 53-58. 16. Brunet, S.; Mey, D.; Pérot, G.; Bouchy, C.; Diehl, F. On the hydrodesulfurization of FCC gasoline: a review Applied Catalysis A: General 2005, 278, 143-172. 17. Ferdous, D.; Dalai, A. K.; Adjaye, J. A series of NiMo/Al2O3 catalysts containing boron and phosphorus. Part II. Hydrodenitrogenation and hydrodesulfurization using heavy gas oil derived from Athabasca bitumen. Applied Catalysis A: General 2004, 260, 153-162. 18. Mapiour, M.; Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Effects of the operating variables on hydrotreating of heavy gas oil: Experimental, modeling, and kinetic studies. Fuel 2010, 89, 2536-2543. 19. Bej, S. K.; Dabral, R. P.; Gupta, P. C.; Mittal, K. K.; Sen, G. S.; Kapoor, V. K.; Dalai, A. K. Studies on the performance of a microscale trickle bed reactor using different sizes of diluent. Energy Fuels 2000, 14, 701-705. 20. Mapiour, M.; Sundaramurthy, V.; Dalai, A. K.; Adjaye, J. Effect of hydrogen purity on hydroprocessing of heavy gas oil derived from oil-sands bitumen Energy Fuels 2009, 23, 21292135. 21. Mears, D. E. Role of axial dispersion in trickle-flow laboratory reactors Chemical Engineering Science 1971, 26, 1361-1366.


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22. Doraiswamy, L. K.; Tajbl, D. G. Laboratory catalytic reactors Catalysis Reviews - Science and Engineering 1974, 10, 177-219. 23. Kumar, V. R.; Balaraman, K. S.; Rao, V. S. R.; Ananth, M. S. Performance study of certain commercial catalysts in hydrodesulfurization of diesel oils Petroleum Science and Technology 2001, 19, 1029-1038. 24. Alzaid, A. H. Impact of conjugated olefins on nickel-molybdenum-sulphide supported on gamma-alumina catalyst deactivation and fouling of naphtha hydrotreaters, University of British Columbia, 2016. 25. Yang, S. H.; Satterfield, C. N. Catalytic hydrodenitrogenation of quinoline in a trickle-bed reactor. Effect of hydrogen sulfide Ind. Eng. Chem. Proc. Des. Dev. 1984, 23, 20-25. 26. Satterfield, C. N. Trickle-bed reactors AIChE J. 1975, 21, 209-228. 27. Fan, Z.; Rahimi, P.; Alem, T.; Eisenhawer, A.; Arboleda, P. Fouling Characteristics of Hydrocarbon Streams Containing Olefins and Conjugated Olefins Energy Fuels 2011, 25, 11821190. 28. Wiehe, I. A. Mitigation of the Fouling by Popcorn Coke Petrol Sci Technol 2003, 21, 673680.


Page 35 of 35

(1-εBed)

Hydrogenation

Dimerization

Gums

32%

68%

NiMoS/Al2O3 H2 at 3.4 MPa 250°°C 30 days

98%

Catalyst Deactivation and Reactor Fouling during Hydrogenation of

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