Article pubs.acs.org/IECR
Determining the Strength of Brønsted Acid Sites for Hydrodewaxing over Shape-Selective Catalysts Stephen J. Miller, Howard S. Lacheen,* and Cong-Yan Chen Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94802, United States ABSTRACT: This paper reports a method for directly determining the effective strength of those acid sites on a molecular sieve catalyst that are actually involved in reactions such as hydrodewaxing. The method is here applied to the conversion of a waxy paraffin over a catalyst containing platinum and a SAPO-11 sieve of high isomerization selectivity, as well as a catalyst containing platinum and a HZSM-5 zeolite (SiO2/Al2O3 = 150) of high cracking selectivity. Results are compared against the measured Brønsted acid site strength on these molecular sieves determined from temperature-programmed desorption of ammonia and Fourier transform infrared analysis of adsorbed pyridine. Characterization of acid sites showed both concentration and apparent acid strength for H-ZSM-5 higher than those for SAPO-11, with the latter having a finite but smaller amount of strong acid sites that adsorbed pyridine at 573 and 673 K. Reactions with n-hexadecane gave significantly higher conversion rates on H-ZSM-5, and it is shown that a strongly adsorbing molecule like ammonia can be used in situ to estimate the strength of acid sites involved in hydrocarbon conversion steps. Using the method, the “effective acid site strength” for the SAPO-11 catalyst is determined to be about 30 kJ/mol less than on the H-ZSM-5 catalyst. The reactivity of these catalysts is discussed in light of the acid sites and other factors, including crystal structure and size.
1.0. INTRODUCTION There has been considerable interest in achieving a better understanding of the conversion of paraffinic hydrocarbons over acidic molecular sieve catalysts, in particular the hydroconversion of long-chain paraffins, which are important for the production of modern transportation fuels and lubricants. Of special interest has been the hydroconversion (hydrodewaxing) of those paraffins over intermediate pore shape-selective zeolite catalysts, such as ZSM-5, to selectively remove waxy molecules allowing those fuels and lubricants to operate at low temperature. Included in that field is the selective skeletal isomerization of long-chain paraffins over a hydrogenation component such as Pt on a 1-D intermediate pore molecular sieve such as SAPO-11, first reported over 20 years ago.1−4 Since that time, a number of commercial processes and catalysts have been introduced, including Chevron’s ISODEWAXING process for reducing the pour point of lubricating oils and middle distillate fuels. This technology made it economically possible to meet the growing demand in the market place for lubricating oils and fuels with advanced performance and environmental and safety benefits from a wide range of waxy feedstocks, including those of high wax content. There have been a number of studies to better understand the catalysis and the relationship between the activity and selectivity of those shape-selective molecular sieves and their physical properties. These studies have focused on two main aspects: (1) the influence of the shape selectivity on the carbenium ion chemistry of the catalysis and (2) the influence © 2016 American Chemical Society
of the acid sites, particularly Brønsted acid site density and strength. For those catalysts of high isomerization selectivity, such as SAPO-11,5−16 researchers have pointed to the constraint of the pores in inhibiting the formation of highly branched reaction intermediates that readily crack, as well as reduced acid site strength as compared to the acid sites in zeolites such as ZSM-5,17−19 where acid site density and strength are typically determined by techniques such as microcalorimetry or temperature-programmed desorption (TPD) of ammonia or pyridine. To date, this has involved measuring site density and strength and then, when there is a distribution of strengths, inferring or guessing which sites take part in the catalysis. In many cases, sites responsible for the catalysis have been labeled as “strong” or “medium” in terms of their acid strengths. One particular drawback to this approach is that there is no general agreement on which sites are “strong” and which are “medium”; the dividing line between the two is often drawn arbitrarily, leading to uncertainty as to which sites are actually involved in the catalysis. Consequently, the current definition of “medium” is arbitrary and of limited value to anyone developing an industrial catalyst. Furthermore, for catalysts such as SAPO-11, where there is a broad distribution of acid site strengths, there has been, to date, no direct Received: Revised: Accepted: Published: 6760
March 18, 2016 May 19, 2016 May 24, 2016 May 24, 2016 DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
Article
Industrial & Engineering Chemistry Research measurement of the strength of the acid sites actually involved in the catalysis. Being able to determine the strength of the acid sites on a catalyst responsible for a particular acid-catalyzed reaction would be of benefit in allowing us to (1) more efficiently screen new materials and new catalyst preparations for specific reactions and (2) design and optimize molecular sieves and other acid catalysts for specific reactions. This paper reports a method for determining the effective strength of acid sites on a catalyst for reactions such as hydrodewaxing. It uses the impact, in a continuous feed reactor, of different dosage levels of ammonia on the reaction acid sites, where all other conditions are constant, including selectivity, such that the only thing that changes upon changing the ammonia level is the catalyst temperature required at that level to maintain a fixed conversion. The method is here applied to the conversion of a waxy paraffin over a catalyst containing platinum and a SAPO-11 sieve of high isomerization selectivity and a broad distribution of acid site strengths, as well as a catalyst containing platinum and a H-ZSM-5 zeolite of high cracking selectivity and a narrow distribution of acid site strengths. The paper also compares those results against acid site distribution and strength of these two molecular sieves, prior to Pt addition, using analytical methods (pyridine titration and ammonia TPD) reported in the literature.
versus 1/T, we should get a straight line with a slope equal to −(Ea + λp)/R, as long as the assumption holds that Kppp ≫ 1+ Kp. If we determine Ea separately, we can then solve for λp, the heat of adsorption of the inhibitor, or that required to desorb the inhibitor so that the hydrocarbon can react. If we choose NH3 as the inhibitor, this will give us a measure of the “effective acid strength” of the active site for the hydrocarbon conversion reaction, assuming adsorption and reaction occur on the same site. We call this the “effective acid strength” because there is likely a distribution of strengths, rather than a single strength, which can catalyze the reaction in question. It should also be pointed out, as noted in a previous publication,2 that the selectivity for hydroisomerizing nhexadecane over Pt-SAPO-11 was unaffected by adding up to 200 ppm nitrogen as n-butylamine to the feed, which then converts to NH3 at the top of the catalyst bed.
3.0. MATERIALS AND METHODS 3.1. Infrared Spectroscopy. Vibrational spectra were measured using transmission Fourier transform infrared (FTIR). Molecular sieves in proton form were ground and pressed into wafers (5−10 mg/cm2) and dehydrated in vacuum at 723 K for 1 h in a steel cell with CaF windows. Spectra were recorded at 423 K with a MCT detector with 128 scans from 400 to 4000 cm−1. Each dehydrated sample spectrum was used as a baseline and subtracted from the absorption spectrum after pyridine exposure to calculate the difference spectra in the 1400−1800 cm−1 range. Pyridine was adsorbed on dehydrated samples at 423 K. Excess pyridine was removed by evacuating at 423 K for 1 h; this also allowed for the removal of the weakly adsorbed pyridine on the sample and infrared cell. Then samples were further evacuated at 573 and 673 K to progressively desorb pyridine. A vibrational mode in the 1540−1550 cm−1 region was attributed to pyridinium ions, providing evidence of Brønsted acidity, and used to calculate the acid site concentration. H-ZSM-5 (SiO2/Al2O3 = 54) was used to calibrate the 1540 cm−1 infrared band. 3.2. Ammonia Temperature-Programmed Desorption. For evaluation of acid strength, ammonia TPD was measured in a quartz flow-through cell with approximately 300 mg of pelletized catalyst (Micromeritics Autochem II 2920). Samples consisted of zeolites in proton form without Pt because the metal could interact with ammonia and also with acid sites of the catalyst in the unreduced state. Samples were dried at 773 K in argon for 1 h, then cooled to 393 K and exposed to a flow of 5% ammonia/argon for 0.5 h. Then the flow was switched to argon to sweep residual ammonia out for 1 h. The sample temperature was then increased at a constant heating rate of 0.17 K/s to a final temperature of 773 K. Flow to weight ratio was varied from 8 × 10−4 to 8 × 10−3 m3/(kg s) in separate tests while heating the sample in argon. Ammonia concentration was monitored by a thermal conductivity detector. A deconvolution procedure was used to separate overlapping peaks using Origin 8.5 software. The baseline was subtracted with a linear function spanning the temperature axis, and the components of the TPD profile were modeled with Gaussian functions. 3.3. Catalysts. The SAPO-11 molecular sieve used in this study was synthesized in our laboratory. It was identified by Xray diffraction analysis and found to contain no impurity crystalline phases. The sieve was dried and then calcined in air at 863 K for 6 h. The molar ratios of oxides were as follows:
2.0. THEORETICAL METHODS The simple rate equation for a unimolecular surface reaction in the presence of an inhibitor can be expressed as Rate = kKp /(1 + Kp + K ppp )
(1)
where k is the rate constant and K and Kp are the adsorption equilibrium constants for the reactant and inhibitor, respectively; p and pp are the partial pressures of reactant and inhibitor, respectively. In the case where the inhibitor is more strongly adsorbed than the reactant and
K ppp > >1 + Kp
(2)
then the rate equation simplifies to Rate = kKp /K ppp
(3)
This equation can then be rewritten as Rate = A e−E / RT A1e λ / RT p /A 2 e λp / RT pp
(4)
where E is the activation energy for conversion of the hydrocarbon and λ and λp are the heats of adsorption of hydrocarbon and inhibitor, respectively; A, A1, and A2 are constants. In this case, any impact of the presence of an inhibitor on the rate may be compensated by adjusting temperature. If p is held constant and the rate is held constant by operating at constant conversion, then pp = C e−(E − λ + λp)/ RT , where C = AA1p/(Rate × A 2)
(5)
pp = Ce−(Ea + λp)/ RT
(6)
or
where Ea is the apparent activation energy, equal to E − λ. If we then vary pp at constant hydrocarbon conversion and plot ln pp 6761
DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
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Industrial & Engineering Chemistry Research 0.24SiO2 : Al 2O3 : 1.02P2O5
The apparent activation energy, Ea, for hydroconversion of nhexadecane over each catalyst was determined by holding n-C16 conversion constant at 90% and measuring the required reactor temperature for that conversion over the range of LHSV between 1 and 8. Because hydrodewaxing normally requires well over 90% conversion of waxy paraffins, we ran at that high conversion to more closely match what would be expected for that process. Based on the Arrhenius relationship, Ea was determined from the slope of the plot of the natural logarithm of the liquid hourly space velocity against the reciprocal reactor temperature.
The ZSM-5 zeolite was CBV1502, obtained from PQ. Its Hform was obtained via ion exchange with a SiO2/Al2O3 molar ratio of about 150 based on ICP. Neutron activation analysis determined a SiO2/Al2O3 molar ratio of 130. External surface areas (ESA) were measured using N2 adsorption at 77 K and calculated from the slope of the straight line obtained by plotting the adsorbed volume versus the de Boer thickness between 0.5 and 0.8 nm. ESA for H-ZSM-5 and SAPO-11 were 65 m2/g and 47 m2/g, respectively. 3.4. Catalyst Testing. For both Pt-SAPO-11 and Pt/HZSM-5 catalysts, 1.0 wt % Pt was loaded to the corresponding sieve via incipient wetness impregnation technique. For example, to prepare the Pt/H-ZSM-5 catalyst, an aqueous solution consisting of 0.1019 g Pt(NH3)4(NO3)2 and 3.4588 g H2O was added to 5.0772 g H-ZSM-5. The resulting mixture was stirred rigorously with a spatula for 10 min, then kept in a sealed glass bottle at room temperature for 48 h, and subsequently dried in a vacuum oven at 393 K for 2 h. The dried sample was then calcined in a calcination oven with flowing air using the following temperature program: from room temperature to 393 at 0.017 K/s and at 393 K for 0.5 h; then from 393 to 450 K at 0.017 K/s and at 450 K for 2 h; subsequently from 450 to 505 K at 0.017 K/s and at 505 K for 2 h; next from 505 to 561 K at 0.017 K/s and at 561 K for 5 h; finally cooling to room temperature. Pt-SAPO-11 was prepared similarly. The calcined Pt/H-ZSM-5 or Pt-SAPO-11 catalyst was then pelletized, crushed, and sieved to 20−40 mesh. For the catalytic testing, 1.0 g of the meshed catalyst was centered in a down-flow stainless steel tube reactor in a split tube furnace. The catalyst was reduced at atmospheric pressure in a H2 flow of 60 standard cm3/min in situ from room temperature to 589 K in 3 h and subsequently kept at 589 K for 5 h. Next, the catalyst was presulfided with 3.5 cm3 of H2S injected in a H2 flow of 60 standard cm3/min over a time of 5 min at 589 K and atmospheric pressure prior to introduction of hydrocarbon feed to minimize hydrogenolysis. Then the reactor was brought to the reaction pressure of 6996 kPa and the preselected reaction temperature (e.g., 561 K). The reaction was then started with pure n-hexadecane (≥99%, Aldrich) feed (containing various amounts of nitrogen and delivered by an Isco pump) at the preselected liquid hourly space veloctiy (LHSV) and molar H2 to hydrocarbon ratio. The inlet H2 to nhexadecane molar ratio was kept at 14 although LHSV varied. To make a feed containing a defined level of nitrogen in nhexadecane (e.g., 20 ppm of N by weight), a stoichiometric amount of n-butylamine was doped into the feed bottle of nhexadecane. n-Butylamine was then converted to NH3 in situ in the reactor under the hydroprocessing conditions applied in this work. Reaction products from the product flow coming from the reactor outlet were injected automatically into an online Agilent 5890 gas chromatograph equipped with a 50 m long HP-1 capillary column and a flame ionization detector. The online gas chromatography (GC) analysis was carried out every 1.5 h. The following GC oven temperature program was used: 5 min at 323 K and then from 323 K for 5 min to 393 K at 0.05 K/s; from 393 to 523 K at 0.1 K/s; and then at 523 K for 20 min. Percent conversion is defined as 100 − wt % n-C16 in the product. Isomerization selectivity is
4.0. RESULTS AND DISCUSSION 4.1. FTIR Acid Site Measurements. Infrared absorption was used to monitor the concentration of hydroxyl species in H-ZSM-5 and SAPO-11 (Figures 1 and 2). Both materials have
Figure 1. Infrared spectra in the hydroxyl region before and after pyridine adsorption on H-ZSM-5 at 423 K () and after pyridine saturation and desorption at 423 K (···), 573 K (- - -), and 673 K (− ·).
Figure 2. Infrared spectra in the hydroxyl region before and after pyridine adsorption on SAPO-11 at 423 K () and after pyridine saturation and desorption at 423 K (···), 573 K (- - -), and 673 K (− ·).
100 × (wt% i‐C16)product /(percent conversion) 6762
DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
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Industrial & Engineering Chemistry Research absorption bands near 3740 cm−1 arising from silanols. Bridging hydroxyls that give rise to acid sites are observed at 3605 cm−1 in H-ZSM-5. In SAPO-11, two vibrations at 3620 and 3520 cm−1 have been assigned to bridging hydroxyls, based on their absence in silicon-free ALPO-11 and their disappearance upon ammonia exposure as reported by Parlitz et al.20 The latter was assigned to bridging hydroxyls distorted by interactions with lattice oxygen. An additional sharp, intense band at 3675 cm−1, not found in aluminosilicates, arises from P−OH groups in SAPO-11. Acid sites were further investigated by adsorption of a base, pyridine, to selectively titrate acid sites. Pyridine that is protonated at Brønsted acid sites or coordinated to Lewis acid sites can be identified by vibrational modes at 1540 and 1450 cm−1, respectively. Adsorption on Brønsted sites was also apparent by the attenuation of the hydroxyl band at 3605 cm−1 for H-ZSM-5 (Figure 1) and 3620 cm−1 in SAPO-11 (Figure 2) after pyridine exposure. The intensity of 3520 cm−1 band was also decreased after pyridine adsorption. At 423 K, evidence of minor pyridine adsorption is found at silanols (3740 cm−1) in both zeolites and other framework hydroxyls at 3675 cm−1 in SAPO-11. Desorption at 573 K resulted in pyridine removal on certain sites, restoring the 3675 and 3740 cm−1 bands. In contrast, the 3620 cm−1 band in SAPO-11 retained a significant amount of pyridine at 573 K. Acid sites remaining at this temperature are sometimes designated as strong acid sites. The 3605 cm−1 hydroxyl peak in H-ZSM-5 was unchanged when heating from 423 to 573 K, further indicating that pyridine remained adsorbed on Brønsted sites. Adsorbed pyridine decreased further after evacuation at 673 K on SAPO-11 and also was lowered on H-ZSM-5. This was also apparent from the pyridine adsorption spectra in Figures 3 and 4, which indicated pyridine on Brønsted sites at 1540 cm−1 was decreased in intensity on both molecular sieves at 673 K. Both structures possessed sites of varying strength, as indicated from the residual surface concentrations with desorption temperature, given in Table 1, including some degree of strong acid sites with pyridine remaining at 673 K. Pyridine is adsorbed at 423 K on H-ZSM-5, 0.23 mmol/g, and
Figure 4. Infrared difference spectra on SAPO-11 after pyridine saturation and desorption at 423 K (), 573 K (- - -), and 673 K (− ·).
Figure 3. Infrared difference spectra on H-ZSM-5 after pyridine saturation and desorption at 423 K (), 573 K (- - -), and 673 K (− ·).
Figure 5. Ammonia TPD profiles during ammonia desorption on SAPO-11 (···) and H-ZSM-5 (). Heating rate for desorption test was 0.17 K/s.
Table 1. Residual Pyridine on Molecular Sieves Calculated from Infrared Spectra sample
pyr. adsorbed, 423 K (mmol/g)
pyr. adsorbed, 573 K (mmol/g)
pyr. adsorbed, 673 K (mmol/g)
H-ZSM-5 SAPO-11
0.23 0.14
0.21 0.07
0.14 0.02
SAPO-11, 0.14 mmol/g, and progressively desorbed with increasing temperature, as it was removed from weaker sites. Pyridine on H-ZSM-5 and SAPO-11 at 573 K was 0.21 mmol/g and 0.07 mmol/g, respectively; these are indicative of the quantity of strong acid sites. 4.2. Ammonia TPD. Acid sites were further investigated by adsorbing ammonia and then measuring desorption profiles (Figure 5). H-ZSM-5 gave two peaks, a low-temperature maxima from weak or nonacidic sites near 473 K and a strong acid site peak centered near 660 K. The deconvoluted peak centered at 660 K in Figure 5 represented 0.20 mmol/g. This is similar to the amount expected from elemental analysis (0.215
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H-ZSM-5 gave ammonia adsorption enthalpies that were nearly independent of heating rate, β, of 150 kJ/mol at 0.08 K/s and 146 kJ/mol at 0.17 K/s. These values are within the reported range of 137−150 kJ/mol for H-ZSM-5 determined by ammonia desorption and calorimetry.28,29 The ammonia heat of desorption for SAPO-11 measured according to eq 7 was 100−109 kJ/mol. The calculated values should be dependent on the distribution of all acid sites in the material. The SAPO-11 may possess sites of higher or lower adsorption enthalpy, as was demonstrated by pyridine adsorption, but the calculated value from eq 7 determined by ammonia TPD appeared to be of lower strength than found in aluminosilicate zeolites, despite the fact that it contains sites with demonstrated alkane isomerization activity. The ammonia TPD profiles contained overlapping contributions from nonacidic or weakly acidic sites that desorbed below 473 K. Peak profile fitting was done by a Gaussian fitting algorithm. It was investigated whether ammonia on the nonacidic sites from the first peak could be preferentially desorbed before the TPD. This was done by desorbing at 463 K in argon after ammonia saturation. This treatment in SAPO-11 gave only one peak, as shown in Figure 7, confirming that the
mmol/g), implying that a small amount of aluminum, less than 10%, in the sample may not be in the zeolite framework. The NH3 TPD profiles for SAPO-11 contained a large peak near 450 K and a shoulder near 550 K. The high-temperature peak occurred at lower temperature than on the H-ZSM-5. It has been attributed to acid sites ranging from medium to strong by other investigators, but there is general agreement that SAPO-11 has weaker acid sites on the average than in zeolites.21−23 Deconvolution of these peaks was not trivial because of the highly overlapping nature of the peaks. The best fit was obtained using Gaussian functions for the lowtemperature and high-temperature contributions. SAPO-11 gave an acid site concentration from the high-temperature peak of 0.19 ± 0.02 mmol/g. It has been demonstrated that the deprotanation energy gives a rigorous description of acid strength.24 The utility of ammonia adsorption enthalpies, however, has been recently shown to be effective for relating the acid site binding enthalpy to reaction rates in the methanol−propene reaction.25 The adsorption enthalpy of acid sites is measurable by changing the conditions of the ammonia desorption experiment, such as testing at variable flow rate, in order to decouple the experimental conditions from the desorption profile.26 Here, we used the method of Sawa et al., to measure acid strength from ammonia desorption profiles by varying flow to sample mass ratio.27 The corresponding heat of desorption is then calculated by eq 7, which was derived under the assumption of equilibrium between adsorbed and gas-phase ammonia. A plot of ln (Tmax) − ln (A0W/F) and inverse temperature (Figure 6) gives a straight line, and the slope is equal to the desorption enthalpy. ln Tmax − ln + ln
A 0W ΔH = F RTmax
β(1 − θmax )2 (ΔH − RTmax ) P 0 exp(ΔS /R )
(7) Figure 7. Ammonia desorption profiles of SAPO-11 with different purge temperatures (393 K, ; 463 K, ···). The desorption step used a heating rate of 0.17 K/s and an argon flow of 25 standard cm3/min.
argon purge removed ammonia from nonacidic sites but may have led to the desorption from acid sites also because the concentration was lower in comparison to the 393 K purge, with an ammonia desorption of 0.12 ± 0.01 mmol/g. The desorption enthalpy was similar as determined by desorption starting at 393 K, about 113 kJ/mol, and this confirmed that the deconvolution procedure used above gave a reasonable model of the aggregate of surface adsorption sites. The SAPO-11 acid site densities detected by ammonia TPD were higher than the 0.07 mmol/g detected by adsorption of pyridine at 573 K. This may be a consequence of the more strongly bonding sites probed by pyridine at 573 K, because pyridine may have already been removed from the surface at lower temperature. It may also be related to contributions of physisorbed ammonia that were not completely excluded under the peak fitting procedure. The acid site density we find for ammonia TPD, after a 463 K purge, is about the same as for pyridine titration at 423 K (Table 1); therefore, we speculate that pyridine adsorption on SAPO-11 at 423 K reflects both medium and strong sites as corroborated by its NH3 desorption profile. A second possibility for the difference is the size of
Figure 6. Dependence of high-temperature maximum on inverse temperature according to eq 7. Plot shows H-ZSM5 at 0.08 K/s (□) and 0.17 K/s (■) and SAPO-11 at 0.08 K/s (○) and 0.17 K/s (●).
The measured parameters in the TPD experiment are the maximum temperature, Tmax, corresponding to the hightemperature peak, and acid site density, A0. The ratio of sample mass, W, to volumetric flow rate, F, is a constant set during the experiment. The rate of temperature increase is β, and θ is the NH3 coverage at peak maximum; ΔH and ΔS are the enthalpy and entropy of NH3 desorption. 6764
DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
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Industrial & Engineering Chemistry Research probe molecules; ammonia can interact with acid sites not approachable by the larger pyridine. The pores in SAPO-11, a medium pore zeolite, have dimensions of 6.5 Å by 4.0 Å and could allow for better access of ammonia than pyridine. Restricted diffusion of pyridine in certain zeolites has been reported previously, such as in ferrierite30 and erionite or offretite with stacking fault.31 Probing acid sites with NH3 or pyridine can be used to characterize zeolites using well-established methods as has been described here. One drawback to this approach is they do not unequivocally distinguish between sites that participate in hydroisomerization reactions and those that do not. In the next section, zeolites will be used in catalytic reactions and the kinetics will be determined with ammonia addition to elucidate the strengths of active sites during realistic conversion conditions. 4.3. Activation Energy Determination. As mentioned in section 3.4, the apparent activation energy, Ea, for hydroconversion of n-hexadecane over each catalyst was determined by holding n-C16 conversion constant at 90% and measuring the required reactor temperature for that conversion over the range of LHSVs between 1 and 8. Based on the Arrhenius relationship, the natural logarithm of the liquid hourly space velocity was plotted against the reciprocal reactor temperature (Figure 8 for Pt-ZSM-5 and Figure 9 for Pt-SAPO-11), giving
4.4. Determination of Acid Site Strength for Reaction. The feed was spiked with different levels of organic nitrogen (as n-butylamine), holding LHSV constant at 2.0 and measuring the required temperature to again obtain 90% conversion, where the catalyst temperature was held constant for at least 24 h at each nitrogen level to ensure equilibration. The last nitrogen level tested was the same as the first level tested to ensure there was no significant deactivation during the testing period. Based on eq 6, the natural logarithm of the nitrogen content in the feed (in ppm by weight) was plotted versus the reciprocal reactor temperature (Figures 10 and 11). The level
Figure 8. Arrhenius plot for n-hexadecane conversion over Pt/HZSM-5 at 6996 kPa and 90 wt % conversion.
Figure 11. Effect of nitrogen on Pt-SAPO-11 at 90% n-hexadecane conversion.
Figure 9. Arrhenius plot for n-hexadecane conversion over Pt-SAPO11 at 6996 kPa and 90% conversion.
of nitrogen added ranged from 200 to 2 ppm. Again, a straight line was obtained at least down to around 10 ppm nitrogen, much below which the assumption that Kppp ≫ 1+ Kp apparently no longer holds. The slope of this line, equivalent according to eq 6 to Ea + λNH3, assuming all the butylamine was converted to NH3 at the top of the catalyst bed, was about 345 kJ/mol for Pt-ZSM-5 (Figure 10) and 315 kJ/mol for PtSAPO-11 (Figure 11). Subtracting out the 180 kJ/mol found for Ea in each case gave an effective acid site strength, λNH3 of about 165 kJ/mol for Pt-ZSM-5 and 135 kJ/mol for Pt-SAPO11. The somewhat higher number for SAPO-11 by this method than the 113 kJ/mol we found via NH3-TPD could be explained if some of the acid sites accounted for in the NH3TPD determination are not substantially involved in the catalysis. It could also be that sites of stronger acid strength have a greater impact on the effective acid strength than sites of lesser strength. It is worth noting that the Pt-SAPO-11 catalyst, prior to amine addition, was around 80 K less active than the Pt-ZSM-5 catalyst. This may be somewhat surprising in light of the results for pyridine titration at 573 K, which shows the Pt-SAPO-11 to have one-third the strong acid site density (adsorbed at 573 K) as the Pt-ZSM-5. If those sites on the SAPO-11 were equivalent
Figure 10. Effect of nitrogen on Pt/H-ZSM-5 at 90% n-hexadecane conversion.
in each case a straight line with a slope, or Ea, of 180−185 kJ/ mol. This value is similar to literature values reported for skeletal hydroisomerization of long-chain n-paraffins.32 The selectivity to branched isomers of n-hexadecane varied little over that range for Pt-SAPO-11, being 89−92%, with the remainder being hydrocracking. Pt-ZSM-5 had much lower isomerization selectivity, also fairly constant over that range at 15−17%. 6765
DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
Article
Industrial & Engineering Chemistry Research
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to those on the ZSM-5, then based on the observed Ea, the activity difference should only be around 15 K. Besides the lower number of acid sites, there are other factors which could contribute to the observed activity difference. One would be a larger crystallite size for the SAPO-11, and lower external surface area. Meriaudeau et al. have already reported a crystallite size effect on the activity of Pt-SAPO-11 for paraffin isomerization, from which the authors suggest that the reaction is mainly occurring at the pore mouths, whose number should be roughly proportional to the external surface area.5 In any event, because both the SAPO-11 and ZSM-5 in our study had about the same external surface area, we do not see this factor as significantly affecting the catalytic results. Second, as reported by Suzuki et al., acid sites with a lower Brønsted acid strength are found to have a lower turnover frequency (TOF) for the catalytic cracking of n-octane, suggesting the same relation might hold for similar acid catalyzed reactions.29 More recently, Wang et al. have shown a good correlation between the TOF for propene methylation and the calculated adsorption enthalpy for NH3 over a range of metal-substituted ALPO-34 catalysts.25 These findings would also suggest that the somewhat lower effective acid strength for SAPO-11, along with fewer acid sites, would also contribute to its lower activity compared to the ZSM-5. Both the Suzuki and Wang references show that a 20 kJ/mol lower acid site strength can reduce TOF by an order of magnitude. Consequently, a somewhat lower acid strength, coupled with fewer acid sites could account for the activity difference observed for SAPO-11 versus the HZSM-5. 4.5. Conclusions. A catalytic test was used to determine an “effective acid strength” for acid-catalyzed hydrodewaxing over both Pt-SAPO-11 and Pt/H-ZSM-5 catalysts. This test showed the effective acid strength for hydroisomerization dewaxing over SAPO-11 to be about 135 kJ/mol, around 30 kJ/mol less than that determined by the same test for hydrodewaxing over Pt/H-ZSM-5. While pyridine titration indicated a portion of the sites on SAPO-11 were of similar strength to those found on H-ZSM-5, the lower hydroconversion activity of Pt-SAPO11 compared to Pt/H-ZSM-5 is consistent with a lower effective acid strength as well as the lower apparent ammonia desorption energy derived from NH3-TPD. The effective acid strength test could serve as a tool to help guide catalyst design.
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AUTHOR INFORMATION
Corresponding Author
*Fax: 1-510-242-2823. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
Article
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DOI: 10.1021/acs.iecr.6b01081 Ind. Eng. Chem. Res. 2016, 55, 6760−6767
