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Acid and base characteristics of molybdenum carbide catalysts
Applied Catalysis A: General 250 (2003) 197–208
Acid and base characteristics of molybdenum carbide catalysts Shyamal K. Bej, Christopher A. Bennett, Levi T. Thompson∗ Department of Chemical Engineering, University of Michigan, 3026 H.H. Dow Building, 2300 Hayward Avenue, Ann Arbor, MI 48109-2136, USA Received 3 July 2002; received in revised form 2 December 2002; accepted 7 December 2002
Abstract The acid and base properties of a high surface area Mo2 C catalyst were characterized using the temperature programmed desorption of CO2 and NH3 , the decomposition of isopropyl alcohol (IPA) as a test reaction and monitoring changes in the associated rates and product selectivities on the addition of acid and base site poisons. The Mo2 C catalyst was prepared using the temperature programmed reaction method and passivated prior to exposure to air. Prior to carrying out the temperature programmed desorption experiments and reaction rate measurements, the Mo2 C catalyst was reduced in H2 at 400 ◦ C. Results obtained for the reduced Mo2 C catalyst were compared with those for MgO, HZSM-5 and 1% Pt/SiO2 catalysts. The study provided evidence for the presence of both acid and base sites on Mo2 C. The base and acid sites on the Mo2 C catalyst were weaker than those on the MgO and HZSM-5 catalysts, respectively. The base and acid sites were likely created as a consequence of charge transfer from molybdenum to carbon. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum carbide; Acid catalysis; Base catalysis; Alcohol decomposition; CO2 TPD; NH3 TPD; Poisoning studies
1. Introduction Transition metal carbides and nitrides possess a number of interesting properties. For example, in high surface area form they have catalytic properties similar to those of Pt-group metals [1]. In addition, carbides are resistant to poisoning by sulfur compounds [2,3]. These materials are known to catalyze reactions including hydrodesulfurization, hydrodenitrogenation, isomerization, dehydrogenation, water gas shift and amination [4–9]. Most transition metal carbides and nitrides are interstitial compounds with the non-metal atoms residing in interstices in the metal lattice. Because of ∗ Corresponding author. Tel.: +1-734-936-2015; fax: +1-734-763-0459. E-mail address: [email protected] (L.T. Thompson).
their significantly different electronegativities, charge transfer from metal to non-metal can occur [10] and this could result in the creation of acid and base sites. In fact, it has been recently reported that some nitrides possess acid and base properties. Kaskel and Schlichte [11] demonstrated that SiN possesses base properties and can catalyze the isomerization of 2,3-dimethylbut-1-ene. Passivated Mo2 N has been claimed to possess acid properties as evidenced by its capability to catalyze the dehydration of octanol to octene [12], although this acidity has been linked to the presence of oxygen. In the present article, we report characterization of the acid and base properties of Mo2 C. Although there are several methods available for characterizing the acid–base properties of solids, two are most commonly used [13–15]. The first method is based on the adsorption and desorption of acidic
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or basic probe molecules [14,16–18]. The second method is based on the performance of the catalysts in test reactions. A number of reactions have been employed to determine the acidity of solids [19,20]. In contrast, only a few reactions have been used to deduce the basicity [13]. The decomposition of alcohols like isopropanol (IPA) and cyclohexanol has been employed to evaluate the acid–base properties of catalysts [13,18]. In addition to thermal desorption and test reaction methods, monitoring changes in the rate and product selectivity for the probe reaction while selectively poisoning the acid and base sites has been used [21,22]. In this study, we used CO2 and NH3 temperature programmed desorption (TPD) to determine the base and acid properties of Mo2 C, respectively. The basicity and acidity were also investigated using the decomposition of isopropanol (IPA) as a test reaction. It is known that IPA undergoes dehydrogenation to form acetone over base sites whereas acid sites favor the dehydration of IPA to propylene [18,23–27]. We also conducted poisoning studies to establish the presence of acid–base properties. The results obtained for the Mo2 C catalyst were compared with those for MgO, a known solid base, HZSM-5, a known solid acid, and a 1% Pt/SiO2 catalyst.
temperature for 2 h. This temperature program was developed based on the results of the thermogravimetric analysis and has been used previously to prepare Mo2 C that is devoid of excess carbon. After completion of the reaction, the sample was quenched to room temperature in the reactant mixture. To prevent pyrolysis on exposure to air, the material was passivated using 1% O2 in He (Scott) flowing at 20 ml/min. Before carrying out the TPD studies or measuring the catalytic activity, the passivation layer was removed by reducing the catalyst. Additional details concerning the synthesis procedure and experimental setup are given elsewhere [8]. The MgO catalyst was prepared according to the procedure described by Di Cosimo et al. [28]. About 250 ml of distilled water was added to 25 g of commercial MgO (Baker) at room temperature. The mixture was then stirred and the temperature was increased to 80 ◦ C. The temperature was maintained for 4 h with constant stirring. The material was dried overnight at 85 ◦ C. The dried mass was then heated at 350 ◦ C for 2 h and 500 ◦ C for 8 h in flowing air (30 ml/min). The calcined sample was kept in an airtight container. The HZSM-5 (Zeolyst International) and 1% Pt/SiO2 (GFS Chemicals) catalysts were used as received. 2.2. Catalyst characterization
2. Experimental 2.1. Catalyst preparation High surface area Mo2 C was prepared by the temperature programmed reaction method. In brief, a vertical quartz flow-through reactor was used for the synthesis. Approximately 1.5 g of ammonium paramolybdate (Alfa Aesar) was placed over a quartz wool plug supported by a quartz frit located inside the reactor. The reactor was placed inside a high temperature furnace. The temperature of the reactor was measured using a thermocouple placed above the precursor inside the reactor and controlled using an Omega CN2010 temperature controller connected to the furnace. A mixture of 15% CH4 and balance H2 (Matheson) was passed through the reactor at a rate of 250 ml/min. The reactor was then quickly heated from ambient temperature to 200 ◦ C in ∼20 min. The temperature was then increased linearly to 590 ◦ C in ∼6.5 h and held at this
The BET surface areas were determined using a Micromeritics ASAP 2010 porosimeter with nitrogen as the sorbate. The materials were degassed in vacuum at 400 ◦ C until the static pressure remained less than 3 mHg prior to analysis. The BET surface areas of the catalysts are given in Tables 1 and 2. Helium, CO2 and NH3 TPD experiments were performed using a Micromeritics ASAP 2910 sorption analyzer. The He TPD experiments allowed us to determine species that desorbed from the freshly pretreated catalysts. The catalysts were pretreated prior to the TPD and activity measurements. The Mo2 C catalyst was heated to 400 ◦ C in flowing H2 , reduced at that temperature for 4 h using a H2 flow of 20 ml/min then flushed with He flowing at 20 ml/min for 30 min at 400 ◦ C. The MgO catalyst was treated at 500 ◦ C for 5 h in a helium flow of 20 ml/min. The HZSM-5 catalyst was pretreated at 500 ◦ C for 4 h in He flowing at 20 ml/min. The 1% Pt/SiO2 catalyst was reduced at 500 ◦ C for 4 h under H2 flowing at
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Table 1 Amounts of CO2 desorbed from the Mo2 C, MgO and 1% Pt/SiO2 catalysts Catalyst
Mo2 C MgO 1% Pt/SiO2 a
BET surface area (m2 /g)
50 212 244
CO2 desorbed below 400 ◦ C
CO2 desorbed above 400 ◦ C
ml/g
Molecules/cm2
ml/g
Molecules/cm2
0.81 16.9 0.22
4.4 × 1013 2.1 × 1014 2.4 × 1012
–a 11.5
1.5 × 1014
CO2 that desorbed at temperatures greater than 400 ◦ C was attributed to decomposition of the Mo2 C catalyst.
20 ml/min before performing the activity and TPD experiments. These experimental conditions are typical of those used to activate HZSM-5 and Pt-based catalysts [29,30]. The pretreated samples were saturated with He, CO2 (Matheson) or NH3 (Scott) at room temperature for 30 min (20 ml/min). The sample was flushed with He flowing at 20 ml/min for 30 min then the temperature was increased to 800–1000 ◦ C at a heating rate of 10 ◦ C/min. Gases that desorbed were monitored using a thermal conductivity detector (TCD) and a mass spectrometer connected on line with the Micromeritics 2910. 2.3. Catalytic activity measurements The decomposition of IPA was performed using a 4 mm i.d. quartz reactor. Approximately 100–300 mg of catalyst was placed on a quartz wool plug inside the reactor. The reactor was then placed inside a furnace. The temperature of the reactor was measured using a K-type thermocouple placed inside the catalyst bed and controlled using an Omega CN2010 temperature controller connected to the furnace. The catalyst samples were pretreated using procedures described in Section 2.2. The reactor was then brought to the desired temperature in order to measure the catalytic activity. For routine catalytic activity measurements, a feed stream consisting of IPA and He was used. Helium
was passed through a series of saturators containing IPA (Fischer) at ambient temperature. The flow rate of He was controlled using a mass flow controller (Tylan FC-260). The He/IPA mixture was mixed with another stream containing pure He coming through a by-pass line. The He flow rate coming from the by-pass line was measured using a rotameter and controlled using a needle valve. The composition of the reactant stream before entering the catalyst bed was 3.0 mol% IPA and 97.0 mol% He. Carbon dioxide, an acid gas, was used to selectively poison the base sites whereas the acid sites were poisoned using NH3 . The temperature for the poisoning studies was determined based on the CO2 and NH3 TPD results. For each temperature, three sets of experiments were carried out. In the first set, the reactant consisting of a mixture of IPA and He was passed over the catalyst. After steady state was reached, pure He coming from the by-pass line was discontinued and an equal amount of either CO2 or NH3 was added to the mixture of IPA and He. The flow rate of CO2 was measured using a rotameter and controlled using a needle valve, whereas, a Tylan FC-260 mass flow controller was used to control the flow rate of NH3 . After the poisoning segment, the CO2 or NH3 flow was discontinued and the He flow was initiated so that the composition was 3.0 mol% IPA and 97.0 mol% He. During the poisoning segment, the reactant stream consisted of 3.0 mol% IPA, 50 mol% He and 47 mol% CO2 or NH3 .
Table 2 Amounts of NH3 desorbed from the Mo2 C and MgO catalysts Catalyst
Mo2 C HZSM-5
BET surface area (m2 /g)
50 625
NH3 desorbed below 300 ◦ C
NH3 desorbed above 300 ◦ C
ml/g
Molecules/cm2
ml/g
Molecules/cm2
4.2 102
2.3 × 1014 4.4 × 1014
44
1.9 × 1014
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The product gas mixture was analyzed using an Hewlett-Packard 5690 gas chromatograph equipped with a flame ionization detector and a packed column containing 28% AT-223 and 4% KOH on Gas Chrom R (Altech). The signal from the detector was analyzed using a Spectra-Physics Chrom Jet integrator, which also controlled injections into the gas chromatograph via a Valco six-way actuator. The quartz reactor and wool were found to have negligible activities under the experimental conditions used.
3. Results 3.1. Helium temperature programmed desorption The He TPD spectrum for the reduced Mo2 C catalyst is shown in Fig. 1. At temperatures less than 580 ◦ C, there was no gas evolution; however, substantial amounts of CO and CO2 were detected above 580 ◦ C indicating that the surface and/or subsurface contained oxygen and this oxygen was reactive. No species other than CO and CO2 desorbed from the re-
duced Mo2 C catalyst. No gas evolution was observed for the MgO, HZSM-5 or 1% Pt/SiO2 catalysts on heating them in He to temperatures as high as 1000 ◦ C. This is not unexpected given that the materials had been reduced or degassed at high temperatures prior to the TPD experiments. 3.2. CO2 temperature programmed desorption The CO2 TPD spectrum for Mo2 C is shown in Fig. 2. A CO2 peak having desorption maxima at 230 ◦ C was observed. In addition, CO and CO2 started desorbing at temperatures higher than 520 ◦ C. These high temperature peaks were probably due to decomposition of the Mo2 C catalyst based on the He TPD results (Fig. 1). The low temperature CO2 desorption peak suggested the presence of base sites. The CO2 TPD spectra for MgO and 1% Pt/SiO2 are shown in Fig. 3. Carbon dioxide desorption peaks for MgO occurred at ∼180, 300 and 525 ◦ C. These results are consistent with those reported in the literature. For example, Diez et al. [18] reported a broad CO2 desorption peak for MgO at temperatures ranging from
Fig. 1. Helium TPD spectrum for the Mo2 C catalyst.
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Fig. 2. Carbon dioxide TPD spectrum for the Mo2 C catalyst.
Fig. 3. Carbon dioxide TPD spectra for the MgO and 1% Pt/SiO2 catalysts.
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180 to 410 ◦ C. During CO2 TPD from the 1% Pt/SiO2 , a very small amount of CO2 desorbed at 90 ◦ C. This peak may have been a consequence of CO2 desorption from sites on the Pt particles or weak base sites on SiO2 . The amount of CO2 desorbed from all of the materials was determined and the results are given in Table 1. Magnesium oxide had a high density of CO2 adsorption sites while the Pt/SiO2 catalyst had the lowest site density. The site density for the Mo2 C catalyst suggests that the base sites accounted for ∼5% of the surface sites (assuming a total site density of 1015 sites/cm2 ). Typically the desorption temperature is an indication of the base site strength. The results suggest that sites on Mo2 C were less basic than those on MgO.
absence of acidity [13]. Ammonia desorbed from the HZSM-5 catalyst producing peaks at ∼110, 170 and 370 ◦ C (Fig. 5). Inui et al. [30] reported similar NH3 TPD results. They found two types of acidic sites present on HZSM-5. The weaker acidic sites had NH3 desorption maxima at about 160 ◦ C while the moderate strength sites produced peaks at about 440 ◦ C. Slight differences in the peak maxima when compared to our results might be due to differences in the carrier gas flow rates. The amount of NH3 that desorbed from the Mo2 C catalyst was significantly lower than that from HZSM-5 (see Table 2). The NH3 adsorption site density was equivalent to ∼20% of the surface of the Mo2 C catalyst. Based on the NH3 desorption temperatures, the strength of acid sites on Mo2 C was lower than those for HZSM-5.
3.3. NH3 temperature programmed desorption 3.4. Isopropanol decomposition The NH3 TPD spectrum for the Mo2 C catalyst is shown in Fig. 4. This catalyst adsorbed significant amounts of NH3 producing a desorption peak at about 125 ◦ C. This result indicated the presence of acid sites. Magnesium oxide did not adsorb NH3 , indicating the
The catalysts were typically tested for IPA decomposition at 140–200 ◦ C. Reaction rates for the Mo2 C, MgO, HZSM-5 and Pt/SiO2 catalysts are compared in Fig. 6. Because it was much less active than the
Fig. 4. Ammonia TPD spectrum for the Mo2 C catalyst.
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Fig. 5. Ammonia TPD spectrum for the HZSM-5 catalyst.
Fig. 6. Isopropyl alcohol decomposition rates for Mo2 C, MgO and 1% Pt/SiO2 catalysts at varying temperatures.
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other catalysts, rates for the MgO catalyst were measured at 260–320 ◦ C. Acetone and propylene were the only products formed under the reaction conditions employed. The selectivities to acetone are compared in Fig. 7. In general the behavior of the Mo2 C catalyst was more like that for MgO than HZSM-5. For all of the materials, the selectivity to acetone decreased with increasing temperature. Figs. 6 and 7 show that the Mo2 C catalyst was significantly more active than the other catalysts. In addition, this material possessed a very high selectivity to acetone indicating that the dehydrogenation of IPA, a base-catalyzed reaction, was predominant. The MgO catalyst was also highly selective for the production of acetone, however, the activity was much lower than that for the Mo2 C catalyst. The 1% Pt/SiO2 catalyst was selective for the dehydrogenation of IPA to acetone, however, it had a very low activity. The HZSM-5 catalyst was highly active for the conversion of IPA. Unlike the Mo2 C catalyst, the HZSM-5 catalyst was selective for the production of propylene. This is a known acid-catalyzed reaction. Isopropanol turnover frequencies based on the total CO2 and NH3 uptakes
were 0.44 and 0.006 s−1 at 200 ◦ C for the Mo2 C and HZSM-5 catalysts, respectively. 3.5. Site poisoning studies 3.5.1. CO2 poisoning The CO2 poisoning studies for the Mo2 C catalyst were conducted at 160 and 260 ◦ C. Based on the CO2 TPD results, most of the base sites should be blocked by carrying out the poisoning study at 160 ◦ C, whereas only the strong sites should be occupied at temperatures higher than 260 ◦ C. Results for the CO2 poisoning studies conducted at 160 ◦ C are shown in Fig. 8. The conversion of IPA was ∼51 mol% and the selectivity for acetone was ∼88 mol% before the addition of CO2 . When CO2 was added, the conversion decreased to 28 mol% and the selectivity to acetone decreased to 72 mol%. This result indicated that base sites on the Mo2 C catalyst were active for IPA dehydrogenation to acetone. While these sites appear to be primarily responsible, they do not account for all of the dehydrogenation activity. It is possible that some base sites remained
Fig. 7. Selectivity toward acetone for Mo2 C, MgO and 1% Pt/SiO2 catalysts at varying temperatures.
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Fig. 8. Effect of CO2 on IPA conversion and selectivity for the Mo2 C catalyst at 160 ◦ C.
available for participation in the reaction or that metal sites contributed. The rate of propylene formation was independent of the absence or presence of CO2 . This observation suggested that IPA dehydrogenation and dehydration occurred on different sites. When CO2 was removed from the feed stream, the activity was essentially restored to its original state indicating that CO2 was reversibly adsorbed on the active sites. Carbon dioxide had no effect on the IPA conversion or selectivity for the Mo2 C catalyst at 260 ◦ C (Fig. 9). This indicated that the stronger basic sites present on the Mo2 C catalyst were not involved in dehydrogenation or dehydration reactions. Carbon dioxide poisoning studies for the 1% Pt/SiO2 catalyst were conducted at 160 ◦ C. Recall that reaction rates were very low for this catalyst. The results of the poisoning studies are shown in Fig. 10. There was only a very slight decrease in the decomposition of IPA upon the introduction of CO2 . Acetone was the only product formed over the 1% Pt/SiO2 catalyst. The results indicated that under the experimental conditions employed, CO2 did not block active sites on Pt or SiO2 . This observation was in agreement with the CO2 TPD studies which showed
negligible amounts of CO2 desorbing from the 1% Pt/SiO2 catalyst. 3.5.2. NH3 poisoning Ammonia poisoning studies for the Mo2 C catalyst were conducted at 180 ◦ C. The effect of NH3 poisoning on IPA conversion and selectivity are shown in Fig. 11. The presence of NH3 decreased the IPA conversion from 40 to 18 mol%, presumably due to blockage of the acid sites. The selectivity towards acetone increased from 84 to 99 mol%. This indicated that acids site were responsible for the dehydration of IPA. Interestingly, the acetone formation rate also decreased indicating that acidic sites may have been involved in the dehydrogenation of IPA to acetone. The catalyst slowly returned to its original activity when the NH3 was removed from the feed stream.
4. Discussion Results from the IPA decomposition studies showed that Mo2 C possessed significant activity for the dehydrogenation of alcohol. These results as well as
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Fig. 9. Effect of CO2 on IPA conversion and selectivity for the Mo2 C catalyst at 260 ◦ C.
Fig. 10. Effect of CO2 on IPA conversion for the 1% Pt/SiO2 catalyst at 160 ◦ C.
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Fig. 11. Effect of NH3 poisoning on IPA conversion and selectivity for the Mo2 C catalyst at 180 ◦ C.
those from the CO2 TPD studies indicated the presence of base sites. Based on the CO2 TPD studies, one would conclude that base sites on Mo2 C were weaker than those on MgO. Nevertheless the rate of the base-catalyzed dehydrogenation of IPA over the Mo2 C catalyst was higher than that over the MgO catalyst. Lahouse et al. [31] reported that IPA dehydrogenation is not only a function of basic site density but also the site strength. Zinc oxide, a material that is typically less basic than MgO, was much more active for dehydrogenation than MgO. Evidently, IPA dehydrogenation is catalyzed by moderate strength base sites. While basic sites alone can catalyze the dehydrogenation of alcohols, several reports have implicated the activity of concerted acid–base pairs in these reactions [21,32,33]. For these acid–base pairs, the presence of Lewis acid sites appear to stabilize the anionic product of hydrogen abstraction steps. The involvement of acid–base pair sites in catalysis by Mo2 C was implied by the TPD and poisoning results. Both CO2 and NH3 reduced the rates of acetone formation. Similar theories have been proposed regarding alcohol dehydrogenation [21]. In particular, the presence of basic
(pyridine or NH3 ) and acidic (CO2 ) in the gas phase feed inhibited the dehydrogenation of alcohol. Because acid and base sites did not appear to account for all of the chemistry, it is likely that metal sites were involved in the decomposition of IPA. Molybdenum carbide and other early transition metal carbides have been reported to possess catalytic properties that resemble those of Pt-group metals [1,34]. Metals are known to catalyze the dehydrogenation of alcohols; however, the rates are typically low [35]. This was confirmed here for the Pt/SiO2 catalyst. The acid and base sites are likely a consequence of charge transfer from metal to non-metal due to their differing electronegativities. Charge transfer would result in electron deficiency (or positive charge) on the Mo atoms and a surplus of electron density (or negative charge) on the carbon atoms. This could lead to the development of Lewis acid and base character over the Mo and C atoms, respectively. Alternately, the acid sites may have been due to oxide domains produced on exposure of the carbides to oxygen [12,36]. The origin of the acidic and basic character will be the subject of a future paper.
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5. Summary The IPA decomposition, and CO2 and NH3 TPD experiments indicated the presence of acid and base sites on the Mo2 C catalyst. The CO2 and NH3 poisoning studies also indicated that dehydration of isopropanol to propylene takes place solely over acid sites, whereas, both acid and base sites might be involved in the production of acetone. The rate of isopropanol dehydrogenation over Mo2 C was higher than that for MgO. In addition to the acid and base sites, metallic sites may have played a role in catalyzing the dehydrogenation reaction over Mo2 C.
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Acid and base characteristics of molybdenum carbide catalysts Shyamal K. Bej, Christopher A. Bennett, Levi T. Thompson∗ Department of Chemical Engineering, University of Michigan, 3026 H.H. Dow Building, 2300 Hayward Avenue, Ann Arbor, MI 48109-2136, USA Received 3 July 2002; received in revised form 2 December 2002; accepted 7 December 2002
Abstract The acid and base properties of a high surface area Mo2 C catalyst were characterized using the temperature programmed desorption of CO2 and NH3 , the decomposition of isopropyl alcohol (IPA) as a test reaction and monitoring changes in the associated rates and product selectivities on the addition of acid and base site poisons. The Mo2 C catalyst was prepared using the temperature programmed reaction method and passivated prior to exposure to air. Prior to carrying out the temperature programmed desorption experiments and reaction rate measurements, the Mo2 C catalyst was reduced in H2 at 400 ◦ C. Results obtained for the reduced Mo2 C catalyst were compared with those for MgO, HZSM-5 and 1% Pt/SiO2 catalysts. The study provided evidence for the presence of both acid and base sites on Mo2 C. The base and acid sites on the Mo2 C catalyst were weaker than those on the MgO and HZSM-5 catalysts, respectively. The base and acid sites were likely created as a consequence of charge transfer from molybdenum to carbon. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum carbide; Acid catalysis; Base catalysis; Alcohol decomposition; CO2 TPD; NH3 TPD; Poisoning studies
1. Introduction Transition metal carbides and nitrides possess a number of interesting properties. For example, in high surface area form they have catalytic properties similar to those of Pt-group metals [1]. In addition, carbides are resistant to poisoning by sulfur compounds [2,3]. These materials are known to catalyze reactions including hydrodesulfurization, hydrodenitrogenation, isomerization, dehydrogenation, water gas shift and amination [4–9]. Most transition metal carbides and nitrides are interstitial compounds with the non-metal atoms residing in interstices in the metal lattice. Because of ∗ Corresponding author. Tel.: +1-734-936-2015; fax: +1-734-763-0459. E-mail address: [email protected] (L.T. Thompson).
their significantly different electronegativities, charge transfer from metal to non-metal can occur [10] and this could result in the creation of acid and base sites. In fact, it has been recently reported that some nitrides possess acid and base properties. Kaskel and Schlichte [11] demonstrated that SiN possesses base properties and can catalyze the isomerization of 2,3-dimethylbut-1-ene. Passivated Mo2 N has been claimed to possess acid properties as evidenced by its capability to catalyze the dehydration of octanol to octene [12], although this acidity has been linked to the presence of oxygen. In the present article, we report characterization of the acid and base properties of Mo2 C. Although there are several methods available for characterizing the acid–base properties of solids, two are most commonly used [13–15]. The first method is based on the adsorption and desorption of acidic
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or basic probe molecules [14,16–18]. The second method is based on the performance of the catalysts in test reactions. A number of reactions have been employed to determine the acidity of solids [19,20]. In contrast, only a few reactions have been used to deduce the basicity [13]. The decomposition of alcohols like isopropanol (IPA) and cyclohexanol has been employed to evaluate the acid–base properties of catalysts [13,18]. In addition to thermal desorption and test reaction methods, monitoring changes in the rate and product selectivity for the probe reaction while selectively poisoning the acid and base sites has been used [21,22]. In this study, we used CO2 and NH3 temperature programmed desorption (TPD) to determine the base and acid properties of Mo2 C, respectively. The basicity and acidity were also investigated using the decomposition of isopropanol (IPA) as a test reaction. It is known that IPA undergoes dehydrogenation to form acetone over base sites whereas acid sites favor the dehydration of IPA to propylene [18,23–27]. We also conducted poisoning studies to establish the presence of acid–base properties. The results obtained for the Mo2 C catalyst were compared with those for MgO, a known solid base, HZSM-5, a known solid acid, and a 1% Pt/SiO2 catalyst.
temperature for 2 h. This temperature program was developed based on the results of the thermogravimetric analysis and has been used previously to prepare Mo2 C that is devoid of excess carbon. After completion of the reaction, the sample was quenched to room temperature in the reactant mixture. To prevent pyrolysis on exposure to air, the material was passivated using 1% O2 in He (Scott) flowing at 20 ml/min. Before carrying out the TPD studies or measuring the catalytic activity, the passivation layer was removed by reducing the catalyst. Additional details concerning the synthesis procedure and experimental setup are given elsewhere [8]. The MgO catalyst was prepared according to the procedure described by Di Cosimo et al. [28]. About 250 ml of distilled water was added to 25 g of commercial MgO (Baker) at room temperature. The mixture was then stirred and the temperature was increased to 80 ◦ C. The temperature was maintained for 4 h with constant stirring. The material was dried overnight at 85 ◦ C. The dried mass was then heated at 350 ◦ C for 2 h and 500 ◦ C for 8 h in flowing air (30 ml/min). The calcined sample was kept in an airtight container. The HZSM-5 (Zeolyst International) and 1% Pt/SiO2 (GFS Chemicals) catalysts were used as received. 2.2. Catalyst characterization
2. Experimental 2.1. Catalyst preparation High surface area Mo2 C was prepared by the temperature programmed reaction method. In brief, a vertical quartz flow-through reactor was used for the synthesis. Approximately 1.5 g of ammonium paramolybdate (Alfa Aesar) was placed over a quartz wool plug supported by a quartz frit located inside the reactor. The reactor was placed inside a high temperature furnace. The temperature of the reactor was measured using a thermocouple placed above the precursor inside the reactor and controlled using an Omega CN2010 temperature controller connected to the furnace. A mixture of 15% CH4 and balance H2 (Matheson) was passed through the reactor at a rate of 250 ml/min. The reactor was then quickly heated from ambient temperature to 200 ◦ C in ∼20 min. The temperature was then increased linearly to 590 ◦ C in ∼6.5 h and held at this
The BET surface areas were determined using a Micromeritics ASAP 2010 porosimeter with nitrogen as the sorbate. The materials were degassed in vacuum at 400 ◦ C until the static pressure remained less than 3 mHg prior to analysis. The BET surface areas of the catalysts are given in Tables 1 and 2. Helium, CO2 and NH3 TPD experiments were performed using a Micromeritics ASAP 2910 sorption analyzer. The He TPD experiments allowed us to determine species that desorbed from the freshly pretreated catalysts. The catalysts were pretreated prior to the TPD and activity measurements. The Mo2 C catalyst was heated to 400 ◦ C in flowing H2 , reduced at that temperature for 4 h using a H2 flow of 20 ml/min then flushed with He flowing at 20 ml/min for 30 min at 400 ◦ C. The MgO catalyst was treated at 500 ◦ C for 5 h in a helium flow of 20 ml/min. The HZSM-5 catalyst was pretreated at 500 ◦ C for 4 h in He flowing at 20 ml/min. The 1% Pt/SiO2 catalyst was reduced at 500 ◦ C for 4 h under H2 flowing at
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Table 1 Amounts of CO2 desorbed from the Mo2 C, MgO and 1% Pt/SiO2 catalysts Catalyst
Mo2 C MgO 1% Pt/SiO2 a
BET surface area (m2 /g)
50 212 244
CO2 desorbed below 400 ◦ C
CO2 desorbed above 400 ◦ C
ml/g
Molecules/cm2
ml/g
Molecules/cm2
0.81 16.9 0.22
4.4 × 1013 2.1 × 1014 2.4 × 1012
–a 11.5
1.5 × 1014
CO2 that desorbed at temperatures greater than 400 ◦ C was attributed to decomposition of the Mo2 C catalyst.
20 ml/min before performing the activity and TPD experiments. These experimental conditions are typical of those used to activate HZSM-5 and Pt-based catalysts [29,30]. The pretreated samples were saturated with He, CO2 (Matheson) or NH3 (Scott) at room temperature for 30 min (20 ml/min). The sample was flushed with He flowing at 20 ml/min for 30 min then the temperature was increased to 800–1000 ◦ C at a heating rate of 10 ◦ C/min. Gases that desorbed were monitored using a thermal conductivity detector (TCD) and a mass spectrometer connected on line with the Micromeritics 2910. 2.3. Catalytic activity measurements The decomposition of IPA was performed using a 4 mm i.d. quartz reactor. Approximately 100–300 mg of catalyst was placed on a quartz wool plug inside the reactor. The reactor was then placed inside a furnace. The temperature of the reactor was measured using a K-type thermocouple placed inside the catalyst bed and controlled using an Omega CN2010 temperature controller connected to the furnace. The catalyst samples were pretreated using procedures described in Section 2.2. The reactor was then brought to the desired temperature in order to measure the catalytic activity. For routine catalytic activity measurements, a feed stream consisting of IPA and He was used. Helium
was passed through a series of saturators containing IPA (Fischer) at ambient temperature. The flow rate of He was controlled using a mass flow controller (Tylan FC-260). The He/IPA mixture was mixed with another stream containing pure He coming through a by-pass line. The He flow rate coming from the by-pass line was measured using a rotameter and controlled using a needle valve. The composition of the reactant stream before entering the catalyst bed was 3.0 mol% IPA and 97.0 mol% He. Carbon dioxide, an acid gas, was used to selectively poison the base sites whereas the acid sites were poisoned using NH3 . The temperature for the poisoning studies was determined based on the CO2 and NH3 TPD results. For each temperature, three sets of experiments were carried out. In the first set, the reactant consisting of a mixture of IPA and He was passed over the catalyst. After steady state was reached, pure He coming from the by-pass line was discontinued and an equal amount of either CO2 or NH3 was added to the mixture of IPA and He. The flow rate of CO2 was measured using a rotameter and controlled using a needle valve, whereas, a Tylan FC-260 mass flow controller was used to control the flow rate of NH3 . After the poisoning segment, the CO2 or NH3 flow was discontinued and the He flow was initiated so that the composition was 3.0 mol% IPA and 97.0 mol% He. During the poisoning segment, the reactant stream consisted of 3.0 mol% IPA, 50 mol% He and 47 mol% CO2 or NH3 .
Table 2 Amounts of NH3 desorbed from the Mo2 C and MgO catalysts Catalyst
Mo2 C HZSM-5
BET surface area (m2 /g)
50 625
NH3 desorbed below 300 ◦ C
NH3 desorbed above 300 ◦ C
ml/g
Molecules/cm2
ml/g
Molecules/cm2
4.2 102
2.3 × 1014 4.4 × 1014
44
1.9 × 1014
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The product gas mixture was analyzed using an Hewlett-Packard 5690 gas chromatograph equipped with a flame ionization detector and a packed column containing 28% AT-223 and 4% KOH on Gas Chrom R (Altech). The signal from the detector was analyzed using a Spectra-Physics Chrom Jet integrator, which also controlled injections into the gas chromatograph via a Valco six-way actuator. The quartz reactor and wool were found to have negligible activities under the experimental conditions used.
3. Results 3.1. Helium temperature programmed desorption The He TPD spectrum for the reduced Mo2 C catalyst is shown in Fig. 1. At temperatures less than 580 ◦ C, there was no gas evolution; however, substantial amounts of CO and CO2 were detected above 580 ◦ C indicating that the surface and/or subsurface contained oxygen and this oxygen was reactive. No species other than CO and CO2 desorbed from the re-
duced Mo2 C catalyst. No gas evolution was observed for the MgO, HZSM-5 or 1% Pt/SiO2 catalysts on heating them in He to temperatures as high as 1000 ◦ C. This is not unexpected given that the materials had been reduced or degassed at high temperatures prior to the TPD experiments. 3.2. CO2 temperature programmed desorption The CO2 TPD spectrum for Mo2 C is shown in Fig. 2. A CO2 peak having desorption maxima at 230 ◦ C was observed. In addition, CO and CO2 started desorbing at temperatures higher than 520 ◦ C. These high temperature peaks were probably due to decomposition of the Mo2 C catalyst based on the He TPD results (Fig. 1). The low temperature CO2 desorption peak suggested the presence of base sites. The CO2 TPD spectra for MgO and 1% Pt/SiO2 are shown in Fig. 3. Carbon dioxide desorption peaks for MgO occurred at ∼180, 300 and 525 ◦ C. These results are consistent with those reported in the literature. For example, Diez et al. [18] reported a broad CO2 desorption peak for MgO at temperatures ranging from
Fig. 1. Helium TPD spectrum for the Mo2 C catalyst.
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Fig. 2. Carbon dioxide TPD spectrum for the Mo2 C catalyst.
Fig. 3. Carbon dioxide TPD spectra for the MgO and 1% Pt/SiO2 catalysts.
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180 to 410 ◦ C. During CO2 TPD from the 1% Pt/SiO2 , a very small amount of CO2 desorbed at 90 ◦ C. This peak may have been a consequence of CO2 desorption from sites on the Pt particles or weak base sites on SiO2 . The amount of CO2 desorbed from all of the materials was determined and the results are given in Table 1. Magnesium oxide had a high density of CO2 adsorption sites while the Pt/SiO2 catalyst had the lowest site density. The site density for the Mo2 C catalyst suggests that the base sites accounted for ∼5% of the surface sites (assuming a total site density of 1015 sites/cm2 ). Typically the desorption temperature is an indication of the base site strength. The results suggest that sites on Mo2 C were less basic than those on MgO.
absence of acidity [13]. Ammonia desorbed from the HZSM-5 catalyst producing peaks at ∼110, 170 and 370 ◦ C (Fig. 5). Inui et al. [30] reported similar NH3 TPD results. They found two types of acidic sites present on HZSM-5. The weaker acidic sites had NH3 desorption maxima at about 160 ◦ C while the moderate strength sites produced peaks at about 440 ◦ C. Slight differences in the peak maxima when compared to our results might be due to differences in the carrier gas flow rates. The amount of NH3 that desorbed from the Mo2 C catalyst was significantly lower than that from HZSM-5 (see Table 2). The NH3 adsorption site density was equivalent to ∼20% of the surface of the Mo2 C catalyst. Based on the NH3 desorption temperatures, the strength of acid sites on Mo2 C was lower than those for HZSM-5.
3.3. NH3 temperature programmed desorption 3.4. Isopropanol decomposition The NH3 TPD spectrum for the Mo2 C catalyst is shown in Fig. 4. This catalyst adsorbed significant amounts of NH3 producing a desorption peak at about 125 ◦ C. This result indicated the presence of acid sites. Magnesium oxide did not adsorb NH3 , indicating the
The catalysts were typically tested for IPA decomposition at 140–200 ◦ C. Reaction rates for the Mo2 C, MgO, HZSM-5 and Pt/SiO2 catalysts are compared in Fig. 6. Because it was much less active than the
Fig. 4. Ammonia TPD spectrum for the Mo2 C catalyst.
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Fig. 5. Ammonia TPD spectrum for the HZSM-5 catalyst.
Fig. 6. Isopropyl alcohol decomposition rates for Mo2 C, MgO and 1% Pt/SiO2 catalysts at varying temperatures.
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other catalysts, rates for the MgO catalyst were measured at 260–320 ◦ C. Acetone and propylene were the only products formed under the reaction conditions employed. The selectivities to acetone are compared in Fig. 7. In general the behavior of the Mo2 C catalyst was more like that for MgO than HZSM-5. For all of the materials, the selectivity to acetone decreased with increasing temperature. Figs. 6 and 7 show that the Mo2 C catalyst was significantly more active than the other catalysts. In addition, this material possessed a very high selectivity to acetone indicating that the dehydrogenation of IPA, a base-catalyzed reaction, was predominant. The MgO catalyst was also highly selective for the production of acetone, however, the activity was much lower than that for the Mo2 C catalyst. The 1% Pt/SiO2 catalyst was selective for the dehydrogenation of IPA to acetone, however, it had a very low activity. The HZSM-5 catalyst was highly active for the conversion of IPA. Unlike the Mo2 C catalyst, the HZSM-5 catalyst was selective for the production of propylene. This is a known acid-catalyzed reaction. Isopropanol turnover frequencies based on the total CO2 and NH3 uptakes
were 0.44 and 0.006 s−1 at 200 ◦ C for the Mo2 C and HZSM-5 catalysts, respectively. 3.5. Site poisoning studies 3.5.1. CO2 poisoning The CO2 poisoning studies for the Mo2 C catalyst were conducted at 160 and 260 ◦ C. Based on the CO2 TPD results, most of the base sites should be blocked by carrying out the poisoning study at 160 ◦ C, whereas only the strong sites should be occupied at temperatures higher than 260 ◦ C. Results for the CO2 poisoning studies conducted at 160 ◦ C are shown in Fig. 8. The conversion of IPA was ∼51 mol% and the selectivity for acetone was ∼88 mol% before the addition of CO2 . When CO2 was added, the conversion decreased to 28 mol% and the selectivity to acetone decreased to 72 mol%. This result indicated that base sites on the Mo2 C catalyst were active for IPA dehydrogenation to acetone. While these sites appear to be primarily responsible, they do not account for all of the dehydrogenation activity. It is possible that some base sites remained
Fig. 7. Selectivity toward acetone for Mo2 C, MgO and 1% Pt/SiO2 catalysts at varying temperatures.
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Fig. 8. Effect of CO2 on IPA conversion and selectivity for the Mo2 C catalyst at 160 ◦ C.
available for participation in the reaction or that metal sites contributed. The rate of propylene formation was independent of the absence or presence of CO2 . This observation suggested that IPA dehydrogenation and dehydration occurred on different sites. When CO2 was removed from the feed stream, the activity was essentially restored to its original state indicating that CO2 was reversibly adsorbed on the active sites. Carbon dioxide had no effect on the IPA conversion or selectivity for the Mo2 C catalyst at 260 ◦ C (Fig. 9). This indicated that the stronger basic sites present on the Mo2 C catalyst were not involved in dehydrogenation or dehydration reactions. Carbon dioxide poisoning studies for the 1% Pt/SiO2 catalyst were conducted at 160 ◦ C. Recall that reaction rates were very low for this catalyst. The results of the poisoning studies are shown in Fig. 10. There was only a very slight decrease in the decomposition of IPA upon the introduction of CO2 . Acetone was the only product formed over the 1% Pt/SiO2 catalyst. The results indicated that under the experimental conditions employed, CO2 did not block active sites on Pt or SiO2 . This observation was in agreement with the CO2 TPD studies which showed
negligible amounts of CO2 desorbing from the 1% Pt/SiO2 catalyst. 3.5.2. NH3 poisoning Ammonia poisoning studies for the Mo2 C catalyst were conducted at 180 ◦ C. The effect of NH3 poisoning on IPA conversion and selectivity are shown in Fig. 11. The presence of NH3 decreased the IPA conversion from 40 to 18 mol%, presumably due to blockage of the acid sites. The selectivity towards acetone increased from 84 to 99 mol%. This indicated that acids site were responsible for the dehydration of IPA. Interestingly, the acetone formation rate also decreased indicating that acidic sites may have been involved in the dehydrogenation of IPA to acetone. The catalyst slowly returned to its original activity when the NH3 was removed from the feed stream.
4. Discussion Results from the IPA decomposition studies showed that Mo2 C possessed significant activity for the dehydrogenation of alcohol. These results as well as
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Fig. 9. Effect of CO2 on IPA conversion and selectivity for the Mo2 C catalyst at 260 ◦ C.
Fig. 10. Effect of CO2 on IPA conversion for the 1% Pt/SiO2 catalyst at 160 ◦ C.
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Fig. 11. Effect of NH3 poisoning on IPA conversion and selectivity for the Mo2 C catalyst at 180 ◦ C.
those from the CO2 TPD studies indicated the presence of base sites. Based on the CO2 TPD studies, one would conclude that base sites on Mo2 C were weaker than those on MgO. Nevertheless the rate of the base-catalyzed dehydrogenation of IPA over the Mo2 C catalyst was higher than that over the MgO catalyst. Lahouse et al. [31] reported that IPA dehydrogenation is not only a function of basic site density but also the site strength. Zinc oxide, a material that is typically less basic than MgO, was much more active for dehydrogenation than MgO. Evidently, IPA dehydrogenation is catalyzed by moderate strength base sites. While basic sites alone can catalyze the dehydrogenation of alcohols, several reports have implicated the activity of concerted acid–base pairs in these reactions [21,32,33]. For these acid–base pairs, the presence of Lewis acid sites appear to stabilize the anionic product of hydrogen abstraction steps. The involvement of acid–base pair sites in catalysis by Mo2 C was implied by the TPD and poisoning results. Both CO2 and NH3 reduced the rates of acetone formation. Similar theories have been proposed regarding alcohol dehydrogenation [21]. In particular, the presence of basic
(pyridine or NH3 ) and acidic (CO2 ) in the gas phase feed inhibited the dehydrogenation of alcohol. Because acid and base sites did not appear to account for all of the chemistry, it is likely that metal sites were involved in the decomposition of IPA. Molybdenum carbide and other early transition metal carbides have been reported to possess catalytic properties that resemble those of Pt-group metals [1,34]. Metals are known to catalyze the dehydrogenation of alcohols; however, the rates are typically low [35]. This was confirmed here for the Pt/SiO2 catalyst. The acid and base sites are likely a consequence of charge transfer from metal to non-metal due to their differing electronegativities. Charge transfer would result in electron deficiency (or positive charge) on the Mo atoms and a surplus of electron density (or negative charge) on the carbon atoms. This could lead to the development of Lewis acid and base character over the Mo and C atoms, respectively. Alternately, the acid sites may have been due to oxide domains produced on exposure of the carbides to oxygen [12,36]. The origin of the acidic and basic character will be the subject of a future paper.
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5. Summary The IPA decomposition, and CO2 and NH3 TPD experiments indicated the presence of acid and base sites on the Mo2 C catalyst. The CO2 and NH3 poisoning studies also indicated that dehydration of isopropanol to propylene takes place solely over acid sites, whereas, both acid and base sites might be involved in the production of acetone. The rate of isopropanol dehydrogenation over Mo2 C was higher than that for MgO. In addition to the acid and base sites, metallic sites may have played a role in catalyzing the dehydrogenation reaction over Mo2 C.
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