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Article

Impact of Co-Fed Hydrogen on High Conversion Propylene Aromatization on H-ZSM-5 and Ga/H-ZSM-5

by
Christopher K. Russell
1,2,*,
Josiah L. Rockey
1,
Rebecca N. Hanna
1 and
Jeffrey T. Miller
1
1
Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA
2
Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(7), 405; https://doi.org/10.3390/catal14070405
Submission received: 7 May 2024 / Revised: 18 June 2024 / Accepted: 20 June 2024 / Published: 27 June 2024
(This article belongs to the Special Issue Research Advances in Zeolites and Zeolite-Based Catalysts)
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Abstract

:
The expanded production of shale gas has increased the desire for developing methods for converting light alkanes, especially propane and ethane, into aromatic compounds (i.e., benzene, toluene, and xylene) for petrochemicals and fuels. The Cyclar process is one example of an industrial process that has been demonstrated for the conversion of butane to aromatics; however, the conversion of lower molecular weight alkanes remains elusive. A multi-step process for the conversion of light alkanes to aromatics may be developed, where the first stage converts light alkanes into olefins and hydrogen, and the second stage converts olefins into aromatics. However, to determine the viability of this process, a better understanding of the performance of olefin aromatization in the presence of equimolar hydrogen is necessary. Herein, H-ZSM-5 and Ga-modified H-ZSM-5 are compared for propylene aromatization in the presence and absence of equimolar hydrogen at 1.9 kPa and 50 kPa partial pressures. The presence of H2 has no impact on the product distribution with H-ZSM-5 at either pressure. At 1.9 kPa with Ga/H-ZSM-5, similar product distributions are observed regardless of the presence or absence of H2 since Ga is not sufficiently active for hydrogenation to inhibit aromatics formation. However, at 50 kPa of H2 with Ga/H-ZSM-5, there is an increased selectivity to C4 products and a decrease in toluene and xylene selectivities at high conversions (i.e., χ > 80%), suggesting that aromatic dehydrogenation of cyclic hydrocarbons has been suppressed.

Graphical Abstract

1. Introduction

Aromatics (i.e., benzene, toluene, and xylene, or BTX) are important feedstocks for the production of liquid fuels and as vital feedstocks to the modern chemical industry. Traditionally formed via the reforming of naphtha, the expanded production of shale gas has increased the desire for developing methods for converting light alkanes, especially propane and ethane, into aromatic hydrocarbons and gasoline [1]. Light alkanes may be converted to aromatics via a two-step process. In the first step, alkanes are converted to olefins and hydrogen through various processes, for example, catalytic dehydrogenation or steam cracking reactions [2,3,4]. Secondly, the olefins may be converted to aromatics on zeolites or bifunctional dehydrogenation and acid catalysts [5,6,7,8].
Brønsted acid catalysts, in particular zeolites, have been broadly investigated for olefin conversion reactions. In particular, H-ZSM-5 has demonstrated the best performance for upgrading olefin feedstocks due to its resistance to deactivation compared to other solid acid catalysts [9]. Studies of propene aromatization generally find that H-ZSM-5 achieves aromatic selectivity around 40% for propene aromatization at 523–573 K. For example, Ono and co-workers report achieving 45.5% aromatic selectivity at 93.6% propylene conversion [10]. By comparison, upon the addition of Zn, aromatic selectivity has been reported to increase to 70.3% (with Zn, the conversion at equivalent space velocity is 97%). Other studies have reported similar increases in aromatic selectivity upon the introduction of Ga or Zn [7,8,9,10,11,12,13].
The addition of a dehydrogenation function to H-ZSM-5 has been reported to increase the aromatic selectivity for a range of hydrocarbon reactions (e.g., alkane dehydroaromatization [9,14,15,16,17,18,19,20,21,22], alkene aromatization [6,10,12,23,24,25], methanol aromatization, and biomass fast pyrolysis [26]). While the initial reactants differ, aromatization of light hydrocarbons generally follows a similar reaction mechanism, where the initial feedstock (i.e., alkanes [9,14,16], methanol [27,28,29], or biomass) are first converted into olefins and then to aromatics by a series of oligomerization and cracking reactions shown in Scheme 1a.
On an exclusively acid catalyst, such as H-ZSM-5, light alkenes oligomerize on the acid sites of H-ZSM-5 to form a mixture of alkenes. Large alkenes can then undergo cracking reactions on the acid sites of H-ZSM-5 to reform light alkenes (e.g., ethylene, propylene, butene, etc.) and light alkanes (e.g., propane, butane, etc.) [3,30,31]. Larger alkenes (i.e., with at least six carbons) have been reported to undergo cyclization reactions, forming cyclic hydrocarbons, and these cyclic hydrocarbons are then converted to one mole of aromatic (BTX) and 3 moles of alkanes, typically propane [9,10].
Contrastingly, when an acid catalyst is combined with or modified by a dehydrogenation catalyst (e.g., Ga [4,5,6,7,9,15,32,33,34], Zn [11,13,16,17,18,23,24], or Pt [14,34,35,36,37,38]), increased production of aromatics has been reported. This may be attributed to an alternative mechanism for aromatics formation similar to that shown in Scheme 1b. The addition of a dehydrogenation catalyst enables the catalytic dehydrogenation of light alkanes (e.g., propane, butane) to reform light alkenes [14]. This results in a lower selectivity to methane and ethane compared to aromatization on pure acid catalysts. Secondly, the addition of dehydrogenation catalysts has been shown to produce 3 moles of hydrogen for each mole of aromatic formed, consistent with catalytic dehydrogenation of cyclic hydrocarbons [9,10].
Previous studies report olefin oligomerization in the absence of hydrogen. However, the production of olefins also gives an equimolar amount of H2, which would require expensive separation. In this study, the product distribution of propylene aromatization on Ga-impregnated H-ZSM-5 is compared to that on H-ZSM-5 in the presence and absence of equimolar co-fed H2 and at 1.9 kPa and 50 kPa partial pressures. Comparisons of the product distribution on both catalysts suggest that Ga enhances aromatic selectivity by enabling the production of aromatic rings via catalytic dehydrogenation. The effect of increasing the partial pressure of C3H6 and H2 shifts the olefin hydrogenation equilibrium to favor paraffin production, thereby reducing the differences observed between Ga/H-ZSM-5 and H-ZSM-5. The effect of equimolar H2 is demonstrated to have minimal impact on the product distribution of H-ZSM-5, although, on Ga/H-ZSM-5, it shifts the product distribution to favor oligomerization products over the formation of aromatics at high conversions in the presence of high partial pressures of propylene and hydrogen.

2. Results

A large number of products are observed (i.e., isomers of alkanes and alkenes containing C4–C7+); thus, products with four or more carbons have been grouped by carbon number, with the exception of benzene, toluene, and xylene. Additionally, products are discussed in the following order: aromatics (i.e., benzene, toluene, and xylene); light alkanes (i.e., methane, ethane, and propane), and reactive intermediates (i.e., ethylene, C4, C5, C6, and C7+ species).

2.1. Product Distribution on H-ZSM-5 with and without H2

The conversion of propylene and other olefins to aromatics and hydrocarbons has been extensively studied. Here, we compare the products of propylene conversion with and without equimolar amounts of co-fed H2 at 723 K by comparing the product distribution of C3H6 aromatization at 50 kPa and 1.9 kPa partial pressure (balance N2). The space velocity was varied to give propylene conversions from about 30% to above 95%. The aromatic distribution (Figure 1) while feeding 50 kPa C3H6 is similar when co-feeding 50 kPa H2 or N2. All three aromatics (i.e., benzene, toluene, and xylene) have selectivities <2% up to conversions about 70% (Figure 1, squares). At propylene conversions above 70%, selectivity to benzene (Figure 1a), toluene (Figure 1b), and xylene (Figure 1c) increase substantially, ultimately reaching selectivities within error of 7%, 15%, and 9%, respectively.
Methane and ethane selectivities (Figure 2a and Figure 2b, respectively) are <5% at all conversions, increasing from selectivities <0.1% until conversions around 80%, at which point they increase to approximately 2%. The low selectivity to methane indicates that monomolecular cracking, the primary mechanism for the formation of methane and ethane, does not contribute significantly to the reaction mechanism until high conversions. This is consistent with high concentrations of olefins inhibiting monomolecular cracking, as reported by Krannila et al. [2]. The selectivity to propane (Figure 2c), however, follows a significantly different trend. Propane selectivity increases from 5% at 45% conversion to 24% at 95% conversion, making it one of the largest by-products of propylene aromatization. The similar selectivity with and without co-fed H2 is consistent with previous reports that Brønsted acid (H+) catalysts are not active for hydrogenation or dehydrogenation reactions. This suggests propane is formed via cracking mechanisms (Scheme 1a), which has been reported as a product of cracking of higher molecular weight hydrocarbons [3].
In addition to aromatics and light alkanes, the remaining products are ethylene and higher molecular weight hydrocarbons (i.e., with four or more carbons). Whereas aromatic and light alkane selectivities monotonically increased (Figure 1 and Figure 2), the selectivity of these products initially increased and then decreased as conversion continued to increase (Figure 3). At 50 kPa, ethylene selectivity increased to 6% at 80% conversion and then decreased to <1% selectivity at conversions >95% (Figure 3a). C4 (Figure 3b), C5 (Figure 3c), C6 (Figure 3d), and C7+ (Figure 3e) all have maximum selectivities at conversions lower than the lowest conversion tested in this study (40%) and ultimately reach 13%, 3%, <1%, and approximately 15%, respectively. At high conversions, the presence of condensed liquids at the outlet of the reactor was observed, suggesting small amounts of high molecular weight products are formed. This may be the reason that significantly more scatter is present in the C7+ selectivity observed at high conversions.
At a lower partial pressure of 1.9 kPa C3H6, the selectivity to each aromatic is <5% until 50% propylene conversion (Figure 1, circles). Benzene selectivity increases from <1% at 45% conversion to 13% at 95% conversion. Toluene is the most abundant aromatic formed, increasing from 3% to 25% across the same range tested. Xylene selectivity at conversions <75% is similar to that of toluene, increasing from 3% to 9% as conversion increases from 45% to 73%; however, as conversion continues to increase, the selectivity to xylene only increases to 13% at 95% conversion. The aromatic distribution is equivalent when co-fed H2. Methane (Figure 2a) and ethane (Figure 2b) selectivity at 1.9 kPa are <5% across all conversions, increasing from <1% at propylene conversions below 70%. As conversion increases beyond 70%, methane and ethane selectivity increase to about 3%. Ethylene, C4s, C5s, C6s, and C7+ species selectivity increases to a maximum value at intermediate conversions and then decreases at higher conversions due to secondary reactions. The maximum selectivity to ethylene (Figure 3a) occurs somewhere between 48% and 73% conversion, while the maxima for the other reactive intermediates occur at conversions ≤ 45% (the lowest conversion tested).

2.2. Characterization of Ga on ZSM-5

To determine the coordination geometry and bond distance between the Ga and neighboring atoms, X-ray absorption spectroscopy (XAS) was performed at the Ga-K edge. XAS can be decomposed into two regions: X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). XANES provides information regarding the coordination geometry and oxidation state of Ga, while numerical fitting of the EXAFS region yields the number of bonds and bond distance between Ga and surrounding atoms. Herein, the XANES of Ga/H-ZSM-5 is compared to gallium acetylacetonate, Ga(AcAc)3 (Figure 4a). Ga(AcAc)3 has octahedrally (Oh) coordinated Ga3+ ions surrounded by six Ga-O bonds with a bond distance of 1.94 Å.
The XANES of Ga/H-ZSM-5 has an edge energy, defined as the inflection point in the initial rise in absorption, of 10,372.6 eV, and a white line energy (i.e., the energy at which the first maximum absorption occurs) of 10,376.2 eV (Figure 4a). Beyond the white line, there are shoulders near 10,381 eV, which has been previously reported to be characteristic of tetrahedral Ga3+ [39,40,41,42]. By comparison, the edge energy and white line energy for Ga(AcAc)3 are 10,376.5 and 10,379.0 eV, respectively.
The white line energy for Ga(AcAc)3 (10,379.4) is consistent with octahedral Ga3+ (Table 1, Figure 4a). Ga/H-ZSM-5 does not have a prominent absorption feature at 10,379 eV, indicating that octahedral Ga3+ is not present. Similarly, Ga/H-ZSM-5 has a lower XANES energy when compared with Ga(AcAc)3, suggesting that the Ga present in Ga/H-ZSM-5 is tetrahedral Ga3+.
Qualitative comparison of the Fourier transform of the EXAFS region of Ga(AcAc)3 and Ga/H-ZSM-5 and quantitative fitting of the EXAFS region of Ga/H-ZSM-5 are consistent with the formation of tetrahedral Ga3+ in Ga/H-ZSM-5. The Ga/H-ZSM-5 first shell is the prominent feature near R = 1.35 Å (Figure 1b, phase uncorrected distance). By comparison, Ga(AcAc)3 has a first shell peak at 1.45 Å, consistent with having a longer bond distance. The intensity of the first shell is significantly higher for Ga(AcAc)3 compared to Ga/H-ZSM-5, indicating that Ga/H-ZSM-5 has fewer Ga-O bonds than Ga(AcAc)3. This is supported by the numerical fitting of the EXAFS (Table 2, Figure 4d), where results confirm the formation of 4.0 Ga-O bonds at 1.84 Å, consistent with previous results indicating the formation of Ga3+ single sites [43]. Tetrahedral Ga3+ with an average bond distance of 1.84 Å has been previously reported as the active form of Ga for propane dehydrogenation and aromatization on Ga/H-MFI [39,44] and on Ga/SiO2 [43].

2.3. Product Distribution on Ga/H-ZSM-5 with and without H2

Ga/H-ZSM-5 is the most widely studied bifunctional catalyst for dehydroaromatization due to its demonstrated higher selectivity to aromatics than H-ZSM-5 [5,9,10,12,13,24]. For this reason, Ga/H-ZSM-5 was also studied for olefin aromatization in the presence and absence of equimolar H2. Previous literature has studied olefin aromatization on Ga/H-ZSM-5 [5,6,7,12,13,25,43,45]. However, since Ga is a known dehydrogenation catalyst [4,39,46], the principle of microscopic reversibility suggests that it is also reactive for olefin hydrogenation, which may affect the product distribution of olefin aromatization by hydrogenating olefins to form paraffin. Here, the product distribution of C3H6 aromatization was determined across a range of conversions achieved by changing the space velocity at 723 K and 1.9 kPa C3H6 or 50 kPa C3H6 in the presence or absence of equimolar H2.
Whereas on H-ZSM-5 feeding either H2 or N2 resulted in no significant differences for any of the products, trends on Ga/H-ZSM-5 differ between co-fed H2 and N2 at 50 kPa. Toluene selectivity (Figure 5b) is similar at conversions below 70% in the presence of H2 or N2, with a selectivity <2%. However, at conversions above 70%, where toluene selectivity becomes more appreciable, the selectivity in N2 increases more rapidly, reaching a selectivity of 20% at 95% conversion compared to only 14% when co-feeding H2. Xylene follows a similar trend (Figure 5c), where xylene selectivity is ≤3% at conversions below 72% and then increases more rapidly in N2 than H2, reaching selectivities of 22% and 14%, respectively. Benzene is produced in smaller amounts at 50 kPa, reaching 5% and 3% selectivity at 95% conversion in N2 and H2, respectively (Figure 5a).
At 50 kPa C3H6 (Figure 6, squares), methane and ethane selectivity both remain <0.3% up to 80% conversion and then increase to 1.4% and 1.2% selectivity at 95% conversion, respectively. Propane selectivity slowly increases from 0.3% selectivity at 47% conversion to 5% at 80% conversion before more quickly increasing to 10% selectivity at 95% conversion. C2H4, C4, C5, C6, and C7+ selectivities decrease as conversion approaches complete conversion (Figure 7). Additionally, with the exception of C4 selectivity at 50 kPa, the selectivity to reactive intermediates is unaffected by the H2 co-feed. The ethylene selectivity is lower, remaining constant (≈3%) from 47% to 62% conversion, then increasing to 8% selectivity at 80% conversion, and then decreasing to 3% at 95% conversion. At 50 kPa (squares), the selectivity trends for C4 are different when co-feeding H2 (red) compared to co-feeding N2 (blue). In N2, C4 selectivity is relatively constant (≈27%) from 47–75% conversion and then decreases to 16% selectivity at 95% conversion. In H2, C4 selectivity remains approximately 27% across the entire conversion range. This 10% difference only occurs at high conversions (≥85%) and is predominantly offset by differences in aromatic selectivity of 3% to each of the aromatics (Figure 5). At 50 kPa, C5 and C6 selectivity are approximately constant at 47–70% conversion (at 27% and 18%, respectively) and then decreases to 5% and 2%, respectively, as conversion increases to 95%. At 50 kPa (squares), the C7+ selectivity decreases from 21% to 13% as conversion increases from 47% to 95%.
At 1.9 kPa, the differences between co-feeding N2 and H2 disappear. Benzene (Figure 5a, blue circles), toluene (Figure 5b, blue circles), and xylene (Figure 5c, blue circles) selectivities increase from 5%, 15%, and 15%, respectively, at 48% conversion, to 20%, 23%, and 20%, respectively, at 94% conversion.
The selectivity to light alkanes (i.e., methane, ethane, and propane) have relatively constant (and low) selectivity until approximately 75% conversion and then begin to increase slightly. At 1.9 kPa C3H6 (blue circles), methane (Figure 6a), ethane (Figure 6b), and propane (Figure 6c) selectivity is constant at ≈ 0.7%, ≈ 0.3%, and 5%, respectively, until approximately 75% conversion, and then increase to 4%, 7%, and 11%, respectively. When co-feeding 1.9 kPa H2 (red circles), the trends are equivalent to that when co-feeding 1.9 kPa N2.
Ethylene selectivity (Figure 7a) at 1.9 kPa C3H6 (circles) is relatively constant (≈17%) until 70% conversion and then decreases to 6% at 94% conversion. C4 selectivity (Figure 7) when feeding 1.9 kPa C3H6 (circles) is relatively constant from 47–70% conversion and then decreases to 7% selectivity at 94% conversion. In 1.9 kPa, C3H6, C5, and C6 selectivity (Figure 7b and Figure 7c, respectively) have maximum selectivity at the lowest conversion measured and steadily decrease to <1% selectivity as conversion approaches 100%. Selectivity to C7+ species (Figure 7e) at 1.9 kPa (circles) is relatively constant at 8% across the conversion range tested. All the trends at 1.9 kPa are equivalent when co-feeding H2 and N2.
In summary, there were little differences in product distribution at 1.9 kPa C3H6 in the presence and absence of H2 on H-ZSM-5 or Ga/H-ZSM-5 at 723 K. At 50 kPa C3H6, there are also no differences in product selectivity on H-ZSM-5 with or without H2. However, for Ga/H-ZSM-5, while there were also few differences in the product distributions below about 80% conversion, at higher conversions, there was a decrease in aromatic selectivity (approximately 3% decrease for benzene, toluene, and xylene, each) and an increased C4 selectivity. Selectivities to the other products were unaffected by the presence of H2.

3. Discussion

3.1. The Impact of H2 on C3H6 Aromatization Product Distribution

The olefin product distributions in the presence and absence of H2 were determined on H-ZSM-5. For H-ZSM-5, no differences in the product distribution were observed at either 1.9 kPa or 50 kPa. This suggests that H2 does not substantially affect propylene aromatization on H-ZSM-5, indicating that Brønsted acid catalysts are ineffective dehydrogenation catalysts. These results are consistent with literature suggesting the formation of aromatics on Brønsted acid protons proceeds via cracking of larger hydrocarbons and subsequent isomerization of the double bond into the cyclic ring, forming alkanes as a by-product. The low selectivity to methane and ethane indicates that these alkanes are not significant by-products from the aromatization process but are rather formed via monomolecular acid cracking of alkanes at higher reaction temperatures.
On Ga/H-ZSM-5, there is also no difference in product distribution at 1.9 kPa (Figure 5, Figure 6 and Figure 7), thus suggesting that at low partial pressures, H2 does not impact propylene aromatization. Previous literature reports that there is an increased selectivity to BTX on Ga/H-ZSM-5 compared to H-ZSM-5, which is due to dehydrogenation of cyclic hydrocarbons, i.e., naphthenes, on Ga. Thus, while it may be initially expected that co-feeding equimolar H2 would suppress aromatics formation, this is not observed at 1.9 kPa. This may be due to Ga being an insufficiently active catalyst to inhibit aromatics formation. The low activity under these conditions is also supported by the similar selectivity observed in propane and ethane in the presence of co-fed H2 (Figure 6b and Figure 6c, respectively), i.e., reactant and product olefins are not hydrogenated. At lower pressures, thermodynamic equilibrium favors dehydrogenation products [47], thus Ga continues to form BTX by dehydrogenation of naphthenes.
At 50 kPa C3H6 and H2, there is a decreased selectivity to toluene and xylene and increased selectivity to C4 products at conversions >80%. At conversions below 80%, product selectivities are similar in the presence and absence of H2. The similar product distribution at low conversions suggests that aromatization occurs on Ga. However, at high conversions, i.e., higher H2 partial pressure, butene hydrogenation leads to increased C4 selectivity. For less reactive olefins, e.g., propylene and ethylene, the selectivities are unaffected. The lower toluene and xylene selectivities also suggest that the rate of aromatic formation is inhibited by the excess H2. At approximately 80% conversion, there is sufficient H2 to suppress Ga aromatization and additional aromatics are formed via cracking on H-ZSM-5. This interpretation is supported by the similar selectivity to toluene and xylene observed at 50 kPa while co-feeding H2 on Ga/H-ZSM-5 and H-ZSM-5 (Figure 8a and Figure 8b, respectively).

3.2. The Impact of Ga on C3H6 Aromatization Product Distribution

By comparing the product distribution of Ga/H-ZSM-5 and H-ZSM-5 under equivalent conditions (i.e., isoconversion and same reaction conditions), the effect of Ga on C3H6 aromatization can be ascertained. At 1.9 kPa, Ga/H-ZSM-5 has a higher selectivity to each aromatic than H-ZSM-5 (Figure 9), in agreement with previous reports that Ga increases selectivity to aromatics for olefin aromatization due to Ga enabling catalytic dehydrogenation of cyclic hydrocarbons [7,9,10,12,13]. On both Ga/H-ZSM-5 and H-ZSM-5, aromatic selectivity initially increases at around 45% conversion (Figure 9a), and similar aromatic selectivity is achieved at near complete propylene conversion. However, aromatic selectivity on Ga/H-ZSM-5 increases more significantly at lower conversions (i.e., 45–80%) compared to H-ZSM-5. At higher conversions (i.e., >80%), the aromatic selectivity increases more rapidly on H-ZSM-5, ultimately achieving a similar selectivity as near-complete conversion is achieved.
The increased selectivity on Ga/H-ZSM-5 is offset by a decreased selectivity to non-aromatic C4+ hydrocarbons (Figure 9b) and propane (Figure 9c) compared to H-ZSM-5. This is consistent with Ga dehydrogenating cyclic hydrocarbons, whereas H-ZSM-5 produces alkanes as a by-product of aromatics formation. Ga-enabling catalytic dehydrogenation converts larger hydrocarbons into aromatic rings, resulting in reduced selectivity to reactive intermediates, especially at intermediate conversions. The increased selectivity to propane on H-ZSM-5 compared to Ga/H-ZSM-5 suggests that Brønsted acid sites form propane during aromatization, whereas on Ga/H-ZSM-5, propane is formed to a lesser extent. Additionally, propane may be converted to propylene on Ga via catalytic dehydrogenation, thus reducing the selectivity on Ga/H-ZSM-5.
As mentioned above, at 50 kPa, the presence of H2 affects the product distribution on Ga/H-ZSM-5 at conversions above 80%; thus, Figure 10 and Figure 11 compare the product distributions in the absence and presence of H2, respectively. Without H2, the Ga/H-ZSM-5 demonstrates an increased selectivity to aromatics (Figure 10a) at conversions above 85% and a lower selectivity to light alkanes (i.e., propane). However, at conversions below 85%, where most industrial processes are operated, the product distribution on Ga/H-ZSM-5 is equivalent to that of H-ZSM-5. When H2 is present, as described above, the product distribution on Ga/H-ZSM-5 and H-ZSM-5 are equivalent (Figure 11). Thus, practically, Ga demonstrates no significant impact on the product distribution of propylene aromatization at 50 kPa, contrasting with the results observed at 1.9 kPa.
The observation that Ga/H-ZSM-5 does not impact aromatics selectivity at 50 kPa contrasts with previous conclusions that Ga/H-ZSM-5 increases the selectivity of aromatics compared to H-ZSM-5 [7,10,12,13]. However, previous studies examined propylene aromatization at different partial pressures of C3H6 [7,10,12,13], compared reaction products at equivalent space velocity as opposed to equivalent conversion [10,12,13], or both. As is evident in this study, the partial pressure of C3H6 shifts the product distribution trends on Ga/H-ZSM-5 and H-ZSM-5 and is discussed further, below. Additionally, propylene is reformed via cracking reactions on Brønsted acid sites [3]; thus, space velocity is not analogous to conversion for aromatization reactions. For example, Shibata et al. compare H-ZSM-5 and Ga/H-ZSM-5 at equivalent contact time (W/F = 10.0 g h mol−1) and achieve 80% on H-ZSM-5 but reach near-100% conversion on Ga/H-ZSM-5 [13].

3.3. The Impact of C3H6 Partial Pressure on Aromatization Product Distribution

On both H-ZSM-5 (Figure 1, Figure 2 and Figure 3) and Ga/H-ZSM-5 (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7), increasing the partial pressure results in a shift in product distribution to form increased amounts of heavy molecular weight, non-aromatic species, and fewer aromatic and light alkane species. Methane and ethane selectivity are lower at 50 kPa than at 1.9 kPa (Figure 2 and Figure 6 for H-ZSM-5 and Ga/H-ZSM-5, respectively). At conversions below 80%, propane selectivity is lower at 50 kPa than at 1.9 kPa; however, as conversion increases, propane selectivity increases more rapidly at 50 kPa, ultimately resulting in a higher selectivity at near complete propylene conversion. This is attributed to shifts in the equilibrium of olefin hydrogenation at higher pressures. Increasing the partial pressure of C3H6 and H2 from 1.9 kPa to 50 kPa, an increase by a factor of 26, shifts the equilibrium of olefin hydrogenation to favor alkanes more significantly [47]. The presence of the Ga functionality has been broadly reported to increase aromatic selectivity by shifting the mechanism for aromatic formation to catalytic dehydrogenation. The suppression of dehydrogenation due to shifting equilibrium may also be in part responsible for the decreased selectivity to aromatics (Figure 5) and increased selectivity to C4+ species at intermediate conversions (Figure 7b–e). As previously discussed, at 50 kPa C3H6 and H2, C4 selectivity on Ga/H-ZSM-5 significantly increases at conversions above 80% due to the hydrogenation of C4 olefins, consistent with the increase in partial pressure shifting the equilibrium towards hydrogenation products. However, due to the variety of isomers present in the higher molecular weight species, determining the relative degree of dehydrogenation was infeasible for this study.

4. Materials and Methods

4.1. Catalyst Synthesis

To produce a 10 g batch of catalyst, 10 g commercial ammonium form ZSM-5 extrudate (SiO2/Al2O3 = 30, H-ZSM-5/Al2O3 = 4, Zeolyst International, CBV 3014) was calcined at 550 °C for 3 h (5 °C/min ramp rate) to convert the catalyst to H+-form ZSM-5 (H-ZSM-5). Ga/H-ZSM-5 was then produced by adding Ga to the H-ZSM-5 via incipient wetness impregnation, targeting 3 wt.% Ga (g Ga/g extrudate, including Al2O3). X-ray Fluorescence results confirmed 2.8 wt.% Ga on Ga/H-ZSM-5. A Ga3+ solution was prepared by dissolving 1.1 g gallium nitrate hydrate (Sigma Aldrich, St. Louis, MO, USA) and 0.83 g citric acid (Sigma Aldrich) in 5 mL Millipore water to achieve a 1:1 mol ratio of Ga:citrate. This solution is then added dropwise to the H-ZSM-5 extrudate while mixing mechanically using a plastic spatula. The solution was then dried overnight at room temperature and calcined at 550 °C for 3 h (5 °C/min ramp rate).

4.2. Reaction Performance Testing

Catalytic performance was evaluated by loading the catalyst in a quartz tube fixed bed reactor (10.5 mm i.d.) equipped with mass flow rate controllers (Parker Porter, CM400) for atmospheric pressure conditions. A furnace (Applied Test Systems series 3210) was connected to a temperature controller to supply the heat and maintain the desired temperature. Reactions were performed at two different partial pressures of propylene and hydrogen: 1.9 kPa and 50 kPa. During a reaction at 1.9 kPa, 3% C3H6 (balance N2, Indiana Oxygen, Indianapolis, IN, USA) and 5% H2 (Indiana Oxygen) were diluted in ultrahigh purity N2 (99.99%, Indiana Oxygen) and co-fed to achieve equimolar 1.9 kPa C3H6 and H2 or 1.9 kPa C3H6 at equivalent space velocities in different trials. At 50 kPa, the reactor is co-fed with pure propylene (≥99.98%, Indiana Oxygen) and pure H2 (Indiana Oxygen). For comparison, equivalent reactions were performed by replacing the H2 with N2 to affect equivalent partial pressures of C3H6 at equivalent space velocities.
Catalysts were supported on quartz wool with a K-type thermocouple placed in the middle of the bottom of the catalyst bed to monitor the temperature in the bed. The reactor effluent was discharged through a line heated to 170 °C using heat tape (Omega, Sydney NSW, Australia) and fed to a gas chromatograph (Agilent 6890A, Santa Clara, CA, USA) equipped with a flame ionization detector (Agilent J&W HP-1 column, 0.320 mm i.d. × 25 m) for reactant and product quantification. The catalytic performances were evaluated at 450 °C and atmospheric pressure. The conversion and product selectivity were obtained at different space velocities corresponding to 25–160 ccm (STP) in combined flow rate.
Product selectivities and reaction conversion were determined based on the relative peak area as measured from gas chromatography assuming conservation of carbon throughout the reaction. Product selectivity to species i , S i , was calculated according to
S i = A i ∑ j A j
where A i represents the area of the peak corresponding to species i , and the denominator represents the total area of the summation of all the peaks excluding the peak corresponding to propylene (i.e., represents the total carbon present in products). As mentioned in the results section, many isomers of hydrocarbons containing four or more carbons are produced. Thus, product selectivities are lumped by carbon number (e.g., the selectivity to all combined butane and butene isomers are represented as C4s).
Similarly, the reaction conversion, χ , was determined according to
χ = A C 3 = ∑ k A k
where A C 3 = represents the area of the gas chromatography peak corresponding to propylene, and the denominator represents the total carbon present in the effluent.
The conversion was also determined by comparing the peak area of the propylene peak when the reaction was run to the peak area of flowing propylene through a bypass line on the reactor, and conversion was consistently found to be within 2–3%. Correlation coefficients for linear alkanes were determined to be similar. Due to the variety of products that are formed from propylene aromatization, the determination of all correlation coefficients is infeasible. Determination of correlation coefficients for linear alkanes and alkenes was attempted, and correlation coefficients were all found to be nearly equivalent; thus, the inclusion of correlation coefficients was neglected in the determination of selectivity and conversion.
This determination of product selectivities assumes that the carbon balance closes, resulting in selectivities potentially being overestimated as large carbonaceous species trapped in the catalyst bed (i.e., coke) and those that liquefy in the lines are not accounted for. No liquid buildup in the lines is observed for experiments conducted while feeding 1.9 kPa C3H6. At 50 kPa C3H6, small amounts of liquid deposition on the walls of the reactor tube at the exit of the clamshell furnace are observed.

4.3. X-ray Absorption Spectroscopy

X-ray absorption spectroscopy (XAS) measurements were taken at the 8-ID beamline of the National Synchrotron Light Source II at Brookhaven National Lab. Measurements were taken at the Ga K-edge (10,367 eV) in fluorescence mode. Samples were ground into a powder, pressed into a wafer, sealed using Kapton tape, and then placed on the beamline for scanning. Scans were taken from 10,167–11,350 eV.
To process data, WinXAS 3.1 software was employed using standard procedures for fitting. Normalization of the absorption spectra was performed by fitting a 1st-degree polynomial to the pre-edge region (10,167–10,315 eV) and a 3rd-degree polynomial to the absorption region above the edge (10,412 eV–11,350 eV). The absorption spectrum was then converted to photon momentum space (k-space), and a cubic spline fit with 5 splines was fit to the absorption spectrum corresponding to the 2–12 k, and the resultant χ(k) spectrum was weighted by a factor of k2. The Fourier transform of the χ(k)*k2 spectrum from k = 2.5–10.5 was then utilized to generate the phase-unshifted r-space spectrum, which fit with the experimental Ga–O scattering path. The phase shift and amplitude of the experimental Ga–O scattering path were determined by isolating the first shell of the r-space spectrum of gallium (III) acetylacetonate, Ga(AcAc)3, standard (Sigma Aldrich) with 6 Ga–O bonds at 1.94 Å. The phase shift and amplitude of this experimental scattering path were then used in combination with the EXAFS equation to perform a least-squares fit of the coordination number and bond distance of the Ga/H-ZSM-5 sample.

4.4. Elemental Analysis

Energy dispersive X-ray fluorescence (EDXRF) data were collected using a Malvern Panalytical Epsilon4 X-ray fluorescence spectrometer equipped with a 15 W silver anode X-ray tube, a 10-sample changer, and helium gas flush option, and an energy dispersive silicon drift detector. Samples were packed as powders into polypropylene XRF cups of appropriate sizes. Prior to data collection, a recalibration standard based on elements Al, Ca, Fe, K, Mg, Na, P, S, and Si was measured for drift correction in the soft X-ray region. Data were collected using the Epsilon 4 software employing the “Omnian” data collection procedure (Malvern Panalytical B.V., The Netherlands, Version 2.1). Data were collected with six different acceleration voltages between 5 and 50 kV and varying thickness Ti, Al, Cu, and Ag filters. Data were analyzed using the standardless Omnian procedure, with data processing parameters being defined prior to analysis for each type of sample using the Epsilon 4 Dashboard software (Malvern Panalytical, Version 2.1.1.10717).

5. Conclusions

By cross-comparing the product distribution of propylene aromatization in the presence and absence of 1.9 or 50 kPa C3H6 co-fed with H2 or N2 on Ga/H-ZSM-5 and H-ZSM-5, the effects of Ga, H2, and pressure were observed. Ga was shown to enhance aromatic selectivity, especially at 1.9 kPa, through catalytic dehydrogenation enhancing rates of aromatization. At higher pressures, this effect is minimized as dehydrogenation equilibrium shifts to favor paraffinic products, thereby reducing the differences in aromatic selectivity on Ga compared to H+. The presence of equimolar H2, which would be produced during the dehydrogenation of alkanes prior to olefin aromatization that could be employed in a two-stage alkane dehydroaromatization process, was shown to have minimal impact on the product distribution of H-ZSM-5, suggesting that Brønsted acid protons are non-reactive for dehydrogenation during aromatization. On Ga/H-ZSM-5 at 1.9 kPa, the product selectivity is also unaffected, suggesting that Ga remains active for catalytic dehydrogenation at these conditions. At 50 kPa, where the dehydrogenation equilibrium favors alkanes more significantly, the product selectivity in the presence of equimolar H2 shifts the product distribution on Ga/H-ZSM-5 to be equivalent to that of H-ZSM-5, corroborating the notion that Ga enhances aromatic selectivity via catalytic dehydrogenation of cyclic hydrocarbons to form aromatic rings. The effect of pressure on C3H6 aromatization served to increase the selectivity of heavy non-aromatic hydrocarbons regardless of the catalyst used or co-fed species.

Author Contributions

Conceptualization, C.K.R. and J.T.M.; methodology, C.K.R.; validation, C.K.R., J.L.R. and R.N.H.; investigation, C.K.R., J.L.R. and R.N.H.; data curation, C.K.R., J.L.R. and R.N.H.; writing—original draft preparation, C.K.R.; writing—review and editing, C.K.R. and J.T.M.; visualization, C.K.R.; supervision, J.T.M.; project administration, C.K.R.; funding acquisition, J.T.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation, Cooperative Agreement No. EEC-1647722, an Engineering Research Center for the Innovative and Strategic Transformation of Alkane Resources.

Data Availability Statement

The data presented in this study are available.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00405 sch001
Scheme 1. A simplified reaction mechanism for aromatization on (a) H-ZSM-5 and (b) Ga-modified H-ZSM-5. Red arrows represent reactions that predominantly occur on H-ZSM-5, and blue arrows represent reactions that predominately occur on Ga3+. C x = and C y = represent alkenes, where x and y are at least 2, C x and C y represent the respective alkanes and C x + y ≥ 6 cyclic represents cyclic hydrocarbons with at least six carbons (i.e., the precursors to benzene, toluene, and xylene).
Scheme 1. A simplified reaction mechanism for aromatization on (a) H-ZSM-5 and (b) Ga-modified H-ZSM-5. Red arrows represent reactions that predominantly occur on H-ZSM-5, and blue arrows represent reactions that predominately occur on Ga3+. C x = and C y = represent alkenes, where x and y are at least 2, C x and C y represent the respective alkanes and C x + y ≥ 6 cyclic represents cyclic hydrocarbons with at least six carbons (i.e., the precursors to benzene, toluene, and xylene).
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Figure 1. The carbon selectivity to (a) benzene, (b) toluene, and (c) xylenes as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
Figure 1. The carbon selectivity to (a) benzene, (b) toluene, and (c) xylenes as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
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Figure 2. The carbon selectivity to (a) methane, (b) ethane, and (c) propane as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
Figure 2. The carbon selectivity to (a) methane, (b) ethane, and (c) propane as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
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Figure 3. The carbon selectivity to reactive intermediates: (a) ethene, (b) butanes/enes (C4), (c) pentanes/enes (C5), (d) hexanes/enes (C6), and (e) larger non-aromatic hydrocarbons, C7+ as a function of propylene conversion in the presence (red) and absence (blue) of equimolar H2 on H-ZSM-5 at 723 K at 1.9 kPa C3H6/N2 (circles) or 50 kPa C3H6 (squares).
Figure 3. The carbon selectivity to reactive intermediates: (a) ethene, (b) butanes/enes (C4), (c) pentanes/enes (C5), (d) hexanes/enes (C6), and (e) larger non-aromatic hydrocarbons, C7+ as a function of propylene conversion in the presence (red) and absence (blue) of equimolar H2 on H-ZSM-5 at 723 K at 1.9 kPa C3H6/N2 (circles) or 50 kPa C3H6 (squares).
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Figure 4. The (a) XANES region (10,350–10,400 eV) and the (b) real and (c) imaginary portions of the Fourier transform of the EXAFS regions for Ga(AcAc)3 (red), Ga/H-ZSM-5 (black), and a model of Ga3+ generated from the isolated first shell of Ga(AcAc)3 reference (blue). (d) The isolated first shell of the EXAFS of Ga/H-ZSM-5 (black) and Ga(AcAc)3. Materials were scanned ex situ in He at ambient temperature.
Figure 4. The (a) XANES region (10,350–10,400 eV) and the (b) real and (c) imaginary portions of the Fourier transform of the EXAFS regions for Ga(AcAc)3 (red), Ga/H-ZSM-5 (black), and a model of Ga3+ generated from the isolated first shell of Ga(AcAc)3 reference (blue). (d) The isolated first shell of the EXAFS of Ga/H-ZSM-5 (black) and Ga(AcAc)3. Materials were scanned ex situ in He at ambient temperature.
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Figure 5. The carbon selectivity to (a) benzene, (b) toluene, and (c) xylene as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
Figure 5. The carbon selectivity to (a) benzene, (b) toluene, and (c) xylene as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
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Figure 6. The carbon selectivity to (a) methane, (b) ethane, and (c) propane as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
Figure 6. The carbon selectivity to (a) methane, (b) ethane, and (c) propane as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
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Figure 7. The carbon selectivity to (a) ethene, (b) butanes/enes (C4), (c) pentanes/enes (C5), (d) hexanes/enes (C6), and (e) larger non-aromatic hydrocarbons (C7+) as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
Figure 7. The carbon selectivity to (a) ethene, (b) butanes/enes (C4), (c) pentanes/enes (C5), (d) hexanes/enes (C6), and (e) larger non-aromatic hydrocarbons (C7+) as a function of propylene conversion in the presence and absence of 1.9 kPa H2 on Ga/H-ZSM-5 at 723 K, 1.9 kPa C3H6, balance N2.
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Figure 8. Comparison of (a) C4, (b) toluene, and (c) xylene selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled). Reaction conditions: 50 kPa C3H6, 50 kPa H2 or N2, 723 K. Reproduced from Figure 1 and Figure 4.
Figure 8. Comparison of (a) C4, (b) toluene, and (c) xylene selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled). Reaction conditions: 50 kPa C3H6, 50 kPa H2 or N2, 723 K. Reproduced from Figure 1 and Figure 4.
Catalysts 14 00405 g008
Catalysts 14 00405 g009
Figure 9. Comparison of (a) combined aromatic (BTX), (b) hydrocarbons with at least four carbons, and (c) propane selectivities as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) in 1.9 kPa H2, 1.9 kPa C3H6, balance N2 at 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6.
Figure 9. Comparison of (a) combined aromatic (BTX), (b) hydrocarbons with at least four carbons, and (c) propane selectivities as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) in 1.9 kPa H2, 1.9 kPa C3H6, balance N2 at 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6.
Catalysts 14 00405 g009
Catalysts 14 00405 g010
Figure 10. Comparison of (a) combined BTX, (b) hydrocarbons with at least four carbons, and (c) combined light gas (i.e., methane, ethane, and propane selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) for C3H6 aromatization). Reaction conditions: 50 kPa C3H6, 50 kPa N2, 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7.
Figure 10. Comparison of (a) combined BTX, (b) hydrocarbons with at least four carbons, and (c) combined light gas (i.e., methane, ethane, and propane selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) for C3H6 aromatization). Reaction conditions: 50 kPa C3H6, 50 kPa N2, 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7.
Catalysts 14 00405 g010
Catalysts 14 00405 g011
Figure 11. Comparison of (a) combined BTX, (b) hydrocarbons with at least four carbons, and (c) combined light gas (i.e., methane, ethane, and propane selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) for C3H6 aromatization in H2). Reaction conditions: 50 kPa C3H6, 50 kPa H2, 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7.
Figure 11. Comparison of (a) combined BTX, (b) hydrocarbons with at least four carbons, and (c) combined light gas (i.e., methane, ethane, and propane selectivity as a function of conversion on H-ZSM-5 (unfilled) and Ga/H-ZSM-5 (filled) for C3H6 aromatization in H2). Reaction conditions: 50 kPa C3H6, 50 kPa H2, 723 K. Data reproduced from Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7.
Catalysts 14 00405 g011
Table 1. Characteristic XANES energies (10,350–10,400 eV).
Table 1. Characteristic XANES energies (10,350–10,400 eV).
SampleEdge Energy
(eV)		White Line Energy
(eV)		Notable Shoulders
(eV)		Coordination Geometry
Ga/H-ZSM-510,372.610,376.210,381 eVTd
Ga(AcAc)310,376.510,379.0-Oh
Table 2. EXAFS Fitting results (10,350–10,400 eV).
Table 2. EXAFS Fitting results (10,350–10,400 eV).
SampleCN
(±10%)		R
(±0.02 Å)		σ2Eo
Ga(AcAc)36 *1.940.005−1.54
Ga/H-ZSM-54.01.840.005 *−1.67
* Values held fixed.
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Russell, C.K.; Rockey, J.L.; Hanna, R.N.; Miller, J.T. Impact of Co-Fed Hydrogen on High Conversion Propylene Aromatization on H-ZSM-5 and Ga/H-ZSM-5. Catalysts 2024, 14, 405. https://doi.org/10.3390/catal14070405

AMA Style

Russell CK, Rockey JL, Hanna RN, Miller JT. Impact of Co-Fed Hydrogen on High Conversion Propylene Aromatization on H-ZSM-5 and Ga/H-ZSM-5. Catalysts. 2024; 14(7):405. https://doi.org/10.3390/catal14070405

Chicago/Turabian Style

Russell, Christopher K., Josiah L. Rockey, Rebecca N. Hanna, and Jeffrey T. Miller. 2024. "Impact of Co-Fed Hydrogen on High Conversion Propylene Aromatization on H-ZSM-5 and Ga/H-ZSM-5" Catalysts 14, no. 7: 405. https://doi.org/10.3390/catal14070405

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