Methanol Usage in Toluene Methylation over Pt Modified ZSM-5

Article pubs.acs.org/IECR

Methanol Usage in Toluene Methylation over Pt Modified ZSM‑5 Catalyst: Effects of Total Pressure and Carrier Gas Yiren Wang,† Min Liu,† Anfeng Zhang,† Yi Zuo,† Fanshu Ding,† Yang Chang,† Chunshan Song,*,†,‡ and Xinwen Guo*,† †

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China ‡ EMS Energy Institute, PSU-DUT Joint Center for Energy Research, Department of Energy and Mineral Engineering, and Department of Chemical Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: The present study investigates the effects of total pressure and carrier gas on toluene methylation using two modified ZSM-5 based catalysts impregnated with or without platinum. Toluene alkylation with methanol was carried out not only at different reaction pressure under nitrogen or hydrogen but also at different total pressure while the partial pressure of reactants remained constant. The amount of coke formed on the catalysts was less under hydrogen atmosphere. However, the hydrogenation reaction catalyzed by Pt modified catalyst promoted methanol conversion to undesired methane and light hydrocarbons, which in turn decreased the toluene conversion. For both carrier gases, it was found that methanol usage toward undesired byproducts increased with total pressure. Such a trend suggests that a higher total pressure results in a lower toluene and methanol adsorption complexes formation rate in the ZSM-5 pores, and therefore the toluene conversion decreased. The hydrogenation reaction and the higher total pressure simultaneously increased the selectivity of methanol converted to light hydrocarbons and methane, which led to the deteriorated catalytic performance of Pt modified catalysts at elevated reaction pressure.

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INTRODUCTION para-Xylene, an important raw material for polyester manufacture, is the most lucrative petrochemical commodities among xylene isomers.1 Global demand for para-xylene has been continuously increasing, reaching 32 million tons/year.2 Conventionally, mixed xylenes are produced via catalytic reforming and thermal cracking of naphtha.1,3,4 para-Xylene is separated from other xylene isomers through a very energyintensive process, such as distillation, adsorption, or cryogenic crystallization.5,6 The high demand for para-xylene has created incentives for technology researchers to develop alternative routes for its production. In particular, the shape-selective toluene methylation process is a very promising way for paraxylene production because toluene is produced beyond market demand3,7 and methanol is expected to be extensively obtained from converting coal and natural gas.3 Moreover, the high concentration of para-xylene in shape-selective toluene methylation products can significantly reduce energy consumption in separation and increase para-xylene productivity in an aromatics complex.5,7 To be commercially competitive, toluene methylation (especially operating in a fixed-bed reactor) requires its catalyst to be highly para-selective, highly active, and highly stable, as well as efficient methanol utilization. Shape-selective toluene © XXXX American Chemical Society

methylation mainly uses medium-pore zeolites (10-membered ring zeolites) as catalyst, such as ZSM-5 and MCM-22.7−14 Pure HZSM-5 or MCM-22 exhibits poor para-selectivity in toluene methylation due to the isomerization on external acid sites.14,15 Therefore, certain modifications are made to improve para-selectivity. Commonly used techniques include (1) addition of an oxide by impregnation with salts like B, P, Mg, La compounds,7−9,11−14,16 (2) silanization by chemical vapor deposition (CVD),17 chemical liquid deposition (CLD),11,18,19 or core−shell synthesis methods,10 (3) precoking,7 (4) tuning crystal size.14,20 Most treatments are aimed at minimizing external acid sites and tailoring relative diffusion rate of xylene isomers.14 However, these effective means for para-selectivity enhancement were achieved at the expense of toluene conversion and catalyst life span. To break the trade-off between selectivity and activity, Ahn and his co-workers19 made a strategy to retain para-selectivity of catalysts without activity loss. The desilication, dealumination, and subsequent SiO2 covering method in their work shortened the diffusion path Received: Revised: Accepted: Published: A

January 22, 2017 March 30, 2017 April 5, 2017 April 5, 2017 DOI: 10.1021/acs.iecr.7b00318 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Catalyst Characterization. Thermogravimetric analysis (TG) was performed on a SDT Q600 thermal gravimetric analyzer (TA Instruments, USA) to analyze coke content of the samples. The TG was first conducted under nitrogen from room temperature to 450 °C at a ramping rate of 10 °C min−1 to monitor the adsorbed aromatic molecules and coke precursor. Then the sample was cooled to room temperature. And the TG was conducted again under synthetic air from room temperature to 900 °C at a ramping rate of 10 °C min−1 (without changing sample) to evaluate the real coke deposits on the samples. Weight loss from 300 to 800 °C in synthetic air was defined as coke deposits of the sample. Temperatureprogrammed desorption of ammonia (NH3-TPD) was used to compare the acid amount and strength of catalyst samples. NH3-TPD was performed on an automated chemisorption analyzer (ChemBET Pulsar TPR/TPD, Quantachrome, USA) from 120 to 650 °C at a temperature ramp rate of 10 °C/min. The desorbed NH3 was detected continuously with a thermal conductivity detector (TCD). Each sample was pretreated at 500 °C in He flow for 1 h, then cooled to 120 °C, exposed to a mixed gas of ammonia (7.88 vol %) with helium for 40 min. The physically adsorbed NH3 was removed by He at 120 °C for 1 h. N2 adsorption/desorption isotherms at −196 °C were recorded in a Quantachrome Quantasorb-SI gas adsorption analyzer (Quantachrome, USA) after evacuation of the samples at 300 °C for 10 h. Isothermal adsorption of n-hexane and cyclohexane was measured on a homemade apparatus by a flow gravimetric method at 25 °C. The sample was dehydrated before measurement at 350 °C under nitrogen flow for 1 h. Catalytic Studies. The gas phase alkylation of toluene (T) with methanol (M) was studied in a downflow stainless-steel fixed-bed reactor containing 1.2 g of 10−20 mesh catalyst. Unless otherwise stated, experiments were carried out at 460 °C and at 0−0.4 MPa (gauge pressure) with a WHSV (weight hourly space velocity of toluene and methanol) of 2.5 h−1. The catalyst was pretreated in situ at 500 °C for 1 h under a flow of hydrogen and then cooled down to 460 °C prior to the introduction of the reactants. Toluene and methanol and water, in molar ratios of nT/nM/nH2O = 4/1/10, were delivered to a vaporizer where the vapors were mixed with carrier gases before passing to the reactor. Hydrogen or nitrogen was used as carrier gas with molar ratios of H2 or N2/(T + M) = 2−32. The reactor effluent was collected in a cold trap. The condensed liquid was separated into two phases and analyzed by gas chromatography. For the organic liquid phase, Agilent GC6890 gas chromatography equipped with an INNOWAX capillary column (60 m × 0.25 mm × 0.25 μm) and a flame ionization detector (FID) was used. A Tianmei 7890 gas chromatography equipped with a HayeSep Q column and a FID was used for the incondensable off-gas and aqueous product analysis. Generally, reaction products were sampled for analysis for about 4−6 h on stream (the time at which the reaction was stabilized and the products collected were representative). The toluene conversion (CT) and the para-selectivity (SPX) were defined in the following equations:

length for reactants and products (promoted catalytic activity) and covered external acid sites (remained para-selectivity).6 Attempts have also been made by researchers to solve the paradox that high para-selectivity leads to poor stability. Modifying toluene methylation catalysts with metals of good hydrogenation properties, like Pt, Pd, Ni, Co, enhanced the catalyst stability remarkably without sacrificing para-selectivity.11,21−25 The reduced alkenes in toluene methylation product by hydrogenating process are the main reason for enhanced life span. Notwithstanding these substantial efforts to make a satisfactory catalyst for toluene methylation, toluene methylation catalysts reported in literature were generally evaluated under atmospheric pressure. Customarily, industrial reactors prefer to be operated under elevated pressure.3,26 This makes the study of pressure effect on catalyst performance essential when reactor scales up. However, few data about toluene methylation catalysts performance under elevated pressure were reported.7,27 The Mobil researchers once mentioned the effect of pressure on the toluene methylation over phosphorus modified ZSM-5 was not significant.7 However, no further studies were reported to investigate the effect of pressure thoroughly. During our toluene methylation research, deteriorated catalytic performance of the promising Pt modified catalyst at elevated reaction pressure was observed. This prompted us to investigate and break down the effects of pressure and carrier gas on catalyst performance of modified ZSM-5 in toluene methylation process. In this work, the effects of total pressure and carrier gas on toluene methylation process, from the perspective of methanol usage, are studied systematically using two representative modified ZSM-5 based catalysts. One is a commercially attractive highly stable ZSM-5 catalyst modified with oxide and platinum. The other one is oxide modified ZSM-5 catalyst. Reactions were carried out at different total pressure under nitrogen or hydrogen while partial pressure of reactants was constant. The study shows that an increase in total pressure results in a lower toluene conversion due to the reduced methanol usage for toluene methylation. Moreover, different carrier gas results in significant differences for methanol usage in toluene methylation over Pt modified ZSM-5 catalyst. Possible solutions for low efficient methanol usage under elevated pressure are discussed as well.

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EXPERIMENTAL SECTION Catalyst Preparation. Two catalysts used in this work were prepared according to procedures described in our previous investigations.12,22 One is Si−P−Mg modified nanoscale HZSM-5 (SiO2/Al2O3 of ∼26, crystal size of ∼100 nm), denoted as mZSM-5. For mZSM-5, SiO2 modifier content is 6 wt %, P2O5 is 5 wt %, and MgO is 3 wt %. To get mZSM-5 catalyst, the nanoscale HZSM-5 was impregnated with tetraethyl orthosilicate (TEOS) dissolved in cyclohexane, aqueous solution of monoammonium phosphate, aqueous solution of magnesium acetate at ambient temperature, followed by drying at 120 °C and calcining in air at 540 °C for 4 h, respectively. The other catalyst was denoted as Pt/ mZSM-5. Pt/mZSM-5 was prepared by loading mZSM-5 catalyst with 1 wt % SiO2 (impregnated with TEOS− cyclohexane solution, dried, and calcined) and then impregnated with aqueous chloroplatinic acid at ambient temperature, followed by drying at 120 °C and calcining in air at 400 °C for 2 h. The Pt content in Pt/mZSM-5 catalyst is 0.15 wt %.

⎛ toluene in product ⎞ ⎟ × 100 C T (%) = ⎜1 − ⎝ toluene in reactant ⎠ SPX (%) = B

p‐xylene × 100 p‐xylene + m‐xylene + o‐xylene

(1)

(2)

DOI: 10.1021/acs.iecr.7b00318 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research Scheme 1. Typical Reactions in Toluene Methylation Process

are utilized to alkylate toluene. In the numerator of eqs 4 and 5, the same rule is applied. For example, in eq 4, “C3” is multiplied by 3 because three methanol molecules are consumed to generate one C3 species. As shown in Scheme 1, benzene in product is mainly generated via toluene disproportionation. It means that the moles of benzene are the moles of xylene formed via toluene disproportionation. This part of xylene should be deducted from methanol usage for alkylation. Therefore, “benzene” is multiplied by −1 in eq 3. Coke deposits in toluene methylation over modified ZSM-5 catalyst are mainly formed from polymerization of olefins and multi-methylbenzenes.31,32 In this research, we assume that toluene and methanol contribute equally in coke formation. And this is the reason that coke formation rate is multiplied by 0.5 in Um‑coke calculation.

Catalytic studies were repeated several times. Error bars for toluene conversion and para-selectivity are ±0.15% and ±0.25%, respectively. Typical reactions in toluene methylation process are presented in Scheme 1. Ideally, one methanol molecule reacts with one toluene to generate one xylene. Virtually, this reaction is accompanied by many side reactions, particularly in ZSM-5 based catalyst.18,28,29 These side reactions include (1) further alkylation of xylenes, (2) conversion of methanol to olefins (mainly the arene cycle in dual-cycle mechanism),30 (3) methane formation, (4) toluene disproportionation, (5) coking. Consuming of methanol in side reactions is the fundamental reason for low toluene conversion. In this research, methanol in toluene methylation process was divided into four different usages: (1) methanol usage for alkylation (denoted as Um‑alky); (2) methanol usage for C2−C4 light hydrocarbon formation (denoted as Um‑LH); (3) methanol usage for methane formation (denoted as Um‑CH4); (4) methanol usage for coke formation (denoted as Um‑coke). These four different fractional uses of methanol were calculated by carbon selectivity of different products.18 The definitions are shown in the following equations:

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RESULTS AND DISCUSSION Catalytic Performance of Modified ZSM-5 Catalysts. Figure 1 shows the catalytic performance of toluene alkylation

Um‐alky (%) =

1 xylene + 1 EB + 2 TriMB + 3 TetraMB + ... − 1 benzene methanolR − (methanol + 2 DME)P

(3)

× 100

Um‐LH (%) = Um‐CH4 (%) =

2C 2 + 3C3 + 4C4 × 100 methanolR − (methanol + 2 DME)P

(4)

1 methane × 100 methanolR − (methanol + 2 DME)P

Figure 1. Toluene conversion and para-selectivity over HZSM-5, mZSM-5, and Pt/mZSM-5 at 460 °C, atmospheric pressure, WHSV = 2.5 h−1, nT/nM = 4, nN2/n(T+M) = 2, nH2O/n(T+M) = 2.

(5) Um‐coke

0.5 coke (%) = × 100 methanolR − (methanol + 2 DME)P

with methanol over parent and modified ZSM-5 catalysts. As shown in the plot, conversion of toluene decreased from 18.4% to 14.8% and 13.7% after Si−P−Mg and subsequent Si−Pt modification. Para-selectivity increased drastically from a nearequilibrium 27.5% to 87.8% after Si−P−Mg modification due to the synergistic effect for tailoring the acid property and pore mouth width of the ZSM-5 catalyst discussed in our previous work.12 Then, the further Si−Pt modification gave mZSM-5 a rise in para-selectivity from 87.9% to 94.5%. The increase in para-selectivity and decrease in toluene conversion after multiple modifications are due to the reduction in acid sites

(6)

The subscripts “R” and “P” indicate the molar flow rate of components in reactants and products, respectively. Each compound in eqs 3−6 is the molar flow rate of this component in products or reactants. In the numerator of eq 3, “xylene” is multiplied by 1 because one methanol is consumed to alkylate toluene (one of the reactants and the starting aromatic molecule). “TriMB” and “TetraMB” in eq 3 are multiplied by 2 and 3 individually because two and three methanol molecules C


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when reaction pressure increased in this experiment, every reactant’s partial pressure increased proportionally and volumetric flow rate in the reactor decreased; i.e., contact time increased. In summary, higher reaction pressure leads to three changes, i.e., higher reactant partial pressure, higher total pressure, and longer contact time between the catalyst and the reactants gas stream. J. Breen and co-workers15 demonstrated that para-selectivity increases in a linear manner with decreasing contact time and they achieved 99% para-selectivity by operating toluene methylation at high space velocity (i.e., low contact time) over a B/ZSM-5 catalyst. Our data are consistent with Breen’s proposed model by assuming para-selectivity has a positive correlation with space velocity. The plot of paraselectivity as a function of contact time between catalyst and reactant gas stream is presented in Figure S7. Furthermore, para-selectivity was higher under nitrogen atmosphere than hydrogen when the contact time was the same. The higher para-selectivity can be attributed to the higher amount of coke deposits when nitrogen was used as carrier gas. The coke deposits could block acid sites and narrow pore width of catalyst which resulted in an improvement in para-selectivity.31,36 The TG curves in Figure S8 confirm that more coke deposits were formed under nitrogen than hydrogen. From the perspective of changes in toluene conversion, increasing reactant partial pressure increases the rate of reaction and longer contact time is favorable for increasing toluene conversion (shown in Figure S9). In the case that nitrogen was used as carrier gas, the increasing toluene conversion was consistent with the presumption. However, the descending toluene conversion was unconventional when hydrogen was used as carrier gas. Furthermore, toluene conversion under nitrogen was always higher than hydrogen within the investigated pressure range. A similar observation has been found for benzene alkylation with methanol at atmospheric pressure.23 The lower benzene conversion under hydrogen over Pt modified catalyst was in agreement with our study. Discrepancies revealed that carrier gas may influence the toluene methylation reaction process. Moreover, among the three changes caused by elevated reaction pressure, the effect of total pressure on toluene methylation is unclear, and that might be a prominent cause for the deteriorated catalytic performance of Pt/mZSM-5 at elevated reaction pressure. Carrier Gas Effect. To elucidate the effect of carrier gas on toluene methylation process, the catalytic performance of Pt/ mZSM-5 and mZSM-5 was evaluated under hydrogen and nitrogen atmosphere, respectively (Figure 3). Each experiment was performed under atmospheric pressure. Results showed that toluene conversion over Pt/mZSM-5 was lower when hydrogen was used as carrier gas. However, toluene conversion on mZSM-5 varied insignificantly with the carrier gas (nitrogen and hydrogen). Over both catalysts, para-selectivity was slightly lower under hydrogen atmosphere instead of nitrogen. This may be owing to the lower amount of carbon deposits formed under hydrogen (TG curves in Figure S8 and Figure S10). The methanol usage in mZSM-5 and Pt/mZSM-5 under nitrogen or hydrogen atmosphere is displayed in Figure 4. For mZSM-5, methanol usages were similar under nitrogen or hydrogen atmosphere (35.6−35.7% for Um‑LH, and 2.7% for Um‑CH4). The difference was that methanol usage for coke formation under nitrogen (1.8%) was a bit larger compared to hydrogen atmosphere (1.4% for Um‑coke). The higher Um‑coke was consistent with the higher para-selectivity under nitrogen and the larger proportion of tetramethylbenzene in product

(shown in Figure S1 in Supporting Information) and the reduction in effective dimensions of the catalyst pore openings (shown in Figures S2−S4). Moreover, Pt modification extended the life-span of mZSM-5 to 500 h 22 or more (shown in Figure S5). The prolonged catalyst’s stability is mainly attributed to the hydrogenation of alkenes to alkanes on Pt clusters and the effective suppression of carbon deposition. Pt/mZSM-5, as a highly stable and high para-selectivity catalyst, shows good prospects for industrial application. Reaction Pressure Effect. Generally, an elevated pressure in reactor is required in industrial scale-up. The effect of modest increase in pressure on the alkylation of toluene with methanol over Pt/mZSM-5 is shown in Figure 2. Under

Figure 2. Toluene conversion and para-selectivity over Pt/mZSM-5 under H2 (a) and N2 (b) atmosphere as a function of reaction pressure at 460 °C, WHSV = 2.5 h−1, nT/nM = 4, n(H2 or N2)/n(T+M) = 2, nH2O/n(T+M) = 2.

nitrogen atmosphere, toluene conversion increased slightly with reaction pressure and para-selectivity decreased as pressure was increased. However, in the case using hydrogen as the carrier gas, both toluene conversion and para-selectivity decreased when pressure was increased from atmospheric pressure to 0.4 MPa gauge pressure. The observation that toluene conversion decreased with increased reaction pressure in toluene methylation was unconventional, since toluene conversion reported in toluene disproportionation33 and toluene alkylation with ethane34 was increasing with reaction pressure. Recently, a similar observation in benzene alkylation has been reported,35 and the decreased benzene conversion was found to be in agreement with our study. However, the explanation for the tendency was not discussed. From stoichiometry, no influence of pressure on reaction equilibrium is expected for ideal gases in toluene methylation. In this experiment, weight hourly space velocity and molar ratio of carrier gas or water to toluene and methanol were maintained when reaction pressure was increased. That is, D


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of products is given in Figure 4 (right). Pt/mZSM-5 exhibited a very low olefins/paraffins ratio of 0.11 which means most olefins formed via MTO process were hydrogenated to paraffins.22,23 In this regard, the hydrogenation reaction may promote MTO process equilibrium to be shifted toward products so that more methanol is used to generate light hydrocarbons. Furthermore, the low olefins/paraffins ratio may suppress the formation of higher alkylated products which can produce xylenes through transalkylation or dealkylation reactions.42−44 The observation of decreased toluene conversion, methanol usage for alkylation, and yield of para-xylene over Pt/mZSM-5 under hydrogen can be explained by both counts. In brief, carrier gas effect on mZSM-5 (a representative ZSM-5 based catalyst modified by oxides) is that more coke deposits are generated under inert nitrogen than reactive hydrogen. Differently, carrier effect on Pt/mZSM-5 (a typical alkylation catalyst modified by metal with good hydrogenation properties) mainly associates with the hydrogenation reaction that can accelerate MTO process in toluene methylation and diminish methanol usage for toluene alkylation and coke formation. Total Pressure Effect. For reactions at a specific elevated pressure, the partial pressure of reactants and contact time between the catalyst and the reactant gas stream are controllable, whereas the total pressure is invariable. In order to investigate the effect of total pressure on toluene methylation process, reactants partial pressure and contact time were maintained constant (i.e., ptoluene of 16 kPa, pmethanol of 4 kPa, pwater of 40 kPa, contact time of approximate 0.55 s) with increasing total pressure in the range of 0−0.4 MPa gauge pressure by varying partial pressure of carrier gas. Total pressure effect on Pt/mZSM-5 and mZSM-5, with inert nitrogen as carrier gas, is shown in Figure 5 and Table 1. For Pt/mZSM-5, it was found that toluene conversion declined with increasing total pressure, while para-selectivity remained unchanged. Methanol usage for methane formation over Pt/ mZSM-5 increased with increasing total pressure, which was in accordance with the growing methanol usage for coke formation. Meanwhile, more methanol was consumed to generate light hydrocarbons (higher Um‑LH under elevated total pressure). Compared with Pt/mZSM-5, mZSM-5 showed a more significant decline in toluene conversion and a rise in para-selectivity with the increasing total pressure (shown in Figure 5b). A big rise in Um‑coke and Um‑CH4 on mZSM-5 was also found when increasing total pressure. In the carrier effect experiments, we noticed that methanol usage toward coke deposits on mZSM-5 was higher under nitrogen. In this experiment, elevated total pressure made carrier gas effect more profound due to the substantial growth of carrier gas partial pressure. The larger amount of coke deposits formed on mZSM-5 (higher Um‑coke) might be the reason that paraselectivity rose with total pressure. From these results, it can be concluded that the increasing total pressure promotes various methanol involved side reactions and lowers the fraction of methanol for toluene alkylation. But the toluene disproportionation (i.e., toluene side reaction) had not been promoted considering the decreasing toluene conversion and insignificant change in benzene content (Table S3) under the studied pressure range. This can probably be attributed to the different impact on methanol and toluene diffusion under higher total pressure. It is known that an increase in total pressure results in a decrease in the apparent diffusivity.45 Compared with methanol, molecular dimension of

Figure 3. Toluene conversion, para-selectivity, and yield of para-xylene over mZSM-5 and Pt/mZSM-5 using N2 or H2 as carrier gas. Reaction conditions: 460 °C, atmospheric pressure, WHSV = 2.5 h−1, nT/nM = 4, n(N2 or H2)/n(T+M) = 2, nH2O/n(T+M) = 2.

Figure 4. Methanol usage (left) and ratio of olefins/paraffins (right) of toluene methylation over mZSM-5 and Pt/mZSM-5 using N2 or H2 as carrier gas. Reaction conditions: 460 °C, atmospheric pressure, WHSV = 2.5 h−1, nT/nM = 4, n(N2 or H2)/n(T+M) = 2, nH2O/n(T+M) = 2.

(0.11% under nitrogen and 0.04% under hydrogen, shown in Table S2, and multi-methylbenzene was considered as coke deposit precursor). The similar methanol usage can explain the similar toluene conversion on mZSM-5 under different carrier gas. Differently, carrier gas had a clear effect on methanol usage in Pt/mZSM-5. Under hydrogen atmosphere, more methanol was converted into light hydrocarbons (40.6% under nitrogen, 44.3% under hydrogen) and methane (1.8% under nitrogen, 4.1% under hydrogen). It was reported that methane could be formed from methanol via possible mechanisms with surface methoxy species as intermediates in methanol-to-olefin (MTO) process, i.e., hydrogen transfer reaction between methoxy group and the hydride donor.37,38 In typical methanol related process, the hydride sources can be methanol39 itself or a large molecule deposited on the catalyst surface, referred to as coke.40 Hydrogen as a reactive carrier gas can be adsorbed and dissociate on platinum particles surface and then migrates to the surface of the zeolite support.41 Therefore, hydride source could also be hydrogen over Pt/mZSM-5 in the present work. As discussed above, coke formation is accompanied by the conversion of methanol to methane. Under hydrogen atmosphere, only a small amount of methanol (0.9) was used to form coke deposits over Pt/mZSM-5. This means hydrogen was a major hydride donor for methane formation over Pt/ mZSM-5. The olefins/paraffins ratio (C2−C4/C2°−C4°) E


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methylated aromatics dealkylation (i.e., arene cycle in dualcycle mechanism for MTO process).18,30 Coke deposits formed from polymerization of olefins and multi-methylbenzenes are promoted as well. Excess methanol in catalyst pores, caused from elevated total pressure, reacts with coke precursor via hydrogen transfer reaction, which produces methane and accelerates coking process. These presumptions and deductions are supported by an observation reported in literature9 that the selectivity of methanol conversion to xylene decreased as the methanol concentration increased over B/ZSM-5. Moreover, experiments shown in Figure S11 indicate that addition of water as a competing adsorbate can suppress the invalid conversion of methanol to byproduct and increase toluene conversion. Similar experiments were conducted under hydrogen to support the proposed interpretation of total pressure effect. Results summarized in Figure 6 and Table 2 show the total pressure effect on Pt/mZSM-5 and mZSM-5 under reactive hydrogen. On both catalysts, toluene conversion decreased with total pressure and no remarkable change was shown in paraselectivity. As for mZSM-5, more methanol was utilized for light hydrocarbons production with the increasing total pressure under hydrogen atmosphere. The increment of methanol usage for light hydrocarbon formation (from 35.7% to 38.3%) and decrement of methanol usage for toluene alkylation (from 60.1% to 57.5%) verified the presumption that higher total pressure increases methanol concentration in catalyst pores and promotes methanol related side reactions. Methanol usage for methane formation dropped and then increased slightly when total pressure was increased. Same trend was found for Um‑coke. Since coking is a dehydrogenation reaction, high partial pressure of hydrogen at elevated total pressure could inhibit the formation of coke deposits. This resulted in a decrease in Um‑CH4 and Um‑coke at 0.2 MPa total pressure (gauge). When using reactive hydrogen as carrier gas, it is possible that small amounts of olefins hydrogenated to paraffins over ZSM-5 under relatively high hydrogen partial pressure.38,50 The increased Um‑CH4 at 0.4 MPa total pressure and the decreased olefins/paraffins ratio support the assumption. It should be noted that a long induction period was found over Pt/mZSM-5 under hydrogen at elevated total pressure. As shown in Figure 7, catalytic performance of Pt/mZSM-5 at total pressure of 0.2 and 0.4 MPa (gauge) reached the stable period at 23−25 h on stream instead of 4−6 h on stream. Thus, the products collected at 23−25 h on stream were considered to be representative and used in Figure 6a. As discussed in the

Figure 5. Effect of total pressure on toluene conversion and paraselectivity over (a) Pt/mZSM-5 and (b) mZSM-5 under N 2 atmosphere. Reaction conditions: 460 °C, WHSV = 2.5 h−1, ptoluene = 16 kPa, pmethanol = 4 kPa, pwater = 40 kPa, pN2 variable.

toluene is similar to the ZSM-5 pore size (configurational diffusion). Therefore, diffusion of methanol into ZSM-5 catalyst pores will become much easier than diffusion of toluene at higher total pressure.46 It is reasonable to assume that toluene concentration could decrease and methanol concentration could increase in ZSM-5 catalyst pores. Studies on toluene methylation mechanism47−49 have proposed that toluene methylation over acid zeolites starts with (a) methanol adsorbed on acid sites and formation of methoxonium ion, then (b) formation of toluene and methanol (methoxonium ion) coadsorption complexes prior to alkylation reactions.47 Such change at higher total pressure decreases the formation rate of toluene and methanol adsorption complex in catalyst pores and promotes the further methylation of toluene and the subsequent generation of light hydrocarbon from the highly

Table 1. Methanol Usage for Toluene Alkylation (Um‑alky), for C2−C4 Light Hydrocarbons Formation (Um‑LH), for Methane Formation (Um‑CH4), and for Coke Formation (Um‑coke), Methanol Conversion, and Ratio of Olefins/Paraffin in Toluene Methylation Using Nitrogen as Carrier Gas: Effect of Total Pressurea Pt/mZSM-5

a

mZSM-5

total pressure (gauge):

0 MPa

0.2 MPa

0.4 MPa

0 MPa

0.2 MPa

0.4 MPa

Um‑CH4 (%) Um‑LH (%) Um‑alky (%) Um‑coke (%) MeOH conversion (%) ratio of olefins/parafins

1.8 40.6 55.8 1.9 99.2 64.8

1.9 42.4 53.6 2.1 99.3 74.7

2.3 45.3 50.0 2.4 99.1 69.5

2.7 35.6 60.0 1.8 99.4 51.3

3.6 34.8 58.6 3.0 99.4 67.5

5.3 33.9 56.4 4.4 98.4 77.2

Reaction conditions: 460 °C, WHSV = 2.5 h−1, ptoluene =16 kPa, pmethanol = 4 kPa, pwater = 40 kPa, pN2 variable. F


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Figure 7. Toluene conversion (solid) and para-selectivity (open) as a function of time on stream at 0 MPa (■), 0.2 MPa (▲), and 0.4 MPa (●) total pressure (gauge pressure) over Pt/mZSM-5 under H2 atmosphere. Reaction conditions: 460 °C, WHSV = 2.5 h−1, ptoluene = 16 kPa, pmethanol =4 kPa, pwater =40 kPa, pH2 variable.

of methanol−toluene coadsorption complex47,49 is relatively slow. The toluene conversion at the early stage increased significantly when toluene was preadsorbed on Pt/mZSM-5 before the introduction of reactants mixture (shown in Figure S13). In summary, four sets of experiments were conducted to assess the effect of total pressure on toluene methylation process. In most cases, toluene conversion decreased with increasing total pressure while para-selectivity remained unchanged. The results suggest that the change of total pressure may change the toluene and methanol concentration in catalyst pores and lead to a lower toluene and methanol adsorption complexes formation rate in the ZSM-5 pores. The higher total pressure promotes methanol usage for different side reactions in toluene methylation process and diminishes toluene conversion. Meanwhile, the unchanged para-selectivity exemplified the assumption that para-selectivity is mainly dominated by contact time.

Figure 6. Effect of total pressure on toluene conversion and paraselectivity over (a) Pt/mZSM-5 and (b) mZSM-5 under H2 atmosphere. Reaction conditions: 460 °C, WHSV = 2.5 h−1, ptoluene = 16 kPa, pmethanol = 4 kPa, pwater = 40 kPa, pH2 variable.

carrier effect experiments, the hydrogenation reaction facilitates MTO process in toluene methylation. According to the presumption stated above, the combination of higher methanol concentration in catalyst pores and hydrogenation effect of Pt cluster would give a tremendous rise in methanol usage for light hydrocarbon and methane formation with the increase of total pressure over Pt/mZSM-5 under hydrogen atmosphere. Methanol usage for light hydrocarbon and methane formation increased from 48.4% to 56.2% and 60.4% (shown in Table 2) and is consistent with inference. Moreover, at the early stage of toluene methylation reaction over Pt/mZSM-5 under elevated total pressure (sampled at 4−6 h on stream), considerable amount of methanol was converted to methane due to the high partial pressure of hydrogen. This observation indicates that hydrogenation of methoxonium ions is quick and the formation

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CONCLUSIONS Reaction pressure effect was rarely considered in toluene methylation reactions. Pt modified ZSM-5 catalyst (Pt/mZSM5) as a commercially attractive catalyst shows super good stability under hydrogen atmosphere. However, it was found that both toluene conversion and para-selectivity decreased with reaction pressure over Pt/mZSM-5. In this work, effects of total pressure and carrier gas on toluene methylation were investigated over Pt/mZSM-5 and mZSM-5 (an oxide modified

Table 2. Methanol Usage for Toluene Alkylation (Um‑alky), for C2−C4 Light Hydrocarbons Formation (Um‑LH), for Methane Formation (Um‑CH4), and for Coke Formation (Um‑coke), Methanol Conversion, and Ratio of Olefins/Paraffin in Toluene Methylation Using Hydrogen as Carrier Gas: Effect of Total Pressurea Pt/mZSM-5

a

mZSM-5

total pressure (gauge):

0 MPa

0.2 MPa

0.2 MPa

0.4 MPa

0.4 MPa

time on stream (h):

4−6

4−6

23−25

4−6

23−25

Um‑CH4 (%) Um‑LH (%) Um‑alky (%) Um‑coke (%) MeOH conversion (%) ratio of olefins/parafins

4.1 44.3 50.8 0.9 99.0 0.112

9.5 46.7 43.2 0.7 99.2 0.005

2.7 49.7 47.0 0.7 99.0 0.005

13.1 47.3 39.0 0.6 99.1 0.009

3.3 51.4 44.8 0.6 98.9 0.009

0 MPa

0.2 MPa

0.4 MPa

2.7 35.7 60.1 1.4 99.1 53.0

2.3 36.9 59.8 0.9 99.2 35.1

3.2 38.3 57.5 1.0 99.2 25.5

Reaction conditions: 460 °C, WHSV = 2.5 h−1, ptoluene = 16 kPa, pmethanol = 4 kPa, pwater = 40 kPa, pH2 variable. G


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ZSM-5 catalyst without Pt loading). Results show that carrier gas and total pressure influenced methanol usage for desired (e.g., toluene alkylation) and undesired reactions (e.g., MTO, methane formation, and coking), but these two variables did not affect para-selectivity. Compared with inert nitrogen, coke deposits were less when using hydrogen as carrier gas. However, the methanol usage toward undesired formation of methane and light hydrocarbons was increased under hydrogen atmosphere due to the hydrogenation property of Pt/mZSM-5. The total pressure effect decreased the methanol usage for toluene alkylation under elevated reaction pressure. The analysis suggests that increasing total pressure decreases toluene concentrations in catalyst pores, resulting in a lower toluene and methanol adsorption complexes formation rate in the ZSM-5 pores. The relatively larger amount of methanol in catalyst pores causes ineffective methanol utilization and reduces toluene conversion under higher total pressure in toluene methylation process. Low methanol utilization efficiency caused by elevated total pressure and reactive carrier gas is a hindering factor in industrial scale-up. To some degree, operating conditions optimization, including adjustment of toluene/methanol ratio and water vapor addition, can tailor the methanol usage in toluene methylation. Further, certain catalyst innovation, which could improve methanol utilization efficiency under elevated reaction pressure, is essential for future commercialization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00318. Characterizations of HZSM-5, mZSM-5, and Pt/mZSM5 catalysts; TG curves of different samples; molar distribution of toluene methylation effluent over Pt/ mZSM-5 and mZSM-5; comparison between the effect of WHSV and pressure; effect of water as a competing adsorbate; and ternary xylene isomer plot illustrating the effect on para-xylene selectivity by changing contact time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*C.S.: fax, +1-814-865-3573; tel, +1-814-863-4466; e-mail, [email protected]. *X.G.: fax, +86-411-84986134; tel, +86-411-84986133; e-mail, [email protected]. ORCID

Yiren Wang: 0000-0002-9935-0806 Yi Zuo: 0000-0002-3368-132X Notes

The authors declare no competing financial interest.

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Methanol Usage in Toluene Methylation over Pt Modified ZSM-5

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