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Kinetics, Catalysis, and Reaction Engineering

Solvents molecular mobility in coked catalyst ZSM-5 studied by NMR relaxation and Pulsed Field Gradient techniques Bingjie Zhou, Zuwei Liao, Carlos Mattea, Siegfried Stapf, Hongqiao Jiao, Lin Wang, Zhuang Zhuang, Binbo Jiang, Jingdai Wang, and Yongrong Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00261 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 3, 2018

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Industrial & Engineering Chemistry Research

Solvents molecular mobility in coked catalyst ZSM-5 studied by NMR relaxation and Pulsed Field Gradient techniques Bingjie Zhoua, Zuwei Liaoa*, Carlos Matteab, Siegfried Stapfb, Hongqiao Jiaoc, Lin Wangc, Zhuang Zhuangc, Binbo Jianga, Jingdai Wanga, Yongrong Yanga a

State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China; b

FG Technische Physik II/Polymerphysik, Technische Universität Ilmenau, D-98684 Ilmenau, Germany;

c

Coal to Liquids Chemical R&D Center, Shenhua Ningxia Coal Industry Group, 750001 Yinchuan, PR China E-mail: [email protected]

Abstract Through gasification and methanol synthesis, biomass can produce methanol and then produce light olefins through methanol-to-hydrocarbon (MTH) process. To make this new biomass based production route profitable, the efficiency of MTH process is important. Catalyst deactivation is the main reason for the decline of conversion and selectivity. NMR was applied to detect the relaxation and diffusion of liquid molecules in a series of coked ZSM-5, investigating the effect of coke on molecular transport properties. N-Heptane was chosen as a probe molecule in describing pore network connectivity, whereas methanol was used for relaxation measurements. Though there were only minor differences of pore connectivity among the samples, the longitudinal relaxation time showed an almost linear relationship with coke contents, suggesting that the interaction between reactants and catalyst surface influences the catalyst performance within low coke contents. PFG-NMR and NMR relaxation, as fast and straightforward measurements, could simulate catalyst behaviors during the reaction process. Key words: ZSM-5; Coke; PFG-NMR; Relaxation; Diffusion. 1. Introduction Light olefins, like ethylene and propylene, are important basic chemical materials. They are usually obtained from steam cracking of naphtha or the fluid catalytic cracking (FCC) units. The process consumes non-renewable feedstock, the fossil fuels. Most greenhouse gas emissions are attributed to the combustion of fossil fuels. Nowadays, a more sustainable and environmentally friendly ways of producing chemicals and transportation fuels is of vital importance 1. Instead of fossil feedstock, biomass is a sustainable material in the long run. Methanol can be produced from biomass through gasification and methanol synthesis 2. Then light olefins are produced through methanol-to-hydrocarbon (MTH) or methanol-to-olefin (MTO) processes 3. Zeolite catalysts are widely used in these MTH or MTO processes. The catalytic property of the zeolite, especially the shape selectivity, is closely related to the molecular motions in the voids of catalysts. Considering that the pore size of zeolite is between 0.4 nm and 0.8 nm, any minor changes of molecular shape and geometry size will have a big influence on diffusion. During the various industrial processes, catalyst pellets suffer from the deposition of coke 4. This has two consequences which are detrimental to catalyst performance: first, pore throats are affected and 1

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overall tortuosity is increased, eventually leading to full pore blockage which makes catalyst sites inaccessible to the reactants. Secondly, surface coverage hides active sites and therefore also reduces the total reactor efficiency 5, 6. While the performance of a reactor is quantified by global conversion data in online monitoring, in order to predict catalyst behaviors and simulate the performance during the whole reaction-regeneration processes, the details of the processes acting on a molecular scale require individual laboratory studies. The motion of molecules is influenced by two factors: one is the size of the pores, the other is the interaction with the surface of zeolites. There are several experimental methods that are available to study the translational mobility in zeolite catalysts. One of the most widely employed methods is gas physisorption, using the unreactive nitrogen as the adsorbate, which can follow mesopore structure evolution for catalysts during deactivation process. However, it may undergo some specific interaction with polar groups (e.g. hydroxyl groups, active metal sites) on surfaces, which will lead to a decrease of the accuracy 7. The gravimetric approach is appropriate for systems where the size of the molecules is close to the pore size 8. NMR, a non-invasive method, is applied to probe transport and interactions across the hierarchy of length scales, which will provide information about the physical structure and chemical properties of coked catalysts without extracting coke or dissolving the framework of the catalyst 9. The test of diffusometry gives a determination of pore size morphology by using suitable probe molecules to measure the tortuosity 10. In addition, relaxation measurement is a powerful tool in the analysis of pore surface properties through the interactions between adsorbates and adsorbents 11. Pulsed field gradient (PFG) NMR is one of the methods that can probe the self-diffusion coefficient of molecules in the porous media directly 11, 12. In 1991 13, PFG NMR has been used to measure hydrocarbon diffusivities in zeolites for the first time. For a small molecule like methane, it showed a bi-exponential spin-echo attenuation, which meant that methane followed both interand intra-crystalline diffusion. Larger molecules like n-butane and n-pentane, on the other hand, only showed intra-crystalline diffusion. The diffusion coefficient was found to decrease sharply as the carbon chain length increases. Weber et al. 14 analyzed the diffusion of 1-octene in the Pd/θ-Al2O3. It was illustrated that the passivation of the pore surface inhibited molecular self-diffusion within the adsorbed surface layer. PFG-NMR can also be used to determine how the coking levels influence the tortuosity of the porous catalyst pellet. The self-diffusivities of probe molecules, pentane and heptane, decreased linearly with increasing coke content 15. Compared to fresh catalyst, the effectiveness factor of the most heavily coked catalyst was found to decrease by 10%. Ren et al. 16 investigated a series of naphtha reforming alumina catalysts from different deactivation and regeneration processes by NMR measurements. An increase of the tortuosity of up to 50% was found and was compared to the evolution of surface area by means of BET measurements. The results indicated that through the regeneration process, a full recovery of the activity could not be reached, and that the presence of coke can affect the fractal dimension of the accessible surface which was confirmed by NMR relaxometry measurements. D’Agostino et al. 17 used the T1/T2 ratio at a magnetic field strength of 7 T to determine the relative affinity of reactant and solvent on different catalytic surfaces during the catalytic oxidation of 1, 4-butanediol. The catalyst with the highest activity showed a higher T1/T2 of the reactant than solvent, which meant a preferential surface affinity for the reactant. NMR relaxation 2


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measurement can be applied to guide selection of proper catalyst and predict catalyst behavior. Combined with NMR cryoporometry, one can obtain the corroborative information of surface interactions in porous media and measure pore size distribution at the same time 11. Neil Robinson et al.18 recently used NMR spin-lattice relaxation as a non-invasive measurement to demonstrate the molecular dynamics within liquid-saturated mesoporous oxides. It showed that the the passivation significantly increased the motional freedom of adsorbed liquid methanol because the passivation eliminated the hydrogen bonding interaction between methanol and the surface of oxides. In most zeolite-catalyzed hydrocarbon transformation reactions, the catalytic performance is influenced by the size, shape and distribution of zeolite catalyst pores19-21. During the reaction process, coke deposition will lead to loss of catalyst activity and then influence the selectivity and lifetime of a catalyst. In industrial processes, the properties of catalyst’s activity and deactivation determine the type of reactor and choice of reaction and regeneration processes22. Existing research focuses on the compositions of the coke and the formation mechanisms, which are influenced by pore structures and reaction conditions23, 24. There is rare to find research in studying how the coke directly influences catalyst behavior on a molecular scale during the petrochemical industrial processes. In this study, MTH was taken as an example for a process where the performance of the catalyst ZSM-5 is heavily affected by the accumulation of coking. A series of ZSM-5 with different coke content was studied with the purpose of investigating the effect of coke on molecular transport properties, which was made under the same reaction conditions within different time on stream (TOS) in a fixed-bed reactor. Thermo-gravimetric analyzer (TGA) and nitrogen adsorption (BET) measurements were used to provide information about coke content, pore size distribution, pore volume, specific surface area, and average pore diameter. In order to choose an appropriate probe molecule in describing the tortuosity of ZSM-5, the diffusion properties of five different test liquids with molecular sizes either smaller or larger than the micropore reference of 0.55 nm were tested and compared. The measurements of the diffusion capabilities and relaxation in coked ZSM-5 were performed by NMR using n-heptane and methanol as probe molecules. The catalyst in this research is within low coke content, those heavily coked catalyst is what we want to research on in the future. 2. Material and methods 2.1 Sample preparation Powder catalysts ZSM-5 (crystal size 200 nm), provided by SINOPEC RIPP, were tableted under the pressure of 30 kg, and then crushed and sieved into 14-20 mesh. The sieved catalyst (S-0) was loaded in the middle of a quartz tube (inner diameter: 80 mm) reactor. The performance of the catalyst in MTH was conducted in a fixed-bed reactor under stable reaction conditions of 495 °C, 0.1 MPa and weight hourly space velocity (WHSV) of methanol 5 h-1. A series of coked catalysts (S-1, S-2, S-3, S-4) were acquired from different TOS (5 h, 10 h, 20 h, 63.5 h). The composition of products was analyzed using an online Agilent 7890A gas chromatograph (GC) equipped with a HP-PLOT Q colume and a flame ionization detector (FID). According to the retention time, they were divided to CH4, C2H4, C2H6, C3H6, C3H8, CH3OCH3, CH3OH, C4, C5+. 3



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The conversion and main products distribution were calculated by equations (1) and (2).

Conversion of methanol = P/E=

ω ( methanol ) inlet − ω ( methanol ) outlet × 100% ω ( methanol ) inlet

ω ( propylene ) outlet ω ( ethylene ) outlet

(1) (2)

ω represents the CH2-based mass fraction, and P/E is the ratio of propylene to ethylene in the outlet stream, aimed at showing the products distribution. 2.2 Thermo-gravimetric analysis A thermo-gravimetric analyzer (METLLER TGA SDTG851E) was used for detecting the amount of coke deposited on the samples. The samples were heated at 398 K for 10 min with N2 to remove water and other impurities in the catalyst, and then the nitrogen flow was changed to a mixture of O2 (20%) diluted with nitrogen at a total flow rate of 50 ml/min. Afterwards, the reactor temperature was increased from 398 K to 1123 K at a heating rate of 10 K/min. 2.3 Nitrogen adsorption-desorption measurements N2 adsorption-desorption experiments were performed using a physical adsorption instrument (Micromeritics, ASAP2020). Before the measurements, the samples were outgassed at 573 K for 24 h to remove impurities. The specific surface area and pore volume were calculated by the BET method and t-plot method. The pore size distribution was calculated by non-local density functional theory (NLDFT) method. 2.4 NMR experiments Methanol, n-heptane, cyclohexane, benzene and toluene were selected as probe molecules for NMR diffusion measurements. The reactant methanol was chosen as the probe molecule for NMR relaxation measurements. Catalyst samples were first placed in a vacuum oven at 110 °C for 24 h and allowed to cool in dried air. Next the catalysts were soaked in 2 ml of liquid to ensure the catalysts were submerged, for 24 h at room temperature. Then a pre-wetted absorbent paper was used to remove excess liquid, and the samples were placed into 5 mm diameter glass NMR tubes (filling height about 1-2 cm). The tubes were sealed with PTFE tape and left to equilibrate for 30 minutes before the measurement. The 1H longitudinal (T1) NMR relaxation and the diffusion experiments were performed at a magnetic field of 7 T on a Bruker Avance III NMR spectrometer, working at a proton resonance frequency of 300 MHz. Uniaxial field gradient coils (DIFF 30) providing a maximum gradient strength of 1200 G/cm were attached for the purpose of diffusion experiments. All measurements were carried out at a temperature of 17 °C. The T1 was measured using the inversion recovery pulse sequence (16 points, 8 scans). The diffusion coefficient was measured using the PGSTE (stimulated echo) sequence (16 points, 64 scans). The values of the self-diffusion coefficient, D, were calculated by fitting the decay of the 4


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signal intensity of the stimulated echo using the following equation 25,

I = I0 exp(−D(γ gδ )2 (∆−δ / 3))

(3)

where I is the intensity of the stimulated echo in gradient field, while I0 is the NMR signal in the absence of the gradient. γ is the gyromagnetic ratio of the 1H nucleus, 4258 Hz/G, g is the gradient strength, δ is the gradient pulse duration, 1 ms. ∆ is the diffusion time, which stands for the separation between the leading edges of the gradient pulses. In this study, ∆ is kept fixed at 20 ms (see Figure 1(a) for a typical decay). The diffusion coefficient is related to the average distance travelled by the molecule during diffusion time ∆ , that is the root mean square displacement (RMSD), calculated by equation (4).

< x 2 > 1/ 2 =

(4)

2D∆

The < x 2 > 1/ 2 for heptane in S-0 over an observation time 20 ms is 6.0 µm, much larger than the size of a single pore (several nanometers) and a single crystal (200 nm), but smaller than the size of catalyst particle (14-20 mesh, that is 0.85-1.4 mm) . For each catalyst particle, it contains lots of catalyst powder. The diffusion may happen in intra-crystalline (micropores, mesopores and macropores), inter-crystalline and between catalyst powder to powder. Diffusion coefficient D, which is calculated from equation (3), is an average value for all the diffusion in the testing system. In order to assess the diffusive behavior of probe molecules in different samples, a dimensionless parameter ξ was defined, given by the ratio of the free bulk liquid self-diffusivity, D0, to the effective self-diffusivity of the same liquid within the pore space, Deff.

ξ=

D0 Deff

(5)

Spin-lattice (T1) relaxation experiments were performed and evaluated for reactant methanol in coked catalyst samples. Signal recovery curves and fittings 26 are shown in Figure 1(b). (a)

(b) Normalized Echo Signal I/I0

1.0

Normalized Echo Signal I/I[0]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

0.6

0.4

0.2

0.8

0.4

0.0

S-0 S-1 S-2 S-3 S-4

-0.4 0.0 0.0

6.0x109

1.2x1010

1.8x1010

0

600

γ2δ2g2(∆−δ/3) s/m2

1200

1800

Time (ms)

5


2400

3000


Figure 1. Typical curves of diffusion and relaxation measurements: (a) echo decay curve of the PFG stimulated echo sequence for the diffusion of methanol on fresh catalyst S-0, the red line is a fit to equation (3); (b)the spin-lattice relaxation curves of methanol in samples, obtained using the inversion recovery sequence.

3. Results and discussion 3.1 Catalytic reaction and characterization of samples Figure 2 shows the performance of ZSM-5 catalyst in the MTH process. The conversion of methanol approaches a constant value after an initial decline, but the ratio of propylene to ethylene (P/E) increases first and then keeps steady. The whole reaction process can be divided into two parts: one is an unsteady stage (I) from the beginning of the reaction to 10 h, followed by a steady stage (II) from 10 h to 63.5 h. All samples were characterized by TG and BET analysis. The results are shown in table 1. Figure 3 shows the pore size distribution of fresh catalyst, calculated by NLDFT method. In such a catalyst, at least four pore size domains become relevant: the micropores of the ZSM-5 repeat unit with a diameter of about 0.55 nm; medium-sized pores between 2 and 4 nm identified by BET porometry indicating larger voids within the microcrystals; several tens to hundreds of nm in accordance to the microcrystal size, and large structures between the catalyst pellets. Stage I lasts for nearly 10 hours and shows a coke content of 1.3%, but Stage II only shows a coke content of about 1.7% accumulated for 53 h. At the beginning, there are many strong acid sites on the surface of catalyst, so it shows a fast carbon deposition speed 27, the decline of methanol conversion and a better products distribution. As the reaction proceeds, the coke preferentially covers these strong acid sites, so the speed of carbon deposition slows down and a steady reaction state is reached.

9 100

6

98

P/E

Conversion of methanol (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3

96

S-0

S-1 S-2

S-3

S-4

94

0 0

15

30

45

60

TOS(h)

Figure 2 Methanol conversion and P/E versus TOS in the MTH reaction.

Table 1 Coke content and BET characterization of samples used in this study. 2

Samples

S-0

TOS（h）

0

S

Coke

BET

-1

3

（m ·g ）

-1

Pore volume（cm ·g ）

（%） 0

Total

Micro

External

Total

Micro

Meso

379

306

73

0.45

0.14

0.31

6


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S-1

5

0.86

367

268

102

0.44

S-2

10

1.31

413

344

69

0.51

0.15

0.36

S-3

20

2.24

403

317

86

0.59

0.14

0.45

S-4

63.5

3.09

419

340

79

0.50

0.15

0.35

1.5

(a)

S-0 S-1 S-2 S-3 S-4

1.2

0.11

0.33

(b)

S-0 S-1 S-2 S-3 S-4

1.2

dV(log d) (cc/g)

1.5

dV(log d) (cc/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

0.6

0.9

0.6

0.3

0.3

0.0

0.0 0

10

20

30

40

0

1

Pore Diameter (nm)

2

3

4

5

6

Pore Diameter (nm)

Figure 3 (a) Pore size distribution catalysts; (b) zoom up of 0-5 nm in graph (a).

3.2 ξ of coked catalyst From equation (5), a dimensionless parameter ξ is calculated. As is shown in Figure 4, different guest molecules lead to significantly different ξ values, which means that ξ cannot easily be considered as tortuosity. Compared to the size of micropore in this porous medium, 0.55 nm, the five liquids can be divided into two groups. On the one hand, are methanol and n-heptane, of which the critical size of molecule is smaller than 0.55 nm. The other group consists of cyclohexane, benzene and toluene with a size larger than 0.55 nm. The results in Figure 4 shows that: 1. As the size of molecules increases, ξ shows a sharp decrease at cyclohexane. This is due to the large geometric size of the cyclohexane molecule which does not allow it to enter the micropores. As a consequence, diffusion is less hindered since it is only affected by translational motion within the larger pore structures. 2. For cyclohexane, benzene and toluene, the value of ξ is found to increase significantly with growing average molecular size. These three molecule types all encounter the system consisting of mesopores and macropores. For benzene and toluene molecules, a conjugated and delocalized large π bond exists in the phenyl ring, which will interact with the cations in the zeolite channels. As a result, the diffusion is slower than cyclohexane 28. The differences of ξ between benzene and toluene is because of the geometrical differences caused by the methyl group of toluene. 3. For n-heptane and methanol, though the size of them are similar, the ξ values still show differences. This can be attributed to the fact that the hydroxyl group of methanol molecule leads to a special interaction with the surface of catalyst 14. The typical literature value of diffusion coefficient in micropores is 10-11-10-14 m2/s29, 30, which is smaller than the results. As is mentioned above, the RMSD is larger than the pore size and crystal size. So the ξ here is an average value of diffusions to evaluate the pore connectivity of zeolite. Taking the molecule size and interaction into consideration, a weakly interacting species 7



0.8

3.3

0.7

3.0

0.6 0.55

2.1

0.3

C yc

M

To lu en e

0.4

Be nz en e

2.4

lo he xa ne

0.5

et ha no l

2.7

Molecular diameter (nm)

3.6

H ep ta ne

ξ=D0/Deff

such as n-heptane is indeed a suitable molecular probe for quantifying the pore network connectivity31.

Figure 4. The critical diameter of molecular and ξ for different probe molecules diffusing in the fresh catalyst S-0.

The results of the self-diffusion coefficient of n-heptane in the catalyst samples with different coke contents are presented in Figure 5(a). The ξ for n-heptane is shown in Figure 5(b). In principle, the ξ is expected to change depending on the location of the deposited coke. While coke blockage of larger pores increases the tortuosity of zeolite, the blockage of micropores has the opposite effect of reducing the tortuosity because small pores cannot be accessed by reactants 16 and the distance travelled by any molecule in a given time is larger, an effect that is used in size-exclusion chromatography. The result shows that the diffusion coefficients fluctuate within a small range. Within experimental error limits, the coefficients in different coked catalysts or the ξ can similarly be considered constant. It is speculated that the low coke content (less than 3%) of these samples is insufficient to significantly change the pore size distribution, as shown in Figure 3. Compared to the total pore volume of S-0 and S-4 in table 1, the coke is firstly covering the surface of the catalyst and may not reach the stage of blocking the channels, so it does not have big influence on self-diffusion of n-heptane. Consequentially, no detrimental geometric effect on the catalytic performance is expected for these coke contents.

9.6x10

-10

4.0 2

Diffusion Coefficient (m /s)

8.8x10

-10

8.0x10

-10

7.2x10

-10

6.4x10

-10

3.5

ξ=D0/Deff

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.0

2.5

2.0 0.0

0.7

1.4

2.1

2.8

3.5

0.0

0.7

1.4

2.1

2.8

3.5

Coke content (%)

Coke content (%)

Figure 5. The self-diffusion of n-heptane in a series of coked catalysts: (a) the relationship between coke contents and diffusion coefficients; (b) the relationship between coke contents and ξ.

8


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3 Interaction between methanol and the surface of coked catalysts Reaction efficiency depends on overall pore structures of zeolite, but also on the accessibility of molecules to the active sites. The coke has two influences on the catalyst: one is making the pore narrower, possibly blocking the micropore eventually; the other is to cover the surface of the catalyst material, especially the active sites, and leading to an alteration of surface structure and chemistry. Although the small content of coke was shown not to have a significant influence on the pore network connectivity, the activity of the catalyst and the products distribution are clearly affected during the reaction process, as shown in Figure 2. We further employ the NMR relaxation measurements to analyze how the presence of coke changes the surface of ZSM-5. In the process of gas solid catalytic reaction, the adsorption of reactant molecules onto the surface of catalysts is an important step. The only reactant, methanol, is chosen as the probe molecule. The spin-lattice relaxation of methanol in this heterogeneous system is a single exponential (Figure 1(b)), which proves effective fast exchange between local sites of possibly different relaxivity. Exchange takes places on a scale corresponding to the root-mean squared displacement of methanol during T1, which is about 20 µm. The T1 of liquid molecules is caused by fluctuating magnetic fields, which are themselves generated by reorientations of the molecules themselves, and by interactions with the surface. Since the bulk liquid relaxation time of methanol is on the order of 4-5 s, the surface interaction dominates. Since in this study, a pure catalyst without binder was used, one can identify two kinds of interactions: one is between methanol molecules and the native surface of the catalyst; the other is between methanol molecules and the surface of coke. As the coke content increases, T1 also shows an increasing tendency, and is approximately proportional to the coke content (Figure 6). This is indicative for the fact that interaction between methanol and the native surface is dominant at this system: as the reaction proceeds, coke covers the active sites and reduces the probability of methanol molecules interacting with active sites. Coke itself apparently has a lower surface relaxivity for methanol. As a consequence, the strength of interaction decreases, which leads to an increasing of T1. This quantitative relation could be used to simulate catalyst behaviors during the reaction process since the determination of T1 times is a fast (less than one minute) and straightforward experiment.

500 450 400

T1 (ms)

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350 300 250 200 0.0

0.7

1.4

2.1

2.8

3.5

Coke content (%)

Figure 6. Influence of the coke content on NMR longitudinal relaxation time T1 for methanol as a probe molecule.

9



4. Conclusions The performance of ZSM-5 in MTH process showed that there was an unsteady stage with a fast coke deposition speed and a steady stage with a slow deposition speed. PFG NMR were applied to detect the diffusion of liquid molecules in coked samples. n-heptane was chosen as an appropriate probe molecule and the diffusion of n-heptane in catalysts within different coke content has been tested. Low coke content has no significant influence on molecular diffusion. NMR relaxation was applied to test the the relaxation of methanol in coked catalyst. T1 shows an almost linear relationship with coke contents. The interaction between reactants and catalyst surface influences the catalyst performance in this system. NMR, as a facile and non-invasive method, can be used to predict catalyst behavior in the deactivation process.

Acknowledgements The financial support provided by the Project of National Natural Science Foundation of China (91434205 & 61590925), the National Science Fund for Distinguished Young (21525627), and the International S&T Cooperation Projects of China (2015DFA40660) are gratefully acknowledged.

References (1) Huber, G. W.; Iborra, S.; Corma, A., Synthesis of transportation fuels from biomass: chemistry, catalysts, and engineering. Chem. Rev. 2006, 106, (9), 4044-4098. (2) Holmgren, K. M.; Berntsson, T.; Andersson, E.; Rydberg, T., System aspects of biomass gasification with methanol synthesis – Process concepts and energy analysis. Energy 2012, 45, (1), 817-828. (3) Campelo, J. M.; Lafont, F.; Marinas, J. M.; Ojeda, M., Studies of catalyst deactivation in methanol conversion with high, medium and small pore silicoaluminophosphates. Appl. Catal., A 2000, 192, (1), 85-96. (4) Bibby, D. M.; Howe, R. F.; McLellan, G. D., Coke formation in high-silica zeolites. Appl. Catal., A 1992, 93, (1), 1-34. (5) Bjorgen, M.; Svelle, S.; Joensen, F.; Nerlov, J.; Kolboe, S.; Bonino, F.; Palumbo, L.; Bordiga, S.; Olsbye, U., Conversion of methanol to hydrocarbons over zeolite H-ZSM-5: On the origin of the olefinic species. J. Catal. 2007, 249, (2), 195-207. (6) Guisnet, M.; Magnoux, P., Coking and deactivation of zeolites: Influence of the Pore Structure. Appl. Catal. 1989, 54, (1), 1-27. (7) Gopinathan, N.; Greaves, M.; Lowe, J. P.; Wood, J.; Rigby, S. P., Determination of the location of coke in catalysts by a novel NMR-based, liquid-porosimetry approach. J. Colloid Interface Sci. 2012, 381, (1), 164-170. (8) Ruthven, D. M., Principles of Adsorption and Adsorption Processes. John Wiley and Sons Ltd: 1984. (9) Gladden, L. F.; Mitchell, J., Measuring adsorption, diffusion and flow in chemical engineering: applications of magnetic resonance to porous media. New J. of Phys. 2011, 13, 035001. 10


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(10) Chua, L. M.; Hitchcock, I.; Fletcher, R. S.; Holt, E. M.; Lowe, J.; Rigby, S. P., Understanding the spatial distribution of coke deposition within bimodal micro-/mesoporous catalysts using a novel sorption method in combination with pulsed-gradient spin-echo NMR. J. Catal. 2012, 286, 260-265. (11) Mitchell, J.; Stark, S. C.; Strange, J. H., Probing surface interactions by combining NMR cryoporometry and NMR relaxometry. J. Phys. D: Appl. Phys. 2005, 38, 1950. (12) Heink, W.; Kärger, J.; Pfeifer, H.; Salverda, P.; Datema, K. P.; Nowak, A., Self-diffusion measurements of n-alkanes in zeolite NaCaA by pulsed-field gradient nuclear magnetic resonance. J. Chem. Soc., Faraday Trans. 1992, 88, (3), 515 - 519. (13) Datema, K. P.; den Ouden, C. J. J.; Ylstra, W. D.; Kulpers, H. P. C. E.; Post, M. F. M., Fourier-transform Pulsed-field-gradient 1H Nuclear Magnetic Resonance Investigation of the Diffusion of Light n-Alkanes in Zeolite ZSM-5. J. Chem. Soc. Faraday Trans. 1991, 87, (12), 1935-1943. (14) Weber, D.; Sederman, A. J.; Mantle, M. D.; Mitchell, J.; Gladden, L. F., Surface diffusion in porous catalysts. Phys. Chem. Chem. Phys. 2010, 12, (11), 2619. (15) Wood, J.; Gladden, L. F., Effect of coke deposition upon pore structure and self-diffusion in deactivated industrial hydroprocessing catalysts. Appl. Catal., A 2003, 249, (2), 241-253. (16) Ren, X. H.; Bertmer, M.; Stapf, S.; Demco, D. E.; Blümich, B.; Kern, C.; Jess, A., Deactivation and regeneration of a naphtha reforming catalyst. Appl. Catal., A 2002, 228, (1–2), 39-52. (17) D'Agostino, C.; Feaviour, M. R.; Brett, G. L.; Mitchell, J.; York, A. P. E.; Hutchings, G. J.; Mantlea, M. D.; Gladdena, L. F., Solvent inhibition in the liquid-phase catalytic oxidation of 1,4-butanediol: understanding the catalyst behaviour from NMR relaxation time measurements. Catal. Sci. Technol. 2016, 6, 7879. (18) Robinson, N.; Gladden, L. F.; D Agostino, C., Exploring catalyst passivation with NMR relaxation. Faraday Discuss. 2017, 204, 439-452. (19) Bleken, F. L.; Barbera, K.; Bonino, F.; Olsbye, U.; Lillerud, K. P.; Bordiga, S.; Beato, P.; Janssens, T. V. W.; Svelle, S., Catalyst deactivation by coke formation in microporous and desilicated zeolite H-ZSM-5 during the conversion of methanol to hydrocarbons. J. Catal. 2013, 307, 62-73. (20) Kim, J.; Choi, M.; Ryoo, R., Effect of mesoporosity against the deactivation of MFI zeolite catalyst during the methanol-to-hydrocarbon conversion process. J. Catal. 2010, 269, 219-228. (21) Rojo-Gama, D.; Signorile, M.; Bonino, F.; Bordiga, S.; Olsbye, U.; Lillerud, K. P.; Beato, P.; Svelle, S., Structure-deactivation relationships in zeolites during the methanol-to-hydrocarbons reaction: Complementary assessments of the coke content. J. Catal. 2017, 351, 33-48. (22) Jiang, B.; Zhou, B.; Jiang, Y.; Feng, X.; Liao, Z.; Huang, Z.; Wang, J.; Yang, Y., Kinetic and regenerator modeling of the coke combustion in the moving bed MTP process. Chem. Eng. Res. Des. 2017, 122, 52-62. (23) Liu, B.; Slocombe, D.; AlKinany, M.; AlMegren, H.; Wang, J.; Arden, J.; Vai, A.; Gonzalez-Cortes, S.; Xiao, T.; Kuznetsov, V.; Edwards, P. P., Advanced in the study of coke formation over zeolite catalysts in the methanol-to-hydrocarbon process. Appl. Petrochem. Res. 2016, 6, 209-215. (24) Schulz, H., “Coking”of zeolites during methanol conversion: Basic reactions of the MTO-, MTP-and MTG processes. Catal. Today 2010, 154, (3-4), 183-194. (25) Kimmich, R., NMR: Tomography, Diffusometry, Relaxometry. Springer-Verlag: Berlin, 1997. (26) Jiang, B.; Yang, Y.; Li, J.; Mattea, C.; Wang, J.; Stapf, S.; Yang, Y. R., Solvent Diffusion in Silica/Poly[styrene-co-(acrylic acid)] Core-Shell Microspheres by Pulsed Field Gradient NMR Techniques. Appl. Polym. Sci. 2013, 40161. (27) Aguayo, A. T.; Benito, P. L.; Gayubo, A. G.; Olazar, M.; Bilbao, J., Acidity deterioration and 11



coke deposition in a HZSM5 zeolite in the MTG process. Stud. Surf. Sci. Catal., Delmon, B.; Froment, G. F., ^Eds. Elsevier: 1994; Vol. 88, pp 567-572. (28) Zhang, J.; Zhao, Z.; Duan, A.; Jiang, G.; Liu, J.; Zhang, D., Chromatographic Study on the Adsorption and Diffusion of Light Hydrocarbons in ZSM-5 and USY Zeolites. Energy Fuels 2009, 23, (2), 617-623. (29) Jobic, H.; Bée, M.; Pouget, S., Diffusion of Benzene in ZSM-5 Measured by the Neutron Spin− Echo Technique. J. Phys. Chem. B 2000, 104, (30), 7130-7133. (30) D'Agostino, C.; Ryabenkova, Y.; Miedziak, P. J.; Taylor, S. H.; Hutchings, G. J.; Gladden, L. F.; Mantle, M. D., Deactivation studies of a carbon supported AuPt nanoparticulate catalyst in the liquid-phase aerobic oxidation of 1,2-propanediol. Catal. Sci. Technol. 2014, 4, 1313-1322. (31) D'Agostno, C.; Mantle, M. D.; Gladden, L. F., Inhibitory effect of oxygenated heterocyclic compounds in mesoporous catalytic materials: A pulsed-field gradient NMR diffusion study. Microporous Mesoporous Mater. 2017.

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Enter into micropores

3.6

Can not enter into micropores

Coke content ~ Longitudinal relaxation time

0.8

0.7

3.0

0.6 0.55

2.7

0.5

2.4

0.4

450 400

T1 (ms)

3.3

Molecular diameter (nm)

500

=D0/Deff

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


350 300 250 200

2.1

0.3 H

ol an

e

e an pt M

ne xa

h et

C

yc

he lo

e

nz Be

e en

en lu

To


0.0

0.7

1.4

2.1

Coke content (%)

2.8

3.5

PDF w - American Chemical Society

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