1. Probing the Effect of Water in Catalytic Reactions by In Situ Solid-State NMR
Maryam Abdolrahmani, Kuizhi Chen and Jeffery L. White*
Department of Chemistry, Oklahoma State University, Stillwater, OK 74078
1. Motivation
2. Experimental section
4. Conclusions
1H MAS NMR spectra of cumene over H-ZSM-5 was an indication of :
Dealkylation reactions,
Reduction of the aliphatic peaks in the opened system,
Small changes in the aliphatic peaks in the closed system.
As global energy demand increases simultaneously with diminishing supplies of petroleum resources, Methanol can be used as a feed to produce different type of hydrocarbons. Methanol-to-hydrocarbon (MTH) chemistry is an example of zeolite-based catalysis where understanding the role of water is critical.
The stronger adsorption of water at the active site increased selectivity to some product molecules in MTH
What is the role of water on the zeolite activity?
2.1. Materials:
In the course of time, chemical shift of cumene peaks changed after injection of cumene in presence and absence of water.
Chemical shifts of cumene peaks were changed by simultaneous injection of water and cumene.
Area under aliphatic peaks decreased by raising temperature in the opened system, while in the closed system, the change in the peak areas was negligible.
Reduction of the aliphatic peak areas was an indication of dealkylation reaction of cumene.
Presence of water in cumene reactions, moved aromatic chemical shifts upfield.
H-ZSM-5:
Each channel is 10-member ring channel. Si/Al=15: 6 acid sites/u.c.(unit cell)
In this poster, 1eq. =1H2O/acid site
Aromatic
-CH
-CH3
PDMS
Cumene
Cumene:
2. Introduction
Fig.2: 1H Solid-state MAS NMR spectrum of cumene
2..2. In-situ MAS NMR spectroscopy:
Fig.5. 1H Solid-state MAS NMR spectra of 1 eq. cumene, opened system
Recently, it was found that the rate constants increase by an order of magnitude at water loadings in the range of ≤1 water molecule per catalyst active site relative to the dry catalyst.
The MAS NMR rotor with catalyst is used as a microreactor to simulate the reaction process under batch-like condition. In situ 1H NMR measurements were carried out on Chemmagnetic
CMX spectrometers operating at 300 MHz, 7.5
mm rotor, Spin speed: 4000 Hz, 90 pulse with
d1:2s, and ns:32.
All samples activated and adsorbed with cumene and/or water outside of the NMR probe.
3.2. Chemical shift of aromatic peaks upon reaction:
5. Future Work
Fig.6a: 1H MAS NMR spectra showing 1 eq. cumene and 1 eq. water reacting on HZSM-5 catalyst as the temperature was raised from r.t. to 220 ͦC and dropped to r.t.
Hot air
Dry
Use d6-benzene and propene over HZSM-5.
Wet
Fig.6b: 1H MAS NMR spectra showing 1 eq. water reacting on HZSM-5 catalyst as the temperature was raised from r.t. to 230 ͦC and dropped to r.t.
(2) Use fixed bed reactor with gas chromatography
Fig. 1a
Fig. 1b
3. Results & Discussion
Reactor
Furnace
Feed
Carrier gas(He) GC
Product
Dahl and Kolboe proposed a ‘hydrocarbon pool’ mechanism in which methanol forms a pool of (CH2)n species within the zeolite pores that produce different types of hydrocarbons. The two catalytic cycles in MTH on H-ZSM-5 has contributed significantly to the general understanding of the hydrocarbon pool mechanism. Six major chemistries occur within the dual cycle mechanism for MTH:
3.1. Chemical shift of cumene after injection of water:
Water effect on initial chemical shift of aromatic peak after injection:
After injection of cumene without water over H-ZSM-5, 1HNMR results show that cumene peaks shift to low field, while in presence of water, cumene peaks shift to up field.
6. References
(1)
3.4. Summary
Olefin methylation,
olefin cracking,
(3) hydrogen transfer,
(4) cyclization,
(5) aromatic methylation
(6) aromatic dealkylation
(2) Chen, K.; Damron, J.; Pearson, C.; Resasco, D.; Zhang, L.; White, J. L. ACS Catalysis 2014, 4, 3039
(3) Ivanova, I; Brunei, D; Nagy, J; Derouane, E; Journal of Molecular Catalysis A: Chemical, 1995, 95,
Fig.3: 1H Solid-stae MAS NMR spectra of 1 eq. cumene with and without water
31. Closed vs. opened system:
7. Acknowledgement
Why are intersected in aromatic alkylation and dealkylation ?
Aromatic centers are very important intermediates in zeolite-based hydrocarbon reactions:
Opened cap (a hole inches)
Closed cap
This material is based on work supported by the U.S. Department of Energy, DOE/EPSCOR (Grant DE-SC ).
Fig.4: 1H MAS NMR spectra showing cumene reacting on HZSM-5 catalyst as the temperature was raised from r.t. to 120 ͦC.