(583a) Di-methyl Ether Conversion into Olefins over HZSM-5: Effect of SiO2/Al2O3 Ratio Effect on Surface Chemistry and Reactivity Properties

Di-methyl Ether Conversion  into Olefins over  HZSM-5:

Effect of SiO2/Al2O3 Ratio Effect
on Surface Chemistry and Reactivity Properties

Abdullah Al-Dughaither ^(1,2) and Hugo I. de
Lasa⁽¹⁾*

^1.  Chemical Reactor
Engineering Centre, Western University, London,
ON, N6A 5B9

^2.  SABIC Technology
Center, Riyadh, (Saudi Arabia)

* Corresponding author: Tel.: +1 519 661 2144;
Email: hdelasa@uwo.ca

Objectives

This study investigates HZSM-5 as a potential catalyst for olefins
production from di-methyl ether (DTO). The aim of this study is to provide an
in-depth understanding of the unique effect of SiO2/Al2O3 ratio on the
physico-chemical and reactivity properties of the HZSM-5 zeolite.

Materials
and Methods

This study was carried out using three commercial NH4⁺ZSM-5 (SiO2/Al2O3
= 30, 80, and 280). The ammonia ZSM5
precursors were calcined to have ZSM-5 in the protonic form (H⁺ZSM-5). N₂ isotherm and XRD tests were performed to assess the
structural and porosity properties. NH₃-TPD experiments were carried
out to establish acidity and provide insights on intrinsic
desorption kinetics. Desorption parameters were estimated numerically using
MATLAB^® solver. Pyridine FTIR was also employed to evaluate the
influence of Brønsted and Lewis acid sites while changing the SiO₂/Al₂O₃
ratios of the HZSM-5. The
reactivity runs were performed in a gradientless Berty reactor unit using
catalyst extrudates that compose 5% Versal-950 and 70% Fused Alumina.  

Results and Discussion

N₂ adsorption
and desorption showed isotherms with hysteresis, with this pointing to
micropores and intercrystalline mesopores. Zeolite SiO₂/Al₂O₃
ratio shows however, no effect on specific surface
area, pore volume and pore size distribution (Table 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003000380034000000
and Figure 1 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003100310030000000
). DFT
with a cylindrical geometry confirmed the HZSM5 5.5 Ǻ channel size. HZSM5
displayed weak and strong acid site distributions. One finding from Table 4 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003100370039000000
is the weaker to strong fraction and less total
acidity with increasing SiO₂/Al₂O₃ ratio. These
findings points towards a correlation between Al content and acidity on HZSM5.
FTIR shows as well a similar trend for Brönsted acidity while the composition of HZSM5s was not noticeably influencing Lewis acidity (Figure 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200300037000000
, Figure 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200360030000000
).

Concerning NH₃ desorption kinetics, the activation energy was higher for the stronger
acid sites than for the weaker sites (Table 2 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003200390039000000
, Table 3 08D0C9EA79F9BACE118C8200AA004BA90B02000000080000000E0000005F005200650066003300340036003600340034003300300038000000
) .
Furthermore, the ratio of the intrinsic desorption rate constants for the weak
and acid sites were found to be consistent with their acidity density fraction.

DTO over
HZSM5 is an ?in series? reaction where formed olefins may continue reacting
yielding heavier paraffin and aromatic hydrocarbons. Reactivity runs in the
present study, show that the ZSM5-30 zeolite exhibits promising 32-67% DME
conversions with 20.6-32% light olefins selectivity.

Conclusions

·        
A
thorough understanding of SiO2/Al2O3 ratio on HZSM-5 physical and reactivity
characterization, will lead in the context of DTO process to a significant
olefin selectivity optimization.

·        
To our
knowledge and in spite of their great significance for DTO future
implementation, there is no reported study addressing these topics in the open
literature.

Table  SEQ Table \* ARABIC 1 N₂
isotherm results (SA m²/g, VP cm³/g, dp Ǻ)

^a at P/P₀ = 0.1

^b Dubinin?Astakhov method, affinity
coefficient for N2 = 0.33

^c Thickness curve type Harkins and Jura: t-plot = [13.99/(0.034 - log(P/P₀ ))]^0.5

^d Barrett-Joyner-Halenda, adsorption mespores
range: 17 < dp < 3000 Ǻ.

^e Horvath-Kawazoe, Method: interaction parameter
for zeolite adsorbent = 3.49×10^-43 ergs/cm⁴

^f Measured at Max
relative pressure: P/P₀ = 0.995

^g Density Functional Theory, Method:
Non-negative Regularization; No Smoothing

^h Cylindrical pore geometry (Saito/Foley)

ⁱ Slit pore geometry

^j Micropore analysis method, hydraulic
diameters' pores range: 2.94 < dp < 38.8
Ǻ

Figure
 SEQ Figure \* ARABIC 1 Pore size distribution of ZSM5 in micro-channel
region (DFT) and meso-intercrystalline region (BJH).

Table  SEQ Table \* ARABIC 2 Optimized kinetic constants for NH3
desorption

Table  SEQ Table \* ARABIC 3 Cross-correlation coefficients for
NH3 desorption                                             Table  SEQ Table \* ARABIC 4 NH3-TPD acidity measurements at b = 15 K/min.

       Figure  SEQ Figure \* ARABIC 2 
Pyradine FTIR Brönsted acidity                                                              Figure  SEQ Figure \* ARABIC 3 
Pyradine FTIR Lewis  acidity