Mechanistic Aspects of Hydrodeoxygenation of p-Methylguaiacol over Rh/Silica and Pt/Silica

Article abstract of DOI:10.1021/acs.oprd.8b00211

The mechanism of p-methylguaiacol (PMG) hydrodeoxygenation (HDO) has been examined over two Rh/silica catalysts and a Pt/silica catalyst at 300 °C and 4 barg hydrogen. Sequential conversion of PMG to 4-methylcatechol is followed by m- and p-cresol formation and finally toluene production, although direct conversion of PMG to p-cresol is favored over a commercial Rh/silica catalyst. Dehydroxylation and hydrogenation are shown to occur over metal functions, while demethylation and demethoxylation are favored over the fumed silica support. A mechanistic pathway for HDO of PMG is proposed.

Full text of DOI:10.1021/acs.oprd.8b00211

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Full Paper
Mechanistic aspects of hydrodeoxygenation of
para-methylguaiacol over Rh/silica and Pt/silica
Florent Bouxin, Xingguang Zhang, Iain Kings, Adam Fraser Lee,
Mark John H. Simmons, Karen Wilson, and S. David Jackson
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00211 • Publication Date (Web): 29 Aug 2018
Downloaded from http://pubs.acs.org on August 29, 2018
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Mechanistic aspects of hydrodeoxygenation of
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para-methylguaiacol over Rh/silica and Pt/silica.
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Florent P. Bouxina, Xingguang. Zhangb, Iain N. Kingsc, Adam F. Leeb , Mark J. H. Simmonsc,
Karen Wilsonb, S. David. Jackson*a
aCentre for Catalysis Research, School of Chemistry, University of Glasgow, Glasgow G12 8QQ,
Scotland
bSchool of Science, RMIT University, Melbourne VIC3001, Australia
cSchool of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
* Author to whom all correspondence should be addressed
david.jackson@glasgow.ac.uk
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Abstract.
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The mechanism of p-methylguaiacol (PMG) hydrodeoxygenation (HDO) has been examined over
two Rh/silica catalysts and a Pt/silica catalyst at 300 °C and 4 barg hydrogen. Sequential conversion
of PMG to 4-methyl catechol is followed by m- and p-cresol formation, and finally toluene
production, although direct conversion of PMG to p-cresol is favoured over a commercial Rh/silica
catalyst. Dehydroxylation and hydrogenation are shown to occur over metal functions, while
demethylation and demethoxylation are favoured over the fumed silica support. A mechanistic
pathway for PMG hydrodeoxygenation is proposed.
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Keywords. Para-methylguaiacol; hydrodeoxygenation; rhodium; platinum; mechanism.
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Introduction.
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Hydrodeoxygenation (HDO) of bio-oils is an active research area in both catalysis and bioenergy,1,2
and offers one of the methodologies capable of upgrading bio-oils into a form suitable for blending
with petroleum. Previously,3 we examined the deactivation of three catalysts for the HDO of para-
methylguaiacol, a common component of pyrolytic bio-oils. The catalysts were 2.5 wt% Rh/silica
supplied by Johnson Matthey (JM), an in-house prepared 2.5 wt% Rh/silica (A) and 1.55 wt%
Pt/silica (A). All three catalysts exhibited an initial deactivation phase, although the Rh/silica (JM)
achieved steady-state after ~6 h on-stream and maintained a constant activity over the subsequent
test period. In contrast, the two in-house catalysts did not reach steady state within the testing
timeframe, both Rh/silica (A) and Pt/silica (A) underwent continuous deactivation, albeit following
different mechanisms.3 Here we focus on the reaction mechanisms over the different catalysts, and
their response to deactivation. There are no other literature studies of p-methylguaiacol, however
guaiacol has been the subject of several investigations. Mu et al4 studied guaiacol HDO over a Rh/C
catalyst at 40 bar hydrogen and 250 °C in a batch reactor, reporting demethoxylation as the dominant
process resulting in phenol as the main product (~35 % selectivity) at a modest conversion of ~13 %;
other major products were cyclohexanone and cyclohexanol (~25 % selectivity). Gutierrez et al.5
examined rhodium and platinum on zirconia supports for guaiacol HDO, predominantly observing
hydrogenated products at 100 °C, with some deoxygenation at 300 °C, although few details were
provided. Platinum catalysts have been somewhat more researched and their HDO mechanism
considered. A detailed reaction network for guaiacol conversion over Pt/alumina was uncovered by
Gates and co-workers,6 wherein a wide product distribution was detected, reflecting HDO in
competition with transalkylation and hydrogenolysis. In the course of our previous work it became
apparent that demethylation, demethoxylation, and hydrogenation pathways were affected differently
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by catalyst deactivation and we were interested to put this into context mechanistically, and through
the use of two metals and two silica supports, determine the active sites for each reaction.
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Results and Discussion.
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The three catalysts were tested for para-methylguaiacol HDO over an extended period of 32 h. As
previously reported, the Rh/silica (JM) catalyst achieved a steady state within the timeframe of the
testing, whereas Rh/silica (A) and the Pt/silica (A) both exhibited continuous deactivation resulting
in significant selectivity variations between 1 h and 32 h on stream (Figures 1-3).
Rh/silica (JM)
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Figure 1. Product distribution for p-Methylguaiacol HDO over Rh/silica (JM); 47 % and 34 %
conversion after 1 h and 32 h time on stream respectively.
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1 h TOS
32 h TOS
Figure 2. Product distribution for p-Methylguaiacol HDO over Rh/silica (A); 50 % and 26 %
conversion after 1 h and 32 h time on stream respectively.
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Figure 3. Product distribution for p-Methylguaiacol HDO over Pt/silica (A): 70 % and 47 %
conversion after 1 h and 32 h time on stream respectively.
In the early stages of the test Rh/silica (A) is selective to toluene (33 %) and para-cresol (30 %) but
after 32 h time on stream the major products are 4-methylcatechol (41 %) and para-cresol (34 %).
This is in marked contrast to Rh/silica (JM), where toluene selectivity (< 5 %) and 4-methylcatechol
selectivity (< 10 %) are both low throughout the test. Pt/silica (A) displays a less pronounced
switchover from meta- and para-cresol to 4-methylcatechol production.
Mechanistically, the results for both Rh catalysts reflect sequential hydrogenolysis reactions as
shown in Scheme 1. As Rh/silica (A) deactivates, it loses hydrogenolysis/HDO activity, such that
the demethylation product 4-methycatechol is increasingly favoured. This selectivity switching
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mirrors the relative ArO-CH3 versus Ar-OCH3 bond strengths of ~381 kJ.mol-1 versus 419 kJ.mol-1
respectively. None of the catalysts promoted C-C bond scission and concomitant benzene
generation, and only products of cresol hydrogenation (p- and m-methylcyclohexanone) were
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Scheme 1. Mechanism of hydrogenolysis/HDO of PMG.
observed, but not methylcyclohexane. 3-Methylanisole was only formed as a minor product over
Rh/silica (A), suggesting that dehydroxylation of PMG to 3-methylanisole (and subsequent
demethylation to m-cresol) is generally an unfavourable pathway consistent with its high energy
barrier of ~431 kJ.mol-1. Demethylation of PMG to 4-methylcatechol was unaffected by
deactivation, presumably reflecting the weaker bonds to be cleaved. p-Cresol may form by either
direct demethoxylation of PMG, or (very energetically unfavourable) dehydroxylation of 4-
methylcatechol. Since p-cresol production was largely unaffected by deactivation, we conclude it
forms through the former (less energy intensive) direct demethoxylation. Rh/silica (JM) is never
effective for complete PMG dehydroxylation (toluene selectivity <5 %), indicating a lack of the
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highly active sites requisite for such a challenging bond cleavage, which may in part underlie its
superior stability. To test the proposed mechanism the catalyst was operated under a range of space
velocities. As the space velocity was increased conversion and selectivity to secondary products
decreased, however selectivity to 4-methylcatechol increased (yield also increased except for the
highest space velocity) as shown in Figure 4, confirming 4-methylcatechol as a primary product.
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Conversion
4-methyl-catechol
Figure 4. Conversion of PMG and selectivity to 4-methylcatechol. Conditions: 280 °C, 4 barg H2.
Although the major products over Pt/silica (A) after 1 h time on stream were m-cresol and p-cresol,
there is also marked hydrogenation activity, evidenced by significant selectivity to hydrogenated
cresols. The initial total selectivity to p-methylcyclohexanone and p-methylcyclohexanol is ~11 %
(with proportionally lower values for the meta-isomers). After 32 h, when the catalyst has
deactivated, the selectivity changes to favour 4-methylcatechol and the hydrogenated form 4-methyl-
2-hydroxy-cyclohexanol. Overall Pt/silica (A) is the most active hydrogenation catalyst, which is
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maintained even during deactivation.3 The mechanism outlined in Scheme 1 can be extended to give
Scheme 2 taking into account the platinum hydrogenation activity.
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Scheme 2. Extended mechanism of hydrogenolysis/HDO/hydrogenation of PMG. The aromatic
species are as in Scheme 1; compound 1, 1,2-dihydroxy-4-methyl-cyclohexanol; compound 2, 4-
methyl-cyclohexanone; compound 3, 4-methyl-cyclohexanol; compound 4, 3-methyl-cyclohexanone;
compound 5, 3-methyl-cyclohexanol.
The above discussion considers the behaviour of the catalysts with time and changes in their
selectivity; however it is also worthwhile to compare the selectivity at equivalent conversion. The
results are shown in Figure 5. What is immediately noticeable is that the yield/selectivity of 4-
methycatechol is the same for both Rh/silica (A) and Pt/silica, the catalysts with the same support.
This suggests that the demethylation reaction (the breaking of the ArO-CH3 bond) is favoured over
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Rh(JM) Rh(A) Pt
Figure 5. Selectivity to products at ~32 % conversion.
the fumed silica support rather than the metal function. In contrast dehydroxylation (breaking Ar-
OH bonds) appears to take place over the metal function. This is suggested at by the selectivity of
toluene, where the Rh catalysts give a similar figure but is confirmed by the selectivity to m-cresol
and m-anisole (Rh/silica (JM) and Rh/silica (A) selectivity 0.6 %, Pt/silica selectivity 0.4 %). This in
agreement with Nimmanwudipong et al. who suggested dehydroxylation was performed over the
metal function.6 It is also consistent with the deactivation study, where demethylation activity was
the least affected by carbon deposition.3 There are two routes to the formation of p-cresol, through
4-methylcatechol by dehydroxylation and by demethoxylation direct from PMG. Given that
dehydroxylation is a difficult process, the high selectivity to p-cresol over the Rh/silica (JM)
suggests that demethoxylation of PMG to give p-cresol takes place over the support.
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In conclusion, we have clarified and simplified the mechanism for PMG hydrodeoxygenation and
hydrogenation. Dehydroxylation and hydrogenation are shown to occur over metal functions, while
demethylation and demethoxylation are favoured over the fumed silica support. Only one catalyst
(Rh/silica (A)) gave significant yields of toluene, the full HDO product but the sites responsible were
rapidly deactivated. Overall rhodium was a more effective HDO metal than platinum.
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Experimental
Three catalysts were used in this study 2.5 % Rh/SiO2 (JM), 2.5 % Rh/SiO2 (A) and 1.55 % Pt/SiO2.
3
All catalysts were prepared by impregnation and details of their preparation is reported elsewhere .
The catalysts were ground and sieved to between 350 and 850 ꢀm before use and all reagents and
solvents were purchased from Sigma-Aldrich and used without further purification.
Conversion and yield are defined in equations 1, 2 and 3
Conversion = ((Σ moles products + moles PMG out)-(moles PMG out))/(Σ moles products + moles
PMG out)
(Equation 1)
(Equation 2)
Selectivity = moles product/Σ moles products
Yield = moles product/(Σ moles products + moles PMG out)
(Equation 3)
3
The catalytic testing and analysis methodology has been reported elsewhere but briefly the tests
were performed in a continuous-flow, fixed-bed reactor. The catalysts (~0.5 g) were pre-reduced in-
situ before reaction at 300 °C for 2 h under 100 mL min-1 of 40 % H2/Ar. After the catalysts were
reduced, p-methylguaiacol (PMG) was pumped into the gas flow and vaporised at 200 °C. The
reaction temperature was 300 °C with a hydrogen partial pressure of 4 barg giving a H2:PMG ratio of
15. The total pressure was made up to 10 barg using argon. Typical weight hourly space velocity
(WHSV) of PMG was 2.5 h-1, while the gas hourly space velocity (GHSV) was 7200 h-1. The
products were trapped in a condenser at 5 °C before sampling.
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A full description of the analysis methodology is reported elsewhere . In brief to fully quantify the
products three distinct solutions were prepared from the same mixture of products. Firstly two
internal standards were added, (decane at 0.86 and heptadecane at 10.2 g L-1) before two aliquots
were silylated. Analysis of the three solutions using this technique permitted a full quantification of
minor and major products. The quantitative analyses were performed on an HP 5890 gas
chromatograph fitted with a Supelco DB-5 capillary column (30m × 0.32 mm, 1 mm thickness).
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Acknowledgments
UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the
UK Catalysis Hub Consortium and funded by EPSRC (grants EP/K014706/1, EP/K014668/1,
EP/K014854/1, EP/K014714/1 and EP/M013219/1).
References
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(2) Lup, A. N. K.; Abnisa, F.; Daud, W. M. A. W.; Aroua, M. K. A review on reactivity and
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(3) Bouxin, F. P.; Zhang, X.; Kings, I. N.; Lee, A. F.; Simmons, M. J. H.; Wilson, K.; Jackson, S. D.
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and platinum catalysts, Appl. Catal. A: 2017, 539, 29–37
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catalyzed aqueous phase hydrogenation and hydrodeoxygenation of lignin-derived pyrolysis oil and
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guaiacol on noble metal catalysts, Catal. Today, 2009, 147, 239–246
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