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Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects

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Abstract

A systematic study of the comparative performances of supported Pt, Pd, Ru and conventional CoMo/Al2O3, NiMo/Al2O3, NiW/Al2O3 catalysts as well as the effects of solvent, H2 pressure and temperature on the hydroprocessing activity of a representative model bio-oil compound (e.g., p-cresol) is presented. With water as solvent, Pt/C catalyst shows the highest activity and selectivity towards hydrocarbons (toluene and methylcyclohexane), followed by Pt/Al2O3, Pd and Ru catalysts. Calculations indicate that the reactions in aqueous phase are hindered by mass-transfer limitations at the investigated conditions. In contrast, with supercritical n-heptane as solvent at identical pressure and temperature, the reactant and H2 are completely miscible and calculations indicate that mass-transfer limitations are eliminated. All the noble metal catalysts (Pt, Pd and Ru) show nearly total conversion but low selectivity to toluene in supercritical n-heptane. Further, conventional CoMo/Al2O3, NiMo/Al2O3 and NiW/Al2O3 catalysts do not show any hydrodeoxygenation activity in water, but in supercritical n-heptane, CoMo/Al2O3 shows the highest activity among the tested conventional catalysts with 97 % selectivity to toluene. Systematic parametric investigations with Pt/C and Pt/Al2O3 catalysts indicate that with water as the solvent, the reaction occurs in a liquid phase with low H2 availability (i.e., low H2 surface coverage) and toluene formation is favored. In supercritical n-heptane with high H2 availability (i.e., high H2 surface coverage), the ring hydrogenation pathway is favored leading to the high selectivity to 4-methylcyclohexanol. In addition to differences in H2 surface coverage, the starkly different selectivities between the two solvents may also be due to the influence of solvent polarity on p-cresol adsorption characteristics.

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Abbreviations

\( a_{\text{b}} \) :

Gas–liquid interfacial area per unit volume of reactor, m2/m3

\( a_{\text{p}} \) :

Liquid–solid interfacial area, m−1

\( C_{\text{A}}^{ *} \) :

Saturation solubility of H2 in liquid phase, kmol/m3

\( C_{\text{AS}} \) :

H2 concentration on the catalyst surface, kmol/m3

\( D_{\text{e}} \) :

Effective diffusivity, m2/s

\( d_{\text{i}} \) :

Impeller diameter, m

\( D_{\text{M}} \) :

Molecular diffusivity, m2/s

\( d_{\text{p}} \) :

Particle diameter, m

\( d_{\text{t}} \) :

Reactor diameter, m

\( H_{\text{e}} \) :

Henry’s law constant, kmol/m3/atm

\( h_{\text{l}} \) :

Height of the first impeller from the bottom, m

\( h_{2} \) :

Height of the liquid, m

\( K_{\text{l}} \) :

Liquid film mass-transfer coefficient, m/s

\( K_{\text{l}} a_{\text{b}} \) :

Overall gas–liquid mass-transfer coefficient, s−1

\( K_{\text{s}} \) :

Liquid–solid mass-transfer coefficient, m/s

\( m \) :

Order of reaction with respect to hydrogen

\( M_{\text{w}} \) :

Molecular weight of solvent, g/mol

\( n \) :

Moles of gas at constant pressure, kmol

\( N \) :

Agitation speed, Hz

\( N_{\text{p}} \) :

Power number

\( P_{\text{H2}} \) :

Partial pressure of hydrogen, MPa

\( R \) :

Universal gas constant, kJ/kmol/K

\( R_{\text{H2}} \) :

Overall rate of hydrogenation, (kmol/m3) s−1

\( r_{ \max }^{{}} \) :

Maximum rate of hydrogenation, (kmol/m3) s−1

\( T \) :

Temperature, K

\( V_{\text{g}} \) :

Volume of the gas in the reactor, m3

\( V_{\text{l}} \) :

Volume of the liquid in the reactor, m3

\( w \) :

Catalyst loading, kg/m3

\( {{\upalpha}}_{ 1} \) :

Parameter defined by Eq. 1

\( {{\upalpha}}_{ 2} \) :

Parameter defined by Eq. 3

\( \phi_{ \exp } \) :

Parameter defined by Eq. 12

\( \rho_{\text{l}} \) :

Density of liquid, kg/m3

\( \mu_{\text{l}} \) :

Viscosity of liquid, centipoise

\( \chi \) :

Association factor

\( {{\upupsilon}}_{\text{M}} \) :

Molar volume of the solute, cm3/mol

\( \rho_{\text{p}} \) :

Density of particle, kg/m3

\( \in \) :

Porosity of the catalyst particle

τ :

Tortuosity

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Acknowledgments

Funding for this work was provided by US Department of Agriculture (Grant 2011-10006-30362) and core funds from the Center for Environmentally Beneficial Catalysis (CEBC) at the University of Kansas. Helpful discussions with Drs Juan J. Bravo Suarez and Debdut Roy are gratefully acknowledged.

Author information

Correspondence to Bala Subramaniam.

Appendix: Criteria for Evaluating Significance of Mass-Transfer Limitations

Appendix: Criteria for Evaluating Significance of Mass-Transfer Limitations

(a) Gas–liquid mass-transfer resistance is considered insignificant if

$$ \alpha_{ 1} = \frac{{R_{{{\text{H}}_{ 2} }} }}{{K_{\text{l}} a_{\text{b}} C_{\text{A}}^{ *} }} < 0.1 $$
(1)

where \( K_{\text{l}} a_{\text{b}} \) is the gas–liquid mass-transfer coefficient and is calculated according to the correlation proposed by Gholap and co-workers [52].

$$ K_{\text{l}} a_{\text{b}} = (1.48 \times 10^{ - 3} )(N)^{2.18} (V_{\text{g}} /V_{\text{l}} )^{1.88} (d_{\text{i}} /d_{\text{t}} ){}^{2.16}(h_{1} /h_{2} )^{1.16} $$
(2)

(b) Liquid–solid mass-transfer limitation is considered unimportant if

$$ \alpha_{2} = \frac{{R_{{{\text{H}}_{ 2} }} }}{{K_{\text{s}} a_{\text{p}} C_{\text{A}}^{ *} }} < 0.1 $$
(3)

where \( a_{\text{p}} \), the external surface area of the catalyst per unit volume for spherical particles is given by

$$ a_{\text{p}} = 6w /\rho_{\text{p}} d_{\text{p}} $$
(4)

and \( K_{\text{s}} \) is the liquid–solid mass-transfer coefficient and is estimated by the correlation proposed by Sano and co-workers [53].

$$ \frac{{K_{\text{s}} d_{\text{p}} }}{{D_{\text{M}} F_{c} }} = 2 + 0.4\left[ {\frac{{e(d_{\text{p}} )^{4} \rho_{\text{l}}^{ 3} }}{{\mu_{\text{l}}^{ 3} }}} \right]^{0.25} \left[ {\frac{{\mu_{\text{l}} }}{{\rho_{\text{l}} D_{\text{M}} }}} \right]^{0.333} $$
(5)

where \( F_{\text{c}} \) is the shape factor (assumed to be unity for spherical particles) and \( D_{\text{M}} \) is the molecular diffusivity calculated by using the correlation proposed by Wilke and Chang [54].

$$ D_{\text{M}} = \frac{{(7.4 \times 10^{ - 8} )T(\chi M_{\text{w}} )^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-\nulldelimiterspace} 2}}} }}{{\mu_{\text{l}} {{\upupsilon}}_{\text{M}}^{ 0. 6} }} $$
(6)

and e is the energy supplied calculated by using the correlation proposed by Calderbank [55].

$$ e = \frac{{N_{\text{p}} N^{3} d_{\text{i}}^{5} \psi }}{{\rho_{\text{l}} V_{\text{l}} }} $$
(7)

where \( \psi \) is the correction factor for the presence of gas bubbles calculated by using the correlation proposed by Calderbank [55].

$$ \psi = 1. 0- 1. 2 6\left[ {\frac{{Q_{\text{g}} }}{{Nd_{\text{i}}^{3} }}} \right]\begin{array}{*{20}c} {} \\ \end{array} {\text{for }}\left[ {\frac{{Q_{\text{g}} }}{{Nd_{\text{i}}^{3} }}} \right] < 3.5 \times 10^{ - 2} $$
(8)
$$ \psi = 0.62 - 1. 8 5\left[ {\frac{{Q_{\text{g}} }}{{Nd_{\text{i}}^{3} }}} \right]\begin{array}{*{20}c} {} \\ \end{array} {\text{for }}\left[ {\frac{{Q_{\text{g}} }}{{Nd_{\text{i}}^{3} }}} \right] > 3.5 \times 10^{ - 2} $$
(9)

where \( Q_{\text{g}} \) is the volumetric flow rate (m3/s) of gas calculated by using the formula

$$ Q_{\text{g}} = r_{ \max } V_{\text{L}} V_{\text{M}} $$
(10)

where \( V_{\text{M}} \) is the molar gas volume (m3/kmol) calculated by using the formula

$$ V_{\text{M}} = V/n = RT/P_{\text{H2}} $$
(11)

(c) Pore diffusion resistance can be considered to be insignificant if

$$ \phi_{ \exp } = \frac{{d_{\text{p}} }}{6}\left[ {\frac{{(m + 1)\rho_{\text{p}} R_{\text{H2}} }}{{ 2D_{\text{e}} wC_{\text{A}}^{ *} }}} \right]^{ 1 / 2} < 0.2 $$
(12)

where \( D_{\text{e}} \) is the effective diffusivity and is calculated by using the formula

$$ D_{\text{e}} = D_{\text{M}} ( \in / {{\uptau}}) $$
(13)

If gas–liquid mass-transfer limitation is significant, \( C_{\text{A}}^{ *} \) is replaced by \( C_{\text{AS}} \) which is calculated by using the formula

$$ C_{\text{AS}} = C_{\text{A}}^{ *} - \frac{{R_{\text{H2}} }}{{\left[ {\frac{ 1}{{K{}_{\text{l}}a_{\text{b}} }} + \frac{1}{{K_{\text{S}} a_{\text{p}} }}} \right]^{ - 1} }} $$
(14)

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Wan, H., Chaudhari, R.V. & Subramaniam, B. Catalytic Hydroprocessing of p-Cresol: Metal, Solvent and Mass-Transfer Effects. Top Catal 55, 129–139 (2012). https://doi.org/10.1007/s11244-012-9782-6

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Keywords

  • Hydroprocessing
  • p-cresol
  • Noble metal catalysts
  • Conventional hydrotreating catalysts
  • Water
  • n-Heptane